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A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in anaqueous solution , and may require a mordant to improve the fastness of the dye on the fiber. Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment , and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments. Dyed flax fibers have been found in the Republic of Georgia dated back in a prehistoric cave to 36,000 BP . [1] [2] Archaeological evidence shows that, particularly in India and Phoenicia , dyeing has been extensively carried out for over 5000 years. The dyes were obtained from 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. Organic dyes The first human-made (synthetic) organic dye, mauveine , was discovered by William Henry Perkin in 1856. Many thousands of synthetic dyes have since been prepared. Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colors, and they imparted better properties to the dyed materials. [3] Dyes are now classified according to how they are used in the dyeing process. Acid dyes are water -soluble anionic dyes that are applied to fibers such as silk , wool , nylon and modified acrylic fibers using neutral to acid dyebaths. Attachment to the fiber is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid dyes are not substantive to cellulosic fibers. Most synthetic food colors fall in this category. Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers , but find some use for wool and silk. Usually acetic acid is added to the dyebath to help the uptake of the dye onto the fiber. Basic dyes are also used in the coloration of paper . Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point , with the addition of either sodium chloride (NaCl) or sodium sulfate (Na2SO4). Direct dyes are used on cotton , paper, leather , wool, silk and nylon . They are also used as pH indicators and as biological stains .

Chemistry Dyes

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  • A dye can generally be described as a colored substance that has an affinity to the substrate to

    which it is being applied. The dye is generally applied in anaqueous solution, and may require

    a mordant to improve the fastness of the dye on the fiber.

    Both dyes and pigments appear to be colored because they absorb some wavelengths

    of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity

    for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and

    based on the salt used they could be aluminum lake, calcium lake or barium lake pigments.

    Dyed flax fibers have been found in the Republic of Georgia dated back in a prehistoric cave to

    36,000 BP.[1][2] Archaeological evidence shows that, particularly

    in India and Phoenicia, dyeing has been extensively carried out for over 5000 years. The dyes

    were obtained from 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.

    Organic dyes

    The first human-made (synthetic) organic dye, mauveine, was discovered by William Henry

    Perkin in 1856. Many thousands of synthetic dyes have since been prepared.

    Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast

    range of new colors, and they imparted better properties to the dyed materials.[3] Dyes are now

    classified according to how they are used in the dyeing process.

    Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and

    modified acrylic fibers using neutral to acid dyebaths. Attachment to the fiber is attributed, at least

    partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid

    dyes are not substantive to cellulosic fibers. Most synthetic food colors fall in this category.

    Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, but find some

    use for wool and silk. Usually acetic acid is added to the dyebath to help the uptake of the dye

    onto the fiber. Basic dyes are also used in the coloration of paper.

    Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or

    near boiling point, with the addition of either sodium chloride (NaCl) or sodium sulfate (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 dyes require a mordant, which improves the fastness of the dye against

    water, light and perspiration. The choice of mordant is very important as different mordants can

    change the final color significantly. Most natural dyes are mordant dyes and there is therefore a

    large literature base describing dyeing techniques. The most important mordant dyes are the

    synthetic mordant dyes, or chrome dyes, used for wool; these comprise some 30% of dyes used

    for wool, and are especially useful for black and navy shades. The mordant, potassium

    dichromate, is applied as an after-treatment. It is important to note that many mordants,

    particularly those in the heavy metal category, can be hazardous to health and extreme care must

    be taken in using them.

    Look up leuco

    form inWiktionary, the free

    dictionary.

    Vat dyes are essentially insoluble in water and incapable of dyeing fibres directly. However,

    reduction in alkaline liquor produces the water soluble alkali metal salt of the dye, which, in this

    leuco form, has an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble

    dye. The color of denim is due to indigo, the original vat dye.

    Reactive dyes utilize a chromophore attached to a substituent that is capable of

    directly reacting with the fibre substrate. The covalent bonds that attach reactive dye to natural

    fibers make them among the most permanent of dyes. "Cold" reactive dyes, such as Procion

    MX, Cibacron F, and Drimarene K, are very easy to use because the dye can be applied at room

    temperature. Reactive dyes are by far the best choice for dyeing cotton and other cellulose fibers

    at home or in the art studio.

    Disperse dyes were originally developed for the dyeing of cellulose acetate, and are water

    insoluble. The dyes are finely ground in the presence of a dispersing agent and sold as a paste,

    or spray-dried and sold as a powder. Their main use is to dye polyester but they can also be used

    to dye nylon, cellulose triacetate, and acrylic fibres. In some cases, a dyeing temperature of

    130 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 dyeing is a technique in which an insoluble azo dye is produced directly onto or within the

    fibre. This is achieved by treating a fibre with both diazoic and coupling components. 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 color is controlled by the

    choice of the diazoic and coupling components.

  • Sulfur dyes are two part "developed" dyes used to dye cotton with dark colors. The initial bath

    imparts a yellow or pale chartreuse color, This is aftertreated with a sulfur compound in place to

    produce the dark black we are familiar with in socks for instance. Sulfur Black 1 is the largest

    selling dye by volume.

    [edit]Food dyes

    One other class which describes the role of dyes, rather than their mode of use, is the food dye.

    Because food dyes are classed as food additives, they are manufactured to a higher standard

    than some industrial dyes. Food dyes can be direct, mordant and vat dyes, and their use is strictly

    controlled by legislation. Many are azoic dyes,

    although anthraquinone and triphenylmethane compounds are used for colors such

    as green and blue. Some naturally-occurring dyes are also used.

    What makes the Dyes coloured?

    This is a very common question that occurs in everybodies mind. The answer to which is explained by the presence of a substance called Chromophore in the dyes. By definition dyes are basically aromatic compounds. Their structures have aryl rings that has delocalised electron systems. These structures are said to be responsible for the absorption of electromagnetic radiation that has varying wavelengths, based upon the energy of the electron clouds.

    It is actually because of this reason that chromophores do not make dyes coloured. Rather it makes the dyes proficient in their ability to absorb radiation. Chromophores acts by making energy changes in the delocalised electron cloud of the dye. This alteration invariably results in the compound absorbing radiation within the visible range of colours and not outside it. Human eyes detects this absorption, and responds to the colours.

    Another possibility is that if the electrons are removed from the electron cloud, it may result in loss of colour. Removing electrons may cause the rest of the electrons to revert to the local orbits. A very good example is the Schiff's reagent. As sulphurous acid reacts with pararosanilin, what happens is that a sulphonic group attaches itself to the compound's central carbon atom. This hampers the conjugated double bond system of the quinoid ring, and causes the electrons to become localised. As a consequence the ring ceases to be a chromophore. As a result, the dye becomes colourless.

    To conclude chromophores are the atomic configurations which has delocalised electrons. Generally they are represented as carbon, nitrogen, oxygen and sulphur. They can have alternate single and double bonds.

    How can the colour of the Dyes be altered?

    The answer lies in the Modifiers. Colour modifiers like methyl or ethyl groups can actually alter the colour of dyes. They do so by altering the energy in the delocalised electrons. It has been found that by addition of a particular modifier there is a progressive alteration of colour. An example can be given for methyl violet series.

    The following diagram explains what happens to the colour of the dyes when modifiers are added.

  • Step A: When no methylgroup is added the original dye Pararosanil as it is called is red in colour.

    Step B: As Four Methyl groups are added the reddish purple dye Methyl Violet is got.

    Step C: With the addition of more groups a purple blue dye Crystal Violet is obtained. It has in it six such groups.

    Step D: Further addition of a seventh methyl group the dye that is got is called Methyl green.

    What gives the Dyes Solubility and Cohesiveness?

    The answer to this riddle lies in substance called Auxochrome. Moreover the Auxochromes has also the abilty to intensify colours. It is a group of atoms which attaches to non-ionising compounds yet has the ability to ionise. Auxochromes are of two types, positively charged or negatively charged.

  • Classification of Dyes

    There are several ways for classification of dyes. It should be noted that each class of dye has a very unique chemistry, structure and particular way of bonding. While some dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. Some of the prominent ways of classification is given hereunder.

    Organic/Inorganic Natural/Synthetic By area and method of application Chemical classification- Based on the nature of their respective chromophores. By nature of the Electronic Excitation(i.e, energy transfer colorants, absorption colorants and

    fluorescent colorants). According to the dyeing methods

    o Anionic(for Protein fibre)o Direct(Cellulose)o Disperse(Polyamide fibres)

    However the most popular classification is the one that is advocated by the US International Trade Commission. This system classifies dyes into 12 types.

    Group Application

    Direct Cotton, cellulosic and blended fibres

    Vat dyes Cotton, cellulosic and blended fibres

    Sulphur Cotton, cellulosic fibre

    Organic pigments Cotton, cellulosic, blended fabric, paper

    Reactive Cellulosic fibre and fabric

    Disperse dyes Synthetic fibres

    Acid Dyes Wool, silk, paper, synthetic fibres, leather

    Azoic Printing Inks and Pigments

    Basic Silk, wool, cotton

    Application of Dyes and PigmentsDyes

  • Have you ever wondered what gives the t-shirt you wear its jazzy colours or the leather shoes its shinning black or for that matter the colour print out you have just taken from your printer. Perhaps there would be a very few applications, where there would be no use of the dyes. Constant researches done over the 20th century and there after has resulted into every imaginable form of color of dye. Modern dyes serves more than just being pretty. They have become indispensable tools for a variety of industries. From acting as colorants for plastics, Textile dyeing industries and the highly sophisticated biotechnology industry dyes are touching our life everywhere. Dyes are also used by industries for inks and tinting. Today various dyes are manufactured to meet the requirements of each type of industries. Dyes are available in various forms. Examples are dry powders, granules, pastes, liquids, pellets, and chips.

    Other industries where dyes are used in a variety of products include paper and pulp, adhesives, art supplies, beverages, ceramics, construction, cosmetics, food, glass, paints, polymers, soap, wax biomedicine etc.

    Dyes that cater to speciality industries often come with specialized properties that include:

    Resistance to heat Resistance to weather conditions Resistance to ultraviolet light (UV) Some products are water soluble Conducts electricity Contain reinforcing fibers Free from heavy metals

    Did you Know?Dyes are an integral part of Microbiology Dyes are used to make the microorganisms distinctly visible and or differentiate them. Crystal violet (C-8650) and safranine (S-0700) are the two dyes that are used in Gram's stain. Gram's stain, is a technique of staining that is used to classify bacteria. In it a bacterial specimen is stained with crystal violet, afterwards it is treated with an iodine solution, decolourized with alcohol, and lastly again counter stained with safranine. It has been found that Gram-positive bacteria is able to retain the violet stain, while gram-negative bacteria is not.

    Did you Know?Organic dyes[edit]Food dyes