Polyurethane Rubber (Evergreen 10)

This polyurethane rubber is a softer and stretchier alternative to silicone rubber. However, whilst silicones are inert, urethane rubbers tend to discolour over time. This sample used to be off-white, but has turned a sludgy brown as a result of exposure to UV light and moisture. Silicones are generally thought to have lower toxicity 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.