Color difference control in the production of the

2022-07-25
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Color difference control in color paper production the market demand for color paper in cultural paper 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 need to exert a certain tension on the publishing house's anti-theft edition, increase the beauty of the pictures, and expand sales. But the manufacturers of color paper are all faced with common difficulties: 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. This paper discusses the color difference of printing and writing paper made from wheat straw pulp and imported wood pulp

cultural continuous dyeing method can adjust the amount of dyes in time to reduce the color difference, but it is necessary to choose the dyes with fast coloring. Most of these dyes are basic dyes, which have the disadvantages of poor sun fastness and easy fading. However, 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 for 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 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. 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 selecting 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 color 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, compare them with the measured value of the sample, and then conduct quantitative test. If the sample paper is soaked with water and compared with the prepared size, the time for determining the ratio of dye to size can be shortened. The consumption of direct dyes is generally 1/10000, while that of basic dyes is 1/100000

3 ingredients

culture should first ensure the stability of beating degree and concentration of the slurry. The washing of self-made straw pulp has a great impact on dyeing. It is required that the hardness be less than 0.06% (600ppm) and the residual chlorine be less than 0.004g/l. When the residual chlorine is too high, the fiber will not even be colored. As the coloring degree of straw pulp and wood pulp is different, and the damaged paper has dyes, the ratio 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 better not to use whitening agent to avoid additional color difference

after the cultural slurry is prepared, the dye shall be added first, and after the circulation time of 10 ~ 15min, rosin gum and bauxite shall be added. It can also be changed to reverse sizing, adding bauxite first and then rosin size, and the effect is better. 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, 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 in dyeing. 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 equal to the dye concentration in the batching tank. Or as follows: when direct dyes are used, the amount of dyes added in the first pool is 1.5 times that of the normal ones, 1.3 times that of the second pool, and 1.1 times that of the third pool, and then it is added normally. If it is basic 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 use amount and concentration of retention and filter aid are required to be stable. The change of retention rate of fine fibers and fillers also causes the change of dye retention. In addition, it should be noted that the circulation amount of white water is stable. In normal production, there are inevitably some interference factors, which cause color fluctuation. 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 with the fiber for dyes, when the color becomes lighter, it can reduce the amount of filler if the belt deformation is too large. 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 China measures brightness). The principle is as follows: after the type and amount of dye are determined, 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 filler concentration should be stable and should be added continuously on the inlet pipe of the head tank or headbox, so as to adjust the flow in time

if continuous dyeing method is adopted for culture, direct dye or basic dye should be selected. The dyeing is fast and easy to operate. For continuous dyeing, it is very important to select the adding point of the dye. 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. The disadvantage is that the adjustment time is too long during startup and shutdown

5 conclusion

it is easy to control the color difference in the process of color paper production by measuring the whiteness of paper and increasing or decreasing the amount of filler; Intermittent or continuous dyeing should be selected according to the speed of the vehicle, the choice of dyes and the specific conditions 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 white water closed circulation can also consider the continuous dyeing method (end)

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