EVALUATION OF RECYCLING POTENTIAL OF PAPER MAKING FIBERS

Abstract

Bagasse and wheat straw are important raw materials for the Indian paper industry, with a potential of high growth. The recycled paper available to the Indian paper industry will have increasing proportions of these fibers in future. Bagasse and wheat straw are significantly different from woods in terms of fiber morphology and chemical composition. Recycled fibers of these raw materials behave differently from the recycled wood pulps. Recycling potential of the pulps made from these fiber sources is evaluated in the present study. The thesis consists of six chapters. In the first chapter, the need and aim of the work have been described. A brief introduction of the methodology adopted and the scope and limitation of the study have been discussed. Chapter 2 covers a review of the available literature in this area. The effect of recycling on different properties of wood pulps has been extensively studied. It is evident that the most detrimental effect of recycling on papermaking fibers is loss in tensile strength. Several mechanisms have been put forth to explain this loss based on studies of physical and chemical changes that take place in the fiber during recycling. The generally accepted view is that the loss in tensile strength is due to loss of fiber swelling on drying of pulps. The loss in fiber swelling reduces the mobility of surface molecules, this in turn decreases inter molecular interaction between two fiber interfaces resulting in a decrease in fiber-fiber bond strength. The relative bonded area decreases for stiffer recycled fibers. The recycling potential of fibers sourced from a given raw material depends on the yield of pulp. The amount of hemicelluloses present in pulp has been found to play an important role in the pulp's recycling potential. Refining of recycled pulps increases both tensile strength and fiber swelling but at the cost of increased drainage resistance. The problems of slow drainage of recycled pulps can be alleviated to some extent by techniques such as fractionation, addition of strength aid, and enzyme treatment of recycled pulps. Chapter 3 describes the pulps used for the recycling experiments. Single specie pulps of bagasse and wheat straw, three chemical pulps and two semichemical pulps, were prepared in the laboratory. A commercial chemimechanical of bagasse was procured from a large Indian newsprint mill. Chapter 4 presents the details of experimental work on recycling of pulps. The pulps were subjected to six cycles of sheet making — drying — reslushing. Various III properties of the pulps were evaluated for the standard handsheets prepared at different cycles. No chemical or mechanical treatment was given to the pulp between the cycles. The wheat straw pulps have shown a higher recycling potential than the bagasse pulps. The loss in strength of bagasse pulp is of the order of the loss reported for wood pulps. Fiber classification remains nearly unchanged on recycling of wheat straw pulps, but shows a high loss of fines (P200) fraction on recycling of bagasse pulps. There appears no major impact of recycling on the fiber length. The proportions of alpha-, beta-, and gamma-cellulose do not change with recycling of bagasse and wheat straw pulps. However, the bagasse pulp with lower gamma cellulose content than the wheat straw pulp suffered a greater loss in tensile strength. Chapter 5 describes the experimental work to study the response of conventional strength enhancement treatments of recycled pulps. The recycled pulps were subjected to alkali treatment, refining, and combination of both. For some experiments, the pulps were fractionated into a fine and coarser fractions, and the coarser fraction was refined separately and mixed back with the fines. Physical and strength properties of the treated pulp have been compared with the properties in untreated recycled and virgin state. Freeness of the recycled pulps decreases sharply on refining. Reduction in freeness due to refining is more severe for wheat straw pulps. Very fine particles (P250) contribute to a great deal in the slowness of the wheat straw pulp, as indicated by a significant rise in freeness when this fraction was removed from the pulp. Chapter 6 summarizes the main conclusions and the recommendations for the future work.