polymers

Article Enhanced Thermal Insulation of the Hollow Microsphere/ Fabric Textile

Jintao Sun 1 , Fei Cai 1, Dongzhi Tao 1, Qingqing Ni 1,2 and Yaqin Fu 1,*

1 Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China; [email protected] (J.S.); [email protected] (F.C.); [email protected] (D.T.) 2 Department of Mechanical Engineering & Robotics, Shinshu University, Ueda 386-8567, Japan; [email protected] * Correspondence: [email protected]

Abstract: Glass fiber fabrics/hollow glass microspheres (HGM)–waterborne polyurethane (WPU) textile composites were prepared using glass fiber, WPU, and HGM as skeleton material, binder, and insulation filler, respectively, to study the effect of HGM on the thermal insulation performance of glass fiber fabrics. Scanning electron microscopy, Instron 3367 tensile test instrument, thermal constant analysis, and infrared thermal imaging were used to determine the cross-sectional morphol- ogy, mechanical property, , and thermal insulation property, respectively, of the developed materials. The results show that the addition of HGM mixed in WPU significantly enhanced thermal insulation performance of the textile composite with the reduction of thermal conductivity of 45.2% when the volume ratio of HGM to WPU is 0.8 compared with that of material without HGM. The composite can achieve the thermal insulation effect with a temperature difference ◦ ◦  of 17.74 C at the temperature field of 70 C. Meanwhile, the tensile strength of the composite is  improved from 14.16 to 22.14 MPa. With these results, it is confirmed that designing hollow glass Citation: Sun, J.; Cai, F.; Tao, D.; Ni, microspheres (HGM) is an effective way to develop and enhance the high performance of insulation Q.; Fu, Y. Enhanced Thermal materials with an obvious lightweight of the bulk density reaching about 50%. Insulation of the Hollow Glass Microsphere/Glass Fiber Fabric Keywords: hollow glass microsphere (HGM); thermal insulation; glass fiber fabric; composites Textile Composite Material. Polymers 2021, 13, 505. https://doi.org/ 10.3390/polym13040505 1. Introduction Academic Editor: Andrea Zille Global warming has become a hot issue since its first proposal in the 1970s. Excessive Received: 18 January 2021 human activities, such as the use of fossil fuels, are one of the main causes [1–5]. Due to Accepted: 4 February 2021 Published: 7 February 2021 the limited energy sources and environmental degradation, energy saving has become compulsory [6]. Energy management through the use of thermal insulation materials,

Publisher’s Note: MDPI stays neutral which is an important method in alleviating the global warming trend, can effectively with regard to jurisdictional claims in reduce energy consumption [7]. Some organic insulation materials, such as expanded published maps and institutional affil- [8–10] and rigid polyurethane [11–14] foams, are flammable and fire hazards, iations. thereby limiting their application. By contrast, the use of inorganic thermal insulation materials has been increasing. The thermal conductivity of composite materials is usually reduced by adding thermal insulation fillers to achieve thermal insulation. The thermal conductivity of air, which is one of the materials with the lowest thermal conductivity in nature, is about 0.023 W/(m·K) [15]. Therefore, the hollow structure of the fillers as an Copyright: © 2021 by the authors. effective way is used for storing a large amount of air to facilitate the preparation of barrier Licensee MDPI, Basel, Switzerland. This article is an open access article thermal insulation materials. distributed under the terms and As a kind of high-temperature-resistant inorganic fibrous thermal insulation material, conditions of the Creative Commons glass fiber fabric is widely used in the chemical industry, construction, fire protection, and Attribution (CC BY) license (https:// other industrial fields [16]. However, as a traditional thermal insulation material, glass fiber creativecommons.org/licenses/by/ fabric has relatively high thermal conductivity, high density, and general thermal insulation 4.0/). performance [17]. Therefore, the enhancement of its thermal insulation performance and

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the reduction in its overall density for lightweight in reducing energy consumption have become a key issue that needs to be resolved. Hollow glass microspheres (HGM), which are spherical and lightweight inorganic capsule, are widely used in some fields, such as coatings [18], plastics [19], aerospace [20,21], storage [22,23], and building materials [24]. Having the advantages of isotropy, low density, good chemical stability, good moisture resistance, small and well-distributed internal stress, and low thermal conductivity, HGM have become a lightweight thermal insulation filler with a wide range of use and excellent performance [25–32]. The positive impacts of HGM addition on lowering the thermal conductivity and flammability of composites have been demonstrated [33–35]. The tiny spherical structure of HGM has the advantages of better fluidity, dispersibility, and isotropy, than flake-, needle-, or irregular-shaped fillers. The addition of HGM as a filler can effectively control the shrinkage of the product and dimensionally stabilize the product in all directions without warping. Combining HGM as a filler and a matrix material reduces the weight of the composite material and improves the thermal insulation performance [36]. Researchers have done a lot of work in related research by using HGM as fillers in combination with polypropylene, phenolic resin, epoxy resin, silicone rubber, high-density polyethylene, and other organic matrices to study the thermal insulation effect [37–42]. Besides, thermal insulation materials are generally able to withstand various physical and chemical changes and mechanical effects at high temperatures. Therefore, good mechanical properties are necessary for the use of thermal insulation composites. In this study, hollow glass microspheres are added to glass fiber fabric to incorpo- rate the superior features of HGM such as low density and low thermal conductivity in conventional textile products to achieve the preparation of lightweight thermal insulation textile composite materials. A series of textile composites with different HGM/waterborne polyurethane(WPU) volume ratios were prepared, and the mechanical properties and thermal insulation properties were evaluated.

2. Materials and Methods 2.1. Materials HGM were purchased from Minnesota Mining and Manufacturing, St. Paul, MN, USA. The physical properties of HGM are shown in Table1. Glass fiber fabrics ( ρ = 1.1 g/cm3) and WPU (solid content = 46%) were purchased commercially. Ethanol was purchased from Aladdin Industrial Corporation, Shanghai, China. All chemicals were used as received without further purification.

Table 1. Physical properties of the hollow glass microspheres.

Density Compressive Strength Thermal Conductivity Model Color (g/cm3) (MPa) (W/(m·K)) VS5500 0.38 37.9 0.127 white

2.2. Methods The masses of HGM required were calculated by different ratios. After washing thrice with ethanol, HGM were dried in a 60 ◦C blast-drying oven. Figure1 shows the preparation of the textile composite. HGM and WPU were mixed, and the mixture was stirred slowly for a while. The ratio of HGM to WPU was referred to as the volume ratio. The mixture was evenly coated on the surface of glass fiber fabrics with a thickness of 0.7 mm through the coating method. The vacuum drying box was used to remove the bubbles from the textile composite. The textile composite was placed in an oven maintained at 60 ◦C for 4–6 h and cooled to room temperature. Afterward, the textile composite could be processed into the required sizes, and the corresponding structural performance was observed. The thickness of the final textile composite was controlled at 2 mm. The volume content of the glass fiber fabric in the textile composite was 35%. Polymers 2021, 13, x FOR PEER REVIEW 3 of 11

Polymers 2021, 13, 505processed into the required sizes, and the corresponding structural performance was ob- 3 of 10 served. The thickness of the final textile composite was controlled at 2 mm. The volume content of the glass fiber fabric in the textile composite was 35%.

Figure 1. PreparationFigure 1. Preparation of glass fiber of fabric/glass fiber hollow fabric/ glass hollow microspheres–waterborne glass microspheres–waterborne polyurethane polyurethane thermal insulation textile composites.thermal insulation textile composites.

2.3. Characterization2.3. Characterization The surface morphologiesThe surface morphologiesof HGM and the of HGM manufactured and the manufactured samples were samples observed were observed us- using field-emissioning field-emission scanning electron scanning microscopy electron microscopy(FESEM, ZEISS (FESEM, VLTRA-55, ZEISS VLTRA-55,Jena，Ger- Jena, Germany). many). The particleThe particlesize of HGM size of was HGM measured was measured using a laser using particle a laser particlesize analyzer size analyzer (Mas- (Mastersizer tersizer 2000, Malvern2000, Malvern City, United City, UnitedKingdom). Kingdom). The The viscosityof the HGM–WPU of the HGM–WPU mixture mixture with with different proportionsdifferent proportions was analyzed was using analyzed a rotary using rheo a rotarymeter (Thermo rheometer Fisher (Thermo Scien- Fisher Scientific, tific, Waltham, Waltham,MA, USA). MA, The USA). thermal The conductivity thermal conductivity of the textile of thecomposite textile compositewas meas- was measured ured using a thermalusing constant a thermal analyzer constant (TPS analyzer 2500S, (TPS Shanghai 2500S, K-Analysis Shanghai K-AnalysisCo., Ltd, Shang- Co., Ltd., Shanghai, hai, China). TheChina). drainage The method drainage was method used was to usedmeasure to measure the bulk the density bulk density of the of textile the textile composite. composite. TheThe tensile tensile strength strength was was investigated investigated by by a universal a universal tensile tensile testing testing machine machine (Instron-3367, (Instron-3367, Instron,Instron, Norwood,Norwood, MA, MA, USA) USA) at roomat room temperature, temperature, with with a cross-head a cross-head speed of 20 mm/min. speed of 20 mm/min.The infrared The infrared thermal thermal imaging imaging (InfraTec, (InfraTec, Dresden, Dresden, Germany) Germany) was used was to test the thermal used to test the insulationthermal insulation performance performanc of thee textile of the composite. textile composite. The working The working distance dis- was 40 cm. The tance was 40 cm.temperature The temperature of the sampleof the sample was detected was detected and recorded and recorded using a using thermocouple a ther- connected to mocouple connecteda temperature to a temperature controller. controller. 3. Results and Discussion 3. Results and Discussion The diameter of the commercially purchased HGM was not uniform, and the particle The diameter of the commercially purchased HGM was not uniform, and the particle size distribution was relatively wide, as shown in Figure2a,b. A laser particle size analyzer size distribution was relatively wide, as shown in Figure 2a,b. A laser particle size ana- was used to determine the particle size distribution of HGM. Figure3 shows that the lyzer was used to determine the particle size distribution of HGM. Figure 3 shows that particle size distribution of VS5500 HGM was 10–100 µm, which was approximately the particle size distribution of VS5500 HGM was 10–100 μm, which was approximately normal. Figure2c shows that the cavity of HGM contained air and some smaller HGM normal. Figure with2c shows multicapsules. that the cavity This special of HGM structure contained allowed air and the externalsome smaller heat to HGM be reflected multiple with multicapsules.times This inside special the HGM,structure and allowed part of the the external heat entered heat to thebe reflected smaller capsulemulti- in the HGM, ple times insidewhich the HGM, further and reduced part of thethe heatheat transferentered efficiencythe smaller and capsule enhanced in the the HGM, thermal insulation which further reducedperformance the heat of the transfer textile composite.efficiency and The enhanced damage in the Figure thermal2d shows insulation that the HGM had a performance ofthin the textile wall and composite. a large cavity The da volume,mage in which Figure had 2d great shows advantages that the HGM as a barrier had type thermal a thin wall and insulationa large cavity filler. volume, which had great advantages as a barrier type ther- mal insulation .

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Figure 2. FESEM images of hollow glass microspheres at the resolution of (a) 100×, (b) 200×, and Figure 2. FESEM images of hollow glass microspheres at the resolution of (a) 100×,× (b) 200×,× and (c,d)Figure 1000×. 2. FESEM images of hollow glass microspheres at the resolution of (a) 100 ,(b) 200 , and (c(c,d,d)) 1000 1000×.×.

Figure 3. The particle size distribution of hollow glass microspheres. Figure 3. The particle size distribution of hollow glass microspheres. Figure 3. The particle size distribution of hollow glass microspheres. The viscosity of the HGM–WPU mixture with different ratios was determined at ◦ 20TheC viscosityThe Figure viscosity4 showsof the of HGM–WPU thethe relationshipHGM–WPU mixture betweenmixture with thewithdifferent viscosity different ratios and ratios was the determinedwas rotation determined time at of20 at the 20 °C Figuremixture°C Figure 4 shows at different4 shows the relationship the formulations. relationship between As between the the volume viscosity the viscosity fraction and of theand HGM rotation the inrotation the time mixture timeof the of increased, mix- the mix- turethe tureat different viscosity at different formulations. of the formulations. HGM–WPU As the As mixture volume the volume gradually fraction fraction of increased. HGM of HGM in Thisthe in mixture findingthe mixture increased, also increased, indicates the viscositythatthe viscosity the fluidityof the of HGM–WPU the of theHGM–WPU mixture mixture graduallymixture graduall graduall decreased,y increased.y increased. which This was Thisfinding notfinding conducivealso indicatesalso indicates to the thatprocessing thethat fluidity the fluidity of of composite the of mixture the mixture materials. gradually gradually The decreased, same decreased, conclusion which which canwas be notwas obtained conducive not conducive during to the the to pro- the sample pro- cessingpreparationcessing of composite of composite process. materials. With materials. increasing The sameThevolume same conclusion conclusion fraction can of becan HGM, obtained be obtained stirring during the during mixture the sample the became sample preparationdifficult.preparation process. Moreover, process. With the With increasing coating increasing became volume volume difficult, fraction fraction thereby of HGM, of affecting HGM, stirring thestirring the final mixturethe molding mixture be- effect be- cameofcame difficult. the compositedifficult. Moreover, Moreover, material. the coating the The coating WPU became hadbecame difficult, an average difficult, thereby viscosity thereby affecting ofaffecting 1711 the mPa final the· s finalmolding and molding certain effectfluidity.effect of the of When compositethe composite the volume material. material. ratio The of HGM/WPUWPUThe WPU had anha was daverage an 1, average the viscosity average viscosity viscosityof 1711 of 1711mPa·s of the mPa·s mixtureand and · certainreachedcertain fluidity. fluidity. 5845 When mPa When thes, resulting volume the volume inratio difficulty ratioof HGM/WPU of inHGM/WPU preparing was 1,was composite the 1, average the average materials viscosity viscosity by of using the of the the mixturecoatingmixture reached method. reached 5845 5845mPa·s, mPa·s, resulting resulting in difficulty in difficulty in preparing in preparing composite composite materials materials by by usingusing the coating the coating method. method.

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FigureFigure 4. 4. TheThe viscosity viscosity of the mixture with differentdifferent volumevolume ratiosratios ofof hollowhollow glass glass microspheres microspheres and andwaterborne waterborne polyurethane. polyurethane.

CommercialCommercial HGM HGM are are generally low in mechanical property and prone to cracking underunder pressure. After mixingmixing HGMHGM withwith the the resin, resin, the the flexibility flexibility of of the the resin resin can can be be used used to torelieve relieve the the cracks cracks caused caused by pressureby pressure to a certainto a certain extent. extent. As a result,As a result, WPU actedWPU asacted a binder as a binderand protected and protected HGM. TheHGM. cross-sectional The cross-sectional morphology morphology of the textile of the composite textile composite is shown inis shownFigure 5in. WhenFigure the 5. ratioWhen of the HGM/WPU ratio of HG wasM/WPU only 0.1,was HGMonly 0.1, were HGM completely were completely wrapped wrappedin WPU andin WPU evenly and dispersed evenly dispersed throughout througho the resinut the matrix resin matrix [43] (Figure [43] (Figure5b). As 5b). such, As such,HGM HGM did not did break not break during during the preparation the preparatio of then cross-sectionof the cross-section sample sample for FESEM. for FESEM. As the Asvolume the volume fraction fraction of HGM of increased, HGM increased, the probability the probability of contact of increased. contact increased. Collision Collision was easy wasand easy caused and HGM caused to rupture,HGM to thereby rupture, affecting thereby theaffecting thermal the insulation thermal insulation effect in the effect textile in thecomposite. textile composite. Insufficient Insufficient resin was presentresin was for present protection for protection due to the excessivedue to the number excessive of numberHGM, resulting of HGM, in resulting the rupture in ofthe the rupture majority of ofthe HGM majority (Figure of 5HGMd). Figure (Figure6 shows 5d). theFigure bulk 6 showsdensity the of bulk textile density composites of textile with composites different ratios. with different When the ratios. ratio reachedWhen the 0.8, ratio the reached volume 3 0.8,density the volume of the textile density composite of the textile (0.54 g/cmcomposite) was (0.54 reduced g/cm by3) 50.9%was reduced compared by 50.9% with that com- of 3 paredmaterial with without that of HGM material (1.1 without g/cm ). HGM The density (1.1 g/cm of HGM3). The was density smaller of HGM than thatwas ofsmaller other thancomponents that of other in the components composite. With in the an composite. increased volume With an fraction increased of HGM, volume the fraction content of HGM,other components the content decreasedof other components under a certain decrea thickness,sed under decreasing a certain the thickness, overall density decreasing of the thecomposite, overall density which is of advantageous the composite, for which the preparation is advantageous of lightweight for the preparation insulation materials. of light- weightMoreover, insulation during materials. the preparation Moreover, of the during textile the composite, preparation the mechanical of the textile mixing composite, of the theHGM mechanical and WPU mixing generated of the a small HGM number and WPU of bubbles. generated Although a small a number vacuum of drying bubbles. box Alt- was houghused to a removevacuum some drying bubbles box was during used the to remo coatingve some molding bubbles process, duri someng the bubbles coating remain, mold- ingthereby process, decreasing some bubbles the bulk remain, density thereby of the composite decreasing material. the bulk density of the composite Figure7 shows the effect of the different volume ratios of HGM/WPU on the thermal material. conductivity of the material. As the volume fraction of HGM increased, the thermal conductivity of the textile composite decreased. At volume ratios of 0.4 and 0.8, the thermal conductivity values of the textile composite were 0.1411 and 0.1154 W/(m·K), respectively, thereby showing a reduction of 32.9% and 45.2%, respectively, compared with that of the material without HGM (0.2104 W/(m·K)). These results indicate that the addition of HGM affected the improvement of thermal insulation performance. However, when the ratio exceeded 0.8, increasing the content did not result in a considerable decrease in the thermal conductivity of the textile composite. Solid heat conduction, heat radiation between adjacent hollow microspheres, and heat convection inside the hollow microspheres occur in the prepared textile composite. When the content of HGM reaches the limit, the three main heat transfer paths in the textile composite no longer changes, and the overall insulation performance of the material remains unchanged [44,45].

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Figure 5. Cross-section FESEM images of composites with hollow glass microsphere/ waterborne Figure 5. Cross-section FESEM images of composites with hollow glass microsphere/ waterborne polyurethaneFigure 5. Cross-section volume ratios FESEM of (a) 0, images (b) 0.1, of (c composites) 0.4, and (d with) 0.7.hollow glass microsphere/waterborne polyurethane volume ratios of (a) 0, (b) 0.1, (c) 0.4, and (d) 0.7. polyurethane volume ratios of (a) 0, (b) 0.1, (c) 0.4, and (d) 0.7.

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FigureFigure 6. Effects 6. Effects of ofthe the hollow hollow glass glass microspheres microspheres volu volumeme fraction fraction on on the the bulk bulk density density of ofthe the tex- textile Figure 6. Effects of the hollow glass microspheres volume fraction on the bulk density of the tex- tilecomposite. composite. tile composite. Figure 7 shows the effect of the different volume ratios of HGM/WPU on the thermal Figure 7 shows the effect of the different volume ratios of HGM/WPU on the thermal conductivity of the material. As the volume fraction of HGM increased, the thermal con- conductivity of the material. As the volume fraction of HGM increased, the thermal con- ductivity of the textile composite decreased. At volume ratios of 0.4 and 0.8, the thermal ductivity of the textile composite decreased. At volume ratios of 0.4 and 0.8, the thermal conductivity values of the textile composite were 0.1411 and 0.1154 W/(m·K), respectively, conductivity values of the textile composite were 0.1411 and 0.1154 W/(m·K), respectively, thereby showing a reduction of 32.9% and 45.2%, respectively, compared with that of the thereby showing a reduction of 32.9% and 45.2%, respectively, compared with that of the material without HGM (0.2104 W/(m·K)). These results indicate that the addition of HGM material without HGM (0.2104 W/(m·K)). These results indicate that the addition of HGM affected the improvement of thermal insulation performance. However, when the ratio affected the improvement of thermal insulation performance. However, when the ratio exceeded 0.8, increasing the content did not result in a considerable decrease in the ther- exceeded 0.8, increasing the content did not result in a considerable decrease in the ther- mal conductivity of the textile composite. Solid heat conduction, heat radiation between mal conductivity of the textile composite. Solid heat conduction, heat radiation between adjacent hollow microspheres, and heat convection inside the hollow microspheres occur adjacent hollow microspheres, and heat convection inside the hollow microspheres occur in the prepared textile composite. When the content of HGM reaches the limit, the three in the prepared textile composite. When the content of HGM reaches the limit, the three main heat transfer paths in the textile composite no longer changes, and the overall insu- Figuremain heat 7. Effects transfer of hollow paths glassin the microspheres textile composite volume no fraction longer on changes, the thermal and conductivity. the overall insu- The Figurelation 7.performance Effects of hollow of the glass material microspheres remains volume unchanged fraction [44,45]. on the ther mal conductivity. The insertlation image performance is a thermal of insulationthe material mechanism remains diagram unchanged of the [44,45]. textile composite. insert image is a thermal insulation mechanism diagram of the textile composite.

Thermal insulation composite materials must not only have excellent thermal insu- lation properties but also have good mechanical properties. The textile composites of the different volume ratios (0–0.8) and the same size have been prepared to investigate the effect of the volume ratios of HGM/WPU on the mechanical properties. A dumbbell- shaped cutter was used to cut the textile composite into a dumbbell-shaped specimen (25 mm × 4 mm × 2mm), and the mechanical properties were tested using a universal tensile testing machine (Instron-3367, Instron, Norwood, MA, USA) under a tensile speed of 20 mm/min at room temperature. At least five effective specimens were tested for each sam- ple. As seen from Figure 8, as the volume ratio increased, the tensile strength of the textile composite gradually increased. When the volume ratio reached 0.6, the best tensile strength was 22.14 MPa, which was 56.3% higher than that of material without HGM. The tensile strength of composite materials depended on the combined effect of WPU and HGM. When the volume ratio was 0, the main role was mainly the WPU matrix. As the amount of HGM continued to increase, the role of HGM continued to highlight. From the stress-strain diagram, it could be concluded that when the volume ratio was 0.6, the bond- ing between the water-based polyurethane mixed with hollow glass microspheres and the glass fiber fabric achieved a good effect. As the volume ratio continued to increase to 0.8, the tensile strength was greatly reduced, even lower than the glass fiber fabric with only WPU added (14.16 MPa). When the amount of HGM added reached a certain value, the combination of WPU and HGM reached saturation. Moreover, when the volume ratio reached 0.8, stirring of the mixture became particularly difficult, more bubbles were gen- erated, and the coating effect was poor. Therefore, the tensile strength of the textile com- posite materials had a downward trend.

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Thermal insulation composite materials must not only have excellent thermal insula- tion properties but also have good mechanical properties. The textile composites of the different volume ratios (0–0.8) and the same size have been prepared to investigate the effect of the volume ratios of HGM/WPU on the mechanical properties. A dumbbell- shaped cutter was used to cut the textile composite into a dumbbell-shaped specimen (25 mm × 4 mm × 2 mm), and the mechanical properties were tested using a universal tensile testing machine (Instron-3367, Instron, Norwood, MA, USA) under a tensile speed of 20 mm/min at room temperature. At least five effective specimens were tested for each sample. As seen from Figure8, as the volume ratio increased, the tensile strength of the textile composite gradually increased. When the volume ratio reached 0.6, the best tensile strength was 22.14 MPa, which was 56.3% higher than that of material without HGM. The tensile strength of composite materials depended on the combined effect of WPU and HGM. When the volume ratio was 0, the main role was mainly the WPU matrix. As the amount of HGM continued to increase, the role of HGM continued to highlight. From the stress-strain diagram, it could be concluded that when the volume ratio was 0.6, the bonding between the water-based polyurethane mixed with hollow glass microspheres and the glass fiber fabric achieved a good effect. As the volume ratio continued to increase to 0.8, the tensile strength was greatly reduced, even lower than the glass fiber fabric with only WPU added (14.16 MPa). When the amount of HGM added reached a certain value, the combination of WPU and HGM reached saturation. Moreover, when the volume ratio reached 0.8, stirring Polymers 2021, 13, x FOR PEER REVIEW 8 of 11 of the mixture became particularly difficult, more bubbles were generated, and the coating effect was poor. Therefore, the tensile strength of the textile composite materials had a downward trend.

FigureFigure 8. 8.(a)( aTypical) Typical stress-strain stress-strain curves curves and and (b ()b tensile) tensile strength strength of of textile textile composites composites with with differ- different ent hollow glass microspheres/waterborne polyurethane volume ratios. (c) Digital photo and (d) hollow glass microspheres/waterborne polyurethane volume ratios. (c) Digital photo and (d) SEM SEM image of the cross-section after a tensile fracture. image of the cross-section after a tensile fracture.

AtAt volume volume ratios ratios of 0.9 of 0.9and and 1.0, 1.0,the thetherma thermall conductivity conductivity of the of textile the textile composite composite re- mainedremained stable, stable, and the and thermal the thermal insulation insulation performance performance did not did change not changeremarkably. remarkably. As a result,As a only result, samples only sampleswith a volume with a ratio volume of 0–0.8 ratio were of 0–0.8selected were for selected the thermal for theinsulation thermal performanceinsulation performancetest. At ambient test. temperature At ambient (23 temperature °C) and relative (23 ◦humidityC) and relative of 50%, humidity the textile of composite50%, the was textile placed composite on a constant-temperatu was placed on a constant-temperaturere heating table. When heating the temperature table. When of the thetemperature sample surface of the became sample stable surface after becamea few minutes, stable afterthe images a few minutes,were obtained the images under werethe infrared thermal imaging system and are presented in Figure 9a. As summarized in Figure 9b, the temperature difference between the surface of the same composite material sample and the heating stage gradually increased with increasing test temperature. At a temper- ature field of 70 °C and a ratio of 0.8, the composite material achieved a temperature dif- ference of 17.74 °C, which was improved by 235% compared with that of the material without HGM. No evident artificial bubble was observed in the prepared composite ma- terial. When the bubble diameter was less than 4 mm, no natural convection was observed in the bubble. Thus, the influence of the bubble generated during the preparation process on the heat convection could be ignored. Similarly, the diameter of HGM we selected was basically below 0.1 mm, and the natural convection of gas in the glass microspheres could be ignored. Besides, polymer composites are usually tested at low temperatures, and the proportion of heat radiation in the total heat transfer is extremely small. Therefore, the solid heat transfer has the greatest effect on the thermal insulation performance [46]. At volume ratios of 0.4 and 0.5, the textile composite had densities of 0.697 and 0.690 g/cm3, respectively and thermal conductivities of 0.1411 and 0.1408 W/(m·K), respectively. Thus, the thermal conductivity of textile composites was almost the same when the bulk density was similar.

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obtained under the infrared thermal imaging system and are presented in Figure9a. As summarized in Figure9b, the temperature difference between the surface of the same composite material sample and the heating stage gradually increased with increasing test temperature. At a temperature field of 70 ◦C and a ratio of 0.8, the composite material achieved a temperature difference of 17.74 ◦C, which was improved by 235% compared with that of the material without HGM. No evident artificial bubble was observed in the prepared composite material. When the bubble diameter was less than 4 mm, no natural convection was observed in the bubble. Thus, the influence of the bubble generated during the preparation process on the heat convection could be ignored. Similarly, the diameter of HGM we selected was basically below 0.1 mm, and the natural convection of gas in the glass microspheres could be ignored. Besides, polymer composites are usually tested at low temperatures, and the proportion of heat radiation in the total heat transfer is extremely small. Therefore, the solid heat transfer has the greatest effect on the thermal insulation performance [46]. At volume ratios of 0.4 and 0.5, the textile composite had densities of 0.697 and 0.690 g/cm3, respectively and thermal conductivities of 0.1411 and Polymers 2021, 13, x FOR PEER REVIEW 9 of 11 0.1408 W/(m·K), respectively. Thus, the thermal conductivity of textile composites was almost the same when the bulk density was similar.

FigureFigure 9. 9. (a)( Infrareda) Infrared thermal thermal images images of textile of textile comp compositesosites with different with different hollow hollow glass micro- glass micro- spheres/waterbornespheres/waterborne polyurethane polyurethane volume volume ratios. ratios. ( (bb)) The The thermal thermal insulation insulation performance ofof compos-com- positesites with with different different hollow hollow glass glass microspheres/waterborne microspheres/waterborne polyurethane polyurethane volume volume ratios ratios at at different differ- enttemperatures. temperatures.

4.4. Conclusions Conclusions InIn summary, summary, HGM HGM were were added added to the to the glass glass fiber fiber fabric fabric through through a simple a simple coating coating pro- cess,process, successfully successfully preparing preparing lightweight lightweight ther thermalmal insulation insulation textile textile composite composite materials. materials. TheThe thermal thermal conductivity conductivity of of the the textile textile comp compositeosite was was reduced reduced by by 45.2% 45.2% when when the the volume volume ratioratio of of HGM HGM to to WPU WPU was was 0.8. 0.8. The The composite composite with with an an obvious obvious lightweight lightweight of of the the bulk bulk density reaching about 50% could achieve the thermal insulation effect with a temperature density reaching about 50% could achieve the thermal insulation effect with a temperature difference of 17.74 ◦C at the temperature field of 70 ◦C. Meanwhile, the tensile strength of difference of 17.74 °C at the temperature field of 70 °C. Meanwhile, the tensile strength of the composite was improved from 14.16 to 22.14 MPa. This work may provide new ideas the composite was improved from 14.16 to 22.14 MPa. This work may provide new ideas for the preparation of new thermal insulation materials in the future. for the preparation of new thermal insulation materials in the future. Author Contributions: Conceptualization, J.S.; methodology, J.S. and D.T.; software, J.S., F.C. and Author Contributions: Conceptualization, J.S.; methodology, J.S. and D.T.; software, J.S., F.C. and D.T.; validation, F.C.; formal analysis, F.C.; data curation, J.S. and F.C.; writing—original draft prepa- D.T.; validation, F.C.; formal analysis, F.C.; data curation, J.S. and F.C.; writing—original draft prep- ration, J.S.; writing—review and editing, Q.N. and Y.F.; supervision, Y.F.; project administration, Y.F.; aration, J.S.; writing—review and editing, Q.N. and Y.F.; supervision, Y.F.; project administration, funding acquisition, Y.F. All authors have read and agreed to the published version of the manuscript. Y.F.; funding acquisition, Y.F. All authors have read and agreed to the published version of the manuscript.Funding: This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No. LY19E030010). Funding: This work was supported by the Natural Science Foundation of Zhejiang Province (Grant No.Institutional LY19E030010). Review Board Statement: Not applicable. 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: This work was supported by the Natural Science Foundation of Zhejiang Prov- ince (Grant No. LY19E030010). The authors are grateful to the scholars cited in this paper, and their research results have given a lot of inspiration and reference to this work. Conflicts of Interest: The authors declare no conflict of interest.

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