Acid‐ and alkali‐catalyzed tannin‐based rigid foams

Abstract Acid‐ and alkali‐catalyzed polyflavonoid tannin‐based rigid foams were prepared. These foams have comparable physical and mechanical properties to the synthetic phenolic rigid foam used as a comparative standard. The fluid polymer phase was based on a mimosa tannin‐formaldehyde resin with a minor addition of a fortifier resin. Expansion of the fluid phase was brought about by a physical blowing agent, whereas dimensional stabilization was achieved through cross‐linking at the desired density. In the case of the acid‐catalyzed foam, a heat‐generating agent in the form of furfuryl alcohol was employed. The polymer composition of tannin–formaldehyde/urea–formaldehyde systems as a function of pH was predicted from the respective gel times and rate constants, i.e., above pH 7, the copolymer proportion will tend to 100% and, that at pH 3.4, the polymer blend proportion will tend to a maximum. © 1994 John Wiley & Sons, Inc.