246 



THE INDIA RUBBER WORLD 



January 1, 1921 



which will tell a good deal about the quality of the product 

 Flocculated pigment readily accounts for many difficulties other- 

 wise inexplicable. Among these is the heating tip of tire treads 

 heavily compounded with carbon black. 



(2) A help in the analysis of rubber goods by showing the 

 compounding ingredients used, such as reclaimed rubber, fiber. 



The compounds were aged in an air bath at 70 degrees C. 

 Tires, however, heat from internal friction while running, and 

 this temperature is often exceeded. In solid tires, in fact, the 

 temperature at times exceeds 100 degrees C. 



The physical properties of the compounds after aging are 

 tabulated in Table II. 



Fig. 9 



Fig. 10 



Fig. 11 



Fig. 12 



Fig. 13 



and pigment. In the case of the pigment, it may even be pos- 

 sible to learn its source, because the same pigment made accord- 

 ing to different processes varies greatly in shape and size. Chem- 

 ical analysis shows no difference between blanc fixe and ground 

 barytes, but the microscope tells at once. 



Most work of this character is done by direct observation under 

 the microscope''without the trouble of photographing. By exam- 

 ining the slide with colored light, increased contrast is shown in 

 certain cases. It is planned to make observations with polarized 

 light, which should open a wide field for further investigation 

 and for identification of the pigments. It should be mentioned 

 also that staining of the section will bring out particles which 

 would otherwise be invisible, as, for instance, glue.* 



Table II — Physical Properties of the Compounds on Aging at 70* C. 



*\V. E. Wiegand, Canadi.in Chemical Journal, 4 (1920). 160. 



THE AGING OF CERTAIN RUBBER COMPOUNDS' 

 By I. R. Ruby and Harlan A. Depew^ 



THERE has been much discussion relative to the merits of car- 

 bon black and zinc oxide treads for automobile tires. The 

 outstanding objection to the black tread tire is that, although in 

 the beginning it wears comparatively v/ell, it later chips away at 

 an accelerated rate. 



Three compounds. A, B and C, were prepared and investigated 

 for the study of this problem. Table I gives their composition 

 by weight and by volume. These compounds are typical fast- 

 curing tread compounds, with the exception of A, which contains 

 a low percentage of pigment. Treads in which XX red zinc 

 oxide is the only pigment generally carry 55 to 58 per cent of 

 zinc oxide instead of only 46 per cent, as in this case. There is 

 no appreciable difference in aging, however, between compounds 

 containing 46 and 58 per cent zinc oxide, as shown by evidence 

 not included in this report. 



Table I — Composition of Compounds 



, By Weight ^ , By Volume > 



.■\ B C A B C 



First latex rubber 92 92 92 100 lOO 100 



Sulphur S.S 5.5 5.5 2.67 2.67 2.67 



Hexamrthylene tetramine 1.5 1.4 1.4 1.38 1.25 1.25 



XX red zinc oxide 84 98 22 15.0 17.5 4.0 



Carbon black 25 . . . . 14.5 



Lithopone 20 . . . . 5.0 



'Read before the Rubber Division of the American Chemical Society, 

 CbicaKO. Illinois. Scptrmbcr 6-10, 1920. 



•Research Laboratory of the New Jersey Zinc Co., Inc., Palmerton, 

 PennsyWania. 



It appears that the compound containing lithopone ages some- 

 what less satisfactorily than a straight XX red zinc oxide com- 

 pound. The carbon black compound is much inferior to the others 

 on the basis of tensile strength and elongation on aging. This 

 is especially serious when it is reaHzed that, as the percentage 

 of carbon black in a tread compound increases, the heat de- 

 veloped by internal friction increases faster than a linear function. 



The test specimens were about 0.17-inch thick, and accordingly 

 the tensile strength and elongation did not record an interesting 

 hardening on the surface as fully as thinner specimens would 

 have done. This hardening penetrated about 0.03 inch in 16 days. 

 The rubber compounded with carbon black showed deep surface 

 cracks from this effect by bending the rubber. There was no evi- 

 dence of this surface hardening in the case of compounds A and 

 B. This particular hardening is probably responsible to a con- 

 siderable degree for the accelerated wear 'of a black tread tire 

 with time, the hard surface being continually cracked and torn 

 away by road abrasion. 



The cause is uncertain, but appears to be oxidation. The ab- 

 sorptive power of carbon for oxygen has long been recognized 

 and it may be reasoned that carbon black acts as a catalyst and 

 oxidizes the soft rubber to hard rubber. 



Rubber to the value of f 16,551 was exported from British 

 East Africa and Uganda in 1919, as against £\4,026 in the 

 preceding year. 



