Powder for CRSWe are currently using polyurethane paint on aluminum panels. We are looking into powder paint. What are the advantages and disadvantages of going with powder vs. polyurethane? Additionally, if we decide to go with CRS (cold rolled steel) panels, will powder paint adhere differently to aluminum compared to CRS? Please take into consideration that our panels are painted with a two-tone color, 90 percent is a cool white color and 10 percent is purple.
There is an obvious environmental benefit as powder coatings are VOC-free and do not generate hazardous waste. Because it is a 100 percent solid there is no need for mixing with solvent and any other additives required to either adjust the viscosity or the dry time. With powder, there is no need for primers such as vinyl wash, commonly used on aluminum. Powder coatings come ready to use and do not require as much application skill as liquid paints. Usually a two- to three- day training program will allow an operator to be comfortable in production. It also translates into fewer rejects. On aluminum as well as CRS, the surface preparation is equally important with both liquid and powder. Failing to properly clean and etch the surface will result in brittleness, poor adhesion and severe field failures due to poor weatherability. In most cases, powder as well as liquid paint will perform well on aluminum but for outdoor application, a chromate conversion (or chrome-free substitutes) would be preferable. Physical and mechanical properties of powder coatings generally exceed their liquid counterpart. Because powder coatings are a solid product, there is no possibility for color adjustment.
More information on your current process and pretreatment and end-use is required to compare the capabilities of these coatings and their performance expectation.
Two-tone finishes can be achieved with powder coatings but may be prone to appreciable amount of masking. Remember, powder coatings are electrostatically charged, therefore powder particles will transfer to the substrate and will wrap around edges and shadow areas. If no powder is wanted in these areas, protection by means of template or heat-resistant tape will be required.
Avoiding StreaksWe apply medium gloss gray powder at around 3 to 4 mils. Sometimes our operators leave streaks on the parts when they apply the powder and some areas are darker than others. These streaks are still visible when the coating is cured. Any advice you can give us on the cause of the streaks and how to avoid them would be appreciated.
Providing that your substrate has an even color following your pretreatment, I recommend that you do a film thickness survey on the rejected parts. Darker areas (shadows) usually indicate less film build, thus possible improper substrate coverage. Check with your coating supplier to find out what film thickness is required for consistency and adequate hiding of the substrate. Light and vibrant colors sometimes require greater minimum film thickness than neutral colors.
The spraying technique also can be the cause as it appears in this case to be done manually. A good spraying technique is to cross your gun patch vertically and horizontally every 10" or so with a gun distance of 10 to 12" from the target. The type of gun attachment is also important as it is directly linked to the width of the spray pattern. A large round deflector will cover a greater area per minute than a flat spray because of a much larger gun opening at the tip thus allowing more powder delivery while maintaining an acceptable powder travel speed. The powder comes out in a cone shape that limits the "tiger stripes" effect as opposed to a flat spray that will concentrate the powder in a long but narrow pattern. Flat surfaces still can be painted using a flat spray gun attachment but will require a little more skill and attention by the operators as it becomes easy to get 4 mils in an area and only 1.5 mils 3" away.
Richard Robidoux is UV/technical sales manager at Protech Chemicals Ltd. Call 514-904-8998 or visit www.protechpowder.com.