214 BELL SYSTEM TECHNICAL JOURNAL 



cotton sufficiently to permit its use instead of silk as far as possible 

 for general purposes. The latter has considerable direct economic 

 importance because of the large difference in cost of insulating silk 

 and cotton and the quantities of the materials involved. 



Within the past three years, two methods of improving the electrical 

 characteristics of textile insulation have been brought into commercial 

 use. The first is the purification of silk and cotton whereby electro- 

 lytic impurities such as sodium and potassium salts inherent in the 

 commercial materials are removed by a simple and inexpensive washing 

 process.^ The second is the treatment of textile insulated wire with 

 cellulose acetate, which is the subject of discussion in this paper and 

 the contemporary paper "Cellulose Acetate Treatment for Textile 

 Insulation-Development of the Manufacturing Process" by Messrs. 

 C. R. Avery and H. Kress. 



Cellulose acetate became of interest in connection with insulation 

 problems several years ago, when it was investigated in the form of 

 artificial silk for use as a substitute for natural silk in wire and cable 

 insulation. At that time, the material was found to possess excellent 

 electrical characteristics and satisfactory stability, but it did not prove 

 to be economically satisfactory as a general substitute for silk because 

 the physical characteristics of the yarn made its use on standard high 

 speed insulating machinery difficult. Application of the material in 

 the form of a lacquer to cotton or silk insulation appeared to offer 

 more promise and has proven advantageous, as will appear from the 

 following discussion. The treatment, as now applied to telephone 

 central office wire insulation, consists in the formation of a coating of 

 the material on the textile insulated conductor, by passing the con- 

 ductor through an acetone solution of pure cellulose acetate and sub- 

 sequent evaporation of the solvent. Pure cellulose acetate without 

 the addition of a plasticizer is used, because thus far it has been found 

 more satisfactory than a compounded material as regards the con- 

 trolling requirements for central office wire insulation, namely good 

 electrical characteristics, slow burning properties and stability. There- 

 fore, this discussion is confined to the characteristics and use of the 

 pure material. 



Properties of Cellulose Acetate 



In the investigation of a material to be used for insulating purposes, 



it is necessary to examine the processes by which the material is manu- 



1 "The Predominating Influence of Moisture and Electrolytic Material upon 

 Textiles as Insulators," R. R. Williams and E. J. Murphy, A. I. E. E. Transactions, 

 April, 1929. "Purified Textile Insulation for Telephone Central Office Wiring," 

 H. W. Glenn and E. B. Wood, .1. /. E. E. Transactions, April, 1929. 



