Emulsion polymerization, PVDF

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

Suspension- and emulsion-polymerized PVDF exhibit dissimilar behavior in solutions. The suspension resin type is readily soluble in many solvents even in good solvents, solutions of the emulsion resin type contain fractions of microgel, which contain more head-to-head chain defects than the soluble fraction of the resin (116). Concentrated solutions (15 wt %) and melt rheology of various PVDF types also display different behavior (132). The Mark-Houwink relation (rj = KM°-) for PVDF in A/-methylpyrrohdinone (NMP) containing 0.1 molar LiBr at 85°C, for the suspension (115) and emulsion... 

Figure lO.IQTransmission electron micrograph of particles of emulsion-polymerized PVDF particles (D = 128 nm).i°>... 

Emulsion polymerization requires the use of free radical initiators, fluorinated surfactants, and often chain transfer agents. The polymer isolated from the reaction vessel consists of agglomerated spherical particles ranging in diameter from 0.2 to 0.5 jm.56 It is then dried and supplied as a free-flowing powder or as pellets, depending on the intended use. If very pure PVDF is required, the polymer is rinsed before the final drying to eliminate any impurities such as residual initiator and surfactants.57... 

Obtained by the polymerization of vinylidene fluoride by two methods (a) Suspension polymerization producing S-PVDF (Typell PVDF)- Solef is of this type and is more crystalline with fewer structural defects (b) Emulsion polymerization producing E-PVDF (Type I PVDF) PVDF can be co-polymerized... 

It is made by emulsion polymerization, spray-dried and sold as a fine particle powder. Pigment and fluoropolymer particles are separately dispersed in a solution of an acrylic copolymer compatible with PVdF, the two dispersions being blended to give the final paint. After application of paint, the coil is heated to 240-260 °C in 30-60 s, when the PVdF particles melt and lose their identify, forming a blend with the acrylic. 

VDF polymerization technology based upon super critical or liquid carbon dioxide as polymerization media has been reported (57-62). This technology offers an advantage in the polymer isolation step where a clean dry polymer is produced simply by depressimzation. The residual monomeifs) and CO2 can be recycled back to the reactor. PVDF is not soluble in CO2 (58-60) and as a result, additional pol5uneric stabilizers are required to produce stable particles. Adequate CO2 density for pol5unerization requires pressure significantly higher (typically >100 bar) than a conventional emulsion polymerization. 

Reaction temperature ranges between 10 and 150°C at pressure of 1 MPa or higher. Similar to TFE, emulsion polymerization of vinylidene fluoride requires a stable fluorinated surfactant and an initiator such as peroxide or persulfate. Suspension polymerization is conducted in an aqueous medium, sometimes in the presence of a colloidal dispersant like hydroxy cellulose. Solution polymerization of VDF in solvents uses free radical initiators. PVDF is commercially produced by aqueous emulsion or suspension processes [72]. 

PVDF-based polymers dissolve easily in polar solvents such as N-methyl pyrrolidone (NMP) and N,N-dimethylformamide (DMF). Their affinity for solvents with polar groups such as the carbonyl group is high, and they do not swell easily in solvents such as ethanol or hydrocarbons. Their solubility in different solvents and the performance of the solutions are related to the polymerization method. The viscosity of the PVDF obtained by suspension polymerization and its solubility in NMP are higher than those of a product such as Kynar, which is obtained by emulsion polymerization. For some as-yet unknown reason, the PVDF solution obtained by suspension... 

PVDF is a linear partially fluorinate polymer containing 59.4 wt% fluorine and 3wt% hydrogen. The high level of intrinsic crystallinity, typically near 60%, provides stiffness, toughness, and creep resistant properties. Incorporation of various fluorinated comonomers at low levels, typically about 5-20 wt%, enhances flexibility and clarity of PVDF by reducing the crystallinity, which in turn reduces the end use temperature rating. PVDF is commercially produced via free radical polymerization either with emulsion or suspension processes. 

PVDF is correctly named poly(l,l-difluoroethylene) and represented by (-CF2CH2-)n- It is a hard, tough thermoplastic fluoropolymer. PVDF is prepared by free-radical initiated polymerization, either in suspension or (usually) in emulsion systems. The basic raw material for PVDF is vinylidene fluoride (CH2=CF2), a preferred synthesis of which is dehydrochlorination of chlorodifluoroethane. 

The PVDF can be polymerized by free radical initiated polymerization in suspension or emulsion (2). Because it offers inherent property advantages to the resin, KYNAR polymers are made using the emulsion technique. The schematic outline of the manufacturing process is shown in Figure 2. The process is based on radical initiated polymerization in emulsion. The polymer recovery and work up follows well known techniques used by the plastics industry. 

Poly(vinylidene fluoride), PVDF or PVF2, is usually manufactured from radical initiated batch polymerization process in aqueous emulsion or suspension of CH2=CF2 monomer. PVDF is a thermoplastic that exhibits interesting properties, such as piezoelectric, pyroelectrical, and ferroelectric behaviors. PVDF has even superior dielectric permittivity arising from the strong polymerization originating from C—F bonds, and the spontaneous orientation of dipoles in the crystalline phases makes it a polar polymer with good compatibility with polar chemicals. 

TFE and VDF can be polymerized in any proportion, thus giving rise to a broad variety of products. The copolymers are obtained in either emulsion or suspension process. An approximate composition of TFE PVDE of 20 80 is the eutectic point in the system, with a melting point of 120 ° C. It is widely used as a technical polymer and is available under the brand names Kynar 7200 and Kynar SL (Pennwalt Corporation). A composition with the an approximate composition of TFE PVDF 29 71) is available as Fluoroplast 42 (Russia). Both copolymers are soluble in ketones and esters but are insoluble in alcohols and chlorinated hydrocarbons and are mainly processed via melt extrusion at temperatures between 190 and 260 °C [27, 28]. The low-wave-number FTIR spectrum of the copolymer is depicted in Figure 3.13. 

The first successful aqueous polymerization of vi-nylidene fluoride was reported in 1948 [83] using a peroxide initiator in water at 50-150°C and 30 MPa. No surfactants or suspending agents were present in the polymerization recipe. PVDF has been polymerized by a number of methods including emulsion, suspension, solution, and bulk. Later, copolymers of vinylidene fluoride with ethylene and halogenated ethylene monomers were also produced [84]. In 1960, a manufacturing process was developed and PVDF was introduced to the market. 

Many analogies exist between PVDF and PVDC at the structural level. However, the strong energy of C-F bonds reduces the possibility to form branches, and thus this polymer is mainly linear. It is obtained by radical polymerization of vinyUdene fluoride monomer (VDF) in emulsion or in suspension. Because the monomer is a gas (Tb = -84°C), the polymerization is carried out under a pressure of about 10 MPa and at 80°C. However, temperatures and pressures higher or lower than this one can also be used, with the choice being determined by the use of a given initiator—that is, by a redox system or a peroxide. 

Comparison of PVDF Manufactured by Emulsion and Suspension Polymerization... 

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