Research On Biological Enzyme Pretreatment To Improve The Softness Of Tissue Paper

Tissue paper consumption is growing rapidly in recent years with the economic development and living standard improvement. Softness of tissue paper is a very important property for the consumers, it will determine the quality and price of the tissue paper. The softness is governed by many factors such as the type of pulp, refining, creping and chemicals. The softness can be maneuvered by adjusting the proportion of long fiber and short fiber at the fixed creping; the other is to add the chemical softener to improve the softness. Chemical softener is a type of synthetic chemicals,which may be harmful to human health because it is easy to attach the skin. It is very important to choose a healthy and safe way to improve softness of tissue paper. The development and application of biological enzyme provide the alternative way to solve the softness issue of the tissue paper.Enzyme pretreatment of pulp fiber not only can increase the softness, but also can provide the decreased refining energy and higher absorbent properties of the tissue paper.In the first part of the thesis, three types of pulps were studied, which were the bleached kraft softwood pulp, bleached kraft hardwood pulp and high yield pulp. The effects of biological enzyme pretreatment on the fiber properties and sheet properties were investigated. In the second part, the model of single fiber flexibility was proposed based on single fiber flexibility changes before and after the enzyme pretreatment.The results of the first part show that: (a) The optimum beating degree of bleached kraft softwood pulp was 31 °SR for the softness properties of the sheet. When the enzyme dosage was 1%,softness(TS7) value decreased by 35 units compared with untreated fiber,tensile strength index decreased by 9.6 N-m/g, the bulk increased by 0.12 cm /g, water retention value increased by 6.5%, the pulp viscosity reduced by 170 mL/g. The length and width of fiber kept the same level, average fiber kink index increased. (b) The optimum beating degree of bleached kraft hardwood pulp was 27 °SR for the softness properties of the sheet. When the enzyme dosage was1%, softness(TS7) value decreased by 19 units compared with untreated fiber, tensile strength index decreased by 6.7 N-m/g, the bulk increased by 0.21 cm3/g, water retention value increased by 5.8%, the pulp viscosity reduced by 61 mL/g. The average fiber kink index increased. (c) When the enzyme dosage was 1%, softness(TS7) value decreased by 9 units compared with untreated fiber for high yield pulp, tensile strength index decreased by 0.4 N-m/g, water retention value increased by 7.6%. The average fiber kink index decreased.The second part was the mechanism of the enzyme treatment to improve the softness of the tissue paper. A model was proposed based on the single fiber flexibility changes before and after the biological enzyme pretreatment of bleached softwood kraft pulp. The result showed that span length (i.e. the position between wire center and fiber contact glass)reduced from 336.77 μm to a minimum of 105.46 μm after the biological enzyme pretreatment of bleached softwood kraft pulp, and the width of paper kept the same. When the enzyme dosage increased 1%, softness(TS7) value of single fiber flexibility increased about 2.0×1012 (1/N·m2).