Process for preparing fluoropolymer dispersions

A process for preparing fluoropolymer dispersions based on tetrafluoroethylene (TFE) homopolymers or TFE copolymers that are substantially anionic fluorinated surfactant free, comprising the following steps:

• step a1) to a fluoropolymer dispersion obtainable by an emulsion or microemulsion polymerization process it is added:
• a nonionic surfactant having cloud point (CP) between 40° and 80° C., in an amount from 1.5% to 50% by weight with respect to the fluoropolymer of the dispersion;
• step a2) heating of the dispersion obtained in step a1) up to a temperature Tc in the range CP±10° C., CP being the cloud point of the used nonionic surfactant;
• step a2″) decantation in a reactor at the temperature Tc, until obtaining the separation of the dispersion in:
• a lower aqueous phase formed of the concentrated fluoropolymer dispersion, wherein the fluoropolymer concentration is in the range 40%-75% w/w, an upper aqueous phase (surnatant) substantially fluoropolymer free;
• step a3) washing of the decanted dispersion obtained in a2″) with a solution of water and nonionic surfactant;
• wherein the concentration by weight of the surfactant in the solution ranges from 0.5% to 50% by weight with respect to the dispersion;
• the washing solution comprising water and surfactant is continuously input into the reactor from its lower part, the rate of the washing solution in the reactor from the bottom upwards being lower than the decantation rate of the fluoropolymer particles;
• in an exchange zone the washing solution is in countercurrent with respect to the decantation motion of the fluoropolymer particles, the amount of the washing solution introduced into the reactor, referred to the amount of fluoropolymer present in the reactor, is higher than 0.5 liters of washing solution/kg of fluoropolymer;
• the temperature Tc at which the process is carried out, and the input rate of the washing solution must be such as to maintain substantially unchanged the separation of the two phases, obtained in step a2″) by decantation, during all the subsequent process steps;
• step a4) the upper aqueous phase is continuously removed from the upper part of the reactor;
• step a5) discharge and recovery of the lower phase containing the fluoropolymer dispersion, the discharge taking place when from the upper part of the reactor at least the amount of washing solution fed in step a3) flows out;
• optionally when the discharge starts, additional dispersion can be fed repeating step a1) and a2).