DETERIORATION OF ORGANIC POLYMERS 1097 



cause poor aging at various times are cobalt, manganese, and iron.^^ Since 

 the "drying" of paint is an oxidative reaction, and since rapid drying is a 

 desirable feature, the paint industry has found it advantageous, to use 

 certain metaUic salts as ''paint dryers".''^ 



Retardation of Oxidation 



Antioxidants 



It was discovered by Moureu and Dufraisse about IQIS''^ that the oxida- 

 tion of many organic compounds could be very greatly retarded by the 

 addition of small quantities of certain other chemicals, which they called 

 "antioxygens." Although the mechanisms which they postulated for the 

 action of these materials were later found to be incorrect, their discovery 

 led to the wide use of such protective agents in industry, particularly in 

 rubber which needs this protection badly. The action of what are now called 

 "antioxidants" becomes clear when one understands the free radical chain 

 mechanism of oxidation outhned above. Antioxidants are chain stoppers.'* 

 By interposing themselves in the chain reaction they terminate it by giving 

 rise to relatively inert free radicals^"- ^^ (stabiUzed by resonance). For ex- 

 ample, the antioxidant, designated HA, could act in the following way: 



ROO- + HA — ROOH + A 



In this case the antioxidant satisfies the peroxidic radical by giving it the 

 hydrogen atom it needs, but the residual radical • A is not sufficiently reactive 

 toward oxygen to continue the chain. A typical antioxidant is |8-phenyl 

 naphthylamine. 



It was pointed out earlier that many ineffective collisions of the radical 

 ROO- with substrate molecules occur before reaction takes place. If the 

 reactivity of ROO- toward HA is sufficient that few ineffective collisions 

 take place, then small concentrations of HA in the substrate will be ade- 

 quate to stop each chain at a very early stage. This not only saves all those 

 substrate molecules which would otherwise have become links in these 

 chains but, by so doing, it limits the number of molecules of peroxide pro- 

 duced and thus keeps the rate of initiation of new chains at a low level. 

 The degree of protection by antioxidants varies with the length of the 

 "natural" chain reaction (which is a function of the ratio of effective to 



