Topics 01: Introduction of Wet Preparatory: Wet Processing 1
Introduction of Wet Preparatory
Where the textile materials are subjected to liquid for some modification or value addition purpose is called textile wet processing. There are 04 main categories under wet-processing, these areas:
- After-treatment or Finishing
Flow chart of wet processing:
1. Inspection: Inspection of grey fabric includes the removal of Neps, warp end breakage, weft end breakage, hole spot etc.
2. Stitching: After inspection, fabrics are stitched to make the larger lengths of fabric.
3. Singeing: Projecting or floating fibers are removed by burning.
4. Desizing: Size materials included in the textiles during sizing are removed by the desizing process.
5. Scouring: Using Alkali (NaOH, Na2CO3) for increasing absorbency & removing oil, wax & other impurities of fabric.
6. Bleaching: To remove natural color & make the fabric permanent white.
7. Mercerizing: To increase the luster & enhance dye affinity of fabric.
8. Dyeing: To produce a color effect on the fabric.
9. Printing: To imply specific color into a localized design or, pattern.
10. Finishing: Finishing is performed to impart a special effect on the fabric for consumer satisfaction. Example- Soft finish, Hard finish, Wrinkle-free finish, Compacting, stentering etc.
Pretreatment in Textile:
Pretreatment is a series of cleaning operations. Natural & synthetic fibers contain primary impurities naturally & secondary impurities added during spinning, knitting & weaving processes. All impurities which cause adverse effect during dyeing & printing is removed in pretreatment. Uneven de-sizing, scouring & bleaching in the pretreatment processes might cause deterioration in the qualities of processed products, such as uneven dyeing & a decrease in fastness.
The objective of Pretreatment:
- To convert fabric from a hydrophobic to a hydrophilic state.
- To remove natural & additive impurities from the fiber.
- To achieve the degree of desire for whiteness.
Fiber impurities refer to describe material other than fiber that is found in raw or processed stock. Fabric obtained after weaving is called grey fabric. It contains both natural and other added impurities.
It is important to remove the impurities present in grey fabric. It makes fabric suitable for dyeing and printing, The process involved in the removal of these impurities is known as fabric pre-treatment.
Impurities could be also found in man-made fibers. But the percentage is much lower than those found in the natural fibers. Here 2 types of impurities in cotton, they are:
a) Natural Impurities.
b) Additive Impurities.
Natural Impurities in Cotton:
a. Fats: Fats are tri-glycerides of fatty acids formed by a reaction between stearic acid & glycerol.
b. Wax: Waxes are esters of fatty acids formed by the reaction between fatty acid & fatty alcohol. Fatty substances act as a lubricant that is essential for the proper spinning of cotton fiber into yarn. But these fatty substances in cotton balls serve as a protective barrier to water penetration which is responsible for the poor absorbency of cotton fabric resulting in uneven dyeing or printing.
c. Pectin: Pectin’s present in cotton are derivatives of pectic acid. Pectic acid with free COOH and its Ca and Mg salts are insoluble in water.
d. Protein: Proteins present in cotton are nitrogenous compounds with amide (-CONH) groups. Proteins are produced by the reaction between amino acids and fatty acids.
e. Coloring matter: The cream color of cotton is due to the presence of two colored pigments named by Morrin & Gossypetine. It is essential to remove these impurities to improve the whiteness of cotton fabric and the brightness of colors after dyeing and printing.
Additive Impurities in Cotton:
a. Seed: The largest type of impurity found in different raw cotton. It may consist of un-ginned seeds with fibers still attached, ginned seeds or under-developed seeds and part of the seed.
b. Chaff: A collection of vegetable fragments. Most of them consist of leaf, bract and stalk. A bract is a form of small leaf growing beneath the cotton boll.
c. Dirt: Sand and soil originate from the cotton field.
d. Dust: During weaving and storage, the fabric attracts dust from the atmosphere. Dust is mechanically clean and gets removed during the chemical processing operation used for the removal of other fiber impurities.
e. Micro dust: The finest of impurities consisting of very fine particles of chaff, dirt, small fiber fragments and mildew spores.
f. Oil stains: Oil stains are accidentally formed on the fabric during the weaving operation due to the negligence of workers while oiling loom parts.
g. Trash: The term trash is often applied to the combination of all the above impurities.
h. Size materials: Size materials, used to give a protective coating on yarn to prevent due to abrasion during weaving, are present as an added impurity on the grey fabric after weaving.
Pretreatments for Cotton:
Pretreatment of Jute:
The presence of oil, wax, pectin and other mineral matters in jute makes some serious problems in dyeing, printing and finishing. For these reasons, various treatments are required for better dyeing, printing and finishing of jute products.
Pretreatments for Wool:
After shearing, wool is mechanically treated to shake out dirt. Then open the fleece to improve the efficiency of subsequent scouring. The raw wool is scoured by aqueous washing solvent scouring. It is less widely practiced. The most common treatments before dyeing wool are:
- Raw wool scouring: aqueous or solvent washing
- Scouring (desizing)
- Fulling or, crabbing or, thermo-fixing
- Easy-care treatments
- Anti-felting anti-shrinking treatments
- Wool Bleaching
Pretreatment of Silk:
The silk filament is coated with a gummy substance known as silk gum. After removal of gum, the filament becomes soft, acquires luster. Naturally, silk is cream in color which is preferred in the market. Therefore, the only pre-treatment required is for the removal of natural gums. If white silk is required, then bleaching with H2O2 is carried out.
- Degumming or Scouring.
Pretreatment of Synthetic Fibers:
Although the synthetics do not need to be given a very strong pretreatment however the possible steps in the pretreatment of synthetics are
- Heat setting.
- Bleaching. (if necessary)
Additional Question for Introduction of Wet Preparatory
- Why pretreatment is important?
- What are the types of impurities? Define with suitable examples.
- Discuss in brief different sections or divisions of wet-processing.
- Compare natural impurities & additive impurities.
Topics: Introduction of Wet Preparatory References: TARIQUL ISLAM Lecturer, Textile Engineering College