Volume 3, Number 3
Aug 1990

Questions on Alum Usage Answered

An extended excerpt from an article of the same title in the December 1989 PIMA (P. 52-53). Reprinted with permission.

A mill manager writes: "I am confused about the use of alum with AKD (alkyl ketene dimer) size. At first we were told not to use alum. Then we learned that alum aids AKD systems. Now an article in the July Tappi Journal (Akpabio, U. D. and Roberts, J. C., 'The Effect of Alum on the Mechanism of Alkylketene Dimer Sizing,' P. 141-145) asserts that alum is detrimental to sizing development with AKD. Does alum help or hinder AKD sizing?" I understand the mill manager's confusion. The article cited just doesn't match current industry usage of alum in alkaline systems.

Early promoters of alkaline sizing warned that alum should not be used because it is incompatible with synthetic sizes. Then it was discovered that small amounts of alum improved runnability and made the system work. Alum provides aluminum ions that enable formulation of non-tacky aluminum salt of the hydrolyzed ASA size rather than sticky calcium or magnesium salts.

Alum addition to ASA and AKD systems, at a rate of three to 10 pounds per ton of furnish, provides many benefits including improved sizing, drainage, and retention (particularly of titanium dioxide) and reduces press picking problem. Alum is an economical source of cationic (positive) charge which can lower the overall anionic (negative) charge of the system and can precipitate natural wood resins and hydrolyzed size onto fibers, so they are carried out of the system.

Sizing can be improved by the proper orientation of the size on the fiber surface. Positively charged colloidal aluminum floc traps anionic fillers and fines and deposits them within the fibrous system. Even small amounts of dissolved or almost colloidal anionic fines can disrupt drainage. Adsorption of various aluminum species on fiber surfaces provides a cationic patch for filler and fines retention.

In the article cited above, the authors performed a series of experiments to show the effects of extreme alum usage. However, these experiments do not mirror real world conditions and do not support the conclusion that alum is detrimental to sizing development with AKD. In these experiments, the alum addition rate was up to 80 percent alum, or 1,600 pounds of alum per ton of furnish. This is 160 times the amount of alum used in alkaline papermaking! In addition, all experiments were done under acid conditions from pH 4.4 to 5.7. AKD is not an effective sizing agent in acid media, so it is hard to understand why an AKD sizing evaluation would be done in these ranges.

The article does illustrate the buffering effect of alum: 40 pounds of alum per ton of furnish dropped the pH to 5.0; 800 pounds of alum dropped the pH to 4.5; and with 1,600 pounds, the pH dropped to 4.4.

Alum is readily adsorbed onto the fiber and fines surfaces under the conditions found in alkaline papermaking. When used in such extreme amounts, alum would interfere with proper size attachment of a cellulose reactive size because fiber surfaces are so densely coated with hydrolyzed aluminum that little surface area is left for the AKD reaction.

This article also decries the fact that alum decreases the anionic charge of the papermaking system, coagulates and flocculates fines and deposits them on the fiber surfaces. These actions, in fact, are desirable as stated above for retention and drainage improvement. Another confusing conclusion was that poor sizing development may be caused by preferential adsorption of cationic starch-stabilized size particles on the positively charged (cationic) aluminum ion in the stock. Actually, preferential adsorption occurs between oppositely charged particles.

The paper also implies that sulfuric acid can be substituted for alum in alkaline paper . This is not true. Sulfuric acid is added to alkaline systems only if it is necessary to maintain a particular pH. This is generally not needed with AKD/calcium carbonate systems that buffer near pH 8.

The paper does show that there can be too much of a good thing. Though alum has been used for more than a century and has often been called the "papermaker's friend," it will restrict size development if grossly overapplied.

This brings to mind another concern expressed to me. Some papermakers say they advertise an acid-free paper and so cannot add alum. Yet even with the use of alum, the overall system is alkaline and so is acid-free. Also, acid formed from hydrolysis reactions or atmospheric conditions is neutralized when carbonate is incorporated into the sheet even in small amounts.