1 Types and characteristics of papermaking dyes

There are many kinds of dyes, and the classification methods are different, and the dyes applied in the paper industry are usually classified by the properties of dyes.

(1) DirectDyes. It is one of the most important dyes in papermaking dyes, with convenient dyeing, complete chromatography and low price. The chemical molecular structure is: most of the direct dyes are sodium sulfonate salts of aromatic compounds, and some are sodium salts of carboxylic acids, and the dye structure is mainly azo or triazo. This kind of dye has a straight line, long chain, same plane and through the conjugate system, so it can be close to the cellulose macromolecules with a straight line and long chain, and rely on the intermolecular attraction to produce a strong bond. In addition, because the structure of the dye also contains hydroxyl, amino or carboxyl groups, it can form hydrogen bonds with the hydroxyl group on the cellulose macromolecule, so that the cellulose fiber has a strong direct dyeing and affinity (direct dyeing refers to the nature of the dye in the aqueous medium by the cellulose fiber adsorption, and affinity indicates the ability of the dye to bind with the fiber).

(2) BasicDyes. The chemical molecular structure is: In 1856, H.W.Perkin synthesized the world's first synthetic dye aniline violet, which is an alkaline dye. Its chemical structure types are diarylmethane, triarylmethane, azotype and heterocyclic compounds containing nitrogen (such as oxanthracene, oxazine and thiazine, etc.). Basic dyes are salts generated by aromatic bases and acids (organic acids, inorganic acids), that is, salts of colored organic bases. Its basic base is generally amino, and after salt formation, it is a -NH2 •HCl salt group, which is dissociated into pigment cation and acid anion in aqueous solution, so it is cationic and has a strong acetic acid taste. It is characterized by bright color, and strong coloring power, with a very small amount of dye can get a deep and bright color, but the color fastness and light resistance is relatively poor. Due to the chemical structure of the basic dye, it does not have a hydrogen bond with the fiber, so it is mostly used for dyeing mechanical pulp, chemical pulp, and local color paste with high lignin content.

(3) AcidDyes. Its structure generally contains soluble groups such as sulfonic acid, carboxyl and hydroxyl, soluble in water, and mostly dyed in acidic media, so it is called acid dyes. Its color is bright, complete chromatography. Chemical molecular structure is: compared with direct dyes, acidic dyes have simple structure, lack of long common double bonds and homoplanar structure, so cellulose fibers generally have no coloring force, not suitable for direct dyeing of cellulose fibers, need to add aluminum ions or cationic fixing agents to help it stay on the fiber surface. Therefore, its immersion fastness is also relatively poor, the color of the backwater is darker, and the color is relatively easy to migrate into the blanket, and the dyeing effect is far less than that of direct dyes and basic dyes. Because of the above properties, acid dyes are commonly used in acid sizing systems, or for surface applications.

(4) Pigments (Pigments). It is a colored fine particle powdery substance, generally insoluble in water, oil, solvents and resins and other media, and can be dispersed in various media. Pigments can be divided into inorganic pigments and organic pigments from their chemical composition, and can be divided into natural pigments and synthetic pigments from their sources. Pigments have no affinity for cellulose fibers and are mostly used for paper coating. It has a strong light fastness, bright color, complete chromatography.

2 Basic application rules of paper dyes

In the dyeing process of papermaking, the object of the dye action is the paper fiber. The mechanism of action between dyes and different slurries is different. For example, alkaline dyes have strong affinity for slurries with more lignin, and are suitable for slurries with high content of lignin. However, acidic dyes with less affinity for fibers need to be fixed on the fibers by the deposition effect produced by color fixing agents or fixing agents. The requirements of the paper industry for dyes, in addition to the chemical structure of the dye, also depend on the end use of the paper. Paper or cardboard for packaging, its fast performance requirements are not high or do not need, so you can choose basic dyes for dyeing, and high-grade writing paper or sanitary paper need good light fastness dyes, paper used in special occasions, such as decorative paper base paper, you can use pigments for dyeing. In the wet end of paper making, there are many factors that have an impact on the dyeing process or the application of dyes, and the following are some common influencing factors:

(1) Slurry. In the paper industry, different types of paper are used in different sizes, so the choice and application of dyes also have different degrees of influence. For the pulp with high lignin content of machine pulp and unbleached pulp, basic dyes are usually used, and direct dyes are suitable for most of the pulp. Bleached softwood pulp is usually darker than bleached hardwood pulp in the same amount of staining. The color paste is yellowish than the bleached paste; The dyeing effect of recycled pulp or secondary pulp is more yellow than that of bleached pulp. Due to the increase of fine fiber content, the dye is more inclined to remain on the blanket surface when bleaching sulfite broadleaf pulp and secondary pulp are dyed.

(2) degree of beating. Reflecting the degree of beating is an indicator to measure the degree of dewatering of the slurry. It can comprehensively reflect the degree of fiber cutting, swelling, spinning and fine fibrosis. Generally, increasing the beating degree of the slurry can improve the dyeing yield of the dye. Viscous beating is darker than free beating. The reason is that the higher the beating degree, the higher the degree of fine fibrosis of the fiber, the larger the specific surface area, the better the brush size can improve the retention rate of dyeing; The density of the pulp increases, the gap between the fibers decreases, and the paper pages are tight, so the exchange surface between the fibers and the air is reduced, thus reducing the amount of reflected light, making the amount of light penetrated is increased, which is conducive to the dye to absorb a certain wavelength of light, therefore, the higher the beating degree, the darker the color of the pulp dyed paper. On the contrary, the non-broom paste has poor adsorption capacity, large dye loss, and the paper color is light under the same amount of dye.

(3) Water hardness. Refers to the total concentration of calcium and magnesium ions in water. The influence of water hardness on different kinds of dyes is also very different, the influence on cationic direct dyes and basic dyes is very small, and the influence on anionic dyes and whitening agents is relatively large. For example, the electrostatic repulsion between anionic direct dyes and cellulose fibers will prevent dyes from being adsorbed on cellulose fibers, but at this time, in addition to increasing the binding force between them by adding cationic fixing agents, this electrostatic repulsion can also be eliminated by increasing the hardness of water to improve the binding force between them. It is worth noting that paper mills are located in various regions of the world, and the water quality in each region is very different. Water hardness is usually higher in the north and lower in the south, and can be increased by adding magnesium sulfate or sodium chloride (calcium chloride, which can also be adjusted in non-sizing systems).

(4) Response time and pH. Usually, the longer the reflection time, the more favorable the dyeing effect of the dye on the fiber. pH has different effects on different dyes. It will make some changes in the color of the dye.

(5) papermaking filler. In the paper industry, the use of fillers can increase the opacity and whiteness of paper, improve printing adaptability and the performance of other papers. Whether the filler is kaolin, calcium carbonate, titanium dioxide or talc, they have an effect on the tone depth of the paper. For a certain quality of paper, the proportion of fillers retained inside the paper should be maintained at a specific level, if for some reasons need to change, it will have a corresponding impact and change on the tone of the paper. Due to the suction effect of the wet paper web by the web dewatering equipment, the filler in the paper is easy to be carried away by white water, and the specific surface area of the filler is greater than the specific surface area of the fiber, and the dye is more easily adsorbed on the filler, which is easy to cause the problem of two sides of the paper color difference. The difference between the two sides is usually reduced by selecting the variety of dye, or by strengthening the retention and filtration system or by adjusting the process such as changing the addition point.

(6) Add points and add order. ① Add points. The paper types and conditions produced by each paper mill are different, so the dyeing method is different, and the adding point of the dye is also different. In batch dyeing, the dye is usually added to part of the slurry pool; In continuous dyeing, the dye is usually added near the flow box; In the subsection dyeing, the dye can be divided into two sections to be added, usually 70% to 80% of the dye is added to the pulp crusher, the rest of the dye is usually added to the suction of the slurry pool, or the slurry tube after the slurry box. In general, the anion direct dye is added to the front, while the cationic direct dye can be added near the flow box. ② Add order. There are usually the following rules to follow: Keep anionic direct dyes away from cationic direct dyes and base dyes. Try to keep dyes and whiteners away from other cationic additives. Try to keep the dye separate from the fixing agent. The requirements of the production conditions are different, and the fixing agent can be added in two ways: one is all added to the front of the dye to make it fully react with the fiber; The other is to add part of the dye before, and the other part of the dye after, to reduce the color of the backwater.

(7) Other wet end paper auxiliaries. Traditionally, because whitening agents and dyes are anionic substances, they are easy to react with cationic substances, causing the problem of color spots. Therefore, the two substances should be added before all chemicals are added, such as the mixing pool or the pulp pool of the paper machine, so that it has enough time to react with the fiber and avoid interference from other cationic substances. On the other hand, some low molecular weight and high charge cationic polymers, wet strength rosin and cationic starch can adsorb anionic dyes and improve the retention of dyes. Retention agents are the most important components of paper fabrication and are generally divided into two categories: low charge density high molecular weight and high charge density low molecular weight. The former has a relatively small impact on dyes, and the possible negative impact is because it improves the retention of fillers and fine fibers, resulting in a double-sided difference in paper color. The latter has a relatively large impact on the dye, which may darken the color of the paper, reduce the light fastness, and cause the dye to flocculate and other negative effects. Therefore, in the dyeing work, it is necessary to understand the types of wet end additives and their addition points in each paper mill served, so as to avoid their negative impact on the application of dyes, so that dyes can play a maximum effect in paper making.