Oxidation of Hevea Vulcanizates Containing Carbon Black

Abstract

Abstract The oxidation of unvulcanized natural rubber and of other Jong-chain polyisoprenes proceeds through cyclic peroxidic intermediates whose detailed structure depends upon the temperature of oxidation. The principal product is a hydroperoxide. At low temperatures high yields of stable hydroperoxides may be obtained. Their structure was characterized by the work of Bolland and Hughes on squalene peroxide. The structure of the low temperature peroxide formed in rubber has not been investigated directly because of analytical difficulties. Moderate yields can be obtained; and its properties are consistent with a structure similar to that formed from squalene. At very high temperatures, the predominant cyclic intermediate has a different structure from that formed at low temperatures. It apparently is not converted to a stable hydroperoxide, but rather an intermediate radical decomposes directly, leading to breakage of carbon-to-carbon bonds in the hydrocarbon chain. This conclusion is not fully established experimentally, but consideration of the structure of the probable intermediate, suggests that a “zipper effect” should be found, leading to high yields of low molecular weight products per apparent scission, if the intermediate RO4 resulting from successive additions of two oxygen molecules has significant stability. The observed yields correspond instead to a primary yield of one molecule of scission products per scission. Associated with this decomposition is a group of low molecular weight compounds, including levulinaldehyde, formaldehyde, formic acid, acetic acid, carbon dioxide, and (by inference) water. The yield of each of the compounds depends to some extent on the experimental arrangement used to study their formation, but under a given set of conditions yields of one or more of these compounds form a good index of scission reactions in the hydrocarbon chain. So far as is known the ratio of scission to other reactions of the polymer with oxygen is determined solely by temperature, through its effect on the ratio of “low temperature” to “high temperature” peroxide intermediates.