Poly(vinyl chloride) (PVC), the second widely used resin in the world (after polyethylene) is made by the polymerization of vinyl chloride monomer (VCM). In theory the chemical structure of the polymer is simple, consisting of the same structure as for polyethylene with one hydrogen in every other —CH2— group being replaced by a chlorine atom
However, as the repeat unit is asymmetrical because of the presence of only a single chlorine atom, two types of linkages, head to tail and head to head, are possible:
~CH2—CHCl—CH2CHCl~ ~CH2—CHCl—CHCl—CH2 -Head to tail Head to head
In general, however, the head-to-tail linkages are predominant (nearly 90%) in the resin. The weight-average molecular weight Mw of commercial PVC resins ranges from about 100,000-200,000 and the polydispersity index is about 2.0. The resin has a glass transition temperature of 75-85°C and a crystalline melting point of 120-210°C. The crystallinity in PVC is due to syndiotactic sequences in the polymer and amount to about 7-20% in commercial resins. Resins with higher levels of crystallinity can be obtained by polymerization under specific conditions.
The polymer is susceptible to both photo- and thermal degradation; and, for products intended for outdoor use, the resin has to be compounded with light stabilizers. Such formulations typically contain other additives (such as a thermal stabilizer package to protect the resin during processing), fillers, and lubricants. The compounds not containing any plasticizers or the rigid PVC materials (also referred to as uPVC) are used extensively in building products such as pipes, fittings, siding, window frames, and rainwater products. In unplasticized formulations of PVC intended for outdoor use, an opacifier, usually rutile titania, that effectively absorbs the damaging ultraviolet (UV-B) radiation is incorporated in the formulation to protect the surface from UV-induced degradation. The solar ultraviolet component, particularly the UV-B radiation (290-315 nm) causes light-induced dehydrochlorination and, to a lesser extent, oxidation reactions in inadequately protected formulations. This leads to uneven yellowing discoloration of white vinyl surfaces, "chalking," or the release of titania pigment from the binder at the exposed surface of the material, and loss of impact strength of the product on prolonged exposure. In the United States PVC is used predominantly in the building sector with nearly half the resin sold going into pipe, fitting, and conduit, followed by residential siding.
PVC resin can also be made into a versatile soft pliable rubbery material by incorporating plasticizers such as organic phthalates into the compound. Plasti-cized PVC (also referred to as pPVC) is used widely as packaging film, roofing membranes, belting, hoses, and cable covering. With pPVC, calendering is employed to produce films and sheets. The resin is also used as a coating on paper or fabric and is made into numerous household products. A small amount of the plasticized film is used in packaging, for instance, in meat wraps where it is approved by the Food and Drug Administration (FDA) for food-contact use. Of the commodity thermoplastics, PVC is the most versatile resin in terms of both the processability and the range of applications.
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