Textile finishing

During textile finishing, properties are added to textile articles so that they will be appreciated by a large public. To obtain this change, the textile article has to pass through preparatory operations which will facilitate the next operations of dyeing and (chemical) finishing.

1. Preparatory operations

The preparatory operations performed are :

1.1 Desizing

Operation during which the sizing product applied to the warp yarns before passing onto the loom is removed.

In the case of woven or knitted fabrics and during spinning, the yarns are often treated with oil or wax to augment the velocity and to assure a better quality. On the other hand, these lubricants have a negative effect on dyeing (they prevent the colorants from penetrating into the fibres.

Cleaning (also called desizing or boiling off) removes the oils, waxes and other dirty spots.

Moreover, because most processes are performed under tension, what tends to elongate the fibres, yarns and fabrics, the cleaning process allows them to take back a desired shape by relaxing them.

There are two cleaning methods: desizing with water and detergent and dry-cleaning.

1.2 Washing

During the washing process, all improper products are removed from textiles such as grease or dust… that usually remain on natural fibres or dirt on chemical fibres.

In the case of wool, carbonising is often associated with washing to remove vegetable materials (thistles, straw…) from woollen fibre flocks.

1.3 Mercerising

This adds a better resistance to cotton, lustre and a higher capacity to absorb water and chemicals by modifying the internal structure (amorphous and crystalline zones).

Mercerised cotton is often used for sewing thread.

1.4 Bleaching

Certain fabrics need to be bleached before dyeing or to arrive at very white products. This is in particular so for cotton, linen, ramie, etc., since they are not white in their natural state.

Textiles that will be dyed in very pale colours deed to be bleached beforehand to obtain the right shade.
Fabrics that are white in their final usage are usually bleached and then treated with an optical azure to obtain a very bright and beautiful white

Bleaching may be done by a dyeing apparatus or on a continuous stenter. On the continuous stenter, the fabric is pulled through a series of bleaching and washing baths in its full width by means of rollers. Bleaching is either done with chloride or peroxides depending on the fibre and applied colorants. However, extreme care is needed to neutralize any residual chloride before dyeing if chloride is used during bleaching.

2. Dyestuffs

Dyestuffs used to add colour to textiles are chemical agents known as dyestuffs or pigments. Dyestuffs are most frequently used. There are hundreds of available dyestuffs. They are subdivided in several categories. Each one of them is defined by its chemical structure. The most frequently used dyestuffs are :

Direct dyestuffs: used on celluloid fibres (such as cotton, linen, rayon…). They offer a wide variety of colours but the colours are not as bright of intense as one may wish. These dyestuffs have a poor fastness to washing.

Reactive dyestuffs: used on celluloid fibres, protein fibres (wool and silk) and polyamide. These dyestuffs offer a good fastness and allow to obtain very bright colours.

Vat dyestuffs: may be used on cotton, acrylics and polyamides. They are regarded as having a better fastness than any other dyestuff class. They are used for textiles requiring colours with an enhanced fastness to commercial laundering against high temperatures and sometimes to bleaching. (Examples: uniforms, commercial tablecloths, etc). These dyestuffs contain two forms: one is reduced, the other oxidized. Both forms very often contain different colorations.

Acid dyestuffs: used on polyamide, elasthane and some specialised acrylics. These dyestuffs offer a wide variety of bright colours, but their fastnesses vary according to the different dyestuffs inside this same classification. According to the dimensions of the dyestuff’s molecule, there are three categories each of them needing a distinct pH value.

Cationic or basic dyestuffs: mainly used on acrylics. These dyestuffs produce bright colours with an excellent colour fastness. Disperse dyestuffs: used on polyester, polyamide, acetate and others. A fine colour variety is available with these dyestuffs but their colour fastness may vary considerably.

Pigments represent another category of applied dyestuffs. Whereas most dyestuffs are diluted in water, absorbed by fibres, and in most cases, chemically bonded with the fibre, pigments are deposited onto the surface. They cannot adhere to the fabric without adding a binding agent. This binding agent is usually mixed with the pigment and acts as an adhesive. Pigments are principally used for printing operations and for the coloration of melted polymers before extruding certain synthetic fibres.

3. Dyeing

Dyeing and printing are applied to colorise fabrics. The quality of the dyeing and printing is characterised by the fastness to light, water etc.

In the dyeing process, the textiles are dyed on their entire surface regardless of their presentation: fibres, slivers, hanks, fabrics or confectioned clothing.

In order to dye, one prepares a bath in which one dissolves the dyestuffs and chemicals that are necessary to the process.

By bringing the textile material into contact with the bath, the dyestuff is absorbed by the material where it remains more or less fixed.

The actual dyeing processes depend on the nature of the textile material and the type of dyestuff.

Dyeing or applying colour may be done during different steps in the production process. This is generally determined by the final use of the product and sometimes by fashion trends. The particular step where the product is dyed determines the coloration process to be used.

The five main dyeing procedures are :
· In-mass dyeing (during the extrusion of synthetic fibres)
· Fibre flock dyeing (the fibres are dyed before spinning)
· yarn-dyeing
· piece-dyeing (after weaving or knitting)
· garment-dyeing or product-dyeing

Rem. : There are special effects that can be obtained by dyeing blends, such as polyester/cotton. These are :
1. cross dyeing – produces multicolour effects in a yarn or fabric blend by selecting dyestuffs with different affinities to different fibres. When the blended fibres are dyed, each of them is dyed in a different colour.
2. Union dyeing – produces an even colour in the yarn or fabric blend. The dye bath contains different dyestuffs producing the same colour on each fibre of the blend.
3. Shade on shade dyeing is carried out when variants of the same fibre are used in the same yarn of fabric to produce different shades of the same colour in a single bath. This method is often used for carpets.

4. Printing

The objective of this technique is to print certain patterns on fabrics.

Prior to printing, one prepares a paste made of dyes, water, chemicals and a thickening agent preventing the paste to smear over the surface of the fabric.

The most widely spread printing systems use rotating machines consisting of perforated cyindres reproducing printing patterns .

The printing paste is introduced into the cylinders. By running through the cylinders’ perforations it is deposited onto the fabric.

Another and more modern printing method is called transfer printing. It consists of transferring at one the complete pattern onto the fabric on the basis of a special paper by applying heat or pressure. The printed patterns may be applied onto yarns, fabrics or confectioned garments.

Most commercial prints are made by one of the four following methods :
o Flat printing
o Roller printing
o Transfer printing
o Ink Jet printing

5. Dyestuff quality

The quality of dyestuffs and prints is determined by fastness.

In this way, a suspended curtain that looses its colour over time has a bad fastness to light and is therefore of a poor quality. A shirt loosing its colour by washing it has a bad fastness and is therefore of a poor quality.

Depending on the use to which a textile article is destined, the particular fastness characteristics are defined. For example: for a curtain a good fastness to light is required, for a shirt, a good fastness to transpiration and washing is demanded, and an upholstery fabric for a chair should possess a good resistance to friction and a good fastness to light .

6. (Chemical) finishing

(Chemical) finishing adds qualities to fabrics which they lack; it eliminates certain flaws or improves their touch and aspect.

First of all, one has to distinguish between functional and aesthetic finishing. Functional finishing improves the product’s performance under conditions of specific use whereas aesthetic treatments improve the appearance or touch (sensation) of the fabric.
Secondly, there is a distinction between chemical treatments (wet) and mechanical treatments (dry).

A third way to classify treatments is done by their degree of permanency. These classifications are :
o Temporary – the finish is removed by washing or dry-cleaning; e.g. calendaring (similar to pressing).
o Renewable – finishes that may be applied again. Examples of this type of treatment are starch and dirt repellent finishes.
o Durable – a treatment that will last the entire life of the product but with decreasing efficiency.
o Permanent – finish remaining entirely the same during the entire life of the product.

6.1 Chemical finishing

Chemical treatment to add particular qualities and characteristics to fabrics. The most commonly used treatments are :
- Crease-resistant treatment, allowing to avoid the tendency to crease of cotton fabrics.
- Shrink-resistant finish limits the tendency to shrink of cotton.
- By applying the water repellent and oil repellent finish, one avoids that fabrics absorb water and oil.
- Other finishes add specific properties to fabrics to starch and reinforce them.
- The softening finish improves the touch of the fabric.

6.2 Finishing

Mechanical or physical treatments to give particular qualities and characteristics to fabrics. The main finishing operation are :
Calendering – consists of submitting the fabric to a high pressure between two cylinders at high temperature, which will give a bright and ironed aspect. There is a variant to calendering called honeycomb by which embossed patterns are engraved on the surface of the fabric.
Raising and sueding consist of cutting certain fibres on the surface of the fabric to give them a soft and velvety aspect. Raising is obtained by grating the fabric with metallic points, and sueding by the friction of sandpaper.
Sanforising allows to prevent the cotton from shrinking when washed. It is carried out by compressing the cotton to reduce its shrinking capacity .

6.3 Coating

Application of a rubber, (polyvinyl chloride) or PU (Polyurethane) paste on one or both faces of a fabric. As soon as the coating is dry, it is firmly bonded to the fabric. As an example of coated fabrics, one can mention tarpaulin. Several coating processes (in solvant or aqueous medium) are possible.