Investigations on the influence of surface structure and modification on ozone resistance of elastomers

Abstract

The adverse effects of ozone on elastomers depicts a crucial part in the degradation of rubber products and therefore sometimes limits the durability of rubber parts used in manifold applications. This attack proceeds mainly on C-C-double bonds of the polymeric main chain, resulting in characteristic ozone cracks, if strain is applied on the rubber material during the attack.The rubber industry applies two main strategies in order to reduce the risk of a premature failure in application due to ozone attack: Primarily, saturated polymers – like EPDM – are deployed in order to slow down the severe ozonolysis reaction of C-C-double bonds. On the other hand, chemical additives like paraffinic waxes or antiozonants are used to actively prevent the ozonation of the material. However, as the usage of saturated polymers is limited to certain fields of application, the second approach may be still the more universal and also less expensive one.Since the degradation process is accelerated if higher ground-level background ozone concentrations prevail, the steady rise of the ozone amount in the atmosphere – due to climate change – results in a shortened lifespan of rubber goods. In addition, not all applications are compatible with antiozonants and the usage of some of them may face restrictions – because of their toxicity – in the near future.Earlier observations at Semperit may suggest alternative mechanisms to influence the ozone resistance of rubber: Firstly, a surface pre-passivation with ozone at unstrained condition, which may affect the ozone resistance in a positive way, was discovered. Secondly, Semperit found a divergence in the ozone resistance of hydraulic hose covers and laboratory test specimens, that were made of the same elastomeric material. This divergence can be attributed to one or more differing production parameters.In this thesis both phenomena were investigated and moreover chemical surface modifications with a positive impact on the ozone resistance of rubber products were developed and tested.With the help of a DOE (Design of Experiments) approach, statistically significant production parameters influencing the ozone stability could be identified. Three-dimensional surface structures and a high surface to bulk ratio emerged as significant adverse parameters.Furthermore, it turned out that the reaction of C-C-double bonds – located at the unstretched rubber surface – with ozone, does not prevent a further attack of ozone on the strained rubber material.Several chemical surface modifications were elaborated and tested on their effect on the ozone resistance of elastomers. These reactions were characterized with ATR/FT-IR, XPS and SFE measurements. Chlorination reactions with short reaction times showed an enhancement of the crack free period with a factor of 3-9, compared to the unmodified blank. Epoxidation reactions with m-CPBA improved this factor even to 25 and higher values.