«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Textiles Industry 87 Chapter 2 All the necessary ingredients are metered (dosed) and mixed together in a mixing station. Since between 5 and 10 different printing pastes are usually necessary to print a single pattern (in some cases up to 20 different pastes are applied), in order to reduce losses, due to incorrect measurement, the preparation of the pastes is done in automatic stations. In modern plants, with the help of special devices, the exact amount of printing paste required is determined and prepared in continuous mode for each printing position, thus reducing leftovers at the end of the run.
It is common practice in many printing houses to filter the printing pastes before application, using for example a filter cloth. This operation is especially important for thickeners to prevent free particles from blocking the openings of the screens.
Printing (paste application) After preparation, the paste is applied to specific areas of the textile using one of the following
· direct printing (which also includes digital and transfer printing) · discharge printing · resist printing.
In the case of direct printing the dye is applied to specific areas of a pretreated textile substrate, which can be white or pre-dyed (in light colours).
It is possible to speak of discharge printing, if in the fixation process that follows the application of the printing paste there is local destruction of a dye applied previously. If the etched (discharge), previously dyed area becomes white, then the process is called white discharge. If, on the contrary, a coloured pattern has to be obtained in the etched area after the destruction of the previously applied dye, then the process is called coloured discharge. In this case the printing paste must contain a reduction-resistant dye along with the chemicals needed to destroy the previous one. As a result the pre-dyed background is destroyed according to a pattern and the dye, which is resistant to reduction, takes its place.
Figure 2.15: Schematic representation of discharge printing[63, GuT/ ECA, 2000]
In the case of resist printing, a special printing paste (called “resist”) is printed onto certain areas of the fabric to prevent dye fixation. In the case of physical resist the material is printed with a difficult-to-wet resin that inhibits the penetration of a dye applied in a second stage. On the other hand, with a chemical resist, dye fixation is prevented by a chemical reaction.
Depending on the way the process is carried out, one can speak of pre-printing, intermediate or over-printing resists. One common procedure is the wet-on-wet process in which the resist paste is initially printed, then the material is overprinted with full cover screen and finally fixed and
washed. Over-printing resists can be applied only if the dye, already present in the previously dyed and dried fabric, is still in its unfixed form, as in the case of developing dyes.
Figure 2.16: Schematic representation of resist printing [63, GuT/ ECA, 2000] The difference between transfer printing and the techniques described earlier is that with this technique the surface of the fabric is not directly printed.
Instead, the pattern is first created on an intermediate carrier (e.g. paper) using selected disperse dyes and then it is transferred from there to the fabric. The dye is usually fixed by placing the printed paper in contact with the fabric into a thermal pressure system. Under the influence of the heat the dye sublimates and diffuses from the carrier into the fibre of the textile substrate. There is no need for further treatment such as steaming, washing, etc. This technique is applied for polyester, polyamide and some acrylic fibres, using selected disperse dyestuffs according to the specific type of fibre.
Fixation After printing, the fabric is dried. Water evaporation leads to an increase in dye concentration and at the same time prevents the colours from smearing when the fabric is transported over the guide rollers. At this stage the dye is not yet fixed.
The aim of the subsequent fixation step is to transport as much as possible of the dye, which is retained by the thickener, into the fibres. This is especially important with dyes, such as vat dyes, for example, that are printed in the insoluble form and are converted into the corresponding soluble state only after reaction with the reducing agents during the fixation process.
Fixation is usually carried out with steam. Water vapour condenses on the printed material, swells up the thickener, heats the print and provides the necessary transport medium for the diffusion of the dye. The distribution of the dye between fibre and thickener is an important factor in determining the fixation degree of the dye, which is called the "retaining power" of the thickener. The thickener, in fact, is often composed of polysaccharides and therefore competes with cellulose in retaining the dye. This is the main reason why the fixation rate of a given dye is 10 % lower in printing than in dyeing.
The last step of the printing process consists in washing and drying the fabric. When printing with insoluble dyes such as vat dyes this operation also serves as a means to re-convert the dye to the original oxidised state. In this case, after an initial rinsing with cold water, the printed material is treated with hydrogen peroxide. The process is completed with a soap treatment with sodium carbonate at the boiling point.
As already explained, washing is not necessary with pigment printing and transfer printing. This holds for any dyeing/ printing system where thickeners are not needed and where the dyestuff is (nearly) completely fixed (e.g. printing carpet tiles with digital jet printing techniques, see Section 4.7.8)
At the end of each batch and at each colour change various cleaning operations are carried out:
· the rubber belt, to which the fabric is glued during printing (see description below), is cleaned in continuous mode with water to remove excess adhesive and printing paste. Some machines are equipped with water re-circulation systems · the printing gears (all systems responsible for feeding and applying the paste to the substrate) are cleaned by first removing as much as possible of the paste residues and then rinsing with water. In some companies the paste residues are directed back to the appropriate printing paste batch containers for re-use · the remaining paste in the containers, in which the paste is prepared (paste vats), are in general previously cleaned up by means of sucking systems before being washed out with water. The residual printing paste collected in this manner is then disposed of.
2.8.2 Printing technology A variety of different machines can be used for printing fabrics. The most commonly used are described below.
Flat-screen printing Flat-screen and rotary-screen printing are both characterised by the fact that the printing paste is transferred to the fabric through openings in specially designed screens. The openings on each screen correspond to a pattern and when the printing paste is forced through by means of a squeegee, the desired pattern is reproduced on the fabric. A separate screen is made for each colour in the pattern.
Figure 2.17: Screen printing with automatic squeegee system[69, Corbani, 1994]
Flat-screen printing machines can be manual, semi-automatic or completely automatic. One type of machine, which is still commonly found in printing houses, can be described as follows.
The fabric is first glued to a moving endless belt. A stationary screen at the front of the machine, is lowered onto the area that has to be printed and the printing paste is wiped with a 90 Textiles Industry
Chapter 2squeegee. Afterwards the belt, with the fabric glued on it, is advanced to the pattern-repeat point and the screen is lowered again. The printed fabric moves forward step by step and passes through a dryer. The machine prints only one colour at a time. When the first colour is printed on the whole length of the fabric, the dried fabric is ready for the second cycle and so on until the pattern is completed.
Figure 2.18: Representation of a flat-screen printing machine
In other fully mechanised machines all the colours are printed at the same time. A number of stationary screens (from 8 to 12, but some machines are equipped with up to 24 different screens) are placed along the printing machine. The screens are simultaneously lifted, while the textile, which is glued to a moving endless rubber belt, is advanced to the pattern-repeat point.
Then the screens are lowered again and the paste is squeezed through the screens onto the fabric. The printed material moves forward one frame at each application and as it leaves the last frame it is finally dried and it is ready for fixation.
Figure 2.19: Representation of "Mechanised screen printing machine with stationary screens mounted in a frame" [69, Corbani, 1994] In both machines the continuous rubber belt, after pulling away the fabric, is moved downward in continuous mode over a guide roller and washed with water and rotating brushes to remove the printing paste residues and the glue, if necessary.
After this, the belt is sent back to the gluing device. In some cases the glue is applied in liquid form by a squeegee, while in other machines the belts are pre-coated with thermoplastic glues. In this case the textile is heated and
then it is squeezed by a roller or simply pressed against the rubber-coated belt, causing the glue to soften and instantly adhere.
After printing, the screens and the application system are washed out. It is common practice to squeeze the colour from the screens back into the printing paste mixing containers before washing them.
Rotary-screen printing Rotary-screen printing machines use the same principle described earlier, but instead of flat screens, the colour is transferred to the fabric through lightweight metal foil screens, which are made in the form of cylinder rollers. The fabric moves along in continuous mode under a set of cylinder screens while at each position the print paste is automatically fed to the inside of the screen from a tank and is then pressed through onto the fabric. A separate cylinder roller is required for each colour in the design.
Figure 2.20: Representation of the rotary-screen printing process [63, GuT/ ECA, 2000]
Figure 2.21: Representation of a rotary-screen printing machine [69, Corbani, 1994] A conventional paste feeding system for rotary-screen printing machines is represented in Figure 2.
22. A suction pipe leads from the paste vat to a pump, from where a printing hose leads to the squeegee (dye pipe with squeegee). From here the paste is directed inside the cylinder roller. The fill volume of this so-called printing paste input system is quite high and as a consequence the amount of paste residue that has to be removed at each colour change is also fairly significant. Various systems have been introduced in order to lower the volume configuration of this equipment, which also reduces the amount of such wastes (see Section 4.7.4). Another possibility, which has also already been implemented in some companies, is to recover and re-use these residues for making up new recipes (see Sections 4.7.5 and 4.7.6).
Figure 2.22: Printing-paste feeding system for a rotary-screen printing machine [69, Corbani, 1994]
Rotary-screen printing machines are equipped with both gluing and washing devices analogous to those described earlier for flat-screen printing. The belt is washed in order to remove the residues of paste and adhesive. Not only the belt, but also the screens and the paste input systems (hoses, pipes, pumps, squeegees, etc.) have to be cleaned up at each colour change.
In roller printing, the print paste is supplied from reservoirs to rotating copper rollers, which are engraved with the desired design. These rollers contact a main cylinder roller that transports the fabric. By contacting the rollers and the fabric the design is transferred to the fabric. As many as 16 rollers can be available per print machine, each roller imprints one repeat of the design. As the roller spins, a doctor blade in continuous mode scrapes the excess of paste back to the colour trough. At the end of each batch the paste reservoirs are manually emptied into appropriate printing paste batch containers and squeezed out. The belt and the printing gear (roller brushes or doctor blades, squeegees and ladles) are cleaned up with water.
Figure 2.23: Roller printing machine [4, Tebodin, 1991] Figure 2.
24: More recent example of roller printing machine [7, UBA, 1994]
Jet printing Jet printing is a non-contact application system originally developed for printing carpets, but now increasingly used in the textile sector.
The first commercial jet printing machine for carpets was the Elektrocolor, followed by the first Millitron machine. In the Millitron printing system, the injection of the dye into the substrate is accomplished by switching on and off a dye jet by means of a controlled air stream. As the carpet moves along, no parts of the machine are in contact with the face of the substrate. Air streams are used to keep continuously flowing dye jets, deflected into a catcher or drain tray.
This dye is drained back to the surge tank, filtered and re-circulated. When a jet is requested to fire, the air jet is momentarily switched off, allowing the correct amount of dye to be injected into the textile substrate. The dye is supplied in continuous mode to the main storage tank to compensate for the amount of dye consumed.
Figure 2.25: Schematic representation of the Millitron system[63, GuT/ ECA, 2000]
Spray printing systems and first generation jet printing methods cannot be controlled to produce a pre-specified pattern. Thus the equipment must first be employed to produce a wide range of effects and only then can selections be made from these by the designer or marketing staff.
An early improvement was made by the first digital carpet printers (Chromotronic and Titan by Zimmer and Tybar Engineering, respectively). These machines are based on the so-called “drop on demand principle”, namely the use of switchable electromagnetic valves placed in the dye liquor feed tubes to allow the jetting of discrete drops of dye liquor in a predetermined sequence according to the desired pattern.
In these machines, although the amount of dye applied can be digitally controlled at each point of the substrate, further penetration of the dye into the substrate is still dependent on capillary
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).