Copolymers of Fluoroolefins
Mary different copolymers of fluoroolefins are possible and were reported in the literature. Commercial use of fluoroolefin copolymers, however, is restricted mainly to elastomers. Such materials offer superior solvent resistance and good thermal stability. The elastomers that are most important industrially are vinylidine fluoride–chlorotrifluoroethylene [260] and vinylidine fluoride–hexafluoropropylene copolymers [282]. These copolymers are amor phous due to irregularities in their structures and can range in properties from resinous to elastomeric, depending upon composition [283]. Those that contain 50–70 mole percent of vinylidine fluoride are elastomers. The Tg ranges from 0 to 15C, also depending upon vinylidine fluoride content [284]. They may becross-linked with various peroxides, polyamines [284], or ionizing radiation. The cross linking reactions by peroxides take place through hydrogen abstraction by primary radicals:
Copolymers of vinylidine fluoride with hexafluoropropylene are prepared in aqueous dispersions using persulfate initiators. Hexafluoropropylene does not homopolymerize but it does copolymerize. This means that its content in the copolymer cannot exceed 50%. Preferred compositions appear to contain about 80% of vinylidine fluoride. The cross-linking reactions with diamines are not completely understood. It is believed that the reaction takes place in two steps [285, 286]. In the first one, a dehydrofluorination occurs:
The above elimination is catalyzed by basic materials. These may be in the form of MgO, which is often included in the reaction medium. In the second step the amine groups add across the double bonds:
Free diamines, used for cross-linking, are too reactive and can cause premature gelation. It is common practice, therefore, to add these diamine compounds in the form of carbamates, like ethylenediamine carbamate or hexamethylene diamine carbamate. The above fluoro elastomers exhibit good resistance to chemicals and maintain useful properties from 50 to +300C.
Copolymers of tetrafluoroethylene with hexafluoropropylene are truly thermoplastic polyperfluoroole fins that can be fabricated by common techniques. Such copolymers soften at about 285C and have a continuous use temperature of 260 to +205C. Their properties are similar to, though somewhat inferior to, polytetrafluoroethylene.
