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.
References:
1. Textilelearner.blogspot.in
2. Repository.tamu.edu
3. About.com
Foam dyeing and finishing of textiles has demonstrated as an economically very
attractive technology to conserve energy. Foam finishing has a wide application on
cotton, synthetic fibres 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 the textile processing industry as a low add-on
technique.
The wet processing of textiles consumes about 60 per cent of the total, primarily to dry fabrics. Wet
processing encompasses those processes which convert fabrics off the loom or knitting machine into
finished goods. The major processes include scouring, bleaching, dyeing, printing and finishing. At each
stage huge quantities of water is also consumed.
In conventional continuous wet processing (except printing), the fabric is immersed in a bath containing a
dilute dispersion of auxiliaries and chemicals. The saturated fabric is then passed through squeeze rolls to
extract excess liquor and finally travels into a dry oven where the fabric is dried and to a high temperature
chamber where it is cured. The amount of heat energy consumed at each stage is quite enormous and
processors are forced to look at alternative sources of energy savings at each stage of processing. Foam
dyeing has come to the centre stage as a result of rapidly escalating energy costs, and the conservation of
water and energy is a subject of debate in all forums.
Modern jet dyeing machines allow dyeing of cotton below a liquor ratio of 8:1, and further reduction with
foam is not practical, because there would be insufficient water for dissolving the dyes and chemicals.
Hydrophobic fibres are dyed with essentially water insoluble disperse dyes, and dyeing at much lower
liquor ratios is practical. Dyeing of textured polyester on a one jet or soft flow machine is quite successful
at a very low liquor ratio, but on a multi-port jet there is insufficient bath to ensure even distribution of
the dyes throughout all the tubes.
Advantages of foam finishing technology
Significant energy savings as there is minimal water
Savings in dyes - auxiliaries and chemicals
Minimised down time as compared to pad dry cure technique
Can be applied on all kinds of fabric blends and compositions
Reduce environmental pollution and solid waste.
Foam dyeing and finishing of textiles has demonstrated as an economically very attractive technology to
conserve energy. Foam finishing has a wide application on cotton, synthetic fibres and their blends.
Special equipments can be designed for foam sizing, bleaching, mercerizing dyeing and finishing. Any
type of finish can be obtained using foam finishing technology.
Wet pick-up, the amount of finish liquor applied to a fabric is expressed as a percentage based on the dry
weight of the fabric. Dry add-on - the amount of solids finish applied to the fabric is expressed as a
percentage of the dry weight of untreated fabric.
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.
