AGING AND YIELDING IN MODEL EPOXY THERMOSETS

The effect of physical ageing on the d.s.c. heat flow and yielding beh aviour has been investigated for two series of epoxy resins in which t he crosslink density is varied without significant compositional varia tion. At low degrees of undercooling, the extent of enthalpic ageing a pproached a thermodynamic limit with increased ageing time, but at lar ger degrees of undercooling, full densification was not achieved in th e time-scale of the experiments due to kinetic control of the ageing p rocess. At constant degree of undercooling, the relaxation enthalpy de creased as the crosslink density was raised, which is attributed to to pological restrictions on the process of segmental re-organization lea ding to densification. However, neither crosslink density nor amine na ture appeared to significantly affect the ageing kinetics when measure d at the same degree of undercooling. The influence of ageing on the r oom temperature yield stress was also investigated. Even though the yi eld stress increased with crosslink density, the influence of ageing o n the yield stress was reduced. The time-scales for enthalpic ageing a nd yield stress enhancement did not appear comparable. (C) 1998 Elsevi er Science Ltd. All rights reserved.