Textile Bleaching

Bleaching of Textiles.

Bleaching is chemical treatment employed for the removal of natural coloring matter from the substrate. The source of natural color is  organic compounds  with conjugated double bonds , by doing chemical bleaching  the discoloration takes place by the breaking the chromophore , most likely destroying the one or more double bonds with in this conjugated system. The material appears whiter after the bleaching.

Bleaching Agent.

A bleaching agent is a substance that can whiten or decolorize other substances.Bleaching agents essentially destroy chromophores (thereby removing the color), via the oxidation or reduction of these absorbing groups. Thus, bleaches can be classified as either oxidizing agents or reducing agents .


Type of Bleaching agents

a.Oxidative Bleaching Agents

b.Reductive Bleaching Agents

c.Enzymatic Bleaching Agents

Bleaching with hypochlorites.

Bleaching Powder

Calcium hypochlorite is usually a white or grayish-white powder, but it is also available as tablets. It is a strong oxidizer and is considered more stable than sodium hypochlorite. It is also believed to provide more chlorine.

Sodium Hypochlorite

Sodium Hypochlorite is a greenish-yellow liquid commonly referred to as "Bleach." The chemical compound formula for Sodium Hypochlorite is NaOCl.Sodium Hypochlorite is prepared by reacting dilute caustic soda solution with liquid or gaseous chlorine, accompanied by cooling. It is used extensively as a bleaching agent in the textile, detergents, and paper and pulp industries.

The active ingredients in hypochlorite bleaches vary with pH. At pH <>2 is the main component in solution; at pH 4 to 6, HOCl is the dominant species; at pH > 9, OCl ⁻ is the only component present. It is the hypochlorite ion in basic solution that is the active ingredient in household bleach, which is typically about 5 to 6 percent NaOCl. The OCl ⁻ ion oxidizes chromophores in colored materials, and is itself reduced to chloride and hydroxide ions.

 
Hypochlorite Bleaching process

The conditions of the bleaching agent varies according to the result required, the concentration depends upon the fabric quality, degree of whiteness required, types of machine and next operation.

Quantity Required:-

Normally 2.5-3.0 gpl of available chlorine is sufficient for good bleaching , but it is necessary to optimize it on a possible lower  value for safe bleaching process.

How to Test available chlorine content in a bleaching bath

ANALYSIS METHOD
Determination of available Chlorine.
Method	:Iodometric(Standard)
Apparatus	:Erlemeyer flask(100 ml)  -I  Burette stand  -I  Pipette(50 ml)  -I  Spatula  -I  Measuring Cylinder  -I  Micro burette(10ml.)  -I  Measuring Cylinder.  -I
Reagent	Sodium Thio Sulphate Solution(.1N) Glacial acetic acid Pottassium Iodide(KI crystal) Starch indicator solution.

Procedure :- 1) Collect sample solution ( Sodium hypo chlorite ) in an air-tight bottle. NOTES / CAUTION a) Keep sample solution away from excessive light and do not agitate the sample solution. b) Start chlorine determination IMMEDIATELY after sampling. ( i.e. do not store sample to be analysed for chlorine ). 2) Introduce 10ml of glacial acetic acid and about 0.5 g of Pottassium Iodide (Spatula full ) crystal into a 100ml erlemeyer flask. 3) Withdraw y ml  of sample solution with bulb pipette and introduce into above Solution. 4) Swirl the flask to well mix the solution. 5) Titrate with 0.1N Sodium Thio Sulphate Solution from upto a light Yellow Colour. 6) Add 1 ml of starch indicator solution and continue to titrate until the last drop of titrant causes the solution to turn colourless. % active chlorine = 3.545 x N x V/y   (i;e 3.545 x N x V divided by y) Where : V = Volume of (N ) Na2S2O3 N = Normality of Na2S2O3 y = Sample amount

How to test caustic gpl in bleaching solution

pH :-General PH range is 10-11 or 10.5-11.5 during bleaching if PH reaches 9 then it is a danger level and at 7 PH the bleaching is worse and causes extreme damage to the cloth.The PH value is maintained by adding sod ash or by buffering agent.

Temperature:-The suggested temperature for hypochlorite bleaching is 37-40oC. Generally reaction is accelerated with  increases of temperature.

Water Quality:-water for bleaching should be soft and even hard water can be used but should be free from Cu++ and Fe++.

Effect of metals

The bleaching equipment should be made of stainless steel , to avoid the catalytic degradation of the cellulose in the presence of copper and iron.

Substrate preparation

The substrate must be pre scoured in the presence of chelating agents , it should be free from rust spots and traces of metallic impurities when bleaching with hypochlorites.

Bleaching Time :-The time factor depends upon the following consideration.
(a) Concentration.

(b) PH value.

(c) Degree of Whiteness.

(d) The type of machine used in bleaching.

Roughly for normal machine the time is 2-3 hrs is required for completion of bleaching process.
antichlor and proper neutralization treatments should be followed by proper wash for removal of reagents after hypochlorite bleaching process.

More About Sodium Hypochlorite

Sodium hypochlorite FAQ

 
Bleaching with peroxides.

The bleaching bath is composed of hydrogen peroxide (35% or 50% by wt.) as the bleaching agent, an activator (usually alkali) and stabilizers.

Bleaching Process with Hydrogen Peroxide

Hydrogen peroxide bleaching can be done by

1.Batch wise, 

2.Continuous 

3.Semi continuous method.

a.Quantity of peroxide required in Bleaching

b.Temperature

Cotton and Bast 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 generally between 2 and 5 hours. In a pressurized high temperature (HT) apparatus cotton can also be bleached at temperatures of 110 - 130°C in only 1 to 2 hours.

c.Time

During the impregnation processes the temperature and as well the retention time varies widely. During a cold bleach process a dwell time of 18 to 24 hours is necessary. In the pad steam process under atmospheric pressure the bleaching time is generally between 1 to 3 hours. The above mentioned processes describe batch processes. Today a lot of continuously, intelligent finishing equipment exists in which the bleaching step is only one of some other treatments and the reaction time of the impregnated material in such steamer is only between 7 to 20 minutes. In general these bleaching process correspond to a preliminary bleach.

d.pH:-The pH value depend on the fibres to be bleached and pre-treatment.

NaOH is used in case of H2O2 bleaching. This is used to bring the PH upto 9-10 because H2O2 become active at this PH or oxidation is start at this PH.

For the bast fibres, such as linen, weaker alkaline or soda alkaline baths are used in order to avoid a cottonizing. Regenerated cellulose fibres are more sensitive. Therefore, they are only bleached in weak alkaline baths.
Alkali sensitive animal fibers must be bleached in very weak alkaline solutions. Phosphates and ammonia are most widely used as alkalization source. With tetrasodium pyrophosphate simultaneously a stabilization of the bleaching liquor can be attained.

e.Water Quality

Soft Water free of iron and copper impurities is recommended for peroxide bleach treatment.

f.Peroxide Stabilizers

High pH and temperature lead to the faster decomposition of peroxide bleaching liquor and degradation of cellulose.The role of the stabilizer is simply to control or regulate these effects the
act as buffers, sequestrates and in special cases, enhancing performance of the surfactant used in the bleach bath.

For caustic alkaline bleach sodium silicate, organic stabilizers or the combination of both are suitable. In weak alkaline baths the addition of tetrasodium pyrophosphates can be used alone or together with an organic stabiliser.

Advantages of Peroxide Bleaching:
1.Among the oxidizing bleaching agents, only hydrogen peroxide provides a high bleaching effect     at reasonable costs, especially if modern short-term bleaching processes are used with only a        few minutes bleaching time.
2.Peroxide bleaching keeps the fibre quality intact.
3.Cotton can be bleached with peroxide in a single stage. Other processes require two or three
   bleaching stages,(desize with scour, scour with bleach and desize with scour and bleach).
4.No separate pre treatment is necessary because hot, alkaline bleaching has not only a bleaching but also a cleaning effect, it therefore combines  the advantages of an alkaline extraction with the bleaching treatment.
5.Animal fibres can only be bleached with peroxide to a high and stable degree of whiteness.
- Corrosion of stainless steel equipment does not occur during peroxide bleaching.
6.The spent peroxide baths still contain residuals of hydrogen peroxide which fever the                  degradation of the organic impurities in the effluent, and this helps to decrease the chemical        oxygen demand (COD).

 Bleaching of wool with hydrogen peroxide.

After scouring, wool may be bleached by immersion or pad and dry techniques, using alkaline or acid solutions. 

Bleaching of silk with hydrogen peroxide.

Prior to bleaching, silk is usually degummed. Hydrogen Peroxide addition assists this process and it is universally used as the bleaching agent for natural silk, usually in an alkaline solution.

Bleaching of synthetic fibres with peroxide

When used alone, synthetic fibers do not normally require bleaching. However, blends of synthetic fibers with natural or regenerated fibers, e.g. cotton-polyester are frequently bleached. The most popular bleaching agent is Hydrogen Peroxide and it is used in both batch and continuous processes.
 

Advantages and disadvantages of peroxide over hypochlorite bleaching.

Bleaching with sodium perborate

Sodium perborate (PBS, NaBO₃.nH₂O where n=1 or 4) can readily be incorporated. It has been described as a stable, solid form of hydrogen peroxide allowing its introduction into the wash at the same time as the detergent. Sodium perborate is a gentler bleach than sodium hypochlorite, causing less damage to fabrics and dyes, but by itself is only effective at high (>60ºC) temperatures. Although solid chlorine bleaches exist, they are rarely used in laundry detergents.
Click Here for detailed chemistry stabilization pH solubilization

Bleaching with sodium chlorite.(NaClO₂ )

The sodium chlorite is available as a powder , and it is applied under strongly acidic conditions to textiles. Its application produce a toxic and corrosive gas.

a. bleaching mechanism

b. effect of pH.

c. effect of temperature.

d. effect of metals

Application process of chlorite bleach on cellulosics and synthetics

Persulphates

Ammonium ,potassium and sodium  Persulphates are oxidizing agent that find use in textile bleaching are

a.       for bleaching of protein fibers in combination with hydrogen peroxide.

b.      Activated cold bleaching of cellulosics.

c.       To oxidize vats and sulphur dyes

d.      To bleach fur and skins in combination with hydrogen peroxide.

Bleaching with peracetic acid.

Peracetic acid is produced by the chemical reaction of acetic acid and hydrogen peroxide. It works in a very narrow pH range of 7 to 8. Below pH 7.0 the bleaching is not proper and above pH 9.0 fiber degradation takes place. Peracetic acid is used a bleaching agent for nylon and acetate where hydrogen peroxide can not be used.

Reductive bleaching systems

Reductive bleaches work by reducing colored impurities into colorless forms.

1.sodium hydrosulphite.

Sodium hydrosulphite 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 purity ranges.

2.sodium sulphide.

Sodium sulphide is also a strong reducing agent

3.sulphur dioxide

Sulphur dioxide was used as a bleaching agent in early 20^th century for bleaching of wool.

Reductive bleaching of wool.

Bleaching of silk with reducing agents.

Commonly Sodium hydrosulphite, Sulfurdioxide and sodium sulphoxylates are the reductive bleaching agents which are used for silk.

Reductive bleaching of nylon

Oxidative bleaching isn’t suitable for polyamides as H2O2 attacks the polymer, instead
reductive dyeing using sodium hydrosulphite is used.

Peroxygen bleaching compounds.

A number of solid peroxygen compounds that release hydrogen peroxide when dissolved in water exist. These include sodium perborate (NaBO 3 z 4H 2 O or NaBO 2 z H 2 O 2 z 3H 2 O) and sodium carbonate peroxyhydrate (2Na 2 CO 3 z 3H 2 O 2 ). The structure of sodium perborate contains the peroxoanion B 2 (O 2 ) 2 (OH) 4 2− , which contains two O–O linkages that join two tetrahedral BO 2 (OH) 2− groups. These peroxygen compounds are used in detergents, denture cleaners, and tooth powders.

Bleaching of jute.

Bleaching of linen.

Bleaching of blended fabrics.

Bleaching of cotton weft knitted fabrics.  

 

Enzymatic Scouring and bleaching

Aside from cellulose, raw cotton contains impurities such as complex organic compounds and complex substances. The purpose of cotton preparation is to remove the non-cellulosic impurities from the cellulosics fibers and increase the wettability and whiteness of the fabric. This improves dyeing performance and subsequent processing steps such as printing and finishing. Conventional time consuming methods used in scouring and bleaching of cellulose-based textile material use large amounts of chemicals, energy, and water. To avoid environmental damages, the effluents require neutralization or dilution prior to discharge. The greatest problem occurring during bleaching with peroxide is in the radical reactions of the bleaching compounds with the fiber. This can  lead to a decline of polymerization and eventually to a decrease in tensile strength, especially in the presence of metal ions that act as activators for hydrogen peroxide. As a remedy, the use of enzymes for bioscouring has been suggested.

The use of enzyme technology is attractive because enzymes are highly specific and efficient, and work under mild conditions. Furthermore, the use of enzymes results in reduced process times, energy and water savings; improved product quality; and potential process integration.

As the purpose of scouring is to remove natural impurities — such as polymeric substances like pectins,waxes and xylomannans, among others —from cotton or other natural fibers, there are plenty of enzymes that can act on such impurities.Alkaline pectinase, which loosens fiber structure by removing pectins between cellulose fibrils and eases the wash-off of waxy impurities, is the key enzyme for a bioscouring process. Compared to the conventional alkaline boil-off, an efficient bioscouring process provides many advantages, such as reduced water and wastewater costs, reduced treatment time and lower energy con-sumption because of lower treatment temperature. Moreover, the weight loss in fabric is reduced, and fabric quality is improved with a superior hand and reduced strength loss.

However, the current enzymatic scouring processes are not effective enough to prepare the fabric for dyeing in batch and especially in continuous processes as the enzyme needs reasonably long reaction times, and controlling its performance in a continuous process is difficult.

As the enzyme does not break down waxes, a high-temperature treatment with surfactants is needed for a sufficient degree of hydrophilicity to guarantee even dyeing or printing.

The whiteness of the bioscoured material has remained lower than the whiteness obtained by using the traditional processes including alkaline wash.

Pectinase has  inability to remove motes — the remainders of cottonseed fragments. Thus, a separate bleaching step would be needed after the bioscouring process. On the other hand, the alkaline boil-off can be combined with simultaneous peroxide bleaching to efficiently remove the motes. As motes are not acceptable on fabrics other than those that will be dyed to dark shades, bioscouring will have limited usage unless a simultaneous mote-removal process is developed.

An efficient bio preparation process should be based on a combination, preferably simultaneously, of enzymes for desizing, scouring and bleaching in one bath. Success in developing such a process would result in a simple process, including savings in water, time and energy consumption. The environmental impact would be minimized by low to no use of harsh chemicals and lowered waste burdens, rendering a fabric of higher quality.

Accordingly, the attempts to combine the bioscouring and bleaching steps are not fully satisfactory, because they still involve alkaline conditions, thus causing strength losses and environmental damage

Full White Process

Bleaching Process for Full Whites and OBA application 
Introduction.

The coloring matter ,whether it is natural or present as a contaminant in the fiber is generally decolorized by different bleaching methods. However the appearance of the textile substrate is somewhat creamish after the bleaching, therefore chemical treatments are become necessary to neutralize the yellow tint of the textile fibers.

There are two methods ,which are generally used for this purpose,

1. By using a blue tinting agent , which absorbs the yellow part of the light and  reflected light appears to be of bluish tint. The total light reflected by this mean is less than the total incident light.
2. By using fluorescent optical brightening agents :- The OBA s (optical brightening agents ) are most widely used in textiles , paper, detergents  and plastics . The optical brightening effect is obtained by the addition of light , which means that the amount of light reflected by the substrate is more than the incident light , due to which the object appears brighter.

 
Desired properties of Good OBA

Before selecting an optical brightener for textile application we must look for following properties,

1. it should have good solubility , should not have its own color and good substantivity for the textile substrate under OBA application.
2. OBA’s should have good light as well as wet fastness properties.
3. Its rate of strike on the substarte.
4. build up and exhaustion properties.
5. requirement of electrolytes and its sensitivity towards different exhausting agents.
6. Effect of temperature on the exhaustion and build up properties.
7. Application pH range and sensitivity towards change in pH.
8. Effect of water hardness.
9. It should have good leveling and penetrating properties.
10. Should not decompose to colored products on exposure to atmospheric conditions as well as storage , and it should not absorb light in the visible region.
11. it should be compatible and stable  with finishing chemicals, auxiliary and process such as heat and temperature.
12. It should be stable and fast to the common oxidative and reductive bleaching chemicals and bleaching systems.

Chemical constitution of optical brighteners.

Optical brighteners are usually derivatives of  

Triazine-stilbenes (di-, tetra- or hexa-sulfonated)

Coumarins

Imidazolines

Diazoles

Triazoles

Benzoxazolines

Biphenyl-stilbenes

Brighteners can be "boosted" by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be "quenched". Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum. Besides the formation of cis isomer in stilbene-containing brighteners (only the trans isomer is optically active), continued exposure to UV-containing light will actually cleave the molecule and start the process of degradation.

Mechanism of fluorescent whitening.

Fluorescent Whitening Agents (also called optical brightener) absorb high energy radiation in the ultraviolet to violet region (330nm-380nm) on the part of characteristic molecules and emit lower energy radiation in blue region in visible spectrum (400nm-450nm), which yields the counteracting the yellowing appearance. FWA should be transparent on the substrate and should not absorb the visible region of the spectrum. The OBAs are effective only when the incident light has a significance proportion (such as daylight) of UV rays. When material treated with OBAs are exposed to UV black light source , it glows in the dark.Anionic OBA’s exhaust on cotton, wool and silk., cationic OBA’s  exhaust on acrylic and certain polyesters and nonionic OBA’s are exhaust on all synthetics.

Application of textile optical brighteners.