Mechanism of Plastics Pyrolysis

A comprehensive treatment of the mechanism of plastics pyrolysis has been presented by Cullis and Hirschler [56]. Four types of mechanisms of plastics pyrolysis have been proposed  

These different mechanisms and prodnct distribntions, to some extent, are related to the bond dissociation energies, the chain defects of the polymers, and the degree of aromaticity, as well as the presence of halogen and other heteroatoms in the polymer chains. Large amount of styrene monomers can be obtained by pyrolysis of PS, while a wide range of hydrocarbons are prodnced by random degradation of PE and PP [3, 59, 60]. 

Thermal degradations are carried ont by a free radical mechanism, while catalytic degradation are realized by carboninm ions, which consist of hydrocarbon ions carrying a single positive charge. 

A mechanisms of catalytic pyrolysis of waste plastics was proposed by Bnekens [61], using PE as an example, in which ECC catalyst is adopted and the main content include  

Initiation may occur on some defect sites of the polymer chains. Eor instance, an olefinic linkage could be converted into an on-chain carbonium ion by proton addition. Then the polymer chain may be broken np throngh (3-scission. Initiation may also take place through random hydride ion abstraction by low-molecular-weight carbonium ions (R+), The newly formed on-chain carbonium ion then undergoes P-scission. 

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