Fluoropolymers powder coatings

This section discusses the coating of surfaces using dispersions and powders of fluoropolymers. A majority of the dispersions consumed are PTFE based some FEP, PFA, PVDF material are also coated on different substrates. Powder coating resins are all melt processible fluoropolymers. Most of the parts used in the chemical processing industry are fabricated by powder coating which is covered in Ch. 8 of this volume. Liquid coatings are used in the rare cases in which powder cannot reach areas of a part with complex geometry. 

Some parts might need to have a coating thickness less than what other lining processes can provide. These might include valves, tubes, pipes, and impellers. These parts may still need to be protected by a fluoropolymer coating. One of the best references is published by the Powder Coating Institute although other sources are also available. 

For chemical process applications, high film buildup is often desired. Powder coating requires many coats and bakes, and gets more difficult for thicker coatings because the fluoropolymer insulates the grounded part from ground. There is a way to apply thick powder films by a process called hotflocking, described below. 

To take advantage of this process for powder coating, a spray gun can be constructed without a high-voltage power supply. For most coating materials, the ideal material for gun construction would be PTFE since it is positioned at one extreme of the triboelectric series. However, for fluorpolymers, a gun constucted of nylon is best. Nylon is the engineering plastic that is furthest away from PTFE in the triboelectric series, so it provides the best charging of a fluoropolymer powder. The interior of the powder gun is constructed of nylon such that there... 

Hot flocking. Powder coating is one of the best ways to coat a large impeller or a mixing blade with a layer of thick fluoropolymer. Usually, the process starts with a cold electrostatic powder coating step... 

Commercial products. There are many grades of powder-coatings provided by the companies that make fluoropolymers. Any melt-processible fluo-ropolymer theoretically can be made into a powder coating. Some coating companies buy granular fluoropolymers and grind them to a particle size suitable for powder coating. 

Special thanks go to our friend Dr. Lawrence McKeen, DuPont Fluoropolymer Solutions, for contributing the powder coating technology section included in Chapter 8. We would like to sincerely thank Dr. Jorgen Bergstrom and Dr. Stewart Brown... 

Comparison of solvent-based fluoropolymer and fluorocarbon powder coating developed in Japan shows that elimination of solvent is not only good for environment but also improves performance (UV stability especially is improved). The study was carried out with a very well designed testing program to evaluate the weathering performance of the material. 

Spray and baked electrostatic powder coating of fluoropolymers, as described under thin linings, can also be applied as thick linings. One such lining is PVDF and glass, or carbon-fiber fabric. 

The thermoplastic fluoropolymers also can be applied as coatings or linings by powder coating and rotational lining. Both processes can provide relatively thick, void-free fluoropolymer layers compared with coatings that can be achieved with PTFE dispersions. 

Fluoropolymers are used in powder coating in much the same way as other polymers. The principle differences are that higher temperamres are required to melt the fluoropolymers, and surface preparation requirements may be more stringent. In powder coating, powdered resin is applied to a surface using an electrostatic process and then heated in an oven so that the resin melts to form a continuous layer. Additional layers are applied to achieve the desired thickness. 

External release agents are lubricants, liquids or powders, which coat a mold cavity to facilitate part removal. Internal release agents can accomplish the same purpose. The identity of release agents is rarely disclosed, but frequently they are fine fluoropolymer powders, called micropowders, silicone resins, or waxes. 

If solid polymer objects are fluorinated or polymer particles much larger than 100 mesh are used, only surface conversion to fluoropolymer results. Penetration of fluorine and conversion of the hydrocarbon to fluoropolymers to depths of at least 0.1 mm is a result routinely obtained and this assures nearly complete conversion of finely powdered polymers. These fluorocarbon coatings appear to... 

Small particles of fluoropol5miers become highly combustible in the presence of metal fines. Aluminum and magnesium produce fire and/or explosion with PTFE powder when exposed to temperatures above 420°C. Practical situations where a similar reaction is possible include pumps with fluoropoly-mer packing used to pump aluminum flake dispersions, or grinding or sanding metal parts coated with fluoropolymers. PI... 

After batching, the powder was attrition milled (Model HD-01 Union Process, Akron, OH) in hexane at 600 rpm for 240 min in a fluoropolymer coated bucket using tungsten carbide media and a tungsten carbide spindle to reduce the starting particle sizes. The powder was again dried by rotary evaporation. 

To overcome these limitations, recent works propose to use fluoropolymers to help binding the coating of ceramic powders to nonwovens [24], or through impregnation of gel-polymer electrolytes into nonwovens [25]. These composite separators may exhibit better C-rate capability and cyclability than a commercialized polyolefin separator. 

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