Patent GB-1210504-A (GB)
Title: Process for the production of microporous, steam-pervious sheet structures
IPC: B29C67/20
Filed: 1967-11-21
Abstract: 1,210,504. Microporous films. FARBENFABRIKEN BAYER A.G. 21 Nov., 1967 [22 Nov., 1966], No. 52843/67. Heading B5B. [Also in Division C3] In a process for the production of microporous, steam-permeable sheet structures, a colloidal solution showing the Tyndall effect and comprising a film-forming polymer which has a tensile strength higher than 40 kilopounds per square centimeter, a Shore A. Hardness higher than 40 and a softening point above 120‹ C., dissolved in a volatile organic solvent, has added thereto a volatile, liquid non-solvent for the polymer which is miscible with the solvent in the necessary mixing range and which has a relative vaporization factor above the vaporization factor of the solvent, and less than 50% of which is taken up by swelling of the polymer in 24 hrs., wherein the non-solvent is added with stirring to the solution of the polymer in a quantity such that the solution, after the addition of the non-solvent, transmits at the most 40% of the amount of light of a wavelength of 662 mÁ transmitted by the polymer solution not containing the non-solvent, at least 60% of the quantity of non-solvent necessary for the gelling of the polymer solution being present in the resulting mixture, and wherein the solvent and the non-solvent are thereafter evaporated from this mixture after the latter has been shaped. Preferably, the non-solvent is applied essentially at the same temperature as that used for the subsequent evaporation. If desired, at least 10% of the amount of non- solvent which is necessary for gelling at the selected evaporation temperature may be introduced into the polymer solution, and the shaped mixture immersed in a bath of the non- solvent for washing the solvent from the polymer and for evaporating the solvent. Suitable temperatures at which to mix the polymer solution with the non-solvent are from about 0‹ C. to about 250‹ C., preferably from 10‹ C. to 150‹ C. and especially from 20‹ C. to 120‹ C. Generally the polymer solution which has been mixed with the non-solvent is dried after shaping at temperatures which are up to 30‹ C. above or up to 100‹ C. below, preferably from 15‹ C. above to 50‹ C. below the temperature at which the mixing was effected and in particular at the temperature of mixing. Preferably, drying is effected under reduced pressure. The drying temperature may be raised after at least 30% and preferably at least 70% of the solvent has been evaporated. The shaped polymer-solvent-non-solvent mixture may be immersed in a bath of the same, or a different, non-solvent which is miscible with the solvent at the temperature to be used for drying, the solvent washed from the polymer and at least 20%, preferably more than 50%, of the solvent evaporated while the mixture is in the bath, the residual solvent and non-solvent being evaporated after removing the sheet from the bath. The polymer solutions used generally contain from 2% to 50%, preferably 5% to 35% and especially from 8% to 25% by weight of polymer. Suitable solvents and non-solvents for all the polymeric materials used are listed. Examples relate to the production of films of the following materials: (1) polyvinyl chloride; (2) polycarbonates; (3) acrylonitrile-butadiene-styrene graft polymer; (4) cellulose acetate; (5) styrene-acrylonitrile polymer; (6) polymethylmethacrylate; and Examples 7 to 12 relate to the production of films of polyurethane prepared by the method given. Sheet structures obtained according to the process of the invention have a good water vapour permeability and may be used as synthetic leather-like material, diaphragms and battery separator plates either directly or after being connected to other porous substrates as e.g. fibre fleece, split leather, wood, paper, felt or fabrics. If the sheets are glued, care has to be taken that the gluing material does not adhere as otherwise the porosity will be diminished. For the methods of preparation of the polymers see C3 Abridgment.