Thermal decomposition of polyethylene

According to current knowledge, the complex process of thermal decomposition of polyethylene is a combination of a number of elementary reactions proceeding through the free radical mechanism [11-13]. 

Conder JR, Fruitwala NA, Shingari MK (1983) Thermal decomposition of polyethylene glycol 20 m and essential oils in gas—liquid chromatography and the effect of traces of oxygen. J Chromatogr A 269 171-178... 

D. Mnnteanu and S. Turcn, Evaluation of kinetic parameters of the thermal decomposition of polyethylene-vinyl acetate graft copolymers, J. Thermal Anal, 20, 281 (1981). 

M. Day and D. M. Wiles, Influence of temperatnre and environment on the thermal decomposition of polyethylene terephthalate fibres, with and withont the flame retardant tri(2,3)-dibromopropyl phosphate., J. Anal Appl J rolysis, 7, 65-82, (1984). 

J. A. Conesa et ai., Thermogravimetric studies on the thermal-decomposition of polyethylene, Journai of Anaiyticai and Appiied Pyroiysis, 36, 1-15 (1996). 

Thermal decomposition of polyethylene in an inert atmosphere starts at about 280° C and occurs mainly following fragmentation and dehydrogenation reactions, the fragmentation being predominant at temperatures below 600° C (see Section 2.2). Hydrocarbons, from 2 carbon atoms up to 90 carbons, were identified in pyrolysates. Three types of fragment molecules are the most common, namely alkenes, alkanes, and a,(B-dienes. Traces of other hydrocarbons also are formed during pyrolysis. Some reactions typical for polyethylene pyrolysis are shown below ... 

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. 

T. Suebsaeng and C. A. Wilkie, SoUd products from thermal decomposition of polyethylene terephthalate Investigation by CP/MAS13C-NMR and Eourier transform-lR spectroscopy. Journal of Polymer Science Polymer Chemistry Edition, 22,945 (1984). 

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. 

Linear polyethylene can further also be considered as polymethylene. Althoguh first prepared by the thermal decomposition of diazomethane,243 244 Meerwein should be credited to have prepared it by catalytic polymerization of diazomethane effected by boron compounds (esters, halides, alkyls)245-247 taking place with concomitant dediazotation ... 

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... 

M. Blazso, B. Zelei, andE. Jakab, Thermal decomposition of low-density polyethylene in the presence of chlorine containing polymers, J. Anal. Appl Pyrol, 35, 221-235 (1995). 

For most industrial polyethylene processes (slurry and gas phase), thermal stability of the cocatalyst is not a factor since most operate in the temperature range 80-110 °C. However, solution processes operate at high enough temperatures where thermal decomposition of the cocatalyst could become a factor. Fortunately, residence times are typically short in solution processes. 

In this reaction two different procedures have been used. The first is the classical Hofmann degradation, which prepares the alkene by thermal decomposition of the quaternary ammonium hydroxide. Hofmann orientation is generally observed in acyclic and Zaitsev orientation in cyclohexyl substrates. The second is the treatment of quaternary ammonium halides with very strong bases, e.g. PhLi, KNH2 in liquid NH3. The formation of the alkene proceeds via an 1 mechanism, which means a syn elimination in contrast to the anti elimination which is observed in most of the classical Hofmann degradations. In some cases this type of elimination can also be accomplished by heating the salt with KOH in polyethylene glycol monomethyl ether. 

The mechanism of thermal decomposition for polyethylene starts with a random scission as follows ... 

Polymerization of Ethylene by Methyl-Lutetium and -Ytterbium Complexes. Products. Ethylene (60 psi, 30-100 C) is polymerized rapidly by 10 to 10" M cyclohexane solutions of the methyl complexes 3, 4, 5, 6, and 8 (Table). Low Mw oligomers were not formed (analysis by GC) in experiments la or 3a, Table. At 160 C both the Lu and Yb etherates produced very little polyethylene ( lO of the Table yields at 40-100 ) reflecting thermal decomposition of the active species. 

Fig. 38. Total weight loss and main species in the liquid phase during dynamic thermal decomposition. (a) Pyrolysis of polystyrene (Anderson and Freeman, 1961) (5°C/min, 1 mmHg). (b) Thermal decomposition of polypropylene (Ranzi et al, 1997a) (10°C/min, 1 atm), (c) Pyrolysis of polyethylene (Ranzi et al, 1997a) (10°C/min, 1 atm). Discrete model (Dashed) and Moment model (-).

The effect of structure on the mechanism of thermal decomposition of saturated hydrocarbon polymers has been studied more recently by Wall and Straus [40]. Linear and branched polyethylene, polypropylene and various copolymers have been investigated and the rates of volatilization compared. 

This section reviews the different aspects of the thermal conversion of those polymers which are the main components of the plastic waste stream polyethylene, polypropylene, polystyrene, PVC and PET, although the thermal degradation of other polymers is also commented on. The discussion focuses on mechanistic and kinetic factors, as well as on the type of products derived from the thermal decomposition of each individual polymer. The thermal degradation of plastic mixtures, which reflects more accurately the phenomena taking place in the thermal conversion of plastic wastes, is analysed and discussed in the next section. 

Chiu (116) used the apparatus previously described to study the thermal decomposition of selected polymers such as polyethylene terephthalate), po y(vinyl fluoride), po y(vinylidene fluoride), and others. The dielectric constant curves of a group of fluorocarbon polymers are shown in Figure 11.33. As illustrated, the more polar polymers such as poly(vinylidinefiuoride) (PVDF) and poly(vinyl fluoride) (PVF) show characteristic dielectric loss peaks that are distinguishable from the relatively featureless and low-loss curves of the other polymers. For PVF, the low-temperature process is due... 

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