Ashok Athalye, Jaydeep Gupte
Ashok Athalye, Jaydeep Gupte
Since the development of commercial Synthetic Vat dyestuff in 1901, this class of dyestuffs has undergone various physico-chemical modifications and still remain first choice of dyers whenever highest degree of fastness standards are desired. Especially in high quality shirting with multi-coloured yarns for designs in stripes/ checks, formal and industrial work wear, uniforms, home upholstery, medical clothes, etc. only Vat dyes can withstand stringent fastness requirements.
Though, generally as a class of dyestuffs it lacks brilliancy of shades, involves complex dyeing process & overall high recipe cost when compared to the subsequently developed other class of dyestuffs for Cellulosic dyeing like Reactives, Azoics, Sulphur, etc. it is still preferred and used world over for the outstanding fastness properties especially, the wet light fastness, ability to withstand repetitive laundry washes, post mercerization/ alkali fastness & Chlorine/ oxidative bleach fastness.
Traditionally Vat dyes are preferred for use in conventional/simple low turbulence application methods on woven cotton processing in Jiggers or Cotton hank dyeing in Tubs/Vats. The textile dyer/processor is generally reluctant to use these dyes on high turbulence, low liquor ratio package dyeing machines for the fear of getting non uniform, unlevel shades.
As a world renowned Vat dyestuff manufacturer and supplier, Atul Ltd has developed a unique dye manufacturing processes and developed application methods to meet the dyers requirements in terms of eco friendly, non polluting, easy to use Vat dyes in super fine dispersion form.
The state of the art reactors and dye manufacturing routes ensures process modifications during development of these Vat Dyes. Given below are some of the Eco -initiatives taken and process modifications made to overcome the inherent problems of accumulation of the heavy metal ions and toxic intermediate compound formation associated with most of these dyes. Further, ATUL has successfully achieved reduction in the Effluent load by modifying the Dyestuff manufacturing processes and controlling the reaction rates so as to significantly reduce release of ecologically harmful heavy metal ion compounds from the effluent stream.
Some of the manufacturing process route modifications are as given below:
Further these dyestuffs undergo various repeated milling cycles and pass through various filters to ensure a uniform and super fine particle size. A simple generalized schematic process route is given below
These Novatic S/D (Supra Disperse) Vat dyes are specially developed and designed for exhaust application on Cellulosic yarns and piece goods by the Leuco or pre-pigmentation method. The Novatic S/D Vat dyes are recommended for dyeing pale to dark shades on cellulosic fabrics and its blends by various exhaust dyeing methods.
Given below are a few case studies I examples of difficulties faced by the Textile processor in package yarn dyeing, the probable reasons, the problem solving approach and solutions provided by Atul technical team by using Novatic S/D vat dyes.
1. A Cotton package Yarn dyeing processor in north India was facing problems of - severe unevenness, wide Inner-Middle -Outer shade variation, off tone shade and dye deposition within layers leading to poor wash & rub fastness.
Machine - Package dyeing machine.
Substrate - Unmercerised Cotton yarn 2180's
Shade - Royal Blue
M L R - 1:10
Vat dyestuffs for some SSI and low cost imported supplies.
C.I. Vat Blue 6 - 5.5%
C.I. Vat Red 13 - 0.15%
C.I. Vat Green 1 - 0.03%
Caustic Soda Flakes - 13 g/l
Sodium Hydrosulphite - 11 g/l
Method - Pigmentationmethod
Ensuring that the ready for dyeing (RFD) yarn package was consistent in terms of pretreatment considering the uniformity of rewetting absorbency, whiteness index, no residual peroxide, uniform pH, adequate demineralization, etc. following recommendations and problem solving solutions were offered:
i. C.I. Vat Blue 6 - is well known for its very high strike rate, further if the dye particle size is not properly controlled, the dye reduction would be non uniform resulting in increasing the chances of shade change and dye deposition, the uncontrolled high presence of heavy metal ions would further augment un-levelness problem.
ii. This dye recipe was replaced by Atul Novatic Blue BC SID (C.I. Vat Blue 6), Novatic Red 6B SID & Novatic Jade Green XBN SID. Where the dye particles are finely dispersed and properly controlled for uniform reduction and the heavy metal ions are within ETAD limit.
iii. This Dye is highly sensitive to water hardness hence addition of a non dye de-metallising sequesterant like 0.5 g/l of Tulachem Texcell DSA or Tulachem Sequestab EP is highly recommended. Addition of a suitable leveling agent is advisable to further control the dye strike rate.
iv. The proper dyeing procedure involves pasting of dyestuff with cold water, then addition of a alkali stable dispersant like 0.5 -1 g/l of Tulachem Texflow LDP with further dilution with warm water with constant stirring to ensure thorough and uniform dye dispersion followed by filtration and then additional/dosing in the dyeing machine. This would ensure non dye agglomeration and avoid any outer layer depositions.
v. Under such conditions of application, the Pigmentation method of dyeing is highly recommended for achieving good level dyeing. The temperature is raised even up to 100 deg C before reduction depending on the type of perforated tubes being used which withstand high temperatures.
vi. Controlled dyeing cycle and precise use of caustic soda and sodium hydrosulphite is advised for which a regular intermittent pH and reduction tests checks are desirable.
vii. Hydros, the reducing agent dosing should be at a temp below 60 deg C. When the temperatures rises above this or the time of reduction is over extended, the solubilised leuco derivative of the dye is transformed to over reduced anthranolic form which results in duller off tone shade. Under such conditions, addition of Glucose or sodium nitrate can be recommended to prevent over reduction.
viii. To improve wash and rub fastness, in case of such dark shades above 3%, a sharpening treatment is recommended. After dyeing the dye bath is drained and then following treatment given at room temperature for 10 minutes
Caustic Soda Flakes - 0.5 g/l
Hydros - 1.0 g/l
Texflow LDP - 0.5 g/l
ix. A good overflow wash after dyeing is very important to achieve correct shade. The optimum pH for Oxidation is preferably at around 9.
x. Finally thorough soaping with a high performance washing off agent like Tulachem Texhwash DET 1.0 g/l is recommended to obtain correct bright reddish Blue shade.
The lab as well as the shop floor trials conducted with these Atul Novatic SID Vat dyes along with the auxiliary chemicals by controlling the proper dyeing procedure resulted in achieving a perfect uniform, level dyeing giving a bright Royal blue shade with desired fastness properties.
2. A cotton package yarn dyer in central India was facing problem of strength loss after Vat dyeing.
The probable reasons and remedial measures to overcome this problem are
i. The first and foremost imp reason for this is presence of heavy metal ions like Iron, Manganese, Lead and Copper particles, corning either from the cotton fibre, process water, impurities from salt or caustic soda or from the Dyestuff itself.
ii. The Grey yarn may contain some of these heavy metal ion impurities and need to be thoroughly demineralized using product like 0.5 g/l of Tulachem Demin C, prior to scouring and oxidative bleaching to avoid localised catalytic decomposition of Hydrogen Peroxide resulting in tensile strength loss.
iii. The use of only softened process water is not sufficient as normally the water softening process takes care of mainly alkali earth metal ions like Ca and Mg but not the problematic heavy metal ions like Fe and Cu. Hence use of a proper and strong heavy metal ion sequesterant having high complexing constants under high temp and high alkaline conditions is very imp.
iv. The water storing tanks and the supply lines I pipes if corroded may aggravate the problem. Use of non metallic water supply lines and a magnetic filter could help.
v. As such generally the shades with Vat dyes are comparatively dull, hence for such medium to dark shades either a very mild bleaching or only a thorough scouring for achieving a good and uniform rewetting absorbency is sufficient. With this the potential danger of pre dyeing strength loss during peroxide bleaching can be.
vi. Next, the potential source of heavy metal ions comes from the Vat dye itself. Use of a suitable Novatic SID Vat dyes having process modificationally controlled and reduced metal ions can further minimize this problem.
vii. Further, addition of adequate amount of caustic alkali, reducing agent and post dyeing Oxidation with Hydrogen Peroxide with proper sequesterant should be used depending on depth of shade and class of vat dye.
viii. After dyeing lot should be centrifuged and dried as fast as possible, especially over-drying should be avoided.
ix. The effect of finishing treatment and chemicals used in terms of acid liberation, high temp thermo mechanical treatment, etc. should be thoroughly analyzed for effect on yarn strength loss.
x. The yarn dyed fabric undergoes treatments of bleaching and mercerization, here again care must be taken to ensure that no heavy metal ion contamination takes place causing strength loss I damage to yarn.
For further details on this unique Novatic Supra Disperse Vat dyestuff range, the general guidelines on the methods of dye application, important process parameters, recommendation on proper usage of auxiliary and basic chemicals, pre-treatment and post dyeing processing treatments, dyestuff selection guidelines for typical Khakhi, Browns, Olives, Grey, Greens, shades and details on various fastness properties, etc. the readers are invited to refer our Shade Card.
About the Authors:
The authors are associated with Atul Ltd (Colors Division), Gujarat.