Plastic Materials in Telephone Use* 



By J. R. TOWNSEND and W. J. CLARKE 



ORGANIC 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 im- 

 portant 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 re- 

 quired that the telephone industry as a whole provide sensitive instru- 

 ments 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 proper- 

 ties 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 com- 

 prises 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. 



Shellac and asphalt plastics, both natural materials, were among the 

 early important plastics employed in the telephone art. A shellac 

 compound is still the best material for a panel system commutator 

 where there are many long and delicate contact segments and where 

 the principal problem is to obtain accurate location between the insula- 

 tion and the brass segments together with uniform wear. The low 

 molding temperatures and pressures for the shellac mica compound 

 contribute to the success of the manufacture of this part. (See Fig. 1.) 



Other early plastics that have found some limited uses are cellulose 



* Presented before the Organic Plastics Section of the Paint and Varnish Division 

 of the American Chemical Society, Baltimore, Md., April 3-7, 1939. 



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