What Are Dyes In Chemistry

Conventional Method Of Dyeing

In the state of Maharashtra, Gujrat and Rajasthan , the people follow conventional method of dyeing of cotton fabric with natural dyes which may be explained with the following process sequences. The fabric is pretreated before dyeing to get the absorbency. The grey fabrics are given dunging treatment followed by washing. The bleaching treatment is given to make the fabric white, after that it is steamed and stepped into alkaline solution, and finally rinsing and washing treatment is given. After thorough pretreatment the fabric is soaked into solution of harda/myrobolan and dried. The dried fabric is premordanted with alum and subsequently dipped into natural dye solution at boiling temperature. After dyeing the fabric is given washing and rinsing treatment and dried in the sun light. Water is sprayed on the fabric to brighten the shade. The process is repeated 2 to 4;days. The dyeing method differs from place to place. Here are some examples:

6.1.1 In Bengal

6.1.2 In Orissa

The sappan wood chips are boiled with alum and turmeric and after boiling it was cooled. In cooled solution of dye, the fabric materials are kept for 34;h. It is a premordanting process. At some places the cold solution of natural dye is taken with sufficient quantity of water, and the fabric is dipped in cold solution for 24;h and finally boiled for 2;h.

6.1.3 In Uttar Pradesh

The application of natural indigo on cotton fabric is done by two methods which are called Khari Mat and Mitha Mat.

Application Of Natural Dyes

Different researchers had proposed different methods of dyeing of natural and synthetic fibres with natural dyes. The dyeing of textile substrates depends on dyeing parameters which are fibre structure, temperature, time and pH of the dye bath and dye molecule characteristics. The fastness properties of dyes on textile substrates depend on bonding of dyes with fibre. Since natural dyes are lacking in the presence of active groups to make bonds with textile fibres, the fastness properties are not very good. The cellulosic fibres are difficult to dye with natural dyes as they have poor affinity and substantivity. The lack of bonding of natural dyes with cellulosic fibre requires mordanting treatment. Protein fibres have ionic groups and get bonded with natural dyes possessing ionic groups in dye structure.

The dyeing of proteins fibre can be done by exhaust method of dyeing. The dyeing process parameters in wool and silk dying is pH at 4.55.5 and dyeing temperature 8090°C.;The exhaustion % of dyes in dyeing is very poor. The longer liquor ratio may be preferred because of poor solubilities of natural dyes in water. Stainless steel-made dyeing machines are suitable in dyeing of wool and silk.

Classification Of Colorants By Their Chemical Structure

The Colour Index assigns dyes of known structure to one of 25 structural classes according to chemical type. Amongst the most important are:

a) azo dyesc) phthalocyanines

Azo dyes

The azo dyes constitute the largest chemical class, containing at least 66% of all colorants. The characteristic feature is the presence in the structures of one or more azo groups,

together with hydroxyl groups, amine and substituted amine groups as auxochromes.

Aromatic azo compounds are produced from aromatic amines via the corresponding diazonium salt.

A diazonium salt is formed when an aromatic amine is treated with nitrous ) acid. The nitrous acid is formed in situ by adding dilute hydrochloric acid to a cool solution of sodium nitrite at ca 278 K. In the following example, a solution of benzenediazonium chloride has been formed from phenylamine , the simplest aromatic amine:

A solution of another compound such as another aromatic amine or a phenol is then added to the cool solution and produces an azo compound which is coloured. One example is the formation of a red dye when an aqueous solution of 4-aminonaphthalenesulfonic acid is added to a solution of 4-nitrobenzenediazonium chloride to form C.I. Acid Red 74:

Azobenzene is the chromophore of these azo dyes,

and the colour of the molecule can be modified and the intensity of colour increased by varying the auxochromes .

Table 3 The molecular structures of some azo dyes showing the auxochromes.

Anthraquinone dyes

Phthalocyanines

Drawbacks To The New Artificial Dyes

There was a crucial problem with the aniline dyesthey were liable to fade.

Books:

• David, Alison Matthews. Fashion Victims: The dangers of dress past and present. London: Bloomsbury, 2015.;
• Garfield, Simon. Mauve: How One Man Invented a Colour that Changed the World. London: Faber and Faber, 2000.;
• Balfour-Paul, Jenny. Indigo: Egyptian Mummies to Blue Jeans. London: British Museum Press, 2012.;
• Dean, Jenny. A Heritage of Colour. Tunbridge Wells: Search Press, 2014.;
• Edmonds, John. Tyrian or Imperial Purple Dye: The Mystery of Imperial Purple Dye. High Wycombe: John Edmonds, 2000.
• Kalba, Laura Anne. Color in the Age of Impressionism: Commerce, Technology, and Art. University Park, PA, USA: The Pennsylvania State University Press, 2017.;
• Morton, Jocelyn. Three Generations in a Family Textile Firm. London: Routledge and Kegan Paul, 1971.;

Dyeing Of Synthetic Fibres

Different synthetic fibres like nylon, polyester and acrylic can be dyed with natural dyes like onion skin extract, babool bark extract and hina. The dyeing can be done either by padding method or exhaust method with or without mordanting. Dyeing is carried out at acidic pH.;High-temperature high-pressure dyeing gives better results in terms of colour strength than other dyeing;methods.

Types And Application Of Dyes

Dyes, either natural or synthetic, are both relatively prevalent in the industry. The types of dyes present in the marketplace include acid, direct, vat, disperse, reactive, solvent, basic and sulfur dyes. These classes of organic dyes cater to the needs of our industries.

The main application of dyes consists of coloring fibers effectively to ensure desired results. Dyes are responsible for the coloring of inks, color concentrates, chemical compounds, detergents, soaps, and oils / solvents. Furthermore, fascinating applications include color photography, histology staining, microscope cell morphology, and antiseptics. The possibilities of color are endless.

Since the chemistry and properties of each type of dye vary drastically, it’s imperative to take customer and application requirements into account ahead of time. One should consider learning dye classifications and uses of dyes before diving into them.

Based On Method Ofapplication

Acid dyes: These are acidic innature and usedfor dyeing animal fibres and synthetic fibres. These canbe used for protein fibre such as wool and silk. E.g. Picric acid, Naphtholyellow-s

Basic dyes: These are basic dyescontainingbasic group . Theyare used for dyeing animal fibres and plant fibres.

Mordant dyes or Indirectdyes: Thesedyeshave a poor affinity for cotton fabrics and hence do not dyedirectly. They require pretreatment of the fibre with a mordant. Mordant is a substance which can be fixed to the fibre and thencan be combined with the dye to form an insoluble complex called lake.Aluminium, chromium, and iron salts are widely used as mordants. E.g. alizarin.

Direct dyes: They have high affinityforcotton, rayon and other cellulose fibre. So they are applieddirectly as they fix firmly on the fabric. E.g. Congo red

Vat dyes: It can be used only oncottonand, not on silk and wool. This dyeing is a continuous processand is carried out in a large vessel called vat. So it is called as vat dye.E.g. Indigo

Application Of Dq Substrates

DQ substrates such as DQ-gelatin and DQ-collagen type IV utilize the self-quenching property of certain fluorophores such as FITC to block fluorescence of an uncleaved protein. Once the protease cuts the protein, the fluorescence rapidly increases and is easily detectable. Various DQ-substrates are commercially available, DQ-gelatin, DQ-collagen type IV, and DQ-collagen type I, which can be mixed to create any combination of ECM. DQ-gelatin is widely used in gel zymography and in situ zymography to detect the presence of collagenases in the sample . However, to image the ECM proteolysis, DQ-substrates should be mixed with nontagged ECM components. Recently, Jedeszko and colleagues provided a set of exhaustive protocols for visualizing protease activity in living cells with ECM composed of DQ-gelatin in gelatin, DQ-collagen type I in collagen type I, and DQ-collagen type IV in recombinant basement membrane.

One of the primary disadvantages of DQ-substrates is their low specificity. They are cleaved relatively indiscriminately by a number of various proteases and therefore do not enable the identification of protease in question. Moreover, DQ-gelatin-based ECM models do not reflect in vivo conditions.

J.W. Austin, F.J. Pagotto, in, 2003

The Chemistry Of Staining

Staining procedures provide conditions which promote the binding of a given dye to specific cellular organelles or extracellular features. The utility of a staining procedure lies in its ability to bind dye only to selected structures, highlighting these structures in contrast with the rest of the section. To accomplish this, each procedure makes use of a subset of possible interactions between the dye and the cellular components. The major classes of interaction are ionic, covalent, and hydrophobic.

Ionic bonding results from the attraction between positive and negative charges. In solution, acidic groups will lose a proton and become negatively charged . Basic groups will accept a proton to become positively charged cations. The pH of the solution determines the extent to which any chemical group is protonated or deprotonated, and a dye or biological molecule may have many such groups on its surface. Thus, altering the pH of a staining solution will alter the charges on the dye and the tissue molecules, and therefore alter the staining pattern. Ionic bonds are the predominant mode of interaction between tissues and dyes.

Fundamental Principles Of Reactive Dyeing

4.2.1Chemistry of reactive dyes

Reactive dyes have been very popular for the dyeing and printing of cellulosic fibre for many years. The fibre-reactive dyes were first introduced as the Procion dyes of ICI in 1956 for the production of fast brilliant colours on cellulosic materials by continuous dyeing methods . Reactive dyes immediately proved attractive to dyers due to the bright colours and the excellent fastness properties of this new dye class. This immediate interest was further increased when batch dyeing methods for these dyes were developed. They are water-soluble anionic dyes and various physical forms of these dyes are available such as pourable granules, finished powders and highly concentrated aqueous solutions, whereas in the past dyes were mainly marketed as powders. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. The general formula for the reactive dyes may be schematically represented as shown in Fig. 4.11.

4.11. Schematic structure of reactive dye.

Reactive dyes consist of four parts:

In a few cases the reactive part remains attached directly to the dye. Substituent groups and the nature of the bridging unit affect the reactivity and dyeing characteristics of dyes.

The majority of reactive dyes are chemically of the azo class, although anthraquinone-based reactive dyes are also available.

4.2.2Classification of reactive dyes

4.12. General structure of cold brand reactive dye.

4.2.3Properties of reactive dyes

Xanthene And Related Dyes

In 1871 the German chemist Adolph von Baeyer discovered a new dye class closely related to the triphenylmethane series and also without natural counterparts. Heating phthalic anhydride with resorcinol produced a yellow compound he named fluorescein, because aqueous solutions show an intense fluorescence. Although not useful as a dye, its value as a marker for accidents at sea and as a tracer of underground water flow is well established. Phthalic anhydride and phenol react to give phenolphthalein, which is similar in structure to fluorescein but lacks the oxygen linking two of the aryl rings. Since phenolphthalein is colourless in acid and intensely red in base, it is commonly used as a pH indicator in titrations and also as the active ingredient in mild laxatives, a property said to have been discovered after it was used to enhance the colour of wine. While these compounds lack fastness, some derivatives are useful dyes. Tetrabromofluorescein, or eosin, is a red dye used for paper, inks, and cosmetics; its tetraiodo analog, erythrosine, is a red food dye .

The oldest, most commonly used acid-base indicator, litmus, is a mixture of several oxazine derivatives, obtained by treating various species of lichens with ammonia, potash, and lime. Archil, orchil, and orseille are similar mixtures of dyes, obtained from lichens by different methods; cudbear is the common name for the lichenOchrolechia tartarea and the dye therefrom.

Standardization Tests And Identification Of Dyes

Colourfastness tests are published by the International Organization for Standardization. For identification purposes, the results of systematic reaction sequences and solubility properties permit determination of the class of dye, which, in many cases, may be all that is required. With modern instrumentation, however, a variety of chromatographic and spectroscopic methods can be utilized to establish the full chemical structure of the dye, information that may be essential to identifying coloured material present in very small amounts.

The Chemistry Of Dyes

The human eye responds to wavelengths of light between 400 and 700 nanometers . The presence of all wavelengths in this spectrum is perceived as white light. The presence of one wavelength alone is seen as a color: Blue for 450 nm light, Red for 600 nm light, etc. Furthermore, if one color is removed from the full visible spectrum, the light is perceived as having the “complementary color.” For example, materials which absorb at 450 nm will appear carmine. In general, dyes appear colored because they absorb a particular wavelength in the visible region. The eye senses the reflected light as the complementary color.

Different Types Of Dyes With Chemical Structure

World appears an aesthetic place due to the colours that are present around us. These are either natural or man-made. The lush green appears to our eyes, so does a blood red sports car. Since industrialization industries have strived to enhance the appearance of man-made products that surround us. The key component that renders colours to the products around us are dyes. Dyes are primarily responsible for the colour of clothes and fabrics we use.

Dyes are compounds that possess the ability to stick to a fabric. While the quality of dyes varies with the manufacturer, the dyes which take less time to colour the fabric and are chemically stable are most preferable. Dying the fabric forms strong chemical bonds between dye molecules and the fabric. Temperature and time are two important factors that determine the durability of the dye.

There are different types of dyes that are present in the market and they can be classified on the following basis,

The Coloration Of Textiles

The chemical nature of a dye is determined by the chemical and physical properties of the fibres of the textile to be coloured. The four main types of fibres are protein, cellulosic, regenerated and synthetic.

Table 1 Classification of textile fibres.

During the process of dyeing a textile, the dye is distributed between the two phases, the solid fibre phase and the aqueous phase, and at the end of the dyeing process the solution is depleted and most of the dye is associated with the fibre. Once the dye molecules penetrate the fibre there is immediate interaction between the two components, which prevents desorption of the dye molecules back into solution. The type of interaction, whether physical or chemical, will depend on the groups on the dye molecules and in the fibre chains .

Table 2 Approximate relative strengths of bonding between a dye and a fabric.

The colour fastness of a coloured textile is defined as its resistance to change when subjected to a particular set of conditions. The dye should not be affected greatly by sunlight , heat when the fabric is ironed , perspiration and when washed .

Fixation Of Natural Dyes

Natural dyes are having poor affinity and substantivity for textile materials. The bonding groups are not present in natural dyes, due to that most of the natural dyes are having poor washing fastness. The fixation of natural dyes on textile materials can be done with the help of mordanting agents. Mordanting agents are dyeing auxiliaries and are salts of heavy metals. The heavy metals Al, Cr, Cu and Sn are having vacant d orbitals and easily make coordinate bonds with natural dyes and fibre-active sites. The formed complex has bathochromic and hyperchromic shift. There are different types of mordanting agents such as metallic mordants, tannins and tannic acid and oil mordants. The different heavy metal salts work as complexing agent and chelate with natural dye colourants. Some metallic salts are toxic in nature, but even after that, they are having application in fixation of natural dyes. The different mordanting agents are:

Most controversial are lead salts and chromates .

6.5.1 Metallic mordants

Metal salts of aluminium, chromium, iron and copper are used as a mordants. The important mordants are potassium dichromate, ferrous sulphate, copper sulphate, stannous chloride and stannic chloride.

6.5.2 Tannins and tannic acid

6.5.3 Oil mordants

The Colourful Chemistry Of Artificial Dyes

Published: 9 April 2019

You are reading in The colourful chemistry of artificial dyes Part of Chemistry

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In the 21st century, we’re used to having a full spectrum of colours in our wardrobes and around our homes. But we owe this cheap availability of a variety of colours to discoveries in chemistry over the last 200 years, which started a synthetic dye boom.

The synthetic dye boom started with mauveine, the purple dye discovered in 1856 by 18-year-old chemist William Henry Perkin.

Within decades synthetic dyes were available in almost any shade you could imaginebringing with them a fashion revolution, but also environmental consequences.;