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.
1, coating white paper yellow back phenomenon formation mechanism
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 affecting the 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.
2.1 coating base paper
2.1.1 Fiber type and composition of the base paper: It is mainly the composition of the surface fiber. The base paper batter of coated white paper is generally composed of bleached chemical wood pulp, so the yellowing of the base paper is mainly due to heat induced yellowing. The yellowing of mechanical pulp is mainly caused by light induced yellowing, so it is more severe than yellowing in chemical pulp during storage.
2.1.2 pH value of base paper: Acid paperboard is more likely to cause yellowing than alkaline paperboard.
2.1.3 Other raw materials of raw paper: such as filling materials, retention aids and other raw materials also have a slight impact.
2.2 Fluorescent Brightener
Fluorescent brighteners are mainly derivatives of stilbene disulfonic acid (or tetrasulfonic acid) because residual lignin in the pulp or some of the pigment components in the paint will cause yellowing of the paperboard, and the fluorescent whitening agent The use is to eliminate the yellowish tint, which is a way to improve the quality of the cardboard. Although the effect of brighteners is short-lived, because it is yellow in itself, more brighteners will also affect the yellowing of the paperboard.
2.3 color pigment
2.3.1 Different types of colorants are often used as tinting pigments for coated white paper coatings, and their requirements must be lightfast. Direct dyes and basic dyes are cheaper, but they have lower light fastness.
2.3.2 The use of more color pigments will exacerbate the yellowing of coated white paper.
2.4 coating adhesive
There are usually three types of adhesives for coating white paper: acetic acid latex, styrene-acrylic latex, and styrene-butadiene latex. Among them, acetic acid latex has the least effect on yellowing, and styrene-butadiene latex has the greatest impact on yellowing due to its special chemical structure.
2.5 Comparison of Effect of Cardboard Paper Fiber Types and Adhesives on Yellowing of Coated Whiteboard Paper
3. Prevention and remedy of yellowing 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. The experiment proved that the pH of the paper surface layer was between 6.5 and 8. O2_ asks better results. 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 coating adhesive proportion: In order to prevent and solve the phenomenon of yellowing of coated white paper in the coating formulation, a single styrene-butadiene latex is used as the coating adhesive under the requirements of balanced formulation cost and product performance. It is almost 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 the paint, 'cleaning paint water, use of coated starch, moisture of coated paperboard, 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. in conclusion
4,l In the production practice of solving the problem of yellowing of coated whiteboard paper, coating base paper, fluorescent whitening agent, color pigment and adhesive are the main factors that affect 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.