Plastic Materials in Telephone Use

RGANIC plastics are used extensively in the manufacture of telephone apparatus and equipment. They belong to the class of materials known as insulators but are very often employed not only for their electrical properties but for their unique manufacturing and structural possibilities. Good insulating materials are very important in the telephone field although the voltage and current used are much smaller than in the power field. Progressive improvement in transmission, especially for long distance telephone service, has required that the telephone industry as a whole provide sensitive instruments and that there be a minimum loss of the electrical impulse due to leakage through insulating materials. Rubber became at one time the most universally used insulating material in telephone apparatus. Where superior insulating properties are required, rubber has been employed not only in the soft vulcanized form as a covering for wire but as hard rubber. Its use was considerably curtailed as a molding material during the period of the world war due to the high price of rubber. This stimulated the substitution of phenol plastics which were found to produce more permanent parts. Although rubber must be classed as an organic plastic, it will not be dealt with here except in passing since it comprises a large field in its own right and quite distinct from that of the synthetic plastics. In recent years rubber has been greatly improved in life, stability, light sensitivity and resistance to cold flow so that its technical uses in the telephone plant are again increasing.