The progress and prospect for sustainable development of waste wool resources

Abstract

With the growth of the world population and the improvement of living standards, the demand for global textiles has been increasing rapidly. Although natural fibers are affected by the development of synthetic fibers, wool still occupies a certain share in the textile industry and is one of the most indispensable materials. However, many postindustrial and post-consumer waste wool textiles will be produced. The conventional treatment method is landfill or incineration, which is not conducive to economic, environmental, and social development. To counter this problem, many measures and methods have been adapted for the reuse of waste resources. This article provides a review on waste wool recycling and summarizes two main directions for reuse. Waste wool can be used for thermal and sound insulation materials, reinforced composite materials, or adsorbent materials to purify contaminated water which rely on fiber properties. Keratin extracted from waste wool can be applied for the production of high-value products such as functional finishing agents, organic fertilizers, regenerated protein films/fibers, or smart wearable electronic devices. Meanwhile, future development trends and the demand of waste wool recycling are also discussed. Continuous research and exploration are still needed for effective management of waste wool resources and to turn them into useful and valuable materials or products.

Keywords

Textile waste wool recycling sustainable application

Introduction

Today, about 100 m tons of waste textiles are produced in the world every year. According to the statistics, waste textiles in China could reach about 26 m tons as early as 2013, while the current production of waste textiles has reached 40 m tons.¹ Theoretically, 95% of these waste textiles can be recycled and reused, but the recycling rate is very low. In Europe and America, the recycling rate is only 20%, while the recycling rate in China is less than 10%.^2–4 Studies have reported that recycling and reuse of waste clothes can reduce greenhouse gas emissions by 53%, decrease pollution caused by the use of chemical agents by 45%, and cut down water eutrophication levels by 95%,⁵ as shown in Figure 1(a). Overall, converting waste wool textiles into useful and valuable materials and products can effectively improve the problems of solid wastes, environmental pollution, and resource waste.

Figure 1.

(a) The benefits of recycle and reuse waste textiles and (b) the yield distribution of major wool producing areas in the world. (Network picture: https://image.baidu.com/).

Wool is a significant natural fiber and one of the earliest fiber materials to be used. It is reported that wool was applied in Egypt and Northern European countries as early as the 14th century BC.⁶ At present, the main wool producing regions in the world include Australia, China, New Zealand, Argentina, South Africa, and Russia, and the yield distribution of major wool producing areas in the world is presented in Figure 1(b). Due to competition from synthetic fibers, the share of wool in the total world fiber supply has declined, which has a certain negative effect on the wool industry. However, wool is still an important and valuable textile raw material because of its merits, such as excellent hygroscopicity, elasticity, warmth, softness, and so on. Wool production can reach about 1.1–1.2 m metric tons of clean wool per year.⁷

Wool involves 82% keratin protein, 17% non-keratin protein, and 1% non-protein substance.^8,9 It is composed of α-amino acid helix macromolecules containing carboxyl groups, amine groups, and hydroxyl groups. Salt bonds and hydrogen bonds can be formed between macromolecules. Besides, disulfide bonds (S-S) are formed by cysteine. The cross-section of wool fiber is divided into scale layer, cortex layer, and medulla layer from the outside to inside. Among various fiber textiles, wool can be identified by microscopy due to its unique scale structure. The chemical composition and morphological characteristics determine the unique properties of wool. Wool textile is favored by customers due to its special fabric style.

It is well known that wool fibers are obtained directly from sheep and can vary in fineness and length. Not all wool fibers enter the textile process chain, short wool and coarse wool are discarded without textile application value. Meanwhile, consumers' ability to purchase the garments has been increased with the improvement of living standards. The garments will also be deserted after a certain period, attributed to the outdated styles and changed preferences. The average lifetime of wool garments is deemed to be about 2.8 years.¹⁰ Therefore, huge amounts of wool waste are generated every year causing a waste of resources and environmental pollution. It is of great significance to find a novel and high-efficient way to recycle and utilize those waste textiles.

Figure 2 shows the number of publications on waste wool textile from 2012 onwards. The date was collected from the Web of Science Core Database. It can be clearly seen that the number of articles on waste wool resources has gradually increased with the improvement of awareness of sustainable development in recent years. The major research directions of the studies focus on material science, engineering, and environmental sciences ecology.

Figure 2.

Number of pubications on waste wool using “waste textile + wool” as the topical keywords (collected from Web of Science Core Database, 1 March 2022).

This review introduces the current various strategies for the recycling and reuse of waste wool textiles. In Figure 3, the recycling methods are summarized according to the different application forms of waste wool: one is the applications based on the properties of wool fiber, and the other is applications based on the properties of keratin extracted from wool. Exploiting waste wool in architecture, environmental protection, agricultural, biomedicine, intelligent electronic components, and other fields has been explored. Moreover, the article outlines the context and puts forward the future development direction of waste wool textiles, providing new ideas for enhancing the value of waste wool.

Figure 3.

The main application fields of waste wool textiles.

Sources of waste wool textiles

The waste wool textiles are generated in three forms throughout the whole textile industry chain, including low grade/low value wool (coarse wool and short wool), waste semi-products from the textile production process (spinning, weaving, dyeing and finishing), and discarded clothing. The main sources of waste wool textiles are presented in Figure 4. Coarse and short wool fibers obtained from shearing the sheep-hair are not suit for textile processing and belong to low grade/low value fibers. The EU coarse wool amounts to more than 200,000 tons and the China coarse wool and short wool about 90,000 tons per year.¹¹ These low value wool fibers are difficult to be fully applied and generally disposed of as a waste material. Some leftovers or scraps will be produced during the process from fiber to product, such as the short fiber, waste yarn, the defective products, clothing scraps, etc., which are also one source of waste wool textiles. The clothing is discarded by the customers due to being worn out, damaged or out of fashion. Even if the garments are worn again after donation or re-sale they will eventually be abandoned after a period of time. Due to the different forms of the waste wool textiles from the three sources, the recycling difficulty, fiber composition, and utilization value are different. Compared with low grade/low value fibers, the recycling of waste semi-products and discarded clothing is more complicated, so different recycling methods should be adopted for waste wool textiles from different sources.

Figure 4.

Sources of waste wool textiles. (Network picture: https://image.baidu.com/).

Unfortunately, most of the above-mentioned wool textiles are considered as a worthless solid waste, and the common treatment methods of those wastes are landfill or incineration. However, many countries have banned excessive landfill ascribed to soil contamination and possible spread of contagious diseases such as anthrax.¹² The waste wool is burned to produce energy, but incineration leads to air pollution owing to the high sulfur content in the wool. Landfill and incineration can cause not only environmental and health problems but also the loss of resources. Hence, it is urgent to find new sustainable treatment technologies to produce more valuable and useful products from waste wool. As recycle and reuse of waste textiles have received considerable attention, waste wool textiles have been studied as a variety of materials in many fields including construction, agricultural, sewage treatment, biomedicine, intelligent electronic components, and others. A detailed summary and introduction are given below.

Applications of waste wool textiles

Application based on the properties of wool fiber

Application in architecture

In the conventional architecture field, most of the materials used offer little or no consideration for environmental and health issues. With new regulations and increasing awareness of environmental protection, more and more research studies have focused on developing eco-friendly, low-cost and sustainable building materials. Natural materials, such as timber, straw, bamboo, hemp, flax, wool, etc. are regarded as alternatives to conventional materials because of their inherent advantages, bio-degradability, non-toxicity and high level of toughness.^13,14 Thermal insulation and sound insulation are important indicators of building materials: thermal insulation contributes to the energy savings in the building and sound insulation can reduce noise pollution.^15,16 Wool has many outstanding properties including excellent thermal insulation and sound absorption.¹⁷ Thus, waste wool textiles can meet the demand for alternative conventional architecture materials and develop thermal and sound insulation materials. There are two categories wool insulation production in the architecture market: one is 100% wool or wool combined with other materials made of mat or felt; another is wool-reinforced composite material.

Murean et al.¹⁴ explored a sound absorbing construction material based on waste wool by simple hot or cool pressing methods without any binder. Results showed that the WH240_0.05 material (hot-pressed wool moistened with water, temperature was 80°C, pressure was 0.05 MPa) had the best sound-absorbing properties at frequencies below 2000 Hz and the sheep wool had a comparable sound absorption performance to mineral wool or recycled polyurethane foam. Romina et al.¹⁸ manufactured a non-woven material for acoustic applications using 80% sheep wool and 20% polyester fibers obtained from recycled polyethylene terephthalate (PET) flakes by a thermofusion method. The results also demonstrated that sheep wool was a good sound absorbing material at medium and high frequencies. Duran¹⁹ investigated the thermal insulation properties of single and double layered nonwovens made by recycled jute, recycled wool, polyester, wool fibers, and their combinations. The nonwoven containing wool or recycled wool was a good candidate to be used in insulation applications. Moreover, there was no difference in the thermal properties of recycled wool and wool. Patnaik et al.²⁰ studied thermal and sound insulation samples developed from waste wool and recycled polyester fibers (RPET). The RPET/waste wool mats absorbed more than 70% incident noise in the frequency range of 50–5700 Hz. There was no obvious influence on the thermal insulation and acoustic absorption of the mats under high humidity conditions and the mats had excellent biodegradation properties.

Wool also has certain applications in fiber-reinforced composite materials. Li et al.²¹ developed sound absorbing materials with a high sound absorption coefficient and wide sound absorption frequency band used the waste wool/ethylene vinyl acetate (EVA) copolymer by the hot pressing method. The sound absorption performance of the composites was prominent in the middle and low frequency region. The sound absorption coefficient reached 0.9 at 1000 Hz, the noise reduction coefficient was 0.65 and the average sound absorption coefficient was 0.6. Murri et al.¹³ designed a sustainable wool-geopolymer composite material with good thermal insulation and flame-resistant properties to be used in partition walls. The mechanical and thermal properties of two formulations with different amounts of wool fibers were tested. The composite with 23 vol% of wool fibers was suitable as a flame-resistant barrier and could be classified as A2-s1 d0-nonflammable material. Kim et al.²² fabricated the polypropylene (PP)-short wool fiber composite through continuous sheet extrusion. Research showed that the wool fiber content has less influence on the mechanical properties due to the possibility of fiber agglomeration. It has been proved the wool has a positive effect on improving fire retardancy of the composite. A similar finding was confirmed by Das’s team.²³ They fabricated the PP composites reinforced with biochar and wool, and thermogravimetry, flammability properties, and limiting oxygen index (LOI) of the composite were investigated. Wool (5 wt%) could delay the onset of ignition and the time to reach peak heat release rate, and it was beneficial for enhancing the LOI.

Exploiting the performance of waste wool textiles to develop environmentally friendly, sustainable, safe architectural materials can be considered as a choice to solve the waste of wool resources and increase the income from sheep farming.

Application in sewage treatment

Water pollution caused by the discharge of industrial waste, such as oil spills, heavy metal pollution, and dyeing and finishing waste water, is also a serious environmental problem. Sewage poses a threat to our living environment and health, causing ecological damage and several diseases. Therefore, finding appropriate and effective methods for sewage treatment is very necessary. Several methods that have been used for treating sewage are chemical precipitation, membrane filtration, ion exchange, reverse osmosis, solvent extraction, photocatalytic, and adsorption.^24–29 Some of these methods are limited by secondary pollution, high cost, or low treatment efficiency. The absorption method using adsorbents is the most attractive and common one due to having advantages of simple operation and lack of pollution. In recent years, utilizing the fibers as adsorbents has been investigated because fibers possess large specific surface area, excellent adsorption capacity, and high adsorption efficiency.^30–32 Wool contains many functional groups, including peptide bonds, disulphide crosslinks, and side chains of amino acid residues, which can be used as an adsorbent to purify contaminated water. Hence the cheap and abundant waste wool textile resource has the great potential in sewage treatment.

An overview of the main application areas of waste wool as an adsorbent for sewage treatment is presented in Figure 5. The literature has confirmed that wool and recycled wool textiles for oil spill cleanup is feasible. The irregular scales and rough surface, crimp, and waxy cuticle of wool fiber have a synergic influence on oil adsorption and diffusion.³³ Dyeing and finishing waste water is complex, containing dyes and various auxiliaries, e.g. acids, alkalis, salts, dispersants, and heavy metal ions. Dealing with wastewater from textile dyehouses has some problems, such as high chroma, hard degradation, and large amount, which have become the bottleneck of industry development. Modified waste wool textiles also can be used for removal of dyes from wastewater. Heavy metal and toxic ions also cause some serious disorders and diseases at low concentrations, so removal of heavy metal and toxic ions from effluent is an urgent issue. Wool fibers, chemically-modified wool fibers, and regenerated wool protein could be applied to adsorb heavy metal or toxic ions from polluted water. The performance of waste wool as an adsorbent for treating oil spills, dyes, and heavy metals, is reported in the existing literature, given in Table 1.

Figure 5.

Main application areas of waste wool as adsorbent for sewage treatment.

Table 1.

The performance of waste wool as adsorbent for sewage treatment

	Adsorbent	Conclusion	Reference
Oil spills	Recycled wool-based nonwoven material (RWNM)	RWNM has high sorption capacity for different oil (diesel fuel, crude, base, vegetable and motor oil)	Radetic et al.^34,35
	Loose natural wool fibers (NWF) and RWNM	Wool-based sorbents had higher sorption capacity (5.56 g/g for NWF, 5.48 g/g for RWNM) compared to sepiolite (0.19 g/g)	Rajakovis et al.³⁶
	Romanian Merino wool	For viscous fuels, the sorption capacity increased with the increase of the pH and decrease of the initial sorbent mass and temperature. The sorption capacity of less viscous pollutants increased with the decrease of sorbent mass, while the effects of pH and temperature were not significant	Ciufu et al.^37,38
Dyes	RWNM	RWNM treated with chitosan and hydrogen peroxide (H₂O₂) can efficiently remove basic, reactive, direct and metal complex dyes	Radetic et al.^39,40
	Treated wool	The treated wool has excellent decolorization effect on the reactive Yellow Brown KGR dye and can be reused many times	Li et al.⁴¹
	TiO₂ nanoparticles (NPs) deposited on RWNM	The photodegradation rate of DB78 dye increases with increasing the TiO₂ NPs content, raising the temperature or decreasing the initial concentration	Radetic et al.⁴²
	Metal-wool complex (waste wool and Fe³⁺ or Cu²⁺ ions)	The complex can be used as catalyst for degradation CI reactive Red 19; The Cu-Fe-wool has higher catalytic activity under visible irradiation	Cui et al.^43–45
Heavy metals	Wool	Compared with the effectiveness of several low-cost adsorbents, wool was the most effective adsorbent for removing Cr(VI); a two-step mechanism for removal of Cr(VI) using wool.	Dakiky et al.⁴⁶Jumean, et al.⁴⁷
	Chemically modified wool chelating fibers (wool-g-PIAH fibers)	The absorption of Cu(II), Hg(II), and Ni(II) ions using modified wool was studied; The affinity Cu²⁺ of wool-g-poly(isatin acrylic hydrazone) (wool-g-PIAH) fibers was better than Hg²⁺ and Ni²⁺	Monier et al.⁴⁸
	Bauxsol onto waste wool	The material exhibited excellent removal efficiency for arsenite, lead(Pb), copper(Cu), and zinc(Zn), especially for Pb and Cu, their removal reached 100% and 96%, respectively.	Hassan et al.⁴⁹
	Wool graft polyacrylamidoxime (W-g-PAO)	The adsorbent properties of W-g-PAO for cationic and anionic toxic ions was investigated. The adsorption capacity followed the order of Hg²⁺> Pb²⁺> AsO₂^－> AsO4³^－> Cd²⁺	Cao et al.⁵⁰
	Fine wool powder	Fine wool powder was more efficient for removing and recovering Co²⁺ from solution than wool fiber	Wen et al.⁵¹
	Oxidized wool powder	Oxidized wool powder was used as a metal ion exchanger for removing Cu and Zn. The affinity of wool powder for Cu and Zn ions was increased three times in comparison with fiber	Atef El-Sayed et al.⁵²

Compared with some cheap plant waste materials for sewage treatment, such as rice husks, straws, etc., waste wool has a better adsorption effect due to its rich functional groups. Thus, waste wool-based adsorption materials still exhibit certain advantages. How to deal with the waste wool after use still needs further research. If the “sustainable closed loop” of waste wool is not formed, it will have a negative impact on the environment.

Other applications

A N-doped wool derived activated porpous carbon (N-WAPC) from KOH activated waste wool upon the urea modification was synthesized by Li et al.⁵³ N-WAPC had high N content (14.48%), specific surface area (862 m²/g), and an appropriate micropore size (0.52 and 1.2 nm), and can be used for gas separation of CO₂/CH₄ and CH₄/N₂.The gas adsorption capacities of N-WAPC followed the order CO₂ > CH₄ > N₂. The selectivities of N-WAPC were predicted through the (IAST) method, the selectivities of equimolar CO₂/CH₄ and CH₄/N₂ at 25°C and 1 bar were 3.19 and 7.62, respectively. The N-WAPC was a promising adsorbent in gas separation. Kabir et al.⁵⁴ studied the conversion of wool textile waste into biogas via anaerobic digestion. To improve the digestibility of the wool textile waste, different pretreatments were investigated, namely thermal treatment, enzymatic hydrolysis, and a combination of these two treatments. One of the wool textile waste materials could produce methane 0.43 Nm³/kg VS after thermal and enzymatic treatment and this was 20 times higher than untreated samples. The research study provides a direction for making full use of waste wool resources.

Applications based on the properties of keratin extracted from wool

Keratin, the main component of wool fiber, is a natural polymer containing several amino acids, with good biodegradability, biocompatibility, non-toxic, and skin-friendly characteristics.^55–57 Even low-grade wool of poor quality and waste wool products are also rich in keratin. Keratin from waste wool can be used in various fields including fertilizer, functional finishing agents, membranes, fibers/nanofibers, biological scaffolds, and electronic components, etc. Therefore, the recovered keratin is processed into value-added materials or products, which provides a broader way for reuse of waste wool textile. How to efficient extract keratin is the prerequisite for utilization of this resource, and there is an increasing interest in converting wool into keratin using various techniques.

The chemical structure of wool is complex. Besides the strong cystine crosslinking among the large peptide chains, there are also ionic bonds, hydrophobic bonds, and hydrogen bonds.⁵⁸ The essence of extracting keratin from wool is to break the chemical bonds between molecular chains, and then the protein fiber structure will swell and disintegrate. The main methods to isolate keratin from wool are steam explosion, microwave irradiation, oxidation, reduction, ionic liquid, biological enzyme, etc. Different methods will cause different changes in keratin properties and affect the application direction and value of the extracted keratin. The specific characteristics of each method are shown in Table 2.

Table 2.

Extraction methods and characteristics of wool keratin

Method	Mechanism	Processing parameters	Degradation/dissolution/extraction yield	Molecular weight	Reference
Steam flash explosion (SFE)	Using high temperature steam and high pressure to destroy disulfide bonds and peptide chains	0.2–0.6 MPa 164.2°C, 2–8 min	Digestion yield 80% reduction in cysteine content 50%		Miyamoto et al.⁵⁹
		220°C, 10 min	Solid product 62.4%, water soluble fraction 18.7%, sediment 1.1%	18–3 KDa	Tonin et al.⁶⁰
		170°C, 60 min		25–3 KDa	Bhavsar et al.⁶¹
Microwave irradiation	Hydrolysis treatment of wool in a microwave reactor/to lower the processing time by heating up the solution	150–170 W 180°C, 7 min	60%		Zoccola et al.⁶²
		Wool in superheated water 180°C, 30 min in a microwave oven	31%	5 KDa	Bertini et al.⁶³
		Wool in superheated water 170°C, 30 min in a microwave oven	Hydrolyzed keratin 31%	14–3 KDa	Rajabinejad et al.⁶⁴
Oxidation	The disulfide bond in keratin was oxidized to sulfonic acid group(–SO₃) by oxidant	24% Peracetic acid 2 days room temperature	Soluble keratin 57% insoluble keratin 40%	40–60 KDa	Shavandi et al.⁶⁵
		2% w/v Peracetic acid 24 h in a Linitest apparatus with Tris 1M, 2 h, 37°C	31%	40–60 KDa	Rajabinejad et al.⁶⁴
		0.9 wt% NaOH, 2 wt% H₂O₂ 90°C, 6 h	Hydrolysis rate 90%		Chen and Li.⁶⁶
Reduction	The reductant destroys the disulfide bond and the subsequent formation of thiol group (–SH)	8 mol/l Urea, 10%(owf) cysteine, pH 10.5, 70°C, 24 h	63%	30–50 KDa	Xu et al.⁶⁷
		0.5 mol/l NaHSO₃ 0, 8 mol/l urea, pH 7, 65°C, 5 h	33%	45–65, 11–28 KDa	Tonon et al.⁶⁸
		LKZ-610: wool 1: 8(w/w), 26g/l LiBr, 30 g/l NaHSO₃, 90°C, 6 h	100%	14.4–22 KDa	Sun⁶⁹
		L-cysteine, 5 h, 75°C	72%,	40–55 KDa	Wang et al.⁷⁰
		180 ml of 7M urea, 6 g SDS, 15 ml of 2-mercapthoethanol, 60°C, 12 h.	53%	40–60 KDa	Shavandi et al.⁷¹
Ionic liquid (IL)	Ionic liquid can disrupt hydrogen bond	[DBNE]DEP, 120°C, 3 h	44.7%		Liu et al.⁷²
		[AMIM]dca, 130°C, 3 h	47.5 %		Idris et al.⁷³
		Urea/[bmim][DMP] mass ratio 2%, nitrogen, 120°C, 4.5 h	100%		Wang et al.⁷⁴
		Wool: [AMIM]Cl: mercaptoethanol 1: 8.5: 1, 120°C, 7.5 h	100%	31, 43, 66.2, 14. 4–22 KDa	Sun et al.⁷⁵
Biological enzyme	Proteases can convert keratin into peptides and amino acids	Esperase/ esperase + L-cysteine/ esperase + urea/ esperase + L-cysteine + urea	Esperase 24.88%, esperase + L-cysteine 91.63%, esperase + L-cysteine + urea 99.50%;	30 KDa	Zhang et al.⁷⁶
		Pretreated by H₂O₂, 3% (owf) alkaline protease 60°C, 2 h, pH 9–10	88.9%		Shi and Yan⁷⁷
		2.0 mg/ml protease, 8.0 l Esperase, 65°C, 2 h	5271 weight-average molecular weight		Du et al.⁷⁸
Others	Reductant-formic acid methodMolten urea	Pretreated by reductant LKS-610, formic acid, 50°C, 5 h	65%	40–50, 26, 14.4 ku	Li et al.^79,80
Others	Reductant-formic acid methodMolten urea	Molten urea, 150°C, 20 min	Water soluble protein 53%, water insoluble protein 10%,	75 KDa	Murate et al.⁸¹

Fertilizers

Waste wool textiles contain elements such as carbon (50%), nitrogen (16–17%), and sulphur (3–4%) which are nutrients essential to crops. Some studies have confirmed that the use of waste wool as fertilizer for tomato plant, sweet pepper, and eggplant enables them to grow faster than the control grass.⁸² However, there are some disadvantages to using waste wool directly as fertilizer, like slow release of nutrients, and inherent handling problems. The cysteine bonds, peptide, and hydrogen bonds between macromolecules result into the higher stability of wool, and soil cannot easily digest the fiber structure, thus limiting its application as fertilizer. The hydrolysis of waste wool and cleavage of those chemical bonds is necessary for the utilization of wool fertilizer.^11,83,84 Nustorova et al.⁸⁵ evaluated the effectiveness of the wool alkaline hydrolysate as a fertilizer. The hydrolysate has a positive effect on the germination and growth of ryegrass and can improve the soil characteristics. Holkar et al.⁸⁴ compared the acoustic cavitation on the alkaline hydrolysis of wool with a conventional alkaline hydrolysis to acquire organic fertilizer. Acoustic cavitation assisted alkaline hydrolysis of wool combined with ultrasonic had a lower amount of β-sheet and can stimuli the growth of plants. It also consumed less energy than the conventional method. Alessia et al.⁶¹ obtained nitrogen fertilizers by superheated water hydrolysis of waste wool. The amount of nitrogen in samples with different treatment conditions ranged from 3.9–5.09% and the hydrolyzed product can be classified as organic fertilizer. The germination rate reached the values of 177% at a concentration of 1 g/L. The results confirmed the possibility of using wool hydrolysates as a nitrogen fertilizer.

Finishing agent

Keratin solution contains many groups and has good bioaffinity, non-toxic, and skin-friendly properties. It can be used as a natural polymer finishing agent instead of a synthetic finishing agent. In our previous research, wool keratin (WK) solution was prepared by the reductant-metal salt method and applied to finish wool fabric and polyester fabric. The shrinkage rate of finished wool woven decreased from 17.85% to 4.1%, and the anti-shrinkage effect was obviously improved.⁸⁶ Garments with unidirectional hydrophobicity performance can improve the thermal and wet comfort of the human body.⁸⁷ Keratin could also be attached to the surface of polyester fabric after amine hydrolysis treatment to form unidirectional hydrophobicity due to the difference in hygroscopicity of the two materials. Moisture Management Tester (MMT) results showed that the hygroscopicity of the keratin-treated fabric was significantly higher than that of the untreated fabric.⁸⁸ The WK macromolecule has a structure like cashmere fiber, which can form a protein membrane on the fiber surface. The protein membrane reduces the friction characteristics of the fiber and has a positive effect on the anti-pilling performance of cashmere fabric.⁸⁹ Jia et al. found that the anti-pilling grade of cashmere reached 4.5 after being treated by 8%(owf) keratin solution and 5%(owf) LKZ-610 reagent. Du et al.⁹⁰ also prepared a novel bio-composite anti-felting agent based on waterborne polyurethane and keratin polypeptides extracted from waste wool. As the keratin polypeptides content increased from 0 to 6 wt%, the area-shrinking rate of the treated wool fabrics decreased from 4.55 to 0.47%, respectively. Proteins have a certain degree of surface activity because they contain hydrophilic and hydrophobic, anionic and cationic amino acids. Thus, proteins could be regarded as surfactants to be used in fabric dyeing. Bhavsar et al.⁹¹ applied keratin hydrolyzate as a foaming agent in the reactive dyeing of cotton and acid dyeing of wool fabric. The bubble size for dye with keratin hydrolyzate was in the range of 0.02–0.1 mm and the hydrolyzed keratin acted as a dye molecule carrier. The K/S value of the foam-dyed wool fabric was higher than conventional padding processes, and the results also showed that keratin hydrolyzate has no impact on fastness. Shavandi et al.⁹² produced a green bio-based and formaldehyde-free wood adhesive from waste wool hydrolysate. The adhesive consisted of wool-hydrolysed (WH) and polyamidoamine-epichlorhyfrin resins (PAEs). The WH-PAE adhesive containing 25% of WH and reaction time of 60 min had good shear strength that could meet the bonding strength requirement for plywood.

Regenerated protein membrane

The molecular weights and molecular chain structures of keratin extracted by various methods are different. Keratin with a short molecular chain and small molecular weight is suitable for preparing finishing agent and fertilizer, while keratin with a large molecular weight can prepare regenerated protein membranes and fibers. There are several methods for preparing regenerated membranes, such as solvent casting, thermal pressing, compression moulding, layer by layer (LBL) deposition, and electrospinning.⁹³ Pure keratin membrane has some limitations due to poor mechanical processing properties. Therefore, natural or synthetic polymers, such as chitosan,^94,95 gelatin,⁹⁶ silk fibroin,^97,98 poly (vinyl alcohol) (PVA),^99–101 and polyethylene oxide (PEO),^102,103 etc. can be incorporated into keratin membranes to improve the properties and develop some functional products. Regenerated protein membranes can be applied in many fields, especially in biomedicine, filtration, and so on.

Keratin is considered as one of the most promising biomaterial candidates. Cui et al.¹⁰⁴ studied transglutaminase (TGase)-modified WK film and investigated the effect of TGase on the properties of the keratin film. The film was treated with TGase (30 U/g keratin) for 18 h at 40°C, its tensile strength increased from 5.18 ± 0.15 MPa to 6.22 ± 0.11 MPa and the elongation decreased from 83.47 ± 1.79% to 72.12 ± 3.02%. Cell culture experiments indicated that the TGase-treated film can be used for tissue engineering applications. Biomatrices (photo-active keratin films) made of WK functionalized with photosensitizer methylene blue (MB) has been proposed by Aluigi et al.¹⁰⁵ The killing rate against S. aureus of the biometrics reached about 99.9% upon irradiation with visible light. Zhang et al.¹⁰⁶ fabricated composited scaffolds using electrospinning of poly(lactic-co-glycolic acid) (PLGA) and WK solutions. Mechanical tests showed that WK increased the tensile strength and elongation of the scaffold at low concentrations. Cell culture results manifested that PLGA/1.5% WK could promote attachment and proliferation of bone mesenchymal stem cells. They further constructed PLGA/WK composite membrane loaded with different amounts of antibacterial agent ornidazole.¹⁰⁷ The 1% ornidazole composite membrane exhibited excellent physicochemical properties, drug release performance, cell compatibility, and antibacterial effects, which means the membrane can be used as guided tissue regeneration material for periodontal tissue repair. Tran et al.¹⁰⁸ synthesized cellulose , WK, and gold nanoparticles (AuNPs) composite by a novel one-pot method. The composite exhibited high antimicrobial activity and was fully biocompatible, and possessed the required properties for use as dressing to treat chronic ulcerous infected wounds. Zhong et al.¹⁰⁹ fabricated WK/ionic liquid/polyacrylonitrile (WK/IL/PAN) composite antibacterial nanofibrous membrane through electrospinning. The inhibition rate of hybrid nanofibrous membrane against E. coli and S. aureus was 89.21% and 60.7%, respectively. The nanofibrous membrane also showed an excellent wetting performance and water transport property.

Keratin-based elestrospun nanofibrous membrane also can be used for adsorption and filtration materials because of its attributes such as functional group, high permeability, and surface area. Aluigi et al.^110–112 prepared WK/polyamide 6 (PA6) nanofibrous membrane and studied the adsorption capacity for Cr^3 +, Cr^6 +, Cu^2 +, and formaldehyde. The adsorption capacity increased with increase of the specific surface area and keratin content. Keratin-based nanofibrous membrane highly absorbed Cu^2 + ions and they also demonstrated the formation of complexes between Cu^2 + and free carboxyl groups of the protein. Jin et al.¹¹³ prepared the electrospun WK/PET composite membrane to adsorb Cr^6 + in acidic aqueous solution. As the content of keratin increased, the Cr^6 + absorption ability increased. Compared with pure PET membrane (27.27 mg/g), the maximum adsorption ability of the composite membrane reached 75.86 mg/g. Ag doped keratin/PA6 nanofiber membrane with air filtration and antimicrobial properties was prepared by Shen et al.¹¹⁴ The Ag-keratin/PA6 composite membrane was obtained by electrospinning. The experimental results showed that keratin could enhance air filtration efficiency and water-vapour transmission. Meanwhile, antibacterial activity of the composite membrane against S. aureus and E. coli was up to 99.62% and 99.10%, respectively. The composite membrane could be applied in the air filtration field.

Regenerated protein fiber

From the development of gelatin-regenerated fiber and casein-regenerated fiber to soybean protein fiber and milk protein fiber that have appeared in recent years, the research on regenerated protein fiber has also achieved some results. Wool contains abundant keratin, which can be used to prepare regenerated protein fibers. However, it is currently not possible to obtain pure protein regenerated fibers with excellent performance due to the extraction method and preparation process. The extraction of keratin needs the destroy macromolecular chain, but the molecular chain is difficult to rebuild and stretch in the process of regenerated fiber preparation, so the property of pure keratin fiber is poor. WK blended with other polymers for spinning is the most common and effective solution.

Ionic liquids as novel solvents can dissolve and blend WK and cellulose, thus some research studies prepared WK/cellulose blended fiber using ionic liquids. WK/cellulose fiber (mass ratio is 6:4) was obtained using 1-butyl-3-methylimidazolium chloride salt ([BMIM]Cl) through wet spinning with self-made equipment.¹¹⁵ The internal structure of the fiber is dense, and the surface is smooth with obvious microfiber structure. The diameters of blended fibers decrease gradually, and the breaking strength and breaking strength increases gradually with the increase of the draft ratio within a certain range. A self-heating fiber was prepared using the protein/viscose fiber (PVF) by the wet spinning method.¹¹⁶ The self-heating fiber was composed of WK, cotton pulp, and silk protein, of which silk protein was used as crosslinking agent to fix the keratin to cellulose. The fiber generated heat by moisture absorption and the secondary structural synergy between WK and silk fibroin. Xu et al.¹¹⁷ dissolved the extracted WK in 0.3 mol/L Na₂CO₃-NaHCO₃ buffer and prepared the regenerated fiber by wet spinning. The fiber was heated at 150°C for 2 h and drawn twice, and then annealed at 120°C for 1 h. The cross-section of the regenerated wool fiber was irregular and indented, which was caused by the uneven shrinking of fibers during solidification in the coagulation bath. The regenerated wool fibers had smaller diameters than the raw wool fibers, which might have provided a softer hand feeling.

Synthetic polymers are often chosen to blend with keratin to prepare fiber. PVA is widely used as a blending component of other materials due to its excellent properties. The cross-section and surface of 10% keratin/PVA blended fiber is oblate and cylindrical accompanying obvious grooves. The tensile properties demonstrate that the tenacity and elongation of fiber gradually decreases with the increase of keratin content. To improve the mechanical properties of fiber, the content of keratin should not exceed 15%.¹¹⁸ Ghosh et al.¹¹⁹ added extracted WK to the polycaprolactone (PCL) matrix and the fiber was extruded using a melt-extrusion technique. To improve phase homogeneity and melt-spinning behavior, 3-aminopropyl trimethoxy silane was added as a chemical coupling agent between keratin and PCL. Compared with pure PCL fiber, the surface of the silane treated keratin/PCL fiber after stretching showed some marking lines along its axis, which might enhance the friction co-efficiency and resist knot slippage. The mechanical stiffness of silane treated keratin/PCL fiber was reduced and the fiber could be better applied as surgical suture. Zhang research team fabricated keratin/multi-walled carbon nanotubes (MWCNTS) composite fiber and keratin/polyethylene glycol-functionalized grapheme oxide (PEG-g-GO) composite fiber and discussed the structure and properties of the composite fiber.^120,121 The structure of the MWCNTS fiber and the PEG-g-GO fiber was dense but the surface was not smooth and has wrinkles because of the double diffusion effect in the wet spinning process. Addition of a certain proportion of MWCNTS or PEG-g-GO had a good interfacial effect with keratin matrix without any agglomeration and induced the transformation of keratin crystal structure, which was beneficial to enhance mechanical properties.

Flexible electronic devices

With the progress of electronic technologies, electronic devices have been massively used, which could generate enormous societal development and economic growth. However, some negative effects also appear, especially as more and more electronic waste (e-waste) is growing rapidly every year. E-waste is mainly composed of some heavy metals, such as Pb, Cu, tin (Sn) etc. which is difficult to degrade and sometimes even toxic.¹²² Sustainable biomaterials used to fabricate electronic devices have drawn more attention because of their biocompatible, nontoxic, and biodegradable properties.^123–125 Particularly, the technological quest for flexible and wearable intelligent textiles has driven the recent fiber-based material used as electronic devices. As reflected by the literature survey, some studies have been done to convert the waste wool into flexible electronic devices, such as sensor, triboelectric nanogenerator, conductive composite, fluorescence probe, etc. as shown in Figure 6.

Figure 6.

Waste wool can be applied in flexible electronic devices.

The sensor is one of the essential components for flexible wearable devices. Recently some studies have reported that the sensors based on waste wool exhibit many functions for detecting humidity, pH, uric acid, or strain. Patil et al.¹²⁶ fabricated pH and uric acid flexible biosensors using WK as a conductive carbon precursor and gold nanoparticles (AuNPs) as an active metal component. AuNPs@WK composite material was prepared at 500°C exhibited high pH sensitivity of 57 mV/pH unit, while, the composite material with sp² structure manufactured at 700°C had good conductivity and electrocatalytic activity for uric acid oxidation. Zhang et al.¹²⁷ constructed a flexible ultra-sensitive and highly recovery strain sensor using 3D crosslinked WK molecular spring networks via a Michael addition reaction. The elastic constant determines the mechanical response of the stress sensor and can be tuned in a wide range by adjusting the concentration of the crosslinker (PEG-4VS). The WK gel sensor can detect and track different human motions and voice recognition, proving the regenerated WK materials could be applied in wearable electronics. The keratin extracted from waste wool can also be considered as a humidity sensor due to moisture absorption and the H⁺, OH^－ formed by water dissociation may drift under an external electric field. Natali et al.¹²⁸ have demonstrated that keratin can be applied for sensing water molecules and ions by Electrochemical Impedance Spectroscopy (EIS) and cyclic voltammetry (CV) measurement data. Hamouche et al.¹²⁹ also prepared a capacitive type humidity sensor by using keratin bio-composite as a sensing film. They compared interdigital and rectangular-spiral shaped humidity sensors, and the results showed that the response and recovery time of the two sensors are reasonably fast.

Heavy metals can be perceived by fluorescent sensing materials, but some organic materials are toxic and have a lack of sensibility and selectivity to the target metal ions. Developing environment-friendly and non-toxic sensing materials is urgent and would be significant. Yu’s team¹³⁰ have designed a highly efficient fluorescence gold nanoclusters (AuNCs) probe for Cu^2 + ions through WK as a chelating and reducing agent. The results showed that AuNCs@WK system with Cu^2 + ions was turned from red to blue under UV light and the fluorescence probe had higher stability of pH and was more sensitive under acidic conditions. Moreover, the AuNCs@WK system was safe for selective imaging of Cu^2 + ions in living cells.

The triboelectric nanogenerator (TENG) has gained tremendous attention for its ability to harvest energy from the ambient environment and human body movements to provide power for wearable electronic devices. A TENG based on cashmere fibers was developed by Wang et al.¹³¹ The TENG has size of 3 × 2.5 cm, thickness of 2.3 mm, and consists of cashmere fabric as the positive triboelecric material, Poly tetra fluoroethylene (PTFE) as the negative counterpart, and aluminum and Cu tape as the top and bottom electrodes, respectively. The electrification output of the Tween 20 treated cashmere fabric was the highest output and could light up 37 light-emitting diodes (LEDs) connected in series, which showed potential application in self-powered wearable electronics.

Also, waste textiles can be applied to fabricate conductive material. Remadevi et al.¹³² prepared a highly nickel (Ni) doped conductive-porous composite from silk, wool waste fiber, and grapheme oxide by the bulk synthesis method. Compared to un-doped composite, the surface area, pore volume and conductivity of the Ni-doped composite was higher. The bio-based composite has the potential as electrode material. Cataldi et al.¹³³ has also demonstrated a new approach to produce protein-based electronic materials. Resistors, plane capacitors, inductors, and cellulose-based electrode were fabricated using the water-based conductive ink obtained from WK combined with graphene nanoplatelets. Those electronic components were applied in the field of flexible and wearable electronics.

“Wastetronics,” the transformation of waste materials into electronic devices, proposes a new direction for the reuse of waste wool. The development of “wastetronics” technology could contribute to reduce the impact of e-waste and waste wool on the environment, and simultaneously create new opportunities for converting waste wool into high added-value materials.

Other applications

The Japanese Research Institute has applied WK extracted from waste wool to the processing of artificial wigs, which is conducive to the wave shape and strength of artificial wigs.¹³⁴ Keratin was found to detect chlorine in drinking water quantitatively. Abou Taleb et al.¹³⁵ extracted keratin and sericin from coarse wool and raw natural silk, and then supported on PVA matrix to prepare protein-based analytical strips. The strips contain starch and potassium iodide reagent that can react with free chlorine to form liberated iodine, and a bluish-colored complex is generated due to liberated iodine which interacts with starch. This detection method has many advantages, such as easy application, simple manufacture, biodegradable, etc.

Summary and outlook

With the rise of synthetic fibers, the proportion of natural fibers in the market has declined, but wool textiles still occupy a certain share. A large amount of waste wool textiles is generated every year due to low grade/low value wool, waste semi-products from the textile processing, or discarded clothing and eventually ends up in the landfill or incineration. However, wool has many excellent properties and is rich in keratin resources, and landfill or incineration not only represents a waste of resources but also brings environmental and economic social problems.

Therefore, many researchers have worked on finding feasible approaches and pathways for converting the waste wool textiles into the valuable materials or products that can be applied in many aspects, such as thermal and sound insulation materials, sewage treatment, functional finishing agent, regenerated protein membrane/fiber, electronic devices, etc. While waste wool textile recycling still faces many challenges, such as use of a large number of chemical reagents in some recycling processes, lack of considering the treatment of waste wool after use, no clear recycling methods for different waste wool sources, and lack of standards for management and utilization.

Waste wool is produced at every processing stage in the entire wool industry chain, which means that waste wool contains fibers, semi-products, or finished products. There will be differences in the form, fiber length, and mechanical properties of the waste wool. In the future, waste wool can be recycled in different grades according to the length and form of the waste wool. For fiber recycling, short fibers with poor mechanical properties are used to extract keratin or made into nonwoven or adsorbent materials, while long fibers with good mechanical properties should be reused for spinning and weaving. For fabric recycling, there is developing some other products such as composite materials or decomposing the fabric into fibers for utilization. Thus, waste wool with different properties can be used in different fields and give full play to the value of waste wool.

The value of products developed by current recycling operations is not high. Protein-based electronic components have high added-value and promising potential, but they are still far from practical applications. Many waste cellulose resources (sawdust, cotton linter, wood pulp, etc.) have been reused to fabricate regenerated cellulose fiber, such as viscose, Lyocell, Tencel, etc. and regenerated cellulose fiber also plays an important role in the textile market. Therefore, the development of regenerated protein fiber is an effective way to reuse waste wool. However, it is necessary to improve the method of keratin extraction, modification, and spinning due to the poor mechanical properties and low protein content of current regenerated keratin fiber. In the future, researchers should develop environmentally friendly methods for extracting keratin with high relative molecular weight, and find appropriate modifications to further stretch the protein peptide chains, increase the orientation and crystallinity of the prepared fibers, and thus adjust the poor plasticity and fragility of keratin fibers. At the same time, colored keratin solution or keratin solution with functional materials can be used to prepare colored or functional regenerated keratin fibers, which is beneficial to reduce the dyeing and finishing processes. Finding a short-process, non-pollution recycling method, and making full use of much waste wool to give a secondary value is still a difficult problem to be solved urgently, and it needs continuous research by scientific and technological workers.

Developing new and improved technologies to maximize the value of waste wool recycling can provide social, economic, and environmental contributions, and accords well with the sustainable development concept.

Footnotes

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge the financial support from Scientific Research Program Funded by Shaanxi Provincial Education Department, China (No. 20JK0651), Natural Science Basic Research Program of Shaanxi (No. 2022JQ-444; No.2021JQ-685), Science and Technology Plan Project of Beilin District (No. GX2142), Natural Science Foundation of China (No.52073224), Innovation Capacity Support Plan of Shaanxi, China (No.2020PT-043), Scientific and Technology Project for Overseas Students of Shaanxi, China (No.12), Doctoral Scientific Research Starting Foundation of Xi'an Polytechnic University (BS201906;BS201962).

ORCID iD

Wei Fan

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