The pH of sulfite treatments has a profound influence on the efficiency of fiber sulfonation of Norway spruce. Compared at the same pulp yield, the extent of sulfonation decreases in the order of pH value 7#xB7;5 9#xB7;5 4#xB7;5; the sulfonation reaction is improved significantly by using a two-stage treatment at pH 4#xB7;5 and 7#xB7;5. The degree of fiber sulfonation plays a dominant role in the tensile and tear strength improvement of these high-yield pulps.
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