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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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