208 



THE INDIA RUBBER WORLD 



[January 1, 1920. 



Volume Increase of Compounded Rubber Under Strain/ 



F. Schippel. 



By H. 



THE FIRST RECOKD of this interesting phenomenon of volume 

 increase in rubber under strain dates back as far as 1884, 

 when Joule recorded the fact that the specific gravity of rub- 

 ber decreased upon stretching it. His test results stated a change 

 of specific gravity of 0.15 per cent for a 100 per cent stretch. 

 This is a very small increase, and therefore his experiments 

 were made upon comparatively pure rubber, unmixed with pig- 

 ments, as the present paper will show. 



In 1889, Mallock made tests upon pigmented rubbers of dif- 

 ferent kinds, but he made the volume elasticity tests upon the 

 samples only by applying pressure to the water in which he 

 immersed them, thereby simply corroborating the results of the 

 previous investigator. 



Again, in 1890, Sir William Thomson stated that a column of 

 rubber when stretched out suffers no sensible change in volume, 

 and that the contraction of any transverse diameter must be 

 sensibly equal to one-half of the longitudinal extension, and 

 rubber may therefore be regarded as an incompressible elastic 

 solid. This also is true of pure rubber. 



While studying the nature of the stress-strain curves for 

 rubber containing different pigments in varying quantities, the 

 writer considered the stability of the rubber surrounding each 

 particle of pigment in the rubber body, and thought that pos- 

 sibly when the rubber body was elongated sufficiently, the rub- 

 ber might pull away from the particles of pigment in their axes 

 of stress, and cause vacua to be formed on both sides of each 

 particle, the net result of which should be a considerable in- 

 crease in the volume of the rubber body as a whole. 



A preliminary test was made by preparing 

 a transparent vulcanized compound contain- 

 ing a fair proportion of medium-sized lead 

 shot. When this compound was stretched. 



5 grams of sulphur, and 337 grams of whiting, and press cured 

 for 30 minutes at 40 pounds' steam pressure, gave a volume 

 increase of 52 per cent at an elongation of 140 per cent, while 

 the estimated volume increase for a similar compound contain- 

 ing 215 grams of barytes in place of the whiting was 120 per 

 cent at the breaking point. In the former case, the average 

 value of Poisson's ratio up to the breaking point was 0.39 and 

 in the latter case 0.31. These values are not at all abnormal, 

 but their cumulative effect in a substance which has the ability 

 to withstand comparatively enormous elastic strains is worthy 

 of serious consideration from a physical standpoint. 



For smaller percentages by volume of barytes, the volume 

 increase was found to be less at the breaking point, and simi- 

 larly for larger volume percentages, due in the latter case to the 

 formation of local contractions similar to that of metals. 



It was also noted that when barytes was substituted by an 

 equal volume of lampblack, the volume increases for any given 

 elongation were smaller. 



Systematic tests were accordingly made upon a series of 

 compounds containing in various volume percentages one of 

 the following pigments in each case: barytes, whiting, zinc 

 oxide, chma clay, red oxide, lampblack, and carbon black. The 

 curves shown give graphically the results obtained. 



The common base was made up of 100 parts by weight of 

 fine Para, 5 parts sulphur and 30 parts litharge, the weight of 

 the test pigment added in any case being the volume index for 

 that case multiplied by the specific gravity of the pigment. 



A blank test was made by stretching the base only without 



the formation of vacua proceeded gradually until each lead shot 

 had its conical vacua on both sides in the direction of strain. This 

 was very satisfactory, and the test was immediately applied to mis- 

 cellaneous samples of rubber compounds, with the result that this 

 integral phenomenon was actually found to take place. It was 

 more noticeable in the red rubber than in the white. Also, after 

 keeping the samples under tension for some time, and then 

 releasing, temporary increase of volume was noted, which was 

 due to sub-permanent set, or the diffusion of gases into the 

 vacua, or both. In certain cases, the observed volume increases 

 under simple strain were 'very remarkable. For example, a 

 specimen made up of 100 grams of fine Para, 30 grams of litharge, 



'Published by courtesy of the .«imeric:in Chemical Society. Paper read 

 before the Hubbcr 'Division of the .American Chemical Society, at Phila- 

 delphia. Pennsylvania, September 4, 1919. 



any other pigment content. The curve obtained is marked "O" 

 on each series of curves for the pigments. 



METHOD OF PROCEDURE. 



Test rings, having a cross-sectional diameter of 14 inch, and 

 an outside diameter of 2% inches, were made from each com- 

 pound up to about 35 volumes content. For volumes above this, 

 the compounds were too stiff to make a perfect fit in the ring 

 mold, and flat slabs about 100 mils, thick were made, from which 

 flat rings were cut. The internal circumference of the rings was 

 approximately four inches. Each ring was stretched consecu- 

 tively over each of a graduated series of steel bars from three 

 to eight inches long, or as far as the ultimate stretch of the 

 rubber would allow, and the volume increase was calculated by 



