Mini-Encyclopedia of Papermaking Wet-End Chemistry
Additives and Ingredients, their Composition, Functions, Strategies for Use

HARDWOOD

Composition: The major component of wood from a deciduous (hardwood) tree consists of libriform fibers. These are about 1 mm in length and about 20 micrometers in width. Wood from deciduous trees is a complicated mixture of these fibers, multicellular vessels (for transport of water from the roots), and other specialized cells. The kraft pulping method chemically dissolves the lignin, a natural phenolic resin that glues fibers together in wood. Kraft pulping yields fibers that tends to have superior bonding ability after they have been made flexible by the compression and shearing action of a refiner. The down side is that kraft pulping may remove more that half of the original mass of wood. By contrast semi-chemical hardwood pulps are cooked under conditions that solubilize only a fraction of the lignin. Some mechanical refining is needed after the semi-chemical cook to liberate fibers from the softened chips. Two such processes are the carbonate cook and neutral-sulfite semi-chemical (NSSC). Because lignin is stiff, these pulps behave differently. Also, the high content of lignin and extractable materials such as fatty acids contributes to a higher density of anionic surface charge of semichemical fibers, compared to most kraft pulps.

Function: Hardwood fibers are especially suited for producing smooth papers for printing and writing. They also can achieve good stiffness and bulking ability, but this depends on the tree species and refining conditions. Hardwood pulps also tend to produce paper products with more uniform formation, compared to softwood fibers.

Strategies for Use: Papermakers are well aware that different blends of fiber types make sense for different grades of paper. If one wishes to manufacture paper bags it would be foolish to consider using hardwood pulps; they simply would not have enough resistance to tearing. On the other hand, if one wishes to make fine writing papers, it would be foolish to consider 100% of the relatively large southern softwood fibers. The sheet would be too rough and too porous. But wait - if one were to use no softwood fiber at all in making that same writing grade, then it is quite likely that the web will be too weak to run efficiently on your paper machine. One of the most interesting cases to consider is that of the common corrugated box. The flat layers (the "linerboard") are made from unbleached softwood kraft pulp to maximize resistance to edge-wide compression failure. However the interior corrugated or fluted part (the "medium") is usually made out of high-yield hardwood semi-chemical pulp. The relative high content of lignin in such fibers means that they will be stiff enough to play their essential role in keeping the two liners spaced at an optimum distance for strength. When two fiber types are to be blended, it is often the case that the best overall mix of properties can be achieved by separate refining of each type. For example, most softwood pulp can benefit from much greater refining energy input, compared to hardwood. Over-refining tends to reduce fiber strength greatly and produce fines that impede dewatering from the paper.

Cautions: Hardwood fibers and slurries of these fibers are usually non-hazardous if one follows usual safe practices.

Fiber type relative lengths    

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, m_hubbe@ncsu.edu .