Waterproofing and breathable polyolefin roofing membrane by extrusion lamination and sequential stretching

A method for forming a reinforced microporous polyolefin sheet, comprising the steps of:

• a) extrusion laminating a non-porous sheet of a polyolefin onto at least one side of a non-woven fabric to produce a non-porous laminate having a thickness of at least 1 mm, the non-porous laminate comprising the non-woven fabric at least partially embedded in the polyolefin, then
• b) subjecting the non-porous laminate to sequential stretching steps to produce micropores in the polyolefin, the stretching steps comprising
• 1) At least one cold stretching step at a temperature in the range of −20° C. to 50° C. and
• 2) At least one hot stretching step at a temperature greater than 50° C. and up to 150° C.,
• wherein the non-woven fabric has an area weight of 100 to 400 g/m2, a thickness of 0.25 to 0.95 mm under a load of 2 kN/m2, and an elongation to break of 20 to 200% in each of machine and cross directions and
• wherein the polyolefin comprises
• (i) 50 to 95 weight percent of one or more polypropylene homopolymer chain segments, based on the weight of the polyolefin, or 43 to 79 mole percent polypropylene homopolymer chain segments based on the mold content of polymerized units of polypropylene in the polypropylene homopolymer chain segments as a percentage of the total mole content of polymerized monomer units in the polyolefin and
• (ii) 5 to 50 weight percent of ethylene-containing copolymer chain segments based on the weight of the polyolefin or 21 to 57 mole percent of ethylene-containing chain segments based on the mole content of polymerized monomer units in the ethylene-containing copolymer chain segments as a percentage of the total mole content of polymerized monomer units in the polyolefin,
• wherein at least a portion of the ethylene-containing copolymer chain segments comprise polymerized units of ethylene in an amount of at least 45 weight percent based on the weight of the ethylene-containing copolymer chain segments, or at least 55 mole percent based on the mole content of polymerized units of ethylene in the ethylene-containing chain segments as a percentage of the total mole content of polymerized monomer units in the ethylene-containing copolymer chain segments.