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The solubility equilibriums of formic, acetic, levulinic, stearic and naphthenic acids in oil-paper (Kraft) insulation systems have been studied using titration techniques. Temperatures and moisture levels were varied. Low molecular weight carboxylic acids strongly favour the cellulose while the heavier stearic and naphthenic acids favour the oil. Paper moisture did not influence solubility in a systematic way. The presence of low molecular acids increases the water content of the oil. Oil and paper from service aged transformers, show that typically 10-15% of the oil acidity stems from low molecular weight, hydrophilic acids. The concentration of the same type of acids is a hundred-fold higher in the cellulose. Thus, most of the total acidity in the solid insulation consists of hydrophilic acids, while they constitute only a minor part of the acidity in the oil. The methods used here for extraction of low molecular weight acids are proposed as an addendum to international standards for determination of oil acidity.
Ingebrigtsen et al. (Thu,) studied this question.