1 MECHANICALPROPERTIESOFSOLIDCOATINGS Mechanical Properties of of many application methods is a liquid coating. After the liquid coating is dried by evaporative means or is Solid Coatings cured (cross-linked) by oxidative, thermal, ultraviolet light or other method it is a solid coating film. This article will deal for the most part with solid coating JosephV.Koleske films and the properties of liquid coating films can be Charleston,USA found elsewhere in the encyclopedia. Coatings can be colorless or colored; they are thin, solid films that are transparent,translucent,oropaqueinnature.Theterms ‘‘coatings’’and‘‘paint’’shouldbesynonymous,butoften 1 Introduction 1 coatingsrefertoindustrialcoatingssuchasthoseusedon 2 History 2 appliances,officefurniture,paper,automobiles,beverage 3 Architectural Coatings 2 cans, etc. and paint refers to architectural coatings such 4 Industrial Coatings 3 as house paints, wall and ceiling paints, trim paints, etc. 4.1 Film Formation 3 Thematerials usedbyartists.1/ arereferredtoasartists’ 4.2 Mechanical Properties 4 paints.Whilethis isnotacleardistinction, itisusefulto 4.3 Flexibility and Toughness 6 beawareofageneraldifferentiation inthetermsandof 4.4 Adhesion 7 thewaytheyareusedonadailybasis.Consideralsothat 4.5 Hardness 8 coatingsareoftenreferredtoasfinishes. 4.6 Abrasion 8 Paints can be defined as dispersions of pigments, 4.7 Slip 9 optionally including fillers, in a fluid vehicle. The fluid 4.8 Stress in Coatings 10 vehiclecomprisesaliquidbinderthatwillsolidifyduring 4.9 Chemical Resistance 11 cure and, if necessary, a liquid carrier that serves as a viscosity reducing aid and will provide desirable 5 End-uses 11 application characteristics. The liquid carrier is lost to 5.1 Tests Required for Specific End- thelocalenvironment orrecoveredduringthedryingor uses 11 curing process. The binder portion of the fluid vehicle Abbreviations and Acronyms 12 is an oil, dissolved polymer, and, when appropriate, a Related Articles 12 cross-linking agent, or a dispersed polymer in latex or References 12 other dispersion form. The binder holds the pigment, fillerparticles,andmiscellaneousformulatingingredients when it solidifies into a film on exposure to air or some othercuringmedia.Varnish,atermthatisoftenreplaced Coatings can be liquid or solid materials; they have been bythetermclearcoating,isaclearortransparentsolution known and used from the time of early man and are of thatsolidifiesintoafunctionalfilm.Lacquersareopaque majorimportanceforprotectinganddecoratingthemyriad and/or colored varnishes. Collectively or individually, ofitemsinusetoday. Thesethinprotectivefilmsareused paints,varnishes,andlacquersaretermedcoatings. tocoatcommercialandresidentialbuildings(architectural Coatingscanbefunctionaland/ordecorativeinnature. coatings) and for many products in use today (industrial One need only to look at almost any surroundings to coatings). This article is concerned with the formation see how widely coatings are used. Many things that can of coating films and the properties of these films. Static beseenarecoatedwithadecorativeand/orafunctional anddynamicmechanicalproperties,flexibility,toughness, material. For example, within an office there might be adhesion, hardness, abrasion resistance, slip, chemical wallsandceilingcoatedwithacoloreddecorativepaint;a resistance, and stress within coatings are discussed and desk coated with a clear, functional finish, which may waystoinvestigatethesecharacteristicsaregiven.Someof be applied over a colored, decorative stain coating; the end-uses and the relationship of coating properties to alternatively the desk could be coated with a colored, theusesaredetailed. opaque,functionalanddecorativecoating;afloorcoated inamanner similar to that ofthe deskwith afunctional and decorative coating; there may also be an aluminum 1 INTRODUCTION beverage can coated on the outside with a decorative coating that serves as an advertising and identification Theterm‘‘coatingorcoatings’’isusedtodesignateliquid mediumandcoatedontheinsidewithafunctionalcoating or solid materials. A product that is to be applied to a that protects the metal from the chemical nature of the substrate in a continuous or discontinuous film by one can’s contents. In addition to the above, there may be EncyclopediaofAnalyticalChemistry EditedbyRobertA.Meyers.(cid:211) JohnWiley&SonsLtd,Chichester.ISBN0471976709 2 COATINGS many books, perhaps a newspaper, and other printed themummificationprocess(thewordmummyisderived material.Someoftheseareprintedoncoatedpaper,and from the Persian word mumia, which means bitumen the printing in itself is a coating that is discontinuous in or pitch).3;4/ and it is said that Noah’s ark was coated nature.Thebookjacketsandsomeoftheillustrationsin with pitch. About 2500 years ago, Egyptians developed the books are continuous-film coatings. Sheets of labels clear varnishes by heating amber and vegetable oils; found in an office are coated with a pressure-sensitive coloredcoatingsorlacquersweremadebyaddingground adhesive. The list could go on, but it is readily apparent mineralssuchasmalachite,azurite,andironcompounds thatwearesurroundedbycoatings. to the clear varnishes. Early Romans developed the Decorative coatings can be brilliant and bright to method of fresco painting in which paints composed of attractvisualsensesquickly;pastelandsofttoprovidea pigments,fillers,andcarriervehicleswereappliedtowet restfulatmosphere; singlecolorormulticolor toprovide plasterforinteriorsurfacecoatingsandpaintings.Ancient avarietyofdramaticeffects;glossy,semi-glossy,ormatt Hebrewsusedmilk-basedpaintsfordecoratingwallsand toprovidedifferentmoodeffectsortoaffectcleanability; ceilings. Over 3000 years ago, the Japanese developed rough or smooth, and so on. Functional coatings often lacquersbasedonsapfromthevarnishtree(theJapanese protectsubstrates–wood,metal,plastic,orother–from sumac,Rhusverniciflua).Incolonialdays,water-slacked the ravages of nature, which often provide a hostile lime–whitewash–was extensively used for a variety of environmentandcancausewearthroughrusting,erosion, coating purposes..5/ Later whitewash was modified with light attack, etc. The protection provided by functional milk and protein-based materials to improve durability coatings saves natural resources and is friendly to the and adhesion. This was followed by the addition of environment because it minimizes corrosion and other pigments, fillers such as clay, and whiting to provide meansofdegradation,thusallowingasubstratetolastfor a variety of products with improved aesthetic appeal a much longer period than it would without protection. and economics. Finally, the lime was replaced with milk Suchcoatingscouldalsoprotectanexpensivepartofan phosphoproteins, and whitewash became casein paint, a assembly that cannot be reached or is difficult to reach forerunneroftoday’sarchitecturalpaints. forrepair,suchastheroleconformalcoatingplaysinthe Although paintswereusedandmodified insuchways electronics industry. Here the coatings protect printed for centuries, it was not until the Industrial Revolution circuitassembliesfromthehostileenvironmentfoundin (1700–1950)thatthepaintandcoatingindustrytookon outer space and sometimes within plants, laboratories, far-reaching importance. This historical period resulted orlivingquarters.Pressure-sensitiveadhesivesappliedto in the production of a multitude of bridges, factories, the back of heavily coated paper or polyester film form manufacturing machinery, and allied equipment, as well widelyusedfunctionallabelproductsofvariousdesigns. as other items; all of these needed to be coated to Coatings canbedivided intotwo broadgroups: archi- provide protection from hostile natural and derived tectural coatings and industrial coatings. Architectural environments. Protection had to be provided from coatings are those used in decorating and protecting moisture, salt water, barnacles, mildew, mold, wind, houses and other buildings. If the coatings are used on rain, hail, heat and cold, sunlight, sulfurous fumes from the portion of the building exposed to atmospheric con- coal fired furnaces, etc. This protection was provided ditions, they aretermedexterior coatings. Suchcoatings by coatings that extended the lifetime of the world’s areusuallymadefrommaterialsthatarenotlight(radia- infrastructure, the manufacturing facilities, and the ever tion)sensitiveorarestabilizedagainstattackbyradiation increasing number of manufactured items. Today, the of different wavelengths. If they are used on the inner same items plus a myriad of other items–packaging, portionsofthebuildings,theyarereferredtoasinterior cars, trucks, trains, boats, furniture, beverage can liners, coatings; this group is further broken down into ceiling wallpaper, etc.–that grew out of the ever expanding paints,wallpaints,varnishes,masonrypaints,andstains. manufacturingbasearecoatedwithaprotectiveandvery often aesthetically pleasing film that is usually thinner thanasheetofwritingpaper.Ifitwerenotforpaintand coatings, our world would certainly be a dull, corroding 2 HISTORY placeinwhichtolive. Earlyhumans usedplant extracts,treesaps,animal fats, berryjuices, and metal oxides to create paints that were 3 ARCHITECTURALCOATINGS usedtodecorateandcommunicatebymeansofpictures, oftenoncavewalls..2;3/Suchpictureshaveremainedwell definedandvibrantforover15000years.EarlyEgyptians Architecturalcoatingsarethosecoatingsusedoninterior coated dead bodies with bitumen and other materials in and/or exterior surfaces such as those found in or 3 MECHANICALPROPERTIESOFSOLIDCOATINGS on commercial, institutional, industrial, and residential changesthatwouldbeencounteredintheenvironment..16/ buildingsaswellasonvariousstructuressuchasbridges..6/ Accelerated exposure test results are often difficult to Thesurfacescoatedarewood,metal,composition,plaster reproduce and may not correlate with actual or natural andwallboard,plastic,ormasonry.Thecoatingsmaybe exposure testing. However, such testing is widely used latex, alkyd, oil, solvent borne, and so on, and they are because natural weathering can take years to effect applied by brushing, spraying, rolling, as well as other changes,andtheacceleratedtestsdogiveagoodindica- methods. Gloss is an important optical characteristic of tion of coatings that will fail early. They areparticularly thesecoatingsandvariesfromlowtohighdependingon important for comparison purposes, for new product any particular end-use. These coatings are often termed development, and to improve the durability of existing tradesalespaints,becausetheyareusuallypurchasedby products. consumers who will apply them on-site under ambient Naturalweathering.17/ isatruemeasureoftheravages conditions. of nature; however, it is reproducible only if properly Standard methods are available to test the character- planned, conducted, inspected, and reported. Natural istics of architectural coatings..7–10/ Abrasion, dry and weathering tests are not carried out by merely placing wetadhesion,flexibility,chemicalresistance,blockresis- atestspecimenoutdoorsandlettingsunlight,rain,andso tance, print resistance, and cleanability are important onfallonit.Rather,thetestingiscarriedoutatselected properties. siteswiththeexposedspecimenssetataparticularangle. Abrasion resistance, adhesion, and flexibility are dis- Conditions such as these affect the four major factors cussed elsewhere in this article. Mechanical properties listed above and allow other factors such as humidity or of architectural coatings are not usually directly mea- lackofit,biodegradation,andpollutiontobebroughtinto sured, but rather such properties manifest themselves the testing scheme. The synergistic interaction of these in the results of an end-use-related test. Testing of factors, which will vary with exposure site, determines architectural coatings and industrial coatings is often a how a coating’s failure through outdoor exposure takes measureofacomplexinteractionofvariouscoatingphys- place. The variation of solar energy radiation dosage icalcharacteristics.Forexample,washabilityandrelated with season and with the angle of the test specimen to characteristics.11–14/ of a wall coating requires the coat- the sun is an important variable to be considered when ingtobechemicallyresistanttowateranddetergentand reproducibletestresultsareexpected.Thecloserthetest tobe sufficiently strong to withstand a scrubbing action conditions simulate the actual use conditions, the more thatappliestensileandshearstressestothecoatingasits accuratewillbethepredictionoflong-termresults. hardness,adhesion,cohesion,andabrasionresistanceare broughtintoplay. Ease of application and aesthetic characteristics are 4 INDUSTRIALCOATINGS moreimportantthanmechanicalcharacteristicsforinte- rior coatings. However, mechanical and other physical Industrial coatings are coatings applied to factory- properties are important to exterior architectural coat- manufactured products. These include, but are not ingsthat are subjected to outdoor exposure. Exterior limitedto,transportationcoatings(thosecoatingsapplied coatingsareexposedtoheatandassociatedtemperature to aircraft, appliances, automobiles, buses, recreational changes, moisture, oxygen, and sunlight. These factors vehicles, trucks, and trains) beverage-can and spray-can individually or in concert contribute to coating failure. coatings, packaging items, business machine and office Temperature changes alter properties and can result furniture coatings, wood cabinet and furniture coatings, in significant alternating strains, along with concomi- pipeline coatings, printed circuit board and assembly tant stresses,.15/ being placed on the coating–substrate coatings, sign coatings, marine coatings, and masonry matrix.This,coupledwiththeeffectsofexposuretorel- coatings. atively high temperatures, results in cracking, checking, embrittlement, and peeling. Moisture can cause blister- 4.1 FilmFormation ing, erosion, loss of adhesion, and mildew. Oxygen can causesurfacedegradationandeventuallyinternaldegra- Coatingfilmsareformedfromeitherthermoplastic(sol- dation,resultinginembrittlement,cracking,andcrazing. uble) or thermoset (insoluble) polymers combined with Sunlight and, in particular, the ultraviolet light compo- other ingredients, including, if desired, but not limited nent of sunlight can cause surface chalking and loss of to, pigments, fillers, colorants, plasticizers, surfactants, gloss, degradation, and embrittlement, with accompany- solvents, catalysts, or initiators..18–20/ Thermoset coat- ingcrackinganddiscoloration. ings are formed from initially soluble ingredients that Effects of outdoor exposure are often measured with react and undergo a change termed cross-linking as a test devices that attempt to simulate and accelerate consequenceofanenergyinput. 4 COATINGS When thermoplastic polymers are dissolved in a offilms,.23/fordeterminationofminimumfilmformation solvent, films are formed by evaporation of the solvent andcoalescencetemperaturesofaqueousdispersions,.24/ underambientconditionsorinthepresenceofcontrolled for permeability,.25/ for block resistance,.26/ and for heating.Sincephysicalcharacteristicsofthefinalcoating numerous other particular physical factors. A number are dependent on the polymer’s properties, polymers ofthephysicalpropertytestsarebrieflydescribedbelow. of high or relatively high molecular weight are used. The degree of cure or solvent resistance of films, Thehighmolecularweightlimitstheamountofpolymer particularly thermoset coatings, is often determined by that can be dissolved because the viscosity of the final means of a solvent rubbing procedure that is formalized system must be sufficiently low to allow the coating to for zinc-rich, ethyl silicate coatings..27/ A gauze cloth be applied by brush, spray, roll-coater, etc. Polymers is made into a pad and saturated with a solvent such thatareusedtoformcoatingfilmsincludenitrocellulose, as methyl ethyl ketone or acetone. Then, using thumb cellulose acetate butyrate, vinyl chloride/vinyl acetate pressure, the solvent-wet pad is rubbed back and forth copolymers and terpolymers, poly(vinyl acetate), and overthecoating.Althoughthetestissaidtobeimprecise poly(methyl methacrylate). Coatings such as these are becauseofvariabilityinthumbpressurefromoperatorto easilyremovablewithasolventormarredbyaplasticizing operator,itdoesprovidequick,usefulresults,particularly compound; in addition particular liquids can attack the on a comparative basis. Other tests can be found to coating,forexamplewateroralcoholcancausearingon determinesolventresistance..28–31/ nitrocellulose-coatedfurniture. Thermosetcoatingsareproducedwhenmultifunctional 4.2 MechanicalProperties low-molecular-weightpolymersoroligomersarereacted with amultifunctional cross-linking compound that con- Mechanical properties such as tensile strength, elonga- tains appropriate functionality. If a solvent is present, it tion,andtoughnessandrelatedparametersareimportant is removed by evaporation to form a film, which is then characteristics of coatings..32/ Outdoor coatings must heatedtoeffectreactionbetweenthedifferentfunction- withstandtherigorsofthetensileandcompressiveforces alities in the presence of (or without) a catalyst. If the that occur and cause expansion and compression during cross-linkingreactionisbetweenhydroxyl-containingand each day and with the changing seasons as the temper- isocyanato-containingorepoxide-containingcompounds, ature changes. Hail and sleet challenge the toughness forexample,thereactionproceedsbyanadditionprocess of coatings. The effects of temperature on coatings for without emissions to form either urethane linkages or wood substrates are further complicated by the differ- etherlinkages(Scheme1). ences in hardness, chemical composition, and expansion If the reaction is between a hydroxyl-containing com- coefficientsofspringwoodandsummerwood.Obviously, pound and, for example, a methoxymelamine, material these changes occur many, many times over the course is lost by emission of a volatile by-product, in this case of a number of years. Many coatings are applied to a methanol(Scheme2). substratebeforeformingoperationsarecarriedout.Con- In addition to forming films by evaporation of sol- siderthemetalwhitecapsthatareappliedtoavarietyof vent from solutions of polymers and oligomers, films packagedfoodstuffs.Thecapsareformedafterthewhite can be formed from aqueous and non-aqueous disper- coatingisappliedtosheetsteel.Thecoatedmetalissub- sions, organosols, plastisols, electrodeposition, powders, jectedtoseveretensileandshearforcesduringthestrong and radiation-activated systems. The American Society bendingandtwistingoperationsrequiredtoformthecap. for Testing and Materials (ASTM) and other organi- The coating–steel composite must have excellent adhe- zations have a variety of test methods for determining sion, strength, and toughness to withstand the torturous various aspects of film formation and the films formed. operation..33/ Three-piecebeveragecansundergosevere These include detailed procedures for preparing.21/ and bending and twisting when the can ends are combined testing.22/organicfilms,fordrying,curing,andformation withthecanbodyinaflangingoperation.Thesameistrue (HO )2 OH + NCO NCO (HO )2 O N C NCO H O Urethane linkage Scheme1 N N N(CH2OCH3)2 + (HO )2 OH (HO )2 OCH2N + CH3OH CH OCH 2 3 Scheme2 5 MECHANICALPROPERTIESOFSOLIDCOATINGS when the lid is attached to a two-piece can. In addition, duringthestressingprocess.Tocircumventthisproblem, whenthefilledcansaretransportedbytruck,thecoated, whenmeasuredunderstaticconditions,thevalueofEis filledcanisagainsubjectedtotwistingandflexingduring calculated by measuring the stress at some stated, fixed handlingandwitheachbumpandturnintheroadduring strain, usually 1% or less, and dividing it by the strain. transportation. The severity of these operations is such Such a measured value ofE is denoted as the secant that pin holes and failure can develop in the container modulus.34/ measuredattheselectedstrain. andcauseproductlossaswellaspotentialliability. Dynamic mechanical analysis (DMA) is a technique Tensile properties are those characteristics that a that enables the two componentsE0 andE00, which material exhibits when a uniaxial forceF is applied togetherareknownasthecomplextensilemodulus,E(cid:3),to to a specimen of lengthL and cross-sectional areaA, bedistinguishedandmeasured..35;36/ InDMA,thestress 0 as depicted in Figure1. Under no-load conditions, the orstraindescribedinFigure1isappliedtothespecimen specimen is at rest. When the force is applied, the in an oscillatory manner that is usually described as specimen experiences a tensile stress,s, that is equal beingsinusoidalinnature,thoughtheexactnatureofthe totheappliedforceperunitareaandanelongationtoa deformation depends on the particular instrument used finallength,L,involvingatensilestrain,e,whichisgiven for the measurements. A sample is held under sufficient by change in length per unit length or .L(cid:0)L /=L . In tensionthatitisnotlimpatthelowestappliedoscillatory 0 0 theregionwherestressisdirectlyproportionaltostrain, strain.Althoughtheimpressedwavemotionisthesame, a tensile modulus,E, can be defined as the slope of the say sinusoidal, for the stress and the strain, because of stress–strainrelationship as shownin Equations(1)and thematerial’sviscoelasticnaturethesepropertiesareout (2). (Similar expressions can be defined for shear and of phase by an amount or angle,d, the phase lag. For bulkdeformationconditions.) an ideal elastic material,d is zero and the stress and strainareinphase.ForaNewtonianliquid,ifitcouldbe Tensilestress ED so tested, dwould be90(cid:176) . For viscoelastic materials, dis Tensilestrain between0and90(cid:176) .Thesecomponentsofthemoduluscan Forceperunitarea be represented as two vectors that are 90(cid:176) out of phase D .1/ Changeinlengthperunitlength witheachother,asdescribedinFigure2. F 1L s Equations(3–6)describethestresswaveastwowaves, ED (cid:4) D .2/ one in phase with the strain (the elastic response) and A L e 0 the other 90(cid:176) out of phase with the strain (the viscous However,polymersandcoatingsactuallyhavenomeasur- response). ableregionwherestressisproportionaltostrainbecause s cosd polymermoleculescanflowunderanappliedforce.That Storagemodulus E0 D t .3/ is,polymersarenotelasticinnature;rathertheyarevis- e coelasticandslowlyflowwhenplacedunderconditionsof s sind Lossmodulus E00 D t .4/ loading.Themodulusofviscoelasticmaterialshasastor- e ageorcompletelyrecoverableelasticcomponent(E0)and Complexmodulus E(cid:3) D[E02CE002]1=2 .5/ aviscous or loss component(E00) that is not recoverable E00 s sind and the energy of which is lost through viscous heating Losstangent D t Dtand .6/ E0 s cosd t Area A Thecomplextensilemodulus,E(cid:3),ortheonethatisactu- No load ally sensed, can be calculated from the components,E0 andE00, by the Pythagorean theorem. The ratio of the L0 viscous response to the storage response is equal to the Length Tensile loaded F F ¢E E* L 0 Applied d L force E¢¢ New length Figure2 Vectorrepresentationofthecomponents,E0andE00, Figure1 Aspecimenatrestandinatensile-loadedcondition. ofthecomplexshearmodulus,E(cid:3),andthephaseangle,d. 6 COATINGS tangentoftheout-of-phaseangleandisknownastheloss (However,there is no indication in the testmethod that tangent.Whentheseparametersaremeasuredasafunc- elongation determined from tensile studies will yield a tionoftemperature,thelossmodulusandthelosstangent valuerelatedtothecracking-failurepoint.)Themandrel exhibitmaximumsattheglasstransitiontemperature,T , diameter at the point where cracking ceases is reported g and at other temperatures if there are other secondary astheresistancetocrackingresistanceorflexibility. loss mechanisms. Below T , materials are glass-like in The cylindrical mandrel test is a pass/fail test that g nature–hardandbrittle–andaboveT theyaresoftand involves placing the coated substrate over a mandrel, g flexibleinnature.Ifthesystemiscross-linked,aboveT coating side up, and bending the specimen about 180(cid:176) g thematerialwillexhibitanextensiverubberynature. around the mandrel by hand at a uniform velocity in To visualize the storage and loss components of a a specified time. Usually six mandrels having diameters material, consider a rubber ball that is dropped from rangingfrom25mm(1.0in)to3.2mm(0.125in)areused. some height onto a hard, elastic surface. The ball will Thepanelisbentoverthelargestdiametermandreland bounce up and down with a decreasing height as each thenimmediatelyexaminedforcracking.Ifnoneoccurs, succeeding bounce takes place until it is finally at rest. the next smaller mandrel is used and so on until failure The elastic nature of the ball causes it to try and return occursorthesmallestdiametermandrelhasbeenpassed. to the release point by means of stored energy, but the The smallest diameter at which cracking does not occur viscous nature of the ball results in it losing energy as is reported. The test can be used to calculate coating heat to the surroundings; consequently, the ball reaches elongation. a height somewhat less than the original height. The TheT-bendtestinvolvesplacingacoatedmetalpanel motion is gradually damped until finally the ball ceases with a 50mm (2in) minimum width in a smooth jaw to bounce. One readily knows that a markedly different bench vise and holding it firmly..38/ The panel must be result is obtained if this experiment is performed on a sufficiently long that the needed number of bends can summer day than on a winter day. Actually one can be made, i.e. about 150mm (6in). Then the panel is measure important physical properties of materials by bent 90(cid:176) with the coating on the outside of the bend, this technique, but a number of more practical methods removed,andfurtherbentbyhanduntilthebentendcan thanthisoneexist..36/ be inserted in the vise; the vise is tightened to complete the 180(cid:176) bend. The apex end of the bend should be as flat as possible. This is termed a 0T (zero-T) bend. The 4.3 FlexibilityandToughness bendisthenexaminedwitha5to10powermagnifierfor Flexibility is the ability of a coating to be bent or flexed cracksandpressure-sensitivetapeisappliedandremoved informingoperationswithout cracking,losingadhesion, todetermineifcoatingcanbepicked off.Theprocessis orfailinginsomeothermanner.Toughnessistheability then repeated by placing the bent end in the vise and ofacoatingtowithstandlargestressforcesimposedover bendingthrough180(cid:176) aroundthe0Tbend.Thisformsthe a short time without cracking, rupturing, shattering, or 1Tbend. This is continued for 2T, 3T, etc. bends. The tearing..37/Coatingsmustproperlyperformduringmanu- lowest T bend at which no cracks are visible and there facturingoperations,duringuse,andoftenduringmisuse. is no pick off of coating is the value reported. Note that Todothis,theymusthavesufficientflexibilityandtough- the radius of curvature of the bend increases with each ness to withstand failure when subjected to bending succeedingbendandcoatingelongationrequiredtomake and twisting, as is encountered in forming operations, thebenddecreaseswitheachsucceedingbend. toexpansionandshrinkingduringtemperaturechanges, Flexibilityofpipelinecoatingsthataretobesubjected andtomechanicalabuse. to short-radius bends is determined by bending the Flexibility is usually measured by a mandrel bend coated pipe around a designed, variable-radius mandrel test.33/ or a T-bend test..35/ The mandrel bend test to produce a range of short-radius bends..39/ Coating involves bending a coated substrate, usually sheet metal failure is apparent by visual and/or electrical inspection or rubber-type materials, over either a conical mandrel ofcrackingorlossofadhesion. or over cylindrical mandrels of various diameters. The Toughnesscanbedefinedastheabilityofacoatingto standard, smooth-steel, conical mandrel has a length of withstand an impact without cracking or breaking. It is 203mm (8in) and a diameter of 3mm (0.125in) at one dependentonthenatureofthepolymerorpolymersused end and 38mm (1.5in) at the other end. The coated inthecoatingandonadhesion.Impactresistance,which substrate, coating side up, is bent around the mandrel is related to formability, can be measured by dropping withaleverdeviceandtheextentofcracking,ifitexists, a weight from various heights through a guide tube is determined. The distance from the small end of the onto an indenter that rests on the surface of the coated mandrel to the crack is determined visually and can be substrate..40/ The test can be made on the coated side used graphically to determine the percent elongation. (face impact) and/or the uncoated side (reverse impact) 7 MECHANICALPROPERTIESOFSOLIDCOATINGS ofthecoatedsubstrate.Crackingorotherfailureisnoted including plastics, wood and other cellulosics, metals, onoraroundthedimplecausedbytheindenter.Thecited ceramics,etc.Therearetwoaspectsinvolvedinadhesion: ASTMtestgivesthreeproceduresforascertainingfailure: ‘‘basic’’ adhesion, which is the combination of all visualinspection,applicationofanacidifiedcoppersulfate intermolecular and interfacial forces, and ‘‘practical’’ solution, anduseof apinhole detector. Severalimpacts adhesion, which is the work needed to disrupt the aremadeatdifferentimpactvaluesandatthesameimpact adheringcombination.Practicaladhesionisalmostalways value. The value where the force required changes the thequantitymeasuredinthecoatingindustry. resultfrommainlypassingtofailingisthetestend-point. Themostcommonmethodoftestingcoatingadhesion The result at this point is reported as kilogram-meters involvesapplyinganadhesivetapetothecoating,which (inch-pounds)impactresistance. iseitheruncutorcutinsomemanner,andthenremoving A wedge bend device is used to determine impact the tape under specified conditions. The cut surface is resistanceandformabilityofmetalstripsthathavebeen observedandthedegreetowhichthecoatingisremovedis factory coated by a roll coating or other application comparedagainststandards.Thetestisconsideredsimple technique..41/Coatedstripsarebent170–180(cid:176) overa3.2- to perform and low in cost. A widely used test method mm (0.125in)cylindrical mandrelthat is attached tothe forcoatingsonmetallicsubstrates.47/ involvesmakingan impact platform. A 1.82-kg (4lb) guided rod with a flat X-cutinthefilm(methodA)ormakingalatticecutwith end is then dropped onto the test specimen. Variation a device that makes six or eleven cuts in each direction in the height of drop allows the force needed to crack (methodB) with each of the cuts made through the film the coating to be measured. A test that involves high- to the substrate. A transparent, pressure-sensitive tape pressure pressing of an indenter ball into a zinc-rich is applied to the cut area and removed in a prescribed primer-coated metal substrate tests the formability of manner.Thecoatingisthenvisuallyexaminedandrated the coated metal..42/ Formability tests that ascertain the on a zero to five comparison scale. On this scale five flexibility and impact resistance of coatings by stamping indicatesnoremovalandzeroindicatesgreaterthan65% adieintocoatedmetalexist..37/ removal from the scored area, with various descriptions The impact resistance of pipeline coatings is deter- forthevaluesbetweentheextremes.MethodAismeant minedbyalimestonedroptest,.43/afallingweighttest,.44/ tobeusedonthejobandmethodBismeantforuseinthe andapenetration resistancetest..45/ Thelimestonedrop laboratory.Thetestandresultsarequalitativeinnature, test involves dropping weighed amounts of a particular and the results are considered reproducible within one type of limestone through a chute onto a coated pipe. unit when the substrate is metal. On plastic substrates, The number of drops required to penetrate the coating reproducibility is poor since the test is not designed for byeithervisualorelectricalinspectionisreportedasthe relatively soft substrates that are usually coated with impactresistance.Thefallingweighttestissimilartothat brittlecoatings. described above.40/ except special pipe-holding devices Adhesion of coatings to flat substrates can be deter- and impacter surface characteristics are involved. Coat- minedbypushingthepanelbeneatharoundedstylusona ingbreaksorpenetrationsaredetectedbymeasurement balance-beamdevicethatisincreasinglyloadeduntilthe of electrical resistance changes; the impact resistance is coatingisremoved..48/Thisscrapeadhesiontestisusedto the amount of energy required to cause a break. The differentiatethedegreeofadhesiontosubstrates.Itpro- penetration resistancetest involves applying a blunt rod videsrelativeratingvaluesforcoatingswithconsiderably loaded with a dead weight to a coated steel pipe. The differentdegreesofadhesion. depth or rate of penetration of the rod into the coating The pull-off strength.49/ or adhesion of a coating is measured as a function of time. This and any failure is measured by applying an increasing tensile force (crackingorotherpenetration)arereported. perpendicular to the coating surface until a plug of material is detached. Alternatively, this test can be a pass or fail test if a prescribed stress is applied and it is 4.4 Adhesion determinedifthesurfaceremainsintactunderthisstress. The importance of adhesion, the ability of a coating to The tests are carried out with a portable device with a resistremovalfromthesurfacetowhichitisapplied,isself loadingfixturethatissecuredtothecoatingsurfacewith evident..46/ Suchadhesioncanbebetweensubstrateand an adhesive. The adhesive is either a two-part epoxide coating, between a primer coating and a top coating, or acrylic system. The fixture is aligned normal to the between coatings applied to an existing coating, etc. surface,andthetensilestressisappliedinaslow(lessthan In addition, the coating must adhere under various 1MPas(cid:0)1,150psis(cid:0)1),continuousmanneruntilaplugof weathering and cleaning, usually aqueous, conditions. material is removed. The force attained at failure or at The adhesion can be between the same–in a chemical maximumforceappliedisreported.Inaddition,theplug sense–materialsorbetweenabroadvarietyofmaterials is examined to determine the percentage adhesive and 8 COATINGS cohesive failures, and the interfaces and layers involved radius) transparent quartz or sapphire indenter that is infailurearereported. in contact with the coating surface. The load is applied for 60s; after that time, with the loaded indenter still in place, the diameter of the circular impression is rapidly 4.5 Hardness measured.Aninstrumentconstant(1.27)isthendivided Coating hardness is the ability to resist permanent bythesquareoftheindentation diameter inmillimeters indentation, scratching, cutting, and penetration by a toobtainthePfundhardnessnumbers(PHN). hardobject..50/Differentmethodsofevaluatinghardness Numerousothermethodsexistthatdeterminehardness yield different results because they measure different by scraping and indenting as well as by marring and qualities of the material. There is no absolute scale and abrasion..50/ Inaddition,hardnesscanbemeasuredwith eachmethodhasitsownscaleofdefinedhardness. pendulumdampingdevices.53/androckerdevices..54/The Determining hardness by gouging or scratching the threemethodsdescribedaboveareincommonusage. coating with drawing leads or wood pencils of different hardness (from 6B to 6H) is simple and inexpensive; 4.6 Abrasion it is widely used in laboratory development work and production control testing..51/ To conduct the test, the Abrasion resistance is the ability of a coating to resist pencil is sharpened with a draftsman-type sharpener. havingitsoriginalappearanceandstructurealteredwhen The sharpened lead point is then held at a 90(cid:176) angle itissubjectedtotheinfluenceoferosion,rubbing,scrap- to horizontal on No.400 grit abrasive paper and rubbed ing, or other ablative action..55/ Both temperature and untilasmooth,flat,circularcross-sectionisobtained.To environment can have an effect on abrasion resistance, carry out the test, the coated panel is firmly held on a but the relationship between these factors and interre- level surface and the hardest, sharpened pencil is held lated mechanical properties is not simple. For example, on the coating at a 45(cid:176) angle. The pencil is then pushed hardnessandmodulusincreasewithdecreasingtempera- awayfromtheoperatorwhileusingsufficientdownward ture, and this may be detrimental to abrasion resistance pressure to either cut through (gouge) or scratch the if the coating loses flexibility or toughness. Increases in filmortocrumpletheedgeofthelead.Thisprocedureis humidity around an object or subjecting an object to a repeatedwithsofterandsofterleadsuntilapencilisfound moistenvironmentasinwashingawallcansoftenacoat- thatwillnotcutthroughorscratchthecoating.Thegouge ingandalteritsresistancetoabrasion.Otherfactorsthat hardnessisreportedasthehardestpencilthatwillleave canhave an effect on abrasion resistanceinclude tough- the coating uncut for a push stroke of at least 3mm ness,strength,andothermechanicalproperties.Because (0.125in).Thescratchhardnessisreportedasthehardest of the complex interrelationship between characteristics pencil that will not scratch the coating. Because of the that affect abrasion resistance, it is important that the nature of this test, it is operator dependent and results testmethodsubjectstestspecimenstoconditionsthatare mayvarybetweendifferentoperatorsandlaboratories. similartothoseencounteredinactualuse. Indentation hardness of coatings is determined with Many, it not almost all, coated items are subjected to sophisticated devices that determine the resistance to some sort of ablative action, and such action can cause penetration by an indenter..52/ Knoop indentation hard- marring and/or wear. These items include appliances, ness (methodA) is determined by bringing a pyramidal automobiles and other transportation equipment, bev- diamondindenterintocontactwiththecoatingandthen erage cans, business machines, farm equipment, floors, applyingaselectedloadtotheindenterandmaintaining furniture, highways (traffic paints), interior and exterior theloadfor18(cid:6)0:5s.Afterthistime,theindenteriswith- structural walls, and so on. The actions include: wind, drawn.TheKnoopdeviceisequippedwithamicroscope rain, hail, and other natural periodic actions; wear that that has a movable micrometer stage; immediately after can be relatively continuous, such as automobile traffic theindenteriswithdrawn,themicroscopeisadjustedand orwalking;polishingorotherrubbingoffurnitureoran focusedsothattheindentationisinthefieldofthemicro- automobile with a harsh fabric; or accidental contact of scope.Withtheindentationsharplyfocused,thelengthof acoatedsurfacewithabutton,atoy,orasimilarobject. the long indentation diagonal is determined. The inden- An example of rubbing effects caused by similar objects tationlengthisconvertedintoKnoophardnessnumbers rubbingagainsteachotherarethosethatareencountered (KHN) with tables supplied by the instrument manu- whenbeveragecansrubagainsteachotherinamultipack facturer. If the tables are not available, KHN hardness duringshippingandhandling..56/ numberscanbecalculated. Abrasion resistance of coatings applied to flat, rigid Pfundindentationhardness(methodB)isdetermined surfacescanbemeasuredbyrotatingthecoatingagainst withadeviceequippedwithamicroscopethatwillapply an abrasive-filled, weighted wheel..57/ The results are a1.0-kg(2.2lb)loadtoahemispherical(3.18mm,0.125in reportedasthenumberofcyclestoremoveaunitamount 9 MECHANICALPROPERTIESOFSOLIDCOATINGS ofcoating(wearcyclesper25.4m m),asthelossinweight guide tube from a specified height onto a coated planar per cycle multiplied by 1000 (wear index), or as the surface..65/ Silica (sand) is a milder abrasive than silicon weight loss determined at a specific number of cycles carbide and the slower rate of abrasion it causes can (weightloss).Althoughthistestmethodisfairlysimpleto be useful in discriminating between different coatings. carryout,reproducibilityispoor.Itisrecommendedthat Basically this test differs from the previous test in that resultsbelimitedtotestinginonlyonelaboratorywhen the ablative contacts the coating under a gravity flow numerical values are to be used. Agreement of results rate rather than an air-forced flow rate. The abrasion between laboratories is markedly improved if different resistanceisdeterminedasthekilogramsofablativeused coatingsaremerelyrankedratherthantryingtocompare per25.4-m mfilmthickness. numericalvalues. Coatingsonnon-planarsurfacessuchasthosefoundon 4.7 Slip pipelinesaretestedforabrasionresistancebyplacingthe externallycoatedpipethroughaspeciallydesignedsteel Usually slip,.66/ meaning the opposite of traction or drumapparatusanderodingitwithanaqueous,abrasive clinging, is not an inherent property of coatings unless slurry contained in the horizontally revolving drum..58/ one is discussing the tetrafluoroethylene polymers and Thespecimensareelectricallyinsulatedfromcontactwith copolymers. Coatings are said to have good slip when the drum. The test is applicable to all types of electrical theyhavealowcoefficientoffrictionandpoorslipwhen insulatingcoatingincludingthermoplasticandthermoset theyhaveahighcoefficientoffriction.Slipindicatesthe coatings and bituminous materials. Measurement of easewithwhichtwocontactingsurfacescanmovebyeach electrical resistance changes between the pipe and the other. Coatings are said to have slip when they have a drumasthecoatingerodesindicatesthecoatingabrasion tack-freesurfaceandbehaveasiftheywerelubricated. resistance. Because of electrical requirements, metallic Slip is an important characteristic of coated objects protective coatings such as zinc are not tested by this for it is the property that allows coated materials to method. However, such coatings and others designed slidebyoneanotherinformingoperations,duringfilling, to function as electrical barriers are tested for cathodic handlingandshipping,aswellasinothermanufacturing disbondingbyothertests..59;60/ and use operations. However, it is worth pointing out Paintedinteriorhousewallsaresoiledneardoorways, that the surface can be too slippery, that is have too windows, play areas, cooking areas, etc. Such soiled lowacoefficientoffriction.Imagineabeveragecanwith areasand often the entirewalls arecleaned byrepeated asurface so slippery that it could not easily be held in a scrubbings,andduringthescrubbingthepaintissubjected person’shand.Also,alowcoefficientoffrictioncanbean to corrosion. The relative erosion resistance of interior, undesirable characteristic in floor coatings, since people flat wall paints to wet scrubbing can be determined by walkingonthesurfacecouldslipandfallorvehiclescould applying the paint to particular black plastic panels and slideandcausedamageorharm.Gymnasiumfloors,porch scrubbing with a nylon bristle brush that is wet with and deck floors, concrete work-area floors, and kitchen an aqueous, detergent-based scrub medium..61/ The wet floorsareareaswherethisisofparticularconcern. brushisdriveninonedirectionacrossthecoatedsurface; Slip can be imparted to films by incorporating a after each set of 400 cycles the brush is removed, the compoundintoacoatingformulationthatisincompatible scrub medium is replenished, and the brush is replaced. with the dried or cured coating; it will then exudeto This procedure is repeated until the paint film has been the surface of the coating. A way of imparting slip removed.Thenumberofcyclestofailureisreported.The toflat,coatedmetalsheetsistospraylightlyaverylow- test is designed for freshly painted surfaces rather than volatilitylubricantontothecoatingjustafterthecoating aged surfaces. The degree of erosion of exterior paints, iscuredandpriortostackingforthenextmanufacturing which occurs mainly by chalking, can be determined by operation. Compounds such as wax esters, fatty esters, comparison of the surface with pictorial standards..62/ alkanolamides,metallicstearates,waxes,andsiliconesare Pictorialcomparisonsarealsousedtoevaluatethewear usedtodecreasefrictionalresistanceortocontrolslip. resistanceoftrafficpaints..63/ Slipis determined bymeasuringthefrictional proper- Abrasion resistance is also determined by air-blasting tiesofcoatings.Frictionistheforcebetweensurfacesthat siliconcarbidegrains atthe coatedpanel ata45gmin(cid:0)1 opposes imposed sliding motion. It is the characteristic flowrate..64/ Theabatingiscontinueduntilthecoatingis that determines the resistance to slip or the magnitude worn through. At this point, the blasting is terminated, ofslip. and the amount of ablative used is determined. The In one method,.67/ the static friction of coatings is abrasionresistanceisdeterminedasthegramsofablative determinedbyaninclinedplaneslidingtestorahorizontal used per 25.4-m m film thickness. A similar test involves pull test. The inclined plane test employs one or more dropping a silica or silicon carbide abrasive through a weightedsledsthatareindividuallyplacedonthecoated 10 COATINGS surface, which is fixed to a flat, movable surface. The coatingthatisformedfromasolutionofpolymer,thisis movablesurfaceistheninclinedfromthehorizontalata atthepointwheretheT ofthesolvent/polymersolution g rate of 1:5(cid:6)0:5(cid:176) s(cid:0)1 until the sled begins to slide down isequaltotheexperimentaltemperature. the inclined coating surface. Thetangent of the angle of If it is assumed that the internal stress is in a plane inclination at this point is reported as the static friction. parallel to the substrate and is isotropic in nature, the The horizontal pull test has a weighted sled placed on a internalstraine canbedescribedbyEquation(7): i specimenthatisfixedtoaflat,horizontalbase.Thesledis V (cid:0)V thenpulledacrossthespecimenwithamechanicalpower e D s t .7/ i 3V unit, and the force required to start the sled movingis s determined. This force divided by the mass of the sled whereV isthecoatingvolumeatthesolidificationpoint s isreportedasthestaticfriction.Staticfrictiondetermined andV isthecoatingvolumeattimetaftersolidification. t by this method is useful for ascertaining the slipperiness It is readily apparent that, as the volume decreases as a of floor polishes, the slip resistance of footwear on floor function of timeduring finalfilm formation, theinternal tiles and floor coatings, the appropriateness of coatings strainand,therefore,theinternalstressincrease. for the exterior of cans, etc. The measurements are also Changes in temperature will cause the dimensions usefultodeterminetheeffectofcoatingadditivesorspray of a coating/substrate combination to change. Since lubricants on the slipperiness of coatings. A number of the expansion coefficients of the coating,a , and the c;T methods for determining friction can be found in the substrate,a , are almost always different, an internal s;T literature..66/ strain,e ,issetup.ThisisdescribedbyEquation(8). T e D.a (cid:0)a /1T .8/ 4.8 StressinCoatings T c;T s;T Stresses can develop within coatings during film forma- Since absorption and desorption of water can cause tion,throughtemperaturechanges,andthroughrelative similar changes in dimensions of the coating,ac;RH, humidity (RH) changes..15/ These internal stresses have andsubstrate,as;RH,Equation(9)similarlyexpressesthe an effect on coating degradation. They affect adhesion internalstrainthatiscausedbyRHchanges. and/or cohesion and have an effect on delamination e D.a (cid:0)a /1RH .9/ and cracking. Thermoset coatings have higher internal RH c;RH s;RH stresses than coatings that do not involve cross-linking Thesestressesacttogetherandmayaugmenteachother compounds,suchaslacquersandalkyds. andbeveryimportantortheymaynegateeachotherand Although internal stresses can have a detrimental besmallandrelativelyunimportant(Equation10). effect on adhesion, they originate through the process of adhesion. This seeming paradox can be readily s Ds (cid:6)s (cid:6)s .10/ total i T RH understood if the following is considered. To protect a substrate adequately, good adhesion between the Thecomponents isalwayspositive,butthecontributions i substrate and coating is required. However, adhesion from temperature and RH effects can be positive or causes immobility of the coating at and near this negative. Positive effects occur in coatings that tend to interfacialarea,which,inturn,doesnotallowthecoating contract and set up internal tensile stresses. Negative to move in a normal manner, for example when the effects occur in coatings that tend to expand and set up temperaturechanges. compressivestresses.Adry,coldwinterdaywillinvolve When a solid coating film forms, a liquid is changed lowtemperaturesandlowRH,withhighresultantinternal intoasolid.Whilethefilmisliquid,thecoatingismobile tensile stresses. Conversely, a humid, summer day will and volume contraction can take place with no stress involve high temperatures and RH, with resultant high development. As a solid coating film forms, in almost internalcompressivestresses. every instance contraction continues to take place but There are a number of ways.15/ to measure internal is restricted by adhesion. As a result of this restriction, stresses, including brittle lacquer materials, cantilever tensile stresses develop within the coating. However, as beams,optical, straingauges, and X-raydiffraction. The soon as stress develops, the molecules seek to relieve cantilever beam method is most widely used and gives the stress and a relaxation process begins. Therefore, as suitablemeasurements.Thismethoddependsonthefact film development continues, stresses within the film can thatacoatingunderstressonasubstratewilldeflectinthe increase,decrease,orremainconstantdependingonthe directionthatwillrelievethestress.Therearetwotypes rate of stress development and of stress relaxation. It of cantilever beam used. A one-side coated substrate is shouldbenotedthatstressdevelopmentbeginswhenthe eitherfixedatoneendorisfreelysupportedontwoknife T of the changing system is reached. In the case of a edges. The deflection in either case can be measured; g