Abstract
The tea plant (Camellia sinensis) is a widely planted cash crop around the world. Rich in tea polyphenols, it can bring enormous economic and health benefits to mankind. Meanwhile, tea extracts have been used to dye fabric for a long time and have multiple advantages in environmental protection and health. Based on the PRISMA method, this research conducts a systematic review of relevant studies on tea dyeing in the textile industry. To be specific, it collected 274 scientific publications after searching through databases like PubMed and Web of Science, and then selected 43 of the publications for the review. This research aims to summarize and analyze the existing research results, and probe into the advantages and potential of tea dyeing. For this purpose, this research first concludes the research progress of tea dyeing in the past 20 years, and visualizes the research hotspots in this field using the software CiteSpace. Furthermore, it analyzes the major components of tea dyes and the ways of detecting them. In the latter part, this study focuses on their performance of dyeing various fabrics, summarizes several ways to effectively improve the dyeing performance, and discusses the properties that tea dyes can impart to fabrics. Finally, this review also discusses the positive impact of tea dyeing on environmental protection and sustainable development, as well as the main challenges faced in terms of technology, cost, market acceptance, etc. when applied in the textile industry. This review can help researchers in this field better understand relevant research dynamics, and provide insights and necessary references for further study.
Introduction
Tea (Camellia sinensis), a member of the Theaceae family, is a healthy beverage, and also an acknowledged cash crop widely planted in Asian, African, Latin American, and Oceanian countries for over 2100 years.1,2 The consumption of tea ranks second globally, surpassed only by water. 3 It can be divided into six types according to processing techniques and the degree of fermentation: green tea, white tea, yellow tea, oolong tea, black tea, and dark tea. 1 Tea is also used in traditional Chinese medicine as it contains many beneficial active compounds. It is recorded that Chinese people have been using tea as herbs since the Tang and Song Dynasties. 4 The major components of tea, the natural herbal plant, are tea polyphenols, amino acids, alkaloids, carbohydrates, proteins, pectin, aromatic compounds, enzymes, and organic acids. Previous research has shown that this plant has many medical effects. For instance, it can help combat cancer, act as an antioxidant, reduce inflammation, fight bacteria, protect the cardiovascular system, and counteract diabetes and obesity.4,5 Therefore, tea is considered a valuable plant resource that is beneficial to the health of human beings. 6
Tea dyeing is a process during which tea pigments are used for coloring, and it was fist applied to textiles several thousand years ago in China. 7 This process has many advantages. Firstly, it uses natural dyes, which are more environment-friendly and healthy than synthetic ones. The safety of natural dyes lies in their renewability and biodegradability, as well as their minimal impact on water and soil pollution, while also being non-allergenic to the human body. 8 Secondly, as a medicinal plant, tea contains abundant bioactive chemicals, 9 which can develop functional fabrics when tea dyes are attached to the textiles. 10 Of course, natural plant dyes also have some limitations, such as a lack of raw materials for mass production, high costs but low output and so on.11,12 In this case, the tea plant (Camellia sinensis), widely planted in China, Japan, Taiwan, and India, can serve as a powerful supplier of natural dyes. 9 China is the leading country in tea production, processing, distribution, and consumption. 13 In 2019, China’s tea planting area reached 3.1021 million hectares, and this number has been increasing year by year. 14 Tea planting may also produce a large amount of waste like tea dust and tea stems, which amounts to several million tons in China every year. 13 It can provide abundant and cheap materials for making natural tea dyes. 10 Above all, tea is seen as a valuable and promising reservoir of natural pigments 15 that is worth researching and developing.
However, current studies on tea are mainly about the tea plant itself, such as how to drink tea or how to apply it to medical care. There are only a few studies of tea plant’s application in the textile and other industries. Messire et al. 16 and Koch et al. 17 discussed the possibility of using tea to produce all kinds of cosmetics. Guo et al. conducted a systematic review on the research progress of tea waste biomass application, finding that thermochemical conversion can utilize tea waste as bioadsorbents and catalysts, and for electrochemical energy storage. Besides, tea waste treated with biological conversion can be used for ensiling and composting. 18 Jayakala et al. also focused on developing economic value-added products from tea waste through thermal and microbial processes, and highlighted the significant ecological, agricultural, social, and economic benefits such waste can bring when it is made into biochar, which functions as an adsorbent, and compost. 19 Some other studies look into the reutilization of tea waste. For instance, Miao et al. investigated new uses of tea wastes in various fields such as food development, environmental restoration, energy production, and composite materials. 20 The conversion and application of spent tea leaves were investigated by Negi et al., who demonstrated their potential in the development of battery electrodes, nanocatalysts, insulation materials, and edible bioactive peptides. 21 Besides, Madikizela et al. systematically reviewed the application of tea-based materials for the removal of pharmaceuticals from contaminated water. 22
Despite these broad reviews about the application of tea in different fields, there have been no reviews on tea dyeing applied to the textile industry. Given this context, the author will explore the application potential of the tea plant (Camellia sinensis) in textile dyeing using the systematic review approach. Compared with traditional literature reviews, systematic reviews can assist scholars in integrating existing information and furnish them with efficacious data for informed decision-making. 23 Furthermore, the process of screening and analyzing certain articles, which is adopted in the systematic review, can help researchers eliminate bias and draw accurate and reliable conclusions. 24 The present review aims to keep track of research frontiers and hot topics, and discuss tea dyeing from different perspectives. Specifically, it will look into the major components of tea dye and ways of identifying them, and the coloring performance of tea dye on different fabrics. Moreover, this systematic review will also investigate the ways of improving dye uptake and color fastness, and new features of fabrics treated with tea dye. This review can provide a reference for further theoretical study and practice in this field. The following research questions (RQs) are proposed to make the research more understandable:
RQ1: What is the research trend in this field?
RQ2: What are the major components of tea dyes and how to identify them?
RQ3: In the existing studies, on what fabrics have dyeing experiments been conducted? Is there any difference between the dyeing performance of tea dyes on various fabrics?
RQ4: Currently, what methods can effectively improve the dye uptake and color fastness of tea dyes on fabric?
RQ5: What functions can tea dyes impart to fabric? What are the reasons for this?
Methods
The present systematic review was carried out in accordance with the PRISMA guidelines. 25 For the literature search, the Web of Science Core Collection database (accessed 18 November 2024) and the PubMed database (accessed 18 November 2024) were screened for information from the earliest study to November 2024, using the following search keywords: (Tea OR “Camellia sinensis”) AND (dye* OR colorant* OR pigment* OR dyestuff*) AND (Fashion OR Garment OR Cloth* OR apparel OR Fabric OR Textile* OR fiber). The search scope included titles, abstracts, and keywords, and only English language articles were considered. The search identified 274 articles, which were subsequently subjected to screening based on the following criteria and process to ensure their relevance:
17 records were excluded because they were comments, book chapters, letters, news, patents, conference papers, and reports. A total of 257 papers advanced to the next round of screening.
Additionally, 15 duplicate articles were removed, resulting in 242 articles for further screening.
Based on the titles and abstracts, 172 records were excluded as their research topics were unrelated to tea dyeing in the textile field. After this stage, a total of 70 papers proceeded to the next round of selection.
Subsequently, the full texts of 70 articles were reviewed and evaluated to determine if they met the conditions defined in the review’s scope. The exclusion of 27 articles was carried out in this step due to the following reasons: (a) the full text is not available; (b) the source of natural dyes is not tea; and (c) the experimental focus is not the performance of tea dying. The selection process and criteria are shown in Figure 1.
The flow diagram of the selective process of the literature in accordance with PRISMA.
Result
Analysis of Research Trends
The Number of Publications
The quantity of research papers produced serves as a pivotal statistical indicator for gauging the advancement of a research field. As depicted in Figure 2, research related to tea dyeing in the textile field began with the publication of the first article in 2006. As of November 2024, the overall number of publications has shown an upward trend, particularly since 2018, with relevant articles being published each year. By 2024, a total of 32 articles had been published, accounting for 74% of the total publication volume. Notably, 2018 and 2023 represent two small peaks in publication numbers, with 9 and 7 articles published, respectively. Overall, since 2018, an increasing number of researchers have engaged in studies related to tea dyeing, and the rise in publication quantity indicates that the importance and influence of this field are growing, highlighting its substantial research potential.
Annual number of publications.
Main Journals
A statistical analysis of the sources of the 43 selected articles reveals that they are published across 32 different journals, of which nine journals have more than one publication, as presented in Table 1.
The top nine productive journals.
NP: the number of publications; JCI: Journal Citation Indicator™ (2023); CQ: category quartile.
Analysis of Research Hotspots and Frontiers
Keywords serve as direct reflections of the research content within literature, offering insights into the primary themes. 26 In this review, CiteSpace (version 6.3.1) was utilized as a tool to conduct visual analysis of the 43 final included articles, aiming to elucidate the research hotspots in this field. Figure 3 displays the keyword co-occurrence network of the literature, comprising 138 nodes representing keywords and 568 links, with a network density of 0.0601. According to Table 2, the most frequently occurring keywords can be categorized into several groups: for fabric-related keywords--wool, silk, cotton, and fabrics; for tea dye-related keywords--natural dye, natural dyes, catechins, green tea, and polyphenols; for functional property-related keywords--UV protection, antimicrobial activity, antioxidant, and functional property; for technical keywords--acid, chitosan, optimization, adsorption, green, pH, fastness, and extraction.
Co-occurrence network of keywords.
Keywords with a frequency ≥3.
Based on the co-occurrence analysis of keywords in Figure 3 and Table 2, the research hotspots of tea dyeing in the textile industry can be summarized as follows: (1) the dying performance of tea dying on various fabrics, especially wool, cotton, silk, and so on; (2) the components of tea dye, such as the major ones like tea polyphenols and catechins, which play an integral role in the dyeing process--besides, there are different kinds of tea, among which green tea is studied more; (3) the process and techniques of tea dyeing. Tea is a kind of natural dyes, so it has more safety and health benefits compared with composite dyes. On the other hand, however, it requires a more complicated extraction and coloring process and has poor color fastness. Therefore, more and more researchers are paying attention to how to optimize the extraction and coloring process and develop environment-friendly technologies that can help improve dye uptake and color fastness like chitosan, an eco-friendly mordant. A further hotspot is (4) the properties tea dyeing can impart to fabric, such as antibacterial, antioxidant and UV protection properties.
“Burst words” are keywords with high frequency of occurrence, reflecting the evolution of hot topics and related disciplines over a period of time. 26 Figure 4 shows the 14 burst keywords in the field from 2006 to 2024, with the red line indicating the burst periods. According to Figure 4, the keywords “pH,”“silk,”“optimization,” and “UV protection” have significantly increased in frequency in recent years, suggesting that the research focus and future trends in tea dyeing have gradually shifted towards the following areas: (1) dyeing effects on different fabrics--since 2010, the keywords “cotton,”“cotton fiber,” and “fiber” reflect changes in research interest in the dyeing effects of different fiber materials over time. Since 2021, “silk” has become a burst keyword, due to the fact that different materials have different chemical structures and properties, which may result in differences during the dyeing process. Therefore, in-depth exploration of the adaptability and effects of various fiber materials in the tea dyeing process is an important direction for current research. In addition, current studies mainly focus on wool, cotton, and silk. Future research should further explore the adaptability and effects of other types and varieties of fiber materials in the tea dyeing process to meet the market’s demand for diverse fabrics. (2) Functional textile development--the keyword “UV protection” indicates that the functional demands in the development of tea-dyed textiles are gradually increasing. With the growing awareness of health and environmental protection, the development of tea-dyed textiles with specific functions, such as UV protection, has become a hot topic. This not only enhances the added value of products but also meets the market demand for functional products. Furthermore, there may be further expansion into the development of new multifunctional textiles for specific purposes, such as combining antibacterial and antioxidant properties for medical use. These multifunctional textiles can effectively prevent infection transmission and promote wound healing. 27 More tea-dyed textiles with specific functions that meet environmental protection standards may emerge, providing better user experience and health protection. (3) Dyeing process optimization--the keywords “optimization” and “pH” reflect research on the tea dyeing process and technology, including adjusting temperature, time, pH values, and other parameters during dyeing to achieve optimal effects. Researchers are working to optimize the dyeing process under different pH values and conditions to improve color fastness and dyeing results28–30 while reducing resource consumption and chemical use, thus promoting sustainable development. Future research will place greater emphasis on developing sustainable dyeing methods, such as finding more efficient, environmentally friendly, and economically feasible extraction and dyeing processes, reducing water and energy consumption, and contributing to green chemistry and waste minimization strategies.
Top 14 keywords with the strongest citation bursts.
Chemical Components of Tea and Ways to Identify them
Components of Tea Dyes
Tea is a significant source of polyphenols, with content levels ranging from 20% to 35%. The predominant component among these polyphenols is catechins, which encompass various forms including (+)-catechin, (−)-epicatechin (EC), (−)epigallocatechin (EGC), (−)-epicatechin gallate (ECG), (−)epigallocatechin gallate (EGCG) and (−)-gallocatechin gallate (GCG), as shown in Figure 5. 31 During fermentation or other oxidation processes, catechins can be oxidized by benzene ring to theaflavins, which then form thearubigins through oxidative polymerization together with other compounds in tea like dihydroflavonols. After that, thearubigins will go through another oxidative polymerization, where theabrownins of relatively high molecular weight, can be formed. Therefore, tea pigments consist of theaflavins, thearubigins and theabrownins, all of which are oxidative derivatives of tea polyphenols,5,15,32,33 as shown in Figure 6.
Chemical structures of of (+)-catechin and (−)-epigallocatechin gallate. 31
Major transformation pathways of catechins into tea pigments.
Identification Method of Chemical Components
Polyphenolic compounds are the major component of tea dye. Several methods to detect these components involved in current literature are given below.
Folin--Ciocalteu Method
After diluting the extracts of tea dye, add the reagent Folin--Ciocalteu, the solution Na2CO3 and distilled water to the extracts, and then color change can be observed. Heat the mixture for a while, then cool it at room temperature and measure absorbance at the wavelength of λmax, where maximum absorbance occurs. To quantify the concentration of polyphenols, draw a calibration curve of gallic acid, and obtain the concentration from the curve, with results reported in milligrams of gallic acid per gram (mg/g).34,35
UV-Vis Analysis
Measure the absorbance of tea extracts at a certain wavelength range using a UV-vis spectrophotometer. Catechin has two characteristic peaks,15,36 with one appearing at about 276 nm, and the other at about 219 nm. 15 If the peaks of tea extracts on the UV-vis spectroscopy are similar to these two numbers, it proves that the major components of the extracts are tea polyphenols. Establish the absorbance/concentration relationship of the epicatechin (EC) solution at λmax (276 nm), and then calculate the concentration of polyphenols in tea extracts. 15
FTIR Analysis
Determine the structure of tea extracts using a Fourier Transform Infrared Spectrometer (FTIR spectrometer). Determine whether tea polyphenols exist or not by analyzing the characteristic peaks of C-O-C, C=C, C=O, C-H, -OH and the aromatic ring on the FTIR spectrum.15,37,38
Ferric Chloride Test
Add the tea solution to the ferric chloride solution, and identify the tea dye by observing color changes. However, there is no detailed color comparison standard mentioned in the literature. According to Zhao et al.’s experiment, a black precipitate was formed when Longjing tea was added to ferric chloride solution. 29
Dyeing Performance of Tea on Different Fabrics
In the literature, the fabrics involved in tea dyeing experiments include wool, silk, cotton, hemp, flax, cellulose acetate, polyacrylonitrile, polyester and polyamide fibers, which can be further divided into two groups: natural fibers (cellulosic fibers and protein fibers) and synthetic fibers. Different fibers also show different dyeing performances, which are analyzed as follows.
Protein Fibers
Protein fibers mainly include wool and silk. 39 As a natural protein fiber, wool is made up of α-amino acids linked by peptide bonds. Keratin is the major component of wool, whose macromolecules can exhibit amphoteric properties through interactions involving basic amino acids, acidic groups, salts, disulfide bonds, and hydrogen bonds. 33 Wool also contains amido linkages which can interact with functional groups or dye. 40 Wool is popular among consumers for its warmth, comfort, good elasticity, and full texture. 33
Silk is a natural polymer whose molecular structure is formed by polypeptide chains. 41 It is popular around the world for many unique advantages, such as lustrous sheen, soft texture, good breathability, and strong moisture absorption. 42 Its major component, fibroin, contains amido linkages which serve as functional units, allowing it to produce color by interacting with mordants and dyes. 43
Weibang Xia’s research compared the coloring performance of tea dyes on wool and silk under the same conditions, showing that the performance on wool is better. Specifically, after dyeing, wool fabric exhibits a greater color intensity than silk fabric. This disparity can be attributed to wool’s significantly lower crystalline structure and its higher concentration of carboxyl and amine groups, in addition to the presence of α-helix chains and side chains, compared to silk. Dyes, which carry negative charges due to their polyphenolic compounds, can penetrate the amorphous regions of the wool fibers more easily than the crystalline regions and can form hydrogen bonds with the fiber, facilitating interactions with the dye anions. In contrast, the highly crystalline structure of silk limits its ability to absorb dyes when compared to wool. 39
Wang et al. drew a similar conclusion, and they offered three explanations. Firstly, both wool and silk are hydrophilic fibers, but the number of amino groups in wool fibers is nearly the same as that of carboxyl groups, while for silk fibers, the former is less than the latter. Therefore, wool exhibits a stronger binding affinity with tea pigments. Secondly, wool is a porous material that demonstrates capillary action, so water-soluble substances like tea pigments are more easily absorbed onto the voids or surface of wool fibers. Thirdly, wool has far more hydrophilic functional groups than silk in terms of both number and types, which makes tea pigments, water-soluble substances made from polyphenols, interact with it more actively. 44
To sum up, tea dye shows a higher adsorption efficiency on wool compared to silk. Therefore, tea dyeing on wool can produce darker color and exhibit a better dyeing performance.
Cellulosic Fibers
Cotton, as a natural cellulosic fabric, is also one of the most commonly used fabrics. It is most widely used in the textile industry due to its biodegradability, good cost performance and easy availablity, especially its flexibility and hydrophilicity. 36 The terminal hydroxyl groups in cotton’s cellulosic unit can interact with colorants or mordants for dyeing. 45 Apart from garment production, cotton can also find application in manufacturing non-implantable medical products and healthcare and hygiene products, 46 so cotton fibers are the most studied fabric in the 43 literatures of this review. In addition to cotton, Erdem et al. also look into the dyeing effect on cotton silvers. Research has shown that using green tea to dye cotton silvers can produce different shades of brown and beige, which are dependent on the type of mordant used. That means that using different mordants may cause the shade of color to change. 46
Hemp, with outstanding physical properties, is highly comfortable and extremely durable. It is one of the highest-quality fibers in ecological production and one of the most eco-friendly and multifunctional natural textile plants. While possessing several excellent characteristics such as intensity, heat retention capabilities, comfortability and durability, hemp is also considered an environment-friendly plant as few insecticides and herbicides are used during its growing process. Budeanu et al. dyed hemp fabrics using the dye extracted from black tea and produced different shades of lighter reddish-yellow. They found that the fabrics possessed darker colors when the dyeing time was increased. 8
Besides, Wang et al. conducted dyeing experiments on flax fibers using Keemun black tea (KBT). The dyed flax fabric showed excellent color fastness to rubbing, washing, and perspiration, but the fastness to light was comparatively worse. The primary reason is that KBT extracts have a limited affinity for flax fabric, resulting in a lighter color on dyed flax. In this case, the dye on the surface of fibers is more easily oxidized by light, leading to lower color fastness to light. 28
Gong et al.’s research compared the dyeing performance of tea polyphenol dye on cellulosic fibers (cotton) and protein fibers (wool and silk). They found that wool and silk showed better dyeing performance than cotton, which was relevant to the combination method between dye and fibers. Tea pigments interact with protein fibers through intermolecular forces and electrostatic forces, while they combine with cellulosic fibers only through weak intermolecular forces including hydrogen bonds and the van der Waals forces. 47 Since the bond energy of ionic bonds is far higher than that of intermolecular forces, 5 the dyeing performance on cotton is worse than that on protein fibers.
In conclusion, compared with protein fibers, tea dye has lower dye uptake and produces lighter colors on cellulosic fibers. Therefore, to improve the dyeing performance of tea dye on these fibers, it is necessary to improve the dye and fiber combination effect through mordants or other methods that can help enhance dye uptake.48,49
Synthetic Fibers
Pavun et al. conducted dyeing experiments on synthetic fibers cellulose acetate (CA), polyacrylonitrile (PAN), polyester (PES), and polyamide (PA) fibers using green tea and black tea respectively, and compared the dyeing performance with that on wool and cotton. According to research results, the tea extracts GT and BT can be used to effectively dye wool, CA, PA, and cotton. However, the shades and intensity of color are different, as the functional groups of chromophoric bioactive compounds interact and combine with the groups on the surfaces of different fabrics in differentiated manners. 50 In a word, natural fibers can produce better dyeing effects and more intense coloration, and wool is more suitable than cotton for dyeing. This finding is also in line with the opinion above that protein fibers exhibit better dyeing performance.
Tang et al. also drew a similar conclusion after comparing the adsorption capacity of tea polyphenols on wool, silk, and nylon, and their dyeing performance on these fabrics with the addition of metal mordants. It was found that although nylon exhibits the highest adsorption capacity for tea polyphenols, its dyeing effect is worse than that of silk and wool. In other words, mordants helped silk and wool produce darker colors than nylon. The primary reason is that nylon has a lower content of amino groups and carboxyl groups, the worst adsorption capacity for metal, and accordingly a poor ability to form the metal ion--tea polyphenol--fiber complex. 10
Among all the 43 articles, most of the studies focus on cotton, wool, and silk. Based on the comparative dyeing experiments of different fabrics in the literatures, the color intensity of tea dyes on different materials is as follows: protein fibers > cellulosic fibers > synthetic fibers.
Methods of Improving the Dyeing Performance of Tea Dyes
Although tea dyes, like other natural dyes, have an edge over synthetic ones in environmental protection and health, their effectiveness in dyeing fibers is relatively low. 51 Therefore, many studies are dedicated to improving the adsorption rate and fastness of tea dyes on fibers in various ways. A detailed analysis is outlined below.
Metal Mordanting
Mordants are substances that enable the dye to bind to the fibers. They can help improve the adsorption rate and color fastness of tea dyes. 51 Mordanting is the primary method for enhancing the effects of tea dyes, and metal is the most frequently used mordant.
Metal mordanting works like this: it helps form metal complexes between the dye and the fibers. These complexes consist of dye molecules, metal ions, and the functional groups of fibers (such as hydroxyl groups, amino groups, etc.), thus becoming insoluble compounds, which can contribute to higher dye uptake and color fastness by enhancing the interaction between fibers and the dye28,51–53 (Figure 7). Moreover, tea polyphenols can be used as good ligands for metal ions as their molecule units contain highly delocalized conjugated systems and oxygen atoms with strong coordination. Therefore, metal mordanting can improve the effectiveness of tea dyeing. 54 In addition, the formation of insoluble complexes between metal--dye--fiber not only enhances the adhesion of the dye but also changes the color characteristics of the dye. Different metal mordants can produce different hues. For example, in the article by Hayat et al., it is mentioned that pre-treating silk with iron salt (3% Fe) before dyeing can produce a deeper hue (L* = 58.55), with a more yellowish tint (b* = 13.47). Similarly, pre-treating silk with aluminum salt (4% Al) before dyeing can result in a lighter hue (L* = 76.89), with a slightly reddish-yellow tint (a* = 6.04, b* = 8.15). These results suggest that metal mordants, through their interaction with the dye and fiber, can significantly alter the dyeing effects of tea dye, achieving better color intensity while also producing new hues. 43
Suggested binding mechanism metal mordant’s action between fiber and catechin: (a) protein fibers and (b) cellulose fibers.
However, metal mordants also have some disadvantages. For instance, these substances may cause environmental problems and pose potential risks when skin is exposed to them.31,55 That explains why more and more researchers are trying to find more eco-friendly alternatives to metal mordants.
Chitosan Mordanting
Chitosan is a biological mordant. 34 Due to its biocompatibility, non-toxicity, and excellent biological performance, this mordant is given due attention as a novel functional textile material. 9 As a polycationic amino polysaccharide, chitosan is chemically composed of β-(1, 4) linked 2-amino-2-deoxy-β-D-glucopyranose, and is basically an N-deacetylated derivative of chitin. The presence of reactive amino and hydroxyl groups along the backbone confers some interesting properties to chitosan for use in textile finishing.9,56 The molecular structure of chitosan is shown in Figure 8. 30
Chitosan molecular structure. 30
For one thing, chitosan introduces primary NH2 groups to the fiber structure, thus increasing the accessibility sites of dye molecules. 9 Through hydrogen bonds and the van der Waals forces, this mordant ensures that the dye and the fibers can fully combine, so as to improve color fastness.34,56 For another, after the fibers are pretreated by chitosan, the surface is coated with protonated primary amines (-NH+3) of chitosan molecules, which can interact with negatively charged catechin coloring components through ionic bonds and enhance the adsorption rate of tea dyes.9,31 The mechanism of chitosan’s action between fabrics and dyes is shown in Figure 9. From the above analysis, it can be concluded that chitosan can effectively improve the dye uptake and color fastness on fabric. For example, in the research by Lambrecht et al., compared to directly dyed cotton fabric, the red tea-dyed cotton fabric treated with chitosan visually appeared darker brown and showed good color fastness after washing, indicating that chitosan enhanced the bonding between the dye and the fabric. 34 Shahid-ul-Islam et al. reached similar conclusions, noting that as the concentration of chitosan increased, the color of the dyed wool became darker and the color fastness further improved. 9
Suggested binding mechanism of chitosan’s action between fiber and catechin: (a) protein fibers and (b) cellulose fibers.
Plant-based Mordanting
Apart from chitosan, some plants are also good biological mordants. During the dyeing process, the active ingredients in plant mordants can react with the functional groups on the fabric (such as hydroxyl groups, amino groups, etc.), forming cross-linked structures. At the same time, the mordants can also form complexes with the dye molecules, increasing the bond strength between the dye and the fibers, and reducing the loss of dye.43,55 The mechanism of action of plant mordants between the fabric and dye is shown in Figure 10. In addition to enhancing the bond between the fabric and the dye, making the dyed fabric exhibit excellent color fastness, plant mordants can also provide new hues. This is mainly due to the diversity of chemical components in plant-based mordants, which contain various functional active molecules, such as tannin in pomegranate, Lawson in henna, and curcumin in turmeric. The types and proportions of these components determine the final dyeing results. 43
Suggested binding mechanism of plant-based bio-mordant’s action between fiber and catechin: (a) protein fibers and (b) cellulose fibers.
Tayyab Hayat explored the possibility of using turmeric rhizomes, pomegranate peels, acacia bark, and henna leaves as biological mordants. He found that all these plant-based mordants can help introduce new shades and enhance color fastness, because the functional sites of bio-mordants, the amido linkages of the protein fibers like -CO and -NH2, and the -OH of tea dyes may interact with each other by forming additional hydrogen bonds. 43 Adeel et al. also applied turmeric, acacia bark and pomegranate peels as biological mordants in the experiment of dyeing wool with black tea. They obtained similar conclusions to Tayyab Hayat, finding that bio-mordants impart high color intensity to the dyed fabric by producing new shades. 40 This reveals the high potential of biological mordants substituting traditional metal mordants due to their sustainable and environment-friendly characteristics.
Banerjee et al. conducted pre-mordanting on Eri silk using Tsuga canadensis (L.) Carrière (commonly known as Snep sohmylleng), and then dyed with tea leaves. The result showed that the dyed silk had excellent color fastness to washing, rubbing, and light. 55 Abdur Rehman and others used aloe vera extract as a natural mordant, applying pre-mordanting, meta-mordanting, and post-mordanting techniques to dye cotton fabric, and evaluated the dyeing effects and various color fastness properties. The results showed that the pre-mordanting technique performed best in terms of color intensity, wash fastness, sweat fastness, and abrasion fastness, while the post-mordanting technique showed the best performance in sweat fastness Abdur Rehman. 57
Microwave Treatment
Through solid--liquid transfer mechanism, microwave radiation enables the biomolecules to maximally interact with the solvent. Microwave treatment can transfer more mass into the solvent through a balanced and steady heating process, thereby improving the effectiveness of dye pigments. Furthermore, this method can make the fabric surface more suitable for accepting dye molecules, thus increasing the dyeing efficiency. 43 The process of microwave treatment is fast and even, saving time and energy. Besides, it is more compatible and requires smaller equipment size. These advantages contribute to the increasing popularity of microwave treatment in natural dyes.43,45
When extracting tea dyes, microwave treatment can facilitate the interaction between the solvent and pigments by rupturing the cell wall, thus contributing to a good yield through the well-working mass transfer kinetics. Besides, microwave radiation can also adjust the fabric structure and improve the fabric’s adsorption capacity. For example, in Adeel et al.’s research, the extracts of black tea waste with 6 min of microwave treatment showed excellent color strength when dyeing wool which was also treated by microwave radiation. This finding indicates that microwave treatment can improve both the extraction efficiency and the dyeing performance of tea dyes. 40
Enzymatic Treatment
Enzymes can catalyze the oxidation and polymerization reactions of tea polyphenols. After being catalyzed, tea polyphenols first turn into quinones, and then into the brownie due to their instability. Therefore, enzymatic treatment can increase the amount of tea pigments.47,44 This polymerization process, supported by enzymes, can help improve the stability and durability of tea dyeing, and avoid the environmental pollution and health hazards of metal mordants. 38
Gong et al. converted tea polyphenols into tea pigments by using a crude enzyme generated by Aspergillus niger. These pigments can engage with wool fibers through hydrogen and ionic bonds, thus contributing to higher Integ (used to evaluate color intensity) than the control group. Moreover, enzymatic treatment can also enhance color fastness to rubbing and washing. 47
Wang et al. conducted a study of the dyeing performance of tea dyes that are formed through the oxidation and polymerization process catalyzed by Laccase. Research shows that the dyeing effectiveness is better under acidic conditions, and both dyeing performance and color fastness are optimal when the pH is 3. The reason is that under acidic conditions, the major component of tea pigments is micromolecular theaflavin, which is relatively stable in this situation, while it will turn into macromolecular thearubigin and even theabrownin under alkaline conditions, which is not likely to penetrate into densely-woven fabrics. Additionally, the phenolic hydroxyl groups present in theaflavin and tea polyphenols have the capacity to ionize hydrogen ions, resulting in the formation of negative oxygen ions in acidic conditions. Similarly, protein fibers can ionize ammonium ions when the pH is below their isoelectric point. The interaction between the fibers and the colorant occurs primarily through electrostatic forces. By comparison, the combination mode under alkaline conditions is intermolecular forces, 44 so the dyeing fixation rate is higher under acidic conditions. Garg et al. use Laccase to polymerize the phenolic compounds of Camellia sinensis in situ on wool, which not only enhances the coloring effect and fastness of washing but also increases the fabric’s antibacterial activity by producing more phenolic polymers. 38
Plasma Treatment
According to the research findings of Gorjanc et al., plasma treatment can exert a huge influence on the adsorption capacity of tea dyes. To be specific, using oxygen plasma to treat cotton fibers can add oxygen functional groups to the surface, thus increasing the negative charge of the fibers, and making the fabric more exclusive to the negatively charged molecules of tea dyes. In this case, the fabric’s adsorption capacity for dyes decreases. By contrast, using ammonia for plasma treatment can introduce amino functional groups to the fibers, and the bonding of the dye to the surface of amino groups can improve the dye’s adsorption capacity. Besides, natural dyes have excellent adsorption properties, which can help enhance the UV protection ability of tea-dyed cotton fabrics. 58
However, Chen et al. found that oxygen plasma treatment can improve the antibacterial activity of cotton fabrics dyed with green tea. The reason is that oxygen plasma may form hydrophilic functional groups on the surface of the cotton, which can easily form chemical bonds with the water-soluble antimicrobial components in plant-based dyes. 59
Fabric Modification
Apart from plasma treatment, many fabric modification methods can also greatly improve the dyeing effectiveness of tea dyes.
UV radiation is a clean processing technique that can be applied to surface modification of textiles. In the research of Chen et al., the wool fabric, first modified by UV radiation and then dyed with Huangya yellow tea, exhibits better dyeing effects compared with the samples without radiation. This phenomenon occurs because some dye-inhibiting substances like crosslinked cystine disulfide bonds and lipids with higher carbon content are removed during the oxidative degradation caused by UV radiation. Meanwhile, the number of hydroxyl groups in fibers increases. These changes, combined together, can help boost dyeing effectiveness. 54
Citric acid, characterized by being eco-friendly, non-toxic and safe, can serve as a cheap alternative to toxic formaldehyde condensates as a fiber crosslinking agent. In the experiment conducted by Maulik et al., citric acid underwent an esterification reaction with the hydroxyl groups in cotton fibers and tea dye molecules under the influence of the esterification catalyst (NaH2PO4). That enabled the dye molecules to combine with the cotton fibers through chemical bonds and enhanced the dye’s ability to adhere to the fibers, thereby improving the color fastness to washing and light. Additionally, the crosslinking also reduced the penetration of oxygen and water, thus inhibiting the photobleaching mechanism. 60
Maulik et al. modified cotton fabrics through graft copolymerization using acrylamide monomer under the influence of a free radical polymerization catalyst such as K2S2O8. This pre-treatment improved dye uptake, tensile strength, and wrinkle recovery angle in a balanced way, and optimally kept the flexibility of the dyed substrates. Moreover, applying ferrous sulfate to the pre-treated cotton fabric and then dyeing it with tea further enhanced the color intensity and fastness. 32
Rehman et al. in their research dyed cotton fabrics using extracts of black tea after cationizing the cotton fabric with (3-chloro-2-hydroxypropyl) trimethylammonium chloride. The result shows that cationized cotton fabric produced significantly higher color intensity. The reason is that the hydroxyl and carboxyl groups of tea polyphenols formed bonds with the cationic surface of cotton, and functionalizing the surface of cotton with cationic surfactants enhanced the dye fixation and color fastness properties of dyes. This dyeing method involves no chemical substances, so it can make the coloring process cleaner and more sustainable, and also implementable on an industrial scale. 37
Erdem et al. enhanced the hydrophilicity and whiteness of cotton silvers using ultrasonic-bioscouring and ozone-based bleaching, which contributes to higher dyeing quality and uniformity. 46
Azoic Dyeing
Azoic dyeing can improve the dyeing performance of tea dyes on silk fabrics by using tea polyphenols to replace traditional synthetic coupling components. This goal can be achieved through two steps: the polyphenols first absorb the coupling components onto the fabric, and then form azo dyes inside the fiber through diazotization of primary aromatic amines. This method can not only avoid using metal mordants, which are harmful to health and the environment, but also overcome the problems of poor dyeing performance and fastness of natural dyes. As is shown in research, azoic dyeing can produce dark colors like intense brown-orange shades even at low concentrations. Meanwhile, this dyeing method can also significantly improve color intensity, and the effect is dependent on the concentration of primary aromatic amine. At 7% owf, the K/S of oolong tea extracts is about six times that of the dyeing samples with aluminum potassium sulfate as mordant. 41
Others
Apart from the above processing ways, the 43 literature studies reviewed in this study present some other factors that can influence the dyeing performance of tea dyes. For example, the study by Ren et al. demonstrates that different values of dye bath PH may result in different dyeing performance, color fastness to light and antibacterial activity. To be specific, dye bath PH values 3.5, 5.5, 7.5, 9.5 correspond with the colors yellow brown, red brown, brown, and dark brown of the dyed wool respectively. The differences can be attributed to the chemical changes of tea polyphenols and pigments during the dyeing process. In this process, some polyphenols are transformed to yellowish-brown theaflavins and bisflavanols, which can turn into reddish-brown thearubigins through oxidative polymerization. When the dye bath is acidic, H+ hinders the above-mentioned reaction, leading to fewer thearubigins. That explains why the wool fabric shows the color yellow brown when pH is as low as 3.5. As pH increases to 5.5–7.5, more thearubigins can be formed, making the fabric redder. When the pH hits 9.5, tea polyphenols may be easily oxidized to quinones, and further turn into dark brown theabrownins, which enables the fabric to show the corresponding color. As PH decreases, dyeing effects can also show up under light because the tea pigments on the dyed fabric are oxidized to quinones, and then generate conjugate quinone chromophore with the help of strong light. Moreover, when the dye bath is acidic or alkaline, especially if it is alkaline, the oxidative polymerization of tea polyphenols leads to the decrease of –OH and consequently significant decrease in the antibacterial activity. 15
Additionally, researchers have being exploring more and more innovative and environment-friendly dyeing techniques. For instance, after adding glycine to the dye solution, Wang et al. found that this substance can promote the oxidation of tea polyphenols and increase the concentration of tea pigments, thus improving the dyeing effectiveness through non-enzymatic browning reaction. 5 Ren et al., based on the oxidative polymerization of catechins, put forward an in-situ polymerization dyeing technology. This technology can be used to dye cellulosic fibers with tea pigments, and can impart properties such as brown color, good color fastness, and antibacterial activity to cotton fibers without using chemical mordants. 49 Wang et al. dyed flax through a pad-dry dyeing strategy, using dyes extracted from Keemun black tea waste with the absence of mordants. This process includes two steps: the first one is to immerse the flax fabric to the dye solution, and the second one is to pad and dye it. The result shows that theaflavin compounds can dye the fibers by interacting with them through the van der Waals forces and hydrogen bonds. The dyed fabric not only exhibits excellent color intensity and fastness but also acquires a good UV protection property and antibacterial activity. 28
Functional Properties Imparted to Fabrics by Tea Dyeing
Tea polyphenols (TP), as the major component of tea, have many health benefits and functional properties, such as antibacterial, antioxidant and anticancer activity as well as UV protection. Therefore, using tea extracts to dye fabrics can bring added value to them. 15 According to the experiment results in the literature studies concerning different functional properties, fabrics treated with tea dyeing have many new functions such as antibacterial activity, antioxidant activity, UV protection, flame retardancy and deodorizing activity, which are detailed as follows.
Antibacterial Activity
Microorganisms can live on the surface of different fabrics for a long time. Among all the fabrics, natural fibers like cotton, wool and silk are more suitable for bacteria and fungi to grow and proliferate, because they can provide nutrients and appropriate humidity. To reduce the threats posed by microorganisms to human health, scholars, scientists, and governments around the world are developing innovative chemicals to achieve effective textile functionalization. 9 However, despite the efficacy of synthetic antibacterial agents such as triclosan, metal and their salts, organometallics in combating bacteria, their potential for causing significant environmental pollution cannot be overlooked. In contrast, natural eco-friendly agents like natural dyes have excellent antibacterial effects without posing additional risks to the environment, which makes them receive more attention nowadays.33,52
The antibacterial activity of tea dyes has been fully demonstrated by many relevant experiments. Tea polyphenol is the primary antibacterial substance of tea extracts, and its catechin content ranges from 60% to 80%. The antibacterial mechanisms of tea polyphenols mainly include the following: (1) DNA damage or inhibition of nucleic acid synthesis--inducing DNA damage or inhibiting nucleic acid synthesis in bacterial cells. Polyphenols can penetrate the DNA helix, form hydrogen bonds with nucleic acid bases, and inhibit the activity of topoisomerases or DNA gyrases.
27
(2) Interaction with proteins--forming complexes with proteins through hydrogen bonds, hydrophobic interactions, or covalent bonds. Their benzene rings have hydrophobic properties and can also bind to the hydrophobic regions of proteins. The multi-site binding of polyphenols to proteins leads to protein denaturation, and due to the loss of protein function, the transmembrane transport of nutrients and metabolites that maintain bacterial growth and reproduction is disrupted, thus inhibiting bacterial growth.15,27,28,30,36,49,56,61 (3) Inhibition of enzyme activity--inhibiting the activity of specific enzymes, such as 1-deoxy-
The antibacterial mechanism of tea dye.
In addition, the antibacterial properties of tea dyes can be further enhanced through mordanting. Shahid-ul-Islam et al. investigated the impact of chitosan on the antibacterial properties of green tea-dyed wool fabrics. They found that chitosan pretreatment significantly enhanced antibacterial activity against E. coli and S. aureus, with increasing effectiveness as chitosan concentration increased. In addition to chitosan’s ability to enhance the absorption and fixation of tea dyes on fabrics, this effect can also be attributed to the electrostatic interactions between the positively charged NH3+ groups of chitosan and the negatively charged residues present on bacterial cell membranes, which alter cell permeability and cause osmotic imbalance. Furthermore, the amino groups can hydrolyze the peptidoglycan present in the membrane, resulting in the leakage of intracellular electrolytes and low molecular weight proteins (such as nucleic acids, glucose, and dehydrogenases). This phenomenon disrupts normal metabolic processes and ultimately leads to cell apoptosis. 9
However, there are some discrepancies in the conclusions when using metal mordants. Shahmoradi et al. demonstrated that pre-mordanting fabrics with aluminum sulfate greatly improved antibacterial activity in tea-dyed wool fabrics against S. aureus, E. coli, and P. aeruginosa. This improvement is attributed not only to the synergistic effect with the dye but also to the inherent antibacterial properties of the metal ions themselves. The antibacterial mechanisms primarily involve two pathways: the binding of metal ions to proteins or the generation of reactive oxygen species (ROS). The first mechanism involves the covalent attachment of metal ions to the -SH groups of cellular enzymes, resulting in the inhibition of their activity and alteration of bacterial metabolism, ultimately leading to cell death. The second mechanism relies on the metal ions’ pro-oxidant activity, where highly reactive oxygen radicals generated during the reaction attack the structures of the bacteria, causing significant damage. 52 However, in the experiments conducted by Cheng et al., metal mordanting led to varying degrees of reduced antibacterial activity. This may be attributed to two main reasons. First, the phenolic hydroxyl groups of natural polyphenols interact with cellular proteins, disrupting the bacterial membrane structure and inhibiting bacterial growth. However, the free hydroxyl groups in phenolic compounds decrease due to coordination with metal ions, thereby diminishing their antibacterial effectiveness. Another reason is that metal coordination improves the wash fastness of phenolic compounds on the fabric, which reduces the leaching of antibacterial agents from fabric to the bacterial culture during the current antibacterial tests. 42 Therefore, although the impact of metal mordanting on antibacterial efficacy is somewhat contentious, the enhanced durability of antibacterial performance has been consistently affirmed.
UV Protection
Overexposure to UV radiation may exacerbate skin problems, which necessitates the development of UV-protective clothes that can effectively reduce or prevent related risks. 28 However, the UV protection efficiency varies with the type and thickness of the fiber, fabric count, yarn count, fabric structure, and color. Natural fibers like cotton are more suitable to wear in summer, but if titanium oxide and zinc oxide, two primary chemical substances that can improve the UV protection properties of the textiles, are to be used, synthetic fibers have to be incorporated during the preparation of spinning dope. A surfacing coating can also help add such UV-protective substances to natural fibers, but may cause damage to human skin or trigger allergic reactions. Therefore, it is necessary to develop appropriate and innovative methods to enhance the UV protection capacity of cotton and many other natural fabrics. 31
Eleven of the 43 cited articles in this review conducted experiments about UV protection, all of which show excellent protection effects. Tea polyphenols are the determinator that enables tea dyes to improve the UV protection effect of fabrics. 58 Tea polyphenols, especially epigallocatechin gallate (EGCG), can protect the DNA of human cells from damage caused by ultraviolet and visible light radiation. They can reduce the penetration of UV radiation and the DNA damage caused by light, and affect photoimmunology. 31 Due to the presence of conjugated systems and functional groups such as -OH, -NH2, and -NH, the compound exhibits excellent ultraviolet absorption ability. High-energy ultraviolet radiation is suppressed through photochemical cleavage or conversion reactions facilitated by hydrogen bonding in the compound, 35 as shown in Figure 12. Therefore, fabrics can acquire higher UV protection performance when adsorbing more tea dyes through mordanting or by other approaches.35,58 For example, experimental data of Lambrecht et al.’s research showed that cotton fibers not dyed or only treated with the mordant chitosan exhibited hardly any UV-protection ability. By contrast, cotton fibers dyed with red tea showed higher UV protection efficiency. Besides, this property was further enhanced after cotton fibers were treated with both chitosan and tea dyeing. 34
Ultraviolet protection mechanism of tea dye.
Antioxidant Activity
Tea dyes can impart fabrics with antioxidant activity. Natural phenolic compounds have excellent antioxidant activity due to their strong ability of electron donating. Free radicals are unstable molecules with unpaired electrons. These molecules try to stabilize themselves by stealing electrons from other molecules, thereby triggering chain reactions that cause cell damage and disease. 62 The phenolic hydroxyl groups in catechins can neutralize free radicals by providing one or more electrons, reducing their activity and protecting cells from oxidative damage. 27 The antioxidant mechanism is shown in Figure 13. So tea dyes with abundant polyphenols and catechins can bring different degrees of antioxidant activity to fabrics, 56 and the activity level is dependent on the concentration. 42 Hence, mordants can contribute to higher antioxidant activity of fabrics by increasing the adsorption rate of dyes.
Antioxidant mechanism of tea dye.
Additionally, Shahid-ul-Islam’s research shows that the antioxidant activity of the dyed wool is enhanced when chitosan is used as a mordant. The reason is that chitosan facilitates the interaction between the dye and the fabric, and that this mordant can be transformed into stable compounds when its active hydrogen and free radicals combine with each other. Therefore, there’s a synergic effect between chitosan and tea extracts. 9 By contrast, Cheng et al. found that the antioxidant activity of dyed fabrics was obviously weakened when they used metal mordants. The reason is that the number of phenolic compounds decreased when their free hydroxyl groups coordinated with metal ions, 42 and this also explained why metal mordanting reduced the antibacterial activity of the dyed fabrics.
Flame Retardant Activity
Apart from antibacterial activity, UV protection and antioxidant activity, Cheng et al. investigated the flame retardancy of tea dyes. Flame retardancy treatment of fabrics can minimize the risk of fire, so organophosphorus flame retardants, fluorotitanate, and fluorozirconate are often used to process fabrics. Natural tea dyes can serve as a new strategy for making flame-retardant fabrics to replace some synthetic flame retardants that may emit formaldehyde, pose health hazards, generate wastewater, harden the fabric, reduce the strength and cause many other problems. 42 Cheng et al. in their experiment tested the flame retardancy of silk dyed by the extracts of tea stem. The result showed that the silk dyed with 20 g/L tea stem extracts had better flame retardancy benefiting from polymerization products in the tea extracts. Tea extracts can improve the char-forming ability of silk, which makes the dyed silk swiftly convert into char, thus saving its texture structure. The char can inhibit the transfer of heat and the generation and release of flammable gases and then hinder the expansion of fire, Figure 14 shows the mechanism diagram of tea dye flame retardant. Moreover, metal post-mordanting can further enhance the flame retardancy of silk by forming natural polyphenol-metal Ion-silk fiber complexes. 42
Flame retardant mechanism of tea dye.
Deodorization Activity
Zhao et al. also investigated the deodorizing property of tea dyes. The experimental results show that chitosan-modified cotton fabric dyed with 4–8% tea polyphenols (o.w.f) exhibits a deodorizing performance ranging from 85.68% to 87.21%. This is because the phenolic hydroxyl groups in tea polyphenols being able to undergo condensation and neutralization reactions with amine and ammonia groups, they can effectively eliminate odor substances present in the environment, 30 and the deodorization mechanism is shown in Figure 15.
Deodorization mechanism of tea dye.
Discussion
The Impact of Tea as a Textile Dye on Environmental Protection and Sustainable Development
The application of tea dyes in the textile industry has a significant impact on environmental protection and sustainable development. In the production of textiles, the fabrics need dyeing to improve aesthetic appearance and gain added value. 38 Nowadays, synthetic dyes are experiencing more and more limitations because they may cause many ecological problems. 41 By contrast, the application of natural dyes in textile dyeing and finishing is getting more and more attention, for the natural ones are more biodegradable and compatible with the environment, and most of them have certain functional properties. Tea contains many active plant components, one of which is tea polyphenols, whose major component is catechin. Polyphenols have many health benefits and functions. To conclude, as a natural dye, tea not only is safe and eco-friendly but also can endow fabrics with new functions such as antibacterial activity and UV protection.34,29
Currently, textile products dyed with natural extracts are gaining favor among environmentally conscious consumers, and their market share is expanding. 10 However, natural dyes might also cause environmental problems as some dyes may be extracted from rare and endangered species. 63 Furthermore, it is difficult to promote industrialized application of natural dyes due to the lack of raw materials for mass production, high costs and low yield.11,12 In recent years, cash crops have been widely planted and are becoming an important source of natural pigments. In this process, people are gradually paying more and more attention to the application value of tea. 49 The tea plant, a renewable resource, is the most popular beverage in many countries, 42 so it has a substantial global production.40,64 China, with a production of 2.74 million tons, is the largest tea-producing country, followed by India (1.33 million tons), Kenya (569 thousand tons), and Sri Lanka (278 thousand tons).21,65 More importantly, a large amount of tea waste like old leaves and tea stems is produced in tea planting and processing, 47 which is the largest source of agricultural plant waste from the food industry. 48 These wastes may not only result in overconsumption of resources but also lead to harmful environmental problems. The reason is that during the composting process, some organic compounds require excess oxygen, thus emitting methane, which is the second most abundant greenhouse gas and is more likely than carbon dioxide to cause global warming. 39 However, if natural fabric dyes are produced using the tea waste, a new way of increasing the resource efficiency of agricultural waste may be found.47,66 In addition to lower costs, 54 their usage as natural tea dyes can solve environmental problems caused by waste disposal and generate enormous economic benefits,15,49,52 which guarantees their huge research value and market potential.
In summary, the application of tea dye in the textile field has the following positive impacts:
Biodegradability--compared to synthetic dyes, natural tea dyes exhibit better biodegradability and environmental compatibility, thus reducing negative impacts on the ecosystem. Using tea dye can help reduce ecological problems such as water pollution and bio-toxicity caused by synthetic dyes, protecting water resources and biodiversity.
Health benefits--the active components in tea are not only environmentally friendly but also enhance the safety and functionality of the dyed fabric, providing a safer and healthier experience during use.
Renewable resource--as a renewable resource, the tea plant is widely grown and considered a mainstream beverage in many countries, ensuring a relatively stable and sustainable supply of raw materials for tea dyes.
Resource recycling--the effective use of large amounts of waste generated during tea processing, transforming it into natural textile dye, not only reduces resource waste and lowers production costs but also alleviates the environmental burden caused by agricultural waste disposal.
Promoting a circular economy--the development of tea dyes contributes to promoting the circular economy, facilitating efficient use of agricultural resources, and forming a more sustainable production and consumption model. Therefore, tea dyes have significant advantages in environmental protection and sustainable development, enhancing the value of textiles while providing new ideas for ecological protection and resource efficiency.
Application Potential of Tea Dyeing in the Textile Industry
As consumers require more eco-friendly and safer textiles, natural tea dyes show huge development and application potential. Firstly, the dyes used in the tea dyeing process are tea polyphenols, a bio-based pigment. Compared with traditional synthetic dyes, they minimally irritate human skin and can help alleviate skin allergy, so are more suitable for underwear. 5 Secondly, tea dyeing imparts good antibacterial activity to fabrics, is more environment-friendly than synthetic antibacterial agents, and won’t cause any environmental risks. Therefore, it is likely to serve as an alternative to synthetic antibacterial agents in producing antibacterial garments. For instance, it can be used to produce all kinds of protective textiles and garments for health care and hygiene.9,52,67 Besides, tea dyeing can provide fabrics with excellent antioxidant capacity. It can remove free radicals, prevent cell damage and facilitate the growth of new cells, which is beneficial for wounds to heal. Hence, apart from protective textiles, this method can also be applied to the production of medical textiles of high added value for therapeutic use. Tea dyeing is also likely to find its application in wound treatment, especially in treating skin wounds that can be easily infected by Staphylococcus aureus. Additionally, it can be used to produce disposable medical bandages and gauze for wounds to heal. 50 Thirdly, the antibacterial and deodorizing 30 properties of tea dye are also suitable for developing sportswear because sweating during exercise can lead to bacterial growth, resulting in odor and health issues. Tea polyphenols can effectively inhibit bacterial growth and reduce odor on sportswear, thus maintaining its cleanliness and hygiene. Fourthly, excessive exposure to UV radiation can exacerbate skin conditions, so the development of UV protective clothing serves as an effective measure in alleviating or preventing such issues. Tea dyeing also has great application potential in this field due to its outstanding UV protection property.31,28 Lastly, tea dye is also very suitable for developing home textiles, especially bedding that comes into contact with the skin. The natural antibacterial components it contains can effectively enhance the hygiene performance of bedding. In addition, its excellent antioxidant properties provide users with a more comfortable experience. Moreover, using fire-retardant tea 42 dye in curtains, carpets, sofa fabrics, and chair fabrics significantly reduces the risk of fires and plays a crucial role in ensuring home safety.
Challenges in the Application of Tea Dye in the Textile Industry
Although the application of tea dye in the textile industry has multiple advantages, there are still several challenges, mainly in the following areas. (1) Dyeing effect and color fastness--first, the color range of tea extract dyes is relatively limited, mainly in varying shades of brown,40,53 which may not meet the market demand for diverse colors. Additionally, like other natural dyes, tea dyes generally have lower color fastness than synthetic dyes and are easily affected by factors such as light exposure and washing, which could impact the aesthetic appeal and market acceptance of the fabric.27,53. (2) Loss of functional activity--while tea extracts impart excellent functional activity to fabrics, controlling the loss of these bioactive compounds during processes such as washing, to maintain their long-term effectiveness, is a challenge. 27 (3) Standardization and stability of colors--the dye extraction process may be inefficient, and the various plant chemicals in tea extracts are affected by multiple factors (such as raw material source, climate, harvesting time, etc.), leading to difficulty in ensuring consistency between products. 68 Variations in color between different batches or even within the same batch present a challenge to color stability. (4) Technical complexity--the extraction and dyeing processes of tea dye are more complex than those of synthetic dyes, requiring specific extraction and dyeing techniques, and often requiring suitable mordants to improve color fastness. 53 Additionally, although tea extracts are environmentally friendly, ensuring the environmental friendliness of the entire production process, including extraction, processing, and application, remains a challenge in large-scale applications. For example, heavy metal mordants pose environmental risks, 69 yet many studies still rely on these mordants. It is necessary to explore mordant-free or environmentally friendly mordanting processes. 30 (5) Dyeing Costs: The cost of producing and extracting tea dye is generally higher than that of synthetic dyes, which may affect its economic feasibility, especially in large-scale production, where additional costs are incurred for extraction and processing, potentially increasing the final product price. (6) Market Acceptance: The market acceptance of natural dyes is still uncertain, and consumer awareness and preference for tea extract dyed products need to be further improved. Therefore, when promoting and popularizing tea extract dyes, there may be challenges in gaining high market recognition.
Research Limitations
This study also acknowledges certain limitations. The systematic review was confined to the WOS and PubMed databases; thus, excluding many valuable studies found in other databases. Future research should include a wider array of databases for a more comprehensive analysis of research trends and outcomes. Second, the scope of this systematic review was limited to scholarly journals, and subsequent research may profit by allowing for other released works, like business journals, books, book chapters, reviews, and industry reports.
Suggestions for Further Research
Tea dyeing may embrace a rising trend and several opportunities. Firstly, there have been many studies on the use of tea as fabric dyes. However, given the characteristics of natural plant dyes, most dyeing processes need mordants or other treatment methods to help improve the dye uptake and color fastness of dyes. Therefore, the optimization and innovation of the techniques in every tea dyeing stage present considerable scope for further research. Currently, metal mordanting is the most commonly used method, but it limits the environmental advantages of tea dyeing. 38 From the above analysis, it can be concluded that further research will focus on how to develop tea dyeing techniques in an efficient and eco-friendly manner. Secondly, current studies show that tea extracts can be used as functional dyes to dye fabrics and modify their functions at the same time. Among all the properties that tea dyes can impart to fabrics, antibacterial activity, UV protection and antioxidant activity have been fully demonstrated. In the future, researchers can dig into and verify more functional properties, and provide new possibilities for the development of functional fabrics. Thirdly, the recycling and reuse of tea wastes in different fields is also an issue worth studying. While reducing environmental pollution and realizing circular economy, this application of tea wastes can improve the livelihood of tea planters. Lastly, consumer demand research for natural dyes is crucial. By understanding consumers’ awareness, preferences, and purchasing intentions regarding natural dyes, businesses can develop targeted marketing strategies and product development directions. Based on this, selecting appropriate strategies to promote the environmental benefits and advantages of tea dye will be key to its popularization.
Conclusions
This review is a systematic summary of the research findings related to tea dyeing and its application. It aims to reveal the application potential of tea plants in textile dying and offer insights for further theoretical study and practice in this field. Generally speaking, the number of published articles about tea dyeing has been on the rise, especially since 2018. This indicates that in recent years, this field has received more and more attention and maintains high research value. The studies focus on the dyeing performance of tea dyes on different fabrics, the ingredients of tea dyes, techniques that can improve dye uptake and color fastness, and the multiple functions of tea dyes.
Through a systematic review of the included 43 articles, the following conclusions are drawn. Firstly, tea polyphenols are the major components of tea dyes, and catechins can turn into theaflavins, thearubigins, and theabrownins through oxidative polymerization. Therefore, tea dyeing generally can produce warm tones such as dark brown, light brown and reddish-brown. Secondly, when it comes to the dyeing effectiveness of tea dyes on different fabrics, more studies concentrate on natural fibers, especially cotton fibers. The color intensity, from high to low, of tea dyes on different materials is as follows: protein fibers > cellulosic fibers > synthetic fibers. The reason is that with different structures, various fibers also interact with tea dyes in different manners, thus having differentiated combination abilities. Thirdly, other materials or techniques can help enhance the adsorption rate and color fastness of tea dyes on fibers, among which metal mordanting is the most frequently used one. This is a traditional mordanting method, but it may cause damage to the environment. Therefore, more and more environment-friendly materials and techniques are being explored, one example of which is chitosan. Finally, tea dyeing can also impart different functional properties to fabrics, such as antibacterial activity, UV protection and antioxidant activity which have been strongly proven. Besides, some other studies have also validated that fabrics treated with tea dyes also have flame retardancy. All these properties can be attributed to tea polyphenols, the functional compounds, which are adsorbed onto the fabrics and give them multiple protective functions.
As the whole world attaches great importance to environmental issues, it is of significant research value to develop green and eco-friendly natural dyeing methods. As a natural plant dye, tea dyes have the advantages of abundant raw materials, a wide range of sources and low costs, thus having a huge development potential. This review can help researchers have a better understanding of the existing research findings, and offer insights and necessary references for further research in the field of tea dyeing.
Footnotes
Appendix
Details of the reviewed studies.
| ID | Paper title | Year | Types of tea | Fabrics | Mordants | Treatment methods | Functional features | Ref. |
|---|---|---|---|---|---|---|---|---|
| ID1 | Meta-mordant dyeing with Camellia sinensis (L.) O. Ktze var. waldensae (SYHu) Chang (yellow-bud tea) extract for wool fabrics treated by UV radiation | 2018 | Yellow-bud tea (by-product) | Wool | Metal mordants | UV radiation | Not applicable | 54 |
| ID2 | Dyeing characteristics and UV protection property of green tea dyed cotton fabrics–focusing on the effect of chitosan mordanting condition | 2006 | Green tea | Cotton | Chitosan | Not applicable | UV protection | 31 |
| ID3 | Modification of cotton fabric with acrylamide in the presence of K2S2O8 for improving dyeability of natural dyes | 2011 | / | Cotton | Metal mordants | Acrylamide modification | Not applicable | 32 |
| ID4 | Study the effect of metal ion on wool fabric dyeing with tea as natural dye | 2010 | Black tea | Wool | Metal mordants | Not applicable | Not applicable | 51 |
| ID5 | UV protection from cotton fabrics dyed with different tea extracts | 2016 | Black, green and red tea | Cotton | Chitosan | Not Applicable | 1. Antioxidant activity 2. UV protection |
56 |
| ID6 | Painting on handloom cotton fabric with colorants extracted from natural sources | 2014 | / | Cotton | Metal mordants | Not applicable | Not applicable | 70 |
| ID7 | Effect of dye bath pH on dyeing and functional properties of wool fabric dyed with tea extract | 2016 | Oolong tea (by-product) | Wool | Not applicable | Bath dye PH adjustment | 1. Antibacterial activity 2. UV protection |
15 |
| ID8 | Chitosan polysaccharide as a renewable functional agent to develop antibacterial, antioxidant activity and colorful shades on wool dyed with tea extract polyphenols | 2018 | Green tea | Wool | Chitosan | Not applicable | 1. Antibacterial activity 2. Antioxidant activity |
9 |
| ID9 | Concurrent dyeing and finishing of cotton with natural color and citric acid in the presence of NaH2PO4 as catalyst under thermal treatment | 2011 | / | Cotton | Metal mordants | Citric acid treatment | Not applicable | 60 |
| ID10 | Experimental researches regarding the ecological dyeing with natural extracts | 2014 | Black tea | Hemp | Not applicable | Not applicable | Not applicable | 8 |
| ID11 | Eco-dyeing with biocolourant based on natural compounds | 2018 | Green tea | Wool, silk, cotton | Not applicable | Non-enzymatic browning reaction | Not applicable | 5 |
| ID12 | Antibacterial evaluation of cotton fabric pretreated by microwave plasma and dyed with Taiwan folkloric medicinal plants | 2007 | Green tea | Cotton | Not applicable | Plasma treatment | Antibacterial activity | 59 |
| ID13 | Adsorption isotherms and mordant dyeing properties of tea polyphenols on wool, silk, and nylon | 2010 | / | Wool, silk, nylon | Metal mordants | Not applicable | Not applicable | 10 |
| ID14 | Dyeing of Eri silk with natural dyes in presence of natural mordants | 2018 | / | Eri silk | Tsuga canadensis (Snep sohmylleng) | Not applicable | Not applicable | 55 |
| ID15 | Preparation of biomass pigments and dyeing based on bioconversion | 2018 | Green tea (by-product) | Wool, silk, cotton | Not applicable | Enzymatic treatment | Not applicable | 47 |
| ID16 | Assessment of antibacterial activity of wool fabrics dyed with natural dyes | 2014 | Green tea | Wool | Metal mordants | Not applicable | Antibacterial activity | 52 |
| ID17 | Preparation of biocolorant and eco-dyeing derived from polyphenols based on laccase-catalyzed oxidative polymerization | 2018 | Green tea | Wool, silk | Not applicable | Enzymatic treatment | Not applicable | 44 |
| ID18 | Ultrasonic-bioscouring and ozone based bleaching of cotton slivers and coloration of them with natural dye sources | 2018 | Green tea | Cotton Sliver | Metal mordants | Ultrasonic-bioscouring and ozone based bleaching | Not applicable | 46 |
| ID19 | Natural dyeing and UV protection of plasma treated cotton | 2018 | Green tea | Cotton | Not applicable | Plasma treatment | UV protection | 58 |
| ID20 | Fabrication of bio-colored and functional wool using natural Pu’er tea extract | 2023 | Pu’er tea | Wool | Metal mordants | Not applicable | 1. Antibacterial activity 2. Thermal stability |
33 |
| ID21 | Chemical-free dyeing of cotton with functional natural dye: a pollution-free and cleaner production approach | 2022 | Black tea | Cotton | Not applicable | Cationization | Not applicable | 37 |
| ID22 | Azoic deep dyeing of silk and UV protection using plant polyphenols and diazonium coupling | 2020 | Oolong tea | Silk | Metal mordants | Azoic dyeing | UV protection | 41 |
| ID23 | Extraction of functional dyes from tea stem waste in alkaline medium and their application for simultaneous coloration and flame retardant and bioactive functionalization of silk | 2019 | /(by-product) | Silk | Metal mordants | Not applicable | 1. Antibacterial activity 2. Antioxidant activity 3. Flame retardant |
42 |
| ID24 | Sustainable recycling of cafe waste as natural bio resource and its value adding applications in green and effective dyeing/bio finishing of textile | 2023 | Black tea (by-product) | Silk, wool | Not applicable | Not applicable | 1. Antibacterial activity 2. Antioxidant activity 3. UV protection |
39 |
| ID25 | Preparation and long-persistent luminescence study on strontium aluminate particles dip-coated compound textile | 2020 | Green tea | Cotton | Metal mordants | Not applicable | UV protection | 67 |
| ID26 | Waste black tea leaves (Camelia sinensis) as a sustainable source of tannin natural colorant for bio-treated silk dyeing | 2021 | Black tea (by-product) | Silk | 1. Turmeric rhizomes, pomegranate peels, acacia bark,henna leaves 2. Metal mordants |
Microwave treatment | Not applicable | 43 |
| ID27 | A copper composite embedded in graphene oxide as an efficient mordant to enhance the properties of natural dyes for cotton fabric | 2023 | / | Cotton | Ternary composite as the mordant | Not applicable | Antibacterial activity | 36 |
| ID28 | Eco-friendly dyeing of cotton fabric with waste tea leaves-based tannin natural dye | 2021 | Black tea (by-product) | Cotton | Metal mordants | Microwave treatment | Not applicable | 45 |
| ID29 | Dyeing of cotton with the natural dye extracted from waste leaves of green tea (Camellia sinensis var. assamica) | 2018 | Green tea (by-product) | Cotton | Not applicable | Not applicable | Not applicable | 48 |
| ID30 | High-binding-fastness dye from functional extracts of Keemun black tea waste for dyeing flax fabric | 2021 | Keemun black tea (by-product) | Flax | Not applicable | No-mordant pad-dry dyeing strategy | 1. antibacterial activity 2. UV protection |
28 |
| ID31 | Characterization of tea aqueous extracts and their utilization for dyeing and functionalization of fabrics of different chemical compositions | 2023 | Green and black tea | Wool, cotton, cellulose acetate, polyacrylonitrile, polyester, polyamide | Not applicable | Not applicable | 1. Antibacterial activity 2. Antioxidant activity |
50 |
| ID32 | Sustainable application of microwave assisted extracted tea based tannin natural dye for chemical and bio-mordanted wool fabric | 2022 | Black tea (by-product) | Wool | 1. Pomegranate peels, Acacia bark, turmeric rhizomes 2. Metal mordants |
Microwave treatment | Not applicable | 40 |
| ID33 | Dyeing cotton with tea extract based on in-situ polymerization: an innovative mechanism of coloring cellulose fibers by industrial crop pigments | 2019 | Oolong tea | Cotton | Not applicable | In-situ polymerization dyeing technology | Antibacterial activity | 49 |
| ID34 | Laccase-assisted coloration of wool fabric using green tea extract for imparting antioxidant, antibacterial, and UV protection activities | 2023 | Green tea | Wool | Not applicable | Enzymatic treatment | 1. Antibacterial activity 2. Antioxidant activity UV protection |
38 |
| ID35 | Use of tea and tobacco industrial wastes in dyeing and antibacterial finishing of cotton fabrics | 2020 | /(by-product) | Cotton | Metal mordants | Not applicable | Antibacterial activity | 66 |
| ID36 | Enhancing polyphenols and tannins concentration on cotton dyed with red tea | 2023 | Red tea | Cotton | Chitosan | Not applicable | UV protection | 34 |
| ID37 | Extraction of tea natural dye and its dyeing properties on cotton fabrics | 2021 | Longjing tea | Cotton | Metal mordants | Not applicable | Not applicable | 29 |
| ID38 | Sustainable dyeing of mulberry silk fabric using extracts of green tea (Camellia sinensis): extraction, mordanting, dyed silk fabric properties and silk-dye interaction mechanism | 2023 | Green tea | Silk | Metal mordants | Not applicable | 1. Antibacterial activity 2. UV protection |
35 |
| ID39 | Harnessing the power of green and rooibos tea aqueous extracts for obtaining colored bioactive cotton and cotton/flax fabrics intended for disposable and reusable medical textiles | 2024 | Green tea | Cotton,cotton/flax blend fabrics | Not Applicable | In situ biosynthesis of copper-based nanoparticles (CuNPs) | 1. Antibacterial activity 2. Antioxidant activity |
27 |
| ID40 | Captivating coloring and antimicrobial properties of tea leaf and eucalyptus bark on jute–cotton union fabric | 2024 | / | Jute–cotton union fabric | Metal mordants | Not applicable | 1. Antibacterial activity | 61 |
| ID41 | Dyeing and performance testing of chitosan-modified cotton fabrics with tea polyphenols | 2024 | / | Cotton | Chitosan | Not applicable | 1. Antibacterial activity 2. UV protection 3. Odor resistance |
30 |
| ID42 | Eco-friendly dyeing of cotton with natural colorants using natural mordants obtained from aloe vera | 2024 | Black tea | Cotton | Aloe vera | Not applicable | Antibacterial activity | 57 |
| ID43 | Utilization of factory tea (Camellia sinensis) wastes in eco-friendly dyeing of jute packaging fabrics | 2024 | /(by-product) | Jute | Metal mordants | Not applicable | Antibacterial activity | 53 |
/: there is no explicit mention in the text.
Acknowledgements
We have no acknowledgments to declare.
Author Contributions
Conceptualization, H.-y.S. and H.X.; methodology, H.-y.S. and H.X.; software, H.X.; validation, H.X.; formal analysis, H.X.; investigation, H.X.; resources, H.X.; data curation, H.X..; writing–original draft preparation, H.X.; writing–review and editing, H.-y.S.; visualization, H.X.; supervision, H.-y.S.; project administration, H.-y.S. All authors have read and agreed to the published version of the manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
Data Availability Statement
All data generated in this review are included in this paper. Further enquiries can be directed to the corresponding author.