Short-Cycle Syntheses of Ultramarine Blue C. A. KUMINS AND A. E. GESSLER Research Laboratories, Interchemical Corp., New York, N. Y .
U
9
LTRAiMARINE is a deep blue pigment which was known and highly prized for its rich color from the dawn of history. Even today its chromophoric properties are so well appreciated that over 20,000,000 pounds per year are sold in the United States alone. Chemically, the color is a complex alumino-silicate having a crystal structure closely related to the zeolites. The color-producing group is thought t o be contributed by the polysulfide linkage held within the alumino-silicate crystal lattice in some, as yet,, undetermined manner. Figure 1 shows in a diagrammatic manner the basic arrangement of silicon, aluminum, and oxygen' atoms in the zeolitic structure of ultramarine blue. The fundamental unit is a tetrahedron with either aluminum or silicon occupying the center. There are four oxygen atoms a t the corners and since aluminum tetroxide and silicon tetroxide tetrahedra more or less alternate with each other, it can be seen that two oxygens of each tetrahedron are held in common. According t o the latest theories (4), all the atoms are in their ionic state and consequently the net change of two tetrahedra is one minus = 7 ; O + + = -8). In order, therefore, f t o neutralize the molecule, one N a + is required. Since there is
no place within the crystal to accommodate it, the alkali metal is distributed statistically so t h a t the net electrical charge is zero. It is the sodium ion not held firmly within the lattice which can be exchanged for other cations that is responsible for the ion exchange properties of zeolites in general and ultramarine in particular. The polysulfide groups of the blue pigment are considered t o be within the rather open crystal network of the alumina and silica tetrahedra. The negative charge of these sulfide linkages is neutralized by other sodium ions thus giving rise t o two types of alkali metal ions: one t o neutralize the minus one charge of the tetrahedra and the other associated with the sulfur. MANUFACTURE
Present day processes for the manufacture of ultramarine blue follow with some variations the method described below (8). Generally a fine grade of specially treated china clay and sulfur are mixed with either well-calcined sodium sulfate or sodium carbonate. To these ingredients carbon, pitch, or rosin is added to provide the reducing agent for the conversion of the sodium salts t o the polysulfides necessary for the attainment of the blue pig-
+
*
-4 -3 -2
-I
After Pauling (5)
After Rice (6)
Figure 1. Basic Arrangement of Atoms in Zeolitic Structure of Ultramarine Blue
567
568
INDUSTRIAL AND ENGINEERING CHEMISTRY
ment. The resulting mixture is placed in fire clay crucibles which are charged into a furnace. The temperature is then brought to about 850" to 900" C. over a period of 4 to 5 hours, uhere it is maintained from 6 to 12 hours depending upon the speed of the reduction of the alkali salts and their subsequent reaction with the china clay. After the reduction is completed the furnaces are allowed to cool over a period of 2 to 5 days, according t o the size of the unit. The cooled reduced ultramarine, sometimes called primary blue, which is green in color, is then ground in preparation for the conversion to the secondary or final blue step. Since the primary pigment tends to be quite hard and fused the giinding step is an important preliminary in the final conversion to the deep and beautiful color of commerce. Too much or too little crushing will yield an inferior blue of poor pigmentary propel ties. The ground pigment is then mixed with about 7 to 10% of sulfur and heated a t bright red heat in a muffle furnace for a 3- to 6-dav period, depending upon the size of the muffles used. Provision must be made for small amounts of air to enter the calciner so t h a t the sulfur mav be oxidized to sulfur dioxide TT-hich, in turn, converts the primary pigment t o the secondary or blue stage. Great care must be taken during thiq step lest the reaction be carried too far and a product which IS all white or partially so be obtained. Thecooled finished blue is then washed and ground in the usual manner. I n another so-called direct process the primary color is converted to blue by allowing controlled amounts of air to be admitted into the reduction calciner while it is in the cooling stage. In either method the entire procedure requires a period of 1 b 2 weeks per batch. EXPERIMENTAL WORK
I n view of the close similarity of the ultramarine crystal structure with that of zeolites, attempts were made by Singer (7) and others t o use these materials as starting points. I n thimethod of attack, sodium polysulfide solutions were boiled in the presence of zeolites, either natural or fused, both a t atmospheiic pressure and higher. However, while products were obtained which had a slight blue coloration, these m-ere unstable and TT ei e never a true ultramarine blue. Accordingly, work was started here with the puipose of determining whether it was possible to make an ultramarine blue of great chromatic power from synthetic zeolites. It was also desired t o develop a process or processes which were not on11 more capable of scientific control than the older methods but also would produce a worth-while color in a shorter time interval than the 1- t o 2-week period now required. Preliminary experiments indicated t h a t an ultramarine could be formed from a mixture consisting of zeolite, sodium polysulfide, and sulfur calcined in a nitrogen atmosphere and then carefully oxidized with air. While the tinctorial strength of the blue so obtained was extremely lorn, these investigations indicated very definitely the feasibility of supplementing china clay M ith zeolite. Therefore more detailed experiments Ti-ere carried out with the following raw material mix:
350/, sodium polysulfide 45% zeolite 20% sulfur Reduction calcination temperatures TT ere arbitrarily kept at 800" C. for a period of 7 hours. Since it was found empirically that the oxidation appeared to be most efficient a t 400" to 600" C. it was decided t o run the secondary reaction a t 500" C. T o prevent burning of the pigment by air during the primary or reduction step the calciner was kept continuously filled with nitrogen gas. The secondary or so-called "oxidation" step was carried out n i t h sulfur dioxide since this is the gas formed in the present day process which employs sulfur and air to convert the green variety t o the blue. An arbitrary reaction time of 3 hours was set for this stage. The resulting product was washed, milled, and tested for color strength according to the American Society for Testing Materials method D387-36. This technique yielded a hard and fused ultramarine with poor pigmentary properties. The sulfur content varicd from 2 t o
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4% as compared with about 15% for the best commercial blues. It is generally believed that, other things being equal, the sulfur content is an indication of the quality of the color; the higher the value the better are the chromophoric properties. Since the sulfur figure was so low, hydrogen sulfide gas was substituted for the nitrogen with the thought that a t the reaction temperature some sulfur would form from the thermal dissociation of the gas and keep the reaction mass in continual contact x-ith this desirable component. This caused an immediate increase in sulfur content to about 10% with an accompanying increase in tinctorial strength. h series of runs was then made with C.P. sodium sulfide nonnhydrate to determine the effect of forming the polysulfide linkage in the presence of the zeolite. It was hoped that during the course of the reaction the sulfur atoms would unite with each other before the crystal lattice of the zeolite was developed with the possibility that the final structure would be formed around this grouping. The same previously mentioned formula was used except that sufficient additional sulfur x ~ a sadded to make up for the difference between the monosulfide and the tetrasulfide. The resulting products showed an increase in tinctorial strength of about 15% and exhibited slightly better and cleaner color characteristics. Accordingly it \Tab decided to form all the sulfide linkages in situ, so to speak, bv the use of a sodium compound which would decompose rapidly under calcination conditions without giving rise to any undesirable residue. Another requirement which this compound was expected to meet was a decomposition temperature sufficiently below that of the boiling point of sulfur so that the reaction could take place in the presence of a large excess of sulfur. This was believed desirable because it was felt that the higher polysulfides produced the best pigments owing to more resonance possibilities and it was considered that these were more readily formed in the presence of a large excess of sulfur. Other advantages accruing from this technique were thought t o be (a) more rapid reaction caused by the formation of sodium polysulfide in almost molecular sized crystals, (b) formation of the polysulfide in intimate contact with the zeolite at a temperature considerably below the actual crystallization of the aluminosilicate with the possibility of having the crystal lattice growing around the alkali polysulfide, (c) the production of a soft product owing to the absence of free alkali which causes fusion. The classes of compoundswhichmeet these requirements are the organic sodium salts. They decompose rapidly and completely before the boiling point of sulfur is reached and they do not leave any undesirable residue. Long chain fatty acid soaps were considered unsuitable because of the high carbon content which would require too great an excess of sulfur to combine n-ith it as carbon disulfide, which was later found t o be formed. On the basis oi lowest cost per sodium atom, sodium formate and sodium acetate were considered equally practical and therefore all further work was carried out with sodium acetate. Substitution of amounts of sodium acetate equivalent to the sodium content of the tetrasulfide used in the formula described previously, produced intense clean blues of extremely fine texture even before milling. Later colors made with the alkali acetate also were of exceptionally high strength some being twice as strong as the best commercially available material. Their sulfur contents ranged between 15 t o 16%. The data below describe the effect of the variables involved in the manufacture of ultramarine blue (1,3). PREPARATION OF ZEOLITE
All zeolites were made by reaction between solutions of sodium aluminate and sodium silicate. The resulting products were washed almost free of alkali and then dried a t 100' C. for 13 hours so that the final aluminosilicate contained approximately 14% water which was mostly bound within the crystal lattice.
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569
An average analysis expressed on a n anhydrous basis showed the zeolite to contain 50.0% silicon dioxide, 31.8% aluminum oxide, and 18.4% sodium oxide. PREPARATION O F THE RAW MIX
After numerous experiments i t was found that the best results were obtained by the use of a mixture, prepared in a ball mill, containing
34y0anhydrous zeolite (hydrate used) 5 5 . 7 % sulfur 9 , 3y0 anhydrous sodium acetate 1,O% soap binding agent
a
The large excess of sulfur employed does away with the need for any extraneous gas t o prevent oxidation of the pigment during the reduction step. The binding agent serves t o help mix the hydrophobic sulfur with the hydrophilic zeolite and sodium acetate, thus preventing formation of white unreacted specks in the finished product. REDUCTION OR PRIMARY CALCINATION
Forty grams of the mixture described above were calcined a t various temperatures for 1.5 hours in order to determine the minimum reaction temperature. Since sodium sulfide is formed during the course of the synthesis, the quantity of sodium which appears in the wash water obtained from the cooled, filtered primary color is a n indication of the extent of the primary reduction step. The results are listed in the following table: Caloination Temp., C.
1000 950 900 850
825 800
775
Charge Weight, Grams 40 40 40 40 40 40 40
Water Soin. Na, Gram 0 2 0.2 0.2
Water S o h . S, Grams 0.5 0 4 0 3 0 4
0 2
03
0 0
14
0 2 0.z
0.3
Under the calcination conditions studied, the reaction goes t o completion at muffle temperatures of 800" C. or better. Calcination for longer time intervals up t o 24 hours permits a lower temperature for completion of the reaction. This is due, no doubt, t o the fact that temperatures were measured inside the muffle furnace and not in the interior of the calciner because of the corrosive action by the sulfur fumes on the thermocouple. Consequently, keeping the muffle temperature below 775" C. requires a longer time fbr the contents of the flask t o reach the point at which reaction is complete. Further experiments under a n external temperature of 750' C. showed that after 8 hours a high-grade pigment was produced. Temperatures much below 750' C. caused incomplete reaction regardless of the length of calcination time. SECONDARY CALCINATION
*
The reduced ultramarine, washed free of salts and dried, is converted to the blue pigment by reaction with sulfur dioxide gas. Since the correct temperature and time interval were not known a series of runs was made in which both were varied. The tinting strength of the resulting pigment was taken as the criterion for the efficiency of the reaction; the higher values naturally denoting the optimum conditions. Table I, containing the results obtained, shows that best products can be expected by conducting the secondary calcination a t 500" C. for 3 hours.
Figure 2. a. Calcilaed clay
b.
X-Ray Patterns e. d.
Ultramarine
Zeolite calcined for 7.5 hours Zeolite calcined for 2 hours
hours a t 800" C. (Figure 3, c ) . I n this way, the large polysulfide groups (S-S linkage in Ss = 1.04 A.) which are thought to reside within the rather open framework of the alumina and silica tetrahedra, are already present before the structure is even formed. Since it is believed t h a t the resonance within the polysulfide group is responsible for the chromophoric properties of ultramarine it follows that the more numerous and the longer the s-S chains t h a t can be forced into the basic framework the more intense will be the color. Examination of Figure 1, b reveals that the openings in the ultramarine structure vary between 2 and 6 A. According t o Pauling, the S-S bond has a radius of 1.04A. and the S=S one of 0.95A. Since resonance would occur
TABLE I. SECONDARY CALCINATIONS Caloination Temp., C. 450 500
Calcination Time, Hours
500 500
600 600 1
700 700
2
Tinting Strength 145 160 170t 170 165 170 155 145
CHEMISTRY OF THE CALCINhTION REACTION
When the raw mix containing essentially a n organic sodium salt, sulfur, and zeolite is heated in a calciner, the alkali compound decomposes a t a low temperature and in the presence of a large excess of sulfur forms sodium polysulfide, probably the tetra and hexa sulfides. The exhaust gases contain carbon monoxide, carbon dioxide, carbon disulfide and, in the case of sodium acetate, acetic acid. As indicated, some of the carbon resulting from the carbonization of the organic salt is removed as carbon disulfide. Since x-ray data (Figure 2, c and d ) indicate that the zeolite lattice is only formed after 7 hours of calcination at 1000' C., the polysulfide can react with the amorphous aluminosilicate to form the ultramarine structure which occurs at the end of 1.5
Figure 3.
X-Ray Patterns
a. High-silica ultramarine prepared by 1.5-hour calcination b. S a m e as above, but calcined for 4 hours e. Regular ultramarine calcined for 1.5 hours
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INDUSTRIAL AND ENGINEERING CHEMISTRY
by shifts between single and double bonds and assuming an average radius of 1.0 A,, it can be seen that for the size opening considered only Szor SB polysulfide could enter within the preformed ultramarine lattice even though the volume within could conceivably accommodate higher polysulfides.
TABLE11. ANALYSISO F ULTRAMARINE BLUEOBTAINED CLAYAND ZEOLITE Ultramarine Prepared from Clay 33.9 26.0 17.9 6.3 15.9
FROM
Ultramarine Prepared from Zeolite Primary Secondary 36.1 36.2 26.8 26.5 23.0 20.7 14.1
0.0 16.6
According to Table I1 the sulfur content of ultramarines prepared either from zeolite or clay is approximately the same. Therefore, in order to account for the increased average chromaticity of about 70% of the zeolite prepared blue, it is necessary to assume the presence of more resonating centers caused by sulfide linkages in excess of 53 which were present before the crystal structure began t o form. Accordingly the reduced or primary ultramarine resulting from the series of reactions is a deep greenish blue or turquoise powder in contrast t o the light yellow green produced by the usual process. An analysis of the data obtained by a study of the secondary reaction in which the final color is obtained by action of sulfur dioxide and air produces some surprising results. Reference is made to Table 111. Regardless of the reduction temperature employed there always occurred a weight increase in the unwashed “oxidized” pigment. Upon washing, the loss in weight was such that the quantity of color was less than the original amount which was subjected to the action of sulfur dioxide. Analyses of the wash water revealed the presence of sodium sulfate. It can be seen that the difference between the weight of the reduced ultramarine and the washed secondary product corresponds exactly to the sodium oxide content (as sodium sulfate) in the wash water.
TABLE 111. MATERIALS BALANCE Wt. Washed Wt. Washed Calc. Red. U. M., OxideU. bf., Temp., C. Grams Grams 14.3 1000 14.7 14.3 950 14.8 ROO 15.0 14.5 14 5 850 15 0 15 0 14 7 825 14 7 800 15 0
Diff., Gram 0.4 0.5
0.5 0 5 0 3 0 3
Nazi304 in Wash Water 1.00 1.30
Na?O Equivalent 0.43 0.57
1.15
0.50
1 05 1 00 0 90
0 46 0 43 0 39
On this basis, therefore, it can be said that the secondary calcination only involves the removal of oxygen from the crystal lattice as sodium oxide. Referring t o the picture of the molecular structure illustrated earlier it can be seen that this causes a vacant space in the lattice thus supplying another resonating center for the chemically similar sulfur and so it would be reasonable t o expect increased chromophoric properties. As it turns out, the secondary product is the deep beautiful blue of commerce having much greater color value than the primary material. The mechanism for the removal of sodium oxide as the sulfate involves the formation of sulfur trioxide and sulfur from two molecules of sulfur dioxide. The optimum temperature for conducting the secondary calcination occurs around 500” C. which is the same temperature a t which sulfur dioxide reacts most efficiently with air t o form sulfur trioxide. It should also be mentioned that sulfur fumes are constantly given off during this latter step
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and these can only come from the reaction of one molecule of sulfur dioxide with another since the sulfur content of the finished blue (as shown in Table 111) is actually higher than that of the primary product. I n order to test the belief that the conversion of the green or greenish blue product to the finished ultramarine involves merely the removal of sodium oxide secondary calcinations were made using nitrogen dioxide, sulfur trioxide, and sulfur dioxide plus air. The resulting products were as good as these obtained by the use of sulfur dioxide alone and, in fact, the employment of sulfur trioxide actually increased the speed of the secondary reaction so that only 15 to 30 minutes were required instead of the usual 3 hours. ULTRAMARINE FROM OTHER SILICACEQUS MATERIALS
I n the foregoing sections a method for the synthesis of high strength ultramarine blue from precipitated zeolite and alkali metal organic salts was described. Based, therefore, on this work and on the similarity between the ultramarine crystal lattice and that of the zeolites, it was believed that any mixture of components capable of forming a zeolitic structure during calcination would be potentially able to react with polysulfides to produce ultramarine blue. This thought was further strengthened by an analysis of a typical raw mixture, noted below, used to produce this color (Table IV).
TABLE IV.
STASDARD FORMULA FOR ULTRAMARINE KAOLIN China clay NarCOa Carbon Diatomaceous earth Sulfur
FROM
PO0 parts 102 parts 21 parts 16 parts 117 parts
When this mixture is heated the soda ash fuses with the china clay and breaks it down to sodium aluminate and sodium silicate. This reaction is well known and used in the analysis of silicaceous materials. However, because of the small amount of carbon present only a proportionate quantity of this sodium carbonate is reduced t o form sodium polysulfide. Upon further heating, the aluminate and silicate combine to form sodium aluminum silicate and free sodium hydroxide if a n excess of sodium carbonate is present. This, then, unites with the previoudy formed polysulfide to yield the primary ultramarine. It was the purpose of this investigation, therefore, to determine whether the synthesis of ultramarine blue could be accomplished by the use of raw materials which would form a zeolite type structure under more favorable conditions. The raw materials used as potential zeolite formers were sodium aluminate, silica, sodium silicate, and clay. SODIUNI ALUMIXATE MIXTURES
I n this series of experiments, mixtures were made with sodium aluminate, diatomaceous earth, anhydrous sodium acetate, and sulfur. The diatomaceous earth was employed as a silica source because its high surface area enhanced the reaction rate. The following basic formula was used in the calcination experiments. 47.5 grams of 90% grade sodium aluminate and silica 13.0 grams of anhydrous sodium acetate 78.0 grams of sulfur Variations were made in the silicon dioxide-aluminum oxide ratio and the resultant effect on tinting strength as measured by the American Society for Testing Materials method, D387-36, is listed in the following table (Figure 5 ) . Mole Ratio SiOz/AhOr 1.37 1.50 1.61 1.71 1 88
Tinting Strength 160 165 180 140 70
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lattice into amorphous silica and alumina and the latter to the crystallization of the aluminum oxide. X-ray diffraction patterns show this breakup to occur, Figure 2,a. Since this is one of the mechanisms believed responsible for the formation of the zeolitic structure of ultramarine by fusion of sodium carbonate, it was believed that precalcination of the clay prior to causticisation would permit the reactioa of the sodium hydroxide with the amorphous silica and alumina to take place more readily and more efficiently. Sodium silicate and aluminate would be formed which would immediately react t o precipitate a zeolite much in the manner described in the first section.
Figure 4.
X-Ray Patterns
Standard pigment made from clay and soda ash b . Pigment made from zeolite a n d sodium acetate C. Pigment made from sodium aluminate, silica, a n d sodium acetate d . Pigment made from ealcined, causticized clay, and sodium acetate a.
All mixtures were converted t o ultramarine and the optimum silica to alumina ratio of 1.61:l produced beautiful blues, 60% stronger than the best material made by the untreated clay process. Calcination temperatures were a little higher, being 850" C. Another item of interest is the rapid decrease in chromophoric characteristics resulting from high silica to alumina ratios. The explanation appears to lie in the fact that silicon dioxide is well-known crystal formation inhibitor when used in conjunction with alumina. I n the previous section, relative t o the use of zeolite and organic sodium salts, x-ray data (Figure 3) indicated incomplete development of the crystal structure of ultramarine when made with excess silicon dioxide. The same observation in an analogous situation was made by Insley and Ewell ( 2 ) who reported the restraining effect of silicon dioxide on the crystallization of aluminum oxide. X-ray patterns of calcined zeolite and calcined mixtures of sodium aluminate and silica showed the same basic crystalline arrangement and the resulting ultramarine also gave a pattern identical with those produced from zeolite (Figures 2 and 4).
Clays were calcined at various temperatures in the 550' to 950" C. range. These were then slurried in 10% sodium hydroxide solution containing 50% of excess alkali, based on the total weight of clay. After boiling for 16 hours the product was filtered, dried, incorporated with the raw ingredients required for the production of ultramarine, and calcined. On this basis it was determined t h a t optimum results were obtained by precalcination of the clay for 3 hours a t 500" C. This product when causticized gave the analysis listed in Table V. It was found advantageous t o bring up the silicon dioxidealuminum oxide ratio with additional silica in the form of diatomaceow earth. This was usually added prior to causticixation and here as in previous work an optimum amount of silicon dioxide is indicated above which there is a sharp drop in the chromophoric properties of the color as measured by the American Society for Testing Materials tinting strength method. The clays exhibited ion exchange properties when causticized.
TABLE VI.
EFFECT OF SILICONDIOXIDE ADDITIONO N PIGMENTARY STRENGTH
Wt. of Calcined Clay, Grams 150 150 150 150 150 150
Wt. of Diatomaceous Earth, Grams
Tinting Strength
CLAY
Kaolin of good color was used in this portion of the investigation. I n order to obtain a zeolitic structure, the kaolin was refluxed for 18 hours with an equal amount of caustic in about 8% solution. As indicated in Table V it was possible t o insert as much as 17.6% of sodium oxide in this manner.
1
TABLE V. EFFECT OF ALKALI TREATMENT OF CLAY Ala08 Si02 Nan0
Ha0
Before Causticisation 39 9 44.0
...
13.0
After Causticiaation 32 9 38 8 17 8 10 8
Calcined Causticized Clay 30 8 39 1 18 1 12.0
Figure 0.
5. X-Ray Patterns
Calcined sodium oluminate plus silica
b. Ultramarine Causticized clay Calcined causticized clay
C.
d.
When this was incorporated into the formula noted previously and calcined, a definite but tinctorially weak ultramarine was pioduced. Insley and Ewell (9) showed by the method of thermal analysis that kaolin exhibited a broad endothermic effect from 550" u p t o 950° C. where a sharp exothermic peak was obtained. They attributed the former phenomenon to the breakup of the crystal
X-ray data showed all ultramarine, regardless of method of production, to be alike (Figure 4). On the other hand, while the causticized clay exhibited a crystal pattern similar to ultramarine, the calcined causticized clay had a different arrangement (Figure 5 ) .
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
572
.
This may be one reason why the former produced poor blues and the latter strong ones. It was believed that the entrance of the higher polysulfides within the crystal is of paramount importance. Therefore in view of the geometry it is better to start with no structure or with compounds which rearrange themselves during the reaction so as to permit the lattice to be built around the polysulfide#. I n this case, the framework being already formed, the polysulfide entered with difficulty and in fact its sulfur content was only about 7%.
Vol. 45, No. 3,
LITERATURE CITED
(1) (2)
Gessler, A. E., and Kumins, C. A, U. S. Patent 2,535,057 (1951). Insley, H., and Ewell, R. H., J . Research iVutZ. Bur. Staizdards, 14%
792 (1935). (3) Kumins, C. A., U. S. Patents 2,544,693-2,544,695 (1952). (4) Pauling, L., “Nature of the Chemical Bond,” p. 364, Utica, pu’. Y., Cornell University Press, 1939. (5) Ibid., p. 367. (6) Rice. O., “Electronic Structure and Chemical Binding,” p. 311: New York, McGraw-Hill Book Co., 1940. (7) Singer, J., 2. anorg. Chem., 204, 232, 237 (1932).
ACKNOWLEDGMENT
(8) Zerr, G., Rubencamp, R., and hiayer, C., “Treatise on Color Manufacture,” pp. 177-200, London, Charles Griffen &- Co.,
The authors wish to express their thanks to the Interchemical Corp. for their encouragement in this project and for permission t o publish the data.
RECEIVED for re\,iew June 7, 1952. ACCEPTEDNovember 20, 1952. Presented before the Division of Physical and Inorganic Chemistry at the 121st Meeting of the ANERICANCHEMICAL S O C I ~ TBuffalo, Y, N. Y.
1908.
High-Quality Deresination of Guayule d
F. E. CLARK, T. F. BANIGAN, JR., J. W. JIEEKS, AZTD I. C. FEUSTEL U . S . Natural Rubber Research Station, Salinas, Calif.
L
ABORATORY and pilot plant investigations are being
carried on a t this station in order to develop guayule so that it might serve as a domestic source of natural rubber, especially for critical uses where Hevea is considered essential. Removal of the deleterious resin fraction, which normally comprises about 20 to 2574 of the weight of the resinous crude, constitutes one means for effecting quality improvement. The work reported in this paper, which presents results obtained in the experimental deresination of guayule rubber, is a continuation of studies reported earlier from this laboratory ( 5 ) . I n the previous report, Chubb, Taylor, and Feustel reviewed the work of earlier investigators in the field and described a method for producing a high-quality deresinatedrubberbya batch countercurrent acetone extraction of comminuted freshly harvested guayule shrub, followed by the usual pebble milling operation for recovery of the rubber. The present paper surveys progress made toward development of a n alternative and possibly more economical process involving acetone deresination of resinous rubber after recovery from pebble milling. Resinous guayule rubber recovered from shrub by the normal milling process is in the form of small spongy particles known as “worms.” The term “worm deresination” is therefore used to designate deresination methods where the milling operation precedes the acetone extraction step, in contrast t o “shrub deresination” where the reverse order is followed. While the current work was in progress, Wood and Fanning ( I S ) reported a method for deresinating wild rubber of high resin content by mastication in an internal mixer simultaneously with extraction by a continuously renewed solvent (acetone). Tests conducted with Mexican resinous guayule and chilte appeared to offer some promise and may point to a possible application of this method for the deresination of certain of the wild rubbers which are obtained in a massive solid form. Guayule rubber worms, which comprise the milling agglomerates of a number of rubber-bearing cells, are by virtue of their small size (ca. 0.5-mm. diameter by 2.0-mm. length) and spongelike structure, so well adapted to solvent deresination that i t is unnecessary to resort to the more rigorous and cumbersome technique of masticative extraction. Acetone deresination of the
worms has also been found to be very rapid, However, these important advantages are lost if the resinous worms are dried or otherwise allowed to become compacted before deresination. MATERIALS AND METHODS
The resin-containing rubber worms were prepared from freshly harvested 5- to 7-year-old guayule shrub, variety 593, by the pebble milling process (IO). Rubber and cork were separated from bagasse in a flotation tank, and then the cork was removed from the rubber by baica treatment (6). A final scrub milling completed the clean-up of the resinous m’orms. Whenever it was not practical t o deresinate the worms soon after scrub milling, they were placed in covered stock pots containing about an equal volume of water and were occasionally stirred with a paddle t o minimize clumping. In certain instances, especially where storage of small quantities was contemplated for longer periods of time, a small amount of formaldehyde war, introduced and/or the worms were refrigerated. Laboratory scale experiments conducted here and elsewhere ( 5 , l a ) have repeatedly shown acetone t o be a practical choice of solvent for the deresination of guayule worms. Hence our recent investigations have largely been limited to this solvent. The acetone used throughout this work was of technical grade. Since this work was undertaken with the dual purpose of providing a background of information to aid in large scale extractor design and to supply deresinated rubber for tire testing, it became desirable to adopt several approaches to the subject. Thus one phase of the program dealt with countercurrent extractions without agitation where acetone was percolated through static beds of worms packed loosely in various containers such as glass tubes, glass boats, etc. Factors such as flow rates a t equilibrium, composition of miscella (resin-solvent mixture), and solvent retention volumes were determined. Another phase of the study included countercurrent extractions with agitation where higher acetone-to-rubber ratios made stirring possible, thereby making it practical to investigate diffusion rates, temperature effect, and miscella composition. The third phase of the current work comprised the preparation of approximately 4000 pounds of deresinated rubber by means of a series of runs
