Light-induced yellowing

The light-induced yellowing of musk ambrette 48 is simulated41 by photolysis of 48 in 0.1 N methanolic sodium hydroxide solution to give the azobenzene 50 (through the intermediacy of azoxybenzene) and by-products 51 and 52, by intramolecular photocyclization (equation 36). 

HEITNER Light-Induced Yellowing of Wood-Containing Papers... 

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). 

More direct evidence for the formation of o-quinones in light-induced yellowing was presented by Lebo et al. (14). They found that trimethyl phosphite reacts with o-quinones to produce oxyphosphoranes, which in turn, react with water to form cyclic phosphate triesters, as shown in Scheme 3. Treatment of yellowed refiner mechanical pulp with trimethyl phosphite decreased the light absorption at 437 nm, as seen in Table I. Similar decreases in specific absorption were observed at 413 and 320 nm. Solid state 31P nmr analysis of pulp treated with trimethyl phosphite was consistent with the formation of the cyclic phosphate triesters. 

The next phase of research focused on the functional groups in lignin that play a role in light-induced yellowing. Carbonyl and phenolic hydroxyl groups have been studied intensively. 

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). 

Tschimer and Dence (32) have demonstrated the role of both the phenolic hydroxyl and carbonyl groups in light-induced yellowing of spruce TMP. For up to two hours irradiation time, complete reduction of carbonyl groups in TMP... 

Brunow and Sivonen (34) obtained similar results on a lignin model system, ethylguaiacylcarbinol. This compound did not undergo any oxidation when irradiated with near-uv light until p-methoxyacetophenone, a triplet sensitizer, was added to the solution. As the molar ratio of p-methoxyacetophenone to ethylguaiacylcarbinol was increased to 0.32 the rate of photooxidation of this benzyl alcohol increased. This result is consistent with the mechanism of light-induced yellowing shown in Scheme 3. 

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