"We used to use a polyester baking enamel for metal office furniture components. This coating had good resistance to 20 rubs with lacquer thinner and MEK [methyl ethyl ketone]. We find that the powder coat we are using doesn’t have similar chemical resistance, and I suspect that most common powders would fall short of baking enamel in this area. What types of powders would give similar chemical performance to the baking enamel, and why aren’t they more commonly used?"
I have developed crackle finishes with both solvent- and water-based liquid systems. Is it possible to develop a crackle finish with powder coatings?
I would recommend starting with an OH-polyester-TMMGU system and modifying it with various other resins to vary the degree of wrinkling that this system produces. TMMGU is tetra methoxy methyl glycoluril and is supplied by Cytec as Powderlink 1174. It requires an acidic catalyst that is usually buffered by a base. The main polyester suppliers (DSM, Cytec, Hexion and Sun Polymer) have recommendations for the most suitable OH-polyesters to use with TMMGU. You typically want a lower OH material (25 to 35 hydroxyl number) because TMMGU can seriously depress the Tg of your powder.
This system provides a fairly tight wrinkle. Dilution with other resins (acrylic, COOH-polyester, etc) opens up the wrinkle, producing more of a crackle finish.
We used to use a polyester baking enamel for metal office furniture components. This coating had good resistance to 20 rubs with lacquer thinner and MEK [methyl ethyl ketone]. We find that the powder coat we are using doesn’t have similar chemical resistance, and I suspect that most common powders would fall short of baking enamel in this area. What types of powders would give similar chemical performance to the baking enamel, and why aren’t they more commonly used?
Solvent resistance is governed by a few parameters, including resin type and crosslink density (how tightly the molecules are bound after baking). Generally epoxies provide the greatest solvent resistance, followed by polyurethanes, hybrids and most acrylics. The least solvent resistant powders are typically polyesters. This isn’t a hard and fast order of solvent resistance because powders can be formulated for better or lesser resistance.
The completeness of cure also is important - an undercured powder will exhibit significantly less solvent resistance. Make sure that you are achieving the time at temperature on your parts as recommended by your coating supplier.
If you are getting only 20 double rubs (MEK or thinner) with your powder, double bake a part and recheck the solvent resistance. If the solvent resistance doesn’t improve, then you may want to investigate a change in product. You also need to determine if you need more than 20 double rubs solvent resistance to meet the performance requirement of your customer. The application might tolerate a lower level of resistance.
I do not have any experience with the powder process. I realize that powder coated parts are supposed to be baked in an oven, but could another heat source, such as IR heating elements or heat lamps, be used? Also, can different colors be mixed together? What would a powder finish look like if the colors were mixed?
To answer your first question, yes, powder can be cured with energy other than a traditional bake oven. Finishers regularly use various types of infrared (IR) either as sole sources of curing energy or as a boost to their convection oven. IR can be delivered at different wavelengths (commonly referred to as short, medium and long wavelength) and intensities (high, medium and low). The IR can be generated electrically with gas-fired emitters or catalytically driven gas emitters. OurBuyers Guidelists a number of IR curing equipment suppliers if you would like to learn more.
Some specially formulated powders can be cured using ultraviolet (UV) radiation. These powders require a melt stage prior to the UV curing phase. The melt can be accomplished with convection or IR energy.
Another curing technology possibility for powder coatings is near infrared (NIR). Specially formulated products use a very specific wavelength range of infrared based on the components of the coating. This process allows for efficient energy use. (See“Instant Curing with Near Infrared Technology,” Finishing Today, October 2007.)
As for mixing different powders, it all depends on what you’re mixing. Typically a mix of compatible (same or similar chemistry) but different-colored powders will produce a cool speckle finish. Mixtures of incompatible powders can result in a textured or “hammertone” appearance, depending on the concentration of ingredients. One thing is for sure - it’s difficult to replicate the speckle look. The quantity of available powders may vary, so the look will change depending on what goes into the mix. Regardless, this technique is sometimes used to coat non-appearance parts at an economical cost.
Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to firstname.lastname@example.org.
FINISHING ANSWERS: Ask Joe Powder
November 1, 2007