498 



THE INDIA RUBBER WORLD 



[May 1, 1920. 



AMERICAN CHEMICAL SOCIETY. 



RUBBER DIVISION MEETING. 



THE SECOND MEETING ot American rubber chemists, organized 

 as the Rubber Division of the American Chemical Society, 

 took place at the fifty-ninth meeting of the society at St. Louis, 

 Missouri, April 14-15, 1920. 



Brief abstracts of the papers presented at the Rubber Division 

 meeting are given below : 



BHOJUNE ADDITION TO RUBBEH. 



When rubber is treated with bromine, both addition and sub- 

 stitution occur but by determining these separately the unsatura- 

 tion is found to be one double bond for each isoprene unit. 

 — W. K. Lewis and William H. McAdams. 



HECOVERY OF VOLATILE SOLVEKTS, 



Solvents can be recovered by absorption, compression or 

 refrigeration. Each method has its own distinctive field. These 

 fields are brought out and the chief features of each method are 

 also emphasized.— W. K. Lewis. 



THE DETERMINATION OF TRUE FREE AND TRUE COMBINED SULPHTIR. 



Sulphur is considered present as free and combined. Acetone 

 soluble sulphur may be partly combined with resins, etc., as 

 may the sulphur insoluble in acetone, heretofore considered as 

 combined with rubber. The total acetone extract is soluble in 

 alcohol, but if alcohol saturated with sulphur is used, none of 

 the truly free sulphur will dissolve and hence can be separated 

 from the remainder of the extract. Results show about 0.4 per 

 cent sulphur combined with resins. 



About 85 per cent of Hevca resins are saponifiable and hence 

 any resinous sulphur compounds insoluble in acetone may be 

 soluble in alcoholic potash. Acetone extracted sample is boiled 

 8 hours with alcoholic potash and about 0.25-0.30 per cent 

 sulphur extracted. 



So far only pure gum stocks have been investigated.— W. J. 

 Kelly. 



The activity of small amounts of extra light magnesia was 

 compared with the eflfect of similar amounts of certain organic 

 substances. The load required to efifect a given extension was 

 found to be fair measure of the rate of cure of the mixture 

 which contained magnesia. This was not found to be true for 

 the mixtures containing the organic accelerators. The accelerat- 

 ing activity of magnesia in small amount was found to be of 

 secondary character, acting in conjunction with certain extrane- 

 ous substances, probably nitrogeneous, present in the rubber. 

 The amount of these extraneous substances was found to limit 

 the activity of magnesia as an accelerator. — G. D. Kratz and 

 A. H. Flower. 



OBSCTTRING POWER OF PIGMENTS. 



The fineness of pigments can be determined by measurements 

 of turbidity and be expressed in absolute units. — 'W. K. Lewis. 



EFFECT OF COMPOUNDING INGREDIENTS. 



Compounding experiments in which up to 50 volumes of 

 filler were added to 100 volumes of rubber are described. The 

 values of tensile strength are also corrected for actual volume 

 of rubber present. Tensile strength calculated on area at rest 

 is unfair to pure gum stocks. Tensile at break is suggested as 

 a better means of comparison. This is calculated on the actual 

 cross section at break rather than on the original cross section. 

 A correction factor, due to volume increase during stretching, 

 is also involved. A visual picture of the structure of rubber is 

 given. This is based on the hypothesis that large colloidal 

 aggregates function as elastic fibers and the smaller as plastic 

 material. Vulcanization is said to lock up these fibers to form 

 a network. — C. O. North. 



PHYSICAL TESTING OF RUBBER GOODS. 



The report of the Committee on the Physical Testing of Rub- 

 ber Goods was accepted as it stands with the understanding that 

 this committee will attempt to correlate its work with the various 

 other technical societies in the country in order to get unit speci- 

 fications agreeable to all. 



A METHOD FOR THE DETERMINATION OF PERMANENT SET ON VUL- 

 CANIZED RUBBER GOODS. 



Before vulcanization, rubber compounds are plastic. During 

 the cure they are changed to a more or less elastic state. How- 

 ever, there is always a certain residuum of plasticity in the 

 cured compounds. In addition to this there is undoubtedly an 

 arrangement of the elastic material which may be likened to 

 a network of fibers which do not all lie in a line with the 

 direction of the stretching. Under tension these fibers are 

 straightened out and do not again return to their original posi- 

 tions. There is evidence that these fibers may be of irregular 

 shapes, having bends or knuckle-like projections such that in one 

 stretch these projections will cause the fibers to catch, but upon 

 release of tension this hold will be loosened so that on another 

 stretch measurably more elongation will be produced by the 

 same tension. If these elastic fibers are stretched beyond their 

 elastic limit, some of the bonds will be broken causing an 

 apparent set which should not be counted against the stock. 

 The set test then should be such that it will bring out the 

 inelasticity caused in the stock, (1) by plastic flow, (2) complete 

 straightening of the fibers, including those tangled. 



Three methods have been considered for accomplishing this, 

 the possibilities of which may be summed up as follows: 



(A) Decrease in tension under given constant elongation. 



. This method shows no advantage and results do not indi- 

 cate any direct association with the thing we wish to meas- 

 ure ; it is a case of measuring length by units of weight. 

 The test involves expenditures of time and money, which 

 render it impractical. 



(B) Increase in elongation against time under constant load. 

 Reliable results can only be obtained by repeated appli- 

 cations of the load, each of which should remain on the 

 piece for a matter of hours. In spite of its merits, this test 

 was therefore abandoned because it was considered im- 

 practical. 



(C) Stretching for a given period of time to a given elonga- 

 tion, releasing, resting, and repeating until there is no 

 further set. 



The most practical method for measuring set reliably by 

 this method was worked out as follows : 



Compounds were selected representative of pure gum, 

 cheap friction, high and low gravity tread, cheap tread, 

 and cheap mechanical goods stocks. Four each of these 

 pieces were stretched to ten per cent of the breaking elonga- 

 tion four times, held for ten minutes each time, rested ten 

 minutes between stretches and finally ten minutes after the 

 final release the set was measured. The results appear in 

 Table I. 



Table I. Percentages of Set. 



Per Cent Elongation. 



10. 



30. 



40. 



50. 



60. 



70. 



Pure gum 0.0 2.0 8.0 6.0 6.0 8.0 16.0 



Cheap friction 0.0 1.0 3.0 6.4 8.5 16.5 27.5 



Friction 0.0 0.0 4.0 0.0 1.0 8.0 20.0 24.0 



Low gravity tread.... 0.0 1.0 7.0 10.0 17.0 25.5 41.0 51.0 



High gravity tread... 00 1.0 7.0 12.6 19.0 29.5 59.0 53.S 



Cheap tread 0.3 2.0 6.0 12.5 27.5 33.5 40.0 it.S 



Mechanical 1.0 3.0 5.0 14.5 23.5 52.0 42.0 S5.0 



In every case a point was reached beyond which the set went 

 up abruptly. This point, we believe, represents the limit of 

 the range. It seemed impractical to make a separate limit for 

 each compound or class of compounds. In the majority of 

 cases the break in the curve occurred between 60 per cent and 70 



