Hereunder is presented the report concerning the experimental studies made of the effect of ultraviolet rays on the electric properties of epoxy resin, and discussion is made on how such an effect has been brought about.
Exposure of specimens to radiation of ultraviolet rays consists of two processes, their exposure in the fademeter and their natural aging. Natural aging of epoxy resin is carried out outdoors by fixing its specimens on the exposure stand at the angle of 45°facing south under glass cover. Three types of epoxy resin specimens have been used, the bisphenol type, the novolac type and the cycloaliphatic type. The specimens have also been tested with respect to their electric properties, such as insulation resistance and arc resistance, and with respect also to their mechanical properties from the practical point of view as electric insulation materials.
The results of these experiments are as follows.
(1) Exposure of the specimens to the radiation of ultraviolet rays in the fademeter has had an effect of reducing their insulation resistance, as shown in the declining curves, whether gradually or abruptly depending on the lapse of exposure time. The decrease in insuration resistance is comparatively smaller with specimens of the cyclialiphatic type epoxy resin.
(2) Exposure of the specimens to the radiation of ultraviolet rays in the fademeter has had an effect of reducing their arc resistance. With specimens of the bisphenol type epoxy resin, arc resistance either drops rapidly for the initial 1000 hours or less, and then remains nearly constant, or it gradually falls all through. With specimens of the cyclialiphatic type epoxy resin, arc resistance is higher than it is the case with specimens of the bisphenol type epoxy resin. and there is but little drop of arc resistance all through in spite of the longer lapse of exposure time. It is considered, as the result of comparative experimental studies, that the discoloring effect on specimens by natural aging outdoors for 8 months is nearly equal to that by their exposure to radiation of ultraviolet rays in the fademeter for beween 700 and 1000 hours. It appears that arc resistance has little to do with discoloration of specimens.
(3) So far as practical industry is concerned the cyclialiphatic type epoxy resin is considered to be the most efficient electric insulation material. If the bisphenol type epoxy is preferred for the purpose, the use of DAT, HHPA or DDSA is recommended as efficientresistor to ultraviolet rays.