5 Thermoplastic Elastomers: Fundamentals and Applications Tonson Abraham and Colleen McMahan Advanced Elastomer Systems, L.P., Akron, Ohio, U.S.A. I. INTRODUCTION In the fifteenth century, Christopher Columbus witnessed South Americans playing a game centered around a bounceable ‘‘ solid’’ mass that was produced from the exudate of a tree they called ‘‘weeping wood’’ (1). This material was first scientifically described by C M. de la Condamine and Francßois Fressneau of France following an expedition to South America in 1736 (2). The English chemist Joseph Priestley gave the name ‘‘ rubber’’ to the material obtained by processing the sap from Hevea brasiliensis,atall hardwood tree (angiosperm) originating in Brazil, when he found that it could be used to rub out pencil marks (2). A rubber is a ‘‘solid’’ material that can readily be deformed at room temperature and that upon release of the deforming force will rapidly revert to its original dimensions. Rubber products were plagued by the tendency to soften in the summer and turn sticky when exposed to solvents. This problem associated with natural rubber was overcome by Charles Goodyear in the 1840s by subjecting the rubber to a vulcanization (after Vulcanus, the Roman god of fire) process. Natural rubber was vulcanized by heating it with sulfur and ‘‘white lead’’ (lead monoxide) (2). In May 1920 the German chemist Hermann Staudinger published a paper that demonstrated that natural rubber was composed of a chain of isoprene units, that is, a polymer (from the Greek poly, many, and mer, part) of isoprene (3). In vulcanization the rubber macromolecules are chemically bonded to one another (‘‘ cross-linked’’ in a thermosetting process) to form a three-dimensional network composing a giant molecule of infinite 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 163 Copyright © 2004 by Taylor & Francis molecular weight. At present the word ‘‘rubber’’ is associated with macro- molecules that exhibit glass transition below room temperature and have ‘‘long-chain,’’ ‘‘organic,’’ carbon-based backbones or ‘‘inorganic’’ back- bones typified by polysiloxanes and polyphosphazenes. ‘‘Elastomer’’ is always used in reference to a cross-linked rubber that is elastic (Greek elastikos,beatenout,extensible).Anelastomerishighly extensible and reverts rapidly to its original shape after release of the deforming force. Entropic forces best describe rubber elasticity (4). However, it should be noted that under relatively much smaller deformation, plastic materials and even metals can exhibit elasticity due to enthalpic factors (4). Gases and liquids also exhibit elastic properties due to reversible volume changes as a result of pressure and/or heat (4). Nevertheless, the term ‘‘elastomer’’ is always used in reference to rubber elasticity. A plastic material is one that can be molded (Greek plastikos), and a thermoplastic can be molded by the application of heat. A rubber compound (a blend of rubber, process oil, filler, cross-linking chemicals, etc.) is thermo- plastic and is ‘‘set’’ after several minutes in a hot mold, with loss of thermoplasticity. A thermoplastic material can be molded in a matter of seconds, and the molded part can be reprocessed. The viscous character of the thermoplastic melt readily allows control of the appearance of the surface of finished goods. In comparison, the effect of ‘‘ melt elasticity’’ of a rubber compound on end product surface appearance is not as readily controlled. The origin of the first thermoplastic material can be traced to Christian Schonbein, a Swiss scientist who broke a beaker containing a mixture of nitric and sulfuric acid and used his wife’s cotton apron to clean up the spillage! Unfortunately for his wife, but fortunately for science, he left the washed apron near a fireplace to dry. The cotton apron soon combusted without leaving any residue! Schonbein realized that the cotton of the apron was converted to ‘‘ gun cotton,’’ a nitro derivative of the naturally occurring poly- mer cellulose (1). This learning may have been instrumental in the preparation of the first plastic by the English chemist and inventor Alexander Parkes in 1862. First called Parkesine, it was later renamed Xylonite. This substance was nitrocellulose softened by vegetable oils and a little camphor. During this time, elephant tusks, which were used to make ivory billiard balls, among other things, became scarce. In 1869, motivated by the need to find a suitable substitute for ivory, John W. Hyatt in the United States recognized the vital plasticizing effect of camphor on nitrocellulose and developed a product that could be molded by heat. He named this product obtained from cellulose ‘‘Celluloid’’ (Greek oid, resembling). Though primarily regarded as a substi- tute for ivory and tortoiseshell, Celluloid, despite its flammability, found substantial early use in carriage and automobile windshields and motion picture film (3). 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 164 Copyright © 2004 by Taylor & Francis A. Definition of Thermoplastic Elastomer A thermoplastic elastomer (TPE) is generally considered a bimicrophasic material that exhibits rubber elasticity over a specified service temperature range but at elevated temperature can be processed as a thermoplastic (because of the thermoreversible physical cross-links present in the material). It offers the processing advantages of a highly viscous melt behavior and a short product cycle time in manufacturing due to rapid melt hardening on cooling. B. Classification of Commercially Available Thermoplastic Elastomers The TPE products of commerce listed in Table 1 are classified in Table 2 on the basis of their polymer microstructure. Representative examples are included for each polymer class. Segmented block copolymers, triblock copolymers, and thermoplastic vulcanizates represent a significant portion of the TPE family. The fundamental aspects of structure–property relationships in ther- moplastic polyurethanes (TPUs), styrenic block copolymers (SBCs) [with emphasis on styrene/ethylene-1-butene/styrene (SEBS) copolymers and SEBS compounds], and thermoplastic vulcanizates (TPVs) produced from polypropylene and ethylene/propylene/diene monomer (EPDM) rubber were selected for review in this chapter, as representative of the most commercially significant and the closest in performance to thermoset elastomers. Table 1 Thermoplastic Elastomer Products of Commerce Product First commercialized (year, company) Plasticized poly(vinyl chloride) 1935, B. F. Goodrich Thermoplastic polyurethane 1943, Dynamit AG PVC/NBR blends 1947, B. F. Goodrich Styrenic block copolymers 1965, Shell Thermoplastic polyolefin elastomers 1972, Uniroyal Styrenic block copolymers (hydrogenated) 1972, Shell Copolyester elastomers 1972, DuPont Thermoplastic vulcanizates (PP/EPDM) 1981, Monsanto Copolyamide elastomers 1982, Atochem PP/NBR TPVs 1984, Monsanto Chlorinated polyolefin/ethylene interpolymer rubber 1985, DuPont UHMW PVC/NBR 1995, Teknor Apex 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 165 Copyright © 2004 by Taylor & Francis Thermoplastic vulcanizates possess sufficient elastic recovery to chal- lenge thermoset rubber in many applications, and insights into TPE elastic recovery and processability are presented based upon the latest developments in the field. The poor elastic recovery of TPEs at elevated temperature is a key deficiency that has prevented these materials from completely replacing their thermoset counterparts. Thermoplastic elastomers owe their existence as products of commerce to the fabrication economics and environmental advantage they offer over thermoset rubber. TPEs, of course, are designed to flow under the action of heat; hence their upper service temperature is limited in comparison to thermoset rubber. Thus a major hurdle to overcome in the replacement of thermoset rubber with TPEs is the improvement in elastic recovery, partic- ularly at elevated temperature, especially compression set, because in many applications elastomers are subjected to compression. The scope of this chapter includes those TPEs that in our opinion come reasonably close in properties to thermoset elastomers, as listed in Table 1. Not included, for example, are plastomers that are ethylene/a-olefin copolymers generally produced using metallocene catalysts (5).* These materials can be rubberlike only at room temperature. They are thermoplastic owing to the thermorever- sible cross-links provided by crystallization of the ethylene sequences in the polymer but are deficient in elastomeric character above room temperature or when under excessive strain. Thermoplastic elastomers based on melt-blended polyolefins, ethylene/vinyl acetate copolymers, and ethylene/styrene co- polymers are also omitted from the list (6,7). Although thermoplastic olefins (TPOs) represent a commercially important class of materials, they are included primarily as comparative points to their more elastomerically per- forming counterparts, TPVs. Plasticized poly(vinyl chloride) (PVC) is used as a flexible plastic and not an elastomer but is included in Table 1 because it was the first commer- Table 2 Thermoplastic Elastomer Classification Segmented block copolymers Triblock copolymers Thermoplastic vulcanizates Polymer blends TPU SBC PP/EPDM PVC/NBR COPE Hydrogenated SBC PP/NBR COPA PP/IIR *Note that Ziegler–Natta-based plastomers are also commercially available. For example, some of Dow’s Flexomer products are based on ethylene/1-butene copolymers. 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 166 Copyright © 2004 by Taylor & Francis cially produced thermoplastic elastomer. PVC, produced by free radical polymerization, contains crystallizable syndiotactic segments, the crystalli- zation of which is enhanced on mobilization of the polymer chain in the presence of a plasticizer (8). However, imperfections in the crystalline phase limit the upper service temperature of PVC. II. THERMOPLASTIC ELASTOMERS: APPLICATIONS OVERVIEW Thermoplastic elastomers are found in thousands of applications, ranging from commodity TPOs used in automotive bumper and facia applications, through plastomers used as impact modifiers for plastics, and TPVs and SBCs in sealing applications, to TPUs and copolyesters in numerous engineering applications. TPEs replace EPDM rubber in many sealing applications, butyl rubber where permeation resistance is required, and nitrile rubber for oil and fuel resistance. World demand for thermoplastic elastomers will grow at over 6% per year through 2006, according to a recent study (9). The 1.6 million metric ton TPE industry will remain concentrated in the United States, Western Europe, and Japan, although underdeveloped markets such as Asia grow at a faster rate. The most important driver for TPE growth through thermoset rubber replacement is cost savings. This is normally achieved through a combination of material selection, part redesign, and fabrication economics. Recyclability and weight reduction provide additional drivers in some markets. Colora- bility is another important TPE attribute that increases design flexibility. Further, use of TPEs allows introduction of designs, processes, and value- added features not possible at any cost with thermoset rubber. Almost all commercial TPEs have one feature in common: they are microphase separated systems in which one phase is hard at room tempera- ture while another phase is soft and elastomeric. The harder phase gives TPE their strength and, when softened, their processability. The soft phase gives TPEs their elasticity. Each phase has its own glass transition temperature, T g , or crystal melting point, T m , and these in turn determine the temperatures at which the TPEs exhibit their transition properties. Thus, the TPE service temperature on the lower end is bounded by the T g of the elastomeric phase, whereas the upper service temperature depends on the T m of the hard phase. Note that the practical service range also depends on the softening point, stress applied, and article design (10). The ability of TPEs to repeatedly become fluid on heating and solidify on cooling gives manufacturers the ability to produce rubberlike articles using 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 167 Copyright © 2004 by Taylor & Francis the fast processing equipment designed for the plastics industry. Scrap can usually be reground and recycled. Output of parts is generally increased and labor requirements reduced compared to parts manufactured from thermoset rubber. Thermoplastic elastomers can be fabricated by conventional thermo- plastic methods including injection molding, blow molding, and extrusion. Injection molding processes range from single- to multiple-cavity, including up to 48 or more cavities per mold, hot runner mold technology for runnerless part production, insert molding with other materials, and coinjection molding of two materials sequentially or simultaneously. Tools such as MoldflowR (11) allow fast development of tooling and process conditions for many TPEs. Another significant advantage is that injection molding of TPEs allows dimensional tolerances not achievable in thermoset rubber. This allows snap fits and ‘‘living hinges’’ to be designed into the parts. Flexible, nonblooming, flashless parts are easily produced on largely auto- mated molding equipment. A compatible thermoplastic can give excellent bond strength with two-shot injection molding. For noncompatible materials, a physical lock or interference fit is used over a rigid substrate of metal, plastic, or even glass (12). Blow molding is practiced by injection blow molding, extrusion blow molding, or press blow molding processes. Complex designs can be easily manufactured by three-dimensional sequential blow molding with multiple materials. Fabrication process equipment is available today that can blow mold three-dimensional parts from combinations of thermoplastic and ther- moplastic elastomer materials in up to seven layers by precise material de- livery, robotic parison manipulation, and perfectly timed mold positioning, all computer-controlled in a largely automated process (13). Extrusion of thermoplastic elastomers includes single-extrusion, co- extrusion, and triple-extrusion processes. Multiprofile dies for extrusions from a single line provide important improvements in efficiency for simple extrusions. Hard–soft combinations with other polymers, including polyole- fins, polystyrene, and other TPEs, are commonly practiced. Recent develop- ments include coextrusion of thermoset EPDM with TPVs (14,15). Special extrusion processes have been developed to produce foamed profiles using water as the blowing agent (16,17) and create low-friction surfaces with a coextruded slipcoat, offering low-cost environmentally friendly alternatives for specific applications. Robotic extrusion of TPVs, through a system composed of a moving die, flexible heated hose, and 3D robot, has been used to apply seals directly to automotive parts (18,19). Secondary processes such as heat welding, thermoforming, coating, printing, and painting add signifi- cant value at moderate cost in many applications. Thermoplastic elastomers can offer the design engineer greater design flexibility as well as part size and weight reduction. In the case of thermoset 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 168 Copyright © 2004 by Taylor & Francis rubber replacement, the part is usually redesigned to leverage the physical properties and processing characteristics of the TPE. The use of TPEs frequently allows designers to reduce the amount of material per part and, combined with the lower specific gravity of TPEs in comparison with thermo- sets, significantly reduce the overall part weight compared with thermoset rubber (20). An important advantage in redesign is the opportunity for parts consolidation through combinations of thermoplastic elastomer and other thermoplastic components. Thermoplastic elastomer grades have been developed that bond to a wide range of engineering thermoplastics, including polypropylene, polyeth- ylene, polystyrene, polyamides, polyesters, acrylonitrile/butadiene/styrene (ABS) rubber modified plastic, cured EPDM rubber, polycarbonates, and copolyesters. The bond is typically formed through an autoadhesion (diffu- sion) mechanism during thermoplastic processing (21,22). In many cases, bond strengths at levels comparable to material strength can be achieved. A. Thermoplastic Elastomers in Automotive Applications The automotive industry has always been a major end-use market for TPEs and accounts for about 60% of the total demand in North America. Tires account for most of the thermoset elastomeric content in a vehicle. The rest is spread over 600 or more elastomer applications from simple grommets to complex constant-velocity joint boots and radial lip seals. Automotive elastomeric parts serve in a wide range of operating environments. They also provide numerous functions such as air, vacuum, and fluid seals; mechanical shock absorption; flexible couplings; and soft-touch interior components. As with any elastomer, TPEs have their limitations. They do not have the combination of abrasion resistance, flexural strength, deformation resistance, and high-temperature use that thermoset elastomers display; therefore, these materials have found no significant use in pneumatic tires. Key automotive trends have provided a demand for increasing use of TPEs. The most important is the drive for cost reduction in every possible component of the vehicle. Even though TPEs are more expensive as a raw material than thermoset elastomers, the cost of the TPE finished part is usually significantly lower than that of a functionally comparable thermoset rubber part through redesign including lighter weight, shorter cycle time, lower energy usage, lower scrap, and recyclability. Another significant automotive trend is the increased level of govern- ment regulations, which has forced the world’s automotive manufacturers to put major emphasis on improving safety and increasing fuel efficiency, recyclability, and the use of environmentally friendly materials. As Germany led the world in reduction of nitrosamine-containing cure package compo- 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 169 Copyright © 2004 by Taylor & Francis nents for thermoset rubber, the European Union leads with respect to legislation requiring higher recyclable content and lower overall vehicle emissions (23). Recyclability has provided a consistent driver in the Japanese market. Thermoplastics and thermoplastic elastomers are key to reaching the target (24). Vehicle manufacturers have taken a lead as well, including targets for increase in recyclable content and elimination of PVC use in certain auto interior skin applications. The relatively low price of PVC compounds, however, makes replacement by olefinic systems difficult from a cost view- point (25). In addition, the automotive industry is trying to respond effectively to an increased level of technical performance requirements. Higher perfor- mance engines, operating at higher temperatures with lowered emissions, coupled with improved aerodynamics due to decreased frontal and grille area, contribute to increasing under-the-hood temperatures. Longer lived automo- biles also require elastomers with improved ultraviolet resistance. Soft-touch, color-matched interior parts, featuring low odor and low fogging, add to esthetics and consumer-recognized value. Engine compartment timing belt covers with a flexible segment of rubber and a rigid segment of polypropylene have successfully employed TPE. Fuel line covers from specially formulated flame-retardant grades, rack- and-pinion boots taking advantage of the outstanding flex fatigue resistance of TPVs, and clean air ducts featuring innovative convolute designs in combination with polypropylene are just a few examples of automotive applications that leverage the unique properties of TPVs. Thermoplastic elastomers, especially thermoplastic vulcanizates, are moving quickly into automotive weatherseal applications; this market provides significant growth potential for TPEs in the future. TPEs are injection molded for glass encapsulation and cutline seals. They are extruded for belt line and glass run channel seals. Extruded seals can be coated with specially formulated low friction TPEs and joined at the corners with specialty molding TPVs to replace flocked thermoset EPDM seals with 100% recyclable parts. B. Thermoplastic Elastomers in Industrial Applications Thermoplastic vulcanizates are found in hundreds of industrial applications. In most cases the drivers for TPE use are the same as in other industries, i.e., thermoset elastomer performance with the advantages of thermoplastic economics. The building and construction industry takes advantage of TPE performance to provide critical sealing in places such as architectural glazing seals, bridge deck seals, pipe seals, and roofing. Industrial hose applications form a growing segment of TPV applications, including fire hose, washdown hoses, and specialty grades for handling potable water and food. Excellent 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 170 Copyright © 2004 by Taylor & Francis TPV resistance to detergents, acids, and bases, combined with superior flex life and weatherability compared to thermoset rubber, drive application in thousands of small sealing parts such as gaskets and bushings in appliances and mechanical devices worldwide. Specialty TPEs featuring low flame retardancy, good abrasion resistance, dielectric strength, and wet electrical performance are used in electrical applications, especially wire and cable coverings, insulators, and flexible connectors (26). Conductive thermoplastic elastomers incorporating carbon or metal powders are used for static dissipative and conductive properties or in electromagnetic interference/radio frequency interference (EMI/RFI) shielding (27). Multilayer coated sheets are used in roofing, and their use is expanding to innovative applications such as pillow tank liners. C. Thermoplastic Elastomers in Consumer Applications Thermoplastic vulcanizates are found in a variety of consumer products, most recognizably those incorporating grips for soft but secure handling of power tools, housewares, and toothbrushes. Good sealing properties and good chemical resistance make them well suited for kitchen appliances (28). Because many TPEs have consistent frictional characteristics over a range of temperatures and in wet and dry conditions, they are well suited for use in this growing market. The ability to adhere to a variety of substrates by two- shot or overmolding allows processing ease with excellent adhesion. Trans- parent and translucent products are readily available. Many ballpoint pens now feature a soft grip made from a TPE. Cosmetic containers, food containers, and water bottles incorporate TPEs for soft-grip feel, color, and design innovation. The demand for thermoplastic rubber soft grips is also growing in sports applications, such as tennis racket or golf club grips. Other sports and leisure applications include toys, ski equipment, and sports balls (e.g., soccer ball inner bladder) made from butyl rubber–based TPVs. Consumer products emphasize good esthetic design as well as functionality, and the ability of TPEs to be decorated is a real advantage. Techniques such as permanent laser marking and the application of hot stamping foils, heat transfer labels, or screen or tampo printing have been used for marking various products, including multicolored flexible labels. Logos can be integrally designed into products by using overmolding of hard–soft combinations. Effects linked to other materials such as minerals can be obtained through the use of innovative pigments; marble and granite are the most commonly imitated materials (29). Newer application areas for TPEs in consumer products include personal electronics and a growing range of household and garden tools. 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 171 Copyright © 2004 by Taylor & Francis III. SEGMENTED BLOCK COPOLYMER TPEs The segmented block copolymer TPEs included in Table 1–3 contain sequen- ces of ‘‘ hard’’ and ‘‘soft’’ blocks within the same polymer chain. Solubility differences between the polymer segments and association and/or crystalliza- tion of the hard blocks produce phase separation in the molten elastomer as it cools. The hard blocks form the thermoreversible cross-links and reinforce- ment (increasing stiffness) of the elastomeric soft phase. The rate of crystal- lization or association of the hard blocks will impact product fabrication time. Polymer microstructure and morphology is depicted in Figure 1. These TPEs are produced by condensation or addition step growth polymerization and have low molecular weight segments. Although this is desirable, segment mo- lecular weight and molecular weight distribution cannot be readily controlled. In a 40 Shore D copolyester (COPE) elastomer based upon poly(butylene terephthalate) (PBT) hard blocks and poly(tetramethylene oxide/terephthal- ate) (PTMO-T) soft blocks, the hard sequence length varies from 1 to 10 (30). PBT molecular weight of sequence length 10 is 2200, whereas high molecular weight PBT that is commercially available could easily have an M n of 50,000! Thus, a sufficient number of hard blocks have to associate to produce a high enough melting crystal phase to provide a reasonably high elastomer upper service temperature. This necessitates increasing the hard-phase content of the TPE, which results in a hard elastomer (‘‘filler’’ effect). Note that for a given hard-phase content, the lower the number of hard domains (more hard segments per domain), the greater the entropic penalty imposed on the elastomeric phase and the less favored the phase-separated morphology. Increased hard phase content also causes more hard segments to be rejected into the amorphous elastomeric phase, thus raising the rubber glass transition temperature (T g ) and therefore also the TPE lower service temper- ature. In the case of an increase in the number of hard domains, the soft-phase T g is also elevated owing to the increased ‘‘cross-link density.’’ These considerations allow the commercial viability of only hard COPEs. This is a major deficiency in this class of TPEs as the softest product available has a hardness of 35 Shore D. Also based on the above discussion, the more or less continuous hard phase in commercially available COPEs where fibrillar crystalline lamellae (due to short hard segments) are connected at the growth faces by short tie molecules can readily be rationalized. The amorphous phase is also continuous (31). It is difficult to produce useful soft elastomeric products from segmented block copolymers except in the case of thermoplastic polyurethanes (TPUs). The strong association of hard blocks even at low hard block content allows the preparation of soft elastomeric TPUs. TPUs with hardness as low as 70 Shore A are available commercially. 4871-9_Rodgers_Ch05_R2_052704 MD: RODGERS, JOB: 03286, PAGE: 172 Copyright © 2004 by Taylor & Francis [...]... 16 1148 904 57 30 — 13 Crumbly G 165 1e 17 1721 1119 66 34 — 6 Crumbly G 165 0 25 1222 791 76 100 50 6 Sticky G 165 2 25 500 560 77 100 93 8 Viscous oil a Ref 102 Ref 94 c Ref 96 d All samples mixed under N2 at 200jC and molded at 210jC except as noted Mixing time approximately 19 min e Mixed under N2 at 250jC and molded at 260 jC b 65 jC (107), in spite of the increased incompatibility between the rubber and. .. and Properties PS (wt%) PEB (wt%) PEB (MwgMn) 33.3 29.0 28 .6 31.8 Kraton G 165 1a Kraton G 165 0b Kraton G 165 2a Experimentalc PS (MwgMn) 29,000 13,500 7,500 3,400 66 .7 71.0 71.4 68 .2 1 16, 000 66 ,400 37,500 14 ,60 0 TODT (jC) 350 142 SEBS properties (50% SEBS, 50% paraffinic oil)d Hardness (Shore A) UTS (psi) UE (%) M100 (psi) CS (%), 22 hr at 70jC CS (%), 22 hr at 40jC TS (%), 10 min at RT Mixer removal G 165 1... hard segment content above about 45 wt% (33,41– 46) The small-angle X-ray studies of Abouzahr and Wilkes (42) and Cooper and coworkers (43) and the small-angle X-ray and neutron scattering analysis of Leung and Koberstein (41) suggested an interlocking hard domain morphology at high hard segment content Depending upon processing conditions and hard phase type and content, crystalline TPU systems may exhibit... (69 –71) Hydrocarbon diols are being promoted for nonelastomeric polyurethane applications, as in the preparation of castable polyurethanes for moisture-resistant adhesives, coatings, and electrical potting compounds (72) Thermoplastic polyurethanes produced with 2 , 6- toluenediisocyanate (2 , 6- TDI) hard segments with BDO as chain extender and PTMO as the soft Copyright © 2004 by Taylor & Francis Figure 3 Schematic... exceptional toughness and resilience, creep and flex fatigue resistance, impact resistance, and low-temperature flexibility All three types are generally used uncompounded, and the final parts can be metallized or painted Thus, they are often used as replacements for oilresistant rubbers such as neoprene because they have better tensile and tear strength at temperatures up to about 100jC Automotive applications. .. to the Tg of styrene (85) Both the polystyrene end block content and polystyrene molecular weight in SEBS is designed to be lower than that of the rubber midblock For example, Kraton G 165 1(SEBS) of Kraton Polymers has a plastic block of molecular weight 29,000 (33 wt%) and a rubber block of molecular weight 1 16, 000 (68 wt%) ( 86) The rubber block is designed to have a 40 wt% butene content to limit... macromolecules is 35.29 Â 1019 (68 / 1 16, 000 = 5.89 Â 10À4 gÁmol = 5. 86 Â 10À4 Â 6. 023 Â 1023 macromolecules) Assuming a molecular weight between entanglements for PEB of 1800, the number of entanglements per chain is 64 (1 16, 000/1800) If entanglements occur only by the crossing of two different rubber chains, the total number of entanglements in the rubber is 1129 Â 1019 (35.29/2 Â 1019 Â 64 ), which results in... MW of 3400 (31.8 wt%) and a PEB midblock MW of 14 ,60 0 (68 .2 wt%) exhibits a TODT of 142jC ( 96) A SBCs as Compounded Materials In elastomer applications, SEBS is never used alone; it is always compounded to improve product processability and performance and to lower product cost Polypropylene (PP), paraffinic oil, and fillers make up the bulk of a SEBS elastomer compound In elastomer applications, high molecular... TPU a two-stage process In the former, the diisocyanate, chain-extender diol, and soft segment diol are mixed and heated to yield the final product, whereas in the latter the soft-segment diol is first ‘‘end-capped’’ by using an excess of diisocyanate and the chain-extending short-chain diol is subsequently added to form the hard segments and to attach them to the soft segments in an alternating manner... based on 4,4V-diphenylmethane diisocyanate (MDI) and 1,4-butanediol (BDO, a ‘‘chain extender’’), with either poly(tetramethylene oxide) (PTMO) glycol, or poly(1,4-tetramethylene adipate) (PTMA) glycol or poly (q-caprolactone) (PCL) glycol as the soft elastomeric segment (32) TPUs can be produced by a ‘‘one -pot ’ method or in Copyright © 2004 by Taylor & Francis Figure 2 Polyether-based TPU a two-stage process . Abouzahr and Wilkes (42) and Cooper and coworkers (43) and the small-angle X-ray and neutron scattering analysis of Leung and Koberstein (41) suggested an interlocking hard domain morphol- ogy at. moisture-resistant adhesives, coatings, and elec- trical potting compounds (72). Thermoplastic polyurethanes produced with 2 , 6- toluenediisocyanate (2 , 6- TDI) hard segments with BDO as chain extender and. for plastics, and TPVs and SBCs in sealing applications, to TPUs and copolyesters in numerous engineering applications. TPEs replace EPDM rubber in many sealing applications, butyl rubber where