INDUSTRIAL A N D ENGINEERING CHEMISTRY
hIay, 1926
539
Effect of Heat Generated during Stressing upon the Tensile Properties of Rubber’,‘ By C. E. Boone and J. R. Newman B u ~ E a r O: F
S T A N D A R D S , ~ A S H I N G T O K , D.
I
T HAS long beeii k n o a n that the temperature at which the tensile strength of rubber is determined has an important bearing upon the results. A summary of an investigation of the effect of temperature upon the tensile properties of rubber states? “It mill be noticed that the rubbers are not all affected to the same extent by equal differences in temperature, but that there is a marked tendency in each case toward decreased strength, decreased set, and increased elongation as the temperature is raised.” The investigation shows that the rubber is affected by temperature and that the effect is modified by the nature of the compound and possibly by other factors. It is also known that when rubber is stressed heat is generated and the temperature rises. Inasmuch as the tensile properties of rubber are affected by the temperature, the change in temperature of the rubber due to the heat generated by the stressing would be expected to change the tensile properties. The temperature rise of the rubber during the test is dependent upon the specific heat and upon the difference between the rates of heat generation and dissipation. Physical Tests Physical tests were made on a series of rubber compounds in order to ascertain the magnitude of these effects as obtained in practice. Test sheets of the following compounds were prepared and press-cured in aluminum molds: Rubber (smoked sheets) Sulfur Zinc oxide Gas black Litharge Barytes Tetramethylthiuram disulfide
‘m.rr.n,
C h a r t 1-Stress-Strain
---Parts Compd. 1 100 2 5
... . ..
0.25
by Weight-Compd. 2 Compd. 3 100 100 2 8 10 .. 20 .. .. 10 .. 200 0.25
..
C.
Rubber breakdown, minutes Mixing, minutes Temperature of cure, ’ C. Time of cure, minutes
Cornpd. 1 Compd. 2 Compd. 3 15 15 15 10 30 20 125.6 125.6 141.7 15, 20, 25 15, 20, 25 20, 30, 40
Stress-strain tests were conducted, first, in the standard manner4 and, second, by dissipating a large portion of the heat generated during the stretching by means of a stream of air from an electric fan blowing directly on the sample. Duplicate tests were made a t 10’ C. and 24’ C. room temperatures. Note-An attempt was also made to measure the effect of the heat generated in the sample while being stretched b y the use of different testing speeds which would change the rate of generation of heat. Io the tests made, however, with speeds of testing varying from 10 inches to 30 inches per minute, the effects were not so marked nor the results so consistent as those obtained b y the above method.
The results of the tests a t 24’ C. are shown in Chart I and the tests a t 10’ C . in Chart 11. It will be noted that in all cases the cooling of the sample during the test causes the stress-strain curve to move toward the stress axis; that is, a higher tensile stress is necessary to cause a given elongation if the sample is cooled during the test. I n all cases but one the ultimate tensile is higher and the ultimate elongation lower when the sample is cooled. The change in tensile stress due to cooling is considerable; a t the breaking point in one case it is 700 pounds per square inch greater with the sample cooled than when tested in the standard manner. The relative results of cooling by the fan are the same a t each room temperature. Temperature Changes during Tests An approximate determination of the rise in temperature of the rubber samples undergoing test was made by means of a copper-advance thermocouple. One junction of the
Nmg.hOn
C u r v e s at Room T e m p e r a t u r e of 2 4 O C.
These compounds cover a fairly wide range, from 1, a pure gum st,ock, to 3, a heavily compounded stock. The following methods of mixing and curing were used: 1
Received February 6, 1926.
1
Bur. Standards, Circ. 38.
* Published by permission of the Director, National Bureau of Standards.
thermocouple was in the air a t room temperature and the other junction was cemented between two test strips a t the mid point with a very thin layer of rubber cement. The double test strip thus made was tested in the usual way and the temperature rise measured by a deflection galvanom4
B u r . Standards, Circ. 333.
540
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 18, No. 5
Chart 111-Rise of Tern erature of Rubber during Stressing a t Room fernperatwe of 24O C.
Chart IV-Rise
eter. Although the temperatures indicated in this way were lower than the actual temperatures due to the lag in the couple reaching the sample temperature and also due to the heat conducted away by the leads (No. 34 wire), the results are a t least comparative and show a decided rise in temperature while the sample is being tested. These temperature measurements are shown in Charts I11 and IV, corresponding with the tensile determinations shown in the previous charts. There is a rise in temperature amounting to from 6' to 12' C. under the usual conditions of testing. Although the air cooling does not maintain the sample a t room temperature, it does prevent as high a temperature rise as occurs when the test is made in still air.
this paper, the following results were obtained: The temperature rise is from 6" to 12' C. when tested in the standard manner. Cooling with the fan reduces this temperature rise so that i t is from 3" to almost 9' C. This cooling changes the stress-strain relations of the rubber, moving the stress-strain curve toward the stress axis. In one case the breaking load is 700 pounds per square inch greater and the ultimate elongation 20 per cent less due to the cooling by the fan. In general, the greater the time of cure of any particular compound the greater the rise in temperature when stressed, but as the cure is increased the rubber becomes less susceptible to the temperature. The pure gum stock (1) usually changes in tensile properties with changes in temperature more than the compounded stocks ( 2 and 3). From the results of these tests it will be seen that, although the room temperature is maintained constant, the temperature of the rubber during stressing gradually rises as it is elongated and, therefore, the tensile values are not values obtained a t constant temperature. As the tensile properties of rubber are used as criteria in the studies of rubber technology, this change in tensile properties due to the heat generated in the stressed rubber should be considered when precision is desired.
Conclusions
When stressed, rubber generates heat at such a rate that the temperature of the rubber rises considerably above room temperature and this temperature rise due to the heat generated in the rubber causes changes in its tensile properties. The effect of the temperature rise upon the tensile properties of rubber is rnodsed by the nature of the compound, the cure, the room temperature, and other test conditions. With the compounds prepared and tested as described in
of Tern erature of Rubbe: during Stressing a t Room g e m p e r a t w e of IO c.
The Backhaus Process for Carbon Dioxide Purification' By William C . Moore u. s.
INDUSTRIAL ALCOHOL CO., BALTIMORE, MD.
T T H E Curtis Bay, Md., plant of this company blackstrap molasses is used as the raw material for alcohol manufacture. Here fifteen fermenters, each with a capacity of more than 125,000 gallons of mash, are in use, the cycle being such that in general five are refilled daily. The gas from these fermenters is collected during the period of most rapid fermentation, forced by Root blowers through a Feld scrubber operating on water, and thence through the purification system to the compressors.
A
1 Presented before the Division of Industrial and Engineering Chemistry a t the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 t o 10, 1925. Received November 14, 1925.
The Feld scrubber removes entrained material and nearly all the alcohol, aldehyde, etc., in the gas. Nevertheless, the very slight amounts of impurities remaining after the thorough washing in such a scrubber must be removed by further treatment before the gas can be used in carbonating beverages. In the Backhaus process this further treatment consists in passing the gas through active carbon.2 The carbon purifiers used are cylindrical vessels 3.5 feet in diameter and about 13 feet high. Within the cylindrical jacket are placed cooling coils, and about these tubes the active carbon is packed. The washed carbon dioxide enters 2
U, S. Patents 1,510,373 and 1,493,183. Other patents applied lor.
