Changes of the Fiber Morphology and Moisture Sorption Behavior of Transformer Kraft Paper

Hydrolysis can considerably contribute to the aging of cellulose, which severely affects the performance of oil-paper insulation. Herein, the relationship between the mesoscopic morphology and moisture sorption behavior of insulating paper was investigated. Laboratory-prepared insulating paper based on kraft pulp and aged insulating paper obtained from two out-ofservice 110 kV transformers were studied. Dried insulating paper with a high specific surface area was prepared using rewetting and critical point drying. Cold-field electron microscopy, dynamic vapor sorption, and nitrogen sorption were used to analyze the mesopores and moisture sorption properties. The average pore area ratio on the surface of each single fiber of swollen kraft pulp, dried insulating paper hand sheet, and aged insulating paper samples extracted by n-hexane were 20.02%, 2.95%, and 0.15%-1.34%, respectively. The pore sizes of the swollen and dried insulating paper were mainly distributed within 10 and 2-3 nm, respectively. The mesopores of the dried insulating paper fibers collapsed owing to the water surface tension, and the mesopores almost completely disappeared after aging. The EMCs of the insulating paper exhibited a negative linear correlation with the density at a relative humidity (RH) <20%. At RH>40%, the mesopores on the fiber surface provided more adsorption sites for water and became the main factor affecting moisture adsorption.