Method for manufacturing resin impregnated paper

Abstract

The present invention relates to a method for manufacturing a resin impregnated paper, the method comprising the steps of: a) providing a lignin enriched pulp suspension; b) forming a wet web by applying the lignin enriched pulp suspension on a wire; c) dewatering and drying the wet web to obtain a paper substrate; and d) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper.

Description

[0001] METHOD FOR MANUFACTURING RESIN IMPREGNATED PAPER

[0002] Technical field

[0003] The present disclosure relates to methods for manufacturing a resin impregnated paper.

[0004] Resin impregnated papers find common applications in furniture and building construction, offering a robust and enduring surface. Resin impregnated paper is a material created by saturating or impregnating a base paper with a resin, typically a thermosetting resin. This process enhances the properties of the paper, making it more durable, moisture-resistant, and sometimes electrically insulating. It is also relatively cost-effective to produce, especially when compared to pure resin or other synthetic materials.

[0005] Several types of resin are commonly used for impregnating paper, each offering specific properties that make the final product suitable for various applications.

[0006] Some of the main types of resin used include:

[0007] Phenolic Resin: One of the most widely used resins for impregnating paper, phenolic resin provides excellent thermal stability, mechanical strength, and chemical resistance. It's particularly used in applications requiring high durability and resistance to heat and chemicals, such as in electrical laminates and industrial applications.

[0008] Epoxy Resin: Epoxy resin offers superior mechanical properties, excellent adhesion, and high resistance to moisture and chemicals. It is used in applications requiring strong bonding, high strength, and good insulating properties, including high-performance electrical insulation and protective coatings. Melamine Resin: Known for its excellent resistance to heat, moisture, and chemicals, melamine resin is used to impregnate paper for applications that demand these properties. Melamine-impregnated papers are often used in decorative laminates for countertops, cabinetry, and flooring, offering durability along with aesthetic appeal.

[0009] Polyester Resin: Polyester resins provide a good balance of mechanical properties and cost-effectiveness. They are used in various applications, including decorative laminates and insulation materials. Polyester-impregnated papers can offer good strength and durability, making them suitable for both industrial and consumer products.

[0010] Silicone Resin: Silicone resin impregnated paper is used in high-temperature applications due to its excellent thermal stability and electrical insulation properties. It's commonly used in electrical insulation, where resistance to high temperatures is crucial.

[0011] Polyurethane Resin: Polyurethane resin offers excellent toughness, flexibility, and resistance to abrasion and chemicals. It's used in applications that require durable, flexible materials, such as in some types of decorative laminates and specialty papers.

[0012] The base papers selected for resin impregnation are typically engineered to possess optimal permeability and fiber composition, facilitating rapid and uniform resin absorption. In order to minimize resin consumption, the base paper should preferably allow for minimal resin impregnation while still meeting the required properties of the impregnated product.

[0013] The problem with existing single ply base paper structures is that the permeability is difficult to control and the base paper may often exhibit poor formation, 2- sideness, etc. Also, conventional pressing and drying may cause densification of the base paper leading to uneven resin impregnation. Other challenges arise when high percentages of recycled fibers are used in the paper, mainly due to the high levels of secondary and tertiary fines in the recycled material. Therefore, there is a need for improved and alternative techniques for manufacturing a resin impregnated paper, which can eliminate or ameliorate at least some of the drawbacks of the existing techniques.

[0014] Description of the invention

[0015] It is an object of the present disclosure to provide a method for manufacturing a resin impregnated paper, which alleviates at least some of the problems associated with prior art methods.

[0016] The above-mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure, are achieved by the various aspects of the present disclosure.

[0017] According to a first aspect illustrated herein, there is provided a method for manufacturing a resin impregnated paper, the method comprising the steps of: a) providing a lignin enriched pulp suspension; b) forming a wet web by applying the lignin enriched pulp suspension on a wire; c) dewatering and drying the wet web to obtain a paper substrate; and d) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper.

[0018] Paper generally refers to a material manufactured in thin sheets or webs from the pulp of wood or other fibrous substances comprising cellulose fibers, used for writing, drawing, or printing on, or as a construction or packaging material.

[0019] Paperboard generally refers a to strong and / or stiff, thick paper. Paperboard is typically a multiply material comprised of two or more cellulose based plies. Paper can either be bleached or unbleached, coated or uncoated, and produced in a variety of thicknesses, depending on the end use requirements. Resin impregnated paper is a material created by saturating or impregnating paper with a resin, typically a thermosetting resin. This process enhances the properties of the paper, making it more durable, moisture-resistant, and sometimes electrically insulating.

[0020] Resin impregnated paper is typically manufactured by passing the paper, which can vary in thickness and type depending on the intended use, through a bath of liquid resin. The resin thoroughly saturates the paper. After impregnation, the resin-infused paper is dried and cured. The resin-infused paper may be either fully cured, or partially cured to a stage where the material is malleable and tacky, making it easier to handle and manipulate for further processing.

[0021] The resin impregnation significantly improves the physical and chemical properties of the paper. It becomes more robust, resistant to chemicals, moisture, and heat, and has improved dielectric strength, making it a better insulator. The specific properties depend on the type of resin used. Common resins include phenolic resins, epoxy resins, and melamine resins, but other resins such as polyester resins, silicone resins, and polyurethane resins are also used.

[0022] Resin impregnated paper is a versatile material that combines the benefits of paper, such as light weight and flexibility, with the enhanced properties provided by the resin, making it suitable for use in a wide range of applications in industrial, electrical, and consumer products.

[0023] A general problem in the manufacture of resin impregnated paper is how to achieve a rapid and uniform resin absorption.

[0024] The inventors have found that lignin or a lignin derivative can advantageously be used as a bio-friendly additive in the pulp suspension used for forming base papers for resin impregnation. Addition of lignin or a lignin derivative can also reduce the amount of resin required for resin impregnation by replacing a portion of the resin, and / or produce resin impregnated paper with higher performance. Replacing a portion of the resin with lignin or a lignin derivative can improve the sustainability of the product by increasing the renewable content and by binding more CO2 in the product. Without wishing to be bound to any specific scientific theory, it is also believed that the use of lignin in base paper for resin impregnation could facilitate the wetting of the base paper by the resin, and binding of the resin to the base paper.

[0025] Accordingly, the method of the present disclosure uses a lignin enriched pulp suspension for preparing the paper substrate. A pulp suspension is an aqueous suspension comprising a water-suspended mixture of cellulose based fibrous material and optionally non-fibrous additives used in the manufacture of paper or paperboard. The lignin enriched pulp suspension disclosed herein is generally a pulp suspension enriched with lignin or a lignin derivative. In other words, a pulp suspension to which an additional amount of lignin or a lignin derivative has been added.

[0026] In some embodiments, the step of providing the lignin enriched pulp suspension comprises: i) providing a pulp suspension, ii) adding lignin or a lignin derivative and optionally a foaming agent to the pulp suspension to obtain a lignin enriched pulp suspension, and iii) optionally foaming the lignin enriched pulp suspension.

[0027] In some embodiments, the lignin or lignin derivative is added in solubilized form. The solvent is preferably water. The term “solubilized” describes a state where a substance has been made soluble through specific means, especially when it would not be soluble under normal conditions.

[0028] In some embodiments, the lignin or lignin derivative is added in at least partially dissolved form. The term “partially dissolved” describes a state in the dissolution process where solubility limits or incomplete mixing results in both dissolved and undissolved solute coexisting. The term at “least partially dissolved” includes fully dissolved as well as partially dissolved. The lignin or lignin derivative may thus be added in the form of an aqueous lignin solution or an aqueous lignin dispersion. In some embodiments the aqueous lignin solution or aqueous lignin dispersion has a pH value above 8, preferably above 9, and more preferably above 10. In some embodiments, the lignin or lignin derivative is added in the form of an aqueous lignin dispersion. In some embodiments, the aqueous lignin dispersion has a solid content in the range of 5-50 wt%. The aqueous lignin dispersion is preferably prepared such that it is substantially free from large particles, such as particles having a diameter above 100 pm, preferably particles having a diameter above 50 pm, and more preferably particles having a diameter above 25 pm. Substantially free in this context means that less than 5 wt% of the total amount of lignin in the dispersion is present in the form of large particles. The particle size can for example be determined by fractionation using e.g. screens or membranes.

[0029] The added lignin or lignin derivative will be deposited on the surfaces of the cellulose fibers of the pulp. The deposition may occur in the pulp suspension before or during forming of the base paper. The added lignin or lignin derivative will thus be present between the cellulose fibers of the pulp when the base paper is formed. This differs from the native lignin which may in some cases exist in the pulp, which will form part of the cellulose fibers.

[0030] In some embodiments, the lignin or lignin derivative is selected from the group consisting of kraft lignin, organosolv lignin, soda lignin, lignosulfonate, or a combination of two or more thereof, preferably kraft lignin. These different types of lignin or lignin derivatives can be obtained by known methods readily available to the skilled person.

[0031] Kraft lignin can be precipitated from black liquor obtained from the kraft pulping process by adjusting pH. There are well known and commercially available methods for this, for example the LignoBoost method and the LignoForce method.

[0032] Organosolv lignin is obtained through the organosolv process, a method for separating lignin from cellulose and hemicellulose using an organic solvent or a mixture of solvents, often in combination with water.

[0033] Soda lignin is obtained through the soda pulping process, which uses an alkaline solution, typically sodium hydroxide (NaOH), as the pulping agent to dissolve lignin and some hemicellulose in biomass, leaving behind cellulose fibers. Lignosulfonates are obtained through the sulfite pulping process, while separating lignin from cellulose fibers using sulfite salts.

[0034] The different processes for obtaining lignin may also be combined with acidic or alkaline hydrolysis, or enzymatic degradation or oxidation, to further improve the efficiency of the processes or the quality of the obtained lignin.

[0035] Examples of commercially available lignins that can be used with the inventive method include, but are not limited to, Lineo (Stora Enso), BioChoise (Domtar), BioPiva (UPM Kymmene), Amallin (West Fraser), and Ecolig or LigFlow (Suzano).

[0036] In some embodiments, the lignin or lignin derivative is added to the pulp suspension in solubilized form. In other words, lignin or lignin derivative is first solubilized and then added to the pulp suspension in solubilized form. In some embodiments, the lignin or lignin derivative is added in solubilized form or in the form of an at least partial solution obtained by at least partially dissolving the lignin or a lignin derivative in an aqueous alkaline solvent, optionally combined with heating. The alkaline solvent may for example be an NaOH or KOH solution. The pH value of the alkaline solvent maybe at least 10, preferably at least 10.5. The heating may comprise heating to at least 50 °C, preferably to at least 60 °C.

[0037] In some embodiments, the lignin or lignin derivative is added to the pulp suspension and then solubilized or at least partially dissolved in the pulp suspension using an aqueous alkaline solvent, optionally combined with heating. The alkaline solvent may for example be an NaOH or KOH solution. The pH value of the alkaline solvent may be at least 10, preferably at least 10.5. The heating may comprise heating to at least 50 °C, preferably to at least 60 °C.

[0038] In some embodiments, the lignin enriched pulp suspension comprises 0.05-30 wt%, preferably 0.1-25 wt%, and more preferably 0.2-20 wt%, of lignin or a lignin derivative. In some embodiments the lignin or lignin derivative has a molecular weight in the range of 500-4000 Da. The molecular weight may be determined by gel permeation chromatography coupled with UV.

[0039] In some embodiments the lignin or lignin derivative has an inorganic content in the range of 1-20 wt% as determined according to ISO 9795:2023.

[0040] In some embodiments the lignin or lignin derivative has a sulfur content in the range of 0.5-5 wt%, such as in the range of 0.75-5 wt%, as determined according to SCAN-CM 57:99.

[0041] The pulp suspension is an aqueous suspension comprising a water-suspended mixture of cellulose fibers and optionally other fibrous or non-fibrous materials, for example hemicellulose. The pulp suspension may also comprise some lignin nonsolubilized lignin as part of the cellulose fibers. The pulp can be mechanical, chemical or semi-chemical pulp, obtained from softwood or hardwood, bleached or unbleached, virgin or recycled, or a mixture thereof. In some embodiments, the pulp suspension is a kraft pulp suspension, preferably an unbleached kraft pulp suspension.

[0042] In some cases, the dosing of lignin pulp suspension for base paper formation involves challenges including poor lignin retention, poor formation, uneven distribution and deposit formation. There is thus a need for improved methods enabling efficient incorporation of lignin in the pulp suspensions and resulting base paper.

[0043] In some embodiments, the lignin enriched pulp suspension provided in a) is foamed, i.e. the lignin enriched pulp suspension is provided in foam form. Providing the lignin enriched pulp suspension in foam form has several important advantages compared to providing the lignin enriched pulp suspension in liquid form, including but not limited to:

[0044] - Improved retention of the lignin or lignin derivative in the pulp suspension during dewatering of the wet web, leading a higher amount of lignin or lignin derivative remaining in the obtained paper substrate, and less lignin or lignin derivative being wasted.

[0045] - Improved distribution of the lignin or lignin derivative in the foamed pulp suspension and in the obtained wet web and paper substrate, leading to more even resin impregnation.

[0046] In more specific embodiments, the method for manufacturing a resin impregnated paper comprises the steps of: a) providing a foamed lignin enriched pulp suspension; b) forming a wet web by applying the foamed lignin enriched pulp suspension on a wire; c) dewatering and drying the wet web to obtain a paper substrate; and d) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper; wherein the step of providing the foamed lignin enriched pulp suspension comprises: i) providing a pulp suspension, ii) adding lignin or a lignin derivative and optionally a foaming agent to the pulp suspension to obtain a lignin enriched pulp suspension, and iii) foaming the lignin enriched pulp suspension.

[0047] The term foam or foamed, as used herein, refers to a liquid, preferably an aqueous liquid, comprising air or gas bubbles dispersed therein. Typically, the volume of gas is much larger than that of the liquid, with thin films separating gas pockets. Three requirements must be met in order for foam to form. Mechanical work is needed to increase the contact area between the gas and the liquid. This can occur by agitation, dispersing a large volume of gas into a liquid, or injecting a gas into a liquid. The second requirement is that a foaming agent, typically an amphiphilic substance, a surfactant or surface active component, must be present to decrease surface tension. Finally, the foam should form more quickly than it breaks down.

[0048] Foams may be open-cell or closed-cell in nature. Pores connect the gas regions in open-cell foams, while closed-cell foams have enclosed cells. The cells are usually disordered in their arrangement, with varying bubble sizes. The cells present minimal surface area, forming honeycomb shapes or tessellations.

[0049] In order for the lignin enriched pulp suspension to be provided in foamed form, the lignin enriched pulp suspension is preferably capable of being foamed to form a stable foam. By “stable foam” as used herein is meant that the foam of the foamed lignin enriched pulp suspension is sufficiently stable to remain in foam form at least until the wet web of the lignin enriched pulp suspension has been formed.

[0050] A foaming agent is required for foaming the lignin enriched pulp suspension. Thus, in some embodiments, the lignin enriched pulp suspension provided in a) comprises a foaming agent.

[0051] The foaming agent is a compound capable of forming and / or stabilizing a foam in the lignin enriched pulp suspension. The foaming agent is typically an amphiphilic substance, i.e. a chemical compound possessing both hydrophilic and hydrophobic (lipophilic) properties. A foaming agent reduces the work needed to create the foam by reducing the surface tension of the liquid and increases the colloidal stability of the foam by inhibiting coalescence of bubbles.

[0052] Lignin and lignin derivatives may act as foaming agents. Thus, in some embodiments the foaming agent is the lignin or lignin derivative of the lignin enriched pulp suspension. Lignin and lignin derivatives capable of acting as foaming agents may also be referred to as lignin based foaming agents. Thus, in some embodiments, the lignin enriched pulp suspension provided in a) comprises a lignin based foaming agent foaming agent. In some embodiments, the foaming agent is lignin or a lignin derivative obtained by at least partially dissolving the lignin or a lignin derivative in an aqueous alkaline solvent, optionally combined with heating.

[0053] In some embodiments, the lignin enriched pulp suspension provided in a) comprises a non-lignin based foaming agent. A non-lignin based foaming agent may for example be used where the lignin or lignin derivative of the lignin enriched pulp suspension is not capable of acting as foaming agents, or provide insufficient foaming.

[0054] The non-lignin based foaming agent is typically a surfactant, such as SDS, or a surface active polymer, or a combination thereof. In some embodiments, the non- lignin based foaming agent is a non-polymeric or polymeric surfactant, or a combination thereof. The surfactant may be anionic, non-ionic, or zwitter-ionic.

[0055] In some embodiments, the non-lignin based foaming agent is a non-polymeric surfactant, preferably sodium dodecyl sulfate (SDS).

[0056] The non-lignin based foaming agent is preferably an amphiphilic polymer, i.e. a polymer possessing both hydrophilic and hydrophobic (lipophilic) properties. In some embodiments, the non-lignin based foaming agent is water-soluble. The foaming agent may for example be a water-soluble polymer with hydrophobic moieties, such as a hydrophilic polymeric backbone provided with hydrophobic sidechains, or a block copolymer comprised of hydrophilic and hydrophobic sections.

[0057] In some embodiments, the non-lignin based foaming agent is selected from the group consisting of optionally hydrophobically modified polysaccharides, proteins, polyvinyl alcohol, polyvinyl acetate and mixtures thereof. The optional hydrophobic modification typically comprises one or more hydrophobic groups, e.g. alkyl groups, covalently attached to the non-lignin based foaming agent backbone.

[0058] In some embodiments, the non-lignin based foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of optionally hydrophobically modified cellulose, starch, hemicellulose and mixtures thereof.

[0059] In some embodiments, the non-lignin based foaming agent is an optionally hydrophobically modified polysaccharide selected from the group consisting of optionally hydrophobically modified cellulose acetate (CA), ethyl(hydroxyethyl)cellulose (EHEC), methylcellulose (MC), ethylcellulose (EC), hydroxyethylcellulose (HEC), hydroxypropylcellulose (HPC), sodium carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), sulfoethylcellulose, starch, and mixtures thereof.

[0060] In some embodiments, the non-lignin based foaming agent is selected from the group consisting of ethyl(hydroxyethyl)cellulose, hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC), hydroxyethylcellulose, hydrophobically modified hydroxyethyl cellulose (HM-HEC), methylcellulose (MC), hydrophobically modified methylcellulose (HM-MC), hydrophobically modified carboxymethylcellulose (HM-CMC), and hydrophobically modified starch (HM- starch). Examples of useful hydrophobically modified starch derivatives include, but are not limited to dialdehyde starch, hydroxypropylated starch, octenyl succinic anhydride (OSA) starch, and dodecyl succinic anhydride (DDSA) starch.

[0061] In some embodiments, the non-lignin based foaming agent is an optionally hydrophobically modified methyl cellulose.

[0062] In some embodiments, the non-lignin based foaming agent is a hydrophobically modified polyvinyl alcohol (PVOH), such as ethylene modified PVOH. In some embodiments, the non-lignin based foaming agent is a polyvinyl alcohol containing at least 2% acetate groups, more preferably at least 10% acetate groups, and even more preferably at least 15% acetate groups.

[0063] In some embodiments, the non-lignin based foamed lignin enriched pulp suspension comprises a polyvinyl alcohol (PVOH). The PVOH may be a single type of PVOH, or it can comprise a mixture of two or more types of PVOH, differing, e.g., in degree of hydrolysis or viscosity. The PVOH may for example have a degree of hydrolysis in the range of 50-99.9 mol%, preferably in the range of 80-99.9 mol%, and more preferably in the range of 88-99.9 mol%. In some embodiments, the PVOH has an average molecular weight Mw in the range of 15 000-150000 g / mol.

[0064] In some embodiments, the lignin enriched pulp suspension further comprises 0.1-5 wt%, based on dry weight, of microfibrillated cellulose (MFC) or highly refined cellulose having a having a Schopper-Riegler (SR) value in the range of 75-100, as determined by standard ISO 5267-1. The microfibrillated cellulose (MFC) or highly refined cellulose may preferably be added to the pulp suspension together with the lignin or lignin derivative, and may support foam forming and stabilization when the lignin enriched pulp suspension is foamed.

[0065] In some embodiments, the foamed lignin enriched pulp suspension comprises the foaming agent in an amount of 0.1-30 wt% based on the total dry weight of the foamed lignin enriched pulp suspension. In some embodiments, the foamed lignin enriched pulp suspension comprises the foaming agent in an amount of 0.1-10 wt% based on the total dry weight of the foamed lignin enriched pulp suspension.

[0066] In some embodiments, the lignin enriched pulp suspension provided in a) is a foamed lignin enriched pulp suspension.

[0067] The foamed lignin enriched pulp suspension may be prepared by shearing or mixing the lignin enriched pulp suspension into a foam using for example a high shear mixer or a foam generator. The foam is stabilized by the foaming agent. In some embodiments, the foaming is achieved using a foam generator. The lignin enriched pulp suspension may be pumped through a foam generator one or several times in order to reach a desired gas content or foam density. In some embodiments, the lignin enriched pulp suspension is pumped via a high shear mixer or refiner which generates the foam. Foam can be generated either offline or inline at the paper machine.

[0068] Typical air content of the foamed lignin enriched pulp suspension is in the range of 50-70 vol% based on the volume of the foam. The air bubbles prevent flocculation of fibers in the headbox. In some embodiments, the foam is brought to an air content of 60-70 vol% before forming the wet web by applying the foamed lignin enriched pulp suspension on the wire. The consistency of the lignin enriched pulp suspension being subjected to foaming may generally be in the range of 0.2-5 wt%, but as the foaming advantageously allows for higher solids, the consistency of the lignin enriched pulp suspension being subjected to foaming may preferably be in the range of 2-5 wt%. The forming of the wet web in b) may be done using conventional paper or paperboard forming techniques known in the art. Although different arrangements for performing the steps of the inventive method could be contemplated by the skilled person, the inventive method may advantageously be performed in a paper machine, more preferably in a Fourdrinier type paper machine, i.e. a paper machine based on based on the principles of the Fourdrinier Machine. The Fourdrinier type paper machine uses a moving dewatering fabric or woven mesh, commonly referred to as a “wire”, to create a continuous web by filtering out the fibers held in the cellulose pulp composition and producing a continuously moving wet web of fiber. This wet web is subsequently dried in the machine to produce paper or paperboard.

[0069] The forming and dewatering of the inventive method are preferably performed at the forming section of the paper machine, also commonly referred to as the wet end. The forming of the wet web in b) may for example comprise water forming or foam forming. Water forming means that the lignin enriched pulp suspension is applied on the wire in liquid suspension form, i.e. in unfoamed form. Foam forming means that the lignin enriched pulp suspension is foamed and then applied on the wire in foam form. When the lignin enriched pulp suspension is applied by water forming to the wire at a consistency in the range of 0.1-1.5 wt%, and more typically below 0.5 wt%. The consistency of the lignin enriched pulp suspension being subjected to foaming may generally be in the range of 0.2-5 wt%, but as the foaming advantageously allows for higher solids, the consistency of the lignin enriched pulp suspension being subjected to foaming may preferably be in the range of 2-5 wt%. When the lignin enriched pulp suspension is foamed, may generally applied by foam forming to the wire at a consistency in the range of 0.2-5 wt%, but it may preferably be applied by foam forming to the wire at a consistency in the range of 2-5 wt%. Thus, when foam forming is used, the pulp suspension may be applied to the wire at a consistency significantly higher than the 0.1-1.5 wt% typical for water forming. Applying the lignin enriched pulp suspension in foamed form has several important advantages compared to applying the lignin enriched pulp suspension in liquid form, including but not limited to:

[0070] - Improved retention of the lignin or lignin derivative in the pulp suspension during dewatering of the wet web, leading a higher amount of lignin or lignin derivative remaining in the obtained paper substrate, and less lignin or lignin derivative being wasted.

[0071] - Improved distribution of the lignin or lignin derivative in the foamed pulp suspension and in the obtained wet web and paper substrate, leading to more even resin impregnation.

[0072] Accordingly, in some embodiments the lignin enriched pulp suspension provided in a) is applied on the wire in foamed form.

[0073] In some embodiments, the foamed lignin enriched pulp suspension when applied to the wire has a foam density in the range of 150-600 kg / m3, preferably in the range of 250-500 kg / m3, and more preferably in the range of 300-450 kg / m3.

[0074] In some embodiments, the foamed lignin enriched pulp suspension when applied to the wire has an average bubble size below 150 pm, preferably below 100 pm.

[0075] The formed wet web is first subjected to dewatering on the wire. Dewatering on the wire may be one-sided or two-sided dewatering, for example using a hybrid former. Dewatering on the wire may be assisted by various dewatering devices such as blade, table and / or foil elements, suction boxes, friction less dewatering, ultra-sound assisted dewatering, couch rolls, or a dandy roll.

[0076] Further dewatering typically comprises pressing the wet web to squeeze out as much water as possible. The dewatering may for example include passing the formed wet web through a press section of the paper machine, where the wet web passes between large rolls loaded under high pressure to squeeze out as much water as possible. The press section may also include one or more extended nip presses or shoe presses. The removed water is typically received by a fabric or felt. In some embodiments, the dry solids content of the wet web after dewatering is in the range of 15-65 wt%, preferably in the range of 18-60 wt%, and more preferably in the range of 22-55 wt%.

[0077] The drying may for example include drying the dewatered wet web by passing the dewatered wet web around a series of heated drying cylinders. Drying may typically reduce the water content in the web down to a level of about 1-15 wt%, preferably to about 2-10 wt%.

[0078] The dried web is referred to as the paper substrate. The obtained paper substrate is particularly well suited for resin impregnation.

[0079] In some embodiments, the grammage of the paper substrate based on dry weight is in the range of 50-400 g / m2, preferably in the range of 100-400 g / m2, and more preferably in the range of 150-400 g / m2.

[0080] In some embodiments, the paper substrate has a density in the range of 400-950 kg / m3, preferably in the range of 500-900 kg / m3, and more preferably in the range of 600-900 kg / m3.

[0081] In some embodiments, the paper substrate has a wet strength in the machine direction (MD) of at least 0.35 kN / m preferably at least 0.5 kN / m, and more preferably at least 0.6 kN / m, as determined according to SCAN-P 20:95.

[0082] In some embodiments, the paper substrate has a Bendtsen roughness of at least 600 ml / min, preferably at least 700 ml / min, according to standard ISO 8791-2.

[0083] In some embodiments, the paper substrate has a Gurley porosity below 50 s / 100 ml measured according to standard SCAN-P19:78.

[0084] In some embodiments, the paper substrate has a specific formation below 1.6 gA0.5 / m, preferably below 1.5 gA0.5 / m, more preferably below 1.0 gA0.5 / m, and more preferably below 0.7 gA0.5 / m, as determined according to SCAN-P 92:09. In some embodiments, particularly wherein the lignin or a lignin derivative is added in solubilized form or in the form of an at least partial solution obtained by at least partially dissolving the lignin or a lignin derivative in an aqueous alkaline solvent, the paper substrate has a high surface pH. In some embodiments, the surface pH of the paper substrate is above 7, preferably above 8, more preferably above 8.5 or above 9. This high surface pH is believed to facilitate the subsequent resin impregnation.

[0085] The inventive method may be used to manufacture a single ply paper substrate or a multiply substrate. In a multiply paper substrate at least one of the plies is formed using a lignin enriched pulp suspension. A multiply paper substrate may be prepared using methods known in the art using one or more headboxes, and one or more wires, respectively. In one example, a multiply paper substrate may be formed using two or more headboxes or a multilayer headbox on a single wire. In another example, the plies of a multiply paper substrate may be formed separately on two or more separate wires with associated headboxes, and then laminated to form the multiply paper substrate. When a multiply paper substrate is manufactured, the method for manufacturing a resin impregnated paper may for example comprise the steps of: a) providing a lignin enriched first pulp suspension; b) forming a first wet web by applying and partially dewatering the first pulp suspension on a first wire; c) providing an optionally lignin enriched second pulp suspension; d) forming a second wet web by applying and partially dewatering the second pulp suspension on a second wire; e) joining the first and second wet web to obtain a multilayer web; f) further dewatering and drying the wet web to obtain a multiply paper substrate; and g) subjecting the multiply paper substrate to resin impregnation to obtain a resin impregnated paper.

[0086] In some embodiments, the second pulp suspension is also lignin enriched. In some embodiments, the grammage of the first and / or second wet web based on the total dry weight of the web is less than 200 g / m2, preferably in the range of 20- 150 g / m2, more preferably in the range of 20-100 g / m2.

[0087] In some embodiments, two or more paper substrates, wherein at least one of the paper substrates is obtained by the inventive methods are combined into a paper substrate laminate before being subjected to resin impregnation to obtain a resin impregnated paper.

[0088] The paper substrate is subjected to resin impregnation to obtain a resin impregnated paper.

[0089] Resin impregnation is done by saturating or impregnating the paper substrate with a resin, typically a thermosetting resin. This process enhances the properties of the paper, making it more durable, moisture-resistant, and sometimes electrically insulating.

[0090] During impregnation, the paper substrate is typically passed through a bath of liquid resin. The resin thoroughly saturates the paper. After impregnation, the resin-infused paper is dried and cured. The resin-infused paper may be either fully cured, or partially cured to a stage where the material is malleable and tacky, making it easier to handle and manipulate for further processing.

[0091] The resin impregnation significantly improves the physical and chemical properties of the paper. It becomes more robust, resistant to chemicals, moisture, and heat, and has improved dielectric strength, making it a better insulator. The specific properties depend on the type of resin used. Common resins include phenolic resins, epoxy resins, and melamine resins, but other resins such as polyester resins, silicone resins, and polyurethane resins are also used.

[0092] In some embodiments, an external crosslinking agent may be added to the resin formulation prior to, during, or after the impregnation process. The use of an external crosslinker can enhance the crosslink density of the cured resin network, thereby further improving the mechanical strength, thermal stability, and chemical resistance of the final resin-impregnated paper. The crosslinking agent may be selected based on compatibility with the resin system in use. For example, amine or anhydride crosslinkers may be employed with epoxy resins, melamine or isocyanate crosslinkers may be used with acrylic or polyester resins, and acid catalysts may be utilized to promote crosslinking in melamine-formaldehyde or urea-formaldehyde systems. The concentration and type of crosslinker can be tailored to achieve desired curing characteristics and performance profiles. In some cases, the inclusion of a crosslinker also enables partial curing of the resin to produce a flexible yet tacky intermediate product, facilitating handling, storage, and subsequent lamination or molding processes.

[0093] Applications: Due to its enhanced properties, resin impregnated paper finds applications in a variety of fields:

[0094] Electrical Engineering: Resin impregnated paper is used in electrical insulation applications, such as in transformers, capacitors, and other electrical components, where high dielectric strength and thermal stability are required.

[0095] Laminates: When multiple layers of resin impregnated paper are pressed and cured together, they form laminates. These laminates are used in countertops, flooring, furniture, and as decorative surfaces, offering durability and resistance to wear and tear.

[0096] Construction and Building Materials: Resin impregnated paper serves as a moisture barrier and adds strength to construction materials.

[0097] Packaging: Resin impregnated paper can be used for industrial packaging that requires durability and resistance to chemicals and moisture.

[0098] Resin impregnated paper offers several benefits, including improved strength and durability, resistance to moisture and chemicals, electrical insulation properties, and versatility in applications. It's also relatively cost-effective to produce, especially when compared to pure resin or other synthetic materials. In some embodiments, the resin impregnation comprises impregnating the paper substrate with a water-based resin.

[0099] In some embodiments, the resin impregnation comprises impregnating the paper substrate with a resin selected from the group consisting of phenolic resins, epoxy resins, melamine resins, polyester resins, silicone resins, and polyurethane resins, preferably wherein the resin is a phenolic resin. In some embodiments, the phenolic resin further comprises lignin or a lignin derivative. The lignin may advantageously be used to replace a portion, preferably 1-50 wt%, of the phenol in order to increase the amount of renewable material in the phenolic resin. The lignin in the phenolic resin may for example be solubilized by NaOH.

[0100] In some embodiments, the grammage of the obtained resin impregnated paper based on dry weight is in the range of 50-400 g / m2, preferably in the range of 100- 400 g / m2, more preferably in the range of 150-400 g / m2.

[0101] According to a second aspect illustrated herein, there is provided the use of a lignin enriched pulp suspension in the manufacture of a resin impregnated paper.

[0102] The second aspect may generally have the same features and advantages as the first aspect. It is further noted that the disclosure relates to all possible combinations of features, unless explicitly stated otherwise.

[0103] Generally, while the products, polymers, materials, plies, layers and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, plies, layers and processes can also “consist essentially of” or “consist of” the various components and steps.

[0104] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

[0105] EXAMPLES

[0106] 3 trials were run on a pilot papermaking machine with a softwood kraft pulp suspension. Targeted grammage of the formed paper substrates was 100 g / m2.

[0107] To the softwood kraft pulp suspension 2 kg / tn of sodium dodecyl sulfate (SDS) was added as a debonder and / or wetting agent. A paper substrate was prepared from the mixture on the pilot papermaking machine. The results are presented in Table 1.

[0108] Example 2 - Preparation of paper substrate from lignin enriched pulp suspension Lignin was partially dissolved in a NaOH solution at pH 10 at a temperature of 50 °C. To the softwood kraft pulp suspension the partially dissolved kraft lignin was added in an amount corresponding to 20 wt% of kraft lignin based on the total dry weight of the pulp suspension. The lignin was then precipitated onto the fibers of the pulp by reducing the pH of the pulp suspension to pH 3. A paper substrate was prepared from the mixture in the same manner as in Example 1.

[0109] The results are presented in Table 1. The properties of the obtained paper substrate were on same level as in Example 1 , but as the pH of the pulp suspension was below 3, the pH of fiber substrate is also expected to be low.

[0110] The example was also repeated with the partially dissolved kraft lignin being added in an amount corresponding to 5 wt% of kraft lignin based on the total dry weight of the pulp suspension. The results wer roughly the same as the results for Example 2 in Table 1. Example 3 - Preparation of paper substrate from foamed lignin enriched pulp suspension

[0111] Kraft lignin was partially dissolved in a NaOH solution at pH 10 at a temperature of 50 °C. To the softwood kraft pulp suspension the partially dissolved kraft lignin was added in an amount corresponding to 20 wt% of kraft lignin based on the total dry weight of the pulp suspension. The mixture was foamed using a foam generator and a paper substrate was prepared from the foamed mixture in the same manner as in Example 1. The foamed mixture gave good runnability on the pilot paper machine and density and permeability in the target, despite that the lignin content was 20 wt% based on fiber amount.

[0112] The results are presented in Table 1. The changes in brightness and opacity confirm that lignin retention was more efficient with foam forming technology than by precipitation.

[0113] Table 1.

[0114] Gurley porosity was measured according to standard ISO 5636-5. Tensile strength, ratio (MD / CD) was measured according to ISO 1924. Brightness R457 0 / 2° + UV was measured according to ISO 2470-1. Opacity was measured according to ISO 2471.

Claims

CLAIMS1 . A method for manufacturing a resin impregnated paper, the method comprising the steps of: a) providing a lignin enriched pulp suspension; b) forming a wet web by applying the lignin enriched pulp suspension on a wire; c) dewatering and drying the wet web to obtain a paper substrate; and d) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper.

2. The method according to claim 1 , wherein the lignin enriched pulp suspension is a pulp suspension enriched with lignin or a lignin derivative.

3. The method according to any one of the preceding claims, wherein the step of providing the lignin enriched pulp suspension comprises: i) providing a pulp suspension, ii) adding lignin or a lignin derivative and optionally a foaming agent to the pulp suspension to obtain a lignin enriched pulp suspension, and iii) optionally foaming the lignin enriched pulp suspension.

4. The method according to claim 3, wherein the lignin or lignin derivative is added in at least partially dissolved form.

5. The method according to claim 3, wherein the lignin or a lignin derivative is added in the form of an at least partial solution obtained by at least partially dissolving the lignin or a lignin derivative in an aqueous alkaline solvent, optionally combined with heating.

6. The method according to any one of the preceding claims, wherein the lignin enriched pulp suspension comprises 0.05-30 wt%, preferably 0.1-25 wt%, and more preferably 0.2-20 wt%, of lignin or a lignin derivative.

7. The method according to any one of the preceding claims, wherein the lignin or lignin derivative is selected from the group consisting of kraft lignin, organosolv lignin, soda lignin, lignosulfonate, or a combination of two or more thereof, preferably kraft lignin.

8. The method according to any one of the preceding claims, wherein the lignin enriched pulp suspension provided in a) comprises a foaming agent.

9. The method according to any one of the preceding claims, wherein the lignin enriched pulp suspension provided in a) is a foamed lignin enriched pulp suspension.

10. The method according to claim 9, wherein the foamed lignin enriched pulp suspension when applied to the wire has a foam density in the range of 150-600 kg / m3, preferably in the range of 250-500 kg / m3, and more preferably in the range of 300-450 kg / m3.

11. The method according to any one of claims 9-10, wherein the foamed lignin enriched pulp suspension when applied to the wire has an average bubble size below 150 pm, preferably below 100 pm.

12. The method according to any one of the preceding claims, wherein the paper substrate has a Gurley porosity below 50 s / 100 ml measured according to standard ISO 5636-5.

13. The method according to any one of the preceding claims, wherein the paper substrate has a specific formation below 1.6 gA0.5 / m, preferably below 1.5 gA0.5 / m, more preferably below 1.0 gA0.5 / m, and more preferably below 0.7 gA0.5 / m, as determined according to SCAN-P 92:09.

14. The method according to any one of the preceding claims, wherein the resin impregnation comprises impregnating the paper substrate with a water-based resin.

15. The method according to any one of the preceding claims, wherein the resin impregnation comprises impregnating the paper substrate with a resin selected from the group consisting of phenolic resins, epoxy resins, melamine resins, polyester resins, silicone resins, and polyurethane resins, preferably wherein the resin is a phenolic resin.

16. Use of a lignin enriched pulp suspension in the manufacture of a resin impregnated paper.

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