Study Of The Process Of Preparing Chlorinated Polyvinyl Chloride By Aqueous Suspension Method

Chlorinated polyvinyl chloride (CPVC) is an important PVC modified product. CPVC is mainly prepared by aqueous suspension method at aboard. But this process doesn’t not mastere well in China, performance in chain scission of PVC always happen during the chlorination process. Consequently, product CPVC get a high surface chlorine content, low molecular weight, bad heat stability, and is not suitable for preparing hard pipe. This paper puts forward a new process by adding a swelling agent and is aimed to solve the domestic aqueous suspension process defect. The process was to prepare the CPVC with large molecular weight, high chloride content and good heat stability.Firstly, CPVC was prepared by aqueous suspension process in this paper using SG5type PVC as materials. CPVC was characterized by Gel chromatography, infrared chromatography, mercury intrusion method, thermogravimetric method and differential scanning calorimetry. The CPVC’s number-average molecular weight and weight-average molecular weight were82051and171248respectively, and both of them were larger than that of PVC. The PDI value of CPVC was just a bit larger than PVC’s, which turned out that the PVC had no chain scission during the process. The infrared chromatography results showed that CPVC’s molecular chain had lots of good heat stability structure units, but no unstable units like tert-butyl chloride, C-C double bond. TGA results also showed that the CPVC’s initial temperature in HCl gas off stage was10℃higher than PVC’s, which meant that the CPVC’s heat stability was better than PVC’s. DSC results indicated that the CPVC’s glass transition temperature was139.28℃with57.78℃higher than PVC’s. And the DSC results further proved that CPVC had good heat resistance.Secondly, the effects of process conditions on CPVC’s molecular weight, chlorine content and heat stability were studied. At60℃, The dissolution rate and the diffusion rate of chlorine gas balanced and CPVC’s molecular weight, chlorine content and heat stability were the best. The membrana dermalis of PVC dissolved completely and particle chlorinated specific surface area was the biggest when the mass ratio between1,2-dichloroethane and PVC were0.5. When chlorine gas flow was0.4L/min, chlorine concentration dissolved in hydrochloric acid was the largest. When using UV and azodiisobutyronitrile as initiators, free radicals concentrations in hydrochloric acid and reaction rate balanced, the reaction rate was highest at this condition, but it had no influence on PDI value of CPVC. We found that when using the organic weak base sodium citrate to wash CPVC materials, CPVC internal residual acid was removed as much as possible.1,2-dichloroethane recovery effect was the best by using solvent extraction process, and recovery rate reached89.5%. In the optimal process conditions, CPVC’s Mn, Mw, PDI, chlorine content and thermal stability time were82932,173825,2.096,70.89%and303s respectively.Lastly, the chlorination of the mass transfer process and the reaction mechanism were studied in the optimal conditions. And then the mass transfer and reaction kinetics model were established. The results showed that the mass transfer process of chlorine in hydrochloric acid fitted Fick’s diffusion law. In the early chlorination reaction period, the reaction kinetics of PVC and chlorine gas fitted quasi level dynamics process, the kinetic equation wasw=W-(W-56.7)×exp(-Kt). The activation energy Ea was27.341kJ/mol. In the late chlorination reaction period, the stability structure units in molecular chain decreased but the the number of unstable units increased. The CPVC thermal stability reduced with the increase of the time.