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For the purposes of this discussion, a very significant property of drying oil films is their ability to mrinlrle under certain conditions. It has been postulated that wrinkling is caused by the ~ s\\-elling of a polymer layer which results from the s l o migration into it of monomer from beloiv the surface. The concentration of polymer a t the surface must bc great enough t,hat the surface mechanically can behave like a solid, and there must be enough monomer beneath the surface to s \ d l the polymer to a physically observable extent. I t is thus clear th:Lt there murt be a direct relation between wrinkling and the polymer distribution gradient. From these considerations it is evident that a film with a very steep gradient should wrinkle inore readily than one xr.ith a flatter gradient and, thu:, should wrinkle a t a lower film thickness.
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Since t,he film thickness required for wrinkling is easily measured, this property is at present one of the best available methods cor estimating the nature of the polymer distribution gradient. Turning to the effect of driers, one which produces a vei'y fast reaction, and therefore a rate ratio approaching unit)., would be more likelj- to produce a wrinkled film than a drier type (or c-oncentration) which acts more sloivly. The anomalous effccts produced hy combinations of driers are probably related to this distribution pattern. I t seems that a very fertile field for future research on dricrs and drying oils lies in the exploration of the effect of the physical processe8 on t,he thermodynamics and kiuetics of these reactions. E. B. F'IT%GER.ICI[)
structure of
e
1
E. G. BOBALEIL L. R. LEBRIS. A . S. POWELL, AUD WILLIAM vox FISCHEK Case InstitiLte of Technology, Cleceland 6 , Ohio
HEORETICAL research on organic coatings has provided a wealth of quantit'ative data regarding the physical chemistry of paint constituents, the sedimentation and rheological characteristics of solid-liquid dispersions, t,he permeability and deterioration charact,eristicsof films, and optical properties of pigments. In most such studies, either the experimental variables have been so controlled, or else such simplifying assumpt,ions have been made, that the end results cannot be applied to r e d paint, films with any degree of certainty. i\lucxh of this research \vas motivated by the hope that such data n-ould provide practical predictions regarding the structure, durability, and appearance of organic coatings. This hope has been realized only in part, and a considerable cause of this disappointment has been uncertainties regarding film structure. It has long been apparent that film properties cannot be explained solely in terms of compoqitional variables. Differences in film structure, as caused by variatioiis in the mechanism of film formation, can affect durability and appearance to an important degree. Qualitative data regarding equivalence or nonequivaIence of film s h c t u r e are essential for proper interpret'ation of other data regarding compositional variables, a t least to the extent t,hat it must be knorvn whether reproducible films can be obtained from coatings of even coiist ant, composition. T h e most direct, but, very difficult approach to such essential qualitative information is through microscopy. Ih discouraging, because either no results can be ohtaiiied. or ( what seem like good results are later proved to contain artii'ac,t? t,hat obscure the true picture. At the present time, we have confidence in two techniques, each of which has its recognized limitiitions. SILVER-SII.ICA TECHXIQCE (6). The paint specimen war coated with a thin film of silver by condenmtion of silver vapor in a vacuum chamber. The coated specimen was recovered, and a film of polyvinyl alcohol (the polyvinyl alcoliol used in this n.ork \vas obtained from E. I . du Pont, de Seniours &- Co. arid is dwignated as Elvanol, grade 51-05) was applied to the silver filni and allon-ed to dry, and the silver film with its polyviiiyl alcohol haclting was stripped mechanically from the paint filml exposing the silver negative. The eilver negative was then coated with a thin film of 4 i e a in the vacuum chamber. The composite polyvini-1 alcohol-silver-silica film was then placed in dilute nitric acid, n-hich dissolved away the polyvinyl alcohol and vilver, leaving t,he clean, transparent silica replica. The silica posit'ive v,m shadowed by evaporation of chromium in vacuum at a fixed angle. The shadowed silica replica )vas then observed in the cloctron microscope. Every detail of the procedure must be executed with great care. Alter successful replicas are obtained, a great number must' be examined over a wide area to ascertain what represents the best average area of the original surface. The silver-silica bechnique gives the finest resolution-for cxample. it can distinguish differences in binder structure as illustrated by two samples of baked enamel containing different alkyd resins (Figure 1). The method, holvever, ha? a critical fault.
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Figure 1. Silver-Silica Replicas of Top Surface Left. Nonoxidizing oil-modified alkyd resin Center. Oxidizing oil-modified alkyd resin R i g h t . White gloss enamel
The heat of condensation of silver is very high, and a t practical evaporation rates, the heat developed in formation of the silver negative is sufficient to cause a flow-out of the surface of thermoplastic resins. Hence, if the detail sought is surface roughness of a paint film, it may be obscured because the flow-out of the film smooths the surface (Figure 1, right). Very sensitive films may actually buckle and distort under the conditions of silver evaporation, or the silver film may become fused to the organic film PO tightly that it can no longer be stripped. No techniques are known to the authors which have the resolution of this method and which also cause no heat distortion. Consequently, this technique is practical only for very hard, thermoset films. Within this limitation, however, there are numerous uses-for example, this method may be useful for studying the resin structure of thermosetting films with respect to such variables as change of catalyst, composition, and baking schedules. POLYVINYL *~LCOHOL-SILICA TECHNIQUE (6). In this instance, the first film negative was formed by coating the surface with a dilute aqueous solution of polyvinyl alcohol. The dried polyvinyl alcohol film was stripped mechanically and then coated with silica by the usual vacuum evaporation technique. The silica replica was obtained by dissolving away the polyvinyl alcohol in water, and was chromium shadowed and observed as before. The procedure is simpler in principle than the silver-silica technique, but great care must be taken in its application. When all details of this technique are correct, the procedure is convenient, rapid, and accurate. Its limits of resolution-that ie, where replica structure obscures the structure of the primary surface being studied-is lese than that of the silver-silica method;
structures smaller than 0.02 micron are lost. However, this degree of resolution is adequate for studying most pigment structures in film surfaces. The method is applicable to the study of a wide variety of paint films, both thermoplastic and thermosetting. In situations where both methods are applicable, identical results are obtained, except that the silver-silira method shows detail more clearly a t finer dimensions. Both of these replica techniques were developed for the air interface of paint films. T o view the underside of the film, the paint film was stripped from its substrate without distortion. This can be done in a variety of ways. One method suggested by Hoag ( 4 ) was used in this work. The films were stripped electrolytically. Hot-dip tin plate containing the adherent film was made the cathode versus a platinum anode in 0.25% sodium carbonate solution. A direct current of 1 to 2 amperes at 35 to 40 volts dislodged the film in less than a minute. The amount of film distortion seemed to be slight, as the mirror image of the structure of the metal surface a t electron microscopic dimensions was reflected accurately in the underside of the stripped paint film. These techniques make possible a picture of both the top and underside surfaces of paint films. To complete the story, a view of cross sections would be desirable, but to date the authors’ efforts have not been successful in this regard. Some success has been reported by Kienle (6). It is difficult to expose a cross section without distorting the structure along the surface of the cut. Experiments are in progress to test the possibilities of a low temperature fracture technique and penetration etching, which may or may not be successful. Up to the present time, by using replicas, reliable pictures of
Figure 2. Polyvinyl Alcohol-Silica Replica of Top Surface L e f t . White gloss enamel Center. White semigloss enamel R i g h t . White gloss enamel
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Figure 3. Transmission Electron Rlicrograph Left. Enamel from centrifuge discards Center. Enamel before centrifuging Right. Enamel after centrifuging
hottom and top sides of paint surfaces were obtained, and pictures of very thin paint films obtained directly without the use of replicas. These techniques are sufficient' to show whether microscopy merits furt,her efforts in its experimental development. I n general, the results are encouraging. LUSTER AVD GLOSS
Gloss and lust'er are difficult to define, because the vi-ual impression cannot be correlated precisely v-ith physical measurements. By definition and for reasons of simplicity, gloss is defined frequently as the fraction of incident light that is reflerted at, a specular angle. It is not unusual, hon-ever, for the angle of maximum reflection to differ appreciably from the angle of incidence. Visually, such a surface might be called glossy, or bc said to have considerable sheen or luster, !Thereas the instrumental gloss rating might be low. Stearns has pointed out that othei, t,ypes of visual gloss can be distinguished, such a3 absence of bloom, t>exture,and binocular lust'er (which comes from sirnultaneously vielT-ed surface image and Pome subsurface body) (9). The angular distrihut,ion of light reflect,ed from a surface is affected by roughness of the surface. The effect of roughness will depend on the size, frequency, and order of the irregularities in the surface. It is a common practice in paint formulation t,o vary this effect by varying the pigment volume concentration ( I ) . This latter effect, is illustrated in Figure 2, shoTTing replicas of the surfacefi of a white gloss enamel (17% titanium dioxide by volume] and a semigloss enamel (38% t'itanium dioxide by volume), \$-here the same grinding paste was let' doir-n with different proponions of the same vehicle. The maximum peak to valley height of the projections is about micron. There seem to be no sigiiificaiit irregularit,ies of larger dimensions in t8he
tendency for clear binder to accumulate a t the air interface in paint? of low pigment concentration (9).
In general, the control over luster accomplished by roughelring the surface through increasing the pigment-volume concentration r i the art as a fornear or past t'he critirial value is vrell k r i o ~ ~in mulation practice. What is of more practical concern, hoJTever, to quality cont,rol in paint manufacture is the problem of variations of visual luqter in identical formulas. -lxiomatically, poorer luster is ascrihed to poorer dispersion---that is! if aggregates are present which are large compared to film thickness, such aggregates of pigment particles will project into the surface to cause greater surface roughness. This in effect is the theory behind the draw-doim types of grind gages which record the film thickneys rating in a wedge of paint xhere texture is first observed hy t h e eye at a small viewing angle (a). If aggregates or flocculat'es are present in a poor dispersion, undoubtedly these will produce in the finished paint some texture that affects the visual, even if not the instrument,al luster rating of the coating. IIoivever, t'he presence of large surface roughnew faults is not the only means of varying luster in low pigment volumo concent,rationfilmsof constant composition. These aggregates can lie removed by filtration or centrifugation, and luster variations (;ail yet occur in some instances. .A. reason for this may be apparent in Figure 2 (right), compared Tvith Figure 2 (left') for the baked white gloss enamels of ident'ical composition. Figure 2 (right) s h o w a surface for the lower luster enamel IThich contains a greater concentration of pigment particles impinging on the surface. The extent of dispersion in the aggregate-free part of the surface seems to be about, the same in both samples. I n t8hecase of the good disperiion, the pigment has been attracted away from the surface to leave more clear hinder a t the interface. -4pparently, the difference b e h e e n so-called good and poor dispersions is more fundamental than degree of disper?ion, and hap to do TTith
Figure 4. Pol?P i n j 1 Alcohol-Silica Replica of Bottom Surface Left. White gloss enamel Right. l T hite semiglo-s enamel
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the other factors that influence separation of pigment from the air interface. This hypothesis is supported also by the following observations. Enamels can be diluted sufficiently so that thin films can be cast which are sufficiently translucent t o elrctrons to allow a transmission micrograph of the film itself. Such studies have been made on a variety of enamels of the same composition having varying luster, prepared b y grinding the pigment in different mills-e.g., roller mill, dough mixer (BakerPerlrins KO.4 laboratory mill), rmd pebble mill (11). Generally speaking, the luster Figure 5. Polyvinyl .4lcohol-Silica Replica of Bottom Surface rating of enamels from the dough miver Left. 40% solids white enamel, film thickness 0.8 mil dispersions is lower; however, this luster R i g h t . 60% solids white enamel, film thickness 1.2 mils rating can be improved to equal or surpass the others by centrifuging the finished enamel. Centrifuging in order t o improve the luster rating of white enamels prepared in intensive mixers of the Baker-Perkins type has been a commercial practice for several years. The explanations of why this process seemed necessary were only guesses. Initially, the authors favored the popular explanation that white pigments contained some very oversize aggregates which were fractured into smaller particles by grinding in other types of mills, but not in the Baker-Perkins mill. Several observations, however, disturbed this seemingly obvious conclusion. For example, the Figure 6. Polyvinyl Alcohol-Silica Replica of Top Surface Baker-Perkins ground enamels failed to have improved luster when the dry L f f t . Lacquer with deflocculating agent added R i g h t . Lacquer without deflocculating agent added titanium dioxide pigments were further refined before grinding by fractionation in an air classifier-a treatment which feiior luster, even though the degree of pigment dispersion was should have reduced the number of very oversize particles, if comparable to that of the glossier enamels. The centrifuging such were present, I n fact, enamels prepared from what should process seemed to reject that fraction of the pigment which was have been the coarse and fine fractions recovered from the air wetted less easily by the vehicle. If the centrifuge rejects were classifier had the same appearance as those prepared from the unoriginally made u p of larger, unwet aggregates, then such aggretreated pigment (10). While this test was not conclusive, it did gates were so fragile that they were easily broken up in the prosuggest that the fault could not be explained easily by assuming cedure of mounting the thin films for microscopic evnmination khat the processes of pigment mansfacture were inefficient in re(Figure 3, left). moving a small percentage of oversize aggregates. Even in replicas of the finished, thick enamel films, a careful I n the search for some better explanation, transmission electron count is necessary to establish that more flocculates occur in the micrographs were made of thin films prepared from white enamels surface of the lower luster dispersion. However, the possibility which varied in luster. These pictures showed that the pigment should be considered that the luster variation is actually due to dispersion in all films mas about the same, irrespective of the flocculation or aggregation. It may be that the large particle mill type used, and it mattered very little in the instance of the clusters settle faster and cause a subsurface structure that is difBaker-Perkins enamels whether the enamel had or had not been ferent from that produced in the settling of a good diqpersion centrifuged (Sharples centrifuge). Examination of a number of Thib might give the impression described by Stearns as binoculai samples provided one clue which may have some importance. luclter. It would be interesting to know something about the I n films prepared from enamels of higher luster, most of the pristructure of a paint film beneath the surface, but this would remary pigment particles and small flocculates showed a feather edge quire techniques for viewing the underside and cross sections of or shadow caused by preferential drawing up of the binder around films. To date some progress has been made only with respect to the pigment particles when the thinner evaporated. By constudying the underside of a film. trast, thin films from low-luster enamels showed a greater number There is no doubt that some pigment tended to settle to the adof primary pigment particles or pigment clusters which lacked hesion interface of the film. Figure 4 shows the bottom side of thc this binder shadow (Figure 3, center and right). The same two white enamels of Figure 2 a t 17 and 38% pigment volume conshadow effect can be produced when surface active agents such as centration, respectively. As the film thickness waa increased (and lecithin are added to pigment dispersions in xylene (11). These hence the time of film formation) the segregation effects became observations suggest that, in the glossier enamels, the pigment is more apparent, and the pigment tended to fractionate, so that better wetted by the binder. Better wetting may lessen the larger primary particles (or very small flocculates) collected preftendency for pigment to float in or near the air-film interface. erentially a t the adhesion interface (Figure 5 ) . Large flocculates Enamels prepared from the pigment discarded b y the Sharples are rare a t the adhesion interface, indicating that aggregates or ceriti ifuge (from Baker-Perkins mamels) definitely possessed in-
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ference in the original paints was the adtlition t'o one of a n agent that increased sedimentation rate (Figures 6 and 7). The film from the SIOK sedimenting lacquer showed more flocculat,ion structure in the surfacc than a t the bottom. I n general, the above pictures on enamels and lacquers favor the following tentative conclusions :
Figure 7 . Poljvinyl Alcohol-Silica Replica of Bottom Surface Left.
Right.
Lacquer with deflocculating agent added Lacquer without deflocculating agent added
1. Primary particles or small flocrulates, when Tvet by the vehicle, tend to settle toward the adhesion interface if the pigment-volume concentration is low enough and time of film formation is long enough to allow settling. 2. The larger flocculates (possibly bemuse they are not wetted by the vehicle) tend to concentrate in or -near the air interface of the film. 3. Between high- and low-luster dispersions of films of identical compositioii, there are fine textural differences of the film surface caused by flotationof pigments that may accompany, but are not caused primarily by the existence of a greater number of flocculates. These effects are: of course, superposed ot'her coarse t,exture differences where such occur, but such texture differencc~ alone are not the only modifiers of luster, and even when coarse texture differences are eliminated by screening out large aggregates or flocc,ulates, the microstructure oi film surfaces can differ because of varying sedimentation properties of tho pigments. All luster variations cannot be attributed to differences in pigment distribution. Resin incompatibility in mi binders is kn0n.n t o cause turb though euch difi'erences may be susceptible to examination microscopically, no data are yet, available regarding this particular binder problem. Another example of binder fault which affects luster is provided by the blushing phenomenon in nitrocellulose lacquers. Figure 8 s h o w the structural situation of the surfaces for unblushed and blushed lacquer films of identical composition. The water-blushed lacquer film s h o w a disintegrat,ion into a cellular structure which is coarser than any irregularities that can be caused by pigment,. Structural effects of this kind are detected readily by elect'ron microscopy, and the presence or absence of such binder defects can be allowed for in any correlation studies regarding the effect of pigment properties on luster and color of paint films. 011
Figure 8. Polyvinyl .klcohal-Silica Replica of Top Surface Left. Unblushed lacquer surface R i g h t . Blushed lacquer surface
Figure 9. Polyvinyl Alcohol-Silica Replica of Top Swface Left. Tinted gloss enamel Right. Tinted semigloss enamel
flocculates of a size near 1 micron or greater are less prone to settle than are many much smaller particles. I n all instances of t'hese variations in film thickness the top surface structure remained relatively constant, and had greater roughness due to pigment than did the underside. Even where sediment,ation differences were induced by vaiying film thickness, it seemed that the larger flocculates were more concentrated in the air interface than a t the bottom of the film. Similar sedimentation effects were demonstrat'ed more obviously in films cast from two nitrocellulose lacquers, pigmented with magneyium silicate and carbon black, vihere the only dif-
MIXED PIGMENT SYSTEMS
The study of sedimentation and flocculation structures in films containing mixtures of pigments of widely different particle size is of special practical importance because such observations are pertinent to t,he practical problems of flooding and floating ( 7 ) . The polyvinyl alcohol-silica replica technique is suitable for such studies, a t least to the ext,ent that both film interfaces can be examined for structure caused by accuinulation of pigment therein. Some sample micrographs of top and bot.tom surfaces of baked alkyd enamel films containing 17 and 38% volume concentration of mixed pigment are shown in Figures 9 and 10. The pigment
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composition consisted of 9 parts of titanium dioxide and 1 part of milori blue (by volume). At both pigment concentration values, the pigment-induced interface structure is greater at the top than the adhesion interface, and considerably more structure is apparent than in the roniparable all-white system illustrated in Figures 2 (left and center) and 4. These enamels were prepared by mixing the required quantity of vehicle with pigment pastes containing the same vehicle (43% soya-modified, alkyd resin). The white pigment paste was prepared on a porcelain pebble mill, while the milori blue was Figure 10. ground in a steel ball mill. When ishegrinding of blue was sufficiently thorough to eliminate coarse t,extureof blue aggregates, enamels containing the blue showed variations in color strength (flooding). ’ However, examination of a number of such color-variant filmsfailed to show any differences with respect to top or bottom microPtructure which could be caused by variations in the distribution of the blue pigment. In contrast, it has been shown that, under the same conditions, the white pigment tended t,o show variable sedimentation to the adhesion interface, as illust.rated in Figure 4. I t is possible that, color Ptrength variation in this series of tinted enamels was affected more by the distribution of titanium dioxide pigment than of the blue. If this is correct, minimization of Figure 11, at lead part of the flooding problem ehould emphasize remedies directed to reproducing the sedimentation characteristics of the white pigment of larger particle size. _ Flooding can be observed even where the surface microstructure caused by the pigments of fine particle size is remarkably constant. Another direction of cure of the flooding problems is suggested in the micrographs of Figures 6 and 7 , where the addition of a deflocculating agent (phosphoric acid) to the lacquer influenced a dense sedimentation of the pigment of smaller particle size as primary particles to the adhesion intei,face, and at the same time eliminated preferential flotation of the coarser magnesium silicate to the top interface. An even more thorough st,udy of pigment distributions would he possible if a cross-section view of the paint films could be examined with the same precision as is possible for the film interfaces. Ilowever, even if the experimental problem of the cross section is not solved, examination of the surface structures of mixed pigment films can provide added information regarding film structure to aid other experiments, such as correlating the appearance of paint films with measurements of sedimentation behavior (8) and of the pigment-packing factor ( 1 ) in liquid paints. Although the information is qualitative, it is, nevertheless, useful to know whether measurements on the liquid paint are representing true predictions of trends of pigment distribution during film formation.
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Polyvinyl .4lcohol-Silica Rcplica of Bottom Surface L e f t . Tinted gloss enamel Right. Tinted semigloss enamel
Polyvinyl Alcohol-Silica Rcplica of Bottom Surface Left. Structure of normal adhesion film Right. Structure of superior adhesion film
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ADHESION OF PAINT FILMS
Incidental to the experimnent’al technique of stripping paint films from tin plate, it was noted t.hat someareasof the filmstripped quickly, whereas others separated from the substrate only after prolonged electrolysis. Similar indications of spotty adhesion have been observed in the procedure of lift’ingfilms by undercutting with mercury. Where the ordinary situation of easy electrolytic stripping prevails, the adhesion interface reproduces t,he image of the adherent surface with considerable fidelity
(Figure 11, left). The sections that are difficult to &rip show a structure a6 illustrated in Figure 11, right, which suggests that the film ruptured by cohesive rather than adhesive failure. The consistent reproducibility of these observations sugge&s that only a fraction of the substrate int,erfaceis bouiid strongly to the organic film These observations suggest some use of both the stripping techniques and electron microscopy in other fundamental studies of the problem of adhesion. LITEHATUHE CITEU (1) Asbock, W. A., and Van Loo, hl., ISD.E m . CIIEM.. 41, 1470 (1949). (2) Bidlack, V. C., and Fasig, E. W., editors, “Paint and 18rnisli Production Manual,” New York, John Wiley & Sons, 1951. (3j Conolly, S., Paint Technol., 11, 469 (1946). (4) Hoag, L. E., American Can Co., private communication. (5) Kienle, R. H., Ofic.Dig. Federation Paint & Varnish Production Clubs, S o . 300, 11 (1950). (6) LeBras, L. R., thesis, Case Institute of Technology, Cleveland, Ohio, 1953. (7) Kew Kork Club, Ofic.Dag. Federation Paint & Varnish Production Clubs, KO.323 (1951). (8) Ryan, L. W., IIarkins. W. D., and Gans, D. M., IKD. Esa. CHEM..24, 1288 (1932). (9) Steams, E. I., Ofic.Dig. Federation Puint & Varnish Production Clubs. No. 336. 45 (19531. (10) Trautman, W. D., von Fikher, W., and Bobalek, E. O., Ibid., No. 328, 329 (1952). (11) ron Fischer, W., Trautman, W. D., and Friedman, J., Ihid., KO.298, 843 (1949).
RECEIVED for review September 8, 1953. ACCSPTED Deoember 12, 1953. Presented before the Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Solid-Liquid Interfacial Phenomena in the Paint Industry, at the 124th LIeeting of the AMERICANCHZMXCAL SOCIETY, Chioago, Ill.
