Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: Softwood fibers (tracheids) come from needle-bearing, conifer trees. The main chemical components of softwood kraft tracheids - cellulose and hemicellulose - are polymers of glucose, with each unit connected by a beta-1-4-glycosidic linkage. These molecules are arranged in a series of layers having different angles relative to the fiber direction. The thickest of these layers (the S2 sublayer) has a fibril angle almost lined up with the fiber, and this accounts for the high dimensional stability of both fibers and wood in the length direction. The cellulose polymers are partly fused together in microscopic crystalline domains, and this accounts for the insolubility of the wood fibers. Two main pulping methods, kraft and mechanical, are used to liberate the fibers. The kraft process dissolves a natural phenolic resin called lignin from between the fibers. In mechanical pulping the lignin is left in place. For example, in thermomechanical pulping (TMP), wood chips are passed between rotating plates having raised bars at elevated temperature and pressure. The heating softens the lignin and makes it possible to separate the fibers in an intact state, obtaining a yield of over 90% of the original solids from the wood. Softwood fibers from commonly used papermaking species such as pine, spruce, alpine fir, and Douglas fir have tracheid fibers approximately 3 mm long and 20 to 30 mm thick. Especially after kraft pulping the refining process tends to delaminate the cell wall, allowing the fiber to collapse into a ribbon, greatly increasing the fibers' ability to bond to each other.
Function: Softwood fibers (tracheids) confer high tearing strength, ability to withstand multiple folding, and a range of other strength-related properties.
Strategies for Use: Softwood kraft pulp is the main component of linerboard for corrugated containers and for bag grades. Bleached softwood kraft is a key, but usually minor, component of printing papers and many other products that can benefit from the reinforcing ability of the longer fibers. Softwood chemithermomechanical pulp (CTMP) is useful in products that can benefit from decreased density. One of the key considerations in the use of softwood pulps is how to achieve a satisfactory formation uniformity. The ratio of length to width (aspect ratio) of softwood fibers is often as high as 100. That means that the fibers have a strong tendency to clump together when suspended in water. In theory one would have to reduce the solids level to less than about 0.01% to give every fiber room to rotate without colliding into an adjacent fiber. Thus far the paper industry has not been able to afford the extremely high pumping requirements that would be neede, in theory, to achieve such levels of dilution. Rather, it is accepted that there will be some level of nonuniformity due to fiber flocculation. Hydrodynamic shear within the paper machine system itself both causes and breaks up fiber flocs. The use of retention aids may greatly increase the nonuniformity of paper when considering areas of 1 cm2 to 100 cm2. The adverse effect of retention aids on formation can be minimized by using a lower dosage or moving the addition point before the pressure screens. Softwood kraft pulps generally can benefit from a higher level of refining, compared to hardwood pulps.
Cautions: There are no special hazards associated with softwood pulps.
PLEASE NOTE: Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification.
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This page is maintained by Martin Hubbe, Associate Professor of Wood and Paper Science, NC State University, firstname.lastname@example.org .