Color difference control in the production of the

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Color difference control in color paper production release date: Source: the market demand for color paper in Chinese paper industry is increasing year by year. On the one hand, it is the need of readers to prevent myopia; On the other hand, it also meets the needs of publishing houses to prevent theft and increase the beauty of pictures, so as to expand sales. But the manufacturers of color paper are all faced with the common difficulty: color difference control. There are two forms of color difference: one is the change of color depth; One is the change of hue. In actual production, after selecting the type and dosage of dyes, if there are no special circumstances, the source of color difference is mainly the former., Today, China's paper industry is discussing the color difference of printing and writing paper produced from wheat straw pulp and imported wood pulp

cultural continuous dyeing method can adjust the dosage of dyes in time to reduce the color difference, but it is necessary to choose dyes with fast coloring. Most of these dyes are alkaline dyes, which have the disadvantages of poor fastness to sunlight and easy fading, while direct dyes or acid dyes are used for light coloring due to short dyeing time. Continuous dyeing is mainly used in paper machines with fast speed or in the production of light colored paper. The main raw material of domestic production of cultural paper is wheat straw pulp, and the speed of paper machine is mostly below 250m/min, so intermittent pulp mixing and dyeing method is more common

1 dyeing mechanism

culture adds selected dyes to the pulp to selectively absorb most of the spectrum of visible light. The spectrum that is not absorbed but reflected is the desired color. Among them, the wavelength of yellow is the shortest, in the range of 565-585nm, so it is more difficult to control the color difference of yellow paper

2 selection and dosage of dyes

when choosing dyes according to the color of the paper to be produced, it is better to choose two or three dyes to mix, which is easier to control the color difference than a single dye, especially when the main color is yellow. Alkaline dyes are easy to stain and bright in color, but their color fastness is poor. Some of them are difficult to dissolve, and color spots will occur if they are not paid attention to. Direct dyes are relatively fast and have good fastness, so they should be preferred, followed by direct sun resistant dyes

when selecting dyes, prepare the pulp according to the proportion of straw pulp and wood pulp in normal production, add dyes according to experience, copy small samples with a tablet making machine, and conduct qualitative tests to verify whether the hues are consistent. The color whiteness meter can be used to measure the l* value, a* value, b* value, c* value and h* value of the sample, which can be compared with the measured value of the sample, and then the quantitative test can be carried out. If the sample paper is soaked with water and compared with the prepared size, the time to determine the proportion of dye and size can be shortened. The consumption of direct dyes is generally 1/10000, while that of basic dyes is 1/100000

3 ingredients

culture first needs to ensure the stability of beating degree and concentration of slurry. The washing of self-made straw pulp has a great impact on dyeing. The hardness is required to be less than 0.06% (600ppm) and the residual chlorine is less than 0.004g/l. When the residual chlorine is too high, the fiber will not even be colored. Because the coloring degree of straw pulp and wood pulp is different, and the damaged paper has dyes, the proportion must be uniform, especially the whiteness of straw pulp cannot change too much. In the production of color paper, if there is no special requirement for brightness, it is best not to use brightener to avoid additional color difference

after the cultural slurry is prepared, dye should be added first, and then rosin gum and bauxite should be added after maintaining a circulation time of 10-15min. It can also be changed to reverse sizing, adding bauxite first and then rosin, which has a better effect. The pH value on the slurry has a great influence on the dyeing effect, which can be controlled by the amount of bauxite. When AKD neutral sizing is used, dye is added first, and then cationic starch is added. In order to enhance the dyeing effect, some salt can be added to reduce the solubility of the dye and play a certain role as a dyeing aid. If possible, the effect of color fixing agent is better, but it must be noted that the dye should be added to the pulp before it can play a better role

due to the large amount of white water in the system, the dye concentration is inevitably reduced at the beginning of production, and the paper color will change from light to dark during normal production

this kind of transition color paper in production causes great waste. The solution is to add some dyes into the white water system according to the dye concentration in the batching tank, and the amount must be equivalent to the dye concentration in the batching tank. Composites will also be the mainstream in the field of materials science. Or use the following method: when using direct dyes, the amount of dyes added in the first pool is 1.5 times that of normal, 1.3 times that of the second pool, 1.1 times that of the third pool, and then add them normally. If it is alkaline dye or compound dye, add it at 1.2 times, 1.1 times and normal amount

4 papermaking

culture in the process of paper machine papermaking, the usage and concentration of retention and drainage aids are required to be stable, and the change of dye retention is also caused by the change of retention rate of fine fibers and fillers. In addition, pay attention to the stability of the circulation of white water. In normal production, there are inevitably some interference factors that cause color fluctuations. At this time, it can be adjusted by adjusting the amount of filler, because the filler itself has a high whiteness and has a strong function of competing for dyes with fibers. When the color becomes lighter, the amount of filler can be reduced. After repeated verification in production, it is a very effective method to use the whiteness color meter to measure the whiteness of paper for adjustment. As long as the whiteness difference is within ± 1%, the color difference can be effectively controlled (in fact, the whiteness standard adopted in our country measures brightness). The principle is as follows: after determining the type and amount of dye, the whiteness of the colored paper is reduced to a certain value because it absorbs more light than the white paper, but the amount of light absorbed and reflected by the paper is also relatively stable. Once the whiteness changes, it is because the amount of dye absorbed by the fiber changes, resulting in the amount of light reflected by the paper. Reducing or increasing the amount of fillers can also reduce or improve the absorption of dyes by fillers, which correspondingly causes the change of dye concentration, thus changing the color depth of the paper. It is worth mentioning that the purpose of filling this project is to optimize the effectiveness and applicability of an innovative, low-cost thermoplastic composite welding process. The material concentration should be stable and should be added continuously on the inlet pipe of the high-level tank or headbox, so as to adjust the flow in time

if the material made by continuous dyeing has the characteristics of hard, brittle and super absorbent, direct dyes or alkaline dyes should be selected. The coloring is relatively fast and easy to operate. Continuous dyeing is adopted, and the selection of dye addition point is very important. It is best to be at the inlet of the flushing pump or rotor screen to ensure that the dye and the slurry have sufficient mixing time. In the process of papermaking, the color difference is also controlled by measuring the whiteness, and the color depth of the paper is adjusted by adjusting the flow of the dye metering pump. Its disadvantage is that the adjustment time is too long during startup and shutdown

5 conclusion

culture uses the method of measuring the whiteness of paper and controlling the color difference in the production process of colored paper by increasing or decreasing the amount of filler, which is relatively simple and easy; The choice of intermittent or continuous dyeing should be based on the speed of the car, the choice of dyes and the specific situation of the equipment. Generally, the paper machine with faster speed and the paper with lighter color should choose the continuous method. The paper machine with better closed circulation of white water can also consider the continuous dyeing method

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