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What is coating latex?
Source: | Author:kesikeen | Publish time: 598 days ago | 260 Views | Share:
The most widely used coating pigment binders in the paper industry are latexes, which are used in paper coatings to increase coating strength, paper gloss, print gloss, and printability.
The properties of coated paper are mainly determined by the coating. The main components of the coating are pigments, adhesives and other additives and water. Adhesive is an important part of the coating. Its role is to make the pigments adhere to each other, and at the same time to make the pigment adhere to the surface of the base paper. Secondly, the adhesive can also adjust the absorption of the printing ink to achieve the balance between ink solidification and ink gloss. In addition, the adhesive plays a role of protective colloid in the coating, and controls the stability and fluidity of the coating. Therefore, the adhesive must first have strong adhesion to the pigment, otherwise it will cause the surface strength of the coated paper to be insufficient, and the phenomenon of powder falling-off will occur; secondly, the adaptability of the adhesive and the pigment is better, and the coating must be guaranteed Appropriate ink absorption and plasticity to improve the calendering effect; Finally, the adhesive also needs proper viscosity, film formation, and fluidity to facilitate the coating operation and improve the coating uniformity to meet the above requirements. The adhesives can be divided into natural products and synthetic polymers. Natural products include starch, casein, and soy protein, among which starch and its modified products are most widely used; high molecular polymers include various synthetic latexes (styrene-butadiene latex, styrene-acrylic latex, polyvinyl acetate latex, etc.) Polyvinyl alcohol, etc.

I. styrene-butadiene latex
In the 1940s, SBR developed abroad as a coating adhesive for the production of processed paper. The styrene-butadiene latex used for paper processing accounts for about 60% of the styrene-butadiene latex production.
The styrene-butadiene latex is a polymer latex made by emulsion copolymerization of butadiene and styrene monomer after adding different chemicals and additives. The styrene-butadiene latex has good adhesion, can produce high solid content and low viscosity coating, has good machine running performance; paper surface strength and printing gloss is better. Because its particles are smaller than those of natural rubber latex, its mechanical stability is superior to that of natural rubber latex and can be mixed with natural rubber latex in any ratio.
The performance of styrene-butadiene latex is influenced not only by the polymerization conditions but also by the proportion of styrene and butadiene. When the content of styrene increases, the plasticity of the latex increases, the hardness increases, and the pressure and temperature are required to be higher when calendering is performed, so that a paper with a higher gloss and smoothness and a good film forming performance can be pressed, but the adhesion of the coating is improved. And stability drops. In recent years, many polar monomers that can impart special properties to coatings have been introduced into polymerization systems, mainly including unsaturated carboxylate monomers that improve coating adhesion, lightfastness, and print gloss, with printing gloss and ink set. The carboxyl groups are highly active due to the fact that they are related to nitrile unsaturated monomers, amidation substrates, hydroxy compounds, epoxy compounds that enhance adhesion and stability, and amino compounds that impart adhesion and gravure printing properties. As the degree of carboxylation of the latex increases, the shear viscosity and water retention of the coated paper also increase, but the coating exhibits shear thinning characteristics under high shear force, which is very suitable for high speed blade coating.

II. styrene-acrylic latex
Styrene-acrylic latex is obtained by emulsion copolymerization of styrene and acrylate monomers, milky white liquid with blue light. The styrene-acrylic emulsion has good adhesion, transparent film, good resistance to water, oil, heat and aging. Styrene-acrylic latex is used as a binder for paper products and can be used in combination with starch, polyvinyl alcohol, sodium carboxymethyl cellulose sodium lamp adhesives.
The paper produced by the styrene-acrylic latex as a binder has the advantages of smooth surface, high whiteness, good anti-aging properties, and good printability. Therefore, in the current coating field, carboxylated styrene-butadiene latex and polyvinyl acetate are also referred to as three major adhesives. Compared with other adhesives, the outstanding characteristic of styrene-acrylic latex is its superior weather resistance. It has the following features:
(1) The latex has good stability. As long as the polymerization conditions are properly controlled, the obtained polymer emulsion particles are fine, and the storage stability is good. Demulsification is not likely to occur during coating, and the film formation is uniform.
(2) Strong acid and alkali resistance, wide pH range, good compatibility with pigments, fillers and other adhesives.
(3) Excellent light resistance and heat resistance.
(4) has good solvent resistance.
(5) Adhesion and film-forming properties can be adjusted as needed, and due to the presence of polar groups such as ester groups in the molecular chain, the combination with the outside fibers and other coating components is good, and the film formation is smooth and smooth. Feel good.
The cross-linked acrylate copolymer latex is divided into cross-linked type and diplomatic type. The internal cross-linked type is a cross-linkable group such as a hydroxymethyl group, an epoxy group, etc. in the molecular chain, and the cross-linking monomer is methylenebisacrylamide, N-methylolacrylamide, divinylbenzene, Hydroxyethyl acrylate and so on. Diplomatic linkage is the addition of a cross-linking agent, such as cross-linking of polyacrylic acid ester-containing macromolecular chains, glyoxal etc. may be added as a cross-linking agent.

III. polyvinyl acetate latex
Polyvinyl acetate emulsion (PVAc) is a vinyl acetate (VAc) monomer with water as the dispersion medium, emulsion polymerization caused by water-soluble initiator (such as persulfate). It has good adhesion to general substances. The prepared coating has high solid content, low viscosity and stability. It has the advantages of fast adhesion, room-temperature curing, non-toxicity, and no environmental pollution. It is widely used in papermaking, leather and textiles. And other fields.

IIII. polyvinyl alcohol (PVA)
Polyvinyl alcohol is a water-soluble polymer and is often used as an adhesive and surface sizing agent in the paper industry because of its good adhesion, film strength and pigment dispersion. Polyvinyl alcohol is used as a coating adhesive to increase the strength of coated paper and reduce the coating off-powder phenomenon. Polyvinyl alcohol is used as a coating auxiliary adhesive to significantly improve the coating filming property and improve the coating performance. Paper strength, gloss, smoothness, etc., and can reduce the amount of latex, effectively reducing production costs.

V. Starch and its modified products
The use of starch for coating adhesives started at the beginning of the 20th century. With the development of in-machine coating technology, starch has rapidly become an important adhesive, and the natural or unmodified starch has high viscosity and its properties are not Stable, easy to age, and rarely used directly as a coating adhesive. The modified starch that meets the requirements of coating adhesives can be prepared by physical, chemical or biological modification. Because starch is a polyhydroxy linear and branched macromolecular structure composed of D-glucose units, to reduce the viscosity, macromolecules must be cut into small molecules or smaller molecular structures, and general oxidation or acid hydrolysis can reduce the viscosity, However, at the same time, the strength of the starch bond decreases greatly. In order to make up for the loss of bond strength caused by deep oxidation and hydrolysis, it is necessary to connect relevant groups with superior performance to the hydroxyl group to increase the bonding strength, and at the same time, Pigments have good compatibility, resistance to wet grinding, etc. Modified starch adhesives are mainly phosphate ester starch, oxidized esterified starch and grafted starch.
The trend in the development of modern coated paper is the large size and high speed of paper machines. Coating speeds of up to 1800-2000m/min, and near-above 3000m/min intermediate speed coating machines have been tested; paints have grown to higher solids content, and the solid content of some coatings has reached over 70%. This puts higher requirements on the properties of the paper coating adhesives, and also causes new problems in the study of coating paper coating formulations. On the one hand, the rapid development of coated paper, such as domestic coated white cardboard, coated paper, and lightweight coated paper, has increased the demand for latex, and the increased secondary fiber reuse has resulted in a substantial decrease in the surface properties of the paper. The demand for coatings, especially those in adhesives, has become higher and higher. On the other hand, the rise in oil prices in recent years has led to significant increases in prices of downstream products such as styrene-butadiene latex and styrene-acrylic latex. Therefore, in order to reduce production costs, many scientific research workers and companies have turned their attention to cheaper coating starch. It is expected that after high-efficiency modification of starch, carboxylated styrene-butadiene latex can be partially or completely replaced, which not only reduces coated paper. The cost of production, but also can save non-renewable petroleum resources, and because the coating no longer contains refractory synthetic polymers, it is of great environmental significance to achieve complete regeneration and reuse of coated paper products.
From the current experimental research and production practices, various modified starches have failed to meet the requirement of replacing synthetic latex by 100%, and the newly developed biological latex can partially or completely replace the synthetic latex. Bio-latex is a new type of starch-based paper coating adhesive that can partially or completely replace petroleum-based styrene-butadiene latex used in the papermaking industry. It uses starch as the main raw material, which is derived from renewable resources and does not contain volatile organic compounds. It is produced by mechanical extrusion and does not use organic solvents. Compared with synthetic latex, bio-latex is superior to synthetic latex in terms of water retention, opacity, dry stripping, paper stiffness, etc., and has strong adhesion, chemical and mechanical stability, no odor, frost resistance Good, and bio-latex instead of synthetic latex for paper coating, can also make the paper has better printing performance.

Biolatex was the first patented product launched by Ecosynthetix Biomaterials in the United States in 2009. The volume of natural starch particles is approximately 30μm. The key technology of Ecosynthetix is to convert these particles into nanoparticles of 50-150nm. In this volume, The surface area of nanoparticles is 400 times more than natural particles. This means that adhesives made from nanobiological latex require less moisture and can be formulated as high solids and low viscosity coatings, so drying takes less time and consumes less energy. At the same time, the drying time required for paper production is reduced, resulting in a higher yield, and due to the absence of high temperature effects, the cardboard deformation phenomenon will be less. And this adhesive product can also improve coating performance, increase whiteness and brightness, reduce costs and reduce carbon emissions.