Chlorofluorocarbonds (CFCs) were representative physical blowing agents in the foam industry for over 30 years. Trichloromonofluromethane (CCl3F),(CFC-11), was the ideal physical blowing agent for both flexible and rigid foams. The advantages included nonflammability, non toxicity, good compatibility with foaming ingredients, low molecular weight and moderate boiling point. It was used extensively for flexible and rigid polyurethane foams. In flexible foam production, CFC-11 reduced reaction exotherm and made possible to produce large slabstocks in low densities. CFC-11 blown rigid foams had excellent thermal insulation properties. However, because of the ODP of CFCs and HCFCs in stratosphere, they were phased out from the industries. HFCs were one of the major candidates for the third generation blowing agents. Because of high global warming potential (GWP), the use of them was restricted to some extent. The cost of HFCs also limited their applications. Some PFCs were also proposed but they were banned because of the ODP. HFCEs showed
that they have better compatibility with polyols during the foaming process. Liquid CO2 appeared as a blowing agent in the 80s. this blowing agent has zero ODP, low GWP and low cost. It is used to produce very low-density, flexible slabstock foams.