Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 179

Fig. 4.11 Time-temperature-transformation diagram of a curing system. (Reproduced from Enns
and Gillham [ 37 ] with permission of Wiley)
E 1 . For an epoxy-amine reaction, this conjecture is supported by density functional
theory calculations [ 36 ]. They suggest that the straight bimolecular reaction be-
tween epoxy and amine has a rather large barrier of ~ 110 kJ mol − 1 . A markedly
lower barrier (~ 85 kJ mol − 1 ) is estimated for the “self-promoted” reaction that in-
volves two amine molecules, of which one forms a hydrogen bond with the epoxide
oxygen and another participates in nucleophilic attack on the epoxide carbon. In
terms of the aforementioned models (Eq. 4.22 or 4.24), this would be the uncata-
lyzed reaction promoted by a hydrogen-bond donor molecule. On the other hand,
the catalyzed reaction that involves the formation of a hydrogen bond between the
epoxide oxygen and the hydroxyl group (product of reaction 4.18) followed by
nucleophilic addition of the amine to the epoxy carbon has the energy barrier of
~ 70 kJ mol − 1 . The fact that E 1 is generally larger than E 2 suggests that epoxy-amine
curing should be characterized commonly by decreasing E ʱ dependencies similar
to that shown in Fig. 4.10 . However, this is not always the case and many of these
reactions barely show any variation in E ʱ in their initial stages.
As curing progresses to more advanced stages, the reaction medium undergoes
two important microscopic changes: gelation and vitrification. The behavior of a
curing system can be understood in the frameworks of a time-temperature-trans-
formation (TTT) cure diagram [ 37 ] shown in Fig. 4.11 . Gelation takes place when