Insulating paper

About forty organic compounds and also sulfur were tested by Berberich and Friedman¹ for effectiveness as stabilizers of chlorinated diphenyl in paper capacitors. Small percentages of each of these materials were dissolved in Aroclor 1254 and capacitors were impregnated with these solutions. In these tests, the impregnated units were kept at 85°C or 100°C and a d-c voltage of 1000 volts per mil was impressed on them. The time required for the units to fail was recorded and this was used as. the criterion of stabilizing effect of the compounds. It was found that among the compounds tested, those which were most effective in prolonging the life of capacitors were azobenzene, azoxybenzene, anthroquinone, benzil, dinitrobenzil, O-nitrodiphenyl, dinitrotoluene and sulfur. The stabilizing effect of several of these compounds on chlorinated diphenyl was also studied in a conductivity cell thus eliminating paper from the system. It was found that when chlorinated diphenyl was saturated with HCl, its specific conductance increased almost 100-fold over that of the pure material. The addition of aluminum to the HCl saturated solution caused the conductance to rise sharply again and when it had increased another 120-fold, some. stabilizer was added to the solution. A small decrease in conductance resulted from this initially, but in a short time, it again increased but at a much slower rate than that prior to the addition of stabilizer. Other experiments were carried out in which aluminum foil was boiled in carbon tetrachloride with and without stabilizers present. The stabilizers produced a marked decrease in the rate of attack on the aluminum. Also in one experiment, aluminum foil was pretreated with benzene solutions of the stabilizers and it was then boiled in carbon tetrachloride where it rapidly dissolved. From these experimental observations, the authors have evolved a theory of the mechanism of stabilization of chlorinated impregnants in aluminum foil capacitors. They contend that chlorinated diphenyl decomposes in a d-c field yielding HCl at a certain rate. The HCl in turn reacts with aluminum to form aluminum chloride which hastens the decomposition of the chlorinated diphenyl. Thus, the series of reactions is autocatalytic. The role of the stabilizer, according to the authors is to interrupt the cycle by forming a complex with the aluminum chloride and inactivating it.