1086 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



The practical significance of the reaction of ozone on rubber is very great 

 since ahnost all rubber articles which undergo any appreciable stretching 

 in service are in some degree subject to attack. Exceptions are articles com- 

 posed of certain specialty rubbers such as silicones, Hypalon*, and some 

 Thiokols. These are saturated materials and hence are not attacked. Neo- 

 prene and Butyl rubber are more resistant than natural rubber or GR-S, 

 Butyl because it is only slightly unsaturated, and neoprene because its 

 double bond is considerably deactivated by the adjacent chlorine atom.^* 



CI H 

 — C=C— 



Fig. 8— Samples of various rubber compounds after exposure to ozone. (1) Silicone; 

 (2) Hypalon; (3) Buna-N; (4) natural rubber; (5) & (6) Neoprene; (7) GR-S; (8) Butyl. 



Large additions of pigments or plasticizers lower the ozone resistance of 

 neoprene. The measure which has been found most effective for protecting 

 rubber compounds from ozone is the inclusion of several percent of wax. 

 The amount required varies with the type of wax, the polymer, and the 

 other compounding ingredients, the absorptive power of any pigments 

 present being an important factor. By proper compounding neoprene can 

 be made extremely resistant to the attack of ozone, and the other unsaturated 

 rubbers can be greatly improved. The chief effect of temperature changes 

 on the cracking of rubber by ozone is in changing the solubility of wax in 

 the rubber. At elevated temperature the wax fibn may redissolve and leave 

 the rubber unprotected. This is illustrated in Fig. 9 which shows a tape 

 wrapping which has been attacked on the sunny side, not by the light, but 

 by ozone enabled to reach the rubber because the sun's heat had redissolved 

 the wax in it. 



* A chlorinated, sulphonated polyethylene manufactured by the Du Pont Company. 



