Trouble with Paint: Internal Stress-Related Coating System ... · coatings. We will conclude with the effects of film thickness and formu-lation on internal stress. Spontaneous Loss

n the last 2 columns, we ex-plored the phenomenon of in-ternal stress development in

coating systems and discussed theeffects of paint film composition,curing, and aging on the magnitudeand dissipation of stress. In conclud-ing last month’s article, we empha-sized the universality of such stress-es in all paint films and identifiedthis stress as a contributing factor ina variety of failure manifestations.This month, we will examine inmore detail several of the morecommon examples of internal stress-related failure in terms of the phe-nomena already introduced. We willconsider the loss of adhesion andcohesion, cracking, lateral cohesivefailure, and overcoating failures ofcoatings. We will conclude with theeffects of film thickness and formu-lation on internal stress.

Spontaneous Loss of Adhesion and Cohesion from Internal Stress Internal stress is rarely the singlefactor that produces failure in a coat-ing film or system. More usually,failure originates from the imposi-tion of additional stresses on a filmthat is already under stress. The cumulative stress effects produce the failure.

Occasionally, however, internalstress effects alone can cause spon-taneous failure. This result is espe-cially possible where little stress isnecessary to overcome either cohe-sive or adhesive strength, i.e., wherethe film is weak or its adhesion ispoor. Here, internal stresses are nor-

internal stress. The internal stressnot only overcomes the weakenedadhesive force, pulling the film fromthe substrate, but also pulls the filmapart. This vertical cohesive failure(cracking) often accompanies severeadhesive failure. Little downwardforce restrains shrinkage, so that allinternal stress is directed against thecohesive integrity of the film, whichsubsequently fails by cracking. Aswas noted in the September 1996column, the phenomenon is intensi-fied as film thickness increases, sothat mudcracking and associated ad-hesion failures may be more evidentin runs (Fig. 2). Mudcracking can beavoided in alkali silicate-based zincsystems by impeccable surfacepreparation of the steel and the

mally derived from film formation orfrom quenching too rapidly throughthe glass transition (Tg) after baking.

An illustration of this failure is the simultaneous adhesion loss andcohesive failure of a curing alkali silicate zinc-rich primer that hasbeen applied over an unsuitablesubstrate. These films require clean,uncontaminated, freshly blasted steel surfaces of good profile toachieve what is probably a primaryvalency-bonded adhesion. Figure 1 shows such a primer applied overa smooth, possibly contaminatedsteel surface.

This cohesively weak film has aratio of high pigment volume con-centration to critical pigment volumeconcentration (PVC/CPVC). Itshrinks on curing, developing high

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I

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Internal Stress-Related Coating System Failures

Fig. 1 - Delamination and associated cracking resulting from internal stress on dryingin an alkali silicate zinc-rich system on improperly scarified steelPhotos courtesy of the author

by Clive H. HareCoating System Design Inc.

Copyright ©1996, Technology Publishing Company


rapid coating of steel surfaces beforethey can become contaminated byenvironmental impurities.

Where a paint film is strong but isapplied over a weak substrate suchas plaster or a contaminant such aschalk, internal stresses, if strongenough, may produce a cohesivefailure in the substrate or contami-nant.1,2,3 In certain films with veryhigh internal stress, failure may alsobe induced in ostensibly strong sub-strates. Very high builds of strong,highly cross-linked epoxy films, forexample, have produced lateral co-hesive fracture of glass panels, thesubstrate being torn into shards bythe curing epoxy.3

Types of Cracking Cracking from Internal StressSpontaneous cracking failures canoccur without associated delamina-tion from internal stress.

In cracking failure, the stresses areuniformly distributed throughout thecoating film thickness. In this case,the vertical cohesive break extendsall the way through the finish to theundercoat or (in single-coat systems)to the substrate. In such instances,the substrate, therefore, becomesvisible and vulnerable to the subse-quent attack of the environment(i.e., corrosion).

Checking FailuresLike spontaneous cracking failure,spontaneous checking failure canoccur without associated delamina-tion from internal stress.

In checking failures, the stressesare centered in the surface layers ofthe film, and the vertical failure doesnot proceed all the way through the film. The substrate is, therefore,not exposed. Even though checkingfailures do not actually expose thesubstrate, protection (in the case of steel substrates, for example) may be diminished because of re-duced film thickness in the bottomsof the checks.

Alligator CrackingUsually occurring during film forma-tion, spontaneous vertical failures ofthese kinds frequently assume acharacteristic pattern known as alli-

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100 / Journal of Protective Coatings & Linings

Fig. 2 - Mudcracking of a zinc-rich film ondrying is more evident in thick film areassuch as runs and drips.

Fig. 3 - Alligatoring of coating.White coating was a fast-drying lacquer topcoat applied before primer coating(a red epoxy) had completelycured. As the epoxy eventuallycured, shrinkage (tensile stresses) in the system exceeded the cohesion (elongation at break properties)of the white lacquer, whichcracked.

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gatoring or crocodiling (Fig. 3). Thecracks form the irregular peripheriesof closed cells or islands of unbro-ken coating, similar to the appear-ance of alligator skin, hence thenomenclature.

We have already discussed spon-taneous alligatoring of asphalticfilms due to oxidation-inducedbuild-up of internal stress on expo-sure to ultraviolet light. A photo-graph of the phenomenon is seen in Fig. 1 of last month’s segment.This distress is more of a checkingtype of failure than true alligatoring,however, for the substrate is rarelyexposed, at least not until the filmhas aged.

More fundamentally, alligatoringmay be described as a cracking phe-nomenon caused by the applicationof an inflexible finish (having lowelongation at break properties) overa more flexible undercoat (havinghigher tensile strength and highelongation at break). In this case,adhesion is maintained as the filmundergoes vertical cohesive failure.It also occurs where a brittle film ofhigh PVC is applied over a moreflexible low PVC primer. (A com-mon example is the use of veryshort oil alkyd finish coats over longoil primer films.)

As Hess4 reports, the phenome-non is intensified if the undercoathas not completely cured (and,therefore, has not finished contract-ing) before it is recoated.

MudcrackingRelated to alligatoring and occurringspontaneously on drying, mudcrack-ing is another failure phenomenonproduced by internal stress (Fig. 2).Unlike mudcracking, however,which occurs because of internalstress in a single, relatively homoge-neous film of high PVC/CPVC, alli-gatoring may occur in a low PVCfilm. It originates from heteroge-neous stresses within a single film orwithin a coating system composite.

Lacquer Designed to Develop Alligator CracksFailures of this type can be inducedto occur on drying, enabling thistype of phenomenon to be exploitedin “crackle” lacquers specially de-signed to alligator for aesthetic ef-fect. In these systems, the “crackle”finish is applied over an adherentbasecoat (or ground coat) havingmarkedly contrasting color. Whileoleoresinous binders and even ther-mosetting baked systems have beenused as binders for crackle finishes,systems are usually based on nitro-cellulose lacquers. The base coat is arelatively conventional lacquerbased on a plasticized nitrocellulosebinder. The crackle coat is a highPVC, non-plasticized lacquer heavilymodified with an embrittling modifi-er such as ester gum (up to 85 per-cent of the nitrocellulose weight). Asthe system dries, internal stress inthe brittle crackle coat quickly

exceeds the elongation at breakproperties of the film, producing severe alligatoring. As the cracklecoat shrinks and separates, the con-trasting color of the ground coat becomes visible, producing an inter-esting novelty effect, again resem-bling the skin of an alligator. Afterthe effect has stabilized, the weakcoating system is given a clear pro-tective coat of lacquer to preservethe effect.

The effects are controlled by thefilm thickness at which the cracklecoat is applied. High film thickness-es produce wide, larger cracks,while lower film thicknesses pro-duce finer cracks in greater number.Similarly, the effect is intensified asrecoat times between the applicationof the ground and crackle coats be-come shorter. Long recoat times,thin crackle films, and films of toolow a PVC are apt to crack much

continued


less or not at all. Film thickness con-trol is critical to achieve a uniformappearance. The clear association ofcrack severity with the film thicknessof thermoplastic crackle finishesdoes not at first sight accord withthe findings of Croll5 and others.6

They found the magnitude of inter-nal stress development in thermo-plastics to be independent of filmthickness. While crackle finishes arethese days less common than once,we can learn much about the practi-cal effects of internal stress on verti-cal cohesive failure from experiencewith these novelty systems.

Coating Design to Minimize AlligatoringAlligatoring is a system design prob-lem that can be avoided by judiciouscoating system design, i.e., carefulmatching of the physical propertiesin the selection of primer and finish.A firm rule of coating system designis that the primer should always beslightly harder and less rubbery than

the finish. The latter should in turnhave somewhat higher elongation atbreak values compared to theprimer.7 Large differentials in themechanical properties of the variouselements of the system should beavoided, however, and for opti-mized mechanical performance incoating systems, the multi-layeredsystem should behave as close to asingle entity as possible. Thus, allcoats should have comparable Tg,tensile strength, internal stress levels,elongation at break, and adhesion.8

If possible, these values should, inall coatings, change minimally.PVC/CPVC ratios from one coat toanother should, if possible, not dif-fer greatly.9 It is poor practice to usethermosetting midcoats and topcoatsover thermoplastic primers, especial-ly where anticipated service may riseabove the Tg of the primer.

Optimized mechanical behavior ofcoating systems is not, however, theonly criterion for their successfulperformance. It is necessary to acco-

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Fig. 4 - Lateral cohesive failure (splitting) of a zinc-rich film, produced by internalstress effects from the shrinkage on curingof a subsequently applied finish. Note inthe tape area that zinc is visible both onthe steel substrate and on the back of thedelaminating film.

Fig. 5 - Recoating failure of an epoxymastic overcoat applied to an oil paintsystem on the web of a bridge beam.Failure in this case has occurred primarily at the old red lead primer/mill-scaled steel interface, which was theweakest interface in the entire composite.


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modate other requirements for maxi-mized adhesion, impermeability orpermeability, electrical conductivity,corrosion resistance, biocidal activi-ty, and many other properties.

These requirements inevitably demand compromise with formula-tion practice based solely on mechanical requirements. The criti-cality of the effects of formulation,and, more important, of system de-sign, on the mechanical behavior ofpaint systems remains impreciselyappreciated. This is evidenced bythe large number of adhesive andcohesive coating failures that occureach year.

Lateral Cohesive Failure in Overcoated Zinc-Rich SystemsDescription and Causes of Cohesive FailureAnother example of internal stress-related failure is the lateral cohesivesplitting of poorly formulated, cured,or applied zinc primers after recoat-ing with systems of high tensilestrength (Fig. 4). High internal stressresulting from the curing of thestrong finish may exceed the cohe-sive strength within the zinc primer.While this internal stress may alonesometimes cause failure, more often,it acts in tandem with severe hy-grothermal and service stresses applied after curing. Having goodadhesion but marginal cohesivestrength, the primer splits laterallyand separates, leaving part of thezinc film on the steel substrate andpart of the zinc film on the back ofthe overcoating film. The failure scenario is often intensified by the premature recoating of the zinc film (i.e., before it has hadenough time to achieve its maxi-mum tensile strength).

The presence of the overcoat filmmay interfere with the curing mech-anism of the zinc-rich primer. Thislocks in a less than optimized cureand therefore a deficient tensilestrength within the primer. Highly

impermeable finish coats cut downoxygen access to oxidizing zincprimers (epoxy esters). They pre-clude the access of moisture to alkylsilicate and moisture-curing ure-thanes or impede the solvent releaseof thermoplastic zinc primers.

Epoxy zinc-rich primers do notrely on the access of environmentalcomponents for completion of cure.

This allows them to cure beneaththe applied topcoat without being asaffected by the premature superim-position of the finish coats.

Dry SprayDry spray application of the primerwill also lead to weak films. Here,spraying in hot or windy weather or

continued


using too great a distance betweenspray gun and work leads to highlevels of solvent loss before the paint droplets reach the work.These circumstances all lead to thedeposition of films having insuffi-cient wet flow to produce optimumfilm continuity.

In its worse form, dry spray phe-nomena may be recognized in

changes in the pattern of light reflec-tion from the films, from the appear-ance of loose or weakly adheringspray particles at the primer surface,or from the rough feel of the primersurface. Dry spray accumulations onthe surface of the film may be re-moved successfully by screening(rubbing the surface with aluminumwire screening). Screening, however,

will do nothing to address cohesiveinsufficiencies relating to diminishedwet flow within the body of thecoating. Zinc-rich films formulated athigh PVC/CPVC ratios are, at best, ofmarginal tensile strength comparedto more highly bound coatings.There is no room for any furthercompromise with film strength.

Preventing Zinc Cohesive FailureLateral cohesive failure is most oftenrectified by addressing the deficien-cy in the zinc film rather than tryingto dissipate stress in the finish. Thisdeficiency is addressed most easilyby allowing sufficient time for theprimer to dry or cure. Phosphoricacid-containing vinyl wash primersapplied over the zinc-rich primermay improve film cohesion. Theysoak into the pores of the primerfilm. The acid may then react withthe zinc particles, thereby reinforc-ing the film with zinc corrosionproduct (probably zinc phosphate).Formulation devices include lower-ing the PVC of the zinc primer tominimal levels consistent with goodcathodic protection and reinforcingthe film with fibrous or acicular ex-tenders (with consequent adjust-ments of PVC to maintainPVC/CPVC values near unity).

Other factors may reduce the ten-sile strength of the zinc film (or partof it) below optimum levels. Thesefactors will also impair the ability ofthe film to withstand shrinkagestresses from the curing of the finishcoat and increase the risks of lateralcohesive failure. These factors in-clude formulation at excessivePVC/CPVC ratio and the applicationof zinc-rich primers from incom-pletely homogenized cans. Theseconditions result (in part) in the de-position of weak films of inappropri-ately high PVC/CPVC ratio.

In addressing lateral zinc splitting,changes to the finish make littlesense, for the fault lies with theprimer. The only non-formulative re-

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sponse with the finish would be todecrease film thickness of the finish,which would both reduce the barriereffect and decrease the level ofstress, especially in thermosettingfinishes. (Decreasing the film thick-ness of the zinc will also mitigatethe problem.) Formulation responsesfor the finish include decreasing theTg by one of a variety of techniques(such as reducing cross-link density,or plasticizing a thermoplastic fin-ish). Replacing any barrier pigmentsand raising PVC/CPVC ratios of athermosetting finish would reducethe barrier effect, allowing moreoxygen and water into the film andeasier migration of solvent out of it.This strategy, however, might alsoincrease stress levels, particularlywhere acicular pigment replace-ments were used.

In thermoplastic systems, it is un-likely that film thickness reductionwould markedly reduce internalstress. It would, however, reducethe influence of the finish on the drying (curing) difficulties in the primer. Unless film thicknesseswere very thin (which would com-promise aesthetics and perfor-mance), it is doubtful whether filmthickness reduction would mitigatedrying and curing difficulties withmost impermeable thermoplasticsused in protective coatings (vinyls,chlorinated rubber).

Most latex finish coats, conversely,should neither substantially interferewith the primer drying mechanismnor add large amounts of stress intothe system.

Lateral Cohesive Failure in Other SystemsThe “locking in” of a pre-cured andweakened state in a base coat bythe application of a relatively imper-meable finish coat is a failure sce-nario with much wider implicationsthan with zinc systems alone. Similarfailures have been noted in films of

continued


thermoplastic block fillers that arerecoated with strong epoxy systemsof high internal stress (thick films ofhigh modulus). These failures areespecially common when high hy-grothermal stresses may subsequent-ly affect exterior or interior systems.This occurs where variable, particu-larly cycling, levels of water and humidity are present. Associated

failures have also been noted instopper (filler) coats used in bakingsystems. Entrapped solvent in athick stopper film is volatilized dur-ing the baking of a subsequently ap-plied finish. The solvent produces ablister occurring within the fillercoat, which is weaker than it ordi-narily should be because of thehigher free volume.

Similar phenomena may alsooccur in very thick single-coat sys-tems, especially those based on highmolecular weight thermoplasticbinders that hold onto largeamounts of high boiling solvents.These films dry more rapidly at thesurface than they do within theirlower strata. Surface drying pro-duces heterogeneity in the mechani-cal response of the different layersof the film, the surface being higherin modulus than the lower layers ofthe film.

Heterogeneity may be exacerbatedwhere applications are made in thecold or onto cold substrates andwhere outer surfaces of films aresubsequently exposed to sunlight orother heat sources. As the surfacelayers harden, the internal stressesproduced within the system weakenthe more vulnerable lower layers ofthe film. In addition, the heat of thesun may actually volatilize the sol-vent entrapped within the lower lay-ers of the film, which may then notbe able to diffuse fast enoughthrough the drier and less perme-able upper layers. The net effect isto produce lateral cohesive failure inthe form of solvent blisters in theapplied film. The phenomenon is re-lated to solvent popping, discussedin the December 1995 column.

Epoxy Mastic Overcoating FailuresThe protective coatings industry hasbeen haunted by internal stress-re-lated coating failures over recentyears, especially with the popularityof high builds of ambient tempera-ture-cured thermosets of high cross-link density. A case in point is thedelamination of old, leaded oxidiz-ing systems from original mill-scaledsurfaces (Fig. 5) after overcoatingwith high-build applications ofepoxy mastics. These epoxy masticfilms have high tensile strength andhigh film thickness (which in ther-mosets increase internal stress). The

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combination results in films that areboth strong and highly stressed. Thetensile strength and adhesion of theepoxy mastic to the old oil-alkydcomposite are too high to allow anystress dissipation by cracking, peel-ing, or viscoelastic deformation.However, the stresses produced bytheir film formation processes arebuilt up within the total coating sys-

tem and transferred through the system to the weakest interfaces, or continua. The weakest interface is most often the interface betweenthe original primer and the mill-scaled steel substrate, the primarylocus of failure.

It is rare in these cases that failureof the composite occurs immediatelyon application of the epoxy from in-ternal stress effects alone. Internalstress alone is in most cases insuffi-cient to bring about the failure.Where spontaneous failure does immediately occur, it is local. Thisfailure is most often interpreted as insufficient local surface prepara-tion, picked up by the painter himself during the applicationprocess or later in some punch list(without dispute).

More often, the stress remainsstored within the composite, per-haps dissipating slowly. Months oryears later, the stress is compoundedby service stresses or, more likely,hygrothermal stresses to bring aboutdramatic failure of the composite.Rapid temperature drops and lowhumidities (in air below freezing)produce egregious, independentlyderived stress conditions in all coat-ing systems. Combined with residualinternal stress from application,these stresses all too often producea total stress (STOT), which breaksthe camel’s back. That is, where SFis the internal stress from film forma-tion, SH is hygroscopic stress, ST is thermal stress, and FA

continued

Fig. 7 - Intercoat adhesion failure patternon a vinyl finish/zinc primer system thatis clearly related to film thickness effects.(Failure is more intense around bolts andangles where the vinyl film thickness ishigher because of multiple passes withthe spray gun.) On thinner film areas,there is less failure. Theoretically, thiswould be surprising in a thermoplasticfilm. In fact, severe dehydrochlorinationof the finish introduces cross-links andproduces a failure pattern more typicalof a thermoset.

Fig. 6 - Internal stress caused delamination of anold oil paint system from amill-scaled substrate. Curingof a high modulus alkyd recoat (pigmented with highlevels of basic pigments) ledto the failure.


is the adhesive strength, then STOT = SF + SH + ST, and

STOT > FA. The problem may be mitigated

by decreasing the Tg of the epoxyand flexibilizing the product. This allows for better dissipation of SF, the internal stress derived fromcuring.

Unfortunately, most users of coat-ings classify all epoxies as merelyepoxies. They disregard or are igno-rant of the very large differences inproperties that specific curing agentsand curing agent types (as well asmodifiers) bring to the ambient-cured, two-pack epoxy. Stress levelsfrom highly cross-linked aliphaticand cycloaliphatic amine systems aremore significant in this type of fail-ure than are those incurred fromlow Tg polyamide, polyamidoamine,and polyglycoldiamine curingagents. Flexibilizing material, gener-ally non-reactive diluents such as bi-

tuminous oils and other aromatic hydrocarbons, may also be used toimprove stress relaxation. Mono-functional reactive diluents are alsoused to chain stop the polymer. Thislimits the degree of cross-linkingand increases free volume.

Also exacerbating the problem is the specification of high filmthicknesses uniformly across the job,with no specified maximum. Highfilm thicknesses were heavily em-phasized by paint manufacturers inthe 1960s and 1970s when films of low modulus were applied. In these instances, the problems described above were not common-ly encountered.

However, the necessity of highfilm thickness was perhaps too welllearned. In the 1990s, this require-ment is often applied dogmaticallyby specifiers, regardless of the coat-ing system. These high film thick-nesses are applied over bare steel

areas, where thickness is needed,but also over existing coatings,where excessive thickness is coun-terproductive. The golden rule inusing this type of coating should beto build up film thicknesses in spotcoats over bare steel areas, wherecorrosion protection via barrier tech-niques is most important, and tominimize film thickness over exist-ing paint films, where stress consid-erations are far more critical.

Notwithstanding these factors, forthe most part, the germ of the failureis found in the condition of the ex-isting composite, particularly thesteel surface condition beneath oldlead paint. This cannot be improvedwith a new overcoat system.10 Thefailure is usually less problematicwhere the surface was originallyblasted than where the original sur-face is not profiled (i.e., where thesteel beneath the original oil paintstill bears mill scale).

The phenomenon is not, ofcourse, restricted to epoxy masticsystems. Other highly stressed sys-tems are also susceptible. Oleo-resinous recoats (typically alkyds)that contain large quantities of basic pigments such as zinc oxidemay also induce stress-related fail-ures in old oil films at similarly mar-ginal interfaces (Fig. 6). These for-mulation effects are discussed inmore detail below.

Examples of Internal Stress fromFormulation EffectsThe correlation of the theory pre-sented here and in the past 2 articleswith practical field failure must beapplied guardedly. The effect ornon-effect of film thickness on inter-nal stress build-up on thermoplas-tics, for example, is also influencedby such phenomena as solvent en-trapment, aging, formulation, andstabilization of molecular structure.Potential post-drying cross-linkingmechanisms, such as dehydrochlori-

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nation in chlorinated species, mayinduce short-term effects that aresimilar to long-term aging. Thesemay be noted in Fig. 7. Here, avinylchloroacetate finish has beenapplied over an inorganic zinc, andthe system exhibits adhesive failureat the zinc/vinyl interface. In this in-stance, the specific cause of the de-lamination is less pertinent to ourdiscussion than the differential andthe degree of failure noted from onearea of the structure to another.

Note in Fig. 7 that the failure is in-tensified in the bolted and rivetedareas of the structure, where the filmthickness was much higher than itwas on the face of the plates. Thisfact was confirmed by field measure-ment and results from an applicationtechnique in which the bolted areaswere hit from multiple directions toensure complete coverage of thebolts. The film was also noted to bevery brittle compared to its condi-tion when first laid down.

Additionally, the paint in storageshowed dramatic evidence of dehy-drochlorination. (Properly formulat-ed coatings have suitable acid ac-ceptors and stabilizers to stop orslow dehydrochlorination.) Dehy-drochlorination explains the embrit-tlement, which is caused by the introduction of cross-links via cross chain interactions between un-saturated linkages formed by theHCl abstraction.

In this case, the aging process inthe thermoplastic binder impartsthermosetting characteristics to thepolymer in short-term service, whichresults in a stress response moretypical of a thermosetting film. Inother words, internal stress, pro-duced as the system ages, increaseswith film thickness. This may not atfirst sight appear to coincide withthe findings of Croll5 and others6,working with more stable thermo-plastics, who show little if any in-crease in the internal stress of ther-

continued


moplastics with increasing filmthickness. In effect, however, thedeteriorated vinyl in this case is act-ing as a thermoset in which increas-ing film thickness does produce in-creases in internal stress.

Formulation effects may also in-tensify the magnitude and rate of in-ternal stress development in ther-mosetting systems. A good exampleof this is in certain oxidizing paints.In these, there is more opportunityfor post-drying polymerization thanis the case with chemical curing sys-tems, such as epoxies and ure-thanes. Newly applied alkyd filmsare normally highly relaxed and donot build up appreciable internalstress levels until they begin to age.This condition can be markedly al-tered by formulating practice.

Reactions may occur between acid groups in alkyds and basic pigments such as zinc oxide or basic inhibitive pigments (especiallythose with high solubility profiles).These reactions may lead to in-creased cross-link density even inlong oil alkyd primers and finishcoats. Consequently, films hardenrapidly, modulus values are higherthan normal, and elongation proper-ties are reduced in relatively young films. In some respects, these films behave as if they weremuch older.

With certain binder systems, thephenomenon may lead to recoatingdifficulties. Internal stress build-upbetween the hardened, less hos-pitable primer and subsequently ap-plied finishes may lead to intercoatdelamination difficulties. The prob-lem may be intensified when highhygrothermal stresses are imposedupon the stressed system (Fig. 8).Careful attention to the design andlevels of binder and reactive pig-mentation is required in these alkydprimers for metal. Systems should becarefully tested under high stressconditions (particularly high tensilestresses produced by rapid cooling

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in water). With systems that are already formulated, reduced inter-coat intervals between primer and finish coat may improve inter-coat adhesion.

This strategy is by no means auniversal remedy, however, becausestress differentials developing be-tween primer and finish on curingwill rapidly build after initial cure.Mechanical scarification of primersbefore recoating, while perhaps im-practical, may be of more value.

ConclusionAlthough there remains much to belearned about stress phenomena andpaint film performance, we are sig-nificantly expanding our awarenessof the importance of internal stressand stresses of all kinds to coatingsystem performance, failure, andlong-term degradation. This aware-ness is appropriate and timely, formodern thermosets are far moreprone to the development of highlevels of stress than paint systemsemployed 25 years ago.

Premature stress-related failurestoday are more likely and morecommon. They can be avoided by understanding the physics of the problem and designing systemsthat are engineered to minimizesuch stress.

We will attempt to further our un-derstanding next month, when webegin to address the hygrothermallyderived stresses with which all coat-ing systems must contend. JPCL

References1. S.G. Croll, “Adhesion Loss Due to

Internal Stress,” Journal of Coat-ings Technology (June 1980), 35.

2. A. Saarnak, E. Nilsson, and L.O.Kornum, “Usefulness of the Mea-surement of Internal Stresses inPaint Film,” JOCCA (1979).

3. C.H. Hare, “Adhesive and Cohe-sive Failure: Definitions andFundamental Macro-Effects,”JPCL (October 1995), 36.

4. M. Hess, Paint Film Defects, 2ndEdition (New York, NY: Reinhold,1965), p. 319.

5. S.G. Croll, “Internal Stress in Sol-vent-Cast Thermoplastic Coat-ing,” Journal of Coatings Tech-nology (March 1978), 33.

6. T.S. Chow, C.A. Liu, and R.C.Penwell, “Polymer Phys. Ed.,”Journal of Polymer Science (July1976).

7. C.G. Munger and R.W. Drisko,“Factors of Uncoated and CoatedSubstrates that Affect CoatingPerformance,” JPCL (May 1990),62.

8. E. Krejcar and O. Kolar, “Agingand Degradation of Paint FilmMedia,” Progress in OrganicCoatings I (1972/1973), 249.

9. O. Kolar and B. Svoboda, Farbe+ Lack (1969), 31.

10. C.H. Hare, “Aluminum BasedPaints for Lead Encapsulation,”paper presented at WashingtonPaint Technical Group 33rd An-nual Symposium, April 13-14,1993, Tysons Corner, VA.

Fig. 8 - Intercoat delamination ofa brittle alkyd finish from a hard,strong alkyd primer formulatedwith large amounts of basic pigments. Premature hardening ofthe alkyd primer from reaction between the basic pigment andacidic binder produces an inhospitable surface that is moredifficult to adhere to. Internalstress build-up between the primerand finish is exacerbated by severe thermal stresses as the system is cooled rapidly.

Documents

Trouble with Paint: Internal Stress-Related Coating System ... · coatings. We will conclude with the effects of film thickness and formu-lation on internal stress. Spontaneous Loss