paprika and annatto.
f. Dye-Sensitized Solar Cells
Dye-sensitized solar cell is third generation photovoltaic (solar) cell that converts any visible light into electrical energy. It is low-cost solar cell belonging to the group of thin film solar cells. Performances of dye sensitized solar cells are mainly based on dye used as a sensitizer [8]. Godibo et al., attempted the preparation of Dye Sensitized Solar Cells using flowers of Amaranthus caudatus, Bougainvillea spectabilis, Delonix regia, Nerium oleander, Spathodea companulata and a mixture of the extracts [9].
In addition to the above mentioned applications, there is a growing interest for using natural dyes to dye leather, stain wood, pulp, some plastics [10–14]. This chapter intends to discuss the application of natural dyes in textile.
1.2 What are Natural Dyes?
Natural dyes can be derived from natural sources such as plants, animals and minerals. A large number of herbs, shrubs, trees, insects, animals, microbes and minerals have been identified for extraction of coloring compounds [15]. Red, yellow, brown, blue, black, green and orange color can be obtained from natural dyes.
1.3 Why Natural Dyes?
Natural dyes are recommended to be applied on textile materials. Following points support the use of natural dyes on a large scale.
1 Eco-friendly: Natural dyes are extracted from natural sources therefore they are environment safe.
2 Biodegradable: These dyes are capable of being decomposed by microorganisms.
3 Renewable: Replaced by the new material obtained from nature.
4 No health hazard/Non-toxic: Natural origin of these dyes makes them harmless.
5 Variety of shades: Varieties of color, shades and hues present in nature itself.
6 Soothing, soft and lustrous color: Natural dyes are soft and relaxing.
7 Utilization of waste material: Many agriculture waste products can be used in the dyeing process.
8 Antibacterial/UV Protective: Naturally dyed fabrics get special properties like antibacterial and UV protection.
As there are many advantages in using natural dyes but they also have some drawbacks:
1 Expensive: Natural dyes are expensive due to being limited in source.
2 Faded easily: Sometimes their poor attachment on fabric makes them fade easily.
3 Difficult to produce/collect: Collection is somewhat difficult in large amounts.
4 Time consuming: The complete process like collection of dye takes long time.
5 Reproducibility of shades is difficult to control: Natural dyes produced by secondary metabolic activities of plants or by very special processes in other animals, which depend on climate conditions, age and seasonal variations. Thus, one particular shade cannot be achieved again and again by a single dye.
1.4 What are Synthetic Dyes?
Synthetic dyes are made by organic molecules. They are derived from coal tar hence also known as coal tar dyes. William H Perkin synthesized “Mauve” the first synthetic dye in 1856 in the United Kingdom. Then, a significant number of dyes were discovered and industries quickly adopted them to grow, mainly in the United Kingdom, Switzerland and Germany [16].
The Sudan I (Solvent Yellow 14) is one of the members of azo-dyes widely used in textile industry [17]. It is enzymatically transformed, through the action of the intestinal flora, into carcinogenic aromatic amines, when present in the bodies of animals or humans [18]. In the case of azo-dyes, especially, carcinogenicity can be produced by both the dye itself and its own converted compounds [19]. The study of National Toxicology Program confirmed the neoplastic liver nodules in rats by the presence of Sudan I dye [20]. The Basic Red 9 dye, used in the textile, leather, paper and ink industries [21], develops carcinogenic potential in humans [22], and high toxicity to environment [23]. Under anaerobic conditions, it breaks down into carcinogenic aromatic amines, and when disposed in water bodies can cause allergic dermatitis, skin irritation, and cancer [24]. According to the in vivo tests on rats, it causes local sarcomas and tumors in the liver, bladder [25], mammary glands and hematopoietic system [26].
The Crystal Violet dye, shows an intense color [27], and is a member of the cationic triphenyl methane group, and is responsible for mitotic poisoning and abnormal accumulation of metaphases [28] as well as the in vitro clastogenic effects observed in Chinese hamster ovules [29], which induce chromosomal damage too [30]. According to Bharagava et al., this powerful carcinogenic agent promotes fish tumors [28, 31] and hepatocarcinoma, reticular cell sarcoma in various organs, such as the vagina, uterus, ovary and bladder [32] as well as hardened gland adenoma and ovarian atrophy in rats. In humans, it is capable of generating respiratory and renal failure, chemical cystitis, skin irritation and digestive tract disorder [28].
Advantages/Merits of Synthetic dyes
1 Easy preparation.
2 Available in large numbers and quantities.
3 Quality of fast colors
4 Cost effective.
Disadvantages/Demerits of Synthetic dyes
1 Production on high temperature
2 Carcinogenic
3 Hazardous to human health.
4 Problem of environmental pollution.
1.5 Sources of Natural Dyes
Natural dyes can be classified in following groups on the basis of sources (Figure 1.2) [33]:
Figure 1.2 Sources of natural dyes.
1 Plants: Roots, leaves, fruits, flowers and barks can be used as a source of natural dyes. Different colors can be obtained from each part such as Sappan-wood tree pods give red, barks give brown and root gives yellow color. Many by-products of plants can also be used to form dyes.
2 Animals: Dyes can be obtained from dried body of insects for example, Lac, Cochineal and Kermes. Cochineal is a brilliant red dye produced from insects living on Cactus plants. Carmine and Tyrian purple dye derived from cochineal, shellfish (Murex spp.) respectively.
3 Minerals: Mineral dyes include iron buff, iron black, manganese bistre, chrome yellow, and Prussian blue.
4 Microorganisms: Natural colorant can be extracted from fungi, bacteria and algae that are fast growing and have the potential of being standardized commercially [34]. Chitosan, Serratia spp., Trichoderma virens and Alternaria alternata were used to obtained dyes [35]. Natural Red color is produced by Monascus anka and also from fungus Echinodontium tinctorium. Phycocyanin is blue pigment extracted from Spirulina plarensis algae.
1.6 Types of Natural Dyes
Natural dyes were classified in many ways at different time periods by researchers on the basis of chemical constitution and method of application [36].
1.6.1 Classification on the Basis of Their Chemical Constitution
1 Indigoid dyes: This group includes Indigo and Tyrian purple dye. Indigo is extracted from