Surfactants are an essential component of papermaking chemicals, widely used in the processes of pulping, wet-end, surface sizing, coating, and wastewater treatment.
Surfactants are used as cooking aids to promote the penetration of cooking liquor into fibrous raw materials, enhance the removal of lignin and resin from wood or non-wood, and disperse resin. Anionic surfactants used as resin removers include sodium dodecylbenzenesulfonate, sodium tetrapropylene benzene sulfonate, sodium dodecyl sulfate, naphthalene sulfonate condensate, and alkylphenol polyoxyethylene ether sulfate. Non-ionic surfactants include alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, fatty acid polyoxyethylene ester, and polyether. When removing resin with non-ionic surfactants, nonylphenol polyoxyethylene ether is the most effective. The combination of anionic surfactants and non-ionic surfactants has a better synergistic effect, which can promote the removal of lignin and resin and improve the pulp yield. For example, a composite of xylene sulfonic acid and sodium naphthalene sulfonate with nonylphenol polyoxyethylene ether, added in a mass ratio of 1: (1-2), can achieve good resin removal effects.
The principle of waste paper deinking is to utilize surfactants to wet, penetrate, expand, emulsify, disperse, foam, flocculate, capture, and wash the fibers and ink. The main process methods include:
Washing method: Emphasizes dispersing function to facilitate the dispersion and removal of ink by forming colloidal lattices.
Flotation method: Moderate foaming followed by ink capture.
Combination of washing and flotation methods.
Chemicals used for waste paper deinking mainly include alkalis, sodium silicate, chelating agents, hydrogen peroxide, surfactants, calcium salts, etc., among which surfactants play an important role. The main surfactants used as deinking agents for waste paper are:
Anionic type: Fatty acid salts, sulfonic acid salts, sulfate salts, phosphate ester salts, sulfosuccinate esters.
Cationic type: Amine salts, quaternary ammonium salts.
Amphoteric type: Betaines, imidazolines, amino acid salts.
Non-ionic type: Alkoxylates, polyol esters, fatty acid esters, alkyl amides, alkyl glucosides.
The selection of surfactants depends on the printing conditions and deinking processes. Strictly speaking, waste paper deinking agents mainly consist of a series of surfactant-based compound formulations.
Sizing agents are important chemicals in the wet end of papermaking, which impart water resistance to paper and paperboard, mostly used for writing, printing, packaging, and construction papers and boards. Sizing agents are mainly divided into two categories: rosin-based sizing agents and synthetic sizing agents.
The preparation of dispersed rosin size is a physical and chemical process. Solid rosin absorbs heat to become liquid rosin, and there is a great interfacial tension between rosin and water. To reduce this interfacial tension, surfactants need to be added. Both emulsifiers and dispersants for dispersed rosin size are surfactants, and the selection of surfactants is crucial for the preparation of dispersed rosin size. Commonly used surfactants include anionic, cationic, and amphoteric surfactants. In China, the most commonly used is anionic dispersed rosin size, with commonly used emulsifiers being polyoxyethylene types, such as fatty alcohol polyoxyethylene ether phosphate esters, sodium styrene naphthalene sulfonate, and sodium alkylphenol polyoxyethylene ether sulfonates. Some use cationic polyacrylamide, polyamide epichlorohydrin, cationic starch, etc., as cationic emulsifiers to prepare cationic dispersed rosin size.
Synthetic sizing agents mainly include alkyl ketene dimers (AKD) and alkenyl succinic anhydrides (ASA). These two types of sizing agents contain active groups and can react with the hydroxyl groups of fibers and remain on the fibers. Therefore, they are also called reactive sizing agents. Because these sizing agents can tolerate higher pH conditions (pH=7.5–8.5) and can use inexpensive calcium carbonate as filler, they are welcomed by the papermaking industry for improving paper strength, whiteness, and improving papermaking performance. Currently, more than 50% of high-grade paper in developed countries is produced under neutral to alkaline conditions. AKD and ASA are insoluble in water themselves. Using polyoxyethylene-type non-ionic surfactants as emulsifiers, stable AKD emulsions can be prepared.
After pulping treatment, residual resins in the pulp will precipitate during the bleaching process. If not separated in time, they will form sticky deposits, adhering to equipment, paper machine copper meshes, felts, and drying cylinders, causing papermaking obstacles, affecting normal papermaking, and even causing paper defects. Additionally, with the widespread use of waste paper today, resinous substances such as adhesives, ink adhesives, and coating binders in waste paper can also cause resinous obstacles, affecting papermaking. Therefore, research and development of resin control agents have become increasingly important.
Commonly used resin control agents include inorganic fillers (such as talc), biocides, surfactants, chelating agents, cationic polymers, lipases, and membrane separation agents. The most commonly used are surfactants, with anionic surfactants being the most widely used. They include alkyl sulfate salts, alkylbenzene sulfonates, higher alcohols, and phosphate esters. Cationic surfactants mainly include alkyl amine salts or quaternary ammonium salts. Non-ionic surfactants mainly include polyethylene glycol types and polyols. In addition, there are amphoteric surfactants and various multi-component compounds.
Release agents are also used as resin control agents to control the adhesion between drying cylinders and paper sheets, lubricate doctor blades and drying cylinders, and control the distribution of adhesives. They mainly include polyamide polymer emulsions, such as polyvinyl alcohol emulsions, mineral oils, surfactant-coated organosilicon emulsions, and polyamine-polyamide cationic polymer emulsions.
During the papermaking process, there are small amounts of saponins, fatty acids, and other natural and artificially added foaming surfactants in the pulp. Additionally, there are synthetic polymers and starches, which act as foam stabilizers. Therefore, foam may occur, leading to problems such as paper breaks or spots on the paper surface. The main active components of defoamers used in papermaking are high carbon alcohols, polyethers, fatty acid esters, organic silicone polymers, etc. They are generally formulated into oil-in-water emulsions.
Softness is relevant to fibers. Surfactants can adsorb in the opposite direction of hydrophobic groups on the fiber surface, reducing the dynamic and static friction coefficients of fiber materials, thus achieving a smooth and soft texture. Anionic surfactants such as acetic acid sulfate and sulfonated castor oil show their softening effects when adsorbed on the fiber surface.
Cationic surfactants have cationic groups that can directly interact with negatively charged fibers, and hydrophobic groups form low-energy surfaces on the outer side of the fibers, resulting in particularly good softening effects. For example, fatty acid amidoamine epichlorohydrin is mainly used for papers with high softness requirements, such as tissues, crepe paper, sanitary napkins, handkerchiefs, and napkins.
Amphoteric surfactants have a wide range of applications. Their cationic groups can interact with fibers, and their anionic groups can combine with fibers through polyelectrolytes or aluminum ions in the pulp, similarly arranging hydrophobic groups outward, thereby greatly reducing surface energy. For example, 1-(2-aminoethyl)-2-octadecyl imidazoline carboxylic acid derivatives are surfactants of this type. Additionally, both cationic and amphoteric surfactants have antibacterial and bactericidal abilities, effectively preventing paper from molding.
Organosilicone surfactants are special surfactants, mainly used as cationic organosilicone quaternary salts in softeners.
Other softeners include varieties such as polyoxyethylene sorbitan monostearate, polyoxyethylene lanolin, and emulsified wax.
In the production of specialty processed paper, antistatic issues may arise. Surfactant treatment solutions can generate a hydrophilic outer surface, effectively countering static electricity. Surfactants used as antistatic agents form a positive adsorption on the material surface, creating a hydrophobic material surface with hydrophilic ends extending into space. This increases the ionic conductivity and moisture absorption conductivity of fibers, resulting in discharge phenomena, lowering surface resistance, and preventing static electricity accumulation. Surfactants used as antistatic agents typically have large hydrophobic groups and strong hydrophilic groups. Among them, cationic surfactants are the most widely used and effective, followed by amphoteric surfactants.
The primary function of fiber dispersants is to reduce fiber flocculation and improve paper formation. Fiber dispersants allow the formation of a bilayer structure on the fiber surface. The polar end of the outer layer of dispersants has a strong affinity for water, increasing wetting, while being repelled by electrostatic forces, resulting in dispersion. Commonly used fiber dispersants include partially hydrolyzed polyacrylamide (PAM) and polyoxyethylene (PlEO). PlEO exhibits high viscosity, good water solubility, and excellent lubricity. Adding less than 0.05% of it to high-grade tissue paper can achieve good dispersion effects.
Application of Surfactants in Paper Surface Sizing and Coating
Surface sizing and coating involve applying chemicals to the surface of paper to improve its surface properties, enhance printing performance, and overall quality. Surface sizing typically only involves adhesives, while coating uses both adhesives and pigments. The adhesive in surface sizing is pressed into the paper, while pigments in coating are applied to the paper's surface.
Surface sizing agents can be classified into natural and modified products and synthetic products, as well as anionic, cationic, and non-ionic categories. They can also be divided into aqueous solution type and emulsion type. Most surface sizing agents have both hydrophobic and hydrophilic groups, making them surfactants in a broad sense. Major surface sizing agents include modified starch, polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and polyacrylamide (PAM). Different surface sizing agents can be chosen based on specific needs. For example, AKD, dispersed rosin, paraffin wax, chromic acid ester of stearic acid, styrene-maleic anhydride copolymer latex, and other synthetic resin emulsions are used to enhance water resistance. Fluorinated organic substances, such as perfluoroalkyl acrylate copolymers, perfluorooctanoic acid chromium complexes, and perfluoroalkyl phosphate salts, can improve oil resistance. Organosilicone resins are used to increase anti-stickiness, while modified starch, CMC, and PVA are employed to enhance printing performance. PAM-modified starches are used to improve dry and wet strength. Sodium CMC and sodium alginate are used to enhance print glossiness. Combining different sizing agents can significantly improve surface sizing effectiveness.
Coating compositions mainly include adhesives, pigments, and other additives. Surfactants play an important role in coating formulation, serving as dispersants, defoamers, lubricants, preservatives, antistatic agents, and synthetic latexes.
Dispersants: Dispersants are the most important additives in coatings, with most being surfactants. They impart charges to pigment particles, causing them to repel each other. They also form protective colloids around pigment particles and create high-viscosity states around particles to prevent aggregation. Common dispersants include phosphates, silicates, ammonium dihydrogen phosphate, condensation products of benzenesulfonic acid and formaldehyde, casein, and arabic gum. Sodium hexametaphosphate, sodium pyrophosphate, and sodium tetrabasic phosphate are commonly used dispersants in low-solids coatings. High molecular weight organic dispersants, such as sodium polyacrylate solutions, sodium polymethacrylate, sodium salts of copolymers of isobutylene and maleic anhydride, and alkyl phenol polyoxyethylene ethers and fatty alcohol polyoxyethylene ethers, are typically used in high-solids coatings.
Defoamers: Foaming often occurs during coating preparation and application and requires the addition of defoamers. Common defoamers include higher alcohols, fatty acid esters, tributyl phosphate, and tributyl phosphate.
Lubricants: In order to improve the fluidity and lubricity of paper coatings, enhance adhesion, impart smoothness and gloss to the paper coating, increase plasticity, prevent cracking, and improve the printability of coated paper, lubricants can be added. Currently, the most widely used lubricants are water-soluble metallic soaps, with calcium stearate being a representative example. Sodium stearate, another type of water-soluble lubricant, also exhibits significant effects. Hydrocarbon waxes and fatty acid amides can also serve as lubricants.
Preservatives: Some natural adhesives are prone to degradation and mold growth, so preservatives are added to paper coatings. Quaternary ammonium salt cationic surfactants, fluorine-containing cyclic compounds, organic bromides, organic sulfur compounds, N-(2-benzimidazolyl) methylamine (carbendazim), and others are widely used in paper coatings.
Antistatic Agents: By adding octadecyl trimethyl ammonium fluoride, polyoxyethylene sorbitan monostearate, alkyl phenol polyoxyethylene ether phosphate, polystyrene sulfonate, and other compounds to coating formulations, paper can be endowed with antistatic properties.
Synthetic Latex: Synthetic latex is an important coating adhesive, and surfactants play crucial roles as emulsifiers, dispersants, stabilizers, and more in its preparation process.
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