1084 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



simultaneously to greater or less extent. The rates of the reactions vary 

 from one material to another, and the same conditions which degrade natural 

 rubber to a soft gum may cause neoprene or GR-S to become harder and 

 stiffer. 



Returning now to the thermoset polymers, one sees that neither occasional 

 chain scission nor occasional cross-linking can have an important effect on 

 the mechanical properties of a thermoset polymer since every part of the 

 structure is tied to the rest of it by many bonds. For this reason the most 

 conspicuous changes of thermoset materials on exposure to weather are on 

 the surface and are of the third type discussed above. 



From the viewpoint of physical structure all of the elements of deteriora- 

 tion are covered in the above paragraphs. However, nothing has been said 

 about the agencies which cause the chemical changes or the mechanisms 

 by which they are brought about. These agencies and mechanisms become 

 the most important objects of study. One type of change — the cross-linking 

 of molecules — in certain cases can occur by self -reaction under the influence 

 of heat or light in complete absence of other chemicals. Self-reaction is 

 not, however, an important effect in materials which are in engineering use. 

 The changes which lead to the loss of utility of polymers during aging are 

 caused by chemical reaction with the environment. Usually this environment 

 is the atmosphere. There are normally three substances in the atmosphere 

 which under various circumstances may be considered reactive toward or- 

 ganic compounds, namely water vapor, ozone, and oxygen. The next section^ 

 will discuss the ways in which these chemicals bring about the destruction 

 or organic polymers. 



Water 



The chemical reaction of water with organic compounds is limited to 

 materials which contain hydrolyzable groups either as part of their original 

 composition or as a result of oxidation. Examples of such groups are esters, 

 amides, nitriles, acetals, and certain types of ketones. The reaction is illus- 

 trated with an amide Unkage, the unaffected portions of the molecule being 

 represented by the letter P: 



N O 



I II 

 P— CH2— H— C— CH2— P + H2O -^ 



O 



II 

 P— CH2— NH2 + HOC— CH2— P 



When these vulnerable groups are present as substituents on a polymer 

 chain composed exclusively of carbon-to-carbon bonds their hydrolysis 



