28 



THE INDIA RUBBER WORLD 



[October 1, 1920. 



Regarding the condition wherein the complete blackening of 

 the whole mass of rubber occurs, this may be due to the composi- 

 tion of the rubber mixing (which should not contain accelerators 

 that have as the principal part of their composition alkali hydrox- 

 ide or sulphide), as these would act on the antimony in a similar 

 way to the iron salts mentioned, causing solution and subsequent 

 precipitation of the antimony as black sulphides. It should be 

 possible with a good antimony and the right kind of iron oxide 

 (if it is desired to use these two pigments in conjunction) to 

 produce a good red tube, although not of as light a color as with 

 antimony alone. In cases, therefore, where these two materials 

 are used in conjunction, and where blackening has occurred, a 

 blank should be tried out, using the antimony alone. It will be 

 generally found, if the vulcanizing conditions are comparable 

 to that of bulk, that the antimony is at fault because of its insta- 

 bility under the vulcanizing conditions. 



Where crimson antimony has been found to be unstable, if a 

 little magnesium oxide is added to the mixing and it is cured in 

 a mold, the antimony color is preserved in its rich bright shade. 



Similarly, when the same mixing is cured in open steam, the 

 outside only becomes darkened. This varies from brown to coal 

 black, according to the excess of magnesium oxide used and with 

 the degree of instability of the antimony, while under the surface 

 the color will be found equal in shade to that of the press-cured 

 result. It has been demonstrated that a good red color for open 

 steam articles with an unstable antimony may be obtained if care 

 is taken to determine by trial the exact amount of magnesium 

 oxide to use in the mixing rather than to use an excess. 



With regard to the suggestion that red iron oxides are prone 

 under vulcanization to blacken with the formation of ferrous 

 sulphide, our experience is that those oxides of brick red, > on- 

 sisting chiefly of Fe^Os, do not change color except for a slight 

 darkening toward a chocolate shade on the surface of open 

 steam cured goods, while the purple oxides and brown umbers 

 keep their color with very little change. In colored rubber work 

 generally, it should be borne in mind that open steam cured re- 

 sults must not be confounded with the results obtained from 

 molded or press-cured, as the conditions are entirely different as 

 regards the effect on the pigments used in the mixings. 



The cause of the antimony blackening when cured either with 

 or without the admixture of iron oxides does not seem to arise 

 from the small trace of acid that is generally present in the anti- 

 mony pigment, though anything in the nature of a real acid excess 

 would tend to have a blackening effect, as explained in the previ- 

 ous cases, by the ultimate solution and reprecipitation of a small 

 part of the antimony. Even with a mixing wherein an acid sub- 

 stitute was used, this would practically be neutralized by such 

 materials as whiting, magnesium carbonate, lime, or calcined 

 magnesia that are generally present in such mixings where white 

 substitutes are used. 



It must be remembered that it is the nature of all red sulphides 

 of antimony to revert to the black tri-sulphide when subjected to 

 sufficient heat. In the inert atmosphere of a gas such as carbon 

 dioxide, the temperature at which such blackening is complete is 

 155 degrees C, the change of color at this temperature being prac- 

 tically spontaneous. Longer periods at lower temperatures (145- 

 150 degrees C.) will ultimately produce the same effect. It is 

 therefore advisable to cure red goods at as low a temperature 

 as possible if the best results as regards color are desired. In 

 this connection it is well to point out that although an antimony 

 may be found to blacken when heated alone in steam at the tem- 

 perature to which it is to be subjected during the ultimate vulcani- 

 zation when compounded, it does not necessarily follow that it 

 will give bad results, because it has been proved in practice that 

 the rubber acts to a large degree as a protective coating to the 

 red antimony particles and so prevents the discoloration taking 

 place. 



In conclusion, to prevent the blackening of all red goods con- 

 taining antimony, (1) see that the mixing is correct, (2) select 



the quality of the materials in direct regard to the specific pur- 

 pose for which they are intended, (3) make trial mixings com- 

 pounded and cured under conditions comparable to those used 

 in the factory. 



PERMEABILITY OF RUBBER TO GASELS" 



By J. D. Edzixirds' and S. h'. Pickering 



THEOBY OF PEBKEABILITY 



One object of this investigation was to establish, if possible, 

 a quantitative relationship between the permeability of a film 

 of rubber to any particular gas and the various factors on which 

 it is dependent. Only a portion of the program was completed, 

 however, before it became necessary to discontinue the work. 



A simple and satisfactory picture of the process is one of 

 dynamic equilibrium in which the gas is dissolved at one side 

 of the rubber at a rate proportional to its solubility and partial 

 pressure, and diffuses through the rubber where it evaporates 

 from the other side. The same process takes place in the op- 

 posite direction so that the net transference of gas is proportional 

 to the difference in the partial pressures at the two faces of the 

 rubber. Because of the lack of data it is not feasible to analyze 

 the relations between solubility and rate of diffusion through 

 the rubber. The permeability in every case investigated increases 

 rapidly with increase of temperature. According to Kayser' 

 the solubility of both carbon dioxide and hydrogen decreases 

 with increase of temperature. If this be true there must be a 

 rapid decrease in the internal resistance of the rubber to the 

 passage of the gas, because the ordinary temperature coefficient 

 of gaseous diffusion is unable alone to account for the facts. 



A rough parallel, with notable exceptions, may be drawn be- 

 tween the permeability of rubber to different gases and to the 

 boiling points of the gases. In general, the higher the boiling 

 point of the gas the greater the rate at which it penetrates rub- 

 ber. The specific chemical characteristics of the gas and of the 

 rubber colloid determine, however, the solubility, rale of pene- 

 tration, etc., and not enough is known of them at the present 

 time to warrant further speculation. There are, however, many 

 interesting fields of investigation opened by this work, and the 

 results should be extremely useful in the many cases where the 

 behavior of rubber in contact with gases is concerned. 



SUMMAHY 



1. The permeability of rubber compounds varies with the com- 

 position as would be expected. The aging of rubber films is 

 accompanied by a decrease in permeability ; a similar decrease 

 may be effected by overvulcanization. The rubber, which shows 

 a very low permeability for these reasons, is usually very much 

 deteriorated and frequently brittle, so that it is a disadvantage 

 from the standpoint of gas-tightness. 



2. The permeability to any gas is found to be directly propor- 

 tional to its partial pressure provided the total pressure is con- 

 stant. The variation of permeability with total pressure de- 

 pends on the thickness of the rubber, the way in which it is 

 supported, etc. 



3. The permeability to hydrogen is inversely proportional to 

 the thickness of the rubber. No other gas was tested in this 

 respect. 



4. The specific permeability to hydrogen at 25 degrees C. of 

 vulcanized rubber similar to the grade known as dental dam 

 is about 20 X 10"° cc. per minute. This value varies somewhat 

 with the age and chemical characteristics of the rubber. 



5. The temperature coefficient of permeability is quite high. 

 For example, in the tests at 100 degrees C. the permeability to 



'Condensation of a comprehensive report to be issued by the United 

 States Bureau of Standards. 



^Physical chemist. .Mumirmm Co. of America. Formerly with United 

 States liurcaii of Stand.ird«. 



"Associate Chemist, United Slates Bureau of Standards. 



*Wied. Ann., volume 43, page 544, 1891. 



