Thermal and photooxidation of high styrene-butadiene copolymer (SBC)

The thermal and photooxidation of high styrene-butadiene copolymer (SBC) with high styrene content (K-resin) have been studied using a variety of analytical and spectroscopic methods including yellowness, luminescence and FTIR spectroscopy coupled with crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. FTIR and luminescence analysis show complex oxidation processes with some distinct features associated with each phase. Rates of thermal oxidation on oven ageing show carbonyl growth increases with increasing temperature and is autocatalytic at 110degreesC while at 90degreesC an initial induction period is evident. Typical autocatalytic growth and decay of hydroperoxides are also observed at both temperatures with higher concentrations being observed at 90 degreesC. High degrees of crosslinking as well as yellowness (discolouration) show a similar pattern. Fluorescence analysis confirms a rapid initial disruption of the polystyrene excimers coupled with the formation of long wavelength emitting polyconjugated chromophores, possibly, stilbene type in nature giving rise to the colour formation. Oxidation is due primarily to the olefinic vinyl groups and acetophenone end groups with FTIR absorptions at 966 and 1695 cm(-1) respectively. Thermal oxidation gives rise to a predominant absorption associated with ester groups at 1730-1740 cm(-1). Anhydrides, aromatic ketones, aldehydes, lactones/peracids and alpha,beta-unsaturated carbonyl species are also formed in this matrix coupled with rapid hydroperoxidation as shown by hydroxyl group formation. Analysis of the gel matrix showed similar functionalities with both styrenic and olefinic/butadiene phases being involved as well as additional conjugated vinyl group formation. Irradiation of the SBC was undertaken with polychromatic light (Microscal), 365 nm and 254 nm light. Under all three conditions long induction periods were observed prior to carbonyl group formation with 254 nm exposure showing only weak formation due to their high photolytic instability under this condition. Hydroperoxide concentration increased slightly initially and then achieved a steady state under 254 run irradiation. The Microscal and 365 nm light gave similar rates of oxidation as determined by carbonyl index coupled with a gradual increase in stable hydroperoxide concentrations. Weak crosslinking was evident only under 254 nm light while excimer aggregates were destroyed rapidly with slower rates for Microscal and 365 run irradiations. Strong discolouration was also evident under 254 nm light compared with the longer wavelength sources. Irradiation gave predominantly carboxylic acid groups at 1716 cm-1 with lesser evident formation of more active anhydrides, aromatic ketones, aldehydes, lactones/peracids and alpha,beta-unsaturated carbonyl species due to their photolytic instability. Phosphorescence analysis shows the presence of initial active acetophenone groups which during irradiation grow rapidly initially followed by their rapid decomposition (instability and reactivity). On thermal oxidation they grow to a steady state. The olefinic vinyl groups at 966 cm(-1) are one of the major causes of the instability associated with hydroperoxidation to form vinyl hydroperoxides. These groups in-turn break down to give unsaturated carbonyl groups and crosslinked products. They will also give rise to end-chain aliphatic macroradicals capable of further oxidation. Thus, end-chain oxidation is a predominant process at the interphase boundary of the soft aliphatic and hard aromatic segments with the immediate autocatalytic formation of high concentrations of primary hydroperoxides during the early stages of oxidation. Phosphorescence analysis also indicated the presence of initial acetophenone chromophores, which are associated with polystyrene end groups formed by chain breakage at the aliphatic links. These species can act as initial active sensitive sites for further breakdown, possibly via a thermally induced hydrogen atom abstraction process to give benzaldehyde and benzoic acid. These active end groups show a typical autocatalytic growth and decay process on irradiation. The end-chain aliphatic radicals then become the sites for further initial rapid hydroperoxidation and crosslinking reactions, which can give rise to complex gel formation. Reaction mechanisms and coloured reaction products are proposed. (C) 2004 Elsevier Ltd. All rights reserved.