Effect of metals, metal oxides, and carboxylates on the thermal decomposition processes of poly (vinyl chloride)

In this work, pyrolysis-mass spectrometry and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was applied to investigate the influence of some metals (aluminium, iron and zinc), metal oxides (aluminium, titanium, copper and iron) and carboxylates (zinc and tin) on the thermal decomposition processes of PVC. The metals and the oxides were not mixed with the polymer in order to observe the effect of the surface contacts only. The evolution of the volatile thermal decomposition products has been monitored by mass spectrometry using low energy ionisation. Metals (aluminium, zinc, iron) and oxides of enough large metal ion radius (ferric oxide and titanium dioxide) reduced the onset temperature of dehydrochlorination, admittedly by attracting chlorine, weakening the C-CI bonds in PVC. Depressed HCl formation was found in those cases when chlorides could formed (iron, zinc, Ca/Zn carboxylate, cupric oxide and titanium dioxide). Benzene formation was hindered by the studied metals, oxides, and carboxylates as far as they are forming chlorides with HCl. The promotion of benzene evolution observed on alumina (and to a lower extent on titanium dioxide) is assumed to be the result of a facilitated hydrogen exchange on the oxide surface, necessary for the detachment of benzene from a dehydrochlorinated PVC segment. The second step of PVC thermal decomposition is also shifted to a lower temperature by the metals tested and by transition metal oxides. This effect is explained by the easier cleavage of the polyenic chain at segments getting contacted to metal surfaces or to transition metal ions. Apparently, fast pyrolysis of PVC is similarly influenced by the materials studied, as the product yield data of Py-GC/MS are consistent with that of pyrolysis/mass spectrometry. (C) 1999 Elsevier Science B.V. All rights reserved.