DETERIORATION OF ORGANIC POLYMERS 1085 



may affect certain properties of the material (dielectric constant, power 

 factor, insulation resistance, water absorption) but in general the molecular 

 weight of the polymer is unaffected. When the vulnerable group is a link 

 in the skeletal chain, however, the result of hydrolysis is much more serious 

 because it constitutes scission of the primary chain and hence a lowering 

 of molecular weight. Polymers which are subject to this kind of scission are 

 polyesters, polyamides, cellulose and cellulose derivatives (ethers and esters). 

 Hydrolysis is accelerated by high temperature and is catalyzed by acids 

 and alkahes, and hence many polymers of the classes listed are stable only 

 when kept neutral. Polyesters in particular are usually easily hydrolyzed 

 and it is this fact w^hich has been the main barrier to their greater commercial 

 utilization. Hydrolysis as such is a well known reaction and is taken into 

 account in current engineering with materials which are subject to it. For 

 example, nylon molding powder is shipped dry in sealed containers to keep 

 the moisture content low until after the molding operation which requires 

 that the nylon be heated to a high temperature,* and cellulose esters undergo 

 repeated careful neutralizations and washes after esterification to reduce 

 acidity.^ The extent to which water plays a role in the deterioration of hydro- 

 carbon materials which are first attacked by oxidation is not yet known, but 

 it is certainly secondary to the oxidation itself. An important effect of rain 

 in outdoor weathering is the washing awaiy of water soluble oxidation prod- 

 ucts with consequent exposure of new surface. Another effect is the removal 

 of water soluble compounding ingredients. This may be distinctly beneficial 

 as in the case of polyester rubbers vulcanized by acid-producing catalysts,^" 

 or harmful as in certain polyvinyl chloride formulations which contain 

 water soluble protective agents. 



Ozone 



Ozone is an extremely reactive chemical which is present in the air in 

 extremely small amounts, ranging from to 10 parts per hundred million. 

 In this low concentration it has not been shown to have any effect on chem- 

 ically saturated materials, but it is a very serious hazard for unsaturated 

 compounds. Natural rubber and several synthetic rubbers fall in this class 

 (Fig. 8). Ozone is a specific reagent for carbon-to-carbon double bonds, 

 forming an ozonide which undergoes rearrangement resulting in chain scis- 

 sion. ^^ When rubber is not being stretched the attack of ozone appears to 

 be negligible, but when it is under stress the attack has very serious conse- 

 quences resulting in transverse cuts which may sever the piece of rubber.^^ • '' 

 Apparently the initial attack, starting in regions of highest local stress, cuts 

 enough chains to cause a crack to open, and this exposes new surface and 

 concentrates the stress so that the crack grows. 



