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1. The yellowing phenomenon of coating white paper
The latest research results show that the phenomenon of yellowing is the reaction of cardboard surface materials with oxygen in the air during the storage process and after sunlight exposure, which changes the chemical structure of the surface layer material, and thus affects the human visual effect. The degree of oxidation determines the severity of yellowing. The oxidation process is mainly due to the following three factors: 1 high temperature, 2 UV, 3 pH.
2. Factors that affect the phenomenon of yellowing of coated white paper
From the promotion factors in the oxidation process and the analysis of the material composition of the coated whiteboard paper, the factors affecting the yellowing of the coated whiteboard paper mainly include the following aspects: coated white paper base paper, fluorescent whitening agent, color pigment, and coating Cloth adhesive and so on.
3. Preventing and remedying yellowing phenomenon of coated white paper
3.1 Adjustment of the process of coating base paper: The surface pulp fibers of the base paper are all bleached chemical pulp, and the amount of batter is increased to increase the coverage of the mechanical pulp of the liner. The use of alkaline sizing process to reduce the amount of aluminum sulfate bake, increase the 13H value of the original paper. It has been proved by experiments that the pH value of the paper surface layer is better between 6.5 and 8.0. In the process of papermaking, the temperature of the drying should be strictly controlled to ensure that the board is dry, the drying temperature should be reduced as much as possible, and the cold paper of the base paper and coated paper should be used.
3.2 Select better-quality liquid whitening agent: The experiment proves that the application of liquid brightener can reduce the yellowing degree of the coated product. It ensures that the self-degree of the product is improved while the effect on yellowing is obviously lower than that of powdery whitening agent. Better quality liquid brighteners must require higher purity. Poorly pure liquid brighteners not only cause unnecessary waste, but also tend to form yellowing. In addition to the use of fluorescent whitening agents in coating formulations, attention should be paid to the use of brightener carriers (adhesives).
3.3 Reasonable application of color pigments: The use of color pigments is to maximize whiteness (CIE), reduce the amount of fluorescent brighteners, and balance different chemical raw materials to achieve the desired hue. In order to reduce the yellowing of coated white paper, the use of color pigments requires not only easy operation, good affinity, but also high lightfastness. Therefore, it is best to use high quality organic pigments, and its performance in various aspects is ideal. More attention should be paid to the amount of hueing pigments, because too high or too low amounts can not achieve the purpose of color hue. Too high will exacerbate the degree of yellowing of the coated white paper; if it is too low, the desired whiteness will not be achieved.
3.4 Choosing a reasonable proportion of coating adhesive: In order to prevent and solve the phenomenon of yellowing of coated white paper in the formulation of the coating, using a single styrene-butadiene latex as the coating adhesive is almost a requirement in balancing the formula cost and the performance of the product. impossible. Therefore, according to product performance requirements, you can choose styrene-butadiene latex and other latex mixed use, such as low-cost styrene-acrylic latex, both to ensure the strength of coated white paper, but also reduce the degree of return to yellow.
3.5 Selection of New Chemical Auxiliaries: In the study of nano-silicon oxides used in coating formulations, the authors found that nano-silicon oxides have a clear tendency to reduce coating yellow due to their unique molecular structure. In the aging test, the absolute value of the color b value decreases very little. In addition, UV absorbers and antioxidants can also reduce yellowing of the coated white paper coating.
3.6 Other factors that should be taken care of: such as the pH value of paint, the purification of coating water, the use of coated starch, the moisture of coated paperboard, the packaging and storage of coated paperboard, etc. should be strengthened. Although these are not the main reasons for the formation of yellowing, as long as prevention is strengthened in all aspects, it is also effective to solve the problem of yellowing of coated white paper.
4 Conclusion
4.l In the production process of solving the problem of yellowing of coated white paper, coating base paper, fluorescent whitening agent, color pigments and adhesives are the main factors affecting the yellowing. Improve the process conditions of raw paper production, use high-quality fluorescent whitening agents, color pigments, adjust the best dosage; scientifically determine the best mix of adhesives, can effectively reduce the yellowing phenomenon of coated white paper.
4.2 The use of new chemical additives, such as nano-silicon oxides, UV absorbers and antioxidants, will also reduce the yellowing of coated whiteboard.
4.3 Strengthen the prevention of coating whiteboard production control and packaging storage conditions is also an advantageous way to solve the yellowing of coated whiteboard paper.
Yellow coating white paper formation mechanism
People have higher and higher quality requirements for packaging paperboard, coated white paper as high-grade packaging materials, and its exquisite appearance quality has also become the goal of people. However, the yellowing phenomenon of coated white paper has seriously affected the aesthetic appearance of the outer packaging material. The yellowing of coated white paper means that after the product is stored for a certain period of time, or after exposure to sunlight, the whiteness of the product will decrease. The phenomenon of the yellowing of the coated whiteboard paper and the mechanism of the occurrence of the yellowing paper were studied. The measures to find the various factors of yellowing and the stability of the whiteness were increasingly attracted by our attention. This article mainly combines the production practice of our company, analyzes the phenomenon of yellowing of coated white paper, and discusses how to prevent and solve the phenomenon of yellowing.