Volume 3, Number 4
Oct 1990

Trends in Neutral Sizing: Excerpts from an Interview with Paul Rostek

[Editor's Note: Paul Rostek is Director, Erco Systems Group at Albright & Wilson Americas. At the time of the interview be was Manager of US Sales and Marketing for their Paper Chemicals Group. Paul is a chemical engineer with 14 years experience in paper and related industries. His specialty is internal and surface sizing chemistry. The interview is conducted by Barbara Wortley, Senior Technical Manager, General Chemical Corporation, Syracuse. It appeared originally in the 'Paper Chemistry' column in PIMA Magazine, May (p. 49-50), June (p. 92-93) and July 1990 (p. 43-44). The following questions from the interview are reprinted here by permission of the publisher. If you would like a full reprint of the three-part interview, send a letter on your company letterhead to Barbara Wortley, General Chemical Corp., 344 West Genesee, Suite 100, Syracuse, NY 13202.1

BW: Can you compare sizing with ASA and AKD sizes.

PR: Both AKD and ASA sizes have had more difficulty sizing paper containing high levels of PCC filler than Europeans have had with these sizes with ground calcium carbonate (chalk). ASA sizes do not develop hard sizing as well as AKD. Many problems with ASA concern runnability due to the deposition of hydrolyzed ASA on the paper machine and other runnability issues. The latest generation of AKD sizes can achieve the sizing required; however in PCC-containing papers, slip type problem are sometimes experienced. Alkaline-sized papers also may have poorer toner adhesion in copier paper when compared with traditional rosin-based paper because of differences in surface energy. Chemical surface treatment can change the coefficient of friction and improve toner adhesion. This treatment adds cost, although some of it can be offset by allowing the use of slightly lower ketene dimer levels.

BW: How much sizing on a sheet is developed on the size press compared to internal sizing?

PR: A balanced AKD system can give 80 to 85 percent of full sizing before the size press. You need good fines and filler retention because AKD size is strongly attracted to fillers and fines. You also need good drying before the size press to get the ketene dimer into a hydrophobic state. Some chemicals, such as polymers, can improve the amount of sizing developed on the machine. Also, the chemical addition sequence influences sizing to such a degree that it can mean the difference between attaining a good level of sizing or almost no sizing at all.

BW: Neutral rosin systems have been used for many years in Europe with grades incorporating ground chalk filler. Do you see a problem with these sizes in the presence of PCC?

PR: Our initial studies suggest that there should be no major problems so long as you have a source of aluminum present that carries a cationic charge at the operating pH. Aluminum sulfate is difficult to use at neutral pH because it tends to form aluminum hydroxide with minimal charge well below the neutral range. Polyaluminum chlorides offer an alternative because they provide cationic aluminum species that fix the rosin into a hydrophobic state at neutral pH.

BW: Some paper grades will stay with clay filler and use carbonate for coating. The lower amounts of calcium carbonate will allow the system to buffer out near 7.0. Do you believe this is a market for neutral rosin sizing?

PR: Some who make coated printing papers produce a brighter sheet by incorporating calcium carbonate into coating formulations, in addition to moderate levels in the base sheet. When fillers are moderate enough, the use of neutral pH rosin sizing system will be possible.

BW: Is it true that calcium carbonate won't be solubilized at neutral pH?

PR: Very little calcium carbonate goes into solution above a pH of 6.9. Rosin systems that use PCC unit have some degree of pH control because PCC causes papermaking systems to buffer at a higher pH than they would with ground limestone.

BW: Can PCC be produced to buffer at a lower pH?

PR: Yes, but problems may arise when paper containing lower pH PCC is recycled. It is likely that the calcium carbonate will, on recycling, buffer at a higher pH.

BW: But purchasers of secondary fiber get some paper with high PCC content and can operate at neutral pH, sometimes without pH control.

PR: The use of neutral pH rosin sizing systems with PCC-containing furnishes is still being evaluated. These system have been run successfully at pH close to 8.0 at one mill, but it is recommended that the operating conditions be controlled at around pH 7.0 to 7.3.

BW: What are the pluses and minuses of neutral rosin system versus synthetic size system.

PR: Neutral rosin sizes could enhance things like toner adhesion in copier papers and conversion for envelope producers. Few commercial papers have been produced with PCC in neutral rosin systems, so it is hard to say what the benefits are at this time.

BW: How will the recycling of paper affect size usage?

PR: Recycling should affect size use and pH conditions at many North American mills, even those that do not make whitepapers. As more of the industry goes alkaline and as recycling increases, mills with rosin acid systems will find that PCC in the recycled fiber increases system variability and operating cost.

This happened in the United Kingdom where some fine paper production went alkaline and used high levels of calcium carbonate. Non alkaline mills that used recycled fiber found their rosin acid sizing systems were no longer effective. They first tried to neutralize the carbonate with sulfuric acid, but this was not easy to do. A great deal of calcium carbonate had to be solubilized to form a furnish that could be treated with alum.

Another problem often experienced was that the sulfuric acid added to reduce pH would neutralize the calcium carbonate particle surfaces. However, on passage through pumps and refiners, fresh calcium carbonate would be exposed and the furnish was once again above pH 7.0. Eventually, mills either switched to a neutral pH with a suitable rosin system or more commonly to an alkyl ketene dimer-based system. In isolated cases, mills discontinued the use of purchased recycled fiber.

BW: Do synthetic sizes interfere with the functioning of photocopiers.

PR: Synthetic sizes have a tendency to impair toner adhesion, probably because they alter the surface energy of the paper. There was also concern that ketene dimer-sized, precipitated calcium carbonate-filled papers gave poor runnability through photocopiers due to a variable coefficient of friction. Both issues can be addressed by using surface treatments at the size press.

BW: Is fugitive sizing as much a problem with AKD as ASA?

PR: Sizing fugitivity is a problem at some locations and not at others with both types of synthetic sizes. Often the paper producer is not aware the problem exists because the paper is tested as it comes off the machine. It may meet specs then, but in two or three some sizing my have been lost.

We are not entirely clear what causes fugitive sizing. It would be unlikely with ketene dimer, if it is truly cellulose reactive and an ester bond is formed. But fugitivity has occurred in AKD--sized papers, which suggest that the ester bond has not been formed and ketene dimer has weak hydrogen bonding in the hydrophobic state. If the weaker bond is broken the hydrophobic orientation is lost.

BW: What difference does it make if you form the hydrophobic surface on cellulose fiber, wood fines or PCC?

PR: That's a good question. I think fugitivity wasn't an issue until the advent of PCC-filled sheets. There is some concern that, over time, size attached to PCC may lose its hydrophobic orientation under certain conditions. More study is needed before we know what mechanisms are involved.