Powder Coatings

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Introduction:

Powder coatings have been developing as an environmentally acceptable finishing method since the mid-1950's. These coatings are essentially dry paints. They are coatings that are formulated with the same resins, pigments, and additives as paints, but without solvents. Solvents are used in paints simply as a vehicle to transport the resins and pigments to the piece that will be finished and then evaporated from the film. By eliminating the solvents, we dramatically reduce organic emission to the atmosphere and virtually eliminate effluent discharge from the factory.

There are two generic classes of powder coatings available in the marketplace: thermoplastic and thermosetting. Thermoplastic coatings are those that melt with the application of heat to form a continuous film. They do not change chemically, but simply harden as they cool. Thermosetting coatings, on the other hand, melt and chemically react with the application of heat. Thermosetting coatings make up the majority of powder coatings in everyday use.

Powder coatings have a specific place in the paint industry, being different in many aspects from the liquid coatings systems. With respect to the manufacturing process, powder coatings should be classified in the group of plastics rather than paint technologies. The application techniques for powder coatings are unique and not applicable to the other organic surface coatings used commonly by the industry.

Process:

Powder coatings are processed by one of three different methods: dry blend, melt mix, or solution. There are four basic methods for applying powder: fluidized bed, electrostatic spray, friction static spray, and electrostatic fluidized bed.

An electrostatic spray system consists of a powder reservoir, the powder feed mechanism, the gun design, a powder generator, the application booth, and powder recycling equipment. From the reservoir, the powder is fed to the gun by means of a Venturi air pump. Arriving at the gun, the powder/air mixture is charged. The corona charge occurs internally, at the tip of the gun, or at an electrode close to the powder exit. The charge from the powder pack varies from 60 to 120 kV and from 100 to 400 microamps.

In the fluid bed process, powder is placed in the bed which is basically a container with a false bottom. The bottom is a porous membrane which does not allow the powder to fall through but does allow, and evenly distributes, an upflow of dried air. The object to be coated is preheated to a temperature above the melting point of the powder coating. The preheated objected then is immersed in the fluidized powder, where the powder particles melt and fuse together to form a film on the object.

Friction or tribo charging relies on the powder particles colliding with the surfaces of the powder spray apparatus as well as each other; these collisions result in a random static charge being deposited on the power particles. A device in the spray apparatus strips either the positive or the negative charge from the powder, allowing uniformly charged particles to exit the spray unit.

Electrostatic Fluidized Bed is a combination of the fluidized bed and electrostatic spray methods. A charged current of air passes through the permeable membrane, causing the powder particles to be repelled by each other and to form a cloud above the bed. The applied voltage determines the density of the powder cloud. As a grounded object passes over the bed, the charged powder is attracted to it.


Advantages

electrostatic spray system

  • Electrostatic powder spray has the ability to apply very uniform, low-thickness coating to a cold part.
  • fluid bed process

  • Simplicity of equipment
  • Low cost
  • Ease of applying heavy film thickness
  • Friction or tribo charging

  • Best employed for parts that are difficult to coat with conventional or electrostatic methods.
  • Disadvantages

    A quick color change is difficult to achieve, as opposed to the liquid coating systems

    electrostatic spray system

  • Prevents powder deposition in small openings and tight angles
  • Thick films can be difficult to deposit
  • fluid bed process

  • It is not possible to apply thin films (less than 5 mils)
  • Objects are limited to the size of the bed
  • Friction or tribo charging

  • All powders do not accept frictional charge equally
  • Volume of powder sprayed is limited due to the frictional heat that is generated
  • Electrostatic Fluidized Bed

  • Prevents powder deposition in small openings and tight angles
  • Long vertical or dimensional surfaces are not coated uniformly
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