EFFECT OF PULPING AND BLEACHING CONDITIONS ON THE STRENGTH OF KRAFT PULP .2. COMPARISON OF 3 COMMERCIAL SOFTWOODS

Kraft, kraft-oxygen and chlorine dioxide bleached pulps were prepared from New Zealand radiata pine, Canadian white spruce and Southern loblolly pine under identical laboratory conditions; handsheet evaluations were performed after each stage of processing. Only the radiata pine pulps could be consistently oxygen delignified and fully bleached in the laboratory without loss of the parent kraft pulp strength potential. Fully bleached pulps from radiata pine were always stronger than their parent kraft pulps. Loblolly pine and white spruce pulps only achieved this if kraft pulping was limited to 30 Kappa number and oxygen delignification limited to less than 50%. In addition, radiata pine was the easiest to pulp to target Kappa number Loblolly pine had a high lignin content and thus required more extended kraft pulping to reach target Kappa number White spruce pulps achieved higher final brightness after uniform bleaching due to greater ability to scatter light.