INDUSTRIAL AND ENGINEERING CHEMISTRY
March. 1923
2.35
Thermal Changes during Vulcanization' By Ira Williams and D. J. Beaver RESEARCH LABORATORIES, FIRESTONE TIRE& RUBBERCo.,AKRON,OHIO
NE O F the several
0
actions which take place when a mixture of rubber and sulfur is heated is a change in the total energy of the mixture. This change is due to the heat of -reaction between sul.. fur and rubber, and may be either positive or negative.
C. 0. Weber2 was the first worker to consider the effect of the change in the energy content of the system on the progress of the vulcanization. He was of the opinion that sulfur chloride was a good vulcanizing agent because its heat of formation (14,260 cal.) was less than the heat liberated when it reacted with the rubber. On the other hand, both sulfuryl chloride and thionyl chloride were not vulcanizing reagents because their heats of formation (89,780 cal. and 47,400 cal., respectively) were greater than the amount of heat that could be liberated in the reaction of rubber with these compounds. He states that any sulfur-chlorine compound with a heat of formation less than that of sulfur chloride will be a good vulcanizing reagent. The next reference to the heat changes during vulcanization is found in the work of Seidl,* who was interested primarily in the mechanism of the accelerating action of litharge. He measured the rise in temperature of different rubber-sulfur-litharge mixes above the temperature of the surrounding oil bath and concluded that the heat liberated was due to the reaction between hydrogen sulfide and litharge. The hydrogen sulfide was said to be produced by the reaction between the sulfur and the resin present in the rubber. He made no measurements with rubber-sulfur mixes, nor did he isolate any hydrogen sulfide from the sulfur-resin reaction products.
The object of the present paper is to describe a method of determining the intensity of the energy changes during vulcanization and to calculate their approximate amount.
APPARATUS AND METHOD OF PROCEDURE I n the first series of experiments the temperature change a t the center of a uniform cylinder of the material studied was recorded when the cylinder was immersed in a constanttemperature bath. The rubber compound was calendered into thin sheets which were cut into strips about 4 in. wide. A thin-walled glass capillary was then placed with the closed end a t the center of the strip of rubber and the strip rolled about the capillary until a cylinder of the desired size was obtained. This was then slipped into a close-fitting test tube, 1.36 em. in diameter, and sealed in place with a cork through which the glass capillary projected. The temperature at the center of the cylinder was measured by means of a copper-constantan thermocouple which was placed in the glass capillary. The e. m. f. produced was measured with a potentiometer reading to millivolts and capable of recording temperature changes of 0.5" C. Heating was done in a glycerol bath of about one-liter capacity which did not fluctuate more than 1' C. In the second series of experiments a calorimeter was constructed which could be supplied with a practically constant amount of heat. The inside container was a copper can 4.5 cm. in diameter and 9.0 em. deep. The outer jackets were 1
Presented before the Division of Rubber Chemistry a t the 64th Meet-
ing of the American Chemical Society, Pittsburgh, Pa., September 4 to 8,
192?. 2
3
made from cardboard and were packed between with loose cotton. A frame was constructed which supported three mica shelves 2.5 em. in diameter, and were placed one above the other. The lower end of the frame sumorted a heating coil of -nichrome wire. On the lower shelf was placed a piece of asbestos paper which prevented direct radiation from the coil. The center shelf supported a watch glass and the top shelf held a small iron ring in which the sample to be tested was placed. Temperature was measured with a small mercury thermometer extending through the top of the calorimeter and into the sample of rubber. The coil was heated by means of 110 volts, d. e., controlled by passing through a rheostat and wattmeter. In conducting the experiment a heating and cooling curve was first obtained with the rubber and sulfur separated, the sulfur being placed in the watch glass and the rubber in the iron ring. A current of 18 watts was maintained until the desired temperature was reached, when the current was broken and the cooling curve obtained. The experiment was then repeated using the same amounts of sulfur and rubber mixed. In each case the total weight of sample was 5 g. Fig. 1 shows the effect of temperature on the measurable heat liberated. Curves 1, 2, 3, and 4 were obtained using Compound 1 with bath temperatures of 151 O , 161 O , 172', and
The data in the following paper show that when rubber is heated with sulfur there is ( I ) at first an evolution of a small amount of heat; ( 2 ) a slight absorption of heat which becomes noticeable near the end of the reaction; (3) heat liberated due to rubbersulfur reaction because there is no measurable heat liberated in the react ion between rubber -accelerator, sulfur-resin, or sulfur-accelerator; ( 4 ) an increase in the velocity of fhe reaction caused by accelerators, increase in per cent of suvur, or increase in temperature.
"The Chemistry of India Rubber," London, 1902, p. 114. Gummi-Ztg., 25 (1911),710, 798.
IhTDUSTRIAL A N D ENGINEERING CHEMISTRY
256
176" C., respectively. It will be noticed that the intensity of the reaction (as measured by the elevation of the temperature of the cylinder above that of the bath) increases as a direct function of this temperature. This is due partly to the increased velocity of the action and partly to the fact that rubber is a very poor heat conductor. If the same amount of heat is liberated during each reaction, the greater velocity should give the greater temperature rise because a smaller part of the heat would be lost through conduction. TABLE ~- I Compound Smoked Hexamethylene- PiperidylthiNo. Sheet tetramine urnmdisulfide 90.0 1 0:; 89.6 a 0:; 89.5 3 94.0 .. 4 .. 86.0 6 0 :5 88.5 6 90.0' 7 .. 23.01 8 5:o .. 9 99:5 0.5 10 0:5 89.51 11 1 Acetone extracted. 8 Extracted resin. ~
.. ..
..
.. .. .. ..
Sulfur 10 10 10
Zinc
6
..
14 10 10 77 95
..
10
.. .. ..
..
1 .. .... .. ..
I n the next experiment the bath temperature was maintained constant while the amount of sulfur in the compound was varied. Fig. 2 was obtained while heating stocks containing 6, 10, and 14 per cent of sulfur, the bath being a t 176" C. This temperature was used because it was necessary to give the action a high velocity in order to minimize loss of heat through conduction. As would be expected, we find the evolution of heat increasing as the sulfur content is raised. Since the increase in temperature seemed to depend on the speed of the reaction, it was thought that a still greater elevation of temperature could be produced by means of an accelerator. Piperidylthiuramdisulfide was chosen as the accelerator because of its rapid action, and a relatively large amount was used. Compound 2 was taken and the curves may be
Vol. 15, No. 3
compared to Fig. 1 because the sulfur content in each case is the same. The results, which are shown graphically in Fig. 3, indicate that the action was much more rapid. Hexamethylenetetramine, when used in the same amounts as the accelerator above, does not produce as rapid an increase in the rate of action, but, since this accelerator was known to require the presence of zinc oxide to become effective, batches were prepared containing no zinc oxide and one per cent of zinc oxide. These two stocks containing hexamethylenetetramine and with the same sulfur content were checked against one another a t a bath temperature of 175" C. The higher temperature is produced in the stock containing zinc. The curves are shown in Fig. 4. In Fig. 5 are shown the results of a comparison between an extracted and a normal rubber stock, both accelerated and unaccelerated. Compounds 1, 2, 7, and 11 were used. The thinly creped sheets of rubber were extracted for 48 hrs. with acetone in a Soxhlet extractor and showed a loss of 3.5 per cent resin. Only a slight evolution of heat was noticed with the extracted sample, while the accelerated normal rubber stock showed the greatest elevation. The curve for Compound 11 lies between the curves for Compounds 1 and 2, which would indicate that the resin functions merely as an accelerator without in any way influencing the action of the t hiuramdisulfide. It was thought possible that the heat liberated was produced either by the action of sulfur on resin, sulfur on accelerator, or accelerator on the rubber. Accordingly, Compounds 8, 9, and 10 were made containing these ingredients in the same proportions as found in the stocks. The sulfuraccelerator and sulfur-resin mixtures were placed in test tubes and handled exactly as all other samples. At 176" C. the sulfur produces a viscous mass which prevents a large loss of heat through convection. In the case of the sulfur-resin
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
March, 1923
mixture, a temperature two degrees above the bath was noted, but the two others showed no elevation of temperature. While the resin-sulfur mixture reacted quite vigorously a t this temperature, no odor of hydrogen sulfide could be detected. Since in every case changes which would produce a more rapid curing stock also gave greater temperature rises, it was thought of interest to determine the combined sulfur figures on some of the samples above. Samples were extracted with acetone, alcoholic potash, alcohol, and water. Stock
Figure
Curve
3 6 1 2
4
4 1 3
1 2 4 3
Time of Expt. Per cent Per cent Min. Total Sulfur Combined Sulfur 23 10 7.82 22 10 8.93 10 9,99 29 29 10 9.92
Stocks 3 and 6 were cured for the same length of time and a t the same temperature so that the combined sulfur may be taken as an approximate measure of the velocity of reaction. This difference in combined sulfur is probably much less than the actual difference in velocity, because the per cent of free sulfur had been reduced to such a small amount. That this is the case is shown’in Stocks 1 and 2 where all the sulfur was combined, even though Stock 2 had a much greater speed of reaction than Stock 1. Combined sulfur figures as well as the temperature rise obtained seemed to indicate that the energy change was quite large. Two samples of pure gum stock containing 6.5 per cent sulfur were cured to combined sulfur figures of 0.7 per cent and 5.64 per cent, and the heats of combustion were determined in a bomb calorimeter. Sample
I 2 3 4
Combined Sulfur Per cent 0.70 0.70 5.64 5.64
Cal./G. 5828 5810 5828 5836
257
These figures show there is no measurable difference in the heat of combustion between the different states of cure.
CALCULATIONS A rough idea of the amount of heat liberated may be obtained from the curves showing the time-temperature relations a t the center of the small cylinders of rubber. I n order to simplify the calculation, it will be assumed that the temperature gradient from the center of the cylinder to the outside is a straight line. If R is the radius of the cylinder, r the distance from the center to any point in the cylinder, and T is maximum temperature difference between the center and outside of the cylinder, the change from bath temperature of point r will be
R-r R
- T.
.
The length of the circle passing through point r will be 2irr, and if the radius increases by an amount dr the area will be
258
INDUSTRIAL A N D ENGINEERING CHEMISTRY
approximately 21rrdr. If unit length of the cylinder is considered, this may also represent volume. If h is the specific heat of the rubber, and D is density, the heat stored in the rubber when the maximum tem’perature is reached will be found by
which reduces to
TrDhR2 tal.
This amount of heat is, then, lost when the rubber returns to bath temperature, and, since the temperature-rise curve and cooling curve each cover the same range of temperature, the heat lost must be proportional to the time in.each case. If A is the time required to reach the maximum temperature after the temperature of the bath has been reached, and B is the cooling period, the total heat liberated will be
This figure is for unit length of cylinder and must be reduced to unit weight or volume. In the following calculations the last flat part of the cooling curve was eliminated because no such condition exists in the heating curves. It is evident that such figures can be only rough approximations. No account is taken of heat that might be liberated before bath temperature is reached, and it is considered that the reaction is finished a t the highest temperature. The figures of greatest accuracy will, therefore, be obtained from the reactions of greatest velocity, because here loss of heat will be reduced to a minimum. The following are some of the calculations made:
Stock 1
2 6
5
Sulfur Content 10
10 10
14
Vol. 15, No. 3
ACCEI,ERATOR None Piperidylthiuramdisulfide “Hexa” ZnO None
+
Bath Temperature Cal./G. 175 7.5 175 10.2
176 176
8.2 17.2
Both the assumption that the temperature gradient is a straight line and the fact that we consider only a limited portion of the curve will tend to reduce the value obtained, and there is no doubt that these figures are much too low; yet, they serve to show that the amount of heat liberated is small.
FURTHER EXPERIMENTS Since there was a possibility that the thermal changes noticed might be due to some reversible action, a calorimeter was constructed and experiments run as described above. The success of the experiment depends upon obtaining the correct balance between rate of heating and radiation loss from the calorimeter. Several different stocks, both accelerated and unaccelerated, with varying sulfur contents were run, with similar results. Fig. 6 shows a typical curve obtained when using this calorimeter. It will be noticed that the curve obtained with the rubber and sulfur separated is perfectly regular, while that obtained with the mixture rises above for a time and then drops below the first curve. After this it rises regularly until an equilibrium temperature is reached, which is the same as that of Curve 1. The two cooling curves are identical, which fact shows that there is no reversible reaction taking place. These curves show that during vulcanization both an exothermic and an endothermic reaction are taking place, the latter being less intense since it is only noticed after the first action is nearly complete. This is another factor which will tend to lower the calculated value of the amount of heat liberated, as shown above.
A Resource of Millions of Tons of Fertilizer It has long been known that the greensand marls of New Jersey contain small quantities of potash, lime, and phosphatethe elements of a good fertilizer. For more than a hundred yeaYs they were dug and marketed for use as fertilizer, and in the late sixties the quantity so used annually amounted to nearly 1,000,000 tons. With the introduction of prepared fertilizers the greensand-marl industry gradually dies, but here and there in New Jersey small quantities of greensand are still dug and used. It has been considered commercially impracticable to extract the potash from greensand because the mineral in which the potash is locked up-glauconite, a silicate of iron and potassium-is relatively insoluble. Of late years, however, many experiments have been made with the view of devising a process of extracting potash from silicates, and the greensand marls of New Jersey have attracted attention because of their accessibility, abundance, and the relative ease with which they may be mined. The scarcity of potash caused by the shutting out of German supplies during the World War gave impetus t o these experiments and encouraged the hope that a potash industry might be established in the United States, in which event the New Jersey greensands would be of high value. The greensand-marl belt of New Jersey extends across the state from the vicinity of Sandy Hook a t the northeast to Delaware River near Salem a t the southwest, a distance of about 100 miles. It ranges in width from nearly 14 miles in Monmouth County to 1 mile or less in parts of Gloucester County. It is crossed a t many places by railroads and by streams that flow into Delaware River.
During the World War, just before the armistice, the United States Geological Survey, in cooperation with the Department of Conservation and Development of New Jersey, began an investigation to determine the potash content of the greensand marl in this belt. Five type areas were explored by borings and the results were used to obtain specific estimates. These areas are a t Salem and Woodstown, in Salem County; Sewell, in Gloucester County; Somerdale, in Camden County; and Elmwood Road, in Burlington County. The data thus gathered were supplemented by published and unpublished well records and by information on file in the state offices a t Trenton. Moderate estimates show that the New Jersey greensands contain 256,953,000short tons of potash (KzO) that could be mined from open pits-enough to supply the needs of the United States, as shown by the pre-war importation, for nearly a thousand years. Limesand also has been found in probable commercial quantities as far north as Wrightstown. Several companies have undertaken to produce potash from hTewJersey greensand, and some of the companies have marketed small quantities of potash, though none are now actually producing. A detailed report on this investigation, entitled “Potash in the Greensands of ATew Jersey,” by George R. Mansfield, has just been published by the United States Geological Survey as Bulletin 727. The report includes several maps and other illustrations, as well as numerous mechanical and chemical analyses of greensand and an account of its commercial development and use, and notes on the possibility of further development.
