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Chemical modification of lignin-rich paper

The effect of two different aging conditions (e.g., 105°C and 80"C, 65% relative humidity) on the heat-induced yellowing of dithionite-bleached and hydrogen-peroxide-bleached spruce thermomechanical pulp (TMP) and hydrogen-peroxidebleached spruce and aspen chemithermomechanical pulp (CTMP) has been examined. The heat-induced discoloration of the bleached pulps was found to proceed in more or less two phases, i.e., an initial phase with a large drop in brightness followed by a slower phase. The phases were, however, not so well-marked during heat-induced aging as during light-induced discoloration. The most heat-resistant pulp was the aspen CTMP followed by the hydrogen-peroxide-bleached spruce TMP. The spruce CTMP and the dithionitebleached spruce TMP were the least stable, although the difference between the softwood pulps was small. Acetylation was found to improve strongly the heat stability of all of the pulps examined even at a low extent of derivatization. These results together with earlier findings, showing that a low degree of acetylation substantially increases the photostability of different types of high-yield pulps and that the order of discoloration under different light sources was approximately the same for the pulps examined in this work, suggest that light-induced and heat-induced yellowing possibly involves very similar reaction pathways in the lignin moiety. The participation of the pulp carbohydrates could, however, not be excluded because the carbohydrates most likely, to some degree, are also derivatized, even at low acetyl levels.