What are the chemicals in the papermaking process?
Papermaking process chemicals include high molecular synthetic substances used for strengthening, retention and filtration. These additives are called strengthening agents, retention agents and filtration aids. Making full use of new and efficient papermaking chemicals is the main means for the papermaking industry to improve production efficiency and product quality, reduce costs and reduce pollution. This chapter introduces dry strength agents, wet strength agents, retention aids and filtration aids.
Section 1 Dry Strength Agents
A class of fine chemicals used to enhance the strength of paper and paperboard is called papermaking strengthening agents. Paper strengthening agents can be divided into two categories, dry strength agents and wet strength agents, according to their different effects, and their strengthening mechanisms are also different. Dry strength agents are another important chemical used in the papermaking industry to increase the strength of paper. Many water-soluble polymers that can form hydrogen bonds with fibers can become dry strength agents. Dry strength agents are usually used to compensate for the decrease in paper strength caused by the addition of fillers or low-grade fibers (such as recycled fibers). Here we mainly introduce several commonly used dry strength agents in the papermaking industry, as well as the latest research and development directions of dry strength agents.
The strengthening mechanism of strengthening agents. Most natural and synthetic dry strength agents are hydrophilic polymers. These polymers are dispersed between fibers to increase the number of inter-fiber bonds, thereby achieving the purpose of improving paper strength. Most dry strength agents contain cationic groups connected to the main chain ring, which increases the bonding force between the polymer and the fiber and improves the retention of the polymer. Currently, commonly used dry strength agents include natural polymers such as starch and its modified products (such as cationic starch, anionic starch), synthetic polymers such as polyacrylamide, glyoxal polyacrylamide and polyvinyl alcohol, and other water-soluble natural product dry strength agents. In most cases, only adding 0.1% to 0.35% of such substances by mass can achieve an effective dry strength effect. At present, anionic polyacrylamide and modified starch are mainly used in my country. The strength of paper is affected by many factors. First of all, it depends on the bonding force between fibers in the finished paper and the strength of the fibers themselves, as well as the arrangement and distribution of fibers in the paper. The most important is the bonding force between fibers. There are generally four types of fiber bonding forces: chemical bonds, hydrogen bonds, van der Waals forces, and fiber surface interweaving forces. Among them, hydrogen bonding is the main way to generate paper bonding strength. Cellulose molecules have quite a lot of hydroxyl groups, and the hydrogen bonding force formed by countless microfibers is very large, which is the main reason for the dry strength. From the characteristics of its molecular structure, dry strength agents are mostly high molecular polymers containing multiple hydroxyl groups, which is the basis for forming hydrogen bonds with cellulose molecules. The hydrogen bond forming groups in the dry strength agent molecules form hydrogen bonds with the hydroxyl groups on the fiber surface. For example, the free glucose hydroxyl groups of starch participate in the formation of hydrogen bonds between cellulose molecules on the fiber surface, so starch increases the bonding force of internal fibers and increases the number of hydrogen bonds on the natural bonding surface between two bundles of fibers. At the same time, the dry strength agent has a certain improvement effect on the paper sheet forming process. The dry strength agent plays the role of an efficient dispersant at this time, that is, the dry strength agent makes the fiber distribution in the pulp more uniform, provides more bonding between fibers and between fibers and polymers, thereby improving the dry strength.
1 Polyacrylamide
Polyacrylamide is a water-soluble polymer with a mass fraction of more than 50% generated by homopolymerization of acrylamide or copolymerization with other monomers. The chemical structure is shown in the figure below. Due to the water solubility of PAM and the activity of the amide group on the main chain, it has a wide range of applications. The largest area of use is tertiary oil recovery in oil fields, followed by water treatment and papermaking. It is a rapidly developing fine chemical product, and there are currently thousands of varieties. There are three main methods for its production: aqueous solution polymerization, inverse emulsion polymerization, and radiation polymerization.
Aqueous solution polymerization is a traditional method for producing polyacrylamide, which can be used to produce polyacrylamide colloid and powder products. Generally, polyacrylamide colloid is obtained by direct polymerization of 8%-10% acrylamide aqueous solution under the action of an initiator; polyacrylamide dry powder is mostly polymerized with 25%-30% acrylamide solution: the polyacrylamide colloid obtained after polymerization is granulated, kneaded, dried, and crushed to obtain the product. The polymerization reaction is the key process. As shown in the figure.
This method has the advantages of safe production, simple process equipment, and low production cost. It is currently a commonly used method for producing polyacrylamide at home and abroad. my country first used this method to produce polyacrylamide by hand-made workshop-style disc polymerization, and later used a kneading machine. In the late 1980s, the conical kettle polymerization process was developed and successfully tested by the Fifth Institute of the Ministry of Nuclear Industry at the Jiangdu Chemical Plant. The polymerization technology introduced from abroad in the 1990s is similar to the domestic technology, except that the reactor can rotate and the volume of the polymerization kettle is also larger, reaching 5,000 liters.
Emulsion polymerization is another important method for the synthesis of polyacrylamide. Its conventional operation is to disperse the aqueous solution of acrylamide in an organic solvent such as gasoline, stir it vigorously, so that the solution forms a uniformly dispersed emulsion system, and then add an initiator to initiate the acrylamide reaction to obtain polyacrylamide. The characteristics of this method are that high relative molecular mass products (latex or dry powder) can be obtained under high polymerization rate and high conversion rate conditions.
Inverse emulsion polymerization refers to the water-soluble acrylamide with the help of surfactants (mostly non-ionic surfactants) to disperse the acrylamide monomer in the oil phase to form an emulsified system, and emulsion polymerization is carried out under the action of the initiator to form a stable high molecular weight and fast-soluble polyacrylamide latex product, which is dehydrated by azeotropic distillation to obtain powdered polyacrylamide. Since the polymerization reaction is carried out in acrylamide particles dispersed in the oil phase, the heat released during the polymerization process is evenly distributed, the reaction system is stable and easy to control, and it is suitable for preparing high molecular weight and narrow molecular weight distribution polyacrylamide latex or dry powder products.
The radiation initiation method is that acrylamide monomer is directly polymerized by ultraviolet light or radiation to obtain solid polyacrylamide products. This method has a simple production process, but the equipment investment is large, and the molecular weight distribution of the obtained product is very wide, so large-scale industrial production has not yet been carried out.
The synthesis conditions of polyacrylamide are complex, and temperature, monomer concentration, initiator type and concentration, system pH value, additives, hydrolysis conditions, etc. all strongly affect the quality of the product. Generally, low temperature (0-10℃), low initiator concentration, appropriate monomer concentration (20%-35%) and pH value (6-11), appropriate hydrolysis degree, hydrolysis time and temperature are necessary conditions for synthesizing polyacrylamide with high relative molecular weight. The addition of initiators and additives is even more important.
At present, the initiators widely used in AM polymerization are mainly redox initiators and azo thermal decomposition initiators. Commonly used redox initiators mainly include persulfate, organic peroxide-sulfite (bisulfite), thiosulfate or metabisulfite combination, persulfate, organic peroxide-organic tertiary amine combination, high-valent metal ion-organic reducing agent combination, etc. Thermal decomposition initiators are compounds containing weak bonds in the molecule, mainly inorganic, organic peroxides and organic azos. Such as azobisisobutyronitrile (ABN) and its water-soluble derivatives. The decomposition characteristics of azo initiators are that they require higher temperatures, the decomposition reaction is a kinetic first-order reaction, only one free radical is produced, and there is almost no chain transfer. Therefore, even if azo initiators initiate AM polymerization at higher temperatures, they will not cause a large loss of the relative molecular mass of PAM. The product with a relative molecular mass of 10 million to 20 million can be obtained by simply using azo to synthesize PAM. ABN is widely used in the synthesis of ultra-high relative molecular mass HPAM in my country.
In order to obtain products with excellent performance in all aspects, it is essential to add various appropriate additives to the reaction system. When synthesizing PAM, ultra-high relative molecular mass and cross-linking are often contradictory. Ultra-high relative molecular mass requires low initiator concentration, which will lead to cross-linking. Sodium formate or isopropanol are widely used chain transfer agents. These small molecules can prevent cross-linking and improve the water solubility of the product, but they will also weaken the growth of relative molecular mass, so there must be a suitable concentration range. Urea has also been found to greatly improve water solubility, and has almost no effect on the relative molecular mass of the product within a large concentration range, so it is also widely used. In addition, some people have also used inorganic salts such as Na2 SO4, (NH4) 2 SO4, and organic surfactants such as Span20, Tween20, OP210, sodium dodecyl sulfate, etc. to improve water solubility, and achieved good results.
It was found that some substances can increase the reaction rate or increase the relative molecular weight of the product, because these additives such as inorganic salts have a complexing effect on the chain growth radicals and can change the activity of the chain growth free radicals. Therefore, the selection of additives is also a field worth studying.
PAM is a water-soluble polymer with a mass fraction of more than 50% generated by copolymerization of acrylamide (AM) or copolymerization with other monomers. According to the type of functional group, PAM can be divided into four types: non-ionic, anionic (APAM), cationic (CPAM) and amphoteric. PAM has the advantages of easy hydrolysis, easy use, and environmental friendliness. It can be used as a paper enhancer, retention and drainage aid, etc. However, PAM is electrically neutral and cannot be effectively adsorbed on the fibers in the pulp. Therefore, it needs to be ionized when used as a paper strengthening agent. Common methods include: obtaining anionic polyacrylamide (APAM) containing some carboxyl groups through amide hydrolysis; generating cationic polyacrylamide (CPAM) through Hofmann degradation reaction or Mannich reaction; or copolymerizing acrylamide and other monomers to generate anionic, cationic or amphoteric polyacrylamide.
At present, most of the domestic and foreign methods use aqueous solution polymerization. However, the product variety is single, the content of effective ingredients is low (mass fraction is about 8%), the use effect is poor, and the actual application cost is too high, which limits the use of PAM in China. The research on modified dry strength agents is very active at present. For example, amphoteric polyacrylamide paper dry strength agent is synthesized using acrylamide, acrylonitrile, acrylic acid and accelerator (MPA) as raw materials. The results show that the dry strength agent produced has better strengthening effect than similar products from other domestic manufacturers. The terpolymer of methacryloyloxyethyl trimethylammonium chloride/acrylamide/maleic acid (DMC/AM/MA) synthesized by aqueous solution copolymerization was used to prepare amphoteric polyacrylamide suitable for closed-cycle papermaking conditions. The papermaking experiment showed that its reinforcing effect under closed-cycle conditions has reached the level of imported amphoteric polyacrylamide reinforcing agents, and it has a significant anti-anionic garbage interference ability compared with cationic polyacrylamide. A water-in-water type cationic polyacrylamide (CPAA) emulsion reinforcing agent was synthesized by free radical copolymerization with acrylamide and dimethyldiallyl ammonium chloride (DDAC) as the main monomers. The results showed that the additive improved the fiber dispersion state on the surface of the paper sheet, and the structural morphology of the fibers at the fracture showed that the interaction between the fibers was strengthened. A cationic polyacrylate emulsion paper reinforcing agent was synthesized by emulsion polymerization with vinyl acetate (VAc), butyl acrylate (BA), styrene (St), dimethyldiallyl ammonium chloride (DAD2MAC), and acrylamide (AM) as monomers. The reinforcing effect of the polymer emulsion enhancer on sedge pulp and waste paper pulp is more significant than that on bleached coniferous wood pulp. At present, CPAM has been successfully developed, but there are problems such as unstable performance and high price. Amphoteric PAM and PAM graft copolymers are still in the research stage, and high-efficiency PAM enhancers suitable for papermaking with grass and waste paper raw materials are urgently needed to be developed.
Example of polyacrylamide preparation.
This experiment uses acrylamide to synthesize polyacrylamide under the initiation of ammonium persulfate. The reaction equation is as follows:
Add 10.0g acrylamide and 80mL distilled water to a 100mL beaker and stir to dissolve. Then place the beaker in a constant temperature water bath, slowly stir and heat to 60℃, accurately weigh ± 0.001g ammonium persulfate, dissolve it with 10mL distilled water, and then pour it into a 100mL beaker, react for ~1h, cool, and discharge the product.
2. Other papermaking dry strength agents
The main dry strength agents used in actual production are also starch. For details about starch, see the surface sizing agent section. Others include chitosan, guar gum, CMC, polyvinyl alcohol (PVA), etc. However, in general, these substances still need to be modified to a certain extent to achieve better effects. For example, polyvinyl alcohol modified with glyoxal can significantly improve the folding resistance, tensile strength and tear resistance of paper when used for surface sizing. It is a type of dry strength agent with great development potential. At present, foreign research on dry strength agents mainly focuses on the development and application of new processes and new products. For example, PAM latex can be used for surface coating and internal addition to obtain good reinforcement effects; the product obtained by emulsion polymerization of PAE, acrylic monomer, styrene and butadiene has good wet strength effect; guar gum or hydroxypropyl guar gum is grafted with vinyl monomer to facilitate the use of the advantages of natural polymers and synthetic polymers to adapt to the green environmental protection concept and the requirements of the circular economy of the papermaking industry [29]. However, my country’s research on latex enhancers mainly focuses on non-ionic and cationic types. Future research focuses on the development and application of efficient and inexpensive latex enhancers. Newer dry strength agents include polyvinylamine (PVAm) obtained by Hofmann degradation reaction of polyacrylamide (PAM), which has a good reinforcing effect on bleached hardwood pulp, bleached coniferous pulp, mixed wood pulp, deinked pulp and waste paper pulp [30]; there are multifunctional papermaking additive copolymers synthesized from acrylamide (AM), dimethyldiallyl ammonium chloride (DDAC), 2,2-methylacrylphthaloyl trimethyl ammonium chloride (DMC) and acrylic acid (AA), which have the effect of significantly increasing the dry strength of paper and significantly shortening the drainage time. There are also methods of combining one or more reinforcing agents with Al2(SO4)3 or sizing agents, which also have a certain effect on increasing the strength of paper. For example, the reaction of phenol, formaldehyde and triethylamine to obtain amino-modified phenolic resin, and then adding cationic starch, has a synergistic reinforcing effect, and the tensile index and ring crush index are greatly improved. HDS is a type of multi-polymer amphoteric polyacrylamide, which is based on acrylamide monomer. Different types and amounts of water-soluble cationic and anionic monomers are added according to different uses, and various additives are added. It is a dry strength agent obtained by alternately copolymerizing under aqueous solution conditions.
Copolymerization under the action of a hair dryer, etc.
Section 2 Wet Strength Agent
The strength of paper depends on the strength of the fiber itself and the strength of the connection between fibers, as well as the arrangement and distribution of fibers in the paper, that is, the bonding force between fibers in the paper and the fibers themselves, among which the most important is the bonding force between fibers. This bonding force is mainly the bonding force of hydrogen bonds. The ability of cellulose fibers to form hydrogen bonds lies in the presence of cellulose hydroxyl groups. The combination of hydrogen bonds between fibers allows the fibers in the paper to bind to each other without adhesives, giving the paper a certain strength.
Paper treated with wet strength agents has both interweaving of fibers and chemical crosslinking between polymers that react chemically through the drying of the paper, as well as chemical crosslinking between polymers and fibers, which makes the paper difficult to swell in water, limits swelling, and produces wet strength. The wet strength mechanism is that polymer molecules crosslink each other to form a network structure wrapped around the fibers, which restrains the swelling of the fibers and maintains the wet strength of the paper; at the same time, these polymers can be combined with fibers by covalent bonds or ionic bonds. These bonds are sufficient in number and strength to overcome the interaction between fibers and water, so that the paper maintains a certain strength.
It should be emphasized that the application of wet strength agent alone cannot achieve the best effect of wet strength agent. It must be used in conjunction with retention and drainage aids so that wet strength aids can be retained in the paper as much as possible and can increase the retention of fillers and fine fibers.
Wet strength agents are mainly divided into two categories, namely acidic ripening resins represented by MF and UF and alkaline ripening resins represented by PPE, the latter of which has more advantages. Here we will discuss several major reinforcing agents: polyamide epichlorohydrin (PAE), cationic polyacrylamide (CPAM), polyamines (PA) and chitosan, formaldehyde resin [can be divided into urea-formaldehyde resin (UF resin) and melamine-formaldehyde resin (MF resin). The mass fraction of wet strength agent added is 0.5%~1.0% (to absolute dry fiber).
1 Cationic urea-formaldehyde resin
Urea-formaldehyde resin belongs to thermosetting resin, which can be divided into non-ionic, anionic, cationic and amphoteric UF resin. The water solubility of ionic UF resin is greatly improved. Anionic and amphoteric UF resins need to be adsorbed on the fiber through aluminum sulfate, while cationic UF resins can be directly adsorbed on the fiber. Due to the precipitation of free formaldehyde in urea-formaldehyde resin, its application has been decreasing in recent years. At present, some researchers have begun to test modified urea-formaldehyde resins. For example, the synthesis conditions of urea-formaldehyde resins synthesized by partially or completely replacing formaldehyde with glyoxal and the wet strengthening effect of the products on paper. The results show that the glyoxal/urea resin synthesized by completely replacing formaldehyde with glyoxal has a significant improvement in the wet strength performance of paper, and at the same time, there is no formaldehyde pollution at all, which is suitable for use as a wet strength agent for paper. Anionic urea-formaldehyde resins can be used to increase the wet strength and dry strength of paper and paperboard produced from unbleached chemical pulp. They are mainly used in kraft pulp sack paper and low-weight crepe kraft paper. An important benefit of using anionic urea-formaldehyde resins is to obtain better economic benefits. When anionic urea-formaldehyde resins are fixed with aluminum sulfate, the demand for aluminum sulfate can be as high as 3% by mass. The mass fraction of anionic urea-formaldehyde resin is usually 0.5% to 3% (expressed as solid, for absolute dry pulp). In order to obtain the best wet strength, the white water pH value must be 4.5 to 5. Cationic urea-formaldehyde resins have a wider range of applicability than anionic urea-formaldehyde resins. Because they carry a positive charge, they can be adsorbed onto pulp fibers, including bleached pulp fibers, without the addition of aluminum sulfate. The addition of aluminum sulfate does increase the wet strength to a certain extent, but the increase is not as great as that of anionic resins.
2 Melamine formaldehyde resin
MF resin is a polymer obtained by the reaction of melamine and formaldehyde. MF resin contains hydroxymethyl groups, which can form an etherified structure between fiber bundles. This cross-linking between different molecules produces water resistance, which enables the paper sheet to achieve a wet strength effect, so it is mainly used as a wet strength agent and water repellent in the papermaking industry. The main agent used in the papermaking industry is trimethylol melamine. However, due to its unsatisfactory stability, water solubility and free formaldehyde content and its adverse effect on the whiteness and durability of paper, modified MF resin is widely used nowadays. MF resin is modified by using prepolymer generated by cyclization reaction of urea and formaldehyde to obtain a high-solid content wet strength agent for papermaking. The modified melamine formaldehyde resin can significantly improve the wet/dry tensile strength of paper, can be miscible with water in any proportion, and has a storage stability of more than half a year. At present, domestic and foreign research in this field mainly focuses on using small molecule alcohols such as methanol and butanol as etherifying agents and a small amount of cationic modifiers under the catalysis of p-toluenesulfonic acid to block its active groups, prevent gelation, and improve product stability. Anion-modified MF resin is mainly sulfonated melamine resin. In contrast, aminosulfonate-modified resins have better effects and are widely used. This type of product can be cured under acidic, neutral or alkaline conditions, and can be added to the pulp or used in coating. At present, since the reaction between the modifier and formaldehyde produces precipitation and crystallization and is difficult to filter out, this type of product needs to be improved. MF resin modified with cationic agents such as triethanolamine and dimethylamine has positive charge, can be adsorbed by fibers quickly, and can be cured at a higher pH, avoiding the adverse effects of ordinary MF resin on paper whiteness and durability. Therefore, cationic resins are widely used as wet strength agents in the papermaking industry. At present, the main research in this field at home and abroad is to use small molecule alcohols, such as methanol, butanol, etc. as etherifying agents and a small amount of cationic modifiers under the catalysis of p-toluenesulfonic acid to block their active groups, prevent gelation and improve product stability. An etherified modified MF resin was prepared. The results showed that the prepared modified MF wet strength agent had a significant effect on the physical properties of paper. When it was compounded with polyamide polyamine epichlorohydrin (PAE) and applied in papermaking, it not only significantly improved the wet strength, dry tensile strength, sizing degree and other properties of paper, but also broadened the pH range of modified MF, reduced the amount of acid, and was conducive to papermaking under neutral conditions. Cationic modified MF resin is also widely used as a water repellent for coated paper, which can give coated paper coated white paperboard higher wet strength and wet friction resistance. In addition, it can also be used as a heat stabilizer for paper. With the increasing demand for various special papers and the maturity of the preparation process of melamine formaldehyde resin and its derivatives, it is believed that this type of material will have a wider application in the papermaking industry. In recent years, the free formaldehyde content in MF resin has received more and more attention. At the same time, its storage stability is the key technology of this type of product, and increasing the solid content can improve the economic benefits of using MF resin. At present, the research hotspots of melamine formaldehyde resin are to reduce formaldehyde content, improve storage stability and increase solid content.
3 Polyamide epichlorohydrin (PAE) wet strength agent—Adipic acid sales
PAE resin is a strong cationic, high molecular weight wet strength agent that can be used in a wide pH range. It has strong self-adhesion and excellent resistance to environments with more anionic impurities or high salt concentrations, so it can play a good wet strength effect under such harsh conditions. While improving wet strength, PAE resin does not lose the softness and absorbency of the finished paper. The whiteness of the finished paper has little yellowing and good heat resistance. It is widely used in the production of paper towels, liquid packaging paper, photographic base paper and other paper types. PAE resin, which can be cured under medium/slightly alkaline conditions and does not contain formaldehyde, has become one of the research hotspots of papermaking additives because of its convenience and good effect. Based on the principle that nitrogen on the imine and amine groups is easy to react with urea for deamination, a polyurea-modified polyamide polyamine epichlorohydrin with high solid content and good stability was prepared. The results showed that the polyurea-modified polyamide polyamine epichlorohydrin (PUAE) with good stability has obvious moisturizing and strengthening effect. PAE was also modified with rosin, and the same ideal effect was achieved. PAE was modified by a two-step method and ethanolamine (EA) and crosslinking agent water-soluble epoxy resin (WEP). Anionic polyacrylamide (APAM) was used as a retention agent and cationic modified PAE was applied to cotton pulp papermaking. The research results showed that the wet strength of the finished paper could reach 39.8%. The PAE resin was modified by replacing part of diethylenetriamine with the cheap modifier M, which reduced the production cost and the performance of the modified resin also met the requirements of the paper mill. PAE resin can be used in combination with other reinforcing agents under certain conditions to achieve better results. For example, the dual reinforcing system composed of amphoteric polyacrylamide and cationic polyamide epichlorohydrin not only has a good reinforcing effect, but also can improve the water filtration rate of the pulp and resist the high shear force in the system. It is a wet end enhancer with broad market prospects. The development and application of two-component papermaking retention enhancers have challenged the traditional use of retention agents and enhancers alone, and have played a positive role in improving papermaking processes and increasing paper production and quality. Its promotion and application will produce good economic, social and environmental benefits. The application of PAE resins needs to further improve the understanding of its safety. PAE products contain a small amount of by-products from epichlorohydrin, which are low molecular weight organic chlorides with high toxicity. PAE products have been developed that can maintain the original moisturizing and strengthening effect, and the content of such compounds in the product is very small.
4. Polyethyleneimine (PEI)
Polyethyleneimine is the most widely used and recognized cationic wet strength agent. It is a macromolecule generated by the polymerization of ethyleneimine in the presence of an acidic catalyst (such as CO2, oxalic acid). The molecule contains primary, secondary and tertiary amino groups at a ratio of approximately 1:2:1. Its synthesis is as follows:
PEI is a water-soluble polymer that can be mixed with water in any proportion. When used, it is usually added directly to the pulp without the need to add alum to increase its retention rate. This is because the polyethyleneimine polymer chain contains multiple cationic groups, which are cationic in the pulp system and can generate electrostatic attraction with the hydroxyl groups of cellulose to form a secondary cross-linking network. As for the mechanism of action of polyethyleneimine, it can be considered that PEI produces a wet strength effect by generating stronger bonds between atoms, rather than forming a network structure through homo-crosslinking or co-crosslinking. Instead, it is believed that the cationic groups of PEI form a strong ionic bridge with the carboxyl groups of cellulose, thereby playing a wet strength effect.
5 Other wet strength agents
Due to the presence of residual free formaldehyde to a greater or lesser extent, and the papermaking industry in various countries is gradually changing from acidic to neutral or alkaline papermaking processes, the UF and MF resins suitable for acidic papermaking are gradually decreasing. Many traditional wet strength resins have adverse effects on the environment. In view of environmental protection factors, the development of a new generation of pollution-free wet strength agents is an inevitable development trend in the future. Polycarboxylic acid is a new type of environmentally friendly wet strength agent that adapts to this trend. As a new type of environmentally friendly wet strength agent, its strengthening effect on paper has been fully affirmed. Although there are still some shortcomings, it is very likely to develop more effective polycarboxylic acid wet strength agents through certain means. Therefore, polycarboxylic acid is a new type of efficient and environmentally friendly wet strength agent that is worthy of further research and is expected to be industrialized and widely used. Chitosan is a nitrogen-containing natural organic compound chitosan that exists in huge quantities on the earth. It has a similar chemical structure to cellulose and is an excellent papermaking chemical additive. For example, by chemically modifying chitosan and introducing hydroxypropyl nonionic groups, water-soluble chitosan derivative hydroxypropyl chitosan can be prepared. Using it as a wet-end additive for papermaking can effectively strengthen the hydrogen bonding between fibers and help improve the physical strength of paper. At present, research on environmentally friendly wet strength agents has begun abroad, and China’s research in this area is still in its infancy. Research on other wet strength agents such as dialdehyde starch, glyoxal-acrylamide graft copolymer, glutaraldehyde, polyethyleneimine and other wet strength agents has also achieved gratifying results. It is believed that with the continuous efforts of scientific researchers, more papermaking wet strength agents will be developed, and research on papermaking wet strength agents will also make greater progress.
Preparation of several major wet strength agents.
Preparation of polyamide epichlorohydrin (PAE):
In the 1960s, PAE resin began to be used in the papermaking industry and has now become the most widely used wet strength agent. Its preparation method is to generate polyamide through the reaction of dibasic acid and triamine, and then treat the polyamide with epichlorohydrin to obtain alkylable secondary amino groups, which will self-alkylate to form 3-hydroxy-azetidinyl group water-soluble, cationic resin. The characteristics of this resin: 1) non-toxic and odorless, no formaldehyde-containing wet strength agent; 2) can be widely used in the pH range of 4 to 10; 3) has a high efficiency wet strength effect, when the dosage is % to %, the relative wet strength can reach about 14%, so the dosage is small; 4) its damaged paper is easy to recycle and can be pulped at pH 10; 5) has good adsorption properties, high dry strength and temporary wet strength; 6) the stiffness of the paper sheet containing PAE is low. The preparation method of this resin can be roughly divided into two categories: one-step method and two-step method. 1 One-step preparation
Polyamine and epichlorohydrin are reacted in an aqueous medium until the desired molecular weight is reached, and then the reaction is diluted or stabilized under acidic conditions by adding an acid.
Specific example: Add mol epichlorohydrin to a beaker with about the same mass of water, and adjust the temperature to 24 °C. Add 1 mol of polyalkylene polyamine (1, 6-hexamethylenediamine) to the mixture within 30 minutes, while maintaining the temperature of the reaction mixture below about 40 °C. Then add water to dilute the reaction to a total solid content of about 54%, and due to the exothermic reaction, the reaction mixture is heated to about 50 °C. The mixture is then heated to about 85 °C, and the viscosity of the mixture increases. At this time, water is added to a solid content of about 46%, and the temperature is adjusted to 76 °C. Maintain this temperature until the desired viscosity is reached. Stop heating and add water to a solid content of about 38%. After cooling to about 35 °C, the pH value is adjusted to about 1.5 with concentrated sulfuric acid. The obtained hexamethylenediamine-epichlorohydrin resin is highly branched, irregularly structured, and contains many different functional groups.
2. Two-step preparation
The first step is to form a polyamide prepolymer with a dicarboxylic acid or a carboxylic acid derivative (such as adipic acid) and a polyamine (such as diethylenetriamine). Then in the second step, the prepolymer is reacted with epichlorohydrin in an aqueous solution. When the desired molecular weight or viscosity value is reached, the reaction is diluted or stabilized by adding an acid.
Depending on the method of synthesizing polyamide prepolymers, the two-step method can be divided into a classical method and an enzyme-catalyzed method.
1) Classical method (generally referred to as the two-step method)
Step 1, synthesis of prepolymer. The most common method for prepolymer synthesis is the polycondensation reaction of dicarboxylic acid and diamine, usually at a temperature of 150-170 °C, with the solvent and reactants in a near stoichiometric ratio. The water formed during the reaction is removed by distillation, and when a certain degree of polycondensation is reached, the reaction is terminated by adding water and cooling.
2
Step 1, the formation of polymer. The prepolymer further reacts with epihalohydrin in aqueous solution to form a polymer solution with storage stability. Due to its economy, wide applicability and ease of use, the most commonly used epihalohydrin is epichlorohydrin. Its dosage is directly related to the secondary amine or tertiary amine functional group of the prepolymer. Theoretically, a 1:1 molar ratio can be used. In fact, the molar ratio of epichlorohydrin and amine functional group can be from 1:1 to 1:1.
2) Enzyme catalysis method
The catalyst used in this method is a type of lipase. People first discovered that the function of this type of enzyme is to catalyze the hydrolysis of carboxylic acid alcohol esters. Later, it was discovered that this type of enzyme can also catalyze the reverse reaction of ester hydrolysis, including esterification and transesterification. The enzyme method is to use the catalytic effect of lipase on the reverse reaction of hydrolysis reaction to prepare the prepolymer required for synthesizing PAE resin.
Prepolymer synthesis specific example: Dimethyl adipate (6.97 g, mol), diethylenetriamine (4.12 g, and Novozym435 (0.5 g) were mixed in a 100 mL flask. Heat to 90 °C in an oil bath. Keep the viscous mixture at 90 °C for 16 h in an open container and introduce nitrogen flow. A light yellow solid appeared and the reaction was completed. Methanol (60 mL) was added to dissolve the polyamide product, while the enzyme was insoluble and the enzyme was separated by filtration. The residual methanol was separated by rotary evaporation. Finally, a light yellow solid was obtained. The yield was 8.4 g, Mw was 6700 u, and Mw/Mn was.
Step 2 is similar to the classical method.
Section 3 Retention and filtration aids
The function of retention and filtration aids is to increase the retention rate of pulp when it is put on the screen, enhance the water filterability, thereby increasing paper production, reducing energy consumption, and facilitating papermaking. The two generally work together. Speaking separately, retention aids refer to chemicals that help fibers and other chemicals to be retained more in the paper sheet and not be lost with water. Filter aids refer to agents that can increase the filtration speed or reduce the moisture content of the filter cake. Filter aids in the papermaking process mainly refer to some cationic polymer polyelectrolytes such as polyethyleneimine, polyacrylamide and cyclopropane resin, etc. After being fully diluted, they are added as close to the screen of the paper machine as possible to make The slurry is in a micro-flocculation state, which accelerates dehydration. Because multi-element retention and filtration aids are better than mono-element retention and filtration aids, we first briefly introduce the ternary retention and filtration aid system, the binary retention and filtration aid system, and the microparticle retention and filtration aid system.
Principle of the ternary retention and filtration aid system: first use a high-charge, low-molecular-weight cationic fixative to fix the anionic garbage on the fiber, then add a high-molecular-weight, low-charge cationic polymer to make it evenly distributed on the fiber and filler with a controllable positive charge, and then add bentonite particles to form a uniform micro-flocculation, improve water filtration performance, and increase retention rate.
Binary retention and filtration aid The filtration system first adds low molecular weight cationic polymers, which are attached to fillers and fibers through electrical action to form micro-flocculation. Then, high molecular weight anionic polymers are added. Anionic polymers are strongly adsorbed on cationic polymers, and the fibers and fillers in the pulp are grafted and copolymerized through bridging connection to form a flocculation system with strong adhesion and shear resistance, which greatly improves the retention of fine fibers and fillers and also improves the strength of the paper web.
The microparticle retention and filtration system first adds high molecular weight cationic polymers to the pulp, which are adsorbed on long fibers, fine fibers and fillers to form initial floccules. Unstable. Then add inorganic substances with very large specific surface area and negative charge (such as colloidal silica, aluminum hydroxide and bentonite), and the particles will agglomerate under the action of electrostatics to form small, dense and uniform floccules, which greatly improves the retention of fine fibers and fillers and improves the water filtration performance.
In the study of retention and drainage aids, first fix a good reinforcing agent and fix the slurry ratio (same as above), select three types of ternary systems, three types of binary systems and three types of microparticle systems, a total of nine retention systems, and conduct comparative studies at different dosages to obtain the best retention and drainage system selection (type and dosage).
Change one reinforcing agent, add the best retention and drainage system obtained to the slurry for papermaking, and compare the difference between the two reinforcing agents based on paper strength and retention rate. If the difference is not big, both reinforcing agents can be used as substitutes for each other, and can be selected according to market prices. At the same time, they can be used flexibly in case of changes in feeding conditions (all the above screening experiments will record 2-3 sub-best options for backup except for the best results).
Since the retention and filtration process is very complicated, there is no universal principle or universal substance to solve it in one step. One kind of additive can often be successfully used on one machine in one paper mill, but not on another paper mill or another machine. Therefore, there are many kinds of additives. With the development of paper machines towards high speed and large-scale, traditional retention and filtration systems are difficult to adapt, and new retention and filtration aids need to be developed. For example, polyacrylamide is a low-charge and high-molecular-weight compound with positive charge. It can produce retention and filtration effects by bridging the fiber flocculation, but the formed floccules are difficult to re-flocculate after being destroyed by high shear forces such as slurry pumps; on the other hand, strong fiber flocculation also deteriorates the uniformity of the paper sheet. Another example is the use of a two-component system. First, A13+ in alum is adsorbed on the fiber surface to make it positively charged, and then a negatively charged polymer compound is added to combine with the fiber and fine fibers with A13+ to produce a retention effect. This two-component system is suitable for acidic papermaking, but not for neutral or alkaline papermaking. Under this background, it is necessary to develop new retention and filtration systems. Here we mainly introduce new microparticle retention and filtration systems, polymer polymers
1. New microparticle retention and filtration systems
This system generally includes two components: one is a cationic polymer, such as cationic polyacrylamide; the other is anionic inorganic particles (such as anionic silica or bentonite) or anionic microparticle polymers. Its mechanism of action is to first add cationic polymers to negatively charged paper materials to form highly flocculated fibers. Then, through the pulp flow process, the high shear force of the pressure screen breaks up into microflocculates. This microflocculate is then formed into a super-flocculated fiber under the action of microparticle anions, which plays a role in retention and filtration. There are currently four main systems, Compzil, Hydrocol, Hydrosil and Integra systems.
1) Compozil system
This is a retention and drainage system developed by EKA of Sweden. It is composed of cationic starch and anionic colloidal silica. When high-charge cationic starch is added to the system, large flocs are generated and destroyed by shear force. When anionic colloidal silica is added, it is adsorbed on small flocs with positive charge. The final flocculation is mainly charge neutralization rather than bridging effect.
2) Hydrocol system
It is a new type of multi-particle retention and drainage system developed by Allied Colloids. Generally, high molecular weight and low charge density cationic polymers are added first, and then special inorganic fillers are added to form a binary control system. The working process is that when high molecular weight cationic polymers are added, they form initial flocs with fibers through bridging action. Under high shear force, the flocs are broken into a large number of small fragments. Then, inorganic fillers are added through adsorption, electrostatic neutralization and coordination with the non-charged segments of cationic polymers. The small fragments are re-crosslinked to form a unique coagulum structure with small size and dense structure. This inorganic filler is usually bentonite, containing an aluminate layer with two layers of silicate between the layers. The surface area in water can reach 700m2/g. It is negatively charged on the surface of the layer plate and is not affected by pH. It also shows good performance in acidic and alkaline conditions, and is suitable for acidic and neutral papermaking systems.
3) Hydrosil system
It was developed by Bovliden Kemi Ab in Sweden and is only used in alkaline systems. It consists of cationic starch and aluminum sulfate, and sodium hydroxide and aluminum sulfate are used together. When the polymer is added, large floccules are first formed by bridging, and then sheared and dispersed. Anionic particles are added before going online to form small and dense floccules. These small floccules are very easy to dehydrate, which is manifested as increased water filtration rate and increased retention rate.
4) Integra system
It is a multi-element retention and drainage system developed by ECC in the UK, which can operate under neutral and acidic papermaking conditions. Its core is to use lignin sulfonate copolymers with excellent dispersibility to disperse micro-flocculates. The process is to first add ultra-high molecular weight polyacrylamide (PAM) to the front of the paper machine pressure screen to form large flocs, split them into small flocs through high shear, and then add lignin sulfonate to use its dispersibility to form these flocs into stable, flexible and uniform small flocs, thereby achieving good retention and paper sheet uniformity. This system does not use inorganic microparticle additives, but all organic additives.
2 Advantages of using microparticle retention and filtration system
Currently, the cost of using microparticle retention and filtration system is higher than that of traditional unit or dual-element retention system, but from a comprehensive perspective, the advantages of microparticle retention and filtration are multifaceted, and its economic benefits are still cost-effective.
In terms of paper machine operation and production capacity
● Improve the water filtration performance of the net table, vacuum section and press section;
● Increase the speed of the paper machine and reduce the steam consumption of the drying section;
● Purify the white water system and reduce the generation of sediment;
● Reduce paper breaks in the wet section.
In terms of product quality
● Improve the uniformity of paper sheets;
● Improve the uniformity of the quantitative moisture distribution of the paper cross section, thereby reducing the warping and curling problems caused by the stress of the paper sheets;
● Improve the strength and physical properties of the paper sheets.
Save raw material consumption
● Reduce steam consumption (increase the dryness of the paper sheets entering the drying cylinder);
● Reduce fiber consumption (due to reduced white water loss).
3 Application of microparticle retention and filtration aids
The development and application of microparticle retention and filtration aid systems have a history of more than 20 years. Generally, they are divided into three types according to the difference in the second composition (microparticle anion). That is: colloidal silica, bentonite, and micropolymer.
At present, there are about 600 paper machines in the world that use microparticle systems, of which 350 paper machines use silica microparticle systems, 250 paper machines use bentonite systems, and a few other paper machines use micropolymer systems.
For example, a newsprint mill uses 50% TMP pulp and 50% waste paper deinking pulp, pH=, and uses cationic PAM and anionic inorganic salt micro-pulled PM810, and the one-time retention rate is increased to 57%.
The following introduces the effect of using microparticle retention and filtration systems in Fujian Qingshan Paper. Qingshan Paper produces kraft paperboard on a three-long web paper machine. The paper machine has an annual output of 150,000 tons, a speed of 300~350m/min, and a cardboard basis weight of 175g/m2. The surface layer is kraft pulp, and the bottom core layer is American waste paper pulp. The cationic polymer (trade name Percal 230HL) is added at the inlet of the pulp pump, and the dosage is ~0.6kg/t paper. The micro-pulled bentonite is added at the outlet of the pressure screen, and the dosage is 2~3kg/t paper.
Adding 0.3kg of cationic polymer and 2kg of micro-pellet bentonite as additives, the total cost increase is ×36＋2×8=yuan/t paper.
Each ton of paper produces 70m3 of white water. Only the bottom layer is considered, and the cost of a ton of pulp is 1,280 yuan/t, which saves a total of (% – %) × 70 × 1,280 yuan = yuan/t of paper.
Excluding the core layer pulp, the bottom layer pulp alone can offset the cost increase, and can also make a profit of about 5 yuan/t, which is profitable.
4 Actively promote the use of microparticle retention and filtration systems
About 20% of the production capacity of China’s papermaking industry has become a modern papermaking industry. This part has an urgent need for micro-pulling retention and filtration, and the technical level of most other parts is the level of the 1970s to 1980s or earlier. Although the use of unit flocculants or dual polymer systems in this part can still meet the low technical requirements, the use of microparticle retention and filtration systems can also achieve better production results, but the equipment level and other supporting conditions need to be appropriately modified.
At present, most of the paper machines in China are about 3m wide or narrower, and the speed of the paper machine is 300m/min. After using the microparticle retention and filtration system, the dewatering capacity of this type of paper machine is enhanced. It is reasonable to create conditions for increasing the speed and increasing production. This is the first choice. However, the capacity of some flushing pumps has reached its limit and there is no potential to be tapped. In this case, a new flushing pump must be replaced, and some even other dewatering components must be replaced to meet the requirements of increasing the speed.
Some products are high-end products with higher product quality requirements. It is very important to improve the uniformity of paper. This may require further reduction of the concentration of the net, and the use of microparticle retention and filtration aids to improve the dewatering capacity will create certain conditions for the production of certain high-end products.
The microparticle retention and filtration system has the dominant role of the change in the charge of the paper stock. When using new additives, the wet part properties of the old system should be understood. If the negative charge of the old system is too strong, it will affect the bridging effect of the cations. If the cations of the old system are too strong, it will cause excessive cations that are not neutralized and will also affect the retention and filtration effect.
The pH value of paper stock is an important factor. Different microparticle components have different applicable pH ranges. For example, although the silica system still has a retention effect at a pH of 5, the best effect is neutral or alkaline, that is, pH 6~9. The suitable pH range of the bentonite system is relatively wide, and it can work at about pH 4~9.
As early as the 1970s, a variety of polyacrylamide products had been developed in China, cationic starch has also been widely used, and my country’s bentonite resources are also very rich. It should not be difficult to develop and fully apply these chemical products.
2. Binary retention and filtration system
This retention and filtration system uses low molecular weight, high charge density CPAM and high molecular weight, low charge density anionic polymers. Generally, anionic polymers are added after CPAM, and electrical reactions are carried out on the fiber surface to play a retention and filtration role.
The retention mechanism is that first, the cationic polymer CPAM covers the fiber surface to form a positively charged patch, which provides a fixation point for the anion. Then, the high molecular weight anionic polymer combines with the patch, but the rest of the molecular chain is repelled by the negative charge around the positively charged patch, forcing the anionic polymer to extend into the surrounding water, causing it to adsorb to the positively charged patch on the surface of another particle, playing a bridging role and combining the two particles together.
Its filter aid principle is to use polymers to entangle fine components to produce large fiber flocculation clusters, increase the porosity of the paper sheet, and remove water from the floccules, but the water in the floccules cannot be removed.
3. Ternary retention and filtration system
When the concentration of various soluble and colloidal substances in the wet end system of the paper machine increases greatly, such as waste paper and mechanical pulp systems, there are a lot of anionic garbage and a high demand for cations, the ternary method has a better effect.
The first step is to add low molecular weight cationic polymers to fix the natural resin and synthetic resin in the slurry on the fiber, which can greatly reduce the negative impact of resin floccules on the wet end, reduce sediments, and give full play to the role of subsequent additives. The second step is to add high-charge cationic polymers, whose charge density is 2-4 times higher than that of general cationic starch, and adsorb on the fiber in large quantities, while also improving the strength of paper. The third step is to add inorganic substances such as colloidal silica to form micro-flocculation. This can solve the problem of excessive anionic garbage in the wet end of high-speed paper machines, improve retention and water filtration, and produce high-quality paper sheets.
derivative. It is easily soluble in water, has good film-forming properties, good biodegradability, and its molecular structure is very similar to cellulose. It is easy to modify. It can improve both wet strength and dry strength at the same time. It has a huge effect on improving the dry strength of straw pulp in particular. In the papermaking process, it is mainly used as dry strength agent, retention and filter aid, and flocculation agent.
agent. But at the same time, it has shortcomings such as poor bridging ability, poor reinforcement effect under alkaline conditions, and high cost. Therefore, chemical modification is needed to make up for these shortcomings. Moreover, the enhancing effect of chitosan on pulp with different viscosities and different degrees of deacetylation is very different. The enhancing effect of chitosan on pulp basically increases with the degree of deacetylation of chitosan.
The law of large and increasing [22]. Sun Zhenqian[23] used propylene oxide as an etherifying agent to graft-modify chitosan under alkaline conditions, prepared the obtained hydroxypropyl chitosan into a glue solution of appropriate concentration, and sprayed it on the surface of the paper pattern. , conduct a series of tests such as tensile strength test, folding endurance test, dry heat accelerated aging test, and gloss test. The results showed that the paper pattern maintained its original texture, gloss, and color, its tensile strength nearly doubled, and its folding endurance also improved. Zhang Jing[24] studied the effect of surface sizing of chitosan-acetic acid solution with different concentrations on the technical properties of paper. The results showed that the thickness, basis weight and tensile index did not change; the tear index and folding endurance increased. ; The bursting resistance index increases significantly; the whiteness decreases; the yellowing value increases. Liu Zhong [25] used glycidyltrimethylammonium chloride as a cationic etherifying agent to modify chitosan. It was found that the prepared cationic chitosan had a good reinforcing effect on bagasse pulp, and the additives increased the fiber content. The combination between them enhances the strength and performance of bagasse pulp. At present, foreign research on modifying chitosan for use as papermaking reinforcing agent has achieved certain results, such as graft copolymerization of PAE, polyethylenimine, acrylic monomers and chitosan. Compared with foreign countries, there is still a certain gap in this aspect between China and abroad. However, with the intervention of more and more researchers, many gratifying results have been achieved in recent years. For example, Ma Yongsheng [26] used inverse emulsion polymerization technology. By grafting polymerization of chitosan and acrylamide, stable chitosan 2g2AM inverted latex can be obtained, which has achieved good application results as wheat straw pulp and deinking pulp. This is a great contribution to China’s secondary resources.
The use of it has great benefits. In addition, cationic starch and chitosan are used as dual additives, which can effectively improve the physical strength and filler retention rate of paper. As the molecular weight of chitosan increases, the effect of the dual additives also increases. Modified chitosan can also be used as a wet strength agent for paper. For example, Qi Guoping [27] studied using homemade carboxymethyl chitosan as an antibacterial agent, using dipping processing method to develop wet wipes, and tested its antibacterial properties against Escherichia coli and Staphylococcus aureus. The experimental results showed that using carboxymethyl chitosan Wet wipes made from sugar have very good antibacterial effects.
