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Monday, January 27, 2014

How to Increase the Dry-Strength Properties of Paper - Part 1

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 proportion of softwood kraft fibers can be increased if one wants to improve dry-strength in general, and tear strength and folding endurance in particular. Virgin kraft pulps generally have a moderate strength advantage over recycled kraft pulps of the same type, especially if freeness is held constant when making the comparison. The difference has been attributed to closing up of pores in the cell walls of the recycled kraft fibers, making them stiffer and less capable of developing bonded area. Thermomechanical pulps (TMP), especially chemithermomechanical pulps (CTMP), are noted for higher tensile strength compared to stone groundwood, since the pulping process is somewhat less destructive of fiber length. An instrument like the Techpap MorFi (online or lab) can be used to determine the fiber mix.

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 ( 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.