Fluoropolymers stabilizers

Vinyl ethers constitute a third class of monomers which have been cationically polymerized in C02. While fluorinated vinyl ether monomers such as those described in Sect. 2.1.2 can be polymerized homogeneously in C02 because of the high solubility of the resulting amorphous fluoropolymers, the polymerization of hydrocarbon vinyl ethers in C02 results in the formation of C02-insoluble polymers which precipitate from the reaction medium. The work in this area reported to date in the literature includes precipitation polymerizations and does not yet include the use of stabilizing moieties such as those described in the earlier sections on dispersion and emulsion polymerizations (Sect. 3). 

The heat resistance and chemical resistance of the fluoropolymers is mainly dependent on the extent of fluorination and stability of the crosslinks. For example, most fluorocarbons have fluorine contents of 50-70%, more chemically resistant types 65-69%. For comparison, fluorosilicones contain about 36% fluorine. 

This research was an attempt to develop new polymers with the mechanical properties of polyarylene ethers and the dielectric properties of fluoropolymers. After initially testing the viability of the [2n+ 2n] cyclodimerization reaction for preparing high-molecular-weight polymers and testing the dielectric properties of these polymers, two polymers (one thermoplastic and one thermoset) were prepared in larger quantities to evaluate the thermal and mechanical performance of these novel compositions. The high Te thermoset was also quantitatively tested for thermal/oxidative stability. 

From this point of view attaching of functional groups to common inexpensive fluoropolymers seemed to be a very attractive way to improve the chemical and thermal stability of the final functional polymer materials in comparison with common hydrocarbon functional polymers, while keeping the costs quite reason-... 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Arguments similar to those stated above can be used to explain the relative chemical inertness of fluoropolymers. Consider the reactivity of alkanes vs. perfluoroalkanes as shown in Table 4.2 (abstracted from Sheppard and Sharts Statistically, FA based materials will have many more types of bonds, in addition to C—F, than fluoropolymers. These bonds will be subject to the same chemical fate during assault by aggressive reagents as bonds in their hydrocarbon counterparts. Similar reasoning can be used to explain the relative thermal stability of FAs compared to fluoropolymers. Thus, incorporation of perfluoroalkyl groups will not make the modified material less stable than the native one. 

All of the unique properties imparted by fluorocarbons can be traced back to a single origin the nature of the C—F bond. These properties include low surface tension, excellent thermal and chemical stability, low coefficient of friction, and low dielectric constant. However, not all of these properties are possessed by the entire inventory of available fluorocarbons. The fluorocarbons can be assigned to two major categories (1) fluoropolymers, which are materials that are comprised mainly of C—F bonds and include such examples as PTFE, and (2) fluorochemicals (FA) based on the perfluoroalkyl group, which are materials that generally have fewer C—F bonds and often exist as derivatives of other classes of molecules (e.g., acrylates, alcohols, esters). In addition, the properties that dictate the uses of fluorocarbons can be classified into (1) bulk properties (e.g., thermal and chemical stability, dielectric constant) and (2) surface properties (e.g., low surface tension, low coefficient of friction). The types of materials available and properties imparted are not exclusive and overlap substantially. From this array of fluorocarbons and attributes, a large variety of unique materials can be constructed. 

The last 20 years have seen enormous progress in the development of high-performance fluoropolymers. Fluorine-containing polyimides stand out as one of the few types of materials that simultaneously possess outstanding thermal stability and mechanical properties, low dielectric permittivity, and thin-film processability. This combination of properties makes them ideal for use as high-performance insulators in electronic devices. 

Poly(vinylidene fluoride) (PVDF) is the second most important thermoplastic within the fluoropolymer family after PTFE. Although, both the thermal and chemical stability of PVDF are somewhat lower compared to PTFE, the hydrogenated polymer can be more easily processed with conventional equipment, and it offers an advantageous compromise between quality and price. When the... 

In general, fluoropolymers possess the unique combination of high thermal stability, chemical inertness, unusual surface properties, low dielectric constants and dissipation factors, low water absorptivities, excellent weatherability and low flammabilities. Therefore there appears to be an ever-increasing market for fluoropolymers in spite of their relatively high cost [211,212],... 

Ceramic materials, including sapphire, have been used extensively in HPLC pumps for more than 20 years as pistons and check valve components. These materials have also been used to construct heads because of their good chemical stability. The use of ceramics is limited, however, because of high cost and brittleness. Although many systems have one material as the primary construction material, the wetted surfaces of a pumping system can contain several other materials. Therefore, for material-sensitive applications, all the materials in the HPLC eluent flow path should be considered. Materials that may be encountered are polymeric materials for pump seals such as fluoropolymers, polypropylene, and Teflon sapphire pump pistons and check valve seats ruby check valve balls Kalrez, KelF, or ceramic washers and spacers polymer-based transducer components and in older systems connections and joints made with silver solder. 

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