Introduction: -
Wet process engineering is the most useful part of textile preparation and processing. It is a great flow in textile engineering, which is under the section of textile chemical processing and applied science. In the textile industry, wet process engineering plays a vital role in the area of pretreatment, dyeing, printing, and finishing of both fabrics and apparel. Any Coloration process in the fiber stage or yarn stage is also considered in the wet processing division.
What
is wet processing: -
Wet means “soggy” or “waterish” and processing means “technology”. According to textile language, wet processing is the process of dyeing yarn, fabric, or readymade garments.
Different
types of wet processing: -
All the processes main
processes of this include;
·
Singeing
- Desizing…
- Scouring…
- Bleaching…
- Mercerizing…
- Dyeing…
- Printing…
- Finishing…
Flow
Chart of Wet Processing for Knit Fabric: -
Grey fabric inspection
↓
Loading in the machine
↓
Scouring
↓
Bleaching
↓
Dyeing
↓
Hydro extractor / Dewatering
↓
Stitching
↓
Stentering
↓
Compacting
↓
Final inspection
↓
Delivery
The knitted fabrics endure a sequence of different chemical processing treatments such as scouring, bleaching, dyeing, hydro extractor or dewatering, stitching,
stentering, compacting, softener padding, relax drying, final inspection. These
processes are carried out to convey an exceptional property related to that
process like scouring for absorbency, bleaching for whiteness, dyeing to impart
color to the fabric. And finishing process also affects GSM and improves the
softness and handle of the fabric.
Chemicals
are used in wet processing: -
Major
Chemicals Used in Wet Processing: Bases, Acids, salts, surfactants, oxidizing agents, and
reducing agents are the most chemicals that are widely used in Wet
processing industry.
·
Basic chemicals: Soda
ash, Hydrochloric, ...
·
Washing agent or
soaping agent: Serafast-CRD, ...
·
Detergent and scouring
agent: Jintex-GD, ...
·
Leveling agent:
Levelex-P, ...
·
Salt: Common Salt, ...
·
Sequestering agent:
Resotext 600S, ...
·
Whitening agent:
Uvitex2B, ...
· Fixing agent: Sandofix EC,
Basic Chemicals: -
1.Acetic Acid
2.Oxalic Acid
3.Soda Ash
4.Sodium Hypo Chloride
5.Sulpuric Acid.
Scouring agent: -
Scouring agent is needed to
sort out the impurities practically such as sizing agent or
lubricating agent, blemish from weaving/knitting process and abrasion stains
which come from greige woven or knitted fabric.
Chemical used in Scouring process: -
The process of shaking the cotton cloth is
performed in a heavy container called kiers. In the alkali medium, fabric is boiled which
makes soap with free fatty acids. A kier is usually linked, so a solution
of sodium hydroxide can be boiled under pressure by excluding
oxygen which reduces the cellulose of the fiber.
caustic, to swell and dissolve the motes of dust and to saponify oils and waxes, surfactant, to lower the bath's surface tension & then it can wet-out the fabric faster and to emulsifies oils and waxes and chelating agent, to create water dispersible complexes with heavy metals.
Bleaching agent: -
Bleaching is an important part in pretreatment process of Textiles. It
helps 'whiten' the textile stuff by ruining unwanted natural coloring elements.
While
reductive method is used for Polyamide and Poly acrylates, the oxidative
chemicals are widely used for bleaching of natural fibers like cotton, linen,
jute, wool.
A ruining bleach works by moving double
bonds in the chromophore into single bonds. These impurities the capability
of the chromophore to absorb visible light. These are the mechanism of bleaches
based on sulfur dioxide.
The
commonly used application methods are:
Ø Batch or
discontinuous - Kier, Jigger, Winch, package, soft flow, jet.
Type
of Bleaching agents: -
A.
Oxidative Bleaching Agent. (Sodium Hypochlorite is NaOCl)
B.
Reductive Bleaching Agent.
C.
Enzymatic Bleaching Agent.
D.
Bleaching with hypochlorite.
E.
Bleaching Powder.
Bleaching Process
with Hydrogen Peroxide: -
Hydrogen
peroxide is made by bleaching process, given some process below-
1.Batch
wise,
2.Continuous
3.Semi
continuous method.
A.
Quantity of peroxide required in Bleaching
B.
Temperature
Cotton
fibers are bleached at 80 - 95°C in bath processes, while blends of cotton and
regenerated cellulose fibers are bleached at 75 - 80°C. The bleaching time is normally
between 2 and 5 hours. In a pressurized high temperature apparatus cotton can
also be bleached at temperatures of 110 - 130°C in only 1 to 2 hrs.
Bleaching with
per-acetic acid: -
Per-acetic
acid is produced by the chemical reaction of acetic acid and hydrogen peroxide.
It works in a very small pH range of 7 to 8. Below pH 7.0 the bleaching is not
proper and above pH 9.0 fiber wasting takes place. Per-acetic acid is used a
bleaching agent for nylon and acetate where hydrogen peroxide cannot be used.
Reductive
bleaching systems: -
Reductive bleaches work by reducing colored
impurities.
1.sodium
hydro sulphite.
Sodium hydro sulphite is available as free flowing
powder and a strong reducing agent. This is explosive in nature when
come into contact with water. It is available in different correctness volume.
2.sodium
sulphide.
Sodium sulphide is a potential reducing agent
3.sulphur
dioxide
Sulphur dioxide was used as a bleaching agent
for bleaching of wool.
Reductive bleaching of wool.
Bleaching of silk with reducing agents.
Commonly Sodium hydro sulphite, Sulfur-dioxide and
sodium sulphoxylates are the reductive bleaching agents which are used for
silk.
Soaping
agent: -
Surfactant
is the core part in the formulation of soaping agent. It has the function as
dispersion, emulsification and solubilization. Anionic surfactants commonly
used are sulfonate, sulphate, carboxylate and phosphate. Nonionic surfactant
has polyethylene glycol group, polyoxyethylene fatty acid, polyol ester and polyoxyethylene
fatty acid esters on its surface.
Soaping agent for
acid dyes: -
Soaping agent for acid dyes with polymer
compound have strong incompactness and chelating ability and strong affinity of
the hydrolysis reactive dyes and reactive unfixed dyes, and can reduce
the production process, lower water consumption, the fastness of the fabric can
likely advance together, but the temperature is 95 degree.
Mechanism of
Washing-off agents:
The washing-off efficiency of dyes would depend upon the amount of unsettled dye and comfort of removal of unfixed dye. The washing–off agent should facilitate the diffusion of unfixed dye molecules from the fiber into the water. It must extract the negative effect of hard forming ions on the solubility of unfixed hydrolyzed dye.
Generally, soaping off includes a first rinse sequence, a
soaping sequence, a second rinse sequence, and a drying sequence. Soaps
have been replaced by surfactants. But the lack of surfactants chelating properties. Thus
surfactants have been replaced by polymeric washing-off agents. they exhibit
excellent washing-off properties, together with low foaming surfactants. The
dispersing properties in the washing-off agent helps to keep the dyes, particularly,
disperse, naphtha’s in the disperse condition, thus not allowing it to
re-deposit on the fabric. In case of high degree ethoxylated products, good
dispersing properties are obtained. In order to know good techniques for
washing-off, one must understand the reasons for poor washing-off as under:
Due to the presence of electrolytes, it is very difficult to
remove the unreacted/unfixed dye from the fabric. Thus it is equitable to rinse
cold before treating with washing-off agents. It should be well noted that the
fabric has to undergo softening treatment further.
Leveling agent: -
Leveling agent is
a chemical element that works on
the dye molecule who helps in fixing the dye particles equally which determines
to obtain uniform shade. Levelling agents are also known as retarding agents.
Leveling for Cellulosic dyeing: -
Pad batch or continuous
dyeing with direct, vat, Sulphur, naphtha and reactive dyes as well as indigo
types, it is necessary to prevent the disturbing effect of inactivated ions or
any prematurely oxidized dye in order to prevent stains, markings and
inequality, during exhaust dyeing of cellulosic.
Mat level DLR liquid is planned to please the needs
which practically combines dispersing and leveling properties to equally dye
the fabric. It accepts the dyer to facilitate the process to shorten the dyeing
time by-
i) Actual stirring and
introduction of the plenty of the electrolyte previously to the dyes and
ii) linear addition of
alkali, without a dispensing process at a approved temperature in max. 30 minutes.
Classification of leveling agents: -
There are classified into two main groups
based on ionization and structure wise. given below one structure-
1. According
to the ionic nature
Ø Anionic
Ø Cationic
Ø Non-ionic [used for PET dyes]
Mechanism of leveling agent: -
Leveling agents are
surfactants & they may be anionic, cationic, non-ionic or amphoteric in
nature. The main mechanism of non-ionic agents is to form water-soluble depths
with the dye. Two main groups of ionic agents can be recognized. They are
either products that have an instinct for the dye or products which have an
affinity for the fiber. In the case of a dye-substantive leveling agent, there is
competition between the leveling agent and the fiber for the dye. Products
that have an instinct for the fiber compete with the dye for dye-sites on the
fiber. The striking forces between agent and dye, create a counterbalancing
mechanism against dye-fiber striking forces, restraining the uptake of dye by
the fiber. The complex serially breaks down releasing the dye for uniform
sorption by the fiber, as the temperature increases.
The Class of chemical used for preparation of
leveling agent has been shown below with an example is illustrated below-
leveling agent used for
dyeing nylon with acid dyes.
Limitation
of leveling agent:
Dense
of the shade is reduced by 10 to 15%. Therefore, in case of Leveling
agent add 10 to 15% more dye from the recipe.
Use of salt in dyeing: -
Industrial salt is used in the
manufacture and processing of fabrics and materials, such as for dyeing cotton
in the textile industry. When used in a dye bath, salt causes the dye
to completely penetrate the fabric, thereby making the dyeing process
easier.
Glauber's
salt is a common name for sodium sulfate dehydrate, Na2SO4.10H2O. It happens as white or colorless monoclinic crystals. These are water soluble, has a salty, bitter
taste, and are sometime used in medicine as light purgatives; that is also used
in dyeing. Void salt is the common
name of sodium chloride (NaCl).
reactive dyes contain sulphonic acid (-SO3H) group
which is insoluble in water. During the manufacturing of the reactive dyes these sulphonic acid groups are converted into the sodium
salt of sulphonic acid (-SO3Na) which is soluble in water.
Reactive dye – SO3H + Na⁺ → Reactive
dye SO3Na
when
the reactive dye goes in the water, it is solubilized giving dye anions and
sodium cations-
Reactive dye – SO3Na
+ Water — → Reactive dye – SO3⁻ + Na ⁺
………………………………………………………(Dye anion) (Sodium
cation)
Dyeing Mechanism:
-
In actual dyeing mechanism vegetable fibers contains cellulose which
ionizes in the water-
Cell – OH — → Cell – O⁻ + H⁺
While reactive dye goes in the water, it is
soubise giving dye anions and sodium cations-
Reactive dye – SO3Na + Water —→
Reactive dye – SO3⁻ + Na ⁺
……………………………………… (Dye anion)
(Sodium cation)
During dyeing both the negative
ions of dye and cellulose repels each other in the absence of salt and thus no
exhaustion or very little exhaustion is done but in the presence of salt, it
will ionize as follows,
NaCl — → Na ⁺ + Cl ⁻ (Common Salt) or
Na2SO4 – → 2Na ⁺ + SO4 ⁻ (Glauber’s Salt)
Thus the salt neutralizes the negative ion of the
cellulose and fascinating the exhaustion,
(Cell – O⁻ + H⁺ )+ (Na ⁺ + Cl ⁻ )– → Cell – ONa
Cell – ONa + SO3⁻ – Reactive dye – → Cell – O – Reactive
dye
……………………………………………………….(Exhausted dye on the substrate)
Sequestering agent
During dyeing for removing hardness of water Sequestering agent used.
It is dyeing auxiliaries. It is combined with calcium and magnesium
ions and other heavy metal ions in hard water. It forms molecules in which the
ions are held so sequestered that it can no longer react.
EDTA is a well-known
sequestering/chelating agent [ Ethylene di-amine tetra-acetic Acid (EDTA)],
fairly large amounts of EDTA must be used.
NTA, and DTPA react
stoichiometrically (on an equal mole basis) with ions.
Wetting agent: -
Wetting agents are lower
the surface tension of a liquid, allowing easier spreading, and lower the
interfacial tension between two liquids. Basically it is a surfactant. The
wetting agents make the fiber surface clean and help improve the solidity of
reactive and disperse dyeing.
Wetting agents are a type of surfactants.
For example, the formation of micelles is a outcome of adding wetting agents to a liquid. Usually, a micelle contains hydrophilic heads, which forms an outer layer around lipophilic tails. In water, the tails of the micelles can surround an oil droplet, whereas the heads are charmed to the water.
Knit Dyeing process: -
Knit dyeing is a technique of dyeing knitted fabrics. The Knit dyeing process is similar to the yarn dyeing process but there is some difference in quality measurement issue. Basically, all types of single jersey, double jersey, and their derivatives & different colors are dyed in a different way. Flow chart & curve of knit dyeing describes briefly.
Dyes 60 min. →
Salt Soda dossing Time (min)
Leveling agent
Sequestering agent
Additional water
Fabric
Figure: - Process curve for
dyeing of cotton fabric with reactive dye.
Dyeing process for critical color: -
PH-5-5.5
↓
Color Dossing 20 min
↓
Run time 10 min
↓
Run time 6 min
↓
Salt Dossing 50% 20
min (Bath water)
↓
Run time 6 min
↓
Salt Dossing 50% 20
min (Bath water)
↓
Run time 10 min
↓
Salt Dossing 50% 20
min (Bath water)
↓
Rise temp. 600c (10c
)
↓
Run time 20 min
↓
Soda rest 30 min (Bath
water)
↓
Run time 40 min
↓
Shade check
↓
After dye, bath drain
↓
Cold wash 10 min
↓
Drain
↓
Acid wash 6 min
↓
Drain
↓
Hot wash 700
x 6 min
↓
Drain
↓
Soaping 950 x
6 min (Temp 80-900)
↓
Shade check
↓
Drain
↓
Hot wash 700 x
10 min
↓
Fixing (If required PH
4.5-5)
Dyeing process for Light Color:
PH-5-5.5
↓
Color dossing 20 min
↓
Run time 6 min
↓
Salt Dossing 45 min
(Bath Water)
↓
Run time 10 min
↓
Soda Dossing 45 min
↓
Rise temp 600c (10/min)
↓
Run time 40 min
↓
Shade Check
↓
After Dye Bath Drain
↓
Cold wash 10 min
↓
Drain.