DETERIORATION OF ORGANIC POLYMERS 1095 



The factor that determines whether or not cross-hnking will be dominant 

 in the aging of an unsaturated material must be the chemical structure of 

 the polymer (and its peroxide.) The mode of decomposition of the peroxide 

 which, of course, is a function of structure probably has the most important 

 effect. While cross-Hnking can occur in saturated materials, as shown by 

 the vulcanization of saturated polyester rubbers with peroxides, its rate is 

 never high enough to result in a condition that could be called deterioration. 

 Both polyethylene and cellulose acetate butyrate can undergo enough 

 gelation on outdoor exposure to become insoluble, but if this were the only 

 change occurring their toughness would be improved rather than degraded 

 by it. Their deterioration in strength is due entirely to chain scission. 



Modification of Side Groups by Oxidation 



All the oxidation reactions discussed result in the introduction of oxygen 

 into the polymer composition. If the polymer is one which already contains a 

 high percentage of oxygen such as cellulose or even nylon, this may have 

 little effect. If the polymer is a hydrocarbon, however, its power factor will 

 be raised markedly. As a matter of fact the measurement of power factor is a 

 very sensitive way of detecting the addition of oxygen to polyethylene. Ex- 

 cept where the polymer is being used for its low power factor, however, the 

 change in side groups will be secondary to the change resulting from chain 

 scission and cross-linking. 



Acceleration of Oxidation 



The foregoing description of the mechanisms of auto-oxidation makes 

 apparent several ways in which oxidation may be accelerated beyond what 

 might be called the natural rate for a pure material. Since oxidation is a 

 free radical process an obvious way to accelerate it is to add free radicals or 

 materials which produce free radicals. Addition of peroxides to organic 

 compounds generally accelerates the rate of oxidation.^^ Similarly the 

 oxidation of a relatively stable material is accelerated if there is left in it a 

 small amount of a chemical which itself is easily oxidized to peroxides. For 

 example, an addition of turpentine greatly accelerates the air-oxidation of 

 paraffin wax.^ The addition to polyethylene of an unsaturated polymer such 

 as natural rubber would probably have a similar effect. 



It is apparent that the amount by which the rate of oxidation of a sub- 

 strate is accelerated by peroxides, whether the latter are added as such or are 

 self -generated, is dependent on the rate of decomposition of the peroxide. 

 The latter rate can be accelerated by the presence of certain metallic ions 

 and hence they act as catalysts for oxidation reactions. Copper is particu- 

 larly active in this regard in natural rubber, and the rubber industry long 

 ago learned to avoid it^^ (Fig. 12). Other metals which have been found to 



