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 polymer into papermaking pulp for reaction, and preparing the reacted papermaking pulp into a paper product, the papermaking pulp contains free starch; the molecular weight of the cationic polymer is 1,000 Da <-1 >, and the molecular weight of the cationic polymer is 1,000,000 Da; the cationic degree of the cationic polymer is not less than 50mol%. 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 and properties of the fiber itself. For example, the longer the fiber, the larger the aspect ratio, and the better the softness, the higher the dry strength. The content of miscellaneous cells and lignin in papermaking fibers will affect the softness of the fibers and reduce the dry strength; hemicellulose contains a large number of hydroxyl groups, which can increase the hydrogen bonding between fibers and help improve the dry strength of paper. For example, in terms of improving the dry strength of paper, chemical pulp is better than mechanical pulp, and virgin pulp is better than recycled waste paper pulp. For the same chemical pulp, coniferous pulp is better than hardwood pulp.

[0003] Secondly, the dry strength of paper also depends on 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 stronger the bonding force between fibers, and the higher the dry strength of the paper. Applying the method of beating to make the fibers fibrillate can increase the specific surface area of ​​the fibers. The larger the fiber surface area, the more hydroxyl groups (-OH) exposed on the fiber 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, using the method of beating to improve the strength of paper also has disadvantages. This is because the beating process will also cut the fibers, which will reduce the physical indicators of paper tear strength, air permeability, opacity and dimensional stability. In addition, increasing the degree of beating will not only increase power consumption, but also make the fiber water filterability worse, which not only increases the energy consumption of dehydration and drying in the papermaking process, but also affects 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 keeping the strength index unchanged, 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, the water filterability of the pulp can be improved, the running speed of the paper machine can be increased, and the output can be increased.

[0005] Dry strength agents used in papermaking can be basically divided into two categories, one is natural polymers and their modified derivatives, and the other is synthetic polymers. Regardless of whether they are natural or synthetic, 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 -NH2) or amide groups (such as -C(O)-NH2) in their chemical structure, so as 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 in the papermaking process, so the dry strength agents must be water-soluble. In order to allow the dry strength agent to remain on the fiber and not be lost with the discharge of white water from 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 carry only a positive charge, namely the so-called "cationic dry strength agent"; some carry a negative charge in addition to the positive charge, namely the so-called "amphoteric dry strength agent". A few dry strength agents carry only a negative charge, namely "anionic dry strength agents". However, anionic dry strength agents usually need to be used in combination with cationic additives to achieve a reinforcing 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 usually 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 dry strength agent for papermaking. Most of the modified starches used as papermaking dry strength agents are cationic starch and zwitterionic starch, and anionic starch is less used. The dosage of starch-based dry strength agents is usually 5kg / t of paper absolute dry weight-20kg / t of paper absolute dry weight.

[0007] Synthetic dry strength agents are mainly copolymers of polyacrylamide, which are composed of acrylamide units and other units with charges. Among them, the amide group [-C(=O)-NH2] of the acrylamide unit can form hydrogen bonds with paper fibers to provide a reinforcing effect; and the charged units 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 zwitterionic dry strength agents. The most commonly used are cationic and zwitterionic 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 (i.e., the molar fraction of cationic and / or anionic units) of the polyacrylamide dry strength agent is usually only about 5%-30%, and at most does not exceed 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 contain 20% dry weight. The dosage is usually 0.75kg / t paper absolute dry weight to 9kg / t paper absolute dry weight.

[0008] In summary, adding dry strength agent to pulp during papermaking is one of the most commonly used methods in the papermaking industry to improve paper strength. In recent years, due to the increasingly serious problem of forest resource scarcity and the need to protect the environment, one of the current trends in the papermaking industry is to further increase the use of recycled waste paper (also known as "secondary fiber") during the papermaking process. This directly leads to a decline in the quality of papermaking fibers. For this reason, people have to use various means to improve the strength of finished paper. In particular, increase the use of papermaking dry strength agent. However, whether it is a starch-based dry strength agent or a polyacrylamide-based dry strength agent, excessive use will not only increase production costs, but also affect the chemical balance of the paper machine system, causing problems such as adhesive deposition, and bring some negative effects to the operation of the paper machine. At the same time, it will also cause an increase in wastewater COD pollution emissions and pollute the environment.

[0009] In order to improve the dry strength of paper, natural starch needs to be chemically modified, such as cationic starch, anionic starch, or zwitterionic starch, to be used as a papermaking dry strength agent. Unmodified natural starch usually cannot be added directly to paper pulp, because natural starch molecules are basically electrically neutral and difficult to be fixed on fibers, so they cannot play their due functions. However, after natural starch is chemically modified, the cost of the product will increase, and the chemical modification process will also cause certain pollution to the environment. Therefore, if a method can be invented to directly use unmodified natural starch as a papermaking dry strength agent added to the pulp, the production cost of the papermaking process can be reduced.

[0010] Some papermaking processes contain starch from the raw materials of papermaking fibers. This is because when the secondary fibers of recycled waste paper are used as the raw materials of papermaking fibers, some waste paper itself contains a certain amount of starch. These starches are either used as binders to process paper into packaging bags, packaging boxes, packaging boxes and other products, or used as spray starch in the papermaking process to improve the interlayer bonding of paper, or used as a carrier for sizing the surface of paper sheets, etc. The papermaking starch used for these purposes is basically electrically neutral. When these starches enter the papermaking system with recycled waste paper, they will dissolve in the pulping process and become free starch in the pulp. These free starches are not only useless to the papermaking process, but also often harmful, so free starch is sometimes called "dead starch". For example, free starch retained in the papermaking system will breed a large number of microbial activities, affecting the normal operation of the papermaking process. Moreover, after the fermentation of microorganisms, free starch will cause the acidity of the papermaking system to increase, the pH to decrease, the conductivity to increase, and produce odor. In addition, free starch will also cause the COD of papermaking white water to increase, increasing the pressure of wastewater treatment. Therefore, if a method can be invented to fix this part of dissolved starch on papermaking fibers so that it can play a reinforcing role, it can not only improve the strength of paper, save the amount of dry strength agent, and further reduce production costs, but also reduce the odor of paper mills and reduce wastewater COD emissions. Summary of the invention

[0011] Based on this, the present invention provides an application of a cationic polymer as a starch fixing agent in improving paper strength and a corresponding papermaking method. The present invention uses a specific cationic polymer as a starch fixing agent to fix and retain substantially electrically neutral starch molecules on papermaking fibers, so that the starch acts as a papermaking dry strength agent, thereby achieving the purpose of improving paper strength.

[0012] The specific technical solutions are as follows.

[0013] A papermaking method for improving paper strength comprises the following steps:

[0014] Adding cationic polymer to papermaking pulp for reaction, and then preparing paper products from the reacted papermaking pulp;

[0015] The papermaking pulp contains free starch;

[0016] The cationic polymer has a molecular weight of 1,000Da-1,000,000Da;

[0017] The cationicity of the cationic polymer is not less than 50 mol%.

[0018] In some embodiments, the cationic polymer has a molecular weight of 10,000 Da-900,000 Da.

[0019] In some embodiments, the cationic polymer has a molecular weight of 13,000 Da-900,000 Da.

[0020] In some embodiments, the cationic polymer has a molecular weight of 50,000 Da-850,000 Da.

[0021] In some embodiments, the cationic polymer has a molecular weight of 150,000 Da-800,000 Da.

[0022] In some embodiments, the cationic polymer has a molecular weight of 180,000 Da-800,000 Da.

[0023] In some embodiments, the cationic polymer has a molecular weight of 180,000 Da-600,000 Da.

[0024] In some embodiments, the cationic polymer has a molecular weight of 200,000 Da-500,000 Da.

[0025] In some embodiments, the cationic polymer includes at least one cationic monomer and, optionally, at least one nonionic monomer.

[0026] In some embodiments, the mole fraction of the cationic monomer in the cationic polymer is 50 mol %-100 mol %.

[0027] In some embodiments, the mole fraction of the cationic monomer in the cationic polymer is 60 mol %-100 mol %.

[0028] In some embodiments, the mole fraction of the cationic monomer in the cationic polymer is 70 mol %-100 mol %.

[0029] In some embodiments, the mole fraction of the cationic monomer in the cationic polymer is 80 mol %-100 mol %.

[0030] In some embodiments, the cationic monomer in the cationic polymer is selected from at least one of dimethyldiallylammonium chloride, 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 and N-(2-dimethylaminoethyl)methacrylamide.

[0031] In some embodiments, the cationic monomer of the cationic polymer is dimethyldiallylammonium chloride.

[0032] In some embodiments, the nonionic monomer in the cationic polymer is selected from 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.

[0033] In some embodiments, the nonionic monomer in the cationic polymer is selected from acrylamide.

[0034] In some embodiments, the cationic polymer is one or more combinations of polydimethyldiallyl ammonium chloride, polyhydroxypropyldimethylammonium chloride, and dimethyldiallyl ammonium chloride / acrylamide copolymer.

[0035] In some embodiments, the cationic polymer is polydimethyldiallylammonium chloride, and its molecular weight is 150,000Da-800,000Da, preferably 180,000Da-800,000Da, preferably 180,000Da-600,000Da, preferably 200,000Da-500,000Da.

[0036] In some embodiments, the cationic polymer is an allyl ammonium chloride / acrylamide copolymer, and its cationicity is 50 mol%-100 mol%, preferably 60 mol%-100 mol%, preferably 70 mol%-100 mol%, preferably 80 mol%-100 mol%.

[0037] In some of the embodiments, based on dry weight, the cationic polymer is added in an amount of 0.1 kg / t-5 kg / t of paper absolute dry weight.

[0038] In some of the embodiments, based on dry weight, the cationic polymer is added in an amount of 0.4 kg / t-4 kg / t of paper absolute dry weight.

[0039] In some of the embodiments, based on dry weight, the cationic polymer is added in an amount of 0.6 kg / t-2 kg / t of paper absolute dry weight.

[0040] In some of the embodiments, based on dry weight, the cationic polymer is added in an amount of 0.7 kg / t-1.5 kg / t of paper absolute dry weight.

[0041] In some of the embodiments, based on dry weight, the cationic polymer is added in an amount of 0.8 kg / t-1.2 kg / t of paper absolute dry weight.

[0042] In some of these embodiments, the free starch is substantially electrically neutral starch.

[0043] In some embodiments, the free starch is natural starch and / or starch contained in the secondary fiber itself.

[0044] 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.

[0045] In some embodiments, the free starch content in the papermaking pulp is 1kg / t-100kg / t of paper absolute dry weight; or, based on 1% pulp concentration, the free starch concentration in the papermaking pulp is 10mg / L-1000mg / L, preferably 30mg / L-500mg / L, and preferably 100mg / L-300mg / L.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] In some of the embodiments, zwitterionic polyacrylamide is further added to the papermaking pulp, and the added amount is 1 kg / t paper absolute dry weight to 5 kg / t paper absolute dry weight.

[0050] In some of the embodiments, zwitterionic polyacrylamide is further added to the papermaking pulp, and the addition amount thereof is 4kg / t-5kg / t of paper absolute dry weight.

[0051] In some embodiments, the reaction time is 30s-10min.

[0052] In some embodiments, the pulp concentration of the papermaking pulp is 0.2-5%, preferably 0.5-4%.

[0053] In some embodiments, the pulp concentration of the papermaking pulp is 0.5-1%.

[0054] The cationic polymer is used in the papermaking process as a free starch fixative to increase the starch content of the finished paper.

[0055] The cationic polymer is used in the papermaking process as a free starch fixative to improve the strength of paper.

[0056] The cationic polymer is used in the papermaking process as a free starch fixative to reduce the COD concentration of discharged wastewater.

[0057] The cationic polymer is used in the papermaking process, and the cationic polymer is used as a free starch fixative to recover the free starch in the secondary fiber papermaking process.

[0058] Natural starch has not undergone any chemical modification, and its molecules are basically electrically neutral, with neither positive charge nor negative charge. It lacks electrostatic attraction with papermaking fibers, and therefore has a weak interaction with papermaking fibers, making it 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 cationic polymers with certain 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 act as 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:

[0059] 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.

[0060] In addition to natural starch, some secondary fibers used in the papermaking process also contain some starch. These starches are either used as binders to process paper into packaging bags, packaging boxes, packaging boxes and other products, or used as spray starch to improve the interlayer bonding strength of paper during papermaking, or used as a carrier for sizing the surface of paper sheets, etc. The papermaking starch used for these purposes is usually basically electrically neutral. When these starches enter the papermaking system with recycled waste paper, they will dissolve during the pulping process and become free starch in the papermaking white water. This part of the free starch from recycled waste paper is also basically uncharged, and due to the influence of various factors in the manufacture and use of paper, the molecular weight of the waste paper starch is usually smaller than the original natural starch, so it is more difficult to be fixed and retained on the papermaking fibers. These free starches are not only useless to the papermaking process, but also often harmful, so the free starch of pulp is sometimes called "dead starch". For example, the free starch retained in the papermaking system will breed a large number of microbial activities, affecting the normal operation of the papermaking process. Moreover, free starch, after fermentation by microorganisms, will cause the acidity of the papermaking system to rise, the pH to drop, the conductivity to rise, and produce peculiar smell. In addition, free starch will also cause the COD of papermaking wastewater to rise, increasing the pressure of wastewater treatment. However, by applying the method of the present invention, this part of unusable "dead starch" can also be fixed and retained on the papermaking fiber during the secondary fiber papermaking process, so that it can play the role of a papermaking dry strength agent. In this way, not only can waste be turned into treasure, the strength of paper is improved, the amount of conventional dry strength agents is saved, and production costs are reduced, but also the pollution of the environment by this part of the originally unusable "dead starch" can be reduced, and the peculiar smell of the paper mill and the COD discharge of wastewater can be reduced. DETAILED DESCRIPTION

[0061] 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.

[0062] 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.

[0063] The present invention uses a specific cationic polymer as a starch fixative to fix and retain substantially electrically neutral free starch molecules on papermaking fibers, thereby allowing the starch to function as a papermaking dry strength agent, thereby achieving the purpose of improving paper strength.

[0064] The "free 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.

[0065] The specific cationic polymer is polymerized from at least one cationic monomer and has a molecular weight of about 1,000 Da to 1,000,000 Da, more preferably about 40,000 Da to 1,000,000 Da.

[0066] The molar fraction of the cationic monomer in the cationic polymer is 50 mol%-100 mol%, and the molar fraction of the nonionic monomer is 0%-50 mol%.

[0067] The cationic monomer of the cationic polymer includes but is not limited to at least one of dimethyldiallylammonium chloride, 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, and N-(2-dimethylaminoethyl)methacrylamide;

[0068] The nonionic monomer in the cationic polymer is selected from 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.

[0069] According to the method of the present invention, the cationic polymer 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.

[0070] According to the method of the present invention, the cationic polymer used as the pulp free starch fixative can also be used in combination of two or more polymers. When two or more cationic polymers are used in combination, all of them can be cationic homopolymers, all of them can be cationic copolymers, or at least one of them can be a cationic homopolymer and at least one can be a cationic copolymer.

[0071] The cationic polymer 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 pool, pulp flushing tank, pressure screen inlet, headbox inlet, etc. of the paper machine. When two or more cationic polymers are used in combination, they can be added to different parts of the paper machine, or to the same part of the paper machine. For the convenience of use, two or more cationic polymers can be mixed and prepared into a single product in advance, and then added together to a certain part of the paper machine.

[0072] The appropriate amount of the cationic polymer added is between 0.1kg / t paper absolute dry weight and 5kg / t paper absolute dry weight. In order to further improve the strength index of paper, the cationic polymer can also be used in combination with a conventional dry strength agent or wet strength agent. Conventional dry strength agents include but are not limited to zwitterionic or cationic polyacrylamide dry strength agents and zwitterionic or cationic starch. Based on dry weight, the amount of the zwitterionic or cationic polyacrylamide dry strength agent is between 0.5kg / t paper absolute dry weight and 10kg / t paper absolute dry weight; the amount of the zwitterionic starch and / or cationic starch dry strength agent is between 1kg / t absolute dry pulp and 20kg / t paper absolute dry weight; the amount of the conventional wet strength agent is between 0.2kg / t absolute dry pulp and 10kg / t absolute dry pulp.

[0073] The varieties of natural starch suitable for the present invention include but are not limited to corn starch, wheat starch, potato starch, sweet potato starch, cassava starch, etc. These natural starches do not need to be chemically modified, and can be directly added to the pulp in the papermaking process after being gelatinized and dissolved with hot water. The concentration of the suitable natural starch solution is 1% to 5%. Based on dry weight, the addition amount is 5kg / t paper absolute dry weight-100kg / t paper absolute dry weight. The conventional dry strength agent can be added before, after, or between two cationic polymers described in the present invention.

[0074] If the free starch in the pulp itself is more than 5kg / t of paper dry weight (equivalent to a pulp with a pulp concentration of 1%, the concentration of free starch in white water=>50mg / L), adding natural starch may be an option, that is, it may be added or not, depending on the actual situation. In this case, the cationic polymer of the present invention can fix and retain the existing free starch in the white water on the papermaking fibers, thereby achieving the effect of improving the strength of the paper.

[0075] The present invention also relates to a papermaking method, which comprises the step of adding the cationic polymer of the present invention; more specifically, it relates to the following steps:

[0076] (1) dispersing papermaking fibers in water to form a fiber suspension;

[0077] (2) determining the concentration of dissolved starch in the fiber suspension;

[0078] (3) Taking the fiber concentration of 1% of the fiber suspension as a reference, if the concentration of dissolved starch in the fiber suspension is less than 50 mg / L, which is equivalent to that the dry weight of starch per ton of absolute dry fiber does not exceed 5 kg / t of absolute dry weight of paper, then 5 kg / t of absolute dry weight of paper to 100 kg / t of absolute dry weight of paper of natural starch is added to the fiber suspension, and at the same time, 0.1 kg / t of absolute dry weight of paper to 2 kg / t of absolute dry weight of paper of the cationic polymer of the present invention is added;

[0079] Taking the fiber concentration of 1% of the fiber suspension as a reference, if the concentration of dissolved starch in the fiber suspension is >50 mg / L, which is equivalent to the dry weight of starch per ton of absolute dry fiber being not less than 5 kg / t of paper absolute dry weight, then 0.1 kg / t of paper absolute dry weight = 5 kg / t of paper absolute dry weight of the cationic polymer of the present invention is added to the fiber suspension. In this case, adding natural starch is an option, that is, it can be added or not, depending on the actual situation;

[0080] (4) Optionally, 0.5 kg / t-10 kg / t of paper absolute dry weight of zwitterionic or cationic polyacrylamide dry strength agent, or / and 1 kg / t-20 kg / t of paper absolute dry weight of zwitterionic or cationic starch dry strength agent, or / and 0.2 kg / t-10 kg / t of paper absolute dry weight of conventional wet strength agent are added to the fiber suspension;

[0081] (5) The fiber suspension is then subjected to conventional papermaking steps such as dehydration, pressing and drying to form a suitable paper product such as paper or paperboard.

[0082] The technology of the present invention has the following advantages:

[0083] The present invention relates to how to fix and retain the free starch molecules which are basically electrically neutral in pulp on papermaking fibers so as to make them play the role of papermaking dry strength agent, thereby achieving the purpose of improving the strength of paper.

[0084] Natural starch, which has not undergone any chemical modification, is basically electrically neutral in molecules, with neither positive charge nor negative charge, 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 papermaking dry strength agent. However, by applying the method of the present invention, the basically electrically neutral natural 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 finished paper. In this way, natural starch can be directly used as a papermaking dry strength agent, which is more cost-effective than using modified starch, such as cationic starch and zwitterionic starch, as a papermaking dry strength agent, and is also more conducive to green environmental protection.

[0085] In addition to natural starch, if the papermaking process uses secondary fibers from recycled waste paper as raw materials, the waste paper itself also contains some starch. These starches are either used as binders to process paper into packaging bags, packaging boxes, packaging boxes and other products, or used as spray starch to improve the interlayer bonding strength of paper during papermaking, or used as a sizing agent for surface sizing, etc. The papermaking starch used for these purposes is usually basically electrically neutral. When these starches enter the papermaking system with recycled waste paper, they will be dissolved and released during the pulping process to become free starch in the pulp. This part of the free starch from recycled waste paper is not only basically uncharged, but also due to the influence of various factors in the manufacturing and use of paper, its molecular weight is usually smaller than the original natural starch, so it is more difficult to be fixed and retained on the papermaking fibers. These free starches are not only useless to the papermaking process, but are often harmful, so this part of the free starch in the pulp is sometimes also called "dead starch". For example, the free starch retained in the papermaking system will breed a large number of microbial activities, affecting the normal operation of the papermaking process. Moreover, free starch, after fermentation by microorganisms, will cause the acidity of the papermaking system to rise, the pH to drop, the conductivity to rise, and produce peculiar smell. In addition, free starch will cause the COD of papermaking white water to rise, increasing the pressure of wastewater treatment. However, by applying the method of the present invention, this part of unusable "dead starch" can be fixed and retained on the papermaking fibers in the papermaking process of recycling waste paper, so that it can play the role of a papermaking dry strength agent. In this way, not only can waste be turned into treasure, the strength of paper is improved, the amount of conventional dry strength agents is saved, and production costs are reduced, but also the pollution of the environment by this part of the originally unusable "dead starch" is reduced.

[0086] Experimental sample reagent

[0087] A: Polydimethyldiallylammonium chloride aqueous solution, solid content 50%, cationic degree 100%, molecular weight ca. 200,000Da, provided by Aisen (China) Flocculant Co., Ltd.

[0088] B: Polydimethyldiallylammonium chloride aqueous solution, solid content 40%, cationic degree 100%, molecular weight ca. 500,000Da, provided by Aisen (China) Flocculant Co., Ltd.

[0089] C: Polydimethyldiallylammonium chloride aqueous solution, solid content 40%, cationic degree 100%, molecular weight ca. 800,000Da, provided by Shandong Luyue Chemical Co., Ltd.

[0090] D: Polydimethyldiallylammonium chloride aqueous solution, solid content 40%, cationic degree 100%, molecular weight ca. 50,000Da, provided by Shandong Luyue Chemical Co., Ltd.

[0091] E: Polydimethyldiallylammonium chloride aqueous solution, solid content 40%, cationic degree 100%, molecular weight ca. 250,000Da, provided by Shandong Luyue Chemical Co., Ltd.

[0092] F: polyhydroxypropyl dimethyl ammonium chloride (commonly known as polyamine) solution, solid content 50%, cationic degree 100%, molecular weight ca. 13,000Da, provided by Wuxi Tianxin Chemical Co., Ltd.

[0093] G: polyhydroxypropyl dimethyl ammonium chloride (commonly known as polyamine) solution, solid content 50%, cationic degree 100%, molecular weight ca. 20,000Da, provided by Wuxi Tianxin Chemical Co., Ltd.

[0094] H: polyhydroxypropyl dimethyl ammonium chloride (commonly known as polyamine) solution, solid content 50%, cationic degree 100%, molecular weight ca. 150,000Da, provided by Aisen (China) Flocculant Co., Ltd.

[0095] I: Polyhydroxypropyl dimethyl ammonium chloride (commonly known as polyamine) solution, solid content 50%, cationic degree 100%, molecular weight ca. 300,000Da, provided by Aisen (China) Flocculant Co., Ltd.

[0096] J: polyhydroxypropyl dimethyl ammonium chloride (commonly known as polyamine) solution, solid content 50%, cationic degree 100%, molecular weight ca. 100,000 Da, provided by Aisen (China) Flocculant Co., Ltd.

[0097] K: polyhydroxypropyl dimethyl ammonium chloride (commonly known as polyamine) solution, solid content 50%, cationic degree 100%, molecular weight ca. 600,000Da, provided by Aisen (China) Flocculant Co., Ltd.

[0098] L: melamine resin solution, solid content 50%, cationicity 100%, molecular weight ca. 35,000, provided by Aisen (China) Flocculant Co., Ltd.

[0099] M: Dimethyldiallylammonium chloride / acrylamide copolymer aqueous solution, solid content 40%, cationic degree 80%, molecular weight ca. 500,000Da, provided by Shandong Luyue Chemical Co., Ltd.

[0100] N: Dimethyldiallylammonium chloride / acrylamide copolymer aqueous solution, solid content 40%, cationic degree 30%, molecular weight ca. 500,000Da, provided by Shandong Luyue Chemical Co., Ltd.

[0101] O: Dimethyldiallylammonium chloride / acrylamide copolymer aqueous solution, solid content 40%, cationic degree 10%, molecular weight ca. 500,000Da, provided by Shandong Luyue Chemical Co., Ltd.

[0102] P: Papermaking dry strength agent: zwitterionic polyacrylamide aqueous solution, solid content 15%, molecular weight ca. 800,000Da, provided by Guangdong Lee & Man Paper Co., Ltd., China.

[0103] Experimental Materials

[0104] Corn starch: provided by Zhucheng Xingmao Corn Development Co., Ltd.

[0105] Brazilian Eucalyptus oleifera pulp (bleached kraft hardwood pulp): Suzano Pulp and Paper

[0106] Waste corrugated cardboard boxes from the United States: provided by Guangdong Lee & Man Paper Co., Ltd.

[0107] Junye Corrugated Paper: Guangdong Dongguan Junye Paper Co., Ltd. provides Brazilian goldfish eucalyptus pulp

[0108] Experimental setup and instruments

[0109] PL12-00 High-Concentration Hydraulic Pulper: Shaanxi University of Science and Technology Machinery Plant

[0110] IMT-VL01 Standard Wally Beater: Dongguan Internason Precision Instrument Co., Ltd.

[0111] GBJ-A Fiber Decomposer: Changchun Yueming Small Testing Machine Co., Ltd.

[0112] IMT-SJ01 Standard Fiber Decomposer: Guangdong Dongguan Internaisan Precision Instrument Co., Ltd.

[0113] IMT-CP01A-3A Paper Forming Machine: Dongguan Internaisan Precision Instrument Co., Ltd.

[0114] IMT-Burst01P Burst Tester: Guangdong Dongguan Internaisan Precision Instrument Co., Ltd.

[0115] IMT-FOLD01 Folding Endurance Tester: Guangdong Dongguan Internaisan Precision Instrument Co., Ltd.

[0116] UV-1800 UV-visible spectrophotometer: Shimadzu Corporation (China)

[0117] Experimental methods

[0118] Preparation of LOCC pulp: Junye corrugated paper is used as the raw material of secondary recycled fiber and cut into 3cm x 3cm paper pieces. Weigh 300g of Guojunye corrugated paper, add warm tap water to a pulp concentration of 13%, soak for 10 minutes, use PL12-00 high-concentration hydraulic pulper to pulp for 15 minutes, add water to dilute to a pulp concentration of 3%, and store for later use.

[0119] Preparation of AOCC pulp: Use American waste corrugated paperboard as the raw material for secondary recycled fiber, cut into 3cm x 3cm paper pieces, weigh 350g of AOCC paper pieces, add warm tap water to a pulp concentration of 13%, soak for 10 minutes, use a PL12-00 high-concentration hydraulic pulper to pulp for 15 minutes, then add water to dilute to a pulp concentration of 3%, and store for later use.

[0120] 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 Valli pulper, add water to 23L, first dissolve for 3 minutes, then add a 5.5kg weight, and beat for about 11 minutes until the decomposition degree is 30 degrees. Finally, dehydrate, measure the moisture content, and store for later use.

[0121] 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.

[0122] 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 the paper was tested under standard relative humidity conditions using an IMT-Burst01P bursting resistance tester.

[0123] Determination of the folding index of finished paper: According to the national standard GB-T457-2002 for the determination of the folding index of paper, the folding index of the above-mentioned paper was tested under standard relative humidity conditions using an IMT-FOLD01 folding index tester.

[0124] 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.

[0125] 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 and oscillate for 30 minutes to fully dissolve the starch in the finished paper; 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 spectrophotometer at a wavelength of 510nm to measure the absorbance of glucose, and then convert it into the starch content of the paper sample.

[0126] Examples 1-4 and Comparative Examples 1-3

[0127] This group of examples and comparative examples demonstrates the effect of adding the cationic polymers A and B of the present invention on the bursting strength of the finished paper when the LOCC pulp contains low and high concentrations of free starch, respectively.

[0128] Experimental method: 667g of pre-prepared LOCC pulp (pulp concentration 3%, total absolute dry weight 20g) was weighed in two plastic glass beakers, one of which was added with 200ppm amylase and reacted in a constant temperature water bath at 50℃ for one hour. The free starch concentration in the pulp sample was determined to be 38mg / L by iodine-starch colorimetry. No amylase was added to the other pulp sample, and the free starch concentration of the pulp sample was measured to be 192mg / L. Then, both pulp samples were diluted with distilled water to 0.5%, and under the medium-speed stirring condition of a mechanical stirrer, a 1% cationic polymer solution was added or not added according to the experimental requirements. After stirring for 1 minute, the above pulp samples were copied with a sheet copying machine according to the method of the sheet copying experiment to have a gram weight of about 85g / m 2 Then the tear resistance index of the paper piece is determined according to the relevant national standard method.

[0129] The experimental results are listed in Table 1. It can be seen from the results that when the LOCC pulp contains a relatively high free starch concentration (192 mg / L), the cationic polymers A and B of the present invention both have a significant effect on improving the bursting index of the finished paper. On the contrary, when the LOCC pulp contains a very low free starch concentration (38 mg / L), although the cationic polymers A and B are also effective in improving the bursting index of the finished paper, since the overall starch content of the finished paper is still relatively low, the effect of the cationic polymers A and B on improving the bursting index of the finished paper is relatively weak. This shows that the cationic polymer A or B described in the present invention can significantly improve the bursting strength of the finished paper after interacting with the free starch in the pulp.

[0130] Table 1

[0131]

[0132]

[0133] Examples 5-8 and Comparative Examples 4-5

[0134] This group of examples and comparative examples demonstrates the effect of adding the cationic polymers A and B of the present invention to LOCC pulp with or without added free starch to improve the bursting index of the finished paper.

[0135] Experimental method: 667g of pre-prepared LOCC pulp (3% pulp concentration, total absolute dry weight 20g) was weighed in two plastic glass beakers respectively, and gelatinized corn starch solution was added to one beaker to make the free starch concentration of the pulp 250mg / L (based on 1% pulp concentration), and starch was not added to the other beaker. Both pulp samples were stirred in a 50℃ constant temperature water bath for 10 minutes, diluted to 1% pulp concentration, and then dispersed with a GBJ-A fiber separator for 30 seconds. Then, according to the experimental requirements, a 1% cationic polymer solution was added or not, and after continuing to stir for 1 minute, the above pulp samples were copied into a gram weight of about 90g / m using a sheet copying machine according to the sheet copying experiment method. 2 Then the tear resistance index of the paper piece is determined according to the relevant national standard method.

[0136] The experimental results are listed in Table 2. It can be seen from the results that even without the addition of free starch, LOCC pulp itself contains a certain amount of free starch. Adding 0.8 kg / t of pulp absolute dry weight of cationic polymer A or B can increase the bursting index of the finished paper from the original 1.65 kPa·m 2 / g increased to 2.04 and 2.03 kPa·m 2 When 250 ml / L free starch (1% pulp concentration) was added, the bursting index of the finished paper was further increased to 2.25 and 2.22 kPa·m 2 This shows again that the cationic polymers A and B of the present invention can significantly improve the bursting strength of the finished paper after interacting with the free starch originally contained in or added to the LOCC pulp.

[0137] Table 2

[0138]

[0139]

[0140] Note: *Based on 1% pulp concentration

[0141] Examples 9-19 and Comparative Example 6

[0142] This group of examples and comparative examples demonstrates the effects of cationic polymers with different chemical structures and molecular weights on retaining free starch in LOCC pulp and improving the bursting index of finished paper.

[0143] Experimental method: Dilute the LOCC pulp with a pre-prepared pulp concentration of 3% with distilled water to 1%, add or not add 1% cationic polymer solution according to the experimental requirements, and continue stirring for 1 minute. Then, use a sheeting machine to copy the pulp sample into a sheet weight of about 95g / m 2Then, the bursting index of the paper sheet was determined according to the relevant national standard method. In addition, 15 g of the pulp after the reaction was taken and the supernatant was separated by a centrifuge, and the free starch concentration of the supernatant was determined by iodine-starch colorimetry.

[0144] The experimental results are listed in Table 3. It can be seen from the results that although the cationic polymers used in this group of examples are all 100% cationic polymers, their molecular structures and molecular weights are different, so the effects on fixing the free starch in the pulp and improving the burst resistance of the finished paper are also different. The cationic polymers used in Examples 9, 10 and 12 are polydiallyldimethylammonium chloride (PDAC), which has a relatively high molecular weight, so the free starch concentration in the LOCC pulp decreases greatly, indicating that the ability to fix the free starch is strong, and the burst resistance index of the finished paper is also significantly improved. Although the cationic polymer used in Example 11 is also polydiallyldimethylammonium chloride, the molecular weight is relatively small and the effect is also relatively poor. The cationic polymers used in Examples 13 to 18 are polyhydroxypropyldimethylammonium chloride (PDCE, commonly known as polyamine), and Example 19 is dicyandiamide resin (DFP). Although these cationic polymers also have a certain effect on fixing the free starch in the pulp and improving the burst resistance of the finished paper, the effect is obviously worse than that of Examples 9, 10 and 12. In general, polydiallyldimethylammonium chloride cationic polymers with moderate molecular weight are better for fixing free starch in pulp and improving the dry strength of finished paper.

[0145] Table 3

[0146]

[0147]

[0148] Note: 1) The dosage of cationic polymer is 0.8kg / t paper dry weight

[0149] 2) PDAC: Poly(diallyldimethylammonium chloride)

[0150] PDCE: Poly(dimethylamine-co-epichlorohydrin)

[0151] DFP: Dicyandiamide-formaldehyde polymer

[0152] Examples 20-24 and Comparative Example 7

[0153] This group of examples and comparative examples demonstrates: the effect of cationic polymers with different cationic degrees on the retention of free starch in LOCC pulp and the influence on improving the bursting index of finished paper.

[0154] Experimental method: After the LOCC pulp with a pre-prepared pulp concentration of 3% was diluted to 1% with distilled water, 1% cationic polymer solution was added or not added according to the experimental requirements, and the amount of cationic polymer added was 0 and 0.8 kg / t paper dry weight respectively. After stirring for 1 minute, the pulp sample was copied into a sheet weight of about 95 g / m2 using a sheet copying machine according to the method of the sheet copying experiment. 2 Then, the bursting index of the paper sheet was determined according to the relevant national standard method. In addition, 15 g of the pulp after the reaction was taken and the supernatant was separated by a centrifuge, and the free starch concentration of the supernatant was determined by iodine-starch colorimetry.

[0155] The experimental results are listed in Table 4. It can be seen from the results that although the cationic polymers used in this group of examples all contain diallyldimethylammonium chloride cationic monomers, their cationic degrees (i.e., the molar fraction of the cationic monomers) are different, and thus the effects on fixing the free starch in the pulp and improving the bursting resistance of the finished paper are also different. The cationic degrees of Examples A, E and M are all 100% or >50%, so they have a more obvious effect on fixing and retaining the free starch of LOCC pulp and improving the bursting resistance index of the finished paper. In contrast, the cationic degree of the cationic polymers used in Examples N and O is <50%, so there is only a weak effect.

[0156] Table 4

[0157]

[0158] Note: * The dosage is 0.8kg / t paper dry weight.

[0159] Examples 25-27 and Comparative Examples 8-10

[0160] This group of examples and comparative examples demonstrates the influence of the cationic polymer A of the present invention on the bursting index of finished paper and the finished paper starch after different concentrations of free starch are added to the bleached chemical pulp containing no free starch.

[0161] Experimental method: weigh 20g of the pre-prepared dehydrated bleached chemical pulp in a plastic beaker, add deionized water to a total weight of 667g, and prepare a 3% pulp concentration. Stir at a medium speed for 10 minutes in a constant temperature water bath at 50℃, add or not add a certain amount of gelatinized starch solution according to the experimental plan, so that the free starch concentration of the pulp is 0, 150 and 250mg / L (based on 1% pulp concentration), continue to stir for 10 minutes, add or not add a 1% cationic polymer A solution according to the requirements of the experimental plan, so that the added amount is 0 and 0.8kg / t paper dry weight, continue to stir for 10 minutes, then add water to dilute to 0.5%, continue to stir for 1 minute, and use a sheet machine to copy the above pulp sample into a gram weight of about 105g / 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".

[0162] The experimental results are listed in Table 5. It can be seen from the results that when there is no free starch in the pulp, cationic polymer A can hardly improve the bursting strength of the finished paper. However, when the pulp is added with corn starch solution so that the free starch concentration of the pulp reaches 150 and 250 mg / L respectively, A can significantly improve the bursting index of the finished paper, and accordingly, the starch content of the finished paper is also increased. Moreover, the higher the free starch concentration of the pulp, the more significant the effect of A, and the higher the bursting strength of the finished paper and the starch content of the finished paper.

[0163] Table 5

[0164]

[0165] Note: *Based on 1% pulp concentration

[0166] Examples 28-30 and Comparative Examples 11-13

[0167] This group of examples and comparative examples demonstrates the effect of the cationic polymer A of the present invention on the bursting index and starch content of the finished paper after adding different concentrations of free starch to AOCC pulp. The main component of AOCC is long fibers such as chemical wood pulp, which is an excellent secondary fiber raw material for producing kraft linerboard. The free starch concentration of 1% AOCC pulp measured by iodine colorimetry is 54 mg / L.

[0168] Experimental method: 667g of pre-prepared AOCC pulp (3% pulp, absolute dry weight 20g) was weighed in a plastic beaker, and stirred at medium speed for 10 minutes in a constant temperature water bath at 50℃. According to the experimental scheme, a certain amount of gelatinized starch solution was added or not added, so that the free starch concentration of the added pulp was 0, 150 and 250mg / L (based on 1% pulp concentration). After continuing to stir for 10 minutes, according to the requirements of the experimental scheme, a 1% dilution of cationic polymer A solution was added or not added, so that the addition amount was 0 and 0.8kg / t paper absolute dry weight, respectively. After continuing to stir for 10 minutes, the above pulp sample was copied into a gram weight of about 105g / m2 using a copying machine according to the method of the sheet copying experiment. 2 Then, the bursting strength 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".

[0169] The experimental results are listed in Table 6. It can be seen from the results that since AOCC pulp itself contains a certain amount of free starch (54 mg / L), even without adding starch, the cationic polymer A of the present invention can increase the starch content of paper and the tear resistance index of paper. Moreover, by comparing Example 28 with Examples 29 and 30, it is not difficult to see that the starch content of paper and the tear resistance index of paper are not significantly improved when only starch is added to AOCC pulp without adding cationic polymer A. However, by adding starch, the free starch concentration of pulp is increased, and when cationic polymer A is added at the same time, the starch content of paper and the tear resistance index of paper are significantly improved.

[0170] Table 6

[0171]

[0172] Note: *Based on 1% pulp concentration

[0173] Examples 31-34 and Comparative Examples 14-16

[0174] This group of examples and comparative examples demonstrates the effect of the cationic polymer A of the present invention on the bursting index and starch of the finished paper after or without the addition of free starch to LOCC pulp. The free starch concentration of 1% LOCC pulp measured by iodine colorimetry is 194 mg / L.

[0175] Experimental method: 667g of pre-prepared LOCC pulp (3% pulp, absolute dry weight 20g) was weighed in a plastic beaker, and stirred at medium speed for 10 minutes in a constant temperature water bath at 50℃. According to the experimental scheme, a certain amount of gelatinized starch solution was added or not added, so that the free starch concentration of the added pulp was 0 and 250mg / L (based on 1% pulp concentration). After continuing to stir for 10 minutes, according to the requirements of the experimental scheme, a 1% dilution of cationic polymer A solution was added or not added, so that the added amount was 0 and 0.8kg / t paper absolute dry weight, respectively. After continuing to stir for 1 minute, the above pulp sample was copied into a gram weight of about 105g / m2 using a copying machine according to the method of the sheet copying experiment. 2 Then, the bursting strength 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".

[0176] The experimental results are listed in Table 7. It can be seen from the results that since LOCC pulp itself contains a certain amount of free starch, even without adding starch, the cationic polymer A disclosed in the present invention can effectively increase the starch content of paper and the paper bursting index. However, after adding 250 mg / L free starch (based on 1% pulp concentration), at the same dosage of A, the paper bursting strength and the starch content of paper are further significantly improved.

[0177] Table 7

[0178]

[0179] Note: *Based on 1% pulp concentration

[0180] ** Example 33 repeats Example 31; Example 34 repeats Example 32; Comparative Example 16 repeats Comparative Example 15.

[0181] Examples 35-41 and Comparative Examples 17-18

[0182] This group of examples and comparative examples demonstrates the effects of the cationic polymer A of the present invention and the conventional zwitterionic polyacrylamide dry strength agent P on the bursting index and folding endurance of the finished paper after being applied to LOCC pulp. The free starch concentration of the LOCC pulp measured by iodine-starch colorimetry is 208 mg / L (based on a pulp concentration of 1%).

[0183] Experimental method: 667g (20g absolute dry weight) of the above 3% pulp was weighed in a plastic beaker. After stirring for 10 minutes in a 50℃ constant temperature water bath, it was diluted to 1%, and then defiberized for 30 seconds using an IMT-SJ01 standard fiber disintegrator, and further diluted to a 0.5% pulp concentration. Then, according to the requirements of the experimental scheme, a 1% cationic polymer A solution was added or not, and the added amount was 0, 0.4, 0.8, 1.0 and 1.2kg / t paper absolute dry weight, respectively, and stirring was continued for 1 minute. Alternatively, after stirring for 10 minutes in a 50℃ constant temperature water bath, a certain amount of papermaking dry strength agent P was first added to reach 4.7kg / t paper absolute dry weight (refer to the conventional amount used in the actual boxboard papermaking process), and stirring was continued for 5 minutes, and then diluted to 1%, defiberized for 30 seconds, and further diluted to a 0.5% pulp concentration. Alternatively, after adding the dry strength agent P, after stirring, diluting, dispersing, and diluting to 0.5%, a 1% cationic polymer A solution is added or not added, and the added amount is 0, 0.4, 0.8 and 1.2 kg / t paper absolute dry weight, respectively, and stirring is continued for 1 minute. The pulp sample treated as above is copied by a copying machine according to the steps of the copying experiment to a grammage of about 90 g / m 2 Then, according to the relevant national standard method, the bursting strength and folding strength of the paper piece are measured. Another paper sample is taken, and the starch content of the paper sample is measured according to the method of "Determination of starch content of paper".

[0184] The experimental results are listed in Table 8. It can be seen from the results that since LOCC pulp already contains 208 mg / L free starch (based on a pulp concentration of 1%), as the amount of cationic polymer A increases, the bursting index and folding endurance of the finished paper also increase. When the amount of A increases to 0.8 kg / t of paper absolute dry weight, the bursting index reaches the maximum value, and when the amount of A increases to 1.0 kg / t of paper absolute dry weight, the folding endurance and starch content of the finished paper reach the maximum value. Moreover, when the amount of A is only 0.8-1.0 kg / t of paper absolute dry weight, the effect of A on improving the dry strength of the finished paper is equivalent to that of the conventional amphoteric polyacrylamide dry strength agent P of 4.5 kg / t absolute dry weight. It can also be seen from the results of Table 8 that when the conventional reinforcing agent P is used alone, although the bursting index is significantly increased, the starch content of the finished paper is almost unchanged. In addition, when polymer A and dry strength agent P are used in combination, the bursting index and average folding endurance of the finished paper are further improved than when A and P are used alone.

[0185] Table 8

[0186]

[0187] *The amount of samples added is based on absolute dry weight

[0188] Examples 42-45 and Comparative Examples 19-20

[0189] This group of examples and comparative examples demonstrates the effects of applying the cationic polymers A and B of the present invention and the conventional zwitterionic polyacrylamide dry strength agent P to LOCC pulp on the bursting index and starch content of the finished paper.

[0190] Experimental method: 667g of LOCC pulp with a concentration of 3% (20g absolute dry weight) was weighed in a plastic beaker. After stirring for 10 minutes in a constant temperature water bath at 50℃, it was diluted to 1%, and then defiberized for 30 seconds using an IMT-SJ01 standard fiber disintegrator, and further diluted to a pulp concentration of 0.5%. Then, according to the requirements of the experimental plan, 1% cationic polymer A and B solutions were added or not, and the added amount was 0 or 0.8kg / t paper absolute dry weight, respectively, and stirring was continued for 1 minute. Alternatively, after stirring for 10 minutes in a constant temperature water bath at 50℃, a certain amount of papermaking dry strength agent P was first added, so that the added amount reached 4.7kg / t paper absolute dry weight (refer to the conventional amount used in the actual boxboard papermaking process), and stirring was continued for 5 minutes, and then diluted to 1%, defiberized for 30 seconds, and further diluted to a pulp concentration of 0.5%. Alternatively, after adding the dry strength agent P, after stirring, diluting, dispersing, and diluting to 0.5%, add or not add 1% cationic polymer A and B, so that the added amount is 0 or 0.8 kg / t paper absolute dry weight, and continue stirring for 1 minute. The pulp sample treated as above is copied by a copying machine according to the steps of the copying experiment to a grammage of about 95 g / m 2 Then, according to the relevant national standard method, the tear resistance index of the paper piece is determined. 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".

[0191] The experimental results are listed in Table 9. From the results, it can be seen that since LOCC pulp already contains 208 mg / L free starch (based on pulp concentration of 1%), adding 0.8 kg / t of paper absolute dry weight of cationic polymer A or B alone can significantly improve the bursting resistance index and starch content of the finished paper. Adding 4.5 kg / t of paper absolute dry weight of conventional amphoteric polyacrylamide dry strength agent P alone has the same effect of improving the bursting resistance index as 0.8 kg / t of paper absolute dry weight of A or B, but has almost no effect on improving the starch content of the finished paper. When 0.8 kg / t of paper absolute dry weight of A or B is used in combination with 4.5 kg / t of P, the bursting resistance index of the finished paper is significantly improved compared with when they are used alone.

[0192] Table 9

[0193]

[0194] 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.

[0195] 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 cationic polymer to papermaking pulp for reaction, and then preparing paper products from the reacted papermaking pulp; The papermaking pulp contains free starch; The cationic polymer has a molecular weight of 1,000Da-1,000,000Da; The cationic polymer has a cationicity of not less than 50 mol%.

2. The papermaking method for improving paper strength according to claim 1, characterized in that: The cationic polymer has a molecular weight of 10,000 Da to 900,000 Da, preferably 13,000 Da to 900,000 Da, preferably 50,000 Da to 850,000 Da.

3. The papermaking method for improving paper strength according to claim 2, characterized in that: The cationic polymer has a molecular weight of 150,000 Da to 800,000 Da, preferably 180,000 Da to 800,000 Da, preferably 180,000 Da to 600,000 Da, preferably 200,000 Da to 500,000 Da.

4. The papermaking method for improving paper strength according to claim 1, characterized in that: The cationic polymer comprises at least one cationic monomer and, optionally, at least one nonionic monomer.

5. The papermaking method for improving paper strength according to claim 4, characterized in that: The molar fraction of the cationic monomer in the cationic polymer is 50 mol% to 100 mol%, preferably 60 mol% to 100 mol%, preferably 70 mol% to 100 mol%, preferably 80 mol% to 100 mol%.

6. The papermaking method for improving paper strength according to claim 4, characterized in that: The cationic monomer in the cationic polymer is selected from at least one of dimethyldiallylammonium chloride, 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 and N-(2-dimethylaminoethyl)methacrylamide.

7. The papermaking method for improving paper strength according to claim 6, characterized in that: The cationic monomer of the cationic polymer is dimethyldiallylammonium chloride.

8. The papermaking method for improving paper strength according to claim 4, characterized in that: The nonionic monomer in the cationic polymer is selected from 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.

9. The papermaking method for improving paper strength according to claim 8, characterized in that: The nonionic monomer in the cationic polymer is selected from acrylamide.

10. The papermaking method for improving paper strength according to claim 1, characterized in that: The cationic polymer is one or more combinations of polydimethyldiallyl ammonium chloride, polyhydroxypropyldimethylammonium chloride, and dimethyldiallyl ammonium chloride / acrylamide copolymer.

11. The papermaking method for improving paper strength according to claim 10, characterized in that: The cationic polymer is polydimethyldiallylammonium chloride, and its molecular weight is 150,000Da-800,000Da, preferably 180,000Da-800,000Da, preferably 180,000Da-600,000Da, preferably 200,000Da-500,000Da.

12. The papermaking method for improving paper strength according to claim 10, characterized in that: The cationic polymer is an allyl ammonium chloride / acrylamide copolymer, and its cationicity is 50 mol%-100 mol%, preferably 60 mol%-100 mol%, preferably 70 mol%-100 mol%, preferably 80 mol%-100 mol%.

13. The papermaking method for improving paper strength according to any one of claims 1 to 12, characterized in that: Calculated on a dry weight basis, the amount of the cationic polymer added is 0.1kg / t-5kg / t of paper absolute dry weight, preferably 0.4kg / t-4kg / t of paper absolute dry weight, preferably 0.6kg / t-2kg / t of paper absolute dry weight, preferably 0.7kg / t-1.5kg / t of paper absolute dry weight, preferably 0.8kg / t-1.2kg / t of paper absolute dry weight.

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

15. The papermaking method for improving paper strength according to any one of claims 1 to 12, characterized in that: The free starch is natural starch and / or starch contained in the secondary fibers themselves.

16. The papermaking method for improving paper strength according to claim 15, 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.

17. The papermaking method for improving paper strength according to any one of claims 1 to 12, characterized in that: The free starch content in the papermaking pulp is 1kg / t-100kg / t paper absolute dry weight; or, based on 1% pulp concentration, the free starch concentration in the papermaking pulp is 10mg / L-1000mg / L, preferably 30mg / L-500mg / L, preferably 100mg / L-300mg / L.

18. The papermaking method for improving paper strength according to any one of claims 1 to 12, characterized in that: Conventional retention and drainage aids, dry strength agents and / or wet strength agents are also added to the papermaking slurry.

19. The papermaking method for improving paper strength according to claim 18, 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.

20. The papermaking method for improving paper strength according to claim 19, 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-10kg / t of paper absolute dry weight; the dosage of the amphoteric starch and / or cationic starch dry strength agent is 1kg / t-20kg / t of paper absolute dry weight; the dosage of the conventional wet strength agent is 0.2kg / paper absolute dry weight-10kg / t of paper absolute dry weight.

21. The papermaking method for improving paper strength according to claim 18, characterized in that: The papermaking pulp is further added with zwitterionic polyacrylamide in an amount of 1 kg / t-5 kg / t of paper absolute dry weight.

22. The papermaking method for improving paper strength according to claims 1-12, characterized in that: The reaction time is 30s-10min.

23. The papermaking method for improving paper strength according to claims 1-12, characterized in that: The pulp concentration of the papermaking pulp is 0.2-5%, preferably 0.5-4%.

24. Use of the cationic polymer according to any one of claims 1 to 12 in a papermaking process, characterized in that: The cationic polymer acts as a free starch fixative to increase the starch content of paper.

25. Use of the cationic polymer according to any one of claims 1 to 12 in a papermaking process, characterized in that: The cationic polymer acts as a free starch fixative to improve paper strength.

26. Use of the cationic polymer according to any one of claims 1 to 12 in a papermaking process, characterized in that: The cationic polymer acts as a free starch fixative to reduce the COD concentration of discharged wastewater.

27. Use of the cationic polymer according to any one of claims 1 to 12 in a papermaking process, characterized in that: The cationic polymer is used as a free starch fixative to recover free starch in a secondary fiber papermaking process.

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