10-13 THERMOPLASTIC ELASTOMERS

10-13 THERMOPLASTIC ELASTOMERS

Thermoplastic elastomers are the block copolymers which exhibit the elastic properties of elastomers at the use temperature but can be processed as thermoplastics at elevated temperatures. Usually. thermoplastic elastomers are two-phase block copolymers containing both hard and soft polymer segments. The hard segments form domains which, at service temperature. prevent the flow of the soft elastomeric segments but, at elevated temperatures. soften and plasticize the rubber segments so that the material can be processed as a conventional thermoplastic. There are two main types of thermoplastic elastomers: modified thermoplastic elastomers and thermoplastic elastomers by polymer blending.

Modified Thermoplastic Elastomers. The modified thermoplastic elastomers are blends of homogeneous polymers having various types of temporary crosslinkings such as glassy block

polymers of polystyrene (PS) in styrene—butadiene—styrene (SBS) copolymers or the crystalline hard phase of tetramethylene terephthalate in a thermoplastic copolyester/polyether (Fig. 10-7).

In SBS thermoplastic elastomers the polybutadiene functions as a soft cIastomeric phase while the polystyrene phase acts as a hard crosslinking component. The latter can be modified with another polymer such as a poly(phenylene oxide) to increase its glass transition temperature. SBS block copolymers are frequently blended with other polymers such as polystyrene (PS), polyethylene (PE), and polypropylene (PP) together with oils and fillers. Generally blends of this type can be modified by the addition of rubbery materials to improve their impact resistance and flexibility.

FIGURE 10-7 Schematic diagram of a thermoplastic elastomer morphology. Hard crystalline or glassy domains function as crosslinks for soft amorphous rubbery phase.

Thermoplastic polyester elastomers (TPE) and thermoplastic polyurethane elastomers (TPU) are alternating block copolymers having segments of a hard. highly polar or crystallizable material linked by segments of amorphous (soft) rubber-like material at normal service temperature. in TPE the crystalline block consists of dimethyl terephthalate segments and the amorphous soft rubbery phase is poly(tetramethylene ether) glycol terephthalate segments randomly distributed along the polymer chain. Crystalline regions are about 10 nm (100 Å) wide and several hundreds nanometers long. The melting point of these domains is around 200°C (400°F).

In TPU the hard, highly polar blocks of 4.4-phenylmethane diisocyanate or 2,4—2,6-toluene diisocyante extended with 1,4-butane diol form hydrogen-bonded crystalline or glassy domains and the soft elastomeric phase consisting of polyol segments.

TPU and TPE are polar materials and the choice of polymers for blending is limited to polar polymers such as PVC, ABS. and polyesters. Such blending improves abrasion resistance and flexibility and gives a product that is less expensive than pure thermoplastic elastomers. Because of their crystalline blocks both TPU and TPE materials show good resistance to fuels, oils, and similar products. TPU are characterized by exceptionally high tensile strength up to 50 MPa (7 ksi), large hysteresis, and stress softening.

Thermoplastic Elostomers by Polymer Blending. The most useful thermoplastic elastomers under this group are polyolefin blends of a crystalline polyolefin such as isotactic propylene or less frequently polyethylene (HDPE) with a polyolefin rubber such as an ethylene—propylene copolymer (EDR) or ethylene—propylenediene terpolymer (EPDM). The rubbery phase is slightly crosslinked during mixing to enhance elastic recovery. At higher temperatures, the crystalline phase melts and flow of the mixture becomes possible. Upon cooling, the crystalline phase rehardens and the original properties return. Other examples of this class of thermoplastic elastomers are butyl rubber—polyethylene blends and silicone—polyethylene blends.