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) forming a first wet web; b) partially dewatering the first wet web; c) forming a second wet web; d) partially dewatering the second wet web; e) joining the first and second partially dewatered web to obtain a multilayer web; f) further dewatering and drying the multilayer web to obtain a paper substrate; and g) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper; wherein at least one of the partial dewatering in b) and the partial dewatering in d) comprises two-sided dewatering.

Description

[0001] METHOD FOR MANUFACTURING RESIN IMPREGNATED PAPER

[0002] Technical field

[0003] The present disclosure relates to methods for manufacturing a resin impregnated paper and more specifically to manufacturing paper substrates suitable for resin impregnation.

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

[0016] 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.

[0017] It is a further object of the present disclosure to provide a method for manufacturing a resin impregnated paper, which facilitates the use of recycled fibers in base paper for resin impregnation.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] Resin impregnated paper is typically manufactured by passing he 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.

[0022] 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.

[0023] 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.

[0024] The present inventors have found that many of the problems with base papers for resin impregnation can be eliminated or ameliorated by using a multilayer web forming technique, wherein the layers of the multilayer web are formed and dewatered separately.

[0025] 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) forming a first wet web by applying a first pulp suspension comprising cellulose fibers on a first wire, wherein at least 15 wt% of the cellulose fibers are recycled fibers; b) partially dewatering the first wet web to obtain a first partially dewatered web; c) forming a second wet web by applying a second pulp suspension comprising cellulose fibers on a second wire, wherein less than 10 wt% of the cellulose fibers are recycled fibers; d) partially dewatering the second wet web to obtain a second partially dewatered web; e) joining the first and second partially dewatered web to obtain a multilayer web; f) further dewatering and drying the multilayer web to obtain a paper substrate; and g) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper; wherein at least one of the partial dewatering in b) and the partial dewatering in d) comprises two-sided dewatering.

[0026] The manufacturing method involves the separate preparation and partial dewatering of two lower grammage wet webs. The partially dewatered but still wet webs are joined to form a higher grammage multilayer web, which is subsequently further dewatered and dried to obtain a paper substrate. The paper substrate is then subjected to resin impregnation to obtain a resin impregnated paper.

[0027] The forming, partial dewatering and joining steps of the inventive method are preferably performed at the forming section of the paper machine, commonly called the wet end. The wet webs are formed on different wires in the forming section of the paper machine. The preferred type of forming section for use with the present invention includes at least two Fourdrinier wire sections, combined with supporting wire. The pulp suspension is typically applied to the wire using a so called headbox.

[0028] The first and second pulp suspensions are aqueous suspensions comprising a water-suspended mixture of cellulose fibers and optionally other fibrous or non- fibrous materials, for example hemicellulose and lignin. The cellulose fibers can be produced from different raw materials, for example softwood pulp or hardwood pulp. The dry solids content of the first and / or second pulp suspension applied to the wire may be comprised solely of the cellulose fibers, or it can comprise a mixture of cellulose fibers and other ingredients or additives. The first and / or second pulp suspension preferably includes cellulose fibers as its main component based on the total dry weight of the pulp suspension. In some embodiments, the first and / or second pulp suspension comprises at least 50 wt%, preferably at least 70 wt%, more preferably at least 80 wt% or at least 90 wt% of cellulose fibers, based on the total dry weight of the pulp suspension.

[0029] In some embodiments, the cellulose fibers of the first and / or second pulp suspension is Kraft pulp. Kraft pulp will typically comprise at least 10 wt% hemicellulose. Thus, in some embodiments the first and / or second pulp suspension comprises hemicellulose in an amount of at least 10 wt%, such as in the range of 10-25 wt%, of the amount of the cellulose fibers. In some embodiments, the kraft pulp has a KAPPA number in the range of 10-100, preferably in the range of 30-90, and more preferably in the range of 40-85, as determined according to standard ISO 302. In some embodiments, the Kraft pulp is a never dried-kraft pulp, i.e. a kraft pulp that has not undergone any drying process after its initial production and washing stages.

[0030] The compositions of the first pulp suspension and the second pulp suspension differ at least in that the first pulp suspension has a higher content of recycled cellulose fibers than the second pulp suspension. Typically, the content of recycled cellulose fibers in the first pulp suspension is at least 2 times higher, at least 3 times higher, at least 4 times higher, or at least 5 times higher, than the content of recycled cellulose fibers in the second pulp suspension.

[0031] Recycled cellulose fibers in paper manufacturing refer to cellulose fibers that are reclaimed from previously used paper products rather than being derived from fresh wood pulp. These fibers come from various sources, including postconsumer waste (like used office paper, newspapers, and cardboard). The utilization of recycled cellulose fibers obtained from post-consumer waste may be associated with certain drawbacks, including variability in fiber composition and the presence of residual contaminants such as printing inks, adhesives, synthetic polymers, and other non-cellulosic materials, which may adversely impact the performance characteristics and suitability of the fibers for use in resin- impregnated paper applications. The present invention has been found especially useful for recycled cellulose fibers obtained from post-consumer waste The present inventors have found that by reducing the amount of “stickies” in the recycled fibers, the resin absorbency of a paper layer comprising the recycled fibers can be increased. The recycled cellulose fibers are typically obtained by collection and sorting of recyclable paper products, shredding and pulping the sorted paper, washing the pulp to remove inks, adhesives, and other contaminants from the pulp, and optionally refining and / or bleaching the cleaned pulp, in order to meet the target specification for use in resin impregnated paper. The recycled cellulose fibers thus preferably have a certain purity. This purity may be represented by the amount of stickies in the recycled cellulose. The amount (in wt%) of stickies in the recycled cellulose fibers of the first pulp suspension, and preferably also in the recycled cellulose fibers of the second pulp suspension, can be determined using solvent extraction with water and tetrahydrofuran (THF) as solvent as described in TAPPI T204. In some embodiments, the amount of stickies in the recycled cellulose fibers of the first pulp suspension, and preferably also in the recycled cellulose fibers of the second pulp suspension, should be less than 5 wt%, preferably less than 3 wt%, and more preferably less than 1 wt%. In some embodiments, the amount of stickies in the recycled cellulose fibers of the first pulp suspension, and preferably also in the recycled cellulose fibers of the second pulp suspension, should be less than 0.9 wt%, preferably less than 0.8 wt%, and more preferably less than 0.7 wt%.

[0032] Stickies, which originate from adhesives, polymeric coatings, and printing residues, can cause severe operational issues, including equipment fouling and defects in the final paper. The TAPPI T204 method using water and tetrahydrofuran (THF) allows precise quantification of stickies and serves as a reliable measure of fiber purification. Embodiments of the invention set strict upper limits on stickies content, ideally below 1 wt%, to achieve the integrity and functionality of the final product. This enables the use of recycled fibers without compromising paper strength, resin interaction, or surface quality.

[0033] Recycled cellulose fibers with a low content of stickies can be obtained from postconsumer waste by a combination of optimized collection, mechanical processing, and chemical treatment steps, well known in the art. Examples of operations include pre-sorting to minimize incoming contaminants, controlled repulpingto prevent over-fragmentation, screening and cleaning to physically remove stickies, washing and dispersion to disperse and wash out impurities, and chemical aids or solvents to enhance stickies detackification and removal.

[0034] In some embodiments, the recycled cellulose fibers come from post-consumer waste, and the amount of stickies in the recycled cellulose fibers of the first pulp suspension, and preferably also in the recycled cellulose fibers of the second pulp suspension, is less than 1 wt% as determined using solvent extraction with water and tetrahydrofuran (THF) as solvent as described in TAPPI T204.

[0035] The target is to have substantial removal of plastic and stickes in the recycled fiber fraction or post-consumer packaging waste (from barrier paperboard). In addition to the overall concentration of stickies, the size and distribution of these contaminants are crucial. The average specific stickies area (Sa) and average stickies area (D) provide measures of the nature of residual impurities. These parameters, determined by standardized testing protocols such as TAPPI T227 and DIN 34 EN ISO 5263, reflect how finely dispersed the stickies are within the fiber matrix. A lower Sa and D indicate finer and fewer stickies, correlating with better resin absorption and fewer localized weaknesses in the final laminated product. In some embodiments, the recycled fibers of the first pulp suspension when repulped according to DIN 34 EN ISO 5263 (Temp 40 °C, pulping time 20 min and velocity 60 000 rpm) have an average specific stickies area, Sa of less than 12 000 mm2 / kg and more preferably less than 7500 mm2 / kg and most preferably less than 5000 mm2 / kg. In these embodiments, the average stickies area, D, of the recycled fiber fraction is preferably less than 4 mm2, more preferably less than 2.5 mm2and most preferably less than 1.5 mm2according to TAPPI T227. Embodiments with Sa below 5000 mm2 / kg and D below 1.5 mm2are especially preferred, as they demonstrate superior fiber purity and reduced interference with the impregnation and curing processes. Sa is determined by the equation Sa = (A / M), where A is total area of stickies and M is total mass of oven dried of hand sheet. The average stickies area, D, is determined from equation D = Sa / Sn, where Sn is determined from equation Sn = N / M, where N is the total stickies count.

[0036] The average specific stickies area, Sa of stickies when determined for the first pulp suspension in the headbox is preferably less than 5000 mm2 / kg and more preferably less than 4000 mm2 / kg, less than 3500 and or even less than 3000 mm2 / kg as determined according to the TAPPI T227 standard.

[0037] By enabling the use of high-purity recycled fibers from post-consumer waste, the invention not only supports sustainable resource utilization but also maintains the mechanical and barrier properties required for technical-grade resin impregnated paper. The ability to incorporate significant proportions of recycled fibers, such as up to 100 wt% in some embodiments, while meeting strict cleanliness and performance criteria contributes to the circular economy.

[0038] At least 15 wt% of the cellulose fibers in the first pulp suspension are recycled fibers. In some embodiments, at least 20 wt%, preferably at least 30 wt%, and more preferably at least 40 wt%, of the cellulose fibers in the first pulp suspension are recycled fibers. In some embodiments, up to 100 wt%, up to 75 wt%, or up to 60 wt% of the cellulose fibers in the first pulp suspension are recycled fibers. In some embodiments, 15-100 wt%, 15-75 wt%, or 15-60 wt% of the cellulose fibers in the first pulp suspension are recycled fibers.

[0039] In addition to the recycled fibers, up to 30 wt% of the cellulose fibers of the first pulp suspension can be cellulose fibers obtained from broke. In some embodiments, up to 20 wt%, or up to 10 wt% of the cellulose fibers of the first pulp suspension are cellulose fibers obtained from broke. Less than 10 wt% of the cellulose fibers in the second pulp suspension are recycled fibers. In some embodiments, less than 7 wt%, preferably less than 5 wt%, of the cellulose fibers in the second pulp suspension are recycled fibers.

[0040] In some embodiments, the total amount of recycled fibers in the formed paper substrate is at least 10 wt%, preferably at least 15 wt%, and more preferably at least 20 wt% based on the total amount of fibers in the paper substrate.

[0041] In some embodiments, the dry solids content of the first and / or second pulp suspension applied to the wire is in the range of 0.1-1.5 wt%, preferably in the range of 0.15-1.2 wt%, and more preferably in the range of 0.2-1 wt%.

[0042] 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- 200 g / m2, more preferably in the range of 20-150 g / m2.

[0043] After being formed, the first and second wet web are partially dewatered. Dewatering of the webs on the wire may be performed using methods and equipment known in the art, examples include but are not limited to vacuum dewatering, dewatering foils, dewatering blades, frictionless dewatering, and top forming. Partial dewatering means that the dry solids content of the wet web is reduced compared to the dry solids content of the pulp suspension, but that the dewatered web still comprises a significant amount of water. In some embodiments, the dry solids content of the first and second partially dewatered web prior to the joining step is in the range of 5-15 wt%, and preferably in the range of 8-13 wt%. A dry solids content of the first and second partially dewatered web in this range has been found to be especially suitable for joining the first and second wet web into a multilayer web. In some embodiments, the dry solids content of one of the partially dewatered webs is in the range of 5-8 wt%, and the other one of the partially dewatered webs is in the range of 8-15 wt%.

[0044] In the inventive method, at least one of the partial dewatering in b) and the partial dewatering in d) comprises two-sided dewatering. Two-sided dewatering is a known concept in the field of papermaking and devices and systems for two-sided dewatering are commercially available. Devices and systems for two-sided dewatering generally includes upper and lower drainage units, each consisting of an array of adjustable drainage elements. The wire, carrying the wet web, passes between these units. The lower unit typically removes water through gravity and vacuum assistance, while the upper unit typically applies a controlled vacuum to extract water from the top side of the web. A complementary wire may in some cases be provided on top of the wet web, such the upper drainage units dewater the wet web through the complementary wire. Such arrangements may be referred to as twin wire arrangements or dual wire arrangements. The synchronization of the upper and lower units is controlled through an integrated management system, ensuring optimal dewatering efficiency and uniformity.

[0045] Using two-sided dewatering in at least one of the partial dewatering in b) and the partial dewatering in d) allows for improved formation and reduced two-sidedness in the formed base paper. The two-sided dewatering is useful both in the partial dewatering in b) and in the partial dewatering in d), but the effects of the two-sided dewatering are especially notable when applied on a wet web, wherein a high proportion of the cellulose fibers, such as at least 15 wt% of the cellulose fibers or more, are recycled fibers. Without wishing to be bound to any specific scientific theory, it is believed that the reason for this is the high levels of secondary and tertiary fines in the recycled material.

[0046] Thus, in some embodiments at least the partial dewatering in b) comprises two- sided dewatering.

[0047] In some embodiments, the partial dewatering in b) and the partial dewatering in d) both comprise two-sided dewatering.

[0048] In some embodiments, the two-sided dewatering uses a twin wire arrangement.

[0049] In two-sided dewatering, several operations are commonly employed to efficiently remove water from both sides of the wet web. These operations use a combination of mechanical and vacuum forces to achieve uniform and efficient dewatering of the wet web, which is important for improving paper quality and production efficiency.

[0050] In some embodiments, the dewatering uses one or more dewatering operations selected from the group consisting of vacuum dewatering, dewatering foils, dewatering blades, frictionless dewatering, top forming, pressing, extended nip pressing and shoe pressing.

[0051] Vacuum dewatering involves the use of vacuum boxes or slots located beneath the wire to remove water from the bottom side of the paper web. For two-sided dewatering, additional vacuum elements can also be placed above the web to extract water from the top side. These vacuum elements are carefully controlled to optimize water removal while maintaining the structural integrity of the wet web.

[0052] Dewatering foils are stationary dewatering elements positioned under the forming fabric. They create a suction effect as the fabric and web pass over them, enhancing water removal. In two-sided dewatering, the action of foils primarily affects the bottom side, but by altering the structure of the wet web and dewatering dynamics, they can also indirectly facilitate top-side dewatering.

[0053] Dewatering blades are, similar to foils, used to enhance the removal of water. They can be adjusted to apply specific pressure on the wet web, improving dewatering efficiency. While primarily impacting the bottom side, their use in two- sided dewatering helps balance moisture content across the web.

[0054] Frictionless dewatering refers to methods of removing water from the wet web with minimal mechanical contact or friction. Examples include air-based dewatering, ultrasonic dewatering, electro-dewatering and contactless vacuum dewatering.

[0055] Air-based dewatering uses air flows or air knives to remove water from the paper web. Air knives can effectively strip water from the surface of the web without contacting it, reducing the potential for fiber disruption or damage. Ultrasonic dewatering applies ultrasonic energy to the paper web to encourage water removal. This method can enhance the dewatering process by causing the water to move away from the fiber network, potentially reducing the need for mechanical pressing.

[0056] Electro-dewatering uses electric fields to encourage water molecules to move out of the paper web. This technique can accelerate dewatering by targeting the water specifically, without exerting physical force on the fibers themselves.

[0057] Contactless vacuum dewatering uses vacuum systems that remove water by creating a differential pressure without the need for the dewatering elements to physically touch the web. This can be achieved through carefully engineered vacuum boxes or slots positioned close to, but not touching, the web.

[0058] Top forming uses a complementary wire arranged on top of the paper web, allowing dewatering from both sides as the web passes through a series of rolls and dewatering elements. This method is particularly effective for two-sided dewatering because it physically enables direct access to both sides of the web.

[0059] Pressing is not exclusive to the forming section of the paper machine, but is an important part of the dewatering process that follows forming. In the context of two-sided dewatering, pressing can be optimized to enhance water removal evenly across the thickness of the web or paper, ensuring uniform density and moisture content.

[0060] Extended nip pressing uses specialized presses designed to provide a longer duration of pressure application on the web or paper, which can enhance dewatering efficiency. They are used in the press section, but they support the two-sided dewatering by promoting more uniform water removal.

[0061] Shoe presses provide an extended, flexible nip that adapts to the thickness of the web or paper, allowing for efficient and uniform dewatering across its width and thickness. They are used in the press section, but they support the two-sided dewatering by promoting more uniform water removal. In some embodiments, the two-sided dewatering uses one or more dewatering operations selected from the group consisting of vacuum dewatering, dewatering foils, dewatering blades, frictionless dewatering, and top forming.

[0062] The partially dewatered but still wet webs are joined to form a higher grammage multilayer web. In some embodiments, the grammage of the multilayer web based on dry weight is in the range of 50-400 g / m2, preferably in the range of 100-300 g / m2, more preferably in the range of 150-250 g / m2. The joining is preferably performed by wet lamination of the first and second partially dewatered web. In some embodiments, the dry solids content of the first and second partially dewatered web when they are joined is in the range of 5-15 wt%, and preferably in the range of 8-13 wt%. In some embodiments, the dry solids content of one of the partially dewatered webs is in the range of 5-8 wt%, and the other one of the partially dewatered webs is in the range of 8-15 wt%. When the pulp suspension is dewatered on the wire a visible boundary line will appear at a point where the web goes from having a reflective water layer to where this reflective layer disappears. This boundary line between the reflective and non-reflective web is referred to as the waterline. The waterline is indicative of a certain solids content of the web. The webs are preferably joined after the water line. Joining the webs while they are still wet ensures good adhesion between the layers. The joining can be achieved by applying one of the partially dewatered webs on top of the other. Joining of the partially dewatered webs may be improved by various additional operations. In some embodiments, the joining of the first and second partially dewatered web to obtain the multilayer web further comprises pressing the first and second partially dewatered webs together. In some embodiments, the joining of the first and second partially dewatered web to obtain the multilayer web further comprises applying suction to the formed multilayer web. Applying pressure and / or suction to the formed multilayer web improves adhesion between the web layers.

[0063] The dry solids content of the multilayer web is typically further increased during the joining step. The increase in dry solids content may be due to dewatering of the multilayer web on the wire with optional pressure and / or suction applied to the web, and also due to drying operations performed during or shortly after the joining, e.g. impingement drying or air or steam drying. The dry solids content of the multilayer web after joining, with optional application of pressure and / or suction, is typically above 8 wt% but below 30 wt%. In some embodiments, the dry solids content of the multilayer web prior to the further dewatering and drying step is in the range of 8-30 wt%, preferably in the range of 10-20 wt%, and more preferably in the range of 14-20 wt%.

[0064] In the further dewatering and drying step f), the dry solids content of the multilayer web is typically further increased. The resulting multilayer film preferably has a dry solids content above 90 wt%.

[0065] The further dewatering typically comprises pressing the web to squeeze out as much water as possible. The further dewatering may for example include passing the formed multilayer web through a press section of a paper machine, where the web passes between large rolls loaded under high pressure to squeeze out as much water as possible. The removed water is typically received by a fabric or felt. In some embodiments, the dry solids content of the multilayer film after the further dewatering is in the range of 15-48 wt%, preferably in the range of 18-40 wt%, and more preferably in the range of 22-35 wt%.

[0066] To even further enhance formation and reduce two-sidedness of the finished base paper, the further dewatering in f) may also be done using two-sided dewatering. Thus, in some embodiments the further dewatering in f) comprises two-sided dewatering.

[0067] The drying may for example include drying the multilayer web by passing the multilayer web around a series of heated drying cylinders. The drying may typically remove the water content down to a level of about 1-15 wt%, preferably to about 2-10 wt%, and more preferably to about 4-6 wt%,.

[0068] The dried web is referred to as the paper substrate. The obtained paper substrate is particularly well suited for resin impregnation. 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 100-300 g / m2, more preferably in the range of 150-250 g / m2.

[0069] 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.

[0070] 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.

[0071] In some embodiments, the paper substrate has a Bendtsen roughness of at least 500 ml / min, preferably at least 700 ml / min, but preferably less than 1500 ml / min, according to standard ISO 8791-2.

[0072] In some embodiments, the paper substrate has a Gurley porosity below 50 s / 100 ml measured according to standard SCAN-P19:78.

[0073] In some embodiments, the paper substrate has a specific formation below 1.6, preferably below 1.5, more preferably below 1.0, and more preferably below 0.7, as determined according to SCAN-P 92:09.

[0074] The paper substrate is subjected to resin impregnation to obtain a resin impregnated paper.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] Applications: Due to its enhanced properties, resin impregnated paper finds applications in a variety of fields:

[0079] 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.

[0080] 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.

[0081] Construction and Building Materials: Resin impregnated paper serves as a moisture barrier and adds strength to construction materials.

[0082] Packaging: Resin impregnated paper can be used for industrial packaging that requires durability and resistance to chemicals and moisture.

[0083] 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.

[0084] 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.

[0085] 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- 350 g / m2, more preferably in the range of 150-30 g / m2.

[0086] Generally, while the products, materials, plies, layers and processes are described in terms of “comprising” various components or steps, the products, polymers, materials, layers and processes can also “consist essentially of” or “consist of’ the various components and steps. It is further noted that the disclosure relates to all possible combinations of features, unless explicitly stated otherwise.

[0087] 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. EXAMPLES

[0088] Example 1 (Comparative) - Commercial single ply substrate

[0089] A commercially available single ply substrate made with 100% unbleached virgin kraft pulp (KAPPA number about 80). The structure and characteristics of the single ply substrate is set out in Table 1.

[0090] Example 2 - Two ply substrate with virgin kraft pulp in top ply and recycled fiber in back ply

[0091] A two ply substrate made with 100% unbleached virgin kraft pulp (KAPPA number about 80) in the top ply, and 20% recycled fiber, 10% broke, and 70% unbleached virgin kraft pulp in the back ply. The structure and characteristics of the two ply substrate is set out in Table 1. At about the same grammage as the commercial single ply substrate of Example 1 , the two ply substrate surprisingly exhibited improved strength properties despite the significant content of recycled fiber. Also, the two layer structure allowed for good density-wet strength ratio as well as not too large difference in Cobb60 between the top side and back side.

[0092] Table 1.

[0093] *MD = measured in the machine direction, CD = measured in the cross direction. Unless otherwise specified, parameters discussed in the present disclosure are measured according to the following standards: Grammage ISO 536 Density ISO 534 Wet Strength SCAN-P 20:95 Tensile strength ISO 1924-2 Cobb60 ISO 535

[0094] Ash content ISO 1762

[0095] Bendtsen roughness ISO 8791-2 Gurley porosity SCAN-P19:78 Specific formation SCAN-P 92:09 KAPPA number ISO 302

Claims

CLAIMS1 . A method for manufacturing a resin impregnated paper, the method comprising the steps of: a) forming a first wet web by applying a first pulp suspension comprising cellulose fibers on a first wire, wherein at least 15 wt% of the cellulose fibers are recycled fibers; b) partially dewatering the first wet web to obtain a first partially dewatered web; c) forming a second wet web by applying a second pulp suspension comprising cellulose fibers on a second wire, wherein less than 10 wt% of the cellulose fibers are recycled fibers; d) partially dewatering the second wet web to obtain a second partially dewatered web; e) joining the first and second partially dewatered web to obtain a multilayer web; f) further dewatering and drying the multilayer web to obtain a paper substrate; and g) subjecting the paper substrate to resin impregnation to obtain a resin impregnated paper; wherein at least one of the partial dewatering in b) and the partial dewatering in d) comprises two-sided dewatering.

2. The method according to claim 1 , wherein at least the partial dewatering in b) comprises two-sided dewatering.

3. The method according to any one of the preceding claims, wherein the further dewatering in f) comprises two-sided dewatering.

4. The method according to any one of the preceding claims, wherein the two- sided dewatering uses a twin wire arrangement.

5. The method according to any one of the preceding claims, wherein the two- sided dewatering uses one or more dewatering operations selected from the groupconsisting of vacuum dewatering, dewatering foils, dewatering blades, frictionless dewatering, and top forming.

6. The method according to any one of the preceding claims, wherein at least 20 wt%, preferably at least 30 wt%, and more preferably at least 40 wt%, of the cellulose fibers in the first pulp suspension are recycled fibers.

7. The method according to any one of the preceding claims, wherein less than 7 wt%, preferably less than 5 wt%, of the cellulose fibers in the second pulp suspension are recycled fibers.

8. The method according to any one of the preceding claims, wherein the recycled cellulose fibers come from post-consumer waste.

9. The method according to any one of the preceding claims, wherein the amount of stickies in the recycled cellulose fibers of the first pulp suspension is less than 1 wt% as determined using solvent extraction with water and tetrahydrofuran (THF) as solvent as described in TAPPI T204.

10. The method according to any one of the preceding claims, wherein the amount of stickies in the recycled cellulose fibers of the first pulp suspension, and preferably also in the recycled cellulose fibers of the second pulp suspension, is less than 1 wt% as determined using solvent extraction with water and tetrahydrofuran (THF) as solvent as described in TAPPI T204.

11. The method according to any one of the preceding claims, wherein 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 20-200 g / m2, more preferably in the range of 20-150 g / m2.

12. The method according to any one of the preceding claims, wherein the dry solids content of the multilayer web prior to the further dewatering and drying step is in the range of 8-30 wt%, preferably in the range of 10-20 wt%, and more preferably in the range of 14-20 wt%.

13. The method according to any one of the preceding claims, wherein 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.

14. The method according to any one of the preceding claims, wherein the paper substrate has a Bendtsen roughness of at least 500 ml / min, preferably at least 700 ml / min, according to standard ISO 8791-2.

15. 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 SCAN-P19:78.

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

17. 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.

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