Magnetic filter medium and method for its production

Abstract

A filter medium for removing metallic particles, in particular for liquid filtration, is provided. The filter medium is comprised of a substrate, an adhesive layer containing magnetic particles applied onto the substrate, and at least one nanofiber layer arranged on the adhesive layer.

Claims

1. A filter element for filtering a fluid and removing metallic particles from the fluid being filtered, the filter element comprising: a substrate layer configured to be flowed thorough by the fluid to be filtered, the substrate layer selected from the group consisting of: a nonwoven polymer fiber layer, a non-woven natural fiber layer, and a cellulose layer; an adhesive layer configured to be flowed thorough by the fluid to be filtered, the adhesive layer containing magnetic ferrite particles having ferromagnetic properties, the magnetic ferrite particles fixedly embedded into adhesive of the adhesive layer, the adhesive layer arranged directly on an upstream side of the substrate layer; a first fiber layer as a first nanofiber layer configured to be flowed thorough by the fluid to be filtered, the first nanofiber layer arranged on an upstream side of the adhesive layer, such that the adhesive layer and magnetic ferrite particles are sandwiched between the substrate layer and the first nanofiber layer; wherein the metallic particles of the fluid entering through the first nanofiber layer are captured by the magnetic ferrite particles of the adhesive layer and thereby removed from the fluid flowing though the filter medium; wherein the substrate layer, adhesive layer and first nanofiber layer together circumferentially close about a hollow interior of the filter element.

2. The filter element according to claim 1, wherein the filter element is a round filter element circumferentially closed about the hollow interior of the filter element.

3. The filter element according to claim 1, wherein the filter medium comprises a second fiber layer arranged directly on the first nanofiber layer; wherein the second fiber layer is a layer of thick nanofibers having fiber diameters greater than fiber diameters of the first nanofiber layer.

4. The filter element according to claim 3, wherein the first nanofiber layer and the second fiber layer have fiber diameters in a range between 50 nm and 800 nm.

5. The filter element according to claim 1, wherein fibers of the first nanofiber layer are selected from the set consisting of: polyamide, aliphatic polyamide, aromatic polyamide, polysulfone, cellulose acetate, polyethersulfone, polyurethane, polyurea urethane, polybenzimidazole, polyetherimide, polyacrylonitrile, polyethylene terephthalate, polypropylene, polyaniline, polyethylene oxide, polyethylene naphthalate, polybutylene terephthalate, styrene butadiene rubber, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidene fluoride, and polyvinyl butylene.

6. The filter element according to claim 1, wherein the filter medium of the filter element is folded into a star shape.

7. The filter element according to claim 1, wherein the magnetic ferrite particles are present in powder form and wherein the magnetic particles have a particle size distribution in the range of 0.1 to 700m.

8. The filter element according to claim 1, wherein the adhesive layer is a hotmelt adhesive.

9. The filter element according to claim 1, wherein the adhesive layer is comprised of a reactive adhesive; wherein the reactive adhesive is comprised of an aqueous dispersion of polyurethane and polyacrylic esters.

10. The filter element according to claim 1, wherein the first fiber layer comprises polyamide fibers.

11. The filter element according to claim 1, wherein said first nanofiber layer selected from the set consisting of: a polymer fiber nonwoven; and a natural fiber nonwoven.

12. The filter element according to claim 1, wherein the filter medium is configured for removing metallic particles from fluids in electrical discharge machining, operable for reducing erosion in eroding processes, in treating electrolytic wastewater and sludges, or in water/drinking water treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in the following in more detail with the aid of the drawings. It is shown in:

(2) FIG. 1 schematically the configuration of a filter medium coated with nanofibers according to the prior art;

(3) FIG. 2 schematically the configuration of a filter medium according to the invention;

(4) FIG. 3 a possible application of the filter medium according to the invention;

(5) FIG. 4 an exemplary graph of time-dependent pressure increase of reference material (only NF) and additional HM layer and F; and

(6) FIG. 5 an exemplary graph of the course of turbidity as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

(7) The invention is based on the idea of combining the known good filtration efficiency of nanofibers with magnetic separation effects in order to separate particularly effectively magnetic metal particles that are formed e.g. in eroding processes.

(8) For this purpose, a filter medium is used in which on a substrate a magnetic particle-containing adhesive layer is applied and this adhesive layer is coated with at least one fiber layer. For example the fiber layer is a meltblown or spunbond nonwoven. Especially the fiber layer is a nanofiber layer. The substrate can be e.g. a polymer fiber nonwoven, a natural fiber nonwoven or a cellulose substrate. The employed adhesive material can be a hotmelt adhesive (hotmelt), 2- or multi-component adhesive or a reactive adhesive wherein the reactive adhesive may be comprised e.g. of an aqueous dispersion. Reactive adhesives are adhesive materials which, over time, will cross-link and form a chemical network, mainly due to the presence of moisture in the air (reactive polyurethanes or polyolefins) as well as adhesive materials that form in the dry state a network, and therefore more a physical one (for example, acrylate dispersions). An example of an aqueous dispersion is the Akrylep 417E adhesive of the company Lear which is comprised of polyurethane and polyacrylic esters.

(9) For the magnetic separation effect, magnetic particles are used which are in particular in powder form, e.g. strontium ferrite. Particularly advantageous is the use of a ferrite powder wherein the particle size distribution is in the range of 0.1 to 700 m. The magnetic particles are either sprinkled onto the adhesive layer or are admixed with the adhesive which has the advantage that no sedimentation occurs during sprinkling. Subsequently, the adhesive layer provided with the magnetic particles is applied onto the substrate. Also possible is a two-sided coating of the substrate with the mixture of adhesive and magnetic particles so that a higher application is possible.

(10) Subsequently, the substrate is coated with at least one layer of nanofibers. The nanofibers are comprised preferably of polyamide but can also be produced of other materials. The nanofibers are preferably produced by electrospinning wherein from solutions or melts, preferably of polymers, continuous fibers with diameters of a few millimeters to a few nanometers can be produced. The fibers can be made of all suitable polymers, including thermoplastic and thermoset polymers. Suitable polymers for producing nanofibers comprise, for example, but are not limited to, polyamide, aliphatic polyamide, aromatic polyamide, polysulfone, cellulose acetate, polyethersulfone, polyurethane, polyurea urethane, polybenzimidazole, polyetherimide, polyacrylonitrile, polyethylene terephthalate, polypropylene, polyaniline, polyethylene oxide, polyethylene naphthalate, polybutylene terephthalate, styrene butadiene rubber, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidene fluoride, polyvinyl butylene, copolymers or derived compounds and combinations thereof.

(11) The filter medium which is produced in this way is thus comprised of a substrate, an adhesive layer with magnetic particles applied to the substrate, and at least one layer of nanofibers.

(12) In a particular embodiment of the filter medium according to the invention, the substrate with the applied adhesive layer is coated with two or more nanofiber layers which have different fiber diameters or fineness. Advantages of a different layering can be optimized pressure losses and/or optimized dust capacities. For example, a layer of thick nanofibers (fiber diameters approximately 240 nm) followed by a layer of thin nanofibers (fiber diameter approximately 90 nm) can be applied onto the adhesive layer. In general, fiber diameters in a range between 50 nm and 800 nm can be used in combination. Reverse layering is possible also.

(13) In addition to the good filtration efficiency of the nanofibers, the filter medium according to the invention has also good magnetic separation effects due to the use of magnetic particles. The problem known from the prior art of the magnetic particles being washed out when introduced into nanofibers or fine fibers due to the fact that the diameters of the magnetic particles are often greater than the diameters of the fibers (compare dissertation Rcker and dissertation by Bistram) is circumvented by introducing the magnetic particles into the adhesive layer according to the invention. After binding of the adhesive, the magnetic particles are fixedly embedded in the adhesive matrix and can therefore not be washed out.

(14) In the following, an embodiment of the method according to the invention is provided. In this context, so-called EDM (electrical discharge machining) media were used that are water-resistant.

(15) As magnetic particles, strontium ferrite powder of the type No. 15 of the company Tridelta was used. This is a hard ferritic material which is permanently magnetic. As a hotmelt the product 614.18 of the company Jowat AG was used. 25 g of the ferrite powder was premixed with 75 g of the hotmelt. The mixture was melted to 200 C., thoroughly mixed with each other, and subsequently cooled to room temperature. The hotmelt ferrite mixture was then applied at a melting temperature of 145 C. by means of a nozzle system. The nozzle diameter was 1 mm. The substrate was uniformly coated in this way with the modified hotmelt so that a hotmelt layer of approximately 25 g/m.sup.2 was produced. The fiber diameter of the hotmelt filaments was on average approximately 0.3 mm. A polypropylene melt-blown was selected as a coating substrate.

(16) The polypropylene nonwoven which was provided with the hotmelt ferrite mixture was subsequently provided with a first nanofiber layer of 0.5 g/m.sup.2. The average fiber diameter was 240 nm. Subsequently, the medium was provided with a second layer of nanofibers. The applied material of this layer was 0.1 g/m.sup.2 and the average fiber diameter 90 nm.

(17) For reference purposes, also a polypropylene melt-blown medium was provided. The latter was however not coated with a ferrite-containing hotmelt layer but only with a first nanofiber layer (0.5 g/m.sup.2) whose average fiber diameter was 240 nm and a second nanofiber layer (0.1 g/m.sup.2) whose average fiber diameter was 90 nm.

(18) For the spinning tests, a lab spinning device of the company Elmarco (NS Lab 500) was used. The distance between wire and counter electrodes was 170 mm. The coating rate was generally 10 Hz (0.26 m/min) at an electrode voltage of 80 KV, the amperage approximately 0.016 mA and the electrode speed 38 Hz (6.1 rpm). The humidity was 58% r.h. at 22 C.

(19) The production of the nanofibers with the diameter of 240 nm was done with the following formulation:

(20) 16% polyamide (BASF Ultramid B24)

(21) 28% formic acid (99%)

(22) 55% acetic acid (96%)

(23) The production of the nanofibers with the diameter of 90 nm was done with the following formulation:

(24) 14% polyamide (BASF Ultramid B24)

(25) 29% formic acid (99.9%)

(26) 57% acetic acid (96%).

(27) Measurement and Comparison of the Filtration Efficiency

(28) FIG. 1 shows schematically the configuration of the reference sample 2 of polypropylene melt-blown substrate 4 with a first layer 6 of nanofibers with a fiber diameter of 240 nm deposited thereon and a second nanofiber layer 8 with a fiber diameter of 90 nm.

(29) FIG. 2 shows schematically the configuration of the sample 10 according to the invention of a polypropylene melt-blown substrate 4 with a hotmelt ferrite layer 12 applied thereon, a first layer 6 of nanofibers with a fiber diameter of 240 nm deposited thereon, and a second nanofiber layer 8 with a fiber diameter of 90 nm.

(30) On an EDM testing apparatus a standardized steel material of the type X210CrW12 with the dimensions of widthlengththickness=10040066 mm was eroded. The eroding removal rate was 120 mm.sup.2/min. The produced particles were taken up with water and at a rate of 17.5 l/h passed through a surface of 528 cm.sup.2 of the filter medium to be tested. The turbidity (NTU, nephelometric turbidity unit) was measured according to ISO 7027 (scattered light measurement 90 angle, wavelength 860 nm).

(31) Since turbidity is caused by the EDM particles, the decrease of the turbidity is a measure for the success of filtration.

(32) Recorded was the pressure increase in bar across the running time in hours as well as the decrease of turbidity, also across the running time in hours.

(33) The following Table 1 shows a comparison of the pressure increase for reference sample and sample according to the invention,

(34) TABLE-US-00001 TABLE 1 time pressure increase pressure increase [hours] reference [bar] invention [bar] 9 0.98 0.42 12 1.29 0.76 14 1.49 0.98 16.5 1.74 1.26 18 1.89 1.42 19 2.00 1.53 22 2.30 1.87 23 2.40 1.98

(35) The diagram of FIG. 4 illustrates the results of Table 1 (REF=reference material; HM=hot melt; F=fibers).

(36) The following Table 2 shows a comparison of the turbidity courses for reference sample and sample according to the invention.

(37) TABLE-US-00002 TABLE 2 time turbidity turbidity [hours] reference [NTU] invention [NTU] 0 0.98 0.42 0.25 1.29 0.76 1 1.49 0.98 1.5 1.74 1.26 3 1.89 1.42 22 2.00 1.53

(38) The diagram of FIG. 5 illustrates the results of the Table 2 (HM=hotmelt; F=fibers).

(39) The embodiment demonstrates that introducing the adhesive (hotmelt) with magnetic particles leads to a significantly faster drop of turbidity; also, the pressure increase is lower. The adhesive layer is used for introducing magnetic particles in order to separate metallic particles more effectively without them possibly being carried away by the fluid to be filtered.

(40) It is apparent to a person of skill in the art that the nanofibers can be optimized for the respective application, for example, with regard to water sensitivity, by using known additives.

(41) The filter medium according to the invention can be used in particular for liquid filtration, for example, for removal of iron and other metallic particles from motor oil. FIG. 3 shows such a possible use of the filter medium 10 according to the invention as a round filter 20 in which the filter medium 10 is folded in a star shape, for example. On the end faces, the folded filter medium 10 is sealed e.g. by end discs 21.

(42) A further possible application lies in the field of electrical discharge machining (EDM). In eroding processes metal particles with a high iron proportion are released which oxidize quickly to iron oxide hydroxide. These magnetic iron oxide hydroxide particles can be removed well by use of the invention. Additional cost savings result because the media that have been employed up to now can be replaced by less expensive media.

(43) A further possible application is opened up in case of treatment of electrolytic wastewater and sludges, e.g. in processing of the anode sludge in copper refining for separation of the magnetic particles and passage of the noble metals. In the treatment of red mud contaminated wastewater, the magnetic iron hydroxides and oxides, containing also heavy metals such as e.g. Hg, Cr, and Pb, can be separated.

(44) Finally, the invention can also be used for water or drinking water treatment for separating rust particles which otherwise would cause a reddish brown color of the water and deposit within the conduit network.