Crosslinking of Vinylidene Fluoride-Containing Fluoropolymers

Fluoropolymers are well-known for their good properties in terms of chemical, thermal and electrical stabilities, inertness to acids, bases, solvents and oils, and high resistance to ageing and oxidation. Polyvinylidene fluoride (PVDF) is useful as a homopolymer endowed with interesting characteristics. It contains a high crystallinity rate, but is base sensitive. In addition, VDF can be co- or terpolymerized with several fluorinated monomers, rendering them suitable as elastomers and various examples of synthesis of VDF-copolymers are also presented. This review also focusses on binary and tertiary systems containing VDF. Several curing systems for these VDF-containing copolymers have been investigated, especially diamines and their derivatives, aromatic polyhydroxy compounds, peroxides with coagents, such as triallylisocyanurate, radiations, and thiol-ene systems. The best vulcanizate properties are obtained by a two-step process. First, the material is press cured at different times and temperatures, then, it is post cured in air or under nitrogen at higher temperature and time, and under atmospheric pressure. Poly(VDF-co-HFP) copolymers can react with primary, secondary or tertiary monoamines, but they are mainly crosslinked by diamines such as hexamethylene diamine (HMDA), their carbamates (HMDA-C), and derivatives. A mechanism of crosslinking is identified by Infrared and 19F NMR spectroscopies, and was evidenced to proceed in three main steps. First, a VDF unit undergoes a dehydrofluorination in the presence of the diamine, then the Michael addition occurs onto the double bonds to form crosslinking, while HF is eliminated from crosslinks in the presence of HF scavengers. The crosslinking mechanism with bisphenols takes place also in three main steps (dehydrofluorination, then substitution of a fluorine atom by a bisphenol, and elimination of HF). The most efficient crosslinking bisphenol is bisphenol AF . A fluoropolymer crosslinked with peroxide/coagent systems needs to be functionalized or halogenated to insure a free radical attack from peroxide. The peroxide is introduced with a coagent that enhances the crosslinking efficiency, and the most efficient one is triallylisocyanurate (TAIC). The crosslinking mechanism of the peroxide/triallylisocyanurate system proceeds in three main steps. The crosslinking reaction occurs from a macroradical arising from the functional or halogenated polymer which is added onto the three double bonds of the TAIC. A fourth way to crosslink VDF-based fluoropolymers deals with high energy radiation, such as X and γ (60Co or 137Cs)-rays, and charged particles (β-particles and electrons). Three different reactions are possible after irradiation of a PVDF, and the one that leads to crosslinking is the recombination between two macroradicals. The irradiation dose used on the VDF-based copolymer has an influence on the thermal and mechanical properties. Finally, a crosslinking system also used to vulcanizate hydrogenated elastomers concerns a thiol-ene system which requires a mercapto function born by the VDF-based polymer. Crosslinking occurs via a non-conjugated diene. The mechanical properties (tensile strength, elongation at break, hardness, elongation modulus, compression set resistance …) of the three main crosslinking systems of fluoroelastomers are compared. Finally, the main applications of crosslinked VDF-based fluoropolymers are summarized which include tubing in the aircraft building industry, sealing, tube or irregular-profile items of any dimension, films with good adhesion to metallic or rigid surfaces, multilayer insulator systems for electrical conductors, captors, sensors, and detectors, and membranes for electrochemical applications.