THERMOPLASTIC ELASTOMERS AND THEIR APPLICATIONS

231

THERMOPLASTIC ELASTOMERS AND THEIR APPLICATIONS GEOFFREY HOLDEN Holden Polymer Consulting, Incorporated, 1042 Willow Creek Road, A 1 1 1 - 2 7 3 , Prescott, AZ 86305. (520)-771-9938, Fax (520)-771-8389, [email protected] Introduction Classification and Structure Production Structure / Property Relationships Applications Economic Aspects and Tradenames Literature Cited

Introduction The use of thermoplastic elastomers has significantly increased since they were first produced about thirty-five years ago. A recent article(1) estimates their worldwide annual consumption at about 1,000,000 metric tons/year in 1995 and this is expected to rise to about 1,400,000 metric tons/year in 2000. Several books(2-4) and articles (5,6) have covered this subject in detail. The first two books (2,3)concentrate mostly on the scientific aspects of these polymers while the other(4) concentrates on their end uses. The properties of thermoplastic elastomers in relation to other polymers are summarized in Table I. This table classifies all polymers by two characteristics - how they are processed (as thermosets or as thermoplastics) and the physical properties (rigid, flexible or rubbery) of the final product. All commercial polymers used for molding, extrusion, etc., fit into one of the six resulting classifications - the thermoplastic elastomers are the newest. Their outstanding advantage can be summarized in a single phrase - they allow rubberlike articles to be produced using the rapid processing techniques developed by the thermoplastics industry. They have many physical properties of rubbers, e.g., softness, flexibility, and resilience. However they achieve their properties by a physical process (solidification) compared to the chemical process (cross-linking) in vulcanized rubbers. In the terminology of the plastics industry, vulcanization is a thermosetting process. Like other thermosetting processes, it is slow, irreversible and takes place upon heating. With thermoplastic elastomers, on the other hand, the transition from a processable melt to a solid, rubberlike object is rapid, reversible and takes place upon cooling (Figure 1).

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Table I. Comparison of thermoplastic elastomers with conventional plastics and rubbers

Thermosettino

Thermoplastic

Rigid

Epoxies Phenol-Formaldehyde Urea-Formaldehyde

Polystyrene Polypropylene Poly(vinyl chloride) High Density Polyethylene

Flexible

Highly filled and/or highly vulcanized rubbers

Low Density Polyethylene EVA Plasticized PVC

Rubbery

Vulcanized Rubbers (NR, SBR, IR etc.)

Thermoplastic Elastomers

Thus thermoplastic elastomers can be processed using conventional plastics techniques, such as injection molding and extrusion; scrap can be recycled. Additionally, some thermoplastic elastomers are soluble in common solvents and so can be processed as solutions. F i g u r e ! Polymer transitions

Thermoplastic Elastomers Heat Strong Elastic Solids

Weak Processable Fluids

^ Cool

Conventional Vulcanizates Heat, Time Weak Processable Fluids

->

Strong Elastic Solids

At higher temperatures, the properties of thermoplastic elastomers are usually not as good as those of the conventional vulcanized rubbers. Applications of thermoplastic elastomers are, therefore, in areas where these properties are less important, e.g., footwear, wire insulation, adhesives, polymer blending, and not in areas such as automobile tires. Classification and structure Thermoplastic elastomers can be divided into four basic types:

233

Thermoplastic Elastomers and their Applications 1. 2. 3. 4.

Styrenic Thermoplastic Elastomers Multiblock Copolymers Hard Polymer / Elastomer Combinations Graft Copolymers

Almost all thermoplastic elastomers contain two or more distinct polymeric phases and their properties depend on these phases being finely and intimately mixed. In some cases the phases are not chemically bonded but in others they are linked together by block or graft copolymerization (Table II). Table 11. Thermoplastic elastomers based on block copolymers

Refs.

Hard Seoment. A

Soft or Elastomeric Seoment. B Structure^^

Polystyrene Polystyrene

Polybutadiene and polyisoprene Poly(ethylene-co-butylene) and Poly(ethylene-co-propylene) Polyisobutylene

T,B T

7-9,11

T, B

10

Polybutadiene, polyisoprene Poly(propylene sulfide) Polydimethylsiloxane Polydimethylsiloxane Polydimethylsiloxane Polydimethylsiloxane Polyester and Polyether Polyether Poly(p-hydroxyalkanoates) Polyester and Polyether Polydimethylsiloxane Polyether Polydimethylsiloxane Poly(alkyl acrylates) Poly(diacetylenes) Poly(a-olefins) Poly{ethylene-co-butylene) and Poly{ethylene-co-propylene) Poly(a-olefins) Polypropylene(atactic)

T T

11 11 12

Polystyrene and Substituted Polystyrenes Poly(a-methylstyrene) Poly(a-methylstyrene) Polystyrene Poly(a-methylstyrene) Polysulfone Poly(silphenylene siloxane) Polyurethane Polyester Poly(P-hydroxyalkanoates) Polyamide Polycarbonate Polycarbonate Polyetherimide Polymethyl methacrylate Polyurethane Polyethylene Polyethylene Polypropylene(isotactic) Polypropylene(isotactic)

T, M

T M M M M M M M M M T. B

M M T

8.9

11,12

13 14 15-17 18-19

20 21,22 23-25 26,27

28 29 30 31,32 11,31

M*

31

M*

31,32

a. T = triblock, A-B-A, B = Branched, (A-B)nX, M = Multiblock, A-B-A-B-AM* = Mixed Structures, including multiblock,

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At least one elastomeric phase and one hard phase must be present, and the hard phase (or phases) must become soft and fluid at higher temperatures so that the material as a whole can flow as a thermoplastic. Styrenic thermoplastic elastomers. These are based on simple molecules such as an A-BA block copolymer, where A is a polystyrene and B an elastomer segment. If the elastomer is the main constituent, the polymers should have a morphology similar to that shown in Figure 2. Here, the polystyrene end segments form separate spherical regions, i.e., domains, dispersed in a continuous elastomer phase. Most of the polymer molecules have their polystyrene end segments in different domains. At room temperature, these polystyrene domains are hard and act as physical cross-links, tying the elastomeric midsegments together In a three-dimensional network. In some ways, this is similar to the network formed by vulcanizing conventional rubbers using sulfur cross-links. The difference is that in themioplastic elastomers, the domains lose their strength when the material is heated or dissolved in solvents. This allows the polymer or its solution to flow. When the material is cooled down or the solvent is evaporated, the domains harden and the network regains its original integrity. Figure 2. Morphology of styrenic block copolymers

Polystyrene Domain Elastomer Mid Segment

0.1 pm

Analogous block copolymers with only one hard segment (e.g., A-B or B-A-B) have quite different properties. The elastomer phase cannot form a continuous interiinked

235

Thermoplastic Elastomers and their Applications

network since only one end of each elastomer segment is attached to the hard domains. These polymers are not thermoplastic elastomers, but are weaker materials similar to unvulcanized synthetic rubbers(7). In commercial applications, three elastomeric mid-segments have been used for many years - polybutadiene, polyisoprene and poly(ethylene-butylene). The corresponding block copolymers are referred to as S-B-S, S-l-S and S-EB-S. Later, polymers with poly(ethylene-propylene) mid-segments (S-EP-S) were introduced. A more recent development, although not yet commercialized, is styrenic block copolymers with an isobutylene mid-segment (S-iB-S) (10). These can also be produced with substituted polystyrene end segments Multiblock copolymers. The multiblock copolymers have structures that can be written as A-B-A-B-A-B-A-B-.... or (A-B) . For those of commercial importance, the hard (A) segments are crystalline thermoplastics while the softer, elastomeric (B) segments are amorphous. In best known types, the hard segments are thermoplastic polyurethanes, thermoplastic polyesters or thermoplastic polyamides and the soft segments are either polyesters or polyethers. Similar materials have been recently introduced in which the hard segments are polyethylene and the soft segments are either homopolymers or copolymers ofa-olefinssuchas 1-butene, 1-hexeneand 1-octene. The morphology of these (A-B)^ multiblock copolymers is shown diagrammatically in Figure 3. Figure 3. Morphology of multiblock copolymers with crystalline hard segments

Soft B Blocks

Hard A Blocks Crystalline Regions Amorphous Regions

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G. Holden

This structure has some similarities to that of a poly(styrene-b-elastomer-b-styrene) equivalents (Figure 2) and also some important differences. First, the hard domains are much more interconnected; Secondly, they are crystalline. Thirdly each long (A-B) molecule may run through several hard and soft regions. Hard polymer / elastomer combinations. Some thermoplastic elastomers are not block copolymers, but instead are fine dispersions of a hard thermoplastic polymer and an elastomer. Some are simple blends while others are produced by dynamic vulcanization (see later). A list of the various polymers used to produce thermoplastic elastomers based hard polymer / elastomer combinations of all types is given in Table III. Table III. Thermoplastic elastomers based on hard polymer / elastomer combinations.

Hard Polymer

Soft or Elastomeric Polymer

Structure^^

Refs.

Polypropylene Polypropylene Polypropylene Polypropylene Polypropylene Polypropylene Nylon Polypropylene

EPR or EPDM EPDM Poly(propylene / 1-hexene) Poly(ethylene / vinyl acetate) Butyl Rubber Natural Rubber Nitrile Rubber Nitrile Rubber Nitrile Rubber + DOP" Ethylene Interpolymer EPDM S-B-S + Oil S-EB-S + Oil

B DV B B DV DV DV DV

31-34 33,35.36

PVC Halogenated Polyolefin Polyester Polystyrene Polypropylene

B, DV

B B, DV

B B

33 33 35,37 35,38

35 35,36 39-41 41,42

33 43 43

a. B = Simple Blend, DV = Dynamic Vulcanizate b. DOP = DIoctyl phthalate. Other plasticizers can also be used.

The two materials usually form interdispersed co-continuous phases with a final morphology similar to that shown in Figure 4. Polypropylene is often chosen as the hard thermoplastic because it Is low priced, solvent resistant and has a high crystal melting point (165^C). Combinations with ethylene-propylene-diene monomer (EPDM) or ethylene-propylene copolymer (EPR) are the most important commercial products based on polypropylene(31 -36); other elastomers that can be used include nitrile(35,36), butyl(37) and natural(38) mbbers. Softer, more impact resistant materials can be produced by using propylene copolymers as the hard phase(31,33). Halogen-containing polyolefins (41) are another option. Two examples are blends of PVC with nitrile rubber(39-41) and blends of halogenated polyolefins with ethylene inter-polymers. Mixtures of the last two polymers are claimed to give a single phase system.

Thermoplastic Elastomers and their Applications

237

In these blends dispersion of the two phases is most often achieved by intensive mechanical mixing but in the polypropylene / EPR combinations, polymerizing the finely dispensed elastomer phase simultaneously with the hard polypropylene is possible(31 -33). Figure 4. Morphology of simple blends based on hard polymer / elastomer combinations.

Elastomer

Hard Polymer

Sometimes, the elastomer phase is deliberately cross-linked during the intensive mechanical mixing. This is described as "dynamic vulcanization"(35,36). It produces a finely dispersed, discontinuous, cross-linked elastomer phase (see Figure 5). The products are called dynamic vulcanizates or thermoplastic vulcanizates. This process is more complex than simple mixing, but the products have two important advantages. First, the cross-linked elastomer phase is insoluble and so oil and solvent resistance is improved. Secondly, cross-linking reduces or eliminates the flow of this phase at high temperatures and/or under stress. This improves resistance to compression set.

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Figure 5. Morphology of dynamic vulcanizates

Elastomer

.Hard Polymer

I

1 tOjum

Graft copolymers. Thermoplastic elastomers have also been produced from graft copolymers. A list of the various polymers used to produce thermoplastic elastomers based graft copolymers of all types is given in Table IV. Graft copolymers may be represented as B-B-B-B-B-B

I (A)„

B

Thermoplastic Elastomers and their Applications

239

This represents a polymer where each elastomeric B chain has (on average) n random grafts of hard A blocks. B chains that do not have at least two A blocks grafted onto them will not be elastically effective, because they cannot form a continuous interlinked network similar to that shown in Figure 2. To ensure that almost all the B chains have at least two A blocks grafted onto them, n should be greater than two, perhaps as high as ten (10). Although much effort has been expended in research on thermoplastic elastomers based on graft copolymers, they have not become commercially Important. Table IV. Thermoplastic elastomers based on graft copolymers'^

Refs.

Hard Pendant Seoment. A

Soft or Elastomeric Backbone Seament. B

Polypivalolactone Polystyrene & Poly (a-methyl styrene) Polyindene Polystyrene & Poly (a-methyl styrene) Polyindene Polystyrene & Poly (a-methyl styrene) Polyindene Polyacenapthylene Poly (para chlorostyrene) Polystyrene Poly(a-methyl styrene) Polystyrene Polymethylmethacrylate

31 Poly(ethylene-co-propylene) 44,45 Polybutadiene, Polybutadiene, 46 47-49 Poly(ethylene-co-propylene) Poly(ethylene-co-propylene) 46 50-52 Polyisobutylene 46.51 Polyisobutylene Polyisobutylene 51 Polyisobutylene 53 Chlorosulfonated polyethylene 54 Polychloroprene 52 Poly(butyl or ethyl-co-butyl) acrylates 55 Polybutylacrylate 56

a) For more detailed information, see Chapters 5, 13 and 14 of Reference 3

Production As noted above, many copolymers and polymer combinations can give thermoplastic elastomers. This section covers the production of only the most significant. Stvrenic thermoplastic elastomers. The block copolymers on which these materials are based are made by anionic polymerization(57-60). In principle, this is a very simple system in which the polymer segments are produced sequentially from the monomers. The first step in the polymerization is the reaction of an alkyl-lithium initiator (R'Li"*") with styrene monomer: R Li.+ + nS

R-(S)n" Li"^

For simplicity, we denote the product as S'Li"*". It is called a "living polymer" because it can initiate further polymerization. If a second monomer, such as butadiene, is added:

240

G. Holden S"Li'*' + mB -^

S-(B)^"Li"^

W e denote this product as S-B 'Li'*'. It is also a "living polymer" and by repeating these steps, block copolymers with multiple alternating blocks (S-B-S-B-S....) can be produced. In practice there are no apparent advantages in going beyond triblocks (i.e.. SB-S). Another variation is to use a coupling reaction to make linear or branched structures such as (S-B)^x (where x represents an n-functional junction point). A typical example is: 2 S-B" Li"*" + X-R-X

-^

S-B-R-B-S + 2LIX

Many coupling agents have been described, including esters, organo-haiogens and silicon halides(59,63). The example above shows the reaction of a difunctional coupling agent, but those of higher functionality (for example SICI^) can also be used. These give branc/hed or star-shaped molecules such as (S-B) x. The third method of producing these block copolymers uses multifunctional initiation(60,62,63). In this method a multifunctional initiator ('*' Li "R" Li '*') is first reacted with the diene (in this case, butadiene). 2nB + * Li 'R' Li *

->

* Li '(B)n-R-(B)n- Li*

W e denote this product as * Li 'B' Li'*' . The final two steps are similar to the con'esponding steps in the sequential polymerization described above. When the reaction to produce the '*' Li 'B" Li"*" is completed, styrene monomer is added and it in turn initiates its polymerization onto the "living" chain ends to give * Li 'S-B-S'Li'*'. A protonating species is then added to stop the reaction and give final product, S-B-S. This example shows the use of a difunctional initiator. There is no reason in principle why initiators of higher functionality could not be used but none appears to have been reported in the literature. All these reactions take place only in the absence of terminating agents such as water, oxygen or CO2; thus they are usually candied out under nitrogen and in an inert hydrocarbon solvent. These conditions produce polymers with narrow molecular weight distributions and precise molecular weights. Only three common monomers - styrene, butadiene and isoprene - are easily polymerized anionically and so only S-B-S and S-l-S block copolymers are directly produced on a commercial scale. In both cases polymerization of the elastomer segments in a non-polar solvent predominantly gives the 1,4 polymeric structures: -fCH2-CH=CH-CH25 ^-

-(CH2-C=CH-CH23 pCH3

Polybutadiene

Polvisoprene

Both these polymers contain one double bond per molecule of the original monomer. These bonds are quite reactive and limit the stability of the product. More stable

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241

analogues can be produced from S-B-S polymers in which the polybutadiene midsegments are polymerized in relatively polar solvents. These conditions produce a random copolymer of the 1,4 and 1,2 isomers. After hydrogenation this gives a saturated elastomer that can be considered a copolymer of ethylene and butylene (EB). 1 ,4

1,2 I

--CH2"CH—CH-CH2"CH2"CH--

I I I I

E H2 ^

B I

--CH2"CH2"Cn2"CH2"CH2"CH--

I

I CH II CH2

Polvbutadiene

I I I

I C2H5

Poly(ethvlene-butvlene)

S-EP-S block copolymers can be produced by hydrogenating S-l-S precursors. Similar block copolymers with polyisobutylene mid-segments (e. g., S-iB-S) are made by carbocationic polymerization(10,66). This is a more complex system than the anionic system described above. The initiators have functionalities of two or more. They have the general formula (X-R) x (where X-R represents a hydrocarbon moiety with a functional group X and x represents an n-functional junction point). X can be a chlorine, hydroxyl or methoxy group. Polymerization is carried out at low temperatures (about 80°C) in a moderately polar solvent and in the presence of a co-initiator (TiC^ or BCI3). As in anionic polymerization, the polymer segments are produced sequentially from the monomers. Thus, an S-iB-S block copolymer would be produced in two stages: X-R-X + 2n(iB) - >

"*" (iB)n-R-(iB)n"

The product, which we can denote as "*" iB" is a difunctional living polymer. It can initiate further polymerization, so if a second monomer, such as styrene, is added. "iB* +2m(S) - > *(S)^.iB-(S)^" After termination, this gives the block copolymer S-iB-S. Polyisobutylene is the only elastomeric mid-segment than can be produced by carbocationic polymerization. There are many aromatic polymers (mostly substituted polystyrenes) that can form the endsegments(IO). Multiblock cpolvmers. The thermoplastic elastomers based on polyurethanes, polyesters and polyamides are produced from pre-polymers by condensation reactions. For those based on polyurethanes, three starting materials are used 1. 2. 3.

A long chain diol, also called a polyglycol ( H O - R L - O H ) A short chain diol, also called a chain extender (HO-Rs-OH) A diisocyanate (OCN-R*-NCO) that can react with the hydroxyl groups In the diols to give a polyurethane.

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G. Holden

The basic reaction can be written: / \ ^ N C O + HO-Ay

-^

/\^NHCOO^V

In the first stage of polymerization, an excess of the diisocyanate is reacted with the long chain diol. This gives a prepolymer terminated with the reactive isocyanate groups, OCN-R*-NCO +

HO-RL-OH

-^

OCN-(R*-y.RL-U)n-R*-NCO Prepolymer

where U represents the urethane linking group, -NHCOO-. We can denote the prepolymer as OCN-Prepoly-NCO. It will further react with the short chain diol and more diisocyanate: OCN-Prepoly-NCO + HO-Rs-OH + OCN-R*.NCO -^ OCN.[Prepoly-y-(R*-y-Rs-y)n-]^.R*-NCO The final product is an altemating block copolymer with two types of segments: 1.

Those formed in the first stage. These are based on the prepolymer. They are alternating copolymers of the long chain diols and the diisocyanate.

2.

Those formed in the second stage. These are altemating copolymers of the short chain diols and the diisocyanate.

The long chain diols have a broad molecular weight distribution. Thus the prepolymers formed from them and the diisocyanate monomers do not have a regular repeating structure and are amorphous. Typical glass transition temperatures of the long chain diols are in the range of -45°C to -100°C (16), so at room temperatures these prepolymers are elastomeric. They form the soft elastomeric phase In thefinalpolymer. In contrast, the short chain diols are single molecular species (e. g., 1,4-butanediol or ethylene glycol). Thus the copolymers formed from them and the diisocyanate or diacid monomers do have a regular repeating structure and so are crystalline. Typical crystallization temperatures of these segments are above 150°C (16), and so at room temperatures they are hard. They form the hard phase in thefinalpolymer. MDI (Diphenylmethane 4,4'-diisocyanate) and TDI (2,4 Toluene-diisocyanate) are the most common diisocyanates used to produce polyurethane thermoplastic elastomers. The long chain diols are usually polyesters (e. g., poly(ethylene adipate) glycol) or polyethers (e. g., poly(oxytetramethylene) glycol). Polycaprolactone glycol is used in premium products. The various possible combinations of all three starting materials (diisocyanates, long chain diols and short chain diols) give a very wide variety of commercial thermoplastic

Thermoplastic Elastomers and their Applications

243

polyurethanes (16). In contrast, although thermoplastic polyesters are produced in a similar way (with diacids or diesters replacing diisocyanates) only three starting materials are used commercially(18). These are: Poly(oxytetramethylene) glycol (the long chain diol) 1,4-butanediol (the short chain diol) Terephthalic acid (the diacid) or its methyl diester There are two ways to produce polyamide thermoplastic elastomers (21). The first is based on the reaction of a carboxylic acid with an isocyanate to give an amide: / \ ^ N C O + HOOC-^V

"^

/\^NHOOC-/\/

+ COj

The reaction scheme is similar to that shown above for the production of thermoplastic polyurethane and polyester elastomers. Again, the product is a copolymer with alternating segments. In the second method of producing polyamide thermoplastic elastomers, a polyamide terminated by carboxylic acid groups, HOOC-PA-COOH (or the corresponding ester, ROOC-PA-COOR) reacts with a long chain diol: HOOC-PA-COOH + H O - R L - O H

"^

HO-(RL-E-PA-E)n-RL-OH + 2nH20

ROOC-PA-COOR + H O - R L - O H

-^

HO-(RL-E-PA-E)n-RL-OH + 2nR0H

where E represents an ester link. Essentially, this amounts to preparing an alternating block copolymer from two prepolymers, one (the polyamide) crystalline, the other (the long chain diol) amorphous. The block copolymers of ethylene with a-olefins are produced using metallocene catalysts (31-33). The a-olefins are typically 1-butene, 1-hexene or 1-octene. These copolymerize with ethylene to give segments with pendant groups, usually arranged atactically. Because of their random and atactic structures, these segments cannot crystallize. Instead, they are amorphous materials with low glass transition temperatures and so are soft and rubberlike at room temperature. They form the soft phase. The remainder of the polymer is polyethylene. Except for a very few side groups, this has a linear, symmetrical structure and therefore does not exhibit tacticity. Thus, the long polyethylene segments in the polymer chain cannot have significant irregularities and so can crystallize. They form the hard phase. In all these multlblock (A-B) polymers, both the number of segments and their individual molecular weights have a very broad distribution, in contrast to the simple A-B-A triblocks in the styrenic thermoplastic elastomers.

244

G. Holder)

Hard polymer / elastomer combinations. There are two types of these materials - simple blends of the hard polymer and the elastomer and the dynamically vulcanized products in which the elastomer is cross-linked during the mixing process. Both the hard polymers and the elastomers used to make these products can be obtained "off the shelf. Thus an almost unlimited range of combinations can be investigated quickly and easily. Similarly, commercial products can be made without the very high capital investment required to produce novel polymers. To produce simple unvulcanized blends, the two polymers are mixed on high shear compounding equipment. For the dynamically vulcanized versions, vulcanizing agents must be added and the temperature controlled so as to cross link the rubber particles during mixing. In both cases, only fine dispersions will produce optimum properties. A good match of the viscosities of the two polymers will aid the production of a fine dispersion, as will a match in solubility parameters. If the two polymers have very different solubility parameters (e.g., one is polar while the other is not), a coarse dispersion with poor adhesion between the phases can result. This can often be avoided by using block or graft copolymers as compatibilizing agents(33,35). Graft Copolymers. Graft Copolymers are typically produced from elastomers with active sites (e. g., EPDM or halobutyl rubbers). These sites can be coupled to small blocks of the hard phase polymer or used to initiate further polymerization of the monomer that will form the hard phase polymer. Structure / property relationships With such a variety of materials, it is to be expected that the properties of thermoplastic elastomers cover an exceptionally wide range. Some are very soft and rubbery where others are hard and tough, and in fact approach the ill-defined interface between elastomers and flexible thermoplastics. Since most thermoplastic elastomers are phase separated systems, they show many of the characteristics of the individual polymers that constitute the phases. For example, each phase has its own glass transition temperature (T ), (or crystal melting point (T^), if it is crystalline). These, in turn, detemiine the temperatures at which a particular thermoplastic elastomer goes through transitions in its physical properties. Thus, when the modulus of a themioplastic elastomer is measured over a range of temperatures, there are three distinct regions (see Figure 6). At very low temperatures, both phases are hard and so the material is stiff and brittle. At a somewhat higher temperature the elastomer phase becomes soft and the thermoplastic elastomer now resembles a conventional vulcanizate. As the temperature is further increased, the modulus stays relatively constant (a region often described as the "rubbery plateau") until finally the hard phase softens. At this point, the thermoplastic elastomer becomes fluid.

Thermoplastic Elastomers and their Applications

245

Figure 6. Stiffness of typical thermoplastic elastomers at various temperatures

10= Hard Elastomers

Flexural Modulus, psi

10^

Service Temperature Range

Weak Fluid Materials

10^ Soft Elastomers

10-^ Stiff Brittle Materials

t Tgof Soft Rubbery Phase

Temperature

f Tg orTm of Hard Phase

Thus, thermoplastic elastomers have two service temperatures. The lower service temperature depends on the Tg of the elastomer phase while the upper service temperature depends on the Tg or T^, of the hard phase. Values of Tg and T^ for the various phases in some commercially important thermoplastic elastomers are given in Table V.

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G. Holden

Table V - Glass Transition and Crystal Melting Temperatures ^^ Soft. Rubbery Phase Tg (^C)

Hard Phase Ig-OrTm C^)

S-B-S

-90

95(Tg)

S-l-S

-60

95(Tg)

S-EB-S

-60

95(Tg)&165(TJ b)

S-iB-S

-60

95 - 240(T )^^

-40 to-60^)

Thermoplastic Elastomer Type Polystyrene / Elastomer Block Copolymers

Multi-block copolymers Polyurethane / Elastomer Block Copolymers Polyester / Elastomer Block Copolymers

-40

Polyamide/Elastomer Block Copolymers

-40 to-60^)

190(T^) 185to220(T^) 220 to 275(T^)

-50

70(T^)^>

Polypropylene/EPDM or EPR combinations

-50

Polypropylene/Nitrile Rubber combinations

-40

165(T^) 165(T^)

PVC/Nitrile Rubber/DOP combinations

-30

80(TJ

Polyethylene / Poly(a-olefin) Block Copolymers Hard Polymer / Elastomer Combinations

a. b. c. d. e.

Measured by Differential Scanning Calorimetry In compounds containing polypropylene The higher values are for substituted polystyrenes and polyaromatics (see Table 3.3) The values are for polyethers and polyesters respectively This low value for Tg is the result of the short length of the polyethylene segments.

As noted above, many different polymers are used to make the hard and soft phases In all these types of thermoplastic elastomers. Their influence on some properties of the products can be summarized as follows: Hard Phase The choice of polymer in the hard phase strongly influences the oil and solvent resistance of the thermoplastic elastomers. Even if the elastomer phase is resistant to a particular oil or solvent, if this oil or solvent swells the hard phase, all the useful physical properties of the thermoplastic elastomer will be lost. In most commercial thermoplastic elastomers, this hard phase is crystalline and so resistant to oils and solvents. Styrenic thermoplastic elastomers are an exception. As pure polymers, they have poor oil and solvent resistance (although this can be improved by compounding - see later). However, this gives them the advantage that they can be applied from solution.

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247

Soft Elastomer Phase In the styrenic thermoplastic elastomers, analogous S-B-S, S-l-S and S-EB-S polymers have somewhat different properties. S-B-S polymers are lowest in cost, S-l-S equivalents are the softest while the S-EB-S polymers are the most stable but also the highest priced. In the thermoplastic elastomers with crystalline hard segments, those with polyester-based elastomer segments are tougher and have better resistance to oils and solvents. The polyether-based materials are more flexible at low temperatures and show better hydrolytic stability. In the hard polymer/ elastomer combinations, resistance to oil and solvents and to compression set are dramatically improved if the elastomer phase is dynamically vulcanized. Oil and solvent resistance can be still further improved if the elastomer is a polar material such as nitrile rubber. Hard/Soft Phase Ratio The hardness of these materials depends on the ratio of the volume of the hard phase to that of the softer elastomer phase. In the styrenic thermoplastic elastomers, this ratio can be varied within quite wide limits. Thus, in an S-B-S block copolymer, as the ratio of the S to B segments is Increased, the phase morphology changes from a dispersion of spheres of S In a continuous phase of B to a dispersion of rods of S in a continuous phase of B and then to a lamellar or "sandwich" structure in which both S and B are continuous. If the proportion of S is increased still further, the effect is reversed in that S now becomes disperse and B continuous. As the polystyrene phase predominates, the block copolymer gets harder and stiffer until eventually it becomes a clear flexible thermoplastic, such as K-Resin(Phillips) and Versaclear (Shell). In hard polymer / elastomer combinations, there are limits on the proportions of the elastomer phase in both the simple blends and in the thermoplastic vulcanizates. In the simple blends, if too much elastomer is used, the morphology changes from an interdispersed structure (in which both phases are continuous) to a dispersion of the hard polymer in the elastomer. Since the elastomer is not vulcanized, it has little strength. Thus when it becomes the only continuous phase, the properties of the blend are unsatisfactory. In the thermoplastic vulcanizates, the dispersed elastomer phase is cross-linked and so cannot flow. It can thus be considered as an elastomeric filler, and when too much is present, processability suffers. Because of these limits on the amount of the elastomer phase, producing very soft products from hard polymer / elastomer combinations is difficult. The multiblock polymers with crystalline hard segments also have limits on softness. The hard phase segments must have high enough molecular weights so that they can crystallize. Softer versions of these polymers require the molecular weights of the elastomer segments to be higher still, so as to increase the soft/hard phase ratio. Thus for very soft products, the total molecular weight of the block polymer is increased to the point where processing can be difficult. Applications Applications of thermoplastic elastomers of all types have been extensively described (4,43). Some highlights are: Styrenic Thermoplastic Elastomers. Only the anionically polymerized versions of these block copolymers (i.e., S-B-S, S-l-S, S-EB-S and S-EP-S) are produced commercially. Thus all the information in this section is based on experience with these materials. If

248

G. Holden

S-iB-S and similar block copolymers are ever produced commercially, they should have similar applications. Styrenic block copolymers differ from the other thermoplastic elastomers In at least two significant ways. First, both the hard and soft phases are amorphous, and thus the pure polymers are soluble in common solvents such as toluene. Secondly, in their various end uses, these polymers are always compounded with large amounts of ingredients such as other polymers, oils, resins and fillers. In the majority of their applications, the styrenic block copolymer comprises less than 50% of the final product Compounding significantly changes many properties - for example, solubility. Thus although the pure styrenic themrioplastic elastomers are completely soluble in solvents such as toluene, compounded products containing insoluble polymers (e.g., polypropylene) are not. The properties of compounded products produced from styrenic thermoplastic elastomers cover an exceptionally wide range and so their applications are more varied than those of the other thermoplastic elastomers. For injection molding, extrusion, etc. (i. e. processing on conventional thermoplastics equipment) end users prefer to buy pre-compounded products, and grades have been developed for the various specialized end uses. Products based on S-B-S are typically compounded with polystyrene, hydrocarbon oils and fillers. In those based on S-EB-S, polypropylene often replaces polystyrene. This polymer gives better solvent resistance and increases the upper service temperature. Typical applications include footwear, wire and cable insulation, automotive and pharmaceutical items. Processing of these compounded products is simple. Usually, compounds based on S-B-S block copolymers are processed under conditions suitable for polystyrene while those based on S-EB-S block copolymers are processed under conditions suitable for polypropylene. Another major application of styrenic thermoplastic rubbers is in adhesives, sealants and coatings. Tackifying and reinforcing resins are used to achieve a desirable balance of properties. Oils and fillers can also be added. These adhesives and sealants can be applied either from solvents or as hot melts. One very important application is in pressure sensitive hot melt adhesives. S-l-S block copolymers are softer and stickier and so they are often used to formulate adhesives of this type - in fact it is probably their largest single end-use. A final application is in blends with thermoplastics or other polymeric materials. Styrenic block copolymers are technologically compatible with a surprisingly wide range of other polymers. Blends with many other thennoplastics have improved impact resistance. These block copolymers can also be used as compatibilizers - that is, they can produce useful products from blends of thermoplastics that othenA^ise have poor properties(65). Multiblock Polvmers with Crystalline Hard Segments. The very tough and relatively hard materials based on polyurethane, polyester or polyamide hard segments are generally regarded as premium products(16-19,21,22). Articles made from them are produced by the typical techniques used to process thermoplastics (e. g., injection molding, blow molding, extrusion). Because of their crystalline hard segments and polar elastomer segments, they have excellent oil resistance. Thus they are used in demanding

Thermoplastic Elastomers and their Applications

249

applications as blow molded boots for automobile steering gear assemblies, grease seals, drive belts and hydraulic hose. They can also be blended with polar polymers such as PVC or used as the hard phase in hard polymer / elastomer combinations (33). The polymers with polyethylene hard segments are lower in cost. Their suggested applications include wire and cable insulation, PVC replacement (66) and blends with polypropylene, either to improve impact resistance or as the soft phase in a hard polymer / elastomer combination Hard Polymer / Elastomer Combinations. Polypropylene / EPDM or EPR combinations are the most important (31,33) and are used to make products such as injection molded bumpers for automobiles, where a combination of toughness, low temperature flexibility and low cost makes them very attractive. However, their use is limited because they can only be used to produce fairly hard products (typically above 60 Shore A hardness). Almost all applications for the polypropylene / EPDM thermoplastic vulcanizates(35,36) are as replacements for vulcanized rubber. They are used in automotive and appliance parts and also in the construction industry for window seals, etc.. Generally, they have better compression set and can give softer products (as low as 35 Shore A hardness). Similar thermoplastic vulcanizates based on polypropylene and nitrile rubber blends have improved solvent resistance. Blends based on halogen containing polymers are also significant (41). Those based on halogenated polyolefin / ethylene interpolymer blends are claimed to be single phase systems (41,42). They are often used where solvent and fire resistance is important. PVC / nitrile rubber / Dioctyl phthalate blends are used in similar applications and in footwear (39-41). Finally, the S-B-S / polystyrene / oil and S-EB-S / polypropylene / oil compounds described above can also be considered as blends of a hard polymer (polystyrene or polypropylene) with a soft elastomer phase (S-B-S / oil or S-EB-S / oil). Economic aspects and trade names Worldwide, about 1,000,000 metric tons of thermoplastic rubbers of all types were estimated to be used in 1995(1), with a value of about $3 billion. Consumption should increase to at least 1,400,000 metric tons by 2000. North America consumed about 43% of this amount. Western Europe about 36% and Japan accounted for most of the rest. The styrenic block copolymers represent about 50% of the total market and polypropylene/ EPDM or EPR combinations (including thermoplastic vulcanizates) about another 30%. The thermoplastic polyurethanes and the thermoplastic polyesters together made up another 15% Major end uses were transportation, footwear, industrial goods, wire insulation, medical (growing very rapidly), adhesives, coatings, etc. Table VI gives values of three important properties (price, specific gravity and hardness) for typical commercially available thermoplastic elastomers. Trade names and suppliers of commercial thermoplastic elastomers of all types are given in Tables VII - IX.

250

G. Holden

Table VI - Approximate Price and Property Ranges for Thermoplastic Elastomers ^^

Price Range (cents/lb.)

Specific Gravity

Hardness

Polystyrene / Elastomer Block Copolymers S-B-S (Pure) S-l-S (Pure) S-EB-S (Pure) S-B-S (Compounds) S-EB-S (Compounds)

85-130 100-130 185-280 90-150 125-225

0.94 0.92 0.91 0.9-1.1 0.9-1.2

65A-75A 32A-37A 65A-75A 40A-45D 5A-60D

Polyurethane / Elastomer Block Copolymers

225-375

1.05-1.25

70A'')-75D

Polyester / Elastomer Block Copolymers

275-375

1.15-1.40

35D-80D

Polyamide / Elastomer Block Copolymers

450-550

1.0-1.15

60A-65D

Polyethylene / Poly(a-olefin) Block Copolymers

80-110

0.85-0.90

65A-85A

Polypropylene / EPDM or EPR Blends

80-120

0.9-1.0

60A-65D

Polypropylene / EPDM Dynamic Vulcanizates

165-300

0.95-1.0

35A-50D

Polypropylene / Butyl Rubber Dynamic Vulcanizates

210-360

0.95-1.0

50A-80D

Polypropylene / Natural Rubber Dynamic Vulcanizates

140-160

1.0-1.05

60A-45D

Polypropylene / Nitrile Rubber Dynamic Vulcanizates

200-250

1.0-1.1

70A-50D

PVC / Nitrile Rubber Blends

130-150

1.20-1.33

50A-90A

Chlorinated Polyolefin / Ethylene Interpolymer Blends

225-275

1.10-1.25

50A-80A

a) These price and property ranges do not include fire retardant grades or highly filled materials for sound deadening. b) As low as 60A when plasticized

Thermoplastic Elastomers and their Applications

251

Table VII - Some Trade Names of Thermoplastic Elastomers Based on Styrenic Block Copolymers

.

Elastomer Seoment

Trade Name (Mfr.^

Type

KRATON^^ D and

Linear and

CARIFLEXTR (Shell)

branched

VECTOR (Dexco)^^

Linear

SOLPRENE^^ (Phillips)

Branched

B

TAIPOL (Taiwan Synthetic

Linear and

Borl

Rubber Company

Branched

\ /

Notes General purpose, soluble.

Borl

Also compounded products Borl

^

General purpose, soluble.

QUINTAC (Nippon Zeon)

Linear

FINAPRENE(Fina)

Linear

B

COPERBO (Petroflex)

Linear

B

TUFPRENE & ASAPRENE (Asahi)

Linear

B

CALPRENE (Repsol)

Linear and branched

B

EUROPRENESOLT (Enlchem)

Linear and branched

Borl

STEARON (Firestone)

Linear

B

High polystyrene content

K-RESIN (Phillips)

Branched

B

Very high polystyrene content. Hard and rigid.

KRATONG (Shell)

Linear

EBorEP

SEPTON (Kuraray) DYNAFLEX (GLS) MULTI-FLEX (Multibase) HERCUPRENE^^J-VON) FLEXPRENE (Teknor Apex)

Linear Linear Linear Linear

EBorEP / B or EB >\ EB \ BorEB

1

TEKRON(TeknorApex)

Linear Linear

EB

ELEXAR'^^) (Teknor Apex)

Linear

EB

C-FLEX (Concept)®)

Linear

EB

I

Not available as compounded products.

>'

1

Improved stability. Soluble when uncompounded

Only compounded products

B

J

Wire and Cable compounds Medical applications. Contains silicone oil

a) Joint venture of Dow and Exxon. b) No longer made In U.S.A. Similar products are produced by Taiwan Synthetic Rubber Company. c) Formerly J-PLAST. d) Formerly produced by Shell, e) Now Consolidated Polymer Technologies Inc.

252

G. Holden

Table VIII - Some Trade Names of Thermoplastic Elastomers Based on Multiblock Copolymers with Crystalline Hard Segments Hard Segment

Trade Name (Mfr) ESTANE (B.F. Goodrich)

Notes

N

MORTHANE^^Morton International) PELLETHANE^\DOW)

Elastomer Segment

) Polyurethane

ELASTOLLAN (BASF)

Polyether

Hard and Tough. Abrasion

or amorphous

and oil resistant. Good tear

Polyester

strength. Fairly high priced

DESMOPANandTEXIN (Bayer)'b)

HYTREL (DuPont) LOMOD (GE) URAFIL (Akzo) ECDEL (Eastman) RITEFLEX(Hoechst) ARNITEL (DSM)

PEBAX(ElfAtochem) VESTAMIDE (Huls) GRILAMID and GRILON (EMS America) MONTAG (Monsanto)^) OREVAC (Atochem)^^

ENGAGE & AFFINITY (Dow) EXACT (Exxon) FLEXOMER (Union Carbide)

V Polyester

Polyether

V Polyamide

Polyether or amorphous polyester

Polyethylene

Poly(a-olefins)

a) Including some with polycaprolactone segments. b) Formerly marketed by Mobay and Miles. c) For hot melt adhesives

Similar to polyurethanes but more expensive. Better low temperature flexibility. Low hysteresis.

Similar to polyurethanes but can be softer. Expensive. Good low temperature flexibility.

Flexible and low cost. Good low temperature flexibility but limited at higher temperatures.

Thermoplastic Elastomers and their Applications

253

Table IX - Some Trade Names of Thermoplastic Elastomers Based on Hard Polymer / Elastomer Combinations Hard Polymer

Elastomer

Notes

Polypropylene

EPDM or EPR

Relatively hard, low density, not highly filled

DV^

Polypropylene

EPDM

Better oil resistance low compression set, softer

TREFSIN (AES) and SARLINK 2000 (Novacor)"'

DV

Polypropylene

Butyl Rubber

Low permeability, high damping

VYRAM (AES)

DV

Polypropylene

Natural Rubber

Low Cost

GEOLAST (AES)

DV

Polypropylene

Nitrile Rubber

Oil resistant

ALCRYN (Advanced Polymer Alloys)®'

Blend

Chlorinated Polyolefin

Single phase, Ethylene Interpolymer oil resistant

SARLINK 1000 (Novacor).d)

DV

CHEMIGUM (Goodyear)

Blend

APEX N(Teknor Apex)

Blend

Trade Name (Mfr.)

Type

REN-FLEX (D&S)^^ ^ HIFAX (Himont) POLYTROPE (Schulmam) \ Blend TELCAR (Teknor Apex) [ FERROFLEX (Ferro) FLEXOTHENE (Equistar)°^ -^ SANTOPRENE (AES)^^ SARLINK 3000 & 4000 (Novacor)^ UNIPRENE (Teknor Apex) HIFAX MXL (Himont)

RIMPLAST (Petrarch Systems) a) b) c) f)

^ ) PVC

Nitrile Rubber

Blends of TPEs with Silicone Rubbers

Oil Resistant

Medical applications

A joint venture between Dexter and Solvay Formerly Quantum. Product is a blend of PP and EPR produced in the polymerization reactor Advanced Elastomer Systems - a joint venture between Solutia (formerly Monsanto) and Exxon Chemical d) Now a part of DSM e) Formerly DuPont Dynamic Vulcanizate - a composition in which the soft phase has been dynamically vulcanized, I.e., cross-linked during mixing

254

G. Holder)

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