People have higher and higher requirements for the quality of packaging paperboard. As a high-end packaging material, coated whiteboard paper's exquisite appearance and quality have also become people's pursuit goals. However, the yellowing phenomenon of coated white board paper has seriously affected the appearance of the outer packaging materials. The yellowing of coated whiteboard paper means that the whiteness of the product will decrease to a certain extent after being stored for a certain period of time or exposed to sunlight. Studying the phenomenon and mechanism of yellowing of coated whiteboard paper, finding out various factors of yellowing and measures and methods to stabilize whiteness have attracted increasing attention from us.

1. The formation mechanism of the yellowing phenomenon of coated whiteboard paper

The latest research results show that the yellowing phenomenon is caused by the reaction of the cardboard surface material with oxygen in the air during the storage process and after exposure to sunlight, changing the chemical structure of the surface material, thereby affecting human visual effects. The degree of oxidation determines the severity of yellowing. The oxidation process is mainly due to the promotion of the following three factors: 

① high temperature, ② ultraviolet light, ③ pH value

2. Factors affecting the yellowing phenomenon of whiteboard paper

From the analysis of the promoting factors in the oxidation process and the material composition of coated whiteboard paper, the factors that affect the yellowing phenomenon of coated whiteboard paper mainly include the following aspects: coated whiteboard paper base paper, fluorescent whitening agent, toning pigment, coating Cloth adhesives, etc.

Coated base paper

Fiber types and composition of base paper:

Mainly composed of surface fibers. The base paper pulp for coated whiteboard paper is generally composed of bleached chemical wood pulp, so the yellowing of the base paper is mainly due to heat induction. The yellowing of mechanical pulp is mainly light-induced yellowing, so during storage, the yellowing is more serious than that of chemical pulp.

Base paper pH value: Acidic cardboard is more likely to cause yellowing than medium-alkaline cardboard.

Other raw and auxiliary materials of the base paper: such as filling substances, retention aids and other raw and auxiliary materials also have a slight impact.

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 paint will cause the cardboard to turn yellow, and the fluorescent whitening agent Use is to eliminate the yellowish tone and is a way to improve the quality of cardboard in appearance. Although the effect of the whitening agent is short-lived, because it is yellow, a larger amount of whitening agent will also affect the yellowing of the cardboard.

Mixing pigments

Different types of colorants are often used as toning pigments for coating whiteboard paper, and they must be lightfast. Direct dyes and basic dyes are cheaper, but have lower light fastness.

Using a larger amount of toning pigments will aggravate the yellowing of the white paper.

There are usually three types of adhesives used to coat whiteboard paper: acetate latex, styrene-acrylic latex and styrene-butadiene latex. Among them, acetate latex has the least impact on yellowing, and styrene-butadiene latex has the greatest impact on yellowing due to its special chemical structure.

Prevention and solutions to the phenomenon of yellowing of coated whiteboard paper

Process adjustment of coated base paper:

The pulp fibers of the base paper are all bleached chemical wood pulp, and the amount of pulp is increased to enhance the coverage of the lining mechanical pulp. Use a medium-alkaline sizing process to reduce the amount of aluminum sulfate in the batter and increase the PH value of the base paper. Experiments have proven that the pH value of the base paper surface layer is better between 6.5 and 8.02. During the papermaking process, the drying temperature must be strictly controlled. While ensuring that the cardboard is dry, the drying temperature should be reduced as much as possible, and the cold cylinders for base paper and coated paper should be used well.

Choose a better quality liquid whitener:

Experiments have proven that the application of liquid whitening agents can reduce the degree of yellowing of coated products. While it ensures to improve the whiteness of the product, its impact on yellowing is significantly lower than that of powdery whitening agents. Better quality liquid whitening agents must require higher purity. Liquid whitening agents with poor purity will not only cause unnecessary waste, but also easily cause yellowing. In addition, fluorescent whitening agents must be used in the coating formula, and attention must be paid to the application of the whitening agent carrier (auxiliary adhesive).

Proper application of toning pigments:

The use of toning pigments is to maximize the whiteness (CIE), reduce the amount of fluorescent whitening agent, and balance different chemical raw materials to achieve the required hue. In order to reduce the yellowing of coated whiteboard paper, the selection of toning pigments must not only be easy to operate, have good affinity, but also have high light fastness. Therefore, it is best to choose high-quality organic pigments, whose performance in all aspects is relatively ideal. Pay more attention to the amount of coloring pigments used, because too high or too low a dosage will not achieve the purpose of coloring. If it is too high, the degree of yellowing of the coated whiteboard paper will be aggravated; if it is too low, the required appropriate whiteness will not be achieved.

Choose a reasonable coating adhesive ratio:

In the coating formula, in order to prevent and solve the phenomenon of yellowing of coated whiteboard paper, it is almost impossible to use a single styrene-butadiene latex as the coating adhesive under the requirements of balancing formula cost and product performance. Therefore, according to product performance requirements, you can choose to mix styrene-butadiene latex with other latexes, such as cheap styrene-acrylic latex, which can not only ensure the strength performance of coated whiteboard paper, but also reduce its degree of yellowing.

Other factors that should be noted:

For example, the pH value of coatings, purification of coating water, use of coating starch, moisture content of coated cardboard, packaging and storage of coated cardboard, etc. must be paid more attention to. Although these are not the main reasons for yellowing, as long as prevention is strengthened in all aspects, the solution to yellowing of coated whiteboard paper is also very effective.

in conclusion

In the production practice of solving the problem of yellowing of coated whiteboard paper, coating base paper, fluorescent whitening agent, toning pigments and adhesives are the main factors affecting yellowing. Improve the process conditions of base paper production, select high-quality fluorescent whitening agents and toning pigments, and adjust the optimal dosage: scientifically determining the best combination of adhesives can effectively reduce the yellowing phenomenon of coated whiteboard paper.

The use of new chemical additives, such as nano silicon-based oxides, ultraviolet absorbers, antioxidants and other chemical products will also reduce the yellowing phenomenon of coated whiteboard paper.

Strengthening the production control of coated whiteboard paper and prevention of all aspects of packaging and storage conditions is also a beneficial way to solve the problem of yellowing of coated whiteboard paper.