Abstract
A polyethylene coating process that uses a flat-die extrusion method has been developed by Nippon Steel Corporation and others to manufacture polyethylene coated large-diameter steel pipes.
They are used in pipelines for the long distant transportation of natural gas or oil.
In this process, the molten polyethylene sheet is wound around the pipes as they rotate and advance on the skew rolls. The new developed process is able to meet changes in pipe diameter and coating thickness with a single size of die. The properties of molten polyethylene give an important influence on the good appearance and uniformity of the polyethylene coating. Accordingly, it is necessary to use polyethylene with a high melt strength, a low melt index and a low shrinkage stress.
Furthermore, rapid cooling after the polyethylene coating is necessary to guarantee excellent impact resistance of the coated pipes at low temperatures.
