Laboratory papermaking method of multilayer paper

Abstract

The application discloses a laboratory papermaking method of multilayer paper, comprising the following steps: providing a first pulp and a second pulp and respectively performing beating treatment; performing forming and dewatering on the first pulp to a first dryness, and then peeling to form a core layer wet paper web; performing forming and dewatering on the second pulp to a second dryness to form a bottom layer wet paper web retained on a forming wire, wherein the second dryness is less than the first dryness; spraying an interlayer bonding agent in atomized form on the bottom layer wet paper web, and attaching the core layer wet paper web on the bottom layer wet paper web to form a multilayer wet paper web; performing press dewatering and drying on the multilayer wet paper web to obtain a multilayer paper sheet, and realizing the laboratory wet-state superposition of the multilayer paper through asymmetric dryness control and atomized sizing, so that the corresponding relationship between the laboratory small test and the production machine table product is established, and the laboratory small test can be used as the basis of the production machine table pilot test.

Description

Technical Field

[0001] This invention relates to a laboratory papermaking method for multi-layered paper. Background Technology

[0002] Multilayer paper (such as white cardboard, grey-backed white cardboard, coated white cardboard, and other packaging papers) typically consists of multiple layers of pulp, including a face layer, a core layer, and a bottom layer. The core layer uses chemimechanical pulp to provide bulk, stiffness, and reduce costs, while the face and bottom layers use chemical pulp to provide strength and surface properties. During the product development of multilayer paper, it is necessary to evaluate the impact of process parameters such as pulp ratio and beating degree on product performance in order to select the optimal raw material combination and process conditions.

[0003] In the industry, laboratory hand-made sheets are commonly used to validate and evaluate pulp. For the evaluation of multilayer paper, the conventional approach is to mix the pulps of each layer in a specific ratio and then make single-layer hand-made sheets to approximate the multilayer product, or to make sheets of each layer separately, dry them, and then glue them together with adhesive. However, the validation capability of single-layer hand-made sheets is insufficient: single-layer mixed sheets lose the layered structure of multilayer paper, resulting in systematically lower measured values ​​for properties such as bulk and stiffness, and making it impossible to distinguish the quality of different core layer pulps; dry lamination causes the interlayer bonding to degenerate into an adhesive mechanism, which differs from the bonding mechanism of multilayer paper produced on production lines. The above methods are all difficult to correlate with products produced on production lines and cannot serve as a basis for pilot-scale production. Therefore, the research and development of multilayer paper still requires extensive validation on production lines, additionally occupying production space.

[0004] Therefore, a new technical solution is needed to solve the above-mentioned technical problems. Summary of the Invention

[0005] Therefore, the present invention provides a laboratory papermaking method for multi-layer paper to solve the above-mentioned technical problems.

[0006] A laboratory papermaking method for multi-layered paper includes the following steps: S1: Provide a first slurry and a second slurry, and perform pulping treatment on the first slurry and the second slurry respectively; S2: The first pulp is shaped and dewatered to a first degree of dryness on the sheet forming machine to form a core layer wet paper web, and the core layer wet paper web is peeled off from the forming wire, wherein the first degree of dryness is 20% to 30%; S3: The second pulp is shaped and dehydrated to a second degree of dryness to form a bottom wet paper web retained on the forming wire. The second degree of dryness is 15% to 20%, and the second degree of dryness is less than the first degree of dryness. S4: Spray interlayer binder onto the bottom wet paper web in the form of atomization, and attach the core wet paper web onto the bottom wet paper web to form a multi-layer wet paper web; S5: Press, dehydrate, and dry the multi-layer wet paper web to obtain multi-layer paper sheets.

[0007] Furthermore, step S1 also includes providing a third pulp and beating the third pulp; in step S3, the third pulp is shaped and dehydrated to a second dryness to form a surface wet paper web retained on the forming wire; in step S4, an interlayer binder is sprayed in an atomized form on the side of the core wet paper web away from the bottom layer to attach the surface wet paper web to the core wet paper web, forming a multilayer wet paper web with at least three layers.

[0008] Furthermore, the first pulp is chemimechanical pulp and / or waste paper pulp; the second pulp and the third pulp are bleached or unbleached chemical pulps, and the multilayer paper sheet is cardboard sheet.

[0009] Furthermore, the first slurry accounts for 30% to 40% of the total slurry volume, the second slurry accounts for 30% to 35%, and the third slurry accounts for 30% to 35%, and the sum of the three is 100%.

[0010] Furthermore, the first pulp is pulped to a freeness of 25-30°SR, the second pulp is pulped to a freeness of 30-40°SR, and the third pulp is pulped to a freeness of 35-45°SR.

[0011] Further, in step S2, an absorbent medium is laid on the wet core paper web, and a light pressure of 5 to 20 kgf is applied for dehydration, and the pressure is maintained for 30 seconds to 2 minutes.

[0012] Further, in step S4, an interlayer binder is sprayed onto the non-wire surface of the bottom wet paper web; the interlayer binder is at least one of starch, polyvinyl alcohol, polyamide-epoxychloropropane, and carboxymethyl cellulose, or a mixture thereof.

[0013] Furthermore, the interlayer binder is a gelatinized starch aqueous solution with a mass fraction of 1% to 3%; the average particle size of the droplets formed by the atomized spray is 10 to 100 μm.

[0014] Furthermore, in step S5, absorbent medium is laid on both sides of the multi-layer wet paper web, and after preliminary dehydration and shaping to a dryness of 20% to 25%, a two-stage pneumatic constant pressure pressing is performed.

[0015] Furthermore, during the two-stage pressing, the pressing pressure is 400–500 kgf, the holding time is 1–5 minutes, and the dryness after pressing reaches 35%–45%.

[0016] Beneficial Effects: This invention discloses a laboratory papermaking method for multi-layer paper, comprising: providing a first pulp and a second pulp and beating them separately; shaping and dewatering the first pulp to a first degree of dryness on a sheet forming machine, and then peeling it off to form a core layer wet paper web; shaping and dewatering the second pulp to a second degree of dryness to form a bottom layer wet paper web retained on the forming wire, wherein the second degree of dryness is less than the first degree of dryness; spraying an interlayer binder onto the bottom layer wet paper web in an atomized form, and attaching the core layer wet paper web onto the bottom layer wet paper web to form a multi-layer wet paper web; pressing, dewatering, and drying the multi-layer wet paper web to obtain a multi-layer paper sheet, specifically achieving the following effects: I. By layering and stacking the layers one by one, the independent fiber ratio and beating degree of each layer are fully preserved, so that each layer can establish a one-to-one correspondence with the corresponding layer of the production machine, forming a structure that roughly corresponds to the multi-layer product of the production machine. Existing single-layer mixed sheeting cannot establish this correspondence because it loses the layered structure and the dry stacking is degraded into an adhesive mechanism due to the interlayer bonding. Therefore, the method of this invention can serve as the basis for pilot production of production machines, reducing the number of trials, lowering development costs, and shortening the development cycle.

[0017] Second, by controlling the dryness of the core wet paper web to 20%–30% and the dryness of the bottom and top wet paper webs to 15%–20%, an asymmetric dryness difference is formed, which solves the technical problem that multi-layer wet paper webs are difficult to stack due to insufficient strength in the laboratory, and enables multi-layer structures to be reliably formed on a laboratory scale.

[0018] Third, by spraying interlayer binder onto the wet paper web in an atomized form, the sizing is applied in a non-contact manner, avoiding the damage to the fragile wet paper web caused by contact sizing methods such as brushing and roller coating. This ensures that the interlayer binder is applied evenly, enhances the interlayer bond between the core layer and the bottom and top layers, and prevents delamination of multiple layers of paper.

[0019] Fourth, by keeping each layer of wet paper web in a wet or semi-dry state when stacked, the forming mechanism of the wet part of the production machine is simulated, so that the interlayer bonding depends on fiber interweaving and hydrogen bonding, rather than the adhesive between dry paper sheets. The resulting multi-layer paper sheet has the same interlayer bonding mechanism as the production machine.

[0020] Fifth, the method of the present invention only requires a laboratory sheeter, a spraying device and a flat press. It has low equipment investment, simple operation and short cycle. It can quickly and cost-effectively evaluate process parameters such as pulp ratio and beating degree on a laboratory scale. It overcomes the shortcomings of the prior art, which can only be approximated by single-layer mixing sheet making or requires additional production equipment for verification. Detailed Implementation

[0021] This invention provides a laboratory papermaking method for multi-layered paper, which uses a laboratory papermaking machine to reproduce the layered structure of multi-layered paper, establishing a correspondence between laboratory pilot-scale tests and production machine products, and serving as the basis for pilot-scale production.

[0022] The applicant's research found that the bulk and stiffness of multilayer paper are mainly determined by its layered structure of a high-density surface layer and a low-density core layer. When the pulps of each layer are mixed and formed into a single layer, the contribution of the core layer mechanical pulp to bulk and stiffness is diluted by the surface layer chemical pulp, and the layered structure disappears. This results in a systematically low measurement of bulk and stiffness, making it impossible to distinguish the quality of different core layer pulps. In other words, even the basic function of pulp screening is difficult to achieve in single-layer mixed papermaking.

[0023] Further research revealed that the interlayer bonding of multilayer paper depends on the interweaving and hydrogen bonding of fibers in the wet state. Therefore, only by keeping the wet paper webs in a wet or semi-dry state when stacking them can the forming mechanism of the wet section of the production machine be better simulated; while drying the layers and then pasting them with adhesives degenerates the interlayer bonding into an adhesive mechanism, which is fundamentally different from the bonding mechanism of multilayer paper on the production machine.

[0024] Further research revealed that the difficulty in wet lamination lies in the insufficient strength of the wet paper web, making it difficult to handle. By controlling the dryness of the core layer wet paper web to 20%–30%, it achieves sufficient wet strength to peel completely from the forming wire and maintain its self-holding; by controlling the dryness of the bottom layer wet paper web to 15%–20%, it remains moist, allowing the fibers to fully swell and conform to the shape of the paper; and by employing atomized non-contact sizing to avoid damaging the fragile wet paper web with contact sizing, the aforementioned problems were solved.

[0025] It is understood that the multi-layered paper is cardboard. It is understood that the cardboard can be white cardboard, gray-backed white cardboard, coated white cardboard, or other packaging paper types, with a typical structure in which the surface and bottom layers are made of chemical pulp and the core layer is made of chemimechanical pulp.

[0026] Specifically, the preparation method includes the following steps: S1: Provide a first slurry and a second slurry, and perform pulping treatment on the first slurry and the second slurry respectively.

[0027] The first pulp is used to form the core layer, which provides basic bulk and stiffness. It is understood that the first pulp can be chemimechanical pulp and / or waste paper pulp, and the core layer typically has a high basis weight in multilayer paper to fully utilize its contribution to bulk and stiffness.

[0028] The second pulp is used to form the underlayer, which provides tensile and tear strength. It is understood that the second pulp is preferably a chemical pulp, and more specifically, it can be a bleached or unbleached chemical pulp, such as bleached sulfate softwood pulp, bleached sulfate hardwood pulp, bleached bamboo pulp, etc.

[0029] Understandably, the first pulp and the second pulp are beating separately, that is, the core pulp and the bottom pulp are beating independently, so as to control the degree of beating of each layer and give each layer of pulp a differentiated beating process, thereby truly simulating the differentiated design of each layer of industrial multilayer paper.

[0030] Furthermore, the first pulp is beaten to a freeness of 25-30°SR to retain fiber length and provide bulk; the second pulp is beaten to a freeness of 30-40°SR to form sufficient tensile strength and tear strength.

[0031] Furthermore, step S1 also includes providing a third pulp and subjecting the third pulp to pulping treatment. The third pulp is used to form a surface layer, which serves as a printing and contact layer and requires certain strength, fineness, and printability. It is understood that the third pulp is preferably a chemical pulp, more specifically, it can be a bleached or unbleached chemical pulp, such as bleached sulfate softwood pulp, bleached sulfate hardwood pulp, bleached bamboo pulp, etc.

[0032] Furthermore, the third slurry is pulped to a freeness of 35-45°SR.

[0033] S2: The first pulp is formed on the sheet forming machine, and after being dehydrated to a first degree of dryness, the core wet paper web is peeled off from the forming wire. The first degree of dryness is 20% to 30%.

[0034] The aforementioned slurry forming device is a commonly used laboratory slurry forming device, such as the TAPPI standard slurry forming device or a manual / semi-automatic slurry forming device of equivalent specifications, used to form slurry on a laboratory scale.

[0035] Specifically, the first slurry is poured into the forming groove of the sheet forming machine for forming to obtain the core layer wet paper web. Then, the core layer wet paper web is dehydrated. Preferably, the dehydration is carried out by water absorption and light pressure, that is, water-absorbing paper or other water-absorbing medium is laid on the core layer wet paper web and light pressure is applied to make the dryness of the core layer wet paper web reach a first dryness, wherein the first dryness is 20% to 30%.

[0036] Understandably, controlling the dryness of the core wet paper web to 20%–30% serves two purposes. Firstly, when the dryness is below 20%, the moisture content in the core wet paper web is too high, resulting in insufficient inter-fiber bonding and excessive softness, making it difficult to peel completely from the forming wire and prone to breakage. Secondly, when the dryness is above 30%, the fiber surfaces of the core wet paper web tend to close, leading to poor interlayer bonding during subsequent lamination with the bottom wet paper web. Therefore, controlling the dryness to 20%–30% ensures that the core wet paper web has sufficient wet strength, allowing it to peel completely from the forming wire without breaking, forming a semi-dry, self-sustaining core wet paper web.

[0037] S3: The second pulp is shaped and dehydrated to a second degree of dryness to form a bottom wet paper web retained on the forming wire. The second degree of dryness is 15% to 20%, and the second degree of dryness is less than the first degree of dryness.

[0038] Specifically, the forming wire is replaced, and the second slurry is poured into the forming trough of the sheet forming machine for forming to obtain the bottom wet paper web. Then, the bottom wet paper web is dehydrated. Preferably, dehydration is carried out using only the vacuum suction of the sheet forming machine without applying additional pressure, so that the dryness of the bottom wet paper web reaches the second dryness, and the bottom wet paper web is retained on the forming wire without being peeled off.

[0039] Understandably, controlling the dryness of the bottom wet paper web to 15%–20%, which is lower than that of the core wet paper web, keeps the bottom wet paper web moist, ensuring that its fibers are fully swollen and its surface is open. This facilitates full adhesion with the core wet paper web in subsequent steps, forming a good interlayer bond. At the same time, the bottom wet paper web is retained on the forming mesh, using the forming mesh as a support carrier, which facilitates subsequent transfer and stacking operations and prevents the wet paper web from deforming or breaking during the transfer process.

[0040] Understandably, by combining steps S2 and S3, the core wet paper web is in a semi-dry state and the bottom wet paper web is in a wet state, forming an asymmetric difference in dryness: the core wet paper web achieves self-holding, peelability, and transferability by being dry, while the bottom wet paper web achieves adhesion by being wet, thereby solving the problem of difficulty in stacking multiple wet paper webs in the laboratory due to insufficient strength.

[0041] Furthermore, when a third pulp is also provided, the forming screen is replaced, and the third pulp is poured into the forming trough of the sheet forming machine for forming to obtain a surface wet paper web. Then, the surface wet paper web is dewatered. Preferably, dewatering is performed only by vacuum suction of the sheet forming machine, so that the dryness of the surface wet paper web reaches a second dryness of 15% to 20%, and the surface wet paper web is retained on the forming screen without being peeled off.

[0042] S4: Spray interlayer binder onto the bottom wet paper web in the form of atomization, and attach the core wet paper web onto the bottom wet paper web to form a multi-layer wet paper web.

[0043] Specifically, the bottom wet paper web is retained on the forming wire with its non-wire side facing upward. An interlayer binder is evenly sprayed onto the non-wire side of the bottom wet paper web in an atomized form using a spraying device. Then, the core layer wet paper web obtained in step S2 is attached to the bottom wet paper web to form a multi-layer wet paper web.

[0044] Understandably, the dryness of the underlying wet paper web is only 15% to 20%, and its texture is fragile. If contact application methods such as brushing or roller coating are used, the surface fiber arrangement of the underlying wet paper web will be damaged, and the underlying wet paper web may even crack or stick to the roller. However, by using atomized spraying, the droplets settle on the surface of the underlying wet paper web in a non-contact manner without disturbing the wet paper web. Therefore, atomized spraying of interlayer binder is a feasible way to apply interlayer binder to the fragile wet paper web.

[0045] Furthermore, the interlayer binder is preferably starch, such as corn starch, tapioca starch, etc. More specifically, the interlayer binder is a gelatinized starch aqueous solution with a mass fraction of 1% to 3%; further, the amount of the gelatinized starch aqueous solution applied is 0.1% to 0.5% of the paper's oven-dry weight; of course, in other embodiments, the interlayer binder may also be at least one of polyvinyl alcohol (PVA), polyamide-epoxychloropropane (PAE), carboxymethyl cellulose (CMC), or a mixture thereof.

[0046] Furthermore, the average particle size of the atomized spray droplets can be 10 to 100 μm to ensure that the interlayer binder is evenly covered on the surface of the wet paper web, while avoiding excessive local coverage due to excessively large droplets or scattering due to excessively small droplets.

[0047] Understandably, the interlayer binder is used to increase the bonding force between layers, overcome the strength reduction caused by the weakening zone of fiber interweaving, and prevent delamination of multilayer paper. Since the core layer is chemimechanical pulp, the residual lignin on its fiber surface and the hydrogen bond force are relatively weak. Furthermore, the core layer, bottom layer, and top layer are formed separately and then laminated, resulting in insufficient fiber interweaving at the interlayer interface, forming a weakening zone of fiber interweaving. The interfacial bonding force is naturally weak, so it needs to be reinforced by the interlayer binder.

[0048] It is understood that the multi-layer wet paper web has at least two layers, that is, it includes at least a core layer and a bottom layer.

[0049] Furthermore, when a third pulp is also provided, an interlayer binder is evenly sprayed in an atomized form on the side of the core wet paper web away from the bottom layer using a spraying device. Then, the surface wet paper web obtained in step S3 is flipped over along with the forming wire so that the non-wire side of the surface wet paper web faces the core wet paper web and is attached to the core wet paper web to form a multilayer wet paper web with at least three layers.

[0050] S5: Press, dehydrate, and dry the multi-layer wet paper web to obtain multi-layer paper sheets.

[0051] Specifically, absorbent paper or pulp board is laid on both sides of the multi-layer wet paper web as an absorbent medium for preliminary dehydration and shaping, so that the dryness of the multi-layer wet paper web reaches 20% to 25%. Then, the multi-layer wet paper web is peeled off from the forming wire and sent to a flatbed press for pressing and dehydration. The pressed multi-layer wet paper web is then dried to obtain multi-layer paper sheets.

[0052] Understandably, the absorbent medium absorbs the water squeezed out during the pressing process, preventing the squeezed water from being drawn back into the paper web.

[0053] Furthermore, the pressing is preferably a two-stage pneumatic constant pressure pressing, the pressing pressure can be 400-500 kgf, the holding time can be 1-5 minutes, and the dryness of the multi-layer wet paper web after pressing reaches 35%-45%.

[0054] Furthermore, the drying process is carried out under vacuum at 60–100°C until the dryness is ≥95%, thereby obtaining the multilayer paper sheet.

[0055] In this invention, the multilayer paper can be cardboard. It is understood that the basis weight of the cardboard is typically 200–400 g / m², wherein the basis weight of the top and bottom layers can be 60–80 g / m², and the basis weight of the core layer can be 100–180 g / m².

[0056] The laboratory papermaking method of the present invention will be further illustrated below through specific embodiments and comparative examples. Example

[0057] S1: Provides core layer slurry, bottom layer slurry and top layer slurry.

[0058] The core layer pulp is a chemimechanical pulp; the bottom layer pulp and the top layer pulp are both mixed pulps of bleached sulfate softwood pulp (NBKP) and bleached sulfate hardwood pulp (LBKP), with an oven-dry mass ratio of NBKP to LBKP of approximately 30:70.

[0059] The oven-dry mass ratio of the core layer slurry, the bottom layer slurry, and the top layer slurry is approximately 40:30:30.

[0060] The core layer slurry was pulped to a freeness of 28°SR, and the bottom layer slurry and top layer slurry were pulped to a freeness of 40°SR respectively.

[0061] S2: Form the core layer pulp on a TAPPI standard paper forming machine (forming area 200 cm²), lay absorbent paper and apply light pressure to dehydrate it, so that the wet web dryness of the core layer reaches 25%, and peel the wet web of the core layer from the forming wire.

[0062] S3: Replace the forming screen to form the bottom layer pulp, and dehydrate it through vacuum suction to achieve a wet paper width of 16%, which is then retained on the forming screen; then replace the forming screen again to form the top layer pulp, and dehydrate it through vacuum suction to achieve a wet paper width of 16%, which is then retained on the forming screen.

[0063] S4: Keep the bottom wet paper web on the forming wire with its non-wire side facing up. Use a spraying device to atomize and spray 2% by mass of gelatinized starch solution onto the non-wire side of the bottom wet paper web. Then attach the core wet paper web onto the bottom wet paper web. Next, atomize and spray the same gelatinized starch solution onto the side of the core wet paper web away from the bottom. Then flip the top wet paper web along with the forming wire so that the non-wire side of the top wet paper web faces the core wet paper web and attaches it onto the core wet paper web to form a multi-layer wet paper web with a three-layer structure.

[0064] S5: Lay absorbent paper on both sides of the multi-layer wet paper web, press lightly to remove water, and then send it into a flatbed press for two-stage constant pressure pressing (both the first and second stages are 450 kgf, each holding pressure for 2 minutes). After pressing, the dryness reaches 40%. Then, vacuum dry the multi-layer wet paper web at 85℃ until the dryness is ≥95%, and then multi-layer white cardboard sheets are made.

[0065] The surface layer has a basis weight of 75 g / m², the core layer has a basis weight of 100 g / m², and the bottom layer has a basis weight of 75 g / m², for a total basis weight of approximately 250 g / m².

[0066] After mixing the core slurry, bottom slurry, and top slurry from Example 1 in the same proportion, they were formed into single-layer sheets using a TAPPI standard sheeter. No layer forming, interlayer spraying of binder, or stacking was performed. The remaining sheet forming, pressing, and drying conditions were the same as in Example 1.

[0067] The core slurry, bottom slurry, and top slurry from Example 1 were each made into sheets and dried to full dryness (dryness ≥ 95%). Then, starch glue was applied between the dry paper sheets, and the sheets were stacked and pressed. The remaining conditions were the same as in Example 1.

[0068] Twenty-five sets of experiments were conducted with different slurry ratios and freeness combinations. Each set of samples used parallel samples, and the mean value of the parallel samples was used as the representative value for that set. Specifically, for each combination, multi-layer sheet preparation was performed according to the method of this invention (example), the single-layer mixed sheet preparation method of Comparative Example 1, and the dry-state superposition method of Comparative Example 2. Data on the bulk and stiffness of the products were collected from the production machine under the same slurry combination, ensuring a one-to-one correspondence between the data from each method and the production machine for each combination.

[0069] Pearson correlation coefficient was used to evaluate the correlation between the performance data of the wafer copying methods and the products from the production machine; if the data did not meet the normal distribution, Spearman rank correlation coefficient was used. A significance test (P-value) was then used to determine whether the two were significantly correlated, where P < 0.05 indicated a significant correlation, and P > 0.05 indicated no significant correlation.

[0070] The results show that: method Correlation coefficient r with production equipment p-value Correlation determination Example 1 ＞0.8 ＜0.05 Significantly related Comparative Example 1 Low ＞0.05 No significant correlation Comparative Example 2 Low ＞0.05 No significant correlation As can be seen, the multilayer sheet forming in the embodiments showed a significant positive correlation with the bulk and stiffness of the products produced on the production machine (r > 0.8, P < 0.05), indicating that the method of the present invention can correctly reflect the influence of variables such as slurry ratio and beating degree on product performance, and has a corresponding relationship with the production machine, and can be used as the basis for pilot production on the production machine; while the single-layer mixed sheet forming in Comparative Example 1 and the dry-state composite sheet forming in Comparative Example 2 showed no significant correlation with the products produced on the production machine (P > 0.05), indicating that the single-layer mixed sheet forming, due to the loss of layered structure, and the dry-state composite sheet forming, due to the degradation of interlayer bonding into an adhesive mechanism, could not correctly reflect the influence of variables on product performance, and had no corresponding relationship with the production machine.

[0071] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A laboratory papermaking method for multi-layered paper, characterized in that, Includes the following steps: S1: Provide a first slurry and a second slurry, and perform pulping treatment on the first slurry and the second slurry respectively; S2: The first pulp is shaped and dewatered to a first degree of dryness on the sheet forming machine to form a core layer wet paper web, and the core layer wet paper web is peeled off from the forming wire, wherein the first degree of dryness is 20% to 30%; S3: The second pulp is shaped and dehydrated to a second degree of dryness to form a bottom wet paper web retained on the forming wire. The second degree of dryness is 15% to 20%, and the second degree of dryness is less than the first degree of dryness. S4: Spray interlayer binder onto the bottom wet paper web in the form of atomization, and attach the core wet paper web onto the bottom wet paper web to form a multi-layer wet paper web; S5: Press, dehydrate, and dry the multi-layer wet paper web to obtain multi-layer paper sheets.

2. The method according to claim 1, characterized in that, Step S1 further includes providing a third pulp and beating the third pulp; in step S3, the third pulp is shaped and dehydrated to a second dryness to form a surface wet paper web retained on the forming wire; in step S4, an interlayer binder is sprayed in an atomized form on the side of the core wet paper web away from the bottom layer to attach the surface wet paper web to the core wet paper web, forming a multilayer wet paper web with at least three layers.

3. The method according to claim 2, characterized in that, The first pulp is chemimechanical pulp and / or waste paper pulp; the second pulp and the third pulp are bleached or unbleached chemical pulps, and the multilayer paper sheet is cardboard sheet.

4. The method according to claim 2, characterized in that, The first slurry accounts for 30% to 40% of the total slurry volume, the second slurry accounts for 30% to 35%, and the third slurry accounts for 30% to 35%, and the sum of the three is 100%.

5. The method according to claim 2, characterized in that, The first pulp is pulped to a freeness of 25-30°SR, the second pulp is pulped to a freeness of 30-40°SR, and the third pulp is pulped to a freeness of 35-45°SR.

6. The method according to claim 1, characterized in that, In step S2, an absorbent medium is laid on the wet core paper web, and a light pressure of 5 to 20 kgf is applied for dehydration, and the pressure is maintained for 30 seconds to 2 minutes.

7. The method according to claim 1, characterized in that, In step S4, an interlayer binder is sprayed onto the non-wire surface of the bottom wet paper web; the interlayer binder is at least one of starch, polyvinyl alcohol, polyamide-epoxychloropropane, and carboxymethyl cellulose or a mixture thereof.

8. The method according to claim 7, characterized in that, The interlayer binder is a gelatinized starch aqueous solution with a mass fraction of 1% to 3%; the average particle size of the droplets formed by the atomized spray is 10 to 100 μm.

9. The method according to claim 1, characterized in that, In step S5, absorbent medium is laid on both sides of the multi-layer wet paper web, and after preliminary dehydration and shaping to a dryness of 20% to 25%, a two-stage pneumatic constant pressure pressing is performed.

10. The method according to claim 9, characterized in that, During the two-stage pressing, the pressing pressure is 400-500 kgf, the holding time is 1-5 minutes, and the dryness after pressing reaches 35%-45%.