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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 11
Theory
Acknowledgment
Throughout the discussion, it has been stressed that the superior properties of USF rubber may be correlated with the production of an unspoiled,and unpolymerized hydrocarbon different from that found in ordinary market grades, Greater softness, greater solubility, improved flex cracking, faster rate of vulcanization, elimination of smoking, reduction of Weping, heavy COmPreSSiOn for shipment, the Presence of formaldehyde as enzyme poison, antioxidant, and possible polymerization retarder-all these factors support the suggestion that USF may contain a more unsaturated rubber molecule than is present in usual market grades.
The writers wish to express their appreciation of the cooperation and help of W. E. Cake, director of the Research Department of the United States Rubber Plantations, and of his efficient staff. Literature Cited (1) Hastings and Rhodes, J. Rubber Research Inst. MuZU~U, 9, NO. 243, 200 (1939). (2) hIcGavack and Linscott (to U. Rubber Products), U. S. Patent 2,213,321 (Sept. 3, 1940). (3) Rubber Research Inst. of Malaya, Ann. Rept., 1938, 153ff. (4) Van Dalfsen, Arch. Rubbercultuur,25, KO.3, 295 (1941).
RISE OF TEMPERATURE ON FAST STRETCHING OF SYNTHETICS
AND NATURAL RUBBERS S. L. Dart, R. L. Anthony, and
Eugene
U n i v e r s i t y of N o t r e Dame, Notre Dame,
Guth
Ind.
HAT rubber warms up when stretched rapidly and cools T down when released is one of the earliest recognized peculiar characteristics of this material. It was observed first in 1805 by Gough (6). Sometime later Page (IO) rediscovered the same effect. Many years later Joule (8) independently observed this effect and made some quantitative measurements of the temperature rise due to fast stretching. Kot much work was done along these lines until after about 1924 when it became of interest t o find the heat given off by rubber during stretching. This heat could be measured in three different ways : (a) direct calorimetric measurement of the heat exchange when rubber is stretched isothermally, (b) determination of the temperature rise during adiabatic stretching, ( c ) measurement of the change in heat content between stretched and unstretched rubber on melting or going into solution. The first method has not yet been utilized successfully. The second was used by Joule ( 8 ) , Chauveau (4), Williams ( I I ) , Ariano (a), Boone and Kewman (5') and Ornstein, Wouda, and Eymers (9); this is the method to be treated in this work. A number of authors have worked on the third method; there has been wide disagreement between the results obtained. Sensory observation (i. e., stretching rubber rapidly or, better, holding it stretched and then letting it retract quickly and touching it t o the lips) shows that the maximum change in temperature should be greater than 10" C. Surprisingly, experiments reported in the literature (6) thus far give only about a third of the expected value. Aside from the fact that an obvious discrepancy exists, this phenomenon is of great interest in tire construction. Experimental Procedure
A schematic diagram of the apparatus employed is shown in Figure 1. The ends of the rubber sample were sandwiched
between pieces of scrap rubber to prevent the clamps from cutting the sample. The elongation was regulated by adjusting the distance the weight was allowed t o fall. The ratchet was used t o hold the weight before extension and to hold the rubber extended before retraction. The rubber samples used were dumbbell-shaped, with a center section 0.25 inch wide and about 1.5 inches long. Two such samples were clamped as a sandwich and given a half twist to ensure good heat contact between the sample and the thermocouple. Then the samples clamped the thermocouple increasingly tighter as greater elongations were attained. It was ascertained that this twist had no influence on the results of the experiment. The thermocouple was
Figure I.
Diagram of A p p a r a t u s
1. Rubber sample 2.
Thermocouple
3. Stops used during re4.
traction Galvanometer
5. Pulley system 6. Ratchet 7. Cables 8. Weak spring
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1942 Fig. 2 L a t e x Stock I
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Accelerated Stock
O 400 Elongation in %
E l o n g a t i o n in %
Fig. 5 U n a c c e l e r a t e d Stock
01 0,
C m
500
a
6.0 3 -n----*--T
OO
100
300 400 Elongation in %
200
500
Elongation i n 'YO I
Fig. 8 Neoprene
910
I
C ._ 0,
m
2
u
0
$5
c
L
placed between the samples at the midpoint. The elongation was determined by measuring with a cathetometer the distance between marks on the center section of the strip. The thermocouple was made of copper and constantan wire 0.004 inch in diameter. The galvanometer was of the double suspension type having
+*
/
1
a period of the order of 0.1 second and a sensitivity of about 10-7 ampere per mm. a t a scale distance of 1 meter. The speed of this instrument was more than ample for our purposes. I n combination with the light thermocouple, this galvanometer made it possible to read a temperature change within a t least 0.2 second.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I n general, the temperature change on extension was different from that on retraction. Thus, a curve mas run for both. I n the graphs the solid curve is for extension, and the corresponding points are indicated by crosses. The dashed curve and the circles are for retraction. The procedure for taking data for a curve was as follows: The weight was released and the temperature change noted. After the elongation had been recorded, the sample was released and the temperature change again noted. This was done on the same sample for the whole curve, beginning a t small elongations and working up to larger elongations until the sample broke. Temperature Effects
The first sample was an undercured latex stock. The temperature effect (Figure 2 ) is reversible up to about 230 per cent. Above this point the cooling effect on retraction is larger than the heating on extension. The change in slope of the curves around 250 per cent marks the beginning of crystallization. The niaximum elongation attained before the sample broke was only about 550 per cent, and the temperature changes seem to be such that the b o c u ~ v e swould cross if higher elongations were attainable. The next sample was also an undercured latex stock, and the curve (Figure 3) is similar to that of Figure 2 except that higher elongations were obtained. It is to be noted that the curves do cross. The highest temperature change for this run was 13.7' C. at 674 per cent elongation. An accelerated gum stock gave a similar curve (Figure 4). An 8 per cent sulfur unaccelerated stock gave a different type of curve (Figure 5 ) . There is no sudden change of slope, and the temperature changes are comparatively small. This implies that there is no crystallization. The curves are very close together, which sugsests that the process is nearly reversible. Also the curve for retraction lies beneath that for extension. Figure 6 was obtained for Hycar OR. This also shows no crystallization. This substance 1% as very plastic and allowed a n elongation of oiily about 250 per cent before breaking. Here the retraction curve is definitely lower than the extension curve. The next sample x a s a heavily loaded stock. Figure 7 shows a sharp change in slope a t the small elongation of 100 per cent. This is probably due largely to internal friction rather than crystallization. The retraction temperature change is only slightly more than half the extension temperature change. Since this means a large heating but small cooling, it is seen that for a complete extension-retraction cycle a comparatively large net temperature rise should result. This is of special interest as tire treads are made from stocks of this type. The last sample tested was a loaded neoprene stock, and the curve (Figure 8) is similar to the loaded rubber curve. This sample was much tougher than any of the other samples. A few variations were tried in order to obtain a general picture. A study was made of the variation in the retraction temperature change as a function of the time the rubber was kept stretched before retraction. For times of 1 to 5 minutes t h e variation was negligible, but for a 2-hour stretch an increase of 10 per cent in the retraction temperature change was noted. If a sample was greatly elongated on the first stretch, it gave a temperature change nearly 30 per cent greater than the curve would indicate. Thus, for rubber of the second type mentioned a temperature rise of 14.7" C. was observed a t 571 per cent on the first stretch. Extension-Retraction Results
In general, accelerated stocks appear to give a higher temperature change on retraction than on extension for the
Vol. 34, No. 11
range 300 to 500 per cent. Belov this range the extension and retraction curves are nearly the same. The temperature changes for extension of loaded stocks are the greatest of any of the types investigated, but the plastic flow was such as to prevent extreme elongations. For the loaded stocks the retraction curve \vas much Ion-er than the exten,'Qioncurve. For the accelerated stocks the curves took a sharp upward trend near 300 per cent. This is assumed to be due to the beginning of crystallization bccause x-ray analysis s h o w that the rubber does begin to form into a crystalline state a t about this point. A large portion of the heat observed above this point is probably latent heat of fusion. It is of interest to note that the temperature response of the rubber was immediate for elongations of lees than 300 per cent, but above this there was a lag of the temperature behind the elongations. This lag amounted to as much as 5 seconds for high elongations and could not possibly be due to slowness in the galvanometer or thermocouple response. The heat contact of the thermocouple to the rubber vas best a t high elongations because of the twist in the sample. This implies that some process which takes a small amount of time must be going on in the rubber during elongation. Tilliams ( 1 1 ) also found this time lag but attributed most of it to the slow response of his set-up, and so he extrapolated back to get his temperature changes. A time lag of 1.2 seconds after stretching before a photographic fiber diagram was obtained with x-rays was reported by Acken, Singer, and Davey ( I ) . However, Hauser and Mark ( 7 ) , as a result of visual observations with the aid of a fluorescent screen, found no detectable time lag. Conclusion
This investigation covered more types of rubber and rubberlike synthetics than were studied by previous investigators. From the results it seems apparent that a t least a part of the disagreement between previous workers on this subject is due to the different types of rubber employed. It is also important to note that the temperature changes observed were of the order predicted by sensory observation. Literature Cited (1) Acken, Singer, and Davey, IND. Exo. CHBM.,24, 54 (1932). (2) Ariano, India-Rubber J., 75, 759 (1926). (3) Boone and Newman, ISD. ENG.CHDM.,18, 539 (1926). (4) Chauveau, Compt. rend., 128, 388, 479 (1899). ( 6 ) Davis and Blake, "Chemistry and Technology of Rubber", p. 364, New York, Reinhold Pub. Corp.. 1937. (6) Gough, Mem. Proc. Manchester Lit. & Phil. Soc., 1 (2). 288 (1805). (7) Hauser and Mark, KolZoidchem. Beihefte, 22, 63 (1936). (8) Joule, Trans. Roy. SOC.(London), 149, 91 (1859). (9) Omstein, Wouda, and Eymers, Proc. Aoad. Sci. Amsterdam. 33. 273 (1930). (10) Page, Am. J . Sci. A r t s , 4 (2), 341 (1847). (11) Williams, IND. ENG.CHEM.,21, 872 (1929).
