Natural Dyes: The Development and Application

natural dyes

Natural dyes refer to pigment substances extracted from animals and plants or obtained from minerals. They have good compatibility with the environment and can reduce the harm of dyes to the human body.

Because of their biodegradability, they greatly reduce the toxicity of dyeing wastewater. The COD value of dyeing wastewater from natural dyes is much lower than that of synthetic dyes. In addition to dyeing functions, natural dyes also have multiple functions such as drugs and perfumes. Most natural dyes are traditional Chinese medicine. During the dyeing process, the medicine and fragrance components are absorbed by the fabric together with the pigment, so that the dyed fabric has a special medical health function for the human body.

Natural dyes have good environmental compatibility and medical health functions, which have attracted the attention of dye research and application institutions in many countries. Japan, India and other countries are conducting research on dyeing with natural dyes. Japan has set up the “Vegetable Dyeing” Research Institute to apply modern science and technology to research and develop natural dyes.

1. Sources and Colors of Natural Dyes

There are three main categories of natural dyes: plant pigments, animal pigments and minerals. Mineral pigments are mostly inorganic and generally insoluble in water. They are usually used as pigments, and only a few can be used as dyes. Most natural dyes come from plants, mainly the roots, stems, leaves, flowers and fruits of plants, and a small amount come from animals, such as lac insects and cochineal insects. The colors of natural dyes are mostly yellow and red, and blue, green and black are less.

1.1 Natural red dyes

Most of the red pigments are found in plant roots, tree bark, or dark gray insects. Although the source of red pigment is limited, it exists in a large amount in plants and is easy to extract. Carmine is the most beautiful red pigment.

It was first discovered by Africans that madder can be dyed. They found that madder is not only delicious, but its roots can dye the lips red, which is difficult to wash off. Therefore, madder became one of the earliest cosmetics. There are two main sources of madder, Indian madder and English madder. The root growth period of madder used to extract dyes should be two or three years, when the root diameter is about 060m and the length is about 60m. The roots that meet the age of growth are dug out, washed, dried, and then put in a thin cloth bag and boiled with water. The red dye will be precipitated from the roots. Generally, 2% of dye can be obtained from dry roots.

The pigment extracted from madder is called alizarin, and its main component is 1,2-dihydroxyanthraquinone. It can be dyed directly with madder. Add madder root to 30℃ warm water, then add the pre-mordanted wool fabric dye solution and slowly increase the temperature to 100℃. After dyeing for 1-1.5h, the temperature will drop to 90℃ immediately, Dye at 90°C for half an hour, stirring regularly. When the required shade is obtained, the wool fabric is cooled in the dyeing solution, rinsed in warm water and clean water, dehydrated and dried. About 120g wool fabric needs 60g madder and 4.5L water. On cotton fabric, madder is complexed with mordant to form an insoluble metal complex dye, which is dyed bright red, commonly known as Turkish red.

Shellac is a resinous protective secretion secreted by shellac insects. Lac insects are parasites that grow on a large number of wild or species plants. The viscous shellac is dissolved in water and sodium carbonate solution, and then precipitated with lime to obtain shellac dye, which is bright red and scarlet. In the production of water-soluble dye shellac, the useless shellac acid should be removed. The red dye extracted is often dusty. Therefore, it must be carefully filtered and removed before dyeing to prevent dust from being stained on the dyed fabric during the dyeing process. When dyeing, the dye solution is boiled first, and then the cotton skein is immersed in it for dyeing until the desired color is obtained. When dyeing cloth, the process is the same, but the color is usually darker. If the cloth is boiled in capers leaves before dyeing, the color fastness will be better.

There are many natural red dyes, such as the monascus pigment secreted by the monascus fungus, which is the pigment produced by the monascus fungus, which can dye silk into a beautiful deep red in an acid dye bath below the isoelectric point. In addition, the beetatin in red beets produced by yeast, as well as many red flowers (henna) can extract red pigment.

1.2  Natural Yellow Dyes

The number of plants containing natural yellow pigments is much greater than that of other colors, and most of them are flavonoids. Some flavonoids also have physiological activities that are beneficial to human health, and some have ultraviolet absorption characteristics and antioxidant properties, and their use value needs to be further explored.

Turmeric dye is a derivative of turmeric, and it is one of the most famous and brightest yellow dyes from natural sources. Turmeric dye is extracted from the new or dried roots of turmeric. The dried roots are boiled in water for 45 minutes, and the dye starts to precipitate. The extract is filtered and it can be used for dyeing. The mordant process is carried out in advance, and the liquor ratio of hair is 1:30 and the liquor ratio of silk is 1:40 at 50~60℃ for 3min. After the mordant dyeing, the fabric samples are cooled and squeezed out evenly. They are directly put into the dyeing solution without washing or drying, and dyed in a boiling state for 45 minutes, and the liquor ratio is 1:40. The dyed sample is cooled in the dye bath, then washed, soaped, washed and dried.

Riboflavin produced by yeast is a microbial yellow pigment for dyeing, with high safety and bright color.

1.3 Natural Blue Dyes

Since ancient times, indigo has been one of the most important and commonly used blue dyes. It is extracted from the leaves of a plant that grows in Asia, Africa, the Philippines, and the United States. The process of extracting indigo from indigo plants is very simple, mainly including the following three processes:

(1) soaking the plant in water for fermentation;

(2) separating the extract and oxidizing in the air;

(3) finally precipitating the dye It is separated and made into a marketable cake or powdered indigo.

It mainly exists in plants in the form of glucoside. It can be fully hydrolyzed under the action of dilute acid, alkali or yeast. The hydrolyzed product is indoxyl and sugar. Doxyphene can be oxidized again to indigo. In the fermentation process, the indigo plant is first decomposed into glucose and then converted into lactic acid, which in turn is converted into butyric acid, carbon dioxide and hydrogen. The hydrogen released in the solution reduces indigo to indigo white. Indigo white forms a yellow-green solution in the presence of alkaline solution for dyeing. As soon as it is exposed to oxygen in the air, the indigo is reconverted into the original insoluble form in the fiber, so the indigo dye has good washing fastness.

Pine blue is an important pigment plant in China, and its leaves contain indigo, which can extract blue pigment; it is also a commonly used medicinal plant. Its root medicinal material is named “Banlangen” and its leaves are “Daqingye”. Pine blue is mainly used for dyeing cotton yarn or cotton cloth, but also for dyeing wool and silk. Its coloring theory is the same as that of indigo.

Indigo dye is the most common natural dyeing method for us to know. In China, indigo dye is one of the most popular natural dyeing way in our history. You still can see many indigo dye clothing in garment shops.

Indigo dye also has a long history all of the world and under the high temperature of sustainable fashion, indigo dye pump up in our eyes again. 

indigo dye

1.4 Natural Dyes of Other Colors

Catechu brown dye is extracted from a kind of wood called catechu, which grows mainly in places such as India. Catechin brown dye mainly contains two coloring components, catechin tannins and catechins. The former is soluble in cold water, and the latter is soluble in hot water. According to the difference of the solubility of the two, catechin tannin and catechin can be separated from catechin brown dye. In catechu palm, the proportion of catechin can be as high as 17%. At present, catechin brown dye is mainly used to increase the weight of cotton and silk.

Hematoxylin is a relatively common black natural dye, which is extracted from the wood of Hematoxylin. The wood that has just been chopped down is not black and will change color when it is oxidized by the air. Black dye is found in the bark of cypress bark and the bark of the fruit of tomato branch Acacia Arabian.

The green natural dye elderberry is extracted from a shrub that grows in the Scottish Highlands. It is extracted from leaves in spring and berries in autumn. The berries are purple at the beginning. After soaking in ammonia and soda water, they become beautiful apple green. After soaking for 24 hours, the aluminum mordant dyed hair can be dyed without heating, as long as it is soaked for 24 hours. Natural sources of green dyes are also plants such as lily of the valley and kudzu.

Orange natural dyes can be extracted from plants, such as the flowers of dahlias; or from mineral sources, such as a red clay.

2. The Dyeing Method of Natural Dyes

Due to the complex molecular structure of natural dyes and a wide variety of types, the dyeing mechanism of fibers varies from type to type, and the dyeing methods are also quite different. For protein fibers and cellulose fibers, the main dyeing methods are The mordant dyeing method, dyeing before mordant method, dyeing after mordant method; for synthetic fibers, it is mainly divided into normal pressure dyeing and high temperature and high pressure dyeing. Generally speaking, the dyeability of cotton and hemp is worse than that of silk and wool.

2.1 Direct Dyeing Method

Some natural pigments of plant dyes have good solubility in water, and the dye solution can be directly adsorbed on the fiber, so direct dyeing method, such as gardenia, can be used. Boil the gardenia fruit with water for 60 min, and extract twice. The extract solution is at 40~45℃, adjusted with acetic acid to adjust the pH value to about 5, put into cotton fabric and dyed for 10min, and then washed with water, it can get grayish yellow. Hydrophilic natural dyes such as carminic acid have good water solubility and cationic chemical structure, which can dye silk directly.

2.2 Mordant Dyeing

Some natural plant dyes have good solubility in water, and the dye solution can also be directly absorbed on the fiber, but the dyeing fastness is not good, so it needs to be dyed by mordant dyeing. The process of mordant dyeing is generally divided into methods such as first dyeing, then mordant, first mording, or first mordant, then dyeing and then mordant. The dyeing steps of the mordant method after dyeing are as follows:

(1) Preparation of dye liquor. Cut the bark, trunk roots and leaves of the plant into small pieces, put them in a container with water or ethanol, and extract the dye solution.

(2) for dyeing. The extracted dye liquor is heated and immersed in the yarn or fabric for boiling dyeing for 20~30mn.

(3) Mordant dyeing. The dyed yarn or fabric is immersed in a solution containing a mordant for 30-40mn. The mordant is used for the color and luster required, and chemicals containing aluminum, copper, iron or tin are used.

(4) Wash and dry with water. For the madder itself, the directness to the fiber is very low, but the molecules with complex coordination groups can be mordant first to make the fiber adsorb the metal ions, and then the dye and the metal ions are fixed on the fiber by complex bonding. The dyeing sequence is: mordant dyeing → washing → dyeing → washing → drying. Whether it is dyed first and then mordant or first mordant and then dyed, the dyeing can be repeated several times as needed.

2.3 Vat Dyeing

Natural dyes such as indigo, which are not soluble in water, need to be reduced to soluble leuco indigo in an alkaline solution with a reducing agent. After being dissolved in water, the dye molecules are adsorbed on the surface of the fiber and dyed. The fiber is then ventilated and oxidized to the fabric, and the blue leuco body becomes insoluble indigo again and is fixed on the fabric. The dyeing process is similar to that of vat dyes.

2.4 Synthetic Fiber Dyeing

For synthetic fibers, normal pressure dyeing and high temperature and high pressure dyeing can be used, and the dyeing process depends on the nature of the dye. Natural dyes such as madder, comfrey and rhubarb have anthraquinone or naphthoquinone in the pigment structure, which is very similar to the structure of disperse dyes. Their relative molecular mass is very small and they are hydrophobic. Using these dyes to dye polyester fibers, the adsorption isotherm is in line with the Nernst isotherm of disperse dyes dyeing polyester, indicating that the dyeing mechanism of these natural dyes is similar to that of disperse dyes.

Polyamide fibers contain amino and carboxyl groups, which can be dyed with ionic dyes, such as acid and metal complex dyes, and their adsorption mechanism is Langmuir mechanism; at the same time, as hydrophobic fibers, they can also be dyed with disperse dyes. When dyeing with naphthoquinone natural dyes such as comfrey and juglone, the adsorption mechanism is consistent with the Nernst isotherm when dyeing polyester with disperse dyes; when dyeing with annatto, because its pigment is linear ionic molecules, it is applied when More than one mechanism appears, but the Langmuir mechanism is dominant.

The berberine contained in the natural dye Phellodendron amurense is a cationic dye. This dye grows on the roots of plants of the genus Berberis and has a bright fluorescent yellow color. Berberine is an alkaloid whose structure is similar to cationic dyes and can be used to dye acrylonitrile fibers. Thermodynamic research results show that the dyeing mechanism conforms to the Langmuir adsorption isotherm, indicating that positively charged dyes can form ionic bonds with negatively charged fibers, so that the dyes can be adsorbed on the fibers.

3. Extraction of Natural Dyes

The industrial production of natural dyes can be leached by aqueous solution. After leaching, continuous decanting, continuous centrifugal precipitation and other treatments are used to separate the liquid from the solid, and the pure suspended particles larger than 5μm pass through the non-woven fabric bubbles. The filter or reverse osmosis system is removed, and then the dye is precipitated, squeezed with a filter or separated by an ion machine, and dried in a vacuum at a low temperature. This can be ground into powder dyes with a particle size of less than 200 meshes to obtain standardized natural dyes. In order to ensure the reproducibility of the results, the raw material should be clear of the humidity, ash content, water or alkali extract and absorption spectrum, and the raw material should be ground into a powder of 50-100 mesh, and through violent expansion, the extraction process uses water without metal impurities (hardness) Less than 50mg/kg) in a corrosion-resistant stainless steel container. Ethanol extraction can also be used for dyes that are difficult to dissolve in water. In addition, new technologies such as ultrasonic-assisted extraction, supercritical extraction, and enzymatic extraction are also being applied here.

The characteristics of various dyes are usually described by water-soluble ash content, humidity, pH value, spectral intensity and dye intensity (through dyeing test) to standardize them.

4. Problems in the Application of Natural Dyes

Natural dyes are conducive to environmental protection, but because natural dyes are mostly derived from animals and plants, the pigment content is small. To obtain enough dyes, a large number of plants need to be harvested or cut down, or animals are hunted. This will cause damage to the ecological environment. Therefore, how to reasonably develop and produce natural dyes is an important topic in this field, such as the artificial cultivation of plant dyes, the production and application of microbial pigments.

At present, the structure of natural dyes is not very clear, and the extraction process is backward, which makes it difficult to carry out standardized production and is not conducive to large-scale application.

In terms of application, with the exception of a few, natural dyes generally have the problems of low dye uptake and poor color fastness. Even if mordants are used, many still fail to meet the requirements, especially the light fastness and soap fastness. Such as the yellow dyed by natural dyes, the light fastness is only 3-4. In addition, traditional dyeing methods with natural dyes still have problems such as low color yield and long dyeing time. Most natural dyes use mordants when dyeing. Most traditional mordants contain heavy metal ions, and many of them are included in the list of banned ecological textiles. Therefore, it is necessary to improve the traditional dyeing methods and develop new mordants to replace the traditional mordants containing heavy metal ions. For example, researchers use rare earth-citric acid complexes as mordants to dye ramie fibers.

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