STUDIES ON THE MECHANISM OF THE PHOTOYELLOWING OF BLEACHED MECHANICAL AND CHEMIMECHANICAL PULPS .3. THE ROLE OF HYDROXYL RADICALS

As part of a continuing study on the mechanism of photoyellowing, the proposed involvement of hydroxyl radicals in the photoreversion of peroxide-brightened high-yield pulps was examined. Specifically, tests were performed to determine 1) if hydroxyl radicals were in fact generated when either lignin-free paper sheets impregnated with lignin model compounds or peroxide-brightened TMP and CTMP were irradiated with visible light and 2) if present, whether these radicals had any effect on photoyellowing. Based on results obtained using a combination of UVNIS detection and trapping methods, hydroxyl radicals were identified in nearly all systems tested except where non-phenolic lignin models were involved. By irradiating peroxide-brightened TMP impregnated with peroxides as hydroxyl radical sources, evidence was obtained indicating that such radicals participate in reactions leading to chromophore formation as expressed by the light absorption coefficient. It was further shown that, in some instances, addition of hydroxyl radical scavengers, particularly ascorbic acid, to peroxide-brightened TMP sheets leads to a reduction in photoyellowing.