The influence of particular barriers on treeing in polyester resin

The changes in treeing behavior brought about by the introduction of a layer of aluminum oxide filling agent in an otherwise unfilled sample of polyester resin have been investigated. The initial observation that increasing the particle size increases the lifetime of samples with aluminum oxide barriers suggested a direct influence of particle size. However, polariscope observations showed increasing mechanical strain in the barrier region with increasing particle diameter. It is considered, therefore, that the particle size is not intrinsically the controlling factor in determining lifetime but rather the associated mechanical strain, which is generated around particles when shrinking occurs during curing of the resin. The strain is greatest when the work of adhesion is high. Reduction of a high work of adhesion by oiling the particles reduces the associated mechanical strain, and this indirectly produces the short lifetime. When aluminum oxide was replaced by PTFE the reduction in work of adhesion again resulted in lower mechanical strain and a consequent reduction in lifetime