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VULCANIZATION OF RUBBER '
ERNST A. HAUSER
Massachusetts Institute of Technology, Cambridge, Massach.usetts AND
D. S . LE BEAU Midwest Rubber Reclaiming Co., East St. Louis, Illinois Received December 4, 1946 INTRODUCTION
A considerable amount of work has been done by many investigators to obtain a better insight into the reaction of vulcanization, but so far no fully satisfactory answer has been obtained. It is known, for example, that the tensile strength at optimum cure increases with increasing chain lengths of the molecules, and it has also been found by chemical analysis that the number of sulfur atoms combined per double bond of the rubber molecule apparently increases with the molecular weight of the rubber subjected to vulcanization (6). The importance of the a-methylenic group for the combination of sulfur has also been demonstrated (1, 2). However, very little is still known in regard to the effect of zinc oxide and the actual function of accelerators for vulcanization. One must even frankly admit that, in spite of all the work done so far, we still do not know whether maximum physical properties of vulcanized rubber depend on the combination of either one, two, or three atoms of sulfur per double bond, or if the number of combined sulfur atoms has any bearing a t all on the differences reported for some properties of vulcanizates. This proves that chemical analysis alone is not sufficient t o elucidate the process of vulcanization and that other methods of investigation must therefore be introduced into any further studies. To avoid unnecessary complications a t the very outset of such attempts, it was considered advisable first t o limit the work to the vulcanization of a pure gum stock, Le., without incorporating zinc oxide or vulcanization accelerators. It was also felt that the first systematic study should be carried out by making use of the Peachey (9, 10) process, for the following two reasons: This method of vulcanization is one in which sulfur becomes available in statu nascendi; the reaction is carried out in solution and a t room temperature, thus permitting its study with various molecular-weight fractions of the rubber hydrocarbon. The possibility of such an approach was demonstrated for the first time when natural rubber deposited from solutions was used in studies with the electron microscope. However, it was soon found that the rubber fibres so formed, when exposed to the electron beam, lose their elasticity, become brittle, and rupture (3). With the introduction of the new microscopic technique using ultra-illumination by incident light (4,5 ) , a systematic study of the vulcanization reaction seemed possible. 171
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172
ERNST A. HAUSER AND D. S. LE BEAU EXPERIMENTAL PROCEDURE
Two hundred grams of prime ribbed smoked sheet rubber (Hevea brasiliensis Muell. arg.) were milled for 9 min. on a 6 x 12 in. rubber mill with a friction ratio of 1: 1.4 and a gap of 0.2 in. (mill temperature = 135°F.). Then the sheet was cut into small pieces and dissolved in benzene. The solution was subjected t o cold vulcanization with the same equipment used for making the preparations for electron-microscope studies (3). The preparation of the samples for microscopic investigation was carried out as previously described ( 5 ) . Since the results obtained with the electron microscope (3) demonstrated that the presence of the acetone-extractable matter in the rubber does not change its morphology, no attempt was made t o remove it (7, 11). The acetone extract of the preparation amounted t o 2.6 per cent. The samples were analyzed for free and total sulfur by the conventional methods.
TOTAL SULFUR CALCULATED FROM GAS REACTION
REST PERIOD BETWEEN GAS REACTION AND EVAPORqTION OF SOLVENT 2 HR. AT 25°C.
Concentration = 1.8 per cent
Total sulfur by analysis
per cent
per cenl
7.0
1.3
13.3
3.8
17.5
6.4
30.0
19.0
1
TOTAL SULFUR CALCULATED FROY GAS REACTION
Free sulfur by analysis
1.8 5.0
Concentration = 1.8 per cent
Total sulfur by analysis
per cent
0.3 0.9
REST PERIOD BETWEEN REACTION AND EVAPORATION OF SOLVENT 24 HR. AT 25'c.
,
per cent
per cent
3.4 5.6 9.4 14.0 24.0
1.0 2.1 4.8 9.1 18.9
~
Free sulfur by analysis
per cent
0.25 0.5 1.1 1.7 5.0
ANALYTICAL DATA
As shown by table 1, the reaction, contrary t o previous assumptions (3), is not instantaneous but depends on the time interval between the introduction of the reacting gases and the evaporation of the solvent prior t o the chemical analysis of the sample (table 1). It was furthermore found that the amount of sulfur produced and combined with the hydrocarbon depends on the concentration of the rubber solution (table 2). I n the 2 per cent solution, analysis reveals that one atom of sulfur is bound per double bond. However, as the percentage of total sulfur increases above 10, analysis becomes more and more difficult. This may possibly be due t o the difficulty in evenly distributing the gases during the reaction, particularly if a great, increase in the viscosity of the solution occurs. As can be seen from the data in tables 1and 2, it was not possible t o account for the differences between the amounts of sulfur actually formed by the reaction and those called for by theoretical calculation. The possibility that this deficiency might be due t o some imperfection of the equipment finally had t o be eliminated, when it was found that even greater amounts of sulfur could not be
173
VULCAXIZATION OF RUBBER
accounted for when working with low concent,rations of rubber. This discrepancy might be explained by assuming that some of t,he gases, particularly at low concentrations of rubber in the solutions, are not adsorbed on the rubber and are thus lost during evaporation. This possibility is indicated by the fact t>hntno sulfur is formed from the gases in pure solvents (8). However, when rubber is introduced into the solvent, the reaction occurs. MICROSCOPIC OBSERV.\TIONS
It>was decided t o study the differences listed in table 1, i.e., to follow the course of progressive vulcanization by the microscopic method recently developed (4, 5 ) . Figure 1 shows the original sample as obtained from a 2 per cent solution of prime ribbed smoked sheet in benzene after milling and before vulcanization. The formation of blobs and strings, characteristic of total rubber, is evident. Figure 2 shows a sample prepared from the same smoked sheet solution after TABLE 2
!
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SULFUR CALCLIL9TED FROM G A S REACTION
per cenl
2.0 3.0 5.0 15.6 20.0
i '
2 PER CENT SOLUTION: 24-HR. REST PERIOD BETWEEN GAS REACTION AND EVAPORATION OF SOLVENT AT 27°C.
0.5 PER CENT SOLUTION: 24-HR. REST SULFUR CALCULATED FROB GAS REACTION
Total sulfur by analysis
Free sulfur by analysis
per cenl
per cenl
per cenl
0.9
0.2 0.3 0.6 2.7 4.4
2.5 6.5 10.0 11.8 25.5
1.6 3.6 13.3 16.8
PERIOD BETWEEN GAS REACTION AND EVAPORATION OF SOLVENT AT 27'C
Total sulfur by analysis
Free sulfur by analysis
per cent
per cenl
0.3 0.7 1 .-I 2.1 9.2
0.1 0.2 0.4 0.5 1.9
enough gases had been passed through t o produce a compound containing 1.7 per cent of sulfur, according to calculation. This solution was deposited on the \\.ire gauze 3 hr. after the gases had reacted. At the time of depositing the solution on the gauze, no visible change in its consistency could be observed. Figure 3 shows a sample prepared from the same solution la hr. after the gas reaction had occurred. The number of blobs has greatly diminished. It also should he stated that the film, although still spreading perfectly, dried much more rapidly than previously experienced. Figure 4 shows a sample prepared from the same solution 3 hr. after the gas reaction. A pronouncedly coarser network results. Spreading of the film is still satisfactory, but drying occurs very rapidly. Figure 5 shows a sample prepared from the same solution 7 hr. after the gas reaction. The film deposited on the wire gauze only breaks to a pronounced network containing some very small blobs in a few places, although the solution still spreads on the water surface. Further prolonged storage of the solution after the gas reaction had taken place resulted in the formation of continuous films only. The solution had
VULCANIZATION O F RUBBER
175
reached the consistency of a very weak jelly, which did not spread too well and dried extremely rapidly. If a sample was prepared from the same smoked sheet solution after enough gases had been passed through to produce a compound containing sulfur in the amount of 2.6 per cent by calculation and the solution was deposited on the wire gauze $ hr. after the gases had reacted, no visible changes in the consistency of the solution could be observed a t the time the sample was prepared. Although a greater amount of network can be observed, this sample is not much different from the corresponding sample containing 1.7 per cent of calculated sulfur (see figure 2). The sample prepared from the same solution but 3 hr. after the gases had reacted again shows similarity with the corresponding sample containing 1.7 per cent calculated sulfur (see figure 4). The sample prepared from the same solution, but 7 hr. after gas reaction, became difficult to spread. Further prolonged storage of the solution between gas reaction and sample preparation resulted in a slimy gel which could not be spread a t all. If deposited on wire gauze, the films stayed homogeneous and would not break. If enough gases had been passed through t o produce a compound containing 15 per cent of calculated sulfur, the film would not spread thin on water even a t a short time interval. After a 14-hr. time interval, the solution was completely gelled and would not spread a t all on water. It can also be noticed that the rubber in this preparation has lost its elastic behavior and has become brittle. Microscopically this phenomenon became noticeable only after approximately 25 per cent of sulfur had been introduced into the solution. One phenomenon still deserves to be mentioned in this connection. When studying a great number of preparations, one is led to the conclusion that increasing cross linkage of the molecules effected either by random polymerization or by vulcanization results in the formation of films which have decreasing tendency to break, and which, if they do break, show different morphological characteristics. This is understandable, because cross linking will reduce the contraction of the film when breaking and result in the formation of a network of decreasing and smaller mesh openings, and the faster drying of the partially vulcanized films upon spreading is readily explainable by the mechanism of syneresis. FIG.1. Original sample as obtained from a 2 per cent solution of prime ribbed smoked sheet in benzene after milling and before vulcanization. Magnification X 3000. FIG.2. Sample prepared from the same solution as in figure 1after enough gases had been passed through t o produce a compound containing 1.7 per cent of sulfur. (a) magnification X 1000; (b) magnification X 2600. FIG.3. Sample prepared from the same solution as in figure 1, I$ hr. after the gas reaction. Magnification X 2600. FIG.4. Sample prepared from the same solution as in figure 1 , 3 hr.after the gas reaction. Magnification X 2600. Fig. 5. Sample prepared from the same solution as in figure 1 , 7 hr. after the gasreaction. Magnification X 2600.
176
JOHN J. MCBRADY AND ROBERT LIVINGSTON SUMMARY
A combination of chemical and microscopic methods has been used to follow the progressive vulcanization of rubber. Microphotographs using ultra-illumination by incident light of rubber vulcanized by the Peachey process, containing different amounts of sulfur, show that vulcanization in solution is not an instantaneous reaction but that the changes in the morphology of the preparation depend on the concentration of the rubber in the solution and on the time which has elapsed between the occurrence of the gas reaction in the solution and the deposition of the rubber film therefrom on the wire gauze. The authors wish to express their thanks t o Dr. F. 0. Schmitt, Department of Biology, Massachusetts Institute of Technology, for the use of the Peachey apparatus. REFERENCES
(1) FARMER, E. H.: Trans. Faraday S O C .38, 356 (1942). (2) FARMER, E.I%.,AND MICHAEL, S.E. : J. Chem. Soc. 1942,513. (3) HALL,C. E., HAUSER, E. A , , LE BEAU,D. S.,SCHMITT, F. O., AND TALALAY, P . : Ind. Eng. Chem. 36, 634 (1944). (4) HAUSER, E . A.,AND LE BEAU,D. S.: Ind. Eng. Chem. Si', 786 (1945). (5) HAUSER, E.A.,AND LE BEAU,D. S.: Ind Eng. Chem., in press. (6) HAUSER, E. A., LE BEAU,D. S., AND SHEN,Y . : Rubber Age 68 (l), 59 (1945). E.A.: India-Rubber J. 108,584 (1945). (7) HAUSER, (8)HOCK, L.,AND SCHMIDT, H . : Kautschuk 10, 82 (1934). S.J.: India-Rubber J. 63,427 (1922). (9) PEACHEY, (10)PEACHEY, S.J., AND SKIPSEY, A . : J . SOC.Chem. Ind. 40,5T (1921). (11) STEVENS, H.P . : India-Rubber J. 108, 325 (1945).
THE FORMATION OF TETRAVALENT URANIUM DURING THE URANYL-SENSITIZED PHOTOCHEMICAL DECOMPOSITION OF OXALIC ACID' JOHN J. McBRADY AND ROBERT LIVINGSTON School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received J a n u a r y 31, 1946
In the absence of air, the uranyl-sensitized, photochemical decomposition of oxalic acid is accompanied by the irreversible formation of tetravalent uranium. This side reaction amounts to 1 per cent of the total reaction, when the actinoThis paper is based upon a dissertation submitted by J. J. McBrady t o the Faculty of the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy. While the experiments were concluded in November 1942,their publication has been delayed, owing t o wartime conditions.
