Continuous dyeing Faults


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Continuous dyeing Faults

  1. 1. Continuous Dyeing Faults
  2. 2. This is probably the most common fault to occur in continuous dyeing. Sometimes these are retained visible folds formed during preparation or the dyeing process itself, but more often they are temporary folds that have appeared and vanished again in a preparatory process, their presence only being revealed later by differential dye uptake.  The source of such problems may be difficult to identify with certainty. Excessive, insufficient or variable tension in running fabrics can be important causes of crease formation.  Tight selvedges, differential shrinkage, the development of bowed or skewed weft , incorrectly bowed expanders and worn or badly rotating rollers can all produce rippled or irregular patterns of creasing.  The build-up of lint, loose threads or other insoluble debris as hard deposits on roller surfaces is another potential source of variability in fabrics running over them. Creasing
  3. 3.  Shade matching in continuous dyeing is a challenge quite different in scale from that of automated batchwise dyeing, where a mis-matched dyelot can often be given a minor correction without removing the dyed batch from the dyeing vessel.  If a continuous dyeing arrives at the delivery end of the range slightly off-shade, some form of reprocessing is usually inevitable.  Even if the dyeing range is equipped with correctly positioned on-line colorimetric control , substantial amounts of off-shade material are produced whilst a colour correction step is being implemented.  Slightly off-shade but otherwise commercially acceptable fabric can often be disposed of by negotiation with the original customer or sale elsewhere as seconds quality.  More serious divergencies from the target shade are usually corrected in batchwise open-width equipment rather than attempting to apply a correction in a second run on the continuous range. However, it may be technically and commercially preferable to cover the faulty goods by re-dyeing to navy or black, because these deep shades are normally in demand and usually readily disposable as seconds. Shade Matching Faults
  4. 4.  Level bulk-scale dyeings that unexpectedly fail to match the relevant laboratory dyeing for shade are often attributable to human error in formulating the dyebath, either in bulk or in the lab. Such errors can be cross-checked by repeating the lab dyeing and testing the bulk-scale liquor in a lab-scale dyeing.  If the same shade has shown satisfactory lab-to-bulk reproducibility in a previous dyelot, the off-shade result may have a different cause, such as a change in substrate dyeability, liquor ratio, water supply, non-standard preparation, or a failure in the bulk-scale control of dyebath pH, temperature or chemical additions.  Modern dyeing control systems are so reliable that substrate variability is far more likely to be involved.  Consistency of substrate dyeability can be readily monitored in a vertical organisation with a limited number of fabric suppliers to the dyehouse, but the wide variety of fabric qualities dealt with in a typical commission dyehouse demands constant vigilance to keep the frequency of off- shade faults down to an acceptable level. Lab-to-Bulk Reproducibility
  5. 5. Two Sidedness  This occurs when one face of the fabric is subjected to a higher temperature than the other during the pre-drying stage.  Dye migration takes place preferentially towards the hotter surface to give a slightly deeper shade than on the cooler one.  If the component water-soluble dyes in a trichromatic combination differ in substantivity, however, the least substantive will tend to migrate more readily than the most substantive component and the two sides of the dyed fabric will show differences in hue as well as depth. Two Sidedness
  6. 6. Listing Listing is the term used to describe weftway differences in hue or depth across the fabric width, normally a gradual shading from one selvedge to the other, or a difference between the centre of the fabric and both selvedges. Possible causes of listing include weftway variation in: 1. nip loading in a preparation treatment 2. residual size content after preparation 3. heat setting before dyeing 4. moisture regain after padding 5. nip loading at the padding stage 6. temperature or moisture content during drying 7. temperature during thermofixation. Listing
  7. 7. Ending Ending is the term used to describe warpway variations in hue or depth along the fabric length of a dyelot. Possible causes of ending include random variations of: 1. desizing, scouring or bleaching conditions 2. heat setting before dyeing 3. moisture regain before padding 4. nip loading at the padding stage 5. migration during pre-drying after padding 6. temperature in drying, thermofixation or steaming treatment 7. dwell time during fixation or aftertreatment. Ending
  8. 8. Tailing This term refers to the depletion of dye concentration in the pad liquor that takes place gradually during continuous running. The higher the dye substantivity and the lower the applied depth, the more pronounced is the depletion or tailing effect.  If the component dyes in a trichromatic combination differ significantly in substantivity, tailing may be more obvious because it manifests itself as a gradual change in hue. This fault can be minimised by rapid recirculation of the pad liquor from the trough back into the stock feed tank.  However, it is also essential to consider carefully the relationship between the laboratory pad, stock tank and pad liquor formulations.  A lab-scale padding gives a similar shade to that of the first few metres dyed in bulk, whereas the equilibrium shade reached after several minutes of bulk-scale running may be significantly paler or off-shade relative to the lab result.  Quantification of these differences can be used to calculate allowance factors, so that the stock feed and lab-scale formulations can be adjusted to ensure that the pad liquor at equilibrium yields the target shade on the finished goods. Tailing
  9. 9. Chemical Pad Bleeding Another substantivity-dependent fault of a similar nature can arise in the pad- dry- thermofix-reducing pad-steam process for vat or sulphur dyes and the pad- dry- thermofix-alkaline pad-steam application of reactive dyes to polyester/cotton blends.  Although the electrolyte concentration of these chemical pad liquors is invariably high, there may be significant desorption of unfixed dyes from the dried goods during immersion.  Lab-scale dip tests may give qualitative confirmation, but as with tailing problems they cannot reproduce the equilibrium state reached on prolonged running.  Nevertheless, quantitative measurements of colour differences do enable allowance factors to be determined for defining the relationship between dye padding formulation, chemical pad composition and target shade on the finished goods. Chemical Pad Bleeding
  10. 10. Staining Faults These are of almost infinite variety and tend to occur randomly, but careful analysis of the processing history of the fabric batch in question often pinpoints the source of the problem .  The fault may appear in a repeat pattern along the fabric length and it is important to record the exact circumference of all rollers and other cylindrical components that come into contact with the running fabric.  Staining faults may be roughly categorised as random staining, resist marks, spotting and foam marks. Staining Faults
  11. 11. Random Staining This may be variously described as mealiness, swealing, patchiness or blotchiness. When these effects are encountered on continuously dyed fabric the source of the fault is frequently migration during pre-drying after padding. The migration of water-soluble dyes at this stage is inversely related to substantivity and can be minimised by careful incorporation of electrolytes and migration inhibitors . The degree of reactivity of reactive dyes is also significant, because highly reactive dyes become partly fixed during pre-drying and this effect competes with migration. Thus high-reactivity dyes with high substantivity are the least prone to migration problems . The migration of an individual disperse dye can be restricted by other dyes present in combination that may have a larger particle size or a tendency to flocculate. However, the higher the concentration of migration inhibitor present, the less apparent are these differences in the migration behaviour of individual dyes . The reduced liquor retention attainable by vacuum impregnation with disperse dyes greatly suppresses the extent of dye migration at the subsequent pre-drying stage . In the absence of a migration inhibitor, vacuum impregnation is more effective than infrared treatment as a means of inhibiting migration but it is important to recycle the extracted dye liquor back to the pad trough . Random Staining
  12. 12. Resist Marks Most of these faults arise from inadequate preparation but it can be quite difficult to deduce precisely where or how the fault originated. Occasionally, chemical analysis may confirm that the poor dye uptake was caused by the presence of residual cotton wax, size polymer, oil or other contaminant before dyeing but far more often the dyeing and washing-off processes extract the offending impurities and such tests are then negative. Localised acidic or alkaline resists are normally caused by soluble contaminants that are soon extracted or neutralised at the dyeing stage. Resist Marks
  13. 13. Light Spots Repeated white or pale-coloured spots may be attributable to localised deposits on roller surfaces that interfere with the application of uniform pressure to the fabric containing absorbed dye liquor. Random light spots can arise as a result of water droplets falling onto the moving fabric after condensation has occurred on roof surfaces within or above hot and wet processing equipment. These wet spots dilute the unfixed dyes and chemicals locally, thus inhibiting full fixation in the spotted region. Light Spots
  14. 14. Dark Spots These spots or specks are usually local concentrations of deposited dye attributable to unsatisfactory dissolution or dispersion of the dyes and inadequate sieving before feeding to the pad trough. Incompatibility between dyes and auxiliaries, or between different classes of dyes when dyeing blends, pH fluctuations, variations in the water supply and desorption of impurities from the fabric have all occasionally been found to contribute to dye spotting problems. Airborne dye particles released when weighing, dissolving or dispersing must not be allowed to contaminate fabric or machinery in the vicinity; this is taken care of in modern dispensary facilities. Low-energy disperse dyes may volatilise and contaminate the interior surfaces of thermofixation equipment, so thorough cleaning between dyelots can be critical. Dark Spots
  15. 15. Foam Marks These faults often arise when a scum or foam on the surface of a dye liquor contains undissolved dye particles that can become airborne or collapse as a random deposit on the fabric surface. The presence of excess migration inhibitor, wetting agent or other surfactant, accompanied by turbulence of the dye liquor when operating at high speeds, may contribute to such problems. Similar faults can arise during the reoxidation of vat or sulphur dyes when leuco compounds desorbed from the fabric surface become reoxidised in particulate form if foam is building up at the liquor surface. Foam Marks
  16. 16. Typical semi- and fully-continuous dyeing of woven fabrics Process Type Example Pad-batch Semi-continuous Reactive on cellulosics Pigment pad-jig develop Semi-continuous Vat, sulphur on cellulosics Pad-batch-beam Semi-continuous Disperse/reactive on polyester/cellulosics Pad (coupler) – jig develop (diazo) Semi-continuous Azoics on cellulosics Pad (leuco ester and oxidant) – jig develop (acid) Semi-continuous Vat leuco ester on cellulosics Pad-dry-wash Continuous High-reactivity dyes on cellulosics Pad-dry-bake Continuous Reactive on cellulosics Pad-dry-thermofix Continuous Disperse on polyester Pad-dry-steam Continuous Reactive, vat, sulphur on cellulosics Dye pad-dry-alkaline pad-steam Continuous Reactive on cellulosics Pigment pad-dry-reducing pad- steam Continuous Vat, sulphur on cellulosics Pad-dry-thermofix-alkaline pad- steam Continuous Disperse/reactive on polyester/cellulosics Pad-dry-thermofix-reducing pad- steam Continuous Disperse/vat or /sulphur on polyester/cellulosics