Chemical modification of lignin-rich paper Part 8. Effect of light source on the accelerated light-induced yellowing of untreated and acetylated high-yield pulps

The effect of three different irradiation sources (e.g., ultraviolet "black light" fluorescent lamps, UV/VIS-fluorescent lamps, and a xenon lamp) on th e photoyellowing properties of dithionite-bleached and hydrogen-peroxide-bl eached spruce thermomechanical pulp (TMP) and hydrogen-peroxide-bleached sp ruce and aspen chemithermomechanical pulp (CTMP) has been examined. The agi ng response of additive-treated (e.g., UV-screen) and/or chemical-modified (acetylated), high-yield pulps toward the tested light sourcss is also high lighted. The order of light-induced discoloration, of the different high-yield pulps examined, was determined to be: hydrogen-peroxide-bleached spruce TMP>hydr ogen-peroxide-bleached spruce CTMP>dithionite-bleached spruce TMP>>hydrogen -peroxide bleached aspen CTMP (pretreated with hydrogen peroxide). The spec tral distribution of the tested light sources did only marginally alter the aging response of the unmodified, dithionite-bleached, and hydrogen-peroxi de-bleached spruce pulps, whereas the high-brightness aspen CTMP showed som e variation in the degree of brightness reversion. The acetylated and UV-sc reen impregnated pulps were, however, strongly dependent on the light sourc e used, which emphasizes the importance of developing a standard method for accelerating testing and measurements of yellowing of lignin-containing ma terials that could be generally accepted by the research community. The improved stability toward light, achieved by acetylation of high-yield pulps, is not only a consequence of retarding the color-forming reactions, but also to some degree an effect of promoting photobleaching reactions, bo th for nonirradiated and pre-yellowed pulps.