Chemical modification of lignin-rich paper

The effect of long-term irradiation on the yellowing of paper made from acetylated and non-treated unbleached and hydrogen-peroxide-bleached spruce TMP has been studied. The simultaneous change in the concentration of phenolic hydroxyl groups that occurs on irradiation has also been determined. The yellowing was more pronounced in the hydrogen-peroxide-bleached TMP than in the unbleached TMP, indicating that new structures that are detrimental to the light stability are generated during bleaching. It is also evident that the brightness reversion proceeds more or less in two phases, i.e. a rapid initial phase followed by a slower phase. Acetylation was found to improve strongly the long-term light-stability of the examined TMP even at a low extent of derivatization. The improvement of the photostability coincided with a reduction in the initial concentration of phenolic hydroxyl groups present in the pulps. Irradiation for a short period of time decreased the number of free phenolic hydroxyl groups in the acetylated TMP paper samples, but new phenolic hydroxyl groups were generated on prolonged irradiation.