Patent GB-1195958-A (GB)
Title: Process for Preparing Porous Sheets and Films consisting of Polyolefins.
Inventor: ROCK MICHAEL
IPC: C08J9/26
Filed: 1968-10-07
Abstract: 1,195,958. Making microporous films and sheets. W. R. GRACE & CO. 7 Oct., 1968 [29 Nov., 1967], No. 47477/68. Heading B5B. Sheets and films which shrink to only a moderate extent when treated to render them porous are formed from a fluxed mixture of 5 to 65% of a polyolefin having a standard-load melt index of 0, or a mixture thereof with a compatible lower molecular weight polyolefin with a small standard-load melt index, the mixture having a standard-load melt index of 0, at least 30% of a plasticizer soluble in water or in a conventional organic solvent and 5 to 60% of a finely divided filler which is insoluble in the plasticizer. All the percentages are percentages by volume. The sheet or film is rendered porous and permeable to liquids by immersing it first in a liquid which is a solvent for the plasticizer but not for the filler and in which the polyolefin component of the sheet or film is substantially insoluble so as to extract the plasticizer, and then, while still wet with the solvent, is immersed in a solvent for the filler so as to extract the filler. Between the extraction of the plasticizer and the extraction of the filler from the sheet or film, the sheet or film is maintained immersed in a liquid in which the sheet or film is substantially insoluble. If the solvent used for extracting the plasticizer is not miscible with the solvent used for extracting the filler, an intermediate solvent should be used with which the plasticizer solvent and the filler solvent are both miscible. The polyolefin used may be a polyethylene, a polypropylene, a polybutene, an ethylene-propylene copolymer, an ethylene-butene copolymer, a propylene-butene copolymer or an ethylenepropylene-butene terpolymer. The standardload melt index indicates the rate of extrusion of the polyolefin as measured under the conditions specified in ASTM D 1238-57T (Condition E). In the example, 19% of a 99% ethylene-1 butene copolymer, 55% of calcium carbonate having an average particle size of 0À10 to 0À35 micron, 25% of a petroleum oil having an SSU viscosity of 547 at 100‹ F. and 1% of stearic are mixed and extruded to form a film 8 inches wide and 2 mils thick. The percentages of the components of the mixture are percentages by weight. The film is immersed successively at room temperature in two hexane baths, then successively in 1 : 1 acetone : hexane, and 1 : 1 water-acetone baths, thereafter successively in a 36% hydrochloric acid bath and two water baths and finally in a 35- 45% by weight aqueous potassium hydroxide bath. The acid bath and the two water baths contain 3% of a surfactant. Wet extracted sheets or films produced according to the invention may be used directly as battery separators in alkaline-type batteries or may be dried and stored until ready for use. Suitable fillers are said to be silica, alumina, calcium oxide, magnesium oxide, barium oxide, titanium dioxide, iron oxide, zinc oxide, tin oxide, magnesium carbonate, molybdenum disulphide, zinc sulphide, and particularly water-soluble metallic salts and sugar. Suitable plasticizers include dibutyl sebacate, stearic acid, hydrocarbon lubricating oils, tricresyl phosphate, paraffin wax, ethylene glycol, glycerol, triethyl phosphate, polyvinyl alcohol and polyacrylic acid.