Polyurethanes are generated by poly-addition of isocyanate and polyoles. These components can be mixed to make 2-component resins. In general, the polymers crosslink during the reaction so that further thermoplastic processes are rules out. However, there is also non-crosslinked polyurethane of the group of thermoplastic elastomers (TPE), which is suitable for thermoplastic processing; the elastic properties range from rigid to jelly-like. Isoyanate reacts with water to release CO2, which is used for easy foaming. Hard PUR-foams have a brittle character. They are widely used as insulation foams and as core materials in sandwich composite components.