December 1, 1920 



THE INDIA RUBBER WORLD 



171 



What the Rubber Chemists Are Doing 



The "Slope" or "Type" of the Rubber Stress- Strain Curve' 



By Dr. O. De Vries 



IN a recent paper' Schidrowitz, Goldsborough and Hatschek 

 have discussed the nature of the stress-strain curves of vul- 

 canized rubber-sulphur mixtures, and especially the mathe- 

 matical solution of tliese curves as belonging to the conchoid 

 fainily. The figure for "slope" or "type" plays a prominent role 

 in these calculations. 



Doubtless the slope of the stress-strain curve is an important 

 figure, representing as it does the increase in load necessary to 

 produce a certain elongation. The fact that the stress-strain 

 curve at high elongations (at least for rubber-sulphui- mixtures, 

 with which this paper deals exclusively) ends in a straight line, 

 so that the slope of this part of the curve is a constant, gives a 

 special importance to this figu;-e, which represents the resistance 

 to stretching, or the distensibility, at liigh elongations. The 

 higher the figure for slope of this part of the stress-strain curve, 

 the more easily the vulcanized product already stretched to ten 

 or more times its original length, yields to a further increase 

 of load, and the weaker it is. 



SLOPE OR TYPE 



Schidrowitz and his coworkers have called this property 

 "type." It shows markedly the differences between higher and 

 lower grades of rubber, and is a better index of the properties 

 of the lower grades than the tensile strength, -the figure for 

 which, in such cases, is often uncertain because of the presence 

 of particles of dirt, etc. The different grades of plantation- 

 rubber, in our testing work, gave the following figures for "slope" 

 or "type," determined by the method of Schidrowitz : 



Normal 



Average Figures Limits 



First quality crepe 35-8 34 — 38 33 — 39 



Smoked sheet 36.7 35 — 39 33 — 40 



Crepe from lump 37.3 .(5— 38V5 33'/i—W 



Crepe from tree scrap 38.8 37 — 41 SS'A—^A'/j 



Crepe frrrni bark rubber 42.9 40—43 38 —47 'A 



Crepe from earth rubber 37.6 36 — 39 34 — 40 



Crene fniin washings 39.1 37 — 41 SSyi — 46 



The highest figure was 53', found in a sample of very inferior 

 crepe from washings; the lowest figure, 32, is sometimes met with 

 in crepe from matured rubber. 



Especially when figures of 40 or higher are found for slope, the 

 sample may be expected to be inferior, and the higher figures 

 generally indicate a rubber which on keeping is liable to become 

 tacky. Whether in the first .grades a difference in slope between 

 35 and 38 has any practical importance, remains to be seen. 

 Tlinugh, theoretically, a rubber with a slope of only 34 to 35 is 

 stronger and therefore preferable, it is not yet clear whether the 

 difference is sufficiently great to affect the manufacture. 



DETERMINATION OF ■SLOPE" 



The determination of "slope" is easy and necessitates no addi- 

 tional testing, as the stress-strain curve obtained in the deter- 

 mination of tensile strength may serve to read the slope also ; 

 and as slope represent a separate property, independent of tensile 

 strength of rate of cure, it certainly deserves more attention than 

 has hitherto been given to it. The more dependent properties 

 one takes into consideration, the !)cttcr a substance with prop- 

 erties so complicated as rubber can be judsed. 



It has been shown* that a close relationship exists between the 

 slope and the permanent set, when both are determined for 

 mixtures of 92'/o rubber and V'A .sulphur, vulcanized to our 

 standard state of cure (length of 990 per cent at a load of 1.30 

 kilograms; coefficient of vulcanization approximately 5). The 

 closer nature of this relationship has not yet been worked out, 

 but there appears to be no doubt that this relationship is founded 



on the intrinsic properties of the vulcanizate, and that there is 

 one factor — be it structure, composition, or some otlier — which 

 causes a certain rubber to stretch easily at high elongations, and 

 that at the same time, after releasing, show large deformation. 



CHARACTERISTICS OF 'SLOPE" 



It is not yet clear which intrinsic property of the rubber is 

 responsible for the slope, and which factors in preparation have 

 an influence on it. From our investigations the following facts 

 are brought out : 



(1) The slope becomes greater (the rubber less resistant to 

 stretching) by prolonged and heavy tapping. 



(2) The slope decreases by maturation* (decomposition on 

 keeping the still wet coagulum, giving a quick-curing rubber) ; it 

 also decreases by the use of sulphite and bisulphite in the latex, 

 which prevent surface-o.xidation and discoloration of the coag- 

 ulum. 



(3) The slope increases by coagulation with alcohol', hy the 

 action of lower organisms causing spots on crepe, and by traces 

 of copper salts, causing tackiness; also by strong heating of fresh, 

 wet coagulum. 



It would seem that the slope is smaller (the rubber betterj, the 

 purer the rubber is, and greater, the more decomposition-products 

 are present. The exact nature of these changes is, however, far 

 from clear. Addition of foreign substances (such as gypsum or 

 talc) to the rubber-sulphur mixture does not alter the slope, 

 which also remains the same for mixtures with different contents 

 of sulphur.* 



"SLOPE " AND INCREASING TIMES OF CURE 



One of the chief points which Schidrowitz and his coworkers 

 bring forward is that tlie upper ends of the stress-strain curves 

 tor different states of cure run parallel, and that the slope, 

 determined by their method, is a constant, not changing with 

 increasing times of cure. 



This is certainly not correct for curves obtained by our method 

 of testing. We have on several occasions' reproduced sets of 

 stress-strain curves which show clearly that the slope of the 

 upper straight part is greater for less cured samples, and becomes 

 smaller and smaller the further the sample is cured. This was 

 regularly found to be the case in our testing of thousands of 

 samples. Table I gives some figures representing this decrease 

 in slope with increasing time of cure. 



The same difference may be noted in the .sets of curves pub- 

 lished by B. J. Eaton and his coworkers for mixtures of 90 parts 

 of rubber and 10 of sulphur; though no figures are given, and 

 the slope of the curve, in whichever form, is not determined in • 

 their testing work, the reproductions often' show clearly enough 

 that the end of the curve becomes less steep the more the cure 



'Journal of the Society of Chemical Industry, September 30. 1920, 308t. 



'Jmirnal of the Society of Chemical Industry, 1919, 347 T; The India 



Rubber World. December 1. 1919, 149 . # », xnt i«dia 



'Tournal of the Society of Chemical Industry, 1919, 92t. 



Industry Xm n6o"' ^' "'""'<''""'"- .T"'"-"^! of <h'e Society of Chemical 



'O. de V'rics, .\rchief voor de Rubbcrcultuur, 1918, 2. 237 97 and S57- 

 1920, 4, 217. ' 



T 'P\y^ ^'/„'fS "5^ "v^I "/llendoorn. Journal of the Society of Chemical 

 Industry 1919, 38t: The India-Rubber Tournal. 1919, S7. 116S; Archilf 

 voor de Rubbcrcultuur, 19IS. 2. 7f?3. 791. -nrtmci 



'Journal of th- Socjety rf Ch-miral Industry. 1919. 91t; India Rubber 



m? 1 217- 1918 2 i?) ' ""■ ^"^'"^ ''°'"' de Rubbercultuur, 



'Journal r,f the Society of Chemical Industry. 1916. 715. 1046: and 

 .^gncultural P.ulletin, F. M. S., No. 27. 



