“investigations on dicarboxylic acid based chain extended...
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CHAPTER 1
INTRODUCTION
This thesis presents the results of “Investigations on Dicarboxylic Acid
Based Chain Extended Polyurethanes” with focus on the effect of extenders
(dicarbocylic acids) on the performance of PUs. An introduction to PUs and factors
contributing to the performance of chain extended PUs, raw materials for PU
preparation, characterizations and applications of PUs are described in this chapter.
This chapter also covers the review of literature on chain extended PUs, motivation,
background of the research investigation, objectives of the present investigation and
present research problem.
All around us polyurethanes are playing a vital role in many industries from
ship building, construction of cars to footwear. They appear in an astonishing variety
of forms, a variety that is continuously increasing. PU is an incredibly resilient,
flexible and durable material that can replace paint, cotton, rubber, metal, and wood in
thousands of applications across all fields. PU might be hard, like fiberglass, squishy
like upholstery foam, protective like varnish, bouncy like rubber wheels, or sticky like
glue. Since its invention in the 40s, PU has been used in everything from baby toys to
airplane wings, and continues to be adapted for contemporary technology.
Polyurethane plastics were initially synthesized by Bayer [1], and they have
been known to users for more than six decades, predominantly as elastomers and
foams. PU plastics belong now to the group of important materials applicable in
numerous fields of engineering [2]. If the volumes of PU products and raw materials
needed for production are considered, PUs shall be put among the prime polymeric
materials, and namely they shall be ranked 5, after the unquestionably dominant
polyolefins (polyethylene (PE), polypropylene (PP)), polyvinyl chloride (PVC),
polystyrene (PS) and diene rubber.
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The very wide applicability of PUs results from the fact that their performance
properties can be widely modified by selecting appropriate raw materials, catalysts
and auxiliary compounds, by employing various production methods and/or by
employing various methods for further processing and/or for shaping the final
products. Resulting from their specific micro-phase structure, which is formed by
rigid chain segments and flexible chain segments PUs offer very good elasticity with
reasonably high mechanical strength and abrasion resistance at the same time and also
controllable hardness. PUs can be available both as relatively rigid elastomers and as
flexible elastomers with compact or foamed structures. There are two inhibiting
factors for applicability of PUs: their limited stability at temperatures above 90 oC,
and their flammability referred to as foamed PUs. PUs are produced in the form of
foamed plastics, structural elastomers and coating elastomers, adhesives, leather-like
materials and auxiliary agents. Flexible cellular PU elastomers became inherent in
comfort-providing elements, especially in furniture making and the automotive
industry. On the other hand, rigid foamed PUs can be converted into lightweight
elements with structural stability and superior thermal insulation performance
(closed-cell foams) and/or acoustic insulation performance (open-cell foams). PUs
offer advantageous performance properties, ease of processing, good resistance to
water, oils, greases, organic solvents, diluted acids and alkalis. All that makes them
applicable in numerous fields of technology, economy and in everyday life [3-4].
These advantages make PUs as modern organic polymeric plastics.
Some PU is categorized as an elastomer. It has elastic properties while
maintaining some rigidity, such as in the wheels of a dolly that absorb shock but don’t
compress too much. It can be extremely flexible when used as a foam insulator in
construction or a foam cushion in upholstery. It can be deformed over and over but
still maintain its original shape; in other words it has a structural memory. Elastomers
have made our home and work environments warm and comfortable.
PU is a thermoplastic that resembles other kinds of plastic, metal, or
fiberglass. Thermoplastics are rigid and smooth with a sealed surface impermeable to
water. These are used when strength and durability are important, such as seats in
airport terminals or packaging crates on a truck. Some thermoplastics are difficult to
recycle, but they can be reused.
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We can find PU in every room of our house and practically everywhere we go.
The populating of these materials grew during World War II; the polymer has
protected, reinvented, joined, or transported countless items. It seals surfaces like
wood, metal and paint to protect from rot, corrosion or fading. As an adhesive, PU
resists moisture and heat, so it is ideal for use in sun or ocean. It insulates walls,
temperature-controlled vehicles and consumer coolers. PU formulations cover an
extremely wide range of stiffness, hardness and densities. PUs are widely used in high
resiliency flexible foam seating, rigid foam insulation panels, microcellular foam
seals and gaskets, durable elastomeric wheels and tires, electrical potting compounds,
high performance adhesives and sealants, spandex fibres, seals, gaskets, carpet
underlay and hard plastic parts.
All PUs are based on exothermic reaction of polyisocyanates with polyol
molecules, containing hydroxyl groups. Relatively few basic isocyanates and a range
of polyols of different molecular weights and functionalities are used to produce the
whole spectrum of PU materials. Additionally several other chemical reactions of
isocyanates are used to modify the range of isocyanate-based plastic materials. The
chemically efficient polymer reaction may be catalyzed, allowing extremely fast cycle
times and quantity production. No unwanted by-products are given off because the
raw materials react completely. No after cure treatment is necessary.
Thermoplastic polyurethane (TPU) elastomers play an important role within
the rapidly growing family of thermoplastic elastomers (TPEs). Since PU elastomers
were the first homogeneous thermoplastically processable elastomers, let us consider
the history which lead to the discovery and development of PU.
Historically, work on PU was carried out by Otto Bayer and his coworkers of
Fabenindustrie at Leverkusen, Germany (now Bayer AG) in 1937 [1]. Their original
target was to duplicate or improve the properties of synthetic polyamide fibres.
Subsequently, the elastomer properties of PUs were recognized by DuPont [2] and by
ICI [5]. By the 1940s PUs were produced on an industrial scale [6]. A PU elastomer
was synthesized which consisted of linear polyesters and 2-nitro-4, 4’-diisocyanato-
biphenyl. Chain extension by short chain diols have proved to be the breakthrough to
PU elastomers, which were trade named by Bayer as Vulkollan. The polyester
urethane elastomer was developed by Seeger et al of the Goodyear Tire and Rubber
Co., [7].
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In 1969, Bayer AG exhibited an all plastic car in Dusseldorf, Germany. Parts
of this car were manufactured using a new process called reaction injection molding
(RIM). RIM technology uses high-pressure impingement of liquid components
followed by the rapid flow of the reaction mixture into a mold cavity. Large parts,
such as automotive fascia and body panels can be molded in this manner.
Polyurethane reaction injection molding (RIM) evolved into a number of different
products and processes. Using diamine chain extenders and trimerization technology.
poly(urethane urea), poly(urethane isocyanurate) and polyurea RIM were produced.
The addition of fillers, such as milled glass, mica and processed mineral fibres gave
rise to reinforced reaction injection molding (RRIM), which provided improvements
in flexural modulus (stiffness) and thermal stability. This technology allowed
production of the first plastic-body automobile in the (Pontiac Fiero) United States, in
1983. Further improvements in flexural modulus were obtained by incorporating glass
mats into the RIM mold cavity also known as SRIM or structural RIM.
1.1 Polyurethanes: Chemistry and technology
Polyurethane chemistry is a very broad field that encompasses a large number
of chemical reactions, including many reactions of isocyanates with active hydrogen
compounds, with other isocyanate groups, with other unsaturated compounds, etc [8].
Only the more important isocyanate reaction, i.e., the formation of chain extended PU
is covered here.
The general aspect of chemistry and applications of various types of PUs used
in a variety of engineering areas are discussed in this section. Polyurethane elastomers
are linear block co-polymers in which one of the two blocks is typically a polyether or
a polyester diol with a molar mass between 300 and 6000 [9]. These blocks comprise
of soft segments because at the service temperature they exist in a rubbery or viscous
state and impart elastomeric properties [10-11]. The other segments are generally
composed of aromatic diisocyanates extended with diols (chain extenders) to produce
blocks with molar mass in the range 500-3000. These blocks form hard segments
because at the service temperature they are in the glassy or semicrystalline state.
Dimensional stability is imparted through microphase separation of the hard segments
into domains, which act as a reinforcing filler and the multifunctional cross-links. PUs
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is thermoplastic because heating above the hard segment, glass transition temperature
(Tg) will allow the material to flow. A wide range of physical properties and
morphologies has been observed, depending on the composition and chemical
structure of hard and soft segments [12-15].
The driving force for phase separation is due to the incompatibility of the hard
and soft segments. Generally, the urethane segments are more polar than either the
polyether or polyester segments. Other factors that influence phase separation are
segment length, crystallisability of segment, composition, thermal history and method
of preparation. Experimental evidence has shown that segmental PUs are not
completely phase separated but exhibit some degree of phase mixing; the polyester
based PUs are less phase separated than those containing polyethers. This is due to
the fact that ester groups are more polar and have better hydrogen bonding
capabilities than ether groups. Interchain attractive forces between rigid segments are
far greater than those present in the soft segments [16-18]; this is due to the high
concentration of polar groups and the possibility of extensive hydrogen bonding. Hard
segments significantly affect mechanical properties such as modulus, hardness and
tear strength and thus the performance of these elastomers at elevated temperature
depends to a greater extent on the structure of the rigid segments [19-20].
A variety of experimental methods have been used to prepare PUs [12-13].
However, the most widely used method is the reaction of di- or poly- functional
hydroxyl compounds, eg., hydroxyl terminated polyesters or polyethers, with di- or
polyfunctional isocyanates. The general structure of linear PU is derived from a
dihydroxy compound (HO-R-OH) and a diisocyanate (OCN-R'-NCO). The
functionality of the hydroxyl group containing compound and of the isocyanate group
can be increased to three or more to form branched or cross linked polymers. Other
structural variations are also possible. For instance, R-group may be changed to
include different types of glycols, ethers, esters, etc., and similarly, the nature of R'
may be altered to include groups from naphthalene diisocyanate (NDI) to the
hexamethylene diisocyanate (HDI). Thus, PUs are almost unique in the cross-linking,
chain flexibility and intermolecular forces can be varied widely and almost
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independently. It is for the reason that PUs are available as fibres, soft and hard
elastomers, flexible and rigid foams, coatings and as highly cross-linked rubbers.
In addition to the above, a number of chemically well defined model PU
structures have been prepared. For instance, Harrel prepared PUs of monodisperse
block unit based on piperazine and poly (tetramethylene oxide) [21]. By controlling
the hard-segment size distribution and the soft-segment molar mass distribution, he
was able to correlate structures of the model PU system with such ultimate properties
as modulus, tensile strength, elongation and extension ratio. However, these polymers
do not have any commercial interest and were not capable of hydrogen bonding,
which is an important factor in determining properties of PU systems. Camberlin et al
[22] prepared model hard blocks based on methylene bis-(phenyl isocyanate) (MDI)
and 1, 4-butanediol (BD) and studied their thermal properties. Recently, Eisenbach
and Gunter [23] and Qin et al [24] have synthesized a limited number of hard blocks
of known structures based on MDI and BD. Eisenbach and Gunter [23] reacted the
hard segment models to obtain segmented PU (SPU) elastomers, whereas Qin et al
[24] prepared monodisperse triblock copolymers with poly (propylene-oxide)
(PPO) as the soft segment. Hwang et al [25] have synthesized monodisperse BD -
MDI hard segments which were later shown to be high melting rod like compounds.
Also, Millar et al [26] have published a study on a series of model PU backbones
containing various hard-segment length distributions. This list continues to infinity
and one can find such details in the literature.
In the industrial area PU is known for its excellent resistance to a wide variety
of organic solvents [27]. Of particular interest is the fact that when exposed to organic
liquids the membrane swells but when removed and allowed to dry out, it returns to
the original dimension. The effect of organic solvents on PU depends to a great
extent on the nature of the solvent and the type of PU segments. Alcohols, acids,
ketones and esters tend to cause swelling and degradation, particularly at high
temperatures. Aliphatic hydrocarbons and esters are generally inert, but aromatic
hydrocarbons are more active and promote swelling at room temperatures and gradual
breakdown at higher temperatures. PUs in contact with such organic liquids upto
service temperatures of around 50 oC, can be considered to be the most resistant
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polymers available [27]. However, chlorinated solvents are known to cause swelling
sometimes even leading to degradation. The tensile and tear strengths may be
reduced to about 25% of the initial values after 6 months immersion in chloroform at
ambient temperature. Almost all PUs are known to be highly resistant to water.
However, acidic or alkaline media accelerate hydrolytic attack and therefore solutions
of salts of weak acids or bases are likely to induce degradation. On the other hand,
strong acids and bases attack PUs rapidly. Polyurethanes are highly resistant to UV
light, hence outdoor weatherability is good. It is generally considered that PUs are
resistant to the effects of damage by high energy radiation.
1.2 Urethane group formation
The basic chemical reaction involved in making any type of PU, including
TPU, is urethane group formation. This is accomplished by causing an organic
isocyanate (-NCO) group to react with an alcoholic hydroxyl (-OH) group as seen
below;
O=C=N-R-N=C=O + HO- R'-OH [ CO -NH-R-NH-CO-O- R'-O ]n Diisocyanate Diol Polyurethane
where, R and R' are alkyl or aryl groups.
1.3 Basic raw materials for the production of linear polyurethanes (TPU)
Chain extended PU elastomer formation requires bifunctional reactants to
enable the building of long linear chains. The types of components used to prepare
PUs are given in Table 1.1. Almost all PU components are liquids at room
temperature or low-melting solids.
Table 1.1. Components to prepare thermoplastic polyurethane
Components General structure
Diisocyanate OCN - R – NCO
Macrodiol HO - R’ – OH
Chain extender H OOC - R” - H OOC or H2N - R – NH2
The basic pool of feeds applicable in the manufacture of PUs is made up of: diisocyanates, polyether polyols or polyester polyols, diols, diamines employed as
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low molecular-weight extenders of isocyanate prepolymer chains, catalysts for polyaddition process of diisocyanates and compounds with unstable hydrogen atoms (water, alcohols, and amines) and auxiliary substances selected for specific processes, e.g. blowing agents for foamed PUs, multi-functional amines or isocyanates as cross-linking agents or organo phosphorus antipyrene compounds which are widely used in foamed PUs. The choice of those materials has been discussed in detail in numerous review papers intended to present production processes of individual PU products [28-30].
1.3.1 Diisocyanates
The only technically reasonable method today for the production of PU plastics is the polyaddition process of diisocyanates and polyols [31-32]. The diisocyanate component is a relatively small molecule of molecular weight ~150 - 250. In TPU its function is two fold. First it acts as a coupling agent for the macrodiol and then it reacts with chain extender to produce PU. Diisocyanates such as MDI, TDI and HDI are extensively used to synthesize a variety of PUs and for their characterization by many scientists [33-40].
Isocyanates are the major constituents of the rigid segments of PUs. With
increasing symmetry of the isocyanate, the following properties increase the ability of the PUs to crystallize, microphase separation, modulus of elasticity, tensile strength, hardness and rubbing resistance. The nearer the angle between the isocyanate groups in the ring (180o) the closer can be the packing of the rigid segments, thus conferring a higher strength and modulus of elasticity on the PUs.
Rigid PU foam is one of the most effective practical thermal insulation
materials, used in applications ranging from buildings to the modest domestic refrigerator. Comfortable and durable mattresses, car and domestic seating are manufactured from flexible foam. Items such as shoe soles, sports equipments, car bumpers and soft front ends are produced from different forms of PU elastomers. Besides many of us rely on PUs for elastic threads found in underwater and other clothing. Polyurethane is a substance categorized as a polymer based on its chemical structure. One manufactures PU by combining a diisocyanate and a diol, two monomers, through a chemical reaction. This makes a basic material whose variations can be stretched, smashed or scratched and remain fairly indestructible. Depending on
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the different diisocyanates and diols or polyols constituents, the resulting PU might take a liquid, foam or solid form, each with advantages and limitations.
The highest mechanical strength is offered by PUs obtained from disocyanates
with symmetrical structures, like aromatic MDI or its alicyclic equivalent HDI [38]. Linear PUs obtained from MDI and BD form a zigzag chain in which benzene rings of MDI are arranged at a right angle against each other [39]. On the other hand, the PU chains synthesized of 2,4-TDI and 2,6-TDI are arranged in one plane because of the coplanar arrangement of both TDI-derived benzene rings and urethane groups attached to those rings [40]. New reactions of isocyanates have been revealed recently and they have been utilized in the production of PU-based powder coatings [41-44].
1.3.2 Polyols Initially polyether polyols are used for the production of linear PUs.
Polyethers impart softness and flexibility to PUs [45-46]. PUs obtained from polyesterdiols are less resistant to hydrolysis than polyether polyurethanes [47-49]. General attention has been attracted recently to one more type of polyesterdiols applicable in the production of PU elastomers, i.e., aliphatic polycarbonates (PCs) obtained from cyclic alkylene carbonates [50-51]. PUs produced from polyoxypropylene glycols (POG) [52] with asymmetrical structures have lower mechanical strength than their analogues derived from poly (oxytetra methylene) glycol (PTMG) [53]. Its hydrophobic nature enhances the phase separation potential and its regular structure is favourable for crystallization.
Among vegetable oils, castor oil (CO) represents a promising raw material due
to its low cost, low toxicity, its availability as a renewable agricultural resource, rising costs of petrochemical feed stocks and partially due to an enhanced public desire for environmentally friendly green products. Its major constituent, recinoleic acid (12-hydroxy-cis-9-octadecenoic acid), is a hydroxyl containing fatty acid [54]. The preparation of polymers from renewable sources such as vegetable oil-based materials is currently receiving increasing attention because of economic and environmental concerns [55-60].
1.3.3 Chain extenders
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Chain extended PU (CEPUs) elastomers largely possess the same outstanding
mechanical properties as other types of solid PU elastomers, such as the castable and
injectable liquids and millable gums. These property levels usually enable less
material to be used in a given application than would be the case with a nonurethane
material and this feature gives TPU an edge in many instances. Further assets of
CEPUs or the TPU are its ability to be fabricated into finished elastomer products
without the need for curing (crosslinking) which makes it possible to reprocess TPU
scrap formed during product manufacture. With so many favourable features, it is no
wonder that chain extended PUs or TPUs have become so attractive to polymer
producers and so well received by plastic fabricators.
Chain-extending agents are the diols and diamines of a small molecule, which
increase the size of the rigid segments as well as the hydrogen bond density. The
corresponding tri or more highly functional compounds act as branching or
crosslinking agents. The effect of an extender on the PU properties is remarkable,
although it usually constitutes a minor part of the polymer. 1,4-Butane-diol is the best
extender of the aliphatic glycols. The glycol-extended PUs are more flexible and less
strong than their amine-extended analogues [61]. Extender with a cyclic structure
increases the PU strength properties to a greater extend than linear extenders.
A chain extender affects PU properties to an extent far greater than suggested
by its mass fraction. Each extender molecule incorporated more than doubles the
length of the rigid segment. Furthermore, amine (or water) chain extenders introduce
urea groups, which are more polar than the urethane group and facilitate phase
separation. Diamines enhance the strength and hardness of the PU more than glycols.
The diamines substituted ortho to the amino group impart to the PU prepolymer
system the best combination of good strength properties of the resulting polymer with
a satisfactory pot life of the formulation. Butane-diol chain extended polymers
showed superior properties, which is ascribed to the regularity in the back bone chain
of the polymers and ease of formation of hydrogen bonds [62]. Ganga et al [63-64]
reported the synthesis and properties of segmented PU (SPU) using phenolphthalein
as chain extender [63]. Ramesh et al reported the effect of several aromatic diamines
and aliphatic diols on mechanical and thermal properties of PUs obtained using
hydroxyl terminated polybutadiene [64].
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Diols make the group of difunctional chain extenders for urethane–
isocyanate prepolymers, which is most widely employed in the production of PU
elastomers. That group comprises ethylene glycol, diethylene glycol, 1,4-butanediol
and 1,6-hexanediol [59, 65-66]. Diamines like 1, 2-ethylenediamine and 1,6-
hexamethylene diamine, can also be used as chain extenders, but in this case urethane
chains are extended through urea groups. Aliphatic amines are used frequently to
extend chains of urethane ionomers [67-68]. Dianoles are applicable for the
conversion of PU elastomers with a higher number of aromatic rings in the production
of chemically resistant coatings [69]. Moreover, there is a pretty numerous group of
low-molecular weight and bifunctional diols and/or amines, which form rigid
mesogens utilized in liquid crystal linear PUs [70-72]. The thermal stability of PUs
can be improved by using diamine chain extenders include hydrazine or aliphatic
[73], aromatic [74] and heterocyclic diamines [75-76]. Very recently diamine based
chain extended PU and has been prepared their structure-property relationships
reported [76a]. James et al have investigated the phase separation of ethylene diamine
or a diamine mixture based chain-extended PUs by FTIR spectroscopy and phase
transitions [77]. Shilov et al [78] synthesized rigid segments separately in the
reaction of MDI with BD, and soft segments separately in the reaction of PTMG with
the pyromellitic anhydride, at the molar ratio of 2:1 in each case. The possibility of
extending with diol and isocyanate alternately, and the use of a trifunctional
isocyanate, is a method employed for the synthesis of PU dendrimers [79].
Dicarboxylic acid chain-extended PUs [i.e., poly (urethane-amide] is of
particular interest in biomedical applications. It is well established that such
copolymers usually phase separate into high Tg (sometimes crystalline) ‘hard’
domains and relatively low Tg ‘soft’ domains [80-83]. The degree to which the hard
and soft segments microphase segregation and the resulting morphology have a
profound effect on the copolymer’s ultimate properties. Despite their importance there
is inadequate understanding of hard segment–soft segment phase separation and the
influence of thermal and process history on phase separation in these materials.
Fluorinated polymers exhibit many interesting bulk and surface properties due
to the unique characteristics of the fluorocarbon chains, including high oxygen
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permeability [84], good stability against hydrolysis [85], excellent thermal stability
and chemical resistance [85-87], low interfacial free energy and good water and oil
repulsion. Fluorocarbon chains have been incorporated into PUs via fluoro-containing
diisocyanates [88], chain extenders [88-94] or soft segments [86-88, 95-99]. For
instance, Ratner et al [89-92] synthesized a series of fluorine-containing aromatic PUs
by using various perfluoro chain extenders and studied their surface and bulk
structure. Kajiyama et al [93] introduced a fluorocarbon-containing diol as chain
extender into a PU and examined its effect on the surface properties of the PU.
Tonelli et al [86-87] utilized a perfluoropolyether as a soft segment to synthesize
fluorinated PUs, which exhibits low-temperature elastomeric behavior, thermal
stability and superior chemical resistance. Ho et al [95-96] prepared fluorinated PUs
based on a series of fluorinated diols to obtain PUs surfaces with minimum adhesion.
Chun et al have investigated the effect on shape memory and mechanical
properties of PU copolymers by changing the chain extender from BD to
ethylenediamine (ED) [100]. Dependence of thermal and mechanical properties on the
selection of chain extender, BD or ED was investigated by IR, XRD, DSC, tensile test
and shape memory test. The morphology, thermal stability, barrier property and
tensile properties of chain extended PU/clay nanocomposites were investigated by
Kim et al [101]. They found that the tensile strength and elongation at break were
enhanced by introducing organoclay and increasing the dispersibility of organoclay
due to the strong interactions between PU matrix and organoclay. Also these
properties showed maximum at 1 wt% organoclay and decreased with the increase of
clay content due to the aggregation of organoclay.
Goldstein et al have synthesized a family of segmented degradable poly(ester
urethane urea)s (PEUURs) from 1,4-diisocyanatobutane, poly(e-caprolactone) (PCL)
macrodiol as a soft segment and a tyramine-1,4-diisocyanatobutane-tyramine as a
chain extender [102]. They demonstrated the suitability of this family of PUs for
tissue engineering applications and established a foundation for determining the effect
of biomaterial modulus on bone tissue development.
Synthesis and characterization of degradable PU elastomers containing an amino-acid based chain extender have been investigated by Skarja and Woodhouse
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[103]. They have also studied the structure–property relationships of degradable PU elastomers containing an amino acid-based chain extender [104]. Debowski and Balas have prepared and characterized the PU elastomers in polar solvent with N,N'-ethylene urea as a chain extender [105]. Caracciolo et al have reported on the synthesis, characterization and in vitro behaviour of segmented poly (ester urethane urea)s obtained from novel urea-diol chain extenders [106].
Recently Tatai et al have investigated the effect of chain extender structure on properties and in-vitro degradation of PUs [107]. The influence of chain extenders on mechanical and the adhesive behaviour of PUs have been reported by Delpech and Coutinho [108]. Pkhakadze et al [109] synthesized PUs containing a chain extender based on symmetric esters of phenylalanine and glycols. They found that chymotrypsin enhanced the degradation of their materials. Yang et al synthesized chain extended polyurethane acrylate (PUA) ionomers using dimethylol propionic acid (DMPA) as chain extender and characterized with FTIR, NMR and GPC [110]. They also reported that these PUA ionomers are photocured to a greater extent than conventional PUs. The photocured PUA ionomers exhibited superior pendulum hardness along with satisfactory adhesion, impact and flexibility.
1.4 Polyurethanes - Preparation and processing
There are two polymerization methods by which TPU can be prepared; the two-step (prepolymer) process and the one-step (one-shot) process. The former involves the preparation of a low-molecular weight, linear, isocyanate-terminated prepolymer followed by its chain extension to a high-molecular-weight linear polymer. A schematic representation of formation of dicarboxylic acid based CEPUs is shown in scheme 1.1.
n OCN-R-NCO+O-R’-OH OCN - R-(NHCO-O-R’-OCONH- R-) n NCO Diisocyanate Macrodiol Prepolymer (n-1) HOOC- R”-COOH Chain extender
[ ( NHCO - O - R’ - O - CONHR ) ( HNOC - R” – CONH-R )n-1 x ] Polyurethane
Scheme 1.1. Formation of dicarboxylic acid based chain extended PU
In the first step the dry macrodiol and diisocyanate react to produce isocyanate
-terminated linear chains, which remain relatively low in molecular weight and melt
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viscosity, thus facilitating subsequent mixing with chain extender. The diisocyanate-
macroglycol chain segments comprise the urethane-spare soft segments in the TPU
chains [31-32]. In the second step, added dry chain extender reacts with prepolymer
terminal isocyanate groups in further urethane link formation to couple the
prepolymer molecules and produce a high-molecular-weight PU system.
In the “one shot” (single-step) process all the TPU components are mixed
together at one time. Here the alternating soft and hard segments are joined end-to-
end through urethane linkages.
Review of the preparation and processing of various polyurethane elastomers is
schematically represented in the scheme 1.2.
Preparation technique Components
Prepolymer
Oligomerol + diisocyanate
One-shot
Oligomerol+diisocyanate(s) +extender + catalyst
Intermediate NCO-terminated (stable) prepolymer
Additional treatment Degassing under vacuum, for 30 min at 80-100oC
Additional component Extender (Glycol or liquid diamine) + catalyst
Additional treatment Degassing (2-5 min.)
Degassing
Processed material Processing technique Additional treatment
Liquid PU (unstable)
Mould casting RIM Post-curing Post-curing(e.g. (e.g.annealing) annealing)
Polyurethane elastomer (stable) Milled Thermoplastic Processing Processing (Press forming, (injection extrusion, moulding, vulcanization) extrusion,
calendering)Type of PU elastomer Cast Cast, injection
moulded
Scheme 1.2. Schematic representation of synthesis and processing of PUs
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1.5 Chain structure and behavior
The urethane linkages in chain extended PUs provides ample opportunity for
inter-chain hydrogen bonding. This occurs to different degrees depending on the
groups involved for example, between the urethane hydrogen atoms of one chain and
the urethane and ester carbonyl groups or the ether oxygen atoms of adjacent chains
(Seymour et al, 1975) [111]. In PU elastomer chains, urethane-urethane association
through hydrogen bonding with attendant ordering (aggregation) of the urethane units
and the greatly intensified process in segmented PU elastomer chains would seem to
account for the elastic nature of TPU [112].
Rather convenient chain extenders are the diamines in which the reactivity of
the amino groups is reduced by neighboring sterically hindering substituents. These
react fairly rapidly, but there is still enough time left for all the necessary processing
operations to be carried out. Some of the aromatic diamines are solids under standard
conditions and must be heated or dissolved in other PU materials before use. Mixtures
of glycol and a diamine are also sometimes used as chain extenders. PUs are
segmented polymers, that is, they are built from alternating rigid and flexible
segments. Unsegmented thermoplastic PUs are made from diisocyanates and low-
RIM glycols.
PU properties are the resultant of the overlap, often in a fairly complex
manner, of a number of parameters related to both molecular and supramolecular
structure. The parameters involved are: segmental flexibility, sizes of flexible and
rigid segments, mutual ratio of both kinds of segments in the polymer, hydrogen
bonds, vander Waals forces, size and symmetry of the aromatic rings, segment
orientation, crosslinking bonds, microphase separation and crystallization. The
advantages of PUs include: (i) high mechanical strength, abrasion resistance (over
160 times that of natural rubber (NR)), (ii) resistance to hydrocarbons (fuels) and (iii)
the capacity to absorb much mechanical energy.
The self-reinforcement of the PU occurs more readily if the flexible chains are
long, rigid segments are absent in the soft phase and the PUs are strictly linear. The
PU properties improve with more complete phase separation and greater ordering of
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the segments in the hard phase [113-122]. The more distinct the separation of the soft
phase flexible segments from the hard phase, the lower is the Tg of the polymer and
the greater the ability to withstand low temperatures.
If the rigid segment content in a PU is high, the spherulites are formed from
the rigid segment domains. This is also the case for a large content of flexible
segments, which also make up a spherulitic structure [113-124]. The presence of
spherulites improves the thermal stability of the polymer [125-130]. The spherulitic
structure may develop in PU during annealing, for example at 240 oC. The properties
of PUs with mixed chain extenders and mixed soft segments have been reported by
Oan Ahn et al [131]. The influence of chain extenders and chain end groups on
properties of segmented PUs have been reported by several authors [132-133]. The
influence of chain extenders on the phase morphology of a novel series of model
segmented PUs have been reported by Savelyer et al [134]. PU membranes have
excellent barrier properties. These PU membranes have been extensively studied for
swelling, sorption and diffusion and chemical resistivity by several authors to
establish structure-property relationship of PUs [135-139]. In view of the importance
of PU as barrier material in several engineering areas, it is important to know its
transport characteristics with respect to common organic solvents [140-142].
Therefore, it is necessary to study their interaction with various commonly used
organic solvents. Some previous studies have been made on solvent transport of the
PU membranes, more experimental data are still needed for better understanding of
thermodynamic interaction between PU and solvent [135-138].
The properties of PUs can be modified by varying its microstructure and by
dispersing inorganic fillers in PU matrix [143]. PUs possesses good mechanical
properties such as high abrasion resistance, tear strength, flexibility and elasticity.
However, they show poor thermal stability and barrier property, which can be
modified by incorporating clay filler [144]. Several researchers have studied the effect
of hard and soft segments on the properties of PU by using a number of analytical
techniques [3, 112, 145]. All such studies suggest that the flexible polyester or
polyether segments possess a lower Tg, than the stiff urethane blocks. Thus, complete
17
information on structure-property relations is necessary for the proper design
applications of PUs.
One of the most easily accessible methods of measuring the microphase
separation in PUs is the measurement of changes in the Tg. Also, such effects can be
demonstrated by electron microscopic techniques where the presence of domains in
polyester or polyether based elastomer is revealed by staining the samples with iodine
and observing the darkened areas by transmission electron microscopy (TEM).
Evidence from x-ray diffraction, thermal analysis, birefringence and mechanical
properties of PUs suggests that these polymers have long flexible (1000-2000nm)
segments with much shorter (150 nm) rigid units which are covalent as well as
hydrogen bonds together (Figure 1.1) [3-4]. Modulus-temperature data usually show
at least two definite transitions, one below the room temperature which is related to
segment flexibility of the polyol and another above 100 oC due to dissociation of the
inter-chain forces in the rigid units [146-147]. Differential scanning calorimetry
(DSC) and Fourier transform infrared spectroscopy (FTIR) have also been used
[4, 148] to investigate different types of structures present in PUs. DSC studies have
shown three transitions: the one below -30 oC associated with the Tg of soft block;
transitions in the region 80-150 oC and those above 160 oC associated with the
thermal dissociation of the hard block aggregates which are crystalline or
paracrystalline [149].
This chapter deals with several aspects of chain extended PUs, but there is
much more published information on the subject than could be cited and discussed
here. What has been attempted is to provide a path through the wealth of information
on TPUs, leading from their inception, progressively through the science and
technology that have attended their development to the uses and markets for these
remarkable materials.
1.6 Applications of chain extended polyurethanes
The most important properties from the viewpoint of materials technology are:
static and dynamic mechanical properties, acoustic, optical and electrical properties,
as well as corrosion resistance, chemical performance and biological performance of
18
PUs. This section, however, is going to present the review of the latest trends in
applications of the linear PUs. Special attention has been paid to elastomers, ecologic
lacquers, adhesives and binders, biomedical materials and modern materials for
electronics [150-152].
Figure 1.1. Schematic representation of chain extended polyurethane
The ease of processing of TPUs or CEPUs by numerous familiar methods;
their outstanding physical properties, which enable effective use in thin cross
sections, the aesthetics they allow in products, their persisting novelty and their
proven value and acceptance have all combined to generate a great number of
applications for TPUs, and the number continues to grow. The following paragraph
gives some idea of the versatility of TPUs in their applications.
The excellent application properties of PUs account for the fact that their use
is economically feasible and that their range of applications is constantly increasing
[112]. In some instances they are even irreplaceable. Among these properties are a
unique combination of a high elastic modulus [153-154], good flexibility (even at
high hardness), exceptional tear and abrasion resistance [155], resistance to mineral
oils and lubricants, resistance to UV radiation and finally the fact that the products
retain these properties as well as provide fairly easy and efficient processing.
The applications of PU enable one: (i) to decrease the weight of the products,
i.e., by elimination or reduction of the use of metals; (ii) to substantially reduce
labour - even large pieces can be obtained in one short cycle; (iii) to secure comfort
and safety in use of the products; (iv) to obtain products of excellent service
performance, difficult to achieve in other ways; and (v) to reduce the energy
19
consumption for the processing operations, coupled with a raised efficiency, a high
degree of automation and a commercial scale of operation.
PUs are mainly used in soles, solid tyres and impellers. The majority of PU
usage is for rigid and flexible foams. About 15% of PU production is for elastomer
applications. This is mainly due to their unique combination of properties and the
ability to be processed, shaped and formed by almost all known manufacturing
techniques. A process that has been responsible for this growth is the RIM of liquid
PU elastomer. Despite the high costs, it has an important market expansion factor.
The use of PU in different segments is shown in Figure 1.2.
8%
4%
6%
7%13%
16% 7%
39%Misc
Shoe Soles
Textile Industry
Coatings
Refrigerator
Construction
Automotive
Furniture Beedings
Figure 1.2. Applications of polyurethanes
The use of PU despite their expense compared with other high-volume
polymers, brings economic benefits often because of the longer service life of the
products and their reliability in service and hence elimination (or shortening) of
stoppages for repair.
1.6.1 Electrical and electronics
PUs have found widespread use in electrical engineering and electronics
mainly on account of their impact and abrasion resistance, considerable adhesion, and
suitability of operation over a wide temperature range of -50 to 150 oC. The electrical
20
properties of PUs are comparable with those of the more expensive epoxide, but they
are more constant with change of temperature and frequency.
One of the major applications of PU in these fields is the sealing of
components and assemblies with a protective coat of solid cast PU. It protects the
capacitors or coils against the adverse effects of aggressive environments as an
impermanent. Several researchers have reported on the preparation of composites of
polyaniline (PAni) with several PU matrices [156-168]. Conducting polymers (PAni
filled) has considerable attention due to its high thermal stability, easy preparation,
cheap monomer, good conductivity and potential applications [168-173].
The liquid crystal linear PUs, which contain rigid aromatic mesogens in their
chains, as sensitizers of mechanical vibrations, as optical materials and as materials
for electronics have so far been completely met [174-176]. The elastomeric PU mixes
with increased contents of aromatic segments, by the use of for example. dianole-type
polyols are useful for the production of chemically resistant coatings and filling
compounds for electronic circuits [69].
1.6.2 Building or structural material
PUs finds an ever-increasing application in the building trades and
construction industry, reducing labour costs and need for skilled labour [27]. Work
progress can be speeded up, consumption of materials lowered, and building materials
of improved quality obtained ensuring thus buildings of better standard, with reduced
running costs. In the building industry, PUs are found in polymeric concrete
components, insulating materials, the cores of lightweight insulating board of
laminate type, floor carpeting and lining, coatings, sealants, and flexible moulds. The
construction industry is the largest consumer of the rigid PU foams which are
currently the best known insulating materials.
1.6.3 Packaging
PUs are used as packing in two forms: as foams and adhesives. The PU foams
have a better strength to-density ratio than other foams and thus are popular as
cushioning materials which protect products against mechanical damage in
21
commercial handling. The simplest way is to use flexible foam scrap or blocks to
secure the goods packed in rigid packing. The foam blocks used may be in the form of
inner boxes matching the product shape.
1.6.4 Modern materials for ceramics and electronics
The polymeric binders applicable in ceramics technologies seem to make an
interesting application of the future for water-dilutable PUs. The powder materials
like corundum (Al2O3) or zirconia (ZrO2) can be press moulded with the use of
environmentally friendly PU ionomers or PU–acrylate copolymers and then subjected
to preliminary machining as the so-called green ceramics. Such elements, e.g.,
corrosion resistant components of machines or chemical process equipment, are given
shapes and dimensions which are very close to the final requirements and thus the
final machining with the use of hard and expensive diamond tools can be minimized
[177-178]. Another outlet for PU dispersions is their use in the form of ceramic-
polymeric slurries for the production of thin films in the tape casting (doctor blade)
method. The films are then used in the manufacture of capacitors [179-180].
1.6.5 Biomedical materials
A few of the noteworthy applications may be mentioned in this section. The
exceptional combination of good physico-mechanical properties, high hydrolytic
stability and biological stability opens up wide application for PUs in contact with
body fluids (biomaterials) and makes it possible to use PUs as highly specialized
purposes [181-182]. They can be utilized to produce end prostheses, cardiac valves
and/or regenerative membranes for damaged internal organs, neither they do induce
any inflammatory condition of tissues, nor undergo any destruction by body fluids,
and no blood components are deposited on them [183-184].
The properly synthesized PU elastomers and PU-PDMS copolymers, owing to
their lowered free surface energy are capable of offering considerable resistance to
biodegradation. They are additionally known for their physiological inertness in
relation to living organisms, hence making them an interesting choice as materials for
medical implants [185-186]. The advantageous reduction of adhesion, which prevents
22
aggregation of blood cells, results from the level of phase separation and hydrophilic
performance of surfaces of such materials [183-184, 187].
For some of the biomedical applications such as vascular prostheses, artificial
skin, pericardial patches, soft-tissue adhesive, drug delivery devices, scaffolds for
tissue engineering, biocompatibility and biodegradability are a must [188-196].
Biodegradability of PUs is generally achieved by incorporating labile and
hydrolysable moieties into the polymer backbone [197-200]. The most common
method for fulfilling this goal is the application of polyols (soft segments) with
hydrolysable bonds as starting materials for the preparation of PUs. Several hydroxyl
terminated polymers such as polycaprolactone (PCL), polyalkylene adipate,
polylactides and polyglycolides were used for the synthesis of hydrolytically
degradable PUs [201-208].
By careful selection of the diisocyanate, chain extender and macrodiol
components, a broad range of physical properties can be achieved. In general, PUs are
biocompatible and have been used in a variety of biomedical applications, including
ligament and meniscus reconstruction [209-210], blood-contacting materials
[211-213], infusion pumps [214], heart valves [215], insulators for pacemaker leads
and nerve guidance channels [216]. For tissue engineering applications a degradable
polymer is desirable and can be achieved by incorporating labile ester linkages into
the polymer backbone [217-218]. Biodegradation to non-cytotoxic components may
be promoted by the use of lysine ethyl ester diisocyanate (LDI) [219] or
1,4-diisocyanato butane (BDI) [220] in place of methylene bisdiphenylisocyanate
(MDI), which has been suggested to degrade into carcinogenic and mutagenic
compounds. In the area of biomedical engineering, because of its excellent blood
compatibility and mechanical properties, segmented poly (urethane urea) (SPUU) has
been used in cardio vascular prosthesis [221-222], intra-aortic balloon pump [223],
pacemaker wire insulation [224], artificial heart valves [225], components of
haemodialysis units and diaphragms. However, adhesion of blood platelets on the film
surface of SPUU and the mechanism of its blood compatibility has not yet been fully
explored. This might be due to the insufficient physical and structural
characterizations of SPUU.
23
In medicine, PUs are used primarily in alloplasty (plastic surgery with the use
of tissues of non-human origin) and in endoprosthetics (organ replacement). Artificial
blood vessel, drains and urine catheters are made from PUs. PU elastomers show a
good compatibility to human skin. The good compatibility of ether-PU elastomers
with human blood and tissues allows catheters and tubes for blood to be made from
TPU. Even microprobes, biodegradable compliant and blood compatible vascular
prosthesis have been developed [226-230]. Their future applications could possibly
include space technology, as a cable sheathing material in telecommunications,
electronics and other burgeoning areas [231].
1.6.6 Dental applications of PUs
Possible applications of PUs in the field of dentistry include denture bases,
false teeth, and fillings. By coating teeth with abrasion-resistant PU coatings
containing over 0.7% fluoride, one can prevent dental caries. The live tooth tissue is
bonded by using urethane prepolymers [232].
1.6.7 Controlled drug delivery system
Whereas most of the applications described relate to mechanical behaviour,
PUs finds several other uses. Anionic and cationic ion-exchange resins have already
been mentioned. A new application involves controlled drug delivery. Conventional
oral dosage forms of water-soluble drugs consist of coated tablets. After dissolution
of the coating in the stomach, they disintegrate more or less rapidly. As a result, drug
concentrations in the blood quickly reach a sharp peak, followed by a decrease in rate
determined by the metabolic half-life in the body. For many purposes, a controlled
steady drug delivery is desirable, which makes use of insoluble hydrogel beads that
are loaded with the drug. The drug delivery depends on the hydrophilicity of the
polymer, the diameter of the bead and the diffusion rate. Use of coatings or shell
structures that form multilayered beads with different diffusion rates can control this
last parameter. Because the retardation of the delivery rate is generally desired, the
outer layer(s) should have a low permeability towards the drug. The other advantages
are: (a) prolonged drug release from the complex; (b) reduced toxicity by slowing
drug absorption; (c) protection of the drug from hydrolysis or other degradation;
(d) improved palatability, and (e) ease of formulation [233]. Ion-exchange resin beads
24
have also been micro encapsulated with acacia and gelatin [234]. Recently
Aminabhavi et al have studied the transdermal drug-delivery system (TDDS) for
castor-oil-based PUs [235].
1.6.8 Biodegradable materials
The use of vegetable feeds; starch [236-237], castor oil [56, 76, 237], soybean
oil [238], palm oil [239], natural rubber (NR) [240], lignin, wood flour, molasses,
cellulose, glucose, gargum [241] or saccharose makes it possible to obtain
components for linear PU plastics with improved biodegradability. That is
advantageous for recycling of some PU goods, e.g. foamed PU packages.
1.7 Scope of the present investigation
The multifunctional role of castor oil and its polymers have been well
documented in the literature in the last few years and have left many options to derive
useful products out of this agricultural by-product. The nature of castor oil has
prompted different researchers to react it with diisocyanate to produce numerous PU
materials on par with those produced from other polyols like PEG, HTPB, etc. A few
research studies have been cited in the previous sections of this chapter and in forth
coming chapters highlighting the utilization of polyol functionality of castor oil.
Literature is in vogue in utilizing the polyol functionality and long unsaturated
hydrocarbon by modifying it with various diisocyanates, chain extenders, vinyl
monomers, etc., polymerizing them to get all kinds of polymer products like
elastomer, thermoplastic and thermoset PUs. Also literature survey reveals some
studies on influence of chemistry of chain extenders on PUs performance and its
morphological behaviors. There are no significant and systematic studies reported on
dicarboxylic acid based chain extended PUs especially transport phenomena and
effect of starch filler on the performance. There is ample scope for extending this
functionality by reacting with dicarboxylic acid based chain extenders.
In the present study, CEPUs is chosen since it has many excellent
characteristics such as transparency, good weathering resistance, abrasion behaviour,
25
biocompatibility etc. The performance of PU can be varied by using different types of
chain extender and hence it widens the application range of PUs.
Chain extended PUs can be used to produce products, which needs high
impact strength, high abrasion resistance and biomaterials for many emerging
engineering applications. The demand for such materials is unfolding from
automobile to aerospace industries. Hence, preparation and characterization of a
series of chain extended PUs with different dicarboxylic acid based chain extenders
have been investigated. Similarly, there is also scope to produce composites of PUs
for a variety of applications like, ecofriendly, structural, antistatic, biomedical
coating, packaging and automobile etc.
1.8 Background and motivation of the investigations
The chemistry and technology of CEPUs has been the subject of intense
research in both academic and industrial aspects. PUs are a key component of current
polymer research and technology to generate new series of materials. PUs are
potentially inexpensive route to prepare new products by using naturally occurring
macrodiols to develop new base polymers. Chain extended PUs or PUs are most
commonly/extensively used polymers because of its unique chemistry. There is an
ample scope to continue research on modifications of PUs to meet the demand of
newer applications in many sectors.
1.9 Objectives of this study
The present investigation intends to study the effect of chemistry of
dicarboxylic acid chain extenders on the performance of PUs.
1.10 Present research problem
The main goal of the present research work is to achieve a comprehensive
study and understanding of the synthesis and characterization of castor oil based chain
extended PUs with different diisocyanates (toluene diisocyanate (TDI) and
hexamethylene diisocyanate (HDI)) and with different chain extenders (phthalic acid
(PA), citric acid (CA), glutaric acid (GA), itaconic acid (IA), tartaric acid (TA) and
maleic acid (MA)). Apart from this starch, zeolite and short silk fibre incorporated
26
PU composite systems have been synthesized and characterized. The molecular
transport of CEPU membranes has been studied with different aromatic penetrants at
different temperatures in order to understand the swelling, sorption and diffusion
behaviours.
The prepared chain extended PUs were characterized by spectroscopic,
physico-mechanical properties, chemical resistivity, optical properties, thermal
properties (thermogravimetric analysis (TGA), differential scanning calorimetry
(DSC) and dynamic mechanical analysis (DMA)). The morphological behaviours and
microstructural parameters of PUs were determined using scanning electron
microscopy (SEM) and wide angle X-ray spectrometer (WAXS).
This research investigation covers the preparation of a new series of
transparent and toughened chain extended PUs and the evaluation of their structure-
property relationship. The results are expected to be beneficial to tailor made
applications as sought by material and polymer technologists. The results have been
systematically presented and analyzed. Since experimental methods are very
important for understanding the results, their description has been included. The
details of the experimental works carried out are presented in Chapter 2. With the
understanding of the experimental equipments, procedures and methods of
measurements (characterizations) on CEPUs, the experimental data are presented in
the forthcoming chapters.
27
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