MOLYBDENUM
Organic Pigments The most important uses of molybdenum in organic pigments lie in the complex acids-phosphomolybdic and phosphotungstomolybdic. These acids along with phosphotungstic acid constitute a group of precipitating agents for several classes of basic and other dyestuffs that produce three valuable related classes of pigments known as phosphomolybdic acid (PMA) colors, phosphotungstic acid (PTA) colors, and phosphotungstomolybdic acid (PTMA) colors. The preparation of a pigment from a dyestuff by this method results in a product lower than the original material in brilliance and strength yet very satisfactory for a pigment and with much improved lightfastness and other related properties. The development of these pigments is reviewed, and the more widely accepted theories on the composition and structure of PMA colors are summarized. Properties, methods of manufacture, and important applications are also discussed.
W. W. WILLIAMS AND J. W. CONLEY Imperial Paper 6% Color Corp., Glens Falls, N . Y .
T
HE principal use of molybdenum in organic pigments is
in the form of the complex heteropoly acids, phosphomolybdic and phosphotungstomolybdic acids. These compounds, along with phosphotungstic acid, constitute precipitating agents for numerous basic and other closely related dyestuffs, resulting in a class’of organic pigments noted for brilliance, strength, and greatly improved stability to light, heat, and water compared with the earlier pigments of this type. Dyestuffs when precipitated with phosphomolybdic acid are known as PMA colors, with phosphotungstic acid as PTA colors, and with a mixture of these acids, phosphotungstomolybdic acid, as PTMA colors. Typical dyestuffs which are employed in the manufacture of PMA and PTMA colors include malachite green, brilliant green, Setoglaucine, methyl violet B, ethyl violet, Victoria pure blue BO, rhodamine B, rhodamine 6G, and thioflavine (1, 18). The 1953 production and sales figures for some of the most important PMA toners (full strength pigments) are given in Table I. The figures for PTMA pigments are not available since they have been combined statistically with the PTA production data (20). Also the production and sales records for flushed, extended, and laked pigments are omitted in the figures given in Table I. HISTORY
Some of the precipitating agents which were used for basic dyestuffs before the discovery of PMA and PTA colors were chestnut bark extract, sumac, quebracho, rosin soap, “green earths,” tannic acid-tartar emetic, and many others (4, 22).
Table I. 1953 Production of PMA Toners (Full Strength Pigments) (20) PMA Toner Setoglaucine Victoria pure blue B Brilliant green Malachite ereen RhodamineB Rhodamine 6 G Methyl violet B
1953, Production, Lb. 8,000 60,000 12,000 12,000 17,000 32,000 282,000
1958 Sales, Lb. 8,000 52,000 11,000 8,000 11,000 31,000 267,000
Even though the colors formed using these materials exhibited in some instances both brilliance and strength yet they were a6 a class extremely sensitive to light, heat, and water. The most important product of these nonpermanent pigments today is made by precipitating methyl violet B with the tannic acid-tartar emetic combination ( 1 8 ) . The initial invention for the preparation of PMA colors in this country was disclosed in a patent granted to Immerheiser in July 1917 (9). A comparable patent covering the use of phosphotungstic acid as a precipitant for basic dyestuffs was issued to Immerheiser and Beyer on the same date (10). Because of the unsettled conditions created by World War I the full application of these processes to the manufacture of pigments was not possible until several years later when molybdenum and tungsten became more readily available (18). In May 1921, two other patents were issued which influenced the progress of the development of this class of pigments. The first, granted to Lendle, was an expansion of the usual basic dyestuffs to include “such acid colors as acid green, erioglaucine, soluble blue, and acid violet (11, 13). The second patent, issued to Linz, disclosed a method for preparing the complex phosphotungstic acid from a soluble phosphate and tungstateplus a s u i t a b l e acid in the presence of the dyestuff to be precipitated. This warn carried out without isolation and purification of the complex acid ( 1 4 ) which had been required heretofore, and this change greatly simplified the methods of preparing these colors resulting in quite an appreciable reduction in the cost of their manufacture
Me2N.q
+
MeJpjP.Oi24 MOO,
NHMe
Figure 1. Phosphomolybdate toner of methyl violet August 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
1507
ENGINEERING, DESIGN, AND EQUIPMENT
b
b
COOEt
"Et Figure 2. Phosphomolybdate toner of Victoria pure blue BO ( 5 )
Figure 4.
Phosphomolybdate toner of rhodamine 6 6
Figure rj.
Phosphomolyhdate toner of thioflavine T
Me
0 Figure 3.
Phosphomolybdate toner of brilliant green
(18). In addition to these four patents, Hartmann discovered that a combination of the two complex heteropoly acids, phosphomolybdic and phosphotungstic, when used together for the precipitation of a dyestuff resulted in a pigment with lightfastness superior to the corresponding product formed when either complex acid was employed alone ( 8 ) . Although variations and minor improvements in these processes have continued to be made during the succeeding years the basic principles disclos6d in the five patents noted are still those which control the manufacture of PMA and PTA colors today. However, the continued progress that has been made in the quality and performance of this class of organic pigments has been accomplished largely through improved techniques and procedures in the particular plants in which the pigments have been manufactured. Although there is not complete agreement as t o the reason, the fact is that from the date of discovery until 1935 only the phosphotungstic colors, or those from phosphotungstic acid modified with a small percentage of phosphomolybdic acid, were of major interest to the pigment manufacturers. Some authorities claim it was scarcity of molybdenum caused by inefficient methods of obtaining the pure compounds which consequently created a high price (7), while others insist that the lakes from the tungsten product were superior in brilliance, strength, and lightfastness ( 1 6 ) . This relative position of the two complex acids changed, however, with an increase in importance for molybdenum during the period 1935 t o 1940 when the supply of tungsten was greatly reduced by conditions in the Far East. Finally the tungsten ore was placed under government control to +ensure proper distribution for critical World War I1 production. When attention did become centered on possible replacements of tungsten by molybdenum, many improved phosphomolybdate colors were developed, some with equal if not superior properties to the corresponding tungsten derivatives ( 16, 18). COMPOSITION AND STRUCTURE O F PMA COLORS
The composition of the complex heteropoly acids is not considered, but those who are interested are referred to the basic work of Wu (21)or the thorough summary of this paper by Pratt (18), and more recent discussions by Pauling (17), Autenrieth and Osterheld (s), Keggin ( T I ) , and others. In the original discussion Wu proposed two principal formulas for the complex phosphomolybdic acids, one containing a ratio of phosphoric anhydride to molybdenum oxide of 1:24 (3Hz0.Pz06, 24MoOs) and a reduced less stable form of 1: 18 ratio (3HzO. P20J.18MoOa). Recent x-ray data indicate that a preferable formula for the 1 to 1508
24 acid probably should be H3P01.1 2 M 0 0 ~ . x H z 0( 1 7 ) . As a basis for the study of the composition of five PMA pigments with regard to their lightfastness and other properties (Figures 1-5) the 1 to 24 acid will be used. There are two main points of view with regard to the structure of the phosphomolybdic acid colors made from various dyestuffs. The first is that most dyes exist as salts, usually chlorides or sulfates, and that the formation of a phosphomolybdate or phosphotungstate color is simply a replacement, in solution of a rather soluble anion by a much heavier, less soluble anion which in this case would be the formation of a phosphomolybdate from a chloride. There is some evidence that this explanation is correct and that approximately 6 ( 1 8 ) or 7 (19) moles of dye are 24Mo03. However, required for each molecule of 3 H ~ 0Pz06. the data available are not sufficient to make this explanation conclusive since the formation of these colors appears a t times to follow erratic paths (16,18). The second point of view contends that the phenomenon is purely one of adsorption and has no stoichiometric basis for its existence ( 1 5 ) . Figure 6a and 6b show first the two extreme resonance configurations of a typical styryl dye (p-dimethylaminostyryl-benzothiazole ethochloride) as the extremely soluble chloride salt ( 9 ) When a solution of this salt is treated with a mole of sodium iodide, the much Iess soluble iodide salt of the dye is precipita,ted a t once (Figure 6c). If the very similar molecule thioflavine T be substituted (Figure 7a, 7b) for that of the styryl dye in Figure 6 the two extreme resonance configurations will be analogous. If now the much heavier and less soluble phosphomolybdate ion be added, it could easily be assumed that B replacement would take place as indicated in Figure 7c. There are several reasons why it is doubtful whether more than one amino nitrogen of a basic dyestuff molecule is combined with a phosphomolybdate ion a t any one time as described by Pratt (18). Figure 81,11, and I11 shows that as the resonance between the three dimethylamino- groups in crystal violet (18) is restricted stepwise, first by the addition of one mole of hydrogen chloride t o change the color from violet t o green (Figure 81, 11) and consequently to tie up one --NMez group as the nonresonating hydrochloride, then secondly, to change the dye solution from green to yellow by forming the hydrochloride of the second -NMe2 group (Figure 811,111)and simultaneously reducing the resonance to a very low order. Since there is generally only a very slight change, if any, in the shade of the basic dyestuff when converted to its corresponding PMA pigment, it is unlikely that more than one amino nitrogen is influenced during a single period of time. Also, there seems to be no substantial difference between the molecular ratio of the complex acid to dyestufl' required to precipitate thioflavine T and brilliant green, which contain only two nitrogens as compared with methyl violet or crystal violet which have three substituted amino groups.
INDUSTRIAL AND ENGINEERING CHEMISTRY
VoI. 4'1,No. 8
MOLYBDENUM MANUFACTURE
The conditions under which these pigments are manufactured vary within rather wide limits. While there are certain generalizations which will apply to all the colors in this class of compounds, the specific conditions for the preparation of any one product are determined very largely by the properties dpsired in the finished color.
fa)[
a:; C
'[*
Et
I
Et
C H=CH+NMe,]CI
-
I
No1
it Figure 6. Replacing more soluble anion with less soluble one in styryl dye
The general method of preparing a PMA color is as follows: 1. The dyestuff is dissolved in water a t 60' to 70" C. a t a dye concentration of approximately 1yo. The p H of the solution is usually brought to 2.5 to 3.5 by means of acetic acid. 2. The complex acid solution is prepared from soluble phosphate and molybdate salts with the addition of a mineral acid to bring the p H within the range of 3 to 4. Usually the temperature of the complex solution is 20" to 30' C.
3. The complex acid solution is added to the dye solution under controlled conditions of agitation and rate of addition. After addition is complete the reaction mixture is heated for stabilization of the final pigment. After the heating period an amount of extra complex solution required to tie up the excess dye which was liberated during the heating period is added. In the procedure given by Pratt for the preparation of a PTM.4 color lake from crystal violet (18), and in the detailed study of conditions for the preparation of toners from ten separate dyestuffs (PMA, PTA, and PTMA colors are included) by Linz and Coffer ( 1 5 ) , there are certain points of agreement which may be summarized 1. Dye Solution. The concentration of the dye solution should be no more than 1% unless the dyestuff is extremely soluble. Care should be exercised to have the dye in solution a t the time of strike (addition of complex solution to the dye solution). Usually it is safer to warm the dye solution to 60" t o 70" C. unless other conditions make this undesirable. The pH of the dye solution should be approximately 3.
2. Complex Acid Solution. The complex acid is normally made with a 1 to 24 ratio of P20ato MOOS,since this is the complex acid that will predominate under the conditions of the procedure. The normal pH range for the PMA complex should be 3 to 3.5. However, there are exceptions to this when the pH may be dropped if lightfastness is desired a t a sacrifice of strength and shade. The complex solution is normally prepared a t room temperature (20" to 30" C.).
3. Formation of Pigment (Strike). The complex solution is added to the warm dye solution under controlled conditions. The actual rate of addition of the complex solution does not appear to be too critical. If maximum strength and brilliance are desired, the ratio of complex acid t o that of dye should be kept close t o theory before heating. After the reaction mixture is heated for stabilization a slight bleed will appear and additional complex will be necessary to tie up the remaining dye. To obtain maximum lightfastness a t the expense of brilliance and strength, the pH of the complex solution should be lowered and the ratio of complex solution to dye should be increased above theory. Heating the reaction mixture is necessary in order to stabilize the product and establish greater resistance t o light. It is important that the end use of a pigment of this type be kept in mind in order to select the optimum conditions of pH, temperature, ratio of complex acid to dyestuff, and other techniques of operation. PROPERTIES
In addition to brilliance and strength, which properties are desired in all organic pigments, the most important characteristics of this class of colors are resistance to light, heat, and water. There is not general agreement on the relative stability of PMA and PTA colors to sunlight. It is an established fact, however, that PTMA colors are superior to either single complex acid type alone in their stability to sunlight. Rather than assume either point of view to be entirely correct the authors prepared representative exhibits of each of the dyes, the formulas of which are given in Figures 1 to 5. Fade-0-hfeter exposures have been made a t 10% strength levels of the 100% PMA, 100% PTA pigments of each of the dyes, and certain mixed (PTMA) examples of the same dyes; rhodamine 6G is shown in Figure 9. I n general the findings are similar to those described by Linz and Coffer ( 1 5 )and Killeffer and Linz (12). To summarize the results it can be said
1. PTA pigments are cleaner and brighter on print tone than PMA colors. PTMA colors are likewise cleaner than the corresponding PMA products. 2. When exposed a t 10% strength levels in a Fade-0-Meter, the PT-4 colors fade but remain clean while the P M A colors darken very rapidly and are less sensitive to change thereafter. 3. The PTMA colors are superior in lightfastness to either PlLA or PTA colors alone. The results of the exhibits are summarized in Table 11.
w
NMe,
he
I
,+obC
VIOLET
L.
I
..
AMeiHCI
I'
lE YELLOW
Me
Figure 7. August 1955
Possible replacement of soluble anion with one much less in thioflavine T
Figure 8. Effect of increased acid concentration on solution of crystal violet
INDUSTRIAL AND ENGINEERING CHEMISTRY
1509
ENGINEERING, DESIGN, AND EQUIPMENT PTMA
PMA
considered nontoxic. It is for this same reason that these products are used in printing of children’s toys and in enamels for spraying toys. Another useful property of phosphomolybdic acid pigments is their low solubility or bleed in melted paraffin. This finds application in the printing of wrapping paper for foodstuffs that must be coated after printing to avoid loss of moisture or penetration of odors. Since these colors are insoluble in the hot paraffin the paper can be coated very soon after printing without streaking or bleeding of the colored ink (6). I n the past 15 years molybdenum colors have been developed as replacements €or tungsten, when the latter was in short supply, and the performance of the PMA pigments has proved so satisfactory that the industry has not returned to the use of the more expensive PTA products. It is possible that even greater improvements in the development of PMA colors may result in the future, strengthening even more their position in the field of printing ink colors.
PTA
ACKN 0W LEDGM ENT
Figure 9. Fade-0-Meter exposure of 10% strength levels of the 100% PIMA, loo$%PTA, and PTMA of rhodamine 6 6 pigment
The authors wish to thank the Engineering Department and the Microanalytical Laboratory of the Imperial Paper and Color Corp. for their assistance in preparing the exhibits, and to express their appreciation to their colleagues in the Research and Development Division for their helpful suggestions in the preparation of this paper. LITERATURE CITED
Table 11. Summary of Fastness to Light of Representative PMA, PTA, and PTMA Pigments Dyestuff Methyl violet
~~~~~l~ :+$;? Lightfastness, 10% Strength Basis Tannic in Exposures, Figure Hr. Ph‘IA PTA PTMA acid 1
Victoria pure
blue BO (6)
13 11 24 6 16
Fair
...
FaL
Fair Fair
Fair
...
Good Good
8
Good Good
4
4
. Good Good
5
4
Brilliant green
3
Rhodamine 6G Thioflavine T C N
8
6
Fair
...
Good . . . Good Fair
,
,
..
. ... ... . Exoeilknt Exikilent Exheilknt
... .
Very poor Very poor
..... .....
.... . ..... . . . .. ,....
. . ...
.....
USES
The principal uses for phosphomolybdic acid pigments, as well as the phosphotungstic acid colors, are in the field of printing inks. Other important applications include tinting and shading of white paper, in water colors, show card inks, in wax crayons, in the manufacture of colored paper, and in enamels for children’s toys. I n the manufacture of white paper it is necessary to use coloring material to offset the yellowness of the natural paper stock. The PMA colors are used extensively for this purpose. I n preparation for this tinting and shading application it is necessary to give the pigment special processing. It must be in a readily dispersed form, in fact its properties need to resemble more nearly those of a water-soluble dye than pigments. By far the largest percentages of PMA pigments are used in printing ink applications, These colors are widely used in all types of printing inks especially in magazine printing, package or boxboard printing, and in label printing. The properties of these pigments which make them valuable for this purpose are their brilliance, and their stability towards light, heat, water, and melted paraffin. The requirements for food wrapper and food container printing inks, for example, are very strict with regard to odor, toxicity, and appearance. The PMA colors are useful in this capacity since they are insoluble in water and 1510
Allen, E. R., in “Encyclopedia of Chemical Technology,” edited by Kirk, R. E., and Othmer, D. F., vol. 10, pp. 674-7, Interscience Encyclopedia, Inc., New York, 1953. Autenrieth, L. F., and Osterheld, R. K., Ibid.,vol. 7,pp. 458-65. Brooker, L. G. S., Keyes, G. H., and Williams, W. W., J . Am. Chem. Soc., 64, 199-210 (1942). Curtis, C. A., “Artificial Organic Pigments and Their Applications,” translated by Fyleman, E., pp. 15-26, Pitman, London, 1930. Dorman. K. L., and Fox, K. R., Am. Duestuff Reptr., 43, X o . 14, 427 (1954). Ellis, C . , “Printing Inks, Their Chemistry and Technology,” pp. 356-7, Reinhold, New York, 1940. Gardner, H. A., Natl. Paint, Varnish Lacquer Assoc., Circ. 513, 234-8 (1936). Hartmann, E. (to Grasselli Dyestuff Corp.), U. S. Patent 1,653,851 (December 27, 1927). Immerheiser, C. (to Badische Anilin- und Soda Fabrik), Ibid., 1,232,551 (July 10, 1917). Immerheiser, C., and Beyer, A.,Ibid., 1,232,552(July 10,1917). Keggin, J. F., Nature, 132, 351 (1933); Proc. Roy. SOC.(Lond o n ) , A144, 75 (1934). Killeffer, D. H., and Lins, A., “Nolybdenum Compounds,” pp. 172-5, Interscience, New York, 1952. Lendle, A. (to Kutroff, Pickhardt & Co.), U. S. Patent 1,378,418 (May 17, 1921). Linz, A. (to The Chemical Foundation), U. S. Patent 1,378,882 (May 24, 1921). Lins, A.,and Coffer, L. W., Am. Ink Maker, 17,No. 9,39-67; No. 10, 29-35; NO. 11, 27-45 (1939). Loughlin, J. E., D y e s t u f s , pp. 217-23 (September 1938). Pauling, L., in “Molybdenum Compounds,” by Killeffer, D. H., and Linz, A., Chap. 9, pp. 95-109. Interscience, New York, 1952. Pratt, L. S., “Chemistry and Physics of Organic Pigments,” pp. 133-79, Wiley, New York, 1947. Richards, K. M., J. SOC.Dyers Colourists, 52, 378-80 (1939). U. S. Tariff Commission, Washington 25, D. C., Rept. 194, “Synthetic Organic Chemicals, United States Production and Sales, 1953.” Wu, H., J. Biol. Chem., 43, 189-220 (1920). Zerr, G., and Riibencamp, R., “Treatise on Color Manufacture,” translated by Mayer, C., pp. 435-62, Griffin, London, 1908. RECEIVED for review January 19. 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDJune 6, 1955.
Vol. 41, No. 8
