NATURAL DYES:-

Natural Dyes, their properties and classification:-

Description of Natural Dyes.

Natural dyes and their use in dyeing is probably the most ancient art of all times. People started using natural dye as their first intellectual tool to portray their surroundings and themselves by this art. Though natural dyes have been used to color textiles from ancient times, but in the 19th century, synthetic dyes replaced natural dyes after discovery of mauve colorant. Natural dye is defined as any dye, pigment, or any other substance derived from natural sources such as plants, animals and minerals are renewable and sustainable bioresource products with minimum environmental impact. They have been known since antiquity for their use in coloring of textiles, food substrate, natural protein fibers like wool, silk and cotton, and leather as well as food ingredients and cosmetics.

Fig of Natural Dyes ref by printerest.

The term ‘natural dye’ is conventionally taken to apply to those products that are derived from plant sources, although dyes derived from insects, such as cochineal, kermes and lac, are also included. The natural dyes are complex mixtures of components derived extracted from natural substances such as plants, animals or minerals. A large number of herbs, shrubs, trees, insects, animals, microbes and minerals have been identified for extraction of coloring compounds. Red, yellow, brown, blue, black, green and orange color can be obtained from natural dyes.

Synthetic dyes are heavily used in textiles and industries while they have many problems like waste disposal. Other problems include they are not biodegradable, have water pollution, environment unfriendly, and carcinogenic. This situation leads to choose natural dyes as a reasonable replacement or solution, although we know that they are not successful in commercial. Natural dyes are biodegradable, fairly less harmful, ecofriendly, produces highly attractive and smooth shades. Natural colorants come from plants or animals, used in industries that are producing different products of human use. Natural dyes not only are biodegradable and biocompatible colorants but also give enormous functional properties to the material that is to be consumed.

Color fastness of natural dyes to washing and light is in general inferior to well-selected and applied synthetic dyes, and normally does not meet consumer demands.

The term ‘natural’ and ‘safe’ are not synonymous; there are many naturally occurring substances, e.g. arsenic and asbestos, which are harmful. Natural dyes are derived from plant, animal or mineral sources, and while some dyes are quite safe to use, some are hazardous. For example, indigo and logwood are skin and respiratory irritants, and plants such as lily of the valley and bloodroot are toxic.

Handling of natural dye and its dyeing process needs scientific interventions at four main steps:

1)Extraction

Isolation of dye molecule

2) Dyeing

3)Dye fixing

Applications of natural dyed textile can be extended to a diverse field such as sportswear and medicinal field due to its various functional properties such as antimicrobial as well as UV protection.

Sources of Natural Dyes:-

Natural dyes are basically elements of natural resources, and these dyes are generally classified as plant, animal, mineral, and microbial dyes based on their source of origin, although plants are the major sources of natural dyes. Natural dyes are sustainable as they are renewable and biodegradable.

Fig: Sources of Natural Dyes ref by printerest.

Natural dye can be classified in following groups on the basis of sources:

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. Chitosan, Serratia spp., Trichoderma virens and Alternaria alternata were used to obtained dyes. Natural Red color is produced by Monascus anka and also from fungus Echinodontium tinctorium. Phycocyanin is blue pigment extracted from Spirulina plarensis algae.

Properties/Characteristics of Natural Dyes:

1) Natural dyes are organic and are derived or extracted from resources found in nature. Natural dyes are mostly eco-friendly, biodegradable, less toxic, and less allergenic as compared to synthetic dyes. However, studies have shown that certain natural dyes may have detectable mutagenic effects e.g., elderberry color and safflower yellow; others, like carmine, can cause asthma by continuous inhalation.

2) Natural dyes are safer from synthetic dyes and since it does not contain harmful chemicals there is a very low possibility for causing skin allergies. Mostly all plant dyes are safe to be worn.

4)Natural dyes are bio-degradable; therefore, it does not harm the water ecosystem when drained into water bodies.

5) Natural dye tends to fade easily, so for making the dyes stick to the fabric dye fixative are used, this can be in the form of starch, seaweed, Alum (hydrated double sulfate salt), table salt, vinegar.

Fabrics dyed with natural dye are more delicate and have to be handled with care, it should not be dried directly in sunlight.

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.

A. Classification on the basis of their chemical constitution:-

1. Indigoid dyes:

 This group includes Indigo and Tyrian purple dye. Indigo is extracted from Indigofera tinctoria and considered the most primitive dye. Woad plant (Isatis tinctoria) also has indigo as the chief blue dyeing component.

2. Anthraquinone dyes:-

 Most of the red natural dyes from both plant and mineral origin are based on the anthraquinoid structure. Madder, Lacs, Cochineal are some examples of this group. Alizarin and purpurin are the main chromophores in Rubia tinctorum.

3. Alpha naphthoquinones:- Lawsone (henna) is a most important member of this class. Another dye is juglone, isolated from the shells of unripe walnuts.

4. Flavonouds: Yellow dyes obtained from this group and can be classified under flavones, isoflavones, aurones and chalcones. These yellows are found in a variety of plants, including Persian berries (Rhamnus spp.), young fustic (Cotinus coggygria), old fustic (Chlorophora tinctoria) and yellow wood (Solidago virgaurea).

5. Di-hydrropyrans:

In chemical structure, di-hydropyrans are similar to the flavones. These natural dyes give dark shades on cotton, wool and silk. Logwood and Sappan-wood are the most common examples.

6. Anthocyanidins:

Orange dye carajurin obtained from leaves of Bignonia chica. Carajurin is a chemical member of this class.

7. Carotenoids:

The class name carotene is derived from the orange pigment found in carrots. In these, the color is due to the presence of long conjugated double bonds. Usually, red, orange and yellow colors come in this category and can be obtained from different plants, e.g. yellow, orange color in sunflower.

Classification of Natural dyes:-

Fig: Classification of natural dyes ref by printerest.

B. Classification based on method of application/preparation:

1. Direct Dyed:-

Direct dye soluble in water can be taken up directly by the material. Direct dye also called substantive dyes because of their excellent substantivity for cellulosic material like cotton and viscose rayon. Turmeric, Chebulic myrobalan and Annatto used in direct dyes.

2. Vat Dyes:

As the name suggests that the dye is prepared in a large container for storing and mixing liquids or wooden vessels commonly known as ‘Vat’. This is a primitive method of dye preparation.

3. Mordant Dyes:

Mordant dyes are attached to textile fibers by a fixing agent “mordant” which can be organic or inorganic substance. Since chromium is used extensively hence, mordant dyes are sometimes called chrome dyes.

4. Acid Dyed:

These dyes performed in acidic medium. Sulfonic or Carboxylic groups of dye molecules can form electrovalent bonds with amino groups of wool and silk.

5. Basic Dyes:

These dyes form an electrovalent bond with the carboxylic group of wool and silk. Berberine has been classified as basic dye.

6. Dusperse Dyes:

 Disperse dye have low aqueous solubility and low molecular weight. These dyes require post mordanting treat6ment with chromium, copper or tin salt.

Availablity of Natural Dyes:

The use of natural dyes in textiles was eliminated since synthetic dyes give variety of reproducible shades and colors. Natural dyes have limited availability. They are subject to the growing seasons of the plants or the lifecycles of the insects from which they are derived. Yields are low in relation to the ground used, and there may be competition with foodstuff production. There is just not enough natural dye production to support commercial demands on a large scale. Synthetic dyes, however, are produced continually on demand.

Natural dyes are used in small-scale craft industries, where the quantity of dyes required is low. Because of the limitations of producing these dyes on a commercial scale, regardless of any technical restraints, they will not replace synthetic dyes for the global mass production of dyed textiles.

Production of Natural Dyes:

Natural dyes are obtained from vegetable, animal and mineral sources. Production flow chart of natural dyes are described below.

Flow chart for the production of natural dyes:

Raw Material

↓

Size Reduction

↓

Batch Extraction

↓

Phase Separation

↓

Fine Filtration

↓

Evaporation

↓

Spray Drying

↓

Finishing

↓

Packing

Raw Material:-

The raw material for the production of natural dye is mostly vegetable matter such as seeds, leaves, roots, bark, heart wood, etc., of the plants. The most important part of the production of natural dyes is the sourcing of the raw material. The raw material selected for the extraction of the coloring matter should be easily available at reasonable price and in large quantity throughout the year.

For the procurement of the raw materials, proper specifications in terms of moisture content, ash content, water or alkali, extractable matter, and the absorption spectra must be specified so as to ensure reproducible results.

Size reduction:

The raw materials are required to be ground to powder form so as to facilitate the extraction. The powder should have a fine and uniform particle size distribution. If the particle size becomes very fine then there are chances of swelling and lump formation. Hence for the proper extraction, optimum size between 50 and 100 mesh is required to be adjusted for each raw material depending on its swelling characteristics. Excess heating of the raw material during grinding should be avoided since these raw materials are sensitive to heat.

Batch extraction:

The batch extraction of the coloring component must be carried out in noncorrosive stainless steel (SS) vessel. The coloring component can form metal complexes with the material of construction of the extractor. Depending on the batch size and the material to liquor ratio, suitable steam jacketed vessel with low speed stutter is recommended to be used for extraction of the coloring matter. It is important that the water used for extraction should be free from metallic impurities. Hence, soft water with less the 50 ppm hardness is recommended for the extraction of the coloring matter. Other important factors for the proper and reproducible results are the time, temperature, and pH of the water used for extraction. This varies from color to color.

Phase separation:

After extraction, the solids and the liquor are required to be separated. This can be done in various ways such as, by continuous decanting, continuous centrifugation, etc. During this process, the solid waste generated after extraction is separated and disposed off.

Fine filteration:

For removing the fine particle, >5 microns, a sparker filter with a now-woven filter cloth is recommended. One also uses the reverse osmosis system for eliminating the fine suspended particles.

Precipitation/concentration/drying:

The precipitation of the dye from the extract is brought about with change in pH from alkaline to acidic pH. The precipitated dye is filtered through the filter press or separated from the liquor by centrifugation. The precipitated dye is dried under vacuum at the low temperature. In some cases, the dye liquor is concentrated under vacuum to a solid content of approximately 30%, and spray dried. The dye powder thus obtained is ground to 200 mesh particle size and formulated to get the standardized natural dye.

Characterization:

Every lot of the dye must be characterized in terms of water soluble matter, ash content, moisture, pH and spectra intensity as well as the strength of the dye (by the dyeing trial).

Benefits or Advantages of Natural Dyes:

Natural dyes are recommended to be applied on textile materials. The natural dyes have several advantages such as: these dyes need no special care , wonderful and rich in tones, act as health cure, have no disposal problems, have no carcinogenic effect ,easily biodegradable, require simple dye house to apply on matrix and mild reactions conditions are involved in their extraction and application. Following points support the use of natural dyes on a large scale.

Eco-friendly: Natural dyes are extracted from natural sources therefore they are environment safe.

Biodegradable:

 These dyes are capable of being decomposed by microorganisms.

Renewable: Replaced by the new material obtained from nature.

No health hazard/Non-toxic:

 Natural origin of these dyes makes them harmless.

Variety of shades:

Varieties of color, shades and hues present in nature itself.

Soothing, soft and lustrous colors:

Natural dyes are soft and relaxing.

Utilisation of waste material:

Many agriculture waste products can be used in the dyeing process.

Antibacterial/UV Protective:

Naturally dyed fabrics get special properties like antibacterial and UV protection.

Demerits or Disadvantages of Natural Dyes:

There are some limitations of natural dyes which includes, lesser availability of colors, poor color yield, complex dyeing processing, poor fastness properties and difficulty in blending dyes. As there are many advantages in using natural dye but they also have some drawbacks:

Expensive;

Natural dyes are expensive due to being limited in source.

Faded easily:

Sometimes their poor attachment on fabric makes them fade easily.

Difficult to produce/collect:

Collection is somewhat difficult in large amounts.

Time consuming:

 The complete process like collection of dye takes long time.

Reproducibility of shades is difficult to control: These 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.

Besides, the water insoluble natural dye can create problems during their application, leading to nonuniform and uneven dyeing.

Conclusion:

Although the natural dyed textiles are promising to provide functional properties such as antimicrobial, UV protection and mosquito as well as moth repellence, still the stability-related issues with natural dyes also need to be significantly addressed. Natural dyes produce an extraordinary diversity of rich and complex colors that complement each other. Dye extracts may look a little expensive at first, but they are very economical when their concentration has been taken to account. So much of work has already been done on most of the old dyes that the need of newer dye sources has emerged to keep alive the use of these dyes. Many scientists/researchers are doing a great deal of work in the area of amelioration of natural dye, and this period can be considered as the renaissance of natural dyes. The increasing knowledge of different natural sources providing beautiful colors is expanding the shade cards of different fabrics. Revamping of this ancient art has another distinctive attribute, that is, the empowerment of such tribes that used to thrive on this form of art.

Ref: https://textileleaner.net