Study On Bleaching Process of Cotton Fabric Dyeing

Bleaching process of cotton fabric dyeing

Bleaching Process:

The bleaching process is designed to produce white fabrics and must be accomplished with minimal damage to the bleached cotton fabric. Bleaching is an oxidation process by which color material is destroyed and cotton is invariably destroyed. In the case of Oxidative bleaching, Sodium hypochlorite, bleaching powder, sodium chloride, and chlorine-based bleaching are very good bleaching agents. Non-oxidative bleaching agents, Hydrogen peroxide, sodium pur sulfate, sodium percarbonate, Sodium perborate, and peracetic acid are very good bleaching agents in textile engineering. Oxidizing and reductive are both methods that are widely used for the bleaching of unbleached cotton. In this process, we use an oxygen bleach process. To get the optimum whiteness index we use hydrogen peroxide chemical. Acetic acid can be used to control the PH instead of sodium carbonate. The full-sized raw cotton fabric needs to be used to get better bleaching performance

Objectives of Bleaching:

1)      To ensure a high degree of whiteness for white goods

2)      To ensure even and stable white for dyed goods

3)      To ensure no or only tendering of the cotton fiber

4)      To increase the absorbency of the fabric for the next process

5)      Complete removal of cotton seeds and motes

Recipe Required for Hydrogen Peroxide Bleaching:

·         Sequestering Agent- 1cc/L

·         H2O2- 15% (Weight of Fabric)

·         Sodium Silicate- 1/3 of H2O2

·         NaOH- 3gm/L

·         Na2CO3- 5gm/L

·         PH- 11

·         Room Temperature – Boiling

·         Time – 30 minutes

·         M:L – 1:80

Functions of Used Chemicals:

1) A Sequestering Agent is used to remove hardness from water

2)      Sodium Silicate- It is a stabilizer which is used to reduce the damage of the fiber

3)      NaOH and Na2CO3 are used to hold the PH at 11

4)      H2O2 – Increase the whiteness of the unbleached cotton fabric

Working Procedure of Bleaching Process:

After measuring the weight of the scouring fabric, we again take water 80 times greater than its fabric weight. We can use a wetting agent to wet the fabric properly. Then the mixer is transferred into an aluminum vessel. After that, there are added specific amount of Sodium Silicate, Sodium Carbonate, and Sodium Hydroxide, and added to the heater to give heat. When the temperature of the mixer is raised to 60C, Hydrogen peroxide is introduced and kept above the heater until boiling for 30 minutes. Then the fabric is taken to the washing machine for wash. At last, the mixer is taken to the dryer for drying for 3-4 minutes. From the weight of scoured and bleached fabric, we find out the amount of bleaching for dyeing in textile engineering.

Calculation of Bleaching:

Fabric Weight = 5.66gm

Weight of Water = 80*5.66

                                     =452.8 mL

Sequestering Agent= 452.8/1000

                                     =0.4528mL

Hydrozen Per Oxide= 15/100*5.66

                                       = 0.849 gm

Sodium Silicate = 1/3*0.849

                              = 0.283 gm

Sodium Hydroxide = 3/1000*452.8

                                     = 1.3584 gm

Sodium Carbonate= 5/1000*452.8

                                    = 2.264 gm

Amount of Bleaching= 5.66-5.22/5.66 *100%

                                           =7.7 %

Results:

Amount of Bleaching Percentage = 7.7 %

Recommendation:

·         During measuring the weight of the fabric, the value of the electric balance must remain zero
·          Amount of Specific chemicals must be taken carefully
·         Hand Gloves need to be worn during the chemical
·         Hydrogen Per Oxide should be added after it raised up to 60 degrees temperature
·         The mixer must be boiling for 30 minutes
·         We should use an aluminum vessel for the mixer
·         Fabric should remain in the dryer max (3-4) min


Bleaching Process in Textile PPT (Precision Processes Textiles):

The aim of bleaching is to remove any unwanted color from the fibers. This may be the grey or yellow tinge of a natural fiber. It may be a consequence of discoloration from the manufacturing process. If bleaching is essential high-quality white goods are being produced. The bleaching process also eliminates impurities remaining from the previous process and improves the absorbency for fabric dyeing and printing. Bleaching agents are usually oxidizing agents. The most common bleaching agent is a stabilized alkaline solution of H2O2. This is a powerful oxidizing agent that destroys the natural coloring matters present in cotton. Any residue of starch in the cotton is rapidly oxidized by the hydrogen peroxide used in bleaching. So that the peroxide is less effective in destroying undesirable colored impurities. In addition, residual starch can also reduce some dyes during dyeing, particularly under alkaline conditions, resulting in reduced color depth.

Hydrogen Peroxide has largely replaced the solution of sodium hypochlorite as a bleaching agent. The latter chemical requires careful PH and temperature control during bleaching to avoid oxidizing the cotton. Hydrogen peroxide has other advantages such as alkali boiling and bleaching can be combined into one process, and continuous operation is relatively easy. Washing of bleached fabric is less critical as traces of residual peroxide are less damaging than those of chlorine from sodium hypochlorite. Again, both alkali boiling and bleaching remove unwanted contaminants from cotton material. As a result, waste liquors and large volumes of washing water produce a dilute effluent.

Bleaching must be distinguished from the destruction of the color of a dyed fabric, which may be necessary if it must be redyed. 

Materials manufactured from man-made fibers are much easier to prepare than those from natural fibers. Artificially made fibers are relatively clean and any other chemical such as size is removable by simple washing with a detergent solution. Bleaching of artificially made fibers is not necessary. It is most common when a fabric made of synthetic fibers gas a yellow cast from overheating or has picked up dirt during manufacture. The process is much milder than for natural fibers.

Since many fabrics require bleaching and tend to be somewhat yellow, they have a stronger absorption of blue and violet light in the 400-500 nm wavelength range. Certain colorless compounds absorb non-visible, near-ultraviolet light at wavelengths between 300 -400 nm and transform some of the absorbed energy into visible blue-violet fluorescence in the 400-500 nm region. This emission of light offsets the absorption of the fibers in this wavelength region, even producing an effect where the reflected light plus the fluorescence causes more visible light to leave the fabric surface than is apparently incident upon it. This gives the impression of a brilliant white. Such components are called fluorescent whitening agents or optical brighteners. They are widely used for textiles, paper, and in domestic detergents

Cellulosic and protein fibers are particularly vulnerable to growth by micro-organisms under warm humid conditions. Even though synthetic fibers are fairly resistant to bacterial degradation, a variety of residual chemicals on the fiber surfaces can serve as nutrients for air-borne microorganisms. The rapid growth of colonies of bacteria produces colored spots and often a distinct odor. The odor of mildew is from the growth of fungi. Once formed, the colored pigments from bacterial metabolism are often resistant to vigorous bleaching. This problem can be minimized by ensuring that stored fabric is completely dyed and relatively clean. The use of anti-bacterial agents such as chlorophenol or a quaternary ammonium compound will prevent this problem.

Effective preparation before dying is essential, no matter what the physical form of the textile material, particularly for fabrics. Uneven preparation invariably results in unlevel dyeing. For loose dyed fibers and to a lesser extent dyed yarn, uneven color is blended by the mixing that occurs in subsequent manufacturing processes. This is impossible for dyed fabric and unlevel color leads to rejection of the goods.




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