Publication: Pulp & Paper International
Issue: September 1993
Author: Ian Barrett

Amphoteric Starches:

Increasing strength across the board

At Mayr-Melnhof's Colthrop mill in the UK, adding an amphoteric starch to the furnish has improved surface strength and ply bonding for coated board grades. Ian Barrett reports.

With printers demanding increasingly high-quality surfaces for faster machine runs, a smooth coating and superior printability are imperative in coated boards. However, internal strength remains one of the primary factors for producers of any type of board.

At the 55,000-ton/yr Colthrop coated board mill near Newbury in the UK, the installation of a new coating section in 1991 had greatly increased surface smoothness of the end product, resulting in a substantially crisper ink impression. However, the latex-based coating caused adhesion between the printing ink and coating surface to increase so dramatically that sheet rupture became a major problem. Performance tests revealed that adhesion levels in the board, particularly between the top plies, were insufficient. The mill therefore began to look for a way to eliminate sheet rupture, by enhancing internal product integrity and strength without sacrificing surface smoothness and printability.

Positive approach has negative effects

Owned by the Austrian papermaker Mayr-Melnhof since 1991, Colthrop had already been using cationic farina and maize starches in its wet-end system for about 15 years. These starches have a cat-ionic charge which forms an electrochemical bond with the anionic fiber. This boosts strength and prevents starch from being lost in the mill effluent.

With cationic starches, however, there is an optimal addition level, beyond which any subsequent starch addition is ineffective. The papermaker also runs the risk of over-cationizing the system. This occurs when too much cationic starch is added and the balance between the charges in the wet-end chemistry becomes unstable. In comparison with other standard papermaking chemicals, cationic starch is a poor competitor for the negative charges with which it has to bond to create strength.

In an attempt to address the problem of inadequate board strength, the mill had been adding ever-increasing amounts of cationic starch, with no improvements in strength. In January 1992, starch consumption, at 66% above the norm, was well above desirable levels and the mill was dissatisfied with the quality of its product.

Cato starch profits from dual charge

Colthrop turned to National Starch & Chemical, which began a mill-based program looking at the internal application of starch. The problem was approached from two angles: the first objective was to retain the starch in the sheet to give superior inter- and intra-ply bonding; the second was to provide better adhesion between the coating and the substrate, by increasing the strength of the bonds at the paper/coating interface. Following an analysis of Colthrop's wet-end chemistry, National Starch advised the introduction of a waxy maize amphoteric starch, Cato 246, to the mill furnish.

Unlike cationic starches, which have only positive charges, amphoteric starches carry both positive and negative charges. This dual nature means that higher starch levels per fiber can be achieved, thus enhancing board strength. In addition to improving the efficiency of other components, amphoteric starches also help to reduce the dependence on very high-molecular-weight polymers which are primarily used as retention aids. This, in turn, improves the efficiency of the entire operation.

To ensure consistent end-product quality, the balance of cationic and anionic charges can be manipulated according to chemical fluctuations in the wet-end chemistry.

More strength with less starch

Following the introduction of Cato 246 into the system in March 1992, starch consumption levels dropped dramatically. The amount of amphoteric starch consumed in June was down 23% on the standard level. This amounts to an overall reduction of nearly two thirds (64%) from the high levels used in January. Unlike the previous system, where all the cationic starch added above a certain point was essentially wasted, the amphoteric starch was more efficiently used to increase bond strength. This resulted in more first-quality board production and a noticeable appreciation in quality, according to mill customers.

Not only has starch consumption at Colthrop been reduced, but there have also been improvements in product consistency. IGT and Scottbond test results have shown significant increases in surface strength and ply bonding. This has meant reduced losses for the mill, since more board meets the stringent quality requirements, and less therefore has to be rejected.

The Colthrop mill uses a wide variety of furnishes, including over 80% recycled fiber, for its different board grades. Wet-end chemistry variations are further compounded by a looped water system. Nevertheless, the new starch is now being used in all board grades at the mill, together with a new slurry-feed cooker from National Starch designed to the company's own specifications. The replacement of the previous dry-feed system has eliminated the problem of unpredictable starch solids levels, the company claims, and, as a result, metering is more accurate.

Doug Reid, managing director at Colthrop, has been very pleased with the results. "We had looked at other areas, such as drainage rates and refining the pulp fiber, but these were less responsive. On changing over to the amphoteric starch and a more efficient cooker system, we have seen marked improvements in controllability and levels of strength."

Ian Barrett is European laboratory supervisor for National Starch & Chemical.

 

 

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© 1993 Miller Freeman, Inc. All rights reserved.