Monday, May 22, 2017

Pretreatment of Textile Wet Processing

Pretreatment processes:
Wet processing is a versatile field in the textile processing. It is carried out in aqueous stage on textile substrate for various processing stages such as scouring, bleaching, dyeing, printing and finishing. Generally textile wet processing means those steps by which the grey fabric from loom or knitting machine is processed to finished fabric which is then used for final target product. Textile wet processing stages are primarily classified under three heading: Pre-treatment, Dyeing/Printing, and Finishing.
Textile pretreatment process
Fig: Textile pretreatment process
Textile pretreatment is the heart of the textile dyeing, printing and finishing process because fibers, yarns or fabrics have many impurities like sized polymers (Starch, PVA, polyacrylate, CMC, CMS), cotton wax, lubricant, silk sericin, wool oil etc.

Flow chart of pretreatment process of textile wet processing:

Inspection of grey fabric
(Assessment of different types of faults)

(Adding two or more than two fabric together)

Searing and cropping
(Removal of excess thread from the fabric)

(Removal of short fiber ends from fabric surface by burning)

(Removal of sizes, which was applied as weaving aid)

(Removal of natural or adventitious impurities)

(Destruction of natural coloring and improving whiteness)
(Neutralize the alkali with an acid)
(Treatment of cotton with strong alkali under tension) 

 (Wash the fabric with hot and cold water)
(Washing with clean water to remove soap, detergent, dirt or impurities from fabric)
(Removal of excess water from fabric

All the above pretreatment process in textile are briefly described below:

1. Inspection of grey fabric:
The fabric found from loom or knitting machine is passed through the inspection table to find any kind of fault such as end breakage, picks, holes, neps etc. and then fault free fabric is selected for subsequent processing. It may be manually or online done.

2. Stitching:
Sometimes the woven fabric from loom is not of enough length as required and in such cases the fabrics are joined to make it larger through stitching.

3. Shearing and cropping:

The excess warp or weft yarn on fabric edges are removed by cutting with blade or scissors, this process is called searing and cropping.

4. Singeing:
A singe is a slight scorching, burn or treatment with flame. The process of removing loose, hairy and projecting fibers from fabric surface by burning is called singeing and that uses gas flame or hot plate in order to get rid of small fibers protruding on the fabrics (these fibers normally come from spun yarns). It has the following objectives:
  • To get rid of the small fibers.
  • To make the fabrics smoother than before.
  • To help the printing ink or dyestuff to make clearer marks on the fabrics.
  • To improve rubbing fastness and washing fastness of the dyed or printed fabrics.
5. Desizing:
In desizing process, sizing material is applied to the warp yarn before weaving is removed to facilitate the penetration of dyes and chemicals in the subsequent wet processing operations. The main purpose of sizing is to form coating around the cotton warp yams so as to stand the tension during weaving and reduce the breakage. The surface coating of sizes are stiff, hard, smooth and less absorbent to water. If this size material is not removed from fabric then dyes and chemicals don’t react with fabric properly. As a result dyes and chemicals become wastage and don’t get actual dyeing shade on fabric. So we can say that desizing is very important process of textile wet processing.

For knowing details about Desizing, you can also read: Desizing Process in Textile Industry

6. Scouring:
Scouring is a process by which all natural and adventitious impurities such as oil, wax, fat etc. are removed to produce hydrophilic and clean textile material. Absorbency of the fabric also increases a greater extent in this process i.e. makes the fabrics highly hydrophilic. It also helps to clean textile material by adding alkali. It is a vital process of wet processing. It also helps to remove natural color and make the fabric for next process.

For knowing details about Scouring, you can also read: Scouring Process in Textile

7. Bleaching:
Textile bleaching is a chemical treatment process. In this process, destructing natural coloring matters and remaining trace impurities are removed from the grey fabric. Bleaching process is done to impart a pure permanent and basic white effects which is suitable for the production of white finishes, level dyeing and desired printed shade with minimum or no degrading or without diminishing the tensile strength.

For knowing details about Bleaching, you can also read: Textile Bleaching Process

8. Souring:
Bleaching is done in the alkaline medium. Since it is difficult to remove alkali from the fabric this is followed to neutralize the alkali with an acid, the excess of which can then be easily washed out. This process is known as “souring” in which the bleached material is treated with dilute H2SO4 or HCl at room temperature.

9. Mercerizing:
Mercerizing is a chemical process to improve the luster and increase strength. It also has some other engineering advantages. In this process the fabric is treated with specific concentration of alkali (NaOH) under tension. This is to make the cotton goods more luster than before using sodium hydroxide solution in cold condition and tension. This will lead the cotton fibers to be swollen and increased strength. Also this will improve dye absorption of the fibers. Jhon mercer was the first chemist in the world who found this phenomenon in 1844. Therefore, the process was called mercerizing.

Causting is the process nearly the same as mercerizing but it uses lower concentration. To make the goods more luster because raw cotton fibers are kidney-shaped. After this process, the fibers change to cylindrical shape resulting more evenness reflected light from the goods. To improve dyeability of the goods about 5-10%.

10. Washing:
Normally after every wet process, the fabric is washed with hot and cold water. Sometimes soap solutions are also used for washing.

11. Rinsing:
The process of washing something with clean water to remove soap, detergent, dirt or impurities from fabric is called rinsing.

12. Drying:
Drying means the removal of excess water from fabric. The wet fabric after various processing stages is dried in different types of drying machines.

After pretreatment process dyeing and printing process are done.

Textile dyeing:
Dyeing is the process of coloration to textile products like fibers, yarns, and fabrics. For dyeing, different types of dyes are used to produce different shades at different types of fabric. Dyeing is carried out in a solution, generally aqueous, known as the dye liquor or dye bath at specific temperature. It can be done by hand or by machine.

For knowing details about Dyes and Dyeing, you can also read: Textile Dyes and Dyeing

Textile printing:
Textile printing is an art of design by mechanical and chemical application. It is the localized application of dyes or pigments and chemicals by any method which can produce particular effect of color on the fabric according to design.

For knowing about Printing Style, you can also read: Styles of Printing or Printing Styles