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Advanced Fiber Materials

, Volume 1, Issue 1, pp 61–70 | Cite as

Designing Textile Architectures for High Energy-Efficiency Human Body Sweat- and Cooling-Management

  • Kun Fu
  • Zhi Yang
  • Yong Pei
  • Yongxin Wang
  • Beibei Xu
  • YuHuang Wang
  • Bao Yang
  • Liangbing HuEmail author
Letter
  • 978 Downloads

Abstract

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.

Graphic Abstract

Keywords

Thermal responsive textile IR transmission Cooling textiles Moisture transfer and management 

Introduction

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 [18]. 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 schematic of the cooling textile design is shown in Fig. 1. The knitted fabric is designed with moisture-responsive yarns outside and PET yarns inside to dynamically modulate IR transmission, moisture evaporation, and airflow. The outside moisture-responsive yarns can absorb the sweat transmitted by the inside PET yarns, which allows the skin to stay more comfortably dry. After absorbing the sweat, the outside moisture-responsive yarns change their structure from loose to dense, and the enlarged loop structure allows more heat and moisture to be released while allowing more air to flow through the fabric to further cool the skin. The moisture-responsive fiber has a bimorph structure consisting of two components made of hydrophilic cellulose and hydrophobic triacetate. Moisture can be absorbed by the cellulose component, which causes swelling and expansion, while the triacetate component remains the same size. The unbalanced swelling behavior causes the fiber to deform, and the resulting deformation causes the yarn to change diameter from a loosely packed structure to a densely packed one, thus enlarging the loop size for better ventilation. The mechanism replies on the moisture-induced bi-component deformation, causing fiber to bend and transform from loose state to a densely packed state. In the knitting loop, when the loop legs become denser, the loop area will correspondingly become bigger [22].
Fig. 1

Cooling textiles for sweat- and cooling-management. a The cooling textile design based on a double layer design enables good human body IR transmittance, moisture transport, and air ventilation when people are having mild sweating. b Schematic of the reversible loop structure change during wetting and drying of the moisture-responsive yarns. Double layer fabric structure consisting of two knitted layers, in which the outer is moisture-responsive to release sweat from human skin, and the inner layer is hydrophobic maintain dry contact between skin and the outer layer of fabric

Figure 2a, b show the fabric dyed with rhodamine red to highlight the cellulose components of the yarn in the fabric, leaving the triacetate undyed. The remaining white region is the PET. The front and back sides of the fabric clearly demonstrate the distribution of the yarns in the fabric pattern, which was made using a knitting loom. Scanning electron microscopy imaging (SEM) demonstrates the morphology of the bimorph cellulosic fibers (Fig. 2c, d). The bimorph fiber has a side-by-side structure consisting of tri-acetate and bi-acetate components. The rhodamine red-dyed part is bi-acetate. Due to the varying hydrophilicity of the two fiber components, the different parts absorb water to different degrees, and it is this difference in swelling that causes the fiber to bend. In fiber bundles, the active bending of the fiber triggers the yarns to have diameter change by densifying or loosing fiber-to-fiber interspace (Fig. 2e). The PET yarns were designed to display a loosely packed structure by using curved fibers, and this porous structure allows better water absorption and skin-touch comfort. In contrast, the cellulose yarns have a dense structure due to the use of round, smooth fibers.
Fig. 2

a, b Photo image of the front and back side of the cooling fabric. Rhodamine red was used to dye the fabric to show the moisture-responsive yarns and the undyed PET. The inset displays a large piece of the undyed cooling textile. c SEM image of the moisture-responsive bimorph fiber, which consists of cellulose and tri-acetate components. d Confocal microscopic image of the bimorph fiber. The red part is dyed by Rhodamine red, indicating the successful transition from di-acetate to cellulose after NaOH treatment. The undyed component of the fiber is tri-acetate. e Birefringence microscopic image of the fabric structure with interlocked PET and bimorph yarns

We tested and compared the air permeability, vertical wicking, and moisture management properties of a commercial control and the cooling textile (Fig. 3). The commercial control was made of 100% cotton weft knitted fabric, obtained from a T-shirt, and featured a similar weight as the cooling textile (difference < 10%, tested by following ASTM D 3776). The air permeability (SDS ATLAS M021A, 38 cm2 Test Head, ASTM D737) provides a measure of the rate of air flow passing perpendicularly through a known area of the fabric and it indicates how open and breathable the fabric structure is, with higher values indicating a more open structure leading to greater air convection and therefore a more cooling sensation for the garment wearer. Figure 3a shows the air permeability of the cooling textile (587.0 cfm) was 580% higher than the commercial control fabric (86.2 cfm). This indicates the cooling textile was much more breathable than the control, and thus should provide better cooling performance.
Fig. 3

Characterization of the cooling fabric in comparison with commercial cotton fabric in terms of air permeability and moisture transport behavior: a air permeability; b wicking test; c OMMC index; and d average wetting time

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.

To show the wicking behavior and the dynamic structure change in wet condition, the sample was dampened with water by attaching the fabric onto a wet Kimwipe™ for 5 min, which was used to simulate human body perspiration. The images in Fig. 4 show the diameter of the cellulose yarns decreased from ~ 400 to 330 μm, which is mainly triggered by the water absorption. The free space within the cellulose yarn becomes filled with water, and the capillary force and hydrogen bonding make the individual fibers more compact closer to each other within the yarn, leading to a smaller yarn diameter. We hypothesized that the diameter change of the cellulose yarns can lead to enlarged empty space between cellulose yarns, allowing more IR and vapor transport, indicating the dynamic volume change of yarn diameter in a response to moisture.
Fig. 4

Wet characterization of the cooling textile. a Dry state of the cooling textile fabric. b Wet state of the cooling textile fabric. c Comparison of the dry and wet parts of the cooling textile. d IR image of the dry and wet regions of the cooling textile on the palm of a hand. e IR transmittance spectrum of the cooling textile and conventional cotton fabric

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.

In order to quantitatively evaluate the effects of porosity on the heat transfer process between the skin and environment, we constructed a simulation model using ANSYS Fluent software. The porosity was calculated by the portion of projection area of the face plain loop over unit loop overall area. Figure 5a shows the schematic for heat transfer, which involves several heat transfer mechanisms. Wet skin has a higher temperature than the fiber and environment, so there is thermal radiation. There is also thermal conduction through the inner air and the fiber, and natural convection caused by gravity and temperature difference. Moreover, the water evaporation process also contributes to heat transfer. All these heat transfer mechanisms are coupled. For real clothes on a human body, natural convection is complex as a result of the complex geometry. Here we simplified the human body surface as a vertical flat plate since the curvature of most area on this surface is very large. To simplify the problem further, we assumed that the inner air is stationary. The reason is that the characteristic length and temperature difference for the inner air region is too small to generate natural convective heat transfer, which is comparable to heat conduction in the inner air [1]. Based on these assumptions, we only considered heat conduction in the inner air. For natural convection on the outer surface, we give a convective heat transfer coefficient in simulation. The natural convection coefficient is calculated by correlations as below [23]:
$$ Nu = \frac{{h_{c} L}}{k} = \left\{ {0.825 + \frac{{0.387Ra^{1/6} }}{{\left[ {1 + (0.492/Pr)^{9/16} } \right]^{8/27} }}} \right\}^{2} , $$
(1)
in which
$$ Ra = GrPr = \frac{{g\beta (T_{w} - T_{\infty } )L^{3} }}{va}, $$
(2)
in which Nu is the nusselt number, hc is the convective heat transfer coefficient (W/m2/K), L is the characteristic length (for the human body L is about 1.65 m), k is the thermal conductivity (W/m/K), Ra is the Rayleigh number, Pr is the Prandtl number, Gr is the Grashof number, g is acceleration of gravity, β is the thermal expansion coefficient, (1/K), Tw is the temperature of the wall (K), \( T_{\infty } \) is the temperature of the bulk air (K), v is the dynamic viscosity (kg/m/s), and a is the thermal diffusivity (m2/s). In simulation, an iterative process was used to determine hc since it depends on Tw, which can only be determined from simulation.
Fig. 5

Thermal simulation study of the cooling textile fabric. a Schematic of heat transfer in the cooling textile. b Geometry of the ANSYS simulation. c Temperature contour for 42% porosity

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.

For the treatment of heat transfer by evaporation, we only considered water vapor diffusion through the hole since it is much stronger than that through the fiber. For heat flux caused by water vapor evaporation, we use the correlations below [24, 25]:
$$ q^{\prime \prime } = h_{\text{m}} (p_{\text{w2}} - p_{\text{w1}} )\phi /1000, $$
(3)
in which
$$ p_{\text{w}} = {\text{RH}}p_{\text{s}} , $$
(4)
$$ p_{\text{s}} = 1000\exp \left( {16.262 - \frac{3799.89}{T + 226.35}} \right), $$
(5)
in which \( q'' \) is the evaporative heat transfer flux (W/m2), hm is the evaporative heat transfer coefficient (W/m2/kPa), pw2 is the water vapor partial pressure at the skin (Pa), pw1 is the water vapor partial pressure of the environment (Pa), ϕ is the porosity of the textile, pw is the water vapor partial pressure (Pa), and RH is the relative humidity. For skin, RH = 100%, for outer air, RH = 50%. Finally, ps is the saturation pressure of water vapor (Pa), and T is the temperature of water (°C).
hm can be calculated by:
$$ {\text{Lewis ratio}} = \frac{{h_{\text{m}} }}{{h_{\text{c}} }} $$
(6)
in which the Lewis ratio is 16.5 for a typical indoor environment.

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.

Table 1 provides the moisture transfer values of the cooling textiles based on three different porosities of the material. The 10% porosity corresponds to conventional cotton fabric, while 42% and 47% porosity was demonstrated for our cooling textile. The heat conduction is not listed because it is an intermediate process. As can be seen, as the porosity of the textile increases from 10 to 47%, the radiation flux also increases from 28.93 W/m [2] to 36.48 W/m2, which is an enhancement of 26%. This is because more thermal radiation from the skin can go through the pore to the environment. The convection heat flux shows a very small decrease. Meanwhile, the heat flux due to evaporation increases from 19.41 to 90.59 W/m2 (367% increase). This can be explained by Eq. (3) which shows the heat flux caused by evaporation is proportional to the porosity of the textile. The total heat flux increases from 74.41 to 152.29 W/m2, which is an enhancement of 105%. For textile porosities of 42% and 47%, the radiation heat transfer does not significantly increase, but the heat flux caused by evaporation rises from 80.95 to 90.59 W/m2. Based on our simulation results, our cooling textile clearly demonstrates better cooling ability as compared with conventional cotton fabric. The reason can be attributed to its larger porosity, which cause stronger heat flux due to evaporation as well as better radiation heat transfer between the skin and the environment.
Table 1

Moisture transfer calculation based on different porosity of cooling textiles

Porosity (%)

qradiation/W/m2

qconvection/W/m2

qevaporation/W/m2

qtotal/W/m2

hc/W/m2/K

10

28.93

26.07

19.41

74.41

2.95

42

35.46

25.34

80.95

141.75

2.93

47

36.48

25.22

90.59

152.29

2.93

Conclusions

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.

Experimental

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.

Notes

Acknowledgements

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.

Supplementary material

42765_2019_3_MOESM1_ESM.docx (143 kb)
Supplementary material 1 (DOCX 142 kb)

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Copyright information

© Donghua University, Shanghai, China 2019

Authors and Affiliations

  • Kun Fu
    • 1
    • 2
  • Zhi Yang
    • 3
  • Yong Pei
    • 3
  • Yongxin Wang
    • 4
  • Beibei Xu
    • 4
  • YuHuang Wang
    • 4
  • Bao Yang
    • 3
  • Liangbing Hu
    • 1
    Email author
  1. 1.Department of Materials Science and EngineeringUniversity of MarylandCollege ParkUSA
  2. 2.Department of Mechanical EngineeringUniversity of DelawareNewarkUSA
  3. 3.Department of Mechanical EngineeringUniversity of MarylandCollege ParkUSA
  4. 4.Department of ChemistryUniversity of MarylandCollege ParkUSA

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