Effects of selective addition of papermaking chemicals to fines and long fibres on strength and runnability of wet paper

Selective addition of papermaking chemicals to fines and long fibre fractions are known to have positive effect on strength properties of dry paper. Practically no information on the effects of selective addition on drainage and wet web properties is available. In this laboratory scale study, the effect of adding cationic starch and synthetic cationic polymer selectively to different fibre fractions was examined. TMP and bleached hardwood kraft pulp were used. The pulps were fractionated to fines and long fibre fractions. Starch was added into the long fibre fraction and synthetic cationic polymer was added into the fines fraction. After a certain time the fractions were combined and hand sheets were prepared. Tensile strength and tension holding capacity of the wet and dry paper were measured using high strain rate. The tension holding capacity (defined as the tension after a fast straining to 1% elongation followed by 0.5 seconds of relaxation) has been found to be a good indicator of wet web runnability in high speed machines. The results showed that the selective addition of chemicals to pulp fractions significantly consolidate the wet web and increase drainage rate. Pulps composed of separately chemically treated fractions gave considerably higher wet web tensile strength. The effect was greater with chemical pulp than with mechanical pulp. With chemical pulp, combining separately treated fractions gave remarkably higher residual tension than chemical addition to original pulp. With mechanical pulp no clear effect on tension holding capacity was found. Generally the results in this study indicate a high potential to improve pulp drainage and wet web runnability by means of separate chemical treatment of pulp fractions. (9 refs.)