Reaction
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PVC is manufactured by polymerising vinyl chloride monomer (VCM). Polymerisation is a chemical reaction which combines small monomer molecules to form large chain-like molecules called polymers.
The liquid vinyl chloride monomer (VCM) is piped to the autoclaves where the polymer is formed in the presence of water, suspending agents, initiators and at a controlled temperature.
The role of each is essential in the reaction process:
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Initiators – on heating, these become a source of free-radicals which start the polymerisation reaction.
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Suspending agents – the amount and type (together with the reaction temperature) determine the size of the droplets of monomer dispersed in the water, and also the porosity of the PVC grains produced.
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Temperature – the reaction temperature fixes the average molecular weight as well as (together with the suspending agents) the size and structure of the polymer particles.
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Water – allows initial VCM dispersion to a mist of fine droplets, then agglomeration to the final PVC grains, as well as helps in removing the heat of reaction.
Approximately equal volumes of water and VCM are piped into the autoclave creating the suspension which begins the polymerisation.
The chemical reaction which creates PVC is exothermic i.e. it gives off heat. As the temperature within the reactor needs to be controlled, this heat must be removed. The cooling jacket maintains the reacting temperature at the control point within the range of 50 – 80° C. The acceptable temperature variation for the required molecular weight of the polymer is +/-0.25° C.
The VCM polymerises to form a slurry of PVC grains in water. After this conversion (usually 3 – 5 hours), the reaction is stopped by discharging the autoclave to the degasser and removing unconverted VCM. The reaction is not completed 100% because it starts to slow down after about 85% conversion.
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