Polyurethane Chemistry
Reaction between isocyanate and alcohol (polyol).
Crosslinking occurs between isocyanate groups (-NCO) and the polyol’s hydroxyl end-groups (-OH)
Thermoplastic PU (TPU) have some crosslinking, but purely by physical means. These bonds can be broken reversibly by raising the material’s temperature, as in molding or extrusion.
Ratio between the two give a range of properties between a flexible foam (some crosslinking) to a rigid urethane (high degree of crosslinking).
In PUR foams density can range from 1 lb/ft3 to 70 lb/ft3.
Foams are produced by chemical blowing agents.
Catalyst are used to initiate reaction.
RIM process is used to produce fenders and bumper covers