The Structure and Properties of Rubbers Produced in Radiation Vulcanization

Abstract

Abstract 1. The structure and properties of irradiation vulcanizates of technical and purified SKB, NR, SKS-30A, SKS-30AM, and SKI rubbers with and without fillers, made by irradiation in a nuclear reactor and in a cobalt source, with integral doses of 107–108 r, have been studied. 2. The density of the network formed in irradiation vulcanization is determined by the quantity of energy absorbed, the type of rubber, the irradiation conditions (medium, temperature), and certain other factors. The yield of crosslinks per 100 ev in air is ∼12 for extracted butadiene-styrene rubber, ∼4 for extracted NR, and 2.5 for technical SKS-30A. The crosslinking effect increases with increase of temperature and decreases in presence of an oxidation inhibitor (phenyl-2-naphthylamine). 3. The thermomechanical stability of irradiation vulcanizates is superior to that of vulcanizates made with thiuram without sulfur, and such vulcanizates undergo the least changes in properties during chemical relaxation. In contrast to sulfur vulcanizates, the relative rate of stress relaxation in irradiation vulcanizates depends on the density of the vulcanization network. This indicates the C—C type bridges are formed in the course of irradiation vulcanization. The thermomechanical stability of irradiation vulcanizates is raised by the introduction of active blacks. 4. The presence of carboxyl, hydroxyl, ether, and ester groups in irradiation vulcanizates of natural, butadiene-styrene, and sodium butadiene rubbers vulcanized in presence of air has been demonstrated by infrared spectroscopy. A dose of 60·106 r causes almost complete disappearance of double bonds from extracted butadiene-styrene rubber. In the technical rubber, containing phenyl-2-naphthylamine inhibitor, this dose reduces the degree of unsaturation by about 30% of the initial value. 5. The tendency to crystallization on stretching was studied in irradiation vulcanizates of natural rubber. The crystallinity of irradiation vulcanizates made by irradiation with relatively small doses, up to 20–30·106 r, is the same as that of sulfur vulcanizates of the same network density. The degree of crystallization decreases with increasing radiation dose. 6. Conditions for the production of massive multilayer specimens by irradiation vulcanization were investigated. It was found that a uniform vulcanization network can be obtained irrespective of the thickness of the specimen (in the range 0.1–40 mm). 7. The physicomechanical and technical properties of irradiation vulcanizates were studied. In comparison with the best sulfur with the same filler contents, irradiation vulcanizates have a higher resistance to aging (4–5 times as high at 130°), low residual deformations, low hysteresis, high endurance to repeated deformation, and high heat resistance. The tensile strength of irradiation vulcanizates, as a function of the radiation dose, passes through a maximum, but does not reach the values found in the best sulfur vulcanizates.