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Dry-Strength Resins (DSR)

[3, 9–11, 22–24] Some of the specific mechanical properties important for paper are tensile strength, tearing resistance, folding endurance, bending resistance, burst and sur¬face strength, internal bond and compression resistance. It is generally accepted that the major factors contributing to dry strength development in paper manu¬facture include Van der Waals forces, hydrogen bonding and ionic bonding. Be¬cause of the special effect of water on paper strength, it is common to distinguish between “dry” and “wet” strength properties of paper. The two subjects are ob¬viously related, but wet strength will be treated as a separate topic. Paper strength is affected by many furnish and process variables. On the furnish side, longer softwood fibers produce stronger papers than shorter hardwood fibers. Fillers re¬duce strength. Alkaline pH conditions in the wet-end produce stronger papers, especially after aging, than do acid pH conditions. On the process side, both in¬creased refining and wet pressing increase paper strength. The basic factors that influence paper strength are: individual fiber strength, interfiber bond strength, the number of interfiber bonds (bonded area) and the distribution of fibers (sheet formation). While the first factor cannot be influenced by strength additives, the remaining three factors can be strongly influenced by such products.

Many water soluble, hydrogen bonding polymers will act as dry strength ad¬ditives. In fact, wood fibers contain their own natural dry strength additive in the form of hemicelluloses. It is well-known that the removal of hemicelluloses from wood fibers makes it more difficult to develop their bonding characteristics.
. • Starch: In general starch derivatives represent the most common and by far the largest amount of dry strength additives (Fig. 3.1).

. • Vegetable Gums: Much less important but also used are water soluble vegetable gums, such as locust bean gum and guar gum. These highly hydrophilic poly¬mers have chemical structures which are similar to cellulose, enabling them to participate in extensive hydrogen bonding with fiber surfaces. The natural mate¬rials are nonionic and are not retained by fibers to any extent. Consequently, successful commercial products all have cationic groups attached to the main chain, which increases the attraction between gum molecules and fibers and results in improved polymer retention. Due to the combination of high retention and effective dry strength enhancement only 0.1–0.35 % of the material has to be added in most instances.

. • Polyacrylamide Resins: The fully synthetic DSR are of growing importance. In Japan they already form the largest proportion of the total consumption of dry strength resins. Their worldwide consumption is an average of 300 g dry poly¬mer per ton of paper or 3 % of the total amount of specialty chemicals (see Fig. 3.3). In the group of synthetic dry strength resins the polyacrylamide-based products are still dominant today. Since anionic polyacrylamides are negatively charged, they are not directly attracted to paper fibers. A cationic substance, such as alum or polyamide epichlorohydrin resin, must be used to promote their retention. To avoid the need for a cationic promoter, it is possible to incorporate cationic groups (e. g. methacryuloyloxethyl trimethyl ammonium methosulfate, dimethyldiallyl ammonium chloride, vinyl benzyl trimethyl ammonium chlo¬ride, 3-acrylamido-3-methyl butyl trimethyl ammonium chloride) directly into the polyacrylamide backbone by copolymerization. When used as dry strength additives, typically 10 % of the monomers will be charged and their molecular weight will be between 100 000 and 500 000. This range is low enough that the polymers will not bridge between particles and cause flocculation, and high enough to retard migration of the polymer into the fiber pores with concomitant loss of activity.

• Polyvinylformamide/Polyvinylamine Resins (PVF/PVAm): Recently developed new types of polymers in the form of polyvinylformamide and polyvinylamine are coming into use. All these water soluble polymers contain primary amino groups that can form hydrogen bonds with surface cellulose molecules in fibers and improve interfiber bonding. These polymers enable the papermaker to achieve combinations of paper properties that cannot be obtained through refin¬ing alone. For example, strength properties can be improved without affecting sheet bulk and/or appearance properties. The new DSRs are produced by polymerizing vinylformamide and then hydro¬lyzing it. This results in a chain type macromolecule with primary amino groups without using an additional monomer. These polymers can be varied within a very wide range of molar mass and charge density to optimize the performance. Medium or low charged polyvinylamines of medium molecular length give high performing dry strength resins. Further improvements in cost-performance can be achieved by product combinations, e. g. low/middle charged cationic poly¬vinylamine plus low molecular, middle/high charged anionic polyacrylamide or cationic polyvinylamide plus anionic polyvinylformamide. These products have no remaining monomers, and do not contain formalde¬hyde or organically bound chlorine, therefore they do not release chlorine to the effluent. The environmental advantages of this new group of DSRs is underlined by the fact that they have been approved by the German health authorities (BfR-Bundesinstitut für Risikobewertung) for food packaging paper and board. PVF and PVAm products also conform to the requirements of the United States Food and Drug Administration (FDA) regulation 21 CFR 176.170 (Components of paper and board in contact with aqueous and fatty foods), according to its cur¬rent status. It may be added at levels of up to 1.5 % solid polymer, expressed as a proportion of the dry, finished paper. Polyacrylamide- and polyvinylamine-based DSRs are mainly supplied as ready to use aqueous solutions or emulsions (10–40 %) or as water soluble powders that must be dissolved prior to use. No other preparatory steps are necessary. The most beneficial way of application is to meter them continuously to the thick paper stock at a point of thorough mixing, e. g. at stock dilution. Addition rates of 0.1 to 0.5 % of solid material are adequate for most uses. Excessive use can overcationize the stock suspension and reduce the effectiveness of the DSR and of other cationic additives.

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