The effects of molecular structure on the thermochemical properties of phenolics and related polymers

Abstract

Phenolic resins and related polymers have been synthesized by established methods to provide different types of char-forming polymers for pyrolysis studies.Certain features of the molecular structure of these polymers which determine the char yield have been characterized.Pyrolysis studies have been carried out under both thermogravimetric conditions and in environments simulating reentry ablation to relate the char yields of these polymers to their molecular structure.It has been possible to account for the char-forming reactions of these phenolic derived resins in terms of a general pyrolysis mechanism for these classes of polymers.An analysis based on this mechanism has been developed which predicts f r o m molecular structure the observed thermogravimetric char yields accurately over a range from 11 to 65 percent yield.makes it possible to control the primary rates of vapor to char production as required f o r particular heat-shield applications. This analysisComposites have been developed from these polymers and have been tested under one-dimensional pyrolysis simulating reentry ablation.Up to applied heating rates of about 23 cal cm-2sec-1, the observed char yields agree quite closely with those obtained in thermogravimetric tests.It is tentatively concluded that the primary char-forming processes with these classes of materials are independent of polymer heating rate over a range of temperature rise rates varying from 2O to 5000' C per minute.heating rates greater than 23 cal em-2sec-l an additional amount of char is formed due to secondary pyrolysis of vapor products within the char matrix induced by high char temperatures.These secondary processes seem to depend on surface temperature, impact pressure, and the chemical nature of the vapor products.