| 摘要 |
PROBLEM TO BE SOLVED: To obtain a tubular foam which is excellent in heat resistance and heat- insulating properties and is usable e.g. as a heat-insulating material for pipe lines used at high temps. by extruding a PP resin having special viscoelastic characteristics while controlling the extrusion shear rate at an extrusion slit to within a specified range. SOLUTION: This tubular foam is produced by a noncross-linking extrusion molding process, i.e., by feeding a PP resin and a nucleating agent to an extruder having a circular slit at the exit of the extrusion channel, forcing and homogeneously mixing a volatile blowing agent into the molten resin in the extruder under a high pressure, extruding the resulting mixture tubularly into a low-pressure zone to allow the mixture to foam, and cooling and solidifying the resulting foam while taking it off, subject to the conditions that the PP resin fed exhibits a biaxial-elongation viscosity of 4.5×10<6> P or higher at a biaxial-elongation strain of 0.2 and a biaxial-elongation strain hardening ratio of 0.3 or higher and that the extrusion shear rateγat the slit is 400-1,000sec<-1> . The obtd. tubular foam has a density of 0.015-0.04g/cm<3> , a wall thickness of 7-20mm, a closed cell content of 70-100%, and a circularity at the outer periphery of 1.0-1.3.
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