GB1066480A - Process for preparing filters having a microporous layer attached thereto and product - Google Patents

Abstract

A process for the manufacture of a corrugated microporous filter element comprises treating a porous substrate sheet in selected areas intended as the bases of corrugation folds, to reduce porosity by at least 10% relative to other areas of the sheet, folding the sheet along these areas and intermediate areas to form corrugations having base folds to a radius of curvature of at least 0.025 inch, applying to the corrugated sheet a dispersion comprising a suspending liquid and a particulate material, and causing the dispersion to flow through the sheet, thus depositing particulate material either on the substrate or within its pores, or both, to form a microporous layer. The maximum pore diameter may be 25 microns. The substrate sheet is preferably paper, optionally in two or more layers, and optionally resin-impregnated. Other materials include: plastics; textiles; felts and the like; porous sintered powder sheets; two wire-mesh sheets sintered together. The "reduction of porosity" may be effected by compression during formation of the corrugations, or by coating or impregnating (e.g. with synthetic resins). In the latter case a backing layer, or strips, having finer pores, may be applied permanently or temporarily to the reverse of the substrate sheet. If the corrugated sheet is to be used flat, the layer-forming suspension is applied to it in that form, but if it is to be used in tubular form its ends are joined and sealed prior to the application. The sheet may be impregnated with a setting agent for the corrugations prior to forming them, if necessary. For maximum adhesion of the microporous layer, a prior anchoring layer is laid down on the substrate sheet. The anchoring layer material consists of a bonding agent and a particulate material in a dispersing liquid. The bonding agent is preferably a liquid, or a solid which may be rendered liquid by appropriate treatment. Liquid thermosetting resins, e.g. polyepoxide resins, are preferred. In such cases, curing agents for these resins (preferably organic amines) are incorporated in the mixture. Amongst thermoplastic solid bonding agents, polyethylene, polytetrafluoroethylene and synthetic rubbers are preferred. The particulate material of the anchoring layer may be any of those used for the microporous layer (see below). When the later microporous layer is to enter the pores of the substrate sheet the anchoring-layer particles are non-fibrous; when coating is intended, the particles are fibrous. The dispersing liquid used in the anchoring layer material can be any of those used in applying the microporous layer, or any inert liquid. The microporous layer dispersion consists of a particulate material and optionally a bonding agent or a wetting agent in the dispersing liquid. Particulate material of a fibrous or like character is preferred, e.g. fibres of mineral, vegetable or synthetic origin; certain inorganic compounds, especially potassium titanate; glass; metals; and clays with needle-like particles. Optionally, non-fibrous particles such as diatomaceous earth may be included, and may even form the major constituent. Bactericides and like substances may be included. The dispersing liquid may be selected from: water, alcohols, glycols, hydrocarbons. The degree of flocculation of the dispersion is controlled, and may be varied by adding a dispersing agent or a flocculating agent before or after deposition. The dispersion may be agitated during deposition. The depth of penetration should not exceed 2 or 3 fibre diameters. Preferably a differential pressure is maintained across the element during deposition, which takes place several times; between one application and the next, the element is removed from the dispersion and a differential pressure applied to set the layer. Adhesion is effected by heating (e.g. a curing oven, or inward hot gaseous flow through the tubular element) or by standing at room temperature. A bactericidal layer (e.g. of Ag Br) may be applied to the element afterwards.ALSO:A process for the manufacture of a corrugated microporous filter element comprises treating a porous substrate sheet in selected areas intended as the bases of corrugation folds, to reduce porosity by at least 10% relative to other areas of the sheet, folding the sheet along these areas and intermediate areas to form corrugations having base folds to a radius of curvature of at least 0.025 inch, applying to the corrugated sheet a dispersion comprising a suspending liquid and a particulate material, and causing the dispersion to flow through the sheet, thus depositing particulate material either on the substrate or within its pores, or both, to form a microporous layer. The maximum pore diameter may be 25 microns. The substrate sheet is preferably paper, optionally in two or more layers, and optionally resin-impregnated. Other materials include: plastics; textiles; felts and the like; porous sintered powder sheets; two wire-mesh sheets sintered together. The "reduction of porosity" may be effected by compression during formation of the corrugations, or by coating or impregnating (e.g. with synthetic resins). In the latter case a backing layer, or strips, having finer pores, may be applied permanently or temporarily to the reverse of the substrate sheet. If the corrugated sheet is to be used flat, the layer-forming suspension is applied to it in that form, but if it is to be used in tubular form its ends are joined and sealed prior to the application. The sheet may be impregnated with a setting agent for the corrugations prior to forming them, if necessary. For maximum adhesion of the microporous layer, a prior anchoring layer is laid down on the substrate sheet. The anchoring layer material consists of a bonding agent and a particulate material in a dispersing liquid. The bonding agent is preferably a liquid, or a solid which may be rendered liquid by appropriate treatment. Liquid thermosetting resins, e.g. polyepoxide resins, are preferred. In such cases, curing agents for these resins (preferably organic amines) are incorporated in the mixture. Amongst thermoplastic solid bonding agents, polyethylene, polytetrafluoroethylene and synthetic rubbers are preferred. The particulate material of the anchoring layer may be any of those used for the microporous layer (see below). When the later microporous layer is to enter the pores of the substrate sheet the anchoring-layer particles are non-fibrous; when coating is intended, the particles are fibrous. The dispersing liquid used in the anchoring layer material can be any of those used in applying the microporous layer, or any inert liquid. The microporous layer dispersion consists of a particulate material and optionally a bonding agent or a wetting agent in the dispersing liquid. Particulate material of a fibrous or like character is preferred, e.g. fibres of mineral, vegetable or synthetic origin; certain inorganic compounds, especially potassium titanate; glass; metals; and clays with needle-like particles. Optionally, non-fibrous particles such as diatomaceous earth may be included, and may even form the major constituent. Bactericides and like substances may be included. The dispersing liquid may be selected from: water, alcohols, glycols, hydrocarbons. The degree of flocculation of the dispersion is controlled, and may be varied by adding a dispersing agent or a flocculating agent before or after deposition. The dispersion may be agitated during deposition. The depth of penetration should not exceed 2 or 3 fibre diameters. Preferably a differential pressure is maintained across the element during deposition, which takes place several times; between one application and the next, the element is removed from the dispersion and a differential pressure applied to set the layer. Adhesion is effected by heating (e.g. a curing oven, or inward hot gaseous flow through the tubular element) or by standing at room temperature. A bactericidal layer (e.g. of Ag Br) may be applied to the element afterwards.