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 so me metals (aluminium, iron and zinc), metal oxides (aluminium, titanium, co pper and iron) and carboxylates (zinc and tin) on the thermal decomposition processes of PVC. The metals and the oxides were not mixed with the polyme r in order to observe the effect of the surface contacts only. The evolutio n of the volatile thermal decomposition products has been monitored by mass spectrometry using low energy ionisation. Metals (aluminium, zinc, iron) a nd oxides of enough large metal ion radius (ferric oxide and titanium dioxi de) reduced the onset temperature of dehydrochlorination, admittedly by att racting chlorine, weakening the C-CI bonds in PVC. Depressed HCl formation was found in those cases when chlorides could formed (iron, zinc, Ca/Zn car boxylate, cupric oxide and titanium dioxide). Benzene formation was hindere d by the studied metals, oxides, and carboxylates as far as they are formin g chlorides with HCl. The promotion of benzene evolution observed on alumin a (and to a lower extent on titanium dioxide) is assumed to be the result o f a facilitated hydrogen exchange on the oxide surface, necessary for the d etachment 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 P VC is similarly influenced by the materials studied, as the product yield d ata of Py-GC/MS are consistent with that of pyrolysis/mass spectrometry. (C ) 1999 Elsevier Science B.V. All rights reserved.