Extrusion of peroxide crosslinkable polymer parts
A process for the extrusion of peroxide crosslinked polymer parts, mainly tubes, in which the crosslinkable polymer is heated in an extruder in a controlled manner to a value above the crystallite melting point of the polymer, but below the crosslinking temperature, and fed continuously to an extrusion die to create the part and in which at least a partial crosslinking of the polymer is achieved in the extrusion die by heating of the polymer above a crosslinking temperature provision.
1. A method for extruding a peroxide crosslinked polymer tube, comprising:
supplying a mixture to a screw extruder, the mixture comprising: a crosslinkable polymer, a crosslinking agent, and a stabilizing agent, wherein the polymer has a crystallite melting point and a crosslinking temperature, and wherein the screw extruder is a conically tapered screw extruder having at least two sections of differing spiral pitch;
heating the mixture in the screw extruder with an external heating unit to a temperature above the crystallite melting point but below the crosslinking temperature;
controlling the temperature of the mixture in the screw extruder with the external heating unit and an internal cooling unit;
continuously feeding the mixture from the screw extruder to an extrusion die, wherein a melting pressure before entry to the extrusion die is approximately 700-1500 bar;
heating the mixture in the extrusion die above the crosslinking temperature to effect at least a partial crosslinking of the polymer in the extrusion die, wherein the temperature (° C.) of the mixture in the extrusion die is at least 15% above the crosslinking temperature (° C.); and
discharging the mixture from the extrusion die, wherein the degree of crosslinking of the polymer on discharge from the extrusion die is above 60%.
2. The method of claim 1 , wherein the crosslinking agent comprises organic peroxide.
3. The method of claim 1 , wherein the melting pressure before entry to the extrusion die is approximately 1200 bar.
4. The method of claim 1 , wherein said extruder is a twin screw extruder.
5. The method of claim 1 , wherein the temperature of the polymer in the extrusion die is achieved by heating the extrusion die externally.
6. The method of claim 1 , wherein the temperature of the polymer in the extrusion die is achieved by the induction of heat from the interior of the extrusion die.
7. The method of claim 1 , wherein the temperature (° C.) of the polymer in the extrusion die is not higher than 60% above the crosslinking temperature (° C.) of the polymer.
8. The method of claim 1 , wherein the temperature (° C.) of the polymer before entering the extrusion die is not higher than 30% above the crystallite melting point (° C.) of the polymer.
9. The method of claim 1 , wherein the crosslinking temperature (° C.) of the polymer is approximately 30% above the crystallite melting point (° C.) of the polymer.
10. The method of claim 1 , wherein the crystallite melting point of the polymer is approximately 125-140° C.
11. The method of claim 1 , wherein the crosslinking temperature of the polymer is approximately 165-185° C.
12. The method of claim 1 , wherein the tube is maintained at a temperature above the crosslinking temperature after discharge from the extrusion die.
13. The method of claim 1 , wherein the tube is cooled after crosslinking.