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Sunday, February 9, 2014

How to Increase the Dry-Strength Properties of Paper - Part 3

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).

There has been some debate as to whether dry-strength chemicals increase the relative bonded area within paper or whether they increase the strength of bonding per unit of bonded area. The answer can depend on some definitions. Usually relative bonded area is defined based on optical tests, comparing the light scattering coefficient of a paper sample with a corresponding sheet that was formed from an organic solvent, such as butanol. The latter sample will have almost zero strength, due to the non-swollen condition of the fibers during the formation process, the inability of hydrogen bonds to form, and the inability of cellulosic macromolecules on the adjacent surfaces to intermingle. But the optical tests cannot sense effects of distances that are less than about a quarter of a wavelength of light, e.g. about 50 nm, which is much larger than the range of a typical chemical bond (except possibly polymeric bridges in the expanded form). That means that the optically bonded area generally is expected to be much higher than the actual area of contact on a molecular level. 
Experiments have shown that dry-strength chemicals such as cationic starch tend to increase the strength per unit of optically bonded area to a greater extent than they increase the optically bonded area [Howard, Jowsay 1989].

Once the pulp has been refined to an optimum extent (most important variable), moderate improvements in strength can be achieved by adding chemicals (secondary effect). The most popular dry-strength additive for the wet end in the U.S. is cationic starch. Cationic versions of corn, potato, and tapioca starches usually are formed by alkaline treatment of slurries of starch grains with an epoxide chemical that contains a quaternary ammonium group. In the case of wet-end products, there is no intentional degradation of the starch molecular mass, and most wet-end starches need to be cooked before use. Batch conditions often involve about 20 minutes of stirring below the boiling point of water. Alternatively, starch can be solubilized in a jet cooker.

The optimum addition point for cationic starch is usually complicated by local situations, including the availability of inlet taps and the need to use many of those addition points for other additives. A general principle is that adsorption of dry-strength agent onto long fibers is expected to yield a greater positive effect on paper strength than an equal amount added to the fines fraction. That means that there is sometimes an advantage of mixing cationic starch with the thick stock before it is diluted with fines-rich white water at the fan pump.

Strength benefits of cationic starch and similar additives (including cationic guar gum) tend to show diminishing returns with increasing dosage. The practical upper limit of starch addition usually is related to the available surface area of the wetted solids in the furnish, and to some extent also on the amount of negatively charged carboxylate groups of those surfaces. Common practical maximum addition levels of cationic starch, depending on the nature of the furnish and the need for strength, often lie between 1% and 1.5% (20 to 30 lb/ton). Evidence that the adsorption capacity of the fiber surface for cationic starch has been exceeded often comes in the form of increased foaming. This is because starch that is in solution, rather than on fiber surfaces, can act as a stabilizer for foam bubbles.

In cases where the dry-strength effects of cationic starch alone are not sufficient, one has options of (a) using a microparticle additive that may make it possible to increase the amount of starch that can be retained and also promote faster dewatering so that the sheet can enter the wet-press section with less water and become consolidated more effectively, or (b) using synthetic dry-strength additives. Anionic and amphoteric acrylamide polymers have been shown to have a superior dry-strengthening ability in some tests. Anionic acrylamide products and carboxymethyl cellulose (CMC) can be added in sequence with a suitable high-charge cationic polymer in order to achieve efficient retention on the fiber surfaces.

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.