Environmental issues

action of the fibre and fibre surface wetting forces. This can lead to problems of reproducibility (e.g. when the substrate is too wet) and means that it is still necessary to use thickeners to control the rheology of the dye liquor.

The latest improvement in jet printing of carpet and bulky fabrics is now represented by machines in which the colour is injected with surgical precision deep into the face of the fabric without any machine parts touching the substrate. Here, the control of the quantity of liquor applied to the substrate (which may vary for example from lightweight articles to heavy quality fabrics) is achieved by varying not only the “firing time” but also the pumping pressure.

This system can be likened to an “injection dyeing” process. The name “injection dyeing” is used as a commercial name to define the technology applied on the latest Milliken’s Millitron machine. Another digital jet printing machine commercially available is Zimmer’s Chromojet.

In the Chromojet system, the printing head is equipped with 512 nozzles. These are magnetically controlled and can open and close up to 400 times a second (see also Section 4.7.8).

The carpet is accumulated into a J-box, and is then steamed and brushed. When it reaches the printing table it is stopped. The jets are mounted on a sliding frame that can itself be moved in the direction of the warp while the carpet remains stationary during the printing process.

Ink-jet printing is another digital printing technique with its origins in paper printing technology that is now also increasingly used in the textile industry. In ink-jet printing, colour is applied to the surface of the substrate without variation in firing time, pressure or velocity. For this reason it can only be applied for flat light fabrics, especially silk (see also Section 4.7.9).

The TAK printing system can still be found in the carpet industry. With this technique irregular patterns can be produced. The carpet, previously dyed with a ground shade, is provided with coloured spots through dripping. The size and the frequency of the coloured spots can be varied by adjusting the overflow groove placed along the carpet width.

Figure 2.26: Schematic representation of the TAK system [63, GuT/ ECA, 2000]

Printing paste residues

Printing paste residues are produced for different reasons during the printing process and the amount can be particularly relevant (Section 3.3.3.5.5 provides information about consumption and emission levels). Two main causes are, for example, incorrect measurements and the common practice of preparing excess paste to prevent a shortfall.

Moreover, at each colour change, printing equipment and containers (dippers, mixers, homogenizers, drums, screens, stirrers, squeegees, etc.) have to be cleaned up. Print pastes adhere to every implement due to their high viscosity and it is common practice to use dry capture systems to remove them before rinsing with water. In this way these residues can at least be disposed of in segregated form, thus minimising water contamination.

Another significant, but often forgotten source of printing paste residues is the preparation of sample patterns. Sometimes they are produced on series production machines, which means high specific amounts of residues produced.

There are techniques available that can help to reduce paste residues (see Section 4.7.4) and techniques for recovery/re-use of the surplus paste (see Sections 4.7.5 and 4.7.6). Their success is, however, limited due to a number of inherent technological deficiencies of analogue printing technology. Most of these deficiencies are related to the analogue transfer of the pattern, the unavoidable contact between the surface of the substrate and the applicator (screen) and the need for thickeners in the formulation (paste rheology), which limits the ultimate potential for paste re-use. Digital printing offers a solution to these problems (see Sections 4.7.8 and 4.7.9).

Waste water from wash-off and cleaning operations

Waste water in printing processes is generated primarily from final washing of the fabric after fixation, cleaning of application systems in the printing machines, cleaning of colour kitchen equipment and cleaning of belts.

Waste water from cleaning-up operations accounts for a large share of the total pollution load, even more than water from wash-off operations.

Emission loads to water are mainly attributable to dyestuff printing processes because in the case of pigment printing, although considerable amounts of waste water arise from cleaning operations, pigments are completely fixed on the fibre without need for washing-off.

Pollutants that are likely to be encountered in waste water are listed in the table below.

Pollutant Source Remarks

Organic dyestuff Unfixed dye The related environmental problems depend on the type of dyestuff concerned (these are discussed in Section 9)

Urea Hydrotropic agent High levels of nitrogen contribute to eutrophication

Ammonia In pigment printing pastes High levels of nitrogen contribute to eutrophication

Sulphates and sulphites

Reducing agents by-products Sulphites are toxic to aquatic life and sulphates may cause corrosion problems when concentration is >500 mg/l Polysaccharides Thickeners High COD, but easily biodegradable CMC derivatives Thickeners Hardly biodegradable and hardly

bioeliminable Polyacrylates Thickeners

Binder in pigment printing Hardly biodegradable, but >70 % bioeliminable (OECD 302B test method) Glycerin and

polyols

Anti-freeze additives in dye formulation

Solubilising agents in printing pastes

m-nitrobenzene sulphonate and its corresponding amino derivative

In discharge printing of vat dyes as oxidising agent

In direct printing with reactive dyes inhibits chemical reduction of the dyes

Hardly biodegradable and water-soluble

Polyvinyl alcohol Blanket adhesive Hardly biodegradable, but >90 % bioeliminable (OECD 302B test method)

Multiple-substituted aromatic amines

Reductive cleavage of azo dyestuff in discharge printing

Hardly biodegradable and hardly bioeliminable

Mineral oils / aliphatic hydrocarbons

Printing paste thickeners (half-emulsion pigment printing pastes are still occasionally used)

Aliphatic alcohols and hydrocarbons are readily biodegradable

Aromatic hydrocarbons are hardly biodegradable and hardly bioeliminable Table 2.17: Pollutants that are more likely to be encountered in waste water from printing processes

Volatile organic compounds from drying and fixing

Drying and fixing are another important emission source in printing processes. The following pollutants may be encountered in the exhaust air [179, UBA, 2001]:

• aliphatic hydrocarbons (C10-C20) from binders

• monomers such as acrylates, vinylacetates, styrene, acrylonitrile, acrylamide, butadiene

• methanol from fixation agents

• other alcohols, esters, polyglycols from emulsifiers

• formaldehyde from fixation agents

• ammonia (from urea decomposition and from ammonia present, for example, in pigment printing pastes)

• N-methylpyrrolidone from emulsifiers

• phosphoric acid esters

• phenylcyclohexene from thickeners and binders.

A more comprehensive list of pollutants potentially present in the exhaust air from heat treatment after printing, with an indication of the potential source, is given in Section 12.

2.9 Finishing (functional finishing)