Papermaking chemicals refer to chemicals used to treat fibrous raw materials, pulp and base paper during pulping, paper making and paper processing.

According to the customary naming and the universality of use in the paper industry, it is roughly divided into two categories :① commonly used chemicals, which are indispensable chemicals in most paper production. For example, sodium hydroxide (caustic soda) and sodium sulfide for sulfate pulping; Liquid chlorine for pulp bleaching (see chlorine gas), calcium hypochlorite; Paper sizing rosin, aluminum sulfate (alum), etc. Fine chemicals (see fine chemicals) are special chemicals added to meet certain needs. For example, when bleaching pulp with bleaching solution, adding sulfamic acid can reduce the degradation of cellulose and maintain the strength of cellulose. This classification is also arbitrary, because a fine chemical can become a common chemical after a long period of use. On the contrary, commonly used chemicals can become fine chemicals with special properties after modification.

According to the performance and use of papermaking chemicals, it is roughly divided into the following dozen categories:

Cooking agent

A chemical added in the cooking process to remove lignin from plant fibers. Different pulping methods use different chemicals. Such as alkali method (including sulfate method, caustic soda method, lime method) to sodium hydroxide, sodium sulfide, lime and other cooking agents; Acid sulfite process using calcium bisulfite (or magnesium) as cooking agent; The neutral sulfite process uses sodium sulfite and sodium carbonate as cooking agents. The oxygen-alkali process uses oxygen and sodium hydroxide as cooking agents. The sulfate process with sodium hydroxide and sodium sulfide as cooking agents has the advantages of high yield, good paper strength, etc., and plays a dominant role in pulping technology. The anthraquinone-caustic soda process developed in recent years can accelerate the lignin removal, shorten the cooking time and increase the pulp yield, and is adopted by many pulp mills.

bleach

A chemical used to remove residual lignins and colored substances from pulp. Chemical pulp bleaching and machine wood pulp bleaching two types. Chemical pulp bleaching commonly used oxidizing bleaching agents, such as liquid chlorine, sodium hypochlorite (or calcium), hydrogen peroxide, etc., for a stage of bleaching, it can also be used for multi-stage bleaching such as liquid chlorine - sodium hydroxide - hypochlorite - chlorine dioxide (or sodium chlorite). When bleaching with hypochlorite, sulfamic acid is added to reduce the degradation of cellulose. Reductive bleaching agents, such as zinc disulfite (or sodium), hydrogen peroxide, sodium peroxide and other oxidizing bleaching agents, need to add sodium silicate as a buffer, magnesium sulfate as a stabilizer, in order to adjust the acid value and prevent the catalytic decomposition of trace heavy metal ions on peroxide.

Sizing agent

A chemical used to delay penetration of paper and board by water or aqueous solutions. It is divided into two categories: inner sizing agent and surface sizing agent. In-pulp sizing agent is used to size fibers before paper sheet formation, mainly rosin soap, wax emulsion and stearic acid soap. This kind of glue must be in the presence of acid alum (aluminum sulfate), the formation of flocculant deposited in the fiber, in order to make the dried paper has water resistance, also known as "acid sizing agent". The disadvantage of using acid sizing agent is that the aging property of paper is poor. The alkyl ketene dimer and alkenyl succinic anhydride and other synthetic adhesives developed in the 1970s do not need to add alum when used, and can react with cellulose hydroxyl under neutral or slightly alkaline conditions, so that the paper has water resistance, known as neutral size agent. The paper sizing with neutral sizing agent has good durability and can be filled with alkaline calcium carbonate to improve opacity, whiteness, feel and printing performance. Therefore, it has been widely valued. The compound used to treat the surface of the paper is called the surface sizing agent and is usually carried out in the sizing press or calender roll of the paper machine. The commonly used surface sizing agents are modified starch, polyvinyl alcohol, animal glue, sodium alginate, etc., which can improve the strength, stiffness and printing performance of paper. Some special surface sized agents, such as styrene-maleic anhydride copolymer and chromium chloride stearate, can significantly increase the water resistance of paper, and methylated melamine resin can improve the morphological stability of paper, so that the deformation is small under large humidity changes.

Dyes and toners

A chemical that dyes paper various colors. Commonly used are alkaline dyes, acid dyes, direct dyes, fluorescent whitening agents. In addition, some dyes can also play a color role, such as using unbleached pulp paper, adding a small amount of blue dye, can partially offset the yellow color of the fiber, improve the whiteness.

fortifier

A chemical used to increase the physical strength of paper and board. It is divided into two categories: drying strength agent and humidifying strength agent:

Early drying strength agents for starch, vegetable gum and other natural products, and later developed into starch derivatives and carboxymethyl cellulose and other semi-synthetic products, but there are large amounts, the effect is not ideal and other shortcomings. In the late 1950s, synthetic polymers were added to pulp, and great progress was made. The main drying strength agent is low to medium molecular weight ionic acrylamide copolymer, in addition to polyurethane, polyamide and other resins. Using a small amount of drying strength agent can obviously improve the physical strength of paper such as breaking resistance, folding resistance and tensile resistance. Under the condition of maintaining the original strength of paper, cheap raw materials such as part of waste paper and machine wood pulp can be used to replace high-quality coniferous wood pulp. Or increase the amount of filler, save fiber raw materials and so on.

Humidifying strength agent can make the paper completely soaked in water, still can maintain part of the strength (called wet strength), China is widely used in melamine formaldehyde resin and urea formaldehyde resin.

Retention aid

Chemicals used only to improve the retention rate of fine fibers and fillers in the papermaking process are called retention AIDS, and if they both speed up the filtration of paper materials, they are called retention AIDS. Early use of inorganic products such as polyaluminum chloride, sodium aluminate, and later developed into the use of cationic starch, cellulose derivatives and other semi-synthetic products, the effect is not very obvious. Although there is a certain retention effect, but the dosage is larger. In recent years, the use of mostly positive charged polymer, such as polyacrylamide, polyethylene imide, polyamide, polyurethane and so on. Their long molecular chains can bridge the gaps between fibers, fine fibers, fillers, etc., and gradually neutralize the negative charge on the fiber surface to form flocculation, thereby improving the retention rate and increasing the water filtration. The negative charge of polyacrylamide in the presence of aluminum ion complex condensation, can also improve the retention rate. The use of retention agents can also reduce the scale of the paper machine, improve operation, and reduce the content of fine fibers and fillers in the discharge wastewater, which is conducive to environmental protection.

defoamer

Also known as anti-foaming agent, it is used to inhibit and eliminate the chemicals formed by various factors in the papermaking process. Often selected according to the specific situation and requirements of each process. If the wastewater treatment requires low cost and does not cause secondary pollution, the surfactant can be mixed with bentonite with certain foaming inhibition ability as filler; The defoamer used for alkaline pulp washing, bleaching and black liquor concentration requires alkali resistance and high temperature resistance, mostly composed of olefin solvents and lipophilic surfactants; The defoamer used in the paper machine requires that the paper surface will not form oil spots and affect the water resistance, and the emulsion defoamer composed of silicone oil or hard fat amide and calcium (aluminum) soap can be used. The coating defoamer should have both defoaming and defoaming functions, without affecting the brightness and printing performance of the coating, and its components are mainly composed of saturated alcohols (such as octanol), fatty alcohols (such as TBP) or polyoxyethylene fatty alcohol ether and other surfactants.

Formaldehyde

A chemical used in the papermaking process to inhibit and eliminate pulp spoilage caused by the growth of microorganisms. Early use of chlorine gas, chloramine, copper sulfate as a preservative, the effect is not obvious. Later, mercury, tin and other heavy metal salts and chlorophenol compounds were used, which had sterilization effects, but were banned because of their toxicity and environmental pollution. In recent years, a series of organic preservatives with high efficiency, low toxicity, wide sterilization spectrum and good biodegradability have been developed, most of which contain groups such as bromine, cyanide, sulfhydryl, benzimidazole and benzothiazole.

Deinking agent for waste paper

Chemicals that can remove printing inks for books and newspapers, or that can separate adhesive resins, plastics, adhesives from fibers. Early with sodium hydroxide, sodium silicate and other alkaline substances heating cooking, some also add alkyl benzene sulfonate, alkyl sulfate and other detergents, the pulp produced dark color, low whiteness, only suitable for the production of general household paper. The deinking agent is mainly composed of non-ionic surfactants such as polyoxyethylene nonylphenol ether and polyoxyethylene alkylphenol ether with high emulsification, and is added with other additives according to the quality requirements of pulp and ink type. After bleaching, the waste pulp can replace some hardwood pulp to produce intermediate paper and high-grade paper. The new deinking agent composed of olefin solvent, penetrant and emulsifier can also make rubber, plastic, resin and other adhesives easy to separate from the fiber.

Resin control agent

Chemicals that can prevent wood pulp free resin precipitation or waste paper residue ink agglutination in the papermaking process and cause production obstacles. At the early stage, starch or gelatin was added to the pulp to protect the colloid from resin condensation, and sodium hydroxide or alum was also used to adjust the acid value to avoid resin precipitation, the effect was not obvious, and then water softener such as ammonia carboxychelate was used to chelate metal ions such as calcium, magnesium and iron in water to eliminate its harmful effects. In recent years, resin control agents have been developed, which are composed of organic dispersants such as sodium polyacrylate, sodium methylene binaphthalene sulfonate and non-ionic surfactants such as polyoxyethylene alkylphenol ether, which can disperse resin particles and reduce adhesion, thereby avoiding obstacles caused by agglutination.

flocculant

A chemical that causes fine particles suspended in water or an aqueous solution to form flocculent deposits. Commonly used are alum, polyaluminum chloride, ferric chloride and other inorganic salts. When used for water treatment, the amount is large and the amount of sludge discharged is large. The polyacrylamide polymer flocculant developed in recent years has a small dosage and obvious effect. Flocculants are widely used in wastewater purification in paper industry. In alkali recovery, the clarification of sodium carbonate and sodium sulfate solution is accelerated, and when sodium carbonate is causticized with lime milk, the precipitation of calcium carbonate is accelerated, and the dryness of sludge can be improved by adding it to the sludge dewatering machine for treatment.

Coating adhesive

A chemical that binds the pigments to each other and to the base paper during coating processing. Traditionally, casein, bean protein, oxidized starch, polyvinyl alcohol and other natural or synthetic compounds are used, but there are some problems such as poor water resistance, variable quality, high viscosity and poor rheology of the coatings. In the 1950s, the development of styrene butadiene latex, acrylate emulsion, polyvinyl acetate and other coatings as adhesives, most of them have superior adhesion and wet friction resistance, as high concentration coatings have good rheology, finished products have bright gloss and so on. Later, carboxylated benzene butadiene latex was developed, which can be used as a coating adhesive to improve mechanical stability and increase the brightness of coated paper and ink after printing, and it has replaced casein as the main adhesive. After the oil crisis, synthetic latex tends to be mixed with starch derivatives such as starch phosphate to ensure the quality of coatings and reduce the cost of adhesives.

Coating processing AIDS

A series of chemicals that can improve the quality of coatings, give special properties, or enable the normal operation of coating processing. The main ones are :① dispersant, which is used to reduce the viscosity of high concentration paint and make it have good rheology. ② Lubricant is used to improve the rheology of the coating, reduce the coefficient of friction, and reduce the sticking phenomenon of the coated paper in the super calender. (3) Solvent resistance, used to improve the water resistance of starch, casein, polyvinyl alcohol and other paint adhesives. It can increase the resistance of the coating to wet friction and the drawing strength of the paper. ④ Paint rheological modifier, used to adjust the viscosity of the paint to adapt to various coating processes. Reducing the viscosity of the coating is called viscosity reducer, such as urea, dicyandiamide, etc.; The viscosity of the coating is called a thickener or water retaining agent, such as acrylate copolymers, cellulose derivatives, etc. The viscosity reducing agent can keep the paint in proper rheology and avoid peeling, streaking or arching of the coated paper. In the coating process, it is also necessary to add defoamer, preservative, fluorescent whitening agent and other additives.

Commonly used papermaking chemicals are blanket cleaning agent, stripping agent, water repellent, softener, flame retardant and so on.