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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 17, No. 10
Twenty-Five Years of Rubber Chemistry’ By William C. Geer2 TR&B. F. GOODRICE Co..AKRON,OHIO
UR knowledge of the chemistry of rubber and its various products has grown coincidentally with the growth of rubber factories. I n 1900 these factories were few, and the chemist as a forward-working research man was rare in the industry. Since then, factories and research laboratories have grown; and during this twenty-fiveyear period, not only in these commercial, but somewhat also in university, laboratories many searching investigations have been performed. Thus our knowledge of the chemistry of rubber is mainly derived from the researches of the last twenty-five years. If the reviewer were to incorporate in one paper references even to the majority of these workers, it would become necessary to expound the work of each, with differences of opinion and findings clearly delineated. I n particular, the revienrer’s own judgment of the weight to be given each would need precise statement. Such a method wouId extend the review to the size of a small volume. He has, therefore, attempted to outline only the generally accepted essential facts of rubber chemistry as they have been developed during the past twenty-five years.
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there has finally developed a more complete theory of the structure of the rubber hydrocarbon, and proved by refractive index measurements,lO*ll chemists now believe it consists of many C S H ~groups with only one double bond in each These polymerized groups are aggregated into larger masses, which show a diminished chemical activity. The structural formula may thus be CHa
written (-CHz-C = &H-CHr),; but whether it is a long chain or a large ring is not known. The future has to do with the x; for precisely the way in which these groups of carbon and hydrogen are united to form the aggregates, and the effect on physical properties of the size of these aggregates, is one of the unsolved chemical problems in this intricate phase of organic chemistry. During these past twenty-five years occurred what one of our well-known chemists, who has been so forceful in humanizing chemical knowledge, called “the race for rubber.” Stimulated by the high price of natural rubber in 1910, the chemist assiduously pursued the chemical synthesis of the rubber hydrocarbon. From several starting points, he succeeded finally in developing a structure and a rubbery subChemistry of the Rubber Hydrocarbon stance which upon analysis proved to be a true rubber. These What is rubber? That is an old question. Crude rubber syntheses were successful from a chemical standpoint, but had been purified and analyzed1~2~* prior to 1900; i t had been they were not so in practice. Synthetic rubber, although distilled as early as 1826, and several hydrocarbons, including vulcanized into numerous rubber products, never has had isoprene, C ~ H Sand , caoutchene or dipentene, CiOHie, were the final touch given to it which Nature adds in the tree. isolated from the distillate. The carbon-hydrogen ratio, The products made from it had neither the life nor the CJ&, was known and accepted before 1900. Likewise, strength possessed by those made from natural rubber. chemists knew the fact of chemical unsaturation, and it Although there was a race for rubber, the best that can be was believed from optical measurements3 that there were said for the chemist is that he ran on a different track than one and one-half double bonds for each C a s group, and did the planters in the Far East, and arrived at a lesser goal. accordingly the fundamental group in rubber was believed Some of the admirable work which has been done in the to be CloHls. adding of hydrogen to r u b b e r , l 8 ~or ~ ~the various products Rubber has been a difficult substance to examine from the made in the oxidation of rubber must be passed over with standpoint of the organic chemist, for it is colloidal; molecular the statement that by the action of oxygenls on rubber several weight measurements are of no account; it has no constant products are claimed to have been isolated, but few of them melting point, and decomposes when distilled. It was are pure materials and much is yet to be done. We know known for many years that it reacted with the halogens, of the nitrositele products, and of methyl and ethyl hydro oxygen, sulfur, and nitric acid. It had been chlorinated3 rubber,I7 which latter are prepared through very difficult and an impure bromide was known. The addition of sulfur reactions with zinc methyl and zinc ethyl. A recent most refers back to the important date of 1839, when Charles interesting development of a theoretical nature has been the Goodyear discovered that sulfur affected rubber, making it preparation of crystalline rubber.*s Certain European chemless sensitive to heat and cold; but until the work of WeberJ4 ists have recently found it possible to obtain actual crystals in 1902, little was definitely postulated regarding the real of rubber hydrocarbon from certain highly purified solutions. action of sulfur upon rubber. The chemistry of these re- We have conceived of rubber as a colloidal plastic; and yet actions is now more clear, for it is well established that two now the chemist has changed our colloid into crystals. The bromine atoms add to each CsHs group forming (C5HsBrz). crystals are quite different from rubber as we know it. They (or as it used to be written, C10H16Br4,40 whence it was called are very sensitive to oxidation, and possess relatively little rubber tetrabromide) ; that chlorine substitutes as well as elasticity. adds, forming a so-called heptachloride5 of the approximate Vulcanization with Sulfur formula (CIOH&lr)r; and that the. halogen acids form true addition products (C5HsX)z.6J Because of these additions, From the practical standpoint, the vulcanization of rubber, and those of sulfur and of sulfur monochloride,8 SSCl?, to- or its combination with sulfur, is the field that has commanded gether with the studies of the oxidation of rubber, and es- the chief interest. Most of our commercial rubber products pecially because of Harries’ work on the action of ozone,g demand the interaction of rubber and sulfur, under the inPresented before the Division of Industrial 1 Received March 16, 1925. fluence of temperature and time. These are the factors and Engineering Chemistry at the 69th Meeting of the American Chemical discovered by Goodyear in 1839, and they persist. Down Society, Baltimore, Md., April 6 to 10, 1925 to 1900 they were still much in the form left them by GoodVice president of The B. F. Goodrich Company. year. But from 1900 on, the energetic work of a number of * Numbers in text refer to bibliography at end of article. f
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IXDUSTRIAL A N D ENGINEERING CHEMISTRY
chemists has led to the improvement, not only in our knowledge of what is vulcanization, but also in changes in the methods by which vulcanization can be carried out. The disputes between the advocates of a chemical addition theory of vulcanization and a physical adsorption theory are well known in rubber literature. Vulcanization is concerned not only with sulfur and its effect on the rubber hydrocarbon, but also with the substance sulfur monochloride, the action of which was discovered in 1846 by ParkesXBin Great Britain. The light shed upon the way sulfur acts is assisted by our knowledge of the changes brought about by halogens, oxygen, etc. Vulcanization is difficult of explanation because even with a given quantity of sulfur we are disturbed in our theories by variations in the finished product. The rubber man deals in “undercure,” “overcure,” and “proper cure.” Did we deal with one unvaried material, with a constant percentage of sulfur, our problem would be simple. We can, however, in the light of our knowledge undertake a few positive statements. There is, during vulcanization, a chemical reaction between rubber and sulfur, and during this reaction the rubber combines with some sulfur. We know that time and temperaturez0 for a given mixing are factors; we know that when a mixture of rubber and sulfur is heated, the rubber dissolves the s ~ l f ~ and r ~chemical ~ ~ ~ action follows; and during this heating process we believe that the rubber aggregates are being broken down by the action of heat, so that the longer the time of heating, the relatively weaker is the product. These two actions go on simultaneously: the addition of sulfur and the breaking down of the rubber aggregate. Thus, in the simplest and shortest explanation that can be made, our theories lead us to believe that, whatever other effect may occur, there IS a t least a definite chemical action taking place during vulcanization, with heat and sulfur playing the major parts.
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a t least, which is most plausible. The original proposer called it the hydrogen sulfide-polysulfide theory.z5 It consists simply (or rather complexly) in the belief that when amines are used as accelerators, hydrogen sulfide is formed and adds to itself sulfur to form a polysulfide in a manner analogous to the formation of ammonium sulfide. This polysulfide sulfur easily splits off a very active form of sulfur, which then quickly attaches itself to the rubber. The accelerator therefore serves to activate the quantity of sulfur in contact with rubber. Our rubber factories are dominated by accelerators, and it is no idle statement to say that the practical results obtained by the chemist in the rubber industry have modified the handling of factories in a marked way. It is an achievement brought about during the last twenty-five years. The manufacturer on his part “turns over” his plant more rapidly, and has needed a smaller investment in order to meet the demands of the consumer of tires and other rubber goods than would have been required had there been none of this chemical work. The consumer, likewise, has profited by increased values. Part, although not all, of the well-recognized additional tire service is due to the qualities made possible by these organic accelerators during rubber vulcanization. ~ In J ~connection with our story of vulcanization is another and more recent development, for chemists have found the role that is played by organic acids, the use of which is old in practice. One of our prominent chemists has analyzed natural rubber and found in it the presence of organic acidsoleic, linoleic, and one hitherto unknown which he calls “heveic” acid.z6 These acids have a definite influence on rubber vulcanization, since without their presence metallic oxides mould fail as accelerators. Certain grades of rubber seem to be deficient in these acids; and this deficient rubber may be improved by the addition of organic acids in the presence of metallic oxides.
Accelerators
Rubber Latex
From a practical standpoint, next to the discovery of the effect of sulfur, the greatest advancement from chemical work has come from the use of those substances that speed the time and lower the temperature a t which sulfur is added to rubber. These substances, broadly speaking, we term “accelerators of vulcanization.” The use of such inorganic powders as white lead, litharge, and so on, was known to Goodyear and his contemporaries, but down to 1906 essentially nothing was done in the use of organic substances. The story of the history of organic accelerators has been given in another p l a ~ e , ~and ~ ~only ~ ~ a‘ brief comment will be repeated here. It was one of our American chemists who found in 1906 that aniline oil and other nitrogenous organic chemicals, when added to mixtures of rubber and sulfur, shortened the time of vulcanization, and gave to the finished mixture higher physical values. The list of these organic compositions is long. Chemists have studied the effect of literally thousands of organic compositions used not only to accelerate vulcanization, but to yield better products. Among the inorganic accelerators, the chief ones of value are litharge, magnesium oxide, and zinc oxide, the latter being in point of fact used as a promoter of the organic accelerator. A large number of groups of accelerators are used, the effect of which upon the temperature of vulcanization is so varied that today it is possible t o add sulfur to rubber and thus vulcanize it a t temperatures ranging from slightly above that of the room up to temperatures which radically affect the aggregation of the rubber hydrocarbon. The most important question worthy of discusaion in this place is-just what is done by the accelerator? Let me state what is probably the most generally accepted theory; the one,
Mother Nature produces rubber in the form of little drops of rubber hydrocarbon and puts them into suspension in water in the glands of the rubber tree. This colloidal suspension, or latex as it is called, has been subject during the past ten years to a study of an intense character, and with many admirable additions to our knowledge. To one interested in a further study of latex, its composition and properties, and the recent work in its industrial applications, no better reference can be made than to that most admirable paper (which to the writer is a classic in presentation) by van R o s ~ e m . ~ ~ Rubber latex is one of the most interesting of the colloidal suspensions. The globules of the rubber hydrocarbon are of various sizes and shapes-globular, .pear-shaped, etc., varying in size from as low as 0.5p in diameter to as high as 3.0,~. One recent investigator** succeeded in pricking some of these particles, and came to the conclusion that each particle is surrounded by an adsorbed layer of nonrubber substance, and each particle consists of two different rubber phases; one, the outer layer, which is scarcely soluble in benzene and of very high viscosity, and an inside fluid phase of low viscosity and soluble in benzene. The Brownian movement of these particles in the latex has been observed, and through cataphoresis experiments the rubber particles can be shown to have a negative charge. Much work has been done on the practical methods of coagulation of latex, and studies of the properties of the rubber obtained by the use of various acids, salts, etc., as coagulants. There seems to be no doubt that the protective colloidz9 in this suspension consists of proteins of an amphoteric char-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
acter; and that the resin acids and soaps are constituents of the protective layer. Our acceptance of the latex as a colloidal system leads us to understand the coagulation process, but there still seems to be some difference of opinion among those who have studied it most closely regarding precisely the part played by enzymes and bacteria, and therefore exactly what happens when acids are added. We do generally agree, though, that coagulation results in a massing of the particles, and that the solid, dry, crude rubber as we get i t consists of particles each surrounded by some proportion of the protective colloid originally in the latex. During the past two years particularly, there has been considerable agitation in the industrial field over the application of latex in the rubber industry. Latex has been mixed with sulfur and powders and dried;30latex has been sprayed3I as a means of obtaining dry rubber; it has been used to impregnate tire fabrics;32it has been used in making dipped goods; latex has been vulcanized without c o a g ~ l a t i o n and ,~~ the resulting vulcanized latex evaporated to form layers upon cloth, sheets of rubber, and other articles. With some degree of success latex has been used in the paper industry.34 It is possible to ship latex from the Far East without coagulation by the use of p r e s e r ~ a t i v e sit; ~is~ possible to concentrate latex from the natural percentage of approximately 35 per cent total solids up to 60 per cent, and even 75 per cent.36t37 The creation of an artificial latex or rubber suspension by mechanical means from solid, dry rubber has been carried From the practical manufacturing standpoint, none of these new processes in the direct application of latex has extended into the industry in any large way. Many of the scientists and some of the technologists seem to believe that the uses of latex industrially have but just begun, and do not represent a mere temporary enthusiasm. Colloid Chemistry of Rubber Cements
The vulcanization of rubber in sol and gel form has attracted the attention of some clever chemists, who have succeeded in vulcanizing rubber solutions in benzene without the formation of a ge1,39-43and subsequently evaporating to a film of vulcanized rubber. They have also vulcanized such solutions to true gels. This has led some chemists to speculate on the colloid chemical nature of vulcanized rubber, and some of them seem to believe that when the evidence is all in, vulcanized rubber will be found to be a gel consisting of a chemical addition product of sulfur and rubber, in the presence of rubber or some physically changed modification.44,44“ Colloid Chemistry of Rubber Mixtures
I n the old days dry powders were added to soft rubber mixtures for the sake of making them stiffer than was possible with sulfur alone, and for cheapening purposes. They were called ‘Tillers.” Today the rubber mixture is considered by the rubber chemist as a disperse system of a very particular ~ h a r a c t e r , ~and 5 each of these dry powders which we now know as “pigments” in any such system is used to obtain defined values. In studies that have been published, two characteristics of great value are shown in such systems. The fineness of subdivision, or, as we now call it, “particle size,” is of prime importance; for particle size affects not only the property of stiffness of the dispersion but also such important characteristics as resistance to abrasion, tensile strength, the modulus of elasticity, and so Particle size alone, however, does not account for the valuable characteristics of some of these more modernly considered pigments such as carbon black and zinc oxide; and we today are looking toward the ultimate solution of the question in the
Vol. 17, No. 10
evaluation in terms of physical constants of the interfacial tension between the rubber and the surface of these particles. 47,48 There is doubtless still another important characteristic in the shape of the particles. It may be a question of total surface which today is not possible of evaluation with our present microscopic methods; but it is true that certain carbon blacks-for instance, the particle size of which is in the order of certain other blacks-yield, when vulcanized in rubber, softer mixtures; yet others of nearly equally fine subdivision yield stiffer mixtures. So we must consider not only particle size but also the unmeasured interfacial tension constants and the degree of plasticity shown by given volumes of given substances. There is a science as well as an art in modern rubber compounding, and while this phase of rubber chemistry is passed over rapidly, this is done largely, not from lack of exact data, for in point of fact it is the rubber mixture that constitutes the rubber business, but from the lack of coordinated conclusions. Mixtures are made to have different properties for each of many thousand different uses. The future rather than the past holds the key to the evaluation and predetermination of characteristics to be given to finished mixtures because of known physical and chemical characteristics possessed by substances to be dispersed into the rubber phase. Many other of our problems might be mentioned for which some solution or theories have been offered, such as that of mastication and the effect of it upon the protective colloid; the effect of heat prior to vulcanization; the effect of acids and alkalies upon rubber; but these numerous subjects must be passed over a t this time. Aging of Rubber
I n the old days a rubber mixture after vulcanization was not expected to last, on ordinary standing, in the light, or a t ordinary temperatures, for more than one or two years; it became hard and evil-smelling. Chemists, however, separated certain products, resinous in character, from this so-called “deteriorated” rubber. It has ever been a study on the part of chemists to determine what to do to prevent this deterioration through aging, and how to test in advance whether any given composition after any given cure would live out a long enough useful life. The question, therefore, in front of the manufacturer has been-how long will it last? Within the last twenty-five years various methods for testing this aging, and various apparatus for working out accelerated aging tests, have become known and used.4g$b0 Apparently, there is no unanimous view as to which particular method of test is superior, but the majority of rubber manufacturing laboratories today use one test or another for this purpose. I n the study of aging more light has been thrown upon the precise chemical mechanism of it. It is generaly conceded a t the present day that aging has to do with oxidation. Whether it is direct oxidation of the rubber or whether it is an oxidation of the free sulfur, and from those oxidation products a catalytic action is developed, chemists do not seem wholly to agree.15+1~52+3 Oxidation products of rubber, however, have been collected when rubber has been submitted in closed vessels to the action of air a t ordinary as well as a t elevated temperatures. To prevent this oxidation a number of substances have been These as a rule have been nonaccelerators of vulcanization, but they do serve to retard the action of oxygen. We are not, however, quite ready to state whether they serve as buffers to take oxygen instead of the rubber taking it, or whether they serve
October, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
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14-Staudinger and Fritschi, Helvefica Chim. Acta, 6, 785 (1922): Hydrogenation. I b H e r b s t , Rer.. 39, 523 (1906); Kirchhof, Kolloid-Z., 13, 40 (1916); Peachey, India Rubber J., 46, 361 (1913); cf. Fisher, Ind. En& Chem., 16, 027 (1924): Addition of oxygen. 16--Harries, Ber.. 34, 2991 (1901); 35, 3256, 4429 (1902); 36, 1937 (1903): Nitrosites. Vulcanization without Sulfur 17-Staudingerand Widmer, Heiwfica Chim. Acta, 7,842 (1924) : Methyl ethyl hydro-rubber. Since sulfur seemed to be the substance which gave to rubber and 18-Pnmmerer and Koch, A n n . , 438, 294 (1924): Crystalline rubber. improved properties by vulcanization, it is natural that many 19-Parkes, English Patent 11.147 (1846) : Sulfur chloride cure. ZC--Weber, Kolloid-Z., 1, 33, 65 (1906): Time and temperature chemists should have studied the problem of x-ulcanization without sulfur.66.6715* Several substances, notably certain factors. 21-I-Iinrichsen, Ibid., 8, 245 (1911): Rubber dissolves S, then reacts. peroxides and certain aromatic nitro c o m p o ~ n d s have , ~ ~ ~ ~ ~ 22-Venable and Green, Ind. Eng. Chem., 14, 319 (1922): Solubility been found definitely to bring about a stiffening, an increase of S in rubber. 23-Kelly and Ayers, Ibid., 16, 148 (19.24): Solubility of S in rubber. in strength, and the other physical changes that we call 24--Geer, Ibid., 14, 369 (1922): History of accelerators. results of vulcanization. None of these, however, so far as 24a-Geer and Bedford, Ibid., 17, 393 (1925). the published literature goes, are known to give properties 25-Bedford and Scott, Ibid., 12, 31 (1920); 13, 125 (1921); cf. Sebrell, that equal, let alone be superior to, the products obtained “Organic Accelerators of Vulcanization,” in Bedford and Winkelmann‘s from the use of sulfur. They are, therefore, pretty largely “Systematic Survey of Rubber Chemistry:” Polysulfide theory. 26-Whitby and Dolid, India Rubber World, 68, 496 (1923); Whitby, only of scientific interest. Brit. Assoc. Adu. Sci., Rapt., 1923, 432; India Rubber J . , 68, 617, 735 (1924): F a t t y acids in rubber. Psychology of Rubber Chemistry 27-Van Rossem, Rubber A g e (London), 6, 523 (1924); J. Soc. Chem. I n d . , 44, 33T (1925): Latex. It would not be in keeping with this review were no comZS-Hauser, India Rubber J . , 68, 19, 725 (1924); Freundlich and Hauser, ment made upon the changed attitude, during the past Kolloid-Z., 36, (Zsigmondy-Festschrift), 15 (1925): Microscopy of latex. 29-Belgrave, Malayan A ~ YJ., . 11, 348 (1923): Protective colloid in twenty-five years, on the part of rubber manufacturing companies, so far as secrecy is concerned. Rubber factories latex.aO-Laub, Bull. Rubber Growers’ Assoc., 4, 209, 321 (1922): Latex S a-ere formerly closed to visitors. Kowadays we have found fillers. 31-Hopkinson, U. S. Patent 1,424,020 (1922): Sprayed latex. the advantages to be gained through somewhat full and free 32--Hopkinson, I n d . Eng. Chem., 15, 1267 (1923): Impregnating tire discussion of our technical affairs. Significant in this change of attitude on the part of the fabrics. 33-Schidrowitz, English Patent 193,451 (1921) : Vulcanized latex. directors of industrial rubber companies has been the growth 34-Kaye, Bull. Rubber Growers’ Assoc., 4, 408, 506 (1922): Latex SOCIETY. paper. of the Rubber Division of the AMERICANCHEMICAL 35-De Vries, Arch, Rubbercultuur, 7, 168 (1923): Preserved latex. It was born in Boston as a section in 1909, authorized as a 36-Gibbons and Shepard (General Rubber CO.), English Patent division in 1919, and has been animated by commendable 218,544 (1924); McGavack (General Rubber Co.), U. S. Patent 1,523,821 enthusiasm. Particularly significant, since the city of Akron, (1925): Concentrated latex. Ohio, is the rubber center of the world, was the organization 37-Hauser (K. D. P., Ltd.), English Patents 213,886 and 219,277 of the Akron Section of the AMERICANC H E h f I c a L SOCIETY (1924): Concentrated latex. 38-Pratt, Italian Patent 225,949 (1923) ; India Rubber World, 69, in 1923. Rubber chemists in Akron meet together freely, 785 (1924): Water dispersion. and although it is admitted that for broadening purposes 39-Helbronner and Bernstein, Re@. 4th Infern. Rubber Congress, we customarily discuss topics other than rubber in our London, 1914, p. 156: Vulcanization in solution. Akron meetings, yet in the Rubber Division of this SOCIETY 4O--Peachey, English Patent 129,826 (19191; 146,734 (1919); India papers have been numerous upon subjects which twenty-five Rubber J . , 69, 1195 (1920): Vulcanization in solution. 41-Stevens, English Patent 164,770 (1920); India Rubber J., 61, years ago would have caused hands of superintendents and 1160; 62, 203 (1921): Vulcanization in solution. directors to be raised in holy horror. 42-Le Blanc and Kroger, Z . Elekfrochem., 27, 335 (1921): VulcanizaWe have come to the conclusion that “we live in deeds, tion in solution. 43--Boiry, Caoufchouc gutta-percha, 21, 12,257, 12,293, 12.335, 12,386, not years,” and an atmosphere of friendly competition and 12,454 (1924); 22, 12,507 (1925): Vulcanization in solution. good sportsmanship now envelops the rubber industry. 44-Ostromuislenskii, J . Russ. Phys. Chem. Soc.. 47, 1453, 1462, 1467 (1915); India Rubber J., 62, 467 (1916): Theory of vulcanization. Bibliography 44a-Twiss, J . Soc. Chem. Ind., 44, 106T (1925): Theory of vulcanization. I-Faraday, Mag. Pharm., 14, 180 (1826) : Analyzed rubber distillate, I b G r e e n , J . Ind. Eng. Chem., 13, 130 (1921); J . Franklin Inst., 192, 2-Williams. Trans. Roy. Soc. London, 241 (1860!: Characterized 637 (1921); Chem. M e f . Ens., 26, 53 (1923): Rubber pigments. isoprene. 46-Green, J . I n d . Eng. Chem., 13, 1029 (1921): Colloid chemistry of 3-Gladstone and Hibbert, J . Chem. Soc. (London), 63, 679 (1888): pigments. Optical me8surements. 47-Wiegand, Can. Chem. J., 4, 160 (1920); cf. Spear, Colloid Sym(-Weber, Gummi-Zfg., 16, 527, 545, 559, 561 (1902): Action of S on posium Monograph (Madison), 1923, p. 321: Colloidal rpigments. rubber. 48-Klein and Parrish, J . Oil Colour Chem. Assoc., 7, 54, 82 (1924): 4a-Weber, Bey., 33, 770 (1000). Colloidal chemistry of rubber pigments. 5-Peachey, English Patent 1894 (1915) (cf. References 3 and 7): 49-Geer and Evans, Rubber Age (London), 2, 308 (1921): Aging. Heptachloride. 50-Bierer and Davis, Ind. Eng. Chem., 16, 711 (1024): Aging. &Harries, Bar., 46, 733 (1913): Addition of halogen acids. 5l-Peachey, J . Soc. Chem. Ind., 31, 1103 (1912); 32, 179 (1913); S7, 7-Hinrichsen, Quensell, and Kindscher, Ihid., 46, 1283 (1913) : Addi55 (1918): Oxidation. tion of halogen acids. 52-Bruni, India Rubber J., 63, 415, 814 (1922): Oxidation. S-TVeber, J . SOC.Chem. I n d . , 13, 11 (1894): Addition of sulfur chloride; Giorn. chim. ind. apfil., 5, 122 (1923): Oxidation. Ea-Marzetti, cf. Hinrichsen and Kindscher, Ber., 46, 1291 (1913); Kolloid-Z., 6, 202 54-Moureu and Dufraisse, English Patent 181,365 (1922): Anti(19101; Bernstein, Ibid., 11, 185 (1912). oxidants. g-Harries, Ber.. 37, 2708 (1904); 38, 1195, 3985 (1905); 45, 936, 943 55-Bayer & Co., D. R . P. 257,813; 264,820 (1911); 259,722 (1922); (1912): Ozone reaction. 329,676: Antioxidants. IO-Macallum and Whitby, Trans. Roy. SOC.Can., 18, 191 (1924): 56-Degen and Kuth, English Patent 9956 (1908): Iodine cure. Refractive index. D. R . P. 260,916 (1912): Selenium, tellurium cures, 57-Klopstock, Refractive index. 11-Twiss, Nature, 113, 822 (1924): 5S-Boggs, J . I n d . Eng. Chem., 10, 117 (1918): Selenium cures. 12-Harries, Ann., 406, 173 (1914): Rubber as aggregate of CaHs 59-Ostromuislenskii, J . Russ. Phvs. Chem. Soc., 47, 1462, 1467, 1885, groups. 1898, 1904 (1915); 48, 1114 (1916); Caoufchouc & gulfa-percha, 14, 9119 13-Pummerer and Burkard, Ber., 55, 3458 (1922); Pummerer and (1917): Cures with active oxidizing agents. Koch, Ann., 438, 294 (1924): Hydrogenation. 60--Gibbons, U.S. Patent 1,291,828 (1919): Nitroanthraquinone cures.
definitely to reduce the ability of rubber to add oxygen. There is still much opportunity for study in the question of aging. But our modern rubber mixtures do last over a reasonable period of years.
