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urrent and anticipatedcoating regulations toreduce atmosphericemissions to protectdrinking water sys-tems are pushing tankowners and coating
specifiers towards the use of 100% solids-by-volume (SBV) coatings.1 The majority ofthe high-solids coatings, whether epoxy,polyurethane, or polyurea, can and insome instances must be applied usingplural-component spray equipment.Specialized and complex, plural-component spray units meter andthen mix the components of thecoating within the equipmentor at the spray gun. In addi-tion to being a large invest-ment for a contractor, plur-al-component equipmentrequires the contractorto be knowledgeable inits set-up, operation,
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troubleshooting, cleaning, and maintenance. Theequipment should be configured to meet the coatingmanufacturer’s recommendations to help ensure aproblem-free application and afford a lining sys-tem that will provide long-term performance.This article will address basic informationabout plural-component spray equipment,including the types of coatings used, the envi-ronmental conditions necessary for success-ful application and cure, considerations forsurface preparation, equipment types andcomponents, and factors affecting appli-cation.
The Basics ofPlural-Component Spray
The Basics ofPlural-Component Spray
by Robin Hasak, Tnemec Company Inc.
C
Photo courtesy of JEO and the City of Wahoo, Nebraska
Proper equipment useis key to lining water tanks
with 100% solids.
03732) is typically recommended whenutilizing 100% SBV coatings for single-coat applications.4
Surface contamination, such as spentabrasive particulate matter or “backsidecontamination” (generated from contami-nants in the metallic abrasive), howeverslight, will affect the adhesion of 100%SBV coatings.5 Airborne particulatematter or statically charged ultra-fineparticles may impede the process ofeffectively blowing down the substratewith clean and dry compressed airbefore coating application. Therefore,vacuuming the substrate may becomethe only option to achieve a clean andcontaminant-free surface.One possible alternative to the daily
cycle of blasting and painting is the useof dehumidification equipment duringinterior work. Ambient temperaturescan be raised or lowered, depending onthe particular need at the time, and thehumidity can be lowered. The applicatorcan then perform surface preparation fora few days longer, spend adequate timeto clean the substrate, and let airbornecontaminants settle out, thus reducingthe risk of poor coating adhesion.
Equipment Types and RequirementsTwo basic types of plural-componentequipment are available today: mechani-cal proportioners, or fixed ratio (Fig. 1);6
and electronic proportioners, or dosingunits (Fig. 2).7 The two equipment sys-tems function basically the same. Thecoating components are fed to the pro-portioner (the pump), circulated andheated, proportioned at the proper mixratio, mixed, and then spray applied.Coating manufacturers, however, may
have special needs for the application of
Coating Types UsingPlural-Component Application
The following three coating types can beformulated as 100% SBV coatings withlittle or no volatile organic compounds(VOCs). Each of the three coatings typescan be applied using plural-componentequipment.• Epoxy: A two-component materialthat mixes a base (resin or epoxy) withan activator (catalyst, hardener, or con-verter).• Polyurethane: A two-component, fast-set coating formed by reacting (or mix-ing) an isocyanate with a polyol resin.• Polyurea: A two-component fast-setcoating that is formed by reacting an iso-cyanate with an amine resin.
Environmental Conditions andHigh-Solids Coatings Application
Generally speaking, the applicator whoapplies high-SBV coatings must knowthe minimum and maximum applicationtemperature and humidity recommend-ed by the manufacturer of the specificcoating. Proper material storage temper-atures, ambient and substrate tempera-tures, and humidity are essential forgood film formation and coating cure.Polyurethanes and polyureas can be
formulated for low-temperature applica-tions and can be tolerant of higherhumidity; however, applicators must stillstrictly follow good painting practice.The dew point must be 5 degrees Fabove the substrate temperature and
stable or ascending, and the substratemust be dry when the coating is appliedand through initial cure.2
Generally speaking, epoxies requiresimilar environmental conditions. In thecase of amine-cured epoxy, the condi-tions of high humidity and low tempera-ture can cause amine exudates, com-monly known as amine blush, to form onthe surface of the coating. If it is notremoved, amine blush will interfere withthe adhesion of subsequently appliedcoatings.Temperatures outside the range rec-
ommended by the coating manufacturercan also hinder good film formation forall three types of coatings.Adequate ventilation during coating
application and through final cure mustbe in place to assure proper film forma-tion of high-SBV coatings throughout thetank. ANSI/AWWAD102-06 (“CoatingSteel Water Storage Tanks”) also statesthat ventilation shall be used for propercure of the coating system as well as forworker safety.3
Surface PreparationThe service life of a coating systemdepends on the degree and quality of thesurface preparation specified andachieved before coating application.Because 100% SBV coatings are formu-lated to contain no solvent, their abilityto wet out the substrate can be dimin-ished significantly. For steel substrates,coating manufacturers typically recom-mend a more aggressive three- to four-mil (75- to 100-micron) angular anchorprofile for single-coat applications. Forconcrete substrates, a concrete surfaceprofile (CSP) range of CSP3 to CSP5(per ICRI Technical Guideline No.
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Editor’s Note: this article is basedon a paper the author presented inJanuary 2008 at PACE 2008, thejoint conference of SSPC and PDCA.
or tote tanks, the coating componentswill need to be transferred and fed to theproportioning unit. A variety of feedpumps come in a wide range of configu-rations and gallon-per-minute (GPM) out-puts that will ensure that the proportion-ing unit has a sufficient volume of eachof the coating’s components to providethe proper mix ratio (Fig. 3). An improp-er feed pump GPM rating, in conjunctionwith an improper pressure setting (eitherof which is too high or too low), cancause improper feeding of the propor-tioning unit. Off-ratio coating can result.Two ways to help prevent improper feed-ing are as follows: make sure that theGPM output of the feed pump is twicethe amount of the GPM out-put of the proportioningpump or spray tip, and donot let the feed pump pres-sure exceed 20% of the pro-portioning pump pressure.The equipment feed sys-
tem should also includematerial agitators (Fig. 4).There is, however, one con-cern to be aware of:polyurea and polyurethanecoatings both include an iso-cyanate component, whichunder most circumstances isnot agitated, due primarily
their coatings that require modificationsor additions to the standard equipmentconfigurations. If the equipment manu-facturer does not have the means tomodify the equipment to meet the coat-ing manufacturer’s needs, the equipmentmaker should be able to direct the coat-ing applicator to an approved distribu-tor or supplier that can.The coating manufacturer should be
contacted regarding the specific equip-ment requirements for the application of
its material. In some cases, applicatortraining may be necessary before thecoating manufacturer will feel comfort-able with an applicator purchasing andapplying the coating. When an applica-tor needs equipment training, the equip-ment manufacturer should be contacted.Seldom, if ever, will a coating manufac-turer assume responsibility for the actu-al operation of the equipment systemused to apply its coatings.
Equipment ComponentsThe individual components in a typicalplural-component setup are the feed sys-tem, heating system, proportioning sys-tem (mechanical or electronic), circula-tion system, hose bundle, mixing system,and spray gun.
Feed SystemWhether supplied in five-gallon (19-liter)containers, 55-gallon (208-liter) drums,
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to its sensitivity to moisture and secon-darily to its viscosity. Excessive agitation,regardless of the generic composition ofthe material, can also force air to becomeentrained in the material (resulting in airbubbles or foaming), which can cause theproportioning system to give impropermaterial ratios. Desiccant filters may alsobe required on material containers, espe-cially those holding an isocyanate.
Heating SystemMaterial heating requirements vary witheach product. The time of year and geo-graphical location of the job also affecthow well one can achieve and then main-tain the proper temperature for material
Fig. 2: Electronic proportioner (dosing unit)Courtesy of Graco
Fig. 3: Feed systemCourtesy of Tnemec
Fig. 1: Mechanical proportioner (fixed ratio)Courtesy of WIWA
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application. In colder regions, forinstance, heating, circulating, and main-taining the material temperature willrequire more time, labor, and heatingequipment than in warmer regions.Heating equipment setups can includedouble-wall heated material tanks, 55-gallon drum heaters (or 5-gallon bucketheaters), in-line heaters mounted on theproportioning unit, and hose-bundle heatsystems (Figs. 5–9). Double-wall heatedtanks normally have a water andantifreeze mixture that is kept at thedesired temperature by separate thermo-statically controlled heaters within thedouble wall of the tanks. Heat for aninsulated paint hose bundle can be pro-vided by either an electric heat trace(similar to the type used to prevent resi-dential water pipes from freezing) or hol-low tubing for circulating hot water froma small heated tank through the tubingby means of a circulation pump. Eitherway, the intent of providing heat to apaint hose bundle is strictly to helpmaintain material temperature from thepump to the spray gun. Either setupshould have thermostats to control thetemperature. The high-solids content andsometimes significantly high viscosity ofsome materials may require a combina-tion of heating methods. When thinkingabout heating, another requirement forconsideration is a source of electricitythat will provide adequate amperage.
Proportioning SystemWith mechanical proportioning sys-tems, the material ratio is set and main-tained by the inside diameter of thefluid cylinders and/or the number of
fluid cylinders. Through the mechanicsof the proportioner, all the fluid cylin-ders stroke at the same time. To changethe material ratio, the inside diameterand/or the number of fluid cylindersmust be changed.Mechanical proportioning systems
can handle materials mixed by staticmixers and applied with standard air-less spray guns, as well as polyureasthat require application using impinge-ment mix spray guns. Most mechanicalproportioning systems can be config-ured to achieve a maximum pressurerating ranging between 3,500 psi and5,000 psi (241 and 345 bar).Mechanical proportioning systems
have a ratio tolerance range of 2%,fewer sensors and valves than electron-ic proportioners, and one air motor.
Fig. 4: Feed pumps and tank agitatorCourtesy of Tnemec
Fig. 5: Double wall heated tanksCourtesy of Tnemec
Fig. 6: Drum heatersCourtesy of Tnemec
recording. However, electronic propor-tioning systems have more sensors,valves, and switches, as well as a high-er ratio tolerance range of 5% (i.e.,more to keep track of, more to gowrong). In addition, the pump mechan-ic must have more comprehensivetroubleshooting skills than thoserequired for mechanical proportioners.
Circulation SystemThe circulation system for plural-com-ponent equipment moves the separatecomponents of the coating materialfrom their heated material containersthrough the proportioning pump andinline heaters, through the paint linesin the heated hose bundle, and thenback to their heated material contain-ers. In the field, the process is referredto as the “recirculation mode.” Thematerial is circulated using a combina-tion of feed pump pressure and pro-portioner pressure. By monitoring andadjusting the heat of the material con-tainer, the inline heaters on the propor-tioner, and the hose bundle heat, thematerial that is being readied for appli-cation can be brought to a uniform andconsistent application temperaturethroughout the entire plural-compo-nent system, as the material manufac-turer recommends.If the material is too cold, it may not
feed the proportioner effectively, thuscausing the proportioner pressure to
Due to the limited number of movingparts, troubleshooting equipment prob-lems can be less complicated than withelectronic proportioners. As with anytype of equipment, however, there arealso disadvantages. With mechanicalproportioning systems, changing thematerial ratio can be difficult becausethe equipment must be shut down andcleaned. The fluid cylinders must thenbe removed from the proportioner,and the correct number or size of fluidcylinders must be installed. Fluid pres-sure gauges must be monitored at alltimes to ensure a balance of pressurebetween the fluid cylinders, or off-ratio material will be applied. Thepump mechanic must know trou-bleshooting procedures that requireinterpretation of differences in fluidgauge pressure.With an electronic proportioner,
material ratio is set and maintained
electronically through metering valves(Fig. 10). The proportioner is made upof two air motors that run separatelyfrom each other, with each motor dri-ving its own fluid section. The materialratio is changed electronically ratherthan mechanically.Electronic proportioning systems are
not recommended for materials such aspolyureas because they require appli-cation using an impingement mix spraygun. Most electronic proportioning sys-tems can be configured to achieve amaximum pressure rating rangingbetween 3,500 and 7,250 psi (241 and500 bar).Electronic proportioning systems
can offer advantages such as the abilityto easily change the material ratio, elec-tronic monitoring of the proportioningpumps and metering valves, automaticsystem shutdown when an error issensed, error reporting, and data
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Fig. 7: Bucket band heaterCourtesy of Tnemec
Fig. 9: Hose bundle heat systemsCourtesy of Tnemec
Fig. 10: Electronic (dosing) systemCourtesy of Tnemec
Fig. 8: In-line heating systemsCourtesy of Graco (left) and WIWA (right)
be set too high to compensate for thecold material and increased viscosity.Therefore, a properly functioning heatingsystem can be considered vital to theapplication. Attempting to force materialthat is not properly heated through theequipment could have an undesirableeffect on the application, such as an off-ratio material or improper spray atom-ization of the material.Occasionally, the recirculation modemay not include the material in the hosebundle paint lines. Instead, after goingthrough the proportioner and then theinline heaters, the material goes directlyback to the heated material containers.When it is time to apply the coating, thematerial in the paint lines that is not atthe proper temperature is discarded dueto an inability to obtain a good spraypattern and proper atomization.
Hose BundleThe hose bundle typically comes in 50-foot (15-meter) lengths and includes thepaint lines (one for each material compo-nent), a solvent purge line, a hose bundleheat source (i.e., an electric heat trace orhot water tubing), hose bundle insulation(for retention of hose bundle heat), and asolvent-resistant protective hose bundlecover. Recirculation lines in the hosebundle may be a standard feature ormay need to be specially ordered. Tomaintain proper material ratios, theinside diameter of the individual materi-al component hoses may need to be off-set (e.g., 1⁄2-inch diameter for the basecomponent and 3⁄8-inch diameter for theactivator component). The inside diame-ter of the material hose will vary fromone product to another, depending onthe material’s individual component vis-cosity and the material mix ratio, and onwhether the atomizing pressure betweenthe two components can be balanced.
Mixing SystemPolyurea is applied using an impinge-ment mix spray gun (Fig. 11). The spraygun is attached directly to the end of the
hose bundle with a special adapter. Inthe event of a spray gun malfunction,check valves inside the spray gun willstop a crossover of material back intothe paint lines, which would result inpremature mixing of the components,and limit the problem to the spray gun.Located at the front of the spray gun, amix module mixes the material together,while the fluid tip controls the fan pat-
tern and supplies the proper amount ofmaterial. To prevent clogging of the mixmodule, material screens in the spraygun filter the material. The coating man-ufacturer should recommend the mixmodule and tip size, which must bepaired together to obtain the proper mix.Coatings that are not applied with an
impingement mix spray gun use a differ-ent type of set up that starts with a mixmanifold (or block) with internal checkvalves, which is attached to the end ofthe hose bundle (Fig. 12). The mix mani-fold will have an assortment of fluidvalves and should, but may not always,have material heat and fluid pressuregauges. The setup includes fluid valvesfor the solvent purge system, separatematerial component fluid valves that areconnected by a bar and are referred toas the “dual control valve,” and, if thehose bundle is so equipped, materialcomponent fluid valves for materialrecirculation lines.The material line from the mix mani-
fold to the spray gun can be configuredin several different ways, depending onthe coating’s generic composition andthe recommendations of the equipmentand material manufacturers. This por-tion of the system can consist of one ormore static mixers (Fig. 13), an integra-tion line, a whip hose, and an airlessspray gun (Fig. 14) with an appropriatespray tip.Static mixer size is designated by
inside diameter measurements—themost common, 1⁄4 and 3⁄8 in.—and by thenumber of “folds”—the most common,
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Fig. 11: Impingement mix spray gunCourtesy of Graco
Fig. 12: Mix manifoldCourtesy of Tnemec
Fig. 14: Airless spray gunsCourtesy of Tnemec
Fig. 13: Static mixersCourtesy of Tnemec
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12 fold and 6 fold. The folds of a staticmixer are the concave blades that mixthe two pressurized material compo-nents together as the materials passthrough the blades. The greater thenumber of folds, the more thoroughlythe material is mixed. Stainless steelstatic mixers are usually preferredover plastic static mixers, which can bedamaged more easily. Should the plasticstatic mixer be damaged, the appliedmaterial may not be mixed adequately.(Contractors should have spare staticmixers on hand). Static mixers aredirectional and must be positioned withthe flow arrow pointed toward thespray gun. The inside diameter, numberof folds, and number of static mixersrequired should be recommended bythe coating manufacturer.An integration line, if required,
begins combining the two coating com-ponents through the friction created bymaterial flow through the line beforethe components flow through the staticmixer(s), ensuring that the coating isthoroughly mixed. The airless spraygun should be rated for the properpressure range and be the proper typefor the application of high-solids materi-al. In addition, the spray tip should besized in accordance with the materialmanufacturer’s recommendations.Generally, the solvent purge system
is comprised of an electric or air-drivenhigh-pressure solvent pump and a dedi-cated paint line from the solvent pumpto the mix manifold (Figs. 15–16). Themix manifold should include a primarysolvent purge shut-off valve and a sep-arate shut-off valve for each side of themixing block. The solvent pump mustbe capable of delivering enough solventunder sufficient pressure to purgemixed material from the integrationline, whip hose(s), static mixer(s), spraygun, and spray tip.Determined by the coating manufac-
turer, material purge time can rangefrom several seconds to a minute orlonger. Failure to purge mixed material
in time can result in the coating settingup and the loss of that portion of theequipment from the mix manifold tothe spray tip.
Factors Affecting ApplicationMany factors can affect the applicationof high-SBV coatings. Some of these fac-tors have been discussed, and others willbe determined after job specifics areknown. Take, for example, the earlier dis-cussion of substrate cleanliness. Personsfamiliar with the lining of potable waterstorage tanks can recognize that, often,abrasive residue is left on lower portionsof the tank to protect those surfacesfrom overspray and is then removedprior to coating application. If spent abra-sive residue is removed from the tankbefore painting and the substrate iscleaned to a degree that will minimize orreduce airborne particulate matter dur-ing the application, then the substrateshould be covered with visquene orclean tarps to prevent the accumulationof overspray. With 100% SBV coatings,the amount of overspray generated canbe significantly more than that generatedby conventional thin-film epoxy and canbe difficult to remove from a preparedsubstrate before the coating is applied.Another consideration, this one
regarding equipment usage, pertains tocross contamination of the plural-compo-nent equipment. Most equipment manu-facturers recommend that plural-compo-nent equipment remain dedicated to aparticular generic coating type. Thus,plural-component equipment for epoxycoatings should be dedicated to epoxycoatings, with the base (resin or epoxy)always on the same side of the propor-tioner and the activator (catalyst, hard-ener, or converter) always on the oppos-ing side. Switching sides on the propor-tioner with material components whenany residual material is left in the systemfrom the last application can cause avery large and very expensive equip-ment problem.With polyurethane or polyurea mater-
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ial, this recommendation should be thesame. What might happen if plural-com-ponent equipment used regularly toapply an amine-cured epoxy wasbrought to the next job to applypolyurethane? Before leaving for thenext job, the plural-component equip-ment setup is flushed, but a residualamount of material is left in the fittingsand miscellaneous hoses in the system.At the next job, the isocyanate portionof the polyurethane material is runthrough the right side of the proportion-er, the same side where the amine por-tion of the epoxy system was run on theprevious job. Enough amine residual inthe system combined with isocyanateyields a chemical reaction similar to thatof polyurea components. Again, the endresult will cause a very large and veryexpensive equipment problem.
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Fig. 15 (top): Electric flush pumpFig. 16 (bottom): Pneumatic flush pump
Courtesy of Tnemec
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ConclusionMuch more information is available onthe use of plural-component equipmentwith high-SBV coatings. However, theintent of this article is not to providereaders with a “how-to” manual but toprovide a basic understanding of plural-component equipment and how it is usedto apply coatings. Remember, the mostimportant starting point is to open com-munication among the applicator, equip-ment manufacturer, material manufac-turer, and specifier to help ensure a prob-lem-free application and afford a liningsystem that will provide the expectedlong-term performance.
References1. See, for example, 40 CFR 59, 400-413, “National Volatile OrganicCompound Emission Standards forConsumer and CommercialProducts,” and “ANSI/NSF 6I—Drinking Water System Components-Health Effects.”
2. SSPC-PA 1, SSPC PaintingManual,Volume 1, Fourth Edition (Pittsburgh,PA: SSPC: The Society for ProtectiveCoatings).
3. AWWAD102.06, “Coating SteelWater Storage Tanks” (Denver, CO:AWWA).
4. International Concrete RepairInstitute (ICRI) Guideline No. 03732,Selecting and Specifying ConcreteSurface Preparation for Coatings,Sealers, and Polymer Overlays (DesPlaines, IL: ICRI)
5. “Metallic Abrasives,” Chapter 2.4 inSSPC PaintingManual, Volume 1,Fourth Edition (Pittsburgh, PA: SSPC:The Society for Protective Coatings).
6. Wiwa Duomix 230/333,WIWA/WilhelmWagner L.P.,3734A Cook Blvd., Chesapeake, VA23323.
7. Graco Extreme Mix, Graco-Gussmer,88-11th Avenue NE, Minneapolis,MN 55413.
Robin W. Hasak is asenior technical servicerepresentative for TnemecCo., Inc. His 28 years ofexperience in the potablewater tank industryinclude work as an appli-cator for Chicago Bridge
& Iron and work as an inspector for a nationaltank inspection company. At Tnemec, his
focus is on coating system recommendations,systems application, new product application,failure analysis, and the use of plural-compo-nent equipment. Hasak develops and leadstraining sessions on a variety of industrialcoating subjects. He is a member of SSPCand AWWA; is a NACE-Certified CoatingInspector; and is a contributing member ofthe AWWA M42 Revision Task ForceCommittee.
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