Overview of traditional wet processing Preparation for coloration

Enzymes can be applied in several steps of textile wet processing and in formulations of detergent powders. Preparation for coloration processes aims to prepare the textile materials to receive dyes or pigments with high fastness properties.

In preparation, all impurities and natural colored pigments have to be removed. Generally preparation for coloration is similar for all colors, but is more stringent for whites and lighter shades. Major processes during preparation are singeing, desizing, scouring, washing-off, bleaching, mercerising, carbonisation, and thermal treatments.

Singeing consists of treatment with flames to burn out fuzz fibers directly from fabrics and is applied mainly on cellulosic materials and their mixtures.

Desizing is the removal of sizes that are added to yarns to prevent breaks and stops during the weaving process. Desizing is only done on woven fabrics. Depending on the chemical nature of the size, removal could be effected by hydrolysis or oxidative processes or both.

Scouring is the removal of natural impurities of natural fibers and can be applied to fibers, yarns, knitted or woven fabrics, and garments. Scouring is done by neutral or alkaline washing with detergents.

Washing-off is the removal of lubricants added during the spinning, knitting, or weaving process to reduce friction and electrostatic energy. Washing-off is also done with detergents. Both the scouring and washing-off processes improve the hydrophilicity of the textile material and help the dyes to penetrate the fibers. Scouring is usually applied to natural fibers and washing-off is usually applied to synthetic fibers.

Carbonisation is a process applied to wool fibers to remove the vegetal soils, by treatment with sulfuric acid.

Bleaching is the removal of naturally colored pigments in natural fibers. Nowadays it is done with hydrogen peroxide in alkaline conditions and it applied to fibers, yarns, fabrics, or garments.

Mercerisation is the treatment of cellulosic fibers with highly concentrated solutions of caustic soda (300 g/L) under tension.

Thermal treatment, also called thermosetting, is used for all synthetic fibers with the aim of giving the same thermal history to the textile in order to achieve even results in further dyeing.

Coloration

Coloration is a major process in textile finishing and consists of the fixation of dyes and pigments in textile materials with high fastness properties.

There are several classes of dyes, depending on the process of application and on the chemical nature of the fiber. Major classes of dyes for cellulosic fibers are direct, vat, and reactive dyes. Major classes for protein fibers are acid and reactive dyes.

After coloration

Processes after coloration may include a variety of chemical and mechanical treatments where an effect can be added to or removed from the fabric: dimensional stability treatments, anti-crease finishing, softening, sanforization, calendering, lamination, carding, and others.

Desizing cotton with amylases

The use of α-amylases for desizing starch and their derivatives from woven fabrics was introduced almost 100 years ago. The enzymes used are mainly of bacterial origin such as Bacillus subtilis.

Enzymatic desizing is the method of choice in wetting processing routes prior to dyeing when high levels of dye fastness are demanded, owing to the fast and very efficient removal of starch.

Incomplete removal of starch might cause friction fastness problems.

Enzymatic removal of H2O2

Catalases were successfully introduced to the textile industry for the removal of hydrogen peroxide bleaching and prior to dyeing at the beginning of the 1990s. The fast decomposition of hydrogen peroxide by catalases leads to a reduction in water consumption during washing. Brighteners can also be added to the bleached cotton and prevents problems in further dyeing.

Cellulose Finishing

Cellulases are the most successful enzymes used in textile processing. They can be used to obtain an aged or renewed look for cotton fabric. Cellulase systems include the individual enzymes endoglucanases (EGs) and cellobiohydrolases.

Depilling/Cleaning Effects

Fabric or garment depilling is usually carried out after heavy processing where pills are raised. Cellulases are used for pilling removal from fabric surfaces in machinery with high levels of mechanical agitation like jets, winches, or drum washing machines.

Ageing Effects

The action of cellulases and mechanical agitation, simultaneously or sequentially, will abrade fiber surfaces, releasing cotton powder and causing defibrillation at the surface. In denim fabrics, because of enzymatic abrasion dye or dye aggregates with cotton will be released from yarns giving contrasts in the blue color.

Key Features of Cellulose Processing

In both applications, mechanical agitation is very important as it seems to create more sites for cellulase attack either because of increased diffusion into the fabric or due to the increased surface area after defibrillation.

Indigo backstaining during enzymatic washing

The redeposition of the removed indigo dye by washing on the reverse side of denim is commonly known as backstaining. A mechanism responsible for backstaining has been proposed which suggests that cellulase proteins interact with indigo, reducing indigo particle size and acting as carriers of fine indigo particles already dispersed in the bulk solution to the cotton fabric.

Cellulosic fibers

Cellulosic fibers are currently the only 'synthetic' fibers treated with enzymes. Cellulase dosages applied to regenerated cellulose fibers are lower than for cotton as the former fibers are more susceptible to enzyme attack.

Enzymes in detergents

Detergents are one of the most important markets for industrial enzymes. The function of the enzymes in detergents is to enhance the removal of soil particles by breaking them into smaller particles which can be more efficiently washed off.