DETERIORATION OF ORGANIC POLYMERS 1099 



the fact that antioxidants are limited in their effectiveness against photo- 

 oxidation. The rate of initiation of chains in a material exposed to sunlight 

 is so great that any antioxidant present is used up relatively fast. Further- 

 more, the oxidation of the antioxidant itself is rapid in sunhght and hence if 

 it were not removed in the one way it would be in the other. 



There are many chemicals in current use as antioxidants and more are 

 being created all the time. Most of them are either phenols or aromatic 

 amines. It is frequently asked why one antioxidant is more effective than 

 another, if indeed such differences do exist. The answer is not altogether 

 clear but certain statements can be made about it. In the first place, for 

 any given substrate there are usually several antioxidants which are equally 

 good. However, gradations of effectiveness of many commercial antioxidants 

 can be demonstrated. Many factors can exert an influence on this. Some 

 are solubiHty in the substrate, volatihty, inertness toward the substrate. 

 Beyond these are the reactivity of the antioxidant toward free radicals, 

 both hydrocarbon and peroxidic, and the relative stability of the free radical 

 left when the antioxidant reacts. Undoubtedly, some intermediate level of 

 reactivity is desirable in an antioxidant^^ • ^^ and this desired level probably 

 varies from one substrate to another. 



Light Screens 



It was mentioned above that antioxidants are of Httle effect against rela- 

 tively strong photooxidation because of the overwhelming rate of generation 

 of chains. The most serious problems of deterioration in the Bell System 

 are, of course, in outdoor applications, and it is quite clear that this is be- 

 cause of exposure to short wave light. The extensive commerical use of 

 unprotected material outdoors came about because of a lack of appreciation 

 of this fact. Once this vulnerability or organic materials to Hght is appreci- 

 ated the remedy is obvious, at least in principle, and that is to shut off the 

 light. For this purpose there are many pigments available as well as many 

 light-absorbing organic compounds. A great deal of work' has been done with 

 various substrates in testing the effects of the absorbers, and this can be 

 summarized as follows: 



In the class of light colored pigments, none offers complete protection. 

 Most of them have a sHght effect; a few are fairly helpful; and a few are 

 actually harmful, acting as photosensitizers. Of the darker pigments several 

 are quite effective but the outstanding ones are lead chromates, iron oxides 

 and carbon black, the last being the best. A study of the effect of various 

 types of carbon black in various concentrations in polyethylene has been 

 reported^^ wherein it is shown that under the most favorable conditions the 

 useful life of polyethylene, as judged by accelerated tests, can be extended 



