Effect of silane adhesion promoters on the crosslinking kinetics of water-borne epoxy varnish systems and the mechanical strength of electrical steel laminates
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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. In order to improve the adhesion to the steel substrate hydroxy groups
are of utmost importance. Hence, 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.
The model formulations with different types and contents of silane AP were based on
a Bisphenol A diglycidyl ether with an epoxy equivalent weight of ~500, a Dicyandiamid
crosslinking agent and the poloxamer 407 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. The coatings were
cured to the partly crosslinked B-stage at a peak metal temperature of 200°C. Coated
steel sheets were stacked, laminated and finally cured at 190°C for 1h.
Rheokinetic measurements were performed to examine the crosslinking kinetics under
non-isothermal conditions. 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-conditioned of the coatings in A-stage revealed a
further reduction of curing onset associated with significantly higher initial viscosity.
This was a clear indication for pronounced pre-crosslinking in A-stage and hence,
limitation of the shelf life of silane-modified varnishes. The glass transition range in the
fully cured state was solely affected by diamine-based AP. Interestingly, the
investigated low molar mass silane APs had no significant positive effect on roll peel
strength of laminates.