Properties of Hard Rubber. IX. Swelling in Various Liquids. (Part XIII of Swelling Of Rubber)
Authors: Parris R. W. , Scott J. R. 
Published: February 2011

Abstract
1. Literature on the swelling of hard rubber is reviewed (Section I). 2. Experiments are described on the swelling of two hard rubbers, with high and low combined sulfur contents, respectively, in a representative selection of 20 liquids at 34° C (Section II). 3. It is shown that at 34° C hard rubber is virtually insoluble in common organic liquids, but that it absorbs considerable amounts (up to 100 per cent) of some liquids, although unaffected by others (II, 4 and 5). 4. The time-swelling curves of hard rubber are usually similar to those of soft rubber, except when the swelling is large; in these cases the absorbed liquid apparently increases the permeability of the rubber and so alters the shape of the curve (II, 6(i)). 5. The swelling properties (swelling maximum, swelling time, increment) of the two hard rubbers in the 20 liquids, so far as they could be determined, are tabulated (Table VI) and discussed in relation to the degree of vulcanization of the rubber and the chemical and physical properties of the liquid, the following being the main conclusions: (a) With increasing combined sulfur content, the amount of liquid absorbed decreases, but the time required to reach saturation usually increases (II, 6(iii)). (b) The swelling action of practically all liquids decreases, as the degree of vulcanization increases, over the whole range from raw rubber to hard rubber, but the rate of decrease varies according to the chemical nature of the liquid. In consequence of this, aromatic hydrocarbons, halogenated hydrocarbons, and carbon disulfide have the strongest swelling action on hard rubber, whereas hydroaromatic hydrocarbons and especially paraffin hydrocarbons (and apparently fatty oils also) have very little or none; polar compounds are intermediate. An important result is that at 34° C hard rubber vulcanized to the stage corresponding to C5H8S is quite unaffected by paraffin hydrocarbons and linseed oil (II,6(iv) (a)). (c) There is no simple relation between the viscosity of a liquid and the time required to reach saturation, as with soft rubber, because strongly swelling liquids are absorbed relatively more quickly than weakly swelling liquids, apparently because they increase the permeability of the rubber (II, 6(iv) (b)). 6. Liquids with a strong swelling action often, but not always, cause hard rubber to, crack; a high combined sulfur content seems to make the rubber more resistant to this cracking (II, 6(v)). Notes are added on the mechanical properties of swollen hard rubber (II, 7). 7. The present results indicate that hard rubber intended to resist the action of organic liquids at temperatures not far above atmospheric should be vulcanized as fully as is consistent with its having the other properties required because, by so doing, its resistance to swelling and cracking is increased. The only possible objection to this at present evident is that a high degree of vulcanization may accentuate the very slow increase in swelling (increment) which continues indefinitely after saturation has been reached, although from the present results (Table VI) this does not appear to be usual, so that the objection does not seem serious. 8. It is recommended that, in future swelling tests on hard rubber, the test-pieces be thinner (1 or 2 mm.) than at present (5.5 mm.), as this would greatly accelerate absorption. By so doing, the experiments would be shortened and an interpretation of the time-swelling curves facilitated, because the distinction between the saturation and increment phases would be clearer. A lower test temperature might be advantageous also, since it would reduce the increment.