A number of PVC processes are employed according to the end application, but polymerisation is normally performed at 40-70oC with the VCM in a liquid state under pressure in a batch reactor. All routes employ free radical initiators which are either soluble in the monomer or in the aqueous solution.
Suspension polymerisation is the most common PVC process - around 89% of vinyl resin produced in North America is obtained using this route - because the resins produced are the most versatile and suitable for a wide range of applications. The vinyl chloride is introduced under pressure to the sealed reactor where it is finely dispersed in water by vigorous agitation. An initiator that is soluble in the monomer is also added. The mixture is then heated to 60-70oC. When around 90% of the monomer is converted to the polymer, the reaction is halted by discharging the slurry into a degasser. The remaining monomer is recycled while the resin is filtered, centrifuged and dried.
Emulsion polymerisation is conducted in an aqueous solution containing water-soluble initiators and emulsifiers. The PVC latex formed has a fine particle size and is more suitable for use in paints, paper and fabric finishes and printing inks.
Mass polymerisation, in which no water is added during polymerisation, usually employs a pre-polymerisation stage containing liquid VCM in the presence of sufficient initiator to allow for 10% conversion. The solution is sent to an autoclave reactor where more initiator and VCM are added and the mixture is heated. The powder resin can be used to make a film with high clarity as well as in other applications.
The vinyl resin produced is inherently hard and brittle, and needs to be mixed with other additives before it can be processed into useful products. For example, the addition of plasticisers makes PVC compounds soft and flexible while adding impact modifiers creates compounds that are tough and impact-resistant.
The initial step in producing vinyl compounds involves dry blending, which takes place in a closed vessel where dry and liquid additives are mixed using blades or paddles. Because the resin particles are porous, the liquid additives are absorbed relatively easily, yielding a dry powder compound. In a certain applications such as pipe manufacture, these powder compounds can be processed directly into the final product. In other applications, such as wire and cable, rigid profiles and injection moulded parts, additional processing may be necessary.
A new process for the recycling of PVC has been developed by Solvay. A 10,000 tonnes/year demonstration plant at Ferrara, Italy, was brought on-stream in early 2002. The Vinyloop process involves using a solvent to dissolve the polymer, separating it completely from other materials in a mixture or composite structure. The PVC is then recovered by precipitation and dried to give PVC resin granules.
Polyvinyl chloride
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Polyvinyl chloride
Uses and Outlook
Two types of polyvinyl chloride (PVC) homopolymer are produced: rigid resins which have considerable strength and hardness; and flexible resins that contain a large proportion of plasticisers to make them soft and stretchable.
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