Physics and Chemistry of Pigments J -
J
C. E. BARNETT The New Jersey Zinc Company (of P a . ) , Palmerton, P a . T h e paper deals first with the optical properties of pigments. A study of the publications of the past 25 years on paints and pigments compels the conclusion that 25 years ago the principal limitation of the paint formulator was the low hiding power of his pigments. Today hiding power i s no longer the limiting factor and the formulator is free to concentrate on other variables that respond to pigmentation. Increase in pigment hiding power constitutes one of the major pigment advances of the past 25 years. Its effects weave in and out through the whole range of paint formulation. Research on surface chemistry of pigments is summarized. This is an attractive field of research in the pigment industry because definite changes in the character of pigments can be made rather simply and the results of small scale experiments can be readily converted to plant scale. Finally, new pigments and the more important improvements in old ones that have been made during the past 25-year period are discussed.
T
his discussion consists of an account of the application of formal chemistry and physics t o the development and understanding of pigments. New and improved pigments have not developed from the simple direct application of the knowledge of chemistry and physics. An investigator working on the development of pigments applies all the techniques and knowledge of chemistry, physics, and other sciences that he can bring to bear on his problem and along with them uses a good bit of knowledge arising entit ely in past experience in his particular field. These skills and techniques are combined in a manner which he himself often does not bother t o analyze. Most new discoveries in the pigment field have come from this type of research. It has followed established scientific principles in systematic planning and critical analysis but definitely is neither pure chemistry nor pure physics. After a new pigment has been originated, a detailed study of the result in the light of traditional sciences often has served to explain what has happened and led t o improvements in the nature of a final polishing. I n a field as broad as that of pigments so many of the important and essential items are specific to a single process, plant, or product that no one person can discuss them adequately. This paper is a n attempt to assess the present state of knowledge of the fundamental' physical and chemical characteristics which are responsible for the usefulness of pigments. It is the nature of industrial research that this sort of knowledge has followed rather than preceded the development of the pigments. A study of publications on paints and pigments compels the conclusion that 25 years ago the principal limitation of the paint formulator was the low hiding power of his pigments. It was necessary for the hiding pigment to be a very high fraction of the total pigment, and lack of hiding set a minimum for pigment concentration. Today hiding power is no longer the limiting factor and the formulator is free t o concentrate on other variables that, respond t o pigmentation. Increase in pigment hiding power constitutes one of the major pigment advances of the past 25 years.
I n some cases the results are directly apparent in increased covering power, but in others, which are even more important, it has enabled the formulator to use new or better binders or a particularly desirable extender or to make other variations to attain an objective not directly related t o the hiding power of his paint. The surface chemistry of pigments is an attractive field of research because definite changes in the character of pigments can be made rather simply and the results of small scale experiments can be readily converted to plant scale. OPTICAL PROPERTIES O F PIGJlENTS WHITE PIGMENTS
Light Absorption. Merwin ( 9 6 ) in 1917 named light abrorption, index of refraction, and particle diameter as controlling the optical characteristic3 of pigments. This was an excellent study and is still a useful reference. So far as white pigments are concerned, the required absence of absorption in the visible spectrum is largely a matter of keeping them sufficiently pure during manufacture. The way reflection is built up in a thick layer of transparent crystals is evtremely interesting. A good metallic mirror may give 90 to 95% reflection a t the surface but that is the limit-the remainder is lost by absorption. I n contrast, the reflection a t a single magnesium oxide-air interface is less than 7% but successive reflections within a thick layer of the powder build up an absolute reflectance in excess of 99% for the visible spectrum. White pigments are expected t o produce good white finishes when used in vehicles that in a few cases may be almost colorless but generally are yellow. It is important to consider light absorption by the vehicle in order to understand what the pigment must do in order to obtain the desired result from the combination. I n Figure 1, A represents a hypothetical white light consisting of equal luminosities of blue, green, and red light being reflected from a nonselective white plate covered with a film of a colorless liquid. I n this case the reflected light has the same spectral distribution as the incident light. B represents the conditions when the plate is covered with a film of yellow-colored vehicle. Because of the selective absorption of blue light by such a vehicle, red light predominates in the reflected radiation, which, therefore, appears yellow. I n C the conditions are the same as in B , except that a nonselective pigment has been added to the film. This decreases the penetration of light into the film, so that the vehicle has less opportunity to manifest its selective absorption in the blue. As a result, the reflected light is greater in intensity than in B but the reflected radiation is still definitely yellow. D represents the ideal case. The pigment has its maximum efficiency in the blue region of the spectrum and therefore can reduce the penetration of blue light into the film and thus turn the blue out of the film before it can be absorbed by the vehicle. At the same time, the greater general opacity of the pigment reduces the over-all penetration of incident light more than in C, therefore decreasing the opportunity for selective absorption of blue light by the vehicle. As a result of these factors, the rcflected light, although reduced somewhat in intensity by absorption of the vehicle, will tend to retain the same spectral tlistribution as the incident light.
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Absorption of light in the ultraviolet and infrared regions of the spectrum is an important property of pigments, though not directly related to their visible colors. Ultraviolet absorption is important in the protection t h a t the pigment provides against the destructive influence of ultraviolet radiation on organic vehicles. It can also be the source of troublesome photochemical reactions. It has additional importance because the index of refraction increases to high values on the long wave-length side of an absorption band. Therefore, owing to the absorption in the long wavelength ultraviolet, the index of refraction of most white pigments increases on going from red toward blue in the visible spectrum. This tends t o increase the efficiency of the pigment in the blue relative to the red as diagrammed in Figure 1, D. About the time Division of Paint, Varnish, and Plastics Chemistry was organized, Pfund (104) and Stutz (114) were showing t h a t most of the high strength white pigments have absorption bands in the ultraviolet (Figure 2), which in some cases may cut slightly into the visible spectrum. Thus some of our best white pigments narrowly miss being colors. The potency of the effect is i1lus:rated in Figure 3, which shows white and ultraviolet (3650 A.) photographs of zinc oxide and lampblack paints. I n the ultraviolet the white pigment is blacker than the black. Index of Refraction. Next t o light absorption, index of refraction is the most important property in determining the optical characteristics of pigments. The basis for the effect of index of refraction is given by the Fresnel equation relating the intensity of the reflected light a t the interface between a medium of refractive index n ~immersed , in a medium of index no
where R is the reflection coefficient or fractional part of the incident light t h a t is reflected. Figure 4 is a plot of the reflection coefficient for pigments of different indexes immersed in an oil medium of refractive index 1.5. It is obvious from this relation t h a t to prevent transmission of incident light, fewer individual reflections will be required in the case of a,high index pigment than for a low one. I n 1931, Kubelka and Munk (77) set up equations to express film brightness or reflectivity in terms of two constants, s, the absorption constant, and r, the “remission” constant. These are specific constants of the material in question. Kubellca and Munk showed that the brightness of an infinitely thick film, H a ,was a function of the ratio s / r
Ha
= 1
+;
S
-
H a increases with decreasing s (absorption) and increasing r (reflection). Further development of the basic differential equations led to a general expression for the brightness of a paint film in terms of the background brightness, the brightness of the paint in a n infinitely thick film, the reflection constant, r, and the film thickness, x. I n this equation r and z appear only as the product which might be called the optical thickness of the film. The coating is thus characterized by the two values of brightness and optical thickness. Judd (72) and others a t the Bureau of Standards have published elaborations of the KubelkaMunk equation in which the brightness of films over black and over white is related to the infinitely thick film brightness of the paint and to the optical thickness. Particle Size. The literature tends to give the erroneous impression t h a t we know all about the effect of the particle size of pigments on paint properties, and how to control the particle size of pigments, and t h a t particle size is the most important factor in determining both the optical and surface characteristics of pigments. Actually, the scientifically acceptable knowledge of the subject as applied to the action of pigments is very meager. It is a less important factor in determining the optical propeities of pigments than either light absorption or index of refraction. A great deal of pigment research time is devoted t o particle size
A
Figure 1.
273 /
Simplified Representation of Absorption of Light within Oil and Paint Films
studies, but this t o a large degree i s because it is the only one of the three factors controlling optical properties t h a t is subject t o extensive controlled variation. I n discussing the size of a particle with respect t o its optical behavior, the wave length of light is the comparison standard. Thus a particle is large or small, depending upon whether the diameter is large or small compared to the wave length of the light in which i t is being examined. The basic studies of the whole range of particle size-from d > X t o d < A-were made by Debye (38) and Mie (87) in 1988 and 1909. ’ Debye’s dissertation, “The Pressure of Light on Spheres of Any Material,” received little attention in connection with the problem until 1931. I n t h a t year, Stratton and Houghton (112) combined the Debve and Mie c o n s i d e r a tions to caloulate the s c a t t e r i n g curve for water d r o p l e t s in air (fogs) from d large to d small with reapect t o A. The calculation was a laborious process i n v o l v i n g halforder Bessel functions for which there were no complete tables. These lfght-scatt e r i n g intensity f u n c t i o n s have since been calculated for a wide range of conditions by the Bureau of Standards’ Mathematical Tables Project and are given in an OSRD report (78). The thecretical scattering curves resulting from these calFigure 2. Ultraviolet Reflectance of S o m e White Pigments c u l a t i o n s are
INDUSTRIAL A N D ENGINEERING CHEMISTRY
214
Figure 3.
Zinc Oxide a n d Lampblack in Visible aud Ultraviolet Light L e f t . Visible light Right. Ultraviolet light
specific for the relative refractive indexes [(nc) = nl/n, (Equation l ) ] of particle and surrounding medium for which they were calculated. However, Bailey (9) found empirically that if the ratio of particle size to wave lengt,h (d/h) was multiplied by the Lorent,z refractive index coefficient, m2 1 p = (const.) where p = density, the resultant scattering m2 2’ curve was independent of the refractive index of the particles or the medium. This was confirmed by LaMer and others (80) and the resulting universal scattering curve is reproduced in Figure 5 .
+
The abscissa in Figure 5 represents the variation in particle diameter, d, multiplied by the refractive index factor, m2 l/m2 2 . The calculation has been made for a constant wave !ength, X = 0.52 p. The ordinate K expresses scattered energy in watts per unit of cross-sectional area per particle for 1 watt per square centimeter illumination. For very large particles (d/A large), the scattering coefficient, K , has the value 2. The light is specularly reflect,ed from the surface, and the angular distribution of the reflected light will lack the uniformity needed in a white pigment. The reflection is nonselective with respect to A. As the diameter is decreased, the value of K remains 2 and the nature of the reflected light remains the same until d and X are the same order of magnitude. As the diameter of the particle decreases from this size, the optical behavior of the particle goes through a transition. When d has become small with respect to the particle behaves according t o Rayleigh’s scattering law. The value of K drops rapidlv to a very low value, the particle shows selective scattering 74th respect t o A, and the optical efficiency becomes low, so that particles in this size range are too fine to make good pigments. I n the transition range as the ratio d/X decreases, K increases to a maximum of 2.7, decreases again, then increases t o a value over 4, and then decreases smoothly as d / A reaches the values corresponding t o Rayleigh scattering. I n this transition region, the individual particles are much more efficient optically than in either of the other two regions.
+
The above discussion considers scatt’ering by a single particle The scattering of a given mass of particles i s the real point of interest and is related t o the scattering by a single particle by t,he function l/d3, as the number of particles in a given mass is proportional to l/da. The changes due t o this transformation are: The maximum is shifted toward smaller ratios, the relative size of the secondary maximum is greatly reduced, and the curve does not level off at large values of d / X but continues t o decrease as the ratio increases. Reflection maxima or transmission minima corresponding to t h a t shown in Figure 5 have been shown experimentally by Stratton and Houghton ( 2 1 . 2 ) for mater drops in air, by Labler and others (‘79, 80) for sulfur sols and smokes, and by Pfund (103) and Barnett (111 for pigments. The essential condition is a high degree of uniformity of particle size and sufficiently small particles if the phenomenon is t o appear in the visible region. This discussion has been confined t o the way the optical properties of a pigment ought t o vary with particle size. Most of the time it is not possible to demonstrate a close dependence of
Vol. 41, No. 2
optical properties on particle size charact’eristics. The situation has been aptly stated recently by an English physicist (Newman, 9 6 ) , “It has for many years been thought that the particle size characteristics of pigments have profound influence on the properties of paints, or a t least there has been a n uneasy feeling that a close correlation ought to exist even though so far the hope of it,s scientific demonstration has in many ways receded the more it has been pursued.” If it has not been possible t o demonstrate a close correlation between the particle size of pigments and their properties in paints, it is not because of any lack of work on methods for measuring particle size. I n the early 1920’s, Green ( 6 7 ) , D u m (SI),Allen ( T ) , and ot,hers were working out methods for cxamination and measurement of the particle size of pigments with the light microscope. Loveland and Trivelli (81) and Hatch and Choate ( 6 7 ) were developing t,he statistics of size frequency curves and the relation t,o chemical reactions yielding precipitates. I n 1930, Haslam and Hall (66) applied the ultraviolet microscope to pigment problems. This improved resolving power slightly and has been useful in distinguishing the coinponents of mixed pigments. Gehman and Morris (66) in 1932 described a method using dark-field illumination to measure the average size of pigments. Carman (29) in 1938 related the surface area of a powder t o the rate of flow of liquid t,hrough a plug of the powder of known porosity. This “permeability” method was adapted t o gases by Askey and Feachem (8) and has bcen studied by many investigat’ors (18, 101, 106). I n 1935, Emmett, and Brunauer ( 4 2 ) proposed the evaluation of the surface area of powders from the adsorption of nitrogen at’ low t>emperat,ures. The method has received a great deal of attention in recent years. The electron microscope (69) was made available as a standardized instrument in the late 1930’s. For the first time, it became possible to observe the true crystalline form of pigment particles and t o be confident of the measurements on fine pigments. The trend of most of the particle size work over these years has been toward methods which are more sensitive t o very fine particles. A zinc oxide having a surface mean diameter of 0.8 micron as measured by t,he light microscope is found to h a m a surface mean diameter of 0.5 micron by the electron microscope and a surface mean diameter of 0.3 micron by gas adsorption. This emphasis on fine particles has been essential in the case of certain colored pigments and carbon blacks. At the same time, it has resulted in a concentration of attention on the very small figures in microns t h a t represent the average particle size of pigments. This ha3 occurred with some neglect of the at least equally important factors of distribution of particle size, number of coarse particles, and effective particle size in a given medium.
FRESNEL REFLECTION IN O I L MEDIUM
as the size is changed.
0
Figure 4.
.02
.04
.06
.08
Reflectance Coefficient vs. Index of Refraction of Some White P i g m e n t s
February 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
Most pigments contain a few particles capable of showing up as texture in paint films, and in fact if enough sample is taken, there will usually be some residue on a 325-mesh screen. These coarse particles or aggregates are the points where failure invariably starts in a rubber test piece. In paints, they show up as texture, lack of gloss, haze, and lower optical strength. As long as such coarse aggregates can be shown t o be present in a pigment, the expression of an average diameter of the order of a few tenths or hundredths of a micron cannot be very significant. Failure to appreciate this is undoubtedly one cause of the lack of correlation between paint properties and particle size values such as decried by Newman. There has been some effort directed a t the mesurement of the size characteristics of pigments as they are used in paints, rubber compounds, or plastics, Gamble and Barnett (44) and Bailey (9) have described light-scattering methods and Martin (84) and Jacobsen and Sullivan (70) centrifugal sedimentation methods to measure the “effective’, size characteristics of pigments.
in spectrophotometric data. Entirely arbitrary formulas. for expression of the results will often correlate better with visual grading than will designations based on the I.C.I. values. In the more general case of colors, the eye is often capable of detecting smaller differences than instruments can measure.
K
’ A
’
COLORED PIGMENTS
Everything that has been said concerning the white pigments applies also to colors. The effects due to light absorption outweigh everything else. Variations in particle size will cause noticeable changes in the undertones of blacks, blues, or reds. In the case of greens, where the absorption is strong at both ends of the visible spectrum, no effect of particle size on the color would be expected The chemistry of the preparation of the organic colors is too involved for this review. The possibilities are illustrated in a discussion of toluidine red by Pratt (105).
If only the above reaction were considered, one would expect toluidine red to be a definite chemical individual with characteristic properties and a perfectly definite color or hue. Such, however, is not the case. A great variety of toluidine reds is available on the market with wide variations in hue, brilliance, tint, strength, and stability to baking in enamels. For example, variations employed in the preparation of the product may modify very greatly the tinctorial properties with no appreciable change in the chemical nature of the final material. Diazotization may be conducted a t different temperatures, and different acid concentrations, the temperature of coupling a8 well as the rate of coupling may be changed, the alkalinity of the naphthol solution in which coupling occurs, as well as the chemical nature of the alkali may be changed, dispersing agents or other agents may be em loyed before, during, or after coupling, the reaction mixture aker coupling may be heated to various temperatures, variations in washing and conditions of dryin may be made, and during all these steps variations may be mafie in the type and rate of agitation to be eniployed. In short, almost an infinite variety of conditions may be used in the preparation of the color and each variation will alter to a greater or less extent its properties.
A number of very important contributions to the field of color measurement have been made during the past 25 years. I n 1922, the colorimetry Committee of the Optical Society (38) was summarizing the available data relating color to its stimulus conditions and comparing the principal methods of color measurement. I n 1928, Wright (191) and Guild (60) in England confirmed the Optical Society data on the tristimulus values of the spectrum colors. By 1931, the subject had assumed such importance that the International Commission on Illumination undertook an international standardization (73). Since 1931, there have been many publications of modifications, transformations, and refinements but no fundamental alteration of these standards. Excellent instruments have been developed and supplied commercially, but neither the instrumentation nor the formulas for expression of color stemming from the I.C.I. system are entirely adequate. Failures occur in both places. In the case of near whites the I.C.I. tristimulus values and transformations based on them often fail to represent significant differences
275
Figure 5.
m‘t2
Universal Scattering Curve
The subject of the optical properties of pigments may be summed up as follows: Light absorption, index of refraction, and particle size are the fundamental characteristics of pigments which control their optical properties. We do not have sufficient scientific knowledge of any of these fundamental characteristics to use them in the specification of pigments for paints. Therefore, we resort to performance tests on paints themselves. Finally, the realization of gloss, sheen, texture, surface smoothness, color, brightness, and most of the other paint film characteristics which come under the head of optical properties involves a sensory and mental process on the part of the user. If we knew everything about the optics of pigments, there would still be a psychological problem in the application of that knowledge in paints. Nevertheless, the fundamental research on the optical properties of pigments has made possible the integration of painting and decorating plans with lighting installations. It has also resulted in brighter colors, greater hiding power, higher gloss, and improved surface texture in enamels and better control of gloss and sheen in other paints. I n short, it has been responsible for increased satisfaction and surety in the use of pigments in paints.
SURFACE CHEMISTRY
A Symposium on the Wetting Power of Paint and Varnish Liquids a t which W. D. Harkins presided was the feature of the first meeting of the Division of Paint and Varnish Chemistry. Since then the division has sponsored other symposia on subjects related to the wetting of pigments by paint liquids and many historic papers on the general subject have been presented before it, No one who heard them is likely to forget the papers a t the 1927 and 1928 meetings by F. E. Bartell and H. J. Osterhof on the adhesion tension cell, the 1930-31 papers by W. D. Harkins and R. Dahlstrom, D. M. Gans, and L. W. Ryan on the wetting and adsorption of pigments and their effect on flocculation, dispersion, and settling in paints, or the 1933 paper by F. E. Bartell and C. W. Walton showing how the wetting characteristics of antimony sulfide could be changed from organophilic to hydrophilic in accordance with the chemistry of the surface layer. When a paint vehicle is added to a pigment, the pigment-air interface of the latter is more or less completely replaced by a pigment-oil interface. Many of the properties of the finished paint, in the can, and as a film, are related in what happens in the replacement of the pigment-air by a pigment-oil interface. The interest of the paint chemist in the subject is shown by some
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23 papers froin the Paint and Varnish Production Clubs between 1929 and 1942. Excellent reviews have been published by Allen, Knoll, Ryan, and Murray (41,Green and Melsheimer (69), and Green (68). The principal sources of fundamental work have been the laboratories of W. D. Harkins and F. E. Bartell. The former has approached the problem by measurement of the heat of wetting and related energy considerations. The latter has measured the angle of contact between liquids and solids, preferential adsorption of liquids by solids, and displacement of one liquid by another when both are in contact with a solid. The published work of Harkins and his associates includes studies on titanium oxide and the extended forms with calcium sulfate and barium sulfate, zirconium oxide, silica, stannic oxide, zinc sulfate, barium sulfate, zirconium silicate, and graphite. This research has recently been reviewed by Harkins (65). In the early part of their investigation, they found that the heat of wetting of the hydrophilic oxides in nonpolar liquids was very low, of the same order as the energy of immersion of water in the same liquids, and the pigments were flocculated. Addition to the liquid of a polar molecule such as butyric acid resulted in a marked increase in the heat of wetting, very near t o that found for pigments in the pure polar liquid. At the same time the pigment became dispersed and settled to a low volume. They explained this on the basis of adsorption from the liquid of polar molecules oriented with the polar ends of the molecules adjacent to the pigment. I n more recent work, Harkins and Jura (66) investigated the distance to which the molecular interaction between a solid and adjacent liquid extends into the liquid. They found that adsorbed films of water, nitrogen, butane, and nheptane onoanatase, TiOt, had minimum thicknesses of 15, 36, 64, and 72 A,, respectively. These values correspond to several molecular layers. Bartell in a series of historic papers with Hatch (le), Lloyd ( 1 4 ) , and Walton ( 1 7 ) described alterat,ions in the surface of antimony sulfide, lead sulfide, sugar charcoal, and carbon black. These substances are normally organophilic, but their surfaces were changed to hydrophilic by various chemical oxidation reactions. The wetting characteristics of the materials changed in accordance with the chemistry of the surface layer. Bartell and Osterhof (16) described a cell to measure the contact angle of liquids against powders and the displacement of one liquid by another from the surface of powders. In other vork, Hartell and his students (IS, 92) studied the relation between liquid absorption and adhesion tension and the interfacial tension of liquids against water. In binary organic liquid systems, that component having the higher adhesion tension against the solid was preferentially adsorbed over the greater portion of the concentration range. Over some part of the range each of the components \\-as selectively adsorbed. The adsorption on silica was almost opposite that for carbon from the same liquid pairs. Bartell and Miller (15) determined sedimentation values for dried zinc oxide in toluene and amyl acetate and their mixtures and for n-butyl alcohol and its mixtures with toluene. Their conclusion was that a low degree of wetting gives a flocculated, high sedimentation volume with rapid settling. ,4 high degree of wetting gives a stable suspension which settles slowly to a sinall but densely packed sediment. This is now well recognized in all studies of the problem of pigrnent settling in paints and other liquid media. ,Miller has shown (88) that the various free surface energies in a given solid-liquid system may assume different values according t o whether the liquid is advancing or receding from the surface. From this hysteresis of contact angle, it follows that the liquid absorption value in a given system may depend on the amount of work done on the system during the test. I n the Gardner-Coleman oil-absorption test, method, involving a minimum of work, the end point may correspond to an advancing angle, while in the A.S.T.M. method, where the system is rubbed with a spatula during the test, the end point, may be nearer a
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receding condition. Miller believes that the dynamic receding contact angle is the fundamental criterion for wetting of solids by liquids, because the magnitude of this angle determines the stability of t'he liquid as a film on the solid and the force of adhesion between liquid and solid. Pigment manufacturers recognize the importance of the nature of the pigment surface. A large number of patents have been issued covering methods of modifying the surface of pigments t o attain a variety of objectives. This is an attractive field of research in the pigment' industry. Definite changes in the character of pigments can be made to improve mixing and dispersion in vehicles and even the optical properties and weathering characteristics of paints may be modified. The results of laboratory scale experiments can usually be introduced into commercial scale operations without the need for expensive semicommcrcial scale work in between and with little alteration or addition to plant equipment. The patents on the subject fall into two categories, so far as processing is concerned. I n one class, a chemical reaction is made to take place a t the surface of the solid in a manner analogous to that mentioned above for Bartell's oxidation of the surface layer of antimony sulfide. I n the second, a dispersing or wetting agent for water or oil is mixed with the pigment and functions directly or after reaction a t the surface. The action of wetting agents is largely on the vehicle, and in the majority of cases they have the same action whether added t o the vehicle or mixed with the pigment. However, addition t o the pigment is often preferred because of convenience and more efficient utilization of the agent. A wetting agent for pigments in aqueous suspensions reduces the surface tension of water and the interfacial tension at the solid-water interface. A dispersing agent may have little effect on the surface tension of water but act.s to overcome the adhesive forces between particles. Surface tension is easily measured in aqueous systems, and it is often assumed that lowering the surface tension will improve the wetting of a pigment. This does not always follow. A practical test, such as sedimentation volume, that involves the actual is a more useful guide. For aqueous systems, one has a choice among a wide variety of organic materials and a few inorganic compounds. Some of the more common of the former are the soluble salt'sof sulfated higher fatt'y alcohols, sulfonated amides, fatty acid ester sulfonates, sulfonated ethers, alkyl aryl sulfonates, and p partially esterified with higher fatty acids. and phosphates are the important inorgani the agent acts as a dispersing or wetting agent, as defmed above, the process is specific. One material may work well in some syst.ems and not in others. The results may be determined by chemical reaction between the agent and reactive ions in the surface layer of the pigment. Thus alkaline earth or other nictal ions in the pigment surface may interchange with t'he sodium ion of the wetting agent,, resulting in the destruct,ion of the wett,ing action or complete reversal of that action. The organic agents function in considerably smaller amounts, but, in general, they are much more expensive and more affected by reactivity with pigments than the inorganic compounds, With sodium silicate as a wetting agent, the reaction with calcium ions will siniply mean a loss of part of the effectiveness of the agent, since the calcium silicate formed is hydrophilic. With a sodium soap as the wetting agent, reaction with caIcium ions may result in a wetting agent for oil, completely the reverse of its intended purpose. A large number of patents (1, 6, 22, 26, S2, 39, 40, 45, 74, 109, 119) claim specific applications of the general principles given above. Rate of mixing, texture, consistency, water absorption, and stability are claimed to be improved in all kinds of watert,hinned paints, paper coat,ings, pigmented paper pulps, and latices-in fact, all applications that are made of pigments in aqueous suspensions. Most pigments have hydrophilic surfaces and behave toward
February 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
organic liquids a good bit like a water surface. If the addition of a wetting agent to an oil reduces the interfacial tension between oil and water, it may also reduce the interfacial tension between the oil and a hydrophilic pigment and improve the wetting of the pigment by the oil. The patent literature is full of claims covering agents to be mixed with pigments in order to improve various characteristics of pigment-oil systems. Practically all the aliphatic and aromatic acids as well as their metallic soaps, esters, and amides (6, 10, 1Q-21,84, 27, 88, 50,31,85,48,65, 68, 85,89, Q7-99,107,115,116,117,118,IrZ0)have been stated to improve mixing, millgrinding, gloss, leveling, texture, and freedom from hard settling. A variety of waxes, stearin pitch, amines, and proteins have been claimed to have similar beneficial effects (5449,QO). Other patents cover the use of fatty acids and soaps as lubricants in the manufacture of metallic flake pigments (68, 61, 69, 76). The coating of titanium dioxide with naphthenates and phthalates is claimed to increase chalk resistance and improve disgersion (60,61,94, 96). In some cases the effectiveness of the agent depends on the subsequent addition of another material to the paint (46, 76, 85). If pigment-vehicle wetting relations are properly adjusted, the introduction of small amounts of water will promote the formation of a type of flocculated pigment structure of marked thixotropic characteristics. A pigment of definite but not too great hydrophilic tendencies is required, and additions to the pigment of water-soluble gums such as gum arabic, or of combinations of water-sensitive and water-repellent agents such as sodium silicate and oleic acid have been used to obtain this type of responsive pigment. The addition of basic magnesium carbonate to pigments (83) depends on the reaction between the added magnesia and fatty acids in the paint vehicle to form magnesium soaps to produce a paint of high consistency. I n a number of the cases listed, the active agent is the result of a reaction between ions in the pigment surface and the treating materials. An example of this is the treatment of zinc oxide with propionic acid (56, 111), which results in the formation of zinc propionate a t the surface of the pigment. A more delicate case is the coating of the particles of zinc oxide by surface reaction with phosphoric acids (47) to form a zinc phosphate coating that will retard the reactivity of the pigment with the weak organic acids found in paint vehicles. The most intriguing types of pigment surface treatments are those involving the adsorption of one reagent a t the surface of the pigment, followed by the addition of a second material to react with the adsorbed ion and form an insoluble compound. A typical example is the addition of sodium silicate which is strongly adsorbed by most pigments, followed by a soluble salt of a metal which will react to form an insoluble silicate a t the surface. The formation of the insoluble silicate is usually accompanied by flocculation of the pigment. Aluminum and other silicates and aluminates on titanium dioxide ( 8 , 3 7 ,54, 66,64,88,QS, 100,108, 108, 110) improve resistance to chalking and color retention. Pigments of high consistency and high dry hiding power, especially when used in flat wall paint formulations, may be produced by the above general methods to form aluminum and barium silicates and phosphates preferably prior to calcination of the pigment (2, 88, 91, 116). The pigments resulting from such treatments appear to be preferentially wet by the volatile constituents of paint vehicles. When the thinner evaporates, voids are left in the film, in effect giving pigment-air interfaces and increased hiding power for the dry paint film. It seems a long way from the fundamental work of Harkins, Bartell, Bingham, and many others to the industrial applications just cited. Yet there are many connecting links. The fundamental studies can be credited in large measure for relief from the difficulty of hard settling of pigment during the shelf storage of paints. Any one who remembers the hard, indestructible cake that settled in the cans of paint containing the early titanium pigments will appreciate this accomplishment. Furthermore,
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the fundamental work served to excite pigment and paint development. One weakness in all the work has been the assumption that all the surfaces of a crystal have the same energy, although i t is well known that this is not true. Harkins includes a section on the subject in his discussion (66). “The observed effects are the statistical sum of the effects of all the faces, edges, and corners present. Since the energy is different for the different crystal faces, and edge, or a corner, it is apparent that the actual concentration of adsorbed material, and the energy r e lations should be different a t the different crystal positions.” In terms of what actually happens when a liquid wets a solid, these local differences in crystals could well be sufficient to make the results of the average measurement for the whole crystal lose much of their significance. Harkins quotes a number of cases. Faraday observed that perfect crystals of sodium sulfate, carbonate, or phosphate would remain stable for years, but if the surface were scratched or broken, efflorescence immediately spread out from the point of injury and soon covered the whole crystal. Crystals of ferric xanthate decompose to ferric oxide along the edges and preferential adsorption a t edges and corners occurs with radioactive indicators. The hemihydrate of potassium acid oxalate loses water a t 0’ C. Under the microscope the reaction is seen to start with the appearance of small dots which spread in such a way as to keep the boundaries parallel to the crystal edges. The transformation of yellow into red mercuric oxide practically always begins a t an edge, preferably a t a corner, but rarely a t a face. The reaction between large crystals of zinc oxide and weak mineral acids always begins at an edge or corner and usually proceeds to one entire face, but the other faces of the crystal remain intact until the crystal breaks up. Reactions with organic acids proceed in the same way but the soaps formed may choke off the reaction. COMPOSITION O F AMATERIALS Xot many new pigment chemicals have been discovered during the past 25 years. Yet the two most notable occurrences concerning pigments in this period probably are the discovery and development of the phthalocyanines and the rise of the titanium pigments as a class to top production in the field. In 1922 Gardner (48) gave only a single reference; “The recent advent of titanium oxide as a constituent of white paints should be of interest to analytical chemists.” The history of titanium pigments during the period is largely one of the growth of an industry and the improvement of the product through research and engineering. On the other hand, the discovery of the phthalocyanines as a chemical type, the recognition of their usefulness as pigments, and the commercial development have all occurred since 1929. Metallic powder and flake pigments have been known for a long time but they have attained a position of importance in the paint industry only within the past 25 years. In 1922 Gardner (48) gave no reference to any of them The present uses of aluminum powder, zinc dust, metallic copper, and the copper bronzes are specialties but they include some highly important ones. The sum total of the improvements in established pigments is as important as the new ones mentioned above. The patents issued in 1922 to Breyer, Croll, and Farber ( 9 6 )for light-resistant lithopone started the ball rolling in white pigments. A further advance came in 1928 with the discovery that small amounts of cobalt retarded the darkening action (71). During the past 15 years, notable improvements have been made in the direction of increased tinting strength and hiding power in that oldest of white pigments-basic carbonate of white lead. The commercial production of pure zinc sulfide as a white pigment dates to the middle 1920’s, but the present hydrogen sulfideprecipitation process is fairly recent (1933). The commercial production of the rutile modification of titanium dioxide in 1941 gave us our white
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
pigment of highest refractive index. Pigment grade antimony trioxide has been manufact,ured in the United States only since 1927. Because of its relatively high cost on a hiding power basis, this excellent pigment has been replaced in many applications by the chalking- and fading-resistant grades of titanium dioxide, At the present time antimony oxide is used for the fire resist,ance it imparts to many coating compositions. The concept that many materials of low refractive index-that is, those often called “inert8” or “extender” pigments-may be used for other purposes than to reduce costs and cheapen paints is definitely a development of this period. These materials are now rightly considered as valuable pigments, which when carefully chosen and correctly used can improve the performance of paints. The quality of practically all of them has been greatly improved and is precisely controlled. These improvements stem directly from the application of the fundamental knowledge discussed under surface chemistry, particle size, and absorption. Space permits only two examples. Water-ground muscovite mica of proper particle size and shape characteristics (ratio of diameter to thickness and freedom from excessive fines) increases the durability of paints by reducing checking and cracking and retarding the rate of eroding in exterior house paints. This factor of particle shape has been found to he important to the optical and weathering properties t,hat other pigments impart to paints. Extremely fine precipitat,ed calcium carbonate is finding many applications. This mat,erial is hardly a pigment in the usual sense. Its usefulness depends on its ability to produce certain properties in association with other constituents of the paint. It appears to aid the dispersion and suspension of other pigments and by so doing aids in the realization of their inherent optical properties. Its high surface area and reactivity combined with the very low optical efficiency provide some interesting uses in paints and allied dispersions. Some of the colored inorganics are the oldest pigments known. Paintings with red and yellolv iron oxide pigments have been discovered which belong to a period of about 16,000 B.C. The past 25-ycar period has been notable for the introduction of an increased amount of science into t,hemanufacture of most of these materials and in association with t,his the introduction of precise quality control of production. These pigments are used by a wide variety of industries and the manufacturers produce a great, many different, types in order to meet apecial requirements. The chemistry involved in the preparation of some of t’hemis far from simple. Through study of their preparation and performance it has become possible to control tinting strength, texture, oil absorption, reactivity, and light-fastness. The expansion in the production of precipitated iron oxides and furnace blacks, the development of molybdate chrome orange with greatly improved color, brightness, hiding power, and tinting strength over regular chrome orange, and also the interest in zinc yellow because of its corrosion-inhibiting properties, particularly for the light metal alloys, deserve special mention. During the late war there was considerable development in tho field of luminescence. This stemmed from the necessity of carrying out nocturnal operations despite severe blackout restrictions. For general guidance applications, luminescent, materials based on the alkaline earth and zinc and cadmium sulfides were used in paint-type coatings. The energy st’orage and light emission from these materials were improved from five- to tenfold and the volume of production made it possible to reduce their cost by a factor of ten. This has brought the products to such a stage of development that civilian peacetime uses are being found in the coating and plastics fields. The development of t,he “daylight fluorescent” dyes used in signal panels deserves mention. If the fluorescent emission has the same color as the light reflected by the dye, the fluorescence excited by the short’wavelength components of sunlight or artificial light sources will he added to the normal reflection. The result is a brighter and more saturated color than can be obtained by reflection alone.
Vol. 41, No. 2
The outstanding contribution in the field of organic colors hss been the discovery and development of the phthalocyanines. Aside from this, the development of the phosphotungstic and phosphomolybdic lakes and toners has increased the light stability of the basic dyes while retaining their brilliance arid strength. The relation between the position of substituent groups in dye molecules and the absorption spectra of the dyes has been extensively studied and increased our knowledge of the chemical and physical characteristics of the products. Limitations and capabilities are more clearly understood and as a result the use of organic pigments is more satisfactory and certain.
FUTURE DEVELOPMENTS Probably the weakest, point in our knowledge of pigment,s is that we do not know how our pigments are formed, how many molecules combine to make t’he initial units to precipitate from liquid or gas, or whether these continue to grow by addition of single molecules or by aggregation of the precipitated units. There have been some excellent physical chemical studies of precipitation and a few fundamental attempts a t application to pigment reactions, but, according to much of t’his work some of our best operating units should he running in reverse. Thanks to the work in Kolthoff’s laboratory, we do have sound knowledge of the mechanism of the aginz of precipitates. Many products must, be calcined to develop pigment properties, and we need to know more about what happens on the calciner. Do the particles grow by fusing together to new particles, do the original units maintain their idemity to a certain extent, or does the growth take place by vaporizat,ion and condensation? Is there any relation between the process of chemical decomposition or of evolution of water and other vapors and the particle growth in the calciner or between the rate of growth and a conversion from one crystal form to another? Just hox are the coarse aggregates formed which cause so much t,rouble, why are these soinctimes soft and relatively easily broken apart in grinding, a t other times hard and resistant t o almost any degree of grinding? I n the case of optical properties we need t o know more exactly the optimum size for pigment particles of any refractive index. Along with this vie need to know a great deal more exactly the effective particle size of our pigments in paints, rubber, plastics, and other use forms. The optical dispersion of most pigment,s is so great that the transparent or white pigments, as well as tht: colors, must probably be treated as materials of complex rcfractive index. This means that before the optimum particle size can be calculated the refractive index must be known at various wavelengths throughout the visible spectrum. As a matter of fact, the commonly quoted values for the refractive indexes of pigments can all be traced to measurements made in the middle 1800’s. Although there is no indication of any serious error, the measurements do depend on the quality of the natural cr available a t that time. I n the field of colored pigments, it is a safe prediction that continued improvement will be made in the light stahilit,g of hoth the organic and inorganic compounds. The discovery of new colors in recent years has been at a much slower rate than during the early years of the dye industry. But so long as it is impossiblo to convince a woman t,hat a certain color shade she has in mind is a physical impossibility, some one is going to t,ry to produce it. Some adjustment of the I.C.I. tristimulus values can be cxpcctod in the future, Undoubtedly some day spectrophotomct’crs will be as sensitive as the eyes of our expert color graders. I t is a characteristic of spectrometric instruments that no m a t h how sensitive they may be one always wants them a little bit betux. There has been a large measure of success in the application of fundamental surface chemistry directly to pigment problems. I n part, this may be traced to a general appreciation of the weiili points in the theory. As a result, the pigment chemist has basod his work on adsorption rather than on more detailed picturt:.; of
Eebruary 1949
I N D U S T R I A L A N D E N G IN E E R I N G C H E M I S T R Y
wetting phenomena. Probably the greatest bar to progress is the lack of’a method for determining the adhesion of a pigment against dry vehicle or of dry paint film against its substrata, vhich is distinct from other properties.
LITERATURE CITED (1) Alexander, J., U. S. Patent 1,259,708(1918).
(2) Allan, B. W., Ibid., 2,357,721 (1944). (3) .411an, B. W,, and Land, W. E., Ibid., 2,297,523 (1942). ’ (4) Bllen, A. O., Knoll, A,, Ryan, L. W., and Murray, C. A,, Vol IV, Chap. I, “Protective and Decorative Coatings,” New York, John Wiley & Sons, 1944. (5) Allen, E. M., and Lynn, G. M., U. S,Patent 2,140,375 (1938) (6) Allen, E. R., and Kaufmann, W. E., Ibid., 1,665,946 (1928). (7) Allen, R. P., IND.ENG.CHEM.,ANAL.ED., 14, 92 (1942). (8) Askey and Fearhem, J . Soc. Chem. I n d . , 57, 272 (1938). (9) Bailey, E. D., IND.EN*. CHEM.,ANAL.ED., 18, 365 (1946). (10) Baldwin, U. S. Patent 1,946,054 (1934). (11) Barnett, C. E., J . Phys. Chem., 46, 69 (1942). (12) Bartell, F. E., and Hatch, G. B., Ihid., 39, 11 (1935). Bartell, F. E., and Hershberger, A., IND.ENG. CHEM.,22, 1304 (1930). Bartell, F. E., and Lloyd, L. E., J . Am. Chem. Soc., 60, 2120 (1938), Bartell, F. E., and Miller, N. F., Sci. Sec. National Paint Varnish Lacquer Assoc., Circ. 523, 303 (1936). Bartell, F. E., and Osterhof, H. J., “Fourth Colloid Symposium Monograph,” p. 240, New York, Chemical Catalog Co., 1926. Bartell, F. E., and Walton, C. W., J . P h y s . Chem., 38, 503 (1934). Blaine, R. L., A . S . T . M . Bull., p. 51 (August 1943). Blumer, F. S.,Ger. Patent 578,469 (1933). Btihme, H. T., Brit. Patent 358,535 (1929). Booge, J. E., U. S. Patent 1,722,174 (1929). Ibid., 2,178,334 (1939). Breyer, F. G., Ihid., 1,832,355 (1931). Ibid., 1,985,076 (1934). Breyer, F. G., Croll, P. R., and Farber, C. W.,Ibid., 1,411,645 (1922). Breyer, F. G., and Koller, J. P., Ibid., 1,949,025 (1934). Brill, H. C., Ibid., 2,220,952 (1940). Burdick, H. E., Ibid., 2,294,381 (1942). Carman, P. C., J . Soc. Chem. Ind., 57, 225 (1938). Carter. H. K., U. S.Patent 2,232,164 (1941). Church, J. W., and McClure, R. R., Zbid., 2,034,797 (1936). (32j Chwala, A., Ger. Patent 504,598 (1928). (33) Colorimetry Committee, J. Optical Sac. Am., 6 , 527 (1922). (34) Coolidge, C., and Holt, H. S., U. S.Patent 2,009,436 (1935). (35) Ibid., 2,009,437 (1935). (36) Cyr, H. M., Ibid., 2,303,329 (1942). (37) Daiger, W. H., Ibid., 2,357,089 (1944). (38) Debye, P., Ann. P h y s i k , 30, 57 (1909). (39) Dreyfuss, H., Brit. Patent 394,657 (1933). (40) Dunham, H. V., U. S. Patent 1,523,182 (1925). (41) Dunn, E. J., Jr., IND.ENO.CHEM.,ANAL.ED.,2, 59 (1930). (42) Emmett, P. H., Am. Soc. Testing Materials, S y m p o s i u m o n hTew Methods for Particle Size Determination in Subsieve Range, 1941, 95. (43) Erskine, A. M., and Perkins, B. H., U. S. Patent 2,294,394 11942). ~--, -.. (44) Gamble, D. L.. and Barnett, C. E., IND. ENG.CEEM.,ANAL. ED., 9, 310 (1937). (45) Gamble, D. L., and Grady, L. D., Brit. Patent 472,001 (1936). (46) Gamble, D. L., and Grady, L. D., U. S. Patent 2,135,936 (1938). (47) Gamble, D. L., and Haslam, J. H., Ihid., 2,251,869-872 (1941). (48) Gardner, H. A., “Physical and Chemical Examination of Paints, Varnishes, and Colors,” Washington Paint Manufacturers Assoc., 1922. (49) Gardner, H. A , , U. S. Patent 1,894,168(1933). (50) Ibid., 2,037,323 (1936). (51) Ibid., 2,172,505 (1939). (52) Gavin, F. J., Ihid., 2,334,258 (1943). (53) Gearhart, F. B., and Steele, F. A., Ibid., 2,065,687 (1936). (54) Geddes, J. A., Ibid., 2,357,101 (1944). ENG.CHEM.,ANAL. (55) Gehman, 5. D., and Morris, V. N., IND. ED.,4, 157 (1932). (56) Giese, H., U. S.Patent 2,260,177 (1941). (57) Green, Henry, IND. ENQ.CHEM.,16, 667 (1924). (58) Green, Henry, J . Applied Phys., 13, 611 (1942). (59) Green, Henry, and Melsheimer, L. A., “Protective and Decorative Coatings,” Chap. 4, New York, John Wiley & Sons, 1944.
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(60) Guild, J., PhiZ. Trans. Roy. Soc. London, 230, 149 (1931). (61) Hall, E. J., U. S.Patent 1,569,484(1926). (62) Ibid., 2,002,891 (1935). (63) Hanahan, M. L., Ibid., 2,296,636-9 (1942). (64) Hanahan, M. L., and McKinney, R. M., Ibid., 2,212,910, 2,212,935 (1940). (65) Harkins, W. D., in “Colloid Chemistry,” J. Alexander, Vol. V (1944), pp. 12-102: Vol. VI (1946), pp. 1-77, New York, Chemical Catalog Co. (66) Haslam, G. S.. and Hall, C. IT., J . Franklin Inst., 209, 777 (1930). (67) Hatch, T., and Choate, 9. P.,Ibid., 207, 369 (1929). (68) Heckel, H., U. S.Patent 1,906,962 (1933). (69) Hillier, James, Am. SOC.Testing Materials, S g m p o s i u m o n N e w Methods for Particle Size Determination in Subsieve Range, 1941, 90. (70) Jacobaen, A. J., and Sullivan, W. F., IKD.ENO.CHEY.,ANAL. ED., 19, 865 (1947). (71) Jantsch, G., and Wolski, P., U. 5. Patent 1,693,902 (1928). (72) Judd, D. B., Bur. Standards J . Research, 19, 287 (1937); R.P. 1020. (73) Judd, D. B., J . Optical Soc. Am., 23, 359 (1933). (74) Kslber, W. 9 . , U. S.Patent 2,056,924 (1936). (75) Kingsbury, F. L , and Schmidt, C. L., Ibid., 2,365,569, 2,365,560 (1944). (76) Kramer, E., Ibid., 1,832,868 (1931)’ 1,930,684 (1933), 1,932,741 (1933). (77) Kubelka, P., and Munk, F., 2.tech. Phys., 12, 593 (1931). (78) LaMer, V. K., U. S. Dept. Commerce OSRD Rept. 1857, PB 944 (1943). (79) LaMer, V. K., and Barnes, M. D., J . Colloid Sei., 1, 71, 79 (1946). (80) LaMer, V. K., et al., U. 9. Dept. Commerce, OSRD, Rept. 4904, PB 32,208 (1945). (81) Lowland, R. P., J . Franklin Inst., 204, 193 (1927). (82) McKinney, R. M., U. S. Patent 2,150,236 (1939). (83) Marcot, G. P., Ibid., 2,255,263 (1941). (84) Martin, S. W., Am. SOC.Testing Materials, S y m p o s i u m o n New Methods for Particle Size Determination in Subsieve Ranoe, 1941, 66. (85) Meister, W. F., U. S.Patent 2,113,539 (1938). (86) Merwin, H. E., Proc., Am. SOC.Testing Materials, 17, 1946 (1917). (87) Vie, G., Ann. P h p i k , 25, 377 (1908). (88) Miller, N. F., J . P h y s . Chem., 50, 300 (1946). (89) Morrison, J. O., and Perkins, B. H., U. S. Patent 2,282,303 (1942). (90) Mowld; K. S.,Ibid., 2,259,483 (1941). (91) Ibid., 2,269,470 (1942). (92) Murray, C. A,, and Bartell, F. E., Sci. Sec., Natl. Paint Varnish Lacquer Assoc., Circ. 568, 251 (1938). (93) Nelson, W. K., U. S.Patent 2,346,322 (1944). (94) Nelson, W. K., and Ploetz, A. O., Ibid., 2,234,681 (1941). (95) New, G. F., Ibid., 2,242,320 (1941). (96) Newman, A. C. C., Society of Chemical Industry, Symposium on Particle Size Analysis, Feb. 4, 1947. (97) O’Brien, W. J., U. S.Patent 1,832,417 (1931). (98) Ibid., 2,068,066 (1937). (99) Patterson, G. D., Ibid., 2,136,313 (1938). (100) Ibid., 2,296,618 (1942). ENQ.CHEW,AINAL. ED., (101) Pechukas, A,, and Gage, F. W., IND. 18, 370 (1946). (102) Peterson, K. W., U. 5. Patent 2,161,975 (1939). (103) Pfund, A. H., J . Optical SOC.Am., 29, 10 (1939). (104) Pfund, A. H., Proc. Am. SOC.Testing Materials, 23, Part 11, 369 (1923). (105) Pratt, .: S., in Mattiello’s, “Protective and Decorative Coatings, Vol. 11, p. 82, New York, John Wiley & Sons, 1942 (106) Rigden, P. J., J . Soc. Chem. I n d . , 66, 130 (1947). (107) Risse, F., U. 8.Patent 1,832,242 (1931). (108) Robertson, D. W., Ibid., 2,378,790 (1945). (109) Scholz, H. A,, Ibid., 2,167,221 (1939). (110) Seidel, G. R., Ibid.. 2,387,534 (1945). (111) Silver, B. R., and Bridgewater, E. R., Ibid., 2,303,330 (1942). (112) Stratton, J. A., and Houghton, H. G., Phys. Rev., 38, 159 (1931). (113) Stutz, G. F. A., Brit. Patent 429,553 (1934). (114) Stutz, G. F. h.,J . Franklin Inst., 202, 89 (1926). (115) Stutz, G. F. A,, U. S. Patent 1,978,727 (1934). (116) Ibid., 2,263,656 (1941). (117) Sullivan, R. W., Ibid., 2,254,630 (1941). (118) Sutton, J. B., Ihid., 2,226,147 (1940). (119) Tucker, G. R., Ibid., 2,046,757 (1936). (120) Wiegand, W. B., Ihid., 1,848,213 (1932). (121) Wright, W. D., Trans. OpLical Soc. ( L o n d o n ) , 30, 141 (192829); 31, 201 (1929-30). RQCEIVED May 24, 1948.
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