The effect of free dihydroxydiphenylmethanes on the thermal stability of novolac resin

Phenolic resins (PRs) play an important role in aerospace due to their low cost, outstanding chemical resistance, mechanical properties, thermal properties and ablative properties. However, the industrially produced PRs are often mixtures of polyphenolic compounds with different structures and molecular weights, among which the low molecular weight components are detrimental to the properties of PRs, especially the thermal stability. In this paper, the dihydroxydiphenylmethane (DHDM) contents in several typical industrial novolac resins (NRs) were measured by liquid chromatography and gel permeation chromatography, and the effect of DHDM content on the viscosity, curing process, glass transition temperature (T_g) and thermal stability of NRs was investigated. The results showed that a significant reduction in viscosity and T_g was observed for the NRs with higher DHDM content. The existence of DHDMs reduced the gel point and crosslinking density of the cured NRs by hexamethylenetetramine, leading to a significant decrease in the 5% weight loss temperature, the maximum weight loss temperature, as well as the char yield at 800 °C. Therefore, in addition to the free phenols, we should pay more attention to the DHDMs existed in the NRs. This study provides an important insight for NR manufacturers that the DHDM contents should be reduced through the proper control of process parameters to further expand their wider application scope.