Effect of adhesion promoters on rheokinetics and roll peel strength of electrical steel stacks

Epoxy varnishes for stacked electrical steel are of high relevance for renewable energy and electric mobility technologies. Water-borne epoxy varnish systems are still under development, especially with regards to the improvement of the adherence to the steel substrate. The main objective of this study was to assess the potential of low molar mass, silane-based adhesion promoters (AP) with amine or glycidyl functional groups as to their effect on the crosslinking kinetics of coatings and the mechanical performance of electrical steel laminates. Model formulations with different types and contents of silane AP or in combination with a pre-crosslinker were based on a Bisphenol A diglycidyl ether, a dicyandiamide (DICY) crosslinking agent and an emulsifier. The amount of water was fixed to 45 m%. Model varnishes were prepared by shear emulsification at elevated temperatures. Varnishes were applied onto electrical steel sheets and partly crosslinked to the B-stage. Finally, coated steel sheets were stacked and laminated in a heated press. To examine the crosslinking kinetics under non-isothermal conditions, rheokinetic measurements were performed. While the curing onset temperature of formulations with amine-based AP dropped by ∼10 °C, the effect of the epoxysilane AP was less pronounced (drop by ∼5 °C). Pre-conditioning of the varnishes in A-stage revealed a further reduction of the curing onset temperature associated with significantly higher initial viscosity. This was a clear indication for pronounced pre-crosslinking in A-stage and hence, a limited shelf life of silane-modified varnishes. The glass transition range in the fully cured state was solely affected by the diamine-based AP. Interestingly, the investigated low molar mass silane APs had no significant positive effect on the roll peel strength of the laminates.