Sequential Diluted Anhydride‐Epoxy Thermosets: Influence of Slow Dual‐Curing Behavior on Thermal and Mechanical Properties

ABSTRACT This article reports the development and characterization of sequential dual‐curable thermosets based on off‐stoichiometric diluted anhydride‐epoxy formulations containing and not containing BYK‐A 530 (BYK) as an air release additive in the presence of 1‐methylimidazole (IMI) as an initiator. An mixture of diglycidyl ether of bisphenol A (epoxy resin) and 1,6‐bis(2,3‐epoxypropoxy)hexane (diluent) (DG) and tetrahydromethylphthalic anhydride (ANHY) is used in order to achieve low viscosity and long pot life at room temperature. The combination of anhydride‐epoxy condensation (esterification) and epoxy homopolymerization (etherification) as two competing reactions enables sequentiality, and their relative contribution, adjusted by the amount of ANHY in the formulation, determines the network structure build‐up and changes in viscosity. The results show that these changes also depend on pre‐curing conditions and absolute mass sample. Differential scanning calorimetry (DSC) of thermosets reveals an additional exothermic region above 176°C, which requires higher final curing temperatures and causes difficulties in post‐curing compared to conventional thermosets. The glass transition temperature of these thermosets, determined by dynamic mechanical analysis (DMA), is therefore approximately 90°C, which is ≈35°C lower than that of conventional thermosets. The dual‐curable thermosets show the tensile strength of approximately 72.6 MPa, which is slightly lower than that of conventional ones (78.9 MPa).