Urethane-Based IPN Foams

These foams include isocyanurate-modified rigid urethane foams, isocyanurate-modified flexible urethane foams, urethane-oxazolidone foams, urethane-based IPN foams, and urethane-based hybrid foams. 

This method, however, did not result in improved flame retardance because the isocyanurate content was too low. In recent years, however, high-functionality and low-viscosity polyether polyols have become available, and therefore the above method has become less important. Even so, the method was sometimes used to make foams having increased crosslink density to improve dimensional stability or chemical resistance. Therefore, a higher isocyanate index, e.g., 150 to 200, was used to incorporate isocyanurate linkages in rigid urethane foams. 

It should be noted that the incorporation of isocyanurate linkages in small quantity does not improve high-temperature resistance nor flame retardance of the resulting foam. Urethane-modified isocyanurate foams which have isocyanate indices of more than 300 show outstanding high-temperature resistance and flame retardance (71). These foams will be discussed in the next section. 

Urethane-Based IPN Foams. Interpenetrating polymer networks (IPNs) are types of polymer alloys composed of the entanglement of at least two cross-linked components (112). An ideal IPN has essentially no covalent bonds between the polymers. The resulting morphology shows 

The first IPN resin was invented by Aylsworth in 1914 (51). The mixture of rubber, sulfur, phenol and formaldehyde, on heating, yields a simultaneous interpenetrating polymer network 

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