GB1049842A - Cellulosic webs - Google Patents

Abstract

The fibres in a web of cellulosic fibres are swollen by at least 50% by volume and are bound together by a binder of tensile modulus (by A.S. T.M. D882-61T) of 106 to 1011 dynes/cm2, distributed in isotropic microcellular configuration in the web. Such a product may be made from an aqueous slurry of fibres by laying the fibres into a sheet by paper-making methods, wet-pressing and then removing remaining water by solvent-exchange techniques so that the fibres remain in their swollen condition. The sheet is then impregnated with a solution or dispersion of a suitable binder in a solvent that is a swelling agent for or inert towards the fibres and the binder coagulated by immersion in a liquid that is miscible with this solvent but is not a solvent for the binder and is inert towards the fibres. The sheets are then dried. Swelling agents referred to are dimethylacetamide, dimethylformamide, dimethylsulphoxide, formamide, ethylene glycol, methyl alcohol, furfuryl alcohol, water and aqueous solutions of the hydroxides of sodium, potassium and lithium. The swelling liquid must be removed without loss of swelling, e.g. by displacement by an inert liquid which is miscible with water and may be then evaporated. If the displacing liquid has an affinity for water it may itself be displaced in turn by a liquid that has not. Liquids referred to for the first displacement are acetone, methyl ethyl ketone, tetrahydrofuran and dioxane, and for the second benzene and xylene. The swelling liquid or displacing liquid may be still present in the web when the binder is applied, or the solvent for the binder may itself serve as the swelling agent. The binder may be a polymer of acrylic or methacrylic acid or an ester thereof, of a vinyl ester, of a vinyl or vinylidene halide, of 2-chlorobutadiene, 2-3-chlorobutadiene, trichlorethylene, tetrafluorethylene or chlorotrifluorethylene, of styrene, an alkylstyrene or a halostyrene, of ethylene, propylene or butene, of vinyl-carbazole, N-vinylformamide, vinyl-pyrrolidone, acrylonitrile or vinylidene cyanide, of styrene-sulphonic, boronic or phosphoric acid, of formaldehyde, acetaldehyde, crotonaldehyde or acrolein, or of copolymers of these monomers with one another or with other monomers such as butadiene, isobutylene, isoprene, maleates, fumarates, maleic, fumaric and itaconic acids, alkyl glycidyl ether, glycidyl acrylate and methacrylate, alkyl vinyl ethers, ethylene oxide, propylene oxide, butylene oxide, cyclohexene epoxide, tetrahydrofuran, sulphur dioxide and carbon monoxide, or chemically modified polymers such as halogenated polyethylene, hydrolysed polyvinyl acetate, halogenated polyethers, halogenated copolymer of ethylene and propylene, chlorosulphonated polyolefines, halogenated or hydrolysed copolymer of ethylene and vinyl acetate, hydrogenated ethylene/carbon monoxide polymers, polyepoxides modified by hydrolysis, amination or phosphonation, polyesters, polyamides, condensation polymers of diisocyanates and glycols, dimercaptans, diamines or polyethers, or graft or block polymers, or starch, casein, gutta percha, gum acacia, guar gum or gum arabic, or hydroxyethyl-, cyanoethyl-, aminoethyl-or carboxymethyl-cellulose, starch or protein. After impregnation, the sheet may be calendered or embossed, or heated to crosslink or otherwise alter the binder. Plasticizers, polymer-curing agents, dyes, pigments and surfactants may be included. The products are normally water-impermeable, but may be made water-absorptive by treatment with surfactants.