Impact of stabilisers on the thermal catalytic activity of micro- and nano-particulate titanium dioxide in oxidizing condensed mediums

Abstract

This investigation was undertaken to elucidate an interplay between stabilisers and titanium dioxide nano-particles in oxidizing condensed mediums comprising of the initiatied (initiator is 2,2′-azobisisobutyronitrile, AIBN) oxidation of cumene and ethylbezene and an intercomparison with the thermal oven ageing of high-density polyethylene (HDPE). As a preliminary study, the catalytic influence of different nano and micro-particle sized anatase and rutile titanium dioxide pigments on the uninitiated oxidation of cumene at moderate temperature (60 °C) has been established. The oxidation rates depend linearly on the square root of the concentration of the titanium dioxides over a sufficiently wide range and thereby follow the mechanism of oxidation with recombination of peroxy radicals. Kinetic measurements of oxidation rates of the model reactions of initiated oxidation of cumene and ethylbenzene in the absence and presence of a hindered piperidine stabiliser, Chimassorb 119 FL, Cyasorb 3529 and sterically hindered Irganox 1010 stabilisers clearly showed that the stabilisers promote over-additional initiating activity of titanium dioxide pigments. This effect was found to be very large, i.e. the values of afforded additional initiation rates are 2–9 times more than those compared with the value of the initial initiation rate induced by titanium dioxides in the system without stabilisers. In the case of hindered amine stabilisers this phenomenon is more pronounced than that for the Irganox phenolic antioxidants. On the basis of susceptibility towards all the examined stabilisers, resulting in a sharp increase of the initial initiating activity, the titanium dioxides can be ordered as nano-rutile untreated > nano-anatase treated hydroxyapatite ≥ nano-anatase untreated ≥ (nano-anatase (75%) + nano-rutile (25%)) untreated > micro-anatase untreated > micro-rutile treated. These additional initiation rates of titanium dioxides which are affected by the stabilisers can be explained in terms of physical absorption of the stabilisers onto the surface of the titanium dioxide particles. The stabilisers being adsorbed onto the surface of titanium dioxides block titania surface-OH groups and thereby prevent the formation of water in the oxidizing system to promote the additional catalytic capacity of the titanium dioxide pigments. Basic amine functionalities of Chimassorb 119 FL and Cyasorb 3529 are energetically more protonated by the titanium dioxides' surface hydroxyls during oxidation than when compared with phenolic hydroxyl and ester groups of Irganox 1010 and thereby the amine functionalities stimulate a relatively higher level of additional initiating activity of the pigments. This explanation is in good accordance with the data of work elsewhere [Allen NS, Edge M, Sandoval G, Ortega A, Liauw CM, Stratton J, et al. Factors affecting the interfacial adsorption of stabilizers on to titanium dioxide particles (flow microcalorimetry, modeling, oxidation and FTIR studies): nano versus pigmentary grades. Dyes Pigments 2006; 70(3):192–203]. Nano-anatase pigment C is shown to exhibit a sensitizing effect at a concentration of 1 and 2% w/w in the model reaction of cumene initiated oxidation whereas it exhibits a different effect in the thermal sensitizing oxidation at 1% w/w to the stabilising at 2% w/w during thermooxidative ageing in HDPE film.