Elastomers, thermoplastics

Thermoplastic elastomers (TPES), as the name indicates, are plastic polymers with the physical properties of rubbers. They are soft, flexible, and possess the resilience needed of rubbers. However, they are processed like thermoplastics by extrusion and injection molding. 

TPE s are more economical to produce than traditional thermoset materials because fewer steps are required to manufacture them than to manufacture and vulcanize thermoset rubber. An important property of these polymers is that they are recyclable. 

Thermoplastic elastomers are multiphase composites, in which the phases are intimately depressed. In many cases, the phases are chemically bonded by block or graft copolymerization. At least one of the phases consists of a material that is hard at room temperature.  

Currently, important TPE s include blends of semicrystalline thermoplastic polyolefins such as propylene copolymers, with ethylene-propylene terepolymer elastomer. Block copolymers of styrene with other monomers such as butadiene, isoprene, and ethylene or ethylene/propy-lene are the most widely used TPE s. Styrene-butadiene-styrene (SBS) accounted for 70% of global styrene block copolymers (SBC). Currently, global capacity of SBC is approximately 1.1 million tons. Polyurethane thermoplastic elastomers are relatively more expensive then other TPE s. However, they are noted for their flexibility, strength, toughness, and abrasion and chemical resistance. Blends of polyvinyl chloride with elastomers such as butyl are widely used in Japan.  

Random block copolymers of polyesters (hard segments) and amorphous glycol soft segments, alloys of ethylene interpolymers, and chlorinated polyolefins are among the evolving thermoplastic elastomers. 

Thermoplastic elastomers (TPEs) are either block copolymers (SBS, SEES, SEPS, TPU, COPA, COPE) or blends, such as TPO (elastomer/hard thermoplastic, also referred to as thermoplastic olefin) and TPV (fhermoplastic vul-canizafe, blend of a vulcanized elastomer and a hard fhermoplastic). These types represent the majority of fhe TPEs other types are either specialty or small-volume materials. 

Normally, TPEs are not cross-linked because the thermoplastic nature is their desired property in most cases. However, in some cases cross-linking is used to improve mechanical properties, influence flow to reduce swelling in oils and solvents, eliminate dissolution of fhe polymer in oils and solvenfs, increase heat resistance, and influence ofher performance characteristics. Examples where cross-linking by ionizing radiation is necessary for fhe given processes are  

Tensile Strength Data from Selected EB-Irradiated Elastomers 

Polymer Additive Amount of Additive, phr Dose, kGy Tensile Strength, MPa (psi) 

Blend of poly chloroprene and NBR Carbon black and multifunctional acrylate 50 15,000 20 (2,900) 

Thermoplastic elastomers (TPEs) are polymeric materials that are processed into fabricated articles in the same manner as a conventional thermoplastic, yet these articles have the properties and functional performance of a thermoset rubber. They have been gaining a significantly larger market share over the past three decades with nontire applications, compared to the conventional thermoset elastomers discussed earlier. 

TPEs are a whole family of rubber materials that exhibit rubber performance properties but can be melted and reprocessed over and over again. This is different from conventional thermoset rubber compounds that cannot be remelted for reprocessing 

The great processing advantage of TPEs over traditional thermoset rubber compounds more than overcomes their higher material cost. This is due to the ease and efficiency of thermoplastic over thermoset processing. Specifically  

The cost of processing TPEs can be significantly less than that for conventional thermoset rubber. No vulcanization is needed. For example, no million-dollar continuous vulcanization unit is necessary for weather-stripping extrusion. Molding cycles are much shorter since all that is necessary is to cool the molten article below its hardening temperature. 

Processing is much simpler and easier. This means that the fabricated product can be produced to tighter dimensions, for better quality at a lower cost. 

Polyurethane-based thermoplastic elastomers are extensively used in applications requiring physical resilience and chemical resistance. In addition to their elasticity, they also exhibit vibration damping, abrasion, tear, and cut resistance. 

The most important physical characteristics of polyurethane elastomers are their elasticity, low permanent set, high tear strength, and good abrasion resistance. Polyester-based elastomers have higher tear strength than polyether-based analogues. Polyether-based elastomers have better recovery. Important chemical characteristics include stability when ejq osed to the elements and resistance to oil and grease. 

Wheels and tires are one of the major uses of cast polyurethane elastomers. We commonly see these on fork lift trucks and shopping carts, where their excellent abrasion resistance, resistance to oil, and good elasticity are valued. In industrial settings we find polyurethane covers on rollers used for paper, steel, and textile conveyor systems. In such applications, their excellent cut and abrasion resistance help prolong their useful life. If used in hot and humid conditions, polyether-based polyurethanes are preferred. 

Other common uses include gaskets and seals for hydraulic systems in harsh environments, such as mining operations, where abrasion is a constant hazard. We take advantage of the 

We take advantage of the shock absorbing characteristics of thermoplastic urethane rubbers in automotive, motorcycle, and bicycle suspensions and power transmission systems. Industrial shock absorbing applications include buffers for elevators and cranes. 

There are different thermoplastic elastomers (TPEs) that meet different requirements (see Table 6-13). As is the case with many TSEs and plastics, their properties can be varied and controlled by varying the ratio of the basic monomers used to compound TPEs, as well as by changing the types and amounts of additives and fillers. 

The first major elastomer that could be processed without vulcanization was the urethanes. The TP polyurethanes (PURs) do not have quite the heat resistance and compression-set resistance of the TS PURs, but most of their other properties are the same. The abrasion resistance among the elastomers is outstanding, their low-temperature flex is good, PURs oil resistance is excellent at 82°C (180°F), and their load-bearing capability ranks them with the best of all the elastomers. Additives can improve their dimensional stability or heat resistance, reduce friction, or increase their flame retardancy, fungus resistance, or weatherability. 

The copolyesters resistance to nonoxidizing acids, some aliphatic hydrocarbons, aromatic fuels, hot oils, and hydraulic fluids ranges from good to excellent. Thus, they compete with such rubbers as nitriles, epichlorohydrins, and acrylates. However, hot polar materials, strong mineral acids and bases, and chlorinated solvents and creosols degrade the copolyesters. Their weather resistance is low but can be improved considerably, with UV stabilizers and carbon-black additives. 

These TPEs are not direct substitutes for TS rubbers in existing designs. Rather, such parts must be redesigned to use their higher strength and modulus and operate within the elastic limit. Thinner parts, from about i to i in., are usually used. 

The styrenics, generally, are the lowest-priced TPEs. They range in hardness from 28 to 95 Shore A. Their tensile strength is lower and their elongation higher than those of SBR or NR, but their weather resistance is about the same. Styrenics properties can be improved by alloying them with such resins as PP and EVA. They resist water, alcohols, and dilute alkalis and acids. They are soluble in or are swelled by strong acids, chlorinated solvents, and ketones. One type has a service-temperature limit of 65 C (150°F), another to 121 C (250 F). Both have excellent low-temperature flex at -85°C (- 120°F). 

All thermoplstic elastomers have in common at use temperature a continuous rubbery phase of soft segments tied together by glassy or crystalline hard segments which soften at elevated temperatures. Thus, thermoplastic elastomers don t have to be vulcanized, and can be processed by the usual economical thermoplastic techniques. 

It has been noted in several places (Sections 4.6,17.1, and 17.5) that certain thermoplastic polymers can be quite elastic (rubbery) at room temperature. They form a distinct class of materials, the thermoplastic elastomers (TPEs), which have been defined (ASTM D1566) as a family of rubber-like materials that, unlike conventional vulcanized rubber, can be processed and recycled like thermoplastic materials. The typical structure of these materials is the coexistence of soft domains that give rubbery behavior with hard domains that act as heat-labile cross-Unks. Although the thermoplastic elastomers compete in many of the applications traditionally assigned to conventional rubbers, the sensitivity of the cross-links to heat makes them unsuitable for an application such as automobile tires, where the 

FIGURE17.7 Tensile yield strength (TYS) of selected molding resins (unreinforced). (Data 

Payne and Rader [28] cite three generic classes of TPEs. In the first class they put the block copolymers. These include the styrene-butadiene-styrene triblocks, copolyesters, polyurethanes, and polyamides. The latter result when aromatic diamines are used in the hard segments and aliphatic diamines are used in the soft segments, both in combination with aliphatic dicarboxylic acids. The mechanical properties, processing conditions, and solvent resistance differ for the various copolymers. Within each type, a spectrum of hardness values is obtainable by variation of the size and chemical composition of the segments. 

The second class is made up of elastomer/thermoplastic blends, abbreviated TEO. Typical of this class is a dispersion of 20-30 parts of rubber based on ethylene-propylene-diene monomer (EPDM) in a continuous phase of 70-80 parts of a plastic such as isotactic polypropylene. Partial covalent cross-linking of the 

Polyimide Tensile Strength (MPa) Flexural Modulus (GPa) Heat Deflection Temperature (°Cat 1.8 MPa) Continuous Use Temperature (°o 

Because of increased production and the lower cost of raw material, thermoplastic elastomeric materials are a significant and growing part of the total polymers market. Wodd consumption in 1995 is estimated to approach 1,000,000 metric tons (3). However, because the melt to soHd transition is reversible, some properties of thermoplastic elastomers, eg, compression set, solvent resistance, and resistance to deformation at high temperatures, are usually not as good as those of the conventional vulcanised mbbers. AppHcations of thermoplastic elastomers are, therefore, in areas where these properties are less important, eg, footwear, wine insulation, adhesives, polymer blending, and not in areas such as automobile tires. 

The classification given in Table 1 is based on the process, ie, thermosetting or thermoplastic, by which polymers in general are formed into useful articles and on the mechanical properties, ie, rigid, flexible, or mbbery, of the final product. AH commercial polymers used for molding, extmsion, etc, fit into one of these six classifications the thermoplastic elastomers are the newest. 

Thermoplastic elastomers are often multiphase compositions in which the phases are intimately dispersed. In many cases, the phases are chemicaUy bonded by block or graft copolymerization. In others, a fine dispersion is apparendy sufficient. In these multiphase systems, at least one phase consists of a material that is hard at room temperature but becomes fluid upon heating. Another phase consists of a softer material that is mbbedike at RT. A simple stmcture is an A—B—A block copolymer, where A is a hard phase and B an elastomer, eg, poly(styrene- -elastomer- -styrene). 

In principle, A can be any polymer normaUy regarded as a hard thermoplastic, eg, polystyrene, poly(methyl methacrylate), or polypropjiene, and B can be any polymer normaUy regarded as elastomeric, eg, polyisoprene, polybutadiene, polyisobutylene, or polydimethjisiloxane (Table 2). 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Polyurethane TPE are block copolymers consisting of alternating hard and soft segments, separated into two phases [54] due to thermodynamic incompatibility. 

Two series of polyether polyurethanes (PU) based on hydroquinone bis (P-hydroxyethyl) ether (HQEE) or 1,4-butanediol (BDO) as a chain extender were prepared by the one step bulk polymerisation process. By varying the mole ratio of poly tetra methylene oxide (PTMO) extender (with Mn = 1000 and Mn = 2000) and 4,4 -diphenylene methane diisocyanate (MDI) the two series of HQEE (PUlOOOHj, PU 1000H2, PU2000Hj, 

Reprinted with permission from L. Zha, M. Wu and J. Yang, Journal of Applied Polymer Science, 1999, 73, 289S. Copyright 1999, John Wiley and Sons, Inc. 

Reprinted from [55] with permission of John Wiley and Sons, Inc., 1999 

Like PU TPE, blends of thermoplastic polyurethanes and polyamide-12 (PA-12) have been studied by Polosmak and co-workers [61]. They have mixed two types of thermoplastic polyurethane (TPU) based on oligoether (polytetramethylene oxide, molecular weight, 1000) and oligoester (polyethylene butylene glycol adipate, molecular weight, 2000) and PA 12 were characterised by IR spectra and thermal analysis. IR spectra of TPU, PA-12 and their blends show that in amide one (Al) carbonyl absorbancy is seen to split [55] into two main bands with maxima at 1705 and 1730 cm 1. At 1730 cm 1, 

Spheres, cylinders, and alternating lamellae were also observed in the crystalline block copolymers, which will be considered in Chapter 6. 

The thermoplastic elastomers are triblock polymers, the central portion being elastomeric, with short, glassy blocks on either side (Holden et a/., 1969 Robinson and White, 1970). While many combinations of monomers are possible, most important commercial systems comprise styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) compositions. Below the glass transition of the plastic component, tough, highly elastic 

The morphology and properties of the TEP are mainly determined by the A/B ratio and the size of the blocks. 

A rubber is generally defined as a material that can be stretched to at least twice its original length, and that will retract rapidly and forcibly to substantially its original dimensions on release of the force. In contrast, an elastomer is a rubber-like material from the standpoint of modulus but one that has limited extensibility and incomplete retraction. The most common example is highly plasticized poly(vinyl chloride). 

The introduction of the so-called thermoplastic elastomers (actually rubbers) has gotten around the requirement for crosslinking in the strictest sense, that of covalent bonding between chains. They are commonly found for applications requiring flexibility in automotives, household appliances, and soft-grip tools. The most common thermoplastic 

Fundamental Principles of Polymeric Materials, Third Edition. Christopher S. Brazel and Stephen L. Rosen. 2012 John Wiley Sons, Inc. Pubhshed 2012 by John Wiley Sons, Inc. 

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Ffigh-density polyethylene (HDPE) Thermoplastic elastomers... 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

Casting resin Thermoplastic elastomer Cast resin, flexible Mineral- and/or glass-filled Epoxy molding and encapsulating compound Polystyrene... 

Properties Thermoplastic elastomers Urea formaldehyde, alpha-cellulose filled Vinyl ... 

ELASTOMERS SYNTHETIC - THERMOPLASTIC ELASTOMERS] (Vol 9) Polyamidesepichlorohydrin resins... 

Considerable work has also been conducted to try to find thermoplastic elastomers that can be used to simplify processing by enabling dry blending and melt casting instead of the conventional mixing and curing process (see Elastomers, synthetic). 

Gun Propellents. Low sensitivity gun propeUants, often referred to as LOVA (low vulnerabUity ammunition), use RDX or HMX as the principal energy components, and desensitizing binders such as ceUulose acetate butyrate or thermoplastic elastomers (TPE) including poly acetal—polyurethane block copolymers, polystyrene—polyacrjiate copolymers, and glycidyl azide polymers (GAP) to provide the required mechanical... 

B. D. Nahloosky and G. A. Zimmerman, Thermoplastic Elastomers for S olid Propellant Binders, AERPL-TR-86-069, Aerojet Tactical Systems Co., Sacramento, Calif., Dec. 1986. 

HDPE, high density polyethylene PP, polypropylene EVA, ethylene—vinyl alcohol SMC, sheet-molding compound ERP, fiber-reinforced plastic LDPE, low density polyethylene PE, polyethylene BMC, bulk mol ding compound TPE, thermoplastic elastomer. 

Uses. Vinyhdene fluoride is used for the manufacture of PVDF and for copolymerization with many fluorinated monomers. One commercially significant use is the manufacture of high performance fluoroelastomers that include copolymers of VDF with hexafluoropropylene (HFP) (62) or chlorotrifluoroethylene (CTFE) (63) and terpolymers with HEP and tetrafluoroethylene (TEE) (64) (see Elastomers, synthetic-fluorocarbon elastomers). There is intense commercial interest in thermoplastic copolymers of VDE with HEP (65,66), CTEE (67), or TEE (68). Less common are copolymers with trifluoroethene (69), 3,3,3-trifluoro-2-trifluoromethylpropene (70), or hexafluoroacetone (71). Thermoplastic terpolymers of VDE, HEP, and TEE are also of interest as coatings and film. A thermoplastic elastomer that has an elastomeric VDE copolymer chain as backbone and a grafted PVDE side chain has been developed (72). 

Aliphatic C-5—C-6. Aliphatic feedstreams are typically composed of C-5 and C-6 paraffins, olefins, and diolefins, the main reactive components being piperylenes cis-[1574-41 -0] and /n j -l,3-pentadiene [2004-70-8f). Other main compounds iaclude substituted C-5 and C-6 olefins such as cyclopentene [142-29-OJ, 2-methyl-2-butene [513-35-9] and 2-methyl-2-pentene [625-27-4J. Isoprene and cyclopentadiene maybe present ia small to moderate quaatities (2—10%). Most steam cracking operatioas are desigaed to remove and purify isoprene from the C-5—C-6 fraction for applications ia mbbers and thermoplastic elastomers. Cyclopentadiene is typically dimerized to dicyclopentadiene (DCPD) and removed from C-5 olefin—diolefin feedstreams duriag fractionation (19). 

EPDM-Derived Ionomers. Another type of ionomer containing sulfonate, as opposed to carboxyl anions, has been obtained by sulfonating ethylene—propjlene—diene (EPDM) mbbers (59,60). Due to the strength of the cross-link, these polymers are not inherently melt-processible, but the addition of other metal salts such as zinc stearate introduces thermoplastic behavior (61,62). These interesting polymers are classified as thermoplastic elastomers (see ELASTOLffiRS,SYNTHETIC-THERMOPLASTICELASTOLffiRS). 

Butadiene—Methacrylic Acid Ionomers. Carboxyl groups can readily be introduced into butadiene elastomers by copolymerization, and the effects of partial neutralization have been reported (63—66). The ionized polymers exhibit some degree of fluidity at elevated temperatures, but are not thermoplastic elastomers, and are very deficient in key elastomer properties such as compression set resistance. 

KRATON Thermoplastic Elastomers bulletins. Shell Chemical Co., Houston, Tex. 

Noncrystalline aromatic polycarbonates (qv) and polyesters (polyarylates) and alloys of polycarbonate with other thermoplastics are considered elsewhere, as are aHphatic polyesters derived from natural or biological sources such as poly(3-hydroxybutyrate), poly(glycoHde), or poly(lactide) these, too, are separately covered (see Polymers, environmentally degradable Sutures). Thermoplastic elastomers derived from poly(ester—ether) block copolymers such as PBT/PTMEG-T [82662-36-0] and known by commercial names such as Hytrel and Riteflex are included here in the section on poly(butylene terephthalate). Specific polymers are dealt with largely in order of volume, which puts PET first by virtue of its enormous market volume in bottie resin. 

As with all thermoplastic elastomers, the copolyesterethers can be processed as thermoplastics. They are linear polymers and contain no chemical cross-links, thus the vulcanisation step needed for thermosetting elastomers is eliminated and scrap elastomer can be re-used in the same process as virgin material (176—180). 

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