The term polyurethane indicates a vast family of thermosetting polymers in which the polymeric chain consists of urethane bonds -NH-(CO)-or-. Polyurethanes are fundamentally obtained by reaction between a diisocyanate (aromatic or aliphatic) and a polyol (typically a polyethylene glycol or polyester). Moreover, catalysts are added to improve the reaction output, as well as other additives which confer certain features to the material, in particular flame retardants and/or blowing agents (natural gas and/or water, which do not damage the ozone layer, are the most commonly used blowing agents for polyurethane production in Italy). Rigid foams are used to produce thermal insulation material sheets.
It is essentially an improvement on polyurethane (PUR). The percentage of methylene diphenyl diisocyanate (MDI) is higher compared to PUR and a polyol polyester is used in place of a polyol polyether in the reaction. The MDI and polyol reaction takes place at a higher temperature than the reaction temperature for PUR production. This polymer has a relatively strong molecular structure, due to the combination of strong chemical bonds. The higher bond force means that they are also more difficult to break, and consequently a PIR foam is chemically and thermally more stable compared to PUR.