Is curtain coating ready for prime time?
COMMERCIAL SCALE TRIAL
Oliver Rudolph of Voith Paper reported on a commercial scale trial making coated freesheet offset printing paper. The trial was done at Papierfabrik August Koehler, Kehl, Germany on off machine coater SM 1. This coater is 4.2 meters (165 inches) wide and runs at 1200 m/min (3900 feet/min) to curtain coat CB carbonless paper. The curtain coating trial objectives included:
Runnability aspects of 66% solids pigment coating formulation at 1200 m/min
* Curtain stability
* Deaeration efficiency
* Operational aspects (passing splices, start/stop sequences)
Quality potential for woodfree curtain coated paper (single coated)
* Influence of precalendering
* Influence of coat weight
* Gloss and printability in comparison to commercial papers
The base paper was unsized 44-gsm woodfree. Precalendering was done with a soft nip calender at 25, 50, 75 and 100 kN/m. The coating formulation was a standard offset coating formulation with 70:30 ratio of fine GCC and fine clay and 12 parts SB latex binder plus additives. Coating was applied at 66% solids and 600 mPas viscosity. Coat weights applied were 12, 15 and 18 gsm. The coated papers were finished in a 10-roll Janus MK 2 calender at 115[degrees]C and loadings of 150, 225, and 350 kN/m. They were heatset web offset printed.
Runnability was good with excellent curtain stability at 1200 m/min for the entire coat weight range. Deaeration efficiency is critical, and large capacity is required. Splices passed with no problems, and stop/start sequence was not problematic.
Precalendering has a positive effect on coated sheet surface roughness. Comparison of samples at 12 gsm coat weight and Janus calendered at 225 kN/m showed a reduction in PP[S.sub.10] roughness from 1.03 to 0.80 [mu] when precalendering was increased from 25 to 100 kN/m. Varying coat weight from 12 to 18 gsm had very little influence on coated/Janus calendered paper roughness, gloss and print gloss. Printability was found to be comparable to commercial blade coated papers.
The Voith presentation also reported on the use of a bubble tube pressure tensiometer to evaluate the effect of surfactant addition--a key to achieving curtain stability. Measurement of dynamic surface tension at very low surface age is needed. Peeyush Tripathi of Western Michigan University reported that the maximum bubble pressure method does not seem to correlate with the curtain coating process. The rate of surface area increase in minimum bubble pressure is much less that that of the curtain. Understanding of surfactant diffusion rate and performance needs further study.
Stretchability of the coating and elimination of film breaks is dependent on extensional viscosity. More study is needed to de-couple extensional and oscillatory measurements. This may be the key to curtain runnability at low flow rates to obtain low coat weights.
DEAERATION IS KEY
Deaeration appears to be the most important process challenge in curtain coating. Sonic deaerators have viscosity limitations, and thin film vacuum deaerators have bubble size (retention time) limitations. John Taylor of Capella Technology reported on development of a centrifugal spinning vane deaerator with vacuum assist that claims to make air-free coating. This device is not yet commercially available. It appears that more research is needed to understand the fundamental mechanisms of aeration and deaeration. Alternative process strategies merit exploration along with research into the effects of surfactants, binders, and rheology modifiers.
More fundamental research on curtain coating will be forthcoming in the near future. Western Michigan University is installing a laboratory scale pilot curtain coater at present and will focus on research in the upcoming year.
MULTILAYER CURTAIN COATING
Multilayer curtain coating is an interesting prospect. This process has been used for decades in production of photographic films and papers. Robert Hirshburg of TSE Troller Schweitzer Engineering AG discussed the potential advantages of multilayer curtain coating including:
* Less expensive facilities
* One coating station vs. several required for sequential coating
* Shorter and simpler web path and dryer
* More robust web conveyance
* Fewer web turns and less steering required
* Better runnability--fewer breaks
* Operational efficiency with reduced labor
Multilayer curtain coating can be done with either slide curtain dies or slot curtain dies. Slide curtain dies are practically unlimited in number of layers and have potential operational advantages. Slot curtain dies have geometric and hydrodynamic limitations when used for multilayer coating. Multilayer curtain coating could optimize coating cost by applying a bottom "holdout" layer, middle "coverage" layer and top "brightening" layer in one operation--providing optimized printing surface while reducing the use of expensive pigments.
BENCH SCALE CURTAIN COATER
Rajan Iyer of Imerys reported on research done on a high-speed bench scale curtain coater. This 12-inch wide coater is designed for 1000 m/min. It has been used to study the effects of coating deaeration, variable pumping rates, adjustable die gaps and height to the web.
The research to date has emphasized the need for adequate deaeration. Relationships among web speed, curtain impingement velocity and Reynolds number have been shown to predict curtain stability. Calculations were done to correlate pressure at the die entrance with flow factors.
This indicates that it may be practical to estimate viscosity from the pressure at the entrance of the die and other parameters as opposed to measuring extensional viscosity separately.
Dr. Iyer's expectations for curtain coating include:
* Application of coatings at low to high solids, sometimes independent of shear rheological behavior
* Ability to use hyper platy (high aspect ratio) pigments that is difficult to do with current coating technology
* Application of ultra-thin functional coatings that would actually hold out on the paper surface
* Application of alternating layers of coatings forming overall coating structure for light interaction or printing.
BENEFITS FOR CONVENTIONAL PAPER
My opinion is that curtain coating could provide significant benefits and advantages in coating of conventional paper grades. Potential benefits for various grades include:
Curtain coated groundwood
* Can run at up to 1500 m/min (5,000 fpm)
* Minimal stress on base sheet
* Can coat a very weak base stock--reduced kraft and higher filler contents at lower basis weights
* Contour coating provides better coverage even at very low coat weights
* Could be the key to economical ULWC
Curtain coated free sheet
* Improved coverage at low coat weights
* Random pigment orientation--improved opacity
* Minimized water transport into the base stock
* Excellent precoater
* Could reduce furnish costs
Curtain coated paperboard
* Ideal precoater
* Minimal water transport into the base stock
* Minimal streak and scratch potential on rough base sheet
* Random pigment orientation for improved opacity
* Obscure darker base stock at lower cost
Curtain coated digital imaging grades
* Excellent coverage at low coat weights
* Random pigment particle orientation
* Maintains coating structure
* Maintains uniform pore structure
* Could be the key to using lower cost pigments, e.g. zeolite, to make high quality coated ink jet papers
Curtain coated barrier and functional coated paper and paperboard
* Has been used for decades in waxing of corrugated
* Pinhole free coating for MVTR, oil/grease resistant and water-resistant grades
* Minimal shear stress for pressure sensitive adhesives
* Uniform coverage at low coat weights
BARRIERS TO COMMERCIALIZATION
Curtain coating has been featured at the last five TAPPI Coating Conferences and other conferences. Nevertheless, the only known commercial applications are on off machine coaters making carbonless, thermal and technical specialty papers. What are the impediments to commercialization?
Lack of readily available pilot curtain coaters is serious impediment. Machine builders in Europe and Japan have pilot curtain coaters, but the focus of customer trials has been on conventional coating methods. It is questionable whether commercial scale curtain coating installations have been fully engineered--especially with staff reductions by both paper companies and machinery builders.
Retrofits to replace existing coaters are likely to be expensive. Existing coating supply systems may not be readily modifiable for use with curtain coaters. Sheet run requirements for curtain may require major surgery on paper machines and off machine coaters at high capital cost. The book value of currently installed coating equipment is also an impediment.
In spite of these impediments, the potential benefits of curtain coating are likely to overcome the impediments to commercialization. When implementing new technology, North American paper companies like to be a "fast third"--after the risk is reduced. Nevertheless, it is likely that a few full-scale successful commercial curtain-coating installations will facilitate rapid growth.
WHAT YOU WILL LEARN
* Results of a commercial scale curtain coating trial.
* Why deaeration is a key to the process.
* Current barriers to commercialization.
* "TAPPI Coating Conference raises the curtain on coating," by Charles P. Klass, Solutions!, September 2004. To access this article, go to www.tappi.org and enter the following Product Code in the search field: 04SEPSO42.
* "TAPPI Coating Conference: A technology update," by Charles P. Klass, Solutions!, September 2003. Product Code: 03SEPS037.
ABOUT THE AUTHOR
Charles P. (Chuck) Klass heads a consulting firm, Klass Associates Inc., Radnor, Pennsylvania, USA. He is also Adjunct Professor of Paper Engineering, Chemical Engineering & Imaging at Western Michigan University. He has been active in TAPPI for many years and is a member of the Solutions! Editorial Board. Contact him at firstname.lastname@example.org
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|Author:||Klass, Charles P.|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Dec 1, 2004|
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