GB1248656A - Process for the production of microporous sheet structures which are permeable to water vapour - Google Patents

Abstract

1,248,656. Microporous sheets. FARBENFABRIKEN BAYER A.G., and MOBAY CHEMICAL CO. 30 April, 1969 [30 April, 1968], No. 21932/69. Heading B5B. [Also in Divisions B2 and C3] In a process for producing microporous sheets which are permeable to water vapour: (a) 90 to 70 parts by weight of a high molecular weight polyurethane which is free from NCO groups and contains from 6 to 20% by weight of urethane groups and which is produced from precursors having an NCO/OH ratio during synthesis of from 0À99 to 1À15, said polyurethane having a tensile strength of at least 100 kp/cm.<SP>2</SP>, a Shore A hardness of 35 to 85 and a softening point of above 120‹ C., and (b) 10 to 30 parts by weight (solids basis) of a predominantly linear high molecular weight polyurethane which is free from NCO groups and contains 8 to 35% by weight of urethane groups and 0À5 to 2À0% by weight of quaternary ammonium groups, in the form of an aqueous dispersion or colloidal solution, dissolved in (c) 200 to 2000 parts by weight of a water soluble strongly polar solvent for (a), having a boiling point of above 100‹ C., shaping the solution to form a sheet, optionally gelling the solution, treating the solution with water or with a mixture of water and solvent (c) to remove solvent (c) from the solution, and subsequently drying the resulting microporous sheet. An additional low boiling solvent may be used together with solvent (c). Specified low-boiling solvents include acetone, methyl ethyl ketone, tetrahydrofuran, ethyl acetate, methylene dichloride and 1,2-dichloropropane, in proportions of up to 100% by weight of the strongly polar solvent. Preferably the amount of water contained in the solution of (a) and (b) in (c) is below 7% by weight, and at least 2À5% by weight based on the amount of (b) of an anionic synthetic tanning agent may be added to the solution of (a), (b) and (c) prior to gelling. Solvent (c) may be removed by using several mixtures of solvent (c) and water having decreasing solvent content and finally pure water, e.g. by passing the gel through successive baths of such solvents, the gel being squeezed between each bath, and the microporous sheet may be subsequently treated with a solution of a synthetic anionic tanning agent. Shaping is suitably effected by casting, using a porous substrate e.g. wood, split leather, cardboard, paper, woven or unwoven textile sheeting or brickwork, or a non-porous substrate e.g. metal, glass or strips of fabric coated with a synthetic resin e.g. polytetrafluoroethylene. The gelling step is preferably achieved by exposure to moving air at from 10 to 40‹ C. and relative humidity 60 to 99%, especially 80 to 98%, for between 1 and 30 min. depending on the thickness of the solution. Still air may be used to gel the solution but a period of some hours is then necessary. The solutions of the invention may be used to produce laminated materials by spreading on porous or non-porous substrates, e.g. woven and knitted fabrics, fleeces, felts, glass or metal. Films prepared on non-porous substrates may be transferred by the reversing process on to such substrates as wood, split leather, cardboard, paper, woven and non-woven fabrics, brickwork and metal.