11 - Thermoplastic Polyether Ester Elastomers

Abstract

Thermoplastic polyester elastomers are multi-block copolymers, which can be represented by a generalized formula (–A–B–)n. They are essentially copolyether esters with alternating, random-length sequences of either long-chain or short-chain oxyalkalene glycols connected by ester linkages. They also contain repeating high melting blocks that are capable of crystallization (hard segments) and amorphous blocks with a relatively low glass transition temperature (soft segments). Typically, the hard segments are composed of multiple short chain ester units such as tetramethylene terephtalate units and the soft segments are derived from aliphatic polyether and polyester glycols. At useful service temperatures these materials resist deformation because of the presence of a network of microcrystallites formed by partial crystallization of hard segments. These microcrystallites function as physical cross-links. At processing temperatures the crystallites melt and form a viscous melt that can be shaped by usual melt-processing methods. After cooling the hard segments recrystalize and their shape. The ratio of hard to soft segments determines the characteristics of the product. Thus, the products range from soft elastomers to hard elastoplastics. Copolyester elastomers can be processed by injection molding, compression and transfer molding, blow molding, and rotational molding on standard thermoplastic processing equipment using mostly conventional techniques. They can be readily extruded and calendered.