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Sunday, February 2, 2014

How to Increase the Dry-Strength Properties of Paper - Part 2

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).

Refining (sometimes called "beating") can be defined as the repeated passage of wood pulp through zones of compression and shearing. Refiners usually consist of pairs of surfaces with raised metal bars that rotate relate to each other. Important variables include the energy input per unit mass of fiber (after subtracting out the energy required in the case of water alone), the rate of rotation, and the total length of bar edges encountered by fibers during one pass. The effects of refining are most often evaluated by freeness tests. This practice is somewhat unfortunate, since the reduction in rate of dewatering of the pulp is an undesired side-effect of refining, not the main goal. It has been shown that fines in the furnish, often produced during the refining process, tend to dominate the observed changes in freeness as pulp is refined. In theory it would be better to evaluate the extent of refining by measuring the strength of test sheets and by measuring the water content remaining in plugs of fiber that have been centrifuged under standard conditions (water retention value). Kraft fibers in particular are known to become more water-swollen during refining, and there is often a high correlation between water retention value and inter-fiber bond strength.

One of the first considerations in improving paper strength ought to be whether the refining conditions are at their optimum. Often there are opportunities to switch to refiner plates with a finer bar pattern, offering a lower energy input per number of bars encountered by a typical fiber (a measure of the intensity of the refining action). A finer pattern often is less energy-efficient in terms of freeness reduction, but there can be a substantially reduced tendency for fiber shortening. Rather, one achieves more of the desired effects of making the fibers more flexible and partially delaminating the outer layers, creating some fibrillation of the surfaces. It is expected that kraft fibers refined at low intensity ought to develop higher strength (especially tensile strength) compared to the same fibers refined to the same freeness at higher intensity. Some rules-of-thumb regarding optimum refining levels are given by Baker [1995].

Before turning to chemical factors, it is worth noting that wet-pressing can have a major impact on paper strength. This factor is sometimes overlooked due to the fact that papermakers generally keep wet-press nips near to their maximum practical pressure, short of crushing the sheet. A sheet that comes into a wet-press nip too wet, relative to the applied pressure may be crushed, meaning that the fiber structure created during formation is disrupted, often resulting in breaks.

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.