troubleshooting, testing, & analysis
TROUBLESHOOTING COMMON (AND
SOME NOT-SO-COMMON) PROBLEMS
RELATED TO PAINT, POWDER
BY RON JOSEPH (1944–2011), FMR PAINT & COATINGS CONSULTANT,
EXPONENT, INC., MENLO PARK, CALIF., AND KEVIN BILLER, POWDER
COATING RESEARCH GROUP, CINCINNATI, OHIO
(Editor’s note: This section is dedicated to the memory of Ron Joseph, Metal Finishing’slong-time organic coatings editor, who passed away in 2011.)
MATCHING HVLP NOZZLE SIZE TO YOUR APPLICATION
Q:What is the best spray gun and tip to use while spraying Imron 5000 & 6000? I amusing an Iwata HVLP with a 1.3 tip, and I’m still having trouble with heavy metallics.
A:First, you didn’t mention the Iwata HVLP model number. The vendor recommendsnozzle sizes for each spray gun model. For instance, I went online andreviewed the nozzle sizes for the LPH 2003 HVLP spray gun. If you scroll downtheir Web page you will find recommended fluid nozzle sizes, given in millimetersand inches. For your particular spray gun you should review the nozzle sizesIwata has recommended.The HVLP spray gun might be perfectly adequate, but you might need to playwith the fluid flow rate and air atomizing pressure. Invariably, painters use toohigh a fluid flow rate and the atomizing air can’t adequately break up the paintstream. I suggest that you set up your spray gun from scratch. Take a piece ofaluminum foil and attach it onto the spray booth wall. Turn the fluid needle,fan control knobs, and the air atomizing control valve all the way closed. Nowopen the air atomizing control value a quarter turn and do the same with thefluid knob. If no paint emerges from the gun, open the air a little more. Perhapsalso open the fluid control knob another quarter turn. Continue opening theair atomizing valve until some paint emerges from the gun orifice. Eventually, awet round circle of paint will deposit on the foil. At this point, start to open thefan control knob to widen the fan pattern. By opening the air valve, fan control,and fluid control knobs in small increments, you should be able to get a settingthat will produce a good-looking paint finish.This will take time and patience, but the process usually works. If these stepsfail, perhaps you need to change to a smaller or larger tip size.
PAINT ODORS AND SPRAY BOOTHS
Q:I own an upholstery company, and every so often my neighbor spray paints outside.The smell is quite horrible. When I leave my warehouse door open, the odor enters and Imust close the door to keep out the obnoxious smell. Are there regulations preventing himfrom painting outdoors?
A:Depending on how much paint is used, the painters might need to install adedicated spray booth. While this will not stop the odors, it will force the solventsthrough an exhaust stack, approximately 30 ft above ground level. Thiswill considerably minimize the odor.If the painters use very small quantities, such as one quart per day, they mightnot be required by regulation to install a paint spray booth. Please bear in mindthat every state has its own regulations regarding when one is required to install aspray booth. However, you can report the nuisance odors to your local air qualitycontrol district. If the inspectors agree that this is a nuisance condition, they canrequire the painters to desist from painting, or they can ask the painters to do theirwork after hours when you and others have gone home for the day.
FEDERAL STANDARD COLOR VS. PRODUCT NUMBER
Q:I often receive blueprints that request the supplier to powder coat per Fed. Std. 595,Color #26231. Can you please explain to me the difference between a Fed. Std. 595 colorand a powder coat color number?
A:Federal standard colors are ones by which the U.S. government specifies itscolors. It is a long-standing color standard used by architects, engineers, andothers. Colors can be purchased in low-, medium-, and high-gloss ranges. Thestandard can be applied to any type of paint, alkyd, acrylic, epoxy, polyurethane,or any other resin type. The paint can be liquid or powder. You might like to readmore about Fed. Std. 595 at wikipedia.org.Powder and liquid coatings can be purchased to the Fed. Std. 595b standard,to a manufacturer’s unique color range, or to a company’s own customcolor. For instance, most large corporations, such as Deere & Co., Caterpillar,Coca-Cola, Pepsi, Federal Express, UPS, United Airlines, etc., design their ownunique colors. Therefore, the color numbers given by paint manufacturers[suppliers] are unique to that manufacturer. If two or more paint manufacturersmake a color for the same corporate customer, the color number might beassigned by the customer. On the other hand, it is equally possible that eachpaint manufacturer will assign its own number to that color. Generally, customcolors do not carry the same nomenclature as Fed. Std. 595 colors.
ACHIEVING A GOOD-LOOKING AUTOMOTIVE FINISH
Q:I am trying to paint my car with Rust-Oleum semi-gloss black paint. The label on thecan says to mix the paint with mineral spirits. How much should I use to get a good finish?
A:You should add as little mineral spirits as possible. I suggest that you try sprayingsome of the paint without adding mineral spirits. If you get too much orangepeel then add a small amount and try again. On the one hand you want to avoidgetting unacceptable orange peel, but on the other you don’t want the paint tobe so thin that it runs down the doors and vertical surfaces. The label on the canshould tell you the maximum amount that you can add. Bear in mind that yourlocal environmental agency might have VOC regulations that prohibit you fromadding any thinners to your paint.
ALTERNATIVES TO CHROMATE-CONTAINING PRETREATMENTS
Q:I am writing to ask if you have any information about specific plumbing fixture companiesthat use alternatives to hexavalent chromium in their finishing processes. We are a “green” design and consulting firm, and we currently have a client who is interested in usingproducts that have such alternative finishing treatments.
A:I don’t have any dealings with plumbing fixture companies, but there are severalnon-chromate pretreatments that can be used in finishing processes. Chemicalcompanies, including Henkel Surface Technologies and Chemetall, sell non-chromateproducts. You can find both companies and many others on the Internet. (Besure to listen to Chemetall’s recent webinar on www.metalfinishing.com.)
CORROSION-RESISTANT PRIMER FOR AUTOMOTIVE REFINISHING
Q:I live in England and I am about to rust proof my 36- year-old Mark 2 Ford (Mercury)Capri. What would be the best primer to use after I have taken it back to bear metal? I canget my hands on red oxide fairly cheaply, but I was wondering what would be the mosteffective primer? After experiencing a few days under my car with a grinder, I never againwish to repeat this tiresome task.
A:An inexpensive red oxide primer is not what you want. In my opinion, I wouldapply an epoxy primer that contains an effective rust-inhibitive pigment, suchas zinc chromate. Bear in mind, however, that the use of zinc chromate is beinglimited in some areas due to its potential toxicity (it is a known carcinogen).Alternatives are strontium chromate, zinc phosphate, and other inhibitors. Isuggest you check with your local automotive paint store.
VISCOSITY OF AUTO REFINISHING PAINT
Q:What is the typical Zahn cup viscosity of paints used in the auto refinishing industry?Would it be possible to coat a sphere evenly on the circumference with paint? Also, whatkind of air guns would you use for this process?
A:The approximate viscosity of solvent-borne automotive paints is in the rangeof 22–30 seconds on a Zahn No. 2 cup. It should be possible to coat a sphereevenly at this viscosity. If you are located in the U.S., you would probably wantto use an HVLP spray gun. Alternatively, if you live outside the U.S. and are notrequired to meet environmental regulations, you can use a conventional air spraygun; either pressure fed or siphon will work.
SELECTING BETWEEN AIRLESS AND HVLP SPRAY GUNS
Q:As part of a low-cost basement-refinishing project, I would like to leave the ceiling unfinishedbut paint it black. I own a 30-gal compressor and will be using a 2 ½ -gallon pressuretank. Which type of spray gun would you suggest for a quick application where coverage ismore important than finish: traditional airless or HVLP? Since it is an enclosed area, I amconcerned about overspray; ventilation is limited.
A:You would be better off using a commercially available airless spray gun ratherthan a high-volume low-pressure (HVLP) spray gun. HOWEVER, I must warnyou to please read the instructions and thoroughly familiarize yourself with itsoperation. Since such guns operate at pressures usually in excess of 1,000 psig,they can be dangerous, and it is critical you understand how to operate it safely.For your situation, the advantage of airless over HVLP is you can get the job donequickly without generating much overspray. HVLP will generate significantoverspray, and you have already said you do not have good ventilation in thebasement.
WHICH PAINT SPRAY APPLICATION METHOD PRODUCES A DRIER
COATING?
Q:I was pondering over two issues: First, if all variables are held constant, and you applyan automotive waterborne paint, which application would produce a drier film (using anESTA bell) one at 35,000 RPM or one at 55,000 RPM? Second, which application is drier:electrostatic or conventional?
A:If all variables are held constant, I would imagine that the faster rotationalspeed of the electrostatic bell would break up the paint into finer particles, leadingto a drier finish. As I have never performed this experiment, this is only aguess. I would imagine that the same applies to your second question: whichevermethod produces smaller particles would produce a drier finish. In this case, Ipresume the electrostatic spray gun may produce smaller particles, as it can beexpected to accelerate evaporation of the solvent compared to a conventionalprocess.
CALCULATING AIR FLOW IN SPRAY BOOTHS
Q:I have a product that I will be coating with Waterborne Camouflage AliphaticPolyurethane Chemical Agent Resistant Coating (CARC). I will apply the coating with anHVLP sprayer at approximately 65 psi with a 0.070” tip. I calculated the sprayer volumeflow rate to be about 4 SCFM.I am trying to calculate air flow needed for my spray booth and the correspondingenergy consumption. If my makeup air is coming from outside, I have to condition the airfor humidity and temperature prior to entering the paint booth.I estimate my booth will have a footprint of100–150 ft2. There will be two operatorsin the booth: one doing prep; one painting. The product will be hanging on a conveyor atworkingheight.I have read about cross-draft and down-draft booths with velocity requirements of 100ft/min and 50 ft/min, respectively. Would one flow rate be preferred over the other forwaterborne CARC? What is the main difference between the two flow rates? To calculatethe volume flow rate needed, do I multiply the velocity spec by the area of the filter area?How do I know how much filter area I need? Can I bring make-up air into the booth fromthe main building in which the spray booth is located? Do I draw the make-up air into thebooth with my exhaust fan or push it in with another fan? If I do have to bring in make-upair from outside, do you know a reputable company that manufactures efficient equipmentfor controlling the temperature and humidity?
A:The air flow rate inside the booth is independent of the spray gun, its pressure,air throughput, etc. According to the National Fire Protection Association(NFPA) Bulletin 33, you need to move air through the booth at approximately100 ft/min. You can calculate this by using the crosssectional area of the booth.Therefore, if the width times the height of the spray booth equals 80 ft2 youwould need an air flow rate of 8,000 ft3/min.If you wish to lower this, you need to calculate the lower explosive limit (LEL)of the WD-CARC, but this is not a simple calculation. With this information,you then need to calculate the air velocity that will maintain the concentrationof solvents at 25% LEL plus a sufficient safety factor. Suppose you were to findthat for the amount of WD-CARC you intend to use, the air velocity wouldonly need to be 20 ft/min to achieve 25% LEL, then the velocity would be so lowthat all the overspray would fall onto the floor and not be carried to the filters.The smaller the filter area, the higher the velocity of air that passes throughthe filters. This then increases the filter resistance, and you will need to purchasea stronger fan to pull the air through. On the other hand, the larger the filterarea, the more efficiently the filters will be able to capture the overspray. Bear inmind, the spray booth is not only there to withdraw the VOCs, but almost equallyimportant is the filters’ ability to capture the sticky, messy paint particles. For anice clean paint job, you would like the overspray to be quickly carried to the filters;otherwise, they can deposit on freshly painted surfaces and result in dry spray.You most certainly can bring air into the booth from the main building, butyou must exhaust the air to the outside. You can pull it into the booth with asingle exhaust fan or push it in with an air make-up fan, whichever you prefer.
MEASURING SPRAY BOOTH AIR VELOCITY
Q:I need to confirm that our spray booth meets OSHA requirements of 100 fpm per 29CFR 1910.107. Where do you take the measurements in a large spray painting booth (25ft wide x 15 ft high x 60 ft deep); at the face of the booth, midway inside the booth or at theopen end?
A:When I measure booth velocity, I stand a few feet from the filters and measurein various locations so that I can calculate an average. You will find that as youget close to the side walls the velocity will usually be close to zero. Therefore, Idon’t stand within 3 ft from the walls. If you have poor seals in the double doorsat the entrance of the booth, you will find that there is a spike in air velocityin the center of the booth. Therefore, try to avoid measuring air flow along thecenterline between the doors.My method measures the air flow at the points where the painter stands,which is, after all, the intention of the regulation. On the other hand, you canmeasure the velocity directly in front of each filter and then average your readings.Multiply this by the area of all the filters to get the volumetric flow rate(CFM) passing through the booth. Since your filter plenum is probably narrowerthan the width of the booth, you can now divide the volumetric flow rate bythe cross-sectional area of the booth (in your case 25 ft W x 15 ft H) to arrive atthe average velocity across the booth. This velocity will usually be considerablyhigher than my measurement, because air often travels along the ceiling of thebooth and not where the painter stands.As you probably know, 29 CFR 1910.107 provides Table G10 that lists the airvelocities for different situations. However, for a typical USCG side-draft spraybooth, 100 ft/min is a good number to work with.
AIRFLOW RATES FOR DOWN-DRAFT SPRAY BOOTHS
Q:I have a project where an existing down-draft spray booth 54ft (L) x 19.3ft (W) x 16ft(H) (with exhaust pit) is used to spray helicopters. It has an airflow rate of 33,000 CFM andprovides a down-draft velocity of approx 32 fpm. I have been told that the velocity should be100 fpm. In reviewing NFPA and industrial ventilation standards, I have found little informationabout reduced flow in down-draft booths. It appears that the airflow meets the LEL.
A:My friend and colleague Rich Thelen of Global Finishing Systems has kindlyagreed to answer this question...“Spray booth velocities are not cited in NFPA-33. They are vestigial requirements from US 29 CFR 1910.107 (OSHA) thatlist velocities in certain circumstances, such as electrostatic painting or otherpainting. Some of these standards have worked their way into other codes invarious fashions. The U.S. Military Guides (UFC-3-410) list different velocitiesfor different spray techniques, including HVLP, electrostatic and airless.OSHA has deferred to NFPA-33 in the design of spray booths and has publishedits interpretation online at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=21875OSHA considers the use of velocities other than those listed in 1910.107 as“de minimis” violations. “De minimis” violations of standards that have nodirect or immediate relationship to safety and health are not included in citations,but the velocities must conform to NFPA-33.The focus for air velocity is on NFPA-33, which simply states that the airflowshall be sufficient to keep the concentrations of solvents below 25% of the LowerFlammable Limit (LFL). NFPA is concerned with fire protection, whereas OSHAis concerned with human health and safety. Jointly, they cover health, safetyand fire issues, and both agree that NFPA-33 is an acceptable rule. So much forlegal issues!“A spray booth must also remove overspray in a manner that gives a goodpaint finish, while at the same time providing high transfer efficiency. Theseare production issues that must be addressed by the owner. Regulations will notguarantee a good finish! As spray booth manufacturers, we recommend airflowsof 75-125 fpm in cross-draft paint booths and 40-50 fpm in down-draft booths.These velocities consider cross-sections that are normal to the booth airflow. Wehave found that these airflow velocities are well above the 25% LFL requirementand they yield good paint jobs.“Velocities below these numbers may also yield good paint jobs, but generallylower velocities are more likely to cause the airflow in the spray booth to stratifyand lead to painting (and heating) problems. This happens particularly in largespray rooms. Stratification in downdraft booths occurs at lower velocities thanin cross-draft booths. Because the direction of overspray particles is downward,and gravity contributes to particulate removal, down-draft booths are moreefficient at removing overspray at low velocities than cross-draft booths. It canbe costly to remedy stratification, because changing the exhaust and supply fansis a major project.”
REDUCING PAINT DEFECTS—AIRLESS SPRAY GUNS
Q:Please give me advice about reducing paint overspray while using a fast-dry paint andan airless spray gun.
A:The only method of which I am aware for reducing paint overspray whileusing an airless spray gun is to lower the hydraulic pressure. However, if youlower the pressure too much you will not get “tails” and you will not be happywith the finish. If your painter stands closer to the surface he is painting, thenhe might be able to lower the pressure sufficiently to make a difference.
CAN I ELIMINATE THE EPOXY PRIMER WHEN APPLYING CARC?
Q:I am curious to know if it is always necessary to utilize an epoxy primer prior to applying[chemical agent resistant coatings] CARC? Would a substrate, such as fiberglass, be able toskip the primer and still meet the military specifications?
A:I do believe it is possible to apply the CARC topcoat directly over fiberglass,especially if you apply sufficient film thickness > 1.8 mils to provide the necessaryIR camouflage properties. However, I am not able to give you permission toomit the epoxy primer. For that you will need to get approval or a waiver fromthe weapons manager of the product you paint. You might even need to firstobtain a letter from the Army Research Lab (ARL) advising that the omissionof the epoxy is acceptable or necessary for adhesion of the polyurethane CARCto the fiberglass. Address your e-mail to Mr. John Escarsega, این آدرس ایمیل توسط spambots حفاظت می شود. برای دیدن شما نیاز به جاوا اسکریپت دارید
GROUNDING OPERATOR WHEN USING AN ELECTROSTATIC SPRAY
GUN
Q:What options are available for grounding the operator in a hand-held electrostatic spraygun system?
A:The handle of all electrostatic hand-held spray guns should be grounded.Therefore, if your painter is experiencing electrostatic shocks, your firstapproach should be to test that the gun handle is grounded. The best optionfor a painter is to hold the handle firmly in the palm of his bare, sweaty handand make good contact with the handle. By wearing gloves the painter isolateshimself from the ground and will tend to build up an electrostatic charge. If hedoes wear a glove, he should cut a hole in the palm, so that he can have physicalcontact with the handle. Alternatively, one can purchase conductive gloves thatare suitable for electrostatic paint application. Additionally, leather-soled shoesare better than rubber soles. I have also heard of painters wearing a grounded waistband, where the band or belt is in direct contact with the painter’s skin.
MEASURING DRY FILM THICKNESS ON VEHICLES
Q:I am wondering if you can help me find the paint thickness specifications for a 2006Pontiac G6. My car measures between 12–15 mils per panel. I measured two other panels,which ranged from 4–6 mils per panel. I think that, for some reason, the vehicle was paintedtwice in the factory.
A:From the film thickness measurement you mentioned, I would agree withyour assumption. A film thickness range of 4–6 mils is more realistic. I havenever seen published data on the film thickness ranges for individual vehicles.Automotive assembly plants develop their own internal specifications for filmthickness and might make this data available to their vendors, but I don’t thinkthe film thickness values are generally available to the public.If you are really diligent (and lucky), you might be able to find similar vehiclesin public parking lots or at dealerships. If you get permission from the owner tomeasure film thickness, you’re in luck. After measuring the film thickness on severalidentical vehicles, you will know for sure if your vehicle’s panels were repainted.
VISCOSITY MEASUREMENTS OF THIN COATINGS
Q:We currently measure the viscosity of our coatings using an S90 #2 Zahn Cup. Typicalefflux times for various products range from 13–17 seconds. I understand this is lowerthan the range typically measured with a #2 Zahn cup. Should we be using a Zahn #1 cupinstead? Note: The product is a solvent-borne coating.
A:A viscosity of 13–17 seconds is so short that a small error in stopping the stopwatch has a significant effect on the measurement. If you were to use a smallerdiameter orifice, such as the #1 Zahn cup that you suggested, you would lessenthe potential error. When you go to a smaller orifice, ensure that toward the endof the measurement the flow of the effluxing paint should not start, stop, start,stop, etc. When almost all the paint has drained from the cup, you should geta clean cut-off of the paint stream.
POLYURETHANE VS. URETHANE
Q:I am hoping that you will be able to help me with this problem. I am a guitar maker anduse polyurethane/standard automotive lacquer for the finish. However, I find this a little soft,plus it is hard to build. Can you suggest an alternative that I can use, but not nitro-cellulose?Also, what is the difference between polyurethane and urethane?
A:Polyurethane is probably the best coating I can suggest. I don’t know why youfind it soft, because it should be extremely mar resistant. Are you sure you aremixing it properly? Also, I don’t understand what you mean by “hard to build.”You can apply approximately 1 mil (0.001 inches) per application. It occurs tome that perhaps you are applying too many coats too soon, and not allowingthe solvents to evaporate. You could consider a hard furniture coating, suchas a catalyzed wood lacquer, but I don’t know if that is any harder than thepolyurethane.There is no difference between urethane and polyurethane.
SETTING POWDER COATING CONTROLS
Q:I am having trouble getting our process of powder coating dialed in. Specifically, onperforated panels for our products. These panels are typically made of sheet steel 16–18gauge thickness. Regardless of the experience of our painters, we seem to continually get adrip that collects at the bottom of the panel. They are typically small but are considered asrejects by our customers.Our oven is running at 415°F on a typical day. I have asked our painters to isolate the panelsin question to one rack. Parts with differing metal thickness are powder coated on other racks.We are using Tiger Drylac as our powder supplier. Any ideas would be greatly appreciated.
A:I asked my colleague, Mike Cravens, to tackle this one, and here is his reply:“The dripping (or heavy edge coverage on the bottom surfaces) is likely causedby the powder material’s gel time characteristics. Powder materials are heatactivated. The powder material, once applied, must melt, flow, gel, and polymerizeor cure. The typical gel time of a normal powder with a normal cure cycleis 20 seconds. Some materials are formulated with extended gel time to reduceorange peel and eliminate minor outgassing. I must also note that if you areapplying the powder on a hot substrate (above the melt temperature of thepowder) you may be forcing the powder through two flow stages.”If your panel hangs vertically inside the oven, then as its surface temperatureincreases the powder will melt and start to flow. Its viscosity will have droppedconsiderably. If this were liquid paint we would expect it to sag. After a few moreseconds, depending on the powder, the viscosity increases dramatically and flowessentially stops while the powder starts to cure hard. The time it takes until youreach the high viscosity is called the gel time. It is possible that you are usingpowder with a long gel time and, hence, there is sufficient time for the powder toform drips. Of course, you can consider looking for a powder of the same color andtexture that has a shorter gel time, or you can better control the coating film thicknessthat your painter applies. The thicker the film, the more prevalent the drips.
DETERMINING VOC CONTENT FOR SEMI-VOLATILE COMPOUNDS
Q:I just ran some paint solids test on our epoxy/amine catalyzed product according toASTM D2369 “Standard Test Method for Volatile Content of Coatings”. The productis a benzyl alcohol containing amine, but essentially no volatile from benzyl alcohol wasdetected. What do we report to EPA for this compound?
A:The ASTM test is conducted at 230°F (110°C) for one hour and at that temperaturemost volatiles evaporate. However, benzyl alcohol has a boiling point ofapproximately 337°F (205°C) and very little might evaporate from the coating atthe 230°F test temperature. The vapor pressure for this compound is approximately0.11 mm Hg at 25°C.Many years ago, probably in the late 1970s or early 1980s, the EPA established0.1 mm Hg as the vapor pressure above which all volatile organic compoundswould be considered as “VOCs”. The EPA was referring to those volatile organicsthat participated in smog (ozone) formation. Compounds with a vapor pressure< 0.1 mm Hg were considered to have negligible potential to form smog and werenot counted in VOC regulations. On the other hand, some volatile organic compounds,such as acetone, methyl acetate and a few others with vapor pressures >0.1 mm Hg were considered to be exempt from regulations because they do notparticipate in the photochemical reactions that lead to smog formation.EPA no longer implements the 0.1 mm Hg guideline and ASTM D2369 (whichforms part of EPA Method 24A) is now the defining test. However, it is worth notingthat benzyl alcohol, with a vapor pressure of approximately 0.11 mm Hg, is onthe borderline of VOC status. Since its boiling point is considerably higher thanthe 230°F temperature at which the ASTM test is conducted, very little will evaporateduring the one hour test period. Therefore, it is understandable that the labthat conducted the test on your behalf did not detect any significant amount ofbenzyl alcohol. Bottom line: even though this compound is volatile at higher temperatures,you need only report the portion that evaporates during the ASTM test.
WASH PRIMERS FOR MILITARY SPECS
Q:I’m not clear on the difference between MIL-C-8514 and DOD-P-15328 wash primers.What are the benefits/drawbacks of each?
A:The two wash primers look very similar to each other, and I cannot discern adifference unless I spend more time comparing the ingredients and their respectivepercentages. MIL-C-8514C is intended for aircraft metal, predominantlyaluminum alloys, whereas DOD-P-15328 is predominantly used on steels. It ispossible that MIL-C-8514C contains less acid to ensure that when it is appliedto aluminum one does not have excess un-reacted acid remaining on the sur-face. When working for a military contractor, I always recommended that whenDOD-P-15328 was applied to aluminum, painters were to dilute the wash primerwith alcohol to reduce the acid concentration.Therefore, my recommendation is as follows: for aluminum surfaces applyMIL-P-8514C; for steel surfaces apply DOD-P-15328. I urge you to call the paintsuppliers from whom you purchase the coatings and ask the chemist in thelaboratory to provide a recommendation.
CLARIFICATIONS ON SUITABILITY FOR POWDER COATINGS FOR
MILITARY APPLICATIONS
Q:We build communications equipment for the military. Most of this equipment is usedin sheltered applications and, thus, is not exposed to the weather. We paint to meet MILDTL-14072 Finishes for Ground Based Electronic Equipment. We currently use one partalkyd enamel paints, and two parts epoxy paints. We are interested in adding powder coatpaints, but this MIL Spec doesn’t reference its use. I have found one MIL Spec on powdercoat paint, MIL-PRF-24712. However, I’m unable to find a paper trail that will allow usto use this paint on our products.I’ve read that the military is interested in the use of powder coat paints, and that companiesare using powder coat paint for military applications. However, it sounds like you haveto get special permission to use it. Can you point me in the right direction?
A:This is a very good and somewhat timely question. Powder coatings are one of(if not the best) coating technologies to protect a vast array of military products.They are tough, extremely durable, can be formulated in all colors and glosses,and are the most regulatory compliant of all industrial coating technologies.The specification you cite, MIL-PRF-24712 was originated in 1989 and revisedin 1995. Surprisingly, there are no qualified products recognized by the militaryagency responsible for this specification.The specification covers a cornucopia of powder coating chemistries rangingfrom epoxy, to polyester, acrylic, and polyurethane. It also describes three differentclasses related to service environment (dry, immersion and immersion withweather exposure) and performance requirements. The military has recognizedthat MIL-PRF-24712A has become obsolete, and it is diligently working on amajor revision. Part of this revision involves separating the immersionserviceclasses from MIL-PRF-24712 and embodying it in MIL-PRF-23236. NAVSEA initiatedthis change to cover powder coatings used primarily as corrosion-controlmaterials. The new version of MIL-PRF-24712 is expected to be published beforethe end of the year (2009).As for whether powder coating technology exists to meet MIL-PRF-24712 andMIL-PRF-23236, the answer is “yes.” It is just a matter of an interested powdercoating manufacturer submitting appropriate products to the governing militaryagency for qualification. I can provide contact information of individualswho may be willing to work with you in qualifying powder coating for thesespecifications.
POWDER COATING MAGNESIUM ALLOYS
Q:I have a cleaner/phosphate that is supposed to treat magnesium, but how should it behandled as far as dry-off and cure temps? I tried a couple of parts this morning, with a lowglossclear coat and they came out looking like Desert Storm camouflage. This was curedfor 12 minutes, at 360°.
A:Magnesium alloys are a tricky substrate to powder coat unless you knowhow to do it. Most magnesium-fabricated products are cast, resulting in acertain degree of porosity on its surface. Cleaning the substrate is a great idea;however, the cleaners/pretreatment can remain harbored in the pores. Indeed,even without cleaning, air resides in the pores. As the powder melts and flows,the cleaners and air escape from the pores. Most powders are curing at thispoint and can’t recover or reseal the holes caused by the volatiles. The result ispinholes, low gloss, and unsightly surface disruptions.My advice is to continue cleaning as you are doing at present, but run theparts through a relatively high-temperature dry-off before you apply the powdercoating. It’s preferable to coat the parts very soon after the dry-off, even whilethey are still warm, so they don’t re-absorb ambient moisture. As for dry-offtemperature, 400°F for 10 minutes is a good place to start.You should also be aware that many powder suppliers offer product lines thatare better suited for porous substrates such as magnesium. It may be best to useone of these with a well-controlled dry-off process.
MEASURING POWDER COATING DENSITY
Q:Is there any method, equation, or software program to calculate powder density?
A:I am aware of two methods used to determine powder density. Both arecovered in detail in ASTM D5965–02(2007) Standard Test Methods for SpecificGravity of Coating Powders.One uses the volume displacement of the powder into a fluid (kerosene orhexane) with a known density. The weight of the powder is known, so the relationshipbetween weight and volume can then be calculated.This method involves introducing the fluid into a graduated cylinder. Thevolume and weight of the fluid is recorded. Next, a given weight of powder ismixed into the fluid and the displaced volume is determined. It is essentialthat you eliminate all air pockets in the mixture to obtain a reasonably accuratemeasurement. Please be aware that this method doesn’t easily account for thesurface porosity common with most powder coatings and typically results in alower-than-true specific gravity. Nonetheless, it can be used as a decent tool tocompare powders.A much more accurate method, based on the Ideal Gas Law, utilizes a gas pyncometerinstrument that measures volume of a known weight of powder by gasdisplacement. These are relativelyexpensive instruments and areavailable from a number of commercialinstrument suppliers. Eachinstrument is slightly different—some measure volume; others canmeasure volume and density. Youwould have to consult the specificprocedure provided by the instrument manufacturer to successfully measurespecific gravity of powders.I recommend you use the simpler fluid method, but always run a controlsample of know specific gravity along with your samples to be evaluated.Powder-Specific GravityWeight of Powder (g)Final Volume – Original Volume (ml)
PROBLEMS COATING OVER CERTAIN SUBSTRATES
Q:We have a problem when we apply metallic silver powder coating over black E-coatedautomobile truck wheels. During tire assembly an iron rod is used to [seat] the tire. Afterfixing the tire when the rod is withdrawn from the parts, we could see a heavy scratch markat the edge of the part. Is it due to compatibility over e-coat.? How can we avoid this? Lastly,is there any need to improve the powder system? Note: My scratch resistance is 3,000 gmsand impact resistance is 250 kg-cm.
A:This sounds like a tough one. First of all, the surface properties of the powdertopcoat do not appear to be a function of the compatibility with the e-coatprimer. Incompatibility with the e-coat might cause intercoat adhesion issues,but not surface slip problems.The scratch resistance and surface slip of the powder coating can be improved.Your powder supplier can increase the crosslink density and, thus, the hardnessof the powder by using a more functional (containing more chemically reactivegroups) resin. They can also increase the surface slip of the coating by incorporatinga polytetrafluoroethylene/wax blend into the formula.Both measures will help. However, I do not think that this will completelyeliminate scratching caused by an iron bar. You may not have the ability to influencethe manner in which the tires are installed, but using a more forgiving toolmay be the best answer. If the tire installer can use a bar that has a softer surface(nylon or PTFE rich) the scratching could be eliminated.
TROUBLESHOOTING PROBLEMS PERTAINING TO OUT-GASSING ON
ALUMINUM
Q:We have been painting these parts for another company. Lately we have had troublewith out-gassing. I think the quality of the aluminum has decreased. We are using Rohm &Haas Midnight Black Wrinkle. Our pretreatment chemicals are from DuBois. We have afive-stage wash. I really think it is the aluminum, since the steel parts painted at the sametime are perfect. What are your thoughts?
A:Indeed, it sounds like your aluminum may be declining in quality. Beforeyou conclude this, you should also take a close look at your own process. Hasthe pretreatment system changed? Is it in control (i.e., pH, solids, temperatures,etc.)? Are you running your production line at the same speed as before? Are yourunning the same amount of parts through the finishing system? Is your ovensteady and in control?Out-gassing is most common with cast alloys (aluminum and magnesium)and galvanized substrates. A high level of porosity can spell trouble. If you suspectthe aluminum is getting worse, I suggest you take a few parts and preheat them,allow them to cool to just above ambient temperature, then powder coat them.The preheating should expel any entrained volatiles, and the finished part shouldnot exhibit any blisters from out-gassing. If this is the case, then you should get intouch with your part supplier to investigate a change in the quality of their parts.Alternately, you can also investigate the use of an “out-gassing-forgiving”powder coating, which many powder suppliers offer. These minimize the effect of inconsistent porosity in substrates.
