Foam dyeing & finishing: A step towards sustainable processing of textiles

Foam dyeing and finishing oftextiles has demonstrated as an economically very attractive technology toconserve energy. Foam finishing has a wide application on cotton, syntheticfibres and their blends. Senior industry consultant, CN Sivaramakrishnan, who has over 35 years'experience in wet processing and specialty chemicals manufacturing industries,explains why foam dyeing & finishing has caught the attention of thetextile processing industry as a low add-on technique.

The wet processing of textiles consumesabout 60 per cent of the total, primarily to dry fabrics. Wet processingencompasses those processes which convert fabrics off the loom or knittingmachine into finished goods. The major processes include scouring, bleaching,dyeing, printing and finishing. At each stage huge quantities of water is alsoconsumed.

In conventional continuous wetprocessing (except printing), the fabric is immersed in a bath containing adilute dispersion of auxiliaries and chemicals. The saturated fabric is thenpassed through squeeze rolls to extract excess liquor and finally travels intoa dry oven where the fabric is dried and to a high temperature chamber where itis cured. The amount of heat energy consumed at each stage is quite enormousand processors are forced to look at alternative sources of energy savings ateach stage of processing. Foam dyeing has come to the centre stage as a resultof rapidly escalating energy costs, and the conservation of water and energy isa subject of debate in all forums.

Depending on fabric construction and fibre composition, an amount of water equal to 70-100 per cent of the weight of the fabric is evaporated in the heating chamber. More than half the energy consumed in drying is used to heat the air which exhausts the evaporated water and vaporised smoke from the dryer. Since there is very less or no residual water and only a minimal amount of smoke generated in the curing zone, it is often curtained off from the drying zones, so that only a small amount of air is heated to the cure temperature.

The major problem associated with low wet pick-up systems has been the difficulty of uniformly distributing a small volume of treating liquor over the large surface area of a fabric. The problem is exacerbated when hydrophilic fibres such as cotton are in the fabric. When conventional fabric saturating systems are coupled with high extraction techniques, economics require that the extracts be recycled. This can lead to problems of contamination of extract by residual materials in the fabric and alteration of the composition of the recycle liquor due to preferential absorption of one or more components of the treating liquor by the fabric.

Foam processing overcomes these problems. Foam is a dispersion of a gas in a liquid. Here the liquid is generally water, and the gas usually is air. A wet processing which uses air in the form of dispersion foam is called foam finishing. By foaming the concentrated treating liquor, its volume is considerably increased thereby minimising the problem of uniform distribution over the fabric. The foam is metered onto the fabric to ensure that there is no excess liquor which must be removed and recycled.

In continuous foam processing, chemicals or dyes are formulated with a foaming agent in a more concentrated dispersion. The formulation is mechanically foamed, increasing its volume 5-20 fold. The resultant foam is applied as a coating on the fabric, the coated fabric is passed through squeeze rolls which collapses 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 15-35 per cent. Through a combination of higher line speeds and lower dryer temperatures, energy consumption can be reduced anywhere between 40 per cent and 70 per cent.

Keeping the energy factor in mind, Sandoz developed the Sancowad process in 1971 to investigate the possibility of dyeing in liquor ratios of the order of 1.5 to 1, by converting the water phase into micro foam. Foam processing is an energy-conserving alternative to the conventional wet processing. Foam application of dyes offers potential savings in materials and energy, faster production and the possibility of no wash-off processes. The process converted water phase into micro foam. The machine was originally developed for solvent dyeing.

In the Sancowad process, the aqueous dye baths are diluted with air using a foaming agent, thus reducing the liquor ratio and the amount of chemicals. The process was for beck dyeing of cotton, using a 3 per cent foaming agent (Sancowad) and reducing the liquor ratio from 20:1 to 8:1. An oval reel to plate the goods in wide folds, and a driven lifting roller were required for adequate circulation of the goods in the foamed dye liquor.

Foam dyeing & finishing has caught the attention of the textile processing industry as a low add-on technique of processing operation. Foam, instead of water, is the medium used to transfer dyes and chemicals, resulting in low energy consumption as minimal drying and thermal operations are involved. The foams used for textile dyeing and finishing are invariably generated from a dispersion of air in the aqueous dyeing or finishing liquor. Other advantages include higher production speeds, lower usage of dyes and chemicals and overall improvement in the quality.

The last two advantages make the investigation of the transport phenomena of water and foam in the textile substrate very exciting. With the exception of foam density, all variables are of great importance in conventional application processes. The wet pick-up and viscosity are of great importance in every operation, because this is where maximum heat energy is consumed. The group of inter-connected variables, fabric velocity; contact length and contact time is very significant. In processes in which the fabric is completely immersed, the time during which immersion takes place is influential in determining the extent to which and exchange of matter can occur.

In a foam dyeing machine, the entire liquor is discharged through a spray into the drum; the rotation is continued in cold state for 25 minutes during which the foam bearing the dye is distributed uniformly throughout the charge. After 25-30 minutes, the temperature of the machine is raised to 90°C or any temperature depending on the selection of dye and type of fabric to be dyed, by steam or hot air and the machine is run for one hour during which fixation of the dyes takes place. The entire cycle may be so programmed to carry out any desired sequence of operations such as dyeing, rinsing, finishing, centrifuging and drying. This results in substantial savings in both water and energy as water consumption is significantly lower than conventional processes.

The continuous methods of foam dyeing have the following steps:

•         Foam generation

•         Foam application to the substrate

•         Foam distribution with simultaneous drainage and diffusion of the liquid into the substrate Foam collapse and release of active substance.

•         Fixation of the active substance.

Foam finishing technology involves the use of a rapidly-breaking low-density foam or froth as the delivery medium for finishing chemicals, clearly defined metering and flow control system for the transfer of foam to the substrate, pressure-driven impregnation of the foam into the substrate, an applicator system to allow uniform high-speed application and collapse of the foam in a single step. The semi-stable foam is necessary to get spontaneous foam collapse and spreading though the substrate, and is in contrast to stable foams specified in various foam-coating processes normally requiring a separate step to break and distribute the foam through the textile material. The process leads 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.

Foam application techniques permit liquor to be applied at the pick-up values of 25-35 per cent for cotton and 10-20 per cent for synthetics. In foam generator or foam mixer, compressed air is driven through a closed system into the liquor, which is foamed forcibly by the pressure. From the time the foam exits from the mixing head, the pressure gradually declines and by the time it flows from the outlet pipe, it has a fine cell structure. Generally, generators are fitted with elaborate setting and control devices, which enable foams of constant composition. Of utmost importance which has to be closely monitored is the wetting power and the rate of foam collapse. Besides collapsing of foam, the arrangement improves penetration of dyes and chemicals.