Bleaching types - Using Hydrogen peroxide (H2O2)

Types of bleaching -

The bleaching process can be done by four methods or with the help of the four chemicals. Scouring is also combined with the bleaching process and varies according to these four types.

(1) Using dilute sodium hypochlorite (NaOCL) at room temperature
(2) Using Hydrogen peroxide (H2O2) at 80 to 85°C
(3) Using Sodium chlorite (NaCLO2) at a boil
(4) Using compounds like peracetic acid

Following is the description of the second process.

Preparation of bleaching

Hydrogen Peroxide bleaching (H2O2) -

(Universal bleaching agent)

(1) Hydrogen peroxide may be a universal bleach capable of application in bleaching with most textile materials including silk, wool and man-made fibres, cotton, jute, etc, Both scouring and bleaching are combined and distributed in one stage because of heat.

(2) The consumption of water is economical in oxide bleaching because there's no further treatment like souring, antichlor, etc as is the case of hypochlorite bleaching.

(3) Superior whiteness is often achieved by peroxide bleaching.
(4) Absorbancy of textile materials is more for peroxide than Hypochlorite Bleaching Absorbancy - oxide > Hypochlorite.

(5) Oxide bleaching is far safer than Hypochlorite Bleaching because there's no chemical damage or degradation.

(6) Continuous scouring and bleaching using peroxide will be exhausted during one operation on an eternal bleaching machine.

(7) This process is eco-friendly.

(8) Properties of oxide (H2O2) -

(a) It is a colourless and syrupy Liquid
(b) It is stable in acidic conditions
(c) It is unstable in alkali conditions
(d) It is sensitive to sunlight
(e) It decomposes if allowed to react with heavy metals
(h) Stability of hydrogen peroxide -
(f) It is powerful oxidizing agents
(g) It is soluble in water

pH of peroxide	Stability
1 to 3	Highly stable
4.5 to 5	Good
7	Medium
11.5 to 13	Lowest

Volume strength of hydrogen peroxide with its percentage values -

Volume strength	Percentage of H2O2 (Weight/volume)
1	0.3
2	0.6
10	3
20	6
30	9
100	30
114	34
130	39
160	48

Preparation of Hydrogen peroxide -

Hydrogen peroxide is made by Dilute Hydrochloric acid or concentrated sulphuric acid gives hydrogen peroxide.

BaO2 + 2 HCl > BaCl2 + H2O2

BaO2 + H2SO4 > BaSO4 + H2O2

H2O2 bleaching method

Conditions for hydrogen peroxide bleaching-

pH - (Unstable at the alkaline condition) Hydrogen peroxide is stable under acidic conditions hence, to decompose hydrogen peroxide we require alkaline conditions.

In addition to alkali (NaOH), the concentration of per hydroxyl (HO2–)ion will increases as (NaOH) sodium hydroxide will activate hydrogen peroxide so, bleaching is best done at pH 10.5 to 11.

Temperature - (Optimum temperature=80-85°C) The optimum temperature used for bleaching is 80-85°C.

(a) If the temperature is less than optimum temperature (80-85°C) then the action of hydrogen peroxide is very less and results in "Under Bleaching".

(b) If the temperature is more than the optimum temperature (80-85°C) then the action of hydrogen peroxide is very strong which results in "Over Bleaching".

Hydrogen peroxide bleaching reactions -

The oxidation potential of Hydrogen peroxide is very low. It gives out oxygen very easily with the generation of heat as shown in the equations below,

(A) Autolysis - H2O2 > H2O + [O] + k cal (heat)

(B) Dissolved in an aqueous medium - In an aqueous medium, hydrogen peroxide dissociates into per hydroxyl ion(HO2–).
Under the above conditions of pH & temperature hydrogen peroxide will librate hydrogen and per hydroxyl ions
H2O2 < - > H+ + HO2–

Hydrogen Perhydroxyl Peroxide Ion

(C) Bleaching Action - This per hydroxyl ion is highly Unstable & decomposed in presence of oxidizable substances (impurities in Cotton)

HO2- > OH– + [O] Perhydroxyl Atomic oxygen Ion (Bleach active)

(D) Bleaching action in alkaline medium - In addition to caustic soda(NaOH), the concentration of per hydroxyl (HO2–) will increase due to hydrogen peroxide is unstable in Alkaline conditions
NaHO2 < - > Na+ + HO2–

(E) Decomposition - Hydrogen peroxide is decomposed into molecular oxygen which is bleach inactive i.e bleaching is not possible with molecular oxygen. hence, we should avoid this reaction or formation of molecular oxygen

2H2O2 > 2H2O + O2 Molecular oxygen (Bleach inactive).

Preparation of H2O2

Questions -

What are the types of bleaching processes?
Describe the hydrogen peroxide bleaching process.
What is the role of a universal bleaching agent in chemical processing?
Describe the preparation of H2O2 for the bleaching process.
What is the condition of the H2O2 bleaching process?
Describe the mechanisms of hydrogen peroxide bleaching.

References

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Vigo, T. L. (2014). Textile processing and properties: Preparation, dyeing, finishing and performance: Preparation, dyeing, finishing and performance (1st ed.). Elsevier Science & Technology. https://www.elsevier.com/books/textile-processing-and-properties/vigo/978-0-444-88224-0?country=GB&format=print&utm_source=google_ads&utm_medium=paid_search&utm_campaign=ukshopping&gclid=CjwKCAiAhqCdBhB0EiwAH8M_Gn3kr_d2paqVzccxckWobOsLcwl0cD_Mu6vA5I-eqFARiZwx32eUOxoC-LMQAvD_BwE&gclsrc=aw.ds

(N.d.). Textilelearner.net. from https://textilelearner.net/flow-chart-of-textile-wet-processing/

Athalye, A. (2014, June 7). Bleach Clean Up - Bleaching process in textile industry. Fibre2fashion.com; Fibre2Fashion. https://www.fibre2fashion.com/industry-article/7349/bleach-clean-up

bleaching | chemistry. (n.d.). In Encyclopedia Britannica.

Gore, S. (2015). Textile bleaching process with hydrogen peroxide. https://www.academia.edu/11975952/TEXTILE_BLEACHING_PROCESS_WITH_HYDROGEN_PEROXIDE

Sayed, A. (n.d.). Textile Bleaching [A to Z]. Blogspot.com. from https://textileapex.blogspot.com/2015/01/textile-bleaching.html

textile-tutorials. (2020, February 28). Types of bleaching in textile. Textile Tutorials. https://textiletutorials.com/textile-bleaching-oxidizing-bleaching-agent-reducing-bleaching-agent/

The steps to safe textile bleaching. (2019, July 16). Visionlinens.com. https://www.visionlinens.com/blog/the-steps-to-safe-textile-bleaching

Further reading,

Introduction to bleaching processing

Part 1 Bleaching types - Using Hydrogen peroxide

Part 2 Stabilizers for the H2O2 bleaching

Part 3 Recipe and ingredients for H2O2 Bleaching

Part 4 J-box for H2O2 bleaching method

Part 5 Scouring process of H2O2 bleached fabric

Bleaching types - Sodium Hypochlorite Bleaching

Bleaching types - Using Hydrogen peroxide

Bleaching types - Using Sodium chlorite

Bleaching process of Polyester materials