Textile Printing Operations

Numerous variations of textile printing processes are found in textile production depending on the type of fiber, applied dyes, desired effect, and fashion.

At present, flat screen printing and rotary screen printing are the main techniques used. Here the dyestuff is dissolved/dispersed in a printing paste containing thickener and chemicals. With every change of color, the filling of the dosing unit and of the screen has to be withdrawn. As such, changes frequently involve considerable amounts of used printing pastes having to be handled. In addition, the equipment (screen, pumps, and containers) have to be cleaned, so a distinct load is released into the wastewater. This amount increases with shorter lengths of printed batch. Table 10 gives two examples for the composition of printing pastes.

Table 10 Composition of 1000 g Printing Pastes for Pigment Printing and Two-Phase Reactive Printing

Pigment printing

Mass Two-phase reactive (g) printing

Mass (g)

Pigment

5-80 Dyestuff

1-100

Thickener (e.g., polyacrylate)

15-45 Urea

50

Emulsifier (e.g., fattyalcohol-polyglycolethers)

5-10 Alginate thickener

400

Binder (e.g., copolymers from butylacrylate,

60-80 m-Nitro-benzene-sulfonic

15

acrylonitrile, styrol)

acid Na-salt

Fixation agent (melamine formaldehyde

5-10 Buffer (e.g., NaH2PO4)

2-3

condensation prod.)

Catalysator (e.g., MgCl2)

0-2

Softener (fatty acid ester)

5-10

Anti-foam agent

0-3

Water

ad 1000 Water

ad 1000

A COD of reactive printing pastes of 150,000-200,000 mgO2/kg for pigment paste values of up to 350,000 O2/kg are realistic. Additional problems arise from the AOX content (chlorine containing dyestuff) and from heavy metal content resulting from metal ions complexed in the dyes (e.g., Co, Cu, Ni). Attention also has to be given to the use of antimicrobial agents in the printing pastes, which are added to block the microbial growth that results in degradation of the thickener and lowering of the viscosity of the printing paste.

Generally, any release of printing pastes into the wastewater should be avoided, and in many countries such action is forbidden. Figure 13 gives an overview of the possible proceedings to minimize chemical load in the wasted water from the release of printing pastes [64,65].

First the consumption of printing pastes has to be minimized by:

• Minimization of the required volumes to fill the equipment, e.g., printing screen, tubes, pumps, and container. By optimization, a filling of up to 8 kg can be reduced to a consumption less than 2 kg per filling.

• Exact calculation and metering of the consumption of printing paste to avoid excess of pastes.

The minimization of the filling of the equipment is of particular importance for the production of short lengths, for example, during sample printing. In particular for the production of very short lengths (e.g., 120 m), a considerable portion of the printing paste is required for the filling of the printing machine. Depending on the coverage factor of a pattern, approximately 55-80% of the paste is used for printing, while 45-20% is spent for the filling of the printing machine, which is considerable with a mass of 5 kg in this example. When a length of 1000m is produced the portion of paste spent for the filling reduces to 10-3% of the total mass of printing paste [66]. The high consumption of printing pastes for the production of short length samples causes high costs for the production of a collection of new patterns and thus at present digital printing techniques are recommended to substitute for the expensive full-scale production of design samples.

The high content of dissolved compounds and the broad variations in the concentration of dyes and auxiliaries make a direct recycling of pastes difficult. Supported by calculation programs, a certain portion of printing pastes can be added for the preparation of new pastes [67]. In the most simple case, the preparation of pastes for the printing of black color is carried out.

If disposal is necessary, various techniques can be used: drying and incineration, binding in concrete, and anaerobic degradation [64,65].

Printing fHKle(dyeitufl, thick* uer, pH-adjunlmtjil, hpdrolropt, 3»]t, MuUUrltl Wash ¡ait of equipment

Printing fHKle(dyeitufl, thick* uer, pH-adjunlmtjil, hpdrolropt, 3»]t, MuUUrltl Wash ¡ait of equipment

Figure 13 Minimization of chemical load from textile printing (from Ref. 57).

Recycling t Rtuic Ratling thkkencr Disposal

Figure 13 Minimization of chemical load from textile printing (from Ref. 57).

A recent technique to achieve a reuse of the thickener is the precipitation of the thickener by addition of organic solvent (e.g., methanol). After removal of the dyes and chemicals the thickener can be reused for the preparation of new pastes. The removed chemicals and dyes are collected and discarded [68]. By this method a considerable part of the COD-forming compounds can be recycled and the AOX and heavy metal content in the wastewater from textile printing can be reduced.

The replacement of classical textile printing techniques by digital printing techniques (ink-jet and bubble jet) is in full progress. Present limitations result from the availability of appropriate formulations of inks/dyes and fixation techniques. The comparable low production speed and limitations with regard to the quality of the textile material can be expected to be overcome within the next 5-10 years.

8.2.6 Finishing Processes

A great part of the variation in the final properties of a textile is adjusted for by finishing procedures, for example, wrinkle resistance, soil repellence, hydrophobic properties, flame retardance and antimicrobial properties [69]. In many cases chemicals are added by padding/ squeezing followed by drying/fixation, for example, in a stenter. Representative groups of chemicals used are:

• urea-formaldehyde resins for crosslinking of cellulose textiles, e.g., dimethylol-

dihydroxyethylene-urea (DMDHEU);

• dispersions of polymers (polyacrylesters, polyethylene, silicones);

• fluorocarbon compounds.

The applied products are fixed on the textile by drying/curing, but similar to the pad batch dyeing procedures, the last filling of the padding unit needs additional attention. A release of such concentrated finishing baths can introduce a COD of up to 200,000 mgO2/L of liquor [70].

In a first attempt the volumes of residual baths have to be optimized and a reorganization of the recipes with regard to feed of residual excess volumes of a finishing bath into similar finishing recipes is recommended [71]. If reuse is not possible, a careful check of recipes with regard to easy biodegradation/bioelimination is necessary.

8.3. END-OF-PIPE TECHNIQUES 8.3.1 First Steps

The application of end-of-pipe technologies as general procedures for the treatment of wastewater has changed from simple procedures to sophisticated concepts, applying a consecutive set of methods that has been adapted to the particular situation of a textile plant [72]. As already discussed in the previous sections, the separation of concentrated wastes and the treatment of small volumes of concentrates are much more efficient compared to a global treatment of mixed wastes.

Numerous techniques and types of equipment have been developed and tested in laboratory tests, on a pilot scale, or in full technical application. The introduction of a technique is always coupled to a general wastewater treatment concept and has to consider the individual situation of a textile producer [73-75].

As a first step, a separation of different types of wastewater into the following groups is recommended:

• Concentrated liquids: fillings of padders (dyeing, finishing), printing pastes, used dyebaths;

• Medium polluted wastes (e.g., washing, rinsing baths);

• Low to zero polluted wastes (e.g., cooling water).

Basic general procedures applied are:

• Collection and mixing of released baths to level pH and temperature maxima in the final wastewater stream;

• Adjustment of pH by neutralization. Cellulose dyeing and finishing companies mainly release alkaline baths, which can be neutralized by introduction of CO2-containing waste gas from the power/steam generation plant [76].

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