Bleachnig_textiles Using Sodium Chlorite

7/31/2019 Bleachnig_textiles Using Sodium Chlorite

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CRRA NEWS

BLEACHING TEXTILES USING

SODIUM CHLORITE

Sodium chlorite is used to bleach textile goods under such conditions that chlorine dioxide,a selective bleaching agent, is generated in-situ. Chlorine dioxide is a non-chlorinatingreagent used to obtain a high degree of whiteness on cotton as well as synthetic fibreswithout degrading them. Using sodium chlorite entails maintaining good control overthe pH of the bleaching stage (slightly acidic) and adding an activation agent. Arkema,a major manufacturer of sodium chlorite in Europe, provides technical support forbleaching textile goods under optimum and controlled conditions.

The two main products used by finishers to bleach textiles are hydrogen peroxide and sodium chlorite.

Sodium chlorite has been used around the world for over 40 years to bleach textile fibres (100% cellulose fibresor blends of chemical and cellulose fibres). Its advantages over other bleaching agents are many, but its mainasset lies in the fact that its reaction selectivity protects textile fibres. Sodium chlorite is also used to partially orcompletely strip colour from textiles before redyeing them.

A process exists that allows desizing and sodium chlorite bleaching to be combined in a single step to boost thebleaching action of sodium chlorite on cotton and cellulose blend fabrics and thereby make preparation morecost-effective.

Bleaching mechanism with sodium chlorite

Sodium chlorite is a light yellow alkaline liquid that is stable at ambient temperature. When broken down by

acids, it forms chlorine dioxide (CIO2), a green gas that is readily soluble in water.

Chlorite breaks down according to the following mechanism:

5 ClO2- + 4 H+ 4 ClO2 + Cl

- + 2 H2O (I)

4 ClO2- + 2 H+ 2 ClO2 + Cl

- + ClO3- + H2O (II)

Reaction I is always the most important of the two; reaction II is promoted by the drop in pH.

Sodium chlorites colour stripping properties used in bleaching textiles are thought to derive from the formation ofchlorine dioxide. Bleaching is performed in a weakly acidic aqueous solution with enough sodium chlorite toallow the chlorine dioxide to be absorbed by the textile fibres as it is formed. This process is completely

mastered.

Cotton and blends

Semi-continuous processes

In the process called Pad-Roll, sodium chlorite and a high-temperature amylase enzyme arecombined in a single bath to strip away both sizing agents and hemicelluloses. It is currently estimatedthat this enzyme must break down at least 75-85% of them in order for the bleaching process to yieldthe desired results. The enzyme is barely degraded by the sodium chlorite during the first few minutesof the process. The sizing agents (e.g. starches, polyvinyl alcohols, CMCs, polyacrylates and waxes)are first removed in a nearly neutral or slightly alkaline medium. The enzyme acts by forming water-

soluble reaction products in a period of 1-2 minutes. Then, during the reaction process, the pHbecomes acidic. The oxygen generated by the chlorite and chlorine dioxide formed in situ destroys theenzyme. The bleaching phase then starts and continues in a hot, acidic medium.


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CRRA NEWSThe impregnation bath is made up of several chemicals. The sodium chlorite (NaClO2) is firstcombined with an activation agent dissolved in warm water, a non-foaming wetting agent, and a smallamount of 35% hydrogen peroxide to control the formation of chlorine dioxide. The high-temperaturedesizing enzyme, an optical brightener compatible with both the fabric and the chlorite, and theremaining water are then added just before the bath is used. The pH is monitored to ensure that it

remains between 6.5 and 8. The bath is then sent to the loading tank.

The fabric (in pieces) first passes through the bath in the saturator tank at a speed of 50-100 metresper minute and exits between two squeeze rolls. An impregnation rate of 70-100% is recommendedfor easier desizing and enhanced whiteness. The fabric is then heated to between 85C and 95C (thetemperature depends on the type of fabric), wound into a box and maintained (at the sametemperatures) for 90 minutes to 2 hours. After this, it passes into an industrial washer with at least fourcompartments through which water flows backwards at a temperature of 95C, then 60C and finally atambient temperature.

The material is then dyed or finished before being stretched on a frame to dry.

QUANTITIES OF CHEMICALS REQUIRED FOR A 1000 LITRE BATH(90% impregnation rate)

100%COTTON

COTTON /LINEN

100%LINEN

50/50COTTON /

POLYBLEND

67/33COTTON /

POLY BLEND

POLYESTER/ VISCOSE

Sodium chlorite25% solution (litres)

60-80 80-106106-130

40-50 32-40 30-40

Activation agent (kg) 6-8 6-11 11-19 4-5 3-4 3-4Enzyme (kg) 2-5 2-5 2-5 2-5 2-5 2-5Wet ting agent (kg) 1-2 1-2 1-2 1-2 1-2 1-2Hydrogen peroxide

35% (litres)2 to 5 2 to 5 2 to 5 2 to 5 2 to 5 2 to 5

The Pad-Batch process is the same except that the fabric is wrapped in a plastic sheet to preventevaporation and left to stand at ambient temperature for 15-20 hours.

Batch processes

Batch processes (e.g. kier, package, winch, jet, overflow, and jigger) consist of two steps without anyintermediate washing. First, the sizing agents are stripped in a bath containing a non-foaming wettingagent and the high-temperature desizing enzyme. The bath is sent to the processing tank and heated

to 80-90C for 10-15 minutes then cooled to 70C.

Step two consists of bleaching without any intermediate washing. The bath contains the activationagent and the sodium nitrate (corrosion inhibitor) dissolved in warm water, a small amount of35% hydrogen peroxide (2-5 l per m

3of bath), the 25% sodium chlorite, the amount of formic acid

needed to keep the pH between 3.6 and 4, and an optical brightener compatible with the chlorite. It isheated to 95C for 45-60 minutes for cotton/linen o r 115-120C for 30-45 minutes for polyester/cotton(temperature ranges for the optical brightener). The bath is then cooled to 80C, discharged, and thefabric is washed at 95C, then at 65C and finally at ambient temperature.


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CRRA NEWS

Batch bleaching with sodium chlorite

QUANTITIES OF CHEMICALS REQUIRED FOR A 1000 LITRE BATH

(example for liquor ratios between 1:7 and 1:10)

100%COTTON COTTON/LINEN

50/50 COTTON-POLY BLEND

67/33 COTTON-POLY BLEND

Wetting agent (kg) 0.5-1 0.5-1 0.5 0.5Enzyme (kg) 1-1.5 1 0.5-1 0.5-1Activation agent (kg) 3-5 4-6 2-4 2-3.5Sodium nitrate (kg) 3-5 4-6 2-4 2-3.525% solution of sodium chlorite(litres)

6-10 8-12 5-8 4-7

35% hydrogen peroxide (litres) 1 to 3 1 to 3 1 to 3 1 to 3

80% formic acid (litres)1-1.5

(pH 3.6-4)

1-1.5

(pH 3.6-4)

1-1.5

(pH 3.6-4)

1-1.5

(pH 3.6-4)

Synthetic fibres

These fibres are made by polymerising chemical monomers and extruding them through a die. Theyare a product of chemical synthesis, hence the name synthetic fibres.

In most cases, chemical fibres are white enough to be dyed after being scoured just once (to removeweaving oils). Bleaching may however prove necessary in cases where very light shades of white aredesired or if the fabric has turned yellow during heat-setting. Sodium chlorite may be the sole means

of bleaching synthetic fibres (in 100% form or blended with other fibres) that are very sensitive toalkaline hydrolysis (used in hydrogen peroxide bleaching). This is particularly the case of polyesterfibres, acrylic fibres and, to a lesser extent, polyamides.

10-15 min

Wetting agent

Anti-foaming agent

Enz me

Discharging

Accelerant and sodium nitrate

ClO2

H2O2Formic acid

Optical brightener

3 whasings

Cotton/linen blendC


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CRRA NEWSIn the case of fibres such as chlorofibres, sodium chlorite yields excellent results without having toexceed the fibres inherent temperature limits (around 50C).

Most of the time these reactions are too slow to be applied in continuous or semi-continuousprocesses. On the other hand, good results are obtained with a batch processes (kier, jet, overflow).

This is particularly true with chlorofibres.

QUANTITIES OF CHEMICALS REQUIRED TO BLEACH SYNTHETIC FIBRES(liquor ratio* between 1:7 and 1:10)

POLYAMIDE POLYESTER ACRYLIC CHLOROFIBRESWetting agent (kg) 0.5-1 0.5-1 0.5 0.5Accelerant UG45(kg)

3-5 3-5 3-5 3-5

Sodium nitrate (kg) 3-5 3-5 3-5 3-5Ammoniumhydrogen fluoride

0.75 1 1 1

25% solution ofsodium chlorite(litres)

3-6 3-6 3-6 3-6

35% hydrogenperoxide (litres)

0.1-1 l 0.1-1 l 0.1-1 l 0.1-1 l

Acid Phosphoric oracetic

pH = 3.5

FormicpH = 3.5

Formic orphosphoricpH = 3.5

FormicpH = 3.5

Temperature (C) 40 then 85-90 90-95 40 then 85-90 50 (Rhovyl)

* Liquor ratio: volume of dyeing bath in litres to the weight of the fibreto be treated in kilograms.

After bleaching, polyamides and acrylics must be rinsed with bisulphite in an acidic medium and

chlorofibres in an alkaline one.

A less polluting process

Protecting the environment is a major concern throughout the industry, which is attempting toeliminate harmful by-products in bleached goods and reduce pollutants in discharges to a minimum.The objective in bleaching operations is to reduce the formation of polluting by-products, particularlyorganochlorinated products.

Studies have been undertaken to check for the absence of chlorinated organic compounds, notablydioxins, in chlorite-bleached waste cotton. Powerful analysis methods (High Performance LiquidChromatography coupled with a high-resolution mass spectrometer) have not detected any dioxins(detection threshold of up to 1 ppb).

Also, AOX levels measured in wastewater from mills where cotton is bleached with sodium chloriteremain below 0.2 kg per tonne of processed cotton. Good control over the bleaching process allowsAOX generation to be decreased even further.

By comparison, the legal limit set for paper industries is 1 kg of AOX per tonne of pulp.


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CRRA NEWS

ADVANTAGES OF SODIUM CHLORITE PROCESSES

Permanent whiteness.

Hulls and aphids are removed from cotton without having to boil it first.

Characteristics of cellulose fibres left intact, particularly in the case of linen, viscose, rayon, cellwool and cotton/linen or polyester/viscose blends. Bleaches cotton of mediocre quality, whichwould be damaged by other methods. Lower weight loss.

Maximum whiteness is obtained with synthetic fibres such as polyester in polyester/cotton blendsor polyamide in polyamide/cotton blends.

Destroys yellowish thermal decomposition products without affecting the polymer structure ofsynthetic fibres (if bleaching is required). As synthetic fibres are delicate polyamide, polyester,acrylic and chlorofibres in particular sodium chlorite is recommended as a safe and effectivebleaching agent.

Excellent hydrophilic properties of treated fabrics for uniform dyeing.

Less energy, labour, water and time are required when sodium chlorite bleaching is used in steamsaturation (J-Box, U-Box, Pad-Roll) and batch mills. The productivity gains are significant and noinvestment is required.

Little chlorine dioxide is released by the combined desizing and bleaching. A ventilation systemshould nevertheless be installed as a precaution.

OTHER USES OF CHLORITE

Sodium chlorite is used in many industries where quality, safety and environmental protection areessential factors the production of chlorine dioxide, a powerful oxidising agent with excellentbactericidal properties.

Bleaching of food products, such as starches, flours, fruits and alginates.

Bleaching and preservation of leathers and skins.

Preservation of food products.

Disinfection of flour and preservation of its properties.

Treatment of drinking waters.

Treatment of industrial process waters.

Removal of colour from industrial waste waters.


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CRRA NEWSSemi-continuous bleaching.Two semi-continuous processes are used for open-width bleaching ofcotton fabrics. In the first, called Pad-Roll, the fabric is saturated with a pad (80-100% delivery rate),placed in a steamer at 95-98C and wound in a box i n which steam is injected at a temperature of 95-98C to keep the air inside moist and warm. The fab ric is spun on its shaft for 1-3 hours.

In the second, called Pad-Batch, the fabric is impregnated with a pad, wound around a shaft atambient temperature, wrapped in a sheet of plastic to avoid evaporation and spun for 15-20 hours.The fabric then passes into the washer, where it is rinsed in hot then cold water and then dried. Beforebeing dried, it may be neutralised with acetic acid. The advantages to this process are its flexibility andlow investment cost.

Continuous bleaching. Continuous bleaching lines generally utilise the Pad-Steam process andinclude continuous desizing and boiling steps. Continuous processes are suitable for large amounts of

fabric of the same, or at least similar, quality. The machines used are a saturator (pad), a preheaterand a steamer in the shape of a U or J (called a U-box or J-Box). The fabric travels through thesechambers for a relatively long period of time (30 minutes on average) in folded (cuttled) rope form(open-width boxes may be used, but they are expensive and bulky) in a 100-102C steam atmosphere.Several units may be placed side by side so that the fabric may be desized, boiled and bleached once ortwice without stopping. A washer is placed at the end to rinse and neutralise the fabric. Hydrogenperoxide

is particularly suited to this type of process.

Batch bleaching. Batch processes are used for small amounts of fabric. The fabric is bleached inmachines called winches with a high liquor ratio (1:20) or jet or overflow machines with a low liquorratio (1:5 to 1:10). If the latter are used, non-foaming auxiliary agents are necessary on account of theagitation. Jiggers are best for crease-sensitive fabrics (heavy fabrics or those with a high threadcount). The liquor ratio is around 1:5. Kiers and beam machines are used more for delicate fabrics such

as gauze and bunting as well as knitted fabrics. The liquor ratio is nearly 1:10.

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CRRA NEWS

CENTRE DE RECHERCHE RHNE-ALPES (CRRA)Rue Henri Moissan

BP 63 - 69493 Pier re -Benite Cedex FRANCEWater, paper, and textile treatment support team

Tel.: +33 (0)4 72 39 80 04 - Fax: +33 (0)4 72 39 80 28

The information given herein is based upon tests conducted by our research centres and backed byselected literature. It does not constitute a guarantee or formal commitment on our part. The extentof our commitment is set out only in our specification sheets. The handling and use of the products as

well as the applications thereof remain subject to the regulations defined by the current legislation ofeach country and shall not give rise to the responsibility of our company.

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Bleachnig_textiles Using Sodium Chlorite