Volume 3, Number 3
Aug 1990

The Genesis of Alkaline Sizing and Alkaline-curing Wet-Strength Resins

Since 1803, rosin-alum paper sizes have served the paper industry's need for inexpensive, effective water repellents for paper. For disposable papers such as packaging, or papers that do not require flexing such as the facing of wallboard, they have served well. However, readers of this publication will not need reminding of the long-term destruction of book and magazine paper by the rosin-size alum complex. The acidic alum slowly degrades the long molecular chains of the cellulose that comprise the paper. As these chains are shortened, so also are the strength and the life of the paper. And as the paper dies.. the information it bears is lost. A half-century ago, alkaline filled paper was foreseen to be a solution to this problem (1,2).

The first practical sizes for alkaline papermaking were introduced commercially by Hercules Incorporated (then the Hercules Powder Company. They were based on alkylketene dimers ("AKDs"),

in which the groups abbreviated "R" are long hydrocarbon chains. AKDs are made from natural fatty acids, R-CO-OH, derived from animal fats or vegetable oils. Just as oil and water do not mix, so the long hydrocarbon portions of the AKD molecule repel water. The square C-C-C-0- ring is strained, because the bonds between its atom are at a sharper angle than usual for linear molecules. This strain makes the molecule reactive, like a coiled spring, and it reacts with cellulose to anchor itself and implant the water-repellent hydrocarbon tails.

Ketene dimer,

has been known for a long time, but alkylketene dimers were first synthesized in the late 1930's (3). As solutions in organic solvents, they were used to make textiles water-repellent (4). Their preparation as water emulsions (5) the key to their successful use as paper sizes (6), since paper is made from a water suspension.

Between 1952 and 1954, AKD size emulsions underwent successful mill trials in alkaline jute liner and fiber drum stock, and especially in calcium carbonate-filled papers. They were first publicized in February, 1955 (7) under the trademark "AQUAPEL®. AKD emulsions gained immediate market acceptance because of their high efficiency and cost-effectiveness vs. rosin-alum size. Less detrimental to paper strength, they allowed the use of higher filler loading.

Emulsified organic materials and mineral filler particles, such as calcium carbonate, etc., do not stick spontaneously to each other or to paper fibers suspended in water because of mutual repelling negative charges on the particle or fiber surface. Retention aids comprising various water-soluble organic polymers can mask or neutralize these charges. Some polymers can also form adherent bridges between particles. The agglomerated particles can then be filtered and retained by the fiber mat. In the initial trials of AKD size in CaCO₃-filled papers at S. D. Warren Co., the employment of polysaccharide gums as retention aids was a crucial part of their success (8).

Wet-strength resins differ from paper sizes. The resins allow paper to retain part of its original strength after absorbing water. This ability is essential in napkins, facial tissue, and towelling. Sizes repel water, printing ink, and other fluids, but once these do penetrate the sheet, the paper's original strength is destroyed. In packaging such as corrugated cartons, both size and wet-strength resins may be used, as a chemical version of "both suspenders and a belt". In printing papers, wet-strength is sought less often than is sizing, although both effects may be needed in some specialty papers such as maps.

During the 1950's, Gerry Keim, the coinventor of AKD sizing of paper, had developed a family of improved urea-formaldehyde wet-strength resins. In order to cure to an insoluble network m heating and drying, these need an acid catalyst--especially alum, the partner of rosin size. Thus, Hercules already had a compatible line of acid-curing sizes and acid-curing wet-strength resins. With the commercial debut of AQUAPEL® alkaline AKD size, Gerry turned his attention to developing a complementary alkaline-curing strength resin. After first investigating bisphenol-epichlorohydrin products and polyamine-epichlorohydrin reaction products analogous to epoxy resin adhesives, Gerry then considered the less expensive products of epichlorohydrin and amino-polyamides derived from polyamines and inexpensive organic diacids. On January 8, 1957, Gerry Keim and Ed Meginnis made the first laboratory sample of the polyamide-epichlorohydrin product that would eventually be known as 557 wet-strength resin (9). Pound for pound, it was far more effective than urea-formaldehyde resin, and it resulted in more absorbent sanitary papers

Kymene 557 resin was soon found to be good not only for wet strength, but also as a retention aid for Aquapel AKD size. As a cationic polymer, it could not only mask but neutralize the negative charges on size particles and fibers, allowing the particles to adhere to the pulp. Moreover, once the size particles were retained in the sheet, the resin appeared to catalyze the development of water repellency. Thus, the complementary alkaline-curing products could be yoked together, and are particularly effective in beverage cartons. Thanks to the combination of AKD size and polyamide resins, the wax-dipped cartons of a generation ago have been replaced completely by the sleek milk and juice cartons of today.

Beverage cartons have high demands for structural strength and liquid hold-out, but their use as culture bearers is ephemeral. The combination of AKD size and polyamide resins was soon to be recognized as a recipe for paper permanence as well. A 1960 news item read (10):

Two PMC [Papermakers Chemical Department, Hercules] products help to make possible a paper with a life span of 400 years. Aquapel® [AKD size] was used for sizing and Kymene® 557 [wet strength resin] for retention of calcium carbonate. This type of permanent paper was developed by a well-known restorer of documents [W. J. Barrow] under a grant to the Virginia State Library by the Ford Foundation's Council on Library Resources.

In the steps of AKD size, other alkaline sizes such as alkenylsuccinic anhydride (ASA) have come on the market. These highly reactive materials have to be emulsified by the customer immediately before use, to avoid loss of activity and formation of deposits by their reaction with water.

Recently, the traditional acidic rosin-alum sizing system has been adapted to impart sizing under neutral conditions. The development of a self-retaining, cationic dispersed rosin size (Hi-pHase® sizing agent) allows the use of rosin-alum with minimal alum and at higher pH. Hi-pHase® sizing agent with low levels of alum is routinely used at pH 6.5, and has been effective in true neutral system containing low levels of calcium carbonate. Meanwhile, AKD size-Kymene 557 resin emulsions have been supplanted by AKD emulsions that contain new generations of cationic polymer retention aids, while maintaining effectiveness and convenience. These new AKD size Emulsions continue to nurture the growth of alkaline-sized paper today.

Acknowledgment

I appreciate the contributions of information by Mr. L. Brown, S.D. Warren Co., and Dr. S.M. Ehrhardt, Hercules Incorporated.

References

(1) M.B. Shaw and M.J. O'Leary, Bur. Standards J. Research 21, 671 (1938).

(2) E. Sutermeister, in Product Bulletin No. 16, S.D. Warren Co., June 1941.

(3) R. Heuter, "Process for preparing ketenes", U.S. Pat. 2,383,863, Aug. 28, 1945.

(4) W. Hentrich and R. Heuter, "Process for rendering textiles, fibers, and the like water-repellent", U.S. Pat. 2,411,860, Dec. 3, 1946.

(5) W.F. Downey, "High alkyl ketene dimer emulsion", U.S. Pat. 2,627,477, Feb. 3, 1953.

(6) G.I. Keim and W.D. Thompson, "Process of sizing paper with an aqueous emulsion of ketene dimer", U.S. Pat. 2,762,270, Sept. 11, 1956.

(7) J.W. Davis, W.H. Roberson and C.A. Weisgerber, "A New Sizing Agent for Paper - Alkylketene Dimers", presentation at TAPPI Meeting, New York, Feb. 23, 1955; published in TAPPI, 39 (1), 21-23, Jan. 1956.

(8) T.H. Werner, W.H. Marra, and W.B. Gilman, "Sized mineral filled paper and method of making same", U.S. Pat. 2,992,964, July 18, 1961.

(9) G.I. Keim, "Wet-strength paper and method of making same", U.S. Pat. 2,926,116, and "Cationic thermosetting polyamide-epichlorohydrin resins and process of making same", U.S. Pat. 2,926,154; both Feb. 23, 1960.

(10) Anon., Hercules Mixer, September 1960.