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K. German¹, K. Kulesza², M. Florack, J. Pielichowski¹

1Department of Chemistry and Technology of Polymers, Cracow University of Technology,Kraków, Poland

2“Blachownia” Institute of Heavy Organic Synthesis, Kędzierzyn-Koźle, Poland, [email protected]

Abstract: Recycling of PVC wastes is a serious problem due to high chlorine content in this polymer. Dehydrochlorination (DHC) of PVC-containing polymer wastes produces solid residue (half-carbonizate) which conversion to pyrolysis oil in petrochemical plant seems to be attractive way of recycling of PVC wastes. Unfortunately, some polymer ad-mixtures cause with HCl chloroorganic compounds formation in half-carbonizate. The article describes infl uence of polycarbonates on PVC wastes, recycled in a two- stage method. It was found that the presence of PC causes formation of small amounts of benzyl chloride and other chloroaryl or chloroalkylaryl compounds. Poly(ethylene terephtha-late) interact with HCl forming plenty of various chlorocompounds, not only chloroethyl esters of terephthalic and benzoic acids.

1. Introduction

The disposal of plastic wastes will be important part of chemical industry because of large quantities of produced plastics and their environmental impact. Chlorine-containing polymers cause problems during material or energetic recycling due to their decomposi-tion with HCl evoludecomposi-tion. The most common used chlorine-containing polymer is PVC, which decomposition limits material recycling considerably due to worsening of proper-ties: -CH2-CHCl- mers eliminate HCl under temperature (or light) infl uence - PVC gets yellowish or even blackish and it lost plasticity. Such wastes can be used as raw material for pyrolysis or for gasifi cation. A choice between those two methods depends on reac-tivity of PVC in reaction with steam and on an amount of chlorine in half-carbonizate residue after HCl elimination from PVC.

Two main, almost not overlapping stages of mass loss are observed during thermal analy-sis of PVC [1, 2]. In one series of experiments 0,5 g sample of pure PVC was undergone dehydrochlorination (DHC) reaction at temperature up to 400 °C - DHC was broken, when the solid residue (half-carbonizate) was free of chlorine (with accuracy of conducto-metric analysis of eliminated HCl). DHC was followed by pyrolysis and it caused mainly formation of aromatic hydrocarbons - chloroogranic compounds were not found by

GC-MS analysis [2]. In other hand Tromp et al. [3] found chlorobenzene and methylchlo-robenzene, However it is not clear weather formation of the above mentioned compounds resulted from admixtures presence in sample or not.

Segregation methods of plastic’s wastes, which are acceptable from economical point of view, do not guarantee selectivity of 100 % - some of remaining „impurities“ can react with HCl and it causes formation of chloroderivates, which then contaminate pyrolysis products of half-carbonizate. A target of our present investigations is determination of such compounds.

If the density difference is applied as segregation factor, PET remains in PVC plastics fraction. This method is unable also to separate some other polymers having lower den-sity, but fi lled with inorganic additives (e.g. polycarbonates).

Thermal degradation of PVC was already subject of numerous investigations. Some at-tention was also paid to thermal degradation of PET [4-6] and PC [7-11], but behaviour of PVC – PET mixtures and especially interaction of PVC with PC were less investigated.

PVC (HCl) infl uence on products composition of PET pyrolysis was discussed in works [12, 13]. PVC and PET interaction was also taken into account in article [14], where py-rolysis of composed mixture (which simulates MPW - Municipal Plastic Wastes) is de-scribed.

Table 1: Density of selected polymers.

Polymer Code Density [g/cm³]

Polyolefi nes PE, PP 0,89 - 0,97

Polystyrene PS 1,04 - 1,08

Polyamide PA 1,03 - 1,15

Polyurethane - elastomers PU 1,10

Poly(methyl methacrylate) PMMA 1,18

Polycarbonate

(fi lled with glass fi bres) PC 1,20

1,42

Poly(vinyl chloride) PVC 1,38 - 1,55

Poly(ethylene terephthalate) PET 1,38 - 1,41

Polytetrafl uorethylene PTFE 2,20

Degradation of PET starts at temperature ca 350 °C. Liquid phase of products from py-rolysis at temperature up to 400 °C, that is in conditions, which allows providing full PVC dehydrochlorination, constitutes, as we have found mainly benzoic acid and acetal-dehyde (paralacetal-dehyde). Volatile products were not obtained during degradation of bisphe-nol A based polycarbonate (BPA–PC) in an inert atmosphere up to 400 °C [7, 8]. It was found that reaction medium infl uences selectivity of individual products. Alkalis catalyse rearrangement reactions, acids and other substances with active hydrogen atoms cause

depolymerisation [9]. Pyrolysis investigations of PVC-PC mixtures were not so far publi-cised. It can be expected, that thermal-chemical degradation of PC (BPA–PC) should be observed in the presence of HCl.

2. Experimental

2.1. Materials

Poly(vinyl chloride) (PVC S-70) from Anwil, W³oc³awek, Poland; BPA based polycar-bonate LEXAN as waste plastic from Telkom-Telos S.A., Kraków, Poland; carbon di-sulfi de, analytical grade, Riedel-de Haën.

2.2. Techniques

Dehydrochlorination and pyrolysis were conducted in an in-house built pyrolysis set.

Sample was 10-50 g. Temperature was measured by Ni-Cr-Ni thermocouple. Pyrolysis products were transported by nitrogen as a carrier gas into glass condensers system (air and water cooler) with active carbon adsorber and water scrubber at off-gas polypropyl-ene pipe. Dehydrochlorination was performed at 350 °C for 90 min and was followed by pyrolysis at 500 °C for 90 min. PC and PET pyrolysis was performed for comparison under the same conditions (90 min at 350 °C followed by 90 min at 500 °C). Condensed pyrolysis products were dissolved in CH2Cl2; products adsorbed on active carbon were extracted by 4cm³ of CS2. The solutions were analyzed by gas chromatography coupled with quadruple mass detector (GC-MS, HP 8590 Series 2 equipped with 30 m HP-1 col-umn) using helium as carrier gas.

3. Results and discussion

Oil fraction from PET pyrolysis contains mainly benzoic acid and biphenyl. Moreover there are identifi ed small amounts of other compounds showed in Fig. 1.

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Figure 1: Selected part of chromatogram of liquid products formed during PET pyrolysis.

Composition of products from PET pyrolysis changes radically in presence of PVC (HCl), which causes formation of great number of organic chloroderivates; some of them are specifi ed in table 2. It is noteworthy that chloroorganic derivates (for example 3- chlo-robenzoic acid and 4-chlochlo-robenzoic acid) are formed not only in transesterifi cation reac-tion of polyester and HCl. Identifi careac-tion of these compounds we described in an earlier paper [13].

Table 2: Chloroorganic products of PVC-PET 1:1 (w/w) mixture pyrolysed at 450 °C.

Chloroorganic compounds Peak Area Share, %

chlorine in aliphatic moiety

methane, bis(2-chloroethoxy) 0.8

chlorine in aliphatic moiety of benzoic acid esters

benzoic acid, 2-chloroethyl ester 11.2

4-methyl and 4-formyl benzoic acid, 2-chloroethyl esters 8.5 chlorine in aliphatic moiety of 1,4-benzenedicarboxylic acid

esters

1,4-benzenedicarboxylic acid, mono-2-chloroethyl, esters 18.8 1,4-benzezedicarboxylic acid, di-2-chloroethyl ester 30.6

chlorine in aromatic ring

3- and 4-chlorobenzoic acid, ethyl ester, 0.3

chlorine in aromatic ring and in aliphatic moiety

4-chlorobenzoic acid, 2-chloroethyl ester 0.4

Chromatogram of oil fraction obtained during PC pyrolysis is presented in Fig. 2. We did not found fl uorenone and xanthone derivatives – that difference with results described in works [9, 11] can be explained by absence of basic catalysts in reaction medium.

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Figure 2: Chromatogram of polycarbonate pyrolysis products.

Oil fraction chromatogram of PC+PVC 1:1 (w/w) mixture pyrolysis is shown in Fig. 3.

Figure 3: Products of PVC:PC (1:1 w/w) mixture DHC followed by its pyrolysis.

Main products of PVC-PC mixture pyrolysis preceded by its DHC are phenol, 4-iso-propylphenol and 1-isopropyl-4-phenoxybenzene. Our result confi rms conclusion about acidic medium infl uence on mechanism of thermal degradation of PC, but our target was identifi cation of chlorine compounds. We did not found chlorine-derivates among con-densed pyrolysis products, while they can be formed with consecutive reactions (Fig. 4).

C O

Figure 4: Potential synthesis pathways of chlorine-derivates products from reaction between PC and HCl.

Some of products predicted by the reaction scheme showed in Fig. 4 were found among products adsorbed on active coal, but some of them were in isomerised form. Chromato-gram of these products is presented in Fig. 5. Peak area of all founded chlorocompounds is about 2-3%, what means, that a ratio of chlorine in oil fraction from pyrolysis of PVC+PC

mixtures can exceed technological threshold of 10 ppm, even when the PC ratio in waste mixture is much lower than in our mixture. Identifi cation of chlorine derivates by means of GC-MS requires hard attention because of masking of Cl-isotope effect by signals re-sulting from other fragmentation paths.

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Figure 5: Chromatogram of products, adsorbed on active coal, from pyrolysis of PVC+PC (1:1) mixtures.

Pyrolysis of PVC+PET+PC (1:1:1) mixtures gave no new chlorine-containing compounds, but analysis has not been completed yet, because of its complexity.

5. Conclusions

PET interacts with HCl forming great amounts of chloroderivatives, mainly chloroethyl esters of terephthalic acid and of benzoic acid. Pyrolysis of BPA-based polycarbonate produces 4-methylphenol, 4-ethylphenol, 4-isopropylphenol, 4-(1-methyl-1-phenylethyl) phenol, 1-isopropyl-4-phenoxy-benzene, BPA and small amounts of other compounds.

HCl evolved during PVC-PC mixture dehydrochlorination changes composition of PC degradation products - mainly phenol, isopropylphenol and 1-isopropyl-4-phenoxy-ben-zene are formed. Polyesters show interaction with HCl at temperatures below 350 ^°C.

These secondary reactions during thermal degradation of PVC-PC mixture produce small amounts of halogen derivatives, such as chlorobenzene, 1,4-dichlorobenzene, benzyl chloride, parachlorophenol and isopropylchlorobenzene. So, any method is necessary to eliminate chlorine content from half-carbonizate or from pyrolysis oil.

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