PLASTIC MATERIALS IN TELEPHONE USE 



483 



nitrate and casein. However, the cellulose nitrate plastics were only 

 sparingly employed for telephone construction because of the serious 

 fire hazard. Casein has long been used for key buttons and similar 

 minor applications. 



The great expansion of the use of molded plastics in the telephone 

 plant really began with the development of organic materials which 

 had superior manufacturing and structural characteristics over other 

 materials. The newer plastics are of value, therefore, as much from 

 the economies of manufacture as from their superiority over the pre- 

 viously used materials. 



Plastics are conventionally divided into two groups: (1) the thermo- 

 plastics and (2) the thermosetting plastics. The first, considered as 



Fig. 1 — Panel system commutator (flash type mold). 



organic materials, are permanently soluble and fusible as well as fairly 

 rigid at normal or working temperatures and may be deformed under 

 heat and pressure. The second are initially thermoplastic and become 

 insoluble and infusible after a period of time upon application of heat 

 and pressure. These important properties are due to the chemical 

 nature and molecular structure of the materials. All synthetic 

 plastics are polymeric substances, that is, they are the result of a 

 polymerization or condensation from simple organic molecules by 

 linkage of the molecules in fairly definite ways. By a polymerization 

 reaction is meant a reaction in which a more or less considerable 

 number of molecules unite to form larger complexes of the same chemi- 

 cal composition. A condensation reaction on the other hand is one 



