VINYL FLUORIDE POLYMERS (PVF) Introduction The base unit of poly(vinyl fluoride) (PVF) is shown by structure (1). DuPont first commercialized a film based on PVF under the trade mark Tedlar in 1961 and is the only known supplier of this polymer.

PVF homopolymers and copolymers have excellent resistance to sunlight degradation, chemical attack, water absorption, and solvent, and have a high solar energy transmittance rate. These properties have resulted in the utilization of PVF film and coating in outdoor and indoor functional and decorative applications. These films have found use where thermal stability, outdoor durbility, stain resistance, adherence, and release properties are required. PVF has a greater tendency to crystallize than poly(vinyl chloride). It is stable at high temperature, which is important in many of its applications. Vinyl fluoride can be polymerized into homopolymers and copolymers with the aid of a free-radical generating catalyst (initiator), usually under high pressure. Ziegler–Natta-type catalysts can be used to prepare vinyl fluoride polymers at lower temperatures and pressures. 438 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

439

Monomer Properties. Vinyl fluoride [75-02-5] (VF), CH2 CHF (fluoroethene), is a colorless gas at ambient conditions with an ether-like odor. VF is insoluble in water under atmospheric pressure but dissolves slightly in some alcohols and ketones, such as ethanol and acetone. Tables 1–5 (1) provide data for physical, thermodynamic, and transport properties of VF. Other than polymerization into homo- and copolymers, VF is used for the introduction of fluorine atoms into organic compounds and polymers, eg, by grafting into fibers. The net consequence of fluorine incorporation is increased chemical and Table 1. Physical Properties of Vinyl Fluoride Property

Value

Molecualr weight Boiling point, ◦ C Freezing point, ◦ C Critical temperature, ◦ C Critical pressure, MPaa Critical density, g/cm3 Vapor pressure at 21◦ C, MPaa Liquid density at 21◦ C, g/cm3 a To

46.046 −72.0 −160.5 54.7 5.1 0.320 2.5 0.636

convert MPa to atm, multiply by 10.

Table 2. Estimated Thermodynamic Properties of Vinyl Fluoride Heat capacity, J/(g · K)a Temperature, ◦ C

Latent heat of vaporization, J/(g · K)a

Saturated liquid

Saturated vapor at 0.1 MPab

171.8 161.5 151.2 140.1 128.0 114.2

2.02 2.05 2.09 2.13 2.27 2.72

0.93 0.95 0.98 1.00 1.03 1.05

−28.9 −17.8 −6.7 4.4 15.6 26.7 a To b To

convert J to cal, divide by 4.184. convert MPa to atm, multiply by 10.

Table 3. Solubility of Vinyl Fluoride in Organic Solvents Solvent Ethyl alcohol Diethyl ether Adipontrile Propionitrile Acetonitrile Butyrolactone Dimethyl formamide a To

Solubility of cm3 VF gas in 1-cm3 solvent

Henry’s law constant at, 30◦ C, 0.1 MPaa

4 5.5 4 10 10.5 5.2 8.9

50.8 32.9 41.9 57.0 33.4

convert MPa to atm, multiply by 10.

440

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

Table 4. Solubility of Vinyl Fluoride in Water Vinyl fluoride pressure in water,a MPab 0.86 1.75 2.75 3.43 6.87

g/100 g Water 27◦ C

79◦ C

100◦ C

0.3 0.5

0.4

0.9 1.5

0.8 1.5

1.1 1.5

a Vinyl fluoride forms a hydrate at 15.6◦ C and 2.22 MPa. b To

convert MPa to atm, multiply by 10.

Table 5. Estimated Thermal Conductivity and Viscosity of Vinyl Fluoride Property Thermal conductivity, kW/(m · K) at −28.9◦ C −17.8◦ C −6.7◦ C 4.4◦ C 15.6◦ C 26.7◦ C Viscosity, Pa · sb at −28.9◦ C −17.8◦ C −6.7◦ C 4.4◦ C 15.6◦ C 26.7◦ C a To b To

Liquid, saturated

Vapor, saturated at 0.1 MPaa

122.6 124.2 127.6 143.9 172.9

10.28 11.11 11.98 12.76 13.35 14.02

0.031 0.022 0.019 0.016 0.015 0.014

0.0093 0.0096 0.0100 0.0104 0.0108 0.0112

convert MPa to atm, multiply by 10. convert Pa · to P, multiply by 10.

thermal stability, decreased solubility, and enhanced resistance to degradation by light. The useful range of application of grafted article at lower temperatures is also improved. Flammability. Interstate Commerce Commission has classified VF as a flammable gas. It is flammable in air between concentrations of 2.6 ± 0.5 and 21.7 ± 1.0 vol% of VF. Mixtures of air and VF have been reported to ignite at a minimum temperature of 400◦ C (2). Preparation. The first preparation of VF dates back to 1901 and the reaction of zinc with 1,1-difluoro-2-bromoethane [359-07-9] (3). Phenylmagnesium bromide in ether and potassium iodide solution can replace the metal for dehalogenation (4,5). Most approaches to VF synthesis have employed reactions of acetylene [74-86-2] with hydrogen fluoride (HF) either directly (5–8) or utilizing a catalyst (6,9–13) based on mercury or aluminum (6,9,12,14). In another process, an acetylene/HF mixture is passed over a mercuric chloride or fluoride catalyst, which also produces vinylidene fluoride and difluoroethane as by-products (10). Another synthesis (11) consists of two steps in which 1,1-difluoroethane (DFE) is

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

441

formed by adding HF to acetylene, followed by pyrolytic dehydrofluorination of DFE over an aluminum salt catalyst. Vinyl fluoride is purified by removing hydrofluoric acid in soda-lime towers and by scrubbing of acetylene in ammoniacal CuCl. Oxygen is removed by distillation. Vinyl fluoride can be made by the reaction of ethylene [74-85-1] with HF in the presence of a palladium and CuCl2 catalyst (14). A mixture of HF and CH2 CH2 at a 2:1 ratio, containing 35% oxygen, is passed over the catalyst at 240◦ C, producing VF with a 20% conversion of ethylene and 92% selectivity. Other routes of preparation include catalytic pyrolysis of DFE (15–18); dehydrochlorination of 1-fluoro-2-chloroethanes at 500◦ C in the presence of ethylene dichloride, with a 15% conversion and 100% selectivity (19–23); catalytic reaction of DFE with acetylene (24,25), where VF is produced by simultaneous dehydrofluorination of CF2 HCH3 and hydrofluorination of C2 H2 in the presence of catalyst. An optimal yield of 77.5% is obtained at a temperature of 340◦ C at a 2.1:1 ratio of DFE to acetylene, and ferric oxide–cadmium oxide–aluminum oxide catalyst with a ratio of 8:4:88 (25). Another procedure involves halogen exchange of vinyl chloride [75-01-4] with HF (26–28). Highest yields were reported at 370–380◦ C with a two-component catalyst system, containing 96% γ -Al2 O3 and 4% Cr2 O3 with a vinyl chloride-to-HF ratio of 1:3 (28). Hydrogen fluoride addition to acetylene and fluorination of vinyl chloride are the most likely commercial routes for the preparation of VF (29), although most details have not been published. Commercial VF is stabilized with terpenes such as d-limonene to inihibit autopolymerization.

Polymer Properties. The physical, chemical, and electrical properties of a poly(vinyl fluoride) [24981-14-4] (PVF) film are shown in Table 6. PVF is a semicrystalline polymer with a planar zigzag chain configuration (30). The degree crystallinity depends on the polymerization method and the thermal history of the polymer; reported values range from 20 to 60% (31). The significant variation of the degree of crystallinity is thought to be primarily a function of defect structures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (32). Similarity to the phase I crystal form of poly(vinylidene fluoride) suggests an orthorhombic crystal (33). The relationship of polymer structure to melting point and degree of crystallinity has been the subject of a number of studies. Head-to-head regio irregularities in PVF are known (31,34,35) and the concentration of such units has been suggested as the source of variations in melting point (36–38). Commercial PVF contains approximately 12% head-to-head linkages by 19 F nmr and displays a peak melting point of about 190◦ C (37,39–41). Both nmr and ir studies have shown PVF to be atactic (31,34,35,37,42–45) and, as such, variations in stereoregularity are not thought to be a contributor to variations in melting point. PVF with controlled amounts of head-to-head units varying from 0 to 30% have been prepared (37,39) by using a chlorine substituent to direct the course of polymerization of chlorofluoroethylenes and then reductively dechlorinating the

442

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

Table 6. Properties of Poly(vinyl fluoride) Film Property Bursting strength, kPab Coefficient of friction with metal density, g/cm3 Impact strength, kJ/md Refractive index, n20 D Tear strength, kJ/md Propagated Initial Tensile modulus, MPae Ultimate elongation, % Ultimte yield strength, MPae Linear coefficient of expansion, cm/(cm · C) Useful temperature range, ◦ C Continuous use Short cycle (1–2 h) Zero strength Thermal conductivity, 1◦ C/cm, W/(m · K) At −30◦ C At 60◦ C Self-ignition temperature, ◦ C Solar energy transmittance, 359–2500 nm, % Chemical resistance after 1-year immersion at 25◦ C in acids (10%), bases (10%), or solvents Chemical resistance after 2-h immersion at boiling temperature in acids (10%), bases (10%), or solvents Electrical f Corona endurance, h at 60 Hz, 40 V/µm, h Dielectric constant 1 MHz at 23◦ C 1 MHz at 100◦ C 1 Hz at 23◦ C 1 Hz at 100◦ C Dielectriici strength Short-term ac, kV/µm Short-term dc, kV/µm Dissipation factor 1 MHz at 23◦ C 1 MHz at 100◦ C 1 kHz at 23◦ C 1 kHz at 100◦ C Volume resistivity, G · m At 23◦ C At 100◦ C Surface resistivity, G · m At 23◦ C At 100◦ C

Value

ASTM test methoda

200–450c 0.18–0.21 1.38–1.72 43–90 1.46

D774 D1894-78 Weighted samples D3420-80 D542; Abbe refractometer

6–22 129–196 44–110 115–250 33–41 0.00005

D1922-67 D1004-66 D882 D882 D882 Air oven, 30 min

−70 to +107 175 260–300 0.14 0.17 390 90 No visible effect

Hot bar

D1929 E427-71

No visible effect

2.5–6.0

D2275 D150-81

6.2–7.7 10.9–12.6 8.5–8.2 14–13 0.08–0.13 0.15–0.19 D150-81 0.17–0.28 0.09–0.21 0.019–0.019 0.21–0.067 D150-81 2000–700 0.7–2 D257 60,000–20,000 7–20

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

443

Table 6. (Continued) Property Permeability Moisture absorption, % Moisture vapor transmission,g nmol/(m2 ·sh) at 7.0 kPa,i 39.5◦ C Gas permeability, nmol/(m · s · GPa) j at 98 kPa,i 23◦ C Carbon dioxide Helium Hydrogen Nitrogen Oxygen Vapor permeability,k nmol/(m2 · s) l Acetic acid at 23◦ C Acetone at 23◦ C Benzene at 23◦ C Carbon tetrachloride at 23◦ C Ethyl acetate at 23◦ C Hexane at 23◦ C Water at 39.5◦ C

Value 0.5 4.65–29.4

ASTM test methoda D570-81 E96-58T

D1434 22.4 302 117 0.5 6.6 E96, modified 4.9 1570 13 3.9 138 10 22

a Unless

otherwise stated. convert kPa to psi, multiply by 0.145. c Range dependent on composition and tensile modification. d To convert kJ/m to ft · lbf/in., divine by 0.534 (see ASTM D256). e To convert MPa to psi, multiply by 145. f When range of electrical properties is given, the first value refers to 54.8-µm transparent film, and the second vlaue to 54.8-µm white pigmented film. g Measurements made on films of nominal 25-µm thickeness. hTo convert nmol/(m2 · 5) to g/(m2 · d), multiply by 1.94. i To convert kPa to mm Hg, multiply by 7.5. j To convert nmol/(m · s · GPa) to (cc · mL)/(m · d · atm), multiply by 7.7. k At partial pressure of vapor at given temperature. l To convert nmol/(M2 · s) to g/(m2 · d), multiply by 1.94 and by the density. b To

products with tributyltin hydride. This series of polymers shows melting point distributions ranging from about 220◦ C for purely head-to-tail polymer down to about 160◦ C for polymer containing 30% head-to-head linkages. This study, however, does not report the extent of branching in these polymers. Further work has shown that the extent of branching has a pronounced effect upon melting temperature (40,41). Change of polymerization temperature from 90 to 40◦ C produces a change in branch frequency from 1.35 to 0.3%, while the frequency of monomer reversals is nearly constant (12.5 ± 1%). The peak melting point for this series varies from 186◦ C (polymerization at 90◦ C to 206◦ C polymerization at 40◦ C). PVF displays several transitions below the melting temperature. The measured transition temperatures vary with the technique used for measurement. T g (L) (lower) occurs at −15 to −20◦ C and is ascribed to relaxation, free from restraint by crystallites. T g (U) (upper) is in the 40–50◦ C range and is associated

444

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

with amorphous regions under restraint by crystallites (46). Another transition at −80◦ C has been ascribed to short-chain amorphous relaxation and one at 150◦ C associated with premelting intracrystalline relaxation. PVF has low solubility in all solvents below about 100◦ C (44). Polymers with greater solubility have been prepared using 0.1% 2-propanol polymerization modifier and were characterized in N,N-dimethylformamide solution containing 0.1 N LiBr. M n ranged from 76,000 to 234,000 (osmometry), and M s from 143,000 to 654,000 (sedimentation velocity). Sedimentation velocity molecular weights can be related to intrinsic viscosity, using the Mark–Houwink equation: η = K Ma Using an a value of 0.80, which is typical of an extended polar polymer in good solvent, K is determined to be 6.52 × 10 − 5 (47). The conformational characteristics of PVF are the subject of several studies (33,48). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. 13 C nmr chemical shifts are in agreement with these predictions (49). The stiffness parameter (δ) has been calculated (50) using the relationship between chain stiffness and cross-sectional area (51). In comparison to polyethylene, PVF has greater chain stiffness, which decreases melting entropy, ie, (S)m =8.58 J/(mol·K) [2.05 cal/(mol·K)] versus 10.0 J/(mol·K) [2.38 cal/(mol·K)]. A solubility parameter of 24.5–24.7 MPa1/2 [12.0–12.1 (cal/cm3 )1/2 ] has been calculated for PVF using room temperature swelling data (52). The polymer lost solvent to evaporation more rapidly than free solvent alone when exposed to air. This was ascribed to reestablishment of favorable dipole–dipole interactions within the polymer. Infrared spectral shifts for poly(methyl methacrylate) in PVF have been interpreted as evidence of favorable acid–base interactions involving the H from CHF units (53). This is consistent with the greater absorption of pyridine than of methyl acetate, despite a closer solubility parameter match with methyl acetate. PVF is more thermally stable than other vinyl halide polymers. High molecular weight PVF is reported to degrade in an inert atmosphere, with concurrent HF loss and backbone cleavage occurring at about 450◦ C (54,55). In air, HF loss occurs at about 350◦ C, followed by backbone cleavage around 450◦ C. More recent work reports the onset of thermal degradation at lower temperatures and provides a clearer picture of the role of oxygen (56–58). In the presence of oxygen, backbone oxidation and subsequent cleavage reactions initiate decomposition. In the absence of oxygen, dehydrofluorination eventually occurs, but at significantly higher temperatures. PVF is transparent to radiation in the uv, visible, and near-ir regions, transmitting 90% of the radiation from 350 to 2500 nm. Radiation between 7,000 and 12,000 nm is absorbed (59). Exposure to low dose γ irradiation produces cross-links in PVF and actually increases tensile strength and etching resistance, whereas the degree of crystallinity and melting point are reduced (60). PVF becomes embrittled upon exposure to electron-beam radiation of 10 MGy (109 rad), but resists breakdown at lower doses. It retains its strength at 0.32 MGy (32 × 106 rad) while poly(tetrafluoroethylene) is degraded at 0.02 MGy (2 × 106 rad) (61).

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

445

Polymerization VF undergoes free-radical polymerization in both head-to-head and head-to-tail configurations. Typically, commercial PVF contains 10–12% of inverted head-tohead and tail-to-tail units (62). VF is more difficult to polymerize than other vinyl halides (60) because of the high electronegativity of fluorine, which is the most electronegative element. Low boiling point (−72◦ C) and high critical temperature (54.7◦ C) of VF necessitate high pressure polymerization similar to that required for polyethylene. The first polymerization involved heating a saturated solution of VF in toluene at 67◦ C under 600 MPa (87,000 psi) for 16 h (63). In a later procedure, benzoyl peroxide was used to initiate polymerization (9,64). The product was a polymer with a density of 1.39 g/cm3 , which was soluble in hot dimethylformamide, chlorobenzene, and other solvents. A wide variety of initiators and polymerization conditions have been explored (64–66). Examples of bulk (30,31) and solution (65,67–69) polymerizations exist; however, aqueous suspension or emulsion methods are generally preferred (64,70–79). VF volatility dictates that moderately high pressures would be required. Photopolymerizations, usually incorporating freeradical initiators, are also known (64,67,68,80). The course of VF polymerizations is dominated by the high energy and hence high reactivity of the propagating VF radical. The fluorine substituent provides little resonance stabilization, leading to a propagating intermediate which is indiscriminate in its reactions. Monomer reversals, branching, and chain-transfer reactions are common. The reactivity of the VF radical limits the choice of polymerization medium, surfactants, initiators, or other additives and makes impurity control important. Species which can participate in chain transfer or incorporate in the polymer can depress molecular weight or degrade the thermal stability characteristics of the final polymer. The combination of triisobutylborane [1116-39-8] and oxygen has been used to polymerize VF at reduced temperature and pressure (81). Polymerization temperature was varied from 0 to 85◦ C, with a corresponding drop in melting point from about 230◦ C (polymerization at 0◦ C) to about 200◦ C (polymerization at 85◦ C). This dependence of melting temperature, and degree of crystallinity, has been interpreted in terms of variations in the extent of monomer reversals during polymerization (82). VF can be polymerized by a number of techniques, including suspension, bulk, and emulsion polymerizations in batch and continuous modes. Graft and radiation-induced polymerizations of VF have also been reported. Suspension Polymerization. In this technique, liquid VF is suspended in water with the help of a dispersion stabilizer (70). Polymerization is initiated by an organic peroxide such as diisopropyl peroxydicarbonate below the critical temperature of VF (71,72). The reaction can also be initiated by uv light and ionizing radiation (64,73). VF dispersions are usually stabilized by water-soluble polymers such as cellulose derivatives, eg cellulose ester and sodium carboxymethylcellulose, and poly(vinyl alcohol). Inorganic salts such as magnesium carbonate, barium sulfate, and alkylsulfoacids are also used. Polymerization is also reported in the presence of a nonionic surfactant (74) using 0.5–3.0 wt% monoalkylphenyl ether of poly(ethylene glycol)s as dispersing

446

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

agent in water and 0.5–2.5 wt% diisopropyl peroxydicarbonate as initiator. Polymer yield of 12–21 wt% are obtained in 14–18 h at 30–40◦ C. Radiation-initiated suspension polymerization of VF in water yields PVF soluble in solvents such as dimethylformamide (73). Increased dosages of radiation give lower molecular weight polymers. Thermal stability of PVF deteriorates with an increase in the concentration of dispersing agent. In a modified suspension polymerization, the pressure requirements are reduced. The reaction is conducted with azo catalysts (75) at 25–100◦ C and pressures of 2.5–10 MPa (25–100 atm) over 18–19 h. A stainless steel reactor is flushed with nitrogen charged with 150 parts acetylene-free VF, 150 parts deoxygenated distilled water, and 0.150 parts 2,2 -azobisisobutyronitrile. The reactor is heated to 70◦ C for 1 h, agitated, and held at 8.2 MPa (82 atm) for 18 h. The product, 75.8 parts of PVF, is collected in the form of a white cake (64). Continuous polymerization of VF is based on the modified suspension process (76). Bulk Polymerization. In the bulk polymerization, VF is polymerized by a peroxide initiation. In an example (67), a glass ampule is filled with VF containing 6.5×10 − 2 mol/L of di-tert-butyl peroxide. The ampule was irradiated by uv light from a mercury lamp below 25◦ C. A highly porous polymer, insoluble in VF, at conversions over 90% is obtained. Emulsion Polymerization. VF can be readily polymerized by the emulsion method at highly reduced pressures and lower temperatures as compared to the suspension technique (77). Polymerization in an aqueous emulsion facilitates the process control and the removal of reaction heat, increases the molecular weight of the resin (74), and permits high rates of reaction and high yields. Emulsifiers such as fatty alcohol sulfates, alkane sulfonates, alkali salts of fatty acids, and others are slightly to marginally effective (77). Fluorinated surfactants, particularly perfluorinated carboxylic acids containing seven or eight fluorine atoms, are specially effective in maintaining a high rate of polymerization after about 40% conversion. Fluorinated surfactants are characterized by low values of critical concentration of micelle formation (74). They are thermally and chemically stable, and their incorporation does not impair the PVF properties. In a typical example (77), 200 parts water, 100 parts VF, 0.6 parts of a perfluorinated carboxylic acid, 0.2 parts ammonium persulfate, and 3 parts water glass (Na2 O/SiO2 = 1:3.3) are introduced into a stirred autoclave. The mixture is heated to 46◦ C, the pressure is brought to 4.3 MPa (42.5 atm), and held for 8 h. Addition of an electrolyte precipitates a white powdery PVF at 95% yield. Radiation-Induced Polymerization. Exposure to uv radiation in the absence of free radicals does not polymerize VF, but decomposes it into acetylene and HF (83,84). Radiation can be used in the presence of initiators (64), which decomposes into free radicals. The first reported photopolymerization of VF in the presence of benzoyl, lauroyl, or acetyl peroxide resulted in a 36% yield after 2 days at 27◦ C under 254-nm irradiation (64). The rate data in radiation-induced bulk polymerization of VF reveal a heterophase process (80). Polymerization is conducted at γ -ray dosage rates of 0.13–1.0 Gy/s (13–100 rad/s) using 60 Co source. The rate of polymerization at 38◦ C is proportional to the dosage rate to the power 0.42. Gas-phase polymerization of VF with γ -ray has been studied in the range of 0.1–1.0 Gy/s (10–100 rad/s). Polymerization rate increases sharply with the increase in dosage, leading

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

447

to generation of active sites in the polymer chain and branches. Radiation polymerization of VF dissolved in solvents such as tetrachloromethane leads to chain transfer and incorporation of solvent in the polymer. VF can also be polymerized in plasma. Graft Polymerization. The most effective method for graft polymerization of VF is radiation polymerization (80). VF has been grafted to low density polyethylene, polyisobutylene, and polyamides (85–87). The density of the graft copolymers is higher than that of the starting homopolymer. Typically, VF is brought in contact with the polymer simultaneously with irradiation. Grafting to cellulose in the absence of solvent (87) is initiated by γ -rays by means of a 60 Co source. The polymer-to-monomer ratio has only a minor effect on the PVF content, which is in the range of 2–5 wt%. In the presence of solvents that cause swelling of cellulose (mercerization), the PVF content increases dramatically. At a radiation of 30 kGy (3 Mrad) in the presence of dimethylformamide, the PVF content of the graft copolymer is 26.6%; in its absence it is 4.1%. Rot resistance of cellulose improves greatly as a result of PVF grafting (87). Some of the changes that VF grafting imparts to the polymers include a reduction in solubility in solvents, enhancement of thermal stability, increased water and oil repellency, hightened resistance to acids, and increased light stability. Adhesion, processibility, and dyeability of PVF have been improved by grafting with other monomers (80). Styrene, methyl methacrylate, vinyl acetate, and vinylidene chloride have been grafted to PVF films by using 60 Co γ -rays or an electron accelerator. Continuous Polymerization. A process for continuous polymerization of VF in aqueous medium has been described (76,78). A mixture of VF, water, and a water-soluble catalyst is stirred at 50–250◦ C and 15–100 MPa (150–1000 atm). A small amount of a monoolefin (C1 –C3 ) is continuously introduced into the reactor to inhibit the bulk polymerization of VF to low molecular weight polmer. The water-soluble catalyst generates free radicals, which initiate the polymerization. Catalysts include ammonium persulfate, organic peroxides, and water-soluble azo initiators. In a two-stage continuous polymerization the polymer particles formed in the first stage act as nucleation sites for the second reaction zone (88). Effect of Polymerization Variables. The polymerization variables include temperature, pressure, medium, impurities, telogen, and catalyst (initiator) (64–66). Temperature. The polymerization has a significant effect on the molecular weight of PVF. Molecular weight generally decreases as the polymerization temperature is raised, because of a more rapid chain termination and increased branching (65,89). Initiator efficiency reaches a maximum and falls off as temperature is increased (Fig. 1). Pressure. Polymerization of VF requires high pressures (64). The rate of polymerization and the intrinsic viscosity of the polymer increase (Fig. 2) when pressure is increased. Initiators, such as benzoyl peroxide, are generally consumed more efficiently at higher pressures. Some azo initiators do not respond to pressure in the same way. Medium. Water is the best medium for the polymerization of VF; organic solvents have also been used. However, chain transfer to solvents leads to a sharp drop in molecular weight (65,68), and hence thermostability decreases. The PVF

448

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

Fig. 1. Polymer yield as a function of temperature. —, 10 MPa (100 atm), 0.2% AIBN;—, 25 MPa (250 atm), 0.1% benzoyl peroxide; ——, 25 MPa (250 atm), 0.1% AIBN. From Ref. 63.

obtained is usually soluble in the solvents at the temperatures used for the polymerization reaction. A mixture of tert-butyl alcohol and water results in a high molecular weight and a high polymerization rate. Impurities. Oxygen, acetylene, and DFE are among the impurities most likely found in VF. Oxgen up to 135 ppm appears to increase the yield, but 500 ppm inhibits the reaction slightly (65). Benzoyl peroxide at 90-MPa (900-atm) pressure is used to initiate the polymerization. At 1000-ppm acetylene content in VF, polymerization is accelerated with over 99% conversion to a highly cross-linked polymer. An increase in acetylene concentration from 0.5 to 1.0% reduced the rate by two-thirds in radiation-initiated polymerization of PVF (80). The presence of 2% acetylene reduced the yield of a low molecular weight PVF to 3%. The product was brittle and readily soluble in cyclohexane (65). VF containing up to 2.5% DFE gives a normal polymer yield; the properties of the film produced from the polymer are not affected. Telogen. The melt viscosity of PVF can be best controlled by a chainterminating compound (telogen), such as isopropyl alcohol or 1,3-dioxolane; toughness is not affected. Catalysts and Initiators. The type of free-radical initiator (65) employed can have a profound effect on PVF properties, such as thermal stability and wettability.

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

449

Table 7. Melting Point and Degree of Crystallinity of Poly(vinyl fluoride) Obtained at Various Polymerization Temperaturesa ,b Polymerization temperature, ◦ C 85 40 30 20 10 0 a Ref.

Melting point, ◦ C

Degree of crystallinity,c %(±2)

197–205 205–215 218–225 222–230 220–235 225–235

37 44 45 45 48 50

86.

b Polymerization

pressure, 30 MPa (300 atm); alkylboron catalyst. the heat of fusion (differential scanning calorimeter) and the averaged H u = 7.5 kJ (1.8 kcal) per monomer unit. c From

Fig. 2. Effect of polymerization pressure on intrinsic viscosity; 0.2% benzoyl peroxide initiator in aqueous medium. From Ref. 63. To convert MPa to atm, multiply by 10.

The polymer yield depends on the initiator and reaction pressure (Fig. 1). Useful water-soluble initiators include salts of inorganic peracids, organic peroxides containing hydrophilic groups, alkali metal salts of carboxylic azonitriles, and inorganic acid salts of azoamidines (76) (Table 7). In general, polymers prepared at higher temperature tend to have low molecular weight and appear to be more branched. Increasing the initiator concentration reduces initiator efficiency and molecular weight, as evidenced by a drop in the intrinsic viscosity of the polymer. Ziegler–Natta and Other Catalysts. Ziegler–Natta-type catalysts are used to polymerize VF. The catalysts are a mixture of transition-metal halides (titanium halides) and trialkylaluminum (triethylaluminum). Originally, olefins were polymerized by Ziegler–Natta catalysts to polymers with a high melting point, crystalline structure, and a high degree of stereoregularity

450

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

(Table 7). In an early application, VF was polymerized with a catalyst composed of diethylaluminum bromide–titanium tetrachloride–carbon tetrachloride (82). A Ziegler–Natta system based on vanadyl acetylacetonate and AIR(OR)Cl compounds gives good yields (82) at polymerization temperatures up to 50◦ C. A modified catalyst prepared from vanadium oxytrichloride, triisobutylaluminum, and tetrahydrofuran was found to be the most effective Ziegler–Natta system (90). However, the activity of VF with this catalyst system is much lower than that of vinyl chloride. A polymerization at low temperature and pressure utilized a triisobutyl boron catalyst activated by oxygen (82). In a similar method, oxygen was replaced by hydrogen peroxide, and polymerization was carried out in water (91). The initiator system, AgNO3 –tetraethyllead, is used for the polymerization of VF in dimethyl sulfoxide (69). Dimethyl sulfoxide permits the polymerization to proceed for longer periods than do other solvents by complexing of silver ions.

Copolymerization Copolymers of VF with vinylidene fluoride [75-38-7] and tetrafluoroethylene [116-14-3] also have been prepared with this initiation system. VF tends toward alternation with tetrafluoroethylene and incorporates preferentially in copolymerization with vinylidene fluoride [see PERFLUORINATED POLYMERS, POLYTETRAFLUOROETHYLENE; VINYLIDENE FLUORIDE POLYMERS]. More recently, interpolymers of VF have been reported with tetrafluoroethylene and other highly fluorinated monomers such as hexafluoropropylene, perfluorobutylethylene, and perfluoroethylvinylether (92,93). Polymerization reaction took place in an aqueous medium using an initiator consisting of water-soluble organic azo compounds or salts inorganic peracids, examples of which include 2,2 -azobis(2-amidinopropane) dihydrochloride and ammonium persulfate. Reaction conditions were relatively mild at temperature of 60–100◦ C and pressure of 1–12 MPa in the absence of any surfactant. Copolymers of VF and a wide variety of other monomers have been prepared (9,81,82,94–98). The high energy of the propagating VF radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price Q value of 0.010 ± 0.005 and an e value of 0.8 ± 0.2 have been determined (99). The low value of Q is consistent with little resonance stability and the e value is suggestive of an electron-rich monomer. Reactivity Ratio. An extensive study covers copolymerization of various monomers with VF over a range of compositions with alkylboron or Ziegler–Natta catalysts (66). Copolymerization was carried out at 30◦ C in ethyl acetate or methylene chloride. Figure 3 shows the monomer–copolymer composition curves in the alkylboron-initiated copolymerization of VF with a number of monomers. The reactivity ratios given in Tables 8 and 9 indicate random arrangements of monomer units for VF–vinylidene fluoride (VF2) copolymer and formation of alternating units for VF–TFE (C2 H4 ) copolymer (100).

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

451

Table 8. Reactivity Ratios in the Copolymerization of Vinyl Fluoride (M 1 ) at 30◦ C with Ziegler–Natta Catalystsa Catalyst

M2

Isobutylisopropoxyaluminum chloride–vanadylbisacetylacetonate (2:1)

Triisobutylaluminum– tetraisopropoxytitanium (3:1) a Ref.

Vinyl chloride

r1

r2

r1 r2

0.07 ± 0.002

9±1

0.63

4.2 ± 0.4 1.1 ± 0.05 25 ± 5

0.18 ± 0.02 0 0.04 ± 0.02

0.75 0 1.0

Vinylidene fluoride Hexafluoropropene Hexafluoropropene

86.

Table 9. Reactivity Ratios in the Copolymerization of Vinyl Fluoride (M 1 ) at 30◦ C with B(i-C4 H9 )3 Catalysta M2 CH2 CH2 CH2 CHCl CH2 CF2 CF2 CF2 CFCl CF2 CF3 CF CF2 CF3 CF CFH (cis) cyclo-C4 F6 CH2 CHOCOCH3 CH2 CHCN a Ref.

r1

r2

r1 r2

0.3 ± 0.03 0.05 ± 0.005 5.5 ± 0.5 0.27 ± 0.03 0.18 ± 0.02 1.01 ± 0.01 0.9 ± 0.05 3 ± 0.06 0.16 ± 0.01 ∼1 × 10 − 3

1.7 ± 0.1 11.0 ± 1 0.17 ± 0.03 0.05 ± 0.02 0.006 ± 0.02 0 0 0 2.9 ± 0.2 24 ± 2

0.51 ± 0.08 0.55 ± 0.10 0.93 ± 0.20 0.013 ± 0.007 0.011 ± 0.005 0 0 0 0.46 ± 0.05 ∼0.024

86.

Fabrication and Processing Commercial PVF is insoluble at room temperature because of the large number of hydrogen bonds and high degree of crystallinity. Some latent solvents solvate PVF at temperatures above 100◦ C. PVF is converted to thin films and coatings. Processing of PVF, eg, by melt extrusion, depends on latent solvation of PVF in highly polar solvents and its subsequent coalescence. An example is plasticized melt extrusion of PVF into thin films (101). Pigments, stabilizers, plasticizers, and other additives can be incorporated in the film by dispersing them with the polymer in the latent solvent. The solvent is recovered by evaporation after extrusion. The extruded film can be biaxially oriented to varying degrees. PVF can be applied to substrates with solvent-based or water-borne dispersions, or by powder-coating techniques. Viscosity modifiers are often needed to obtain a coatable dispersion. Dispersions can be applied by spraying, reverse roll coating, dip coating, and centrifugal casting. Other methods include casting on a continuous belt, extrusion into a hot liquid (102), and dipping a hot article into the dispersion (103). Adherability of the film may be enhanced by its treatment with flame, electric (corona) discharge, boron trifluoride gas, activated gas plasma, dichromate sulfuric acid, and a solution of alkali metal in liquid ammonia (104–107). A

452

VINYL FLUORIDE POLYMERS (PVF)

Vol. 4

Fig. 3. Monomer–copolymer composition in the copolymerization of vinyl fluoride at 30◦ C with O2 /B(i-C4 H9 )3 = 0.5 initiator (based on total monomer charged) and ethyl acetate as solvent. Comonomers: vinyl chloride (VCl), acrylonitrile (AN), vinyl acetate (VA), chlorotrifluoroethylene, ethylene, tetrafluoroethylene, cis-1-hydropentafluoropropene (C3 F5 H), hexafluoropropene (C3 F6 ), hexafluorocyclobutene (c-C4 F6 ), and vinylidene fluoride (VF2 ). From Ref. 86.

coating of polyurethane, an alkyl polymethacrylate, or a chlorinated adhesive can be applied to PVF surfaces to enhance adhesion (102,108,109).

Economic Aspects PVF is available from DuPont both as transparent and pigmented films and as a resin under the trademark Tedlar PVF film. Films are available in nonoriented and oriented grades in several tensile modifications and thicknesses, with either adherable or nonadherable release-grade surfaces. Nonorieted films exhibit extensive conformability to various shapes. PVF films are available in single layer and integrated multilayers where the top layer of the latter is clear and the bottom layer may possess aesthetic effects such as color, metalic appearance, or pearlescence. The 2000 prices ranged from $30 to $73 per kg. Prices for specially tailored films were significantly higher.

Health and Environment A number of studies and reviews of the toxicological effects of exposure to VF have been conducted (110–112). A 1995 report (113) by International Agency for Research on Cancer (IARC) has evaluated the preceding data for VF and a classification of “probably carcinogenic to humans” was assigned to VF. In 1998,

Vol. 4

VINYL FLUORIDE POLYMERS (PVF)

453

American Conference of Government Industrial Hygienist (ACGIH) classified VF as an A2 carcinogen defined as “suspected human carcinogen.” These classifications are based only on animal data since sufficient epidemiology data do not exist. The oncogenic potential of VF has been studied in male and female rats and mice that were administered VF via inhalation. Exposure concentrations ranged from 0 to 2500 ppm for 6 h/day, 5 days/week for up to 2 years (94). Under the conditions of this study, VF was found carcinogenic, similar to other monohaloethylenes, at concentrations of 25 ppm or greater. Mice were more susceptible than rats to the carcinogenic effects of VF and were found to metabolize VF more readily than rats or humans (114). A metabolite of VF is the suspected carcinogenic species. VF is metabolized to the suspected carcinogenic intermediate at a rate approximately one-fifth that of vinyl chloride (115–119). VF is flammable in air between the limits of 2.6 and 22 vol%. Minimum ignition temperature for VF and air mixtures is 400◦ C. A small amount (