being chased by a bear

7/31/2019 Being Chased by a Bear

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Being chased by a bear ?(An overview of yarn dyeing from the perspective of a textile auxiliarysupplier)

1 . Size and Importance of the Textile IndustryThe Textile Industry is still one of the industries in the world economy that is common toalmost all countries. The Textile Industry is one of the major battlegrounds betweenindustrialised countries and developing countries, and its importance can be measured by itssize in billions of US dollars (1).This only serves to illustrate how competitive our industry is.

2. The BearIn the competitive field of textile exports , textile manufacturers continually search for

markets with lower labour costs and higher productivity. (2)


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Source: Werner International


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Labour costs still vary widely between textile and garment producing countries, buttechnological progress tends to reduce the impact of labour related costs, and factors likeresponse time, import duty benefits and incentives can offset marginally higher labour costs.(3)There is a saying : "If you are being chased by a bear you don't need to run fast - justfaster than the man next to you "

3. Package Dyeing of Yarn - Why package dyeing ?

Available statistics have generally indicated that yarn dyeing accounts for a relatively smallproportion of coloration processes - of the order of 15% of the total material dyed.Yarn can be dyed either in hank form or as packages, packages being dyed in either verticalor horizontal spindle-type machines. The advantages and disadvantages of the differenttechniques have been stated as (4) :

Hank dyeingyarn has fuller bulk and handle - loft

tangling may occur

reeling and back-winding costly and potentially wasteful

levelness often inferior to package methods

machine loading is less for a given machine size.

Package Dyeingleaner yarn, but this gives stitch clarity in finished fabric


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larger (and consistent) machine loadings for a given machine size

faster back-winding and less waste

high levelness and reproducibility even with dyes possessing high fastness

savings in energy, water etc. and space.

A contributory reason for the success of package dyeing relative to hank dyeing is the

almost infinite extent to which the operation can be automated , removing the need formanual intervention and reducing the impact of labour costs.Robotisation of raw material warehousing, loading and unloading of the spindles of thedyeing machine, transporting of dyeing frames from one machine to another,hydroextraction, drying, and finished product warehousing , is increasingly commonplace (5)(6) (7)High flow rate pump development, design of new circulation systems for flow reversal,automation , and multifunction microprocessor controls, have made possible drasticreductions in cycle times and new operating techniques.These techniques depend on the principle of blind dyeing, which in turn involves completecontrol over all aspects of the operation, including :

limits of accuracy of temperature control,

dispensing of dyes and chemicals,

quality of water, dyes and chemicals,

control of the weight, density and uniform dyeability of the dyeing packages.

4. Developments in package dyeing technologyThe developments that have taken place in package dyeing technology have largelyfocused on quality differentiation at the same time as productivity (8), and those producersthat are "running faster than the man next to them" have made quality improvements withrespect to:

Surface appearance

productivity per machine and man-hour improving cost effectiveness

Off-winding properties

Levelness

Shade consistencyHaving established that the only way to stay in business is to perform better than yourcompetitors, how can these quality improvements be made ?4.1 Surface appearanceAutomated variable flow control has been introduced in package dyeing of yarn using theprinciple behind developments to reduce energy transmission to the substrate proposed byCarbonell (9). As has been done in rope dyeing of fabric on a jet dyeing machine, the liquorflow may be changed at different stages in the preparation and dyeing process. Thus a highflow rate will be used at the more critical stages, and the pump flow will be reduced at theless critical stages. This results in a lower energy transmission to the yarn and therefore asmoother , less hairy, surface appearance and less yarn breakages in subsequent use.The general liquor flow rate for a package dyeing is approximately 30 litres/kg/minute.The liquor circulations or number of contacts per minute may be calculated as follows :Interchanges per minute = Flow Rate

Liquor Ratiofor conventional liquor ratio machines ( 10:1 ) = approx. 3for Ultra Low Liquor Ratio machines ( 5:1 ) = approx. 6.Dye liquor interchange through the free volume of the package will be approximately 8 - 12times per minute when processing cotton yarn.Beckmann and Hoffmann (10) found that flow reversal every 4 - 8 circulations gave superiorlevelness during the exhaustion phase, but was of little benefit during the migration phase ofpackage dyeing .

4.2 Productivity per machine and man-hour


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Before the 1940's the only instruments fitted to yarn dyeing machines were mercury-in-steelor bourdon thermometers, and simple timers for reversing flow. Metal cams operated thefirst rate-of-rise temperature controllers, like the Bristol Dyemaster. During the 1960's moresophisticated digital controllers such as the Celcon became available, but all these havenow been replaced by the new generation of computer controllers.Productivity improvements have focused on :

Automation- automation of dye process- automation of total dyeing operationShortened Processing TechniquesPackage Dyeing Technology

4.2.1 Automation of the Dye ProcessRecent developments have seen the introduction of microprocessor controlled addition ofdye and chemicals, and the advent of multifunctional microprocessors has allowed dyecycles to be shortened by virtue of their ability to permit several operations at the same time,e.g. temperature rise , dye & chemical addition, flow rate change , etc. rather than in onestep at a time as was previously the rule.

4.2.2 Automation of total dyeing operationClever computer control also makes possible bleaching processes that achieve considerablewater/effluent savings ( up to 70% is claimed) by re-using both the water and energy contentof the peroxide bleaching rinsing baths by recycling the treatment and/or the rinsing liquors..Conventional machines must be fitted with several liquor storage tanks to make this possible(11).

4.2.3 Shortened Processing Techniques


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Time, water and energy savings are also made possible by high temperature peroxidebleaching of cotton, instead of bleaching at the boil. Techniques involving combinedmicroprocessor cooling and rinsing can be used so that by the time the machine has beencooled from 115 deg C to 80 deg C, the cotton yarn has had the equivalent of two discreetfresh water rinses.With hot water supply at 50 C and 70 C for controlled rinsing, it is possible to complete theoptimised high temperature preparation, reactive dyeing, and rinsing processes in 6 hours(12)(13)

Shortening the total dyeing process is impossible without the control needed to minimisenon-dyeing time, filling, emptying , heating and cooling etc.Increasing productivity by blind dyeing necessitates a high degree of reproducibility of shadeand levelness, impossible without a uniform preparation.


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In some dyehouses it is common practice to pre-scour only before dyeing rather than pre-bleach, perhaps 80% of production being in shades not requiring a bleach.The Nearchimica Minimum Prepare Scour makes possible a rapid process for pre-scouringand dyeing cotton with reactive dyes.Scour/dye methods have failed in the past because they have reduced dye yield, given poorpenetration, precipitation of calcium and magnesium salts, and perhaps dye spots.The Nearchimica Minimum Prepare makes no attempt to remove paraffin wax lubricants orthe natural lubricants found in cotton, and relies on the hot dyeing process to remove theseusing the excellent emulsifying properties of Lubrifil LAF in the dyebath.The typical target process time for Minimum Prepare can perhaps save 2 hours compared toconventional pre-scouring at the boil.

4.2.4 Package Dyeing TechnologyIt is often said that "Well prepared is half dyed" , and in yarn package dyeing, if preparationincludes winding , that is certainly true.(14)Advances in package dyeing technology that have led to productivity improvements include:

Precision WindingDigital or Step WindingNew package centresAutomation } see section 3. aboveRobotics }

With traditional wild or random cross winders the winding angle remains the same, thenumber of revolutions per traverse reducing, as the package diameter increases. The lengthof yarn per traverse is constant, producing an even package density.Advantages Disadvantages+ stable package - areas of ribboning are possible+ constant winding density ( usually approx. 350 g/l) - liquor flow characteristics not optimum

+ low capital cost


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Precision Winding

In "Precision Winding" the winder reduces its surface speed as the package increases insize, so maintaining a fixed number of threads per traverse.

In Precision Winding the winding traverse tends to increase as the package diameter grows.This results in the ends of the package forming a concave profile, which in turn leads to thecolumn of packages requiring some compression when using interlocking centres to achievea good seal.Advantages+ no ribboning+ good liquor flow characteristicsDisadvantages

- fragile package, must be handled with care- density varies from inside to outside ( but higher compared to random - 400-450 g/l )- high capital cost.

Digital or Step Winding

Digital Precision Winding was pioneered by Schweiter with their Digicone system and is

characterised by a drive system with a microprocessor speed controlled drum.


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Initially a precision winding is produced, but as the diameter of the package increases, thecrossing angle is reduced. When a certain minimum value of the crossing angle is reached,there is an instant transition to a smaller winding ratio. The package is built in layers, eachhaving a slightly lower crossing angle than the previous, and each layer having a different butconstant winding ratio, between 20 and 40 layers building up to form a single package.

Digital precision wound packages , with a constant package traverse, are designed to haveflat ends, which permit a minimum of compressionAdvantages+ good liquor flow characteristics ( soft shoulder by periodically varying traverse)+ no ribboning+ stable homogeneous package without pattern zones+ uniform, homogeneous density with excellent off-winding properties (400-450 g/l)+/- moderate capital cost.The increased package density, 20 - 25% higher loading on the package, larger packageweights and consequent higher weight machine loadings , gives lower liquor ratios andconsequent increases in productivity and cost savings .

New Package CentresMore modern centres, designed to increase machine loading and improve off-windingperformance, are gradually replacing conventional centres such as the 4 20 stainless steelcone with spacers and compressible stainless steel springs.

Plastic centres interlock on compression making a seal and avoiding the use of spacers. Upto 20-25% higher loading is made possibleThe rigid reusable, or axially compressible non-reusable polypropylene centres usuallyfeature a protected yarn reservoir, which facilitates off-winding and reduces waste.Liquor flow through the package must be uniform in order that preparation and dyeing isuniform. The 4 20' conical packages are the least suitable for level dyeing from a hydraulicflow point of view, whilst the parallel sided package gives more uniform liquor flow (15).

Going back to our analogy ofbeing chased by a bear, the optimum economic benefits willresult from uniformly wound packages, at the highest densities feasible, on centres that givecolumns without leakages and present the best chance of a level dyeing.


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4.3 Levelness4.3.1 Blame the auxiliary !From the standpoint of the textile auxiliary supplier , the important considerations regardingwinding technology and package centres are the common dyeing problems resulting from thepresentation of a poor package for dyeing which may wrongly be attributed to the choice ofauxiliary .Such problems (14) include :Poor 'batch-to-batch' reproducibility

high levels of reproducibility only possible using same raw materials andprocess conditionsliquor flow rate must be same from batch to batch

Package deformation and yarn abrasionwinding density must be the same from package to package and within apackagepackages must be well wound and tube perforations coveredpressed density of compressed tubes must be the sameleakage and package deformation can arise due to damaged tubes, faulty

spacers and defective locking caps.leakage between packages can be caused by shrinkage and deformation ofplastic tubes subjected to high temperatures. Use self locking caps with followdown devices.

No differential pressureyarn carrier and packages may be leakingwinding density may be too soft for the differential pressure control

Exploding warp beamsfaulty beam density and patterning - for 100 % cotton beam suggest

winding density - 360 - 400 g/lwinding tension - 35 - 40 g/mtraverse - 4 - 6 mm

Uneven dyeing caused by too low a liquor flowincrease flow rate or reduce package densitycheck for leaking package columns

Pressure and lustre marks on inner yarn layers.too high winding density or pressed density ( or high residual shrinkage)careful control needed for yarn tension and package buildcover tube or cone with paper or non-woven sleeve before winding

Leakage in the package columnfit properly sealing cap, spring loaded or self locking.plastic or metal tubes must be stable to process temperature and pressurein pressed systems compression of 10 - 35% should be usedreduce height of column

Package to package colour variationmixed batches or blendslarge variation in package to package density

Difference in shade inside - to - middle - to - outsideuniformity of winding and pressed densityselect correct liquor flow rate for the packages used.

On the other hand - the choice of auxiliary product can sometimes make all the differencebetween a level dyeing and an unlevel dyeing.


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4.3. 2 NEARGAL LU-SRV - a levelling agent for dyeing cottonIn package dyeing of 100% cotton yarn, the problem most often encountered must surely belevelness from inside - to - middle - to - outside of the package, which shows itself as shadevariation when panels are knitted with a single feeder knitting machine from the differentregions of the package.In considering level dyeing it is proposed that we must first consider the mechanism by whichcotton is dyed with anionic dyes, and , in particular, why we add electrolyte to a cottondyeing. There are 3 principal reasons :

(1) Anionic dye molecules will tend to repel each other, and because the cellulose moleculetends to take on a small negative charge when immersed in water, there will be a naturalrepulsion of the anionic dye by the similarly charged fibre.Adding electrolyte , e.g. sodium chloride, to the system, will flood the system with bothnegatively charged and positively charged particles , and will form a compact electricaldouble layer on the dye ion so that it is electrically neutral towards a second ion. It will maskthe negative charge on the cellulose, effectively eliminating the repulsion effect.

(2) Solubility. If we make it more difficult for the dye to remain dissolved in the bath bydissolving more salt in the bath, we shift the equilibrium in favour of the fibre, and the dyewill prefer to absorb onto the cellulose.


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(3) Aggregation. Perhaps most important from the point of view of level dyeing is theaggregation of the dye. Addition of electrolyte influences the aggregation of cotton dyes,increasing the extent to which hydrogen bonds form between neighbouring dye molecules,and creating larger, less mobile, less soluble particles, long planar more hydrophobicparticles with more affinity for the fibre than the parent monomolecular form.

Here we see a typical, simple direct dye and the possible mode of aggregation encouragedby the addition of electrolyte.If we examine a solution of any reactive dye with a UV-VIS spectrophotometer, the trace willshow two peaks, one for the monomolecular form of the dye, one for the n-mer or aggregateof the dye.

If we add common salt to the dye solution we can see that the amplitude of the n-mer oraggregated peak is increased with respect to the peak of the monomolecular form.Similarly, if we add urea , there is more of the monomolecular form of the dye, and less of theaggregate.


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If we now compare a dye solution to which we have added the Nearchimica levelling agent,Neargal LU-SRV, we see that the effect on aggregation is similar to that given by urea, therebeing more of the monomolecular form of the dye relative to the aggregated form.

We have begun with the knowledge from practical experience in production that Neargal LU-SRV does indeed function as a levelling agent, and we have sought to explain themechanism by which it functions. We think this disaggregating effect is significant, andexplains why we should observe greater penetration of the substrate, and more migration .

However, the disaggregating effect cannot be permanent because the final exhaustion is thesame with the levelling agent as without. Neargal LU-SRV has no restraining effect.


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This migrating effect is most easily demonstrated in the laboratory with vat dyes.If we begin with a yarn package composed of half vat dyed yarn and half undyed yarn, wecan clearly see the extent to which Neargal LU-SRV improves migration from the dyed to theundyed fibre (16).

With reactive dyes , we can show the effect of adding Neargal LU-SRVRD to a packagedyeing in production , knitting yarn from the inside of a package next to yarn from theoutside, and comparing with a batch dyed with a competitor's agent.

4.4 Shade consistency

Untreated packages of yarn madewith undyed cotton and cotton

dyed with 2% C.I. Vat Red 13

Same yarn after attempt to level packages using 4g/l Neargal LU-SRV

25 ml/l caustic soda 38 B liq.10 g/l sodium hydrosulphite

at 60 C for 45minutes


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Testimonials from a major Nearchimica customer dyeing knitted cotton fabric on jet dyeingmachines with reactive dyes, support the claim that colour difference from piece to piecewithin one dye lot can be considerably improved by incorporation of Neargal LU-SRV in thedyeing recipe .Total colour differences of less than Delta E - 0.5 have been attained consistently.A useful check list has been proposed where problems of poor 'batch-to-batch reproducibility'are encountered (14) :

Only by using the same raw materials and the same dyeing process can highlevels of shade reproducibility be obtainedCheck that the substrate to be dyed has the same dye affinity as the previousbatch.Ensure preparation is standard.The liquor ratio must be the same from batch to batch.The liquor flow rate must be the same from batch to batch .If a different batch or source of dyestuffs or auxiliaries is used, check inlaboratory.Avoid standing times during the process, be sure that necessary dyes and

auxiliaries are prepared ready for addition at the right time.When dyeing with vat dyes, any oxygen in the system must be neutralised byaddition of Hydros, or displacement with nitrogen. For each cu. metre of airadd:

1.7 kg Hydros1.7 litres caustic soda 38 B

5. SummaryIt has been shown that the textile industry of today is still a significant world-class, but highlycompetitive industry.Developments in package dyeing technology include moves to higher machine loading,higher package density, lower liquor ratio, and shortened preparation and dyeing cycles. All

of these offer significant economies and productivity benefits, but all require carefulconsideration of the textile auxiliary products involved if shade reproducibility and leveldyeing are not to be sacrificed.Nearchimica would propose that in whichever stage of wet processing improvements aresought, we have products that are worthy of consideration.The bear that is chasing all of us is the bear of competition. Sometimes lower pricecompetition. Trying to ensure that we differentiate our production in terms of improvingquality, efficiency, and productivity is the only way that each of us can run faster than theman running next to us and escape from the bear.

References


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1. OECD / Gherzi Organisation, Zurich2. Gesamttextil / Gherzi Organisation, Zurich3. Werner International / Gherzi Organisation , Zurich.4. Chaplin, Park, Thompson, JSDC Vol 96 Nov. 19805. E.Bocus, JSDC Vol. 107 May/June 1991.6. J.F.Gaunt, JSDC Vol.109 Jul/Aug 1993.7. Knitting International Feb. 1989, Fully Robotic Yarn Dyeing.8. P.S.Collishaw, D.T.Parkes , AATCC symposium , Apr 1990 .World Overview of packagedyeing.9. Carbonell , Hasler, Walliser, & Knobel, Melliand Textilberichte.,54, Jan.197310. Beckmann & Hoffmann , AATCC Conference 1976.11. B.Seidl , Sympotex 96 - South Africa12. P.S.Collishaw , AATCC Conference 1997.13. Heetjans JH, Thies GmbH, International Dyer, Vol 181, Nov. 199614. Heetjans & Tindall. A Handbook for the Yarn Dyer.Thies GmbH.15. Denton, J.T.I., 54 . 196316. R.Gordon. Nearchimica Sales Aid , Neargal LU-SRV.

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