Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 142 of 331

Does this mean that you don't have to change anything in order to con- vert to waterbornes? No, it doesn't. Even though the basic application process may not change, some of the specific pieces of equipment used for that process may not be suitable for waterborne materials. Metal parts may corrode. Seals may swell or leak. CONSTRUCTION MATERIALS Waterbornes rust plain steel and in some cases attack aluminum. Even stain- less steel parts can be damaged by some formulations. For example, 400 series stainless steel can dissolve over time in contact with a highly acidic formula- tion. On the other hand, parts made from 316 stainless steel hold up well with most waterbornes. This means that at least some of the application equipment will need to be re- placed when a system is converted to waterbornes. Piston pumps made of plain or alloy steel have to be replaced with pumps made of stainless steel. Pipes and distribution systems need to be made of corrosion-resistant materials such as stain- less steel. Atomizers should contain only stainless steel or plastic wetted parts. Parts made from aluminum will perform satisfactorily for some waterborne ma- terials, but will corrode quickly in contact with others. Some waterborne for- mulations can even become "explosive" in contact with aluminum. Seals in atomizers and pumps need to be changed if they are not compatible with the waterborne material. There is no single best seal material for water- bornes because the formulations vary so much. In some cases, the seals in equip- ment used for solvent-based coating materials are also suitable for waterbornes. For example, Buna-N is suitable for some solvent-based paints and is also a good choice for many waterbornes. One caution about reusing equipment from a solvent-based coating operation for waterbornes, a surprising amount of "dirt" from the old coating material can turn up in the new coating material after the conversion to waterbornes. A few filters in the fluid lines can prevent a lot of downtime due to plugged noz- zles and orifices. As with the coating application process and most of the equipment, the phys- ical plant does not necessarily need to change in order to convert to waterbornes. Often the formulation of the waterborne material can be tweaked a little to ac- commodate the facility. For example, the drying time for a waterborne primer may need to be adjusted for the time available before the color coat is applied. The TE can drop after converting to waterbornes even though the application process is the same and much of the equipment is unchanged. This is especially true if the application process includes atomizing the material. ELECTROSTATICS All spray guns and centrifugal applicators, like rotaries and disks, atomize the coating materials and propel the atomized particles toward the part being coated. With these devices, all the particles that are not aimed directly at the part will miss it and be wasted. The waste can be minimized and TE improved if the atomized coat- ing material is given an electrical charge that is opposite in polarity to the charge on the part being coated. Opposite electrical charges attract and some of the ma- terial that would miss the part entirely instead gets drawn to it by the electrical forces. The technique is called electrostatics and has been used for years by painters and 141

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