Diffusion of Sulfur in Rubber RELATION TO VULCANIZATION A. R . KEMP, F. S. MALM, G. G. WINSPEAR, ..iXD B. STIRATELLI Bell Telephone Laboratories, New York, N. Y .
The absorption of sulfur by various types of rubber was determined at 23' to 86'' C. The absorption technique is unsatisfactory for determining the solubility of sulfur in rubber. The rate of diffusion of sulfur through masticated crepe and vulcanized rubber was determined by the cell method at 58' to 96" C. The solubility of sulfur in masticated crepe rubber was investigated in milling experiments, and the results agree closely with those of Williams. The rate of solution of sulfur in rubber during
A
SURVEY of the literature failed to disclose any previous attempt to determine the rate of diffusion of sulfur through rubber. The present paper is intended to fill this gap and to show the importance of diffusion and solubility of sulfur in rubber upon the control of the vulcanization reaction. The vulcanization of rubber cannot be considered as a homogeneous chemical reaction since the reactants-i. e., sulfur, accelerators, and activators-are seldom completely in solution or uniformly dispersed. The limited solubility of sulfur, many accelerators, and zinc soaps in rubber leads to the presence of these substances partly in solution and partly in the dispersed state during vulcanization. Local high concentrations of these substances occur in solution around the dispersed phase, owing to their increased solubility which results from the temperature rise during vulcanization, coupled with their slow rate of diffusion through rubber. This paper presents the results of a detailed study of the effects of solubility, diffusion, and crystallization of sulfur on heterogeneity of vulcanization. Results showing the effect of various methods of mixing and handling on the physical properties of certain rubber compounds are presented. The diffusion of sulfur into rubber to form a solution during vulcanization, followed by the separation of globular sulfur upon cooling and its transition into dendritic and finally into rhombic sulfur upon standing, was observed under the microscope by Loewen (9, I O ) and Endres ( 3 ) . Several factors which influence the migration and crystallization of sulfur internally and as a surface bloom on rubber were studied by Rimpel ( I S ) , Endres (3, 4),Twiss (go), Loewen ( I O ) , and Graffe ( 5 ) . The absorption of sulfur by rubber a t elevated temperatures was studied by Skellon ( 1 7 ) and by Venable and Green (dl), and the solubility of sulfur a t various temperatures was investigated by Venable and Green ( e l ) , Endres (S), Kelly and Ayers ( 7 ) , Loewen ( I O ) , Morris ( l l ) ,and Williams ($3).
milling was determined. Heterogeneous or spotty vulcanization results from undissolved and nonuniformly dispersed sulfur in the unvulcanized mix. This heterogeneity affects adversely the physical characteristics of the vulcanized rubber and results in a decrease in tensile strength to less than 10 per cent of the best value in extreme cases. The effect of variations in mixing and storage on heterogeneity and physical tests are discussed, and the need for further study in this field is indicated.
Absorption of Sulfur by Rubber Venable and Green (21) studied the absorption of sulfur by sheets of rubber embedded in powdered sulfur a t various temperatures to determine the solubility of sulfur in rubber. It was assumed in this work that the rubber would absorb sulfur until saturation was established and then stop. Since this work was of a preliminary character and quite inconclusive, the present authors deemed it advisable to repeat it. The procedure used in the present investigation was to place thin sheets of rubber, 3.8 cm. square, in a powdered sulfur pack and heat for various periods a t a controlled temperature. Following each period of heating the sheet was removed and cooled and the sulfur was brushed off uniformly with a camel's-hair brush. The sheet was weighed after each period of heating to determine the increase in weight resulting from the absorption of sulfur. The amount of adhering sulfur was found to be fairly constant and was determined initially in order to make a correction for i t in subsequent weighings. Typical data for this type of experiment are given in Table I, which shows the sulfur absorption of molded sheets of masticated crepe rubber after various periods of heating a t 56" c. n a pack of powdered sulfur. Examination of these data indicated that equilibrium is not reached even after heating for 3215 hours. Since the amount of sulfur absorbed greatly exceeded the known solubility of sulfur in rubber, internal crystallization was suspected. Microscopic examination showed the presence of a large quantity of crystalline sulfur in the rubber a t the end of the long heating period. The absorption data are plotted logarithmically in the upper part of Figure 1, along with those obtained in similar experiments where temperature and sheet thickness were varied. Following an initial period of higher absorption rate, all of the results fall on a straight line with slopes varying from 0.20 to 0.22. When the sulfur absorption values for masticated crepe determined a t various temperatures are plotted against the 1075
INDUSTRIAL AND ENGINEERING CHEMISTRY
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VOL. 32, NO. 8
Taylor and Kemp (19) found the following relation to hold for water absorption by rubber:
c = c,* 10
5
*
El L n
5
1
z
50
10
7
100 TIME IN HOURS
0
1000
3000
I
Y)
4U
500
20
; a
18
Y,
16
I
I
I 1
~
0
I
3
2
I
In order to determine the effect of long time absorption without frequent removal and weighing, a specimen of masticated crepe, 0.053 cm. thick, was placed in a sulfur pack for 3000 hours and kept undisturbed a t 25" * 0.3" C. When weighed it showed a sulfur absorption of 2.88 per cent or over twice the sulfur saturation value for rubber a t this temperature. Fluctuating temperatures and disturbance of the rubber would be expected to promote internal crystallization. However, experiments conducted a t 56" * 0.05" C. gave practically the same absorption values as were obtained in the experiments already recorded.
4
5
6
%E
T I M E IN
OF TEMPERATURE AXD THICKNESS os FIGURE 1. EFFECT
THE
A4BSORPTION OF SULFUR BY MASTICATED CREPE
RUBBER Temp., 86 56 56 25
Curve N o . 1 2 3 4
'C.
Thickness, Cm. 0 .0584 0.0610 0.203 0,0572
c = c,iK where C1 = absorption in some unit time such as 1 hour or 1 day TABLEI. ABSORPTIONOF SULFUR BY MASTICATED CREPEAT 56" C. r-7c Weight G a i n 0 7 I
I1
I11
Heating Period, Hr. 44 ._
0.5 1.5 3
90 165 263 7 352 495 9 687.5 11 763.75 13 1340 15 2078 21 2562 27 3215 43 Samples h a d the following properties: Weight, Thickness. Sample No. Gram Cm. 7
a
I
I1 I11
0.8213 0.8464 0.8662
0.0554 0.0610
0.0635
~7~ Weight GainQI
I1
I11
6.10 6.84 7 70 8.45 9.26 9.84 10.52 10.87 11.80 12.83 13.94 14.95
6.45 7.16 8.03 8.73 9.44 10.09 10.67 10.98 11.96 13.08 14.03 15.08
6 10 6.73 7.61 8.34 9.00 9.54 10.25 10.69 11.55 12.57 13 76 14.72
Initial Adhering S, Gram
0 0430 0.0432 0.0440
200
300
400
500
600
700
T I M E IN H O U R S
fifth root of time as in the lower part of Figure 1, straight lines result which pass through the origin. The slope is increased by increased temperature and decreased by increased thickness of the rubber sheet, as would be expected. Although the earlier values up to 21 hours given in Table I are lower than is indicated by curve 2 in the lower part of Figure 1, the one-hour absorption values must be taken from the plotted straight lines if they are used to calculate absorption values for longer periods using the following formula :
Heating Period, Hr.
100
0
FIGURE2. ABSORPTION OF SULFUR BY M.4STICATED CREPE,LATEXGEL, ASD SOFTVCLCANIZEDRUBBERA T 56" C. Curve No.
Rubber
1
Masticated crepe Washed latex film Soft vulcanizeda
2 3
+
Thickness, Cni 0,0610 0.0457 0,0559
a 95 crepe 5 sulfur, vulcanized 4 hours at 141' C. combined sulfur.)
(2.8.3%
The absorption of sulfur by rubber appears to be osmotic in nature and similar to the swelling of rubber in organic solvents. Since the rubber swells in the process of absorbing sulfur, it might be expected that the rate of absorption would be reduced with increased stiffness of the rubber. This was found to be the case with latex gel hydrocarbon, soft vulcanized rubber, and ebonite, all of which show a lower rate of absorption than masticated crepe. Sulfur absorption data on masticated crepe, gel hydrocarbon, and soft vulcanized rubber are compared in Figure 2. The absorption of sulfur a t 86" C. by an ebonite sheet 0.08 cm. in thickness was found to be less than 0.2 per cent after heating in a sulfur pack for 670 hours. This would indicate either a low rate of diffusion or a low solubility of sulfur in ebonite, ,or both, since the solubility value should be exceeded in this period. It is probable that the high resistance of ebonite to swelling accounts for its low sulfur absorption. The same explanation has been given for the low water absorption of ebonite ( 2 5 ) . S o reliable data are available on the solubility of sulfur in ebonite, although it has been assumed by some to be high because blooming of sulfur does not take place when a high content of free sulfur is present. The conclusion to be drawn from this work is that the determination of the absorption of sulfur by rubber in contact
r y w
