Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 141 of 331

application methods CONVERTING TO WATERBORNES BY RONALD KONIECZYNSKI NORDSON CORP., AMHERST, OHIO Just as government regulations concerning gasoline mileage in the '70s helped spur today's improved automobile engines, so government regulations con- cerning VOC emissions from coating lines have spurred the development of some remarkable waterborne and water-based coating materials. Now manu- facturers need to know how to convert their operations to use these new mate- rials efficiently. This means that they need to know what application equipment is suitable and how best to use it to apply waterbornes. A likely first question is "What is the best way to apply a waterborne material?" The answer is "There is no best way." The process that works best in a particular application with a solvent-based coating will usually continue to be the best after converting the application to a waterborne coating. The dynamics of getting ma- terial from the applicator to the part are similar whether the material contains mostly solvents or mostly water. ATOMIZER SELECTION Most coating material applicators break the material up into fine droplets or par- ticles, which are then carried through the air to the part being coated. The process of breaking up the material is called atomization, and the equipment that does the atomization is called an atomizer. Typical atomizers are air spray guns, ro- tary atomizers, and disks. None of these is "best" for all waterborne applica- tions. Instead, the shape of the part being coated, the coverage required, and the production rate determine the best atomizer for a particular application us- ing waterbornes, just as they do with solvent-based materials. To illustrate the importance of part configuration on atomizer selection, con- sider a simple box-like part, open on one end, requiring paint on both the inside and outside surfaces. The outside of the box might best be painted with a soft spray using electrostatics to get good part coverage, high transfer efficiency (TE), and good wrap. A rotary atomizer with electrostatics would be a good choice for the outside of the box. The rotary atomizer and electrostatics would be a poor choice, however, for the inside of the same box because the Faraday cage effect caused by the electrosta- tics and the walls of the box would actually keep most of the paint out of the box. A better choice for the inside of the box would be an airless spray atomizer. Air- less spray uses the momentum of the paint particles to get the paint to the part, rather than electrostatic attraction. The point is that a manufacturer who has spent a lot of time perfecting the coating application process for solvent-based material should stick with that process when converting to waterbornes, if the latest technology and good equipment are already in place and good TE is being obtained. Sometimes a particular waterborne coating formulation may need to be modified slightly to accommodate the atomizer. For example, an emulsion may tend to separate when subjected to severe centrifugal force on a spinning rotary atomizer cup. 140

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