Effect of CaCO3 modified epoxy varnishes on curing kinetics and mechanical performance of electrical steel laminates

Epoxy coatings for stacked electrical steel are of high relevance for renewable energy and electric mobility technologies. Waterborne epoxy varnish systems for electrical steel laminates are still under development. The main objective of this study was to assess the effect of fillers on the crosslinking kinetics of epoxies and the mechanical performance of electrical steel laminates. Model varnishes based on bisphenol-A diglycidyl ether (DGEBA) with an epoxy equivalent weight (EEW) of ~ 500 g/mol were modified with CaCO₃ fillers. The filler content was ranging from 1 to 20 wt%. The onset of gelation for CaCO₃ modified epoxy varnishes was reduced by up to 5 °C. This effect was primarily attributed to enhanced thermal conductivity and reduced heat capacity. Interestingly, no significant effect on the glass transition temperature of the fully cured epoxy was observable. By mechanical testing of electrical epoxy laminates better roll peel strength values were deduced for laminates with CaCO₃ modified epoxies. Moreover, crack growth rates in the stable regime and the threshold strain energy release rate were positively affected.