Fundamentals of Dyes in Dyes and Pigments for MSc students

Fundamentals of Dyes in Dyes and Pigments for MSc students

1. Unit - I Fundamental of Dyes M.Sc. Semester - III Presentation by Dr. Nandan C. Pomal Assistant Professor, Faculty of Science, Sigma University, Vadodara, Gujarat, India

2. Dye Dye , substance used to impart colour to  textiles, paper, lather, and other materials such that the colouring is not readily altered by washing, heat, light, or other factors to which the material is likely to be exposed. Dyeing textiles dates back to the Neolithic period (around 10,200 BC). Dyes were primarily extracted from plants, animals, and minerals. India, Phoenicia, and Egypt were early centers of dyeing. Indigo (blue) , madder (red) , and saffron (orange) were popular choices. History Definition

3. A revolutionary discovery: William Perkin accidentally creates the first synthetic dye in 1886, named mauve , while experimenting with coal tar . Rapid development: This breakthrough sparks a surge in synthetic dye production. Advantages: Synthetic dyes offer a wider range of colors, better colorfastness, and are often cheaper than natural dyes. Impact: Synthetic dyes revolutionize the textile industry and lead to advancements in other fields like medicine. Age of Synthetic Dye

4. Introduction As per the report of unlike the most organic compounds, dyes possess colour because; They absorb light in the visible spectrum (400–700 nm) Have at least one chromophore (colour-bearing group) Have a conjugated system , i.e. a structure with alternating double and single bonds Exhibit resonance of electrons , which is a stabilizing force in organic compounds .

5. When any one of these features is lacking from the molecular structure the colour is lost . In addition to chromophores , most dyes also contain groups known as  auxochromes  (colour helpers) , examples of which are carboxylic acid , sulfonic acid , amino , and hydroxyl groups . Examples carboxylic acid , sulfonic acid , amino , and hydroxyl groups . While these are not responsible for colour, their presence can shift the colour of a colourant and they are most often used to influence dye solubility . 

6. Visible Region – Colour – Wavelength

7. Classification of Dyes

8. Classification Based on Structure Azo Dyes: Most commercially important class, containing azo group (-N=N-). Wide color range, good fastness. Ex: Methyl Orange, Congo Red Anthraquinone Dyes: High fastness to light, heat, and chemicals. Used for cotton, wool, and synthetic fibers. Ex: Alizarin, Indanthrone Triphenylmethane Dyes: Brilliant colors but poor light fastness. Used in paper, inks, and some textiles. Ex: Malachite Green, Crystal Violet Phthalocyanine Dyes: Excellent light and weather fastness, copper complex structure. Used in paints, inks, and textiles. Ex: Copper Phthalocyanine Blue

9. Classification Based on Application Direct Dyes: Directly applied to cellulose fibers without mordants. Good substantivity but moderate fastness. Ex: Congo Red, Direct Blue Acid Dyes: Applied to wool and silk in acidic medium. Good levelness and fastness properties. Ex: Acid Red, Acid Blue Basic Dyes: Cationic dyes, applied to acrylic and modified polyester fibers. Brilliant colors but poor fastness. Ex: Basic Red, Basic Blue Vat Dyes: Insoluble dyes reduced to soluble leuco forms, then oxidized back to insoluble color. Excellent fastness properties. Ex: Indigo, Indanthrone Reactive Dyes: React with fiber forming covalent bonds. Good fastness properties and wide color range. Ex: Reactive Red, Reactive Blue Disperse Dyes: Insoluble in water, applied to polyester fibers at high temperature. Good fastness properties. Ex: Disperse Red, Disperse Blue

10. Classification Based on Solubility Water Soluble Dyes: Most common, used in textile dyeing, paper, and ink industries. Oil Soluble Dyes: Soluble in organic solvents, used in printing inks, plastics, and oil-based coatings. Pigments: Insoluble dyes, dispersed in a binder to form paints and coatings.

11. Theories of dye Structure

12. Understanding the structure of dyes is fundamental to the field of chemistry. This knowledge is essential for developing new dyes with desired properties, predicting color, and understanding the interaction between dyes and substrates. This response will delve into the key theories underpinning dye structure. Chromophores and Auxochromes: The foundation of dye structure lies in the concepts of chromophores and auxochromes. Chromophores: These are groups of atoms responsible for the color of a compound. They typically contain conjugated systems of double bonds, allowing for the absorption of visible light. Common chromophores include: Azo (-N=N-) Nitro (-NO₂) Carbonyl (C=O) Azomethine (-CH=N-) Quinoid structures

13. Auxochromes: These groups enhance the color intensity and modify the color produced by the chromophore. They do this by increasing the electron density on the chromophore. Common auxochromes include; -OH, -NH 2 , -NHR, -NR 2 , X (Cl, Br or I), COOH. 1,3-Dinitronapthalene (Figure 1) is pale yellow but the dye Martius Yellow (2,4-Dinitro-1-naphthol) is orange-red (Figure 2) . (Figure 1) (Figure 2)

14. The color of a dye is directly linked to its molecular structure. Key factors influencing color include: Conjugation: An extended conjugated system in a chromophore leads to a bathochromic shift (red shift), resulting in deeper colors. Auxochromes Effect: The presence of auxochromes can intensify color (hyperchromic effect) and alter the hue. Resonance: The ability of a molecule to delocalize electrons through resonance contributes to color depth and stability. Steric Hindrance: The spatial arrangement of atoms can affect conjugation and color. Colour and Structure Relationship

15. Theories Explaining Colour and Structure Several theories provide insights into the relationship between dye structure and color: 1. Witt's Theory Proposed by German chemist Otto Witt in 1876. Introduced the concepts of chromophores and auxochromes. Successfully explained the color of many dyes. However, it lacked a quantitative explanation for color.

16. 2. Valence Bond Theory (VBT) Describes the electronic structure of molecules in terms of covalent bonds. Explains color based on the energy difference between ground and excited states. Provides a qualitative understanding of color but has limitations in predicting exact wavelengths.

17. 3. Molecular Orbital Theory (MOT) Offers a more accurate and quantitative description of electronic structure. Explains color in terms of electronic transitions between molecular orbitals. Allows for calculations of absorption spectra and prediction of color shifts. This theory is widely used in modern dye research.

18. Colorimetry: The quantitative measurement of color is essential for dye characterization and quality control. Dyeing Processes: The interaction between dyes and fibers is influenced by dye structure and properties. Environmental Impact: The development of environmentally friendly dyes requires a deep understanding of dye structure and reactivity.

19. APPLICATIONS OF DYES

20. Acid dye is a type of dye that is commonly utilized in textile applications under conditions of low pH. Primarily, their application is directed toward the coloring of wool rather than cotton textiles. Certain acid dyes find application as food colorants. Certain substances can also be employed for purposes of marking organelles within the medical domain. Applications of Acid Dyes Basic dyes are widely utilized in the dyeing process of various types of fibers, including wool, silk, and acrylic. The dye bath is usually mixed with acetic acid to make it easier for the fiber to absorb the color. Also, it should be mentioned that these dyes are used to color paper. Applications of Basic Dyes

21. Cellulosic fibers can be dyed directly. A majority of them also dye wool and silk in addition to viscose rayon. They don’t dye rayon, acetate, or synthetic fabrics. Because direct dyes may be applied at low temperatures, they are appropriate for dyeing and discharge printing tasks. In general, these dyes are used in situations where high wash fastness is not necessary. Applications of Direct Dyes Metal-complex dyes find extensive utility across many applications, including wood staining, leather finishing, printing inks for stationery, and coloring agents for metals and plastics. The metallic elements employed in this study encompass copper, chromium, cobalt, and nickel. The fabric treated with metal-complex dyes exhibits favorable light-fastness characteristics. The dyes undergo dyeing processes in conditions of neutral pH, slightly acidic pH, or occasionally severely acidic pH. Applications of Metal Complex Dyes

22. While several reactive dyestuffs have been specifically modified for wool dyeing, their main use is for the coloring of cotton, linen, and viscose rayon fibers. Cold-water fiber reactive dyes work well with a variety of textile components, including cotton, silk, jute, rayon, and hessian, making them suitable for dyeing. Reactive dyes are employed in situations where there is a need for vibrant coloring with exceptional levels of colorfastness and resistance to fading during washing. The technique of cold dying is widely employed in the practice of batik. While several reactive dyestuffs have been specifically engineered for wool dyeing, their primary application is in the dyeing of cotton, linen, and viscose rayon fibers. Applications of Reactive Dyes Dyeing cotton fabrics with vat dyes mainly adopts the dip dyeing process. The vat dye suspension pad steaming process is used for continuous dyeing, which is suitable for the processing of large quantities of fabrics such as work clothes, coats and bedding. The most common method of vat dye printing is one-phase printing with potassium carbonate and anti-migration agent. In addition. The UPS method for mass processing uses two-phase printing, which is most suitable for direct printing on outer covers and covers. Applications of Vat dyes

23. Synthetic dyes are used in a wide range of applications, including textiles, paper, leather, plastics, and food. In the textile industry, synthetic dyes are used to color fabrics and fibers. They are also used in the production of printed textiles, where they are applied to the surface of the fabric in a design. Synthetic dyes are also used in the production of paper for coloring and packaging. In the food industry, synthetic dyes are used to color food and beverages. Textile manufacturers could now produce fabrics in an array of brilliant colors, enabling fashion trends and transforming everyday clothing. Additionally, the availability of consistent and affordable dyes facilitated the growth of industries such as printing, packaging, and art materials. Other Applications

24. Preparation of Some Common Dyes 1. Preparation of Methylene Blue

25. Methyl orange is an example azo dye containing one azo (-N=N-) group. It contains sulphonic acid (-SO 3 H) group and hence it is acidic azo dye. This -SO 3 H group makes the dye more soluble and is also used as reactive point for fixing the dye. This -SO 3 H group acts as auxochrome . The azo dyes are good examples of ingrain dyes (on the basis of mode of application). Methyl Orange

26. Preparation of Methyl orange (dimethyl amino azobenzene sulphonic acid): Step-1 Diazotization of sulphanilic Acid:

27. ( dimethyl amino azobenzene sulphonic acid): Step-2 Coupling of diazo compound (or salt) with dimethyl aniline:

28. Methyl orange is yellow in alkaline solution (base) and red in acid solution.

29. Preparation of Crystal Violet Structure of Crystal Violet

30. Step-1 Preparation of Michler’s ketone: When Dimethyl aniline (two equivalents) is reacted with Carbonyl Chloride; to form Michler’s ketone.

31. Step-2 Condensation of Michler’s ketone with dimethyl aniline

32. It is soluble in water and gives deep blue colour . This dye forms large crystals which are violet in colour . Hence the name Crystal violet These dyes have intense colours but these colours fade rapidly in light, so these are not useful in textile industry

33. Phthalein Dyes The term phthalein is used to represent the dyes formed from phthalic anhydride and phenols; in presence of dehydrating agents like fused ZnCl 2 or conc.H 2 SO 4 . The characteristic chromophoric group present in these dyes is triphenyl-methane structure. For example, Phenolphthalein

34. Preparation of Phenolphthalein Phenolphthalein is formed from phthalic anhydride and phenols; in presence of dehydrating agents like fused ZnCl 2 or conc.H 2 SO 4 .

35. When phenol (2 equivalents) is heated with phthalic anhydride in presence of ZnCl 2 or con.H 2 SO 4 at 120 o C, undergoes condensation; to form Phenolphthalein. Phenolphthalein is not a dye

36. It is used as an acid-base indicator than a dye. In alkali it gives a pink colour and in acid it is colourless . Phenolphthalein is a colourless crystalline solid Phenolphthalein is insoluble in water but soluble in alcohol and alkali; to form deep red solutions.

37. Preparation of Indigo by Heumann’s Synthesis (in 1896) Indigoid dyes contain the group as a carbonyl chromophore. Indigo is obtained naturally from plants of indigofera group. Structure

38. When anthranilic acid and chloroacetic acid (Cl-CH 2 COOH) is undergoes condensation; to form N- phenylglycine -o-carboxylic acid , which is fused with sodium hydroxide ( NaOH or KOH) and sodamide ; to form unstable indoxylic acid , which undergoes further decarboxylation (-CO 2 ); to form indoxyl . Oxidation of indoxyl (2 molecules) by air; to form Indigo dye . Indigo is as a dye for cotton yarn, which is mainly for the production of denim cloth for blue jeans. Preparation of Indigo

39. Indigo is a good example of a vat dye (on the basis of mode of application) .

40. Indigo is a dark-blue Crystalline compound, Insoluble in water Indigo is as a dye for cotton yarn, Which is mainly for the production Of denim cloth for blue jeans. It is used for dyeing cotton By vat process.