Weekly Questions and Answers

By George Trigg, GRT Engineering
and Nick Liberto, Powder Coating Consultants

Welcome to Powder Coating magazine's Weekly Questions & Answers column. Questions for this column are submitted by powder coaters just like you who are seeking ways to improve efficiencies and solve every day problems on their powder coating lines.

1/16/2017 - Q: We’ve just started a powder coating operation. We had mostly sprayed powder on structured components. Recently, a customer required a plain polyester powder coating in star white. With this type of powder, however, we had pinholes on the surface. We tried some tactics to solve the problem. For example, we checked the dryer for the powder coating plant, and it works fine. We checked the spray application equipment, and cleaned the booth and the area thoroughly. We’re still getting pinholes. Please help us. P.B., Navi Mumbai, Mahape, India

A: You left out some information. What is the substrate? Is it a material that might be out-gassing? Or is the cleaning method leaving something on the surface that causes pinholes? And how old and under what conditions is the powder stored? It might be old or gone bad. This isn't likely, but it’s something that should be eliminated from your search for an answer. I don't know with any certainty what you mean by "structured" components. All things being proper, you shouldn't have any problems applying polyester. Maybe you have an equipment contamination problem that just doesn't show up to the naked eye when you examine the parts that are "structured." —G.T.

1/9/2017 - Q: I need to coat copper tubing (refrigeration) for immersion in water (tube and drum water chiller). The coating needs to be durable, yet able to flex with the copper tubing. Any ideas? Thank you. B.A., Wrangell, Ark.

A: Years ago I coated some copper tube for use in a refrigeration unit. I used an epoxy material for that situation, partly because the epoxy was what I had on hand and partly because the epoxy was flexible enough to move with the tubing, yet retain its adhesion, and it was never going to see sunlight, so UV wasn't an issue. Just in case bad things started to grow in the water, the epoxy was pretty resistant to whatever might take life. Okay, so now you’ve heard a tale from the past. It would still apply though: An epoxy material will hold up while immersed in water, and you won't need any UV resistance either. Bear in mind that the copper must be very clean for good adhesion. —G.T.

1/2/2017 - Q: I apply a super-durable TGIC-based (triglycidyl isocyanurate) polyester powder on extruded aluminum parts. Currently, I claim that the coating meets American Architectural Manufacturers Association (AAMA) standard specification 2604-02. This states that a panel exposed to South Florida weathering will retain a minimum gloss of 30 percent and change a maximum of 5 delta E units over a 5-year period. Recently, I began using a new powder supplier, and I can’t wait 5 years for test results. I would like to develop a specification that uses accelerated weathering as the test method, but I'm not sure what performance objectives to claim or which accelerated test to use. Are standard specs already written for this type of weathering? If not, are there performance benchmarks I could use as a starting point (perhaps in the automotive industry)? A.G., Philadelphia, Pa.

A: Several papers have been written in regards to Florida weathering studies on super-durable TGIC-based polyesters. One of these papers compares and correlates between Florida weathering data and artificial weathering with equipment such as QUV A-340 and Weather-O-Meter. There are claims that a given number of QUV A-340 exposure hours are equivalent to 1 year of Florida weathering. It’s possible that 1,000 hours of Weather-O-Meter exposure with the 102/18 program, which is 102 minutes of ultraviolet (UV) light and 18 minutes of deionized (DI) water sprinkling, is equivalent to 1 year of Florida exposure. So, you could look at somewhere around 200 days of artificial weathering test duration, which is about 7 months. This is a long time to wait on results. You should compare the high outdoor durability of the powder from the previous supplier side by side with the powder from the new supplier. We discourage the use of the faster and higher intensity artificial weathering test procedure (QUV B-313 ). It has been proved that these tests don’t correlate well with actual outdoor weathering. The automotive industry is replacing these tests with artificial weathering testing as mentioned above. Ask the new supplier to guarantee the powder against the AAMA 2604-02 weathering specification. If the supplier doesn't want to do that, then you better go back to what has been proved to work. You know the old story: You get what you pay for! —G.T.

12/27/2016 - Q: I have some automotive car parts (sheet metal fog lights) in need of powder coating. I’ve bead-blasted the housings, smoothed the surfaces with 380-grit sanding disks, and blended and smoothed with a Scotch-Brite abrasive wheel. Is it possible to fill the remaining rust pits with a metal-filled epoxy? How will the electrostatic attraction be affected, or do you have suggestions? B.P., Tampa, Fla.

A: The metal-filled epoxy should attract the powder. However, in a finite diagnosis, if the metal fillers don't touch one another and they don’t ever touch the base metal, it’s conceivable that the coating would be light. You could consider pre-heating the part to about 110°F; however, it will cause some heavier film build on the remainder of the part. Actually, I think you won't have any problems with what you’re doing. The filler will be pretty thin, so coverage should be uniform. —G.T.

12/19/2016 - Q: We’re currently using epoxy powder coating and testing for sufficient cure by using methyl ethyl ketone (MEK). We have an initiative to get MEK out of the facility for environmental health and safety reasons. We have received recommendations of toluene and xylene, which aren’t acceptable either. Do you know of any chemical that will be aggressive enough to verify cure without being a total toxic organic or a carcinogen? C.B., Reading, Pa.

A: Plain and simple: No, there’s no substitute for a reasonably quick check on cure. If you follow the Powder Coating Institute’s “#8 Recommended Procedure for Solvent Cure Test,” you’ll use very little MEK, and it can be stored safely in a quart can in a safety cabinet. Several other tests are available, although each has its drawback. • The glacial acetic acid test uses 96 percent, or higher, concentrated acetic acid. It’s applied in one drop on the film surface. After 60 seconds, the surface is wiped off, and the film is judged for loss of gloss, softening, discoloration, and so forth. It’s very similar to the standard MEK test, but glacial acetic acid is very corrosive, and it can cause exposed skin to blister after a short exposure. It seems to be worse than the MEK test. • Another test that is destructive and not very accurate is the scrape test. In this case, you would cut into the paint film down to the substrate and then drag the blade along the film for about an inch. Undercured films will break away easily from the substrate. It takes an experienced person to establish a value to this test, and even then, it’s a bit subjective. • The most accurate test, one which will hold up in a court of law should it ever come to that, is a differential scanning calorimetry test. In this case, 5 to 10 milligrams of scrapings are removed from the film and tested against a known uncured sample of the same powder material. The results will actually give you the percent of cure by comparing the exothermic reaction of the undercured material with the suspected film. The standard coating performance tests for hardness, impact resistance, color fading, and so on will also give you a clue as to whether the coating is cured. Of course, all of this takes time. If I were you, I’d give all of this information to the department that has insisted on the removal of a very, very small quantity of MEK, and tell them that they can bear the burden of the budget that will give you an alternate method of testing parts for cure. —G.T.

12/12/2016 - Q: We’re looking for a powder that meets Mil d 24712. Do you know any suppliers who do the testing on their powder to meet this specification? M.B., Holliston, Mass.

A: I think the spec you’re referring to is Mil-C-24712, not Mil d 24712. This is the generic powder coating specification developed by the US government to describe most all common powder coatings. Most powder coatings should meet this spec, but most powder coating suppliers don’t bother trying to “qualify” their material to this spec. It covers the following generic powder formulations: epoxy, epoxy-polyester hybrid, polyester, triglycidyl isocyanurate (TGIC) polyester, polyester urethane hybrid, acrylic, acrylic-polyester hybrid, acrylic urethane hybrid, urethane, polyurethane, vinyl, and nylon. That about covers the entire gamut of materials in our industry other than polyvinylidene fluoride (PVDF) and Teflon.

Go to this publication’s Web site www.pcoating.com and click on Online Buyers Guide at the left. Search under Powder Coatings for military spec powder coatings to contact a supplier that probably has materials to meet this very generous spec. —N.L.

12/5/2016 - Q: I’ve been having some trouble with clear powder coating over a veined powder coating. After the part leaves the shop, customers complain about the clear cracking or spider-webbing. I’ve talked with the powder supplier, and I was told that this is a common occurrence. Could you also discuss salt-spray testing. I would appreciate any help with this. B.C., Grants Pass, Ore.

A: Most often, this condition is caused by surface tension because of incompatibility between the clear and the basecoat. The two must get along with one another, and your powder supplier should be able to correct this for you. If your supplier didn't mention it, maybe you’re using different suppliers for the two coatings. What is it you want to know about salt-spray testing? The longer the hours in the test without failure, the longer the service life of the coating as far as adhesion is concerned. You can test panels with a scribe or without a scribe. The most severe test is an X scribe across the panel. You then check for creep into the coating from the scribe mark. The creep allowance may be no creep, 1/8-inch creep, or ¼-inch creep. The more liberal the specification, the shorter the adhesion time of the coating. This isn’t a test for abrasion, dents, gloss resistance, or some other physical characteristics of powder coatings. —G.T.

11/28/2016 - Q: Q1: I’d like to powder coat the frame of an experimental aircraft, but I’ve heard that the powder coating process can weaken the metal. The metal involved is 6061 T-6 aluminum. I’ve also read that the temperature and cure times can be set to avoid any change in the alloy. What are the correct parameters for maintaining the structural integrity of this aluminum alloy? R.M., Bradenton, Fla.

Q2: Do you have any information, recommendation, or advice regarding the concerns that powder coating and the temperatures used will weaken or affect the strength of forged rims. T.T., Kapolei, Hawaii

A: I’ll answer both R.M. and T. here because their questions are related. Organic thermoset powder coatings (what most people know as standard decorative powder coatings) have cure temperatures between 250°F and 450°F. The cure times at this temperature vary but can be as much as 30 minutes for the lower temperatures and as fast as 10 minutes for the higher temperatures. Pretty much all ferrous materials (steel and iron--tempered or not) have no difficulty with these temperatures and times. However, tempered aluminum (6061 T-6 is tempered aluminum) and softer metals (copper, lead, tin, etc.) can have problems with temperatures above 350°F. The metal temper can be annealed (softened), or the metal may begin melting if it’s subjected to temperatures higher than 350°F. Therefore, any products that have these materials must be cured at temperatures below 350°F, even if it takes longer to complete the cure of the particular powder coating. @b1:T., you have no worries with your forged steel. However, R.M., you shouldn’t have your parts cured with powder coating above 325°F just to be sure you don’t soften your aircraft structure. You should also be aware that powder coatings are heavier than liquid coatings because they’re applied much thicker. This fact may also cause some issues with adding unnecessary weight to your aircraft as well. —N.L.

11/21/2016 - Q: When I apply a second coat of powder onto a part, the powder doesn’t cover the side of it. I have an inexpensive hobby-type gun, and I think the voltage isn’t enough for make a good job on the second coat. Is this possible or not? C.T., Victoriaville, Que.

A: The equipment you’re using isn’t intended for anything more than small hobbyist applications. If you need to get a bit more sophisticated in your coating results, then you should look into an industrial application unit that’s capable of doing everything you want. You can find this equipment used sometimes, but it will take you some searching to find one. They are available. —G.T.

11/14/2016 - Q: We’ve been custom coaters in Phoenix since the 1980s. We’ve shot powders from the RAL palette since the beginning. We’ve seen improvements in powder quality across the board from all major powder manufacturers, in all formulations. But what has not improved is the uniformity in matching RAL colors from supplier to supplier. They’re all kind of close, but all do not match. Is there any effort by powder manufacturers who sell the RAL palette to come up with standardized RAL colors (similar to Federal Standard) so they all match from supplier to supplier? R.Q., Phoenix, Ariz.

A: RAL color standards can be very useful in obtaining the same color from different suppliers. However, you must also specify the tolerance you allow the supplier to drift from these standards. For this, we normally use a Cie-LAB standard of 1 ΔE, which is undetectable to the naked eye under most lighting conditions. Some colors may also require a maximum value for each of the L, A, & B values in addition to the ΔE value to ensure color accuracy.

Work with your suppliers to establish your own color standards and tolerances to ensure you obtain consistent results. —N.L.

11/7/2016 - Q: I’m using sandblasting media, air blowing with compressed air to clean sand out, then cleaning with a solvent, like a paint thinner or alcohol. I then heat the part dry for 15-30 minutes at 370°F (to outgas and dry the part), and then I remove the part from the oven and let it cool to around 150°F or sometimes cooler (tried different temps). The aluminum is usually cast, mostly motorcycle parts. I've been told that the pinholes were caused from out-gassing so that’s why I started doing the heating before powder application. It seemed to help, but it hasn’t eliminated the problem. C.S., Windsor, Ont.

A: Castings, be they ferrous or non-ferrous, will usually cause out-gassing because the metal is porous, as you know. And if the metal has been subjected to a chemical pretreatment, it’s entirely possible that some of the liquid has entered and become trapped in the pores. When heat gets to the part, as in a cure oven, the air will expand and seek a way out, and the liquid trappings can approach boiling and erupt through the coating. Pre-heating may or may not solve this problem. It will certainly reduce the amount of bubbles in the coating. Some years back, I worked with a research type engineer, and his company had a lot of problems with out-gassing. He determined that time was as important as temperature in the preheat stage. Given enough time, you can virtually eliminate the problem. However, in a production mode, you don't have the time to remove all of the gasses/liquids. So, a compromise is necessary. Sometimes, coating thickness will inhibit the out-gassing, but that adds to cost and can affect appearance. A primer will serve the same purpose and could be the best means of solving the problem. —G.T.

10/31/2016 - Q: I have a customer who wants us to put THREE coats of our TGIC powder paint onto his sheet metal parts. That seems like a disaster waiting to happen. We cure the paint at around 350°F (the max our oven will go to). Do three coats sound reasonable? What is the maximum paint thickness recommended with these powder paints? J.B., Tulsa, Okla.

A: This question is better directed to your powder supplier, as they know the particulars about your coating. However, suffice it to say three coats may be difficult to provide, as each coat after the first will be difficult to apply to a cold part, considering the reduced electrostatic attraction that will occur with the reduced part ground. The next problem is cure; as each coat is cured the preceding coat is over-cured. Because most powders have 100 percent over-bake resistance, you’ll have to cure the first coat only half way before applying the second coat and curing it half way. Even doing this and fully curing the final coat, the first coat will be 150 percent cured, 50 percent higher than recommended. At this point the coating will start becoming brittle.

Therefore, discuss with your customer what is the final film build they want. If it’s 6 mils or less, then consider applying it in two coats. You won’t have over-bake problems, and it’ll perform better than three coats at 2 mils each. —N.L.

10/24/2016 - Q: Just wondering if you can help with a problem we’re having with our powder not attracting to our parts. As far as we can tell, we’re getting a good ground on our parts that are hanging in our booth, but when we spray the powder on the parts, you can see powder just kind of falling off. And if we try to spray in corners or anywhere there is a bend, the powder won’t stick. This is causing us to overspray the parts to get full coverage, and the powder is very thick after the parts are baked. Our operator for our powder coating said it looks like the powder wants to attract directly to the booth instead of the parts. We’ve tried adjusting our settings on the powder coating machine, but nothing seems to help. Your help would be appreciated. C.M., Lewiston, N.Y.

A: From here, this is a classic case of little or no ground. It bothers me that you say "as far as you can tell, we have good ground." You should know precisely whether you have good ground or not. You need a megger meter, or megohm meter, to easily take a reading at the various points along the part stack. If you have 1.0 megohm or less, you have a good ground. Anything over that will gradually be less attractive to the powder. Another way to check ground is to make up a ground wire with a set of alligator clips on each end, and connect to the part and a KNOWN ground. You should see a marked difference. If that isn't the answer, then the problem immediately gets a bit weird. It could be a powder problem. If the powder is old, improperly stored, or damp (you should see other problems if this is the case), or had a problem right from the start, then there are other ways to test for the cause. However, it sure sounds like a ground problem to me. —G.T.

Further reading on the problems discussed in this column can be found in our Article Index and Bookstore.

George R. Trigg is president of GRT Engineering, 6314 Hughes Road, Prospect, OH 43342; 740/494-2496. He has been involved in the powder coating industry for more than 38 years. He holds a BSBA degree from Muskingum College, New Concord, Ohio. His email address is molly95@earthlink.net.

Nick Liberto is president of Powder Coating Consultants (www.powdercc.com), a division of Ninan Inc., 1529 Laurel Avenue, Bridgeport, CT 06604; 203/366-7244. He has more than 3 decades of experience in the powder coating industry. A registered professional engineer in Connecticut, he holds a bachelor’s of science degree in mechanical engineering with a minor in physics. His email address is pcc@powdercoat.com.

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