studies revealed that microwave assisted extraction of Padauk (Pterocarpus sp.) wood are more efficient as compared to conventional extraction methods [54]. Natural dye from Sorghum husk extracted with the help of ultrasound-microwave-assistance [55].4 iv. Fermentation: Indigo extraction is the best example of fermentation method of extraction. In presence of indimuslin enzyme, glucoside indican breaks into glucose and indoxyl [56]. The enzymes produced by the microorganisms present in the atmosphere or those present in the natural resources used in fermentation for assisting the extraction process.
5 v. Solvent extraction: Natural dyes can also be extracted depending upon their solubility by using organic solvents such as acetone, petroleum ether, chloroform, ethanol, methanol, or a mixture of solvents such as ethanol and methanol, mixture of water with alcohol, and so on.
6 vi. Supercritical fluid extraction: Supercritical fluid extractions have become popular in recent years to isolate the organic compound from herbs and dyes as well as dye from natural sources. In supercritical fluid extraction a dense gas as a solvent that usually has carbon dioxide above its critical temperature (31 °C) and critical pressure (74 bar) for extraction is used [14].
1.10 Dyeing Process
1.10.1 Preparation of Fabric Before Dyeing
Raw textile materials are required to prepare base before processing such as dyeing and printing etc. Natural and man-made fibers contain undesirable matter like dirt or stains which are regarded as impurities. Textile materials in this “raw” state are to be “cleaned” and “finished” to make gray yarn or gray fabric.
1 a) Weighing: The weight of textile material helps us to know about the amount of soap for washing, the quality and quantity of chemicals and dye stuff to use in the mordanting and dyeing processes. Therefore, the first step is to weigh the yarn of fabric while it is still dry [57].
2 b) Soaking: Fiber or fabric is soaked for 12 h in tap water to remove the water soluble impurities.
3 c) Scouring: Fibers contain oil and fats on their surface; they are hydrophobic in nature which affects the absorbency of the fibers. The outer hydrophobic layer has to be removed before dyeing. The process by which this water resistance layer is removed from the fabric is called “Scouring”.
4 d) Bleaching: Bleaching is the process of discoloration or removal of natural and other coloring matter from fibers. This is a process of whitening fibers using oxidizing agents.
1.10.2 Mechanism of Dyeing
1 a) Adsorption: Firstly the dye molecules in the dye bath move towards the fiber and those that are nearest to the fiber get “adsorbed” on to the fiber surface. They form a very thin layer of molecules on the surface of fiber.
2 b) Penetration: Secondly the adsorbed dye molecules adhered to the outer surface of fiber gradually penetrate or infiltrate into the pores or canals of the structure.
3 c) Fixation: The final step is one where the dye molecules find suitable locations according to dye size where they get “fixed” or “anchored”.
1.10.3 Process of Dyeing
1 a) Pre-Soaking the Material: Textile stuff, whether it is fabric, yarn or loose fiber is thoroughly wet in water before dyeing begins. Such wetting is achieved by soaking for hours. A thoroughly wet textile dyes well.
2 b) Enzyme Assisted Dyeing: Enzyme assisted dyeing is also performed for textile coloration [58]. Ultrasonic dyeing on cotton and silk fabric is performed with Terminalia arjuna, Punica granatum and Rheum embodi dye. In pretreatment enzyme protease, amylase, diastase and lipase are complexed with tannic acid. Both fabrics showed rapid dye adsorption kinetics and total higher adsorption [59]. Raja and Thilagavathi demonstrated that alkaline protease enzyme process improve the quantity of natural dye exhausted [60].
3 c) Sonicator Assisted Dyeing and Plasma Treatment: Ultrasonic dyeing technique is also called Sonicator dyeing that improves the penetration of dye in fiber or fabric and increases color strength. It is a rapid dyeing process and can be run under mild conditions and low temperatures. Dyeing of wool fabrics carried out with natural dye “lac” through conventional and ultrasonic techniques [61]. In another study Eclipta leaves were taken as natural dye for cotton fabric using both conventional and sonicator methods. Results revealed that Ultra-sonication method showed higher color strength values [62]. Vankar et al. demonstrated the sonicator dyeing method to improve dye uptake on cotton, silk and wool [63]. Plasma treatment in dyeing is conducted for improving the dye uptake of fabric. It is a surface modification technique that performs before dyeing on textile materials [64]. Low-temperature plasma is widely used in non-destructive surface modification of textiles where a wide range of properties can be obtained. Plasma treatment performed on silk fabric and dyeing done with natural dye extracted from Phytolacca decandra [65]. Plasma treatment was conducted to improve the adhesion of chitosan on cotton fibers. After that the cotton is dyed with natural dye extracted from pomegranate rinds. The results exhibited that plasma treatment can enhance the color strength of the dyed sample [66].
4 d) Printing: Printing on textile in India has been a part of India’s cultural identity for thousands of years [67]. Printing produces more colorful effect on the fabric. Printing is a process where colorful designs are created which can be done by hand or machine. Hand printing is done by two methods viz., block printing and screen printing. Boruah and Kalita revealed that turmeric dye produced various soft and stable natural print on eri silk. Three different mordants alum, stannous chloride and ferrous sulfate were selected for printing [68]. Kavyashree investigated the efficiency of natural dye in screen printing on cotton and silk fabrics. Three natural dyes indigo, madder, and sappanwood were selected for screen printing. The results revealed that these dyes can be considered as the recommendable alternative to harmful synthetic dyes [69]. Jimmy et al., investigated the color resistant material from flour of Colocasia esculenta using Acacia catechu as natural dye for batik technique [70].
5 e) Dyeing Condition
i. Dyeing Condition for Cellulosic Material
Cotton is the most popular textile material. Many researchers have attempted to dye on cotton with the natural dyes. Each fabric performs differently in dye bath on the basis of their chemical structure. Dyeing parameters such as dyeing time, temperature, pH, material liquor ratio, dye and mordant concentration play an important role in dyeing. Several studies standardized the dyeing condition for cotton and reported the results as dyeing temperature, 70–100 °C, dyeing time, 60–120 min, material to liquor ratio, 1:20–1:100, and pH, 10–12 may be required for natural cellulosic material.
Vankar et al. used Eclipta as natural dye for dyeing cotton fabric by conventional and sonicator methods [62]. Teli and Paul attempted to dye cotton fabric with extraction of coffee seed coat. Dyeing was done by pre, meta and post-mordanting methods using various mordants. The results showed that coffee seed extract can develop a range of shades with good fastness properties [71]. In another study only coffee seeds were used for dyeing purposes. Some mordants such as FeSO4, CuSO4 and SnSO4 were applied for improvement of color strength of cotton fabric [72]. Shanker and Vankar applied dye extracted from Hibiscus mutabilis using 1:40 for M:L ratio on cotton fabric. Dyed cotton fabric exhibited good fastness properties [73]. Dayal et al. isolated dye from Parthenium hysterophorus and employed on cotton fabric. The dyeing done with M:L ratio 1:100 at 95–98 °C for 60 min on dyebath [74]. Indi and Chinta the fruits of Phyllanthus reticulatus utilized for dye extraction and application. Premordanting was done with alum (8%) and tannic acid (4%) at the
