Abstract
A mathematical model is proposed to calculate the expansion coefficient of the elementary volume of intumescent polymer material at an arbitrary change of temperature. It implies that parallel reactions of cure and gas formation take place simultaneously in the volume of the polymer composition. It was assumed that after attainment of some threshold values of the composition viscosity the gas released may be retained in the foam. Influence of the rate of heating, composition of the blend, and functionalities of the initial monomers on the effectiveness of foaming was investigated for the system phenol—formaldehyde resin with boron oxide. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 329–338, 1997