Dr.N.N. MAHAPATRA
C.Col FSDC ( UK),CText FTI ( Manchester ), FRSC ( UK ),FAIC(USA)
Cold pad batch is known since many years to the dyers and they have been practicing this method as well, but due to the energy crises and the legislation for the protection of environment which is becoming more and stricter towards the dye houses. It has become the necessity of the day to look for the easy and safe methods of dyeing. It is being practised in Textile Dyeing sectors like Palli, Balotra, Jetpur ,Jaipur etc. Besides in the Corporates also it is suitable for Woven Fabrics. Nowadays Tirupur is also thinking to switch over to CPB for dyeing of Cotton Knits.
Cold pad batch dyeing method gives you the required fastness standards with ease and minimum costs, and less energy; however dyers have been reluctant to use this method in general because of following reasons given by them:
1- Difficulty of lab. To bulk reproduction.
2- Large space required for batching.
3- Control of temperature during fixation phase.
4- Longer batching time.
5- Long time required to asses correct shade.
6- Listing and tailing.
Before we discuss the above mentioned points, I would like to mention the salient features of this method:
a- Significant cost saving and waste reduction, as compared to other dyeing methods.
b- Total elimination of the salt and other specialty chemicals such as anti-migrant, levelling agents and fixing agents.
c- Optimum utilization of dyes which reduces the chemical cost and waste load of effluent.
d- Excellent wet fastness properties.
e- This process works at low temperature and low liquor ratio, thus saving energy and water considerably.
f- A uniform dye quality with even color absorbency and color fastness.
g- Ease of running small batches.
h- A high reliability and reproducibility as compared to other continuous methods.
i- It improves the fabric quality.
j- It shows added luster, brighter shade.
k- A soft hand feel and far better penetration of dyes.
After considering the above mentioned facts cold pad batch method is most economical and workable.However for any dyeing methods selection of dyes and process is very important.
We would like to discuss the dyes first as the selection of dyes is the foremost thing in any dyeing process. The dyer must consider following factors before selecting the dyes.
1. Dye chemistry
2. Substantively
3.Reactivity
4 Diffusion coefficient
5 Solubility
Dye chemistry there are two main factors which govern the dye chemistry of reactive dyes chromophores and reactivity.
The chromophores are responsible for the Color gamut, light fastness, bleach fastness, solubility, affinity and coefficient of diffusion.
The reactivity is responsible for the application properties such as dye fibre bond stability, efficiency of the reaction with the fibre, affinity and alkali requirements of dyeing
Substantively
Mainly dependent on the nature of chromospheres. A higher substantively will have low solubility of dyes, high fixation rate, lower diffusion of coefficient, less sensitive towards temperature, pH and dyeing conditions, high risk of unlevel dyeing. Difficult to remove unfixed dye.
Reactivity
The reactivity of the dye can be responsible for low and high dyeing time, temperature and pH alteration can modify the reactivity and two dyes of different reactivity can be dyed at the same conditions by adjusting pH and temperature.
Diffusion coefficient
High diffusion coefficient dyes has better levelling and less dyeing time.
Solubility
Dyes of better solubility can penetrate quickly in the fiber and gives better fixation and leveling, the dye solubility can be increased by increasing temperature, adding urea and reducing electrolytes.
This knowledge of factors controlling the reactive dyes helps in producing the correct first time dyeing specially the cold pad batch dyeing.
For cold pad batch dyeing method the foremost thing is the preparation of the fabric which should have following parameters:
• Complete removal of size
• Good singeing
• Very good absorbency throughout width and length
• Uniform whiteness
• Crease free
• Relaxed fabric
Then the selection of dyes which should include the dye range, which should have medium reactivity and substantively, in my opinion Bi functional dyes of VS and MCT combination will prove best as these dyes will comply in best way with requirements of dyeing method.Temperature of the fabric, dye bath and the batching atmosphere should not be more than 35 C and with minimum variations.
The trough of the padding liquor should be around 30 litters so that the turnover of the padding liquor is less than the dye hydrolysis rate.
The fabric should pass through the expander and spreader rolls.Hydraulic batching should be uniform.Dosing pumps should work properly and should be checked before the start of dyeing.
Fabric should be wrapped properly after the padding of the dye so that no air come in contact with the fabric, also prevents the drying of selvages.The batch should rotate not more than 40 rpm.The batching time should be 16 hrs and for turquoise 24 hrs.This way we can achieve good reproducible and perfect dyeing.I would like to discuss the arguments given by the dyers for this method,the main argument for this method is the lab. to bulk matching, off course this creates difficulty in the initial state but when you have prepared your library than the reproduction of the shades are easy. A correlation can be made with the experience to minimize the difference in matching.The space needed for batching is necessary but when we consider continuous machines like Pad- steam and Thermoses dying ranges, they also require quite a big area.Control of temperature while padding and batching is important, this can be controlled by adjusting the padding temperature with cold water circulation and batching at the area where temperature is somewhat constant and not very high.
Longer batching time and longer time required to asses correct shade are weak points of this process but manageable.
Listing and tailing are the points which need attention in all continuous methods. Apart from preparations in cold pad batch use of silicate should be avoided as much as possible because this product give scaling on paddlers and rollers which causes listing.To overcome this and when it is unavoidable the use of silicate such as in tropical countries one should clean the machine immediately after dyeing with 10 g/l of hexametaphosphate solution at 60 C.The use of combination of soda ash and soda bi carbonate. Gives much better results as compared to silicate and caustic soda. This gives buffering effect which produces better fixation ultimately better wash off.Tri sodium phosphate can also be used as alkali.
