TAPPI Coating Conference a technology update: new technology is making coated papermaking faster and more efficient, and the end product lighter and stronger.

Although kaolin clay remains the dominant coating pigment, the use of ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC) is growing rapidly. Amy Dimmick of Specialty Minerals Inc. reported on the influence of average particle size of aragonitic PCC on coated paper properties. The optimum average particle size for opacity for acicular aragonitic PCC is 0.4-0.557[micro]m. Coated paper smoothness improves and sheet gloss and print gloss both increase as the average particle size of aragonite PCC becomes smaller. Ink set rate increases as average particle size of PCC decreases. David Willoughby of Luzenac America reported on an experimental coating talc with improved theology. It has had successful use in pilot coater trials at up to 40 parts to produce high quality LWC rotogravure paper. Anthony Hiorns of IMERYS discussed the potential advantages of kaolin addition to calcium carbonate precoats, including improved light scattering and better top coat holdout.

HIGH YIELD PULPS IN COATED WOODFREE PAPERS

Xuejun Zoo of PAPRICAN discussed the growing use of high yield pulps in coated paper grades that are normally woodfree. Papermakers are now lacing market demand for products with higher performance at a lower cost. High-yield pulps offer cost effective opportunities in achieving the desired end-use performance, particularly for traditional "woodfree" papers. High-yield pulps can have strength and brightness similar to bleached hardwood kraft pulp but with other unique features such as high bulk, large surface area, and high fines content. These characteristics can facilitate the following:

* Increase bending stiffness of lightweight coated woodfree paper to solve a fluting problem

* Maintain good coating coverage and surface quality while reducing cost

* Develop a new coated grade with a higher bulk.

METERED SIZE PRESSES

Loss of internal strength is a major concern on machines that switch from conventional to metered size press operations, especially when making coating base stock. Juha Lipponen and Johan Gron of Metso Paper discussed the potential improvement in internal strength from reducing press draws to 2%. This effect was more prominent at lower basis weights. Lowering the press draw too far decreased elastic modulus and stiffness.

Mashiro Sugihara of Mitsubishi Heavy Industries characterized the misting phenomena in film coating. Steps to reduce misting include the following:

* Decrease the film thickness at the nip inlet and web separation point from the applicator roll

* Increase color surface tension

* Decrease color density.

Martin Kustermann and Michael Trefz of Voith Paper discussed quality related aspects, process control, and important operational challenges such as time efficiency, threading, and roll covers from various recent metered size press installations.

Rex Robertson of Metso Paper discussed on-machine calendering techniques used with metered size press coated papers. New on-machine multi-nip calenders may eliminate the need for off machine supercalendering and improve product quality.

COATED BOARD BASE STOCK

Matti Lares of Metso Paper presented a detailed study showing the influence of base stock characteristics on coated board quality and printability, especially mottle. Baseboard Bendtsen-roughness and topography (0.5-4 [micro]) had the best correlation to final product surface structure. Precoating mass distribution in smaller wavelength areas also had a good correlation coefficient with final product surface structure.

SIMULTANEOUS TWO-SIDE BLADE COATING

Pasquale Seca, mill manager of Cartifico Ermolli S.p.A., Crevacuore, Italy, discussed experiences with startup of the Mirroblade couter that allows simultaneous blade coating on both sides of a sheet using a single coating station with a small space requirement. The paper quality achieved is comparable to traditional blade coated papers and in some respects is even better due to the fiber coverage the soft tip blades provide. The key to success--good runnability--is to have good control over wet end chemistry and process parameters to avoid impurities and defects in the base paper.

After some initial struggles, Ermolli Crevacuore has been able to reach the primary objectives of the rebuild project--reduced surface roughness, improved gloss two-sidedness, and increased paper machine speed. The 2-sigma values in cross direction and machine direction have improved, although the maximum coating weight levels have increased from 11 to 22 gsm per side after the rebuild.

CURTAIN COATING

Curtain coating has generated feverish interest. Although the meeting did not include formal papers on this topic, the subject arose in the CIG Lunch and Learn, "Emerging Technologies and Markets," and in a roundtable on non-impact coating technologies. Sam Iuliano of Extrusion Dies Industries LLC discussed the comparison of proximity slot die and curtain coating techniques.

John Taylor of Capella Technology Inc. discussed two new air removal hydrocyclones. The Vacuum Degasser generates its own internal vacuum and does not require an external vacuum source. The Foam Blaster combines fine air bubbles into larger ones and thus destroys stable foams. Taylor also described a new laboratory technique for measurement of viscosity and surface tension at deformation rates, process durations, and surface areas that closely mimic the curtain coating process.

Klaus Straenger of Voith Paper described the Direct Fountain (DF) Coater--a curtain coater that is successful at widths up to 4.5 meters (177 inches) and speeds up to 1300 m/min (4265 fpm). Curtain coating achieves excellent coverage while minimizing mechanical stress on the substrate.

COATED PAPER TESTING METHODS

A study at Western Michigan University demonstrates the need for a new testing method for coated papers containing optical brighteners. Ali Bashey of Specialty Minerals reported use of X-ray Photoelectron Spectroscopy (XPS) to measure fiber coverage on coated papers.

BIOLOGICAL CONTROL IN COATING SYSTEMS

Janet Woodward of Buckman Laboratories Inc. discussed experiences at two mills with biological control in coating systems. Many coating components did not have biocide treatment. Microbial growth led to the loss of pH and viscosity change of coating colors. Monitoring the coating additives for aerobic and anaerobic populations detected the sources of high microbiological activity and provided the basis for appropriate treatment to improve coated product quality and operating efficiency.

TVOC IN PAPER COATINGS AND COMPONENTS

Increased regulatory visibility and process and product odor concerns have highlighted the need to quantify accurately the total volatile organic compound (TVOC) content in low TVOC samples such as paper coatings and coating components. EPA Method 24 has been the historical method of choice, but it is totally inadequate when TVOC levels drop below approximately 2%. Kevin Sweeney of BASF described a new method for TVOC analysis and compared it with EPA Method 24. The new method is in use in Europe and will probably replace EPA Method 24.

WHAT YOU WILL LEARN

* Trends in GCC end PCC use in coating formulations

* A report on simultaneous two-side blade coating

* New developments in curtain coating

ADDITIONAL RESOURCES

* "Coating 2003: A research review," Solutions!, May 2003, p. 34

* Papers from the TAPPI Spring Conference are available for viewing on the following TAPPI web link: http://www.tappi.org/content/tp/03spring.htm

* In 2004, this conference will be held in conjunction with Paper Summit, May 3-5 in Atlanta, GA Continue to check the TAPPI web site (www.tappi.org) to learn more Technical program information as well as conference fees will be available in the coming months

Editor's Note: The 2003 TAPPI Spring Technical Conference and Exhibit was May 11-15 at the Sheraton Chicago Hotel & Towers. This was the first of TAPPI's new mega-conferences, combining conference programs from the former Coating, Finishing & Converting, Papermakers, and Process & Product Quality Conferences with sessions from the Fluid Mechanics & Water Removal and Microbiology & Microbial Technology Committees. The Recycled Paperboard Technical Association joined TAPPI us a cosponsor. The program was broad in scope, and the meeting provided networking opportunities for technical professionals from various industries and specialties. This article will focus on advances in coating technology included in the program.

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]