Volume 5, Number 2
May 1992

The Trouble with Alkaline Chemistry

For years it has been apparent that alkaline papermaking was not always easy and that customers were not always happy with the way alkaline paper behaved in their presses or copiers. But there was no literature in print about these general problems, and it was hard to guess from verbal reports how widespread they were. The industry seemed optimistic that solutions would eventually be found for all problems, and indeed many solutions have been found as experience grows, but it has been a mixed picture.

Some mills have had picture-perfect alkaline conversions and happy customers, while others have struggled for years to convert all their grades, looking for the right combination of additives and conditions to make each grade a quality product. Some printers swear by alkaline paper, while others have tried everything they knew to n-take alkaline paper work in their presses, and concluded it was impossible. Some alkaline papers have passed runnability tests on high-speed laser printers and copiers ("Alkaline Recycled Papers>," APA July 1991, p.32), but there have also been vague, persistent reports of slipperiness, limpness and feed problems with alkaline paper in various high-speed machines. The sizing (water-resistance) of paper made with alkaline sizes (AKD and ASA) is slow to develop and has been known to disappear altogether a few weeks or months after the paper is made. Papers filled with precipitated calcium carbonate (PCC) and made at a pH around 8 become increasingly alkaline after manufacture, and usually have a pH between 9 and 10 by the time they are used.

At this year's TAPPI Papermakers Conference, there was a new openness in discussing these problems and characteristics, perhaps because the chemical suppliers (who always give most of the papers) feel they are finally getting a handle on them. Case in point: "The Interactions of AKD with Other Wet-End Additives," by Anthony R. Colasurdo and Ian Thorn of Eka Nobel (p. 135-142 in the preprints). The authors addressed the issue of sizing loss due to AKD hydrolysis. Why, they asked, was this an American problem but not a European one? In the United States, PCC is widely used as a filler, while in Europe it is mainly chalk. Chalk, like ground limestone and marble, has a negligible alkalinity and a stable pH. PCC, on the other hand, starts with a pH of 8.5 but it contains unreacted lime inside the carbonate particles, which eventually emerges and affects the paper's pH.

Although the surface area of the popular scalenahedral (spiky) CaCO₃ is six times that of ground chalk, and although AKD size is attracted to the increased surface, this by itself did not cause size hydrolysis. It seems the PCC, because of its higher pH, caused certain other additives to hydrolyze the AKD. Two of the authors' recommendations are: Choose additives carefully, and plan addition points to maximize AKD reaction with the long fiber rather than the fines. (No recommendations were made to address the alkalinity of the PCC; however, this is under investigation.)

Another paper, "Surface Bonding/Sizing Agents: Keys to Improved Performance and Runnability under Alkaline Conditions," by Thomas Tompkins of Akzo Chemicals (p. 169-174 in the preprints), frankly discusses manufacturers' and printers' problems with alkaline paper, and the role of various surface sizes in solving them. Problems addressed are:

Deposits in press felts, dryer hoods, etc.
Slipperiness of finished sheet
Milking, scumming, etc.
Linting, dusting, etc.
Poor toner adhesion

Slipperiness, Tompkins says, is not as much of a problem as it used to be, because mills have found several ways of controlling it, including reduction of amount of internal alkaline size added.

There seems to be general agreement that alkaline chemistry is more complex, less stable (especially at today's machine speeds and high operating temperatures), and different in nearly every respect from acid chemistry. Because all the mills operate under different conditions (water quality, equipment combinations, and so on), there are no standard solutions. Suppliers and papermakers working together can usually find a way to make an alkaline system work, but no one should fool themselves that it is intrinsically easy.

It will be interesting to see whether the problems described at this year's TAPPI Papermakers will melt away with development of new technology, or whether they will ultimately define the limits of the possible.