Printing Processes

Printing, like dyeing, is a process for applying color to a substrate. However, instead of coloring the whole substrate (cloth, carpet or yarn) as in dyeing, print color is applied only to defined areas to obtain the desired pattern. This involves different techniques and different machinery with respect to dyeing, but the physical and chemical processes that take place between the dye and the fiber are analogous to dyeing.
A typical printing process involves the following steps:

Color paste preparation

when printing textiles, the dye or pigment is not in an aqueous liquor, instead, it is usually finely dispersed in a printing paste, in high concentration

Printing

The dye or pigment paste is applied to the substrate using different techniques, which are discussed below

Fixation

Immediately after printing, the fabric is dried and then the prints are fixed mainly with steam or hot air (for pigments). Note that intermediate drying is not carried out when printing carpets (too much energy would be needed for removing the highly viscous liquor)

After-Treatment

This final operation consists in washing and drying the fabric (it is not necessary when printing with pigments or with other particular techniques such as transfer printing).

Printing with Pigments

Pigment printing has gained much importance today and for some fibers (e.g. cellulose fibers) is by far the most commonly applied technique. Pigments can be used on almost all types of textile substrates and, thanks to increased performance of modern auxiliaries, it is now possible to obtain high-quality printing using this technique.

Pigment printing pastes contain a thickening agent, a binder and, if necessary, other auxiliaries such as fixing agents, plasticizers, defoamers, etc.
White spirit-based emulsions, used in the past as thickening systems, are used only occasionally today (mainly half-emulsion thickeners).
After applying the printing paste, the fabric is dried and then the pigment is normally fixed with hot air (depending on the type of binder in the formulation, fixation can also be achieved by storage at 20°C for a few days). The advantage of pigmentprinting is that the process can be done without subsequent washing (which, in turn, is needed for most of the other printing techniques).

Printing with Dyes

Printing Paste Preparation

The process traditionally starts with the preparation of the paste. Compared to pigment printing, the composition of the pastes is more complex and variable, being determined not by the dye used, but by the printing technique, the substrate, the application and the fixation methods applied.

Apart from the dye, printing pastes contain a thickening agent and various other auxiliaries, which can be classified according to their function as follows:

• Oxidizing agents (e.g. m-nitrobenzenesulphonate, sodium chlorate, hydrogen peroxide)

• Reducing agents (e.g. sodium dithionite, formaldehyde sulphoxylates, thiourea dioxide, tin(II) chloride)

• Discharging agents for discharge printing (e.g. anthraquinone)

• Substances with a hydrotropic effect, like urea

• Dye solubilisers, which are polar organic solvents like glycerine, ethylen glycol, butyl glycol, thiodiglycol, etc.

• Resists for reactive resist printing (e.g. sulphonated alkanes)

• Defoamers, (e.g. silicon compounds, organic and inorganic esters, aliphatic esters, etc.).

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 ofprinting 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 techniques:

• 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).

Discharge Printing

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.

Reset Printing

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.

Fixation

After printing, the fabric is dried. Water evaporation leads to an increase in dye concentration and at the same time prevents the colors 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 fibers. 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 fiber 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.

After-Treatment

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)

Ancillary Operations

• 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 appropriateprinting 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 residualprinting paste collected in this manner is then disposed of.

Overview of Printing

Textile Printing

Introduction to Textile Printing

Textile printing involves the production of a predetermined coloured pattern on a fabric, usually with a definite repeat. It can be described as a localised form of dyeing, applying colorant to selected areas ofthe fabric to build up the design.

The greatest change in fashion and design that has ever occurred in European textiles was the general introduction of printed fabrics. The first printed fabrics were produced in India and China over four thousand years ago. European textile printing dates from about the tenth century. Until relatively cheap printed fabrics became available, patterns on European dress were the result of weaving or embroidery. Such clothes could only be afforded by the wealthy.

Modern procedures for printing textile goods may be traced back to the block printing of silks in ancient China. In this method a wooden block with a raised pattern on the surface was dipped into the printing colorant and then pressed face down on to fabric. The desired pattern was obtained by repeating the process using different colors. Printing by brushing colorant through thin metal stencils and the transfer of illustrations to the printed page from engraved rollers in a printing press were also widespread by the fifteenth century.


Block printing remained a practical proposition until the roller printing machine was invented by James Bell in 1783. This enabled six colors to be printed at a rate equivalent to that of 40 hand-block printers. The success of the machine depended on the hard rollers, each of which bore an engraving (i.e. an intaglio engraving, in which the depth of the recess on the roller determines the intensity of the print produced) corresponding to a particular color component of the design. The machines were capable of continuously printing six different colors in sequence, with the rollers pressed againstthe fabric.
One of the earliest methods of textile printing was the painting of a design on the cloth using a bamboo pen or brush.The technique was widely used in India to produce a range of elaborate and beautiful designs. Indian printed fabrics began to be imported into Europe in the late sixteenth century. The designs were based upon floral motifs and these formed the basis for early European printed designs. Pencilling was used in Europe until the early nineteenth century. The method supplemented hand-block printing.

From about 1752 engraved flat copper plates were used to produce printed fabrics. The design was cut into the surface of the plate and it was these lines which held the color.The fabric and plate were tightly clamped together in a press to transfer the design to the cloth. Plate prints were made in a single color, usually either red, purple or blue. Extremely detailed images could be produced with the process.

Most plate prints were used as furnishing fabrics, although some small patterns were produced as dress material. Handkerchiefs commemorating political or public events were also produced in quantity. By the end of the eighteenth century plate printing had been almost completely replaced by copper roller printing.This type of printing was used industrially from the 1920s to produce high quality prints in small quantities.

Initially the process was a manual one with two printers passing a rubber-edged squeegee across the screen to force the paste onto fabric. The screen then had to be lifted and moved to print the next section of the pattern. In the 1950s semi-automatic and fully automatic flat screen printing methods were developed. These cut production costs significantly.

This technique is the most common form of textile printing and it involves the application of the printing paste through a fine screen placed in contact with the fabric to be printed. A design is created in reverse on the screen by blocking areas of the screen with a material such as an opaque paint. The screen is then placed overthe fabric and the printing paste is forced through the open areas of the screen using a flexible synthetic rubber or steel blade known as a squeegee.

In hand screen printing the squeegee is drawn steadily across the screen by hand at a constant angle and pressure. However, screen printing is now usually automated, with hand screen printing confined to the high fashion industry. Fully automatic screen printing involves the continuous rotation of a cylindrical screen which is kept in constant withthe fabric, ensuring continuous movement of the fabric through the machine. As the screen rotates, printing paste is forced through the design (open) areas of the screen with the aid of stationary squeegee. Printing paste is pumped into the inside of the screen from a container at the side of the machine at an automatically controlled rate.

In the mid 1950s a new type of screen printing method involving a cylindrical screen was developed.Rotary screen printing involves a series of revolving screens, each with revolving screens, each with a stationary squeegee inside which forces the print paste ontothe fabric . Twenty or more colors can be printed at the same time. The process is much quicker and more efficient than flat screen printing. Since the 1970s it has grown to dominate thetextile printing market.

Both dyes and pigments may be used as colorants in the printing process, although the mechanisms by which they are fixed to the textile are quite distinct. The same forces of dye-fiber association apply to both dyeing and printing and, in principle, the dyes used to give a plain-coloured fabric could be used to print that fabric. However, there are three important characteristics a dye must possess in order to be used in the printing process.

• The dye must first be able to dissolve in the small amount of water used in the printing paste.

• The dye must be able to diffuse at a reasonable rate from the printing paste on to the fiber, leading to preference for dye molecules with a low relative molecular mass.

• The unfixed dye must be capable of being washed off satisfactorily without staining the unprinted areas of the fabric.

Pigments are widely used in textile printing, with about 45% of all textile prints produced using pigments. Unlike dyes, they do not directly associate with the textile fibers but are fixed to the textile with a so-called binding agent. The binding agent is usually a copolymer which is incorporated into the printing paste and forms a three-dimensional film when heated.

If a textile print is washed soon after printing and drying, most of the colorant will be washed away. An appropriate fixing technique is therefore necessary. Fixing techniques are seldom completely successful and it is usually necessary to follow fixing with removal of the unfixed dye, thickeners and other auxiliary chemicals by a washing process. The efficacy of the fixing and subsequent washing process is extremely important to the quality of the print, and mistakes made at this stage of the printing process can be very costly.

Printed dyes are usually fixed by a steaming process, the steam providing the heat and the vehicle for transfer of the dye from the printing paste to thetextile fiber.

Pigment prints are fixed to the fabric simply by baking printed fabric. When the fabric has been printed to an adequate temperature the binder forms a continuous film that incorporates the pigment particles and sticks to the fiber surface. At the same time, if the temperature and pH conditions are suitable, cross-linking between the binder molecules is achieved.

During the last 20 years a digital revolution has occurred that has touched everyone's life. We are now surrounded by digital telephones, audio equipment, cameras, camcorders, TV, laser barcode readers, etc., and many homes possess a computer with access to the Internet. Digital technology has greatly affected many industries including the textile printing market, not only by the introduction of full-width jet printing machines but also in every aspect of conventional print production, from the design stage, through coloration and recipe formulation, screen manufacture and print paste preparation to the final control of the printing machine itself.

Traditional textile printing techniques may be broadly categorized into four styles:

• Direct printing, in which colorants containing dyes, thickeners, and the mordants or substances necessary for fixing the color on the cloth are printed in the desired pattern.

• The printing of a mordant in the desired pattern prior to dyeing cloth; the color adheres only where the mordant was printed.

• Resist dyeing, in which a wax or other substance is printed onto fabric which is subsequently dyed. The waxed areas do not accept the dye, leaving uncoloured patterns against a coloured ground.

• Discharge printing, in which a bleaching agent is printed onto previously dyed fabrics to remove some or all of the color.

Resist and discharge techniques were particularly fashionable in the 19th century, as were combination techniques in which indigo resist was used to create blue backgrounds prior to block-printing of other colors. Most modern industrialized printing uses direct printing techniques.

Printing Technology

Textile printing was introduced into England in 1676 by a French refugee who opened works, in that year, on the banks of the Thames near Richmond. Curiously enough this is the first print-works on record; but the nationality and political status of its founder are sufficient to prove that printing was previously carried on in France. In Germany, too, textile printing was in all probability well established before it spread to England, for, towards the end of the 17th century, the district of Augsburg was celebrated for its printed linens, a reputation not likely to have been built up had the industry been introduced later than 1676.

On the continent of Europe the commercial importance of calico printing seems to have been almost immediately recognized, and in consequence it spread and developed there much more rapidly than in England, where it was neglected and practically at a standstill for nearly ninety years after its introduction. During the last two decades of the 17th century and the earlier ones of the 18th new works were started in France, Germany, Switzerland and Austria; but it was only in 1738 that calico printing was first, practiced in Scotland, and not until twenty-six years later that Messrs Clayton of Bamber Bridge, near Preston, established in 1764 the first print-works in Lancashire, and thus laid the foundation of what has since become one of the most important industries of the county and indeed of the country. At the present time calico printing is carried on extensively in every quarter of the globe, and it is pretty safe to say that there is scarcely a civilized country in either hemisphere where a print-works does not exist.

From an artistic point of view most of the pioneer work in calico printing was done by the French; and so rapid was their advance in this branch of the business that they soon came to be acknowledged as its leading exponents. Their styles of design and schemes of color were closely followed-even deliberately copied by all other European printers; arid, from the early days of the industry down to the latter half of the 10th century, the productions of the French printers in Jouy, Beauvais, Rouen, Alsace-Lorraine, &c., were looked upon as representing all that was best in artistic calico printing. This reputation was established by the superiority of their earlier work, which, whatever else it may have lacked, possessed in a high degree the two main qualities essential to all good decorative work, viz., appropriateness of pattern and excellency of workmanship.

If, occasionally, the earlier designers permitted themselves to indulge in somewhat bizarre fancies, they at least carefully refrained from any attempt to produce those pseudo-realistic effects the undue straining after which in later times ultimately led to the degradation of not only French calico printing design, but of that of all other European nations who followed their lead. The practice of the older craftsmen, at their best, was to treat their ornament in a way at once broad, simple and direct, thoroughly artistic and perfectly adapted to the means by which it had to be reproduced. The result was that their designs were characterized, on the one hand, by those qualities of breadth, flatness of field, simplicity of treatment arid pureness of tint so rightly prized by the artist; and, on the other, by their entire freedom from those meretricious effects of naturalistic projection and recession so dear to the modern mind and so utterly opposed to the principles of applied art.