Gaining insight into the oxidation of hardwood kraft fibres using hydrogen peroxide at mildly acidic conditions was the main aim of this study. The oxidized hardwood pulp had an increased number of carbonyl groups and, when formed into sheets, an enhanced durability in water was seen due to the formation of cross-links, known as hemiacetals. The carbonyl groups formed were found to be mainly ketones with the exception of the case with longer reaction times (60–90 minutes) at 85 °C, where aldehydes were detected. Through compositional analysis it was found that mainly xylan was oxidized, likely due to the higher amount of xylan close to the surface of the fibre wall. The influence of xylan on the oxidation process was investigated using cold caustic extraction (CCE) performed prior to oxidation. When the CCE pulp was oxidized, there was an increased rate of introduced carbonyl groups and degradation was more pronounced. This is likely due to the accessible surface area being increased, caused by the formation of pores when the lower molecular weight xylan was being extracted during CCE.

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.

Oxidation of kraft pulp using hydrogen peroxide under mild acidic conditions can be applied in order to obtain new functionality of the fibres, in the form of carbonyl groups. The hydrogen peroxide concentration must, however, be higher than consumed by the oxidation reactions meaning that the liquid must be recirculated to fully utilize the hydrogen peroxide. This paper investigates the consequences of recirculation of the oxidation liquor. It was found that recirculation results in an accumulation of ions of transition metals (copper, iron and manganese) in the oxidation liquor. The transition metal ions are known for catalytic decomposition of hydrogen peroxide, producing radicals which may react with carbohydrates, forming carbonyl groups as well as causing carbohydrate degradation. This was confirmed through the recirculation of oxidation liquor as well as through controlled addition of transition metals. At high transition metal ion concentration the reactions were fast and a severe degradation of carbohydrates was observed, accompanied by a rapid hydrogen peroxide consumption. The consequence of this, in an industrial context, is that the concentration of metal ions must be carefully controlled in order to add functionality to the cellulose without causing excessive degradation of carbohydrates or consumption of hydrogen peroxide.

The objective of this study is to gain further insight into the compositional and physical changes of kraft pulp caused by oxidation with hydrogen peroxide under acidic conditions. A softwood kraft pulp has been subjected to varying degrees of oxidation. Changes in the composition, as well as resulting physical properties of laboratory sheets were investigated. Findings indicated that the oxidation resulted in an increased carbonyl group content, a decrease in intrinsic viscosity and a change in hemicellulose composition. The oxidized pulps were found to have a reduced water retention value and a slightly reduced tensile strength (4–11 % at the investigated conditions). However, a substantial increase in wet web tensile strength (6–23 %) and wet tensile strength (134–242 %) of the sheets was obtained. The sheets could easily be repulped and an increased wet strength maintained to a large extent, even after recycling of the sheets.