Designing Textile Architectures for High Energy-Efficiency Human Body Sweat- and Cooling-Management
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Thermal management of textiles requires local microclimate control over heat and wet dissipation to create a comfortable thermal-wet environment at the interface of the human body and clothing. Herein, we design a fabric capable of both sweat- and cooling-management using a knitted fabric featuring a bilayer structure consisting of hydrophobic polyethylene terephthalate and hydrophilic cellulose fibers to simultaneously achieve high infrared (IR) transmittance and good thermal-wet comfort. The IR transmission of this cooling textile increased by ~ twofold in the dry state and ~ eightfold in the wet state compared to conventional cotton fabric. When the porosity changes from 10 to 47% with the comparison of conventional cotton fabric and our cooling textile, the heat flux is increased from 74.4 to 152.3 W/cm2. The cooling effect of the cooling fabric is 105% greater than that of commercial cotton fabric, which displays a better thermal management capacity for personal cooling. This bilayer design controls fast moisture transfer from inside out and provides thermal management, demonstrating high impact not only for garments, but also for other systems requiring heat regulation, such as buildings, which could mitigate energy demand and ultimately contribute to the relief of global energy and climate issues.
KeywordsThermal responsive textile IR transmission Cooling textiles Moisture transfer and management
Personal cooling have attracted great attentions for their ability to provide thermal comfort by locally controlling the temperature of human beings in a low-cost and energy-saving manner [1, 2, 3]. The combination of personal cooling with textiles is one of the most promising strategies for incorporating personal cooling into daily life [4, 5, 6]. There are several commercially available textiles that can provide different levels of personal thermal regulation. Moisture management textiles, as the most common thermal regulation technology in the industry, cool the human body by removing excess moisture [7, 8] However, the thermal regulation mechanism in such textiles can only be triggered when the microclimate between the body and fabric is at a high humidity level, which limits its practical applications where the humidity level is low. Other technologies based on phase-changing materials, and air- and liquid-cooling techniques have their limitations, such as the bulky size of cold packs, massive consumption of power, and high cost [9, 10, 11].
Recently, many efforts to develop thermal-regulating textiles have been undertaken and reported [12, 13, 14, 15, 16, 17, 18] For personal cooling, a nanoporous polyethylene membrane was developed to enable mid-infrared transparency for efficient human body cooling. For building cooling, a glass-polymer hybrid composite has been demonstrated to exhibit excellent daytime radiative cooling effects . Despite most of the recent technologies and concepts are promising for reducing local temperatures, however, it is far from perfect when real considerations are placed in human beings in a dynamic and complex condition, such as sweating. When sweat is generated in the microclimate close to textiles, a new state of textiles in terms of infrared (IR) emission and moisture will be rebalanced and so far, limited success has been achieved with fabrics that use direct textile structure design to enable dynamic control of human IR and sweat from the body to the ambient environment [19, 20, 21].
In this work, we design a fabric demonstrating effective sweat- and cooling-management of the human body. This cooling fabric is designed with moisture-responsive yarns outside and polyethylene terephthalate (PET) yarns inside to dynamically modulate IR transmission, moisture evaporation, and air flow through active loop-formed pore size change as a function of moisture. The outside moisture-responsive yarns can absorb the sweat transmitted by the inside PET yarns and release the sweat from skin to keep the surface dry, enabling good thermal-wet comfort. The outside moisture-responsive yarns after absorbing sweat change in structure from loose to dense, and the enlarged loop structure can allow more heat and moisture to be released from the body while simultaneously allowing air to flow inside. Our fabric exhibits enhanced IR transmission in dry (2-times higher) and even wet conditions (8-times higher) when compared to conventional cotton fabric, as demonstrated by an IR camera. In comparison with conventional cotton fabric and designed cooling textile, the heat flux is increased from 74.4 to 152.3 W/cm2 with the increased porous area from 10 to 47%. As a result, the cooling effect of the cooling fabric is 105% greater than commercial cotton, displaying better thermal management capacity for personal cooling.
Results and Discussion
The vertical wicking behavior (AATCC 197) indicates a textile’s ability to transport sweat and dry quickly. In this instance we measure the vertical wicking behavior of the fabrics in the warp direction using a lab wicking tester. After 30 min, the wicking distances were 12.6 cm for the commercial cotton control and 10.0 cm for the cooling textile, demonstrating comparable moisture transport in the two materials (Fig. 3b), which were measured according to the standard GB/T 21688.1-2008.
Similarly, the overall moisture management capacity (OMMC) is an index to indicate the overall capability of the fabric to manage the transport of liquid moisture, in which a higher value indicates faster moisture evaporation, which is significant when considering the ability of a textile pull sweat through the fabric and keep skin dry (Moisture Management Tester, SDL ATLAS M290, AATCC 195). Figure 3c shows that the OMMC of the cooling textile (0.492) is 15% higher than the commercial cotton control (0.429), which implies the cooling textile has a better one-way transport of sweat.
Finally, the wetting time refers to the time period in which the top and bottom surfaces of the fabric just start to get wetted respectively after the test commences, with a short wetting time indicating faster sweat absorption. The experimental was carried out according to AATCC Liquid Moisture Management Properties of Textile Fabrics. The tester was MMT SDL ATLAS. The bottom layer of the fabric is defined as the side closer to skin, and the top layer is the one away from skin. The short wetting time on the bottom side of cooling textile indicates fast water absorption. Our results showed that the cooling textile has a shorter wetting time (3% on top and 18% on bottom compared with the control), which means it will wet faster and therefore be better at keeping skin dry (Fig. 3d). The fast wetting time in cooling textile is due to the highly porous structure and bilayer structure that enables fast water transport.
An IR camera was used to show the enhanced IR transmission of the knitted cooling fabric in dry and wet conditions, placed on the palm of a hand (Fig. 4d). In the dry region, we can clearly see the bright color transmitted out of the textile fabric though the pores of the fabric, indicating a higher temperature, which is thermal radiation from the human body. In contrast, for the wet sample, due to the expanded pore size, bright spots can be detected largely through the fabric. The darker color indicates the part absorbed by water, and the brighter part indicate the open-pores part of the fabric. As demonstrated in Fig. 4c, it is easy to see through the open-pore structure of the wet fabric. Note that due to the existence of water in the fabric, since water absorbs IR as well, the wet fabric exhibited a less bright color. When we average the collected temperature on human body, we can see a 1.8 °C average temperature higher for cooling textile than that of conventional cotton fabric (Figure S1). The cooling textile maintained a similar temperature due to the open pores of layer, allowing more skin surface as well as IR to expose to the air. In comparison, conventional cotton fabric has a lower temperature in we state, indicating that human body heat was absorbed by the wet cotton textile. The result indicates that cooling textile has a better human body IR transmission capability and could provide a better thermal-wet comfort.
As shown in the IR image, only dark color is detected when the wet cotton fabric is placed on the human body, indicating that there is no additional thermal radiation being exposed directly to the outer air environment. We compared the IR transmittance of cooling textile and cotton textile in wet and dry state (Fig. 4e). Note the IR transmittance test of fabric was performed in a fixed area. The volume shrinkage of knitting fabric in wet is not considered in this case. Cooling textile and cotton textile exhibited different trend of IR transmittance. For the commercial cotton T-shirt fabric, the IR transmittance decreases from dry to wet. For the knitted textile, the IR transmittance increases from dry to wet, due to the increased open space between the cellulose yarns. Transmission of the cooling textile was ~ 2-times higher in the dry state and ~ 8-times higher in wet state compared to conventional cotton fabric. Note that in real case, knitting fabric inevitably suffers volume shrinkage in wet state, therefore, our cooling textile design will not be suitable for whole garment use, and instead, it is good to implement other fabrics to enable the cooling effect.
Since there are innumerable pores on a piece of textile, it is impossible for us to consider all the pores. But due to the periodicity of the structure, we only need to consider the geometry one pore (Fig. 5b). The lower and upper transparent cubic structures have an edge length set to 1 mm to mimic the real fiber diameter. The lower cube represents the inner air, while the upper transparent part represents the fiber. The green box represents the pore, which is full of air. The emissivity of the skin and fiber was set to 0.95 and 0.5, respectively. The temperature of the skin and environment was set to be 34 °C and 22 °C, respectively. The thermal conductivity of the fiber and air was set to 0.047 W/m/K and 0.027 W/m/K, respectively.
Figure 5c demonstrates the temperature contour for 42% fabric porosity, in which the temperature of the outer surface is about 304 K. Meanwhile, the temperature gradually decreases from the skin to the outer surface. Since thermal radiation heat transfer is between surfaces, the radiation flux from the skin to the fiber will not be very large since the temperature difference is small. However, since the thickness of the fiber and inner air layer between the skin and fiber is so small, the temperature gradient is large. As a result, the heat flux caused by heat conduction is relatively significant. It should be noted that the heat conducted to the outer surface is then removed by natural convection and radiation.
Moisture transfer calculation based on different porosity of cooling textiles
In summary, we have designed a bilayer structure consisting of hydrophobic PET fibers and hydrophilic cellulose fibers to achieve high IR transmittance and good thermal-wet comfort, which provides an effective cooling response to increases in temperature and the resulting sweat that incurs on the human body. In our design, PET fibers have direct contact with human skin and the porous structure provides good water absorption and touch-skin feeling. Meanwhile, the cellulose fibers in the double-layer knit material are located far from the skin, and because of their excellent hydrophilicity, perspiration on the skin can be transported from the PET to the cellulose components of the fabric. In response to this moisture, the cellulose yarn changes diameter due to its bimorph fiber structure, leading to a more open pore structure that allows greater IR transmission and moisture vapor transport. The IR transmission of this cooling textile was ~ 2-times higher in dry state and ~ 8-times higher in the wet state compared to that of conventional cotton fabric. With the comparison of conventional cotton fabric, the cooling textile has an increased heat flux from 74.4 to 152.3 W/cm2 with the increased porous area from 10 to 47%. The bilayer design largely improves the IR transmittance of the fabric in both dry and wet conditions, resulting in a cooling effect that is 105% greater than commercial cotton fabric. This bilayer design that regulate moisture transfer from inside out and thermal management provides a great impact, not only for garment, but also for those with good thermal management requirement, such as buildings, which can greatly mitigate the energy demand, and ultimately contribute to the relief of global energy and climate issues.
Triacetate-diacetate fibers were purchased from Mitsubishi Rayon Textile Co. LTD. PET fibers were provided from Unifi, Inc. The triacetate-diacetate fibers were knitted into fabrics of the desired patterns with PET fibers as the frame to support the actuation of triacetate-cellulose fibers in the knitting lab in the College of Textiles at North Carolina State University. After the textile fabrication, triacetate-diacetate fibers in the fabric went through the same treatment discussed in our previous manuscript [References: science, 2019, dynamic gating of infrared radiation in a textile]. Briefly, and then chemically converted to the triacetate-cellulose bimorph fibers through saponification reaction in an aqueous solution of 0.125 M sodium hydroxide at 60 °C for 0.5 h. The fabric morphology was studied via electron microscopy using a Hitachi SU-70 field emission SEM microscope and a JEOL JEM 2100 TEM. Air permeability test of fabrics is followed by ASTM D737. Details of procedures are referred to ASTM D737. Wicking and moisture management of fabrics were tested according to AATCC 197 and 195, respectively. The wetting time was measured using MMT SDL ATLAS. Transmittance FTIR measurements were performed over a 7–22 μm wavelength range with a Bruker Vertex 70 FTIR spectrometer coupled with a Hyperion 1000 IR microscope, using a liquid N2-cooled mercury cadmium telluride detector. A Schwarzschild reflective objective (15×, NA 0.4) was used to focus the incident light and collect the reflected light.
This project was made possible by financial support from the Delivering Efficient Local Thermal Amenities (DELTA) Program of the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy.
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