With the improvement of people's material and cultural living standards, the development of science and technology and industrial production, the requirements for paper quality and quantity are getting higher and higher. In order to obtain high-quality paper, in addition to papermaking chemicals are indispensable, optical brighteners also play an important role in improving the whiteness of paper and improving the quality of paper.
Pulp fibers are generally yellow or off-white, because the lignin in the pulp absorbs light with a wavelength of 400-500nm. The method of adding dyes and pigments or chemical bleaching is limited to the improvement of pulp whiteness. If strong bleaching methods are used purely to pursue whiteness, the strength of paper fibers will be greatly damaged.
High whiteness paper can be produced through bleaching and optical brightener process. After adding the fluorescent whitening agent, the whiteness of the paper produced, such as offset printing paper, painting newspaper, high-grade writing paper, typing paper, copy paper, etc., can be increased by about 10%.
The application methods of fluorescent whitening agent in the paper industry include: ① directly added to the pulp for whitening; ② used for surface sizing, that is, added to the glue in the surface sizing; ③ used for surface coating of paper, That is, it is added to the coating solution for processing paper.
Fluorescent brighteners used in the paper industry must meet the following requirements:
① It has good compatibility with various papermaking fillers, additives and adhesives.
②It can withstand a certain pH and does not interact with aluminum sulfate used in papermaking.
③ It has high whiteness to paper fiber.
The glue for surface sizing is mainly composed of oxidized starch, polyvinyl alcohol or carboxymethyl cellulose, and fluorescent whitening agent and corresponding additives are added to the glue. The surface sizing paper has fine texture, easy printing and high whiteness. This type of paper is usually used as printing paper, art paper and photographic paper.
Coating a layer of paint containing fluorescent whitening agent on the surface of the formed base paper can produce paper with high whiteness, such as coated paper, printing paper, art paper, etc.