Process for producing a polyethylene composition comprising bimodal or multimodal polyethylene

A process for preparing a polyethylene composition comprising bimodal or multimodal polyethylene in an extruder device comprising the steps of

• a) supplying a bimodal or multimodal polyethylene having a density determined according to DIN EN ISO 1183-1:2004 at 23° C. from 0.940 g/cm3 to 0.968 g/cm3, in the form of a polyethylene powder to a mixing device;
• b) supplying an additive, which is not an organic peroxide, to the mixing device;
• c) mixing the polyethylene powder and the additive at a temperature in the range from 10° C. to 100° C. without melting the polyethylene powder, thereby forming a mixture;
• d) transferring the mixture from the mixing device into the extruder device;
• e) melting and homogenizing the mixture within the extruder device, thereby forming a molten polyethylene composition;
• and
• f) pelletizing the molten polyethylene composition, and
• further adding a liquid comprising an organic peroxide to (i) the polyethylene powder or (ii) the mixture comprising the polyethylene powder, in an amount from 20 ppm to 200 ppm of organic peroxide with respect to the polyethylene powder, wherein the liquid is added at a position where mixing occurs and the polyethylene powder has a temperature of from 10° C. to 100° C.