SPecific Heat of Strained Rubber CHARLES G. BOISSONNAS University of Geneva, Switzerland
The specific heat of two samples of rubber and its variation with extension were determined. One of the samples was highly vulcanized, the other unvulcanized. The measurements were made at room temperature (19' to 21' C.) and involved temperature changes of less than 0.1' C. The specific heat of neither sample varied much with extension. This result is not in agreement with previous data which indicated a decrease in specific heat of about 30 per cent at 100 per cent extension.
curve for the specific heat is most interesting, as the strong variation of this quantity with the elongation must be of utmost importance for the understanding of the molecular state of rubber.'' I n order to check these results by another method, the writer measured the specific heat of rubber a t room temperature (19' to 21" C.) by a process involving temperature changes of less than 0.1" C. Two samples of rubber were chosen. Sample I was highly vulcanized for 30 minutes under 3 atmospheres pressure. Its composition was as follows : Smoked sheet
Sulfur
Accelerator
ZnO, carbon blaok
Calorimeter
2
-resuIts $hey obtained are presented in Figure 1, where the specific heat of one gram of rubber is plotted against extension : I - Eo lo
where I = length of strained sample Eo = length of same sample unstrained As Figure 1 shows, the specific heat diminishes to about two thirds of its original value when the extension is increased from 0 t o 100 per cent (AZ = 1); it increases again on further extension. According to Ornstein et al., "the form of the 1
8.3
The heating unit (Figure 2) is made of constantan wire, 0.2 mm. in diameter, with a resistance of 7.6 ohms; it is coiled around the thin-walled copper tube, A (10 mm. in diameter, 75 mm. long, 0.1 mm. thick) and insulated with Bakelite. One end of the band-shaped rubber, B, is held by a steel pin, C, soldered transversely through the copper tube. The rubber is then wound over the heating unit under a constant strain produced by a weight attached to the free end. The latter is finally attached also to the s a m e pin. O n e o f t h e copper-constantan J unctions of the thermocouple (constantan 0.2 mm., copper 0.15 mm.) is thus placed between the heating unit and the rubber band; the other junction, D, is forced into a hole in the center of a 70-gram copper block. The calorimeter is placed in a glass container with a ground joint and can be evacuated. The rubber and the internal face of the glass container are covered by light aluminum foils, E, which reduce the losses by radiation. The whole apparatus is immersed in a Dewar flask. The thermocouple is connected to a Kipp and Zonen galvanometer, type Zc. The heating coil and its connections (which serve also to suspend the heating unit) are not shown in Figure 2. FIGURE 2. CALORIMETEE
FIGURE 1. SPECIFIC HEAT OF STRAINED RUBBER,AFTER ORNSTEIN, WOUDA, AND EYMERS
AI =
0.9
Sample I1 was a patented, pure crepe, milled, pressed, calendered rubber; it was unvulcanized and contained no fillers. A band-shaped piece of rubber was cut from each sample, the same piece was studied at several extensions.
HE specific heat of unstrained rubber as a function of temperature has been the subject of many investigations; b'ut only one set, of data has been published on the s p e cific heat of strained rubber.' Ornstein and his co-workers heated samples of strained vulcanized rubber to 80" C. and dropped them into a calorimeter a t room temperature. "-The
1
89 1.8
Omstein, Wouda, and Eymers. Proc. Acad. Sci. Amsterdam, 8 9 , 273
1930).
761
INDUSTRIAL AND ENGINEERING CHEMISTRY
762
Calibration and Measurements The two junctions of the thermocouple are attached to the bulbs of two Beckmann thermometers and placed in two Dewar flasks filled with water of slightly different temperatures. During temperature equalization, readings of the arbitrary temperatures indicated by the two thermometers and of the position of the spot on the galvanometer scale are made every 2 minutes. The difference between the readings of the two thermometers is plotted against those of the galvanometer
0
E 1.15 L u
u
.-
v. +
-6 L
c 1
2 1.10
VOL. 31, NO. 6
(Figure 3). The slope of the curve thus obtained gives the calibration. I n the present case a 100-mm. displacement of the spot corresponds to a temperature difference of 0.092" C. with an accuracy better than one per cent. One hour after the rubber band is mounted, readings of the galvanometer are made at intervals of 30 seconds over a period of several minutes. A current of known intensity (between 0.08 and 0.18 ampere) is sent through the heating coil for 5 to 30 seconds, and the readings are continued for 5 minutes. (It was found that a regular slope of the time-deflection curve is practically attained after 2 minutes.) The increase of temperature corresponding to a known energy input and thus the total heat capacity of the rubber and the heating unit can be easily deduced. The heat capacity of the heating unit, calculated from its components (copper, constantan, aluminum, Bakelite), was found to be 0.97 calorie per O C., about 20 per cent of the heat capacity of the rubber band. By deducting 0.97 from the total heat capacity and dividing by the weight of the rubber, we obtain the specific heat of the rubber.
0
P
Results
:
c
.
l 0 0 m m . =O..0SZ0C.
!+ 1.05
-5 0 FIGURE 3.
0
CALIBRATION OF THE
4-50 GALVANOMETER
The experimental data are given in Table I and summarized in Figure 4, where the values of Ornstein are reproduced for comparison. SAMPLE I (580 X 11 X 2 mm., weight 12.5 grams) shows practically no permanent deformation after straining. The specific heat does not change with extension between 0 and 1.9.
a-
TABLE I. SUMMARY OF DATA Expt.
No.
AI
Cm.
Energy Input Calories
Total AV. Specific Specific Temp. Heat Heat Increase Capacity Heat C. Gal./' C. Gal./' C./grarn
Sample I
1 2 3 4 5 6 7
8 9 10 11 12 13
68
0
96 (58) 0.65 166 (58) 1.9
64
0
7 8 9 10
170 (64) 1.7
11 12 13 14
300(64) 3.7
15 16 17 18
67
0
\
/ \
'
0.275 0.275 0.275 0.275 0.426 0.426 0.873
0.0049 0.0049 0.0048 0.0050 0.0072 0.0075 0.0151
5.65 5.65 5.75 5.54 5.95 5.72 5.78
0.375 0.375 0.38 0.365 0.40 0.38 0.385
0.38
0.393 0.565
0.0069 0.0098
5.70 5.80
0.38 0.385
0.385
0.56 0.56 0,273 0.79
0.0102 0.0100 0.049 0.0140
5.50 5.58 5.62 5.65
0.365 0.37 0.37 0.375
0.37
-.
'-/
/'
----
0.285 0.287 0.448 0.670 0.407 0.573
0.0058 0.0061 0.0100 0.0140 0.0083 0.0119
4.92 4.73 4.47 4.78 4.92 4.83
0.395 0.375 0.35 0.38 0.395 0.385
0.38
0.573 0.330 0.560 0.447
0.0109 0.0063 0.0109 0.0086
5.27 5.28 5.16 5.22
0.43 0.43 0.42 0.425
0.43
0.447 0.289 0.440 0.534
0.0083 0.0054 0.0082 0.0100
5.40 5.33 5.37 5.37
0.445 0.435 0.44 0.44
0.44
0.673 0.420 0.482 0.206
0.0135 0,0081 0.0097 0.0041
4.97 5.18 4.99 5.18
0.40 0.42 0.40 0.42
0.41
19 20 21 22 23 24
105 (69) 0.5
0.527 0.407 0.374 0.475 0.622 0.289
0.0106 0.0079 0.0075 0.0100 0.0128 0.0057
4.98 5.13 4.98 4.78 4.86 5.07
0.40 0.415 0.40 0.38 0.39 0.41
0.40
25 26 27 28 29
220 (71) 2.1
0.425 0.462 0.425 0.613 0.712
0.0083 0.0089 0.0084 0.0117 0.0141
5.13 5.17 5.08 5.25 5.07
0.415 0.42 0.41 0.43 0.41
0.42
~rnstsin stud
- present
Extensioh
Sample I1
1 2 3 4 5 6
1
\
At
1
1 2 3 4 HEATOF SAMPLES I AND I1 FIGURE 4. SPECIFIC AS A FUNCTION OF EXTENSION
SAMPLE I1 (640 X 14 X 1 mm., weight 10.0 grams) shows a marked permanent deformation after straining. At the end of the series of experiments, the length of the band was 710 mm., as compared with 640 mm. a t the beginning. To give consideration to the possible influence of this change on the specific heat, extensions were tried in the following order: 0, 1.7, 3.7, 0, 0.5, 2.1. The interpolation in Figure 4 seems to indicate a slow increase of the specific heat with the extension, but this increase is near the limits of reproducibility. The results obtained with both samples do not confirm the large decrease in specific heat with extension, found by Ornstein. Ornstein and his eo-workers measured a "mean specific heat" over a temperature interval of 60" C. The transformations which may occur in strained rubber when it is heated to 80" C. and then suddenly cooled may be accompanied by considerable evolution of heat.
