October, 19%
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INDLLYTIZIA L .hVU ENQINEEEING CHEMISTRY
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1051
Colloid Chemistry of Color Varnish' IILPitting, Seeding, Silting, and Surface Dulling By F. E. Bartell and M. Van Loo' U N I W U Y I I Yur M i c i i i , i n ~ .A N X Arrnon. .Mica.
A number of the common defects observed in color varnishes are believed to be due to vortex ring action caused by evaporation of volatile. Various defects, such as pitting, seeding, silting, and surface dulling, are discussed from this paint of view. Pitting is discussed from three standpoints-as arising (a)from high interfacial tension between the varnish and the surface to which it is applied, (a) from the repulsion of the varnish body from nuclei precipitated from the oil body, with the same electrical charge, and (e) from vortex action, which may leave the centers of the cells as open pores in the films. Seeding is believed to be due to (a)the accumulation of
the pigment on immiscible solids suspended in the oil, ( b ) insufficientwetting of the pigment by the oil body, and ( c ) vortex action, due to the piling up of the denser or larger particles a t the centers or cornera of the cells. Silting is considered to he a result of the regular alignment of cells in the varnish film, coupled wich the vortex action described under seeding. Surface dulling is described as due to (uj the interference of light from the uneven celled surface of the film, ( b ) the accumulation of the pigment or other immiscible phase a t the air interface, and ( c ) the addition of water to the mixed varnishes.
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and each particle or group may act as a nucleus of the same electrical charge as the bulk of the medium. Repulsion around such nuclei would result, giving rise to pitting. The explanation is reasonable, though the writers' pfates have shown nothing which they could presume to call nuclei in the pits. Another type of pitting, for which a logical explanation and remedy can he offered, is that presented in films from which the volatile has evaporated from the centers of the vortices at such a rate that the body of the varnish becomes too viscous to flow into and to fill these voids. As a result the film is left with many of these openings, which detract from the appearance of the film, and render i t readily permeable to moisture and weatbering effects. Each pit, of course, represents a vortex cente?naniely, the center of a cell.
HE first paper in this series' described the celled surface
presented by color varnishes on drying, and showed that this structure is caused b y the vortex action occurring during the volatilization of the thinner used in varnish. The various stagea of the secondary flow occurring subsequent to vortex action, tending to eradicate cell structure in a varnish film, werealsodescribed. I n the present paper certain defects of varnishes, and more particularly the relation of the vortex motion during the drying of varnish films to these defects, will be discussed. Pitting Pitting, the formation of pores or open points in the film, %nayarise from any one of several different causes. The first is the withdrawal of the film from certain points on the snrface to which it has been applied, pointing to a high interfacial tension at these spots. Morrell and de Waele4 have offered such an explanation, as have others. If the surface to which the varnish is applied is either damp or greasy, we may expect a high interfacial tension between the two surfaces in contact. This type of defect is not due to the varnish itself, but rather to the mode and place of application, and consequently does not particularly coneern us here. The writers have found varnishes that consistently "pit," regardless of how or where they are applied. Figure 1 shows a carbon black oil varnish that was rejected for its pitting. I n making this plate i t was necessary to use a very thin fihn, which accounts for the sparseness of the pigment. All plates of blacks iu these papers are perforce of thin films, as ot.herwise they would be meaningless. Figure 1 shows numerous open areas throughout the film from which both the oil body and pigment. have receded, leaving undesirable pits. The varnish was applied to a clean glass slide, free from grease, dirt, or moisture, therefore the foregoingexplanation of high interfacial tension does not liold io this case. Morrell' offers another explanation which at times may apply. With certain new varnishes microscopic dispersed particles may deposit on evaporation of the volatile solvent, Received May 4, 1925. Holder or the Acme white Lead and Color Works Fellowhip. * Txrs Jausrrr., 11, 925 (19251. "Rubber, Resins, Paints. and Varnishes," 1920, pp. 197 snd 188. D. V a n Nuhirand Company t
2
'
Fibure 1 X 150
In another papers the writers have discussed the preparation of membranes with uniform distribution of pores which are formed by vortex action in collodion. Such membranes are commonly used in osmotic work. Since there are so many points of similarity between such membranes and a badly 1J
. Pkys. Chcm., 18. 161 (1924).
INDUSTRI.4L AND EN(X N E E M N C CHE,lIISTltY
1052
celled fihn of varnish, it is very possible that similar osmotic effects may he obtained in some faulty varnish films. If any moisture were to collect on the surface of the varnish, the presence of electrolytes, either on the scirface to which the varnish is applied or in the varnish itself from tlie driers, etc., would set up the necessary condition? for osniasis. As a result the varnish would lose its qualities as a pniteetive eoating and the water would pass through it and loosen the varnish from the surface covered, so that rotting or rusting could proceed. The dried film might then act as a gel and swell by imbibition of water. When the film dried once more, it would be free from the surface to which it had been applied, and the expansions and cont.ractions would usually result in a network of fine cracks which vould grow as time progressed. As t.lio light impinging on the varnished surface would then suffer reflection from the numerous surface planes, dulling by interference would invariably result.
Figure 2 X 150
Since pitting of t.he type last described is a result of vortex action which produces permanent pores in the films, the eradication of this defect lies in the control of this same vortex action. I n overcoming a defect of this nature, one must consider the relative concentrations of the volatile and nonvolatile constituents, the rate of evaporation of the volatile, and the rate of hardening of the film. All these factors influence the degree of definiteness and permanency of the cell structures arising from the vortex action. A varnish that will not show this defect must have such properties that after the volatile has passed off and vortex action has ceased the oil body still retains sufficient fluidity to permit the secondary flow to eliminate the cell centers or pores. A smooth and impermeable film can thus be obtained. Morrel16 states that films of shellac in ordinary spirit varnish are very porous when dried in tho usual way hut are quite impermeable when the shellac is applied in the molten state. I n the first ease active vortex action would occur, and we would expect pores to he formed, whereas in the latter case no such action would be expected. Hence we can readily foresee a difference in permeability of the films prepared by these two methods. Seeding Seeding, the objectionable accumulation of the pigment at certain points in the film, may arise from two or three distinct causes. A common cause is the adherence of the pigment to some foreign or immiscible solid in the oil body, such as una Discussion on "Colloidal Chemistry a( Feints and varnishes." Chcm. Trndc I..66,237 (ISZU).
Vol. 17, No. 10
dissolved or precipitated gums or resins, or some other solid impurity. In a certain carhon black mniish examined by the writers, seeding was excessive; the pigment was piled up badly in certain spots at the expense of the surrounding area, which was quite free from pigment. It appeared that seeding in this film was due to the accumulation of the pigment around certain insoluble solids suspended in the oil body. To test their theory, the writers added to a sample of a good carbon black varnish, a small amount of aluminium palmitate, which was quite insoluble in t.he oil body and at the same time was a type of compound that one might expect to find in a varnish. Figure 2 shows the result very clearly; each bit of palmitate is heavily surrounded by pigment, while around these spots may he seen areas almost denuded of pigment. The seeding thus obtained was alinost like that exhibited by the defective varnish, and the theory seemed substantinted. In the aging of certain varnishes exhibiting t,his defect, a Iicary skin or film formed at the surface. If this skin was disintegrated and dispersed tiiroiigh the oil body, each fragment s e r d as a niiclcos for seeding, but if the skin was removed entirely, tlie tondcncy towards seeding was reduced. The solids mising the defect were present in this film or skin. The rcsin-free acids niny lie the constituents causing this aggregnt.ioii of tlie pigment, because thcsc acids can react with bnsio pigments to produce soaps of varying degrees of insolubility,' lording t,o seeding as cited above. The mechanics of the accinirulation of the pigment u n the foreign matter caii be explained from either of two standpoints-nnmely, that of relati7.e interfacial tension, or of relative wetting or adsorption; in the ultimate analysis both explanations amount to t.he same thing. The distribution of solids in a liquid medium is often considered a function of relative surface or interfacial tensions. Our varnish system contains, among other things, an oil body, a snspended pigment, and, in the case of this defective varnish, a certain immiscible organic solid. Hence, within the varnish the following interfaces will come into play in seeding effects: immiscible solid-oil, pigment-oil, and immiscible solid-pigment. At first the two solid phases, the pigment and the immiscible solid, are both covered with oil. However, if tlie interfacial tension immiscible solid-oil is greater than the sum of the interfacial tensions pignent-oil plus immiscible solid-pigment, then the interfam immiscible solid-oil must disappcar. Since, within the varnish itself it can only he replaced by the interface immiscible solid-pigment, a certain amount of the pigment must adhere to the immiscible solid, displacing the oil. I n other words, in case an interfacial tension of a system can he lowered by the accumulation of a constituent of the system at the interface, that constituent will tend to go to the interface. This will account for tho accumulation of the pigment around a solid shown in Figure 2. The same relationships can be considered from the second standpoint, perhaps the better one-that of relative wettabilities. Ry wettahility of a solid we mean the tendency of the solid phase to adsorb the liquid phase surrounding it; in other words, it is a measure of the force of adhesion between the respective solid and liquid phases. Hence, in the light of the foregoing discussion, if the amount of adsorption is low, which means that the force of adhesion is low, the interfacial tension must he high; and conversely, if the amount of adsorption is high, the force of adhesion must be high, and the interfacial tension low. Thus it is evident that the discussions from the two standpoints of interfacial tensions and relative wettabilities are essentially the same. The immiscible solid has a tendency to adsorb the oil 2 Storey,
Drugr, Oils. Paints, 81, 392 (1922).
points in its path the force of gravity will pull it out from one circuit into the next in continually increasing magnitudes. The friction on the particle is greatest at the wall ncar tlie bottom of the cells, and so cventually tlie centrifugal current.s will deposit the heavier particles in a heap at the center, or at the corners, of the cell. Vortex seeding may bo readily identified by the fact that the heaps of pigment are quite uniform as to space distrihut.ion and size. The renicdy for this defect may depend on one or both of two faotors. Equalization of the relative densiiibies of the two phases usually reduces the seeding, or, as before stated, a rcduction of the size of the suspended particles may minimize the gravitational cffeet,s and eliminate the trouble. Again the importance of careful and uniform grinding in producing a stable suspension required for a good varnish is apparent. Even where ample serondary flow follows sccding by vortex action, it is difficult. to flow out the heaps of pigment to urriform dispersion of color, and at best a streaky film usually rcs!iits. Hence vismsity and oxidation velocity do ?!at coilecrn os so rnoeli in this case as thry have in others. The pigment itself is the disturbing factor, and on it depends the success of the elimination of seeding of the last type.
tinctly orientated about individual centers, but all along a line marking the common centers of all the adjoining cells in the series. If there is any tendency for the pigment to pile up at the centers of the cells, because of its density or the large size of the dispersed particles, it is evident that under tlie present circumstances the pigment will pile up along the line running through the whole series, and silting will result. When vortex action has ceased, following the evaporation of tlie major part of tlie volatile, momentarily at least the line of centers nil1 leave a depression in the film. I n order for the oil body to flow into this depression in its secondary flow, it must Aow partially from the outside cell walls. This will accentuate the variations in density of color and increase the grained appearance. In case the coalescence of the cell centers has not occurred to any marked degree, individual pits will remain iminediately following the ceswtion OF vortex action. During mvndazy flow, the varnish will usually Aow down through the individual series as streamlines. These pits may catch and retain more than their proportionate amount of pigment, and thus increase the silting effect. Another point, which has not yet been investigated, concerns a possible relation between the cracking of varnishes Silting and cell structure. deuendent noon tlie effect of Diement on film. ’If this factor enters, because Silting, the acculnulatioll of the pigm:mellt along certain lines tho elasticity of a &;ish of the unequal distribution of pigment, such Brns will be inin the varrlish body, resulting in a grained of the film, seems to be associated with a certain regular alignment clined to crack and to If the cell Walls are permanent, since they are practically of cells. Instead of the honeycomb arrangemeIlt of hemg. tend to free from pigment their coefficient of elasticity will undoubt(jna~cells usually exhibited, the pigmented become rectangular through tlie repression of two sides of the edly differ from that Of the more hexagon. These cells then line up with their longest sides in These inequalities may lead to cracking due to volume changes contact, forming a uniform series of considera& length in during aging, and due to expansions and contractions during cases. ~i~~~~ 4 sholvs this effectin an exaggerated temperature variations. This would be true e w c i a l 1 ~of such films as exhibited the regular alignment of cells just described, since in such a varnish the pigmented and unpigmented areas are in regular sequence. The forces in such a film are not so well-balanced as in the average film, and cracks of considerable length could occur. A varnish film first shows a very minute and intricate network of fine cracks, which grow as time progresses until they are large and visible t.o the unaided eye. It is apparent that we are dealing liere, not only with the elasticity of the film, but with cell structure as well, and both must be reckoned with in overcoming excessive cracking. _
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Surface Dulling
PiBure
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X 150
form; it is a photomicrograph of a varnish film to which a high volatile had been added. The cells tend to be reetangulnr, althongh they still show traces of the double Y centers discussed in the previous paper. Silting, of at least oiie type, appears to be the result of two effects in the film-namely, the regular alignment of cells, and the accumulation of the pigment particles at the vortex centers, as discussed above. When the cells line up as illustrated in Figure 4 the cell walls sometimes are all pernianeut, but usually, when associated with energetic vortex action, the transverse cell walls in cont.act across the series tend to disappear. If this occurs, the vortex action is no longer dis-
(1)
Surface dulling was the defect which originally engaged the attention of the writers in this work. It would appear from their observations that such dullingrnight arisefrnmmore than one cause. The first would be due to the more or less complete interference of light rays impinging on the celled surface of a defective varnish. Figure 5 shows an irregular cell formationin an ultramarine blue varnish film. From the discrams given in the first paper of this series i t is apparent that a varnish surface which is permanently celled is very uneven and made up of the small surface planes of the cells, which are at augles to each other. There is usually no difficulty in observing this ripply surface on a dull varnish, even without a microscope. The principle involved is simple. Maximum reflection would he obtained from a plane surface (refractive indices being constmt) where light interference would be at a minimum. A high gloss on a varnish at least indicates that the surface is smooth. Such a surface is desirable not only because it sheds moisture readily, but also because it is easily cleaned. Again we find the problem of improving dull films of this type centering about the gaging of secondary flow leading to the eradication of the cell configurations. The oil body must
October, lY25
INDUSI'IZIBL A N D EN
