Perfluoropropylvinyl ether

Commercial polymer with an overall CTFE-to-ethylene ratio of 1 1 contains ethylene blocks and CTFE blocks of less than 10 mol% each. The modified copolymers also produced commercially exhibit improved high-temperature stress cracking. Typically, the modified copolymers are less crystalline and have lower melting points.88 Modifying monomers are hexafluoroisobutylene (HFIB), perfluorohexylethylene, and perfluoropropylvinyl ether (PFPVE).89... 

The need for highly fluorinated thermoplastic polymers that, unlike PTFE, could be fabricated by conventional melt-processing methods led to the development of a group of resins that are copolymers of tetrafluoroethylene (TFE) with other perflu-orinated monomers. Commercially, the copolymer of TFE and hexafluoropropylene (HFP) is commonly known as fluorinated ethylene propylene (FEP). Copolymerization of TFE with perfluoropropylvinyl ether (PPVE) leads to PFA resins, and copolymerization of TFE with perfluoromethylvinyl ether (PMVE) produces MFA resins. 

Modified PTFE represents a relatively new technology that is designed to overcome the limitations of conventional PTFE, namely, poor creep resistance (i.e., tendency to cold flow), difficult welding, and high level of microvoids [35], These changes of properties are accomplished by addition of small amount comonomer in amounts less than 0.1%. Such modifier is most frequently perfluoropropylvinyl ether (PPVE). The copolymerization is carried out in aqueous suspension under practically the same conditions as the homopolymerization of tetrafluoroethylene, that is, at TEE pressures in the range 5 to 20 bar (72 to 290 psi) and temperature range 35°C to 90°C (95°E to 194°E) [36]. There is more on modified PTFE in Section 7.7. 

Perfluoroalkylvinyl ethers form an important class of monomers in that they are used as comonomers for the modihcation of the properties of homofluoropolymers in addition to their broad nse in copolymers with TFE and other monomers. They are capable of snppressing the crystallization of PTFE efficiently, which imparts usefnl mechanical properties to lower molecular weight of polytetrafluoroethylene polymers. Copolymers of PAVEs and tetrafluoroethylene are thermally stable as PTEE homopolymers. Commercially significant monomers are perfluoropropylvinyl ether and perflnoromethylvinyl ether (PMVE), used for the production of a variety of perflnoroalkoxy resins. 

Major research efforts resulted in the development of a modified PTFE, which contains a small amount (0.01 to 0.1 mol%) of a comonomer (see Table 7.5). The most suitable comonomer was found to be perfluoropropylvinyl ether (PPVE) [94]. The comonomer reduces the degree of crystallinity and the size of lamellae [95]. The polymerization process is similar to that for standard PTFE except additives to control the molecular weight are used [96]. 

Perfluoropropylvinyl ether (PPVE) Monomer used for the production of perfluoroalkoxy (PFA). 

Specifically mentioned olefinically unsaturated organic molecules are butenenitrile (all isomers), pentenenitrile (all isomers), methyl butenecarboxylate (all isomers), ethyl butenecarboxylate (all isomers), propyl butenecarboxylate (all isomers), butyl butenecarboxylate (all isomers), 2-ethylhexyl butenecarboxylate (all isomers), methyl pentenecarboxylate (all isomers), ethyl pentenecarboxylate (all isomers), perfluoropropylvinyl ether, methyl cinnamate, ethyl cinnamate, propyl cinnamate (all isomers), cinnamonitrile, methylmaleic anhydride, cyclopenten-... 

WAXD scans from unhydrolysed polymer, -SO F, reveal that the polymer is semicrystalline. (Figure 2) This result is similar to the behavior observed in copolymers of tetrafluoroethylene, (TFE) with hexafluoropropylene or perfluoropropylvinyl ether. As the EW increases, the molar ratio of TFE to comonomer increases and the amount of crystallinity also increases. The amount of crystallinity, deduced from the relative intensity of the amorphous halo and crystalline peak, ranges between 0 and 40%, however, these values represent qualitative estimates rather than quantitative results. 

Perfluroalkoxy polymer or PFA is one of the most important meltprocessible fluoroplastics due to its relative ease of processing and high service temperature equivalent to polytetrafluoroethylene (260°C). It also has the same excellent chemical resistance and low friction properties as PTFE. Perfluroalkoxy polymers are prepared by copolymerization of a perfluoroalkylvinyl ethers (Rf—O—CF=CF2, where Rj is a perfluorinated alkyl group) with tetrafluoroethylene. Examples of commercially utilized ethers include perfluoromethyl-vinyl ether (CFg—O—CF=CF2), perfluoroethylvinyl ether (C2F5—O—CF=CF2) and perfluoropropylvinyl ether (C3F7—O—CF=CF2). Several percent of ether is incorporated in a copolymer. 

A jacketed stirred autoclave was the polymerization vessel, which was evacuated prior to charging with the ingredients. Freon 113 (1,1,2-trichloro-1,2,2-trichloroethane) and perfluoropropylvinyl ether were first loaded in the vessel. The mixture was heated to the reaction temperature, followed by introduction of tetrafluoroethylene. Finally, the initiator solution was eharged to the autoclave. Reactor temperature was maintained by circulating water in the jacket. Pressure was kept constant by TFE addition. At the end of the reaetion, the autoclave was vented and the contents were dried at a temperature of200°C under vacuum (1 mm Hg) for one hour. 

Table 5.8 and Fig. 5.1 illustrate the effect of hexafluoropropylene concentration in the monomer mixture on the incorporation of PPVE. The incorporation of PPVE is at a low 1.1 % by weight without the addition of HFP. The extent of incorporation of perfluoropropylvinyl ether increases slowly with the addition of HFP. It accelerates significantly beyond 4% hexafluoropropylene in the ternary monomer mixture up to a point (between 14.7 and 23.3% HFP) and then begins to decrease. The melting point decreases with HFP and begins to increase in this same range. 

Table 5.8. Effect of Hexafluoropropylene on Perfluoropropylvinyl Ether Incorporation in Terpolymers of Tetrafluoroethylene ...

A terpolymer of tetrafluoroethylene, hexafluoro-propylene, and perfluoropropylvinyl ether was re-ported " ] to have superior stress crack resistance than TFE/HFP copolymer. The terpolymer in this development was prepared by the nonaqueous polymerization process described in US Patent numbers 3,528,954 and 4,029,868.In this procedure a halogenated solvent, in which perfluoropropylvinyl ether and a chain transfer agent had been dissolved, acted as the polymerization medium. Methanol was a common example of an effective chain transfer agent. Polymerization was carried out in a stainless steel pressure vessel. The polymer contained 0.2-2% perfluoropropylvinyl ether and 9-17% hexafluoropropylene. 

Table 5.64. Copolymerization of Tetrafluoroethylene and Perfluoropropylvinyl Ether in Supercritical Carbon Dioxide ...

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