Thermoplastic elastomers polycaprolactone

Among the polyurethane, polyester, and polyamide thermoplastic elastomers, those with polyether-based elastomer segments have better hydrolytic stabihty and low temperature flexibiUty, whereas polyester-based analogues are tougher and have the best oil resistance (43). Polycaprolactones and aUphatic polycarbonates, two special types of polyesters, are used to produce premium-grade polyurethanes (12). 

Most thermoplastic elastomers are stable materials and decompose only slowly under normal processing conditions. If decomposition does occur, the products are usuaHy not particularly ha2ardous and should not present a problem if good ventilation is provided. Extra caution should be exercised when processing polyurethanes, especiaHy those containing polycaprolactone segments. In these cases the decomposition products may include isocyanates and caprolactam, both of which are potential carcinogens. 

Diphenylmethane 4,4 -diisocyanate (MDI) and 2,4 toluene-diisocyanate (TDI) 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. 

In the case of the elastomers, formulations that use diisocyanates (O Fig. 14.16) and mixtures of long diols (oligomers of polybutadiene, polycaprolactone, or polaxyethylene) and short diols lead to thermoplastic elastomer behavior, with physical cross-links (rigid sequences) finding themselves associated, dispersed in micro-doinains, with a continuous elastomer phase (flexible sequences). 

Thermoplastic polyurethane elastomers have now been available for many years (and were described in the first edition of this book). The adipate polyester-based materials have outstanding abrasion and tear resistance as well as very good resistance to oils and oxidative degradation. The polyether-based materials are more noted for their resistance to hydrolysis and fungal attack. Rather specialised polymers based on polycaprolactone (Section 25.11) may be considered as premium grade materials with good all round properties. 

THERMOPLASTIC (LINEAR) POLYURETHANE ELASTOMERS 247 Calculation of equivalent weights Polycaprolactone... 

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