Abstract: A heat insulating pipe has a foamed heat insulating layer on its outer circumferential face in which the heat insulating layer has an expansion ratio of at least 4 and is of a three-layered structure consisting of a middle foamed layer and lower and upper foamed layers formed at both sides of the middle foamed layer. The cells of the middle foamed layer are relatively large. The cells of the upper foamed layer and the lower foamed layer are relatively small. The shapes of the cells of the middle foamed layer are such that, for the average cell diameter (.mu..sub.1) measured in the radial direction of the pipe and the average cell diameter (.mu..sub.2) measured in the axial direction of the pipe, the relation of .mu..sub.1 /.mu..sub.2 =0.3-0.9 holds.

Abstract: In producing a crosslinked polypropylene foam, 0.5 to 4 parts by weight of a compound selected from triacrylate and trimethacrylate of aliphatic polyhydric alcohol and a blowing agent are mixed with 100 parts by weight of propylene homopolymer or copolymer. Then, the mixture is shaped under heat, followed by irradiation with an ionizing radiation such that the dose of the radiation absorbed by the shaped mass ranges between 0.1 and 10 Mrad. Finally, the irradiated mass is heated for foaming.

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.

Abstract: In producing a crosslinked polypropylene foam, 0.5 to 4 parts by weight of a compound selected from triacrylate and trimethacrylate of aliphatic polyhydric alcohol and a blowing agent are mixed with 100 parts by weight of propylene homopolymer or copolymer. Then, the mixture is shaped under heat, followed by irradiation with an ionizing radiation such that the dose of the radiation absorbed by the shaped mass ranges between 0.1 and 10 Mrad. Finally, the irradiated mass is heated for foaming.

Abstract: A method of crosslinking poly-.alpha.-olefin series resins, comprising the steps of subjecting a resin composition to graft reaction under a volume ratio of oxygen to the resin composition set at less than 0.1 and at temperatures ranging between the thermal decomposition temperature of the radical generating agent and 230.degree. C., said composition comprising at least one kind of poly-.alpha.-olefin series resin of polypropylene series resin, poly(butene-1) series resin or poly(4-methyl pentene-1) series resin; a radical generating agent of dibenzoyl peroxide or t-butyl perbenzoate; and a silane compound having the general formula of RR'SiY.sub.