How to prevent and solve the yellowing phenomenon of coated whiteboard paper?
People are increasingly demanding the quality of packaging cardboard. Coated whiteboard paper is used as a high-end packaging material, and its exquisite appearance quality has also become the pursuit of people. However, the yellowing phenomenon of coated whiteboard paper has seriously affected the appearance of the outer packaging materials. The yellowing of coated whiteboard paper refers to the phenomenon that the whiteness of the product will decrease after a certain period of storage or after sunlight exposure. Research on the phenomenon of yellowing of coated whiteboard paper and the mechanism of its occurrence, to find out the various factors of yellowing and the measures and methods for stabilizing the whiteness have increasingly attracted our attention. This article mainly combines our company's production practice to analyze the yellowing phenomenon of coated whiteboard paper, and discusses how to prevent and solve the yellowing phenomenon.
1. Formation mechanism of yellowing phenomenon of coated whiteboard paper
The latest research results show that the yellowing phenomenon is that the cardboard surface material reacts with oxygen in the air during storage and after sunlight exposure, which changes the chemical structure of the surface material, which affects the human visual effect. The degree of oxidation determines the severity of yellowing. The oxidation process is mainly due to the promotion of the following three factors: high temperature, ultraviolet light, and pH.
2. Factors affecting the yellowing phenomenon of coated whiteboard
From the analysis of the promoting factors in the oxidation process and the material composition of coated whiteboard paper, the main factors affecting the yellowing phenomenon of coated whiteboard paper are the following aspects: coated whiteboard paper base paper, optical brightener, color pigments, coating Cloth adhesive, etc.
2.1 Coated base paper
2.1.1 Fiber types and composition of the base paper: mainly the composition of the surface layer fibers. The base paper surface pulp coated with whiteboard paper is generally composed of bleached chemical wood pulp. Therefore, the yellowing of the base paper is mainly due to the heat-induced printing technology. The yellowing of mechanical pulp is mainly induced by light, so it is more serious than the yellowing of chemical pulp during storage.
2.1.2 pH value of base paper: Acid paperboard is more likely to cause yellowing than medium alkaline paperboard.
2.1.3 Other raw materials and auxiliary materials of base paper: raw materials and auxiliary materials such as fillers and retention aids also have a little influence.
2.2 Fluorescent Brightener
The fluorescent whitening agent is mainly a derivative of stilbene disulfonic acid (or tetrasulfonic acid), because the residual lignin in the pulp or some pigment components in the paint can cause the yellowing of the cardboard, while the fluorescent whitening agent The use is to eliminate yellowish tones and to improve the appearance of the cardboard. Although the effect of the whitening agent is short-lived, because it is yellow in itself, a larger amount of whitening agent will also affect the yellowing of the cardboard.
2.3 Toning pigments
2.3.1 Different types of colorants are often used as toning pigments for coating whiteboard paper, and their requirements must be lightfast. Direct dyes and basic dyes are cheaper, but have lower light fastness.
2.3.2 A large amount of toning pigments will aggravate the yellowing degree of coated white paper.
2.4 Coating adhesive
There are generally three types of adhesives used for coating whiteboard paper: acetate latex, styrene-acrylic latex, and styrene-butadiene latex. Among them, acetic acid latex has the least effect on yellowing, but styrene-butadiene latex has the biggest effect on yellowing due to its special chemical structure.
2.5 Comparison of the effects of cardboard base paper fiber types and adhesives on yellowing of coated whiteboard paper
3. Prevention and solution of yellowing phenomenon of coated whiteboard paper
3.1 Process adjustment of coated base paper: All the pulp fibers of the base paper are made of bleached chemical wood pulp, and the amount of surface pulp is increased, and the coverage of the mechanical pulp of the lining is strengthened. The medium-alkaline sizing process is used to reduce the amount of aluminum sulfate in the paste and increase the 13H value of the base paper. It has been proved by experiments that the pH value of the surface layer of the base paper is better between 6.5 and 8.0. In the papermaking process, the drying temperature must be strictly controlled to ensure that the drying temperature of the cardboard is as low as possible, and the cold cylinders of the base paper and the coated paper are used well.
3.2 Selection of better quality liquid whitening agent: It has been proved by experiments that the application of liquid whitening agent can reduce the yellowing degree of coated products. While ensuring the improvement of product self-degree, it has a significantly lower impact on yellowing than powdery whitening agent. A better quality liquid brightener must require higher purity. Liquid whitening agents with poor purity not only cause unnecessary waste, but also tend to form yellowing. In addition, the fluorescent whitening agent should be used well in the coating formulation, and attention should be paid to the application of the whitening agent carrier (auxiliary adhesive).
3.3 Reasonable application of coloring pigments: The use of coloring pigments is to maximize the whiteness (CIE), reduce the amount of fluorescent whitening agents, and balance different chemical raw materials to achieve the desired hue. In order to reduce the yellowing of coated whiteboard paper, the use of toning pigments requires not only ease of operation, good affinity, but also high lightfastness. Therefore, it is best to choose high-quality organic pigments, and its performance in all aspects is relatively ideal. Pay more attention to the amount of toning pigments, because too high or too low amounts can not achieve the purpose of color mixing. If it is too high, it will increase the degree of yellowing of the coated whiteboard paper; if it is too low, it will not reach the desired whiteness.
3.4 Choose a reasonable ratio of coating adhesive: In the coating formulation, in order to prevent and solve the phenomenon of yellowing of coated whiteboard paper, under the requirement of balancing the formulation cost and product performance, using a single styrene butadiene latex as coating adhesive is almost impossible. So according to product performance requirements, you can choose to use styrene-butadiene latex mixed with other latexes, such as inexpensive styrene-acrylic latex, which can not only ensure the strength performance of coated whiteboard paper, but also reduce its yellowing degree printing technology.
3.5 Selection of new chemical additives: The authors researched the application of nano-silicon-based oxides in coating formulations and found that nano-silicon-based oxides have a clear potential for reducing yellow coating due to their unique molecular structure. In the eye aging test, the absolute value of the chromaticity b value decreases very little. In addition, ultraviolet absorbers and antioxidants can also reduce the yellowing printing technology coated with white paper.
3.6 Other factors that should be paid attention to: such as the pH of the paint, the purification of the paint water, the use of coated starch, the moisture of the coated cardboard, the packaging and storage of the coated cardboard, etc., should be paid more attention. Although these are not the main reasons for the formation of yellowing, as long as the prevention is strengthened in all aspects, the solution to the yellowing of coated whiteboard paper is also very effective.
4 Conclusion
4.l In the production process of solving the problem of yellowing of coated whiteboard paper, the main factors affecting the yellowing are coated base paper, fluorescent whitening agent, toning pigments and adhesives. Improve the process conditions of base paper production, select high-quality fluorescent whitening agents and coloring pigments, and adjust the best dosage; scientifically determine the best combination of adhesives, which can effectively reduce the yellowing phenomenon of coated whiteboard paper.
4.2 The selection of new chemical additives, such as nano silicon-based oxides, ultraviolet absorbers and antioxidants, will also reduce the yellowing phenomenon of coated whiteboard paper.
4.3 Strengthening the production control of coated whiteboard paper and the prevention of packaging and storage conditions are also an advantageous way to solve the yellowing of coated whiteboard paper.
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