Thermal Degradation of Unvulcanized and Vulcanized Rubber in a Vacuum

The history of distillation of rubber under various conditions of temperature, pressure, and atmosphere goes back more than a century. As far back as 1860, Williams distilled rubber in an iron retort at relatively low temperatures and obtained some 5 per cent of crude isoprene. In 1922, Staudinger and Fritschi distilled rubber at 275° to 320° C at 0.1- to 0.3-mm. pressure and obtained 3.1 per cent of isoprene. In 1926 Staudinger and Geiger distilled rubber below 300° C at ordinary pressure in an atmosphere of carbon dioxide and obtained 4.3 per cent of crude isoprene. Distillation of rubber at higher temperatures and under atmospheric pressure yielded as much as 58 per cent of isoprene. In addition to isoprene, dipentene and higher terpene compounds were also identified in the distillation products of natural rubber. More recently, distillations of pure synthetic polyisoprene (a mixture of cis- and trans-polyisoprene), purified natural rubber (cis-polyisoprene), and purified gutta hydrocarbon (trans-polyisoprene) were carried out by the present authors in a vacuum under conditions of molecular distillation. The volatile products were fractionated and analyzed both qualitatively and quantitatively, using the mass spectrometer for the lighter fractions. The volatiles consisted of about 3 to 4 per cent of isoprene and 13 to 20 per cent of dipentene, the rest being large terpene fragments of average molecular weight of about 600. The present paper describes an investigation of the effects of various additions to natural rubber, with or without subsequent vulcanization, on the degradation process during pyrolysis in a vacuum, with the view that such a study might throw some light on the structure of vulcanized rubber. In addition to pyrolysis, a study was also made with unvulcanized and vulcanized rubber of the rates and activation energies of thermal degradation in a vacuum, using techniques and apparatus that were previously developed by the authors in connection with an extensive study of thermal degradation of a number of polymers.