While talking about Dyes; many questions arise in reader's mind; like: What is exhaustion? What is a bi-functional dye? What are hot brand, medium brand and cold brand dyes? and many more...


These are explained in brief along with many other frequently asked questions in fabric dyeing:


What is an alternative production process?


Alternative, cleaner production techniques provide an effective way to examine an existing production system to pinpoint areas where changes could be made which will optimize the production process.


Thus it can help to realize the benefits of process optimization, quality control, energy conservation and preventive management. It can also lower energy and costs associated with:


  • raw materials
  • energy
  • labour
  • treatment and disposal
  • insurance and liability


This is an integral part of the environmental management system.


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.


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.




 

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 and MCT.


What is a vinyl sulphone dye?


These dyes are moderately reactive. The dyeing temperature is 600C and pH 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.


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.


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.


Bi functional 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.


What are hot brand, medium brand and cold brand dyes?


It is not common practice to specify dye types in this manner, for example, monochlorotriazine dyes have been given the name 'hot brand' whilst vinyl sulphone dyes have been termed 'warm brand' using this system it is not possible to give a term to mixed bi-functional dyes since they have one monochlorotriazine group and one vinyl sulphone group.


  • Cold brand dyes


The batch wise dyeing temperature of these dyes for optimal fixation is 300C to 400C. These dyes are very much reactive and show high fastness properties. These dyes require very careful control and should only be used where full automation of the dyeing process is possible. Examples of cold brand dyes include: Dichlrotriazines ometimes these dyes are called "M" dyes. Typical Brand name for these dye types are Procion MX.


  • Warm or medium brand dyes


Recommended dyeing temperature of these dyes for the maximum fixation for exhaust dyeing is 600C. The reactive groups for these types of dyes are of vinyl sulphone. The required pH for these dyes is 11.5. Typical brand names of these dyes are Remazol (HOE).

 


  • Hot brand dyes


The dyeing temperature for these types of dyes is 800 C. If the reactive group is monofunctional such as monochlorotriazine it will be called hot brand dyes.


Typical Brand name for these dyes is Procion H E Dyes.


Why do dyes have different colour strengths between batches?


This can happen for a number of reasons. Dye manufacturers mix the dyes with other compounds which are not colorants to give them various properties. The proportions that are added are not always identical from batch to batch. Depending on the quantity of colorant or other compounds in the dye the colour strength for a certain weight of dye will be different (so all new batches of dye should be tested).


Does the quality of dye decrease over time?


Yes, the quality of dyes can decrease in two main ways. Firstly, dyes can also absorb water which makes them heavier, so when they are weighted out for a recipe, less colorant is actually weighted out than needed. Secondly, dyes can react with water in the atmosphere and hydrolyze. This means that the reactive group has reacted with the water and will not be able to react with the cellulose (fabric), the desired depth of shade may not be achieved in the dyeing. To address the problem test dyeings should be carried out in the laboratory with dyes before they are used, especially if they are not used regularly. The recipe may then need to be adjusted.


How do you prevent dye quality from decreasing?


Dyes should be kept in airtight containers and lids should be replaced immediately after use. Storage rooms should be kept as cool and dry as possible.


What are the requirements for selecting compatible dyes?


Compatible dyes are dyes which build up on shade in that they increase in depth as the dyeing progresses to one single shade. Incompatible dye mixtures build up off shade making shade matching very difficult. The dye supplier should be able to specify which dyes in his range are compatible.


Fixation should be what for reactive dyes?


Fixation for reactive dyes will depend on the type of reactive group, dye quality, process parameters and process optimization. After considering all these factors, fixation can typically be in the region of 77%.


How can fixation be measured?


Fixation can be determined by the analysis of the amount of dye in the dye bath at the start and end of dyeing. The analysis requires the use of a spectrophotometer which gives values of dye concentration within the dye bath. The percent exhaustion and percent fixation can be calculated by using the figures obtained from the spectrophotometer.


What are the parameters that a dye manager should consider while selecting any dyeing process?


While selecting any dyeing process the parameters the dye manager should consider:


  • Whether the process is compatible with all the dyes he has selected
  • Whether the process is suited for the machine in terms of temperature, pH, and material of construction and that he is able to control these necessary parameters adequately that the dyes he has selected give the fastness requirements specified by the buyer


What is uneven dyeing?


'Uneven' or more correctly un level dyeing is a term used to describe the distribution of dye over the whole substrate. If a dyeing is unlevel light and dark patches or areas of dye can be seen when comparing different areas of the substrate. These patches may be of the same hue being light and dark or of a different hue if an incompatible dye mixture has been used. Unlevel dyeing may arise from a variety of causes, some are highlighted below:

  • Temperature rise too rapid so dye has been adsorbed too quickly
  • Inadequate salt mixing causing areas of highly positively charged areas on the fabric for which the dye will have high affinity
  • Poor dissolving of dye before addition to dye bath, large aggregates of dye on the fabric will cause darker areas or dye 'spots'
  • Poor fabric transport, e.g. the fabric is moving too slowly around the machine or that the liquor fabric interchange is too low
  • Poor preparation of the fabric before dyeing, if the fabric has not been scoured efficiently completely the dye may not penetrate the 'unscoured' areas.


What is 'right first time' dyeing?


Right first time is an important term in dyeing process. It means to get the right shade of fabric first time without the need for re-shading or washing off to reduce the depth of shade. When using optimised dyeing recipes and practices, less dye, auxiliaries and energy are used.


Why it is necessary to get "right first time"?


Right first time has many advantages including:


Saving money in terms of energy, labour and reprocessing costs Less dye will be in the effluent and thus they can reduce pollution Increased productivity since there is no re-processing so time to dye more production Increased customer confidence


Is the effect of pH in dyeing? What is the optimal pH?


Every chemical reaction works best at a specific pH , each dye class requires a certain pH at which it will be most efficient.


How do fibre reactive dyes work?


The dyeing of cotton does not work in the same way as dyeing protein fibres. Cotton is negatively charged in water and so are the reactive dyes which mean that there is ionic repulsion, the dye will not be attracted to the fibre. To overcome this repulsion salt is added to the dye bath. The positively charged ions from the salt are attracted to the negatively charged cotton so 'mask' the negative charge, this will allow the dye to be adsorbed onto the fibre. Alkali is necessary because it facilitates the reaction between the dyestuff and the fibre. The important point is not the type or amount of alkali but rather the pH of the dye bath since there are various reactive groups on different reactive dye classes each requiring a specific pH to allow reaction of this reactive group with the fibre.


For example vinyl sulphone reactive dyes require a pH of 11.5, whereas monochlorotriazine dyes require a pH of 10.5, therefore the pH must be closely supervised.


The most suitable pH for dyeing varies with the temperature, being approx. 11.5 for common warm dyeing dyeing at 60, 10.5 for hot dyeing 80, there are dyes which can be dyed at low temperatures of 40oC (cold brand) but these dyes require sophisticated equipment and very good control of the dyeing process since these dyes are very susceptible to hydrolysis making them unreactive so will increase effluent loading. It is always advisable to contact the dyestuff supplier for information on the correct application conditions for the dyes one have purchased.


What are the factors that influence the performance of reactive dyes?


There are many factors that influence the performance of specific reactive dyes, some general factors are listed below:-


  • Molecular structure of dye, will dictate the applications conditions required
  • Material being dyed, including the preparation before dyeing
  • Application conditions, e.g. temperature, pH, time, liquor ratio etc
  • Quality of the dye
  • Quality of the dyeing auxiliaries
  • Quality of the water being used to dye the material, soft water is preferred
  • Condition of the dyeing machine, machines in good condition will be more reliable that machines with broken or missing parts


What are some important points to consider during the determination of a dye recipe for exhaustion dyeing with reactive dyes?


The optimum dyeing temperatures and dyeing conditions for reactive dyes differ according to the type of reactive group involved, so the first thing that must be decided is the type of dye that will be used.


The substantivity to cellulose of reactive dyes is lower than that of direct dyes, but the addition of inorganic salts to the dye bath can raise its substantivity. Usually, Glauber salt is used at a rate of 50g/L, but this rate should be increased in the dyeing of deep shades, and can be reduced with dyes that can be used in low salt dyeing. Inorganic salts can be added incrementally to dye baths for level dyeing, and the preliminary addition of Glauber salt, that is, implementation of a dye bath with this pre-dissolved inorganic salt, can be used for dyes with low primary exhaustion rates and with dyeing machinery that provide effective agitation and circulation in the dye bath.


Alkali is employed to promote the reaction between the dye and the fibre and the important point here is not the type of alkali used but the need for the pH of the bath after addition of the alkali, for example vinyl sulphone dyes require a pH of 11.5 for the reaction with the fibre to occur. Soda ash is easily used as the alkali because it is easily weighed as a powder and easily dissolved in water. Combinations of soda ash and caustic soda (sodium hydroxide) can be used to obtain a pH of 11.5. Because the dyeing rate of reactive dyes depends on a combination of exhaustion through substantivity and fixing through reaction, quantitative adjustments through the control of the dyeing rate by adjusting the rate of temperature increase, as are used for other classes of dyes, are not effective, but adjustments with the incremental addition of inorganic salts or alkalis (pH) are effective. Consequently, isothermal dyeing through the incremental addition of alkali or alkali dosing is used. The specifics of incremental addition and dosing are decided by the agitation efficiency of the dyeing apparatus. To prevent rope marks in the use of piece-dyes, a dye bath lubricant can be used in the bath, and if hard water is used in the dyeing process, a sequestering agent can be added.


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