INDUSTRIAL AND ENGINEERING CHEMISTRY
hDtember. 1925
925
Colloid Chemistry of Color Varnish' I-Behavior of Surface Films By F. E. Bartell and M. Van Loo2 U N I V E R S I T Y OF M I C H I G A N ,
ucts largely prohibits the statement of fundamental generalizations. A varnish represents a very complex colloid system, in which a number Of different may be simultaneouslypresent. It usuauJ' contains an Oil, a9 linseed or China wood oil; a resin; a thinner, 3uch turpentine Or a petroleum such as lead, cobalt, or mangan e s e compounds; and a
ANN ARBOR, MICH.
The importance of the application of the principles of colloid chemistry to the study of paints and color varnishes is discussed. Vortex-ring action resulting from the evaporation of the volatile has been observed in microscopic examinations of drying varnish films. Surface cell formation is described, as well as the various stages of subsequent flow up to the point of uniform pigment dispersion. The paper indicates the dependence of the type and quality of the varnish film on the character of the vortex action and the extent of flow.
such systems, gradually increasing the Scope of Work
One of the writers has been conducting researches along this line for about four years. Considerable qualitative data as well as quantitative data have been collected. Extension of the quantjtative data is now under way. The preliminary work has brought out certain observations which
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They not o d y ilifliicm the stability of variiiiiii duriirg st,orag(>, but also may have a pron(,uriced eikct upon the varnish film as a protective and as a derorat,ive coating. Behavior of Disperse Phase during Drying of Varnish
The work on defective variii.lies lras been chiefly in the fiald of ultramaritre blue mid c a r h i i black color varnishes.
FiOure I---Ceil S f r u ~ f u c in e a Dried Film of liltromnrine Blue varnish. x 1.50
Vol. 17, No. '3
Vortex Action in Varnish Films
Wlinn a microscopic examination of the drying film is iiradc immediately after spreading the varnish on the glass,
the film is fairly iiiiiform in appearance. The pigmeut seems to lie evenly distributed, but is visibly in motion, apparently in every direction. This has erroueously been called a Urownian movement. Almost at once, however, there is an accumulation of the pigment at t,he centers of various roiinded surfaces vhich n o v appear iu the film. It is apparent that wliat actually is occurring is a vortex action, more cxaotly vortex-ring action, with the apexes of the rounded surfaces as centers. The dark spots nom, disappear owiiig to the motion of the pigment radially outward along the ring circuit, and the pigment is seen to he moving in a regular manner. The centers thus appear free from pigment aiid appear as clear spots in the film. The rounded surfaces, through their mutual pressures on each ot.her, give rise to polygonal cells instead of spheres or disks. At this stage of the process the motion of the pigment is usually rapid and tlie cells vary considerably in oonfiguratiori and intensity of action. This stage is but transitory and is followed by one in which the large number of smaller cells give way to comparatively few larger cells of approximately the same size and intensity of action. It is obvious that if all the vortexes were of equal force, disks or cylinders of equal radii would be set up. Eaih cell would then be in contact with six others, and tlirough their mutual pressure on each other would produce hexagonal cells, as demanded for the condition of closest packing. Figure 1 shows that in some varnisli filnis this state is closely approached.
I l i e original work lending to tlie discovory of the vortex phciiomenon, to be diserissed later, was conducted with the intent of studying the defect of surface dulliiig of a certain ultramariiie blue varnish. Tlir nti(.roscope, which has not received the attention that it deserves in such ~v.ork,G played ai1 important part in the observations. A small drop of the thoroughly st.irred varnish was pla.ced on a clean glass slidt! Ercc from grease and moisture, and was then smoothcd out riipidly by tilting and rotating the slide. With a little experience a fairly t,liin and uniform film of the varnish was t,hos obtained witliout brushing, and was admirably suited for niicroscopic study. These slides were examined at once, and then at intervals until the varnish had dried and set, the periods varying with the thinner used. This procedure furnished an excellent means f i x studying the process of drying, and gave valuable information as t o the eharaeter aud quality of the varnish itself. Care had to be taken t,o aroid distorting effects due to failure to level the slide, or to undue heat from the source of illumination in case intense illumination became necessary. It was found that with both good and poor color uarnislies the surface of practically every film exhibited a t some stage in it,s drying a distinctly cellular structure, somewhat of a iiiirioycornb nat.ure. The only apparent variations were in the regularity of size, shape, and permanency of the cells. Blsure 2-Uirgram of Surface of Film, Showing Formation of Meragonst cciis Figure 1 shows the characterist.ie cell structure in a dried film of an ultramarine blue varnish. The clear lines mark the ecll walls, tlie open spots within them are tlie cell cent This Iiexagoiial structure resulting from vortex-ring action iLnd the dark Ppecks are the sunpeuded particles of ultramarine in system of a rather related character has bcen reported bliie pigment,. Practically all the varnishes studied have by er:vr!riil observers. Grant' describes the vortex phecxsliihited some type of surface forinat,irii? similnr to this, iimiicuoii iii emulsions uf radioact,ive minerals prepared for :uid appnrentiy the iiivariable Rppearaiire of this (.cllular X-ray examinatioii, as xell as with cdrborundurn, graphite, striirtare a t some tirnc diiriiie the murso of drying depends l.yr:opodium, etc., io ether, alcohol, and molten paraffin. mi some fundamental property or properties of t,lie system He holds that the action is due to vertical doublets at the H r l i w an investigatioii was mad(? of tlu, re:isoiis for such renters of the d l s . Thornsons ascribes tlie cellular effects that, IIC ol~t.ainedin soapy water to convection currents. cr~nfi:.iirations. I >
i : : i r d o w paint >firs ~ r r o c .I' .lid Ins , 28, 33 (I'Jaa,
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(ireen. Chem.
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In IS55 Weber9 uhiervcd this cellular arraogemerit of w n - or aft,er the varnish lilni itself Iias become sufficiently viscous vection currents in suspensions of gamlmge in a mixturc of to resist such motion. When a varnisli. composed as it is alcohol and water. Wagerlo suggests that this same plie- of volatile and nonvolatile i.onstituents, is spread out into nomenon, which he discovered with minute mobile organisms a thin film, evaporatioii takes place a t the very first from in water and with rarious types of sediments in water and practically every point of the iilm. This initial evaporation other liquids, may represent "cohesion figures," and suggests that the f o r m causing the motion may he due to cohesion. suriace tension, diffusion currents, and gravity. Petrie" and Keen'2 report the same surface arrangement in suspensions of finely powdered oriental coffee, in milk and wat.er, and in suspensions of clay in ammonia and in alcolioi. B4nard'a obtained uniform cellular surface configiimtiom in permanent media, such as paraffin, etc., wing coiivectioii currents as the agent ior producing the vortex act,ion. Amoiig other workers, I.edid4 and Woodl6 might be mentioned. The earliest. rcicrerice that \ire could find to this phenomrrro,i under conipsrahle conditions was that given by Kortcwcg" in the notes of Christiiin Iluygens, reprinted in his "Oeuvres." n4rere lie descrihix wit,li the aid of a simple diagram i,xnctly this same phenomeiioir in drops of urine on Lalc, uiider tlic date of Scptembcr 5, 1678. No record, however, f:oold be found of work of this type being carried out upon varnislies. where the writers believe it has a distinct conirncrcial application. This vortex molioii e m easily he demonstrated by siispending some colored pigment, such as ultramarine b h q Pieure 4-Vortex Action Occurring LISn Double Y I . X 150 in chloroform or nretoiie, 2nd pouring a thin layer on a slide. Under the microscope the swift but regular motiiin of the rather sharp and iluite localized pigment indicates a violent vortex action. Remarkable of part of t.hc volatile cii,ii dei:rcase in temperatiire e snriace of the film. Conpled vorkxes may be prodneed during tlie drying of colored collowith this is a loi::ihaed increase in reliltive ilcnsit,p at the surdion membranes." face due to the decrease in conca,tratioii o i tlic less dense i:oniponent, the volatile. Both of then: eff tm-e gradient and the density gradient,, a i rection to produce a flow of the warmer and less dense medium a t the lower portions of the film to the surface. In other words, vertical convection currents are produc!c
