Application methods of fluorescent whitening agents in the papermaking industry (2):
Paper surface coating whitening
The surface coating whitening of paper is a method of processing pigment-coated paper. It uses a coating device to coat one or both sides of the formed base paper with a layer containing fluorescent whitening agent, adhesive and white. The coating of inorganic pigments and other additives is then dried and calendered to make paper with bright surface gloss and high whiteness, such as coated paper, printing paper, art paper, wax paper, etc., while making the paper have good non-toxic properties. Transparency, water resistance, and ink absorption to meet the needs of high-performance paper, often used in the production of high-end printing paper.
The coating is usually composed of a mixture of white pigments, adhesives, fluorescent whitening agents and water. Commonly used white pigments are kaolin, calcium carbonate, satin white, aluminum hydroxide, barium sulfate, titanium dioxide, etc. Binders are usually a mixture of natural and synthetic binders. Natural binders are often casein and starch (modified starch, starch derivatives). Synthetic binders are polyvinyl alcohol, acrylate polymers and styrene-butadiene. Polymers etc.
Binders have a great influence on the whitening of coating slurries. Generally, adhesives composed of high-grade casein are better, while soybean casein and starch are worse. Starch and casein have different effects on various fluorescent whitening agents. Casein is yellower than starch, but optical brightener coatings containing casein are whiter than coatings containing starch. The use of synthetic binders results in better whiteness of the coating. When testing the whitening effect of optical brighteners on coating components, the experimental conditions must be carefully determined if the results are to be compared. If truly high-grade white coated paper is desired, optical brighteners must be added to the pulp.
The dosage of fluorescent whitening agent is generally 0. 01% ~ 0. 2% of the coating dosage. The appropriate pH value for adding the fluorescent whitening agent solution into the paint is 7 to 8.5.
C.I. fluorescent whitening agents 28, 71, 103, 114, 134, 205, 220, 264, 357, etc. can be used to whiten the surface coating of paper. According to the whiteness requirements of the paper and the selected carrier (or so-called auxiliary adhesive), di-, tetra-, or hexa-sulfonic acid-based varieties can be used, among which tetra- and hexa-sulfonic acid-based varieties are more effective.
Paper surface coating whitening process flow:
Preparation of coating slurry containing fluorescent whitening agent → paper coating → drying of coated paper → finished paper
Application example 1:
Optical brightener PRS is used for coating whitening.
Formula (mass fraction):
Fluorescent whitening agent PRS. 0. 05% ~ 0. 5%
Adhesive 1% ~ 20%
Oxidized starch 8% ~ 10%
White filler 20% ~ 30%
Whitening process: Weigh the required amount of fluorescent whitening agent, oxidized starch, adhesive, and white filler. Dissolve the fluorescent whitening agent with an appropriate amount of hot water; add 1:8 to 1:10 water to the oxidized starch in another container, and heat it to above 90°C to form a slurry; then add the fluorescent whitening agent solution, Starch slurry, adhesive, and white filler are mixed into a whitening coating slurry with a solid content of 40% to 50%. Adjust the pH to 5.5 to 6 and set aside.
Use a coater to apply the coating slurry to a paper area of 10 to 20g/m2 at room temperature. The coated paper is dried at 90 to 120°C to obtain the finished paper.
Application example 2:
Fluorescent whitening agent Tinopal ABP liq. Used for coating whitening.
Formula (mass fraction):
Tinopal ABP liq. 0. 2% ~ 1. 2%
Auxiliary adhesive 0. 5% ~ 1. 5%
Coated with porcelain clay. 40% ~ 60%
Calcium carbonate 60% ~ 40%
SR-B latex 12% ~ 15%
Whitening process: Weigh the required amount of fluorescent whitening agent, latex, adhesive, and white filler. The optical brightener Tinopal ABP liq. can be added directly in the form of original solution at any stage of preparing the coating mixture. Its addition will not affect the rheology of the coating mixture and can be used in the range of pH= 7 ~ 11. Mix the above ingredients evenly and set aside.
Use a coater to apply the coating slurry to a paper area of 10 to 20g/m2 at room temperature. The coated paper was dried at 170°C for 5 seconds to obtain the finished paper.
