I

STATE OF THE ART

Eco-efficient composite materials

OCTOBER 2010

Laura Laine 1 and Liva Rozite

2

1 Tampere University of Technology, Department of Material Science/Plastics and Elastomer

Technology, Kokkola unit, Finland

2 Luleå University of Technology, Division of Polymer Engineering, Sweden

II

II

CONTENTS

1 INTRODUCTION ................................................................................................................... 1

2 MATERIAL CANDIDATES .................................................................................................. 3

2.1 Bioresins ........................................................................................................................... 3

2.2 Biofibers ........................................................................................................................... 5

2.3 Bio core materials........................................................................................................... 11

3 PERFORMANCE IN HARSH CONDITIONS .................................................................... 13

3.1 Definition of harsh conditions ........................................................................................ 13

4 TESTING AND CHARACTERIZATION METHODS FOR DURABILITY

EVALUATION............................................................................................................................. 14

4.1 Mechanical testing.......................................................................................................... 14

4.2 Physical testing ............................................................................................................... 16

4.2.1 Moisture and water sorption testing ........................................................................ 17

5 SURFACE TREATMENT METHODS FOR BIOFIBERS ................................................. 18

5.1 Chemical treatment ........................................................................................................ 18

5.2 Physical treatment .......................................................................................................... 20

5.3 Effects on long term properties ...................................................................................... 21

6 CONCLUSIONS ................................................................................................................... 22

6.1 Best choice material candidates ..................................................................................... 22

6.2 Identification of most critical harsh conditions .............................................................. 22

6.3 Surface treatment options and recommendations........................................................... 22

REFERENCES ............................................................................................................................. 23

III

1

1 INTRODUCTION

This state-of-the-art is part of EU INTERREG IV A North -project called ANACOMPO

(Application of natural fiber reinforced composites in harsh environments) and contains

discussion about eco-efficient composite materials, test methods and fiber surface treatment

methods. The aim is to find commercially available biobased resins and natural fibers, make a

literature search of composite testing and fiber surface treatments and define the project

objectives based on the findings. In this report, only thermoset resins used in structural

composites are discussed, thermoplastic resins are excluded.

Increased interest in using natural fibers as reinforcement in polymer composites is due to

higher environmental awareness and the great potential of natural fibers. Natural fibers have

many advantages to be used as reinforcements in polymer composites. They are considered as

biodegradable and sustainable alternatives to other fibers. They are CO2 neutral and leave no

residues when they are incinerated. Their mechanical properties are very good and they are not

abrasive or toxic. Natural fibers have low density and low price. However, they also have

disadvantages like inhomogeneous quality and supply cycles. They have low water resistance

and dimensional stability. Natural fibers absorb moisture and lack good properties in toughness.

[1] In this report, the term „biofiber‟ stands for natural fiber.

Resins used in polymer composites are typically thermoset resins, like epoxies and

unsaturated polyesters, and thermoplastic resins, like polypropylene. Their raw material is

normally crude oil. The resin material can also be biobased and in that case it is based on

renewable resources. Biobased resin means that some amount of the raw materials is derived

from biomass, in other words renewable resources. The rest is crude oil based raw materials. The

motive to develop biobased resins is to have more environmentally friendly composite materials

and to use less crude oil.

On the market, also biodegradable resins are available. These are thermoplastics which

decompose by microorganisms into carbon dioxide and water in a short time. Biodegradable

resins, like polylactic acid, are mainly used in short lifetime applications, such as packaging. In

structural composites, biodegradation is not a main issue.

The percentage of the bio-derived content in a thermoset resin varies greatly and only a few

are nearly 100 % biobased. The biobased content varies from 12 % to 90 %, according to

manufacturers‟ own reports. The characterization of true biobased content is more difficult but

there is also a standard for defining it, ASTM D6866. In this state-of-the-art, biobased

thermosetting resins are of interest and bioresin is used as a synonym for biobased resins for

simplicity. However, the word „bioresin‟ is used in different meanings in other sources.

2

In Chapter 2, bioresins, biofibers and bio core materials are reviewed. In Chapter 3, the

definition of harsh conditions is discussed. Characterization methods for durability evaluation

and literature values are presented in Chapter 4. Surface treatment methods for natural fibers are

discussed in Chapter 5 and conclusions are made in Chapter 6.

3

2 MATERIAL CANDIDATES

Possible material candidates in this project are environmentally friendly composite materials that

have potential to be used in structural applications and in difficult environmental conditions. The

aim is that both resins and reinforcements are to some extent made from biobased raw materials

or constituents with the ideal target a composite made from 100 % biobased raw materials.

Concerning reinforcements, natural fibers have shown very promising mechanical properties.

Biobased resins are quite comparable with crude-oil resins, except the price is higher and

therefore the usage of biobased resins is growing slowly. In this chapter, commercial bioresins,

biofibers and bio core materials are discussed.

2.1 Bioresins

The term „bioresin‟ means that a resin is composed of raw materials, which are wholly or partly

derived from renewable resources. Bioresin can be a thermoplastic, a thermoset or a

biodegradable plastic. Here, only thermoset bioresins are discussed. A thermoset polymer is

highly cross-linked and is cured by using a curing agent and heat or heat and pressure, and the

result is a material with high strength, modulus and durability [2].

In a review article [2], thermosetting bioresins are divided into phenolics, epoxies,

polyurethanes, polyesters and other resins. The article contains detailed information on raw

materials used in different resins, including chemical formulas. To produce bioresins, commonly

used renewable raw materials are plant oils (for instance soybean oil, castor oil, pine oil),

polysaccharides (cellulose, starch) and proteins. Currently, petroleum-based raw materials are

added to renewable raw materials to make bioresins.

In Table 1, current manufacturers for commercial thermosetting bioresins are presented with

information about raw materials, chemical formula or manufacturing method and application

areas. As can be seen, used raw materials are diverse, including soybean oil, epoxidised pine oil

waste, castor oil, furfuryl alcohol and lactid acid. In this group, biobased content varies from

18% to 50-90%. Three of them are unsaturated polyester based, one is an epoxy and others are

varied.

4

Table 1 Commercially available bioresins, their raw materials and applications.

Manufacturer (trade name)

Raw

materials Chemical formula Applications Sour

ce Ashland e.g.

ENVIREZ®

1807

Unsaturated

polyester Soybean oil Bio 18%

e.g. tractor panels [3]

Amroy Europe

Oy EpoBioXTM

Natural

phenols

distilled from

forest industry

waste stream,

eg. epoxidised

pine oil waste Bio 50-90%

kayaks, boats, tent

poles, glues,

electrical cars

[4]

DSM Palapreg

ECO P 55-01 Unsaturated

polyester Bio 55%

Not available SMC/BMC

applications [5]

TransFurans

Chemicals

bvba BioRez™

furfuryl resin

Furfuryl

alcohol based

resins from

biomass

Furfural (FF) is produced from

hemicellulosic agricultural wastes and

source for FA production via catalytic

hydrogenation.

Varied applications [6]

JVS-Polymers

Ltd. LAIT-X /

POLLITTM

Lactic-acid

based Aliphatic hydroxyl acids, such as lactic

acid, are polymerized into poly(lactic

acid) by direct polycondensation with

a use of suitable catalyst

composites,

impregnated

products, coatings

and biomedical

applications

[7]

Bioresin Bioresin

castor oil Not available Automotive,

marine [8]

Reichhold ENVIROLITE

™ Unsaturated

polyester Soya oil Bio 25%

Not available SMC/BMC and

pultrusion

applications

[9]

Cognis Tribest acrylate

functional

resin system

derived from

soya oil

[10]

University of

Borås Soybean oil: Methacrylated,

Methacrylic

anhydride

modified,

Acetic

anhydride

modified

Eg. Methacrylated soybean oil

O

O

O

O

O

O

2 3

3 3

7

O

O O

OH

O

OH

O

O

OH

5

2.2 Biofibers

Natural fibers can be divided in groups of bast, leaf, seed, fruit, wood and grasses. Flax and

hemp fibers belong to bast fibers. [11] Here, the inspection is limited mainly to flax and hemp.

Some of commercial natural fiber and biofiber reinforcement producers are presented in Table 2.

Natural fibers for composite applications are available typically in the form of yarns and fabrics,

which can be woven or non-woven. Some manufacturers provide fibers as preimpregnated to

different resins or plastics. As shown, flax providers are easier to find than hemp providers.

Table 2 Manufacturers of natural fibers and natural fiber reinforcements.

Manufacturer Fiber Webpage Libeco-Lagae, Belgium flax www.libeco.be Engtex AB, Sweden flax www.engtex.se Composites Evolution Ltd, UK flax www.compositesevolution.com Group Depestele, France flax e.g. www.lbn-lin.com Lineo NV, Belgium flax www.lineo.eu Safilin, France/Poland flax www.safilin.fr Fimalin, France flax www.fimalin.com Stemergy: Renewable fibre

technologies, Canada hemp www.hempline.com

HP Johannesson Trading AB,

Sweden jute www.hpjtrading.se

CORDENKA GmbH, Germany regenerated

cellulose www.cordenka.com

The natural fiber has a very complex multi-scale internal structure: it consists of elements

with different size scales. The main constituents involved in composition of plant fibers are

polymers themselves: cellulose, hemicellulose, lignin and pectin (see Table 3). [12]

Figure 1 shows a schematic picture of composition and built of a flax stem (see also the

micrograph in Figure 2). Technical fibers (approximately 1 m long) are isolated from the flax

plant and consist of elementary fibers with length generally between 2 and 5 cm, and diameters

between 19 and 25 μm. The elementary fibers are glued together by a pectin interface. These

fibers have polyhedron shape (see Figure 3) with 5 to 7 sides to improve the packing in the

technical fiber. [12]

6

Table 3 The chemical composition of different natural fibers. [13]

Fiber Type Cellulose, % Hemicellulose, % Lignin, % Pectin, %

Flax 81 14 3 4

Hemp 74 18 4 1

Wood ~ 46 ~ 27 ~ 27 -

Sisal 73 13 11 2

Figure 1 Composition and built of flax stem. [14]

7

Figure 2 Micrograph showing multi-scale structure of the flax stem. (Constructed from

individual images published in [15]).

Figure 3 SEM image of elementary flax fiber. [46]

Elementary fibers are single plant cells and the most common chemical material in plant cell

walls is cellulose (C6H10O5)n. The chemical structure of cellulose monomer is showed in Figure

4. Most of the elementary fibers consist of oriented, highly crystalline cellulose fibrils and

amorphous hemicellulose. The crystalline cellulose fibrils in the cell wall are oriented at an angle

±10º with the fiber axis and give the fiber its high tensile strength (see Figure 5). [12]

8

Figure 4 The chemical structure of cellulose monomer. [12]

Figure 5 Different layers in cell wall. Middle lamella (ML), Primary wall (P), Secondary

wall (S), Lumena (L).

Natural fibers have many properties that make them interesting as reinforcement in

composites. These fibers are renewable, recyclable, have high specific properties and good

isolation properties. Also these fibers have low bulk cost and low weight. However, natural

fibers have several disadvantages compared with synthetic fibers. The main drawback is quality

variations, depending on growth conditions, processing and other reasons, which may also

influence the price of fibers. Natural fibers are sensitive to moisture and have lover durability.

9

The maximal processing temperature is lower than for synthetic fibers. Table 4 shows some

properties for the most popular natural fibers, E-glass and wood fibers.

Table 4 Properties of different fibers. [12,16,17]

Fibers Density

(g/cm3)

Modulus

(GPa)

Strenght

(MPa)

Strain

(%)

Diameter

(μm)

Specific

modulus

Specific

Strenght

E-glass 2.54 72 3530 1.8-3.2 10 28.2 1390

Wood 1.54 30-40 400-800 - 20-40 ~ 25 ~ 390

Flax 1.4-1.5 50-70 500-900 1.5-2.4 10-30 ~ 41 ~ 480

Hemp 1.48 30-60 300-800 1.1-2 10-50 ~ 30 ~ 370

Sisal 1.45 9-20 510-700 2.2-2.9 10-40 ~ 10 ~ 420

Cordenka 1.5 ~20 700-800 13-15 12.5 ~ 13 ~ 470

The process to make flax or hemp plant into a reinforcement fiber has several stages. Fibers

are in fiber bundles, so first the bundle is broken down by retting. Retting is done by spreading

the crop in the fields and leaving there for 3-8 weeks or immersing the crop in water for 1-2

weeks. At present, also many industrial retting methods are being developed, like enzyme

retting. When the retting has broken down the bundle into single fibers, fibers are dried and

mechanically separated from the straw by stripping and combing. [18]

Natural fibers have been used in lines, ropes and other one dimensional products, textiles,

canvas, and papers. As natural fiber mechanical properties can be competitive to glass fibers,

they can substitute glass fibers in composites. However the adhesion between natural fibers and

polymer is a problem. Adhesion can be improved by fiber treatment or by the use of more

compatible polymers (for example bio-based resins). Also the length of natural fiber and

orientation is an issue for using them in the composites.

10

Figure 6 The viscose process. [19]

One of the latest revelations in the type of reinforcement with plant origin is regenerated

cellulose fibers. These fibers, similar to flax and hemp fibers, have high cellulose content, but

unlike flax and hemp they are manmade. “Terms regenerated cellulose, rayon, and viscose rayon

tend to be used interchangeably” [19]. The flow diagram for the viscose process is given in

Figure 6.

First wood pulp is dissolved in caustic soda. Then steeping is performed - for a specified

period of time composition is shredded and allowed to age. The period of aging determines the

viscosity of the viscose. The longer is the ageing time the higher will be the viscosity of solution.

Afterwards aged pulp is treated with carbon disulphide to form a yellow-colored cellulose

xanthate, which is dissolved in caustic soda. This is the starting stage of viscose formation [20].

11

Although these are manmade fibers, they are made out of the natural polymer directly on

contrary to the fibers made out materials with fossil origin. These fibers are continuous and it is

easy to arrange them into fabrics with stable orientation and geometry. At the moment, the main

application for these fibers is in textile industry. However, currently there are many on-going

studies dealing with the evaluation of these fibers as perspective reinforcement for bio-based

polymer composites.

Figure 7 Stress-strain curves of CA (Cordenka EHM), CB (Cordenka 1840), CC (Enka

Viscose), CD (Cordenka 700), CE (Alternative cellulose) and CF (Lyocell) regenerated

cellulose fibers and steam exploded flax and field retted hemp fibers. [21]

There are several types of regenerated cellulose fibers. Typical stress-strain curves for

different fibers are presented in Figure 7. One of the regenerated fibers considered for this

particular project are Cordenka fibers. These fibers are produced by the German company,

Cordenka GmbH, and the main application is for reinforcing rubber in car tires.

2.3 Bio core materials

The most used natural core material in composites is balsa wood. Commonly balsa is used as a

plate which consists of balsa pieces cut to the same thickness. Balsa has good strength properties.

[22] For example, 3A Composites (Switzerland) manufactures balsa for sandwich structures with

12

trade name BALTEK® (more information: www.corematerials.3acomposites.com/baltek-

balsa.html).

In this project, also other biobased core materials are interesting. For instance, Saarpella Oy

(Finland) produces non-woven flax felt as an insulation material, which could be used as core

material. Bioresin (Brazil) has derived polyurethane foam, BIOFOAM, from castor oil.

Interesting is also the possibility to use the bioepoxy of Amroy Europe Oy (Finland) as foam.

In literature, Burgueño et al. [23] studied load-bearing natural fiber composite cellular beams

and plates made of hemp or flax fibers and unsaturated polyester. Dweib et al. [24] used

acrylated epoxidized soybean oil (AESO) resin and flax mats to produce biocomposite sandwich

beams for structural applications. Faruk et al. [25] has made a review of microcellular foamed

wood-plastic composites.

13

3 PERFORMANCE IN HARSH CONDITIONS

Reinforced composites are normally used in applications, where good mechanical and other

properties are required, such as technical and structural applications. They can also be exposed to

difficult or extreme environment conditions in their applications. These conditions can affect the

properties of composites and decrease their performance. Therefore, studies in difficult

conditions are relevant and aid to choose a right application and predict the performance of

composite. In this chapter, the definition of harsh conditions of biobased composites is discussed.

3.1 Definition of harsh conditions

Biobased composites can be used in applications, where the conditions are challenging to

composites, in other words harsh conditions. In marine industry for instance, composites are

exposed to water and in sea areas to saline water. Also humidity can sometimes be very high and

have influence on composites. Biofibers tend to absorb moisture. In a report [26], harsh weather

conditions are said to contain humidity, elevated temperature, UV radiation and rain. Low

temperatures can be added to the list, because outdoor temperature in the Nordic countries is

several degrees below zero in winter. UV radiation from sunlight is at its highest in summer.

Rarely these conditions are present separately and thus they have a combined effect on

composites. During this project, the effect of these conditions on biobased composites is going to

be studied.

In literature, Mehta et al. [26] tested hemp reinforced UPE composite samples in an

accelerated weatherometer. They used the measurement cycle of 48 cycles of UV treatment (340

nm) at 60 °C for 2.5 h followed by water spray for 30 min and condensation at 45 °C for 24 h.

They repeated it 12 times with total duration 2016 h. Weight loss was less than 2% for untreated

hemp composite and 1.25% for acrylonitrile treated hemp composite, surface roughness

increased with exposure time and color changed. Storage modulus decreased only slightly

(values 6-8 GPa) and Tg decreased less than 3°C.

14

4 TESTING AND CHARACTERIZATION METHODS

FOR DURABILITY EVALUATION

Testing and characterization methods in order to evaluate the durability of composites are

divided into mechanical tests, physical tests and chemical tests. Mechanical testing is the most

used test method of these. Mechanical tests provide information on strength, compression,

impact, bending and shear properties. In many applications, the strength properties of composite

are of interest. Physical tests give knowledge of thermal, absorption and structural properties.

Chemical tests give insight into the chemical properties or structure of composites. In this

chapter, numerical data from various studies are collected mainly in tables to show their

performance.

4.1 Mechanical testing

Typical mechanical tests for composites are tensile, flexural, impact and fatigue tests. In

structural applications, composites are required to carry load and good tensile properties are

demanded. In Table 5, data from various studies are collected for flax reinforced composites and

hemp reinforced composites.

The composite structure of test samples varies and that has effect on their properties.

Reinforcements are in the form of mats, woven fabrics, fibers or non-woven mats. Also the resin

is varied: epoxies, polyesters, soybean oil resin and acrylated epoxidized resins. Mechanical test

results differ quite lot, probably partly due to the composite structures. For instance, tensile

strength for flax reinforced composites is between 15 to 90 MPa. In this project, one purpose is

to study, if biofibers are suitable for being used in applications, where good strength properties

are required.

15

Table 5 Test data from the literature for flax and hemp reinforced composites. X means

that tests are done but figure is not found.

Fiber/ Matrix

Processing Tensile

strength Tensile

modulus Flexural Fatigu

e Impact

strength Refere

nce

Flax /

Soybean

oil resin

(MMSO)

(60:40)

Compression

moulding, air

laid / woven

fabric

(5kN) Strength

90 MPa, Mod. 5 GPa

(Charpy) 24-29

kJ/m2

[27]

Flax /

Vinylester

or

Modified

acrylic

resin

RTM, mats (25kN, 2

mm/min) ≈ 70-90

MPa

≈ 7.5-9.5

GPa [28]

Flax /

Acrylated

epoxidized

soy oil

(AESO)

RTM, fiber (5mm/min) ≈ 15-30 MPa

≈ 3.2-4.7

GPa Strength 42- 64

MPa, Mod. 2.7 - 4.2

GPa

[29]

Flax / EP yarns X [30]

Flax / EP Laminates, non-woven

mat

(1mm/min) ≈ 47 MPa

(untreated) ≈ 60-75 MPa

(treated)

≈ 4.5-4.8

GPa [31]

[32]

Hemp /

Acrylated

epoxidized

soy oil

(AESO)

RTM, mats 35 MPa 4.4 GPa Strength 35.7 ±

5.9 MPa / 51.3 ±

2.7MPa, Mod. 2.6 ± 0.2

GPa / 2.7 ± 0.2

GPa

[29]

Hemp /

UPE Compression

moulding,

non-woven

mat

≈ 70 MPa

(AN treated

hemp)

≈ 8 GPa

(AN

treated)

X (notched

Izod) 26

J/m

[33]

Hemp /

UPE

RTM, mat

Strength 22 ± 3

MPa / (Fungal

treated) 26 ± 3.5

MPa, Mod. 3.3 ± 0.3

GPa / (Fungal

treated) 3.7 ±

0.5 GPa

[34]

16

4.2 Physical testing

Data from physical tests, including thermal analysis, are collected in Table 6. Typical physical

tests are water absorption, swelling and moisture content. They are discussed in chapter 4.2.1.

Thermal analysis methods can be thought as physical testing. In the table, the numerical values

of tests from differential scanning calorimetry (DSC), thermogravimetry (TG), dynamic

mechanical analysis (DMA) and scanning electron microscope (SEM) are used to give the scale

to physical properties.

In addition to the table, Gulati et al. [35] used inverse gas chromatography (IGC) to

investigate acid-base characteristics of hemp fibers. They found that highest improvements in

acid-base interactions between fiber and matrix are correlated with actual improvement in the

mechanical properties of RTM manufactured hemp reinforced unsaturated polyester composites.

Table 6 Physical test data from the literature.

Fiber/ Matrix

Processing DSC TG DMA SEM Refere

nce

Flax/Soyb

ean oil

resin

(60:40)

Compression

moulding, air

laid / woven

fabric

Relative

low

thermal

stability

(≈370°C)

Shorter fiber

pullout with

styrene

[27]

Flax /

Acrylated

epoxidize

d soybean

oil

(AESO)

Vacuum

RTM /

infusion, mat

(different

kinds of vol-

%)

(3-point

bending) E‟ ≈ 1500 -

2000 MPa E‟‟ ≈ 200 -260

MPa Tg ≈ 57 – 70°C

[32]

Hemp /

UPE Compression

moulding,

non-woven

mat

Tg = 95°C (max

tan δ) Smaller degree of

fiber pullout (AN

treated)

[33]

Hemp /

Acrylated

epoxidize

d soybean

oil AESO

Vacuum

RTM /

infusion, mat

(3-point

bending) E‟ ≈ 2200 MPa E‟‟ ≈ 270 MPa Tg ≈ 65°C

[32]

Hemp /

Water-

based

acrylic

thermoset

(20-100°C)

heat

capacity of

cured resin

1.98-2.7 J g

-1 K

-1 /

of hemp

2.2-3.4 J g

-1 K

-1

[36]

17

In Table 6, when comparing DMA results, hemp reinforced AESO composite has better

strength than flax reinforced according to higher storage modulus, E‟, value. The larger E‟, the

better strength composite has. Glass transition temperature, Tg, of flax and hemp reinforced

AESO composites can be somewhat lower than it could be due to an incomplete curing process.

Otherwise, the values seem to be moderate. Typically, biofibers are thought to have a limited

processing temperature at approximately 200 °C, which must be taken into account when

choosing a processing method.

4.2.1 Moisture and water sorption testing

Moisture and water sorption tests are important in order to study the water resistance of biofiber

reinforced composites. Natural fibers are commonly known as to easily absorb moisture. In

Table 7, rather different values for water sorption are collected from literature. Water uptake

varies from 2 % to 18 % increase in composite weight. The reason for variation can be found in

different matrices and surface treatments. Moisture absorption for hemp is found to be about 1 to

3 weight-%. Fiber surface treatments can decrease water and moisture absorption.

Table 7 Moisture and water test data from the literature.

Fiber/ Matrix

Processing Water sorption Moisture

absorption Refere

nce

Flax /

Acrylated

epoxidized

soy oil

(AESO)

RTM, fiber (24h) ↑ 2.3 - 4.1 wt% (7 weeks) ↑ 10.4 - 12.4 wt%

[29]

Hemp /

UPE Compression

moulding, mat (30°C, 90% RH)

↑0.7 wt% (untreated) ↑0.3 wt% (AN treated)

[26]

Hemp /

UPE RTM, randomly

oriented mat

(different kinds of

vol-%)

(23°C, saturation

200 days) ↑ ≈2 – 4 wt%

(23°C, 94% RH,

200 days) ↑ ≈1.5 – 2.7 wt% No saturation

[37]

Flax / EP Hemp / EP

(816 h) ↑17.2% (flax) ↑18.4% (hemp)

[38]

18

5 SURFACE TREATMENT METHODS FOR BIOFIBERS

Natural fibers have typically poor adhesion and wettability with resin materials because biofibers

are hydrophilic in nature and resins are hydrophobic. If the reinforcement is not properly adhered

to the matrix, it does not add the strength of the composite. Therefore, different surface treatment

methods for biofibers have been invented and tested to improve the adhesion between biofibers

and polymer matrices. Treatments naturally raise the price of end products.

Generally, treatment methods can be divided into chemical treatments and physical

treatments. Chemical treatment can be defined as a chemical reaction between some reactive

constituents of chemical reagent and biofiber to form a covalent bond [39]. Physical treatment

methods do not change the chemical structure of biofibers, only the surface properties. In this

chapter, the literature search results of treatment methods are presented and their effect on the

long term properties of composites is discussed.

5.1 Chemical treatment

Common chemical treatments are dewaxing, mercerization, bleaching, cyanoethylation, silane

treatment, benzoylation, peroxide treatment, isocyanate treatment, acrylation, acetylation, latex

coating and steam-explosion. [39] John et al. [40] also adds the condensation of coupling agents

onto the cellulose surface.

Many studies have been made to investigate the effect of different surface treatments on the

properties of fibers. Good review articles [39-41] are found in the scientific literature. For

example, possible silanes to be used in silane treatment are various and Xie et al. [42] have made

a table with references about “Silanes used for the natural fiber/polymer composites”.

The main things from the literature articles are collected into Table 8 and Table 9. The

articles that studied only fibers [42,43] are left out of the tables. Surface treatments for flax and

for hemp are separated into different tables.

19

Table 8 Effect of different chemical treatments on hemp reinforced composite properties.

Fiber/ Matrix

Chemical

treatment Conditions Processing Effect on

composite

properties +

Effect on

composite

properties -

Referenc

e

Hemp/

EP Mercerizati

on 22% NaOH, 60

min, 10°C Filament

winding,

UD

Flex. strength ≈ 45% Flex.mod.

of elast. ≈ 100%

[44]

Hemp/

UPE Alkalizatio

n /

Acetylation

6% NaOH for

48h, room

temp. / glacial

acetic acid for

1h, room temp.

RTM,

contains

also glass

fiber mats

Higher

improvement with

alkali in flex.

mod. and strength

[35]

Hemp/

UPE Fungal -

modificatio

n

0.5% glucose

and 0.1% yeast, in rotary shaker

for 4, 6 or 8

days

RTM, mat Flex. strength

21%, flex. mod.

12 % (improved

interfacial

adhesion)

[34]

Hemp/

UPE Acrylonitril

e grafting 3% acrylonitrile,

0.5% dicumyl

peroxide, 96.5%

ethanol, 15 min

Compressio

n

moulding,

non-woven

mat

Tensile strength

80%, tensile mod.

25%, elast.mod.

30%, impact

strength 50%,

storage mod. &

loss mod.

increased

[33]

Hemp/

UPE Alkali /

Silane

treatment

Alkali: 2%

NaOH, 23°C,

1h, dried Silane: 1% 3-

aminopropyltriet

hoxysilane, 30

min, dried

RTM,

randomly

oriented

mat

Treatment

not

significantly

improve the

water

resistance

[37]

Hemp/

UPE Esterificati

on Methacrylic

anhydride +

pyridine: 100°C,

48h, dried / Pyridine

(same)

RTM, non-

woven mat Better interfacial

adhesion Higher flex. mod.,

flex. stress at

break no change

except in mode of

failure

Lower

impact

strength,

Low

toughness Pyridine: no

effects on

properties

[45]

20

Table 9 Effect of different chemical treatments on flax reinforced composite properties.

Fiber/ Matrix

Chemical

treatment Conditions Processing Effect on

composite

properties +

Effect on

composite

properties -

Refere

nce

Flax/Acryl

ated

epoxidized

soybean oil

+ styrene

Lignin

treatment Aqueous

NaOH solution

of kraft lignin

VARTM,

mat Tensile and flexural

properties and fiber

wettability

improved

[46]

Flax/EP Ethylene

diamine

tetraacetic

acid

NaOH, 60 °C,

3 h Laminates, non-woven

mat

Tensile strength ≈ 50%

[31]

Flax/EP Alkalizatio

n NaOH 1% -

3%, 20 min,

room

temperature

Autoclave,

UD mat (Improved interface

quality) Increased

longitudinal and

transverse

flex.mod. and

strength

(fiber

strength

decreased)

[47]

Flax/EP Alkali / Silane / Isocyanate

NaOH 1% / 3-

aminopropyltri

ethoxysilane / Phenyl

isocyanate

Autoclave, UD mat /

Random

non-woven

mat

Tensile strength

17%, tensile mod.

25% No change in

impact toughness

[48]

As can be seen in the tables, the surface treatments influence mainly positively to the

properties of natural fiber reinforced thermoset polymer composites. The comparison of results is

quite challenging, because the results depend on fiber content, composite manufacturing method

and test methods. In generally, in many studies tensile strength, tensile modulus, flexural

strength and flexural modulus increased because of the surface treatment. Mercerization, also

called alkalization, seems to be the most used treatment for flaw and hemp fibers.

5.2 Physical treatment

Typical physical treatment methods are stretching, calendaring, thermotreatment, production of

hybrid yarns, corona and cold plasma. Also other interesting methods have been tested. Gouanvé

et al. [49] studied the effect of autoclave treatment and helium cold plasma treatment on the

water sorption of flax fiber (no matrix). The moisture resistance increased after autoclave

treatment, but the plasma treatment had no effect on the resistance. Hepworth et al. [50] used a

urea treatment to penetrate epoxy resin into the cell walls of flax fibers. Treated fiber reinforced

composites had higher tensile modulus (15 GPa) than untreated fiber composites (11.5 GPa)

although strength was not affected (118 MPa).

21

5.3 Effects on long term properties

Composites are typically used in applications, whose time of use is long, from years to decades.

Hence, the effect of surface treatments on the long term properties of composites should be

studied. The question, how biofiber reinforced composites maintain their properties during a long

period, is interesting. For instance, water and fungus can cause harm to biofibers within time.

Many studies cover short or relatively short term property changes. Changes in long term

properties are one of the main objectives to be explored in this project.

22

6 CONCLUSIONS

The purpose of this state-of-the-art is to give insight into eco-efficient composite materials and

make conclusions that operate as guidelines to further actions in the ANACOMPO project.

Materials that are chosen to be good material candidates tested in this project are discussed,

likewise the recommended surface treatment to be used. The most critical harsh conditions

concerning the use of biocomposites are considered.

6.1 Best choice material candidates

A few from commercially available bioresins are selected for further characterization. The

bioepoxy, EpoBioXTM

, is a good material candidate because epoxies typically have very good

properties and this bioepoxy also has a large bio content percent. Epoxies perform presumably

well in harsh conditions. It is also produced in Finland and hence quite near to all project

partners. ENVIREZ® unsaturated polyester bioresins can also be purchased from Finland and

ENVIREZ® resins have been in the market for almost a decade, so they are worth to take as good

candidates. Also the unsaturated polyester bioresin, Palapreg ECO P55-01, from DSM has quite

high bio content percent, a half of the raw materials. These four resins are selected for further

tests.

6.2 Identification of most critical harsh conditions

Data from harsh condition testing is quite limited available. Probably a good way is to start tests

in all typical condition areas that are known to be difficult. UV radiation, water, humidity and

temperature and also the combined effect of those are commonly of interest.

6.3 Surface treatment options and recommendations

In tables 8 and 9, the most commonly used surface treatment is chemical treatment with sodium

hydroxide. This treatment is quite easy and inexpensive.

23

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