MOLYBDENUM
Inorganic Pigments The principal molybdenum-containing inorganic pigment is molybdate orange. This pigment is a deep, reddish orange approaching a light red in hue, and is characterized by brilliancy, high tinctorial strength, very good opacity, good permanency, and excellent application properties. Chemically, molybdate orange is a solid solution of lead chromate, lead molybdate, and lead sulfate. The production of molybdate orange involves all the conventional techniques for the manufacture of chemical precipitated pigments and presents some specialized problems ; there is involved not only a three component system but also a metastable system, since the color valuable phase of this material is of different crystal structure from the stable phase of lead chromate.
W. G. HUCICLE AND E. LALOR I m p e r i a l Paper & Color Corp., Glens Falls, N . Y .
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HE principal molybdenum-containing inorganic pigment is molybdate orange. Chemically, molybdate orange is a solid solution of lead chromate, lead molybdate, and lead sulfate. Physically, this pigment is a fine powder, a dark reddish orange, almost a light red in shade. Molybdenum blue has been mentioned in the literature as a coloring compound, but this material is unstable and is not used commercially as a pigment. Crude molybdic oxide has also been mentioned as a pigment, but again it is not commercially important. Lead molybdate is used in the ceramic industry but as a flux or as a means of improving adhesion rather than as a pigment (6). Molybdate orange was first introduced as a commercial pigment in late 1934 or early 1935. In Figure 1 is shown the United States production of this pigment over the past 20 years compared with production of the other lead chromate pigments, chrome yellow and chrome orange. This shows the relatively rapid growth and acceptance of this molybdenum pigment; over 9,500,000 pounds of molybdate orange were produced in 1954. In some of its industrial applications molybdenum first came into wide usage as a replacement for tungsten, as a result of economic or wartime shortage pressures; such is not the case with molybdenum orange pigment. From its initial commercial development this color has been molybdenum based; pigments of the general type containing tungstate instead of molybdate can he made, but they have never reached commercial development. HISTORY
Probably the first description of material similar to that which now is called molybdate orange was by Schultze in 1863 ( I d ) . He reported that the mineral wulfenite (PbMoOd) was sometimes colored strongly red when it occurred near the mineral crocoite (PbCr04). Schultze made fusions of molybdate and chromate with lead chloride and sodium chloride. He found that up t o 42Oj, lead chromate could crystallize in the tetragonal form of lead molybdate, forming a series of homogeneous salts, and that these salts were a deep, dark red in color, much darker than he had ever obtained from pure lead chromate. Further work along the same line was described by Jaeger and Germs in 1921 (6). They reported on the binary systems of the sulfates, chromates, and molybdates of lead, and did their work by fusion a t high temperatures. They also found that mixed crystals of lead molybdate and lead chromate could be made and said that the mixed
August 1955
crystals contained about 44y0 lead chromate. Wagner, Haug, and Zipfel (16)have described the three allotropic forms of lead chromate: 1. Rhombic-the light yellow unstable form 2. Monoclinic-the normal, stable form of lead chromate which is a medium yellow shade 3. Tetragonal-found by Jaeger and Germs (6) to exist a t high temperatures, stable only above 783” C.
The first description of useful precipitated pigments based on mixed crystals of crocoite (PbCrOd) and wulfenite (PbMoOd) was that of Lederle and Grimm (IO). The first patent application for this process was made in Germany in August 1930 and in the United States on September 12, 1933 (IO). These patents claimed “mixed crystals suitable as yellow to red coloring matters comprising lead chromate with molybdate or tungstate.” Also described were the addition of lead sulfate and salts of barium and strontium. The first commercial molybdate orange pigment appeared on the market in this country in late 1934 or early 1935. COMPOSITION, FORMATION, AND PHYSICAL PROPERTIES
The composition range covered by commercial molybdate oranges is shown by the circles in the three-phase diagram of Figure 2; each point represents an actual analysis of a commercial product. These pigments fall in a rather small area-10 to 15 mole % PbMoOI and 3 to 10 mole % PbSOd. Products can be made over a much wider range of compositions-notably at higher sulfate and molybdate levels-but they have had little or no commercial acceptance. A t higher molybdate levels, there is no improvement in quality to justify the increased cost and the materials made with higher sulfate contents have lower tinting strengths. The compositions of a number of other reported mixed crystals in this system are shown on the diagrams a8 crosses. These are not double or triple salts but solid solutions and hence of variable composition. Molybdate orange pigments can he made with essentially no lead sulfate in solid solution (2, l d , I J ) but a small amount of lead sulfate is an aid in control. Molybdate orange pigment is essentially lead chromate which has been made to crystallize in the tetragonal system. At normal temperatures the stable form of lead chromate is monoclinic, not tetragonal, and a number of attempts have been made to explain the formation of the unstable tetragonal phase. The most probable mechanism of the formation of this product
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Figure 1.
Comparison of U. S. production of molybdate orange with chrome yellow and orange for the last 20 years
in an aqueous precipitation-i.e,, the inducement of PbCrOc to crystallize in its high temperature tetragonal structure, was described by Linz ( l a ) and Killefer and Linz (6). Linz (1.2) and later Coffer and McCoy ( 2 ) reported that in the acid conditions under which this product is precipitated, lead molybdate is the least soluble phase present. If a solution of chromate, molybdate, and sulfate be added t o a lead solution, lead molybdate, rhombic lead chromate, and lead sulfate are immediately precipitated, Since these precipitates would all be extremely fine, their solubilities would be somewhat greater than those of coarse particles, but it is reasonable to assume that the relative solubilities will not be appreciably changed. Also important is the fact that the lead chromate is present in the rhombic form which is unstable a t room temperature. Apparently, then, the unstable lead chromate and the lead sulfate will dissolve and reprecipitate on the lead molybdate crystals, building into the tetragonal lead molybdate crystal Rtructure. When such a precipitation is made, the first precipitate is a pale, light yellow which develops into the deep orange as the precipitation proceeds or as the product is aged. If the precipitation is followed microscopically, it will be seen that the first particles precipitated are indeed very small and that as the color becomes redder the particles become larger. The electron micrographs shown in Figure 3 illustrate this particle growth. Also, x-ray diffraction patterns of the first product of the precipitation show a considerable proportion of the rhombic lead chromate pattern. As the particles grow and the precipitate becomes redder this rhombic lead chromate pattern disappears and is replaced by the typical molybdate orange diffraction pattern. By mixing separately precipitated rhombic lead chromate, lead molybdate, and lead sulfate, and aging at the proper pH, appreciable conversion of this mixture to the molybdate orange
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mixed crystal can be obtained. I n general, however, more complete conversions and much better pigments are obtained by coprecipitation. As a side light on this conversion, Kolthoff and Eggertsen (7) found that, although flocculated lead chromate reacted readily with molybdate and turned orange, colloidal lead chromate reacted much more slowly.
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Figure 2. Composition range of commercial molybdate oranges shown in three-phase diagram
In Figure 4 are shown x-ray diffraction powder patterns for the various compounds which might be present in the final productmonoclinic lead chromate, lead sulfate, basic lead chromate, rhombic lead chromate, lead molybdate, and molybdate orange.
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MOLYBDENUM The pattern of the molybdate orange is definitely different from those of basic lead chromate, lead sulfate, rhombic lead chromate, or monoclinic lead chromate. The molybdate orange pattern is most like the pattern of lead molybdate except that some of the lines appear to be doubled. Such patterns indicate that the structure of molybdate orange is different from that of the normal forms of lead chromate. These patterns confirm the theory of PbCrOd building into the PbMo04 lattice and also suggest the possibility that molybdate orange contains two phases both very similar to lead molybdate in structure, one being somewhat strained.
Figure 3.
stabilizers and other additives are used to improve either the working properties, the permanency, the wetting, the ease of dispersion, the degree of thixotropy in paint systems, or the behavior in other specialized applications. The molybdate orange pigment is no exception. The lead chromate must be induced to crystallize in the tetragonal system. Since this is an unstable phase, proper conditions must be maintained; first, to develop the phase desired and, secondly, to hold the desired phase and particle size. Stabilizers are used in order that the pigment will not revert to the more stable monoclinic lead chromate during subsequent processing. Stabilizers
Particle growth of molybdate orange pigment
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Many other constituents of molybdate orange have been proposed-barium and strontium as replacements for lead (8, 9), and borate, perborate, phosphate, silicate, cyanide, nitroprusside, chloride, and carbonate for anions (2, 11-25). As far as is known, none of these have been developed commercially. MANUFACTURE
In the manufacture of chemical pigments, obtaining the chemical compound desired is only the first step. This compound must be precipitated in the proper purity; the particle size, particle shape, and size distribution must be controlled to an optimum value; the product must be precipitated in the most desirable crystal structure which may not be the most stable phase of that compound; and finally the product must be treated and handled so that the desired size, shape, and structure of the particles are maintained during subsequent drying, grinding, and use. In general, the particle size of a chemical pigment is determined by the precipitating conditions, not by the final grinding. A common misconception is that the particle size of a pigment is adjusted by the final dry grinding; this is not true for chemical precipitated pigments. Particle sizes of chemical pigments always are less than 5 p, usually less than 1 p . Most pigments must be given one or more of a wide variety of aftertreatments; August 1955
or holding agents used include silicate and hydrous oxides such as aluminum hydrate ( 2 , 12, 23, 16). Pigment manufacturers have developed to a fine art the use of such stabilizers so as to allow the pigment development to proceed to the optimum point and then to hold the color and avoid degradation of the product. Like some other aspects of pigment technology, the use of stabilizers does not lend itself to precise description for three reasons: 1. In some cases it is not well understood 2. A practice developed for a particular process is often so interlocked with other variables that it is not readily adapted for use in another laboratory or plant 3. The pigment industry specific formulations are not generally available for publication. Figure 5 shows electron micrographs of improperly stabilized molybdate oranges, to be compared with Figure 3. Some of the lead chromate of the pigment shown in Figure 5 has reverted to its normal monoclinic structure and appears as needlelike crystals; as a result of this change, the hue of this pigment would be less red than normal and the strength would be lower than normal. As in any pigment, the properties of molybdate orange are markedly affected by the conditions of precipitation and after-
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ENGINEERING, DESIGN, AND EQUIPMENT treatment. Some of the variables which must be closely controlled include batch size, agitation, temperatures, and particular lead salts used, pH, presence of other salts, concentration, and rates of addition. Coffer and McCoy (6)and Wagner, Haug, and Zipfel (16)have described the proper conditions for precipitation of the metastable rhombic lead chromate, and Coffer and
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Figure 4. X-ray diffraction powder patterns for various compounds in molybdate orange
McCoy ( 2 ) and Linz ( l a , IS) have described the optimum conditions for developing the molybdate orange color. These involve the presence of excess soluble lead, room temperature or lower, pH in the range of 2.5 to 4.5. h general manufacturing procedure is as follows: 1. Add a solution of sodium chromate, sodium molybdate, and sodium sulfate to a lead nitrate solution a t temperatures of the order of 15" to 30" C., using efficient stirring and maintaining excess lead a t the end of the addition 2. Stir as necessary to develop the desired color 3. Add stabilizer such as alum 4. Filter, wash, dry, and grind
Figure 5 .
such as litho1 rubines, resinated lithols, or other bluish tone reds, to make light red enamels of good opacity, low cost, and good flow for use where maximum color retention on exterior exposure is not required. In printing inks, where chrome oranges cannot be used satisfactorily because of poor printing properties, molybdate oranges find very considerable use because of their good working properties, fine particle size, opacity, and brightness. Early molybdate oranges had relatively poor permanency, tending to darken on exposure to sunlight, and until very recently they were not recommended for use in outdoor applications. This property has been steadily improved, however. Within the past 2 years molybdate oranges which equal or surpass chrome oranges (basic lead chromates) in resistance to darkening have become available, and these improved types ran be used outdoors. The recent improvements in the permanency of commercial molybdate orange pigments have been obtained principally by treatments with trivalent antimony compounds, a patented process ( 4 ) . Of course, these products do not represent an ideal pigment. As in the case of any real pigment, molybdate oranges have disadvantages; these include a sensitivity to sulfide fumes, a tendency to darken on heating which limits their use in high temperature plastics, and a lack of alkali resistance. The question is often raised as to why there are so many colored pigmentswhy not have just a few all-purpose pigments. Although such a state of affairs would be desirable, it is not yet practical and every real pigment represents a compromise. -4s an example of the choices a pigment user must resolve, consider the orange pigments a paint or enamel formulator has to choose from: chrome orange (basic lead chromate), molybdate orange (lead chromate-molybdate-sulfate), cadmium red (cadmium sulfide-selenide, barium sulfate), and dinitroaniline orange (coupling of o-dinitroaniline and 8-naphthol). Chrome Orange. Lowest cost per pound, but lowest strength pigment of this group; very good permanency, both in full tone and in tints; not suitable for high temperature applications
Improperly stabilized molybdate oranges
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USES
Molybdate orange pigments as a group have excellent brilliancy, strength, and opacity. They find use in a wide variety of applications-paints, enamels, finishes for farm tools and construction machinery, printing inks of many types, Plastics, floor coverings, papers, coated fabrics, and pigmented leather finishes. One of the new applications is in internally pigmented synthetic fibers, such as cellulose acetate and nylon. In addition to its use as the prime color pigment, molybdate oranges have considerable use in combinationa with organic reds, 1504
sensitive to sulfides; for many uses a blend of molybdate orange with chrome yellow is superior to chrome orange. Cadmium Red. High cost, excellent permanency in. full tone, but poorer glosq retention on weathering, more alkali fast and heat resistant than lead chromate Molybdate Orange. Lowest cost on opacity basis, very good permanency in full tone, sensitive to alkali and sulfides, not stable to high temperatures, best gloss of pigments of this group, much higher tinting strength than chrome orange or cadmium red. Organic Oranges. High cost per pound but offset for tinting u8e by high strength, permanency excellent in full tone, most types tend to fade in tints, sensitive to some solvents (bleeding), may have poor heat stability, color is more intense than inorganic
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MOLYBDENUM
pigments, resulting in a brighter appearance in applications such a8 awning paints.
For fields of application other than paints and enamels, the formulator must evaluate still other properties, and other pigments may have to be considered. Molybdate orange has been discussed as a pigment; actually it is a general class of pigments. In this class a number of different pigments are available which vary in redness of shade, tinting strength, cleanness, and permanency. Many of these are especially treated for improved working properties or special applications, such as ease of dispersion in vinyl resins, special adaptation for inks for lithographic printing, or specific netting or bodying properties in special vehicles. Some of the specialized properties are effected by varying aftertreatments of the pigment; others are the result of chemical variations in the coupling. Differences in shade and strength are produced by controlled changes in the particle size. As an illustration of the effect of particle size, Figure 6 shows electron micrographs of a number of different molybdate orange pigments which vary from the lightest, strongest shade available to the darkest red which can be readily made.
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In Figure 7 are shown curves showing the relation of the average particle size of molybdate orange pigments to relative tinting strength and relative hiding power. The points labeled A through F correspond with actual measurements on the pigments shown in Figure 6. Data from another similar series of pigments varying in particle size and redness are also included. The particle size data are in the form of average particle size with respect to surface, measured from electron micrographs, and calculated according to ASTM Method E20-51T ( 1 ) . The hiding power measurements were made according to the procedure developed recently by the Technical Committee of the New York Production Club ( 3 ) . The tinting strength determinations were made by measuring the amount of pigment required to adjust the color of a standard amount of white ink to a standard intensity as judged byeye. A s the particle size of the molybdate orange increases, the tinting strength decreases; in this pigment as the particle size increases, the tint hue also becomes redder. The hiding power verms particle size curve shows a maximum in the range of 0.3 to 0.4 micron; pigments with an average particle size either larger or smaller than this have lower hiding- power. These curves are _ on a relative basis; variations in the method of measuring either property might change the position or slope of the curve although not their general shape. This is another example of the cornpre mising required; for any use one must weigh redness, strength, and hiding power when chooeing a specific molybdate orange. In Figure 8 are shown comparisons of spectrophotometric cur\Tes of molybdate orange pigments with an organic orange and a cadmium orange, as well as a comparison of chrome orange with
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a blend of molybdate orange and chrome yellow made to approximate the color of the standard chrome orange pigment. T o meet the variety of applications and required properties, over 20 different molybdate orange pigments are available on the market. SUMMARY
Molybdate orange is the only important molybdenum-containing inorganic pigment. The history of this product, the theory of its formation, and the properties which make it valuable have been discussed. The production of this class of pigments presents some specialized problems in addition to those involved in the manufacture of chemical precipitated pigments in general. A metastable system, as well as a three component system, is involved and suitable stablizers must be used in the manufacture of the pigment to maintain its desirable properties. ACKNOWLEDGMENT
The authors appreciate and the assistanceand advice of the research and development staff of the Imperial Paper 8~ Color COrP. in their studies of molybdate orange Pigments and in the preparation of this paper. LITERATURE CITED
(1) ASTM Tentative Recommended Practice E20-51T, ASTM
Standards, Pt. 4,p, 1103 (1952).
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(2) Coffer, L. W.,and McCoy, A. L., Am. I n k Maker, 55, 23-7 (January 1947); 27-35 (February 1947); 25-35 (March 1947); 29-35 (April 1947). (3) Dunn, E. J., Jr., Bair, C. H., and associates, Ofic.Dig., Federation Paint & Varnish Production Clubs, No. 346, 793803 (November 1953). (4) Huckle, W.G., and Polzer, C. G. (to Imperial Paper & Color Corp.), U.S. Patent 2,316,244(April 13, 1943). (5) Jaeger, F. M., and Germs, H. C., Z . anorg. u. allgem. Chem., 119, 145-73 (1921). (6) Killeffer, D. H., and Linz, A., “Molybdenum Compounds,” p. 163 ff., Interscience, New York, 1952. (7) Kolthoff, I. M., and Eggertsen, F. T., J. Phys. Chem., 46, 6 1 6 20 (1942). (8) Lederle, E. (I. G. Farben), U. S. Patent 2,063,254 (Dec. 8, 1936). (9) Lederle, E. (unassigned), Ibid., 2,030,009(Feb. 4,1936). (10) Lederle, E.,and Grimm, H. G. (I. G. Farben), German Patent 574,379 (April 12, 1933); German Patent 574,380 (April 13, 1933); U. S. Patent 1,926,447 (Sept. 12,1933). (11) Linz, A,, Belgian Patent 327,250 (Feb. 27, 1937). (12) Linz, A., IND. ENG.CHEM.,31, 298-306 (1939); privately printed with some modification in June 1938. (13) Linz, A., and Coffer, L. W. (Climax Molybdenum CO.), U. 9. Patent 2,176,819 (Oct. 17, 1939). (14) Schultze, H., Liebigs Ann., 126,49-57 (1863). (15) Wagner, H., Haug, R., and Zipfel, M., 2. anorg. u. allgem. Chem., 208,249-54 (1932). RECEIVED for review January 1 9 , 1956.
ACCEPTED June 3, 1956.
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