The high cost of cotton and changes in fashion have contributed to the enhanced production of dissolving pulps which differ from more common, �paper grade pulps� in requiring a high cellulose content, uniform molecular weight distribution and a high reactivity. However, the few �traditional� sulfite mills that still produce dissolving pulp are inefficient, result in low pulp yields and would not be built today. One response to this demand is that an increasing number of Kraft pulp mills are being converted to allow them to produce dissolving grade pulps.However, some Kraft pulp characteristics that enhance their papermaking properties, such as high hemicellulose retention and cellulose degree of polymerization of cellulose (DP), are not desirable dissolving pulp characteristics. Thus a repurposed Kraft pulping operation which hopes to make dissolving pulps requires the addition of several extra steps to remove more hemicellulose and tailor the cellulose to these more �speciality pulps� applications. These include pre-hydrolysis, cold caustic extraction and hypochlorite bleaching stages. Various specific chemicals and enzymes have been assessed for their ability to help produce these novel dissolving pulps. For example, dicarboxylic acids such as oxalic acid and xylanases are effective in removing some of the residual hemicellulose from pulps. In the work that will be described, xylanases and oxalic acid were compared for their ability to remove the last vestiges of hemicellulose from hardwood derived dissolving pulps while assessing the resulting pulps cellulose purity, molecular weight and accessibility. We found that xylanases typically performed better than oxalic acid in the removal of residual hemicellulose, reducing the xylan content of the final pulp to about 1-2%. It is likely that the desirable properties resulting from the solubilisation of residual hemicellulose must be balanced against a reduction in the cellulose reactivity and accessibility of the dissolving pulp.

Enzymatic modification of the hemicellulose in paper grade pulps and its influence on cellulose purity, reactivity, and accessibility

247th National Meeting (2014)

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