A study of the products of PVC thermal degradation

Abstract

PVC thermal degradation in vacuum up to 500 °C has been followed by recording the relative rate of volatile product formation by thermal volatilisation analysis while monitoring by mass spectrometry the formation of the main products: HCl, aromatic and aliphatic hydrocarbons, and non-condensable gases (CH 4, H 2). The material balance after pyrolysis has been evaluated. The liquid fraction collected during pyrolysis was analysed by GC-MS and its composition determined by the integration of the ion current under the peaks due to different compounds. After HCl (53% of the PVC sample), the tar is the major fraction (24%). The liquid fraction (of which 80% is benzene) accounts for 7% of the original polymer. The other fractions are the char (9.5%) and gas fraction (6.6%). 10% of the Cl remained trapped in the polymer until higher degradation temperatures giving rise then to the chlorinated compounds which account for 1.75% of the liquid fraction and 0.14% of the polymer. PVC shows two stages of degradation: during the first stage, between 200 and 360 °C, mainly HCl and benzene and very little alkyl aromatic or condensed ring aromatic hydrocarbons are formed. It was evaluated that 15% of the polyene generates benzene, the main part accumulating in the polymer and being active in intermolecular and intramolecular condensation reactions by which cyclohexene and cyclohexadiene rings embedded in an aliphatic matrix are formed. Alkyl aromatic and condensed ring aromatic hydrocarbons are formed in the second stage of degradation, between 360 and 500 °C, when very little HCl and benzene are formed. In this stage the polymeric network formed by polyene condensation breaks down in the process of aromatisation of the above C 6 rings. The mechanism of benzene formation at different temperatures was considered.