Modern Dyeing Machinery and Equipment in Dyeing Process

Modern dyeing machines are made from stainless steels. Steels containing up to 4% molybdenum are favored to withstand the acid conditions that are common.

A dyeing machine consists essentially of a vessel to contain the dye liquor, provided with equipment for heating, cooling and circulating the liquor into and around the goods to be dyed or moving the goods through the dye liquor. The kind of machine employed depends on the nature of the goods to be dyed. Labor and energy costs are high in relation to total dyeing costs: the dyers aim is to shorten dyeing times to save steam and electrical power and to avoid spoilage of goods.

The conical-pan loose-stock machine is a widely used machine. Fibers are held in an inner truncated conical vessel while the hot dye liquor is mechanically pumped through. The fiber mass tends to become compressed in the upper narrow half of the cone, assisting efficient circulation. Leveling problems are less important as uniformity may be achieved by blending the dyed fibers prior to spinning.

The Hussong machine is the traditional apparatus. It has a long, square-ended tank as a dye bath into which a framework of poles carrying hanks can be lowered. The dye liquor is circulated by an impeller and moves through a perforated false bottom that also houses the open steam pipe for heating. In modern machines, circulation is improved at the points of contact between hank and pole. This leads to better leveling and elimination of irregularities caused by uneven cooling. In package-dyeing machines dye color may be pumped in rather two directions:
Through the perforated central spindle and outward through the package or
By the reverse path into the outer layers of the package and out of the spindle. In either case levelness is important.
Some package-dyeing machines are capable of working under pressure at temperatures up to 130C.

The winch is the oldest piece of dyeing machine and takes its name from the slated roller that moves an endless rope of cloth or endless belt of cloth at full width through the dye liquor. Pressurized-winch machines have been developed in the U.S.

In an entirely new concept, the Gaston County jet machine circulates fabric in rope form through a pipe by means of a high-pressure jet of dye color. The jet machine is increasingly important in high-temperature dyeing of synthetic fibers, especially polyester fabrics. Another machine is the jig. It has a V-shaped trough holding the dye color and guide rollers to carry the cloth at full width between two external, powered rollers, the cloth is wound onto each roller alternately, that is, the cloth is first moved forward, then backward through the dye color until dyeing is complete. Modern machines, automatically controlled and programmed, can be built to work under pressure.

Direct Dye | Types of Direct Dyes | Application of Direct Dyes |After Treatment of Direct Dyed Material

Direct Dyes


Direct Dyes are molecules that adhere to the fabric molecules without help from other chemicals. Direct dyes are defined as anionic dyes with substantivity for cellulosic fibres, normally applied from an aqueous dyebath containing an electrolyte, either sodium chloride (NaCl) or sodium sulfate (Na2SO4)..
The dyeing process with direct dyes is very simple, Direct dyeing is normally carried out in a neutral or slight alkaline dyebath, at or near boiling point , but a separate aftertreatment such as cationic dye fixing , to enhance wet fastness has been necessary for most direct dyeing .

Direct dyes are used on cotton, paper,leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.

Chemicals nature of direct dyes:

Chemically they are salts of complex sulfonic acids.

Structure:-More than 75% of all direct dyes are unmetallised azo structures, great majority of them are disazo or polyazo types.

Ionic Nature:-Their ionic nature is anionic.

Solubility:-They are soluble in water .

Affinity:-They have an affinity for a wide variety of fibers such as cotton ,viscose, silk jute ,linen etc.. They do not make any permanent chemical bond with the cellulosic fibers but are attached to it via very week hydrogen bonding as well as vander waals forces. Their flat shape and their length enable them to lie along-side cellulose fibers and maximize the Van-der-Waals, dipole and hydrogen bonds.

Types of direct dyes:

The SDC classification of direct dyes is follows

(1) Class A – dyes that are self-levelling, i.e. dyes of good migration or leveling properties.

(2) Class B – dyes that are not self-levelling, but which can be controlled by addition of salt to give level results; they are described as salt-controllable.

(3) Class C – dyes that are not self-levelling and which are highly sensitive to salt, the exhaustion of these dyes cannot adequately be controlled by addition of salt alone and they require additional control by temperature; they are described as temperature-controllable.

Application of Direct Dyes

Direct dyes are usually applied with the addition of electrolyte at or near the boil in the machines capable of running at atmospheric pressure .But in HTHP dyeing machines it is carried out at temperatures above the boil in case of pure as well as blended yarns.

An addition of alkali, usually sodium carbonate, may be made with acid-sensitive direct dyes and with hard water as well as to enhance the dye solubilisation. When cellulose is immersed in a solution of a direct dye it absorbs dye from the solution until equilibrium is attained, and at this stage most of the dye is taken up by the fibre. The rate of absorption and equilibrium exhaustion vary from dye to dye. The substantivity of the dye for cellulose is the proportion of the dye absorbed by the fibre compared with that remaining in the dyebath.

Dyeing Method

The color is pasted well and dissolved in boiling water to get a lump free solution .An addition of 0.5–2 g l–1 sodium carbonate may be advantageous when applying dyes of only moderate solubility in full depths.

The dyebath is set at 40°C,
Raise to the boil at 2 degC min–1
Hold at the boil for 30–45 min,
During hold add 10–15 g l–1 of sodium chloride or calcined Glauber’s salt. Light shades are dyed without or lesser addition of salt.
Improved yields can be achieved when applying full depths by cooling to 80°C at the end of the period at the boil, adding a further 5 g l–1 salt and rising to the boil again
Dye bath variables which must be considered for level dyeing,

1.Temperature of Dyeing and rate of heating
2.Electrolyte concentration and addition
3.Time
4.Dye solubility
5.Use of leveling agent

After treatment of Direct dyed material

The wet fastness properties (particularly washing, water and perspiration) of virtually all dyeing of direct dyes are inadequate for many end uses but notable improvements can be brought about by after treatments.
Diazotisation and development
Metal salt treatments
Cationic fixing agents
Formaldehyde treatment
Crosslinking agents and resin treatments

Stripping:- Most direct dyes can be stripped of the use of stripping salts (Sodium Hydrosulphite) and/or by using a chlorine bleaching agent such as sodium hypochlorite, without harmful effects on the fibres.

Color fastness properties of Direct Dyed material:- Generally these dyes are used where high wash fastness is not required.

Wash Fastness:- poor unless treated with suitable dye fixing agent and/or fastness improving finishing agent.
Light Fastness:-Good
Rubbing Fastness:- Moderate to Good
Chemical Wash Fastness:- Poor