1100 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



at least 30 fold. It was shown in this work that for best results the carbon 

 black should be finely divided and well dispersed. The use of polyethylene 

 as a sheath material on outdoor cable would not have been practical without 

 the protective effect of carbon black. The efficacy of carbon black as a light 

 screen is apparently quite general although detailed studies have been made 

 only with polyethylene, rubber,^^ cellulose esters,^^ and polyvinyl chloride, ^^ 

 in all of which it is effective. 



Fig. 14— Cellulose Ace late Butyrate panels exposed to concentrated beams of UV 

 light filtered through pyrex bottles filled with water. Sample at left contains 1% carbon 

 black. Sample at right contains 1% Salol. 



In many applications of plastics and rubbers, colors are desirable and 

 for these the use of carbon black is, of course, precluded. Some success has 

 resulted from the use of organic materials which are transparent to visible 

 light but which absorb in the ultraviolet. For example, phenyl salicylate, 

 known as Salol, at a concentration of 1% is a fairly effective protective agent 

 for transparent cellulose esters.^" Such effects appear to be quite specific, 

 since Salol is not nearly so effective in most other polymers (it is reported to 

 be effective in Saran^*), and many other compounds which are even better 



