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Abstract

The pollution caused by petroleum-based waste has motivated the development of ecologically friendly bio-based materials. Natural fibers are becoming increasingly popular as a renewable and biodegradable resource. In this study, we prepared a kapok/waste silk nonwoven (KSN) by wet-laid and hot-pressing, which had adequate mechanical strength, hydrophilic, lipophilic, and biodegradable properties. The effects of the kapok fiber proportion on the morphology, air permeability, softness, mechanical property, wettability, and biodegradability of KSN were studied. The results demonstrated that the thickness, porosity, air permeability, and softness decreased as the proportion of kapok fiber increased. The mechanical property was significantly enhanced, and the breaking strength increased from 0.58 MPa to 10.85 MPa. With the increase in the proportion of kapok fiber, the spreading time of oil and water increased and decreased respectively, and the static contact angles were below 50°. Degradation rate improved with the increase of kapok fiber proportion, with a maximum degradation rate of 49.41% (30 days). Therefore, the multifunctional, low-cost, easily prepared, and biodegradable kapok/waste silk nonwovens is a promising bio-based material with superior application prospects in dressings, facial mask substrate, packaging, and related fields.

Graphical Abstract

Keywords

Fibrous materials wetting chemistry diffusion chemistry nonwoven fabrication wetlaid fabrication performance materials

Introduction

With the rapid increase of the population, the demand for dressings, facial mask substrate, packaging and related products made from petroleum-based materials has increased dramatically. However, petroleum product waste is increasingly accumulating in the environment due to its long life (non-biodegradable), low recycling rates and inadequate waste management.^1,2 It has been reported that microplastics in the ocean can move through the food chain and accumulate, which has gradually become a significant hazard to the environment and human health.^3–5 Therefore, a replacement with biodegradable, biocompatible, affordable, and environmentally friendly materials is urgently needed. Cellulose-based materials have been widely considered as alternatives to petroleum-based products. Due to the green degradability and biocompatibility of natural fibers, it has been the subject of much research.

Kapok fiber, derived from the fruit of the kapok tree, is a renewable agricultural by-product. As a typical cellulose plant fiber, it is mainly composed of cellulose, hemicellulose, lignin, and a small amount of waxy coating.^6,7 Kapok fiber is well known for its light weight (bulk density: 0.30 g/cm³) and its unique large lumen structure gives it the highest hollow rate (>84%) among natural fibers.^8–10 These physicochemical properties contribute to its excellent moisture absorption (moisture regain rate: 10.75%) and warmth retention (thermal conductivity: 0.045 W·m⁻¹k⁻¹).^11,12 Additionally, kapok fiber exhibits interesting anti-bacterial, anti-mite, biodegradability, and biocompatibility properties. Although challenges such as short length, brittleness, and low cohesive force limit its direct application in spinning, kapok fibers can be blended and spun with other fibers.¹³ Meanwhile, kapok fiber as a filler wadding can be well applied to thermal insulation, sound absorption and buoyancy life-saving materials.^14,15 Due to the hydrophobic properties of waxes, many scholars have focused their research on kapok fibers for oil-water separation, dye adsorption and wastewater treatment.^16–19

Silk fiber is a natural protein fiber that was discovered by chance in China more than 5,000 years ago. It consists mainly of sericin and fibroin, followed by waxy matter, carbohydrates, and inorganic matter.²⁰ Due to the unique elegant beauty, softness, and wearing comfort of silk products, silk fiber has long been used in the production of high-grade textiles. Moreover, silk is widely used in medicine, cosmetics, biological tissue engineering, and smart wearable materials because of its excellent biocompatibility, biodegradability, mechanical strength, and non-toxicity. However, some silk cannot meet the processing needs and become industrial waste, including damaged cocoons, reeling waste, and waste silk fibers from the spinning process. Since waste silk has similar properties to normal silk, its recycling and utilization is considered to be highly valuable.²¹ It is often remade into yarn or fabric in the textile industry, and the modified waste silk can be used to prepare functional textiles.^22–24 Waste silk with exactly the same chemical composition is also expected to replace silk as a raw material for the extraction of regenerated silk fibroin protein.^25,26 Due to the cost of processing, environmental pollution, and safety considerations, there is still a large waste of waste silk resources. As a result, turning waste silk into goods with a high value-added will reduce the waste of biological resources to a large extent.

In recent years, kapok fiber has garnered more attention from scholars in the textile field. Because of its inherent length and chemical composition limitation, kapok fiber is typically blended with other fibers and processed into nonwoven fabrics using needling techniques. The samples prepared by this method tend to be hydrophobic samples with a certain weight (>80 g/m²) and thickness (>4.86 mm).^27–29 On the other hand, the pure silk nonwovens (100% silk) face challenges in large-scale industrial production due to cost constraints, and the choice of waste silk or blending with other fibers is one viable solution.^30–33 To the best of our knowledge, there is currently no research exploring the preparation of nonwoven using a wet-laid technique combining kapok fiber and silk. This may be attributed to the difficulties in dispersing both fibers in water. Herein, we propose an industrially scalable route for the development of lightweight, amphiphilic, high value-added and environmentally friendly kapok/waste silk nonwovens by processing low-cost kapok fiber and industrial waste silk. The alkali treatment and beating process employed in this study aim to obtain a homogeneous fiber suspension, enabling direct formation of nonwoven through wet-laid and hot-pressing. The properties of the obtained KSN were systematically characterized and investigated, and the results showed its potential for applications in dressings, facial mask substrate, packaging.

Materials and methods

Materials

The experiment's kapok fiber came from Pate County in Java Tengah, Indonesia. Its diameter and length were 18.15 ± 4.82 µm and 25.00 ± 2.1 mm (Table S1), respectively. The waste silk was the off-cut from the silk spinning process and provided by a textile factory in China. It belonged to domestic silk with a diameter of 7.09 ± 2.19 µm and a length greater than 10 mm. Sodium hydroxide (NaOH) (purity ≥96.0%) was provided by Sinopharm Chemical Reagent Co., Ltd. Deionized water and vegetable oil were purchased in the local market.

Preparation of kapok/waste silk nonwovens (KSN)

First, the kapok fibers (15 g) were put into a beaker with a 2% mass fraction of NaOH solution (1 L), then placed in a constant temperature water bath (HH-6, Lichen, China) at 70°C for 6 h. Similarly, the waste silk fibers were immersed in the NaOH solution with a mass fraction of 0.6% and subjected to a constant temperature water bath at 99°C for 30 min. After alkali treatment, the fibers were washed with deionized water until the pH value became neutral (pH = 7) to obtain alkali-modified kapok fibers (AKF) and alkali-modified waste silk fibers (ASF). Subsequently, the AKF and ASF were weighed and beaten to create a fiber suspension. Then, the fiber suspension was poured into a customized wet former, and the excess water was removed using a vacuum pump to obtain the wet fiber web. Finally, the fiber web was dried at 100°C by a hot-pressed web to obtain the KSN. The various samples (detailed in Table 1) were marked as KSN-1, KSN-2, KSN-3, KSN-4, and KSN-5.

Table 1.

Basic parameters of KSN.

Sample	Mixing ratio (AKF:ASF)	Dry weight of fiber material (g)	Gramme/Square Meter (g/m²)
KSN-1	0:4	0.6	34
KSN-2	1:3	0.6	34
KSN-3	2:2	0.6	34
KSN-4	3:1	0.6	34
KSN-5	4:0	0.6	34

Characterizations

The morphology of samples was observed under a DXS-10ACKT scanning electron microscope (SEM mode: 10.0 kV, WD: 5.0 mm, magnification: 50/100/1000/3000). The chemical compositions were tested by Fourier transform infrared spectrometer (Spectrum Two, America) in the range of 4000∼400 cm⁻¹. Porosity refers to the percentage of pore volume to the total volume of nonwovens. Each sample was measured 5 times. The calculation formula is: $ε = (1 - \frac{m_{n}}{\bar{ρ_{f}} \cdot δ}) \times 100 %$ (1)Where $ε$ is the porosity (%), $m_{n}$ is the surface density of sample (g/cm²), $\bar{ρ_{f}}$ is the weighted mean density of fibers (g/cm³), and δ is the thickness of sample (cm). Among them, the fiber density was tested according to the pycnometer test method in GB/T 41063-2021.

Physical and mechanical test

The air permeability of samples was tested by an automatic air permeability tester (YG461E, China), according to the standard method of GB/T 5453-1997. The bending stiffness and bending hysteresis moment were measured based on GB/T18318.5-2009. The samples were cut into the shape of 100 mm × 50 mm and tested on a Pure Bending Tester (KES-FB-2, Japan). Tensile test was carried out on a multifunctional electronic fabric strength machine (YG026MB-250, China), according to GB/T 3923.1-2013. The samples were cut to the shape with a dimension of 70 mm × 50 mm, and tensile test was conducted at the drawing speed of 100 mm/min with a clamping distance of 40 mm. Each sample was measured 5 times in the air permeability test and 3 times in the bending and tensile tests. All the above tests were conducted in an environment with temperature and humidity of 25 ± 2°C and 60 ± 5%, respectively.

Wettability test

The wettability was evaluated using an optical contact angle goniometer (OCA15EC, Germany). During the testing process, the samples were firmly attached to glass slides using double-sided tape. In the case of the sessile drop method, both oil and water droplets exhibited spreading behavior on the KSN surface. To measure the static contact angle, therefore the captive bubble method was employed. This method involved releasing small bubbles (5 µl) onto the sample surface immersed in the test liquid using a U-shaped needle syringe. After the bubble stabilized, the contact angle was measured using an optical microscope equipped with a CCD camera.^7,34 Each sample was measured 5 times. Additionally, the dynamic spreading behavior of the test liquid was recorded using the sessile drop method, which follows the same principle as the captive bubble method. However, in this case, a 5 µl droplet of the test liquid was directly dropped onto the surface of the sample using a straight-needle syringe.^35,36 Each sample was measured 3 times. The schematic of the two test methods can be found in Figure S1, and the properties of the liquids used in the aforementioned test were shown in Table 2.

Table 2.

Properties of liquids for experiment.

Solution type	Density (g/cm³)	Viscosity (mPa s)	Surface tension (mN/m)
Water	1.01	1.07	68.38
Vegetable oil	0.93	68.85	33.49

Soil burial test

ASTM-D5988-12 standard method was adopted in the soil burial test. First, the soils used in the experiment were collected in different environments (woods, bushes, and lakeside), with a sample depth over 200 mm. To control soil particle size below 2 mm, the mixed soil was screened by an experimental standard sieve (10 mesh) to remove stones, plant roots, and other non-soil material. Then, samples were cut into a shape with dimensions of 50 mm × 50 mm, buried in beakers with mixed soil (1 kg) at 30∼50 mm, and placed in a constant temperature and humidity environment (25 ± 2°C and 60 ± 5%). 75 mL of deionized water was added every 5 days. The samples were removed from the soil every 15 days, washed with deionized water to clean the soil adhering to the surface, and dried at 80°C for 6 h. Each sample was measured 3 times. The calculation formula is: $W (%) = \frac{({W_{1} - W}_{2})}{W_{1}} \times 100 %$ (2)Where $W$ is degradation rate (%), the $W_{1}$ is the weight of sample before burial (g), and $W_{2}$ is the weight of sample after n days of burial (g).

Statistical analysis

Statistical analysis was conducted on the KSN performance data using one-way analysis of variance (ANOVA). The independent variable was the mixing ratio, and a significance level of p < .05 was set. Additionally, a post-hoc Tukey test was performed to identify the specific differences. The statistical analysis in this study was carried out using GraphPad Prism.

Results and discussion

Characterization

The morphology of KSN was illustrated in Figure 1. SEM images in the surface direction (Figure 1(a)–(e)) revealed that numerous fibers compressed into a flat surface and intertwined together to form nonwoven. It can be seen that the KSN-1 had more pores (Figure 1(a)), and the number of pores decreased as the proportion of kapok fiber increased (Figure 1(b)–(e)). The reasonable explanation was that the fiber diameter of the kapok was larger than waste silk, and a single kapok fiber can cover a larger area (Figure 1(c)). In addition, the fibrillation of waste silk was more obvious compared with kapok fiber. SEM images of the cross-sectional direction were shown in Figure 1(f)–(j). It was shown that the thickness of KSN-1 was 0.287 mm, and the thickness of the samples decreased as the proportion of kapok fiber increased (0.242 mm, 0.214 mm, 0.157 mm, and 0.094 mm for KSN-2, KSN-3, KSN-4, and KSN-5, respectively). The fiber cross-section of waste silk in KSN was almost round (Figure 1(k)), while the kapok was flattened (Figure 1(l)). As a result, the nonwoven's thickness has decreased because the flattened kapok fibers have taken on the role of the nonwoven's support structure by securely tying the neighboring fibers together. To our knowledge, the previously reported nonwoven fabrics of the same type containing kapok fiber or silk generally have a certain mass (>80 g/m²) and thickness (>1 mm).^27–31,33 In comparison, KSN has the advantage of lighter weight and less thickness (Table S2).

Figure 1.

SEM images of KSN at 100 and 1000× magnifications. (a), (b), (c), (d), (e) Surface direction of KSN, (f), (g), (h), (i), (j), (k), (l) Cross-sectional direction of KSN.

The air permeability of KSN was shown in Figure 2(a). Among the samples, KSN-1 exhibited maximum permeability of 113.62 mm/s. With the increase in the proportion of kapok fiber, the permeability of KSN-2, KSN-3, KSN-4, and KSN-5 decreased to 101.40 mm/s, 37.86 mm/s, 18.32 mm/s, and 8.42 mm/s, respectively. The ANOVA test results showed that there was a significant difference (p < .0001) among the samples of the five mixing ratios. The air permeability of nonwovens depended on many factors, such as porosity, thickness, and pores.^37–39 And these factors were affected by the nonwoven structure as well as the morphology and scale of the fibers.^40–42 Based on GB/T 41063-2021 standard, we obtained the fiber densities of ASF and AKF, which were 1.30 g/cm³ and 1.05 g/cm³, respectively. As shown in Figure 2(a), the porosity of KSN decreased from 89.99% to 67.44% with the increase of kapok fiber proportion. This was due to the flattened form of kapok fibers in KSN, as opposed to the round shape like waste silk, and the increased proportion of kapok fiber, the nonwoven’s structure denser. The dense structure not only results in a reduction of the nonwoven's thickness but also in its porosity. In this study, as the proportion of kapok fiber increased, the thickness decreased, the porosity decreased, and the pores were reduced. Due to a combination of factors, the air permeability deteriorated.

Figure 2.

Characterization of air permeability and softness (a) Air permeability and porosity, (b) Bending stiffness and bending hysteresis moment.

The softness of KSN was characterized by testing its bending stiffness and bending hysteresis moment. The bending stiffness reveals the nonwoven's capacity to withstand bending deformation, which is related to the thickness and porosity of the nonwovens.^43,44 And the bending hysteresis moment reflects the amount of material viscosity. As shown in Figure 2(b), the bending stiffness increased from 0.078 to 0.387 gf·cm²/cm as the proportion of kapok fiber increased (p < .0001), and the bending hysteresis moment showed a similar trend (p < .0001). In other words, the sample with pure waste silk has good softness, and as the proportion of kapok fiber increases, KSN loses some softness and becomes stiffer to the touch. The reason for this phenomenon was that the kapok fibers in KSN were stacked tightly and cannot move freely, whereas waste silk in KSN created a loose tangled structure, leading to fiber rotation and displacement during bending.⁴⁵

Mechanical property

According to the stress-strain curves shown in Figure 3(a), the samples were typically linearly deformed before tensile failure. With the proportion of kapok fibers increased, the fracture stress increased from 0.58 MPa to 10.85 MPa (p < .0001), while the elongation decreased (p = .686 > 0.05) (Figure 3(b)). This phenomenon is also supported by the fracture morphology of the specimens. Figure 3(c) showed that the occurrence of pull-out of fibers was much more obvious in the samples with a comparatively low breaking strength.⁴⁶ As seen from the SEM images, many long waste silk fibers were pulled out from KSN-1, explaining the long tail in the stress-strain curve (Figure 3(a)). The length of the pulled-out fibers shortened as the proportion of kapok fiber increased, and the fracture of KSN-5 had some shorter kapok fibers with a jagged end, indicating the breakage of fibers. This led to a sharp rise in the stress-strain curve and a maximum value at partial fiber breakage (Figure 3(a)). Overall, the failure mode of KSN changed from pull-out to breaking as the proportion of kapok fiber increased.⁴⁷ This change in failure mode may be due to the difference in morphology and properties of fibers. Compared with Kapok fiber, the fineness of waste silk is smaller, which favors the formation of inter-fiber tangles.⁴⁸ And kapok fiber is a cellulose fiber which contains a large number of hydroxyl groups that can easily form hydrogen bonds.⁴⁹ Therefore, we next analyzed the strengthening mechanism of KSN in terms of both inter-fiber entanglement and inter-fiber bonding.

Figure 3.

Fracture mechanism of KSN (a) Stress-strain curves, (b) Fracture stress and fracture strain, (c) SEM images of fracture morphology (magnification: 50 & 3000), (d) schematic diagram of fiber entanglement, (e) FTIR spectrum of fibers before and after alkali modification.

Effective entanglement between fibers can increase the slippage resistance during stretching, which enhanced the strength of the nonwovens.^45,50 As shown in Figure 3(d), three types of entanglement can be formed between waste silk and kapok fibers, including silk & silk (S-S), silk & kapok (S-K), and kapok & kapok (K-K). And the contact area of the three entanglements (S-S, S-K, and K-K) increased sequentially. Among them, S-S entanglement not only contained entanglement of waste silk, but also some fibril entanglements, which may provide weak strength. In addition, the rough surface after alkali treatment could increase the contact friction between fibers, which is beneficial to the stability of KSN structure and improve the failure strength.⁵¹

The specific FTIR spectrum data of fibers before and after alkali treatment was shown in Figure 3(e) and Table 3. As for waste silk, similar FTIR spectrum indicated that the main structure did not change of waste silk before and after alkali treatment due to the fact that their sericin had been removed during the spinning process of the silk.⁵² For kapok fiber, the absorption peaks at 3400–3200 cm⁻¹ and 3000∼2800 cm⁻¹ were attributed to OH stretching vibration and C-H stretching vibration, respectively. These peaks were characteristics of the existence of cellulose.^10,53 The absorption peaks at 1737 cm⁻¹ and 1236 cm⁻¹ in the FTIR spectrum of kapok fiber were due to the presence of C=O in the ester bonds. The stretching vibration of C=O was thought to be related to ketones, carboxylic groups and esters in lignin.^54,55 As can be seen from Figure 3(e), the absorption peak at 1737 cm⁻¹ started to disappear and the absorption peak at 1236 cm⁻¹ became weaker, indicating that lignin was partially removed after the alkali treatment.

Table 3.

Infrared transmittance peak of untreated kapok fiber and waste silk (cm⁻¹).

Peaks (cm⁻¹)	Characteristic	Bond type
1618	Fibroin protein (amide I)	C=O stretching
1512	Fibroin protein (amide II)	C=N stretching and N-H bending
2927	Sericin protein	C-H stretching
3400–3200	Cellulose	OH stretching vibration
3000–2800	Cellulose	C-H stretching vibration
1737 and 1236	Lignin	C=O stretching vibration

In addition to the entanglement between fibers, the breaking strength of nonwovens is also determined by the bonding between the fibers.^56–58 Previous studies have demonstrated that alkaline substances can effectively remove surface waxes from plant fibers, thereby exposing cellulose with abundant hydroxyl groups.⁵⁹ Under the Campbell effect during hot pressing, cellulose fibers can come close to each other and form hydrogen bonds, thus enhancing the bonding between the fibers. This phenomenon has also been extensively documented in the literature.^60,61 In this study, the incorporation of a higher proportion of kapok fiber introduced a greater number of hydroxyl groups, thereby facilitating the formation of hydrogen bonding networks. There was no doubt that this does contribute to the improvement of the fracture strength of nonwovens. Both the flattened shape of kapok and the hot-pressing process contributed equally to favorable interlocking and bonding between the fibers. Additionally, FTIR analysis confirmed the presence of residual lignin in the alkali-treated kapok fibers, which may serve as a natural binder and contribute to the overall strength enhancement of KSN.⁶²

Wettability

To characterize the wettability of KSN, the static contact angle and dynamic spreading process were measured. As shown in Figure 4(a)–(b) and Table 4, the water contact angle (WCA) and oil contact angle (OCA) of KSN were below 40° and 50°, respectively, and fluctuated within 10°. The difference between the left and right contact angle was less than 8° (Table S3-S4). This indicated that KSN has hydrophilic and lipophilic properties. The ANOVA test results showed significant differences between the water contact angle values (p = .039 < 0.05) and oil contact angle values (p = .033 < 0.05) of the five samples. However, the post hoc test results indicated that there was no significant difference in the contact angle values between any two samples (p > .05). A water contact angle of 129.2° has been reported for raw kapok fibers.¹⁰ In this study, the transition from hydrophobic to hydrophilic kapok fibers originated from the alkali treatment to remove the surface waxes. Kapok fibers feature many hydrophilic hydrogen bond donors on the cellulose chains, which have hydrophobic pyranose rings on the donors.⁶³ And the waste silk contains both hydrophobic and hydrophilic amino acids.²⁰ The co-existence of hydrophilic and lipophilic components may provide an explanation for the amphiphilicity exhibited by KSN. Existing reports on kapok-based or silk-based nonwovens have shown common properties of hydrophobicity (WCA >90°).^27,29,30 In contrast, the distinctive oil-water amphiphilicity of KSN endows it with enhanced skin-friendly properties (Table S2).

Figure 4.

Characterization of KSN wettability (a) Water contact angle, (b) Oil contact angle, (c) Liquid spreading time, (d) Dynamic water contact angle, (e) Dynamic oil contact angle, (f) Photos of liquid spreading on KSN.

Table 4.

Contact angle data of KSN.

	KSN-1	KSN-2	KSN-3	KSN-4	KSN-5
WCA (˚)	33.89 ± 1.85	34.63 ± 1.65	34.22 ± 0.45	32.46 ± 0.48	31.61 ± 2.55
OCA (˚)	46.26 ± 2.31	42.18 ± 1.86	43.55 ± 1.32	41.64 ± 3.97	45.59 ± 2.36

As shown in Figure 4(c), the water spreading time of the samples decreased and then increased with the increase of the proportion of kapok fiber, and the water spreading times of KSN-1, KSN-2, KSN-3, KSN-4, and KSN-5 were 69.82 s, 18.00 s, 12.73 s, 7.09 s, and 9.73 s, respectively (p < .0001). Oil spreading time increased from 8.07 s to 78.09 s with the increase in the proportion of kapok fiber (p < .0001). According to the principle of “like dissolves like”, kapok fiber is more affinity with water than oil, and silk is just the opposite. In addition, as the proportion of kapok increased, the structure of the nonwoven became more compact and therefore the difficulty of liquid spreading increased. The spreading rate was characterized by plotting the dynamic contact angle as a function of spreading time, there was a linear relationship between spreading time and water contact angle (Figure 4(d)), and an exponential decay with oil contact angle (Figure 4(e)). The change in contact angle during spreading in Figure 4(f) showed this more visually. The reason is that the low surface tension of vegetable oil promotes its spreading, while its high viscosity makes it easy to block the pores between fibers.⁶⁴

Biodegradability

The biodegradability of KSN was tested by a 30-day soil burial experiment. As shown in Figure 5(a), the degradation rates of KSN-1, KSN-2, and KSN-5 after 15 days of soil burial were 1.44%, 7.77%, and 13.08%, respectively. Figure 5(b) showed that some mycelium appeared on the surface of the samples at this time, and a few tears were observed on KSN-5. When buried for 30 days, the degradation rates of KSN-1, KSN-2, and KSN-5 reached 4.31%, 12.68%, and 49.41%, respectively (Figure 5(a)). Among them, the KSN-1 and KSN-2 remained intact and a large number of mycelia were found on their surfaces, while the KSN-5 was broken down into small pieces by microbes (Figure 5(b)). This result suggested that the biodegradability of KSN improved as the proportion of kapok fiber increased. It can be attributed to the difference in the degradation mechanism of the two fibers. Related studies had shown that cellulose fibers tend to exhibit a faster degradation rate than protein fibers.^65–69

Figure 5.

Characterization of the degradability of KSN (a) Degradation rate of KSN, (b) Photos of KSN after being buried in the soil for 0,15,30 days, respectively, (c) Schematic diagram of KSN degradation.

We next explained the degradation process of KSN based on the available literature. There were a lot of bacteria, fungi, and other microorganisms in the soil, some of which can secrete cellulase or protein enzymes, such as aspergillus, penicillium, and bacillus.⁷⁰ In this study, because waxes were resistant to biodegradation,⁶⁶ the removal of waxes from fibers by alkali treatment not only transformed the fiber surface from hydrophobic to hydrophilic but also accelerated fiber disintegration. The degradation mechanism of KSN was shown in Figure 5(c). The cellulose in kapok fiber can be degraded by the synergistic action of cellulases (endoglucanases, exoglucanases, and β- glucosidases).⁷¹ The glycosidic bonds in the amorphous region of cellulose can be hydrolyzed by endoglucanases, resulting in long-chain oligomers. The long-chain oligomers were then hydrolyzed by exoglucanases to produce cellobiose, which was further hydrolyzed by β- glucosidases to glucose. Similarly, the crystalline region of cellulose can be eventually hydrolyzed to glucose by the synergistic action of cellulases.⁷² Contrarily, the biodegradation of waste silk was based on the microbial attack on their protein structure, and the presence of water in the soil contributed to the loosening of silk protein chains.^20,73 The attack of microorganisms occurred first in the loose amorphous region and then extended to the crystalline region, and the final degradation products were small peptides and amino acids.^74,75

Conclusion

In this study, a degradable bio-based nonwoven was prepared from kapok fiber and waste silk, and it was systematically characterized and studied. The results showed that KSN is lightweight, thin, and oil-water amphiphilic. The unit grammage of KSN was 34 g/m², and the thickness was less than 0.3 mm. As the proportion of kapok fiber increased, the structure of KSN became denser and the bending stiffness increased. The air permeability decreased from 113.62 mm/s to 8.42 mm/s, and the bending stiffness increased from 0.078 gf·cm²/cm to 0.387 gf·cm²/cm. Meanwhile, the fracture strength increased from 0.58 MPa to 10.85 MPa, resulting in significant improvements in mechanical properties. KSN also exhibited oil-water amphiphilicity, with oil/water contact angles all less than 50°. The maximum 30-day degradation rate reached 49.41%. The higher silk content can add high value to the product, making KSN-1 and KSN-2 excellent candidates as potential substitutes for dressing and facial mask substrate. Furthermore, KSN-3, KSN-4, and KSN-5, which exhibited good mechanical properties, hold promise for packaging applications. Meanwhile, the application of biodegradable natural fibers can effectively mitigate the “white pollution problem”, and the recycling of waste silk would avoid the waste of resources.

Ethical statement

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Supplemental Material

Supplemental Material - Development and characterization of kapok/waste silk nonwoven as a multifunctional bio-based material for textile applications

Supplemental Material for Development and characterization of kapok/waste silk nonwoven as a multifunctional bio-based material for textile applications by Hongchang Wang, Liyao Cao, Hang Yuan, Yuling Li, Run Wen and guangbiao Xu in Journal of Industrial Textiles.

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: This work was supported by the Science and Technology Commission of Shanghai Municipality (20015800200),the National Key R&D Program of China (2022YFC3006100),and the University-Industry Collaborative Education Program (220606429135509).

ORCID iD

guangbiao Xu

Supplemental Material

Supplemental material for this article is available online.

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