860 .
I-YD U S T R I A L A N D ENGINEERING C H E i I S T R Y
This tabulation is a frank statement of findings, and definitely contraverts the loose statement that the majority of injuries are caused by the negligence or ignorance of the man injured. One wonders what would be shown in a similar tabulation of the experience of a large corporation which had not consistently expended money and effort in accident prevention. Next in importance to proper discipline is the careful selection, assignment, and instruction of new employees. Du Pont Company experience shows that the accident-frequency rate for employees of less than one month’s service is twice that of employees of one year’s service or over. If there are special hazards from toxic materials, new employees should be physically examined, graded, and assigned only to employment to which they are suited. At the Dye Works, for example, accepted applicants are physically graded into three active classes and a fourth class to which employees are assigned for light work only. Employees of the first two classes are physically examined each month. The various operations are also graded into four corresponding classes. The first class involves exposure to nitro and
Vol. 1.5, No. 8
amido bodies; in the second class, these bodies are present but direct contact with them is unlikely; in the third class, such bodies are absent or contact with them is practically impossible; the fourth class covers miscellaneous light work. No employee of one class is permitted to work in a process of a more hazardous class, this restriction applying also to mechanical and labor department employees, who are graded in the same manner as the process workers. Much might be written of other essentials of a safe chemical plant, the least of which is by no means scrupulous cleanliness in mechanical, structural, and human equipment. The same outstanding deduction, however, is to be drawn in every case-that what reduces accidents makes also for increased efficiency. It is sometimes difficult to show in book values the profits that accrue from organized safety work, properly conceived and conscientiously carried out, because such profits, though real, are often intangible. On the other hand, no one will gainsay the economy of ‘‘efficiency,]’and if safety and efficiency go hand in hand, what greater encouragement to prevent accidents should we demand?
Recent Important Investigations in t h e Chemistry of Rubber, and Substantiation of t h e C5H, Ratio’ By Harry L. Fisher THB B. F. GOODRICH Co., AKRON,OHIO
A review has been given of the more important recent investigations on the chemistry of rubber showing the following results: ( I ) Substantiation of the CSHSratio and the C6H8 nucleus as the fundamental grouping in the rubber hydrocarbon, and the presence of a double bond for each c&8 nucleus. ( 2 ) Hydrogenation of rubber directly and in solution has been accomplished.
( 3 ) Studies in oxidation have produced new products, especially (CSHSO),. (4) Viscosity measurements have shown that the change during the heating of a solution of rubber is reversible, provided air is excluded. ( 5 ) Concentrated sulfuric acid on a rubber solution produces what appears to be an isomerized rubber.
N
the rubber all the time by keeping it in an atmosphere of carbon dioxide. The results of their analyses are so good and so important t h a t it seems worth while to give them here. Harries’ best result is also given for the sake of comparison.
OT since the publication of Harries’ work on the ozonides and hydrohalides of rubber a decade ago has so important
a paper on the chemistry of rubber come forth as that by Pummerer and Burkard.2 These chemists have accomplished the “impossible”-they have hydrogenated rubber a t ordinary temperatures. Furthermore, by extremely careful work they have substantiated by analysis t h a t the empirical formula of the rubber hydrocarbon is C6H8, and by their method of hydrogenation that each C5Hs nucleus does contain a “double” bond.
THE C6Hs RATIO It is interesting that their paper should come out a t the same time that serious questions were being raised concerning the empirical formula of rubber. Kirchhof3 has given much time in reviewing the work of many authors, including himself, in an effort to show that the empirical formula is ClOHl7 (instead of CloHl6, which is the same ratio as CbHs), and indeed the array of evidence is considerable. However, he presents the figures only and does not a t the same time compare the methods of preparation of the samples for analysis. Pummerer and Burkard were unusually careful in this regard. They recognized the ease of oxidation of pure rubber and how readily a small amount of oxidation which nil1 vitiate an analysis can occur. Therefore, although they used Harries’ procedure in general, they protected 1 Received February 23, 1923. Presented before t h e Division of Rubber Chemistry at the 65th Meeting of the American Chemical Society, New Haven, Conn , April 2 t o 7, 1923 2 Ber., 6 6 , 3458 (1922), C A , 17, 898 (1923). 3 Kollozdchem. Bezhefle, 16, 47 (1922); C. A , , 16, 4363 (1922).
-C-Pummerer and Burkard.. . . . . ,
. .
Harries‘
Calculated ,
. . .Found
. . . . , . . , . . , , , , Found
87.99 87.99 87.89
H -88.15 87.96
87.85
8:7;11.81
1
11.85 11.82 12.28
DOUBLEBONDSSHOWN BY ADDITIONOF HYDROGEN The other serious question settled by Pummerer and Burkard is the nature of the unsaturation. Recently, Boswell6 has proposed a new structural formula for rubber largely on the basis of its containing no double bonds. His argument is as follows: With double bonds existing in the rubber molecule, it should be possible to add hydrogen directly and produce a saturated hydrocarbon. The endeavors of Harries and of Hinrichsen and Kempf to accomplish this failed. Likewise, all attempts made in this laboratory were unsuccessful. This would seem to indicate that rubber contains no ethylene linkages a t all. Belief in the unsaturated character of rubber depends on the observations t h a t rubber adds on approximately four bromines and two hydrochloric acid mols for each CloHle. However, as these are admittedly very drastic actions, almost certainly accompanied by deep-seated depolymerization of the rubber molecule, it is conceivable that the rubber mol itself contains no double bonds PRESENCE OF
4 “Untersuchungen fiber die naturlichen und kunstlichen Kautschuk . arten,” p. 7. 6 Can. Chem. M e t . , 6, 237 (1922); India Rubber J., 64, 981 (1922).
INDUSTRIAL A N D ENGINEERING CHEiWISTRY
August, 1923
whatever, and that these are only produced by the breaking up of the complex rubber,molecule by the action of bromine or hydrochloric acid. That bromine and hydrochloric acid add to a polymerized compound like rubber constitutes no proof that ethylene linkages are present in the original rubber molecule. Pummerer and Burkard give a series of fifteen experiments very successfully demonstrating t h a t a t the ordinary temperature in the presence of a catalyst rubber adds two atoms of hydrogen for each C6Hs nucleus. The purified rubber was dissolved in either hexane or hexahydrotoluene, and a concentration of less than 1 per cent had t o be used in order to obtain a complete reaction, 0.2 to 0.6 per cent being the best. Higher concentraLions would not work well, possibly, according to the authors, because of the inactivation of the catalyst by absorption of rubber. The catalyst was platinum black activated by oxygen according to the method OF Willststter and Waldschmidt-Leitz.6 The total time necessary for the complete absorption of the gas varied from 3 to 170 hours, the shorter time usually a t temperatures of 70' to 80' C. The agreement with the theory by volume measurements was exact in one case and generally not more than 1 per cent off-that is, not more than 2 cc. in approximately 200 cc. of gas absorbed. These figures were checked by the actual isolation of the hydro-rubber and analysis by combustion. The hydro-rubber is similar to rubber itself in t h a t it is highly elastic, but it is almost colorless and is soluble in ether, giving a colloidal solution. It is extremely susceptible to air oxidation, especially in the presence of the catalyst. Oxygen transforms it into a substance with the same empirical formula as natural rubber. This new rubber is not the same as natural rubber and is called iso-rubber H. It is soluble in ether, and, like rubber, on hydrogenation adds two atoms of hydrogen for each C6Hs nucleus. This new hydrogenated rubber is also readily oxidized, but it has not been investigated further. The authors are aware of the fact t h a t the ease of oxidation of the hydro-rubber brings up the question as to whether the hydrogen atoms are bound by primary valences. They state t h a t the :stability of the substance in a high vacuum (1 mm.) argues against a secondary valence combination,with the colloid. To the writer this unusual reaction-oxidation resulting in the formation of an unsaturated compound-recalls the unique case of l,Z-dihydrobenzene, which, even though it contains two double bonds, will on mild oxidation go over to benzene, a stable though in some respects more highly unsaturated system. Pummerer and Burkard give the equation for the addition of hydrogen as follows: (CJIg)z xH2 = (CJLo), where x is the number of isoprene residues in the rubber molecule. Moreover, an equally long open chain must take up one mol more of hydrogen-i. e., x 1 mols, or:
+
+
+
( C ~ H S$. ) ~(x 1)Hz = C&ioZ+z I t is readily seen t h a t the number of mols of hydrogen consumed 1 in the case of the open chain is - times greater than the corre-
86 1
succeeded in completely hydrogenating rubber. Their paper was actually published a month earlier.3 Their method also comprised the use of a catalyst, but they found it necessary to use very high temperatures and pressures. They employed ordinary platinum black, which was incorporated into purified rubber by mixing it with the benzene solution, then precipitating with alcohol and drying in "absolute vacuum." This platinized rubber without any solvent was placed in a glass container in a rotating autoclave filled with hydrogen a t 93 atmospheres and heated for 10 hours a t 270" C. or a t 102 atmospheres and 280" C. Lower temperatures gave no or only partial hydrogenation. At ordinary pressure but the same high temperature only partial hydrogenation was effected after 7 days while hydrogen was passing all the time. hTickelworks much like platinum, but not so rapidly and completely. The hydro-rubber is a colorless, transparent, tough mass entirely without the elastic properties of rubber. The loss in color is said to be very characteristic of the reaction. The analyses check the empirical formula, (CSH10),, very closely. The authors state that according t o the properties of the hydro-rubber it can be considered as a saturated hydrocarbon with so great a molecular weight that, so far as can be shown by analysis, no distinction can be made and C52H1~s + 2 . The product is soluble in between C6rH102 benzene, chloroform, and ether, and insoluble in alcohol and acetone. I t s solutions are colloidal and in benzene showed no depression of the freezing point. Its solutions do not decolorize bromine solution, but on standing in the sunlight a substitution product is formed with the evolution of hydrogen bromide. The hydro-rubber is unattacked by sulfur monochloride in solution, being recovered unchanged. It is therefore very stable toward chemical reagents, as would be expected from a saturated ring or straight-chain hydrocarbon of high molecular weight. The authors apparently did not find it necessary to guard against oxidation or they would have mentioned it. Their hydro-rubber is evidently quite different from the hydro-rubber obtained by Pummerer and Burkard. Staudinger and Fritschi wished to avoid using the large amount of solvents necessary For purification of crude rubber according to the method of Harries. They found Wildermann's method9 excellent. It consists in extracting the crude rubber with a mixture of two solvents, of which one dissolves rubber and some of the resins, the other dissolves the resins and precipitates the rubber. This allows a more thorough extraction of the resins. Mixtures of chloroform and acetone, 20 to 80 up to 40 to 60 parts, were used for the extraction lasting 8 to 14 days. During this time air was not permitted to enter the apparatus, in order to avoid autoxidation. The authors make the significant statement that although nitrogen was of course present in the crude rubber, only traces or none at all could be detected in the purified rubber. HEAT DECOMPOSITION
Staudiiiger and Fritschi thoroughly reinvestigated the heat decomposition of purified rubber in a vacuum of 0.1 to 0.3 mm. sponding number of isoprene groups of the ring system. Now, pressure. The final temperature of the metal bath was 350" C., the authors are certain t h a t a difference of as much as 10 cc. in but the temperature of the distilling vapors rose only to 320 C. excess could and would have been detected in their apparatus. 36 5 per cent remained as a resinous mass which was found to be If the diflerence were as great as this it would correspond to a only about one half as unsaturated as rubber. Fractionation of value of 20 for x, where the volume absorbed was about 200 cc., the distillate in a high vacuum, determinatlon of the boiling point, as it was in several of their best experiments. The actual excess molecular weight, refractive index, bromiqe absorption, etc , was never greater than 2 cc. They therefore feel t h a t they have of the chief constituents showed the presence of isoprene, 3.1 proved that the rubber molecule must contain a ring system or per cent; dipentene, 8 8 per cent; a CISH24 hydrocarbon with an extremely long chain in which x>20. two double bonds and two rings, possibly a hydronaphthalene STAUDINGER AND FRITSCHI'S METHODOF HYDROGENATION derivative, 4.4 per cent; CnoHs~containing three double bonds and two rings; and C ~ S Hcontaining ~D four double bonds and two Staudinger and Fritschi? working independently of Pummerer rings. No open-chain terpenes were found. and Burkard, reported a t almost the same time t h a t they had X
Be?., 64, 122 (1921). 7 Helz. Chim. A c t a , 5, 785 (1922); J . Chem. Sac. (London), 122, 1043 (1922); J . SOC.Chem. I n d . , 41, 868A (1922). 6
8 Pummerer and Burkard, however, presented their paper a t t h e Centenary of German Scientists and Physicians, Leipzig, September 17 t o 24, 1922,and a n abstract of their paper appeared in Chem Z t g ,46,883 (1922). D D. R. P. 229,386; C. A , , 5, 2439 (1911).
INDUSTRIAL AND ENGINEERING CHEMISTRY
862
Vol. 15, No. 8
Staudinger believes t h a t the splitting of the carbon atoms tells of the marked falling off in viscosity when a solution is boiled takes place in the 1,4 position, or in multiples thereof, like 1,s; for a long time and then cooled, and accounts for the change on 1,16; etc. This is also in accord with his earlier worklo on the the ground that depolymerization has taken place. Lichtenheat decomposition of dipentene into isoprene, in which there berg13 showed that such solutions may give no precipitate with was the same type of splitting. This experimental evidence is alcohol and t h a t the action of hydrochloric acid gas gives a hydroagainst Boswell’s contention t h a t t h e splitting should take place chloride which contains less chlorine than that obtained in the at the double bonds. It is true that the double bonds are “usual same way from the unheated solutions. There is no question points of weakness,” b u t t h a t condition is in connection with the as t o the change in viscosity on heating, but there has been a action of chemical reagents and not necessarily of heat. question concerning the reversibility of the change. Pummerer The hydro-rubber was subjected t o a similar heat decomposiand Burkard carried out the heating in the presence of carbon tion. While the rubber was practically completely decomposed dioxide, also providing against the loss of solvent. Their experiat 300” C , it was necessary t o raise the temperature of the ments, involving 8 hours boiling followed by slow and by rapid metal bath to 350” C., and finally t o 400 C. in order t o complete cooling, indicate t h a t no appreciable change takes place. They the decomposition of the hydro-rubber. Only about 0.6 per cent also kept rubber solutions protected with carbon dioxide in brown remained behind. The distillate contained products entirely flasks exposed to the sun for several months during the unusually different from those from rubber, most of them apparently being warm summer of 1921, and found t h a t alcohol precipitated the straight-chain compounds containing one double bond each, rubber in a normal fashion and t h a t the precipitate was not such as pentene, C6H10, a C16H30 compound, C45HQ0, and C~OHIOO. soluble in ether. Without these precautions Harries had found t h a t alcohol precipitated a n oily mass which was soluble in HARRIES’a-HYDRO-RUBBER ether. I n an in$ccessible journal” Harries and Evers have described GEL FORMATION O F RUBBER I N BOILINGBENZENESOLUTION the preparation of a partially hydrogenated rubber by t h e reA 2 per cent rubber solution boiled under protection of carbon moval of the chlorine from t h e “dihydrochloride” of rubber with dioxide showed no change, but with a 4 per cent solution under zinc in ethylene chloride solution in the presence of hydrogen chloride. The analyses vary but the best agree with t h e formula similar conditions there occurred a separation of a yellowish gel on the bottom and walls of the flask as soon as t h e boiling (CloHls)x. The substance has the general appearance and some point was reached, according to Pummerer and Burkard. Evapof the properties of gutta-percha, forms colloidal solutions, and oration of the solvent was guarded against. A 10-hour experiis unsaturated. It forms an ozonide which is nonexplosive and on hydrolysis gives no levulinic aldehyde. Furthermore, heat ment a t 50” C. gave no separation. The gel formation is reversible, since on quick cooling followed by long standing at decomposition of t h e or-hydro-rubber a t a low pressure gives room temperature i t goes back into solution. The amount no isoprene. separated was about 10 per cent. OXIDATION Pummerer and Burkard found t h a t very dilute solutions of pure rubber absorbed oxygen gas at room temperature, the amount corresponding t o 0 5 mol of oxygen for each isoprene residue agreeing with the formula ( C ~ H S O ) ~The . result was the same with or without platinum black and the absorption was complete in 40 to 50 hours. The product was not isolated. They also found that perbenzoic acid, CsH6CO .OOH, reacted normally toward rubber, in chloroform solution, in absence of moisture, one mol being used for each C6Hs nucleus. This reaction was followed b y titration of t h e reagent and verified by the isolation of t h e rubber oxide formed. It is a white, tough substance, much less elastic than rubber, and insoluble in all the ordinary solvents. Their analysis shows t h a t it has the empirical formula (CSHSO),: Calculated..
...... .....
C Per cent 70.83
H
Per cent 9 53 9.77
This is believed t o be a new oxide of rubber and, like the hydrorubber, shows again that the fundamental grouping in the rubber as was assumed for many years. molecule is C5H8, and not C1~H18, Boswellj and his students have carried out some interesting work on oxidation with potassium permanganate, hydrogen peroxide, air, and a mixture of hydrogen peroxide and iodine. The analyses of t h e products correspond, respectively, in general OZ, and t o t h e empirical formulas : C26H400, C ~ ~ H ~ OCioH160, Cp~H~oIOs.The substances are mostly resin-like. The author starts with C80Has and gives hypothetical structural formulas for rubber and all these oxidation products. VISCOSITY Considerable work has been done on the viscosity of rubber solutions, especially in studying the effect of heat. Harries12 Staudinger and Klever, B e y . , 44, 2212 (1911). Wissenschaflliche Verdfenllichungen aus dem Siemens-Konzern, I, Zweites Heft, 87 (1921); C. A , , 16, 3232 (1922). 12 Loc C i f . , p. 7. 10
11
ACTIONO F CONCENTRATED SULFURIC ACID Kirchhof l 4 has studied the action of concentrated sulfuric acid on rubber both in and out of solution. A precipitate is formed in solution which is insoluble in the ordinary solvents and appears to be a n isomerized rubber which is less unsaturated than rubber itself. Analysis showed only 0.9 per cent sulfur present. Its specific gravity is 1.094/20”. Kirchhof believes t h a t some of the double bonds have saturated each other, with the formation of tetramethylene rings, and proposes a spiral formula t o account for this. On t h e basis of his work he also proposes a new empirical formula, (C10H17)z,for the rubber hydrocarbon in Para rubber,15 as mentioned above, b u t the later work of Pummerer and Burkard shows conclusively t h a t his contention is not correct. Rubber on long standing in concentrated sulfuric acid gives oxidation products from which Kirchhof has isolated an acid with the molecular formula CzoHm03. 13An%., 406, 238 (1914). 14
16
Kolloid Z . , SO, 176 (1922); C. A . , 16, 1885 (1922). Kirchhof states that African rubbers have the usual CIOHIE ratio.
Heat Transfer Symposium A symposium on Heat Transfer is being arranged for the 1924 Spring Meeting of the AMERICANCHEMICAL SOCIETY. Those interested are requested t o write t o the chairman, W. H. McAdams, Massachusetts Institute of Technology, Cambridge, Mass., at their earliest convenience, indicating provisional title where possible, T h e final manuscript must be in the hands of the chairman by March 1, 1924. A trust fund for establishing a fellowship in biological chemist r y in the College of Physicians and Surgeons, Columbia University, t o be named in honor of the founder of t h a t department, William John Gies, will be presented a t its twenty-fifth anniversary, October 1, 1923. T h e committee will also present to Professor Gies an illuminated book containing testimonial letters of appreciation from former students, and from friends in this country and Europe.
