Principal and types of dyeing

Principles of dyeing:
The objective of dyeing is to produce uniform colouration of a substrate usually to match a pre-selected colour. The colour should be uniform through
out the substrate and be of a solid shade with no unlevelness or change in
shade over the whole substrate. There are many factors that will influencethe appearance of the final shade, including: texture of the substrate, construction of the substrate (both chemical and physical), pre-treatments applied to the substrate prior to dyeing and post-treatments applied after the dyeing process. The application of colour can be achieved by a number of methods, but the most common three methods are exhaust dyeing(batch), continuous (padding) and printing.

Exhaust dyeing :
In exhaust dyeing the dye, which is wholly or partially soluble in the dyebath, is transported to the fibre surface by the motion of the dye liquor or by motion of the substrate being dyed. The dye is adsorbed onto the fibre surface and ideally diffuses into the whole of the fibre. Depending upon the dye being
used, the interactions between the dye and the fibre can be either chemical
or physical. Exhaust dyeing is usually conducted using dilute solutions of
dyes, normally termed long liquor dyeing, and can involve liquor to substrate
ratios from 8:1 up to 30:1. As described above there are two main phases to
exhaust dyeing, the adsorption phase and the diffusion phase. Most exhaust
dyeing involves a temperature gradient whereby the dyeing is commenced at
a fairly ambient temperature (30–40°C) with the temperature being increased
slowly up to a final temperature which is
Depending upon the dyes being used, during the diffusion phase, changes
to the dyebath pH may be made to facilitate covalent fixation of the dye
which has diffused into the substrate.
Exhaust dyeing recipes, including auxiliaries together with the dyes, are
traditionally made up by percent weight relative to the weight of substrate
being dyed. The auxiliaries are introduced first into the dyebath and allowed
to circulate to enable uniform concentration throughout the dyebath and Temperature
Adsorption phase Diffusion phase
Time Typical dyeing profile on the substrate surface. The dyes are then introduced into the dyebath and
again allowed to circulate before the temperature is raised in order to obtain
a uniform concentration throughout the dyebath. Gaining uniform concentrations of both auxiliaries and dyes is paramount since non-uniform concentrations on the substrate surface can lead to unlevel dye uptake. The speed of dye uptake (exhaustion) of individual dyes can vary and will depend upon
their chemical and physical properties together with the type and construction of substrate being dyed. The dyeing rate also depends upon the dye concentration, the liquor ratio, temperature of the dyebath and the influence
of the dyeing auxiliaries. Rapid exhaustion rates lead to unlevelness of dye distribution over the substrate surface, so dyes have to be carefully selected when used in multi dye recipes; many dye manufacturers produce information stating which dyes from their ranges are compatible to achieve level
buildup of dye during dyeing. Dyers wish to achieve the highest exhaustion possible to minimize dye remaining in the effluent and increase batch
to batch reproducibility, whilst still obtaining the shade required by the customer. The dyeing process will eventually end in equilibrium, whereby the dye concentration in the fibre and the dyebath do not change significantly. It
is envisaged that dye adsorbed onto the substrate surface has diffused into
the whole of the substrate resulting in a uniform shade required by the customer and that there is only a small concentration of dye left in the dyebath.
This is where the final shade of the substrate is checked against the standard.
If there is any deviation from the required shade, small additions of dye may
be made to the dyebath to achieve the required shade.
Dyers wish to achieve the correct shade the first time of dyeing in order
to minimize further processing and reduce costs. In order to do this uniform
dyeing rates and high exhaustion rates of dyes are required. To achieve short
dyeing cycles, thereby maximizing production, most modern dyeing equipment is enclosed ensuring that the dyebath is maintained at the required
temperature and that there are no temperature variations within the dye-
bath. Some dyeing machines can be pressurised enabling the dye liquor to
be heated to 130°C allowing substrates, such as polyester, to be dyed without the requirement of carriers.
There are two types of machinery available for exhaust dyeing: circulating machines whereby the substrate is stationary and the dye liquor is circulated, and circulating goods machines in which the substrate and the dye liquor are circulated.

Circulating machines
The substrate, in the form of loose stock, sliver, tow, yarn or fabric, is packed
into canisters, wound onto cones or perforated beams and placed inside the dyeing vessel. The dye liquor is pumped through the substrate in two directions, in to out and out to in so that all the substrate has uniform contact with
the dye liquor. Pump speeds can be varied to obtain a range of liquor flow
rates, since the density of the compacted material will influence the liquor
flow rate. Evenness of packing is crucial in obtaining levelness throughout
the substrate. Inconsistencies in packing may lead to channeling of the dye
liquor which would result in some parts of the substrate being dyed to a
greater depth than other areas.

Circulating goods machines
Some hank dyeing machines and particularly fabric dyeing machines fall
into this category of dyeing machine. Fabric dyeing was traditionally carried out on a winch, whereby the fabric was drawn in rope form through
the dyebath via a driven winch reel, the speed of which could be adjusted
depending on the fabric length and construction. However, these have been
largely superseded by pressurized jet dyeing machines. These machines circulate both the fabric, via a winch reel, and the dyebath via the jet. The
dyebath is ‘injected’ into the fabric as it passes through the jet nozzle. Jet
dyeing machines are capable of running at very low liquor ratios, typically
8:1, although some may run as low as 4:1, enabling savings in water and
energy to be made by the dyer. Jiggers were also commonplace with winch
machines, where the fabric is mounted in whole width on a beam and guided
back and forth through the dyebath in open width.

Continuous dyeing :
Continuous dyeing is a process whereby dyeing the fabric and fixation of
the dye are carried out continuously in one simultaneous operation. This is
traditionally accomplished using a production line system where units are
assembled into lines of consecutive processing steps; this can include both
pre- and post-dyeing treatments. Fabric is usually processed in open width,
so care must be taken not to stretch the fabric. The fabric running speed
dictates the dwell time of the fabric through each treatment unit, although
dwell times can be increased by using ‘festoon’ type fabric transport. The
main disadvantage to continuous processing is that any machinery breakdown can cause ruined fabric due to excessive dwell times in specific units
whilst the breakdown is being rectified; this can be a particular problem
when stenters running at high temperatures are employed since fabrics may
be severely discoloured or burnt.
The application of dye can be conducted either by direct application, whereby the dye liquor is sprayed or printed onto the substrate, or by continuous immersion of the fabric in a dyebath and excess dye liquor
removed by squeeze rollers (padding).
Padding involves passing the substrate through a pad trough containing
the dye liquor. It is imperative that the substrate is wet out thoroughly as it
passes into the dye liquor to minimize unlevelness. The amount of dye liquor
retained by the substrate after squeezing is governed by the pressure of the
squeeze rollers and substrate construction. The amount of liquor retained is
termed ‘pick up’, a low pick up being preferable since this minimises migra-
tion of dye liquor in the substrate and saves energy during drying.
In order to obtain a uniform fi xation of dyes on the substrate, it is pref-
erable to dry the fabric after padding and before it passes on to the next
process. Drying equipment is normally infrared heat or by hot air stream
and should be contact-free to avoid marking of the substrate and soiling of
the drying equipment.
After drying, the dye is only deposited on the surface of the substrate, it
must penetrate into the substrate during the fi xation step and become part
of the substrate via chemical reaction (reactive dyes), aggregation (vat and
sulphur dyes), ionic interaction (acid and basic dyes) or solid solution (dis-
perse dyes). Fixation is performed under a number of conditions depending
upon the dye and substrate involved. Generally saturated steam at 100°C is
used for the majority of dyes. Disperse dyes are fi xed in polyester substrates
by the Thermasol Process whereby the substrate is heated to at 210°C for
30–60 s in order for the dyes to diffuse into the substrate. After fi xation sub-
strates are usually washed to remove unfixed dye and auxiliaries.

Printing :
There are a wide number of methods for applying dyes by printing tech-
niques. The most common method is direct printing whereby the dyes are
applied in the form of a print paste containing thickeners and auxiliaries.
The print paste is applied to the fabric via a roller, in the case of engraved
cylinders, or by screens in the case of fl at or rotary screen printing. Following
printing the prints are dried and steamed similar to the processes used
for padding, depending upon the dyes being printed. Each colour of the
design requires its own screen so printing machines can take up a signifi cant
amount of space, especially a fl at screen machine. Pigment printing involves
the use of binders to adhere the pigment to the substrate surface. Pigment
prints usually only require drying/curing after the printing process.
Ink-jet printing is rapidly becoming a popular method of producing pat-
terned designs on fabrics. Production speeds are not as fast as rotary screen
printing, with machines running at 20–30 m/min depending on the complex-
ity of the design being printed. Ink-jet printing poses no problems with solu-
ble dyes for printing substrates such as cotton, nylon, wool, etc., but disperse
dyes and pigments must be finely dispersed within the printing ink to avoid
blocking of the ink-jet nozzles which can be costly to replace and time consuming to unblock.