Theoretical Studies On The Thermal Degradation Mechanism Of Polyvinyl Chloride

Polyvinyl chloride?PVC?is one of the most common plastics.It is widely used because of its degradation,versatility,light weight and low price,which results in a large amount of PVC waste plastics need to be properly disposed of.Among these,thermal degradation has become an important and effective way to transform waste plastics into energy resources.The mechanisms of thermal degradation of polyvinyl chloride model compounds were investigated using density functional theory at the B3LYP/6-311++G?d,p?level.We explored the possible thermal degradation reaction paths for forming the main thermal degradation products of HCl,aromatic compounds and small molecular hydrocarbons such as C₂H₄ and CH₄.The geometry optimization and frequency calculations were performed in the reaction process to obtain the standard kinetic parameters and thermodynamic parameters of the thermal degradation paths.The main research results are as follows:1.The thermal degradation of PVC can be divided into two stages:the first stage is the removal of HCl and to form a conjugated olefin;the second stage is the further degradation of the conjugated olefin to form an aromatic compounds and small molecular hydrocarbons.The mechanism of HCl removal during thermal degradation of polyvinyl chloride model compounds are theoretically calculated.The results show that the formation of HCl undergoes by the concerted reaction mechanism in the first stage of thermal degradation of the polyvinyl chloride dechlorination.It is inferred from kinetic analysis that the branched-chain?—CH₂CH₂Cl,—CHClCH₃ and—CH₃?has a significant effect on the activation energy.The thermal degradation product HCl produced during the thermal degradation reaction catalyzes the thermal degradation reaction.What's more remarkable is that allyl group?—CH=CH—CHCl—?generated after a HCl elimination can obviously reduce the activation energy of HCl elimination,however,butenyl group?—CH=CH—CH₂—CHCl—?has little influence on the activation energy of HCl elimination.2.The evolution mechanism of aromatic compounds during the second stage of polyvinyl chloride thermal degradation is calculated.The calculation results show that conjugated olefins are formed after completing removal of HCl,the formation of aromatic compounds is formed via the molecular rearrangement and cyclization of the conjugated alkenes,a conjugated olefins free radicals is more easily converted into aromatic compound through a series of isomerizations,cyclization and dehydrogenation,but it is difficult to form the radical.Paths 4,5,and 6 may be the main pathway for the formation of aromatic compounds,it is that the formation of aromatic compounds is formed via the molecular rearrangement and cyclization of the conjugated alkenes,and at the same time,some hydrogen radicals can be formed.In addition,compared with the molecular rearrangement and cyclization of the conjugated alkenes,the small molecular hydrocarbons formed by linear alkenes C-C bonds need to overcome a higher energy barrier.Therefore,the small hydrocarbon molecules may be formed by the fracture of conjugated olefin.3.The theoretical calculation of the formation of small hydrocarbon molecules,it is easy to find that the C-C bond of the conjugated olefin is directly broken or the bond energy required to cleave the C-C bond after cyclization of the conjugated olefin is relatively high,so it is difficult to form small molecule products C_mH_n.However,the presence of hydrogen can obviously reduce the energy barriers in formation paths of small molecule products.In the evolution processes of aromatics,more hydrogen radicals can be formed,these contribute to the formation of small molecule products,which are further involved in the degradation of conjugated olefins small molecule hydrocarbons.Therefore,in the second stage of PVC pyrolysis,the conjugated polyene formed in the first stage could be decomposed in parallel reaction channels:one is the evolution of aromatics,another is the formation of small molecule products.The calculated results show that the thermal degradation products of polyvinyl chloride are mainly include:HCl,aromatic compounds and small molecule hydrocarbons such as CH₄,C₂H₄,which are consistented with the relevant experimental results.