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THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



ethylene/^ nylon and cellulose esters,^** for example, causes crazing, cracking, 

 embrittlement, and in extreme cases granulation of the sample. (Fig. 10) 

 In polyvinyl chloride it leads to hardening and discoloration.^^ In natural 

 rubber, GR-S, and neoprene it causes the development of "mud-crack" pat- 

 terns or "alligatoring" of the surface and loss of elongation. Thermal oxida- 

 tion leads to embrittlement of thermoplastics, to "shortening" or loss of 

 elongation in neoprene,^^- ^^ nitrile rubbers, and GR-S, and to reversion or 

 the development of tackiness in Butyl rubber and sometimes in natural 

 rubber. As pointed out earher, these varying effects result from the relative 

 rates of cross-linking and chain-scission reactions. The mechanisms by 

 which oxygen can attack polymers are discussed in the next paragraphs. 



Mechanism of Oxidation Leading to Chain Scission 



The reaction of organic compounds with atmospheric oxygen, frequently 

 called "auto-oxidation" or "autoxidation", has been of interest to chemists 

 for a long time and a voluminous literature on the subject has accumu- 

 lated.^- ^^' ^® While most of the work done hks been on small molecules 

 rather than on polymers it is becoming apparent that much of the mecha- 

 nism of oxidation is the same and what has been learned on small molecules 

 can be applied to large.^^- ^^' ^^ This is fortunate since polymers do not lend 

 themselves readily to normal chemical manipulations. While it might be 

 expected that different compounds would be attacked by oxygen in different 

 ways a general mechanism has emerged which appears to be characteristic 

 for aliphatic hydrocarbon structures and is probably applicable to many of 

 the polymeric materials in current engineering use. It can be described as an 

 autocatalytic free radical chain reaction.^" • ^^ • '^ 



The sequence of events is believed to be as follows: Free radicals are 

 produced in the substrate from the energy of heat or of light. They may 

 arise from the decomposition of unstable groupings such as the — O — O — 



