Is curtain coating ready for prime time?There is a high level of interest in curtain coating--even though there are no current commercial installations making pigmented coated paper Coated paper is paper which has been coated by an inorganic compound to impart certain qualities to the paper, including weight and surface gloss, smoothness or ink absorbency. Kaolinite is the compound most often used for coating papers used in commercial printing. or paperboard. That was the message from the Gorham International Curtain Coating Conference, held October 6-8, 2004 in Orlando, Florida The city of Orlando is a major city in central Florida and is the county seat of Orange County, Florida. According to the 2000 census, the city population was 185,951. A 2006 U.S. , USA. (I had the pleasure of serving as chairman of the Conference.) The presentations and discussions at this conference brought the potential benefits of curtain coating into sharper focus. 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 Deaeration is the removal of air molecules from another gas, usually steam, using a deaerator. Deaeration also refers to degasification, the removal of dissolved gases, such as oxygen, from liquids. 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 CALENDER. An almanac. Julius Caesar ordained that the Roman year should consist of 365 days, except every fourth year, which should contain 366, the additional day to be reckoned by counting the twenty-fourth day of February (which was the 6th of the calends of March) twice. at 25, 50, 75 and 100 kN/m. The coating formulation was a standard offset coating formulation with 70:30 ratio of fine GCC GCC: see Gulf Cooperation Council. (compiler, programming) GCC - The GNU Compiler Collection, which currently contains front ends for C, C++, Objective-C, Fortran, Java, and Ada, as well as libraries for these languages (libstdc++, libgcj, etc). 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 cal·en·der n. A machine in which paper or cloth is made smooth and glossy by being pressed through rollers. tr.v. cal·en·dered, cal·en·der·ing, cal·en·ders 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 ten·si·om·e·ter n. 1. An instrument for measuring tensile strength. 2. An instrument used to measure the surface tension of a liquid. [tensio(n) + -meter. to evaluate the effect of surfactant Surfactant Definition Surfactant is a complex naturally occurring substance made of six lipids (fats) and four proteins that is produced in the lungs. It can also be manufactured synthetically. 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 Western Michigan University, at Kalamazoo, Mich.; coeducational; founded in 1903 as Western State Normal School, became accredited in 1927 as a college, gained university status in 1957. 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 oscillatory characterized by oscillation. oscillatory nystagmus see pendular nystagmus. 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 John Taylor, or Johnny Taylor may refer to: Academic figures
British pharmacologist. He shared a 1982 Nobel Prize for research on prostaglandins. vane the membranous or main part of the contour feather in birds as distinct from the shaft. 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 aeration /aer·a·tion/ (ar-a´shun) 1. the exchange of carbon dioxide for oxygen by the blood in the lungs. 2. the charging of a liquid with air or gas. aer·a·tion n. 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 Fujifilm [1] [2] Velvia 50
TSE 1. See Tokyo Stock Exchange (TSE). 2. See Toronto Stock Exchange (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 hy·dro·dy·nam·ic also hy·dro·dy·nam·i·cal adj. 1. Of or relating to hydrodynamics. 2. Of, relating to, or operated by the force of liquid in motion. limitations when used for multilayer coating. Multilayer curtain coating could optimize coating cost by applying a bottom "holdout hold·out n. One that withholds agreement or consent upon which progress is contingent. Noun 1. holdout - a negotiator who hopes to gain concessions by refusing to come to terms; "their star pitcher was a holdout for six " layer, middle "coverage" layer and top "brightening" layer in one operation--providing optimized printing surface while reducing the use of expensive pigments. [ILLUSTRATION OMITTED] 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 Reynolds number [for Osborne Reynolds], dimensionless quantity associated with the smoothness of flow of a fluid. It is an important quantity used in aerodynamics and hydraulics. 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 Refers to being "opaque," which means to prevent light from shining through. For example, in an image editing program, the opacity level for some function might range from completely transparent (0) to completely opaque (100). * 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 zeolite Any member of a family of hydrated aluminosilicate minerals that have a framework structure enclosing interconnected cavities occupied by large metal cations (positively charged ions)—generally sodium, potassium, magnesium, calcium, and barium—and water , 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 cor·ru·gate v. cor·ru·gat·ed, cor·ru·gat·ing, cor·ru·gates v.tr. To shape into folds or parallel and alternating ridges and grooves. v.intr. * Pinhole free coating for MVTR MVTR Moisture Vapor Transmission Rate MVTR Multi-Stage Vacuum Tube Receiver MVTR Mobile Videotape Recorder , oil/grease resistant and water-resistant grades [ILLUSTRATION OMITTED] * 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 TAPPI Technical Association of the Pulp and Paper Industry 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 North American named after North America. North American blastomycosis see North American blastomycosis. North American cattle tick see boophilusannulatus. 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. ADDITIONAL RESOURCES * "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: 03SEPS SEPS Subfascial Endoscopic perforator Surgery SEPS Shortstop Electronic Protection System SEPS Styrene-Ethylene-Propylene-Styrene SEPS Southeastern Pharmacology Society SEPS Standard Electronic Processing System SEPS Sprint Email Protection Services 037. 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 cklass@compuserve.com [ILLUSTRATION OMITTED] |
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