Cellulose Acetate Treatment for Textile Insulation — 

 Development of the Manufacturing Process 



By C. R. AVERY and H. KRESS 



Equipment was developed and a plant constructed for coating textile in- 

 sulated wire wit h a film of cellulose acetate. The wire is treated at a speed of 

 240 feet [)er minute. Recovery of more than 85 per cent of the acetone used 

 as a solvent is effected with carbon adsorbers. Thorough precautions have 

 been taken to prevent fire and explosion and to render them harmless if they 

 should occur. 



Introduction 



IN the telephone plant of the Bell System a large quantity of textile 

 insulated wire is employed for wiring the switchboards and con- 

 necting them to the incoming lead covered cables. The insulation on 

 this wire varies with its use in the plant, common constructions in the 

 past being tinned copper wire insulated with two servings of silk and 

 one of cotton, or, where the requirements are not so exacting, enameled 

 wire with two servings of cotton. 



The constant demand for better electrical characteristics in tele- 

 phone circuits has led the engineers of Bell Telephone Laboratories to 

 seek an improved insulation for this wire as discussed in the contem- 

 porary paper, "Cellulose Acetate Treatment for Textile Insulation, 

 Engineering Development," by E. B. Wood and D. R. Brobst. The 

 old design wire was affected by the variation in the dielectric proper- 

 ties of the textile insulation with its moisture content which in turn 

 varied with the surrounding humidity. It was found that the appli- 

 cation of a thin film of cellulose acetate to the textile insulation con- 

 siderably stabilized its properties. 



The action of the cellulose acetate coating is illustrated in Fig. 1. 

 When the surface of untreated wire is magnified, it is evident that it 

 is a mass of extending fibers and when two conductors lie adjacent, these 

 fibers interlace. Under humid conditions the textile becomes moist 

 and the interlacing fibers afford a path for current leakage. The 

 moist fibers also have a considerably higher dielectric constant than 

 the dry fibers and the electrical capacitance between adjacent wires 

 is increased, therefore, with high humidity. In the lower half of the 

 figure, the treated wire is shown, with the fibers ironed and sealed 

 down by the cellulose acetate process. It is not claimed that the film 

 is free from cracks, for cracks do form in the subsequent twisting and 

 forming operations permitting moisture to enter, but no matter how 



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