Composition of paper
The papermaking process can be divided into several processes, such as beating → mixing → seasoning → purification and screening → papermaking → finishing and drying. Each process has different functions and purposes for papermaking. Among them, seasoning is to add various excipients to the paper, which includes sizing, adding fillers, whitening, dyeing, mixing and adding auxiliaries. The purpose is to improve the quality and performance of paper and paper from different angles.
Pulp fibers are generally yellow or gray white, which is due to the fact that lignin in pulp will absorb light with a wavelength of 400-500 nm. The improvement of pulp whiteness by adding dyes and pigments or chemical bleaching is limited. If strong bleaching is used simply to pursue whiteness, the strength of paper fibers will be greatly damaged.
High whiteness paper can be produced by bleaching and optical brightener process. After adding optical brightener, the whiteness of the produced paper, such as offset printing paper, painting newspaper, advanced writing paper, typing paper, copy paper, etc., can be increased by about 10%.
Role of optical brightener in Paper
Optical brightener plays an extremely important role in improving the whiteness of paper, which can not be replaced by other chemicals, especially in the production of high-grade paper. The optical brightener can not only emit visible light, but also absorb ultraviolet light and convert it into visible blue or purple optics. Therefore, the pulp fiber whitened by the optical brightener can reflect more visible light than the original, and the generated optical brightener counteracts the yellowish color on the fiber and plays the role of optical whitening.
Because the optical whitening effect of optical whitening agent can significantly improve the whiteness of paper products and make them bright and pleasing to the eye, without causing chemical loss of pulp, small dosage and convenient use, it plays a role that can not be achieved by any bleach and other dyes. Therefore, the use of optical brightener has become an important means to improve the whiteness of pulp after chemical bleaching. It is widely used in the whitening treatment of paper with high whiteness requirements.
Application of optical brightener in papermaking industry
① Directly added to the pulp for whitening;
② Used for surface sizing, that is, in the sizing solution added to the surface sizing;
③ It is used for surface coating of paper, that is, it is added to the coating solution for processing paper.
Requirements for optical brighteners used in paper industry
① It has good compatibility with various papermaking fillers, additives and adhesives.
② It can withstand a certain pH and has no effect on aluminum sulfate for papermaking.
③ It has high whiteness to paper fiber.
The optical brightener is injected into the surface glue and painted on the paper
The sizing solution for surface sizing is mainly composed of oxidized starch, polyvinyl alcohol or carboxymethyl cellulose. The optical whitening agent and corresponding additives can be added to the sizing solution. After surface sizing, the paper is fine, easy to print and high whiteness. This kind of paper is usually used as printing paper, art paper and photographic paper.
A layer of coating containing optical brightener is coated on the surface of the formed base paper to produce paper with high whiteness, such as coated paper, printing paper, art paper, etc.