Rubbone

Of all the reactions which natural rubber undergoes, oxidation next to vulcanization, may be said to rank in the first place. For not only does the life and usefulness of all raw and vulcanized rubber depend on the course of this reaction, but attempts to create useful oxides may be regarded as based on the same reaction. It has already been shown that, by the catalytic oxidation of rubber, solutions of greatly reduced viscosity and/or increased concentration are obtainable. Thus, a 50 per cent solution of rubber which will flow at normal temperatures can be made by incorporating 2.5 per cent of cobalt linoleate on the mill, subsequently “letting down” in white spirit or other solvent by means of an internal type of mixer. The mechanism of the oxidation of rubber is a complex and still incompletely solved problem, but a reasonably acceptable explanation of this reduction in viscosity is that it is due to a disaggregation of the rubber micelle, whereby a very small amount of chemically combined oxygen is enabled to effect a very large physical change in the character of the colloid. This change is promoted by catalysts, such as the ordinary paint driers, and there appears to exist an analogy between the drying of paint and the disaggregation of rubber solutions, but it is interesting to note that whereas copper soaps have substantially no effect on the drying of linseed oil, they do reduce the viscosity of rubber solutions appreciably. Solutions of rubber containing catalysts have been “blown” by bubbling air, oxygen or ozonized air through them while maintained at raised temperatures by suitable means. The experiments covered a great variety of conditions, including different solvents, different types and concentrations of rubber and catalyst, mixed catalysts and a variety of temperatures and blowing conditions. It was found that under suitable conditions the oxidation of the rubber could be taken to a stage where the cobalt linoleate itself became oxidized and was precipitated from the solution. After filtering, the resulting solutions were evaporated to yield an oxidized rubber resin. A standard procedure has been developed for the production of this rubber resin, or “Rubbone” as it is conveniently termed, as follows. A solution of milled blanket crepe in white spirit is made by dissolving 20 parts by weight of rubber in 80 parts by weight of solvent, adding 0.5 part of cobalt linoleate, and aerating at 80°C. until a sample drawn off from the bulk shows a quick separation of sediment and a clear supernatant solution of the resin. The solution is clarified by “tanking” or centrifuging and is then distilled by steam or in vacuo to separate the resin.