After greige fabrics have been received in the greige room by the processing mills the bales are opened and inspected. It is checked and verified weather the greige fabrics meets the specific standards, and also checked all weaving defects are marked out. Fabric inspection involves following three processes
Perching: It is a visual inspection and it is carried out through the inspection table which consists of a frosted glass with lights behind and above it. The fabric imperfections like thick places, defects, yarn knots, stains and other flaws are marked.
Burling: It is the removal of yarn knots or other imperfection from the fabric.
Mending is obviously, the actual repair of imperfections. Knotting should be carefully and thoroughly so that the repair or holes is not visible.
The recent innovation by the use of laser beams. The imperfections like stains, holes, hick places, loose ends and protruding threads of warp and weft are detected. The apparatus works by the on-line system and eliminates human error.
After the goods have been inspected, checked and classed in the greige room, according to quality and stamped, they are sewn together, end to end, by sewing machines especially constructed for this purpose. Stitching should be done in such a manner that the creases in fabric at the time of stitching should be avoided.
The use of proper stitching thread is necessary to avoid stitch marks during colour padding. For heavy fabrics intended for mercerizing and continuous operations, the seam should be wider (15 mm) and stronger. Sometimes, glueing or bonding of fabrics to bind the end pieces by fusing polymer films under the action of high temperature is used instead of sewing to reduce the amount of rags. Goods of similar weight, width and construction and the goods which will receive a similar treatment are batched together and each lot is given a number.
The main objectives of preparation treatments of textile materials are;
- Remove from the fibers all impurities, both naturals and / or those added during production that may interfere in subsequent dyeing and / or finishing process.
- Improve the ability of the fibbers to absorb water solutions of dyes and chemicals.
- Impart the proper brightness or whiteness to fabrics according to need, especially when brilliant or certain pastel shades are desired.
Preparation of Cotton Fabrics
Cotton fibers on the average consist of 90-96% cellulose when bone-dry. The approximate amounts of other materials present in the fiber are: 1.1 – 1.9% proteins, 0.7-1.2% pectin, 0.4-1.0% waxes, 0.7-1.6% ash, and 0.5-1.0% other impurities.
In addition to the natural impurities, a considerable amount of fine particles of broken seeds and other fragments of the cotton plant are still left attached to the fibers. During the processing of fibers into yarns and during weaving or knitting operations, additional impurities are accumulated. Spinning oils are used in knitting, and sizing materials are applied to the warp so that it won’t break easily during weaving. Woven fabrics coming out of the loom may contain as much as 20% foreign substances, natural and added materials, most of which has to be removed during the preparation o the fabric, prior to dyeing.
The preparation of fabrics made of cotton or polyester / cotton blends may include some or all of the following operations: singeing, desizing, scouring, bleaching, and mercerizing. With increasing demands for savings in chemicals, energy, and water, certain treatments are often combined. An example of combined application is the so-called ‘oxidative desizing’ where desizing and scouring are taking place at the same time.
Before the removal of impurities by wet processing begins, fabrics are usually singed. In this process fuzzy and protruding fiber ends are removed by burning them off. Singeing is done to fabrics that require a clear and smooth surface such as broad cloth and batiste, and fabrics to be printed, and it is usually performed on both sides of the fabric.
- Singeing improves surface appearance of fabrics and also reduces soiling of fabrics.
- Singeing reduces the fogginess caused by differential light reflection by protruding fibres from the surface of the fabric and improves the appearance of the dyed fabrics.
- The singeing of a fabric containing thermoplastic fibers such as polyester will also reduce its tendency to pill.
In s singer, a fabric is passed over open flame at very high rates (e.g. 300 yards per min.) to prevent scorching, after which the singed fabric is passed through water (or introduced directly into the desizing bath) to extinguish any sparks that may cause it to ignite.
Uneven singeing may lead to un-level dyeing. Therefore, the fabric is passed through the singer flat, in open with and under a slight tension. The gas burners should be properly controlled and maintained; otherwise streaky dyeing may occur. Also, the same level of singeing must be applied to face and back of fabric, to avoid fact to back shading.
Prior to weaving, cotton warp yarns are coated with sizing materials to provide them with additional strength, and resistance to abrasion. The most common sizing materials used is different kinds of starches, and various grades of partially hydrolyzed polyvinyl alcohol.
Types of Desizing
1. Rot Steeping, 2. Acid Steeping 3. Oxidative Desizing, 4. Enzymatic Desizing
- Acid Steeping:
In this method, dilute hydro choleric acid or sulphuric acid or a mixture of both is used to hydrolyze the starch from the sized fabric. The fabric is steeped in 0.25% (10 g/l) of the acid, passed through padding mangle and kept for 6 hrs. The temperature rises to 50C.
The starch present in the fabric is liquefied and is easily removed by washing. Since, acid can attack cellulose care has to be taken to avoid damage of cotton.
The fabric should be washed with hot water after desizing.
Advantage: The time consumption is reduced compare with rot steeping
Disadvantage: There is no control over the process.
- Oxidative Desizing:
In this process starch is oxidized. It is an instant desizing carried out within 2 to 4 minutes. It is carried out under temperature of 95C. The commonly used oxidative desizing agents are hydrogen per oxide or potassium per sulphate of sodium bromide.
In this process the time required for desizing is less and the process is continuous. The major draw back of this process is expensive and slightly complex procedure. There may be possibilities of penta chloro phenol content in the desized fabric that may be harmful.
- Enzymatic Desizing:
The most commonly used and the most effective method of removing starch from the fabric is by the use of enzymes. The positive feature of this process is there is no possibility of damaging the fabric and also imparts softness and improves the wet ability of fabric.
The enzymes are complex proteins found in vegetable and animal cells. Enzymes are sensitive to their environment. They have a limited self life of only several months, and should be stored at temperature below 21C. When used at temperatures or pH other than those recommended they will be destroyed.
The commonly used desizing enzymes are
- Alpha amylase,
- Pancreatic enzymes (animal origin) e.g. Novoformosol,
- Malt enzymes (vegetable origin) e.g. Diastofor,
- Bacterial enzymes (bacterial origin) e.g. Repidase
Most desizing enzymes have their optimum temperature operation in the range of 38-68C. They are, however, enzymes that operate well at the boil and the above. Within the recommended range of a specific enzyme, there is a temperature where the enzyme shows maximum activity. The pH of the impregnating bath, which is usually in the range of pH 4.5 – 7.5 has to be adjusted properly so that the enzyme will perform with high efficiency.
Fabric is impregnated with de-sizing bath stored 8-12 hrs. Impregnating bath contains Required amount of enzyme, Wetting agent and Nacl. Fabric thoroughly washed with hot water.
New Development in Enzyme desizing:
Enzyme application at high temperature 96’-100’C Fabric padded with enzyme bath then passed Wet steam 96’-100’C temp. De-sizing completed less than 1 minute The main advantage of de-sizing with enzymes is that there is no risk of damaging the fibres. However, it is relatively expensive.
This cleaning treatment, also referred to as alkali boil-off, removes most of the cottons impurities. This includes sizing residues, wax, oil, protein, and other foreign substances like sand, dust and leafy matters. The scouring efficiency is mainly depends on maturity, fineness and origin of cotton fibers.
Kier boiling is most commonly used for scouring operation. A kier is a cylindrical vessel capable of holding 1 to 5 tons of fabric. It is provided with pressure gauge and safety valve. For kier boiling, a mixture of 5-10 g/l caustic soda solution, 1% sodium silicate, 1% soap along with wetting agent. Boiling takes place at 101c temperature for 6-8hrs under pressure.
After completion of boiling the liquor is completely drained out and immediately follows washing with cold water. It is the best way of scouring cotton fabrics as the maximum impurities are removed; moreover, the traces of PCP are also removed. However, it is a time consuming batch process and not preferred now a days.
In continuous machines, scouring can be carried out through a series of J-Boxes for progressive desizing and scouring. The desized and washed fabric is padded with strong alkali solution (5-10 gm/lit NaOH or mixture of NaOH & sodium carbonate), emulsifying agent and wetting agent. After padding the saturated cloth, it enters in to J-Box where it remains for 40-60 mins in plaited form at a temperature of about 100°C. The cloth before entering J-Box is rapidly preheated by means of steam in U- shaped heating tube. The internal surface of the entire J-Box is very smooth to avoid any friction in the moving cloth.
Bleaching is the process of removing natural coloring matters that cannot be removed from fibers by extensive scouring. Bleaching is also required for fabrics that are to be dyed in pastel shades, in particular light blues and violets, and for materials to be dyed in colors with maximum brightness. The main objective of bleaching process is, to achieve the desirable level of whiteness with minimum damage to the fibers, and within the shortest possible time.
Currently, hydrogen- peroxide is by far the most widely used bleaching agent. Sodium chlorite (Na C1O2), sodium hypochlorite, bleaching powder, per acetic acid and some lesser extent sulfur- di- oxide (SO2) gas, sodium hydrosulphite (Na2 S2 O3) ozone (O3)
Sodium hypochlorite Bleaching
The yarn or cloth after scouring is thoroughly washed before bleaching and is squeezed or hydro extracted to remove excess of water as otherwise it would dilute the bleach liquor or “chemicking”
In typical batch treatments of cotton fabrics with sodium hypochlorite in kiers, the bleaching bath is prepared as follows:
Sodium hypochlorite 1-3 gm/lit of available chlorine
Ph of the bath 11 – 11.5
Temperature Room temperature (30 – 40 c)
Time 45 min – 2 hrs
Since the bleaching is done in alkaline medium (pH 10-11) the alkali, which is present in the material has to neutralized with dilute mineral acid. It is referred as “souring”
- Powerful and economical method of processing
- Free from the defects of bleaching powder
- Excessive chlorine may cause pollution
- All protein impurities must be completely removed before bleaching otherwise the fabric may turns yellowish.
- Residual chlorine must be removed.
To remove the residual chlorine, the fabric is to be washed with sodium – thio – sulphate (chlorine destroyer) or hydrogen peroxide (weak solution) the chlorine will wash away. This treatment is called “antichlor”
Advantages of bleaching with hydrogen peroxide
- Hydrogen per oxide is a universal and is used extensively for the bleaching of cotton materials. It can be employed for bleaching fibers like wool, silk, polyester and other man- made fibers also, under a wide range of application conditions.
- It repairs less manipulation of fabric and hence less labor.
- The loss in weight of fabric in bleaching is less than that with hypochlorite bleaching.
- Per oxide bleached are more absorbent than hypochlorite bleached fabrics.
- No possibilities of yellowing of fabric.
- Bleaching with hydrogen per oxide can be carried out by batch or continuous methods, and at room or high temperatures.
- In addition, hydrogen per oxide relatively inexpensive, does not release toxic chemicals or unpleasant odour, and does not cause corrosion of equipment.
CONTINOUS BLEACHING PROCESS
In this process desizing, scouring and bleaching are combined together. These are very attractive options since savings in time, energy, labor, etc., are possible. However, combined operations are not always sufficient for preparation of certain cotton fabrics for dyeing and finishing.
- 》In this method, the fabric is padded with the enzyme bath and then passed into a wet steam chamber at 96C- 100C where the desizing takes place in less than a minute.
- The fabric after desizing is impregnated a solution of 4-6% NaOH, 1-3 g/l wetting agent, and 1-3 g/l emulsifier at the period of 2-3 minutes.
- The fabric after impregnation is taken out and piled into J-box at 98C and allowed to remain there for two hours during which period the alkali reacts with the impurities.
- The fabric is then rinsed and impregnated with 1% hydrogen peroxide at pH 10.5-using1% sodium silicate as a stabilizer and wetting agent.
- The cloth after impregnation is again heated to 90-95C and stored for an hour in J- box.
- It is then rinsed thoroughly.
Since the bleaching is carried out in alkaline medium the alkali, which is present in the material, has to be neutralized with dilute mineral acid. This treatment with dilute hydrochloric acid or sulphuric acid is known as souring.
The fabric is impregnated with dilute hydrochloric acid or sulphuric acid at room temperature for 30-60 minutes; the fabrics are then thoroughly washed with water and then dried.
The treatment of cotton fabrics or yarns with a cold concentrated solution of sodium hydroxide solution for one minute or less is called mercerization: cotton fibers swell, untwist and their bean shaped cross section changes into round form.
Due to swelling of cellulose in NaOH solution many hydrogen bonds are broken and molecular structure tends to become de-crystallized. Cellulose I is formed with sodium hydroxide at concentration of 12-19 (by weight). Soda cellulose II is formed at concentrations between 20 and 45% by weight. Cellulose I exists in a parallel chain conformation while cellulose II exists in anti parallel conformation.
Currently, mercerized cotton is produced mainly for its increased dyeability and luster. The need for these qualities is evident when trying to obtain deep shades with the bright reactive dyes. Also the acquired luster will provide a lustrous background for the dyes that will further enhance their brightness.
Mercerization improves the following properties.
- Strength would be increased 15-25%;
- Luster further enhanced;
- Greater affinity to water dyes and other chemical finishes.
Fabric is padded with about 20-25% NaOH solution containing a wetting agent and then passed over several cans to allow a doweling time of approximately one minute. During this time NaOH will penetrate the fibres and react with them. At this stage the tension is applied lengthwise.
The fabric is then placed on a stenter frame (tension now is applied in both warp and weft directions) and is pulled to its desired dimensions. While on the tenter frame, the fabric is washed by spraying water until the amount of sodium hydroxide on the fabric is reduced to only a few present.
Now, the fabric is removed from the tenter frame, and passing the fabric through several washers, one of which contains a diluted solution of sulfuric or acetic acid removes the remaining alkali.
The fluorescent brightening agents called ‘optical brightening agents, increase the apparent whiteness or brightness of materials, and are used in textiles, detergent, paper, paints, etc.
Fluorescence is the emission of light caused by radiation. Fluorescent compounds used for whitening have the property of absorbing ultra –violet light and re-emitting energy in the form of weaker energy, i.e. visible light. A part of the absorbed U.V. energy is transferred into heat. Sunlight contains U.V. radiations in the range of 330-380 nm (near U.V.) that are absorbed by optical brighteners used for whitening
Optical whitening agents are applied to textile at the time of scouring, bleaching or finishing. The florescent brighteners should be stable in the bleaching bath and also should be stable on the fibre at the heat setting temperature in the case of polyester. Florescent brighteners can be applied to cotton materials prior to resin treatment or it can be added to the resin formulation bath. For blended fabrics, a mixture of two whiteners those are suitable for both the component fibres used.
Application of Optical Brighteners
The application of optical brighteners are depends on the types of fibres on which is applied and accordingly can be classified as direct, disperse and cationic types. The direct brightening agents are mainly used for the brightening of cotton, linen, viscose and nylon materials. The acid florescent brightening agents serve mainly for the brightening of silk and wool fabrics. The disperse florescent brightening agents are used primarily for polyester, acetate and acrylic materials.
CLEANING OF WOOL
Raw wool contains the following impurities; Wool grease, soil, dust, cellulose fragments and saint (dried perspiration, mostly soluble potassium salts
Scoured with detergent solution at pH of up to 11 at below 60’ C temperature with anionic/non ionic detergent and small amount of weak base (0.4% Na2Co3) sodium carbonate / ammonia. This cleaning is sufficient for preparing wool for spinning. However wool will be scoured again prior to dyeing
Cellulose material such as leaves, grass, seeds, and vegetable fibers are removed. The fabric (some time’s loose fibers) is treated with a dil. Solution. Of H2So4, then dried, and cured. Cellulose materials decompose to form a very fine black powder, which can be easily dusted out.
Two methods of bleaching are commonly used, viz. (i) the oxidation method using hydrogen peroxide and (ii) the reduction method employing sulphur dioxide, sodium bisulphate or sodium hydrosulphite. Sometimes the wool is bleached with peroxide followed by bleaching with sulphur dioxide.
A method of peroxide bleaching under acidic conditions has been devised; the bath is made up of 4 volume of H2O2 containing 0.25% formic acid and adjusted to pH 3 to 3.5 at room temperature. The scoured material is impregnated with the peroxide solution on a padding mangle, and allowed to stand overnight, it is then washed.
CLEANING OF SILK
Degumming of Silk
Silk fiber in its natural state, contains a heavy coating of a glue-like substance or gum called sericin which gives it a harsh and stiff feel and hides the rich luster and whiteness of the real fiber. This silk-gum amounts to about 25 per cent of the weight of raw silk. Wild silks such as Tasar silk, Eri silk and Muga silk also contain gum. Unless the gum is removed, silk does not exhibit its characteristic luster, smoothness as well as softness and tends to dye unevenly.
The removal of gum from silk is known as “degumming” or “boiling-off” and is a very important preparatory process. When the gum is completely removed, silk loses 22 to 30% of its weight.
Degumming with Synthetic Detergents
Synthetic detergents are now being increasingly used in place of soap. Their main advantage is that they permit continues processing of piece goods and the time of treatment is shorter viz. 40 minutes.
In this method, the material is treated with 2.5 to 5 g/l detergent (Sandopan SRS liq. of Sandoz) at pH 11.2 to 11.5 for 30 to 60 monutes at boil and then rinsed thoroughly first with hot water and then with cold water.
The disadvantage of this method is that soft feel is not always obtained.
Degumming with Enzymes
This treatment is longer than that with synthetic detergents and involves a special pre-treatment before enzyme treatment requiring swelling of the gum.
The material is first treated with
0.5 g/l soda ash ; 0.5 g/l Glauber’s salt ; 3 g/l penetrating agent like Imerol XN liq. (Sandoz); at boil for about 1 hour.
It is then treated with 1-5 g/l protein enzyme; 1 g/l soda ash and ; 2 g/l Glauber’s salt; at 45°C for about 1 hour.
It is further treated in a third bath containing 0.5 g/l soda ash; 2 g/l sodium silicate; 2 g/l Imerol XN liq. (Sandoz) 0.5 g/l Glauber’s salt at boil for 1 hour.
It is a three-step process. Improper degumming can give rise to mechanical damage causing chafe marks at the surface; as a result, the ends dye a lighter shade.
HEAT – SETTING OF SYNTHETIC FABRICS
Heat setting of thermosetting is a heat treatment applied to fabrics made of thermoplastic fibers such as polyester or nylon to impart dimensional stability. Heat – setting temperatures are well above the glass transition temperature (Tºg) of the fiber, and heat is applied by means of hot air, heated cans, or steam. The treated fabric acquires dimensional stability, i.e. a memory to the shape it was during the heat setting.
It will resist shrinkage and creasing, and will have the ability to maintain pleats present in the garment during the heat – setting treatment. In a typical heat – setting of polyester, the fabric is placed on a tenter frame set to the desired final width, and introduced into a hot – air oven at a constant temperature chosen in the range of 190 – 215ºC for 30 to 90 seconds.
Heat – setting affects the dye ability of the fiber. Usually it decreases its dyeablity, and therefore when performed before dyeing it is extremely important to apply the heat – setting uniformly. Uneven temperature in the oven may cause difference in the fabric from the selvedge-to-selvedge and/ or from selvedge to center, which will show later as uneven dyeing.
SELECTION OF PROCESS SEQUENCE
The selection of correct process sequence is most essential on the performance of further processing such as dyeing printing, finishing etc. the major problems are mainly due to wrong and inadequate selection of process sequence. The fabric preparation depend on type of fibres and fibre blend composition ,yarns, fabric quality parameters, size type, colour fastness properties requirement and endues.
Cotton knitted fabrics are subjected to a modified routine in which singeing and desizing stages are some times omitted, where as for woven fabrics desizing can not be omitted. Synthetic fibers are produced under controlled conditions and contain only added impurities, there cleaning is comparatively easy. However, when they are used in admixture with natural fibers, the problems aggravate. Preparation of fabric also depends on the end requirement. For example , “ full white bleach” is done in the case of goods which are supplied white, ‘half beach’ for over dyeing or when ever big blotch and fully covered designs are to be printed and ‘full bleach’ whenever pastel shades are to be dyed. Cotton Fabrics to dyed in deep shades will most likely require mercerization in order to improve their dye ability
Fabrics preparation is a costly series of energy consuming processes. Economic consideration, thus have led to considerable modification of the existing processing sequences. The traditional sequence of pre – treatment is shortened by single stage bleaching, where kiers are still in use. It is there for necessary to balance the cost of preparation by optimizing the process sequence, conditions and development for shorter routes. Furthermore, several new types of machineries and newer process have been evolved to meet the requirement of efficient but economical and rapid preparation of cloth. All these factors necessitate the alteration of process sequences to get the optimum results and the technologists can decide as and when any of the stages are to be by-passed without compromising the quality of the goods.
Process sequence for cotton fabric to kier
Method A: for white / printing: (Poplin, Cambric, Rubia, Mulls etc..) greige mercerize → scour in kier → Rinse in kier → Wash → Chemicking in Cistern → Wash → Open boil kier using H2O2 → Rinse in kier → wash → scotch →Dry.
Method B: for dyeing: (Poplin, Cambric, Rubia, Mulls etc.) greige mercerize → Desize in Cistern → Wash → scour in under pressure → Wash → Open boil in kier using H2O2 → Rinse in kier → Wash → Scutch → Dry.
Process sequence for cotton fabric on J – Box
Method A : For white / Printing : greige mercerize → Desize in J – Box Cistern → Wash → Caustic saturator → steam in J – Box → Wash → Chemick saturator → Store in J- Box → Peroxide saturator → Steam in J – box → Wash → Scutch → Dry.
Method B : For dyeing : greige mercerize →Wash → Caustic saturator → steam in J – Box → Wash → Chemick saturator → Store in J- Box → Wash → Antichlor → Wash → Neutralise → Scutch → Dry.
Process sequence for cotton fabric on pad – Roll/ Thermoreaction Chamber
( T.R.C )
Method A : For white / Printing : Singeing cum Desizing → Wash – Scour on pad –Roll (T.R.C) → Wash → Dry → Mercerize → Bleaching on pad –Roll (T.R.C) → Steaming → Wash → Dry.
Method B : For Dyeing : greige mercerize → – Scour on pad –Roll (T.R.C) → Wash → Bleach on pad –Roll (T.R.C) → Wash → Dry.
Process sequence for knitted cotton goods
Method A : For white goods : Hypochlorite bleach → Reduction or peroxide bleach + Fluorescent Whitening Agent (FWA)
Method B : For white goods : Demineralization acid medium (metals) → peroxide bleach + Fluorescent Whitening Agent (FWA)
Method C : For white goods : Alkaline treatment + Demineralization (oils, paraffin spots + metals ) → peroxide bleach + Fluorescent Whitening Agent (FWA)
Process sequence for woollen fabric
Method A : For dyeing in worsted routine: Greasy wool → Scouring → Carding → Gilling → Combing → Combed Tops → (Top Dyeing) → Gilling → Recombing → Drawing and Spinning ( yarn Dyeing ) → Weaving /knitting → Fabric Dyeing → Finishing
Method B : For dyeing in worsted routine: Greasy wool → Scouring (loose stock dyeing ) → Oiling→ Willeying → Carding → Spinning → Yarn Dyeing → Scouring and milling → Fabric Dyeing → Finishing.
Process sequence for Silk fabric
Method A : greige Inspection and Stitching l → Singeing → Washing → Degumming → Bleaching → Dyeing → Padding mangle (apply some adhesive also) → Stentering → Felt calendaring → Curing → Decatising ( to impact fluffy & soft feel)
Process sequence for Polyester fabric
Method A : Heat setting → Scouring → Bleaching → Weight reduction → Dyeing → Drying on Stenter.
Process sequence for Polyester/ Cotton blends
Method A : For yarn: Scouring → Bleaching → Dyeing → Anti- static finishing.
Method B : For white goods: Desize → Scour → Mercerizing → Heat setting → Anti pilling i.e. Brushing, Cropping and singeing → Bleaching and Optical whitening → Stentering and Sanforizing.
Method C : For Dyeing : Desizing → Scouring → Mercerizing → Heat setting → Bleaching → Dyeing → Anti pilling i.e. Brushing, Cropping and singeing → Stentering or Sanforizing.
Process sequence for Polyester/ wool blends
Method A : General : greige Inspection → Removal of stains → Scouring → Pre – setting ( setting of wool component by crabbing ) → Drying → Heat – setting → Dyeing → Brushing and cropping → Singeing → Light source ( if necessary ) → Steam or Damp → Decatising or Pressing.
Method B: Knitted goods (Crisp Handle): Open steam (Allow full relaxation ) → Light brushing → Close Cropping on face side of fabric → Decatising.
Method B: Knitted goods (Soft Handle): Heavy scour → Cropping on face side of fabric → Open Steam → Decatising.