Textile Application of the Color Sensitivity of a Dye Mixture

If we want to measure reactive dye sensitivity, we must know the conditions, type of dyes added, nature of dyeing PDC, PB, Exhaust, what auxiliaries are added, time, temperature, liquor

Javaid Mughal, Mansoor Iqbal and Muhammad Naeem

(A Team of Textile Scientists at PCSIR Lab Complex Karachi)

INTRODUCTION:

Cotton is the backbone of the world’s textile trade It has many qualities and countless end uses, which make it one of the most abundantly, used textile fibres in the world It is a seed hair of plant of genus gossypium, the purest form of cellulose found in nature However, cotton is one of the most problematic fibres as far as its general wet processing or dyeing is concerned Quite frequently, the problems in dyed cotton materials are not due to the actual dyeing process but due to some latent defects introduced from previous production and processing stages Often, the root-cause of a problem in the dyed material can be traced as far back as to the cotton field

The dyeability variations are cotton obtained from different sources It has been suggested that the substrate should be obtained from a single source, wherever possible, in order to keep the dyeability Variations than other, those dyes should be selected for dyeing which are less sensitive to dyeability variation In dyeing, if resultant shade for a dye mixture passes the quality examination after its first dyeing, the product is called a right-first-time product The process producing the right-first-time product is thus called a right-first-time process: one that

is economical because it consumes the least energy, labor, and time etc To produce a first-time product, process control is essential for dyeing and, to enable this, many modern dyeing control systems have been developed Unfortunately, errors in the dyeing operation will still sometimes occur Therefore employing a low-sensitivity recipe in dyeing may be an alternative approach Should dyeing errors occur, the less sensitive the recipe is to such errors, the more chance there is that the resultant shade will successful

Target Shade

Dye Brand Sensitivity

Figure 1: Procedure for predicting colour sensitivity

Normal Concentration

Sensitivity

Temperature Change database

Time Change database

Liquor Ratio Change database

Auxiliaries Change database

Modified Dye

Temperature Sensitivity

Time Sensitivity

Auxiliaries Sensitivity Liquor Ratio Sensitivity Total

Sensitivity

The research that has been done in this field has been limited to establishing and applying a method of predicting the colour sensitivity of a matching recipe This research help us the selection of dyes in a mixture recipe for dyeing However, in practice, the effectiveness of the recipe solution by the concentration sensitivity could be only marginal and other parameters might be more important to the recipe performance than an error in the dye concentration Therefore it is important to consider colour sensitivity in its widest possible context Dye sensitivity is collection of various functional parameters If we want to measure reactive dye sensitivity, we must know the conditions, type of dyes added, nature of dyeing (PDC, PB, Exhaust), what auxiliaries are added, time, temperature, liquor ratio, fabric etc so many factors affected on the dyes sensitivity

Sensitivity measuring terminology:

Sensitivity has two major parts first, part is to do the changes in dyeing parameters chemically or mechanically, in second part is to measure the sensitivity by colour measuring instruments in the terms of K/S, CIE Lab, %E, %F, Build-up properties, Strength comparison

or by fastness properties

There are many diverse reasons for measuring colour, and instrument requirements vary widely with each application Relative measurements at a single location are obviously less stringent than absolute measurements that must provide comparable data when measured at different locations Spectrophotometers, colorimeters, and instruments that combine various features of both are available to meet most of these requirements Although simple, low-cost colorimeters and spectrophotometers are usually adequate for relative colour measurements

on specific problems, the more complex and costly double-beam recording spectrophotometers, interfaced to a computer, is the preferred instrument system for the general analysis of synthetic dyes

Colour is three dimensional, a property that is apparent in various ways Colour atlases arrange colours using three scales (Hue, value and chroma) in the case of the Munsell system

To accurate specification of object colours and colour differences, CIE recommended dimensional uniform colour spaces-CIE Lab and CIE LUV in 1976 Since the equations are long, they are omitted here These are called the CIE 1976 (L* a* b*) colour space or CIE Lab colour space, and the other, CIE 1976 (L* U* V*) colour space or CIE LUV colour space, and have similar structures as the Munsell colour solid In imaging applications, CIE Lab space is commonly used In CIE Lab space, L* shows the lightness, and (a* b*) the colour as shown in fig, 1

three-Figure 1 CIE Lab

The coordinate (L*, a*, b*) is calculated from the (x, y, z) of the given light stimulus and (Xn, Yn, Zn) of the white point Therefore, the CIE Lab space has a function of correcting for chromatic adaptation to the white point, and is intended for object colour and displays The colour difference in the CIE Lab space is calculated as the Euclidean distance between the points in this three-dimensional space, and is given by,

In CIE Lab, the selections of light sources are also available in the system

• An artificial day light source, D65

• A tungsten-filament sources

• A three-band fluorescent sources TL84

• A UV source (For enhancing fluorescent white)

Steps in the determination of a dyeing recipe:

A recipe of dyes consists of one or many dyes, the quality of each dye (in the quantity of product that should be used at the beginning of the dyeing) and the application process Recipe

of dyes also indicates all the parameters of dyeing process, such as the temperature, time, liquor ratio, quantity of chemicals and auxiliaries

The determination of a dyeing recipe takes place in three main steps:

First step:

The first step consists of selection of the dyestuffs and of the dyeing process that may be used

to calculate the recipes of dyes

a mean of testing alternative dye combinations to find the optimum mixture that will minimum cost and /or color difference relative to the target shade

The number of recipes to be predicted rises very rapidly as the number of possible dyes is increased Then, it is very important to eliminate as soon as possible, all the objects that will not permit the requirement to be fulfilled Moreover, in the case of article made of several fibers, such as cotton/polyester articles, a recipe of dye has to be calculated for cotton fiber, and another one for the polyester fibers

Third step:

In the third step “best” calculated recipe of dyes is selected The complete dyeing recipe is determined Then a sample is dyed and if the dyed sample has the desired color and if the client requirements are fulfilled, this dyeing recipe is accepted for production If the desired color is not obtained, then the recipe of the dyes is to be modified (i.e the concentration of each dyes), until the color difference between target and the match sample is negligible Finally if the requirements are not achieved by the dyeing recipe (for instance the washing fatness is too low), then the dyer must choose another recipe of dyes Whenever there is some difficulties with all the calculated recipes of dyes, it means that the textile processing mills can not fulfill the client specifications due to dyes or process

At present, there are tools that permit the recipe of dyes to be calculated by using the Kubelka

– Munck’s relationship or K / S values

Fig Process of determination of a dyeing recipe

COLOR SENSITIVITY OF DYE MIXTURE:

Formulating colorant recipes to match target colours is not an easy task Manual color prediction often uses a trial and error method, for which the experience of the colourist is essential The majority of color matching also required color to be matched not only under daylight, but also under other artificial light sources such as cool white fluorescent (CWF),

TL84, D65, etc a previous recipe archive is very useful for manual color matching A colourist

would firstly search the previous recipe archive to find out the closest colour matching target

or standard and then make sure adjustment to the recipe if the recipe color is not the exact

match to the target However this trial and error process is lengthy and arduous even for a professional colourist

Computer color matching is an alternative method Alderson first discovered the commercial

application of computer color recipe formulation in textile in 1961.the computer color matching becomes a necessity for a modern dye house to install a computer colorant formulation system Color matching to a target shade depends not only on the formulation system but also on the:

- Accuracy of the recipe preparation

- The repeatability of the dyeing process

- The color measurement process

There is a need of quality control at each step in the coloration and the measuring process

The core of the formulation system is based on the theory developed by Kubelka & Munk

known as Kubelka – Munk theory

Recipe correction:

After recipe formulation, a new color sample can be produced according to the colorant concentration suggested Because of the influence of the various variables in the dyeing process, the produced sample may not be acceptable and a recipe correction process may be needed

Cu = recipe used in dyeing equal to Cp

Cb = recipe back – predicted for the batch dyeing results

Correction method in equation 1 is called weighted or ratio correction and that is given in equation 2 is called additive correction

If a batch is already too dark compared with standard this correction for exhaust dyeing will fail, mean that the absorbed dyes should be stripped before correction for continuous dyeing diluents need to be added to dilute the dye liquor, However for a slight dark color, the production correction for exhaustion dyeing should correct huge difference and try to reduce the overall color difference

Visual assessment of colour:

For the visual determination of the specification of an unknown specimen, a colour atlas is viewed alongside the specimen under prescribed conditions An approximate specification of the specimen is given by the denotation of the chip juded closest to its colour The human visual system can distinguish several million colour, however, where as most colour atlases contain at best a few thousands chips Even the most comprehensive systematic sampling of colour space yet produced, whilst the human visual system is an excellent null detector (being very capable of assessing whether or not colours match), it is not good at estimating the relative magnitude of differences in colour If several assessors interpolate to arrive at specification their reports usually differ, after significantly, and even the some assessor assessing on different occasions usually gives a variety of specifications In an experiment, the ICI colour atlas coordinates of six specimens were assessed, to the nearest chip, by five experienced assessors under identical conditions on each of six occasions, not one assessor chose the same three coordinates for any specimen on all occasions, and one assessor did not choose the same three coordinates, for three of the specimens, on any occasion

Lack of correlation between interpolations is one of the principle problems of visual methods

of specifying colour Nevertheless colour atlases continue to be used because it is necessary

in same applications, and desirable in others, to have available a collection of chips systematically sampling colour space

Variables in the analytical method:

Specific details concerning sampling, solvents, weights, volumes, and special techniques to cope with solution and measurement idiosyncrasies must be established individually for each species of colorant Although several of these problems are minimized simply by good analytical technique, some problems are unique to the spectrophotometeric measurement of

colorants in solution A few of the pitfalls encountered in solution coloristics are discussed below

Solution variables:

Solution variables may have a pronounced effect on colour measurement Each effect should

be considered from its dual aspect; by measuring the material the effect may be studied, and

by controlling the effect the material may be determined In developing an analytical method each new colorant should be screened to determine its behaviour towards each of the solution variables If a solution variable is found to have a significant effect on the coloristics, conditions must be changed to maximize analytical precision Shade and strength parameters may both vary with solution conditions, one parameter may change without any significant effect upon the other

Concentration effect:

Variation in absorptivity with concentration represents non-conformance to beer’s law rather than a failure of the law Beer’s law holds for single molecular or ionic species of the colorant being analysed If aggregate molecules or ions are present in equilibrium, the equilibrium may shift with changes in the concentration and the system does not conform to Beer’s law

If the aggregate equilibrium is constant with changes in concentration, the system conforms Conformance to Beer’s law is required to obtained reliable coloristics data Non conformance

is usually dealt with by narrowing the concentration range or by changing the solvent strength alone can be accurately determined in a non conforming system either measuring the absorbance at an isobestic point or by making appropriate corrections from a predetermined plot of absorbance versus concentration

Temperature effect:

At very low temperature the molecular vibrations of a colorant may be so restricted that the solution becomes colourless At room temperature and higher, colour variation with temperature is usually attributed to isomerization, aggregation, or chemical reaction Temperature effects on coloristics properties are either reversible or irreversible A reversible effect during heating to dissolve the dye is unimportant because the original form is obtained after cooling If an irreversible change occurs during heating for dissolution conditions can be changed to avoid the temperature effect If temperature affects the dilute solution it should be thoroughly investigated to determine whether the change is reversible and at what temperature the changed occurs

pH effect:

The initial pH of the dyebath will be lower at the end of the dyeing by one half to whole unit, indicating that some alkali has been used up during dyeing The cellulosic fibre is responsible for some of this reduction, while a smaller part is used by the dyestuff as it hydrolysis The effect of pH, account must be taken of the internal pH of the fibre as well as the external pH

of the solution The internal pH is always lower than external pH of the solution Since the decomposition reaction is entirely in the external solution, the higher external pH favors decomposition of the dye rather than reaction with the fiber pH influences primarily the concentration of the cellusate site on the fibre It also influences the hydroxyl ion concentration in the bath and in the fibre Raising the pH value by 1 unit corresponds to a temperature rise of 20oc; the dyeing rate is best improved by raising the dyeing temperature

once a pH of 11-12 is reached Further increased in pH will reduce the reaction rate as well as the efficiency of fixation Different types of alkalis, such as caustic soda, soda ash, sodium silicate or a combination of these alkalis, are used in order to attain the required dyeing pH The choice of alkali usually depends on the nature of dye

The colour of many dyes varies quite drastically with pH, and in most instances this behaviour is predictable from chemical structure To provide reliable coloristics data for products that vary with pH, it is necessary to adjust the pH prior to measurement Adjustment

is made either by titration or by using a solvent buffered to a specific pH For case of operation the buffered solvent is preferred, provided its buffering capacity is capable of suppressing pH variation from one sample to another

Electrolyte effect:

The addition of electrolyte results in an increase in the rate and extent of exhaustion, increase

in dye aggregation and a decrease in diffusion The electrolyte efficiency increases in the order: KCl<Na2SO4< NaCl There may be impurities present in the salt to be used, such as calcium sulphate, magnesium sulphate, iron, copper and alkalinity that can be source of many dyeing problems

Ion effect:

Many dyes are good chelating agents are evidenced from their structures and the extensive commercial use of metalized dyes Thus it should be no surprise that dyes scavenge metal ions from solution and that the metal ions change the coloristics properties This effect decreases the strength and change the shade Some dyes are sensitive to electrolytes and changes with the concentration of ions in solution

Interfering ions present many problems in correlation solution coloristics with shade and strength determination in end use test The effect of interfering ions can be minimized, and sometimes eliminated, by adding chelating agents, surfactants, or protective colloids to the colorant solution, however, it must be remembered that these measures are not being taken in the application tests

Impurity analysis:

Impurity is defined here as any undesirable colorant, or combination of colorants, that adversely affects the end use properties of a dye If the impurity is identified as a single species and significant amounts of it are present, it can be quantitatively determined by analytical technique or by separation technique

The methods of impurity measurement apply only when the level of impurity is significant large so that it can be seen to influence the profile of a spectral curve It is also possible to have impurity present at such low concentration that they are not detectable by conventional analysis yet create problems in dyeing If these impurities absorb in an area of low absorbance for the dye being measured their absorbance can be exaggerated by measuring a more concentrated solution (the stock solution is frequently used) in this particular area of the spectrum

The most convenient way with simple instrumentation is to determine the absorbance of sample and standard at specific wavelength and normalize for concentration, path length, and

AI according to the following equation

I.I = Aλa

BC AI Where I.I is the impurity index, A is the absorbance at wavelength a, b is the sample path length in centimetres; C is the dye concentration in grams per liter (as decimal fraction) of the sample being measured

I.I = f A1 (sample) - f A2 (standard)

Application to dyeing problems:

Solution colourist can be as important to dye application, as it is to dye synthesis It was recognized during early instrumental measurement of dyed fabric that the accuracy of color specification was limited more by the dyeing process than by the method of measuring As a result, solution colorists in one form or another has been used to study all aspects of the dyeing process Solution spectra are usually obtained when the dye is received Although most solutions measurements are made to determine strength, other colorists criteria may also

be used to determine acceptability Before solution colorists can be used in this way it must

be established that solution data correlate with application testing Solution colorists is also used in controlling the dye bath For proper controlling it is necessary for the solubility of the dye to be known, because poor solubility causes weak dyeings and/or specking

Solution colorists is often used in the maintanance of dye standards Dye standards may change with time owing the hygroscopicity of powders, evaporation of liquids, or the general degradation of quality with time Periodic color analysis of dye standards and comparision with data obtained from earlier analysis will indicate quality changes Standard drift is not easily detected from routine analysis where samples are compared to standard in a relative sense

Influence of Dye characteristics in reactive dyeing:

The major dye variables that affect reactive dyeing are dye chemistry, substantivity,

Reactivity, diffusion coefficient and substativity, each of these will be discussed below:

Dye chemistry:

Reactive dye has a wide variety in terms of their chemical structure The two most important component of a reactive dye are the chromophour and the reactive group The characteristics governed by the chromophour are color gamut, light fastness, chlorine / bleach fastness, solubility, affinity and diffusion The chromophour of most of the reactive dyes are azo, anthraqnone, Pthalocynine Azo dyes are dischargible, diazo dyes have the disadvantage of being much more sensitive to reduction and many of them are difficult to wash-off Most of

them possess excellent fastness to light and to crease resistant finish, but they are not dischargeable Pthalocynine dyes diffuses slowly and are difficult to wash off Meta complex dyes containing copper possesses rather dull hues, but shows a high degree of fastness to light and to crease resistant finishes There substantivity is fairy high 1:2 complexes diffuse relatively slowly So a longer time is needed to wash out unfixed dye completely

The dye characteristics governed by reactive group are reactivity, dye-fiber bond stability, and efficiency of reaction with the fiber and affinity Dyeing conditions, especially the alkali requirements and temperature as well as the use of salt also depends upon the type of reactive group Dyes based on s-triazine do not have good fastness properties in acidic media and, due

to their high subtativity, have poor wash off properties Similarly dyes having a vinyle sulphone reactive system have poor alkaline fastness The chemical bond between the vinyle sulphone dyes and cellulosic fiber is very stable to acidic hydrolysis The substantively of hydrolyzed by products of vinyl sulphone is low, so washing off is easy Monochlorotriazine have good fastness to light, perspiration and chlorine The turquoise reactive dye shows an optimum dyeing temperature that is generally about 20°C higher than that of other dyes with the same reactive group The flourotriazine groups forms linkages with cellulose that are stable to alkaline media Reactive dyes of dichloroquinoxaline, monochlorotriazine and monoflorotrizine types show a tendency for lower resistance to peroxide washing and dye-fiber bond stability A lower sensitivity to changes in dyeing conditions (particularly temperature) is the most important characteristics feature of the monochlorotriazine – vinyl sulphone heterobifunctional dyes

Substativity:

Substantivity more depends upon chromophour as compared to reactive system A high substantivity may results:

• Lower dye solubility

• High primary exhaustion

• A high reaction rate

• Lower diffusion coeffecient

A low sensitivity of dyes to the variation in the processing conditions such as time, temperature, pH, material to liquor ratio may results:

• Less diffusion

• Less migration and levelness

• More difficult to the removal of unfixed dyes

Substantivity is also the best measure of the ability of a dye to cover dead cotton or immature cotton fibers Covering power is best when the substantivity is either high or very low An increase in the dye substantivity may be affected by:

• Lower concentration of dyes

• Higher concentration of electrolyte

temperature and pH over which the dye can be applied Altering the pH or temperature, two dyes of intricsically different reactivity may be made to react at a similar rate can modify reactivity of dye

Solubility:

Dyes of better solubility can diffuse easily and rapidly into the fibers, resulting in better migration and leveling Increasing the temperature, adding urea and decreasing the use of electrolyte may affect on increase in dye solubility

Dyes and chemicals:

Commercial samples of Rifafix reactive dyes (Red, Blue and yellow) and Sumifix dyes (Red, Blue and yellow), wetting agent Sandozin MRN, which is poly glycol ether derivative, sodium Alginate were obtain from market Sodium hydroxide sodium Bicarbonate, Soda Ash, Urea, DAP are AR Grade chemicals

Equipment:

Pad dying was carried out by using a laboratory scale vertical Padder (Rapid) The samples were cured on the curing machine (Rapid) The colour coordinates were recorded on the Data Color Spectraflash SF 650X Wash fastnesses were carried out on the IR dyeing machines (Aiba) Light fastness was carried out on Weather-o-meter

Experimental work:

For the determination of colour sensitivity of two different brands of dyes three primary colours (Red, Blue and Yellow) has been used and by the combination of three colours a trichoromacity diagram has been developed Standard recipe of PDC dyeing to measure colour sensitivity

By the combination of three colours we develop 66 shades of individual dye brand Out of these 66 shades we chose three shades from 2 sets of combination, which we consider as target shades (Attached sheet) On similar pattern we produce these target shades by combination of two dye brands

Similarly we determine the colour sensitivity of target shades of individual dye The factors that effect colour sensitivity of target shades are curing time, urea concentration, pH and use

of DAP

To check the sensitivity of the dyes with reference to specific target shades standard Recipe

of pad dry cure dyeing is given bellow

Standard Recipe of Pad dry cure dyeing:

Dye stock solution = 10 gm/l

Sodium Carbonate = 20 gm /l

Urea = 150 gm/l

Sodium Alginate = 20 ml of 2 % Stock solution

Standard conditions for Pad dry cure dyeing:

COLOUR SENCITIVITY

COLOUR MATCHING TRIANGLE

BY PAD – DRY – CURE METHOD Tri chromacy dyeing Recipe