rubber. By reducing the particle size of a filler as dispersed in rubber the reinforcing effect is correspondingly increased. In presenting data on the reinforcing value of fillers it is therefore of greatest importance to state definitely the particle size of the material as present in the compound.Owing to an alignment of the particles during the milling and calendering operations, anisotropic fillers show abnormalreinforcingeffects.These effects are manifest, even though the filler is completely dispersed, and are not obliterated when the particle size is reduced to a state comparable with the finer fillers.The best reinforcing fillers are therefore those of smallest particle size, completely dispersed in and wet by the rubber and isotropic in particle shape. Any material, whatever its chemical nature, that fulfils these requirements will be a good reinforcing filler.
By processes of cyclization and chlorination, synthetic elastomers can be converted into derivatives which differ from the parent product in the following respects: 1. Higher specific gravity. 2. Higher softening point. 3. Greater hardness and rigidity. 4. Less flexibility and elongation. 5. Greater chemical resistance. 6. Lower solution-viscosity. 7. Lower moisture-vapor transfer. 8. Lower moisture absorption. 9. Less tackiness. 10. Greater solubility in polar solvents. Thus, many of the desirable features of the commercially important derivatives of natural rubber can be duplicated in synthetic products. It is, furthermore, logical to assume that the field of usefulness of synthetic derivatives will be extended by selection of the proper type of synthetic rubber for cyclization, chlorination, and hydrochlorination to bring out certain desirable characteristics which are not inherent in products made from natural rubber.
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