There are four major reasons for considering powder coating over liquid paint:
- To reduce VOC emissions,
- To produce better coatings with improved corrosion protection,
- To reduce production costs,
- To improve Employee safety.
Solvent-based paint processes produce both air and water pollutants and water-bourn coatings produce hazardous waste. The air pollutants contribute to photochemical smog.
There are two ways to reduce pollution: by installing expensive equipment to treat the pollutants produced; or by eliminating the source of the pollutants. Obviously the second method is more economical. Equipment installed merely to prevent the discharge of pollutants to the environment represents an extra expense which contributes nothing to profitability. Production processes which minimize the production of pollutants at their source are a more positive solution and are less expensive in the long run.
Powder coating eliminates the air and water pollutants produced by solvent paint lines. As a bonus, working conditions are cleaner, healthier and safer.
Nevertheless, although everyone is more aware of emissions problems and anti-pollution laws and regulations are becoming tougher, there are two better reasons for powder coating. As we shall see, powder coatings are superior in quality to solvent-based paint coatings and their cost is competitive.
According to the United States Department of Commerce, the annual loss due to corrosion in this country is ten billion dollars. Powder coatings can help reduce this figure. Coatings formed from dry powders are homogeneous, coherent and free from porosity caused by solvent evaporation. Many experts believe that this is one of the reasons why dry powder coatings exhibit greater corrosion resistance than coatings formed from the same resins in liquid coatings systems. This is further complimented by the user’s selection of predetermined chemicals.
An example of the superior performance of powder coatings over liquid paint systems has been reported by a Canadian manufacturer of steel battery boxes for electric trucks. They found that even the highest quality liquid paint provided protection for only six months. The paint film on the batteries scratched and chipped in service. Battery acid would then penetrate beneath the paint film at these points and attack the otherwise unexposed substrate.
The manufacturer switched to an epoxy powder coating. This coating also gets scratched and chipped during service as batteries are removed for charging and replaced. But, although battery acid attacks the exposed metal, it does not penetrate between the intact film and its substrate. After 18 months there were no signs of deterioration of the epoxy coating or its bond to the substrate.
In order to achieve sufficient thickness to provide the necessary corrosion resistance, most liquid paints must be applied in two passes. Electrostatic spray powder coatings from 1- to 6-mils thick can be applied in one pass. This eliminates the need for two coating booths.
An automotive parts manufacturer had difficulty passing a 96-hour salt spray test with a one-coat liquid paint system. Since converting to an epoxy powder coating, also applied in one pass, the manufacturer gets over 200 hours in the same test. The powder coating also withstands subsequent forming operations.
Powder coatings produce no drips, runs or sags. Edge coverage and coverage inside holes and recesses is excellent. With the electrostatic spray process, the coating powder envelops the part. In addition, the insulating effect of the powder already adhering to the surface limits build-up, thus helping to control coating thickness. Thickness is relatively uniform even on complex shapes.
Powder coatings are cost competitive with liquid paint. In fact, many powder coatings cost less than equivalent coatings formed from liquid paint, because:
- Powders are 100 percent solids.
- Efficient powder recovery provides nearly 100 percent material utilization.
- The coating line can be completely automated and requires very little maintenance.
Continual development of new, lower cost coating powders should reduce costs even more.
Continuous recovery of over-sprayed powder is key to economical production of powder coatings. Powder recovery systems can reclaim up to 98 percent of overspray for reuse.
In contrast, liquid paint overspray is lost and cannot be reclaimed. The deposition efficiency of liquid paint spray systems ranges from 30 to 90 per cent, and seldom averages more than 65 percent.
The true cost of any coating material is the cost of covering a given surface area with a dry film of specified thickness. Let’s compare the applied material cost of a liquid paint system to that of a typical powder coating system.
Only the solids in a liquid paint system form the finished coating. The solvents are just a vehicle to apply the solids. They evaporate during the baking process. If solvents don’t completely evaporate, they cause subsequent problems in the finished coating.
Thus, the coverage of any coating is based solely on its solids content. The following formula gives the coverage of a gallon of liquid paint:
231 x % solids x 1,000 = coverage, sq. ft. per mil
Powder is 100 percent solids and is sold by weight. This must be converted to volume, using the specific gravity (s.g.), in order to calculate coverage. The following formula gives the coverage of one pound of powder:
1,728 x 1,000 = coverage, sq. ft. per mil
62.4 x 144 x s.g.
Each of these formulas is for a coating 1-mil thick and ignores losses. Obviously if the coating is thicker and the system is not 100 per cent efficient, coverage will be reduced accordingly.
For example, assume a liquid paint costs $12.85 per gallon, contains 35 percent solids, and that deposition efficiency is 65 percent. As shown in Table II, actual coverage per gallon is 365 square feet, and the cost per square foot for each mil-thickness of coating is 3.52 cents.
Now assume an epoxy coating powder costs $2.95 per pound and has a specific gravity of 1.5. With a good recovery system, material utilization efficiency should be 98 per cent. Referring to the table, actual coverage of the powder is 128.8 square feet per pound and the cost per square foot of a 1-mil thick dry film is 2.29 cents. Thus, the actual cost of material for the epoxy powder coating is less than that of the liquid paint coating by 1.23 cents per square foot. Assuming a production rate of 1,000,000 square feet of surface to coat per year. This would represent a savings of $12,300 per year just in material (other savings can include hazardous waste disposal, energy and insurance).
Of course, this is just a hypothetical case. But it is not unrealistic. The assumptions are consistent with costs in industry for both paint and powder, powder coating based on the cost of the powder, its specific gravity and the thickness of the finished coating. It assumes 100 percent efficiency, so correction must be made for any powder loss. The formula shown is the same as above. It appears to be different because it has been simplified and the intermediate step of computing coverage has been eliminated. The graph permits the computation of the cost per square foot based on actual coating thickness.
In the example shown, the cost of a 2.5-mil thick coating using a powder costing $2.50 per pound and having specific gravity of 1.4 is 4.6 cents per square foot. This assumes 100 percent efficiency. If overspray recovery is 98 percent, the cost would be about 4.69 cents per square foot.
Electrostatic powder spray coating provides other out-of-pocket savings, too. For example:
- No solvents or thinners are required at all; not for mixing, not for cleanup.
- In many systems, simple degreasing is sufficient pretreatment. However, iron or zinc phosphate treatment will improve adhesion.
- There are no paint-arrestor filters, no water-wash chemicals.
- Cooling or heating of recovery air is not required as clean air is returned to the plant after filtering.
It is no exaggeration to say that the development of the electrostatic powder spray process is a fourth reason for the current powder revolution.
In the electrostatic spray process (Fig. 3) powder is drawn from its container and carried to the spray gun by clean, dry compressed air. Individual particles of powder are electro-statically charged as they pass through the gun. The part to be coated is grounded, and therefore at a lower potential than the charged particles, so an electrostatic field is generated between the tip of the gun and the workpiece. Particles projected from the gun are attracted to the surface of the part and adhere to it until they are fused to the surface and heat cured in the bake oven into a homogeneous coating.
Electrostatic spray can apply powder coatings as thin as 1-mil. As “thin film” refinement continues in the development of powder coatings, lower film build can be expected. The heat retention properties of the part are not a factor. The same thickness can be applied to thin as well as thick sections.
- “First High-Volume Epoxy Powder Coating Line in Auto Industry,” Industrial Finishing, August 1971, pp.20-23.
- Breton, “Experience with Powder Coating,” Proceedings First North American Conference on Powder Coating, February 1971, Mclean-Hunter Ltd., pp. 85-89.
- Widdifield, G., “Economics Involved in the Use of Thermosetting Powders,”
Proceedings First North American Conference on Powder Coating, February 1971, Mclean-Hunter Ltd., pp. 69-76.
- Azzam, H.T.., “Coatings Without Solvents,” Machine Design, March 18, 1971, pp. 91-95.