Thinning and Gelation of Rubber Cements Practical Methods of Inhibiting the Effects of Light and Heat C. K. NOVOTNY, Firestone Tire & R u b b e r Go., Akron, Ohio
T
HE causes of changes in the viscosity of rubber bolutions under various conditions of aging have been the subject of considerable investigation. The natural deterioration of rubber and the action of copper in accelerating this deterioration was early recognized, Fickendey (6) showed that the degradation of a rubber solution occurring in the sunlight was arrested if the oxygen was removed. Bernstein (4) concluded that the action of ultra-violet light on rubber solutions brought about a reduced viscosity in the absence of oxygen. This theory was widely supported. Van Rossem, about this time, concluded as a result of his experiments that oxidation of rubber in solution was always a secondary change preceded by depolymerization and that this depolymerization by heat is catalyzed by oxygen. Porritt (9), wliile investigating balloon fabrics, used solutions to follow the effect of light on rubber. He found that the use of Soudan I11 as a dye in a rubber solution greatly improved the resistance to the degrading effects of light. I n 1925 Asano ( I ) conducted a research on the action of ultraviolet light on rubber. According to his experience, raw rubber became partially insoluhle in all rubber solvents when exposed to ultra-violet light. He carried out exposures in
solutions to ultra-&let light and the change brought about by heating the rubber solutions to 38" C. in the dark were studied by Garner (8). Both conditions caused a lowering of the viscosity, but the light brought it about a t a much more rapid rate. The degradation of rubber solutions as a function of time a t different temperatures was followed by Bary and Fleurent (W),whose work shows that with increase in temperature there is an increase in the rate of degradation, and the viscosity approaches that of the solvent. Fujihara ( 7 ) followed the viscosity change of rubber solutions in benzene, exposed to ultra-violet light, and found them to decrease to a constant viscosity which was very near that of the solvent. He found that rubber sols in various other solvents behaved siniilarly, but that the change occurred a t different rates of speed. Where carbon tetrachloride was used, he obtained a precipitate during the exposure. Following publications by other investigators, Bary and Fleurent ( 3 ) carried out experiments on rubber solutions which were placed in tubes that had been carefully evacuated to free them from oxygen. I n spite of all precautions, there were a few cases where some oxygen was present, as was made apparent by the behavior of the solutions. The tubes were stored a t elevated temperatures (48" and 98" C.). They found that the degradation of rubber solutions no longer approached a limit corresponding to a relative viscosity of unity or that of the solvent, as had been previously observed in the presence of oxygen, but approached a limiting degradation. The results show that the greater the concentration of the rubber solution and the lower the temperature of heating, the smaller is the magnitude of the total degradation. Recently Dufraisse (6) published a review, with an excellent bibliography, on the oxidation of rubber, including work done in solutions. I n this article he pointed out the practical difficulties of carrying out experiments with the exclusion of air, and the fact that traces of oxygen of about 1 1 part in 100,000 are sufficient to start the degradation of t h e -. rubber. The difficulty in reducing the concentration of OXYI OL0 04 8 /Z /6 2.0 y gen below this point may account for some of the conflicting Gum ConfenC ( Z j results which have appeared in the literature on the degradaFIGURE 1. VISCOSITIES OF CEMENTSEXPOSED TO DIFFUSED tion of rubber, in the past. Present-day investigators agree DAYLIGHT FOR 52 DAYS concerning the harmful effects of minute quantities of oxygen inert gases and in air, and found that in all cases the rubber on the stability of rubber solutions. I n the present investigation, the experimental work was was rendered slightly insoluble. Separating the soluble portion in each case and preparing a solution of known directed toward practical ways and means of retarding the strength, he found that the viscosity was the same for all thinning of rubber patching cements under ordinary atmosrubbers which had been exposed under the different condi- pheric conditions. Two cements were used: a noninflammable cement contions, but that their viscosities were considerably lower than t h a t of the original rubber. He stated that light below taining 2 per cent of Gristly crepe rubber by weight, the 2250 A. had a special action on rubber, giving a white, opaque, solvent being composed of 70 per cent carbon tetrachloride polymerized rubber, while above 2250 A. an oxidized trans- and 30 per cent gasoline; the other containing 6 per cent by parent rubber was formed. I n the course of his investigation, weight of slightly milled Gristly crepe rubber in gasoline. he used yellow, orange, and red filters as a means of protecting The viscosity of these cements, as made up, was equivalent to approximately 11 poises. This viscosity will hereafter be rubber from the effect of light. Porritt and Frye ( I O ) found that heating rubber solutions referred to as the standard and all viscosities given will be in the presence of air permanently lowered the viscosity, relative to it. while heating after thorough evacuation of the containers EXPERIMENTS IK STSLIGHTAXD DIFFUSED DAYLIGHT did not bring about any appreciable change. They clearly Samples of the carbon tetrachloride and gasoline cements showed that a small quantity of oxygen was sufficient to were put up in clear glass bottles. Half of the bottles in cause a great deal of change. The change of viscosity brought about by exposing rubber each group were exposed to direct sunlight on the roof of the 170
Februarl-, 1934
ISDL-STRIIL IYD E N G I N E E R I N G CHELIISTRY
171
laboratory and the other half were placed in the diffused day- from becoming thin ab a r e d t of the action of light. Allight of the laboratory. The results in direct sunlight showed though brown bottles are not? a> effective for this purpose as that the carbon tetrachloride cement thickened to a rigid gel red bottles, they are more readily available and lover in after 2 hours' exposure, while the gasoline cement became cost. solvent-thin after 7 days' exposure. I n a few cases, during a EFFECTO F HEAT series of tests, samples of gasoline cements were mcountered which tended to gel after 7 days' exposure. However, the The next work was done on a means of stabilizing rubber general tendency for gasoline cements was to become thin cements not only against light but also against lieat a t the when exposed to direct sunlight. It has been suggested that same time, Although the thinning in cements caused by heat the cause of gelation may be the action of ultra-violet light on is not as troublesome as that caused by light, this factor must traces of sulfur or sulfur-containing compounds normally be taken into account in hot climates. The use of filters or found in gasoline, thus bringing about a cure. The author metal containers mill not eliminate the trouble caused by has no evidence in substantiation of this hypothesi.. Whether heat. Carbon tetrachloride cements stored for 21 days in these occasional gelations of gasoline cements may he due to the dark a t 50" C. (122" F.) showed a decrease t o 0.3 of the polymerization is open to question. I n the case of carbo11 original viscosity. Gasoline cement (containing 6 per cent tetrachloride, the cause of the gelation which took place rubber) stored for 21 days a t 50" C. had a viscosity 0.4 that every time the cement was exposed to sunlight can be ascribed of its original. The trend of these results is in agreement n-ith to a cure brought about by ultra-violet light and the chlorine or those already appearing in the literature. chlorine compounds produced by the decomposition of carbon tetrachloride under the action of the actinic rays. Positive MASTICATIOX OF THE RUBBER tests for water-soluble chlorides were obtained on samples of Many investigators have mentioned the fact that the more carbon tetrachloride and the carbon tetrachloride cements rubber is milled, up to a certain limit, the lower will be the after they had been exposed to the sunlight, n-liereas there viscosity of the solution prepared from a definite amount of the r e r e no indications of these materials in identical tests made milled rubber. It seemed that if cements were made from hefore the exposure. rubber which had been milled for various lengths of time, In experiments carried out in diffused daylight, carbon and increased amounts of rubber were used to make the cetetrachloride cements became solvent-thin after 21 clays of ments up to the desired viscosity, their stability might be exposure, and began to gel if allowed to stand for about 110 greater against light and heat, because the rubber had already days. The action here is very similar to that which takes been subjected to a certain amount of breakdown. place in direct sunlight, but occurs a t a very much slower rate, h series of cements having practically identical viscosities and a degradation of the rubber takes place, permitting the was made up, using gasoline as the solvent for Gristly crepe cement to become exceedingly thin prior t o gelation. I n the rubber which was given the following treatments: case of the gasoline cement, it was found that after 21 days' GUMCONTENT TREATMENT exposure in diffused daylight, the viscosity was 0.7 of the bv weight original, and in 45 days it wa3 only 0.1 of the original. 3.5 Gristly crepe
EFFECTO F LIGHTF I L T E R S
O S VISCOSITY CHANGES
Since the carbon tetrachloride cement was very suweptible to light, it was decided to test the effect of various light filters upon it. Although it is obvious that a metal can or some other nontranslucent container would eliminate the trouble caused by light, a transparent bottle was considered a more attractive package. The substances used as filters were 4ieets of colored Cellophane, in purple, blue, green, and red. The cement was placed in clear glass bottles, which were then wrapped with sheets of the colored Cellophane. A brown bottle filled with the same cement was also tested in this experiment. The samples were placed in direct sunlight during the middle of the summer. The results are shown in Table I. T4BLE
I. OBSERVATIONS
FILTER Cuntrol, no filter Blue Cellophane Purple Cellophane Brown lase Green 8ellophane Red Cellophane
D suble layer of red Cellophane
CEMENTS EXPOSED TO DIRECT SUKLIGHT ON
EFFECT O N V I S C O S I T Y
Gelled after 2 hours' exposure Gelled after 45 hours' exposure Gelled after 45 hours' exposure Stringy after 100 hours' exposure Thinned t o 0.15 of original viscosity after 200 hours' exposure Thinned t o 0.59 of original viscosity after 200 hours' exposure Thinned t o 0.82 of original viscosity aiter 200 hours' exposure
Following the above test, two clear glass bottles were filled nith gasoline cement and one was wrapped in a sheet of red Cellophane. The bottles were placed in direct sunlight, and it was found that the control became solvent-thin in 6 days, whereas after 30 days the cement with the red Cellophane filter had a viscosity 0.9 of its original. Since the conditions of this test were severe, it is safe to say that a red wrapper, red-coated bottle, or red bottle of the proper shade would be highly beneficial in preventing cements
3.5 3.8
One pass through tight cold rolls Two nasses throueh tight cold rolls
Samples of these cements were placed in clear glass bottles and exposed to diffused daylight for 52 days. The results clearly demonstrate the effect and value of milling rubber to a certain extent before making it up into a cement, in order to slow up considerably the thinning or decrease of viscosity. d graphical representation of the behavior of thew cements may be seen in Figure 1. A numerical interpretation shows that the relative viscosities of the cements were: Unmilled rubber, solvent thin One pass through mill, 0.1 of original viscosity T r o passes through mill, 0.3 of original viscosity Four pasres through mill, 0.53 of original viscosity Eight passes through mill, 0.73 of original viscosity Ten passes through mill, 0.86 of original viscosity Sixteen passes through mill, 0.96 of original viscosity When the cements with increased gum content which had heen more stable under diffused daylight were subjected to direct sunlight or storage a t 50" C. (122' F.),their stability in relation to their rubber content followed very closely, on a relative basis, those results obtained on aging in diffused daylight. Extensive tests were not run on the cements stored at cxlevated temperatures, but a few runq were made to check w n e of the results obtained on aging in diffused daylight. USE O F h T I 0 X I D a 4 S T S I N C E J f E N T S
Another attempt t o stabilize cements against the degradation brought about by elevated temperatures was made by incorporating various compounds in cements. The corn-
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INDUSTRIAL AND ENGlNEERING CHEMISTRY
pounds tried were: p-amidophenol, m-toluylenediamine, p-hydroxydiphenyl, hydroquinene, phenyl-@-naphthylamine, triacetin, nitrobenzene, and symmetrical di-b-naphthyl-pphenylenediamine. Although some of the materials, such as p-amidophenol, m-toluylenediamine and triacetin, caused the cements to thickell \%,henstored at 250 C,, they did not retard the degradation brought about by storing at 50" C. Di-pnaphthyl-pphenylenediamine showed possibilities of retarding thinning caused by elevated temperatures, but precautions had to be taken to exclude light a t all times, since otherwise a photochemical reaction took place which caused the cement to thin rapidly and turn a dark brown color. This material is usable in cements which are put up in metal cans or tubes.
Vol. 26, Xo. 2
mission and exceedingly helpful suggestions made this paper possible. LITERATURE CITED Asano, K., India Rubber J . , 703 307, 337, 389 (1925). Bary, P., and Fleurent, E., Rubber Chem. Tech., 4, 505 (1931). Ibid,, 6 , 111 (1933). Bernstein, G . , K o u o ~ ~ z12, . , 183 (1913). ( 5 ) Dufraisse, C., Rubber C h e m . Tech., 6, 157 (1933). (1) (2) (3) (3)
k;
(1832j,
(8) G
~T, L,,~lnst. ~~ ~~Ind,bT~ ~b ~,4, 413 ~~ ~(1927). ~. , (9) Porritt, B. D., India Rubber J . , 60, 1159 (1920). (10) Porritt, B. D., and Frye, J. D., Inst. Rubber I n d . Trans., 3, 203 (1E)23).
ACKNOWLEDGMENT The author wishes to express his sincere appreciation to N. A. Shepard, J. N. Street, and C. R. Park, whose per-
RECEIVEDSeptember 20, 1933. Presented before the Division of Rubber Chemistry a t the 86th Meeting of the American Chemical Society, Chicago, 111.. September 10 t o 15, 1933.
Removal of Thiophene from Benzene HARRY N. HOLMES AND NORVILBEEMAN, Oberlin College, Oberlin, Ohio
E
VER since Meyer discovered thiophene in 1882 (IS), there has been a demand for thiophene-free benzene which is needed, as pointed out by Ardagh and Furber ( 2 ) ,chiefly for the manufacture of dyes and for the preparation of certain c. P. chemicals. The first mention made of sulfuric acid as a means of removing thiophene from benzene appeared in the report of Meyer in 1882 ( I S ) . He called attention to the fact that common coal-tar benzene, after boiling for 10 hours with concentrated sulfuric acid on a water bath with a reflux condenser, failed to give the familiar Baeyer indophenin reaction with isatin. Since that time the sulfuric acid treatment has been used extensively. Deniges ( 5 ) , Dimroth (6), Paolini and Silberman (14), and Ardagh and Furber ( 2 ) have made use of such salts as mercuric acetate and mercuric stearate in thiophene removal. Dutt and Hamer (7) claim that chlorination will free benzene from thiophene. A light treatment with sulfuric acid, followed by chlorination, is said to be used now in the United States. ANHYDROUS ALUMINUM CHLORIDE TREATUENT In 1894 the Soci6t6 Anonyme des MatiBres Colorantes de St. Denis (Paris) took out patents for the process of removing thiophene from commercial benzene by warming it with 5 to 20 per cent anhydrous aluminum chloride (15). Two years later Haller and Michel (9) reported the results of refluxing, fur a half-hour, benzene containing thiophene with from 0.1 to 5 per cent of aluminum chloride. They stated that refluxing is not essential, for they agitated benzene, as free as possible from water, with aluminum chloride in the cold and obtained a red substance which deposited as a viscous liquid. Since they preferred refluxing, it would seem that their method of agitating in the cold must have been less effective. Heusler ( 1 1 ) described a process of heating benzene containing thiophene for approximately 9 hours in a reflux apparatus with about 5 per cent aluminum chloride. He called attention t o the reddish mass which separated and to the fact that the reaction appeared to start even before heating. At the end of his report, announcement was made of a British patent ( I O ) covering the use of anhydrous aluminum chloride and other metal chlorides for the removal of thiophene from benzene, with concentrations between 5 and 20 per cent over a temperature range of 100" to 600" C.
NATUREOF
THE
REACTION BETWEEN ALUXINUM CHLORIDE AND
THIOPHENE
Heusler suggests that an intermediate compound is formed (probably red in color) between the thiophene and the aluminum chloride, a product somewhat soluble in benzene and decomposed by water with regeneration of thiophene. Boedtker (4) observed, in the Friedel-Crafts reaction with benzene, the liberation of hydrogen sulfide and hydrogen chloride, and attributed these products to the reaction between the thiophene in the benzene and the aluminum chloride. I n the present work it was observed that, when benzene containing thiophene, after drying several days over fused calcium chloride, was treated with anhydrous aluminum chloride, there was an evolution of hydrogen sulfide arid hydrogen chloride a t the time when the mixtures were distilled after decanting from the reddish sludge a t the bottom of the reaction flasks. This was observed following contact over a range of 6" to 65" C.
ESTIMATION OF THIOPHENE IN BENZENE The first test for thiophene was the indophenin test discovered by Baeyer, depending upon the blue color obtained by reaction between thiophene and the isatin dissolved in sulfuric acid. It was applied erroneously as a test for benzene until Meyer isolated thiophene and demonstrated that it was this compound, present as a n impurity in coal-tar benzene,' that was responsible for the color reaction. This test was later modified and improved until it can now be used to detect concentrations as low as 0.0005 per cent ( I , 3, 16). The sensitiveness of the test depends upon the presence of some oxidizing agent, such as ferric chloride or nitric acid. It has been the authors' experience that isatin solution shaken with air, but without nitric acid, produces a satisfactory color reaction within an hour. I n testing for the last traces of thiophene it was found advisable to shake thoroughly 25 cc. of the benzene to be tested with 2 cc. of c. P. concentrated sulfuric acid. When the layers separated, 1 cc. of the sulfuric acid layer mas drawn off, and to this was added by pipst 0.1 cc. of a sdution of isatin 1 Very impure bensene may contain as much as 0.6 per cent by weight of thiophene (Richter, V. von, "Organic Chemistry," Vol. 111, p. 21, Blakiston, 1923), b u t t h e usual water-white benzene purchased by this laboratory was seldom found t o run higher than 0.05 per cent.
