TREATMENT FOR TEXTILE INSULATION 215 



factured to determine whether there is anything inherent in these 

 processes which would affect the use of the product. This is particu- 

 larly pertinent in the case of an insulating material which is to be 

 used for a period of twenty years or more, as in a telephone exchange. 



The manufacture of cellulose acetate is described briefly by the 

 following operations: 



In the first or acetylation process, cellulose fibers, usually cotton, 

 are treated with glacial acetic acid and acetic anhydride, together with 

 a catalyst such as sulphuric acid, until the fibers are completely ace- 

 tylated and pass into solution. The acetate obtained at this stage is 

 brittle, of low tensile strength and insoluble in the commercial solvents. 

 Therefore, the solution is subjected to a hydrolizing process in which 

 water is added and the mixture allowed to stand until hydrolysis has 

 been carried to the point at which the cellulose acetate becomes acetone 

 soluble. The acetyl content is somewhat reduced in this step and 

 serves as an index to the extent of hydrolysis. The solution is then 

 poured into water and the cellulose acetate precipitated, after which 

 it is given a purification treatment until the mass is free from acid and 

 then dried in warm air. 



The completed product is a porous, flaky mass, white in color, which, 

 when dissolved in acetone, gives a solution nearly colorless but with 

 a slight amber tinge. 



From the above outline of the processes of manufacture, the impor- 

 tance of the acetyl content of the product is obvious. If the acetyl 

 content is too high, the material is not soluble in acetone and if the 

 acetyl content is too low, the hydrolysis has been carried too far and 

 the acetate becomes partly soluble in water. Such an acetate would 

 be unsatisfactory for insulation on account of inferior electrical char- 

 acteristics under humid atmospheric conditions. 



By changes in control of the acetylation and hydrolizing processes 

 various kinds of cellulose acetate may be obtained which, with the 

 same general composition and acetyl content, give different viscosities 

 of solution when dissolved in a solvent. For example, films for experi- 

 mental purposes have been made from cellulose acetates which vary 

 in viscosity as much as a hundred fold with the same proportions of 

 cellulose acetate and solvent. For lacquer and films, a low viscosity 

 acetate is employed, while for plastics, cellulose acetates of high vis- 

 cosity are usually specified. 



For use as insulation, it is necessary that the acetate be stable 

 throughout the life of a telephone exchange. In other words, it must 

 retain its good electrical characteristics and transparency for a period 

 of twenty years or more though exposed, as it will be, to variations of 



