The Low Temperature Bleaching concept is a major development in the pre-treatment of Cotton. The process is simple to implement and does not require special equipment or monitoring. It can be used over a wide range of equipment and gives major improvements in the quality of the cotton produced. It is suitable for application on pure cotton and synthetic blends with cotton. When Lycra is present additional scouring systems may be required. Compatibility with these systems is not an issue.
The EURODYE-CTC Low Temperature Bleaching (LTB) is a very effective and gentle process to bleach cotton at a much lower temperature than the traditional 98°C. Typically the bleaching is conducted between 60°C and 70°C with each mill deciding depending upon their individual requirements. Bleaching at 70°C will obtain a slightly improved whiteness. The process itself is easy to implement not requiring specialist equipment or constant monitoring. It is very robust simple to apply and does not require pH monitoring and control. The main benefits of the system are the improved quality of the cotton and better reproducibility. Mills which are running the system report better batch to batch repeatability especially in cone dyeing. The system may be used in a wide variety of machines such as Jet and Cone (Yarn) dyeing machines over a wide range of liquor ratios. In general terms the advantages of the EURODYE-CTC Low Temperature Bleaching can be described as below.
GENERAL BENEFITS
- Better handle
- Very little fibre damage
- Reduction in water usage; no wasted water or time cooling from 98°c
- Smaller carbon footprint due to less heating and reduction in radiated heat from the machine during processing
- Better reproducibility
- Paraffinating agents can be reduced or eliminated in yarn dyeing
- Yarn packages experience less stress fractures so the dye liquor circulates more evenly and results in better dyeing
- Better wearability as the fibres do not soak up moisture and it is wicked to the surface to evaporate
- Yarn has been observed to dry quicker
- Cones in package dyeing wet out and settle much more evenly ensuring an even flow through the package and a very much reduced risk of cone splitting
As with all systems it is essential that there is some fine tuning to the system due to local conditions and requirements. The process can be used under a wide range of liquor ratios. Adjustment from a general formulation should be made to allow for different liquor ratios. However it is essential that the caustic level should be set at 2 g/l solid caustic regardless of liquor ratio. This is because the caustic is needed to soften the cotton seeds which is achieved by pH and not by a relationship between the amount of caustic and the cotton.
It is very important to accept that this is a new approach and not to run a comparison against the traditional system. The aim is to achieve a dyed fabric/yarn that is suitable for its end use and not to measure comparisons during the process of achieving it. The Low Temperature Bleaching system will gram for gram yield a fabric with a few degrees of whiteness lower than normal. However the difference is small and we are talking about dyed material not whites. Practical experience has shown us that despite the couple of percentage of reduction in whiteness a perfectly good dyeing which is on shade is achieved simply by continuing the dyeing process.
Bleaching in the range of 60°C to 70°C gives a fibre that is not inherently hydrophilic because some of the waxes remain. It is a myth to believe that you cannot dye without good Hydrophility. The proof of this is that Scour dyeing is a well-recognised established dyeing method. In this process as in LTB the waxes are still present In the case of LTB some of the waxes are removed by the action of the alkali and scouring agent and some by saponification. In addition to this the fabric is already wet out and there is no barrier to the transport of dyestuffs into the fibre.
It is important that the CROSPREP CAT (activator) stock should not become contaminated with other products due to poor housekeeping. The product itself is inherently stable but can be adversely affected if contaminated.
LIMITATIONS OF APPLICATION
The system is not suitable for application to uses demanding high Hydrophility such as towels.
The Eurodye-CTC Low Temperature Bleaching system is not intended for use for full whites.
PRACTICAL APPLICATION
The enclosed recipe is a generic one for the Low Temperature Bleaching application. It will need some adjustment dependent upon local requirements and conditions.
- For Jets
3-6 % owf Hydrogen Peroxide 50%
2.0 g/l Caustic (Solid) Regardless of L:R
1.0 % owf CROSPREP HES Multifunction wetter/Stabiliser/emulsifier
1.0 % owf CROSPREP CAT Activator for peroxide bleaching at lower temperature
Dyebath lubricant such as DYELUBE NF in Jets if creasing is normally a problem. The LTB system may reduce or eliminate the need for a lubricant as the fabric is inherently softer.
Note that the concentration of caustic is in g/l and does not vary according to L:R.
As the liquor ratio is reduced the g/l concentration increases and this gives rise to the possibility of optimisation of the recipe. It will not always be necessary to hold the same level of % owf in very low L:R such as 5:1. However the above is a good starting point.
The fabric should be loaded into the machine and the CROSPREP HES added to wet out the fabric. The lubricant if needed should also be added now. Then the caustic should be added so that it becomes diluted before contact with any other strong chemicals. The Hydrogen Peroxide should then be added and finally the CROSPREP CAT added preferably with clean water in the addition tank rather than liquor removed from the bath.
It is important that the CROSPREP CAT stock should not become contaminated with other products due to poor housekeeping. The product itself is inherently stable but can be adversely affected if contaminated.
The fabric is typically bleached for 45-60 minutes dependent upon local requirements. The longer the bleaching time the higher the reflectance. However as normal after initial 45 minutes the increase in whiteness is small but on occasions can be important.
Similarly there is a relationship between temperature and whiteness. Bleaching at 60°C is common amongst the many companies who have adopted the system. However it is perfectly safe to bleach at 70°C should it be desired in order to achieve a few extra degrees in whiteness.
After bleaching ……… please see comments below after Cone dyeing section
- For Cone (Yarn) dyeing
3-6 % owf Hydrogen Peroxide 50%
2.0 g/l Caustic (Solid) Regardless of L:R
0.5-1.0 g/l CROSCOLOR PTM Deaerating agent
1.0 % owf CROSPREP HES Multifunction wetter/Stabiliser/emulsifier
1.0 % owf CROSPREP CAT Activator for peroxide bleaching at lower temperature
The procedure then follows a similar routine as described for Jets and with a few minor modifications is repeated below.
The CROSCOLOR PTM and CROSPREP HES should be added to the liquor as soon as the water is charged into the vessel.
CROSCOLOR PTM is included in this recipe to provide efficient deaeration as opposed to wetting and emulsifying. CROSCOLOR PTM is also very good for foam control which is essential with powerful pumps and an enclosed system, regardless of foaming properties of auxiliaries.
After the yarn has wet out and the air expelled then the caustic, peroxide and Crosprep CAT is added in that order.
Once again ensuring that the Crosprep CAT does not come into contact with concentrated peroxide by adding it on its own as the final chemical.
Note : there are systems where the full bleach liquor is prepared in in a large side tank and pumped into the dye vessel, as opposed to direct chemical feed to the dye machine. In this case the mix should be prepared following the same order for addition of chemicals as outlined above. It is important that the caustic should be diluted by mixing before the peroxide is added and similarly the peroxide mixed in before the CROSPREP CAT is added.
As commented above when the liquor ratio is reduced the g/l concentration increases and this gives rise to the possibility of optimisation of the recipe. It will not always be necessary to hold the same level of % owf in very low L:R such as 5:1. However the above is a good starting point.
After bleaching
When the bleaching is completed with either system (Jet or Package) the fabric/yarn should be washed as normal. It is not necessary to use higher temperatures than 60°C. after the initial washing is completed the pH needs to be adjusted and the peroxide destroyed using a catalyst such as CROSZYME PEK.
Typically this would be as follows :-
Adjust pH to be in the range 6-8. The optimum performance is around neutral.
0.4-0.6 g/l CROSZYME PEK
Temperature : 50°C
Time : 15 minutes
When the time is complete, check for residual peroxide. If the peroxide is clear then the dyeing can be commenced. If preferred the bath can be dropped and re-filled, it is a personal choice of the local technical team.
Dyeing
The dyeing procedure should be followed as normal. However as a best practice and as an aid for level and reproducibility it is recommended that 0.5 – 1.0 g/l of CROSCOLOR SLR NEW or CROSCOLOR ADM are added to the dyebath to ensure level and reproducible dyeings. CROSCOLOR ADM is recommended for particularly difficult shades such as Turquoise. For less difficult shades the CROSCOLOR SLR NEW is ideally suitable and offers a more economical alternative to CROSCOLOR ADM.
For cone dyeing 1.0 g/l CROSCOLOR PTM is recommended in addition to the above to ensure there is no entrapped air in the package during the dyeing process.
Washing off after dyeing
After dyeing is completed the liquor should be drained and a rinse given with warm water to reduce the hydrolysed dye and electrolyte and the pH reduced to near neutral.
Dosage levels vary according to the liquor ratio but also according to the depth of tone.
In general, the following dosages can be applied:
- 0.20 to 1.0 % (owg) (0.20 – 1.0 g/l for a L.R of 1/10)
Darker shades require the use of a slightly higher amount of product, depending on the rate of fixation.
The soaping-off bath should be kept at the boil for 10 to 15 minutes. Or could be replacing by 2 soaping-off at 60°C-70°C for 10 to 15 minutes each.
For Pale to medium shades a single ‘soaping’ at 60°C with CROSCOLOR BCSR (GOTS) or CROSCOLOR VIKTOR (not GOTS) is usually enough if followed by the usual rinsing
For dark and difficult shades such as Dark and deep Red shades Two ‘Soaping’s’ with CROSCOLOR BCSR or CROSCOLOR VIKTOR at 60°C should be sufficient to give a good colour fastness.
