Method and device for applying a curtain of foam on an airlaid nonwoven fibre web

Abstract

This invention provides a method for applying a foamed additive layer to a moving web of airlaid nonwoven fibres, characterized by dispensing the foamed additive (223) from at least one foam curtain applicator (300) positioned above the web (225).

Description

[0001] Method and device for applying a curtain of foam on an airlaid nonwoven fibre web

[0002] Field of the invention

[0003] The invention relates to devices and methods for applying additives to airlaid nonwoven fibre webs by means of a foam curtain.

[0004] Background of the Invention

[0005] In the manufacturing of airlaid nonwoven fibre materials, various additives, like binders and other chemicals, are often applied to the web to enhance strength and functionality. Traditionally, these additives are applied using spray methods, which have notable limitations. Spray application easily results in uneven, streaky or cloudy coverage, in particular when small amounts of spray are used. Spray applications are prone to nozzle clogging, messy overspray, and produce undesired airborne particles and mist. Spray methods are also limited to low-viscosity binders, excluding many eco- friendly, high-viscosity options. The need for frequent maintenance and cleaning further adds to operational inefficiencies. Furthermore, as the additives are in aqueous solution or suspension, the application of them to the web increases the moisture content of the web and leads to increased energy consumption when the web is subsequently dried.

[0006] US4348251 discloses a method and system for applying a foamed binding agent to a fibrous web, by use of an extruder having an upper lip extending beyond the lower lip, and by the drawing of a vacuum beneath the applied foam. It is suggested that the extension of the upper lip beyond the lower lip and the vacuum are critical to the formation of a uniform foam layer atop the web.

[0007] There is nevertheless a desire to provide an improved way of producing a web of nonwoven fibres.

[0008] Summary of the Invention

[0009] The object of the invention is to provide an improved way of producing a web of nonwoven fibres. The object is achieved with a method according to claim 1. Thus, the invention provides a method for applying a foamed additive layer to a moving web of airlaid nonwoven fibres, which method comprises dispensing the foamed additive from at least one foam curtain applicator positioned above the web. Preferably, dispensing the foamed additive comprises dispensing the foamed additive with a uniform falling curtain of the foamed additive. Thereby, the foamed additive may reach the web purely by being gravity fed.

[0010] This invention introduces a foam curtain coating technology that addresses at least some of these challenges by offering a cleaner, more precise, more clog-free, and sustainable alternative to spray applications. The foam curtain method supports the application of a wide range of binder materials, including bio-based materials, facilitating cleaner application, greater additive consistency, and enhanced compatibility with eco-friendly binders.

[0011] Tests performed by the applicant show that, compared to a spray application, the foamed additive curtain application of the invention provides a more uniform film. In particular, a more even distribution of the additive over the lateral direction of the web is provided. (The lateral direction is also known as the "cross direction" or "CD".) In addition, as opposed to an extruder which applies an additive to a web by the use of pressure which pushes the additive out of an extruder outlet and onto the web, the foam curtain applicator of the invention provides a curtain in which the foamed additive falls into the web without the application of a dispensing pressure. Such a curtain requires a relatively low viscosity of the foam. This means that the foam will have a relatively low air-to-liquid ratio. Such an air-to-liquid ratio will enhance its penetration into the web leading to improved bonding. Thereby, in addition to providing a uniform film, the foam curtain application will provide a better penetration giving improved bonding.

[0012] Preferably, the foam curtain applicator comprises a hollow elongated foam applicator duct, wherein an elongated outlet slot is formed in a lower portion of the duct, wherein the duct contains some of the foamed additive before the foamed additive is dispensed through the slot and onto the web, wherein, while some of the foamed additive is dispensed, the pressure of the foamed additive in the duct is substantially the same as the pressure of the atmosphere surrounding the foam curtain applicator. Thus, preferably, there is no additional pressure applied to the foam in the foam applicator duct. Thereby, the foamed additive is dispensed from the applicator duct purely by gravity. Specifically, preferably, the foamed additive exits the duct through the slot purely by means of gravity. Thereby, the foam will pour out of the duct. Preferably, the duct extends in the cross direction of the web to be treated. In some embodiments, the slot is replaced by a series of holes formed in the lower portion of the duct. Nevertheless, a slot may be better for securing a stable uniform curtain.

[0013] Preferably, the viscosity of the foamed additive, when it is dispensed, is no more than 10000 mPa-s, more preferably no more than 8000 mPa-s, as measured with a Brookfield viscometer RV4 or RV-5 at 10 rpm. Preferably, the viscosity of the foamed additive, when it is dispensed, is no more than 5000 mPa-s, more preferably no more than 4000 mPa-s, as measured with a Brookfield viscometer RV4 or RV-5 at 30 rpm. Preferably, the viscosity of the foamed additive, when it is dispensed, is no more than 3000 mPa-s, more preferably no more than 2500 mPa-s, as measured with a Brookfield viscometer RV4 or RV-5 at 100 rpm. Thereby, the foamed additive may have such a low viscosity that it is able to exit the foam curtain applicator purely by gravity, and to penetrate the web, but thick enough to form a stable curtain. The viscosity with said limits is preferably the dynamic viscosity of the foamed additive. Nevertheless, preferably the viscosity of the foamed additive, when it is dispensed, is at least 500 mPa-s, more preferably at least 1000 mPa-s, as measured with a Brookfield viscometer RV4 or RV-5 at 10 rpm. Preferably the viscosity of the foamed additive, when it is dispensed, is at least 300 mPa-s, more preferably at least 500 mPa-s, as measured with a Brookfield viscometer RV4 or RV-5 at 30 rpm. Preferably the viscosity of the foamed additive, when it is dispensed, is at least 200 mPa-s, more preferably at least 300 mPa-s, as measured with a Brookfield viscometer RV4 or RV-5 at 100 rpm. By avoiding a viscosity that is too low, it is possible to avoid a situation where the web becomes oversaturated with liquid. Preferably, the foamed additive is applied with a viscosity and a flow rate sufficient to form a continuous curtain above the web. The viscosity in of said limits is preferably the viscosity of the foamed additive at 20 °C.

[0014] Preferably, the additive comprises a natural or synthetic polymer-based binder. The additive may comprise starch, native starch, modified starch, cationic starch, carboxymethylcellulose (CMC), microfibrillated cellulose (MFC), nanofibrillated cellulose (NFC), highly refined cellulose, AKD, ASA, SAE, pigments (GCC, PCC, kaolin, talc), barrier chemicals (e.g., synthetic or bio-based polymer latices, polyvinyl acetate (PVA), vinyl acetate ethylene copolymer, polyvinyl alcohol (PVOH)) or mixtures of these or other agents, other natural or synthetic polymer-based binders or strengthening agents.

[0015] Preferably, the method comprises, before dispensing the foamed additive onto the web, providing the additive in an un-foamed state, and foaming the un-foamed additive. Foaming the un-foamed additive may be done by means of a foam generator producing the foam by mixing the additive, which may be a binder liquid, and air. Alternatively, or in addition, the additive may be foamed by adding a foaming agent to it. Preferably, the foamed additive is dispensed onto the web no more than 60 minutes after the step of foaming the un-foamed additive. Thereby, the stability of the foam may be ensured, since the surface tension of bubbles in the foam decreases with bubble age.

[0016] Preferably, the method comprises, before foaming the un-foamed additive, forming the un-foamed additive by diluting solids with a diluting liquid, such as water, wherein, after said dilution, the un- foamed additive contains 10-40 wt% of diluted solids. The solids may form at least a portion of a binder. The dilution may be done by dissolving or suspending the solids with the diluting liquid. In some embodiments, the additive is formed by diluting a binder in the form of a liquid with water or another diluting liquid.

[0017] Preferably, the viscosity of un-foamed additive, e.g. after said dilution, is no more than 20 mPa-s. Such a viscosity may contribute to a stable curtain. Increasing the un-foamed additive viscosity, which may be a liquid phase viscosity, will increase the viscosity of the foamed additive, and if the foamed additive viscosity is too high, a stable curtain may be prevented. Nevertheless, preferably the viscosity of un-foamed additive is at least 2.0 mPa-s.

[0018] Preferably, the air-to-liquid ratio of the foamed additive, when dispensed, is no more than 15, preferably no more than 10. Thereby, where the foam is produced in a foam generator mixing the additive with air, the ratio of a flow of air to a flow of additive in liquid form to the foam generator may be 1.5-15. Such an air-to-liquid ratio may contribute to a stable curtain. Increasing the air-to- liquid ratio will increase the viscosity of the foamed additive, and, as mentioned, if the foamed additive viscosity is too high, a stable curtain may be prevented. Preferably, the air-to-liquid ratio of the foamed additive, when dispensed, is no more than 7.0. In particularly preferred embodiments, the air-to-liquid ratio is no more than 5.0, or in even more preferred embodiments below 2.4. Nevertheless, preferably the air-to-liquid ratio of the foamed additive, when dispensed, is at least 0.5, preferably at least 1.0, more preferably at least 1.5, even more preferably at least 2.0. By providing a sufficient amount of air, it is possible to avoid a situation where the web becomes oversaturated with liquid. The air-to-liquid ratio is preferably defined as the volume of air divided by the volume of liquid. In a continuous process for the foaming of the additive, the air-to-liquid ratio may be defined as the flow of air divided by the flow of liquid.

[0019] Preferably, the average bubble size in the foamed additive is relatively large. The reason is that decreasing the average bubble size will increase viscosity, and, again, if the foamed additive viscosity is too high, a stable curtain may be prevented. The average bubble size in the foamed additive, when dispensed, may be at least 10 pm, preferably at least 30 pm, more preferably at least 50 pm. The average bubble size in the foamed additive, when dispensed, may be no more than 200 pm, preferably no more than 150 pm, more preferably no more than 100 pm.

[0020] Preferably, the temperature of the foamed additive when dispensed is 10-40°C, preferably around 20°C. Preferably, additive penetration into the web is enhanced by providing and operating a suction box positioned below the web. The suction box is preferably positioned, in the direction of travel of the web, below the foam curtain applicator and / or after the foam curtain applicator. The suction box may serve to suck foam into web. The suction box may further serve to suck air out of the foam. Preferably, after dispensing the foamed additive from the foam curtain applicator, the weight of additive applied to the web, per unit area, is 1-15 % of the weight of dry web per unit area. More preferably, the weight of additive applied to the web, per unit area, is no more than 10 % of the weight of dry web per unit area. Thereby, it may be avoided that the web gets too saturated with the additive. Preferably, the weight of additive applied to the web, per unit area, is at least 5% of the weight of dry web per unit area. Thereby, it may be possible to obtain a stable curtain, in particular where a foam curtain applicator outlet slot has a width within preferred limits as specified herein, and the air-to-liquid ratio of the foamed additive, when dispensed, is within preferred limits as specified herein. The dry web is understood as the web before the foamed additive has been applied to it. The weight of additive, or of dry web, per unit area may be referred to as the specific weight of the additive and the dry web, respectively.

[0021] Preferably, the method comprises providing the foam curtain applicator with an elongated outlet slot with a length in cross direction which is no more than 0.5 m wider or narrower than the treated moving airlaid nonwoven web. A slot that is wider than the web ensures full and even coverage. On the other hand, a slot that is narrower than the web ensures an economical use of additive.

[0022] Preferably, the method comprises providing the foam curtain applicator with an elongated outlet slot, wherein the width of the slot is greater than or equal to 0.2 mm, preferably greater than or equal to 0.5 mm, preferably greater than or equal to 1.0 mm. If the slot is too narrow, the texture of the foam may worsen significantly. Nevertheless, preferably, the width of the slot is equal to or less than 6.0 mm, preferably equal to or less than 4.0 mm, preferably less than or equal to 3.0 mm. In some embodiments, the width of the slot is less than or equal to 2.0 mm.

[0023] Preferably, the velocity of the moving web when the web passes the foam curtain applicator (300) is at least 100 m / min, preferably at least 150 m / min, more preferably at least 170 m / min. In some embodiments, the velocity of the moving web when the web passes the foam curtain applicator (300) is at least 250 m / min, at least 300 m / min, or at least 400 m / min. Preferably, the velocity of the moving web when the web passes the foam curtain applicator (300) is no more than 1000 m / min, preferably no more than 800 m / min, more preferably no more than 700 m / min. In some embodiments, the velocity of the moving web when the web passes the foam curtain applicator (300) is no more than 400 m / min, no more than 350 m / min, or no more than 300 m / min.

[0024] Preferably, where the foam curtain applicator is provided with an elongated outlet slot, the width of the slot is greater than or equal to 2.0 mm and equal to or less than 4.0 mm, and the velocity of the moving web when the web passes the foam curtain applicator is at least 150 m / min and no more than 350 m / min, and the air-to-liquid ratio of the foamed additive, when dispensed, is no more than 5.0 and at least 1.0.

[0025] The invention also involves a system for applying a foamed additive to a moving web of airlaid fibres, which system comprises a foam curtain applicator device arranged to treat a moving web of airlaid nonwoven fibres with a foam additive (for example a binder). The applicator device includes a foam curtain applicator positioned above the web. The applicator device is provided with a flow of foam from a foam supply and dispenses the foam through an elongated outlet slot extending in the cross direction of the web, in order to form a continuous foam curtain that applies a substantially even layer of additive across the web width.

[0026] Preferably, the viscosity of the foamed additive and the design of the foam curtain applicator are such that the foamed additive is dispensed from the foam curtain applicator onto the web as a uniform falling curtain. Thereby, the system may comprise the foamed additive.

[0027] Preferably, the system comprises a barrier next to the foam curtain applicator to protect the curtain from disturbing air movements in the surroundings. This is particularly advantageous since the velocity of the web may be relatively low in a system involving airlaid fibres.

[0028] Preferably, the system comprises a suction box underneath the foam curtain applicator. The suction box is preferably positioned, in the direction of travel of the web, below the foam curtain applicator and / or after the foam curtain applicator. Preferably, the system includes a calendering unit positioned upstream of the foam curtain applicator. Preferably, the system includes a drying or curing unit positioned downstream of the foam curtain applicator. Preferably, the foam curtain applicator comprises an elongated slot of adjustable width. The slot may also be of adjustable length and / or adjustable height above the web.

[0029] Brief description of the figures

[0030] Figure 1: Schematic view of prior art airlaid fibre web production line arrangement with a spray application system.

[0031] Figure 2: Schematic view of an airlaid fibre web production line in accordance with an embodiment of the present invention.

[0032] Figure 3: Schematic lateral cross-sectional view of a foam curtain applicator in accordance with an embodiment of the present invention.

[0033] Figure 4: Process flow diagram in accordance with one embodiment of the invention.

[0034] Figure 5: Schematic view along a direction of movement of a web, of a foam curtain applicator in accordance with an embodiment of the present invention.

[0035] Detailed Description of embodiments of the Invention

[0036] Figure 1 shows schematically a prior art airlaid nonwoven fibre web manufacturing line with a conventional spray application system 100. It comprises a raw material source 101 which contains raw material, for example rolls of fluff pulp 103 positioned on an unwinder 105. A transport system 107 transports a web 109 of unwound fluff pulp to one or more fluff processing machines, such as one or more hammer mills 111, which chop and hammer the web in order to produce fibres. These fibres are transported by an air flow 113 to one or more forming units 115 where they are laid onto the upper surface 117 of an endless loop of forming fabric 119. Laying of the fibres can be assisted by one or more suction boxes 121 provided inside the fabric loop 119. This forms a porous web 125 of airlaid fibres and this can optionally be made denser by calendering between calender rolls 127.

[0037] Additives, such as binder polymers, are applied to the upper surface 129 of the web of airlaid fibres by a conventional spray system 131, which sprays a layer of additive 133 onto the web. A suction box 134 may be used to aid penetration of the liquid dispersion layer into the web.

[0038] The sprayed web is then cured by heating, for example in a through air dryer (TAD) 135. If necessary, a further additive layer is applied to the lower surface 137 of the web 125 by a further spray system 139 with suction box 140 above the web.

[0039] This treated web is then cured by heating, for example in a through air dryer (TAD) 141.

[0040] The treated web is then scanned in a conventional profile scanner 143 before being conventionally wound onto a reel 145 or the like.

[0041] Figure 2 shows schematically a new airlaid nonwoven fibre web manufacturing line with foam curtain application system 200 in accordance with an embodiment of the present invention. It comprises a raw material source 201 which contains raw material, for example rolls of fluff pulp 203 positioned on an unwinder 205. A transport system 207 transports a web 209 of unwound fluff pulp to one or more fluff processing machines, such as one or more hammer mills 211 which chop and hammer the web in order to produce fibres. These fibres are transported by an air flow 213 to one or more forming units 215 where they are laid onto the upper surface 217 of an endless loop of forming fabric 219. Laying of the fibres can be assisted by one or more suction boxes 221 provided on inside of the fabric loop 219. The fibres form a porous web 225 of airlaid fibres and this web can optionally made denser by calendering between calender rolls 227.

[0042] A foamed additive is applied to the upper surface 229 of the web of airlaid fibres by a foam curtain applicator 231 (described in more detailed below in connection with figure 3) which deposits a foam layer 233 onto the web. A suction box 234 may be used to aid penetration of the foam layer into the web.

[0043] The treated web is then cured by heating, for example in a through air dryer (TAD) 235.

[0044] If necessary, a further foamed additive layer is applied to the lower surface 237 of the web 225 by a spray system 239 positioned below the web or the web is inverted so that the previously lower surface faces upwards and a further additive layer is applied by a further foam curtain applicator. A suction box 240 may be used to increase penetration of the additive into the web.

[0045] If necessary, this treated web is then cured by heating, for example in a through air dryer (TAD) 241.

[0046] The treated web may then be scanned in a conventional profile scanner 243 before being conventionally wound onto a reel 245 or the like.

[0047] Figure 3 shows a schematic cross-sectional view through a foam curtain applicator 300. The applicator comprises a hollow elongated foam applicator duct 301 which extends at least the width of the web in the cross direction of the web to be treated. The duct contains foamed additive 233 which is to be applied to a web 225 passing beneath it. The foam is supplied via a supply line 237 from foam source 235, which can be a foam generator producing the foam by mixing foaming liquid (binder liquid) and air. The amount of foam supplied can be controlled by a pump 239 integrated to the foam mixer. An elongated slot 305 is formed in a lower portion of the duct, for example between the nozzle lips 307 and 309 at the base of the duct. If applicable, the slot may extend beyond the edges of the web in order to avoid any edge effects of a curtain of foam from effecting the application of foam onto the web. Preferably the slot width W in the machine direction (MD) can be changed by means of a movable nozzle lip 309 which can be moved in the machine direction MD to cover or expose more of the slot 305 in the machine direction. Slot width W can be greater or equal to 0.2 mm and equal to or less than 4.0 mm. Preferably the slot gap is greater or equal to 0.5 and less than or equal to 2.0 mm. The width W is dependent on the desired flow rate, flow pressure and initial curtain velocity criteria. This enables the width in the machine direction of the foam curtain 311 descending from the slot to be adjusted. A suction box 313 may be provided under the web 225 and the forming fabric 219 supporting it in order to aid penetration of the foamed additive into the web 225. Preferably the height H of the foam curtain 311 from the web 225 can be adjusted by raising or lowering the duct 301. Height H can be greater than or equal to 5 mm and equal to or less than 400 mm. Preferably height H is equal to or less than 200 mm, more preferably equal to or less than 150 mm, even more preferably equal to or less than 100 mm. Preferably height H is greater than or equal to 10 mm, more preferably greater than or equal to 20 mm, even more preferably greater than or equal to 30 mm. Thereby it is possible to ensure that the applicator 300 does not touch the web, whereby the risk of a web breakage is reduced. This makes embodiments of the invention advantageous over methods with extruders, which normally have to be placed very close to the web.

[0048] Figure 4 shows the steps of a method for forming a nonwoven airlaid web with a foam curtain application from web formation through additive application.

[0049] The method comprises the following steps in sequence:

[0050] 401 provide a source of bundled fibres

[0051] 403 process fibre bundles to form loose fibres e.g. with at least one hammer mill

[0052] 405 form an airborne flow of fibres and form air laid web of fibres on a conveyor fabric

[0053] (406 optionally calendar the fibre web)

[0054] 407 apply an additive layer by a foam curtain

[0055] 409 cure / dry top side additive (410a optionally treat lower side of the web by liquid spraying or a further foam curtain application)

[0056] (410b cure / dry lower side additive)

[0057] The airlaid non-woven web of fibres with a foam additive can then undergo:

[0058] 411 quality check e.g. web profile scan

[0059] 413 winding

[0060] In more detail, a method for forming an airlaid, non-woven web of fibres with a foamed binder or other additive comprises the following steps in order:

[0061] Forming fibres: One or more hammer mills and forming units can be used to refine the fibre stock and ensure uniform web formation and the required web basis weight.

[0062] Web Formation: The airlaid nonwoven web is formed by feeding airborne fibres into forming units positioned on / above air permeable conveyor fabrics, optionally using suction boxes under the conveyor fabrics to suck the fibres onto the conveyor fabric.

[0063] Optional calendering: The web may undergo calendering to compact and smoothen the porous web structure before the foam is applied.

[0064] Foam Curtain Application: The web passes beneath the foam curtain applicator, where the foam is applied uniformly across the web. The foam may contain various binders, including latex, starch, or bio-based polymers, or additional functional chemicals, depending on the specific application.

[0065] Optional Suction-Assisted Foam Application : An optional suction box may be provided under the web, whereby the suction pulls the foam deeper into the fibre web, enhancing binder penetration and resulting in improved fibre bonding. However, pulling foam too deeply into and through the web might cause sticking of the web to, and contamination of, the supporting wire.

[0066] Preferably, it is sufficient to pull foam half-way or nearly through, in particular if in a further step binder application is done on the back side.

[0067] Drying and Quality Control: After coating, the web is dried, measured for quality, and reeled for further processing.

[0068] The foam curtain applicator is designed to dispense foam through an elongated slot positioned above the airlaid nonwoven fibre web. The curtain application area (width) in the web cross direction (CD) may be arranged slightly narrower than the width of the web to which it is being applied, i.e. the curtain application is run in so-called "in-board" -mode, leaving narrow untreated dry web strips at the edges, thus avoiding wet edges, runnability issues and contamination of the supporting wire. Preferably, the in-board curtain application mode is arranged to leave from 2 cm to 5 cm wide narrow untreated dry edges, which can be subsequently trimmed (cut) away later. Thus, the length L of curtain applicator slot width in the CD direction can be arranged to be slightly narrower than web width W, to accommodate the desired application width. It is also possible to use applicators with slots wider than web (i.e. L > web width) (or desired binder application width) by using special edge collectors to take away excess foam curtain at the edges, thus narrowing the curtain width.

[0069] Viscosity and Application Rate considerations: The foam viscosity has an impact on foam penetration into the porous airlaid web. In addition, foam viscosity must remain within suitable limits, so that foam flows easily through the foam supply line, foam applicator, creates a stable curtain and settles on the moving web properly. Since it is difficult to define exact limits to foam viscosity, the proper conditions can be found by simple experiments. Foam viscosity is affected by additive (binder liquid) product viscosity, additive solids content in the foaming liquid, foam density (liquid-to-air ratio) and foam bubble size, among other factors. The foam used can have any viscosity capable of forming a curtain, allowing for high-viscosity, bio-based binders to be used in the application - something which is difficult to achieve with spray applicators. The application amount of foam applied additives can be typically about 1-15% dry binder per dry web weight. The actual application mass flow rate is adjusted based on desired dry add-on and binder solids content in the foaming liquid, wet add-on being for example in the range 10-100 g / m2, more advantageously 10-50 g / m2, for typical airlaid products. Actual curtain flow rate to get the desired application rate can be adjusted based on web speed.

[0070] Curtain Formation and Stability considerations: The stability of the curtain means that the falling curtain remains integral, uniform and stable, and produces an even and uniform film layer on the target surface throughout the application process. An unstable curtain is manifested by quickly growing holes appearing in the falling curtain, starting from small initial disturbances or pinholes, and finally producing uneven "coating" result, possibly leaving uncoated spots. Foam curtain stability is a complicated phenomenon, and is influenced by multiple factors, including foam's material properties, like rheology, surface tension, density, foam homogeneity, and flow geometry and flow properties, like initial curtain flow velocity, flow rate, curtain thickness and curtain height (the vertical height of the slot above the web surface). Achieving a stable curtain may require trial-and- error adjustments, such as modifying the additive's chemical composition, foaming process and the application geometry, like curtain height and foam curtain initial flow velocity and curtain thickness at the nozzle exit point. Preferably, the vertical distance from the slot to the web is adjustable between 0.5 and 40 cm , with the lower value preferred for avoiding the risk of holes growing too much during the curtain falling time. The curtain initial velocity and thickness (W) can be adjusted by setting the nozzle gap width. In practice, slot gap width is pre-set by mounting suitable thickness shim plates between two halves of the slot die applicator. Another way for nozzle gap adjustment is to via movable / adjustable nozzle lips, for example by mechanically bending one of the lips. The nozzle gap width is preferably set to a value between 0.2- 4.0 mm, most advantageously between 0.5-2.0 mm.

[0071] Optional Suction Box: For applications that require deeper penetration of the binder into the fibre web, one or more suction boxes may be placed below the web to draw the foamed additive deeper through the fibre web. This feature is optional, although highly desirable, providing additional flexibility based on production needs but is not required for basic operation.

[0072] Binder Compatibility: This invention supports a range of binder types, including highly viscous, sticky polymers, and bio-based options, which are often incompatible with traditional spray systems, for example, due to too high viscosity compromising spray atomization and due to stickiness creating accumulation, contamination and nozzle clogging. The foam curtain method avoids clogging risk of tiny nozzle openings and eliminates mist formation, therefore providing better compatibility with current binder materials as well as enabling sustainable manufacturing by allowing the use of new eco-friendly but more demanding materials without sacrificing application quality or operational reliability.

[0073] Benefits and Advantages:

[0074] Nozzle Clogging Eliminated: Foam curtain technology replaces separate spray nozzles which have narrow orifices, which are easily blocked, with a slot die nozzle having bigger continuous extending nozzle gap opening and therefore preventing clogging and reducing maintenance needs.

[0075] Cleaner Application Process: Foam curtain technology does not produce atomization or small airborne particles, and avoids overspraying past the web edges, therefore eliminating misting, reducing material waste and maintaining a cleaner workspace.

[0076] Enhanced Material Compatibility: Foam curtain technology provides a wider operation window for material solids and viscosity, thus supporting the use of eco-friendly, high-viscosity binders, expanding binder options for sustainable manufacturing.

[0077] Improved Bonding and Penetration: When used with the optional suction box, the foam curtain achieves enhanced binder penetration and improved bonding More effective material and energy use: Since spray application usually provides somewhat streaky, cloudy and uneven applied film, the application amount needs to be increased for better coverage (so called overspraying) respectively. However, foam curtain technology provides more a uniform film and enables the use of lower binder application dosings for the same coverage. Owing to the increased penetration of a foam into a web compared to a water-based additive, less foamed binder is needed to achieve the desired penetration of the web. As foam contains less water than similar volume of water, the amount of water applied to the web is reduced. This means that less water needs to be removed in subsequent drying stages, thereby reducing the amount of heating required and reducing energy consumption.

[0078] Lower application amounts mean material savings as well as drying energy savings.

[0079] Reference is made to fig. 5. In some embodiments one or more barriers 502 are provided next to the foam curtain applicator 300, to protect the curtain 311 from disturbing air movements in the surroundings. Fig. 5 shows barriers offset from the applicator 300 in the web cross direction. Preferably these barriers overlap the applicator in the web movement direction. In addition or alternatively, one or more barriers may overlap the applicator in the web cross direction, and may thereby be offset from the applicator in the web movement direction. The barriers may be made in metal, plastic or glass. The barriers may be transparent for improved visibility. On some embodiments, the barriers are formed by air-knives. Such disturbances can for example occur due to a draft in the surrounds, e.g. caused by fans in a factory in which the manufacturing line is installed. In this embodiment, the barrier 502 forms a skirt between the surroundings and the applicator 300. Disturbances in the surrounding air can also be caused by the moving web. Thereby, barriers formed by air-knives may be particularly beneficial, since such barriers will not have any solid part close to the web which may create a risk of contact between the solid part and the web. Barriers formed by air-knives may serve to reduce or eliminate air disturbances created by the web movement, by a counter blow against the web.

Claims

ClaimsClaim 1. A method for applying a foamed additive layer to a moving web of airlaid nonwoven fibres, characterized by dispensing the foamed additive (223) from at least one foam curtain applicator (300) positioned above the web (225).Claim 2. A method according to claim 1, wherein dispensing the foamed additive comprises dispensing the foamed additive with a uniform falling curtain of the foamed additive.Claim 3. A method according to any of the previous claims, wherein the foam curtain applicator (300) comprises a hollow elongated foam applicator duct (301), wherein an elongated outlet slot (305) is formed in a lower portion of the duct, wherein the duct contains some of the foamed additive (233) before the foamed additive is dispensed through the slot and onto the web, wherein, while some of the foamed additive is dispensed, the pressure of the foamed additive in the duct is substantially the same as the pressure of the atmosphere surrounding the foam curtain applicator.Claim 4. A method according to any of the previous claims, wherein the foam curtain applicator (300) comprises a hollow elongated foam applicator duct (301), wherein an elongated outlet slot (305) is formed in a lower portion of the duct, wherein the foamed additive exits the duct through the slot purely by means of gravity.Claim 5. A method according to any of the previous claims, wherein the additive comprises a natural or synthetic polymer-based binder.Claim 6. A method according to any of the previous claims, comprising before dispensing the foamed additive (223), providing the additive in an un-foamed state, and foaming the un-foamed additive.Claim 7. A method according to claim 6, wherein the foamed additive is dispensed onto the web no more than 60 minutes after the step of foaming the un-foamed additive.Claim 8. A method according to any of claims 6-7, comprising, before foaming the un-foamed additive, forming the un-foamed additive by diluting solids with a diluting liquid, such as water, wherein, after said dilution, the un-foamed additive contains 10-40 wt% of diluted solids.Claim 9. A method according to any of claims 6-8, wherein the viscosity of un-foamed additive is no more than 20 mPa-s.Claim 10. A method according to any of the previous claims, wherein the air-to-liquid ratio of the foamed additive, when dispensed, is no more than 10, preferably no more than 7.0, more preferably no more than 5.0, even more preferably no more than 2.4.Claim 11. A method according to any of the previous claims, wherein the air-to-liquid ratio of the foamed additive, when dispensed, is at least 0.5, preferably at least 1.0, more preferably at least 1.5, even more preferably at least 2.0.Claim 12. A method according to any of the previous claims, wherein the average bubble size in the foamed additive, when dispensed, is at least 10 pm, preferably at least 30 pm, more preferably at least 50 pm.Claim 13. A method according to any of the previous claims, wherein the average bubble size in the foamed additive, when dispensed, is no more than 200 pm, preferably no more than 150 pm, more preferably no more than 100 pm.Claim 14. A method according to any of the previous claims, wherein additive penetration is enhanced by providing and operating a suction box (313) positioned below the web (225).Claim 15. A method according to any of the previous claims, wherein, after dispensing the foamed additive (223) from the foam curtain applicator, the weight of additive applied to the web, per unit area, is 1-15 % of the weight of dry web per unit area.Claim 16. A method according to any of the previous claims, comprising providing the foam curtain applicator (300) with an elongated outlet slot (305), wherein the width (W) of the slot is greater than or equal to 0.2 mm, preferably greater than or equal to 0.5 mm.Claim 17. A method according to any of the previous claims, comprising providing the foam curtain applicator (300) with an elongated outlet slot (305), wherein the width (W) of the slot is equal to or less than 4.0 mm, preferably equal to or less than 2.0 mm.Claim 18. A method according to any of the previous claims, wherein the velocity of the moving web when the web passes the foam curtain applicator (300) is at least 100 m / min, preferably at least 150 m / min, more preferably at least 170 m / min.Claim 19. A method according to any of the previous claims, wherein the velocity of the moving web when the web passes the foam curtain applicator (300) is no more than 1000 m / min, preferably no more than 800 m / min, more preferably no more than 700 m / min.Claim 20. A system for applying a foamed additive (233) to a moving web of airlaid fibres (225) characterized in that it comprises a foam curtain applicator (300) positioned above the web (225).Claim 21. A system according to claim 20, wherein the viscosity of the foamed additive and the design of the foam curtain applicator are such that the foamed additive is dispensed from the foam curtain applicator onto the web as a uniform falling curtain.Claim 22. A system according to any of claims 20-21, comprising a barrier next to the foam curtain applicator to protect the curtain from disturbing air movements in the surroundings.Claim 23. A system according to any of claims 20-22, wherein the system comprises a suction box (313) underneath the foam curtain applicator (300).Claim 24. A system according to any of claims 20-23, further including a calendering unit positioned upstream of the foam curtain applicator (300). Claim 25. A system according to any of claims 20-24, further including a drying or curing unit positioned downstream of the foam curtain applicator (300).Claim 26. A system according to any of claims 20-25, wherein the foam curtain applicator (300) comprises an elongated slot (305) of adjustable width (W).

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