PARTICLE AGGREGATION A Factor in the Reflection of Light by Paints DONALD F. WILCOCK', University of Cincinnat'i, Cincinnati, O h i ~
During the development of paints to reflect ultraviolet light, it was noted that the reflection factors of certain paints decreased as the time of grinding in a pebble mill was increased. The decrease i n reflection is accompanied by a decrease i n the proportion of aggregates. The effect is greater, the smaller the difference i n refractive index between the pigment and the vehicle. This suggests that the effect is due to the presence of entrained air i n the aggregates.
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CCH attention has been devoted, both theoretically
and practically, to the influence of particle size on the efficiency of the reflection of light by pigments and to the measurement of particle size. A white pigment, whether in a paint or dry, reflects light by a process of successive reflections and refractions of incident light by its particles until it emerges as reflected light. Shoulejkin ( S ) , among others, showed the theoretical relation between the size of a particle and the manner in which it scatters light; and Casperson's calculations (1) showed that there is a n optimum particle size for scattering and reflection which is of the order of magnitude of the wave length of the incident light. The practical result of such studies is the knowledge that a pigment with particles of the optimum size for scattering will have better hiding power, less light absorption, and greater over-all reflection than the same pigment with larger or smaller particles. Relatively little attention has been devoted to the effects of aggregates or clusters of pigment particles in paints on hiding power and light reflection. Indeed, efforts have usually been directed toward their elimination. One purpose of the grinding or dispersing mills used in the manufacture of paints is to disperse the aggregates into single particles in so far as possible. It is the intention here to show a t least one aspect of the problem of reflection in which the presence of aggregates may be desirable.
Experimental I n the course of work on the development of paints to reflect ultraviolet light (4),i t was noticed that the ultraviolet reflection factors of certain paints were lower when they were ground for 70 hours in a pebble mill with pebbles than when they were mixed for 70 hours in the same mill without pebbles. One would have expected that grinding in the pebble mill with pebbles would result in a better dispersion of the particles. Since the particle sizes of the pigments being used were larger than the optimum, the better dispersion should have caused 1
little change in the reflectances of the paints, and that change should have been in the direction of higher reflectance.
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A series of paints containing ma nesium carbonate as pi ment and a 60-40 mixture of ethylceflulose and tri-(p-tert-but$phenyl) phosphate (Dow plasticizer 7) dissolved in 80-20 tolueneethanol as vehicle was prepared in order to study the effect of grinding time on reflection. The pigment constituted 40 per cent of the solids by volume. The paints were identical in every respect except for the len h of time thev were ground in an 8-inch porcelain pebble m$ at 90 r. p. m. The ground paints atered much more readily through cloth varnish filters than did the unground paint. All paints were sprayed on steel panels to such a thickness that practically complete hiding and maximum light reflection were obtained. The reduction in reflection factor with increase in time of rindin is shown in Figure 1 for both ultraviolet and visible tght. %he ultraviolet reflection measurements were made in the band 2800-3200 d. (4). The visible reflection measurements were made with a sphere reflectometer giving relative values based on a magnesium carbonate block as a reflection standard, and using a Weston photronic cell and a copper chromate filter. The points at 0 time of grinding are for the paint mixed in the mill for 70 hours without pebbles.
u 20
40
60
2o0
HOURS GROUND IN PEBBLE MILL
FIGURE1. EFFECT OF TIMEOF GRINDINQ ON REFLECTION OF MILLED PAINT G-10 Microscope slides were made from the 0 and 70-hour paints in order to study their structures. Each paint was diluted 5 to 1 and 50 to 1 with a resin solution of the same viscosity as that used in the paint. A drop of diluted paint was then placed on a slide and drawn out in one stroke by the edge of another slide to a thin film, Photomicrographs of the slides (Figure 2 ) were made by transmitted light. Single particles appear with a clean line border and a clear center. Some a gregates appear perfectly black, indicating that they are relecting practically all the light, while others are less opaque and the fact that they are composed of a number of small t,ransparent particles can be seen. The phot'omicrographs illustrate the increase in the degree of dispersion of this magnesium carbonate paint as the result of grinding, which had been indicated by the rate of 51-
Present address, T h e Sherwin-Williams Company, Chicago, Ill.
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July, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ground in pebble mill, 5 :1 dilution
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Unground, 5 :1 dilution
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Ground in pebble mill, 50 :1 dilution Unground, 50 :1 dilution FIGURE2. MAGNESIUM CARBONATE PAINT,G-10 ( X 173) tering. The ratio of number of aggregates to number of single showed the much greater proportion of aggregates in the particles in the ground and in the unground paints was deunground paint. termined from the photomicrographs, and is shown in Table A comparison of the reflection factors of several ground and I. The number of particles counted on each photomicrounground paints, together with the refractive indices (for graph was from 1000 to 4000. Because the very small particles do not show on the plates, because there are borderline cases where it is not certain whether a group of particles is TABLEI. AGGREGATE COUNT a n aggregate or a naturally occurring coincidence of particles, Ratio of Aggregates t o Single Particles and because some aggregates are probably swept off the slide A gregates smaller Aggregates larger or disoersed during its preparation, the count cannot be conPaint Dilution t f a n 10 miorons than 10 microns sidereb a measure-of the actual proportion of aggregates in G-1Oakround) 5:l 0.03 0.001 G-lOa(ground) 50:l 0.02 0.0007 any one paint but rather an approximate G-lob unground) 5:l 0.07 0.007 G-1Oblunground) 50:l 0.04 0.006 measure of the relative proportion of aggregates in a series of paints being compared. The proportion of large aggregates in the TABLE11. EFFECTOF GRINDING ON PAINTSCONTAINING DIFFERENT unground paints is probably larger than inPIQMENTS Ultraviolet Reflecdicated. Nevertheless, the greatest difference Refractive Index tion Factor, % between the ground and unground paints is Pigment V ehic1e Pigment Vehicle Ground Unground found in the fraction of large aggregates, hlg carbonate Ethylo?llulose-Dow 7 1.5-1.54 1.5 32 64 Acryloid 1.5-1.54 1.5 36 66 showing, as might be expected, that these are ~ ~ $ ~ ~ ~ a $ a k e Ethylcellulose-Dow r ) 7 1.74 1.5 47 70 the first to be affected by the grinding process. MgO, heavy oarbonate (Sauibb) white Ethylcellulose-Dow 7 1.74 1.5 35 59 Visual comparison in a Zeiss-Thoma bloodlead Acryloid 2.0 52 1.5 51 count chamber @?)under the microscope also
INDUSTRIAL AND ENGINEERING CHEMISTRY
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visible light) of the pigments and resins from which they are made, is shown in Table 11. The decrease in reflection factor on grinding is greater, the smaller the difference in refractive index between the pigment and the vehicle.
Conclusions The decrease in the reflection factor of certain paints on grinding is accompanied by a decrease in the proportion of aggregates. Since the paints are otherwise identical, it is concluded that a large part of the reflective power of such paints must be attributed to the presence of aggregates or clusters of pigment particles. The further fact that aggregates are much more effective when the pigment has almost the same refractive index as the vehicle suggests that air trapped within each aggregate may be the cause of their effectiveness. A necessary condition for this would be moderately poor wetting of the pigment by the vehicle so that the aggregate interstices would not be penetrated by the vehicle until aided by the grinding process. Other vehicle combinations (e. g., ethylcellulose-Vinylite, isobutyl methacrylate-butyl stearate) did not show the effect of aggregates with the same pigments; their paints retained high reflection factors after grinding, which indicates very poor wetting of the pigment. A vehicle which wet the pigment readily, on the other hand, might be expected to have low reflection factors, both before and after grinding. Inert pigments, which have an index of refraction about the same as that of most vehicles and which consequently scatter light poorly when immersed in such vehicles, would be much
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more effective when in contact with air within an aggregate and could then make a paint with an improved reflection factor. Paints containing high hiding pigments, which have a refractive index already large compared with most vehicles, would increase in reflectance relatively little as the result of the presence of aggregates containing air. The presence of air is suggested as offering a reasonable explanation of the facts, but it has not been definitely demonstrated. Whatever the explanation may be, this phenomenon shows the possibility, under the proper conditions of dispersion and wetting, of making a more efficient use of inerts in white paints.
.4cknowdedgment The writ’er thanks The Lowe Brothers Company for its support of the fellowship which made this work possible and for permission to publish the results. Grateful thanks are due Walter Soller of the University of Cincinnnati, and R. W. Kewish of The Lowe Brothers Company for their assistance and advice
Literature Cited (1) Casperson, Torbjorn, Kolloid-Z., 60, 151 (1932): 65, 162 (1933). (2) Kithn, Curt, 2. ungev. Chem., 28, 126 (1915). (3) Shoulejkin, W-as.,Phil. M a g . , 48, 307 (1924).
(4) Wilcock, D. F., and Soller, W., IND.ENG.CHEM.,32, 1446 (1940). PARTI1 of a dissertation submitted t o the faculty of t h e Institute of Scienti60 Research, University of Cincinnati, in partial fulfillment of the requirements for the degree of doctor of engineering science.
Derivatives of Allvlic Chlorides Beta-Methylglycerol and Its Derivatives G. HEARNE AND H. W. DE JONG Shell Development Company, Emeryville, Calif. excess, the reaction at room temperature is complete within a few seconds. I n fact, it can be used as an analytical method for determining the amount of the chloro alcohol in a solumethallyl chloride by chlorohydrination was described in tion. I n order to obtain the maximum yield, the P-methyla previous article (I)of this series (I,8, IO). I t s structure is epichlorohydrin should be recovered from the aqueous solution similar to that of glycerol dichlorohydrin (CH,ClCHOHCHgas soon as possible, for it hydrates to P-methylglycerol monoCl); therefore we are not surprised that i t has somewhat chlorohydrin. This is particularly rapid a t elevated temperasimilar chemical properties. Thus i t has served as the starting tures, so it is advisable either to distill under reduced pressure point for the synthesis of a number of new compounds which, or to use a continuous stripping column whereby the contact by analogy with the well-known glycerol derivatives, might time at elevated temperatures is reduced. The epichlorohybe termed “P-methylglycerol derivatives”. The present drin is obtained as an azeotrope with water. paper discusses the methods of preparation and some of the A large proportion of the dichloro-tert-butyl alcohol from reactions of these compounds. the chlorohydrination of methallyl chloride is recovered as a Preparation of P-Methylepichlorohydrin dilute aqueous solution containing hydrochloric acid. This P-Methylepichlorohydrin (l-chloro-2-methyl-2,3-epoxy- can also be used for the synthesis of P-methylepichlorohydrin by supplying additional alkali to neutralize the hydrogen 0 chloride. Lime can be used instead of sodium hydroxide. /\ The following are data from a typical experiment: propane, CH2C1-C - CH2), was previously synthesized by \ Dichloro-tert-butyl alcohol (0.4mole) was added at a rate of 2 cc. per minute t o a lime slurry consisting of 120 grams of calCHa cium hydroxide in 300 cc. of water in a flask equipped with a the action of diazomethane on chloroacetone (1). stirrer and distillation column. A t a temperature of about 60” C. Dichloro-tert-butyl alcohol reacts with alkali readily to and a pressure of 135 mm., a constant-boiling mixture of the form P-methylepichlorohydrin with the elimination of one epichlorohydrin and water was distilled over as rapidly as the chloro alcohol was added to the flask. The yield was 93 per cent. molecule of hydrogen chloride (4).If the alkali is added in
HE synthesis of dichloro-tert-butyl alcohol [2-methyl1,3-dichloro-2-propanol,CH2C1C(CHs)OHCH2C1] from
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