SE548285C2

SE548285C2 - Production of bonded air-laid blanks

Info

Publication number: SE548285C2
Application number: SE2430511A
Authority: SE
Inventors: Maria Törnblom
Original assignee: Stora Enso Oyj
Priority date: 2024-10-07
Filing date: 2024-10-07
Publication date: 2026-05-13
Also published as: SE2430511A1; WO2026078532A1

Abstract

A method and system (100) for producing a bonded air-laid blank (10) are disclosed. PVOH-treated natural fibers and a polymer binder are introduced into a forming head (110). The PVOH-treated natural fibers and the polymer binder are captured as an unbonded air-laid web (20) on a conveyor (120) arranged in connection with an outlet (113) of the forming head (110). The unbonded air-laid web (20) to at least partly melt the polymer binder and bind the PVOH-treated natural fibers to form a bonded air-laid

Description

[0001] The present invention generally relates to bonded air-laid blanks, and to a method and system for producing such bonded air-laid blanks.

[0004] BACKGROUND

[0005] An air-laid blank, sometimes also referred to as dry-laid, dry-formed blank, air-laid bat, dry-laid bat, airlaid mat, or dry-laid mat, is formed by a process known as air-laying, in which natural fibers and a polymer binder are mixed with air to form a porous fiber mixture deposited onto a support and consolidated or bonded by heating. During the heating the natural fibers are bonded by the polymer binder. The air-laid blank is characterized by being porous, having the character of an open cell foam. Air-laid blanks are produced in a so-called dry forming method, i.e., generally without addition of water. The air-laying process is described in, for instance, U.S. patent no. 6233787.

[0007] In the production of air-laid blanks, non-bonded natural fibers and fines present on or within the air-laid blanks or detached therefrom during production, are perceived as lint or dust. Such lint or dust may constitute an aesthetic problem for products formed from the air-laid blanks. Furthermore, in larger quantities, such dust can cause inconvenience and irritations for persons handling the air-laid blanks during and following production and converting.

[0009] U.S. patent publication no. 20220290344 discloses non-woven materials having low dust or lint content and methods of making the same. Such non-woven materials can include cellulose fibers pre-treated with a plasticizer or include the addition of plasticizer during the process of forming the material.

[0011] There is a need to produce air-laid blanks with less lint or dust present on or detaching from the produced air-laid blanks.

[0014] SUMMARY

[0015] It is a general objective to provide a method and system of producing bonded air-laid blanks having improved linting characteristics.

[0017] These and other objectives are met by embodiments as disclosed herein.

[0018] The present invention is defined in the independent claims. Further embodiments of the invention are defined in the dependent claims.

[0020] An aspect of the invention relates to a method of producing a bonded air-laid blank. The method comprises contacting natural fibers with a solution comprising a polyvinyl alcohol (PVOH) to form PVOH-treated natural fibers and introducing the polyvinyl alcohol (PVOH) treated natural fibers and a polymer binder into a forming head. The method also comprises capturing the PVOH-treated natural fibers and the polymer binder as an unbonded air-laid web on a conveyor arranged in connection with an outlet of the forming head. The method further comprises heating the unbonded air-laid web to at least partly melt the polymer binder and bind the PVOH-treated natural fibers to form a bonded air-laid blank.

[0022] Also disclosed herein is a system for producing a bonded air-laid blank. The system comprises a conduit arranged to convey an air flow of natural fibers or a mixture of natural fibers and a polymer binder. The system also comprises a spray nozzle arranged in the conduit to spray a solution comprising a PVOH onto the natural fibers or the mixture of the natural fibers and the polymer binder to form PVOH-treated natural fibers or a mixture of PVOH-treated natural fibers and the polymer binder. The system further comprises a forming head comprising an inlet in fluid communication with the conduit and arranged to receive the PVOH-treated natural fibers or a mixture of the PVOH-treated natural fibers and the polymer binder. The forming head also comprises an outlet. The system additionally comprises a conveyor arranged in connection with the outlet to capture the PVOH-treated natural fibers and the polymer binder as an unbonded air-laid web. The system further comprises a heating device arranged to heat the unbonded air-laid web to at least partly melt the polymer binder and bind the PVOH-treated natural fibers to form a bonded air-laid blank.

[0024] Also disclosed herein is a bonded air-laid blank comprising PVOH-treated natural fibers and a polymer binder binding together the PVOH-treated natural fibers.

[0026] Generally, in the production of bonded air-laid blanks, non-bonded natural fibers, fragments and particles thereof, present on or in the bonded air-laid blanks or detached therefrom during production are perceived as lint or dust. Such lint or dust may constitute an aesthetic problem for products formed from the bonded air-laid blanks. Furthermore, in larger quantities, such dust can cause inconvenience and irritations for persons handling the bonded air-laid blanks during and following production. The lint or dust may also cause problems for electronics and electronic equipment, if for instance, the bonded air-laid blank is employed as cushioning or insulation insert for packaging the electronics or electronic equipment. The present invention reduces such lint and dust problems by treating the natural fibers with a PVOH. The PVOH acts as an adhesive and captures and binds loose fibers, fragments and particles thereof, which are not effectively bonded within the bonded air-laid blank by the polymer binder. The treatment of the natural fibers by the PVOH thereby substantial improves the linting behavior of the bonded air-laid blank and products produced therefrom.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The embodiments, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

[0032] Fig. 1A is a perspective view of a bonded air-laid blank according to an embodiment;

[0034] Fig. 1B is side-view of a portion of a bonded air-laid blank according to an embodiment;

[0036] Fig. 2 is a flow chart illustrating a method of producing an air-laid blank according to an embodiment;

[0038] Fig. 3 is a flow chart illustrating additional, optional steps of the method in Fig. 2 according to an embodiment (not being part of the claimed invention);

[0040] Fig. 4 is a flow chart illustrating additional, optional steps of the method in Fig. 2 according to another embodiment;

[0042] Fig. 5 is a flow chart illustrating an additional, optional step of the method in Fig. 2 according to an embodiment;

[0044] Fig. 6 is a flow chart illustrating an additional, optional step of the method in Fig. 2 according to another embodiment; and

[0046] Fig. 7 is a schematic illustration of a system for producing an air-laid blank according to various embodiments (not being part of the claimed invention).

[0049] DETAILED DESCRIPTION

[0050] The present invention generally relates to a method for producing bonded air-laid blanks.

[0051] Bonded air-laid blanks are characterized by being porous, having the character of an open cell foam. They are resilient and have great damping and insulation capacity. These characteristics of bonded airlaid blanks make the material suitable to replace polymer foams and formed in-place fossil-based materials in packaging solutions. A common way of protecting the goods is to include cushioning or insulation elements or products, such as inserts of suitable form into the packaging. These cushioning or insulation elements or products are typically made from a foamed petroleum-based polymer, of which expanded polystyrene (EPS) is by far cheapest and most common. EPS is, however, one of the most questioned plastic materials and many brand owners are looking for more sustainable solutions for these packaging applications. Bonded air-laid blanks are useful for production of more environmentally friendly replacements to corresponding cushioning inserts made of or from foamed polymers, for instance EPS or foamed polyurethane (PU). Bonded air-laid blanks also find uses where there is a need for providing insulation, such as thermal or sound insulation. Illustrative, but non-limiting examples, of such applications include thermal insulation of heated or cold food products or other articles that need to be kept within defined temperature ranges. Furthermore, sound absorbing panels or elements could be produced from the bonded air-laid blanks.

[0053] In the production of bonded air-laid blanks, non-bonded natural fibers present on or in the bonded air-laid blanks or detached therefrom during production are perceived as lint or dust. This lint, to a major part, consists of natural fibers, fiber fragments or smaller particles commonly referred to as "fines" in the art that have not been sufficiently bound by the polymer binder either on the surfaces or within the bonded air-laid blanks. Further, if the content of the polymer binder used to bind the natural fibers is decreased, such as to promote repulpability of the bonded air-laid blanks in board mills and thereby to increase the environmental friendliness of the bonded air-laid blanks, then the linting tendency of the bonded air-laid blanks is generally increased significantly.

[0055] The lint or dust may constitute aesthetic problems for products formed from the bonded air-laid blanks. Furthermore, in larger quantities, such dust can cause inconvenience and irritations for persons handling the bonded air-laid blanks during and following production. The lint or dust may also cause problems for electronics and electronic equipment if these are packaged using cushioning or insulation elements or inserts made by the bonded air-laid blanks. In such a case, non-bonded natural fibers may cause short circuits if reaching the electronic circuitry within the electronics or electronic equipment. Such non-bonded natural fibers might also be a risk during operation of the electronics or electronic equipment causing heat development that might ignite the non-bonded natural fibers.

[0056] The present invention solves or at least reduces the linting problem of prior art air-laid blanks by pretreating the natural fibers prior to the air-laying process. In more detail, the natural fibers are treated with a polyvinyl alcohol (PVOH). The PVOH has strong adhesion and thereby acts as an adhesive for loose natural fibers and fragments and smaller particles thereof, i.e., so-called fines. The PVOH will thereby bind the fibers, fragments and particles to another. Hence, the PVOH interconnects the natural fibers and the fragments and particles thereof and thereby restrict release thereof from the air-laid blank during handling, converting operations or use.

[0058] The present invention therefore relates to a method of producing a bonded air-laid blank 10, see Figs.

[0059] 1A, 1B, 2, and 7. The method comprises introducing, in step S1, PVOH-treated natural fibers and a polymer binder into a forming head 110. The PVOH-treated natural fibers and the polymer binder are captured in step S2 as an unbonded air-laid web 20 on a conveyor 120 arranged in connection with an outlet 113 of the forming head 110. The method also comprises heating, in step S3, the unbonded airlaid web 20 to at least partly melt the polymer binder and bind the PVOH-treated natural fibers to form a bonded air-laid blank 10.

[0061] The heating as applied in step S3 will partly melt the polymer binder to become tacky and adhere to the PVOH-treated natural fibers in the unbounded air-laid web 20 to thereby form the bonded air-laid blank 10. Most of the PVOH-treated natural fibers in the unbounded air-laid web 20 will thereby be bonded together by the polymer binder forming the porous, open cell foam like structure of the bonded air-laid blank 10. However, some natural fibers will not be bonded by the polymer binder. Further, there is generally a distribution in the dimensions of the natural fibers especially if the natural fibers are produced by mechanical pulping processes. Some natural fibers may be broken into smaller parts during the production of the bonded air-laid blank 10. Such shorter fibers or indeed fiber particles or debris, also referred to as fines in the art, may then, if not bonded by the polymer binder following step S3, escape through the fiber structure of the bonded air-laid blank 10. Additionally, natural fibers may be at least partly bonded by the polymer binder, but the bond might be far from sufficient so that such natural fibers become detached from the fiber structure of the bonded air-laid blank 10. This is in particular a problem for the natural fibers present at the surfaces 11, 12, 13, 14 of the bonded air-laid blank 10, see Figs. 1A and 1 B, as these surface fibers generally have fewer bonds, formed by the polymer binder, to neighboring natural fibers in the bonded air-laid blank 10 as compared to natural fibers present within the bulk of the bonded air-laid blank 10. The PVOH-treatment of the natural fibers prior to entering the forming head 110, however, significantly reduces the amount of loose natural fibers, and fragments and particles thereof in the bonded air-laid blank 10 since the previously added PVOH acts as an adhesive and binds such natural fibers, and fragments and particles thereof within the bonded air-laid blank 10.

[0063] The bonded air-laid blank 10 as produced in the method of Fig. 2 therefore has significantly less nonbonded natural fibers, fiber fragments and particles (fines) as compared to a bonded air-laid blank 10 produced using non-treated natural fibers and polymer binder rather than using PVOH-treated natural fibers and the polymer binder. The PVOH promotes, by acting as an adhesive, interconnection between the natural fibers and any fiber fragments and particles in the bonded air-laid blank 10. As a consequence, such natural fibers, fiber fragments and particles will thereby, even if not bonded by the polymer binder to other natural fibers in the bonded air-laid blank 10, remain at the surface of or within the bulk of the bonded air-laid blank 10.

[0065] This means that the amount of lint or dust from the bonded air-laid blank 10 of the invention produced using PVOH-treated natural fibers, is significantly reduced. This in turn means that the bonded air-laid blank 10 and products produced therefrom, such as cushioning inserts or other packaging elements, will contain less lint and dust. The bonded air-blank 10 of the invention has a reduced tendency to contaminate packaged goods and other products, including food or beverages, in contact with the bonded air-laid blank 10 or products produced therefrom with such lint and dust. These effects, as achieved by the bonded air-laid blank 10, not only solve any aesthetic problems caused by the lint and dust, but also reduce the risk of any inconvenience that could otherwise be caused by inhaling fine fiber-based lint and dust. Furthermore, the risk of problems with non-bonded natural fibers contaminating sensitive equipment, such as electronics, is reduced by the invention.

[0067] Figs. 1A and 1B schematically show a bonded air-laid blank 10 produced by the method as shown in Fig. 2. As is shown in Fig. 1A, a bonded air-laid blank 10 is typically in the form of a sheet having a length L, a width W and a thickness T. Generally, lint and dust are present on and released from the surfaces 11, 12, 13, 14 of the bonded air-blank 10. Fig. 1A illustrates main surfaces 12, 14 of the bonded air-laid blank 10 that are the two surfaces defined by the length L and the width W. The main surfaces 12, 14 are substantially parallel with the upper surface of the conveyor 120 (Fig. 7) with one of the main surfaces 14 facing the conveyor 120 (Fig. 7) and being positioned thereon during production of the bonded air-laid blank 10 and with the other main surface 12 facing in a direction opposite to the conveyor 120 (Fig. 7). The main surfaces 12, 14 of the bonded air-laid blank 10 have a respective surface area that is typically substantially larger than the surface area of the longitudinal sides 11 or end sides 13 of the bonded airlaid blank 10.

[0068] Fig. 3 is a flow chart illustrating additional method steps of Fig. 2 according to various embodiments (not being part of the claimed invention). Step S10 of Fig. 3 comprises mixing natural fibers and the polymer binder to form a mixture of the natural fibers and the polymer binder. In an embodiment, the method continues from step S10 to step S13, thereby omitting the optional method steps S11 and S12. Step S13 comprises contacting the mixture of the natural fibers and the polymer binder with a solution comprising the PVOH to form a mixture of the PVOH-treated natural fibers and the polymer binder. The method then continues to step S1 in Fig. 2. In this embodiment, step S1 comprises introducing the mixture of the PVOH-treated natural fibers and the polymer binder into the forming head 110.

[0070] This embodiment thereby involves pre-mixing the natural fibers and the polymer binder prior to introducing them into the forming head 110 and prior to contacting the natural fibers with the PVOH. In step S13, the PVOH will interact with and thereby bind to the natural fibers and fragments and particles thereof in the mixture of natural fibers and polymer binder. The PVOH will not significantly affect the binding properties of the polymer binder during the heating step S3 in Fig. 2. This means that the PVOH can be contacted with the mixture of the natural fibers and the polymer binder prior to the introduction into the forming head 110 and still achieve an efficient treatment of the natural fibers.

[0072] In an embodiment, contacting the mixture of the natural fibers and the polymer binder with the solution comprising the PVOH (PVOH solution) in step S13 comprises spraying the solution comprising the PVOH onto the mixture of the natural fibers and the polymer binder.

[0074] As is shown in Fig. 7, a conduit 170 is preferably arranged in fluid communication with an inlet 111 of the forming head 110. In such an embodiment, the method comprises the additional step S11 of Fig.3, which comprises transporting the mixture of the natural fibers and the polymer binder by an air flow in the conduit 170 to the forming head 110. The conduit 170 then comprises an equipment, typically a spray nozzle 180, arranged to spray the solution comprising the PVOH onto the mixture of the natural fibers and the polymer binder as transported or conveyed by the airflow in the conduit 170. Fig. 7 schematically shows the direction of the air flow by an arrow in the conduit 170. This air flow flowing in the conduit 170 towards the forming head 110 carries the mixture of the natural fibers and the polymer binder.

[0076] In an embodiment, step S13 of Fig. 3 comprises spraying the solution comprising the PVOH in a flow direction of the airflow in the conduit 170. In this embodiment, the spray nozzle 180 is thereby arranged in the conduit 170 facing the forming head 110 and the inlet 111 therein. The solution comprising the PVOH is thereby sprayed onto the mixture of the natural fibers and the polymer binder as they are transported, by the air flow, past the spray nozzle 180 in the conduit 170 towards the forming head 110. Such an embodiment achieves an efficient treatment and coating of the natural fibers, and any fiber fragments and particles ("fines") carried by the air flow past the spray nozzle 180. Furthermore, such an arrangement of the spray nozzle 180 in the conduit 170 reduces the risk of clogging of the spray nozzle 180 by the natural fibers and the polymer binder.

[0078] In an embodiment, the solution is an aqueous solution of the PVOH. In a preferred embodiment, the solution is water comprising the PVOH. For instance, the PVOH could be dissolved or dispersed in the aqueous solution, preferably water.

[0080] In an embodiment, the solution comprises the PVOH at concentration selected within an interval of from 0.1 up to 5 % by weight of the solution. In a preferred embodiment, the solution comprises the PVOH at concentration selected within an interval of from 0.5 up to 5 % by weight of the solution.

[0082] Reference to an interval of from X up to Y herein includes the range of values between X and Y including the end points of the interval, i.e., X and Y.

[0084] The spray nozzle 180 is preferably arranged in a central position in the conduit 170 to achieve an efficient spraying of the solution comprising the PVOH onto natural fibers as they are transported by the air flow past the spray nozzle 180. As an example, the spray nozzle 180 could be arranged at a center of a cross section of the conduit 170, such as at a center or origin of a circle for a conduit 170 having a circular cross section or at an intersection of diagonals for a conduit 170 having a rectangular or quadratic cross section.

[0086] The embodiments are, however, not limited to a central arrangement of the spray nozzle 180 in the conduit 170. Alternatively, the spray nozzle 180 could be peripherally arranged in the conduit 170 at or in connection with one of the inner wall(s) of the conduit 170.

[0088] The spray nozzle 180 is preferably arranged to deliver the solution comprising the PVOH as a spray or aerosol.

[0090] In Fig. 7, a single spray nozzle 180 is arranged in the conduit 170, which is generally sufficient for most applications. The embodiments are, however, not limited thereto. Thus, multiple, i.e., at least two, spray nozzles 180 could be arranged in the conduit 170 to spray the solution comprising the PVOH onto the mixture of the natural fibers and the polymer binder.

[0092] The treatment and coating of the natural fibers with the solution comprising PVOH by the spray nozzle 180 can, in an embodiment, be promoted by locally inducing turbulence in the airflow in a section 172 of the conduit 170 comprising the spray nozzle 180. For instance, one or more turbulence inducing elements 175 could be arranged in the conduit 170 to act as obstruction(s) for the air flow and thereby locally induce turbulence in the air flow. Such a local induction of turbulence in the air flow achieves a turbulence of the natural fibers as they are transported past the spray nozzle 180. As a consequence, a more efficient treatment and coating of the natural fibers with the solution comprising the PVOH is achieved.

[0094] Another form of turbulence inducing element is an element arranged to induce a pressure pulse or wave in the air flow to induce turbulence in the air flow and thereby in the natural fibers or the mixture of the natural fibers and the polymer binder transported by the air flow.

[0096] Instead of, or as a complement to, having one or more turbulence inducing elements 175, turbulence in the air flow in connection with the spray nozzle 180 could be achieved by other means. For instance, there could be a turn in the conduit 170 inducing turbulence in the air flow in connection with the spray nozzle 180. A further variant is to have a change, such as an increase or a reduction, in the crosssectional area of the conduit 170, such as an increase or reduction in the inner diameter for a conduit 170 with circular cross section.

[0098] Hence, in an embodiment, the method comprises an additional step S12 as shown in Fig. 3. This step S12 comprises locally inducing turbulence in the airflow in a section 172 of the conduit 170, in which the solution comprising the PVOH is sprayed onto the mixture of the natural fibers and the polymer binder.

[0100] In the above-described and in Fig. 3 shown embodiments, the natural fibers are first mixed with the polymer binder and then contacted with the solution comprising the PVOH. According to the invention, the natural fibers are first treated with the PVOH and then mixed with the polymer binder. Such an approach is shown in Fig. 4. The method comprises step S22 of Fig. 4, which comprises contacting natural fibers with a solution comprising the PVOH to form the PVOH-treated natural fibers. A preferred embodiment of this step S22 is to spray the solution comprising the PVOH onto the natural fibers.

[0101] In an embodiment, the method comprises an additional step S20. This step S20 comprises transporting the natural fibers by an air flow in a conduit 170 to the forming head 110. The various embodiments described in the foregoing in connection with Fig. 3 also apply to the embodiments shown in Fig. 4 but with the difference that in Fig. 3 the mixture of the natural fibers and the polymer binder is transported by the air flow in the conduit 170 and treated by the spray nozzle 180 spraying the solution comprising the PVOH onto the mixture. In Fig. 4, the natural fibers are transported by the air flow in the conduit 170 and treated by the spray nozzle 180 spraying the solution comprising the PVOH onto the natural fibers. Flence, in an embodiment, step S22 comprises spraying the solution comprising the PVOH in a flow direction of the air flow.

[0103] The method may also involve locally inducing turbulence in the air flow in step S21 in a section 172 of the conduit 170, in which the solution comprising the PVOH is sprayed onto the natural fibers.

[0105] In an embodiment, the method also comprises mixing the PVOH-treated natural fibers and the polymer binder in step S23. The method then continues to step S1 in Fig.2. In this embodiment, step S1 comprises introducing a mixture of the PVOH-treated natural fibers and the polymer binder in the forming head 110.

[0107] Flence, in an embodiment, the natural fibers are first treated with the PVOH, such as by spraying the solution comprising the PVOH by a spray nozzle 180 arranged in a conduit 170, through which the natural fibers are transported by an air flow towards the forming head 110. The PVOH-treated natural fibers are then mixed with the polymer binder downstream, in the direction of the airflow, of the spray nozzle 180 but upstream of the forming head 110. The so-formed mixture between the PVOH-treated natural fibers and the polymer binder is then introduced into the forming head 110.

[0109] In an alternative embodiment, the mixing of the PVOH-treated natural fibers and the polymer binder is taking place first within the forming head 110. In such an embodiment, the forming head 110 comprises multiple inlets 111, i.e., at least one inlet 111 for the PVOH-treated natural fibers and at least one inlet for the polymer binder. As an example, the system 100 as shown in Fig. 7 could comprise at least one conduit 170 as shown in Fig. 7 for the natural fibers and where the at least one conduit 170 comprises a spray nozzle 180 arranged to spray the solution comprising the PVOH. The system 100 also comprises at least one conduit (not shown) for the polymer binder. This at least one conduit for the polymer binder does not need to contain any spray nozzle 180. The conduits 170 then enter the forming head 110 at separate inlets 111 and the PVOH-treated natural fibers transported by at least one conduit 170 and the polymer binder transported by at least one other conduit are mixed with each other in the forming head 110.

[0111] The PVOH-treated natural fibers and the polymer binder are introduced in step S1 into the forming head 110, also referred to as forming chamber in the art, as discrete input streams and/or as one or more mixed input streams at one or more inlets 111. The PVOH-treated natural fibers and the polymer binder are mixed and blended during the passage through the forming head 110 ultimately forming an unbonded air-laid web 20 on the conveyor 120. The forming head 110 may include equipment arranged inside the forming head 110 to promote separation and mixing of the PVOH-treated natural fibers and the polymer binder during the passage through the forming head 110. Such equipment may comprise, for instance, rolls with interlocking spikes, one or more drums, such as slit drums, and/or one or more strainers.

[0113] The PVOH-treated natural fibers and the polymer binder or the mixture thereof pass(es) through the forming head 110 to the outlet 113, such as arranged in connection with a lower end 114 of the forming head 110 and is further mixed through this passage. The mixture of the treated natural fibers and the polymer binder is then captured on the conveyor 120. In an embodiment, the conveyor 120 is an airpermeable conveyor 120 and the mixture of the treated natural fibers and the polymer binder is captured at least partly by a vacuum, i.e., an air suction or under-pressure, applied across the air-permeable conveyor 120 that is disposed in connection with the outlet 113 of the forming head 110. Hence, in an embodiment, the method of Fig. 2 preferably comprises an additional step of passing the mixture of the PVOH-treated natural fibers and the polymer binder to the outlet 113 of the forming head 110 while applying a gas suction through the air-permeable conveyor 120 in connection with the outlet 113 of the forming head 110.

[0115] Such a gas suction or vacuum is applied through the air-permeable conveyor 120. The gas suction or vacuum applied across the air-permeable conveyor 120, thus, draws the mixture of the PVOH-treated natural fibers, and the polymer binder down onto the air-permeable conveyor 120. For instance, the airpermeable conveyor 120 could comprise a plurality of openings, through holes or channels allowing air to be sucked or drawn through the air-permeable conveyor 120. As an illustrative, but non-limiting, example, the air-permeable conveyor 120 could be a mesh conveyor, a wire conveyor or a belt conveyor with a belt comprising a plurality of minute through holes. However, any such openings are preferably small enough to prevent the PVOH-treated natural fibers and the polymer binder from passing through the air-permeable conveyor 120. Hence, the PVOH-treated natural fibers and the polymer binder are instead deposited as a mixture onto the air-permeable conveyor 120 in the form of an unbound air-laid web 20.

[0117] In an embodiment, the conveyor 120 is an endless air-permeable conveyor. As an example, the conveyor 120 could comprise an endless conveyor belt 122 running along driver rollers 124, 126 as shown in Fig. 7. An endless conveyor belt 122 is a conveyor belt 122 that has been made into an endless belt 122 without joints. Such an endless conveyor belt 122 is also referred to as jointless conveyor belt in the art.

[0119] In an embodiment, step S3 comprises heat treating the unbound air-laid web 20 to at least partly melt the polymer binder and form the bonded air-laid blank 10. The heat treatment applied in step S3 performs a bonding operation, in which the unbound air-laid web 20 is introduced into or otherwise passes a heating device 140, also referred to as a bonding oven or heater, see Fig. 7, where heat, such as in the form of heated or hot air, is blown into, sucked into and/or circulated through the unbound air-laid web 20 to melt or partially melt the polymer binder. The polymer binder thereby becomes tacky and adheres to the PVOH-treated natural fibers and, thus, holds the fiber material together and thereby results in a bonded air-laid blank 10.

[0121] The heating or bonding operation in step S3 may also comprise, and/or be accompanied by, a densification to create a larger number of binding points in the fiber structure and, thus, a stronger and denser bonded air-laid blank 10. Such a densification operation could be applied either before the bonded air-laid blank 10 has been allowed to cool after the heating device 140 or upon renewed heating, such as in a heated calender. It is also possible to perform the densification operation in the heating device 140, e.g., as a combined heating and densification operation. In this latter case, step S3 comprises heat treating the unbound air-laid web 20 to at least partly melt the polymer binder and simultaneously applying pressure onto the unbound air-laid web 20 to form the bonded air-laid blank 10. The densification can include various types of operations including, but not limited to, calendering and/or pressing operations.

[0123] Fig. 5 is a flow chart illustrating an additional, optional step of the method shown in Fig. 2 according to an embodiment. In this embodiment, the method continues from step S3. A next step S30 comprises cooling the bonded air-laid blank 10 by blowing a gas or gas mixture through the bonded air-laid blank 10. The method then ends or continues to step S40 in Fig. 6.

[0125] In this embodiment, the system 100 comprises a cooling device 150 arranged downstream of the heating device 140. Such a cooling device 150 is then arranged to blow a gas or a gas mixture, typically air, through the bonded air-laid blank 10 to cool the bonded air-laid blank 10 as output from the heating device 140. The cooling device 150 could then cool the air-laid blank 10 to a temperature at or slightly above ambient temperature or to a temperature above ambient temperature but below the temperature inside the heating device 140, such as to a temperature at which the polymer binder solidifies sufficiently.

[0127] In an embodiment, the method also comprises cutting, in step S40 as shown in Fig. 6, the bonded airlaid blank 10 following heating in step S3 in Fig.2 or the cooling in step S30 in Fig.5. The cutting operation in step S40 could be performed using any suitable cutter or cutting device 160. Illustrative, but non-limiting examples, of such cutting device 160 include a saw, a punch, a knife, etc. The cutting device 160 is preferably in the form of a cross-cutting device 160 that cuts through the whole thickness of the bonded air-laid blank 10.

[0129] The cutting in step S40 divides the (continuous) bonded air-laid blank 10 into suitable sizes for downstream handling and processing. The cutting could be across the width of the bonded air-laid blank 10 to get, for instance, rectangular or quadratic bonded air-laid blank pieces.

[0131] The cutting in step S40 is preferably performed while the bonded air-laid blank 10 is transported on the conveyor 120. Hence, it is generally preferred if the cutting device 160 is moved in synchrony with the bonded air-laid blank 10 during the cutting in step S40. For instance, the cutting device 160 is starting the cutting in step S40 from a start position and then moves in synchrony with the bonded air-laid blank 10 in the longitudinal direction of the bonded air-laid blank 10 until the cutting is completed at a stop position. The cutting device 160 is then preferably transported back to the start position to be ready for a next cutting operation.

[0133] As shown in Fig. 7, the conveyor 120 could include bend rollers 121 and at least one take-up roller 123 arranged to divert the conveyor belt 122 away from the cutting device 160. This means that the conveyor belt 122 turns away from the cutting device 160 to enable the cutting device 160 to cut through the complete thickness of the bonded air-laid blank 10 without the risk of engaging and damaging the conveyor belt 122. The bend rollers 121 could be in the form of bend pulleys or bend idlers and the takeup roller(s) 123 could be in the form of take-up pulley(s) or take-up idler(s).

[0135] In an embodiment, the cutting device 160 and the bend rollers 121 and take-up roller(s) 123 are preferably movable relative to the conveyor 120 to be moved, preferably in synchrony, with the bonded air-laid blank 10 transported by the conveyor 120. This is schematically illustrated by the hatched arrow in Fig. 7.

[0136] Also disclosed herein is a system 100 for producing a bonded air-laid blank 10, see Fig. 7. The system 100 comprises a conduit 170 arranged to convey an airflow of natural fibers or a mixture of natural fibers and a polymer binder. The system 100 also comprises a spray nozzle 180 arranged in the conduit 170 to spray a solution comprising a PVOH onto the natural fibers or the mixture of the natural fibers and the polymer binder to form PVOH-treated natural fibers or a mixture of PVOH-treated natural fibers and the polymer binder. The system 100 further comprises a forming head 110 comprising at least one inlet 111 in fluid communication with the conduit 170 and arranged to receive the PVOH-treated natural fibers or the mixture of the PVOH-treated natural fibers and the polymer binder. The forming head 110 also comprises an outlet 113. The system 100 also comprises a conveyor 120 arranged in connection with the outlet 113 to capture the PVOH-treated natural fibers and the polymer binder as an unbonded air-laid web 20. The system 100 further comprises a heating device 140 arranged to heat the unbonded air-laid web 20 to at least partly melt the polymer binder and bind the PVOH-treated natural fibers to form a bonded air-laid blank 10.

[0138] In an embodiment, the spray nozzle 180 is arranged to spray the solution comprising the PVOH in a flow direction of the air flow, which is schematically shown in Fig. 7.

[0140] The spray nozzle 180 is preferably arranged substantially at a center of a cross section of the conduit 170. Such a placement of the spray nozzle 180 within the conduit 170 achieves an efficient distribution of the sprayed solution, such as in the form of an aerosol, onto the natural fibers or the mixture of the natural fibers and the polymer binder transported by the air flow past the spray nozzle 180. The embodiments are, however, not limited to a central arrangement of the spray nozzle 180 in the conduit 170. Alternatively, the spray nozzle 180 could be peripherally arranged in the conduit 170 at or in connection with one of the inner wall(s) of the conduit 170.

[0142] In another embodiment, the conduit 170 comprises multiple spray nozzles 180 arranged within conduit 170 to spray the solution comprising the PVOH. In such an embodiment, the multiple spray nozzles 180 could all be arranged at or in connection with a center of the (cross section) of the conduit 170. Alternatively, the multiple spray nozzles 180 could be arranged in connection with the periphery of the conduit 170 and are then preferably arranged and directed to spray the solution comprising the PVOH into the air flow with the natural fibers or the mixture of the natural fibers and the polymer binder. It is also possible to combine one or more centrally positioned spray nozzles 180 with one or more peripherally positioned spray nozzles 180.

[0143] In an embodiment, the system 100 also comprises a turbulence inducing element 175 arranged in a section 172 of the conduit 170 comprising the spray nozzle(s) 180. In this embodiment, the turbulence inducing element 175 is arranged to locally induce turbulence in the airflow. This induction of turbulence achieves a turbulence of the natural fibers that results, in combination with spraying the solution comprising the PVOH from the spray nozzle(s) 180, in an efficient and even treatment or coating of the natural fibers with the solution comprising the PVOH.

[0145] Any turbulence inducing element 175 that could induce such a turbulence of the airflow and thereby of the flow of the natural fibers or the mixture of the natural fibers and the polymer binder in the conduit 170 could be used according to the invention. As an example, the turbulence inducing element 175 could be in the form of one or multiple obstructions or elements protruding from the inner wall(s) of the conduit 170 into the conduit 170. In another example, the turbulence inducing element is arranged to induce a pressure pulse or wave in the air flow to induce turbulence in the air flow and thereby in the natural fibers or the mixture of the natural fibers and the polymer binder transported by the air flow.

[0147] Instead of, or as a complement to, having one or more turbulence inducing elements 175, turbulence in the air flow in connection with the spray nozzle 180 could be achieved by other means. For instance, there could be a turn in the conduit 170 inducing turbulence in the air flow in connection with the spray nozzle 180. A further variant is to have a change, such as an increase or a reduction, in the crosssectional area of the conduit 170, such as an increase or reduction in the inner diameter for a conduit 170 with circular cross section.

[0149] In an embodiment, the system 100 comprises a single conduit 170 arranged to convey the airflow of the mixture of the PVOH-treated natural fibers and the polymer binder into the inlet 111 of the forming head 110. In another embodiment, the system 100 comprises multiple conduits 170 arranged to convey the air flow of the mixture of the PVOH-treated natural fibers and the polymer binder into separate inlets 111 of the forming head 110. In such an embodiment, each such conduit 170 preferably comprises a respective spray nozzle 180 as described herein. In a further embodiment, the system 100 comprises at least one conduit 170 with a spray nozzle 180 and arranged to convey an airflow of the natural fibers and at least one conduit without any spray nozzle and arranged to convey an air flow of the polymer binder. In this embodiment, the natural fibers conveyed by the at least one conduit 170 are treated by the respective spray nozzle 180 spraying the solution comprising the PVOH onto the natural fibers so that the natural fibers entering the forming head 110 are PVOH-treated. The polymer binder is then conveyed by at least one other conduit into the forming head 110 and the mixing of the PVOH-treated natural fibers and the polymer binder is taking place within the forming head 110. In yet another embodiment, the system 100 comprises at least one conduit 170 with a spray nozzle 180 and arranged to convey an air flow of the natural fibers. In this embodiment, the polymer binder is added to the air flow of PVOH-treated natural fibers at a position downstream of the spray nozzle 180 but upstream of the forming head 110. In this embodiment, the at least one conduit 170 comprises, for instance, a junction, such as a T-junction, at which the polymer binder is introduced into the air flow of PVOH-treated natural fibers. This means that the PVOH-treated natural fibers and the polymer binder are mixed, in this embodiment, downstream of the spray nozzle 180 but prior to reaching the forming head 110.

[0151] The conduit 170 in Fig. 7 is illustrated as a horizontal conduit 170 connected to the forming head 110. The embodiments are, however, not limited thereto. The conduit 170 could be a vertical conduit connected to the forming head 110 or being angled with an angle from 0° (vertical conduit) up to 90° (horizontal conduit) relative to the forming head 110. In these various embodiments, the air flow flowing through the conduit will incident into the forming head 110 with an angle of incidence from 0° (vertical conduit) up to 90° (horizontal conduit). The conduit 170 can also comprise multiple conduit sections separated by a turn. As an example, an upstream section of the conduit 170 could be an upstream vertical conduit section, which is followed by a turn and then a downstream horizontal conduit section that is in fluid communication with the inlet 111 of the forming head 110. Another example is a conduit 170 with an upstream horizontal section followed by a turn and then a downstream vertical conduit section. In the examples of the multiple conduit sections, then the spray nozzle 180 could be arranged in the upstream conduit section or in the downstream conduit section. It is further possible to have at least one spray nozzle 180 arranged in the upstream conduit section and then another at least one spray nozzle 180 arranged in the downstream conduit section.

[0153] The section 172 of the conduit 170, in which the spray nozzle 180 is arranged, could thereby be a horizontal conduit section 172, a vertical conduit section 172 or a conduit section 172 angled in between 0° (vertical conduit section) and 90° (horizontal conduit section). The conduit section 172 with the spray nozzle 180 could be arranged at or close to the end of the conduit 170 that is in fluid connection with the inlet 111 of the forming head. Alternatively, the conduit section 172 could be a more distant or upstream part of the conduit 170 that does not necessarily have to be in connection with the forming head 110.

[0154] The outlet 113 of the forming head 110 is typically arranged in connection with the lower end 114 of the forming head 110 and in vicinity of the conveyor 120. This means that the PVOH-treated natural fibers and the polymer binder passing through the forming head 110 are captured as an unbonded air-laid web 20 on the conveyor 120.

[0156] In an embodiment, the conveyor 120 is an air-permeable conveyor 120. Such an air-permeable conveyor 120 allows air to pass through the conveyor belt 122.

[0158] In an embodiment, the system 100 comprises a vacuum source 130 arranged beneath the air-permeable conveyor 120 to provide a gas suction through the air-permeable conveyor 120 in connection with the outlet 113 of the forming head 110. In such a case, the vacuum source 130 is arranged to draw the PVOH-treated natural fibers and polymer binder onto the air-permeable conveyor 120 to deposit them thereon and at least partly compact them forming the unbonded air-laid web 20.

[0160] The vacuum source 130 is arranged to apply a vacuum or gas suction over the air-permeable conveyor 120 and, thus, draws the PVOH-treated natural fibers and the polymer binder down onto the airpermeable conveyor 120.

[0162] The air-permeable conveyor 120 could be any type of conveyor 120, over which a vacuum or gas suction can be applied by the vacuum source 130 and onto which the PVOH-treated natural fibers and polymer binder can be captured to form the unbonded air-laid web 20. Typical examples of such air-permeable conveyors 120 that could be used include belt conveyors, in which the belt comprises a plurality of openings, through holes or channels for allowing air to be sucked or drawn through the belt, wire conveyors and mesh conveyors with meshes that are small enough to allow capturing of the PVOH-treated natural fibers and polymer binder. In such a case, the belt, wire network or mesh is preferably an endless or jointless belt, wire network or mesh running between drive rollers 124, 126, also referred to as tail pulley 124 and head pulley 126.

[0164] The captured unbonded air-laid web 20 is then transported by the conveyor 120 towards the heating device 140. The heating device 140 may, for instance, be in the form of a bonding oven. The heating device 140 is arranged to provide heat, such as in the form of hot air that is circulated through the unbonded air-laid web 20 to melt or partly melt the polymer binder. The polymer binder thereby becomes tacky and adheres to the PVOH-treated natural fibers and, thus, holds the fiber material together and thereby results in the bonded air-laid blank 10.

[0165] In a particular embodiment, the heating device 140 is arranged to heat the unbonded air-laid web 20 to a temperature selected within an interval of from 100°C up to 210°C, preferably within an interval of from 100°C up to 190°C, and more preferably within an interval of from 100°C up to 165°C. A too high temperature may damage and deteriorate the PVOH-treated natural fibers in the unbonded air-laid web 20.

[0167] The heating device 140 may also be arranged to densify the unbonded air-laid web 20 to create a larger number of binding points in the fiber structure and, thus, a strongerand denser bonded air-laid blank 10. The heating device 140 could perform such a densification by applying the heat and simultaneously applying pressure onto the unbonded air-laid web 20 to form the air-laid blank 10.

[0169] Alternatively, a separate compression device, such as calender roll, (not shown) could be arranged downstream of the heating device 140 to perform the densification on the bonded air-laid blank 10 before it has been allowed to cool after the heating device 140.

[0171] The densification performed by the heating device 140 or the separate compression device can include various types of operations including, but not limited to, calendering and/or pressing operations.

[0173] In an embodiment, the system 100 may comprise a cooling device 150 arranged to cool the bonded airlaid blank 10 by blowing gas or gas mixture through the bonded air-laid blank 10. The cooling device 150 is, thus, arranged downstream of the heating device 140. The cooling device 150 may then blow gas, such as air, through the bonded air-laid blank 10 to reduce the temperature thereof. The bonded air-laid blank 10 leaving the cooling device 150 preferably has a temperature close to or slightly above ambient temperature, such as room temperature (20-25°C), or at least a temperature below the preferred heating temperature inside the heating device 140, preferably below the softening temperature of the polymer binder. For densification, a heated calender could be arranged downstream of the cooling device 150.

[0175] A cutting device 160 is arranged downstream of the heating device 140 and the optional cooling device 150 to cut the bonded air-laid blank 10. As mentioned in the foregoing, the cutting device 160 could be any type of cutter that is capable of cutting air-laid blanks. Illustrative, but non-limiting examples, of such cutting devices 160 include a saw, a punch, a knife, etc.

[0176] Also disclosed herein is a bonded air-laid blank 10 comprising PVOH-treated natural fibers and a polymer binder binding together the PVOH-treated natural fibers.

[0178] The PVOH applied to natural fibers as disclosed herein promotes adherence and binding of any "loose" natural fibers, and fragments and particles thereof, not bound by the polymer binder. The PVOH thereby reduces the amount of linting and dusting from the bonded air-laid blank 10 as compared to prior art bonded air-laid blanks lacking any pre-treatment of the natural fibers.

[0180] In an embodiment, the natural fibers are or comprise wood fibers. In an embodiment, the natural fibers are or comprise cellulose and/or lignocellulose fibers. Hence, in an embodiment, the natural fibers contain cellulose, such as in the form of cellulose and/or lignocellulose, i.e., a mixture of cellulose and lignin. The natural fibers may also contain lignin, such as in the form of lignocellulose. The natural fibers may additionally contain hemicellulose. In a particular embodiment, the natural fibers are cellulose and/or lignocellulose pulp fibers produced by chemical, mechanical and/or chemi-mechanical pulping of softwood and/or hardwood. For instance, the cellulose and/or lignocellulose pulp fibers are in a form selected from the group consisting of sulfate pulp, sulfite pulp, thermomechanical pulp (TMP), high temperature thermomechanical pulp (HTMP), mechanical fiber intended for medium density fiberboard (MDF-fiber), chemi-thermomechanical pulp (CTMP), high temperature chemi-thermomechanical pulp (HTCTMP), and a combination thereof.

[0182] The natural fibers, such as cellulose and/or lignocellulose pulp fibers, could be bleached natural fibers, such as bleached cellulose and/or lignocellulose pulp fibers, unbleached natural fibers, such as unbleached cellulose and/or lignocellulose pulp fibers, or a combination of bleached and unbleached natural fibers, such a combination of bleached and unbleached cellulose and/or lignocellulose pulp fibers.

[0184] The natural fibers can also be produced by other pulping methods and/or from other cellulosic or lignocellulosic raw materials, such as flax, jute, hemp, kenaf, bagasse, cotton, bamboo, straw, or rice husk. It is also possible to use natural fibers that are a mixture of fibers from different raw materials, such as a mixture of wood and any of the materials mentioned above.

[0186] The bonded air-laid blank 10 may also comprise a minor portion of synthetic material or fibers that are mixed with the natural fibers. Such synthetic material or fibers that may be mixed with the natural fibers include, for instance, glass or mineral wool, and/or carbon fibers. Any such synthetic material or fibers may be added at an amount of no more than 10 % (w/w) of the bonded air-laid blank 10, preferably no more than 8 % (w/w), such as no more than 6 % (w/w), or preferably no more than 4 % (w/w) of the bonded air-laid blank 10.

[0188] In an embodiment, the natural fibers have a length weighted average fiber length of up to 10 mm, preferably of up to 8 mm, more preferably of up to 6 mm, and most preferably up to 5 mm. In a particular embodiment, the natural fibers have a length weighted average fiber length selected within an interval of from 1 mm up to 10 mm, preferably selected within an interval of from 1 mm up to 8 mm, more preferably selected within an interval of from 1 mm up to 6 mm, and most preferably selected within an interval of from 1 mm up to 5 mm.

[0190] Length of fibers, such as natural fibers, as referred to herein is length weighted average fiber length. Length weighted average fiber length is calculated as the sum of individual fiber lengths squared divided by the sum of the individual fiber lengths as described in e.g., ISO 16065-1:2014, Pulps - Determination of fibre length by automated optical analysis- Part 1: Polarized light method, or ISO 16065-2:2014, Pulps - Determination of fibre length by automated optical analysis - Part 2: Unpolarized light method.

[0192] The polymer binder is included to bind the bonded air-laid blank 10 together and preserve its form and structure during use, handling, and storage. In an embodiment, the polymer binder may also assist in building up the foam-like structure of the bonded air-laid blank 10. The polymer binder is, in such an embodiment, intermingled with the PVOH-treated natural fibers during the air-laying process forming a fiber mixture. The polymer binder may be added in the form of a powder but is more often added in the form of fibers that are intermingled with the PVOH-treated natural fibers in the air-laying process.

[0194] In a particular embodiment, the polymer binder is selected from the group consisting of a polymer powder, polymer fibers and a combination thereof.

[0196] The polymer binder could be a natural or synthetic polymer binder, or a mixture of natural polymer binders, a mixture of synthetic polymer binders, or a mixture of natural and synthetic polymer binders, but is preferably a thermoplastic polymer binder.

[0198] In an embodiment, the polymer binder is selected from the group consisting of a thermoplastic polymer powder, thermoplastic fibers, and a combination thereof. In a preferred embodiment, the polymer binder is in the form of thermoplastic polymer fibers.

[0199] In an embodiment, the polymer binder is made from i) a material selected from the group consisting of polyvinyl alcohol (PVOH), thermoplastic starch (TPS), polyethylene (PE), ethylene acrylic acid copolymer (EAA), ethylene-vinyl acetate (EVA), polypropylene (PP), polystyrene (PS), such as styrene-butadiene rubber (SBR) or styrene acrylate copolymer, polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), polylactic acid (PLA), polyethylene terephthalate (PET), polycaprolactone (PCL), polyvinyl alcohol (PVA), polyethylene glycol (PEG), poly(2-ethyl-2-oxazoline) (PEOX), polyvinyl ether (PVE), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), polymethacrylic acid (PMAA), polyvinyl acetate (PVAc), polyurethane (PU), copolymers thereof and mixtures thereof, and ii) optionally one or more additives.

[0201] Hence, in an embodiment, the polymer binder is made of a material selected from the above-mentioned group. In another embodiment, the polymer binder is made of a material selected from the abovementioned group and one or more additives.

[0203] In an embodiment, the polymer binder is or comprises, such as consists of, mono-component and/or bicomponent polymer fibers. Bi-component polymer fibers, also known as bico fibers, comprise a first polymer, copolymer and/or polymer mixture and a second, different polymer, copolymer and/or polymer mixture. Most often the bi-component polymer fiber comprises a core made of the first polymer, copolymer and/or polymer mixture and a sheath made of the second polymer, copolymer and/or polymer mixture, although other combinations of two or even more polymers, copolymers and/or polymer mixtures are possible.

[0205] In an embodiment, the polymer binder is a thermoplastic polymer binder and the thermoplastic polymer binder is or comprises, such as consists of, mono-component thermoplastic polymer fibers made of i) a material selected from the group consisting of PVOH, TPS, PE, EAA, EVA, PP, PS, PBAT, PBS, PLA, PET, PCL, PVA, PEG, PEOX, PVE, PVP, PAA, PMAA, PVAc, PU, copolymers thereof and mixtures thereof, and ii) optionally one or more additives. In another particular embodiment, the thermoplastic polymer binder is or comprises, such as consists of, bi-component thermoplastic polymer fibers having a first material, such as a core made of i) a first material, selected from the group consisting of PVOH, TPS, PE, EAA, EVA, PP, PS, PBAT, PBS, PLA, PET, PCL, PVA, PEG, PEOX, PVE, PVP, PAA, PMAA, PVAc, PU, copolymers thereof and mixtures thereof, and ii) optionally one or more additives, and a second material, such as a sheath made of i) a second material, typically a different material, selected from the group consisting of PVOH, TPS, PE, EAA, EVA, PP, PS, PBAT, PBS, PLA, PET, PCL, PVA, PEG, PEOX, PVE, PVP, PAA, PMAA, PVAc, PU, copolymers thereof and mixtures thereof, and ii) optionally one or more additives. In a further embodiment, the thermoplastic polymer binder is or comprises, such as consists of, a combination or mixture of mono-component thermoplastic polymer fibers made of i) a material selected from the group consisting of PVOH, TPS, PE, EM, EVA, PP, PS, PBAT, PBS, PLA, PET, PCL, PVA, PEG, PEOX, PVE, PVP, PM, PMM, PVAc, PU, copolymers thereof and mixtures thereof, and ii) optionally one or more additives, and bi-component thermoplastic polymer fibers having i) materials, such as of the core and/or sheath, selected from the group consisting of PVOH, TPS, PE, EM, EVA, PP, PS, PBAT, PBS, PLA, PET, PCL, PVA, PEG, PEOX, PVE, PVP, PM, PMM, PVAc, PU, copolymers thereof and mixtures thereof, and ii) optionally one or more additives.

[0207] The thermoplastic polymer binder could be made of a single type of thermoplastic polymer fibers, i.e., made of a same material in the case of mono-component thermoplastic polymer fibers or made of the same materials in the case of bi-component thermoplastic polymer fibers. However, it is also possible to use a thermoplastic polymer binder made of one or multiple, i.e., two or more, different mono-component thermoplastic polymer fibers made of different materials and/or one or multiple different bi-component thermoplastic polymer fibers made of different materials.

[0209] An advantage of using bi-component thermoplastic polymer fibers is that they can have a core with a higher melting point that keeps its fiberform during the binding operation, whereas the sheath melts and becomes tacky. The intact core will support the three-dimensional structure of the bonded air-laid blank 10 and, thus, promote porosity while the melted or tackified sheath will attach to the natural fibers and preserve the strength of the bonded air-laid blank 10.

[0211] In an embodiment, the polymer binder is a polymer powder, preferably a thermoplastic polymer powder, made of i) a material selected from the group consisting of PVOH, TPS, PE, EM, EVA, PP, PS, PBAT, PBS, PLA, PET, PCL, PVA, PEG, PEOX, PVE, PVP, PAA, PMM, PVAc, PU, copolymers thereof and mixtures thereof, and ii) optionally one or more additives.

[0213] It is also, as mentioned in the foregoing, possible to use a thermoplastic polymer binder that is a combination of thermoplastic polymer fibers and thermoplastic polymer powder.

[0215] In an embodiment, the bonded air-laid blank 10 comprises the PVOH-treated natural fibers at an amount or concentration of at least 70 % by weight of the bonded air-laid blank 10.

[0216] In a preferred embodiment, the bonded air-laid blank 10 comprises the PVOH-treated natural fibers in an amount or concentration of at least 72.5 %, more preferably at least 75 %, such as at least 77.5 %, at least 80 %, at least 82.5 %, at least 85 % by weight of the bonded air-laid blank 10. In some applications, even higher amounts or concentrations of the PVOH-treated natural fibers may be used, such as at least 87.5 %, or at least 90 %, at least 92.5 %, at least 95 % or at least 97.5 % by weight of the bonded air-laid blank 10.

[0218] In an embodiment, the bonded air-laid blank 10 comprises the polymer binder at an amount or concentration selected within an interval of from 2 up to 30 % by weight of the bonded air-laid blank 10.

[0220] In some embodiments, the bonded air-laid blank 10 comprises the polymer binder at an amount or concentration selected within an interval of from 2 up to 25 % by weight of the bonded air-laid blank 10, preferably within an interval of from 2 up to 20 %, such as from 2 up to 15 % by weight of the bonded airlaid blank 10, or more preferably within an interval of from 2 up to 10 % by weight of the bonded air-laid blank 10.

[0222] In an embodiment, the PVOH-treated natural fibers comprise the PVOH at an amount or concentration selected within an interval of from 0.5 up to 5 % by dry weight of the natural fibers. Percentage by dry weight as used herein refer to the percentage of the weight of dry PVOH of the total weight of dry natural fiber.

[0224] In a preferred embodiment, the PVOH-treated natural fibers comprise the PVOH at an amount or concentration selected within an interval of from 1 up to 5 % by dry weight of the natural fibers.

[0226] Thus, relatively low amounts of PVOH are needed to achieve the reduction of linting and dusting from the bonded air-laid blank 10.

[0228] PVOH is a water-soluble polymer compound with strong adhesion. It is an environmentally friendly material that can be degraded into water and carbon dioxide by bacteria and enzymes.

[0230] The PVOH may be PVOH, a PVOH derivative or a PVOH analogue. The PVOH could be a single type of PVOH or it can comprise a mixture of two or more types of PVOH. In such a case, the different types of PVOH may, for instance, differ in degree of hydrolysis, viscosity, and/or molecule weight. In an embodiment, the PVOH has a degree of hydrolysis in the range of 80-99.9 mol%, preferably in the rage of 88-99.9 mol%. In an embodiment, the PVOH may have a molecular weight within an interval of from 10 up to 200 kDa, such as within an interval of from 20 up to 150 kDa.

[0232] The bonded air-laid blank 10 may comprise one or more additives in addition to the PVOH-treated natural fibers and the polymer binder. One or more additives could be added to the polymer binder and/or added when producing the polymer binder. Alternatively, or in addition, one or more additives could be added to the PVOH-treated natural fibers. Alternatively, or in addition, one or more additives could be added to the mixture of the PVOH-treated natural fibers and the polymer binder, such as during the air-laying process or prior to the air-laying process.

[0234] Illustrative, but non-limiting, examples of such additives include electrically conducting or semiconducting fillers, coupling agents, flame retardants, dyes, impact modifiers, hydrophobization agents, etc.

[0236] The bonded air-laid blank 10 produced according to the invention has preferably an average thickness W, see Fig. 1A, of at least 5 mm, and preferably an average thickness of at least 5 mm and at most 200 mm.

[0238] In an embodiment, the bonded air-laid blank 10 has an average density selected within an interval of from 10 up to 200 kg/m<3>, such as from 20 up to 200 kg/m<3>.

[0240] In an embodiment, the bonded air-laid blank 10 has an average grammage selected within an interval of from 300 up to 10000 g/m<2>.

[0242] The bonded air-laid blank 10 as produced according to the method and by the system 100 of the present invention comprises significantly less non-bonded natural fibers as compared to bonded air-laid blanks 10 produced according to prior art technologies, i.e., without any pre-treatment of the natural fibers or the mixture of the natural fibers and the polymer binder. This means that the bonded air-laid blank 10 produce less linting.

[0244] The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations, and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.

Claims

1. A method for producing a bonded airlaid web (10), said method comprising: contacting (S22) natural fibers with a solution comprising a polyvinyl alcohol (PVOH) to form PVOH-treated natural fibers, introducing (S1) said PVOH-treated natural fibers and a polymeric binder into a forming head (110); collecting (S2) said PVOH-treated natural fibers and said polymeric binder as an unbonded airlaid web (20) on a conveyor (120) disposed adjacent an outlet (113) of said forming head (110), and heating (S3) said unbonded airlaid web (20) to at least partially melt said polymeric binder and bond said PVOH-treated natural fibers to form a bonded airlaid web (10).

2. The method of claim 1, wherein contacting (S22) said natural fibers comprises spraying (S22) said solution comprising said PVOH onto said natural fibers.

3. The method of claim 2, further comprising: transporting (S20) said natural fibers with an air stream in a conduit (170) to said forming head (110), wherein spraying (S22) said solution comprises spraying (S22) said solution comprising said PVOH in a flow direction of said air stream.

4. The method of claim 3, further comprising inducing (S21) local turbulence in said air stream in a section (172) of said pipeline (170), in which said solution comprising said PVOH is sprayed onto said natural fibers.

5. The method of any one of claims 1 to 4, further comprising mixing (S23) said PVOH-treated natural fibers and said polymeric binder, wherein introducing (S1) said PVOH-treated natural fibers and said polymeric binder comprises introducing (S1) a mixture of said PVOH-treated natural fibers and said polymeric binder into said forming head (110).

6. A method according to any one of claims 1 to 5, wherein said solution is an aqueous solution of said PVOH.

7. A method according to any one of claims 1 to 6, wherein said solution comprises said PVOH in a concentration selected within a range from 0.1 up to 5% by weight of said solution, preferably selected within a range from 0.5 up to 5% by weight of said solution.

8. Method according to any one of claims 1 to 7, wherein said PVOH-treated natural fibers comprise said PVOH in an amount selected within a range from 0.5 up to 5% by weight of a dry weight of said natural fibers, preferably selected within a range from 1 up to 5% of said dry weight of said natural fibers.

9. Method according to any one of claims 1 to 8, wherein said natural fibers comprise wood fibers, preferably cellulose and/or lignocellulosic fibers and more preferably fibers of cellulose and/or lignocellulosic pulp produced by chemical, mechanical and/or chemi-mechanical pulping from softwood and/or hardwood.

10. The method according to claim 9, wherein said natural fibers are fibers of cellulose and/or lignocellulosic pulp in a form selected from the group consisting of kraft pulp, sulfite pulp, thermomechanical pulp (TMP), high-temperature thermomechanical pulp (HTMP), mechanical fiber intended for medium-density fiberboards (MDF fiber), chemi-thermomechanical pulp (CTMP), high-temperature chemi-thermomechanical pulp (HTCTMP), and a combination thereof.

11. A method according to claim 9 or 10, wherein said natural fibers are fibers of bleached cellulose and/or lignocellulosic pulp, fibers of unbleached cellulose and/or lignocellulosic pulp, or a combination of fibers of bleached cellulose and/or lignocellulosic pulp and fibers of unbleached cellulose and/or lignocellulosic pulp.

12. A method according to any one of claims 1 to 11, wherein said polymeric binder is selected from the group consisting of a polymer powder, polymer fibers and a combination thereof, preferably selected from the group consisting of a thermoplastic polymer powder, thermoplastic polymer fibers and a combination thereof, and more preferably thermoplastic polymer fibers.

13. A method according to claim 12, wherein said thermoplastic polymer fibers are selected from the group consisting of monocomponent thermoplastic polymer fibers, bicomponent thermoplastic polymer fibers and a mixture thereof, preferably bicomponent thermoplastic polymer fibers.

14. The method according to any one of claims 1 to 13, wherein said polymeric binder is made from i) a material selected from the group consisting of (PVOH), thermoplastic starch (TPS), polyethylene (PE), ethylene acrylic acid copolymer (EAA), ethylene vinyl acetate (EVA), polypropylene (PP), polystyrene (PS), polybutene adipate terephthalate (PBAT), polybutene succinate (PBS), polylactic acid (PLA), polyethylene terephthalate (PET), polycaprolactone (PCL), polyethylene glycol (PEG), poly(2-ethyl-2-oxazoline) (PEOX), polyvinyl ether (PVE), polyvinyl pyrrolidone (PVP), polyacrylic acid (PAA), polymethacrylic acid (PMAA), polyvinyl acetate (PVAc), polyurethane (PU), copolymers thereof and mixtures thereof, and ii) optionally one or more additives.