EFFECT OF PULPING AND BLEACHING CONDITIONS ON THE STRENGTH OF RADIATA PINE KRAFT PULP

The strength of bleached kraft pulp is crucial to its performance in a wide range of paper grades. However, the maximum strength potential of kraft pulp can be lost during pulping and bleaching. This paper describes the investigation of the pulp strength response of a typical slabwood sample of radiata pine to kraft pulping, oxygen delignification and final bleaching over a range of conditions of practical interest to New Zealand kraft pulp producers. Kraft pulps at three Kappa number levels were prepared, then each was oxygen delignified to three levels. Each kraft-oxygen pulp was fully bleached with D(EO)D and (DC)(EO)D sequences. Increasing the extent of kraft delignification from 31 to 22 Kappa number reduced the pulp viscosity as well as unbleached and bleached pulp handsheet strength. Increasing the extent of oxygen delignification from 35 to 58% Kappa number reduction also decreased pulp viscosity but had little effect on pulp strength. Pulp bleachability and ease of refining, on the other hand, were improved by extending kraft pulping and oxygen delignification. In comparison with (DC)(EO)D bleaching, elemental chrlorine free (ECF) bleaching with a D(EO)D sequence was less effective resulting in lower final brightness and lower pulp strength. Overall, the handsheet strengths of fully bleached pulps were greater than those of the parent kraft pulps. Results confirmed that radiata pine kraft pulp was resistant to pulp strength loss when bleached in the laboratory.