One cannot imagine a world without textiles. The primary function is to clothe, protect, embellish and insulate the human body. Textile industry, as with any industry today, is concerned with quality of the finished article. It is expected of a processor to produce quality goods, while maintaining the cost of production. Fabric faults or defects are responsible for nearly 85% of the defects found in the processing industry and manufacturers do not recover anything by selling fabrics on seconds and as off-quality goods. It is imperative, therefore, to detect, identify, and to prevent defects from reoccurring.
Defects can occur due to any reason and at any stage of fabric processing. Spinning, weaving and knitting faults are normally identified during the pre-inspection stage. Grey fabric is checked for faults such as hole, scratch, dirt spot, cracks, loom oil and mud stains etc. The task of fabric defect detection is carried out by human visual inspection, in most of the traditional dye houses. Manual inspection is usually a difficult task due to the small scale of detects and the large scale of inspected surface. In the case of the weaving sector, inspection is performed at the end of the manufacturing stage.
Fabric defect detection is an important part of quality control in the textile industry if these defects are not detected and plugged early, it can create serious problems at the final stage. Manual detection can be detrimental many a times, as small defect go unnoticed. To overcome these drawbacks, automated checking with magnet can be installed and implemented. The processor not only has the responsibility to cover up these defects, but also ensure that these defects do not get compounded during subsequent wet processing.
Proper selection of dyes and auxiliaries play an important role to prevent processing defects. Dyes vary widely in their ability to effectively eliminate white or off-shade specks and it is advisable to consult dyestuff manufacturers to cover immature cotton which are capable of aggravating the problem of uneven and unlevel dyeing. Pretreatment with caustic soda cannot cover the defect of dead cotton but can give partial relief to obviate during subsequent processing. Many a times, mercerization after dyeing are effective, but should be tried in extreme cases as there is a danger of change of shade with dyes which are not resistant to strong alkalis.
Ideal cotton should have minimum 90 – 95% cellulose content for trouble free processing. Incomplete removal of natural impurities like pectins, waxes, proteins, and other organic compounds can lead to poor dye pick up, patchy dyeing, uneven dyeing, unlevel dyeing, shade change, yellowing etc. Metal contaminants such as calcium, magnesium, iron etc. can pose serious issues to compound the problems. Metal content in cotton may vary depending on origin of cotton cultivation and production. Enzymes, petroleum sulphonates, solvent mixtures, and phosphonic acid derivatives should be part of the scouring formulations to overcome the problems.
Preparations of yarn packages play an important role during yarn dyeing both in controlling levelness and yarn quality. A well wound package not only increases level dyeing but also minimizes risk associated with subsequent dyeing and finishing processes. A powerful wetting agent having excellent rewetting action also helps in uniform dyeing.
Unlike synthetic sizes, natural sizing agents like starch derivatives are used for coating of warp yarns. Starch based sizing agents are used because of economical advantages and satisfactory weaving performance. The insoluble starches have to be converted in to soluble sugars either by oxidative or enzymatic treatments as incomplete removal can lead to serious processing problems. Extreme precautions need to be taken during oxidative de sizing, as excess chemical and temperature can lead to oxy cellulose formation leading to strength loss of cotton. The sizing material present on warp yarns can resist dyes and chemicals and its complete removal is essential. Proper selection of enzymes depending on the type of size is very critical and it is therefore necessary to identify the type of size on the fabric before de sizing and can be determined by spot tests.
Hydrogen peroxide is the safest bleaching agent for cotton. Controlled bleaching, regular checking of bath composition during bleaching operation can prevent early detection of defects and failures. The two concentrations to be taken in to accounts are: Hydrogen peroxide quantities based on the weight of the fabric and based on weight of the solution. All factors being equal, the concentration on the weight of the goods determine the final degree of whiteness. In order to get adequate bleaching effect, there must be enough peroxide present from the beginning. Periodical checking of Ph, peroxide release will determine the bleaching rate. Greater the solution concentration, faster will be the bleaching action. No peroxide bleaching system ever uses up its entire peroxide charge for active bleaching, as some is always ‘lost’ during bleaching process.
Reactive dyes are one of the most commonly used classes of dyes for cotton. Two important aspects of reactive dyeing namely dye variables and system variables, along with important characteristics of reactive dyeing such as exhaustion, migration and leveling, fixation and colour yield, and washing-off and fastness have to be taken in to account. The removal of unfixed dye is an important step after dyeing and takes place in three phases. These are dilution of dyes and chemicals in solution and on the surface of the cellulose; diffusion of the deeply-penetrated, unfixed, hydrolyzed dye to the fiber surface; and dilution and removal of the diffused out dye. Dyed goods have to be rinsed cold twice to remove electrolyte, then rinsed hot to remove the hydrolyzed dye from the fiber prior to a soaping at 95ºc. Polymeric dispersing agent having re depositing properties helps in near or total removal of unfixed dyes from the fabric.
Despite environmental concerns, sulphur dyes still a choice of dyers for deep black, navy blue, khaki and olive green shades. Sulphur dyes containing sulphur are water-insoluble form both as an integral part of the chromophore and in attached polysulphide chains. They are normally applied in the alkaline reduced (leuco) bath from a sodium sulphide solution and subsequently oxidized to the insoluble form on the fiber. Sulphur dyes differ from vat dyes in being easier to reduce but more difficult to re-oxidise, different oxidants producing variations in hue and fastness properties. A leuco sulphur dye has the same colour index (CI) number as the parent sulphur dye but exists as the soluble leuco form of the parent dye together with a reducing agent in sufficient quantity to make it suitable for application either directly or with only a small addition of extra reducing agent.
Two serious problems associated with sulphur dyes are acid tendering and bronziness. In severe conditions of heat and humidity, some sulphur dyeings, notably black, can generate sulphuric acid within the cellulosic fibers, leading to tendering. Thorough washing, good rinsing before soaping, use of alkaline bath in the final rinse, storing the fabric at low temperature and humidity, use of sodium acetate and soda ash for neutralization can prevent tendering of sulphur dyed goods. Bronziness is another common problem often faced after sulphur dyeing. Bronziness can be well controlled by using optimum quantity of sodium sulphide, salt, liquor ratio, proper water quality and after treatment with sulphonated castor oil after dyeing. Dull shades due to incomplete reduction of dyes is another serious defect which can erupt in the dyeing of both sulphur and vat dyes as both class of dyes are applied in the leuco form with the aid of reducing agents.
Defect free processing of textiles is a vast subject and a real challenge to processors. Countless problems can erupt during different stages of processing as there are many variables. Proper understanding of fiber chemistry, chemicals and auxiliaries, machineries can prevent and solve many problems.