Polyurethanes are versatile materials that are widely used as engineering thermoplastics, elastomers, coatings and adhesives. They generally have higher tensile strength than natural or synthetic rubbers and higher elongation than nylons or polyesters, and, depending on the composition, can span a hardness range from very soft, flexible elastomers to strong, hard castings. The morphology of a polyurethane, a hard segment chemically linked to a soft segment, and the thermodynamic phase separation between the hard and soft segments leads to many of the outstanding mechanical properties. The hard segments act as physical or chemical crosslinks and give the material strength and stiffness, while the soft segments impart elastomer properties such as rebound and flexibility.
Thermoplastic polyurethanes (TPUs) make up a small segment of the polyurethane market but are among the highest-performing materials. They are linear, segmented block copolymers and are typically synthesized via condensation polymerization of a di-isocyanate (TDI or MDI), a diol chain extender (such as butane diol) and a longer-chain polyol (polyester or polyether). The di-isocyanate plus the chain extender make up the hard block and the polyol makes up the soft block.1 These materials are true thermoplastics in that they can be dissolved in solvents, melted, reformed and reshaped multiple times.