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.

2/13/2017 - Q: Is e-coating (electro-deposition coating) similar to powder coating? If different, what’s different about the applications? Is there anything different between the two applications with the Faraday cage? R.L., Oriskany, N.Y.

A: The only similarities between e-coating and powder coating is the word coating. I assume you know what powder coating is, and I won’t bore our readers with information they already know about the finishing process that’s the very name of this magazine.

E-coating is an immersion finishing process in which parts are dipped into a tank of paint (often epoxy), and an electrical charge is sent through the paint to attract the paint to the part. The part may have a negative or positive polarity, depending on if the e-coat tank is anodic or cathodic. The coatings are limited in formulation and color for this process, unlike powder coatings, which come in numerous formulas and colors for just about any application.

E-coat is applied in very thin films (often around 0.2 mil). It can be used as an inexpensive topcoat but is most often used as a primer. There are no Faraday cage issues with e-coat, but there are limitations to where the coating can be deposited. These areas are defined as low current density areas and areas where “throw power” limits coverage. This limitation is significantly less of an issue in e-coating than the Faraday coverage issue in powder coatings.

Finally, e-coating can be applied very inexpensively. If the average applied material cost for powder coating is $0.15 per square foot, the average applied material cost for e-coat is $0.02 per square foot. —N.L.

2/6/2017 - Q: Can powder coating replace nickel plating? We already have the equipment to apply powder coating. We’re having problems with corrosion (rust) in nickel-plated parts (housing bearings) during overseas shipments to a customer. That’s why we think we can replace the nickel plating with powder coating. The parts are sent to a vendor to be nickel-plated, but we’re having problems with the coating. The vendor is probably not doing it right, so we thought we could use powder coating to protect the parts better. The housings are made of carbon steel. C.B., El Salto, Jalisco, Mexico

A: Well, you can, but it would depend on what the coating is supposed to do. I would need to know what the coating performance specifications are to pass any type of judgment. Powder would certainly be a cheaper coating, and in this country nickel presents environmental problems. Nickel is certainly a hard surface coating, and tends to be a heavier coating than powder coating, so part tolerances would be an issue as well. I certainly wouldn't make a change until I understood the coating performance specifications. By the way, there are ways to protect metal parts that are shipped via boats to prevent oxidation. But based on the information you’ve told me, powder would do nicely if the film build doesn't create any problems. I suspect that the film thickness will have to be 2.0 to 2.5 mils. If that works for you, then go for it. —G.T.

1/30/2017 - Q: We currently powder coat our aluminum brake calipers red and black. They frequently see temperatures around 300°F and are subjected to hot DOT 3 brake fluid. We’re using bright red polyurethane and black polyester TGIC. We apply the colors separately and cure them at 400°F for 40 minutes. After they’ve been cured, we do a hot pot test where we submerge the caliper in hot DOT 3 brake fluid at 250°F for 2 hours, remove, and let cool, wiping off excess brake fluid. We then bake at 375°F for 20 minutes. After that time, the red is still rock hard, and the black is soft and tacky. What would cause that? C.S., Camarillo, Calif.

A: There is some speculation on my part, but I would think that calipers of aluminum would reach cure temperature in less than 15 minutes. I don't know how much less, but less. That would mean for instance that the powder cure is ±30 minutes at 400°F. You should check your powder cure schedule. I think you’re over-baking, and that may be the problem with the black. You need to get your hands on a temperature recorder for the oven, and rather quickly. Your powder supplier should have one that you can use for a brief time to develop a cure chart. Without one, you’re groping in the dark. You’ll need it for about a day. Run more than one test. These instruments fit into a heat-resistant thermal box. Depending on the type of unit, it may have from three to six or more probes. The probes measure metal temperature on several locations on the part, usually a high point and a low point, and if so equipped, a mid-point. Another probe measures oven air. These instruments are critical in detailing information about what’s going on in an oven. You may have hot or cold spots due to improper adjustment of the dampers. The thermocouples may be bad or failing and giving false information. You get the point. Typically, the instrument is encased inside a thermal protective unit and has a graph ribbon on which the various probes will make their color-coded lines. The probes are usually numbered so that you can tell which line is air temperature and which lines are metal temperature. In theory, you should have a nice ramp-up set of lines for metal temperature. The air reading will go up much quicker, and the lines should be fairly straight to the exit point. If your production levels are at least moderate, and you’re planning to grow your business, then you should consider purchasing a temperature recorder for your oven. Frankly, I don't know how you could operate without one. —G.T.

1/23/2017 - Q: We’re somewhat new to powder. We’re having issues with clumping due to humidity here in Indiana. We don’t always have our materials stored in our air-conditioned areas, and in some cases, we have $10-per-pound material that is used infrequently clumping between days it’s used. We’re looking for ways to rescue clumped material and any other ideas to use, such as desiccants, when the powder isn’t in the air-conditioned storage. C.J., Markle, Ind.

A: For some things, there are no substitutes. However, if you have agglomerated material, you can run it through a sieve, be it handheld or powered, and remove a great deal of the lumps, but not all. In the long run, the cheapest way to avoid agglomerated powder (due to moisture) is to place the material is an environmentally controlled atmosphere. Maybe that's the bosses' office, or whatever. You’re going to spend a lot of time (and money) trying to eliminate it from the system. For the boxed powder, you should get some air conditioning. Desiccant absorbers don't work very well in this situation. That’s because the preferred storage and usage humidity is in the 20 to 60 percent range, and desiccant canisters or bags aren't that sophisticated. The most common way to condition the powder that’s in the hoppers is to leave the fluidization air on all the time, including weekends and overnight. This is predicated on your having good, dry compressed air. If the compressed air is damp, don't bother because it’ll make the situation worse. Hey, suck it up, take the bosses’ credit card and go to Wal-Mart or H.H. Gregg to buy a room air conditioner and put it in the storage room. And make sure you size the AC for the room. —G.T.

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.

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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