Military and Aerospace Applications for Powder Coating.pdf

8/12/2019 Military and Aerospace Applications for Powder Coating.pdf

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MILITARY AND AEROSPACE APPLICATIONSFOR POWDER COATING T 5545

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Larry Brown Do n ald R Mart inHughesMissile Systems Company

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MILIT RY and EROSP CE PPLIC TIONS for POWDER CO TING

INTRODUCTIONRegulations restricting Volatile Organic Compound (VOC) emission along with hazardous wastegeneration and disposal are beginning to seriously impact the painting of aerospace hardware. Useof many of the traditional aerospace paint systems such as Mil-P-23377 epoxy primer and Mil-C-83286 urethane topcoat has been effectively prohibited in many areas of the country since theyexceed 420 g d i t e r VOC content allowed by regulation. Some users of the new compliant paintsystems have experienced significant paint-related cost increases due t o increased rework, morestringent record keeping, and additional hazardous waste disposal costs.

Historically, the use of powder coatings in military and other ae rospace high-performanceapplications has been limited. This is primarily the result of demanding specification requirements,test and developm ent expenses, reluctant customer acceptance of powders, and relatively lowproduction rates as compared to typical commercial uses. How ever, pow ders are rapidly gainingnotice as environmental regulations and cost constraints become tighter. At Hughes AircraftCompany, pow ders have been investigated for use, principally on defense systems, since 1983.Today, pow der coating is successhlly used on a number of company programs.

Hughes Missile Systems Company HM SC)Hughes Missile Systems Company is a subsidiary of Hughes Aircraft Company, a unit of GeneralMotors. Primary company facilities, including Headquarters, Research and Developm ent,Engineering, and Manufacturing are located in Tucson, Arizona. Products produced in Tucsoninclude missiles and other weapon systems as well as a wide range o f high-performance electronicequipment, subsystems, and devices.420 Powder Coating 94 Proceedings


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The standard liquid paint system used at the HMSC Tucson plant site is a Mil-P-23377 epoxyprimer followed by a Mil-C-83286 urethane topcoat. Mil-C-85 14 wash primer is also used on somealloys, primarily Inconel. Prior to 1986, all production hardw are was painted using these or similarliquid paints which all have a high solvent content. HMSC began evaluating pow der coating in1983 and implemented several epoxy pow der applications on he Phoenix and Maverick missilesystems between 1986 and January 1991.

A typical conventional wet paint process flow chart is presented in Figure 1with all VOC emissionpoints and hazardous material HAZMAT) generation points identified. n ubstrates requiring awash primer several additional opera tions would be included which fi rth er add VOC emission andhauirdou s w aste generation points to the flow chart. At present, HM SCs conventional paintingprocess consists of at least thirteen operations, twelve of which generally produce VOC emissionsand eight of which are likely to produce hazardous waste.

THIRTEEN STEPSTWELVE VOC EMISSION ACTIVITIESEIGHT HAZMAT PAINT, PAINT COMPO NENTS, AND SOLVENTS ) GENER ATION ACTIVITIES

Figure 1 Conventional Liquid Paint System Process Flow


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Figure 2shows the epoxy powder coating process which has replaced conventional wet painting onseveral applications at the HMSC Tucson facility. A high pressure aqueous cleaning system hasbeen implemented to replace solvent based cleaning so even cleaning related VOC and HAZMATgeneration poin ts have been eliminated. This finishing process has no VOC emissions orhazardous material generation points.

SIX STEPS

Figure 2 Epoxy Powder C oat Process

Finish Selection an d Co ntra ct Requ irements

Virtually all contracts for governm ent-purchased or military aerospace equipment imposespecifications which define the performance requirements of the particular system. Theserequirements include operational parameters which must be demonstrated under specifiedenvironments. Details on the conditions the system will be stored and deployed under are alsogiven. Warrantee information, deployed and operational life, and m aintenance and repairexpectations are provided as well.


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llof these factors have an impact on the selection of finish systems for the hardw are. Thecontract itself often imposes requirements in the form of upper-level design specifications whichcover the entire system. Tw o common examples seen at HM SC are MIL-STD-1587 MaterialsandProcesses For Air Force Weapons Systems), and MIL-STD-454 ElectronicEquipment,Srrmdard Genera l RequirementsFor). These specifications, and others similar to them, invoke amyriad of lower-level drawings and specifications which flow dow n and define the designconstraints of a particular program.

At the onset of design all finish requirement must be fully understood and evaluated by thedesigner. It i s often the lower-level requirements which are most obvious to the specifier of thefinish. They may be general in nature, such as those contained in MIL-F-18264 Finishes, Organic,W e p n s System, Application and ControlOf , r they may be specific to a particular material,such s MIL-C-83286 Co din g, Urethane,Aliphatic Isocyanate, For Aerospace Applica tions).The designer must also be aw are of any other specifications or documents invoked in the contractflowdown which define the performance of the finish system. Commonly encountered examplesinclude specifications covering corrosion prevention and dissimilar metals issues. Contrac ts alsomay impose program-specific design standards, and HM SC has its own complete set of internalspecifications, standards , and p ractices.

Clearly, the designer o r specifier of an aerospace finish is faced with num erous stringent limitswithin which to cho ose a material or a finishing process. At HM SC, a series of design standardshave been developed which considerably simplify the selection process. These standards arearranged such that a finish system can be chosen based on the customer (or branch of service),applicable specification requirements, and other unique aspects of the program. They identify

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powder coating s a preferred, environmentally-friendly process. In addition, a completeTechnical Package covering powder coating has been written under a Hughes esignfor theEnvironment (DFE) effort. This Technical Package offers the designer comprehensive technicaldata which can be used to evaluate the suitability of powder coatings in a particular application.

Historically, the standard HMSC finish system on most aerospace contrac ts has been MIL-C-83286polyurethane topcoat over MIt-P-23377 epoxy primer. These specifications tightly contro l thefinish produced by imposing tests and requirements on the material, the application process, and theresulting coatings. For example, MIL-C-83286 contains 2 separate requirements applicable to thecoating material proper, 9 requirements pertaining to the admixed coating and its application, and19requirements applicable to the applied and cured coating on the finished hardware.

It is these last requirements, which define the performance of the as-applied finish, which haveprecluded the use of powder on most HMSC military programs, even though powder has beenidentified a s a preferred alternate in internal HMSC standards for some time. The requirements arequite varied and range from such intrinsic film properties as color and gloss, through filmapplication evaluations including adhesion and flexibility testing, to in-service performancerequirements such as humidity and corrosion resistance, fluid resistance, and acceleratedweathering.

The problem is not that pow ders fail to meet the perform ance requirements of specifications suchas MIL-C-83286 (which, of course, is the primary concern of the customer), but that the propertype of certified performance data is typically not available. Since most powder manufacturers andpow der applicators provide powder primarily to commercial industry they only test for propertiesimportant in a commercial application. These might include items such as adhesion, color, and424 Powder Coating 94 Proceedings


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gloss. It would be unusual for a powder supplier to evaluate parameters of interest in theaerospace industry, su ch as extreme low temperature flexibility (-65F or lower), resistance tomilitary-type fluids (such as particular fuels and hydraulic fluids), and performance under extremecorrosion conditions (like acidified salt fog). Even when aerosp ace test results a re available it isusually found that t he tests w ere not performed in accordance with the required MIL-specs, orcertain test pro cedures and conditions were not followed exactly. In the aerospace world certifyingto the correct testpro ced ure s is as important as certifying compliance to manufacturing andperformance standards.

HMSCApplication Examples

A contractor desiring to use pow ders on an aerospace program, then, is faced with evaluating theperformance of pow ders under the applicable contract and specification requirements. At H MSC ,the approach has been to subject pow der finishes to exactly the same perform ance requirementsimposed by specifications govern ing solvent-based systems. or example, on the M averick missileprogram the standard finish for the airframe was one coat o f MIL-P-23377 primer followed by tw ocoats of flat-gray or olive drab MIL-C-83286 topcoat. Because of environmental constraints onthe finishing process an alternate was desirable, and a powder was recognized as a preferredcandidate. The powde r selected, an epoxy with a &sing temperature of about 325 F, wassubjected to all the performance requirements of the MIL-P-23377 and MIL-C-83286specifications. These requirements included adhesion, flexibility, corrosion resistance, and o thers.Other requiremen ts such as viscosity and pot life are obviously geared towards the application of atwo-com ponen t reactive liquid systems These do not apply to powders, and were not considered inthe M averick evaluation.

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Proving powders as a replacement for traditional liquid systems at HM SC may also involve testingbeyond material specificationrequirements. On Maverick, a FinishSpecification existed as a separatecontractual docum ent. This finishspecification defined color andmarking of Maverick variants,cleaning and pretreatmentprocedures, and standard finishes(both organic and inorganic) for allprogram substrates. Likewise, aunique Maverick CorrosionPrevention and Control Plan wasimposed at the contractual levelwhich defined othe r performancerequirements and the exact approachto corrosion prevention. Fig ur e3 Powder Paint ing a Maverick Airframe

Complying with requ irements imposed by these additional documents mandated a number of extratests to prove the use of powder on Maverick at HM SC. Testing not only confirmed conformanceto the salt spray and humidity criteria contained in MIL-C-83286, but also demonstratedcompliance to additional Maverick Finish Specification and the Corrosion Prevention and ControlPlan corrosion requirements. Since the new powder system did not include a sacrificial primer(such as M IL-P-23377 used in the previous design) the Maverick customer was particularly


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concerned about under-film corrosion, especially filiform corrosion. In this case a filiform testseries was included in the Maverick powder qualification, even though such a requirement hadnever been applied to the system before.

During he evaluation of contract requirements and the performance o f tests, issues may arise whichare unique to the hardware and finish at hand. These may require a fresh look and an open mind onthe part of both contractor and customer. On Maverick this situation was encountered with respectto the powder cure temperature. Since the primary Maverick substrate to be coated w as aluminum,the relatively high cure temperature of the selected powder (340' F) was a concern due t o potentialmechanical degradation of the hardw are. This was an issue which had not been encoun tered onMaverick before. Tests showed that there w s a slight metallurgical change in the aluminum duringcure. After careful review both HMSC and the Maverick customer agreed that the change did notprod uce significant mechanical property degradation and was negligible.

Another out of the ordinary concern, at least for HMSC, was weathering o f the finish. Early inthe evaluation program it became clear an epoxy powder was the leading contender. Epoxies,howw er, tend to chalk when exposed to ultraviolet light, especially in sunlit, outdoorenvironments. This chalking often leads to the formation of a light-colored, powdery film on thefinish surface. Whether this phenomena is detrimental, aside from appearance, is a source of somedisagreement within the finishing industry.

HMSC aced a dilemma with respect to this weathering and chalking. On the one hand, thephysical properties of the leading contender (epoxy) w ere very good, generally exceeding those o fthe current solvent-based finish. On the o ther hand, it was clear the epoxy would chalk if exposedto long-term weathering. Fortunately , this type of exposure is not very likely for a tactical missilePowder Coating '94 Proceedings 427


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system such as Maverick. Mavericks are normally transported and stored in closed containerswhich offer protection from the external environment. The only exposure to weathering is at thepoint o f use, as the missile is loaded onto an aircraft, flown to the target, and fired. This exposureis much too short to produce chalking of the finish. In this light HMSC felt the chalking problemwas inconsequential, and the customer ultimately agreed.

With any new and developing technology a sound global knowledge of the intricacies of theproposed application is required. Although for tactical Mavericks weathering was not a concern,this has not been the case on all HMSC hardw are. For example, many variants of a missile aredesigned and manufactured, including training missiles which are used to simulate everything fromloading and maintenance to actual flight and combat conditions. These training missiles have long,multi-mission lives and are subject to normal flightline environments for periods approachingcontinuous outdoor exposure. In this case chalking might be a grea ter issue, even when it not aconcern for the tactical versions (which are in protected storage). At HMSC the p ractice has beento remain with conventional solvent-borne coatings for these long-exposure products, althoughevaluations of improved-weathering powders are on-going.

An underlying aspect of aerospace work illustrates other problems which are encountered withpowder coatings. Compared to most commercial applications aerospace programs, especiallyweapons systems, involve relatively low production rates and low total production. Thedevelopment of a new missile system, for example, may span 5 to 1 years from the time of initialconcept until i l l production is reached. During this time research, development, testing andproofing, and various stages of low-rate production may yield no more than a few hundred missiles.

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s n example, HM SC is developing the seeker (the eyes and the brain ) of the ASRAAM(Advanced Short-Range Air-to-Air Missile). The first deliveries, for evaluation hardware include arelatively small number of missiles. Even at full-rate production the total number of systemsdelivered over the life of the program does not even approach that of a typical commercial product.

From the beginning HM SC considered powder the preferred finish on ASRAAM. With someforesight, HMSC asked t o change, or tailor, the contract to allow the use o f powders in lieu ofthe originally-required wet finishes. Even so, a number of problems were encountered prior toeventual use of a powder finish on the program. The two most important related to the quantity ofpowder required and the payback of development and test costs to powder suppliers.

On many aerospace programs the total amount of powder used throughout full-rate productionmight not even equal that used by a large commercial furniture manufacturer in a m6nth.Quantities early in n aerospace program are even lower. On ASRAAM few powdermanufacturers were interested in providing the small quantities of powder required for the firststages of the program. Complicating this was the requirement for a color that met an imposedBritish standard which was not available off he shelf' from any powder manufacturer. This madeany powder a custom formulation, with unique compounding and dedicated production runs.

Eventual pay-back of research and development costs (both for the contractor and the powdermanufacturer) is also often a stumbling block for powders in aerospace applications. With typicalcommercial end-items the powder maker can expect sales to increase to relatively high rates veryquickly, generating profit in a fairly short order. Under these circumstances, the manufacturer canafford to be somewhat flexible in pricing, often carrying forward development costs into a time



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period in which delivery rates are high. At this point, the percen tage of the total system cost whichrepresents the finish is relatively low, even if R&D costs are included. An aerospace program, onthe other hand, presents less opportunity to be flexible in powder pricing as full-rate productionmay be several years away, and the quantity of powder sold in any case might be relatively low.

Additionally, the market fo r ae rospace products is currently rather uncertain. There are no hardguarantees a program will ever go into high-rate production, or even be produced at all. Normally,the colors required are not commercial standard, and there is little or no demand for them outsideof aerospace work, so the m anufacturer cannot justify developing the powder as an addition totheir com mercial product line. In this light most powder manufacturers understandably try torecover their up-front c osts during the first few powder deliveries. In essence, the research anddevelopm ent and dedicated manuhcturing costs which might have been substantially borne by themanufacturer on a commercial or product line powder is forced on the defense contractor at theworst possible time early in the produc t life cycle.

This early cost leads t o a significant problem for the contractor who is struggling to sell the conceptof powders to its customer. It is difficult to dem onstrate the overall cost advantage of powderswhen raw material costs, in low volumes, are high. Program viability depends more on cost todaythan ever before due t o intense competition and cost pressures related to aerospace work. It mustbe recognized, by both the contractor and customer, that due to their environmental benefits, lowlabor and energy costs, and other advantages powder finishes are cost-effective in the long run

HMSC has been fortunate to team with powder manufacturers who have been willing to custom-formulate powders in the correc t colors and who understand the realities of aerospace contractingrelated to co st. In many casesHMSC as found it advantageous to work with smaller suppliers430 Powder Coating '94 Proceedings


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who are trying to gain a foothold in the expanding niche market of aerospace powder coatings.Even so, the initial price per pound o f production ASRAAh4 powder was approximately an orderof magnitude grea ter than a commercial, offhe shelf powder.

Recumng Savings

Overall, HMSC as found pow der coating offers several potential economic advantages whencompared to conventional liquid finishing technologies . Savings can be realized in the areas oflabor, energy, material, hazardous waste disposal, and safety. Although the savings in certain areasare sometimes difficult to quantify, the powder system usually offers a significant cost advantageover so lvent-borne coating systems.

Labor savings associated with powder coating result from several factors. First, powder coatingsare delivered ready-to-use and require no mixing with solvents or catalysts. This eliminates anenti re ope ration usually associated with liquid coatings. Additionally, monitoring and maintainingmany process parameters associated with liquid painting (viscosity, pH, solvent content, percentsolids, etc.) is unnecessary. Powder coating is usually a one coat application, and significant laborsavings result from eliminating multiple applications of primer and topcoat. Since the powderparticles can be removed from the part with compressed air prior to curing it is very easy to reworkparts. After the parts are cured, any required touch-up can be performed using a compatible liquidpaint. Clean-up of the spray gun and booth is much easier with powder coatings. It requires only abroom, compressed air, and a vacuum cleaner instead of the solvents and wipe cloths required toclean-up d e r solvent-based liquid painting. In certain applications powder coating can eliminateup to 75 of the labor required for liquid painting.

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Significant energy savings can be realized from the implementation of powder coating. Since thequantity of volatiles in powder coating is minimal and no low-flashpoint solvents are present,makeup air requirements can be dramatically reduced. Also, no makeup air is required for thepowder coating booth and only a small amount is required to vent the ovens used for curing. Thistransla tes into significant savings in air conditioning andlor heating. While the oven temperaturesrequired for powder coating are normally higher than those used to cure liquid coatings, the curetime is usually shor ter. This results in a energy savings on a per-part basis which favors powderover traditional solvent-borne finishes.

The nature of powder coating also leads to material cost advantages due to nearly 100 percentmaterial consumption. An electrostatically applied powder coating will achieve approximately 65percent first-pass material utilization. The over-sprayed powder can be collected, screened andthen mixed with virgin powder for reuse. Since less than one percent of the as-delivered powder isvolatile, powders easily achieve material utilization rates of 95 percent or better. Liquid systemsusually achieve overall material utilization rates of between 20 to 60 percent, since the sprayedmaterial contains large quantities of volatile solvents which are lost up the stack and since liquidcoating overspray cannot be reclaimed.

Since drips and runs a re nonexistent with powder coating, the reject rate is low, resulting in higheryields. The cured powder paint film also usually has better abrasion characteristics than liquidpaints. This enables powder coated parts to withstand handling and assembly with less paintdamage, again resulting in reduced rework. Additionally, cleanup of spray guns and paint boothsused for powder coating requires no solvents, firt he r reducing costs.

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Significant quantities of flammable hazardous materials, including cleaning and reducing solvents,mixed paint, and catalys ts are associated with liquid painting systems. These require specialhandling, storage, and disposal. The need for solven ts and other hazardo us materials is eliminatedwhen using powder coatings. The quantity and type of hazardous waste generated from a powdercoating operation is dependent on the type of powder, resin formulation and pigmentation(especially metallic) constituents. Pow der coating formulation sources indicate that m odempow der coating are expected to pass Toxicity Characteristic Leaching Procedure (TCLP) testing.In this case alternatives to disposing of excess or used powder as hazardous waste may beavailable, depending on local regulations. To ensure proper disposal the w aste classification orlisting of spent or unused pow ders must be determined on a case by case basis.

Pow ders have significant environmental advantages when compared to solvent-based liquid coatingsystems. These advantages, including grea tly reduced solvent use, low er fire hazard, reducedhandling and disposal costs, and increased operator safety, provide a competitive advantage whencomparing pow der coating to other finishing systems. The cost and liability associated with wastegenerated from a powder coating process may be considerably less than a solvent-based paintingsystem.

In many areas, the use of certain liquid coatings is either prohibited or requires the installation andoperation of expensive adsorption systems to remove VOC's from paint booth and oven exhausts.Th e lack of volatiles in powder coating eliminates this problem and may significantly reduce the firerisk. Pow der coatings also result in a much cleaner environment for paint shop employees, and maylower labor costs by reducing the overhead associated with safety equipment. In addition, in somecases a less skilled ope rator is required to apply powder, resulting in further labor cost savings.Powder Coating '94 Proceedings 433


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Powder Coating Specifications

T o d ate the only MIL-Spec covering powder coating is MIL-C-24712 Cmtings, PowderedEpoxy) issued by the Naval Sea Systems Command in February 1989. This specification isintended for both interior applications (such as steel and aluminum equipm ent, furniture, andelectrical boxes), as well as exter ior steel and aluminum surfaces exposed to marine environm ents.The first revision of this specification is scheduled to be issued in the near future and will addpolyester powder coatings for improved exterior UV resistance. Paints meeting E - C - 2 4 7 1 2 areavailable from several sources. Othe r government specifications such as WS 2235 1 for the M ark48 torpedo cover powder coating for specific weapon systems. This specification was issued by theNaval Underwater Systems Center and cove rs the powder materials, application processes and testrequirements for the M ark 48 Advanced Capability ADCAP ) torpedo.

In most cases the aerospace contractor is faced with writing a unique specification which cove rs apowder coating application, since no MIL-spec applies. At HMSC number of specification andsource control drawings have been genera ted covering a variety of powders. One significantdrawback to writing in-house specifications is that they are normally limited to use on a singleprogram. This requires duplicate specifications be w ritten for each program , at considerableengineering expense.

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CONCLUSIONS

Increasingly stringent environmental regulations tend to favor the long term development andimplementation of powder coating. The elimination of VOC emissions and the reduction ofhazardous wastes gene rated are significant advantages. Although there are problems working withpowder in the aerospace environment, especially with test and documentation issues, experience atthe HMSC Tucson plant site has demonstrated powder coating consistently reduces the cost offinishing a product. The cost savings and quality improvements associated with pow der coatinghave enabled HMSC to be proactive in developing and implementing this environmentally-friendlyprocess in today's competitive environment.



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BIOGRAPHY

Lany Brown is n Engineering Specialist with the Advanced Manufacturing TechnologyDepartmen t at Hughes Missile Systems Company.Engineering from Texas A M University. During the last 15 years at HM SC he has specialized inthe areas o f material handling and manufacturing engineering. Since 1988 he has been the projectmanager or principal inves tigator for several powder paint projec ts concentrating on developingand implementing powder paint technology in aerospace applications.

He received his BS degree in Industrial

Don M artin is n Engineering Specialist with the Components and Materials EngineeringDepartmen t at HMSC, and leads the Materials and Processes Applications Engineering Group. Hespecializes in environmental aspects of engineering design, and represents HMSC on the HughesCorporate Design for the Environment Working Group. Don has been involved in the selectionand testing of powder coatings in aerospace applications since the mid-1980's.

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Military and Aerospace Applications for Powder Coating.pdf