Method for extrusion coating of a pipe with a foamed cover layer having

Abstract

A method for manufacturing a heat insulating pipe having a foamed insulating layer around the outer surface of said pipe. The foamed insulating layer has an expansion ratio of at least 4. It has three integral sub-layers, said sub-layers including a cellular middle foamed sub-layer which is integral with a cellular inner foamed sub-layer and a cellular outer foamed sub-layer, said inner and outer sub-layer being on opposite sides of said middle sub-layer, said cellular inner foamed sub-layer being the innermost sub-layer with respect to said pipe and surrounding the outer surface of said pipe. The cells of said inner and outer sub-layers are smaller than the cells of said middle cellular sub-layer, the ratio .mu..sub.1 /.mu..sub.2 of the cell dimensions of said cellular middle sub-layer are between 0.3 and 0.9, wherein .mu..sub.1 is the average cell dimension measured in the radial direction of said pipe and .mu..sub.2 is the average cell dimension measured in the axial direction of said pipe. The method for producing said 3 sub-layered foamed insulating layer comprises passing a pipe heated to a temperature of between 60.degree. C. and 120.degree. C. through the nipple of a cross-head die and simultaneously extruding a mixture of a propylene-ethylene block polymer having an ethylene content of between 1 and 20% by weight and a Melt Index of between 0.1 and 4 together with a foaming agent over the outer surface of said pipe to cause said mixture to expand and form said 3 sub-layered foamed insulating layer on said pipe, the relationship between the overall thickness t of the 3 sub-layered foamed insulating layer, the expansion ratio M of the 3 sub-layered foamed insulating layer, and the die-nipple distance d of a cross-head die being ##EQU1## wherein K=0.75 to 0.20.