A Photoyellowing Simulation of Bleached High Yield Wood Pulp by Lignin Models Included in a Solid Carbohydrate Matrix

Abstract

A series of lignin model molecules was prepared, adsorbed and grafted (for the most appropriate one) on to bleached chemical pulp. The rates of brightness reversion of these solid materials under simulated solar light exposure were established and compared to that of bleached mechanical pulp (containing lignin). 4-hydroxy-3-metboxy acetophenone (acetovanillone or acetoguaiacone) 1 and 3,5-dimethoxy-4-hydroxy acetophenone (acetosyringone) both incorporating α-carbonyl and phenolic functions which have been claimed essential for an efficient coloration, did not prove to be very active in the solid state. In contrast, α-carbonyl β-O-4 and β-1 structures with whether phenolic function or not, display an intense yellowing in this medium, and mimic well the brightness reversion of bleached mechanical pulp (especially the non phenolic β-O-4 chromophore adsorbed and grafted). These results are in contrast with those found in liquid ethanolic solution where acetovanillone 1 (conc.=10⁻²M) and the α-carbonyl 4- O-methylated β-O-4 dimer 3 (conc.=3.7×10⁻³M) were shown to undergo a similar yellowing. Inter and intramolecular mechanisms were put forward to explain this difference. Moreover, the reduction of the carbonyl chromophore into benzyl alcohol appears to prevent discoloration to some extent. However, all the studied structures were found light sensitive, even the 4-O-methylated α-O-4 dimer 11 which contains neither carbonyl nor phenolic groups.