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Printing involves localised coloration. This is usually achieved by applying thickened pastes containing dyes or pigments onto a fabric surface according to a given colour design. In particular, the viscosity of a print paste is critical. It determines the volume of paste transferred to the fabric and the degree to which it spreads on and into the surface yarns. The paste must colour all the visible fibres on the printed surface, so it must penetrate somewhat into the yarn structure. If the paste is too ‘thin’, it will spread, giving poor print definition, and penetrate too far into the yarns decreasing the colour yield.
Printing was originally done by hand using wooden blocks with a raised printing surface, much as children do potato printing. The two main techniques used for transferring paste onto fabrics involve engraved rollers carrying paste in the recesses corresponding to the colour pattern, or screens with the open mesh in the pattern areas. There will be one roller or screen contacting the fabric surface for each colour to be printed.
The most important printing method today is pigment printing. This involves printing the coloured pattern onto the fabric surface and curing the printed areas by heating in air. The print paste contains coloured pigments and a binding agent. On curing in hot air, the binder forms a solid film of transparent polymer that holds the pigments in place on the yarn surfaces. The great advantage of pigment printing is that the fabric does not require washing after the fixation process. Soluble dyes used in printing a fabric are of the same types as those normally used to dye it a solid colour. Apart from in pigment printing, the usual sequence of operations is printing, drying, steaming and washing. Dyes for printing must have high solubility because there is only a limited amount of water in a thickened print paste and after drying the dyes must re-dissolve in a limited amount of condensed steam. The paste must dissolve the dyes to allow their diffusion into the fibres. It will also contain all the other required chemicals for fibre wetting and dye fixation.
The final washing removes the thickening agent, unfixed dyes and other auxiliary chemicals from the printed surface. During washing, it is critical that the dyes removed do not stain any white ground or other printed areas. For this reason, dyes for printing often have relatively low molecular weights so that their substantivity is not very high. When a manufacturer sells the same dye for both dyeing and printing, the two product formulations will invariably be different. Pigment and dye printing are both direct printing methods. Two other important indirect printing methods called discharge and resist printing also give coloured designs. In discharge printing, a uniformly dyed fabric is printed with a paste containing chemicals that destroy the colour leaving a white pattern. If the paste contains other dyes, stable to the chemical that discharges the ground colour, they can dye the treated area. In this way, multi-colour effects are also possible. In resist printing, the fabric is dyed after printing. The printed areas resist dye absorption so that these design areas are reserved. After removal of the resist agent, the design may be white or coloured if other dyes and appropriate chemicals were present in the original paste.
The rapid development of CAD computer systems for print design has had a significant impact on this activity. The development of digitised textile printing using, for example, ink jet printers is well underway. Such computer assisted manufacturing will considerably influence the textile printing industry in the near future. Its other preoccupation, as for the dyeing industry in general, is that of reducing the amounts of biodegradable and potentially harmful chemicals in the effluent leaving the works so that its environmental impact is limited.
Printing was originally done by hand using wooden blocks with a raised printing surface, much as children do potato printing. The two main techniques used for transferring paste onto fabrics involve engraved rollers carrying paste in the recesses corresponding to the colour pattern, or screens with the open mesh in the pattern areas. There will be one roller or screen contacting the fabric surface for each colour to be printed.
The most important printing method today is pigment printing. This involves printing the coloured pattern onto the fabric surface and curing the printed areas by heating in air. The print paste contains coloured pigments and a binding agent. On curing in hot air, the binder forms a solid film of transparent polymer that holds the pigments in place on the yarn surfaces. The great advantage of pigment printing is that the fabric does not require washing after the fixation process. Soluble dyes used in printing a fabric are of the same types as those normally used to dye it a solid colour. Apart from in pigment printing, the usual sequence of operations is printing, drying, steaming and washing. Dyes for printing must have high solubility because there is only a limited amount of water in a thickened print paste and after drying the dyes must re-dissolve in a limited amount of condensed steam. The paste must dissolve the dyes to allow their diffusion into the fibres. It will also contain all the other required chemicals for fibre wetting and dye fixation.
The final washing removes the thickening agent, unfixed dyes and other auxiliary chemicals from the printed surface. During washing, it is critical that the dyes removed do not stain any white ground or other printed areas. For this reason, dyes for printing often have relatively low molecular weights so that their substantivity is not very high. When a manufacturer sells the same dye for both dyeing and printing, the two product formulations will invariably be different. Pigment and dye printing are both direct printing methods. Two other important indirect printing methods called discharge and resist printing also give coloured designs. In discharge printing, a uniformly dyed fabric is printed with a paste containing chemicals that destroy the colour leaving a white pattern. If the paste contains other dyes, stable to the chemical that discharges the ground colour, they can dye the treated area. In this way, multi-colour effects are also possible. In resist printing, the fabric is dyed after printing. The printed areas resist dye absorption so that these design areas are reserved. After removal of the resist agent, the design may be white or coloured if other dyes and appropriate chemicals were present in the original paste.
The rapid development of CAD computer systems for print design has had a significant impact on this activity. The development of digitised textile printing using, for example, ink jet printers is well underway. Such computer assisted manufacturing will considerably influence the textile printing industry in the near future. Its other preoccupation, as for the dyeing industry in general, is that of reducing the amounts of biodegradable and potentially harmful chemicals in the effluent leaving the works so that its environmental impact is limited.
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