POLY VINYLIDENE FLUORIDE

Poly(vinylidene fluoride) (PVDF) is a fluoropolymer which shares some properties, including high cost, with PTFE. It contains the repeat unit 

The resistance of PVDF to y-radiation is exceptional the material is little affected even by a dose of 300 Mrad from a Co source. In the reprocessing of nuclear waste, reliability of the equipment is of the utmost importance. Reliability has to be maintained during the handling of corrosive liquids such as nitric and hydrofluoric acids at 105-1 lO C, while exposure to high-energy radiations is also a major consideration in this application. On all these counts, PVDF shows satisfactory behaviour, making it a recommended material for pipework in this exacting application. 

Its resistance to ageing and weathering is excellent, and so it finds ready use in polluted industrial locations eight years exposure in such an environment 

Copolymers of vinylidene fluoride and tetrafluoroethylene are available, characterized by lower crystallinity and melting point, with greater flexibility, extensibility and toughness. Otherwise they are generally similar to the homopolymers. 

This melt-processable homopolymer was first introduced in 1961 as Kynar by the Pennsalt Chemical Corporation (the company name being subsequently changed to Pennwalt). Other companies now manufacturing similar polymers are Dynamit Nobel (Dyflor), Kureha (KF), Solvay (Solef) and Atochem (Foraflon). 

The monomer is a gas boiling at -84°C which may be made by dehydrochlorination of l-chloro-l,l-difluoroethane  

The membrane surfaces have also been grafted or coated with polyacrylamide, poly(acrylic acid) [70, 71], poly(vinyl alcohol) and cellulose derivatives [72]. Another possibility for improving the membrane properties is the use of polymer blends. Blends of PVDF/PVP [73, 74], PVDF/poly(ethylene glycol) (PEG) [75], PVDF/sulfonated polystyrene [76], PVDF/poly(vinyl acetate) [77] and PVDF/ poly(methyl methacrylate) [78] have been used in the preparation of micropor-ous membranes. 

Poly(viaylidene fluoride) [24937-79-9] is the addition polymer of 1,1-difluoroethene [73-38-7], commonly known as vinylidene fluoride and abbreviated VDF or VF2. The formula of the repeat unit in the polymer is —CH2—CF2—. The preferred acronym for the polymer is PVDF, but the abbreviation PVF2 is also frequently used. The history and development of poly(vinyhdene fluoride) technology has been reviewed (1 3). 

There is growing commercial importance and escalating scientific interest in PVDF. The World Patent database, including the United States, Hsts 678 patents that cite the term poly(vinyHdene fluoride) for the period 1963—1980 and 2052 patents for the period 1981—1992 Chemical Abstracts files covering the years 1967—1992 contain 5282 references for the same term. Thirty years ago there was only one commercial producer of PVDF in the world now there are two in the United States, two in Japan, and three in Europe. 

Properties. VinyHdene fluoride is a colorless, flammable, and nearly odorless gas that boils at —82°C. Physical properties of VDF are shown in Table 1. It is usually polymerized above its critical temperature of 30.1°C and at pressures above 3 MPa (30 atm) the polymerization reaction is highly exothermic. 

Preparation. Thermal elimination of HCl from l-chloro-l,l-difluoroethane (HCFC-142b) [75-68-3] is the principal industrial route to VDF covered by numerous patents (8—19). Dehydrohalogenation of l-bromo-l,l-difluoroethane (20), or 1,1,1-trifluoroethane (HFC-143a) (21—25), or dehalogenation of l,2-dichloro-l,l-difluoroethane (26—28) are investigated alternative routes (see Fluorine compounds, organic-fluorinated aliphatic compounds). 

The commercially preferred monomer precursor HCFC-142b has been prepared by hydrofluorination of acetylene (29), vinyHdene chloride (30—32), or 1,1,1-trichloroethane (33—39). 

1-Difluoro-l-chloroethane (obtained from acetylene, vinylidene chloride or 1,1,1-trichloroethane) is mostly used as the starting material for the synthesis of vinylidene fluoride  

CH3CF2CI + 2HC1 1,1-Difluoro-l-chloroethane is then dehydrochlorinated CH3CF2CI CH2—CF2 + HQ 

Vinylidene fluoride (b.p. — 84°C) is free-radically polymerized in suspension or emulsion at 10-300 bar and 10-150°C. Suspension-polymerized material contains less branching and consequently a narrower molecular-weight distribution than the emulsion-polymerized material. For this reason, the suspension-polymerized material has higher crystallinity, greater mechanical strength, and better chemical stability. Materials from both polymerization methods contain a considerable proportion of head-to-head linkages. 

After vulcanization with BPO in the presence of ZnO, amines, or diisocyanates, copolymers of vinylidene fluoride with hexafluoropropylene (70 30) or trifluorochloroethylene (70 30 and 50 50) give oil-resistant, chemically inert elastomers which can be used at temperatures up to 150-200°C. However, these elastomers can be used down to temperatures of only — 10 to — 50°C. 

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Cakmak M, Teitge A, Zachman FI G and White J L 1993 On-line small-angle and wide-angle x-ray scattering studies on melt-spinning poly(vinylidene fluoride) tape using synchrotron radiation J. Polym. Sc/. 31 371- 81... 

Poly(vinylidene Fluoride). Poly(vinylidene fluoride) consists of linear chains in which the predominant repeating unit is... 

Fluorinated ethylene-propylene resin Poly(vinylidene fluoride) Ethylene-tetrafluoroethylene copolymer Ethylene- chlorotrifluoro- ethylene copolymer Cellulose- filled Glass-fiber- reinforced... 

PVDF in [FLUORINE COPTPOUNDS, ORGANIC - POLY(VINYLIDENE FLUORIDE)] (Vol 11)... 

Poly(vinylidene fluoride), tangential flow. Regenerated cellulose hoUow fiber. Polyacrylonitrile hoUow fiber. 50-nmhead, 150-nm tail. 

The equimolar copolymer of ethylene and tetrafluoroethylene is isomeric with poly(vinyhdene fluoride) but has a higher melting point (16,17) and a lower dielectric loss (18,19) (see Fluorine compounds, organic-poly(VINYLIDENE fluoride)). A copolymer with the degree of alternation of about 0.88 was used to study the stmcture (20). Its unit cell was determined by x-ray diffraction. Despite irregularities in the chain stmcture and low crystallinity, a unit cell and stmcture was derived that gave a calculated crystalline density of 1.9 g/cm. The unit cell is befleved to be orthorhombic or monoclinic (a = 0.96 nm, b = 0.925 nm, c = 0.50 nm 7 = 96%. 

Table 3. Electrical Properties of Poly(vinylidene fluoride)...

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