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Two approaches in pulping to achieve lower kappa numbers while maintaining pulp quality were compared in lab scale. In ASAM pulping of southern pine the inorganic chemical charge, temperature and time at temperature were varied to find suited conditions to obtain pulps with kappa levels of 25, 20 and 15. Corresponding kraft pulps were produced using conventional batch cooks for the high kappa number and EMCC simulations for the lower kappa numbers. Differences in the process and in product quality were identified. ASAM pulping resulted in 3 % higher yields than kraft pulping. ASAM pulps had much higher viscosities, 10 - 20 % better strength properties and 10 points higher brightnesses.
Bleachability was investigated using OD (E+O+P)DP and OZEP sequences (brightness >85 % ISO) . Strength losses in TCF bleaching could be attributed in part to the unoptimized bleaching conditions. It was shown that ASAM pulps responded better to chlorine-free bleaching, leading to significant strength advantages over kraft pulps.
In a second part of this study TCF bleaching of ASAM pulps was optimized. By using a two stage oxygen treatment the ozone charge could be reduced resulting in higher viscosity and better strength properties at even higher brightness. This demonstrated that ASAM pulps can be bleached TCF with an increase in tear strength and 5 - 10 % losses in tensile strength.
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