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

BLEACHED KRAFT PULP

Composition: Bleached kraft pulp consists mainly of cellulose and hemicellulose, two polymers that are composed of sugar molecules. The glucose units in each of these two chemicals are linked together in such a way (beta-1-4) that they are much more resistant to chemical hydrolysis than starch molecules. The cellulose chains have a great tendency to form crystalline domains that involve internal hydrogen bonding. These domains are responsible for the fact that the fibers do not dissolve in water. Hemicellulose has little tendency to crystallize due to the presence of side groups along the molecular chains. For this reason it functions like a glue, holding the fiber strands together and aiding in inter-fiber bonding when paper is produced. Kraft fibers are highly porous. The most important group of pores are slit-like and have a width between about 5 and 20 nm (based on a method called solute exclusion). Drying of the refined fibers causes some of these pores to close irreversibly and the fibers never regain all of their original flexibility and bonding ability when they are rewetted. Native cellulose fibers also have a central void area called a lumen. This void area usually collapses when the fibers are refined, formed into a sheet, pressed, and dried. Kraft fibers usually have a small amount of residual lignin and extractives (see pitch).

Function: Often the main component of paper, providing strength and other properties.

Strategies for Use: There is an economic incentive to use kraft pulp in the making of paper at the same site where it is originally produced. Such a practice avoids having to either (a) dry the fibers twice, or (b) ship large amounts of water. Also there is a loss of bonding ability each time that fibers are dried. Although this loss is moderate when drying unrefined kraft pulps, subsequent refining is likely to produce a greater level of fiber fines, and this will tend to slow the drainage. It is recommended to take care in the final washing of bleached kraft pulp to minimize carryover of highly anionic byproducts of oxidation. Market pulps are usually received as bales having a moisture content in the range of 10 to 20%, and these are dispersed with strong agitation in a hydropulper tank. Most grades of paper require the pulp to be refined by passing a slurry of 2 to 5% solids content between rotating metal bars of a refiner. The refining can be optimized by controlling the pH to the weakly alkaline range, controlling the energy content per mass of fibers (dry basis), and controlling the freeness of the resulting pulp. Excessive refining tends to hurt dewatering and bulk. By use of suitable dry-strength chemicals it is often possible to decrease the refining level and still achieve the specified strength levels.

Cautions: Pulp production involves a great many hazards. These include explosions of black-liquor recovery boilers, release of toxic gases such as chlorine and chlorine dioxide, and very high pressure steam. There are also many immediate hazards involved in handling of pulp bales with fork lifts, cutting of baling wires, and tossing bales into the maelstrom of a hydropulper.

Refining schematic: collapse of the lumen and fibrillation   Illustration of how repeated compression and shearing of fibers in a refiner weakens the cell wall (shown here in cross-section), allowing the fiber to collapse into a ribbon with fibrillation at its surface.

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 .