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Abstract
The aim of this work was to provide softwood kraft pulp fibres with new functionalities by the introduction of carbonyl groups. Carbonyl groups are known to affect properties such as wet strength through the formation of covalent bonds, i.e. hemiacetals. The method developed involves oxidation using hydrogen peroxide at mildly acidic conditions. It was found that the carbonyl group content increased with both increasing temperature and residence time when oxidized at acidic conditions. The number of carboxylic groups, however, remained approximately constant. There was virtually no increase in carbonyl groups when oxidation was performed at alkaline conditions. The maximum increase in carbonyl groups was found at a residence time of 90 min, a reaction temperature of 85 °C and a pH of 4. These conditions resulted in an increase in carbonyl groups from 30 to 122 µmol/g. When formed into a sheet, the pulp oxidized at acidic conditions proved to maintain its structural integrity at aqueous conditions. This indicates the formation of hemiacetal bonds between the introduced carbonyl groups and the hydroxyl groups on the carbohydrate chains. Thus, a possible application for the method could be fibre modification during the final bleaching stage of softwood kraft pulp, where the wet strength of the pulp could be increased.
Highlights
Wet strength is an important property in some paperbased products, e.g. various packaging and absorbing applications
Another wet strength agent used is dialdehyde starch which, is linked to effluent emissions of iodine as a result of the sodium periodate used for oxidation (Bates et al 1999)
Oxidation of cellulose in systems without a pH buffer The pH of the suspension changes over time during oxidation. This is due to the formation of both hydroxide ions and organic acids, such as formic acid (Jung et al 2009; Wen et al 2019)
Summary
Wet strength is an important property in some paperbased products, e.g. various packaging and absorbing applications. The production of polyamide or epoxidebased additives are associated with emissions of adsorbable organic halides (AOX) These additives can cause issues when the fibre material is repulped, as they result in apermanent increase in wetstrength, which requires oxidizing treatments to successfully re-pulp (Bates et al 1999; Epsy and Geist 1993). Another wet strength agent used is dialdehyde starch which, is linked to effluent emissions of iodine as a result of the sodium periodate used for oxidation (Bates et al 1999) The action of these aldehyde-based additives relies on the introduction of covalent bonds, i.e. hemiacetals, between the aldehyde groups of the additives and the hydroxyl groups in the carbohydrate structure. The addition of the above-mentioned wet strength agents could be avoided altogether if aldehyde groups could be introduced to the cellulose structure itself by appropriate oxidation
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