Why does clothes' colour fade when they are dried in the sun?

Why does clothes' colour fade when they are dried in the sun?

Colour is imparted to clothes by impregnating dyes into the fabric structure. There are a number of different types of dyes that are used in the textile industry which differ from each other in their basic chemical structure and their mode of application to the fabric. The fact that the dyes are brightly coloured is itself an indication that they absorb light. The absorbed light can often degrade the dye molecules, a process referred to as photochemical degradation. The extent of photochemical degradation depends strongly on the chemical nature of the dye used. A class of dyes called `direct dyes' are in general strongly degraded and hence have poor light fastness, while another class of dyes called the `vat dyes', for example, have superior light fastness. While certain additives are available, that claim to improve light fastness of some dyes, their use is not universal. An easy way to minimize fading due to light would be to dry clothes on an indoor line rather than in a hot sunny ambience!

Source: thehindu.com

How Is Dry Cleaning Of Clothes Done?

How Is Dry Cleaning Of Clothes Done?

Dry cleaning means cleansing of textiles in non-aqueous (non-water) solvents. Highly vapourising organic solvents are commonly used as dry cleaning fluids. Many dirt, soil or stains producing substances on the fabric are organic in nature (sticky oily compounds). There is a common role in this regard that like dissolves like.

In wet cleaning, washing is achieved by the combined action of soap and water. Soap acts as a mediator between oil (present in dirt) and water, which are otherwise immiscible and non-reacting, in other words unlike in nature.

In dry cleaning, cleansing is achieved by the action of a solvent, which dissolves out the sticky matter due to its like nature. Dry cleaning fluids serve as a vehicle to carry away the soil. Camphene, naphtha, benzene, benzol, petrol, carbon tetrachloride, trichloroethylene and tetrachloroethylene are some of the frequently used dry cleaning fluids.

Dry cleaning neither changes nor alters the colour of the garment but retains all the finish and all of its new appearance, whereas the wet method makes the garment fade, flimsy, soft, loose and out of shape. Dry cleaned garments facilitate the subsequent pressing of many types of garments that are exceedingly difficult and tedious to press after they have been washed in water.

In spite of these advantages, dry cleaning is not widely undertaken, as the solvents used are costly and extremely combustible.

Published in The Hindu on Jan 24, 2002.