Estimation of the Stability of Rubber Solutions

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Abstract Various types of rubber products, such as seamless goods and rubberized fabrics, are prepared from rubber solutions and are vulcanized either by the hot or cold process. “Cold vulcanization” is effected either by sulfur chloride vapor or by very dilute solutions of sulfur chloride in solvents such as carbon disulfide, carbon tetrachloride, or benzine. “Hot vulcanization” is carried out by heating with sulfur, organic accelerators, and inorganic substances such as colloidal zinc oxide, to “activate” the accelerators. To safeguard against any unforeseen difficulties, it is necessary not only to ascertain how much tendency mixtures containing powerful accelerators have to vulcanize, but above all to determine for how long a time a solution remains stable, i. e., does not commence to vulcanize during ordinary use. To obtain an insight into the practical utility of a mixture, it is in general necessary merely to heat the mixture at 110° C. in a press. A mixture can be regarded without hesitation as satisfactory for practical use if it shows no indication of vulcanization after one hour at this temperature. On the other hand, products prepared from such a solution should as a rule vulcanize fully in 30 mm. at 125° C. or perhaps in 40–50 mm. at 110° C. In view of this, the practical utility of a mixture is judged by the tendency for incipient vulcanization to take place when the mixture is heated at 70° C. in a press. In the case of solutions of rubber mixtures which contain all ingredients milled into the rubber before the solution is prepared, incipient vulcanization is evident by gelation of the solution, i. e., a change from sol to gel. With mixtures of this kind, incipient vulcanization at 70° C. in a press may be accepted as a reasonable criterion, but in no case should it be regarded as a trustworthy indication of the stability of a solution up to the point of gelation, for the rate of vulcanization of the solution to the point of gel formation, and during gel formation itself, is influenced considerably by the constitution and the concentration of the solvent. The concentration of the solution and the degree of plasticity of the rubber mixture likewise influence gel formation. A test of the dry mixture at 70° C. in a press for incipient vulcanization does not take account of these factors.