High-strength paperboard

Abstract

The application relates to the field of packaging materials, and particularly discloses a high-strength paperboard which comprises a core layer and a surface layer, the core layer comprises a core outer layer and a core inner layer, the core outer layer comprises the following components: unbleached kraft softwood pulp, cotton pulp, calcined kaolin, carboxymethyl cellulose and cationic polyacrylamide, and the core inner layer comprises the following components: hardwood pulp, mechanical pulp, microfibrillated cellulose and cationic starch. The double-layer differential structure design is adopted, the core outer layer is constructed with a high proportion of unbleached kraft softwood pulp and cotton pulp to form a long-fiber rigid skeleton, the calcined kaolin and the carboxymethyl cellulose are matched to fill the fiber gaps, a main bearing layer with high density and high rigidity is formed, the core inner layer is formed with the hardwood pulp, the mechanical pulp and the microfibrillated cellulose to form a buffer layer with low density and high elasticity, so that the paperboard has excellent compression resistance, bending resistance and impact buffering capacity under the same basis weight, and the warping deformation is inhibited, and the strength, toughness, dimensional stability and cost control are synergistically improved.

Description

Technical Field

[0001] This application relates to the field of packaging materials, and more specifically, to a high-strength cardboard. Background Technology

[0002] Corrugated cardboard has become one of the most widely used packaging materials due to its advantages such as light weight, low cost, easy processing and recyclability. It is widely used for the storage and transportation packaging of various products such as food, home appliances and handicrafts. However, with the development of modern industry, the requirements for the mechanical properties of cardboard are increasing. In many application scenarios, cardboard is required not only to have good compressive strength, but also to have high overall structural strength to meet the needs of harsh working conditions such as heavy goods packaging, high-strength cushioning and building formwork.

[0003] In related technologies, increasing the thickness of the face paper and core paper is usually used to improve the strength of paperboard. However, this method will increase the weight of the paperboard, which will not only increase the consumption of raw materials and production costs, but also increase energy consumption during transportation. This is not in line with the industry development trend of lightweighting, energy conservation and environmental protection, and needs to be improved. Summary of the Invention

[0004] To address the aforementioned issues, this application provides a high-strength cardboard.

[0005] The high-strength paperboard provided in this application adopts the following technical solution:

[0006] A high-strength paperboard includes a core layer and two face layers, the core layer being located between the two face layers. The core layer comprises two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layers comprise the following components: 60-78 parts unbleached sulfate softwood pulp, 10-20 parts cotton pulp, 5-15 parts calcined kaolin, 0.5-1 part carboxymethyl cellulose, and 0.05-0.2 parts cationic polyacrylamide. The inner core layer comprises the following components: 50-70 parts hardwood pulp, 30-50 parts mechanical pulp, 1-3 parts microfibrillated cellulose, and 1-3 parts cationic starch.

[0007] By adopting the above technical solution, the outer core layer utilizes a high proportion of unbleached sulfate softwood pulp and cotton pulp to provide a long fiber skeleton, combined with calcined kaolin and carboxymethyl cellulose to fill the fiber gaps, forming a high-density, high-rigidity main load-bearing layer. The inner core layer utilizes the coarse fiber structure of hardwood pulp and mechanical pulp, combined with the spatial network support of microfibrillated cellulose, retaining a large number of pores to form a low-density, high-elasticity buffer layer. Under the same basis weight, the high-density outer layer closely adheres to the surface layer to bear the main load, enhancing the compressive and bending resistance of the paperboard. At the same time, the low-density inner layer provides buffer space, which can absorb impact and suppress warping deformation of the paperboard caused by changes in environmental humidity by adjusting the shrinkage difference between the inner and outer layers. While controlling the overall material cost, it achieves a synergistic improvement in strength, toughness and dimensional stability.

[0008] Preferably, the inner core layer further includes 0.8-1 parts by weight of oxidized modified sodium alginate, wherein the carboxyl content of the oxidized modified sodium alginate is 3.5-5 mmol / g.

[0009] By adopting the above technical solution, the oxidized modified sodium alginate has both carboxyl and aldehyde groups. The aldehyde groups in its molecular chain form covalent bonds with cellulose fibers, while the carboxyl groups interact electrostatically with cationic starch in the system. A flexible cross-linked network is constructed in the large-pore structure of the low-density core inner layer, which improves the bonding strength and structural resilience between fibers, thereby improving the impact fatigue resistance of the paperboard.

[0010] Preferably, the preparation method of the oxidized modified sodium alginate is as follows: Sodium alginate is dissolved in deionized water to prepare a 1wt% solution. At room temperature, 1% of the dry weight of sodium alginate as TEMPO oxidant and 5% of the dry weight of sodium alginate as sodium bromide are added to the solution sequentially. The pH is adjusted to 10.5 with 1mol / L NaOH solution and kept constant. Then, a sodium hypochlorite solution with an effective chlorine mass fraction of 10% is added dropwise, the amount of which is 15% of the dry weight of sodium alginate. After reacting for 4 hours, 1mol / L HCl solution is added dropwise to adjust the pH to 7.0. An equal volume of anhydrous ethanol is added to the reaction solution. The precipitate is separated by centrifugation, the supernatant is discarded, the precipitate is redispersed in deionized water, and ethanol is added again to precipitate. The washing-centrifugation process is repeated 3 times. The final precipitate is dried at 40℃ for 24 hours to constant weight, and then pulverized to obtain oxidized modified sodium alginate powder.

[0011] By adopting the above technical solution, the hydroxyl groups in sodium alginate molecules can be converted into carboxyl groups under mild conditions, thereby increasing the number of active sites. By controlling the pH value and reaction time, excessive oxidation leading to molecular chain degradation can be avoided, ensuring the molecular weight and structural stability of the oxidized sodium alginate.

[0012] Preferably, in the preparation method of the oxidized modified sodium alginate, after reacting for 4 hours, 5% of the dry weight of sodium alginate in methanol is added dropwise. After stirring evenly, a drop of the reaction solution is placed on starch-potassium iodide test paper. If the test paper turns blue or black, 2% of the dry weight of sodium alginate in methanol is added dropwise. After stirring evenly, the test paper is used for detection again. This process is repeated until the test paper retains its original yellow or light brown color. Then, 1 mol / L HCl solution is added to adjust the pH to 7.0.

[0013] By adopting the above technical solution, methanol is added dropwise in stages and the reaction endpoint is monitored with starch-potassium iodide test paper. This can quench the residual sodium hypochlorite in the system, avoid excessive oxidation leading to degradation of the sodium alginate molecular chain, and ensure that the structure of the oxidized modified sodium alginate is stable and the carboxyl content is controllable.

[0014] Preferably, the outer core layer further comprises 0.05-0.1 parts by weight of quaternary ammonium chitosan.

[0015] By adopting the above technical solution, quaternary ammonium salt chitosan can form electrostatic adsorption and hydrogen bonding synergistic effects with fibers, carboxymethyl cellulose and oxidized modified sodium alginate, further improving the bonding strength and structural density between the core and outer layer fibers, enhancing the interfacial bonding force between the core layer and the surface layer, and thus improving the overall strength and toughness of the paperboard.

[0016] Preferably, by weight, the surface layer comprises 30-50 parts bleached sulfate softwood pulp, 20-40 parts bleached sulfate hardwood pulp, 5-15 parts nano-calcium carbonate, 5-15 parts calcined kaolin, 3-8 parts oxidized starch, 2-5 parts styrene-butadiene latex, 0.5-2 parts polyvinyl alcohol, 0.2-0.8 parts alkyl ketene dimer sizing agent, and 0.1-0.5 parts cationic polyacrylamide.

[0017] By adopting the above technical solutions, using a blend of bleached sulfate softwood pulp and hardwood pulp as the surface layer fiber matrix, the structural strength, surface uniformity, and smoothness of the surface layer can be balanced. Adding a blend of nano-calcium carbonate and calcined kaolin filler effectively improves the surface fineness, opacity, and dimensional stability of the surface layer, enhancing the smoothness of the paperboard surface and its adaptability to subsequent printing and processing. The synergistic effect of oxidized starch, styrene-butadiene latex, and polyvinyl alcohol strengthens the bonding strength between fibers and improves the flexibility and film-forming properties of the surface layer, preventing surface cracking. Alkyl ketone dimer sizing agents impart good water resistance to the surface layer, reducing moisture absorption and deformation during paperboard use. Cationic polyacrylamide provides excellent retention and filtration, increasing the retention rate of fillers and adhesive components, resulting in a more uniform and dense surface layer structure.

[0018] Preferably, the surface layer further includes stearic anhydride by weight, and the mass ratio of stearic anhydride to alkyl ketene dimer sizing agent is 1:4.

[0019] By adopting the above technical solution, stearic anhydride and alkyl ketene dimer are compounded and applied in a specific ratio, which can exert a synergistic effect, further improve the water resistance and moisture absorption capacity of the surface layer, and reduce the water absorption and deformation of the cardboard during use.

[0020] Preferably, the surface layer further comprises 0.1-0.4 parts by weight of sodium carboxymethyl cellulose.

[0021] By adopting the above technical solution, sodium carboxymethyl cellulose can adjust the water retention and film-forming properties of the slurry system, making the surface layer dry shrinkage more uniform, reducing the tendency of paperboard to curl and warp, and improving dimensional stability.

[0022] In summary, this application has the following beneficial effects:

[0023] 1. Due to the use of a differentiated double-layer structure design in this application, the outer core layer is constructed with a high proportion of unbleached sulfate softwood pulp and cotton pulp to form a rigid long fiber skeleton, and calcined kaolin and carboxymethyl cellulose to fill the fiber gaps, forming a high-density, high-rigidity main load-bearing layer. The inner core layer is formed with hardwood pulp, mechanical pulp and microfibrillated cellulose to form a low-density, high-elasticity buffer layer. Thus, under the same basis weight, the paperboard has both excellent compression and bending resistance and impact cushioning ability, while suppressing warping deformation, and achieving a synergistic improvement in strength, toughness, dimensional stability and cost control.

[0024] 2. In this application, oxidized modified sodium alginate is preferred. Oxidized modified sodium alginate has both carboxyl and aldehyde groups. The aldehyde groups in its molecular chain form covalent bonds with cellulose fibers, while the carboxyl groups interact electrostatically with cationic starch in the system. A flexible cross-linked network is constructed in the large-pore structure of the low-density core inner layer, which improves the bonding strength between fibers and the structural resilience, thereby improving the impact fatigue resistance of the paperboard.

[0025] 3. The preparation method of the oxidatively modified sodium alginate in this application can convert the hydroxyl groups in the sodium alginate molecule into carboxyl groups under mild conditions, thereby increasing the number of its molecular active sites. By controlling the pH value and reaction time, excessive oxidation leading to molecular chain degradation can be avoided, thus ensuring the molecular weight and structural stability of the oxidatively modified sodium alginate. Detailed Implementation

[0026] The present application will be further described in detail below with reference to the embodiments.

[0027] cotton pulp Average fiber length: 1.5-3.0 mm mechanical pulp Thermomechanical pulp Calcinated kaolin Particle size: 1250-2500 mesh Microfibrillated cellulose Length: 50-200μm, Diameter: 20-100nm cationic starch Degree of substitution: 0.02-0.07 Sodium alginate Mesh size: 120-200 mesh, viscosity of 1wt% aqueous solution: 200-500 mPa·s Quaternary ammonium salt chitosan Degree of substitution: 0.60-0.85; Viscosity of 1wt% aqueous solution: 50-200 mPa·s Nano calcium carbonate Particle size: 30-80nm Oxidized starch Degree of substitution: 0.01-0.05 Polyvinyl alcohol Degree of hydrolysis: 87-89 mol%, viscosity: 4-6 mPa·s Sodium carboxymethyl cellulose Viscosity of 1wt% aqueous solution: 100-300 mPa·s Light calcium carbonate Particle size: 1250-2500 mesh

[0028] It should be noted that wt% refers to weight percentage. The basis weight of the paperboard in each embodiment and comparative example of this application is 300g / m², and the thickness is controlled at 1mm, with the surface layer at 0.15mm, the outer core layer at 0.25mm, and the inner core layer at 0.2mm. All samples were placed in a constant temperature and humidity chamber at (23±1)℃ and (50±2)% relative humidity for 24h before testing to achieve moisture balance.

[0029] Unless otherwise specified, all raw materials used in the following embodiments are commercially available.

[0030] Preparation Example 1

[0031] Preparation method of oxidized modified sodium alginate: Sodium alginate was dissolved in deionized water to prepare a 1wt% solution. At room temperature, 1% of the dry weight of sodium alginate (TEMPO oxidant) and 5% of the dry weight of sodium alginate (sodium bromide) were added to the solution sequentially. The pH was adjusted to 10.5 with 1mol / L NaOH solution and kept constant. Then, sodium hypochlorite solution with an effective chlorine mass fraction of 10% was added dropwise, the amount of which was 15% of the dry weight of sodium alginate. After reacting for 4 hours, 1mol / L HCl solution was added dropwise to adjust the pH to 7.0. An equal volume of anhydrous ethanol was added to the reaction solution. The precipitate was separated by centrifugation, the supernatant was discarded, and the precipitate was redispersed in deionized water. Ethanol was added again to precipitate, and the washing-centrifugation process was repeated 3 times. The final precipitate was dried at 40℃ for 24 hours to constant weight, pulverized and passed through a 200-mesh sieve to obtain oxidized modified sodium alginate powder. The carboxyl content of oxidized modified sodium alginate was 3.5-5 mmol / g.

[0032] Preparation Example 2

[0033] Preparation method of oxidatively modified sodium alginate: Dissolve sodium alginate in deionized water to prepare a 1 wt% solution. At room temperature, add 1% TEMPO oxidant (dry weight of sodium alginate) and 5% sodium bromide (dry weight of sodium alginate) sequentially to the solution. Adjust the pH to 10.5 with 1 mol / L NaOH solution and maintain a constant pH. Then, add sodium hypochlorite solution with an effective chlorine mass fraction of 10% (15% of the dry weight of sodium alginate). After reacting for 4 hours, add methanol (dry weight of sodium alginate) dropwise. After stirring evenly, take one drop of the reaction solution and place it on starch-potassium iodide test paper. If the test paper turns blue or black, continue adding 2... Add methanol to sodium alginate by dry weight, stir well, and then test with test paper. Repeat this process until the test paper retains its original yellow or light brown color. Then, add 1 mol / L HCl solution to adjust the pH to 7.0. Add an equal volume of anhydrous ethanol to the reaction solution. Centrifuge the precipitate, discard the supernatant, redisperse the precipitate in deionized water, add ethanol again to precipitate, and repeat the washing-centrifugation process 3 times. Dry the final precipitate at 40℃ for 24 hours to constant weight, pulverize it through a 200-mesh sieve, and obtain oxidized modified sodium alginate powder. The carboxyl content of oxidized modified sodium alginate is 3.5-5 mmol / g.

[0034] Example 1

[0035] This application discloses a high-strength paperboard, comprising a core layer and two face layers, with the core layer located between the two face layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 60 parts unbleached sulfate softwood pulp, 10 parts cotton pulp, 5 parts calcined kaolin, 0.5 parts carboxymethyl cellulose, and 0.05 parts cationic polyacrylamide.

[0036] The core inner layer comprises the following components: 50 parts hardwood pulp, 30 parts mechanical pulp, 1 part microfibrillated cellulose, and 1 part cationic starch;

[0037] The surface layer comprises the following components: 30 parts bleached sulfate softwood pulp, 20 parts bleached sulfate hardwood pulp, 5 parts nano-calcium carbonate, 5 parts calcined kaolin, 3 parts oxidized starch, 2 parts styrene-butadiene latex, 0.5 parts polyvinyl alcohol, 0.2 parts alkyl ketene dimer sizing agent, and 0.1 parts cationic polyacrylamide.

[0038] Example 2

[0039] This application discloses a high-strength paperboard, comprising a core layer and two face layers, with the core layer located between the two face layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 78 parts unbleached sulfate softwood pulp, 20 parts cotton pulp, 15 parts calcined kaolin, 1 part carboxymethyl cellulose, and 0.2 parts cationic polyacrylamide.

[0040] The core inner layer comprises the following components: 70 parts hardwood pulp, 50 parts mechanical pulp, 3 parts microfibrillated cellulose, and 3 parts cationic starch;

[0041] The surface layer comprises the following components: 50 parts bleached sulfate softwood pulp, 40 parts bleached sulfate hardwood pulp, 15 parts nano-calcium carbonate, 15 parts calcined kaolin, 8 parts oxidized starch, 5 parts styrene-butadiene latex, 2 parts polyvinyl alcohol, 0.8 parts alkyl ketone dimer sizing agent, and 0.5 parts cationic polyacrylamide.

[0042] Example 3

[0043] This application discloses a high-strength paperboard, comprising a core layer and two face layers, with the core layer located between the two face layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 70 parts unbleached sulfate softwood pulp, 15 parts cotton pulp, 10 parts calcined kaolin, 0.8 parts carboxymethyl cellulose, and 0.1 parts cationic polyacrylamide.

[0044] The core layer comprises the following components: 60 parts hardwood pulp, 40 parts mechanical pulp, 2 parts microfibrillated cellulose, and 2 parts cationic starch;

[0045] The surface layer comprises the following components: 40 parts bleached sulfate softwood pulp, 30 parts bleached sulfate hardwood pulp, 10 parts nano-calcium carbonate, 10 parts calcined kaolin, 5 parts oxidized starch, 4 parts styrene-butadiene latex, 1 part polyvinyl alcohol, 0.6 parts alkyl ketone dimer sizing agent, and 0.3 parts cationic polyacrylamide.

[0046] Example 4

[0047] The difference from Example 1 is that, by weight, the inner core layer also includes 0.8 parts of oxidized modified sodium alginate, which was prepared in Preparation Example 1.

[0048] Example 5

[0049] The difference from Example 4 is that, by weight, the inner core layer also includes 0.8 parts of oxidized modified sodium alginate, which was prepared in Preparation Example 2.

[0050] Example 6

[0051] The difference from Example 4 is that, by weight, the outer core layer also includes 0.05 parts of quaternary ammonium chitosan.

[0052] Example 7

[0053] The difference from Example 1 is that the surface layer also includes stearic anhydride, and the mass ratio of stearic anhydride and alkyl ketene dimer sizing agent is 1:4.

[0054] Example 8

[0055] The difference from Example 7 is that, by weight, the surface layer also includes 0.1 parts of sodium carboxymethyl cellulose.

[0056] Example 9

[0057] This application discloses a high-strength paperboard, comprising a core layer and two face layers, with the core layer located between the two face layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 60 parts unbleached sulfate softwood pulp, 10 parts cotton pulp, 5 parts calcined kaolin, 0.5 parts carboxymethyl cellulose, 0.05 parts cationic polyacrylamide, and 0.05 parts quaternary ammonium chitosan.

[0058] The core inner layer comprises the following components: 50 parts hardwood pulp, 30 parts mechanical pulp, 1 part microfibrillated cellulose, 1 part cationic starch and 0.8 parts oxidized modified sodium alginate, which was prepared by Preparation Example 2;

[0059] The surface layer comprises the following components: 30 parts bleached sulfate softwood pulp, 20 parts bleached sulfate hardwood pulp, 5 parts nano-calcium carbonate, 5 parts calcined kaolin, 3 parts oxidized starch, 2 parts styrene-butadiene latex, 0.5 parts polyvinyl alcohol, 0.2 parts alkyl ketene dimer sizing agent, 0.1 parts cationic polyacrylamide, 0.1 parts sodium carboxymethyl cellulose and stearic anhydride, wherein the mass ratio of stearic anhydride to alkyl ketene dimer sizing agent is 1:4.

[0060] Example 10

[0061] This application discloses a high-strength paperboard, comprising a core layer and two face layers, with the core layer located between the two face layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 78 parts unbleached sulfate softwood pulp, 20 parts cotton pulp, 15 parts calcined kaolin, 1 part carboxymethyl cellulose, 0.2 parts cationic polyacrylamide, and 0.1 parts quaternary ammonium chitosan.

[0062] The core inner layer comprises the following components: 70 parts hardwood pulp, 50 parts mechanical pulp, 3 parts microfibrillated cellulose, 3 parts cationic starch and 1 part oxidized modified sodium alginate, wherein the oxidized modified sodium alginate is prepared from Preparation Example 2;

[0063] The surface layer comprises the following components: 50 parts bleached sulfate softwood pulp, 40 parts bleached sulfate hardwood pulp, 15 parts nano-calcium carbonate, 15 parts calcined kaolin, 8 parts oxidized starch, 5 parts styrene-butadiene latex, 2 parts polyvinyl alcohol, 0.8 parts alkyl ketene dimer sizing agent, 0.5 parts cationic polyacrylamide, 0.4 parts sodium carboxymethyl cellulose and stearic anhydride, wherein the mass ratio of stearic anhydride to alkyl ketene dimer sizing agent is 1:4.

[0064] Example 11

[0065] This application discloses a high-strength paperboard, comprising a core layer and two face layers, with the core layer located between the two face layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 70 parts unbleached sulfate softwood pulp, 15 parts cotton pulp, 10 parts calcined kaolin, 0.8 parts carboxymethyl cellulose, 0.1 parts cationic polyacrylamide, and 0.08 parts quaternary ammonium chitosan.

[0066] The core inner layer comprises the following components: 60 parts hardwood pulp, 40 parts mechanical pulp, 2 parts microfibrillated cellulose, 2 parts cationic starch and 0.9 parts oxidized modified sodium alginate, which was prepared by Preparation Example 2;

[0067] The surface layer comprises the following components: 40 parts bleached sulfate softwood pulp, 30 parts bleached sulfate hardwood pulp, 10 parts nano-calcium carbonate, 10 parts calcined kaolin, 5 parts oxidized starch, 4 parts styrene-butadiene latex, 1 part polyvinyl alcohol, 0.6 parts alkyl ketene dimer sizing agent, 0.3 parts cationic polyacrylamide, 0.2 parts sodium carboxymethyl cellulose and stearic anhydride, wherein the mass ratio of stearic anhydride to alkyl ketene dimer sizing agent is 1:4.

[0068] Example 12

[0069] The difference from Example 4 is that the oxidized modified sodium alginate is replaced with sodium alginate.

[0070] Comparative Example 1

[0071] The difference from Example 1 is that the paperboard is a single-layer structure (conventional paperboard) and the paperboard includes the following components: 60 parts bleached hardwood pulp, 40 parts bleached softwood pulp, 15 parts light calcium carbonate, 3 parts oxidized starch, 0.3 parts alkyl ketene dimer sizing agent, and 0.2 parts cationic polyacrylamide, which serve as a blank control group.

[0072] Comparative Example 2

[0073] The difference from Example 1 is that the core layer has a single homogeneous structure and, by weight, the core layer comprises the following components: 60 parts unbleached sulfate softwood pulp, 10 parts cotton pulp, 5 parts calcined kaolin, 0.5 parts carboxymethyl cellulose, 0.05 parts cationic polyacrylamide, 50 parts hardwood pulp, 30 parts mechanical pulp, 1 part microfibrillated cellulose, and 1 part cationic starch.

[0074] Comparative Example 3

[0075] The difference from Example 1 is that microfibrillated cellulose is replaced with hardwood pulp.

[0076] Performance testing

[0077] (1) Compression strength test (ring crush strength): 12.7 mm × 152 mm samples were cut from each of Examples 1-12 and Comparative Examples 1-3 and tested according to standard GB / T 2679.8-2016 "Determination of ring crush strength of paper and paperboard". The test results are shown in Table 1 below.

[0078] (2) Strength test (bursting strength): Examples 1-12 and Comparative Examples 1-3 were tested according to the standard GB / T 454-2020 "Determination of paper bursting strength". A pneumatic bursting strength tester was used to record the bursting index. The test results are shown in Table 1 below.

[0079] (3) Toughness test (tensile strength): Examples 1, 4-6, 12 and comparative examples 1-3 were tested according to the standard GB / T 12914-2018 "Determination of tensile strength of paper and paperboard by constant speed tensile test (20 mm / min)". The test results are shown in Table 1 below.

[0080] (4) Folding endurance test: Examples 1, 4, 5 and 12 were tested according to the standard GB / T 457-2008 "Determination of folding endurance of paper and paperboard". The test results are shown in Table 1 below.

[0081] (5) Water absorption test: Examples 1 and 7 were tested according to standard GB / T 1540-2002 "Determination of water absorption of paper and paperboard (Cobb method)". The contact time between the sample and water was 30 min. The test results are shown in Table 1 below.

[0082] (6) Dimensional stability test: Examples 1, 7, 8 and comparative examples 1-3 were tested according to standard GB / T 459-2002 "Determination of the extensibility of paper and paperboard". The sample size was 220mm×220mm. The dimensional change rate after moisture absorption was calculated. The test results are shown in Table 2 below.

[0083] Table 1. Test results for compressive strength, strength, toughness, flexural endurance, and water absorption.

[0084] Example 1 3.95 2.9 5.75 185 51.0 Example 2 3.88 2.8 / / / Example 3 4.02 2.9 / / / Example 4 4.28 3.2 6.35 262 / Example 5 4.35 3.2 6.52 288 / Example 6 4.52 3.4 6.70 / / Example 7 4.02 2.9 / / 35.2 Example 8 3.98 2.9 / / / Example 9 4.50 3.3 / / Example 10 4.47 3.3 / / Example 11 4.55 3.4 / / Example 12 4.06 2.9 5.82 210 / Comparative Example 1 2.75 2.1 4.52 / / Comparative Example 2 3.48 2.6 5.12 / / Comparative Example 3 3.62 2.7 4.72 / /

[0085] The " / " in the table indicates that no test was performed.

[0086] Table 2 Dimensional stability test results

[0087] Example 1 0.16 0.19 Example 7 0.13 0.16 Example 8 0.08 0.11 Comparative Example 1 0.26 0.31 Comparative Example 2 0.21 0.26 Comparative Example 3 0.23 0.28

[0088] In conclusion, the following conclusions can be drawn:

[0089] 1. As can be seen from Example 1 and Comparative Examples 1-3, and Table 1-2, the core layer adopts a differentiated double-layer structure design, which can improve the strength, toughness, and dimensional stability of the paperboard. The reason may be that the outer core layer uses a high proportion of unbleached sulfate softwood pulp and cotton pulp to provide a long fiber skeleton, and calcined kaolin and carboxymethyl cellulose to fill the fiber gaps, forming a high-density, high-rigidity main load-bearing layer. The inner core layer uses the coarse fiber structure of hardwood pulp and mechanical pulp, combined with the spatial network support of microfibrillated cellulose, retaining a large number of pores, forming a low-density, high-elasticity buffer layer. Under the same basis weight, the high-density outer layer closely adheres to the surface layer and bears the main load, enhancing the compressive and bending resistance of the paperboard. At the same time, the low-density inner layer provides a buffer space, which can absorb impact and suppress warping deformation of the paperboard caused by changes in environmental humidity by adjusting the shrinkage difference between the inner and outer layers, thus achieving a synergistic improvement in strength, toughness, and dimensional stability.

[0090] 2. As can be seen from Examples 1, 4, and 12 and Table 1-2, adding oxidized modified sodium alginate to the core inner layer can improve the strength, toughness, and folding endurance of the paperboard. The reason may be that oxidized modified sodium alginate has both carboxyl and aldehyde groups. It utilizes the aldehyde groups in its molecular chain to form covalent bonds with cellulose fibers, and at the same time utilizes the carboxyl groups to conduct electrostatic interactions with cationic starch in the system. A flexible cross-linked network is constructed in the large-pore structure of the low-density core inner layer, which improves the bonding strength and structural resilience between fibers, thereby improving the impact fatigue resistance of the paperboard.

[0091] 3. Combining Examples 1, 4, and 5 with Table 1-2, it can be seen that in the preparation method of oxidatively modified sodium alginate, adding methanol dropwise in stages and monitoring the reaction endpoint with starch-potassium iodide test paper can promote the improvement of the strength, toughness, and folding endurance of the paperboard. The reason may be that by adding methanol dropwise in stages and monitoring the reaction endpoint with starch-potassium iodide test paper, residual sodium hypochlorite in the system can be quenched, avoiding excessive oxidation that leads to degradation of the sodium alginate molecular chain, and ensuring that the structure of the oxidatively modified sodium alginate is stable and the carboxyl content is controllable.

[0092] 4. As can be seen from Examples 1 and 4-6 and Table 1-2, adding quaternary ammonium salt chitosan to the core outer layer can improve the strength and toughness of the paperboard. The reason may be that quaternary ammonium salt chitosan can form electrostatic adsorption and hydrogen bonding synergistic effects with fibers, carboxymethyl cellulose and oxidized modified sodium alginate, further improving the bonding strength and structural density between fibers in the core outer layer, enhancing the interfacial bonding force between the core layer and the surface layer, and thus improving the overall strength and toughness of the paperboard.

[0093] 5. As can be seen from Examples 1 and 7 and Table 1-2, adding stearic anhydride to the surface layer can improve the moisture resistance of the paperboard. The reason may be that using stearic anhydride and alkyl ketene dimer in a specific ratio for sizing can play a synergistic sizing effect, further improving the water resistance and moisture resistance of the surface layer, and reducing the water absorption deformation of the paperboard during use.

[0094] 6. As can be seen from Examples 1 and 7-8 and Table 1-2, adding sodium carboxymethyl cellulose to the surface layer can improve the dimensional stability of the paperboard. The reason may be that sodium carboxymethyl cellulose can adjust the water retention and film-forming properties of the pulp system, making the surface layer dry shrinkage more uniform, reducing the tendency of paperboard to curl and warp, and improving dimensional stability.

[0095] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high-strength cardboard, characterized in that, The product comprises a core layer and two surface layers, the core layer being located between the two surface layers. The core layer includes two outer core layers and an inner core layer located between the two outer core layers. By weight, the outer core layer comprises the following components: 60-78 parts unbleached sulfate softwood pulp, 10-20 parts cotton pulp, 5-15 parts calcined kaolin, 0.5-1 parts carboxymethyl cellulose, and 0.05-0.2 parts cationic polyacrylamide. The inner core layer comprises the following components: 50-70 parts hardwood pulp, 30-50 parts mechanical pulp, 1-3 parts microfibrillated cellulose, 1-3 parts cationic starch, and 0.8-1 parts oxidized modified sodium alginate, wherein the carboxyl content of the oxidized modified sodium alginate is 3.5-5 mmol / g. The preparation method of the oxidized modified sodium alginate is as follows: Sodium alginate is dissolved in deionized water to prepare a 1wt% solution. At room temperature, 1% of the dry weight of sodium alginate as TEMPO oxidant and 5% of the dry weight of sodium alginate as sodium bromide are added to the solution sequentially. The pH is adjusted to 10.5 with 1mol / L NaOH solution and kept constant. Then, a sodium hypochlorite solution with an effective chlorine mass fraction of 10% is added dropwise, and the amount is 15% of the dry weight of sodium alginate. After reacting for 4 hours, 1mol / L HCl solution is added dropwise to adjust the pH to 7.

0. An equal volume of anhydrous ethanol is added to the reaction solution. The precipitate is separated by centrifugation, the supernatant is discarded, the precipitate is redispersed in deionized water, and ethanol is added again to precipitate. The washing-centrifugation process is repeated 3 times. The final precipitate is dried at 40℃ for 24 hours to constant weight, and then pulverized to obtain oxidized modified sodium alginate powder.

2. The high-strength paperboard according to claim 1, characterized in that: In the preparation method of the oxidized modified sodium alginate, after reacting for 4 hours, 5% of the dry weight of sodium alginate in methanol is added dropwise. After stirring evenly, a drop of the reaction solution is placed on starch-potassium iodide test paper. If the test paper turns blue or black, 2% of the dry weight of sodium alginate in methanol is added dropwise. After stirring evenly, the test paper is used to detect the solution again. This process is repeated until the test paper retains its original yellow or light brown color. Then, 1 mol / L HCl solution is added to adjust the pH to 7.

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3. The high-strength paperboard according to claim 1, characterized in that: The outer core layer also includes 0.05-0.1 parts by weight of quaternary ammonium chitosan.

4. The high-strength paperboard according to claim 1, characterized in that: By weight, the surface layer comprises 30-50 parts bleached sulfate softwood pulp, 20-40 parts bleached sulfate hardwood pulp, 5-15 parts nano-calcium carbonate, 5-15 parts calcined kaolin, 3-8 parts oxidized starch, 2-5 parts styrene-butadiene latex, 0.5-2 parts polyvinyl alcohol, 0.2-0.8 parts alkyl ketene dimer sizing agent, and 0.1-0.5 parts cationic polyacrylamide.

5. The high-strength paperboard according to claim 4, characterized in that: The surface layer also includes stearic anhydride by weight, wherein the mass ratio of stearic anhydride to alkyl ketene dimer sizing agent is 1:

4.

6. The high-strength paperboard according to claim 5, characterized in that: The surface layer also includes 0.1-0.4 parts sodium carboxymethyl cellulose by weight.