Any of a class of very versatile polymers that are made into flexible and rigid foams, fibres, elastomers (elastic polymers), surface coatings, and adhesives. They are produced by reacting a diisocyanate (a compound with two functional groups of the type NCO) with a diol (a compound with two hydroxyl, or OH, groups). Foamed polyurethanes, used for insulation and mattress and upholstery filler, are made with organic compounds containing carboxyl groups, causing a reaction that liberates carbon dioxide bubbles throughout the product. Spandex fibres are highly elastic and have replaced natural and synthetic rubber fibres for many textile purposes. Polyurethane elastomers are made into auto parts, rollers, flexible molds, medical equipment, and shoe soles. Polyurethane surface coatings are applied as sealants to wood, concrete, and machine parts and as linings for tanks and pipes; moisture-curing polyurethane resin is used as a general-purpose waterproof glue.