WO2024132736A1

WO2024132736A1 - Process for manufacturing a multi-layer paper or cardboard

Info

Publication number: WO2024132736A1
Application number: PCT/EP2023/085471
Authority: WO
Inventors: Stefan Erren; Martin Schachtl; Dieter Schoenhaber
Original assignee: Basf Se
Priority date: 2022-12-21
Filing date: 2023-12-13
Publication date: 2024-06-27

Abstract

A process for manufacturing a multi-layer paper is provided, wherein the process comprises a step: a) spraying onto a surface side of a first fibrous layer to be joined with a surface side of a second fibrous layer with an additive formulation; wherein the first and the second fibrous layer have dry matter content of from 25 to 65 wt%; and the additive formulation comprises (a-1) water and (a-2) a salt containing a CO3 2- or a HCO3 - anion, and the amount of water is at least 75 wt%, based on the total weight of the additive formulation.

Description

Process for manufacturing a multi-layer paper or cardboard

Description

The present invention relates to a process for manufacturing a multi-layer paper or cardboard using an additive formulation containing a salt of carbonate or hydrogencarbonate, to the paper or cardboard, obtainable by said process, and to a paper machine comprising said additive formulation. Further, the invention relates to the use of said formulation as a spraying additive in a process of manufacturing a multi-layer paper or cardboard.

Background of the invention

Multi-layer papers or cardboard may be obtained from paper stock mixtures or fiber stock mixtures with the same or different stock compositions by pressing together individual, still wet paper webs or layers of paper.

Main costs for producing paper and cardboard are caused by fiber material, energy and chemicals. Thus, the specific volume of a paper is an important property for cost optimization. Reducing only the raw material, however, often leads to undesired properties, like less strength or bending stiffness, for example, when folding paper or cardboard. Said properties should not be negatively affected.

Additives for volume expansion of a paper are little known. Carbonate or hydrogencarbonate salts, known as blowing agents in different fields, have already been used in the paper or fiber industry.

DE 102004054224 A1 discloses a process for preparing a biodegradable material from lignocellulosic fiber material, starch and water, wherein sodium carbonate and adipic acid are added as blowing agents and mixed in order to form a foam-like structure, which may then be shaped into a flat or strand-like form.

CN 111019194 A discloses a foamed packaging material with at least 3 layers, obtainable by a mixture of bamboo fiber pulp, starch, water, glycerol and ammonium hydrogencarbonate, wherein the density of each layer changes in gradient along the thickness direction.

CN 104074105 A discloses a process for preparing a molded paper product made by injection molding a mixture comprising pulp fibers, starch, a synthetic resin, diethyl-phthalate, water and a foaming agent like sodium carbonate or ammonium hydrogencarbonate.

WO 2019/076702 A1 discloses a process for producing a multi-layer paper, wherein an aqueous formulation containing a water-soluble polymer based on N-vinylformamide is sprayed onto an inner layer of the multi-layer paper. The known processes for producing multi-layer paper or cardboard do not yet fully meet the requirements. There is still a need for multi-layer papers having mechanical properties like strength at a lower basis weight.

It is therefore an object of the invention to provide a process for preparing a paper or cardboard having a lower basis weight, i.e., a higher specific volume, while other mechanical strength properties like bursting strength, tensile stretch and/or short span compression strength are essentially not significantly negatively affected and/or are even improved.

A further object is to provide a paper or cardboard having a lower basis weight, i.e., a higher specific volume, while other paper properties like bursting strength, tensile index, tensile stretch and/or short span compression strength are essentially not negatively affected and/or are even improved.

A further object is to provide a formulation to be used in a paper or cardboard manufacturing process as an additive in paper manufacturing, especially to increase the specific volume of a multi-layer paper or cardboard.

Summary of the invention

It has now been found that a specific additive formulation may be used in a process for preparing a multi-layer paper or cardboard having an increased specific volume, resp., while properties like bursting strength, tensile index or compression strength should not be significantly negatively affected and/or should be even improved, especially in cross direction.

Therefore, in a first aspect, the invention relates to a process for manufacturing a multi-layer paper or cardboard, the process comprising: a) spraying onto a surface side of a first fibrous layer to be joined with a surface side of a second fibrous layer with an additive formulation; wherein the first and the second fibrous layer independently of one another have a dry matter content of from 25 to 65 wt%; and the additive formulation comprises a-1) water, and a-2) a salt containing a COa^2- or a HCOa' anion, and the amount of water is at least 75 wt%, based on the total weight of the additive formulation.

In a further aspect, the invention relates to a multi-layer paper or cardboard, obtainable or obtained by a process, as defined in any aspect herein.

In a further aspect, the invention relates to the use of a formulation comprising a-1) water, a-2) a salt containing a COa²' or a HCOa' anion, and a-3) optionally a water-soluble or water-dispersible organic polymer P, as defined in any aspect herein, as a spraying additive in a process of manufacturing a multi-layer paper or cardboard. In a further aspect, the invention relates to a paper machine, equipped with a wire section containing a first wire and a second wire, a spraying device, a press section and a dryer section, said sections are arranged in the paper machine in the order of the wire section, followed by the spraying device, the press section and the dryer section, wherein the spraying device contains an additive formulation, as defined in any aspect herein.

Detailed description of the invention

The term “dry matter content”, as used herein, means the ratio of the mass of a sample after drying to the mass of the sample before drying, expressly understood in percentages by weight (wt%). The dry matter content is preferably determined according to DIN EN ISO 638-1:2022-07 and DIN EN ISO-2:202, resp..

The term “solids content of an organic polymer” in wt%, as used herein, means the ratio of the mass of a sample after drying to the mass of the sample before drying, multiplied by 100. The solids content may be determined from a material sample of the organic polymer by drying this sample in a circulating air-drying cabinet at 140°C for 120 minutes. For example, in the case of an aqueous polymer solution, suspension or emulsion, the sample (0.5 to 1.5 g) is placed in a metal lid for drying. Drying is carried out at ambient pressure, usually at 1013 mbar.

The term “ethylenically unsaturated monomer”, as used herein, means a non-aromatic ethylenically unsaturated monomer, wherein the C=C double bond is susceptible to radical polymerization.

The term "(meth)acryl" or similar terms, as used herein, encompasses acryl, methacryl and a mixture thereof.

The term “latex” or “polymer latex”, as used herein, means a dispersion or emulsion of polymer particles formed in the presence of water and optionally a surfactant.

The term “any combination thereof”, as used herein, means two or more combinations thereof, either different kinds of one constituent or one group, i.e. , different subgroups, or different kinds of a list of constituents or groups.

As used herein, the indefinite article “a” comprises the singular but also the plural, i.e., an indefinite article in respect to a component of a composition means that the component is a single compound or a plurality of compounds. If not stated otherwise, the indefinite article “a” and the expression “at least one” are used synonymously.

Figure 1 shows a schematic overview of a part of a test paper machine used in the Examples of the pilot plant. The production of paper is a process, wherein a solid phase comprising fibrous material and various paper ingredients is separated from an aqueous phase. Said separation is usually effected in two or more steps and may be modulated within these steps through the choice of mechanical parameters and/or the choice of admixing chemical ingredients.

The first fibrous layer, used in step a), is usually obtained by dewatering a first fiber suspension. The second fibrous layer, used in step a), is usually obtained correspondingly. The first aqueous fiber suspension is understood to be a composition comprising (i-1) water and (i-2) a first fibrous material (pulp) containing cellulose fibers.

Mechanical and/or chemical methods may be used to obtain the aqueous fiber suspension. Virgin and/or recycled fibers may be used as the pulp. All fibers from wood, like softwood or hardwood, or annual non-wood plants commonly used in the paper industry may be used. Suitable annual non-wood plants for producing pulp are, for example, rice, wheat, sugar cane and kenaf. Mechanical pulp, e.g., from pine or deciduous wood, includes, for example, stoneground wood (SGW), pressure groundwood (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) and chemo-thermomechanical pulp (CTMP). Chemical pulp, e.g., from pine or deciduous wood, includes the chemically digested sulfate, sulfite or soda pulp. Pulp may also be bleached or used in unbleached form. Recycled fibers, for example, may come from wastepaper. The wastepaper may optionally be subjected to a deinking process beforehand. Recycled fibers from wastepaper may be used alone or in a mixture with other fibers, especially native fibers.

An aqueous fiber suspension may be obtained, for example, by recycling paper or cardboard, for example by mechanically treating wastepaper in a pulper together with water, until the aqueous fiber suspension has the desired consistency. Another example of the combination of two fiber sources is the mixing of a primary fiber suspension with recycled scrap of a coated paper, which is produced using the primary fiber suspension.

In addition to water, the aqueous fiber suspension may contain further components which may optionally be added to the fiber suspension or may be present through the use of wastepaper or existing paper.

A possible component of the first aqueous fiber suspension is (i-3) an organic polymer that is different from a fiber. The organic polymer (i-3) contained in the first aqueous fiber suspension may be any polymer, known in the field of paper and cardboard manufacturing. The organic polymer (i-3) may be non-ionic, cationic or anionic and may be natural, modified-natural or synthetic.

A non-ionic organic polymer (i-3) may be uncharged-neutral or amphoteric-neutral. A non-ionic organic polymer (i-3), for example an uncharged-neutral organic polymer generally contains no monomer units with a functional group that carries a charge at least at pH 7. Examples of a non- ionic organic polymer (i-3) include polyacrylamide, poly(acrylamide-co-acrylonitrile), poly(vinyl alcohol) or poly(vinyl alcohol-co-vinyl acetate). A cationic organic polymer (i-3) may be purely cationic or amphoteric-cationic. A cationic organic polymer, for example a purely cationic polymer generally contains monomer units with a functional group that carries a positive charge at least at pH 7, but it does not contain a monomer unit with a functional group that carries a negative charge at least at pH 7. Examples of a cationic organic polymer (i-3) include poly(allylamine), poly(diallylamine), poly(diallyl- dimethylammonium chloride), poly(acrylamide-co-diallyldimethylammonium chloride) or poly(acrylamide-co-2-(N,N,N- trimethylammonium ethylacrylate chloride).

An anionic organic polymer (i-3) may be purely anionic or amphoteric-anionic. An anionic organic polymer, for example a purely anionic polymer generally contains a monomer unit with a functional group that carries a negative charge at least at pH 7, but it does not contain a monomer unit with a functional group that carries a positive charge at least at pH 7. Examples of an anionic organic polymer (i-3) include poly(acrylic acid), poly(styrene-co-n-butyl acrylate-co- acrylic acid) or poly(acrylamide-co-acrylonitrile-co-acrylic acid).

An amphoteric organic polymer usually has a monomer unit with a functional group that carries a positive charge at pH 7 and a monomer unit carrying a negative charge at pH 7, either in excess of one kind of charge or essentially in balance of the anionic and cationic charge.

The organic polymer (i-3) may be a linear, branched or cross-linked polymer. Cross-linking may take place, for example, by adding a cross-linking agent already during the polymerization of the starting monomers or by adding a cross-linking agent after the polymerization has taken place, in particular also only shortly before the addition of the organic polymer (a-c) to the aqueous fiber suspension. If necessary, both types of cross-linking may be combined. A cross-linked organic polymer having a high degree of cross-linking, typically already during the monomer polymerization, may be present in the first aqueous fiber suspension as particles, as so-called organic microparticles.

The organic polymer (i-3) may also be natural, modified-natural or synthetic. A natural organic polymer is usually obtained from nature, where appropriate isolation steps are used, but no specific chemically synthetic modification. An example of a natural organic polymer (i-3) is unmodified starch. A natural organic polymer (i-3) does not include cellulose, which is a fibrous material (i-2). A modified-natural organic polymer may be modified by a chemically synthetic process step. An example of a modified natural organic polymer (i-3) is cationic starch.

The fiber suspension may also include any mixture of two or more organic polymers (i-3).

Examples of a further component may be a filler (i-4), preferably an inorganic particle, in particular one or more inorganic pigments. Examples of an inorganic pigment include pigments based on metal oxides, silicates and/or carbonates conventionally used in the paper industry.

The first aqueous fiber composition may further comprise one or more paper additives (i-5), different from the afore-mentioned components (i-2), (i-3) and (i-4). Examples of a paper additive (i-5) include a mass sizing agent, a water-soluble salt of a trivalent metal cation, a defoamer, a non-polymeric wet strength agent, a biocide, an optical brightener, a paper dye and any combination thereof. Such paper additives are well known in the art. The choice of paper additives (i-5) in the first fiber suspension is usually dependent on the desired performance in a specific paper or cardboard grade. The instant process may use customary amounts of the typical paper additives.

Usually, more than one organic polymer (i-3) and more than one filler (i-4) are added to the first aqueous fiber suspension. In the case of an organic polymer (i-3) or filler (i-4), these may serve, for example, to influence technical properties of the paper or cardboard manufacturing process itself or technical properties of the paper or cardboard produced, for example, as a retention agent, a drainage agent, a wet strength agent or a dry strength agent. For example, a cationic polyacrylamide as organic polymer (i-3) may also act as a retention agent.

The amount of the organic polymer (i-3) may vary, for example of from 0.001 to 2 wt%, based on the total weight of the first fibrous material (i-2) in the first fiber suspension. The total weight of the first fibrous material (i-2) relates to the dry matter content of the first fibrous material (i-2).

The amount of the organic polymer (i-3) relates to the solids content of the organic polymer (i- 3).

The dry matter content of the first aqueous fiber suspension may be of from 0.1 to 5 wt%.

Generally, a headbox equally distributes the first aqueous fiber suspension onto a first wire having a first upper side, a first lower side and meshes as openings, allowing for producing a uniformly thin, as homogeneous as possible fibrous layer. After application of the first fiber suspension, a part of the water (i-1) of the first aqueous fiber suspension run through the meshes, wherein a first fibrous layer is formed. The fibrous material of the fiber suspension as well as possible other components, which should be present in the paper finally produced, for example a filler, are ideally retained entirely or at least essentially in the fibrous layer formed. Possible further components of the fiber suspension, which are added to support the retention of the other components, to support dewatering the fiber suspension or to support uniform layer formation, for example an organic polymer, develop their effect in this process. Mostly, these possible further components of the fiber suspension remain entirely or at least essentially in the resulting fibrous layer. The dry portion of the fibrous layer, which determines the dry matter content of the fibrous layer, contains the retained fibrous material, possible other components which should be present in the paper finally produced, and the possible further components as constituents. Depending on their retention behavior, these constituents are, for example, the fibrous material, the organic polymers, the fillers and the paper additives. The fibrous layer, to be sprayed in step a), is solid enough to be able to remove it from the wire.

The first wire usually contains an endless wire, preferably of plastic material. After the resulting fibrous layer is separated from an endless wire, said wire runs back to the material application, in which a new fiber suspension is applied to the endless wire. Several principles for forming a fibrous layer are known, for example using a Fourdrinier wire, a mold former, a twin wire hybrid former, a twin gap former, an inclined wire or the like.

Generally, the first fibrous layer is obtained by dewatering an aqueous fiber suspension on a first wire. Usually, an organic polymer (i-3) may be added to the first fiber suspension as a retention agent prior to the dewatering step to form the first fibrous layer.

Dewatering of the fiber suspension may occur on one or both sides of the web layer. Dewatering of the fiber suspension on the upper side of the wire may be supported by applying a vacuum to the lower side of the wire. The vacuum is understood to be a lower pressure than the pressure on the upper side of the wire, corresponding usually to the ambient pressure.

The dry matter content of the first fibrous layer is of from 25 to 65 wt%, preferably 30 to 65 wt%. The dry matter content is usually dependent on the process conditions of the paper machine.

The basis weight (grammage) of a fibrous layer is defined herein as the mass of components per square meter of fibrous layer that remains on drying, preferably as a constant mass in the dry matter content determination at 105°C drying temperature. The basis weight of a fibrous layer may vary in a wide range and may be of from 20 to 150 g/m². The sum of all basis weights of the fibrous layers is not the basis weight of the dried multi-layer paper or cardboard finally produced therefrom, as at least one of the fibrous layers is still sprayed with a small increase in grammage, the layer composite at dewatering by pressing and drying could lose some of the above-mentioned components with a low decrease of grammage or the dried multi-layer paper or cardboard or its moist precursors could be stretched or compressed at said dewatering steps or further steps. In the latter case, one square meter of the fibrous layer would no longer correspond to one square meter of the dried multi-layer paper or cardboard. However, approximately, the basis weight of the first fibrous layer may correspond to the proportion of the layer, resulting from said fibrous layer in the further process, of the total grammage of the dried multi-layer paper.

A second aqueous fiber suspension, resulting in a second fibrous layer, is understood to mean a composition comprising (ii-1) water and (ii-2) a second fibrous material (a pulp) containing cellulose fibers. The explanations and preferences for obtaining the first fibrous layer and defining the dry matter content apply mutatis mutandis to the step of obtaining the second fibrous layer, optionally with an organic polymer (ii-3), a filler (ii-4), a paper additive (ii-5), a second wire, which has a second upper side and a second lower side, and to the dry matter content of the second fibrous layer.

Generally, the second fibrous layer is obtained by dewatering an aqueous fiber suspension on a second wire. The dewatering step of the second fiber suspension may be carried out in analogy to the dewatering step of the first fiber suspension, usually by applying vacuum.

The dry matter content of the second fibrous layer may be essentially the same as that of the first fibrous layer or may be different. The dry matter content is usually dependent on the process conditions of the paper machine. The dry matter content is preferably in the same range. The dry matter content of the second fibrous layer is of from 25 to 65 wt%, preferably from 30 to 65 wt%.

For example, in case of a multi-layer paper or cardboard with two layers, the dry matter content of the first and the second fibrous layer may be the same or may be different. For example, in case of a multi-layer paper or cardboard with more than two layers the dry content of the first fibrous layer may be the same or may be different to the second fibrous layer as well as to the further fibrous layers.

The second fibrous material (ii-2), the composition of the second fiber suspension and the basis weight of the second fibrous layer may be the same or may differ from that of the corresponding first fibrous layer.

Preferably, the first aqueous fiber suspension is based on wastepaper. More preferably, the first and second aqueous fiber suspension are based on wastepaper.

Accordingly, a process for manufacturing a multi-layer paper or cardboard is preferred, wherein the first fibrous layer is obtained by dewatering an aqueous fiber suspension, and the first aqueous fiber suspension is based on wastepaper.

More preferred is a process for manufacturing a multi-layer paper or cardboard, wherein the first fibrous layer is obtained by dewatering a first aqueous fiber suspension, the second fibrous layer is obtained by dewatering a second aqueous fiber suspension, and the first aqueous fiber suspension and the second aqueous fiber suspension are based on wastepaper.

In step a) at least one surface side of the first fibrous layer is sprayed with an additive formulation. This creates at least one sprayed fibrous layer with a sprayed surface side. The sprayed surface side of the first fibrous layer is the surface side which is intended to be joined with a surface side of a second fibrous layer.

Spraying is preferably carried out with a spraying device. The spraying device contains, for example, one or more nozzles suitable to spray the additive formulation homogenously. The additive formulation is usually sprayed from the nozzle or nozzles onto the surface side of the fibrous layer to be sprayed. The additive formulation is preferably under an overpressure relative to the ambient pressure, for example 0.5 to 15 bar, preferably 0.5 to 5 bar, more preferably at 0.8 to 3 bar. The overpressure is built up shortly before it leaves the nozzle. A container for storing the additive formulation may be part of the spraying device.

In step b), joining of the first fibrous layer with the second fibrous layer ensures the formation of the layer composite. A surface side of the first fibrous layer comes into permanent contact with a surface side of the second fibrous layer. At least the first surface side of these two surface sides is a sprayed surface side. When joining, the surface sides usually come into contact at least to such an extent that the fibrous layers adhere weakly to one another, wherein the fibrous layers are arranged or merged such that the fibrous layers overlap in their entire width. The step of joining corresponds to a complete overlapping of the first fibrous layer and the second fibrous layer. The step of joining takes place, for example, in terms of space and time almost immediately before pressing step c).

In step c), the layer composite is generally pressed, which leads to further dewatering and a corresponding increase of the dry matter content. Step c) usually starts when the layer composite of step b) reaches the so-called couching line.

At couching, dewatering usually takes place under mechanical pressure on the layer composite. Removing water by mechanical pressure is more energy efficient than removing water by adding heat or drying. By applying the layer composite onto a water-absorbent belt, e.g., a feltlike fabric, the dewatering step may be supported by absorbing the water obtained by pressing. A roll may suitably be used for applying pressure onto the layer composite. Passing the layer composite through two rolls is particularly suitable, wherein the rolls may be rested onto the water-absorbent belt. The surface of the roll may consist of steel, granite or hard rubber. The surface of a roll may be coated with a water-absorbent material, which usually have a high degree of absorbency, porosity, strength and elasticity. After contact with the layer composite, the water-absorbent materials are usually regenerated by dewatering.

At the end of step c), a partially dewatered layer composite is generally be formed. The partially dewatered layer composite is firm enough to be able to be fed to the next step without mechanical support. The partially dewatered layer composite, for example, has a dry matter content higher than the first fibrous layer and the second fibrous layer of step a) by at least 5 wt% (absolute value). The partially dewatered layer composite preferably has a dry matter content of at least 35 wt% up to about 70 wt%.

In step d), further dewatering of the partially dewatered layer composite obtained in step c) is carried out by drying to form a dried multi-layer paper or cardboard. Drying may be carried out by any drying device known in the art, for example by supplying heat to the partially dewatered layer composite. Drying may be carried out by a heated cylinder, an infrared dryer, an air dryer, for example, using warm air, or by any combination thereof.

For example, drying may be carried out by drying cylinders. Typical cylinder temperatures are of from 120 to 160°C. A cylinder may be coated, wherein a better surface quality of the dried multilayer paper or cardboard may be obtained.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, the process further comprising: b) joining the first fibrous layer with the second fibrous layer to form a layer composite; c) dewatering the layer composite obtained in step b) by pressing; and d) drying the layer composite obtained in step c) to form a dried multi-layer paper or cardboard. The process for manufacturing a multi-layer paper or cardboard may include further steps generally known in the art. For example, step d) may be followed by calendaring, by coating by conventional means, dependent on the final use, and/or by foreshortening or cutting to a predetermined size.

A dried multi-layer paper or cardboard is defined herein as a sheet material that has a grammage, i.e. , a basis weight of the dried paper or cardboard, of up to approximately 600 g/m². The designation “paper” in the narrower sense is typically used for grammages up to 225 g/m², while the designation “cardboard” is used for grammages of 250 g/m² or higher. The grammage of the dried multi-layer paper or cardboard is preferably of from 20 to 400 g/m², more preferably from 40 to 280 g/m².

The dry matter content of the dried multi-layer paper or cardboard, as finally manufactured, is, for example, at least 88 wt%, preferably of from 89 to 98 wt%.

The multi-layer paper or cardboard may have two or more layers. The multi-layer paper or cardboard preferably has two, three or four layers and optionally a final coating layer. More preferably, the multi-layer paper or cardboard has two or three layers.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the multi-layer paper or cardboard contains 2, 3 or 4 layers, preferably 2 or 3 layers.

The first fibrous layer to be sprayed may also be a layer composite, wherein another fibrous layer has been joined with the first fibrous layer to be sprayed.

In case of two layers, the process contains exactly one first fibrous layer and one second fibrous layer, having optionally a sprayed surface side. In case of three layers, an additional fibrous layer is present as a third fibrous layer. In case of four layers, another additional fibrous layer is present. An optional third or fourth fibrous layer may be joined with or without spraying to the layer composite of the first fibrous layer and the second fibrous layer. The step of joining of more than two layers may be carried out simultaneously or subsequently. Preferably, the step of joining of more than two layers, having a sprayed or non-sprayed surface side, is carried out simultaneously. Such joining step(s) is/are usually followed by the further dewatering step c) and the drying step d).

In case of three or more layers, the first fibrous layer and/or a further fibrous layer, for example, the second fibrous layer, may also be sprayed on both surface sides.

The instant process of manufacturing a multi-layer paper or cardboard may contain a further fibrous layer, wherein a surface side thereof is sprayed with the additive formulation.

For example, a surface side of the second fibrous layer may be sprayed with the additive formulation. The surface side may be the one which is intended to be joined with the sprayed surface side of the first fibrous layer or may be the other surface side of the second fibrous layer. In said case, a further third layer is needed to be joined to the sprayed surface side of the second fibrous layer. The second fibrous layer may be sprayed with the additive formulation onto the surface side which is intended to be joined with the sprayed surface side of the first fibrous layer. Alternatively, the second fibrous layer may be sprayed with the additive formulation onto the surface side which is opposed to the surface side intended to be joined with the sprayed surface side of the first fibrous layer.

Especially, the additive formulation is sprayed simultaneously onto a surface side of the second fibrous layer and onto a surface side of the first fibrous layer.

In case of two or more fibrous layers, the step of spraying may preferably be carried out simultaneously with the spraying step a).

The additive formulation may only be applied by spraying to an internal surface side of any fibrous layer within the multi-layer composite, i.e., any sprayed surface side needs to be in contact with another surface side of a fibrous layer within the multi-layer composite.

The additive formulation is preferably applied as a spraying solution or spraying suspension.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the additive formulation is applied as a spraying solution or spraying suspension.

The additive formulation, used in step a), comprises (a- 1 ) water and (a-2) a salt containing a carbonate (COs^2-) anion or a hydrogencarbonate (HCOa') anion, wherein the amount of water is at least 75 wt%, based on the total weight of the additive formulation.

The cation of the salt may be suitably selected from NH4⁺ and an alkali metal cation and any combination thereof. The alkali metal cation is preferably Na⁺, K⁺ or any combination thereof. Na⁺ is more preferred.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the salt containing a COa²' or a HCOa' anion contains a cation selected from an alkali metal cation, NH4⁺ and any combination thereof.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the cation is NH4⁺ or Na⁺, preferably NH₄ ⁺.

The additive formulation contains water (a- 1) in an amount of at least 75 wt%, based on the total weight of the additive formulation, preferably at least 80 wt%, more preferably at least 85 wt%. The amount of water in the additive formulation is usually dependent on the processing conditions of the paper machine. The additive formulation usually contains the salt containing a COs^2- or a HCOa' anion (a-2) in an amount of from 0.5 to 25 wt%, based on the total weight of the additive formulation, preferably from 0.5 to 20 wt%, more preferably from 0.5 to 15 wt%.

The additive formulation usually contains the salt containing a CCh²' or a HCOa' anion (a-2) in such an amount, that the applied quantity of the salt containing a COa²' or a HCOa' anion (a-2) is of from 2 to 70 g/m², based on the solids content of the salt (a-2) of the spraying solution or spraying suspension and based on the sprayed area, preferably from 3 to 45 g/m², more preferably from 3 to 30 g/m².

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the additive formulation contains the salt containing a COa²' or a HCOa' anion (a-2) in an amount of from 0.5 to 25 wt%, based on the total weight of the additive formulation, preferably from 0.5 to 20 wt%, more preferably from 0.5 to 15 wt%.

The additive formulation may comprise one salt containing a CCh²' or a HCOa' anion or any combination thereof. A combination may comprise a salt having different cations and/or different anions. Preferably, one salt is used in the instant process.

The additive formulation preferably contains

(a-1) water,

(a-2) a salt containing a COa²' or a HCOa' anion, and

(a-3) at least one organic polymer P.

Preferably, the organic polymer P (a-3) is water-soluble or water-dispersible.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the additive formulation contains (a-1) water,

(a-2) a salt containing a COa²' or a HCOa' anion, and

(a-3) at least one organic polymer P, which may be water-soluble or water-dispersible.

An organic polymer P is water-soluble, if its solubility in water under standard conditions (20°C, 1013 mbar) and at pH 7 is at least 5 wt%, preferably at least 10 wt%. The weight percentages relate to the solids content of a polymer P.

The spraying solution, as used herein, is a solution of the organic polymer P in the solvent water. If another liquid is present which does not mix sufficiently with water to dissolve, this mixture is also referred to herein as a spraying solution. Contrary thereto, solid particles are typically not present in the spraying solution. Solid particles are usually also absent down to colloidal dimensions, i.e., <10^-5 cm.

The spraying suspension, as used herein, is a solution of the organic polymer P in the solvent water, wherein typically water-insoluble solid particles are additionally present. If another liquid is present, which does not mix sufficiently with water to dissolve, this mixture is also referred to herein as a spraying suspension. The temperature in this connection is 23°C, and the pressure is ambient pressure of approximately 1013 mbar.

Typically, the spraying solution or spraying suspension has a pH > 5, preferably > 6, more preferably > 7. Preferably, the spraying solution or spraying suspension has a pH of 5 to 12, more preferably of 7 to 11.

Due to the high content of water, the density of the spraying solution or spraying suspension may be assumed to be approximately 1 g/cm³.

The organic polymer P may be a non-ionic polymer, a cationic polymer or an anionic polymer. Preferably, the organic polymer P is a non-ionic polymer or an anionic polymer.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the organic polymer P is a non-ionic polymer or an anionic polymer.

Especially, the organic polymer P is an anionic polymer.

The organic polymer may be a synthetic organic polymer P1 comprising at least one unit derived from an ethylenically unsaturated monomer. The organic polymer P may also be a natural or a modified-natural polymer P2, preferably a polymer P2 based on a polysaccharide.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the organic polymer P is selected from a synthetic polymer P1 comprising a unit derived from an ethylenically unsaturated monomer, a natural or modified natural polymer P2 based on a polysaccharide and any combination thereof.

The organic polymer P1 preferably comprises at least one unit derived from an ethylenically unsaturated monomer, preferably selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof.

The ethylenically unsaturated monomer may be at least one ethylenically unsaturated monomer selected from a non-ionic monomer M1, an anionic monomer M2, a cationic monomer M3, an amphoteric monomer M4 and any combination thereof.

An anionic monomer M2 carries at least one negative charge at pH 7. A non-ionic monomer M1 carries no charge at pH 7. A cationic monomer M3 carries at least one positive charge at pH 7. An amphoteric monomer M4 carries at least one positive charge and at least one negative charge at pH 7.

Examples of a non-ionic vinyl or allyl monomer include - an ester of vinyl alcohol or allyl alcohol with a Ci-C20-monocarboxylic acid, such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate or vinyl laurate;

- an ether of vinyl alcohol or allyl alcohol with a Ci-C2o-alkanol, such as methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, isobutyl vinyl ether, 2-chloroethyl vinyl ether, hydroxyethyl vinyl ether, octyl vinyl ether or phenyl vinyl ether; or a monoether of polyethylene oxide or Ci-Ce-alkylpolyethyleneoxide with vinyl alcohol or allyl alcohol;

- a heterocyclic vinyl compound, such as N-vinylpyrrolidone, N-vinylpiperidone, N-vinylcapro- lactam, N-vinyl-5-methyl-2-pyrrolidone, N-vinyl-5-ethyl-2-pyrrolidone, N-vinyl-6-methyl-2- piperidone, N-vinyl-6-ethyl-2-piperidone, N-vinyl-7-methyl-2-caprolactam, N-vinyl-7-ethyl-2- caprolactam, N-vinylpyridine, N-vinylimidazole or 5-methyl-3-vinyl-2-oxazolidinone (VMOX);

- a vinyl aromatic compound, such as styrene, o-, m-, p-methylstyrene or 4-n-butylstyrene;

- a vinyl halide, such as vinyl chloride or vinyl fluoride;

- a vinylidene halide, such as vinylidene chloride or vinylidene fluoride; and

- a C2-C8-monoolefin, such as ethylene, propylene, isobutylene, 1-butene, 1-hexene or 1- octene.

Examples of a non-ionic (meth)acrylic monomer include

- a monoester of a a,p-ethylenically unsaturated monocarboxylic acid with a Ci-C2o-alkanol, such as methyl (meth)acrylate, methyl ethacrylate (= methyl 2-ethyl acrylate), ethyl (meth)acrylate, ethyl ethacrylate (= ethyl 2-ethyl acrylate), n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-octyl (meth)acrylate, 1 ,1 ,3,3-tetramethyl-butyl (meth)acrylate, or 2-ethylhexyl (meth)acrylate;

- a monoester of a a,p-ethylenically unsaturated monocarboxylic acid with a C2-C2o-alkanediol, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxyethyl ethacrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4- hydroxybutyl (meth)acrylate, or 6-hydroxyhexyl (meth)acrylate;

- a diester of a a,p-ethylenically unsaturated dicarboxylic acid with a Ci-C2o-alkanol or a C2- C2o-alkanediol;

- a primary amide of a a,p-ethylenically unsaturated monocarboxylic acid, such as (meth)acrylamide;

- a N-alkylamide of a a,p-ethylenically unsaturated monocarboxylic acid, such as N-methyl (meth)acrylamide, N-isopropyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-(n-propyl) (meth)acrylamide, N-(n-butyl) (meth)acrylamide, N-(tert-butyl) (meth)acrylamide, N-(n-octyl) (meth)acrylamide, N-(1 ,1 ,3,3-tetramethyl-butyl) (meth)acrylamide, or N-(2-ethylhexyl) (meth)acrylamide;

- a N,N-dialkylamide of a a,p-ethylenically unsaturated monocarboxylic acid, such as N,N- dimethyl (meth)acrylamide;

- a nitrile of a,p-ethylenically unsaturated monocarboxylic acid, such as (meth)acrylonitrile; and

- a dinitrile of a a,p-ethylenically unsaturated dicarboxylic acid.

Examples of a diene monomer include C4-C -olefins with exactly two double bonds that are conjugated, such as butadiene or isoprene. Examples of an anionic monomer M2, i.e., an anionic monoethylenically unsaturated monomer M2, include

- a Cs-Cs-carboxylic acid or a salt thereof, such as (meth)acrylic acid, dimethacrylic acid, ethacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, mesaconic acid, citraconic acid, methylene-malonic acid, vinyl acetic acid, allyl acetic acid, crotonic acid or salts thereof;

- a monoethylenically unsaturated sulfonic acid or a salt thereof, such as vinyl sulfonic acid, 2- acrylamido-2-methylpropanesulfonic acid (AMPS), 2-methacrylamido-2- methylpropanesulfonic acid, allylsulfonic acid, methallylsulfonic acid, sulfoethyl (meth)acrylate, sulfopropyl (meth)acrylate, styrene sulphonic acid or salts thereof;

- a monoethylenically unsaturated phosphonic acid or a salt thereof, such as vinylphosphonic acid, vinylphosphonic acid monomethyl ester, allylphosphonic acid, allylphosphonic acid monomethyl ester, acrylamidomethylpropylphosphonic acid, acrylamidomethylene- phosphonic acid or salts thereof; and

- a monoethylenically unsaturated mono- or diester of phosphoric acid or a salt thereof, such a monoallyl phosphoric acid ester, methacrylethylene glycol phosphoric acid or salts thereof.

Salts of monomers M2 include alkali metal, alkaline earth metal or ammonium salts, preferably, sodium, potassium, magnesium, calcium or ammonium salts, more preferably sodium or potassium salts.

Preferred anionic monomers M2 include a Cs-Cs-carboxylic acid and a salt thereof, especially (meth)acrylic acid and maleic acid.

Examples of a cationic monomer M3 include

- a quaternized, monoethylenically unsaturated monomer, such as [2-((meth)acryloyloxy) ethyl]trimethylammoniumchloride, [3-((meth)acryloyloxy)propyl]trimethylammoniumchloride, or 3-((meth)acrylamidopropyl)trimethylammoniumchloride;

- a monoethylenically unsaturated monomer which carries at least one secondary or tertiary amino group and whose at least one secondary or tertiary amino group is protonated at pH 7, such as an ester of (meth)acrylic acid with an optionally mono- or dialkylated amino-C2-Ci2- alkanol, for example, N-methylaminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, or N,N-dimethylaminocyclohexyl (meth)acrylate; a mono- and diester of an a,p-ethylenically unsaturated dicarboxylic acid with an optionally mono- or dialkylated amino-C2-Ci2-alkanol, for example based on fumaric acid, maleic acid, monobutyl maleate, itaconic acid or crotonic acid; an amide of an a,p-ethylenically unsaturated monocarboxylic acid with a dialkylated diamine, for example, dialkylaminoethyl (meth)acrylamides or dialkylaminopropyl (meth)acrylamides, like N-[2-(dimethylamino) ethyl] (meth)acrylamide, N-[3- (dimethylamino) propyl] (meth)acrylamide, N-[4-(dimethylamino)butyl] (meth)acrylamide, or N-[2-(diethylamino)ethyl] (meth)acrylamide; vinylimidazole or alkylvinylimidazole; and

- a diallyl-substituted amine which has exactly two ethylenic double bonds and is quaternized or protonated at pH 7, or its salt form, such as diallylamine or diallyldimethylammonium chloride.

The amino-C2-Ci2-alkanols may be Ci-Cs-mono- or Ci-Cs-dialkylated at the nitrogen atom.

In the case of a cationic monomer, salt form means that a corresponding anion ensures charge neutrality in case of a quaternized nitrogen or protonation. Such anions are, for example, chloride or hydrogensulfate.

Preferred quaternizing agents may be dimethyl sulfate, diethyl sulfate, methyl chloride, ethyl chloride or benzyl chloride. Methyl chloride is more preferred.

Examples of an amphoteric monomer M4 include 3-(dimethyl(methacryloylethyl)ammonium)- propanesulfonate, 3-(2-methyl-5-vinylpyridinium)propanesulfonate, N-3-methacrylamidopropyl- N,N-dimetyl-p-ammonium-propionate, N-2-acrylamidoethyl-N,N-dimethyl-p-ammonium- propionate, 3-vinylimidazol-N-oxide, 2-vinylpyridine-N-oxide and 4-vinylpyridine-N-oxide.

The synthetic polymer P1 may also comprise a cross-linking monomer M5 having at least 2 ethylenically unsaturated double bonds, which are not conjugated. Examples include triallylamine, methylenebisacrylamide, glycol di(meth)acrylate, glycerol triacrylate, pentaerythritol triallyl ether, N,N-divinylethylene urea, tetraallylammonium chloride, a polyalkylene glycol or a polyol, like pentaerythritol, sorbite or glucose, which are at least twice esterified with (meth)acrylic acid.

The synthetic polymer P1 is preferably a copolymer comprising at least one unit derived from a non-ionic monomer M1, selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof, and optionally at least one anionic monomer M2 or at least one cationic monomer M3.

The synthetic polymer P1 is preferably a copolymer, obtainable by polymerizing at least one non-ionic monomer M1, selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof, and optionally an anionic monomer M2 or a cationic monomer M3.

The total amount of all monomers of the synthetic polymer P1 is 100 mol%.

The synthetic polymer P1 may comprise

50 to 100 mol% of a unit derived from a non-ionic monomer M1, preferably 70 to 100 mol%; 0 to 50 mol% of a unit derived from an anionic monomer M2 or cationic monomer M3, preferably 0 to 30 mol%;

0 to 10 mol% of a unit derived from an amphoteric monomer M4; and 0 to 2 mol% of a unit derived from a cross-linking monomer M5, wherein the total amount of all monomers is 100 mol%.

The synthetic polymer P1 is preferably a non-ionic or an anionic copolymer, preferably an anionic copolymer.

The synthetic polymer P1 may comprise

50 to 100 mol% of a unit derived from a non-ionic monomer M 1 , preferably 70 to 100 mol%; 0.5 to 50 mol% of a unit derived from an anionic monomer M2, preferably 0.5 to 30 mol%, more preferably 1 to 20 mol%, especially 1 to 10 mol%;

More preferred is a synthetic polymer P1 , which is a copolymer, obtainable by polymerizing at least one non-ionic monomer M1 , selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof, and an anionic monomer M2.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the synthetic polymer P1 is obtainable by polymerizing at least one non-ionic monomer M1 , selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof, and optionally an anionic monomer M2.

Especially, the synthetic polymer P1 is obtainable by polymerizing at least one non-ionic monomer M1 , selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof, and optionally an anionic monomer M2 selected from an ethylenically unsaturated Cs-Cs-carboxylic acid and a salt thereof.

The organic polymer P1 is preferably applied to the additive formulation in form of a latex, usually obtainable by emulsion polymerization.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the synthetic polymer P1 is added to the additive formulation as an aqueous dispersion.

Generally, the average particle diameter of the particles of the synthetic polymer P1 is of from 0.01 to 1 pm, preferably from 0.15 to 0.20 pm, usually determined by dynamic light scattering (DLS) at 22°C according to EN ISO 22412:2017.

The aqueous dispersion of the organic polymer P1 may have a solids content of from 30 to 70 wt%, preferably from 40 to 60 wt%.

A preferred synthetic polymer P1 is a water-dispersible copolymer made of - a vinyl aromatic compound, an Ci-Ci2-alkyl (meth)acrylate and optionally an ethylenically unsaturated carboxylic acid or a salt thereof; or

- a Ci-Ci2-alkyl (meth)acrylate and (meth)acrylonitrile; or

- a Ci-Ci2-alkyl (meth)acrylate and vinyl acetate; or

- a vinyl aromatic compound and an ethylenically unsaturated carboxylic acid; or

- a vinyl aromatic compound, butadiene and optionally an ethylenically unsaturated carboxylic acid; or

- ethylene, vinyl acetate and optionally an ethylenically unsaturated carboxylic acid; or

- Ci-Ci2-alkyl (meth)acrylate, a vinyl aromatic compound, and (meth)acrylonitrile; wherein the vinyl aromatic compound is selected from styrene, o-, m-, p-methylstyrene, 4-n- butylstyrene and any combinations thereof, and wherein the ethylenically unsaturated carboxylic acid is selected from (meth)acrylic acid, maleic acid or maleic anhydride, fumaric acid, itaconic acid and any combinations thereof.

A more preferred synthetic polymer P1 is selected from

- a copolymer made of styrene and Ci-Ci2-alkyl (meth)acrylate, preferably Ci-Ci2-alkyl acrylate, more preferably n-butyl acrylate, and optionally (meth)acrylic acid or a salt thereof;

- a copolymer made of Ci-Ci2-alkyl (meth)acrylate and acrylonitrile;

- a copolymer made of Ci-Ci2-alkyl (meth)acrylate and vinyl acetate;

- a copolymer made of styrene, butadiene and optionally (meth)acrylic acid or a salt thereof;

- a copolymer made of styrene and maleic acid,

- a copolymer made of ethylene and vinyl acetate; and

- a copolymer made of Ci-Ci2-alkyl (meth)acrylate, styrene and acrylonitrile.

An especially preferred synthetic polymer is selected from

- a copolymer made of styrene and Ci-Ci2-alkyl (meth)acrylate, preferably Ci-Ci2-alkyl acrylate, more preferably n-butyl acrylate, and (meth)acrylic acid or a salt thereof;

- a copolymer made of styrene, butadiene and (meth)acrylic acid or a salt thereof; and

- a copolymer made of styrene and maleic acid.

Also, any mixture of synthetic polymers P1 may be used.

The synthetic polymers P1 may be commercially available, for example, under the trade names Acronal, Styronal or Basoplast.

The synthetic polymer P1 may be obtained by conventional methods known in the art, for example, by solution, precipitation, suspension or emulsion polymerization. Solution or emulsion polymerization in aqueous media is preferred, especially a free-radical aqueous emulsion polymerization, using, for example, radical polymerization initiators, like peroxides, hydroperoxides, redox catalysts or azo compounds. The procedure for free-radical aqueous emulsion polymerizations of monomers in an aqueous medium has been extensively described and is therefore sufficiently familiar to the skilled person, as, for example, described in D. Diederich, Chemie in unserer Zeit 24, 1990, pages 135 to 142. The organic polymer P may be a natural polymer or a natural-modified polymer P2, preferably a polysaccharide or a derivative of a polysaccharide, also called herein a modified polysaccharide, more preferably a derivative of a polysaccharide.

The polysaccharide or a derivative thereof may be a homopolysaccharide, a derivative thereof, a heteropolysaccharide or a derivative thereof.

The homopolysaccharides are usually homopolysaccharides based on glucose and derivatives thereof, preferably cellulose, a derivative thereof, starch or a derivative thereof, more preferably a derivative of a homopolysaccharide, especially a cellulose derivative.

Examples of a derivative of a homopolysaccharide include a chemically modified starch, such as methylated or ethylated starch, or a cellulose ether.

A chemically modified starch may be obtained by oxidation or by functionalization of a natural starch by covalently attaching a chemical group or breaking covalent bonds in the starch, for example, by esterification or etherification of a natural starch followed by starch degradation. Preferably, a chemically modified starch is anionic starch, cationic starch or a starch ether. A dispersion containing a starch derivative, for example cationic starch, and an alkylketene dimer may also be used.

A cellulose ether is a derivative of cellulose, wherein the hydroxy groups of cellulose are partially or completely substituted by ether groups. The cellulose ether may be an alkyl ether or an arylalkyl ether, which alkyl or arylalkyl groups may be further substituted by hydroxy, carboxy or carboxylate groups. Corresponding counterions for carboxylate groups may be alkali metal ions, such as sodium or potassium, or ammonium ions. Cellulose ethers may carry one type of substituent on the cellulose ether molecular chain, while mixed ether may carry two or more different substituents, like methyl-hydroxyethyl cellulose.

Preferred cellulose ethers are methyl cellulose, ethyl cellulose, propyl cellulose, carboxymethyl cellulose (CMC), hydroxyethyl cellulose, hydroxypropyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methylhydroxybutyl cellulose, ethylhydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, benzyl cellulose and any mixed cellulose ethers. Among the carboxymethyl celluloses, the sodium compound is preferred.

The cellulose ethers are, for example, commercially available or may be prepared by usual methods. The cellulose ethers may be prepared, for example, by the action of alkyl halides or arylalkyl halides, epoxides or activated olefins on cellulose that has been activated with bases, for example, with an aqueous sodium hydroxide solution.

Typically, the substitution degree is of from 0.5 to 0.95, i.e., 0.5 to 0.95 carboxymethyl groups per 10 anhydroglucose units.

Cellulose ethers may be available under the tradenames Texturecel or Finnfix. Heteropolysaccharides may be selected from alginic acid, salts or derivatives thereof, like sodium alginate, potassium alginate, ammonium alginate, calcium alginate, propane-1, 2-diol alginate, agar, carrageenan, processed Eucheuma algae, locust bean gum I carob gum, guar gum, gum tragacanth, gum arabic, xanthan gum, karaya gum, tara gum, gellan gum, konjac gum, konjac glucomannan, soybean-hemicellulose, pectin or a derivative thereof like amidated pectin, and any combinations thereof.

The heteropolysaccharides are typically obtained by fermentation or by isolation from natural sources.

A preferred heteropolysaccharide is a heteropolysaccharide having a COOH group, a derivative thereof or a heteropolysaccharide having a OSO3 group, more preferably an anionic heteropolysaccharide. Examples include a heteropolysaccharide based on galacturonic acid, mannuronic acid, guluronic acid, glucuronic acid or any derivative thereof.

Preferred is a process for manufacturing a multi-layer paper or cardboard, wherein the organic polymer P is selected from a heteropolysaccharide having COOH groups, a derivative thereof, a heteropolysaccharide having OSO3 groups, a derivative thereof, a derivative of cellulose and any combination thereof.

A preferred polymer P2 is a derivative of cellulose, especially a cellulose ether.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the natural or modified natural polymer P2 is a derivative of cellulose.

A preferred derivative of cellulose is a cellulose ether, especially an anionic cellulose ether, in particular sodium carboxy methyl cellulose.

Also, any mixture of natural or modified natural polymers P2 may be used.

Further, any mixture of a synthetic polymer P1 and a natural or modified natural polymer P2 may be used.

Suitable organic polymers P1 and P2 are, for example, polymers and polymer dispersions (latices) suitably employed in the paper manufacturing process, for example as a surface sizing agent or as a polymer in a paper coating.

The viscosity of the organic polymer P may vary. The organic polymer P preferably has a viscosity of from 3 to 1000 mPa s, more preferably from 3 to 500 mPa s or 5 to 500 mPa s, especially from 10 to 400 mPa s, in particular from 20 to 300 mPa s. The viscosity, preferably of the organic polymer P1, is usually determined according to DIN EN ISO 2555:2018. The viscosity of the natural or a modified-natural polymer P2 is preferably measured in 1 wt% solution, 2 wt% solution or 4 wt% solution.

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the organic polymer P has a viscosity of from 3 to 1000 mPa s, preferably 3 to 500 mPa s.

If the viscosity is lower than 3 mPa s, an additional effect may not be observed. If the viscosity is greater than 1000 mPa s, the additive formulation may have not the desired characteristics of sprayability.

The molecular weight of the organic polymer P is typically not critical. The organic polymer P preferably has a weight-average molecular weight M_w of from 30,000 to 1 ,000,000 g/mol, preferably from 50,000 and 500,000. The weight average molecular weight may be determined with static light scattering.

Typically, the additive formulation contains the organic polymer P (a-3) in an amount of from 0.1 to 10 wt%, based on the total weight of the additive formulation, preferably from 0.5 to 10 wt%, more preferably from 0.5 to 8 wt% The weight of polymer P in the additive formulation relates to the solids content of polymer P.

The additive formulation usually contains the organic polymer P (a-3) in such an amount, that the applied quantity of the organic polymer P is of from 2 to 30 g/m², based on the solids content of the organic polymer P (a-3) of the spraying solution or spraying suspension and based on the sprayed area, preferably from 3 to 25 g/m², more preferably from 3 to 20 g/m².

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the additive formulation comprises the organic polymer P in an amount of from 0.1 to 10 wt%, based on the total weight of the additive formulation.

Optionally, the additive formulation may contain one or more further components.

The additive formulation may contain an agent for releasing carbon dioxide (a-4). Examples include phosphoric acid or disodium pyrophosphate (Na2H2P2O?). The agent for releasing carbon dioxide may be suitably used in connection with sodium hydrogencarbonate.

The additive formulation may contain the agent for releasing carbon dioxide in an amount equivalent to the amount of bound CO2 in the salt (a-2). Preferably, the molar ratio of the salt (a- 2) to the agent for releasing CO2 (a-4) is about 0.45 : 0.55 to 0.55 : 0.45.

Further, the additive formulation may contain a spraying aid (a- 5), which is different from a polymer P. The spraying aid (a-5) is, for example, a viscosity regulator, a pH regulator, a defoamer or a biocide. The additive formulation may contain the spraying aid in an amount up to 2 wt%, based on the total weight of the additive formulation, preferably from 0 to 1 wt%. If present, the amount of spraying aid (a- 5) may be of from 0.001 to 1 wt%, based on the total weight of the additive formulation, preferably from 0.005 to 0.8 wt%, more preferably from 0.01 to 0.5 wt%.

Further, the additive formulation may contain further components derived from the polymerization process in case of an organic polymer P1 , preferably from the emulsion polymerization. Examples may be an emulsifier or a seed latex.

The applied quantity of spraying solution or spraying suspension is preferably of from 2 to 70 g/m², based on the solids content of the spraying solution or spraying suspension and based on the sprayed area, more preferably from 3 to 65 g/m².

Accordingly, in a preferred aspect, the invention relates to a process for manufacturing a multilayer paper or cardboard, wherein the applied quantity of the additive formulation is of from 2 to 70 g/m², based on the solids content of the spraying solution or spraying suspension and based on the sprayed area.

In a further aspect, the invention relates to a multi-layer paper or cardboard, obtainable or obtained by a process, the process comprising: a) spraying onto a surface side of a first fibrous layer to be joined with a surface side of a second fibrous layer with an additive formulation; wherein the first and the second fibrous layer independently of one another have a dry matter content of from 25 to 65 wt%; and the additive formulation comprises a-1) water, and a-2) a salt containing a COa²' or a HCOa' anion, and the amount of water is at least 75 wt%, based on the total weight of the additive formulation.

The instant process is preferably for manufacturing a cardboard, especially for a folding box.

In a further aspect, the invention relates to the use of an additive formulation comprising a-1) water, a-2) a salt containing a carbonate or a hydrogencarbonate anion, and a-3) optionally an organic polymer P, as defined in any aspect herein, as a spraying additive in the process of manufacturing a multi-layer paper or cardboard, preferably as defined in any aspect herein, especially to increase the specific volume of a multi-layer paper or cardboard.

Preferably, the term “to increase the specific volume of a multi-layer paper or cardboard” means that the specific volume of a multi-layer paper or cardboard is higher if compared to a specific volume of a multi-layer paper or cardboard, prepared without spraying with the additive formulation or spraying with water. The process of manufacturing of a multi-layer paper or cardboard is preferably carried out in a paper machine. The paper machine preferably comprises the following sections and units:

- a wire section with a first wire, which has a first upper side and a first lower side and a second wire, which has a second upper side and a second lower side;

- a spraying device containing the additive formulation;

- a press section; and

- a dryer section.

Said sections and units are arranged in the paper machine in the order of the wire section, followed by the spraying device, the press section and then the dryer section. The spraying device is preferably located at the end of the wire section. In the paper machine, step a) takes place before the press section, preferably at the end of the wire section, step b) takes place before or at the beginning of the press section, step c) takes place within the press section and step d) takes place within the dryer section.

The spraying device preferably comprises at least one nozzle, more preferably one or more nozzles, which make it possible to spray the additive formulation under an overpressure, for example, of from 0.5 to 5 bar compared to the ambient pressure. The first fiber suspension and the second fiber suspension pass through the paper machine by subjecting to dewatering in a wire section, being sprayed onto at least one surface side, joining, dewatering by pressing and drying to form a multi-layer paper in the direction from the wire section to the dryer section.

Accordingly, in a further aspect, the invention relates to a paper machine, equipped with a first wire section with a first wire and a second wire, a spraying device, a press section and a dryer section, said sections and device are arranged in the paper machine in the order of the wire section, followed by the spraying device, the press section and the dryer section, wherein the spraying device contains an additive formulation, as defined in any aspect herein.

The paper machine is suitable for the process, as defined in any aspect herein.

The preferences for the process for producing a multi-layer paper or cardboard applies to the other objects of the invention.

The instant process provides an easy and economic process for manufacturing a multi-layer paper or cardboard to minimize costs, by increasing the specific volume essentially without negatively influencing mechanical strength properties like tensile strength, tensile stretch, burst index and/or short span compression strength. These mechanical properties are advantageously suitable for manufacturing a cardboard, preferably based on wastepaper. Especially, the instant process is suitable for cardboard used for folding boxes.

The instant additive formulation may be easily sprayed in form of an aqueous formulation, especially having a low viscosity and a good stability. Further, the instant additive formulation allows for a good workability without the nozzles becoming clogged. In case the organic polymer P is a water-dispersible polymer, the viscosity of an aqueous dispersion is low although the dispersion contains a high concentration of polymer P, as the polymer is stably dispersed as fine particles.

All percent, ppm or comparable values refer to the weight with respect to the total weight of the respective composition except where otherwise indicated. The terms “% by weight” and “wt%” are used herein synonymously. All cited documents are incorporated herein by reference.

The following examples shall further illustrate the present invention without restricting the scope of this invention.

Examples

Components

Ammonium carbonate (BASF, industrial grade)

Ammonium hydrogencarbonate (BASF, industrial grade)

Sodium hydrogencarbonate (Sigma-Aldrich)

Disodium pyrophosphate (Sigma-Aldrich)

Acronal® Pure 2728 (BASF); aqueous anionic copolymer dispersion based on n-butyl acrylate and styrene; solids content ~ 50 wt%; viscosity 100-280 mPa s according to DIN EN ISO 2555, 23°C

FINNFIX® 10 Sodium Carboxymethylcellulose (CPKelco; viscosity 50-200 mPa s (Brookfield, LV, 4% aqueous solution (dry basis))

Dry matter content

The dry matter content was determined in accordance with DIN EN ISO 638-1 :2022-07 and DIN EN ISO 638-2:2022-06, resp., using the oven-drying method. The dry matter content of the sheet of paper is to be understood as meaning the ratio of the mass of a sample dried to constant mass at a temperature of (105 ±2)°C under defined conditions, to the mass of the sample before drying. The dry matter content is reported as percentages by weight (wt%).

Ash content

The ash content was determined according to ISO 2144:2019.

Sample preparation: The papers and cardboards were conditioned at a temperature of 23±1 °C and a relative humidity of 50±2% for 24 hours and analysed under these climate conditions.

Basis weight / thickness / specific volume

The basis weight (g/m²) was determined gravimetrically according to DIN EN ISO 536:2020.

The thickness of a paper or board (pm) was determined with a micrometer. The pressure acting between the pressure surfaces was 100 ± 100 kPa on an area of 200 mm². The specific volume (cm³/g) is the ratio of thickness to basis weight. Burst index

The bursting strength of cardboard was determined with a Bursting Strength Tester SE 002 J 5- 3 (Lorentzen & Wettre GmbH) according to DIN EN ISO 2759:2014-10. An average value of 10 single measurements (5 per each side) were evaluated. The burst index results from bursting strength per unit width (measured in kPa divided by basis weight g/m²).

Tensile stretch in cross direction

The tensile stretch (breaking elongation) in % was determined with a Tensile Tester SE 062/064 (Lorentzen & Wettre GmbH) according to DIN EN ISO 1924-2:2009-05. A sample strip (15 mm width) was tested in cross direction at a constant speed of 20 mm/min and clamping length of 100 mm. An average value of 5 single measurements were evaluated.

Short span compression strength index

The compression strength was determined with a Compression Strength Tester STFI 93381 3-1 (Lorentzen & Wettre GmbH) according to DIN 54518:2004-03. 10 strips per direction (cross direction) with a width of 15±0.1 mm were cut and measured to determine an average value (measured in kN/m). The compression strength index (cross direction) results from compression strength per unit width (expressed in Nm/g).

Preparation on a laboratory scale

Examples 1 to 3 a) Preparation of an outer layer (basis weight 60 g/m²)

100% deinked pulp (DIP), 17% ash content

40 g of DIP in 2 I of water were treated in a disintegrator for 20 min using 3000 rpm to obtain a suspension (consistency 2%), which was diluted with water to 10 I in a diffuser under stirring (consistency 0.4%). 471 ml of the resulting homogenous suspension were diluted to 5 I of water in a sheet former (consistency 0.04%) and treated with air. After allowing to stand the suspension was dewatered by suction. The wet fibrous layer had a dry matter content of approximately 35 wt%. b) Preparation of an intermediate layer (basis weight 120 g/m²) 100% unbleached sulfate pulp

The procedure of a) was repeated with the exception that 80 g of 100% unbleached sulfate pulp was used. The wet fibrous intermediate layer has a dry matter content of approximately 35 wt%. c) Preparation of the additive formulation I and II

Additive formulation I 3 wt% solution of NH4 HCO3 in tap water

Additive formulation II 3 wt% solution of (NH4)2 CO3 in tap water d) The intermediate layer and one outer layer were joined in wet state to form a layer composite. The intermediate layer of the layer composite was sprayed with the additive formulation I or II in different amounts, followed by joining with a second outer layer, prepared according to the procedure a).

Sprayed amount (applied quantity):

Example 1 1 .6% of NH4 HCO3, corresponding to 3.84 g/m²

Example 2 16.1% of NH4 HCO3, corresponding to 38.64 g/m²

Example 3 15.0% of (NH4)2 CO3, corresponding to 36 g/m²

The resulting layer composite was dried on a wire at 120°C for 10 min in a flow-through dryer and at 130°C for 10 min in a drying cabinet.

Comparative Example 1

The procedure of Example 1 was repeated with the exception that the layer was not sprayed.

The dried layer composites were analysed, and the results are listed in Table 1.

Table 1

Comparative Example 2

The procedure of Example 1 was repeated with the exception that 2.36 g of NH4 HCO3 (2.95%, corresponding to 7.08 g/m²) were mixed into the ingredients of the intermediate layer instead of spraying onto the composite.

Table 2

The incorporation of ammonium hydrogencarbonate in mass does not lead to an increase of the specific volume but leads to a densification of the paper.

Preparation on pilot scale Examples 4 to 7 a) Preparation of a cardboard having two layers (each layer having a basis weight of 65 g/m²) with corrugated board paper (100% wastepaper, PF Varel), cationic starch (Cargill 35844) and retention aid (Percol 540, cationic polyacrylamide).

The 2-layer cardboard was prepared using a test paper machine (pilot plant, at Papier- technische Stiftung in Heidenau), which spraying part is schematically shown in figure 1.

1 headbox 1 (fiber suspension for lower fibrous layer)

2 headbox 2 (fiber suspension for upper fibrous layer)

3 wire 1 (lower wire, Fourdrinier wire)

4 wire 2 (upper wire, Foudrinier wire)

5 spraying device, using a two-component jet (Schlick), ca. 15 cm before the couching line

6 running direction

7 press section

The following conditions and parameters were used:

Working width: 0.42 m

Working speed: 2 m/min

Headbox: central distributor tank (perforated roll with turbulance chamber, wiper and adjustable outlet slit)

Spraying width: 35 cm

Press section: Pick-up suction press

1^st press nip: up to 14 kN/m

2^nd press nip: up to 20 kN/m

Dryer section: 8 heat cylinders; oil heating T_max 140°C

Calender section: 2 rolls, pressure (max) 20 kN/m b) Preparation of the additive formulations

Comparative formulation I 2 wt% Acronal® Pure 2728 in tap water (dry basis)

Additive formulation III aqueous solution containing 3 wt% of (NH4)2 CO3 and 3 wt% of latex Acronal Pure 2728 (dry basis)

Additive formulation IV aqueous solution containing 6 wt% of (NH4)2 CO3 and 3 wt% of latex Acronal Pure 2728 (dry basis)

Additive formulation V aqueous solution containing 3 wt% of NH4 HCO3 and 3 wt% of latex Acronal Pure 2728 (dry basis)

Additive formulation VI aqueous solution containing 6 wt% of NH4 HCO3 and 3 wt% of latex Acronal Pure 2728 (dry basis) c) The pressure to open the jet valve and spraying the additive formulation was 1 bar. The spraying width was 35 cm. The layer on the lower wire was sprayed in wet state (45 wt% dry matter content) with the additive formulations and comparative formulations, resp., such to obtain the applied quantities, as mentioned in Table 3. The amount (volume) of each sprayed additive formulation was the same. In Comparative Example 4, the layer was only sprayed with the same amount (volume) of water.

Table 3

The evaluation of the dried cardboards did not consider 5 cm on each edge. The dried cardboards were cut to A3 and analysed, and the results are listed in Table 4. The error of measurement is ± 0.01.

Table 4

The specific volume of the Comparative Examples 5 and 6 are approximately the same but increases when ammonium carbonate or ammonium hydrogencarbonate is added.

Examples 8 to 10 a) Preparation of a cardboard having two layers (each layer having a basis weight of 65 g/m²) with pulp (made of 50% long fiber - pine/spruce (Stendal ECF) and 50% chopped fiber - birch (Soedra birch gold Z), cationic starch (Cargill 35844) and retention aid (Percol 540)). The 2- layer cardboard was prepared using a test paper machine (pilot plant, at Papiertechnische Stiftung in Heidenau), as described herein-before. b) Preparation of the additive formulations

Comparative formulation II 2 wt% Finnfix 10 in tap water

Additive formulation VII aqueous solution containing 3 wt% of NaHCOs, 8 wt% Na2H2P2O? and 3 wt% of Finnfix 10

Additive formulation VIII aqueous solution containing 3.75 wt% of NH4 HCO3 and 3 wt% of

Finnfix 10

Additive formulation IX aqueous solution containing 3 wt% of NH4 HCO3 and 3.75 wt% of

Finnfix 10 c) The layer on the lower wire was sprayed as described herein-before. The applied quantities are mentioned in Table 5. The amount (volume) of each sprayed additive formulation was the same. In Comparative Example 7, the layer was only sprayed with the same amount (volume) of water. The results of the analysed cardboards were listed in Table 6.

Table 5

Table 6

Claims

1. A process for manufacturing a multi-layer paper or cardboard, the process comprising: a) spraying onto a surface side of a first fibrous layer to be joined with a surface side of a second fibrous layer with an additive formulation; wherein the first and the second fibrous layer independently of one another have a dry matter content of from 25 to 65 wt%; and the additive formulation comprises a-1) water, and a-2) a salt containing a CCh²' or a HCOa' anion, and the amount of water is at least 75 wt%, based on the total weight of the additive formulation.

2. The process according to claim 1, the process further comprising: b) joining the first fibrous layer with the second fibrous layer to form a layer composite; c) dewatering the layer composite obtained in step b) by pressing; and d) drying the layer composite obtained in step c) to form a dried multi-layer paper or cardboard.

3. The process according to claim 1 or 2, wherein the salt containing a COa²' or a HCOa' anion contains a cation selected from an alkali metal cation, NH₄ ⁺ and any combination thereof.

4. The process according to claim 3, wherein the cation is NH₄ ⁺ or Na⁺, preferably NH₄ ⁺.

5. The process according to claim 1, 2 or 3, wherein the additive formulation comprises a-3) an organic polymer P.

6. The process according to claim 5, wherein the organic polymer P is a non-ionic or anionic polymer, preferably an anionic polymer.

7. The process according to claim 5 or 6, wherein the organic polymer P is selected from a synthetic polymer P1 comprising a unit derived from an ethylenically unsaturated monomer, a natural or modified natural polymer P2 based on a polysaccharide and any combination thereof.

8. The process according to claim 7, wherein the synthetic polymer P1 is obtainable by polymerizing at least one non-ionic monomer M 1 , selected from a vinyl monomer, an allyl monomer, a (meth)acrylic monomer, a diene monomer and any combination thereof, and optionally an anionic monomer M2.

9. The process according to claim 7, wherein the natural or modified natural polymer P2 is a derivative of cellulose.

10. The process according to any one of the claims 6 to 9, wherein the organic polymer P is added to the additive formulation as an aqueous dispersion.

11. The process according to any one of claims 6 to 10, wherein the organic polymer P has a viscosity of from 3 to 1000 mPa s, preferably 3 to 500 mPa s.

12. The process according to any one of the preceding claims, wherein the additive formulation comprises the salt containing a COa²' or a HCOa' anion in an amount of from 0.5 to 25 wt%, based on the total weight of the additive formulation, preferably 0.5 to 20 wt%.

13. The process according to any one of the preceding claims, wherein the additive formulation comprises the organic polymer P in an amount of from 0.1 to 10 wt%, based on the total weight of the additive formulation.

14. The process according to any one of the preceding claims, wherein the applied quantity of the additive formulation is of from 2 to 70 g/m², based on the solids content of the spraying solution or spraying suspension and based on the sprayed area.

15. A multi-layer paper or cardboard, obtainable or obtained by a process, as defined in any one the claims 1 to 14.

16. The use of a formulation comprising a-1) water, a-2) a salt containing a COa²' or a HCOa' anion, and a-3) optionally an organic polymer P, as defined in any one of claims 1 or 3 to 14, as a spraying additive in a process of manufacturing a multi-layer paper or cardboard.

17. A paper machine, equipped with a wire section containing a first wire and a second wire, a spraying device, a press section and a dryer section, said sections and device are arranged in the paper machine in the order of the wire section, followed by the spraying device, the press section and the dryer section, wherein the spraying device contains an additive formulation, as defined in any one of claims 1 or 3 to 14.