Polyolefins thermal decomposition

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

P.Y.17 is also frequently used in polyolefins, sometimes in the form of pigment preparations. Its heat stability in these media was said to be about 220 to 240°C, but must now, as a result of the detected thermal decomposition of diarylide yellow pigments in plastics, be limited to 200°C. This tendency to decompose excludes P.Y.17 from recommendation for use in polystyrene, in which the pigment largely dissolves under the processing conditions. The same is true for ABS. 

The majority of packaging plastic materials consists of polyolefins and vinyl polymers, namely polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). Obviously, these polymers have many other applications not only as packaging materials. Chemically they are all composed of saturated hydrocarbon chains of macro-molecular size their typical thermal decomposition pathway is free radical one initiated by the homolytic scission of a backbone carbon-carbon bond. In spite of the basic similarity of the initial cleavage, the decomposition of the hydrocarbon macroradicals is strongly influenced by fhe nafure of the side groups of the main chain. 

The poor selectivity of the thermal decomposition of polyolefins has promoted the development of catalytic cracking. Catalytic cracking lowers the pyrolysis process temperature and lowers the boiling temperature range of the resultant liquid products. The use of molecular sieves and amorphous silica-alumina catalysts for the cracking of waste polymers into a range of hydrocarbons has been widely studied (see Chapters 3-5, 7, 8). 

Recently the pyrolysis of polymer mixtures has become a focus of interest due to the increasing role of plastics recycling. Many researchers have investigated the thermal decomposition of various polymers in the presence of PVC. Kniimann and Bockhom [25] have studied the decomposition of common polymers and concluded that a separation of plastic mixtures by temperature-controlled pyrolysis in recycling processes is possible. Czegfny et al. [31] observed that the dehydrochlorination of PVC is promoted by the presence of polyamides and polyacrylonitrile however, other vinyl polymers or polyolefins have no effect on the dehydrochlorination. PVC generally affects the decomposition of other polymers due to the catalytic effect of HCI released. Even a few per cent PVC has an effect on the decomposition of polyethylene (PE) [32], HCI appears to promote the initial chain scission of PE. Day et al. [33] reported that PVC can influence the extent of degradation and the pyrolysis product distribution of plastics used in the... 

Table 6.1.12. Summary regarding literature information on thermal decomposition of several other polyolefins.

The polymer generates by thermal degradation a high yield of monomer. Other perfluorinated polyolefins are known in practice. Table 6.3.7 indicates the results of some literature reports regarding the results of thermal decomposition of several perfluorinated polyolefins. 

Table 6.3.7. Summary regarding literature on thermal decomposition of some perfluorinated polyolefins.

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

The products obtained by PE thermal decomposition largely depend on the degradation temperature and the reactor type. Most of the studies appearing in the literature on PE thermal decomposition focus on pyrolysis of the polyolefin by treatment at high temperatures (usually above 600 °C) only a few papers deal with the thermal cracking of PE at lower temperatures. 

The rate of the thermal decomposition of polyolefins is related to the presence of branches and side substituents in the polymer backbone. Accordingly, the following order of thermal degradation is usually observed HDPE < LDPE < PP < PS. Depending on the temperature, up to four fractions can be produced by the thermal decomposition of these plastics gases, oils, waxes and a solid residue. PVC degradation follows a different pathway HC1 is first removed at a low temperature to yield a polyene residue, which is then decomposed at a higher temperature. 

The polymer molecules start to break down in the presence of catalysts at considerably lower temperatures than in thermal decomposition. A significant catalytic conversion of polyolefins into volatile products has been detected at temperatures as low as 200 °C, compared with the value of 400 °C which is necessary in the thermal degradation of PE and PP to observe the formation of the first gases. As a consequence, catalytic treatments of plastic materials are usually carried out at low temperatures, in contrast with the range of 500-800 °C, typical for thermal cracking and pyrolysis. 

Foaming with dry gases generated by the thermal decomposition of a dihydro-oxadiazinone + azodicarboxylic acid amide or ester PPE-polyolefin graft copolymer and NBR Epoxy-terminated liquid PB, with either PP-MA or SEBS SBR and SBS copolymer ABS and SAN Hydroxynaphthoic acid... 

BSH is used for foaming rubbers, polystyrene, epoxy resins, polyamides, PVC, polyesters, phenol-formaldehyde resins, and polyolefins. However, the thermal decomposition of BSH yields not only nitrogen but also a nontoxic residue (disulfide and thiosulfone) which may degrade to give thiophenol and thus an unpleasant odor to the foams. 

Az, Dewald and Schnaitmann identified 3,3 -DCB as one of the primaiy products of thermal decomposition when polyolefin resins containing PY13 or PY83 pigments are subjected to processing temperatures above 200°C. The thermal decomposition mechanism is thought to proceed via bond scission reactions at the two azo linkages to produce non-azo derivatives... 

Group 6 metal (chromium, molybdenum, tungsten) carbonyls trapped in zeolites have been found to undergo partial thermal decomposition to produce M(CO)3 species which react with phosphines, polyolefins, and arenes to form the expected complexes, e.g. ( / -C6H6)Cr(CO)3 [210]. 

Some effort has recently been made to study copyrolysis of wood biomass and polyolefins.The effects of reaction temperature, wood-polymers mixture composition, and catalysts on the mixture s conversion into liquids and gases were established and discussed. The optimum temperature of wood—plastic mixture conversion, which corresponded to the maximum total liquid products yield, was close to 400°C. In the cohydropyrolysis processes the non-additive increase of the wood—plastic mixture conversion degree and of the distillable fractions yields took place as a result of the chemical interaction between radical fragments of wood and the thermal decomposition of polyethylene. 

Kiran, E. and Gillham, J. (1976) Pyrolysis-molecular weight chromatography A new on-line system for analysis of polymers. II. Thermal decomposition of polyolefins Polyethylene, polypropylene, polyisoprene J. Appl. Polym. Sci. 20, 2045-2068. 

Although more powerful computers are now available, the kinetics is not in an advanced phase and able to generalize all experimental results. In order to summarize the development of the kinetic treatment for the thermal decomposition of polyolefins, the following simplified scheme is proposed ... 

Some values for the elementary steps of thermal decomposition of polyolefins have been also evaluated [41, 290] as follows ... 

Jakab E., Omastova M., Thermal decomposition of polyolefin/carbon black composites . Journal of Analytical Applied Pyrolysis, 2005 74 204—214. 

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