Papermaking method for improving paper strength

Abstract

The invention relates to a papermaking method for improving paper strength, which comprises the following steps: adding a cationic copolymer into papermaking pulp for reaction, and preparing the reacted papermaking pulp into a paper product, the papermaking pulp contains free starch; the cationic copolymer is formed by polymerizing at least one cationic monomer and at least one nonionic monomer; the molecular weight of the cationic copolymer ranges from 2,000 Da to 20,000 Da, and the molecular weight of the cationic copolymer ranges from 2,000 Da to 2,000 Da; and the cationic degree of the cationic copolymer is 5 mol%-50 mol%. According to the method, free starch molecules which are basically electrically neutral can be fixed and retained on papermaking fibers, so that the starch plays a role of a papermaking dry strength agent.

Description

Technical Field

[0001] The invention relates to the field of papermaking, in particular to a papermaking method for improving the strength of paper. Background Art

[0002] Paper strength is one of the most important properties of paper. Usually, paper strength refers to the strength property of air-dried paper, which is also called dry strength. The main dry strength properties of paper include: tensile strength, breaking length, bursting resistance, folding resistance, interlayer bonding, tearing strength, stiffness, ring pressure, etc. The factors that affect the dry strength of paper are first the morphology, properties and composition of the fiber itself. For example, the longer the papermaking fiber is, the larger the aspect ratio is, and the softer it is, the higher the dry strength of the paper is, and vice versa. For another example, the miscellaneous cells and lignin in the papermaking fiber will reduce the dry strength of the paper. The hemicellulose in the papermaking fiber is conducive to the hydrogen bonding between the fibers, which is conducive to improving the dry strength of the paper. In addition, different types of pulp have different dry strengths of paper due to different fiber characteristics. For example, the strength of chemical pulp is better than that of mechanical pulp, and virgin pulp is better than recycled waste paper pulp. Coniferous wood pulp in chemical pulp is better than broadleaf wood pulp.

[0003] Secondly, the factor that affects the dry strength of paper is the bonding force between fibers. The bonding force between fibers in a paper sheet comes from hydrogen bonding. The more hydrogen bonds between fibers, the greater the bonding force between fibers, and thus the higher the dry strength of the paper. The beating method is used to make the fibers fibrillate and increase the specific surface area of ​​the fibers. The larger the fiber surface area, the more hydroxyl groups (-OH) on the surface, and thus the more hydrogen bonds between fibers, and the greater the dry strength of the paper. Generally speaking, the higher the degree of beating, the higher the strength of the paper. However, there are also limiting factors in using the beating method to improve the strength of paper. This is because the beating process will also cut the fibers, which will have a negative impact on the dry strength and other physical indicators of the paper. For example, although increasing the degree of beating can increase the tensile strength of the paper, it will also reduce the tear strength, air permeability, opacity and dimensional stability of the paper. In addition, increasing the beating degree not only increases the power consumption of beating, but also makes the fiber water filterability worse. This will not only increase the energy consumption of dehydration and drying in the papermaking process, but also affect the running speed of the paper machine.

[0004] In addition, adding reinforcing agents to improve the strength of paper is also a common method in the papermaking industry. Reinforcing agents refer to chemical additives that can improve the strength of paper, also known as "dry strength enhancers" or simply "dry strength agents". The papermaking dry strength agent mentioned here refers to the agent added to the pulp during the papermaking process. Adding dry strength agents to pulp can achieve the following economic benefits: 1) Cheaper secondary fiber raw materials can be used to replace more expensive superior fibers, thereby saving production costs; 2) While maintaining the original strength indicators, the paper sheet quantity can be reduced, thereby saving raw materials and reducing costs; 3) Improving paper sheet strength can improve product quality, enhance product grade, and increase product value; 4) While achieving the same paper strength, the pulping energy consumption can be reduced, and at the same time, water filtration can be improved, vehicle speed can be increased, and production can be increased.

[0005] Dry strength agents used in papermaking can be divided into two categories: one is natural polymers and their modified derivatives, and the other is synthetic polymers. Regardless of whether they are natural polymers or synthetic polymers, the most basic feature of these dry strength agents is that they contain a large number of hydroxyl groups (-OH) or amine groups (such as -NH 2 ) or amide (such as -C(O)-NH 2 ) in order to form a large number of hydrogen bonds with the hydroxyl groups on the fiber surface, thereby achieving the effect of improving the strength of the paper sheet. Dry strength agents are usually added to the pulp during the papermaking process. Therefore, dry strength agents must be water-soluble. In order for the dry strength agent to remain on the fiber and not be lost with the white water of the paper machine, the dry strength agent molecules usually need to carry a certain amount of charge. Since the fiber surface is negatively charged, dry strength agents usually carry a positive charge. Some are only positively charged, called "cationic dry strength agents"; some are negatively charged in addition to positive charges, called "amphoteric dry strength agents". A few dry strength agents are only negatively charged, called "anionic dry strength agents". However, anionic dry strength agents usually need to be used in conjunction with cationic additives to achieve an enhancing effect.

[0006] Natural polymer dry strength agents mainly include starch, guar gum, chitosan, water-soluble cellulose, and so on. Among them, starch is the most commonly used and used in the largest amount. Natural starch is basically electrically neutral, with neither positive charge nor negative charge (potato starch is an exception, which carries a small amount of negative charge). If natural starch is added directly to the pulp, it will be difficult to remain on the papermaking fibers. Therefore, natural starch usually needs to be chemically modified before it can be used as a papermaking dry strength agent. Most of the modified starches used as papermaking dry strength agents are cationic starch and amphoteric starch, and anionic starch is less used. The dosage of starch dry strength agents is usually 5kg / T to 20kg / T of paper dry weight.

[0007] Synthetic dry strength agents are mainly copolymers of polyacrylamide, which are composed of acrylamide monomers and other charged monomers. 2 ] can form hydrogen bonds with the hydroxyl groups of paper fibers to provide a reinforcing effect; while charged monomers can assist the dry strength agent molecules to be adsorbed on papermaking fibers through electrostatic attraction. According to the different charges they carry, polyacrylamide dry strength agents can be divided into cationic, anionic and amphoteric dry strength agents. The most commonly used are cationic and amphoteric types. Anionic polyacrylamide dry strength agents are less used. Non-ionic polyacrylamide is rarely used because it is difficult to remain on papermaking fibers. In order to ensure that the dry strength agent molecules can form enough hydrogen bonds with the fibers, the ionicity of the polyacrylamide dry strength agent (i.e. the molar fraction of cationic monomers and / or anionic monomers) is usually only about 5%-30%, and no more than 50%, and the molecular weight is usually not more than 1,000,000Da. Commercially available amphoteric or cationic polyacrylamide dry strength agents usually contain 15% dry weight, and a few are 20% dry weight. The dosage is usually 0.75kg / t to 9kg / t of paper absolute dry weight.

[0008] As for starch dry strength agents, the papermaking industry usually does not directly add unmodified natural starch to the pulp, but uses chemically modified starch, such as cationic starch, anionic starch, and amphoteric starch as papermaking dry strength agents. This is because natural starch molecules are basically electrically neutral and cannot generate electrostatic attraction with papermaking fibers, making it difficult to be fixed on the fibers, and thus difficult to play its due function. However, chemical modification of natural starch will not only increase the cost of the product, but the chemical modification process will also cause certain pollution to the environment. Therefore, if a method can be invented to directly add unmodified natural starch to the pulp as a papermaking dry strength agent, it can not only reduce the production cost of the papermaking process, but also be beneficial to environmental protection. Summary of the invention

[0009] Based on this, the present invention provides an application of a cationic copolymer as a natural starch fixing and retaining agent in improving paper strength and a corresponding papermaking method. The present invention uses a specific cationic copolymer as a natural starch fixing and retaining agent to fix and retain substantially electrically neutral starch molecules that are difficult to fix and retain on papermaking fibers, thereby allowing these starches to act as a papermaking dry strength agent, thereby achieving the purpose of improving paper strength.

[0010] The specific technical solutions are as follows.

[0011] A papermaking method for improving paper strength comprises the following steps:

[0012] Adding a cationic copolymer to a papermaking pulp to react, and then preparing a paper product from the reacted papermaking pulp;

[0013] The papermaking pulp contains free starch;

[0014] The cationic copolymer is polymerized from at least one cationic monomer and at least one nonionic monomer;

[0015] The cationic copolymer has a molecular weight of 2,000,000Da-20,000,000Da;

[0016] The cationic degree of the cationic copolymer is 5 mol% to 50 mol%.

[0017] In some embodiments, the cationic copolymer has a molecular weight of 4,000,000 Da to 15,000,000 Da.

[0018] In some embodiments, the cationic copolymer has a molecular weight of 6,000,000 Da to 12,000,000 Da.

[0019] In some embodiments, the cationic copolymer has a molecular weight of 8,000,000 Da-10,000,000 Da.

[0020] In some embodiments, the cationic copolymer has a cationicity of 8 mol % to 45 mol %.

[0021] In some embodiments, the cationic copolymer has a cationicity of 8 mol % to 35 mol %.

[0022] In some embodiments, the cationic copolymer has a cationicity of 8 mol % to 25 mol %.

[0023] In some embodiments, the cationic copolymer has a cationicity of 10 mol % to 20 mol %.

[0024] In some embodiments, the cationic monomer in the cationic copolymer is selected from at least one of acryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyltrimethylammonium chloride, acryloyloxypropyltrimethylammonium chloride, methacryloyloxypropyltrimethylammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate, acryloyloxyethyldimethylbenzylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, acryloyloxypropyldimethylbenzylammonium chloride, methacryloyloxypropyldimethylbenzylammonium chloride, acrylamidopropyltrimethylammonium chloride, methacrylamidopropyltrimethylammonium chloride, acrylamidoethyltrimethylammonium chloride, methacrylamidoethyltrimethylammonium chloride, N-(3-dimethylaminopropyl)acrylamide, N-(3-dimethylaminopropyl)methacrylamide, N-(2-dimethylaminoethyl)acrylamide, N-(2-dimethylaminoethyl)methacrylamide and dimethyldiallylammonium chloride.

[0025] In some of the embodiments, the nonionic monomer in the cationic copolymer is at least one of acrylamide, methacrylamide, N-methylacrylamide, N-methyl(meth)acrylamide, N-isopropylacrylamide, N-isopropyl(meth)acrylamide, N,N-dimethylacrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethylacrylamide, N,N-diethyl(meth)acrylamide, N-methyl-N-ethylacrylamide, N-methyl-N-ethyl(meth)acrylamide, N-hydroxyethylacrylamide, N-hydroxyethyl(meth)acrylamide and N-vinylformamide.

[0026] In some embodiments, the cationic copolymer is polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer.

[0027] In some embodiments, the molecular weight of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 4,000,000Da-15,000,000Da.

[0028] In some embodiments, the molecular weight of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 6,000,000Da-12,000,000Da.

[0029] In some embodiments, the molecular weight of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 9,000,000 Da-110,000,000 Da.

[0030] In some embodiments, the molecular weight of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 9,500,000Da-105,000,000Da.

[0031] In some embodiments, the molecular weight of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 8,000,000Da-10,000,000Da.

[0032] In some embodiments, the cationicity of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 8 mol %-45 mol %.

[0033] In some embodiments, the cationicity of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 8 mol %-35 mol %.

[0034] In some embodiments, the cationicity of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 8 mol %-25 mol %.

[0035] In some embodiments, the cationicity of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 10 mol %-20 mol %.

[0036] In some of the embodiments, the cationic copolymer is two polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymers with different molecular weights and cationic degrees; wherein the molecular weight of the first polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 7000,000Da-9000,000Da, and its cationic degree is 8mol%-12mol%; the molecular weight of the second polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 9000,000Da-11000,000Da, and its cationic degree is 15mol%-25mol%.

[0037] In some of the embodiments, the cationic copolymer is two polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymers with different molecular weights and cationic degrees; wherein the molecular weight of the first polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 7500,000Da-8500,000Da, and its cationic degree is 9mol%-11mol%; the molecular weight of the second polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 9500,000Da-10500,000Da, and its cationic degree is 18mol%-22mol%.

[0038] In some embodiments, the ratio of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer to the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 1:0.1-20.

[0039] In some embodiments, the ratio of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer to the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 1:0.5-10.

[0040] In some embodiments, the ratio of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer to the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 1:0.8-5.

[0041] In some embodiments, the ratio of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer to the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 1:1-2.

[0042] In some embodiments, the ratio of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer to the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 1:1.3-1.5.

[0043] In some of the embodiments, the cationic copolymer is added in an amount of 0.05 kg / t-2 kg / t of paper absolute dry weight.

[0044] In some embodiments, the cationic copolymer is added in an amount of 0.08 kg / t-1.5 kg / t of paper absolute dry weight.

[0045] In some embodiments, the cationic copolymer is added in an amount of 0.1 kg / t paper absolute dry weight to 1 kg / t paper absolute dry weight.

[0046] In some embodiments, the cationic copolymer is added in an amount of 0.15 kg / t paper absolute dry weight to 0.5 kg / t paper absolute dry weight.

[0047] In some embodiments, the cationic copolymer is added in an amount of 0.18 kg / t paper absolute dry weight to 0.3 kg / t paper absolute dry weight.

[0048] In some embodiments, the free starch is substantially electrically neutral starch.

[0049] In some of the embodiments, it is characterized in that the free starch is natural starch.

[0050] In some embodiments, the natural starch is selected from at least one of corn starch, wheat starch, potato starch, sweet potato starch and tapioca starch.

[0051] In some of the embodiments, the content of free starch in the papermaking pulp is 0.5 kg / t paper dry weight to 100 kg / t paper dry weight.

[0052] In some of the embodiments, the content of free starch in the papermaking pulp is 5 kg / t-50 kg / t of paper absolute dry weight.

[0053] In some of the embodiments, the content of free starch in the papermaking pulp is 10 kg / t paper absolute dry weight-30 kg / t paper absolute dry weight.

[0054] In some of the embodiments, the content of free starch in the papermaking pulp is 15 kg / t paper absolute dry weight - 25 kg / t paper absolute dry weight.

[0055] In some of the embodiments, conventional retention and drainage aids, dry strength agents and / or wet strength agents are further added to the papermaking slurry.

[0056] In some of the embodiments, the conventional dry strength agent is an amphoteric polyacrylamide dry strength agent, a cationic polyacrylamide dry strength agent, amphoteric ionic starch and / or cationic starch; the conventional wet strength agent is urea-formaldehyde resin, melamine formaldehyde resin and / or polyamide epichlorohydrin resin.

[0057] In some embodiments, based on dry weight, the amount of the amphoteric polyacrylamide dry strength agent and / or cationic polyacrylamide dry strength agent is 0.5kg / t-10kg / t of paper absolute dry weight; the amount of the amphoteric starch and / or cationic starch dry strength agent is 1kg / t-20kg / t of paper absolute dry weight; the amount of the conventional wet strength agent is 0.2kg / 10kg / t of paper absolute dry weight.

[0058] In some embodiments, the reaction time is 30s-10min.

[0059] In some embodiments, the pulp concentration of the papermaking pulp is 0.2-5%, preferably 0.5-4%.

[0060] In some embodiments, the pulp concentration of the papermaking pulp is 0.5-1%.

[0061] The cationic copolymer of the present invention is used in a papermaking process, wherein the cationic copolymer is used as a free starch fixative to increase the starch content of the finished paper.

[0062] The cationic copolymer of the present invention is used in a papermaking process, wherein the cationic copolymer is used as a free starch fixative to improve the strength of paper.

[0063] Application of the cationic copolymer of the present invention in a papermaking process, wherein the cationic copolymer is used as a free starch fixative to reduce the COD concentration of discharged wastewater

[0064] Natural starch, which has not undergone any chemical modification, has basically electrically neutral molecules, with neither positive nor negative charges, lacks electrostatic attraction with papermaking fibers, and is difficult to be fixed and retained on papermaking fibers. This means that natural starch cannot usually be directly added to paper pulp as a dry strength agent for papermaking. The inventors of the present invention unexpectedly discovered during long-term papermaking experiments that some cationic copolymers with specific molecular weights and ionicity can be used as starch fixatives, which can fix and retain basically electrically neutral starch molecules on papermaking fibers, thereby allowing natural starch to play the role of a papermaking dry strength agent, thereby achieving the purpose of improving paper strength. Therefore, the papermaking method of the present invention has the following advantages:

[0065] The method of the invention can fix and retain the basically electrically neutral starch molecules on the fibers, so that they can play the role of papermaking dry strength agent, thereby significantly improving the strength of paper. In this way, natural starch can be directly used as a papermaking dry strength agent, which is more cost-effective and more environmentally friendly than using modified starch, such as cationic starch and amphoteric starch, as a papermaking dry strength agent. DETAILED DESCRIPTION

[0066] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the embodiments, and preferred embodiments of the present invention are given below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. The purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0067] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0068] The present invention uses a specific cationic copolymer as a fixing agent for substantially electrically neutral free starch to fix and retain substantially electrically neutral starch molecules on papermaking fibers, thereby allowing the substantially electrically neutral starch to act as a papermaking dry strength agent, thereby achieving the purpose of improving paper strength.

[0069] The "free starch" or "dissolved starch" mentioned in the present invention refers to starch molecules dissolved in the water phase of the pulp. These starch molecules are either natural starch added to the pulp during the papermaking process, or starch contained in secondary fibers, but released again during the pulping and papermaking process.

[0070] The specific cationic copolymer is copolymerized by two or more different monomers and includes at least one cationic monomer and at least one nonionic monomer.

[0071] The cationic copolymer has a molecular weight of 2,000,000 Da to 20,000,000 Da, and a cationicity of 5 mol % to 50 mol %, preferably 8 mol % to 45 mol %, and more preferably 8 mol % to 35 mol %.

[0072] The cationic monomers of the cationic copolymer include, but are not limited to, acryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyltrimethylammonium chloride, acryloyloxypropyltrimethylammonium chloride, methacryloyloxypropyltrimethylammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate, acryloyloxyethyldimethylbenzylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, acryloyloxypropyldimethylbenzylammonium chloride, methacryloyloxypropyldimethylbenzylammonium chloride, acrylamidopropyltrimethylammonium chloride, methacrylamidopropyltrimethylammonium chloride, acrylamidoethyltrimethylammonium chloride, methacrylamidoethyltrimethylammonium chloride, N-(3-dimethylaminopropyl)acrylamide, N-(3-dimethylaminopropyl)methacrylamide, N-(2-dimethylaminoethyl)acrylamide, N-(2-dimethylaminoethyl)methacrylamide and dimethyldiallylammonium chloride.

[0073] The nonionic monomers of the cationic copolymer include, but are not limited to, acrylamide, methacrylamide, N-methylacrylamide, N-methyl(meth)acrylamide, N-isopropylacrylamide, N-isopropyl(meth)acrylamide, N,N-dimethylacrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethylacrylamide, N,N-diethyl(meth)acrylamide, N-methyl-N-ethylacrylamide, N-methyl-N-ethyl(meth)acrylamide, N-hydroxyethylacrylamide, N-hydroxyethyl(meth)acrylamide and N-vinylformamide.

[0074] According to the method of the present invention, the cationic copolymer used as a pulp free starch fixative can be used alone or in combination with a conventional papermaking dry strength agent and / or wet strength agent. The conventional dry strength agent is an amphoteric polyacrylamide dry strength agent, a cationic polyacrylamide dry strength agent, an amphoteric starch and / or a cationic starch; the conventional wet strength agent is a urea-formaldehyde resin, a melamine-formaldehyde resin and / or a polyamide epichlorohydrin resin.

[0075] According to the method of the present invention, the cationic copolymer used as a fixative for substantially electrically neutral starch can be used alone or in combination of two or more different cationic copolymers.

[0076] The cationic copolymer can be added to different parts of the paper machine, such as the pulper and discharge tower of the pulping workshop, or the pulp storage tower, pulping tank, pulp flushing tank, pressure screen inlet, headbox inlet, etc. of the paper machine. Alternatively, the cationic copolymer is added at two or more different adding points at the same time. When two or more different cationic copolymers are used in combination, they can be added to different parts of the paper machine respectively, or added to the same part of the paper machine. Even for the convenience of use, two or more different cationic copolymers can be mixed together in advance and then added to a certain part of the paper machine.

[0077] The amount of the cationic copolymer added is between 0.05kg / t-2kg / t of paper absolute dry weight. In order to further improve the strength index of paper, the cationic copolymer can also be used in combination with conventional dry strength agents or wet strength agents. Conventional dry strength agents include but are not limited to amphoteric or cationic polyacrylamide dry strength agents and amphoteric or cationic starch. The amount of the amphoteric or cationic polyacrylamide dry strength agent is between 0.5kg / t of paper absolute dry weight and 30kg / t of paper absolute dry weight. The amount of the amphoteric or cationic starch dry strength agent is between 1kg / t of paper absolute dry weight and 50kg / t of paper absolute dry weight. The conventional wet strength agent includes but is not limited to urea-formaldehyde resin, melamine formaldehyde resin, polyamide epichlorohydrin resin, and the amount is between 0.2kg / t of paper absolute dry weight and 20kg / t of paper absolute dry weight.

[0078] The substantially electrically neutral starch suitable for the present invention is natural starch, including but not limited to corn starch, wheat starch, potato starch, sweet potato starch, cassava starch, etc. These starches do not need to be chemically modified, and can be directly added to the pulp in the papermaking process according to the method of the present invention after being gelatinized and dissolved with hot water. The appropriate concentration of starch solution is 1% to 5%. The addition amount is 0.5kg / t paper absolute dry weight-100kg / t paper absolute dry weight. The conventional dry strength agent can be added before or after the cationic copolymer of the present invention, or between two cationic copolymers.

[0079] The present invention also relates to a papermaking method, which comprises the step of adding the cationic copolymer of the present invention; more specifically, it relates to the following steps:

[0080] (1) dispersing papermaking fibers in water to form a fiber suspension;

[0081] (2) adding 0.5 kg / t paper absolute dry weight to 100 kg / t paper absolute dry weight of gelatinized and dissolved natural starch to the fiber suspension to fully mix the fiber suspension and the natural starch;

[0082] (3) adding 0.05 kg / t paper absolute dry weight to 2 kg / t paper absolute dry weight of the cationic copolymer of the present invention to a fiber suspension containing natural starch;

[0083] (4) Optionally, 0.5 kg / t paper absolute dry weight to 30 kg / t paper absolute dry weight of amphoteric or cationic polyacrylamide dry strength agent, or / and 1 kg / t paper absolute dry weight to 50 kg / t paper absolute dry weight of amphoteric or cationic starch dry strength agent, or / and 0.2 kg / t paper absolute dry weight to 20 kg / t paper absolute dry weight of a conventional wet strength agent are added to the fiber suspension;

[0084] (5) The fiber suspension is then subjected to papermaking steps such as dehydration, pressing and drying to form paper products such as paper or paperboard with suitable dry strength.

[0085] The technology of the present invention has the following advantages:

[0086] The present invention relates to how to fix and retain substantially electrically neutral starch molecules on papermaking fibers, thereby achieving the purpose of improving the dry strength of paper.

[0087] Natural starch, which has not undergone any chemical modification, has basically electrically neutral molecules and cannot form electrostatic attraction with papermaking fibers, so it is difficult to be fixed and retained on papermaking fibers. For this reason, natural starch cannot usually be directly added to paper pulp as a papermaking dry strength agent. However, by applying the method of the present invention, the basically electrically neutral starch molecules can be fixed and retained on the papermaking fibers, so that they can play the role of a papermaking dry strength agent, thereby achieving the purpose of improving the dry strength of the paper sheet. In this way, natural starch can be directly used to improve the dry strength of the finished paper. This is more cost-effective than using modified starch, such as cationic starch and amphoteric starch, as a papermaking dry strength agent, and is also more conducive to green environmental protection.

[0088] Experimental samples

[0089] A: Polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer powder, cationicity 10 mol%, molecular weight about 8,000,000 Da, provided by Aisen (China) Agent Co., Ltd., diluted with deionized water to 0.1%, and set aside.

[0090] B: polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer emulsion, cationic degree 20 mol%, molecular weight about 10,000,000 Da, solid content about 45%, provided by Solenis (Shanghai) Chemical Co., Ltd., diluted with deionized water to 0.1%, and set aside.

[0091] Experimental Materials

[0092] Corn starch: provided by Zhucheng Xingmao Corn Development Co., Ltd.

[0093] Brazilian Eucalyptus oleifera pulp (bleached kraft hardwood pulp): Suzano Pulp and Paper

[0094] Experimental setup and instruments

[0095] PL12-00 High-Concentration Hydraulic Pulper: Shaanxi University of Science and Technology Machinery Plant

[0096] IMT-VL01 Standard Wally Beater: Dongguan Internason Precision Instrument Co., Ltd.

[0097] GBJ-A Fiber Decomposer: Changchun Yueming Small Testing Machine Co., Ltd.

[0098] IMT-SJ01 Standard Fiber Decomposer: Guangdong Dongguan Internaisan Precision Instrument Co., Ltd.

[0099] IMT-CP01A-3A Paper Forming Machine: Dongguan Internaisan Precision Instrument Co., Ltd.

[0100] IMT-Burst01P Burst Tester: Guangdong Dongguan Internaisan Precision Instrument Co., Ltd.

[0101] UV-1800 UV-visible spectrophotometer: Shimadzu Corporation (China)

[0102] Experimental methods

[0103] Preparation of dewatered bleached chemical pulp: Weigh 390g of absolute dry Brazilian goldfish eucalyptus pulp, add tap water to a pulp concentration of 13%, soak for 10 minutes, and use a PL12-00 high-concentration hydraulic pulper to pulp for 15 minutes. After pulping, move it into an IMT-VL01 Valley beater, add water to 23L, first beat it for 3 minutes, then add a 5.5kg weight, beat it for about 11 minutes, and make the pulp have a decomposition degree of 30 degrees. Finally, dehydrate it, measure the moisture content, and store it for later use.

[0104] Papermaking sheeting method: Take a certain amount of prepared pulp and use IMT-CP01A-3A paper sheet former to make paper sheets with a diameter of 20 cm and a certain gram weight. Unless otherwise specified, the wet paper sheets are dry-fried at 105°C and 0.095MPa vacuum for 5 minutes without pressing.

[0105] Determination of paper bursting resistance index: According to the national standard GB-T454-2002 for the determination of paper bursting resistance, the bursting resistance of paper sheets was tested using an IMT-Burst01P bursting resistance tester under standard relative humidity conditions.

[0106] Preparation of starch solution: weigh 32g corn starch into a 1000ml glass beaker, add deionized water to 800g, and adjust to a 4% starch solution. Then place the starch solution beaker in a 95℃ constant temperature water tank, stir and gelatinize for 1 hour. Then keep it warm at 60℃ for later use.

[0107] Determination of starch content in finished paper: Weigh a certain weight of absolutely dry paper sample in a container, add deionized water to make the slurry concentration 2% (w / w), then put the container containing the paper sample suspension into a boiling water bath oscillator, oscillate for 30 minutes, so that the starch in the finished paper is fully dissolved; then use the Megazyme total starch assay kit method (K-TSTA 04 / 2009) to determine the total amount of starch. This method allows the starch molecules in the paper extract to be hydrolyzed into glucose under the catalysis of amyloglucosidase (AGS). Then use a Shimadzu UV-1800 ultraviolet-visible spectrophotometer at a wavelength of 510nm to determine the absorbance of glucose, and then convert it into the starch content of the paper sample.

[0108] Example 1

[0109] This example tests the effect of the cationic copolymers A and B of the present invention on the bursting index and starch content of the finished paper after 15 kg / t of natural starch is added to the bleached chemical pulp.

[0110] Experimental method: Weigh 20g of each of the 3 pre-prepared dehydrated bleached chemical pulps, add deionized water to a total weight of 667g, and prepare a 3% pulp concentration. Add 15kg / t paper absolute dry weight of gelatinized corn starch solution under a constant temperature water bath of 50℃ and medium speed stirring. Continue stirring for 10 minutes, then add water to dilute to a pulp concentration of 0.5%. Then weigh 650g of the diluted pulp, place it on a mechanical stirrer at a speed of 1000rpm, and treat it in three conditions: in the first condition, do not add any cationic copolymer and stir for 1 minute; in the second condition, after stirring for 30 seconds, add 0.2kg / t paper absolute dry weight of cationic copolymer A dilution, and continue stirring for 30 seconds; in the third condition, first add 0.1kg / t paper absolute dry weight of cationic copolymer A dilution, stir for 30 seconds, then add 0.14kg / t paper absolute dry weight of cationic copolymer B dilution, and continue stirring for 30 seconds. After completing the above treatment, the pulp sample was made into a sheet with a gram weight of about 105 g / m 2 Then, the tear resistance index of the paper piece is determined according to the relevant national standard method. Another paper sample is taken and the starch content of the paper sample is determined according to the method of "Determination of starch content of paper".

[0111] The experimental results are listed in Table 1. It can be seen from the results that when 15 kg / t paper absolute dry weight of corn starch was added to the pulp, and then the cationic copolymer A (0.2 kg / t paper absolute dry weight) of the present invention was added, the starch content and bursting index of the paper were increased from the original 1.08 kg / t paper absolute dry weight and 2.45 kPa·m 2 / g increased to 4.32g / t paper dry weight and 2.88kPa·m 2 / g; if two cationic copolymers A (0.1kg / t paper dry weight) and B (0.14kg / t paper dry weight) are used together, the starch content and bursting index of the finished paper are further increased to 5.78g / t paper dry weight and 3.29kPa·m 2 This indicates that the cationic copolymers A and B of the present invention can fix and retain the substantially electrically neutral corn starch on the fiber, thereby improving the dry strength of the paper.

[0112] Table 1

[0113]

[0114] Example 2

[0115] This example tests the effect of the cationic copolymers A and B of the present invention on the bursting index of the finished paper and the finished paper starch after adding 25 kg / t of natural starch in an absolute dry weight of paper to the bleached chemical pulp.

[0116] Experimental method: Same as Example 1, but the amount of corn starch added to the pulp is 25 kg / t paper absolute dry weight. The experimental results are listed in Table 2. It can be seen from the results that when 25 kg / t paper absolute dry weight of corn starch is added to the pulp, and then the cationic copolymer A (0.2 kg / t paper absolute dry weight) of the present invention is added, the starch content and bursting index of the finished paper are increased from the original 1.50 kg / t paper absolute dry weight and 2.51 kPa·m 2 / g increased to 5.57kg / t paper dry weight and 2.84kPa·m 2 / g; if two cationic copolymers A (0.1kg / t paper dry weight) and B (0.14kg / t paper dry weight) are used together, the starch content and bursting index of the finished paper are further increased to 8.23g / t paper dry weight and 3.37kPa·m 2 This again shows that the cationic copolymers A and B of the present invention can fix and retain substantially electrically neutral corn starch on the fibers, thereby improving the dry strength of the paper.

[0117] Table 2

[0118]

[0119] Comparative Example 1

[0120] This comparative example tests the effect of the cationic copolymers A and B of the present invention on the bursting index of the finished paper when the bleached chemical pulp is in the absence of natural starch.

[0121] Experimental method: Same as Example 1, but no corn starch was added to the pulp. The experimental results are listed in Table 3. It can be seen from the results that when no corn starch was added to the pulp, whether copolymer A (0.2 kg / t paper absolute dry weight) was used alone or cationic copolymer A (0.1 kg / t paper absolute dry weight) and B (0.14 kg / t paper absolute dry weight) were used together, the bursting resistance index of the finished paper was basically unchanged. This shows that if there is no natural starch in the pulp, cationic copolymers A and B themselves do not have the function of improving the dry strength of paper.

[0122] Table 3

[0123]

[0124] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0125] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A papermaking method for improving paper strength, characterized in that: The steps include: Adding a cationic copolymer to a papermaking pulp to react, and then preparing a paper product from the reacted papermaking pulp; The papermaking pulp contains free starch; The cationic copolymer is polymerized from at least one cationic monomer and at least one nonionic monomer; The cationic copolymer has a molecular weight of 2,000,000Da-20,000,000Da; The cationic degree of the cationic copolymer is 5 mol% to 50 mol%.

2. The papermaking method for improving paper strength according to claim 1, characterized in that: The molecular weight of the cationic copolymer is 4,000,000 Da to 15,000,000 Da, preferably 6,000,000 Da to 12,000,000 Da, preferably 8,000,000 Da to 10,000,000 Da.

3. The papermaking method for improving paper strength according to claim 1, characterized in that: The cationic copolymer has a cationic degree of 8 mol% to 45 mol%, preferably 8 mol% to 35 mol%, preferably 8 mol% to 25 mol%, preferably 10 mol% to 20 mol%.

4. The papermaking method for improving paper strength according to claim 1, characterized in that: The cationic monomer in the cationic copolymer is selected from at least one of acryloyloxyethyl trimethyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, acryloyloxypropyl trimethyl ammonium chloride, methacryloyloxypropyl trimethyl ammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropyl acrylate, dimethylaminopropyl methacrylate, acryloyloxyethyl dimethylbenzyl ammonium chloride, methacryloyloxyethyl dimethylbenzyl ammonium chloride, acryloyloxypropyl dimethylbenzyl ammonium chloride, methacryloyloxypropyl dimethylbenzyl ammonium chloride, acrylamidopropyl trimethyl ammonium chloride, methacrylamidopropyl trimethyl ammonium chloride, acrylamidoethyl trimethyl ammonium chloride, methacrylamidoethyl trimethyl ammonium chloride, N-(3-dimethylaminopropyl)acrylamide, N-(3-dimethylaminopropyl)methacrylamide, N-(2-dimethylaminoethyl)acrylamide, N-(2-dimethylaminoethyl)methacrylamide and dimethyldiallylammonium chloride.

5. The papermaking method for improving paper strength according to claim 1, characterized in that: The nonionic monomer in the cationic copolymer is at least one of acrylamide, methacrylamide, N-methylacrylamide, N-methyl(meth)acrylamide, N-isopropylacrylamide, N-isopropyl(meth)acrylamide, N,N-dimethylacrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethylacrylamide, N,N-diethyl(meth)acrylamide, N-methyl-N-ethylacrylamide, N-methyl-N-ethyl(meth)acrylamide, N-hydroxyethylacrylamide, N-hydroxyethyl(meth)acrylamide and N-vinylformamide.

6. The papermaking method for improving paper strength according to claim 1, characterized in that: The cationic copolymer is polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer.

7. The papermaking method for improving paper strength according to claim 6, characterized in that: The molecular weight of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 4,000,000Da-15,000,000Da, preferably 6,000,000Da-12,000,000Da, preferably 8,000,000Da-10,000,000Da; and / or, The cationicity of the polyacrylamide-acryloyloxyethyltrimethylammonium chloride copolymer is 8 mol%-45 mol%, preferably 8 mol%-35 mol%, preferably 8 mol%-25 mol%, preferably 10 mol%-20 mol%.

8. The papermaking method for improving paper strength according to claim 7, characterized in that: The cationic copolymers are two types of polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymers with different molecular weights and cationic degrees; wherein the molecular weight of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 7000,000Da-9000,000Da, and its cationic degree is 8mol%-12mol%; the molecular weight of the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 9000,000Da-11000,000Da, and its cationic degree is 15mol%-25mol%.

9. The papermaking method for improving paper strength according to claim 8, characterized in that: The cationic copolymers are two types of polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymers with different molecular weights and cationic degrees; wherein the molecular weight of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 7500,000Da-8500,000Da, and its cationic degree is 9mol%-11mol%; the molecular weight of the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 9500,000Da-10500,000Da, and its cationic degree is 18mol%-22mol%.

10. The papermaking method for improving paper strength according to claim 7, characterized in that: The ratio of the first polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer to the second polyacrylamide-acryloyloxyethyl trimethyl ammonium chloride copolymer is 1:0.1-20, preferably 1:0.5-10, preferably 1:0.8-5, preferably 1:1-2, and preferably 1:1.3-1.

5.

11. The papermaking method for improving paper strength according to any one of claims 1 to 10, characterized in that: The addition amount of the cationic copolymer is 0.05kg / t-2kg / t paper absolute dry weight, preferably 0.08kg / t paper absolute dry weight-1.5kg / t paper absolute dry weight, preferably 0.1kg / t paper absolute dry weight-1kg / t paper absolute dry weight, preferably 0.15kg / t paper absolute dry weight-0.5kg / t paper absolute dry weight, preferably 0.18kg / t paper absolute dry weight-0.3kg / t paper absolute dry weight.

12. The papermaking method for improving paper strength according to any one of claims 1 to 10, characterized in that: The free starch is electrically neutral starch.

13. The papermaking method for improving paper strength according to any one of claims 1 to 10, characterized in that: The free starch is natural starch.

14. The papermaking method for improving paper strength according to claim 13, characterized in that: The natural starch is selected from at least one of corn starch, wheat starch, potato starch, sweet potato starch and tapioca starch.

15. The papermaking method for improving paper strength according to any one of claims 1 to 10, characterized in that: The content of free starch in the papermaking pulp is 0.5kg / t-100kg / t of paper absolute dry weight, preferably 5kg / t-50kg / t of paper absolute dry weight, preferably 10kg / t-30kg / t of paper absolute dry weight, preferably 15kg / t-25kg / t of paper absolute dry weight.

16. The papermaking method for improving paper strength according to any one of claims 1 to 10, characterized in that: Conventional retention and drainage aids, dry strength agents and / or wet strength agents are also added to the papermaking slurry.

17. The papermaking method for improving paper strength according to claim 16, characterized in that: The conventional dry strength agent is an amphoteric polyacrylamide dry strength agent, a cationic polyacrylamide dry strength agent, amphoteric ionic starch and / or cationic starch; the conventional wet strength agent is a urea-formaldehyde resin, a melamine formaldehyde resin and / or a polyamide epichlorohydrin resin.

18. The papermaking method for improving paper strength according to claim 17, characterized in that: Calculated by dry weight, the dosage of the amphoteric polyacrylamide dry strength agent and / or cationic polyacrylamide dry strength agent is 0.5kg / t-30kg / t of paper absolute dry weight; the dosage of the amphoteric starch and / or cationic starch dry strength agent is 1kg / t-50kg / t of paper absolute dry weight; the dosage of the conventional wet strength agent is 0.2kg / paper absolute dry weight-20kg / t of paper absolute dry weight.

19. The papermaking method for improving paper strength according to claims 1-10, characterized in that: The reaction time is 30s-10min.

20. The papermaking method for improving paper strength according to claims 1-10, characterized in that: The pulp concentration of the papermaking pulp is 0.2-5%, preferably 0.5-4%.

21. Use of the cationic copolymer according to any one of claims 1 to 10 in a papermaking process, characterized in that: The cationic copolymer is used as a free starch fixative to increase the starch content of paper.

22. Use of the cationic copolymer according to any one of claims 1 to 10 in a papermaking process, characterized in that: The cationic copolymer acts as a free starch fixative to improve paper strength.

23. Use of the cationic copolymer according to any one of claims 1 to 10 in a papermaking process, characterized in that: The cationic copolymer is used as a free starch fixative to reduce the COD concentration of discharged wastewater.

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