MECHANISMS OF ANTIOZONANT PROTECTION OF ELASTOMERS

Ozone cracking in elastomers can be characterised by two parameters: a threshold condition and a characteristic rate of growth. Many chemica l additives can reduce the growth rate but the most potent also effect ively raise the threshold by forming a protective layer on the surface . Earlier studies with the chemical antiozonant N,N'-bis(1-ethyl-3-met hylpentyl)-p-phenylene diamine have suggested that protective layer fo rmation comes about primarily as a result of direct reaction between t he antiozonant and ozone and that the rate of layer formation, and pro tective action, are governed by diffusion processes. Consistent with t his, elastomers that have relatively high glass transition temperature s, and hence high internal viscosity (which retards diffusion), such a s nitrile rubber or epoxidized natural rubber, are found to be more di fficult to protect than lower glass transition materials, such as natu ral rubber or styrene-butadiene rubber. Recently two further di-alkyl para-phenylenediamines have been studied in natural rubber and 50% epo xidized natural rubber. Marked differences in rates of layer formation have been found with the different antiozonants but these do not corr elate with protective action. It appears that relatively small differe nces in the paraphenylenediamine substitution can lead to marked diffe rences in the nature (and effectiveness) of the layer that is formed.