Table Of Content1
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
