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Impact of water quality and other parameters on Right-First-Time dyeing withreactive dyes: Based on note by Peter John Gibson. May 2003.

Detailled info available in TI on chapters marked with*

Main quality criteria for process water in order to achieve good dyeing/textile

processing performance of reactive dyes are:

1) Neutral initial pH ideal range: 6-7.

2) Low Hardness elimination of alkaline earth metal salts (Ca, Mg). Ideally less

than 3 degrees German total hardness (approx 50 ppm). *

3) Bicarbonate content as low as possible. Check and correct (if necessary)alkali conditions in order to achieve a pH sufficient for dye fixation (more NaOHto correct for bicarbonate buffering power which would otherwise keep pH toolow & speed of fixation insufficient).

4) Chlorine content as low as possible. An issue with municipal water in someareas. Major impact on many dyes particularly dioxazine blue chromophores like Cibacron bri.Blue FN-G. Worst case scenario, could need dosing of minute

quantities of thiosulphate dosing in process water to neutralise chlorine. *

5) Metal content - e.g. Fe and Cu. Can impact certain metal complex dyes moreparticularly certain direct dyes. Does not have major effect on current Cibacronrange, red chromophores however often turn more bluish in presence of copper

ions. *

6) Colour of process water not normally a major impact except in very paleand/or very bright shades. An indicator of presence of other impurities inwater( Iron salts for example).

7) Water treatment chemicals if carried over into the process water, can have anadverse effect on dyeing performance e.g. presence of Al salts and otherchemicals from, for example, boiler treatment. (i.e. water treatment does notalways resolve all problems it can create some if not well controlled).

But even ideal process water is, of course, no guarantee of dyeing

performance, since many impurities can be introduced into the dyeing systemby the CEL fibre itself (hardness, reducing agents, metal ions), salt (impact onhardness and pH), alkali (hardness, bicarbonate and pH effects includingbuffering at wrong pH.)

Whilst all the above have an impact on dyeing (Right-First Time) performance,experience shows that the following factors have a much greater influence:

Compatibility of the dyes in trichromy. Importance of this fundamentalparameter is a key part of Cibacron promotion and extensively discussed

elsewhere.

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Carry over(and lack of neutralisation) of pre-treatment chemicals.Inadequate neutralisation of alkali (internal fibre pH even if dyebath pH

seems OK) and of peroxide on fiber *. Both have major impact on dye yielddue to premature dye hydrolysis. Carry over of thiosulphate-based "peroxide

quenchers" * such as Invatex, can have a serious negative impact on yield ofreactive dyes.Enzyme based neutralisers seem to have a lesser negative effect, but wehave not systematically checked the impact on all our Cibacron dyes.

Hygroscopicity of dye commercial form. All powder commercial dyeformulations, including reactive dyes, are hygroscopic, and therefore,when exposed to humid air (due to incorrect storage conditions), willabsorb up to 20% moisture per weight of dye! This absorbtion will starton the exposed top layers of the container, leading to different, unreliablecolor strength of dye samples.

Dye dissolving is a potential problem area since this activity is often left tothe customer even in "Ciba supervised trials". Particularly with customers usedto handling poorly soluble dyes requiring dissolving at/near to the boil (such asProcion HE types and some Remazols), this can have disastrousconsequences for some of our "warm dissolving" dyes, particularly if processwater is slightly/strongly alkaline. Worst case scenario, in pale shades forexample: up to 50% of the dye can be hydrolysed even before it reaches thedye bath (time of dissolving before addition to dye bath also has a strongimpact, if dissolving is carried out under such negative conditions).

3) Liquor Ratio the most common cause of poor reproducibility. Mostcustomers base their chemical additions on data provided by the machinerymanufacturer, on the physical dimensions of the dyeing vessel or on liquorlevel indicators on the dyeing machine itself. Lately, there has been someimprovements, but experience shows that these liquor indicators can easily be25% out, compared to actual liquor ratio (liquor in dead volume in circulationtubes, heat exchanger, expansion tanks, etc., often not taken intoconsideration; also wet pretreated fabric content vs. measurements with wateralone lead to wrong assumptions). If liquor ratio is wrong (e.g. higher thananticipated), salt concentration is wrong (less than required resulting in lowerexhaustion) and alkali concentration is wrong (lower than required in lower

fixation of a lower exhausted amount of dye). Our Ciba "robustness test" doesnot take into account these cumulative effects we simply test 20%-0-+20%conditions for individual effects not for cumulative effects. Take a typicalCibacron dyeing, supposedly to be made at 10:1 LR, with 80 g/l salt and 20g/lsoda ash. If liquor ratio is actually 12,5:1 (+25%), then salt concentrationbecomes 64g/l (-20%) and soda ash 16g/l (-20 %). If you make a lab dyeingunder former and latter conditions you will see a massive difference far awayfrom the delta E 0.5 differences shown in our data. In bulk, proper checking ofsalt concentration, with hydrometer and salt graphs, is vital to ensure goodreproducibility correcting, where necessary, salt and alkali additions tomatch the actual liquor ratio.

4) Lab to Bulk Reproducibility assuming the above factors are well controlled,actual lab practice is another factor which can seriously impact RFT and lab to

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bulk reproducibility: Quite often, lab dyeing is carried out on pre-bleached, dryfabric, which can lead to immediate errors, both from a liquor ratio and from adye concentration point of view. In bulk practice, we may start with grey fabric,which can contain up to 10% impurities. After pre-treatment, a 100 kg batch canactually weight as little as 90 kg. However, everything is calculated for 100 kg

dye, salt, alkali, etc. If we start in the lab with dry pre-bleached fabric and baseall calculations on this, we have immediate errors exacerbated if impurity levelis high. Ideal lab practice is to take customer's grey fabric and cut into, say, 10 gportions; then bleach and dry (check weight to confirm amount of impurities ifsay 9.5 g, you know that fabric contains 5% waxes etc) and then carry out thelab dyeing on this fabric assuming the original weight of 10 g. An alternativemethod is to check quantity of impurities first and then if impurities are say6%, use 9.4 g of dry pre-bleached fabric for your lab dyeing instead of 10 g, butstill basing all your calculations on 10 g.

4) Other Factors even if all the above factors are well controlled, there

remains a number of other factors which affect RFT performance: sampling ofthe bulk dyeing soaping off, conditioning and finishing (or not) of the sampletaken at the end of the fixation phase can impact performance.

a. Conditioning is critical an over-dried sample can appear to belacking 20% yellow and 10-15% red compared to a fully conditioned

sample (steaming is advisable) *. Many wrong shading additions havebeen made based on poorly conditioned "sample bits". Yellow NP hasthe best performance in this respect. It is important to compare "likewith like" and therefore if compared to a finished standard, the "samplebit" should also undergo similar finishing if not, you should compare

an unfinished sample with "an unfinished standard".b. Residual pH of the dry dyed fabric, can impact the final shade. Mostred chromophores for example give a slightly more bluish shade if thealkali has not been properly washed off and if CEL pH remains a trifletoo alkaline. It is sometimes advisable to add minute quantities of aweak acid in final rinsing bath of washing off process.

c. Photochromy. Quite frequent in khaki and earth tones, containing bestselling golden yellow reactive chromophores. Finishing, moisture andpH conditions affect photochromy. (See Training module on phototropy ofreactive dyes)

d. Thermochromy. Many dyed samples show a slighty different shade

immediately after drying (or any othe hot treatment), recoveringprogressively their final permanent shade.