Formulating coatings for direct print flexco without drying.

Application of pigmented coatings improves appearance and printability. Also, various combinations of sheet surface properties are necessary for different printing methods. Those methods are changing and evolving.

While offset lithography has the widest use as a printing process, flexography is the fastest growing impact printing process. It is typography that uses resilient plates. Most flexographic inks are aqueous, so surface smoothness in paper and board is a desirable characteristic. When a flexographic press includes dryers--as with label or preprint presses--high holdout is beneficial.

HIGH QUALITY GRAPHICS

Where direct post-printing uses corrugated board such as a flexographic folder-gluer, no drying occurs. The sheet surface must absorb the ink vehicle to minimize dot gain and prevent smudging. The demand for high graphic quality corrugated is burgeoning due to changes in retailing--especially at "big box" stores. Improvements in the flexographic process have made a 120-150 line 4-color process the norm in high quality post-printing. Since white (fiber) top liner is not adequate for these applications, coated linerboard use is growing. Coated linerboard must also resist cracking, offer scuff resistance--no "metal marking"--and provide excellent glueability.

Coated solid bleached sulfate (SBS) board producers usually design their formulations for offset printing. Double blade coating is the dominant process in North America. For many years, blends of No. 2 coating clay and 60% <2[micro] ground calcium carbonate (GCC) had use as a precoating. The primary pigment for topcoats was No. 1 or extra-fine coating clay with some use of plastic pigments for gloss. Acrylic and acetate latices provided glueability.

Over the past decade, SBS producers have changed to higher carbonate levels in the precoating. Numerous SBS mills now run 100 parts 60% < 2[micro] GCC in their precoating. Styrene butadiene (SB) and styrene acrylate (SA) latexes find use as precoatings. Carbonates now have common use in a topcoating, with levels as much as 50 parts of 90% <2[micro] GCC being common. SA latex use in topcoating formulations is growing. These formulations provide a surface with good strength and excellent holdout.

Attempts to directly post-print boxes made with linerboard coated with these types of formulations have failed due to problems with dot gain, smudging, and metal marking. What changes does a coating formulator need to make?

To reduce dot gain and prevent smudging, the coating structure requires "opening" to provide pores for rapid capillary transport of ink vehicle from the surface. The coating structure must provide the proper amount of void volume. If the coating is too porous, print gloss will suffer.

Preventing metal marking requires limiting the use of abrasive pigments such as calcium carbonate, calcined kaolin, and titanium dioxide (Ti[O.sub.2]).

Two primary processes are used to make coated linerboard:

* Single coating white top or mottled white linerboard: apply approximately 9-12 g/[m.sup.2] (1.8-2.5 1b/1000 [ft.sup.2]) coating

* Double coating brown linerboard: apply 8 g/[m.sup.2] (1.5 lb/1000 [ft.sup.2]) precoating and 8-10 g/[m.sup.2] (1.8-2.0 lb/1000[ft.sup.2]) topcoating.

When precoating brown linerboard with a blade or smooth rod, using calcined clay or Ti[O.sub.2] causes optical mottle. Carbonate in a linerboard precoating can cause an undesirable hue in double-coated board. The precoating pigment is usually kaolin. Chemically structured kaolin has given better results than regular No. 2 coating clay.

To minimize metal marking, the total parts of GCC, calcined kaolin, and Ti[O.sub.2] should be below 40 in a single coating or top coating. The balance should be premium No. 1 coating clay.

The amount of Ti[O.sub.2] and extender depends on the brightness required in the coated product. Specialty precipitated calcium carbonate (PCC) has had use up to 45 parts in this application with good results.

Acetate and acrylic latices were the most commonly used binders in this application. New styrene-butadiene-acrylate (SAB) latexes seem to provide a desirable balance of strength and structuring. Soy protein polymer at 4 parts can help to keep the coating structure open, especially when crosslinking with ammonium zirconium carbonate (AZC).

The most promising new product for this application is high brightness, specially modified, natural zeolite pigment. It has low abrasion and significantly improves print quality at levels of 10-15 parts.

About the author: Charles P. (Chuck) Klass heads a consulting firm, Klass Associates, Radnor, Pennsylvania, USA. He has been active in TAPPI for many years and is a member of the Solutions! Editorial Board. Contact him at [e-mail]