Chemical Knowledge 04:Thermal Degradation Mechanism of PVC
Update time: 26-01-19 Views: 29
Thermal Degradation Mechanism of PVC
PVC decomposes significantly at 100–150°C. Ultraviolet light, mechanical forces, oxygen, ozone, hydrogen chloride, and certain reactive metal salts and metal oxides can greatly accelerate its decomposition. The thermal-oxidative aging of PVC is quite complex, with literature reports dividing its thermal degradation process into two steps:
(1) Dehydrochlorination: The active chlorine atoms on the PVC polymer chain are eliminated, producing hydrogen chloride (HCl) while generating conjugated polyenes.
(2) Formation of longer-chain polyenes and aromatic rings: As degradation proceeds further, the allylic chlorine atoms become extremely unstable and are readily eliminated, forming longer-chain conjugated polyenes—this is the so-called "zipper-like" dehydrochlorination. Simultaneously, a small amount of C–C bond cleavage and cyclization occurs, generating a small quantity of aromatic compounds. Among these, dehydrochlorination is the primary cause of PVC aging. The degradation mechanism of PVC is relatively complex without a unified conclusion; the main mechanisms proposed by researchers include the free radical mechanism, ionic mechanism, and unimolecular mechanism.
