Lifetime assessment of epoxies by the kinetics of thermal degradation

Abstract

ABSTRACT:The kinetics of thermal degradation of 4,4Ј-diaminodiphenylsulfone (DDS)-cured glycidylethers (DGBA) and glycidylamines (TGDDM) have been studied by isothermal annealing. Thermal degradation proceeds via first-order kinetics with activation energies of 138 (DGBA/DDS) and 95 kJ/mol (TGDDM/DDS). From these data the temperaturedependent lifetime can be calculated. Based on the heat distortion temperature and 20 years' service as criteria, the upper service temperature is found to be around 115-120°C.In spite of the different activation energies and bond stabilities, only marginal differences in the upper allowed service temperatures between these resins were found. Only slight improvements can be expected by optimizing the cure conditions.

Theoretical evaluation and the effect of thermal degradation on the Time–Temperature–Transformation (TTT) diagram of bisphenol A epoxies

“…For the system discussed here (difunctional resin and tetrafunctional curing agent at stoichiometric equivalence), it is obtained from statistics by Finally, according to ref. 12, the thermal degradation can be considered as a reversal of curing, its kinetics proceeds via a first‐order mechanism and can be characterized by where p t is the actual glass temperature after some degradation time t and p i is the initial degree of cure as obtained from eqs. (1)–(3).…”

“…All data used for calculation are reported elsewhere9, 12; they are as follows: k = 6.7E5 (min −1 ) E A 1 = 64 (kJ/mol) k = 4.7E5 (min −1 ) E A 2 = 59 (kJ/mol) E s = 0.3 (kJ/mol) T g ∞ = 383 (K) K 1 = 117 (K) K 2 = .777 k = 6.89E9 (min −1 ) E A ( d ) = 138 (kJ/mol) …”

“…Finally, according to ref. 12, the thermal degradation can be considered as a reversal of curing, its kinetics proceeds via a first-order mechanism and can be characterized by…”

“…The data for thermal degradation were obtained following the procedure reported in ref. 12. All data used for calculation are reported elsewhere 9,12 ; they are as follows:…”

“…So, since appropriate theoretical models for both the kinetics of curing9–11 and thermal degradation12 are available, it seems more promising to calculate all data, i.e., the times to gel and to vitrify, and that of devitrification, from an independent basis, and to compare them with experimental data.…”