Improving the quality and printability of coated paperboard.

Coated paperboard is a global business--especially in the coated solid bleached sulfate board (SBS) market.

U.S. exports are increasing as the dollar weakens against foreign currencies. U.S. exports of coated SBS board have traditionally been approximately 20% of total U.S. production. Exports are forecast at 27%-29% of U.S. production in 2003 and 2004. Previously, European coated SBS brightness has been higher than in North America due to higher penetration of calcium carbonate and optical brighteners in the European market.

The use of higher levels of calcium carbonate in European paper has produced lower sheet gloss, but European printers use ultraviolet (UV) varnish to achieve high package surface gloss. Environmental regulations in Europe are altering the focus. German printers are moving away from UV varnishing for environmental reasons. This is creating a need for higher sheet gloss.

A tendency exists to remove optical brighteners for environmental and economic reasons. The widespread use of gravure printing by European packagers also created a need for smoother board with low parker print surf (PPS).

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To be successful in exporting, American SBS producers needed to develop smoother, higher gloss, higher brightness board with a bluer shade. As they did so, they found a preference for this type of product among their domestic customers. Target values for coated SBS boards are as follows:

* ISO Brightness: 86%-90%

* Hunter B value: 3.0

* Hunter Gloss: 60%-70%

* PPS10: 1.0-1.2 [Micro].

Improvement of any coated product begins with improvement of the base stock--starting with furnish. Most coated SBS manufacture occurs in the Southern United States to take advantage of the stiffening properties of bulky southern pine fibers. Over the past decade, American SBS producers have increased their hardwood percentage to achieve a smoother sheet.

Most mills still run at acidic pH, but several mills have converted to alkaline papermaking. Board makers have been reluctant to use fillers because of the adverse effect on stiffness. Some coated SBS producers are looking at the bulking benefits of 5%-8% scalenohedral precipitated calcium carbonate (PCC) as a key to achieving brightness and smoothness without significant bulk loss.

American mills have traditionally manufactured base stock for coated SBS board on conventional fourdrinier machines with a conventional head box, dandy roll(s), conventional size press, and calender wet stack before the coater section. Recent paper machine improvements at several coated SBS mills include the following:

* Dilution control head box (improved profile)

* Top former to improve formation

* Metered size press.

Some board makers are including pigment in the surface size on conventional and metered size presses.

Coating and calendering equipment is also undergoing upgrades. The traditional coating section on a bleached board machine consisted of two applicator roll blade coaters. The gloss calender was a traditional steam-heated gloss calender. Most mills have added backside coaters not only to make coated two side (C2S) grades but also to reduce linting and improve curl control. Several mills are now triple coating. Installation of jet applicator blade coaters can improve smoothness by eliminating the applicator roll film split pattern. High temperature, oil heated, soft nip calendars are replacing conventional gloss calenders to develop higher gloss.

Medium ground calcium carbonate (GCC) with 60% material less than 2[micro] has become the dominant precoating pigment. Precoating binders have switched from acetates and acrylics to high strength styrene butadiene latices (SBR). Another precoating trend is running at higher solids (70% or higher).

Top coating pigment formulations have traditionally contained premium No. 1 coating clay, fine GCC, and a limited amount of plastic pigment with an acetate or acrylic latex binder system. Current trends include:

* Use of premium ultra-fine high glossing clay to replace No. 1 clay

* Use of narrow particle size ultra-fine ground carbonate to improve glossing

* Use of aragonite PCC for improved coating structure and improved glossing

* No plastic pigment to reduce cost

* Use of styrene acrylate latex to replace acetate and acrylic latex without sacrificing gluability.

ABOUT THE AUTHOR

Chuck Klass is president of Klass Associates, Radnor, Pennsylvania, and is a member of the TAPPI Editorial Board. Contact him at [e-mail] or by phone at [phone].