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LIGNININ CmLTJLosE-The lignin content of the cellulose prepared by the two methods agrees within the experimental error. This is to be expected, for in either case the chlorination was carried on until the sodium sulfite solution used for extracting the chlorinated product showed no discoloration. CY-, p-, AND Y - ~ E L L U L O S EIN CELLuLosE-The only marked difference between the effect of the two methods on acellulose is shown in black locust. P-Cellulose is considerably lower in black locust and catalpa cellulose prepared by the short chlorination method. From the limited amount of data available it appears that short-chlorinated cellulose is more stable than that prepared by the long method. A comparison of the results obtained from the short and long chlorination methods indicates that the reaction between chlorine and lignin is fairly rapid. The substance “lignin chloride,” soluble in sodium sulfite, must be removed from the surface of the wood particles. The “lignin chloride’’ seems impermeable to chlorine, for prolonged contact of the gas with the chlorinated sawdust has little effect in further removing lignin. This statement is based on the data of (1) the last samples of basswood, tanbark oak, and incense cedar
Vol. 16, No. 2
(Table I), in which two, three, and four sulfite extractions were made, respectively, and (2) the cellulose prepared by the short and long methods (Table 11) in which the number of extractions are equal. Prolonged chlorination does, however, break down the a-cellulose. CONCLUSION In preparing pulp for paper commercially, it is found that from 40 to 45 per cent of the wood is utilized. Approximately 60 per cent should be utilized, for that amount of cellulose can be isolated from the wood by the chlorination method. Thus, 25 per cent of the paper-making material is lost in commercial operations. It is this lost cellulosic material that the progressive paper manufacturer is striving to recover by modifying his pulping process. There are two objections to the chlorination method-the number of steps involved and the cost of the chlorine. If the amount of chlorine can be reduced in a commercial operation in the same proportion as reported in this paper, one of the objections against introducing the chlorination process for preparing pulp commercially will have been overcome.
The Solubility of Sulfur in Rubber’ By W. J. Kelly and K.B. Ayers GOODYEAR TIRS & R U B B E RCo.,A K R O NOHIO ,
*
N THE discussions of ratios of the concentrations The question as to whether or not rubber adsorbs sulfur has been the theories of vulcanin the two phases are condiscussed quite extensively in the literature. I n the following work stants, the system obeys ization which have been it is proved by a study of the distribution of sulfur between rubber and Henry’s law and the sulfur published in the past sevanother phase that the system obeys Henry’s law and hence the sulfur is dissolved by the rubber. eral years, many references is dissolved in and not adsorbed by the rubber. Figures are given are made to the fact t h a t Thus by plotting the correshowing the solubility at diflerent temperatures and degrees of vulsponding values of the conthe first step in the process canization. The bearing of this on theories of vulcanization is incentrations a straight line mas either the solution of dicated and the possibility of using the laws of chemical kinetics to passing through the origin sulfur in or its adsorption study the reaction between sulfur and rubber is pointed out. would be obtained. Even if by the rubber. No definite the sulfur were dissolved in Tv-ork t o show which of these processes actually took place was done until Skellon2 pub- the rubber it would be possible to have a curved line passing lished his experiments on the migration of sulfur in rubber. through the origin if the molecular weight of the sulfur in the As a result of this work Skellon concluded that the rubber rubber were different from that in the other phase. Assumdissolved the sulfur and hence did not adsorb it. Venable ing for the moment that in the liquid phase sulfur were present and Green3 also m’ade some similar experiments, but went as SSand in the rubber as Sd, we should have the reaction further and determined the maximum amount of sulfur which expressed by the equation the rubber would take up under certain conditions. Both sa e 2% these methods had one thing in common. The sulfur was taking place a t the interface, and instead of expressing allowed to diffuse into the rubber-in Skellon’s work from another piece of rubber containing a large excess of sulfur, Henry’s law as Cl = K and in Venable and Green’s experiments from the elementary (1) sulfur itself. Both authorities conclude from their work we should have to write that the rubber dissolves the sulfur. The conditions of the experiments do not permit the definite conclusions that the sulfur is not adsorbed, becausein both cases the sulfur might have diffused through the vapor phase and been adsorbed by the rubber. The best way to and the result would be a curve of the second degree. I n this determine whether sulfur is dissolved or adsorbed by the case it would be difficult to distinguish between solubility rubber is to study the distribution of the sulfur between and adsorption without resorting t o other means, as the Tubber and a known solvent for the sulfur in which the rubber adsorption isotherm is expressed by the equation is insoluble. In this way if an equilibrium exists between (3) t h e two phases it can be approached from both sides. If the 1 Presented before the Division of Rubber Chemistry a t the 65th Meetwhich, if n = 2 or 0.5, gives exactly the same curve as Equai n g of the American Chemical Society, N e w Haven, Conn., April 2 to 7. tion 2. 1922. For the experiments to be described in this paper n-butyl 2 I n d i a Rubber J . , 46, 251 (1913). alcohol was chosen as the liquid, and it was found that when THISJOURNAL, 14, 319 (1922).
I
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INDUSTRIAL A N D ENGINEERIhrG CHEMISTRY
February, 1924
the corresponding concentrations were plotted the result was a straight line passing through the origin. Thus Equation 1 is applicable to this system\ and it must be concluded that the sulfur is actually dissolved in tlie rubber and not adsorbed by it. Taken in conjunction with the work of Endres4 the proof of solubility is complete.
EXPERIMEKTAL Two methods were employed in this work and the equilibrium was approached from both sides. I n the first method the sulfur was mixed with the rubber and the mixture placed in amyl a coho1 and allowed to come to equilibrium. Portions of tlie solution were then removed and the concentration of the sulfur was determined. The total amount of sulfur in the alcohol could then be calculated, and this subtracted from the amount originally present in the rubber gave the concentration of the sulfur in the rubber. In the second method different amounts of rubber were placed in a definite volume of saturated solution of sulfur in butyl alcohol, and after equilibrium had been reached the solution was again analyzed. The sulfur lost from the solution was taken up by the rubber and its concentration could be easily calculated. These experiments were all carried out in a thermostat controllable to * 0.01" C.
148
According t o Henry's law, if one of the phases is saturated the other is also, and hence the intersection of the equilibrium line with the coordinate corresponding to saturation of the alcohol will give the solubility of sulfur in rubber a t the temperature employed. For butyl alcohol a t 30" C. the value a t saturation is 1.87 mg. per gram, and hence the solubility of sulfur in rubber is 10.1 mg. per gram. The solubility of the sulfur in amyl alcohol a t 40" C. was not determined, as immediately after the data in Table I were obtained amyl alcohol was discarded in favor of n-butyl alcohol, which was used in all the later experiments.
I
I AmOH
1 40"
0.8
+
.',
FIRSTMETHOD PROCEDURE-TO determine the amount of sulfur in the alcohol a known weight was taken and evaporated to dryness on the steam bath. The residue was then oxidized with nitric acid saturated with bromine and potassium chlorate and the sulfur precipitated and weighed as barium sulfate. REsmTs-The results of these experiments are given in the tables and figures. TABLE I-SULFUR ORIGINALLY PRESENT IN RUBBER (Samples in amyl alcohol a t 40' C.) Per cent Weight Total Total Sulfur in Rubber Rubber and Amyl Sulfur Sulfur Sulfur OrigiSulfur Alcohol Present in Amyl Rubber nally G. G. Ci0 G. G. G. C,b 19 3 4 0 . 0 0 . 3 9 5 0.0806 0.0158 0.0648 3 . 3 6 0'42 {20:0 3 9 . 7 0 . 3 9 4 0.0840 0.0156 0.0684 3 . 4 2 ll "
.--
0.466 0.455 0.497 0.499 0.681
0.0564 0.0576
0.0475 0.0493 0.0610 0.0708
0.783 gram alcohol. gram rubber.
0,0186 0.0176 0.0190 0.0195 0.0243 0.0282
0.0378 0.0400 0.0285 0.0298 0.0367
0.0426
5.36 5.56 5.70 5.92 7.29 8.50
8.68 11.5 12.6 11.5 11.8 10.7 10.7
TABLE 11-SULFUR ORIGINALLY PRESENTIN BUTYLALCOHOL (Different weights of rubber added t o 100 cc. of butyl alcohol saturated with sulfur a t 30" C. 25-cc. samples for analysis) Rubber K =G G. C la C2b c1 2 5.0 4 5.3 6 5.3 8 5.4 10 5.0 12 5.2 5.3 14 a Ci == mg. sulfur per gram alcohol. Cp =: mg. sulfur per gram rubber. ' I n d i a Rubber Would, 68, 635 (1923).
BETWEEN RUBBERA N D BUTYLAND AMYLALCOHOLS
OF S U L R U R
K'z
I n the experiment reported in Table I the sulfur was originally present in the rubber and was partially removed by the alcohol, and in Table I1 the equilibrium was approached from the opposite direction. These results are plotted in Fig. 1,the lines I and I1 corresponding t o the data in Tables I and 11, respectively. It is obvious that the points all fall fairly well on a straight line passing through the origin, and hence the conditions of Henry's law as stated in Equation 1 are fulfilled.
-.
DISTRIBUTION
Since this method is too cumbersome for the determination of the solubility of sulfur in a large number of samples, it was used only to establish the fact that the rubber dissolves C and does not adsorb the sulfur. ._ SECOND METHOD 8.50
Qll
7.05 39.8 7.20 40.0 5.0 40.0 5.18 39.6 5.03 1.21 39.8 1.41 5.02 39.8 a CI = mg. sulfur per Cz = mg. sulfur per
FIG.
As was stated above, if the alcohol is saturated with sulfur and equilibrium has been established between the sulfur in the alcohol and that in the rubber, then the rubber is also saturated with respect to the sulfur. Hence, it is only mcessary to leave a strip of rubber in contact with a saturated solution of sulfur in the alcohol for a sufficient length of time to establish equilibrium and then remove and analyze the rubber for sulfur. This method was followed in the experiments to be described and good results were obtained a t 40" and 55" C. At higher temperatures other complications arose which could not be eliminated, and these will be discussed later. PROCEDURE-%bS of rubber, vulcanized or unvulcanized, approximately 2 mm. thick and weighing about 2 grams, were first extracted with acetone and then suspended in a saturated solution of sulfur in butyl alcohol. The solution was contained in an aluminium vessel equipped with a cqver and having a stirrer on which the samples were hung. The rubber was kept near the top of the solution and an excess of sulfur was always present. The surface of the rubber sample was kept free of solid sulfur by employing a deep vessel with the butyl alcohol about 15 em. (6 inches) deep. If the solution is stirred slowly it will be kept saturated throughout without causing the solid sulfur to be agitated sufficiently to come in contact with the rubber. If any solid sulfur adheres to the rubber the determination is spoiled, as it cannot be completely removed. I n this way several samples could be suspended in the same solution as each piece of rubber formed its own system with
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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the solution. After the samples were in the solution long enough for equilibrium to be established, they were removed and placed directly in weighing bottles and weighed. They were then vacuum-dried and weighed again in order to determine the amount of butyl alcohol in them and thus make a correction for the sulfur in this amount of alcohol. The rubber was then extracted with acetone and the sulfur thus obtained was oxidized, precipitated, and weighed as barium sulfate. The acetone extraction was made so that the sulfur taken up by the rubber from the alcoholic solution could be separated from the sulfur combined with the rubber, for which a correction would have to be made if the total sulfur of the sample were determined. REsums-The solubility of sulfur is calculated on parts of sulfur per 1000 parts of rubber. TABLE111-SOLUBILITYOB SULFUR IN RUBBER Rubber Temp.
c.
Sulfur Mix
40
100 : 10
55
100 :10
55
100 : 30
Cure at 40 Lbs. Hours
Coefficient of
Vulcanization Curve 1, Fig. 2 1 1.46 2 2.98 3 4.34 4 6.84 5 7.95 Cttrve 2, F i g . 2 0 0 1 1.46 2 2.95 4.34 3 4 6.54 Curve 3, F i g . 2 3 5.47 8.72 4 13.92 5 Curve 4. Fig. 2
Soltrbility
G./1000 G . Rubber 14.8 14.5 15.3 16.1 16.9
19.6 21.3 22.4 23.9 25.6 23.6 26.4 30.0
The figures for 75" C. are included in the table for one mixing and are plotted in Fig. 2 , Curve 4, although there is some doubt as t o their accuracy. By tbe method employed it is not possible to obtain reliable results at the higher temperatures because the rubber depolymerizes or is oxidized and
Vol. 16, No. 2
becomes soluble in the butyl alcohol. This complicates the analysis of the samples to such an extent that no confidence can be placed in the results at prespt. It is hoped that these difficulties may be overcome and reliable figures obtained for the solubility of sulfur in rubber at higher temperatures. Working according t o Venable and Green's method it is not possible to determine the solubility of the sulfur onsccount of the extreme slowness of diffusion when the coefficient of vulcanization is more than about 7. The results recorded here, however, prove that the solubility of sulfur increases linearly with the coefficient up to a value of the latter of 17, or up to the point where about 35 per cent of the double bonds of the rubber are saturated with sulfur. There is a slight difference in Curves 2 and 3, which is possibly due to experimental error. However, it may also be due to differences in degree of aggregation of the rubber, in which case the mixing which is disaggregated more should dissolve more sulfur. I n these two stocks the one for which the points on Curve 2 were obtained contained less sulfur, originally and hence would vulcanize more slowly. This would result in a more disaggregated rubber and thus the solubility of the sulfur should be greater. This explanation is offered only tentatively until sufficient data are a t hand to prove it. The valae for the solubility of sulfur in rubber changes very slightly at room temperature as the cure is prolonged. At 30" C. it is about 1 part per 100 of rubber, and hence if a stock is vulcanized so that it contains not over 1 per cent of true free sulfur it cannot bloom out.
CONCLUSION The fact that rubber dissolves and does not adsorb the sulfur has a direct bearing on the theories of vulcanization. I n the f i s t place it is not possible for one substance to dissolve and at the same time adsorb a second substance. Thus there is only one way to have sulfur adsorbed by the rubber and that is to convert it to the form which is insoluble in the rubber. This might happen in rubber through the reactions between sulfur and the resins, proteins, or accelerators. It has, however, been shown by Twiss6 that there is no difference in the rate at which S, and SA vulcanize, and also that when a stock containing only the insoluble form is vulcanized, all the free sulfur is soluble in acetone after vulcanization. This is also in agreement with the results obtained by end re^,^ who showed that the rhombic or soluble form is the most stable of the forms of free sulfur in rubber. Hence, instead of insoluble sulfur being formed in rubber, the opposite takes place and the insoluble form is converted to the soluble. Thus the sulfur-rubber mixture, being a solution, must be homogeneous with respect to these two substances at all concentrations of sulfur below the saturation point, and as this point is rather high at vulcanizing temperatures it should be possible t o apply the laws of chemical kinetics to the system, and thus obtain some insight into the mechanism of the reaction. 6
J. SOC.C h e n . I n d . , 36, 787 (1917).
Thomas T. Read Honored
Coefficient of Vulcanizahon FIG.2-~0LUBILITY
OF
SULFUR
IN
RUBBER
Thomas T. Read, supervising mining engineer of the Bureau of Mines, Department of the Igterior, has been awarded a gold medal and a commemorative diploma by the Government of Brazil. Dr. Read was the representative of the Department of the Interior with the United States commission t o the celebration of the 100th anniversary of the independence of Brazil at Rio de Janeiro last year, and was also the special delegate of the American Institute of Mining and Metallurgical Engineers t o the Federated American Engineering Congress held in connection with the celebration.
