Light-induced yellowing effect

Lin and Kringstad (7) confirmed Leary s results in an experiment with a solution of milled wood lignin (MWL) in methylcellusolve water. Irradiation of this solution in a vacuum produced no colour. This solution, opened to air and irradiated, produced the same amount of colour as a solution of milled wood lignin initially irradiated in air. The effect of oxygen on light-induced yellowing of newsprint is further illustrated by the accompanying effect on methoxyl content. Leary found that the methoxyl content of newsprint after irradiation in a vacuum decreased by only 0.1% whereas the same irradiation in air decreased the methoxyl content by 0.4% (6). 

The effect of sodium borohydride on the light-induced yellowing of hydrogen peroxide bleached TMP is different from that of newsprint made from unbleached mechanical pulp. Treatment of bleached TMP with sodium borohydride does not change the extent or the rate of light-induced yellowing (15). 

Schmidt, J.A., Heitner, C., "Light-Induced Yellowing of Mechanical and Ultra-High Yield Pulps. Part I. Effect of Methylation, NaBH Reduction and Ascorbic Acid on Chromophore Formation , J. Wood Chem. Technol., 1991,11(4) 397. 

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. 

Effect of Aromatic Carbonyl Croups The action spectra of Forsskahl and Tylli show that light of 320-330 nm induces yellowing most efficiently, consistent with the hypothesis that aromatic carbonyl groups are the most important sensitizers of the reactions. However, several authors [124,125] have noted that thorough borohydride reduction of mechanical pulps reduces light-induced yellowing only marginally. 

M Ek, H Lennhokn, and T Iversen. A Comment on the Effect of Carbonyl Groups on the Light-Induced Yellowing of Groundwood Pulp. Nordic Pulp Paper Res. J. 5 159-160, 1990. 

JA Schmidt, C Heitner. Light-induced yellowing of mechanical and ultrahigh yield pulps. Part 1. Effect of methylation, sodium borohydride reduction and ascorbic acid on chromophore formation. JWood Chem Technol 11(4) 397 18, 1991. 

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