PLASTIC MATERIALS IN TELEPHONE USE 493 



Typical Applications of Thermosetting Plastics 

 Phenol Plastics 



A typical phenol plastic telephone part is the handle for the handset. 

 This part is molded in a multiple semi-positive die of the kind de- 

 scribed above. 



In addition to the laboratory tests on the raw material, samples of 

 the molded handles must withstand a dropping test. After being 

 conditioned, handles representative of a given lot are equipped with 

 transmitters and receivers and dropped down a nearly vertical chute 

 to strike on a steel block. The test is made by dropping first at 36 

 inches and then increasing the drop in increments of 2" until the handle 

 breaks.^ Normal product handles will withstand a drop of 55 inches 

 on a steel block without failure. 



The electrical properties of phenol plastic compounds are adequate 

 for most uses in the telephone plant. Two grades are recognized, 

 however, the mechanical and the electrical. Fully 90 per cent of the 

 uses involve the mechanical grades. For certain high-frequency in- 

 sulation purposes special mica-filled phenol plastics are used in place 

 of the regular wood and cotton filled varieties. 



One of the outstanding disadvantages of a phenol plastic is the 

 ease with which it carbonizes on exposure to electrical arcing. For 

 this reason phenol plastic compounds have only a limited use for com- 

 mutators and similar applications. However, in addition to handsets 

 they have proved of value for mouthpieces, receiver cases, subset 

 housings, non-magnetic coil forms, coil cases, jack mounting blanks 

 and terminal blocks. 



Phenol Fiber 



Phenol fiber for telephone apparatus is made of alpha cellulose 

 paper, Kraft paper and rag paper by the usual impregnation with a 

 suitable phenol resin varnish and lamination of a number of sheets 

 under heat and pressure. The most important requirement for the 

 paper is that it shall be pure, clean and free from electrolytes. The 

 paper is carefully tested for chlorides, conductivity of water extract 

 and for alcohol soluble materials. Several grades of phenol fiber are 

 necessary to meet the requirements, some of which are largely me- 

 chanical and others electrical. 



The principal tests for phenol fiber are cold flow and shrinkage, 

 insulation resistance, corrosion tendency, arc resistance, transverse 

 strength and impact. Arc resistance applies to the case where wiping 



' "The Impact Testing of Plastics," Robert Burns and Walter W. Werring, Proc. 

 A.S.J.M., 19, Vol. 38, 1938. 



