Method of making a fluid transmitting porous tube or sheet

A method of making a tube or sheet-shaped fluid transmitting porous material having an extension of 1.2 to 10 times and a thickness of 0.15 to 3 mm. which comprises mixing a polyolefin (A) and a secondary polymer (B) and a decomposition type chemical blowing agent, said secondary polymer (B) being a member selected from the group consisting of:

• 1. an ethylene-vinyl ester copolymer of which the vinyl ester content is at least 10% or more by weight based on the copolymer,
• 2. a copolymer of ethylene with an unsaturated carboxylic acid ester of which the unsaturated carboxylic acid ester content is at least 10% or more by weight based on the copolymer,
• 3. a rubber selected from the group consisting of a natural rubber, a butadiene rubber, a solution-polymerized random copolymer type styrene-butadiene rubber, an emulsion-polymerized random copolymer type styrene-butadiene rubber, a butadiene-acrylonitrile rubber, an iosprene-butadiene rubber, a polyisoprene rubber and an isobutylene-isoprene rubber,
• 4. A thermoplastic elastomer selected from the group consisting of a styrene-butadiene block copolymer and a styrene-isoprene block copolymer, and mixtures thereof, said secondary polymer being employed in an amount of ##EQU2## and continuously subjecting the resulting polymer mixture to extrusion while foaming, to form a film by passage through a die of an extruder;
• said extrusion moulding being effected under conditions defined by the formula:
• 0.15 <
• S/t.sub.2 10
• wherein
• t.sub.2 = t.sub.1 /a1/3
• 1.2 <
• a <
• 10,
• S represents an average die slit clearance (mm.),
• t.sub.2 represents an average thickness of the foaming polymer composition in the state before foaming (mm.),
• t.sub.1 represents an average thickness of the porous material (mm.), and
• a represents an expansion based on the volume of the foaming polymer composition in the state before foaming (times.)
• Said extrusion further satisfies the following conditions:
• 7 .ltoreq. V = Q/W
• wherein
• Q represents an extrusion volume of the extruded porous material from which the contained space volume is subtracted (cm.sup.3 /min.),
• W represents a cross section area of the die slit (cm.sup.2), and
• V represents an extrusion speed calculated from Q and W (cm./min.).