The Appearance of Atmospheric Cracks in Stretched Rubber

Abstract

Abstract It was established that two factors are necessary for the formation of cracks: (1) stretching of rubber; (2) the action of ozone. Experiments at night proved without doubt that light is not necessary for the formation of cracks, in fact it often has a negative influence as a result of the formation of a surface skin. The formation of cracks begins at about 8 per cent elongation and reaches its maximum, i. e., the formation of largest cracks, which thickly cover the whole surface, at about 10 to 20 per cent elongation, above which degree of elongation the cracks become gradually smaller. Artificially ozonized air causes the same cracks as the open air, only much more rapidly. Nitrogen oxides, sulfur dioxide, and chlorine form no cracks. Moreover, no cracks were formed in unstretched rubber subjected to the highest concentrations of ozone. The influence of sunlight or the light from a mercury quartz lamp is not necessary to the formation of cracks; in fact, an oxidized skin may be formed by the light, and this skin may even prevent the formation of cracks. Chemical factors, too, which promote the oxidation of rubber (iron and copper compounds) have no influence on the formation of cracks. The influence of the degree of vulcanization on the formation of cracks is to be attributed solely to the mechanical properties, and indeed to the modulus of vulcanized rubber. The smaller the modulus the larger are the cracks.