Defence of doctoral thesis: Axel Martinsson – Oxidation of Kraft Pulp Fibres – Hydrogen Peroxide Oxidation under Acidic Conditions and its Influence on the Properties of Pulp

Abstract:

An increase in the use of bio-based products, combined with a decrease in the use of fossil-based products, is important for fulfilling the demands of a growing population in a sustainable manner. Moreover, reducing the usage of single-use plastics is vital in combating the plastic pollution that is afflicting our environment. A partial solution to these problems is to replace plastic products with products based on cellulose fibres. Their hydrophilic nature, however, makes utilization of fibre-based materials difficult under wet or moist conditions: the hydrogen bonds that contribute to the majority of the material’s strength are broken in wet conditions. The wet strength can be increased through the addition of wet strength agents or mechanical treatments or, alternatively, the carbohydrate fraction of the fibres may be modified to improve their wet strength. One such modification is to introduce carbonyl groups in the carbohydrate fraction through oxidation, which would improve the wet strength by forming inter-fibre hemiacetal bonds. Methods often used on a laboratory scale to introduce carbonyl groups in the carbohydrate structure may, however, have economical or environmental drawbacks. In a kraft pulp mill producing bleached pulps, oxidative chemistry is typically utilized in the bleaching step in which the processes are optimized to remove residual lignin and chromophoric groups. Optimizing the oxidation operation to introduce carbonyl groups to the carbohydrate structure has the potential of producing a kraft pulp with inherent wet strength using the existing infrastructure of the mill, e.g., in the final bleaching step.

The main aim of this work was to investigate the feasibility of using a common bleaching agent, namely hydrogen peroxide, to introduce carbonyl groups into the carbohydrate fraction of kraft pulp. It was found that oxidations carried out under mildly acidic conditions (pH 4, 85 °C) could increase the carbonyl content by a factor of about 4 at a residence time of 90 minutes. Also, the content of transition metal ions in the liquid phase requires to be controlled, which is important in industrial applications where the liquid fraction is recirculated. Oxidation was found to be successful in increasing the wet-web tensile strength (+ 6-23%) and wet tensile strength (+ 134-242%), with a slight reduction (– 4-11%) occurring in the dry tensile strength.

Keywords: Hydrogen peroxide, kraft pulp, oxidation, wet strength, carbonyl groups