The Photovulcanization of Butyl and SKB (Polybutadiene) Rubbers

Abstract

Abstract It is known that the mechanical properties of raw and vulcanized rubbers change markedly under the action of light in which not only oxidation processes but also photovulcanization processes in which sulfur is involved play an essential part. In view of the wide interest shown lately in questions of radiation vulcanization of polymers, as well as the possibility of reducing the photodegradation effect upon the mechanical properties of some polymers, we made studies of the photovulcanization of butyl rubber (degradation by irradiation) and SKB (polybutadiene) rubber (structure formation by irradiation). The study was made on films 200µ thick of purified butyl rubber with a molecular weight of about 42,000 and an unsaturation of about 1%. Both ordinary (twice crystallized) sulfur and radioactive S35 were used. The films obtained from a solution of rubber plus sulfur on cellophane were irradiated with a PRK-7 lamp through a Pyrex glass light filter and distilled water at a distance of 25 cm. Then determinations were made of the absorption spectrum and the static modulus 30 minutes and 24 hours after applying the load. The amount of free sulfur remaining in samples extracted previously was determined from the intensity of the S35 radiation. SKB not containing PBNA was reprecipitated from a benzene solution. When films of SKB (200µ thick) were irradiated, the light filters UFS-1, ZhS-4 and OS-5 were used. After the irradiation of the SKB films their static modulus was determined 30 minutes after the application of the load. When we irradiated butyl rubber containing 0.25 to 2.5% sulfur, we observed a distinct photovulcanization optimum (see Figure 1) in the modulus as a function of irradiation time like that which occurs in thermal vulcanization, and an optimum of the photovulcanization rate as a function of sulfur concentration.