WO2008000049A2 - A sintered metal fiber medium and a method to provide a sintered metal fiber medium - Google Patents

A sintered metal fiber medium and a method to provide a sintered metal fiber medium Download PDF

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Publication number
WO2008000049A2
WO2008000049A2 PCT/BE2007/000070 BE2007000070W WO2008000049A2 WO 2008000049 A2 WO2008000049 A2 WO 2008000049A2 BE 2007000070 W BE2007000070 W BE 2007000070W WO 2008000049 A2 WO2008000049 A2 WO 2008000049A2
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WO
WIPO (PCT)
Prior art keywords
metal
slurry
fibers
primary slurry
porous metal
Prior art date
Application number
PCT/BE2007/000070
Other languages
French (fr)
Other versions
WO2008000049A3 (en
Inventor
Frank Verschaeve
Berend Degrieck
Gerrit Pieter Van Betsbrugge
Original Assignee
Nv Bekaert Sa
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Publication date
Application filed by Nv Bekaert Sa filed Critical Nv Bekaert Sa
Publication of WO2008000049A2 publication Critical patent/WO2008000049A2/en
Publication of WO2008000049A3 publication Critical patent/WO2008000049A3/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F7/00—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression
    • B22F7/002—Manufacture of composite layers, workpieces, or articles, comprising metallic powder, by sintering the powder, with or without compacting wherein at least one part is obtained by sintering or compression of porous nature
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/14—Other self-supporting filtering material ; Other filtering material
    • B01D39/20—Other self-supporting filtering material ; Other filtering material of inorganic material, e.g. asbestos paper, metallic filtering material of non-woven wires
    • B01D39/2027—Metallic material
    • B01D39/2041—Metallic material the material being filamentary or fibrous
    • B01D39/2044—Metallic material the material being filamentary or fibrous sintered or bonded by inorganic agents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/002—Manufacture of articles essentially made from metallic fibres
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

  • the present invention relates to a sintered metal fiber medium and a method to provide a sintered metal fiber medium.
  • the method relates more particularly to a method to provide a sintered metal fiber medium using metal fiber slurry comprising short metal fibers.
  • Sintered metal fiber media are well known in the art for numerous applications, such as e.g. liquid or gas filtration.
  • a first method for providing sintered metal fiber medium is to provide a metal fiber web by air lay down, and sintering this air laid web in appropriate furnaces.
  • a disadvantage of this air lay down web is the fact that the web is usually relatively inhomogeneous, especially when relatively thin sintered metal fiber media are to be provided. This is because it is difficult to provide the air laid webs sufficiently homogeneous. Therefore, to have a sintered metal fiber medium with homogenous properties over its surface, usually several air laid webs are stacked (so-called doubled).
  • Another method to provide a web, prior to sintering operation is to use the so-called wet lay down method or paper making method, as described in
  • the metal fibers are brought in a slurry, which slurry is poured on a screen.
  • the water is extracted, e.g. sucked, from the slurry through the screen.
  • the remaining dewatered slurry is then sintered.
  • a binding agent may be used to temporarily bind the metal fibers to each other and so to make the dewatered slurry transportable. This dewatered slurry is then sintered, optionally first debinding the binding agent.
  • a disadvantage of the wet webbing is that in case thin and relatively short fibers are used, some of the shorter fibers are sucked through the screen, together with the water being removed from the slurry. In case of thin webs made prior to sintering, the dewatering step may suck small or larger holes in the web where few or no fibers are retained for sintering.
  • the dewatered slurry and thus the sintered metal fiber medium may have inhomogeneous zones where less fibers are present, even when several layers of the freshly dewatered webs are stacked one to the other prior to sintering.
  • fibers with small equivalent diameter e.g. 2 ⁇ m to 6 ⁇ m
  • the phenomena of sucking out fibers along with the water during dewatering is noticed. This is because usually the amount of fibers with smaller lengths is larger, the finer the fibers are. As a result, more fibers with a short length are sucked out along with the water during dewatering in case of fibers with small equivalent diameter.
  • a method for providing a sintered metal fiber medium where a slurry of metal fibers is tape cast, sintered after debinding and optionally compressed.
  • a combination is made with other porous metal structures such as wire mesh or expanded metal sheets, by combining debound and optionally sintered foil comprising metal fibers, which combination is then to be sintered to couple the porous structure to the metal fibers. It has been found that the products obtained by this method may suffer from decoupling of the metal fibers from the porous structure during use of the medium e.g. in filtration applications. It is noticed that, e.g.
  • the above objective is accomplished by a method to provide a sintered metal fiber medium and a sintered metal fiber medium according to the present invention.
  • a method for manufacturing a sintered metal fiber medium from metal fibers and a porous metal structure having a first surface and a second surface comprises the steps of • making a primary slurry comprising metal fibers, a solvent and a binding agent by mixing the metal fibers and the binding agent; • applying the primary slurry on either the first surface or the second surface of the porous metal structure; • solidifying the primary slurry by removing the solvent yielding a composite structure comprising the metal fibers, the binding agent and the porous metal structure; and
  • debinding of the binding agent and sintering the composite structure may be done in one step or in two separate steps.
  • the removal of the solvent may be by evaporation of the solvent, e.g. a drying step.
  • the evaporation may be assisted by providing dry air and/or heated air to the primary slurry, which may accelerate the evaporation.
  • the removal of the solvent during solidifying of the primary slurry may be done by evaporating the solvent, such as by thermo-evaporation, i.e. executed or assisted by providing heated air.
  • evaporating the solvent may be forced, e.g. by heating the primary slurry, e. g. by forcing heated air over the surface of the primary slurry, or by radiating, e. g. microwave- or IR-radiating.
  • Solidifying of the slurry by applying evaporation of the solvent e.g. by drying such as air drying or heating of the slurry by microwave or IR-radiation, prevents the metal fibers of the primary slurry from penetrating into the pores to a too large extent, such as through the pores, to such an extent that the slurry, and hence the metal fibers become present at the other of the first or second side of the porous metal structure.
  • the depth of penetration may be limited to only a part of the thickness T of the porous metal structure, e.g. providing metal fibers being present in the pores located between the metal fiber covered surface and a depth of T/2. This effect is obtained in particular in embodiment by a combination of removing the solvent by evaporation of the solvent and applying the slurry by tape casting of the slurry.
  • the solvent may be removed from the slurry by having the solvent reacting, such as polymerizing, e.g. under influence of UV- illumination or heat, optionally or by causing a chemical reaction, e.g. a polymerisation, with other components in the slurry, which may harden the solvent.
  • the solvent thus may optionally function as binding agent once solidified.
  • the solvent may be a UV-curable component, causing the component to solidify under UV-illumination.
  • the solvent may be a heat-curable component, causing the component to solidify under application of heat.
  • the solvent may be a bi- or multi-component solvent, causing the component to solidify by reaction between the two or more components when the solvent is subjected to appropriate conditions, such as appropriate illumination or temperature to cause the reaction to occur.
  • the application of the primary slurry may be done by non-contact or contact deposition techniques.
  • the application of the primary slurry may be done by tape casting the primary slurry onto the surface of the porous metal structure.
  • the application of the primary slurry may be done by spraying the primary slurry onto the surface of the porous metal structure.
  • the application of the primary slurry may be done by plastering the primary slurry onto the surface of the porous metal structure.
  • the application of the primary slurry may be done by brushing the primary slurry onto the surface of the porous metal structure.
  • the application of the primary slurry may be done by printing, e.g. screen printing, of the primary slurry onto the surface of the porous metal structure.
  • a method for manufacturing a sintered metal fiber medium from metal fibers and a porous metal structure having a first surface and a second surface may comprise the steps of
  • the porous metal structure may be a structure or a combination of structures, this or these structures being chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
  • the structure or the combination of structures may be subjected to a compression step.
  • the porous metal structure may be a combination of structures.
  • Each of the structures may be chosen from the group consisting of wire meshes, such as a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
  • Each of the structures may be a wire mesh, optionally chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh.
  • Each of the structures may be subjected to a compression step prior to making the combination of structures.
  • the combination of structures may be sintered prior to applying the primary slurry on the combination of structures.
  • the porous metal structure may be a combination of a plurality of wire meshes, which combination is optionally subjected to a compression step such as calendering, rolling or pressing.
  • a compression step such as calendering, rolling or pressing.
  • This provides a flattening of the outer surfaces of the combination of the plurality of wire meshes, providing a flat surface to the combination of the plurality of wire meshes.
  • This flattened or flat surface provides a larger contact surface for the slurry to contact the combination of the plurality of wire meshes.
  • the porous metal structure is optionally a combination of a plurality of compressed wire meshes, i.e. wire meshes being individually subjected to a compression step such as calendering, rolling or pressing prior to making the combination, such as by stacking.
  • a compression step such as calendering, rolling or pressing prior to making the combination, such as by stacking.
  • two flat surfaces will contact each other along contacting zones.
  • Each of the wire meshes, to be individually compressed may be chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh.
  • Meshes in a combination of structures may have the same or different properties, such as identical or different mesh sizes, identical or different wire diameters or wire alloys.
  • the wire mesh with the finest mesh size is used to provide the surface of the porous metal structure to which the slurry is provided.
  • the method may comprise the additional step of impregnating the porous metal structure with a solution of the solvent and the binding agent prior to applying the primary slurry.
  • the debinding may be done by thermally debinding the binding agent.
  • the method further may comprise the step of applying at least a secondary slurry on the composite structure.
  • the secondary slurry may comprise secondary metal fibers.
  • the secondary slurry may be different from the primary slurry.
  • the secondary metal fibers may be different from the metal fibers of the primary slurry.
  • the method further may comprise the step of compressing the porous metal structure prior to applying the primary slurry.
  • the solvent of the primary slurry may be water.
  • the present invention also includes a sintered metal fiber medium obtained by a method according to this first aspect of the present invention.
  • a sintered metal fiber medium comprises metal fibers and a porous metal structure, which porous metal structure has two surfaces.
  • the fibers are present on only one of the surfaces, providing, the fibers are present in at least part of the pores of the porous metal structure.
  • the porous metal structure may be a structure or a combination of structures, which structures or structures is or are chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
  • the structure may comprise two flat outer surfaces.
  • the porous metal structure may be a combination of structures, each of the structures being wire meshes, such as chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh.
  • each of the structures may be provided with two flat surfaces, such as by individually compression of each of the wire meshes.
  • the metal fibers may be present at only one of the outer surfaces of the porous metal structure.
  • the porous metal structure may have a thickness T, and the metal fibers are present in the pores located between the metal fiber covered surface and a depth of T/2.
  • Fig. 1a and Fig. 1 b are schematical views of a sintered metal fiber medium as subject of the present invention.
  • Fig. 2 is schematically a view of an alternative sintered metal fiber medium as subject of the present invention.
  • a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
  • equivalent diameter of a metal fiber is meant the diameter of an imaginary circle having the same surface as the surface of a radial cross section of the fiber.
  • the bubble point pressure of a filter medium is measured according to the ISO 4003 testing method.
  • P porosity
  • d (weight of 1 m 3 sintered metal fiber medium)/ (SF)
  • S F specific weight per m 3 of alloy out of which the metal fibers of the sintered metal fiber medium are provided.
  • the mean flow pore size (also referred to as MFP) can be measured using a "Coulter Porometer I I" testing equipment, which performs measurements of the mean flow pore size according to ASTM F-316-80.
  • the "bubble point pressure” (also referred to as BBP) is determined according to ASTM E128-61 , being he equivalent of ISO4003.
  • the "Air permeability" (also referred to as AP) is determined according to NF 95-352, being the equivalent of ISO 4002.
  • the Ra value is defined as the arithmetic mean deviation of the surface height from the mean line through the measured profile from the measured length.
  • the mean line is defined so that equal areas of the profile lie above and below the line.
  • the term "slurry” is to be understood as a mixture of a solvent, a binder and insoluble matter, insoluble in this solvent.
  • the solvent may be water.
  • flat surface of a wire mesh is to be understood as the wire of the wire mesh being provided with, at at least one of the outer surfaces of the mesh, flat zones, which flat zones are substantially coplanar.
  • equivalent diameter of a particular fiber is to be understood as the diameter of an imaginary fiber having a circular radial cross section, which cross section has a surface area identical to the average of the surface areas of cross sections of the particular fiber.
  • metal fibers are provided.
  • the metal fibers are for example made of steel such as stainless steel.
  • Preferred stainless steel alloys are AISI 300 or AISI 400-serie alloys, such as AISI 316L or AISI 347, or alloys comprising Fe, Al and Cr, stainless steel comprising Chromium, Aluminum and/or Nickel and 0.05 to 0.3 % by weight of Yttrium, Cerium, Lanthanum, Hafnium or Titanium, such as e. g. DIN 1.4767 alloys or Fecralloy®, are used. Also Cupper or Cupper-alloys, or Titanium or Titanium alloys may be used.
  • the metal fibers can also be made of Nickel or a Nickel alloy.
  • Metal fibers may be made by any presently known metal fiber production method, e.g. by bundle drawing operation, by coil shaving operation as described in JP3083144, by wire shaving operations (such as steel wool) or by a method providing metal fibers from a bath of molten metal alloy.
  • the metal fibers may be cut using the method as described in WO02/057035, or by using the method to provide metal fiber grains such as described in US4664971.
  • the metal fibers used to provide the sintered metal fiber medium are characterized in having an equivalent diameter D and an average fiber length L.
  • the equivalent diameter D of the metal fibers is less than 100 ⁇ m such as less than 65 ⁇ m, e.g.
  • the equivalent diameter of the metal fibers is less than 15 ⁇ m, such as 14 ⁇ m, 12 ⁇ m or 11 ⁇ m, or even more preferred less than 9 ⁇ m such as e.g. 8 ⁇ m.
  • the equivalent diameter D of the metal fibers is less than 7 ⁇ m or less than 6 ⁇ m, e. g. less than 5 ⁇ m, such as 1 ⁇ m,1.5 ⁇ m, 2 ⁇ m, 3 ⁇ m,3.5 ⁇ m, or 4 ⁇ m.
  • the metal fibers all have an individual fiber length. As some distribution on these fiber lengths may occur, due to the method of manufacturing the metal fibers, the metal fibers, used to provide a sintered metal fiber medium as subject of the invention, will be described as having an average fiber length L but this does not prevent the length of individual fibers differing considerably. This length is determined by measuring a significant number of fibers, according to appropriate statistical standards.
  • the average fiber length of the metal fibers is smaller than 10 mm, e.g. smaller than 6 mm, optionally smaller than 1 mm, such as smaller than 0.8 mm or even smaller than 0.6 mm such as smaller than 0.2 mm.
  • substantially all fibers used during the method of manufacturing the sintered metal fiber medium will occur in the sintered metal fiber medium, the average fiber length L can be measured in a similar way on the sintered metal fiber medium.
  • the metal fibers in the sintered metal fiber medium thus may have a ratio of average fiber length over diameter (UD) being optionally less than 110, more preferred less than 100, but usually more than 30.
  • An L/D of about 30 to 70 is preferred for metal fibers with equivalent diameter in the range up to 6 ⁇ m, in case the metal fibers are obtained by the process as described in WO02/057035, hereby incorporated by reference.
  • a primary slurry which slurry comprises metal fibres, a solvent and a binding agent.
  • the primary slurry comprises an amount of metal fibers in the range of 2% weight to 40% weight of the primary slurry, more preferred between 5% weight and 15% weight of the primary slurry.
  • concentration combined with the tape casting action to provide substantially flat layers of primary slurry, causes metal fibers to be distributed more homogeneously.
  • Too much metal fiber in the primary slurry may cause conglomeration of the fibers, causing on its turn inhomogeneous metal fiber distribution throughout the sintered metal fiber medium.
  • Too little metal fiber in the primary slurry may cause problems during debinding, where too much debinding causes disturbing of the sintering of the metal fibers. Further, such production of sintered metal fiber medium becomes uneconomic, as too much energy is to be consumed for debinding the binding agent, and a large volume of binding material is to be removed. In each cast layer, the metal fiber distribution over the surface may become irregular.
  • the primary slurry comprises a binding agent, optionally a polymer binding agent, and metal fibers.
  • a binding agent for the purpose of the invention is to be understood as a product for thickening the primary slurry.
  • the primary slurry comprises a solvent for dissolving the binding agent, which solvent is removed by evaporation during solidification of the primary slurry, such as e.g. water.
  • a solvent for dissolving the binding agent such as e.g. water.
  • a water soluble binding agent is used, e. g. polyvinyl alcohols, methyl cellulose ethers, hydroxypropylmethylcellulose, polyethers from ethyleneoxide, acrylic acid polymers or acrylic copolymers.
  • the binding agent is added to the solvent, in a concentration of optionally between 0.5% weight and 30 % weight of the primary slurry. Most preferred, a binding agent is chosen which requires a concentration of less than 20% weight or even less than 15 % weight or even less than 10% weight of the primary slurry, in order to provide the required viscosity. A viscosity range between 1.000 cPs and 600.000 cPs is optionally used for the slurry.
  • the components of the primary slurry are blended using appropriate mixing equipment. In case foaming of the primary slurry occurs, small amounts of a defoaming component are added.
  • the slurry may comprise viscosity changers.
  • the fibers are dispersed in the solvent, optionally water, mixed with a dispersant and/or wetting agent by soft stirring.
  • a defoaming agent is added.
  • the binding agent is added and after some minutes of stirring, a neutralization agent may be added to block the fluid, i.e. the primary slurry.
  • a shear thinning slurry may be obtained, having a viscosity of more than 300.000 cPs.
  • a porous metal structure is provided.
  • the porous metal structure is optionally a structure or a combination of structures, such as a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet or a perforated metal sheet.
  • a compression step e.g. a rolling, calendering or pressing operation, in order to smoothen the surface of such structure.
  • the porous metal structure has optionally a substantially flat structure and two substantially flat surfaces, and a substantially uniform thickness T being the average distance between the two outer surfaces.
  • a metal wire mesh of AISI316L wires having a diameter of 0.1 mm is used, having a thickness of about 0.21 mm and a surface weight of 493 g/m 2 .
  • the mesh may be e.g. a metal wire mesh of AISI316L wires having a diameter of 0.25 mm. This mesh has a thickness of about 0.5 mm and a surface weight of 1220 g/m 2 .
  • a first metal wire mesh such as a wire mesh of AISI316L wires having a diameter of 0.1 mm, having a thickness of about 0.21 mm and a surface weight of 493 g/m 2 is used.
  • the first wire mesh was compressed by pressing the wire mesh.
  • a second wire mesh such as a wire mesh of AISI316L wires having a diameter of 0.25 mm is provided. Also this second wire mesh is compressed, such as pressed or calendered.
  • This second wire mesh has a thickness of about 0.5 mm and a surface weight of 1220 g/m 2 .
  • the first and second wire mesh are stacked and sintered to each other, thereby providing an alternative porous metal structure.
  • the porous metal structure will be oriented and provided in such a way that the slurry is provided to the porous metal structure by applying it to the outer surface of the porous metal structure, which outer surface is provided by the first wire mesh.
  • the primary slurry is applied to only one of the two outer surfaces of the porous metal structure, whereby a composite structure is provided.
  • This application can be done by many different techniques, such as tape casting, spraying, plastering, brushing or printing, such as screen-printing.
  • the metal fibers anchor better to the porous metal structure, especially if the porous metal structure is provided with a flat surface, e.g. by compressing wire meshes being part of the porous metal structure.
  • a flat surface e.g. by compressing wire meshes being part of the porous metal structure.
  • the metal fibers migrate into the pores of the porous metal structure.
  • the fibers may migrate through the whole thickness of the porous metal structure and be present in the pores at both sides of the porous metal structure.
  • no layer of metal fibers will be formed at the opposite, second side of the porous metal structure.
  • the migration is not deeper than a part of the thickness T of the porous metal structure, such as only half the thickness T of the porous metal structure. This effect is obtained in particular when the slurry is provided by tape casting the slurry onto one of the outer surfaces of the porous metal structure.
  • these migrated fibers have the additional effect that, when the sintered metal fiber medium is used as surface filtration medium, the medium has a limited back pressure during filtration, because usually metal fibers are only present along a flow path for the fluid along the thickness of the metal fiber layer, together with preferable at maximum half the thickness of the porous metal structure. Further, the risk on flowless zones under or in the medium (as compared to media having metal fibers at both sides of the porous metal structure) or between the metal fiber layer and the porous metal structure (as compared to media being provided by coupling a sintered or debound metal fiber layer to the porous metal structure) is significantly reduced or avoided.
  • the clearance of the applicator is kept relatively small, such as between 0.2 mm and 6 mm, more preferred between 0.2 mm and 3 mm.
  • the speed of movement of the applicator is chosen according to the viscosity of the slurry and the composition of the primary slurry.
  • the clearance and thus the thickness of the layer of the primary slurry is chosen in function of the amount of metal fibers in the primary slurry, the required weight per surface unit of the sintered metal fiber medium, and the required density of the metal fiber layer of sintered metal fiber medium.
  • the slurry is pumped into the slurrybox, which is mounted in front of the blade.
  • the slurry which may comprise a wide range of metal fibers, is provided on a porous metal structure, a wet layer of slurry of 100 ⁇ m to 3000 ⁇ m may be provided.
  • the thickness is influenced by the substrate, gap and shape of blade; the casting speed and the reological behavior of the slurry.
  • the porous metal structure is usually supported itself by a belt or flat surface.
  • a thin layer of the primary slurry may be provided using paint coating nozzle guns.
  • the nozzle of the gun is optionally at least 0.1mm diameter.
  • thick- sprayed primary layers are provided using a body coater gun.
  • the body coater gun has a nozzle diameter of more than 4 mm, e.g. 5 mm. The slurry can be sucked by the vacuum of the paint coating gun or body coater gun, or can be provided to the gun under pressure.
  • the primary slurry When the primary slurry is provided by means of plastering, the primary slurry can be provided by using the spoon filling technique, which is done using a flat or straight spoon.
  • the primary slurry is to have a shear thinning reological behavior with a rather high viscosity in the range of 10.000 cPs to 1.000.000 cPs.
  • the primary slurry When the primary slurry is provided by means of brushing, optionally the primary slurry has a viscosity ranging from 1.000 cPs to 700.000 cPs. Most preferred a soft brush is used. There must always be a large amount of slurry on the brush.
  • the primary slurry When the primary slurry is provided by means of screen printing, optionally the primary slurry has a viscosity ranging from 3.000 cPs to 700.000 cPs, having good flow behavior and gratisisation effects.
  • the primary slurry is solidified, forming a composite structure which comprises the binding agent, the porous metal structure and the metal fibers.
  • the solvent is removed from the primary slurry.
  • This may be done by evaporating the solvent.
  • a solvent may be used which evaporates easily at ambient temperature.
  • the evaporation may be executed as a drying step in case water was used as solvent.
  • the drying or evaporating may be thermo-evaporation, i.e. executed or assisted by air-drying or may be forced by heating the primary slurry, e. g. by forcing heated air over the surface of the primary slurry, or by radiating, e. g. microwave- or IR- radiating.
  • the thickness of the primary slurry is reduced up to some extent, as the volume of the primary slurry is reduced to provide the volume of the composite structure.
  • the L/D ratio of the fibers in the solidified and later on the sintered metal fiber medium is identical to the L/D of the metal fibers used to make the primary slurry.
  • the composite structure so obtained may be subjected to a compression step, e.g. by a plate press or by means of a roller press or calendering. This to reduce the thickness of the composite structure and to densify the metal fiber layer provided by solidification of the primary slurry
  • additional layers of secondary slurry may be applied on the composite structure, i.e. on the surface of the composite structure provided by the solidified primary slurry.
  • Such provision may be done in a substantially identical or similar way as the application of the primary slurry to the porous metal structure.
  • the secondary slurry can be solidified itself in a similar or substantially identical way as described above for the primary slurry.
  • the secondary slurry may be identical to the primary slurry, but it may be a different slurry as well
  • the different solidified secondary slurry layers need not comprise identical metal fibers, nor need they be of an identical metal fiber content per surface unit or volume.
  • the different secondary slurry layers may differ from each other in metal fibers, metal fiber content, thickness, weight and other properties, the provision of several layers , optionally comprising different metal fibers, further has the advantage that during compression of the metal fiber medium, either prior to or after sintering, provide a more equal density or porosity over the whole surface of the medium.
  • the risk on having the porous metal structure surfacing again at the outer side or sides of the sintered metal fiber medium after compression is reduced or avoided in case of compression prior to sintering.
  • the risk on having more dense zones above the warp and weft elements of the mesh is reduced or avoided in case of compression after sintering.
  • additional porous metal structures may be stacked to one or more solidified slurry layers.
  • a metal wire mesh, an expanded metal sheet or one or more layers of air laid web, wet laid web or a layer of metal powder may be added to the solidified slurry layers comprising metal fibers and a binding agent.
  • a metal foil or a metal plate is added to the stack.
  • such porous metal structure or metal foil or plate may be added to the sintered metal fiber medium as subject of the invention, e.g. by sintering such porous metal structure or metal foil or plate to the sintered metal fiber medium as subject of the invention in a second sintering operation.
  • the composite structure is subjected to thermal treatment, for debinding of the binding agent, and consecutively sintering the metal fibers to each other.
  • Such debinding and sintering may be done in one thermal operation, or may be executed as two consecutive operations, not necessarily being done immediately one after the other.
  • the sintered metal fiber medium may further be subjected to compression, e.g. rolling or calendering, in order to further reduce the thickness of the sintered metal fiber medium, or to smoothen the surface of the sintered metal fiber medium.
  • compression e.g. rolling or calendering
  • Fig. 1a and Fig. 1 b show schematically views of a sintered metal fiber medium 1000 as subject of the present invention.
  • the sintered metal fiber medium 1000 as shown in Fig. 1a is a general representation of the media 100 and 200, which are provided according to the embodiments of the method to provide a sintered metal fiber medium as subject of the present invention, and of which some physical properties are set out further in Table 1.
  • the sintered metal fiber medium 1000 as shown in Fig. 1 b is a general representation of the media 300, 400, 500, 600 and 700 which are provided according to the embodiments of the method to provide a sintered metal fiber medium as subject of the present invention, and of which some physical properties are set out further in Table 1.
  • the sintered metal fiber medium 1000 comprises metal fibers 1001 and a porous metal structure 1002.
  • the porous metal structure has two surfaces 1100 and 1200. As set out above, the fibers are present on only one of the surfaces, i.e. on surface 1100, which surface is understood as the fiber covered surface.
  • the fibers 1001 are present in at least part of the pores 1300 of the porous metal structure 1002.
  • the porous metal structure has a thickness T, and the metal fibers 1001 are present in the pores 1300 located between the metal fiber covered surface 1100 and a depth of T/2, as indicated by the plane 1500.
  • this plane 1500 is defined by the centre lines of the different weft elements 1004.
  • the porous metal structure 1002 is a woven metal wire mesh.
  • the pores 1300 are provided between the different warp elements 1003 and weft elements 1004.
  • the porous metal structure i.e. a structure being a wire mesh, was subjected to a compression, which caused the warp elements 1003 of the woven structure to be flattened, providing the mesh with flat surfaces 1005.
  • the porous metal structure 1002 is a combination of a plurality, i.e. in this embodiment two, woven metal wire meshes 1021 and 1022.
  • the first wire mesh 1021 is a finer wire mesh as compared to the second wire mesh 1022, i.e.
  • the pores of the first wire mesh 1021 have a smaller volume and are more numerous per surface unit of the wire mesh, as compared to the second wire mesh 1022.
  • the pores 1300 of the porous metal surface comprise the pores provided between the different warp elements 1213 and weft elements 1214 of the first wire mesh 1021 , the pores provided between the different warp elements 1223 and weft elements 1224 of the second wire mesh 1022, and the void spaces created along the contact zones 1023 at the surface 1053 of the meshes 1021 and the surface 1054 of the meshes 1022.
  • the surfaces 1053 and 1054 are oriented towards each other.
  • the porous metal structure has a thickness T, and the metal fibers 1001 are present in the pores 1300 located between the metal fiber covered surface 1100 and a depth of T/2, as indicated by the plane 1500.
  • each of the wire meshes 1021 and 1022 was subjected to a compression prior to providing the meshes one to the other to create the combination.
  • This compression provides the warp elements 1213 and 1223 of the woven wire mesh structures with flat surfaces 1005.
  • the two wire meshes 1021 and 1022 were sintered one to the other before the metal fiber were provided, in this particular case by applying a slurry comprising the metal fibers to the combination of the plurality of compressed wire meshes.
  • the porous metal structure 1002 is provided.
  • the flat surfaces of both wire meshes create larger contact zones 1023 between the meshes. This has as an effect that larger and stronger sinterbonds are obtained at the contact zones 1023.
  • the porous metal structure thus comprises two flat outer surfaces 1051 and 1052.
  • the effect of the outer surface 1052 being flat and being in contact with the metal fibers 1001 cause the provision of more sinter bonds between metal fibers and outer surface 1052.
  • the meshes E*, K* and S* have been calendered.
  • the mesh type "E*+K*" is to be understood as an E*-mesh and a K*-mesh which were sintered together, optionally as in the present embodiments, after being calendered.
  • FIG. 2 An alternative sintered metal fiber medium 2000 is shown in Fig. 2.
  • the same reference numbers refer to the same or similar technical or functional features as in Fig. 1a and Fig. 1 b.
  • a primary layer of metal fiber slurry On the porous metal structure 1002, a primary layer of metal fiber slurry
  • a secondary layer of metal fiber slurry 1011 is provided by using tape casting techniques, e.g. using a doctor blade to tape cast the slurry 1011. This layer 1011 is solidified.
  • the composite structure of porous metal structure and solidified slurry layers are further processed by debinding, sintering and optionally, compression prior to and/or after sintering.
  • possibly some metal fibers present in the primary slurry 1010 may migrate beyond half the thickness T of the porous metal structure 1002.
  • Some metal fibers from the primary layer 1010 may be present in the pores of the porous metal structure at the second outer surface 1200. At this second surface, the metal fibers present are believed to be not functional for the filtration properties of the sintered metal fiber medium 2000.
  • the side on which the primary and secondary slurry are applied, is understood as the fiber covered surface.
  • the presence of metal fibers from the primary layer of slurry applied to the porous metal structure, in at least part of the pores of the porous metal structure, which is believed to be a characterizing feature of a sintered metal fiber medium according to the present invention have however the advantage that it improves the anchoring of the metal fibers to the porous metal structure after sintering, especially when the primary slurry is provided by spraying or direct casting to the porous metal structure.
  • the metal fibers of the primary metal fiber slurry are present throughout at least part of the thickness of the porous metal structure.
  • a sintered metal fiber medium obtained by using a method as subject of the invention has an improved homogeneity of its physical properties such as air permeability, filtration efficiency, pore size, bubble point pressure and pore distribution.
  • the average thickness of the sintered metal fiber medium may vary over a large range, but relatively thin sintered metal fiber medium may be obtained, e. g. sintered metal fiber medium with thickness less than or equal to 0.2 mm or even less than or equal to 0.1 mm. Even more surprising, it was found that sintered metal fiber media having such thickness less than 0.2 mm or less than 0.1 mm, a bubble point pressure of more than 10000 Pa may be obtained. It was also notices that a high filtration efficiency may be obtained when such sintered metal fiber media having a thickness less than 0.2 mm or less than 0.1 mm are used as a liquid filter.
  • the weight of the sintered metal fiber medium as subject of the invention is optionally less than 1200 g/m 2 , more preferred less than 900 g/m 2 or even less than 600 g/m 2 , such as about 100 g/m 2 .
  • the porosity of the sintered metal fiber medium may vary over a large range, but it was found that such sintered metal fiber medium may have a porosity in the range of 40% to 99%, more preferred in the range of 55% to 80%, such as in the range of 55% to 70%. Without applying a rolling or pressing operation to the solidified primary slurry or sintered metal fiber medium, porosities of 80 to 99% may be obtained. Lower porosities may be obtained by applying a rolling or pressing operation to either the solidified primary slurry and/or the sintered metal fiber medium.
  • a sintered metal fiber medium as subject of the invention may have a mean flow pore size of less than 2 times the equivalent diameter D.
  • the sintered metal fiber medium optionally has a mean flow pore size of less than 1.5 times the equivalent diameter D. More preferred, the mean flow pore size of the sintered metal fiber media is equal or less than the equivalent diameter D of the metal fibers of the sintered metal fiber medium, increased by one ⁇ m.
  • UO average fiber length over diameter
  • an L/D of about 30 to 70 is preferred for metal fibers with equivalent diameter in the range up to 6 ⁇ m, in case the metal fibers are obtained by the process as described in W002/057035, hereby incorporated by reference.
  • the outer surface of the sintered metal fiber medium, to be used as inflow side of the medium when used for solid-liquid surface filtration has a substantially flat surface.
  • substantially flat is meant that the Ra value measured over a statistically relevant length less than three times the equivalent diameter D of the metal fibers of the sintered metal fiber medium. More preferred, Ra value of the first outer surface of the sintered metal fiber medium is less then the equivalent diameter D, for example less than 0.5 times the equivalent diameter D.
  • a sintered metal fiber medium obtained by the method as subject of the invention may advantageously be used as filter medium, for filtration of particulates from fluids, either gas or liquid, e. g. by surface filtration.
  • the sintered metal fiber medium may be used for soot filtration, or for filtration of beverages, such as beer, wine, or for filtration of oils or coolants.
  • the sintered metal fiber medium may also be used in fuel cells.

Abstract

A sintered metal fiber medium according to the present invention comprises metal fibers and a porous metal structure, which porous metal structure has two surfaces. The fibers are present on only one of the surfaces, providing, the fibers are present in at least part of the pores of the porous metal structure. A method for manufacturing a sintered metal fiber medium from metal fibers and a porous metal structure having a first surface and a second surface according to the present invention, comprises the steps of • making a primary slurry comprising metal fibers, a solvent and a binding agent by mixing the metal fibers and the binding agent; • applying the primary slurry on either the first surface or the second surface of the porous metal structure; • solidifying the primary slurry by removing the solvent yielding a composite structure comprising the metal fibers, the binging agent and the porous metal structure; • debinding the binging agent and sintering the composite structure.

Description

A SINTERED METAL FIBER MEDIUM AND A METHOD TO PROVIDE A SINTERED METAL FIBER MEDIUM
Technical field of the invention
The present invention relates to a sintered metal fiber medium and a method to provide a sintered metal fiber medium. The method relates more particularly to a method to provide a sintered metal fiber medium using metal fiber slurry comprising short metal fibers.
Background of the invention
Sintered metal fiber media are well known in the art for numerous applications, such as e.g. liquid or gas filtration.
A first method for providing sintered metal fiber medium is to provide a metal fiber web by air lay down, and sintering this air laid web in appropriate furnaces.
A disadvantage of this air lay down web, is the fact that the web is usually relatively inhomogeneous, especially when relatively thin sintered metal fiber media are to be provided. This is because it is difficult to provide the air laid webs sufficiently homogeneous. Therefore, to have a sintered metal fiber medium with homogenous properties over its surface, usually several air laid webs are stacked (so-called doubled). Another method to provide a web, prior to sintering operation, is to use the so-called wet lay down method or paper making method, as described in
WO98/43756, EP933984A, JP11-131105, JP61 -225400 and JP61-223105.
The metal fibers are brought in a slurry, which slurry is poured on a screen.
The water is extracted, e.g. sucked, from the slurry through the screen. The remaining dewatered slurry is then sintered. A binding agent may be used to temporarily bind the metal fibers to each other and so to make the dewatered slurry transportable. This dewatered slurry is then sintered, optionally first debinding the binding agent.
A disadvantage of the wet webbing is that in case thin and relatively short fibers are used, some of the shorter fibers are sucked through the screen, together with the water being removed from the slurry. In case of thin webs made prior to sintering, the dewatering step may suck small or larger holes in the web where few or no fibers are retained for sintering.
Also, an imprint of the supporting net, used to support the wet slurry during dewatering, is obtained. The net pattern is noticed on the dewatered web as repetitive thinner spots.
As a result, the dewatered slurry and thus the sintered metal fiber medium, may have inhomogeneous zones where less fibers are present, even when several layers of the freshly dewatered webs are stacked one to the other prior to sintering. Especially in case fibers with small equivalent diameter, e.g. 2 μm to 6 μm, are used, the phenomena of sucking out fibers along with the water during dewatering is noticed. This is because usually the amount of fibers with smaller lengths is larger, the finer the fibers are. As a result, more fibers with a short length are sucked out along with the water during dewatering in case of fibers with small equivalent diameter.
As described in WO2005/099940, a method for providing a sintered metal fiber medium is provided, where a slurry of metal fibers is tape cast, sintered after debinding and optionally compressed. A combination is made with other porous metal structures such as wire mesh or expanded metal sheets, by combining debound and optionally sintered foil comprising metal fibers, which combination is then to be sintered to couple the porous structure to the metal fibers. It has been found that the products obtained by this method may suffer from decoupling of the metal fibers from the porous structure during use of the medium e.g. in filtration applications. It is noticed that, e.g. when the medium is used in a filter system, wherein the surface filtration medium is cleaned by applying a flow of fluid in opposite direction as compared to the filtration, the coupling of porous structure and sintered metal fibers may be ruptured. This causes the sintered metal fibers no longer be supported by the porous structure, leading to rupture of the sintered metal fiber structure. Hence the filter system has to be shut down and maintained, causing significant down time of the filter system and considerable production losses. Summary of the invention
It is an object of the present invention to provide an alternative sintered metal fiber medium and method for manufacturing a sintered metal fiber medium. It is an advantage of embodiments of the present invention to provide a sintered metal fiber medium, which has metal fibers applied directly on a porous structure, thereby penetrating into the porous structure. It is an advantage of embodiments of the present invention to improve the anchoring of the metal fibers onto or into the porous structure. It is an advantage of embodiments of the present invention to improve the mechanical strength of the sintered product. It is an advantage of embodiments of the present invention that loosening of the metal fiber layer during cleaning of the sintered metal fiber medium when used as a filter medium is reduced or even prevented. It is an advantage of embodiments of the present invention to provide a sintered metal fiber medium for surface filtration purposes, having a low back pressure as compared to sintered metal fiber media having two sides being provided with a layer of metal fibers. It is an advantage of embodiments of the present invention to provide a sintered metal fiber medium for use as a surface filter medium, having a reduced number of obstructed zones or flowless zones in the medium in which bacteria can grow during use for filtration of edible fluids or even avoiding such zones altogether.
The above objective is accomplished by a method to provide a sintered metal fiber medium and a sintered metal fiber medium according to the present invention.
According to a first aspect of the present invention, a method for manufacturing a sintered metal fiber medium from metal fibers and a porous metal structure having a first surface and a second surface, comprises the steps of • making a primary slurry comprising metal fibers, a solvent and a binding agent by mixing the metal fibers and the binding agent; • applying the primary slurry on either the first surface or the second surface of the porous metal structure; • solidifying the primary slurry by removing the solvent yielding a composite structure comprising the metal fibers, the binding agent and the porous metal structure; and
• debinding the binding agent and sintering the composite structure. According to embodiments of the present invention, debinding of the binding agent and sintering of the composite structure may be done in one step or in two separate steps.
In case water is used as solvent, the removal of the solvent may be by evaporation of the solvent, e.g. a drying step. The evaporation may be assisted by providing dry air and/or heated air to the primary slurry, which may accelerate the evaporation.
According to embodiments of the present invention, the removal of the solvent during solidifying of the primary slurry may be done by evaporating the solvent, such as by thermo-evaporation, i.e. executed or assisted by providing heated air. Alternatively evaporating the solvent may be forced, e.g. by heating the primary slurry, e. g. by forcing heated air over the surface of the primary slurry, or by radiating, e. g. microwave- or IR-radiating.
Solidifying of the slurry by applying evaporation of the solvent, e.g. by drying such as air drying or heating of the slurry by microwave or IR-radiation, prevents the metal fibers of the primary slurry from penetrating into the pores to a too large extent, such as through the pores, to such an extent that the slurry, and hence the metal fibers become present at the other of the first or second side of the porous metal structure. The depth of penetration may be limited to only a part of the thickness T of the porous metal structure, e.g. providing metal fibers being present in the pores located between the metal fiber covered surface and a depth of T/2. This effect is obtained in particular in embodiment by a combination of removing the solvent by evaporation of the solvent and applying the slurry by tape casting of the slurry.
As an alternative, the solvent may be removed from the slurry by having the solvent reacting, such as polymerizing, e.g. under influence of UV- illumination or heat, optionally or by causing a chemical reaction, e.g. a polymerisation, with other components in the slurry, which may harden the solvent. The solvent thus may optionally function as binding agent once solidified.
The solvent may be a UV-curable component, causing the component to solidify under UV-illumination. The solvent may be a heat-curable component, causing the component to solidify under application of heat. The solvent may be a bi- or multi-component solvent, causing the component to solidify by reaction between the two or more components when the solvent is subjected to appropriate conditions, such as appropriate illumination or temperature to cause the reaction to occur. According to embodiments of the present invention, the application of the primary slurry may be done by non-contact or contact deposition techniques.
According to embodiments of the present invention, the application of the primary slurry may be done by tape casting the primary slurry onto the surface of the porous metal structure. According to alternative embodiments of the present invention, the application of the primary slurry may be done by spraying the primary slurry onto the surface of the porous metal structure. According to alternative embodiments of the present invention, the application of the primary slurry may be done by plastering the primary slurry onto the surface of the porous metal structure. According to alternative embodiments of the present invention, the application of the primary slurry may be done by brushing the primary slurry onto the surface of the porous metal structure. According to alternative embodiments of the present invention, the application of the primary slurry may be done by printing, e.g. screen printing, of the primary slurry onto the surface of the porous metal structure.
According to embodiments of the present invention, a method for manufacturing a sintered metal fiber medium from metal fibers and a porous metal structure having a first surface and a second surface, may comprise the steps of
• making a primary slurry comprising the metal fibers, a solvent and a binding agent by mixing the metal fibers, the solvent and the binding agent; • applying the primary slurry on either the first surface or the second surface of the porous metal structure by tape casting the primary slurry onto the surface of the porous metal structure;
• solidifying the primary slurry by removing the solvent by evaporating the solvent, yielding a composite structure comprising the metal fibers, the binding agent and the porous metal structure; and
• debinding the binding agent and sintering the composite structure.
According to embodiments of the present invention, the porous metal structure may be a structure or a combination of structures, this or these structures being chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
Optionally, the structure or the combination of structures may be subjected to a compression step.
According to embodiments of the present invention, the porous metal structure may be a combination of structures. Each of the structures may be chosen from the group consisting of wire meshes, such as a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet. Each of the structures may be a wire mesh, optionally chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh.
Each of the structures, optionally chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh, may be subjected to a compression step prior to making the combination of structures.
According to some embodiments of the present invention, the combination of structures may be sintered prior to applying the primary slurry on the combination of structures.
The porous metal structure may be a combination of a plurality of wire meshes, which combination is optionally subjected to a compression step such as calendering, rolling or pressing. This provides a flattening of the outer surfaces of the combination of the plurality of wire meshes, providing a flat surface to the combination of the plurality of wire meshes. This flattened or flat surface provides a larger contact surface for the slurry to contact the combination of the plurality of wire meshes. When the slurry is solidified, debound and sintered to the combination of the plurality of wire meshes, a stronger sinterbond is obtained between metal fibers provided by the slurry and the flattened surface of the combination of the plurality of wire meshes which are to contact the metal fibers.
The porous metal structure is optionally a combination of a plurality of compressed wire meshes, i.e. wire meshes being individually subjected to a compression step such as calendering, rolling or pressing prior to making the combination, such as by stacking. This has as an effect that the wires present at the outer surface of the structure are flattened, providing a flat surface to the porous metal structure. This has further as an effect that the wires present at the outer surface of each of the structure are flattened, providing flat surfaces to each of the wire meshes. When combining the compressed wire meshes, two flat surfaces will contact each other along contacting zones.
During sintering of the combination of a plurality of compressed wire meshes, either prior to providing the slurry or after the slurry has been provided to the combination of a plurality of compressed wire meshes, a stronger sinterbond is obtained between the meshes of the combination of a plurality of compressed wire meshes along the contact zones between adjacent wire meshes, which are flattened zones.
Each of the wire meshes, to be individually compressed, may be chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh.
Meshes in a combination of structures may have the same or different properties, such as identical or different mesh sizes, identical or different wire diameters or wire alloys. Optionally, in case at least two different wire meshes are used, the wire mesh with the finest mesh size is used to provide the surface of the porous metal structure to which the slurry is provided.
According to embodiments of the present invention, the method may comprise the additional step of impregnating the porous metal structure with a solution of the solvent and the binding agent prior to applying the primary slurry.
According to alternative embodiments of the present invention, the debinding may be done by thermally debinding the binding agent.
According to alternative embodiments of the present invention, the method further may comprise the step of applying at least a secondary slurry on the composite structure. According to alternative embodiments of the present invention, the secondary slurry may comprise secondary metal fibers. According to alternative embodiments of the present invention, the secondary slurry may be different from the primary slurry. According to alternative embodiments of the present invention, the secondary metal fibers may be different from the metal fibers of the primary slurry.
According to alternative embodiments of the present invention, the method further may comprise the step of compressing the porous metal structure prior to applying the primary slurry.
According to alternative embodiments of the present invention, the solvent of the primary slurry may be water.
The present invention also includes a sintered metal fiber medium obtained by a method according to this first aspect of the present invention.
According to a second aspect of the present invention, a sintered metal fiber medium comprises metal fibers and a porous metal structure, which porous metal structure has two surfaces. The fibers are present on only one of the surfaces, providing, the fibers are present in at least part of the pores of the porous metal structure.
According to some embodiments of the present invention, the porous metal structure may be a structure or a combination of structures, which structures or structures is or are chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
According to some embodiments of the present invention, the structure may comprise two flat outer surfaces.
According to some embodiments of the present invention, the porous metal structure may be a combination of structures, each of the structures being wire meshes, such as chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh. According to some embodiments of the present invention, each of the structures may be provided with two flat surfaces, such as by individually compression of each of the wire meshes.
According to some embodiments of the present invention, the metal fibers may be present at only one of the outer surfaces of the porous metal structure.
According to some embodiments of the present invention, the porous metal structure may have a thickness T, and the metal fibers are present in the pores located between the metal fiber covered surface and a depth of T/2.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.
The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.
Brief description of the drawings
Fig. 1a and Fig. 1 b are schematical views of a sintered metal fiber medium as subject of the present invention.
Fig. 2 is schematically a view of an alternative sintered metal fiber medium as subject of the present invention.
Description of illustrative embodiments
The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term "coupled", also used in the claims, should not be interpreted as being restricted to direct connections only.
Thus, the scope of the expression "a device A coupled to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
The following terms are provided solely to aid in the understanding of the invention. These definitions should not be construed to have a scope less than understood by a person of ordinary skill in the art.
With equivalent diameter of a metal fiber is meant the diameter of an imaginary circle having the same surface as the surface of a radial cross section of the fiber.
The bubble point pressure of a filter medium is measured according to the ISO 4003 testing method.
The term "porosity" P is to be understood as P= 100*(1 - d) wherein d = (weight of 1 m3 sintered metal fiber medium)/ (SF) wherein S F = specific weight per m3 of alloy out of which the metal fibers of the sintered metal fiber medium are provided.
The mean flow pore size (also referred to as MFP) can be measured using a "Coulter Porometer I I" testing equipment, which performs measurements of the mean flow pore size according to ASTM F-316-80. The "bubble point pressure" (also referred to as BBP) is determined according to ASTM E128-61 , being he equivalent of ISO4003.
The "Air permeability" (also referred to as AP) is determined according to NF 95-352, being the equivalent of ISO 4002. The Ra value is defined as the arithmetic mean deviation of the surface height from the mean line through the measured profile from the measured length. The mean line is defined so that equal areas of the profile lie above and below the line. The term "slurry" is to be understood as a mixture of a solvent, a binder and insoluble matter, insoluble in this solvent. The solvent may be water.
The term "flat surface of a wire mesh" is to be understood as the wire of the wire mesh being provided with, at at least one of the outer surfaces of the mesh, flat zones, which flat zones are substantially coplanar. The term "equivalent diameter" of a particular fiber is to be understood as the diameter of an imaginary fiber having a circular radial cross section, which cross section has a surface area identical to the average of the surface areas of cross sections of the particular fiber.
In a first step of the method to provide or manufacture a sintered metal fiber medium as subject of the present invention, metal fibers are provided.
Any type of metal or metal alloy may be used to provide the metal fibers. The metal fibers are for example made of steel such as stainless steel. Preferred stainless steel alloys are AISI 300 or AISI 400-serie alloys, such as AISI 316L or AISI 347, or alloys comprising Fe, Al and Cr, stainless steel comprising Chromium, Aluminum and/or Nickel and 0.05 to 0.3 % by weight of Yttrium, Cerium, Lanthanum, Hafnium or Titanium, such as e. g. DIN 1.4767 alloys or Fecralloy®, are used. Also Cupper or Cupper-alloys, or Titanium or Titanium alloys may be used. The metal fibers can also be made of Nickel or a Nickel alloy. Metal fibers may be made by any presently known metal fiber production method, e.g. by bundle drawing operation, by coil shaving operation as described in JP3083144, by wire shaving operations (such as steel wool) or by a method providing metal fibers from a bath of molten metal alloy. In order to provide the metal fibers with their average length, the metal fibers may be cut using the method as described in WO02/057035, or by using the method to provide metal fiber grains such as described in US4664971. The metal fibers used to provide the sintered metal fiber medium are characterized in having an equivalent diameter D and an average fiber length L. Optionally the equivalent diameter D of the metal fibers is less than 100 μm such as less than 65 μm, e.g. less than 36 μm such as 35 μm, 22 μm or 17 μm. Optionally the equivalent diameter of the metal fibers is less than 15μm, such as 14 μm, 12 μm or 11 μm, or even more preferred less than 9 μm such as e.g. 8 μm. As an example, the equivalent diameter D of the metal fibers is less than 7 μm or less than 6 μm, e. g. less than 5 μm, such as 1 μm,1.5 μm, 2 μm, 3 μm,3.5 μm, or 4 μm.
The metal fibers all have an individual fiber length. As some distribution on these fiber lengths may occur, due to the method of manufacturing the metal fibers, the metal fibers, used to provide a sintered metal fiber medium as subject of the invention, will be described as having an average fiber length L but this does not prevent the length of individual fibers differing considerably. This length is determined by measuring a significant number of fibers, according to appropriate statistical standards. The average fiber length of the metal fibers is smaller than 10 mm, e.g. smaller than 6 mm, optionally smaller than 1 mm, such as smaller than 0.8 mm or even smaller than 0.6 mm such as smaller than 0.2 mm. As according to the present invention, substantially all fibers used during the method of manufacturing the sintered metal fiber medium will occur in the sintered metal fiber medium, the average fiber length L can be measured in a similar way on the sintered metal fiber medium.
The metal fibers in the sintered metal fiber medium thus may have a ratio of average fiber length over diameter (UD) being optionally less than 110, more preferred less than 100, but usually more than 30. An L/D of about 30 to 70 is preferred for metal fibers with equivalent diameter in the range up to 6 μm, in case the metal fibers are obtained by the process as described in WO02/057035, hereby incorporated by reference.
According to a second step of the method of the present invention, a primary slurry is provided, which slurry comprises metal fibres, a solvent and a binding agent.
Optionally, the primary slurry comprises an amount of metal fibers in the range of 2% weight to 40% weight of the primary slurry, more preferred between 5% weight and 15% weight of the primary slurry. Apparently, when this primary slurry is used for applying the slurry to a porous metal structure by means of tape casting, as set out in a subsequent step, such concentration combined with the tape casting action to provide substantially flat layers of primary slurry, causes metal fibers to be distributed more homogeneously.
Too much metal fiber in the primary slurry may cause conglomeration of the fibers, causing on its turn inhomogeneous metal fiber distribution throughout the sintered metal fiber medium.
Too little metal fiber in the primary slurry may cause problems during debinding, where too much debinding causes disturbing of the sintering of the metal fibers. Further, such production of sintered metal fiber medium becomes uneconomic, as too much energy is to be consumed for debinding the binding agent, and a large volume of binding material is to be removed. In each cast layer, the metal fiber distribution over the surface may become irregular. The primary slurry comprises a binding agent, optionally a polymer binding agent, and metal fibers.
A binding agent for the purpose of the invention is to be understood as a product for thickening the primary slurry. It is further preferred that the primary slurry comprises a solvent for dissolving the binding agent, which solvent is removed by evaporation during solidification of the primary slurry, such as e.g. water. This has a further advantageous effect on the metal fiber distribution homogeneity over the surface and in depth of the sintered metal fiber medium which results from the further process. Optionally a water soluble binding agent is used, e. g. polyvinyl alcohols, methyl cellulose ethers, hydroxypropylmethylcellulose, polyethers from ethyleneoxide, acrylic acid polymers or acrylic copolymers. The binding agent is added to the solvent, in a concentration of optionally between 0.5% weight and 30 % weight of the primary slurry. Most preferred, a binding agent is chosen which requires a concentration of less than 20% weight or even less than 15 % weight or even less than 10% weight of the primary slurry, in order to provide the required viscosity. A viscosity range between 1.000 cPs and 600.000 cPs is optionally used for the slurry. The components of the primary slurry are blended using appropriate mixing equipment. In case foaming of the primary slurry occurs, small amounts of a defoaming component are added. Optionally the slurry may comprise viscosity changers.
For preparation of the slurry, optionally the fibers are dispersed in the solvent, optionally water, mixed with a dispersant and/or wetting agent by soft stirring. Advantageously a defoaming agent is added. The binding agent is added and after some minutes of stirring, a neutralization agent may be added to block the fluid, i.e. the primary slurry. A shear thinning slurry may be obtained, having a viscosity of more than 300.000 cPs.
In another step of the method to provide a sintered metal fiber medium as subject of the present invention, a porous metal structure is provided. The porous metal structure is optionally a structure or a combination of structures, such as a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet or a perforated metal sheet. In order to further improve the coupling of metal fibers, which will be applied to this porous metal structure in a subsequent step of the method as subject of the present invention, the porous metal structure is subjected to a compression step, e.g. a rolling, calendering or pressing operation, in order to smoothen the surface of such structure. As an example, by compression of a wire mesh, the wires at the outer surfaces of the mesh are flattened. The porous metal structure has optionally a substantially flat structure and two substantially flat surfaces, and a substantially uniform thickness T being the average distance between the two outer surfaces. As a first example, a metal wire mesh of AISI316L wires having a diameter of 0.1 mm is used, having a thickness of about 0.21 mm and a surface weight of 493 g/m2. Alternatively, the mesh may be e.g. a metal wire mesh of AISI316L wires having a diameter of 0.25 mm. This mesh has a thickness of about 0.5 mm and a surface weight of 1220 g/m2.
Alternatively, a first metal wire mesh, such as a wire mesh of AISI316L wires having a diameter of 0.1 mm, having a thickness of about 0.21 mm and a surface weight of 493 g/m2 is used. The first wire mesh was compressed by pressing the wire mesh. A second wire mesh, such as a wire mesh of AISI316L wires having a diameter of 0.25 mm is provided. Also this second wire mesh is compressed, such as pressed or calendered. This second wire mesh has a thickness of about 0.5 mm and a surface weight of 1220 g/m2. The first and second wire mesh are stacked and sintered to each other, thereby providing an alternative porous metal structure. During provision of the slurry, as will be set out further, the porous metal structure will be oriented and provided in such a way that the slurry is provided to the porous metal structure by applying it to the outer surface of the porous metal structure, which outer surface is provided by the first wire mesh.
In a subsequent step, the primary slurry is applied to only one of the two outer surfaces of the porous metal structure, whereby a composite structure is provided. This application can be done by many different techniques, such as tape casting, spraying, plastering, brushing or printing, such as screen-printing.
It was found that by the provision of the slurry in liquid or viscous state to the porous metal structure, the metal fibers anchor better to the porous metal structure, especially if the porous metal structure is provided with a flat surface, e.g. by compressing wire meshes being part of the porous metal structure. Although the theory behind this phenomenon is not completely understood at present, and without being limited by theory, it is believed that the penetration of at least a part of the metal fibers in the pores of the porous metal structure, and optionally the presence of flat surfaces for contacting the metal fibers, causes the better anchoring of the metal fibers once the structure has been sintered later on. It was noticed that when the primary slurry is applied to a first side of the porous metal structure, the metal fibers migrate into the pores of the porous metal structure. The fibers may migrate through the whole thickness of the porous metal structure and be present in the pores at both sides of the porous metal structure. However, no layer of metal fibers will be formed at the opposite, second side of the porous metal structure. Optionally, the migration is not deeper than a part of the thickness T of the porous metal structure, such as only half the thickness T of the porous metal structure. This effect is obtained in particular when the slurry is provided by tape casting the slurry onto one of the outer surfaces of the porous metal structure. Once sintered, these migrated fibers have the additional effect that, when the sintered metal fiber medium is used as surface filtration medium, the medium has a limited back pressure during filtration, because usually metal fibers are only present along a flow path for the fluid along the thickness of the metal fiber layer, together with preferable at maximum half the thickness of the porous metal structure. Further, the risk on flowless zones under or in the medium (as compared to media having metal fibers at both sides of the porous metal structure) or between the metal fiber layer and the porous metal structure (as compared to media being provided by coupling a sintered or debound metal fiber layer to the porous metal structure) is significantly reduced or avoided. This has the advantage that the risk of contamination with bacteria is reduced or even avoided when the sintered metal fiber medium is used in filtration processes of edible fluids such as edible oils, beverages, wine, fruit juice and beer. It was found advantageous to first apply a binding agent to the porous metal structure, e.g. to impregnate the porous metal structure with a solution of solvent and binding agent, prior to applying the primary slurry. The metal fibers from the primary slurry migrate in the solution of solvent and binding agent, which was already provided to the porous metal structure.
When the primary slurry is applied by tape casting, e.g. by using a doctor blade applicator for casting on a substantially flat surface of the porous metal structure, the clearance of the applicator is kept relatively small, such as between 0.2 mm and 6 mm, more preferred between 0.2 mm and 3 mm. The speed of movement of the applicator is chosen according to the viscosity of the slurry and the composition of the primary slurry.
The clearance and thus the thickness of the layer of the primary slurry is chosen in function of the amount of metal fibers in the primary slurry, the required weight per surface unit of the sintered metal fiber medium, and the required density of the metal fiber layer of sintered metal fiber medium. When using a doctor blade applicator, the slurry is pumped into the slurrybox, which is mounted in front of the blade. The slurry, which may comprise a wide range of metal fibers, is provided on a porous metal structure, a wet layer of slurry of 100μm to 3000μm may be provided. The thickness is influenced by the substrate, gap and shape of blade; the casting speed and the reological behavior of the slurry. The porous metal structure is usually supported itself by a belt or flat surface.
When the primary slurry is provided by means of spraying, a thin layer of the primary slurry may be provided using paint coating nozzle guns. The nozzle of the gun is optionally at least 0.1mm diameter. Optionally, thick- sprayed primary layers are provided using a body coater gun. Optionally, the body coater gun has a nozzle diameter of more than 4 mm, e.g. 5 mm. The slurry can be sucked by the vacuum of the paint coating gun or body coater gun, or can be provided to the gun under pressure.
When the primary slurry is provided by means of plastering, the primary slurry can be provided by using the spoon filling technique, which is done using a flat or straight spoon. Optionally, the primary slurry is to have a shear thinning reological behavior with a rather high viscosity in the range of 10.000 cPs to 1.000.000 cPs.
When the primary slurry is provided by means of brushing, optionally the primary slurry has a viscosity ranging from 1.000 cPs to 700.000 cPs. Most preferred a soft brush is used. There must always be a large amount of slurry on the brush.
When the primary slurry is provided by means of screen printing, optionally the primary slurry has a viscosity ranging from 3.000 cPs to 700.000 cPs, having good flow behavior and egalisation effects.
It was found that the provision of the slurry by means of one of the steps as set out above, and in particular using tape casting of the slurry on the porous metal structure, the presence of metal fibers at the second surface of the porous metal structure, on which no slurry was provided, may be avoided.
In a next step, the primary slurry is solidified, forming a composite structure which comprises the binding agent, the porous metal structure and the metal fibers. During solidification, i.e. removing enough solvent to render the slurry solid, the solvent is removed from the primary slurry.
This may be done by evaporating the solvent. A solvent may be used which evaporates easily at ambient temperature. Alternatively, the evaporation may be executed as a drying step in case water was used as solvent. The drying or evaporating may be thermo-evaporation, i.e. executed or assisted by air-drying or may be forced by heating the primary slurry, e. g. by forcing heated air over the surface of the primary slurry, or by radiating, e. g. microwave- or IR- radiating.
It is understood that only the solvent, e.g. water is removed, which solvent was not chemically bound to the binding agent.
It is understood that, in case solvent is evaporated, the thickness of the primary slurry is reduced up to some extent, as the volume of the primary slurry is reduced to provide the volume of the composite structure.
As it is clear that during solidification of the primary slurry, no fibers are removed, the L/D ratio of the fibers in the solidified and later on the sintered metal fiber medium is identical to the L/D of the metal fibers used to make the primary slurry. After solidification of the primary slurry layer, the composite structure so obtained may be subjected to a compression step, e.g. by a plate press or by means of a roller press or calendering. This to reduce the thickness of the composite structure and to densify the metal fiber layer provided by solidification of the primary slurry Optionally, additional layers of secondary slurry may be applied on the composite structure, i.e. on the surface of the composite structure provided by the solidified primary slurry. Such provision may be done in a substantially identical or similar way as the application of the primary slurry to the porous metal structure. After the provision of one or more layers of secondary slurry, the secondary slurry can be solidified itself in a similar or substantially identical way as described above for the primary slurry. The secondary slurry may be identical to the primary slurry, but it may be a different slurry as well The different solidified secondary slurry layers need not comprise identical metal fibers, nor need they be of an identical metal fiber content per surface unit or volume. The different secondary slurry layers may differ from each other in metal fibers, metal fiber content, thickness, weight and other properties, the provision of several layers , optionally comprising different metal fibers, further has the advantage that during compression of the metal fiber medium, either prior to or after sintering, provide a more equal density or porosity over the whole surface of the medium. The risk on having the porous metal structure surfacing again at the outer side or sides of the sintered metal fiber medium after compression is reduced or avoided in case of compression prior to sintering. In case of use of a metal mesh as porous metal structure, the risk on having more dense zones above the warp and weft elements of the mesh, is reduced or avoided in case of compression after sintering.
Optionally, other additional porous metal structures may be stacked to one or more solidified slurry layers. As an example, a metal wire mesh, an expanded metal sheet or one or more layers of air laid web, wet laid web or a layer of metal powder may be added to the solidified slurry layers comprising metal fibers and a binding agent.
Optionally, a metal foil or a metal plate is added to the stack. Alternatively, such porous metal structure or metal foil or plate may be added to the sintered metal fiber medium as subject of the invention, e.g. by sintering such porous metal structure or metal foil or plate to the sintered metal fiber medium as subject of the invention in a second sintering operation.
In a final step, the composite structure, optionally provided with additional layers of solidified secondary slurry or additional layers of metal fiber web, is subjected to thermal treatment, for debinding of the binding agent, and consecutively sintering the metal fibers to each other.
Such debinding and sintering may be done in one thermal operation, or may be executed as two consecutive operations, not necessarily being done immediately one after the other.
After sintering, the sintered metal fiber medium may further be subjected to compression, e.g. rolling or calendering, in order to further reduce the thickness of the sintered metal fiber medium, or to smoothen the surface of the sintered metal fiber medium.
It is understood that time and temperatures of these steps are to be chosen in function of the alloy and volume of the sintered metal fiber media to be obtained. Fig. 1a and Fig. 1 b show schematically views of a sintered metal fiber medium 1000 as subject of the present invention. The sintered metal fiber medium 1000 as shown in Fig. 1a is a general representation of the media 100 and 200, which are provided according to the embodiments of the method to provide a sintered metal fiber medium as subject of the present invention, and of which some physical properties are set out further in Table 1. The sintered metal fiber medium 1000 as shown in Fig. 1 b is a general representation of the media 300, 400, 500, 600 and 700 which are provided according to the embodiments of the method to provide a sintered metal fiber medium as subject of the present invention, and of which some physical properties are set out further in Table 1.
The sintered metal fiber medium 1000 comprises metal fibers 1001 and a porous metal structure 1002. The porous metal structure has two surfaces 1100 and 1200. As set out above, the fibers are present on only one of the surfaces, i.e. on surface 1100, which surface is understood as the fiber covered surface. The fibers 1001 are present in at least part of the pores 1300 of the porous metal structure 1002.
The porous metal structure has a thickness T, and the metal fibers 1001 are present in the pores 1300 located between the metal fiber covered surface 1100 and a depth of T/2, as indicated by the plane 1500. In case of a wire mesh, this plane 1500 is defined by the centre lines of the different weft elements 1004.
In the embodiments as shown in Fig. 1 b, the porous metal structure 1002 is a woven metal wire mesh. The pores 1300 are provided between the different warp elements 1003 and weft elements 1004. The porous metal structure i.e. a structure being a wire mesh, was subjected to a compression, which caused the warp elements 1003 of the woven structure to be flattened, providing the mesh with flat surfaces 1005. In the embodiments as shown in Fig. 1a, the porous metal structure 1002 is a combination of a plurality, i.e. in this embodiment two, woven metal wire meshes 1021 and 1022. The first wire mesh 1021 is a finer wire mesh as compared to the second wire mesh 1022, i.e. the pores of the first wire mesh 1021 have a smaller volume and are more numerous per surface unit of the wire mesh, as compared to the second wire mesh 1022. The pores 1300 of the porous metal surface comprise the pores provided between the different warp elements 1213 and weft elements 1214 of the first wire mesh 1021 , the pores provided between the different warp elements 1223 and weft elements 1224 of the second wire mesh 1022, and the void spaces created along the contact zones 1023 at the surface 1053 of the meshes 1021 and the surface 1054 of the meshes 1022. The surfaces 1053 and 1054 are oriented towards each other.
The porous metal structure has a thickness T, and the metal fibers 1001 are present in the pores 1300 located between the metal fiber covered surface 1100 and a depth of T/2, as indicated by the plane 1500.
As shown in Fig. 1a, each of the wire meshes 1021 and 1022 was subjected to a compression prior to providing the meshes one to the other to create the combination. This compression provides the warp elements 1213 and 1223 of the woven wire mesh structures with flat surfaces 1005. The two wire meshes 1021 and 1022 were sintered one to the other before the metal fiber were provided, in this particular case by applying a slurry comprising the metal fibers to the combination of the plurality of compressed wire meshes. Once the wire meshes were sintered, the porous metal structure 1002 is provided. The flat surfaces of both wire meshes create larger contact zones 1023 between the meshes. This has as an effect that larger and stronger sinterbonds are obtained at the contact zones 1023. The porous metal structure thus comprises two flat outer surfaces 1051 and 1052. The effect of the outer surface 1052 being flat and being in contact with the metal fibers 1001 cause the provision of more sinter bonds between metal fibers and outer surface 1052. TABLE 1
Figure imgf000024_0001
Details of the type of mesh are hereafter enclosed in Table 2
Table 2
Figure imgf000024_0002
Figure imgf000025_0001
The meshes E*, K* and S* have been calendered. The mesh type "E*+K*" is to be understood as an E*-mesh and a K*-mesh which were sintered together, optionally as in the present embodiments, after being calendered.
An alternative sintered metal fiber medium 2000 is shown in Fig. 2. The same reference numbers refer to the same or similar technical or functional features as in Fig. 1a and Fig. 1 b. On the porous metal structure 1002, a primary layer of metal fiber slurry
101 is applied by spraying the primary slurry to the porous metal structure 1002. After solidification of this primary metal fiber slurry 1010, a secondary layer of metal fiber slurry 1011 is provided by using tape casting techniques, e.g. using a doctor blade to tape cast the slurry 1011. This layer 1011 is solidified.
The composite structure of porous metal structure and solidified slurry layers are further processed by debinding, sintering and optionally, compression prior to and/or after sintering. As shown in Fig. 2, possibly some metal fibers present in the primary slurry 1010 may migrate beyond half the thickness T of the porous metal structure 1002. Some metal fibers from the primary layer 1010 may be present in the pores of the porous metal structure at the second outer surface 1200. At this second surface, the metal fibers present are believed to be not functional for the filtration properties of the sintered metal fiber medium 2000. The side on which the primary and secondary slurry are applied, is understood as the fiber covered surface.
The presence of metal fibers from the primary layer of slurry applied to the porous metal structure, in at least part of the pores of the porous metal structure, which is believed to be a characterizing feature of a sintered metal fiber medium according to the present invention have however the advantage that it improves the anchoring of the metal fibers to the porous metal structure after sintering, especially when the primary slurry is provided by spraying or direct casting to the porous metal structure. As an example the metal fibers of the primary metal fiber slurry are present throughout at least part of the thickness of the porous metal structure.
Surprisingly it was found that a sintered metal fiber medium obtained by using a method as subject of the invention, has an improved homogeneity of its physical properties such as air permeability, filtration efficiency, pore size, bubble point pressure and pore distribution.
The average thickness of the sintered metal fiber medium may vary over a large range, but relatively thin sintered metal fiber medium may be obtained, e. g. sintered metal fiber medium with thickness less than or equal to 0.2 mm or even less than or equal to 0.1 mm. Even more surprising, it was found that sintered metal fiber media having such thickness less than 0.2 mm or less than 0.1 mm, a bubble point pressure of more than 10000 Pa may be obtained. It was also notices that a high filtration efficiency may be obtained when such sintered metal fiber media having a thickness less than 0.2 mm or less than 0.1 mm are used as a liquid filter.
The weight of the sintered metal fiber medium as subject of the invention is optionally less than 1200 g/m2, more preferred less than 900 g/m2 or even less than 600 g/m2, such as about 100 g/m2.
The porosity of the sintered metal fiber medium may vary over a large range, but it was found that such sintered metal fiber medium may have a porosity in the range of 40% to 99%, more preferred in the range of 55% to 80%, such as in the range of 55% to 70%. Without applying a rolling or pressing operation to the solidified primary slurry or sintered metal fiber medium, porosities of 80 to 99% may be obtained. Lower porosities may be obtained by applying a rolling or pressing operation to either the solidified primary slurry and/or the sintered metal fiber medium.
As the sintered metal fiber medium may be used for surface filtration in solid-liquid filtration. A sintered metal fiber medium as subject of the invention may have a mean flow pore size of less than 2 times the equivalent diameter D.
It was found that the sintered metal fiber medium optionally has a mean flow pore size of less than 1.5 times the equivalent diameter D. More preferred, the mean flow pore size of the sintered metal fiber media is equal or less than the equivalent diameter D of the metal fibers of the sintered metal fiber medium, increased by one μm.
In the preferred case, when the mean flow pore size of less than 2 times the equivalent diameter D and when the metal fibers in the sintered metal fiber medium have a ratio of average fiber length over diameter (UO) which is optionally less than 110, more preferred less than 100, but usually more than 30, surprisingly it was found that such sintered metal fiber media can be cleaned repetitively, e.g., back flush or back pulse, with high efficiency and apparently with a restricted or even no particles retained after cleaning. Especially when the method is used in which all the solvent is removed by evaporation.
An L/D of about 30 to 70 is preferred for metal fibers with equivalent diameter in the range up to 6 μm, in case the metal fibers are obtained by the process as described in W002/057035, hereby incorporated by reference. Advantageously the outer surface of the sintered metal fiber medium, to be used as inflow side of the medium when used for solid-liquid surface filtration, has a substantially flat surface. With substantially flat is meant that the Ra value measured over a statistically relevant length less than three times the equivalent diameter D of the metal fibers of the sintered metal fiber medium. More preferred, Ra value of the first outer surface of the sintered metal fiber medium is less then the equivalent diameter D, for example less than 0.5 times the equivalent diameter D.
A sintered metal fiber medium obtained by the method as subject of the invention may advantageously be used as filter medium, for filtration of particulates from fluids, either gas or liquid, e. g. by surface filtration. As an example, the sintered metal fiber medium may be used for soot filtration, or for filtration of beverages, such as beer, wine, or for filtration of oils or coolants. The sintered metal fiber medium may also be used in fuel cells The invention has been described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured in other ways according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims. It is also to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention.

Claims

1.- A method for manufacturing a sintered metal fiber medium from metal fibers and a porous metal structure having a first surface and a second surface, comprising the steps of • making a primary slurry comprising said metal fibers, a solvent and a binding agent by mixing said metal fibers and said binding agent;
• applying said primary slurry on either said first surface or said second surface of said porous metal structure;
• solidifying said primary slurry by removing said solvent yielding a composite structure comprising said metal fibers, said binding agent and said porous metal structure; and
• debinding said binding agent and sintering said composite structure.
2.- A method as in claim 1 , wherein removing the solvent during solidifying of the primary slurry is done by evaporating the solvent.
3.- A method as in any one of the claims 1 to 2, wherein said applying of said primary slurry is done by tape casting said primary slurry on said surface of said porous metal structure.
4.- A method as in any one of the claims 1 to 2, wherein said applying of said primary slurry is done by spraying said primary slurry on said surface of said porous metal structure.
5.- A method as in any one of the claims 1 to 2, wherein said applying of said primary slurry is done by plastering said primary slurry on said surface of said porous metal structure.
6.- A method as in any one of the claims 1 to 2, wherein said applying of said primary slurry is done by brushing said primary slurry on said surface of said porous metal structure.
7.- A method as in any one of the claims 1 to 2, wherein said applying of said primary slurry is done by printing said primary slurry on said surface of said porous metal structure.
8.- A method as in claim 7, wherein said printing is done by screen printing.
9.- A method as in any one of the preceding claims, wherein said porous metal structure is a structure or a combination of structures, said structures being chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
10.- A method as in claim 9, wherein the structure or the combination of structures is subjected to a compression step.
11.- A method as in any one of the claims 9 or 10, wherein said porous metal structure is a combination of structures, each of the structures is subjected to a compression step prior to making the combination of structures.
12.- A method as in claim 11 , wherein the combination of structures is sintered prior to applying said primary slurry on the combination of structures.
13.- A method as in any one of the preceding claims, wherein said method comprises the additional step of impregnating said porous metal structure with a solution of said solvent and said binding agent prior to applying said primary slurry.
14.- A method as in any one of the preceding claims, wherein said debinding is done by thermally debinding said binding agent.
15.- A method as in any one of the preceding claims, wherein said method further comprises the step of applying at least a secondary slurry on said composite structure.
16.- A method as in claim 15, wherein said secondary slurry is different from said primary slurry.
17.- A method as in any one of the claims 15 to 16, wherein said secondary slurry comprises secondary metal fibers.
18.- A method as in claim 17, wherein said secondary metal fibers are different from said metal fibers of said primary slurry.
19.- A method as in any one of the preceding claims, wherein said method further comprises the step of compressing said porous metal structure prior to applying said primary slurry.
20.- A method as in any one of the preceding claims, wherein said solvent of said primary slurry is water.
21.- A sintered metal fiber medium comprising metal fibers and a porous metal structure, said porous metal structure having two surfaces, said fibers are present on only one of said surfaces providing a fiber covered surface, said fibers are present in at least part of the pores of said porous metal structure.
22.- A sintered metal fiber medium as in claim 21 , wherein said porous metal structure is a structure or a combination of structures, said structures being chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh, a knitted metal wire mesh, an expanded metal sheet and a perforated metal sheet.
23.- A sintered metal fiber medium as in claim 22, wherein the structure comprises two flat outer surfaces.
24.- A sintered metal fiber medium as in claim 22, wherein said porous metal structure is a combination of structures, each of the structures is chosen from the group consisting of a woven metal wire mesh, a braided metal wire mesh and a knitted metal wire mesh and each of the structures is provided with two flat surfaces.
25.- A sintered metal fiber medium as in any one of the claims 21 to 24, wherein said metal fibers are present at only one of the outer surfaces of the porous metal structure.
26.- A sintered metal fiber medium as in any one of the claims 21 to 25, wherein said porous metal structure has a thickness T, said metal fibers are present in the pores located between said metal fiber covered surface and a depth of 112.
27.- A sintered metal fiber medium obtainable by a method according to any of the claims 1 to 20.
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