DYEING PREPARATORY PROCESSING FOR COTTON AND WOOL
Submitted To: Mr. Krishnaraj G
Submitted By :AnjaliM F tech-Ist sem Apparel Production Roll No. 05
CONTENTS Processing of Cotton1) Introduction 2) Impurities in grey fabric 3) Standard Process Sequence 4) Grey Checking 5) Stitching 6) Shearing and Cropping 7) Singeing 8) Desizing 9) Scouring 10) Bleaching 11) Mercerization 4 5 6 7 8 9 10 14 19 24 29
Processing of Wool1) 2) 3) 4) 5) 6) 7) Introduction Cropping Scouring Crabbing Carbonizing Milling Bleaching 35 36 37 39 40 41 42 44
PROCESSING OF COTTON
INTRODUCTION After the grey fabric comes out of the loom shed, for a layman it is a ready cloth but it still contains heap of impurities, defect and need to be given some physical and chemical treatment so as to make wearable. It is here that the work of textile chemist begins. This processing of grey fabric into a wearable cloth is mainly chemical in nature and termed as wet processing. The word wet is used because the processing involved in the conversion of the grey fabric into the finished cloth is generally done with wet treatment as in dry state it is troublesome. So the work of processor is a vital and key piece in getting the cloth from the loom to market and finally on the body. A generalized component in the cotton fabric is: COMPONENT PROPORTION Cellulose 80-85% Waxes and fatty acid 0.4-1% Proteins 0.8-1.8% Pectins 0.3-1% Mineral matter 1-1.8% Moisture 6-8% Others 1-4% (Gums and lignous substance)
IMPURITIES IN GREY FABRIC There are three types of impurities present in the grey fabric. They are as follows: 1) Natural impurities: - The various types of natural impurities present in
grey fabric are: a) Waxes b) Pectins c) Pigments d) Colloid substances e) Color These mask the absorbency of the fabric.2) Added impurities: - The various added impurities are deliberately
added for improvement of certain properties during weaving operations. Some are: a) Gums b) Poly vinyl alcohol c) Fatty compound3) In process impurities: - These are produced in the grey
fabric due to the development of the cellulose material during process. These are: a) Broken cotton seeds known as kitties (black spots on the fabric) b) Leafy material c) Oil stains d) Lubricants
SEQUENCE PROCESS STANDARD A generalized sequence that can be followed for processing is: Grey Checking Stitching Shearing and Cropping Singeing Desizing Scouring Bleaching Mercerization The end use of the textile will determine whether all of those stages are necessary. In some cases the stages may occur in a different order or two stages may be combined into one. Above sequence is only generalized one.
GREY CHECKING When a grey fabric enters a process house then the first process that is carried out is grey checking and mend able defects are removed. The whole process is carried out on a fabric inspection table. Various parts of a fabric inspection table are: 1) Beam stand: - Manually or automatic which holds the fabric beam on it 2) Fabric beam: - It is roll of fabric from loom 3) Guide rollers: - They help in fabric movement and are negatively driven by the surface contact of the fabric 4) Inclined plate: - This is the main part and is made of wood or metal on the two ends and of glass in the middle, as illuminator works better on the glass. It is adjustable and generally set at an angle of 45. 5) Rollers: -They grab the fabric coming and pull it and are positively driven by the motor 6) Meter Reading device: - This device works for noting the fabric length 7) Illuminator: - These are light sources either below or above the glass plate Whenever some defect is seen on the fabric then the flow of the fabric is stopped by pressing down the handle at the lower end of the plate and after mending the defect the machine is started by lifting the handle up. The two main defects are mended at the fabric inspection table: 1) Loose ends: - The grey fabric contains several loose ends along the selvedge of the fabric which are removed by cutting them at the inclined plate 2) Stains: - The grey fabric may have some stains on it, which can be removed by using stain removers of two types. The two types are: a) Water based: - In this water itself is used as solvent b) Solvent based: - In this some solvents are used to emulsify the stains out. They are efficient but the particular portion of which the stain is removed becomes highly absorbent which creates problem in dying as it gets darker than rest of fabric7
STITCHING After grey checking the next process in sequence is stitching. In order to maintain the continuity of the process long length of the fabric is required and the several length of the fabric of same type are stitched using stitching machine. Different types of stitching are done. These are: 1) Loop stitch: - This is the most common type of stitching used for ordinary fabrics. The machine is stationary and the fabric moves. 2) Flag stitch: - This is used for delicate fabrics; in this a small piece of fabric is folded and put in between the two ends of the two fabrics. It provides strength so that fabric withstands the processing tension. 3) Narrow stitch: - In this type the fabric is stationary and the stitching machine moves. This is done for export quality fabrics as stitching is absolutely straight and crease free. SORT NUMBER: - The different length of either same or different quality fabrics are stitched and are marked with a number for identification by yellow marker on both ends of a length of the fabric known as sort no. LOT NUMBER: - The total length of fabric of same quality that is processed in single day or shift is given lot no. in order to identify the fabric and is written in yellow marker which is not removable in further processing.
SHEARING AND CROPPING The fabric after grey checking and stitching still have certain defects which are to be removed mechanically. There are certain loose fibers and loose yarns over the fabrics. The maximum length of the loose yarns that are removed here is about 8cm. Shearing means cutting off and Cropping means cutting the loose yarns as short as possible. The process of shearing and cropping is done in a sequence. The fabric in its full width is subjected to the curved blade roller cutter and before this the fabric is passed over brush roller so that: a) Any small loose fibers which can be taken up are taken up by the brush roller b) The loose yarns are all raised up by the brushes After passed through a pair of brush roller and a cutter the fabric is again passed through another pair of the brush rollers and cutter. This helps in shearing both sides of the fabrics. A problem arises if the stitched part passes over the cutter with the same speed, hence a seam detector is placed about 3-4m before the cutter. When a seam is detected than cutter is being stopped and it move back so that the stitch portion passes unsheared. More the no. of seams less is the efficiency of the cutter. Cropping efficiency (%)= A-B 100 A A is no. of yarns originally present. B is no. of uncut yarns.
SINGEING After having done the shearing and cropping still very small fibers are left out on the surface of the fabric. These are termed as Protruding fibers. This process of removing these short fibers by means of heat or burning is termed as SINGEING. Cotton and cellulosic fibers behave ideally on singeing as they form light pale dusty ash on singeing as they can be easily removed. In case of blend fabrics composed of cellulosic and synthetic fibers, the pills are formed due to abrasion and singeing can minimize this. The protruding fibers if left in fabric may cause the following problems: 1) They may cause capillary action during dying and hence they will look darker than the rest of the fabric 2) In case of roller printing when the blade remove the extra color than these fibers may cause the slippage of the blade and hence uneven printing 3) They give harsh feel to the finished fabric 4) They also cause problem in printing of fabric. There are some operations that must be performed before singeing. They are: 1) The fabric should be completely dried before singeing by passing over drying cylinders. 2) The anti mildew agents should be removed from the fabric otherwise at high temperature gets converted into HCl and degrades the fabric. Singeing is continuous process carried out on dry open width of the fabric. Speed of 150-400 meters per minute is typical to avoid the scorching of the fabric. The aim is to burn down the protruding fibers and not the fibers locked inside the yarn, which are not protruding. The exact running speed is largely determined by the amount of hairiness and construction of yarn and fabric.
There are mainly three types of singeing: 1) PLATE SINGEING This machine consists of a pair of curved copper plate on which the fabric is passed in full width. The plates are heated from bottom by oil or gas oven. When the fabric comes in contact with the red hot plates the protruding parts of fibers burns off. The transverse motion of the fabric is arranged so that the fabric remains in contact with the plate but at different parts so that local cooling of the plate doesnt take place. The main drawback of this process is that protruding fibers lying in the interstices are not singed. It also causes uneven singeing due to uneven heating. 2) ROLLER SINGEING In this machine roller or cylinder made up of copper or cast iron, which are internally heated, are used. The fabric moves over the heated cylinder in direction opposite to the direction of the cylinder. Here effect of local cooling is less. But here also fibers in the interstices are not singed and fabric also gets some lusture. Also due to the passage of same width of the fabric for long time,