Inhibition of the photo-induced reversion of high-yield pulps and model lignin compounds by mercaptans

Lignin-rich pulps, like TMP, have been limited to the manufacture of short-life papers like newsprint. This is because the lignin in these pulps tends to yellow rapidly and extensively when exposed to sunlight (reversion). Researchers have attempted to inhibit this reversion. Some mercaptans exhibit excellent inhibition abilities, but little is known about the mechanism. This thesis explores the inhibition of photo-induced reversion of TMP and model lignin compounds (MLCs) using mercaptans, along with some mechanisms for this inhibition. Inhibition studies required a quick and accurate simulation of the natural reversion process. Ultravitalux sunlamps were shown to induce rapid photo-reversion of TMP and peroxide bleached TMP (BTMP) handsheets, closely matching sunlight-induced reversion. Mercaptans, particularly ethylene glycol bisthioglycolate (EGB), efficiently inhibited this reversion. Their initial bleaching effect resembled hydrogen peroxide bleaching, possibly via removal of coniferaldehyde structures. Inhibition required the mercaptans to remain within the paper sheet during irradiation, and for unbleached TMP results indicated that this inhibition occurred mainly via a removal of yellow irradiation products as they were formed. Studies on model lignin compounds (MLCs) extended knowledge of both reversion processes, and modes of mercaptan inhibition. Studies confirmed that a free phenolic hydroxyl group was essential for reversion. A C=C or C=0 group, in conjunction with the phenolic hydroxyl group, may have sensitised reversion. When C=C and C=0 groups were conjugated on a side chain, however, reversion was found to be blocked (e.g. 3- methoxy-4-hydroxy cinnamaldehyde). Mercaptan inhibition of photo-induced reversion in MLCs was shown to be diverse. While some MLCs were completely inhibited (e.g. guaiacol), others were only partially inhibited (e.g. vanillin), or not inhibited at all (e.g. eugenol). For compounds containing a side chain C=C-C=0 conjugation (e.g. 3-methoxy-4-hydroxy cimiamaldehyde), mercaptan treatment actually enabled reversion, by destroying the conjugation which had previously blocked reversion. This diversity in MLC inhibition suggests that mercaptans are likely to inhibit via several modes, which are determined by interacting species. This was confirmed by different mechanistic studies. For example, three mercaptans had a strong potential to inhibit reversion of isoeugenol mainly by removing coloured products of irradiation as they formed. For methoxyhydroquinone and acetovanillone, such a mechanism was supported in only two mercaptans, while for other MLCs (e.g. vanillic acid) this mechanism was possible only as a partial mode of inhibition. An alternative mode was also found to occur. This involved an inhibition of photosensitising reactions, and results again reflected a diversity in effectiveness. For example, while two photo-sensitisers (acetophenone and acetoveratrone) sensitised and enhanced photo-reversion of guaiacol, EGB inhibited only one of these photo-sensitisers (acetoveratrone). A third mechanism was also demonstrated, which involved a \UV-shielding\" effect. Here the mercaptan did not require contact with the MLC to inhibit reversion. Again results were varied. Whereas each mercaptan shielded vanillin no mercaptan shielded guaiacol. Furthermore EGB shielded methoxyhydroquinone but the other two mercaptans did not. Thus the inhibition of reversion by mercaptans was shown to be a complex process involving several different modes of action which are prevalent or not depending on both the mercaptan and the species it is inhibiting."