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3.6.1.3 Dyeing Processes

3.6.1.3.1 Stock Dyeing

This is also called internal dyeing and is the most widely used paper dyeing proc¬ess. Because of clean working conditions and the most efficient usage, the dyes are now mostly added continuously and fully automatically into the stock flow. More seldomly the dyes are metered batchwise in the pulper or mixing chest.
The choice of dye and the fixing and dyeing conditions largely depend on the raw materials used in papermaking (recycled fibers, stone groundwood, TMP, CTMP, unbleached or bleached chemical pulp, type and portion of filler) and on its prepa¬ration process e. g. a higher degree of beating of the pulp results in a deeper coloring. Fillers increase the required amount of dyes because they absorb dyes and, at the same time, reduce the coloration owing to their covering power and brightness. Fillers also lead to an increase in the two-sidedness of the paper sheet.
In continuous dyeing, the point of addition is also determined by a few factors
e. g. high consistency dyeing at a stock consistency of 3–4 % (before mixing with white water ahead of the headbox) and alternatively low consistency dyeing at a stock consistency of 0.5–1.5 % (in front of the mixing pump or pressure screens). The pH conditions are very important. The addition of aluminum sulfate usually promotes the absorption of dyes and yields less colored waste water and effluent. In general, there is a trend towards paper production in the neutral or alkaline pH range. These conditions need dyes with a very good affinity to the paper stock in a neutral medium and/or very effective fixatives and retention aids.
The two stock dyeing processes have the following advantages and disadvan¬tages:
Batch addition has the advantage of thorough mixing of the additives with the paper stock and optimal fixation due to longer contact time between the fiber and dye. The disadvantages are that the time required for color correction and color change is relatively long (loss of productivity). The handling of the dyes is more problematic with regard to clean working conditions and to an exact and regular metering control system. An integration of a continuous on-machine dye shade measurement with the metering of the dyes is not possible (less productivity).
Continuous addition has the advantages of a short length in the stock line that must be cleaned when the color is changed, and of less broke because the desired shade is attained more quickly (higher productivity). However, a lower color yield (low contact time) is obtained for intensely colored papers. The more complex equipment required for this dyeing process must be taken into consideration as well. On the other hand, control of the shade of the paper produced by continuous color measurements in the paper machine, and fast adjustment of the feeding pumps, lead to less broke and thus higher productivity.

A specialty of stock dyeing is the so-called tinting, mainly used for printing and writing papers. This procedure basically consists of counteracting the slight yellow tinge of all paper stocks by adding a violet dye or a combination of pure blue and a brillant red dye, which leads to a slightly blue shade. The human eyes perceive this shade as more bright.

3.6.1.3.2 Surface Coloring
Here liquid dyes are added e. g. during the size preparation for size press or film press application. Other additives e. g. starch, synthetic sizing agents, optical brighteners are also applied in this way. For such product combinations negative interaction of any kind must be avoided. An essential prerequisite for uniform dyeing is adequate and, above all, uniform absorbency of the paper. The advan¬tages of this process are: quick changeover of shades, the possibility of only one-paper-side dyeing (typical paper grades for this are liner and testliner), absence of dyes in the water circuit, and, in the case of papers with higher basis weights (> 80 g m–2), the saving of dyes. Nevertheless, compared with stock dyeing – the classical dyeing process – surface dyeing has gained acceptance only in individual cases because really uniform dyeing of the paper is difficult to achieve. It is occa¬sionally advisable to combine stock and surface dyeing, e. g. to correct two-sided-ness. The bleed fastness of surface dyed papers is generally lower than for internal dyed papers.

3.6.1.3.3 Dip Dyeing
A small group of specialty papers, called effect papers (flower crepe paper, tissue paper), is noted for its intense brilliant shades. The paper is passed through a dipping bath containing an aqueous solution of the dye or dye combinations. The excess dye liquor is pressed off between two rolls and the wet paper is creped, if required, before drying. Acid dyes are usually used because they have high sol¬ubility and bright shades. The low affinity of these dyes for fibers results in uni¬form dyeing, even in the case of papers with greatly varying fiber composition. The bleed fastness of dip dyed paper is poor, corresponding to that of surface-colored paper and even poorer than stock dyed paper.

3.6.1.3.4 Surface Coloring by Coating
In the usual coating process, the surface of the paper or board is covered with white pigments. In the case of colored coatings, the starting material is also a white pigment coating mixture, and the desired shade is attained by adding a dispersion of a colored organic or inorganic pigment. This coloring method and these colored pigments are mainly used for specialty paper and board e. g. for labels, documents, impressive image brochures and packaging materials. E. g. for a bronze-glazed paper surface aluminum or brass powder is added to the coating color, which produces a silver or gold effect or, in combination with soluble dyes, a metallic effect. Water-soluble dyes can be used only to a limited extent because, even with the use of fixatives, bleeding cannot be prevented (migration into the base paper and into moist surfaces). Also, the inadequate lightfastness compared to pigments limits the use of these dyes.

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