An Evaluation of Tellurium Compounds as Polymer Stabilisers

Abstract

Some new organotellurium compounds were synthesised and their ability to act as stabilisers for poly (vinyl chloride) (PVC) and polypropylene (PP) assessed. Molecular weight measurements and mass spectra data indicate that diphenyl tellurium maleate and diary tellurium β-mercaptopropionate exist as dimers, whilst in the case of dicetyltellurium maleate there was shown to be an equilibrium between monomer and dimer. The dimer was shown by IR to contain an ester-like bridge which would allow ready dissociation to monomer species. The organotellurium compounds have been shown to neutralise hydrogen chloride, replace chlorine atoms of chlorohydrocarbons with carboxylate groups and also have the ability to stoichiometrically decompose peroxides. The effect of organotellurium compounds on PVC during processing, thermal ageing and UV irradiation was examined by measuring unsaturation, colour and carbonyl formation. These compounds were found to be effective stabilisers during processing and thermal oxidation but they do not lead to good UV stability during photo-oxidation. When used in combination with 4-octyloxy-2-hydroxvbenzophenone (HOBP) or when analogues of HOBP were incorporated into the organotellurium molecule the UV stability was dramatically improved. Tetrakis (diethyldithiocarbamato) tellurium (IV), Te(dtc)4' and bis (diethyldithiocarbamato)tellurium (II), Te(dtc)2' were found to be efficient melt stabilisers for PP and was also effective during thermal and photo-oxidation. Increasing the processing time for PP containing Te(dtc)4 led to an increase in the life-time for the PP but showed an initial pro-oxidant effect during photo-oxidation. This is due to the formation of tetraethylthiuram disulphide, TETD, by the decomposition of Te(dtc)4  into Te(dtc)2 and TETD. During the thermal oxidation Te(dtc)4 was a better stabiliser than Te(dtc)2' this is due to decomposition of Te(dtc)4 into Te(dtc)2 and TETD. By analogy with other metal dithiocarbamates, it seems likely that Te(dtc)4 and Te(dtc)2 act as a stabiliser for PP by destroying hydroperoxides into non-radical products.