Chemical modification of lignin-rich paper - Part 9. Effect of dry heat and moist heat on the accelerated yellowing of untreated and acetylated high-yield pulps

The effect of two different aging conditions (e.g., 105 degrees C and 80 de grees C, 65% relative humidity) on the heat-induced yellowing of dithionite -bleached and hydrogen-peroxide-bleached spruce thermomechanical pulp (TMP) and hydrogen-peroxide-bleached spruce and aspen chemithermomechanical pulp (CTMP) has been examined. The heat-induced discoloration of the bleached pulps was found to proceed i n more or less two phases, i.e., an initial phase with a large drop in brig htness followed by a slower phase. The phases were, however, not so well-ma rked during heat-induced aging as during light-induced discoloration. The m ost heat-resistant pulp was the aspen CTMP followed by the hydrogen-peroxid e-bleached spruce TMP. The spruce CTMP and the dithionite-bleached spruce T MP were the least stable, although the difference between the softwood pulp s 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 togeth er with earlier findings, showing that a low degree of acetylation substant ially increases the photostability of different types of high-yield pulps a nd that the order of discoloration under different light sources was approx imately the same for the pulps examined in this work, suggest that light-in duced and heat-induced yellowing possibly involves very similar reaction pa thways in the lignin moiety, The participation of the pulp carbohydrates co uld, however, not be excluded because the carbohydrates most likely, to som e degree, are also derivatized, even at low acetyl levels.