COTTON YARN DYEING:-

Cotton Yarn Dyeing:-

Introduction:-

The practise of cotton yarn dyeing was popular even in ancient days. During those days, cotton yarn dyeing was carried out in hand form manually in hank form. It was dyed in a wooden vat where cotton hanks were loaded on bamboo sticks which was rotated by two people inside the dye liquor.

Nowadays cotton yarn dyeing is on the boom. New units are coming up all over the country. Technicians are busy in adjusting to the new scenario. The main dyed cotton yarn market is Tirupur, Ludhiana and Delhi. It is one of the most challenging jobs. It has been found that besides imported yarn dyeing machines and precision soft winding machines there are lot of factors responsible for cotton yarn dyeing. The water and the cotton yarn itself play a major role. The RFD (ready for dyeing) package becomes the heart of cotton yarn dyeing. A good technician will select the appropriate chemical to be used in cotton scouring. This varies from place to place and cotton to cotton. Enormous variety of grey cotton qualities can cause different pre-treatment results.

Metal analysis for good cotton is as follows:-

Calcium – 650 ppm

Magnesium – 420 ppm

Iron – 12 ppm.

Manganese – < 2

Copper -< 1.

But the cotton we receive is as follows:-

Calcium –1643 ppm.

Magnesium – 870 ppm,

Iron- 88 ppm,

Manganese -12 ppm and

Copper –5 ppm.

It has been found that Maharashtra and Gujarat cotton is better than Punjab cotton. South cotton is also better. After all it is an art because in cotton yarn dyeing there are numerous variables in respect to fibres, dyes and chemicals along with process conditions and application techniques. other factors like package density, angle of winding, package size, package compression, flow-rate/flow direction, drying, perforation of spindle, M.L ratio, pump pressure, entrapped air, chemical dozing, foam profile, pH, temp, and flow cycle/ time substrate variation, substrate pretreatment variation, dyeing cycle, washing off cycle. Also play a greater role in cotton yarn dyeing.

Following are the general problems faced in in cotton package dyeing today;

1) Strength loss

2) Uneven whiteness

3) Uneven absorbency

4) Harsh handle

5) Channel building

6) Unlevel dyeing

7) Poor batch to batch reproducibility

8) Package deformation and yarn abrasion

9) Shade variation in inner, middle and outer layer

10) Differential dyeing of varying cottons from different origins/mixing.

Selection of chemicals is very crucial. It has to be decided as per the circumstances. It has to be established by hit and trial methods. Firstly sufficient trials should be made in sample dyeing machines. Then converted into bulk process. Soft water is essential for cotton yarn dyeing. Small amounts of calcium and magnesium can affect migration and diffusion properties of reactive dyes. It will cause restraining or even precipitation resulting in reduced yields, unlevelness, poor reproducibility and reduced fastness.

The maximum recommended level for dyehouse water is ;

Total hardness – 50 –55 ppm.

pH -7. 0 +/- 0. 5

Copper –0. 05 mg/l

Iron – 0. 05 mg/l

Chloride ions –300 mg/l

The quality of salt used has a large influence on the dyeing performance. The quality of commercially available common salt (sodium chloride) and glaubers salt (sodium sulphate) can vary more often in pH, hardness and moisture content. If appropriate corrective actions are not taken the potential savings in cost of salt will be more than the cost of re-processing.

Fig: Cotton yarn dyeing.

1) Highlights of wetting and scouring aid:

Nowadays all-in-one chemical is available for scouring of cotton yarn. It should be based on combinatorial chemistry. It should be best suitable to remove any and all kind of impurities present in cotton. It must be a low foaming wetting agent and detergent with emulsifying, dispersing and extractive action while scouring and bleaching. It is APEO –free mixture of sequestering and surface active. It must have stabilizing and anti catalytic effect in alkaline pretreatment bath. It is better to have excellent wetting and rewetting properties. And having sustained low foam profile.

Its extractive leaching action should break the impurities into smaller compounds and subsequently assist in keeping them suspended in the bath and prevent any redeposition of the same. Complete removal of seed husks should take place. It should have a modified molecular structure and can attack and remove pectins, proteins, seed husk (kitty) and any other hydrocarbon in the form of insecticides, pesticides etc. with minimum weight loss. It will extract both water soluble and insoluble impurities present in cellulose due to its dispersing and emulsifying properties. Due to its sequestering properties it removes harmful alkaline earth salts which would otherwise tend to deposit on the yarn. It must have high penetration power. And excellent detergency. And effective carrying capacity for soil and fats.

It should provide instant absorbency in cotton which will indicate complete removal of impurities. It will assure uniform, even and equal capillary action. It will enhance whiteness index with brilliancy and luster. The degree of whiteness (CIE) will be between 70 to 75 acc berger the DP values (grey; 2380) should be more than 2000. The fibre damage factor should be less than 0. 1. The absorbency should be less than 2 secs as per TEGEWA drop test. The hydrophillicity must be 3 secs/cm. It will open up the cellulosic chain for greater affinity of the same towards dye molecules. It will prevent any undesired loss in tensile strength.

It should be non-ionic. In nature it should be of low foaming. It must be stable to high alkalinity and high temperatures (beyond 100 deg c) CIBA has come out with a SMART PREP process which is one step discontinuous peroxide bleach process. it reduces water usage up to 75 %. Cycle time is also less. CLARIANT has also introduced BLACK MAGIC process. Having the same parameters but it is on a slightly higher cost.

Enzymatic scouring is now considered to avoid strength loss and weight loss. In cotton yarn scouring, rossari biotech, Mumbai has come out with a product called SCOURENZ CBE POWDER. Which is giving good results in some units. The weight loss is only 1 % as compared to 4-5 % in alkaline scouring process.

2. Highlights of Stabilizer:

It should be a unique blend of organic and inorganic components that synergistically will stabilize the hydrogen peroxide bleach baths. It will control the decomposition of hydrogen peroxide with little or no sodium silicate. It controls the rate of release of peroxy ions during bleaching. Eliminates ca ++ and mg ++ powdery precipitates on yarn. It effectively controls metal hydroxides and metal impurities in bleach bath. In addition to the stabilizing action essential for effective bleaching it should also have high sequestering action for tramp ions of the heavy metal class that can cause catalytic damage during this stage of wet processing. It should give outstanding chelation of ferric (+ 3) ion under conditions encountered in peroxide bleaching. In other words it will facilitate the formation of hydroperoxy ion over the atomic oxygen for effective stabilization of hydrogen peroxide. It reduces the risk of damage by deactivating the destabilizing action of heavy metals. It will inhibit formation of insoluble waxes and fats. It is silicate free and capable of dispersing salts and colloids. It must have crystallization and precipitation –inhibiting action. It contains no surfactant and anionic in nature.

3) Highlights of Peroxide killer:

It is of two types. One is organic hydrogen peroxide decomposition inhibitor for pre dyeing treatment. And the other is enzyme based which removes any traces of hydrogen peroxide specifically by splitting it into oxygen and water. It is also called as peroxide scavenger. It can be added as is to the bath prior to dyeing step.

It improves shade reproducibility without loss of colour yield with dyes that are sensitive to oxidation processes outstanding biodegradability dyeing. The advantage of enzyme based killer is it has no effect on textile fibers or dyestuffs and hence dyeing can be continued in the same bath. It must be pH flexible and temperature flexible. After pretreatment and before peroxide kill pH of bath should be 5.5 to 6.5. Temperature should be below 50°C. In other words it is high powered state–of–art bio-catalyst. It starts deactivating peroxide from the room temperature. Reduces the effluent load substantially.

4) Highlights of leveling agent:

On a longer term processing without sequestrant also damages the machinery by scale formation in the most undesirable impurities in yarn, water, common salt, glaubersalt, caustic soda and soda ash are the di- and trivalent cations, eg ca ++, mg ++, cu ++, fe ++ etc. these ions increase hardness of the process bath. Ca ++ and mg ++ reacts with alkali and precipitates as sticky substances on the yarn which creates patchy dyeing. So it should be a combination of dispersing, leveling and sequestering nozzles and base. Dyebath pH should be adjusted to 6.5 before making dye addition. Hardness in dyebath should be around 100 ppm and TDS in dyebath should be less than 1000 ppm. In case of light and sensitive shades soda addition should be done in fresh water. Controls the strike rate in the adsorption phase leading to level dyeing. Buffer action prevents shock during the alkali phase and ensures uniform fixation improves dyestuff solubility.

5) Highlights of washing off agent:

This is non-foaming acrylic co-polymer product having excellent washing-off properties desired in after-treatments giving appropriate fastness properties. It should have the power of dispersing and sequestering and protective colloid. It is anionic in nature due to its moderate chelating property it suppresses the formation of calcium salts of reactive dye hydrolysates which are difficult to wash-off leading to poor rubbing fastness. Due to its efficient dispersing action (protective colloid) unreacted and hydrolysed reactive dye remains in finely dispersed states which are drained out.

In other words a tailor made wash-off agent for the effective removal of unfixed or hydrolysed reactive dyestuff from the surface of the yarn. It works in a broad pH range from 5.5 to 8.5. Eliminates adverse effects of ca ++, mg ++ salts of reactive hydrolysates, which are difficult to wash out.

6) Highlights of dyefixing agent;

It is non-formaldehyde based and cationic in nature. it combines with loosely held hydrolysed. Conventional dyefixing agents are formaldehyde based condensation products which contain free formaldehyde. It is banned in the export market. It is not eco-friendly. Dyestuffs forming a complex molecule which is insoluble, thus reducing the bleeding tendency of such dyeing. Improves both wet and perspiration fastness of reactive dyes. Has minimal effect on the light fastness. Produces good fixation with little or no change in final shade of dyeings. Before addition of dyefixing agent pH of bath should be adjusted to 5.5.

7) Highlights of softening agent:

It must be a universal cationic softener. It improves gliding and running properties of yarn assisting the process of unwinding without breakages. It is an emulsion of paraffin, polyethylene and fatty acid amide. Before addition of softener, pH of bath should be adjusted to 5.5. Before addition of softener in bath it should be checked for its compatibility (precipitation) by making small addition separately. Nowadays it is a specially modified cationic softener with a special addition of a reactive specialty silicone softener. It impacts a pleasing silky non-greasy, full, soft handle to the yarn. It gives good lubrication, improves sewability properties. It also imparts good lubricity and excellent soft handle to yarns, thus facilitating further operations such as twisting, rewinding, weaving and knitting.

In some mills flakes are giving good results. Like MAGMA CS FLAKES of rossari chemicals, Mumbai.

8) Selection of dyestuffs:

Industry s most versatile performing dyestuff range which will meet the most stringent fastness specifications has to be selected. According to the ever increasing demands of international fabric buyers like Adidas, Nike, Gap, Marks & Spencer, John Lewis, Columbia sports, Timberland, Kohl, Levis, Norm Thompson, Sara, Esprit.

Apart from basic requirements like compatibility, consistency, reproducibility and right first time approach, it should have added values like:

Ideal profile of substantivity, exhaustion and fixation rate, which enable to have every similar performance and least sensitivity to small processing variables when used in combination.

Excellent level dyeing properties due to homogenous fixation behavior and high migration index.

High fastness to post mercerization, chlorine and oxidative wash.

Meet the most stringent specifications of fastness to light.

High performance non flaring properties in different illuminants.

Excellent washing off properties, which will minimize effluent costs and improve productivity.

Meets the requirements of major ecology standards.

Ref:-https://textilelearner.net