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. In this article I will discuss bleaching process of different blended fibre fabrics.
Bleaching of Blended Fibre Fabrics:
In tropical countries synthetic fibers blended with cellulosic fiber fabrics are very popular due to their excellent combination of aesthetic properties and easy-care properties. Thus, bleaching of blended fibre fabrics before further processing is an important step. Many of the preparatory processes used for natural and synthetic fibres have little or no application in preparation of blended fibre fabrics.
List of Bleaching of Blended Fabric:
- Bleaching polyester/cotton blended fabrics
- Bleaching of polyester/wool blended fabrics
- Bleaching of nylon/cellulose blended fabrics
- Bleaching of nylon/wool blended fabrics
- Bleaching of acrylic/cellulosic blended fabrics
- Bleaching of acrylic/wool blends
- Bleaching of acetate/cellulosic blends
- Bleaching of polyester/linen blended fabrics
- Bleaching of wool/viscose blends
- Bleaching of viscose/cotton blended fabrics
- Bleaching of Cotton Weft Knitted Fabrics
1. Bleaching polyester/cotton blended fabrics:
Polyester fiber in blends with cellulosic fibres in the ratios of 65/35 and 50/50 are common construction. When cellulose portion is rayon, the blends rarely require bleaching, but when cotton is present bleaching is usually necessary. Bleaching treatments of such blends are normally required to remove the natural colours of cotton, sighting colours and if the polyester portion is turned yellow at the time of heat-setting operation.
Chlorine bleaching can be done at 1-2-degree Tw bleaching powder solution for 2-3 h, which is followed by dechlorinating treatment. Sodium thiosulphate, acidic sodium sulphate, ammonia, H202 etc. can be used as dechlorinating agents.
Alkaline hydrogen peroxide bleaching is the most preferred system for polyester/ cotton blends and bleaching can be carried out on various equipment using batch wise, semi-continuous and continuous method.
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The most effective method of bleaching polyester/cotton is sodium chlorite, which may be followed by peroxide bleaching. Chlorite bleaches the husk, but does not destroy them completely. Polyester/cotton blends may be bleached with sodium chlorite in long liquors and also by pad-steam process.
Peracetic acid can also be used for bleaching polyester/cotton blended fabrics with a solution containing 4 g/1 peracetic acid, 1 g/1 tetrasodium pyrophosphate (stabiliser) and 1 g/1 wetting agent with a liquor ratio of 5 : 1. Peracetic acid has a pH of about 1.5 and the bleaching bath solution is adjusted to pH 5.5 with the help of dilute alkali. The material is entered cold and mn for 10 min. Then the temperature is raised to 65-70 C in 15 min and bleaching is continued at this temperature till the concentration of peracetic acid drops to 0.09 g/1. The fabric is then washed thoroughly with hot and cold water. The bleaching can be performed in the kier or in the jigger.
2. Bleaching of polyester/wool blends:
The wool portion contained in the blend show reversion to a creamy colour and yellowing of the fabric. In general, blends containing wool and polyester fibres can be bleached with hydrogen peorxide either in acid or alkaline medium without risk of damage.
In acid medium, the fabric is treated with a solution containing 30-40 ml/1H202 (35%), 2-4 g/1 organic stabiliser, 0.25 g/1 wetting agent and 0.25 g/1 detergent at pH 5.5-6 (acetic acid) for 40-60 min at 80 C or 2-2.5 h at 65 C The treated fabrics are then given warm and cold rinse.
In alkaline medium, the bath comprises of H202 (35%), 30-40 ml/1; sodium pyrophosphate, 2-4 g/1 ; ammonia to maintain the pH between 8.5-9.0. The bath is set at 40 C and the goods are treated for 2-4 h, and rinsed well in warm and cold water.
As peroxide-bleached goods tend to show reversion to a creamy colour, it is usual to follow with a treatment in reducing agent to stabilise the bleach in a bath containing 3-4 g/1 stabilised hydrosulphite and 1 g/1 synthetic detergent at 50 C for 30-40 min, rinsed and dried.
Polyester/silk blends can be bleached by a similar manner to that of polyester/ wool blends.
3. Bleaching of nylon/cellulose blends:
Blends of nylon and cellulosic fibres may be bleached with either H202 or NaC102, using batch wise or continuous method. H202 does not bleach nylon and normal methods of bleaching degrade nylon and cause yellowing. Blends containing 30% or less of nylon may be bleached by the continuous H202 method, and in such cases, cotton will absorb the peroxide preferentially and so protect the nylon from damage. The use of protective agents which prevent undue damage to the nylon poriton of the blend is reported.
The goods are entered into a bath containing 2-3 volume H202, 1 g/1 sodium hydroxide flake, 0.2 g/1 peroxide stabilizer, 0.25 g/1 sequestering agent and 0.002 to 0.05 g/1 free radical suppressor at 40 C the temperature is raised to 85 C and then the treatment continued for 1 h. The treated goods are then cooled and rinsed thoroughly. When appropriate, selected optical brighteners may be incorporated in the peroxide bleach bath.
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Hypochlorite does not damage nylon but it has got no bleaching action on it. Sodium chlorite causes no degradation of either cellulosic or polyamide and is a better bleaching agent than peracetic acid for cotton. For batchwise bleaching the fabric is treated with a solution containing sodium chlorite (2-5 g/l) at pH 3 to 4 at 90 C for 1 – 1/2 to 2 h. This is followed by a treatment in a 2 g/1 solution of sodium carbonate at 40-50 C and finally hot and cold rinses are given in water. Pad-roll and continuous processes are also used for bleaching polyamide/cellulose mixture.
4. Bleaching of nylon/wool blends:
It is difficult to bleach this blend since the method normally used for nylon degrade wool. The usual method is either to bleach the wool portion with H202 at low temperature or to carry out reduction bleaching process. Alkaline H202 bleaching always damage the polyamide fibres to some extent. Normal alkaline H202 bleaching process may be used with safety on blends containing up to 25% polyamide, but an acid bleach must be used when proportion exceeds this figure. The fabric can be bleached with a solution containing 12-15 ml/1 H202 (35%), 2 g/1 tetrasodium pyrophosphate, 1 g/1 EDTA (30%) and 0.25 g/1 protective agent at 60-65 C for 45-60 min and then rinsed well in water.
5. Bleaching of acrylic/cellulosic blends:
If the cellulosic portion is cotton, bleaching is invariably required for this fibre. If the acrylic portion does not require bleaching, then a peroxide treatment can be done at pH 9.5. The alkaline condition should not be high as otherwise it would cause degradation of the fibre. The fabric is treated with a solution containing 7.5-10.0 g/1H202 (35%), 3 g/1 sodium silicate (79-degree Tw and 1 g/1 sodium carbonate at 90 C for 45-60 min. After bleaching the bath is cooled slowly to 500C rinsed and neutralised.
When acrylic fiber also requires bleaching, then mild chlorite treatment will act on both the fibres in the blend. The bath is prepared at 35~ with 1.5 g/1 sodium chlorite, 2 g/1 oxalic acid, 1 g/1 tetrasodium phosphate and 1 g/1 corrosion inhibitor.
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The bleach bath should give a pH of about 3.5 to 4.0. The temperature of the bath is raised to 90 C over 30 min and processing continued for 30-45 rain at this temperature. The bath is cooled slowly to 50 C and then rinsed thoroughly. An antichlor treatment is given in a bath containing sodium bisulphite (1.5 g/l) and tetrasodium phosphate (1.5 g/l) at 60 C for 20-30 min, cooled the bath and rinsed well. The temperature of drying should not exceed 80 C.
6. Bleaching of acrylic/wool blends:
Hydrogen peroxide is not suitable for acrylic fibre at highly alkaline condition and moreover acrylic fibre turns yellowish on alkaline peroxide treatment. The discolouration can be improved by after-treatment with formic acid in presence of detergent. Acrylic/wool blends can also be bleached by a reduction bleach or by combination of peroxide and reduction bleaching process.
7. Bleaching of acetate/cellulosic blended fabrics:
Diacetate/viscose blends have been used in dresswear, shirting and underwear. These blends may be bleached with hydrogen peroxide or sodium hypochlorite, preferably the latter. The goods may be treated with a solution containing 5 g/1 H202 (100 vol.), 2 g/1 sodium silicate and 1 g/1 soap at 70-75 C for a minimum time of 30 rain.
Bleaching can be done by treatment with a solution containing sodium hypochlorite (2-3 g/1 available chlorine) adjusted to pH 10 at room temperature for 30 min, and then the fabric is given cold treatment with 1 ml/1 HC1 and thorough rinsing. Alternatively, an acid solution is prepared with 10 ml/1 sodium hypochlorite (50-degree Tw and 2 ml/1 hydrochloric acid adjusted to pH 3. The treatment is carried out cold for 40 min, goods are well rinsed and then treated in a second bath with 3 g/1 sodium bisulphite at 40 C for 20 rain.
8. Bleaching of polyester/linen blends:
Polyester/long-staple fibres are used in the linen industry, where yams may be of either the “stretch broken” or “unbroken” type, but more commonly of the latter. The linen component of the blend may be of bleached or unbleached fibre and yams spun from unbleached fibre may be bleached before weaving. Most fabrics in this blend are woven on sized (singles) warps. Unmodified warp sizes are removed by enzyme treatment and non-cellulosic matter is removed by an alkaline scour.
Goods made from bleached yarns or fibres require only a light scour with 2 g/1 soda-ash along with 1 g/1 detergent. Goods prepared from unbleached yarns or fibre are padded with dilute caustic soda solution at 70 to 80 C batched on a roll and allowed to rotate at this temperature for 24 h. The goods are then rinsed, scoured and bleached with hydrogen peroxide or with sodium hypochlorite. Sodium chlorite is not normally used for bleaching this blend.
9. Bleaelling of wool/viscose blends:
Bleaching is usually carried out by immersion of the material in a liquor containing 1-2 vol. H202 and 5 g/1 sodium silicate or sodium pyrophosphate at 30~ overnight or at 40-50 C for 4 h. The bath is adjusted to pH 8 and it is advisable to add 0.25-0.5 g/1 of a suitable sequestering agent. For treatment by the shorter time, the concentration of H2O2 may be increased to 3 to 3.5 vols. The amount of H202, however, depends on the quality of wool or proportion of viscose in the blends.
10. Bleaching of viscose/cotton blends:
Viscose/cotton blends can be bleached either by batch method on jig and winch or by a continuous process using J-Box. Bleaching is done on a winch with sodium hypochlorite (2 g/1 available chlorine) adjusted to pH 10-11 with sodium carbonate, for 1 h at 25 C or alternatively, with 5 g/1 sodium chlorite adjusted to pH 4 with acetic acid for 30 min at 80 C Alternatively, the bleaching treatment may follow with alkaline hydrogen peroxide at 85 C.
In the continuous method the fabric is saturated with bleach liquor consisting of H202 and potassium persulphate, passed through a J-Box, followed by short boiloff, rinsing and drying over cans with a total processing time of 15 min. The temperature in the J-Box approximates 70 C and that of wash liquor 80 C.
11. Bleaching of Cotton Weft Knitted Fabrics:
Knitted fabrics can be produced from a wide range of fibres and blends, either as flat fabric or garments. Garments, ranging from outerwear (including sport and leisure wear) to hosiery, are generally weft knitted. Warp knits usually require filament yams which are 100% synthetic and do not normally require bleaching.
The weft knitted fabrics produced from 100% cotton and synthetic fibre/cotton blends may be called jersey, rib or fleece depending on how the loop lie. While weft knitted fabrics are comfortable to wear, since they are light and pleasing to the skin, they do have disadvantages in that they are easily deformed by mechanical stresses and wet creased than woven fabrics.
The bleaching of knitted fabrics-like that of woven fabrics-should result in high whiteness, low chemical or abrasion damage, low crease formation and high absorbancy to water. Sodium hypochlorite is generally not suitable for bleaching of knitted fabrics on account of alkaline nature of the bleach process and recent restriction on adsorbable organo-helogen compound (AOX) generation. However, hypochlorite may be used at pH 11 and at temperature not exceeding 30-35~ in presence of effective wetting agent. Then the fabric is given an antichlor treatment, the whole process taking 3-4 h.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.