TEXTILE WET PROCESSINGTHE BASIC OPERATIONS We can consider the wet processing sector to have three distinct types of process included within it. These are represented along with their aims. 1PREPARATION PROCESS: Exists to ensure that the textile has the right physical and chemical properties to enable it to be coloured or finished. EXAMPLE: Desizing, Singeing, Scouring, Bleaching etc. 2COLOURATION PROCESSES: Exists to provide the textile with colour either for aesthetic reasons or for some functional purpose determined by the end-use of the product. EXAMPLE: Dyeing, Printing. FINISHING PROCESSES: Exists to provide the textile with the properties that the end-use demands and which have not already been provided by any earlier processes. EXAMPLE: Water Repellency, Flame Repellency, Antisoiling finish etc.
DEFINITION:Pretreatment means any treatment, which is done before actual (dyeing and printing) process. Textile pretreatment is the series of cleaning operations. All impurities which causes adverse effect during dyeing and printing is removed in pretreatment process.
AIM OF PRE-TREATMENT: Conversion of fabric from hydrophobic to hydrophilic state. To remove dust, dirt etc from the fabric. To achieve the degree of desire whiteness.
CONSTITUENTS OF COTTON:As discussed earlier pretreatment is the series of cleaning operations. In pretreatment all the impurities present in cotton are removed. Cotton fiber by nature contains: Cellulose 86.8% Natural Impurities: Oil and Waxes 0.7% Pectins 1.0% Carbohydrates 0.5% Proteins 1.2% Salt 1.0% Water 8.5% Colour Pigments: Others 2.0% Except cellulose and water all the impurities are removed in pretreatment.
IMPURITIES REMOVED DURING PRE-TREATMENT:Short Fibres Applied Impurities Artificial Impurities Natural Impurities Colour Pigments (Size Material) (Oil, Strains, Dust, Dirt) (Oil, Wax, Pectins, Proteins) (Naturally present in cotton) Singeing Desizing Scouring Scouring Bleaching 2
PRETREATMENT PROCESS OF COTTON FABRIC:12345678Inspection of grey fabric. Marking of grey fabric. Stitching. Shearing / Cropping. Singeing. Desizing (only for woven fabric) Scouring. Bleaching. All process discuss in detailed one by one.
INSPECTION OF GREY FABRIC:
After receiving the cloth from grey godown, it should be thoroughly checked and inspected before subjecting it to wet processing. The cloth is examined for the following:
Oil, rust, stains, holes, damage selvedge and weaving faults like floats and read marks etc.
Width, length, yarn count, ends and picks per inch and fabric composition.
MARKING OF GREY FABRIC:Before carrying out any process, marks are made on the material for identification. For marking special inks are used capable of resisting all subsequent treatment which the material may have to go undergo.
Small pieces of fabric are stitched together so as to form a continuous length, which is workable in the subsequent processes. The stitching must be very secure so that the fabric may run smoothly on various machines.
SHEARING / CROPPING:
Removal of fibres, tufts and loose threads from the surface of material by cutting them with blades on shearing or cropping machines. This process is carried out in open width. In this process tufts are not removed as closed as by singeing.
Singeing is the process in which protruding fibres present on the surface of the fabric are removed. In order to achieve the smooth surface which facilitate printing.
DESIZING:Desizing is the process in which size material is removed.
Scouring is the process in which natural as well as artificial impurities are removed. 3
BLEACHING:Bleaching is the process in which we remove the colour pigments in order to achieve the degree of whiteness.
MERCERIZATION:Mercerization is the optional process or in on the customer requirement. Mercerization is done to achieve the luster, strength, more absorbent etc.
PRETREATMENT OF WOOL FIBRES:(i) (ii) (iii) Sequence of pretreatment of wool fibres a given below: Scouring. Carbonizing (remove the vegetable impurities) Bleaching.
PRETREATMENT OF MAN-MADE FIBRES:(i) (ii) (iii) Desizing Scouring. Heating Setting. (if size material is applied) (dimensional stability)
INTRODUCTION:This is usually the first stage in pre-treatment; consist of destroying by burning all the tufts, fibers and hairs protruding form the surface of the material (yarn and fabric). There by giving it smooth and cleaning face. Singeing is generally done o grey goods before any other treatment. The fabric is singed on one or both sides by passing either over plates or heated cylinder or by gas flame.
OBJECTIVES OF SINGEING: Surface hairs trape air in the fabric when it is immerged in water. This means that it takes longer time for water to enter the fabric, singeing therefore indirectly helps to increase the fabric wet ability. 5
To prominent the woven structure of the fabric. To create smooth surface for printing. To prevent firstly or cloudy after dyeing. To prevent or minimize the tending of landed fabrics composed of cellulose and synthetic fiber (mainly polyester) the form pills after abrasion during usage.
METHODS OF SINEING:Usually three types of singeing can be carried out: 1) Plate Singeing Machine. 2) Rotary Singing Machine. 3) Gas Singeing Machine.
PLATE SINGEING MACHINE:Plate singeing essentially consist of two covered copper plates thick and slightly barreled which way be set in the fire clay and heated to bright redness by a furnace below or by a suitable heating arrangement. The cloth in open with passes and it contact with the hot plates at a speed varying between 135 to 225 meter/minute. The protruding fibers are burnt during the passage of the cloth through the machine. However, the fiber ends present in the interstices of warp and weft are not signed. Since they are not made assessable to the action of intense heat. Secondly since there is actual contact between the cloth and a hard and hot surface of the metal a certain amount of luster is produced in the cloth due to friction. The main objective to plate singeing is that it may not be possible to maintain the plates at a uniform temperature and this cause unnerve singeing.
ROLLER SINGEING MACHINE:
Some of the defects of plate singeing machine are over come in roller singeing machine which way be considered as a modification of the formers. In this machine, the singeing surface is a rotating cylinder instead of stationary curved plate. It may be made of either copper or cast-iron and provided with the internal fairing system. A fresh not surface of the cylinder is presented into the coming fabric by the slow rotating of the cylinder. In this case, the surface temperature of the cylinder is more uniform at all places. Than in the case of hot plates. The reaction of the cylinder is in the opposite direction to fabric
PRE-REQUISITE Fabric should be completely dry, otherwise yellowish colour with heating take place on the fabric. POST-CARE Used water to extinguish the sparks. 6
movement which rises the naps-ensuring efficient singeing. As in plate singeing the fabric end in the interstices are not removed from the fabric and luster is imported to the fabric to some extent due to the contact with the hard and not-metal surface in roller singeing.
GAS SINGEING MACHINE:
Gas Singeing is intended for single and double face singeing of fabric with passes through the guide roller. Then in brushing zone which rotate in the sense opposite to fabric to be singed. The brushes are placed in a special chamber from which dust and fluf removed form the fabric are exhausted by a fan.
Then the fabric passes over gas burner with ceramics nozzles where singing is covered out. Coming out of the gas singeing machine a box containing water. This bath may also contain desizing agent. In this way after singeing we can desize and batched the fabric.
INTRODUCTION:The major portion of the size consists of starch, wax and tallow. All these remain on the warp yarn even after weaving the cloth. It is necessary to remove the size from the cloth, otherwise the hydrophobic of the wax and tallow constraint the subsequent dyeing and printing process. Thus desizing is the term usually restricted to the process of removal of starch size can be performed efficiently and completely if are taken to degrade the starch in some way and to convert it to the product with a higher degree of water solubility. DESIZING Size has to be solubility and the very steps in the process are as given below: 1) Impregnation the fabric with desizing agent. 2) Allow the time for the agent to the absorbed into the size and degrade or solubilized it. 3) Wash out the degrade size material.
OBJECTIVES OF DESIZING: To prepare the fabric for dyeing and printing by removing the coating of size from the surface of the fabric. The chemical required for desizing depends upon the nature of the size applied.
OXIDATIVE DESIZING:Some oxidizing agents, particularly hydrogen peroxide and some persulphate will degrade. Starch will out serious attack of the cellulose as long as reasonable care is taken with the process. The fabric is impregnated with hot solution of oxidizer and batched up for several hours or steamed for 20min at 100-105oC. Since hydrogen peroxide is used as bleach for cellulosic substrate as a bleaching process would normally follow desizing but H2O2 is used for desizing with size of PVA at 9PH. It is reasonable to suppose that the two processes might be combined into one. This is some time done, particularly where the amount of size to be removed is hot large. However, a more common practice is to economic on enzyme desizing process by using condition which are milder the removal of size is then completed action of the hydrogen per oxide bleach.
HYDROLYTIC DESIZING:Starch are the mixture of two carbohydrates. (a) Amylose a long straight chain. (b) Amylopectin a complex branch. The thickening power of starch is largely due to the amylopectin. These compound are in soluble in water but can be solubilized by hydrolysis of these polymers. Hydrolysis is generally brought about by using one of the following processes. (a) Rot or Bacteria Desizing. (b) Acid Desizing. (c) Enzymatic Desizing.
C H 2OHO
C C C O C
H H C C O
CH 2 C C C O C C C
Starch contain 19% amylose and
1:4 AmyloseC O C O C
81% amylopectin 9
1:4 & 1:6 Amylopectin
In this process, the grey cloth is dipped in a tank full of water for 24 hours and a give temperature range of 25-40oC. The bacteria present in water produce enzyme, which react with starch and converted them into glucose unit, which are soluble in water and on washing the fabric can easily be removed. This is slow but economic process.
(b) ACID DESIZING:
The process of acid steeping is also called as grey scouring. In this process grey cloth is placed in a weak solution(0.5-1%) of H2SO4 for 4-6hours at a maximum temperature of 40oC starch become soluble in water.
Enzymes are biochemicals which a specific action on only one type of compound and which bring about a chemical change in it. They are contained with in all living system and carry out many functions. For Example: Their role in digestion, the modern agencies used for desizing into three groups.
ENZYMES(i) (ii) (iii) Melt Preparation Pancreatic Amylose Bacterial Amylose
ORIGINVegetable or cellulose enzyme Waste of slaughter house. Obtain from micro-organism.
The only amylose family of enzyme have an action on starch (Amylose and Amylopectin) and nothing else these amylose converted the starch mixture very rapidly to soluble dextrin and then more slowly convert this to reducing substances and sugar such as maltose. These water soluble product could be easily washed out in actual practice the grey cloth may first be passed through hot water and then through desizing solution, whose
Starch (Insoluble) Dextrin (Insoluble) Soluble Dextrin Maltose (soluble)
Alpha glucose (soluble)
concentration, PH and temperature may be vary according to the type of enzyme.
ENZYMESMelt Extract Pancreatic Bacterial
CONCENTRATION3-20gm/litre 1-3gm/litre 0.5-1gm/litre
TEMPERATURE50-60oC 50-60oC 60-70oC
PH6-7.5 6.5-7.5 5.5-7.5
Enzymes suffer from one disadvantage, that if the condition of temperature and PH not favorable their desizing activity is destroyed. After passing fabric from desizing solution fabric is batched for 8-12hours. During this time starch is converted from insoluble starch to soluble -glucose and than simple washing is done.
SCOURINGINTRODUCTION:Cotton is based on 90% of cellulose and the remaining portion consists of impurities such as fats, waxes & organic compounds. The main impurities which must be removed are fats and waxes which are insoluble in water thus decreasing the water absorbency of fabric raw cotton contains: Cellulose 86.8%
Natural Impurities:Oil and Waxes Pectins Carbohydrates Proteins Salt Water
0.7% 1.0% 0.5% 1.2% 1.0% 8.5%
Others 2.0% These are three major processes in scouring to remove impurities. (i) Soaponofication. (ii) Emulsification. (iii) Solvent Extraction (Detergency). These processes are discussed in detail.
(i) SOAPONOFICATION:Soaponofication is the process in which fats are treated with caustic and lead the formation of hydrophilic soaps (sodium salts of fatty acids).CH 2 OOCH | CH OOCH | CH 2 OOCH35
CH 2 OH | CH OH + 3 (C17 H 35 COONa ) | CH 2 OH
C17 + NaOH C17
Waxes present in the fabric cannot be removed in saponification. These are esters of higher fatty alcohol & fatty acids similarly mineral oils, lubricants oil etc cannot be converted into water soluble product by boiling with NaOH solution. The process of emulsification is used in the scouring of cotton material containing non-saponifated oil such product can be removed by emulsifier. Thus the scouring solution should also contain an emulsifying agent in addition to Sodium hydroxide and wetting agent ordinary soap (washing) is good emulsifying agent.
(iii) DETERGENCY OR APPLICATION OF DETERGENT IN SCOURING:After removing the starch (by desizing) vegetable oil (by saponification) mineral oil and waxes (by emulsification), the remaining constituents of the size and other solid particles including dust and dirt particle have to be remove from the fabric. When the binding material such a starch, oil and waxes are removed by process described before the solid particle are loosened from the fabric...