The precursor to the polymer PVC is vinyl chloride monomer (VCM).
The monomer is produced in a five stage process:
- Chlorine (Cl) is extracted from sea salt via electrolysis.
- Ethylene (C2H4) is derived from hydrocarbon raw materials.
- Ethylene and chlorine react to produce ethylene dichloride (EDC).
- Ethylene dichloride is decomposed through a thermal ‘cracking’ process resulting in vinyl chloride monomer (VCM) and hydrogen chloride (HCl).
- Hydrogen chloride is reacted with ethylene and oxygen to produce more EDC and water.
There are four key steps in PVC manufacture:
- Packaging and Dispatch
Explore the PVC manufacturing process below.
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:
Initiators – on heating, these become a source of free-radicals which start the polymerisation reaction.
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.
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.
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.
In the Recovery Section, unconverted VCM is removed from the PVC slurry. VCM gas is vented from the degasser to the gasholder, and remaining VCM dissolved in the polymer is removed by live steam in the stripper. The recovered VCM gas is compressed, cooled and liquefied; water is separated, and the VCM liquid is returned to VCM storage for re-use. Almost all of the VCM that has not been used in the PVC batch is recovered and reused.
The next step in PVC manufacturing separates the PVC from the slurry to complete the process. Most of the water is removed when the slurry passes through the centrifuge. A damp 'cake' of polymer leaves the centrifuge and is conveyed into the fluid bed dryer. Here, the remaining water contained in the porous grains evaporates as a stream of heated air bubbles through the polymer powder.
In order to minimise emissions, entrained powder is collected in a cyclone and the moist air is wet-scrubbed before discharge into the atmosphere.
Any oversize particles are screened from the dried resin before conveying to silo.
The final product is a fine white powder known as PVC resin.
The resin is packed in a warehouse into either 25 kg bags for dispatch on one tonne pallets, into one tonne bulk bags or lined shipping containers for bulk deliveries.