Color Difference Control in Colored Paper Production

The market demand for color paper in cultural papers is increasing year by year. This aspect is the need for readers to prevent myopia; on the other hand, it also meets the needs of publishers for anti-piracy, increasing the beauty of pictures and thus expanding sales. However, manufacturers of colored papers face common difficulties without exception: color control problems. There are two forms of color aberration: one is the change of shades of color; the other is the change of hue. In actual production, after selecting the type and amount of dye, if there is no special situation, the source of color difference is mainly the former. This paper discusses the color difference of printing and writing paper produced by wheat straw pulp and imported wood pulp.

Although the continuous dyeing method of culture can adjust the amount of dye in time to reduce the color difference, it is necessary to choose a coloring dye. Most of these dyes are basic dyes, which have the disadvantages of poor light fastness and fading, and direct dyes or acid dyes are lighter because of shorter dyeing time. Continuous dyeing is mainly used for faster speed paper machines or for producing lighter-colored paper. The main raw material for domestic production of cultural paper is wheat straw pulp, and the speed of paper machines is mostly below 250m/min. Therefore, batch dyeing methods are more common.

1 staining mechanism

Cultures add selected dyes to the pulp so that it selectively absorbs most of the spectrum of visible light, and the unreflected spectrum is the desired color. Among them, the wavelength of yellow is the shortest and is in the range of 565 to 585 nm, so that the color difference of producing yellow paper is more difficult to control.

2 dye selection and dosage

When the culture selects the dyestuff according to the color of the paper to be produced, it is better to choose two or three dyes to mix. It is easier to control the color difference than a single dye, especially if the main color is yellowish. Alkaline dyes are easy to be colored and have a good color but poor color fastness, and some are hardly soluble, causing slight stains when they are not noticed. Direct dyes are relatively fast and have good fastness and should be preferred, followed by direct fast dyes.

When choosing the dyestuff for culture, the pulp is first mixed with pulp pulp in normal production, dyes are added according to experience, and the sample is made with a sheet-fed machine to make a qualitative test to verify whether the hue is consistent. The color whiteness meter can be used to determine the L*, a*, b*, c*, and h* values ​​of the sample and compare it with the measured value of the sample before performing the quantitative test. If the sample paper is soaked with water and compared with the prepared slurry, the ratio of the dye to the paste can be shortened. The amount of direct dyes is generally 1/10,000 units, and basic dyes are 1/100,000 units.

3 ingredients

Culture must first ensure the stability of the pulp's beating degree and concentration. The washing of self-made grass pulp has a great influence on the dyeing. It requires the hardness to be less than 0.06% (600ppm) and the residual chlorine is less than 0.004g/l. When the residual chlorine is too high, it may even cause the fibers not to color. Due to the different degree of coloring of straw pulp, and the loss of paper with dye, it is required that the ratio must be uniform, especially the whiteness of straw can not be changed too much. Production of colored paper, if there is no special requirement for brightness, it is best not to use brighteners, so as to avoid extra color.

After the culture slurry is well prepared, dye should be added first, and after 10 to 15 minutes of cycle time, rosin and bauxite should be added. It can also be changed to reverse sizing, first adding osmium and then adding rosin, better. The pH of the slurry on the grid has a great influence on the dyeing effect and can be controlled by the amount of rammed earth. With AKD neutral sizing, dyes are added first followed by cationic starch. In order to enhance the dyeing effect, some salt can be added to reduce the solubility of the dye and to help the dyeing. If the condition is better, the effect of the fixing agent is better, but care must be taken that the dye should be added into the slurry before it can play a better role.

Culture Due to the large amount of white water in the system, the concentration of dyes is inevitably reduced at the beginning of production, and the color of the paper will be shallower and darker during normal production.

This transitional color paper in production causes great waste. The solution is to add a portion of the dye to the white water system based on the dye concentration in the dosing pool, which must be equivalent to the dye concentration in the batch pool. Or as follows: When using direct dyes, the amount of dye added in the first tank is 1.5 times normal, 1.3 times in the second tank, 1.1 times in the third tank, and it is normally added later. If it is a basic dye or a compound dye, it is added in 1.2 times, 1.1 times and normal amounts.

4 Make a copy

In the process of paper machine copying, culture, the usage and concentration requirements of retention aids and filter aids are stable, and the change in the retention of fine fibers and fillers also causes changes in dye retention. Also note that the circulation of white water is stable. In normal production, there are inevitably some disturbing factors that cause color fluctuations, which can be adjusted by adjusting the amount of filler added, because the filler itself has a high degree of whiteness, and it has a very 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, using a whiteness color meter to measure the whiteness of paper is a very effective method. If the whiteness is within ±1%, the color difference can be effectively controlled (in fact, our country adopts The whiteness standard measures brightness. The principle is as follows: After determining the type and amount of dyes, the copied paper absorbs light more than white paper, causing the whiteness to drop to a certain value, but the amount of light absorbed and reflected by the paper sheet is also relatively stable. Once the whiteness changes, This is because the fiber absorbs the change in the amount of dye that causes the amount of light reflected by the paper to change. Decreasing or increasing the amount of filler can also reduce or increase the absorption of the dye by the filler, which in turn causes changes in the concentration of the dye, thereby changing the color depth of the paper. It is worth mentioning that: the filler concentration should be stable and choose to continuously add in the high tank or headbox inlet pipe in order to adjust the flow in time.

Culture, such as the use of continuous dyeing method, should use direct dyes or basic dyes, coloring faster and easier to operate. With continuous dyeing, it is important to select the dye addition point, preferably at the inlet of the slush pump or rotor screen, to ensure that the dye and slurry have sufficient mixing time. In the papermaking process, the color difference is also controlled by measuring the whiteness, and the color depth of the paper is adjusted by adjusting the flow rate of the dye metering pump. The disadvantage is that the adjustment time is too long when the machine is on and off.

5 conclusions

Culture measures the whiteness of paper and uses the method of increasing or decreasing the amount of filler to control the color difference in the production process of colored paper. It is relatively simple and convenient; the choice of batch or continuous dyeing should be based on the speed of the vehicle, the choice of dyestuff, and the specific conditions of the equipment. General speed The faster paper machine and paper with lighter color, the continuous method, and the white water closed cycle, the better paper machine can also consider continuous dyeing.

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