Polyurethane Rubber (Evergreen 20)

This polyurethane rubber is a softer and stretchier alternative to silicone rubber. However, whilst silicones are inert, urethane rubbers tend to discolour as a result of exposure to UV light and moisture - this sample used to be blue. Silicone rubbers are generally less toxic than urethane rubbers, but the Evergreen range of urethane rubbers (now discontinued) was so-called because it used no suspected carcinogens or substances harmful to human health (mercury, isocyanate, etc) in its formulation. Regardless of the end-product or even type of polyurethane you are using, the way in which you use polyurethane resins is always the same: the raw materials come in two parts, a liquid isocyanate (a very reactive chemical derived from petroleum) and a polyol (a long-chain alcohol), which are mixed to a specified ratio to create a reacting liquid, which is then poured into a mould or onto a surface and left until it cures to form a solid object. This relatively low-tech process makes it a useful material for tinkering and making prototypes.