Low-Cure High-Solids Coating

With the country on the verge of another energy crisis, now is the time to explore the possibility of using low-cure, high-solids coatings...

If you had sticker shock this winter with the soaring cost of home heating bills, you are not alone. But, homeowners aren't the only ones affected. If you are responsible for running a compliant liquid finishing operation that employs a gas-fired cure oven, you probably have seen your gas bills increase 30-50% or more in the last several months.

Most high-performance coatings achieve optimum chemical and physical properties when baked 10-45 min at 300-350F. These coatings are available in VOC levels ranging from 2.8-3.5 lb/gal and can be applied with most conventional air atomizing and electrostatic spray equipment. The best results are obtained when the coatings are applied over substrates such as treated steel and aluminum. Also, the coatings provide for a cost-effective finish, especially when a finisher is spraying multiple colors that require quick color change schedules.

However, there is one significant disadvantage to these coatings—high energy consumption for final cure. Other less significant drawbacks include high application viscosity requiring special spray application equipment and limited applications over heat-susceptible substrates such as plastics.

Low-cure high-solids coatings are an alternative to previous high-performance coatings that can overcome the drawbacks mentioned in the previous paragraph.

Coating Properties

A low-cure, high-solids polyester thermoplastic system capable of coating substrates such as treated cold-rolled steel, aluminum and fiberglass-reinforced plastics has been developed. This low-cure, high-solids coating has properties that are characteristic of typical higher cure, high-solids coatings:

  • 2.5-3.5 lb/gal VOC;
  • HAPs-free systems available;
  • Low viscosity systems—20-35 sec with a #3 Zahn cup at 70F;
  • Wet film thickness 1.3-3.0 mils;
  • Single coat coverage; and
  • Excellent flow and leveling properties.
TABLE I—Solvent Resistance/Film Hardness Tests
Cure Temperature (F) D-MEK Rubs Pencil Hardness
210 18 F
220 50 H
230 100 3H
240 120 3H
250 150 5H
270 170 5H+
300 200+ 5H+
330 200+ 5H+

During development of this low-cure, high-solids coating, two properties were evaluated to ensure maximum performance—solvent resistance and film hardness.

Solvent resistance is the ability of a coating to resist solvent attack or film deformity. Rubbing the coating with a cloth saturated with an appropriate solvent is one way to assess when a specific level of solvent resistance is achieved. All rubbing tests were conducted using methyl ethyl ketone (MEK) and employed a double rub technique, one complete forward and backward motion over the coated surface. This test was performed until the double rubbing action cut into the film or a noticeable film disorder was evident.

Film hardness is the ability of the coating to resist cutting, sheering or penetration by a hard object. One method of measuring the coating's hardness is to scratch the film with pencil leads of known hardness. The result is reported as the hardness lead that will not scratch or cut through the film to the substrate. While this test is quite subjective, it does provide a quick and rather reliable method to determine film hardness.

TABLE II—Time/Temperature to 100 D-MEK Rubs and H Pencil Hardness
Cure Time (min) Cure Temperature (F)
20 220
15 230
10 240
7 250
5 270
4 300

The first series of solvent resistance/hardness tests were performed using a set bake time (15 min) with variations in curing temperature (210-330F) over controlled iron phosphate steel substrate panels. Table I shows the number of solvent rubs until film disorder and the pencil hardness at which the coating was scratched.

The second series of solvent resistance/hardness tests set out to determine the minimum time required for complete cure at a given temperature that would achieve 100 MEK double rubs and H pencil hardness (see Table II).

Gas Savings

Now that we know a low-cure, high-solids coating can meet the same performance requirements as a standard high-performance coating, we can now examine the cost savings of using a low-cure, high-solids coating. To illustrate the savings in natural gas consumption employing a low-cure coating, a standard, direct-flame oven capable of curing several substrates for a period of 20 min at various oven settings was used in the study. Energy losses, expressed in Btus, were calculated based on part load, specific heat of the part, conveyor load, oven area and recalculation. The following data was used in calculating the Btu/hr.

TABLE III—Low-Cure Energy Savings
  Aluminum Fiberglass
Specific Heat (Btu/lb - F) 0.214 0.116
Conveyor Chain-on-edge Chain-on-edge
Line Speed (fpm) 15 15
Parts/hr 450 450
Part Weight (lb) 30 30
Cure Time (min) 20 20
Oven Temperature (F), standard coating 350 300
Oven Temperature (F), low-cure coating 300 220
Oven Load (Btu), standard coating 1,367,400 1,563,700
Oven Load (Btu), low-cure coating 1,131,400 1,023,500
Energy Savings (%) 17.25 34.55

From the data in Table III, it is evident that by lowering the bake temperature from 350F to 300F for aluminum parts and 300F to 225F for fiberglass parts, a significant gas savings can be realized using a low-cure coating. Since natural gas costs vary in each geographical region, exact dollar figures were not employed in this study.

The soaring cost of natural gas is sending an unwelcome jolt to many OEMs operating finishing systems that use gas-fired cure ovens. Energy saving products, such as low-cure or forced-cure coatings, have not been evaluated since the energy crisis in the 1970s. Suppliers and users of baked finishes have a renewed interest in developing energy saving coatings to control the increasing costs of maintaining a finishing operation. Fortunately, the technology and coating systems that minimize the effects of rising energy costs are available.