Sage Journals: Discover world-class research

Abstract

Textile clothing coated with silica aerogels has the potential of thermal insulation performance for heating and cooling. This work investigated the thermal isolation properties of untreated and treated three-layered weft-knitted spacer fabrics with different thicknesses (2 mm, 3 mm, and 4 mm) by using silica aerogels. Three samples of spacer fabrics (300GSM, 350GSM, 540GSM) were coated with nanoporous silica aerogel at a 26°C temperature and then kept for aging, exchanging the solvent, surface modification. The characteristics, for example, thermal resistance, thermal conductivity, yarn arrangement angle, porosity, and air permeability of spacer fabric samples, were investigated. Scanning electron microscopy analysis and Fourier transform infrared spectroscopy–attenuated total reflection test were conducted to explore the surface morphology and surface changes initiated by the silica coating. The experimental results indicated that the treated weft-knitted spacer fabrics with 350GSM have a higher thermal resistance of 0.09131 m² K W⁻¹, higher porosity ratio, higher air permeability, higher arrangement angle, and lower density. The statistical analysis also verified the significant performance (p = 0.000) of treated fabric samples at the 0.05 level.

Keywords

Thermal isolation weft-knitted spacer fabric thickness density silica aerogels

Introduction

In recent years, the use of high-performance textiles has been increased due to the high demand in comfort, health care, and safety of living beings.¹ Therefore, consumption of multifunctional textile materials (ultraviolet (UV) radiation protection, antimicrobial protection, dye fastness, anti-wrinkle finishing, superhydrophobicity, biomolecules immobilization, etc.) has made expanding trend in the worldwide clothing, including casual wear, sportswear, work wear, military wear, medical wear, active wear, underwear, and so on.^2,3 However, the dynamic interactions of textile clothing with human skin have led to the reduction of skin maceration, such as discomfort most specifically due to the thermo-physiological factor.⁴ Thus, it is necessary to investigate the optimization of insulation that enables human to remain in a good physiological state comfort, especially in cold weather.

Thermo-physiological comfort is associated with thermal properties, water vapor transmission, sweat absorption, and drying ability of fabrics.⁵ These are affected by the numerous major parameters like fiber geometry, cross section, structure, pore distribution, pathways, molecular structure, surface tension, fabric structure, thickness, density, and so on.^6,7 Similarly, its functionality and durability are also prompted by the finishing treatments.⁸ In the past few years, extensive research has been carried out on the evaluation of thermo-physiological characteristics of fabrics.^9–11 A lot of researchers also have developed a neural network, statistical, mathematical, and theoretical models for the determination of the thermal properties of textile knitted fabrics.^12–14 Among all, weft-knitted spacer fabrics have gained significant importance due to the special 3D structure with high porosity and open apertures in the surface.¹⁵ Moreover, they are regarded as eco-friendly textile materials since they can be recycled.

Weft-knitted spacer fabric is constructed on a double jersey circular knitting machine having a rotatable needle cylinder and needle dial.¹⁶ The fabric generally consists of two outer layers of elastic yarn (an upper layer and a bottom layer) and one middle layer of spacer yarn.¹⁷ The two outer layers are made up of knitted fabrics; however, the middle layer is constructed by spacer filaments (monofilament or multifilament) tightly connecting the two outer layers. This helps to reduce the air permeability and minimize heat loss through convection.¹⁸ Thus, it becomes excellent for tailor-made attires because of softness and comfort qualities compared to the other (warp and woven) spacer fabrics. Moreover, they have excellent transversal compressibility and planar elasticity.¹⁹ The characteristics of the weft-knitted spacer fabrics were further enhanced with the addition of even small amount of additives in particular micro-sized and nano-sized bio-ceramic additives (BCAs), such as silica (SiO₂), zirconia (ZrO₂), magnesium oxide (MgO), ferric oxide (Fe₂O₃), silicon carbide (SiC), and germanium (Ge) derivatives.^20–25

Silica aerogels (SAs) are super-insulating nanoporous materials, although they have inadequacies due to brittleness, moisture instability, and high cost.²⁶ However, the integration of these kinds of additives improves the attenuation of textile fabrics. SAs have a large internal surface area, low density, small pore size, and large pore volume.²⁷ It possesses very low thermal conductivity and high thermal insulation properties and performance due to the reduction in the conduction, convection, and radiation.²⁶ Therefore, it has become very popular in establishing desired modifications in textiles, that is, weft-knitted spacer fabric surfaces, and used in aerospace suits, sea, and biological purposes, and so on.²⁸ Many researchers have applied SAs to the fibers and fabrics as functional agents (super-thermal insulating, superhydrophobicity, protection from UV radiation, static biomolecules, colorfastness of materials and antimicrobial preservation) through sol-gel method.^29,30

The relationships between the knit structures and the thermal isolation properties of the weft-knitted spacer fabrics are still very limited. The authors have not found significant research work on the weft-knitted spacer fabric with the SAs coating for investigating its thermal isolation properties. Therefore, in this research work, SiO₂ tenuous gels were made from tetraethylorthosilicate with some other important substances through the sol-gel method. The 3D spacer fabric samples (uncoated and coated with SAs) were investigated. The influence on thermo-physiological characteristics of weft-knitted spacer fabrics was evaluated rigorously through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy–attenuated total reflection (FTIR-ATR) analysis test. Further, porosity, thermal insulation, and air permeability test were also conducted to determine their effectiveness. These fabrics have low weight, excellent porosity ratio, and good thermal resistance simultaneously. Thus, weft-knitted spacer fabrics with SA coating were investigated as an alternating source for thermal isolation material.

Experimental work

Materials

Three kinds of weft-knitted spacer fabric samples (92% polyester + 8% spandex) were collected from Tianbin Textile Co. Ltd., Changshu, China. All weft-knitted tuck spacer fabrics were made on a circular knitting machine of gauge 28. The first and second fabric samples were supplied with similar yarn count (75D) while the third sample with 100D. The yarn count of the middle layers of the three spacer fabrics was uniform (40D). The thickness of the three samples was different, that is, 2 mm, 3 mm, and 4 mm, respectively. The detailed specifications of fabric samples have been given 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%, EtOH), N,N-dimethyl-formamide (>98%, DMF), Hydrochloric acid (37%, HCl), and ammonia (25%, NH₄OH) were procured from Shanghai Union Chemical Industry Co. Ltd., Shanghai Lingfeng Chemical Reagent Co. Ltd., Algae Group Chemical Reagent Co. Ltd., and Shanghai Macklin Biochemical Co. Ltd. HMDS (hexamethyldisilazane) was bought from Shanghai Code Group Chemical Brake Co. Ltd. All the chemicals and reagents were of analytical grade and applied without any further rectification.

Table 1.

Identification of weft-knitted spacer fabrics.

Fabric characteristics	Sample 1	Sample 2	Sample 3
Fabric composition	92% polyester, 8% spandex, 75D	92% polyester, 8% spandex, 75D	92% polyester, 8% spandex, 100D
Thickness (mm)	2	3	4
GSM (g m⁻²)	300	350	540
Fabric density (kg m⁻³)	150	117	135
Spacer yarn composition	100% polyester, 40D	100% polyester, 40D	100% polyester, 40D
Spacer yarn arrangement, θ = tan⁻¹ h/w	71.53°	79.96°	75.65°
Wales (W) & Courses (C) per inch	W-47 & C-67	W-38 & C-60	W-44 & C-65
Stitch density per inch	1.43	1.58	1.48
Porosity	88%	92%	90%

Silica sol preparation

The silica sol-gel process was carried out in two stages, that is, primarily acid-catalyzed TEOS hydrolysis and finally base-catalyzed gelation. Initially, 111.1 mL of TEOS were dissolved in 694.4 mL of EtOH. This mixture was stirred at 400 r/min to form a homogeneous solution. After that, 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. Second, 27.8 mL of NH₄OH was added into the (TEOS/EtOH) solution with magnetic stirring of 30 min. Then, 27.8 mL of DMF was added to form the homogeneous nanoporous structure of the aerogels. Finally, 27.8 mL of HMDS were dissolved in the solution (TEOS/EtOH) for 2 h at 60°C with magnetic stirring. The ultimate silica concentration of the sol solution was 11.11 wt%. Figure 1 has shown the stages for the preparation of a silica sol-gel process.

Figure 1.

Preparation of silica sol-gel process (acid-catalyzed TEOS hydrolysis and base-catalyzed gelation).

Fabric samples treated with silica gels

All the weft-knitted spacer fabric samples were cut into 35 cm² pieces and immersed into the silica sol for 15 min. 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 structures (shown in Figure 2). The wet-gelled samples were washed with ethanol for 8 h. The washing was repeated thrice with the exchanging into n-hexane. The wet-gelled films were washed with n-hexane for 8 h to remove the ethanol-containing fluid. It was then dried in an oven at 40°C and 60°C systematically for 5 h, followed by further drying at 100°C for 5 h.

Figure 2.

Fabric samples treated with (a) silica aerogel during dip-coating and (b) forming gel network structure.

The fundamental test for samples

Examine of silica gel on fabric samples

The treated and untreated samples of spacer fabrics were dried at 107°C for 60 min. The add-on % of silica on spacer fabric samples was calculated according to equation (1)

Add-on % of silica = \frac{W_{2} - W_{1}}{W_{1}} \times 100

(1)

where W₁ is the weight of untreated samples in kg and W₂ is the weight of treated samples in kg.

SEM analysis

The SEM was completed with a Flex-SEM 1000 (SU1000, Hitachi Ltd. Japan). All 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.

FTIR analysis

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

Spacer yarn arrangement angle

Weft-knitted spacer fabrics were prepared with two distinctive layers (upper layer and bottom layer) fabrics that were joined together by a connecting layer (middle layer). Figure 3 has shown the critical yarn arrangement angle of the spacer fabric. The spacer yarns were formerly settled perpendicular to the two outer layer fabrics. The yarn arrangement angle of spacer fabrics was calculated by the equation (2)

θ = \tan^{- 1} \frac{h}{w}

(2)

where θ is the spacer yarn arrangement angle in degree, h is the thickness in mm of the spacer fabric, and w is the segment width in mm.

Figure 3.

The spacer yarn arrangement angle of weft-knitted spacer fabric.

Porosity test

In this test, the dry samples were measured in weight, then dipped in water for 24 h and re-measured their weights. The porosity of the samples was found as the percentage of the ratio of the difference in the dipped and the dry weight to the dry weight. The porosity measurements were accomplished for the average of 10 readings. Therefore, porosity percentage was calculated as follows equation (3)

P = \frac{W_{t} - W_{r}}{W_{r}} \times 100

(3)

where P is the porosity ratio in %, W_t is the dipped weight in kg, and W_r is the dry weight in kg.

Thermal insulation test

Thermal insulation properties of spacer fabrics were verified by the thermal resistance tester (YG606E) of China. The temperature of the testing plate was kept at 35°C. Each sample was measured three times, and the average values were calculated. The heat transferred from one part of the medium to another (medium). Thermal resistance substantiated heat properties and amounts of temperature variance by which a material resists heat flow. The fabric density and entrapped air inside the fabric structure caused higher thermal resistance. The surface of spacer fabric is self-possessed of pores enclosed by the yarns. The thermal resistance is connected with the thermal conductivity and thickness of the spacer fabric. The relationship between thermal resistance and the thermal conductivity was defined by equation (4)

R = \frac{h}{λ}

(4)

where R is thermal resistance in m² K W⁻¹, h is the fabric thickness in mm, and λ is thermal conductivity in W m⁻¹ K⁻¹.

Air permeability analysis

The air permeability of weft-knitted spacer fabrics was investigated through air permeability tester (YG461E, China). The test was conducted under specific conditions, that is, air pressure 100 P_a and the circular test area was 20 cm². Air permeability defined as the volume of air passing in 1 s through 100 mm² of fabric samples at a pressure variance of 100 P_a. In this experiment, each sample was measured 10 times in several places, and the average results were recorded.

Measurement of density

Density compares the amount of material of an object to its volume. The densities of untreated samples and treated samples were measured by equation (5)

ρ = \frac{m}{V}

(5)

where ρ is the density of spacer fabrics samples in Kg m⁻³, m is the mass of samples in Kg, and v is the volume of samples in m³.

Statistical analysis

The statistical analysis was also carried out to examine the level of significant impact. With a certainty interim set at 95%, p value smaller than 0.05 was viewed as a statistically significant distinction. Noteworthy contrast information (Figures 6 and 8) were entitled by various characters, that is, A, B, C, D, E, F and a, b, c, d, e, f.

Results and discussion

Measurements of thermo-physiological comfort and its effectiveness in three types of weft-knitted spacer fabrics integrated with SAs were performed under standard conditions, BS1051, at 20°C and 65% relative humidity. To verify the performances of these fabric samples, the results were analyzed through add-on %, SEM, FTIR, yarn arrangement angle, porosity, thermal insulation, and air permeability test. The influence of SAs on the fabric structure features has also been statistically analyzed. Moreover, the effects of density and thickness on isolation properties have also been discussed respectively. Table 2 has shown the face side, a back side, wales-wise and course-wise view of weft-knitted fabric samples.

Table 2.

Different visualizations of weft-knitted spacer fabrics.

Fabric characteristics	Sample 1	Sample 2	Sample 3
Front/face
Back
Wales wise
Courses wise

Influence of add-on % of SA coating on the fabric samples

SAs were made through the sol-gel method and applied to the fabric samples. It was found with the help of equation (1) that the add-on % of silica on the three samples was 49%, 62%, and 55% respectively. The Si-OH in silica particles vastly interconnected with the fibers by thermodynamic (hydrogen and dipolar-dipolar bonds) interaction. The polar chain structure of polyester (PET) fibers have the initial issue for the hydrogen, and dipolar-dipolar interaction thus caused the higher add-on % of silica on the samples. The tight adhesion between SAs accumulations and fabric structures attributed the formation of hydrogen bonds among spacer fabric samples and silica nanoporous due to the surface hydroxyls. It was observed that the higher the add-on % of silica, the higher the thermal resistance of samples (as in sample 2).

Influence of SA coating on the thermal resistance

The surface particle morphology and microstructure of the untreated and treated samples with silica gels were observed by the SEM test, as shown in Figure 4. Figure 4(a) has not shown any effect of the silica gel on the fabric surface, while Figure 4(b) has displayed a dense surface morphology and incessant structure. The silica gels have a nanoporous, tantamount, and well-structured network on the surface of treated fabric samples. After treated with silica gels, the morphology and uniformity coating of all treated fabrics was exaggerated by the substrate surface. Furthermore, the surfaces of treated fabrics coated by silica particles were found thicker than untreated fabrics. Thus, the treated fabrics have the existence of the SiO₂ network in their structures.

Figure 4.

SEM images of (a) untreated and (b) treated samples with silica gels (scale bars: 1 mm).

In addition, several large bridges and crusts of interphase substantial were noted in the spaces of the fibers. The surface of untreated spacer fabric samples was smooth while stubs and burls were observed on the surface of treated fabrics. Hence, the treated fabric samples covered with silica sol nanoparticles made the surface severe and harsh. Among all the three treated fabric samples, sample 2 had the most silica gel on the surface. Therefore, it had fewer friezes and more porosity ratio on the surface. Consequently, it caused the highest thermal resistance as compared to sample 1 and sample 3. Sample 1 has the lowest amount of silica on its surface; therefore, it was smoother than the other two samples and, as a result, has the lowest thermal resistance.

Influence of SA coating on the molecular structure

Figure 5 has shown the outcomes of FTIR analysis. All the fabric samples have indicated the availability of specific functional group at the absorption peaks. The occurrence of the absorption peak at around 1050 cm⁻¹ has the stretching and bending of C–O bonds. It was present in all untreated and treated samples. The absorption peak at 1050 cm⁻¹ was due to the unbalanced stretching and vibration of Si–O–Si bonds that confirmed the formation of nanoporous silica network structure on the samples. Due to the two kinds of the peak at the same position, the spectra were observed among the untreated and treated samples at 1050 cm⁻¹. The presence of an absorption band at nearly 1350 cm⁻¹ and 2950 cm⁻¹ were owing to the stretching and bending of C–H bonds. The strong peak at 1700 cm⁻¹ has indicated that all the samples have C=O groups in their structures. The peak around 2200 cm⁻¹ was due to the asymmetric stretching of –C≡C– bonds. Thus, the existence of Si–O–Si bonds in the spectra ensured the improvement of the thermal insulating properties of the materials (fabric samples).

Figure 5.

The FTIR spectra of untreated samples (UTS) and treated samples (TS).

Influence of SA on yarn arrangement angle

The yarn arrangement angle of three spacer fabric samples (S1, S2, and S3) was calculated and found 71.53°, 79.96°, and 75.65°, respectively. It was observed that the SA coating had no effect on the yarn arrangement angle. However, the fabric sample that had a higher spacer yarn angle had higher thermal resistance, that is, in our case, sample 2 had shown the highest thermal resistance since having the largest yarn arrangement angle (79.96°) as compared to the other two samples. Therefore, thermal resistance has a direct relationship to the spacer yarn arrangement. Greater the yarn arrangement angle, higher would be the thermal resistance. Moreover, it was revealed that the thermal resistance was higher when the spacer yarns placed into “v” form (Table 2 wales-wise and courses-wise figures) between the same two fabric faces rather than the spacer yarns perpendicular to the fabric faces.

Influence of SA coating on the porosity

The relationship among the porosity ratio, thermal conductivity, and thermal resistance of all spacer fabrics has been displayed in Table 3. The untreated fabric samples have higher porosity (88%, 92%, and 90%) and thermal conductivity levels (0.03519, 0.04739, and 0.06867 W m⁻¹ K⁻¹). However, when the fabric samples were treated with SAs, the porosity (85.67%, 90.33%, and 87.67%) and thermal conductivity (0.02706, 0.03286, and 0.05200 W m⁻¹ K⁻¹) were reduced. Among all untreated fabric samples, the samples that had lower porosity level also had lower thermal conductivity. The similar trends were observed in the treated fabric samples, that is, the samples with lower porosity level (85.67%) had decreased thermal conductivity (0.02706 W m⁻¹ k⁻¹). The untreated and treated fabric samples with higher porosity ratio have a higher thermal resistance.

Table 3.

Thermal insulation properties of weft-knitted spacer fabrics.

Sample	Thickness (h) mm	Fabric weight (w) g m⁻²	Air permeability (q_v) m ms⁻¹	Porosity (p) %	Fabric density (ρ) Kg m⁻³	Thermal conductivity (λ) W m⁻¹ K⁻¹	Thermal resistance (R) m² K W⁻¹
UTS 1	2	300	473.81	88	150	0.03519	0.05683
TS 1	2	392	269.99	85.67	196	0.02706	0.07390
UTS 2	3	350	456.83	92	117	0.04739	0.06330
TS 2	3	449	224.03	90.33	150	0.03286	0.09131
UTS 3	4	540	363.95	90	135	0.06867	0.05825
TS 3	4	620	189.83	87.67	155	0.05200	0.07692

UTS = untreated samples; TS = treated samples.

It was found that the decreasing porosity rates were 2.64%, 1.82%, and 2.59% after coating with silica gels. With the decrease in porosity (%), there was an increase in thermal resistance. Sample 2 had lost the lowest (1.82%) porosity after coating with SAs, and consequently, it had a higher thermal resistance. After the SA coating, the thermal resistance of three samples increased, that is, 0.07390, 0.09131, and 0.07692 m² K W⁻¹, respectively. It was also observed that after the treatment with SA, the fabric sample (S2) had the most thermal resistance among all the entire weft-knitted spacer fabric samples. This was due to the greater spacer yarn arrangement (79.96°), higher porosity (92%) level, and lower density (117 Kg m⁻³). High porosity ratio indicated the more spaces in the sample structure, which entrapped the huge air. The entrapped air in the fabric directed the high thermal resistance. The highest porosity ratio possessed the highest thermal resistance, since with the porosity ratio of 92% and the thermal resistance of 0.09131 m² K W⁻¹ in the fabric sample (S2). Thus, among the three fabric samples, the fabric sample (S2) has the highest porosity and the maximum thermal insulation properties. The higher values of thermal resistance signpost a lower heat transmission from the skin to the fabric surface. This is related to a warmer feeling.

However, when the porosity ratio decreased from 88% to 85.67%, 92% to 90.33%, and 90% to 87.67%, the thermal conductivity values also decreased from 0.03519 to 0.02706, 0.04739 to 0.03286, and 0.06867 to 0.05200 W m⁻¹ k⁻¹ for sample 1, sample 2, and sample 3, respectively. These SAs treated samples possessed lower thermal conductivity due to the reduced conduction, convection, and radiation. The circuitous path for heat transfer from the treated solid nanoporous structure and low density minimized the conduction. Notable tiny pore sizes reduced the convection. The radiative thermal conductivity with dissipating dopants has made the impact to diminish the aggregate thermal conductivity. The multi-porous structure of the treated spacer fabrics with SAs assumed an essential job in diminishing the thermal conductivities contrasted with the untreated spacer fabrics. Aerogels were astonishing thermal protectors since they verged on invalidating these methods of heat transfer. Henceforth, the lower thermal conductivity of the aerogel has made an alluring thermal protecting material.

Influence of SA coating on the air permeability

Figure 6 presented the pictorial relationship between the air permeability (pink color bar) and thermal resistance (green color line) in the selected samples. Moreover, these different characters (A, B, C, D, E, F and a, b, c, d, e, f) indicated the significant differences in the figures. It has expressed an indirect relationship (inversely proportional) between air permeability and thermal resistance, that is, after the treatment with SAs; the air permeability decreased while the thermal resistance increased. When the air permeability ranges were from 189.83 to 473.81 m ms⁻¹, the thermal resistance of the untreated and treated fabrics was 0.05683 to 0.09131 m² K W⁻¹. After the chemical treatment of sample 1, sample 2, and sample 3, the air permeability values decreased to 56.98%, 46.04%, and 52.16% with the increase in thermal resistance of 30.05%, 44.25%, and 32.05%, respectively. Thus, there was an increase in thermal resistance with the decrease in air permeability. Moreover, sample 2 lost the lowest air permeability rate (46.04%) and achieved the highest thermal resistance rate (44.25%) as compared to sample 1 and sample 3. Higher the air permeability, lower the thermal resistance. The error bars appeared in Figure 6 represented the standard deviation. Among all the three fabric samples, sample 2 displayed the highest thermal resistance value as compared to sample 1 and sample 3. It was due to the less decrease rate of air permeability with the silica gels coating that also indicated sample 2 possessed higher air permeability than the other two samples.

Figure 6.

The relationship between the air permeability (q_v) and the thermal resistance (R).

The air permeability and thermal resistance curve has been illustrated in Figure 7 of the three untreated and treated weft-knitted spacer fabrics. In addition, there was a positive relationship between air permeability and thermal resistance from 189.83 to 473.81 m ms⁻¹. The square relationship coefficient of first-order linear fitting was 0.612. The intercept and the slope of the curve were 859.63 and −7560.64, respectively. The relationship between air permeability and thermal resistance has been expressed by equation (6)

y = 859.63 - 7560.64 x

(6)

Figure 7.

The linear fit curve of air permeability (q_v) and the thermal resistance (R).

Since a higher value of thermal resistance specified a lower heat transferred from the skin to the fabric surface, this is generally allied with a warmer feeling.

Influence of SA coating on the fabric density

The correlation between fabric density (red color bar) and thermal resistance (blue color bar) has been illustrated in Figure 8. The different characters (A, B, C, D, E, F and a, b, c, d, e, f) indicated the significant differences in the figures. It has demonstrated that thermal resistance increased with the increase in fabric density of every individual sample after the chemical treatment. Thus, both have a direct relationship (directly proportional). The fabric density of three treated samples was 196, 150, and 155 kg m⁻³, wherein thermal resistance was 0.07390, 0.09131, and 0.07692 m² K W⁻¹. In the table, sample 2 has the lowest density and the highest thermal resistance as compared to sample 1 and sample 3. Among all the fabric samples, it has the best thermal resistance since it has a higher space to trap the air inside. Higher fabric density leads to greater thermal resistance and vice versa. The error bars appeared in Figure 8 represented the standard deviation. It validated that the density of all treated samples was increased after coating with silica gel. In addition, when the fabric density values altered from 117 to 196 kg m⁻³, the thermal resistance of the fabric samples changed from 0.05683 to 0.09131 m² K W⁻¹. This was due to the increase in fabric density, in accumulation to the fabric thickness.

Figure 8.

The relationship between fabric density (ρ) and thermal resistance (R).

It was observed that the density of sample 1 was 150 kg m⁻³ before treatment, and the density of sample 2 was 150 kg m⁻³ after treatment with SAs. All the fabric samples gained more density after coating with SA; hence, SAs played a vital role in increasing the fabric density and hereafter the thermal resistance. In sample 1, there was no silica particle before SAs coating, whereas sample 2 had silica particles after coating with SAs, although both sample 1 and sample 2 have the same densities but different thermal resistances. This was due to the amount of SA on the fabric samples.

Influence of SA coating on the fabric thickness

It was observed that the thicknesses of samples were unaffected by the silica treatment. Thus, the thickness had not made any effective influence on thermal conductivity. However, the thermal conductivity of all the treated samples was decreased after coating with silica gels. This was due to the influence of other fabric characteristics as, for example, the thermal conductivity decreased with the decrease in porosity. The values of thickness and thermal conductivity have been displayed in Table 3. The thermal conductivity decreased in every individual sample after coating with SAs. The thermal conductivity values decreased with SAs coating from 0.03519 to 0.02706, 0.04739 to 0.03286, and 0.06867 to 0.05200 W m⁻¹ k⁻¹ for sample 1, sample 2, and sample 3, respectively. The decreasing rates of thermal conductivities of three samples after coating were 23.10%, 30.66%, and 24.28%. Sample 2 has shown the lowest thermal conductivity. It also indicated that sample 2 possessed the highest thermal resistance among all the three samples.

The fabric sample with more thickness (sample 3 = 4 mm) had a less thermal resistance (0.07692 m² K W⁻¹). Similarly, the fabric sample with less thickness (sample 1 = 2 mm) had less thermal resistance (0.07390 m² K W⁻¹) than sample 3. Sample 2 (with thickness 3 mm) exhibited the highest value of thermal resistance (0.09131 m² K W⁻¹). Thus, sol-gel silica coating had no effect on fabric thickness; therefore, it had not any direct or indirect relationship with the change in thermal resistance. However, in all the samples, the thermal resistance values were improved to 0.07390, 0.09131, and 0.07692 m² K W⁻¹, respectively, after the SAs coating. The reason was porosity, density, and air permeability factors. Moreover, the stitch density of sample 1, sample 2, and sample 3 were 1.43/in, 1.58/in, and 1.48/in, respectively. Sample 2 had a higher stitch density compared to sample 1 and sample 3. It also played an important role to enhance the thermal resistance. Likewise, the change in fabric tightness changed the effect of thermal resistance. Sample 2 showed higher thermal resistance among all other samples since holding higher SAs percentage in the structure after coating. The higher the add-on % of silica gels, the higher the thermal resistance.

Statistical analysis

The performance analysis of untreated and treated spacer fabric samples was also statistically analyzed using the t test. Table 4 has shown the spacer fabric samples results in thermal conductivity, thermal resistance, yarn angle arrangement, porosity, air permeability, fabric density, and fabric thickness. It was observed a statistically significant difference (p-value ⩽ 0.05) in treated samples of thermal conductivity, thermal resistance, porosity, and air permeability at the level of 0.05, while yarn angle arrangement, fabric density, and fabric thickness have no statistically significant results (p = 1) at the level of 0.05. Thus, treated samples have better results in fabric characteristics as compared to the untreated spacer fabric samples. According to the results of thermal isolation properties (thermal resistance and thermal conductivity) of silica coating, it is observed that the structures of fabric samples and the chemical composition of SA have in primitive influence to the higher thermal resistance of silica coating on the surfaces of the spacer fabrics samples.

Table 4.

Statistical analysis of untreated and treated spacer fabric samples.

Parameters	Samples	Mean	SD	T-value	P-value
Thermal conductivity	UTS	0.05042	0.01471	−2.12	0.050
Thermal conductivity	TS	0.03730	0.01134	−2.12	0.050
Thermal resistance	UTS	0.05946	0.00338	−7.09	0.000
Thermal resistance	TS	0.08071	0.00833	−7.09	0.000
Yarn angle arrangement	UTS	3	1	0.00	1.000
Yarn angle arrangement	TS	3	1	0.00	1.000
Porosity	UTS	90.000	2.175	3.66	0.001
Porosity	TS	87.89	2.285	3.66	0.001
Air permeability	UTS	431.53	53.13	17.15	0.000
Air permeability	TS	227.95	37.51	17.15	0.000
Fabric density	UTS	134	16.5	−1.89	0.154
Fabric density	TS	167	25.2	−1.89	0.154
Fabric thickness	UTS	3	1	0.00	1.000
Fabric thickness	TS	3	1	0.00	1.000

UTS = untreated samples; TS = treated samples.

Conclusion

In this article, the thermal isolation properties of weft-knitted spacer fabrics treated with SA were investigated. This is a novel approach for the insulation of SA on the spacer fabric structure. In the investigation, it was observed that SAs had a significant influence on the thermal resistance, thermal conductivity, porosity, and air permeability. It did not influence fabric thickness and yarn angle arrangement. The important results of the study under discussion can be summarized as follows:

It was a successful application of SA by generating SiO₂ network on the weft-knitted spacer fabric structures. All the three fabric samples have shown higher thermal resistance (0.05683 to 0.07390, 0.06330 to 0.09131, and 0.05825 to 0.07692 m² K W⁻¹) after coating with SAs and could be used for warmer feelings in winter. The performance analysis of thermal resistance was also statistically significant (p = 0.000) at the 0.05 level. Sample 2 (with greatest stitch density and lowest wales and courses) has the highest thermal resistance.

The three spacer fabric samples showed lower thermal conductivity (0.03519 to 0.02706, 0.04739 to 0.03286, and 0.06867 to 0.05200 W m⁻¹ k⁻¹) after coating with SAs. The statistical analysis of treated and untreated samples was also statistically significant (p = 0.050) at the 0.05 level.

The weft-knitted spacer fabrics have high porosity ratio (85.67%, 90.33%, and 87.67%) after treated with SAs. The air could be trapped more to keep the body warmer. The performance analysis of the porosity ratio between treated and untreated fabric samples was also statistically significant (p = 0.001) at the 0.05 level.

The air permeability of the spacer fabrics was decreased after the SAs treatment. The performance analysis of air permeability between treated and untreated fabric samples was also statistically significant (p = 0.000) at the 0.05 level.

The fabric density was increased after the SAs in the weft-knitted spacer fabrics. However, the performance analysis of density between treated and untreated fabric samples was not statistically significant (p = 0.154) at the 0.05 level.

The spacer fabric samples showed no change in fabric thickness and yarn angle arrangement after the SA treatment. The performance analysis was not statistically significant (p = 1.000) at the 0.05 level.

Sample 2 has the lowest fabric density, highest porosity ratio, and highest air permeability, among all samples. It has the highest thermal resistance and resists more heat transfer between the objects. Therefore, it is the most suitable in an industrial application having good insulating properties such as space suits, down jackets, and cold protective materials.

Thus, changing parameters of weft-knitted spacer fabrics through SAs, the fabrics, or garments could be used in a severe situation.

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 work was supported by the National Key R&D Program of China (2016YFC0802802).

References

Varnaitė-Žuravliova

Sankauskaite

Stygiene

et al . The investigation of barrier and comfort properties of multifunctional coated conductive knitted fabrics. J Ind Text 2016; 45(4): 585–610.

Miraftab

Comparison of air permeability and moisture management properties of the jersey, interlock, and pique knitted fabrics. J Text I 2012; 2: 1–5.

Zhu

Gao

Guo

et al . Modified silica sol coatings for highly hydrophobic cotton and polyester fabrics using a one-step procedure. Ind Eng Chem Res 2011; 50(10): 5881–5888.

Ziaei

Ghane

Thermal insulation property of spacer fabrics integrated by ceramic powder impregnated fabrics. J Ind Text 2013; 43(1): 20–33.

Onofrei

Rocha

Catarino

The influence of knitted fabrics’ structure on the thermal and moisture management properties. J Eng Fiber Fabr 2011; 6(4): 10–22.

Onal

Yildirim

Comfort properties of functional three-dimensional knitted spacer fabrics for home-textile applications. Text Res J 2012; 82(17): 1751–1764.

Sampath

Aruputharaj

Senthilkumar

et al . Analysis of thermal comfort characteristics of moisture management finished knitted fabrics made from different yarns. J Ind Text 2012; 42(1): 19–33.

Xing

Ding

Silica xerogel coating on the surface of natural and synthetic fabrics. Surf Coat Tech 2008; 202(19): 4721–4727.

Arumugam

Mishra

Militky

et al . Investigation on thermo-physiological and compression characteristics of weft-knitted 3D spacer fabrics. J Text I 2017; 108(7): 1095–1105.

10.

Barauskas

Sankauskaite

Abraitiene

Investigation of the thermal properties of spacer fabrics with bio-ceramic additives using the finite element model and experiment. Text Res J 2018; 88(3): 293–311.

11.

Ertekin

Marmarali

Heat, air and water vapor transfer properties of circular knitted spacer fabrics. Tekst Konfeksiyon 2011; 21(4): 369–373.

12.

Majumdar

Modelling of thermal conductivity of knitted fabrics made of cotton–bamboo yarns using artificial neural network. J Text I 2011; 102(9): 752–762.

13.

Demiryürek

Uysaltürk

Thermal comfort properties of Viloft/cotton and Viloft/polyester blended knitted fabrics. Text Res J 2013; 83(16): 1740–1753.

14.

Chen

et al . Analysis of physical properties and structure design of weft-knitted spacer fabric with high porosity. Text Res J 2018; 88(1): 59–68.

15.

Mao

Russell

SJ.

The thermal insulation properties of spacer fabrics with a mechanically integrated wool fiber surface. Text Res J 2007; 77(12): 914–922.

16.

Willmer

Circular knitting machine, especially for the production of spacer fabric. USPTO Patent Full Text and Image Database. Patent 6915666B2, USA, 2005.

17.

Liu

Compression property and air permeability of weft-knitted spacer fabrics. J Text I 2011; 102(4): 366–372.

18.

Pereira

Anand

Rajendran

et al . A study of the structure and properties of novel fabrics for knee braces. J Ind Text 2007; 36(4): 279–300.

19.

Taurino

Fabbri

Messori

et al . Facile preparation of superhydrophobic coatings by sol–gel processes. J Colloid Interf Sci 2008; 325(1): 149–156.

20.

Bae

Min

Jeong

et al . Superhydrophobicity of cotton fabrics treated with silica nanoparticles and water-repellent agent. J Colloid Interf Sci 2009; 337(1): 170–175.

21.

Bae

Jang

Jeong

et al . Superhydrophobic PLA fabrics prepared by UV photo-grafting of hydrophobic silica particles possessing vinyl groups. J Colloid Interf Sci 2010; 344(2): 584–587.

22.

Yang

Deng

Preparation and physical properties of superhydrophobic papers. J Colloid Interf Sci 2008; 325(2): 588–593.

23.

Chen

Zhou

et al . Preparation of super-hydrophobic surface on stainless steel. Appl Surf Sci 2008; 255(5): 3459–3462.

24.

Xiu

Hess

Wong

UV and thermally stable superhydrophobic coatings from sol–gel processing. J Colloid Interf Sci 2008; 326(2): 465–470.

25.

Zheng

Shan

Bai

et al . Assembly of silica aerogels within silica nanofibers: towards a super-insulating flexible hybrid aerogel membrane. RSC Adv 2015; 5(111): 91813–91820.

26.

Daryabeigi

Heat transfer in high-temperature fibrous insulation. J Thermophys Heat Tr 2003; 17(1): 10–20.

27.

Stark

Fricke

Improved heat-transfer models for fibrous insulations. Int J Heat Mass Tran 1993; 36(3): 617–625.

28.

Wang

Fang

Cheng

et al . One-step coating of fluoro-containing silica nanoparticles for universal generation of surface superhydrophobicity. Chem Commun 2008; 7: 877–879.

29.

Hoefnagels

de With

et al . Biomimetic superhydrophobic and highly oleophobic cotton textiles. Langmuir 2007; 23(26): 13158–13163.

30.

Xing

Dai

Superhydrophobic surfaces prepared from water glass and non-fluorinated alkylsilane on cotton substrates. Appl Surf Sci 2008; 254(7): 2131–2135.

Influence of silica aerogels on fabric structural feature for thermal isolation properties of weft-knitted spacer fabrics

Abstract

Keywords

Introduction

Experimental work

Materials

Silica sol preparation

Fabric samples treated with silica gels

The fundamental test for samples

Examine of silica gel on fabric samples

SEM analysis

FTIR analysis

Spacer yarn arrangement angle

Porosity test

Thermal insulation test

Air permeability analysis

Measurement of density

Statistical analysis

Results and discussion

Influence of add-on % of SA coating on the fabric samples

Influence of SA coating on the thermal resistance

Influence of SA coating on the molecular structure

Influence of SA on yarn arrangement angle

Influence of SA coating on the porosity

Influence of SA coating on the air permeability

Influence of SA coating on the fabric density

Influence of SA coating on the fabric thickness

Statistical analysis

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References