TREATMENT OF INDUSTRIAL FABRICS

Abstract: A method of treating industrial fabrics such as papermachine clothing, wherein at least one surface of the fabric, or of the yarns, fibres, strips or membranes from which such a fabric is made, is modified by using high energy electromagnetic radiation, from excimer or ablation lasers. This may be used to alter the hydrophilic/hydrophobic properties of the surface by etching on a micron scale. Selective masking at any desired scale may be used.

Description: TREATMEIMT OF INDUSTRIAL FABRICS This invention relates to the treatment of industrial fabrics. The term "industrial fabrics" includes within its scope all forms of papermaking machine fabrics, including dryer fabrics, press felts (including extended nip press belts), shoe press sleeves, corrugator machine press belts, and also to conveyer belts, printing blankets, silicon wafer grinding belts, and filter cloths. As discussed in our co-pending British Patent Application No 971 5508.9, the surface characteristics of such fabrics may be influenced by treatment, for example to adjust the wetting characteristics of the fabric, or in a composite fabric of two or more layers, of at least one layer of the fabric. The treatment may for example render at least one surface of the fabric hydrophobic, or alternatively hydrophilic. Either of these alternatives has advantages in particular circumstances for handling paper webs, filter cakes, or drained or expressed water. In the aforesaid co-pending application, we propose subjecting one or more layers or surfaces of an industrial fabric to plasma treatment. It has been found that the surface conditioning resulting from such plasma treatment suffers from some disadvantages. For example the conditioning may be insufficiently durable in certain conditions, especially wet conditions, and most particularly where high temperatures are experienced. In addition, plasma treatment requires the provision of expensive apparatus for maintaining an atmosphere in the treatment area below atmospheric pressure, as it has so far only been found to be possible to carry out plasma treatment at atmospheric pressure on a restricted scale under laboratory conditions. Also, all and the whole of any material subjected to plasma treatment is treated during exposure to the plasma; it is not possible to selectively expose certain surfaces or areas to treatment, and leave other areas untreated. In our co-pending Application No. PCT/GB97/63180 (WO 98/27277) there is disclosed a method of making a papermakers fabric such as an embossing felt wherein a batt of fibres is subjected to ultrasonic energy to at least partially melt the surface fibres of the batt, and imprinting a pattern into the molten or softened batt whilst the fibres are molten. The process of the above application is relatively cumbersome, being limited in machine speed by the time taken for absorbtion of ultrasonic energy to cause the required degree of melting, in the fineness of the patterns which can be produced, and the fact that a change of pattern involves substituting one embossing roller for another in a time consuming operation. An object of the invention is to provide a method of treatment of industrial fabrics, as above defined, which enables at least some of the mentioned limitations of plasma treatment to be overcome and can also be applied to yarns, fibres, strips and membranes, etc. Preferred objects are to enable treatment to be carried out at atmospheric pressure, and to enable surfaces and areas of surfaces to be selectively treated or untreated, including contiguous areas. According to this invention, a method of treating an industrial fabric, as hereinbefore defined, comprises modifying at least part of at least one surface of such fabric or other material of the kinds suggested above, or of a layer in or to form part of such a fabric, by treatment with high energy electromagnetic radiation. The invention also provides an industrial fabric, as hereinbefore defined, wherein at least part of at least one surface of such fabric, or of a layer in or to form part of such a fabric has been modified by treatment with high energy electromagnetic radiation, and such a fabric including a least some yarns, fibres, strips or membranes or the like which have been so treated. The high energy electromagnetic radiation is preferably UV excimer radiation which may be applied using an excimer or an ablation laser, or a UV excimer lamp. The modification of the surface or part of a surface is preferably carried out at a submicron scale, by etching the surface using the laser. The etching may effect pitting or scoring the surface to provide grooves, hollows or protrusions at the selected scale or expose ionised material. Such modification can alter the wetting characteristics of the surface, for example by providing keying or electrostatic attraction for water adhesion to render a surface hydrophilic; alternatively such modification can micro- planarise a surface by removing small scale irregularities, removing such keying points, to enhance the hydrophobic properties of the surface. Such detailed modification can be carried out without charring of polymeric materials used in the fabric, or incurring other thermal damage to surrounding non-radiated regions. The intensity of laser irradiation may be varied to thereby alter the nature of the surface e.g. by making it more hydrophobic or hydrophilic depending on the type of material being treated. The process may be carried out in selected atmospheres, such as those containing a silane, e.g. Si(CH 3 ) 4 ; a siloxane (Si (OCH 3 ) 4 ) or a perfluorocarbon (eg 1 -6C perfluoroalkane); or tetrafluoroethylene; ammonia, or a combination of any one or more of the above. Surface modification according to the invention may be used to remove localised areas, on any required scale, of coatings to expose a substrate surface. According to a further preferred embodiment of the invention there is provided a method of treating an industrial fabric, as hereinbefore defined, or of a yarn, comprising modifying at least part of at least one surface of such a fabric or yarn, comprising exposing said part of said surface to radiation from an excimer UV emitting device. The UV emitting device is preferably one with a relatively wide beam to enable simultaneous treatment of a substantial area of such fabric or yarn. A plurality of such UV emitting devices may be provided in an array to direct their respective beams each over part of a greater area of the fabric or yarns to be treated, e.g. subjecting the whole width, and a few metres of the length of a papermachine fabric, to simultaneous treatment. The fabric or a multiple yarn feed may be advanced at a preselected speed through the zone effected by the UV emitting devices, to subject all parts of the device to treatment for a dwell period governed by the advance speed of the fabric or yarns and the length of the treatment zone in the directions of advance. As the UV radiation is propagated in direct linear paths, pattern masks of any desired detail down to the limit imposed by diffraction effects may be used to control the arrangement of areas exposed to and shielded from the radiation. As the wavelength of UV radiation is typically around 200-250nm, details such as spots on a micron or submicron scale may be created by using a mask with apertures of a suitable scale. The patterns may of course be provided on any suitable scale, for example allowing myriad pin-point, or full area exposures over certain areas and completely masking others to provided patterns of hydrophilic or hydrophobic treated areas to effect the properties of the paper web being produced. The UV treatment need not be undertaken under a special atmosphere, and thus may be carried out under standard pressure using ambient air composition. Safety considerations may require provision of shielding to prevent exposure of workers to excessive dosages of UV radiation. The UV treatment proceeds by way of dissociation of the molecules of the surface, and provision may be needed for confinement or evacuation of the radicals generated. The UV excimer emitter or emitters can produce radiation in a narrow waveband, and may typically be emitted at 222nm. The treatment may for example create hydrophilic surfaces by dissociating carboxylic bonds, leaving unsaturated carboxylic and/or ketone bonds exposed at the surface. The UV radiation may also or instead etch the surface in a submicron scale due to molecular breakdown, and this can be used to produce micron or submicron scale pitting using a pin-hole mask. Preferred examples of process and fabric according to the invention will now be described by way of example with reference to the accompanying drawings, wherein:- Fig. 1 is a diagrammatic view of apparatus used in the treatment method according to the invention; Fig. 2 is an enlarged diagrammatic cross-section view of a first sample fabric of the invention which has been treated by the method according to the invention; Fig. 3 is a view similar to Fig 2 of a second sample fabric of the invention, which has been treated by the method according to the invention; Fig. 4 is a diagrammatic view of apparatus used in the treatment method according to the invention; Fig. 5 is a slightly enlarged view of a fragment of a mask for partially covering a fabric being treated to selectively expose some areas to radiation, and mask other areas from the radiation; Fig. 6 is a much enlarged view of a fragment of an alternative mask, with pin-hole apertures, for producing micron or submicron scale pitting in the fabric surface; and Fig. 7 is a view of a reflector device for enabling yarns to be treated in both sides using only a single bank of UV emitters. Referring first to Fig. 1 , treatment apparatus for use in the method of the invention comprises a fabric dispensing roll 10, and a fabric take up roll 1 1 . Untreated fabric 1 2 is unwound from roll 10, and passes through a treatment station 1 3. After treatment in station 1 3, the treated fabric is wound up on roll 1 1 . Treatment station 13 is shown diagrammatically as comprising an enclosure or cabinet through which the fabric is passed, with an inlet slot 1 and an outlet slot 15, which preferably have provision for sealing. This enables an atmosphere to be maintained in the enclosure other than ordinary air, and/or at below or above ambient atmospheric pressure or temperature, as required by the particular treatment. Laser means 16 are disposed at the treatment station 13, and may comprise a bank of laser projectors directed towards at least the upper surface of the fabric 1 2. A further bank of lasers, not shown, may be mounted below the fabric, to be directed at the underside of the fabric 12. The lasers preferably comprise excimer lasers, but may be ablation lasers, and are preferably fourth harmonic UV lasers with a beam wave length of up to 320nm, typically 224-226nm, with a beam of size 20 microns minimum size 1 micron, preferably masked to be larger or smaller, e.g. submicron dimensions; and may fire 10 pulses per second. If ablation lasers are used, these may be a harmonic YAG laser apparatus, preferably a Q-switched YAG laser used in conjunction with frequency tripling or quadrupling crystals, e.g. Nd:YAG lasers. YAG lasers need no gas environments and emit e.g. UV frequency light. Excimer lasers require a gas environment in which to ablate. The lasers operate in accordance with a predetermined program to provide e.g. pitting or scoring of the fabric surface, in predefined areas or over the entire surface, or localised removal of a coating over such areas. As an alternative to programming of the lasers, areas where treatment is not required may be masked e.g. by a foil cut-out pattern. Fig. 2 shows a cross-section through a sample of treated fabric, the fabric comprising a single woven layer made up of warp threads 20, and interwoven weft threads 21 a, 21 b, 21 c in each repeat, each floated over two warp threads and below one warp thread 20. Weft thread 21 a is shown in cross-section and is etched at 23, (scale exaggerated) by incident laser beams 24. The other threads 21 b, 21 c etc. are similarly etched by the incident laser beams. This provides a surface of the fabric as a whole which is modified by the pits 23 to provide a roughened surface at e.g. the submicron scale. This can provide a key for adhesion of a water layer by surface tension, and thus increase the hydrophilic properties of the surface. Fig. 3 is a similar view of another sample of treated fabric, comprising a single woven layer made up of warp threads 30 and interwoven weft threads 31 a, 31 b, 31 c in each repeat. The woven layer is provided on its upper surface with a coating 32. After treatment using lasers 1 6 in accordance with the invention, localised areas 33 are removed by incident laser beams 34 (the scale is again exaggerated) . This has the effect of modifying the properties, e.g. the adhesion or water retaining properties of the coating 32 by exposing the underlying fabric layer in treated areas of the fabric. Fabrics treated by lasers in accordance with the method of the invention may be used as filter fabrics, e.g. in belt filters, barrel-neck filters and the like, and also as papermachine clothing, notably press felts, dryer belts, shoe press sleeves, corrugator belts, and also conveyer belts and silicon wafer grinding belts. It is for example advantageous to provide hydrophilic surfaces for filter media to obtain improved cake release where moisture is present in the material being filtered. This moisture will form a very thin film of water of the surface of the filter which improves cake release. Conversely, in high temperature gas filtration there will be little or no moisture, and the best cake release is then achieved using a hydrophobic coating. Another use for hydrophilic media is in papermachine forming fabrics or dryer fabrics to improve adhesion between the paper web and the fabric. For papermachine clothing a hydrophobic fabric conversely gives improved pick-up or transfer of the paper, pulp or tissue web, since web and contaminant release is improved. This is particularly important in high¬ speed machines for making fine paper grades (e.g. newsprint) or tissue. These requirements can be met by a filter belt or papermachine clothing appropriately treated by the method of the invention, wherein for example the laser means etch away the top surface layer or coating, and any contaminants, leaving a chemically activated or deactivated surface on e.g. a polymer substrate which is either hydrophilic or hydrophobic, or which can be rendered so by subsequent exposure to e.g. air, silanes, ammonia or fluorocarbons. The method according to the invention can also for example be used to treat the underside of a papermachine forming fabric to render the surface hydrophilic and thereby reduce rewetting of the paper web, particularly towards the end of the forming section prior to the pick-up position where the web is transferred to the press section. This can increase web dryness by 0.5-3.0%, which results in significant cost savings for the papermaker as less energy has to be used in the dryer section to remove water. In a further example selected areas on the paper-contacting side of a papermachine fabric may be laser treated by the method according to the invention, to form a pattern of hydrophilic areas. This creates a corresponding textured pattern in the paper or tissue product. GB-A-2,233,334 describes the use of excimer lasers to ablate and etch the surfaces of polymeric materials, and describes the physical effects of laser treatment on a micron- and submicron- scale on the treated surface. The present invention makes use of these effects to provide useful surface treatment for industrial fabrics. The invention may also be used to laser etch yarns, fibres, strips or membranes or he like which may subsequently be made up into fabrics, or composite belt structures. A further embodiment of the invention is illustrated in Figs. 4 to 6. Referring first to Fig. 4, apparatus for UV excimer lamp treatment of industrial fabrics, or yarns, is shown in extemely diagrammatic and simplified form. The fabric 1 10 is payed out from a dispensing roll 1 1 1 , and is passed through a treatment station 1 1 2, and wound up after treatment in a take-up roll 1 13. Treatment station 1 12 comprises shielding 1 14, to prevent excessive exposure of workers to ultra violet radiation reflected or diffuse from the treatment station. The shielding encloses a first bank of UV excimer lamps 1 1 5 located above the path of the fabric 1 10. A second bank of UV excimer lamps 1 16 is located below the part of the fabric 1 10 so that both surfaces of the fabric may be treated in one pass of the fabric through the treatment station 1 1 2. An optional mask 1 17 is located over the fabric 1 10, and this is used to provide for selective exposure and masking of areas of the fabric from the lamps 1 15 so that only selected areas of the fabric are treated by the excimer UV light. No corresponding mask is shown below the fabric, but may be provided if desired. Further, either of the banks 1 1 5 or 1 16 of UV excimer lamps may be deactivated if only one surface of the fabric 1 10 is to be treated. Instead of a web of fabric such as 1 10, the path may be occupied by multiple yarns, dispensed from and taken up by banks of bobbins. Similarly to the fabric 1 10, the yarns can be subjected to UV excimer radiation on one or both sides. Fig. 5 shows a small part of a mask 1 17, in the form of a net of diamond shaped openings 1 1 8, and an interconnected grid of lands 1 19. The lands 1 19 serve to mask areas of the fabric from the UV radiation, which is however admitted by the openings 1 18. This produces a patterned array in the fabric surface of areas, in this case in a network, having differing properties, for example hydrophilic properties in the exposed areas and more hydrophobic properties in the unexposed areas. This pattern influences the properties of any paper web processed in the fabric and produces patterning in the paper or tissue produced. Fig. 6 shows a fragment of a second mask 1 17, much enlarged. This comprises a sheet 1 20 of e.g. metal foil provided with myriad of pin hole apertures 121 . These allow UV excimer radiation to effect very small areas of the fabric, producing micron and submicron scale pits due to dissociation of molecules leading to erosion of the fabric surface. This can effect the surface properties of fabrics or yarns by for example improving mechanical keying for retention of surface water, thus improving hydrophilic properties. Fig. 7 suggests how a single bank of UV excimer lamps may be used to treat both sides of yarns 1 22, using a reflector such as a silvered or aluminised mirror 1 23 to reflect UV radiation passing between the yarns back to the undersides of the yarns. The reflector may be formed with flutings or channels on its reflective surface to direct reflected rays and focus them on the yarns 1 22. The UV excimer lamps 1 15, 1 16 provide narrow band radiations, e.g. centred on 222nm, this will dissociate surface molecules of e.g. PET, to increase hydrophily of the surface, by generating unsaturated carboxylic acid groups on the surface. Other materials which can be treated include aramids, PEEK, etc. The invention also provides an industrial fabric as defined hereinbefore treated by the method of the invention, yarns when treated by the method of the invention, and an industrial fabric made from such yarns.

Claims: CLAIMS 1 . A method of treating an industrial fabric, comprising modifying at least part of at least one surface of such fabric, or of yarns, fabric, strip, membrane or the like in or to form part of such a fabric, by treatment with high energy electromagnetic radiation. 2. A method according to claim 1 , wherein high energy electromagnetic radiation is produced by a UV excimer or ablation laser. 3. A method according to claim 2, wherein the surface or surfaces are etched using said laser. 4. A method according to claim 3, wherein the etching effects pitting or scoring of said surface to provide grooves hollows or protrusions, or to expose ionised material. 5. A method according to claim 3 wherein a coating is provided on said surface, the coating being selectively removed to expose a substrate surface, by the etching step. 6. A method according to claim 3 wherein the etching step effects removal of surface irregularities. 7. A method according to any preceding claim wherein the treatment is carried out in an atmosphere containing one or more of a silane, a siloxane, a perfluorocarbon, tetrafluoroethylene or ammonia. 8. A method according to any proceeding claim wherein said etching is carried out at a micron-or submicron-scale. 9. A method according to any preceding claim wherein areas which are not to be treated are masked by a shaped foil mask. 10. A method according to any preceding claim wherein the treatment makes the said surface hydrophobic. 1 1 . A method according to any of claims 1 to 10, wherein the treatment makes the said surface hydrophilic. 1 2. A method according to any preceding claim wherein a coating or matrix material is applied to the treated fabric, yarns or fibres. 1 3. A method according to my preceding claim wherein said, yarns or fibres comprise or include a polymer such as a polyester, polyamide, or polyolefin. 14. An industrial fabric wherein at least part of at least one surface of such fabric, or of a layer in or to form part of such a fabric has been modified by treatment according to any one of claims 1 to 13. 15. A fabric according to claim 14 comprising a filter cloth, conveyor belt, or papermachine clothing, or comprising a component thereof. 16. A method according to claim 1 wherein said high energy electromagnetic radiation is produced by a UV excimer lamp. 17. A method according to claim 1 6 wherein said lamp has a relatively wide beam to enable simultaneous treatment of a substantial area of said yarns or fabric. 18. A method according to claim 17 including a plurality of such lamps provided is provided in an array to direct their respective beams each over part of a greater area of the fabric or yarns to be treated. 19. A method according to claim 16, 17 or 18, wherein pattern masks are used to control the arrangement of areas exposed to and shielded from the radiation. 20. A method according to any one of claims 1 6 to 19, wherein said radiation is produced in a narrow waveband centred on 222nm.

Inventors: STROEM JAN, STROEM, JAN
Applicants: SCAPA GROUP PLC, STROEM JAN, STROEM, JAN
Application year: 1998
Main classification: D21F1
IPC codes: D06M10, D21F1