trouble with paint - adhesion (2)

ast month, we reviewed thechemical aspects of adhesion.This month, we will consider

the physical aspects of this phenom-enon. The column begins with anintroduction to mechanical adhe-sion; describes the roles of substratecontamination and surface prepara-tion; discusses adhesion over conta-minated surfaces and ways to en-hance adhesion; and gives examplesof adhesion failure.

Mechanical Adhesion and PenetrationFor many years, it was thought thatthe increase in adhesion achieved byscarification techniques (abrasiveblasting and sanding, for example)was derived from mechanical entan-glements of the coating film withinthe pores and fissures of the scarifiedsurface. Current thinking proposesthat such techniques merely removesaturating impurities from potentiallyreactive sites on the substrate and si-multaneously increase the real sur-face area with respect to the appar-ent (planar) area. Expansion of thesurface area increases the number ofpotentially reactive sites on the sub-strate for either primary or secondarybonding. An increased number of re-action sites, rather than purely me-chanical effects, is the principal rea-son for improved adhesion.

Mechanical adhesion is, however,entirely possible on porous surfacessuch as wood, paper, leather, andconcrete, where penetration by, andabsorption of, the coating into thesubstrate play a significant role inadhesion. Similar submicroscopic ef-

mon over some zinc-rich primersand will be discussed in more detailin a later segment of this series.

Coatings that fill narrow-mouthedcavities can be dislodged only byproducing lateral cohesive failure inthe coating film or the substrate (Fig.1). While extreme, this porosity isnot unusual on naturally poroussubstrates such as wood, paper (in-cluding paper-surfaced drywall), andleather. It is also found on plaster,concrete, cinder block, and otherporous masonry surfaces.

fects are also possible on scarifiedmetal surfaces; plastics; and othersanded, etched, or otherwise rough-ened substrates (including existingpaint films). In achieving such adhe-sion, the coating must flow into thesurface porosities and, for maxi-mized effect, completely wet out theinterior cavities, displacing occludedair. In some highly viscous or fastdrying films, such air displacementfrom the substitute may producebubbling of the recent film as the airbecomes entrapped in the dryingcavity. This type of defect is com-

JULY 1996 / 79

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continued

TROUBLE with PAINT

Adhesion: Part 2by Clive H. Hare,Coating System Design

Table 1System Design Requirements for Maximized Adhesion

Substrate

High Cohesive Strength

High Surface Energy

Increased Surface Area

Pure SurfaceNo Contamination

Presence of Active SitesUnbound Metal Oxides,Hydroxyls, Etc.Polar Groups

Porosity

Solubility or Partial Solubility in Coating Solvent

Coating

Low Surface Energy

Polar Groups

Reactive GroupsCarboxylic AcidsHydroxyls

Low Resin Solution ViscosityLow SolidsLow Molecular Weight ResinsPolymerizing after Application

Slow Evaporating Solvent Systems

Slower Conversion (Cure, Polymerization) Rate (Compared to Rate of Solvent Loss)

Low Internal StrainReduced Shrinkage on Polymerization

Good Flexibility

High Tensile Strength and/or Large Work to Break Values

Low PermeabilityUse of Barrier Pigments/Chlorinated BinderUniform High Cross-link DensityElimination of Hydrophilic Components

Coupling AgentsSilanes, etc.

Copyright ©1996, Technology Publishing Company


It is more likely that the coatingwill only partially fill such cavities.When the coating polymerizes, sub-sequent shrinkage may cause void-ing and cracks in the film or thesubstrate as the coating pulls awayfrom the interior surface of the cavi-ty. These cracks and weaknesses aregenerally the sites of initial disbond-ment, from which more widespreadfailure is propagated.1 Incompletelyfilled cavities on coated metal sub-strates may also provide sites forwater to begin to accumulate be-neath the film. Water accumulationmay lead to lateral adhesion loss,the accumulation of still more water,and eventual corrosion. Salt can alsolodge easily in these cavities on cor-roded steel and cause difficulty incleaning.2

The penetration of paint into cavi-ties is a capillary phenomenon facili-tated by the following: • surface energetics (e.g., low sur-face tension binders and high sur-face energy substrates), • an increasing size of the opening, • low viscosity of the paint’s contin-uous (resin solution) phase, and • reduced rate at which solvent islost and viscosity increases. Wicks et al.1 notes that the diameterof the pigment particles may belarge relative to small surface irregu-larities, so that only the liquid phaseof the paint may enter the substrate.A similar phenomenon is likelywhere the paint is applied over aporous surface, such as old, chalk-ing films from which the originalbinder has been degraded by ultra-violet light and eroded by rain. Theconsequent partition of resin andpigment is shown in Fig. 2. The lin-seed oil binder wicks away from thebulk phase of the paint film and intothe chalking surface of an existingfilm that has been primed.

The simplest way to reduce thesolution viscosity of the resin is toreduce solids by adding solvent. Un-fortunately, this approach is incom-

patible with trends in environmentalregulations on solvent emissions.Also, for high molecular weight lac-quers, penetration of even lowsolids solutions of such polymersinto small porosities may be imped-ed by the size of the binder mole-cule. Therefore, resin solution vis-cosity is best reduced through resindesign. In most systems, the viscosi-ty of the resin solution phase is re-lated to the molecular weight of thesolid binder (the resin). Resins withlow average molecular weight andminimal high molecular weight frac-tions at the time of application3 willmore readily penetrate and adhereto irregular surfaces. A study3 notesthe negative effect of increased mol-ecular weight on wet adhesion. Mi-croscopically, most surfaces are farfrom planar, especially after me-chanical or chemical preparation.Therefore, it may be reasonable todiscuss adhesion in terms of sub-strates as well as coatings (Table 1).

Paint films that slowly build mole-cular weight after application are in-herently more suited to good adhe-sion than high molecular weightlacquers. For this reason, linseed oil-based red lead primers of low mole-cular weight adhere well to rustysteel. The surface energetics and theslow rate at which viscosity increas-es (through both solvent evapora-tion and polymerization) also play aconsiderable part. The longer thepaint can stay against the cavity in awet, low viscosity state, the morepenetration will occur. Thus, highboiling solvents provide better adhe-sion than fast evaporating solvents.

It has also been suggested1 thatsome of the improvement in adhe-sion in baked systems results fromthe initial reduction in viscosity pro-duced by the rise in film tempera-ture before cure begins. Similar phe-nomena (along with glass transition[Tg] and free volume effects dis-cussed in the December 1995 and

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80 / Journal of Protective Coatings & Linings

Fundamental Techniques for Cleaning Substrates

SolubilizationSoluble inorganics (e.g. Salts) dissolved in water. Soluble organics (e.g. Oils) dissolved in solvent.

EmulsificationLifts and emulsifies insoluble organic and inorganic soils in detergent solutions.

SaponificationChemically hydrolizes and renders water soluble non-soluble esters and salts.

Chelation and SequestrationRemoves calcium and magnesium from hard water, iron, and other metal oxides from metal surface by chelating mechanisms.

DeflocculationWets, lifts, and disperses contaminating dirts, surrounding particles with surfactant barrier which suspends dirt residues as a dispersion and prevents resettlement and recontamination.

General PrecautionsMaterials applied by dipping, spraying, wiping, or brushing (spreading techniques less efficient in removing residues).Efficiency increases with temperature and pressure (e.g., as in steam cleaning).Rinsing (in clean water or pure solvent) is an important necessary final step.

Copyright ©1996, Technology Publishing Company JULY 1996 / 81

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January 1996 issues) are involved inthe increased adhesion of post-baked thermoplastic coatings.

However, the shrinkage that oc-curs on curing is counterproductiveto adhesion.4 Giving superior adhe-sion, epoxies shrink far less than dotrue condensation systems, such asamino-cured baking coatings, andfree radical-induced systems, such asunsaturated polyesters.

Substrate ContaminationMany conditions may compromisethe nature and uniformity of a sur-face. Substrates may be contaminat-ed with oils, greases, waxes, dirt, lai-tance, loose powders, rust, scales,and surface chalk (pigmentaryresidues from the weathering-in-duced degradation of old paintfilms). In 1 study of salt contamina-tion on 78 bridges in Germany,Gross5 detected 20 types of residualsalts, mainly sulfates. Similar de-posits are likely to occur on mostexterior surfaces, particularly inareas prone to acid rain. In coastalareas, chloride salts are also on mostpaintable exterior surfaces. Roaddirt, a combination of dirt, oils, andthese salts, is found on the road-fac-ing surfaces of many bridge struc-tures, while paintable surfaces inother industries may be plagued byother residues common to miningand manufacturing processes. Looseresidues from a carelessly preparedsurface can include unremoved blastdebris (and debris from sanding)and chlorides from muriatic acidetching procedures. These residuesare as damaging to subsequent ad-hesion as the deterioration of theoriginal surface that required thesurface preparation. Chalk residuestypical of old paint films may beparticularly difficult to recoat withsome coatings (e.g., latex paints)and may be quite difficult to removeby washing alone.

All of these conditions may beconsidered as more or less poorly

bound films of little cohesivestrength. Problems in coating suchsurfaces are similar to those experi-enced in dealing with the paintingof old whitewash, calcimine, andsimilar loosely bound paints calleddistempers. Failure to remove thesefilms will lead to adhesion problems.Subsequent delamination is actuallya cohesive failure within the contin-uum of the loose film. The adhesionof the new coating to the surfacelayers of the loose film is good. As aconsequence of such cohesivebreak-up, substantial debris will beleft on the backside of the delami-nating coating film and on the re-ex-posed substrate itself. The drivingforce of the delamination may bethe shrinkage stresses incurred inthe system as the new film dries andpolymerizes. Another cause is thehygrothermal stresses produced asthe system is put in service, expand-ing and contracting under the influ-

ence of environmental moisture andtemperature.

Less often, delamination may re-sult from cohesive insufficiencies inthe substrate, more so in relativelyweak wood and cementitious sur-faces than in metal.

Surface PreparationThe purpose of surface preparationis to remove all anomalous sub-stances and conditions and renderthe surface a better approximationto the theoretically “pure” substrate.The efficacy of surface preparationdepends on the type of contamina-tion and the surface preparationmethod. Various cleaning tech-niques may be used, but none istruly universal. (See box on page80.) Optimized surfaces will dependon the cleaning methodology. Sim-ply washing the surface with water,for example, will have a nominal ef-

continued


fect on removing low energy organ-ic contaminants such as oil films andgrease deposits. Solvent washingand solvent vapor degreasing, whichmay more effectively remove oilsand greases, will have little effect oninorganic salts. These contaminantsmay be more easily removed withwater. The efficacy of cleaning inboth cases depends on the solubilityof the contaminant in the cleaningmedia. Fortunately, solubilization ofthe contaminant is not the onlyprocess for cleaning the surface.Emulsification of the contaminant isan alternative. Detergent washing,for example, may effectively emulsi-fy non-soluble contamination whiledissolving water-soluble matter.

Heat and mechanical energy willimprove the efficiency of the clean-ing operation when applied withwater, solvent, or detergent solu-tions. High temperature detergentcleaning techniques, such as steamcleaning, are very effective. Hotcleaning solutions are more effectivethan cold ones because of the in-creased kinetic energy of the mole-cules (contaminant and cleaningagent) at high temperature. Mechan-ical force helps remove contamina-tion even more effectively. Wiping,rubbing, scrubbing, sanding, andwater and abrasive blasting repre-sent increasing levels of force thatcan be used to remove surface cont-aminants. Techniques that providesufficient energy to remove the cont-amination and scarify the surfacemay be particularly useful becausethey will simultaneously clean thesurface and increase the true surfacearea of the substrate compared to itsapparent planar area. High energymechanical cleaning methods suchas abrasive blasting may not, howev-er, be suitable for weak substratessuch as plaster and wood.

Mechanical force alone may notentirely remove organic solubles.For example, heavy grease depositswill normally have to be removed

from surfaces with solvent washesbefore abrasive blasting. Nor willdry blasting completely remove inor-ganic salts that have formed saltnests on old, corroded steel sur-faces. Wet blasting with abrasiveand water may be required to moreeffectively remove inorganic salts(chlorides and sulfates) from old,rusting steel surfaces, especiallywhere the surface is porous. Salt-contaminated high alloy steelsbelow bridge decks have in somecases displayed porosity after severalyears in service.2

Chemical cleaning techniques6,7,such as acid etching and alkaline de-oxidation, accomplish the same thingas blasting for metal, although alka-line cleaning gives no profile. Abra-sive blasting and acid pickling willalso chemically remove tightly bond-ed oxides and other scales that can-not be removed by other techniques.These techniques strip away oxidefilms that chemically saturate metalsurfaces and free up reactive groupson the surface for subsequent reac-tion with potentially complementarygroups on the paint binder. If we re-move heavy oxide deposits, we leavenascent steel surfaces with thin, ad-herent metal oxide and hydroxidefilms. These will then react with thecoating. Chemical cleaning, includingthe acid etching of concrete surfaces,must be followed by an adequaterinsing step to remove loose salts andother residues left after cleaning.

Adhesion to Contaminated SurfacesIn practice, optimum surface prepara-tion may not always be feasible.Cost, access difficulties, environmen-tal concerns, and substrate sensitivitymay force coating application overimperfectly prepared surfaces.Whether inadvertent or deliberate,application of paint over surface con-taminants often produces catastroph-ic peeling failures. However, riskmanagement, environmental con-

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TROUBLE with PAINT

cerns, and the cost ofhazardous waste dis-posal encourage at-tempts to apply newcoating systems overmarginally preparedsurfaces.

Just as the effective-ness of any method ofsurface preparationdepends on the typeof contamination andthe nature of themethod, the success oflimited preparation islargely governed bythe specific nature ofthe surface to be coat-ed and the coating itself. Unless thecoating can assimilate the contami-nation (a rare circumstance), thesuccess of the system must dependon the ability of the coating to dis-place or penetrate the contamination

and reach sound sur-face beneath. Assimila-tion is possible, for ex-ample, where a water-borne paint is appliedover a damp surface orwhere a brush is usedto work a long oil alkydinto a dusty surface.

Displacement oc-curs when a high wet-ting resin system (e.g.,epoxy) is applied to

wet steel (in extreme cases, belowwater). The resin or resin and sol-vent mix associates with the sub-strate more strongly than doeswater, and thus displaces the water.

continued

Fig. 2 - The lateral portion of an oil binderwicks away from the bulk phase of a redlead/linseed oil primer into a layer of surfacechalk on an old paint film.Photo reproduced from The Painting of Steel Bridges and Other Structures by Clive H. Hare with permission from Van Nostrand Reinhold

a. Porous substrates such as wood and evenmetal may contain narrow-necked cavities.

b. High wetting, low viscosity paints may be ableto penetrate these cavities, displacing air and fillingthem. This provides excellent anchorage for thefilm. Subsequent adhesive failure must necessarily,therefore, involve additional cohesive failure of thefilm across the neck of the cavity.

c. Complete air displacement is rarely possible,however, and some reopening of cavities mayoccur as films shrink on polymerization. Solventdiffusion from these areas may also be very slow.On metal, cavities may also contain corrosionproduct and soluble salts, which are not easily ac-cessible for removal by surface cleaning.

d. In service, unfilled, reopened, or contaminatedcavities will form sites at which penetrants (such aswater) passing through the film may accumulate.This may be particularly problematic on metal inwater service or condensing environments, whereosmotic and electroendosmotic gradients may leadfirst to blistering and then to underfilm (intra-cavity)corrosion. Where the cavities contain depassivat-ing chlorides and sulfates, these highly aggressiveelectrolytes may lead to high corrosion rates.

Figure 1 - MechanicalAdhesion and PotentialTroubleFigures courtesy of theauthor.

Partitioned oil binder wicking into chalk of old film

Bulk phase of newly-

applied oil primer

Substrate (old chalking finish)

a.

b.

c.

d.


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The displacement of water allows thecoating to adhere in the presence ofthe external aqueous bulk phase.Polyamide- and amidoamine-curedepoxies are particularly effective inthis regard because of the hydropho-bicity of the curing agent. Displace-ment cannot occur when the surfacetension of the coating is higher thanthat of the contaminated surface.Therefore, water-borne paints ciss,crawl, and bead up over oily sur-faces, while a low energy oil paintmay wet and achieve satisfactory ad-hesion over the same surfaces.

An example of penetration iswhen a red lead linseed oil paint isapplied to a rusty steel surface andsoaks into and through the rust.However, it would be virtually im-possible for a high molecular weightvinyl lacquer to adhere to a dustysurface. The molecular weight of thecoating is too high to allow goodpenetration of the resin solution intothe dust, and the solvent systemgenerally evaporates too quickly.Similarly, latex paints have muchless success in penetrating old,chalking films than alkyds, particu-larly those with high oil content.

On surfaces contaminated withloose particles, chalk, rust, and dirt,the effects of penetration are thesame as those noted in the discus-sion of mechanical adhesion. Thesame may also apply to porous,poorly bound substrates, such as oldcalcimine films and certain plasters.The relative surface energetics of thepaint and substrate are critical. Thelow surface tension of oil paints isresponsible for the excellent servicerecord of these coatings over conta-minated surfaces. Oil paints, howev-er, also have a low continuousphase viscosity and a highly pro-tracted rate of conversion. Bothproperties allow the wet coating tosoak into and through the intersticesof the contamination, eventuallyreaching and wetting out sound sub-strate and also wetting and binding

much of the contamination. As thesurface tension of the binder in-creases, wetting becomes increasing-ly less likely. The ability of the coat-ing to enter and penetrate theinterstices of the contamination is re-duced as the viscosity of the contin-uous phase of the paint increases.Reduced penetration is commonwhere drying times are short or mol-

ecular weight and, therefore, viscosi-ty increase rapidly during cure.

Latex paints have particular diffi-culty penetrating contamination andwetting the substrate. Although latexpaints have poorer wetting charac-teristics than solvent-borne systems,it might appear that the low viscosi-ty of the high molecular weight dis-

continued


TROUBLE with PAINT

persions would facilitate penetrationcompared to high molecular weightsolutions. In fact, the individual latexparticles may be larger than the in-terstices of many porous films (thechalking surfaces of existing paintfilms, for example). This size differ-ence prevents the latex from enter-ing the interstices and pores of thechalking layer, although the watermay do so. Therefore, coalescenceof the latex particles occurs over, in-stead of within, the contaminatinglayer, resulting in a dry film thatrides on top of the chalk. The film isthen vulnerable to adhesive difficul-ties. An apparent adhesive failure inthis film may be a cohesive failurewithin the chalk layer.

Attempts have been made to uselatexes of very fine particle size(<0.1µ).8 Presumably, fine particlesize latexes are more likely to fit intothe existing porosities of the porouschalk than resins of larger particlesize. Carboxylated latexes are nowused as grinding vehicles in surfac-tant-depleted pigment dispersionphases. They are used as latex emul-sions for chalk binding and adhe-sion over marginal surfaces becauseof the improved wetting and bindingproperties of these vehicles.

High wetting coalescents such asn-methyl-2-pyrrolidone can also im-prove the penetration of chalkingfilms and the adhesion by solvationof the more substantial underlayersof the old film. In this respect, hy-drophilic coalescents such as thealiphatic glycol ethers may be moreefficient than water-immiscible coa-lescents (e.g., the aromatic glycolethers and trimethylpentanediolmonoisobutyrate). Water-immisciblecoalescents associate more stronglywith the non-continuous (resinous)phase of the new latex being ap-plied and are not present in the pen-etrating water.9,10 Paradoxically, hy-drophilic solvents are less efficientas coalescents than less miscible ma-terials that partition into the latex

because they tend to remain in thewater phase.

None of these techniques for en-hancing adhesion to chalk is as ef-fective as the traditional approach ofmodifying the latex binder with upto 20 percent by weight of low mol-ecular weight (very long oil) alkyds,epoxy esters, vegetable oils, or poly-esters. The modifiers are emulsifiedby the surfactant system of the latexpaint to form a barely compatiblewhole. On application, these slowdrying, high wetting modifiers parti-tion themselves from the bulk latexbinder and penetrate the loosechalking layer. They may partiallysolvate the substrate and markedlyimprove the adhesive strength of thelatex paint to the contamination. Themodifiers may also increase the co-hesive strength of the loose chalkingcontamination.

Alkyd modification compromisesthe long-term exterior durability,

mildew resistance, color retention,and drying time of latex paints.However, the benefits of oil andalkyd modification usually outweighthese disadvantages.

Latex de-adhesion over porous,chalking, and otherwise contaminatedfilms is also affected by the Tg andmechanical properties of the finallatex film. Latex systems with high Tgare similar to other strong films ofhigh modulus that do not readily dis-sipate strain from either internal orexternal stress, unless they cohesivelyor adhesively break. Therefore, theytend to transfer the stress into otherparts of the composite and induce co-hesive failure in the loose substratelayer. More flexible films of reducedTg and modulus deform more readilywith stress and produce less strainwithin the weak chalk layer.

The strength and nature of the re-coat system will also markedly affect

continued


the success or failure of any coatingapplied over marginally preparedsubstrates. This factor becomes in-creasingly significant as the interfa-cial adhesion between substrate andrecoat is further compromised. Final-ly, the adhesion is also influencedby the nature of the contaminationand the surface beneath it.

Enhancing Adhesion with Coupling AgentsThe most effective techniques forpromoting adhesion are enhancingthe substrate condition, removingcontamination, and, particularly, in-creasing the real surface area per ap-

parent area of surface. Whetherthese results are achieved by me-chanical (sanding or abrasive blast-ing) or chemical techniques (acidpickling or alkali deoxidation) is lessimportant than the results.

On the other side of the interface,the selection and design of the resin,coating, and coating system can sig-nificantly affect adhesion. Unfortu-nately, the selection of coatings in-volves many other aesthetic andengineering considerations in addi-tion to adhesion, such as dryingspeed, hardness, chemical resis-tance, gloss, and color. These prop-erties are not always compatible

with optimum adhesion.There is, of course, no universal

potion that will transform a coatingthat does not adhere into one thatdoes. In some cases, acids, usuallyphosphoric acid, have been used to-wards this end on suitable substrates(e.g., steel), but results vary fromsystem to system.

Over the past several decades,however, formulators have success-fully designed additives to upgradeadhesion to certain substrates. Anexample is the effect of silane modi-fication on the adhesion of coatingsto glass and siliceous surfaces.

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continued

Fig. 3 - Adhesion Promotion with Silanes. 1) Alkoxy silane is hydrolyzed by water to form silanol groups; 2) Silanol groups on hydrolyzed silane react with silanol groups on the substrate to form siloxane bridges, which anchor the silane backbone to the substrate; 3) Organo-reactivegroups (e.g., amines) on the opposite end of silane react with complementaryorgano-functional groups (e.g., epoxy) on the polymer. The polymer is thus covalently bonded to the substrate.

OH H OH

Ho — Si N — C — C

OH

OR

RO — Si — OR + HOH

NH2

Alkoxy Silane

1.

2.

OH

HO — Si — OH + ROH

NH2

Hydrolyzed Silane

OH

HO — Si NH2

OH

OH

HO — Si NH2

OH

+

Si — OH

Si — OH

Si — OH

Si — OH

Si — OH

Si — OH

OH

Ho — Si NH2

OH

OH

Ho — Si NH2

OH

Si — OH

Si — O —

Si — OH

Si — OH

Si — O —

Si — OH

Glass substrate

Hydrolyzed Silane Substrate bonded to Silaneby Siloxane bridge

3. OH

Ho — Si NH2

OH

OH

Ho — Si NH2

OH

Si — OH

Si — O —

Si — OH

Si — OH

Si — O —

Si — OH

OH H OH

Ho — Si N — C — C

OH

Si — OH

Si — O —

Si — OH

Si — OH

Si — O —

Si — OH

Polymer Substrate covalently bonded to polymer

+

O

C — C

O

C — C


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In some respects, silanes are simi-lar to surfactants in structure. All arecharacterized by the presence of tri-alkoxysilyl groups on one end of ahydrocarbon chain, the other end ofwhich is terminated by an organo-reactive group such as an epoxy, anamine, a vinyl group, or a mercaptofunctionality. Typical materials areshown in Table 2. After hydrolysiswith surface water, at least one ofthe resultant silanol groups on thesilane triols reacts with silanolgroups on the substrate to form asiloxane bridge. The organo-reactivegroup on the other end of the silaneis available to react with the binderto give a chemically bonded bridgeacross the interface between binderand substrate (Fig. 3). For example,an amino-terminated silane mayreact with an epoxy, and a vinylsilane with an unsaturated polyester.Because the silane has trifunctionali-ty, remaining silanol groups on adja-cent silane molecules may intercon-dense to give a thin, polysiloxanefilm along the immediate surface ofthe glass.11

Unlike most examples of molecu-larly engineered adhesive improve-ment that require modification of thepaint binders, silane modificationcan be used by the paint formulatorto improve adhesion. Silanes im-

prove the adhesion of coatingsthrough their use as pretreatments,in resin modification before the in-corporation of pigments, or in modi-fication of the primer during paintmanufacture. Pretreatments are 1 to2 percent solutions of silanes inwater and suitable solvents that areapplied to the surface before prim-ing. The silane may also treat thepigment before its incorporation intothe paint. While this techniquewould seem more likely to improvethe cohesive strength of the coatingfilm, it apparently also upgrades wetadhesion. Improved wet adhesionmay be related to the closer associa-tion of the pigment and binder.

In addition to modifying surfacecoatings on glass, this technique isclaimed to upgrade adhesion tometal surfaces. While success here isless universal than on glass, there ismuch evidence to support theseclaims.12,13,14

Not all silanes bear reactivegroups that complement groups onthe polymer. However, there is evi-dence that these silane materials toomay upgrade adhesion. In this case,physical entrapment of the pendantsilane chains within the polymermay be responsible for the im-proved adhesion.12

The question of permanence of

the -Si-O-Si- bonds at the siliceoussubstrate (and possibly -C-O-Si-bonds at the metal interface) is inter-esting. The bonds are susceptible toreversible hydrolysis. Here, thesiloxane hydrolyzes back to silanolgroups in the presence of any waterthat may reach the interface underwet conditions. Performance of coat-ings modified under wet conditionsdoes not indicate that this reversal iscomplete enough to be problematic,however. Walker12 has shown thethe importance of using silane modi-fication to improve wet adhesion,which has great significance for thecontrolling of corrosion. It has alsobeen claimed that, in direct conse-quence of reversible hydrolysis,silane-bonded systems are stress at-tenuating. Under stress, the siloxanebonds at the substrate will cleave inthe presence of water, allowing thesystem to slip across the substrateplane to form again at new (lessconstrained) sites on the substrateafter the stress has been removed(Fig. 4).11,15,16

The value of similar materials asadhesion promoters has also beenclaimed. These substances includecertain organo titanates17, such asisopropyl tri (n-ethylamino-ethyl-amino) titanate, and zircoalum-

continued

Fig. 4 - Stress Attenuation Through Silane Slippage - In the presence of interfacial moisture, a reversible silanol reaction allows the siloxanelinkages of the interface to break and rapidly reform. Under shear stress, this allows the siloxane bridges to slip from silicon atom tosilicon atom along the substrate surface. The bonded coating can thus move along the substrate, dissipating strain without catastrophicdisruption in adhesion.

NH

OH

OH

O O

Si

OH NH NH NH

OH

O

Si

O O OH

Si O

HO

O OH

Si

OH

Si

H2O

1 2 3 4 5 6 7 8 9 10

H2O H2O H2O

Si Si Si Si Si Si Si Si Si

NH

OH

OH

O O

Si

OH NH NH NH

OH

O

Si

O O OH

Si O

HO

O OH

Si

OH

Si

1 2 3 4 5 6 7 8 9 10

Si Si Si Si Si Si Si Si Si

ShearingStress

Polymer

Substrate Substrate

Polymer


inates.18 These materials seem evenmore system-specific than thesilanes, however, and inappropriateselection of the titanates can dramat-ically reduce adhesion.17 Silanesand, to a lesser extent, the titanatesand zircoaluminates have also beenused to treat siliceous pigments,such as silica, wollastonite, andmica.11,19 Here, the silane reactswith the pigment surface so that thepigment presents the organo func-tional tail of the silane to its environ-ment. In effect, the pigment be-comes covered with a mechanicallybonded “hairy” layer (Fig. 5). Whenused to pigment judiciously selectedbinders, the organofunctional end ofthe silane will react with the binder,producing a chemically bonded in-ternal matrix between pigment andresin (analogous to that betweenresin and substrate in adhesion pro-motion). The cohesion of the result-ing paint film is augmented. In theseapplications, the pigment is effec-tively encased in the mono layer oforganic treatment. Depending on thetype of treatment used, the pigmentmay present reactive amine, epoxy,

vinyl, and mercapto groups to thepotential binder.

Improved pigmentary interfacesnot only improve the physical prop-erties of the coating, but may alsoreduce permeability of water andother penetrants through the inter-stices of the pigment and binder. A separate advantage of such treat-ment is that the treated pigments are much lower in oil absorptionthan their untreated counterparts,which translates into high criticalpigment volume concentrations,lower viscosity, and applications in coatings with low volatile organiccompounds. Commercial examples of pigments embodying this tech-nology are used with and without inhibitive pigments in anti-corrosiveprimers.19

Adhesion, Cohesion, and CoatingSystem Response to StressThe adhesion of a coating film to asubstrate is part of a complex systemof forces. These forces must main-tain a satisfactory equilibrium in thepresence of an equally complex sys-tem of stresses that play upon (and

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Table 2Typical Organo-Reactive Silane Adhesion Promoters

Formula Functionality Typically Used With

CH2 = CHSi (OC2H5)3 Vinyl Unsaturated PolyestersCH2 = CHSi (OC2H4OCH3)3 Vinyl Acrylics, Vinyls, Alkyds,

and Vinyl EstersCH3 0

CH2 = C – C – OC3H6Si (OCH3)3 Methacryl

HSC3H6 Si (OCH3)3 Mercapto Urethanes and EpoxiesHSC2 H4 Si (OC2H5)3 Mercapto

H2 NC3 H6 Si (OC2H5)3 Amino Epoxies, Alkyds,H2 NC2 H4 NHC2 H6 Si (OCH3)3 Amino and Urethanes

Amines, AmidesUrethanes, and

CH2 CHCH2 OC3 H6 Si (OCH3)3 Epoxy Formaldehyde Systems

0

Copyright ©1996, Technology Publishing Company 93

TROUBLE with PAINT

strains that play within) the total ap-plied composite. To attempt to accu-rately characterize or predict coatingsystem performance by measuringthe adhesion of the primer alone isas simplistic as attempts to charac-terize the mechanical behavior ofthe total applied composite paintsystem by studying the stress andstrain response of the unsupportedfinish coat.

While our understanding of the in-terplay of forces within the multi-component system remains incom-plete, we can appreciate the effectsof film strength and thickness of amulti-coat (or even a single-coat)paint system on paint film failure. Itis no accident that wash primers andpre-treatments are applied andcured as ultra-thin film coatings.Ultra-thin films minimize the effectsof high film cohesion on adhesion.As the film thickness increases from1 to 5 to 50 mils (25 to 125 to 1,250micrometers), the negative impact ofhigh cohesive strength and in-creased internal stress on adhesionbecomes more extreme. In our zealfor good corrosion protection, it hasbecome popular to advocate thicker,rather than thinner, films. Unfortu-nately, the disadvantages of thethicker films, such as their negativeeffects on the adhesion and cohe-sion of the system, have been unrec-ognized, underplayed, or ignored.

The author advised a contractormany years ago that as long as hisanchor pattern was deep enough, heneed not worry about putting toomuch coal tar epoxy on the interiorwalls of a water cooling plant. Sev-eral weeks later, the same contractorreturned with several paint chips tocomplain that a six-mil (150-microm-eter) anchor pattern did not preventthe film from sheeting from thenewly blasted steel shortly after ithad been put into service. The paintchips were 7⁄8 in. (22 mm) thick.

In a separate instance, anothercontractor had been convinced to

do a quick repaint job on the ceilingof an old mill. The job had hardlystarted when the newly applied filmbegan to delaminate profusely fromthe ceiling, which bore heavy de-posits of calcimine. Calcimine is oneof a series of underbound paintsused on ceilings in the early years ofthis century. The mill owner wasoutraged, not least with the author,who suggested in lieu of a completeremoval of the existing calcimine, toabandon the alkyd flat being used infavor of the cheapest, most under-bound paint that he could find. Hap-pily, the author’s recommendationprevailed, and the ceiling was satis-factorily recoated (aesthetically, ifnot from an engineering standpoint)without further peeling, at least untilthe next repainting.

The thick, strong, cohesivelybonded epoxy film of high modulus,unlike the cheap ceiling paint, wasunable to dissipate strains that hadbuilt up within the continuum oncuring. Consequently, adhesiveforces had to be of a very high orderto avoid adhesive failure response.In the old, cohesively weak cal-cimine, the adhesive forces that heldthe film to the substrate were infi-nitely weaker than the epoxy. Filmcohesion was also much weaker.The strain that developed within thecoating as a result of solvent lossand curing could be effectively dissi-pated by cohesive microcrackingalong the pigmentary and vehicle in-terfaces. In short, the coating filmcracked because it was too weak topeel. The cracks were too fine to bevisible to the unaided eye.

Problems could still occur whenthe weak calcimine was subsequent-ly recoated with strong alkyd finishin the first abortive repainting thatproduced the original failure. Whilethe cohesion of the initial calciminedid not change, the cohesivestrength of the total composite didbecause the coating applied as a re-coat was much stronger. Alternative-

ly, recoating with the weak finishdid not substantially add to the co-hesive strength of the upper strata ofthe system. Therefore, internal strainwas minimized and dissipated in amicrocracking of the new film. Co-hesive failure in the lower strata wasavoided. Eventually, as more andmore coats come to be applied overthe same weak underbound paint,the total system response to curingand service stresses will produce somuch strain within the system thatthe cohesive and adhesive integrityof the early poor film (by far theweakest element of the entire sys-tem) will disintegrate. The weakpaint, or part of it, along with everycoat applied over it, will delaminate.

ConclusionIn the above examples, the majorityof the total stresses producing adhe-sive (or cohesive) failure are derived

continued


from the application of new coatingsover old ones, not from the serviceenvironment.

These types of stresses, called in-ternal stresses, will be the subject ofthe next article in this series. JPCL

References1. Z.N. Wicks, F. Jones, and P. Pap-

pas, Organic Coatings Scienceand Technology, Vol. II (NewYork, NY: Wiley Interscience,1994), p. 154.

2. G. Tinklenberg, “Corrosion ofUnpainted Weathering Steel—Causes and Cure,” Proceedingsof 2nd World Congress—CoatingSystems for Steel Bridges, Octo-ber 26-27, 1982, LaGuardiaMarriott Hotel, New York City,NY (Rolla, MO: University of Mis-souri).

3. P.S. Sheih and J.L. Massin-gill, “Fundamental Studies ofEpoxy Resins for Can and CoilCoatings I: Adhesion to Tin FreeSteel,” Journal of Coatings Tech-nology (Vol. 62, Number 781,1990), 25.

4. A.F. Lewis and L.J. Forrestal,“Adhesion of Coatings,” in Char-

acterization of Coatings: PhysicalTechniques, Part I, Vol. 2 ofTreatise on Coatings, ed. R.R.Myers and J.S. Long (New York,NY: Marcel Dekker, 1969), p. 57.

5. H. Gross, “Examination of SaltDeposits Found Under GermanPainted Steel Bridge Decks,” Ma-terials Performance (October1983), 28.

6. S. Spring, Preparation of Metalsfor Painting (New York, NY: Rein-hold, 1965).

7. B.M. Perfetti, Metal Surface Char-acteristics Affecting OrganicCoatings, Federation Series onCoatings Technology (Philadel-phia, PA: Federation of Societiesfor Coatings Technology, May1977), p. 47.

8. G.G. Schurr, Exterior HousePaint, Unit 24 of Federation Se-ries on Coatings Technology(Philadelphia, PA: Federation ofSocieties for Coatings Technolo-gy, May 1977), p. 38.

9. M.A. Winnik and Y. Wang,“Latex Film Formation at theMolecular Level: The Effect of Co-alescing Aids on Polymer Diffu-sion,” Journal of Coatings Tech-

nology (August 1992), 51.10. K.L. Hoy, “Estimating the Effec-

tiveness of Latex CoalescingAids,” Journal of Paint Technolo-gy (April 1973), 51.

11. M.R. Rosen, “From Treating Solu-tion to Filler Surfaces and Be-yond—The Life History of aSilane Coupling Agent,” Journalof Coatings Technology (Septem-ber 1978), 70.

12. P. Walker, “Organo Silanes asAdhesion Promoters,” in Silanesand Other Coupling Agents, ed.K. Mittal (Utrecht, The Nether-lands: VSP, 1992), p. 21.

13. C. Kerr and P. Walker, in Adhe-sion II, ed. K.W. Allen (Barking,UK: Elsevier Applied SciencesPublishers, 1987).

14. R.G. Schmidt and J.P. Bell, in Advances in Polymer Science,ed. K. Dusek, Vol. 75 (New York,NY: Springer Verlag, 1986).

15. P.W. Erickson and E.P. Pluedde-mann, Chapter 6 in CompositeMaterials, Vol. 6, (New York, NY:Academic Press, 1974).

16. E.P. Plueddemann, Journal ofPaint Technology, (Vol. 42,Number 550, 1970), 600.

17. C.A. Kumins, et al. (The Cleve-land Society Technical Commit-tee), “Study of Organic Titanatesas Adhesive Promoters,” Journalof Coatings Technology (August1979), 38.

18. L.B. Cohen, “Corrosion Reduc-tion in High Solids and WaterBorne Coatings Using Zircoalu-minate Adhesion Promotion,”Proceedings of the FifteenthWater Borne and Higher SolidsCoatings Symposium, February3-5, 1988, New Orleans, LA, ed.G.L. Nelson et al. (Hattiesburg,MS: University of Southern Missis-sippi), p. 155.

19. C.H. Hare, “The Evolution ofCalcium Metasilicate in Paintand Coatings,” Modern Paintand Coatings (November 1993),32.

JULY 1996 / 95

TROUBLE with PAINT

Fig 5 - Structure of Surface-Modified Pigment

trouble with paint - adhesion (2)

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