Application methods of powder coating

Excerpt: Powder coating is a type of coating that is applied on the object as a free-flowing, dry powder.

POWDER coating is a type of coating that is applied on the object as a free-flowing, dry powder. Powder coating is an environmentally acceptable finishing method since the mid – 1950's. These coatings are mainly dry paints. The coatings are formulated with the same resins, pigments and additives that are used in liquid paints without the use of solvents. These dry paints do not require solvent to keep the binder and filler in a liquid suspension form. By eliminating the solvent we can reduce the organic emission to the atmosphere. It can also reduce effluent discharge from the factory. The powder coatings are especially divided into two classes in the market place such as thermoplastic and thermosetting. Thermoplastic coatings melt under heating and form a continuous film. They simply harden as they cool without undergoing any chemical change. On the other hand, thermosetting coatings melt and undergo chemical change upon heating and form a continuous film. The powder coating create a hard finish which is tougher than liquid paint. Powder coating is mainly used for coating of metals, like automobile and bicycle parts, aluminum extrusions, drum hardware, household appliances and also medium-density fiberboard (MDF), plastics etc. The most common polymers used for powder coating are polyester, polyurethane, polyesters epoxy (i.e. hybrid) and acrylics etc.

Advantages of powder coatings

The potential advantages of powder coatings are as follows :-

1) Minimizes air pollution and also water contamination.

2) Coating increases performance.

3) Improves economics to the user in application and handling.

4) 100% usage of material that is purchased for coating purpose.

5) No need to handle unusable material like sludge from the paint booth.

6) Film build up is fast than that of liquid paint.

7) No need for paint mixing and adjustments for viscosity as powder coating does not use solvents.

8) Color drift, cracking, sagging or blistering problems often seen in liquid paint are also eliminated.

9) Reduction of labor costs.

10) Eliminates specialized and lengthy training in the application technique.

11) Reduced number of application units and simplifies the automation process.

12) Reduced floor space for finishing.

13) Lack of solvents, eliminates paint line fires which can lead to lower insurance premiums.

14) Fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.

15) Wide range of specialty effects are easily accomplished than that of liquid systems.

Disadvantages of powder coatings

1) Powder coating is basically a chemical coating. Hence, it has many of the problems of solution paint.

2) If it is not properly formulated, the powder coating will exhibit sagging at high film thickness.

3) It can show poor performance if it is not cured properly.

4) It can show film imperfections like craters and pinholes.

5) It can show poor hiding power than solution paint at low film thickness.

The powder coating process

The powder coating process involves three basic steps,-

1) Part preparation or the pre-treatment.

2) The powder application.

3) Curing.

Part preparation or the pre-treatment

Prior to powder coating process, oil, lubrication greases, metal oxides, welding scale must be removed properly. Various chemical and mechanical methods are available for the removal of above impurities. The selection of the powder coating method depend on the following factors such as –

a) Size / thickness / configuration of part.

b) Material of the part.

c) Line speed being run.

d) Type of impurities to be removed.

e) Performance requirement of the finished product.

Chemical pre-treatment is carried out with the help of phosphates or chromates in multiple stages such as degreasing, etching, de-smutting, different types of rinses and finally chromating or phosphating of the product. In chemical pretreatment, both cleaning and improving the bonding of the powder to the metal is carried out. However, the chromates are toxic to the environment. Hence, recent additional processes of pretreatment have been developed.

In some high end performance characteristic applications, after the pretreatment process, the product is electrocoated and then it is powder coated.

Another method of surface pretreatment involves abrasive blasting or sand blasting or shot blasting.

The Powder Application Processes

There are various methods are available for powder application. Some of the important application methods are fluidized bed, electrostatic (corona) spray, friction static spraying, electrostatic fluidized bed etc.

Fluidized Bed Coating

In this process, there is a container with a false bottom. The bottom is a porous membrane. There is an inlet for the upflow of dried air. Powder is placed in this bed on the porous bottom. The bottom does not allow the powder to fall through. It allow and evenly distributes the powder due to the upflow of dried air, through the bottom.

Fig: Fluidized Bed Coating :-

In the process, air rises through the permeable membrane on to which the powder contained in the bed. This results in a fluid action of air and powder. This action is necessary to give an even distribution of powder. This action is also necessary to allow an object to enter the bed.

The object to be coated is preheated to a temperature (at least 350°F) above the melting point of the powder to be coated on the object. The preheated object is immersed in the fluidized powder bed. The powder particles in the contact with an object melt and fuse together to form a film on the object. A post-heat sometimes is required to give the film more flow or to improve the appearance of the powder coating on the object or to complete the curing process.

The fluidized bed process is used for coatings between 10-15 mils. The final thickness dependent upon part temperature and dip time in the powder.

Typical uses

Buss bars, transformers, fabricated wire, metal furniture, raceways.

Advantages

1) Uniform and thick coatings,

2) Good process control.

Disadvantages

1) Preheat and post-heat ovens required.

2) Thinner substrates do not hold heat and difficult to coat.

Electrostatic Spraying

Most powders are insulators with relatively high volume resistivity values. Therefore, they accept either positive or negative charge and are attracted towards the grounded or oppositely charged object.

In the actual process, the powder from the powder reservoir is fed to the gun with the help of an air pump. A high voltage generator is used to charge an electrode placed at the tip of the powder gun. This creates an electrostatic filed (or corona) between the gun and the substrate. Gas Molecules from the air pick up electrons emitted from the corona. The negative charge from the gas molecules is transferred to the powder particles, as they are pushed from the gun towards the substrate. The substrate to be coated is earthed. The charged powder particles have a strong attraction to the grounded part and deposit there. Virtually all resins except nylon can be applied easily with this process.

This process can apply coatings having thickness between 0.8 and 10 mils. Electrostatic spraying can be used for decorative as well as functional coatings.

Advantages

1) Heavy coatings can be applied to the substrate.

2) Applies quickly to the substrate.

3) Process can be automated.

4) Minimum operator training requirement.

Disadvantages

1) Requirement of high voltage source.

2) Thickness control becomes sometimes difficult.

3) Capital cost higher than other application methods.

Friction Static Spraying (Tribostatic Spraying)

Tribomatic static charging is the second most common method of spraying powder coating. This method depends on the powder to develop a charge (either positive or negative) while passing through special hoses and guns. While passing the powder particles through the noses and guns, they collide with the surfaces of the powder spray equipment as well as each other. A device in the spray equipment strips either a positive or the negative charge from the powder. The powder particles get uniformly charged while exit the spray unit. These frictionally charged particles will then be attracted to the substrate and get deposited on it.

(Fig. 3)Friction static spraying

Advantages

1) No high voltage source required.

2) Better penetration of particles into deep recesses.

3) Low capital cost.

Disadvantages

(1) Slower rate of application.

2) Not all powders accept frictional charge equally. Even powders of the same generic type may show differences in charge retention.

3) Wears out parts faster.

Electrostatic Fluidized Bed

This method is a combination of the fluidized bed and electrostatic spray method. A charged current of air passes through the permeable membrane contained in the bed. The powder particles get repelled by each other and form a cloud above the bed. The density of powder cloud is determined by the applied voltage. A grounded object is passed over the bed. The charged powder is attracted to it.

Advantages

1) Conductive surfaces can be easily coated.

2) Speed line is high.

3) Process can be automated easily.

4) Thin films can be applied on it.

Disadvantages

1) Object with deep recesses are difficult to coat.

2) Long vertical or dimensional surfaces are not coated uniformly.

3) Coating area limited to 3-4 inches above the bed.

Curing

When a thermoset powder is exposed to elevated temperature, it melts, flows out and undergoes chemical reaction into a high molecular weight network like structure of polymer. This process is called as cross linking process.

This process requires a typical temperature and a certain length of time in order to cure fully and to establish full film properties. Normally the cure temperature is about 2000C (3900F) for a period of 15 minutes. The curing conditions could vary according to the powder manufacturer's specifications. The energy required for curing process is applied through conventional ovens, infrared cure ovens or by laser curing process. The laser curing process offer significant reduction in cure time.

References

1) http://www.pfonline.com/articles/powder-application-methods 2) http://www.cfpowdercoating.com/powder-coating-process/ 3) https://www.tcipowder.com/powder-coating-troubleshooting-guide/chapter-7.html 4) http:/www.powdercoating.org/page/whatsPC 5) https://en.wikipedia.org/wiki/powdercoating 6) Berins Michael L, 'SPI Plastics engineering handbook', Chapman and Hall, P. no. 459 to 503, (1991) 7) http://www.misumi-techcentral.com /tt/en/surface/ 2014/06/186-powder-painting-fluidized-bed-coating-method.html 8) http://www.misumi-techcentral.com/tt/en/ surface/2014/06/187-powder-painting-air-electrostatic-fluidized-bed-coating-method.html 9) https://www.chinapowdercoating.com/tribo-charging/ 10) http://www.sppcinc.com/powdercoating 11) Literature survey of various internet resources.

Author Details

N. L. Phadke

Dept. of Plastics Engineering, Government Polytechnic, Miraj