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Abstract

The interest in multifunctional textile materials has been increased due to the health and safety measures of living beings, especially in severe conditions. Therefore, this study investigated the hydrophobicity, oil sorption capacity, and bending properties of untreated or uncoated and treated or coated 3D weft-knitted spacer fabric samples (92% polyester/8% spandex), i.e. sample 1, sample 2, and sample 3, having thicknesses of 2 mm (300 gm⁻²), 3 mm (350 gm⁻²), and 4 mm (540 gm⁻²), with silica aerogels (SAs) through the sol-gel method. SEM, FTIR-ATR, and surface roughness test of fabric samples were analyzed to comprehend the influence of SAs. The experimental results revealed the excellent hydrophobicity and oleophilicity of all the treated 3D weft-knitted spacer fabric samples, providing a higher water contact angle (CA) 142 ± 0.84° and an oil sorption capacity 7.51 ± 0.08g/g and 6.88 ± 0.06g/g for vegetable oil and engine oil, especially of sample 2 owing to the most silica particles. The statistical analysis also demonstrated a significant performance (P < 0.05) of treated spacer fabric samples at the 0.05 level. Thus, these fabrics are suitable for an industrial application of hydrophobic and oleophilic properties.

Keywords

hydrophobicity oleophilicity weft-knitted spacer fabrics silica aerogels sol-gel method

Introduction

In recent years, industrial discharges such as dyeing and printing chemicals, leather treatments, and oil spills, etc. have caused severe challenges to the world environment [1 –3]. They have high toxicity which has provoked detrimental effects to the ecological system, marine life, and public health [4]. Therefore, the optimizations in material characteristics for high-performances are in great demand of clean up, recovery, and safety measures [5]. For example, hydrophobicity and oleophilicity affect the silica aerogel wettability (contact angle) and surface tension (surface energy) and their accumulation lead efficiently towards many ecological applications, i.e., adsorption of organic liquids, oil spill remediation, thermal insulation, catalyst supports, water repellent, aeronautics, and aerospace, etc. [5 –8].

Amongst all aerogels (silica, cellulosic, carbon, graphene, resorcinol-formaldehyde, and inorganic), silica aerogels (SAs) exhibited conspicuous potential due to their unique molecular structure, high porosity, hydrophobicity, oleophilicity, low density, and thermal conductivity [9,10]. Therefore, SAs actively react for hydrophobic modification through synthesis and make a strong association with 3D structures which helps to control the structural collapse during the critical condensation process [10]. Thus, hydrophobicity could be enhanced by increasing the surface roughness and reduction in the surface energy [11]. Many research approaches have recently been applied to textile hydrophobic treatments with SAs through chemical crosslinking, interpenetrating network structure (IPNS), and combining silica with 3D nanofibers under ambient conditions, using various precursors, co-precursors, and silylating agents [12 –15].

Rao et al. prepared silica aerogels by methyltrimethoxysilane (MTMS) precursor [16]. Polydimethylsiloxane (PDMS) sponge was studied by Choi et al. for the absorption of oil from water [17]. Gurav et al. investigated the desorption time of twelve dissimilar organic liquids from TEOS-based silica aerogels [18]. Yu et al. presented flexible and oleophilic MTES-based aerogels through a two-step process [19]. Similarly, textiles with polyaniline and fluorinated alkyl silane [20], nitric acid solution [21], synthetization of polyacrylamide hydrogel, and cross-linked onto polyester fabric via UV-initiated polymerization [22], and incorporation of lignin in polypropylene via thermally induced phase separation (TIPS) [23 –25]. Most of these methodologies have their own weaknesses, for instance, costly precursors, limited lifespan, complicated procedures, and also time-consuming fabrications. Thus, the challenge remains for robust [26] and elastic textile materials with improved hydrophobic properties [27].

3D weft-knitted spacer fabric (92% polyester/8% spandex) is typically hydrophobic and therefore the best alternative to cotton and traditional moisture-sensitive textile materials [28 –32]. Moreover, it has good transversal compressibility, planar elasticity, higher strength, thermal insulation, and can be recycled as well [32 –38]. The features of spacer fabrics have been improved through minor quantities of nano-sized and micro-sized bio-ceramic additives [39 –44], however, their hydrophobic and oleophilic studies are very limited. Recently, the formation of super-hydrophobic surfaces through the sol-gel technique has become renowned and applied extensively in numerous studies [45 –48]. Therefore, in the present study, SAs were prepared from tetraethylorthosilicate (TEOS) based on sol-gel technology. The influence of SAs on hydrophobic appearances of uncoated and coated 3D weft-knitted spacer fabrics was assessed thoroughly by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy–attenuated total reflection (FTIR-ATR) analysis test. The effect of SAs on hydrophobic, oleophilic, and bending properties of spacer fabrics was also investigated. In this study, these 3D weft-knitted spacer fabrics with SAs coating were explored as an alternating source for hydrophobic and oleophilic materials.

Experimental work

Materials

Three kinds of 3D weft-knitted spacer fabric samples (92% polyester/8% spandex) were provided by Tianbin Textile Co. Ltd., Changshu, China. The two samples have a similar yarn count (75D) whereas the third sample was delivered with 100D. The yarn count of the middle layer of the three spacer fabric samples was kept uniform (40D). All kinds of 3D weft-knitted tuck spacer fabrics were manufactured on a circular knitting machine (gauge 28). The thicknesses and weight of the three samples varied i.e. 2 mm (300 gm⁻²), 3 mm (350 gm⁻²), and 4 mm (540 gm⁻²), respectively. The comprehensive specifications of 3D weft-knitted spacer fabrics used in this study are detailed in Table 1. Tetraethyl orthosilicate (TEOS) and N-hexane (>99%) were purchased from National Drug Group Chemical Reagent Co. Ltd. and Yangyuan Chemical Texnology Co. Ltd. Ethyl alcohol (99%, E_tOH), N, N-dimethyl-formamide (>98%, DMF), Hydrochloric acid (37%, HCl), and Ammonia (25%, NH₄OH) were procured from Shanghai Union Chemical Industry Co. Ltd., Lingfeng Chemical Reagent Co. Ltd., Algae Group Chemical Reagent Co. Ltd., and Shanghai Macklin Biochemical Co. Ltd., Shanghai, China. HMDS (hexamethyldisilazane) was bought from Shanghai Code Group Chemical Brake Co. Ltd., Shanghai, China. All the chemicals and reagents were applied without any further rectification. Two types of oils, namely vegetable oil and engine oil were used for the oil sorption test. The densities, surface tensions, and viscosities of the two oils were measured by dynamic contact angle tester (DCAT1) and digital rotary viscometer (ASNB2). The test temperature was maintained at 20 ± 2°C. The basic properties of these two oils are presented in Table 2.

Table 1.

3D weft-knitted spacer fabric specifications used in this study.

Fabric characteristics	Sample 1	Sample 2	Sample 3
Wales	47in⁻¹	38in⁻¹	44in⁻¹
Courses	67in⁻¹	60in⁻¹	65in⁻¹
Stitch density	1.43in⁻¹	1.58in⁻¹	1.48in⁻¹
Fabric density	0.0024kgin⁻¹	0.0019kgin⁻¹	0.0022kgin⁻¹
Spacer yarn arrangement angle	71.53°	79.96°	75.65°

Table 2.

The basic properties of vegetable oil and engine oil.

Oil properties	Vegetable oil	Engine oil
Viscosity (mPa.s)	62.7 ± 12.50	233.5 ± 15.70
Density (g.cm⁻³)	0.89 ± 0.20	0.85 ± 0.10
Surface tension (mN.m⁻¹)	32.45 ± 0.30	29.7 ± 0.50

Silica sol preparation

The silica sol-gel process was carried out in two stages i.e. firstly, acid-catalyzed TEOS hydrolysis and secondly, base-catalyzed gelation (see Figure S1, Supporting file) [44]. Primarily, 111.1 ml of TEOS were dissolved in 694.4 ml of EtOH. This mixture was stirred for 30 min at 400 rpm with a magnetic stirrer to form a homogeneous solution. Afterward, the acid catalyst solution was prepared by dissolving 27.8 ml of HCl in 83.3 ml of H₂O. It was then added dropwise into the (TEOS/EtOH) solution with magnetic stirring which was continued for 60 min at 60 °C. Secondly, 27.8 ml of NH₄OH was added into the (TEOS/EtOH) solution with magnetic stirring of 30 min. Then, certain amounts of DMF were added to form the homogeneous nano-porous structure of the silica aerogels. Finally, 27.8 ml of HMDS was dissolved in the solution for 120 min at 60 °C with magnetic stirring.

Fabrication of samples with silica aerogels

3D weft-knitted spacer fabric samples were conditioned at 20 °C and 65% relative humidity for 24 h. Then, the samples were cut into 35 cm × 35 cm pieces and immersed into the silica sol for 15 min (see Figure S2, Supporting file). Subsequently, the samples were solidified into a tightly sealed container. After the formation of the gelled layer on fabric samples (20 min), the wet-gelled samples were further kept for 24 h at 25 °C to secure the gel network in the fabric structures. The wet-gelled samples were washed with ethanol for 8 hours. The washing was repeated three times and exchanged into n-hexane. The wet-gelled films were washed with n-hexane for 8 hours to eradicate the ethanol-containing fluid. Finally, all samples were dried in an oven at 40 °C and 60 °C methodically for 5 h, followed by additional drying at 100 °C for 5 h. Figure 1 has shown different stages during the preparation of hydrophobic spacer fabric samples.

Figure 1.

Different stages during the preparation of hydrophobic weft-knitted spacer fabric samples.

SEM analysis

The scanning electron microscopy (SEM) was completed with a Flex-SEM 1000, (SU1000, Hitachi Ltd. Japan). All 3D weft-knitted spacer fabric samples were coated with a gold layer using a vacuum sputter coater before analysis. The surface morphology of fabric samples was examined by SEM (5.00KV × 550SE).

FTIR test

The Fourier transform infrared spectroscopy (FTIR) was accomplished with an American Nicolet TM 5700 FT-IR spectrometer. The spectra were generated in the range of 400–5000 cm⁻¹ based on Attenuated Total Reflection Technique (ATR). The test was used in 64 scans and encoded at 8 cm⁻¹ resolution.

Surface energy test

The surface energies of samples or sorbents were determined based on the Owens-Wendt-Rabel-Kaelble (OWRK) method. In this test, water, ethanol, and ethylene glycol were used as the liquid [49]. The test temperature was reserved at 20 °C. The surface tension of these liquids is listed in Table 3.

Table 3.

Characteristics of liquids solution for surface energy measurement of samples.

Liquids	Polar component (mN.m⁻¹)	Dispersion component (mN.m⁻¹)	Surface tension (mN.m⁻¹)
Water	52.20 ± 0.40	19.90 ± 0.10	72.10 ± 0.50
Ethylene glycol	29.00 ± 0.39	19.00 ± 0.28	48.00 ± 0.67
Ethanol	4.60 ± 0.21	17.50 ± 0.25	22.10 ± 0.46

Surface roughness test

The surface roughness was assessed by Kawabata Evaluation System for fabrics (KES-FB-4, Japan). 3D weft-knitted spacer fabric samples were cut into 20 cm × 20 cm for this test. Every test was done three times to achieve an average value. All fabric samples were conditioned at 20 °C and 65% relative humidity for 24 h before measurement and performance testing.

Water contact angle test

The water contact angle was estimated by an optical video contact angle instrument (Model OCA 40, Germany). The water contact angle was assessed for 60 sec after a water droplet of 5 µL was dropped on the fabric samples. All the contact angles exhibited an average of five separate areas of each sample. The test temperature was kept at 20 °C.

Oil sorption capacity test

3D weft-knitted spacer fabric samples were cut into 3 cm × 3 cm pieces and immersed into the oils for 15 min. The sorption capacity (weight gain) of untreated and treated weft-knitted spacer fabrics towards oil (vegetable oil and engine oil) was taken by dipping the dry tested spacer fabrics in a beaker filled with oil. After reaching the saturation level, the spacer fabrics were placed on wire mesh to drain out for one min and were removed for weight measurements. Each test was repeated three times to attain an average value. The test temperature was maintained at 20 °C. The oil sorption capacity test was performed by using ASTM standard test method (ASTM D281-12, 2016) that was calculated by equation (1)

N = \frac{W_{2} - W_{1}}{W_{1}}

(1)

where N is the sorption capacity, W₂ is the weight absorbed by fabric and W₁ is the initial weight of the fabric.

Bending test

The bending test was done using a self-fabricated test stage since spacer fabrics are too thick to be tested through Fabric Assurance by Simple Testing (FAST) or KES. In any case, the test rule was equivalent to FAST. The applied force (Newton) was applied to bend the samples for output in millimeters (mm). Bending test for ductility provided a modest way to evaluate the quality of materials by their ability to resist other surface irregularities during one continuous bend. The fabric samples were cut into 25 cm × 2.5 cm for this test. The typical test condition for this test was 20 °C and relative humidity was 65%. Bending lengths were attained in two distinct directions (transverse and longitudinal) of the spacer fabrics.

Statistical analysis

In this study, the statistical analysis of variance with two pair-wise various comparisons (t-test) was used to analyze the data. With a certainty interim set at 95%, p-value < 0.05 was viewed as a statistically significant difference. Noteworthy difference (shown in Figures 4 and 8) was entitled by various characters, which are, a, b, c, d, e, and f.

Results and discussion

Physical properties of the untreated and treated 3D weft-knitted spacer fabric

The surface and physical properties (particle morphology and microstructure) of the untreated and treated samples with silica aerogels were perceived by the SEM test as shown in Figure 2. Figure 2(a) has displayed the 3D weft-knitted spacer fabric surfaces without silica aerogels coating; whereas Figure 2(b) has exposed compressed surface morphologies and incessant structures on the 3D weft-knitted spacer fabric surfaces due to silica aerogels coating.

Figure 2.

SEM images of (a) untreated and (b) treated samples with silica gels (scale bars 100 µm).

The silica aerogels have the presence of an identical, nano-porous, and well-formed SiO₂ network on the surface of coated spacer fabric samples. After coating with silica gels, the morphology and uniformity coating of all treated fabrics were inflated by the substrate surface. The surface of spacer fabric coated with silica particles was denser than untreated spacer fabric. The coating also severely and harshly transformed the spacer fabric surface.

Additionally, several crusts and large bridges of interphase substantial were found in the spaces of the fibers. Sample 2 had the most silica gel nanoparticles on the surface among three types of treated spacer fabric samples since it had lower fabric density and greater arrangement angle. Hence, it had fewer friezes and a more porosity ratio on the fabric surface. Therefore, it triggered the highest water contact angle as compared to sample 1 and sample 3. Sample 1 (with the highest fabric density and lowest arrangement angle) had the lowest quantity of silica nanoparticles on its surface; thus, it was smoother than the other two samples and, as a result, has the lowest water contact angle. The increasing water repellency of spacer fabrics as a result of the existence of the silica nano-porous particles on their surfaces was attributed to both changes in the surface conformation and increased roughness on the surfaces [32]. Henceforth, the surface roughness of the spacer fabrics with silica aerogels coating has made an alluring hydrophobic and oleophilic material. Consequently, the nano-porous coating shaped by silica aerogels played a vital role in the mechanical properties.

FTIR spectra

The FTIR spectra of untreated and treated weft-knitted spacer fabric samples are shown in Figure 3. All characteristics absorption peaks were signifying the availability of a specific functional group. The occurrence of the absorption peak at around 1050 cm⁻¹ has the stretching and bending of C-O bonds. It was present in both untreated and treated samples. Moreover, the absorption peak at 1050 cm⁻¹ was owing to the unbalanced stretching and vibration of Si-O-Si bonds has confirmed the formation of nano-porous silica network structure on the treated samples [12]. These two kinds of peaks at the same position displayed the spectral difference between the untreated and treated samples at 1050 cm⁻¹.

Figure 3.

FTIR spectra of untreated (UTS) and treated (TS) weft-knitted spacer fabrics.

The existence of an absorption band at approximately 1350 cm⁻¹ and 2950 cm⁻¹ was due to the stretching and bending of C–H bonds. The strong peak at 1700 cm⁻¹ directed that all the fabric samples have C = O groups in their structures. The peak at 2200 cm⁻¹ was owing to the asymmetric stretching of -C≡C- bonds in the treated samples. The presence of Si-O-Si bonds in the spectra increased the surface roughness, thus, improved the hydrophobic and oleophilic properties of treated samples.

Surface energy test

The surface energies of untreated and treated 3D weft-knitted spacer fabrics along with their contact angles with water, ethylene glycol, and ethanol are shown in Table 4. There was a noticeable difference in surface energy observed among the three untreated and treated samples. The surface energy values increased with SAs coating from 24.15 ± 0.55 to 41.55 ± 0.98 mN.m⁻ ¹ , 21.97 ± 0.27 to 34.33 ± 0.60 mN.m⁻ ¹ , and 22.99 ± 0.37 to 39.33 ± 0.80 mN.m⁻ ¹ for sample 1, sample 2, and sample 3, respectively. Sample 2 showed lower surface energy i.e. 21.97 ± 0.27 to 34.33 ± 0.60 mN.m⁻ ¹ relatively close to the surface energy of oils (20–30mN.m⁻¹) and far lower of water (72 ± 0.50 mN.m⁻¹), triggering the fabric’s excellent hydrophobicity with water contact angle up to 142 ± 0.84°. Sample 1 and sample 3 had relatively higher surface energy at 24.15 ± 0.55 to 41.55 ± 0.98 mN.m⁻ ¹ and 22.99 ± 0.37 to 39.33 ± 0.80 mN.m⁻ ¹ ; however, it also showed good hydrophobicity with its water contact angles up to 132 ± 0.95° and 136 ± 0.89°. Moreover, while comparing with the compositions of surface energies; immense changes were detected in three samples. Sample 2 showed a higher ratio of dispersion component to the polar component which was (9.17, 6.78), followed by (8.09, 5.65) for sample 3 and (7.13, 4.84) for sample 1. Surface energy and its dispersion and polar composition played a vital role for fabrics as a sorbent to absorb oil [50].

Table 4.

The surface energy of untreated (UTS) and treated (TS) weft-knitted spacer fabrics.

	Contact angle (°)			Surface energy (mN.m⁻¹)
Samples	Water	Ethylene glycol	Ethanol	Dispersion components	Polar components	Total
UTS1	86 ± 1.14	51 ± 0.52	39 ± 0.60	21.18 ± 0.33	2.97 ± 0.22	24.15 ± 0.55
TS1	132 ± 0.95	95 ± 0.90	28 ± 0.41	34.44 ± 0.56	7.11 ± 0.42	41.55 ± 0.98
UTS2	90 ± 1.00	59 ± 0.31	41 ± 0.32	19.81 ± 0.17	2.16 ± 0.10	21.97 ± 0.27
TS2	142 ± 0.84	109 ± 0.65	32 ± 0.15	29.92 ± 0.39	4.41 ± 0.21	34.33 ± 0.60
UTS3	88 ± 1.10	55 ± 0.43	40 ± 0.54	20.46 ± 0.23	2.53 ± 0.14	22.99 ± 0.37
TS3	136 ± 0.89	100 ± 0.74	30 ± 0.26	33.42 ± 0.47	5.91 ± 0.33	39.33 ± 0.80

In terms of ratios between dispersion component and polar component, it was concluded that sample 2 had the better hydrophobic and oleophilic properties among the three samples. The lower the surface energy the higher the hydrophobic and oleophilic properties. On the other hand, the higher the ratio (dispersion component to polar component), the better the hydrophobic and oleophilic properties.

Surface roughness test

The surface roughness of treated 3D weft-knitted spacer fabrics was noticed higher than untreated weft-knitted spacer fabrics (shown in Figure 4). Moreover, these different characters (a, b, c, d, e, and f) indicated the significant differences in the figure. The error bars that appeared in Figure 4 represented the standard deviation.

Figure 4.

Surface roughness of untreated and treated weft-knitted spacer fabrics.

The surface roughness of untreated 3D weft-knitted spacer fabric was 0.69 ± 0.03 µm, 0.82 ± 0.01 µm, and 0.76 ± 0.02 µm; whereas the surface roughness of conforming coated 3D weft-knitted spacer fabric was 0.87 ± 0.02 µm, 1.12 ± 0.01 µm, and 0.99 ± 0.01 µm for sample 1, sample 2, and sample 3, respectively. The increasing rates of water contact angle of three samples after coating were 26.09%, 36.59%, and 30.26%. This was due to the presence of burls and stubs in the spacer fabric structures. The higher the surface roughness of samples, the better the water contact angle. Sample 2 showed a better contact angle due to its higher surface energy among all samples that triggered a better hydrophobic surface towards the water.

Water contact angle test

The water contact angle of treated 3D weft-knitted spacer fabrics was discovered higher than that of their concerning untreated 3D weft-knitted spacer fabrics. The water contact angle of untreated and treated samples is represented in Figure 5. The static water contact angles of the untreated and treated samples were determined by dropping 5 µL, water droplets on the surface of the different positions. The droplet readily permeated into the untreated samples to a full extent, indicating the excellent hydrophilicity of untreated samples, whereas, the water droplet stayed on the surface of the treated samples for a long time with higher contact angles. The water contact angle improved from 86 ± 1.14° to 132 ± 0.95°, 90 ± 1.00° to 142 ± 0.84°, and 88 ± 1.10° to 136 ± 0.89° after silica aerogels coating for sample 1, sample 2, and sample 3 respectively. The improving rates of water contact angle of three samples after coating were 53.49%, 57.78%, and 54.55%. It was due to numerous crusts and large bridges of interphase substantial were observed in the spaces of the fibers. Moreover, burls and stubs were noted on the surface of treated spacer fabrics that also indicated the surface roughness. Though all treated samples improved the water contact angles significantly, however, sample 2 had the most prominent water contact angle results followed by sample 1 and sample 3 since holding the higher SAs percentage in the fabric structure after coating [51]. The higher the add-on % of silica gels, the higher the water contact angle.

Figure 5.

The water contact angle of (a) untreated and (b) treated weft-knitted spacer fabrics.

On the other hand, when the untreated and treated samples were placed onto the water surface, the untreated samples quickly drowned to the bottom, while the treated samples floated on the water surface for a longer time as shown in Figure 6(a). This was caused due to the harsh and rough surface of treated samples with silica sol nanoparticles. Thus, the treated 3D spacer fabric samples showed higher hydrophobic behavior. The higher hydrophobicity of the silica aerogels coated samples demonstrated the successful surface modification of 3D weft-knitted spacer fabrics.

Figure 6.

The modified weft-knitted spacer fabrics: (a) the treated fabric floating on the surface whereas untreated sunk down, and (b) water droplet (dyed with blue) and oil droplet (dyed with red).

Oil sorption capacity test

The hydrophobic modification of 3D weft-knitted spacer fabric enabled to repel of water and absorbed oil shown in Figure 6(b). The sorption capacity study was conducted for untreated and treated 3D weft-knitted spacer fabric for two different pure oils without water, presented in Table 5. The oil sorption capacity of untreated 3D weft-knitted spacer fabric for vegetable oil was 5.17 ± 0.15 g/g, 5.49 ± 0.12 g/g, and 5.35 ± 0.14 g/g, while the oil sorption capacity of corresponding treated 3D weft-knitted spacer fabric was noticed 6.82 ± 0.13 g/g, 7.51 ± 0.08 g/g, and 7.15 ± 0.11g/g for sample 1, sample 2 and sample 3, respectively. The oil sorption capacity of untreated 3D weft-knitted spacer fabric for engine oil was 4.78 ± 0.14 g/g, 5.06 ± 0.09 g/g, and 4.98 ± 0.12 g/g, while the oil sorption capacity of treated 3D weft-knitted spacer fabric was observed 6.25 ± 0.11 g/g, 6.88 ± 0.06 g/g, and 6.58 ± 0.08 g/g for sample 1, sample 2 and sample 3, respectively. Hence, there was a significant improvement in the oil absorbency of the treated or coated textile fabrics. For vegetable oil, the improving rates of oil sorption capacity of three coated samples were 31.91%, 36.79%, and 33.64%, while, for engine oil, the improving rates of oil sorption capacity of three coated samples were 30.75%, 35.97%, and 32.13%.

Table 5.

Results of contact angle and oil sorption capacity of UTS and TS spacer fabrics.

Samples	Contact angle (°)		Coefficient of variation (CV %)		Oil sorption capacity for vegetable oil (g/g)		Coefficient of variation (CV %)		Oil sorption capacity for engine oil (g/g)		Coefficient of variation (CV %)
Samples	UTS	TS	UTS	TS	UTS	TS	UTS	TS	UTS	TS	UTS	TS
Sample 1	86 ± 1.14	132 ± 0.95	1.33	0.72	5.17 ± 0.15	6.82 ± 0.13	2.98	1.93	4.78 ± 0.14	6.25 ± 0.11	2.91	1.70
Sample 2	90 ± 1.00	142 ± 0.84	1.11	0.59	5.49 ± 0.12	7.51 ± 0.08	2.10	1.03	5.06 ± 0.09	6.88 ± 0.06	1.80	0.90
Sample 3	88 ± 1.10	136 ± 0.89	1.24	0.66	5.35 ± 0.14	7.15 ± 0.11	2.61	1.51	4.98 ± 0.12	6.58 ± 0.08	2.46	1.18

UTS: untreated samples; TS: treated samples.

From the resultant data, it was evident that the vegetable oil sorption capacity of both untreated and treated samples was higher than the engine oil. This was due to the low viscosity of vegetable oil rapidly moved into a capillary network of sorbents or samples. In contrast, the high viscosity of engine oil significantly affected the capillary system during the absorption. Moreover, the density and surface tension of vegetable oil was higher than the engine oil (shown in Table 2). The sorbents or fabric samples with more nano-porous silica gel particles absorbed more vegetable oil as compared to engine oil. Among all the entire treated spacer fabric samples, sample 2 had the most oil sorption capacity as compared to sample 1 and sample 2 for both oils. This was due to the higher add-on % of silica gels and the higher porosity ratio [51]. The higher the add-on % of silica gels, the higher the oil sorption capacity. Overall, the higher oil pickup ability was due to hydrophobic interaction and van der walls force between 3D weft-knitted spacer fabric sample or sorbent and oil. Therefore, this strategy was beneficial for developing oil sorbent with better oil sorption performance.

The relationship between water contact angle and oil sorption capacity

The water contact angle was used to express the oil sorption capacity of 3D weft-knitted spacer fabrics. The water contact angle and oil sorption capacity curve have been illustrated in Figure 7 by increasing the order of water contact angle of the three untreated and treated 3D weft-knitted spacer fabrics. When the water contact angle ranges from 86 ± 1.14° to 142 ± 0.84°, the oil sorption capacity of the three types of untreated and treated spacer fabrics changed from 5.17 ± 0.15 g/g to 7.51 ± 0.08 g/g for vegetable oil and 4.78 ± 0.14 g/g to 6.88 ± 0.06 g/g for engine oil. This means that 3D weft-knitted spacer fabrics with a higher water contact angle have higher oil sorption capacity. Besides, it is interesting that there was a positive relationship between water contact angle and oil sorption capacity from 86 ± 1.14° to 142 ± 0.84°, where the square correlation coefficient of first-order linear fitting was 0.985 and 0.983 for vegetable oil and engine oil, respectively. The slope and the intercept of the curves were 26 and –50.14 for vegetable oil, and 29 and –54.76 for engine oil, correspondingly. Hence, the relationship between water contact angle and oil sorption capacity has been expressed by equations (2) and (3)

y = 26 x - 50.14

(2)

y = 29 x - 54.76

(3)

where y is the water contact angle and x is the oil sorption capacity.

Figure 7.

The linear fit curve of water contact angle and oil sorption capacity: (a) vegetable oil and (b) engine oil.

The higher the water contact angle, the better the oil sorption capacity of the spacer fabric. In this study, while comparing the two, i.e. water contact angle and oil sorption capacity, the outcomes anticipated that water contact angle was the main influencing factor on oil sorption capacity.

Bending properties of 3D weft-knitted spacer fabrics

The bending properties of untreated and treated 3D weft-knitted spacer fabrics were led along with the transverse and longitudinal directions, separately. The treated 3D weft-knitted spacer fabrics were revealed higher than untreated 3D weft-knitted spacer fabrics. The outcomes are shown in Figure 8. The different characters (a, b, c, d, e, and f) indicated the significant differences and the error bars that appeared in Figure 8 represented the standard deviation. The bending rigidity of spacer fabrics with a similar structure expanded (impact) with the silica aerogel coatings. The bending rigidity increased from 3.87 ± 0.07 cN.cm to 4.75 ± 0.05 cN.cm, 4.25 ± 0.03 cN.cm to 5.03 ± 0.02 cN.cm, and 4.50 ± 0.05 cN.cm to 5.60 ± 0.04 cN.cm for sample 1, sample 2 and sample 3, respectively in the transverse direction after silica aerogels treatment. The increasing rates of bending rigidity of three samples after coating were 22.74%, 18.35%, and 24.44%, respectively.

Figure 8.

The bending properties of weft-knitted spacer fabrics: (a) transverse direction and (b) longitudinal direction. [Characters indicated the significant differences.]

Similarly, the bending rigidity of 3D weft-knitted spacer fabrics increased from 3.98 ± 0.06 cN.cm to 4.88 ± 0.04 cN.cm, 4.30 ± 0.04 cN.cm to 5.10 ± 0.02 cN.cm, and 4.45 ± 0.05 cN.cm to 5.53 ± 0.04 cN.cm for sample 1, sample 2, and sample 3, gradually in the longitudinal direction after silica aerogels treatment. The increasing rates of bending rigidity of three samples after coating were 22.61%, 18.60%, and 24.27%, respectively. From both transverse and longitudinal directions, it was apparent that sample 3 gained the highest rigidity % and sample 2 possessed the lowest rigidity % among the three samples. Moreover, sample 3 has the most notable bending rigidity results pursued by sample 1 and sample 2. With the thickness ranges from 2 to 4 mm, the measures of bending rigidity for untreated and treated fabrics were 3.87 ± 0.07 to 5.60 ± 0.04 cN.cm and 3.98 ± 0.06 to 5.53 ± 0.04 cN.cm for transverse direction and longitudinal direction, respectively. That was due to the impact of the fabric structures and composition on bending properties. Additionally, the fact of the silica aerogels was also huge on bending properties. Thus, the bending properties of 3D weft-knitted spacer fabrics could be altered through structural changes and the chemical composition of silica aerogels, in order to meet the firmness necessities of various attires.

Statistical analysis

A statistics t-test (shown in Table 6) was conducted between untreated (UTS) and treated samples (TS) of spacer fabrics for four different variables i.e. surface roughness, contact angle, sorption capacity (vegetable oil and engine oil), and bending rigidity (transverse direction and longitudinal direction). It was found that there are statistically significant differences between UTS and TS samples in respect of all four variables (5% significant level surface roughness: t = −12.212, p = 0.000; contact angle: t = −64.832, p = 0.000; sorption capacity: (vegetable oil: t = −25.308, p = 0.000; engine oil: t = −24.309, p = 0.000); bending rigidity: (transverse direction: t = −18.594, p = 0.000; longitudinal direction: t = −18.700, p = 0.000). The t-test proves that the UTS samples yield better results in all independent variables. Hence, the treated samples have superior results of hydrophobic, oleophilic, and bending properties as compared to the untreated spacer fabric samples since the chemical composition of SAs has the main influence on the surfaces of the 3D weft-knitted spacer fabrics samples.

Table 6.

The statistical analysis of untreated and treated weft-knitted spacer fabric samples.

Test Methods		Samples	No	Mean	Std. deviation	Std. error mean
Surface roughness		UTS	9	0.76	0.05698	0.01899
Surface roughness		TS	9	0.99	0.11036	0.03679
Contact angle		UTS	15	88	2.02661	0.52327
Contact angle		TS	15	136.67	4.35343	1.12405
Sorption capacity	Vegetable oil	UTS	9	5.34	0.20017	0.06672
	Vegetable oil	TS	9	7.16	0.32012	0.10671
	Engine oil	UTS	9	4.94	0.17660	0.05887
	Engine oil	TS	9	6.57	0.28705	0.09568
Bending rigidity	Transverse direction	UTS	9	4.21	0.27973	0.09324
	Transverse direction	TS	9	5.13	0.37685	0.12562
	Longitudinal direction	UTS	9	4.24	0.21442	0.07147
	Longitudinal direction	TS	9	5.17	0.28892	0.09631
Paired sample t-test (paired differences).
Methods		t	Sig. (2-tailed) P-value	Mean difference	Std. error difference	95% Confidence interval of the difference
Methods		t	Sig. (2-tailed) P-value	Mean difference	Std. error difference	Lower	Upper
Surface roughness		−12.212	0.000	−0.2300	0.01965	−0.28532	−0.19468
Contact angle		−64.832	0.000	−48.6667	0.75066	−50.27667	−47.05666
Sorption Capacity	Vegetable oil	−25.308	0.000	−1.8256	0.07213	−1.99189	−25.308
Sorption Capacity	Engine oil	−24.309	0.000	−1.6289	0.06701	−1.78341	−24.309
Bending rigidity	Transverse direction	−18.594	0.000	−0.9211	0.04954	−1.03535	−0.80688
Bending rigidity	Longitudinal direction	−18.700	0.000	−0.9267	0.04955	−1.04094	−0.81240

UTS: untreated samples; TS: treated samples.

Conclusions

In this study, uncoated and coated 3D weft-knitted spacer fabrics (sample 1, sample 2, and sample 3) with silica aerogel (SA) nanoparticles through the sol-gel method were investigated for hydrophobicity, oleophilic, and bending properties. The performance analysis of surface roughness, hydrophobicity (higher contact angle), oil sorption capacity, and bending properties between treated and untreated fabric samples, was statistically significant (p = 0.000) at the 0.05 level. Amongst all, sample 2 (with the most silica gel particles), has indicated excellent results in properties. Thus, the surface roughness of treated samples was improved from 0.69 ± 0.03 µm to 0.87 ± 0.02 µm, 0.82 ± 0.01 µm to 1.12 ± 0.01 µm, and 0.76 ± 0.02 µm to 0.99 ± 0.01 µm, and water contact angle was upgraded from (86 ± 1.14° to 132 ± 0.95°, 90 ± 1.00° to 142 ± 0.84°, and 88 ± 1.10° to 136 ± 0.89°) and therefore could be used for water-repelling materials. Likewise, the oil sorption capacity of treated samples was enhanced from (5.17 ± 0.15 g/g to 6.82 ± 0.13 g/g, 5.49 ± 0.12 g/g to 7.51 ± 0.08 g/g, and 5.35 ± 0.14 g/g to 7.15 ± 0.11 g/g) for vegetable oil and (4.78 ± 0.14 g/g to 6.25 ± 0.11 g/g, 5.06 ± 0.09 g/g to 6.88 ± 0.06 g/g, and 4.98 ± 0.12 g/g to 6.58 ± 0.08 g/g) for engine oil. However, the results of vegetable oil sorption capacity were better than engine oil. Similarly, the bending properties of treated 3D weft-knitted spacer fabric samples were observed higher (3.87 ± 0.07 cN.cm to 4.75 ± 0.05 cN.cm, 4.25 ± 0.03 cN.cm to 5.03 ± 0.02 cN.cm, and 4.50 ± 0.05 cN.cm to 5.60 ± 0.04 cN.cm) for transverse direction as compared to longitudinal direction i.e. 3.98 ± 0.06 cN.cm to 4.88 ± 0.04 cN.cm, 4.30 ± 0.04 cN.cm to 5.10 ± 0.02 cN.cm, and 4.45 ± 0.05 cN.cm to 5.53 ± 0.04 cN.cm. Hence, hydrophobicity and oleophilicity properties of 3D weft-knitted spacer fabrics have been improved significantly and appropriate to use in an industrial application such as space suits, seas, and protective materials or clothing. Moreover, with the altering factors of both weft-knitted spacer fabrics and silica aerogels, the treated fabrics could be used in a severer condition.

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 financially supported by the Shanghai Natural Science Foundation of Shanghai Municipal Science and Technology Commission (20ZR1400600), the Fundamental Research Funds for the Central Universities (2232021G-06).

ORCID iDs

Syed Rashedul Islam

Abeer Alassod

Jinhua Jiang

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The study of hydrophobicity and oleophilicity of 3D weft-knitted spacer fabrics integrated with silica aerogels

Abstract

Keywords

Introduction

Experimental work

Materials

Silica sol preparation

Fabrication of samples with silica aerogels

SEM analysis

FTIR test

Surface energy test

Surface roughness test

Water contact angle test

Oil sorption capacity test

Bending test

Statistical analysis

Results and discussion

Physical properties of the untreated and treated 3D weft-knitted spacer fabric

FTIR spectra

Surface energy test

Surface roughness test

Water contact angle test

Oil sorption capacity test

The relationship between water contact angle and oil sorption capacity

Bending properties of 3D weft-knitted spacer fabrics

Statistical analysis

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References