Far-Infrared Emission Properties and Thermogravimetric Analysis of Ceramic-Embedded Polyurethane Films

polymers

Article Far-Infrared Emission Properties and Thermogravimetric Analysis of Ceramic-Embedded Polyurethane Films

Ashik Md Faisal 1,2,3,4,* , Fabien Salaün 2,3 , Stéphane Giraud 2,3 , Ada Ferri 1, Yan Chen 4 and Lichuan Wang 4

1 Politecnico di Torino, DISAT, Corso Duca Degli Abruzzi 24, 10129 Turin, Italy; [email protected] 2 University of Lille Nord de France, F-5900 Lille, France; [email protected] (F.S.); [email protected] (S.G.) 3 ENSAIT, GMTEX, F-59100 Roubaix, France 4 College of Textile and Clothing Engineering, Soochow University, Suzhou 215123, China; [email protected] (Y.C.); [email protected] (L.W.) * Correspondence: [email protected]

Abstract: The far-infrared ray (FIR) is one kind of electromagnetic wave employed for numerous bio-interactive applications such as body thermoregulation, infrared therapy, etc. Tuning the FIR- emitting property of the functional textile surface can initiate a new horizon to utilize this property in sportswear or even smart textiles. Ceramic particles were studied for their unique ability to constantly emit FIR rays. The purpose of this research is to characterize the FIR emission properties and the thermogravimetric analysis of ceramic-embedded polyurethane films. For this purpose, ceramic particles such as aluminum oxide, silicon dioxide, and titanium dioxide were incorporated (individually) with water-based polyurethane (WPU) binder by a sonication technique to make a thin layer of film. Significant improvement in FIR emissive property of the films was found when using Citation: Faisal, A.M.; Salaün, F.; different ceramic particles into the polyurethane films. Reflection and transmission at the FIR range Giraud, S.; Ferri, A.; Chen, Y.; Wang, L. Far-Infrared Emission Properties were measured with a gold integrating sphere by Fourier-transform infrared (FTIR) spectrometer. and Thermogravimetric Analysis of The samples were also characterized by thermogravimetric analysis (TGA). Different physical tests, Ceramic-Embedded Polyurethane such as tensile strength and contact angle measurements, were performed to illustrate the mechanical Films. Polymers 2021, 13, 686. properties of the films. The study suggested that the mechanical properties of the polyurethane films https://doi.org/10.3390/ were significantly influenced by the addition of ceramic particles. polym13050686 Keywords: far-infrared ray; ceramic particles; polyurethane; emissivity; functional textiles Academic Editor: Begoña Ferrari

Received: 17 January 2021 Accepted: 22 February 2021 1. Introduction Published: 25 February 2021 Polyurethane (PU) has been effectively used as a coating material due to its unique properties of corrosion resistance, microbial resistance, and durability against wear and Publisher’s Note: MDPI stays neutral weathering [1,2]. Nowadays, PU binder gains a great deal of attention due to the potential- with regard to jurisdictional claims in ity in selection of monomeric materials, which allows for a wide range of applications [3,4]. published maps and institutional afﬁl- iations. Water-based polyurethane (WPU) binders are the composed of low or non-volatile organic compounds, and they are widely used as coating materials since they are environmentally friendly and non-toxic [5,6]. Unfortunately, PU coatings are often hygroscopic, which causes permeation of aggressive ions such as oxygen and chloride; long exposure in ultraviolet (UV) radiation causes photochemical degradation of PU, which makes the coating Copyright: © 2021 by the authors. yellowish. Both of this process gradually causes PU disintegration which affects its me- Licensee MDPI, Basel, Switzerland. chanical and optical properties [7,8]. Several studies suggest that the addition of inorganic This article is an open access article particles (such as cerium oxide, titania, and zinc oxide) may improve the chemical and distributed under the terms and conditions of the Creative Commons mechanical resistance of the PU (hybrid) coatings [9–13]. Attribution (CC BY) license (https:// Ceramic particles would be a great choice in order to improve the functionality as well creativecommons.org/licenses/by/ as the chemical and mechanical resistance of the PU coatings. Ceramic particles, such as 4.0/). aluminum oxide, silicon dioxide, titanium dioxide, were studied for their unique property

Polymers 2021, 13, 686. https://doi.org/10.3390/polym13050686 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 686 2 of 9

to constantly emit FIR rays [14]. The application of aluminum oxide could significantly increase the surface emissivity of coated composite material [15,16]. Silicon dioxide-based finishing shown excellent thermal insulation properties on cotton fabrics even though the emission properties of silicon dioxide were not so far deeply analyzed [17]. Ceramic coatings containing titanium dioxide showed a greater infrared emissivity value (about 90%) in the range of resonance wavelength [18,19]. Ceramic particles have also shown good antimicrobial properties when incorporated into textiles as a coating material [20]. Polymer-based ceramic composites were proposed for applications like nano-membranes and optical applications due to their FIR-emissive properties [21,22]. The utilization of FIR ray-associated textile products has shown interest from the sportswear industry [23] because of its potential bio-interactive application [24–26] and body thermoregulation [27–30]. In order to amplify the desired emissive properties of the engineered textile surface, researchers have successfully exhibited several methods, including the use of dyes and pigments, electro-spinning of different materials, insertion of inorganic materials, or even modification of the cross-sectional shape of the fiber [31]. FIR is one kind of invisible electromagnetic wave with the wavelength range from 0.75 µm to 1000 µm. FIR has been claimed for the generation of resonance to the human body and is also known as “thermal ray” [32]. The electromagnetic spectrum is divided into several wavelength regions (from 10−5 nm to 103 m) by methods of detecting and producing radiation. All electromagnetic radiation is primarily the same and subordinate by the same laws, and the only difference is the wavelength. The absorption wavelength range of most organic compounds is from 6 µm to 14 µm, which is also known as the resonance wavelength (life rays) [33,34]. The human body absorption wavelength is around 10 µm and according to Wien’s law, the peak wavelength radiation (λpeak) of human skin is estimated to be 9.34 µm [35]. Resonance occurs when the elements closer to the human body emit or reflect a similar wavelength as the human body. It is reported that resonance enhances blood circulation, which eventually generates metabolic heat [32]. Therefore, the addition of FIR-emitting ceramic particles as a filler into the PU coating is a promising way to improve the functionality and the performance of the composites. In the work presented here, ceramic particles have been incorporated into the water-based polyurethane binder to utilize the FIR radiation properties of ceramics. The emissive properties (in FIR range) of thin-layer polyurethane films incorporated with different ceramic particles have been investigated. Enhancement of FIR emissive properties, especially at the peak wavelength region, was found due to the addition of ceramic particles into the PU composites. Thermogravimetric analysis (TG) and different physical tests were performed to characterize and illustrate the mechanical properties of the films. The study suggested that the addition of ceramic particles have a significant influence on the mechanical properties of the PU composite.

2. Materials and Methods In this study, three different ceramic particles were individually incorporated with a water-based polyurethane binder to independently investigate their FIR emissive properties. The WPU binder Lurapret N5657 liq (with 40% solid content) was supplied by Archroma (Reinach, Switzerland) and ceramic nanoparticles (aluminum oxide, silicon dioxide, titanium dioxide) were purchased from Evonik (Essen, Germany). The detailed specification of the samples are shown in Table1. Based on the solid content of the end product, 5 wt.% of ceramic particles (weight- percentage) were incorporated with a water-based polyurethane (PU) binder. The concen- tration of ceramic particles in water was kept at 10 g/L. Ceramic particles were initially dispersed to water using the probe-sonication technique for one hour and then the water- based polyurethane binder was slowly added to the mixture. For a uniform mixture of the solution, further probe-sonication and magnetic stirring were applied. Then, the mixture was put into a Teflon mold to make a thin layer of polyurethane film. The films were kept to the Teflon mold at room temperature (~20 ◦C) for 24 h and then dried in an incubator at Polymers 2021, 13, 686 3 of 9

Table 1. Compositions and characteristics of the samples.

Name of Ce- Ceramic Particle Size of Ce- Ceramic Content Thickness of Sample ID ramic Type ramic (nm) (wt. %) Film (mm) PU - - - 0 0.6 ± 0.02 Polymers 2021, 13, 686 PU/Al2O3 Aluminum oxide Hydrophobic 7~20 5 0.6 3± of0.04 9 PU/SiO2 Silicon dioxide Hydrophobic 5~50 5 0.6 ± 0.03 PU/TiO2 Titanium dioxide Hydrophilic 7~100 5 0.6 ± 0.03

95 ◦C for 30 min. The ﬁlms were then cured at 130 ◦C for 6 min to ﬁnish the crosslinking of Based on the solid content of the end product, 5 wt.% of ceramic particles (weight- the polyurethane. Crosslinking was preliminarily observed by the extent of swelling (sol- ventpercentage) up-take). were The thicknessincorporated of each with sample a water-based was measured polyurethane around 0.6 (PU) mm. binder. An illustration The concen- oftration the sample of ceramic preparation particles technique in water can was be seenkept in at Figure 10 g/L.1. Ceramic particles were initially dispersed to water using the probe-sonication technique for one hour and then the water- basedTable polyurethane 1. Compositions binder and characteristicswas slowly added of the samples. to the mixture. For a uniform mixture of the solution, further probe-sonication and magnetic stirring were applied. Then, the mixture Particle Size of Ceramic Content Thickness of Film Sample ID Name of Ceramicwas put into Ceramic a Teflon Type mold to make a thin layer of polyurethane film. The films were kept Ceramic (nm) (wt. %) (mm) to the Teflon mold at room temperature (~20 °C) for 24 h and then dried in an incubator PU - - - 0 0.6 ± 0.02 at 95 °C for 30 min. The films were then cured at 130 °C for 6 min to finish the crosslinking PU/Al2O3 Aluminum oxide Hydrophobic 7~20 5 0.6 ± 0.04 PU/SiO2 Silicon dioxideof the polyurethane. Hydrophobic Crosslinking 5~50 was preliminarily 5 observed by the 0.6 extent± 0.03 of swelling PU/TiO2 Titanium dioxide(solvent up-take). Hydrophilic The thickness 7~100 of each sample was 5measured around 0.6 0.6± 0.03 mm. An illustration of the sample preparation technique can be seen in Figure 1.

Sonication

PU Ceramic O H O H H C N C N C O C C O H H H H H n Drying/Curing

Cross-linking PU/Ceramic Film

Water/PU/Ceramic FigureFigure 1. 1.Sample Sample preparation preparation scheme scheme for for polyurethane polyurethane composites composites with with different different ceramics. ceramics.

AA Fourier-transformFourier-transform infrared infrared (FTIR) (FTIR) spectrometer spectrometer from from Pike Pike Technologies Technologies (Fitchburg, (WI, USA) WI,with USA) an integrated with an integrated reflective reflective gold sphere gold wa spheres used was [1] used to measure [1] to measure the spectral the spectral transmit- τ ρ µ µ transmittancetance (τ) and (reflectance) and reflectance (ρ) at the ( ) wavelength at the wavelength range rangefrom 6 from μm 6to m14 toμm 14 in mstandard in standard atmospheric conditions. The emissivity (ε) of the films were then calculated atmospheric conditions. The emissivity (ε) of the films were then calculated by Kirchhoff’s by Kirchhoff’s Law of thermal radiation: ε = 1 − τ − ρ [36]. The surface morphology Law of thermal radiation: ε = 1 − τ − ρ [36]. The surface morphology of the samples was of the samples was investigated using the LEO 435 VP scanning electron microscope (SEM)investigated from Leica using (Richmond, the LEO 435 IL, USA).VP scanning At first, electron the sample microscope was electroplated (SEM) from and thenLeica (IL keptUSA). onto At first, the SEM the sample specimen was holder electroplated and photographed and then kept from onto above. the SEM A field specimen emission holder scanningand photographed electron microscope from above. (FESEM) A field was emissi used toon achievescanning well-defined electron microscope images down (FESEM) to thewas micrometer used to achieve range. Thermogravimetricwell-defined images analysis down of to the the samples micrometer was carried range. out Thermogravi- using a TGAmetric 3+ analysis (Mettler Toledo,of the samples Columbus, was OH, carried USA). out Thermogravimetric using a TGA 3+ (Mettler analysis Toledo, was performed Columbus, atOH, the USA). temperature Thermogravimetric range from 20 ◦analysisC to 700 was◦C, atperformed a heating rateat the of temperature 10 ◦C per minute, range at from an 20 inert°C to atmosphere 700 °C, at a (inert heating nitrogen rate gasof 10 condition °C per minute, at a constant at an flow inert of atmosphere 50 mL/min during(inert nitrogen the experiment).gas condition The at a contact constant angle flow of theof 50 samples mL/min for during different the liquids experiment). was measured The contact by the angle DIGIDROPof the samples contact for different angle meter. liquids Tensile was properties measured were by the measured DIGIDROP according contact to angle the ISO meter. 527-1Tensile standard properties method were with measured a cell force according of 1 kN to and the a ISO drawing 527-1 speed standard of 1 mm/min.method with The a cell ± ◦ testsforce were of 1 performedkN and a atdrawing standard speed atmospheric of 1 mm/min. conditions The (a tests temperature were performed of 20 2 atC standard and relative humidity of 65 ± 5%). atmospheric conditions (a temperature of 20 ± 2 °C and relative humidity of 65 ± 5%). 3. Results and Discussion 3.1. SEM Analysis of Composite Films The surface morphology of the samples was investigated via FESEM images and shown in Figure2. By magnifying the samples at 9000 times and comparing Figure2a (PU without ceramic particles) with Figure2b–d, the presence of ceramic particles were clearly observed. Ceramic particles have the tendency to agglomerate while using water as a solvent due to the high polarity of water [37]. In this work, a sonication technique was used to disperse the ceramic particles uniformly and to reduce the agglomeration of the ceramic particles. According to the SEM observation, the ceramic particles were seen to be well Polymers 2021, 13, 686 4 of 9

3. Results and Discussion 3.1. SEM Analysis of Composite Films The surface morphology of the samples was investigated via FESEM images and shown in Figure 2. By magnifying the samples at 9000 times and comparing Figure 2a (PU without ceramic particles) with Figure 2b–d, the presence of ceramic particles were clearly observed. Ceramic particles have the tendency to agglomerate while using water as a sol- Polymers 2021, 13, 686 vent due to the high polarity of water [37]. In this work, a sonication technique was4 ofused 9 to disperse the ceramic particles uniformly and to reduce the agglomeration of the ceramic particles. According to the SEM observation, the ceramic particles were seen to be well disperseddispersed in in the the polyurethane polyurethane matrix. matrix. The mean observedobserved aggregate aggregate size size of of the the ceramics ceramics waswas roughly roughly 100 100 nm nm to to500 500 nm. nm. The The presence presence of spherical of spherical particles particles of low of low diameters diameters were documentedwere documented by the SEM by the microphotograph, SEM microphotograph, which which formed formed larger larger agglomerates agglomerates and aggre- and gates.aggregates.

FigureFigure 2. Photos 2. Photos of ofdifferent different samples samples and and scanning scanning electron electron microscopemicroscope (SEM)(SEM) images images of of the the PU, PU, PU/ceramic PU/ceramic embedded embedded films: (a) PU, (b) PU/Al2O3, (c) PU/SiO2, and (d) Pu/TiO2. ﬁlms: (a) PU, (b) PU/Al2O3,(c) PU/SiO2, and (d) Pu/TiO2.

3.2.3.2. FIR FIR Emission Emission Properties Properties of of Composite Composite Films ToTo make make a aqualitative qualitative analysis analysis of the FIRFIR emissionemission properties properties between between the the PU PU films films (with(with and and without without ceramicceramic particles), particles), FTIR FTIR spectra spectra was performedwas performed at the wavelengthat the wavelength range rangeof 6 µofm 6 toμm 14 toµ m14 (resonance μm (resonance wavelength). wavelength). The results The results of reflectance, of reflectance, transmittance, transmittance, and andemissivity emissivity of different of different samples samples can be can seen be in seen Figures in Figures3–5. It was 3–5. observed It was observed that the overall that the overallemissivity emissivity of the ceramicsof the ceramics embedded embedded PU films PU were films significantly were significantly higher than higher the ordinary than the ordinary(films without (films without ceramic) ceramic) ones. At ones. the peak At the wavelength peak wavelength (λpeak = 9.34(λpeakµ m)= 9.34 of humanμm) of bodyhuman bodyradiation, radiation, the emissivitythe emissivity of the of aluminumthe aluminum oxide oxide (Al2O (Al3)2 incorporatedO3) incorporated film wasfilm foundwas found to tobe be 82.7%, 82.7%, which which was was higher higher than than the emissivity the emissivity of the commonof the common polyurethane polyurethane film (81.8%, film Polymers 2021, 13, 686 µ µ 5 of 9 (81.8%,shown shown in Figure in3 Figurec). At the 3c). mid-IR At the spectral mid-IR range, spectral from range, 10.5 mfrom to 14 10.5m, μ them to influence 14 μm, of the added ceramic particles on the emissivity was even greater. influence of added ceramic particles on the emissivity was even greater.

[a] [b] [c] 100 100 100 PU PU PU 90 90 90 PU/Al O PU/Al2O3 PU/Al2O3 2 3 80 80 80

70 70 70

60 60 60 86

λ peak= 9.34 µm 50 50 50

84 Total emissivity = 82.7% 40 40 40

Emissivity (%) Emissivity 82 Reflectance (%) Reflectance 30 (%) Transmittance 30 30 Total emissivity = 81.8% 80 20 20 20

78 10 10 10 9.1 9.2 9.3 9.4 9.5

0 0 0 67891011121314 67891011121314 6 7 8 9 10 11 12 13 14 Wavelength (µm) Wavelength (µm) Wavelength (µm) FigureFigure 3. 3.FIRFIR property property characterization characterization of of alum aluminuminum oxide incorporatedincorporated PUPU ﬁlms. films. Fourier-transform Fourier-transform infrared infrared (FTIR) (FTIR) measuredmeasured (a) ( areflectance,) reﬂectance, (b (b) )transmittance, transmittance, ( (cc)) emissivity. emissivity.

At the peak wavelength of human body radiation, the emissivity of the silicon diox-

ide (SiO2) incorporated film was found 83.6% (shown in Figure 4c), which is higher than the ordinary PU film. The addition of silicon dioxide to the PU film provided good emissive properties, especially at the mid-IR spectral range.

[a] [b] [c] 100 100 100 PU PU PU 90 PU/SiO 90 PU/SiO 90 2 2 PU/SiO2 80 80 80

70 70 70

60 60 60 86 λpeak= 9.34 µm 50 50 50 84

40 40 40 Total emissivity = 83.6% 82 Emissivity (%) Emissivity Reflectance (%) 30 (%) Transmittance 30 30 Total emissivity = 81.8% 80 20 20 20

78 10 10 10 9.1 9.2 9.3 9.4 9.5

0 0 0 6 7 8 9 1011121314 6 7 8 9 1011121314 6 7 8 9 1011121314 Wavelength (µm) Wavelength (µm) Wavelength (µm) Figure 4. FIR property characterization of silicon dioxide incorporated PU films. FTIR measured (a) reflectance, (b) transmittance, (c) emissivity.

The emissivity at the peak wavelength of the titanium dioxide (TiO2)-added PU film was found 83.7% (shown in Figure 5c), which is also higher than the emissivity of the common PU film (81.8%).

[a] [b] [c] 100 100 100 PU PU PU 90 PU/TiO 90 PU/TiO 90 2 2 PU/TiO2 80 80 80

70 70 70

60 60 60 86 λpeak= 9.34 µm 50 50 50

40 40 40 Total emissivity= 83.7% 82 Emissivity (%) Emissivity Reflectance (%) Reflectance 30 (%) Transmittance 30 30 Total emissivity= 81.8% 80 20 20 20

78 10 10 10 9.19.29.39.49.5

0 0 0 67891011121314 67891011121314 6 7 8 9 10 11 12 13 14 Wavelength (µm) Wavelength (µm) Wavelength (µm) Figure 5. FIR property characterization of titanium dioxide incorporated PU films. FTIR measured (a) reflectance, (b) transmittance, (c) emissivity.

The addition of ceramic particles into PU composite offers good emissive properties, especially at the mid-IR spectral range. The emissivity of the titanium dioxide added PU film was shown better results compared to other ceramic particles added PU films. There- fore, the addition of ceramic particles improved the FIR emissive properties of the PU

Polymers 2021, 13, 686 5 of 9

[a] [b] [c] 100 100 100 PU PU PU 90 PU/Al O 90 PU/Al O 90 PU/Al O [a] 2 3 [b] 2 3 [c] 2 3 100 80 10080 80100 PU PU PU 90 70 9070 7090 PU/Al O PU/Al2O3 PU/Al2O3 2 3 80 60 8060 6080 86

λ peak= 9.34 µm 70 50 7050 5070

84 Total emissivity = 82.7% 60 40 6040 4060 86

Emissivity (%) Emissivity 82 Reflectance (%) Reflectance λ peak= 9.34 µm 50 30 50(%) Transmittance 30 3050

84 Total emissivityTotal emissivity = 81.8% = 82.7% 80 40 20 4020 2040

Emissivity (%) Emissivity 82

Reflectance (%) Reflectance 78 9.1 9.2 9.3 9.4 9.5 30 10 (%) Transmittance 3010 1030 Total emissivity = 81.8% 0 0 0 80 20 20 20 67891011121314 67891011121314 6 7 8 9 10 11 12 13 14 78 10 Wavelength (µm) 10 Wavelength (µm) 10 9.1Wavelength 9.2 9.3 (µm) 9.4 9.5 0 0 0 Figure67891011121314 3. FIR property characterization of alum67891011121314inum oxide incorporated PU films.6 Fourier-transform 7 8 9 10 11infrared 12 13(FTIR) 14 measured (a) reflectance,Wavelength (µm) (b) transmittance, (c) emissivity.Wavelength (µm) Wavelength (µm) Figure 3. FIR property characterization of aluminum oxide incorporated PU films. Fourier-transform infrared (FTIR) measured (a) reflectance, (b) transmittance,At the peak (c) emissivity.wavelength of human body radiation, the emissivity of the silicon dioxide (SiO2) incorporated film was found 83.6% (shown in Figure 4c), which is higher than the Atordinary the peak PU wavelength film. The addition of human of silicon body dioxide radiation, to the the PU emissivity film provided of the good silicon emis- diox- sive properties, especially at the mid-IR spectral range. Polymers 2021, 13, 686 ide (SiO2) incorporated film was found 83.6% (shown in Figure 4c), which is higher5 of than 9

[a] the ordinary PU [b]film. The addition of silicon dioxide[c] to the PU film provided good emis- 100 100 100 sive properties, PU especially at the mid-IR spectralPU range. PU 90 PU/SiO 90 PU/SiO 90 2 2 PU/SiO2 80 80 80 [a] [b] [c] 100 70 10070 70100 PU PU PU 90 60 PU/SiO 9060 PU/SiO 6090 86 PU/SiO 2 2 λpeak= 9.34 µm 2 50 50 80 50 80 80 84

Total emissivity = 83.6% 70 40 7040 4070 82 Emissivity (%) Emissivity Reflectance (%) 30 (%) Transmittance 30 30 60 60 86 60 Total emissivity = 81.8% 80 λpeak= 9.34 µm 20 20 20 50 50 50 84 78 10 10 10 9.1 9.2 9.3 9.4 9.5 40 40 40 Total emissivity = 83.6% 82 Emissivity (%) Emissivity Reflectance (%)

0 (%) Transmittance 0 0 30 6 7 8 9 101112131430 6 7 8 9 1011121314 306 7 8 9 1011121314 Total emissivity = 81.8% 80 Wavelength (µm) Wavelength (µm) Wavelength (µm) 20 20 20

78 Figure10 4. FIR property characterization of silicon10 dioxide incorporated PU films. FTIR10 measured9.1 9.2(a) reflectance, 9.3 9.4 9.5 (b) trans- 0 0 0 mittance,6 7(c) emissivity. 8 9 1011121314 6 7 8 9 1011121314 6 7 8 9 1011121314 Wavelength (µm) Wavelength (µm) Wavelength (µm) The emissivity at the peak wavelength of the titanium dioxide (TiO2)-added PU film FigureFigure 4. FIR 4. FIR property property characterization characterization of of silicon silicon dioxide dioxide incorporated incorporated PUPU ﬁlms.films. FTIRFTIR measured measured (a ()a reﬂectance,) reflectance, (b )(b trans-) transmittance, (c) emissivity. was found 83.7% (shown in Figure 5c), which is also higher than the emissivity of the mittance, (c) emissivity. common PU film (81.8%). The emissivity at the peak wavelength of the titanium dioxide (TiO2)-added PU film [a] [b] [c] 100 100 100 was found PU 83.7% (shown in Figure 5c), PUwhich is also higher than the emissivity PU of the 90 PU/TiO 90 PU/TiO 90 PU/TiO common PU2 film (81.8%). 2 2 80 80 80

70 70 70 [a] [b] [c]

100 60 10060 60100 86 λ = 9.34 µm PU PU peak PU 90 50 9050 PU/TiO 5090 PU/TiO 84 2 2 PU/TiO2 80 40 8040 4080 Total emissivity= 83.7% 82 Emissivity (%) Emissivity Reflectance (%) Reflectance 30 (%) Transmittance 30 30 70 70 70 Total emissivity= 81.8% 80 20 20 20 60 60 60 86 78 λpeak= 9.34 µm 10 10 10 9.19.29.39.49.5 50 50 50

84 0 0 0 40 40 Total emissivity= 83.7% 67891011121314 67891011121314406 7 8 9 10 11 12 13 14 82 Emissivity (%) Emissivity Reflectance (%) Reflectance

Wavelength (µm) (%) Transmittance Wavelength (µm) Wavelength (µm) 30 30 30 Total emissivity= 81.8% 80 Figure20 5. FIR property characterization of20 titanium dioxide incorporated PU films.20 FTIR measured (a) reflectance, (b) 78 Figure 5. FIR property characterization of titanium dioxide incorporated PU films. FTIR measured9.19.29.39.49.5 (a) reflectance, (b) trans- transmittance,10 (c) emissivity. 10 10 mittance,0 (c) emissivity. 0 0 67891011121314 67891011121314 6 7 8 9 10 11 12 13 14 Wavelength (µm) Wavelength (µm) Wavelength (µm) AtThe the addition peak wavelength of ceramic ofparticles human into body PU radiation, composite the offers emissivity good ofemissive the silicon properties, dioxide Figure 5. FIR property characterization(SiOespecially2) incorporated atof thetitanium mid-IR film diox spectral waside found incorporated range. 83.6% The (shown PUemissivity films. in FigureFTIR of the measured4 c),titanium which (a isdioxide) reflectance, higher added than (b the PU) transmittance, (c) emissivity. ordinaryfilm was shown PU film. better The results addition compared of silicon to dioxide other ceramic to the PU particles film provided added PU good films. emissive There- properties,fore, the addition especially of ceramic at the mid-IR particles spectral improv range.ed the FIR emissive properties of the PU TheThe addition emissivity of atceramic the peak particles wavelength into PU of thecomposite titanium offers dioxide good (TiO emissive2)-added properties, PU film especiallywas found at 83.7%the mid-IR (shown spectral in Figure range.5c), The which emissivity is also higher of the than titanium the emissivity dioxide added of the PU filmcommon was shown PU film better (81.8%). results compared to other ceramic particles added PU films. There- fore, theThe addition addition of of ceramic particlesparticles into improv PU compositeed the FIR offers emissive good emissiveproperties properties, of the PU

especially at the mid-IR spectral range. The emissivity of the titanium dioxide added PU film was shown better results compared to other ceramic particles added PU films. Therefore, the addition of ceramic particles improved the FIR emissive properties of the PU composite films. These films can be used as an FIR emissive coating which is similar to currently available FIR-emitting materials [4,38].

3.3. TG Properties of Composite Films The effect of ceramic particles on the thermal stability of the polyurethane ﬁlms was measured using thermogravimetric analysis (carried out in inert conditions). The results of residual mass and the derivative residual mass are shown in Figure6. The thermal behavior of both PU and PU/ceramics displayed multi-step degradation. The main step of degradation of microcapsules containing ceramic particles occurred between 368 ◦C and 408 ◦C. The decomposition of PU and PU/ceramics is characterized by the onset ◦ temperature at 5% weight loss (Tonset5%), starting at around 316 C. Polymers 2021, 13, 686 6 of 9

composite films. These films can be used as an FIR emissive coating which is similar to currently available FIR-emitting materials [4,38].

3.3. TG Properties of Composite Films The effect of ceramic particles on the thermal stability of the polyurethane films was measured using thermogravimetric analysis (carried out in inert conditions). The results of residual mass and the derivative residual mass are shown in Figure 6. The thermal behavior of both PU and PU/ceramics displayed multi-step degradation. The main step of Polymers 2021, 13, 686 degradation of microcapsules containing ceramic particles occurred between 368 °C6 and of 9 408 °C. The decomposition of PU and PU/ceramics is characterized by the onset temperature at 5% weight loss (Tonset5%), starting at around 316 °C.

100 0 (a) PU (b) PU 90 PU/Al O PU/Al O 2 3 -5 2 3 80 PU/SiO 2 PU/SiO2 PU/TiO 70 2 PU/TiO2 -10 60

50 -15

Residual Mass (%) Mass Residual -20 30

20 -25 10 (%/min) Mass Residual Derivative

0 -30 0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700 Temperature (°C) Temperature (°C) Figure 6.6. ThermogravimetricThermogravimetric (TG) analysisanalysis ofof differentdifferent PU/ceramicPU/ceramic composites; ( a): residual mass (%) and ((b)) derivativederivative residualresidual massmass (%/min).(%/min).

The main decomposition of PU correspondingcorresponding toto PU/ceramicsPU/ceramics took took place place at at 420 °C.◦C. Thermogravimetric data of different films films are shown in Table2 2.. ThereThere isis almostalmost nono charchar residue remaining in thethe PUPU samplesample atat overover 480480 ◦°C.C. TheThe weightweight lossloss ofof PU/ceramicPU/ceramic ◦ ◦ ◦ (PU/Al2O2O3:3 :315°C, 315 C, PU/SiO PU/SiO2: 3182: 318 °C, C,and and PU/TiO PU/TiO2: 3122: 312°C) compositesC) composites began began around around the ◦ samethe same temperature temperature of PU of (T PUonset5% (Tonset5%: 316 °C).: 316 ThisC). suggests This suggests that the that thermal the thermal stability stability of poly- of polyurethaneurethane films films is not is notadversely adversely affected affected by by the the the the incorporation incorporation of of ceramic ceramic particles. However, thethe thermal thermal degradation degradation process process should shou beld differentbe different as the as ceramicthe ceramic particles particles have havean effect an effect on the on structure the structure of PU of composite PU compos [39ite]. [39]. Around Around 5% of5% char of char residue residue was was found found for forPU/ceramic PU/ceramic films, films, while while for PUfor film,PU film, the residuethe residue was was 0.2%. 0.2%. This This indicates indicates that thethat addition the ad- ditionof ceramic of ceramic particles particles increases increases the residual the residu mass ofal themass PU of composites. the PU composites. It was also It observedwas also observedthat the content that the of content the ceramic of the particle ceramic and particle the method and ofthe sample method preparation of sample hadpreparation minimal hadeffect minimal on Tmax effectand Tononset Tmax. and Tonset.

TableTable 2. 2.Thermogravimetric Thermogravimetric data data of of different different PU/ceramic PU/ceramic composites. composites.

Mass Losses forMass Each Losses Step for Each Step Residue Tonset5% Step IStep I Step II Step II Step III Step III Residue◦ Sample ID Sample Tonset5% at 600 C (◦C) Tempera- Mass Tempera- Mass Tempera- Mass at 600 TemperatureID (Mass°C) Temperature Mass Temperature Mass (%) Range (◦C) Losses (%) tureRange (Losses◦C) Lossesture (%) LossesRange (◦C) tureLosses Losses (%) °C (%) PU 316 261–270 2.5Range ( 368–376°C) (%) Range 28.6 (°C) (% 416–425) Range (°C 68.1) (%) 0.2 PU/Al2O3 316 261–270PU 316 2.3 261–270 351–360 2.5 368–376 20.8 28.6 416–425 416–425 71.268.1 5.3 0.2 PU/SiO 319 278–286 3.0 351–360 17.8 416–425 73.2 5.6 2 PU/Al2O3 316 261–270 2.3 351–360 20.8 416–425 71.2 5.3 PU/TiO2 313 278–286 3.3 368–376 33.4 408–416 57.3 5.6 PU/SiO2 319 278–286 3.0 351–360 17.8 416–425 73.2 5.6 PU/TiO2 313 278–286 3.3 368–376 33.4 408–416 57.3 5.6 3.4. Mechanical Properties of Composite Films The mechanical properties of polymer composites can be influenced by the reinforcing effects of a kind of filler added to the polymer [40]. Properties of fillers, such as the particle size, aggregate size, degree of dispersion, shape, surface characteristics, and aspect ratio can influence the mechanical properties of polymer composites. The effect of ceramic particles on the tensile properties of the polyurethane films are listed in Table3. The addition of ceramic particles can affect the crystallization of the polyurethane matrix. The ceramic particles can change the structural characteristics of the semi-crystalline matrix which can affect the tensile strength of the films [4]. The addition of ceramic particles resulted in the increment of tensile strength for the films which indicates the uniform dispersion of ceramic particles into the films. The elongation at break of the PU/ceramic composites Polymers 2021, 13, 686 7 of 9

decreased significantly compared to the ordinary PU film. The addition of ceramic particles into the PU composite seems to cause the failure of the films at low elongation, most likely due to the shape, surface, and brittle fracture as seen in Figure2.

Table 3. Mechanical properties of different PU/ceramic composite ﬁlms.

Tensile Properties PU PU/Al2O3 PU/SiO2 PU/TiO2 Tensile strength (MPa) 18 ± 0.2 21 ± 0.4 19 ± 0.3 20 ± 0.8 Elongation at break (%) 439 ± 1.9 233 ± 0.8 225 ± 0.5 228 ± 0.5

3.5. Contact Angle Analysis of Composite Films The comparison of contact angles for different measuring liquids of the samples can reveal the changes in the hydrophilic nature of the ﬁlms, as the polyurethane was incorporated with different ceramic particles. The contact angle of the samples for different measuring liquids is shown in Table4. The contact angle of a solid surface may vary due to several factors, such as the wettability of the surface, nature of the material (hydrophilic or hydrophobic), and roughness of the surface [41]. From observing the samples, it was found that the contact angles for different measuring liquids increased with the addition of ceramic particles into the PU composites; which revealed that the harshness developed by the ceramic particle increases the hydrophobicity of the PU composites.

Table 4. The contact angle of the samples for different measuring liquids.

Average Contact Angle (◦) Sample ID Diiodomethane Water PU 40 ± 0.2 44 ± 0.3 PU/Al2O3 50 ± 0.3 60 ± 0.4 PU/SiO2 49 ± 0.3 54 ± 0.3 PU/TiO2 50 ± 0.3 54 ± 0.2

4. Conclusions The aim of this work was to analyze the FIR emissive properties of ceramic-embedded PU films and the effect of ceramic particles on the mechanical properties of PU films. The results showed that the addition of ceramic particles has a significant influence on the FIR emissive properties of the polyurethane films. The reinforcing effect of ceramic particles also influenced the mechanical properties of the polyurethane films. This work will provide useful guidance towards the implementation of the ceramic particle as an FIR material to composites.

Author Contributions: Conceptualization, F.S., S.G., A.M.F. and A.F.; formal analysis, F.S., S.G. and A.M.F.; methodology, F.S., S.G. and A.M.F.; software, A.M.F.; data curation, A.M.F.; writing—original draft preparation, A.M.F.; writing—review and editing, F.S., S.G., A.M.F., A.F., Y.C. and L.W.; supervision, F.S., S.G., A.F., Y.C. and L.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the European Commission under the framework of Erasmus Mundus Sustainable Management and Design for Textile (SMDTex). Project no. SMDTex-2017-43 entitled “Assessment of thermal comfort of FIR functionalized garments”. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Authors are grateful to the European Commission and the China Scholarship Council for providing the full Ph.D. scholarship of Ashik Md Faisal. ENSAIT-France, Polito-Italy, and SUDA-China are also acknowledged as the host of this ongoing research project. Polymers 2021, 13, 686 8 of 9

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Tielemans, M.; Bleus, J.-P. New radiation-curable polyurethane dispersions for outdoor application on wood. In Proceedings of the 5th International Woodcoatings Congress, Prague, Czech Republic, 17–18 October 2006. 2. Saha, S.; Kocaefe, D.; Boluk, Y.; Pichette, A. Surface degradation of CeO2 stabilized acrylic polyurethane coated thermally treated jack pine during accelerated weathering. Appl. Surf. Sci. 2013, 276, 86–94. [CrossRef] 3. Patti, A.; Acierno, D. The effect of silica/polyurethane waterborne dispersion on the perforating features of impregnated polypropylene-based fabric. Text. Res. J. 2019, 90, 1201–1211. [CrossRef] 4. Xiong, Y.; Zou, Y.; Cai, S.; Liu, H.; Huang, S.; Li, H. Processing and characterization of polymer-based far-infrared composite materials. Polymers 2019, 11, 1451. [CrossRef][PubMed] 5. Qiu, F.; Xu, H.; Wang, Y.; Xu, J.; Yang, D. Preparation, characterization and properties of UV-curable waterborne polyurethane acrylate/SiO2 coating. J. Coatings Technol. Res. 2012, 9, 503–514. [CrossRef] 6. Caminero, M.Á.; Chacón, J.M.; García-Plaza, E.; Núñez, P.J.; Reverte, J.M.; Becar, J.P. Additive manufacturing of PLA-based composites using fused filament fabrication: Effect of graphene nanoplatelet reinforcement on mechanical properties, dimensional accuracy and texture. Polymers 2019, 11, 799. [CrossRef][PubMed] 7. Heidarian, M.; Shishesaz, M.R.; Kassiriha, S.M.; Nematollahi, M. Characterization of structure and corrosion resistivity of polyurethane/organoclay nanocomposite coatings prepared through an ultrasonication assisted process. Prog. Org. Coat. 2010, 68, 180–188. [CrossRef] 8. Rosu, D.; Rosu, L.; Cascaval, C.N. IR-change and yellowing of polyurethane as a result of UV irradiation. Polym. Degrad. Stab. 2009, 94, 591–596. [CrossRef] 9. Saha, S.; Kocaefe, D.; Krause, C.; Larouche, T. Effect of Titania and zinc oxide particles on acrylic polyurethane coating performance. Prog. Org. Coat. 2011, 70, 170–177. [CrossRef] 10. Figueira, R.B.; Silva, C.J.R.; Pereira, E.V. Organic–inorganic hybrid sol–gel coatings for metal corrosion protection: A review of recent progress. J. Coat. Technol. Res. 2015, 12, 1–35. [CrossRef] 11. Wang, X.; Hu, J.; Li, Y.; Zhang, J.; Ding, Y. The surface properties and corrosion resistance of fluorinated polyurethane coatings. J. Fluor. Chem. 2015, 176, 14–19. [CrossRef] 12. Bautista, Y.; Gomez, M.; Ribes, C.; Sanz, V. Relation between the scratch resistance and the chemical structure of organic–inorganic hybrid coatings. Prog. Org. Coat. 2011, 70, 358–364. [CrossRef] 13. Saadat-Monfared, A.; Mohseni, M.; Tabatabaei, M.H. Polyurethane nanocomposite films containing nano-cerium oxide as UV absorber. Part 1. Static and dynamic light scattering, small angle neutron scattering and optical studies. Colloids Surf. A Physicochem. Eng. Asp. 2012, 408, 64–70. [CrossRef] 14. Kubiliene, D.; Sankauskaite, A.; Abraitiene, A.; Krauledas, S.; Barauskas, R. Investigation of thermal properties of ceramic- containing knitted textile materials. Fibres Text. East. Eur. 2016, 24, 63–66. [CrossRef] 15. Bartl, J.; Baranek, M. Emissivity of aluminium and its importance for radiometric measurement. Meas. Sci. Rev. 2004, 4, 31–36. 16. Abbas, A.; Zhao, Y.; Ali, U.; Lin, T. Improving heat-retaining property of cotton fabrics through surface coatings. J. Text. Inst. 2017, 108, 1–7. [CrossRef] 17. Rosace, G.; Guido, E.; Colleoni, C.; Barigozzi, G. Influence of textile structure and silica based finishing on thermal insulation properties of cotton fabrics. Int. J. Polym. Sci. 2016, 2016, 1–10. [CrossRef] 18. Wang, Z.; Wang, Y.; Liu, Y.; Xu, J.; Guo, L.; Zhou, Y.; Ouyang, J.; Dai, J. Microstructure and infrared emissivity property of coating containing TiO2 formed on titanium alloy by microarc oxidation. Curr. Appl. Phys. 2011, 11, 1405–1409. [CrossRef] 19. Hanada, T.; Aikawa, T.; Soga, N. ChemInform abstract: Physical properties and structure of rf-sputtered amorphous films in the system titania-silica. Chem. Inf. 1984, 15, 52–56. [CrossRef] 20. Faisal, A.M.; Salaün, F.; Giraud, S.; Ferri, A.; Chen, Y.; Wang, L. Analysis of the thermal comfort properties and FIR infrared emission characteristics of ceramic nanofillers imbedded fabrics. In Proceedings of the 19th World Textile Conference-Autex, Ghent, Belgium, 11–15 June 2019; p. 3. 21. Wang, H.-F. Bone and joint protection ability of ceramic material with biological effects. Chin. J. Physiol. 2012, 55, 47–54. [CrossRef] [PubMed] 22. Leung, T.-K.; Lin, Y.-S.; Lee, C.-M.; Chen, Y.-C.; Shang, H.-F.; Hsiao, S.-Y.; Chang, H.-T.; Chao, J.-S. Direct and indirect effects of ceramic far infrared radiation on the hydrogen peroxide-scavenging capacity and on murine macrophages under oxidative stress. J. Med. Biol. Eng. 2011, 31, 345–351. [CrossRef] 23. Worobets, J.T.; Skolnik, E.R.; Stefanyshyn, D.J. Apparel with far infrared radiation for decreasing an athlete’s oxygen consumption during submaximal exercise. Res. J. Text. Appar. 2015, 19, 52–57. [CrossRef] 24. Conrado, L.A.L.; Munin, E. Reduction in body measurements after use of a garment made with synthetic fibers embedded with ceramic nanoparticles. J. Cosmet. Dermatol. 2011, 10, 30–35. [CrossRef] 25. Vigneshwaran, N.; Kumar, S.; Kathe, A.A.; Varadarajan, P.V.; Prasad, V. Functional finishing of cotton fabrics using zinc oxide–soluble starch nanocomposites. Nanotechnology 2006, 17, 5087–5095. [CrossRef] 26. Birol, H.; Rambo, C.R.; Guiotoku, M.; Hotza, D. Preparation of ceramic nanoparticlesvia cellulose-assisted glycine nitrate process: A review. RSC Adv. 2013, 3, 2873–2884. [CrossRef] Polymers 2021, 13, 686 9 of 9

27. Vatansever, F.; Hamblin, M.R. Far infrared radiation (FIR): Its biological effects and medical applications. Photon. Lasers Med. 2012, 1, 255–266. [CrossRef] 28. Hsu, P.-C.; Song, A.Y.; Catrysse, P.B.; Liu, C.; Peng, Y.; Xie, J.; Fan, S.; Cui, Y. Radiative human body cooling by nanoporous polyethylene textile. Science 2016, 353, 1019–1023. [CrossRef] 29. Faisal, A.M.; Salaün, F.; Giraud, S.; Ferri, A.; Chen, Y.; Wang, L. Characterization and thermographic analysis of fir emitting ceramic nanoparticle embedded films. In Proceedings of the 19th World Textile Conference-Autex, Ghent, Belgium, 11–15 June 2019; p. 7. 30. Pooley, M.A.; Anderson, D.M.; Beckham, H.W.; Brennan, J.F. Engineered emissivity of textile fabrics by the inclusion of ceramic particles. Opt. Express 2016, 24, 10556–10564. [CrossRef] 31. Anderson, D.M.; Fessler, J.R.; Pooley, M.A.; Seidel, S.; Hamblin, M.R.; Beckham, H.W.; Brennan, J.F. Infrared radiative properties and thermal modeling of ceramic-embedded textile fabrics. Biomed. Opt. Express 2017, 8, 1698–1711. [CrossRef] 32. Park, C.H.; Shim, M.H.; Shim, H.S. Far IR emission and thermal properties of ceramics coated fabrics by IR thermography. Key Eng. Mater. 2006, 849–852. [CrossRef] 33. Warner, S.B. Fiber Science; Pearson College Division: London, UK, 1995. 34. Tao, Y.; Li, T.; Yang, C.; Wang, N.; Yan, F.; Li, L. The influence of fiber cross-section on fabric far-infrared properties. Polymers 2018, 10, 1147. [CrossRef] 35. Halliday, D.; Resnick, R.; Bowen, G.H. Fundamentals of physics. Phys. Today 1972, 25, 53. [CrossRef] 36. Hsu, P.-C.; Liu, C.; Song, A.Y.; Zhang, Z.; Peng, Y.; Xie, J.; Liu, K.; Wu, C.-L.; Catrysse, P.B.; Cai, L.; et al. A dual-mode textile for human body radiative heating and cooling. Sci. Adv. 2017, 3, e1700895. [CrossRef][PubMed] 37. Kamiya, H.; Gotoh, K.; Shimada, M.; Uchikoshi, T.; Otani, Y.; Fuji, M.; Matsusaka, S.; Matsuyama, T.; Tatami, J.; Higashitani, K.; et al. Characteristics and behavior of nanoparticles and its dispersion systems. In Nanoparticle Technology Handbook; Elsevier: Amsterdam, The Netherlands, 2008; pp. 113–176. [CrossRef] 38. Hu, X.; Tian, M.; Qu, L.; Zhu, S.; Han, G. Multifunctional cotton fabrics with graphene/polyurethane coatings with far-infrared emission, electrical conductivity, and ultraviolet-blocking properties. Carbon 2015, 95, 625–633. [CrossRef] 39. Soares, A.R.; Pontón, P.I.; Mancic, L.; D’Almeida, J.R.M.; Romao, C.P.; White, M.A.; Marinkovic, B.A. Al2Mo3O12/polyethylene composites with reduced coefficient of thermal expansion. J. Mater. Sci. 2014, 49, 7870–7882. [CrossRef] 40. Salaün, F.; Vroman, I.; Bedek, G.; Lewandowski, M. Effects of microparticles on isotactic polypropylene: Thermomechanical and thermal properties. J. Polym. Sci. Part B Polym. Phys. 2008, 46, 2566–2576. [CrossRef] 41. Abbas, A.; Zhao, Y.; Wang, X.; Lin, T. Cooling effect of MWCNT-containing composite coatings on cotton fabrics. J. Text. Inst. 2013, 104, 798–807. [CrossRef]