Coating 2003: a research review: the latest in coating R&D, presented at the TAPPI Coating Conference.

Coating formulations, equipment, and processes continue to evolve as demands grow from end users for higher quality and better performing coated papers. The evolution is mapped in current research and development, much of which is being presented at the 2003 TAPPI 8th Advanced Coating Fundamentals Symposium May 8-10, 2003 and the TAPPI Spring Technical Conference & Trade Fair, May 12-15, 2003 in Chicago, Illinois, USA. For more information, consult the "Additional Resources" section at left. The following is a review of selected abstracts of papers.

Paper from the Advanced Coating Fundamentals Symposium:

SURFACE TENSION DRIVEN LEVELING OF SPRAY DROPLETS

Douglas W. Bousfield, University of Maine, and Martti O. Toivakka, Abo Akademi University

Spray coating has the potential to be a major non-contact coating method. However, to obtain good coverage and uniformity of coating layer on paper, sufficient leveling of droplets must take place. Though there has been some work on the leveling of scratches in coating layers due to blades, little has been done to understand the leveling of droplets right after impact.

A model of surface tension driven leveling is developed that can start with the shape after the impact, and follow the leveling of the fluid layer as a function of time. The model accounts for surface tension of fluid, viscosity, and base paper absorption characteristics, such as void fraction, pore size, and contact angle. Various parameters, such as coating solids content, filter cake resistance, and droplet size, are investigated.

Papers from the Spring Technical Conference:

INFLUENCE OF THE AVERAGE PARTICLE SIZE OF ARAGONITIC PRECIPITATED CALCIUM CARBONATE (PCC) ON COATED PAPER PROPERTIES

Amy C. Dimmick, Specialty Minerals Inc. Changes in pigment average particle size have been shown to strongly influence coated paper properties, especially in the areas of gloss and opacity. An earlier study by Donigian, et al., had shown increasing sheet gloss and decreasing print gloss trends as the average particle size decreased when using calcitic precipitated calcium carbonate (PCC) pigments with a prismatic morphology. Some work had been done by Legrix, et al., examining aragonite PCC with two different average particle sizes, but the PCC pigments were limited to only a fine and coarser size, without any continuum to determine definitive trends. Earlier work by Zeller has shown that the optimum pigment average particle size with prismatic PCC pigments for light scattering is between 0.4 [micro]m and 0.6 [micro] m. Introducing a change in the pigment morphology from prismatic to the acicular or needle shape and adding a series of aragonitic PCC pigments with a range of average particle sizes has extended the trends established in previous work.

The acicular morphology has an inherently higher aspect ratio, which should play a role in changing the coating structure and thus coated paper performance with respect to ink setting and optical properties. The focus of this work was to determine whether the sheet gloss and print gloss trends observed with the lower aspect ratio prismatic PCC pigments were also true for acicular aragonitic PCC pigments. The second objective was to determine whether the optimal particle size range for opacity predicted by Zeller for calcite PCC would hold for aragonite PCC particles.

The pigment portion of the coating color has been shown to contribute significantly to the sheet gloss and print gloss performance of the coated paper. Initial studies have shown that the particle size of the pigment affects the pore structure of the coating, which directly affects the print gloss. It was postulated that, by changing the morphology of the PCC pigment from a prismatic to an acicular nature or from low aspect ratio to a higher aspect ratio, the pore structure should change. This change in pore structure would affect ink set rates, potentially providing different print gloss trends than initially established with lower aspect ratio pigments.

CHARACTERIZATION OF MISTING PHENOMENA IN FILM COATING

Toshiaki Miyakura, Masahiro Sugihara, Hiroshi Miura, and Kenji Yamada, Mitsubishi Heavy Industries Ltd.

Misting that occurs at the outlet of applicator roll nip is a signature problem in high-speed film coating. This phenomenon causes the decline of production quality and loss of natural resources and production efficiency. Several works[....]were carried out on this phenomenon, but the general role of mist amounts cannot be seen in those works. In this paper, we mainly focused on the characterization of misting phenomenon in high-speed film coating. To confirm the hypothesis on the origin of misting, we carried out the flow-visualization at the outlet of the applicator nip region. In this experiment, the origin of misting, i.e. filament formation and its break up at the outlet of applicator nip, have been visualized. Such photographs cannot be seen in previous works.

Non-dimensional, experimental formulas that show the specific role of misting have also been derived though our experiments and analysis. In this paper, we summarize the results of fundamental studies and the concept of our new film coating technologies.

INFLUENCE OF PRECALENDERING ON THE PROPERTIES OF SPRAY COATED WOODFREE PAPER

Olli Suontausta, M-real Corp,

Spray coating technology is a new coating method where the coating process is based on high-pressure spray application of coating or surface size directly onto the surface of paper or board without any direct machine contact. This means that the pressure pulse toward the paper during the coating application process remains lower than in film transfer coating and especially in blade coating, and weaker base papers can be used. This should improve runnability of the coater by reducing the number of web breaks. Spray coating technology has some other interesting properties also. The spray application unit can be expected to be very versatile, because it offers good possibilities for wet-on-wet application and reduces the time needed for a grade change. Depending on the size of the nozzles and number el nozzle rows, the amount of applied material can be varied from a few grams up to 20-30 g/[m.sup.2]

As the name of the technology reveals, in spray coating application coating color is sprayed on the paper surface. As a consequence the behavior of coating color in the nozzle and on the paper surface after impact is the key issue for the quality of the spray coated paper. The roughness of the paper surface, together with porosity and absorption speed of the water phase of the coating color; can be expected to be critical properties for leveling of the coating color droplets when they immobilize after the impact on the paper surface.

When the smoothness of coated paper is considered to be one of the most important factors in promoting print quality, calendering of the base paper before coating can be seen as one possible tool for improving the smoothness of the paper. All the studies where the roughness of the base paper has been reviewed are based on trials where blade or film transfer coating units have been used as a coating unit. The target of this study was to find out what is the importance of paper surface roughness on the properties (especially roughness) of spray coated paper. In this study both open (uncoated base paper) and closed (blade precoated base paper) surface were used as base materials. Trials with base paper and precoated paper were performed as two separate trial runs. For this reason the results are not comparable with each other. Part of the result presented here are based on results obtained by Pasi Pulkkinen. S!

IN THIS ARTICLE, YOU WILL LEARN:

* New research in spray coating

* Methods to control misting

* How pigment sizes affect coated paper properties

ADDITIONAL RESOURCES:

* For books and CD-ROM materials about coating and coated papers, visit www.tappi.org and click on "TAPPI Press."

* For information about or proceedings of TAPPI's 2003 Advanced Coating Fundamentals Symposium and the TAPPI Spring Technical Conference and Trade Fair in Chicago, contact Karen Van Duren, [e-mail]

* To learn more about TAPR member groups and discussion boards for coated papers, visit www.tappi.org and click "Paper Coating" on the menu bar below the logo.