Interplay of Physical and Chemical Aging in Graphite/Epoxy Composites

Effects of pressure and temperature on time-dependent changes in physical/mechanical properties of graphite/epoxy composites were investigated. Samples from an eight-ply thick laminate of commercially used composites were cured, post-cured and then quenched to environments of various temperature and pressure. Time- dependent changes in their properties were analyzed by calorimetric and dynamic mechanical measurements. An increase in the glass transition temperature was found to occur at longer aging times. The rate of this process was enhanced by an increase in temperature and/or a decrease in pressure. An explanation was offered in terms of types and mechanisms of molecular events that occur in the glassy state. Physical aging, i.e. a time-dependent decrease in free volume (and enthalpy) takes place during the first stage of aging, but is not the sole mechanism responsible for the observed in crease in T g. Here, experimental results were checked against theoretical predictions based upon the free volume concept. After a certain period of time (which depends on T and P of the environment), however, chemical aging, i.e. additional crosslinking, ap pears to take place.