Enzyme Washing

In enzyme washing, cellulase enzymes are used. Hydrolysis of the cellulose, which is catalysed by cellulase, causes the surface fibres to become weakened and later they get removed when there is either fabric-to-fabric abrasion or fabric-to-stone abrasion during washing. The temperature and the pH used are specific to the type of cellulase employed. Usually neutral cellulases are applied at pH 6–7, while acid cellulases are applied at pH 4.5–5.5. However, the latter result in a greater extent of back staining, being more penetrative. An enzyme dose of 2–4 g/L is normally enough, provided that the enzyme activity is not impaired. In general, the colour of the enzyme washed goods is more uniform, particularly when stone is not added. Because cellulases are reactive only on cellulose, any size materials or other impurities must be removed before the cellulase treatment.

Temperature is the most important factor in enzyme treatment. Enzyme reaction increases with temperature, but it is only activated within a temperature range in which the enzyme structure remains stable and unchanged. Beyond this optimum range, the enzyme activity decreases sharply as the protein structure of enzyme is tangled through thermal agitation. There is a trend of increase of colour fading effect when temperature increases from 50°C to 60°C. This is because the enzyme reaction is activated by higher temperature within the optimum temperature range and thus more surface fibres are hydrolysed by cellulase and the weakened fibres are removed by abrasion of fabrics and mechanical agitation. The indigo dye particles are also removed along with the cotton fibres.

The mechanical action can provide colour fading effect of denim garments in enzyme washing. The specimen treated without any agitation has better colour fading effect than that treated with mild and vigorous agitation because the fibres are weakened after treating with enzyme alone and are not well removed by mechanical agitation. The surface of fabric becomes hairier and a layer of fuzz gets formed on the surface. For specimens treated under vigorous agitation, a biopolishing effect is obtained. The weakened fibres are well removed by strong mechanical agitation, and thus a cleaner and smoother surface is produced, and the fabric has a lighter shade as dye particles are also removed with cellulose fibres.

Longer enzyme treatment time prolongs enzymatic degradation of cellulose and the time for further abrasion. It is shown that the colour fading effect increases with increased treatment time. The increase in colour fading effect is mainly due to desorption of dye particles that are weakly adsorbed on fabric. In addition, the increased colour fading effect with more time is also due to the fuzziness of fabric caused by prolongation of the enzyme treatment. With a longer treatment time, cellulase effectively hydrolyses fragments of cotton fibrils and thus the fabric is less fuzzy than the original. Usage of cellulase has some distinct advantages, such as:

• Cellulase is economical and environmentally friendly as compared with stone washing using pumice stones. The percentage of fabric damages is reduced with cellulase treatment. The enzymatic treatment of denim fabric ensures the same result but consumes less water and time, resulting in less waste and damage to machines. The pollution, quality variability and imperfections are also reduced in enzymatic treatment.
• Enzymes can be recycled.
• The productivity of washing is increased due to the space formerly taken up by the pumice stones; the same washing machines can handle more jeans.
• The time consumption for removing stone fragments from the denim garments is eliminated in the case of cellulase treatment.
• The duration or number of rinses washing after enzyme treatment is less than pumice stone washing.
• A small quantity of enzyme can replace several kilograms of pumice stones during washing, which ultimately leads to less damage to garments and machines.
• Washed garments with a softer feel and better appearance is achieved in cellulase treatment.

Cellulase treatment of denim fabric has some disadvantages, too. There may be chances of back staining in cellulase treatment. To remove the back staining, the garments are rigorously washed. However, this adds to usage of water for the washing.