Scanning Electron Microscopy of Rubber Tear

A relatively simple technique has been described which allows observation of the deformation zone at tear tips in rubber specimens. Investigation of various rubbers revealed qualitative information about the type of yielding that occurs in the deformation zone under different conditions. The initial appearance of the tear tip in nitrile, polybutadiene (both with and without carbon filler), and natural rubber suggests a process of stretching followed by rupture and then relaxation onto the tear walls. Only in the case of a fluoro-elastomer was there evidence of cavitation, although microcavitation (of sizes ≪1 µm) may be occurring in the other rubbers at much higher magnification. On the other hand, aging in laboratory air had the dramatic effect of producing fibrous and nodular networks in the tear tip material of all the rubbers except the fluoroelastomer. In the first few hours, the network structure developed as cavities (Figure 9), but after 24 h the network was fully developed across the tear. The aging process is very likely an ozone attack not only on the rubber under stress at the tear tip, but also on the rubber on the tear wall. It is quite possible that there were residual stresses in the material on the wall which facilitated attack. Chain scission appeared to be allowing the rubber to relax by viscous flow under the action of surface forces. These results point up the need to distinguish between failure controlled by pure stress and failure assisted by oxidation/ozonolysis in studies of slow tear growth in rubbers. The SEM technique also provides information about fillers. Their elemental composition can be determined by XES, and some qualitative information can be gained about their adhesion to the matrix and their effect on tear tip yielding.