Reaction thermodynamics of amine-cured epoxy systems: Validation of the enthalpy and heat capacity of reaction as determined by modulated temperature differential scanning calorimetry

The reaction enthalpy and reaction heat capacity of three aromatic epoxyamine systems have been determined with modulated temperature diffential scanning calorimetry (MTDSC), mostly in quasi-isothermal conditions, over a wide temperature range (33-140 degreesC) and for different mixture compositions. The reaction enthalpy is only slightly dependent on the epoxy-amine chemistry, from -111 to -98 kJ/mol epoxy functionality. With the model system phenyl glycidyl ether (PGE)+ aniline, the reaction enthalpy of the secondary amine-epoxy reaction step is equal to that of the primary amine-epoxy reaction. Group contributions needed to calculate the reaction heat capacity with an additivity approach are evaluated, and a new value of 37.2 J mol(-1) K-1 for the group N--H)(C)(CB) is proposed. With this group contribution, the additivity method predicts almost equal values for the reaction heat capacity of both amine-epoxy reaction steps at 298.15 K (Delta(r)C(p,prim) = 15.7 J mol(-1) K-1 and Delta(r)C(p,sec) = 14.6 J mol(-1) K-1), whereas the experimental value of Delta(r)C(p,sec) is about three times larger than that of Delta(r)C(p,prim) at 100 degreesC. These results are confirmed experimentally for PGE+aniline as a different temperature dependence of both reaction heat capacities. MTDSC therefore is potentially interesting for differentiating between reactive species in an epoxy-amine reaction, a benefit previously assigned to spectroscopic methods only. (C) 2003 Wiley Periodicals, Inc.