Degassing method, degassing device and use of screw elements

A process for removing volatile compounds from a polymer-containing medium which contains at least one polymer and at least one volatile compound, the process comprising:

• a) introducing the polymer-containing medium into a devolatilisation apparatus comprising:
• a barrel and n barrel holes Bn having associated hole diameters Dn where n is an integer,
• one or more screws Wn arranged concentrically in each of the barrel holes Bn and configured to be rotatable within the barrel holes, wherein each screw has an axis of rotation An and is equipped with treatment elements, the treatment elements having a cross-sectional profile orthogonal to the axis of rotation, wherein the cross-sectional profile in a circumferential direction has:
• m relative maxima Rmmax n in respect of a radial dimension of the cross-sectional profile to the axis of rotation An of the screw Wn, where m is an integer from 1 to 8,
• a maximum value Rmax n in respect of the radial dimension of the cross-sectional profile to the axis of rotation An of the screw Wn, where Rmax n fulfils:
• Rmax n<=(Dn/2)
• at least one feed zone,
• one or more devolatilisation zones comprising in each case at least one devolatilisation opening which is suitable for discharge of volatile constituents from a polymer-containing medium from the extruder, and
• at least one discharge zone,
• wherein the treatment elements comprise screw elements having a pitch t, and the screw elements are configured so that each of the following three conditions are met:
• S1) the cross-sectional profile has at least one relative maximum Rmmax n based on the radial dimension of the profile curve, for which:
• 0.420 Dn<Rmmax n<0.490 Dn,
• S2) 1.38 Dn<t<5.00 Dn, and
• S3) the cross-sectional profile of the respective screw element has no tangential angle β greater than 30°, on the active flanks located at the front in the direction of rotation in the range of the radial dimension from 0.95 Rmax to Rmax, where the tangential angle β is defined as
• a smaller of the two angles formed on drawing tangents at any point on the cross-sectional profile of the treatment element at which the cross-sectional profile cannot be continually differentiated, and
• is 0° at any point on the cross-sectional profile of the treatment element at which the cross-sectional profile can always be differentiated;
• and
• b) operating the devolatilisation apparatus to remove volatile compounds from the polymer-containing medium through the devolatilisation openings of the devolatilisation zones to deplete the polymer-containing medium of volatile compounds and isolate polymer as product P from the polymer-containing medium on discharge from the devolatilisation apparatus, wherein the product P has a lower proportion of volatile compounds than the polymer-containing medium introduced into the devolatilisation apparatus.