COATED PIGMENTS TECHNOLOGY E. J. D U N N , JR.,
AND M A R T I N KUSHNER
National Lead Go., Hightstown, AT. J . Inorganic coatings may b e produced on fine particles like silica by performing chemical reactions a t the surface of the substrate particles. Seven types of reactions have been used for the fabrication of various types of coatings. Ten different compounds are illustrated as coatings on silica core particles. Several physical and chemical techniques are used to establish the type of coating and the compound formed on the surface of the substrate particles. These materials should prove of interest in the fields of paints, plastics, and ceramics, and in the battery industry. HE PRODUCTION or fabrication of inorganic coatings on Tfine particles such as S O 2is a relatively new commercial technique ( 8 ) , although the reactions involved have been known for a long time. Some of these coatings are chemically bonded, some are chemisorptions, some appear to produce a physical bond, but all are firmly attached to the substrate. Physical organic coatings have been put on powders for many years to reduce dusting or to aid dispersion of particles in vehicles. Harkins ( 3 ) , Bartell ( I ) , Zettlemoyer ( g ) , and others have studied pigment particle surfaces in attempts to improve the incorporation of pigments into vehicles and to assist in developing auxiliary properties for paint. plastics, and inks. hlonomolecular layers of various organic coatings have been applied and oriented on the surface of materials for specific purposes by Langmuir (5),McBain ( 6 ) ,and others. However, the application of inorganic coatings to the surfaces of fine powders is a relatively new field, but the products appear suitable for wide industrial application. The use of silica as a core or substrate for coatings has been studied extensively by National Lead Co. (7, 8 ) . Substrates other than silica have also been investigated, but only silica core materials are covered here.
Types of Surface Reactions
To produce coated silica cores, chemical and physical reactions must be performed at the surface of the silica particles. The silica core will comprise the bulk of the particle, while the inorganic coating will form a thin surface layer around the silica core. Some general types of chemical reactions that aid the development or the bonding of coatings to a silica core are as follows : Raising the state of valence by oxidation. Formation of a small amount of silicate. Production of basic compounds. Use of activating agents that catalyze reactions. Application of elevated temperatures and calcination. Use of materials that enhance sintering or fusion. Dehydration of a colloid. Combinations of these principles are used in practice to obtain good coatings. Figure 1 shows a center core or particle substrate with a coating of another substance bonded to it. All the coated particles referred to are of this type. One type of coating that can be put on silica is an oxide coating such as lead, antimony, or zinc oxide. Raising the valence state of the coating material while the material is at the silica surface normally produces an adherent coating on the silica particles. As an illustration, Sbz03 ground with silica and calcined at 600' C. for 2 hours will produce a good coating on silica particles. The Sbz03 is converted to SbzOi 4
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
at this calcination temperature, and the reaction is fairly rapid. Similarly, litharge (PbO) may be converted to red lead (Pb304) while at the silica surface. The transition from a lower state of oxidation to a higher level of oxidation indicates a valence change. This creates nascent. active material which develops the strong bond to the silica core. Coatings formed by this principle cannot be separated from the silica core by hammer mill grinding, which is normally used to break up agglomerates. Similarly, grinding or incorporating such products in the usual paint vehicles or plastics by the use of regular grinding equipment will not separate the coating from the substrate. In the above examples of oxide coatings, no silicate is formed. At most, the quantity formed is so low it cannot be detected by the usual analytical techniques. However, for some coating materials, the formation of appreciable amount of silicate is another means of anchoring or bonding the coating to the surface of the silica particle. This is an excellent way to bond a coating containing lead. Kational Lead Co.'s white pigment, basic silicate white lead. and its orange-red pigment, basic lead silico-chromate, both have appreciable ?-tribasic lead silicate in the coating. The 7 tribasic lead silicate is an important factor in producing a good coating with these products. Activating agents are helpful in forming coatings on silica, just as catalytic agents help to form some compounds. The presence of a small amount of KOH during the initial wet grinding of the silica seems to initiate a bonding action for some coating materials. I n the preparation of an antimony oxide coating on silica, O.2Y0 KOH by \veight helps to bond the coating to the silica. However, a reasonably satisfactory coating may be formed without KOH. Another important factor in producing a bond for the antimony oxide coating is calcination or heat. During calcination, Sbz04forms, in situ, at the substrate surface, and a good coating is obtained. Optically, this coating appears thicker than theoretical calculated values suggest. A photomicrograph
CORE
COATiNG' MATERIAL Figure 1 . particles
Schematic diagram of coated
of SbzO,-coated silica is shown in Figure 2 in comparison with uncoated silica particles. A phenomenon that was unexpected but proved helpful in this coatings technique was the slight sintering or fusion action obtained a t temperatures well below those a t which compound formation usually occurs. As the rate of reaction is much slower at lower temperatures, the amount of reaction can be limited to just that required to produce bonding for some coatings. If the fusion was allowed to go to completion through higher temperatures, the product would be spoiled. One would end up with a hard refractory mass or a glass that would be difficult to grind. At temperatures 100' to 300' C. helow those usually required for compound formation, reactions with fusible materials can be regulated to cause the silica to become coated. Fusing silicates to silica particles is a good example of such a reaction. Elements such as magnesium and sodium seem to activate this reaction. I n the cement industry, it is understood that the dehydration of a colloid produces a cementing action. This same action was beneficial to the production of some coatings on silica. As an illustration, when Cu(OH), is precipitated as a colloidal material around suspended silica particles, a very thick viscous-type slurry is obtained. Dehydrating this slurry by filtration and low temperature drying gives a smooth coating of copper oxide on the silica core. Such a procedure normally produces a mixture of CuO and CUZOin the coating. However, the CuO may be reduced to CuzO, and the final coating may he predominately CurO. Coating Aids
The condition or state of the silica particle surface affects the degree of coating. I n this study, very coarse silica particles seem more resistant to coating than the fine particles. Grinding the silica apparently activates the particles for reaction, and creating these activated surfaces helps the substrate particles accept the coating materials. Wet grinding of silica tends to produce some colloidal silicic acid. Silicic acid drying in the batch seems to help develop a band for the coating material. The ground silica should be used relatively soon after grinding and not allowed to age unduly for best coating reaction. The silica will settle out from the slurry and produce a very hard cake if it is aged too long before use. Mechanically, it is possible to form a smear type of coating on silica. If an extremely fine particle size material is ground with silica in a hall mill, there is a tendency for the fines to cover or smear the surface of the coarser particles of silica. This is a helpful first step in producing a good coating. Some coating materials that have a natural affinity for silica may be applied directly in a completely dry phase reaction. No water is needed. Water vapor may be adsorbed on the surface of these materials, but no liquid water or free water is involved. I n organic vehicle paint technology, adsorbed water on the surface of particles is normally considered objectionahle. I n the application of inorganic coatings, physically adsorbed water or chemisorbed water could he beneficial. Coating materials that are difficult to hand to silica particles usually combine best under precipitation conditions rather than by dry phase reactions. Coaling Thickness
For many years, National Lead Co. has been interested in the measurement of the particle size of pigments. Various techniques are used, but one of the most informative procedures
..
-
Silica particles, I I uuux
Figure 2. 0.
Uncoated
b.
Antimony oxide-coated
seemed to be the microscopical method. An idea of the general fineness of pigmentary particles measured by the microscopical method is illustrated in Table I. These data are primarily indicative of the general fineness, as these materials are produced in ranges of particle size. Perhaps the specific area values do not appear large, but they actually represent from 1 to 10 acres of surface per solid gallon of pigment. This may give a better idea of the tremendous area that has to be coated.
Toble I. Sire of Pigment Particles
White lead
Ti02 Red lead-furnace Red lead-fume Ground silica
1.5 0.5 3.0 1.1
5.0 Relation of uolurne and frequcncy and D is medim of 0
3.7 11.1 2.0 1.9 5.0 5.4 1.2 1.1 mrfmt of particle$, FD8JFDawhere F is sizinE intemols. 1,172
30,000 723 3,572 300
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The values in Table I are computed on the basis ot smooth surfaced spherical particles of the average diameters listed. Harkins (3) and Herring (4) have pointed out that crystal faces are not smooth and that normally they have a hill and valley structure. However, these values give a good relative measure of surface area. The above area measurements show that if 50% by weight of coating material is applied to silica, it will have to be a thin coating, as the surface area per solid gallon of pigment is so large. For a 50% by weight red lead coating on silica, the coating would be less than 1 micron thick. When the specific gravity of the coating is closer to the specific gravity of the silica, the coating will he thicker. The thickness of a coating on silica for pigr.entary use need be only a fraction of a micron to give the properties characteristic of the straight pigment. Basic lead silico-chromate is a thin coating of monobasic lead chromate and y-tribasic lead silicate on silica. A photomicrograph of original silica particles and hasic lead s:
Figure 3. (1.
Original
Silic:a par1 b.
Baricleod
DEGREES
I
29
Figure 4. X-ray diffraction patfern of antimony oxidecoated silica 6
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particles before and after staining Unstained leaded zinecwted 0.
rinrcoated
b.
H,S-stained
leaded
30
(I.
Red lead-coaled
b. e.
Zinc chromate-cooled C. Calcium plumbole-coated
Copper borafe-coated d. f. Copperoxide-cooted
Lead chromolecoated
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The basic lead silico-chromate coating is formed by grinding silica in a ball mill to appropriate pigmentary particle size. The ground slurry is pumped to a reaction tank, and litharge and Cr203 are added. When mixing is complete, most of the reactions are initiated, and the pigment is physically adhering to the SiOz substrate particles. The slurry is filtered, and the filter cake, as a pasty mass, enters a rotary kiln where it is calcined to form the y-tribasic lead silicate and complete the formation of monobasic lead chromate in the coating. Examination of these basic lead silicate and basic lead chromate coated silica particles in paint films after four years’ exposure at Yational Lead Co.’s Sayville outdoor exposure station shows that the coating on the particles is still intact. These aged particles closely resemble the original unexposed material, just as is shown in Figure 3. In general, the thickness of the coating can be decreased or increased, depending on how much of the reactive coating material is put into the process. For some end uses, a thick coating might be required, while for other uses a thin coating would suffice. Technique to Establish Coatings
Dunn (2) has described eight different techniques used to establish that compounds were formed on the surface of the silica particles. These techniques compared light microscopy, x-ray, electron microscopy, electron diffraction, chemical reactivity tests, settling tests with high specific gravity fluids, pigment staining tests, and pigment extraction tests to remove the coating. Of these various procedures, microscopy, x-ray, pigment staining, and pigment extraction techniques to remove the coating proved to be the most helpful. These are the principal techniques used to study and verify coatings in this Lsork. The technique normally used to verify what compounds are formed in the coatings processing is x-ray diffraction analysis. A typical plot of the x-ray data, giving the comparative intensities of the interplanar spacings for an antimony oxide coated silica, is shown in Figure 4. Lines in Figure 4 typify those for silica and Sb204. The strong peaks for silica are at the interplanar spacings of 3.34 and 4.25 A. The other peaks visible on the x-ray pattern typify the strongest lines for Sb204 at 3.07, 3.44, 2.94, and 2.65 A. A small amount of Sbz03 oxide is indicated at line 3.15 A. The analysis of this pattern would indicate approximately 50% silica, 49% Sbz04, and less than 1% Sbz03. This shows a rather complete conversion from Sb203 to S b z 0 4 . S o antimony silicate was detected by the x-ray analysis. Staining particles with a suitable agent such as HzS or H 2 C r 0 4 usually produces a colored or darkened surface on the particles. The color change would not occur with silica alone. This further indicates that an element exists throughout the surface of the particles as a coating. Figure 5 illustrates the appearance of a leaded zinc oxide coating on silica before and after staining with HzS. Staining makes the particles practically opaque. The lead compounds darken with H2S: forming PbS which is uniformly dispersed in the coating. If the lead were not uniformly dispersed throughout the coating: some particles would remain fairly transparent just like unstained particles. Further confirmation that coatings exist on the silica core may be found by the extraction technique. For example, the SbzO4 coating was stripped with HCl: and the original ground silica particles (Figure 2) were reproduced. 8
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PRODUCT RESEARCH A N D DEVELOPMENT
Some Coatings Produced
By the use of the coating techniques mentioned or illustrated above, a large number of coatings on silica cored particles have been prepared ; some of these are : Red lead- and lead oxide-coated silica Calcium plumbate-coated silica Zinc chromate-coated silica Copper oxide-coated silica Antimony oxide-coated silica Leaded zinc- and zinc oxide-coated silica Copper borate-coated silica Monobasic lead chromate-coated silica Combination coating consisting of both red lead and monobasic lead chromate Photomicrographs of some of these coated materials (Figure 6) show that good coatings of these materials have been obtained. I n effect, the unevenness of the surfaces of these particles as compared with the smoothness of the original silica particles indicates that rather pronounced chemical reactions of chemisorptions occurred at the surface of the silica particles. Most of these cored pigments are under evaluation tests. Many have been exposed in paints on the Sayville atmospheric fences and the company’s marine station; some tests are being conducted in Florida. Some of these materials are being evaluated in plastics, some in ceramics, and some in the storage battery industry. Consideration has also been given to other end uses. I t takes many years to evaluate products, and results of evaluation tests will probably be reported at some future date. Such a broad series of products will probably be changed and improved after the initial evaluations. Conclusions
Many chemical reactions can be performed at the surface of fine particles like silica. The reactions to produce coatings on silica cores are achieved by a wide variety of techniques already common to industry. Proof of the development of various inorganic coatings on silica has been illustrated with various physical and chemical techniques. The art of applying coatings to fine particle substrates should prove of great economic advantage to the paint, plastics, and allied industries.
Acknowledgment
The authors thank the Sational Lead Co. for permission to publish this work and F. L. Cuthbert, technical director, for his guidance and encouragement.
literature Cited
(1) Bartell, F. E., J . Phys. Chem. 5 8 , 36 (1954). (2) Dunn, E. J., Jr., Paint Varnish Prod. 42, No. 9! 19 (1952). (3) Harkins, W. D., “Physical Chemistry of Surface Films,” ’p. 291, Reinhold, New York, 1952. (4) Herring, C., Phys. Reo. 8 2 , 87 (1951). ( 5 ) Langmuir, I., J . Am. Chem. SOC. 3 9 , 1848 (1917). (6) McBain, J. W.:“Colloid Science:” p. 39, Reinhold, New York, 1950. (7) Taylor, N. W.,LVilliams, F. J., Bull. Geol. SOC.Am. 46, 1121 (1935). ( 8 ) Williams, F. J., Pitrot, A. R., Znd. Eng. Chem. 40, 1948, (1948). (9) Zettlemoyer, A. C., O ~ CDig. . Federation SOC.Paint Technol. 2 9 , 1238 (1957). RECEIVED for review October 22, 1962 ACCEPTED January 7, 1963 Division of Organic Coatings and Plastics Chemistry, 142nd Meeting, ACS, Atlantic City N. J., September 1962.
