Common types of papermaking chemicals
1. Definition: Papermaking chemicals refer to the general term for various chemical agents and additives used in the papermaking process. It covers a wide range of content, including pulping chemicals (such as cooking aids, deinking agents, etc.), papermaking chemicals (such as sizing agents, wet strength agents, etc.), paper processing chemicals (such as bactericides, defoamers, coating agents) and pollution control chemicals. Its purpose is to improve the quality and production efficiency of paper, improve operating conditions, reduce manufacturing costs, increase economic benefits and develop new paper types.
2. Glue:
(I) Classification:
1. Plant glue: rosin, oxidized starch, carboxymethyl cellulose, etc.
2. Animal glue: bone glue, skin glue, casein, stearic acid, etc.
3. Mineral glue: paraffin, water glass, etc.
4. Synthetic glue: vinyl ketone dimer, alkenyl succinic anhydride cationic acrylate, etc.
Currently, rosin is still the most widely used in the world.
(ii) Rosin – is the solid obtained by distilling rosin and removing turpentine. It generally contains 80-90% rosinic acid and its molecular formula is C19H29COOH. Grade 4-5 rosin is orange-yellow and transparent, and is most suitable for papermaking industry as a sizing agent. Rosin used in the papermaking industry is required to have a chemical value of 160-180 mg KOH. The unsaponifiable content should be ≤9%, the volatile matter should be ≤0.5%, and the softening point should be 73-78°C.
Rosin glue – rosin is insoluble in water, but it can react chemically with soda ash and caustic soda to produce sodium rosinate. This reaction is called saponification of rosin, and the resulting glue particles are called rosin glue. Including neutral glue (completely saponified), white glue (10-45% unsaponified), high-free rosin glue (70-90% unsaponified), and reinforced rosin glue (also called maleic rosin glue, a high-efficiency modified rosin glue).
Reinforced rosin glue – has high softening point, high acid value, high saponification value, strong antioxidant stability. As a papermaking sizing agent, it has low dosage and good sizing effect.
(III) Sizing agent – Sizing agent is a papermaking additive that can give paper and paperboard anti-ink, water resistance, emulsion resistance, corrosion resistance and other properties to improve smoothness, strength and application period. It can be divided into internal sizing agent and surface sizing agent. Internal sizing agent can be roughly divided into three categories: rosin sizing agent, synthetic sizing agent and neutral sizing agent. Surface sizing agent generally uses anionic polymer substances.
Sizing agent, also known as internal sizing, refers to chemicals added to pulp, paper and paperboard to obtain the performance of fluid resistance. This type of chemical has hydrophilic groups and hydrophobic groups. On the fiber surface, the outer side is oriented to hydrophobic groups, which gives the paper hydrophobicity and fluid resistance. Except for blotting paper, filter paper, wax paper, cigarette paper, daily paper and other paper types, almost all papers need sizing. Sizing on paper can improve the paper’s water resistance, oil resistance, printing ink resistance and other properties, and at the same time improve its smoothness, hydrophobicity and printing adaptability.
There are two methods of sizing. One is to add the sizing agent to the pulp before the paper sheet is formed (during pulping or mixing), which is called internal sizing. The other is to apply it on the surface of paper or paperboard after the paper sheet is formed, which is called surface sizing. The internal sizing agents mainly include rosin sizing agents, synthetic sizing agents, paraffin sizing agents and neutral sizing agents. Surface sizing agents and neutral sizing agents. Surface sizing agents are mainly used to improve the smoothness of the paper sheet surface, reduce porosity, and prevent surface linting and hair loss. Modified and unmodified starches are more commonly used, and PVA is also used. Others include paraffin emulsion, animal glue, carboxymethyl cellulose, polyurethane, styrene-maleic anhydride copolymer, acrylic acid and acrylic acid ester copolymers, dihydroxy fatty acid compounds, etc.
III. Fillers:
1. Overview – some solid particles that are basically insoluble in water added to the pulp. In order to achieve the purpose of filling, papermaking fillers should have small and uniform particle size, high whiteness and large relative density, stable chemical properties, not easy to react with acids and alkalis, no oxidation or reduction, insoluble or difficult to dissolve in water, large hiding power, refractive index and scattering coefficient, abundant resources, low price and other characteristics. Fillers are generally inorganic minerals, and organic synthetic fillers have also been developed in recent years. Commonly used are talcum powder, kaolin, titanium dioxide, calcium carbonate, etc. There are also special fillers added due to special requirements, such as conductive paper with carbon black as filler to make it conductive.
2. Function-It has opacity, improves the opacity of paper, and makes thin paper more suitable for double-sided printing; it can improve the whiteness of paper; improve the flatness and uniformity of paper, increase the softness of paper, and make the paper surface flat and smooth, which is more suitable for printing and writing; it can reduce the moisture absorption and deformation of paper; meet the special requirements of paper, etc.
3. Classification – Natural fillers: talcum powder, white clay (kaolin), calcium carbonate, asbestos, gypsum, barite (barium carbonate), calcined gypsum), etc.;
Artificial fillers (precipitated calcium carbonate, calcium magnesium white, precipitated calcium sulfate, calcium dioxide, zinc barium white, zinc white, sodium aluminosilicate, calcium silicate), etc.
Fourth, colorants: There are two categories: pigments and dyes. Pigments are mostly inorganic compounds (i.e. inorganic pigments), which do not come into direct contact with fibers and are only suitable for paper
coating. Dyes are mostly organic compounds, soluble in water, and include alkaline, acidic, direct dyes and fluorescent brighteners.
Fluorescent brightener: A special dye that can increase the whiteness of paper. It can absorb invisible ultraviolet light and turn it into visible light, eliminate the yellowness in pulp, and increase the visual whiteness of paper. Domestic brighteners mainly include brightener R (with red light), brightener B (with blue), and brightener VBL (blue light).
5. Papermaking auxiliaries:
1. Cooking auxiliaries – Chemicals that can assist the cooking agent in improving the cooking efficiency during the pulping process. They have the chemical effect of removing lignin and the colloidal effect of retaining fiber strength, shortening the cooking time, reducing the amount of alkali and bleaching liquid, and improving the whiteness and filterability of pulp. The main ones are anthraquinone, dihydrodihydroxyanthracene disodium salt, quinone, etc. Dihydrodihydroxyanthracene disodium salt can accelerate the delignification speed, shorten the cooking time, make pulp quickly, consume less energy, use less alkali and bleaching chemicals, have a high fiber yield, reduce waste liquid load, and have little environmental pollution. Odorless. The addition amount is preferably 0.02% to 0.05%.
2. Deinking agent – Chemicals or biological products that can separate waste paper fibers and ink. The main function is to destroy the adhesion of ink to paper fibers, so that the ink is peeled off from the fibers and dispersed in water. Basic composition:
① Surfactant: has the functions of penetration, emulsification, dispersion and cleaning, has foaming and stability during flotation, and has anionic/non-ionic types.
② Bleaching agent: improves the whiteness of pulp after deinking.
③ Alkali: plays the role of saponification and cleaning.
④ Sodium silicate: plays the role of buffering, washing and dispersing.
Classification: washing method, flotation method, washing-flotation method deinking agent, etc.
Biological enzyme deinking agent: The main component is alkaline lipase, which directly decomposes the ink, coating and colorant on the waste paper, and can also remove the resin in the pulp.
Advantages of bio-enzyme deinking agent:
① Directly reduce deinking cost, with obvious comprehensive cost advantage;
② Bio-enzyme deinking conditions are mild, with less chemical dosage, less fiber loss, and good fiber properties;
③ Greatly reduce sewage COD and BOD, which is beneficial to environmental protection and reduces sewage treatment capacity;
④ Effectively deal with adhesive problems and improve paper machine operation efficiency;
⑤ No or less use of chemicals such as sodium silicate caustic soda to prevent pipe scaling;
⑥ Significantly improve pulp yield and paper strength;
⑦ Clean production, energy saving, and reduced material consumption
⑧ Simple to use, suitable for most current deinking processes
3. Defoamer – also known as antifoaming agent, used to inhibit and eliminate chemicals that form foams due to various factors in the papermaking process.
In-pulp defoamer: non-ionic silicone emulsion, dimethyl silicone oil and fumed silica, etc.
Papermaking defoamer: fatty acid esters and non-ionic surfactants, eliminate the foam generated during papermaking ingredients, and do not affect paper strength and
sizing effect; emulsified silicone defoamer, methyl silicone oil water emulsion.
4. Retention aids – chemicals that improve the retention of fibers and fines on the paper machine wire.
A. Types of retention aids:
(1) Conventional retention aids – including unit and dual systems. Their characteristic is that polymers are added to the outlet of the paper machine pressure screen. Conventional retention aids are mainly polymers, and alum, polyaluminium chloride (PAC) or fixing agents can be added before. Commonly used fixing agents are polyamines, polydiallyldimethylammonium chloride (Polydadmac) and polyethyleneimine.
(2) Microparticle retention aids – composed of polymers such as cationic polyacrylamide and microparticles, can more effectively overcome the contradiction between flocculation, retention and paper uniformity, and are particularly suitable for large-scale high-speed and web-type paper machines. Microparticles are the last chemicals added.
Commonly used microparticles are: bentonite, colloidal silica and organic micropolymers.
Polymers include: ① Anionic/cationic starch, starch ether type: white powder, used for wet-end addition in papermaking to improve the retention rate of fibers and fillers and improve paper strength.
② Cationic polyacrylamide: colorless, transparent, viscous liquid.
③ Cationic starch and acrylamide graft copolymer.
④ Acrylamide and sodium acrylate copolymer.
(3) Ultrafine particle retention aid system – organic particles and inorganic particles are used together to obtain better retention and drainage effects. This retention method using two particles is called an ultrafine particle retention aid system.
B. Characteristics of retention aids:
(1) Wet-end addition: retention aids are only used in the wet end and added to the pulp.
(2) Retention and drainage: retention aids must produce retention and drainage effects, otherwise they are not retention aids.
(3) Flocculation mechanism: “flocculation” is the mechanism of action. Flocculation is divided into “flocculation” and “coagulation”. The former refers to the process in which high molecular polymers use long chains to “bridge” between different particles to form flocs; the latter refers to the process in which aluminum salts, fixatives or microparticles rely on charge to aggregate different particles into flocs. Flocculation pulls particles together, which is exactly the opposite of the requirement that paper uniformity requires fibers to be dispersed. Therefore, retention aids generally have a negative impact on paper uniformity. When the effect is too strong, it will make the uniformity worse.
(4) High molecular weight: The main role in the retention aid system is played by polymers. If there is no polymer to produce flocculation, it is difficult to obtain the single-pass retention rate required by general paper machines by relying solely on other retention chemicals. Therefore, the retention aid system usually includes at least one polymer product. Other chemicals, such as aluminum salts, fixatives and microparticles, are called coagulation aids, which mainly play an auxiliary role and are used in conjunction with polymers.
(5) Post-screen addition: The flocs produced by chemical flocculation are easily destroyed by shear force. Therefore, the appropriate addition point is an important guarantee for the retention aid system to play its best role. The equipment that generates the greatest shear force in the paper machine sizing system is the pulping pump and the pressure screen. In order to avoid strong shear force from destroying the flocs, at least one chemical needs to be added after the pressure screen when using retention aids. Generally, not all retention aid products can be added before the pressure screen.
5. Filter aids – In the filtering operation, an auxiliary powdery granular substance added to reduce the filtration resistance, increase the filtration rate or obtain highly clarified filtrate is called a filter aid. Usually some hard powdery or fibrous solids. Filter aids are only used for filtering operations for the purpose of obtaining clean filtrate. There are two types of filter cakes formed by filtration: one type has considerable rigidity, and its structure does not change with the increase of operating pressure, which is called incompressible filter cake; the other type of filter cake has a structure that changes with the increase of operating pressure, resulting in the narrowing of the filtrate flow channel in the filter cake and a sharp increase in the flow resistance of the filtrate, which is called compressible filter cake. For compressible filter cakes, adding filter aids can enhance their rigidity and prevent the increase of filtration resistance. When filtering a suspension containing sticky fine particles, a dense filter cake layer is often formed. Adding filter aids can change the filter cake structure and reduce the filter cake resistance. The use of filter aids: one is to mix the filter aid with the suspension to be filtered in a certain proportion, and then filter together; the other is to prepare a suspension containing only filter aids, filter it first, form a pre-coat on the filter medium, and then filter the filter slurry. The filter aid pre-coat can withstand a certain pressure without deformation, and can prevent the filter cloth from increasing resistance due to clogging. Classification: Commonly used filter aids include diatomaceous earth, paper pulp, activated carbon, metal chips, acid clay, asbestos, cellulose and slag. Diatomaceous earth has a large specific surface area, can adsorb colloidal substances, and form a filter cake with a high porosity. It is the most widely used filter aid. Paper pulp is mostly used as a pre-coat to prevent early clogging of the filter cloth. Basic requirements: ① Rigid particles that can form porous cake layers, so that the filter cake has good permeability and low fluid resistance. ② Chemical stability.
③ Incompressible within the operating pressure range.
6. Enhancer – Applicable to neutral, acidic sizing system or non-sizing system, with a retention rate of more than 95%. After using the enhancer, some low-grade pulp raw materials can be used to replace high-grade wood pulp, improve the grade of pulp, save wood pulp, and reduce production costs.
Dry strength agent – At present, the main dry strength enhancer is low to medium molecular weight ionic acrylamide copolymer, in addition to polyurethane, polyamide and other resins. Using a small amount of dry strength enhancer can significantly improve the physical strength of paper such as tear resistance, folding resistance, and tensile strength; while maintaining the original strength of paper, some waste paper, machine wood pulp and other cheap raw materials can be used to replace high-quality coniferous wood pulp; or increase the amount of filler to save fiber raw materials, etc.
Wet strength agent – Adding in the wet part of papermaking can obtain good wet strength, used as wet enhancement of absorbent paper. After paper or paperboard is completely soaked with water, it can still maintain 20-40% of its original dry strength. Many special papers such as photographic paper, banknote paper, frozen food packaging, tea paper, etc., must have a certain wet strength. Mainly thermosetting synthetic resins, such as melamine-formaldehyde resin, urea-formaldehyde resin, polyvinyl alcohol, polyamine polyamide epichlorohydrin resin, etc.
7. Resin control agent-a chemical that can prevent the precipitation of free resin in wood pulp or the agglomeration of residual ink in waste paper during the papermaking process and cause production obstacles. In the early days, starch or gelatin was added to the pulp as a protective colloid to avoid resin agglomeration, and sodium hydroxide or alum was used to adjust the acid value to avoid resin precipitation, but the effect was not obvious. Later, water softeners such as aminocarboxylic acid chelates were used to chelate metal ions such as calcium, magnesium, and iron in water to eliminate their harmful effects. In recent years, resin control agents have been developed. They are composed of organic dispersants such as sodium polyacrylate and sodium methylene bisnaphthalene sulfonate and non-ionic surfactants such as polyoxyethylene alkylphenol ethers. They can disperse resin particles and reduce adhesion, thereby avoiding obstacles caused by agglomeration.
8. Coating processing aids – a series of chemicals that can improve the quality of coatings, impart special properties, or enable coating processing operations to proceed normally. The main ones are: ① Dispersant: used to reduce the viscosity of high-concentration coatings to give them good rheological properties.
② Lubricant: used to enhance the rheological properties of coatings, reduce the coefficient of friction, and reduce the sticking of coated paper during super calendering.
③ Insolvent: used to improve the water resistance of coating adhesives such as starch, casein, and polyvinyl alcohol. It can increase the wet friction resistance of the coating and the roughness strength of the paper surface.
④ Coating rheology improver: used to adjust the viscosity of the coating to adapt to various coating processes. Reduce the viscosity of the coating.
The viscosity reducer is called a viscosity reducer, such as urea, dicyandiamide, etc.; the thickener or water retainer is called a thickener, such as acrylate copolymers, cellulose derivatives, etc. The viscosity reducer can keep the coating in proper rheology and avoid peeling, streaking or bulging of the coated paper. Other additives such as defoamers, preservatives, fluorescent brighteners, etc. must also be added during the coating process.
9. Coatings for coating – Water-based coatings are commonly used. The composition of water-based coatings can be roughly divided into four categories: pigments, adhesives, additives and water. The combination of these four components is closely related to the quality of the paper after coating.
① Pigments – There are white clay (white clay), calcium carbonate, sardine white, aluminum hydroxide, titanium dioxide and plastic pigments. Generally, it can be divided into three types: binder type, solid type and hollow type, and the latter two are more commonly used. Characteristics of pigments:
⑴ The particle size should be appropriate. Generally speaking, the smaller the particle size, the better it is for improving the whiteness, opacity and gloss of the paper. However, if the particle size is too small, its surface area will increase, resulting in increased viscosity and reduced fluidity of the pigment coating, and the amount of adhesive required will also increase accordingly.
⑵ The shape of the pigment particles should also be appropriate. Needle-shaped pigment particles have poor fluidity, while hexagonal and spherical pigment particles have better fluidity and can form a porous coating layer to improve the ink absorption of the paper.
⑶ It has high whiteness and opacity (stronger shielding ability).
⑷ It is easy to disperse in water and has good stability and fluidity.
⑸ It has better chemical stability and the amount of adhesive is reduced as much as possible.
⑹ Lower wear resistance (reduces wear of the coating machine and printing plate).
② Adhesives – Adhesives are divided into natural and synthetic products. Natural products include starch, cheese, soy protein, etc. Synthetic products include various synthetic latexes, polyvinyl alcohol, etc. The most widely used one is a mixture of starch and synthetic latex. The proportion of synthetic latex is increasing, and all-latex adhesives have been developed recently, which is consistent with the development of high-concentration coatings. The following requirements are imposed on any adhesive: ⑴ Strong adhesion to pigments. ⑵ Good adaptability and stability to pigments, but no chemical reaction with pigments. ⑶ Appropriate viscosity and film-forming properties. ⑷ Appropriate fluidity to facilitate coating operations and improve coating uniformity. ⑸ The color should be light, free of impurities and foreign matter, and have good coating operability. ⑹ Since general adhesives, whether natural or synthetic, still have some properties that cannot be improved, the so-called auxiliary adhesive (co-Binder) has appeared in recent years to make up for the lack of water retention and fluidity of adhesives. Its dosage is only about 10-30% of the total adhesive dosage. Now some factories are trying to use this to meet the demand of reducing the total adhesive dosage. Because the dosage is small, it can sometimes be regarded as an additive. Currently, commonly used auxiliary adhesives are soy protein and rheology modifier. ③ Additives – dispersants, lubricants, preservatives, fluorescent agents, water-resistant agents, dyes, ink absorbents and improvers, etc., which mainly play a role in making up for the deficiencies of pigments and adhesives and reducing operational troubles.
VI. Chemical additives:
1. Softening and swelling agent – improves the fluffiness of paper, special for toilet paper and superstitious paper. Increase the volume of toilet paper, improve water absorption, the water absorption should be large, and the quality should be good. Appearance: white solid powder, effective content ≥ 95%, non-ionic. Used for softening, it can significantly improve the flexibility of daily paper and cultural paper. The fried paper has a good feel, fine and uniform wrinkles, and the tension and whiteness are also improved to varying degrees.
2. Waterproofing agent –
3. Surface sizing of corrugated paper Starch cross-linking agent – a substance that can bridge between molecules so that multiple linear molecules can be bonded and cross-linked into a network structure, commonly known as “curing agent”, “hardening agent”, “maturation agent”, etc. A composite reinforcing agent specifically for curing and reinforcing surface sizing starch. Adding it to gelatinized starch can increase the ring crush strength, tension, retention rate and water resistance of corrugated paper. It is an important additive for corrugated base paper. The ring crush strength is generally increased by 20-30% (increased by 1-2 ring crush indexes). White powder. More than 96%. Cationic, pH value 4-6, easily soluble in cold water.
4. Papermaking polyester defoamer – a broad-spectrum defoamer made of polyether fatty acid ester liquid, which can eliminate or inhibit the generation of foam in the aqueous phase system, especially for water-soluble polymers and medium and high viscosity systems.
5. Ink adsorbent – used in ink regeneration fiber and secondary fiber recycling, to overcome the adhesive substances and ink resin problems in the fiber that cause paper diseases. It is an ideal additive for secondary fiber recycling. Powdered solid, insoluble in water, contains ionicity, and can form a dispersion in water. It has good dispersibility and sizing effect, as well as retention, which can improve various physical properties and retention rate of paper products. Increasing the dosage will not increase the production cost.
6. Cationic polyacrylamide – a linear polymer copolymerized by cationic monomers and acrylamide, with functions such as decolorization, adsorption, deturbidity removal, and bonding. Uses: sludge dehydrating agent, papermaking additive, sewage and organic wastewater treatment, oilfield chemical agent, etc.
7. Honeycomb paper flame retardant – can be used to treat various honeycomb paper products, natural wood, artificial boards, wood products and processed parts; it can also be used for fabrics such as bamboo, hemp, cotton, and plant fiber materials or products. The treated materials do not change the original appearance gloss and physical and mechanical properties. When it encounters an open flame, it will not burn, only partial carbonization will be shown, which can effectively inhibit the spread of flames and prevent the spread of fire.
8. Stiffener – used in the production process of various types of paper and paperboard to significantly increase the dry strength of paper and paperboard, reduce the consumption of high-quality raw materials, reduce production costs, or improve product grades. Synthetic modified polyamide resin, amphoteric composite, nearly neutral. The effect of increasing the dry strength of corrugated paper is particularly obvious. Enhance the moisture-proof performance of paper.
9. Special scale inhibitor for papermaking black liquor evaporator – During the alkali recovery process of papermaking black liquor, due to the high temperature, strong alkalinity, high content of silicon, magnesium, calcium, oxalate, solid matter and high viscosity of papermaking black liquor, the evaporator is severely scaled and even blocked, and it has to be stopped for cleaning, which increases costs and affects production. It has always been a problem that plagues alkali recovery.
The special scale inhibitor for papermaking black liquor evaporator is compounded with multiple components such as surfactants, dispersants, and viscosity reducers. When the scale inhibitor is added to the papermaking black liquor, the viscosity reducer in the scale inhibitor reacts with silicon, oxalate, and various sugars in the black liquor to make it lose viscosity and suspend in the black liquor. Surfactants and dispersants disperse the scale-forming ions such as calcium, magnesium, and silicon in the black liquor to the maximum extent, increase the precipitation resistance and fluidity, and make it difficult to precipitate and adhere to the surface of the evaporator, while keeping the surface of the evaporator clean and scale-free. The dispersant will also react chemically with the dirt on the surface of the evaporator, dissolving the old dirt and removing it, thus achieving the purpose of removing the old dirt. As the viscosity of the black liquor is reduced, the growth of new dirt is prevented, and the removal of old dirt increases the heat conduction speed, the evaporation rate of water in the black liquor is accelerated, and the time required for the evaporation of the black liquor is shortened, so that the entire alkali recovery process is energy-efficient and has significant benefits.
Solid content ≥40%, pH value (1% aqueous solution) 8.0-10.0%.
10. Pulp dispersant – anionic or composite polymer, which can effectively disperse pulp fibers. It improves the uniformity of pulp and the softness of paper, and is an important auxiliary agent for toilet paper production. It is also widely used in the papermaking of cultural paper, glossy thin paper and newsprint. It can avoid holes in the paper, have high uniformity and good feel, and can also increase the speed of papermaking, save pulping time, reduce energy consumption, and have significant economic benefits. White bright small particles and slightly yellow bright particles, effective content ≥98%, molecular weight 15-20 million units. With its extremely small dosage, it can exert a unique dispersing effect and save pulp, and increase the paper output by 5-10%. It can increase the retention rate of fine fibers and additives, and reduce the concentration of filtered water under the screen. It can be used under a wide range of pH values of 4-10. In the use of paper coating, paper dispersants have high dispersing efficiency, stable coating viscosity, less foam, non-toxicity, non-corrosiveness, can increase the solid content of the coating, and have good fluidity and scrub resistance, can maintain the gloss of paper products, and are not easy to mold. Paper dispersants can also increase the speed of papermaking, save pulping time, reduce energy consumption, and have significant economic benefits.
