Effect of elevated temperatures on the strength of cured resins and resin-based materials

Measurement of the loss of weight and ultimate compressive strength of phenol-formaldehyde resins under the influence of elevated temperatures has shown that during thermal degradation there is an abrupt transition from a first to a second and third structural stage with steadily increasing stabilization of the strength properties. Apart from the dependence on the heat treatment conditions, all the resins investigated were found to give a straight-line relation between the relative change in ultimate compressive strength and the relative change in weight for each structural stage. The investigation was conducted on standard specimens obtained by molding a mixture of hardening resin and the same resin in precured powdered form.