October 1, 19-'0.] 



THE INDIA RUBBER WORLD 



19 



end of each cycle was proportional to the logarithm of the num- 

 ber of the cycle in question. Also, and naturally, these workers 

 found that the shorter the extension the narrower was the 

 hysteresis loop. 



Another general rule laid down by Bouasse and confirmed by 

 Schwartz is that the greater the speed of generation of the cycle 

 the greater will be its area. You will at once appreciate the 

 significance of this in regard to the internal heating of solid 

 tires. Not only do excessive driving speeds multiply the num- 

 ber of hysteresis loops per second and, therefore also the heat 

 liberated, but they also actually increase the calories of heat 

 generated per revolution of the wheel 



One aspect of hysteresis is at least encouraging, namely, that 

 the area of the loop diminishes with increased temperature. We 

 may be thoroughly thankful that the converse is not the case. 

 Solid tires and tlie breaker strips in pneumatic casings would 

 go to pieces in no time if there were not this compensating law. 

 Incidentally this temperature relationship strongly suggests the 



effect is now generally known as the Joule effect. Interestingly 

 enough, the very first stages of extension are accpmpanied 

 by a slight cooling effect. The corresponding cooling which 

 accomparjies retraction of the stretched rubber is definitely less 

 than the heating effect on the extension. This difference repre- 

 senting the net increase in thermal content of the sample is the 

 exact equivalent of the hysteresis loop, to which I have already 

 referred. This heat must be attributed to internal friction in 

 the rubber. 



It may be of interest to compare the thermodynamical behavior 

 of vulcanized rubber with that of better understood systems. 

 Gases when expanded or compressed isothermally develop pro- 

 nounced thermal effects. In fact, the energy expanded during 

 compression, for example, is all turned into heat. Steel springs, 

 on the other hand, are examples of systems which develop 

 practically no thermal effects when deformed. All of the work 

 done on the system appears as potential energy of strain. 



Vulcanized rubber is intermediate between a gas and a steel 



800 

 §.,-600 



Ipioo 

 OiS;?oo 



800 

 600 

 400 

 200 



800 

 600 

 400 

 ?00 



.Base 



800 

 600 

 400 

 ZOO 



STRESS STRAIN CURVES 



■Base 



600 

 400 

 200 



400 800 IZOO 1600 ?00O 2400 

 Load Grams per5q. MM. 



Sase 



800 

 600 



400 

 ?00 



n 400 800 1200 1600 2000 2400 2800 



•3ase 



400 800 ItOO 1600 2000 2400 2800 

 Base 



400 800 1200 



1600 2000 2400 

 ■ Base 



400 800 1200 1600 2000 2400 

 ■Base 



RED OXIDE 



800 1200 1600 2000 2400 2800 

 ;Base 



400 



800 1200 1600 2000 2400 



BLACK 



600 

 400 

 200 



800 1200 1600 2000 2400 2800 



400 800 1200 1600 2000 2400 2800 



ENERGY ABSORPTION CURVES 



.£■800 

 ^600 

 ^400 

 ^200 



800 

 600 

 400 

 ?00 



50 100 150 



Volume Percentages of Pigment 



100 



150 



800 

 600 

 400 

 200 



800 

 600 

 400 

 200 



600 

 400 

 200 



800 

 600 

 400 

 200 



100 



150 



600 



400 

 200 







800 

 600 



400^ 

 200 



800 

 600 

 400 

 200 



50 



100 



150 



100 



150 



5LUE 



100 



150 



100 



stress Strain Curves 

 Elonqafion percentages plotted 

 against load m qrams per 

 square MM. (Numbers adjacent 

 to circles indicate volume per- 

 centages of pigment.) 



Energy Absorption Curves 

 _^ Foot pounds per cubic incti plofled 

 ^0 against volume percentages of 

 pigment added io ttie base. 



resemblance in many respects of rubber to a viscous liquid. In 

 fact, Shcdd and Ingcrsoll use the term "viscosity loop" rather 

 than hysteresis loop for this reason. 



THERMAL PHENOMENA 

 In the year 1805, Gough recorded in the memoirs of the Man- 

 chester Literary and Philosophical Society, that when he stretched 

 a strip of rubber and held it to his lips it felt warmer than 

 before stretching. Page, in 1847, made the same observation. 

 Finally Joule also recorded the fact that while metals and other 

 materials cooled on stretching, rubber, on the contrary, warmed. 

 Lord Kelvin applied Le Chatelier's principle of equilibrium and 

 predicted that if this was so, stretched rubber must contract on 

 .heating. Joule confirmed this by actual experiment, and the 



spring. When rubber is stretched the work done turns partly 

 into potential energy of strain and partly into heat. In the case 

 of an ideal rubber or compound (that is, one which shows com- 

 plete reversibility), all of the heat liberated during extension 

 will be reabsorbed during retraction and likewise all of the 

 work done will be regenerated, thus leaving the sample in the 

 same thermal as well as meclianical state as it was before 

 stretching. Such a rubber would not heat up in a casing or solid 

 tire any more than would a perfect gas, when alternately ex- 

 panded and compressed. There would be no hysteresis. Actually, 

 of course, the heat is not completely reabsorbed and the energy 

 is correspondingly reduced. It is thus convenient to keep in 

 mind the two thermal values involved in any cycle of extension 



