March 1, 1921 



THE INDIA RUBBER WORLD 



425 



Vulcanized Rubber Energy' 



By William B. Wiegand- 



IT IS PROPOSED to discuss very briefly and nonmathematically turn to substantially its original length, it will differ from its 



some of the many interesting energy relationships of vul- uriginal state only by the total amount of frictional heat de- 



canizcd rubber. veloped. By the law of conservation of energy, we can at once 



say that this frictional heat is exactly represented by the differ- 



ENERGY STORAGE CAPACITY ^^^^ between the mechanical energy input and output of our 



In Table 1 is shown what is known as the "proof resilience" system. This phenomenon is, of course, known as hysteresis, 



of the chief structural materials. This is defined as the number and is exhibited by all structural materials. The fact that in 



of foot-pounds of energy stored in each pound of the material (he case of rubber the energy storage capacity is several hundred 



when it is stretched to its elastic limit. You will observe that times greater than in the case, for example, of steel, explains 



tempered spring steel has less than one one-hundredth the re- \vhy hysteresis phenomena become relatively of such cardinal 



silience of vulcanized rubber, and that even hickory wood, its importance to rubber technologists. 

 nearest rival, also shows less than one per cent of the resilience 



of rubber " REVERSIBLE HEAT AND THE JOULE EFFECT 



T-i. • . r • J- ii J r • • 1 Suppose we extend a rubber sample and allow the reversible 



Phis property of course is directly made use of m airplane , , , ,. t-^. ^ ... > ^. ... ^ 



,. 1.. »u» i.f ..-i.- 1 heal thus generated to disappear. In other words, we stretch 



shock absorbers, etc., but our present reference to it is made ... ,, ,,, , , ,. . 



•.u ■ 4. J- • c 1 f *u 1. 4 r ^i.- . 1 't isothermally. We are then dealing with a system substantially 



with a view to discussion, first, of the character of this stored .,., . -^ _, . ^ . , ■: u oi l = j 



J •» . - f _ »■ ■ » .u 1 ! J. 1 ■ J "1 equilibrium. Ihe two factors governing this equi ibrium are, 



energy and its transformation into thermal enersy of two kinds; . , , , , 



J J .1 „ IT *■ J ■ c 4 „ 1 1 1 • hrst, the load on the rubber, and, second, the thermal condition, 



and, second, the modihcation and in fact remarkable increases . .,,,.. ^^"^ "^.i. 



1 11^1 1 11 J ■ 1 Any change in the equilibrium requires a change in these two 



m energy storage capacity made possible through the admixture , ^ , , ... , , , 



r ■, u, J- ■ J- » factors. Conversely, a change in either of these factors will shift 



of suitable compounding ingredients. , ji' , , r , , '■^ ^y 



the equilibrium. Now one of the fundamental properties of any 



Table I — Proof Resilienxe equilibrium is that when any factor is changed the equilibrium 



,, . , „ ^'a '-'',*• will be shifted in such a wav as to offset the change in the factor. 



Material Per Cu. In. .j. ^, , j . . ' , , .,, 



Gray cast irrn 0.37.1 I hus, if the load IS increased, the sample will stretch and become 



Ralrsted' .!'".';; '.'.'.'.'.. '.'.'.'.'.'.V.'.'.WWW'.W.'. iIa stlffer so as to resist the increased load. Similarly, if the tempera- 

 Tempered spring steel 95.3 ture of the sample is increased, the rubber will contract, since in 



structural nickel steel 14.7 . . , . , . . , . , ,. , ... , 



Rolled aluminium ; 7.56 SO doing heat IS used up and in this way the disturbance minimized. 



ffickorr«o'o°d"" .■.'.■.■'.'.■.".■.■.■.:::::::::.■'::;:: ut's^ '^^'^ contraction on heating was predicted by Lord Kelvin, after 



Rubber 14,600.00 Joule had discovered, or rather rediscovered, the development of 



heat during extension. Metals and most other rigid bodies be- 



THERMAL EFFECTS , . . . ,, j/v . r i ■ t j r 



have, or course, in a totally different fashion. Instead of generat- 



What happens to the mechanical work done on a rubber i„g ^eat on extension they consume heat and become cooler, 



sample when it is stretched to any given point? Is it in the ,,.ith the result that the application of heat to a stretched metal 



form of potential energy of strain, as in the case of a steel ,,,ire causes it to expand instead of contract, as in the case of 



spring? No. Has it all been irrecoverably lost in the form of rubber 



heat, as when a lump of putty is flattened out? No. Or lastly, t~i t , a- , l l- , 



, . . , ,, , , , , Ihe Joule effect has been subjected to many mismterpreta- 



as when a perfect gas is isothermally compressed, has the work ^- , , , ,. -t .■ . , . 



, ^, , , J • . • , c tions, such, for example, as attributing it to a huge negative 



done on the sample been turned into an equivalent amount ot , . ai ■ , r ■ , • , • • ■ 



, ..,,,. , , • ,• -, TT • temperature cocthcient of expansion, which is incorrect, since 



heat, convertible back into work during retraction? Here again i , i. • ^ . , •,■ ai ■ ^,. 



. -, rubber has in fact a large positive coefficient. Others have at- 



the answer is. No. . » j . , ■ ., , l 



tempted to explain the phenomenon bv assuming an increase in 



The fact is that rubber has all three properties combined. Young's modulus. Bouasse, the French investigator, who has 



When you stretch a rubber band, some of the energy is stored ^^^^ ju^h masterly work on the elastic properties of rubber, dis- 



as potential energy of strain, exactly as when you stretch a steel proved this hypothesis, however, and showed in fact that Young's 



spring. Another fraction of the energy input is turned into modulus decreased with increased temperature. 



what may be called reversible heat. You can easily feel this t-u v i . j • , , , 

 , ,. ,, 1 , , ... ,. Ihe writer has not done any experimental work on the re- 

 heat on stretching a rubber thread and touching it to your lips. •, i i ^ i • , , t , n- . , , , 

 _, ,,, • ,. ,,L versible heat which governs the Joule effect, but there can be 

 The rest of the energy input or work done on the rubber ap- i u^ . •.. . i • , • , t-, r , , 



, , ,./.,•,, no doubt as to its technical importance. Thus, for example, the 



pears m the form of frictional heat. • ^ i ^ . r ,-, ,• , ,, ,_ , ,. . 



internal state of a solid tire tread as well as breaker conditions 



RETRACTION in large pneumatics is clearly bound up with the reversible thermal 



We will suppose that the extension was made rapidly (i. e., ^^^'=^ ^' ^^" '"'^ ^''h the frictional thermal effect. Every time 



adiabatically) and consider what happens on retraction, which '^e tire tread impacts upon the road surface each part of the 



we will assume to take place rapidlv and immediately. First, ''"''''" '*°'^'^ traverses a stress-strain cycle. Even if we admit 



the potential energy of strain will nearly all be returned in the ''^^^ "^^ reversible heat generated during extension is reabsorbed 



form of useful work, exactly as in the case of the steel spring. ''"■"'"" C'^'ifnction. we have to consider the gradual building up 



Second, the reversible heat which on extension acted to increase °^ "iternal temperatures due to accumulation of frictional heat, 



the temperature of the sample will be reabsorbed, transformed '^^" "'"'^'''*<^ '" temperature, acting through the Joule effect, will 



into useful work, and therefore cause no energy loss. Finally, '«''«" "'^ extensibility of the heated rubber as compared with 



the frictional heat developed during extension will be increased adjacent regions at lower temperatures, thus setting up strains 



bv a further amount on retraction, at the expense of the poten- ^'^'^'^ doubtless play a role in breaker separation, the bane of 



tial energy of the stretched sample. large-size pneumatics. It is therefore highly desirable to work 



_,, , , , , ■ , out rubber compounds which will develop not only minimum 



Ihus, when the rubber has been stretched and allowed to re- , . ,. , v ,. i. . , ■ • -l, , ^ A .-, .• 

 frictional heat, but also minimum reversible heat. Quantitative 



'Presented before the Rubber Division at the meeting of the American measurements of the Joule effect with different compounds and 



Chemical Society, Chicago. Illinois. .September 6-10, 1920. j'cr„_ t u • i i .i • i'^. 



'Ames Holden McCready, Limited, Montreal, Canada. different cures would serve as an index to this quantity. 



