tion of the formaldeOR many years it hyde solution. It has has been recogalso been proposed to nized that protein add what might be must be hardened or tanned in some mantermed “dormant hardening agents” to the ner in order to attain m a x i m u m value for s o f t plastic, agents industrial application. which would not act Animal skins and hides until activated by the are tanned to produce heat of pressing or other agency. after leabher; glues and the molding operation cements are set or had been performed. cured to increase water resistance; casein is Among these might be mentioned trioxyhardened to produce casein plastic material. methylene ( l o ) , hexamethylenetetramine (I, In the case of proa), a1d e h y d e a m i n o tein plastic material, reaction products ( l 8 ) , this process is time c o n s u m i n g a n d is diphenylolpropane and therefore rather inf o r m a l d e h y d e (16), phenol and formaldeefficient. For example, hyde (27), dicyanodip o w d e r e d c a s e i n is amide and formaldemixed with 25 to 40 hyde (22,dS), methylol per cent water (6), d e r i v a t i v e s of u r e converted to the plastic by heat and pressure ureas and thane (%I), formed into thedesired, m e t h y l o l u r e a s (IS), GEORGE H. BROTHER AND LEONARD L. McKINNEY glyoxals (9,14),formalshape, usually sheets, S. Regional Soybean Industrial Products Laboratory, U. dehyde in mixture with rods, tubes, or button Urbana, 111.1 hydroaromatic alcohols blanks, and then hard(26)or ketones, a-ethylened by immersion in /3-propylacrolein (W8), a formaldehyde solufurfural ( l a ) , etc. In no case, however, is there any mention tion, The hardening requires from 1 week to 6 months. of attempting to produce thermoplastic material by the reThe seasoning or drying requires about an equal period, action of a powerful hardening agent directly with protein. depending upon a number of factors; the most important On the contrary, one finds numerous statements to the effect is the thickness of the material. that protein material treated with a strong aldehyde such as Numerous attempts have been made to shorten or elimiformaldehyde (l2,66,97,88) is so changed as to be no longer nate this hardening period which has been recognized as the thermoplastic. It has long been a recognized fact that most serious weakness in the manufacturing process of protein hardened casein plastic material can be softened enough by plastics. It has been proposed to incorporate certain salts moist heat to permit bending and embossing to a limited (8, 17) into the soft plastic in order to speed up the penetraextent, but not molding to shape. This fact has made the economic use of waste or scrap casein plastic material practically impossible. Protein that is mixed with resinous matter to form molding Protein hardened by an aldehyde at or mixtures, such as casein with urea-formaldehyde resin (91), near its isoelectric point is thermoplastic. casein with phenol-formaldehyde resins (87), etc., is usually The best plastic with the lowest water considered merely a filler material. I n no sense can it be absorption is produced by treating the considered the principal plastic or binding agent. At most f t exerts a somewhat modifying effect on the resinous binder. protein with an aldehyde in its isoelecThis is the case with the applications to plastics manufacture tric range. Water absorption increases that have been made with soybean meal to date (6).
F
Protein Plastics from Soybean
Products
Action of Hardening’or
-
Tanning Agents
on Protein Material’
sharply on either side of this range. The most effective hardening agents are some of the aldehydes, and of these, formaldehyde is the best. Commercial soybean protein is the best protein material of those studied for the production of plastic material. Acid caseins rank a close second with rennet casein a rather poor third. Zein does not react with aldehydes analogous to casein or soybean protein. As a base for the preparation of plastic material, it is better without an aldehyde treatment.
Thermoplastic Protein-Formaldehyde Material The preliminary study (3), which indicated t h a t added water was not necessary for the plasticization of soybean protein material, was followed up, and a t the same time the protein-hardening reaction was considered from a new angle. Instead of assuming that protein material, hardened with strong agents, is rendered nonthermoplastic, it was proposed to investigate this treatment and study the reaction as completely as possible, broadening the study to include commercially available aldehydes, ketones, chromium salts, aluminum For the first article in this series, see. literature citation ( 8 ) . A 006perative organization participated in by the Bureaua of Chemistry and Soils and of Plant Industry of the United State8 Department of Agriculture, and the Agricultural Experiment Stations of the North Central States of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin. 1
1
1236
NOVEMBER, 1938 salts, tannin, and a syntan. Although the study is primarily concerned with soybean meal and with commercial soybean proteins, for purposes of comparison some phases of the problem were studied on commercial milk caseins and on zein, the prolamine derived from corn. Satow (24) and Hull (1%’) were the only previous workers who even suggested such an approach. The former was primarily concerned with “glutinizing” soybean protein by means of treatment with alkali or acid and suggests the possibility of adding formaldehyde t o the glutinized mass before it is formed. H e states, however, that formaldehyde added t o the curd before glutinization spoils it for plastic purposes and later (26) confirms and emphasizes this statement as follows: “By using formalin, therefore, as a precipitant, the resultant proteid becomes useless as a material for the manufacture of plastic products.” His processes are obviously what are known in the casein plastics industry as wet processes. They have no industrial importance and have produced no commercial material. I n 1908 Betts (4) had suggested a similar treatment of casein with formaldehyde, which also has proved impractical. After stating that “the reaction between formaldehyde and proteins is so rapid that there has never been enough time to mold articles of this type when formaldehyde was compounded with the other ingredients,” Hull describes a process for the production of a molding powder from protein and furfural. He emphasizes the point that the casein is treated with “a substantially anhydrous aldehyde” and intimates that the reaction proceeds between anhydrous materials. It has been found by experiments in this laboratory that at least 4 per cent water must be present in order for hardening t o occur. Furthermore, the claim by Hull that the furfural-casein plastic is more water resistant than the caseinformaldehyde plastic is not borne out by data given later in this paper. A preliminary experiment conducted in this laboratory consisted of soaking soybean meal in 40 per cent formalde-
INDUSTRIAL AND ENGINEERING THEMISTRY
1237
hyde solution overnight, draining off the excess formaldehyde solution, drying the hardened protein a t 60” to 80” C. to about 15 per cent moisture content, and pressing. Selfsoured (lactic acid) casein was subjected t o the same procedure. I n both cases the material was found to flow readily. The test pieces were broken u p and successfully remolded, thus demonstrating that these proteinformaldehyde combinations are thermoplastic. An x-ray diffraction pattern taken of the caseinformaldehyde plastic failed t o show any essential change in the protein structure. A study was made of the possible effect on the structure and plastic behavior of the protein distended with caustic a t the same time that it was hardened by formaldehyde. Accordingly a small piece of sodium hydroxide was dissolved in the formaldehyde solution t o effect a pH, in this case, of 9.0. Casein treated overnight with this solution produced an equilibrium p H close to the isoelectric point of the casein. The hardened material was somewhat lighter in color and more translucent than that produced by treatment with formaldehyde alone. An x-ray diffraction pattern of this material shows the diffraction pattern of paraformaldehyde. This would indicate that formaldehyde in intimate association with the casein swelled by caustic had been condensed t o paraformaldehyde.
Effect of pH o n ProteinFormaldehyde Reaction This study was extended to cover a broad pH range in order to determine the most favorable pH conditions for the protein-formaldehyde reaction. Commercial soybean “alpha” protein was used. A specific characteristic of protein plastic material is its tendency to absorb water. The water absorption, under strictly controlled and reproducible conditions, was used to evaluate the plastic and to determine the completeness of the hardening reaction. The pH of the formaldehyde solution (commercial formalin) was controlled
by the addition of formic acid or sodium hydroxide as required. The pH of the solid protein and of the protein-formaldehyde complexes was determined by suspending 5 grams of the solid in 20 CC.
FIGURE1. X-RAY DIFFRACTION PATTERNS FOR CASEIN-FORMALDEHYDE AND PARAFORMALDEHYDEof distilled water until no further change in pH occurred. ThepH of the water was then determined ( T o p ) casein-formaldehyde. (center) casein-caustic-formaland designated the pH of the exdehyde; (bot&) paraformaldehyde
1238
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 30, NO. 11
tract of the solid. All pH determinations were made with a portable glass electrode pH meter. One part of the soybean protein was added to two parts by weight of the hardening solution and allowed to stand 24 hours at room temperature (25' C., 77' F.). The excess hardening solution was then washed out with water, and the hardened protein air-dried to a moisture content of about 10 per cent. The pH of the extract of thc original unhardened protein was 4.4; that of commercial formalin, 3.0. The pH values of the formaldehyde solutions used, the mother liquors, and the extract of the washed protein-formaldehyde complexes were determined and are recorded in Table I. The hardened powder was pressed into disks 2 inches in diameter and l/s inch thick, bp molding in a positive die of proper dimensions at 120' C. and 351 kg. er sq. cm. (250" F. and 5000 pounds per square inch). These 8sks were tested for water absorption for 24 and 48 hours. The results are recorded in Table I.
absorption showed 10.5 per cent in 24 hours and 15.2 per cent in 48 hours. When this experiment was repeated, using water-formic acid solution in place of the formaldehydeformic acid solution, identical results were obtained. Table I shows that this hardened material, produced by a formaldehyde solution with a pH of 4.1, gave a plastic showing a water absorption of 10.7 per cent in 24 hours and 15.7 per cent in 48 hours; the plastic from protein hardened by formaldehyde solution with a pII of 1.5 gave 26.5 and 36.1 per cent, respectively. These results indicate that the proteinformaldehyde compound, once formcd, is stable in mild acid a t room temperature, whereas the presence of the same concentration of acid in the formaldehyde solution profoundly affects the hardening reaction. The protein-formaldehyde produced in the isoelectric range of the protein readily dried to a light tan friable powTABLE I. EFFECT OF PH ON WATER ABSORPTION OF HARDENED der; that produced on either the acid or basic side of the SOYBEAN PROTEIN isoelectric point was glutinized, brown in color, and difficult c pH MeasurementsMother Hardened Water Absorpti0n.a yo Sample Formaldeto dry or pulverize, Although it is appreciated that mo!ded liquor protein ext. 24 hours 48 houra No. hyde solu. material made from this friable powder without extrudon or I
4
I
I
I
Average of two determinations.
Since there is no official method for the determination of water absorption by protein plastic material, it was decided to adapt A. S. T. M. specification D-45-33 to this purpose. This specification outlines three methods for conditioning samples or of obtaining the dry weight on the same-viz., 24 hours at 100' C., 24 hours at 50" C., or 24 hours in a desiccator. Obvious1 results by the three methods could not be expected to ched. It is doubtful whet her protein-formaldehyde would be stable at 100' C. in the absence of water, so it was decided t o employ the 50" C. temperature for 24-hour pretreatment. The test pieces were then weighed, immersed in distilled water at room temperature (25" C.) for 24 hours, weighed, immersed for a second 24 hours, and weighed again. A comparison of the pH values given for the mother liquor with those for the formaldehyde solution shows that the protein evidently exerts a considerable buffering action, even on the acid side of its isoelectric point. The hardened proteins obtained in samples 4 and 5 are quite similar in appearance and plastic properties and are the best of the series. They were formed a t practically the same mother liquor p H which is in the isoelectric range of this protein, although there is an initial difference of an entire pH unit in the formaldehyde solutions used. These results appear to indicate that a small increincnt of acid near the isoelectric point does not prevent formaldehyde from blocking off the active amino groups of the protein. Figure 2 shows the relation between the plastic produced from protein hardened a t various pH values and the water absorption of the respective plastics. It has been shown that the most significant p H is that of the mother liquor, and this pH was used. Twice the amount of sodium hydroxide was used in solution 7 as in 6, but the pH of formaldehyde solution 7 did not show a proportionate rise. It has been found that dilute caustic solutions will not increase the p H of formaldcliyde solutions to more than 11.0, probably because of salt formation, such as Na-0-CHrOH (IO). In order to study the effect of acid on the hardening reaction as comparcd to that on the hardened protein, 25 grams of material hardened in formaldehyde solution (pH 4.1) mere immersed in 50 cc. of formaldehyde solution (pH 1.5) for 24 hours a t room temperature. Plastic disks tested for water
I 0 .
I
1
5
I
I 4
I I
5
I
1
6
pH of Mother L i q u o r
FIGURE2. VARIATION IN WATER ABSORPTION OF HARDENED SOYREAN PROTEIN WITH PH OF HARDENING
milling on calender rolls might not be expected to have the strength or appearance of true plastic material, the contrast of the molded samples was striking. The tan material produced' near the isoelectric point flowed rcadily to form fairly tough resinouslike material; the glutinous material darkened in color and increased in brittleness towirds the extremes of pH until the test pieces could be released whole from the die only with difficulty. At the end of the 48-hour soaking, samples 3, 4, 5, and 6 were in good condition and dricd without fracturing or marping perceptibly. Samplrs 1, 2, 7, and 8 fracturrd on drying, and sample 0 completely disintegrated in the water in less than 20 hours. The differonce noted in the protein-formaldehyde material produced at or near the isoelectric point of the protein and on the acid or alkaline side of this point is most striking from a consideration of the water absorption of the material. According to the data in Table T and Figure 2, this is a minimum in the case of the first, but increases in the second to a maximum at either pH extremc.
Comparative Action of Commercial Hardening Agents In order to study the comparative efficiency of commercially available hardening agents for the possiblc production of protein plastic material, it was decided to continue the use of commercial soybean "alpha" protein, treating it a t the most favorable p H (4.3)with 20 per cent solutions of com-
NOVEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
T A B L11 ~. COMPARATIVE ACTIONOF HARDENING AQENTS ON COMMERCIAL SOYBEAN AwnA PROTEIN Agent Blank0 FormaXdehyde Acetaldehyde Acetaldol Propionaldehyde Crotonaldehyde Furfural Chromium sulfate Aluminum sulfateb Acetone Methyl ethyl ketone Methyl isobutyl ketone Methyl n-amyl ketone Tannin Syntan 0
b
Water Absorption, % 24 hours 48 hours 43.0 17:9 10.0 22.3 33.6 18.5 28.4 21.9 30.8 20.8 31.4 37.4 37.6 45.2 40.0
Condition on Drying Fractured Very good Fractured Fractured Very good
ETd
Shattered
I?ntrented commerrial soybean alpha protein. Test piece shattered on conditioning.
c Test pieces from these agents sliowed the properties of untreated protein.
mercially available aldehydes and ketones and with favorable concentrations of chromium salts, aluminum salts, tannin, and a syntan. The criterion was to be the same as that of the preceding study-i. e., the comparative water absorption of material that could be molded into a plastic test piece. The water absorption a t the end of 24- and 48-hour immersion, together with the condition of the test pieces on drying after the immersion, is recorded in Table 11. Butyraldehyde, isobutyraldehyde, and n-octyl aldehyde had no hardening action on the protein, while positive results were obtained with all other aldehydes considered in this study. Acetaldehyde and crotonaldehyde produced dark brown opaque material which was not as free flowing as the protein-formaldehyde, and hence required 15 rather than 10 per cent water for adequate flow properties. Acetaldol produced a protein plastic quite similar in appearance to that produced by acetaldehyde, but the material flowed more like the protein-formaldehyde complex. Propionaldehyde produced a light red translucent protein plastic, quite free flowing and resinous, but so brittle that it was necessary to use 15 per cent water in order to produce test pieces which could be removed unbroken from the die. Furfural produced a dark colored opaque protein plastic having properties quite similar to those of the protein-propionaldehyde. Of the ketones investigated, only the methyl n-amyl ketone produced a large enough piece of the moldcd material to test; it gave a water absorption of 40.3 per ccnt in 24 hours and disintegrated before 48 hours. The blank, untreated protein gave a figure of 43 per cent water absorption in 24 hours. Obviously the ketones did not improve the material to any marked extent. Chrome protein was prepared in a manner somewhat analogous to that used in the formation of chrome collagen in the tanning of leather. The protein was treated with about a 10 per cent aqueous solution of chromium sulfate of initial pH 1.5for 24 hours a t room temperature rather than the elevated temperature used in the tan wheel. The pH quickly came to 2.5 after the addition of the protein and remained a t this point throughout the rest of the period. The chrome was set by the addition of sodium bicarbonate solution to a pH of 6.5, in which the protein remained a second 24 hours a t room temperature. Filtering and drying produccd a green powder which was stable even in boiling water. Fifteen per cent water was used to plasticize the material. A dark green, translucent, hard, and. somewhat brittle plastic resulted. The water absorption was rather high, b u t other properties of the plastic indicated that further investigation of this material might lead to important results. Alum protein has already been investigated (3) in connec-
1239
tion with the production of water-plasticized protein-plastic material and gave some interesting resiilts. The alum protein with 20 per cent water gave nothing but hard brittle material which was difficult to mold and shattered badly on drying. It is difficult to account for such behavior of this material; the results can merely be recorded and possibly explained when more is known concerning the hardening reaction. Similar procedures with tannin or the syntan did not lead to positive results, but further investigation may determine more favorable conditions for their reaction with proteins. From the data recorded in Table I1 it is easy to understand why formaldehyde has been so universally used 8s a hardening agent for protein material in spite of its disagreeable characteristics. The properties of the other protein-aldehyde complexes are of considerable interest, particularly the difference shown by the protein-propionaldehyde and protein-furfural complexes as compared with the others. These results are by no means absolute but are merely comparative and hold only for the conditions of this experiment. It is possible that under different conditions the agents which gave negative results here would respond favorably.
Action of Forinaldehyde or Furfural on C o m mercial Protein Material Materials selected for this study included expeller soybean flour, solvent-extracted soybean meal, and commercial products known as “gamma” protein, “alpha” protein, and “protein 70,” together with rennet and self-soured casein and zein. A partial chemical analysis of the samples used is given in Table 111. TABLE111. ANALYSISOF PROTEIN MATERIALS (IN PER CENT) Material Expeller soybean flour Solvent-extd. soybean flour Commercial gamma protein Commercial protein 70 Commercial alpha protein Rennet casein Lactic acid casein Zein li
Crude Protein0 53.19 48.69 53.44 69.40 88.30 82.50 85.25 92.90
Oil 4.63 0.35 0.40 0.42 0.12 0.11 0.15 0.16
Sugar 7.0
Moisture 6.00 9.28 9 0 7.58 10.37 5.50 1.09 8.27 7.77 7.28 ... 7.15
... ...
Per cent N multiplied b y 6.25.
The treatment with formaldehyde followed the procedure already outlined and was carried out with the solution a t a pH of 4.1. While this pH is most favorable for soybean protein, it may not be best for the other proteins. However, i t is probably not enough a t variance with their isoelectric ranges to make significant differences. Seven per cent furfural was mixed well into the samples, and the mixtures were allowed to stand 2 days a t room temperature before pressing. The protein-furfural reaction, completed in the press, produced an almost black plastic with every sample except the zein. There was no evidence of any reaction having taken place with the zein. As in the preceding studies, the criterion for judging results of this study was the comparative water absorption of uniform molded disks. The results are recorded in Table IV. The formaldehyde-hardened soybean products, with the exception of alpha and gamma protein, disintegrated in water; the corresponding furfural products did not. I n view of the consistently favorable results obtained with the soybean alpha protein and formaldehyde, i t is somewhat surprising to find that soybean meal reacts so unfavorably. This difference is possibly caused by a chemical difference in the protein, a result of the treatment it has received in the process of separating it from the meal.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Another difficulty presented by the expeller meal flour is the presence of uncombined oil. If this is in excess of 2 or 3 per cent, it will tend to be squeezed out when the plastic is pressed. It will also tend to protect the protein from the action of the hardening agent. Until some method of avoiding these difficulties is discovered, it will not be possible to consider expeller soybean flour seriously for plastics production.
superior to the casein in flow properties, translucency, and strength. Although a thermoplastic material has been prepared from protein and aldehydes, especially formaldehyde, and studied comparatively from several angles, this material is not in itself a molding compound. It is believed, however, that practical molding materials can be developed from the protein-aldehyde material as a base by plasticizing it with agents other than mater. Investigations are being conducted along these lines.
TABLE IV. COMPARATIVE WATERABSORPTIONOF PROTEIN MATERIALSTREATED WITH FORMALDEHYDE AND FURFURAL Material Soybean expeller flour Soybean extracted flour Soybean gamma protein Soybean protein 70 Soybean alpha protein Rennet casein La.ctic acid casein Zeina
-Formaldehyde-treated - Water Absorption % Furf;ral-treated . hours hours hours 24 hours
Disintegrated Disintegrated 53.8 Disintegrated 10.0 14.8 10 7 31.5
48
Disintegrated Disintegrated 56.7 Disintegrated 17 9 22 7 14 6 34.7
24 57.1
48 59.3
51.7
52.0
56.9 39.4
56.1 50.1
37.4 25.0 24.6 23.0
37.6 33.9 36.5
..
a Zein without treatment: 17.0 per cent in 24 hours, 30.1 per cent i n 48 hours.
The solvent-extracted soybean meal and the commercial products, gamma soybean protein and soybean protein 70, present eome possibilities when hardened with furfural. The plastic will have to be limited to black or very dark colors, but it is well knit, strong, and free flowing, and the raw materials are very low in cost. The water absorption is too great for some applications, but there are indications that it can be corrected fairly easily by proper study. It has been found that a small percentage of aluminum stearate in the plastic mixture reduces the water absorption sharply. No hardening action was produced by either formaldehyde or furfural on zein, contrary to the recently published statement that zein reacts with formaldehyde more readily than with casein (16). The water absorption of untreated zein is less than that of the treated product. This is quite in accord with the theory which states that the formaldehyde reaction takes place with the free amino groups of the protein molecule (7). Since zein has no free amino groups (11) it cannot be expected to give the reaction. Rennet casein, which is quite generally used in the commercial manufacture of casein plastic material, gave definitely poorer results than the acid casein. Not only was the water absorption higher, but a much less satisfactory plastic resulted which was poorer in flow, much weaker, and less translucent. Of the two, the rennet casein is probably less hydrolyzed than the acid casein. The water absorption of the product of soybean alpha protein was about the same as that of the acid casein, but the soybean plastic was definitely
VOL. 30, NO. 11
Acknowledgment The x-ray diffraction patterns were made with the assistance and collaboration of G. L. Clark and his associates. The pictures were taken and interpreted in the chemical laboratories of the University of Illinois. ,
Literature Cited (1) Bartels, Armandus, French Patent 420,543 (April 6, 1911), 1st addition of Jan. 23, 1911. (2) Bartels and Miech, U. S. Patent 1,560,368 (Nov. 3, 1925). (3) Beckel, A. C., Brother, G. H., and McKinney, L. L., IND.ENG. Chem., 30, 437-40 (1938). (4) Betts, P. W. F. G., U. 9. Patent 893,129 (July 14, 1908).
(5) Brother, G. H., in Sutermeister’s “Casein and Its Industrial Applications,” A. C. 8. Monograph Series, Chap. 6 , New York, Chemical Catalog Go., 1927. (6) Chase, H., Brit. Plastics, 7, 516 (1936). (7) Cubin, H. J., Biochem. J.,23, 28 (1929). (8) Dumont, H., U. S.Patent 1,992,478 (Feb. 26, 1935). (9) Ernst, O., and Sponsel, K., Ibid., 1,841,797 (Jan. 19, 1932). (10) Franzen, Hans, J. prakt. Chem., 199, 261 (1915); German Patent 277.437 (Ami1 19. 1912). (11) Hoffman, W. F., and Gortner; R. A., Colloid Symposium Monograph, 2, 209-368 (1925). (12) Hull, S. M., U. S. Patents 1,648,179 (Nov. 8, 1927), 1,711,026 (April 20, 1929). (13) I. G. Farbenindustrie Akt.-Ges., British Patent 268,804 (March 31, 1926). (14) Ibid., 279,863 (Feb. 26, 1929); French Patent 719,891 (July 9, 1931). (15) Imperial Chemical Industries, Ltd., French Patent 738,156 (June 3 , 1932); British Patent 379,739 (Sept. 5, 1932). (16) International Patents Development Co., French Patent 820,043 (Oct. 30, 1937). (17) Internationale Galalith-Ges. Hoff L Co., German Patent 593,224 (Feb. 23, 1934). (18) Manfred, Otto, British Patent 281,223 (Sept. 7, 1928). (19) Morin, French Patent 588,441 (May 30, 1907). (20) Ott, K., and Schussler, H., German Patent 575,927 (May 5, 1933). (21) Redman, L. V., U. S. Patent 1,732,533 (Oct. 22, 1929). (22) Ripper, K., Ibid., 1,952,941 (March 27, 1934). (23) Rossiter, E. C., Ibid., 2,097,895 (Nov. 2, 1937). (24) Satow, S., Ibid., 1,245,975-6 (Nov. 6 , 1917). (25) Satow, S., Tech. Repts. T6hoku I m p . Univ., 2, 93-6 (1921). (26) Stock, P., German Patent 489,438 (March 4, 1926). (27) Sturken, O., U. S. Patent 2,053,850 (Sept. 8, 1936). (28) Sturken, O., and Woodruff, J. C., Ibid., 2,040,033 (May 5, 1936).
RECEIVED June 28,
1938.
