Formation and Curing of Rigid Polyurethane Foam

PUR are a broad class of highly cross-linked plastics prepared by multiple additions of poly-functional hydroxyl or amino compounds. Typical reactants are polyisocyanates [toluene diisocyanate (TDI)] and polyhydroxyl molecules such as polyols, glycols, polyesters, and polyethers. The cyanate group can also combine with water this reaction is the basis for hardening of the one-part foam formulations. 

Most rigid polyurethane foams have a closed cell structure. Closed cells form when the plastic cell walls remain intact during the expansion process and are not ruptured by the increasing cell pressure. Depending on the blowing process a small fraction (5-10%) of the cells remain open. Closed cell structures provide rigidity and obstruct gaseous or fluid diffusional processes. 

One component formulation consists of prepolymers that are intermediate between monomers and the final polymer product. When released from a pressurized container the foaming gas expands and the prepolymer (containing unreacted cyanate groups) reacts with the moisture (water) in air to complete the polymerization reaction and cure. Because curing depends on the presence of moisture, when foam forming reactants are applied to occluded areas, such as cavities, 

Processing of rigid foams from two part formulations involves combining measured quantities of the polyisocyanate with a polyhydroxyl such that there are no or limited reactive isocyanate functional groups. Moisture is not required to complete the cure. Once the reactants are combined the mixture is poured into a form where expansion and polymerization take place simultaneously. Cure times are usually very fast, on the order of minutes. 

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