How can a thermoplastic elastomer be both thermoplastic and elastomer?

This month, polymer expert and technical blog author, Dr Charlie Geddes, takes a look at the unique properties and best uses of TPEs  

The class of polymers known as thermoplastic rubbers (TPR), or thermoplastic elastomers (TPE), describes pretty well what it says on the label. 

As thermoplastics these materials soften with heat and can be melt processed, by extrusion or injection moulding, but regain their soft solid properties when cooled.  As elastomers they conform to the ASTM definition of materials that “can be stretched repeatedly to twice their length and, on release of the stress, will recover, with force, to the original length”.

Most linear polymers above their glass transition temperature can be easily stretched but few have the ability to recover elastically. Traditional vulcanised rubbers achieve elastic recovery by inserting strong chemical bonds between the polymer chains (irreversible crosslinks) to create loose networks. Thermoplastic elastomers acquire recovery from reversible ‘physical’ crosslinks (domains, crystalline regions or ionic bonds), created by close association of the ‘hard’ parts of the polymer chains.

Although TPEs have been synthesised to mimic the properties (mechanical and chemical) of traditional vulcanised elastomers they perform less well at elevated temperatures, as one might expect.  However, the considerable savings in processing costs make them attractive in many applications. The real boost to TPE development came when designers discovered their ‘soft touch’ properties for hand held appliance such as electric shavers and keypads.

TPEs also struggle to match the compression set resistance and creep resistance of vulcanised elastomers.  I have witnessed the rejection of TPEs as replacement for vulcanised elastomers because they could not match the compression set resistance imposed in the specification, not because compression set had any relevance in the application but because it happened to be on the data sheet of the material being replaced.