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


Composition: Kraft pulp is what you get after you place a chip of wood in a pressurized vessel in the presence of hot caustic soda and sodium sulfide. The cooking process attacks and eventually dissolves the phenolic material called lignin that glues the fibers to each other in the wood. The word "kraft" means "strong" in the language of its origin. German manufactures of pulp discovered that addition of the sulfur to a "soda cook" improved the selectivity of the process - dissolving the lignin with less damage to the cellulose. The fibers initially liberated by kraft pulping usually are brown, similar to a typical grocery bag (paper type) or corrugated box. Bleaching of kraft pulps requires a series of aggressive chemical treatments with oxidizing agents and caustic soda. In either case, kraft pulps tend to be more flexible than fibers produced by purely mechanical refining of pulp. The superior bonding potential of kraft pulps becomes evident when they are refined sufficiently to allow collapse of the lumen, delamination of the cell wall, and partial fibrillation of the surfaces. Kraft fibers tend to be more porous, compared to mechanical fibers. Slit-like pores having widths in the range of 5 to 20 nm appear to occupy the space formerly occupied by lignin. Each time that paper is dried, many of these pores appear to close irreversibly. Under ordinary condition of repulping, the fibers do not regain their original ability to swell with water.

Function: Main structural element in many paper grades, including corrugated boxes, office papers, liquid cartons, high-grade folding boxboard, labels, bags, cups, and many more. Grades such as magazine paper, even if they contain mostly mechanical fibers, are likely to contain some softwood kraft pulp to achieve needed levels of fold resistance and tear strength.

Strategies for Use: One of the keys to successful use of kraft pulps is optimization and uniformity of refining conditions. Refining usually is needed in order to meet bonding strength objectives. However, too much refining causes excessive damage to individual fibers, yielding paper that may be too thin or too easy to tear. In such cases dry-strength additives such as cationic starch may be used in order to meet bonding strength requirements with less loss of bulk and tear strength. Uniform refining can be achieved by a combination of approaches. These include control of the consistencies and flows to a refiner, control of the amount of energy expended per dry-mass of pulp, and on-line freeness control (usually not available). Higher pH conditions (in the range up to about 8) favor more rapid refining with less damage to the fibers, especially in the case of high-yield kraft pulps. Hardwood and softwood can be refined separately, then recombined, since the softwood usually can benefit from more severe or prolonged mechanical treatment. In general, a kraft pulp blend that is rich in softwood fibers (average length about 3 mm) is easy to run on a paper machine and the paper is tough. A kraft pulp blend rich in hardwood fiber is less forgiving of non-ideal tensions in open draws on the paper machine and the product is more easily torn.

Cautions: Kraft fibers and slurries of these fibers are usually non-hazardous once they are delivered to the paper mill. Various toxic materials such as sodium sulfide and chlorine dioxide are used in their preparation.

Fiber composition versus pulping yield   Composition of wood fibers as a function of the extent of kraft pulping

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, .