208 .\NNXJAL REPORT SMITHSONIAN INSTITUTION, 19 40 



to oxidize spontaneously, which made it useful in paint, undoubtedly 

 contributed a great deal to the difficulty which was experienced by 

 the proofing industry in those days. It was a common event for 

 rubber compounds to turn to a sticky, gummy condition and later 

 to become brittle and worthless. Mackintosh, Hancock, and Good- 

 year all had a great deal of trouble on this account. Goodyear hoped 

 to restore rubber to its original condition by vulcanizing, and he was 

 surprised when the improvement went beyond that. Although vul- 

 canization did greatly improve matters, rubber goods were a source 

 of trouble both to the manufacturer and to the user because of their 

 highly variable life. 



As the knowledge of the structure and properties of organic chem- 

 icals developed, the reason for the rapid deterioration of rubber 

 became more evident. This is summed up as follows by Semon (16) : 



From a purely chemical viewpoint, it is not difficult to understand why 

 rubber should oxidize in air, for both raw and vulcanized rubber show about 

 85 percent more unsaturation than linseed oil. The iodine number of linseed 

 oil is from 175 to 200, whereas that of raw or vulcanized rubber is from 

 350 to 372. The surprising fact, therefore, is not that rubber deteriorates, but 

 that it is so stable. 



It was recognized many years ago that the oxygen of the air played 

 an important role in the deterioration of rubber. It was also found 

 that deteriorated rubber always contains combined oxygen. Later 

 accurate measurements showed that a relatively small amount of oxy- 

 gen, combined in this way with the rubber, produced a profound ef- 

 fect on the properties of the product. 



The theoretical aspects of the oxidation of rubber were studied 

 by a number of scientists, including Moureu and Duf raisse, Kirchhof , 

 and Ostwald. Important contributions were also made by workers 

 in industry, particularly with respect to the effects of certain im- 

 purities, such as copper and manganese, which greatly accelerate 

 the deterioration of rubber by oxygen. The phenomenon of oxygen 

 deterioration has received more attention at the hands of scientists 

 than the phenomenon of vulcanization. 



According to current theories, rubber is depolymerized when it oxi- 

 dizes. This is borne out by the fact that the viscosity of a solution 

 of unvulcanized rubber is reduced by oxidation. Vulcanized rubber 

 oxidizes more readily than unvulcanized rubber. Although the mech- 

 anism of the oxidation reaction is still being investigated, it is gen- 

 erally believed to be autocatalytic. 



While a number of substances had previously been proposed as pre- 

 ventives of oxidation, the first materials put to large-scale use were 

 the aldehyde-amines. Aldol-a-naphthylamine, proposed by Win- 

 klemann and Gray, and acetaldehyde-aniline, proposed by Cadwell, 

 are the first representatives. The aldehyde-amines have already been 



