As the name PVC (Poly-Vinyl Chloride) indicates, Vinyl chloride is the monomer (VCM) – starting unit, which is polymerized to obtain PVC.
Manufacturing of VCM is generally a 2 stage process. Viz.
a) Direct chlorination of Ethylene to obtain Ethylene Dichloride (EDC)
CH 2 =CH 2 (ethylene) + Cl 2 (chlorine) → CH 2 ClCH 2 Cl (EDC)
b) EDC cracking (thermal decomposition at high pressure and high temperature) to obtain VCM
The HCl thus generated is used in Oxychlorination process to convert ethylene to EDC
CH 2 =CH 2 (ethylene) + 2HCl (hydrochloric acid) + ½O 2 (Oxygen) → CH 2 ClCH 2 Cl (EDC) + H 2 O (water)
The process also includes EDC and VCM purification stages and a by-product separation and disposal
stage. A by-product of the EDC cracking phase is acetylene due to the EDCL cracking all the way.
Consequently, the HCl recycle stream is usually passed through a hydrogenation reactor to convert it
back to ethylene. This in turn produces more EDC.
A new patented technology, based on ethane-to-VCM, has also developed and is based on ethane (which is much cheaper than ethylene) reacting with HCl or chlorinated hydrocarbons and oxygen using a catalyst system.
VCM is also manufactured through the Calcium Carbide route that is not affected by the cost-cycles routinely associated with the petrochemicals based production. In China, the dominant process to make VCM is based on acetylene produced from calcium carbide. Coke made from coal and calcium oxide from limestone are heated to over 2000 o C to make calcium carbide and carbon monoxide. Water is then mixed with the calcium carbide to make acetylene which reacts with anhydrous hydrogen chloride to form VCM.
The attraction of this technology in China is that it does not need ethylene and all the raw materials are domestically available. Other advantages include lower capital costs to build an integrated vinyl complex and a simpler technology. However, the process is highly polluting to the environment and has a high energy requirement.
3C + CaO → CaC 2 + COThe process is free radical polymerization and involves initiators like organic peroxides. The
polymerization process is highly exothermic reaction carried out in thick walled, high-pressure rated
steel reactors jacketed for removal of the heat of reaction.
Different Polymerization Techniques Used –
- Suspension polymerization,
- Emulsion polymerization,
- Bulk or mass polymerization,
- Microsuspension polymerization.
In this process, VCM is polymerized in large-scale batch reactors (autoclaves) in the presence of water, initiators that are soluble in VCM, and a protective colloid/suspending agent (e.g., polyvinyl alcohols) to stabilize the suspension. Other additives are chain-transfer/chain-extending and chain-terminating agents, pH regulators, and anti-foaming agents.
The dosage and nature of the protective colloid and the stirring conditions decide the extend of monomer droplet agglomeration in the course of the polymerization. The reaction is normally allowed to run until about 90% of the VCM has been polymerized. Once the desired level of conversion has been achieved, the reaction is stopped using the short-stops i.e. chain terminating agents.
The polymerization reaction takes place within each of the suspended droplets of liquid monomer which are progressively converted to solid or porous PVC particles which are insoluble in the monomer. Unreacted VCM is extracted from the polymer and recycled. The PVC slurry thus obtained is centrifuged to remove water. The polymer thus obtained is then dried. Before taking up the next batch of polymerization, the reactor is cleaned and coated with an anti-fouling agent to prevent the generation of gels/high molecular weight particles.
The average particle size of suspension PVC is between 100–180 μm with a range of 50–250 μm. The particles are complex and irregular in shape with a dense semi-permeable skin.
More than 80% of total PVC produced worldwide is made by suspension polymerization technique. The resin can be used in both rigid and flexible applications and grades are formulated to meet an extensive range of requirements, such as high plasticizer absorption for flexible products, or high bulk density and good powder flow required for rigid extrusion. Typically, the grades made by suspension polymerization technique fall in 50 – 100 K value range.
This process involves polymerization of VCM in an autoclave in presence of water, a water soluble initiator and an emulsifier (anionic surfactants). At the end of the polymerization stage, the autoclave contains a stable dispersion of fine PVC particles in water. PVC is separated from the water by evaporation in spray drying equipment. The final size classification is achieved by milling the agglomerates.
The primary particles are solid, smooth-surfaced spheres which are clustered into irregularly shaped aggregates after drying, with a typical particle size of 5.50 μm. When broken down in the paste mixing process, the particle size is in the region of 0.5 μm with a range of 0.1–3.0 μm.
When emulsion PVC resin (also known as dispersion resins) is mixed with a liquid plasticizer, a paste (or plastisol) is produced due to the surfactant layer around the grain surface. Hence these resins are called paste resins which are used in a wide range of specialty applications such as coating, dipping, or spreading. Resins are available for differing viscosity and flow requirements, speed of gelation, clarity, and gloss level. K values normally range between 60 and 85. Emulsion PVC accounts for < 10% of total PVC produced.
This process involves polymerization of VCM in the absence of a carrier, such as water. The polymerization is carried out by a two-stage process. In the first stage of prepolymerization, monomer and initiator are charged and polymerization proceeds to about 10% conversion at which time the formed particles are dispersed in the bulk of the VCM. Vigorous agitation is necessary to obtain the desired particle size distribution. In the second stage of polymerization, this material is slowly agitated and additional monomer and initiator are added. The reaction continues to approximately 20% conversion at which time all of the liquid VCM is absorbed into the porous structure of the grains leaving only dry powder. Polymerization continues to 70–90% conversion and the unreacted monomer is recovered.
Particle size and range are similar to suspension polymer but the particles have highly spherical shapes, with higher bulk density and porosity. The heat stability and fusion properties are superior. These grades are highly suited for critical applications like transparent products, electrical and medical sectors.
The cost benefits of this process arising out of absence of a drying stage, are offset to some extent by difficulties in removing VCM traces from the polymer and the level of oversized material that is generated. Also absence of a medium in the polymerization reaction sometimes makes it difficult to control.
The number of suppliers offering PVC produced by this process is restricted to licensees of the original technology, and accounts for < 10% of world PVC production.
This process involves polymerization of VCM in water in presence of an initiator soluble in VCM and an emulsifier. These components are circulated through a mixing pump (homogenizer) which causes the mixture to disperse into very fine droplets, before being placed in an autoclave. The droplets are coated with surfactant which stabilizes them during the reaction. Initiation and polymerization occur within the droplets. After polymerization, the autoclave contains a stable dispersion of fine particles of PVC in water. As the name of the process implies, the PVC particle diameter is smaller, within the range 0.2–3 μm. Porosity of these PVC particles is very low.
The subsequent operations for obtaining the final product are similar to those of the emulsion polymerization process.
Microsuspension-polymerized PVC is used for producing plastisols and paste in combination with emulsion-polymerized PVC.
