The Language of Rubber – Part 8 (More on Ozone Attack of Rubber Products) - By John Woon (Senior Latex Consultant)

Ozone in the air will normally attack the double bonds present in polymer molecular chains. Examples of polymers or rubbers most prone to ozone attack are natural rubber, polybutadiene, styrene-butadiene rubber (SBR) and nitrile rubber (NMR).

Nitrile rubber is a copolymer of butadiene and acrylonitrile units with very high proportion of butadiene as in the case of SBR, hence ozone attack on NBR is quite common.

Ozone attack is common especially when the rubber is being stressed during use and is normally indicated by the presence of varied sizes of cracks perpendicular to the direction the force applied to the rubber. The density or number of cracks varies with ozone gas concentration. The characteristic cracking is known as ozone cracking.

Applications where ozone resistance is of utmost importance, the right choice of rubbers is important. Such ozone-resistant elastomers include EPDM, butyl rubber, fluoroelastomers like Viton and polychloroprene rubbers like Neoprene because with most of these rubbers, double bonds form only a very small proportion of the molecular chains. In the case of polychloroprene, the presence of a large chlorine atom prevents ozone from approaching the double bonds in the chains.

A method to determine the ozone resistance of vulcanised rubber products is described in ASTM. In one test, thin strips of test pieces are clamped with an elongation of 20% and placed in a chamber containing 25 pphm of ozone at a given elevated temperature.

The test pieces are then removed and observed for cracking periodically.