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

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Barnett, Vicky Gail (1997) Inhibition of the photo-induced reversion of high-yield pulps and model lignin compounds by mercaptans. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
Keywords: Wood-pulp products, Newsprint, Thiols
Copyright Holders: The Author
Copyright Information:

Copyright 1997 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:48
Last Modified: 08 Jun 2016 03:23
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