AMPTIAC is a DOD Information Analysis Center Adminis tered by the Defense Information Sys tems Agency, Defense Technical Information Center
Special Issue:
Ships Navy Experts Explainthe Newest Material &Structural Technologies
The issue you hold in your hands has been 14 months in themaking. It began with a simple idea: turn the spotlight on theage-old art of building ships. We wanted to show the excitingnew technologies that are offering novel materials for ship con-struction, changing the way ships are built, and indeed creatingone of the most fundamental shifts in Navy combatants sincesteel replaced wood.
This simple mission turnedout to be much more complex.The project underwent a num-ber of different iterations, butfinally settled in and cametogether. It has been a labor oflove for yours truly, for I reallydo believe that even though air-planes and tanks often grab thespotlight, Navy ships are still the most challenging structuraland materials engineering systems fielded in today’s military.Nothing has the complexity, impact, size, and sheer force of afighting vessel, nor can many things capture the imagination inquite the same way.
So here it is, finally, and I am thankful that it is done. Notjust because it is off my desk and I can get on to the next proj-ect, but we are proud because AMPTIAC has compiled some-thing that probably has not existed before: an overview of thenewest technologies being incorporated into structures andmaterials for use aboard Navy combatants. And the people pro-viding the perspective are the experts at the Office of NavalResearch, NSWC-Carderock, and the Naval Research Lab. Youwon’t find this level of detail, variety, and expert contentfocused on this subject anywhere else.
That all being said, there is one critical feature of this publi-cation that needs some attention: the DOD center behind it.Some of you out there have been reading this publication forseven years now. You undoubtedly remember about two yearsago when we shifted over to our current layout format and full
color reproduction. You also have probably noticed that we arepublishing these large special issues fairly often. It is all a partof our mission to bring you the most in-depth, focused, andtechnologically exciting coverage of Defense materials and pro-cessing advances available anywhere.
But the side effect of the more noticeable and attention-grabbing Quarterly, is thatAMPTIAC itself has lost someattention. The reality is that thecenter has grown with numer-ous projects, focused reports,and database efforts over thepast few years, but there aremany out there that may readthis publication and not evenknow that the center exists.
We want to put more emphasis on the other efforts AMPTIAC is involved in, and let our customers and potentialcustomers know that we are here for you. We help with ques-tions, assist in materials selection, and provide consultation ona variety of materials and processing-related issues. We havemore than 210,000 DOD technical reports in our library anddirect access to hundreds of thousands more throughout DOD,DOE, NASA, and other US Government agencies. We havedozens of focused reports tailored to specific technology areasand many more compiling vast amounts of data into hand-book-style resources.
The basic message here is to take note of this magazine, readit, and enjoy. But if you think AMPTIAC is just the Quarterly,Think Again.
Wade BabcockEditor-in-Chief
Editorial: There’s More to AMPTIAC
than the Quarterly
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The AMPTIAC Quarterly, Volume 7, Number 3 27
INTRODUCTIONIn 2001, the US Navy embarked on an ambitious plan to sig-nificantly reduce the costs associated with shipboard mainte-nance and corrosion control. Sponsored by the Office of NavalResearch (ONR) and through the technology transition leader-ship of the Naval Sea Systems Command (NAVSEA), theNaval Research Laboratory (NRL) is developing and institu-tionalizing rapid cure coatings to achieve these goals. The keyto this program’s success is the identification of new rapid curecoating technologies, certifying them, and transitioning themto use in the fleet.
In this program, the identification of new technology andmaterial developments are managed through a cooperativearrangement between ONR and NAVSEA. Under the direc-tion of ONR, NRL is responsible for naming candidate mate-rials and for the development of new technology products.Under the role of technology transition, NAVSEA performs areview of candidate materials and newly developedproducts, then issues the directives for proceedingwith laboratory qualification testing.
The technology of rapid cure coatings has been inexistence for many years, but the materials and appli-cation technology were not fully exploited until veryrecently. For many years rapid cure coatings were lab-oratory curiosities and remained in the realm of futuremarketing concepts. The research for these coatingtechnologies has typically focused on two main areas:application technology and component materials.Both of these issues will be addressed in this article.
THE NEED FOR RAPID CURE COATINGSInternal areas of ships are under constant attack fromcorrosive elements. Internal tankage areas must betreated to protect against severe and possibly criticaldamage. This is commonly done with any number ofspecialized coatings. Figure 1 shows the typical corrosion pres-ent inside a seawater ballast tank.
Fully one-half of the time for a ship’s overhaul is consumedwaiting for coatings to cure before work can resume either inthe treated area or in adjacent areas. For example, in any oneday of a ship’s availability, there can be more than 10 to 15 large
scale (>3000 ft2) surface preparation and painting activities inprogress. The scheduling of these operations must take intoaccount the need to retain access to surrounding areas, as wellas allowing concurrent work to progress in order to minimizescheduling impacts. Fully three-fourths of these painting andpreservation activities directly affect work in adjacent areas, andthus careful scheduling is required to minimize conflicts.
Balancing work efforts throughout the ship is extremely timeconsuming and constantly changes to reflect items such asstructural repairs, material availability, and personnel schedul-ing. Perhaps the greatest factor affecting the time required formaintenance processes like painting is weather. Althoughequipment and procedures are available to minimize the impactof the surrounding environment, painting (especially cure anddrying times) is still clearly affected to a significant degree byweather conditions. Delays due to unfavorable weather createsubsequent delays in additional tasks, whether they are addi-
tional painting, structural repairs, or equipment installation.One of the most problematic issues of painting in a shipyard
is premature damage to a freshly applied coating. Some exam-ples of these scenarios are included in Table 1. Most of thesepremature damage issues stem from the fact that the majorityof coatings require lengthy cure times.
Arthur Webb, Marine Coatings Section HeadCenter for Corrosion Science and Engineering
Naval Research Laboratory Washington, DC
Figure 1. Corrosion in a Seawater Ballast Tank on the USS Whidbey Island (LSD 41).
The AMPTIAC Quarterly, Volume 7, Number 328
Typical solvent-free, high solids-content coatings employed bythe Navy have cure times ranging from 8-12 hours at ambienttemperature (~25 ºC). This means that for approximately 8-12hours the coating system is at the mercy of the environment andmust be protected from every conceivable source of contamina-tion. Many applicators stage quality assurance personnel toguard the application, however, they must be trained, licensed,and paid. Having to hire and pay benefits to someone who sim-ply stands and watches a coating dry is not popular within theindustry.
RAPID CURE TECHNOLOGYA simple solution would be to employ a coating system thatcures much more quickly, and thus not prone to the onslaughtof time-controlled risks associated with slower curing coatings.At present there is no formalized definition of a rapid cure coat-ing, although the concept can generally be applied to a coatingsystem that cures either instantly or within 20 minutes of appli-cation. Fast cure is a broader term that has been in use for sever-al years; however it usually refers to systems that cure in 3-4hours. Rapid cure coatings in this program are based on twotypes of materials: polyurethanes and polyureas. All rapid curecoatings still require substrates to be prepared in the same fash-ion that they are for traditional coatings.
PolyurethanesPolyurethanes are versatile materials and are used as coatings, lin-ings, structural resins, sealants, and foams. Since their discovery,the materials employed to make polyurethane products havechanged significantly. In general, a polyurethane coating containsa polyol and one or more of the following: a monomeric diiso-cyanate, a polymeric isocyanate, or an isocyanate prepolymer.Isocyanates also fall into two types: aromatic and aliphatic. In therealm of polyurethane coatings, the actual formulation and com-ponents used depend on the target end use. Aromatic isocyanatesare generally limited to structural applications, chemical resistantliners and coatings, composites and foams. They are fast reactingbut exhibit poor exterior weathering performance. Aliphatic iso-cyanates, on the other hand, are used in exterior applicationswhere UV resistance is required but they are less chemically resist-ant and slower reacting.
Traditionally, polyurethane coatings have been employed inarchitectural applications where aesthetic properties were thedriving factor. For this reason, their market was limited to niche
applications that focused not on coatings but on structuralapplications, namely insulation, foams and composites. Thedevelopment of plural component application equipment (tohandle the quick setting foams and sealants) has recently sparkedthe creation of rapid cure polyurethane coatings. (In general,plural component equipment is any style of machine that iscapable of metering and mixing multiple components, andapplying them.) The advances in chemically resistantpolyurethanes for foams and sealants have also led to the slowbut steady development of chemically-resistant polyurethanecoatings
In the case of tank coatings,systems employing aromaticpolyisocyanates are used forchemical resistance. Figure 2shows the seawater ballast tankfrom Figure 1 after applicationof a polyurethane rapid curecoating.
Rapid Cure Polyurethanes forShipboard Corrosion ControlThe focus of this program hasbeen on the identification,development and use of rapid cure polyurethane coatings basedon aromatic isocyanates for tank linings. The chemical resistanceand inherent high degree of chemical crosslinking leads to a hard,resilient and chemically resistant coating ideally suited for ship-board tanks and voids. Thus far, all of the coatings employedmake use of high molecular weight (polymeric) aromatic iso-cyanates which have a very low vapor pressure. Figure 3 shows abefore and after comparison illustrating the application of a rapidcure polyurethane coating system.
PolyureasEarly work with polyurethanes resulted in the concurrent dis-covery of polyureas. Polyurea is the product of the very rapidreaction between isocyanate and an amine. The reaction is sorapid that mixing a pure low molecular weight diisocyanate witha low molecular weight amine is nearly explosive. Their discov-ery was overshadowed by the successes of polyurethanes, andtherefore polyureas remained a little known corner of the largerworld of isocyanate technology.
In general, the lack of interest in polyureas remained as aprobable consequence of their inherently rapid cure times, poorfilm forming properties, and lack of adequate application tech-nology. Polyurethanes had developed into a relatively maturetechnology area with a proven history of success and perform-ance, but polyureas did not enjoy the same exposure. However,interest in polyureas began to surface when plural componentequipment was introduced.
Rapid Cure Polyureas for Shipboard Corrosion Control Thefocus of this program has been on the identification, develop-ment and use of rapid cure polyurea coatings based on aromaticisocyanates and non-polyether polyamines. The chemical resist-ance and inherent high degree of chemical crosslinking leads to ahard, resilient and chemically resistant coating ideally suited for
Figure 2. Seawater Ballast Tankon USS Whidbey Island (LSD 41)after a Polyurethane Rapid CureCoating is Applied.
Table 1. Examples of Tank Coating Contamination.• Equipment is reinstalled in a freshly coated tank, only to
have that equipment fail soon thereafter.• A tank is coated and during a subsequent shift another
repair task creates dust, shavings, or metal chips which areallowed to enter the tank, embedding themselves in the still-curing coating.
• Water from washing of interior decks and passage-ways isinadvertently allowed to run into a freshly coated tank.
• Sea valves where lines have been removed are accidentallytest cycled resulting in hydraulic fluid being sprayedthroughout the tank.
The AMPTIAC Quarterly, Volume 7, Number 3 29
shipboard tanks and voids. Thus far, all of the coatings employedmake use of high molecular weight (polymeric) aromatic isocyanates which have a very low vapor pressure to minimizehazardous exposure to workers.
MATERIAL PERFORMANCE EVALUATIONThe key to success in this program has been the rapid progres-sion from laboratory evaluation of candidate products to ship-board demonstration. At the present time two rapid curepolyurethane coatings have been fully evaluated at NRL’s KeyWest Marine Corrosion Facility. (This consists of performanceevaluation in accordance with MIL-PRF-23236.) Some of therequired performance tests include but are not limited to:• Cyclical seawater/fuel/air exposure in simulated ballast
tank conditions,• 1000 hours condensing humidity,• Cathodic disbondment, and• Jet fuel immersion
In addition, a unique test protocol is employed for evaluatingthe performance of candidate coatings for collection, holding andtransfer (CHT) tank service. To date, two candidate rapid curepolyurethane products have passed the rigorous CHT protocol.
In general, the performance evaluation program is designed toinitiate accelerated testing (based on the tests just listed) imme-diately upon or shortly after receipt and application of the can-didate coating(s). These tests are initiated while further applica-tion is performed for the longer-term simulated ballast tanktests. The scheduling is such that all of the accelerated testing iscompleted by the time the simulated tank tests have reached thesix-month mark. NAVSEA is then notified of the results andallows for satisfactory coatings to be scheduled for sea trials.
The installation of a new coating system aboard an active plat-form can require a significant lead time (six to eight months),thus, the candidate coating in many cases has completed the fullMIL-PRF-23236 evaluation in the laboratory before it is installedaboard ship. In other instances, a rapid insertion may be per-formed, and thus a candidate coating can actually be appliedbefore the full scale performance testing has been completed. Ineither case however, no coating can be applied until the six-monthperformance mark has been achieved.
If a candidate coating has met the six month qualification, hasbeen successfully installed on an active platform, and has demon-strated a passing grade in the one year test sequence and/or has
demonstrated one year of satisfactory in-service per-formance aboard ship, the coating then receivesapproval for incorporation into the QualifiedProducts List (QPL). This is provided that all otherperformance, manufacturing, environmental andhealth and safety requirements have been met.
TECHNOLOGY AND PROCESSDEMONSTRATIONEquipment RequirementsFor rapid cure coatings, plural component applicationequipment is required which can deliver multiplecomponents to create a specific formula. The basic elements of these systems are a metering or propor-tioning pump, mixing chamber, and the application
gun. There are other features such as heating, material feed or sup-ply, and material process recordation, but these features may ormay not be necessary for a particular application.
Proportioning Pumps Plural component equipment falls intotwo general categories, fixed ratio and variable ratio. As theterminology implies, a fixed ratio machine requires a manualchangeover of the metering portion of the unit when a propor-tion ratio change is necessary. In general, the proportioning ratiofor fixed ratio machines is very accurate and is set at the factoryusing standards. For variable ratio machines, the proportioningratio can be adjusted rather rapidly, but is typically less accurate.Consequently, this may require the user to perform multipleratio checks and make small or fine adjustments to establish andverify a proper proportioning ratio.
Plural component pumps are typically powered by air,hydraulics or electricity. Air-operated plural component pumpsdominate the heavy industrial market where solvent-free epoxiesare applied, whereas hydraulic-operated pumps are generallyemployed in lower volume markets such as in-situ foam or smallscale, specialized coating applications. The choice of an air- orhydraulic-operated machine is more of a preference than anindustry norm.
Mixing Chambers or Stations Mixing chambers or stations areavailable in two basic types; pump-mounted (also called remotemixing) and impingement mixing. For pump or remote mixing,the A and B components of the coating are metered at the cor-rect ratio by pump-mounted proportioner and delivered to thegun as a mixed material through a single feed line. This type ofconfiguration is ideal for solvent-free coatings and where coatingworking time is greater than 20 minutes. It also works well forcoatings which require a brief dwell time before application dueto slight component incompatibilities in the first few momentsof mixing.
Impingement mixing is different from pump-mounted or re-mote mixing stations in that the coating components are supplied in individual feed lines all the way to the applicationgun. When the gun is triggered, the individual components aremixed in a chamber immediately behind its tip or nozzle. In thismanner, the coating experiences a brief or instantaneous periodof mixing before exiting the gun and making its way to the substrate. For impingement mixing, it is imperative that the two
Figure 3. Before and After Pictures of Rapid Cure Coating Application.
The AMPTIAC Quarterly, Volume 7, Number 330
components of the coating are as mutually compatible as possi-ble. For fast or instant reacting coating systems, impingementmixing is the only available mode of application.
Shipboard DemonstrationDuring FY02 and 03 there have been four successful applicationdemonstrations of rapid cure, chemically resistant polyurethanecoatings on active ships.Three of the four demonstra-tions occurred on 1) USS Gunston Hall (LSD 44) (one seawater ballast tank)2) Whidbey Island (two seawater ballast tanks)3) USS George Washington(CVN 73) (one damage control void)
A fourth ship, USS Tortuga(LSD 46) was being prepared for application in June/July ’03and will host application of two rapid cure, solvent-freepolyurethanes in four seawater ballast and two potable watertanks. Figure 4 shows pictures of the polyurethane rapid curecoating technology as applied to seawater ballast tanks onWhidbey Island.
For the first four demonstrations, the application was performed using an air-operated, plural component propor-tioning pump that was modified by the manufacturer to handle rapid cure polyurethane coatings. The pump was fittedwith an impingement mixing gun, having a material deliveryrate of approximately 3 gallons per minute. The configurationperformed satisfactorily, and allowed the applicator to achieveproper film thickness control. The applicator was able toprogress at a fairly rapid rate yielding a coverage area of approx-imately 15-20 ft2 per minute.
Although mobility of the gun was limited by physical size(large compared to a single feed gun), its delivery lines weremore flexible due to lower material pressures. This essentiallygranted the same coverage of tight spaces and complex geome-tries within the demonstration tanks, as would be experiencedusing a single feed gun (which requires higher operating pres-sures resulting in less gun-to-supply-line mobility.) The gunmanufacturer has introduced a smaller gun with a delivery rateof about 1 to 1.5 gallons per minute. It will be employed onthe next demonstration using a newly approved coating systemfor demonstration in combination with a hydraulically-operat-ed, proportioning pump.
Cost Savings At present, a cost savings based on time required to complete thecoating application has been demonstrated. However, as with anynew material and process scenario, there is a significant learningcurve. For example, much of the savings from reduced time-to-curewere quickly consumed with process learning and material control.However, when compared to the initial costs associated with the
Navy’s successful demonstra-tion of solvent-free epoxy coat-ings in the early 1990s, the rel-ative increases in preservationcosts or those associated with alearning curve are similar.
For the first two demon-strations, the manpower andmaintenance costs were high-er than would be expected fora “traditional” job. However,much of this was based on
increased quality assurance and control, equipment, material,surface preparation, and environmental control. The preserva-tion costs were also higher in that there was only one certifiedapplicator. (This was also the case when solvent-free coatingswere introduced.) Efforts are currently underway to bring inmore certified applicators with materials and process experience;this will reduce the ultimate job cost significantly. Although theactual application, which would include equipment preparationand painting, will be higher for single coat application versus asingle feed or conventional solvent based coating application,such factors as surface preparation remain the same. The signif-icant cost reductions, however, are in the time critical operationssurrounding coating cure time. These include work stoppage inadjacent areas, maintaining dehumidification and heating, andconstant rework due to premature damage incurred to a wet orinsufficiently cured coating.
SUMMARYThe US Navy has recently been identifying, developing andevaluating rapid cure coatings in order to reduce the costs ofshipboard corrosion control painting. The coatings themselveswill cure in significantly less time than current, traditional coat-ings; about 20 minutes or less compared to 8-12 hours. Whilecost of application and preparation are similar, subsequent sav-ings created from shipboard repair schedulers who no longerhave to “protect” curing coatings, or rework damaged coatings,will be significant. Additionally, the Navy is streamlining theprocesses to get new coatings certified for use in the fleet, thusputting these materials in ships much sooner.
Figure 4. Seawater Ballast Tanks after Application of Rapid CureCoatings.
Arthur Webb has more than ten years of experience in the coatings industry and holds BS and MS degrees inChemistry from the California Polytechnic State University, San Luis Obispo. His focus areas were the investigation ofrapid reaction kinetics and polymer chemistry for coatings and composites. Mr. Webb is the head of the MarineCoatings Section at the Center for Corrosion Science and Engineering, Naval Research Laboratory where he isresponsible for the testing and evaluation of corrosion control and specialty coatings for the US Navy. Mr. Webb isalso actively involved in polymer resin research and development and his team is currently developing rapid cure, sol-vent free resins for corrosion control and antifouling coatings.
