Abstract
1. A method of measuring the tear resistance of rubber compounds was developed. This method has a number of advantages over other methods (Goodrich, Heidensohn, Goodyear, etc.), including the following. (a) The symmetrical shape and the large surface of tearing (20 sq. cm.). This excludes the possibility of short, accidental tears, and enables better observation of the nature of the tear. (b) Owing to the small size of the clamped portion of the specimen compared to the size of the tearing surface, “end effects” are largely eliminated. (c) Tearing forces are registered periodically (every 10 seconds), and it is possible in this way to determine the total energy expended on tearing, and the nature of its changes. (d) The experiments are easy to perform, and the apparatus is simple. 2. The addition of carbon black (10–100 per cent) to synthetic rubber compounds increases considerably the tear resistances of the vulcanized products. The best results are obtained with compounds containing between 50 and 75 per cent of carbon black. 3. A zinc oxide content of between 8 and 14 per cent improves the tear resistances of compounds of both synthetic and natural rubbers. 4. Compounds with increased fibrous structure show increased abrasion resistance when the fiber direction is parallel to the movement of the abrading surface. 5. Compounds cured for short times only show low tear resistances when made of synthetic rubbers of both low (0.31) and high (0.87) plasticity. The best results are obtained with compounds made of synthetic rubber of medium plasticity (0.53 and 0.40) (see Figure 4). 6. When comparing tear resistances, it is of the utmost importance to maintain the thickness of test-specimens within narrow limits. It is desirable to keep variations within 10–15 per cent. 7. The weakening of the uncut portion of a test-specimen with increase in depth of cut is less pronounced with compounds of synthetic rubber than it is with natural rubber compounds, particularly in the case of overcured samples. With synthetic compounds, this weakening effect varies 2–3 times; with natural rubber compounds it is 3.5–6.5 times. The relation between tear resistance and depth of cut is shown in Figure 5 and Table VI. 8. Carbon black compounds of both synthetic and natural rubbers exhibit various structural forms of tearing. Synthetic rubber compounds with low tear resistances show simple and smooth tearing surfaces, usually in the prolongation of the cut, or at a slight angle to it (Figure 1, type A or intermediate between A and B). Synthetic compounds with high tear resistances show complicated tearing surfaces (Figures 6 and 7, types C, D, E and F).
Historical and Recent Achievements in the Field of Microbial Degradation of Natural and Synthetic Rubber