Tests on Crepe Soling. III. Water Absorption

The main results obtained are summarized below: (1) The following observations made by previous workers are confirmed. The absorption by crepe immersed in pure water continues for a very long time, if not indefinitely; the initial rate of absorption and the amount absorbed after long immersion increase rapidly with rise of temperature. The influence of temperature is probably due largely to its effect on the mechanical properties of the rubber, the softening induced by heat facilitating the distension of the rubber. (2) Soling crepes vary enormously in rate of water absorption; this result is ascribable to a variety of causes, as there is direct evidence that water absorption is influenced by the number of plies and the ease of penetration of water between the plies, while the porosity (permeability) of the rubber and its content of water-soluble matter must also come into play; the rigidity of the rubber would be expected to have some influence, although this is not evident in the present experiments. (3) Water absorption does not differentiate sharply between estate and factory-prepared soling crepes, though there are indications that on the average factory crepes absorb water somewhat less rapidly. (4) Among the soling crepes examined, water absorption bears no obvious relationship to mechanical properties or solution viscosity. (5) Addition of solutes to the water in which rubber is immersed reduces the water absorption, and this occurs quantitatively in accordance with the osmotic theory of water absorption. These results indicate that water-absorption tests do not in general yield any information as to the mechanical or physical properties of crepe soling or its mode of manufacture, and are therefore of value only in relation to the water-absorbing properties of the material. Absorption at atmospheric temperatures is so slow that under normal conditions of use it probably does not affect more than a thin surface layer. This layer, however, is responsible for the grip of the soling on the pavement, and if absorption of water influences this gripping power, the control of water-absorbing capacity becomes important. The wide variation at present found between different crepes suggests that an investigation of the factors governing water absorption would enable this property to be closely controlled over a wide range. Penetration of water between the plies, which will occur especially when the soling is somewhat worn, may be more serious than absorption at the outer surface, since water cannot readily dry out from between the plies, and the absorption may therefore become cumulative and lead to weakening of the ply-adhesion.