Improved pulp yield and strength by retained glucomannans in kraft pulping of softwood

The possibilities of improving yield and strength properties of softwood bleached kraft pulps by retaining a higher content of glucomannan during kraft cooking using additives that decrease the rate of carbohydrate degradation were looked upon. In addition the effect of an increased alkaline concentration, favouring stopping reactions over peeling reactions was explored. Yield increases in the range of 2 to 4%-units were achieved using additives. In the case of the higher alkali charge instead a small yield decrease was noticed. Higher alkali charge in general resulted in a larger loss of xylan in the pulp. When examining the effects of the application of shearing forces at the end of the cook, mimicking industrial pulps, all cooks using high alkali conditions were affected by a large decrease in strength properties, both in tensile index and tear index as well as in fibre strength measured as re-wet Zero-span. For pulps cooked with polysulphide and H2S additions, stabilising the glucomannan degradation, the strength reductions were smaller than for the reference pulps. This resulted in pulps with both a higher yield and similar or better strength properties than those for the reference pulp. These pulps also had better beatability, i.e. the tensile strength increased faster during PFI-beating than for the reference pulp.