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Kinetic processes in alkaline peroxide bleaching

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Wright, P. J.(Philip James) (1993) Kinetic processes in alkaline peroxide bleaching. PhD thesis, University of Tasmania.

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Abstract

The response of a commercial hardwood mechanical pulp, E. regnans stone
groundwood, to alkaline peroxide bleaching has been studied by examining the
kinetics of the bleaching process under conditions of constant reagent concentrations.
Several kinetic models have been tested for their ability to describe the observed
kinetic behaviour in terms of a minimum number of chemically meaningful
parameters. A previously reported kinetic model, developed to describe the peroxide
bleaching of softwoods, was found to be inadequate. As a consequence, two new
kinetic models have been proposed, both of which are defined in terms of first order
processes. The first model type assumes two distinct categories of chromophores
which are eliminated to give colourless products. The chromophoric groups are
divided into peroxide susceptible or peroxide resistant groups on the basis of their
kinetic behaviour. The second model type assumes a single class of chromophores
which are eliminated to give colourless products and potential chromophores. In this
'equilibrium' model, chromophore elimination is opposed by a chromophore creation
route arising from conversion of potential chromophores to chromophores. An
understanding of the main chemical processes occurring in peroxide bleaching has
been achieved by analysing the behaviour of pseudo-first order rate constants for each
model. Both of the proposed models are able to describe a number of kinetic features
observed under constant reagent concentrations, such as a maximum in the rate of
bleaching in the pH 11-12 range. However, only the model containing a reversible
step can describe the full range of experimental behaviour observed when reagent
concentrations are altered during bleaching, suggesting that colour formation is an
important process during alkaline peroxide bleaching.
The kinetics of reactions between alkaline peroxide and model lignin chromophores
have been examined to lend further interpretation to the kinetic models developed for
the bleaching of pulp. Cinnamaldehyde has been studied as a model for a,ß~­unsaturated aldehyde chromophores while the reactions of ortho quinone chromophores have been investigated using 4-tert-butylorthoquinone as a model.
Reactions with cinnamaldehyde have shown that a,ß-unsaturated aldehydes are
readily attacked by perhydroxyl anions (H02-) to give colourless epoxide
intermediates. The epoxides are further degraded to benzaldehyde derivatives leading
to an irreversible removal of colour. Reactions between alkaline peroxide and 4-tertbutylorthoquinone
indicate that ortho quinones undergo competing colour removing
and colour creation reactions with perhydroxyl anions and hydroxide ions
respectively. The colour creation processes result in the formation of peroxide
resistant hydroxy para quinones which lead to incomplete removal of colour during
bleaching.
The kinetic behaviour of model a,ß~-unsaturated aldehydes and ortho quinones can be
related to the reaction steps in the two chromophore kinetic model described
previously, and can explain certain kinetic features observed during the bleaching of
pulp. However, the process of chromophore formation contained in the equilibrium
kinetic model was not observed in studies on model a,ß~unsaturated aldehydes and
ortho quinones, and further work is required to identify possible colour causing
reactions under peroxide bleaching conditions.

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

Copyright 1993 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, 1994. Includes bibliographical references

Date Deposited: 04 Feb 2015 23:21
Last Modified: 22 Aug 2016 05:57
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