The Determination of True Free Sulfur and the True Coefficient of Vulcanization in Vulcanized Rubber

Abstract The methods so far proposed for the determination of the free sulfur in vulcanized rubber depend upon the removal of the sulfur by extraction of the rubber with hot acetone, and subsequent oxidation to sulfuric acid of the sulfur in the extract. The extract is liable however to contain, in addition to sulfur in the elementary state, organic compounds containing sulfur derived from the rubber resins, accelerator, or antioxidant. In the subsequent oxidation this sulfur will be oxidized to sulfuric acid to an extent which depends on the method of oxidation used. In the present work, a method of analysis has been developed which is more rapid than the existing methods, and in which it is considered that the determination of the sulfur present in the elementary state is less likely to be affected by organic compounds containing sulfur. It has been found that when vulcanized rubber placed in contact with a metal, such as tin or aluminum, is boiled in hydrochloric acid, hydrogen sulfide is generated by the action of the nascent hydrogen produced. This reaction occurred with rubber which had been previously treated with acid alone to decompose metallic sulfides, but no hydrogen sulfide was obtained from rubber which had been previously extracted with acetone and was free from metallic sulfides. As these results showed that the reactions were not due to the presence of mineral sulfides, or of sulfur combined with the rubber, it was concluded that the hydrogen sulfide was formed by the reduction of the free sulfur.

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The Determination of Free Sulfur by a Volumetric Method Part II

Rubber Chemistry and Technology ◽

10.5254/1.3543561 ◽

1933 ◽

Vol 6 (4) ◽

pp. 512-517

Author(s):

W. D. Guppy

Keyword(s):

Hydrogen Sulfide ◽

Organic Compounds ◽

Sulfur Compounds ◽

Acetone Extract ◽

Volumetric Method ◽

Organic Sulfur Compounds ◽

Vulcanized Rubber ◽

Volumetric Determination ◽

Free Sulfur

Abstract 1. The method previously described for the volumetric determination of free sulfur in vulcanized rubber has been compared with the older gravimetric methods in cases where other organic compounds containing sulfur are present in the vulcanizate. 2. The volumetric method gave lower results than the methods involving oxidation of the acetone extract in the case of vulcanized rubber containing aldehydeamine condensation products of thiouram disulfide compounds. This indicated that the sulfur combined with some organic compounds was not reduced by the reagents used. 3. The results with the accelerator tetraethylthiouram disulfide showed that in some cases part of the sulfur in organic compounds was reduced under the conditions of the reaction. 4. The acetone-soluble and the acetone-insoluble portions of brown substitute contain organic sulfur compounds. Part of the sulfur in these compounds was reduced to hydrogen sulfide by the action of nascent hydrogen. 5. The sulfur compounds present in white substitute were stable toward the reducing agents used in the estimation of free sulfur. 6. The sulfur compounds formed by the vulcanization of ebonite were in part reduced to hydrogen sulfide with tin and acid. Variations in the composition of the mixing and in the vulcanizing conditions altered the amount of these reducible compounds. 7. The volumetric method previously described cannot be used for the determination of the free sulfur in ebonite, brown substitute or in vulcanized rubber containing brown substitute. In the case of vulcanized rubber containing brown substitute or of ebonite the method can be used to determine the amount of sulfur in the acetone extract. 8. The volumetric method can be employed for the determination of the free sulfur in soft vulcanized rubber containing white substitute and in reclaimed rubber.

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Determination of Free Sulfur in Stocks Cured with Sulfur Containing Accelerators

Rubber Chemistry and Technology ◽

10.5254/1.3539921 ◽

1962 ◽

Vol 35 (2) ◽

pp. 498-500

Author(s):

E. P. Khehaskova ◽

N. A. Okhapkina ◽

V. N. Provorov

Keyword(s):

Sodium Sulfite ◽

Sodium Stearate ◽

Cadmium Acetate ◽

Strontium Nitrate ◽

Vulcanized Rubber ◽

Normal Method ◽

Sulfur Containing ◽

Sulfite Solution ◽

Free Sulfur

Abstract For the determination of free sulfur in vulcanized rubber the normal method is the sulfite method, consisting of the heating of a sample of rubber with a solution of sodium sulfite. The free sulfur reacts with the sulfite, forming thiosulfate, which is determined by iodometric titration. The unreacted excess of sulfite is removed with formalin. The method gives good results in the analysis of stocks which do not contain sulfur-containing accelerators. In the presence of these accelerators the results are high: in some cases the method is quite inapplicable, as it is impossible to establish the endpoint of titration accurately. This may be explained by the circumstance that the accelerators and the products of their decomposition during vulcanization can pass into the solution of sodium sulfite and react further with the iodine. The method of determination of free sulfur proposed by Hardmann, consisting of the acetone extraction of the rubber in the presence of copper gauze, is protracted and complicated. A certain improvement in the sulfite method of Bolotnikov and Gurova was introduced in the ASTM method (1952). Sodium stearate is added to the sodium sulfite solution for better wetting of the rubber, and a certain amount of paraffin to prevent the formation of foam. After termination of heating the solution the stearic acid is precipitated with strontium nitrate and certain accelerators with cadmium acetate. For the majority of stocks this method gives correct results, but in a number of cases it is difficult to establish the end of titration at all.

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Determination of True Free Sulfur in Vulcanized Rubber. A Modification of the Thiocyanate Method

Rubber Chemistry and Technology ◽

10.5254/1.3539712 ◽

1936 ◽

Vol 9 (1) ◽

pp. 172-177

Author(s):

Syukusaburô Minatoya ◽

Itirô Aoe ◽

Idumi Nagai

Keyword(s):

Silver Nitrate ◽

Nitrate Solution ◽

Potassium Cyanide ◽

Silver Nitrate Solution ◽

Potassium Thiocyanate ◽

End Point ◽

Modified Method ◽

Vulcanized Rubber ◽

Free Sulfur

Abstract The limit of quantity of potassium cyanide was determined as regards the interference with the end point of the titration of potassium thiocyanate with silver nitrate solution. Acetone was found to be a suitable solvent for the separation of potassium cyanide from potassium thiocyanate. A new modified method was proposed for the determination of true free sulfur in vulcanized rubber, by the use of acetone to remove the cyanide which renders obscure the end point of the titration of thiocyanate with silver nitrate solution. The proposed method was applied to the analysis of pure sulfur and some rubber goods, and was ascertained to be accurate and easy to carry out, requiring no special apparatus.

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Determination of Free Sulfur in Vulcanized Rubber Mixtures

Rubber Chemistry and Technology ◽

10.5254/1.3543401 ◽

1952 ◽

Vol 25 (2) ◽

pp. 365-370 ◽

Cited By ~ 1

Author(s):

Horst Frey

Keyword(s):

Acetone Extraction ◽

Vulcanized Rubber ◽

Widespread Acceptance ◽

Free Sulfur

Abstract Numerous papers on the determination of free sulfur in rubber mixtures have been published. Most of these are in the older literature, and they offer various methods for determining free sulfur. In principle the methods can be divided into two general groups. The procedures described in the first and larger group involve preliminary acetone extraction before the actual determination of the free sulfur, whereas the methods comprising the second group, which include only two different procedures, do not involve acetone extraction. Of these two methods, only that of Oldham, Baker, and Craytor has found widespread acceptance.

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Determination of True Free Sulfur in Vulcanized Rubber

Industrial & Engineering Chemistry Analytical Edition ◽

10.1021/ac50098a021 ◽

1935 ◽

Vol 7 (6) ◽

pp. 414-416 ◽

Cited By ~ 6

Author(s):

Syukusaburô Minatoya ◽

Itirô Aoe ◽

Idumi Nagai

Keyword(s):

Vulcanized Rubber ◽

Free Sulfur

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Analysis of Vulcanized Rubbers. Influence of Accelerators on the Determination of Free Sulfur by the Copper Spiral Method

Rubber Chemistry and Technology ◽

10.5254/1.3543454 ◽

1952 ◽

Vol 25 (4) ◽

pp. 956-958

Author(s):

J. Mann

Keyword(s):

Hydrogen Sulfide ◽

Copper Sulfide ◽

Acetone Extract ◽

British Standard ◽

Standard Methods ◽

Vulcanized Rubber ◽

Potential Source ◽

Source Of Error ◽

Free Sulfur

Abstract The copper spiral method is being inserted in a revised edition of the “British Standard Methods of Testing Vulcanized Rubber”, and this raises the question of what sulfur compounds react as “free” sulfur. In this method, “free” sulfur is considered to be that part of the sulfur which is present in the acetone extract and which reacts with a copper spiral, placed in the acetone during the extraction, with the production of copper sulfide, the amount of which can be estimated after its removal from the acetone. Since most accelerators contain sulfur, it is obvious that the presence of accelerators or accelerator fragments is a potential source of error. Fourteen accelerators of various types were therefore examined, and it was found that some react with copper, producing compounds which evolve hydrogen sulfide on treatment with hydrochloric acid.

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