What is PVC?

Polyvinyl chloride, (IUPAC Poly(chloroethanediyl)) commonly abbreviated PVC, is a thermoplastic polymer.

It is a vinyl polymer constructed of repeating vinyl groups (ethenyls) having one of their hydrogens replaced with a chloride group.

Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene.^[1] PVC is widely used in construction because it is cheap, durable, and easy to assemble. PVC production is expected to exceed 40 million tons by 2016.^[2]

It can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is used in clothing and upholstery, and to make flexible hoses and tubing, flooring, to roofing membranes, and electrical cable insulation. It is also commonly used in figurines and in inflatable products such as waterbeds, pool toys, and inflatable structures.

Preparation

Polyvinyl chloride is produced by polymerization of the vinyl chloride monomer (VCM), as shown. Since about 57% of its mass is chlorine, creating a given mass of PVC requires less petroleum than many other polymers. However, because PVC also has a much higher density than hydrocarbon polymers, and chlorine production has its own energy requirements, this ends up being of little practical relevance in the production of most solid objects.

By far the most widely used production process is suspension polymerization. In this process, VCM and water are introduced into the polymerization reactor and a polymerization initiator, along with other chemical additives, are added to initiate the polymerization reaction. The contents of the reaction vessel are continually mixed to maintain the suspension and ensure a uniform particle size of the PVC resin. The reaction is exothermic, and thus requires a cooling mechanism to maintain the reactor contents at the appropriate temperature. As the volumes also contract during the reaction (PVC is denser than VCM), water is continually added to the mixture to maintain the suspension.

Once the reaction has run its course, the resulting PVC slurry is degassed and stripped to remove excess VCM (which is recycled into the next batch) then passed though a centrifuge to remove most of the excess water. The slurry is then dried further in a hot air bed and the resulting powder sieved before storage or pelletization. In normal operations, the resulting PVC has a VCM content of less than 1 part per million.

Other production processes, such as micro-suspension polymerization and emulsion polymerization, produce PVC with smaller particle sizes (10 μm vs. 120-150 μm for suspension PVC) with slightly different properties and with somewhat different sets of applications.

The product of the polymerization process is unmodified PVC. Before PVC can be made into finished products, it almost always requires conversion into a compound by the incorporation of additives such as heat stabilizers, UV stabilizers, lubricants, plasticizers, processing aids, impact modifiers, thermal modifiers, fillers, flame retardants, biocides, blowing agents and smoke suppressors, and, optionally pigments.^[3]

Applications

PVC's intrinsic properties make it suitable for a wide variety of applications. It is biologically and chemically resistant, making it the plastic of choice for most household sewerage pipes and other pipe applications where corrosion would limit the use of metal.

With the addition of impact modifiers and stabilizers, it becomes a popular material for window and door frames. By adding plasticizers, it can become flexible enough to be used in cabling applications as a wire insulator.

Pipes

PVC pipes in use with intumescent firestops at Nortown Casitas, North York, Ontario.
PVC pipes inside of a well, the top one is 20 mm drinking water quality and the bottom is for irrigation, with connectors from 48 mm to 32 mm pipeRoughly half of the world's polyvinyl chloride resin manufactured annually is used for producing pipes for various municipal and industrial applications.^[4] In the water distribution market it accounts for 66% of the market in the US, and in sanitary sewer pipe applications, it accounts for 75%. Its light weight, high strength, and low reactivity make it particularly well-suited to this purpose. In addition, PVC pipes can be fused together using various solvent cements, or heat-fused (butt-fusion process, similar to joining HDPE pipe), creating permanent joints that are virtually impervious to leakage.

In February, 2007 the California Building Standards Code was updated to approve the use of chlorinated polyvinyl chloride (CPVC) pipe for use in residential water supply piping systems. CPVC has been a nationally accepted material in the US since 1982; California, however, has permitted only limited use since 2001. The Department of Housing and Community Development prepared and certified an Environmental Impact Report resulting in a recommendation that the Commission adopt and approve the use of CPVC. The Commission's vote was unanimous and CPVC has been placed in the 2007 California Plumbing Code.

In the United States and Canada, PVC pipes account for the largest majority of pipe materials used in buried municipal applications for drinking water distribution and wastewater mains.^[5] 

References

1. ACC Resin Statistics Annual Summary, http://www.americanchemistry.com/s_acc/sec_policyissues.asp?CID=996&DID=6872 , retrieved 2009-11-18.
2. Ebner, Martin (2008-11-18), "Ceresana Research Releases New Comprehensive PVC Market Study", Newswire Today, http://www.newswiretoday.com/news/42864/ , retrieved 2009-11-18.
3. Polyvinyl Chloride (PVC) 07/08-7 Report, ChemSystems, November 2008.
4. Shah Rahman (June 19-20 2007). "PVC Pipe & Fittings: Underground Solutions for Water and Sewer Systems in North America" (PDF). 2nd Brazilian PVC Congress, Sao Paulo, Brazil. http://www.institutodopvc.org/congresso2/ShahRahman.pdf .
5. Shah Rahman (October 2004). "Thermoplastics at Work: A Comprehensive Review of Municipal PVC Piping Products" (PDF). Underground Construction: 56–61. http://www.oildompublishing.com/uceditorialarchive/october04/oct04utech.pdf .