24. Polymerization of vinyl chloride General aspects

Polymerization processes of polyvinyl chloride (PVC) can be either batch or continuous processes, but in the industrial scale predominantly batch systems are used.

Repetitive unit in the structure of PVC is given by:

H H where i is the degree of polymerization or the number of basic units in the [ C C ] i molecular chain H Cl General aspects

Basic units have mostly attached to each other according to the head-to-tail mechanism. Then a chlorine atom occurs regularly in alternating carbon atoms. Only about 0,5-1 % of additions have occured according to the head-to-head mechanism. Sterically chlorine atom can be placed either iso-, syndio- or atactically.

The average polymerization degree of commercial PVC polymers is 500-1500 in which case the weight average molecular weight is around 50000-500000 g/mol and the number average molecular weight is about 30000-90000 g/mol. Indicated numbers correspond to K values 50 - 80. Though PVC is partly crystalline 2-10 %, it can be essentially considered as an amophous polymer. General aspects

Radical polymerization can induce an intramolecular radical transfer reaction in which case the formation of a side chain occurs. Side chains have attached to about 0,05-0,2 % of carbon atoms in a polymer chain.

Several different end groups exist in PVC polymer. The end groups consist of either substances present during polymerization, such as initiators, or the effects of thermal reactions. General aspects PVC exists in powder form after polymerization. Suspension PVC exists as resin granules that have a diameter of about 130 m. The granules are protected by a about 10 nm colloidal surface cover. Morphological structure of PVC particles: General aspects

Below the cover of a polymer particle lie agglomerates with a diameter of about 10 m. The agglomerates are composed of porous about 1 m primary particles or globules which are presumed to consist of 100 nm domains that can be divided into smaller microdomains having a diameter of about 10 nm.

Emulsion and mass PVC have no corresponding cover like suspension PVC, because of which they have significantly finer granular size than suspension PVC. Otherwise the internal structure of particles is similar with all PVC grades. Production processes (suspension polymerization)

Suspension polymerization is performed in a jacket-equipped stirred autoclave. Polymerization reaction is performed at a constant temperature which also defines the molecular weight or K value of the produced product.

Process engineering problems with suspension polymerization: . Mixing technique to control water-vinyl chloride monomer (VCM) dispersion . Autoclave fouling . Settling of slurry . Foaming during stripping Production processes (suspension polymerization)

Block diagram for suspension polymerization: Production processes (suspension polymerization)

Suspension polymerization reactor system: Production processes (mass polymerization)

Polymerization is conducted in monomer that functions both as dispersion and heat transfer medium. Polymerization reaction occurs without water or any protective colloids or emulsifiers. This way certain product properties can be improved, such as porosity, water absorption and transparency. Production processes (emulsion polymerization)

Block diagram for the production process of emulsion PVC: Production processes (emulsion polymerization) Emulsion polymerization reactor system: Production processes (emulsion polymerization)

Typical problems: . Weak mechanical stability of latex . Foaming of emulsifiers . Pronounced fouling of autoclave . Repeatability of batches . Possible impurities in feedstock

Microsuspension polymerization:

Similar to emulsion polymerization. The most significant difference between the production processes is the monomer- soluble initiator used with microsuspension polymerization. The initiation occurs inside a monomer droplet.