Patent ID: 7169451

Claim:
A gas-permeable membrane which is useful in the packaging of respiring biological materials and which comprises: (a) a microporous polymeric film comprising a network of interconnected pores such that gases can pass through the film, and (b) a polymeric coating on the microporous film, wherein (1) the pores in the microporous film have an average pore size of less than 0.24 micron; and (2) the microporous film was prepared by a process comprising the steps of (A) preparing a uniform mixture comprising a polymeric matrix material in the form of a powder, a finely divided, particulate, substantially water-insoluble filler, and a processing oil; (B) extruding the mixture as a continuous sheet; (C) forwarding the continuous sheet, without drawing, to a pair of heated calender rolls; (D) passing the continuous sheet through the calender rolls to form a sheet of lesser thickness; (E) passing the sheet from step (D) to a first extraction zone in which the processing oil is substantially removed by extraction with an organic extraction liquid which is a good solvent for the processing oil, a poor solvent for the polymeric matrix material, and more volatile than the processing oil; (F) passing the sheet from step (E) to a second extraction zone in which the organic extraction liquid is substantially removed by steam or water or both; and (G) passing the sheet from step (F) through a forced air dryer to remove residual water and organic extraction liquid; and (3) the polymeric coating has a thickness such that the membrane; (i) has a P 10 ratio, over at least one 10° C. range between −5 and 15° C., of at least 1.3; (ii) has an oxygen permeability (OTR), at all temperatures between 20 and 25° C., of at least 775,000 ml/m 2 .atm.24 hrs (50,000 cc/100 inch 2 .atm.24 hrs; and (iii) has a CO 2 /O 2 permeability ratio(R) of at least 1.5; the P 10 , OTR and R values being measured at a pressure of 0.035 kg/cm 2 (0.5 psi).