Rigid foams are highly crosslinked. A continuous belt process is used. The difference between rigid and flexible foams is the degree of crosslinking. A high degree of crosslinking is achieved by using relatively low molecular weight polyols for coupling with the isocyanate. Flexible foams are mainly based on polyethers.
Flexible foams use carbon dioxide gas.
Diphenylmethane diisocyanate is the preferred isocyanate for rigid foams on account of its low volatility. Other blowing agents besides carbon dioxide include freon and pentane. Both polyesters and polyethers can be used. Epoxies are crosslinked polymers in which the crosslinking is derived from reactions of the epoxy group. Bisphenol A precipitate is filtered off and washed with toluene to remove unreacted phenol.