Textile finishing encompasses all the treatments the fabric is subject to, i.e. scouring, bleaching, dyeing and/or printing and the final chemical-mechanical operations, in order to improve its dyeability, printability, absorbent qualities, colour, hand and appearance.
The various finishing processes can be classified as follows, depending on the type of fibre:
- Colorouring (dyeing – printing)
- Final finish
The materials to ennoble are prepared for dyeing, printing or finishing. The auxiliary substances used in spinning, weaving or heddling (oils, waxes, sizes, etc.) and the natural impurities still in the fibres are eliminated in this phase so as to obtain the degree of purity required for the next process.
Singeing: this process removes the protruding hairs to have a smoother fabric surface.
Sizing: this is done on orthogonally woven fabrics so as to completely remove the size from the warp yarns. It makes the fabric uniformly absorbent for the following processing baths.
Washing/Scouring: the purpose of this process is to eliminate the fatty and peptic substances, to soften the shells and make the material suitable for absorbing the next set of treatments.
Bleaching: its purpose is to eliminate any impurities and unwanted colours. The aim is to obtain pure whites and to prepare the bases for light dyes. All the fibre's natural colourings disappear with bleaching.
Mercerising: a typical treatment of cotton fabrics and yarns, performed in a caustic soda solution. This operation shortens and swells the fibre which becomes translucid and increases its strength. It has the effect of making fabrics shinier, improving their dye and they have a more consistent hand making them much more resistant.
Dyeing: to dye a textile product it has to be treated with an aqueous solution or with colour dispersions with the addition of various substances: salts, alkalis, acids and other auxiliaries. The dyes dissolved in the bath must be absorbed and penetrate the fibre.
The type of fibre to dye has an important role in this phase. Synthetic fibres, for example, absorb the colour only at temperatures higher than 100°C.
Printing: print is a localised dye, the purpose of which is to have different coloured patterns on the fabrics. To have a sharply defined pattern special dyes are used called "printing pastes".
After dyeing, printing and other ennobling processes, other operations are necessary to prepare the fabric for the next finishing process.
Fixing/Steaming: the dyestuffs applied must be fixed meaning they have to become one with the fibre. This is achieved mainly by the use of steam treatment whereby the fabric is quickly heated and penetrated by the steam that accelerates the spreading of the dyestuff in the fibre.
Washing: washing is necessary in the preliminary and intermediate treatments for eliminating impurities, residues of oils and size, also residues of dye that has not fixed and other substances used for printing.
Heat setting: the mechanical tensions created inside the fibres are regularised with heat setting. It can be done as a preliminary, intermediate and final treatment. With a hot (180°C) followed by a controlled cold treatment, the fibre is set at minimum tension conditions so the fabric becomes unshrinkable, it has good dimensional stability and stays smooth after washing.
This operation is only done for fabrics containing Polyester, Nylon and Lycra.
Finishing or dressing comprises a series of operations that prepare the textile material for the use wanted. The final finish is often a combination of different finishing processes but which cannot be randomly combined because some of them are not compatible. In dry finishing (mechanical), the effect wanted is achieved by means of physical processes.
On the other hand however, chemical substances are used to achieve the effects wanted with wet finishing.
Raising: this is an operation for wool and cotton fabrics and it helps form a certain hairiness on the surface of the fabric.
Calendaring: this makes the fabric smooth, compacting it by compression, imparting glaze to it and, if wanted, printing decorative patterns on it. Depending on the conformation of the surface, on the temperature and on the different cylinder speeds, different aesthetic effects can be obtained.
Shrinkage (Sanfor): the fabric goes through a rubber ring and heated steel cylinder that, by exerting pressure on the fabric, compacts the wefts and, as a result, stabilises the warp. Drying on the shrinkage plate and final cooling guarantee that the fabric will not shrink when subject to the following wet treatments. Finishing the fabric with this procedure imparts high dimensional stability to the garments even after numerous washes.
[Source: Cotonificio Albini]