basic coatings technology

1

excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials

BASIC COATINGS TECHNOLOGY

Lloyd M. Smith, Ph.D.Vice President

Corrosion Control Consultants and LabsHerndon, Virginia, USA

Coatings have many uses in industrial situations.They are used for corrosion control, chemicalresistance, heat resistance, temperature control,identification, decoration, camouflage, fireretardation, noise control, anti-fouling protection,and many other reasons.

Corrosion protection is one of the primary usesof coatings in industrial applications. Corrosionis the destruction of material by chemical orelectrochemical reaction on exposure to theenvironment. All materials, including metals,concretes, and plastics, will eventually corrode.However, coatings are a cost-effective method ofcontrolling corrosion. Alternatives include theuse of alloys or composite materials that may bemore corrosionresistant but not as cost-effective.

This book is organized by generic types ofindustrial protective coatings rather than by end-uses. Different generic types of coatings may beused for the same application. Alternatively, ageneric coating type may have many end-useapplications. The user or specifier mustdetermine the most appropriate or cost-effectivecoating system for particular structures andexposure environments, and weigh theadvantages and disadvantages of each type ofpaint or coating.

Terminology used in the industry can be confusing.The terms paint, coating, and lining sometimes areused interchangeably, but there are differencesin their meanings. The Paint/Coatings Dictionary1

defines paint and coating as follows:

• Paint - Any pigmented liquid, liquefiable or

mastic composition designed for application to asubstrate in a thin layer which is converted to anopaque solid film after application. Used forprotection, decoration, or identification, or toserve some functional purpose such as the fillingor concealing of surface irregularities, themodification of light and heat radiationcharacteristics, etc.

• Coating - A liquid, liquefiable, or masticcomposition which is converted to a solidprotective, decorative, or functional adherentfilm after application as a thin layer.

Based on these definitions, the major differencebetween paint and coating is that paint ispigmented, while no such requirement ismentioned for coating. They both are liquid,liquefiable, or mastic compositions that areconverted to a film after application as a thinlayer. Therefore, varnishes and clear coats arecoatings but not paints. Processes such asgalvanizing and metallizing also meet thedefinition of coating. With galvanizing, forexample, zinc is heated to a liquefiable state andapplied as a thin layer. It is converted to a solidfilm with a decorative or functional purpose.

While the distinctions between paint and coatingappear to be minor, it is common in the industryto distinguish between them, even though mostmaterials used are pigmented and meet thedefinition of paint. Coating generally refers tomaterials used for protective or functionalpurposes, while paint refers to materials used foraesthetic or decorative purposes. Thus, astructure is coated while a room is painted. This

2

Basic Coatings Technology by Lloyd M. Smith, PhD.

differentiation is emphasized further by somewho refer to the materials used in industrialsituations as protective coatings.

The definition of lining from the IndustrialMaintenance Coatings Glossary2 is “a materialused to protect a container against corrosion and/or to protect the contents of a container fromcontamination by the container shell material.”Liners commonly are thought of as thick, built-up systems containing matting or similarreinforcing material. However, the definitiondoes not exclude coatings from use as linings. Infact, coatings are the standard material for liningthe inside of water storage tanks.

There are many similarities in composition,handling, and use of paints, coatings, and linings.There also are some differences. The basicconcepts and terminology of the coatingsindustry are presented here to help the readerbetter understand the information presented aboutthe various generic types of coatings in this book.

COATING COMPONENTS

A coating can contain as few as three or fouringredients or as many as twenty or thirtyingredients, depending on the formulation. Thethree main components of a coating are the resin,pigment, and solvent. Resin and solvent comprisethe liquid portion of a coating. Together, they arecalled the vehicle. Resin and pigment arereferred to as the film solids, since they are thematerials left after the coating has dried.

Resins

Resin is the binder that holds the pigmentparticles together and provides adhesion of thecoating to the surface. Most coatings are namedby the generic type of resin (i.e., vinyl, epoxy,acrylic, polyurethane, etc.). The resin componentof most coatings is a mixture or chemical blendof materials. There are different types of epoxyresin, for example, and many combinations of

epoxy resin and hardener. So, the resincomposition of an epoxy coating may bedifferent for epoxy coatings from differentsuppliers or even for different products from thesame manufacturer. In addition, other resin typescan be modified with epoxy resin. Therefore,referring to a coating as an epoxy—or any othertype of resin— provides only limited information.

The resin or binder is responsible for most of acoating’s physical and chemical properties,including hardness, abrasion resistance, chemicalresistance, weather resistance, adhesion, andcohesion. The type of resin system alsodetermines a coating’s curing mechanism.

Resins can be classified as thermoplastic orthermosetting. Thermoplastic resins can berepeatedly softened by heating and hardened bycooling. They also can be dissolved by theoriginal solvent used in the coating. Vinyls aresuch a resin. Coatings based on thermoplasticresins usually are packaged in one container.Thermosetting resins, however, undergo achemical reaction by the action of heat, catalysts,ultraviolet light, etc., that makes them relativelyinfusible. They do not harden and soften byheating and cooling or redissolve in solvent.Epoxies are such a resin. Coatings based onthermosetting resins usually are packaged in twoor more containers, although thermosetting resinsthat cure by such methods as heating, ultravioletlight, or reaction with constituents in theatmosphere come in one container.

Pigments

Most pigments are inorganic compounds,although some bright color pigments areinsoluble organic compounds. Prime pigmentsprovide opacity, a term for hiding power. Theyalso can provide improved durability,weathering, and protection to light-sensitiveresins. Titanium dioxide is a commonly usedprime pigment for white and light tints. Otherpigments are added for color.

3


Some coating types contain anti-corrosivepigments for corrosion protection. Lead- andchromium-based compounds historically used forthis purpose are being replaced by nontoxicmetal compounds such as phosphates, chromates,phosphosilicates, phosphites, some organiccompounds, and other compounds. Flat, plate-like pigments, such as mica and aluminum flake,are used to decrease moisture permeability. Stillanother group of pigments, known as extender orfiller pigments, add mechanical strength to thefilm, control viscosity, reduce settling, reducegloss, and improve film-build. Examples ofthese pigments are talc, silica, and clays.

A critical parameter in a coating film is thepigment-to-resin ratio, known as the pigmentvolume concentration (PVC). Properties of acoating film change dramatically near the criticalpigment volume concentration (CPVC), the pointat which there is just sufficient binder to coat thepigment particles and to fill the voids between them.

Coatings formulated below the CPVC (i.e., thosecontaining an excess of resin) have high gloss,low moisture permeability, and a potential toblister. Adding more pigment reduces gloss,increases moisture permeability, and decreasesthe tendency to blister. When the CPVC isexceeded so there is insufficient binder to wet thepigment particles and fill in the void spaces, thecohesive and adhesive strength may be reduced.Some coatings, such as inorganic zinc-richprimers, are intentionally formulated above theCPVC. This is necessary because of the mechanismby which they provide corrosion protection.

The coatings formulator determines theacceptable PVC for a particular product. Theproperties of the dry film depend uponmaintaining the proper ratio of pigment to resin.This is why properly mixing a coating prior touse is so important. If settled pigments are notredispersed in the mixing process, the dry filmwill not have the properties or performancedesigned by the formulator.

Solvents

The main function of the solvent is to provideease of coating application. Solvents dissolve ordisperse the resin, provide flow-out and levelingduring application, and control adhesion anddurability of the dry film.

A coating formulation usually contains a blend ofsolvents. The resin is dissolved or dispersed inthe primary solvent. However, if one of the resinconstituents is not soluble in the primary solvent,a co-solvent may be needed. Other solvents maybe added to control the evaporation rate or toprovide adequate flow-out and leveling. Thesolvent can control the rate of chemical reactionin some coatings.

Solvents are not part of the dry film. Theyevaporate during the drying or curing process.Organic solvents contribute to the production ofphotochemical smog. As a result, federal, state,and local air quality regulatory agencies setlimits on the volatile organic compound (VOC)content allowed in coatings. The maximumVOC content allowed varies by locale and bycoating type or use. For example, zinc-richprimers, heatresistance coatings, and swimmingpool coatings have a higher VOC limit thanindustrial maintenance coatings. These varianceswere obtained due to the unique characteristics oruses of these coatings, with no viable alternativesavailable. Currently, the VOC content limit forindustrial maintenance coatings varies from 2.0Ibs/gal (240 g/L) to 3.8 Ibs/gal (450 g L),depending on locale.

VOC regulations are having a profound effect oncoatings formulation. Coating manufacturers areconcentrating their research and formulationefforts on developing VOC-compliant coatings.The main technologies currently being used arehigh-solids and water-borne coatings, becausethey are low in VOC content. There even aresome coatings on the market that contain novolatile organic compounds. High-solids

4


coatings contain a high percentage of solids,hence less solvents. Water-borne coatingsconsist of two classes of materials. There arelatex or emulsion coatings, in which water is themain solvent. The other class is water-reduciblecoatings, which contain a solvent blend that canbe thinned with water.

Additives

In addition to resins, pigments, and solvents,many coating formulations contain additives—specialty materials that vary widely dependingon the resin type. Oil-based coatings, forexample, contain driers to promote curing. Hard,brittle resins such as vinyls contain plasticizers toproduce a more flexible film. Emulsion systemsemploy a number of additives, including wettingagents, dispersants, freeze-thaw stabilizers, anti-microbial agents, and film-forming aids. Otheradditives may be incorporated into a coatingformulation to control consistency and pigmentsettling or improve sag resistance.

PRODUCT DATA SHEET

Users and specifiers need not know coatingformulation. However, proper use of a coatingrequires them to follow information supplied bythe manufacturer. Product data sheets contain awealth of information about the proper selection,use, and application of a particular coating. Theamount of information and the form in which it ispresented vary by manufacturer.

Information on selection and use of the coatingis presented mainly for specifiers. This infor-mation typically includes the generic type ofcoating, intended uses of the product (i.e.,primer, intermediate coat, or topcoat), acceptablesubstrates, and recommended exposureenvironments. The product data sheet also mayindicate where the coating should not be used. Ifit is unclear whether the intended use falls withinthe recommended uses of the material, themanufacturer should be consulted.

Performance data—information about chemicalresistance and physical properties of the coating—may be presented on a data sheet either assubjective ratings or as the results of test methodsdeveloped by consensus standards organizationssuch as ASTM. One such common test is for saltfog resistance, which is the topic of ASTM B117, Method of Salt Spray (Fog) Testing.Interpreting these test results requires knowledgeof the test methods and the performance ofcoating materials of similar generic type. Weldon3

discussed interpretation of the results of some ofthese tests. Note there are no standard definitionsin the industry for subjective ratings such asexcellent, good, and fair, and, therefore, theyshould be interpreted with caution.

Data sheet information on compatible coatingsindicates acceptable primers (or whether thecoating can be applied directly to a substrate) andtopcoats. Some manufacturers present informationon recommended coating systems. Others includethis information in a separate document such as acoating system selection guide or specifier’s guide.

Other categories of product data sheetinformation include the color, gloss, and basicphysical characteristics of the coating, such asdensity, percent solids by weight, percent solidsby volume, flash point, viscosity, andrecommended dry film thickness. Manymanufacturers also report the VOC content of theproduct, including the VOC content when thecoating is thinned with the recommended thinner.Density, percent solids by weight, and viscosityare useful if there is a need to quantitativelyverify that a batch of paint was manufactured totolerance. These measurements are determinedby simple laboratory tests. The percent solids byvolume can be used to calculate how muchcoating material to apply to achieve the desireddry film thickness or to determine the expecteddry film thickness for the applied wet filmthickness. Multiply the percent solids by volumeby the wet film thickness to determine theexpected dry film thickness.

5


Coatings must be applied within or near the manu-facturer’s recommended thickness or thicknessrange, or problems may occur. Specifying acoating too thin may affect the life of the coatingsystem. Rust may form sooner than expected onsteel substrates, for example. On the other hand,specifying a coating too thick may require twocoats, adding significantly to the cost ofapplication. Likewise, applying a coating in onecoat at a dry film thickness significantly abovethe manufacturer’s recommendation can result inlack of curing, solvent entrapment, blistering,mudcracking, delamination, or other defects.

Product data sheet information on surfacepreparation, storage, mixing, application, anddrying all relates to the use of the coating. Thisinformation is as important to the user andspecifier as it is to the applicator.

The cleanliness of the surface to which thecoating will be applied is usually presented withreference to surface preparation standards fromconsensus organizations such as SSPC: TheSociety for Protective Coatings, NACEInternational, or ASTM. Surface preparationspecifications may range from solvent cleaningfor application of a topcoat to white metal blastcleaning for complete removal of all previouslyapplied coating material. The requirements forprimers applied to steel will include bothcleanliness and surface roughness or anchorprofile. The user/specifier must determine thatthe minimum surface cleanliness required isachievable before using/specifying a particularproduct. The surface cleanliness level indicatedon the data sheet is the minimum. Higher levelsof cleanliness are acceptable, lower levels are not.

Storage conditions are important becausecoatings are complex mixtures of chemicals. Thequality of a coating can deteriorate with time dueto heat, cold, or moisture. Like otherperishables, coatings have a shelf life, which istheir storage limit in an unopened can.Depending on the coating, shelf life can be as

long as a few years or as short as several weeks.The product data sheet should indicate storageconditions, such as a specific temperature range

Proper mixing of a coating is required to make ithomogeneous. The way a coating is mixed canaffect its performance. Some coatings can bemixed by stirring, shaking, or mechanical means.Others can be mixed by mechanical means only.Even then, mixing must be done carefully toblend the materials without overagitation, whichcan introduce air or moisture into the coating andresult in application or curing problems.Coatings that are supplied in multiple containersmust be mixed in the proper order and in theproper proportions after the individualcomponents are mixed. Multicomponentcoatings are packaged so correct chemicalproportions will result if all contents of eachcontainer are combined. If not, the result may bea substandard film.

Thinning instructions generally are included withmixing instructions. Both the type and amountof thinner to be used are important, because thethinner must maintain a proper balance ofproperties in the coating. The recommendedthinner should not be replaced with anotherthinner without the manufacturer’s approval.Using the wrong type or amount of thinner cancause a variety of application or curing problems.

Successful use of a coating requires adhering toany relevant induction time and pot liferequirements listed on the product data sheet.Induction time is how long a mixed coating mustpre-react in the can before it can be applied. Potlife is the maximum amount of time a coatingcan be applied after it is mixed. Induction timeand pot life are temperature-dependent—shorterin warm temperatures, longer in cooltemperatures. Users and specifiers should checkthese requirements to determine if they arereasonable for expected conditions or if carefulplanning of the timing of mixing and applicationis needed.

6


Application conditions generally listed on datasheets include materials temperature, ambienttemperature, surface temperature, relativehumidity, and dew point. Temperature andrelative humidity requirements are productspecific. Good painting practice requires thatsurface temperature be at least 5˚F (3˚C) abovethe dew point. Site ambient conditions can limitthe choice of coatings. Site conditions that falloutside the specified range may be altered, suchas by dehumidifying a contained area or warmingit with indirect heat in cold weather.

If a coating requires a specific type of applicationequipment, that information should be on theproduct data sheet as well. Some coatings canonly be applied by spray methods, so they wouldnot be suitable where only manual methods canbe used. Likewise, factors such as siterestrictions on spray painting, skill of the workforce, and availability of specialized equipmentmay mean that certain types of coatings would beunsuitable in particular circumstances.

Product data sheets may include a drying schedule,which should be checked before selecting acoating. Different types of drying times may bepresented, including dry-to-touch (the time for acoating to be tack-free), dry-to-handle (the timewhen a coated piece can be moved carefully),dry-torecoat (the time when another coating layercan be applied), and final cure. Drying times andcure generally are presented as a minimumamount of time at a specific temperature. Thisinformation should be reviewed in light of anysite and project limitations. There also may be amaximum dry-to-recoat time that may requirespecial attention or planning.

CURING MECHANISMS

Cure refers to the length of time before a coatingcan be put into service. There are four curingmechanisms for coatings: air oxidation, solventevaporation, chemical reaction, and hydrolysis.This section explains the characteristics and

differences of each method.

Air Oxidation

Coatings that cure by air oxidation containdrying oils. These include oilbased coatings andsome hybrid coatings. Crosslinking of the resinpolymers occurs by reaction with oxygen in theair. The solvent evaporates when the coating isapplied, but it takes longer for oxygen topermeate the film. Therefore, recoat times arerelatively long—days for some formulations.The slow-drying characteristic of oil-basedprimers can be an advantage since the coatingcan flow into the surface.

Air-oxidizing coatings generally are applied atlow film builds, such as 2 to 3 mils (50 to 75micrometers) dry film thickness per coat. Theycan surfacecure if applied too thickly in one coat.When this occurs, the top of the coating cures,limiting oxygen permeation to the rest of thecoating. As a result, the coating is hard on topand soft on the bottom. A thickly applied layermay cure through with additional time, but anexcessively thick coating layer will not.

Oxidation continues after a coating has cured.This causes the coating to become brittle withtime. Air-oxidizing coatings are easilyrecoatable, but sooner or later the system willbecome too brittle, too thick, or lose adhesion.Then, it must be totally removed and replaced.

Coating films that contain oils may undergo aprocess called saponification, which is thehydrolysis of an ester by an alkali with theformation of an alcohol and a salt of the acidportion. The oil-containing portion of thecoating is an ester. The alkali may come fromthe substrate. Portland cement concrete is analkaline surface, as are reaction products of someactive metals, notably zinc. The salt formed is ametallic salt of a fatty acid, better known as soap.In severe cases of delamination resulting fromsaponification, it actually may be possible to wet

7


a surface and form a lather. Because ofsaponification, oil-containing coatings should notbe applied to concrete, galvanized surfaces, orzinc-rich primers.

Air-oxidizing coatings have moderate moisturepermeability. Therefore, anticorrosive pigmentsusually are added to the primer formulation forproducts applied to steel. Air-oxidizing coatingsgenerally are formulated in one component andhave unlimited pot life. They are easy to applyby brush, roller, or spray.

Solvent Evaporation

Coatings that cure by this method only requirethat the solvent evaporate from the film. Thesecoatings are made by dissolving the resin in anappropriate solvent. No crosslinking or chemicalreaction occurs during film formation, whichinvolves attraction and entanglement of the resinmolecules to the point where their movement isrestricted. These types of coatings contain thermo-plastic resins. Vinyls, chlorinated rubbers, andasphaltics are examples of coatings in this class.

Solvent-evaporating coatings have relatively lowsolids. VOC-compliant formulations are difficultto make. Solvent-evaporating coatings have lowmoisture permeability and protect by a barriermechanism. They have good water and sunlightresistance but poor solvent resistance. They areeasy to repair because the topcoat solvent softensthe existing film, giving a good bond.

Solvent-evaporating coatings are applied byspray methods only. They become viscousquickly, so they cannot be worked with a brushor roller. The curing mechanism does allow lowtemperature application, although drying isretarded.

Latex and other water-borne coatings also cureby solvent evaporation. In these cases, thesolvent is water. The resin is present asemulsified particles, which coalesce to form a

film as the water evaporates. The coalescingreaction is temperature-dependent, and there is aproper temperature range in which they can beapplied. Application methods include brush,roller, or spray.

Latex films have relatively high permeability.Therefore, they are useful on substrates such asconcrete and wood that are porous, or “breathe.”Anticorrosive pigments are used in latex coatingsystems designed for steel.

Although water is the main solvent in water-borne coatings, organic cosolvents usually areincluded in the formulation so the emulsifiedresin particles are compatible with the water.The amount of organic solvent used is relativelysmall, so these coatings generally have reducedVOC levels.

A number of resins that cure by other mechanismscan be made compatible with water. However, adistinction must be made between emulsioncoatings and water-reducible coatings. Water-reducible coatings contain a solvent blend that canbe thinned with water, but the curing mechanismdoes not change. Therefore, water-reduciblealkyds still cure by air oxidation, and water-reducible epoxies still cure by chemical reaction.

Chemical Reaction

Coatings that cure by chemical reaction arepackaged in two or more containers. One cancontains the resin. The other can contains thecrosslinking agent, referred to as the hardener orcuring agent. The resin, after mixing, becomesthermosetting, which means the componentsform a film by a chemical crosslinking reaction.Epoxies, polyurethanes, and polyesters arecommon coatings that cure by this mechanism.

Chemically cured coatings have a pot life andmay have an induction time. The chemicalreactions are temperature-dependent, and thematerials can only be applied and cured above

8


the minimum application temperature. Thecrosslinking reaction that occurs usually resultsin a smooth, hard film, which is likely to have amaximum recoat time. Most chemically curedcoatings can be applied by brush, roller, or sprayequipment.

These films have low permeability and protect bybarrier formation. The properties of the barrierfilm provide these coatings with good chemicaland Solvent resistance. There are a number ofresins and hardeners that can be combined,which gives the formulator great latitude indesigning VOCcompliant systems.

Hydrolysis

Hydrolysis means reaction with water. Coatingsthat cure by hydrolysis require a sufficientamount of moisture in the air to react with thechemical groupS to form the film. Self-curing,solvent-borne inorganic zinc-rich primers andmoisture-curing polyurethanes are the two maincoating types that cure by this mechanism

Humidity is as important—if not more important—than temperature in the cure of these coatings.Warm air can hold more moisture than cool air.For example, air at 70˚F (21˚C) will have a higherabsolute humidity (grains of water per unitvolume of air) than air at 50˚F (10˚C) when therelative humidity is the same at both temperatures.

Moisture-cure coatings have a pot life. Sincethey are exposed to air during mixing andapplication, moisture in the air will react with theresin, causing it to polymerize.

PROTECTION MECHANISMS

The three mechanisms by which coatings canprotect a substrate are barrier protection,inhibitive pigment protection, and sacrificialprotection.

All coatings protect partially or solely by barrier

protection. They separate the substrate from theenvironment, especially sunlight and moisture.One indicator of a coating’s ability to act as abarrier is its moisture vapor transmission rate,which is the rate at which moisture vaporpermeates a coating and reaches the substrate.Good barrier coatings have very low moisturevapor transmission rates.

Coatings with high moisture vapor transmissionrates incorporate anticorrosive pigments forapplication to steel substrates. These pigmentsare slightly soluble, and a small amount dissolvesas moisture permeates the film. They are carriedto the substrate where they passivate the steel.Some pigments react with certain species thataccelerate corrosion, forming an insoluble com-pound to protect the substrate from those species.

Corrosion theory states that when two dissimilarmetals are in contact and corrosion conditionsexist, the more active metal will corrode toprotect the less active metal. Zinc metal is moreactive than steel. Therefore, sacrificial coatingscontain zinc in electrical continuity with thesteel. Zincrich primers, for instance, containelemental zinc dust as the major pigment. Zincmetal also can be spray-applied by a processcalled metallizing, moltenapplied by hot-dipgalvanizing, or mechanically applied by amethod known as sherardizing.

Understanding protection mechanisms is an assetin the proper design of coating systems.

DESIGN OF A COATING SYSTEM

A coating system consists of surface preparationand one or more coats of material applied in aspecific order. Surface preparation includes bothlevel of cleanliness and roughness (anchor profile).

The first coat of material applied to a surface isthe primer. The function of the primer is toprovide adhesion to the surface for subsequentcoats. Some primers for steel contain anti-

9


corrosive pigments, but this is a secondaryfunction. An important concept is that the entirecoating system provides corrosion protection.

The second coat in a multicoat system is theintermediate coat. An intermediate coat providesthickness for increased barrier protection as wellas specific chemical resistances. An intermediatecoat also can serve as a tie coat between anincompatible primer and topcoat.

The last coat of material applied is the topcoat.Its main function is resistance to weather,sunlight, and chemicals. It also adds to thebarrier protection of the coating system.Aesthetics, such as color and gloss, are importantproperties of topcoats in many situations.

However, a coating system does not alwaysconsist of three coats of material. It may includeone coat, two coats, or three or more, dependingon such factors as severity of the exposure environ-ment, expected life of service, and appearancerequirements. There is a relationship betweencoating life and total thickness of a coating system.

Designing a coating system should take intoaccount the curing and protection mechanism ofthe coating layers. It make no sense to place aninhibitive pigment system on top of a barriercoating, for example.

It is rare that only one coating system is availablefor a specific use. In most cases, there are alter-natives that can be used. A wealth of informationis available to assist the user/specifier in selectinga coating system. This information can be foundin technical literature about industrial maintenancecoatings as well as in government-sponsoredresearch or evaluations. Also, many coatingsmanufacturers recommend coating systems intheir data sheets, specification guides, orindustry-oriented literature.

Designing a coating system starts withidentifying desired properties for a particular

service and identifying limitations that may exist.Desired properties may include one or more of thefollowing: corrosion resistance, color and glossretention, abrasion resistance, water resistance,chemical (including fuel) resistance, heatresistance, cleanability, and ease of maintenance.

Corrosion Resistance

Corrosion resistance is a desired property ofmany coating systems. The life of a corrosion-resistant coating system depends on surfacepreparation, the coating material selected foreach layer, and the thickness of each layer. Inselecting coating material, the major decision isthe type of primer, which must be compatiblewith the substrate.

Color and Gloss Retention

Color and gloss are functions of the topcoat.Important considerations in specifying color andgloss are the initial gloss desired and theexposure environment. Deterioration of colorand gloss will result from exposure to sunlight,with direct exposure being the most aggressive.Coatings protected from sunlight, on the otherhand, will have very little deterioration.

Abrasion Resistance

Abrasion resistance in a coating may be neededfor a variety of reasons. For instance, a floorcoating may need resistance to vehicle traffic,such as forklifts or carts. Alternatively, thesource of abrasion may be air-borne particulates.For heavy exposures, such as vehicular traffic,abrasion resistance is a function of the completecoating system. For light exposures, such as air-borne particles, abrasion resistance is primarily afunction of the topcoat.

Water Resistance

Water resistance can involve many differentsituations, including immersed structures such as

10


the inside of water storage tanks, locks and dams,or ship bottoms. It also involves structures thatare constantly wet because of location, such asbeing downwind from a cooling tower. Wettingcan be intermittent. This can include structuresthat are exposed to water for periods of time orlong times of wetness due to condensation. Waterresistance is a function of the total coating system.

Chemical Resistance

Chemical resistance is a primary considerationof linings that may be exposed to liquids, such asfor primary or secondary containment structures,although chemical contaminants can also be inthe air. For instance, structures downwind froma chemical plant, refinery, pulp and paper mill,or power plant may need to be protected withcoating materials that are resistant to a specifictype or class of contaminants, such as acid.A chemically resistant coating also may beneeded for splash and spillage areas that receiveintermittent exposure to a chemical. (There is nouniform definition of splash and spillage.To some manufacturers, it refers to constantintermittent exposure; to others, it refers tosituations when an upset occurs and is cleaned upin a timely manner.) Chemical resistance is afunction of the total coating system for primarylinings. In other exposures, it may be a functionmainly of the topcoat and intermediate coat.

Heat Resistance

Heat resistance may be needed for constant orintermittent heating conditions. Most coatingsare based on organic chemicals, which limitsconstant heat resistance to the range of about 150to 250˚F (66 to 121˚C), depending on generictype and formulation. Special coatings are neededfor higher temperature exposures. Heat resistanceis a function of each layer of a coating system.

Cleanability

Cleanability of a coating system refers to the

removal of dirt, grime, and other contaminants.Cleanability of the coating system on fuel storagetanks is important, for example, because a lightcolor must be maintained for temperature controlto reduce evaporation of the stored product.Cleanability may be necessary for sanitaryreasons in some circumstances, and the ability todecontaminate a coated surface is a primaryconcern in certain nuclear and militaryapplications. Cleanability should not beconfused with scrub resistance, which isresistance to coating thickness erosion from thecleaning process. Cleanability and scrubresistance are mainly functions of the topcoat.

Ease of Maintenance

Maintenance is the process of making repairs to acoating system, either with or without applicationof a full topcoat. Some coating systems areintended to be replaced once degraded, whileother coating systems are intended to bemaintained one or more times during their lifecycle. Some situations may require almostconstant maintenance to a coating system.Limited opportunities to perform the work oradverse working conditions may makemaintenance a high priority in the selection of acoating system.

Other Factors

A variety of other factors also may influence thechoice of coating materials and limit the numberof coating systems to consider. Some commonlimitations include surface preparation, access tothe surface, drying times, weather, equipment,applicator skills, safety and environmentalregulations, and budget.

• Surface preparation — It may not be possibleto achieve the desired level of substrate cleanlinessfor certain types of coatings. For example, itmay only be possible to perform hand-toolcleaning. Therefore, only primers intended forsurfaces cleaned with hand tools can be used.

11


• Access to the surface — Access for coatingmaintenance may be limited due to physicalconstraints of construction or layout. Limitedaccess usually relates to the ability to performsurface preparation, although limited access alsocan relate to coating application methods.

• Drying times — Drying time is important whencoating projects are under time constraints. Timepressures may result in a coating being appliedbefore the previous coat has dried sufficiently ora unit being put into service before the coatingsystem has cured. Such problems can be avoidedby selecting the appropriate coating materials.Drying time also may be important for newconstruction where coatings are applied in theshop, since the pieces cannot be moved until dry-to-handle times are met.

• Weather — Weather conditions, includingtemperature and humidity levels, are otherfactors to consider. Minimum drying and curingtemperature requirements are a more commonconcern than high temperatures. If the ambienttemperature is near the minimum required level,the area might possibly be heated to bring thetemperature within an acceptable range. Highhumidity can be a limitation with certain coatingtypes. Polyurethanes, for example, will notproduce the expected gloss when applied atrelative humidity levels above the maximumlisted on the data sheet. Low humidity is alimitation with moisture-cured coatings, althoughit may be possible to introduce moisture or humidityby other means, such as spraying with water.

• Equipment — Available equipment can affectsurface preparation as well as coatingapplication. For example, spray-applied coatingsshould not be used when brushes and rollers arethe only types of equipment available. Somespecialty coatings may require plural-componentspray equipment in which the components aremixed at the spray gun. Such coatings should notbe applied with any other type of spray system.Coatings that require agitation should not be used

if an agitation pot is not available. If the properequipment is not available, consideration shouldbe given to acquiring it.

• Applicator skills — Certain high-performancecoatings require skilled applicators to apply thecoatings correctly. Coating work to be performedby unskilled labor should involve the “friendliest”materials available.

• Safety and environmental regulations — Safetyand environmental regulations may limit surfacepreparation as well as coating selection. Forinstance, lead paint removal regulations mayfavor coating maintenance. Therefore, overcoatingmaterials would be preferred to primers that mustbe applied to bare metal. As another example,flammable solvents may be especially hazardousin a particular work area, so water-borneformulations may be indicated instead.

• Budget — Budget is a limitation on everyproject. However, an expensive coating projectmight still be cheaper than the alternatives, suchas losing use of the structure, constructing theunit or structure of a more resistant material, orusing a different type of protective system.Practicalities dictate a limited amount of fundsfor coating needs. Various options may beavailable, such as using funds for certainstructures while delaying work on otherstructures, or using an overcoating system ratherthan total removal and replacement. In othercases, budgetary considerations may dictate atwo-coat system rather than a three-coat system.Budgetary influence on the choice of coatingmaterials is usually related to surface preparationand the cost of the entire project rather than justthe coating materials themselves.

A more informed choice of coating systems canbe made once the properties and limitations ofvarious materials have been determined. Eventhen, there usually are alternatives. It is goodpractice to use the minimum number of systemspossible. This makes it easier to keep track of

12


the coatings applied to various surfaces and alsomakes it simpler for field personnel. In somecases, only a small number of coating systemsmay be needed because of limited exposureenvironments and functions. In other cases,twenty or more coating systems may be requireddue to varying exposure conditions and multiplecoating functions.

COATINGS SPECIFICATION

Once the decision on a particular coating systemhas been made, the next task is to specify andpurchase the materials. The most commonspecifying methods are formula specification,performance specification, and user experience.

Formula Specification

Formula specifications are coatings recipesdeveloped by government agencies, especiallythe federal government, and technicalorganizations such as SSPC: The Society forProtective Coatings. The advantage of formulaspecifications is supplying the user with aknown, proven coating material that should beidentical from each supplier. The disadvantage isthat the formula specification may be outdatedand not cover all generic types of coatings. Thefederal government, the largest preparer offormula specifications, has cut back considerablyon the maintenance and updating of specifications.Many formulas have been rescinded becausethey contained hazardous materials. Others havebeen abandoned due to similarity with otherformulations. Only some have been updated tobe VOC-compliant. There are no federalgovernment formula specifications for certaintypes of coatings, such as epoxy mastics.

Performance Specification

Performance specifications allow more latitudein the formulation of the coating than formulaspecifications, with performance requirementsgreatly increased. Performance usually is

specified as the minimum requirement for abattery of accelerated laboratory tests.Organizations such as SSPC, which have turnedto performance-type specifications, may requirea field history as part of the acceptance criteria.Performance specification enables a coating typeto be specified, even for VOC compliance,without severely limiting the ingenuity of theformulator in a time of changing technology andregulations. However, with performancespecification, there is no correlation betweenhours of exposure in an accelerated laboratorytest and years of exposure in the field, orexposure in one field environment and successfulperformance in another. A properly writtenperformance specification will eliminateunacceptable coatings, but will not differentiateamong acceptable coatings.

Coatings that pass the requirements of aperformance specification are often included on aqualified products list (QPL). A test program isdeveloped and coatings from differentmanufacturers are evaluated. Those that meet thecriteria or minimum levels of the test programare placed on the QPL. The federal governmenthas QPLs associated with some specifications.Some government agencies, such as the NationalAeronautics and Space Administration (NASA),have evaluated specific coating systems fromdifferent suppliers and reported the results in aform that can be used as a QPL. Other largeusers also have developed QPLs for coatingsystems they use. Michigan Department ofTransportation, for example, has a QPL for asystem consisting of an epoxy polyamide zinc-rich primer/epoxy polyamide intermediate coat/aliphatic polyurethane topcoat based on a batteryof accelerated laboratory tests. Privatecompanies also have developed QPLs based onexposure tests at their facilities.

As an alternative to QPLs, some users haveestablished lists of qualified suppliers or vendorsbased on criteria such as technical service, productavailability, and limited performance testing.

13


User Experience

Another method for specifying paints is selectingproducts from manufacturers with which theuser/specifier has had good experience. Thismethod is not as formalized as a QPL test, but itdoes provide a level of confidence in the qualityof the product. It is important that coatingperformance and not applicator performance beevaluated. Even a test patch program can beuseful. Test patches of coatings from differentsuppliers can be applied to a structure andevaluated regularly. Unacceptable coatings willbe evident relatively quickly, so usefulinformation can be generated within a few years.Another evaluation method is to contactreferences obtained from the coatingmanufacturer or supplier for similar projectswhere the recommended coating system was used.

COST

There are a number of methods for evaluating thecost of coating materials. All too often, thechoice of coating material is based on the costper gallon. However, this cost can be misleading.

Coatings are applied to achieve a certain dry filmthickness. Therefore, the percent solids byvolume affects the true cost of the material. Forexample, a coating that costs $20 per gallon($5.28 per liter) at 70 percent solids and isapplied at 3 mils (75 micrometers) dry filmthickness has a theoretical materials cost (100percent transfer efficiency on a flat surface) of$0.053 per square foot ($0.57 per square meter).However, a coating that costs $18 per gallon($4.76 per liter) and is applied at the samethickness but has 50 percent solids has atheoretical materials cost of $0.067 per squarefoot ($0.72 per square meter). The lower solidsformulation is about 25 percent more expensivethan the higher solids formulation.

Actually, the cost of the coating material isusually a small percentage of the cost of the

overall project. Surface preparation and coatingapplication are much larger cost factors. Breevortand Roebuck4 have gathered information on thecosts of coating projects from around the UnitedStates. For example, using a zinc-rich primer/epoxy polyamide intermediate coat/polyurethanetopcoat over a near-white blast-cleaned surface,the average costs per square foot (square meter)in 1992 dollars were

Surface preparation (field) $1.00 ($10.76) 46%Coating application $0.85 ($9.15) 38%Coating materials $0.34 ($3.66) 16%Total $2.19 ($23.57) 100%

The coating materials represent about 16 percentof the cost. If the project had included the removalof a lead-based coating system, environmentaland worker protection items would have increasedthe cost of the project by a factor of 2 or 3. Thecost of the coating materials would then be about5 to 7 percent of the cost of the project.

The preferred method for selecting a coatingmaterial is not based on cost per gallon (or costper liter). Rather, it is based on the life cycle costof the installed coating system per year of service(cost/ft2/yr or cost/m2/yr). While it is possible toobtain a fairly accurate estimate of the cost persquare foot (cost/ft2) or cost per square meter(cost/m2), determining the years of service can bemore difficult. In many cases, records aboutyears of service are not kept, the coatingstechnology currently in place is out of date, ornewer coatings have not been on the market longenough to determine their life. To compensatefor lack of data, estimates have been made of theexpected life of coating systems. Theseestimates include information gathered byBreevort and Roebuck4 for industrial exposuresand estimates made by Hare5 for bridges.

CONCLUSION

Basic knowledge of coatings technology willassist the user and specifier in implementing a

14


cost-effective coatings program. This appendixhas presented important concepts used incoatings. Generalizations have been presentedon classes of coatings materials. These weremeant for illustrative purposes. The rest of thisbook contains more detailed information aboutthe major generic types of coatings commonlyused in industrial maintenance painting.

NOTES

1. Paint/Coatings Dictionary (Philadelphia, PA:Federation of Societies for Coatings Technology,1978).

2. Joseph A. Bruno Jr., ed., IndustrialMaintenance Coatings Glossary (Pittsburgh, PA:Technology Publishing Company, 1994).

3. Dwight Weldon, “Understanding Test Datafrom Coatings Manufacturers’ Product DataSheets,” Journal of Protective Coatings andLinings (Pittsburgh, PA: Technology PublishingCompany, May 1993), 52-59.

4. Gordon H. Brevoort and A. H. Roebuck, “AReview and Update of ‘The Paint and CoatingsCost and Selection Guide,’” MaterialsPerformance (Houston, TX: NACEInternational, April 1993), 31-45.

5. Clive H. Hare, “Protective Coatings forBridge Steel,” NCHRP Synthesis of HighwayPractice 136 (Washington, DC: TransportationResearch Board, December 1987).

ACKNOWLEDGMENTS

The editors gratefully acknowledge Stephen G.Pinney and Kenneth B. Tator for their time andeffort in reviewing this chapter.

basic coatings technology

Documents