Resin impregnated filter media

Abstract

The present invention discloses a filter medium and a method of manufacturing a filter medium. The filter medium is created using a fabric substrate of a woven or non woven material. The substrate is impregnated with a thermosetting resin that is subsequently heat cured thereby encapsulating the fibers of the substrate. Further a rheology modifier may be mixed with the resin before impregnation to control the flow properties of the resin.

Description

[0001] This invention relates to improved filter media of the kind comprising a woven or non-woven substrate.

[0002] EP-A-0,741,815 discloses a method of making a fabric which is suitable for use as a filter medium by applying a film of a reticular polymer to a suitable substrate from a release sheet. The resultant filter medium provides a micro porous filter, which has a good wear resistance so long as the film endures. When the film is abraded however, the substrate becomes vulnerable to wear and the filter medium loses its micro-filtration capabilities.

[0003] In order to improve the effective lifetime of filter media, it is desirable to improve the wear resistance of the filter cloths, belts sleeves or the like, and if possible to reduce the rate of deterioration of filtration capacity which tends to occur where a relatively coarse base substrate is surface treated or partially impregnated from one side or the other. There is a general tendency for coatings, however applied, to sit on the substrate as a discrete layer, or to penetrate only a comparatively short distance into the substrate, so that when the coated substrate is subjected to abrasion, the filter medium is impaired quickly once the coating has been worn, even if only locally.

[0004] Japanese Patent Application No. 06301439, published under No. 08131735, discloses filter medium comprising a blend of two types of fibres, filter fibres and heat fusible fibres. The fibres are joined together by partial fusion of the heat fusible fibres as the filter medium is heated and shaped. Reinforcing materials are attached to the Intersections of the two types of fibres, and the reinforcement material may be for example a water-soluble phenol, an epoxy resin, unsaturated polyester or a polyamide. This type of filter medium is intended for example for cylindrical cartridge filters or the like. It is not clear how the reinforcement materials are introduced nor how they are caused to adhere only at the intersections of the two types of fibre used. The use of a non-woven substrate comprising a blend of two different fibre tpes raises production costs, as the fibres first have to be blended. Also the presence of a fusible component in the fibre blend restricts the utility of the filter medium to lower temperature uses, since high temperatures could cause further fusion of the fusible fibres, leading to blinding of the filter pores by melted fibre material.

[0005] It is an object of the invention to provide a filter medium material and a process for manufacture of a filter medium material with improved wear resistance, and greater dimensional stability and which does not substantially suffer from the disadvantages noted.

[0006] From a first aspect, the invention provides a process for manufacturing a filter medium, comprising impregnating a woven or non-woven subsrate with a thermosetting resin combined with subsequent curing of the resin by heating the impregnated substrate to a curing temperature appropriate for the resin used, whereby the yarns or fibres of the substrate are encapsulated by the resin so that the yarns or fibres are mutually bonded where they cross each other.

[0007] In a preferred process according to the invention, the resin is mixed with a rheology modifier before application to control the viscosity and flow properties of the resin, and may be added to the resin system which is preferably diluted to below 15% solids content, for example 10% solids content, in an amount of below 5% by weight of the rheology modifier for example 3% by weight. An example of a rheology modifier, which may be used is hydroxy ethyl cellulose.

[0008] From a second aspect, the invention provides a filter medium, comprising a substrate of woven or non-woven fabric impregnated with a thermosetting resin cured by subsequent heating of the impregnated substrate to a curing temperature appropriate for the resin used, the yarns or fibres of the substrate being encapsulated by the resin so that the yarns or fibres are mutually bonded where they cross each other.

[0009] The thermosetting resin may be any one or a mixture of any two or more selected from:--a phenolic, epoxy, formaldehyde, amino/furan, melamine, silicone, unsaturated polyester, polyurethane, polyamide, fluorocarbon or cross linked thermoplastic based thermosetting resin systems.

[0010] Further, other known additives can be added to the resin to enhance supplementary propeties. Examples may include--carbon (to give conductive or anti-static qualities), grafting binders which attach PTFE onto the chain terminator of phenol end groups (to improve cake release), or antibacterial agents such as 3-trimethoxysilyl, poly-dimethyloctadecyl ammonium chloride, and silane quaternary salts (to minimise crystal or fungi build up).

[0011] The thermosetting resin preferably penetrates the substrate, to a depth, which will give the desired balance of dimensional stability and flexibility and effectively encapsulates or coats the fibres or yarns within the impregnated part of the fabric without clogging the void spaces between the woven or felted yarns or fibres. The void spaces may then be used if required to hold micro porous materials such as flocculated, coagulated, foamed or other micro porous materials.

[0012] The substrate may be a woven or non woven felt, spun bonded, thermo bonded, or melt blown include polymeric yarns or fibres, and these may be anyone or a blend of two or more of polyester (e.g. PET) or polypropylene or other materials, such as polyamide (nylon) or polyethylene or polyurethane. The substrate may also include a metal or ceramic mesh or screen.

[0013] The resin may be applied as a liquid, by known means such as knife-coating, spraying or lick coating or by immersion of the base fabric in the coating material. The exposure of the fabric to the resin liquid is preferably sufficiently prolonged to ensure the desired depth penetration of the liquid into the base fabric. The resin bearing liquid may be an aqueous or non-aqueous dispersion, or emulsion, or the like of the resin in the liquid phase. Before curing, the coated fabric is optionally subjected to an intermediate heating, below the curing temperature to drive off moisture or other solvent, and to effect fibre encapsulatlon that is to ensure that the resin adheres to and `wets` the fibres or yarns, without substantially impeding the voids of the fabric structure. Curing of the resin is then effected, with typical curing times from 5 minutes to one hour or more, and typical curing temperatures from 100.degree. C. to 200.degree. C. Intermediate heating and curing may be effected during a single pass, or the steps may be carried out separately and not necessarily immediately subsequently.

[0014] Examples of the method according to the invention, and of the filter media produced thereby will now be particularly described by way of example.

EXAMPLE I

[0015] A base fabric comprising a needled non-woven felt of PET (polyethylene terephthalate) fibres was lick coated with an aqueous phenolic thermosettable resin.

[0016] The felt was then heated for 10 to 20 minutes to close to 100.degree. C. In order to dry the felt by evaporating the aqueous phase and soften the resin to cause it to wet the fibers effectively, thereby encapsulating the fibres in the resin.

[0017] Following completion of the drying step the coated felt was then heated to 106.degree. C. and maintained at this temperature for 10 minutes, thereby effecting curing of the thermosetting resin.

[0018] Tests showed that the resultant filter medium has only slightly impaired flexibility and air permeability as compared to the untreated felt further, dimensional stability and resistance to abrasion was significantly improved.

EXAMPLE II

[0019] A base fabric comprising a woven polypropylene cloth was coated with an aqueous phenolic thermosetting resin. The resin was applied using a knife coating mettod.

[0020] The coated cloth was then heated to about 130.degree. C. and maintained at this temperaure for 1 hour, thereby ensuring that the resin wets the fibres effectively, causing encapsulation of the fibres with the resin and also effecting subsequent curing of the resin.

[0021] Again the resulting filter medium had improved dimensional stability and abrasion resistance, and only slightly impaired flexibility and air permeability as compared to the untreated fabric.

[0022] The Method is also applicable to woven fabrics to coat the yarns or fibres comprising the fabric.

[0023] FIG. 1 illustrates in a considerably magnified view what is believed happens to the structure of the impregnated region of a filter medium produced from a non-woven felt as described by Example I.

[0024] FIG. 2 is a scanning electron micrograph of a coated polyester needle felt as prepared by the Example I as set out above;

[0025] FIG. 3 is a scanning electron micrgraph of uncoated polypropylene cloth before treatment in accordance to the Example II set out above;

[0026] FIG. 4 is a scanning electron micrograph of coated woven multi filament polypropylene cloth in accordance with Example II set out above;

[0027] FIG. 5 is a magnified view of two adjacent coated fibres within a multi filament yarn and meniscus of the binding resin;

[0028] FIG. 6 is micrograph of a cross section through a coated multifilament yarn within a coated woven polypropylene cloth;

[0029] FIG. 7 is a graph showing the effect on permeability and filter throughput of a coating according to the invention;

[0030] FIG. 8 is a graph showing the improvement in dimensional stability of a polypropylene belt material after treatment by coating according to the invention; and

[0031] FIG. 9 is a graph showing the abrasion resistance of a phenolic resin treated woven polypropylene compared to an untreated substrate.

[0032] The impregnated fabric resulting from Example 1 as shown in FIG. 1 consists of an array of randomly orientated fibres 20, some of which are shown in sectional view. The coated fibres each comprise a core 20 consisting of the fibre itself and a coating or sheath 22 of the thermosetting resin produced by wetting of the fibres by the resin during the drying stage of the method described in the examples. Where the fibres 20 cross and contact each other, the resin forms bridges or pools 24, which connect the fibres firmly to each other. Once the resin has been cured the bonding remains permanent.

[0033] FIG. 2 shows in a scanning election micrograph, part of a coated polyester needled felt produced by method 1 set out above. As shown, the coating 22 tends to collect in pools 24 about the intermeshing zones of the fibres 20, with thickend sheath parts extending along the fibres.

[0034] FIG. 3 shows uncoated yarns in a polypropylene cloth, which have substantially no medium between yarns to give dimensional stability and to prevent abrasion, which is already evidenced by the presence of fibrils 25 on the yarn surfaces.

[0035] FIG. 4 shows by contrast shows such yarns after coating, and it will be noted that fibrillation is less marked, and that bridges of resin, e.g. 26, connect the yarns and inhibit them from moving against each other, increasing stability and hereby reducing abrasion. The meniscus of resin between yarns is show enlarged in FIG. 5.

[0036] FIG. 6 shows how resin penetrates between the fibres of a multiflament yarn, reducing internal wear within the yarn.

[0037] FIG. 7 compares, the filtrate through put (a measure of permeability) of filter cloths which have and which not been impregnated with a phenolic resin by the method of the invention. It is noted that the decrease in permeability is marginal.

[0038] FIG. 8 similarly compares the dimensional stability of treated and untreated polypropylene belt material, when tested by tensioning in the direction of the weft yarns.

[0039] The treated belt (full line) shows greater dimensional stability than the untreated belt, demonstrated in that a greater force is necessary to produce the same "stretch". For example, when the applied stress is 20 kgf/2.5 cm, then the treated belt shows a strain (or elongation) of approximately 7% compared to greater than 12% for the untreated belt.

[0040] Finally FIG. 9 shows the result of comparative tests of a treated and an untreated woven polypropylene substrate. The untreated material showed a marked increase in permeability over 2000 abrasion cycles, signifying a loss of micro filtration, but the treated material exhibited no noticeable deterioration.

[0041] The bonding thus produced increases the dimensional stability hence the propensity of the yarns to be displaced and to rub against each other is reduced lessening internal wear and ultimately yarn breakage within the fabric. A similar effect is present in woven base fabrics where the yarns are coated and pools bridges are formed within the weave knuckles or in the case of multi-filament or staple yarns, where parallel fibres contact each other.

[0042] A further benefit is that the resin bridges or pools lock the fibres, and the openings between fibres, relative to one another thus providing more accurate and consistent pore sizes. The pore sizes will remain constant throughout the life of the filter media.

[0043] The properties of filter media according to and produced by the method of the invention can be controlled by varying conditions and materials.

[0044] The speed and degree of penetration of the resin into the substrate can be controlled by varying the viscosity of the coating. The solids content of the coating may be changed to vary the thickness of the film encapsulating the yarns or fibres.

[0045] The properties of the resin used for coating are preferably controlled depending upon the properties of the substrate by dilution and rheology modifiers (which modify the flow properties of the resin).

[0046] In an example a first sample of a 373 gm-2 nylon 66 monofilament duplex weave cloth was subjected to application of a neat (unmodified) resin comprising 60% solids, 40% solven and having a viscosity of 200 cPs.

[0047] This was found to have an excessive coat weight and low permeability, due to the low surface area monofilament yarns being encapsulated in a thick layer of resin.

[0048] A second sample of the cloth was subjected to application of resin diluted to 10% solids, of lower viscosity (about 20cPs). This resin bled through the entire cloth thickness and produced an insufficient coat weight, with the yarns being incompletely encapsulated.

[0049] A third sample of the cloth was subjected to the application of a specially prepared resin mixture.

[0050] This resin sample was prepared by dilution of the resin to 10% solids and then thickening by addition of hydroxy-ethyl cellulose as a rheology modifier (i.e to modify the flow properties of the resin). The hydroxyl ethyl cellulose rheology modifier had been prepared previously in accordance to manufactures guidelines and was added slowly to the dilute resin with vigorous stirring to give 3% by weight of rheology modifier in the resin mixture. The viscosity of this mixture as applied to the third cloth sample was 4500 cPs. The coated fabric was cured in the usual manner in a single pass through an oven at 160.degree. C., with dwell time 10 minutes.

[0051] With use of a rheology modifier and dilution of the resin it was found that the resin system had an optimum solids content for increased abrasion resistance and optimum viscosity for substrate penetration, with minimal loss of substrate flexiblity and permeability.

1 Sample 1 Sample 2 Sample 3 substrate 373 gm-2 nylon 66 as sample 1 as sample 1 monofilament duplex weave resin 60% solids 10% solids 10% solids rheology nil Nil hydroxy ethyl modifier cellulose 3% viscosity 200 cPs 20 cPs 4500 cPs permeability low High good encapsulation thick layer coat poor coat bled good weight excessive through fabric encapsulation leaving without exces- insufficient resin sive thickness

[0052] Drying conditions can determine the way or extent to which the fibres are encapsulated with shorter drying times so that, the coating will migrate less.

[0053] Advantages of the invention for example in relation to EPA 0,741,815 mentioned above include the fact that a single process can provide a coating throughout a variable depth within the substrate, and possibly the whole substrate matrix by if necessary completely soaking the fabric with the liquid phase resin. Using the method of the European Patent, if the abrasion resistance of both sides of the fabric are to be improved, the resin has to be applied separately to each side in a repeated process.

[0054] Experimental results show as noted above that the process of the invention does not significantly change the filtrate throughput of the filter medium. Air and liquid permeability is only slightly reduced. Further the method of the invention can be used with low surface energy materials such as polypropylene due to encapsulation of individual fibres. The coating applied by the method known from the said European Patent can be easily peeled off such surface, as it does not adhere to them.

[0055] Because of the depth of penetration, the coating is permanent, with a residue of coating throughout the full depth of the coating and the cloth will continue to resist abrasion during the cloth lifetime.

[0056] The void spaces between the yarns or fibres of the base cloth are not substantially impeded by the coating, as evidenced by the air permeability, and these can be filled to render the filter medium micro porous by coagulated, flocculated, or micro porous foamed materials as known in the art. The filter media of the invention may be used In liquid or gas any filtration application as cloths for drum, cartridge, and disc filter sleeves and bags or other filters also conveyor belts, pulp dewatering belts, corrugators belts, tower press belts and filter press cloths.

Claims

1. A process of manufacturing a filter medium, comprising: impregnating a substrate with thermosetting resin said substrate being formed from intertwined yarns; and subsequently curing the thermosetting resin by heating the impregnated substrate to the curing temperature of the resin used, to thereby encapsulate the yarns of the substrate with the resin so that the yarns are mutually bonded where they contact each other.

2. A process according to claim 1 said resin is mixed with a rheology modifier before application to control the viscosity and flow properties of the resin.

3. A process according to claim 2 wherein the rheology modifier comprises below 5% of the resin composition by weight.

4. A process according to claim 2 wherein the rheology modifier comprises hydroxyl-ethyl cellulose.

5. A process according to claim 2 wherein the resin is diluted to below 15% solids content before addition of the rheology modifier.

6. A process according to claim 2 wherein the thermosetting resin is selected from the group consisting of: phenolic, epoxy, formaldehyde, amino furan, melamine, silicone, unsaturated polyether, polyurethane, polyamide, fluorocarbon based thermosetting resin, cross linked thermoplastic based thermosetting resin and mixtures thereof.

7. A process according to claim 1 wherein the substrate is selected from the group consisting of: woven fabric, non-woven fabric, spun bonded yarns, thermo-bonded yarns, and melt blown polymeric yarns.

8. A process according to claim 7 wherein the polymeric yarns are a blend of two or more materials selected from the group consisting of: polyester, polypropylene, polyamide, polyethylene, and polyurethane.

9. A process according to claim 7 wherein the substrate includes a metal or ceramic mesh screen.

10. A process according to claim 1 wherein the coated fabric is subjected to an intermediate heating, before curing to a temperature below the curing temperature to remove water or other solvent.

11. A filter medium comprising a substrate of a fabric impregnated with a thermosetting resin cured by subsequent heating of the impregnated substrate to the curing temperature of the resin.

12. A filter medium according to claim 11 wherein said resin contains a rheology modifier to control the viscosity and flow properties of the resin.

13. A filter medium according to claim 12 wherein the rheology modifier comprises below 5% of the resin composition by weight.

14. A filter medium according to claim 12 wherein the rheology modifier comprises hydroxyl-ethyl cellulose.

15. A filter medium according to claim 12 wherein the resin is diluted to below 15% solids content before addition of the rheology modifier.

16. A filter medium according to of claim 11 wherein the thermosetting resin is selected from the group consisting of: phenolic, epoxy, formaldehyde, amino furan, melamine, silicone, unsaturated polyether, polyurethane, polyamide, fluorocarbon based thermosetting resin, cross linked thermoplastic based thermosetting resin and mixtures thereof.

17. A filter medium according to claim 11 wherein the substrate is selected from the group consisting of: woven fabric, non-woven fabric, spun bonded yarns, thermo-bonded yarns, and melt blown polymeric yarns.

18. A filter medium according to claim 17 wherein the polymeric yarns are a blend of two or more materials selected from the group consisting of: polyester, polypropylene, polyamide, polyethylene, and polyurethane.

19. A filter medium according to claim 17 wherein the substrate includes a metal or ceramic mesh screen.