dyes and pigment
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Dyes and PigmentsDyesAre substances that can be used to impart color to other materials, such as textiles, foodstuffs, and paper. Unlike pigments, dyes are absorbed to a certain extent by the material to which they are applied. The colors from some dyes are more stable than others, however. A dye that does not fade when the material it was applied to is exposed to conditions associated with its intended use is called a fast dye. Contrariwise, a dye that loses its coloring during proper usage is referred to as a fugitive dye. Some of the conditions that could cause such a change in the properties of a dye include exposure to acids, sunlight, or excessive heat as well as various washing and cleaning procedures. Certain dyes may be considered both fast and fugitive, depending on the material with which they are used. The process of dyeing is carried out in a variety of ways depending on the specific dye utilized as well as the properties of the material. Silk, wool, and some other textiles may, for instance, be directly dyed by simply dipping them into the colorant. Much more often, however, the use of a reagent known as a mordant is necessary to fix dyes to materials. A number of different compounds may be used as mordants, but metallic hydroxides of tin, iron, chromium, or aluminum are most common. Oftentimes, the color that a particular dye imparts is dependent on the mordant it is utilized with. Another method of dyeing involves the use of vats. For instance, the dye indigo begins as a colorless soluble substance that is dissolved in water in a vat before cloth is dipped into it. When oxygen from the air or another chemical added to the vat comes into contact with the indigo solution, an insoluble blue color results. Batik dyeing, a process that was invented during antiquity in Java, can be used with silks or cottons and involves the application of wax to the cloth before dye treatment in order to create unusual designs and color patterns. A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. The dye is usually used as an aqueous solution and may require a mordant to improve the fastness of the dye on the fibre. (In contrast, a pigment generally has no affinity for the substrate, and is insoluble)
Archaeological evidence shows that, particularly in India and the Middle East, dyeing has been carried out for over 5000 years. The dyes were obtained from either animal, vegetable or mineral origin with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale. The first man made organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many thousands of dyes have since been prepared and because of vastly improved properties imparted upon the dyed materials quickly replaced the traditional natural dyes. Dyes are now classified according to how they are used in the dyeing process.
Acid dye - Water soluble anionic dyes that are applied to fibres such assilk, wool, nylon and modified acrylic fibres from neutral to acid dyebaths. Attachment to the fibre is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fibre. Acid dyes are not substantive to cellulosic fibres.
Basic dye - Water soluble cationic dyes that are applied to wool, silk,cotton and modified acrylic fibres. Usually acetic acid is added to the dyebath to help the take up of the dye onto the fibre. Basic dyes are also used in the coloration of paper.
Direct (Substantive) dye - Dyeing is normally carried out in a neutralor slightly alkaline dyebath, at or near the boil, with the addition of either sodium chloride (NaCl) or sodium sulphate (Na2SO4). Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.
Mordant dye - As the name suggests these dyes require a mordant. Thisimproves the fastness of the dye on the fibre such as water, light and perspiration fastness. The choice of mordant is very important as different mordants can change the final colour significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques.
Vat dye - These dyes are essentially insoluble in water and incapable ofdyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye. In this leuco form these dyes have an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye.
Reactive dye - First appeared commercially in 1956 and were used todye cellulosic fibres. The dyes contain a reactive group that, when applied to a fibre in a weakly alkaline dyebath, form a chemical bond with the fibre. Reactive dyes can also be used to dye wool and nylon, in the latter case they are applied under weakly acidic conditions.
Disperse dye - Originally developed for the dyeing of cellulose acetate.They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent then sold as a paste or spray dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibres. In some cases a dyeing temperature of 130 deg C is required and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.
Azoic dye - A dyeing technique in which an insoluble azo dye isproduced directly onto or within the fibre. This is achieved by treating a fibre with a diazo component and a coupling component. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique in that the final colour is controlled by the choice of the diazo and coupling components. One other class which describes the role dyes have rather than their mode of use is food dyes. This is a special class of dyes of very high purity. They include direct, mordant and vat dyes. Their use is strictly controlled by legislation. Many are azo dyes but anthraquinone and triphenylmethane compounds are used for colours such as green and blue. Some naturally occurring dyes are also used.
Pigments:Are the basis of all paints, and have been used for millennia. They are ground colored material. Early pigments were simply as ground earth or clay, and were made into paint with spit or fat. Modern pigments are often sophisticated masterpieces of chemical engineering. In biology, pigment is any color in plant or animal cells. Nearly all types of cells, such as skin, eyes, fur and hair contain pigment. Creatures that have deficient pigmentation are called albinos. In the coloring of paint, ink, plastic, fabric and other material, a pigment is a dry colorant, usually an insoluble powder. There are both natural and synthetic pigments, both organic and inorganic ones. Pigments work by selectively absorbing some parts of the visible spectrum (see light) whilst reflecting others. A distinction is usually made between a pigment, which is insoluble, and a dye, which is either a liquid, or is soluble. There is no well-defined dividing line between pigments and dyes, however, and some coloring agents are used as both pigments and dyes. In some cases, a pigment will be made by precipitating a soluble dye with a metallic salt. The resulting pigment is called a "lake". Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths. Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs --organisms which make their own food using photosynthesis. In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more of the sun's energy.
There are three basic classes of pigments. Chlorophylls are greenish pigments which contain a porphyrin ring. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll "captures" the energy of sunlight. There are several kinds of chlorophyll, the most important being chlorophyll "a". This is the molecule which makes photosynthesis possible, by passing its energized electrons on to molecules which will manufacture sugars. All plants, algae, and cyanobacteria which photosynthesize contain chlorophyll "a". A second kind of chlorophyll is chlorophyll "b", which occurs only in "green algae" and in the plants. A third form of chlorophyll which is common is (not surprisingly) called chlorophyll "c", and is found only in the photosynthetic members of the Chromista as well as the dinoflagellates. The differences between the chlorophylls of these major groups was one of the first clues that they were not as closely related as previously thought. Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color. These compounds are composed of two small six-carbon rings connected by a "chain" of carbon atoms. As a result, they do not dissolve in water, and must be attached to membranes within the cell. Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae as well as the diatoms. Phycobilins are water-soluble pigments, and are therefore found in the cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria and Rhodophyta. The picture at the right shows the two classes of phycobilins which may be extracted from these "algae". The vial on the left contains the bluish pigment phycocyanin, which gives the Cyanobacteria their name. The vial on the right contains the reddish pigment phycoerythrin, which gives the red algae their common name.
Phycobilins are not only useful to the organisms which use them for soaking up light energy; they have also found use as research tools. Both pycocyanin and phycoerythrin fluoresce at a particular wavelength. That is, when they are exposed to strong light, they absorb the light energy, and release it by emitting light of a very narrow range of wavelengths. The light produced by this fluorescence is so distinctive and reliable, that phycobilins may be used as chemical "tags". The pigments are chemically bonded to antibodies, which are then put into a solution of cells. When the solution is sprayed as a stream of fine droplets past a laser and computer sensor, a machine can identify whether the cells in the droplets have been "tagged" by the antibodies. This has found extensive use in cancer research, for "tagging" tumor cells.
REACTIVE DYEReactive dyes first appeared commercially in 1956, after their invention in 1954 by Rattee and Stephens at the Imperial Chemical Industries Dyestuffs Division site in Blackley, Manchester, United Kingdom Various developments including new chemical types 1980s Mixed bifunctional dyes (esp. Sumitomo Sumifix Supra dye
About Reactive DyeReactive dyes are used to dye cellulosic fibres. The dyes contain a reactive group, either a haloheterocycle or an activated double bond, that, when applied to a fibre in an alkaline dye bath, forms a chemical bond with an hydroxyl group on the cellulosic fibre. Reactive dyeing is now the most important method for the coloration of cellulosic fibres. Reactive dyes can also be applied on wool and nylon; in the latter case they are applied under weakly acidic conditions. Reactive dyes have a low utilization degree compared to other types of dyestuff.
General Features of a Reactive Dye Molecule:
W = water solubilising group D = chromophore B = bridging group RG = reactive group X = leaving group
Types of Reactive Dyes 1. Sulphatoethyl sulphone dyes:O NH2 SO3Na
N H SO2CH2CH2OSO3Na
Vinyl sulphone dye (Remazol Brilliant Blue R, C.I. Reactive Blue 19)
2. Monochloro-s-triazine dyes:Cl SO3Na N OH HN N N N N N H
Monochloro-s-triazine dye (Procion Red H-3B, C.I. Reactive Red 3)
3. Bis (monochloro-s-triazine) dyes:Cl SO3Na N OH HN N N N N N H N H N N Cl N NH OH N SO3Na NaSO3 NaSO3 N
Bis(monochloro-s-triazine) dye (Procion Red HE-3B, C.I. Reactive Red 120
4. Mixed Bifunctional reactive dyes:
Cl N Dye NH N N HGeneral structure of Sumifix Supra dyes MCT-SES or MCT-VS [Reactron Supra F dyes are similar
Reactions of Reactive Dyes Nucleophilic addition:H C H H C H OSO3Na
O DYE S O
- NaHSO 4 DYE
O S O CH CH2
O DYE S O
H C H H2O O Cellulose
O S O CH
O DYE S O
H C H
H C H O Cellulose
-elimination of -sulphatoethylsulphone to vinyl sulphone and reaction with cellulose.
Nucleophilic substitutionAromatic rings are electronically very stable, and will attempt to retain this. This means that instead of the nucleophilic addition that occurs with alkenes, they undergo nucleophilic substitution, and keep the favourable electron system. However, nucleophilic subsitutions are not very common on aromatics, given their already high electron density. To encourage nucloephilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack. For example2:
But this requires harsh conditions. To improve the rate under mild conditions, powerful electron-withdrawing groups such as -NO2 may be added2.
However, this will only work if there is a good leaving group, such as -Cl or -N2. The major fibre-reactive group which reacts this way contains six-membered, heterocyclic, aromatic rings, with halogen substituents. For example, the Procion dye2: (This is the same as the chime molecule at the top of the page)
Where X = Cl, NHR, OR. Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilised as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens2:
One problem is that instead of reacting with the -OH grous on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolysed. The two reactions compete, and this unfavourable because the hydrolysed dye cannot react further. This must be washed out of the fabric before use, to prevent any leakage of dye, and not only increases the cost of the textile, but adds to possible environmental damage from contaminated water2.
Continuous: eg. Pad - Thermofix
Semi-Continuous: eg. Pad - Batch
Batchwise Exhaustion: eg. Winch, Jet, Package and Beam Dyeing
Printing: eg. Print - Thermofix
Advantages and Disadvantages Advantages: Full Colour Gamut Brilliant, bright colours Colvalent fixation high WashFastness (WF) Varying reactivities Various temperatures including low energy (cold dyeing) Various methods of application Inexpensive to apply (but dyes expensive)
Disadvantages: Incomplete fixation (problem with hydrolysis) Need for wash-off (for high WF) Need for high concentrations of salt Affect natural balance of watercourses High pH Some dyes are AOX potentially harmful to the environment
UsageReactive dyes are used to dye cellulosic fibres. The dyes contain a reactive group, either a haloheterocycle or an activated double bond, that, when applied to a fibre in an alkaline dye bath, forms a chemical bond with an hydroxyl group on the cellulosic fibre. Reactive dyeing is now the most important method for the coloration of cellulosic fibres. Reactive dyes can also be applied on wool and nylon; in the latter case they are applied under weakly acidic conditions. Reactive dyes have a low utilization degree compared to other types of dyestuff, since the functional group also bonds to water, creating hydrolysis.
Hot And Cold Dyeing Brands Cold Dyeing Brand:Reactive Cold Brand Dyestuffs are fibre reactive dyes which form a chemical linkage with Hydroxyl groups of Cellulose and thus give dyeing and printing good fastness to wet treatments.
Hot Dyeing Brand:He Dyes are Reactive Dyes for Cellulosic material and designed to give high fixation by exhaust dyeing method when applied at the temperature 75C - 95C.
The other misconception that people have is that they assume the Hot dye is more colour-fast than the Cold one because you have boiled the colour in. This is not so. Cold dyes are more robust and colours will remain brighter for longer than Hot dyes. The cooler process is not only a little easier, it is also more lasting. Cold reactive dyes are very reliable and used throughout the global clothing and textile industries to permanently colour fabrics made from plant fibres. The dyes react with the fibre on a molecular level to produce a permanent bond that withstands wash after wash. The colour becomes part of the fabric.
Disperse DyeDisperse dye is originally developed for the dyeing of cellulose acetate. They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent then sold as a paste or spray dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibres. In some cases a dyeing temperature of 130 deg C is required and a pressurized dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding. Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. Their main use is the dyeing of polyesters, and they find minor use dyeing cellulose acetates and polyamides. The general structure of disperse dyes is small, planar and nonionic, with attached polar functional groups like -NO2 and -CN. The shape makes it easier for the dye to slide between the tightly-packed polymer chains, and the polar groups improve the water solubility, improve the dipolar bonding between dye and polymer and affect the colour of the dye. However, their small size means that disperse dyes are quite volatile, and tend to sublime out of the polymer at sufficiently high temperatures. The dye is generally applied under pressure, at temperatures of about 130oC. At this temperature, thermal agitation causes the polymer's structure to become looser and less crystalline, opening gaps for the dye molecules to enter. The interactions between dye and polymer are thought to be Van-derWaals and dipole forces. The volatility of the dye can cause loss of colour density, and staining of other materials at high temperatures. This can be counteracted by using larger molecules or making the dye more polar (or both). This has a drawback, however, in that this new larger, more polar molecule will need more extreme forcing conditions to dye the polymer2.
The most important class is the azo class. This class of azo disperse dyes may be further sub-divided into four groups, the most numerous of which is the aminoazobenzene class. This class of dye can be altered as mentioned before, to produce bathochromic shifts. A range of heterocyclic aminoazobenzene dyes are also available. These give bright dyes, and are bathochromically shifted to give blues. The third class of disperse dye is based on heterocyclic coupling components, which produce bright yellow dyes. The fourth class are disazo dyes. These tend to be quite simple in structure. Other than these, there are disperse dyes of the carbonyl class, and a few from the nitro and polymethine classes. Common and Generic Names for Disperse DyesColour Index Names for PROSperse Disperse DyesPRO chem # Name Colour Index Name
D118 D225 D333 D350 D360 D426 D450 D459 D460 D650 D770 D773 D880 D885 D125
Bright Yellow Clear Orange Fuchsia Flame Scarlet Bright Red Turquoise National Blue Bright Blue Deep Navy Cool Black Meadow Sage Iris Lilac Buttercup
Disperse Yellow 218 Disperse Orange 25 Disperse Violet 33 Disperse Red 325 Disperse Red 60 Disperse Blue 26 Disperse Blue C-4RA (manufacturer's mix?) Disperse Blue 56 Disperse Navy 35 Disperse Black C-MDA (manufacturer's mix?) In House Mix In House Mix In House Mix In House Mix In House Mix
APPLICATION OF DISPERSE DYES:
Dyeing Polyester with Disperse Dyes Polyester requires the use of disperse dyes. Other types of dyes leave the color of polyester almost entirely unchanged. While novices happily charge into dyeing with acid dyes (for wool or nylon) and fiber reactive dyes (for cotton and rayon), often with excellent results, the immersion dyeing of polyester is a different story. However, disperse dye can be used by even young children to make designs on paper, which can then be transferred to polyester fabric, or other synthetics, with a hot iron. The possibilities are endless, using fabric crayons, rubber stamps, painting, and even screen printing. Stamp Pad Ink Disperse dye can be applied to paper with rubber stamps, and then ironed on to polyester, just like the crayons. You can use special, large-scale fabric stamps to apply other dyes to fabric, but only disperse dyes allow such fine lines that almost any rubber stamp designed for use on paper will work, if your fabric is smooth enough. Look for a product called "Heat Set Ink" at companies that sell rubber stamping supplies. Caroline Dahl's wonderful book Transforming Fabric gives source information for this material, in addition to many project ideas and beautiful inspiring photographs of works made with disperse dye on polyester.
PROPERTIES OF DISPERSE DYE:Fastness to light is generally quite good, while fastness to washing is highly dependent on the fibre. In particular, in polyamides and acrylics they are used mostly for pastel shades because in dark shades they have limited build-up properties and poor wash fastness.
Chemical characteristics and general application conditionsDisperse dyes are characterised by the absence of solubilising groups and low molecular weight. From a chemical point of view more than 50 % of disperse dyes are simple azo compounds, about 25 % are anthraquinones and the rest are methine, nitro and naphthoquinone dyes. The dye-fibre affinity is the result of different types of interactions: hydrogen bonds dipole-dipole interactions Van der Waals forces. Disperse dyes have hydrogen atoms in their molecule, which are capable of forming hydrogen bonds with oxygen and nitrogen atoms on the fibre. Dipole-dipole interactions result from the asymmetrical structure of the dye molecules, which makes possible electrostatic interactions between dipoles on the dye molecules and polarised bonds on the fibre. Van der Waals forces take effect when the molecules of the fibre and colourant are aligned and close to each other. These forces are very important in polyester fibres because they can take effect between the aromatic groups of the fibre and those of the colourant. Disperse dyes are supplied as powder and liquid products. Powder dyes contain 40 - 60 % of dispersing agents, while in liquid formulations the content of these substances is in the range of 10 - 30 %. Formaldehyde condensation products and ligninsulphonates are widely used for this purpose.
Dyeing with disperse dyes may require the use of the following chemicals and auxiliaries: dispersants: although all disperse dyes already have a high content of dispersants, they are further added to the dyeing liquor and in the final washing step carriers: for some fibres, dyeing with disperse dyes at temperatures below 100 C requires the use of carriers. This is the case with polyester, which needs the assistance of carriers to enable an even penetration of disperse dyes below boiling temperature. Because of environmental problems associated with the use of these substances, polyester is preferably dyed under pressure at temperature >100 C without carriers. However, carrier dyeing is still important for polyester-wool blends, as wool must not be submitted to wet treatment at temperatures significantly above 100 C thickeners: polyacrylates or alginates are usually added to the dye liquor in padding processes. Their function is to prevent migration of the dye liquor on the surface during drying reducing agents (mainly sodium hydrosulphite): they are added in solution with alkali in the final washing step. Disperse dyes are widely used not only for dyeing, but also for printing synthetic fibres.
Environmental issues:The environmental properties of disperse dyes are assessed under the following parameters Parameters of concern Bio-eliminability Comments Owing to their low water-solubility, they are largely eliminated by absorption on activated sludge in the waste water treatment plant Some disperse dyes can contain organic halogens, but they are not expected to be found in the effluent after waste water treatment (because they are easily eliminated by absorption on the activated sludge) (see also Section 18.104.22.168) The following disperse dyes potentially have an allergenic effect: Disperse Red 1, 11, 17, 15; Disperse Blue 1, 3, 7, 26, 35, 102, 124; Disperse Orange 1, 3, 76; Disperse Yellow 1, 9, 39, 49, 54, 64. Heavy metals Aromatic amines Unfixed colourant Effluent contamination by additives in the dye formulation These dyes are still offered by some Far East dealers and manufacturers [294, ETAD, 2001] Level of fixation is in the range of 88 - 99 % for continuous dyeing and 91 - 99 % for printing Conventional dispersants (formaldehyde condensation compounds, lignosulphonates, etc.) are poorly biodegradable (