How to solve the phenomenon of yellowing of coated whiteboard paper
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People have higher and higher requirements for the quality of packaging paperboard, and coated whiteboard paper as a high-grade packaging material, and its exquisite appearance quality has become the pursuit goal of people. However, the yellowing phenomenon of coated whiteboard paper has seriously affected the aesthetics of the outer packaging material. The yellowing of coated whiteboard paper refers to the phenomenon that the whiteness of the product will decrease after being stored for a certain period of time or after being exposed to sunlight. Studying the phenomenon of yellowing and the mechanism of coating whiteboard paper, and finding out various factors of yellowing and measures and methods for stabilizing whiteness have attracted more and more attention. This paper mainly analyzes the phenomenon of yellowing of coated whiteboard paper in combination with the production practice of our company, and discusses how to prevent and solve the phenomenon of yellowing.
1. Formation mechanism of yellowing phenomenon of coated whiteboard paper
The latest research results show that the yellowing phenomenon is that the paperboard surface material reacts with the oxygen in the air during storage and after sunlight, which changes the chemical structure of the surface layer and affects the human visual effect. The degree of oxidation determines the severity of the yellowing. The oxidation process is mainly due to the following three factors: 1 high temperature, 2 ultraviolet light, 3 pH.
2. Factors affecting the phenomenon of yellowing of coated whiteboard paper
From the analysis of the promoting factors in the oxidation process and the materials of the coated whiteboard paper, the factors affecting the yellowing phenomenon of the coated whiteboard paper mainly include the following aspects: coated whiteboard paper, fluorescent whitening agent, coloring pigment, and coating. Cloth adhesive, etc.
2.1 coated base paper
2.1.1 Fiber type and composition of the base paper: mainly the composition of the surface layer fibers. The base paper surface coated with whiteboard paper is generally composed of bleached chemical wood pulp, so the yellowing of the base paper is mainly caused by heat induced yellowing. The yellowing of mechanical pulp is mainly caused by light-induced yellowing, so it is more serious than yellowing of chemical pulp during storage.
2.1.2 Base paper pH value: Acid paperboard is more likely to cause yellowing than medium alkaline paperboard.
2.1.3 Other raw materials of the base paper: The raw materials such as filling materials and retention aids also have a slight influence.
2.2 fluorescent whitening agent
The fluorescent whitening agent is mainly a derivative of stilbene disulfonic acid (or tetrasulfonic acid), because the residual lignin in the pulp or certain pigment components in the coating will cause the yellowing of the paperboard, while the fluorescent whitening agent The use is to eliminate the yellowish hue, which is the way to improve the quality of the board. Although the effect of the brightener is short-lived, because it is yellow in itself, a higher amount of brightener will also affect the yellowing of the board.
2.3 color pigment
2.3.1 Different types of colorants are often used as color-coated pigments for coating whiteboard papers, and their requirements must be lightfast. Direct dyes and basic dyes are relatively inexpensive, but have low light fastness.
2.3.2 The amount of coloring pigment is more, which will increase the degree of yellowing of coated white paper.
2.4 coating adhesive
There are generally three types of adhesives used to coat whiteboard paper: acetate latex, styrene-acrylic latex, and styrene-butadiene latex. Among them, the acetic acid latex has the least influence on the yellowing, and the styrene-butadiene latex has the greatest influence on the yellowing due to its special chemical structure.
2.5 Comparison of the influence of paperboard base paper fiber type and adhesive on the yellowing of coated white paper
3. Prevention and solution of the phenomenon of yellowing of coated whiteboard paper
3.1 Process adjustment of coated base paper: The surface pulp fibers of the base paper are all bleached chemical wood pulp, and the amount of the surface pulp is increased to enhance the coverage of the mechanical pulp of the lining. The medium alkaline sizing process is adopted to reduce the amount of aluminum sulfate in the surface and increase the 13H value of the base paper. It has been proved by experiments that the pH value of the base paper layer is better between 6.5 and 8.0. In the process of making paper, the drying temperature should be strictly controlled to ensure that the drying temperature of the paperboard is as low as possible, and the cold paper of the base paper and the coated paper is used.
3.2 Use better liquid brightener: It has been proved by experiments that the application of liquid brightener can reduce the yellowing degree of coated products. While ensuring the self-degree of the product, it has a significantly lower effect on the yellowing than the powdery whitening agent. Liquid brighteners of better quality must require higher purity. A liquid brightener with poor purity not only causes unnecessary waste, but also tends to form a yellowing phenomenon. In addition, the use of fluorescent whitening agents in the coating formulation, but also the application of brightener carrier (auxiliary adhesive).
3.3 Reasonable application of coloring pigments: The use of coloring pigments is to maximize whiteness (CIE), reduce the amount of fluorescent whitening agent, and balance different chemical materials to achieve the desired hue. In order to reduce the yellowing of the coated whiteboard paper, the selection of the coloring pigment not only requires easy handling, good affinity, but also high lightfastness. Therefore, it is best to use high-quality organic pigments, and its performance in all aspects is ideal. More attention should be paid to the amount of coloring pigment, because the amount of too high or too low can not achieve the purpose of coloring. Too high will increase the degree of yellowing of the coated whiteboard paper; if it is too low, the desired whiteness will not be achieved.
3.4 Selecting a reasonable ratio of coating adhesive: In the coating formulation, in order to prevent and solve the phenomenon of yellowing of coated white paper, under the requirement of balancing formula cost and product performance, using a single styrene-butadiene latex as coating adhesive is almost impossible. Therefore, according to product performance requirements, you can choose to mix styrene-butadiene latex with other latexes, such as inexpensive styrene-acrylic latex, which can ensure the strength properties of coated white paper and reduce the yellowing degree.
3.5 Selection of new chemical auxiliaries: In the research of the application of nano-silicon-based oxides in coating formulations, the nano-silicon-based oxides have obvious potential in reducing coating yellow due to their unique molecular structure. In the gaze aging test, the absolute value of the chromaticity b value decreases little. In addition, UV absorbers and antioxidants can also reduce the yellowing of the coated white paper coating.
3.6 Other factors that should be noted: for example, the pH of the coating, the purification of the coating water, the use of coated starch, the moisture of the coated paperboard, and the packaging and storage of the coated cardboard. Although these are not the main reasons for the formation of yellowing, as long as the prevention is strengthened in all aspects, it is also effective to solve the problem of yellowing the coated white paper.
4 Conclusion
4.l In the production practice of solving the problem of yellowing of coated whiteboard paper, the application of base paper, fluorescent whitening agent, coloring pigment and adhesive is the main factor affecting the yellowing. Improve the process conditions of raw paper production, select high-quality fluorescent whitening agent, coloring pigment, adjust the optimal dosage; scientifically determine the best combination of adhesive, 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 white paper.
4.3 Strengthening the prevention of various aspects of production control and packaging and storage conditions of coated whiteboard paper is also an advantageous way to solve the yellowing of coated whiteboard paper.