Mini-Encyclopedia of Papermaking Wet-End
Additives and Ingredients, their Composition, Functions, Strategies for Use
Composition: "Gee, everybody knows what a fiber is?" But, in fact, not everyone does. And since they contribute the majority of dry mass to a paper sheet, it is worth spending some time to describe them. The first issue involves the source of fiber. Most of the fibers from conifers (softwood trachieds) are about 3 millimeters in length and about 20 to 35 micrometers in width. By contrast libriform fibers from hardwood (deciduous) trees are typically about 1 mm in length and not as thick. Within these classes there is wide variety. Also there are effects due to the age of the tree, the environment, the season in which a particular fiber grows, the presence of shade, etc. Wood from deciduous trees is, in fact, a complicated mixture of these fibers, multicellular vessels (for transport of water from the roots), and other specialized cells. The next point of differentiation between papermaking fibers is the means by which the fibersare liberated from the wood. The kraft pulping method chemically dissolves the lignin, a natural phenolic resin that glues fibers together in wood. Kraft pulping yields fibers that tend 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 and some of the bonding ability and strength is lost when the fibers are recycled. By contrast mechanical pulps retain most of the original dry content, including lignin. Because lignin is stiff, these pulps behave differently. Also, the high content of lignin and extractable materials such as fatty acids and resin acids contributes to a higher density of anionic surface charge of mechanical fibers, compared to most kraft pulps.
Function: Fibers form the main structure of a sheet of paper and their nature determines the potential for strength development, opacity, density, and porosity, etc.
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 U.S. 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 would 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. 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 and produce fines that impede dewatering from the paper.
Cautions: Each facility needs to develop safe practices that minimize the chance of accidents. Potential hazards involving pulp are too diverse to be listed here.
|Layered structure of a papermaking fiber in wood. Note that the fibril angle of cellulose chains differs for each sublayer|
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, email@example.com .