Studies on Biological Treatment of Mechanical Pulps (IX)

Abstract

To investigate the effect of phenolic hydroxyl groups in mechanical pulps on depression of light-induced color reversion of mechanical pulps, thermomechanical pulps (TMP) and groundwood pulps (GP) were acetylated, and examined to what extent the color reversion of the pulps were prevented by the acetylation. When about 11% of the phenolic hydroxyl groups in unbleached GP (UGP) and unbleached TMP (UTMP) were acetylated, the color reversion of the pulps was completely depressed. On the contrary, the color reversions of bleached GP (BGP) and bleached TMP (BTMP) were completely depressed when about 16% and 29% of the phenolic hydroxyl groups in the BGP and BTMP, respectively, were acetylated. The differences in the depression of the color reversion between the unbleached pulps and the bleached pulps were estimated as the differences in the degree of changes in lignins formed by the different pulping conditions, and as the differences in the degree of formation of quinones and hydroquinones, chromophore and leuco-chromophore responsible for the color reversion, respectively, generated during hydrogen peroxide bleaching of the unbleached pulps. When the acetylated pulps were reduced with potassium borohydride accompanied with phase transfer catalyst (PTC), the effects of the depression of the color reversion were lowered in comparison with those of the color reversion in the acetylated pulps. It was thought that a part of deacetylation of the acetyl groups in the pulps was conducted during the reduction.On the basis of the results, the phenolic hydroxyl groups in mechanical pulps seems to contribute mainly to the depression of the color reversionof mechanical pulps.