structure-property correlations in thermoplastic starch

Péter MüllerPhD Thesis

Structure-Property Correlations in Thermoplastic Starch Blends and Composites

© Péter Müller, 2015

Supervisor: Béla Pukánszky

All Rights Reserved

ISBN 978-963-313-191-6

Published by Laboratory of Plastics and Rubber Technology,Department of Physical Chemistry and Materials Science

Budapest University of Technology and EconomicsH-1111 Budapest, Mûegyetem rkp. 3. H ép. 1

Budapest University of Technology and Economics

Structure-Property Correlations in Thermoplastic Starch Blends and Composites

PhD Thesis

Prepared byPéter Müller

Supervisors:Béla Pukánszky

Laboratory of Plastics and Rubber TechnologyDepartment of Physical Chemistry and Materials Science

Budapest University of Technology and Economics

Institute of Materials and Environmental ChemistryResearch Center for Natural Sciences

Hungarian Academy of Sciences

2015

Contents

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Contents Chapter 1……………………………………………………………………………… 9 INTRODUCTION

1.1. Introduction……………………………………………………...……………… 9 1.2. Background………………………………………………………….………...... 10 1.2.1. Starch………………..………………………………………………………… 12 1.2.2. Thermoplastic starch……..…………….... …………………………………... 13 1.2.3. TPS nanocomposites…..………………………………………………….……. 14 1.2.4. TPS reinforced with natural fibers……………………………………….……. 14 1.2.5. TPS blends…….……………………………………………………..…... 15 1.3. Scope…………………………………………………………………………….. 15 1.4. References………………………………………………………………………. 18

Chapter 2…………………………………………………………………………….. 23 THERMOPLASTIC STARCH/LAYERED SILICATE COMPOSITES: STRUC-TURE, INTERACTION, PROPERTIES

2.1. Introduction……………………………………………………...……………… 23 2.2. Experimental………………………………………………………….………... 23 2.3. Results and discussion..………………………………………………………… 24 2.3.1. Structure and exfoliation……………….... …………………………………... 24 2.3.2. Mechanical properties…………..……………………………………….……. 30 2.3.3. Interaction……………………………………………………………….……. 32 2.3.4. Other properties…………………………………………………………..…... 35 2.4. Conclusions…………………………………………………………………….. 36 2.5. References………………………………………………………………………. 36

Chapter 3……………………………………………………………………………… 39 EFFECTS OF PREPARATION METHODS ON THE STRUCTURE AND MECHANICAL PROPERTIES OF WET CONDITIONED STARCH/MONT-MORILLONITE NANOCOMPOSITE FILMS

3.1. Introduction……………………………………………………………….…….. 39 3.2. Experimental……………………………………………………………………. 40 3.2.1. Preparation of plasticized starch/montmorillonite nanocomposite films…….... 40 3.2.2. Characterization……........................................................................................... 41 3.3. Results and discussion.………………………………………………………..... 42 3.3.1. Structure ……………………………..…….…................................................... 42 3.3.2. Moisture content of conditioned nanocomposite films ………………………. 47 3.3.3. Mechanical properties …………... ………..…………………………………. 48 3.3.4. TPS/clay interaction …………………………………………………………. 51 3.4. Conclusions…………………………………………………………………….. 53 3.5. References……………………………………………………………………… 53

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Chapter 4……………………………………………………………………………… 55 THERMOPLASTIC STARCH/WOOD COMPOSITES: INTERFACIAL INTER-ACTIONS AND FUNCTIONAL PROPERTIES

4.1. Introduction…………………………………………………………………….. 55 4.2. Experimental……………………………………………………………………. 56 4.3. Results and discussion………………………………………………………….. 57 4.3.1. Mechanical properties ….…………………………………………………….. 57 4.3.2. Reinforcement ………………………………………………………..………. 59 4.3.3. Interfacial adhesion …………………………………………………………... 60 4.3.4. Functional properties ………………………………………………………… 65 4.4. Conclusions…………………………………………………………………….. 68 4.5. References……………………………………………………………………… 69

Chapter 5…………………………………………………………………………….... 71 INTERACTIONS, STRUCTURE AND PROPERTIES OF PLA/PLASTICIZED STARCH BLENDS

5.1. Introduction…………………………………………………………………….. 71 5.2. Experimental……………………………………………………………………. 72 5.2.1. Materials……………………………………………………………………… 72 5.2.2. Sample preparation…………………………………………………………… 72 5.2.3. Characterization……………………………………………………………… 72 5.3. Results and discussion…………………………………………………...……... 73 5.3.1. Interactions, miscibility….………………………………………...…………. 73 5.3.2. PLA/glycerol interaction.………………………….………………...……….. 77 5.3.3. Modeling………………………………………………..………...…………... 79 5.3.4. Structure………………………………………………………………………. 84 5.3.5. Properties……………………………………………………………………… 85 5.4. Conclusions…………………………………………………………...………… 88 5.5. References……………………………………………………………...……….. 88

Chapter 6…………………………………………………………………………….... 91 PHYSICAL AGEING AND MOLECULAR MOBILITY IN PLA BLENDS AND COMPOSITES

6.1. Introduction…………………………………………………………………….. 91 6.2. Experimental……………………………………………………………………. 92 6.2.1. Materials……………………………………………………………………… 92 6.2.2. Sample preparation…………………………………………………………… 93 6.2.3. Characterization……………………………………………………………… 94 6.3. Results and discussion…………………………………………………...……... 94 6.3.1. PLA/TPS blends…………………………………………………...…………. 94 6.3.2. PLA/glycerol blends…………………………….…………………...……….. 99 6.3.3. Comparison, other blends and composites…………..…………...…………... 101 6.3.4. Physical ageing, interactions…………………………………………………. 104 6.4. Conclusions…………………………………………………………...………... 106

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6.5. References……………………………………………………………………. 106 Chapter 7…………………………………………………………………………..….109 SUMMARY LIST OF SYMBOLS………………………………………………………………..... 113 ACKNOWLEDGEMENTS………………………………………………………….. 117 PUBLICATIONS………….…………………………………………………………. 119

Introduction

9

Chapter 1 1.1. INTRODUCTION

Following a decline as a result of the 2008 financial crisis, the production of plas-tics increases continuously, and in 2010, it reached the value of 265 Mt worldwide and 57 Mt in Europe [1]. In the same year, European plastics converters processed 46.4 mil-lion tons into products, approximately 40 % of these being short service life applications, mainly for packaging purposes, resulting in 24.7 Mt of post-consumer waste [1]. Not sur-prisingly, the related environmental concerns have also increased in recent decades strengthening efforts to decrease the ecological effect of polymeric materials. There are various ways to decrease the environmental impact of plastics and one of the most advan-tageous methods is composting [2]. Accordingly biodegradable and compostable poly-mers have found application in various fields. Bio-based polymers, i.e. polymers pro-duced from renewable feedstock, biomass in general, will at least partially replace fossil sources resulting in considerable environmental benefits like decreased carbon-dioxide emission.

Although the term "biopolymer" is used in several different ways depending on the application area, the generally accepted definition covers polymers that are either re-newable-based, biodegradable or both. The global production capacity of these materials shows dynamic growth [3,4] and the ratio of biodegradable polymers compared to non-degradable biobased types has also increased recently [4]. One of the reasons leading to this trend might be the considerable changes in legislation related to compostable prod-ucts in recent years. The relative importance of bio-based and biodegradable grades in polymer production might further increase in the future as production technology im-proves and becomes more cost-effective. According to various estimates, only less than 4 % of world biomass is utilized by humanity, the majority for food-related, while only a fraction for chemical applications and plastics production [3] indicating that there is tre-mendous room for the further increase of capacity yet.

Biopolymers have much potential and several advantages, but they possess some drawbacks as well. In spite of increasing production capacity, they are still quite expen-sive compared to commodity polymers and their properties are also often inferior, or at least do not correspond to the expectation of converters or users. Although natural poly-mers are available in large quantities and they are also cheap, their properties are even farther from those of commodity plastics. As a consequence, biopolymers must be often modified to meet the expectations of the market. To utilize their potentials and penetrate new markets, the performance of biopolymers must be increased considerably. Conse-quently, the modification of these materials is in the focus of scientific research, but also industrial development. In contrast to the development of novel polymeric materials and new polymerization routes, modification is a relatively cheap and fast method to tailor the properties of plastics. As a result, this approach may play a crucial role in increasing the competitiveness of biopolymers.

Chapter 1

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The Laboratory of Plastics and Rubber Technology of the Department of Physical Chemistry and Materials Science at the Budapest University of Technology and Econom-ics together with the Institute of Materials and Environmental Chemistry at the Hungarian Academy of Sciences have considerable experience in the modification of synthetic and natural polymers. Polymers can be modified in many ways including chemical modifica-tion, like copolymerization or grafting, or by the addition of another polymer, a filler or fiber to the polymer. The combination of a polymer with another component always leads to a new material the properties of which must be optimized in order to meet the require-ments of a potential application. However, successful optimization requires the deep knowledge of structure-property correlations and since biopolymers are relatively new materials relatively limited knowledge is available on them. The experience of the group in this field led to various projects related to natural and biodegradable materials. As early as 1992, the group worked on the plasticization of starch and plasticized starch/polyolefin blends together with a Swiss company, Fluntera. Later it participated in various European projects including a Eureka and an FP7 project on wood reinforced polypropylene and poly(lactic acid) which is a biopolymer produced from natural raw materials. Starch is a bio based and biodegradable polymer which is available in large quantities and very cheap. Starch plasticized with small molecular weight materials (TPS) behaves like a plastic and can be processed with technologies used for thermoplastics. Unfortunately similarly to other biopolymers, starch also has drawbacks and needs modification. The experience of the group in plasticized starch and the general interest in fully biodegrada-ble materials led to a project which focused on the modification of TPS with the goal to improve its properties and compensate for its deficiencies like poor mechanical proper-ties, physical ageing, water sensitivity and large shrinkage. The project resulted in several BSc and MSc theses, publications and the idea of a patent which might find practical application as well. This thesis summarizes the most important scientific results of the project, its main conclusions and further potentials. 1.2. BACKGROUND

By the end of the millennium, research on biopolymers became a very important area. Bio-based polymers, i.e. polymers produced from renewable feedstock, biomass in general, might replace fossil sources and also have considerable environmental benefits like decreased carbon-dioxide emission. Natural polymers represent a specific class of materials among polymers based on natural resources. Fig. 1.1 presents worldwide bi-opolymer production capacities according to the type of the polymer [4] indicating that starch and its blends, poly(lactic acid) and various types of polyhydroxyalkanoates (PHAs) are of the largest importance among bio-based and biodegradable polymers. The structure of the most important biopolymers is presented in Fig. 1.2.

Introduction

11

6 %

5 %

7 %

8 %12 %

16 %

28 %

2 %

1 %

15 %

BioPET BioPA Cellulose based PLA blends Other

Bio-PE Starch blends PLA PHA Biodegradable polyester

Figure 1.1 Worldwide production capacities of bioplastics in 2010 [4].

Figure 1.2 The chemical structure of the most important biopolymers: a) polyhydrox-

yal-kanoates; b) poly(lactic acid); c,d) the main components of starch, am-ylose and amylopectin, respectively [5].

Chapter 1

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1.2.1. Starch

Starch is a semicrystalline polymer and it represents the major form of stored car-bohydrate in plants. Starch is composed of repeating α-d-glucopyranosyl units, a mixture of two substances, an essentially linear polysaccharide, amylose and a highly branched polysaccharide-amylopectin. In amylose the repeating units are linked by α-(1-4) link-ages; the amylopectin has an α-(1-4)-linked backbone and ca. 5 wt% of α-(1-6)-linked branches [6-9]. The relative amounts of amylose and amylopectin depend upon the bo-tanical source. Corn starch granules typically contain approximately 70 wt% amylopectin and 30 wt% amylose [10]. The properties of starch depend strongly on the ratio of these two components. Starch has numerous advantages and several drawbacks. It is cheap and available in abundant quantities, produced from renewable resources and completely bi-odegradable. On the other hand, it is sensitive to water, its processing is difficult and its properties are usually inferior to commodity polymers. Because of strong hydrogen bonds developing among the molecules, neat starch cannot be melted and processed without further treatment [11,12]. Neat starch has a high glass transition temperature, and its rel-ative large modulus and strength is accompanied by poor deformability and impact re-sistance due to the limited conformational mobility of its stiff chains [13].

Depending on the amount of water used during processing, starch is either gelati-

nised or melted. In the bakery industry, partly gelatinised starch products are obtained as a result of the large amount of water used in dough. Also in other food applications, like in puddings or soups and in non-food applications like paper sizing agents, adhesives and coatings, starch is gelatinized. When applied in bioplastics, either as filler in plastics, grafted onto other polymers or as a starch based polymer, small water contents are used during processing and starch is partially melted.

Processingat small watercontent

Amount of remaining structureExtent of destructuring

Processingat large watercontent

gelatinisedstarch

Bakery products(bread, cakes)

BioplasticsThermoplasticstarch

destructurizedstarch

Figure 1.3 Classification of starch based systems.

Introduction

13

In Figure 1.3 starch products are classified with respect to the water content ap-plied and the extent of destructuring occurring during processing. 1.2.2. Thermoplastic starch (TPS)

Thermoplastic starch is produced when granular starch is processed at small water contents using thermal and mechanical forces in the presence of (polyol) plasticisers that do not evaporate during processing. The procedure yields basically amorphous starch. Several other additives like lipids, lecithin and glycerol monostearate are also used to improve flow properties. The term destructurised starch (DS) is used for granular starch that is turned into a homogeneous, mainly amorphous starch mixture using thermal and mechanical forces, irrespective of the additives used [14].

Chemical modification and plasticization partially resolves some of the technolog-ical problems and plasticized starch can be processed by extrusion, injection molding or other processing methods [15-19]. The nature and concentration of plasticizer strongly influence the rheological and mechanical properties of TPS. Numerous studies and pa-tents describe the plasticization of starch using either low molecular mass or polymeric compounds such as water, glycerol, urea and formamide [20].

Increasing plasticizer content brings about a decrease in the tensile strength of thermoplastic starch, while its elongation-at-break increases. Starch is a natural polymer with numerous hydrogen bonds forming among the hydroxyl groups of its molecules, which leads to reasonable tensile strength values. Glycerol, sorbitol, or glycol behave as diluents and decrease the interaction among starch molecules and thus tensile strength decreases as well. At the same time macromolecular mobility increases leading to larger deformability [21,22].

The increase in elongation-at-break with increasing plasticizer content occurs in a certain range of glycerol content. Above 35 wt%, elongation-at-break decreases. This ef-fect caused by excessive amounts of glycerol results from the plasticization effect of the compound, from the decrease of intermolecular forces among starch molecules [21,23-25]. Similarly, an increase in water content in the blend induces a decrease in the tensile strength of TPS and an increase in its deformability. Similarly to plasticizers, if water content exceeds 35 wt%, elongation-at-break decreases again [26].

During the storage of TPS, amylose and amylopectin recrystallizes in some extent. At longer storage periods, resulting in larger crystallinity of TPS, tensile strength in-creases and elongation-at-break decreases with storage time. Increased moisture content during storage intensifies the changes in mechanical properties [27,28].

Besides the advantages (processability, flexibility, biodegradability, etc.) of plas-ticized starch, it also has disadvantages like poor mechanical properties, water sensitivity, poor dimensional stability, etc. To improve the mechanical properties of TPS based ma-terials, other additives can also be applied including emulsifiers, cellulose, plant fibers, bark, kaolin, pectin, and others [29-34].

Chapter 1

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1.2.3 TPS nanocomposites

One way to improve the properties of natural polymers is to modify them by add-ing layered silicates. The usual claim that the hydrophilic clay is not compatible with hydrophobic polymers [35-37] is not valid in this case; the active hydroxyl groups of starch are strongly hydrophilic and may interact with layered silicates. In the past decades numerous attempts were reported in the literature on the preparation of nanocomposites with bio-based matrices [38-40].

Polymer/clay nanocomposites are assumed to exhibit improved barrier, thermal and mechanical properties compared to traditional composites. In most cases TPS/mont-morillonite nanocomposite films were prepared by melt blending (in internal batch mixer or in a twin screw extruder) [40-45] or solution mixing (film casting) [41,43,46,47]. The results clearly demonstrated that the incorporation of organophilic montmorillonite with apolar character led to the formation of conventional microcomposites, while due to the polar nature of both starch and Na-montmorillonite (NaMMT) the application of such a clay results in an intercalated/exfoliated structure of TPS nanocomposites [41,43]. In spite of the incomplete exfoliation of the silicate, TPS/NaMMT nanocomposites have im-proved properties compared to neat TPS. The properties of these latter strongly depend on the type of the starch and the montmorillonite used, as well as on the amount of MMT and glycerol.

Mostly various aliphatic polyesters are combined with layered silicates organ-ophilized with a variety of surfactants. Usually extensive intercalation or exfoliation is claimed resulting in significant improvement in properties including stiffness, strength and most importantly water resistance [48-51]. Much less information is available about nanocomposites prepared from thermoplastic starch and related materials. 1.2.4. TPS reinforced with natural fibers

Another way to improve the properties of TPS is to reinforce it with natural fibers. [40,52-57]. A considerable number of papers deal with the effect of natural fibers on the properties of thermoplastic starch. Most of them focus on the influence of fiber type and amount, but usually determining properties only at one or two fiber contents. All kinds of fibers have been used as reinforcement in TPS including various forms of cellulose [52]; (cellulose fibers from leafwood); [53] (wheat straw fibers, hemp fibers, cotton linter fi-bers); [58] (bleach pulp fiber); [59] (micro winceyette fibers); [60] (bacterial cellulose and vegetable cellulose fibers); [61,62] (bacterial cellulose); [63] (jute and flax fibers); [64]), sisal [55-57], wheat straw [53], hemp [53,65,66], cotton [53,67,68], flax [63,69], ramie [64,70], etc. Somewhat less papers deal with TPS/wood composites [54,71-73], although wood is cheaper and simpler to handle during processing. Some of the papers cited investigates the effect of fiber characteristics on mechanical properties and conclude that stiffness and strength increase both with increasing fiber length and content. [52].

Introduction

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1.2.5. TPS blends

The preparation of polymer blends is a further approach to improve the poor prop-erties of thermoplastic starch. The research on TPS combined with synthetic polymers like polyethylene (PE) or polystyrene (PS) had been started decades ago resulting in a large number of papers about the properties, structure, interactions and biodegrability of such blends. Even though the blends of TPS with synthetic polymers are only partially biodegradable – which occasionally can be more harmful than useful – several companies offer such products and numerous papers have been published in this research area [74,75].

The biocompatibility and biodegrability of TPS can be retained if it is blended

with biopolymers. The biopolymers most often used for the preparation of biodegradable TPS blends are polycaprolactone (PCL) [76-80], poly(lactic acid) (PLA) [81-86], poly(vi-nylalcohol) (PVA) [87,88], poly(hidroxyalcanoates) (PHA) (poly(hydroxybutyrate) (PHB), poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) [89-93], poly(butylene suc-cinate-co-adibate) (PBSA) [94,95], poly(butylene adipate-co-terephthalate) (PBAT) [94,96-99] and polyesteramide (PEA) [94,99,100].

One of the biopolymers used and studied the most frequently in such blends is PLA. PLA is produced from renewable resource [101] and the increase in production capacity decreased its price to reasonable levels. PLA also has several drawbacks. Its slow crystallization [102,103] and fast physical ageing [104,105] results in instable structure and rapidly changing properties, which might result in brittleness, fast deterioration of properties and finally catastrophic failure. As a consequence, PLA is often modified by various means including copolymerization [106-108], plasticization [86,109,110], blend-ing [111,112] and the incorporation of various fillers [113-116] and reinforcements [117-119]. These components modify all properties of the polymer often in an unpredictable manner; contradictory results are reported on the effect of such modifications in the liter-ature. One of the drawbacks of PLA is its poor impact resistance, which might be im-proved by blending with thermoplastic starch. Accordingly, the structure and the proper-ties of TPS/PLA blends are of theoretical and practical interest and the investigation of interactions between PLA and TPS is amply justified. 1.3. SCOPE

As described in the introductory part of this Thesis and in the background infor-mation supplied, the interest in biobased and biodegradable polymers increased consid-erably in recent years and the tendency points towards even further development in this area. As a consequence, the use of natural and biodegradable polymers as matrix materials or at least components of plastic products is in the forefront of research and it is industrial practice in certain areas. However, the structure and behavior of these materials are com-plex and complicated thus intensive research is needed for further development and to obtain materials with acceptable properties. Because of its expertise, our group was in-vited quite frequently to participate in various projects related to biopolymers. We worked on plasticized starch at an early date, on the modification of a natural polymer and then

Chapter 1

16

spent considerable time on the development of polymer composites reinforced with wood fibers. The work resulted in numerous BSc, MSc and PhD theses during the years. One of our first project was related to the modification of cellulose acetate by reactive pro-cessing in which we studied the chemistry and the resulting structure and properties of the material obtained [120]. Subsequently, our attention was focused on the use of wood as reinforcement in various polymers. The first thesis in this area dealt with interfacial interactions in PP/wood composites [121], while the second on the study of microme-chanical deformation processes in heterogeneous polymers systems generally and in wood reinforced PP particularly [122]. The demand to prepare fully biodegradable mate-rials led to the investigation of PLA/wood composites; another thesis is under preparation in this area. The information obtained in these studies resulted in extensive knowledge on such materials and paved the way for the current Thesis.

The use of natural and biodegradable synthetic polymers only as part of a product does not satisfy the demand of complete biodegradability thus PP/wood composites can be of economic interest for the industry, but further development is directed towards en-vironmentally more benign materials. The combination of aliphatic polyesters and natural polymers, like PLA/wood composites is one step into this direction yielding fully biode-gradable materials. Cellulose and starch are produced in very large quantities each year, thus it seems to be obvious to use them for the production of a larger number of products. Starch is especially advantageous, since it can be plasticized and the resulting TPS can be processed very efficiently with the technology of thermoplastic polymers. On the other hand, starch is a complex material with a complicated structure, strong interactions and a number of deficiencies. Similarly, even PLA produced by a synthetic route has draw-backs, like fast physical ageing, brittleness resulting in poor impact resistance. Intensive research is going on to overcome such problems all over the world and considering the expertise of our group it seemed to be obvions to join this effort and work on the modifi-cation of biopolymers. In this Thesis attention is focused mainly on plasticized starch. The results of our effort to modify starch with various strategies from the use of nano-fillers, through wood fibers and blending are reported in this Thesis.

Major deficiencies of plasticized starch products are their poor mechanical prop-erties and water sensitivity. Poor mechanical properties result from the use of a large amount of small molecular weight plasticizer which replaces hydrogen bonds among starch molecules thus make them flexible and allow their processing. On the other hand, the stiffness of the amylopectin chain prevents the formation of entanglements thus the deformability as well as strength of plasticized starch are small. Plasticizers are usually molecules containing numerous active hydroxyl groups, thus water sensitivity does not decrease, but increases as a result. One obvious way to improve both mechanical proper-ties and water sensitivity is to combine plasticized starch with nanofillers. Layered sili-cates are claimed to improve the mechanical properties of polymers already at very small amounts, at several percent silicate content. However, the condition of property improve-ment is the complete, but at least extensive exfoliation of the clay in the polymer matrix that is very difficult to achieve. In order to facilitate exfoliation the surface of the clay is modified to decrease interactions among the individual layers. Using previous experience with layered silicate nanocomposites, we attempted the improvement of TPS properties

Introduction

17

through this route. The results obtained on TPS/layered silicate nanocomposites are re-ported in Chapters 2 and 3. First silicates with various surface modifications were ho-mogenized into TPS by melt processing in an internal mixer. Neat sodium montmorillo-nite was used as reference and it was compared to silicates modified with amino acid, hydroxyl functional and aliphatic surfactants. The structure and properties of the nano-composites were characterized extensively and the effect of competitive interactions act-ing among the components was analyzed in detail. The results and conclusions of the study are reported in Chapter 2. Although the type of surface modification had a large effect on the structure and properties of the composites, complete exfoliation could not be achieved and thus the desired improvement in properties was not reached either. In order to come closer to our goal, another approach was selected in the next step. Unmod-ified sodium montmorillonite exfoliates completely in water. Glycerol, used as plasticizer for TPS is miscible with water, while at elevated temperature starch can also be dissolved in water. As a consequence, nanocomposite films were prepared by solvent casting and their structure and properties were compared to those of plates produced by traditional melt blending. The results are reported in Chapter 3 and show that preparation technology has a major impact on the extent of exfoliation, transparency and mechanical properties of the composites.

Although modification of TPS with a layered silicate through the solvent casting approach improved properties considerably, the method lacks the benefit of easy pro-cessing by a thermoplastic technology. Another possibility to improve the properties of thermoplastics starch is to reinforce it with wood fibers. The approach has the additional benefit of yielding an extremely cheap material. However, the type and amount of fibers must be optimized and the interaction of the components is an unknown, but important factor strongly influencing properties. The results of experiments carried out to investi-gate the possibility of modifying starch with wood are reported in Chapter 4. Starch plas-ticized with 36 wt% glycerol was reinforced with three wood flours of different particle characteristics. The results clearly showed that particle size and aspect ratio are important factors determining properties and the right combination of characteristics leads to mate-rials with advantageous properties. The results obtained in this project opened up the pos-sibility to produce encapsulated fertilizers and the application for a patent is explored at the moment.

As mentioned before, plasticized starch has relatively poor mechanical properties; its stiffness, strength and deformability are quite small. Some groups try to overcome this deficiency by using less, around 20 wt%, plasticizer. However, processability is quite limited for such materials and they are also very brittle. PLA, on the other hand, has con-siderable stiffness and strength. One may hope that the combination of the two materials, i.e. TPS and PLA, might result in materials that combine the benefits of both polymers and result in blends with useful properties. Many examples are found in the literature which report the blending of plasticized starch with polymers, mainly with aliphatic pol-yesters to preserve the benefit of full biodegradability. In a project, we selected to follow this approach and prepared plasticized starch/PLA blends in the entire composition range. The results are reported in Chapter 5. The goal of the study was to investigate the phase structure of the blends, the interaction of the components and the resulting properties. Plasticized starch is said to phase separate above a certain plasticizer content resulting in

Chapter 1

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a two phase material. Blending a third phase into this polymer would result in complicated structure and unpredictable properties. Another issue that is largely ignored in the litera-ture is the role and possible partitioning of the plasticizer in starch/polymer blends. Glyc-erol is a mobile, small molecular weight compound that can diffuse into the second pol-ymer, but TPS is treated as a uniform, single material in the literature. Researcher do not even consider the diffusion of glycerol into the aliphatic polyester, or any other polymer, combined with starch. As a consequence, we explored these question, i.e. the competitive interaction of the components, the possible partitioning of glycerol, phase structure and mutual reinforcement of the phases.

The next chapter of the Thesis differs somewhat from the rest. PLA was modified with TPS in this project, but we combined it also with other materials during the years, with a CaSO4 filler, wood fibers, glycerol and various polymers. Considerable infor-mation was accumulated during these projects about the thermal properties and behavior of PLA and similarities were observed in spite of the very dissimilar characteristics of the components. These observations are summarized in Chapter 6 of the Thesis. The detailed analysis of the glass transition and cold crystallization behavior of PLA in the presence of these second components allowed us drawing of general conclusions about the molec-ular mobility of the polymer and its effect on structure and properties. Although we could not arrive to an unambiguous correlation between increasing mobility and additive prop-erties, the observations might be useful in the explanation of the behavior of PLA and also in material development. In the final chapter of the Thesis, in Chapter 7, we briefly summarize the main results obtained during the work, but refrain from their detailed discussion, because the most important conclusions were drawn and reported at the end of each chapter. This chapter is basically restricted to the listing the major thesis points. The large number of experimental results obtained in the research supplied useful information and led to sev-eral conclusions, which can be used during further research and development related to the preparation of modified plasticized starch materials, but also in other areas of hetero-geneous polymers. As usual, quite a few questions remained open in the various parts of the study, their explanation needs further experiments. Research continues in this area at the Laboratory and we hope to proceed successfully further along the way indicated by this and by previous Theses. 1.4. REFERENCES 1. Plastics Europe. Plastics – The Facts 2011 – An analysis of European plastics

production, demand and recovery for 2010. <http://www.plasticseurope.org/docu-ments/document/20111107101127-final_pe_factsfigures_uk2011_lr_041111 .pdf>

2. Kale, G., Kijchavengkul, T., Auras, R., Rubino, M., Selke, S. E., Singh, S. P., Macromol. Biosci. 7, 255-277 (2007)

3. Shen, K., Haufe, J., Patel, M., Product overview and market projection of emerging bio-based plastics. Final report of Utrecht University to European Bioplastics 2009.

4. European Bioplastics. Driving the evolution of plastics. <http://en.european-bioplas tics.org/wp-content/uploads/2012/publications/imagebroschuere_Dec2012.pdf>

Introduction

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PhD Thesis, Budapest 2010

Thermoplastic starch/layered silicate composites

23

Chapter 2 Thermoplastic starch/layered silicate composites: structure, interac-

tions, properties1

2.1. INTRODUCTION

Recently several attempts were reported in the literature for the preparation of nanocomposites with biodegradable matrices [1-3]. McGlashan and Halley [4] combined a polyester with TPS in order to prepare biodegradable nanocomposites. They blew films from TPS/polyester blends and found a singnificant improvement in processability as well as mechanical properties. The extent of property improvement depended very much on the dispersion of the clay. The information on nanocomposites with all-TPS matrices are scarce and contradictory. Fischer [5] reported the preparation of the perfect material from TPS and clay. The homogenous incorporation of clay intensified the destructuriza-tion process of starch, led to easier processing, decreased the hydrophilic character of TPS and improved stiffness as well as strength. Unfortunately, he failed to give any details on the composition, preparation or actual characteristics of this wonderful material in his review paper [5]. Huang et al. [3] prepared composites from sodium montmorillonite (NaMMT) and thermoplastic starch. The crystallinity of starch decreased as an effect of modification, which they explained with the strong interaction of –OH groups attached to starch and NaMMT, respectively. They reported also an improvement of water resistance, which meant a decrease of water absorption from 23 to 19 %. Apart from the fact that –OH groups can be found only at the edges of MMT platelets constituting less then 5 % of the total surface of the clay, some other results are also difficult to explain. The mechan-ical properties of their TPS, for example, are similar to those of our material, although the glycerol content of their starch was three times larger. They also achieved a tenfold in-crease in strength as an effect of modification.

In view of the increased interest in natural polymers and related materials and be-

cause of the contradictions mentioned above, the goal of our study was to prepare TPS/clay composites from NaMMT and various organophilized silicates and to determine their structure and properties. We were also interested in the influence of component in-teractions on intercalation and/or exfoliation and on the resulting properties of the com-posites prepared. 2.2. EXPERIMENTAL

High quality wheat starch produced in Hungary (Szabolcs Keményítő Ltd.) plas-ticized with 36 wt% glycerol (Reanal, Hungary) was used in the experiments. This glyc-erol content was selected in preliminary experiments and was kept constant throughout the study. A sodium montmorillonite with a specific surface area of 780 m2/g and ion ex- ___________________________________________________________ 1Bagdi K., Müller P., Pukánszky B.: Thermoplastic starch/layered silicate composites: structure, interaction, properties, Composite Interfaces, 13, 1 (2006)

Chapter 2

24

change capacity of 90 mEq/100 g, as well as three organophilic clays were used as fillers. These later were obtained from Süd Chemie GmbH (Germany) and they were treated with aminododecanoic acid (Nanofil 784), stearyl dihydroxyethyl ammonium chloride (Nan-ofil 804) and distearyl dimethyl ammonium chloride (Nanofil 948), respectively. Silicate content changed between 0 and 7 vol% in seven steps.

The silicate was swollen in glycerol for 1 day and then homogenized with the

starch powder. The blend was introduced into an internal mixer (Brabender W 50 EH) and homogenized at 150 C for 10 min. 1 and 2 mm thick plates were compression molded from the melt at 150 C, for 5 min. The plates were stored under dry conditions until further study. Specimens were cut from the plates for the determination of various char-acteristics.

The crystalline structure of TPS and the gallery structure of the fillers were studied

by X-ray diffraction (XRD) using a Phillips PW 1830/PW 1050 equipment with CuK radiation at 40 kV and 35 mA. The measurements were done on 2 mm thick compression molded plates. The light transmission of the samples was determined using a UV-VIS spectrometer (Spekol 840) at various wavelengths. Only results obtained at 700 nm are reported here. The color of the plates was measured with a ColorQuest 45/0 apparatus (Hunterlab). Color coordinates were determined and standard yellowness index was cal-culated to characterize the extent of discoloration. Dynamic mechanical (DMTA) spectra were recorded using a Polymer Labs MKII spectrometer at 1 Hz frequency between -100 and +100 C with 3 C/min heating rate. Tensile properties were determined using an Instron 5566 apparatus at 10 mm/min cross-head speed on specimens with 10 x 1 mm cross-section. Gauge length was 50 mm. Five parallel specimens were measured to gen-erate the data reported in the Chapter. Water absorption was measured by storing the samples in 50 % relative humidity and following the change of their weight. 2.3. RESULTS AND DISCUSSION

Results are reported in several sections. First we discuss the structure of the TPS matrix and that of the silicate. The possibility and extent of intercalation and/or exfolia-tion is analyzed in this section. Subsequently we present various characteristics of the composites and discuss the effect of competitive interactions on composite structure and properties. 2.3.1. Structure, exfoliation

The hydroxyl groups in starch form hydrogen bonds resulting in a stiff structure with high glass transition temperature and viscosity. Starch starts to degrade before it melts [6] thus its processing with the usual technologies of thermoplastic polymers is possible only after plasticization. Starch can be plasticized by most hydrogen bonding compounds including water, glycerol, amines, etc [7,8]. The combined effect of plastici-zation and processing leads to the disruption of its crystalline structure, to a completely amorphous material. Amorphous starch recrystallizes upon storage and the developed crystal form as well as crystallinity depends on the water content of the starch [9-11].


25

XRD patterns were recorded on plasticized starch and its composites in order to follow changes in the crystalline structure. The XRD traces of starch powder, TPS and a compo-site containing 2 vol% Nanofil 948 (N948) silicate are presented in Fig. 2.1.

5 10 15 20 25 30

b)

c)

a)

Inte

nsity

(cou

nts)

Angle of reflection, 2

Figure 2.1 XRD pattern of a) starch, b) TPS and c) that of one of its composites. Silicate:

2 vol% Nanofil 948.

Plasticization and processing change the structure of starch completely. The ma-jority of TPS is amorphous, but a certain amount of crystalline phase is also present, which differs completely from that of neat starch. A more detailed study of the patterns and comparison to literature references indicate that the peaks detected in the XRD pat-tern of TPS correspond to the VA modification of starch forming at low water content [9]. Crystalline form or crystallinity is influenced only slightly by the presence of the silicate as shown by the comparison of the two traces belonging to TPS and to its composite, respectively. This statement is valid for all the composites prepared, thus we refrain from the presentation of the traces detected at higher 2 angles (>10 ) as well as from the discussion of starch crystallinity. We may conclude that the incorporation of silicates af-fect the post-crystallization of starch only in a limited extent, which contradicts the ob-servation of Huang et al. [3].

The characteristic reflection of organophilic montmorillonite can be also observed

at small 2 angles, i.e. at 2.9 and 6.4 , in Fig. 2.1. Although the presence of the peak is unambiguous, it strongly overlaps with the shift of the baseline in this region.

Chapter 2

26

2 4 6 8 10

A)

Inte

nsity

(cou

nts)


Figure 2.2 Difference XRD traces of TPS/layered silicate composites containing various amounts of N948 clay. Silicate content: 0.5, 1.0, 2.0, 3.0, 5.0, and 7.0 vol% from bottom to top. The pattern of N948 was added as reference (A).

In Fig. 2.2, the XRD pattern of TPS was deduced from that of the composites in order to facilitate evaluation. The appearance of the silicate reflection with relatively high intensity indicates that the clay is not exfoliated. Intercalation or at most partial exfolia-tion may occur during processing. Moreover, the intensity of the reflection seems to in-crease proportionally with clay content as shown in the figure. The very intensive reflec-tion of the organophilic silicate is a consequence of a high degree of order and/or orien-tation of the clay layers. The location of the reflection is at larger 2 angle (2.6-3.0 ) in the composites than in the neat N948 (see reference pattern, A), but shifts towards smaller angles with increasing silicate content indicating that the gallery structure of the clay changes slightly as an effect of processing. A similar observation could be made also for the rest of the silicates, although only small changes could be observed practically in each case.

The XRD pattern of the composites containing the various silicates is compared

to each other at 2 vol% clay content in Fig. 2.3. Considerable differences can be observed both in the location and the intensity of the silicate reflection. The peak of N784 and N804 is located at relatively large 2 angles, i.e. at 5.3 and 5.8 , respectively, and the intensity is rather small. This indicates a small degree of surface modification and the absence of much surplus surfactant in the galleries of the silicate. A larger amount of the aliphatic


27

amine was used to prepare Nanofil 948, its gallery distance is larger and also the intensity of the reflection is stronger than that of the rest of the silicates.

The behavior of neat NaMMT is very interesting. The silicate reflection is located

at around 5 , which is unusual for MMT even at relatively high water content. The gallery distance of dry montmorillonite is approximately 1 nm [12], which would result in a re-flection at around 9 . Moreover, the intensity of the silicate content of dry MMT is very small, usually a broad small intensity peak is detected in composites prepared with such fillers [13]. The XRD traces of TPS/NaMMT composites indicate that swelling the sili-cate in glycerol results in the adsorption of some of the plasticizer and an extension of gallery distance to around 1.8 nm, which might be further modified during processing. This result is in accordance with the finding of Ishida et al. [14,15], who showed that the use of a small amount of swelling agent may facilitate intercalation or even lead to com-plete exfoliation.

2 4 6 8 10

d)

c)

b)a)

Inte

nsity

(cou

nts)


Figure 2.3 Comparison of the gallery structure of TPS/layered silicate composites. Sil-icate content: 2 vol%. Clay: a) NaMMT, b) N784, c) N804, d) N948.

Further information is obtained about the possible interaction of the components

from Fig. 2.4, in which we plotted the gallery distance of the silicates against the clay content of the composites. The gallery distance of NaMMT increases from the initial value of 1.25 nm to 1.78 nm, then a further slight increase occurs with increasing filler content. The gallery distance of N784 increases slightly as an effect of processing, but decreases significantly from this maximum with increasing filler content.

Chapter 2

28

0 2 4 6 81.5

1.6

1.7

1.8

1.9

Gal

lery

dis

tanc

e (n

m)

Silicate content (vol%)

Figure 2.4 Dependence of the gallery distance of the silicates on composition in

TPS/clay composites. Symbols: ( ) NaMMT, ( ) N784, ( ) N804.

The layer distance of N804 decreases drastically upon composite preparation and also with increasing filler content. The results indicate that competitive interactions take place during mixing. The polymer or glycerol may enter into the galleries of the filler or surfactant diffuses out of it dissolving in TPS. Obviously mostly this latter occurs in the case of N784 and N804, which might interact with glycerol and TPS because of their chemical structure. N784 treated with a surfactant containing an acid functionality may even react chemically with the active –OH groups of the components. On the other hand, glycerol obviously diffuses into the galleries of neat NaMMT. We omitted the filler treated with the aliphatic amine from the figure, because of its much larger gallery dis-tance and the slight changes in it. The surfactant with the long aliphatic chains probably does not dissolve in glycerol and TPS very well, but compete with the plasticizer for the active places on the surface of the silicate instead. Such competitive processes were ob-served also by Li and Ishida [15], who studied the intercalation of clay by polymers in the presence of solvents. They found interesting changes in gallery distance as a function of the concentration of the solvent.

Fig. 2.5 presents the dependence of the intensity of silicate reflection on clay con-

tent. Intensity increases almost linearly with filler content, which indicates that exfolia-tion does not take place during processing or always the same fraction of the silicate ex-foliates independently of clay content. The differences in the intensity of the various sil-icates may be caused by dissimilarities in gallery distance, order or orientation.


29

0 2 4 6 80

500

1000

1500

2000

2500

3000

Inte

grat

ed in

tens

ity


Figure 2.5 Integrated intensity of the silicate reflection plotted against the composition

of TPS/clay composites. Symbols: ( ) NaMMT, ( ) N784, ( ) N804, ( ) N948.

Further information can be obtained about the structure of the composites from their light transmission. Particles, which are smaller than the wavelength of incident light are transparent, while larger ones or aggregates scatter light, make the material opaque. Published results indicate that the transparency of nanocomposites increases with increas-ing extent of exfoliation [16-18]. Fig. 2.6 shows the transparency of our composites as a function of silicate content. The light transmission of PVC/NaMMT composites is also plotted as reference [18]. We cannot expect any exfoliation to occur in this latter case. Large differences can be observed in the light transmission of the composites containing the various clays. The transparency of composites prepared from N948 and N804 remains large in the entire composition range, while that of the other two materials decreases sig-nificantly. We must call attention here to the behavior of N784. Its composites are trans-parent up to 3 vol% silicate content but light transmission decreases considerably at larger concentrations. The change might be related to the collapse of galleries shown in Fig. 2.4. NaMMT obviously does not exfoliate at all in spite of its glycerol absorption and increase of gallery distance. Obviously we must not overemphasize either the changes in gallery distance or transparency, but we can safely conclude that during the processing of TPS composites interactions take place among all components including the compounds used for the organophilization of the clay. Complete exfoliation definitely does not take place under the conditions used in this study, but some intercalation or limited delamination cannot be excluded completely.

Chapter 2

30

0 2 4 6 80

20

40

60

80

100

Tran

spar

ency

(%)


Figure 2.6 Effect of silicate content on the transparency of TPS/clay composites. Wave-

length: 700 nm. Symbols: ( ) NaMMT, ( ) N784, ( ) N804, ( ) N948, ( ) PVC/NaMMT.

2.3.2. Mechanical properties

The main goal of the preparation of composites and especially of layered silicate nanocomposites is to increase the stiffness and strength of the matrix polymer. Dynamic mechanical spectroscopy is often used for the characterization of layered silicate nano-composites and information about reinforcement as well as interaction is drawn from the spectra. The stiffness of any polymer increases when inorganic fillers are incorporated into it [19], thus the extent of reinforcement is difficult to judge from changes in this property. Intercalation may restrict the movement of polymer chains leading to an in-crease in Tg and the decrease of mobility, i.e. the intensity of the tg peak [20]. Occasion-ally, some increase in mobility was observed in cases when the surfactant interacted with the matrix polymer [18] or the presence of the clay restricted crystallization [21].

The DMTA spectra of TPS and of the composite containing 2 vol% Nanofil 948 are presented in Fig. 2.7. The glass or transition of plasticized starch appears at around 44 C. The identification of the other transition is more difficult, some relate it to the movement of –CH2OH groups, which we do not find very probable.


31

-100 -50 0 50 1000.0

0.1

0.2

0.3

0.4

0.5

0.6

5

6

7

8

9

10

11

12

Loss

tang

ent,

tg

Temperature (°C)

log

Mod

ulus

, E',

E" (P

a)

Figure 2.7 DMTA traces of TPS ( ) and that of one of its layered silicate composites

( ). Silicate: 2 vol% Nanofil 948.

According to McCrum at al. [22] it is related to the relaxation process of some smaller groups inside the chains or attached to them, while some others [23] assign it to the formation of a heterogeneous, two-phase structure. The dynamic mechanical spectra of all our composites are very similar; all characteristics change only slightly as an effect of modification. The intensity of relaxation increased, while that of decreased some-what, but no tendency could be found in the changes as a function of silicate content. Similarly, systematic change could not be observed in the Tg of TPS either. The small differences observed might have been the result of changing processing conditions or water content. Especially the latter factor influences the mechanical properties of TPS composites very much [9,23-25] and it is very difficult to control. Since far reaching con-clusions cannot be drawn from the results of DMTA measurements we refrain from their further discussion.

The composition dependence of the tensile strength of two of the composite series is presented in Fig. 2.8. We omitted the other two series for the sake of clarity; the corre-sponding values are situated between the two boundary cases, which are presented in the figure. The standard deviation of the measurement is relatively large because of the rea-sons mentioned in the previous paragraph.

The silicate treated with the amino acid (N784) and with the aliphatic amine (N948), respectively, represent two boundary cases. The use of the latter compound re-sults in a decrease of strength, while the treatment of the clay with the acid leads to sig-nificant reinforcement. Obviously, interactions play a significant role in the determination of properties as indicated by the change in gallery structure and transparency.

Chapter 2

32

0.00 0.02 0.04 0.06 0.084

5

6

7

8

9

Tens

ile st

reng

th (M

Pa)

Silicate volume fraction

Figure 2.8 Effect of composition on the tensile strength of TPS layered silicate compo-

sites. Symbols: ( ) N784, ( ) N948. 2.3.3. Interactions

Interfacial interactions considerably influence, occasionally determine the proper-ties of all heterogeneous materials. They play an important role also in the studied TPS/clay composites. The dependence of torque recorded in the internal mixer during homogenization is plotted against filler content in Fig. 2.9. This quantity depends on the fusion rate or TPS. The same composites represent the boundary behavior as above. Torque increases with filler content for the silicate treated with N 784, while it decreases in the presence of the clay covered with the aliphatic amine (N 948). Interactions are very different in the two cases; the acid functionality may even react with either glycerol or starch, while the aliphatic amine covered filler acts as a lubricant. Interactions are rela-tively strong also in the other two cases, which is probably caused by the high surface free energy of neat NaMMT and the similar chemical structure of the surfactant to that of glycerol and starch for N804, respectively.

Reinforcement, i.e. the load-bearing capacity of the second component or the

strength of interaction can be estimated also from the strength of the composites. The use of a simple semi-empirical model developed earlier [26,27], allows us to calculate a pa-rameter (B) which is proportional to the load carried by the dispersed component. The model takes the following form for tensile strength

exp 2.5 1

1 0 BnTT

(2.1)


33

where T and T0 are the true tensile strength ( T = and = L/L0) of the composite and the matrix, respectively, n is a parameter taking into account strain hardening, is the volume fraction of the fiber and B is related to its relative load-bearing capacity, i.e. to the extent of reinforcement, which, among other factors, depends also on interfacial in-teractions. We can write Eq. 2.1 in linear form

ln 1

5.2 1 ln ln 0 BTnT

Tred (2.2)

and the plot of the natural logarithm of reduced tensile strength against fiber content should result in a linear correlation, the slope of which is proportional to the load-bearing capacity of the reinforcement and under certain conditions to the strength of interaction

0 2 4 6 830

35

40

45

50

55

Torq

ue (N

m)


Figure 2.9 Effect of interfacial interactions on the viscosity (maximum torque) of

TPS/layered silicate composites. Symbols: ( ) NaMMT, ( ) N784, ( ) N804, ( ) N948.

The tensile strength of the composites is plotted in this form in Fig. 2.10. Rela-

tively good straight lines are obtained for the two selected cases; standard deviation and changing water content may account for the scatter. The slope of the lines gives B. The difference between the two sets of composites is obvious; the silicate covered with N784 has a strong reinforcing effect, while the aliphatic amine results in a very moderate load-bearing capacity of the filler.

Chapter 2

34

0.00 0.02 0.04 0.06 0.081.5

1.7

1.9

2.1

2.3

2.5

ln(r

educ

ed st

reng

th,

red)

Silicate volume fraction

Figure 2.10 Reinforcing effect of two organophilic silicates in TPS composites [26,27].

The slope of the straight lines is proportional to reinforcement. Symbols: ( ) N784, ( ) N948.

The parameters of the model, i.e. B and the intersection of the line, which corre-sponds to the calculated strength of the matrix, 0c, are listed in Table 2.1. Also the good-ness of the linear fit is given. This latter is very small in some cases, because of the large standard deviation of the determination and the small slope in the case of N948. As men-tioned above, the reinforcing effect of the filler organophilized with N784 is strong; the value of parameter B is larger than 8, a value usually obtained for short fiber reinforced composites. The B value of a PP composite containing a commercial CaCO3 filler is somewhere around 1. Two main factors influence the value of B: the size of the contact surface between the components and the strength of interaction. The former may increase considerably as a result of exfoliation, while the second is determined by the character of interaction (secondary forces, covalent bonds). Since we did not investigate these ques-tions in detail, we refrain from further speculation. Nevertheless, we may conclude that significant differences exist in the performance of the various silicates, which are proba-bly related to differences in their interaction with TPS. This statement is further corrobo-rated by the fact that the results of Table 2.1 are in complete agreement with those pre-sented previously.


35

Table 2.1 Reinforcing effect of the silicates used in the study and other parameters of

the model applied to estimate it [26,27]

Silicate Treatment 0 (MPa)

0c

(MPa) B R2

MMT none

5.6

5.6 7.01 0.9493

Nanofil 784 -amino acid 5.6 8.42 0.9972

Nanofil 804 hydroxyl amine 6.0 4.03 0.8228

Nanofil 948 octadecyl amine 5.9 1.55 0.5366

0 2 4 6 850

70

90

110

130

150

170

Yel

low

ness

inde

x


Figure 2.11 Discoloration of TPS/layered silicate composites during composite prepara-

tion. Effect of silicate content. Symbols: ( ) NaMMT, ( ) N784, ( ) N804, ( ) N948.

3.3.4. Other properties

Starch is a natural polymer which degrades easily already at the processing tem-perature of TPS [24-27]. Although the degradation mechanism of natural polymers is rather complicated, we followed eventual changes in the chemical structure of the poly-mer by measuring its color. Degradation, but also other chemical reactions often lead to

Chapter 2

36

the formation of conjugated structures, which result in the yellowing of the polymer. The yellowness index of the composites is plotted against silicate content in Fig. 2.11. Alt-hough it is somewhat surprising, the color of the composites correlates very well with their mechanical properties (see Fig. 2.10 and Table 2.1). Color change supplies a further proof that the amino acid reacts with TPS and the reaction leads to discoloration as well as to improved interfacial interaction. We must call attention here to the deviating point at 7 vol% N784 content. Although we do not know the exact reason for the deviation, but it appears both in strength and color (compare Figs. 2.10 and 2.11).

The resistance against water is claimed to be one of the advantages of TPS com-

posites containing layered silicates [3,5]. Equilibrium water uptake in an atmosphere of 50 % relative humidity is around 9.5 % for TPS, which decreased by about 0.5-1.0 % for composites containing 5 vol% silicate, i.e. water adsorption does not decrease in the pres-ence of the various clays, which contradicts claims of some literature sources [3,5]. The largest decrease was observed in the case of the neat NaMMT, while the smallest for N948. This result also indicates that only limited extent of exfoliation occurred during the preparation of the composites, but also that all the investigated TPS composites are very active; their components interact with each other and with the environment. 2.4. CONCLUSIONS

Our experiments focused onto the preparation and study of thermoplastic starch/layered silicate composites revealed that no or only limited exfoliation took place during the homogenization of the composites in spite of the fact that clays with different organophilization were used. On the other hand, practically all components of the system enter into interaction with each other. Competitive dissolution and adsorption of the sur-factant and the plasticizer take place in the composites. The occurrence of chemical reac-tions cannot be excluded either in the case of the amino acid modified silicate. The type and strength of interactions determine the properties of the composites. The reinforcing effect of the silicates vary in a wide range, the amino acid modified clay improves strength almost ten times as much as the silicate treated with an aliphatic amine. Changes in all other properties including viscosity, color and water adsorption correspond to those ob-served in strength and are dominated by interactions. 2.5. REFERENCES 1. Walter, P., Mader, D., Reichert, P., Mulhaupt, R., J. Macromol. Sci.-Pure Appl.

Chem. A36, 1613-1639 (1999) 2. Mani, R., Bhattacharya, M., Eur. Polym. J. 37, 515-526 (2001) 3. Huang, M.-F., Yu, J.-G., Ma, X.-F., Polymer 45, 7017-7023 (2004) 4. McGlashan, S. A., Halley, P. J., Polym. Int. 52, 1767-1773 (2003) 5. Fischer, H., Materials Science & Engineering C-Biomimetic and Supramolecular

Systems 23, 763-772 (2003) 6. Simon, J., Müller, H. P., Koch, R., Müller, V., Polym. Degrad. Stabil. 59, 107-

115 (1998)


37

7. Smits, A. L. M., Kruiskamp, P. H., van Soest, J. J. G., Vliegenthart, J. F. G., Carbohydr. Polym. 51, 417-424 (2003)

8. Biliaderis, C. G., Lazaridou, A., Arvanitoyannis, I., Carbohydr. Polym. 40, 29-47 (1999)

9. vanSoest, J. J. G., Hulleman, S. H. D., deWit, D., Vliegenthart, J. F. G., Ind. Crop. Prod. 5, 11-22 (1996)

10. vanSoest, J. J. G., Hulleman, S. H. D., deWit, D., Vliegenthart, J. F. G., Carbohydr. Polym. 29, 225-232 (1996)

11. Hulleman, S. H. D., Kalisvaart, M. G., Janssen, F. H. P., Feil, H., Vliegenthart, J. F. G., Carbohydr. Polym. 39, 351-360 (1999)

12. Theng, B. K. G., The Chemistry of Clay-Organic Reactions, John Wiley & Sons Inc.: London (1974)

13. Pozsgay, A., Fráter, T., Papp, L., Sajó, I., Pukánszky, B., Journal of Macromolecular Science-Physics B41, 1249-1265 (2002)

14. Ishida, H., Campbell, S., Blackwell, J., Chem. Mat. 12, 1260-1267 (2000) 15. Li, Y. Q., Ishida, H., Polymer 44, 6571-6577 (2003) 16. Wan, C. Y., Qiao, X. Y., Zhang, Y., Zhang, Y. X., Polym. Test 22, 453-461 (2003) 17. Manias, E., Touny, A., Wu, L., Strawhecker, K., Lu, B., Chung, T. C., Chem. Mat.

13, 3516-3523, (2001) 18. Pozsgay, A., Csapo, I., Százdi, L., Pukánszky, B., Mater. Res. Innov. 8, 138-139

(2004) 19. Nielsen, L. E., Mechanical Properties of Polymers and Composites, Marcel

Dekker: New York (1974) 20. Pinnavaia, T. J., Beall, G. W., Polymer-Clay Nanocomposites, Wiley: New York

(2001) 21. Rácz, L., Pukánszky, B., Pozsgay, A., Pukánszky, B., in From Colloids to

Nanotechnology (2004) Vol. 125, pp.96-102. 22. McCrum, N. G., Read, B. E., Williams, G., Anelastic and dielectric effects in

polymeric solids, Wiley New York (1967) 23. Forssell, P. M., Mikkila, J. M., Moates, G. K., Parker, R., Carbohydr. Polym. 34,

275-282 (1997) 24. Forssell, P., Mikkila, J., Suortti, T., Seppala, J., Poutanen, K., J. Macromol. Sci.-

Pure Appl. Chem. A33, 703-715 (1996) 25. Myllymaki, O., Eerikainen, T., Suortti, T., Forssell, P., Linko, P., Poutanen, K.,

Food Sci. Technol.-Lebensm.-Wiss. Technol. 30, 351-358 (1997) 26. Pukánszky, B., Turcsányi, B., Tüdős, F., in Interfaces in polymer, ceramic, and

metal matrix composites, Ishida, H. (ed.), Elsevier: New York (1988) pp.467-477. 27. Pukánszky, B., Composites 21, 255-262 (1990)

TPS/montmorillonite nanocomposite films

39

Chapter 3 Thermoplastic starch/wood composites: Interfacial interactions and

functional properties2

3.1. INTRODUCTION

Recently growing interest has been shown in the application of biopolymers as packaging materials in order to reduce the environmental pollution caused by plastic waste and to achieve sustainable development.

Recently several attempts were reported in the literature for the preparation of TPS

nanocomposites. In most cases TPS/montmorillonite nanocomposite films were prepared by melt blending (in internal batch mixer or in a twin screw extruder) [1-11] or solution mixing (film casting) [1,3,12-19]. Large extent of exfoliation was achieved using only water or less than 10 wt % glycerol as plasticizer [1,2,7]. Several studies proved that the use of glycerol contents larger than 10 wt% led to the formation of intercalated structures with interlayer basal spacing (d001) increasing from 1.2 to 1.8 nm [1,10,12]. It is difficult to verify whether the starch or the glycerol molecules intercalate between the clay layers, because both have a tendency to penetrate into the silicate layers, but penetration of glyc-erol is favored owing to its smaller molecular size [19,20]. Several investigations confirm the strong influence of the polyol plasticizer on the exfoliation process and thus on the resulting morphology. This effect is likely related to the hydrogen bonds established be-tween glycerol and MMT platelets, which could decrease the attractive forces between starch and clay [1,10,12]. Exfoliated/intercalated morphology is found to be dependent also on NaMMT content. Exfoliation is the predominant mechanism of clay dispersion at small filler content [15], while increasing the clay content above 5 wt% favours the for-mation of an intercalated structure.

In spite of incomplete exfoliation TPS/NaMMT nanocomposites have improved

properties compared to TPS. Their properties strongly depend on the type of the starch and the montmorillonite used, as well as on the amount of MMT and glycerol. Papers published so far indicate that larger extent of exfoliation results in better properties [1,2,5,7,11,15,16,20]. Besides their barrier properties packaging materials should possess also proper mechanical characteristics. Although several papers discuss the stiffness, strength and deformability of TPS nanocomposite films [2,4,6,7,11,14-16,20,21], only a limited number of them reports systematic experiments carried out as a function of filler content in a wide composition range [2,4,5,9,16,20], and often very poor mechanical properties are published compared to commodity polymers. Since packaging materials are not usually applied under dry conditions, the mechanical properties of dry TPS/clay composites investigated generally are not relevant, because it is well known that humidity can strongly influence the strength and the stiffness of TPS nanocomposite films. In spite of this effect, relatively few papers have been published on TPS composites studied under ambient conditions (RH = 30-60 %) [2,4,7,9,11,21]. Furthermore the effect of processing ______________________________________________________________________ 2Müller P., Kapin E., Fekete E.: Effects of preparation methods on the structure and mechanical properties of wet conditioned starch/montmorillonite nanocomposite films

Chapter 3

40

technology on the properties of TPS nanocomposites of the same composition has not yet been thoroughly elucidated. Although Aouada et al. [20,22] prepared TPS nanocompo-sites by the combination of intercalation from solution and melt-processing and they found that the applied method resulted in intercalated/exfoliated structure and good ther-mal, mechanical properties as well as decreased hydrophobicity and water absorption, indeed, the measured mechanical properties were very poor and the different effect of the individual processes on the morphology and properties of TPS nanonocomposites was not investigated at all.

As a consequence, the goal of our work was to prepare TPS/NaMMT nanocom-posite films with different glycerol and clay content using a melt blending as well as a solution mixing procedure and to determine the structure and properties of dry and wet (conditioned) films in a wide composition range. Considerable attention is paid also to interactions developing among the components. 3.2. EXPERIMENTAL

High quality corn starch produced in Hungary (Hungrana Ltd.) was used in the experiments. Glycerol was purchased from Aldrich, Hungary. Sodium montmorillonite (Cloisite Na+) with a cation exchange capacity (CEC) of 92.6 meq/100 g clay was sup-plied by Southern Clay Products (Rockwood Additives Ltd.). 3.2.1. Preparation of plasticized starch/montmorillonite nanocomposite films

TPS nacomposite films were prepared by solution casting and melt blending. So-lution casting was carried out in the following way: native starch was dispersed in excess amount of distilled water containing 30 and 40 wt% of glycerol. Then the suspension was continuously stirred at 80 °C for 30 min to gelatinize corn starch granules. The starch concentration of the solution was 4.5 wt%. Sodium montmorillonite (NaMMT) was dis-persed in distilled water at the concentration of 0.8 wt% by sonication for 30 min at room temperature. The clay dispersion was added to the aqueous gelatinized starch and the mixture was stirred for another 30 min at 90 °C. Films were obtained by casting the hot suspension into Petri dishes covered by a Teflon sheet and dried in an oven at 40 °C for 24 hours. Clay content changed between 0 and 25 wt% based on the amount of dry starch. The thickness of the films was 0.10 ± 0.02 mm. Before characterization the films were stored at 23 °C and 52 % RH until constant weight was reached. Nanocomposite films were prepared also by melt intercalation. During the process dry starch was premixed with glycerol (40 wt%) and montmorillonite in a Petri dish and the mixtures were intro-duced into an internal mixer (Brabender W50 EH) and homogenized at 150 °C for 10 min. 0.10 mm ± 0.02 mm thick plates were compression molded from the melt at 150 °C and for 5 min. Some of the films prepared by melt mixing were stored under dry condi-tions, while others were stored at 23 °C and 52 % RH until further study. Table 3.1 con-tains the list of nanocomposite films, their compositions and the methods used for their preparation.


41

Table 3.1 Preparation method and composition as well as designation of TPS nanocom-

posites

Sample Method Glycerol content (g/100 g starch)

Clay content

g/100 g starch (vol%) S30 solution 30 0 - 25 0 - 8 S40 solution 40 0 - 25 0 - 7 M40 melt 40 0 - 25 0 - 7

3.2.2. Characterization

The crystalline structure of TPS and the gallery structure of the filler were studied

by X-ray diffraction (XRD) using a Philips PW1830/PW1050 equipment with CuKα ra-diation at 40 kV and 35 mA. Samples were scanned in the diffraction angle range of 2–35° in 0.1° steps. Diffractograms were recorded on powders (montmorillonite and starch) or films using a multipurpose sample stage. The basal spacing of the silicate layers was calculated using the Bragg’s equation. The extent of starch crystallinity was estimated by dividing the crystalline area by the total (crystalline + amorphous) area [23].

The morphology of the samples was examined by scanning electron microscopy

(SEM) using a Jeol JSM 6380 apparatus. Micrographs were taken from surfaces created by cutting with an ultramicrotome. The light transmission of the films was determined using an UV-VIS spectrometer (Unicam W500) at various wavelengths. Only results ob-tained at 700 nm are reported here.

The equilibrium water content of the conditioned (at 23 °C at 52 % RH) film sam-

ples was determined by thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer TGA 6 equipment from 35 to 700 °C, at a heating rate of 10 °C/min, under nitrogen flow. The total weight loss up to 160 °C was identified as the water content of the samples.

The tensile properties of the samples were measured using an Instron 5566 appa-

ratus. Young’s modulus was determined at 0.5 mm/min while ultimate properties at a cross-head speed of 5 mm/min. All characteristics were derived from five parallel meas-urements.

Properties of TPS nanocomposites were investigated as a function of volume frac-

tion of clay. In order to calculate the volume fraction of the filler we estimated the density of the nanocomposites from the compositions (NaMMT, starch, glycerol and water con-tent) assuming the additivity of densities of the components.

Chapter 3

42

3.3. RESULTS AND DISCUSSION

TPS nanocomposite films prepared by solution casting or melt intercalation were more or less transparent and showed homogeneous appearance without breaks, fractures, insoluble particles or bubbles (Fig. 3.1). The films with smaller plasticizer content (water and glycerol) appeared to be stiffer and more brittle than films containing more plasti-cizer. Cast films were white, while films prepared by compression molding showed brown color.

Figure. 3.1 Different appearance of TPS nanocomposites prepared by solution casting (left) and melt blending (right).

3.3.1. Structure Crystallinity

Fig. 3.2 shows the XRD patterns obtained for the conditioned TPS (S30-0, S40-0

and M40-0) films prepared by different methods together with the XRD trace of the native starch. The amylopectin side chains of native starch can crystallize in three crystalline forms, namely, the A-type for cereal starches, the B-type for tuberous and amylose rich starch and the C-type which has a structure between those of the A- and B-types [24]. These structures are completely or partially destroyed during processing resulting in an amorphous matrix. According to van Soest and Vliegenhart [25] two types of crystallinity can be distinguished in thermoplastic starch after processing: residual crystallinity and process-induced crystallinity. The residual crystallinity is caused by incomplete melting or solution of starch during processing and can be A-, B- or C-type, as occurs in native starches. The induced crystallinity is associated with the crystallization of amylose and identified as VH-, VA- or EH-types.


43

5 10 15 20 25 30 35

S30 conditioned

S40 conditioned

Inte

nsity

(cou

nts)

2 (degree)

Starch

M40 conditioned

Figure 3.2 X-ray patterns of starch and TPS samples prepared by different methods.

It is well known that native corn starch crystallizes in the A-form and the charac-teristic diffraction peaks with strong reflections at the 2 angles of about 15° and 23° and an unresolved doublet at 17° and 18° can be easily identified in the XRD pattern of native starch in Fig. 3.2 indeed. The absence of these peaks in the XRD patterns of the cast TPS films clearly proves that the original crystalline structure was completely destroyed dur-ing the solution mixing processing. Some minimal residual A-type crystallinity may be assumed in the TPS film prepared by melt process. The characteristic peaks at 2 = 12.9°, 17.3°, 19.7° and 22.2° indicate the formation of the VH-type structure in all films. VH-type crystallinity is typical for TPS samples in which the water content is larger than 10 wt% [24,26]. The extent of starch crystallinity was also calculated for all TPS nanocom-posites and the results are presented in Fig. 3.3. From Fig. 3.3 it is obvious that nanocom-posites containing 40 wt% glycerol have somewhat larger crystallinity than samples with 30 wt% glycerol content which is probably caused by the larger mobility of starch chains in TPS composites containing more plasticizer. Although the results indicate that the crys-tallinity of the matrix polymer increases with clay content, the overlapping of the charac-teristic peak of the clay at 2 = 20.05° [27] and that of the starch at 2 = 19.7° must be considered here.

Chapter 3

44

0 2 4 6 80

3

6

9

12

15

S30 conditioned S40 conditioned M40 conditioned

Cry

stal

linity

(%)

NaMMT content (vol%)

Figure 3.3 Effect of clay content on the crystallinity of TPS nanocomposites.

2 4 6 8 10

Inte

nsity

(cou

nts)

degree)

TPS

4 wt % NaMMT

10 wt% NaMMT

15 wt% NaMMT

25 wt% NaMMT

NaMMT

Figure 3.4 X-ray diffraction patterns of NaMMT, TPS and S30 nanocomposites.


45

Dispersion of NaMMT in the TPS matrix

The 2 range between 2° and 10° was analyzed in order to obtain information about the dispersion of the nanoclay in TPS quantitatively. Fig. 3.4 shows the XRD pat-terns of Cloisite-Na+, TPS (S30-0), and TPS–cloisite-Na+ nanocomposites (S30) pre-pared by solution casting at different NaMMT contents. Starch does not have any char-acteristic reflection in the studied range while NaMMT exhibits a single 001 diffraction peak at around 7.3°. In the composite films the 001 diffraction peak of NaMMT (1.21 nm) shifts to smaller angles (5.2° 0.2°) corresponding to an interlayer basal spacing (d001) of 1.69 0.06 nm independently of clay content. Similar results were obtained also for the S40 and M40 samples. These results indicate that either the glycerol or the polymer chains or both entered into the silicate layers forming intercalated starch/MMT nanocomposites without reaching complete exfoliation. The similar size of glycerol and glucose units in starch makes it difficult to assess, on the basis of the change in the inter-layer spacing, whether starch, glycerol or both entered into the galleries.

0 2 4 6 80

200

400

600

800

1000

1200

1400

Inte

grat

ed In

tens

ity

Clay content (vol%)


Figure 3.5 Integrated intensity of the clay reflection plotted against clay content.

Figure 3.5 presents the dependence of the intensity of clay reflection on clay con-

tent for nanocomposite films prepared by different methods. Intensity increases almost linearly with filler content up to approximately 3.0 vol% (10 wt% related to dry starch) in all samples, which indicates that exfoliation does not take place during processing or always the same fraction of the silicate exfoliates independently of clay content. Although the scattering of measured intensities of the M40 samples (prepared by melt intercalation)

Chapter 3

46

is quite large we can conclude that above 3 vol% clay content the extent of exfoliation is smaller in these nanocomposites than in samples prepared by solution casting.

0 2 4 6 80

20

40

60

80

100

Tr

ansp

aren

cy (%

)


S40 conditioned PVC/NaMMT S30 conditioned M40 conditioned

Figure 3.6 Effect of clay content on the transparency of different TPS/clay and PVC/clay composites.

Further information can be obtained about the structure of the composites from

their light transmission. Fig. 3.6 shows the transparency of our composites as a function of clay content. The light transmission of PVC/NaMMT composites is also plotted as reference [28]. We cannot expect any exfoliation to occur in this latter case. According to Fig. 3.6 large differences can be observed in the light transmission of the investigated TPS nanocomposites prepared by different procedures. The transparency of nanocompo-sites prepared by solution mixing remains large in the entire composition range, while that of the samples prepared by melt intercalation decreases significantly. Attention should be paid here to the small transparency of the TPS film prepared by melt blending without NaMMT (M40-0). The small transparency as well as the brown color of this sample might be related to the degradation of starch during melt homogenization. NaMMT obviously does not exfoliate at all in nanocomposites prepared by melt blending, while we may assume some exfoliation to occur in samples prepared by solution mixing. The significant difference in the composition dependence of the transparency of nano-composites prepared by various techniques does not coincide with their similar gallery distances and scattering intensities determined from the XRD patterns of the samples. Obviously we must not overemphasize either the changes in gallery distance or transpar-ency, but we can safely conclude that during the preparation of TPS composites interac-tions take place among all components. Complete exfoliation definitely does not take


47

place under the conditions used in this study, but intercalation and limited delamination cannot be excluded completely.

a) b) Figure 3.7 Scanning electron micrographs recorded on the cryo-cut surfaces of S40-10

(a) and M40-10 (b) TPS/NaMMT nanocomposites.

The degree of dispersion of NaMMT in the TPS composites was also investigated with the help of SEM micrographs. SEM micrographs taken from the S40-10 and M40-10 composites are presented in Fig. 3.7. These samples were prepared by different meth-ods, but with the same glycerol and filler content. Several large clay aggregates of around 10 m in diameter were observed in the starch film produced by melt intercalation, while the dispersion of NaMMT was homogeneous in the cast films and only very small parti-cles could be seen in these samples. Similar homogeneous clay distribution was observed in all nanocomposite films (S30 and S40 samples) prepared by film casting. 3.3.2. Moisture content of conditioned nanocomposite films

The large water sorption capacity of TPS is well-known and better resistance against water is claimed to be one of the advantages of TPS composites containing layered silicates [1,29]. Absorbed water acts as plasticizer, thus influencing composite character-istics, mainly mechanical properties. TGA measurements were carried out on the TPS nanocomposite films prepared by solution casting or melt mixing and conditioned at 23 ° and 52% relative humidity in order to determine the exact composition. The results are presented in Fig. 3.8 and indicate small differences in water content, the original water content of TPS decreases by 3-4 wt% as the filler content increases by 7-8 vol%. The comparison of the water sorption capacity of various cast films shows that TPS nanocom-posites containing 40 wt% glycerol can absorb more water than samples with 30 wt%

Chapter 3

48

plasticizer content because of the strong hygroscopicity of glycerol. The comparison of the films prepared by unlike methods is difficult, since different interactions can form between starch and water depending on the method of preparation.

0 2 4 6 80

5

10

15

20

Equi

libriu

m w

ater

con

tent

(%)



Figure 3.8 Effect of filler and glycerol content as well as preparation technique on the

equilibrium water content of TPS nanocomposites.

3.3.3 Mechanical properties

Stiffness, strength and deformability of conditioned cast films as well as the same properties of dry and conditioned films prepared by melt intercalation were determined to characterize the mechanical behavior of the composites. The modulus, tensile strength and elongation-at-break values of the investigated TPS nanocomposites are plotted against clay content in Figs. 3.9-3.11. The figures clearly show that the mechanical prop-erties of the samples depend strongly on their plasticizer (water and glycerol) and clay content. The standard deviation of the measurements is relatively large even though the films showed homogeneous appearance. According to Fig. 3.9 stiffness increases from around 0.70 GPa up to approximately 2.60 GPa for S30 and dry M40 films and from 0.10 GPa up to 0.45 GPa for S40 TPS nanocomposites, i.e. clay reinforce starch strongly. The S30 and dry M40 samples have almost the same plasticizer content, i.e. 40 wt% glycerol for M40 and 30 wt% glycerol + around 10 wt% water for S30 samples, thus the similar modulus of M40 and S30 films containing the same filler amount means that the prepa-ration method has limited effect on stiffness and water seems to have stronger plasticizing


49

effect than glycerol. Preparation technique, plasticizer content and the type of plasticizer influence tensile strength and deformability more than modulus.

0 2 4 6 80

1

2

3

4

You

ng's

mod

ulus

(GPa

)


S40 conditioned S30 conditioned M40 conditioned M40 dry

Figure 3.9 Effect of NaMMT content on the stiffness of TPS/NaMMT nanocomposites.

The largest tensile strengths and the smallest elongation-at-breaks were measured

for dry M40 nanocomposites in the 0-6 vol% NaMMT range. According to Fig. 3.10, tensile strength increases from 11.0 MPa up to 20.3 MPa. Above 3-4 vol% clay content composite strength does not increase further, which indicates the influence of some struc-tural effect probably the inhomogeneous distribution of the filler discussed above. Pre-sumably the strong plasticizing effect of water results in the smaller strength and larger deformability of the conditioned S30 samples compared to the dry M40 composite. With larger glycerol content tensile strength is smaller and elongation-at-break is larger. The tensile strength of the S40 nanocomposites is smaller than that of the S30 samples and the change with increasing NaMMT content is small (only approximately 3 MPa from 2.1 MPa to 5.2 MPa). It is important to note, however, that the stiffness and strength of conditioned M40 films are very poor, the worst among all samples in spite of the compa-rable glycerol and water content of S40 and conditioned M40 TPS nanocomposites. Prob-ably the water is differently bonded during solvent casting and conditioning and has dif-ferent effect on mechanical properties. We mentioned earlier that the small transparency and brown color of the M40 samples might be related to the degradation of starch occur-ring during melt homogenization. If this assumption is true, it can explain the poorer me-chanical properties of the conditioned M40 samples.

Chapter 3

50

0 2 4 6 80

5

10

15

20

25

30

35 S40 conditioned S30 conditioned M40 conditioned M40 dry

Tens

ile st

reng

th (M

Pa)


Figure 3.10 Effect of NaMMT content on the strength of TPS/NaMMT nanocomposites.

0 2 4 6 80

20

40

60

80

100

120

Elon

gatio

n at

bre

ak (%

)


S40 conditioned S30 conditioned M40 dry M40 conditioned

Figure 3.11 Effect of NaMMT content on the elongation-at-break of TPS/NaMMT nano-composites.


51

3.3.4. TPS/clay interaction Interfacial interactions play an important role in the studied TPS/clay composites.

Reinforcement or the strength of interaction can be estimated from the composition de-pendence of the tensile strength of the composites. The use of a simple semi-empirical model developed earlier [30], allows us to calculate a parameter (B) which is proportional to the strength of interaction and the load carried by the dispersed component.

0.00 0.02 0.04 0.06 0.080

1

2

3

4

5

6

ln(r

educ

ed te

nsile

stre

ngth

)

Volume fraction of NaMMT

S40 conditioned S30 conditioned M40 dry

B=16.80

B=15.50

B=11.97

Figure 3.12 Tensile strength of TPS/NaMMT nanocomposites plotted against filler con-

tent in the linear representation of Eq. 2.1.

In Fig. 3.12 the reduced tensile strength of the composites is plotted against filler content in the form indicated by Eq. 2.2. Relatively good straight lines are obtained for the three selected cases; standard deviation account for the scatter. The slope of the lines gives B. The parameters of the model, i.e. B and the intersection of the line, which corresponds to the calculated strength of the matrix, σT0, are listed in Table 3.2. The goodness of the linear fit, i.e. determination coefficient, is also listed in the table, in column four.

The value of B ranges from 12.0 to 16.8 for the conditioned S30 and S40, as well

as for the dry M40 nocomposites, and they are very large compared to usual particulate filled commodity polymers; B is often smaller than 1 for PP/CaCO3 composites e.g. [30,31].

Chapter 3

52

Table 3.2. Calculated tensile strengths of TPS matrices, load-bearing capacities of clays and the goodness of linear fits

Sample 0

(MPa) B R2

S30 nanocomposites 6.8 15.47 0.9612

S40 nanocomposites 2.2 11.97 0.9783

Dry M40 nanocomposites 10.8 16.80 0.9752

Similar, but slightly smaller B parameters (9.6-12.0) were determined for TPS/wood composites (see Chapter 4). We could not calculate parameter B for the con-ditioned M40 nanocomposite samples, because their tensile strength practically does not change with increasing filler content. The strength of the unfilled conditioned M40 TPS film was very poor (1.0 MPa).

Three main factors influence the value of B: the size of the contact surface between

the components, the strength of interaction and the tensile strength of the matrix ( 0). The first may increase considerably as a result of exfoliation, while the second is determined by the character of interaction (secondary forces, covalent bonds). B is defined as

0

ln1 iffAB (3.1)

where Af and f are the specific surface area and density of the reinforcing component, ℓ the thickness of the interphase forming spontaneously in the composite, while i and 0 is the strength of the interface and the matrix, respectively. We could see in Fig. 3.5 that the extent of exfoliation is similar in the three investigated TPS nanocomposites in the range of 0-3 vol % clay content. It has also been proven earlier that the properties of the matrix ( 0) play an important role in the actual value of B [11,32]. The softer is the matrix, the larger is the reinforcing effect of filler. Comparing the B parameters determined for the S30 and S40 nanocomposites we can see that B calculated for the S30 composites is larger than the one for S40. This is rather surprising, since the tensile strength of the S40-0 TPS matrix is much smaller than that of the S30-0 sample. Assuming similar extent of exfoliation of the montmorillonite in the different nanocomposites we can conclude that somewhat stronger interfacial interaction forms between the starch and the clay in the S30 samples than in the S40 composites. The probable reason for the stronger interaction is the smaller amount of glycerol applied for plasticization. As described in the introduc-tory part, competitive interactions develop among starch, glycerol and clay and several authors [1,10,12] pointed out that the presence of glycerol may hinder the interaction between starch and montmorillonite. The comparison of the B value of the M40 samples


53

with those of the S30 and S40 nanocomposites is difficult because of the different prepa-ration techniques used and because of dissimilar plasticizer contents. 3.4. CONCLUSIONS

According to the results of X-ray diffraction analysis, scanning electron micros-copy and light transmission measurements all nanocomposites possess intercalated struc-ture, but at larger clay content (above 10 wt%) nanocomposites prepared by melt interca-lation contained aggregated particles as well. VH-type crystallinity was found in all nano-composites, which is typical for TPS containing more than 10 wt% water. Somewhat larger crystallinity was observed in nanocomposites containing more plasticizer, which is probably due to the larger mobility of starch molecules in these composites. The applied clay content was much larger than the usual 1-5 wt%, and the results proved that these nanocomposites can offer good properties in spite of the lack of complete exfoliation. Clay reinforces TPS considerably in nanocomposites prepared by solution casting. Simi-larly good mechanical properties were determined also on dry samples prepared by melt blending, but conditioning of these samples resulted in very poor stiffness and strength, the worst among all samples studied. We assume that water either adsorbed or bonded in the composites in conditioning or solution casting, respectively, has different effect on mechanical properties. With the aid of a simple model we could prove the strong interac-tion between starch and montmorillonite and also that an increase in glycerol content decreases starch/clay interaction. 3.5. REFERENCES 1. Chivrac, F., Pollet, E., Averous, L., Mater. Sci. Eng. R-Rep. 67, 1-17 (2009) 2. Dean, K., Yu, L., Wu, D. Y., Compos. Sci. Technol. 67, 413-421 (2007) 3. Ray, S. S., Bousmina, M., Progress in Materials Science 50, 962-1079 (2005) 4. Huang, M. F., Yu, J. G., Ma, X. F., Polymer 45, 7017-7023 (2004) 5. Chen, B. Q., Evans, J. R. G., Carbohydr. Polym. 61, 455-463 (2005) 6. Avella, M., De Vlieger, J. J., Errico, M. E., Fischer, S., Vacca, P., Volpe, M. G.,

Food Chem. 93, 467-474 (2005) 7. Tang, X. Z., Alavi, S., Herald, T. J., Carbohydr. Polym. 74, 552-558 (2008) 8. Magalhaes, N. F., Andrade, C. T., Carbohydr. Polym. 75, 712-718 (2009) 9. Chivrac, F., Pollet, E., Dole, P., Averous, L., Carbohydr. Polym. 79, 941-947

(2010) 10. Chiou, B. S., Wood, D., Yee, E., Imam, S. H., Glenn, G. M., Orts, W. J., Polym.

Eng. Sci. 47, 1898-1904 (2007) 11. Muller, C. M. O., Laurindo, J. B., Yamashita, F., Carbohydr. Polym. 89, 504-510

(2012) 12. Pandey, J. K., Singh, R. P., Starch-Starke 57, 8-15 (2005) 13. Kampeerapappun, P., Aht-Ong, D., Pentrakoon, D., Srikulkit, K., Carbohydr.

Polym. 67, 155-163 (2007)

Chapter 3

54

14. Cyras, V. P., Manfredi, L. B., Ton-That, M. T., Vazquez, A., Carbohydr. Polym. 73, 55-63 (2008)

15. Schlemmer, D., Angelica, R. S., Sales, M. J. A., Compos. Struct. 92, 2066-2070 (2010)

16. Majdzadeh-Ardakani, K., Navarchian, A. H., Sadeghi, F., Carbohydr. Polym. 79, 547-554 (2010)

17. Masclaux, C., Gouanve, F., Espuche, E., J. Membr. Sci. 363, 221-231 (2010) 18. Kelnar, I., Kapralkova, L., Brozova, L., Hromadkova, J., Kotek, J., Ind. Crop. Prod.

46, 186-190 (2013) 19. Chaudhary, D., Liu, H. H., Ultrason. Sonochem. 20, 63-68 (2013) 20. Aouada, F. A., Mattoso, L. H. C., Longo, E., Ind. Crop. Prod. 34, 1502-1508 (2011) 21. Chung, Y. L., Ansari, S., Estevez, L., Hayrapetyan, S., Giannelis, E. P., Lai, H. M.,

Carbohydr. Polym. 79, 391-396 (2010) 22. Aouada, F. A., Mattoso, L. H. C., Longo, E., Ind. Crop. Prod. 50, 449-455 (2013) 23. Liu, H. S., Yu, L., Simon, G., Zhang, X. Q., Dean, K., Chen, L., Carbohydr. Res.

344, 350-354 (2009) 24. vanSoest, J. J. G., Hulleman, S. H. D., deWit, D., Vliegenthart, J. F. G., Ind. Crop.

Prod. 5, 11-22 (1996) 25. vanSoest, J. J. G., Vliegenthart, J. F. G., Trends Biotechnol. 15, 208-213 (1997) 26. Mitrus, M., in Thermoplastic Starch, Wiley-VCH Verlag GmbH & Co. KGaA:

(2010) pp.77-104. 27. Zahedsheijani, R., Faezipour, M., Tarmian, A., Layeghi, M., Yousefi, H., Eur. J.

Wood Wood Prod. 70, 565-571 (2012) 28. Pozsgay, A., Csapo, I., Szazdi, L., Pukanszky, B., Mater. Res. Innov. 8, 138-139

(2004) 29. Averous, L., J. Macromol. Sci.-Polym. Rev C44, 231-274 (2004) 30. Pukánszky, B., Composites 21, 255-262 (1990) 31. Kiss, A., Fekete, E., Pukánszky, B., Compos. Sci. Technol. 67, 1574-1583 (2007) 32. Százdi, L., Pozsgay, A., Pukánszky, B., Eur. Polym. J. 43, 345-359 (2007)

Thermoplastic starch/wood composites

55

Chapter 4 Thermoplastic starch/wood composites: Interfacial interactions and

functional properties3

4.1. INTRODUCTION

The constant threat of depleting fossil fuel sources and the increasing environmen-tal awareness of the public resulted in increasing interest in natural polymers in all fields of life [1]. Cellulose, lignin and starch are renewable materials produced in very large quantities each year. The use raw materials or additives in plastics have become industrial practice recently and as a consequence, scientific interest increased as well. Cellulose and wood is used as reinforcement, lignin to produce various chemicals, while under some conditions starch is applied as a thermoplastic polymer. Besides the advantages (processa-bility, flexibility, biodegradability, etc.) of plasticized starch, it also has disadvantages like poor mechanical properties, water sensitivity, dimensional instability, etc. In order to overcome these drawbacks thermoplastic starch (TPS) is often modified by blending with other polymers (see Chapter 5 and 6), by the addition of fillers or reinforcements or the preparation of nanocomposites (see Chapter 2 and 3). In order to maintain one of the most important advantages of starch, i.e. biodegradability, modification is carried out mostly with aliphatic polyesters and natural fibers.

A considerable number of papers deal with the effect of natural fibers on the prop-

erties of thermoplastic starch. Some of the papers cited investigates the effect or fiber characteristics on mechanical properties and conclude that stiffness and strength increase both with increasing fiber length and fiber content [2]. Others, but much fewer in number, also mention interfacial interactions and based mostly on SEM micrographs [2-4], the shift of absorption bands in FTIR [3] and changing glass transition temperature deduce that interfacial adhesion is strong between natural fibers and starch [2,5-8]. Very few attempts exist which estimate interfacial adhesion quantitatively [5]. Since water sensi-tivity is a drawback of starch composites, its change with fiber modification is studied relatively often, and the results generally indicate that water absorption decreases with increasing fiber content. This change is explained with the smaller water uptake of the fibers compared to that of starch. Much fewer attempts are made to determine the dimen-sional stability of starch/fiber composites, although shrinkage might be considerable hin-dering application in some areas. The few papers existing indicate that shrinkage de-creases with increasing fiber content, i.e. modification is beneficial from this aspect as well.

The study of existing literature indicated that relatively few papers have been pub-

lished on TPS/wood composites. Even less report systematic research, experiments car-ried out as a function of fiber content in a wide composition range. As a consequence, the goal of our work was to prepare TPS/wood composites with three wood fillers having ______________________________________________________________________ 3Müller P., Renner K., Móczó J., Fekete E., Pukánszky B.: Thermoplastic starch/wood composites: Interfacial interactions and functional properties, Carbohydrate Polymers 102., 821-829 (2014)

Chapter 4

56

different particle characteristics, determine reinforcement and its effect on functional properties. Considerable attention is paid to interfacial interactions, and an attempt is made to estimate adhesion quantitatively in comparison with commodity polymer/wood composites. 4.2. EXPERIMENTAL

The corn starch used in the experiments was supplied by Hungrana Ltd. Hungary and its water content was below 1 %. Glycerol with low, 0.5 % water content was obtained from Molar Chemicals Ltd. Hungary and it was used without further purification or dry-ing. Three different wood fibers were used as reinforcement, all three obtained from Ret-tenmeier and Söhne GmbH, Germany. The particle characteristics of the fibers are listed in Table 1. Since in several of our previous projects aspect ratio turned out to be one of the most important characteristics of wood fibers [9,10], we included it in the abbreviation of the samples used. Accordingly, WP126 indicates a wood fiber with an aspect ratio of 12.6. Composites were prepared with 5, 10, 15, 20, 30 and 40 vol% wood content.

Thermoplastic starch containing 36 wt% glycerol was prepared by dry-blending

in a Henschel FM/A10 high speed mixer at 2000 rpm. The wood fibers were introduced into the mixer after plasticization. TPS granules were produced by processing the dry-blend on a Rheomex 3/4" single screw extruder attached to a Haake Rheocord EU 10 V driving unit at 140-150-160 °C barrel and 170 °C die temperatures, and at 60 rpm screw speed. The granules were injection molded into standard ISO 527-1 tensile bars using a Battenfeld BA 200 CD hydraulic machine at 140-160-170 °C barrel, 180 °C nozzle and 40 °C mold temperatures, 140 bar injection pressure, 54 mm/s injection rate, and 35 s cooling time.

Table 4.1 Particle characteristics of the studied lignocellulosic fibers

Fiber Abbr. D[4,3]a ( m)

Lengthb ( m)

Db ( m)

Aspect

ratiob

Spec. surface (m2/g)

Arbocel CW 630 W35 39.6 93.5 33.3 3.5 1.44

Filtracel EFC 1000 W68 213.1 363.4 63.9 6.8 1.50

Arbocel FT 400 W126 171.2 235.2 21.8 12.6 3.50

a) volume average particle size b) average values of wood particles diameter determined from scanning electron micro-graphs


57

The mechanical properties of the specimens were characterized by tensile testing using and Instron 5566 universal testing machine at 5 mm/min cross-head speed and 115 mm gauge length. Micromechanical deformations during tensile testing were followed by recording acoustic emission signals with a Sensophone AED 40/4 apparatus with 20 db threshold. Particle characteristics of the fibers and composite structure were characterized by scanning electron microscopy (SEM) using a Jeol JSM-6380 equipment. SEM micro-graphs were taken from the fracture surface of composite samples created during tensile testing. The crystalline structure of the TPS matrix, the wood fibers and the composites was characterized by X-ray diffraction using a Phillips PW 1830/PW 1050 equipment with CuKα radiation at 40 kV and 35 mA anode excitation. Water absorption was deter-mined at 23 °C and 52 % relative humidity by the measurement of the weight increase of the specimens. The desired relative humidity was achieved with saturated solution of Mg(NO3)2. Shrinkage was followed by the measurement of the length of the samples as a function of time. The specimens were kept in a desiccator over silica gel to avoid the complicating influence of water absorption. 4.3. RESULTS AND DISCUSSION

The results are presented in several sections. First mechanical properties and struc-ture are presented and then the reinforcing effect of the fiber is discussed in the next section. Special attention is paid to interfacial interactions, and the effect of wood rein-forcement on functional properties is discussed in the final section of the paper. 4.3.1. Mechanical properties

Stiffness is used the most frequently to estimate the effect of fillers and fibers on the properties of composites. The modulus of our TPS/wood composites is plotted against fiber content in Fig. 4.1. According to the figure stiffness increases from around 0.35 GPa up to almost 5 GPa, i.e. modification results in very strong reinforcement. The initial steep increase slows down at larger fiber contents indicating some structural effect. In wood composites this latter was shown to be the mere physical contact of the particles due to large wood content [9-11] or a kind of network formation in the case of fibers with large aspect ratio [12]. The results support this explanation since the largest deviation from the tendency observed at small fiber content occurs for the small (W35) and the long (W126) fibers (see Table 4.1). This latter has larger reinforcing effect at small fiber contents than the other two.

The tensile strength of composites containing the three wood fibers is plotted against wood content in Fig. 4.2. The same conclusions can be drawn here as from the composition dependence of modulus. Strength increases at least as strongly as stiffness and the different fibers influence also this property dissimilarly. Small and long particles reinforce TPS more than the third wood flour, but structural effects are also stronger in these two cases. Deformability, i.e. the elongation-at-break of the specimens decreases drastically with increasing wood content (not shown), but such an effect is usually ex-pected both in particulate filled and fiber reinforced composites.

Chapter 4

58

0.0 0.1 0.2 0.3 0.4 0.50

1

2

3

4

5

6

You

ng's

mod

ulus

(GPa

)

Volume fraction of wood

Figure 4.1 Effect of wood fiber type and content on the stiffness of TPS/wood composites;

( ) W126, ( ) W68, ( ) W35.

0.0 0.1 0.2 0.3 0.4 0.50

10

20

30

40

Tens

ile st

reng

th (M

Pa)


Figure 4.2 Dependence of the tensile strength of TPS/wood composites on wood content

and type; ( ) W126, ( ) W68, ( ) W35.


59

We can conclude from these results that wood reinforcement is beneficial if stiff-ness and strength are the targeted properties, considerable reinforcement occurs, but its extent is difficult to judge from the qualitative evaluation of the results. 4.3.2. Reinforcement

Reinforcement, i.e. the load-bearing capacity of the second component, can be es-timated quantitatively by the use of the simple model developed earlier to describe the composition dependence of tensile yield stress [13], tensile strength [14] or fracture re-sistance [15]. The composition dependence of tensile strength is plotted in the linear form of Eq. 2.1 in Fig. 4.3.

0.0 0.1 0.2 0.3 0.4 0.51

2

3

4

5

6

ln(r

educ

ed te

nsile

stre

ngth

)


Figure 4.3 Tensile strength of TPS/wood composites plotted against wood content in the

representation of Eq. 2.2. ( ) W126, ( ) W68, ( ) W35.

We obtain straiglit lines at small fiber loadings indeed, but deviations are observed from linearity at large wood contents. Such deviations usually indicate structural effects, i.e. they confirm our tentative explanation described in the previous section. The slope of the straight lines is parameter B, which expresses the load-bearing capacity of the second component, i.e. reinforcement. The value of B changes from 9.4 to 12.6 for the three wood fibers used, and these values are very large compared to usual particulate filled commod-ity polymers; as mentioned earlier B is often smaller than 1 for PP/CaCO3 composites, for example [14,16].

Chapter 4

60

The reinforcing effect of a filler or fiber depends on interfacial interactions, but also on the properties of the matrix. We can see that the strength of the matrix plays an important role in the actual value of B. This role can be seen even better if we plot B values as a function of the natural logarithm of reciprocal matrix strength, i.e. ln(1/ 0). The correlation is presented for various thermoplastic wood composites in Fig. 4.4. We can see that the inherent property of the matrix dominates in the determination of the extent of reinforcement. However, reinforcement depends also on interfacial adhesion, thus we need further study and analysis to define its strength.

-4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.50

3

6

9

12

15

PP

Para

met

er B

Matrix property, ln(1/

TPS

PLA

PVC

Figure 4.4 Influence of matrix strength on the reinforcing effect of wood fibers

(see Eqs. 2.1-2.2).

4.3.3. Interfacial adhesion

As described in the introductory part, several approaches are used for the estima-tion of the strength of interfacial interactions in TPS/natural fiber composites. FTIR and the determination of the glass transition temperature were mentioned as specific exam-ples. The temperature dependence of the loss modulus of TPS and that of the composites containing the three wood reinforcements at 30 vol% is plotted in Fig. 4.5. Both TPS and the composites exhibit two transitions, one at low temperature which can be assigned to the relaxation of the glycerol-rich phase [17,18], while the second above room tempera-ture which is identified as the glass transition temperature of TPS. Reinforcement with wood results in considerable increase of this latter indicating large decrease in the mobil-ity of starch molecules. This is rather surprising, since very small changes can be seen in


61

the Tg of particulate filled polymers and only at large contact surfaces [19,20]. The prob-able reasons for the large effect are strong hydrogen bonds and the stiffness of starch molecules; a few secondary bonds are sufficient to decrease mobility. Even the presence and competitive interaction of the small glycerol molecules do not influence this effect. Wood properties do not seem to play a role here, but we must mention that the specific surface area of the three fillers is very similar. We can conclude that strong interactions develop at the segmental and molecular level, but we do not know much about macro-scopic interfaces.

-100 -50 0 50 1000

20

40

60

80

100

120

140

Loss

mod

ulus

, 10-6

E"

(Pa)

Temperature (°C)

TPS

composites

Figure 4.5 Temperature dependence of the loss modulus of TPS and its wood composites

at 30 vol% wood content; decreased mobility in the presence of wood; ( ) W126, ( ) W68, ( ) W35.

Various deformation processes may take place around the inclusions in heteroge-neous polymer systems including composites. Besides debonding, i.e. the separation of the interface between the matrix and the reinforcement, several other processes including matrix yielding, the pull-out or fracture of the fibers, cavitation, etc. may also take place during the deformation of the composites [11,21-24]. Some of these are burst like pro-cesses emitting elastic waves, which can be picked up by appropriate sensors.

Chapter 4

62

0 2 4 6 8 10 12 140

2

4

6

8

10

12

0

1000

2000

3000

4000

5000

6000

7000

8000

Tens

ile st

ress

(MPa

)

Elongation (%)

Cum

mulative N

o. of hits

AE

Figure 4.6 Acoustic emission testing of a TPS/W126 composite. Wood content: 5 vol%.

( ) individual acoustic events. Stress and cumulative number of events vs. elongation traces are plotted as reference. Determination of characteristic stress ( AE).

The result of such an acoustic emission measurement is presented in Fig. 4.6. The small circles are individual acoustic events, one of the continuous lines shows the cumu-lative number of events (right axis), while the other is the stress vs. elongation trace (left axis) shown as reference. We can see that no acoustic activity can be detected below certain deformation or stress, and a characteristic stress ( AE) can be determined in the way indicated in Fig. 4.6. This stress is related to the initiation of the dominating micro-mechanical deformation process taking place around wood particles upon deformation. The composition dependence of this characteristic stress is plotted against wood content in Fig. 4.7.

We can see differences in the effect of the three fibers; the longest and largest fibers initiate the process at larger, while the small particles at smaller stress. We cannot identify the dominating deformation process either from the value of the characteristic stress or the shape of the cumulative number of signal trace; it can be either debonding or the fracture of the wood particles, as shown earlier [9-11].


63

0.0 0.1 0.2 0.3 0.4 0.50

10

20

30

40

W35

W68

Cha

ract

eris

tic st

ress

, A

E (MPa

)


W126

Figure 4.7 Characteristic stress of the dominating micromechanical deformation mech-

anism plotted against fiber content; ( ) W126, ( ) W68, ( ) W35.

If debonding is the dominating process, interfacial adhesion can be estimated quantitatively with the help of a model developed earlier[25]. According to the model debonding stress can be expressed as

2/1

21 RFE

CC aTD (4.1)

where D and T are debonding and thermal stresses, respectively, E the Young's modulus of the matrix, Fa strength of adhesion, R the radius of the particles, while C1 and C2 are geometric constants related to the debonding process. If we know the parameters of the equation, which were calculated from measurements done on polymer/filler pairs with known characteristics (E, R, Fa), as well as the stiffness of the matrix and the size of the particles, which we usually do, we can calculate the strength of adhesion (Fa) [26].

In our case we can assume that D = AE and calculate Fa from Eq. 4.1. The results are listed in Table 4.2 for the three sets of composites. Polypropylene (PP) reinforced with one of the fillers (W35) is used as reference. In the absence of coupling interaction is poor in PP, while covalent bonds are assumed to form when a functionalized coupling agent (MAPP) is used. We can see one order of magnitude difference between the two cases. The adhesion calculated in the TPS composite containing the small particles (W35)

Chapter 4

64

is similar as in PP at good adhesion, but considerably larger values are obtained for the other two fibers indicating very good adhesion. Table 4.2 Comparison of estimated interfacial adhesion in TPS/wood and PP/wood com-

posites

Matrix Fiber Coupling Adhesion, Fa (mJ/m2)

B Bln( 0-1)

PP W35 no 150 2.3 -6.8

PP W35 MAPP 1190 4.7 -13.8

TPS W35 no 1200 11.6 -21.1

TPS W68 no 3800 9.4 -17.1

TPS W126 no 3900 12.6 -22.9

Since parameter B in Eqs. 2.1-2.2 is also related to interfacial adhesion, we can also use it for the estimation of the strength of interaction, but we must compensate for the effect of changing matrix property. We did that by multiplying B by ln(1/ 0) and the results are also listed in Table 4.2. They convey practically the same message as Fa, i.e. interfacial adhesion is very strong in TPS/wood composites. Even if not debonding, but some other mechanism dominates in TPS composites, it is certain that strong interfacial interaction develops between the components considerably influencing properties.

We could not define the dominating micromechanical deformation process taking

place during the deformation of TPS/wood composites from the composition dependence of properties and acoustic emission measurements. We hoped that SEM micrographs taken from fracture surfaces created during tensile testing would offer further information about interfacial adhesion and processes occurring around the particles. Two micrographs are presented in Figs. 4.8a and b taken from composites containing 10 vol% fibers. The fracture surface of the TPS/W68 composite presented in Fig. 4.8a shows a very large broken particle which is firmly embedded in the matrix. A larger number of broken fibers can be observed on the surface of the TPS/W126 composite. Most of the fibers fracture perpendicularly to their axis, which occurs only at very strong interactions. The SEM study obviously confirms that interaction is strong in our TPS/wood composites and not debonding, but fiber fracture is the dominating process. Apart from the firm embedding of the particles into the TPS matrix very few characteristics features can be seen on the fracture surface of TPS/W35 composites (not shown).


65

a) b) Figure 4.8 SEM micrographs taken from the fracture surface of TPS/wood composites at

10 vol% wood content (a: W68, b: W126). The surfaces were created in tensile testing.

4.3.4. Functional properties

In order to fulfill their function, products prepared from TPS composites must sat-isfy certain conditions. The adsorption of excessive water modifies properties, stiffness and strength decrease and the part may fail completely. Similarly, shrinkage may result in unacceptable dimensional changes. Earlier studies showed that the reinforcement of TPS with natural fibers decreases its water uptake, i.e. results in the improvement of this characteristic. The equilibrium water uptake of the composites is plotted against wood content in Fig. 4.9. Equilibrium water absorption was determined from adsorption iso-therms by fitting the following form of Fick's law to the experimental results

tatataMMt 25exp251 9exp

91 exp81 2

(4.2)

where Mt is time dependent weight increase, M the final (equilibrium) water uptake reached after infinite time, t the time of adsorption and a a constant characterizing the overall rate of water adsorption.

Fig. 4.9 shows that water absorption decreases with wood content, as reported in the literature, and the effect of the three wood fibers is very similar. The broken line in Fig. 4.9 indicates additivity, i.e. the value of water uptake calculated from that of TPS and wood for each composition. We can see that the actual water absorption of the com-posites is smaller than the additive value. A probable explanation might be the strong interaction between the components discussed in the previous section. We must call the attention here to the fact though, that water absorption is rather large even at 40 vol% wood content, i.e. it might hinder the application of these materials in certain fields.

Chapter 4

66

The shrinkage of TPS is another important factor which may limit its application. Fig. 4.10 shows the photo of injection molded TPS tensile bars at different wood contents. A HDPE bar is used as reference. Shrinkage is extremely strong and some warpage also occurs. Such shrinkage obviously cannot be tolerated in certain applications. The reason of these large dimensional changes is the limited flexibility of the starch molecules and their long relaxation times. Injection molding results in non-equilibrium structure and the presence of the plasticizer allows relaxation, but only on a very long time scale. The figure clearly shows that shrinkage decreases drastically with increasing wood content.

0.0 0.1 0.2 0.3 0.4 0.58

10

12

14

16

Equi

libriu

m w

ater

abs

orpt

ion

(%)


Figure 4.9 Effect of wood content and fiber type on the equilibrium water absorption

(M ) of TPS/wood composites. The broken line indicates additivity; ( ) W126, ( ) W68, ( ) W35.

The time dependence of shrinkage is shown in Fig. 4.11 for the composites con-

taining the large aspect ratio fibers (W126). The presence of already 5 vol% fiber de-creases shrinkage considerably, while above 10 vol% shrinkage can be practically ne-glected, at least compared to that of neat TPS. We should like to call the attention to the long time scale of the experiment. The effect of wood content can be seen much better in Fig. 4.12 together with the effect of fiber characteristics. In accordance with literature data, fibers with larger aspect ratio hinder shrinkage in a larger extent than short particles. Obviously, wood reinforcement has very advantageous effect on the shrinkage of TPS and it may extend the application area for these materials considerably.


67

Figure 4.10 Shrinkage of injection molded TPS tensile bars.

0 100 200 300 4000

2

4

6

8

10

12

Shrin

kage

(%)

Time (day)

TPS

5 vol%

10 vol%

Figure 4.11 Time dependence of the shrinkage of TPS/W126 composites at different wood

contents; ( ) 0, ( ) 5, ( ) 10 vol%, (♦) ≥ 20 vol%.

Chapter 4

68

0.0 0.1 0.2 0.3 0.4 0.50

2

4

6

8

10

12

Equi

libriu

m sh

rinka

ge (%

)


Figure 4.12 Dependence of equilibrium shrinkage on wood content and fiber type; ( )

W126, ( ) W68, ( ) W35. 4.4. CONCLUSIONS

The modification of TPS with wood particles improves several properties consid-erably. Stiffness and strength increases, and the effect is stronger for fibers with larger aspect ratio. Wood fibers reinforce TPS considerably due to poor matrix properties and strong interfacial interactions, the latter resulting in the decreased mobility of starch mol-ecules and in the fracture of large wood particles during deformation. Strong interfacial adhesion leads to smaller water absorption than predicted from additivity, but water up-take remains relatively large even in the presence of wood particles. The shrinkage of injection molded TPS parts is very large around 10 % and dimensional changes occur in a long period of several hundred hours. Shrinkage decreases considerably to a very low level already at 15-20 vol% wood content rendering the composites good dimensional stability. Wood reinforcement of TSP is generally advantageous for most application ar-eas.


69

4.5. REFERENCES 1. Markarian, J., Plastics, Additives and Compounding 10, 22-25 (2008) 2. Averous, L., Fringant, C., Moro, L., Polymer 42, 6565-6572 (2001) 3. Prachayawarakorn, J., Ruttanabus, P., Boonsom, P., J. Polym. Environ. 19, 274-

282 (2011) 4. Lu, Y. S., Weng, L. H., Cao, X. D., Carbohydr. Polym. 63, 198-204 (2006) 5. Averous, L., Boquillon, N., Carbohydr. Polym. 56, 111-122 (2004) 6. Soykeabkaew, N., Laosat, N., Ngaokla, A., Yodsuwan, N., Tunkasiri, T., Compos.

Sci. Technol. 72, 845-852 (2012) 7. Martins, I. M. G., Magina, S. P., Oliveira, L., Freire, C. S. R., Silvestre, A. J. D.,

Neto, C. P., Gandini, A., Compos. Sci. Technol. 69, 2163-2168 (2009) 8. Curvelo, A. A. S., de Carvalho, A. J. F., Agnelli, J. A. M., Carbohydr. Polym. 45,

183-188 (2001) 9. Renner, K., Móczó, J., Pukánszky, B., Compos. Sci. Technol. 69, 1653-1659 (2009) 10. Renner, K., Kenyó, C., Móczó, J., Pukánszky, B., Composites Part a-Applied

Science and Manufacturing 41, 1653-1661 (2010) 11. Dányádi, L., Renner, K., Móczó, J., Pukánszky, B., Polym. Eng. Sci. 47, 1246-1255

(2007) 12. Faludi, G., Hári, J., Renner, K., Móczó, J., Pukánszky, B., Compos. Sci. Technol.

77, 67-73 (2013) 13. Turcsányi, B., Pukánszky, B., Tüdős, F., Journal of Materials Science Letters 7,

160-162 (1988) 14. Pukánszky, B., Composites 21, 255-262 (1990) 15. Pukánszky, B., Maurer, F. H. J., Polymer 36, 1617-1625 (1995) 16. Kiss, A., Fekete, E., Pukánszky, B., Compos. Sci. Technol. 67, 1574-1583 (2007) 17. Forssell, P. M., Mikkilä, J. M., Moates, G. K., Parker, R., Carbohydr. Polym. 34,

275-282 (1997) 18. Avérous, L., Pollet, E., in Environmental Silicate Nano-Biocomposites SE - 2,

Avérous, L., Pollet, E. (eds.), Springer: London (2012) pp.13-39. 19. Bleach, N. C., Nazhat, S. N., Tanner, K. E., Kellomäki, M., Törmälä, P.,

Biomaterials 23, 1579-1585 (2002) 20. Mortezaei, M., Famili, M. H. N., Kokabi, M., J. Appl. Polym. Sci. 115, 969-975

(2010) 21. Renner, K., Henning, S., Móczó, J., Yang, M. S., Choi, H. J., Pukánszky, B., Polym.

Eng. Sci. 47, 1235-1245 (2007) 22. Renner, K., Móczó, J., Suba, P., Pukánszky, B., Compos. Sci. Technol. 70, 1141-

1147 (2010) 23. Imre, B., Keledi, G., Renner, K., Móczó, J., Murariu, M., Dubois, P., Pukánszky,

B., Carbohydr. Polym. 89, 759-767 (2012) 24. Faludi, G., Dora, G., Renner, K., Móczó, J., Pukánszky, B., Carbohydr. Polym. 92,

1767-1775 (2013) 25. Pukánszky, B., Vörös, G., Compos. Interfaces 1, 411-427 (1993) 26. Renner, K., Móczó, J., Vörös, G., Pukánszky, B., Eur. Polym. J. 46, 2000-2004

(2010)

PLA/plasticized starch blends

71

Chapter 5

Interactions, structure and properties of PLA/Plasticized starch blends4

5.1. INTRODUCTION

In order to maintain one of the most important advantages of starch, i.e. biodegra-

dability, it is modified mostly with aliphatic polyesters and very often with PLA [1-5]. The components are immiscible, heterogeneous blends form upon mixing, but the state-ments about the compatibility of TPS and aliphatic polyesters are often contradictory. Averous and Fringant [6,7] found dissimilar compatibility in various starch/aliphatic pol-yester blends. Martin and Averous [8] claimed low level of compatibility in PLA/TPS blends, but in another part of their paper they state that changes in Tg indicate some inter-action between TPS and PLA. The degree of compatibility claimed varies in a wide range depending on blend components and the authors of the paper. Ma et al. [9,10], for exam-ple, found complete immiscibility in PLA/TPS blends when the plasticizer was glycerol, while good compatibility if formamide was used. However, the statements on compati-bility or miscibility are almost invariably qualitative in nature and based on the observa-tion of SEM micrographs, changes in Tg or mechanical properties, but rarely on thermo-dynamic consideration. Poor compatibility of the components is also indicated by at-tempts to improve interactions by preparing hybrid blends [11], adding amphiphilic mol-ecules [12] or a coupling agent [13].

Another interesting question that is never discussed or even mentioned in relation

with TPS blends is the role of the plasticizer. Although many papers have been published on the most diverse plasticizers in TPS, their efficiency and the structure formed, TPS is treated as a single, homogeneous material in blends, in spite of the fact that TPS plasti-cized with glycerol was claimed to phase separate above a certain plasticizer content [6,7,14-17]. The distribution of a third material, e.g. the aliphatic polyester, in a two phase polymer, TPS, is not straightforward, several structures may form and must be consid-ered. Moreover, the plasticizer is a small molecular weight substance which is quite mo-bile at the temperature of processing, but also under ambient conditions. Although origi-nally it is located in starch, it may diffuse into the polymer during processing and partition between the two polymer components. Although Ma et al. [9,10] claimed improvement in compatibility upon the use of formamide compared to glycerol, they did not explain the effect or its mechanism either. In view of these questions, the goal of this part of the project was to estimate interactions in PLA/TPS blends by thermomechanical analysis and thermodynamic modeling, to study the role and partitioning of the glycerol plasticizer in the components and to determine the structure and properties of the blends in a wide composition range. ______________________________________________________________________ 4Müller P., Móczó J., Bere J., Erika F., Mester D., Kállay M., Gyarmati B., Pukánszky B. Interactions, struc-ture and properties of PLA/plasticized starch blends

Chapter 5

72

5.2. EXPERIMENTAL 5.2.1. Materials

The PLA used was obtained from NatureWorks (USA). The selected grade (Ingeo

4032D, Mn = 88500 g mol-1 and Mw/Mn = 1.8) is recommended for extrusion. The poly-mer (<2 % D isomer) has a density of 1.24 g cm-3, while its melt flow index (MFI) is 3.9 g/10 min at 190 °C and 2.16 kg load. The corn starch used for the preparation of TPS was supplied by Hungrana Ltd., Hungary and its water content was 12 wt%. Glycerol with 0.5 wt% water content was obtained from Molar Chemicals Ltd., Hungary and it was used for the plasticization of starch without further purification or drying. Thermoplastic starch samples containing 36 and 47 wt% glycerol (TPS36 and TPS47, respectively) were pre-pared and used in the experiments. The composition of the PLA/TPS blends changed from 0 to 1 volume fraction in 0.1 volume fraction steps. Glycerol was added to PLA also alone as a "plasticizer" in 1, 3, 5, 7 and 10 vol%.

5.2.2. Sample preparation

Corn starch was dried in an oven before composite preparation (105 °C, 24 hours).

Thermoplastic starch powder was prepared by dry-blending in a Henschel FM/A10 high speed mixer at 2000 rpm. TPS was produced by processing the dry-blend on a Rheomex 3/4" single screw extruder attached to a Haake Rheocord EU 10 V driving unit at 140-150-160 °C barrel and 170 °C die temperatures, and 60 rpm screw speed.

PLA and the second component were homogenized in an internal mixer (Bra-

bender W 50 EHT) at 190 °C and 50 rpm for 12 min. Before homogenization poly(lactic acid) was dried in a vacuum oven (110 °C, 4 hours). Both temperature and torque were recorded during homogenization. The melt was transferred to a Fontijne SRA 100 com-pression molding machine (190 °C, 5 min) to produce 1 mm thick plates for further test-ing. 5.2.3. Characterization

The glass transition temperature of the phases and other thermal transitions ap-

pearing in the blends were determined with dynamic mechanical analysis (DMA) using a Perkin Elmer Pyris Diamond DMA apparatus in tensile mode with constant amplitude (10 μm) and frequency (1 Hz) in the temperature range between -100 and 80 °C. Heating rate was 2 °C/min, while the size of the specimens was 50 x 5 x 1 mm. Mechanical prop-erties were further characterized by tensile testing using an Instron 5566 universal testing machine. Specimens of 150 x 10 x 1 mm were used, the gauge length was 115 mm. Ten-sile modulus was determined at 0.5 mm/min, while properties measured at larger defor-mations at 5 mm/min cross-head speed. Five parallel measurements were carried out at each blends composition. The structure of the blends was analyzed by scanning electron microscopy (SEM) using a Jeol JSM 6380 LA apparatus. Samples were broken at liquid nitrogen temperature and then a smooth surface was created by cutting the sample with a microtome. Surfaces were etched according to the matrix polymer, chloroform was used in the case of PLA, while 1 mol HCl for the TPS matrix. Light transmittance through the


73

samples was measured on 1 mm thick specimens using a Unicam UV-500 spectropho-tometer at 700 nm wavelength. Water absorption was determined at 23 °C and 52 % rel-ative humidity by the measurement of the weight increase of specimens. The desired rel-ative humidity was achieved with a saturated solution of Mg(NO3)2. 5.3. RESULTS AND DISCUSSION

The results are presented in several sections. The question of component interac-tion and miscibility is discussed first and then the effect of the plasticizer on PLA prop-erties is presented subsequently. Conclusions drawn from these results are supported by model calculations in the next section followed by the discussion of structure and prop-erties. 5.3.1. Interactions, miscibility

One of the most often used approaches to estimate the miscibility of two compo-nents is the determination of the composition dependence of their glass transition temper-ature. Complete miscibility results in a single glass transition temperature in between those of the components, while complete immiscibility results in two transition tempera-tures which correspond to those of the two polymers in question [18]. A shift in glass transition temperatures indicates partial miscibility. Martin and Averous [8] claimed par-tial miscibility of PLA and TPS based on this approach.

-90 -60 -30 0 30 60 90 120105

106

107

108

109

1010

10-2

10-1

100

101

Mod

ulus

, E',

E" (G

Pa)

Temperature ( C)

Loss tangent, tg

E'

E"

tg

Figure 5.1 DMA spectra showing relaxation transitions in the PLA/TPS47 blend contain-

ing 50 vol% of both components.

Chapter 5

74

The DMA spectra of a PLA/TPS47 blend containing 50 vol% of both components is presented in Fig. 5.1. The spectra are rather complicated exhibiting several transitions. Plasticized starch were shown to present two transitions, one corresponding to its glass transition, while the second was claimed to result from phase separation and was assigned to a glycerol rich phase [6,7,14-17]. The glass transition temperature depends very strongly on the type and amount of the plasticizer and it may cover a wide range of tem-peratures from -30 to 90 °C. The glass transition temperatures of our TPS samples are -3 and 33 °C for the plasticized starch TPS47 and TPS36, respectively. The second transition assigned to thermoplastic starch can be also identified in the spectra allegedly indicating the presence of a glycerol rich phase [6,7,14-17]. However, some doubts may arise about the assignment of this transition. Some authors argue that the appearance of two peaks proves a priori phase separation [15] that is not true; several polymers are known to show

relaxation peaks without the presence of a second component (e.g. PMMA, PVC). The possibility of phase separation was mentioned only as a possibility anyway in the much cited paper of Kalichevsky [14], later it was treated as fact [6,7]. Basically none of the papers mentioning phase separation shows any other evidence, but reference to earlier papers and the appearance of two peaks in DMA spectra. Occasionally, the claim might be supported by the analysis of the composition dependence of Tg or the determination of the activation energy of the transition, but no direct evidence. This transition temperature is around -50 or -60 °C that is claimed to be close to the Tg of glycerol at -78 °C and used as a strong argument for the formation of a glycerol rich phase. Phase separation was said to occur above 27 wt% glycerol content. Unfortunately, a number of facts contradict or at least question the hypothesis of phase separation. First of all we do not believe that -50 °C is close to -78 °C. The peak appeared also at 15 wt% glycerol content in the work of Mikus et al. [17] below the claimed composition of phase separation. Moreover, the transition was detected at around 0 and -10 °C in TPS plasticized with sorbitol and man-nose, respectively, as shown by the same paper [17]. Finally, similar transition was observed in grafted cellulose acetate and benzylated wood which do not contain any ex-ternal plasticizer [19,20]. Accordingly, we think that the transition belongs to smaller units of the amylopectin chain, like a single glucose ring, the movement of which be-comes possible upon plasticization. The position of the peak depends on the type and amount of the plasticizer which forms strong secondary interactions with starch mole-cules. However, the possible phase separation of the glycerol might be important for the phase structure and properties of TPS blends and cannot be ignored. The limited misci-bility of starch and glycerol is shown by the exudation of the plasticizer at large glycerol content and Stading et al. [21] showed heterogeneous structure in their plasticized starch films.

The remaining two transitions observed in the spectra of the PLA/TPS blend shown in Fig. 5.1 can be assigned to PLA. The glass transition of the polymer results in a sharp peak on the tan trace and the cold crystallization of the polymer results in an-other, smaller transition as well. Independently of the debate on the assignment of the transition of TPS, the glass transition temperatures of the two polymers can be identified unambiguously. Conclusions about miscibility are often drawn from the composition de-pendence of the glass transition temperature of the blend or its components as mentioned above.


75

0 20 40 60 80 100-20

0

20

40

60

80

Rel

axat

ion

tem

para

ture

(°C

)

TPS content (vol%)

PLA

TPS36

TPS47

Figure 5.2 Effect of composition on the glass transition temperature of the components

in PLA/TPS blends; ( , ) TPS36, ( , ) TPS47; full: PLA, empty: TPS.

The Tg of the two blend series is plotted against composition in Fig. 5.2. Separate transition temperatures can be identified for the two polymers. The Tg of PLA is detected at around 65 °C, it does not seem to depend on the amount of plasticizer in TPS and decreases slightly with increasing TPS content. Similar changes were observed by others and led them to the conclusion of partial miscibility [8]. However, the Tg of PLA was shown to decrease also in the presence of particulate fillers [22] and we cannot claim partial miscibility in that case. Moreover, if the components are partially miscible, the Tg of TPS should increase, instead of decreasing with increasing PLA content. The reason for the decrease is unclear and needs further study and considerations. A tentative expla-nation might be that the presence of PLA chains increases the mobility of amylopectin, but this hypothesis needs further proof. The only conclusion that we can draw from the analysis of DMA spectra and the composition dependence of the Tg of the components is that they are immiscible with each other and even partial miscibility must be very small.

Other measurements may supply further, indirect evidence about the miscibility or immiscibility of the components. The light transmittance of the blends is plotted against composition in Fig. 5.3. PLA is transparent, 68 % of the incident light passes through it. Transmittance of TPS is smaller 14.7 and 20.2 % for TPS47 and TPS36, respectively, indicating a slightly heterogeneous structure. However, the transparency of the PLA/TPS blends is extremely small, it is below 1 % at most compositions indicating very poor miscibility and a heterogeneous structure.

Chapter 5

76

0 20 40 60 80 1000

4

8

12

16 TPS36 TPS47

Ligh

t tra

nsm

ittan

ce (%

)

TPS content (vol%)

68.1 %

Figure 5.3 Effect of blend composition on the light transmittance of PLA/TPS blends; ( )

TPS36, ( ) TPS47.

0 20 40 60 80 1000

2

4

6

8

10

12

Equi

libriu

m w

ater

upt

ake

(%)

TPS content (vol%)

Figure 5.4 Independence of the water absorption of PLA/TPS blends of blend composi-

tion and glycerol content; ( ) TPS36, ( ) TPS47.


77

Obviously, interactions between the two blend components cannot be very strong and phase separation occurs at all compositions. Another indication for poor interactions is supplied by the results of water absorption measurements (Fig. 5.4). If the interaction between two polymer components is strong, water or any other solvent must compete for active sites and absorption will be smaller than that dictated by additivity. As Fig. 5.4 shows, water absorption in PLA/TPS blends follows almost perfectly additivity indicating the weak interaction of the components.

0 2 4 6 8 10 120

20

40

60

80

Ligh

t tra

nsm

ittan

ce (%

)

Glycerol content (vol%)

Figure 5.5 Effect of glycerol content on the light transmittance of PLA/glycerol binary

blends. 5.3.2. PLA/glycerol interaction

Similarly to others we treated TPS as a single, homogeneous material up to now. However, to check the possible effect of glycerol diffusion into PLA, measurements were done on PLA modified with different amounts of glycerol. The transmittance of light through 1 mm thick PLA plates containing various amounts of the plasticizer is plotted against composition in Fig. 5.5. PLA is transparent, as mentioned above, but transparency decreases considerably with increasing glycerol content. According to the correlation the solubility of glycerol cannot be more than 1 or 2 vol% in PLA, above this amount a sep-arate phase forms. Dispersed glycerol droplets scatter light resulting in inferior transpar-ency. Obviously, the solubility of glycerol in PLA is quite small, thus one cannot expect considerable diffusion of the plasticizer from starch into PLA in the blends.

Chapter 5

78

-120 -80 -40 0 40 80 120106

107

108

109

1010

10-2

10-1

100

101

Mod

ulus

, E',

E" (P

a)

Temperature ( C)

Loss tangent, tg

E'

E"

tg

Figure 5.6 DMA spectra of the PLA/glycerol binary blend containing 10 vol% of the plas-

ticizer. Strong glycerol transition at low temperature.

Limited solubility of glycerol in PLA is further confirmed by the results of DMA measurements. The spectra recorded on PLA containing 10 vol% glycerol is presented in Fig. 5.6. The spectra show the typical behavior of PLA, glass transition around 65 °C and the subsequent cold crystallization of the polymer. However, an additional transition also appears on the spectra at very low temperatures, around -80 °C. The transition can be assigned to glycerin and its temperature corresponds to that determined for glycerol by Averous et al. [6] using DSC. The intensity of the tg peak increases with increasing amount of glycerol in the blend, further confirming the assignation (Fig. 5.7). Since the transition can be detected already at the smallest glycerol content, below 1 vol%, we must assume that the solubility of glycerol in the blend is smaller even than this amount, since dissolved glycerol would shift the glass transition temperature of PLA, but would not appear as a separate peak on the tg vs. temperature trace. The study of PLA/glycerol blends confirmed our previous conclusions and indicated that the interaction between PLA and the plasticizer is relatively weak resulting in very limited solubility.


79

0 2 4 6 8 10 120.00

0.01

0.02

0.03

0.04

0.05

tg

inte

nsity


Figure 5.7 Influence of composition on the intensity of the tg peak assigned to the glass

transition of glycerol. 5.3.3. Modeling

Density functional theory (DFT) and local coupled-cluster calculations with sin-gle, double and perturbative triple excitations [LCCSD(T)] have been performed to in-vestigate hydrogen bond interactions between poly(lactid acid) and glycerol, amylose and glycerol, as well as PLA and amylose. In order to reduce time and computer capacity to a reasonable level, simplified computational model systems for linear PLA and amylose were introduced. A trimer structure of three L-lactic acid monomer units was created for the former in which the terminal hydrogens of both OH groups were replaced by methyls in order to avoid the formation of hydrogen-bonded complexes by them during geometry optimization. The amylose model system was a singe monomer unit, that is, a single -D-glucose molecule, in which (1,4) OH groups were replaced by methoxy groups for the same reason. Fig. 5.8 shows the corresponding structures indicating hydrogen bond do-nors (in blue) and acceptors (in red) as well as the acronym for each site used for the identification of the various hydrogen-bonded complexes. All possible donor-acceptor pairs were considered.

Chapter 5

80

a) b)

c)

Figure 5.8 Structure of the computational model systems and acronyms used for the iden-tification of hydrogen bond donors (in blue) and acceptors (in red) studied in this work: (a) trimer of L-lactic acid with terminal methyl protection; (b) glyc-erol; (c) -D-glucose with methyl protection on the (1,4) OH groups.


81

Table 5.1 Hydrogen bonding interaction energies and enthalpies calculated with the DFT and LCCSD(T) methods

M06-2X/6-311++G** LCCSD(T)/aug-cc-pVTZ

Species E (kJ/mol)

H (kJ/mol)

E (kJ/mol)

H (kJ/mol)

GLY(1H)-PLA(CO) -66 -56 -59 -49

GLY(2H)-PLA(CO) -78 -68 -69 -59

GLY(1H)-PLA(COC) -78 -67 -71 -60

GLY(2H)-PLA(COC) -79 -68 -75 -64

GLY(1H)-GLU(1OMe) -61 -52 -55 -46

GLY(2H)-GLU(1OMe) -82 -71 -72 -61

GLY(1H)-GLU(2OH) -87 -75 -77 -65

GLY(2H)-GLU(2OH) -91 -79 -82 -70

GLY(1H)-GLU(3OH) -76 -64 -66 -55

GLY(2H)-GLU(3OH) -77 -65 -71 -59

GLY(1H)-GLU(4OMe) -88 -76 -75 -63

GLY(2H)-GLU(4OMe) -58 -48 -53 -43

GLY(1H)-GLU(6OH) -75 -65 -69 -58

GLY(2H)-GLU(6OH) -56 -46 -51 -41

GLY(1H)-GLU(COC) -56 -46 -51 -41

GLY(2H)-GLU(COC) -83 -72 -72 -61

PLA(CO)-GLU(2OH) -65 -58 -60 -53

PLA(CO)-GLU(3OH) -56 -50 -53 -47

PLA(CO)-GLU(6OH) -52 -45 -52 -44

PLA(COC)-GLU(2OH) -58 -52 -54 -47

PLA(COC)-GLU(3OH) -50 -43 -45 -38

PLA(COC)-GLU(6OH) -47 -42 -45 -39

Equilibrium structures for the individual donor and acceptor molecules were ob-tained separately from fully unconstrained optimizations using DFT with the M06-2X functional of Zhao and Truhlar [23], which accurately describes weak non-covalent in-teractions. For these calculations the 6-311++G** basis set [24] was used. The donors and acceptors were then linked into hydrogen bonded complexes, and unconstrained ge-ometry optimizations were performed for these structures subsequently.

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The total energies of the species were converted into enthalpies using the rotational constants and harmonic vibrational frequencies calculated at the reference geometries. Using the DFT equilibrium structures, additional single point energy calculations were also carried out at the LCCSD(T) [25] level with the aug-cc-pVTZ basis set [26,27], where diffuse functions on hydrogen atoms were excluded. LCCSD(T) energies were also converted to enthalpies using corrections obtained from DFT calculations. Particu-larly, the difference between enthalpy and energy calculated with DFT was added to the energy obtained with LCCSD(T). Finally, the enthalpy of the hydrogen bonding interac-tion at both levels of theory was derived as the enthalpy difference between the complex and its constituent donor and acceptor sites. The Gaussian suite of programs [28] were invoked for the DFT calculations; all LCCSD(T) calculations were performed with MRCC.

a) b)

c)

Figure 5.9 Formation of hydrogen bonds between various pairs of the components in the

ternary system PLA/glycerol/starch; a) glycerol/lactic acid trimer, b) glyc-erol/glucose, c) lactic acid trimer/glucose.


83

Table 5.1 lists the calculated hydrogen bonding energies and enthalpies for the various donor-acceptor pairs defined by linking the corresponding model systems to each other. Figs. 5.9a-c show the three hydrogen bonded complexes with the smallest interac-tion enthalpies, that is, those structures of the three interacting pairs in which the strongest hydrogen bonds form.

According to our results presented in Table 5.1, the strongest interaction develops

between glycerol and the model system of glucose. The hydrogen bonding enthalpies calculated with DFT and LCCSD(T) are -79 and -70 kJ/mol, respectively, and, as it can be seen in Fig. 5.9b, three hydrogen bonds can form, in which both glucose and glycerol can act as donor and also acceptor at the same time. The interaction between glycerol and our PLA model is considerably weaker, binding enthalpies of -68 and -64 kJ/mol were obtained with DFT and LCCSD(T), respectively. In this case (Fig. 5.9a), two hydrogen bonds may form between the model PLA and glycerol. As shown in Fig. 5.9c, the for-mation of only a single hydrogen bond is possible between amylose and the polylactid acid model with a bonding enthalpy of -58 kJ/mol and -53 kJ/mol, respectively.

In order to estimate the amount of glycerol dissolving in PLA and starch, let us

assume that the number of binding sites in both phases is significantly larger than the number of glycerol molecules available. Accordingly, the glycerol molecules can be at-tached to the site with the strongest binding enthalpy, i.e. using the notation of Table 5.1, the GLY(2H)-PLA(COC) and GLY(2H)-GLU(2OH) complexes can form in PLA and amylose, respectively. The ratio of concentrations can be approximated by the equilib-rium constant of the GLY(2H)-PLA(COC) + GLU PLA + GLY(2H)-GLU(2OH) re-action. If we assume that binding entropy is approximately identical for the two com-plexes, the Gibbs free energy of the above reaction (ΔrG) equals the difference in the binding enthalpies of the complexes. From Table 5.1, using the more accurate LCCSD(T) enthalpies, we obtain the value of ΔrG = -6 kJ. Relying on the well-known formula for the equilibrium constant, K = exp(-ΔrG/RT), we obtain about K = 11 at room temperature indicating that the concentration of glycerol is by an order of magnitude larger in the amylose than in the PLA matrix. Accordingly, our results suggest that the interaction between glycerol and amylose is considerably stronger than that between glycerol and PLA, thus the glycerol molecules are most likely located within the starch phase of the blends thus confirming our conclusions drawn in the previous two sections.

Using the enthalpies determined in these calculations a lattice model was also cre-

ated for the estimation of the mutual solubility of the polymer phases and the effect of glycerol on it. One glucose, one glycerol and three lactide units were placed in the cells of the lattice and the entropy of mixing was calculated with the usual Boltzman probabil-ity function. The molecular weight of PLA was taken as 90000 g/mol, while 2000000 g/mol was used for starch assuming that it consist of pure amylose. Interactions were assumed to form only through hydrogen bonds, dipole-dipole and dispersion forces were neglected. Because of numerous simplifications and neglecting a number of factors, the results allow the drawing of only qualitative conclusions. A phase diagram generated from the calculated free energies is presented in Fig. 5.10. It indicates that PLA does not dissolve any TPS at all, while it can be dissolved in a small extent in plasticized starch. Glycerol facilitates the dissolution of PLA in TPS, a conclusion which is in agreement

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with those of Ma et al. [9,10] who found that a plasticizer can improve the compatibility of PLA and TPS. On the other hand, these calculations also confirm that the majority of glycerol molecules is located in the TPS phase and does not diffuse into PLA. We must emphasize here again that these calculations are very qualitative in manner shown also by the small glycerol concentration used. At larger glycerol content, it may form a sepa-rate phase, as discussed earlier, that would overly complicate calculations.

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

5

6

starch phase

PLA content, PLA

PLA phase

Gly

cero

l con

tent

(vol

%)

Figure 5.10 Phase diagram of PLA/TPS blends; effect of glycerol on miscibility. 5.3.4. Structure

The phase structure of the PLA/TPS blends was studied by scanning electron mi-croscopy. Etched surfaces were prepared in order to help the clear distinction of the phases. Three micrographs are presented in Fig. 5.11 to demonstrate the effect of compo-sition on structure. In accordance with previous results, heterogeneous structure forms at all compositions. PLA is the continuous phase at large PLA and small TPS content in which TPS is dispersed in the form of droplets of a few micron size (Fig. 5.11a). The opposite occurs at the other end of the composition range, PLA is dispersed as small particles in the continuous TPS phase here (Fig. 5.11c). A more or less co-continuous structure develops at around 50 vol% of both components as shown by Fig. 5.11 b. The concentration range of co-continuous structure is very narrow, dispersed structure was observed at both sides of 50/50 composition, i.e. both at 40 and 60 vol% TPS content. Both the dispersed structure and the narrow range of the interpenetrating network like structure are in strong agreement with the rest of our conclusions about the immiscibility of the two components and the development of only weak interactions between the phases.


85

a) b)

c)

Figure 5.11 Effect of composition on the dispersed structure of PLA/TPS blends, SEM micrographs at magnification 500x; a) 10, b) 90, c) 50 vol% TPS.

5.3.5. Properties

Conclusions about compatibility are often drawn from the composition depend-ence of mechanical properties. However, the properties of the components differ consid-erably from each other in our case, composition dependence is dominated by this differ-ence and it is very difficult to arrive to any reasonable conclusion as a consequence. This statement is demonstrated adequately by Fig. 5.12 showing the composition dependence of the Young's modulus for the two sets of blends. The stiffness of PLA is around 3 GPa, while it is around 60 MPa and less than 1 MPa for TPS36 and TPS47, respectively. Young's modulus decreases quite rapidly with increasing TPS content and the stiffness of the blends is always smaller than the one predicted by additivity, indicating again weak interactions.

Chapter 5

86

0 20 40 60 80 1000

1

2

3

4

You

ng's

mod

ulus

(GPa

)

TPS content (vol%)

TPS36

TPS47

Figure 5.12 Composition dependence of the Young's modulus of PLA/TPS blends; ( )

TPS36, ( ) TPS47.

0

10

20

30

40

50

60

0 20 40 60 80 1000

10

20

30

40

50

60

70

Tens

ile st

reng

th (M

Pa)

TPS content (vol%)

TPS36

TPS47

Elongation-at-break (%)

Figure 5.13 Tensile strength and elongation-at-break of PLA/TPS blends plotted against

TPS content; ( , ) TPS36, ( , ) TPS47; full: strength, empty: elongation-at-break.


87

The composition dependence of mechanical characteristics measured at yielding and at break is very similar to each other. Tensile strength and elongation-at-break are plotted against TPS content in Fig. 5.13. The correlations offer limited information again. The strength of PLA is deteriorated rapidly with increasing TPS content almost inde-pendently of glycerol content. The negative deviation from additivity indicates strong incompatibility of the components. The deformability of PLA and most of the blends is very small forecasting also poor impact properties that was confirmed by independent measurements. Changes in the elongation-at-break values of the blends shows that TPS properties dominate above 80 vol% PLA content, but PLA decreases the deformability of TPS quite fast.

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

TPS matrix

ln(r

educ

ed te

nsile

stre

ngth

)

Volume fraction of TPS

PLA matrix

Figure 5.14 Reinforcing effect of the second component expressed by the slope of the

straight lines in PLA/TPS36 blends; ( ) PLA matrix, ( ) TPS36 matrix.

The mechanical properties of blends also allow the estimation of the compatibility of blends. The simple model introduced in Chapter 2, which describes the composition dependence of tensile properties contains a term related to the load-bearing capacity of the dispersed phase and thus to the strength of interfacial adhesion [29-31]. Elimination of the influence of effective load-bearing cross section allows the calculation of reduced strength (see Eq. 2.2) the composition dependence of which should result in a straight line with the slope indicating the reinforcing effect of the dispersed component. The tensile strength of the PLA/TPS36 blends is plotted in this form in Fig. 5.14. Two lines are ob-tained depending on the matrix polymer. TPS does not have any reinforcing effect in PLA, the slope of the line is close to zero. This result is in accordance with previous conclusions saying that TPS does not dissolve in PLA at all and the interaction between

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the two phases is poor. PLA reinforces TPS in some extent, but this is a natural conse-quence of the much larger stiffness and strength of PLA. Any filler would reinforce TPS in a similar extent, it is purely a mechanical response of the weak matrix. Although ac-cording to the model calculations some PLA dissolves in TPS, interfacial adhesion is weak, since starch molecules are surrounded by glycerol, while entanglements cannot form either because of the stiffness of the amylose and amylopectin molecules thus stress cannot be transferred across the phases. Obviously the properties of these PLA/TPS blends are moderate at most and some coupling strategy much be applied in order to de-velop a material for practical use. 5.4. CONCLUSIONS

A detailed analysis of experimental results obtained on PLA/TPS blends supported

by model calculations showed that the interaction of the two components is weak. The investigation of the possible partitioning of glycerol in the two phases indicated that most of the plasticizer is located in the TPS phase and does not diffuse into PLA. Thermody-namic modeling predicted some dissolution of PLA in TPS which was assisted by the presence of the plasticizer, but TPS does not dissolve in PLA at all. As a consequence of weak interactions properties are moderate at most. Blending of the two components re-sulted in heterogeneous, two phase structure at all compositions. No tangible proof was found for the formation of a glycerol rich phase in TPS, the relaxation transition assigned to this phase was rather explained with the movement of smaller structural units of starch molecules. Weak interfacial adhesion does not allow stress transfer through the interface resulting in poor strength and small deformation. TPS deteriorates the properties of PLA considerably and although this latter polymer reinforces TPS somewhat, blends with a starch matrix are extremely weak. Useful materials can be produced from PLA and TPS only with the development of an appropriate coupling strategy. 5.5. REFERENCES 1. Shirai, M. A., Grossmann, M. V. E., Mali, S., Yamashita, F., Garcia, P. S., Müller,

C. M. O., Carbohydr. Polym. 92, 19-22 (2013) 2. Chakraborty, A., Sain, M., Kortschot, M., Cutler, S., J. Biobased Mater. Bio. 1, 71-

77 (2007) 3. Teixeira, E. D., Curvelo, A. A. S., Correa, A. C., Marconcini, J. M., Glenn, G. M.,

Mattoso, L. H. C., Ind. Crop. Prod. 37, 61-68 (2012) 4. Huneault, M. A., Li, H. B., Polymer 48, 270-280 (2007) 5. Li, H., Huneault, M. A., J. Appl. Polym. Sci. 119, 2439-2448 (2011) 6. Averous, L., Moro, L., Dole, P., Fringant, C., Polymer 41, 4157-4167 (2000) 7. Averous, L., Fringant, C., Polym. Eng. Sci. 41, 727-734 (2001) 8. Martin, O., Averous, L., Polymer 42, 6209-6219 (2001) 9. Wang, N., Yu, J. G., Chang, P. R., Ma, X. F., Carbohydr. Polym. 71, 109-118

(2008) 10. Ning, W., Jiugao, Y., Xiaofei, M., Polym. Compos. 29, 551-559 (2008) 11. Zhang, K., Mohanty, A. K., Misra, M., Acs Appl. Mater. Inter. 4, 3091-3101 (2012) 12. Yokesahachart, C., Yoksan, R., Carbohydr. Polym. 83, 22-31 (2011)


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13. Ren, J., Fu, H., Ren, T., Yuan, W., Carbohydr. Polym. 77, 576-582 (2009) 14. Kalichevsky, M. T., Jaroszkiewicz, E. M., Blanshard, J. M. V., Polymer 34, 346-

358 (1993) 15. Forssell, P. M., Mikkila, J. M., Moates, G. K., Parker, R., Carbohydr. Polym. 34,

275-282 (1997) 16. Lourdin, D., Coignard, L., Bizot, H., Colonna, P., Polymer 38, 5401-5406 (1997) 17. Mikus, P. Y., Alix, S., Soulestin, J., Lacrampe, M. F., Krawczak, P., Coqueret, X.,

Dole, P., Carbohydr. Polym. 114, 450-457 (2014) 18. Olabisi, O., Robeson, L. M., Shaw, M. T., Polymer - Polymer Miscibility, Academic

Press: New York (1979) 19. Szamel, G., Klebert, S., Sajo, I., Pukanszky, B., J. Therm. Anal. Calorim. 91, 715-

722 (2008) 20. Dominkovics, Z., Dányádi, L., Pukánszky, B., Composites Part A. 38, 1893-1901

(2007) 21. Stading, M., Rindlav-Westling, A., Gatenholm, P., Carbohydr. Polym. 45, 209-217

(2001) 22. Molnár, K., Móczó, J., Murariu, M., Dubois, P., Pukánszky, B., Express Polym. Lett.

3, 49-61 (2009) 23. Zhao, Y., Truhlar, D. G., Theor. Chem. Acc. 120, 215-241 (2008) 24. Krishnan, R., Binkley, J. S., Seeger, R., Pople, J. A., J. Chem. Phys. 72, 650-654

(1980) 25. Rolik, Z., Szegedy, L., Ladjanszki, I., Ladoczki, B., Kallay, M., J. Chem. Phys. 139

(2013) 26. Kendall, R. A., Dunning, T. H., Harrison, R. J., J. Chem. Phys. 96, 6796-6806

(1992) 27. Dunning, T. H., J. Chem. Phys. 90, 1007-1023 (1989) 28. Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A.,

Cheeseman, J. R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G. A., Nakatsuji, H., Caricato, M., Li, X., Hratchian, H. P., Izmaylov, A. F., Bloino, J., Zheng, G., Sonnenberg, J. L., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Montgomery Jr., J. A., Peralta, J. E., Ogliaro, F., Bearpark, M. J., Heyd, J., Brothers, E. N., Kudin, K. N., Staroverov, V. N., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A. P., Burant, J. C., Iyengar, S. S., Tomasi, J., Cossi, M., Rega, N., Millam, N. J., Klene, M., Knox, J. E., Cross, J. B., Bakken, V., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R. E., Yazyev, O., Austin, A. J., Cammi, R., Pomelli, C., Ochterski, J. W., Martin, R. L., Morokuma, K., Zakrzewski, V. G., Voth, G. A., Salvador, P., Dannenberg, J. J., Dapprich, S., Daniels, A. D., Farkas, Ö., Foresman, J. B., Ortiz, J. V., Cioslowski, J., Fox, D. J.; Gaussian, Inc.: Wallingford, CT, USA, 2009.

29. Pukánszky, B., Turcsányi, B., Tüdős, F., in Interfaces in polymer, ceramic, and metal matrix composites, Ishida, H. (ed.), Elsevier: New York (1988) pp.467-477.

30. Pukánszky, B., Composites 21, 255-262 (1990) 31. Fekete, E., Pukanszky, B., Peredy, Z., Angew. Makromol. Chem. 199, 87-101

(1992)

Physical ageing and molecular mobility in PLA

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Chapter 6

Physical ageing and molecular mobility in PLA blends and composites5

6.1. INTRODUCTION

PLA has created much interest recently, because of the combination of its biodeg-

radability with numerous advantageous properties. PLA is modified in many ways and modification changes all properties of the polymer, often in an unpredictable manner; contradictory results are reported on its in the literature. Similarly to other polyesters, PLA crystallizes relatively slowly [1,2]. Most products prepared by traditional melt pro-cessing technologies are amorphous as a result [3], although the properties of crystalline PLA might be more advantageous in several respects [4]. Occasionally partially crystal-line parts can be also produced depending on processing conditions and composition. Because of the importance of crystallinity and crystalline structure in the determination of properties, a very large number of studies have been dedicated to their investigation and many attempts have been made to modify them [1,5-9]. However, the determination of unambiguous correlations between crystallization conditions, composition and struc-ture is difficult because of the complicated structure and many transitions appearing on the DSC trace of PLA. During the heating of amorphous PLA the polymer goes through glass transition first (Tg), and then cold crystallization (Tcc, Hcc) occurs. The crystals formed in the latter process melt at higher temperature (Tm, Hm). PLA can crystallize in

, ', and forms [1], and crystal perfection, recrystallization and change of modifica-tion may result in multiple peaks during heating and sometimes also during crystallization [1]. Even the amorphous phase of semicrystalline PLA has a complicated structure; it is claimed to contain three types of amorphous molecules: the rigid amorphous fraction, the interspherulitic mobile amorphous phase and the intraspherulitic mobile amorphous phase [10].

Attempts have been made to modify the crystalline structure by nucleation. Talc

was claimed to nucleate PLA efficiently [2,11-14], and the combination of the filler with organic compounds was said to improve crystallization characteristics, i.e. nucleus den-sity and crystalline growth rate, even more [11,15]. However, the results are often ambig-uous and contradictory. Difficulties of interpretation are further increased by the fact that the nucleation efficiency of a certain substance is sometimes evaluated by its effect on the temperature of cold crystallization [6-8,16-20], while in other cases during cooling from a quiescent melt [13,14,21]. Crystallization can be accelerated also by the use of plasticizers [2,22-27] which increase the mobility of PLA chains. Often this approach and effect are also called nucleation [27].

______________________________________________________________________ 5Müller P., Imre B., Bere J., Móczó J., Pukánszky B. Physical ageing and molecular mobility in PLA blends and composites. J. Therm. Anal. Calorim. available online

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The addition of another polymer, i.e. blending, or the use of fillers or reinforce-ments further complicate this already complex picture. For example, the incorporation of organophilic clay increased Tg in one case [18], decreased it in another [8], while glass transition temperature remained constant in a third [19]. The same can be said about all crystallization characteristics including the temperature and enthalpy of cold crystalliza-tion, melting or crystallization and about the effect of all kinds of additional components including plasticizers, elastomers, polymers, fillers, etc. [2,6,19,22,28-32].

Blending PLA with other biodegradable polymers like TPS might result in cost

reduction while maintaining or enhancing its biodegradability. In spite of the large num-ber of publications dealing with PLA/TPS blends, only a few studies are available on the crystallization and melting behavior of the these blends. Most of the authors claim pro-gressive melting, glass transition and cold crystallization temperature decrease with in-creasing TPS content[23,33], while others report that glass transition temperature remains constant [34]. Contrary to our results presented in Chapter 5 some of publications claims that the plasticizer of TPS can migrate into the PLA, increasing chain mobility, which leads to increased crystalline growth rate [35,36]. Because of the simultaneous effect of nucleation and molecular mobility, the clear effect of the additive is difficult to define. Since physical ageing also affects molecular mobility and thus crystallization, this process influences practically all characteristics determined by thermo-analytical methods. More-over, time and thermal history also plays a role, a factor which is neglected in most ex-periments.

In a series of experiments we prepared poly(lactic acid)/thermoplastic starch blends in the entire composition range to determine the effect of composition on proper-ties. DSC measurements yielded interesting results which were difficult to explain. In order to be able to interpret the results better and to draw general conclusions, if possible, the results obtained on PLA/TPS blends were compared to other heterogeneous PLA blends (PBAT, PC) and composites (CaSO4, wood). This paper reports the most important results and conclusions of these studies, during the interpretation of which also the effect of physical ageing was considered. 6.2. EXPERIMENTAL 6.2.1. Materials

The PLA used as matrix was obtained from NatureWorks (USA). The selected

grade (Ingeo 4032D, Mn = 88500 g mol-1 and Mw/Mn = 1.8) is recommended for extrusion. The polymer (<2 % D isomer) has a density of 1.24 g cm-3, while its melt flow index (MFI) is 3.9 g/10 min at 190 °C and 2.16 kg load. The corn starch used for the preparation of TPS was supplied by Hungrana Ltd., Hungary and its water content was 12 wt%. Glyc-erol with 0.5 wt% water content was obtained from Molar Chemicals Ltd., Hungary and it was used for the plasticization of starch without further purification or drying. Thermo-plastic starch samples containing 36 and 47 wt% glycerol (TPS36 and TPS47, respec-tively) were prepared and used in the experiments. The composition of the PLA/TPS blends changed from 0 to 1 volume fraction in 0.1 volume fraction steps. Glycerol was added to PLA also in itself as a "plasticizer" in 1, 3, 5, 7 and 10 vol%.


93

Polycarbonate (Macrolon 2658, Bayer Material Science AG, MFI = 13 g/10 min at 300 °C, 1.2 kg, density = 1.2 g cm-3) and poly(butylene-adipate-co-terephtalate) (Eco-flex F BX 7011, BASF, MFI = 3.5 g/10 min at 190 °C, 2.16 kg, density = 1.26 g cm-3) were used as blend components. Compositions covered the entire composition range from 0 to 1 volume fraction in 0.1 volume fraction steps.

Five different lignocellulosic fibers were used as reinforcement, three wood flours

(W), a microcrystalline cellulose (MC) and a ground corn cob (CC). Two of the wood flours, Filtracel EFC 1000 (W68, D[4,3] = 213.1 m, aspect ratio, AR = 6.8) and Arbocel FT400 (W126, D[4,3] = 171.0 m, AR = 12.6), were acquired from Rettenmaier und Söhne GmbH, Germany, while the third, Lasole 200/150 (W54, D[4,3] = 280.8 m, AR = 5.4) from La.So.Le. Est Srl, Italy. All three were soft woods; further information was not supplied by the producer. The microcrystalline cellulose, Vivapur MCC (MC29, D[4,3] = 138.0 AR = 2.9), was acquired from Rettenmaier und Söhne GmbH, Germany. The fifth filler was the heavy fraction of corn cob (CC, D[4,3] = 143.4 m, AR = 2.3) obtained from Boly Kft., Hungary. Particle size D[4,3] was determined by laser light scattering, while aspect ratio (AR) manually from SEM micrographs. The abbreviation used indicates the origin of the fibers (wood, W; microcrystalline cellulose, MC; or corn cob, CC) and ten times their aspect ratio, i.e. corn cob with an aspect ratio of 2.3 is referred to as CC23. The amount of the reinforcement usually changed from 0 to 60 wt%, but sometimes composites could not be processed with the largest fiber content because of technological reasons.

The CAS-20-4 calcium sulfate (CaSO4) used as filler was supplied by the United

States Gypsum Co. (USA). The filler, manufactured from high purity gypsum rock using controlled calcination and fine grinding, has a volume average particle size of 4.4 μm, specific gravity of 2.96 g cm-3 and calcium sulfate content >99%. The filler was surface coated with 1.5 wt% stearic acid, CaSO4(StAc), resulting in monolayer coverage [7] in order to modify interfacial interactions. Coating was carried out at 120 °C and 100 rpm for 10 min in a Haake Rheomix 600 mixer fitted with blades for dry-blending. The CaSO4 content of PLA composites, both with coated and uncoated fillers, changed from 0 to 30 vol% in 5 vol% steps.

6.2.2. Sample preparation

Corn starch was dried in an oven before composite preparation (105 °C, 24 hours). Thermoplastic starch powder containing 36 and 47 wt% glycerol was prepared by dry-blending in a Henschel FM/A10 high speed mixer at 2000 rpm. TPS was produced by processing the dry-blend in a Rheomex 3/4" single screw extruder attached to a Haake Rheocord EU 10 V driving unit at 140-150-160 °C barrel and 170 °C die temperatures and 60 rpm screw speed.

PLA and the second component were homogenized in an internal mixer (Bra-

bender W 50 EHT) at 190 °C and 50 rpm for 12 min. Before composite preparation poly(lactic acid) was dried in a vacuum oven (110°C, 4 hours). The wood fibers and the polymers (PBAT, PC) were also dried in an oven (105 °C, 4 hours, 80 °C, 4 hours and

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120 °C, 3 hours, respectively) before processing. Both temperature and torque were rec-orded during homogenization. The melt was transferred to a Fontijne SRA 100 compres-sion molding machine (190 °C, 5 min) to produce 1 mm thick plates for further testing.

To study the effect of physical ageing on the mechanical properties of PLA, stand-

ard specimens (ISO 527 1A) were produced by injection molding (Demag IntElect 50/330-100) at 180-190-200-210 °C barrel and 20 °C mold temperature, using 900 bar injection and 700 bar holding pressure (decreasing to 0 bar in 40 s), after drying the pol-ymer according to the procedure described above. All specimens were kept in a room with controlled temperature and humidity (23 °C and 50 %) after injection molding.

6.2.3. Characterization

Thermal transitions including glass transition, melting and crystallization of the blends and composites prepared were studied by differential scanning calorimetry (DSC) using a Perkin Elmer DSC 7 equipment. 3–5 mg samples were heated to 200 C at 10 C min-1 heating rate, kept there for 5 min to erase thermal history and then cooled down to 50 C with 5 C min-1 cooling rate to record crystallization characteristics. After 1 min holding time the samples were heated again to 200 C at 10 C min-1 rate to determine the same thermal characteristics as in the first run.

The effect of physical ageing of PLA on its mechanical properties was followed

by tensile testing on standard 4 mm thick ISO 527 1A specimens using an Instron 5566 apparatus. Stiffness (E) was determined at 0.5 mm/min crosshead speed (up to 0.3 % elongation) and 115 mm gauge length. Tensile strength ( σ), and elongation-at-break (ε) were calculated from force vs. deformation traces measured on the same specimens at 10 mm/min cross-head speed.

6.3. RESULTS AND DISCUSSION

The results are reported in several sections. The thermal behavior of PLA/TPS blends is presented first, then the possible role of glycerol is discussed in the next section. The results obtained on starch blends are compared to those determined on other blends and composites subsequently, while the possible role of physical ageing in the determi-nation of thermal transitions, including crystallization, is discussed in the final section.

6.3.1. PLA/TPS blends PLA is frequently modified with thermoplastic starch in order to decrease the brit-

tleness of the polymer, to increase impact resistance and to facilitate biodegradation [33,37-40]. The mechanical properties of the blends change accordingly, stiffness de-creases and deformability increases with increasing TPS content [23,33,34,40,41]. DSC traces recorded in the first heating run on PLA blends prepared with the TPS containing 36 wt% glycerol (TPS36) are presented in Fig. 6.1. The comparison of the traces obtained


95

at various TPS levels offers some information. The glass transition temperature of PLA does not seem to change or it may increase slightly as a function of TPS content. It is even more interesting that the intensity of the transition varies strongly. A distinct peak corresponding to the glass transition appears at 20 vol% and similar peaks can be ob-served also at higher concentrations of TPS except at 60 vol%. The dissimilar behavior of this blend is difficult to explain, but it is in line with the sometimes unpredictable be-havior of PLA blends and composites [7]. The distinct peak detected at glass transition results from the physical ageing of PLA. These changes in the glass transition process of the polymer indicate that blending might influence its physical ageing and modify the state of the molecules compared to their equilibrium state.

20 50 80 110 140 170 200

Hea

t flo

w (m

W)

e

ndo

e

xo

Temperature (°C)

PLA

10 vol%

20 vol%

30 vol%

40 vol%

50 vol%

60 vol%

70 vol%

Figure 6.1 DSC traces recorded during the first heating run on PLA/TPS36 blends. Effect

of TPS content.

Chapter 6

96

Considerable changes can be observed in the other transitions as well. The tem-perature of cold crystallization definitely moves towards lower temperatures. Possible changes in intensity are difficult to estimate by the qualitative observation of the traces; quantitative analysis is needed. Similarly, changes can be observed in the temperature range of melting as well. First a small exothermic peak or shoulder appears in the trace and then the crystalline polymer melts [14,42]. The peak temperature of fusion moves towards lower temperatures with increasing TPS content, but compared to the neat poly-mer its intensity increases visibly indicating larger crystallinity in the blends. Obviously, blending with thermoplastic starch profoundly changes the thermal behavior of PLA; all transitions are modified from glass transition through cold crystallization to melting. TPS seems to change the mobility of PLA molecules thus modifying physical ageing and crys-tallization.

20 50 80 110 140 170 200

Hea

t flo

w (m

W)

e

ndo

e

xo

Temperature (°C)

PLA

10 vol%

20 vol%

30 vol%

40 vol%

50 vol%

60 vol%

70 vol%

Figure 6.2 Effect of TPS content on the crystallization of PLA in the PLA/TPS36 blends

of Fig. 6.1.


97

DSC traces obtained during the cooling of the polymer from the quiescent melt state are shown in Fig. 6.2. The strong effect of TPS is seen clearly in the figure. PLA crystallizes very slowly, only a small undefined peak appears on the DSC trace. The in-tensity of the crystallization peak increases with increasing TPS content; the phenomenon is demonstrated well by the trace obtained on the blend containing 20 vol% TPS36. Dou-ble peaks appear on most of the traces, which might result from the formation of various modifications [6]. Different crystalline structures have been reported for PLA, the for-mation of which depends on the conditions of crystallization. The most common -mod-ification forms in conventional melt and solution crystallization conditions. However, only the ' crystal appears at crystallization temperatures below 100 C, while crystalli-zation between 100 and 120 C results in the simultaneous formation of the ' and the crystal structures [42,43]. The and the modifications occur under more special condi-tions, the first during the orientation of the form, while the second through epitaxial crystallization. We may safely assume that under the conditions of our experiments first the less perfect ' form develops, which recrystallizes during heating to the more ordered

form. However, further study is needed to explain the appearance of multiple peaks unambiguously and relate them to structure. Such change of the melting process is usually explained by nucleation. However, the temperature of crystallization seems to remain the same or changes only slightly, thus increased crystallinity must result from increased mo-lecular mobility thus supporting our earlier assumption.

The qualitative analysis presented above is strongly supported by quantitative

evaluation. The glass transition temperature of the two sets of blends is plotted against TPS content in Fig. 6.3. Tg increases slightly as deduced from the primary DSC traces. The values are very similar for the two series; the glycerol content of TPS does not seem to influence them much. We can see several points deviating from the general tendency. Such behavior was observed in PLA/CaSO4 blends before [7], for which a clear and un-ambiguous explanation could not be given. Even the increasing glass transition tempera-ture of PLA is difficult to explain, since the Tg of the two TPS materials is 10 and 40 C, respectively, thus a decrease in PLA Tg would be expected instead of the increase ob-served. Both the unexpected change in Tg and the large scatter of some experimental val-ues must be related to the physical state of PLA not controlled sufficiently by experi-mental conditions.

The composition dependence of the temperature of cold crystallization is presented in Fig. 6.4 for both series of blends. The figure confirms the qualitative observation that Tcc decreases with increasing TPS content. The decrease is very similar for the two series independently of glycerol content. The plot contains several deviating points again, which seems to be a typical characteristic of PLA irrespectively of composition or the quantity investigated. Melting characteristics determined in the first heating run are shown in Fig. 6.5. The results corroborate our previous statements and conclusions even stronger than Figs. 6.3 and 6.4. The temperature of fusion decreases strongly with increasing TPS con-tent, while crystallinity increases at the same time. The composition dependence of ther-mal characteristics seems to support our assumption that these changes result from the modification of molecular mobility of PLA, which changes upon blending.

Chapter 6

98

0 20 40 60 80 10050

55

60

65

70

Gla

ss tr

ansi

tion,

Tg (°

C)

TPS content (vol%)

Figure 6.3 Effect of TPS content on the glass transition temperature of PLA in PLA/TPS

blends. Symbols: ( ) TPS36, ( ) TPS47.

0 20 40 60 80 10080

90

100

110

120

Col

d cr

ysta

lliza

tion,

Tcc

(°C

)

TPS content (vol%)

Figure 6.4 The peak temperature of cold crystallization plotted against the TPS content

of PLA/TPS blends. Symbols: ( ) TPS36, ( ) TPS47.


99

Since the temperature of crystallization remains constant (not shown), they cannot result from nucleation. Although the facts are clear, the explanation is still missing. Since glyc-erol is a polar molecule and PLA is also capable of forming H-bonds, the possible effect of glycerol must be considered specifically.

0 20 40 60 80 100155

160

165

170

175

180

185

M

eltin

g te

mpe

ratu

re,

m (°

C)

TPS content (vol%)

25

30

35

40

45

50

55

Enthalpy of fusion, H

m (J/g)

Figure 6.5 Dependence of melting characteristics of PLA on TPS content. First heating

run. Symbols: ( , ) TPS36, ( , ) TPS47; ( , ) peak temperature of melt-ing, Tm, ( , ) enthalpy of fusion, Hm.

6.3.2. PLA/glycerol blends

Since our primary assumption is that modification changes the mobility of PLA molecules, we must analyze the possible influence of glycerol on the thermal transitions of the polymer. As mentioned in the introductory part, plasticization is frequently used for the modification of PLA crystallization and crystalline structure [1,2,22-27]. We re-frain from presenting all DSC traces and characteristic values and show only a few typical ones which can be evaluated in comparison to the previous section. Fig. 6.6 shows the crystallization traces of PLA containing various amounts of glycerol. The traces are very similar to those presented in Fig. 6.2. Glycerol clearly enhances the rate of crystallization and crystallinity. The “plasticizer” has the same effect on glass transition temperature and melting characteristics as well, in fact the composition dependence of characteristic val-ues changes in the same way as shown in Figs. 6.3-6.5 for PLA/TPS blends. An example is presented in Fig. 6.7, in which we plotted the peak temperature of melting and the heat of fusion as a function of glycerol content. The first decreases, while the second increases with increasing amount of the plasticizer, just like in the case of the TPS blends.

Chapter 6

100

20 50 80 110 140 170 200

Hea

t flo

w (m

W)

en

do

ex

o

Temperature (°C)

PLA

1 vol%

3 vol%

5 vol%

7 vol%

10 vol%

Figure 6.6 DSC traces recorded during the cooling of PLA containing various amounts

of glycerol. From the two figures and the results presented above one may conclude that glyc-

erol plasticizes PLA thus increasing molecular mobility, which finally leads to the changes observed. However, we must consider several facts here. First of all, the effect is completely independent of the glycerol content of TPS. Moreover, the solubility of glycerol is very limited in PLA shown by the transparency of PLA plates containing var-ious amounts of glycerol (see Fig. 5.5). Transparency does not change up to 1 vol%, but decreases sharply above 4 vol%. The composition dependence of the characteristics plot-ted in Fig. 6.7 differ considerably from that presented in Fig. 5.5; melting temperature and the enthalpy of fusion change also above 7 vol% at which transparency is practically constant. The relatively poor solubility of glycerol in PLA is confirmed by the molecular modelling calculations presented in the previous chapter showing much stronger interac-tions between glycerol and starch, than between glycerol and PLA. Although some plas-ticizing effect of glycerol cannot be excluded, the changes presented in sections 6.3.1 and 6.3.2 seem to be of more general character.


101

0 2 4 6 8 10155

160

165

170

175

Mel

ting

tem

pera

ture

, m (°

C)


30

35

40

45

50

55

60

Enthalpy of fusion, H

m (J/g)

Figure 6.7 Effect of glycerol content on the crystallization characteristics of PLA. Sym-

bols: ( ) peak temperature of crystallization, Tc, ( ) enthalpy of crystalliza-tion, Hc.

6.3.3. Comparison, other blends and composites

Further blends and composites have been also prepared and studied at our labora-tory; in order to increase the predictive power of our conclusions, DSC results have been compared for all of them. Selected crystallization traces are compared to each other in Fig. 6.8. Almost all combinations of materials result in the same effect as presented above irrespective of their type or quality. The dissimilar behavior of PBAT is difficult to un-derstand and needs further investigation. Practically every modification result in in-creased rate of crystallization and larger crystallinity. This result confirms our conclusion presented above that not glycerol specifically, but modification generally changes the thermal characteristics of PLA and the quantities derived from DSC measurements.

The glass transition temperature of PLA is plotted against the amount of selected

materials used as second component in Fig. 6.9. Tg increases for the TPS blends, but de-creases for all other materials. We could not explain the increase unambiguously, but the decrease is similarly difficult to understand especially in the case of CaSO4 having the highest surface energy. One would expect that PLA molecules adsorb onto the high en-ergy surface of the filler leading to decreased molecular mobility and increased glass transition temperature. Apparently thermal history and molecular mobility play more im-

Chapter 6

102

portant roles in the determination of relaxation processes and glass transition than inter-actions. The correlation presented in Fig. 6.10 further confirms these relationships. The peak temperature of melting is plotted against composition for the same materials as in Fig. 6.9. Tm decreases for all materials irrespectively of their physical form or chemical composition indicating again easier crystallization kinetics caused mainly by increased mobility. The rest of the characteristics, i.e. the enthalpy of crystallization and fusion, change in the same way as presented in previous sections. The only difference in the effect of the materials used for modification is basically in the extent of the changes, which are influenced by undefined factors and must include ageing time, thermal history and probably also interactions.

30 60 90 120 150 180

Hea

t flo

w (m

W)

e

ndo

e

xo

Temperature (°C)

TPS36

W54

W126

CaSO4

PLA

PBAT

Figure 6.8 Effect of various materials used for the modification of PLA on the crystalli-zation of the matrix polymer in PLA blends and composites. The amount of the modifying component is 10 vol%.


103

0 20 40 60 80 10050

55

60

65

70

composites

glycerol

Gla

ss tr

ansi

tion,

Tg (°

C)

Additive content (vol%)

TPS

blend

Figure 6.9 Effect of modification on the glass transition temperature of PLA in various

blends and composites. Symbols: ( ) TPS36, ( ) TPS 47, ( ) glycerol, ( ) W126, ( ) CaSO4,( ) CaSO4(StAc) ( ) PBAT.

0 20 40 60 80 100150

155

160

165

170

175

180

Mel

ting

tem

pera

ture

, m (°

C)

Additive content (vol%)

blend

TPS

wood

Figure 6.10 Influence of the amount of the second component on the melting temperature

of PLA blends and composites. Symbols are the same as in Fig. 6.9.

Chapter 6

104

0 20 40 60 80 100 1202.4

2.6

2.8

3.0

3.2

3.4

3.6

You

ng's

mod

ulus

(GPa

)

Ageing time (h)

0

50

100

150

200

250

300

Elongation-at-break (%)

Figure 6.11 Effect of ageing time on the mechanical properties of PLA. Symbols: ( )

Young's modulus, ( ) elongation-at-break. 6.3.4. Physical ageing, interactions

Physical ageing is a known disadvantage of PLA, which proceeds relatively fast because of the low glass transition temperature of the polymer and which results in con-siderable changes of properties in a relatively short time [16,44,45]. We discussed the possible effect of molecular mobility on the thermal transitions of PLA, and physical ageing is closely related to and depends on mobility. Fig. 6.11 demonstrates the consid-erable modification of properties as an effect of physical ageing. Stiffness and deforma-bility are plotted against ageing time in the figure. Modulus increases from about 2.7 to 3.2 GPa, while elongation-at-break decreases from more than 250 % to less than 10 %. This large decrease in deformability results in the well-known brittleness of PLA that limits its application in certain fields.

The physical ageing process is accompanied by decreased Tg (Fig. 6.12), although

the opposite effect, i.e. increased Tg was also observed by others [46]. The ageing process can be followed by the measurement of enthalpy relaxation; an endotherm peak appears at glass transition, the intensity of which increases with ageing time (Fig. 6.12). The peak is very similar to those shown in Fig. 6.1 during the glass transition of PLA thus we can safely assume that the modification of the polymer accelerates the ageing process by in-creasing mobility and bringing the molecules closer to their equilibrium state. All latter


105

processes are determined by these changes, i.e. faster cold crystallization, larger crystal-linity, less ordered crystals, and lower melting temperature. Increased mobility is shown also by faster crystallization; nucleation has not been observed in our experiments indi-cated by the practically constant value of crystallization temperature (Tc).

0

1

2

3

4

5

6

0 200 400 600 80060

62

64

66

68

Enthalpy relaxation (J/g)

G

lass

tran

sitio

n, T

g (o C)

Ageing time (h)

Figure 6.12 Changes in glass transition temperature and the intensity of enthalpy relax-

ation during physical ageing. Symbols: ( ) Tg, ( ) enthalpy relaxation.

Table 6.1 Surface energy of the second component and its estimated interfacial adhe-sion with PLA

Second component Surface tensiona,

sd (mJ/m2)

Work of adhesion

Wa (mJ/m2)

CaSO4 79.8 243.0

CaSO4(StAc) 21.6 121.0

wood 45.0 97.7

TPS 37.0 98.2

a) dispersion component of surface tension

Chapter 6

106

Although the role of increased mobility and physical ageing seem to be rather cer-tain, the reason is completely unclear and difficult to understand. PLA chains are rather flexible [47-49] shown also by the relatively low glass transition temperature of the pol-ymer. Interactions among the molecules must be also quite weak since only van der Waals forces can develop among them, which is in line with the low Tg. One might expect strong interactions to develop between certain materials and PLA; it is capable of forming hy-drogen bonds with starch and glycerol, for example, or adsorb onto the high energy sur-face of inorganic fillers. The dispersion component of surface tension and the estimated reversible work of adhesion with PLA are listed in Table 6.1 for selected materials. Inter-actions may play a role in the determination of the ageing process and molecular mobility, but it cannot be the determining factor. The Tg of PLA increased when TPS with small surface energy and weak interactions was added, while decreased when the filler having a surface with considerably higher energy was used for modification. More materials, further analysis and much more control of ageing history are needed in order to obtain the necessary correlations to control PLA properties sufficiently. 6.4. CONCLUSIONS

The detailed analysis of the transitions of PLA/thermoplastic starch blends indi-

cated that all are determined by the molecular mobility of PLA chains. Blending modifies molecular mobility thus often decreases glass transition temperature and changes the in-tensity of enthalpy relaxation. All other transitions and characteristics, i.e. cold crystalli-zation, melting and the corresponding enthalpies change accordingly. Increased molecu-lar mobility accelerates also the physical ageing of the polymer. The comparison of the results to those obtained on other PLA blends (PBAT, PC) and composites (wood, CaSO4) confirmed the general character of the phenomenon. Nucleation was not observed in the heterogeneous PLA blends and composites studied; crystallization temperature recorded during cooling from the quiescent melt remained constant. The interaction between PLA and the various components used for modification changed in a wide range, but no direct correlation was found between the strength of interaction and molecular mobility. The reason for the observed increase in mobility is unclear and needs further study and expla-nation.

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Summary

109

Chapter 7 7.1. SUMMARY

In recent years the interest of both the public and the scientific community turned towards natural based and biodegradable materials. This tendency was clearly reflected by the projects related to these materials in our laboratory. We participated in several international and domestic projects on plasticized starch, wood and natural fiber rein-forced composites, and more recently on lignin. The goal of these projects was either to obtain basic knowledge about the factors determining the properties of such materials or to develop new, better or cheaper materials and/or processes. This Thesis focusing on the modification of thermoplastic starch used the knowledge obtained in one of these pro-jects, but also in earlier studies, built on it, and extended it further. Plasticized starch is a promising, cheap material, but a complex one as well, which needs much study and de-velopment work before obtaining materials of practical use. The Thesis reports the pro-gress that we achieved during this work. Although we summarized the most important results at the end of each chapter, we briefly repeat them here to give a concise overview of the results. At the end of this chapter we compile our most important new findings in a few thesis points.

Our first experiments focusing onto the preparation and study of thermoplastic starch/layered silicate composites, revealed that no or only limited exfoliation takes place during the homogenization of the composites irrespectively of the organophilization of the clays used. On the other hand, practically all components of the system enter into interaction with each other. Competitive dissolution and adsorption of the surfactant and the plasticizer take place in the composites. The occurrence of chemical reactions cannot be excluded either in the case of the amino acid modified silicate. The type and strength of interaction determine the properties of the composites. The reinforcing effect of the silicates vary in a wide range, the amino acid modified clay improves strength almost ten times as much as the silicate treated with an aliphatic amine. Changes in all other proper-ties including viscosity, color and water adsorption correspond to those observed in strength and are dominated by interactions.

TPS/NaMMT nanocomposite films were prepared by solution casting and melt blending in the second stage of the work. Clay content changed between 0 and 25 wt% based on the amount of dry starch (0 – 8 vol% in the composites). Filler content was much larger than the usual 1-5 wt%, because results published earlier proved that these nano-composites can offer good properties in spite of the lack of complete exfoliation. X-ray diffraction analysis, scanning electron microscopy and light transmission measurements showed that all nanocomposites possess intercalated structure, but at larger clay content (above 10 wt%) nanocomposites prepared by melt intercalation contained aggregated par-ticles as well. VH-type crystallinity was found in all nanocomposites, which is typical for TPS containing more than 10 wt% water. Somewhat larger crystallinity was observed in nanocomposites containing more plasticizer, which is probably due to the larger mobility of starch molecules in these composites. In spite of incomplete exfoliation the clay rein-forces TPS in nanocomposites prepared by solution casting, both stiffness and strength

Chapter 7

110

increased considerably. Similarly good mechanical properties were determined on dry samples prepared by melt blending, but conditioning of these samples resulted in very poor stiffness and strength, the worst among all samples studied. We assume that water bonded during conditioning or the solvent csating process has different effect on mechan-ical properties. With the aid of a simple model we could prove that the increase of glycerol content decreases starch/clay interaction.

The modification of TPS with wood particles improves several properties consid-erably. Stiffness and strength increases, and the effect is stronger for fibers with larger aspect ratio. Wood fibers reinforce TPS considerably due to poor matrix properties and strong interfacial interactions, the latter resulting in the decreased mobility of starch mol-ecules and in the fracture of large wood particles during deformation. Strong interfacial adhesion leads to smaller water absorption than predicted from additivity, but water up-take remains relatively large even in the presence of wood particles. The shrinkage of injection molded TPS parts is very large, around 10 %, and dimensional changes occur on a very long timescale of several hundred hours. Shrinkage decreases considerably to a low level already at 15-20 vol% wood content rendering the composites good dimensional stability. Wood reinforcement of TPS is generally advantageous for most application ar-eas.

A detailed analysis of experimental results obtained on PLA/TPS blends supported by model calculations showed that the interaction of the two components is weak. The investigation of the possible partitioning of glycerol in the two phases indicated that most of the plasticizer is located in the TPS phase and does not diffuse into PLA. Thermody-namic modeling predicted some dissolution of PLA in TPS which was assisted by the presence of the plasticizer, but TPS does not dissolve in PLA at all. As a consequence of weak interactions properties are moderate at most. Blending of the two components re-sulted in heterogeneous, two phase structure at all compositions. No tangible proof was found for the formation of a glycerol rich phase in TPS, the relaxation transition assigned to this phase was rather explained with the movement of smaller structural units of starch molecules. Weak interfacial adhesion does not allow stress transfer through the interface resulting in poor strength and small deformation. TPS deteriorates the properties of PLA considerably and although this latter polymer reinforces TPS somewhat, blends with a starch matrix are extremely weak. Useful materials can be produce from PLA and TPS only with the development of an appropriate coupling strategy.

The detailed analysis of the transitions in PLA/thermoplastic starch blends indi-cated that all are determined by the molecular mobility of PLA chains. Blending modifies molecular mobility thus often decreases glass transition temperature and changes the in-tensity of enthalpy relaxation. All other transitions and characteristics, i.e. cold crystalli-zation, melting and the corresponding enthalpies change accordingly. Increased molecu-lar mobility accelerates also the physical ageing of the polymer. The comparison of the results to those obtained on other PLA blends (PBAT, PC) and composites (wood, CaSO4) confirmed the general character of the phenomenon. Nucleation was not observed in the heterogeneous PLA blends and composites studied; crystallization temperature recorded during cooling from the quiescent melt remained constant. The interaction between PLA and the various components used for modification changed in a wide range, but no direct

Summary

111

correlation was found between the strength of interaction and molecular mobility. The reason for the observed increase in mobility is unclear and needs further study and expla-nation.

The most important conclusions of this Thesis can be summarized briefly in the following main points: 1. By the detailed analysis of the properties of thermoplastic starch/layered silicate

nanocomposites we showed that practically all components of the system enter into interaction with each other. Competitive dissolution and adsorption of the surfactant and the plasticizer take place in the composites and the occurrence of chemical reac-tions cannot be excluded either. The type and strength of interaction determine the properties of the composites (Chapter 2).

2. Further study of TPS/clay nanocomposites proved that all nanocomposites possess intercalated structure irrespectively of preparation technology. In spite of incomplete exfoliation layered silicates reinforce TPS in nanocomposites prepared by solution casting, both stiffness and strength increase considerably (Chapter 3).

3. During the modification of TPS by wood fibers we found that wood reinforces TPS considerably due to poor matrix properties and strong interfacial interactions that result in the decreased mobility of starch molecules and in the fracture of large wood particles during deformation (Chapter 4).

4. We could also prove that the shrinkage of TPS decreases to a very low level already at 15-20 vol% wood content resulting in composites with good dimensional stability (Chapter 4).

5. We investigated the possible partitioning of glycerol in TPS/PLA blends the first time and proved by measurements and model calculations that most of the plasticizer is located in the TPS phase and does not diffuse into PLA (Chapter 5).

6. Contrary to numerous statements published in the literature we did not find any proof for the formation of a glycerol rich phase in TPS, the relaxation transition assigned to this phase was explained with the movement of smaller structural units of starch molecules (Chapter 5).

7. With the detailed analysis of the transitions of PLA/thermoplastic starch blends and comparison to other modified PLA systems we pointed out the first time that all tran-sitions are determined by the molecular mobility of PLA chains. Blending modifies molecular mobility thus often decreases glass transition temperature and changes the intensity of enthalpy relaxation. All other transitions and characteristics, i.e. cold crystallization, melting and the corresponding enthalpies change accordingly (Chap-ter 6).

List of symbols

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List of symbols and abbreviations

a Overall rate of water adsorption Af Specific surface area of fillers (m2/g) B Load bearing capacity of fillers

rG Gibbs free energy of a model reaction

cc Enthalpy change of cold crystallization (kJ/mol)

m Enthalpy change of melting (kJ/mol) E Young's modulus (GPa)

Elongation at break (%) E” Loss modulus (GPa) Ec Young's modulus of composite (GPa) Em Modulus of a matrix polymer (GPa) Fa Strength of adhesion (N)

Volume fraction of the filler in the composite L Length of the specimen measured at the moment of failure (mm)

Relative elongation ℓ Thickness of the interface (mm) L0 Gauge length (mm) M∞ Equilibrum water uptake (%) R2 Accuracy of fitting Rf Radius of spherical particles (mm)

f Density of the filler (g/cm3) Tensile strength (MPa) Strength of the matrix

Characteristic stress (MPa) Debonding stress (MPa)

i Strength of the interface (MPa)

Thermal stress (MPa)

True tensile strength of the composite (MPa)

T True tensile strength of the matrix (MPa)

Tred Reduced tensile strength t Time of absorption (s)

List of symbols

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T Temperature (K) Tc Crystallization temperature (°C) Tcc Cold crystallization temperature (°C) Tg Glass transition temperature tgδ Loss factor Tm Melting temperature (°C) CaCO3 Calcium carbonate CaSO4 Calcium sulphate DFT Density functional theory DMA Dynamic mechanical analysis DRIFT Diffuse reflectance infrared spectroscopy DS Destructurised starch DSC Differential scanning calorimetry FTIR Fourier transform infrared spectroscopy

LCCSD(T) Local coupled-cluster calculations with single, double and perturbative triple excitations

MAPP Maleic anhydride grafted polypropylene MFI Melt flow index (g/10 min) MMT Montmorillonite N784 Organophilic clay treated with aminododecanoic acid N804 Organophilic clay terated stearyl dihydroxyethyl ammonium chloride N948 Organophilic clay treated distearyl dimethyl ammonium chloride NaMMT Na-montmorillonite PBAT Poly(butylene adipate-co-terephthalate) PBSA Poly(butylene succinate-co-adibate) PC Polycarbonate PCL Polycaprolactone PEA Polyesteramide PHA Polyhydroxyalkanoate PHB Poly(hydroxybutyrate) PHBV Poly(hydroxybutyrate-co-hydroxyvalerate) PLA Poly(lactic acid) PP Polypropylene PVA Poly(vinylalcohol) PVC Poly(vinyl chloride)

List of symbols

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RH Relative humidity (%) SEM Scanning electron microscope TGA Thermogravimetric analisys TPS Thermoplastic starch XRD X-ray diffraction

Acknowledgements

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Acknowledgements

First of all I should like to thank my supervisor, Béla Pukánszky for his atten-tion and relentless support of my studies. I appreciate very much his guidance towards a more scientific way of thinking. I am grateful for my friends and colleagues at the La-boratory of Plastics and Rubber Technology at the Budapest University of Technology and Economics, as well as at the Institute of Materials and Environmental Chemistry, Chemical Research Center, Hungarian Academy of Sciences for their help and for the pleasant atmosphere they created. I would like to express my gratitude to Erika Fekete and János Móczó, who helped me. And at last but not the least, I should like to thank my family especially my wife, my son and my parents for their continuous support in reaching my goals.

Publications

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List of Publications Papers used for the preparation of the Thesis 1. Bagdi K., Müller P., Pukánszky B.: Thermoplastic starch/layered silicate compo-

sites: structure, interaction, properties, Composite Interfaces, 13, 1 (2006) (IF: 0.690; H:35)

2. Bagdi K., Müller P., Pukánszky B.: Termoplasztikus keményítő (TPS) alapú nano-

kompozitok előállítása és jellemzése, Műanyag és Gumi, 43, 496 (2006) 3. Kapin É., Müller P., Dr. Fekete E.: Szerkezet/tulajdonság összefüggések TPS/ré-

tegszilikát nanokompozit filmekben, Műanyag és Gumi, 47, 369 (2010) 4. Müller P., Kapin E., Fekete E.: Effects of preparation methods on the structure and

mechanical properties of wet conditioned starch/montmorillonite nanocomposite films, Carbohydrate Polymers 113., 569-576 (2014) (IF: 3.916; H:1)

5. Müller P., Renner K., Móczó J., Fekete E., Pukánszky B.: Thermoplastic

starch/wood composites: Interfacial interactions and functional properties, Carbo-hydrate Polymers 102., 821-829 (2014) (IF: 3.916; H:6)

6. Imre B., Müller P., Móczó J., Fekete E., Pukánszky B.: A keményítő módosítása -

A faliszt hatása a termoplasztikus keményítő mechanikai és funkcionális tulajdon-ságaira, Műanyag- és Gumiipari Évkönyv XII: pp. 57-66. (2014)

7. Müller P., Imre B., Bere J., Móczó J., Pukánszky B. Physical ageing and molecu-

lar mobility in PLA blends and composites. J. Therm. Anal. Calorim. available online (DOI: 10.1007/s10973-015-4831-6) (IF: 2.042)

Other publications 1. Iosip M. D., Bruma M., Ronova I., Szesztay M., Müller P.: Compared properties of

related aromatic poly(1,3,4-oxadiazole-amide)s, European Polymer Journal, 39., 2011-2021, (2003) (IF: 1.086) (H:29)

2. Pozsgay A., Fráter T., Százdi L., Müller P., Sajó I., Pukánszky B.: Gallery structure

and exfoliation of organophilized montmorillonite: effect on composite properties, European Polymer Journal, 40., 27-36, (2004) (IF: 1.419) (H:55)

3. Bruma, M., Damaceanu, M., D., Muller, P.: Comparative Study of Polyimides

Containing Oxadiazole and Ether Groups, High Perform. Polym. 21 (5), 522-534 (2009) (IF: 0.988) (H:13)

Publications

120

4. Banhegyi, Gy., Vargha, V., Muller, P., Zelenyanszki, E.: Estimation of thermal ageing

acceleration factors of epoxy insulations using thermogravimetry and intuitive kinetic model, IEEE T. Dielect. El. In., 16 (5), 1420-1429 (2009) (IF: 0.848) (H:1)

Conference presentations 1. Bagdi K., Müller P., Klébert Sz., Pukánszky B.: Lágyított keményítő és kompozitja-

inak szerkezete és tulajdonságai, MTA Természetes Polimerek Munkabizottságának ülése, 6. April 2004, Debrecen

2. Bagdi K., Müller P., Klébert Sz., Pukánszky B.: Lágyított keményítő és kompozitja-

inak szerkezete és tulajdonságai, MTA KK Kutatóközponti Tudományos Napok, 3. June 2004, Budapest

3. Bagdi K., Müller P., Klébert Sz., Pukánszky B.: Properties and structure of

thermoplastic starch and its composites, 5th International Symposium on "Materials made from Renewable Resources", naro.tech 2005, Messe Erfurt, Germany, 1st – 2nd September 2005 (poszter)

4. Müller P., Móczó J., Pukánszky B.: Természetes szállal erősített termoplasztikus

keményítő kompozitok: Határfelületi kölcsönhatások és funkcionális tulajdonságok, MTA Műanyag és Természetes Polimerek Munkabizottságának ülése, 14. May 2013, Budapest

121

Nyilatkozat

Alulírott Müller Péter kijelentem, hogy ezt a doktori értekezést magam készítettem

és abban csak a megadott forrásokat használtam fel. Minden olyan részt, amelyet szó szerint, vagy azonos tartalomban, de átfogalmazva más forrásból átvettem, egyértelmű-en, a forrás megadásával megjelöltem.

Budapest, 2015. augusztus 25. Müller Péter

structure-property correlations in thermoplastic starch

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