Wet processing

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  • 1.Textile Chemical ProcessingWhat is the need of Textile Chemical Processing?In Textile Chemical Processing the chemical treatments are given to the fabric afterbeing manufactured. Actually the fabric coming from the loom is not having properties likeabsorbency, softness etc and the most important is that the appearance of the fabric is dirty orpale yellow; we cannot use it directly for making apparels or clothing. So, it is necessary togo for chemical processing of the material to make it wearable.The general sequence of processes carried out on gray clothGray InspectionStitchingMechanical CleaningSingeingDesizingScouringBleachingMercerising Dyeing/Printing FinishingGray InspectionIt is checked whether the grey fabrics are inconformity with standards, and all weaving faultsare marked out. Fabric inspection involves threepossible steps: perching, burling and mending.Perching is a visual inspection and the namederives from the frame, called a perch, of frostedglass with lights behind and above it. The fabricpasses through the perch and is inspected. Flaws,stains or spots, yam knots and otherimperfections are marked. Burling is the removalofyam knots or other imperfections from the fabric.The faults are then mended and any knots in thematerial are then pushed to the back. Mending isobviously, the actual repair of imperfections. Knotting should be done carefully andthoroughly so that the repair or holes is not visible.StitchingAfter the goods have been inspected and checked they are classed in the grey room,according to quality and stamped. Goods of similar weight, width and construction and thegoods which will receive a similar treatment are sewn together, end to end, by sewingmachines especially constructed for this purpose and each batch is given a number called lot number. The fabrics are usually sewn on circular machine. 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. The pre-cleaning of grey fabrics may be carried out in a separate unit just before cropping and shearing operations. The purpose of brushing is to

2. remove the short and loose fibres from the surface of the cloth. It also removes husk particlesclinging to the cloth. Brushing is mainly done to fabrics of staple fibre content, as filamentyams usually do not have loose fibre ends. Cylinders covered with fine bristles rotate overthe fabric, pick up loose fibres, and pull them away by either gravity or vacuum. The raisedfibre ends are cut off during shearing operation. Brushing before cropping minimizes pilling. Shearing is an operation consists of cutting the loose strands of fibres from either surfaceof a fabric with a sharp edged razor or scissors. By manipulating the shearing it is alsopossible to cut designs into pile fabrics. Good cropping is perhaps, the simplest way ofreducing the tendency of blended fabrics to pill. In the case of cotton fabrics, in particular,care should be taken to see that the shearing blades do not scratch the surface of the fabric,which otherwise can cause dyeing defects during subsequent dyeing.SINGEING The objective of singeing is to remove projecting fibres or protruding fibres, which givesit a fuzzy appearance, from the surface of the fabric so as to give it a smoother, cleanerappearance. The reason for which singeing is necessary:(i) Singeing improves the end use and wearing properties of textiles.(ii) The burning-off of protruding fibres results in a clean surface which allows the structure of the fabric more clear.(iii) Singeing reduces the fogginess caused by differing reflection of light by theprojecting fibre and the dyed fabrics appear brighter.(iv) Singeing is an effective means of reducing pilling in blended fabrics containingsynthetic fibres.(v) Unsinged fabrics soil more easily than singed fabrics.(vi) A closely singed fabric is essential for printing fine intricate patterns.(vii) Singeing process facilitates and speeds up desizing, if the fabric is impregnated with desizing liquor immediately after singeing.On the other hand there are singeing faults which are not visible and once occurred can nolonger be repaired. They are:(i) Uneven singeing effect can cause streaks when the fabric is dyed, or bubbles whenthe fabric is finished.(ii) In the cotton system singeing is done on the grey cloth, but for blended fabrics containing synthetic fibres grey state singeing is not advisable because small globules of melted synthetic fibres absorb dye preferentially, giving cloth a speckled appearance.(iii) There is a possibility of thermal damage to temperature sensitive fibres, forinstance polyester.(iv) Stop-offs can cause heat bars on fabrics. Creasing produces streaks which are magnified when dyed. Generally, singeing is done on both sides of the fabric. No chemical change occurs inthe fabric during singeing and the reaction is basically one of oxidation. Singeing anddesizing can be frequently combined by passing the singed cloth through the water bath 3. which includes enzymes. The enzymes digest the various sizing agents, making it easy toremove them during the scouring operation.Singeing Machinery:Singeing machineries are mainly based on direct and indirect singeing systems. Thedirect singeing may be done either on a hot plate, or on a rotary cylinder, or on a gas singeingmachine or on a machine combining plates and gas burners. The special features of indirectsingeing systems are no flame contact, uniform singeing, heat retention zone and singeing bymeans of heat radiations. The indirect system produces fabrics which have a softer touch ascompared to other methods. Gas singeing is more convenient, more economical and moreeffective than other methods and is well accepted commercially. The plate singeing and rollersingeing machines are now out of date. 1. Plate Singeing MachineFigure - A Plate singeing Machine with alternating arrangementIn plate singeing machine there are two curved Copper plates, 1-2 inches thick. TheseCopper plates are set in the fire clay and heated to bright redness by the furnace below or bysuitable heating arrangement. The cloth in open width passes over and in contact with the hotplates, at a speed of 135 225 m/min. Automatic transverse motion is fitted to the machine,which constantly changes the part of the hot plate in contact with the fibre, to avoid the localcooling and non-uniform heating of the plates. The protruding fibres are burnt during the passage of the cloth through the machine.However the fibres present in the interstices of the warp and weft are not singed. In this case,as there is actual contact of the fabric with the hard hot surface of the metal, a certain amountof lustre is impregnated in the cloth due to friction. The drawback of this machine is it maynot be possible to maintain plates at uniform temperature and this causes uneven singeingwhich gives rise to faint streaks when the cloth is dyed. 4. 2. Roller Singeing MachineFigure- A Roller Singeing Machine The fabric is made to pass over, in contact with rotating cylinders. These cylinders aremade up of cast iron or copper and are heated by internal firing system. In this case, surfacetemperature is more uniform than in the case of hot plates. However, the problem of localcooling of the cylinder by contact with the cloth causes uneven singeing. The rotation of thecylinder is in the opposite direction to the fabric movement, which erases the neps on fibreensuring an efficient singeing.In this case also, the fibre present in the interstices of warp and weft are not removedfrom the surface and the lustre is impregnated to the fabric due to the contact of the hot andhard metal surface. 3. Gas Singeing Machine In this process, the fabric is passed rapidly over a row of gas flames at a high speedand then immediately into the quench bath to extinguish the sparks. And cool the fabric. Themachine consists of one or more gas burners giving continuous flat or vertical flames. Herethe flame is produced by compressed air and coal gas. The flame is adjusted with respect tothe width of the fabric. The angle of the burners and the height of the flame can also beadjusted. When the cloth is drawn over flame at a high speed, the protruding fibres burnwithout damaging the cloth. The speed of transmission of the cloth through the singeingmachine has to be adjusted to suit the amount of singeing required without the risk ofburning the cloth. The position of the burner must be taken into consideration according to the fabric tobe singed (Fig. 2-11). There are three different methods of applying flame to the material bychanging the position of the flame to offer: 5. Figure- Gas Singeing Machine1. Tangential singeing Fig. 2-11 (a)2. Singeing onto water cooled rollers Fig. 2-11 (b)3. Singeing into the fabric Fig. 2-11 (c) 6. Singeing of Different Fibres materials:In singeing the short fibres are burnt off from the surface of the fabric by direct orindirect heating systems without damage to the cloth by scorching or burning. The thermalbehavior of different kinds of fibres is different and singeing at higher temperature isnaturally associated with greater hazards on excessive contact period and may cause thermaldegradation of the fibre.In case of vegetable fibres, grey singeing is necessary as it leads to slight yellowingwhich needs subsequent bleaching to get high degree of whiteness. Grey singeing is alsoeconomical as singeing at any other stages of processing requires additional washing anddrying. Vegetable and regenerated fibres fabrics can be singed very strongly with maximumburner intensity to obtain good results. Regenerated fibres normally bum to a little less easilythan natural fibres. Wool has poor combustion properties and are very sens