Wet Strength (Low, Repulping of)
The ability of paper products to maintain a substantial proportion of their original strength after being completely saturated with aqueous solution is known as wet strength. Sometimes the criterion for defining a wet-strength grade of paper is that the ratio of wet to dry tensile or burst strength is at least ten or fifteen percent. Wet strength can be critical to the performance of certain paper towels, bags, beverage containers, and containerboard products that will be used under wet conditions.
The primary means of increasing the wet strength of paper consists of reactive, polymeric chemicals. Most prominent among wet-strength additives is the family of polyamidoamine-epichlorohydrin resins. The performance of such resins can be maximized by adding them at a point in the process where the pH is within the range of about 6 to 9, the additive is well mixed with the furnish, and the charge of the furnish is sufficiently negative so that the system does not become excessively positive in surface charge upon addition of the wet strength resin. In some cases the performance of cationic wet-strength resin can be improved by sequential addition of carboxymethylcellulose or other negatively charged additives. Also in some cases there can be an advantage of adding the wet-strength resin to thick stock before it becomes diluted at the fan pump with fines-rich white water.
Factors that favor a high uptake and high effectiveness of wet-strength resin on kraft fibers include (a) the presence of negative charge at the fiber surfaces, usually associated with relatively high yield or unbleached nature, (b) increased refining, which increases the available surface area for adsorption of wet-strength resin, and (c) increasing pH in the range of 3 to 8, since the carboxylate groups at the fiber surface will tend to become increasingly dissociated, making the surfaces more and more negative.
A wide range of wet-strength levels are required in different paper products. What might be considered "good" wet strength in one situation will be considered as "too low" in another. Wet-strength evaluation of competitive products is recommended to get a sense of whether wet-strength targets are reasonable, especially in the case of a new product that is being introduced by a company to compete with or replace existing paper products.
LOW WET STRENGTH can be due to insufficiencies in any of the factors mentioned in the previous paragraphs. Assuming that a polyamidoamine-epichlorohydrin resin is being used, some key possibilities to test are (a) the pH is too low for efficient retention and cure of the resin, (b) the pulp is not sufficiently refined to allow all of the wet-strength resin to adsorb effectively, (c) the system contains too much cationic starch, in addition to the wet-strength resin, so that a lot of the cationic polymer additives remain in the water phase, (d) wet-strength resin is being degraded by chlorine dioxide or other oxidizing agents added for slime control, (e) the paper is not being dried sufficiently to cure the product, or (f) the product needs to be evaluated only after it has been oven-cured, or after it has had additional time to cure during storage as part of a hot roll of paper.
One of the tricks that papermakers use to achieve wet-strength targets, especially when the customers have specified certain test criteria, is to use relatively high treatment levels of internal sizing agents such as AKD. Though there is a big difference, in principle, between making the paper resist water penetration and making it retain some of its strength after complete wetting, these distinctions are sometimes blurred. If the sizing treatment is enough to keep the paper from completely wetting under the specified conditions of a wet-strength test, then it is likely that a high value will be recorded for the apparent wet strength.
REPULPING DIFFICULTIES WITH WET-STRENGTHENED PAPER
Flakes of poorly dispersed paper often plague papermaking operations that have to deal with repulping of wet-strength paper, whether it consists of broke from their own process or whether it arrives at the mill as waste paper. The main tools used by papermakers to repulp broke include (a) high intensity shearing in large pulpers, (b) elevated temperatures, within the limits of what has been found to work well as an overall system temperature of the wet end, and (c) time. In some cases the furnish may be passed through a refiner to break down any remaining flakes.
Strategies for repulping of broke differ for bleached kraft pulp, versus other furnish that contains substantial amounts of unbleached kraft or high-yield fiber. Bleached kraft is often treated with sodium hypochlorite or other oxidizing agents to help break down the wet-strength resin. Those options are not practical for the unbleached or high-yield pulps, since the oxidant would be consumed by lignin in the fiber. Rather, one would have to rely on the combined effects of time, increased temperature, high shear, and possibly increased pH to swell the fibers.
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 (firstname.lastname@example.org). 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. Go to top of this page.