textile dyeing

Textile dyeing is concerned with organic (that is, carbon-based) compounds that can be dissolved in appropriate solvents, usually water. The dyes in solution are absorbed on the surface of the textile fibre then pass into the interior of the material by a process called diffusion.

The process of transferring the dye from solution to the fibre is called exhaustion, with 100% exhaustion meaning that there is no dye left in the dyebath solution. An important property of a dyeing is its levelness, in other words when the same depth of colour can be seen all over the material.

Another factor is good penetration, when the dye has penetrated deeply into the structure of the fibre, colouring it from the outer surface of the fibre to its interior.

Dye molecules are attracted by physical forces at the molecular level to the textile. The amount of this attraction is known as 'substantivity': the higher the substantivity the greater the attraction of the dye for the fibre.

Think of all the garments you own and imagine the things that they have to go through before you buy them and during use. Their colour has to resist fading when they are used or left out in sunlight. They have to be suitable for repeated washing without the colour running. But there are other factors as well.For example, the colour in your swimwear must also be fast to sea water and the chlorinated water used in the swimming pool. The blouse and shirt you wear next to your skin should not discolour because of the effects of perspiration (which can be either acid or alkali).

These are the sort of issues that dye manufacturers need to consider before launching dyes onto the market.Because of the wide range of end uses of coloured textiles, many tests have been developed to assess fastness. Testing involves comparing a dyed sample that has been exposed to an agency, for example to light or to washing, with an original, to assess accurately any change in shade or change in depth of colour. Up to a certain level, changes are considered by industry to be acceptable, depending on the end use of the dyed material. If these levels are exceeded the product fails the test.In washing fastness a sample is tested with so-called adjacent fabrics to assess the extent of staining of a piece of white fabric, similar to what happens when coloured garments are washed together with whites in a washing machine.