Dielectric loss features of some epoxy resins

Abstract At temperatures far below the heat distortion point, the dissipation factor (and therefore power factor) of epoxy resins is comparably low—in the order of 0.001–0.01. Internally flexibilized, polyester‐modified resins may have higher loss tangents, in the order of 0.01–0.10, even at room temperature. At temperatures far above the heat distortion point, anhydride curing agents give lower dissipation factors than do amines or acids. With anhydride cures, the higher molecular weight resins investigated cure faster than the lower molecular weight resins, give lower elevated temperature power factors and loss tangents, and are tougher. With any hardener–epoxy resin system, the optimum elevated temperature power factor, weight loss on aging and heat distortion point will probably depend on a close examination of the optimum hardener/resin ratio. With certain anhydride‐cured low molecular weight epoxy resin systems, an increased rate of cure and lower elevated temperature power factor may be obtained by use of a small amount of higher molecular weight (polyhydroxy) epoxy resin. Attempts to compare the effect on power factor of changing the hydroxyl groups to less mobile dipoles by acetylation were unsuccessful, since the resulting acetoxy groups changed the cure mechanisms. However studies with resins of different molecular weights indicated that the effect of hydroxyl content on elevated temperature power factor is subordinate to other effects such as the rigidity of the cured resin system, rate of cure and ionic impurities. Acetylation of hydroxyl‐containing epoxy resins may inhibit cure with some curing agents, due to side reactions of the resulting acetoxy group with specific hardeners.