20 



THE INDIA RUBBER WORLD 



[October 1, 1920. 



and retraction, namely, QR or reversible heat and QF or fric- 

 tional heat, which is non-reversible and which accumulates when 

 a rubber article is subjected to repeated strains. 



The production of rubber compounds and cures for which 

 QF is a minimum should be one of the focal researches for us 

 all. The profound eflfcct of mineral additions upon this quantity 

 is already a part of technical knowledge. The fundamental 

 reasons for this inter-relationship are deserving of our best 

 thought. They lead us into the arcana of rubber structure. 

 INVERSION POINTS 



In 1898 Lundal discovered that if a given load was applied 

 to a rubber sample under gradually increasing temperatures, 

 there was one temperature at which the addition of the load in 

 question would cause no heating, and in fact above which it 

 would produce cooling. Conversely at any given temperature 

 he found that there was a critical load at which there would 

 be no thermal effect. The lower the temperature the lower was 

 the value of this critical load. Thus in a particular example 

 raising tlie temperature from 18 degrees C. to 58 degrees C. in- 

 creased the critical load from 44 to 102 grams. 



The technical importance of these points of inversion in the 

 Joule effect is manifestly very great. Suppose, for example, 

 we could so adjust the thickness of the friction and skim coat 

 between the plies of fabric in our casings as to bring the actual 

 strains on the gum stock in actual service on the road to the 

 critical point of inversion. Under such conditions there would 

 be no thermal effect, no heating up of the stock, no perishing, 

 no ply separation. Obviously it will be worth millions of dollars 

 to our industry to conduct a successful research along the lines 

 of determining the situation of these critical or neutral points 

 as the state of cure and composition of the various mixings are 

 systematically varied." 



TESTING CRUDE RUBBER 



Reference is made to the method of Schidrowitz which will 

 be found in detail in The India Rubber World, December 1, 

 1919, page 149. Briefly, Schidrowitz first notes that the stress- 

 strain curves for the same mixing, but with advancing cures, 

 come regularly down the paper and never intersect. The stiff 

 parts of the curves are, moreover, parallel to each other. 



Second, the inclination or s'.ope of the final part of the curve 

 is an index of the quality of the crude rubber under test. The 

 flatter the curve, that is, the less the slope, the better the crude 

 as ordinarily estimated. 



Joule, Kelvin, Bouasse, Rontgen and the other master physi- 

 cists were interested mainly in the correlation of the properties 

 of rubber, as rubber, with those of other materials. We, on the 

 other hand, are vitally interested in knowing how one rubber 

 and one compound differs from another in its physical proper- 

 ties. Reference will therefore now be made to some of our own 

 experiments designed to bring out the comparative behavior of 

 a few of the more generally used inorganic compounding in- 

 gredients. These included carbon and lampblack, china clay, red 

 oxide, zinc oxide, glue, whiting, fossil flour, and barytes. 

 BASIC MIXING 



In order to avoid the tedium of doing a series of cures for 



each mi.\ing, a base mix was developed containing. 



Parts by weight 

 Fine Para lOO 



Litharge 30 



Sulphur 5 



By volume this is : 



Volumes 

 Rubber 100 



Litharge 3 



Sulphur 2'/! 



In this mixing the accelerator and sulphur are so balanced 

 as to preserve a practically flat curing condition over a range 



^For a discussion of the cause of Joule effect and many other matters. 

 reference is made to a forthcoming volume by Professor G. S. Whilby •{ 

 McGill University, Montreal, Canada. 



of cure from IS to 45 minutes at 40 pounds of steam. To this 

 base mi.xing increasing amounts of each pigment were added 

 on the volume basis. The additions were continued until the 

 stock grew so dry and leathery on the mill as to be unworkable. 

 The cures were made in an ordinary laboratory press and the 

 test pieces stretched on a Scott machine. 

 BARYTES 



X"ote the unchanged curvature of the base mixing curve. In- 

 creasing additions of this pigment have merely the effect of 

 shortening the curve. Barj-tes is nothing but a diluent. It 

 adds no useful property to any compound, but on the other hand 

 detracts from both the tensile strength and elongation. For 

 this very reason very large proportions (up to 150 volumes) 

 could be incorporated into the hundred volumes of rubber before 

 the stock became unmanageable. 



FOSSIL FLOUR 



This pigment shows signs of disturbing the basic stress-strain 

 curve. There is less curvature. The curve has moved toward 

 the "load" axis. A compound containing fossil flour is definitely 

 stiffer than one containing barytes. However, after 30 volumes 

 have been added there is no more rotation of the curve, which 

 merely shortens, as in the case of barytes. The fossil flour parti- 

 cle is smaller than the barytes particle, and, in our opinion, the 

 change in behavior after 30 volumes is most simply explained 

 by assuming an agglomeration of the fossil flour particles, at 

 this stage, into larger complexes, generating less rubber surface. 

 The total quantity absorbed by the gum was in this case only 

 75 volumes. 



WHITING 



Here again the curve shows some displacement (stiffening) 

 up to an addition of 20 volumes of the pigment. Thirty volumes 

 adds nothing to the effect, and we assume that between these 

 two volumes agglomeration has set in. The ma.ximum absorp- 

 tion of pigment was in this case 125 volumes. Those who use 

 more than 20 volumes of whiting in a compound must disclaim 

 any beneficial effects on the physical properties. 



GLUE 



This was added in the jelly state. Up to 20 volumes there 

 was a definite displacement of the curve indicating that glue is 

 not a mere diluent, like barytes, but exerts a definite stiffening 

 or toughening action in a compound. The tensile at break is, 

 however, lowered. 



ZINC OXIDE 



This pigment shows a marked reinforcing or stiffening effect 

 on the compound. The tensile strength at rupture is maintained 

 undiminished up to a volume addition of 20 volumes. 



Beyond this the curve recedes, as in the case of barytes. Ag- 

 glomeration of particles has set in. Up to 20 volumes zinc im- 

 proves wearing power. Beyond this it partakes more and more 

 of the characteristics of a diluent. The best white treads 

 contain not much more than 20 volumes of zinc to 1(X) of rubber. 



The maximum absorption was in this case about 125 volumes. 

 RED OXIDE 



This useful (and sometimes treacherous) pigment shows a. 

 reinforcing action up to 15 volumes. Beyond this is agglomera- 

 tion. The tensile docs not hold up so well as with zinc. 

 CHINA CLAY 



China clay vies with zinc oxide as a re-inforcing agent. The 

 rotation of the curve is even more marked than with zinc, 

 although the breaking tensile is less well maintained. Twenty 

 volumes again represents the maximum loading without detract- 

 ing from the physical properties. Naturally, clays differ marked- 

 ly according to their origin and colloidal condition. The above 

 result must be regarded as only an individual finding. 



LAMPBLACK 



We now approach royalty in the pigment realm. Note the 

 steady, clear-cut, downward progression of the curves toward 



