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UBC Theses and Dissertations

Elucidating the pulp properties that influence the ability of enzymes to facilitate the conversion of hardwood Kraft pulp to dissolving-grade pulps Dou, Xiaoli


Dissolving pulp is characterized by its high cellulose/low hemicellulose content, minor amounts of residual lignin/extractives, high brightness and a uniform molecular weight distribution. Dissolving pulp can be produced through acid sulfite cooking or alkaline Kraft cooking. However, due to issues with chemical recovery and pollution, the predominant pulping process has globally shifted to the Kraft process. Kraft pulps retain hemicellulose and high molecular weight cellulose, which are undesirable for dissolving pulps. Therefore, steps such as prehydrolysis (PHK) and cold caustic extraction (CCE) aimed at removing hemicellulose and decreasing cellulose molecular weight are typically employed. However, these processes are chemically intensive, non-specific and pose operational challenges for mills. The use of enzymes (hemicellulases and cellulases) is one potential alternative to chemical methods of facilitating mill conversion due to the high specificity of enzymes and their ability to function under more benign conditions. Initially, xylanase and oxalic acid treatments were assessed for their potential to convert Kraft-to-dissolving pulp. It was apparent that the accessibility of hemicellulose and cellulose to chemical or enzymatic reagents was critical. Compared to oxalic acid, enzymes were more specific in removing hemicellulose while boosting cellulose reactivity. Model substrates, varying in their hemicellulose accessibility and cellulose properties, were used to assess the influence of various pulp characteristics on enzymatic pulp modification. The influence of pulp characteristics imparted by PHK and CCE on the ease of enzymatic modification was also assessed. It appeared that CCE negatively impacted the accessibility of hemicellulose due to the solubilisation of low molecular weight carbohydrates fragments which acted as “spacers” between cellulose microfibrils, preventing fibril aggregation. Lowering the acidity of the prehydrolysis or the alkalinity of Kraft pulping conditions increased the ease of enzymatic removal of the hemicellulose, presumably by increasing hemicellulose accessibility. Separating the fibres into various size fractions indicated that the shorter fibres within the Kraft pulp were more susceptible to enzymatic modification, likely due to their increased porosity. It was apparent that Kraft pulping conditions played a significant role in governing enzyme accessibility to the various pulp carbohydrates and thus the potential of using enzymes to enhance dissolving pulp production and properties.

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