(661e) UV-Vis Online Analysis and Modeling of Chlorination Process of PVC Particles Using a Gas-Solid Method With UV Intensification

Chlorinated poly (vinyl) chloride (CPVC) is a high performance thermoplastic, produced by further chlorination of PVC resin. As the chlorine content rises from 56.8 wt% to 63-71 wt%, CPVC products have better physicochemical properties of heat stability, flame retardant and so on in comparison with PVC. The aqueous suspension method has been successfully applied to manufacture CPVC using a batch operation. However, inevitable discharge of waste water and gases is encountered. From the viewpoint of environmental concern, gas-solid method is acknowledged as a much cleaner one, because dry chlorination process does not need water and is easy to handle CPVC products and effluent gases.

Dry chlorination of PVC particles is known as a gas-solid, non-catalytic reaction, which can be intensified by heat, UV light[1] and cold plasma[2]. A chlorination platform was established in our lab, including a vibrated-bed photoreactor and a UV-Vis online analysis system to quantitatively record instantaneous reaction rate from the very beginning to the end of chlorination process. Influences of operating parameters such as temperature, chlorine concentration and UV intensity can be illustrated vividly from the measured dynamic characteristics. Firstly, temperature and chlorine concentration are the two key factors determining the reaction rate and the chlorination degree. Secondly, UV radiation has significant intensification effect on the process, where both 253.7 and 350 nm UV lights work well to enhance the chlorination reaction. It can be deduced from the curves of chlorination rate that the chlorination process obeys a reaction-limited regime, but the intrinsic kinetics derived by radical quasi-steady state hypothesis of Cl· and -CH· in the former work[3] cannot be well-fitted. Theoretical analysis and characterization results by Pyrolysis GC-MS further demonstrated in different ways that the activation energy (E) in the rate constant (k) may not be constant as a result of steric hindrance in dense solid-state polymer phase. We therefore proposed a Steric-hindrance Active Energy (Eσ) related to chlorination degree to correct Ein order to characterize its change during the chlorination. A theoretical model considering the reaction, diffusion and solution of chlorination in the polymer phase was accordingly established, showing good accordance between the model predictions and experimental results.

References:

[1] Weben JP, Cloud S, Assenat M, Fons S, Vrillon C, Superchlorinated polyvinyl chlorides and methods of preparation. 1966. US3532612.

[2] Lu W, Yang Q, Yan B, Cheng Y, Plasma-assisted synthesis of chlorinated polyvinyl chloride (CPVC) characterized by online UV–Vis analysis. Chem. Eng. J. 2012, 207-208, 923-930.

[3] Barriere B, Glotin M, Leibler L, An analysis of the reaction-diffusion mechanism governing the chlorination process of poly(vinyl chloride). J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 3201-3209.