Process for extrusion

Abstract

A process for the extrusion of polyethylene having a broad molecular weight distribution wherein the polyethylene is prepared in pellet form, said extrusion taking place in a pelletizing extruder having one or more zones essentially filled with polyethylene and two or more zones partially filled with polyethylene including a feed zone comprising (i) introducing the polyethylene into the extruder at a temperature sufficient to melt the polyethylene; (ii) introducing a mixture of an inert gas and oxygen into at least one partially filled zone with the exception of the feed zone, said mixture containing about 1 to about 21 percent by volume oxygen based on the volume of the gaseous mixture; (iii) passing the molten polyethylene through each zone at melt temperature; and (iv) extruding the polyethylene into pellets and cooling same.

Description

[0001] This application is a continuation-in-part of application Ser. No. 09 / 022,793 filed on Feb. 13, 1998, which is a continuation of application Ser. No. 08 / 671,166 filed on Jun. 26, 1996.[0002] This invention relates to a process for the extrusion of polyethylene into pellets useful in the production of blown film.BACKGROUND INFORMATION[0003] Ziegler-type catalysts have undergone development over the years to improve the economy and quality of various polyethylene products. This development has tended towards narrowing the molecular weight distribution of the resin; however, narrow molecular weight distributions are not desirable for blown film resins, i.e., the narrow molecular weight distribution does not improve the processability of the resins into blown film in terms of bubble stability. Rather, bubble stability is a characteristic of broad molecular weight distribution resins.[0004] A particularly good technique for producing broad molecular weight distribution polyethylen...

AI Technical Summary

Problems solved by technology

This development has tended towards narrowing the molecular weight distribution of the resin; however, narrow molecular weight distributions are not desirable for blown film resins, i.e., the narrow molecular weight distribution does not improve the processability of the resins into blown film in terms of bubble stability.

While the resins produced by these two stage processes can be turned into products having superior mechanical strength and other advantageous physical and chemical characteristics, the resins generally do not achieve a high level of bubble stability.

Unfortunately, excessive amounts of free radical generators can cause chain scission, which is characterized by a rupture of chemical bonds in the backbone and side chains of the polymer.

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