A process for treating an impure thermoplastic polymer, including raw and recycled polymers, during extrusion to separate low molecular weight compounds therefrom, including: introducing a separating agent into plasticized impure thermoplastic polymer passing through a first high pressure zone, the low molecular weight compounds of the plasticized polymer being at least partially soluble in the separating agent; extracting at least a part of the low molecular weight compounds from the plasficized polymer into the Separation agent during passage of the plasticized polymer through an extraction zone; removing the separating agent charged with low molecular weight compounds from the extruder as the plasticized polymer passes through a low pressure zone having a residual pressure, P, which is lower the introductory pressure, Pe, and for which 1 bar >=P>=Pc; collecting the charged separating agent as it is removed from the extruder; thermal vacuum treating the plasticized polymer as it passes through a last low pressure zone to vacuum degas the plasticized polymer at a pressure, Pr for which Pr<0.5 bar and thereby further remove separating agent and low molecular weight compounds from the plasticized polymer; and passing the plasticized polymer through a last high pressure zone to an extruder head. The combination of solvent extraction in the extruder with subsequent vacuum degassing results in removal of large amounts of low molecular weight compounds with a relatively small expenditure of energy since energy-intensive vacuum degassing is needed only for removal of residuals.

A raw polyamide-6 with a caprolactam content of 9.3 weight % is purified in an equal-twist twin-screw kneader, Model ZSK 40, in accordance with FIG. 2 by means of solvent extraction with water. At a flow rate of 22 kg/h of the mixture of substances, 8 kg/h of water are supplied to the extruder at a pressure of 4 bar absolute via the steam feed line 19 and removed to a great extent from the first low pressure Zone 5 at a pressure of 3 bar absolute. A second extraction follows the first extraction. In the process, water is supplied to the extruder via the steam feed line 33 at a pressure of 2 bar abs., whereupon the added water flows opposite the polymer melt, in the process is increasingly charged with residual contaminants to be extracted and is removed from the second low pressure zone 25 at atmospheric pressure. A portion of the added water is absorbed by the polymer melt and, together with the possibly remaining small amounts of contaminants, is removed in the succeeding last low pressure zone 9 at a vacuum of 1 mbar. Subsequently, the purified polymer melt 24 is pelletized and analyzed as to caprolactam content. The caprolactam content could be reduced to 640 mg/kg, corresponding to a 99.3% reduction of caprolactam.

In accordance with the invention, the extracted Caprolactam always remains dissolved in the aqueous phase and does not lead to obstructions of pipelines. Accordingly, a trouble-free, reliable, continuous operation is assured.