The Tackiness of Unvulcanized Rubber
Authors: Garner T. L. 
Published: February 2011

In ordinary cases of exposure of rubber to air and light, the processes of tackiness and oxidation run concurrently, but it was possible to reduce the viscosity of a rubber solution in the absence of oxygen by exposure to ultra-violet light. Such a solution gave on evaporation a depolymerized rubber which readily absorbed oxygen. The introduction of a small quantity of oxygen into a tube containing rubber in an atmosphere of nitrogen gave, on exposure, a rubber apparently as tacky as if exposed in air. The only difference was that part of the rubber was rendered insoluble. The action of light was partially polymerization and partially depolymerization, and the extent of each was determined by the conditions obtaining. The presence of oxygen was necessary to secure a tacky film of rubber, and thus confirmed Porritt and Fry's experiment. Little change took place in the solution-viscosity of rubber or its solution when heated to 40° C. in the absence of light. Under the conditions of these experiments no insoluble rubber was obtained by exposing solutions or uncompounded rubber in air, but in inert gases the formation of insoluble rubber of the polymerized, oxygen-free type, noticed by Asano, was observed to increase with the length of the exposure. The insoluble rubber formed when rubber, protected by the incorporation of an antioxidant, was exposed to light did not seem to be of this type, but the transparent, oxidized rubber obtained by Asano in some of his experiments. At the same time the rate of oxidation judged by the increase in acetone extract did not progress to the same extent as with unprotected raw rubber. The protection afforded by antioxidants from the point of tackiness (judged by solution-viscosity) appeared to bear no relation to their value in prevention of oxidation changes. One important point must be borne in mind in connection with the technical application of antioxidants. It has been shown that the latter will protect a rubber in such a way as to prevent depolymerization and oxidation changes to some extent, thereby assisting the formation of an insoluble and presumably polymerized rubber, which is not observed in ordinary cases of exposure. Thus an insoluble rubber layer is formed on the surface of the exposed rubber and destroys the tackiness. Consequently, the use of much antioxidant in a mixing which is required to retain its tackiness while exposed in a factory to light for some time, does not appear to be good practice. Samples of mixed rubber containing 1 per cent of antioxidant were found to lose their surface tackiness much more rapidly than the blank mixings on exposure to light; the surfaces of the former became hard and dry. The following theory is suggested. On mastication of raw rubber globules are partially burst and also depolymerization of the hydrocarbon occurs, resulting in reduced solution-viscosity. The tackiness of masticated rubber is due to the action of oxygen upon the depolymerized hydrocarbon and cannot appear until the latter is available. Thus on exposure of unmasticated rubber, the surface layer must first be depolymerized and then oxidized. Antioxidants in protecting the rubber from depolymerization and consequent oxidation allow the polymerizing action of light to become the superior effect. Consequently, in a tacky rubber, repolymerization of the oxidized and depolymerized layer occurs to some extent and in proportion as polymerization occurs, so tackiness is lost. In ordinary masticated rubber, the oxidized depolymerized rubber is distributed through the whole, but in special cases only the surface may be affected. Tackiness will be destroyed by substances which exude to the surface and cover the tacky layer with a greasy film, e. g., stearic acid, or by substances which assist in any way the polymerizing action of light. Hence, where tackiness is desired, circumstances leading to these results should be avoided.