(PVC and other Chloropolymers)
Chloropolymers are produced from alkenes in which one or more hydrogen atoms have been replaced by chlorine. The most important member of this polymer class is poly(vinyl chloride), commonly called PVC or vinyl. It is the third largest-selling commodity thermoplastic after polyethylene and polypropylene with more than 40 million tons a year produced worldwide.
Commercial PVC is a clear, moderately tough, mostly amorphous polymer with good flame resistance. Chemically pure PVC, on the other hand, is a very brittle plastic with low impact strength. PVC is compatible with many plasticizers, which greatly improve the flexibility and tougheness of PVC. In the past, phthalates were the plasticizers of choice. Because of health concerns, the low-molecular weight phthalates have been gradually replaced with high-weight phthalates in Europe.
A major drawback of PVC is its poor resistance to heat and light. It easily decomoposes under moderate heat (dehydrochlorination), leading to intense color formation and detoriation of the polymer properties over time. For this reason, stabilizers are added to the PVC to prevent or delay this process.
More than 90 percent of PVC is manufactured as the homopolymer. The rest is copolymerized with other monomers, mainly to improve the flexibility and processability, necessary, for example, for flexible film products. Common copolymers are vinyl acetate, vinylidene chloride, diethyl fumarate and diethyl maleate. The addition of vinyl acetate improves the processabilty, enhances the stability and leads to better color retention and clarity but lowers the softening point. Copolymerization with 10 to 20 percent diethyl fumarate and diethyl maleate leads to better workability and improved tougheness while retaining the high softening temperature.
Generally, there are six major types of PVC available:
PVC-U (rigid or unplasticized)
is a hard and rigid thermoplastic with an ultimate tensile stress between 38 and 55 MPa at 20°C. It is resistant to most chemicals and has a continuous service temperatures of 55 to 60°C, although the actual maximal service temperature will depend on stress and environmental conditions. It is also easy to weld using thermoplastic welding equipment and easy to bond using solvents or adhesives.
is rigid thermoplastic with improved impact toughness. The elastic modulus, yield strength and ultimate tensile strength are generally lower than those of PVC-U. The properties generally depend on the type and amount of modifier added.
PVC-P (flexible or plasticized)
is much more flexible and has high impact strength and is easier to extrude, calender and mold. On the down side, it has lower temperature and chemical resistance depending on the plasticizer added. It also has lower ultimate tensile strength due to softening effect of the plasticizers, i.e. it has a much greater variability in performance.
is produced by chlorination of PVC resin. It has similar properties to PVC-U but has a higher service temperature than PVC-U of up to ca. 95°C but a similar ultimate yield stress at room temperature.
PVC-O (Oriented PVC)
also called high strength PVC is manufactured by a special extrusion process, which results in a preferential orientation of the polymer chains in stretch direction. It has similar chemical resistance as PVC-U but markedly enhanced mechanical properties in the preferred direction. For example, the ultimate tensile strength is up to twofold of PVC-U, which can result in significant material savings.
are dispersions of PVC resins in plasticizer.1 Being liquid, these pastes can be pressure less molded at room temperature. When heated at about 150 - 210 °C, the thermoplastic PVC swells and absorbs the plasticizer which fuses or gels the plastisol into a homogenous thermoplastic. Often metal stabilizers, fillers, rheology aids (pyrogenic silicas, bentones etc.) and epoxidised soy bean oil (ESBO) are added to enhance the stability and to modify the properties. The latter also acts as a plasticizer. In addition, blowing agents are sometimes added to foam the plastisol during the gelation process.
Besides PVC, polyvinylidene chloride (PVDC) and chlorinated polyvinyl chloride (CPVC) have found commercial uses. CPVC is considerably more ductile than PVC, and PVDC has much improved barrier properties against water, oxygen and gases (aromas), which makes it ideal for food packaging. Both thermoplastics have improved corrosion resistance compared to PVC due to the higher chlorine content. For example, CPVC can withstand corrosive water at temperatures greater than PVC, which makes this polymer more suited for water piping systems in residential and commercial construction than PVC.
For many applications other than building and construction, PVC is often plasticized to improve the flexibility (PVC is rather brittle). In the past, phthalates have been mainly used as softeners. Very common phthalate include di(2-ethylhexyl) phthalate (DEHP), dimethyl phthalate (DMP), di-n-butyl phthalte (DBP), diethyl phthalate (DEP), diisobutyl phthalte (DIBP), diisononyl phthalate (DINP), and di-n-octylphthalte (DNOP). Because of health concerns, low-molecular weight phthalates have been gradually replaced with high-molecular weight phthalates in the European Union, United States, Japan, and several other countries. Non-phthalate plasticizers are currently being investigated for compliance with domestic and international regulatory inventories as well. Noteworthy phthalate-free plasticizers are trimellitic, benzoic, and adipic acid esters, as well as adipic acid polyesters. Non-phthalte plasticizers of low toxicity are highly recommended to plasticize PVC for toys, food utensils, and food containers.
Several countries have prohibited the use of phthalates in childcare products (products for children under three years) and children toys or have limited the concentration of these chemicals to 0.1 percent or less. Most likely, these products will be manufactured with other resins or phthalate-free PVC in the near future.
COMMERCIAL POLYVINYLCHLORIDEThree major producers of PVC are: PolyOne Corp. (Geon™, Fiberlock™), Formosa Plastics (Formolon®), and Solvay ( Benvic®).
Poly(vinyl chloride) finds extensive use is in the building and construction industry. This industry accounts for about 75 percent of all PVC consumption in the United States and for about 60 percent in the European Union.2 It is extensively used for water and sewage pipes, sidings, window frames, flooring, and wire and cable insulations. Other important applications are footwear, sporting goods, toys, and automotive parts like upholstery, floor mats, auto tops, and automotive wires and plastic films.
1The most common
plasticizers are chemically stable diesters and triesters of
aromatic or aliphatic acids such as phthalates and aliphatic carboxylic acid esters
as well as polyesters and phosphates.
2Source: California Environmental Protection Agency, Office of Environmental Health Hazard Assessment, October 2006