Ozone ageing: experimental methods on pristine and protected natural rubber

Abstract

Many industrial applications require natural rubber (NR) as an irreplaceable polymer for its unique behaviour as its resistance to crack growth. The damage caused by ozone, seen as an ageing accelerator, influences the lifetime of rubber components. The data on ozone-induced ageing experiments are often incomplete or remain unpublished, whereas comprehensive databases for other environmental loads as oxygen do exist. A variety of experimental methods is used to investigate the ageing mechanism of ozone. The ultimate scope is to collect physically based data suitable to include it in thermo-mechanical modelling of the material’s full behaviour. Therefore, NR mixtures, without and with antiozonants: p-phenylenediamine (PPD) and paraffinic wax, are analysed. First, an accelerated, artificial ageing method is developed to reconstruct the real ageing in the laboratory. Experiments conducted henceforth are microhardness tests, uniaxial tensile tests and IR spectroscopy to determine the elastic modulus, the stress response and molecular change due to ageing. Independent of static deformation during the ageing process, both antiozonants show a significant protection effect up to the maximum loading with 75 pphm ozone concentration for 111 h at 50% relative humidity and 40 ^circ C. Paraffinic wax completely prevents measureable mechanical change and no surface cracks are visible, though IR spectra reveal ageing-induced molecular reactions, whereas the pristine and soley 6PPD protected compounds are clearly distinguishable by their surface crack picture. Neither of the material compounds, loaded with or without strain during ozone-ageing, contains cracks of a depth further than 300 upmu {textrm{m}}. The data generated on ozone ageing of rubber helps to distinguish it from thermo-oxidative ageing that is described comprehensively in the literature. In conclusion, the data proves the degradation and quantifies some characteristic material changes caused by ozone loading.