What are optimum recipes?
An optimum recipe is the recipe for which we need to use minimum dyes and chemicals but will achieve the correct shade first time with the maximum fixation. This will mean that there will be less wastage of dyes and chemicals and reduce the effluent load. This will also include optimizing the dyeing process.
How can we get optimum recipes?
Yes, optimum recipes can be achieved but require the evaluation, implementation and control and monitoring of various dyeing parameters; this will include, dye selection, measuring the exhaustion and fixation of dyes, monitoring dyeing temperatures, pH, liquor ratio and time. In addition to controlling the dyeing process, careful attention must be paid to the quality and storage of dyes and chemicals in order to avoid variation in materials between batches. Careful monitoring of lab to bulk recipes needs to be undertaken together with establishing good communication between the lab and the plant.
What is an azo dye and which ones are banned?
Azo dyes are a group of organic compounds which contain the N=N functional group, known as the azo group. Under aerobic conditions (in the presence of oxygen) azo dyes are resistant to degradation but under anaerobic (in the absence of oxygen) conditions, they can be reduced to certain aromatic amines called arylamine. This can be a problem because a number of these aromatic amines have been found to be carcinogenic and thus can pose a risk to health. Only 22 azo dyes are considered to be harmful and are banned in textile coloration under the EU Directive 2002/61/EC. These can be found on the Azo Dye briefing note written by the project.
Why is knowledge of dye chemistry important?
While selecting dyes it is sometimes difficult to select compatible dyes if little information is given by a dyestuff supplier. It is the utmost duty of dye manager or production manager to select the best dyes for his production. Various parameters to be considered including: dye cost, quality of dyes, dyeing process, and shade matching. There are various types of dyes for many textile fibres; for example, reactive dyes, vat dyes for cotton, acid dyes for wool, silk and polyamide fibres. Cotton reactive dyes such as vinyl sulphone or mono chloro triazine have a different reactive group which will require distinctive application conditions. A knowledgeable dyer will be able to recognize the reactive group on the dye molecule from its chemical structure and be able to specify the correct application conditions for the specific dye and be able to select compatible dyes for 3 dye combination shades.
What is exhaustion?
In exhaust dyeing, all the material contacts all the dye liquor and the fibre absorbs the dyes. The dye concentration in the bath therefore gradually decreases. The degree of dye bath exhaustion is therefore a function of time describes the rate and extent of the dyeing process. For a single dye, the exhaustion is defined as the mass of the dye taken up by the material divided by the total initial mass of dye in the bath, but for a bath of constant volume:
exhaustion calculation
C0 = concentration of dye initially
C1= Concentration of dye after dyeing
The concentration can be calculated from measuring the absorbance. This absorbance can be measured for any single dye by plotting a graph of absorbance versus concentration of any dye. The slope of this curve will show the rate of absorbance and can be calculated using the formula:
Absorbance(A)= CEl
Where C= concentration of the dye
E= extinction coefficient of the dye
l= path length of the cell used for containing the dye liquor when taking the absorbance measurement. So from here one can easily find out the concentration of dye in a solution after measuring the absorbance or any particular dye and putting the concentration data in the formula above one can find out the exhaustion.
What is fixation?
Fixation is basically how much the dye reacts with the fabric. It depends on how much dye is fixed onto the fabric after considering all the washes.
For the measurement of fixation one can do the same as for exhaustion and also measuring the absorbance of all the washes and then he can find out easily the fixation for the fixation
Formula will be:
Fixation % = [{C0-(C1+CW1+CW2+…..)}/C0] x 100
C0 = concentration of dye initially
C1= Concentration of dye after dyeing
CW1, CW2 are concentrations of the dye in each wash.
What types of reactive dyes are there?
There are three main types of reactive dyes: vinylsulphones (VS), monochlorotriazines(MCT) and hetero-bifunctional groups that are a mixture of both VS andMCT.
What is a vinylsulphone dye?
These dyes are moderately reactive. The dyeing temperature is 600C andpH 11.5 is used by utilising a mixture of soda ash and caustic soda. These dyes show excellent fixation efficiency under the correct alkaline conditions.
Remazol dyes which contains 2-Sulphatoethyl sulphonamide. This is capable of either reaction with cellulose or hydrolysis to the hydroxyethylsulphonamide according to the usual nuchleophilic addition mechanism.
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Example: Remazol Black B (CI Reactive Black 5), Remazol Brilliant Blue R (CI Reactive Blue 19),Remazol Brilliant Violet 5R
What is a monochlorotriazine dye?
These dyes are less reactive than vinylsulphone dyes. They require more energetic reaction conditions, typically 800C and pH 10.5, are necessary for fixation on cellulosic fibres.
A typical monochlorotriazine dye is shown below.
monochlorotriazine graphic
Monochloro-s-triazine dye (Procion Red H-3B, C.I. Reactive Red 3)
What is a bi-functional dye?
A bi-functional dye is a reactive dye that has more than one type of reactive group in the molecule. These reactive dyes are designed to have the ability to react with the fibre in more than one way. This increases how much of the dye in the bath is actually fixed to the fibre, rather than being wasted through hydrolysis.
Bifunctional dyes contain two reactive systems of the same type or different type. The objective to produce these dyes for exhaust dyeing was for higher substantivity exhaustion and fixation values. Homo bifunctional dyes contain two of the same reactive group.
