Inhibition of light-induced yellowing

Inhibition of Light-Induced Yellowing of Lignin-Containing Paper... 

Until recently, the mechanism of the inhibition of light-induced yellowing was subject to speculation. However, solid state ESR and CIDEP have provided insight into the mode of inhibition by thiols (Wan, J.K.S., et al, J. Wood Chem. Technol., in press). Near-uv irradiation of unbleached and peroxide bleached thermomechanical pulp impregnated with thiols caused a rapid increase of the thiol radicals. The time resolved CIDEP spectrum, however, shows a symmetric broad band characteristic of the polarized phenoxy radical. This result suggests that thiols quench triplet generated phenoxy radicals in a secondary thermal process. 

JX Guo, DG Gray. Inhibition of light-induced yellowing of the high-yield pulp by sodium hydroxymethylphosphinate. J Pulp Paper Sci 22(2) 64-70, 1996. 

Ascorbic acid inhibits light-induced yellowing for a finite time, 1 to 2 hours when irradiated with near-uv light with an intensity of 9.2 mW/cm2 (6). This limitation has been attributed in part to photooxidation of ascorbic acid. In addition to air oxidation, ascorbic acid is oxidized by photochemically produced peroxyl radicals, superoxide radical anion and singlet oxygen (27,28). If ascorbic acid is to be an effective inhibitor of light-induced yellowing it s oxidation must be slowed. 

Ascorbates. The fact that ascorbic acid and sodium ascorbate inhibit light-induced yellowing of lignin-containing papers was first reported 25 years ago (15). For a limited time ascorbic acid stops yellowing, but eventually yellowing proceeds at the same rate as the untreated paper (6,17), as seen in Figure 2. 

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