A chemiluminescence study of micron and nanoparticle titanium dioxide: effect on the thermal stability of metallocene polyethylene

The thermal activity of a series of nano and micron particle grade anatase and rutile titanium dioxide pigments, with various densities of surface treatments, particle size and surface area, have been determined in monomodal metallocene polyethylene. Their performance has been undertaken by chemiluminescence (CL), differential scanning calorimetry and hydroperoxide analysis. Chemiluminescence study under nitrogen on polyethylene containing nano and micron titania allows one to determine the thermal activity of pigment during processing. Such results show that for pigmented polymers, a marked suppression of emission is observed, which correlates well with the lower initial determined hydroperoxide content. This effect is enhanced with pigment content and coated grades. For untreated pigment, more oxidation is induced by the greater surface area of the pigment particle, which, in turn influences the dispersion and enhances the pigment–polymer contact. At temperatures below the polyethylene melting point, low chemiluminescence was observed due to the restricted mobility of hydroperoxides to give the disproportionation reaction responsible for the emission. Under oxygen, the high activity of Irganox 1010 as a long-term thermal stabiliser is observed, and the activity of titanium dioxide has been seen to be dependent on the particle size. In general, nanoparticles exhibit a greater thermal sensitising effect, which is reduced by the coating. Increasing the content of pigment, the oxidation induction times (OIT) are greatly reduced and are not detected by DSC. It is concluded that the CL method can be used to determine differences in the oxidation level of pigmented polyethylenes.