Anna M Dlugaj, David May
The main drawback of sequential dual-curable off-stoichiometric epoxy-anhydride thermosets diluted with 1,6-bis-(2,3-epoxypropoxy)-hexane (HDDGE), which is low glass transition temperature after final curing at 150 °C, was overcome by using 1,4-bis-(2,3-epoxypropoxy)-butane (DD) as a difunctional aliphatic epoxy-based reactive diluent. In addition to this diluent, diglycidyl ether of bisphenol A, tetrahydromethylphthalic anhydride and 1-methylimidazole were mixed and used as epoxy, anhydride comonomer, and anionic initiator, respectively. The quasi-sequential curing behavior of off-stoichiometric formulations arises from two reactions: the anhydride-epoxy condensation (esterification) and epoxy homopolymerization (etherification). Although these reactions are designed to occur sequentially, the overlap in their activation temperature ranges leads to partial competition between them. The relative contribution of both curing stages, as well as precuring time, determine the changes in viscosity during the curing process. It is therefore difficult to precisely separate these two curing stages, which makes dual-curing formulations quasi-sequential. The final off-stoichiometric material showed a T g of 137.1 °C, determined from the peak temperature of the damping factor (tan δ) vs temperature curve obtained by Dynamic Mechanical Analysis, and the temperature at which the mass reduction of 5 wt % occurred under air atmosphere (T -5wt-%) was 365.6 °C, determined by Thermogravimetric Analysis. For comparison, the dual-curable off-stoichiometric epoxy-anhydride thermosets containing HDDGE showed a T g of 89.0 °C and a T -5wt-% of 343.3 °C. However, the use of DD as a diluent comes with the drawback that the developed off-stoichiometric formulations have a reduced mechanical strength of 65.5 MPa, which is 14% less than comparable off-stoichiometric thermosets containing HDDGE.