Nano-synergy boosts printing performance: advanced nano-solutions, which consider the wet end, sizing and coating in a holistic way, can improve surface characteristics.

When you can deliver properties to your customer's customer such as enhanced sheet uniformity, the ability to control inks, and minimized release of carbonates onto their presses, why not do it? Proprietary surface treatments are doing just that, becoming the logical extension of nanoparticle-based retention/drainage systems to improve quality and add value.

One surface treatment breakthrough is a patented system that includes the use of starch, zirconium crosslinkers (AZC or KZC), and surface size polymers in a size press solution. The system works by immobilizing the starch and surface size at the sheet's surface. Zirconium crosslinkers can react with the starch, surface size and fibers of the sheet. This allows the size press solution to remain on the surface and not get lost deep in the sheet. One benefit is the ability to use lower grade starches, which are not good film formers, and still achieve high print quality with a lower tendency for dusting.

In recent trials at one coated fine paper mill using latex-based paper coatings, the mill added zirconium crosslinkers to the coating to minimize latex penetration into the sheet. This considerably lowered latex dosages without hurting the strength of the coating. Print trials showed no deterioration of the paper's print quality.

SYNERGY PAYS

The synergy between wet-end silica nanoparticle systems (to improve formation), surface sizing, and crosslinkers like zirconium (to enhance the surface) is important. Before the development of tools like advanced silica nanoparticle formulations, which are designed to work in concert with sizing and coating combinations, coating interactions between latexes and pigments were less predictable.

The application of silica nanoparticle technology results in better retention and dewatering, which in turn provide better formation for smoothness and hole reduction, as well as savings in OBA (optical brighteners) and Ti02. It is important for wet end, sizing and coating specialists to work together, especially when the savings from base sheet gains can more than pay for the surface enhancements achieved through sizing and coating.

Surface enhancement can also come from better control of the interior of the sheet, utilizing ASA (Alkenylsuccinic anhydride) to control pick up of starch or coating. More effective internal sizing, including ASA's on-machine curing capability, means less surface sizing, which improves printability.

Ammonium zirconium carbonate (AZC) is viewed as the most efficient crosslinker in coating formulations, because it imparts a high degree of wet pick resistance. Faced with the risk of moisture absorption when printing 6 to 10 color jobs, printers need to minimize the progression of "pick up" from one printing unit to the next. For an offset sheet, multiple wetting means that a sheet must survive a lot of water absorption on the blanket. With multiple rewetting, coating wet strength is considerably enhanced with AZC.

Crosslinkers can make a real difference for printers facing increasingly precise demands in terms of shade, tone, and accents, as well as polycoat or UV cured coatings. The best evidence is a reduction of complaints from printers, when papermakers apply the combination of silica nanoparticle technology and this surface enhancement approach.

The growth of surface treatments of this kind is closely associated with recent efforts to improve the metered size press. Several coated paper producers have overcome traditional concerns like misting and patterns on the surface, and delayed cure.

One producer of bond paper for offset printing faced excessive dust buildup on the print blankets. when technicians applied the new technique, they saw a ten-fold increase in runnability, requiring clean up after 70,000 copies instead of 7,000.

About the authors:

Craig White, is market development manager-coating; Chris Glasgow is market development specialist-sizing; and Paul Fish is market development manager-retention for Eka Chemicals/Akzo Nobel. Email to [e-mail];