Review: Light-Induced Yellowing of Lignocellulosic Pulps – Mechanisms and Preventive Methods

Abstract

This review will focus on recent progress regarding the mechanisms of light-induced discoloration of mechanical and chemimechanical pulps and on the proposed preventive treatments. It is evident that the mechanisms behind photoyellowing of lignin-rich pulps are complex and that several types of reaction pathways may coexist. Photoyellowing proceeds via one initial fast phase and a slower following phase. The fast phase has been ascribed to oxidation of free phenolic groups and/or hydroquinones and catechols to photoproducts of mainly quinonoid character. A multitude of reactions involving several lignin subunits are possible. Important intermediates are phenoxyl radicals, and to some extent ketyl radicals. The importance of the phenacyl aryl ether pathway might be more important than previously thought, even though the original content of such groups is low in lignin. Even though many preventive methods against photoyellowing have been suggested, no cost-efficient treatment is available to hinder photoreversion of lignincontaining paper permanently. Suggested methods for stabilization include chemical modification (etherification and esterification), coating the paper product, addition of radical scavengers, excited state quenchers, or ultraviolet absorbing compounds.

Surface photo-discoloration and degradation of dyed wood veneer exposed to different wavelengths of artificial light

“…The interaction of UV energy with the molecular chain of the organic polymer distributed on the surface of dyed wood involves complex reactions [6,29]. The quantum energy associated with UV light is sufficient to break down many of the internal chemical bonds such as carbonyl, carboxyl, phenolic hydroxyl, unsaturated double bonds [30] and dyes chemical composition.…”

Xylanase Treatment Suppresses Light- and Heat-Induced Yellowing of Pulp

“…The tendency to yellow, also called “brightness reversion,” is one major drawback to HYP and straw pulp application1112. In general, pulp yellowing includes both light- and heat-induced yellowing.…”

Effect of mechanical activation on structure changes and reactivity in further chemical modification of lignin

“…6. Similar to photooxidation reaction of lignin [41], the phenolic hydroxyls may also be converted to quinone and the C=O in side chain was converted to C−OH (Fig. 7).…”