Fabrication of reinforced hydrophobic coatings for the protection of silk fabric

Materials

White Bombyx mori silk fabrics were purchased from Hangzhou dream show Co., Ltd. BS, a solvent-free silane/siloxane silicone concentrate, was kindly supplied by Guangdong Longhu Sa. & Tech. Company Limited, and it was diluted in distilled water of 8.3 wt.% Fumed silica (SiO₂) nanoparticles of 12 nm in mean diameter were purchased from Shenzhen Chemical Co., Ltd. EGDE was selected as the crosslinking agent to increase the crosslinking between the silk fabric and SF.

Silk fibroin

Degummed silk fibers were dissolved in 9.3 mol/L LiBr solutions at 60℃ with a bath ratio of 1:6 for 4 hours. After being dissolved completely, the solutions were dialyzed in a cellulose tubular membrane (molecular weight cutoffs of 3500) against deionized water (replaced every 6 hours) for 3 days and filtered. SiO₂ nanoparticles were dispersed in BS solution, and the resulting solutions of 2.0% w/w were ultrasonicated for 30 minutes to obtain a homogeneous dispersion.

Silk fabric modification

The SF, diluted to 1.0% with deionized water, was sprayed on clean fabrics with a mount of 0.05 mL/cm ² . After 30 minutes the same amount of 3 wt.% EGDE was sprayed and the samples were dried at 30℃. Next, the BS/SiO₂ solution was sprayed in three layers on the samples treated with SF and EGDE at intervals of 10 minutes and every layer had a spray amount of 0.05 mL/cm ² . This sample was marked as Sample BS/SiO₂.

For comparison, pure BS solution (without SiO₂ nanoparticles) was sprayed on the samples treated with SF and EGDE in the same manner as Sample BS. Moreover, BS solution were sprayed on the original silk fabrics directly and marked as Sample S*, as shown in Table 1. All specimens were kept at room temperature for 2 days.

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Table 1.

The complete description of the samples used in the next discussion

Sample name	Complete description
SF	Silk fabric was sprayed 1% concentration of SF solution.
SF/EGDE	Silk fabric was firstly sprayed 1% concentration of SF solution and then sprayed 3% concentration of EGDE.
BS/SiO2	Silk fabric was treated in the following manner: (1) 1% SF and 3% EGDE solution; (2) BS and SiO2 mixed solution.
BS	Silk fabric was treated in the following manner: (1) 1% SF and 3% EGDE solution; (2) BS solution.
S*	Silk fabrics coated with only BS solution.

Characterization and measurement

The surface morphologies of samples were observed with a scanning electron microscope (SEM, Quanta250). The chemical composition element distribution and content of the surface of the coated fabric were characterized by an energy-dispersive spectrometer (EDS-SEM, Quanta250). The static contact angle was tested by a contact angle analyzer (2120 KRUSS DSA30) and droplets of distilled water. The volume of the droplets was 5 µL. The Fourier transform-infrared spectroscopy (FTIR) method was used to monitor the chemical changes on the surface of the samples on a Nicolet 5700 FTIR spectrometer. Abrasion resistance was investigated using a fabric abrasion tester (Martindale, YG401E). The samples were measured at a gauge length of 4 cm and the crosshead rate was 5 mm/min in a mechanical testing machine (YG026MB) to collect the tensile strength. The tested samples were cut into 9 cm × 2.5 cm. The bending stiffness of the coated fabric was characterized using an electronic stiffness tester (LLY-01) with a sample size of 15 cm × 2.5 cm. The air permeability of the coated samples was characterized with an automatic permeability tester (YG461) and the differential pressure used in this test was 100 Pa. The coated fabric was washed by a wash fastness tester (SW-8A).

The water vapor permeability was observed by a typical test. Three beakers contained 50 mL water were placed in an oven preheated to 100℃ for 30 minutes, and then samples were covered on these beakers and then the same number of allochroic silica gels was placed on the surface.

Effect of SF and SF/EGDE

The FTIR spectra of uncoated, Sample SF and Sample SF/EGDE silk fabric were investigated to confirm the interaction between silk fabric with SF and EGDE, as shown in Figure 1. FTIR spectra of both samples showed characteristic bands for silk: C=O carbonyl stretching with a small contribution from N-H in-plane bending of the peptide at around 1620 cm^–1 for amide I, N-H bending with a minor contribution from the C-N stretching vibration at around 1520 cm^–1 with β-sheets at 1520 cm^–1 and α-helix/random coil at 1545 cm^–1 for amide II and N-H bending and C-N stretching of the peptide group at around 1230 cm^–1 for amide III.^62–64 From the secondary structure, comparison of the amide I band deconvoluted according to the literature ⁶⁴ and Sample SF/EGDE has the highest β-sheet percentage compared to other samples due to the mesoscopic molecular network reconstruction by hydrogen bonds and crosslinking of EGDE to form β crystals, as shown in Figure 1(b). Meanwhile, the percentages of α-helix, random coil and β-turn accordingly decrease, indicating that the secondary structure of SF undergoes a transition due to the addition of EGDE. For only the addition of SF, the β-sheet decreases in contrast to the uncoated sample. This may be because, in the process of spraying, the β-sheet is difficult to reconstruct. When the EGDE is added, the chemical crosslinks and more crystals are formed between the SF and EGDE. Furthermore, due to the increase of β-crystals, the stress of silk fabric accordingly increases. Moreover, the EGDE can also build crosslinking between the silk fabric and SF, which causes elongation of the silk fabric, as shown in Figure 2. After calculation, the stress and strain of the fabric after SF/EGDE treatment were increased by 6.67% and 6.83%, respectively, compared with the untreated samples, demonstrating the formation of a crosslinked structure. Figure 3 illustrates the crosslinking mechanism of SF and EGDE according to the above results. OPEN IN VIEWER

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Figure 2.

Mechanical properties of the uncoated samples and Sample SF/EGDE. SF: silk fibroin; EGDE: ethylene glycol diglycidyl ether.

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Figure 3.

Schematic illustration for the crosslinking mechanism of silk fibroin and ethylene glycol diglycidyl ether (EGDE).

Contact angle and self-cleaning properties

The surface of silk fabric can be easily reinforced by the deposition of silk sericin over the interface via a spraying routine. Photographs of the contact angle of uncoated and coated samples are shown in Figure 4. The measured contact angle of water content is 5 µL. The static contact angle of the uncoated sample is 43.27° (Figure 4(a)), which shows that the uncoated samples have good hydrophilicity; thus, when the colored water droplets were placed on the sample surface, they were absorbed in a very short time (Figure 4(b)). However, Figures 4(c) and (e) show that the contact angles of Samples BS/SiO₂ and BS are 145.77° and 138.72°, respectively, indicating that the addition of BS and SiO₂ produce a strong water-resistance in which the hydrophilic silk fabric changed into hydrophobic fabric. The hydrophobic agent BS could reduce the surface energy of the sample, and nano-scale SiO₂ increases the surface roughness. Moreover, it is obvious that the contact angles of Sample BS/SiO₂ are superior to those of Sample BS. The results clearly suggest that the coatings with SiO₂ nanoparticles have an excellent function to better protect silk fabrics from water. OPEN IN VIEWER

The original silk fabrics treated with only BS have contact angles of about 154°, which attain the superhydrophobic effect, as evidenced in Figures 4(g) and (h). However, if only using BS treatment, the silk fabric will be prone to damage when the fabric withstands the friction. In order to better repair and protect silk fabric, SF is selected as the first coating to afford more tensile strength.

It is necessary to study the self-cleaning function of textile fabrics for the protection of fabrics. The random network Teflon nanofiber membrane developed by Wong et al., ⁶⁵ which is lubricated with a perfluorinated liquid having a low surface tension, can repel almost any immiscible solution. We prepared a fluorine-free, reinforced hydrophobic coating directly on a silk fabric substrate with large pores. A macromolecular network structure and a nanometer-scale rough surface were established to protect the silk fabric from the external environment. The rolling pattern of the water droplet of Sample BS/SiO₂ at tilting angles of 10° and 15° is shown in Figure 5, and it can be seen that the blue water droplet easily rolls down. The self-cleaning tests of Sample BS/SiO₂ are presented in Figure 6. As shown in Figures 6(a) and (b), when the dirt was put on the sample surface, the water droplets rolled off from the surface free of residue, taking dirt particles with them. Figure 6(b) shows that the blue water droplets form a sphere while removing dirt, indicating that Sample BS/SiO₂ has an excellent self-cleaning property. The hydrophobic surface exhibits a low attraction force to drops of water and droplets could roll off under a small force. OPEN IN VIEWER

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Figure 6.

Self-cleaning test of Sample BS/SiO₂. Dirt was put on the surface of the sample and blue water droplets removed the dirt (from (a) to (b)). (Color online only).

SEM and energy-dispersive X-ray spectroscopy analysis of the coated fabrics

The surface morphologies of the uncoated and coated samples were characterized by the SEM and the results are presented in Table 2. As shown in pictures (a) and (b) in Table 2, the surfaces of the uncoated sample are smooth and there is no any attachment. After coated by SF film and BS coatings, the silk surface looks slightly rough compared with the uncoated sample (pictures (c) and (d) in Table 2). Due to the increase of SF and BS, the surface roughness of the silk fabric increases, which in turn induces a rough surface at the nanometer scale. A highly rough structure is observed from the surface of Sample BS/SiO₂, which is displayed as pictures (e) and (f) in Table 2. The surfaces of Sample BS/SiO₂ are covered by a larger and higher coating, which reveals that the hydrophobic properties of silk fabrics are improved. ⁶⁶ Consequently, according to pictures (c)–(f) in Table 2, the composite coatings project into the pores of the fibers, and this could increase the crease recovery and the flexural rigidity, as shown in Figure 7. OPEN IN VIEWER

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Table 2.

Scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy mapping dots and element percentage of samples

	Uncoated	Sample BS	Sample BS/SiO2
SEM
C element mapping dots and percentage (wt%)
	56.88	57.11	49.95
N element mapping dots and percentage (wt%)
	20.45	14.86	2.57
O element mapping dots and percentage (wt%)
	22.67	26.15	35.95
Si element mapping dots and percentage (wt%)
	0	1.88	11.54

The chemical composition of the surface of the sample before and after treatment was characterized using the energy-dispersive method. The energy-dispersive X-ray spectroscopy (EDS) images and elemental contents of the original silk fabric (uncoated), Sample BS and Sample BS/SiO₂ are shown in Table 2. Only carbon, nitrogen and oxygen elements were detected from the EDS of the original silk fabric. The presence of carbon, nitrogen and oxygen can be attributed to silk fabrics. A hydrophobic Si component was detected on the surface of the fabric after coating, which confirms its hydrophobicization. The percentage of N in the coated fabric is significantly reduced and the distribution is sparse, which is consistent with the infrared measurement, indicating that the hydrophobic coating evenly covered the fabric. The increase in the percentage of the O element is due to the formation of Si-O and Si-O-Si groups after the coating. Therefore, when SiO₂ is added to the coating liquid, the mapping points of Si and O elements are significantly increased.

Bending stiffness of the coated fabrics

In order to evaluate the softness of the fabric after coating, we tested the bending length of the sample to obtain the bending stiffness according to Equation (1)

G = m × C 3 × 10 - 3

(1)

After testing, the original fabric has a bending stiffness value of 0.65, and the bending stiffness is found to increase significantly after coating, as shown in Figure 7. The stiffness of the treated silk fabric is increased because SF is crosslinked by the EGDE on the yarn, and further protection of the hydrophobic coating increases the tangential sliding resistance at the warp and weft interlacing points, thereby increasing the bending stiffness. This method increases the stiffness of the fabric and the crease recovery, which can be interpreted as the crosslinking of SF by EGDE onto the silk fabric, and the coating is evenly applied.

FTIR analysis of the coated fabrics

Characterization of uncoated samples, Sample BS and Sample BS/SiO₂ was carried out with FTIR and the results are shown in Figure 8. The spectra of Samples BS/SiO₂ and BS exhibit a wide and intense band at 1030 cm^–1 generated by Si-O-Si asymmetric stretching vibration, and the intensity of this band increases significantly in Sample BS/SiO₂. Meanwhile, a sharp peak at 1230 cm^–1 can be assigned to Si-CH₃ stretching vibration, the band at 910 cm^–1 corresponds to Si-O stretching vibration and the absorbing at 788 cm^–1 belongs to Si-O-Si symmetric stretching vibration.^67–69 These observations demonstrate that hydrophobic agent BS and SiO₂ nanoparticles have been deposited on the silk surface. Two distinct bands can be observed at 1627 and 1520 cm^–1, which can be assigned to the C=O carbonyl stretching and the N-H bending, respectively. However, the intensity of these two bands for Samples BS and BS/SiO₂ shows a marked decrease. This is because the composite coatings almost cover the fabric surface, which results in these two bands being undetectable. This further confirms the presence of hydrophobic components. OPEN IN VIEWER

Permeability of air and water vapor of the coated fabrics

The breathability of the fabric is one of the evaluations of comfort, and the result is shown in Figure 9(a). It can be seen that the air permeability of the uncoated samples and Sample BS are 263.25 and 446.47 mm/s, respectively. The air permeability of Sample BS increases by 70% compared to the uncoated silk fabric, which is attributed to the hydrophobic agent BS having high gas permeability. However, with the addition of SiO₂ nanoparticles, the air permeability of Sample BS/SiO₂ decreases to 262.89 mm/s. Generally, the spraying of SF and BS solutions forms transparent films on the sample surface, as shown in Figure 9(b), and binds fibers together, which results in a more loose arrangement between the silk yarns. However, spraying hydrophobic solutions dispersed SiO₂ nanoparticles for Sample BS/SiO₂, which may fill the pores of the fabric and reduce the transmittance of air to some extent, as shown in Figure 9(a). We can also see that Sample BS/SiO₂ has a coating covering a portion of the pores of the silk fabric in the SEM images in Table 2, resulting in the decrease of air permeability compared with that of Sample BS. OPEN IN VIEWER

The moisture permeability of the uncoated sample, Sample BS and Sample BS/SiO₂ is presented in Figure 9(b). These results indicate that most of the allochroic silica gels on the surface of the uncoated samples and Sample BS changed from blue to pink at 10 and 20 minutes, while Sample BS/SiO₂ has a slower speed. At 30 minutes, the surface of Sample BS/SiO₂ still retains a few blue silica gel particles. The results suggest that the water vapor permeability of Sample BS/SiO₂ is slightly worse due to SiO₂ particles filling the pores of silk fabric. ¹⁸

Mechanism simulation and demonstration of waterproof/breathable performance

In order to investigate the relationship between the performance and the structures, it is necessary to first have a clear idea of the mechanism of the waterproof and breathable performance of composite silk fabrics. Sheng et al. ¹⁸ used polyacrylonitrile to form nanofiber membranes by electrospinning to form nano-scale pores, which highlights the advantages of electrospinning tiny pores with high porosity. Different from Ding et al., the woven silk fabric we selected has large gaps and holes itself, and the warp and weft yarns of the base fabric form a convex and concave structure at the interlacing. When SF and EGDE are sprayed onto the silk fabric, the EGDE crosslinked SF forms a network on the silk fabric, establishing a capillary channel between them. The structure of the silk fabric changes from a macrostructure to a microstructure. When BS or BS/SiO₂ is used to treat the surface, a finer interconnecting channel can be constructed due to the pore effect compared to the unique SF treatment, as shown in images (a)–(f) in Table 2. In addition, SiO₂ nanoparticles form a nano-scale roughness on the surface of the reinforced silk fabric, enhancing the hydrophobic effect. The simulation of the mechanism of waterproofness and water vapor permeability for silk fabric coating is shown in Figure 10(a). OPEN IN VIEWER

Comprehensively considering the waterproofness, breathability and surface performance, Sample BS/SiO₂ was chosen for exhibiting the waterproof and breathable performance, which was carried out using distilled water and allochroic silica gel particles, referring to the method of demonstrating water repellency and breathable properties in the literature. ¹⁸ Herein, water droplets stained with methylene blue dye were used to indicate the waterproofness, and silica gel particles were utilized to confirm the water vapor transforming through the coatings, as presented in Figure 10(b). The water droplets still stood on the fabrics after half an hour, indicating the water-resistance of the modified fabrics. Moreover, the color of the allochroic silica gel particles changed from blue to pink, which suggests there was a large amount of water vapor transferred through the coatings. The results show that the sample can be waterproof and, at the same time, breathable.

Abrasion resistance of the coated fabrics

Figure 11 shows the measurements of contact angles of Samples BS/SiO₂ and BS of treated silk fabric as a function of the cycles of friction by the abrasion test. The silk sample treated with BS/SiO₂ corresponds to 145.77° and decreases to 132.31° after 200 cycles, then increases to 144.48° for 400 cycles and, subsequently, no clear changes are observed. After 1000 cycles of friction, the contact angles are 140.81°, a decrease of 3.4% compared with the uncoated sample. Because the behavior reported in Figure 11(a) for 200 cycles is rather surprising, Sample BS/SiO₂ was abraded for 150 and 250 cycles, and the results indicate the contact angles are 135.09° and 135.90°, respectively. It can be speculated that at the beginning of abrasion, the surface rough structure was damaged and some holes were formed, as shown in Figure 11. With the increase of the friction cycles, such as 400 cycles, the nanosilica particles rearranged on the sample surface, which produced a new nano-scale rough structure, as shown in Figure 11(a). When the treated samples after 1000 cycles were administered three drops of water on its surface, it can be seen from Figure 11(d) that the droplets still retain a perfect sphere. OPEN IN VIEWER

As shown in Figure 11(c), there is a small fall of contact angles of Sample BS with the increase of friction cycles. After abrasion for 1000 cycles, the contact angles decrease from 138.72° to 134.45°, a decrease of 3.1% compared with the uncoated sample. From Figure 11(d) we can see that although silk yarns are broken, water droplets still take a spherical shape on the worn out sample.

The components contained in BS solutions, such as siloxane, are high hardness, scratch-resistant and could offer excellent hydrophobic and abrasive properties to silk fabrics by applying coatings. Moreover, SiO₂ nanoparticles with high activity are easy to react with SF molecules and lead to the formation of a network structure, which could enhance the strength of the hydrophobic coatings. ⁷⁰ All of the results above suggest that the hydrophobic composite coatings of Samples BS/SiO₂ and BS possess good stability. Moreover, Sample BS/SiO₂ has better abrasion-resistance.

Washing durability of the coated fabrics

In order to evaluate the washing durability of the coated fabric, Samples BS and BS/SiO₂ were subjected to 5, 10, 15 and 20 cycles of washing according to the AATCC 61-2006 methods, as shown in Figure 12. The contact angles of the samples after repeated cycles of washing show a decreasing tendency, indicating that the adhesion of water droplets to the fabric increased, possibly due to some coating on the surface of the coated fabric being washed away during the washing process. After washing for 20 cycles, the contact angle of Sample BS/SiO₂ is reduced by about 4°, and the contact angle of Sample BS is reduced by about 15°. The result shows that coated fabric Sample BS/SiO₂ has high washing durability. OPEN IN VIEWER