Features Research & Innovation
Paper Chemicals: Graphic pop, smooth print runs from “nano-paper”

Printers want more “pop” in their photos and graphics produced on their high-speed, 10-color presses, and the combination of retention/drainage, and surface enhancement technologies are making the difference. In addition to nanoparticle-based retention/drainage systems to improve base sheet uniformity, many coated paper producers are boosting sheet performance with “nano” surface treatments. These include latex formulations enhanced with zirconium based crosslinkers and engineered pigments. Much of the innovation at the size press revolves around nano-combinations applied there, such as surface sizing and specialty coating additives to improve sheet properties.

According to Bill Waddell, Domtar’s new product development manager for coated papers, helping printers notch up print quality and run smoothly connects with their commitment to maximizing coating performance. Referring to one specialty additive, Waddell notes, “AZC (ammonium zirconium carbonates) is the most efficient crosslinker available for coating formulations, because it imparts a degree of wet pick resistance that you would not have otherwise. Wet pick strength is something to be concerned about with a 6- or 8-colour job, where the pick up by the by 4th or 6th or 8th unit creates a moisture absorption risk.”

Waddell points out that the demand for shades, tones, and accents in printing, topped off with a polycoating or UV-cured coatings at ever increasing speeds, means that coating strength must be better than ever. As for their continued development efforts and their success with printers, he sums it up by saying, “Sometimes, no comment from printers is the best comment and, when it comes to surface strength, we hear very little.”

Waddell believes that quality coating builds upon a well-formed sheet. He points to their long history with silica nanoparticle retention/drainage systems at the wet end to achieve this objective.

Domtar Cornwall’s Jay Van Wagner, a coating specialist, says that many innovations are coming with world class machines, where high-speed coaters are running at 5,000 ft per minute vs. 3,000 fpm 25 years ago. The new marvels include machines like Sappi’s newest PM at Gratkorn, Austria and Burgo’s new MSP LWC machine at Verzuolo, Italy. Double and triple coats on both sides are notable, indeed. At the same time, he states that much has been done to improve the metered size press, where innovation with a wide range of additives is increasingly the norm. He notes that issues like misting and patterns on the surface have been overcome. Some mills are using SymSizers and the like as pre-coaters.

In accord with Waddell, Van Wagner says that AZC as a wet strength agent for offset printing was a good move. As he puts it, “An offset sheet must survive a lot of water absorption due to multiple wetting on non-image areas of the blanket. With multiple rewetting, coating wet strength became an issue. Delayed cure is a no longer an issue with AZC.”

Another point Van Wagner makes is that “Some additives deliver an environmental and workplace safety edge. When we changed to AZC at Cornwall, we eliminated environmental concerns and improved workplace safety.”

Van Wagner emphasizes the importance of a uniform base sheet, made possible with advanced nano-chemistry. In his words, “many North American coated sheets are single pass, so you don’t have any opportunity to cover anything up. Even two passes on each side requires a quality base sheet.”

Creativity

with mineral combinations

Gary McCaig, technical director with NorkseCanada, believes that much has been achieved with minerals, and the combination of carbonates and clays.

NorskeCanada’s PM 5 at Port Alberni, Vancouver Island, produces lightweight-coated grades, which are being designed to be even lighter. McCaig points out that unique mineral combinations and additives support product development. He says, “our on-line film press coating and hot soft calendering call for special attention with coating formulations.”

Like many others, their development work aims at improving quality and reducing costs. McCaig says on the cost side that “minimizing Ti02 has been achieved through a series of trials which optimize combinations of minerals — such as Capim from Brazil, and more traditional kaolin. New combinations of minerals, including GCC (ground calcium carbonate) and PCC (precipitated calcium carbonate) may be the best answer for economy and quality in the future. Trial work with major suppliers like IMERYS and Dow Chemical, who have extensive pilot plant capabilities, has been crucial to our continued success. Misting on the film press has been eliminated, and we are coating with very little lost time.”

McCaig adds that PCC has been important as an economic and efficient filler for uncoated grades to maintain opacity and raise brightness.

Pointing out that filler for fibre substitution has been valuable, McCaig emphasizes that a quality base sheet is a must, as coating cannot cover up all mistakes. He says “moving to lighter basis weights, formation and fiber issues become more obvious. The biggest change I’ve witnessed is on-line instrumentation to better control retention. Whitewater consistency and charge control help with uniformity and basis weight.”

Synergy from

papermaking to coating

Bruno Bolduc, applications specialist with Eka Chemicals, stresses the importance of synergy between wet-end silica nanoparticle systems to improve formation, surface sizing and crosslinkers like zirconium to enhance the surface. He notes that “the demand for better surface treatment in paper is quite strong, driven by printing technology. Our role as developers of complimentary nano-products can be a vital asset to the competitiveness of our customers.”

Bolduc believes that the metered size press will continue to produce innovations, such as the application of surface sizing agents and zirconium. Says Bolduc, “some printers have already seen less dusting at multiple print stations and enhanced performance, resulting from novel size press combinations.”

What Eka Chemicals refers to as their Multi-Print system includes the use of starch, zirconium crosslinkers and surface size polymers in a size press solution, where the system works by immobilizing the starch and surface size at the surface of the sheet. Says Bolduc, “zirconium crosslinkers have the ability to react with the starch, surface size and fibers of the sheet. This action allows for the size press solution to remain on the surface and not dive into the sheet, where the solution is not of use. This makes it easier to use lower grade starches, and still get good print quality and lower the tendency for dusting.”

Nano understanding

enhances coatings

Often advances in technology represent overcoming technical hurdles, or perceived obstacles, before taking off. According to Martha Newall, manager Pacific Northwest for IMERYS, the more important issue today for papermakers and their coatings suppliers is to understand the nanostructures of kaolins, carbonates, process chemicals and fiber — and the ways they can be combined. According to Newall, “Innovative, sometimes revolutionary formulations, are helping coated paper producers meet the demands of their high speed converting and printing customers — delivering desired properties at lower basis weights at lower overall cost.”

As she puts it, “The demand for increased productivity through speed increases has posed new challenges for the Canadian coating operations. It requires a close-in focus on the structure and flow properties of their existing coating formulations, and the role the pigments, binders and additives as a whole. And we have only just begun along this high value path.”

Dial-in performance

According to Femi Kotoye, tech service and development manager for emulsion polymers for paper coating with Dow Chemical, nanotechnology is surging ahead in coating. Says Kotoye, “when you can dial in the properties you want, such as sequencing polymers, why not do it? Today, you can develop a film with a rigid outside and a very soft outside with n
anotechnology.”

He adds, “Coating interaction between latexes and pigments are determined by surface activities. Before nanotechnology, the bulk properties dictated what end use performance looked like. Not any more.”

As Dr. Richard W. Siegel, the Robert W. Hunt professor of materials engineering at the Rensselaer Polytechnical Institute in Troy, New York reminds us, “Paper made with nanoparticle technology has great potential to recreate itself as a versatile end product.”

Clearly, nano-papermaking, surface treatment and coating will continue to evolve in the coming years, and without question, papermakers will have a greater ability to design properties into the sheet. Lower overall cost, fewer problems in converting and on the printing presses, more dynamic printing…not a bad way for the industry to progress.

White House report web site. http://itri.loyola.edu/nano/toc.htm

Pulp & Paper Canada, Paper Innovation Ramps Up With Nano-Chemistry, 1/2001