Yellow, Pink, Orange, Red....so on and so forth. The world is full of colours and when it comes to the textile industry, importance of colours increases manifold. In the world of textile and apparel, colours are one of the factors that attract customers. This makes textile dyeing more of an art than a process. The world of textile dyeing is not oblivion to the changes in technology, consequent to this several new methods of dyeing are being developed. The new kinds of fabrics, detergents, environmental concerns are some of the issues that are kept in mind to develop new dyes in order to replace the conventional and more harmful dyes.
Another important factor is the fact that almost all the products are subjected to seasonal demand and variation. Industrial textile dyes must rise up to meet all these new and specific technical requirements.
One of the additions to the textile dyeing is foam dyeing process. As the name suggests, in foam dyeing, the main dyeing element is foam. In foam dyeing, foam is obtained from an aqueous solution and then spread on a fabric. Foaming agent and a carrier for dye stuff are also used in the process. After the fabric is covered with foam, it is put through high temperature so that the dye bonds well.
An aqueous solution is any solution in which water is the solvent i.e. it has the maximum quantity of water. Foam is a dispersion of a gas in a liquid. Here the liquid is generally water and the gas is generally air but it may also be an inert gas.
This process has an added advantage over other processes, as it involves treating a fabric with foam at low wet pick-ups. Wet pick-ups mean the amount of finish liquor applied to a fabric. Basically there are two types of foam: dispersion foam and condensation foam. Dispersion is a heterogeneous system which is made up of dispersed phase and dispersion medium. In colloidal dispersion one substance is dispersed as very fine particles in another substance called dispersion medium. A condensation reaction involves the loss of a small molecule such as water or hydrogen chloride from the reactants to form a connecting bond.
There are also two types of foam processing. In conventional continuous wet processing, the fabric web is immersed in a bath containing a dilute dispersion of the chemicals or dyes. The saturated fabric is then passed through squeeze rolls to extract excess liquor and finally travels into an oven where the fabric is dried. When necessary, the dried fabric passes into a higher temperature zone where the finish' is cured or fixed.
In continuous foam processing, the chemicals or dyes are formulated with a foaming agent in a more concentrated dispersion. The formulation is mechanically foamed, increasing its volume five to twenty fold. The resultant foam is applied as a coating to the fabric, the coated fabric is passed through squeeze rolls which collapse the foam and distribute the chemicals uniformly through the fabric. The fabric then enters the drying oven as before but the water to be evaporated is only 15 to 35%.
The foam dyeing process involves pre-washing the fabric to remove any oils or dirt that might be on the fabric. After this, soda ash is dissolved in hot water. The quantity depends on the length of the fabric. The fabric is soaked in the soda ash solution for 5-10 minutes and then wringed lightly and dried.
Following this, dye powders are dissolved in appropriate amount in the warm water and stirred into a smooth paste. Then foaming agent is added to the filtered dye solution. The mixture is whisked until well blended and thick and creamy. The mixture is then poured into a tray until about an inch deep. The dye or the foaming agent is squirted in the mixture over the mixture in the tray in a desired pattern. A tool is used to swirl and spread the dye into pattern desired. Air bubbles are removed by pushing down gently with a tool or a gloved hand. Later, the fabric is allowed to sit on the surface of the dye for 5 minutes, then it is removed carefully. A flat surface is placed with the foaming agent side up and is allowed to sit for 2 to 24 hours to develop the dye. The piece needs to remain moist for the activation to take place.
Subsequent to this, the fabric is rinsed in cool running water to remove the soda ash, and the temperature is increased to hot. Rinsing is continued until water is almost clear, then again it is washed in hot water to remove any excess dye. It is dried, ironed and the perfectly dyed fabric is ready!
The foaming agent used in the process of foam dyeing must generate foam readily and offer a good wetting capability. These apart, the foaming agent should exert fast uniform wetting action and should show little or no effect on colour fastness. It should be compatible with other products of the finishing mix and be least effected by water hardness. Another important aspect is that it should not cause yellowing of white materials and be capable of producing various bubble sizes to meet specific requirements.
Within the past few years, procedures have been developed where foams are used in different forms. The new process shows the foam could be applied on the surface of a fabric or material and still it can obtain uniform distribution or penetration of the colour in the interior of the yarn or fabric.
Foam dyeing technology is not hard on the environment like other processes. The advantages of the foam dyeing process include improved dye prefixation and migration of the dye into the fibre, higher colour yields in the fabric even after relatively short dyeing times and improved dimensional stability of the dyed fabrics. It is an energy-conserving alternative to the conventional wet processing, i.e., dyeing, printing and finishing, of textiles.
The water is ordinarily used as a medium to apply dyes or chemicals to a fabric, up to 75% of the water can be removed from the formulation and the more concentrated mix applied to the fabric as foam where air serves as the dispersing medium instead of water. Since there is less water to evaporate, less energy is needed to dry the fabric. The use of foam has led to doubling of line speeds.
Foam dyeing has also led to reduction in the consumption of chemical agents per unit of fabric because foam finishing provides for more uniform distribution of chemicals within the fabric. Depending on fabric construction and fibre composition, an amount of water equal to 70 to 100% of the weight of the fabric is evaporated in the oven.
Even before the energy crisis of 1973-74, efforts were being made to reduce water consumption in textile processing because of a decrease in the availability of acceptable quality water and the increasing costs of treating water and disposing of waste water.
The major problem associated with low wet pickup systems has been the difficulty of uniformly distributing a small volume of treating liquor over the large surface area of a fabric. Foam processing overcomes these problems. By foaming the concentrated treating liquor, its volume is considerably increased thereby minimizing the problem of uniform distribution over the fabric. The foam is metered onto the fabric, there is no excess liquor which must be removed and recycled.
Foam dyeing of textiles has been demonstrated as an economically very attractive technology to conserve energy. It is being widely used in several countries including the United States and South America.
Foam dyeing proves that a method does not need to be difficult or harsh on pockets to be called 'good'. It is actually a good alternative to other textile dyeing processes. Whatever the colour choice is from light to bright to dark, this process of textile dyeing is worth every penny.