Vinyl chloride is an organochloride with the formula H2C=CHCl that is also called vinyl chloride monomer, VCM or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC).
Nearly all production of VCM is based on ethylene, which is first reacted with chlorine to make ethylene dichloride (EDC). There are two routes commonly used to make EDC: direct chlorination using pure chlorine and ethylene, and oxychlorination in which the ethylene reacts with chlorine in hydrogen chloride. The EDC is then converted to VCM by thermal cracking and the hydrogen chloride by-product can be recycled to an oxychlorination plant to make more EDC. Many EDC/VCM complexes use an integrated chlorination-oxychlorination process which proceeds in three stages: the chlorination of ethylene in the liquid or vapor phase to make EDC; the thermal cracking of EDC to form VCM and hydrogen chloride; and the oxychlorination of ethylene with recycled hydrogen chloride to make more EDC.
VCM is sent through a catalyst-containing reactor where polymerization occurs. Chemicals cause the VCM molecules to react until they link together into Polyvinyl Chloride (PVC). The linking of VCM molecules creates PVC resin – which is where all vinyl compounds begin.
The suspension polymerization process is most widely used process to manufacture PVC. First, the raw material VCM is pressurized and liquefied, and then fed into the polymerization reactor, which contains water and suspending agents in advance. Through high-speed agitation within the reactor, small droplets of VCM are obtained.
Next, the initiator for polymerization is fed into the reactor, and PVC is produced by reaction under a few bar at 40 - 60° C. PVC obtained through suspension polymerization is suspended in water as particles of 50 - 200μm diameter (in slurry form).
Thereafter, the slurry discharged from the polymerization reactor is stripped of residual monomer, dehydrated, dried and the particle size controlled by screening to yield PVC in the form of a white powder. The un-reacted VCM is entirely recovered through the stripping process, and after purification, recycled as raw material for reuse in this process. PVC resin produced via this ‘suspension’ process is referred to within the industry using the abbreviation S-PVC.
Emulsion polymerization and bulk polymerization are alternative, much less extensively employed, technologies to manufacture PVC. Emulsion polymerization produces finer resin grades having much smaller particles, which are required by certain applications. This type of resin is sometimes called ‘paste’ PVC and referred to within the industry using the abbreviation P-PVC to distinguish it from S-PVC.
PVC comes in two basic forms: rigid (sometimes abbreviated as RPVC) and flexible. The rigid form of PVC is used in construction for pipe and in profile applications such as doors and windows. It is also used for bottles, other non-food packaging, and cards (such as bank or membership cards). It can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is also used in plumbing, electrical cable insulation, imitation leather, signage, inflatable products, and many applications where it replaces rubber.
PVC Production Process in China
The calcium carbide route used in China involves heating lime and coal-derived coke in an electric furnace at a temperature of 2,000 °C to obtain calcium carbide. Acetylene is generated by the hydrolysis of this calcium carbide. This early part of the process is labor intensive, requires a lot of energy, and generates vast quantities of a watery calcium hydroxide slag. With the use of a catalyst that is usually based on mercuric chloride, the acetylene is then reacted with anhydrous hydrogen chloride to produce vinyl chloride.
Major Technology Providers & Producers
- Ineos Technologies
- Hanwa Chemical
- Ineos Technologies