(2) TPE Properties and Development Trends

   Thermoplastic polyurethane elastomers (TPEs), whether blends of two or more polymers or manufactured by polymerization, have a microphase separated structure consisting of rubbery soft segments and hard segments. A hard segment exhibits plasticity at high temperatures; but when it cools and becomes hardened, it constrains the soft segment and causes the material to exhibit the elastic response of rubber.

   Generally, TPEs get their names and classifications, such as TPO or TPS, based on the polymer used in the hard segments.

   The advantages of TPEs for automotive parts depend on what they are being used to replace. If a part was originally vulcanized rubber, the plasticity of TPEs results in higher productivity, energy saving, and recyclability. If they are replacing general-purpose thermoplastic resins such as polypropylene (PP) or polyethylene (PE), their advantage is in the rubber-like flexibility. Accordingly, TPEs are generally positioned as materials occupying the middle ground between rubber and resins. Typical TPEs used for automotive parts are shown in Table 1 along with their properties and issues. Major applications of each type of TPE in automotive parts are shown in Table 2.

   The chief reasons for adoption of TPEs in automotive parts can be classified broadly into the four given in (1) to (4) below.

1. （1）As a replacement for vulcanized rubber: Productivity, low cost, design freedom, light weight, recyclability (environmental measure)
   Applications: Weather stripping, boots of various kinds (for constant-velocity joints, rack & pinion mechanisms, etc.), glass run channels
2. （2）As a replacement for soft resins (flexible PVC, etc.): Light weight, flexibility, recyclability, non-fogging
   Applications: Interior covering on instrument panel, door trim, etc.
3. （3）As a replacement for thermosetting resin: Safety, flexibility, tactile impression
   Applications: Assist handles, knobs
4. （4）As new parts requiring soft materials: Flexibility, safety, cold resistance
   Applications: Airbag covers (driver, passenger, curtain, knee airbags, etc.)

   The discussion below focuses on TPO and TPS, for which high demand in automotive applications is expected to continue.

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