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ribbon around the wire, dry the insulation, and reel up the finished product.” This process is now supplying a large percentage of the fine-gage insulated wire for telephone cable work. It is expected that a little later it will be possible to apply this type of insulation to larger gage cables and for circuits other than those in the telephone industry.
PAPER AND RUBBER There is another interesting development which has been making progress quietly during recent years and which gives promise of developing into a specialized industry. It involves integral combinations of paper with rubber, the rubber usually being in the form of latex. There are two general processes for making these products. One involves mixing the latex with pulp in the beater, while in the other the paper or board is made first and saturated with rubber latex afterward. The various products made in this way are finding increasing use in the manufacture of shoes, particularly as inner soles and midsoles; in the automobile industry as rim strips, body shims, washers, etc.; and as a base for pyroxylin coatings in the manufacture of imitation leather. LATESTDEVELOPMENTS During the last ten years there has been considerable reference in the patent literature to the subject of cellulose ethers, and, during the past year or two, materials which belong in this chemical classification have made their appearance on the market in a limited way. The simplest ether of cellulose is the methyl ether. It can be made water-soluble and is being offered under several trade names in connection with textile h i s h i n g and printing operations. Considerable study has also been devoted to ethyl cellulose, and this also is available on the market for special purposes. Perhaps the most promising of the cellulose ethers a t the present time is the benzyl, which is being manufactured in Europe and can be purchased in this country. It is reported that a t least one American manufacturer is studying the feasibility of producing this ether here. Benzyl cellulose is unusually stable toward chemical action. It is not decomposed by alkalies of mercerizing strength, nor
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is it affected by sulfuric acid up to 50 per cent concentration. Furthermore, benzyl cellulose is unusually resistant to water and moisture. It is claimed that films made of benzyl cellulose are practically impermeable to water vapor. It will probably be necessary to improve somewhat on the methods of production which have so far been published, but with benzyl chloride available a t low cost, it is reasonable to expect that benzyl cellulose will be available in the near future a t moderate prices for such purposes as thin transparent wrapping materials, electrical insulation, lacquers, and industrial finishes, molding powders, and for conversion into the innumerable articles which are now fabricated from pyroxylin plastics. There is also, a t the present time, considerable interest in mixed cellulose esters in which the hydroxyl groups instead of being all esterified by the same acid, are esterified partly by one acid and partly by another. It is claimed, for example, that cellulose acetopropionate offers several advantages over any of the cellulosic compounds which are now available. With relatively inexpensive propionic acid promised by the organic chemical manufacturer, it seems probable that in the near future cellulose acetopropionate also will be available for the many purposes for which cellulose compounds are now being used. Moreover, all of these latest cellulose compounds are to be regarded only incidentally as replacing cellulosic or other materials which have been available for a longer time. Their chief importance lies in the probability that their novel properties will greatly extend their use.
ACKNOWLEDGMEXT The information and figures given in Table I11 regarding cemented shoes were furnished through the courtesy of the United Shoe Machinery Corporation and its subsidiary, the Boston Blacking and Chemical Corporation. LITERATURE CITED (1) Hyden, W. L.,IND.EKQ.CHEM.,21, 405 (1929). (2) Little, J. S., Tech. Assoc. Papers, Series 16, No. 1. 150 (1933). (3) Pfund, A. H., J. Optical SOC.Am., 30. 23 (1930). RECEIVED September 8, 1933.
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The Synthetic Rubber Problem’ WALLACE H. CAROTHERS, E. I. du Pont de Nemours & Company, Wilmington, Del. This paper is a n attempt to indicate the nature If this were true it should be WO o b j e c t i v e s i n atand current status of the synthetic rubber problem possible to infer the structure tempts to s y n t h e s i z e by a study of chemical behavior r u b b e r are (l) to disf r o m the standpoint of organic chemistry. I t and then to make a r a t i o n a l cover or demonstrate completely includes a n outline of unsolved problems and synthesis-that is, from known the structure of rubber and exsome new data bearing o n the relation between starting materials to build up plain its properties in terms of the structure of dienes and their suitability as by known and d e l i b e r a t e l y this structure, and (2) to procontrolled steps the supposed duce artificially a commercially starling materials f o r the synthesis of rubber. acceutable s u b s t i t u t e . The structure. If the synthetic product was then exactly identical second of these objectives is now an accomplished fact, but the first is far from having been in all its properties with the natural product, the structure of the latter would be proved. achieved. What is sometimes referred to as the synthesis of rubber The difficulties in this connection can be illustrated by first viewing the facts in the light of the assumption that consists essentially in this: Isoprene on standing passes rubber hydrocarbon is a chemical individual in the usual slowly into an elastic solid having the chemical composition s e n s e i . e., that it is made up of identically similar molecules and many of the chemical reactions of rubber. But alcapable of being represented by a single definite formula. though this material is elastic, it is not physically identical with rubber; few experts in the field would mistake it for 1 Because of the oomprehensive historical article on “Synthetic Rubber” rubber. The product synthesized, then, is probably not publiahed by Whitby and Kats [IND.ENQ.CEBM.,25, 2105. 1338 (1933)l rubber; and, even if it were, the synthesis is not rational. since thia paper wae preaented in Chicago, it has been considerably revised It is a spontaneous or accidental transformation of unknown with the elimination of historical matter.
T
Jaounry, 1931
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mechanism from whicli little can be inferred concerning the The complete description of rubber in t.er1ns of molecular structure of the product. structure (distribution of lengths, geometrical state, etc.) The question then arises as to how one can determine has not yct been acconrplished; the mechanism of the polywhether a dven sample of material is rubber. Tho deter- merization of isoprene is unknown. Practical approach to the mination of identity is complicated by the fact that rubbcr is synthetic rubber problem must therefore be highly empirical. lacking in easily measured and sharply characteristic pliysiciil Starting with the historically important observation that propert.ies. Moreover, those properties that can be meas- isoprene on standing is transformed to an elastic solid, the ured-. g., plasticity-are not necessarily the same for attempt is made to modify favorably the properties of the different samples or even for the same sample at different product by controlling the conditions under which the transtimes. The result is that there must be considerable latitude formation occurs. The enormously extensive experiments in the specifications as to what constitutes rubber, and it is in this direction with isoprene, and with the related conialmost impossible to set any definite numerical limit for a pounds butadiene and dimethylbutadiene, have no doubt given property. This lack of definition unfortunately makes led to improved products, but, so far as any published iriit possible to designate as rubber-like almost any material formation gocs, they have never led to anything closely 91)having any appreciable degree of resilience and elastic ex- proaching rubber. The possibility was tensibility; partially early recognized that polymerized styrene some dienes might be is said to be rubberf o u n d whose polylike, and so are some m e r i z a t i o n would aqueous dispersions yield products much of starch, gelatin, and superior to those obsodiumsilicate. It is tained from isoprene, in t h i s s e n s e t h a t but the exploration many of the so-called of this possibility was s y n t h e t i c rubbers hampered b y the are physically rubberdearth of s u i t a b l e like. Chemically those derived f r o m methods for synthesizing dienes. isoprene resemble rubber very closely, Revolutionary but so also do chicle, nroeressin this direcStretohed poiychloropfene Stretobed poiybrornoprene b a l a t a , a n d guttako: recently became nercha which also Ficunx 1. X-RAYDIPFRACTION I’AT~IINS wssible with the disliave resilience a n d iovery ( 9 ) of a simple clastic extensibilitv hut are not called “rubher” because thev process for preparing vinylacetylene. The latter is an especially suitable starting material for the synthesisof new typesof do not possess t i m e properties in sufficient degree. I n spite of the looseness with which the term “rubher- dienes. Among the new dienes obtained from it are chlorolike’’ is used, rubber is a unique material. When vulcanized prene and bromoprene (0, 7) : i t possesses an extraordinary degree of elastic extensibility CHzd-CH=CH2 HCI 4 CHp=CCH=€Hn combined with great strength. No other type of material I CI (with the exception of those noted below) remotely apVinylscctyiene Chloroprene proaches i t in the magnitude of these combined properties. Rubber is also unique in the nature of the patterns that it These compounds polymerize several hundred tunes as furnishes when examined by x-rays. Unstretched samples rapidly as does isoprene, and they lead directly to products yield a dBuse pattern characteristic of amorphous materials; which are equal to rubber in strength and elastic extensibility. stretched samples furnish a point diagram charaderistic of Moreover the products eshibit instantaneously reversible fibrous crystals. The fiber diagram disappears instantly fiber orientation (Figure 1). Polychloroprene and polywhen the sample is released from stress. The unique proper- bromoprene are therefore the first synthetic products that ties of rubber are: great strength combined with an elastic equal rubber in those qualities which make rubber unique, extensibility of about 900 per cent, and instantaneously and in this imnortant sense thev are the onlv true svnthetic reversible fiber orientation. rubbers yet kniwn. It is now generally recognized that the physical variability T h e field of svnthetic rubber still Dresents many unsolved of rubber is inherent in (or at least reconcilable with) its or iucomplotely solved problems. Among these may be mcnchemical structure. Rubber molecules are long chains or tioned the following: threadlike structures built up by the regular 1,hombination DEvELoPaieNT O F SATIBFACTORY METHODS FOR fI3YSICALLY of units having the formula -CIIrC(CH3)=CII-CHr. CEAILACPERIZING RUBBERIN SICXIPICANT NUMERICAL UNITS. Chemical properties are conditioned by the presence of this If there were, for example, two properties, x and y, simply unit, hut the physical properties are determined by the measurable on small samples and such that together they average length of the chains and the way in which the in- furnished a numerical indication of quality, cumulative dividual lengths me distributed above the average. Geo- progress in the study of rubbers would not be so difficult. metrical isomerism due to the presence of the double bonds Obviously important practical indications of quality are may also come into play. When isoprene polymerizes, its extensibility and breaking strength, hut these properties molecules apparently unite to form chains similar to those must be measured on the rubber after it is vulcanized, and indicated above. But if the product is to approach rubber the values obtained. will depend partly upon a very compliin its properties, the unions must occur regularly 1,4-(without cated set of factors which constitute the conditions of vulinversions) and the configurations about the double bonds canization. The difficulties here are to a large cxtent illmust he the same as those in natural rubber, as must also the licrent in the nature of rubber and are presented also by average length of the chains and the distribution about tlre other macromolecular materials. The problem of physically average. characterizing such materials in significant numerical units
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ic of the utmost importance to the whole field of resins, tion are thus immediately dispensed with and a clearer plastics, and fibers, and it should receive more attention theoretical approach is made possible. It may be expected jointly from physicists and chemists. Meanwhile, in future that this method of approach will ultimately throw considerstudies of synthetic rubber, quantitative data must be ob- able light on the relation between structure and rubber-like tained whenever possible, since the accumulation of sys- properties. tematic information is impossible without quantitative IXFLUENCE OF STRUCTURE ON DIEKEPOLYJIERIZATIOK data. COMPLETE DETERMINATION OF THE MOLECULAR STRUCTable I contains data concerning the influence of strucTURE OF RUBBER. This will probably have to await the tural variations on the relative speeds of polymerization of development of more information concerning macromolecular various dienes and on the nature of the products. Very compounds generally. little quantitative significance can be attached to the numbers RELATION BETWEEN PHYSICAL BEHAVIOR AND MOLECULARpresented because they are derived by calculations sometimes STRUCTURE. It is quite certain that high strength requires involving extrapolations from data of uncertain reproducithe presence of very long molecules while elastic extensibility bility. The numbers are, however, useful for indicating implies low internal viscosity, but these conditions are cer- relative degrees of magnitude. tainly not sufficient. Solution of this problem will obviously require more knowledge concerning the structure of rubber, TABLEI. ESTIMATED RELATIVE SPEEDS OF POLYMERIZATION OF VARIOUSDIENESAT 25" C. COMPARED WITH ISOPRENE and the study of synthetic materials of more or less known structure may be expected ultimately to throw considerable C o r - POSITION A N D N.ATURE OF SUBESTD. POUND STITUENT SPEED"CHARACTER OF POLYMER light on it. c-cC 4 INFLUENCE OF STRUCTURE ON DIENE POLYMERIZATION. 1 2 3 4 1 ... c1 C1 . . . 2000 Hard, not exteneible Dienes vary greatly in their ease of polymerization and in the 2 ... I . . . . . . 1500 Rubber-like under cernature of the product. The fact that chloroprene and tain conditions 3 ... Br . , , ... 1000 Good rubber bromoprene are so much superior to isoprene emphasizes the 4 ... c1 . . . .., 700 Excellent rubber 5 ... C1 CHa . . , 500 Fair rubber but low eximportance of completely exploring the relation between tensibility 6 C1 structure and polymerization of dienes. Some data bearing Cl c1 ... 120 S(ift, elastic 7 ... CaHs ... ... 9osc on this point are presented in Table I. If the inferences from 8 CHa C1 ... ... 30Sc 9 CzHa C1 ... ... 30Sc these data can be relied upon, it appears that the best dienes 10 GHs C1 . . . ... 1osc will be of the type CH2=CX--CH=CH2 in which X is an 11 C:His C1 . . . . . . 1osc 12 . .. CiHis activating group other than alkyl or aryl. 1" a L I ... ... i D O I C , SI 14 3 Fair ru ex. .. CH3 CH3 CONTROL OF DIENE POLYMERIZBTTON. The product obtenaibility tained spontaneously and accidentally from isoprene is so 15 CHI CH3 c1 ... 1 . 5 Soft 16 . .. CHI . .. . . . 1.O Fair rubber much like natural rubber in some ways that it is difficult to 17 ... ... . . . .., 0 . 8 Fair rubber 18 CHI ... . . . ... 0.3 ...... believe that it will always be impossible to control the reac19 CHI CHI ... . . . Probtion in such a way as to obtain a product fully equal to natural rubber. I n the absence of simple objective measures of physical quality, progress in this direction must of necessity be slow. Another complication arises from the fact that dienes are enormously susceptible to catalytic effects not only . . . (CI in the speed of their polymerization, but also in the physical The eetimated speeds are based on calculated unimolecular reaction properties of the products. Exact reproducibility is there- velocity constants. The data from which the c o n s h n t s are derived suffer fore possible only under the most elaborately controlled from a very large factor of uncertainty owing to the fact that, f o r dienes, the rate of olymerization varies considerably with the history of the sample conditions, and one of the conditions that must be con- and ,witK conditions (amount of exposure to light and air) which are not specified or controlled in all of the experiments. These uncertainties are trolled is the amount of exposure of the diene to air and light. hoyever, thought to be insufficient to affect the order of magnitude of th; I n spite of the enormous amount of work implied by the indicated numbers. The constant taken for isoprene (1) was 0.0000048 (in hours), and was derived (by extrapolation) from d a t a presented by Whitvoluminous patent literature, the factors indicated above by and Crozier (IO). The value for compound 14 is also derived from their For com ound 18, data of Lebedev and hferzhkovski, quoted by have been generally ignored. Exact indications concerning data. Whitby and Gafiay (If) were used. Compounds 19 t o 26 which are prethe effect of conditions on the nature of products obtained sented in decreasine order of rate without anv attemDt to estimate numerical from diene polymerizations are exceedingly meager. The possibility still remains of so controlling such reactions as to obtain improved products. Carothers Collins and Kirby (6). 4 Carothers Williams Collins and MECHANISM OF DIENE POLYMERIZATION. If the mecha- 3Kirby (7). 15 Carbthers and CoffAan' ( 5 ) ' 7 12' Carother; and B e k h e t ( 4 ) 8-1i, iacdbson and Carothers ( 8 ) . a'nd' 13,' unpublished. Whitby nism of diene polymerizations were sufficiently understood, and ; Gallay ( I f ) reach conclusions s i m i l k to those stated below as t o the a theoretical attack on the control of such reactions would effect of substitution on the rate of diene polymerization. become possible. The three types of substituent groups present are halogen, SYFJTHESIS OF RUBBEH-LIKE MATERIALS BY OTHER REACTIONS THAN DIENE POLYMERIZATION. One method of phenyl, and alkyl. Comparison of the @-monosubstituted deattacking the synthetic rubber problem would be to syn- rivatives shows that all of these groups have an accelerating thesize giant molecules of known structure, study the rela- effect; methyl is very feeble, heptyl is appreciably stronger, tion between physical behavior and structure, and from in- phenyl is much stronger, and the halogens are very much ferences thus established proceed to the synthesis of ma- stronger in the increasing order chlorine, bromine, iodine. terials having the required structure. The deliberate and The full activating effect is manifested only if the group is on rational synthesis of sufficiently large molecules having a the p- or y-carbon atom. Alkyl on the terminal carbon completely known structure is at present impossible, but an inhibits, and it depresses the activating effect of a group on approach to this ideal is found in reactions of condensation the @-carbon. It appears that to obtain a high rate of polymerization, which proceed by a definitely known mecha- polymerization the terminal carbons must be free of any nism and lead to products whose general structural plan substituents. A substituent a t the y-carbon atom generally can be certainly inferred ( 2 ) . Some of the complications reenforces the effect of one already present a t the P-carbon,but and obscurity inherent in reactions of addition polymeriza- these disubstituted dienes (Cl, C1; CHB,C1; and CH3, CHI)
January, 1934
INDUSTRIAL AND EKGINEERING
alliyield product< that are deficient in extencibility. Another specific effect is that due to phenyl. P-Phenylbutadiene polymerizes quite rapidly, but the product is mostly the crystalline dimer. The small amount of higher polymer formed is soft and probably has a relatively low molecular weight. (This effect of phenyl is further illustrated by observations on other aryl butadienes made by Xhitby and Gallay, 11 .) The formation of dimers is ah-ays a competing reaction in the synthesis of rubber from dienes, and it rapidly becomes more serious the higher the temperature used. At ordinary temperatures the rate of dimer formation from isoprene and chloroprene is roughly the same, but the temperatures required t o obtain 50 per cent polymerization of the two dienes in 10 days are respectively about 90" and 25' C., and the percentages of dimer in the products at these temperatures are about 40 and < l . This fact gives addi-
CHEXlISTKY
33
tional emphasis to the importance of a high rate of polymerization. (1)
LITERATURE CITED Berchet and Carothers, J. Am. Chem. Soc., 55, 2034
-
(1933).
(2) Carothers, Chem. Rev., 8, 364 (1931). (3) Carothers and Berchet, J . Am. Chem. Soc., 55, 2507 (1933).
(4) Ibid.,55, 2813 (1933). ( 5 ) Carothers and Coffman, Ibid., 54, 4071 (1932). (6) Carothers, Collins, and Kirby, Ibid.,55, 786 (1933). (7) Carothers, Williams, Collins, and Kirby, Ibid., 53, 4203 ( 1 9 3 1 ) . (8) Jacobson and Carothers, Ibid., 55, 1622 (1933). 19) Sieuwland, Calcott, Downing, and Carter, Ibid., 53, 4197 (1931). (10) TThitby and Crozier, Can. J . Research, 6, 203 (1932). (11) Whitby and Gallag, Ibid., 6, 280 (1932).
RECEIVED November 23, 1933. Thie paper is Contribution 138 from the Experimental Station of E. I . du Pont de Kernours h Company.
Physical Properties of DuPrene Compounds E. R. BRIDGWATER E. I. du Pont de Nemours & Company, Wilmington, Del. black per 100 volumes of rubber HERE is perhaps no Although DuPrene differs chemically f r o m or DuPrene. The two DuPrene e n g i n e e r i n g material natural rubber, it bears a closer physical resemcompounds each contain 2 per that has so wide a range blance to rubber than any of the previously cent of the antioxidant, phenylof properties and so great a diknown synthetic rubbers. I t differs f r o m nafural / ? - n a p h t h y l a m i n e . This is versity of uses as v u l c a n i z e d rubber in inany respects which makes it more suitperhaps the best antioxidant for rubber. By appropriate selecDuPrene compounds, and 2 per tion and proportioning of acable f o r certain purposes and less suitable f o r cent of it is completely soluble in celerators, antioxidants, reenothers. Its physical properties are susceptible DuPrene, both before and after forcing agents, etc., and by varyof wide vuriation depending on the nature and vulcanization. However, this ing conditions of vulcanization amounts of vulcanizing agents, reenforcing piga m o u n t of phenyl-/?-naphthylone can modify the properties of ments, etc., that are compounded with it, and it amine is not completely soluble rubber in a l m o s t any desired in rubber and, if used in rubber, direction. S e r v i c e conditions fills the need f o r a rubber-like material having would bloom to the surface after that a few years ago could not greater resistance to the action of oils and solvents vulcanization. T h e r e f o r e , an be fulfilled with any known type inand greater resistance to the deteriorating equal weight of phenyl-a-naphof rubber compound are today fluences oj' heat and oxjdation than natural rubthylamine was used in the rubbeing met with modern rubber ber. ber c o m p o u n d s since it is an compounds which may not even eauallv efficacious antioxidant involve any new raw materials but merely an artful combination of compounding ingredients for rubber and has the advan&ge"of being more soluble in rubber and, consequently, nonblooming. The DuPrene that have long been in use. The rubber chemist has even greater latitude in selecting and the corresponding rubber compounds also differ in other compounding ingredients for DuPrene than for rubber. respects. For example, compound D-10 contains rosin and Consequently, there is no one ideal DuPrene compound, but magnesia which are desirable ingredients for DuPrene comrather there are hundreds of DuPrene compounds, each of pounds but are not of particular value in a rubber compound which represents some chemist's effort to develop the composi- of this type. R-10, on the other hand, contains the accelertion best suited to meet a particular set of service conditions. ator mercaptobenzothiazole, stearic acid, which functions Likewise, results that we are today unable to obtain with as an activator for the accelerator, and, of course, sulfur. DuPrene we may next year be able to realize, not through im- TABLEI. COMPOSITION OF DUPRENE AND NATURAL RWB~R provement in DuPrene itself but through growth in our knowlCOMPOONDS edge of DuPrene compounding. D-10 R-10 D-11 R-11 Duprene type D 100.0 100.0 The purpose of this paper is to compare the physical propSmoked sheets 100.0 100.0 erties of DuPrene and natural rubber. The most one can do Channel carbon black 36.0 45.0 Zinc oxide. 10.0 6.0 10.0 6.0 is to compare the properties of certain popular types of DuLight-calcined magnesium oxide 10.0 10.0 Prene compounds with similarly compounded rubber stocks. Phenyl-B-naphthylamine 2.0 2.0 Phenyl-a-naphthylamine 2.0 2.0 The tests reported in this paper were conducted on two DuWood rosin 5.0 5.0 Sulfur 3.0 1.0 3.0 Prene compounds, D-10 and D - l l , and on two rubber comhlercaptobenaothiazole 0.5 0.7 pounds, R-10 and R-11, whose compositions are given in Stearic acid 1.0 4.0 Pine tar 2.0 Table I. Compounds D-10 and R-10 are of the pure gum Cottonseed oil 3.0 type; that is, they contain only such compounding ingrediCURINGRANGE ents as are essential for vulcanization. Compound R-11 is of the type commonly used for tire treads and D-11 is its One of the most desirable properties of a rubber compound DuPrene counterpart, both containing 25 volumes of carbon is the ability to produce a good vulcanizate over a wide range
T
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