Evaluation on Thermal Protection Performance of TiO₂@ATO Coated Aramid Nonwoven

Abstract

Aramid nonwoven (AN), with lightweight and flexible features, is highly attractive as a thermal insulation material to prevent or minimize skin burn damage. However, it has a finite protective effect due to the restriction of the polymer material and the surface hairiness of nonwoven fabrics. This study aimed to introduce different shapes of TiO₂ particles coated with Sn(Sb)O₂ (ATO) so as to promote protective performance by taking advantage of the high refraction of TiO₂@ATO in the visible light (Vis) and near-infrared (NIR) range. It is demonstrated that, compared with AN, the thermal protection effect of the obtained TiO₂@ATO-coated AN (TiO₂@ATO/AN) was significantly improved owing to the excellent radiation and conduction heat-shielding performance of the TiO₂@ATO coating. The micro-morphology, Vis–NIR spectral features, heat-shielding performances, and thermal protective performance (TPP) values of different composite nonwovens were measured to explore their thermal insulation mechanisms and performances. Compared with pristine AN and AN coated with commercial nano-ATO (ATO/AN), the heating rate of TiO₂@ATO/AN heated using an alcohol lamp was much lower. The maximum TPP values of TiO₂@ATO/AN were enhanced to 1457.874 kW·s/m², while the values for AN and ATO/AN were only 432.23 kW·s/m² and 945.054 kW·s/m², respectively. This study proposes an attractive solution for protecting fabrics against heat damage and opens up tremendous possibilities for heat-resistant nonwoven materials applied in the thermal protection field.

1. Introduction

As functional protective materials, heat insulation materials can ensure the safety of internal objects against elevated temperatures by hindering the processes of heating convection, heat conduction, and heat radiation. They are widely used in safety protection, such as flame-protective clothing [1], spacesuits [2], the external structures of buildings or tankers [3,4], and so forth. In practical terms, among the three heat transfer paths, namely, heating convection, heat conduction, and heat radiation, the latter is a kind of noncontact transfer mode that transfers energy between objects separated by electromagnetic wave radiation, causing maximum damage in high-temperature environments [5]. For instance, heat radiation takes up almost 80% of the total radiation energy of a burning fire. According to the information reported, the energy of the solar radiation spectrum tends to concentrate in the visible light (Vis) and near-infrared (NIR) ranges [6]. Wien’s displacement law recognizes that the ripple crest of black-body radiation tends to shift to a shortwave direction as temperature increases [7]. Similarly, the thermal radiation of a flame is also mainly shortwave radiation (wavelength: ≤2.5 µm). These findings indicated that the use of heat insulation materials is an effective approach to prevent heat injury by introducing a low-transmittance coating, especially one with high reflectivity, in the shortwave range as the outer layer of heat-protective materials. Additionally, thermal conductivity has a significant influence on the protective effect.

Over the years, two aspects have been considered to improve the heat protection performance of flame-retardant and heat insulation fabrics. One is the development of low thermal conductivity and a high specific heat capacity via the composite structure design of refractory fibrous materials, such as multi-layer structures [8], porous structures [9], and hollow structures [10]. However, it is unrealistic to use heavy multi-layer fabric and brittle porous aerogel for heat-protective clothing. Meanwhile, the preparation process of a hollow structure is complex and involves high costs. Hence, nonwoven materials with a porous, fibrous structure and good mechanical performance have been found to be ideal candidates for heat protection clothing [11]. By contrast, the use of a thermal barrier coating is a common approach for improving the protective effects of flame-retardant and heat insulation fabrics owing to the advantages of high thermal protection efficiency, simplicity, and low cost. For example, polymeric coatings with self-extinguishing abilities were used to prevent flame-induced damage to the inner material [12]; a coating with phase change materials also showed a cooling effect by phase transitions [13].

In addition, some heat-resistant inorganic materials have also been introduced as heat-insulating additives due to the advantages of high heat resistance, high stability, low cost, and no toxicity. Some of them could effectively inhibit heat conduction due to their low heat conductivity coefficients [14]. The others could achieve the same effect by reflecting off radiant heat, such as Sn(Sb)O₂ (ATO), Al₂O₃, TiO₂, and so forth [15,16]. For noncontact flame or sources of intense heat, a heat-insulating coat with high reflectivity plays a more significant role. Having the advantages of high-cost performance, visible light transmittance, and NIR diffuse reflection, the ATO film has become one of the most promising heat insulation materials. However, the high thermal conductivity and aggregation effect of nano-ATO powders in matrix restrict their widespread application in thermal insulation fields. On this basis, numerous studies have explored pathways to improve the heat resistance and increase the dispersity of ATO [17,18]. Another low-cost option for heat-barrier coating is TiO₂ which has a high refractive index and high heat resistance. However, as a typical photocatalytic semiconductor, TiO₂ may degrade the organic substrate, resulting in a loss of gloss and chalking; it can also shorten the life of heat protection coatings [19]. To solve this problem, inert substances should be used to cover TiO₂ particles so as to inhibit light-catalyzed reactions.

In this study, aramid nonwoven (AN) was used as a heat insulation inner lining and covered with a high-reflectivity coating by the knife coating method. For the sake of high reflectivity in the Vis–NIR range, high heat resistance, and no photocatalytic degradation, rodlike TiO₂@ATO and spherical TiO₂@ATO (R-TiO₂@ATO and S-TiO₂@ATO) particles with core–shell structures were applied as additives in heat insulation coatings. The radiative heat transfer was blocked effectively owing to the excellent NIR diffuse reflection of both ATO and TiO₂. Micro-nano TiO₂@ATO was easily dispersed in the coating using high shear force, especially for R-TiO₂@ATO. Moreover, the ATO-encapsulated TiO₂ matrix improved heat resistance considerably compared with nano-ATO. Experiments were performed to verify the improvements in thermal insulation performance and thermal protective performance (TPP) of the obtained coated nonwoven.

2. Experimental

2.1. Preparation of TiO₂@ATO and Coated Nonwoven with TiO₂@ATO/Silicone Coating

R-TiO₂@ATO powders with a core–shell structure were synthesized by the chemical co-deposition method, as reported in our previous work [20]. Specifically, TiO₂ whiskers were first prepared by hydrolysis reaction of tetraisopropyl orthotitanate (analytical grade, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) in an ethylene glycol medium in a modified microwave oven (700 W, 2.45 GHz) with a refluxing apparatus. After washing and drying, the decomposition and crystallization of hydrolysis products occurred at high temperatures. The conductive ATO layer was obtained after the synchronous hydrolysis reaction of tin and antimony salts (SnCl₄·5H₂O and SbCl₃) on TiO₂ whiskers and the calcination treatment of precipitates. As for S-TiO₂@ATO, a similar co-deposition process was conducted on TiO₂ nanoparticles (average diameter: 200 nm) purchased from Shanghai S&D Fine Chemical Institute Co., Ltd. (Shanghai, China). All chemicals (unless specifically noted otherwise) were analytic-grade reagents purchased from the Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, and used without further purification.

The laboratory-made TiO₂@ATO particles of different shapes and ATO (average diameter: 10–20 nm; Zhi-Ti Purification Technology Co., Ltd., Zhejiang, China) were transferred into silicone resin solution (part A of Ausbond 195, Shenzhen Ausbond Co., Ltd., Shenzhen, China) and mixed at a speed of 1000 rpm for 2 h using a high-speed and high-torque blender (WB3000-C, Wiggens, Baden-Württemberg, Germany). The curing agent (part B of Ausbond 195, Shenzhen Ausbond Co., Ltd., Shenzhen, China) was added to the homogenous solution at a ratio of 1:10 relative to the resin matrix. The mixture was continuously stirred for 20 min, and the solution was then casted on AN (Aramid 1313, 250 g/m², Qisheng New Material Technology Co., Ltd., Shandong, China) using a coating machine (ST-203DH, Sitai Instrument Co., Ltd., Beijing, China). After the blade coating process, the coated nonwoven was cured in the stove at 120 °C for 24 h. The concrete steps and the surface microstructures of coatings with different TiO₂@ATO are shown in Figure 1. All operations were carried out at room temperature. The basic specifications of coated nonwoven samples prepared with different parameters are listed in

Table 1. Parameters of fabric samples.

Fabric Code	Filler Type	Filler Content	Times of Blade Coating
A1	ATO	40 wt%	Twice
A2	S-TiO2@ATO	40 wt%	Twice
A3	R-TiO2@ATO	40 wt%	Twice
B1	S-TiO2@ATO	40 wt%	Once
B2	S-TiO2@ATO	40 wt%	Three times
C1	S-TiO2@ATO	30 wt%	Twice
C2	S-TiO2@ATO	50 wt%	Twice

.

The parameters of filler contents and coating times were chosen according to the concrete condition of application. With regard to filler content, when the content is too low, the radiation heat-shielding performance of the coating is dissatisfactory, while for excessive content, thermal conductive pathways would form by mutual contact of ATO. For coating times, the thickness of a coating depends on coating times. A longer coating time means a thicker coating and better heat insulation performance. Nevertheless, a thicker coating is more unfit to wear. Hence, a univariate analysis of appropriate parameters was utilized (see Table 1).

2.2. Characterization of TiO₂@ATO and Coated Nonwoven with TiO₂@ATO/Silicone Coating

A morphological analysis was performed using a scanning electron microscope (SEM, SU-1510, Hitachi, Tokyo, Japan) and a transmission electron microscope (TEM, FEI Tecnai F20, Hillsboro, TX, USA). The SEM was operated at an acceleration voltage of 10 kV and a working distance of 10.6 mm. Prior to the examination, the samples were gold sputter-coated under argon to render them electrically conductive. The crystalline phases of the nano-ATO and TiO₂@ATO coatings were identified using an X-ray diffractometer (XRD, D8 Advance, Bruker, Karlsruhe, Germany) with Cu-Ka radiation in the 2θ range of 10–80° and a step of 0.01°. A tensile testing machine (RTC1250A, A&D Co., Ltd., Tokyo, Japan) was used to investigate the mechanical properties of the coated nonwoven. The rectangular specimens were stretched at a speed of 5 mm/min at room temperature. UV–Vis–NIR diffuse reflectance and absorption spectra of TiO₂@ATO/silicone coatings were investigated using a Shimadzu UV-3600 spectrophotometer, with an integrating sphere attachment (Shimadzu ISR-3100, Kyoto, Japan).

2.3. Evaluation of TPP

The TPP value reflects the integrated effect on thermal radiation and the convection protective performance of the fabric. The larger the value, the better the protective performance achieved. The test process referenced the internationally recognized set of standards, such as NFPA 2112, ISO17492, ASTM F2703, and GB 8965-2009 [21]. The test apparatus (RPH-III, Fang Ke, Shanxi, China) is shown in Figure 2. The tester consisted of two Meker burners and nine heated quartz tubes, which worked as one radiative heat source and two convective heat sources, respectively. The convection heat source came from the combustion of propane. These heat sources could produce mixed heat fluxes consisting of convective and radiant heat fluxes. The tester was equipped with a water-cooled shutter to insulate the fabric samples from heat sources before testing and ensure accurate exposure times. A thermocouple temperature sensor was mounted on a piece of insulating board composing a calorimeter. The calorimeter was connected to the computer processing system.

As for the test process, the samples were cut in sizes of 15 cm × 15 cm and placed on the hollowed-out plane after instrument calibration. The coated side faced the heat source, and the other side was close to the calorimeter, which simulated the real wearing condition. The test was run until the calorimeter sensed a heat source that could cause a second-degree burn. The time to reach a second-degree burn was recorded and used to calculate TPP values of the different fabric samples together with heat flux. The calculation formula was as follows:

TPP = t_n × F

(1)

where t_n is the time to reach a second-degree burn and F is a nominal heat flux of (83 ± 2) kW/m².

3. Results and Discussion

3.1. Micro-Morphology of TiO₂@ATO

The TEM figures of R-TiO₂@ATO and S-TiO₂@ATO are displayed in Figure 3. The average diameter of these two kinds of TiO₂@ATO particles was approximately 200–250 nm. The length–diameter ratio of the R-TiO₂@ATO was more than 100. The inset shows that the thickness of the ATO layer was approximately 10 nm. It is well known that ATO has high near-infrared reflectance and optical transmittance, especially when assembled as a thin film. Consequently, internal TiO₂ with a high refractive index would achromatize the outer ATO layer. The cooperating results of TiO₂ and ATO particles with core–shell structures showed the promotion of reflectance in the visible range compared with pure ATO particles that would absorb visible energy from the heat source. As a result, the TiO₂@ATO coating might possess high visible reflection and a better thermal insulation effect.

The crystal structures of prepared TiO₂@ATO and ATO coatings are shown by their XRD patterns in Figure 3c. The standard rutile TiO₂ (PDF#21-1276) and SnO₂ (PDF#41-1445) patterns are shown above and below the samples, respectively. It was obvious that almost all the characteristic peaks of TiO₂ and ATO were perfectly indexed to the standard pattern, and the peak intensity of ATO was much lower than that of TiO₂, indicating the formation of abundant TiO₂ and few ATO particles. Moreover, the average size and the crystallinity of the polycrystalline particles of ATO were much lower than those of the TiO₂ particles. The reason underlying this phenomenon was the solid solution generated by Sb⁵⁺ substituting for Sn⁴⁺, reducing the purity of SnO₂, even without a significant variance in structure [22].

3.2. Mechanical Properties

The stress–strain curves in Figure 4 provide information about the deformation characteristics of coated nonwovens with different coating additives. TiO₂@ATO/AN with a content of 40 wt% displayed better comprehensive performance compared with other specimens. The reinforcing effect of coated nonwovens with different fillers was reflected in the modulus, breaking strength, and elongation results. The toughening effects were more obvious in specimens with S-TiO₂@ATO and ATO, while specimen failure was more brittle when rigid R-TiO₂@ATO was added. Theoretically speaking, the smaller the nano-filler, the stronger the reinforcement. However, the reinforcement property of ATO nanoparticles was similar to those of submicron S-TiO₂@ATO particles due to the aggregation in silicone resin coatings despite the much smaller diameter. Furthermore, the fracture toughness decreased with an increase in the content of TiO₂@ATO, followed by first a rise and then a fall, with an increase in the thickness of the coating layer. Qualitatively, the stress–strain curves confirmed good dispersion and strong filler–matrix interactions of TiO₂@ATO/AN achieved by the solution-mixing approaches. As for filler content, the fracture stress was raised and elongation at break was reduced as filler content increased. The change in coating thickness did not have such a significant impact on the mechanical properties of TiO₂@ATO/AN.

3.3. UV–Vis–NIR Spectrophotometric Analysis

Figure 5a illustrates the UV–Vis–NIR diffuse reflectance spectra of coated nonwovens with different fillers. The reflectivity of TiO₂@ATO/AN was much higher than that of the ATO coating in the range of 480–1500 nm, while the reverse was the case in the UV region. The maximum reflectivity of the R-TiO₂@ATO and S-TiO₂@ATO coatings was, respectively, 99.36% and 84.28%; it then gradually decreased with the increase in wavelength. The reflectivity of the R-TiO₂@ATO coating in the region of 1500–2500 nm was around 40%, which was also higher than that of the S-TiO₂@ATO and ATO coatings. As a result, R-TiO₂@ATO/AN exhibited a superior reflective insulation effect. The absorption spectra of coated nonwovens with different filler types are exhibited in Figure 5b. Despite the UV region being below 400 nm, TiO₂@ATO/AN in white color was shown to have low absorption in the Vis–NIR region. Specifically, in the wavelength range above 400 nm, the specimen with rodlike fillers had the lowest absorption and the low-absorption region was also broader among all specimens. However, the low absorbance for the S-TiO₂@ATO coating was in the range of 400–1500 nm. In contrast, the ATO-filled coating in dark-blue color presented a high absorption spectrum from 200 to 2500 nm.

In consequence, the temperature of ATO/AN might increase easily by heat absorption in the thermal environment. The result indicated that the TiO₂ core of TiO₂@ATO played a significant role in the reflectivity property, especially the rodlike TiO₂. The reason was probably because TiO₂, with high dispersion and a high refractive index, would reflect the light penetrating through the ATO nanolayer. More importantly, a relatively high length–diameter ratio of R-TiO₂@ATO rendered a better light-scattering effect, resulting in higher diffuse reflection spectra compared with those for S-TiO₂@ATO. Given the aforementioned UV–Vis–NIR spectral analysis, it was reasonable to conclude that the TiO₂@ATO coatings played a key role in thermal radiation resistance to provide thermal protection for the underlying AN.

3.4. Thermal Insulation Performances

Thermal insulation is another crucial index for practical applications of TiO₂@ATO/AN in the thermal protection area. A setup was intentionally designed to detect the top-surface temperature of the nonwovens on an alcohol lamp using a thermal infrared radiation image tester to visually assess the thermal insulation properties of composite specimens (Figure 6a). The heating curves of different samples are shown in Figure 6b. The temperature of the pristine AN on the flame quickly increased from room temperature (26.5 °C) to above 250 °C within 20 s, while the temperature of ATO/AN increased quickly in the first few seconds, which was even more than the heating rate of AN, and then the rate of increase slowed. In comparison, the temperature of TiO₂@ATO/AN slowly increased under heating, especially in the first few seconds. The difference in the heating rate in the first stage might have been caused by the difference in absorption rates, especially in the visible range. The infrared photos in Figure 6c were also used to further exhibit the thermal insulation of the AN before and after coating treatment. The maximum temperatures of different samples followed the order AN > ATO/AN > S-TiO₂@ATO/AN > R-TiO₂@ATO/AN. This result was primarily related to the reflection and absorption spectra in the UV–Vis–NIR range. The light in the visible and infrared regions contributed more than 90% of the total solar heat [23]. Hence, the reflection and absorption in the visible and infrared regions played key roles in thermal insulation performance and resulted in a temperature difference of approximately 84.5 °C between AN and R-TiO₂@ATO/AN in 20 s.

Low thermal conductivity was also advantageous for thermal insulation materials. The thermal conductivity primarily relied on the concentration and migration rate of free electrons [24], so that the thermal insulation property of TiO₂, which was a wide-bandgap semiconductor, was much higher than that of ATO with an effective carrier. As a result, the thermal insulation of TiO₂@ATO/AN was enhanced when TiO₂ was introduced. Eventually, the temperature of R-TiO₂@ATO/AN slowly increased to 170.8 °C in 20 s while that of ATO/AN increased to 204.7 °C for the same dosage. The enhanced thermal insulation property of TiO₂@ATO/AN provided favorable conditions for the thermal protection system.

3.5. TPP Analysis

The time to reach a second-degree burn and corresponding TPP values of different specimens are shown in

Table 2. TPP value of AN before and after coating treatment.

Fabric Code	tn (s)	TPP Value (kW·s/m2)
AN	5.31	432.23
A1	11.61	945.054
A2	13.51	1099.714
A3	13.31	1083.434
B1	8.41	684.574
B2	17.91	1457.874
C1	9.91	806.674
C2	14.21	1156.694

. The TPP value for AN was only 432.23 kW·s/m², while an obvious improvement was noted after the coating treatment because the time to reach a second-degree burn was extended. The result showed that the thickness of the coating, filler dosage, and filler type affected the TPP value. Although the thickness of the coating positively correlated with protection performance, it was impractical when lightweightness and breathability were taken into consideration in relation to wearability. A higher filler content was also beneficial for the TPP value within a certain dosage range. This trend was obvious at a low content, and the difference disappeared with a continuous increase in the content. In terms of the filler type, both R-TiO₂@ATO/AN and S-TiO₂@ATO/AN exhibited significant improvements in protection performance compared with ATO/AN, which was consistent with the thermal insulation performance of different specimens. Considering the large aspect ratio of R-TiO₂@ATO, the heat transfer paths were more easily established by overlapping with each other, resulting in better thermal conductivity of R-TiO₂@ATO/AN, which had a serious influence on the performance of thermal protective materials. Consequently, R-TiO₂@ATO/AN and S-TiO₂@ATO/AN exhibited similar TPP values despite the higher reflexivity of R-TiO₂@ATO/AN in the Vis–NIR spectral region.

The state of surface damage of different samples after the TPP test is shown in Figure 7. The heated area was severely carbonized owing to the relatively poor heat resistance of pristine AN with abundant surface hairiness. In contrast, the ATO coating on the AN surface was completely destroyed, and the TiO₂@ATO coating yielded various char layers. In comparison, both R-TiO₂@ATO/AN and S-TiO₂@ATO/AN with 40 wt% filler dosages exhibited minimized thermal damage, as evidenced by their high diffuse reflectance abilities. With an increase in the thickness and filler content of the coating layer, the damage to underlying AN became smaller. Meanwhile, agglomeration of powders and cracks occurred gradually on the char layers. The results clearly indicated that the TiO₂@ATO coating treatment significantly enhanced the TPP of AN, making it a good candidate in the thermal protection field.

4. Conclusions

In summary, core–shell TiO₂@ATO particles with high solar reflectance and low thermal conductivity were synthesized by depositing an ATO nanolayer on TiO₂. The incorporation of TiO₂ and ATO met the requirements of thermal insulation coatings with low thermal conductivity, high reflection, and thermal radiation stability. After coating TiO₂@ATO on the AN matrix, the as-prepared TiO₂@ATO/AN coating improved the mechanical, heating insulation, and thermal protection properties of AN. Specifically, the maximum diffuse reflectance of R-TiO₂@ATO/AN and S-TiO₂@ATO/AN was, respectively, 99.36% and 84.28% in the region of 1500–2500 nm. Compared with AN, the maximum temperature of R-TiO₂@ATO/AN dropped to about 84.5 °C after treatment with the alcohol lamp for 20 s. The result of a comprehensive investigation of TPP was consistent with the previous results and a remarkable improvement in protective effects was displayed after the TiO₂@ATO coating. This study provided an attractive solution for protecting garments against heat damage in fields related to aerospace and emergency rescue.