This concept has been bandied about rather vigorously in the past year or so. It can be described as consecutively applying two distinct layers of two different powder coatings and using a single bake cycle to cure both layers. The concept of “two powders—one bake” offers attractive operational economics versus the more conventional “two powders—two bakes” process. Productivity is nearly double and capital equipment utilization is high.
There are a couple reasons one might consider two layers of powder. The most obvious is the ability to apply a primer plus topcoat system thereby providing excellent corrosion resistance and edge coverage with an outdoor durable topcoat. This approach is appropriate for finishes requiring performance in extreme environments such as tropical climates and northern locales that use deicing salt on roadways. Basecoat-clearcoat is another two-coat system commonly encountered. In this scenario, a color coat—often a metallic type—is applied, then followed by the application of a glossy clear powder. The clearcoat provides an attractive finish and also protects the metallic pigments from the elements.
The challenge of dry-on-dry involves keeping the powders separate as the second layer is applied. Electrostatic charge and air velocity tend to move particles between the dry layers.The key to making this process work resides in careful deposition of each layer. This means excellent grounding of parts, racking and part conveyors. It also requires management of electrostatic charge. Powder spray equipment should have charge limiting technology to reduce excess build-up of charge. In addition the air flow to the gun should be as low as practical. High velocity air will disturb the first layer of powder causing the interspersing of particles. One last application issue – a higher ratio of powder to air is preferred to deliver more powder and less air to the coated substrate.
Another consideration involves material characteristics. It is advantageous to use two powder coatings that have a similar curing speed. A large differential in cure can cause wrinkling, texture and a reduction in gloss. In addition it is wise to use powders with similar particle size. A large difference in particle size distribution will increase the tendency for the powders to intermix.
Dry-on-dry powder technology requires careful selection of materials and application conditions. While the powder coating material technology may be rather conventional it is wise to perform a fair amount of testing before committing to a system on a commercial scale. (This concept was presented by both Rodger Talbert of TCI Powder Coatings and Ronald McMahon of Sherwin-Williams.)
There were a number of innovative technology presentations at the conference that were touted as new; however, many had been around for a few years. These include:
- Infrared Curing of Powder Coatings
- Powder Coatings on Composites
- High Temperature Resistant
- Low Temperature Cure Fluoropolymer Based Technology
- Thin Film Powder Coatings
- Anti-Microbial Techniques
- Alternates to Phosphate Based Pretreatments
- Bluetooth™ Connectivity for
- Film Thickness Instrumentation
These were all very good presentations that offered value to attendees, but for long-time members of the industry, they were not necessarily novelty topics. Some of these technologies have been commercial for five years and some have been around for 25 or more years.
There is a bright side, however. In spite of the dearth of new technology, there has been an undercurrent of a rebirth of product development. Powder manufacturers are being driven to reignite their R&D programs by factors in the market. OEMs are demanding novel powder approaches for emerging substrate technology. Researchers are finally exploring avenues to deposit powder on non-traditional substrates such as molded plastics and ceramics. These new opportunities will undoubtedly spur the innovation needed to expand powder’s share of the industrial coatings market.