The decomposition of plastics can be considered as depolymerization of polymer into low-molecular product. The general reaction mechanism for the thermal degradation is described with the following steps, and also shown in Figure 5.1.
The decomposition of plastics depends on the plastic type, with different reaction mechanisms of plastics proposed with four types .
DEGRADATION OF HDPE Initiation
CH2-CHX + CH2-CHX
End-chain scission : CH2-CHX-CH2-CHX
Hydrogen chain transfer
Intermolecular : w CH2-ChX + ^ CH2-CHX-CH2-CHX-CH2 w
V^CH2-CHX + ^ CH2-CHX-CH2 va->► wCH2-CH2X + w CH2-CX-CH2v
Intramolecular : «CH2-CHX-CH2-CHX-Ch2-- wCh2-CHX=CH-CHX-CH3
b-cleavage: >» CHX-CH2-Cx-CH2 ^-- ^ ChX+CH2=CX-CH2 ^
Formation of branches
v^ch2-Chx + ^ ch2-Cx-ch2 w —► ch2-cx-ch2 ^ w ch2-Cx-ch2 ^ + w ch2-cx-ch2 w —>► v-a ch2-cx-ch2 w wch2-cx-ch2w
Bimolecular coupling: w CHX-Ch2 + ChX-CH2 w-► w cHX-CH2-CHX-CH2w
^ch2-chx-ch2 + chx-ch2-ch2^-- v^ch2-chx-ch3 + chx=ch-ch2v^
These different mechanisms are related to the bond dissociation energies, the chain defects of the polymers, the aromatic degree and the presence of halogen and other hetero-atoms in the polymer chains. For the reaction mechanism of the main components in waste thermoplastics, the pyrolysis of PVC occurs by the chain-stripping mechanism with much less monomer recovery, whereas that of PS with cyclic structure occurs by both end-chain and random-chain scission mechanism and the monomer recovery is very high. Especially, PE and PP which comprise the main polymers in waste plastics pyrolyze by random-chain scission, which yields a wide range of hydrocarbon products. The oil products consist of higher-boiling-point hydrocarbons with low valuable products as well as lower-boiling-point hydrocarbons. Thus, in the pyrolysis process the more cracking of high-boiling-point hydrocarbons to obtain valuable light oil product with high yield must be taken into consideration in a large-scale plant.
A new macroscopic degradation mechanism of polymers studied by Murata et al.  was suggested with two distinct mechanism in the thermal degradation of PE, PP and PS. One is a random scission of polymer links that causes a decomposition of macromolecules into the intermediate reactants in liquid phase, and the other is a chain-end scission that caused a conversion of the intermediate reactants into volatile products at the gas-liquid interface. There are parallel reactions via two mechanisms. The random scission of polymer links causes a reduction in molecular weight of macromolecules and an increase of the number of oligomer molecules. The chain-end scission causes a dissipation of oligomer molecules and a generation of volatile products.
The cracking reactions of heavy hydrocarbons using a catalyst such as solid acid and bifunctional catalyst, etc. have been explained with the difference of simple thermal degradation of the polymer. In the depropagation of the polymer chain using the catalyst, the molecular weight of the main polymer chains may be rapidly reduced through successive attacks by acid sites on the catalyst, yielding a high fraction of low-molecular product. Also, the carbonium ion intermediates in the catalytic reaction progress can undergo rearrangement by hydrogen or carbon atom shifts with producing the isomers of high quality and can undergo cyclization reactions, by means of the intramolecular attack on the double bond of an olefinic carbonium ion.
In the case of a bifunctional catalyst playing different active site roles, this catalyst consists of both acidic and metal material as reforming catalyst. The metallic sites catalyze hydrogenation/dehydrogenation reactions, while the acidic sites on the support catalyze isomerization reactions, as shown in Figure 5.2. This catalyst can promote the isomeriza-tion of straight-chain paraffins into branched-chain molecules, the dehydrocyclization of straight-chain paraffins into cycloparaffins and also the dehydrogenation of naphthenes into metal site
Figure 5.2 Reaction mechanism of hydrocarbon on bifunctional catalyst . (Reproduced with permission from Elsevier)
aromatics. These reactions improve the octane numbers of light hydrocarbons. However, this catalyst is very expensive. Thus, its use in oil recovery from waste plastics containing contaminated material must be taken into careful consideration.
Characteristics of thermal and catalytic degradation of heavy hydrocarbons can be described with the following items, respectively.
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