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Preparation of trifluorochloroethylene polymer
Films based on ethylene/trifluoroethylene polymers are known in the prior art. US4623670 (ASAHICHEMICAL IND [JP] (Japan Asahi Kasei Industry Co., Ltd.)) 18/11/1986 and US 4702836 (ASAHICHEMICAL IND [JP] (Japan Asahi Kasei Industry Co., Ltd.)) 27/10/1987 disclose a method for preparing a film made from a fluoropolymer comprising a copolymer of ethylene and trifluorochloroethylene, the method comprising extruding the plasticizing compound into a pellet by volume;
The pellets are then melt-moulded into products such as hollow fibres; the plasticiser is removed from the moulded product by a first extraction; the inert compound is then removed by a second extraction with a strong acid or a strong base. This method has several disadvantages: the polymer needs to be worked at temperatures between 230°C and 250°C during the plasticising and moulding steps; the extensive use of chlorotrifluoroethylene oligomer plasticisers, which are removed with chlorinated solvents such as 1,1,1-trichloroethane; the addition of inert compounds during the moulding step to give the polymer sufficient mechanical properties, and the fact that these compounds are also removed by extraction. This method is therefore complex and expensive.
Vinyl chloride trifluoride is in particular an ethylene/vinyl chloride trifluoride polymer melt-blended with a high-boiling substance used as a plasticiser (e.g. trinonyl trimellitate); the product is optionally formed by moulding or melt-spinning in the presence of a support; it is cooled to induce film formation and then extracted to remove the high-boiling plasticiser. Wherein the mixture of polymer and solvent (e.g. glyceryl triacetate or acetyl citrate) is prepared at temperatures in excess of 200°C; the blends are moulded into suitable shapes, cooled until the polymer is cured and then the solvent is extracted from the cured film.
Diffusion-induced precipitation of vinyl chloride trifluoride, also defined as "diffusion-induced phase separation", is a well known method of preparing polymeric films by dissolving the polymeric material in a suitable solvent to form a solution, casting the film layer and then immersing it in a non-solvent. The contact between the film-forming layer and the non-solvent results in the formation of two phases. A polymer-rich solid phase that forms the membrane structure and a solvent-rich liquid phase that forms the pores of the liquid-filled membrane.
The diffusion-induced precipitation method does not require the polymer to be operated at temperatures above 200°C, i.e. close to the temperature at which the polymer may start to degrade. The method does not require the use of a plasticiser for vinyl chloride trifluoride, which is extracted with a suitable solvent and then recovered at the end of the film production process. The solvents generally used can be used to prepare the polymer solution and water can be used as a non-solvent to induce precipitation of the polymer from the solution while removing the solvent.
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