The Behavior of Ebonite towards Organic Liquids

Abstract

Abstract (a) Experiments are described on the swelling of rubber-sulfur ebonites in numerous organic liquids, with special reference to the influence of the nature of the liquid, the ratio of rubber to sulfur and time of cure of the ebonite, and the temperature. (b) The rubber-sulfur compound constituting ebonite is not appreciably soluble in any of the liquids examined at 34° C. or in aliphatic hydrocarbon liquids at 100° C. (c) Ebonite may absorb as much as its own volume of some liquids but is quite unaffected by others. (d) The swelling curve often shows the same features as that for soft vulcanized rubber, but an important difference arises when the absorption is large, because the absorbed liquid progressively increases the diffusion constant and so changes the shape of the curve. For the same reason, also, the swelling capacity of the liquid has a greater influence than its viscosity on the time required to reach maximum absorption. (e) Increasing the combined sulfur content of the ebonite reduces the amount of liquid that it will absorb and renders the absorption much slower. In making ebonite to withstand the swelling action of organic liquids, therefore, the aim should be to obtain, by using an adequate proportion of sulfur and sufficiently long vulcanization, the highest combined sulfur content consistent with the required mechanical properties; over-vulcanization, however, may be detrimental. It should be noted that slowness of absorption is in itself an important advantage in uses involving only intermittent periods of contact with volatile liquids which can dry out again between these periods. (f) The extent to which swelling is reduced by increasing the combined sulfur content from the soft rubber stage to the ebonite stage varies greatly with the nature of the swelling liquid. Two important consequences of this are: firstly, with aliphatic hydrocarbon liquids the swelling action practically vanishes when the ebonite stage is reached; and secondly, conclusions regarding the swelling of ebonite cannot be based on data for soft rubber, because the relative swelling effects of various liquids are quite different in the two cases. (g) The inertness of aliphatic hydrocarbons such as paraffin and transformer oil towards ebonite persists at temperatures up to 100° C, that is, far above the “yield temperature” or softening point of the ebonite.