Cationic photocurable epoxy compositions with metallic filler: Influence of monomers and photoinitiators on electrical conductivity

Abstract

Cationic curable epoxy formulations with metallic fillers are suitable for electrically conductive photocurable systems, due to the shadow cure mechanism efficiently curing material hidden from irradiation between the filler grains. The goal of this study was to examine how the electrical resistivity of the material depends on monomer combinations and photoinitiators (PIs) used, with an emphasis on obtaining low resistivity materials from low viscosity mixtures. A dendritic, silver-coated copper filler was dispersed in mixtures of cycloaliphatic epoxy, oxetanes, and an epoxy reactive diluent with two different PIs. Rheology of the mixtures was studied to distinguish between the influence of the geometric configuration of the filler grains and that of electric contact resistance between them. Temperature scans were run to understand the influence of thermal expansion and glass transition on the resistivity. It was shown that in the intermediate range of the filler content value, between 30 % and 50 %, the resistivity strongly varies with the species and the combination of the monomers used, the PI type, and its concentration. Large part of this variability originates from the contact resistance between the filler grains. Hypothetic mechanisms of variability of the contact resistance were defined, based on chemical and thermal shrinkage in the photocuring, and its possible variability along the path of the curing front propagating between the filler grains. It explains how the effects may emerge but does not tell why different characteristics of the particular PIs used directly influence the effect. Additional experiments were proposed to gather insight into this ambiguity.