Light-Induced Bleaching of High-Yield Pulps Part I. Sensitized Bleaching of Pulp Sheets

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Abstract
Bleached high-yield pulps undergo a much higher rate of light-induced brightness reversion than their chemical counterparts. This tendency to discolor is one of the main factors limiting the application of high-yield pulps to short-term products such as newsprint, tissue, toweling, and foodboard. The present study was conducted to determine whether the light absorption responsible for aging could be utilized to activate and subsequently destroy the chromophores in high-yield pulps. This would permit production of a bleached pulp. Aspen thermomechanical pulp (TMP) and white oak chemimechanical pulp (CMP) sheets were bleached by irradiation with near ultraviolet light (300–400 nm) while saturated with alcohols in the presence of oxygen. Addition of benzophenone and/or hydrogen peroxide as photosensi-tizers to the alcohols prior to irradiation accelerated the bleaching rates and increased the sheet reflectance (457 nm) 20 to 30 points. High light intensity enhanced bleaching; alkali and water retarded the brightening process. Benzophenone and hydrogen peroxide accelerate bleaching, presumably by utilizing light energy to synthesize significant amounts of organic peroxides (α-hydroxyhydroperoxides) for light-induced bleaching. It is suggested that in the absence of added benzophenone or hydrogen peroxide, chemical groups in lignin (i.e., α-carbonyl groups) are responsible for sensitizing the formation of adequate quantities of these peroxides for bleaching to occur. The light also accelerates bleaching, possibly by activating lignin chromophores, and this enhances their reactivities towards the alcohol peroxides.