216 BEJ.L SYSTEM TECHNICAL JOURNAL 



temperature, indoor sunlight exposure and atmospheric moisture. 

 From the standpoint of stabiHty, especially as regards a possible in- 

 creased rate of deterioration with time, it is important that the mate- 

 rial shall be essentially free from impurities which might be introduced 

 in the acetate manufacturing process. 



Cellulose acetate film has very desirable electrical properties char- 

 acterized by high dielectric strength, low conductivity and low a-c. 

 capacitance and conductance. It absorbs much less moisture than 

 silk, cotton or wool. It has a specific gravity of about 1.25 and a 

 dielectric constant of from 5.5 to 6.0 at 1,000 cps. under atmospheric 

 conditions of 70° F. and 50 per cent relative humidity. 



The acetate film is strong and tough and not easily injured by han- 

 dling. The transparency of the film is such that the colored threads 

 used in the color scheme for identification purposes in telephone wires 

 and cables can be readily seen through the acetate coating. 



Cellulose acetate film is very stable under normal conditions and 

 when exposed to artificial aging tests. Tests made in the Labora- 

 tories with acetate film exposed to high humidities and high tempera- 

 tures for several years indicated that there was very little deterioration 

 of the film in its electrical or other physical properties. The electric 

 characteristics of the film were not appreciably affected by this expo- 

 sure and no discoloration of the film was apparent. 



As compared to cellulose nitrate, cellulose acetate is a much more 

 desirable material on account of its slow-burning characteristics, and 

 the fact that the gases given off on the combustion of the acetate are 

 comparatively non-toxic. From these standpoints the hazards in- 

 volved in the use of the nitrate preclude its use in the telephone central 

 office. Acetate film does not turn yellow with age to the same extent 

 as the nitrate film. 



Pure cellulose acetate film is somewhat hard and brittle. This is, of 

 course, a disadvantage because it tends to make the treated wire less 

 flexible than wire with untreated insulation which introduces new 

 problems in the handling of the treated wire. A large amount of work 

 has been done with a view to obtaining a plasticizer for cellulose 

 acetate which will add the property of flexibility to the film without 

 affecting the desirable characteristics which the pure acetate film now 

 possesses. The problem of obtaining such a plasticizer is difficult 

 inasmuch as the general tendency of such materials is to impair the 

 electrical characteristics, lower the tensile strength, and increase the 

 inflammability of the film when used in amounts sufficient to produce 

 a film of desired flexibility. 



