Method for the separation of volatile compounds from viscous products by means of a thin-film evaporator, and polylactide resin

A method for removing compounds in gaseous aggregate state from a polymer melt containing polylactide and/or a poly(co)lactide, said melt comprising said compounds in liquid or solid aggregate state, with a thin-film treatment apparatus comprising:

• a housing having a heatable and/or coolable housing jacket which surrounds a rotationally symmetrical treatment chamber extending in an axial direction,
• at least one inlet port, arranged in an inlet region of the housing, for introducing material to be treated into the treatment chamber,
• at least one outlet port arranged in an outlet region of the housing, for discharging the material from the treatment chamber, and
• a drivable rotor shaft arranged in the treatment chamber and extending coaxially therewith, for generating a film of the material on an inside face of the housing jacket and for conveying the material from the inlet region to the outlet region, the rotor shaft comprising a central rotor shaft body with rotor blades arranged on the periphery of said body, radially outermost ends of said blades being spaced a distance from the inside face of the housing jacket, and
• at least one temperature sensor arranged on the rotor shaft in the treatment chamber for measuring the temperature of the film of material, wherein the at least one temperature sensor is positioned on the rotor shaft for submersion in the film during operation of the thin-film treatment apparatus, the method comprising:
• carrying the polymer melt in a viscous aggregate state having a viscosity in a range of 1 to 50,000 Pa·s into the treatment chamber via the at least one inlet port,
• generating a film of the polymer melt on the inside face of the housing jacket,
• converting at least a portion of the compounds present in the liquid or solid aggregate state in the polymer melt into the gaseous aggregate state,
• discharging at least a portion of the compounds in the gaseous aggregate state from the thin-film treatment apparatus,
• discharging the polymer melt after treatment in the thin-film treatment apparatus via the at least one outlet port from the thin-film treatment apparatus, and
• determining a local temperature value of the film at one or more location with the at least one temperature sensor that is disposed on the rotor shaft and is submerged in the film.