Bark Extracts as Multifunctional Finishing Agents for Technical Textiles: A Scientific Review

Abstract

Bark extracts are important sources of natural dyes. They possess many functional properties of potential interest to the textile industry. Currently, textiles with eco-friendly functional finishing are increasingly sought for in medical and protective clothing due to stringent environmental laws and the associated toxicity of synthetic agents. In view of this, recent studies on bark extracts for multi-functional finishing of textiles, particularly for antimicrobial and UV protective finishing, is reviewed. Bark extracts from various trees are able to effectively impart antimicrobial resistance and UV protection properties to treated fabrics; however, their long-term sustenance and strength depend on a multitude of factors. However, the application of bark extracts on several types of textile fabrics have no significant impact on textile quality.

Introduction

Textiles with multifunctional finishing are a subject of intense research in the recent past due to increase in demand for hygienic and protective clothing for healthy living. In this direction, the application of synthetic agents to impart antimicrobial functional finishing to textiles was developed. Pertaining to antimicrobial textile materials, various synthetic products, like triclosan (C₁₂H₇C_l3O₂), chlorine-based products, and positively-charged poly(hexamethylene biguanide) (C₈H₁₉N₅), have been popularly used in the industry for many years. However, these synthetic agents are hazardous due to their toxicity, side effects, and environmental pollution. Recently, as scientists are exploring the use of nano particles for gaining functionality at a lower percent of add-on, nano-silver has emerged as a popular and effective antimicrobial. It is true that nano-silver is not as toxic as the previously mentioned chemicals, but cost, difficulties in nano particle preparation, nano toxicity, durability, and other issues are major obstacles to its use.

UV-protective materials also have importance in daily life as harmful UVC (directly reach to the earth from sun) radiation can cause cancer due to the high energy of irradiation. Most textile materials easily allow UV-ray transmission to our bodies through their structure and may cause serious health problems. From the last few decades, researchers have tried to use different organic and inorganic synthetic UV-protective substances for textile treatment. Among the various developments, metal compounds (e.g., zinc oxide, titanium oxide, alumina, stannous chloride, and their nano forms) based finishes, synthetic polyphenolic compounds like benzophenone, 2-hydroxy phenyl benzotriazoles, benzoic acid esters, and hindered amines) have been used popularly for natural and synthetic textiles. Most of them are colorless, costly, are not easily available, and also have toxic effects.

Taking into account that natural dyes are non-allergic, non-toxic, easily available, comparatively cheaper, and eco-friendly for use on textiles, with some having inherent additional properties (e.g., antimicrobial and UV absorbance), these dyes can serve the dual purpose of a dye and a functional agent. This reduces the number of processing steps in industrial textile fnishing with the added benefit of environmental safety.^1-4 Among natural dyes ⁵ from plant sources, bark extracts from the barks of different trees are being increasingly used for multifunctional finishing due to the sustainability of this resource, multi-functionality associated with variation in chemical constituents, and a variety of natural sources of origin.

The bark is the outermost region of the tree trunk formed due to the secondary growth of the cork cambium, which protects the tree from physical factors such as rain, wind, high temperatures, and from attacks by microbes, insects, and other organisms. Since trees are abundant in nature and bark is shed by trees on a regular basis, using bark for multifunctional finishing of textiles can be economically cheap and environmentally sound. Bark extracts as natural dyes are traditionally known to have several useful properties with the potential to be used in innovative textile finishing. The multifunctional properties in bark extracts are due to the presence of secondary metabolites of chemical constituents such as phenolics, tannins, flavones, flavonoids, hydroxyquinones, alkaloids, terpenes, and terpenoids. The diversity in bark extracts gives opportunities for screening their multifunctional potential. Eco-friendly natural dyes from bark extracts having antimicrobial effects ³ and UV protective properties ⁶ are desired for technical textiles and for protection against UV rays. This study reviews the progress of bark extract use for imparting antimicrobial and UV-protection functionalities on textiles.

Bark Extracts for Textile Finishing

The main source of natural dyes is either plants or microorganisms. The dyes from plant sources have different origins. Their overexploitation may lead to the depletion of natural resources and may even result in the extinction of certain plant species. Thus, Global Organic Textiles Standards (GOTS) was created to protect these endangered plants from being used in the extraction of natural dyes. ⁷ On the other hand, extraction of dyes from the bark sources is an ecologically sustainable approach as it uses the bark, which is regularly shed of by trees without harming the trees. Bark is easy to collect, store, and transport. Natural dyes with antimicrobial properties are useful in textiles, in addition to imparting colors. Natural dyes are useful for making medicinal garments, sanitary napkins, socks, and other textiles, as they provide both attractive color and antimicrobial properties on the dyed fabric.^2-5 A summary of the bark extracts and their applications covered in this review paper are given in Table I.

Table I

Technical Summary of Bark Extract Textile Treatments

Plant Bark Extract	Chemical Name of the Active Species	Main Function	Antimicrobial Value (% Inhibition or mm) or UPF Value	Fiber Application	Reference
Azadirachta indica (Neem)	Azadirachtin A, Salanin, Saponin	Antimicrobial	>94%-99% reduction for both Gram-positive and Gram-negative test bacteria for both extracts in conventional as well as microwave cured samples	Cotton and 10% (w/v) of the bark extract	1
Acacia arabica wild	Tannin	Natural dye, antimicrobial	Bromophenol Blue test for dyed fabric. resistance to Gram-positive S. aureus was 99.9% for mordanted and dyed fabric.	Cotton fabric	3
Casaurina equisetiflia	Ellagic acid, tannin, quercetin gallic acid, kaemferol	Natural dye with mordant, Antimicrobial activity	Reduction of 45% and 58% against E. coli and S. aureus, respectively.	Silk 10% owf at MLR 1:20 and pH 5	12
Elm	-	Natural dye with mordant, antibacterial	Reduction in bacteria for dyed fabric are: Untreated, None; mordanted, 99.4%; Cationization 99.9%, Al 97.5%, Cu 99.9%, Fe 77% against Methicillin resistant S. aureus (MRSA) ATCC 33591.	Concentration of dye in elm bark extract was 1.85% owf and applied at a ratio of 1:50 for cotton dyeing.	24
Betula verrucosa (birch tree)	Betulin and betulinic acid	Antibacterial, hydrophobic	Bacterial reduction of up to 99.9% for both E. coli and S. aureus	For cotton, betulin/betulinic acid solution is applied with toluene at a concentration of 0.16 M and grafted by crosslinking.	22
Malabar Neem, Melia composite Willd. syn M. Dubia Cav	Modified triterpenes, limonoids, terpenoids, and phenolics	Natural dye and antifungal activity	Tested against five fungi: Aspergillus flavus, A. niger, A. parasiticus, Fusarium monoliforme, and Penicillium canescens. Highest growth reduction is observed against A. parasiticus (85.35%, 81.63%, 75%) for three dyed fabrics of wool, silk, and cotton, respectively.	Wool, silk, cotton	25
Acacia arabica Wild	-	Natural dye and copper sulfate mordant, antibacterial	Resistance against both Gram-positive and gram-negative bacteria. Growth of organism was measured by bromophenol blue test.	Cotton fabric 1:50 is the dye solution.	3
Cassia fistula (Amaltus)	Rich in tannins, phlobaphenes, and oxyanthraquinone substance with emodin and chrysophanic acid, along with fistuacacidin, barbaloin, and rhein. The main coloring component of stem bark is 1,8-dihydroxy-6-methoxy-3-methyl anthraquinone	Natural dye, mordanted, chitosan treated, antimicrobial	Increase in chitosan concentration leads to increased inhibition zone of all tested microorganisms compared to untreated colored wool. E. coli, B. subtilis are highly affected, meanwhile P. aeruginosa and S. aureus shows a moderate effect.	Wool fabric; 2% or 3% dye concentration is used for dyeing, 4% solution post mordant tannic acid, optimum pH 5, temperature 90 °C	11
Ventilago mdraspantana (Venpadam)	-	Natural dye with pre-mordant, antimicrobial properties	The antibacterial activity zone of dyed bamboo with alum and myrobalan mordant for S. aureus was 33 mm and 37 mm and for E. coli 32 mm and 36 mm, respectively. antimicrobial activity is durable above 20 wash cycles	Viscose fabric from bamboo, 10% extract used for dyeing.	21
Rhizophora apiculata Blume (Magnifera or Mangrove)	-	Natural dye, pre-mordant, UV protection	UPF: 26.7 fabric in 60% dye owf with UPF protection, for CuSO4 (40%) and 40% and 60% dye owf, the UPF is 39.5 and 41.5 respectively. Control silk has 4.4 UPF value (no protection)	Silk fabric, dyed at a liquor ratio of 1:40 and pH 5.4, at 90 °C for 60 min.	36
Garcinia dulcis (Roxb.) Kurz Bark (Maphuut) or Mundu, of the Family Guttiferae	-	Pale yellow natural dye, with mordant more colors, UV protection	Dyed fabric without mordant UPF value of 15.6. Silk fabric dyed with AlK(SO4)2, CuSO4, and FeSO4 mordants with excellent UV protection, while with SnCl2 mordant rated as very good UV protection.	Silk fabric, dye concentration is 20 g/L prepared by spray dried dye powder extracted from bark. Mordant concentration is 5, 10, 20 g/L.	35
Rhizophora apiculata Blume (Mangrove)	Bark rich in tannins (15%-36%), condensed tannins occur composed of four flavonoids: catechin, epicatechin, epigallocatechin, and epicatechin gallate.	Natural dye, with mordant (pre, post, and meta mordanting), UV protection	Dyed fabric without mordant had very good UV protection (UPF 25 to 39) and dyed fabric with mordant had excellent UV protection in most cases studied. (UPF>40)	Cotton fabric; dyed at a fabric to liquor ratio of 1:40 and pH 5.4 at 90 °C for 60 min.	37
Xylocarpus granatum (Cannonball mangrove)	Tannin rich, condensed tannin catechol bark contains approximately 20%-34% tannin on a dry matter basis.	Natural dye, UV protection properties	UPF 50+ for all dyed fabrics indicating excellent UV shielding.	Cotton fabric	38
Quercus (Oak bark)	Gallotannin, ellagitannin, quercetin, and quercetin-3-O-glucoside	Natural dye, UV protection, antimicrobial	UV protection factor values of mordant dyed sample >40. Reduction in bacterial count S. aureus and E. coli reached 85% and 80%, respectively. Good washing durability for both properties even after 50 wash cycles.	Tussah silk fabric. Dyed in liquor ratio of 50:1 at pH 5.	40
Terminalia arjuna (Arjun tree)	Baicalein and ellagic acid. High tannin content.	Natural dye, antimicrobial and fluorescence properties	>85% microbial reduction for Gram-positive bacteria with 5 to 10 mg/mL dye concentration. Unmordanted dyed wool showed more activity than mordanted samples.	Woolen yarn	31

Natural fabrics absorb more moisture that synthetic fabrics. Hence they are susceptible to growth of microorganism, causing unhygienic conditions and the microbial degradation of fabrics. Patel and Tandel have used natural dyes from bark extracts from Acacia arabica (Babool), Acacia catechu (Catechu), and Terminbalia chebula (Harda). The wild Acacia arabica (Babool) tree is commonly found throughout India. This bark extract, when treated with a mordant, imparts a brown color to the textile, along with microbial resistance. Fabric dyed with the extract of Acacia arabica and copper sulfate mordant showed very good resistance against both the Gram-positive and Gram-negative microorganisms. Tannin polyphenols present in the dye molecule assist in the formation of bridges between the chromophoric group of the dye and the fiber surface. ³

Bark extracts are increasingly being screened for use in the antimicrobial finishing of natural fiber fabrics (e.g., cotton, wool, silk, and linen) to protect these fabrics from microbial degradation. Recently, studies on the application of natural dyes as antimicrobial finishing on textiles are growing due to bans on some synthetic dyes and to address the challenges of antibiotic resistant microbes in medical garments that can lead to and hospital cross-infections.^4,7

Researchers have studied the antibacterial finishing of cotton textiles using neem (Azadirachta indica) extract from the bark and seeds of the neem tree. A major biologically active phytochemical group known as limonoids, containing many anthraquinone compounds, was isolated from the neem bark extract. The limonoids (Fig. 1), namely azadirachtin A, nimbin, and salanin, were isolated and characterized. The three major active ingredients present in the bark extract of neem also include tannin and other polyphenolic anthraquinoids. These compounds showed antimicrobial activity against both Gram-positive and Gram-negative representative bacteria in various studies.^1,2 The mechanism of bactericidal action on Gram-positive Staphylococcus aureus includes rupturing the bacterial cell wall, thereby causing the loss of cytoplasm including nucleic acids. Also, the decrease in UV absorbance at 260 nm indicates the breakdown of nucleic acids due to membrane leakage. The bark extract is more effective than seed extract in their bactericidal action. The neem bark extract is chemically attached through cross-linking agents, such as the non-formaldehyde based glyoxal/glycol combination, to render durable antibacterial finishing to the cotton fabric. ¹ The finished fabric has antibacterial durability for up to five machine washes. The application of the extract has no significant effect on the properties of the fabric such as the appearance, tensile strength, crease recovery angle, and so forth. A variety of triterpenoids are present in neem; the final biosynthetic product is azadirachtin A. Other biologically important components represented in Fig. 1 are less complex intermediaries of the biosynthetic pathway. Another role of azadirachtin A is as an insecticide, through the severe growth and molt disruption, and antifeedant effects in insects. Azadirachtin A is structurally analogous to ecdysone, an insect hormone that regulates insect pupation. Application of azadirachtin A interferes with the hormone, blocking the development process leading to pupation, thereby killing the insects. Other components have synergetic effects in effectively keeping insects from developing resistance. Azadirachtin A as an insecticidal agent has different targets ranging from organs and tissues to the cellular level causing multiple effects (e.g., antifeedant action, growth regulation, sterility, and cellular inhibition) on insects. ⁸

Fig. 1

Structure of the active components present in neem bark extract.^1,8,9

Neem seeds have been prevalently used as a source of bio-insecticide, being rich in azadirachtin A compared to other parts of the plant, ¹⁰ The efficacy of neem bark and leaves extract as a natural dye and its anti-moth properties on woolen fabric are evaluated. Results showed that bark extract is efficient as an anti-moth agent without mordants. Improved efficacy is noticed in fabric treated with alum and tin chloride as mordants. This is attributed to high concentrations of azadirachtin A and cyanogenic glucosides (HCN) in bark extracts. The mechanism of action is through hydrolysis of cyanogenic glucosides, forming hydrogen cyanide and aldehydes or ketones. Simultaneously, the high level of azadirachtin A makes the fabric antifeedant. The anti-moth property is similar to commercial anti-moth agents. ¹⁰

The bark extract of Cassia fistula (Amaltus), as both a natural dye and antimicrobial agent for wool fabric, improves the antibacterial activity of wool fabric. The dye component that imparts color is 1,8-dihydroxy-6-methoxy-3-methyl anthraquinone (Fig. 2). The dye is rich in tannins. The dyed wool fabric treated with chitosan shows improved inhibition to Escherichia coli, Bacillus subtilis, and Pseudomonas aerogenosa compared to untreated dyed wool. However, the inhibition is moderate against S. aureus (Gram-positive bacteria) by chitosan-treated dyed wool and no inhibition is reported with dye treated wool samples. ¹¹ However, the authors did not mention the antimicrobial activity of the control wool fabric used for their study.

Fig. 2

Structure of 1,8-dihydroxy-6-methoxy-3-methyl anthraquinone, the main dye component in Cassia fistula bark extract. ¹¹

In another study by Narayanaswamy et al., the properties of Casuarina equisetifolia (Australian pine tree) bark extract on protein fabric (silk fabric) is studied. ¹² Unlike cellulosic cotton fiber, silk fiber contains different amino acids and its chemical structure is composed of amine and carboxylic groups. Because of these oppositely charged groups, silk is amphoteric in nature. It has been recommend by technical analysts to process silk fabric at the isoelectric point (pH 4.5) of the fiber. The study suggests that pre-mordanted dyeing with alum, tannic acid, and tartaric acid improved the color values and fastness properties of the silk fabric. The study reveals that tannin may help the fixation of the dye with the fiber as tannins tend to become insoluble in the presence of mordants. It is reported that the dyed silk exhibits antibacterial activity against both Gram-negative and Gram-positive bacteria. This antibacterial activity may be attributed to the presence of gallic acid, ellagic acid, and tannin components (quercetin, gallic acid, kaempferol) present in the bark extract (Fig. 3). ¹² Active species and the secondary metabolites of C. equisetifolia contain polyphenolic and branched chain tannin based compounds.

Fig. 3

Structure of secondary metabolites in Casuarina equisetifolia bark extract. (a) Ellagic acid, ¹⁵ (b) quercetin, ¹⁶ (c) gallic acid, ¹⁷ and (d) kaempferol. ¹⁸

The study by Borges reveals the mechanism of gallic acid action, ¹³ a hydroxybenzoic acid and a hydroxycinnamic acid, on E. coli, P. aeruginosa, S. aureus, and Listeria monocytogenes. The antimicrobial activity causes irreversible changes in membrane properties (e.g., charge, and intra- and extracellular permeability) and physicochemical properties through hydrophobicity changes and decreases in negative surface charge. Occurrences of local rupture or pore formation in the cell membranes with consequent leakage of essential intracellular constituents are confirmed through various tests.

Very recently, Salim et al. proposes that Aleppo pine (Pinus halepensis) bark extract contains high level of flavonoids and gallic acid. As a result, the crude extract shows 95% antibacterial activity against Gram-positive and Gram-negative bacteria. ¹⁴ From all results, the antibacterial activity of pine bark extract varies, depending on its variety, soil, and the environment where it is grown. These conditions determine the quantity of antibacterially-active ingredients present in the extract.

Sahoo et al. dyed silk fiber by using the bark extract of sal (Shorea robusta gaertn. F.) and mordants. Color intensity and wash fastness properties of the dyed fabric depended on the type of mordant used for the experiment. Copper sulfate (3%) as mordant showed the maximum level of shade change after treatment. Antimicrobial efficacy of the treated fabric also was reported by the author. ¹⁹ Another research group described the effect of votiyar tree (Odina wodier) bark extract on cotton fabric as an antimicrobial as well as an antifungal agent. ²⁰ Further study is needed for conclusive status of antimicrobial activity on dyed fabric, as the functional properties of the dye are not the true indication of properties on the fabrics. The properties of dyes on fabrics depend upon the nature of the fabric, dye, and mordant.

Javakabanu and Manimekalai studied the antimicrobial activity of bamboo fiber made fabric treated with venpadam (Ventilago mdraspantana) bark extract and showed that the dyed fabric exhibits durability of the activity up to 10 wash cycles. ²¹ In a unique study, Huang et al. show that cellulosic fabrics treated with birch tree (Betula verrucosa) bark extract, containing the active species betulin/betulinic acid (Fig. 4), exhibited hydrophobic and antimicrobial properties. The non-polar methyl group (-CH₃) present in the structure of betulin contributes to the hydrophobic property of the treated fabric. ²² In a recent review ²³ on the mechanism of betulin as an antimicrobial agent, betulin enhances the electron transport chain activity, resulting in increased reactive oxygen species generation, Fenton reaction, lipid peroxidation, and bacterial DNA fragmentation. This generates oxidative stress in pathogenic bacteria (e.g., E. coli, P. aerogenosa, and S. aureus) leading to bacterial cell death. Betulin/betulinic acid could be bound to the cellulosic fabric through ether or ester linkages, respectively, improving the functional properties of the cellulose.^21,22 This would promote its application in functional textiles for sportswear, medical mattresses, and bandages, where waterproof antibacterial activity is needed. ²²

Fig. 4

Structure of betulin and betulinic acid active ingredients present in birch tree bark. ²¹

In another study, a cotton knit was dyed with elm bark (EB) extract. The knit is preprocessed with a cationizer. Dyeing with a copper mordant displayed high levels of antimicrobial activity that resisted super bacteria. The bacterial reduction rate of cotton knits dyed with EB extracts (after 24 h incubation) were studied: for untreated (0%), for mordanted (99.4%), for cationization (99.9%), and were simultaneously dyed and mordanted with aluminum sulfate (97.5%), copper sulfate (99.9%), and ferrous sulfate (77%) in separate baths. The concentration of dye in the elm bark extract was 1.85% owf applied at a liquor ratio of 1:50 for dyeing test cotton. The super bacteria used is methicillin resistant S. aureus (MRSA) ATCC 33591. ²⁴

Pal et al. studied the antifungal activity of natural colorants from Malabar Neem (Melia composite) bark on cotton, silk, and wool against five strains of pathogenic fungi. The antifungal activity was effective in cotton fabrics treated with natural dye at concentrations greater than 100 mg/mL. ²⁵ Among the fabrics studied, wool fabrics exhibited remarkable antifungal activity. The study proposed that the property of the dye may be due to the presence of many chemical constituents like limonoids, terpenoids, and phenolics. Limonoids are tetranortriterpenes, and terpenoids are compounds containing multiple isoprene units (Fig. 5). Phenolics are phytochemical components, and aromatic compounds (containing benzene rings) with hydroxyl groups and three carbon side chains. The bark extract is not only a good source of dye, but is also a sustainable alternative for imparting protective finishing to different kinds of textile materials.

Fig. 5

Chemical structure of the (a) prototypical limonoid and (b) the terpenoid taxol, an anticancer drug obtained from bark of Taxus brevifolia.^2,26

Although the role of secondary metabolites as antimicrobial agents are traditionally well known and experimentally proven, the studies on mechanism of action of secondary metabolites are still a subject of intensive research. These mechanisms of secondary metabolites are important as they are being increasingly explored as alternatives for antibiotics in light of multiple drug resistance to pathogenic bacteria, as substrates for innovative drug design and development, and sources of components for health and personal care products. ²⁸

Studies have reported the existence of more than one possible mechanism of action for secondary metabolites. This is possibly due to the structural complexity and other properties such as the hydrophobic nature of secondary metabolites. ²⁹ Scalbert ³⁰ has reviewed the antimicrobial activity of tannins, which is one of the major components of almost all bark extracts. The mechanism includes both direct and indirect modes of action. The inhibition of extracellular microbial enzymes, substrate deprivation for microbial growth, and iron deprivation constitute indirect mechanisms (bacteriostatic), while inhibition of oxidative phosphorylation is a form of direct action (bactericidal) on microbial metabolism.

Another recent work studied the antimicrobial and fluores-cent properties of Terminalia arjuna natural dye bark extract when applied on woolen yarn. ³¹ As per the report of the researchers, the main coloring components present in arjuna bark extract are baicalein and ellagic acid, structures given in Fig. 6. The absorption of dye onto woolen yarn increases with decreased pH and the maximum absorption is reported at pH 3.5. Under acidic pH conditions, the amino groups of wool is protonated, causing ion-dipole interactions with the hydroxyl group of the dye chemical components, whereas the carboxyl groups are un-ionized. In addition, the possible hydrogen bonding between hydroxyl groups of baicalein and ellagic acid and the protonated amine functionalities of woolen yarn is depicted in Fig. 6.

Fig. 6

Structures of the main coloring components of T. arjuna and schematic representation of the hydrogen bonding and ionic interaction of the natural coloring group with wool polymer. ²⁹

The antibacterial activity of the arjuna bark extract is attributed to its high tannin-based polyphenol content. Although its mechanism of action is not precisely known, it is predicted that tannin polyphenols are responsible for the loss of structural integration and protein functionality of the bacterial cell wall. This phenomenon occurs due to the hydrogen bonding, reaction with sulfydryl groups, or by covalent bond formation between tannins and proteins. In the study, Gram-positive bacteria were more susceptible to the antimicrobial effect of the dye than Gram-negative bacteria, with maximum inhibition reported by Bacillus subtilis (MTCC 736) and S. aureus (MTCC 902). The difference of the antimicrobial susceptibility was explained on the basis of differences in cell wall components of the two types (Gram-positive and Gram-negative) of bacteria. The outer phospholipid layer in Gram-negative bacteria, with highly negative-charged structural lipopolysaccharide components, acts as an additional barrier for preventing the entry of bioactive polyphenolic components. The extract shows variations of color ranging from light green to dark green using different solvents and reagents under daylight and UV exposure. The dye solution shows an intense peak at 509 nm under an excitation wavelength of 277 nm, and wool fiber shows no fluorescence properties, as it registered a broad peak at 270-290 nm. Dyed wool yarn showed photoluminescence and the observed peak was recorded at 344 nm. The shift is the combination of a hypsochromic shift of 165 nm (with respect to dye solution) and a bathochromic shift of 64 nm (with respect to undyed wool fiber). The researchers have corroborated the shifting phenomenon with the linkages developed between the dye components and woolen yarn. Thus, the bark extract of T. arjuna is a potential eco-friendly natural dye for protein fibers and also has the potential to act as an efficient antimicrobial and fluorescent finishing agent.

Bark Extracts for UV Protection

UV-protective textiles are important for use in our daily life as it guards us from UV rays that may potentially damage skin or cause cancer due to over exposure. UV rays have been classified as UVA, UVB, and UVC, depending on the intensity of the exposure. UVC has wavelengths less than 280 nm. This highly-energized ray has different detrimental effects on the human body. Generally, the ozone layer of the atmosphere protects the earth from solar UVC rays. UVC rays may also reach the earth's surface due to a small depletion in the atmospheric ozone layer. For this reason, protection against UVC rays is one of the utmost important measures for maintaining good human skin care.

The UV-protection performance of textiles is measured by the UV Protection Factor (UPF). Textile materials having a UPF value > 50 are UV resistant, and materials having a UPF < 10 exhibit low resistant against UV rays. ⁶ Textile coverings having lower UPF values can increase the risk of skin disease. ³² Scientifically, the UV protective property of any textile depends on its cover factor (low cover textiles can easily transmit UV rays to underlying material), color (dark colors can absorb more UV rays than light colors), the chemical groups present in the dyes used to color textiles, and structural properties of the textile materials (e.g., the UPF of virgin wool and silk is greater than virgin cotton with the same cover factor). A UPF factor of 50+ for any textile material is excellent for protecting the underlying surface from harmful UVC rays. Most research studies have shown that tannins as mordants were efficient in absorbing UVA radiation as compared with natural dyes used. This indicates that tannin rich dyes are potential sources for imparting UV protection properties to fabrics. The efficiency of UV radiation absorbance by tannins was similar to carotenes and anthocyanins (Fig. 7). ³³

Fig. 7

Chemical structures of (a) carotene ³³ and (b) anthocyanins. ³⁴

UV-protective properties of silk fabric could be improved by incorporating the bark extract of kurz (Garcinia dulcis). In addition, this extract has also provided an attractive dyeing effect on the fabric surface. Color brightness of the treated fabric was increased with the use of mordants like ferrous sulfate and copper sulfate. ³⁵ The same research group also engineered UV-protective silk fabric by using the extract of magnifera (Rhizophora apiculata Blume). They revealed that the UPF factor of the treated silk fabric depended on the concentration of the dye and the mordant used. Moreover, the type of mordant (metallic group present in the mordant) is also an important factor for determining the UPF of the finished fabric. The bark extract, when used with various mordants such as aluminum potassium sulfate, stannous chloride, and copper sulfate, showed different color shades, from medium to dark reddish brown. Among the mordants, copper sulfate exhibited better protection against UV (with a UPF of 38.6 versus 4.6 of the control silk) and gave a darker color to the silk fabric. ³⁶ Furthermore, Mongkholrattanasit et al. studied the dye extracted from the same magnifera species for its UV-protective property on cellulosic (cotton) fabrics. Cotton fabrics treated with the dye had excellent UV-protective properties. The application of mordants enhanced the UPF of the dyed fabric. The bark is rich in tannins, particularly the condensed tannins, and the structure of the four monomers is shown in Fig. 8. ³⁷

Fig. 8

Four flavonoid monomers of condensed tannin in mangrove bark.^36,37

Extract of the mangrove plant (Xylocarpus granatum) was used by researchers for dyeing cotton fabric.^36,37 The dyeing effect on the fabric surface was developed due to the presence of tannin components in the extract. The tone of the dyed shade also varied with use of a variety of mordants, due to the formation of tannate complexes. The nature of the metal ion in the mordant is responsible for the different colors of the dye. This is due to the presence of condensed tannins, which are the most abundant components in the bark. The O-dihydroxy (catechol) groups of condensed tannins (Fig. 9) form complexes with metal ions (metal chelates, or more specifically, metal tannates), which deviate in shade depending on the mordant used. The wavelength of the absorbed light depends on redox potentials and hence on the nature of the metal ion. The K/S (color strength) value of the dyed fabric changes with the mordant used, and the maximum K/S value is exhibited by ferric sulfate. This may be due to the combination of the green-black color dye complex of the condensed tannins. It is postulated that the dye molecules form charge transfer complexes with metal ions and is responsible for the absorption of light in the visible spectrum. The study also showed that the mordant or the dye had no significant impact on the stiffness of the fabric or its mechanical properties, except in the case of sodium dichromate mordanting. The dyed samples exhibited excellent UV-protective properties (UPF 50+) due to the presence of tannin molecules which contain functional groups that absorb UV radiation. Increased tannin uptake as a result of the complex formation with metallic salt mordants further strengthened this property. Hence, the dye is reported to be a promising colorant for the natural dyeing of cotton. ³⁸

Fig. 9

Structure of catechol and its chelating mechanism with metal ions. ³⁹

Jia et al. studied the effect of dyeing and functionality of tussah silk fabric treated with oak bark (Quercus) extract. ⁴⁰ The study revealed that after dyeing the fabric with oak bark extract, UV protective and antimicrobial properties could be transferred to the silk fabric along with its natural dyeing effect. The dyeing quality further improved with the application of mordant and it was found that the color strength value was higher for the post-mordant dyed samples than the pre-mordant dyed ones. The color of the oak bark extract imparted on the fabric varied with the type of mordant used in the experiment. It was also confirmed by the same research group that the said functional properties of the fabric were stable and durable, even after repeated washing. The coloring components of the bark extract were identified as gallotannin, ellagitannin, quercetin, and quercetin-3-O-glucoside (Fig. 10). The UV-protective property of the bark extract improved with the use of mordants. Ferrous sulfate, as a mordant, exhibited the highest UV-protective ability. This property was attributed to the complexation between the massive o-phenolic hydroxyl groups in the oak bark dye (e.g., coming from the gallotannin and ellagitannin) and the metal cations of the mordant. The treated silk fabric exhibited antimicrobial activity against both Gram-positive and Gram-negative bacteria due to the presence of a large amount of polyphenol in the tannin of the bark extract, which complexes with protein. When in contact with a microbial cell surface, the tannin in the dyed tussah silk fiber potentially destroys the lipids of the microbial cell membrane by combining with the proteins of the cell wall, ultimately resulting in the leakage of intracellular components. The combined properties had washing durability even after 50 wash cycles. ⁴⁰

Fig. 10

Structures of (a) gallotannin, ⁴² (b) ellagitannin, ⁴³ (c) quercetin, ¹⁵ and (d) quercetin-3-O-glucoside from oak. ⁴⁰

The UPF value of untreated wool (15-20) and silk (10-15) is higher than other natural fibers of cellulosic and ligno-cellulosic origin, having UPF values less than 10. ⁴¹ It means the efficacy of blocking the UV rays is greater for these protein fibers compared to cellulosic fibers. However, any textile material having UPF factors greater than 50 is considered to be UV resistant in terms of transmission of harmful UV rays. Therefore, in spite of the comparatively higher inherent UPF values of the protein fibers, they need additional treatment to reach UPF values greater than 50. However, relatively less external treatment is required for UPF factor improvement due to the comparatively higher UPF factor of the base polymeric material itself.

Han et al. reported that control wool fabric is 20% resistant against E. coli and 50% resistant against S. aureus. They treated wool fabric with curcumin extract and achieved 99.9% antimicrobial activity against both types of bacteria. ⁴³ However, Rather et al. reported that control wool showed no antimicrobial activity against S. aureus, P. aeruginosa, E. coli, and B. subtilis, whereas wool fabric, mordanted and dyed with 10% concentrations of arjuna bark extract, showed antimicrobial activity of 90 to 95%. ³¹ This contradiction in the antimicrobial activity of control wool may be due to different varieties of wool fiber (e.g., angora and pashmina) used in the research. Pal et al. reported that both wool and silk treated with the bark extract of melia composite showed an antifungal activity of 75-85% against different types of fungi. However, they did not mention the antifungal activity of control wool and silk fabrics. ²⁵

Conclusion

Research on multi-functional finishing of textiles is gaining momentum as the unexplored properties of bark extracts from many bark sources, with their special constituents, are being revealed. However, the problems of poor binding, poor fastness, unreliability, non-repeatability, and the wastage of dye extract exist. Bark is a renewable resource that is shed by trees and is also a waste product in timber processing. Tough available in abundance, it is not currently being used effectively. Also, the presence of a wide variety of tree species in varied geographical regions provides a wide diversity in sources of bark extracts and their properties. The screening and characterization of bark extracts for functional properties on textiles is a challenging task due to the variety of resources and their complex chemical constituents. Owing to their great diversity in multi-functional properties, barks of different trees can be promising candidates for the creative and green technical finishing of textiles in the coming years.