Rates and Chemistry of Cure Reactions

The reaction rates of the three principal cure reactions of TGDDM-DDS epoxies are as follows  

The rate constants for the S.A.-E and E-OH reactions are 10 times slower than the rate constant for the P. A.-E reaction. From FTIR and DSC studies Moacanin et al. 64) report the E-OH reaction is 10 times slower than the P.A.-E reaction at 177 °C. We previously mentioned that the S.A.-E rate constant for aromatic amines similar to DDS, such as methylene dianiline have been reported to be 7-12 times slower than the P.A.-E reaction 66 . 

In Fig. 20a plot of the % of epoxides consumed by each cure reaction vs. total percent epoxide consumed for a TGDDM-DDS (25 wt.-% DDS) epoxy cured at 177 °C is illustrated. The P.A.-E reaction dominates the early stages of cure, until all the P.A. is depleted. The E-OH reaction dominates the later stages of cure. 

-E reaction is illustrated in Fig. 21, with the sites for further cure reactions via the (i) E-OH and (ii) S.A.-E reactions indicated by arrows. The E-OH and S.A.-E reactions can occur (a) intermolecularly to form crosslinks or (b) intramolecularly 

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Propenal and the S.A. group formation must form from decomposition of the P.A.-E, S.A.-E, and E-OH cure reaction products. However, because we observe the S.A. group formation is most prevalent for low DDS concentration TGDDM-DDS epoxies, which contain a higher concentration of E-OH cure reaction products, then we suggest propenal and S.A. group formation most likely occurs by decomposition of non-cyclized (10) and cyclized (11) E-OH reaction products, (see next Sect. Rates and Chemistry of Cure Reactions)... 

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