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coatings do not require investment in expensive cleanup and effluent monitoring systems. In addition, far less wastewater is generated, resulting in fewer problems and expenses associated with waste disposal. 6. Cost savings in operations: The savings in energy, labor, rework, material, line efficiency, waste disposal, air processing, and cleanup can be substantial compared to liquids. 7. More environmentally friendly: Powders enable the elimination of solvents and hazardous wastes. Unless the coatings are accidentally overheated during the fusing process, powder coatings emit zero or near- zero VOCs during the coating process. Drawbacks vs liquid coatings Powder coatings are particularly well suited to applications where thicker films are desirable because of their increased durability and resistance to corrosion. 1. Thick-films: Powder coatings are ideal for applications where thick liquid coatings of similar properties would ordinarily have been specified, but where VOCs are unacceptable. Also, powder coatings should be considered when the necessary buildup of film thickness with liquids may result in blistering. 2. Corrosion: Powder coatings are applied to retard or eliminate corrosion in many industrial applications. Corrosion is an electrochemical process with three components: a cathode, an anode, and an electrolyte, and there are many circumstances in which all are present. Common examples include metal parts used in or near sea water or acid solutions; parts made from dissimilar metals that are joined together; and vibrating parts that are tightly pressed together. Specific examples include components of CPI systems such as valves, pipes, hangers, unions, joints, filters, and housings for motors and pumps. The key to protecting these parts is total encapsulation of the surface, which can be ensured by applying the coating in multiple layers, if possible. The reason is that, in any single coating layer, small voids or pin holes can serve as an electrolytic path for corrosives. Multiple coating layers can eliminate essentially all of these voids by overlapping them (see diagram next page). 3. Durability: This is a common requirement in applications where frequent contact is made with the coated surface. For example, the surfaces of equipment used for materialshandling, packaging, sealing, molds, underbody parts of vehicles, stone crushing, pulpwood, grain processing, and building ma- -4- Not all advantages, however, lie on the side of powder. Liquid coatings are often preferred because of their ability to form thin films or to be cured at lower temperatures. 1. Liquids are well suited to thin coatings: Thinner coatings are advantageous in many applications, particularly on small mechanical parts where assembly or operations would be impeded by a coating that is thicker than about 38 microns (1.5+ mils). 2. Higher fusing temperatures: Liquid coatings fuse through drying and curing, in which the volatile carriers are driven off by heat, leaving only the solids behind. Volatile components can be removed over a wide range of temperatures, from about 65˚-370˚C (150˚- 700°F), in a time/temperature relationship. So some liquid systems can be processed at low temperatures and still incorporate high-melting components into the cured coating. On the other hand, a powder coating such as PFA must reach its specific melt temperature before it can flow and fuse. Yet some fusion temperatures are incompatible with certain substrate materials. For instance, fusing a die-cast part at 370°C (700°F) is likely to produce eruptions from subsurface voids. Also, forged aluminum parts should not be fused above 205˚C (400°F) because they may soften. Thus, when selecting a powder coating, it is important to ensure that the fusing temperature of the powder is compatible with the temperature resistance of the substrate. When to specify powder
Topcoat/ Second coat Two coats to avoid corrosion Prime/ First coat Pin holes Overlapping layers of the topcoat fill in and cover any minute pin holes in the prime coat. terials can often be eroded by the materials that are being processed. Given enough time, these surfaces can be completely destroyed by the materials passing over them. Thick films of certain powder coatings can slow down this process. Although a powder coating may not stop the erosion entirely, it is likely to prolong the life of parts that are used in these environments. 4. Release: Liquid coatings are often preferred for release applications, although fluoroplastic powder coatings are frequently used when extra durability is required. For example, powder coatings are specified on a number of paper-handling and mold surfaces where long-lasting release is critical to the process. Other examples include slides and chutes for printing equipment and wear surfaces of xerographic equipment. A word on fusing Substrate Powder coatings are fused by heating the coated parts in an oven. In some cases, the parts are heated prior to powder application. • Preheating parts: Preheating of parts, often called “hot flocking” is, of course, necessary when coating via the (non-electrostatic) fluidized-bed method. However, it can also be quite useful in the electrostatic process. For example, in cases where unusually high builds of powder are required, the parts can be preheated to the fusion temperature of the powder. This will cause the powder to fuse on the part as soon as it is applied, enabling a thicker-than-normal coating to be attained. • Oven profiling: Every oven has a unique -5- heating “profile”. If an oven is set at a temperature of 205°C (400°F), the real oven temperature may be 215°C (420°F) in the upper-left corner and 185°C (365°F) in the lower-right corner, which can damage the quality of finished coatings if the profile is not known. For instance, parts in one section of the oven might be withdrawn before fusing is complete or, conversely, could become overheated and burned or blistered. Therefore, prior to long production runs, a thermal profile of the oven should be obtained in order to enable adjustment of, or compensation for, temperature imbalances. • Processing temperatures: The processing (curing or fusing) temperature in the oven depends on the resin used. Many polymers can be cured in the range of 160°-210°C (320°-410°F); however, certain high-performance resins such as fluoropolymers require processing temperatures in the range of 300°- 400°C (600-750°F). How this heat is applied to the part depends on the size of the part, its composition and shape, as well as the number of units to be coated. Large, thick items, such as castings, tend to be fused in convection ovens where heat is supplied remotely, via gas or electric heaters. Small parts, such as stampings and small wrought parts tend to be cured by banks of IR heaters. Choosing the process The choice of how to powder-coat a part depends on (1) the mass and size of the part, (2) its thermal conductivity, (3) its shape, and (4) the number to be coated. 1. Mass and size: For a powder coating to fuse on the surface, the part must be heated to the melt (fusion) temperature of the polymer and remain there for as long as is required for the powder to melt and flow. This is a time/ temperature relationship (see next page). The larger and thicker the part, the longer it takes to reach the fusion temperature of the coating. Extreme examples: a cast valve body and a stamped shell of an automotive oil filter. If a large, thick steel casting is to be
Page 1 and 2: Getting the most out of powder coat
Page 3: This technique requires little equi
Page 7 and 8: cases where the width-to-depth rati

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