Trouble With Paint - Adhesion (1)

revious articles in this serieshave reviewed the molecularmorphology of coating films

and the effects of physiochemicalstructure on cohesive behavior. A paint film without a substrate,however, is hardly a paint film. Discussing the mechanical proper-ties of paint films without consider-ing the substrate and the forces that hold the films in place is equal-ly meaningless. The mechanicalstresses and abuses of service are directed against the forces of adhe-sion as much as they are directedagainst the forces that hold the film together.

This article will consider adhesionand the techniques for optimizing it.

The Nature of AdhesionPaint film adhesion is generally con-sidered a molecular phenomenon.1

Beyond this concept, numerous the-ories have been put forward, but nosingle theory explains all associatedphenomena. Theories include theweak boundary layer concept ofBickerman2, the electrostatic theoryof Derjaguin3,4, the molecular diffu-sion theory of Voyutski5,6, and ad-sorption theories such as Schorn-horn’s7 and Guerra’s.8 As will bediscussed below, bulk phase consid-erations include the absorption andentanglement of the paint film intothe porosity of the substrate, whichis sometimes termed “mechanicaladhesion.” This type of adhesion,also referred to as “lock and key”adhesion, will be discussed in moredetail next month.

MAY 1996 / 77

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Adhesion I

Fig. 1 - Paint film wetting–vector quantities at equilibriumFigures courtesy of the author

by Clive H. HareCoating System Design Inc.

Copyright ©1996, Technology Publishing Company

Copyright ©1996, Technology Publishing Company MAY 1996 / 79

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It is likely that all of the above ef-fects may play some part in paintfilm adhesion. On some substrates,such as paper, wood, and concrete,the mechanical effects may be sub-stantial. On metals, glass, and plas-tic, however, molecular bonding ef-fects predominate. When molecularbonding predominates, it resultsfrom chemical linkages (primary va-lency bonds) between reactivespecies on binder and substrate ormolecular associations (secondaryvalency bonding) between the two.Such forces are operative onlyacross very limited distances. For ad-hesion of any kind, the coating andthe substrate must approach eachother extremely closely. Separationmust be no greater than approxi-mately 5Å units (1Å=10-10 meter),which are molecular dimensionsabout 3 times the diameter of theoxygen atom. These attracting forcesalso diminish at the sixth power ofthe distance between attracting moi-eties. (That is, if the distance dou-bles, the force is diminished by afactor of 64 [26].) To achieve thislevel of approach, the coating mustwet the substrate. The coating mustspread across the substrate, eitherspontaneously or with the aid of theforces of application. It must notbead up or draw back from the sur-face after application.

WettingWetting and its relevance to adhesion have been discussed com-prehensively by a number of author-ities, including Zisman9 and Adam-son.10 Reference 11 is a goodintroduction to the subject. Wettingis an effect of surface energy thatdepends on the relative surface freeenergies, or surface tension, of thecoating and the substrate. For anygiven coating and substrate model,the coating will wet the substrateonly when the interfacial energeticsare appropriate.

All surfaces have surface free en-

ergy (also known as surface ten-sion). Molecules at the surface of aliquid tend to orient themselves tominimize the surface area of the liq-uid. The phenomenon is more visu-ally recognizable in liquids than insolids. In liquids, surface tension isdemonstrated in the spherical shapethat is naturally assumed by the un-restricted volumes or drops of liq-uid. Here, the surface tension of theliquid constricts the surface of thedrop to the smallest surface areapossible that will contain the liquidvolume. Solids also have surface freeenergy, however, and will minimizetheir own surface area by spreadingliquids across this surface. This ten-dency is opposed by the surface ten-sion of the liquid (such as a paint)that is applied to the surface and bythe interfacial free energy that mini-mizes the area of the interface be-tween the paint and the substrate.

Contact of the paint (p) with the

substrate (s) is represented in Fig. 1.Here, γsv is the surface free energyof the substrate in air (expressed asv [vapor]), γpv is the surface tensionof the paint in air, and γsp is the in-terfacial tension between the paintand the substrate. Treating the ener-getics as vector quantities, at equilib-rium these forces may be represent-ed by Young’s equation:

γsv=γsp + γpvcosØ. Theta (Ø) is the contact angle between the liquid on the surfaceand the surface itself (Fig. 1). If the coating is to completely wet the substrate, i.e., if the liquid is not to bead up at all, then the con-tact angle Ø must equal 0 (i.e.,cosØ= 1). Wetting will occur, there-fore, when γsv ≥ γpv + γsp.

The surface energy of the sub-strate (γsv) should never be lowerthan the surface energy of the paintbeing applied (γpv). On clean met-

continued


als, the surface free energy of themetal oxide film is normally veryhigh (several hundred dynes/cm). Itis not usually a problem for coatingswith relatively low surface energies(20-70 dynes/cm) to wet such sub-strates. On certain low-energy plas-tics, however, surface energies mayrange from 15 to 20 dynes/cm. (Theinherently low surface energy of flu-

oropolymers is utilized in non-stickcookware applications).

Even some low surface energycoatings may not wet these plasticsproperly and will have problemswith adhesion. Adhesion problemscan occur on polyolefins and fluo-ropolymers or surfaces that havebeen contaminated with oils orother hydrocarbon impurities, sili-

cone, or fluorochemical fluids be-cause these contaminants reduce thesurface energy of the substrate. Amonomolecular layer of oils and sili-cone fluids will prevent the paintfilm from wetting the substrate. Be-cause of their low energy, these ma-terials will spread themselves readilyover high energy surfaces, loweringthe surface energy of the surfacefrom several hundred dynes/cm toless than 20 dynes/cm. These sub-stances must be carefully removedto ensure good adhesion of subse-quently applied coatings. The effectof such minute quantities of hydro-carbon oils and dimethyl siliconefluids on the wettability of all kindsof surfaces is the foundation fortheir use as lubricants.

Adhesion to High Energy SurfacesThe key to obtaining good adhesionto metals and other high energy sur-faces is, therefore, to ensure that thetrue substrate is presented to thepaint. Usually this involves the re-moval of foreign materials that inter-fere with adhesion. These materialsinclude oils and other contaminationthat reduce surface energy and loosematerials that restrict access of thepaint to all areas of the substrate.The positive consequences of sur-face renewal are magnified when itcan be done simultaneously with theexpansion of the real surface area ofthe substrate, compared to the ap-parent planar area. This result oc-curs with scarification techniquessuch as sanding and abrasive blast-ing. In air, thin oxide and hydroxidefilms will reform almost instantly onmost structurally important metalsubstrates. Unlike rust, these filmsare well bonded to the metal andwill serve as reactive sites that attractpolar and chemically reactive groupson the polymer binder of the paint.Molecular associations between thesubstrate and the paint can thenoccur on these sites.

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80 / Journal of Protective Coatings & Linings


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Good surface preparation will alsoremove damaging contamination,such as soluble salts and other hy-drophilic species. If coated, hy-drophilic species will reduce adhe-sion and may reduce corrosionprotection. This phenomenon willbe discussed in a later column.

Adhesion to Low Energy SurfacesLow Energy Plastic SurfacesOn low energy, highly crystallineplastics, cleaning will have little orno effect. The removal of mold re-lease residues and machining lubri-cants from high energy plastics, suchas epoxies, phenolics, and acrylics,may result in better adhesion of low energy paint films. Cleaningpolyethylene, polypropylene, andpolytetrafluoroethylene, however,will only bare a substrate that is in-herently low energy and, therefore,cannot be wetted by coatings. Tech-niques for improving the ability ofthese widely used plastics to becoated involve surface treatmentsthat change the chemistry of the sur-face by oxidation.12

These techniques were traditional-ly achieved with oxidizing agentssuch as chromic acid. Because of thecosts and difficulties associated withwaste disposal, these traditional pro-cedures, which were effective, arenow being replaced by other oxidiz-ing methodologies. New approachesinclude treatments using propanetorches, corona discharge techniquesin which the surface is oxidized inan electrically generated field of ion-ized air or oxygen, and even ultravi-olet-generated free radical attack onthe plastic surface. In one importanttechnique, free radicals are generat-ed in the surface of the plastic byabstracting hydrogen atoms withelectrically excited rare gases. Thistechnique, known as CASING (cross-linking by activated species of inertgases), is now used extensively.These oxidative techniques generatepolar groups, such as ethers and hy-

droxyls, on the surface of the non-polar plastic. The polar groups in-crease the surface energy of thesolid and provide sites for subse-quent primary valency-bonded ad-hesion with coating systems bearingcomplementary functionalities (i.e.,isocyanate groups).

In some cases, low energy sub-strates (such as polypropylene) arecopolymerized with amorphous,rubber-like materials to improvetheir ability to be painted. Evenhere, the immediate surface of theextruded plastic may remain highlycrystalline.13 For subsequent adhe-sion, this surface skin must be pene-trated by the solvent system of theprimer, which may then more easilyswell the rubber-rich layers beneaththe immediate surface. Thin coats ofchlorinated polyolefin- (CPO) based“wash primers” are commonly usedto help the paint system adhere tothese surfaces. It has been postulat-

ed that adhesion is affected by diffu-sion of solvents of the CPO pretreat-ment through the crystalline skinand into the more susceptible rub-ber domains. The CPO binder is car-ried with the solvent, which resultsin a “mechanical interweaving” ofprimer and substrate14 (Fig. 2).

Thermoplastic substrates with re-duced crystallinity may be coated bycarefully selecting a coating with an appropriate solvent system. Highboiling solvents that are soluble in the plastic will penetrate thesolid. They will reduce the glasstransition temperature of the surfacelayers by expanding the free volumeof the surface, allowing more sol-vent and molecules of the coatingresin into the plastic surface. As thesolvents are desorbed, the coating,resin, and plastic surface remainlocked together, and a type of me-chanical bond forms.

continued


Low Energy Organic Coating SurfaceThis phenomenon underlies the“solvent weld” effect that ensures in-tercoat adhesion between successivecoats of vinyl- and chlorinated rub-ber-based coating systems. The trickwith some plastics is to use the ap-proach without crazing the plasticsurface. Crazing is a fine, randommicro-checking that occurs in thesurface of high Tg thermoplastics,such as polystyrene and polymethyl-methacrylate, as strong solventsfrom the applied coating film are ab-sorbed into the plastic and subse-quently desorbed during drying. Thephenomenon is thought to becaused by shrinkage stresses in thesurface of the plastic. Presumably,initial solvent release from the sur-face of the plastic and surface dryingentraps solvent residues just belowthe plastic surface. As this solventthen diffuses through the drier sur-face layers, the resultant volumetricshrinkage in the lower layers causesstresses that result in cohesive fail-ure of the surface layers.

A similar solvent welding device isoften used to improve the intercoatadhesion between successive ther-mosetting paint films, particularlywhere long recoat times haveelapsed. Here, existing film is rela-tively insoluble in the solvents of therecoat system. but with judicious se-lection of a solvent system, somediffusion may occur. Adhesion isprobably achieved by the migrationof binders from the recoat systemalong with solvent into the surfacelayers of the existing film. As thesolvent evaporates, the binder of therecoat remains entrapped within thesurface of the substrate film. The ef-fect is not unlike that noted abovewhere crystalline plastics are coated.With alkyd and oil thermosets, theeffect may be great enough to liftthe existing system. (See the paintremover effect noted in the January1996 JPCL , pp. 78-80.) In other

cases, the effects may be so slowthat the solvent must be appliedseparately to the coating and be al-lowed to sit and swell the surface ofthe existing film before the new filmis applied. Excessive delays in re-coating hard, coal tar epoxy filmshave thus been overcome withoutsweep blasting using applications ofn methyl 2- pyrrolidone. The authorused a similar technique many yearsago, adding methylene chloride tointerior latex paints to improve theiradhesion to glossy, aged, and welloxidized alkyd films.

Primary Valency BondingChemical adhesion is related to themolecular structure of the paint filmbinder and the substrate. For prima-ry valency reactions to occur acrossany interface, both binder and sub-strate must have mutually reactivefunctional groups. Metal and metaloxide surfaces, for example, willreact with acid groups. Polymerswith this type of functionality makegood binders for coatings applied tometal. The value of this functionalityis exemplified by the adhesion prop-erties of maleic acid-modified vinylchloroacetate lacquers and by thephosphoric acid-based etch primers.The adhesion of etch primers (a pri-mary valency bond) has been mea-sured to be 6 to 24 times that of asecondary valency bonded primer(12,000 psi [840 kg/cm2]) as opposed to 500 psi (35 kg/cm2) fora polystyrene-based system and2,000 psi (140 kg/cm2) for apolyvinyl acetate-based system).15

Vinyl chloroacetate copolymers areso highly cohesive that, when un-modified, they display poor adhe-sion to metals.

If small amounts of a reactive carboxylic acid (i.e., maleic acid) are interpolymerized into this poly-mer, however, the resultant vinylchloroacetate structure is inter-spersed with pendant acid groups.Although the modification is very

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low (about 1 percent), the acidgroups profoundly improve the vinylbinder’s adhesion to metal. Themetal maleic acid linkages anchorthe film to the substrate.

Other examples of primary valen-cy adhesion are as follows. Primaryvalency linkages may also beachieved in urethane coatings onwood. Here, isocyanate groups reactwith cellulosic hydroxyls. When acetoacetylated resins are used,there is some evidence of chelationwith iron and possibly other metal surfaces.

Primary valency linkages may alsobe used in adhesion to certain plas-tics and other coating films. Inter-coat adhesion between urethane fin-ish coats and epoxy primers may befacilitated by using the cross interfa-cial reactions between isocyanates inthe finish and the pendant hydroxylson the epoxy molecules of theprimer or unreacted amine groupson the curing agent. In some cases,excess functionality may be used de-liberately in a primer to be taken upby complementary groups in the fin-ish. Munger16 associates the tena-cious adhesion of inorganic zinc-richprimers on virgin steel to iron sili-cate linkages formed between theiron substrate and the silicic acid ofthe coating binder.

Secondary Valency BondingMost coating binders attain adhesionby secondary valency associations,such as hydrogen bonds and Vander Waals forces. The bonding ener-gies of this type of bonding are con-siderably reduced compared to pri-mary valency or chemical bonds.(See the October 1995 JPCL, pp.177-187.) With most coating bindersapplied and dried at room tempera-tures, there is far more opportunityfor this type of chemical bonding,sometimes referred to as specific ad-hesion, than there is for primary va-lency adhesion on most substrates.

Polarity in substrates and paint

binders is the key to secondary va-lency attraction. Binders rich inether, ester, and amide linkages aswell as other more reactive species

(such as hydroxyls, carboxylic acidgroups, and amines) adhere betterthan less polar systems such as poly-

continued

Fig. 2 - Adhesion to polyolefins with solvent-borne CPO (chlorinated polyolefin) tie coats.


styrene and vinyl toluene copoly-merized with butadiene. The pre-ponderance of such groups inalkyds, epoxies, and polyurethanecoatings has much to do with theexcellent adhesion of these films.

Unfortunately, the very groupsthat improve adhesion, such as hydroxyls, favor the uptake ofwater, which can adversely affectcoating system performance. This di-chotomy and the appropriate formu-lation response will be discussed ina later column.

Most substrates are rich in polargroups, which interact with the paintfilm binder to attain good bonding.The surfaces of all metals, exceptthe noble metals, are chemically different from their bulk phase andare naturally covered by reactionproducts of the metal and its environment. Where these productsare tightly adherent, such as theoxide on aluminum, they will gener-ally contribute to good adhesion.Where the oxide surface is less ad-herent (e.g., on iron) as notedabove, it must be removed beforecoating. The thin, impermeableoxide layer that immediately reformson the newly bared iron surface is,however, when newly formed, ad-herent and suitable as a substrate forgood adhesion.

Glass and siliceous substrates arewell endowed with silanol (-SiOH)groups. Wood, paper, and other cel-lulosics are rich in hydroxyls. Asnoted above, one key to obtainingadhesion on polyethylene plastics isthe oxidation of its surface to intro-duce polar groups. These may serveas hydrogen bond donors and ac-ceptors for polar groups on coatings.

The next article will continue thediscussion of adhesion, focusing onnon-ideal surfaces and techniquesfor upgrading adhesion.

References1. Paul Swaraj, “Physico-Chemical

Interpretation of Paint Film

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Adhesion,” JCT (September1982), 59.

2. J.J. Bikerman, The Science ofAdhesive Joints, Second Edition(New York, NY: Academic Press,1968).

3. B.V. Derjaguin, Nature (Number4480, 1955), 495.

4. B.V. Derjaguin, Research 870. 5. S.S. Voyutskii, Journal of Polymer

Science (Vol. 32, Number 528,1958).

6. S.S. Voyutskii, Autohesion andAdhesion in High Polymers,(New York, NY: Wiley, Inter-science, 1963) 138.

7. H. Schonhorn, Polymer Surfaces,Chapter 10, ed. Clark & Feast(New York, NY: Wiley, 1978),213.

8. C.R. Guerra, T.W. Healy, andD.W. Fuerstenau, Nature (1965),207, 518.

9. W.A. Zisman, Adhesion and Co-hesion, ed. P. Weiss (Amsterdam:Elsevier, 1962), 176.

10. A.W. Adamson, Physical Chem-istry of Surfaces (New York, NY:Interscience Publications, 1960).

11. D.J. Shaw, Introduction to Col-loid and Surface Chemistry,Fourth Edition (Oxford, Eng-land: Butterworth, Heinmann,1992).

12. R.A. Ryntz, The Painting of Plas-tics, Federation Series on Coat-ings Technology (Blue Bell, PA:Federation of Societies for Coat-ings Technology, 1994).

13. R.A. Ryntz, Q. Xie, and A.C. Ra-mamurthy, “Effect of CoatingSolvents on the Morphology ofThermoplastic Polyolefins(TPO),” Journal of CoatingsTechnology (April 1994).

14. Y. Aoki, Journal of Polymer Sci-ence, Part C (1968), 23, 855.

15. T.R. Bullet and A.T.S. Rudram,JOCCA (November 1959), 789.

16. C.G. Munger, “Inorganic ZincCoatings—Past, Present and Fu-ture,” Materials Performance(May 1975), 25.

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