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Saturday, February 15, 2014

How to Increase the Dry-Strength Properties of Paper - Part 4

The primary tools by which papermakers can increase the dry-strength properties of paper are selection or purchase of a suitable quality and type of fibers, increased refining, the use of dry-strength additives, and changing the conditions of wet-pressing (if possible, given the equipment).

The size press is a very important tool for increasing paper strength, partly because of the fact that the practical addition levels are typically much higher, compared to wet-end addition. Also, it is possible to apply relatively inexpensive starch products. "Unmodified" corn starch, which is probably the major size-press additive used in the U.S., needs to be reduced in molecular mass by treatment with enzymes or oxidizing agent just before use in order to reduce the viscosity. Though the degradation decreases the strength of the resulting starch film, this is a necessary compromise that papermakers make in order to run the equipment effectively.An instrument like the Emtec PDA & EST can help you measure your surface sizing.

Sometimes poor performance of size-press starch can be traced to an undesired process of crystal formation, known as retrogradation. Retrogradation is most prominent in the linear component of most starch products, the amylose. Retrogradation is much less of a problem in the case of wet-end starches, since the molecules usually are substituted with cationic groups. Hydroxyethylated starch products for the size press are noted for high strength efficiency, as well as high resistance to retrogradation, and their performance often justifies their higher cost.

See the page related to hold out at the size press for a discussion of how to achieve a balance between internal bonding (if the starch penetrates into the paper) versus surface strength (if the starch is held out effectively).

Conditions needed to maximize tensile strength of paper will not necessarily maximize either the compression strength or stiffness. Such differences can be expected, due to the fact that the latter properties demand less flexibility of the overall product. By contrast, tensile strength can benefit from some ability of the paper to stretch and deform so that the load can be borne more evenly among fibers in the paper.

This information comes from NC State University with the following disclaimer.

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (m_hubbe@ncsu.edu). 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.