- 1 Introduction
Natural dyes are dyes which are derived from plants, dried bodies of insects or minerals, mostly they are extracted from several parts of natural plants like roots, stems, leaves, flowers, and fruits etc. The discovery of man-made synthetic dyes in the mid-19th century triggered a long decline in the large-scale market for natural dyes.
Earlier synthetic dyes which were produced on a large scale got replaced by natural dyes for commercial textile production enabled by the revolution of industries. The above article demonstrates the extraction, evolution, fastness properties, advantages and disadvantages of natural dyes.
A dye is a colored substance that has an affinity for the substrate to which it is being applied. Dyes are applied in an aqueous solution and may require a mordant to improve the fastness of the dye on the fibre. Natural dyes can be defined as those colors which are extracted from a combination of vegetable, mineral and insect sources. They are simply dyed substances extracted from natural sources.
The majority of natural dyes are vegetable dyes from plant sources roots, berries, bark, leaves, and wood and other organic sources such as fungi and lichens. There are more than 500 plant species identified as sources of dyes. Natural dyes are mostly non-substantive and must be applied on textiles with the help of mordant, usually a metallic salt, having an affinity for both the coloring matter and the fibre.
Most of the natural dyes have no substantively on cellulose or other textile fibres without the use of a mordant. To generate an affinity between the fibre and molecules of natural dyes they need the mordanting chemical.
Aluminum sulfate or other metallic mordant anchored to any fibre; chemically combine with certain mordant able functional groups present in the natural dyes and bound by coordinated/covalent bonds or hydrogen bonds and other interactional forces. Thus, for proper fixation of natural dyes on any textile fibre, mordanting is essential in most of the cases.
As compared to synthetic dyes, natural dyes are mostly biodegradable, eco-friendly and less allergic in nature. Research studies resulted that certain natural dyes have mutagenic effects e.g. safflower yellow; many mutagens leads to cancer; other like carmine can cause asthma by inhaling continuously. But it can be said that most of the natural dyes are safe and some even have curative effects e.g., curcumin in turmeric has antibacterial properties.
Even though natural dyes are having several advantages as compared to synthetic dyes, the use of natural dyes is very limited because of unavailability of standard shade cards and standard application procedures on textile materials. Natural dyes don’t have substantivity towards textile material and hence it becomes difficult to apply natural dyes. In order to overcome this problem natural dyes are used in conjunction with mordants.
A mordant which consists of metallic salts is a chemical which gets fixed itself on the textile fibre and it also combines with the dyestuff. It forms a bond between the dye and the fibre, which allows certain dyes with little or no affinity for the fibre which is to be fixed.
Importance of Natural Dyes
1. Evolution of Natural Dyes
Natural dyes have been used in most of the ancient civilizations of the world, like India, China, Mesopotamia, Egypt, Greece, Aztec, and others. The use of natural dyes for aesthetic and other purposes is at least 15000 years old which was known from the discovery of red ochre in ancient burial sites.
The art of dyeing cloth is believed to have been known since 3000 BC in China and 2500 BC in India. In China, dyeing with plants, barks, and insects has been traced back more than 5000 years.
The written record of the use of the use of natural dye was found in China dated in 2600 BC. At relatively the same period (2000 BC), dyeing of cloth in yellow, red, Blue, and green was also practiced in Egypt.
The oldest and most widely used natural dye is Indigo, since last 4000 years, it has been known in India. From the bronze age, a blue dye known as the ‘woad’ has been used in northern European countries like Denmark, Estonia, Finland, Ireland, and UK etc.
Another ancient dye, the Tyrian purple derived from the Mediterranean shell fish of the genera Purpura, and Murex was probably the most expensive dye stuff in history. Tyrian purple may first have been used by the ancient Phoenicians as early as 1570 BC.
The production of this dye was tightly controlled in Byzantium and was subsidized by The Phoenician towns of Tyre and Sidon were the centers of this dye industry in about 800 BC, and the Greek dye factories that produced purple existed all along the Mediterranean coasts.
The Tyrian purple was so precious that an extract of it was often dyed over the purple made from a Lichen genus Rocella. Gradually, the use of Murex died out, until the Lichen alone provided the purple dye.
2. Natural Dyes And Their Extraction
- Natural Indigo:
Indigo is among oldest natural dye used for textile dyeing and printing. Indigo is about 2-3 feet long plant, cultivated on thousands of acres of the land in India. The whole plant is used for extraction of dye and the extract form of the dye is supplied in the powder form.
Lac is probably most ancient of all Animal Dyes. The dye is taken from insect Kerria Lacca. This insert is cultivated on kusumbh and Tesu trees. The principle coloring component in this dye is identified as Laccaic Acid. The dye shade exhibit very good fastness to washing, rubbing, and light.
Madder is one of the oldest Natural Dyes. It requires sub tropical climate and moist soil for the cultivation of madder. They are cultivated in the feet of Himalayas in large quantity.
It is most largely consumed fruit in Asia. The is extracted from the ring of pomegranate fruit. The color which is obtained from pomegranate has good rubbing, washing and light fastness. Pomegranate is also used as mordant and is over dyed with other natural dyes to improve the fastness properties.
Mostly natural dyes are obtained from various parts of plants like stems, flowers, roots etc. They are also extracted from minerals and dried bodies of insects. Some plants may have more than one color depending on which plant which is used for extraction.
The extraction method of vegetable natural dyes basically depends on the method in which the dye is extracted. There are mainly four methods used in the extraction of natural dyes.
- Aqueous method: The dyestuff is boiled at the temperature of 100°C in soft water and the dye solution obtained after boiling is filtered and the optical density is recorded.
- Alkaline method: Initially 1 % alkaline solution is prepared by adding sodium carbonate or sodium hydroxide in water. The material is entered in it and boiled at 100°C. The dye solution is filtered and optical density is recorded.
- Acidic method: Prepare 1% of an acidic solution by adding HCL in soft water. Enter the dye material and boil it at 100°C. Filter the dye solution and record the optical density.
- Alcoholic method: By adding an equal amount of alcohol and water the alcoholic solution is prepared. The dye material is entered and boiling is carried at 100°C and the dye solution is filtered.
The extraction can be carried out in the aqueous, acid or alkaline medium. At present in India, mostly small-scale producers and manufactures of natural dye are using these methods. Even the local dyers are using more crude methods for extraction using metallic flax and crude process in a refined way using blender condenser, distillation plant and drier, and crystallization unit with the capacity of 300 tons per year.
The modern techniques of extraction are carried out with the use of extraction plant, reverse osmosis process, and the latest is supercritical fluid extraction method. This method is very common in developed countries.
A dye comprises two principal groups: the chromophore and auxochromes. Chromophore (usually an aromatic ring) is associated with the coloring property and has unsaturated bonds, whose numbers decide the intensity of color. The dye molecule gets combined with the substrate by auxochrome and hence it leads to impart color to latter. Some animal fibres like wool and silk can be dyed by dipping in the dye solution, but another fibre like cotton requires mordanting. Mordants are the substance which is used to fix a dye to the fibres. Mordants also improve the take-up the quality of the fabric and help to improve color and light fastness. Natural dyes require an element to create a bonding between the fabric and the dye particle. The mordant is known as the element which aids the chemical reaction that takes place between the dye and the fibre so that the dye is absorbed. It is a chemical treatment of fabric with metallic salts or other complex forming agents due to which the natural mordantable dyes are bound onto the textile fibre. Mordanting can be achieved by either pre-mordanting, simultaneously mordanting and post-mordanting.There are three types of mordants:
- Metallic mordants- Metal salts of aluminum, chromium, iron, copper, and tin
- Tannins – Myrobalan and sumac
- Oil mordants- Turkey red oil
4. Fastness Properties of Natural Dyes
1) Color fastness:
Color fastness is the resistance of a material to change in any of its color characteristics or extent of transfer of its colorants to adjacent white materials in touch or both for different environmental and use conditions or treatments like washing, dry cleaning etc. or exposure to different agency heat, light etc.
It is a property of colorant which allows it to retain its different characteristics despite degradation conditions such as exposure to light and dry cleaning. The color fastness is usually rated either by loss of depth of color in original sample or it is also expressed by staining scale.
The color fastness properties of natural dyes do not depend only on the type of natural colorants and its chemical nature, it also depends on the type of mordants used during the mordanting process.
2) Wash fastness
Natural dyes show moderate wash fastness on wool, as assessed by the ISO II test. Logwood and indigo dyes are found to be much faster. Indigo and logwood were having superior fastness properties to that of native natural dyeings, such as Persian berries and water lily root which was achieved with the result of ISO 11 test.
3) Light Fastness
Light fastness of many natural dyes, particularly which are extracted from flower petals are found to be poor to medium. Most of the natural dyes have poor light stability as compared to the best available synthetic dyes, and the color in museum textile are often different from their original color.
4) Rubbing Fastness
In general rubbing fastness of most of the natural dyes is found to be moderate to good and does not require any after treatments. Rubbing fastness depends on nature of color and depth of shade.
Advantages of Natural Dyes
- Natural dyes are biodegradable, non-toxic and non-allergic. They do not cause any health hazards and hence they can be used easily without many environmental concerns.
- The shades produced by natural dyes/colorants are usually soft, lustrous and soothing to the human eye.
- Natural dyes are easy to extract the colors by boiling the plants, leaves, bark or flower heads in water.
- Natural dyes have the subtle beauty that cannot be recreated by synthetic dyes.
- Natural dyestuff can produce a wide range of colors by mix and match system.
- Higher UV absorption is exhibited from the fabrics which are dyed with natural dyes.
- Many natural dyes have antimicrobial properties, they are moth proof and can replace synthetic dyes in kids garment and food stuff for safety.
- Natural dyes are free from carcinogenic components.
- Clothes dyed with natural dyes yield higher profits to dyers and manufacturers.
- Natural dyes do not contain chemicals that are harmful to health and more reliable than the chemical synthetic dyes.
Disadvantages of Natural Dyes
- It is difficult to reproduce shades by using natural dyes/colorants, as these agro products vary from one crop season to another crop season, the place to place and species to species, maturity period etc.
- In case of natural dyes mordant is required to fix the dye in the fabric.
- It is difficult to standardize a recipe for the use of natural dyes, as the natural dyeing process and its color development depends not only on color component but also on Materials.
- Natural dyes are very expensive and color and light fastness are low.
- The dyed textile may change color when exposed to the sun, sweat, and air.
- Traditionally yield and concentration are lower than synthetic dyes.
- Skilled specialists are required to produce natural dyes.
- It requires vast areas of land for its production.
- The time consumed for dyeing with natural dyes is more.
Application of Natural Dyes
- Natural dyes are most widely used for the colouring of textile materials, food substrate, and leather etc.
- The indigo leaves are used to make hair dye as well as prepare medicated hair oil. Leaf powder is used as the natural black color dye for hair.
- Natural dyes are commonly used in the cosmetic industry due to side effects, UV protection, and anti-aging properties.
- Natural dyes are also used as pH indicators.
- Natural dyes are used for food coloration.
This article on “Natural Dyes and its Importance reveals the overview of Evolution, Extraction, fastness properties, advantages, limitations and applications of natural dyes. Natural dyes have a wide variety of application in textile as well as other sectors.
Companies produce natural dyes in India and abroad. Since the last decade, application of natural colorants to cotton, silk, wool, and jute has been gaining popularity all over the world, possibly due to the ban on some synthetic azo dyes which are found to be carcinogenic or allergenic.
In light of above there is still great scope for further research and development to maximize the benefits of natural dye systems; even so, the development and use of natural dyes, has to rate as one of the most important achievements of the color chemists.