Textile Printing | What is Textile Printing | Pigment Printing | Process of Textile Printing | Printing Paste Preparation

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 of the fabric to build up the design.Textile Printing, like Textile 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

Textile 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).

Pigment Printing:

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 pigment printing is that the process can be done without subsequent washing (which, in turn, is needed for most of the other printing techniques).

Printing Paste Preparation:

Dye Printing 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 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 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).