What to Do When Coated Whiteboard Paper Turns Yellow?
The phenomenon of yellowing in coated whiteboard paper refers to a decrease in whiteness after a certain period of storage or exposure to sunlight. Research on the yellowing phenomenon of coated whiteboard paper and its mechanisms, as well as identifying various factors causing yellowing and measures to stabilize whiteness, has been receiving increasing attention. This article primarily analyzes the yellowing phenomenon of coated whiteboard paper based on production practices and discusses preventive and corrective measures.
1. Mechanism of Yellowing in Coated Whiteboard Paper
Research indicates that yellowing occurs when the surface material of the paperboard reacts with oxygen in the air during storage or after exposure to sunlight, altering the chemical structure of the surface substance and affecting visual perception. The degree of oxidation determines the severity of yellowing, primarily influenced by three factors: ① High temperature, ② Ultraviolet light, ③ pH value.
2. Factors Affecting Yellowing of Coated Whiteboard Paper
Analyzing the factors promoting oxidation and the material composition of coated whiteboard paper, several factors affecting yellowing include: raw materials of coated whiteboard paper, fluorescent whitening agents, color pigments, coating adhesives, etc.
3. Preventive and Corrective Measures for Yellowing of Coated Whiteboard Paper
3.1 Adjusting the process of coating the base paper: Use bleached chemical pulp for the surface pulp of the base paper and increase the amount of surface pulp, enhancing coverage of the mechanical pulp. Adopt a mildly alkaline sizing process, reduce the amount of aluminum sulfate in the surface pulp, and increase the pH value of the base paper. Experimental results show that a pH value between 6.5 and 8.0 is optimal for the surface layer of the base paper. During the forming process, strict control over drying temperature is necessary to minimize it while ensuring proper drying of the paperboard, and utilize good cold cylinders for both base paper and coated paper.
3.2 Selecting high-quality liquid brightening agents: Experimental evidence suggests that the application of liquid brightening agents can reduce the degree of yellowing in coated products. While ensuring improved product opacity, its impact on yellowing is significantly lower compared to powdered brightening agents. High-quality liquid brightening agents require high purity. Low-purity liquid brightening agents not only cause unnecessary waste but also contribute to yellowing. Additionally, proper use of fluorescent whitening agents and attention to the application of brightening agent carriers (auxiliary adhesives) in coating formulations are important.
3.3 Rational application of color pigments: The use of color pigments aims to maximize whiteness (CIE), reduce the amount of fluorescent whitening agents, and balance different chemical raw materials to achieve the desired hue. To reduce yellowing of coated whiteboard paper, the selected color pigments should not only be easy to use with good affinity but also have high resistance to sunlight. Therefore, selecting high-quality organic pigments with ideal properties is crucial. Attention should also be paid to the amount of color pigments used since excessive or insufficient amounts fail to achieve the desired color. Excessive amounts exacerbate yellowing of coated whiteboard paper, while insufficient amounts do not achieve the desired whiteness.
3.4 Selecting appropriate ratios of coating adhesives: In coating formulations, it is almost impossible to use a single styrene-butadiene latex as the coating adhesive to prevent and solve the yellowing phenomenon of coated whiteboard paper while balancing formulation costs and product performance requirements. Therefore, depending on product performance requirements, a mixture of styrene-butadiene latex and other latices, such as inexpensive acrylic latex, can be chosen. This ensures the strength performance of coated whiteboard paper and reduces its degree of yellowing.
3.5 Choosing novel chemical additives: Research on the application of nano silicon-based oxides in coating formulations has shown that nano silicon-based oxides have a significant potential in reducing coating yellowing due to their unique molecular structure. In accelerated aging tests, there is minimal decrease in the b value of chromaticity. Additionally, UV absorbers and antioxidants can also alleviate coating yellowing in coated whiteboard paper.
3.6 Other factors to consider: Factors such as the pH value of coatings, purification of water used in coatings, use of coating starch, moisture content of coated paperboard, and packaging and storage of coated paperboard should be given attention. While these may not be the primary causes of yellowing, strengthening preventive measures in all aspects is effective in addressing yellowing of coated whiteboard paper.
4. Conclusion
4.1 In the production practice of addressing yellowing in coated whiteboard paper, factors such as coated base paper, fluorescent whitening agents, color pigments, and adhesives are the primary influencers of yellowing. Improving the production conditions of base paper, selecting high-quality fluorescent whitening agents and color pigments, and adjusting the amount of coating materials scientifically can effectively reduce the yellowing phenomenon of coated whiteboard paper.
4.2 Selecting novel chemical additives, such as nano silicon-based oxides, UV absorbers, and antioxidants, can also reduce the yellowing of coated whiteboard paper.
4.3 Strengthening preventive measures in various aspects of production control and packaging and storage conditions of coated whiteboard paper is also a beneficial approach to addressing yellowing.