Products And Mechanisms During Poly (Vinyl Chloride)Pyrolysis Based On The Minimization Of Secondary Reaction

Since PVC was discovered in19th century, for the widely usage and strongly demands, its industrial production was increased continually. Meanwhile, the relative waste treatment such as incineration, liquefaction and melting regeneration also need to pay much attention. Furthermore, to improve and ameliorate these technologies need the most substantial mechanism about the PVC pyrolysis. Dehydrochlorination (DHC) and hydrocarbon release were commonly recognized as two main stages during the PVC pyrolysis. Furthremore, the first stage was divided into3steps (chain initiation,[polyene growing and termination). However, lots of arguments still exist regarding the degradation mechanism of PVC pyrolysis. The detailed characterization of polyvinylchloride (PVC) thermal degradation was investigated in a wire-mesh reactor where the secondary reactions were minimized. The nascent products (char, gas and tar) were collected as various temperatures, holding time and heating rate to investigate the relevant mechanisms.For the first stage (DHC stage), both char and non-condesing gases were examined which collected by a wire-mesh reactor. Firstly, char samples were collected at a slow heating rate of1K/s and analyzed using an X-ray photoelectron spectroscopy (XPS) to investigate the functional group transformation on the surface. Meanwhile, an element analyzer was also employed to determinate the bulk composition of the identical char samples. The results with holding time from0-300s at200℃provide plenary evidences that first stage (DHC) of PVC initiated by the isomerism on tertiary carbon. Very because of this isomerism, a small amount of chlorine transformed into the internal structure of char, which probably resulted in the organic chlorine release in PVC tar. Besides, non-condensing gases were collected at a heating rate from1-1000K/s with temperatures from300℃to800℃and a holding time from0to300s. EPA-method26A was employed to determinate both Cl2and HC1. The results show that HC1and Cl2releases were increased continually with the increasing temperature and holding time. Lower heating rate could increase the total gas yield but do not change the chlorine distribution.Cl2seem as an accompany product during HC1release, which indicated that a bunch of free Cl radicals were involved in PVC thermal degradation. The free radical mechanism in polyene growth was more suitable for our results. Lastly, DHC stage of PVC was terminated with the formation of aliphatic polyene rather than aromatic polyene. Cyclization reactions were most unlikely to occur before the termination of polyene growth.For the second stage (tar release stage), all the nascent tar samples were collected using a quench system in wire-mesh reactor where the secondary reactions of the evolved volatiles were minimized. The small compounds such as benzenes and alkanes were not detected in nascent tar in wire-mesh reactor, which components are quite different from those of other tars in tube type reactor and vacuum reactor. At a heating rate of1000K/s, the quasi-3rings and3rings group aromatics were the major components in nascent tar, while the content of2rings groups aromatics increased from7.02%to31.75%with increasing peak temperature from500to800℃. At a longer holding time of300s, an increase of2rings group aromatics from7.02%to50.33%was also observed for the nascent tar at500℃, indicating the tar composition significantly changes at different stages of PVC pyrolysis. It seems that3-4rings compounds form in the early stage and then2rings compounds release in the later stage of PVC pyrolysis.Based on our experimental above, the PVC degradation mechanism could proposed as, i.e.(1) isomerism on tertiary carbon;(2) free radical polyene growth;(3) cyclization;(4) aromatic chain scission and quasi-3rings or3rings group releases;(5)2rings group release.