Advanced Functional Fibrous Materials for Enhanced

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Advanced functional fibrous materials for enhanced thermoregulating performance Esfandiar Pakdel, Maryam Naebe, Lu Sun, and Xungai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19067 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Materials & Interfaces

Advanced functional fibrous materials for enhanced thermoregulating performance Esfandiar Pakdel*, Maryam Naebe, Lu Sun, Xungai Wang* Deakin University, Institute for Frontier Materials, Geelong, VIC, Australia [email protected], [email protected], [email protected], [email protected]

Corresponding authors: 

Prof. Xungai Wang (Email: [email protected] ), Tel: +61 3 5227 2894, Fax: +61 419 525 434



Dr. Esfandiar Pakdel (Email: [email protected]), Tel: +61 3 5247 9383

Postal address: Deakin University, Waurn Ponds Campus, Locked Bag 20000, Geelong, VIC 3220, Australia

Keywords: Textiles, thermal properties, passive cooling and heating, phase change materials (PCMs), functional coatings

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Abstract The concept of thermoregulating textiles capable of providing personal thermal management property (PTM) has attracted significant attention in recent years. It is considered as an emerging approach to promote the comfort and general well-being of wearers and also to mitigate the energy consumption load for indoor living space-conditioning. Regulating the heat exchange between human body and environment has been the core subject of many studies on introducing the PTM functionality to textiles. This paper provides an overview of latest literature, summarizing the recent innovations and state-of-the-art approaches of controlling the heat gain and loss of textiles. To this end, methods to control the fundamental aspects of heat gain and loss of fabrics such as using the near-infrared reflective materials, conductive nanomaterials, designing photonic structures of fabrics, and engineered nanoporous structures for passive cooling and heating effects will be discussed. Moreover, specific attention is given to the application of phase change materials in textiles, their integration methods and the associated mechanisms. Several commercial methods such as adapting the innovative designs, introducing moisture management capability and using air/liquid thermoregulating systems will also be discussed. This review article provides a clear picture on the concept of thermoregulating textiles and recommends some future research trajectories for this emerging field.


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Contents Abstract ......................................................................................................................................1 1.

Introduction.........................................................................................................................4

2.

Novel methods of developing thermoregulating textiles....................................................7 2.1.

Cool coatings with NIR reflective materials ...............................................................7

2.2.

Cooling effect with conductive nanomaterials ..........................................................10

2.3.

Cooling textiles based on photonic structures ...........................................................13

2.4.

Passive cooling and heating textiles ..........................................................................16

2.4.1.

Textiles developed based on nanoporous polymeric films ................................16

2.4.2.

Textiles developed based on porous fibers ........................................................26

2.5.

Phase Change Materials (PCMs)...............................................................................30

2.5.1.

Working principle of PCMs ...............................................................................30

2.5.2.

PCMs incorporation in textiles...........................................................................32

2.5.3.

Coating textiles with PCMs ...............................................................................33

2.5.4.

Fibrous structures containing PCMs ..................................................................38

3.

Some other methods to develop thermoregulating garments............................................41

4.

Safety concerns of using nanomaterials with future outlook............................................44

5.

Conclusion ........................................................................................................................47

6.

Acknowledgement ............................................................................................................47

7.

Conflicts of Interest ..........................................................................................................47



1. Introduction Rapid expansion of the world’s population and energy shortage have motivated scientists to find new measures to reduce the dependency on fossil fuels 1. Increased greenhouse gases and global warming are among primary reasons to reduce the consumption of these types of fuels. Reduction in using heating, ventilation and air conditioning (HVAC) systems can be an efficient remedy to alleviate the increasing pace of energy consumption 2. It is estimated that buildings consume 32-40% of the total global energy and contribute to around 30% of greenhouse emission, highlighting the need for alternative methods for indoor thermal management

3, 4.

In an indoor space, the desirable set-point temperature (21.1-23.9 °C) can

usually be achieved by using HVAC facilities. However, adjusting the temperature of entire indoor space using heaters, fans or air conditioning systems consumes a significant amount of energy 5. This plays an important part in escalation of carbon footprint and generation of greenhouse gases, thereby contributing to global warming. Using clothing with personal thermal management (PTM) property can reduce energy usage through providing advanced thermal comfort for wearers over a wide range of temperatures. Manipulation of interactions between incoming and outgoing heat with textiles can potentially result in novel products with advanced thermoregulating effects. The main idea of PTM-based textiles is providing favorable thermal comfort in the microclimate next to the body skin instead of using costly cooling and heating facilities to adjust the temperature of entire indoor space. Apart from indoor applications, textiles with PTM functionality can also be used to develop new types of textiles suitable for outdoor environment such as garments required for farmers, athletes and firefighters. Main functions of textiles include provision of comfort and protection to the wearer against warm and cold environments, ultraviolet ray (UV), sunlight, insect bites, chemical hazard, and fire

6, 7.

Providing adequate comfort is a crucial factor in achieving expected performance of 4 ACS Paragon Plus Environment

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textiles 8. The concept of comfort in clothing can be classified into: thermophysiological (thermal), sensory, ergonomics and psychological. Among these classified types of comfort, thermal comfort is the most important one where developing textiles with the capability of providing thermal comfort for the wearers has attracted considerable attention

8, 9.

Thermal

comfort is related to the regulation of heat gain and loss by body in the presence of textiles. It is achieved when there is a thermal balance between heat generated by the body and total heat lost from it

7, 10, 11.

The heat transfer and dissipation rate from body through textiles is

determined based on different pathways of convection (skin-to-fiber, fiber-to air), conduction (fiber-to-fiber), radiation (skin-to-fiber, fiber-to-air), and, in wet states, evaporation (Figure 1) 8, 12, 13.

Heat transfer characteristics of textiles depend on factors such as: i) fabric properties

(surface morphology, yarns structure, porosity, and finishing treatments), ii) garment design (size, ventilation, number of layers, and weight), iii) body posture and movement, and iv) environmental conditions (air velocity, relative humidity, and temperature) 11, 12, 14.

Figure 1: A schematic illustration of heat transfer mechanisms from skin to environment through textiles Textiles play a crucial role in providing thermal comfort in the absence of any heaters or air conditioning systems and also maintaining the body core temperature 5, 11. Therefore, products with PTM functionality are important due to their potential to provide localized thermal 5 ACS Paragon Plus Environment


comfort for the wearers. Two types of passive and active thermoregulating textiles have been developed based on different mechanisms: i) passive thermoregulating products based on engineered fibrous materials and novel photonic structures, and ii) active thermoregulating textiles capable of active (sense-react-adapt) thermoregulation 7. The former group can continuously warm and cool depending on their nature and environment conditions, and their efficiency relies on suppressing or facilitating the body heat dissipation from the fabrics surface 7, 15.

This type of clothes can have dual applications in indoor and outdoor environments

depending on their structure and the finishing process. Moreover, some other techniques such as incorporating phase change materials (PCMs) either in the surface coating formulations or within the structure of fibers can introduce active thermoregulating property to textiles. Some techniques such as attachment of air/liquid cooling systems have also been used but they have rarely been compatible with fast-paced daily lifestyle 16. This paper reviews different methods of developing thermoregulating functional materials, with a focus on textiles coated with near-infrared (NIR) reflective pigments, highly thermal conductive fibrous materials, engineered photonic structures, textiles with passive radiative cooling and heating effects, textiles containing phase change materials (PCM), and phase change fibers (PCFs). Also, attention will be paid to some other approaches of developing thermoregulating textiles such as specific cooling garment designs, moisture management systems on fabrics and air/liquid cooling attachments. These methods have been used to introduce cooling fabrics and garments for different applications. This paper encompasses different aspects and fundamental principles of developing novel thermoregulating textiles, delineating the latest scientific accomplishments of the field.


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2. Novel methods of developing thermoregulating textiles 2.1.

Cool coatings with NIR reflective materials

Solar radiation which reaches the earth surface consists of a wide range of wavelengths including ultraviolet (100-400 nm) (5%), visible light (400-700 nm) (46%) and infrared (> 700 nm) (49%)17, 18. Some part of the infrared radiation mostly in the region of 700-1100 nm is classified as NIR radiation and is the main portion of solar energy which heats the earth surface. It has been demonstrated that developing cool coatings with high NIR reflectance on different surfaces can minimize the heat gain from sunlight

19.

There are some biological species in

nature which use unique fibrous structures as a protective layer against extreme heat from sunlight and they have been the inspiration for researchers to develop cool coatings. An example is the reflective white fibers covering the back of poplar leaves that reflect up to 55% of the solar spectrum, protecting the plant against wilting. In another example, the Saharan silver ant covers its body with tiny silver hairs which can reflect up to 67% of NIR radiation of sunlight. Also, the diameter and triangle cross-section of hairs play crucial roles in enhancing the emissivity and heat dissipation from ant body which is in the mid-infrared (MIR) region (7-14 μm) 20. Inspired by these structures, cool coatings have been developed on the roofs and outer surface of widow glasses of buildings in hot urban areas to reduce the heat gain from the incident solar irradiation

21, 22.

The cooling effects achieved through this method can

significantly reduce the heat build-up in the living spaces thereby reducing the energy consumption load of air conditioning systems 23. This concept can also be applied to textiles to minimize the heat gain from ambience and provide cooling effect for wearers

24.

The NIR reflective pigments can be included into the

coating process of textiles, reducing the NIR radiation absorption rate. Different types of reflective materials including transition metals (e.g. Ag, Ti, Al), inorganic or organic compounds (e.g. TiO2, Fe2CO3, antimony doped tin oxide, azo pigments) and natural 7 ACS Paragon Plus Environment


compounds (e.g. Chlorophyll) have been used to develop cool coatings 17, 23-26. The presence of NIR reflective materials increases the sunlight reflection from the textile surface, thereby reducing the heat gain through radiation. In preparation of reflective NIR coatings, several parameters such as particle size, shape, dispensability, concentration, and thickness of coating should be considered 25. In order to get the highest possible NIR reflectance, the particle size of reflective pigments is required to be in the ranges of 1/3 to 1/2 of the wavelength of the incident light 26. Another influential parameter is the ratio between reflective indices of applied pigments (Np) and the adjacent medium (Nm) which is air, based on the formula of Np/Nm 27. The higher the reflective index of the materials, the more efficient is the NIR reflectance 27. Therefore, to achieve the best reflectance, it is essential to optimize the particle size of pigments and also to control the crystallinity and crystallite sizes. For example, among various crystalline structures of TiO2 nanoparticles, the rutile one has the highest refractive index making it a suitable option to develop cool coatings. Application of a thin layer of TiO2 nanoparticles on cotton fabric increased the NIR reflectance and provided a cooling effect 27. To achieve the highest NIR reflectance, two parameters of particle size and crystallinity were optimized. To this end, the nanoparticles synthesized through the sol-gel method underwent a calcination process at the controlled temperature (8001000 °C). Under this condition, the particle sizes of TiO2 nanoparticles increased to around 570 nm and the crystalline phase transformation occurred from anatase to rutile with reflective indices of 2.55 to 2.73, respectively. The NIR wavelength is mainly in the range of 700-1100 nm; therefore, optimum particle size to efficiently reflect the NIR is more likely to be in the range of 350-550 nm. The presence of TiO2 nanoparticles annealed at 990 °C for four hours with average particle sizes of 563 nm led to a reduction in cotton fabric surface temperature exposed to sunlight by 1.4-3.3 °C 27. Spherical silica particles can also be used for loading TiO2 nanoparticles and thus increasing the average particle size of reflective pigments. TiO2/SiO2 8 ACS Paragon Plus Environment

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core-shell structure enhanced the cooling effect of cotton fabric by reducing its surface temperature by around 4.3 °C

26.

Similarly, it has been reported that the presence of 2wt%

Janus TiO2/SiO2 nanoparticles on the surface of cotton fabrics led to 79% reflectance of incident NIR radiation in the wavelength range of 800-1100 nm

28.

This is while the fabric

coated with pure commercial TiO2 nanoparticles showed 63% NIR reflectance. The synergistic effect of silica spheres was due to their role in increasing the average size of particles to around 550 nm on the fabrics surface which was an ideal size for reflecting NIR radiation. Also, having TiO2 nanoparticles with a crystalline structure plus two materials (TiO2 and SiO2) with different refractive indices in the system increased the boundary interactions with the incident NIR radiation and enhanced the overall reflectance of the coating (Figure 2) 28.

Figure 2: The proposed mechanism of NIR reflection with TiO2/SiO2 particles applied to cotton fabric (Reproduced with permission from ref (28). Copyright 2016 Elsevier) Other types of nanoparticles such as ZnO and AZO (aluminium doped zinc oxide) can also been used to improve the cooling effects of textiles

29, 30.

It has been reported that there is a

direct relationship between the electrical conductivity and IR reflection of the films 31. This is related to the interactions of the incident electromagnetic waves with existing free electrons in the film. In a comparative study, two coatings of AZO/Ag/AZO and AZO/Cu/AZO were developed on polyester fabrics using magnetron sputtering technique. The results showed that the AZO/Ag/AZO film showed higher IR reflectance which was due to its higher electrical 9 ACS Paragon Plus Environment


conductivity. Little information is available on washing durability of the applied coatings developed through this method. All in all, the poor stability and washing fastness remains one of the main barriers in developing the application of NIR reflective nanoparticles on textiles 32. This issue can be improved through using polymeric binders, surface pretreatment of fabrics and also modification of nanoparticles. The presence of a layer of chitosan between TiO2 nanoparticles and cotton surface improved the washing fastness of nanoparticles on cotton where only 3.7-6.3% reduction in NIR reflectance of fabrics was observed after 50 washing cycles 26, 27. Some methods such as plasma pretreatment, fabrics surface oxidation, and using cross-linking agents are among common methods of improving the durability of applied nanoparticles on textiles

15, 33, 34.

Surface modification of nanoparticles with silane coupling

agents has also been reported effective in improving the durability of nanoparticles 15. Adding silane agents in the synthesis process can result in generation of more reaction sites on the surface of nanoparticles. The presence of silanized ZnO nanoparticles mixed with poly methyl methacrylate (PMMA) on the surface of cotton fabrics led to 52% NIR reflectance on black cotton fabrics coated for seven times 29. Apart from the NIR reflection, this finishing process also imparted superhydrophobic and antifungal properties to the cotton fabrics. 2.2.

Cooling effect with conductive nanomaterials

Most of conventional textiles have very low thermal conductivity making them unsuitable for cooling garments. For instance, the thermal conductivities of conventional textile materials such as cotton, wool, nylon, and polyester fibers are 0.07, 0.05, 0.25, and 0.14 W/m.K, respectively 9, 35. Due to the porous structures, textiles trap a significant amount of air in their pores resulting in low thermal conductivity. Some parameters such as porosity, thickness, air permeability, and water absorption features of fabrics are effective in determining thermal properties of fabrics

36, 37.

One of the methods of introducing the cooling effect to textiles is

accelerating the rate of heat dissipation to environment through increasing the thermal 10 ACS Paragon Plus Environment

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conductivity. Application of thermally conductive materials as coatings on the surface or embedded in the structure of fibers can enhance the cooling effect. Using highly conductive materials such as carbonous nanofillers in the polymeric composite structures can result in a higher thermal conductivity 38. One of the conductive nanomaterials which has been used in the coating of textiles is multi-walled carbon-nanotubes (MWCNTs)

39, 40.

Applying a resin

coating containing MWCNTs on the surface of cotton fabrics resulted in a cooling effect which was due to high thermal conductivity and surface emissivity of coated fabrics 40. The presence of only 11.1% MWCNTs in the coating formulation boosted the thermal conductivity of cotton fabrics by up to 78%, and further increasing its content to 50% enhanced the thermal conductivity of fabrics by 1.5 times 40. The heating test at 50 °C showed that the surface of uncoated fabrics reached the maximum temperature of 45.2 °C within five seconds while the cotton fabrics coated with 11.1% and 50% MWCNT showed the maximum temperature of 43.2 °C and 41.3 °C, respectively. As the concentration of MWCNTs increased, the cooling effect enhanced and the temperature of the fabrics surface dropped. Although the coating process led to a higher thermal conductivity, it had some negative impacts on air permeability and hand feel of the fabrics. In addition to the coating approach, the thermally conductive nanoparticles can be embedded into the fiber structures 41. The spun fibers can be woven or knitted as fabrics with enhanced cooling effects. Although not much works have been carried out in this area, it can be of some merits compared to cool coating method due to fabrication of durable products. In a recent research study, boron nitride nanosheets (BNNSs)/poly(vinyl alcohol) (PVA) composite fibers (a-BN/PVA) were prepared through 3D printing method followed by hot stretching 41. BNNSs were added to the mixture of PVA and dimethyl sulfoxide (DMSO) and the fibers with diameter of 300 µm were prepared after passing through a methanol solution coagulation bath. The printed fibers underwent a hot-stretching process (200 °C) and their diameter decreased to 95 11 ACS Paragon Plus Environment


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µm. This step created a uniform and well-oriented BNNSs in the structure of fibers (Figure 3). The well aligned BNNSs in fibers played a role as energy routes for thermal dissipation. In addition, mechanical properties enhanced significantly (tensile strength: 355 MPa and stiffness: 12.38 GPa) making the printed fibers suitable candidates for wearable applications. The main cooling mechanism of fabrics woven from a-BN/PVA fibers was based on taking away the body heat through high thermal conductivity of fibers (Figure 4)

41.

Analysis of

thermal properties of different fibers by laser-IR camera system revealed that the BN/PVA composite fiber and cotton fibers reached the maximum temperatures of 39.8 ̊C and 55.9 ̊C, respectively. Simulation results on ANSYS suggested that the a-BN/PVA fabric had 2.2 higher thermal conductivity compared with conventional cotton fabric 41.

Figure 3: Schematic illustration of printing process of thermally conductive fibers containing BNNSs (Reproduced with permission from ref (41). Copyright 2017 American Chemical Society)


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Figure 4: The schematic illustration of structure and working principle of printed boron nitride/poly(vinyl alcohol) composite fibers as cooling textiles (Reproduced with permission from ref (41). Copyright 2017 American Chemical Society) 2.3.

Cooling textiles based on photonic structures

Through manipulating and controlling the photonic structures of textiles, the radiative cooling effect can be obtained

42.

In general the radiative cooling is defined as the dissipation of

blackbody radiation of a warm object, mainly in the wavelength of 8-13 μm, through the atmospheric window to the surrounding space with a lower temperature 43. Researchers have tried to employ this phenomenon to develop cooling materials and films

43, 44.

The passive

radiative cooling effect can be achieved by making textiles transparent to the heat radiation generated by the wearer’s body. The conventional textiles such as wool, cotton and polyester are mainly opaque to the thermal infrared (IR) radiation. Therefore, the fibers absorb the generated heat from the human body which is in MIR range. This will result in heat build-up next to the skin, resulting in thermal discomfort. The opacity of natural fibers to the IR radiation is due to the presence of vibrational modes of the molecular bonds such as C―O, C―N, S=O, and C―H in the fiber chemical structure. The absorption peaks of these chemical bonds are in the same region as the human body’s IR peak absorption (9.4 µm). In contrast, there are no



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similar chemical bonds with overlapping absorption peaks in the structure of IR transparent fibers such as polyethylene and nylon. By considering these features, it is possible to design new structures of textiles to facilitate the transmittance of IR radiation and at the same time possess the wearing requirements like comfort, warmth and moisture absorption. One of the methods of enhancing the body heat dissipation is blending the IR transparent fibers into the structure of conventional textiles and engineering their orientation

42.

For instance, thermal

analysis modelling of IR transparent nylon and cotton (17%) blends demonstrated an enhanced body heat dissipation capability compared to 100% cotton 42. The calculations were conducted based on a full-vector electromagnetic field using rigorous coupled wave analysis (RCWA). With an extension in the thermal comfort set-point from 23.9 to 26.1 °C, this composition can be used to develop comfortable cooling cloths. In this design, the square cross-sectioned nylon and cotton fibers were put together as a multilayered periodic array of parallel fibers and their cooling effect was analyzed. The nylon and cotton fibers were modelled as inner and outer layers to enhance the IR transmittance and wearing comfort 42. The blended structure of cotton and nylon fibers had much higher IR transmittance compared to pure cotton while still opaque to visible light. Moreover, the net total radiative heat flux of engineered cotton/nylon blend at 26.1 °C was higher than that of cotton at 23.9 °C. This means that the thermal comfort level and cooling effect provided by textiles made of cotton/nylon blend are higher than pure cotton, though the set-point temperature is 2 °C higher 42. Optimizing the fibers diameter and understanding its impact on IR transmittance of the fabrics can also be an effective method to design cooling textiles. The structure of an infraredtransparent visible-opaque fabric (ITVOF) has been simulated based on this method using analytical modelling through full-wave finite element method (FEM)

45.

It has been

corroborated that controlling the fiber diameter is crucial in determining the heat dissipation rate from the wearer’s body 46. In ITVOF, the fiber diameter was selected in the range of visible 14 ACS Paragon Plus Environment

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light wavelength to reduce the Rayleigh scattering, while maintaining the opacity of yarns in the visible region due to the Mie scattering

45.

This engineered structure made the fabrics

transparent to mid-IR and far-IR radiations promoting the passive radiative cooling effect. Also, the model evaluated the IR transmittance (τc) and reflectance (ρc) requirements of ITVOF at 26.1 °C (Figure 5). It was reported that an absorptive surface of a fabric can be more efficient in heat dissipation (q) than a reflective surface due to its higher emissivity. In conventional textiles, heat conduction and radiation are both equally important in providing the thermal comfort. Therefore, by increasing the thermal conductivity of textiles, the cooling performance and thermal comfort can be enhanced

45.

But in ITVOF, the heat dissipation is mainly

controlled by the heat radiation; therefore, unlike normal cloth, the heat conductivity of air gap between skin and cloth does not play the key role 45. The IR transmittance and reflectance of analyzed cloths were optimized through varying the fiber and yarn diameters. The reduction in fiber diameter resulted in lower backscattering of incident IR hence a higher transmittance 45. Decreasing the fiber diameter from 10 µm to 1 µm led to a significant increase in mid-far IR transparency and therefore decrease in reflectance of the ITVOF. This variation was because of the reduction in the density of cavity resonances in the fiber structure. Similarly, reducing the yarn diameter from 100 to 30 µm resulted in reduction of yarn volume and the number of fibers which exist in the yarns. This resulted in increasing the IR transmittance and decreasing the IR reflectance. The polyethylene-based ITVOF possessed the reflectance values of 0.021 and transmittance of 0.972 which both surpassed the minimum requirements of an efficient cooling fabric at an elevated ambient temperature of 26.1 ̊C 45.



Figure 5: Heat transfer model to analyze the heat convection, conduction and radiation dissipation from the clothed human body to environment (Reproduced with permission from ref (45). Copyright 2015 American Chemical Society) 2.4.

Passive cooling and heating textiles

2.4.1. Textiles developed based on nanoporous polymeric films Electromagnetic waves of thermal radiation are released from all objects in temperatures higher than absolute zero due to the motion of particles and quasiparticles 47. Being considered as a perfect thermal emitter, the human body generates the mid-infrared radiation 47, 48. To provide passive cooling and heating effects, the radiative heat generated by human body should be facilitated and suppressed, respectively

48.

It is proven that the heat radiation accounts for

around 50% of heat loss from human body 49. As mentioned above, through using engineered fiber structures, the body dissipation can be accelerated. However, introducing these thermal functional properties to textiles through coating methods warrants new approaches. Controlling the heat dissipation rate from textiles and also emissivity levels of outer surface of fabrics are two methods of developing novel thermoregulating textiles. Through increasing the surface emissivity of coatings, the cooling effect can be achieved as it enhances the radiative heat dissipation from the product surface. In contrast, decreasing the emissivity results in a heating 16 ACS Paragon Plus Environment

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effect on the fabrics. These mechanisms have been employed by researchers to produce fabrics with passive cooling and heating effects. It was already pointed out that the IR absorption peaks of some synthetic polymers such as polyethylene do not overlap with the MIR radiation emitted from the human body (7-14 µm). Therefore, these IR transparent polymers can be the right candidates to fabricate the cooling textiles. Recently, nanoporous polyethylene (NanoPE) textile has been developed to enhance passive radiative cooling performance

49.

The essence is based on facilitating the heat

dissipation from the human body skin using polyethylene-based materials which are IR transparent (Figure 6a). The prepared NanoPE showed air permeability, opacity to visible light and high IR transmittance. This can result in enhancing the thermal comfort of the wearers even in higher set-point living temperatures without using air conditioning systems. Increasing the set-point temperature only by 1-4 °C results in a significant 7-45% energy saving of cooling facilities usage

50.

Several parameters of NanoPE such as the pore sizes and thickness of

NanoPE were found effective in determining the IR transmittance rates. At a constant thickness of NanoPE samples, optimizing the pore sizes played a crucial role in determining the radiative cooling effect. The presence of pores in the ranges of 50 to 1000 nm in the structure of NanoPE made the polyethylene films opaque to visible light due to their Mie scattering effect (Figure 6b). However, these pores were too small to interact with the MIR radiation generated from body; therefore, they behaved as an IR transparent layer (Figure 6c). Through increasing the pore size from 200 nm to 4.8 µm, the IR scattering effect of pores escalated. At the pore size of 1.2 µm, the transmittance amount of body IR radiation started to decline and bottomed out at the pore size of 2.4 µm. In a simulated thermal experiment, NanoPE film and cotton increased the temperature of skin surface by 0.8 and 3.5 °C, respectively. In order to use NanoPE films as wearable textiles, the cooling films were laminated on a cotton mesh core to get bolstered in terms of mechanical properties (Figure 6d). Also, the surfaces of NanoPE films 17 ACS Paragon Plus Environment


were coated with hydrophilic polydopamine for better hydrophilicity and wicking effect. The air and water vapor permeability was also improved by creating 100 µm holes on the film using microneedle punching. The prepared sample showed the air permeability (40-60 cm3/s cm2 Pa) and tensile strength (45 N) comparable with cotton fabric. However, some other wearability parameters such as washing fastness, coloration, next-to-skin comfort, and UV protection warrant further investigations.

Figure 6: NanoPE cooling textiles, a) Comparing the interactions of normal textile, developed nanoporous PE, and normal PE film with heat, b) SEM image of nanoporous PE, c) IR 18 ACS Paragon Plus Environment

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transmittance rates of cotton, polyethylene film, and developed nanoporous PE film, d) the procedure of preparing NanoPE cooling textiles (Reproduced with permission from ref (49). Copyright 2016 American Association for the Advancement of Science) As it was mentioned, the NanoPE textiles can enhance the heat dissipation rate from the human body. However, to get an efficient outdoor cooling effect the heat gain from the surrounding environment should also be minimized. Therefore, a better cooling effect can be achieved through dovetailing the IR transparency of polymers with IR reflective properties of nanoparticles. Based on this mechanism, the cooling textile consisting of PE polymer and ZnO nanoparticles has been reported (Figure 7)

51.

The cooling textile developed based on this

method was capable of decreasing the skin temperature by 10 °C compared with conventional cotton fabric in hot summer days. The presence of ZnO particles with particle size ranges of 0.1-1 µm in the PE polymeric matrix induced selective infrared reflection due to the Mie scattering effect. This gave rise to the reflectance of sunlight NIR radiation without significant impact on MIR heat dissipation from the human body 51. As it was mentioned earlier, the NIR scattering effect can be achieved by introducing holes with optimized size into the structure of NanoPE

49.

However, the presence of ZnO nanoparticles further augmented the scattering

effect leading to a more efficient cooling performance

51.

Some parameters such as the

thickness of PE and the size and content of nanoparticles were optimized to get the maximum cooling effect. The ZnO nanoparticles were mixed with the melted PE at a weight ratio of ZnO:PE = 2:5 in paraffin and then processed as a thin film or spun as fibers through meltpressing and melt-spinning processes, respectively 51. The simulated experiments demonstrated that PE/ZnO composite system provided a daytime cooling effect where the skin temperatures of 33.5 °C and 45.6 °C were recorded for PE/ZnO and cotton textiles, respectively. The cooling performance of PE/ZnO outperformed the cotton fabric despite its lower sweat evaporation rate which was due to the hydrophobic nature of the developed product. Washing test results 19 ACS Paragon Plus Environment


demonstrated the high durability of embedded ZnO nanoparticles in PE matrix. However, it is required to optimize the mechanical properties and other wearability features such as hydrophilicity and enhanced wicking to promote the cooling performance of these innovative materials.

Figure 7: PE/ZnO cooling textiles, a) schematic illustration of cooling mechanism of PE/ZnO, and b) SEM images of the surface and cross-section of PE/ZnO composite structures, and embedded ZnO nanoparticles in PE matrix, c) simulated (dashed-line) and measured (solid line) selective reflectance and transmittance of incident IR radiation (Reproduced with permission from ref (51). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


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Further research has been carried out to fabricate textiles with dual modes of cooling and heating functionalities 50. The research has shown that apart from MIR transmittance and NIR reflectance of textiles, surface emissivity along with surface temperature of textiles play important roles in achieving radiative cooling and heating effects (Equation 1) 50. qrad= σεtex (T4tex─T4amb)

(1)

Where εtex is the emissivity of textile surface, σ is the Stefan-Boltzmann constant and Ttex and Tamb are the temperatures of textile surface and ambiance, respectively. It can be seen that through increasing the temperature and emissivity of textiles surface, the radiation heat flux will increase and as a result the radiative cooling effect can be achieved. This implies that the thickness of the film can be effective on temperature of the fabric as it determines the heat pathway from skin to the fabric surface 50. Therefore, NanoPE films with different thicknesses of 24 µm and 12 µm were used for this purpose. In order to obtain high emissivity levels on the outer surface of textiles, a highly porous layer of carbon (4-9 µm) and to get a low emissivity surface a layer of shiny metallic material such as copper (50-150 nm) were applied to NanoPE samples using doctor blade and magnetron sputtering techniques, respectively. The high and low emissivity coatings layers were applied to thick and thin NanoPE samples, respectively. These two coated NanoPE samples were attached to each other, providing different thicknesses of NanoPE in each side. As a result, two sides of the produced fabric possessed different emissivity levels and different NanoPE thicknesses; therefore, dissimilar heating and cooling effects were achieved (Figure 8). Switching between cooling and heating modes was possible only by a simple flipping in and out of the fabric 50.



Figure 8: The heating and cooling mechanisms of dual-mode textiles (Reproduced with permission from ref (50). Copyright 2017 American Association for the Advancement of Science) Varying the NanoPE thickness inside and outside of the developed textile was effective in controlling the surface temperature. This was due to altering the heat conduction pathway from skin to the fabric’s surface because of different thicknesses of NanoPE. The higher the thickness of NanoPE, the lower was the surface temperature of the textile due to its lower heat conduction. Therefore, for cooling purposes the thickness of NanoPE layer next to skin was thin to get a faster dissipation of body heat, and vice versa for heating mode. Moreover, the emissivity levels of fabric surface were high and low for cooling and heating purposes, respectively. The applied coatings controlled the emissivity levels on fabrics surface and the thickness of NanoPE controlled the temperature of fabrics surface (Figure 9). The results showed that the carbon side of the fabric had the surface emissivity of 0.8 to 1.0 while the copper side emissivity was 0.3. The porous structure of applied coatings provided the required air and water vapor permeability for the developed textiles. Simulation experiments revealed that the traditional clothes increased the skin surface temperature from 31 to 36.9 °C. Placing the developed dual mode textiles while the high emissivity layer was towards outside, slightly increased the skin temperature to 33.8 °C which was less than that of conventional fabrics. By flipping out the fabric and exposing the low emissivity surface towards the environment, the 22 ACS Paragon Plus Environment

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skin surface temperature increased to 40.3 °C. These results corroborated that the outer surface emissivity plays more important role than that of inner side in adjusting the temperature of skin.

Figure 9: Fabrics with dual thermal functionalities, a) carbon coated side, b) the SEM image of carbon coating, c) copper coated side, d) SEM image of copper coating, e) schematic structure of developed fabric with dual cooling and heating capabilities, f) emissivity of developed coatings (Reproduced with permission from ref (50). Copyright 2017 American Association for the Advancement of Science) Consuming around 47% of the global energy for heating purposes highlights the importance of developing novel textiles with advanced heating functionality. It is a generally held view that the presence of IR reflective materials in the inner side of textiles can be effective to reflect the heat radiation back to the body. Based on this concept, the coatings containing mesh-like silver nanowires were developed on the surface of cotton fabrics through dip-coating method

52, 53.

The distances between the nanowires (200-300 nm) were set to be smaller than the IR radiation wavelength generated by human body (9 µm) and larger than water vapor molecules. Under this condition, the coating layer provided a passive heating effect by trapping the generated 23 ACS Paragon Plus Environment


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heat radiation from body while still maintaining the intrinsic breathability of the fabric 52. The heat set-point temperature expansion through this method was 0.9 °C 52. Through applying a voltage (0.9 V) to the conductive coated fabric, an active Joule heating property was also observed 52. Recent analyses demonstrated that reducing the emissivity of fabric’s outer surface plays more important role in introducing passive heating effect (Figures 10a and b)

54.

To

develop novel heating textiles, the surface of IR transparent NanoPE was modified with polydopamine (PDA) and then coated with IR reflective metallic layers of silver (Ag), copper (Cu), and nickel (Ni) nanoparticles through electroless plating method 54. The prepared film was laminated onto the cotton fabric to get the required physical properties (Figure 10c). Similar to cooling NanoPE textiles, the pore size of IR transparent polyethylene layer (50-1000 nm) was responsible for scattering the visible light and causing the opacity of the samples. The porous structure of metallic layer had an interconnected mesh-like structure with approximate pore sizes of 50-300 nm and provided low emissivity and also required breathability 54. In a simulated experiment, it was observed that the developed photonic structure showed a better heating performance compared to conventional fabrics. The calculated set-point temperature for cotton/Ag/NanoPE fabric was 15 °C which was 7.1 °C lower than that of pure cotton fabric (22.1 °C). Figures 10e and f clearly show that a less amount of heat is emitted from the body of the person who is wearing the cotton/Ag/NanoPE compared with cotton fabric. It indicates that the presence of a metallic layer with a low emissivity on the surface of fabrics successfully has suppressed the heat dissipation to environment. In terms of wearability, it was claimed that the developed heating textiles possessed tensile-strength (50 N), water vapor permeability (0.012 g cm-2 h-1) and wicking distance (~1.3 cm s-1) which were comparable with cotton fabric. The emissivity level of the cotton/Ag/NanoPE fabric increased by 10% after 10 washing cycles in pure water which was due to the release of Ag 54.


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Figure 10: NanoPE heating textiles, a and b) Schematic illustrations of heat dissipation mechanisms in traditional and functional heating textiles, c) the procedure of preparing NanoPE heating textile, d) photographs and SEM images of two different sides of NanoPE heating textile, e and f) comparing the thermal imaging of the body heat dissipation in the presence of conventional cloth (left) and the developed heating cotton/Ag/ NanoPE fabric (right) (Reproduced from ref (54). Published under Creative Commons Attribution 4.0 International License by Nature Publishing Group)



2.4.2. Textiles developed based on porous fibers The cooling textiles developed based on polymeric NanoPE films were promising in providing passive cooling and heating effects; however, their wearability aspects still warrant further modifications. Although it has been reported that some wearability parameters of the developed samples are close to those of traditional textiles, it is still necessary to improve some other aspects of cooling/heating textiles such as hand feel (e.g. softness), wrinkle resistance, and electrostatic discharge capability. Therefore, developing thermoregulating textiles using engineered porous fiber structures has been put forth as an efficient remedy to tackle the reported issues. For instance, the knitted/woven fabrics made of porous PE microfibers (170 μm ± 20 μm in diameter) have been effective in providing the required passive cooling effect 55.

The porous PE microfibers, as IR transparent materials, were prepared from the mixture of

PE and paraffin oil as a solvent using an industrial extruder. The extraction of paraffin oil by methylene chloride after the spinning process led to the formation of fibers with hierarchical porous structures. The pore size achieved through this method was in the range of 100-1000 nm, which was adequate to provide the required opacity for the fibers, without altering the IR dissipation rate. The presence of pores in the structure of fibers played the same role as the created holes in the structure of NanoPE films discussed earlier 49. The ratio between PE and paraffin oil was an effective factor on adjusting pores size, mechanical strength and softness of the spun fibers. The higher content of paraffin in the spinning process, the higher number of pores in the fibers structures. This led to a better softness but lower mechanical strength of spun PE fibers; therefore, the pores size and content in the fiber structure should be optimized to achieve the best performance 55. The knitted PE fabric with the thickness of 425 µm transmitted 70% of incident IR radiation, and induced 1.2 °C increase in the skin temperature; while the cotton fabric with the same thickness blocked all the incident IR radiation and increased the skin temperature by 3.6 °C 55. The calculated set-point temperature for PE fabric was 2.3 °C 26 ACS Paragon Plus Environment

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higher than that of cotton fabric, leading to almost 20% energy saving in indoor space cooling 55.

The PE fabric showed enhanced air permeability and mechanical characteristics compared

with NanoPE film and also showed a cotton-like softness. Besides the cooling effect, the developed PE fabric showed higher air/ water vapor permeability, and water wicking capability compared to the NanoPE film, implying a better wearing comfort 55. The porous structure of fibers can also be used to introduce the passive heating effect to textiles through increasing the heat insulation property of fibers. Fibers with hierarchical structures can minimize the thermal heat escape through radiation and conduction from body to environment. Based on this strategy, the biomimicked fibrous structures have been developed to produce products with enhanced thermal insulation property

56.

Fibers with aligned porous structure

were fabricated through a freeze spinning process to mimic the insulating hairs of polar bear. The fibers produced through this method showed outstanding thermal insulation property. The produced fibers were woven into the fabric structures where good breathability and wearability were obtained. The fibers were spun out of a relatively viscose solution of silk fibroin containing chitosan and passed through a freezing copper ring. Next, the collected fibers underwent a freeze drying process to maintain and stabilize their porous structure. Due to the freezing temperature of the ring, the solutes in the spun fiber structure were frozen creating a porous morphology inside the fibers. The freezing temperature (−40, −60, −80, −100, and −196 °C), during the spinning process was used to control the morphology and pore size of porous structures. Through increasing the spinning freezing temperature, the average pore sizes inside the fibers increased which was due to different nucleation rate of ice crystals. The fibers produced through this method possessed around 87% porosity and had axially aligned porous structures which were helpful in bolstering the mechanical characteristics of fibers (Figure 11). The porosity of the fibers as well as the number of fabric layers were effective in determining the thermal insulation of woven fabrics, where the smaller pores and thicker layers led to a 27 ACS Paragon Plus Environment


better thermal insulation. The fabric woven with biomimicked fibers provided much higher thermal insulating property compared with the conventional polyester fabric with the same thickness (0.4 mm) (Figure 12). The high thermal insulation property of fabric made of porous fibers was due to its low thermal convection and conduction, and high radiative heat reflectance due to the aligned pore structure of the generated fibers 56.

Figure 11: Cross-sectional SEM images of fibers spun at different freezing temperatures (Reproduced with permission from ref (56). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


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Figure 12: Thermal insulation and thermal stealth properties of biomimicked fibrous structures, a) optical and thermal imaging of rabbit before and after covering with different fabrics, b) thermal imaging of rabbit at different background temperatures c) the insulating mechanism of developed biomimicked fabric, d) IR reflectance of different fabrics (Reproduced with permission from ref (56). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)



2.5.

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Phase Change Materials (PCMs)

2.5.1. Working principle of PCMs Incorporating phase change materials (PCMs) into different products has been promising as an effective latent heat energy storage system

57.

In general, PCMs are materials with high

enthalpy of fusion capable of absorbing and releasing thermal energy through the phase changing process. The phase changing process can be done from solid to solid, solid to liquid, liquid to gas, solid to gas and vice versa, depending on the surrounding temperature

58, 59.

Among the latent heat storage forms, the solid to gas and liquid to gas systems are too complicated and almost impractical to use due to their large phase changing volume 60. In solid to liquid systems, through exposing the PCM material to the temperatures higher than their melting point, it initially behaves like sensible heat storage materials and its temperature increases without any phase transition 60. After reaching the temperature to the melting point, PCMs absorb a huge amount of heat energy, known as latent heat fusion, to complete the liquidification process at a constant temperature 59. Then again, the material’s temperature rises while the material is in the liquid phase 59. In temperatures lower than their crystallization point, PCMs are solidified, releasing the absorbed latent heat into the surrounding environment (Figure 13) 61. By absorbing the heat from environment or releasing it, the PCMs can provide heating, cooling, thermoregulating, and temperature buffering effects 61.


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Figure 13: Thermoregulating mechanism of PCMs Diverse ranges of materials including inorganic substances (e.g. salt hydrates), organic compounds (e.g. paraffin waxes, fatty acids), and their eutectic mixtures have been utilized as solid-liquid PCMs (Figure 14)

58, 62, 63.

The inorganic PCMs have high thermal conductivity

and storage capacity; however, they have some disadvantages such as high degree of supercooling, corrosiveness, phase segregation and lack of thermal stability 64. Organic PCMs in turn can be classified into two main groups of paraffinic and non-paraffinic. They can go through congruent melting, i.e, they undergo melting and freezing processes numerous times without any alteration in their characteristics or any phase segregation

60, 63.

However, the

organic PCMs have some intrinsic drawbacks such as flammability, low thermal conductivity and low phase change enthalpy 63. Paraffin waxes which are linear hydrocarbons with formula CnH2n+2, such as n-eicosane, n-octadecane, n-hexadecane, or the mixtures such as ntetradecane/n-hexadecane and n-hexadecane/n-eicosane are the most commonly used PCMs in different applications 7. The PCMs based on fatty acids are synthesized through a simple process from vegetable and animal oils; therefore, they are considered as sustainable alternatives

60.

Of the common fatty acid PCMs, stearic acid, palmitic acid, lauric acid and

myristic acid are noteworthy. The PCMs are encapsulated into thin and resilient shells through some physical, chemical, and physiochemical methods

63.

Some techniques such as spray

drying, fluidized-bed encapsulation technology, and centrifugal extrusion are examples of physical methods for encapsulation of PCMs

57.

The main chemical encapsulation methods

have been developed based on polymerization which can be categorized as suspension, dispersion, in-situ, and interfacial polycondensation

63.

Similarly, some methods such as

coacervation, layer-by layer assembly and sol-gel encapsulation are among the common physiochemical encapsulation methods

63, 65.

The encapsulation process prevents the PCM

materials from evaporation or reaction with surrounding materials. Also, it facilitates the use 31 ACS Paragon Plus Environment


of PCMs in textiles as it prevents the interactions of PCM materials with fibrous materials and possibly changing their intrinsic properties 66. Some materials such as melamine formaldehyde, urea formaldehyde, diacid silicone, polystyrene and polymethacrylate are among common compounds which can be used as shells in the encapsulating process 15, 67.

Figure 14: The main classifications of PCMs 2.5.2. PCMs incorporation in textiles Application of PCMs in textiles provides a thermal barrier effect against the environmental temperature fluctuations, regulating the heat flux from the textiles. The thermoregulating effect of PCM-modified textiles is based on phase transition of PCMs due to the temperature change 61.

The heat required for melting the PCMs can be provided by human body or surrounding

environment. Absorption of heat from the wearer’s body reduces the perspiration rate and thereby promoting the thermal comfort in clothes. If the temperature falls below the crystallization point of PCMs, the absorbed heat will be released again to the wearer’s body, providing heating effect

61.

The phase change temperature of selected PCMs should be 32 ACS Paragon Plus Environment

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compatible with the products’ final applications. In textile applications, PCMs should meet some requirements including: i) melting point between 15-35 °C, ii) narrow melting and solidification temperature intervals, iii) high heat conductivity, high specific heat capacity, and high heat of fusion, iv) high latent heat per volume, and v) small phase transition volume. They should also be non-flammable, non-corrosive, eco-friendly, low in cost, easily available, chemically stable and non-toxic

59, 64.

PCMs can be incorporated into textile through several

methods such as adding in coating formulation, spinning the PCMs-fiber polymer mixture solution, electrospinning the phase change fibers (PCFs), mixing into a foam formulation, cross-linking, and laminating pre-prepared PCM-polymer films to substrate 15, 68. The content of PCMs in a product should be optimized based on the required breathability, flexibility and other mechanical characteristics 57. Thermoregulating textiles for different applications such as beddings, seating, outdoor jackets, footwear have been manufactured by the Outlast® Company and are currently available in the market. 2.5.3. Coating textiles with PCMs In order to apply PCMs to textiles as coatings, PCMs should be dispersed into the coating formulations consisting of surfactant, dispersant, antifoam agent, and polymeric binder 61. Then the coating formulation can be applied to textile surface using conventional coating methods such as blade coating, transfer coating, and dip coating 15. Some factors such as polymer binder type, PCM type, the mass ratio between binder and PCM, the structure of substrate, and coating method are influential parameters on the obtained thermoregulating performance of fabrics coated with encapsulated PCMs 57, 66, 69, 70. The type of binder is an important factor in determining the stability, thermoregulation, and physical characteristics of coated fabrics. The assessment of the effects of ten different types of commercial polymer binders containing 30 wt% microcapsules (paraffin-PCMs



encapsulated in polystyrene shells) confirmed the significant impact of binder type on handfeel and heat capacity of coated fabrics. The heat capacities of 0.92-3.5 J/g were reported for the coated fabrics 71. However, the washing process deteriorated the thermal storage capacity of fabrics by 50-80%, which was due to the leaching out the microcapsules from the coated fabrics, depending on the binder type. Beside the binder type, the amount of microcapsules in the coating formulation should be optimized to achieve the expected thermoregulating performance. Through increasing the content of PCMs in the coating formulation (15-35 wt %), the heat capacity of coated cotton fabrics experienced an increasing trend. The heat capacities of 5.89 and 7.6 J/g were achieved for the cotton samples coated with 30 and 35 wt% of microcapsules, respectively

71.

Similarly, it was realized that the knitted polyester fabric

coated with different amounts (5-23%) of melamine–formaldehyde microcapsules (containing eicosane synthesized through in situ polymerization) showed the heat storage capacities in the ranges of 0.91– 4.44 J/g. The heat storage retention rates of 66%, 41%, and 26% were observed for coated polyester fabrics after 1, 5, and 20 washing cycles 72. To understand the effect of PCMs type, five types of commercial paraffin-based PCMs, including n-tetradecane (C14H30), n-hexadecane (C16H34), n-octadecane (C18H38), n-eicosane (C20H42), and their blends (40 wt% in binder) were used for coating knitted polyester fabric samples

66.

The results showed that the paraffin type and the microcapsules size were both

effective in determining the melting and crystallization temperatures and enthalpies. In a general trend, the melting point of paraffins is a function of carbon atoms number in their chemical structures and paraffins with a longer chain have a higher melting point 57. Moreover, mixing different types of PCMs with various phase transition temperatures in the coating systems led to an extended thermoregulating performance on fabrics, having a longer energy exchange duration 66.


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The structure of fabrics is another parameter which influences the performance ranges of PCMs on the surface of fabrics (Figure 15). The fabrics structure is effective in thermoregulating effect by determining the PCMs loading on the surface and also the amount of heat which can be transferred to them 73. A higher melting temperature and lower crystallization onset were obtained on woven polyester fabrics compared with knitted one

69.

This variation stemmed

from the thinner structure of woven fabrics and higher PCM microcapsules loadings on the knitted fabrics surface

69.

The effect of substrate type and fabric structure was further

investigated by applying the microcapsules of n-hexadecane with poly (methyl methacrylate) (PMMA) shell to the surface of three types of fabrics including cotton, cotton-/polyester and microfiber polyester fabrics using a polyurethane binder

74.

The PMMA is a thermoplastic

polymer and has shown a good mechanical properties which is suitable for coating applications 63.

For the cotton, cotton/polyester, and microfiber polyester fabrics coated with formulations

containing 50g/l microcapsules, the calculated heat storage capacities were 4.95, 10.02, and 8.35 J/g, respectively 74. The different heat capacities were because of different structures of fabrics and also the nature of the materials of tested fabrics. Due to the looser structure of cotton/polyester than pure cotton, a higher amount of microcapsules were attached to the samples surface, resulting in higher heat storage capacity. The hydrophilic nature of cotton fabrics reduced the adhesion between the substrate and hydrophobic microcapsules. In addition, the presence of ester groups in the structure of polyester fibers made it more attractive substrate for the microcapsules with acrylic base shell. These findings highlighted once again the role of substrate type on final heat storage capacity of products. The effect of substrates type is mainly because of their role in determining the amount of microcapsules attached to the surface.



Page 36 of 73

Figure 15: PCMs on the surface of knitted and woven PET fabrics, a and b) coated side and back of knitted fabrics, c and d) coated side and back of woven fabrics, (Reproduced with permission from ref (69). Copyright 2017 Elsevier) To understand the effect of coating method, the formulations containing 20 wt% microcapsules (n-octadecane with melamine–formaldehyde shell) in polymer paste were applied to the knitted polyester fabrics through different methods of roller padding, knife coating, and screen printing 70.

Through the DCS analysis it was shown that while the fabrics prepared through printing had

the highest performance in absorbing and releasing heat, the lowest performance was demonstrated by the fabrics prepared by padding method. This was related to the existence of different amounts of PCMs on the surface of treated fabrics

70.

Cross-linking agents can

improve the stability of PCMs on the surface of textiles 75. This method has already been used to affix different types of nanoparticles on fabrics

39, 76, 77.

Pectin/PCMs were prepared and

applied to cotton fabrics using the cross-linking method. First, the pectin surface was modified with fatty acids through esterification process and then the pectin-fatty-acid composite was used to encapsulate the paraffin based PCMs (octadecane). The prepared PCMs were applied to the surface of cotton fabrics in the presence of cross-linking agent, 1,2,3,4- butane tetra


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carboxylic acid (BTCA), and a catalyst, sodium hypophosphate (SHP), under ultrasonic waves 75.

Although there are some advantages in the encapsulation process, it brings about some issues such as high processing cost, increased weight, extending the thermal response time and reducing the absorption latent heat capacity of PCMs. Therefore, the fabrication of shapestabilized PCMs has been put forth as an alternative approach. The shape-stabilized PCMs are capable of maintaining the PCMs entrapped in a porous supporting structure in the solid state even in temperatures higher than melting point (Figure 16) 78, 79. This technique has been used to prepare aerogel/eicosane microparticles 79. The microcapsules were prepared through three methods of melt-infiltration, solvent-dissolving, and combined melt-dissolving and then were applied to meta-aramid woven fabrics as thermoregulatory coatings

79.

The PCM-aerogel

particles were prepared through adding silica aerogels into the n-eicosane. Due to the capillary force and the high surface area of silica aerogels, the eicosane was stabilized on aerogel particles. Among three employed methods, the melt-infiltration approach showed highest latent heat of fusion (ΔH) where ΔH=198.38 J/g was recorded based on DSC analysis findings. The presence of PCM-aerogel structures on fabrics resulted in a slower temperature rise of fabrics in the range of 35–39 °C compared to untreated samples 79.



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Figure 16: Encapsulation and infiltration procedures to stabilize PCMs (Reproduced with permission from ref (79). Copyright 2016 Elsevier) 2.5.4. Fibrous structures containing PCMs Applying PCMs with polymer binders as coating layers to textiles comes with some of its downsides such as lack of adequate washing fastness, deteriorated moisture absorption and air permeability, not to mention other drastic changes in intrinsic physical characteristics of textiles. Alternatively, the prepared microcapsules can be mixed with the polymer solutions of fibers prior to the spinning step. In this method, the added microcapsules are embedded into the structure of fibers and permanently trapped into the fibers structure 73. Researchers have tried to generate fibers containing PCM microcapsules via different methods such as melt spinning, wet spinning, and dry spinning. Although there are some advantages in producing thermoregulating fibers through this method, the impacts of microcapsules addition on other physical characteristics of spinning polymers and spun fibers should be considered

73.

Polypropylene fibers containing microencapsulated PCMs (n-octadecane) were produced through the melt-spinning process

80.

Four parameters including PCM content (2-12 wt%),

temperature, metering speed and the extruder speed were found influential on latent heat absorption capacity of fibers

80.

DSC results showed that through increasing the content of

PCM microcapsules in polypropylene monofilaments, the latent heat increased and reached to the maximum value of 9.2 J/g in the presence of 12% PCMs. Increasing the content of microencapsulated PCMs in the fibers resulted in increasing the latent heat of fibers and decreasing the tenacity of the prepared yarns

81.

Moreover, integrating the spun fibers

containing PCMs into the structure of cotton fabrics led to a reduced heat transfer rate compared with pure cotton fabric.


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Another commonly used method is spinning PCMs into the form of ultrafine fibers via electrospinning process, producing phase change fibers (PCFs) 82. Electrospinning is a versatile approach to produce fibrous structures with high surface-to-volume ratio from diverse ranges of polymers using an electrostatic force

83.

Three main procedures to fabricate PCFs are i)

uniaxial electrospinning, ii) coaxial electrospinning and iii) multi-fluidic compound-jet electrospinning (Figure 17) 82. Of these methods, the coaxial and multifluidic electrospinning methods produce PCFs with core-sheath structure with single and multichannel tubular microstructures to encapsulate PCMs, respectively 82. There are some parameters such as the selected polymer type (solubility, molecular weight, melting point), the properties of polymer solvent (viscosity, temperature, surface tension), and electrospinning set-up (humidity, electric field, feed rate) which are effective in electrospinning process 83. Fabricating PCFs can be of some advantages such as obviating the need for encapsulation process, easier preparation approach, controllable dimensions, better heat storage efficiency, and cost-efficiency 82. The fibers produced through these methods usually have two components of polymeric shell as supporting matrix and liquid-solid PCM inner part 84. Through changing the characteristics of shell, PCM and electrospinning procedure, the heat absorption capacity of PCFs can be altered.

Figure 17: Setups for a) uniaxial electrospinning, b) coaxial electrospinning and c) multifluidic electrospinning (Reproduced with permission from ref (82). Copyright 2017 Elsevier)



Ultrafine composite fibers of polyethylene glycol (PEG) and cellulose acetate (CA) were generated using uniaxial electrospinning 85. Different amounts of PEG were mixed with CA and the composite fibers were spun with the feed rate of 5 ml/ h from the mixture solution. Spinning fibers with low concentrations of PEG resulted in PEG/CA fibers with smooth surface (Figure 18a). However, increasing the content of PEG in the mixture solution resulted in nonuniform fibers (Figure 18b). Also, the content of PEG played the key role in thermoregulating performance of the spun fibers. In another example, core-shell nanofiber composites consisting of polyethylene glycol (PEG1000) as the core and polyamide 6 (PA6) as the shell were produced using coaxial electrospinning method (Figures 18c and d) 86. The effect of PEG flow rate change on thermal properties, morphology and structures of produced fibers was investigated, while different ratios of PEG1000/PA6: 36, 50, 55, 60, and 70% were used. Through increasing the concentration (viscosity) of PEG in the inner part, the diameter of fibers increased. This in turn resulted in a thicker shell in the structures of spun nanofibers. In the main, the flow rates of core and shell materials played the key role in fibers’ morphology and porosity. Increasing the flow rate led to a thicker nanofibers because of over feeding of polymers into the spinning process. This resulted in appearance of unfavorable beads in the structure of nanofibers due to the shorter time of drying process of spun fibers. It was observed that the flow rate and PEG viscosity both were effective in determining the onset and offset of fibers melting process. Increasing the PEG concentration led to higher peak temperature and latent heat enthalpy. The maximum latent heat enthalpy of 121.86 kJ/kg was reported for the nanofibers made of PEG1000/PA6 70% 86.


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Figure 18: SEM images of a) composite PEG/CA fibers containing 30% PEG, b) PEG/CA fibers containing 60% PEG, (Reproduced with permission from ref (85). Copyright 2011 Elsevier) c) core-shell PEG1000/PA6, d) TEM image of the core-shell composition of PEG1000/PV6 fiber (Reproduced with permission from ref (86). Copyright 2017 Elsevier) 3. Some other methods to develop thermoregulating garments In addition to the aforementioned methods, there are some other common approaches which have already been used in commercialized products to enhance their thermoregulating performance. Innovative garment designs: Through adjusting the design of garments based on their applications the heat exchange between body and environment can be regulated. Clothing design has a direct impact on evaporative resistance and also thermal insulation of garments. Through optimizing some parameters such as garment fit, ventilation and air gap between skin and fabric the air circulation and convective heat transfer can be adjusted 87, 88. For instance, it has been reported that the loose-fitting trouser can provide a higher thermal insulation 41 ACS Paragon Plus Environment


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compared with the tight-fitting one. However, loose-fitting trouser can provide a better air circulation and heat convection. Therefore, the air gap is effective in determining thermal insulation and evaporative resistance

89.

Some products such as PFG Tamiami™ and PFG

Bahama™ from Columbia Sportswear have been developed to promote the adequate air ventilation for wearers. Moisture management of fabric: This technique is mostly used for cooling applications and the main concept is based on accelerating the moisture transfer outward and preventing any moisture intake from environment. This group of fabrics can absorb moisture from skin, facilitate its transmission to outer side of fabric and keep the skin surface dry

90, 91.

Sweat

evaporation is a suitable means to provide cooling effect where it can contribute to body heat dissipation up to 0.58 kcal heat for each gram of sweat. However, moisture evaporation from fabric surface and resultant cooling effect only takes place if the ambience has lower humidity and higher temperature than that of skin and also enough sweat is available to wet the fabric surface. Therefore, this technique is suitable to be applied to fabrics used for physical activity (e.g. sportswear) in hot and dry environment. This mechanism was employed to develop SportwoolTM fabrics by Australian Wool Innovation Ltd. This product consists of an inner layer from fine merino wool next to skin and a top layer made of synthetic fibers. It has been claimed that the developed product can enhance the moisture wicking to surface and keep the skin dry. Another commercial product is Gore-Tex fabric membrane which is made of stretched polytetrafluoroethylene. This product is breathable and waterproof and it is widely used in the textile industry. Some other products such as CoolCore, SPORTINGTEX®, BEAT THE HEAT™, Omni-Freeze™ ZERO, and Trail Turner™ Shell, to name but a few, can also provide cooling effect for wearers through moisture management technique. Attachment of wearable devices: It is sometimes very difficult to achieve the required moisture evaporation and air circulation in protective technical clothing. This can cause the 42 ACS Paragon Plus Environment

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accumulation of metabolic heat in the microclimate next to body skin and also condensation of moisture to water droplets, causing discomfort of wearers. Some researchers tried to improve the thermal comfort of these clothes through external attachment of air-cooled and liquid cooled equipment to the garment. For instance, the ambient air can be circulated next to the body skin through some lightweight battery-powered fans attached at different locations of garment (Figure 19a) 92. Although these types of garments can improve thermal comfort, they bring about some other issues such as bulkiness, more complexity in manufacturing, additional weight, annoying noise, and non-uniform wind blowing around the body. These issues have restricted the use of air-cooling garments in daily applications. Similarly, the liquid cooling cloths can provide cooling effect through circulating the liquid coolant in flexible tubes embedded in the textiles around the body (Figure 19b). The components of this system are a fluid reservoir, liquid coolant (water, propylene, and ethylene glycol) pump, sensors and hoses connected to different parts of the garment. This technology was initially developed by NASA to provide the thermal comfort for astronauts. The main cooling mechanism is based on transferring body heat through conduction to cold liquid running in the embedded tubing networks 93. Therefore, direct contact of tubes containing coolant with skin is required which potentially can cause some discomfort feeling. The main challenge is designing products with high cooling efficiency, low coolant and power costs and also compact and portable structure. Moreover, some parameters such as thickness of inner textile layer in contact with skin, tubes diameter, tubes materials, liquid temperature and circulation speed should be optimized for a better cooling efficiency 94.



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Figure 19: Cooling garments, a) air cooling vest equipped with lightweight fans (Reproduced with permission from ref (92). Copyright 2013 Elsevier), b) liquid cooling garment on manikin (Reproduced with permission from ref (93). Copyright 2015 Elsevier) 4. Safety concerns of using nanomaterials with future outlook The concept of thermoregulating textiles is an emerging research field and has attracted a great deal of scientific attention due to their important role in energy-saving, energy storage and also providing thermal comfort for end-users in modern lifestyle. Researches have resorted to different methods to manipulate the interactions of heat radiation generated from body and sun with textiles to promote the thermal comfort of users. The applications of current technologies in developing functional thermal textiles have some downsides which warrant further explorations. For instance, there are some concerns on the safety aspects of incorporating nanomaterials in textile products. Some of the approaches discussed in this review article are based on using nanomaterials such as NIR reflective particles, carbon-based and metallic coatings to achieve the expected thermoregulating effects. However, it is worth bearing in mind that there is a debate on potential impacts of nanomaterials on users and also environment. Some concerns have also been raised about the occupational health and safety of workers who handle these materials on a daily basis. Currently, there are thousands of products containing nanomaterials in different fields such as textiles, cosmetics, and food containers 44 ACS Paragon Plus Environment

95.

For

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instance, sunscreens containing TiO2 and ZnO nanoparticles are widely applied to skin regardless of their potential implications. Despite the significant progress in introducing new products, there is still a vast knowledge gap on assessing nano-safety. Research findings differ significantly on the toxicity of nanomaterials. The conflicting results come from testing the safety of nanomaterials based on different methods and experimental conditions. Some parameters such as size, surface area, shape, dispersibility, aggregation, crystallinity, surface functionalization, wettability, and surface coating can lead to different biophysicochemical interactions with living beings and environment 96. There are significant amounts of research and funding devoted to understanding the basic effects of nanomaterials in living cells and establish general guidelines in this regard. But only with a slight alteration in the synthesis process, structure or morphology of nanomaterials, new outcomes and effects have been observed 97. Therefore, it is highly important to define standard screening protocols to follow suitable testing methods such as in vitro (cellular and molecular) versus in vivo (animal or whole organism), and to identify responsible organizations for testing nanomaterials and developed products 96. Apart from thermoregulating textiles, nanomaterials have applications in developing smart textiles with novel features such as self-cleaning 98-102, UV protection 103106,

antimicrobial activity 103, 107, 108, fire retardancy 109, 110, hydrophilicity and hydrophobicity

111-113

among others. Therefore, it is required to expand our knowledge on the characteristics,

benefits and risks of using different types of nanomaterials and mitigate the risk of exposure by selecting more benign methods and materials for different applications. The method of application and attachment of nanoparticles influences their release rate during washing process and also their potential toxicity

114.

Incorporation of nanomaterials in the

polymeric matrix of fibers before spinning process can be used to permanently affix the nanoparticles in the fibers structure. The fibers developed through this method are stable during washing; however, the embedded particles may fall off through abrasion and it is applicable 45 ACS Paragon Plus Environment


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only for synthetic fibers 115. Developing functional textiles through finishing (coating, padding and exhaustion) is more feasible, but it results in weaker adhesion of nanoparticles to textiles. Using polymeric binders has been effective in improving the durability of nanoparticles on finished products. The applied nanomaterials should be durable against abrasion, body fluids, water, dry-cleaning solvents, washing detergents, UV and temperature fluctuations

115.

The

main exposure to nanomaterials from functional textiles will be through skin. Therefore, all safety aspects related to manufacturing workers, dealers, consumers and recycling processes of these products should be taken into account

115.

Passing the nanomaterials through skin

depends on their size. Nanoparticles below 4 nm can go through skin barrier easily while the larger particles can penetrate only into injured skins 116. As discussed in this review, most of the nanoparticles such as TiO2 and ZnO which have been used for developing thermoregulating coatings are in 350-550 nm and 0.1-1 µm size ranges, respectively. Therefore, the likelihood of their penetration through skin is not very high. Similarly, PCM microcapsules are in the micron size range making them a safer option than individual nanoparticles. However, the possibilities of entering these particles to nature should be minimized by developing new finishing approaches which can lead to mitigating their potential aftereffects on environment. In addition to using nanomaterials, the applications of IR-transparent polymers in the forms of films or fibers with engineered photonic structures are effective methods to obtain the cooling and heating effects. However, the use of such pure polymers as wearable textiles is not common at the current market. Therefore, further research is warranted to improve the wearability of these materials. Also, the possible impacts of finishing processes, colorations and adding other types of fabrics to the performance of heating-cooling fabrics should be considered. Therefore, further research is still required to assimilate these findings and technologies in the actual textile products. In applying PCMs to textiles, some research areas such as improving the washing fastness, enhancing comfort level and wearability, increasing the heat storage 46 ACS Paragon Plus Environment

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capacities of the treated textiles, developing new testing methods, and expanding the methods of incorporating PCMs in textile materials are some possible future research trajectories. 5. Conclusion This article provided a concise overview on recent research findings and developments in the area of thermoregulating textiles. Some novel methods of fabricating textiles with thermoregulating functionalities along with the influential parameters and their mechanisms were discussed. Of these methods, diverse aspects of using near-infrared reflective nanoparticles, conducting materials, photonic engineered structures, radiative heating and cooling materials and phase change materials were all brought to light mostly based on the most recent literature. Understanding different aspects of these methods along with the influential parameters can be helpful for researchers to eliminate the current drawbacks and improve the functionality of next generation of thermal textile materials. 6. Acknowledgement We wish to acknowledge the support of the Alfred Deakin Postdoctoral Research Fellowship awarded to E. Pakdel by Deakin University. We would also like to acknowledge support from the Australian Research Council World Class Future Fiber Industry Transformation Research Hub (IH140100018). 7. Conflicts of Interest The authors declare no conflict of interest



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Fig 1: A schematic illustration of heat transfer mechanisms from skin to environment through textiles

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Figure 2: The proposed mechanism of NIR reflection with TiO2/SiO2 particles applied to cotton fabric (Reproduced with permission from ref (28). Copyright 2016 Elsevier)


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Figure 3: Schematic illustration of printing process of thermally conductive fibers containing BNNSs (Reproduced with permission from ref (41). Copyright 2017 American Chemical Society)



Figure 4: The schematic illustration of structure and working principle of printed boron nitride/poly(vinyl alcohol) composite fibers as cooling textiles (Reproduced with permission from ref (41). Copyright 2017 American Chemical Society)


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Figure 5: Heat transfer model to analyze the heat convection, conduction and radiation dissipation from the clothed human body to environment (Reproduced with permission from ref (45). Copyright 2015 American Chemical Society)



Figure 6: NanoPE cooling textiles, a) Comparing the interactions of normal textile, developed nanoporous PE, and normal PE film with heat, b) SEM image of nanoporous PE, c) IR transmittance rates of cotton, polyethylene film, and developed nanoporous PE film, d) the procedure of preparing NanoPE cooling textiles (Reproduced with permission from ref (49). Copyright 2016 American Association for the Advancement of Science)


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Figure 7: PE/ZnO cooling textiles, a) schematic illustration of cooling mechanism of PE/ZnO, and b) SEM images of the surface and cross-section of PE/ZnO composite structures, and embedded ZnO nanoparticles in PE matrix, c) simulated (dashed-line) and measured (solid line) selective reflectance and transmittance of incident IR radiation (Reproduced with permission from ref (51). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)



Figure 8: The heating and cooling mechanisms of dual-mode textiles (Reproduced with permission from ref (50). Copyright 2017 American Association for the Advancement of Science)


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Figure 9: Fabrics with dual thermal functionalities, a) carbon coated side, b) the SEM image of carbon coating, c) copper coated side, d) SEM image of copper coating, e) schematic structure of developed fabric with dual cooling and heating capabilities, f) emissivity of developed coatings (Reproduced with permission from ref (50). Copyright 2017 American Association for the Advancement of Science)



Figure 10: NanoPE heating textiles, a and b) Schematic illustrations of heat dissipation mechanisms in traditional and functional heating textiles, c) the procedure of preparing NanoPE heating textile, d) photographs and SEM images of two different sides of NanoPE heating textile, e and f) comparing the thermal imaging of the body heat dissipation in the presence of conventional cloth (left) and the developed heating cotton/Ag/ NanoPE fabric (right) (Reproduced from ref (54). Published under Creative Commons Attribution 4.0 International License by Nature Publishing Group)


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Figure 11: Cross-sectional SEM images of fibers spun at different freezing temperatures (Reproduced with permission from ref (56). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)



Figure 12: Thermal insulation and thermal stealth properties of biomimicked fibrous structures, a) optical and thermal imaging of rabbit before and after covering with different fabrics, b) thermal imaging of rabbit at different background temperatures c) the insulating mechanism of developed biomimicked fabric, d) IR reflectance of different fabrics (Reproduced with permission from ref (56). Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


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Figure 13: Thermoregulating mechanism of PCMs



Figure 14: The main classifications of PCMs


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Figure 15: PCMs on the surface of knitted and woven PET fabrics, a and b) coated side and back of knitted fabrics, c and d) coated side and back of woven fabrics, (Reproduced with permission from ref (69). Copyright 2017 Elsevier)



Figure 16: Encapsulation and infiltration procedures to stabilize PCMs (Reproduced with permission from ref (79). Copyright 2016 Elsevier)


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Figure 17: Setups for a) uniaxial electrospinning, b) coaxial electrospinning and c) multi-fluidic electrospinning (Reproduced with permission from ref (82). Copyright 2017 Elsevier)



Figure 18: SEM images of a) composite PEG/CA fibers containing 30% PEG, b) PEG/CA fibers containing 60% PEG, (Reproduced with permission from ref (85). Copyright 2011 Elsevier) c) core-shell PEG1000/PA6, d) TEM image of the core-shell composition of PEG1000/PV6 fiber (Reproduced with permission from ref (86). Copyright 2017 Elsevier)


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Figure 19: Cooling garments, a) air cooling vest equipped with lightweight fans (Reproduced with permission from ref (92). Copyright 2013 Elsevier), b) liquid cooling garment on manikin (Reproduced with permission from ref (93). Copyright 2015 Elsevier)


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