Patent GB-1000038-A (GB)
IPC: B01D39/20
Abstract: 1,000,038. Porous material. LLOYD & HILLMAN. Nov. 24, 1961 [Dec. 6, 1960], No. 42125/61. Headings C3C, C3H, C3P and C3R. [Also in Divisions B1 and D2] A process for preparing porous material comprises preparing at least two separate suspensions comprising material suspended in a fluid, the separate suspensions severally comprising material of different size, blending the suspensions to vary the proportions of material of each size sequentially to produce blends containing differing proportions of the sizes of material, and depositing the blends upon a base to form a porous material having a pore diameter and pore configuration varying from surface to surface of the material. The suspended materials may be granular or fibrous. The base on which the materials are deposited may be removed prior to use, or remain as part of the porous layer, either distinct and removable or being modified by penetration and impregnation with suspended material. When suspended material used is fibrous, the fibres may be of any length. Suitable are fibres of glass, asbestos, potassium titanate, aluminium silicate, mineral wool, regenerated cellulose, polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyethylene, polypropylene, rubber, poly (glycol terephthalate), polyamides, casein, zein, cellulose acetate, viscose rayon, hemp, jute, linen, cotton, silk, wool, mohair and metal. Suspended particulate materials which can be used are diatomaceous earth, fuller's earth, silicon, magnesia, silica, talc, silica gel, alumina, quartz, carbon, clays, synthetic resins, cellulose derivatives and metals such as aluminium, iron, copper, nickel, chromium, beryllium, copper and alloys. Combinations of fibres and particles are usable. The suspending medium may be water, polyethylene glycols, and their ethers, lubricating oils, vegetable oils, xylene and petroleum ethers. Non-volatile fluids can be removed from the layer by washing. A layer formed by blending fibres A and B by varying the proportions across the mat, one side consisting of 100% A fibres, the other of 100% B fibres to give a "meon pore" which is in effect tapered. A bonding agent or binder for the materials can be incorporated in the fluid media, or may be flowed through the deposited layer as a final operation. Thermosetting and thermoplastic binders suitable for use are listed. Some deposited materials can be bonded by the action of heat or solvent, with some pressure while the material is tacky. An apparatus for depositing the layers comprises reservoirs 1 and 2 containing stirred suspensions of fine and coarse particles respectively. Pumps 6 and 8 deliver slurry to blending point 11 and line 12. The rate of feed of the slurries is controlled by regulating the speed of the pumps by cams 15 and 16 and followers 19 and 20 which operate the variable speed gears 23 and 24, driven by motors 25 and 26. In Example (1) suspensions in a petroleum base hydraulic fluid of glass fibres having mean diameters 0À35 micron and 3À5 microns, respectively are deposited on a 3 micron nitrocellulose membrane, the distribution ratio of fine to coarse particles being as shown in Fig. 2 and the overall ratio R of fine to total fibres being 0À25. In other Examples (2) to (16), the mixtures used consisted of blends fine and coarse glass fibres or this mixture together with diatomaceous earth. In Example (18) blends of different asbestos fibres are deposited on filter paper. The products are used as filters, coalescers for suspended liquids, gas diffusers, battery separators and dialysing membranes.