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N-octadecane infused TDI-PEG-grafted halloysite nanotube for thermal management of unidirectional moisture/breathable smart textiles
* Corresponding author: E-mail address: weizhang2999@163.com (W. Zhang)
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Received: ,
Accepted: ,
Abstract
Porous materials are ideal carriers for preparing shape-stable composite phase change materials (CPCMs), with their pore size and capillary force determining the adsorption and fixation ability of phase change materials (PCMs). Herein, acid etching and polymer modification were used to enhance the adsorption efficiency of n-octadecane by halloysite nanotubes (HNTs) via vacuum impregnation. The maximum loading capacity of the two modified HNTs for n-octadecane was similar, while the shape stability of the CPCMs prepared by grafting was superior, with a latent heat energy of 64.72 J·g-1. These CPCMs were applied to the cotton fabric to form a hybrid layer via wet coating, which maintained an air permeability of 5.97% and a photothermal conversion efficiency of 32.4%. Moreover, the coating layer also displayed hydrophobicity and unidirectional moisture performance. The smart fabric exhibited excellent heat storage and photothermal conversion performance, with the surface temperature being 15.2°C higher than that of raw cotton after sunlight irradiation and absorption saturation, while the cooling time to room temperature was extended by 410 s, highlighting its smart temperature-regulating properties. Overall, this study provided a novel approach for developing smart fabrics with optimal photothermal conversion efficiency.
Keywords
Composite phase change materials
Halloysite nanotubes
Moisture transport
Photothermal conversion efficiency
Smart textiles
1. Introduction
The primary function of clothing is to not only cover and decorate but also provide comfort and protection from external discomfort [1, 2]. Traditional fabrics composed of cotton, wool, hemp, polyester, and other fibers create a stable microclimate near the skin through passive heat insulation, thereby maintaining the body in a comfortable state [3]. When the outside temperature changes drastically, the ability of these fabrics to effectively regulate temperature fails. However, their combination with phase change materials (PCMs) endows fabrics with the ability to adjust temperature by actively storing or releasing heat owing to their buffering effect on temperature changes [4]. Indeed, PCMs alter their physical state during heating and cooling to store and release latent heat energy. Based on the phase transformation mechanisms, PCMs can be categorized into solid-solid, solid-liquid, solid-gas, and liquid-gas [5]. Among them, solid-liquid PCMs are widely used, attributed to their large latent heat capacity and small volume variation during phase change [6].
Paraffin-based PCMs, especially n-alkane such as octadecane, have a melting point of approximately 30°C, which is within the comfort temperature range of the human body [7]. At the same time, they have high latent heat energy, a relatively wide phase transition temperature range, and chemical stability and display potential applications in the fields of thermal energy storage and smart textiles [8]. However, organic PCMs are prone to phase leakage, limiting their applications as textile functional materials [9]. To enhance the practicability and safety of solid-liquid PCMs, several complex methods have been developed [10], including porous adsorption[11], microencapsulation [12], melt impregnation [13], vacuum impregnation [14], sol-gel [15], and chemical grafting/block [16]. For example, octadecane was encapsulated into core-shell cellulose-silica microcapsule to promote its dispersion into waterborne silicone resins, forming a multi-protective coating for fabric and imparting thermal insulation, superhydrophobicity, and other properties to fabrics [5, 17]. PCM microcapsules based on hexadecane and octadecane as cores, encapsulated in poly (urea-formaldehyde) shells via in situ interfaces, and then modified by silver nanoparticles allow for dynamic thermal management in textiles [18]. Compared to other methods, impregnation is simple to operate, and the available carriers are versatile in shape and size. The desired physical and structural characteristics of an appropriate carrier include specific surface area, pore radius, mechanical strength, thermal conductivity, and so on.
The loaded components, PCMs, are generally solely distributed on the surface of the carrier, with a high utilization rate, low dosage, and cost. However, porous materials allow PCMs to impregnate the inner structure, resulting in an ideal loading capacity. Organic polymers [19], cellulose-based porous materials [20], carbon nanotubes [21], expanded graphite alkene [22], copper foa [23], and silicon-based HNTs [24] all serve as carriers for composite PCM (CPCM) preparation. The carrier used in the impregnation method is not only effective in attenuating leakage linked to PCMs but also enhances the stability of the prepared CPCMs. Zhang et al. employed carbon nanotubes and expanded graphite as carriers to adsorb paraffin during the preparation of shape-stable CPCMs. Their powder-like morphology allowed the formation of a coating layer on the surface of textiles, manufacturing functional textiles with photothermal conversion properties [21-23]. Meanwhile, Zhang et al. modified copper foam with graphene oxide and reduced graphene oxide, and the active sites resulting from the modification of carbon materials facilitated the adsorption of paraffin and polyethylene glycol (PEG), with the prepared stable and leakproof CPCM displaying a photothermal conversion efficiency of 86.68% [21-23]. Thanakkasaranee et al. prepared CPCMs with stable shapes by loading molten PEG with HNTs and achieved an excellent heat transfer rate, showing promise for use in food packaging to avoid food loss induced by temperature change [24].
Silicon-based halloysite nanotubes (HNTs) with hollow nanotube structures and large surface area exhibit favorable adsorption performance and are 24% less costly than other carrier materials. Their surface hydroxyl concentration is low, leading to weaker interaction forces and superior dispersion in polymers compared to nanoparticles such as carbon nanotubes. This increases the reaction area between HNTs and adhesives, which is conducive to binding with textiles. Moreover, hydroxyl groups on the surface also provide a basis for further modification [21-23]. Tas and Unal introduced HNT/PEG(400) and HNT/PEG(600) mixtures into a polyethylene (PE) matrix, yielding a stable HNT/PCM hybrid film in a continuous molten state across the temperature range of -22°C-22°C [25]. Gu et al. used acid modification to achieve a maximum load ratio of 54%, compared to only 45wt% for unmodified HNTs, with significantly improved shape stability [26]. Moreover, Xiang et al. concluded that acid etching increased the specific surface area and pore volume of HNTs and prepared CPCMs with an enthalpy value of 112 J/g using a twin-screw extruder, which displayed outstanding thermal reliability and stability, making them ideal for preparing phase change fibers via high-temperature melt spinning [27].
In this paper, n-octadecane, with a high latent heat and a melting point of roughly 28°C, was used as the heat storage material, and HNTs were selected as the supporting material for the load to prepare powdered CPCMs through vacuum impregnation. In order to enhance the adsorption capacity of HNTs to n-octadecane, the inner alumina layer of HNTs was etched with acid to widen their porosity and concurrently improve the impregnation rate of n-octadecane, or 2, 4-toluene diisocyanate (TDI) and PEG were utilized as grafting materials to increase the hydroxyl group density on the outer surface and form a larger gap. The prepared CPCMs were subsequently incorporated into polyurethane agents and applied on the surface of cotton fabric by wet coating to fabricate smart textiles with moisture absorption and photothermal transformation properties. Finally, the latent heat energy and thermal stability of CPCMs, as well as the photothermal conversion and moisture absorption performance of smart textiles, were comprehensively characterized and analyzed.
2. Materials and Methods
2.1. Materials and reagents
Cotton fabric (yarn count: 35*35, yarn density: 130*70, twill) was procured from Hebei Ningfang Group Co., Ltd. HNTs were purchased from Huideli. n-octadecane, TDI, PEG6000, and N, N- dimethylformamide (DMF) of analytical reagent grade were sourced from Macklin. Concentrated H2SO4, toluene, and polyurethane were acquired from Sinopharm.
2.2. HNT etching
HNTs and 1 M H2SO4 were evenly mixed in a round-bottomed flask at a mass ratio of 1:3.92 and reacted at 80°C for 2 h with gentle stirring. Next, the flask was allowed to cool to room temperature, and the reactant was filtered and washed until a neutral pH was achieved. After drying in an oven, acid-purified HNTs (a-HNTs) were obtained.
2.3. HNT grafting
HNTs (10 g) were added into a three-necked flask containing toluene solution, following which 10 g of TDI was gently dropped into the flask and allowed to react at room temperature for 24 h while stirring at 200 rpm. Upon the reaction temperature rising to 80°C, 2.5 g PEG was introduced into the system, and the resulting mixture was stirred for an additional 8 h. Finally, the reactant was cooled to room temperature, filtered, and washed until neutral, and then freeze-dried to yield grafted HNTs (g-HNTs).
2.4. CPCM preparation
An ethanol solution (10 mL) containing dispersed octadecane was thoroughly mixed with a-HNTs or g-HNTs (2 g) in a three-necked flask. The mass ratio of octadecane to HNTs was 1:9, 2:8, 3:7, and 4:6, respectively. After the absolute ethanol completely evaporated (50°C, 200 rpm), the three-necked flask was sealed and vacuumed for 30 min (-0.1 kPa). Afterward, the CPCMs were poured onto filter paper and heated in an oven at 80°C for 10 min, and the unabsorbed n-octadecane on the surface would be adsorbed onto the filter paper.
Where H-CPCM represents the CPCMs prepared using HNTs, P-CPCM is the CPCMs prepared from a-HNTs, G-CPCM denotes the CPCMs prepared from g-HNTs, and post percentage represents the impregnation ratio of octadecane in the CPCMs.
2.5. Preparation of smart fabrics via wet coating
Polyurethane accounted for 35% of the total mass of the solution, and DMF was thoroughly mixed and dissolved at 60°C. HNTs with a mass ratio of 1:2 were added and gently stirred at a rate of 100 rpm for 10 min to prepare the finishing solution. Next, 31.5 g of molten PEG was evenly coated on 5×5 cm cotton fabric through the wet coating method, quickly immersed in deionized water for a pre-defined duration, and then dried in an oven at various temperatures (90°C, 110°C, and 130°C). The pore density and size of the fabric surface coating were adjusted by controlling the soaking time and drying temperature. The specific preparation process is illustrated in Figure 1.
- Preparation of unidirectional moisture/breathable smart fabrics.
2.6. Characterization of HNTs
The microscopic morphology of HNTs was observed under a scanning electron microscope (SEM, S-4800-I, HITACHI, Japan). The structure and diameter of HNTs were visualized under a transmission electron microscope (TEM, JEM-2100, JEOL, Japan). The crystal patterns of HNTs before and after acid etching were characterized using an X-ray diffractometer (D/MAX-2500, Rigaku, Japan). A Fourier transform infrared spectrometer (FTIR) (Nicolet6700, Thermo-fisher, American) was used to monitor the grafting process of HNTs. The pore size of HNTs, a-HNTs, and g-HNTs was measured using a nitrogen adsorption-specific surface area analyzer (Kubo1200, Bjbiaode, China).
2.7. Characterization of CPCMs
A block sample with a length, width, and height of 1, 1, and 0.5 cm was evaluated for thermal conductivity using a thermal conductivity tester (TC300E, XIATECH, China). CPCMs with different components were placed in the center of the filter paper and incubated in a 50°C incubator for 30 min. The adsorption of n-octadecane liquid on the filter paper was observed to assess phase leakage. The latent heat energy, melting point, and phase transition starting temperature of CPCMs were measured using a DSC-214 (Netzsch, Germany) within the temperature range of 0-60°C and a temperature change rate of 10°C/min. A thermal constant analyzer (TPS2500S, Hot Disk, Sweden) was employed to examine thermal conductivity at 20°C.
2.8. Characterization of smart fabrics
The breathability of the coated fabric was investigated using a fabric breathability tester (YG461G, DARONG, China), whilst the contact angle was measured by capturing images of water droplets on the fabric surface using a contact angle measuring instrument (JC-2000D1, POWEREACH, China). The fabrics were placed in outdoor sunlight (36°C) under windless conditions, and the surface temperature was recorded every 30 s using an infrared thermal imager (Ti10 FLUKE America) to evaluate photothermal conversion performance. Thereafter, the fabrics (with a length, width, and height of 1, 1, and 0.5 cm) were placed on a constant temperature table (47°C), and the surface temperature of the sample was recorded using an infrared thermal imager (every 5 s) to determine the heating rate.
The smart fabric was placed 30 cm below an iodine tungsten lamp (500 W), and the accelerated thermal cycle test was carried out, which consisted of heating for 180 s and cooling for 170 s. The temperature was recorded every 5 s using an infrared thermal imager.
Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Thermo-Fisher, American) was utilized to analyze the chemical components of g-HNTs and CPCMs. A capillary flowmeter (CEP-CEP-1100AX, Porous Materials, Inc., American) was used to analyze the flow rate of gas through the fabric to determine the pore size and distribution of the coating.
2.9. Calculation of photothermal conversion efficiency
The solar thermal energy storage efficiency (η) of the functional fabric under the simulated solar radiation of an iodine tungsten lamp was calculated using Eq. (1) [10].
Where: η: photothermal conversion efficiency, %; Qt: the storage heat of the sample, j; Q: received light energy, j; m: mass of CPCM in the temperature-regulating fabric, g; P: intensity of simulated sunlight, lux; international unit of solar intensity; A: the light exposure area of the sample, m2; t, the illumination time from the start temperature to the end temperature of the phase change s.
3. Results and Discussion
3.1. TEM and SEM of HNTs
As displayed in Figure 2, the HNTs had a tubular structure with a clear and smooth outer wall. Upon magnification at 2.5 times, the inner wall of HNTs was clear and smooth, with the presence of a tubular structure (Figure 2d). After acid etching, the alumina layer of HNTs dissolved, and a large amount of free Al and Si accumulated around the HNT [28], which enhanced the dispersion of a-HNTs in absolute ethanol and concomitantly maintained the integrity of the outer wall of HNTs (Figure 2b). Besides, the HNT wall was thin and rough (Figure 2e). Acid corrosion largely occurred in the inner cavity of HNTs. HNTs had smooth surface morphology, uniform wall thickness, and regular cavity holes, thereby facilitating acid penetration and dissolution of the alumina layer of HNTs. Notably, the increase in the inner diameter of the tube improved porosity, which laid a foundation for impregnating more octadecane.
- Low- and high-magnification TEM images of: (a) HNTs×3000, (b) a-HNTs ×3000, (c) g-HNTs ×5000, (d) HNTs ×10000, (e) a-HNTs ×10000, and (f) g-HNTs ×10000.
After grafting modification, HNTs were aggregated, as depicted in Figure 2(c), and retained their tubular structure. The g-HNTs were closely interconnected, and the distance between holes became smaller (Figure 2f), which provided not only a foundation for impregnating octadecane but also enhanced capillary adsorption for octadecane.
In Figure 3, the morphology and structure of g-HNTs were further analyzed using SEM, with results presented in Figure 3(c). Compared with the original HNTs (Figure 3a), the surface of the tubes was coated with the polymer, and the nozzle was covered. HNTs aggregated and combined into an irregularly spherical shape. The HNTs were intertwined, forming large gaps within the aggregates and providing impregnation space for octadecane.
- Low- and high-magnification SEM images of: (a) HNTs×50000; (b) HNTs×80000; (c) g-HNTs×50000; (d) g-HNTs×50000.
3.2. Analysis of structure and properties of HNTs and their CPCMs
3.2.1. Chemical structure of CPCMs
As delineated in Figure 4(a), the HNTs had a relatively wide and strong reflection at 2θ of 12.03°, which was formed by the reflection of the HNT base. Its wide shape could be ascribed to the small crystal size and curved structure of multilayered HNTs. Additional reflections were observed at 18.06°, 24.85°, 35.01°, 38.28°, and 62.42° of 2θ, corresponding to the lattice spacing of 4.44, 3.59, 2.56, 2.35, and 1.49, demonstrating that the HNTs had an octahedral lamellar structure. After acid treatment, the characteristic reflection intensity of the HNTs at 12.03, 18.06, 24.85, 35.01, 38.28, and 62.42 degrees of 2θ decreased, principally due to the dissolution of the alumina in a-HNTs, resulting in a modification of its crystal structure [29].
- (a) XRD pattern before and after purification of HNTs; (b) FTIR spectrum of HNTs graft modification; (c) results of specific surface area testing of HNTs, a-HNTs, and g-HNTs; (d) FTIR spectrum of CPCMs.
The FTIR spectra of HNTs, TDI, PEG, and g-HNTs have been shown in Figure 4(b). The absorption peaks with wave numbers of 3400 cm-1 and 3550 cm-1 corresponded to adsorbed and crystal water in HNTs, respectively. At the same time, the characteristic peaks at 1100 cm-1, 1031 cm-1, and 985 cm-1 could be attributed to the stretching vibration of Si-O-S [30]. Compared with HNTs, peaks at 1660 cm-1 and 1540 cm-1 in the FTIR spectrum of the g-HNTs corresponded to the double bonds -C=O and -N-H of -NHCOO-, validating the formation of carbamate groups [31, 32]. In addition, a new absorption peak at 2277 cm-1 from-N=C=O functional group and absorption peaks at 1600 cm-1 and 1400 cm-1 related to aromatic hydrocarbons also indicated that TDI was successfully grafted onto the surface of HNTs. In the spectrum of the g-HNTs, besides the aforementioned peaks, the additional peaks at 2800-2900 cm-1 corresponding to -CH2 stretching vibrations and the disappearance of the absorption peak of isocyanate (2277 cm-1) collectively suggested that PEG was successfully grafted onto the HNTs via TDI coupling.
3.2.2. Structural analysis of g-HNTs
The N2 isothermal adsorption/desorption curve of HNTs has been shown in Figure 4(c). In the adsorption isotherms of HNTs and g-HNTs, the curve was convex in the low P/P0 area, with capillary condensation of the adsorbate in the high P/P0 area, leading to a rise in the curve. When P/P0 was close to 1.0, adsorption occurred on macropores, resulting in a rapid increase in the curve and indicating the presence of micropores and mesopores in both samples. However, in the adsorption isotherm of g-HNTs, no change was noted in the low P/P0 region, with the curve rising rapidly when P/P0 was close to 1.0, indicating that the g-HNTs lacked micropores and only contained mesopores.
The specific changes in pore diameter have been summarized in Table 1. The inner diameter of g-HNTs was increased due to the dissolution of the alumina layer in the lumen. Compared with the original HNTs, the specific surface area of the modified HNTs was reduced by 66.62%, given that the lumen of the g-HNTs was closed by grafting materials. However, due to the accumulation of HNT, a large gap was formed between them.
| Samples | Specific surface area (m2·g-1) | Average hole radius (nm) |
|---|---|---|
| HNTs | 16.66 | 9.60 |
| a-HNTs | 31.83 | 13.99 |
| g-HNTs | 5.56 | 18.10 |
The chemical compositions of G-CPCM, along with comparative g-HNTs and n-octadecane, were characterized using FTIR spectroscopy and displayed in Figure 4(d). G-CPCMs possess spectral features very similar to those of g-HNTs and n-octadecane, showcasing a series of characteristic absorption peaks. The absorption bands for alkyl C-H stretching vibrations at 2925 cm-1, 2855 cm-1, alkyl C-H bending vibrations at 1469 cm-1 are clearly observed in the infrared spectra of G-CPCMs and n-octadecane, thereby indicating the presence of n-octadecane in G-CPCM [33].
3.2.3. Determination of the best impregnation ratio
The DSC test results for each sample have been summarized in Table 2, where H-CPCM represents CPCMs prepared based on HNTs, P-CPCM denotes CPCMs prepared based on a-HNTs, G-CPCM is CPCMs prepared based on g-HNTs, and the post percentage represents the impregnation ratio of octadecane in CPCMs. As anticipated, the latent heat energy of the three types of CPCMs increased with an increase in the octadecane impregnation ratio. At an impregnation ratio of 10%, the content of n-octadecane is minimal, allowing the types kinds of HNTs to completely adsorb n-octadecane. Consequently, the latent heat energy of the three types of materials was comparable. With an increase in the proportion of octadecane, the difference in latent heat energy gap steadily increased. Compared with H-CPCM 40%, the larger inner diameter of a-HNTs allowed the absorption of a greater amount of octadecane; thus, the latent heat energy of P-CPCM 40% was increased by 17.62%. While the modified HNTs encapsulated the HNTs lumen, the larger voids generated by HNT accumulation could still adsorb octadecane; therefore, the latent energy of 40% G-CPCM was increased by 18.19%, comparable to that of 40% P-CPCM, signaling that the two types of HNTs had similar load capacity. Noteworthily, the melting points of the three types of CPCMs were higher than those of octadecane after the impregnation ratio of octadecane reached 30%. This could be attributed to HNTs being impregnated with an excessive amount of octadecane, and the pores restricted the free movement of octadecane molecules, thereby enhancing the interaction between octadecane fluid and pore surface and resulting in a 35% increase in phase transition temperature [34, 16].
| Samples | Heating up | Cooling | ||||
|---|---|---|---|---|---|---|
|
Onset (°C) |
ΔTm (°C) |
ΔHm (J/g) |
Onset (°C) |
ΔTm (°C) |
ΔHm (J/g) |
|
| n-octadecane | 12.72 | 28.23 | 250.43 | 24.20 | 20.74 | -237.97 |
| H-CPCM 10% | 14.37 | 28.36 | 13.87 | 24.59 | 20.21 | -11.62 |
| H-CPCM 20% | 11.89 | 26.37 | 28.29 | 24.28 | 20.17 | -25.58 |
| H-CPCM 30% | 11.46 | 29.06 | 39.34 | 24.31 | 20.07 | -39.30 |
| H-CPCM 40% | 9.42 | 33.91 | 54.76 | 24.56 | 17.67 | -55.14 |
| A-CPCM 10% | 15.23 | 27.06 | 14.45 | 24.75 | 19.86 | -11.20 |
| A-CPCM 20% | 11.25 | 27.75 | 37.65 | 24.15 | 19.48 | -35.13 |
| A-CPCM 30% | 9.46 | 31.68 | 43.75 | 24.24 | 19.37 | -42.12 |
| A-CPCM 40% | 10.76 | 30.58 | 64.41 | 24.31 | 18.14 | -61.08 |
| G-CPCM 10% | 12.38 | 27.37 | 14.74 | 24.37 | 19.44 | -11.35 |
| G-CPCM 20% | 9.51 | 28.09 | 39.01 | 24.01 | 21.78 | -35.75 |
| G-CPCM 30% | 8.24 | 28.51 | 50.15 | 24.37 | 20.97 | -50.17 |
| G-CPCM 40% | 7.73 | 30.97 | 64.72 | 24.49 | 20.47 | -62.84 |
At impregnation ratios of octadecane in the three CPCMs below 30%, no phase leakage was noted after heating at 50°C. In contrast, at an impregnation ratio of 30%, phase leakage was detected in some samples, as shown in Figure 5(a). Both H-CPCM 30% and P-CPCM 30% displayed phase leakage after the test, but with phase leakage of h-CPCM 30% being more pronounced. At an octadecane impregnation ratio of 40%, severe phase leakage was noted in both H-CPCM 40% and A-CPCM 40%. This is due to the large length and diameter of the HNTs, where its diameter is only a few tens of nanometers, which posed challenges in evacuating air from the lumen in a vacuum environment, resulting in air being sealed in the lumen by octadecane. Heating CPCM expanded air in the cavity, and the octadecane transitioned from solid to liquid. Therefore, the expanded air was forced into the cavity, thereby causing leakage. The pore structure of the grafted HNTs was significantly different from that of the original HNTs, with larger and shallower pores. As a result, minimal air remained in the pores during preparation, which significantly enhanced thermal stability. Interestingly, phase leakage was not observed in 30% G-CPCM, whilst mild phase leakage was detected in 40% G-CPCM. Overall, 40% G-CPCM prepared using G-HNTs exhibited the best performance. Thus, this material was selected for further experiments.
- (a) Phase leakage test for different composite phase change materials; (b) Thermal conductivity test of n-octadecane, HNTs, and their CPCMs.
Optimal temperature-regulating materials should not only exhibit excellent heat storage performance but also possess adequate heat insulation properties. The thermal conductivity of the tested materials has been presented in Figure 5(b). The thermal conductivity of octadecane and the HNTs was 0.2148 W/mK and 0.1398 W/mK, respectively. After HNTs modification via grafting, the thermal conductivity of the HNTs was further reduced to 0.0976 W/mK, which was 30.19% lower than that of the HNTs, given that the trapped air in the lumen restricted its movement. Importantly, increasing the octadecane impregnation ratio increased the thermal conductivity of the CPCM. The thermal conductivity of 40% G-CPCM was 0.1518 W/mK, which was only 8.58% higher than that of the HNTs and 29.33% lower than that of octadecane, reflecting its excellent thermal insulation performance.
3.3. Air permeability and unidirectional moisture conductivity of the fabrics
As shown in Figure 6(a-c), the pore size distribution of cotton fabric is mainly concentrated in the range of 5-10 μm, accounting for 41.61% of the distribution. The distribution of pore size was the lowest in the range of 30-40 μm, with the maximum pore size being 46 μm. PU was coated on the surface of cotton fabric through wet coating. Compared with pure cotton fabric, the proportion of pores smaller than 5 μm increased from 11.65% to 23.88%, the pore size distributed in the 5-10 μm range decreased to 27.01%, and the maximum pore size decreased to 41 μm. This can be ascribed to the low viscosity of the PU solution during the wet coating process, resulting in PU coating on the fiber surface partially covering the pores and the DMF dissolved in water forming smaller pores, resulting in an overall decrease in the pore size of cotton fabric. Following the introduction of G-CPCM during the wet coating process, the viscosity of the coating solution increased. The coating solution was evenly applied to the fabric surface, DMF was removed via dissolution in water, and PU was cured at high temperature to form a film with small holes that covered the fabric, which substantially reduced the pore diameter of the coated fabric. The pore diameter distributed in the 2-4 μm range accounted for up to 24.45% of the distribution, and the maximum pore diameter was 23 μm.
- Pore size distribution of the fabrics: (a) cotton; (b) cotton + PU; (c) cotton+G-CPCM and (d) air permeability of the fabrics.
Owing to the presence of a large number of pores in the coating of the photothermal conversion temperature-regulating fabric, an air permeability test was performed. The results have been illustrated in Figure 6(d), wherein C+PU represents PU wet-coated fabric and C+G-CPCM denotes the photothermal conversion heat-regulating fabric. The air permeability of cotton fabric was 48.87%, which decreased to 32.22% after being covered with PU coating. However, the pore size of the photothermal conversion temperature control fabric coating was the lowest, with an air permeability of merely 5.97%. However, compared with other direct coating processes without air permeability, the fabric demonstrated significant improvement and achieved air permeability effects.
The uncoated cotton fabric in the photothermal conversion fabric was not impacted by the coating and retained excellent hygroscopicity. As shown in Figure 7, 5 μm water droplets were placed on the fabric surface and were completely absorbed within 9 s. However, after applying a wet coating on the coating surface, numerous pore structures were formed on the coating surface, resulting in a rough texture. The structure exerted hydrophobic effects, with the hydrophobic angle reaching 121.22 °.
- Unidirectional conductions of the coated fabrics.
Cotton fabric, C+PU, and C+G-CPCM were exposed to sunlight for 360 s, and temperature changes for each sample have been depicted in Figure 8. Importantly, the temperature of the three samples rapidly increased during the initial stage of illumination, with the temperature of cotton fabric and C+PU increasing to about 30°C after 30 s of illumination. Temperature changes during the subsequent test were comparable. Nevertheless, the temperature of C+PU was marginally higher than that of cotton fabric. After 360 s of illumination, the temperature of cotton fabric was 32°C. Of note, PU coating did not promote photothermal conversion but resulted in a slightly higher temperature compared to the cotton fabric. This result can be attributed to the PU coating covering cotton fibers and creating several small holes, thereby decreasing the air permeability of cotton fabric decrease and compromising the internal air circulation in cotton fabric, eventually leading to a temperature rise. Meanwhile, the temperature of C+G-CPCM increased to 40.4°C after 30 s of illumination, followed by a slower heating rate, eventually reaching 48.1°C after 360 s of illumination, which was 15.2°C higher than that of cotton fabric. This can be attributed to the darker brown color of HNTs promoting the absorption of sunlight, thereby improving their photothermal conversion performance.
- Photothermal effects on cotton fabric, PU-coated fabric, and G-CPCM coated fabric: (a) sample temperature; (b) infrared thermography of samples (outdoor temperature 36°C).
Temperature-regulating fabric should possess not only an active temperature-regulating function but also a passive heat-insulating function to achieve superior temperature regulation. The sample was laid on a constant temperature heating table at 47°C. The test results have been shown in Figure 9. The temperature of the upper surface of the cotton fabric rose to 43.4°C in 10 s. Then, the heating rate decreased, and the temperature stabilized at 46°C at 80 s. At the same time, the upper surface temperature of C+G-CPCM was 34.2°C at 10 s. After 20 s, the heating rate progressively decreased, and the temperature stabilized at 44.5°C after 125 s, which was 1.5°C lower than the highest temperature on the upper surface of cotton fabric. Compared with cotton fabric, the heating time of C+G-CPCM was 45 s longer. This is due to the coated surface of C+G-CPCM containing G-CPCM with low thermal conductivity. Considering that G-CPCM can store energy, the heat at the bottom of C+G-CPCM was steadily transferred to the upper surface. During this process, the slower rate of thermal transfer resulted in increased heat loss, leading to the surface temperature of C+G-CPCM being lower than that of cotton fabric.
- Thermal insulation tests on cotton fabric and thermo-regulating fabric.
As shown in Figure 10(a), the heating rate gradually decreased after 80 s of irradiation in a high-intensity light and heat environment provided by an iodine tungsten lamp. At that point, the temperature of the cotton fabric was 37.4°C, with the final temperature stabilizing at 43.2°C. During the heating process of C+G-CPCM, the temperature rapidly rose during the initial 20 s, and the final temperature reached 35.7°C, following which the heating rate decreased. Throughout the period of 20-100 s, the heating rate of PCMs remained relatively unaltered, following which the heating rate progressively decreased until a final temperature of 52.0°C was reached, which was 19.3% higher than that of cotton fabric C+G -CPCM.
- Photothermal conversion test with tungsten iodine lamp: (a) cotton fabric and the thermo-regulating fabric; (b) thermal cycling stability of the thermo-regulating fabric.
After the heating phase, the temperature of the cotton fabric dropped to room temperature (29.8°C) after 730 s, indicating suboptimal heat storage capacity. C+G -CPCM has a heat storage function owing to the presence of octadecane, and its temperature drop was significantly lower during the cooling process due to the release of heat stored in octadecane delaying the temperature drop of the fabric. At 730 s, the temperature of C+G -CPCM was 32.1°C, which was 7.7% higher than that of cotton fabric. At 1000 s, the temperature dropped to room temperature. In addition, comparing the temperature curve of cotton fabric with that of c+g -CPCM unveiled that after 590 s, the temperature decline for C+G -CPCM significantly slowed. Therefore, C+G -CPCM can prolong the temperature reduction time by 410 s compared with that of cotton fabric, highlighting its favorable heat storage performance.
The mass of G-CPCM in the photothermal conversion temperature-regulating fabric coating was 0.5 g, with a latent heat energy of 64.72 J/g, an iodine tungsten lamp power of 500 w, and a coating area of 0.0025 m2. The energy storage duration of the PCM was 80 s. Substituting these data into formula (2), the photothermal conversion efficiency of C+G-CPCM was determined to be 32.4%.
The thermal performance of C+G-CPCM was assessed four times. As depicted in Figure 10(b), the temperature rise and fall curves were consistent throughout the four tests. However, after each thermal cycle, the initial and final temperatures of the sample gradually rose owing to the strong light and heat emitted by the iodine tungsten lamp increasing the ambient temperature time. Nonetheless, this increase in ambient temperature did not affect the heating trend of C+G -CPCM, implying that the sample had excellent thermal stability.
4. Conclusions
In this study, a CPCM with photothermal conversion ability was prepared by adsorbing octadecane onto g-HNTs. The CPCM was subsequently applied to the surface of cotton fabric by a wet coating method to yield a temperature-regulating fabric with photothermal conversion functions. Compared with HNTs, the pore radius and surface area of g-HNTs were significantly higher, making them an effective carrier for octadecane. The CPCM prepared by the impregnation method effectively mitigated leakage associated with a single PCM, achieving maximum latent heat energy of 64.72 J·g-1. Notably, g-CPCM 40% demonstrated superior stability, thermal conductivity of 0.1518 W/mK, and favorable thermal storage and thermal insulation properties. The photo-thermal conversion temperature-regulating fabric prepared by mixing the CPCM with PU via wet coating displayed a high photo-thermal conversion rate of 32.4%, as well as a smaller pore size and improved hydrophobicity. Compared with cotton fabric, the fabric not only delayed temperature rise but also enhanced thermal stability. Although the air permeability of the photothermal conversion phase change temperature-regulating fabric prepared in this experiment was higher than that of other wet-coated fabrics, the air permeability was only 5.97%. Therefore, future investigations are warranted to develop photothermal conversion phase change temperature-regulating fabrics with ideal air permeability.
Acknowledgment
This research was funded by the Innovation Capacity Improvement Plan Project of Hebei Province (Grant number: 24451501K).
CRediT authorship contribution statement
Jiatong Guo: Concept, experimental studies and manuscript preparation. Honglin Liu: Data acquisition, Data analysis. Yibo Zhang: Data analysis. Yibo Yin: Literature search. Shang Hao: Experimental studies. Wei Zhang: Funding acquisition, manuscript editing, and manuscript review.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Generative AI and AI-assisted technologies in the writing process
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
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