Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 203 of 843

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 stainless steel parts can be damaged by some formulations. For example, 400 series stainless steel can dissolve over time in contact with a highly acidic formulation. 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 replaced 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 stainless steel. Atomizers should contain only stainless steel or plastic wetted parts. Parts made from aluminum will perform satisfactorily for some waterborne materials, but will corrode quickly in contact with others. Some waterborne formulations 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 waterbornes because the formulations vary so much. In some cases, the seals in equipment 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 nozzles and orifices. As with the coating application process and most of the equipment, the physical 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 accommodate 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 coating 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 material 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 coating applicators to improve the TE of their operations. Until recently, most coating materials were solvent based and did not conduct electricity readily. This made it easy to use electrostatics with these materials by simply placing a high-voltage electrode in the coating material at the atomizer nozzle. The coating material picks up a static charge of electricity as it is atomized. Waterborne coating materials conduct electricity much more readily than 198

Articles in this issue

view archives of Metal Finishing Guide Book - 2013