INTRODUCTION:
The main innovation of the project lies in the combination of two or more continuous processes in a single step, as would be the desizing, scouring, bleaching. A semi-continuous process has been designed in which desizing and scouring are done simultaneously using a commercial enzyme cocktail. Immediately afterwards bleaching of cotton piece is performed in a separate bath at high temperatures. Peroxide removal has been done using the enzyme catalase without the need of high rinsing water consumption. Finally, a polishing step was carried out using the first bath where initial enzymes for scouring and desizing have been temperature inactivated.
The development of this semi process allows the reuse of water from the first bath and water reuse of bleaching. This has a direct impact on water savings and energy as enzymatic processes are developed at lower temperatures than conventional chemical processes. This process also involves a reduced pre-treatment time as it reduces the time required for rinsing in case of bleaching, and although the time required for enzymatic desizing & scouring is greater, it is compensated with a lower lost in intermediate processes.
The implementation of these changes has not affected the quality of the fiber with respect to conventional treatments. We evaluated the residual starch content, the degree of bleaching (using optical test), tissue stiffness). No significant differences were observed compared to conventional chemical process. Besides the fibers after enzyme treatment has been analyzed by SEM and no differences have appeared with respect to conventional chemical processing.
Pretreatment process:
Cellulosic fibers, whether they are natural or regenerated need some form of treatment to make them suitable for dyeing or finishing. This treatment, which removes natural or added impurities, is called preparation pretreatment. It can be carried out on loose fibers, yarns or fabrics.
Unless the fibre is uniform in whiteness, absorbency and chemical composition and has law levels of impurities it is unlikely that it will take up dye or finish in a uniform way or to the maximum extent possible. The prepared fabrics must have ‘fitness for purpose’. For example, a fabric to be given a water-repellent finish must be free from residual surfactant. Fibers used for medicinal end uses must have a law ash content.
All such requirements must be met against economic constraints relating to the costs of water, labour, plant, chemicals and energy. The first, water, is particularly important. Not only can the purity of the water affect many of the requirements but the volume and the environmental impact of effluent can, and is, adding increasingly to the cost per kilogram of textile produced. The effluent with the highest biological/chemical oxygen demand stems from pretreatment.
Successful preparation depends on four factors:
Ø The level and type of impurities present.
Ø The chemicals used in the various stages of preparation.
Ø The water supply.
Ø The type of machinery used.
The flow chart of pretreatment process in case of woven cotton fabric:
Singeing
↓
Desizing
↓
Scouring
↓
Bleaching
↓
Mercerizing
The flow chart of pretreatment process in case of knitt cotton fabric:
Demineralization
↓
Single stage scouring bleaching
↓
Enzyme treatment
↓
Mercerezing
Chemicals for pretreatment process:
Ø Caustic soda.
Ø Surfactants/wetting agent.
Ø Detergents.
Ø Sequestering agent/chelating agent.
Ø Sodium silicate.
Ø Soda ash.
Ø Solvent.
Ø Enzyme.
Ø Stabilizers.
Ø Sodium hypochlorite..
Ø sodium chlorite.
Ø Hydrogen peroxide. etc.
Singeing
Singeing is a finishing method for smoothing surfaces of fabrics by burning and removing fuzz thereon, and visualizing fabric textures. A gas-singeing machine is normally employed. The gas-singeing machine is typically equipped with four burners, and is capable of singeing one or both surfaces of the fabrics. A water-cooled roller is provided at a location opposite to the burners, thereby enabling singeing to be performed without undermining the strength of even thin fabrics. It is important to set a drain temperature of the water-cooled roller in a range of 50 to 55 C. Cautions are required because a dew-point is generated when the water-cooled roller is cooled down too much, and results in increased amount of remaining fuzz or adhered carbon. The fabric feed speed is preferably set at around 100 to 150m/minute. Object of singeing: v The purpose of removing loose hairy fiber from the surface of the fabric. v Giving its smooth even and clean looking face. Precautions: v Fabric should be dried properly before singeing. v Fabric should not contain anti-mildew agent like zncl which may be liberate acid at hich temparature and tender the cotton cloth by formation of hydrocellulose causes loose of strength. Types of singeing machine: 1. Plate singeing machine. 2. Roller singeing machine. 3. Gas singeing machine. Desizing
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Process controll in textile parameters for desizing:
Nature of Size: Prior to desizing spoofing test.
Action: Select correct method of desizing
%Wet pickup during Desizing Action:Pick up should not be less than 110%. If pickup variation is there then adjust pressure.
Concentration of desizing agent and other ingredients: HCl-5gpl, Cellulase enzyme 3-5gpl.
Impregnation and dwell time: Non ionic wetting agent 3-5gpl. Dwell time checked during padding. Ensure optimum dwell time.
Temperature: During desizing in the bath (exothermic reaction may rise the temperature. Acid desizing is done at room temperature. Enzyme desizing is done depending on the enzyme.
Action: Regulate the steam supply. In acid desizing replenish the acid solution or provide water cooling system.
pH: Depending on the Enzyme
Desizing efficiency: Standard- Not less than 85%.
Scouring
To remove pectic substances and cotton wax contained in cotton woven fabrics, the use of caustic soda (NaOH), strong alkali, is the most effective. The caustic soda has the effects of not only converting water-insoluble pectose contained in pectic substances into water-soluble pectic acid soda, but also of hydrolyzing (saponifying) cotton wax. However, a single use of the caustic soda does not provide enough penetration, emulsion, washing, and scouring powers, and so a surfactant is generally used together. A chelating agent may also be used together to prevent insolubilization caused by calcium and magnesium. Also, a method of scouring with an enzyme instead of with the caustic soda is being considered in recent years. In this case, protopectinase is used for degrading pectin, and lipase and the like are used for degrading oil.
Objects of scouring:
v To make the fabric highly hydrophilic.
v To remove impurities such as oils, waxes, gum, husks,as nearly as posible.
v To produce a clean material by adding alkali.
v To make the fabric ready for next process.
v To remove non-cellulosic substance in case of cotton.
Scouring process depends on:
v The type of cotton.
v The color of cotton.
v The claanlliness of cotton.
v The twist and count of cotton.
v The construction of fabric.
v The shade % and type of shade of finished product.
Process of scouring:
1) Kier Lining: Prior to filling the fabric in kier.
i) STANDARD: No cheeping of interior wall of kier.
ii) ACTION: Ensure proper cementing of interior wall of kier.
iii) FILLING DENSITY: 7-8Kg/Cubic feet during filling of kier.
2) Removal of Air:
i) ACTION: Air removal before raising pressure
ii) STANDARD: No air, ensure complete removal of air.
3) Circulation:
i) During scouring, circulation is tested by flow rate method.
ii) STANDARD: Smooth circulation.
iii) ACTION: Cleaning of circulation pump.
4) Recipe:
i) Concentration of scouring liquor: Check by titration before and during process.
ii) ACTION: Adjust the concentration.
iii) PRESSURE: 15psi or as per quality and regulate steam supply.
iv) TIME: Previous time record or as per quality.
v) CONCENTRATION OF BLEND LIQUOR: Standard 90% must be consumed
Bleaching
Bleaching of textile material is a commercial, chemical process can be defined as- “Destruction of natural coloring matters to impart a pure permanent and basic white effects suitable for the production of white finishes, level dyeing and desired printed shade with the minimum or number tendering or without diminishing the tensile strength.”
Bleaching agent:
Hydrogen peroxide.
Calcium hypochlorite.
Bleaching agent.
Sodium hypochlorite.
Sodium chlorite.
per acetic acid.
Ozone.
Zinc dust.
Staneaous chloride.
Ferrous sulphate.
Sulphur di-oxide.
Hydrogen Peroxide (H2O2) Bleaching: The H2O2 bleaching is the most common bleaching method, and is environmentally friendly (degradable into water and oxygen). The best bleaching whiteness can be obtained at pH 11 adjusted using caustic soda. However, if metal ions, such as iron ions and copper ions are present in fabrics or in water, catalytic action of these metal ions causes abnormal degradation of H2O2, which may sometimes cause embrittlement of fabrics until holes (pin-holes) are generated due to the excessive degradation. Usually, the abnormal degradation of H2O2 and the resulting generation of pin-holes are prevented by adding a chelating agent. Also, as a stabilizer for H2O2, sodium silicate has been conventionally employed. The obtained bleaching whiteness is favorable. However, the use of sodium silicate has the following disadvantages. Silica scales are generated due to calcium ions and magnesium ions reacted with silica ions. The generated silica scales are adhered to machinery, damaging fabrics, or undermining hand of the fabrics. This case, however, suffers from a contradiction that complete removal of calcium ions and magnesium ions will undermine the stability of H2O2. To solve the disadvantages and contradiction, a non-silicate type stabilizer containing no sodium silicate has been developed, although the cost is higher than sodium silicate. Furthermore, when dyeing is performed without a drying process after the H2O2 bleaching, the problem of residual H2O2 arises. 10ppm of residual H2O2 or more causes dyeing problems, although it depends on a type of dye. To degrade the residual H2O2, a reducing agent or the like, or an enzyme (catalase) can be used. The enzyme appears as a current mainstream. Sodium Chlorite (NaClO2) Bleaching: The NaClO2 bleaching is performed after pH adjustment to 3.5 with an acid such as a formic acid. It provides soft hand with less embrittlement of cotton. Compared with the H2O2 bleaching, some insist that even bluish (?) whiteness can be achieved with the NaClO2 bleaching. Also, the NaClO2 bleaching is free from worries about pin-hole generation, as often occurs in the H2O2 bleaching. However, a harmful chlorine dioxide gas is generated during the bleaching process, and therefore, suitable measures including the equipments need to be taken for metallic corrosion problems and dechlorination purposes. After bleaching, a dechlorination process needs to be provided. A trace chlorine which exists in fabrics reacts with nitrogen compound to form chloroamine, or cause yellowing of the fabrics when exposed to light and heat. Note here that a special reducing agent, sodium thiosulfate (hypo), or hydrogen peroxide (so-called, `cosmetic bleaching`) is used for the dechlorination purposes.
Sodium Hypochlorite (NaClO) bleaching: The NaClO bleaching is characterized in that the bleaching is performed at room temperatures without requiring a heating apparatus such as a steamer, and that the chemical cost is low. However, the disadvantages of the NaClO bleaching include relatively low bleaching whiteness and embrittlement of fabrics. No particular pH adjustment is required in the NaClO bleaching. The NaClO bleaching is usually employed for bleaching fabrics after Kier-scouring, or for bleaching towels. For towels, the two-step bleaching comprising the NaClO bleaching and the H2O2 bleaching is preformed to obtain full whiteness. The NaClO bleaching is also employed for jeans fade adjustment, etc. Note here that a dechlorination process needs to be provided as in the NaClO2 bleaching. Since `chlorine free` is a recent trend, like PRTR, a method without using chlorine is being examined. The same applies to NaClO. Process of bleaching: |
By Kinetic study of bleaching, take 3 titration at 5 minutes. If decomposition in 2nd and 3rd titration is fast then metallic impurity is present.
Remedy: Hexametaphosphate (7-8gpl to 15-20gpl is taken. It does not chelate Fe ion. If Fe ion is present
then EDTA is used. If hardness of water is around 250-300 PPM, then 3gpl EDTA is sufficient, but in
presence of Fe Ion 6-8gpl is needed at 250-300 PPM Hardness. EDTA chelate is stable at higher
temperature).
Concentration of Chemical:
Concentration is selected from uniformity and reproducible results of previous processing. Adjust the
feeding concentration of chemical as required.
PROCESS CONTROL AND SAFETY IN CHEMICAL PROCESSING Prof.Y.M.Indi
Stock Solution Quantity:
Take study of MLR of process or nature of fabric quality.
STANDARD: Full consumption of Stock solution should be there, nothing should remain back as wastage.
Temperature:
Check temperature during process. Temperature depends on bleaching process and bleaching chemical.
Regulate steam supply as needed.
Time:
METHOD: Dwell time record, concentration of bleaching method and process selected. Give adequate
treatment time.
pH: Adjust the pH during the process by pH indicator or pH paper.
STANDARD: For Hypochlorite 9.5-10.5 and for Hydrogen peroxide 10.5-11.5.
Concentration of Drain: After bleaching, with the help of titration ensure optimum utilization.
Mercerizing
Mercerization of fabrics is performed using NaOH with the concentration normally being in a range of 25 to 30°Be', at low temperatures (15 to 25°C). Tension is applied to the fabrics in the vertical direction with a tension cylinder, and in the horizontal direction with a clip tenter. The processing time by the cylinder and the tenter in total is 30 to 60 seconds. To prevent the fabrics from shrinking after going through the tenter, the NaOH concentration in the fabrics needs to be decreased sufficiently (down to 7Be' or lower) when the fabrics leave the tenter.
Also, since the piling on thick fabrics in a wet state leaves creases on the fabrics, the thick fabrics need to be dried promptly.
Also, since the piling on thick fabrics in a wet state leaves creases on the fabrics, the thick fabrics need to be dried promptly.
(Reference)
Change in a cross-section of a cotton fiber during a mercerization treatment.
Change in a cross-section of a cotton fiber during a mercerization treatment.
1-5 | Swelling process in an alkaline solution |
6 | Removal of alkali by water substitution |
7 | After drying |
Process of mercerization:
Moisture Control: More moisture pickup by the fabric means more NaOH and hence more effect. Drying cylinders are kept before mercerization tank to have same moisture content in the fabric throughout for uniform results. Other technique is wet on wet mercerization, where fabric is pre wet but it requires high precaution.
STANDARD: Free from moisture.
METHOD: By sufficient steam in drying cylinder.
Caustic soda solution while padding:
STANDARD: 25% or 50-520Tw.
METHOD OF CHECKING: Twadle meter or titration.
NECESSARY ACTION: Adjust the concentration according to the requirement. Concentration of NaOH will
never change in tank from start to end but then also at a later stage the concentration on Twadle meter
increases because the density of solution may increase due to the impurities from the fabric like
thickening agent etc.
Temperature of Padding Solution:
Ideally it is carried at room temperature. If the temperature is more, then it is because the moisture in
the fabric is more. Water and NaOH leads to exothermic reaction which will increase the temperature.
So dry the fabric properly. If still temperature increases then check water cooling line.
Dwell time: 45-60 Seconds.
Wet pick up of NaOH: Wet pick up is generally 120-125% but it should be perfectly uniform throughout the width and length.Pick up is studied by taking original weight of fabric and weight of fabric after padding. Pick up is checked randomly.
Bio-treatment
While caustic soda is used for a weight-reducing treatment of polyester, a cellulase is used for a weight-reducing treatment of cotton. In case of treating cotton with a cellulase, the weight reduction of around 1 to 5% is usually achieved. The principle object of the weight-reducing treatment is to improve hand feeling, but it is also utilized for other purposes such as removing fibril. The most important point in the weight-reducing treatment with cellulase is that a physical action applied on fabrics will greatly improve the weight-reducing ratio. This feature cannot be seen in other treatments with enzymes, such as a desizing treatment with amylase, and a boil-off treatment with protease. Accordingly, machines suitable for the weight-reducing treatment are a washer, a jet dyeing machine, etc. in the priority order. Balls may be used in the washer to further increase the physical action on the fabrics. One thing you have to be careful in the weight-reducing treatment with cellulase is a possible decrease in fabric strength. The extent of the decrease depends on a fabric type and an employed machine, and so they need to be checked every time. |
Washing
First compartment after mercerization tank is recuperator. Here the caustic soda should not be more than 100Tw. If its % is very low then take less water. (because if more water is present then NaOH associates itself with more water molecule and its size becomes bigger which is very difficult to remove from the core of the fibre while washing and chances are that we do not get proper washing).
ACTION: Adjust the flow of water.
Temperature of Recuperator: Here the hot washing is carried out. Live steam is blown in water. So the temperature should not be less
than 900C.
Caustic soda left on the fabric: Removal of 100% caustic is very difficult and uneconomical. It should not be more than 1% on fabric and if the quantity is more than 1% then check the washing efficiency, efficiency of recuperator and adjust
the flow of water.
Souring: This treatment is carried out when bleaching is performed on the fabric. In grey mercerization it is not performed. Removal of NaOH is very difficult and hence acid neutralization is very cheap and easy. Removal of alkali by water consumes very high amount of water.
STANDARD: Extract of fabric must be neutral after the souring treatment.
Fabric after washing: Check the extract of fabric by pH indicator or pH paper
STANDARD: pH must be neutral. If fabric is acidic then adjust flow of water, if alkaline in nature than adjust scouring process.
Conclusion:
At last we can say that the pretreatment process is very important for cotton fabric. The field of ultrasonic’s is still making strides towards perfection, but already many applications of ultrasonic energy have been found in science and technology. Ultrasonic is the science of sound waves above the limits of human audibility. The aim of this study was to investigate the effects of ultrasound on textile pretreatment processes especially on enzymatic processes. Use of enzymes in the textile industry has become more popular in recent years. Although enzymatic processing offers many advantages, there are a few drawbacks when compared to traditional methods, namely, expensive processing costs and relatively slow reaction rates. Introducing ultrasonic energy during enzymatic treatment of cotton fabric significantly improves enzyme efficiency without affecting the strength of the fabric.
