A method for increasing the filler content and distribution uniformity in paper sheets and enhancing the strength of paper sheets

Abstract

The present invention provides a method for increasing the filler content in paper, improving the distribution uniformity and enhancing the paper strength, belonging to the technical field of papermaking. The method for increasing the filler content in paper, improving the distribution uniformity and enhancing the paper strength comprises the following steps: preparing a modified composite nanofiber dispersion liquid, preparing a cationic buffer solution, preparing a cationic composite sustained-release solution, preparing a filler particle suspension, and mixing and papermaking. The method for increasing the filler content in paper, improving the distribution uniformity and enhancing the paper strength of the present invention can gently and effectively control the flocculation rate of the filler, controllably adjust the floc size, can increase the filler content in the paper while improving the distribution uniformity of the filler in the paper, and effectively avoid the adverse effects of the increase in the filler dosage on the mechanical properties of the paper, and further enhance the paper strength.

Description

Technical Field

[0001] The present invention relates to the technical field of papermaking, and particularly relates to a method for increasing the filler content and distribution uniformity in a paper sheet and enhancing the strength of the paper sheet. Background Art

[0002] In the papermaking process, adding fillers (such as calcium carbonate, talc, titanium dioxide, etc.) is one of the key steps to optimize the properties of paper. Adding fillers can not only replace part of the fiber raw materials, reduce the fiber usage, and lower the production cost, but also endow the paper with certain properties, such as filling the voids between fibers in the paper, improving the formation and smoothness of the paper, enhancing the opacity and whiteness of the paper, and improving the printability. Among them, the fillers fill the voids between fibers in the paper by embedding into the pores between fibers through the filling effect, improving the compactness of the paper structure, and thus improving the physical properties of the paper; the fillers improve the formation and smoothness of the paper by reducing the "bridging" phenomenon between fibers through the filling effect, reducing the surface unevenness, and thus improving the formation and smoothness of the paper; the fillers improve the opacity and whiteness of the paper by the scattering, refraction, and absorption of light after the fillers are filled in the paper, causing the light to scatter and refract at the interface between the fillers and fibers, reducing the proportion of transmitted light, and adjusting the original light yellow tone of the fibers through the own hue of the fillers, thereby achieving the above functions; the fillers improve the printability of the paper by filling the fiber gaps, improving the surface flatness of the paper, and combining the absorption and color fixation of the printing ink by the fillers, reducing the "dot loss" during printing, and improving the image clarity.

[0003] However, adding fillers also has adverse effects on the paper itself. Specifically, during the dewatering process in the wire section of papermaking, affected by gravity and vacuum suction, the fillers will quickly move downward with the white water, resulting in a large amount of filler particles gathering near the back side of the paper sheet (i.e., near the forming wire) when viewed from the Z direction of the paper cross-section, and relatively less near the front side of the paper sheet (i.e., far from the forming wire). The distribution of the fillers in the Z direction of the paper cross-section is uneven, which directly leads to the deterioration of the mechanical properties, surface quality, stability, etc. of the paper. At the same time, to improve the distribution of the filler particles near the front side of the paper sheet, if the filler dosage is directly increased to increase the filler content in the paper sheet, the strength of the paper will decrease significantly, and there is a negative correlation between the two; avoiding the loss of paper strength while increasing the filler content in the paper is a technical problem in the papermaking field.

[0004] In the prior art, it is disclosed that after pretreating filler particles with sodium polyacrylate, flocculating them with a polymer flocculant composed of cationic starch and guar gum flocculant, and then combining them with a copolymer of dimethylaminoethyl methacrylate and acrylamide to form filler flocs; by mixing and papermaking the filler flocs with raw pulp, the addition amount of filler in the paper is increased. However, in actual production, the coating effect of the coagulant used on the filler particles is not easy to control: when the addition amount of the coagulant is small, the flocs formed by the filler particles are small, and the flocculation effect is not obvious; as a result, the increase in the addition amount of the filler in the paper is not obvious, and the filler content in the paper is not significantly increased. When the addition amount of the coagulant is large, due to the large molecular weight and strong positive charge of the positively charged flocculating component, it will quickly and massively coat the filler particles, forming super-large filler flocs locally, which will further lead to more uneven distribution of the filler in the paper; even worse, when the filler flocs into super-large flocs, filler spots will be formed in the paper, affecting the physical properties and printing properties of the paper, resulting in unqualified product quality. Summary of the Invention

[0005] To solve the technical problems existing in the prior art, the present invention provides a method for increasing the filler content, distribution uniformity in the paper sheet and increasing the paper sheet strength, which can gently and effectively control the filler flocculation speed, controllably adjust the floc size, can increase the filler content in the paper sheet while improving the distribution uniformity of the filler in the paper sheet, and effectively avoid the adverse effects of the increase in filler dosage on the mechanical properties of the paper sheet, and further improve the paper sheet strength.

[0006] To solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] A method for increasing the filler content, distribution uniformity in the paper sheet and increasing the paper sheet strength, which consists of the following steps: preparing a modified composite nanofiber dispersion, preparing a positively charged buffer solution, preparing a positively charged composite sustained-release solution, preparing a filler particle suspension, and mixing and papermaking;

[0008] The preparation of the modified composite nanofiber dispersion consists of the following steps: pre-composite and modification treatment;

[0009] For the pre-composite, carboxymethyl cellulose nanofibers are dispersed in deionized water to obtain a nanofiber dispersion; an aqueous solution of zinc nitrate is added dropwise to the nanofiber dispersion, heated to 35-40 °C, and an aqueous solution of 2-methylimidazole is added dropwise; after holding the reaction and standing, the solid is separated, and the solid is washed and dried to obtain composite nanofibers;

[0010] For the modification treatment, the composite nanofibers are treated with a positively charged modification solution to obtain a modified composite nanofiber dispersion;

[0011] The positively charged modification solution is an aqueous solution of dimethyldiallylammonium chloride;

[0012] To prepare the cationic buffer solution, acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride are heated and reacted in the presence of an initiator to obtain the cationic buffer solution;

[0013] To prepare the cationic composite sustained-release solution, the modified composite nanofiber dispersion, the cationic buffer solution, and amphoteric high molecular weight polyacrylamide are mixed evenly to obtain the cationic composite sustained-release solution.

[0014] Furthermore, to prepare the filler particle suspension, ground calcium carbonate is put into deionized water to obtain a ground calcium carbonate dispersion; the cationic composite sustained-release solution is added to the ground calcium carbonate dispersion to obtain the filler particle suspension;

[0015] For the mixed papermaking, the papermaking pulp and the filler particle suspension are mixed evenly and then diluted, and a retention aid is added and then papermaking is carried out to obtain the base paper.

[0016] Preferably, in the pre-composite, the mass concentration of the nanofiber dispersion is 2.9 - 3.3 wt%;

[0017] The concentration of the zinc nitrate aqueous solution is 0.2 - 0.3 mol / L;

[0018] The concentration of the 2-methylimidazole aqueous solution is 0.4 - 0.6 mol / L;

[0019] The heat preservation reaction time is 10 - 12 h, and the standing time is 6 - 8 h.

[0020] Preferably, in the pre-composite, the weight ratio of the carboxymethyl cellulose nanofibers in the nanofiber dispersion to the zinc nitrate in the zinc nitrate aqueous solution is 5 - 5.5:1;

[0021] The molar ratio of the zinc nitrate in the zinc nitrate aqueous solution to the 2-methylimidazole in the 2-methylimidazole aqueous solution is 1:4 - 4.2.

[0022] Furthermore, the method for the modification treatment is to disperse the composite nanofibers in deionized water to obtain a composite nanofiber dispersion with a mass concentration of 8 - 8.5 wt%; then a cationic modification liquid with a mass concentration of 11 - 12 wt% is dropped in, and treatment is carried out at room temperature to obtain a modified composite nanofiber dispersion.

[0023] Preferably, in the modification treatment, the room temperature treatment time is 4 - 5 h;

[0024] The weight ratio of the composite nanofibers in the composite nanofiber dispersion to the dimethyldiallylammonium chloride in the cationic modification liquid is 1:0.28 - 0.31.

[0025] Further, the method for preparing the cationic buffer solution is to disperse acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride in deionized water to obtain a mixed monomer solution with a monomer mass concentration of 10-12 wt%, and after adding an initiator, raise the temperature for reaction to obtain the cationic buffer solution;

[0026] In the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride is 2.3-2.5:0.6-0.7:0.5-0.6.

[0027] Preferably, in the preparation of the cationic composite sustained-release solution, the weight ratio of the modified composite nanofiber dispersion, the cationic buffer solution, and the amphoteric high-molecular-weight polyacrylamide is 1-1.2:1-1.2:2.9-3.1.

[0028] Preferably, in the preparation of the filler particle suspension, the mass concentration of the ground calcium carbonate dispersion is 30-33 wt%;

[0029] The added weight of the cationic composite sustained-release solution is 0.1-2% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion.

[0030] Preferably, in the mixed papermaking, the absolute dry weight ratio of the ground calcium carbonate in the filler particle suspension to the papermaking pulp is 30-50%:50-70%.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] (1)The method for increasing the filler content, distribution uniformity in the paper sheet and enhancing the paper sheet strength of the present invention is as follows: By setting a gradient positively charged composite sustained-release liquid containing small molecular weight, medium molecular weight, and high molecular weight components, mixing it with ground calcium carbonate to form a filler particle suspension, and then mixing it with papermaking pulp for sheet forming. Among them, when the ground calcium carbonate contacts the positively charged composite sustained-release liquid, the small molecular weight modified composite nanofibers in the positively charged composite sustained-release liquid are positively charged and have a small molecular weight, with good mobility and flexibility. They can preferentially contact the filler particles, weaken the negative charge of the filler particles, and more gently flocculate with the negatively charged filler particles. At the same time, in cooperation with the medium molecular weight cationic modified polyacrylamide polymer (i.e., the positively charged buffer solution), it can effectively buffer the flocculation rate of the high molecular weight polyacrylamide and the filler particles, effectively avoid the formation of super-large flocs, and effectively control the size of the filler flocs and their distribution uniformity in the papermaking pulp. At the same time, the modified composite nanofibers have the characteristics of a large specific surface area, many chemical bonds, high surface energy, and high activity, and have a strong binding force with the filler and pulp fibers, capable of forming a more firm fiber network structure, thereby effectively enhancing the paper sheet strength. Further, the medium molecular weight cationic modified polyacrylamide polymer (i.e., the positively charged buffer solution) in the positively charged composite sustained-release liquid has a stronger positive charge and a larger molecular weight compared to the small molecular weight modified composite nanofibers, and can achieve a further buffering effect on this basis. Through the cooperation of the small molecular weight modified composite nanofibers and the medium molecular weight cationic modified polyacrylamide polymer, it can improve the Z-direction distribution uniformity of the cellulose fibers and filler flocs in the paper, increase the filler content in the paper sheet, and simultaneously improve the paper sheet strength under the condition of high filler content. The above-mentioned various technical means cooperate with each other and act synergistically, can gently and effectively control the filler flocculation rate, controllably adjust the floc size, can increase the filler content in the paper sheet while improving the distribution uniformity of the filler in the paper sheet, and effectively avoid the adverse impact of the increase in filler dosage on the mechanical properties of the paper sheet, and further improve the paper sheet strength.

[0033] (2)The method for increasing the filler content, distribution uniformity in the paper sheet and enhancing the paper sheet strength of the present invention controls the absolute dry weight ratio of the filler to the papermaking pulp to be 35%:65%, and the white cardboard with a basis weight of 180 g / m 2 obtained by sheet forming has a bursting index of up to 2.47 (kPa·m 2 ) / g, a longitudinal Taber stiffness of up to 4.91 mN·m, a transverse Taber stiffness of up to 3.17 mN·m, a transverse folding endurance of up to 119 times, and an ash content of up to 25.4%.

[0034] (3)The method for increasing the filler content, distribution uniformity in the paper sheet and enhancing the paper sheet strength of the present invention controls the absolute dry weight ratio of the filler to the papermaking pulp to be 45%:55%, and the white cardboard with a basis weight of 180 g / m 2 obtained by sheet forming has a bursting index of up to 2.40 (kPa·m2 ) / g, the longitudinal Taber stiffness can reach 5.52 mN·m, the transverse Taber stiffness can reach 3.36 mN·m, the transverse folding endurance can reach 113 times, and the ash content can reach 28.6%.

[0035] (4) The method for increasing the filler content, distribution uniformity and paper strength in the present invention controls the absolute dry weight ratio of the filler to the papermaking pulp to be 35%:65%. The offset paper with a basis weight of 70 g / m² obtained by papermaking 2 has a tensile index that can reach 60.61 N·m / g, a transverse folding endurance that can reach 36 times, a printing surface strength (front side) that can reach 3.39 m / s, a printing surface strength (back side) that can reach 3.30 m / s, and an ash content that can reach 26.1%.

[0036] (5) The method for increasing the filler content, distribution uniformity and paper strength in the present invention controls the absolute dry weight ratio of the filler to the papermaking pulp to be 45%:55%. The offset paper with a basis weight of 70 g / m² obtained by papermaking 2 has a tensile index that can reach 69.26 N·m / g, a transverse folding endurance that can reach 34 times, a printing surface strength (front side) that can reach 3.86 m / s, a printing surface strength (back side) that can reach 3.08 m / s, and an ash content that can reach 28.0%.

[0037] (6) The method for increasing the filler content, distribution uniformity and paper strength in the present invention has strong applicability and can be effectively applied not only to the production of white cardboard but also to the production of offset paper, and can effectively increase the filler content, distribution uniformity and strength of white cardboard and offset paper. Specific Embodiments

[0038] In order to have a clearer understanding of the technical features, objectives and effects of the present invention, the specific embodiments of the present invention are now described. It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanations of the present invention. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0039] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, "first", "second", etc. are used to distinguish similar objects and are not used to describe a specific order or sequence. In addition, it should also be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0040] An embodiment of the present invention provides a method for increasing the filler content and distribution uniformity in a paper sheet and enhancing the strength of the paper sheet, which consists of the following steps: preparing a modified composite nanofiber dispersion, preparing a positively charged buffer solution, preparing a positively charged composite sustained-release solution, preparing a filler particle suspension, and mixing and papermaking.

[0041] The preparation of the modified composite nanofiber dispersion consists of the following steps: pre-composite and modification treatment.

[0042] The method of pre-composite is as follows: Put carboxymethyl cellulose nanofibers into deionized water, and after ultrasonic dispersion for 20 - 30 min, obtain a nanofiber dispersion with a mass concentration of 2.9 - 3.3 wt%. Under the condition of stirring at room temperature, drop an aqueous solution of zinc nitrate with a concentration of 0.2 - 0.3 mol / L into the nanofiber dispersion. After the dropping is completed, continue stirring for 20 - 40 min. Then, stir and heat up to 35 - 40 °C, keep stirring and drop an aqueous solution of 2-methylimidazole with a concentration of 0.4 - 0.6 mol / L, and control the dropping rate of the 2-methylimidazole aqueous solution to be 0.4 - 0.5 mL / min. After the dropping of the 2-methylimidazole aqueous solution is completed, keep stirring for 10 - 12 h, and then stand for 6 - 8 h. Then, centrifuge to obtain a solid, and after washing the solid with deionized water, dry it to obtain composite nanofibers.

[0043] In the pre-composite, the weight ratio of carboxymethyl cellulose nanofibers in the nanofiber dispersion to zinc nitrate in the aqueous solution of zinc nitrate is 5 - 5.5:1;

[0044] The molar ratio of zinc nitrate in the aqueous solution of zinc nitrate to 2-methylimidazole in the aqueous solution of 2-methylimidazole is 1:4 - 4.2;

[0045] The diameter of the carboxymethyl cellulose nanofibers is 50 - 100 nm.

[0046] The method of modification treatment is as follows: Put the composite nanofibers into deionized water, stir evenly to obtain a composite nanofiber dispersion with a mass concentration of 8 - 8.5 wt% for standby; put dimethyldiallylammonium chloride into deionized water, stir evenly to obtain a positively charged modification solution with a mass concentration of 11 - 12 wt% for standby; in a room temperature environment, under the stirring condition of 200 - 300 rpm, drop the positively charged modification solution into the composite nanofiber dispersion at a dropping rate of 0.9 - 1 mL / min; after the dropping of the positively charged modification solution is completed, continue stirring at room temperature for 4 - 5 h to obtain a modified composite nanofiber dispersion.

[0047] In the modification treatment, the weight ratio of the composite nanofibers in the composite nanofiber dispersion to dimethyldiallylammonium chloride in the positively charged modification solution is 1:0.28 - 0.31.

[0048] The method for preparing the cationic buffer solution is as follows: acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride are put into deionized water and stirred evenly to obtain a mixed monomer solution with a total mass concentration of 10-12 wt% of monomers (acrylamide, dimethyldiallylammonium chloride, 3-acryloylpropyltrimethylammonium chloride) for standby; ammonium persulfate is put into deionized water and stirred evenly to obtain an ammonium persulfate solution with a mass concentration of 1-1.2 wt% for standby; the mixed monomer solution and the ammonium persulfate solution are added into a reaction kettle, and under the stirring condition of 150-250 rpm, the temperature is raised to 55-60 °C and kept warm and stirred for 3-4 h to obtain the cationic buffer solution.

[0049] In the preparation of the cationic buffer solution, in the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride is 2.3-2.5:0.6-0.7:0.5-0.6;

[0050] The added weight of ammonium persulfate in the ammonium persulfate solution is 1.7-2 wt% of the total weight of the mixed monomers in the mixed monomer solution.

[0051] The method for preparing the cationic composite sustained-release solution is as follows: the modified composite nanofiber dispersion liquid, the cationic buffer solution, and amphoteric high molecular weight polyacrylamide (molecular weight 1,000,000-5,000,000) are mixed evenly according to the weight ratio of 1-1.2:1-1.2:2.9-3.1 to obtain the cationic composite sustained-release solution.

[0052] The method for preparing the filler particle suspension is as follows: ground calcium carbonate GCC is put into deionized water and stirred evenly to obtain a ground calcium carbonate dispersion liquid with a mass concentration of 30-33 wt%; under the stirring condition at room temperature, the cationic composite sustained-release solution is added into the ground calcium carbonate dispersion liquid and stirred evenly to prepare the filler particle suspension.

[0053] In the preparation of the filler particle suspension, the added weight of the cationic composite sustained-release solution is 0.1-2% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion liquid; preferably 0.3-1.2 wt%.

[0054] The method for mixed papermaking is as follows: according to the ratio of the absolute dry weight of the filler (ground calcium carbonate) to the papermaking pulp of 30-50%:50-70%, the papermaking pulp and the filler particle suspension are mixed evenly, the concentration of the paper stock obtained after mixing is diluted to 1-2 wt%, after adding a retention aid, the original paper sheet is obtained by papermaking on a machine.

[0055] In the mixed papermaking, the retention aid is cationic starch, and the addition amount of the retention aid is 0.8-1 wt% (calculated based on the weight of the absolute dry pulp).

[0056] A method for increasing the filler content and distribution uniformity in paper sheets and enhancing the paper sheet strength. In the pre-composite step of preparing the modified composite nanofiber dispersion, in-situ composite is adopted to contact the nanofiber dispersion containing carboxymethyl cellulose nanofibers with an aqueous zinc nitrate solution, adsorb zinc ions onto the carboxymethyl cellulose nanofibers, and then continue to contact with a 2-methylimidazole solution to composite ZIF-8 in an in-situ composite manner to obtain composite nanofibers. Then, in the modification treatment step, the composite nanofibers are treated with a cationic modification liquid containing dimethyldiallylammonium chloride to obtain a modified composite nanofiber dispersion. During the contact process between the modified composite nanofiber dispersion and the filler, through the high specific surface area and high porosity characteristics of ZIF-8, a porous network structure is formed by composite with the carboxymethyl cellulose nanofibers and is uniformly dispersed in the pulp, providing more binding sites for the filler. At the same time, through cationic modification, while further improving the dispersion uniformity of the modified composite nanofibers, the binding performance between the filler and the modified composite nanofibers is enhanced, and the filler content and distribution uniformity are improved. In the step of preparing the cationic buffer solution, dimethyldiallylammonium chloride and 3-acryloylpropyltrimethylammonium chloride are used in combination, and in the presence of an initiator, they are contacted with acrylamide for free radical copolymerization to obtain a cationic modified polyacrylamide polymer with medium molecular weight (i.e., the cationic buffer solution). In the step of preparing the cationic composite sustained-release solution, the modified composite nanofiber dispersion, the cationic buffer solution, and an amphoteric high molecular weight polyacrylamide are mixed to obtain a gradient cationic composite sustained-release solution containing low molecular weight, medium molecular weight, and high molecular weight (i.e., the cationic composite sustained-release solution). In the preparation of the filler particle suspension, ground calcium carbonate GCC is contacted with the cationic composite sustained-release solution. The modified composite nanofibers in the cationic composite sustained-release solution, which are cationic and have a small molecular weight, can flocculate with the anionic filler particles more gently. At the same time, in combination with the cationic modified polyacrylamide polymer with medium molecular weight, it can effectively buffer the flocculation rate of the high molecular weight polyacrylamide and the filler particles, effectively avoiding the formation of super-large flocs, and thus obtaining uniform filler flocs. At the same time, the modified composite nanofibers have the characteristics of large specific surface area, many chemical bonds, high surface energy, and high activity, and have a strong binding force with the filler and pulp fibers, and can form a more solid fiber network structure, thereby effectively enhancing the paper sheet strength. Further, the cationic modified polyacrylamide polymer with medium molecular weight (i.e., the cationic buffer solution) has stronger cationicity and a larger molecular weight compared with the modified composite nanofibers, and can achieve a further buffering effect on this basis.Furthermore, in the hybrid papermaking step, during the hybrid papermaking process of papermaking pulp and filler particle suspension, a gradient cationic composite slow-release solution of low molecular weight modified composite nanofibers, medium molecular weight cationic modified polyacrylamide polymer, and high molecular weight polyacrylamide is used. Due to its low molecular weight characteristics, the modified composite nanofibers are more flexible and can preferentially contact the filler particles, weaken the negative charge of the filler particles, buffer the flocculation of the filler particles and the high molecular weight polyacrylamide, effectively control the size of the filler flocs and their uniform distribution in the papermaking pulp, improve the Z-directional uniformity of the cellulose fibers and filler flocs in the paper, increase the filler content in the paper sheet, and simultaneously improve the paper strength under the condition of high filler content. The above-mentioned technical means cooperate with each other and work synergistically to gently and effectively control the filler flocculation rate, controllably adjust the floc size, increase the filler content in the paper while improving the uniform distribution of the filler in the paper, and effectively avoid the adverse effects of increased filler dosage on the mechanical properties of the paper, further improving the paper strength.

[0057] The following further illustrates the present invention in conjunction with some specific embodiments.

[0058] Example 1

[0059] A method for increasing the filler content, distribution uniformity in the paper sheet and increasing the paper strength consists of the following steps:

[0060] 1. Prepare a modified composite nanofiber dispersion

[0061] (1) Pre-compounding

[0062] Put carboxymethyl cellulose nanofibers into deionized water. After ultrasonic dispersion for 20 min, a nanofiber dispersion with a mass concentration of 2.9 wt% is obtained; under the condition of stirring at room temperature, an aqueous solution of zinc nitrate with a concentration of 0.2 mol / L is dropped into the nanofiber dispersion. After the dropping is completed, continue stirring for 20 min; then stir and heat up to 35 °C, keep stirring and drop an aqueous solution of 2-methylimidazole with a concentration of 0.4 mol / L, and control the dropping rate of the 2-methylimidazole aqueous solution to be 0.4 mL / min; after the dropping of the 2-methylimidazole aqueous solution is completed, keep stirring for 10 h and then stand for 6 h; then centrifuge to obtain a solid, and after washing the solid with deionized water and drying, composite nanofibers are prepared.

[0063] Among them, the weight ratio of the carboxymethyl cellulose nanofibers in the nanofiber dispersion to the zinc nitrate in the zinc nitrate aqueous solution is 5:1.

[0064] The molar ratio of zinc nitrate in the zinc nitrate aqueous solution to 2-methylimidazole in the 2-methylimidazole aqueous solution is 1:4.

[0065] The diameter of the carboxymethyl cellulose nanofibers is 70 nm.

[0066] (2)Modification treatment

[0067] Put the composite nanofibers into deionized water and stir evenly to obtain a composite nanofiber dispersion with a mass concentration of 8 wt%, for later use; put dimethyldiallylammonium chloride into deionized water and stir evenly to obtain a positively charged modification solution with a mass concentration of 11 wt%, for later use; in a room temperature environment, under the stirring condition of 200 rpm, drop the positively charged modification solution into the composite nanofiber dispersion at a dropping rate of 0.9 mL / min; after the dropping of the positively charged modification solution is completed, continue to stir at room temperature for 4 h to obtain a modified composite nanofiber dispersion.

[0068] Among them, the weight ratio of the composite nanofibers in the composite nanofiber dispersion to the dimethyldiallylammonium chloride in the positively charged modification solution is 1:0.28.

[0069] 2. Preparation of positively charged buffer solution

[0070] Put acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride into deionized water and stir evenly to obtain a mixed monomer solution with a total mass concentration of 10 wt% of the monomers (acrylamide, dimethyldiallylammonium chloride, 3-acryloylpropyltrimethylammonium chloride), for later use; put ammonium persulfate into deionized water and stir evenly to obtain an ammonium persulfate solution with a mass concentration of 1 wt%, for later use; add the mixed monomer solution and the ammonium persulfate solution into the reaction kettle, under the stirring condition of 150 rpm, heat up to 55 °C, and keep stirring for 3 h to obtain a positively charged buffer solution.

[0071] Among them, in the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride is 2.3:0.6:0.5.

[0072] The added weight of ammonium persulfate in the ammonium persulfate solution is 1.7 wt% of the total weight of the mixed monomers in the mixed monomer solution.

[0073] 3. Preparation of positively charged composite sustained-release solution

[0074] Mix the modified composite nanofiber dispersion, the positively charged buffer solution, and amphoteric high molecular weight polyacrylamide (molecular weight 5 million) evenly according to the weight ratio of 1:1:2.9 to obtain a positively charged composite sustained-release solution.

[0075] 4. Preparation of filler particle suspension

[0076] The ground calcium carbonate (GCC) is put into deionized water and stirred evenly to obtain a ground calcium carbonate dispersion with a mass concentration of 30 wt%. Under the condition of stirring at room temperature, the positively charged composite sustained-release liquid is added to the ground calcium carbonate dispersion and stirred evenly to prepare a filler particle suspension.

[0077] Among them, the added weight of the positively charged composite sustained-release liquid is 0.3 wt% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion.

[0078] 5. Mixed papermaking

[0079] According to the ratio of the absolute dry weight of the filler (ground calcium carbonate) to the papermaking pulp being 35%:65%, the papermaking pulp and the filler particle suspension are mixed evenly. The concentration of the obtained paper stock after mixing is diluted to 1.5 wt%. After adding a retention aid, the original paper sheet is obtained by papermaking on a machine.

[0080] Among them, the retention aid is cationic starch, and the addition amount of the retention aid is 0.9 wt% (calculated based on the weight of the absolute dry pulp).

[0081] Example 2

[0082] A method for increasing the filler content, distribution uniformity and paper strength in a paper sheet, which consists of the following steps:

[0083] 1. Preparation of a modified composite nanofiber dispersion

[0084] (1) Pre-composite

[0085] The carboxymethyl cellulose nanofibers are put into deionized water. After ultrasonic dispersion for 25 min, a nanofiber dispersion with a mass concentration of 3.1 wt% is obtained. Under the condition of stirring at room temperature, an aqueous solution of zinc nitrate with a concentration of 0.25 mol / L is dropped into the nanofiber dispersion. After the dropping is completed, stirring is continued for 30 min. Then, the temperature is raised to 37 °C with stirring, and an aqueous solution of 2-methylimidazole with a concentration of 0.5 mol / L is dropped while maintaining the temperature. The dropping rate of the aqueous solution of 2-methylimidazole is controlled at 0.45 mL / min. After the dropping of the aqueous solution of 2-methylimidazole is completed, stirring is maintained for 11 h and then left standing for 7 h. Then, solid matter is obtained by centrifugation. The solid matter is washed with deionized water and dried to obtain composite nanofibers.

[0086] Among them, the weight ratio of the carboxymethyl cellulose nanofibers in the nanofiber dispersion to the zinc nitrate in the aqueous solution of zinc nitrate is 5.2:1.

[0087] The molar ratio of the zinc nitrate in the aqueous solution of zinc nitrate to the 2-methylimidazole in the aqueous solution of 2-methylimidazole is 1:4.1.

[0088] The diameter of the carboxymethyl cellulose nanofibers is 70 nm.

[0089] (2)Modification treatment

[0090] Put the composite nanofibers into deionized water and stir evenly to obtain a composite nanofiber dispersion with a mass concentration of 8.2 wt%, for later use; put dimethyldiallylammonium chloride into deionized water and stir evenly to obtain a positively charged modification solution with a mass concentration of 11.5 wt%, for later use; in a room temperature environment, under the stirring condition of 250 rpm, drop the positively charged modification solution into the composite nanofiber dispersion at a dropping rate of 0.95 mL / min; after the dropping of the positively charged modification solution is completed, continue to stir at room temperature for 4.5 h to obtain a modified composite nanofiber dispersion.

[0091] Among them, the weight ratio of the composite nanofibers in the composite nanofiber dispersion to dimethyldiallylammonium chloride in the positively charged modification solution is 1:0.29.

[0092] 2. Preparation of positively charged buffer solution

[0093] Put acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride into deionized water and stir evenly to obtain a mixed monomer solution with a total mass concentration of 11 wt% of the monomers (acrylamide, dimethyldiallylammonium chloride, 3-acryloylpropyltrimethylammonium chloride), for later use; put ammonium persulfate into deionized water and stir evenly to obtain an ammonium persulfate solution with a mass concentration of 1.1 wt%, for later use; add the mixed monomer solution and the ammonium persulfate solution into the reaction kettle, and under the stirring condition of 200 rpm, heat up to 58 °C and keep stirring for 3.5 h to obtain a positively charged buffer solution.

[0094] Among them, in the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride is 2.4:0.65:0.55.

[0095] The added weight of ammonium persulfate in the ammonium persulfate solution is 1.8 wt% of the total weight of the mixed monomers in the mixed monomer solution.

[0096] 3. Preparation of positively charged composite sustained-release solution

[0097] Mix the modified composite nanofiber dispersion, the positively charged buffer solution, and amphoteric high molecular weight polyacrylamide (molecular weight 5 million) evenly according to the weight ratio of 1.1:1.1:3 to obtain a positively charged composite sustained-release solution.

[0098] 4. Preparation of filler particle suspension

[0099] The ground calcium carbonate (GCC) is put into deionized water and stirred evenly to obtain a ground calcium carbonate dispersion with a mass concentration of 31 wt%. Under the condition of stirring at room temperature, the positively charged composite sustained-release liquid is added to the ground calcium carbonate dispersion and stirred evenly to prepare a filler particle suspension.

[0100] Among them, the added weight of the positively charged composite sustained-release liquid is 0.8 wt% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion.

[0101] 5. Mixed papermaking

[0102] According to the absolute dry weight ratio of the filler (ground calcium carbonate) to the papermaking pulp being 45%:55%, the papermaking pulp and the filler particle suspension are mixed evenly. The concentration of the paper stock obtained after mixing is diluted to 1.5 wt%. After adding the retention aid, the original paper sheet is obtained by papermaking on the machine.

[0103] Among them, the retention aid is cationic starch, and the addition amount of the retention aid is 0.9 wt% (based on the weight of the absolute dry pulp).

[0104] Example 3

[0105] A method for increasing the filler content, distribution uniformity in the paper sheet and enhancing the paper sheet strength consists of the following steps:

[0106] 1. Preparation of modified composite nanofiber dispersion

[0107] (1) Pre-composite

[0108] The carboxymethyl cellulose nanofibers are put into deionized water. After ultrasonic dispersion for 30 min, a nanofiber dispersion with a mass concentration of 3.1 wt% is obtained. Under the condition of stirring at room temperature, an aqueous solution of zinc nitrate with a concentration of 0.25 mol / L is dropped into the nanofiber dispersion. After the dropping is completed, stirring is continued for 40 min. Then, the temperature is raised to 40 °C with stirring, and an aqueous solution of 2-methylimidazole with a concentration of 0.5 mol / L is dropped while maintaining the temperature. The dropping rate of the 2-methylimidazole aqueous solution is controlled at 0.5 mL / min. After the dropping of the 2-methylimidazole aqueous solution is completed, stirring is carried out at a constant temperature for 12 h and then left standing for 7 h. Then, solid matter is obtained by centrifugal separation. After the solid matter is washed with deionized water, it is dried to obtain composite nanofibers.

[0109] Among them, the weight ratio of the carboxymethyl cellulose nanofibers in the nanofiber dispersion to the zinc nitrate in the zinc nitrate aqueous solution is 5.3:1.

[0110] The molar ratio of the zinc nitrate in the zinc nitrate aqueous solution to the 2-methylimidazole in the 2-methylimidazole aqueous solution is 1:4.1.

[0111] The diameter of the carboxymethyl cellulose nanofibers is 70 nm.

[0112] (2)Modification treatment

[0113] Put the composite nanofibers into deionized water and stir evenly to obtain a composite nanofiber dispersion with a mass concentration of 8.2 wt%, for later use; put dimethyldiallylammonium chloride into deionized water and stir evenly to obtain a positively charged modification solution with a mass concentration of 11.5 wt%, for later use; in a room temperature environment, under the stirring condition of 250 rpm, drop the positively charged modification solution into the composite nanofiber dispersion at a dropping rate of 1 mL / min; after the dropping of the positively charged modification solution is completed, continue to stir at room temperature for 5 h to obtain a modified composite nanofiber dispersion.

[0114] Among them, the weight ratio of the composite nanofibers in the composite nanofiber dispersion to dimethyldiallylammonium chloride in the positively charged modification solution is 1:0.3.

[0115] 2. Preparation of positively charged buffer solution

[0116] Put acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride into deionized water and stir evenly to obtain a mixed monomer solution with a total mass concentration of 11 wt% of the monomers (acrylamide, dimethyldiallylammonium chloride, 3-acryloylpropyltrimethylammonium chloride), for later use; put ammonium persulfate into deionized water and stir evenly to obtain an ammonium persulfate solution with a mass concentration of 1.2 wt%, for later use; add the mixed monomer solution and the ammonium persulfate solution into the reaction kettle, under the stirring condition of 250 rpm, heat up to 60 °C, and keep stirring for 3.5 h to obtain a positively charged buffer solution.

[0117] Among them, in the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride is 2.4:0.65:0.6.

[0118] The added weight of ammonium persulfate in the ammonium persulfate solution is 1.9 wt% of the total weight of the mixed monomers in the mixed monomer solution.

[0119] 3. Preparation of positively charged composite sustained-release solution

[0120] Mix the modified composite nanofiber dispersion, the positively charged buffer solution, and amphoteric high molecular weight polyacrylamide (molecular weight 5 million) evenly according to the weight ratio of 1.2:1.2:3 to obtain a positively charged composite sustained-release solution.

[0121] 4. Preparation of filler particle suspension

[0122] Put the ground calcium carbonate (GCC) into deionized water and stir evenly to obtain a ground calcium carbonate dispersion with a mass concentration of 32 wt%. Under the condition of stirring at room temperature, add the positively charged composite sustained-release liquid to the ground calcium carbonate dispersion and stir evenly to prepare a filler particle suspension.

[0123] Among them, the added weight of the positively charged composite sustained-release liquid is 1 wt% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion.

[0124] 5. Mixed papermaking

[0125] Mix the papermaking pulp and the filler particle suspension evenly according to the ratio of the absolute dry weight of the filler (ground calcium carbonate) to the papermaking pulp being 45%:55%. Dilute the concentration of the obtained paper stock after mixing to 1.5 wt%. After adding the retention aid, make the original paper sheet by papermaking on the machine.

[0126] Among them, the retention aid is cationic starch, and the addition amount of the retention aid is 0.9 wt% (based on the weight of the absolute dry pulp).

[0127] Example 4

[0128] A method for increasing the filler content, distribution uniformity in the paper sheet and increasing the strength of the paper sheet, which consists of the following steps:

[0129] 1. Preparation of modified composite nanofiber dispersion

[0130] (1) Pre-composite

[0131] Put the carboxymethyl cellulose nanofibers into deionized water. After ultrasonic dispersion for 30 min, obtain a nanofiber dispersion with a mass concentration of 3.3 wt%. Under the condition of stirring at room temperature, dropwise add an aqueous solution of zinc nitrate with a concentration of 0.3 mol / L to the nanofiber dispersion. After the dropping is completed, continue stirring for 40 min. Then stir and heat up to 40 °C, keep stirring and dropwise add an aqueous solution of 2-methylimidazole with a concentration of 0.6 mol / L, and control the dropping rate of the 2-methylimidazole aqueous solution to be 0.5 mL / min. After the dropping of the 2-methylimidazole aqueous solution is completed, keep stirring for 12 h and then stand for 8 h. Then carry out centrifugal separation to obtain a solid. After washing the solid with deionized water, dry it to prepare composite nanofibers.

[0132] Among them, the weight ratio of the carboxymethyl cellulose nanofibers in the nanofiber dispersion to the zinc nitrate in the zinc nitrate aqueous solution is 5.5:1.

[0133] The molar ratio of zinc nitrate in the zinc nitrate aqueous solution to 2-methylimidazole in the 2-methylimidazole aqueous solution is 1:4.2.

[0134] The diameter of the carboxymethyl cellulose nanofibers is 100 nm.

[0135] (2)Modification treatment

[0136] Put the composite nanofibers into deionized water and stir evenly to obtain a composite nanofiber dispersion with a mass concentration of 8.5 wt%, for later use; put dimethyldiallylammonium chloride into deionized water and stir evenly to obtain a positively charged modification solution with a mass concentration of 12 wt%, for later use; in a room temperature environment, under the stirring condition of 300 rpm, add the positively charged modification solution dropwise to the composite nanofiber dispersion at a dropping rate of 1 mL / min; after the addition of the positively charged modification solution is completed, continue to stir at room temperature for 5 h to obtain a modified composite nanofiber dispersion.

[0137] Among them, the weight ratio of the composite nanofibers in the composite nanofiber dispersion to dimethyldiallylammonium chloride in the positively charged modification solution is 1:0.31.

[0138] 2. Preparation of positively charged buffer solution

[0139] Put acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride into deionized water and stir evenly to obtain a mixed monomer solution with a total mass concentration of 12 wt% of the monomers (acrylamide, dimethyldiallylammonium chloride, 3-acryloylpropyltrimethylammonium chloride), for later use; put ammonium persulfate into deionized water and stir evenly to obtain an ammonium persulfate solution with a mass concentration of 1.2 wt%, for later use; add the mixed monomer solution and the ammonium persulfate solution into the reaction kettle, under the stirring condition of 250 rpm, heat up to 60 °C, and keep stirring for 4 h to obtain a positively charged buffer solution.

[0140] Among them, in the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride, and 3-acryloylpropyltrimethylammonium chloride is 2.5:0.7:0.6.

[0141] The added weight of ammonium persulfate in the ammonium persulfate solution is 2 wt% of the total weight of the mixed monomers in the mixed monomer solution.

[0142] 3. Preparation of positively charged composite sustained-release solution

[0143] Mix the modified composite nanofiber dispersion, the positively charged buffer solution, and amphoteric high molecular weight polyacrylamide (molecular weight 5 million) evenly according to the weight ratio of 1.2:1.2:3.1 to obtain a positively charged composite sustained-release solution.

[0144] 4. Preparation of filler particle suspension

[0145] Put ground calcium carbonate (GCC) into deionized water and stir evenly to obtain a ground calcium carbonate dispersion with a mass concentration of 33 wt%. Under the condition of stirring at room temperature, add the cationic composite sustained-release liquid to the ground calcium carbonate dispersion and stir evenly to prepare a filler particle suspension.

[0146] Among them, the added weight of the cationic composite sustained-release liquid is 1.2 wt% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion.

[0147] 5. Mixed papermaking

[0148] According to the ratio of the absolute dry weight of the filler (ground calcium carbonate) to the papermaking pulp being 50%:50%, mix the papermaking pulp and the filler particle suspension evenly, dilute the concentration of the obtained paper stock to 2 wt%, add a retention aid, and then perform papermaking on the machine to obtain the base paper sheet.

[0149] Among them, the retention aid is cationic starch, and the addition amount of the retention aid is 1 wt% (calculated based on the weight of the absolute dry pulp).

[0150] Comparative Example 1

[0151] Adopt the technical solution of Example 2, the difference is: omit the step of preparing the modified composite nanofiber dispersion; and omit the addition of the modified composite nanofiber dispersion in the step of preparing the cationic composite sustained-release liquid.

[0152] Comparative Example 2

[0153] Adopt the technical solution of Example 2, the difference is: omit the pre-composite step in the modified composite nanofiber dispersion.

[0154] Take softwood pulp, thermally bleached chemi-mechanical pulp and bleached hardwood pulp as raw materials, and make papermaking pulp according to the weight ratio of 1:3:6; respectively adopt the technical solutions of Examples 1-4 and Comparative Examples 1-2, and in the mixed papermaking step, uniformly control the ratio of the absolute dry weight of the filler (ground calcium carbonate) to the papermaking pulp at 35%:65%, mix the papermaking pulp and the filler particle suspension evenly, dilute the concentration of the obtained paper stock to 1.5 wt%, add a retention aid (the addition amount is 0.9 wt%), and then perform papermaking on the machine respectively to produce white cardboard with a basis weight of 180 g / m 2 of white cardboard.

[0155] The bursting index, Taber stiffness, folding endurance (transverse), and ash content of white cardboard were measured respectively. Among them, the detection of the bursting index refers to the standard GB / T 454-2020 "Paper - Determination of bursting strength"; the detection of Taber stiffness refers to the standard GB / T 22806-2008 "White cardboard"; the detection of folding endurance (transverse) refers to GB / T 457-2008 "Paper and board - Determination of folding endurance"; the detection of ash content refers to GB / T 742-2018 "Paper, board and pulps - Determination of ignitable loss (ash)". The specific results are shown in the following table:

[0156]

[0157] Furthermore, softwood pulp, thermally bleached chemi-mechanical pulp, and bleached hardwood pulp were used as raw materials to prepare papermaking pulp in a weight ratio of 1:3:6; the technical solutions of Examples 1-4 and Comparative Examples 1-2 were respectively adopted, and in the mixed papermaking step, the absolute dry weight ratio of the filler (ground calcium carbonate) to the papermaking pulp was uniformly controlled at 45%:55%. The papermaking pulp and the filler particle suspension were mixed evenly, and the concentration of the obtained paper stock was diluted to 1.5 wt%. After adding a retention aid (addition amount: 1 wt%), white cardboard with a basis weight of 180 g / m 2 was respectively made on the machine. The bursting index, Taber stiffness, folding endurance (transverse), and ash content of the white cardboard were measured according to the aforementioned method. The specific results are shown in the following table:

[0158]

[0159] It can be seen that in the step of preparing the filler particle suspension in Examples 1-4, the added weights of the cationic composite sustained-release liquid were 0.3 wt%, 0.8 wt%, 1.0 wt%, and 1.2 wt% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion respectively. The bursting index, Taber stiffness, folding endurance (transverse), and ash content indexes of the finally made white cardboard with a basis weight of 180 g / m 2 increased successively with the increase of the addition amount of the cationic composite sustained-release liquid; and according to the changes of each detection index, it can be seen that the more preferred addition range of the cationic composite sustained-release liquid is 0.8-1.2 wt%; within this addition range, the bursting index, Taber stiffness, folding endurance (transverse), and ash content of the white cardboard can be significantly improved.

[0160] After omitting the steps of the modified composite nanofiber dispersion liquid in Comparative Example 1, the cationic composite sustained-release liquid lacks the mobilizing effect of small-molecule modified composite nanofibers, cannot pre-contact the filler particles and weaken their negative charge, cannot effectively buffer the flocculation process of the filler particles, and the size of the filler flocs and the distribution uniformity in the slurry both show obvious deterioration. It cannot effectively improve the Z-direction distribution uniformity of cellulose fibers and filler flocs in the paper, and cannot effectively avoid the adverse effects of high filler content on the mechanical properties of the paper. Specifically, the burst index, Taber stiffness, and transverse folding endurance of the white cardboard are significantly deteriorated compared with Example 2.

[0161] In Comparative Example 2, the pre-composite step in the modified composite nanofiber dispersion liquid is omitted; the small-molecule modified composite nanofibers cannot increase the binding sites with the filler particles through the composite synergistic effect of ZIF-8 and carboxymethyl cellulose nanofibers, nor can they further improve the binding performance between the filler and the modified composite nanofibers, and cannot improve the filler content and distribution uniformity. Specifically, the burst index, Taber stiffness, and transverse folding endurance of the white cardboard are deteriorated to a certain extent compared with Example 2.

[0162] Furthermore, softwood pulp, thermally bleached chemi-thermomechanical pulp, and bleached hardwood pulp are used as raw materials to make papermaking pulp according to a weight ratio of 1:3:6; the technical solutions of Examples 1-4 and Comparative Examples 1-2 are respectively adopted, and in the mixed papermaking step, the absolute dry weight ratio of the filler (ground calcium carbonate) to the papermaking pulp is uniformly controlled at 35%:65%. The papermaking pulp and the filler particle suspension are mixed evenly, the concentration of the obtained paper stock after mixing is diluted to 2 wt%, and after adding a retention aid (the addition amount is 0.9 wt%), they are respectively made into offset paper with a basis weight of 70 g / m 2 2.

[0163] The tensile index, transverse folding endurance, printing surface strength, and ash content of the offset paper are respectively detected. Among them, the detection of the tensile strength refers to GB / T 12914-2018 "Paper and Board - Determination of Tensile Strength - Constant Rate of Elongation Method (20 mm / min)"; the detection of the transverse folding endurance refers to GB / T 457-2008 "Paper and Board - Determination of Folding Endurance"; the detection of the printing surface strength refers to GB / T 22365-2008 "Paper and Board - Determination of Printing Surface Strength"; the detection of the ash content refers to GB / T 742-2018 "Paper Making Raw Materials, Pulp, Paper and Board - Determination of Ignition Residue (Ash Content)". The specific results are shown in the following table:

[0164]

[0165] Furthermore, softwood pulp, thermally bleached chemi-mechanical pulp and bleached hardwood pulp are used as raw materials to prepare papermaking pulp in a weight ratio of 1:3:6. The technical solutions of Examples 1-4 and Comparative Examples 1-2 are respectively adopted. In the mixed papermaking step, the absolute dry weight ratio of the filler (ground calcium carbonate) to the papermaking pulp is uniformly controlled at 45%:55%. The papermaking pulp and the filler particle suspension are mixed evenly. The concentration of the obtained paper stock after mixing is diluted to 2 wt%. After adding a retention aid (the addition amount is 1 wt%), the offset paper with a basis weight of 70 g / m 2 is made by machine. The tensile index, crosswise folding endurance, printing surface strength and ash content of the offset paper are detected according to the above method. The specific results are shown in the following table:

[0166]

[0167] It can be seen that in the step of preparing the filler particle suspension in Examples 1-4, the added weights of the cationic composite sustained-release liquid are 0.3 wt%, 0.8 wt%, 1.0 wt% and 1.2 wt% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion liquid in turn. The burst index, Taber stiffness, crosswise folding endurance and ash content indexes of the finally made offset paper with a basis weight of 70 g / m 2 increase in turn as the addition amount of the cationic composite sustained-release liquid increases; and according to the changes of each detection index, it can be seen that the more preferred addition range of the cationic composite sustained-release liquid is 0.8-1.2 wt%. Within this addition range, the tensile index, crosswise folding endurance, printing surface strength and ash content of the offset paper can be significantly improved.

[0168] After omitting the step of the modified composite nanofiber dispersion liquid in Comparative Example 1, the cationic composite sustained-release liquid lacks the mobilizing effect of the small molecular weight modified composite nanofibers, and cannot pre-contact the filler particles and weaken their negative charge, and cannot effectively buffer the flocculation process of the filler particles. Both the filler floc size and the distribution uniformity in the pulp show obvious deterioration, and the Z-direction distribution uniformity of the cellulose fibers and filler flocs in the paper cannot be effectively improved, and the adverse effects of high-content fillers on the mechanical properties of the paper cannot be effectively avoided. Specifically, the burst index, Taber stiffness and crosswise folding endurance of the offset paper are significantly deteriorated compared with those of Example 2.

[0169] In Comparative Example 2, the pre-composite step in the modified composite nanofiber dispersion liquid is omitted; the small molecular weight modified composite nanofibers cannot increase the binding sites with the filler particles through the composite synergistic effect of ZIF-8 and carboxymethyl cellulose nanofibers, and cannot further improve the binding performance of the filler and the modified composite nanofibers, and cannot improve the filler content and distribution uniformity. Specifically, the burst index, Taber stiffness and crosswise folding endurance of the offset paper are deteriorated to a certain extent compared with those of Example 2.

[0170] Unless otherwise specified, the percentages used in the present invention are all mass percentages.

[0171] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength, characterized in that: The method comprises the following steps: preparing a modified composite nanofiber dispersion, preparing a positively charged buffer solution, preparing a positively charged composite slow-release solution, preparing a filler particle suspension, and mixing and papermaking; The preparation of the modified composite nanofiber dispersion comprises the following steps: pre-compounding and modification treatment; The pre-compounding comprises dispersing carboxymethyl cellulose nanofibers in deionized water to obtain a nanofiber dispersion; dropping a zinc nitrate aqueous solution into the nanofiber dispersion, and dropping a 2-methylimidazole aqueous solution at a temperature of 35-40° C.; keeping the mixture warm for reaction, allowing the mixture to stand, separating and obtaining a solid, and washing and drying the solid to obtain composite nanofibers; In the modification treatment, the composite nanofibers are treated with a positively charged modification liquid to obtain a modified composite nanofiber dispersion; The positively charged modification liquid is an aqueous solution of dimethyldiallylammonium chloride; In the preparation of the positively charged buffer, acrylamide, dimethyldiallyl ammonium chloride, and 3-acryloylpropyltrimethylammonium chloride are reacted at elevated temperature in the presence of an initiator to obtain a positively charged buffer; The preparation of the positively charged composite sustained-release liquid comprises uniformly mixing the modified composite nanofiber dispersion, the positively charged buffer solution, and the amphoteric high molecular weight polyacrylamide to obtain the positively charged composite sustained-release liquid; The filler particle suspension is prepared by adding ground calcium carbonate into deionized water to obtain a ground calcium carbonate dispersion; and the positively charged composite slow-release liquid is added to the ground calcium carbonate dispersion to obtain a filler particle suspension; In the preparation of the filler particle suspension, the mass concentration of the ground calcium carbonate dispersion is 30-33wt%; The mixed papermaking comprises mixing the papermaking pulp and the filler particle suspension evenly, diluting the mixture, adding a retention agent, and then papermaking to obtain base paper; In the mixed papermaking, the absolute dry weight ratio of the ground calcium carbonate in the filler particle suspension to the papermaking pulp is 30-50%:50-70%.

2. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 1, characterized in that: In the pre-compounding, the mass concentration of the nanofiber dispersion is 2.9-3.3wt%; The concentration of zinc nitrate aqueous solution is 0.2-0.3 mol / L; The concentration of 2-methylimidazole aqueous solution is 0.4-0.6 mol / L; The heat preservation reaction time is 10-12h, and the standing time is 6-8h.

3. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 1, characterized in that: In the pre-compounding, the weight ratio of the carboxymethyl cellulose nanofibers in the nanofiber dispersion to the zinc nitrate in the zinc nitrate aqueous solution is 5-5.5:1; The molar ratio of zinc nitrate in the zinc nitrate aqueous solution to 2-methylimidazole in the 2-methylimidazole aqueous solution is 1:4-4.

2.

4. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 1, characterized in that: The modification method is to disperse the composite nanofibers in deionized water to obtain a composite nanofiber dispersion with a mass concentration of 8-8.5wt%; then drop a positively charged modification liquid with a mass concentration of 11-12wt% and obtain a modified composite nanofiber dispersion after treatment at room temperature.

5. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 4, characterized in that: In the modification treatment, the room temperature treatment time is 4-5h; The weight ratio of the composite nanofibers in the composite nanofiber dispersion to the dimethyldiallylammonium chloride in the positively charged modification liquid is 1:0.28-0.

31.

6. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 1, characterized in that: The method for preparing the positively charged buffer comprises dispersing acrylamide, dimethyldiallylammonium chloride and 3-acryloylpropyltrimethylammonium chloride in deionized water to obtain a mixed monomer solution with a monomer mass concentration of 10-12 wt %, adding an initiator, and heating the solution to obtain the positively charged buffer; In the mixed monomer solution, the weight ratio of acrylamide, dimethyldiallylammonium chloride and 3-acryloylpropyltrimethylammonium chloride is 2.3-2.5:0.6-0.7:0.5-0.

6.

7. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 1, characterized in that: In the preparation of the positively charged composite sustained-release liquid, the weight ratio of the modified composite nanofiber dispersion, the positively charged buffer solution, and the amphoteric high molecular weight polyacrylamide is 1-1.2:1-1.2:2.9-3.

1.

8. The method for increasing filler content and distribution uniformity in paper sheets and increasing paper sheet strength according to claim 1, characterized in that: In the preparation of the filler particle suspension, the added weight of the positively charged composite sustained-release liquid is 0.1-2% of the weight of the ground calcium carbonate in the ground calcium carbonate dispersion.

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