Marianne Blazsó

Pyrolysis and debromination of flame retarded polymers of electronic scrap studied by analytical pyrolysis

Abstract Two typical components of electronic scrap have been investigated by analytical pyrolysis: an electronic junction (phthalic polyester filled with ceramic fibres, flame retarded with brominated polystyrene) and a printed circuit board (epoxy resin on woven glass fibre support, flame retarded with brominated epoxy resin). Copyrolysis of these plastic materials has been carried out with inorganic solids of basic character. The volatile thermal decomposition products were analysed on line using pyrolysis-gas chromatography-mass spectrometry. The product analysis revealed that the decomposition reactions of the polymer constituents are significantly altered in the presence of the strong inorganic bases. Due to debromination of the dibromo- and tribromophenyl groups of brominated polystyrene, copyrolysis with sodium hydroxide and with basic zeolites resulted in a considerably reduced yield of dibromo- and tribromostyrenes. From brominated epoxy resins pyrolysed with sodium hydroxide an enhanced bromomethane evolution, while a depressed brominated phenol formation was observed. A diminished production of bromophenols takes place in pyrolysis with sodium-containing silicates as well.

Recent trends in analytical and applied pyrolysis of polymers

Abstract The latest works are reviewed on analytical and applied pyrolysis of polymers, copolymers and blends. Improved identification, component analysis and structural elucidation were performed on several new polymers and copolymers. Various additives, catalysts and residual oligomers were analysed in plastics and the emission of toxic compounds under pyrolysis and combustion were monitored. The development of analytical pyrolysis methods (pyrolysis on line coupled to gas chromatography, mass spectrometry and/or infrared spectroscopy) is closely related to the advances in instrumental chemical analysis and to their combination possibilities. Publications on applied pyrolysis are concerned with the conversion of polymeric precursors into high performance materials (e.g. carbon fibers and ceramics) and the production of useful chemicals (e.g. monomers or fuels) from polymer wastes. In both analytical and applied pyrolysis, knowledge about the chemical reactions taking place is essential. Recent studies on the thermal decomposition mechanism of polyolefins, polystyrenes, acryl polymers, polyesters, polyethers, formaldehyde resins, polyamides, sulfur-and silicon-containing polymers are critically reviewed.

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

Abstract 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–Cl 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.

Thermal decomposition of mixtures of vinyl polymers and lignocellulosic materials

Abstract The effect of wood, cellulose, lignin and activated charcoal on the thermal decomposition of polystyrene (PS) and polyethylene (PE) has been studied in order to investigate the thermal behavior of these materials occurring in municipal waste. Thermogravimetry/mass spectrometry and pyrolysis–gas chromatography/mass spectrometry revealed that these materials had a similar influence on PS and PE thermal decomposition under slow and fast heating conditions respectively. The effect is related to the char-forming capability of the wood-derived additives; thus cellulose had the least and pure charcoal had the greatest influence on the decomposition of the polymers studied. Polystyrene is more sensitive to the presence of additives than the two PE polymers investigated. The thermal decomposition of PS shifts to higher temperature and the product distribution changes significantly in the presence of wood-derived additives. The yield of monomer, dimer and trimer decreases and the formation of other products (e.g. toluene, ethyl benzene and α-methyl styrene) increases. The effect of additives is interpreted in terms of the free radical mechanism of the thermal decomposition of PS. Wood, cellulose and lignin have a small effect on the thermogravimetric curves of PE. Charcoal promotes the hydrogenation of the unsaturated products and the hydrogenated products evolve at higher temperature.

Thermal decomposition of low-density polyethylene in the presence of chlorine-containing polymers

Abstract The thermal decomposition of low-density polyethylene was studied on samples containing a few per cent of poly(vinyl chloride), poly(vinylbenzyl chloride) or poly(chlorostyrene), using thermogravimetry/mass spectrometry, pyrolysis-gas chromatography/mass spectrometry between 400 and 1000 °C, and FT-IR analysis of the pyrolysis tars. Reactive species evolving from the chlorine-containing polymers during their thermal degradation affect the decomposition reactions of polyethylene. Hydrogen chloride, formed from chloropolymers prior to the beginning of the PE thermal decomposition, promotes the initiating steps of thermal degradation and inhibits β-scission of macroradicals. Chlorine evolves at a relatively higher temperature, and enhances dehydrogenation reactions leading to aromatic products.

Natural rubber pyrolysis: Study of temperature-and thickness-dependence indicates dimer formation mechanism

Abstract The technique of pyrolysis—gas chromatography (Py—GC) has been used to study the thermal degradation of natural rubber. The monomer/dimer (M/D) ratio has been measured over the temperature range 300 to 500°C, using sample sizes of the order of 0.3 mg. The M/D ratio has also been measured as a function of sample thickness, using samples in the range 30 nm to 3 μm (0.1 μg to 10 μg). The dependence of M/D on temperature may be interpreted in terms of (a) different activation energies for depropagation versus intramolecular cyclisation, (b) dissociation of dimer to monomer at higher temperatures, or (c) reduced residence time of monomer in the melt at higher temperatures allowing less opportunity for its recombination. The results for the dependence of M/D on thickness (at constant temperature) indicate that interpretation (c) is the most probable. The present work therefore suggests that monomer recombination, possibly by a Diels-Alder mechanism is an important contributor to dimer formation in rubber pyrolysis.

Polyaromatization in common synthetic polymers at elevated temperatures

Abstract Thermal decomposition reactions of polyethylene and phenolformaldehyde novolac were studied at 800–1000°C by pyrolysis—gas chromatography/mass spectrometry. The main high temperature products of both polymers are benzene, naphthalene, acenaphthene and phenanthrene; moreover, dibenzofuran and xanthene are also formed from phenolformaldehyde novolac. A high temperature decomposition mechanism is proposed for polyethylene through conjugated polyene formation, aromatic ring closure and fusion. For phenolic novolac several other reactions are considered such as condensation or reduction of phenolic hydroxyl groups, decomposition of phenol rings into carbon monoxide and pentadienylene biradical, and attachment of the biradical to an aromatic ring resulting in bi- and tricyclic aromatic volatiles. High temperature pyrolysis experiments have also been carried out in the presence of some inorganic materials. The effect of additives has been interpreted in terms of promoting or hindering the reactions yielding polyaromatic volatiles. Polyaromatization and condensation are advanced by additives of high surface area (γ-alumina and activated charcoal) and retarded by potassium carbonate, iron, ferric oxide, copper and cupric oxide.

The effect of carbon black on the thermal decomposition of vinyl polymers

Abstract The effect of carbon black on the thermal decomposition of various polymers has been studied in an inert atmosphere by thermogravimetry/mass spectrometry and pyrolysis–gas chromatography/mass spectrometry. It has been established that the nature of substituents on the hydrocarbon chain of the polymers affects the thermal behavior of the mixtures with carbon black. Carbon black exhibits no influence on the decomposition of poly(methyl methacrylate) (PMMA) which has quaternary carbon atoms in the polymer chain and decomposes by depolymerization. The decomposition of polypropylene (PP) is promoted, whereas, that of polyethylene (PE), polystyrene (PS) and polyacrylonitrile (PAN) is hindered in the presence of carbon black. The char yield of PAN is increased significantly, however, carbon black has no impact on the amount of residue of non-char-forming polymers. Analysis of the pyrolysis products indicates that carbon black has influence through the chain cleavage and H-transfer reactions. The promotion of the chain scission reactions in PP is indicated by the lower decomposition temperature and the increased formation of products originating from the primary macroradicals. It appears that carbon black participates in the termination of the chain reactions, too, thus, the yield of oligomers is significantly reduced from the vinyl polymers. The increased yield of hydrogenated products also confirms that carbon black participates in the H-transfer reactions.

Thermal decomposition of polymers modified by catalytic effects of copper and iron chlorides

Abstract Effects of copper and iron chlorides were studied on the thermal decomposition reactions of polymers using pyrolysis-gas chromatography/mass spectrometry. Copper and iron chlorides found to depress the radical transfer reactions of lower probability in polypropylene and polystyrene at 500°C through the interaction of the radicals and the transition metal ions. Cu(I) and Fe(II) chlorides enhance the production of aromatic and polyaromatic compounds from polyethylene at 1000°C promoting hydrogen elimination. Partial decomposition of the methyl ester side groups takes place in poly (methyl methacrylate) at 450°C, leading to chloromethane and carbon dioxide, when Cu(II), Fe(II) or Fe(III) chloride is present. Heterolytic cleavage of the methoxy group is presumably promoted by the electrophyl transition metal ions. Similar effect was proposed explaining the enhanced phenol evolution from phenol-formaldehyde resin, polycarbonate and epoxy resin at 500–600°C in the presence of Cu(II) and iron chlorides. The rupture of phenolic rings and the reduction of the phenolic hydroxyl groups leading to polyaromatic pyrolysis products from phenol-formaldehyde resin at 1000°C was reduced by Cu(II) and iron chlorides presumably due to a protective interaction of the phenol rings and the transition metal ions. Pyrolysis of polymers was performed also in the presence of PVC and copper or iron. The results lead to the conclusion that the chlorides of lower oxidation state are forming from PVC and copper or iron when co-pyrolysed with polyethylene or phenol-formaldehyde resin.

Thermal decomposition of polyamides in the presence of poly(vinyl chloride)

Abstract Thermal decomposition of aliphatic and aromatic polyamides (polyamide-12, polyamide-6,6 and poly(1,4-phenylene terephthalamide) (Kevlar)) with poly(vinyl chloride) (PVC) was examined by pyrolysis-gas chromatography/mass spectrometry. Polyamide-PVC mixtures (typical mass ratio 1:1) were pyrolysed at 700 and 900°C. It was found that the presence of PVC promoted the hydrolytic decomposition routes of amide groups and volatile nitrile formation from all examined polyamides due to the hydrogen chloride eliminated from PVC under pyrolysis. In the presence of PVC an elevated yield of alkenenitriles was observed from polyamide-12. Benzeneamine is produced from Kevlar instead of benzenediamine in the presence of PVC, and the evolution of benzoic acid, benzenenitrile and benzeneisocyanate is promoted. At 900°C pyrolysis temperature enhanced hydrogen cyanide evolution was observed from polyamide-12 and polyamide-6,6 in the presence of PVC. Mechanisms are proposed for the hydrogen chloride assisted formation of nitriles.