Patent ID: 7462307

Claim:
A process for producing polyolefin foam coated steel pipes with improved compression resistance, comprising a steel pipe core, an intermediate polyolefin foam layer and an unfoamed outer polymer layer, comprising coating by extruder/rotating steel pipe processing or crosshead die pipe coating processing, the polyolefin foam layer having a density of from 400 to 850 kg/m 3 and the polyolefins for the polyolefin foam coating being mixtures of A) 5 to 80% by weight of a compound A, which is selected from the group consisting of modified propylene polymers with a melt index of 0.05 to 10 g/10 min at 230° C./2.16 kg, said modified propylene polymers having strain hardening behaviour, and mixtures of said modified propylene ploymers; and B) 20 to 95% by weight of a compound B which is selected from the group consisting of propylene homopolymers with a stereospecifity index >98% and a melt index of 0.05 to 10 g/10 min at 230° C./2.16 kg, copolymers from 80.0 to 99.9% by weight of propylene and 0.1 to 20.0% by weight of ethylene or α-olefins with 4 to 18 carbon atoms, with a stereospecifity index of the propylene homopolymer matrix of >96% and a melt index of 0.1 to 10 g/10 min at 230° C./2.16 kg, and mixtures of said propylene homopolymers and/or copolymers, wherein the melt of said mixtures in the foam coating steel pipe process contains up to 12% by weight, based on the polyolefin mixture, of chemical blowing agents that split off gas, or hydrocarbons, halogenated hydrocarbons and/or gases as blowing agents and the steel pipes are preheated to a temperature ranging from 170 to 230° C. and the coating extruder has a temperature profile ranging from 175 to 250° C.