Ways to improve pulp dyeing

Nowadays, due to the improvement of people’s lives and culture, more and more people prefer to use colorful paper, which makes the variety of paper consumption more diversified, resulting in increasing demand for color paper. In this way, many paper mills have made process improvements to produce colored paper to meet the needs of society. The production method of colored paper is mainly to dye the pulp (the dyeing is to add dye to the pulp to make it colored), and then copy the colored paper on the paper machine. Some manufacturers have experienced problems such as light color, color unevenness, uneven color, and environmental pollution due to improper operation or lack of experience in producing colored paper. This not only affects the normal production process, but also reduces the quality of the product. The author analyzes the theory of dyeing and the actual situation of some manufacturers, and has improved from the following aspects, and achieved good results.

First, choose the right dye

There are many different types of dyes, including direct dyes, basic dyes, and acid dyes. There are many kinds of dyes in each category. Different dyes have different chemical structures, chemical properties, and suitability for dyeing. When determining the dyeing process, they should first be based on the type and characteristics of the paste, the use of the paper, and various conditions for dyeing (eg, temperature, acid, etc.). Alkaline, time, etc.) choose the best dye.

Second, improve the dyeing process

1. Methods: There are two kinds of methods: dry method and wet method. The dry method is the dyeing method in which the dry powdery dye is directly put into the pulp; and the wet method is to dissolve the dye and fully use the hot water with a certain temperature. Disperse and add the dissolved dye to the pulp for dyeing. It was found in the production that the wet dyeing effect was better than the dry method. The dyes used 80 to 90 degrees hot water to fully dissolve the dyes and perform wet dyeing, which can increase the dyeing rate, improve the uniformity of dyeing, and reduce the amount of dyes used. .

2. Temperature: Increase the temperature of dyeing, the dissolution and dispersion of dye in water is enhanced, and it is beneficial for the dye molecules to be evenly and firmly combined with fibers to improve the dyeing effect. In production, according to the actual situation, the temperature of the pulp during dyeing can be increased to enhance the coloring effect.

2. Water quality: The dyeing of pulp is carried out in water, so the water quality has a certain influence on the dyeing. Water with high hardness and high content of suspended solids will have a negative effect on dyeing. In the production of dyed paper, it is necessary to strengthen the treatment of production water and try to remove the impurities in the pulp.

4, the quality of the pH: a variety of dyes have a suitable pH range, strict control of pH, is to achieve the desired dyeing effect is an important aspect. For most dyes, the pH is maintained at a high value between 4.5 and 5.5, and the coloring effect is good.

5, sizing process: commonly used rosin acid sizing, help pulp coloring, but not the same for different dyes. For acid dyes and direct dyes, rosin gum and bauxite can both cause mordanting, but it will cause direct dyes to bring darker shades and poor light resistance. Rosin gum and a small amount of alumina can also promote the coloring of basic dyes. However, if too much alumina is used and the pH drops below 4.5, it is not conducive to the use of basic dyes. The last addition of bauxite in the sizing and dyeing can better guarantee the sizing and dyeing effects.

Third, improve the quality of pulp

1. Stable Pulp Hardness: The hardness of the pulp is a reflection of the residual lignin content in the pulp, which has a certain influence on the affinity between the fiber and the dye. Under certain dyeing conditions, the color of the pulp will change with the fluctuation of the hardness. In the production, the hardness of the pulp should be strictly controlled to avoid the difference in the color of the pulp.

2. Improve whiteness of pulp: Whiteness mainly influences the visual effect of pulp dyeing. Regardless of the kind of pulp, under the same dyeing conditions, the pulp after bleaching will look brighter than the unbleached pulp and will show better coloration. effect.

3, improve the degree of pulp washing: Many acidic or alkaline chemicals will affect the color of the dyeing paste, the reason is because some dyes are indicators, changes in pH value will cause color changes. Residual oxidants or reducing agents in pulp bleaching have a greater influence on the dye, such as residual chlorine. To remove the adverse effects of residual chemical substances, it is necessary to enhance the washing of the pulp before dyeing so that these chemical substances that affect dyeing are separated from the pulp and the effect of dyeing is improved.

4. Reducing the content of impurities in the pulp: Impurities in the pulp, especially fibrous, impurities or pulp groups are dyed darker. After the formation of the paper sheet, spots with very clear color differences are formed on the surface of the paper, which affects the quality of the product. These fibrous impurities or slurries should be removed as much as possible during the pulp screening process.

5. Control the degree of beating: In the case of the same amount of dye, increasing the degree of beating can enhance the degree of paper dyeing. This is because the degree of freeness increases the tightness of the paper and the sorption of the dye, reduces the reflection of light and increases the degree of transmitted light. In addition, when the fibers are fully dispersed during the beating, more active groups and dyes can act, so the fine fibers have a stronger adsorption force on the dye. According to the type of paper and the conditions for papermaking, reasonably controlling the degree of beating and dispersing the fibers as evenly as possible can also improve the dyeing effect.

Fourth, the use of chemical additives

Acid dyes must have obvious dyeing effect after addition of alum and other cationic mordants, and after adding urea-formaldehyde resin or cationic PEI as mordant, it has a repellent effect on direct dyes containing cationic groups, thus making direct dyes difficult and Fiber binding. Alkaline dyes use cationic polyelectrolytes as mordants, which have better effects than alum. For example, paper can maintain its special strength, air permeability, and softness, and the color tone becomes deeper due to the increase in the amount of PEI.

V. Improving the retention of dyes and fine fibers

In the dyed pulp suspension, a large amount of free-form dye particles are suspended in water. In the net part of papermaking, free dye particles and dyed fine fibers are filtered out with water, which not only reduces the dyeing effect, but also results in Dye waste, environmental pollution and other issues. For this reason, by increasing the retention rate of the net, like improving the retention rate of fine fibers and fillers, the dehydration conditions of the nets can be improved, and more dyes can be retained in the paper by using filter aids and retention aids.

Author: Liu mountain Source: Journal