Dear Joe,
We've been having a nightmare with craters in our powder shop over the last few months. We only spray polyesters in our shop. However, one of our neighbors in our industrial park applies a silicone-based military coating. I'm convinced the overspray mist from our neighbor is causing our trouble. Am I correct in my thinking, and what can I do to remedy this situation?Todd
Dear Todd,
Serious contamination of powder coatings can occur from contact with even minute concentrations of silicone-based paints. Mist from a finishing operation involving silicone-based paints can carry long distances and become entrained in the ambient air of a powder coating operation. The mist of silicone-based paint can also contaminate a powder coating finish by depositing itself on the surface of parts before or during the powder coating process. Furthermore, the mist from a silicone-based paint can be deposited on air handling equipment surfaces such as filters and thereby be distributed to the ambient air, compressed air and/or finishing surfaces.
Silicone-based paints cause craters in powder coatings due to the inherently large difference in surface tension between the silicone-based paint and the powder coating. Craters form when a region of very low surface tension resides within the melted film of a powder coating. The surface tension of silicone-based paints is typically measured at 15 to 20 dynes per centimeter. The surface tension of powder coatings is typically 40 to 50 dynes per centimeter. This is a large enough discrepancy to cause fish eyes or cratering. Laboratory studies have shown that concentrations of silicone-based products as low as 0.001% can cause widespread cratering.
Remediation of silicone contamination of a powder coating system requires exhaustive measures. If the source of the contaminant can be identified, the first action is to isolate the cause. The next steps involve the painstaking procedure of cleaning the entire finishing shop. You should thoroughly clean all air handling equipment, including compressed air systems, booth exhaust and oven air handling systems. You should also thoroughly clean all powder handling and application equipment, including guns, hoppers, hoses (these should be discarded if possible) and booth surfaces. The use of denatured alcohol is recommended to clean plastics parts. It can dissolve the silicone residue while leaving the plastic intact. A test application can be conducted at this point. If contamination is not eliminated, then all surfaces in the shop should be thoroughly washed with an industrial cleaner.
Diligent root cause analysis is vital, as is thorough cleaning of the entire powder application system.
Dear Joe,
We are creating a woodgrain finish in our powder coatings by covering the coating with a specially made, paper-like foil prior to curing. I've heard that a woodgrain finish can also be achieved by applying a single powder, somewhat like a dry sublimation technique. Can you please tell me how to make this type of powder coating?Shiva
Dear Shiva,
I am familiar with the process you are using with the paper. It can create very appealing images on the surface but is rather process-intensive.
Another technique involves applying two dry layers of powder - first a dark brown layer, followed by a lighter brown layer. The dry coatings are then disrupted by dragging a comb-like tool over the deposited powders. The coated object is placed into an oven to affect flow-out and cure of the materials. The resultant finish will have a woodgrain look. This technique requires the use of compatible powders of the proper color and gloss. It also helps to experiment with the technique of "combing" the dry surface. The comb apparatus can be fabricated by fixing stiff strands of plastic to a rigid handle. A successful technique requires a little artistry on the part of the applicator. Some of the major powder manufacturers have this technology and would be happy to provide details and materials.
Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to askjoepowder@yahoo.com.
