Chlorinated pyrolysis products of co-pyrolysis of poly(vinyl chloride) and poly(ethylene terephthalate)

doi:10.1016/S0165-2370(02)00057-8

Journal of Analytical and Applied Pyrolysis

Volume 67, Issue 1, March 2003, Pages 123-134

https://doi.org/10.1016/S0165-2370(02)00057-8 Get rights and content

Abstract

The influence of poly(vinyl chloride) on poly(ethylene terephthalate) pyrolysis was investigated. Products of degradation of this two-polymer mixture were separated by gas chromatography and chloroorganic components were identified. Chloroesters of terephthalic and benzoic acids were found. Mass spectra and fragmentation pattern for identified compounds have been shown. The signals from two fragments at m/z 211 and 213 were proposed to be used for a fast identification of such compounds.

Introduction

Poly(vinyl chloride) (PVC) has to be separated from plastic wastes due to hydrogen chloride (HCl) evolution even at low temperature. This reactive gas makes recycling of other polymers difficult. Separated PVC wastes can be considered as material for two-stage recycling [1], consisting of dehydrochlorination, and then degradation of the residue. We investigate how precise should be or have to be a PVC separation method. Components, which react with HCl, will reduce the efficiency of dehydrochlorination and limit the recycling of the residue. A simple density difference separation of poly(ethylene terephthalate) (PET) is not possible, because its density (1.33–1.44 g cm⁻³) is similar to the PVC density (1.33–1.55 g cm⁻³). Other methods as electrostatic or froth flotation separation of these two polymers are expensive [2].

Thermal degradation of PVC was and still is a topic of numerous studies. Literature concerning this problem was discussed in works of Pielichowski et al. [3], [4] and Knümann and Bockhorn [5]. Literature concerned PET degradation isn't comprehensive. A list of papers about PET pyrolysis was given by Dzięcioł and Trzeszczyński [6]. Investigations of Masuda et al. [7] showed that in the presence of steam, PET degradation starts at temperature about 350 °C. Products of this reaction are oligomers, terephthalic acid, acetaldehyde and CO₂. We additionally found paraldehyde and benzoic acid in experiments conducted at temperature of 300–450 °C, in an inert gas atmosphere. It is noticed a lack of works concerning thermal degradation of PVC–PET mixtures. Knümann and Bockhorn [5] showed results of TG studies on PVC mixtures with polyolefins, polystyrene and polyamide 6, but there are no data on PET. Sakata et al. [8] examined mixtures of PVC/PE and PET/PE; they did not investigated PVC/PET mixtures either. We have not found any paper dealing with a possibility of chloroorganic compound formation during a heating of PVC–PET mixtures. This is very important, because it limits a choice of recycling method of PVC wastes. Moreover, we have not found in literature a reliable method for detection of compounds containing chlorine in mixtures of polymer pyrolysis liquid products of (apart from sophisticated methods with a sample combustion).

Section snippets

Materials

In our studies on PVC/PET mixtures degradation we used commercial, suspension-grade PVC from ‘Anvil’ Factory in Włocławek, Poland and commercial PET, provided by ELANA Inc. in Toruń, Poland. PET is produced in the form of granules by polycondensation of ethyl terephthalate, obtained from transesterefication of dimethyl terephthalate and ethylene glycol.

Thermogravimetric measurements

Thermogravimetric measurements have been done in atmosphere of argon using a thermal analyzer Derivatograph – C, MOM (Hungary). The heating rate

Discussion

Fig. 2 shows a comparison of TG curves of the two polymers in the temperature range that is appropriate for dehydrochlorination process. One can see that PET alone looses its mass at temperature above 300 °C in inert gas atmosphere. It is about 100 °C higher than in case of PVC. As a consequence of this, HCl evolved from PVC can react with PET before its degradation. Concentration of HCl will be a function of PVC decomposition rate. The dehydrochlorination rate increases significantly from 230 °C.

Conclusions

The presented results of experiments shows mutual influence of PVC and PET on the course of dehydrochlorination process and decomposition products. This influence is a consequence of reaction between HCl, eliminated from PVC, and PET. HCl accelerates PET degradation rate and results in the formation of a series of chloroorganic compounds. Interaction between HCl and PET decreases the rate of dehydrochlorination process because a part of HCl is reacted thus the extent of autocatalysis is

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