TECHNICAL TERMS OF TEXTILE DYEING
Author: Muhammad Awais Imran
Indus University, Pakistan
Author: Muhammad Awais Imran
Indus University, Pakistan
This paper highlights the basic dyeing terminologies which are often used in dyeing process. The central theme of this paper is to provide authentic knowledge to textile students and practical dyers in complete scientific manner because most of the students & dyeing related people have misunderstanding regarding basic dyeing terminologies. Some explanations of basic terms are also discussed in this paper.
INTRODUCTION TO DYEING
Dyeing is process in which textile substrate is colored by suitable substances that is dyes or pigment (pigment dyeing). The former is widely used in textile wet processing. A basic knowledge of dyeing process and related terms are quite important to understand dyeing theory. Dyeing can be done at any stage of the manufacturing of textile- fiber, yarn, fabric or a finished textile product including garments and apparels. Textile materials can be dyed using batch, continuous or semi-continuous processes. The type of process used depends on several things including type of material (fiber, yarn, fabric, fabric construction, and garment), generic type of fiber, size of dye lots and quality requirements in the dyed fabric .
A dye is an organic colored compound which when applies to textile for imparting color & strictly adheres with the textile substrate . Dyes should resistant to light, heat and wet agencies. In textile terms, a soluble colorant that attaches in molecular form to the fibers . All dye is a colored substance whereas all colored substances are not dyes. The dyes applied to textile fibers should possess the following characteristics,
- Stable & attractive color i.e. should not undergo structural changes easily
- Posses affinity to the fibers
- Soluble in water or any suitable medium otherwise it should form stable dispersion with water or solvents
- Fastness to light, washing, perspiration, dry cleaning agents and rubbing
Tendency of a dye to move from a solution out of fibers in the dye solution . It is a quality of dye. Without substantivity, most of dyes would simply remain in solution or dispersion in the bath.
The substantivity of dyes depend upon,
- Molecular structure (shape)
- Size of molecule dye
- Dye bath conditions
MORE SUBSTANTIVE DYE
LESS SUBSTANTIVE DYE
This property of a dye determines how much dye is exhausted on to the fiber under neutral conditions .
Attraction between two items, in dyeing affinity essentially means the preferential attraction of the dye for the fiber rather than for the solution of the dye bath. It is quantitative expressing of substantivity. Technically, affinity is expressed in term of energy . Generally, more substantivity dyes have low affinity characteristic, it is due strike or rush of dye molecule.
The process of transfer of dyestuff from the dye bath on to the fiber or material is known as exhaustion.The ratio between the amount of dye taken up by the substrate and the amount of dye originally available .
* Relative molecular mass
Co = initial concentration of dye in dye bath
Cs = concentration during the process
Exhaustion is overall broad term and can be further categorized into two phases.
Primary exhaustion is the phase where dye moves toward the substrate from the solution under neutral conditions in the presence of electrolyte. It is also known as substantive phase. The term secondary is typical movement of dye molecule after addition of dye molecules after addition of suitable alkali for the completion of the dye fiber bonding.
The following graph shows the relation between changes in exhaustion percentage with respect to time function. See fig. 1.
- Concentration of dye
- Concentration of salt
- liquor ratio
Dyes molecules from solution are taken up by certain textile substrates which have porous surface i.e. cotton by the process adsorption. Distribution of the dye stuff on to the surface of the fiber is known as adsorption. Adsorption is depend on
- Dye nature
- Surface area
Distribution of liquor containing is dye stuff on to the whole of the fiber that is on to the surface and inside of the fiber surface.
The some factor are mentioned below which affects the rate of absorption,
- Dyeing Auxiliaries
- Liquor ratio
Transfer of dyestuff from the surface of the fiber in the liquor is known as desorption. Desorption is reverse of dyeing process.
This is a process by which dye move from a surface of the fiber in to the fiber itself and vise versa. The diffusion of given dyestuff is heavily influenced by temperature the higher the temperature the greater the degree and rate of diffusion. Diffusion rate is also depending on the crystallinity of the fabric structure .
The expression is known as Fick’s law and it gives a quantitative value to the diffusion of dye molecules from the outside layers into the interior of the fiber :
ds/dt = -D dc/dx
s = amount of dye diffusing across unit area;
t = a small interval of time;
c = concentration of dye at a specified point x;
x = the distance diffused;
D diffusion coefficient – it is preceded by a negative charge b/c the amount of diffusion is inversely proportional to the value of dc/dx.
Migration is the process by which are dye move around the fiber or level itself. Transfer of dyestuff from heavily dye to light dye portion of the same material during dyeing is known as migration . The migration process comprises adsorption of dye on to the fiber surface, migration through the dye liquor, re-adsorption onto the fiber surface since diffusion is the rate determining step, in this dynamic process, migration itself is also heavily influenced by temperature.
Dye stuff in a solution
Dyestuff on the surface of fiber
(Diffusion & Absorption)
Dyestuff in to the inside and whole the fiber
Dyestuff on the surface of fiber
(Diffusion & Absorption)
Dyestuff in to the inside and whole the fiber
It is a difference in the electrical potential across the interface (a diffuse double layer) of a solid surface contact with a liquid .
The initial rate of dyeing (the initial slope of exhaustion versus time) . See fig. 1. Rapid strike by a dye often results in initial unlevelness and must be avoided for those dyes that cannot subsequently migrate from heavily to lightly dyed areas of the fabric.
The strike depends on the
- Dyeing temperature
- Dyeing pH
- Addition of chemicals
Formation of the “final” bond between the dye and the fiber with the mechanisms such as ionic bonding and hydrophobic forces. Disperse and vat dyes are fixed in the fibre largely by physical entrapment of insoluble dye within the fibre. The bond that causes final fixation is not necessarily the same type of bond is first made as the dye exhaust onto the fibre .
This term describes the rate at which a dye reacts with fibre. High reactivity dyes react rapidly at relatively low temperature, where as low reactivity dyes generally require relatively high temperature for dye fixation .
Dye-uptake is a property or ability of textile fiber to capture the given amount of dye. This property varies fiber to fiber and depends on dye sites, orientation in a fiber.
Rate of dyeing under any set of conditions (i.e. temperature, pH and time) is proportional to concentration of dye in the solution at given time t. rate of dyeing can be written as 
Rate of dyeing = [Dye]s,t . kdye
Subscript s means ‘solution’
Subscript t means ‘time after the start of dyeing’
kdye, it is a proportionality constant called the dyeing rate constant for the particular set of conditions of dyebath.
Rate of dyeing curves can then be constructed similar to that shown in figure. 2 below, in which different dyeing rates of three reactive Remazol dyes. By plotting percentage exhaustion against time of dyeing; these are very useful indicators of dyeing efficiency.
|Fig. Rate of dyeing of different dyes|
Dyeing can be perceived as a two way process or equilibrium reaction, in which dye (D) and the fiber (F) are in equilibrium with dyed-fiber (DF).
D + F ↔ DF
As dyeing proceeds, more dye is taken up by the fiber than is lost from it back into bath; i.e. there is net increase of dye on fiber. Eventually dyeing reaches the equilibrium point, where there is no further in dye on the fiber. However, this does not imply that dye movement has now ceased; at the equilibrium point; dye still enter and leave the fiber but it does not at the same rate.
RATE OF DYEING REACTION
The rate of reaction of dyes means the rate at which chemical bonds between dyes and textile fibers are formed. Although the rate of reaction varies with dyes, it is largely affected by ,
HEAT OF DYEING
The heat of dyeing is the measure of the strength of the bonds by which the dye is held to the fiber. Since the transfer of dye from the solution to the fiber involves a decrease in free energy of the system, heat is given out, and the heat of dyeing is therefore a negative quantity. the converser is, of course, also true, namely that the migration of dye from the fiber to the liquid phase is accompanied by the absorption of heat. Thus an increase of temperature favours desorprtion and consequently decreases the concentration of the dye molecules in the fibre at equilibrium.
It is difficult to measure the direct calorimetric measurement of heat of dyeing b/c the thermal change is so small.
The effectiveness of a given amount of dye in coloring a given mass of fiber.
This decrease the ability of a dye combination usually with similar substantivity, exhaustion and fixation profile to behave as a single dye combination of compatible dye will generally give good reproducibility and sustainability to variable in the process .
Generally it can never be predicted whether the high or low substantive dyes will produce better result the overall result are always based on S.E.F profiles of the combination use for desire shades.
A dye would never be appropriate if it highly reactive and low in substantivity most importantly there should be a minimum difference between the exhaustion and fixation curve irrespective of its high or low substantive character .
The cloud point is the temperature at which the detergent reaches its limiting solubility and start to precipitate out, causing the solution to appear cloudy. In general when comparing two detergents with similar structures, the one have more hydrophilic will have higher cloud point than the other. This is most common in non ionic surfactants.
DEPTH OF SHADE (DOS)
It is a ratio of weight of dye to weight of goods dyed, usually expressed as percentage; amount of dye, owf (over the weight of fabric). Depth of shade (DOS), in these terms, is not really a very good way of comparing the darkness or intensity of color of finished fabrics, due to inherent differences in the hues of different dyes within a family, difference
between dye families, and differences due to the nature of the fabric. Dye manufacturers’ shade cards are typically show one or two depths of shade for a particular dye, often between 1% and 4%, except for black, which is typically 3% to 6% .
It is a reverse of migration, the dyestuff is move out from the fiber surface and goes into the dye bath solution
Anti-migration → decrease in affinity
Each dye class have its critical temperature, above this critical temperature the dye stuff anti-migrate instead of migration.
An additive use in dye or pigment mixture to prevent undesired movement or spreading of the wet dye on fabric . Sodium alginate is the anti-migrating agent used in reactive dyeing, retard the rate of migration by producing physical hindrance and by increase of viscosity of solution. The function of levelling agent or retarder is antimigration.
A system consisting of finely divided particles and the medium in which they are distributed.
The resistance ability of dye molecule to any agency named as washing. Light, rubbing and crocking. The different terms comes under this term such as wash fastness, light fastness etc.
Fading is seen as a colour loss by the dyed or printed fabric textile material. It is result of some changes in the structure of dye molecule due to absorption of light, reaction with air pollutants, laundering, dry cleaning and other agency.
Fabrics dyed with certn blue & violets disperse dyes conaining anthraquinone structure become fade in presence of nitrous oxide. This nitrous oxide may be made in nature from various sources such as open gas fire, electric heating arrangement.
During the exhaust dyeing process the oligomers may diffuse out of the Polyester ﬁber and form grey deposits (white dust) on the ﬁ ber surface and also on machinery and pipes during cooling down of the dyebath.
During the production of polyethylenterephthalate (PET) ﬁbers short chains, consisting of only a few monomers units, are also formed, these are so called oligomers.The maincomponent is a cyclic trimmer. All Polyester ﬁ bers contain small quantities of oligomers, Approx. 0.5% - 3% of the ﬁber .
Uniform in shade over the surface of a piece of dyed fabric or along the length of dyed yarn Level dyeing is usually the objective in commercial processes.
Some dyes, such as leveling acid dyes, are easy to accomplish level results with. They do not bind tightly to the fabric in the dye bath, and dye can leave the fibre and re-enter the dye bath. Other dyes, such as reactive dyes, don’t level as easily, and greater care is required to achieve level results. In general, level dyeing is promoted by good agitation, careful control of the rate of rise of the temperature of the dye bath, control of pH, and sometimes by use of special leveling agents or retarders. It is often the case that the dyes that level most easily are the least washfast .
It is also called restraining agents; a dye bath auxiliary that is typically used to prevent one fiber in a blend from taking up dye intended for the other fiber, or to equalize the uptake When blends are dyed, one fiber may be truly dyed while the other is stained (colored, but with very poor fastness). Reserving agents can be used to significantly reduce the undesired staining. In blends such as wool nylon, reserving agents can act to reduce the dye uptake by the nylon, so that the nylon and wool ultimately are colored similarly .
OWB (or O.W.B)
On weight of bath; usually expressed as percentage; omb is on mass of bath - preferred modern usage. The amount of some constituent of a dyebath or other process bath based on the weight of the bath. For example, something specified as 6% owb would require 0.06 pounds of that item per pound of bath. Since the bath is invariably mostly water, which weighs 1 kilogram per litre, calculations in the metric system is much easier.
OWF (or O.W.F.)
On weight of fibre, usually expressed as percentage; omf is on mass of fibre - preferred modern usage often this is synonymous with owg but distinction may be appropriate when considering a particular fibre in a blend.
OWG (or O.W.G.)
On weight of goods; usually specified as percentage; omg is on mass of goods - preferred modern usage. The amount of dye or auxiliary chemicals used is often based on ratio to the weight of the goods to be dyed. For example, if a formula calls for 3% dye owg, and 400 grams of fabric are to be dyed, the required amount of dye would be 3% of 400 grams, or 12 grams. Owf may be more accurate when blended fibres are considered.
This is the most important component of semi or continuous dyeing machines. It is a dyeing method with very low liquor to goods ratio, where typically only enough strong dye solution is used to saturate the fabric. Padding can have the advantage of high dye yield. Padded goods are usually “batched” - wrapped in plastic and left for some period of time for the dye to attach to the fibre, or steamed to fix the dye quickly.
It is a ratio of amount pick or uptake by fabric when it passes through the solution or dye liquor.
W2 = after padding weight of fabric
W1 = before padding weight of fabric
It is the ratio of the weight of the dyebath or other processing bath to the weight of the goods being dyed or processed for immersion dyeing in art dyeing processes, common liquor to goods ratio is 20:1. That is, for each kilogram of fiber to be dyed, 20 kilograms
of dyebath are used. In the metric system, this is easy to calculate, since one liter of water weighs one kilogram. High liquor ratios are generally avoided, since they often cause poor exhaustion of the dye, though this is not true for all dye types. Modern commercial dyeing equipment often works with low liquor ratios. Very low ratios may be used for methods where essentially all of the dye solution is to be absorbed by the fiber, such as padding .
A solution of known strength, made up with the intent of dilution or mixing before final use Stock solutions are a convenient way of avoiding the need to weigh chemicals each time you need to use some. For example, you need 2% shade of Red of 5 gram of fabric and you have 1% stock solution of dye. Use this formula and take amount of dye in ml.
Dye (ml) = (Wt. Of fabric * Shade %) / Stock Solution %
The term dye blocking is used when one dyestuff blocks the exhaustion of the other dyestuff on to the fabric when combined in the same bath. It is due to the different r.m.m and affinities of dyes molecules in combination shades.
It is a process of redyeing of dyed fabric in response of faulty dyeing or increse the dpth of shade.
Tailing is refers to a phenomenon in which depth & shade occur on dyed fabric owing to worst distribution of dyes from top to bottom (first meters and last meters). This is a problem of semi & continuous dyeing. Tailing behavior is dependent upon the following factors ,
- Kind of fibers
- Padding operation & substantivity of dyes
- Particle size of dyes
- Concentration of dyes
- Dispersibility or solubility of dyes
Listing refers to a phenomenon in which variations in depth & shade occur on dyed fabric owing to uneven dye uptake from side to side and side to center. As causes of listing, the following may be given :
- Uneven drying before padding of the fabric from side to centre
- Uneven squeezing
- Inadequate pretreatment
- Uneven padding liquor temperature
- Uneven fabric temperature
- Variation in dye migration due to air speed
- Temperature dependence of dyes of thermosoling
This problem occurs due to improper dissolution of dye powder while preparing the dye solution . Formation of specks during padding is mostly attribute to the particle size of dyes, compatibility with migration inhibitor & penetrating agents, re-dispersibilty of dispersion liquor (Vat/disperse dyes) .
A larger factor causing specks in practical dyeing is compatibility of dyes with auxiliaries used together. When poor compatibility, dye aggregation is caused and solubility or dispersibility of dyes markedly lowered, a formation of specks but also the tailing are caused.
COLOUR SPOTS / BLOTCHES
This is formation of dark colored area on the dyed fabric, which causes faulty dyeing. The probable reasons are ,
- Filtration of dye solution is not carried out, especially when mixture of dyes are used, before adding into the machine or padding trough.
- Inadequate pretreatment
- Presence of contaminants in water, salt, alkali etc
- Incompatibility of dyes with auxiliaries
Photochroism is refers to the phenomenon in which dyed substrate changes in color under irradiation of light but returns to original color when the irradiation stops. This is said to be tautomerism occurring to molecules under exposure to light . However, the color is restored after the substrates are left overnight.
This phenomenon appears markedly in viscose rayon and tends to occurs increasingly easily as the concentration of urea formaldehyde resin increases. The reversion of color is depending on the relative humidity and light intensity.
Thermochromism is refers to phenomenon in which dyed substrate changes in color under application of heat but returns to original color when the heat stops or reduced.
As to reactive dyes, this phenomenon varies from dye to dye .
Stripping is refers to the removal of dyestuff either partial or complete from the dyed fabric in case of too high dye in depth, unlevel dyed or faulty dyeing.
Complete removal of dyestuff is generally difficult. The stripping of dyes can be done by reducing agents or oxidizing agents, in most cases reducing agents are preferable.
As the name indicate the term washing off is to remove the unfix dyes (in or on the substrate) and other auxiliaries from the dyed substrate. Substances in the second category can often be swollen & removed quite quickly by vigorous washing at low temperature, but removal of dyes from within the fiber will be much slower unless high temperature are used . The removal of soluble materials from the fabric is accomplished by two mechanisms,
- Liquid interchange
- Hard water
- Electrolyte content of water
- Washing process at a low liquor ratio
- Too Low temperature
Coloring fabric with ground pigments mixed with a binder rather than a true dye. Pigment Dyes although pigments are not dyes in a true sense, they are extensively used for coloring fabrics like cotton, wool and other manmade fibers due to their excellent light fastness. They do not have any affinity to the fibers and are affixed to the fabric with the help of binder. After dyeing, the fabrics are subjected to high temperatures.
Clustering of individual particles of a substance that is dyes r pigment to colloidal properties . Aggreagtion of dyes is the dye-dye self association in solution, called dye aggregation, which is importants term where dye molecules or inos takes part. In general, the term aggregation is used for dye-dye interaction & dye association for interaction of dyes with other compounds e.g polymers. Generally, dye molecules form aggregatiom in aqueous solution at room temperature and to extent which depend on size of molecules, No. of solubilzing groups in the dye molecule. In dye aggregation multiple equilibria need to be considered i.e. diametric, trimetric etc, aggregates are formed.
The reasons of aggregation of dyes are,
- Dyes are consists of, a.Hydrophobic aromatic portion b.Polar groups (OH, amino etc.) for water solubility and charged groups (sulfonic or positive charged groups) for rendering molecule water soluble
- When dye molecules dissolved in water a new interface is created between the hydrophobic portion and water. Dye can reduce the size of the interfacial water by overlapping of the hydrophobic areas and there will be a tendency to aggregate.
- Usually linear and planar dye molecules should tend to stack one molecule upon another with the ionized groups arranged so as to give minimum free energy condition causes aggregation.
- Dyes with long aliphatic chains form micelles of a spherical form in which the flexible chains associate in the interior with the sulfonic acid groups exposed on the surface of sphere.
- Aggregation of dimer is more obvious as aromatic ring system have maximum overlap (van der waals forces) because the distance between the anionic charges is larger (minimum electrostatic repulsion).
A dye that is a mixture of two or more different classes of dye, used typically to dye blends of fibres “Household” dyes, of the sort sold in grocery stores, are usually union dyes containing a direct dye which will work on cellulose fibres, and an acid dye which will work on wool or nylon. Industrially, union dyes may be other combinations, such as reactive and disperse dyes for dyeing cotton-polyester blends (often with two distinctly different sub-processes).
PREPARED FOR DYEING
It is a term used for a fabric or garment that is specially made to be dyed; sometimes “preferred for dyeing”; usually abbreviated pfd or p.f.d. PFD fabrics have been desized, scoured, and fully bleached, but have been processed without optical brighteners or softeners which can interfere with dye uptake.
SOME MISCELLANEOUS DYEING METHOD
Color is applied to fabric by different methods and at different stages of the textile manufacturing process. The processes are given below,
SOLUTION PIGMENTING OR DOPE DYEING
Dye is added to the solution before it is extruded through the spinnerets for making synthetic filaments. Simply, Coloration of the polymer prior to manufacturing of fibers.
Garment dyeing Dye is applied to finished products such as apparels and garments.
Stock dyeing is used to dye fibers. In this process, the staple fibers are packed into a vessel and then dye liquid is forced through them. Although the dye solution is pumped in large quantities, the dye may not penetrate completely into the fibers and some areas may be left without dyeing. However, the following blending and spinning processes mix up the fibers in such a thorough way that it results in an overall even color. Woolens are usually stock dyed.
Top is the combed wool sliver. It is wound on perforated spools and the dye solution is circulated through it. This method results in very even dyeing.
When dyeing is done after the fiber has been spun into yarn, it is called yarn dyeing. In this method, the dyestuff penetrates the fibers to the core of the yarn. There are many forms of yarn dyeing- Skein (Hank) Dyeing, Package Dyeing, Warp-beam Dyeing, and Space Dyeing.
SKEIN (HANK) DYEING
The yarns are loosely arranged in skeins or coils. These are then hung over a rung and immersed in a dyebath in a large container. In this method, the colour penetration is the best and the yarns retain a softer, loftier feel. It is mostly used for bulky acrylic and wool yarns.
The yarns are wound on spools, cones or similar units and these packages of yarn are stacked on perforated rods in a rack and then immersed in a tank. In the tank, the dye is forced outward from the rods under pressure through the spools and then back to the packages towards the center to penetrate the entire yarn as thoroughly as possible. Mostly, the carded and combed cotton which are used for knitted outerwear is dyed through this method.
It is similar to package dyeing but more economical. Here, the yarn is wound on to a perforated warp beam and then immersed in a tank for dyeing it applying pressure.
In this method, the yarn is dyed at intervals along its length. For these two procedures- knit- deknit method and OPI Space-Dye Applicator- are adopted. In the first method, the yarn is knitted on either a circular or flat-bed knitting machine and the knitted cloth is then dyed and subsequently it is deknitted. Since the dye does not readily penetrate the areas of the yarn where it crosses itself, alternated dyed and undyed spaces appear. The OPI Space-Dye Applicator technique produces multi coloured space- dyed yarns. The yarns are dyed intermittently as they run at very high speeds through spaced dyebaths. They are continuously subjected to shock waves produced by compressed air having supersonic velocities.
The constructed fabrics are piece dyed for the flexibility they provide. The textile manufacturer can dye the whole fabric in batches according to the fashion demands of the time thus avoiding wastage and resultantly loss. There are several methods prevalent or piece dyeing.
In this method, small batches of constructed natural colored fabric are dyed according to the demands for a given color.
It is used for dyeing long yards of fabric. The fabric is passed in rope form through the dyebath. This rope of the fabric moves over a rail onto a reel which immerses it into the dye and then draws the fabric up and forward and brings it to the front of the machine. This process is repeated many times until the desired color intensity is obtained.
RECENT DYEING TERMINOLOGIES 
ULTRASONIC ASSISTED DYEING
The use of ultrasound in the dyeing of textile can be explained as: when ultrasound waves are absorbed in the liquid system the phenomenon of cavitations takes place. Cavitations can liberate entrapped gases from liquid or porous material like textiles, dyebath etc. The influence of ultrasound on dyeing is explained to have thee-ways effects:
Microwave dyeing takes into account only the dielectric and the thermal properties. The dielectric property refers to the intrinsic electrical properties that affects the dyeing by polar rotation of the dye & influences the microwave field upon the dipoles.
The aqueous solution of dye has two components which are polar, in the high frequency microwave field oscillating at 2450MHz. It influences the vibrational energy in the water molecules and the dye molecules. The heating mechanism is through ionic conduction, which is a type of resistance heating. Depending on the acceleration of the ions through the dye solution, it results in collision of dye molecules with the molecules of the fiber. The mordant helps and affects the penetration of the dye and also the depth to which the penetration takes place in the fabric. This makes microwave superior to conventional dyeing techniques.
The vat and sulphur dyes are insoluble in water; therefore for their application it is necessary to convert them into water-soluble form using suitable reducing agent and alkali. Different reducing agents use for vat and shulphur dyes are briefly reviewed with emphasis on the emerging technique of electro chemical reduction.
PLASMA APPLICATION FOR DYEING OF TEXTILE SUBSTRATE
Dyeability of Cotton Substrate
It has been reported that plasma treatment on cotton in presence of air or argon gas increases its water absorbency. This report was concerned with the effect of air and oxygen plasma on the rate and extent of dye uptake of Chloramine Fast Red K on cotton print cloth. The effect of plasma treatment in two different gas atmospheres (air and oxygen) for different treatment times was studied by applying 2% of Chloramine Fast Red K.
The effect of plasma treatment in air and oxygen appears to increase both the rate of dyeing and the direct dye uptake in the absence of electrolyte in the dye bath. Oxygen treatment is more effective than air plasma treatment. This shows that the increase in the rate and extent of dye uptake for the direct dye studied depends more on the oxygen component of the air than on the nitrogen component, which supports an oxidative mechanism of attack on the cotton.
The contributory factors leading to this increase in dye uptake can be:
- The change of the fabric surface area per unit volume due to the surface erosion.
- The etching effect of the plasma effect on the fibred mages the fiber surface and also removes surface fiber impurities (e.g. cotton wax or any remaining warp size, etc.)
- The chemical changes in the cotton fiber surface (leading to carbonyl and carboxyl groups in the fiber.
- The possibility of the formation of free radicals on the cellulosic chains of cotton.
- Thus the action of oxygen and air plasma treatments modifies the surface properties of cotton and leads to an increase in the rate and extent of uptake of direct dye.
SUPERCRITICAL CARBON DIOXIDE (CO2) DYEINGWater is a valuable raw material which is not unlimitedly available. It must be protected by appropriate legal measures. Usage of water as solvent for chemicals is mostly because of its abundant availability and low cost. Problems associated with usage of water are effluent generation and additional step is needed to dry the fabrics after each step. The amount of energy spent to remove the water is also huge adding to the woes of processors, making processing the weakest link among the entire textile chain. The unspent dyestuffs remain in liquor, thus polluting the effluent. It leads to additional pollution of waste water.
To eliminate the disadvantages it is proposed that certain gases can replace water as solvating medium. High pressure and temperature are needed to dissolve the dyes. Of all the gases being possible of converted into super critical fluids, CO2 is the most versatile and prominently used. Because of their high diffusion rates and low viscosities that allow the dye to penetrate into the fiber. Moreover, by reducing the pressure at the end of the process, dye and CO2 can be recycled.
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