Investigation of the Thermal Protective Performance of Shape Memory Fabric System: Effect of Moisture and Position of Shape Memory Alloy

Abstract

A prototype of temperature-responsive protective fabric assembly with shape memory alloy (SMA) spring was developed. The effect of moisture on the thermal protective performance of fabric was investigated under radiant heat exposure and hot surface contact. The thermal liner of fabric system was pretreated with moisture amount of 25%, 50%, and 100%. Meanwhile, the thermal protection of fabric assembly with SMA springs in different positions between the fabric layers was explored. The results showed that moisture above 25% had a positive influence on thermal protective performance of both traditional and SMA fabric assembly under two hazardous environments. The effect of moisture in SMA fabric assembly was more remarkable than that in fabric without spring. And the SMA spring located between thermal liner and moisture barrier provided better thermal protective performance. The research findings will be beneficial for manufacturing high-performance temperature-responsive fabric.

Keywords

shape memory alloy thermal protective clothing thermal protective performance moisture content

Thermal hazards faced by firefighters and emergency rescuers often include flame, radiation, and hot surface contact (HSC; Holmer, 2006; Sun et al., 2000). For common firefighting and rescue without entering flash fire, thermal radiation produced by fires is in the range of 5–10 kW/m² (Foster & Roberts, 1995). However, in an emergency like flash fire, it may reach up to 200 kW/m² (Holcombe, 1981; Schoppee et al., 1986). According to Abbott and Schulman (1976), the thermal environment faced by firefighters is mainly divided into three categories: normal, dangerous, and emergency situations. Firefighters are exposed to a low radiation condition up to 80% of the working time, which causes the majority of burn injuries compared with occasional HSC and flash fire (Lu et al., 2014; Song et al., 2010). Traditional multilayer protective clothing has low air permeability, which might cause low flexibility and too much metabolic heat to be stored in the body (Lu et al., 2013, 2015). When heat stress exceeds human physiological endurance, pathological damage—such as heat exhaustion, heat syncope, collapse, and heat cramps—may threaten human life (Wang, 2006). Therefore, thermal protective clothing (TPC) should not only provide basic functions but also balance the contradiction (i.e., enhancing thermal protective performance [TPP] and reducing physiological burden). This requires that TPC be dynamically adaptable to external environmental change.

Shape memory material is an important research development in the field of intelligent material science in recent years; it has been widely used in smart textiles in the form of shape memory alloy (SMA) and shape memory polymer (Gök et al., 2015). In 1999, Congalton (1999) presented a completely new solution for thermal insulation at high environmental temperatures. In his study, SMA springs made by NiTi alloy with two actuation temperatures (T _a; i.e., 45 °C and 57 °C) were each embedded in TPC. The springs effectively reduced the temperature of internal fabric, and the springs with a T _a of 45 °C showed superior thermal insulation. Hendrickson (2011) placed shape memory rings (SMR) into a fabric system to form a 7-mm air gap, extending the rescue time for an additional 30 s. Hendrickson further constructed a heat transfer model for an SMR fabric system to predict its protective performance. Subsequently, White (2012) validated Hendrickson’s model and investigated the effects of the SMR position between the fabric layers and the T _a of SMR (50 °C and 90 °C). White (2012) found the protective performance was improved when SMR was placed in the external surface of the moisture barrier, and the SMR’s T _a of 45 °C showed a good potential to decrease heat transfer to human skin, which was consistent with Congalton’s (1999) finding. Yates (2012) further performed human trial tests based on Hendrickson’s study. The subjects dressed the TPC with an assembly of SMR in the pockets at shoulder and upper arm and placed the TPC in a combustion chamber. The SMR significantly reduced the heat flux and the temperature increase rate of TPC, reducing the risk of burns. Park et al. (2012) incorporated two-way NiTi SMA springs into a thermal liner and found that the springs caused around a 30% increase in protection time. Bartkowiak et al. (2017) suggested that the T _a of SMA should be in the range of 50–60 °C when they were applied in clothing to prevent thermal radiation, convection, and molten metal splashes in the industry. In recent years, Ma et al. (2017) evaluated the fabric combination incorporated with SMA springs in six different arrangements under HSC and found that the arrangement mode of springs had a great impact on the protective performance. He et al. (2018) demonstrated the impacts of exposure condition, arrangement mode, and SMA size on thermal protection of flame-retardant fabric systems with SMA springs. Salej et al. (2019) designed weft-knitted fabrics made from commercially available NiTi monofils with a T _a of 75 °C, which could be used in firefighting protective clothing to protect the human skin from overheating or burns.

The TPP of TPC may be affected by both external moisture and internal moisture from the body’s perspiration. Moisture increases the thermal conductivity and heat capacity (Keiser & Rossi, 2008) and thus affects heat transfer through the fabric. Lee and Barker (1986) found that thermal radiation intensity affected the thermal protection of a humid, single-layer fabric. With exposure to a radiation of 21 kW/m², moisture had a positive effect on the thermal protection, but an opposite effect was observed under radiation of 84 kW/m². Keiser and Rossi (2008) used temperature measurements to predict the evaporation speed within multilayer protective clothing. The temperatures within the clothing layers containing a wet layer never rose higher than the temperatures within dry clothing. Zhang et al. (2018) investigated the influence of moisture on TPP using a TPP tester and found that a moisture content of 15% had the greatest negative impact on thermal protection. Barker et al. (2006) discovered that a certain amount of moisture would improve the TPP of fabric system, and 15%–20% water would cause an early burn injury in a low radiation of 6.3 kW/m². Lawson et al. (2004) found that under flame exposure, external moisture decreased heat transfer through the fabric, whereas internal moisture did the opposite, increasing heat transfer. Under low-level radiant heat exposure (RHE), internal moisture decreased heat transfer. Wang et al. (2012) demonstrated that moisture increased the TPP of a fabric system when the air gap between outer shell and moisture barrier was less than 4 mm.

Taken together, previous researchers have shown through their studies the potential for the application of SMA in TPC and investigated related influencing factors. However, only a dry fabric system has been investigated; whether an SMA fabric system with different moisture contents could provide similar performance is unclear. Further, there is a lack of investigation of SMA position effect on thermal protection. In this article, we explored the effect of moisture (in simulation of human perspiration) on the protective performance of fabric assembly under low radiation and HSC as well as the impact of the position of SMA springs between the fabric layers. The research findings will benefit engineers of high-performance, temperature-responsive TPC.

Experimental Methods

Materials

SMA springs

The SMA springs were ordered and manufactured by Xi’an Saite Metal Material Ltd., China. The SMA springs were composed of a copper-based alloy that was made of copper, aluminum, and zinc. The diameter of monofil was 1.5 mm. Two types (i.e., No. 1 and No. 2) of SMA springs were designed according to their diameter. The No. 1 spring had a diameter of 19 mm, a height of 16 mm, and a mass of 1.88 g. The No. 2 spring was 23 mm in diameter, 24 mm in height, and 3.21 g in mass. Considering that skin damage will occur when the temperature exceeds 44 °C, the T _a of springs was set to approximately 45 °C. To keep the fabric flat, the shape of the springs was consistent with Wang et al. (2020) research at the temperature below T _a. To reduce extra pressure on the skin and to change only minimally the original thickness of the air gap between layers, the spring will deform into a cone (Wang et al., 2020) at any temperature above T _a.

Fabric system design

According to the NFPA 1971 (National Fire Protection Association, 2018), the fabric system for firefighter protective clothing consists of a flame-resistant outer shell, a moisture barrier, and a thermal liner. It was cut into 15 cm × 15 cm to conduct bench-scale tests. The basic properties of each fabric layer are displayed in Table 1. SMA springs were stitched in the center of the thermal liner’s external surface (i.e., Position A) by using para-aramid threads and stuck in the center of the moisture barrier (i.e., Position B) by using inorganic, high-temperature-resistant adhesive. Fabric layers were stapled together at diagonal positions to avoid relative movement between each layer.

Table 1.

Basic Properties of the Testing Fabrics.

Layer	Component	Structural Features	Mass (g/m²)	Thickness (mm)
Outer shell	98% Meta-aramid/2% para-aramid	Twill	186.7	0.41
Moisture barrier	100% Meta-aramid/PTFE film	Water thorn felt with PTFE	106.3	0.69
Thermal liner	100% Meta-aramid	Needle-punched nonwoven with meta-aramid woven face cloth	200.0	2.06

Test Apparatus and Conditions

The TPP of the fabric system was measured in simulated rescue environments of low radiation and HSC.

RHE

The test method was in accordance with the previous study by Song et al. (2010). The TPP tester (Mode 701-D-163-1, Precision Products LLC, Richmond, VA, USA) was used to simulate the low radiation exposures (Wang et al., 2020). Radiant heat was generated by a bank of nine translucent quartz infrared lamps placed horizontally beneath a specimen; the external surface of the outer shell was positioned toward thermal exposure (Barker et al., 2006). The exposure duration was set as 77 s to allow the back surface of all specimens’ thermal liners to reach a first-degree burn level (i.e., temperature rise of 12 °C) and to prevent serious damage to the fabric system during prolonged exposure. Temperature recording ended at 150 s to observe the heat dissipation of the fabric system, as the temperature decrease rate during 77 and 150 s was stable. The radiant heat flux was maintained at 0.39 ± 0.015 cal/cm²·s (i.e., 16.4 ± 0.6 kW/m²).

HSC

The TPP of fabrics under HSC exposure was measured according to a modified ASTM F 1060 (American Society for Testing and Materials, 2018). The fabric assembly specimen was horizontally placed in contact with a hot surface plate of electrolytic copper (Precision Products LLC). The temperature of the hot surface was controlled at 400 °C. The exposure duration of the fabric and time of temperature record were set at 22 s to allow the back surface of all specimens’ thermal liners to reach a first-degree burn level (i.e., temperature rise of 12 °C).

Temperature acquisition system

The data were collected by a temperature acquisition system consisting of Type-T thermocouples with a wire diameter of 0.274 mm (Omega Engineering, Norwalk, CT; accuracy: ±0.5 °C) and a data acquisition system (National Instruments, NI 9213, Austin, TX; Zhang et al., 2018). The detecting point of the thermocouple was placed in the center of the thermal liner’s internal surface, and the sensor data were collected by the data acquisition system. The temperature was measured at sampling frequency of 0.5 s.

Experimental conditions

The materials were preconditioned for at least 24 hr in a standard climatic chamber of 20 ± 2 °C and 65% ± 4% relative humidity prior to the test. To simulate varying amounts of sweat absorption, the internal surface of the thermal liner was prewetted with distilled water and kept in a plastic bag. The moisture content of the thermal liner was set at 0%, 25%, 50%, and 100% of its weight in a standard condition, respectively. The fabric system including No. 1 springs and control groups without springs (CON) were both tested. SMA springs were placed in the center of the thermal liner’s external surface.

In order to analyze the impact of position of the SMA springs between the fabric layers, the SMA spring was either fixed in the thermal liner’s external surface (Position A) or the moisture barrier’s external surface (Position B). The two types of SMA springs were involved. Three samples of each fabric assembly were tested. The samples of moisture experiments all incorporated the SMA spring in Position A.

Evaluation Indices and Statistical Analysis

In the two experimental conditions, the protective performance of the fabric system was evaluated based on temperature histories and the time taken to reach temperature increases of 12 °C (ht12) and 24 °C (ht24) in accordance with ISO 6942:2002 (International Organization for Standardization, 2002). The maximum temperature difference from the original temperature (ΔT _max) was examined as well. In the RHE condition, the evaluation indices also included the final temperature at the end of 77 s of RHE (T ₇₇) and the rate of heat dissipation (ST), which were indicated as

ST = (T_{max} - T_{150}) / (150 - t_{max}),

where T ₁₅₀ (°C) is the temperature at 150 s and t _max (s) is the time to reach the maximum temperature.

Descriptive statistics (means and SDs) were calculated for all dependent variables: ht12, ht24, ΔT _max, and ST. All statistical analysis was processed using SPSS Version 21.0 software (SPSS Inc., Chicago, IL). The one-way analysis of variance was used to distinguish differences in the dependent variables. Post hoc tests using least significance difference were conducted as well. The symbol * in statistic figures indicates that a significant difference was observed at the level of p < .05.

Results

Effect of Moisture Content

TPP under RHE

Figure 1A describes the temperature curves of fabric assemblies with different moisture contents under RHE. Except for CON-0% and No. 1-0%, all curves exhibited a similar change trend (i.e., a rapid increase at first, followed by a slow rise, with T ₇₇ in the range of 49.6–60.1 °C). After exposure, temperature continued to increase before reaching its maximum (T _max) and subsequently decreased gradually until the end of the test. A continuous quick rise was observed for the CON-0% and No. 1-0%, exhibiting 122.6 °C and 72.7 °C at the end of exposure. In all control groups, the T ₇₇ in the three wet conditions (i.e., CON-25%, CON-50%, and CON-100%) was 59.1–63.8 °C lower than that of the dry condition (i.e., CON-0%), showing a decrease by 49.0%–52.4%. When the fabric assemblies were incorporated with SMA springs of No. 1, the T ₇₇ of the three wet conditions decreased by 25%–32% to a temperature range of 18.2–23.1 °C compared with that of the dry condition (i.e., No. 1-0%). The temperatures for the fabric assemblies with SMA springs were always lower than the corresponding ones without springs in different moisture contents. However, the differences in the wet conditions were significantly smaller than those in the dry conditions (i.e., 49.9 °C).

Figure 1.

The thermal protective performance of fabric assemblies in different moisture contents under radiant heat exposure condition: (A) temperature histories and (B) ht12 and ht24.

Figure 1B compares the time to reach temperature increases of 12 °C and 24 °C in different scenarios. Generally, the ht12 and ht24 increased as the wetness increased. For the CON groups, significant differences in ht12 and ht24 were observed only between CON-0% and CON-100% (p < .05). When the No. 1 spring was incorporated, No. 1-50% and No. 1-100% had a much higher ht12 of approximately 35 s, which were significantly different from No. 1-0% and No. 1-25% (p < .05). For the ht24, a 31% increase was observed in the No. 1-100% scenario. No. 1-50% and No. 1-100% provided significantly higher ht24 than No. 1-0% and No. 1-25% (p < .05). Comparing CON with SMA groups, the significant differences in ht12 and ht24 were observed in all moisture contents (p < .05).

As shown in Table 2, for the CON groups, the ΔT _max of wet conditions decreased by 59.6–62.4 °C, which was significantly different from the dry condition (p < .05). The ST of CON-0% was the highest and exhibited significant differences from the other CON samples (p < .05). No obvious difference in ΔT _max and ST was found among CON samples in the three different moisture contents. For the SMA samples, the ΔT _max decreased as the moisture content increased, exhibiting 17.5–21.4 °C decrease in the wet samples; there was a significant difference when the dry condition was compared with the three wet conditions and No. 1-25% compared with No. 1-100%. A remarkable difference was detected only in the ST between No. 1-0% and No. 1-100%. Compared to the CON, the incorporation of SMA (i.e., No. 1) obviously decreased the ΔT _max by 48.8 °C and 3.6–8.4 °C, respectively, under dry and wet conditions. The ST showed a remarkable difference only in the dry condition between the CON and SMA scenario (p < .05).

Table 2.

ΔT _max and ST of the Fabrics System Under the RHE and HSC Conditions.

Scenario	RHE Condition		HSC Condition	Scenario	RHE Condition		HSC Condition
Scenario	ΔT _max/°C (SD)	ST/(°C/s) (SD)	ΔT _max/°C (SD)	Scenario	ΔT _max/°C (SD)	ST/(°C/s) (SD)	ΔT _max/°C (SD)
CON-0%	100.0 (11.3)^b	1.012 (.168)^b	43.6 (3.2)^c	No. 1-0%	51.2 (2.0)^c	.289 (.038)^b	31.1 (3.2)^b
CON-25%	37.6 (4.6)^a	0.349 (.069)^a	40.0 (1.0)^bc	No. 1-25%	33.7 (2.3)^b	.282 (.035)^ab	32.1 (1.6)^b
CON-50%	40.4 (1.9)^a	0.346 (.025)^a	38.8 (2.1)^b	No. 1-50%	33.1 (0.8)^ab	.234 (.027)^ab	25.4 (1.7)^a
CON-100%	38.2 (4.5)^a	0.304 (.051)^a	32.0 (0.3)^a	No. 1-100%	29.8 (2.3)^a	.216 (.041)^a	20.4 (3.4)^a

Note. RHE = radiant heat exposure; HSC = hot surface contact; CON = control groups; ST = rate of heat dissipation.

^a,b,c Testing samples with the same superscript letter do not differ significantly from other samples (p > .05); otherwise, significant differences determined between other samples using least significance difference post-hoc tests (p < .05).

In order to further explore the relationship between the moisture content and ΔT _max of fabric assemblies incorporated with a No. 1 SMA spring, the fitting analysis is displayed in Figure 2. A strongly negative correlation was observed (adj. R ² = .98588). Equation 2 indicated that the moisture content sharply decreased the ΔT _max, but as the moisture content increased, the change rate gradually declined.

Figure 2.

Relationship between moisture content and ΔT _max of fabric assemblies incorporated with No. 1 shape memory alloy spring under radiant heat exposure condition.

Δ T_{max} = 30.29801 \times (w + 0.00238)^(- 0.08684),

where w is the moisture content.

TPP under HSC

Figure 3A presents the temperature history through the fabric assemblies in different moisture contents under the HSC. Temperatures observed for CON-0% and CON-25% quickly increased to a range of 62.9–68.9 °C and then slightly decreased to 61.4–61.6 °C. Temperatures for other conditions continuously rose throughout the exposure, showing an ending temperature of 42.8–61.3 °C. The temperature difference at 22 s among CON-0%, CON-25%, and CON-50% was only 0.11–0.18 °C, showing no significant difference (p > .05). When the moisture content rose to 100%, the temperature at 22 s decreased by 5.36 °C compared to the dry condition, showing a significant difference between CON-100% and CON-0% (p < .05). For the fabrics that incorporated springs, the temperature at 22 s of No. 1-25% was only 0.6 °C lower than the dry condition of No. 1-0%, showing no significant difference (p > .05). Temperatures for No. 1-50% and No. 1-100% exhibited 11.4%–21.1% decrease, comparing with No. 1-0%. The temperatures observed for the fabric assemblies with SMA springs were always lower than those without springs in different moisture contents.

Figure 3.

The thermal protective performance of fabric assemblies in different moisture contents under hot surface contact condition: (A) temperature histories and (B) ht12 and ht24.

Figure 3B compares TPP under HSC condition. For both CON and No. 1 scenarios, the ht12 and ht24 increased as moisture content increased. There was no significant difference in ht12 between CON-0% and CON-25% and between No. 1-0% and No. 1-25%. The other comparison groups all showed significant differences. There was no ht24 observed for No. 1-100% (i.e., the temperature did not increase 24 °C during the HSC exposure). The ht12 and ht24 of specimens incorporating SMA springs exhibited higher values than those of CON groups in each moisture content.

As shown in Table 3, the ΔT _max decreased as the moisture content increased, which was similar to the RHE condition. For the CON, temperature for the wet conditions decreased by 3.6–11.6 °C in comparison to the dry condition CON-0%. For the No. 1, the wet conditions exhibited a −1 °C to 10.7 °C decrease compared with the dry condition No. 1-0%. The No. 1 group was significantly different than the CON group under each moisture condition (p < .05). When moisture content was 100%, the decrease in ΔT _max was 38.8% due to the application of the SMA spring. The minimum effect of the SMA spring was found when 25% of moisture was absorbed, showing a slight increase.

Table 3.

Thermal Protective Performance of the Fabric Systems With Two Springs in Two Positions Under RHE and HSC Conditions.

Number	RHE Condition					HSC Condition
Number	ht12/s (SD)	ht24/s (SD)	ΔT _max/°C (SD)	t _max/s (SD)	ST/(°C/s) (SD)	ht12/s (SD)	ht24/s (SD)	ΔT _max/°C (SD)
No. 1-A	21.1 (2.5)	43.6 (3.2)	51.2 (2.0)	87.9 (1.9)	.289 (.038)	7.0 (.5)^b	12.9 (0.2)^ab	31.1 (3.2)^b
No. 1-B	19.8 (0.3)	37.3 (4.6)	59.5 (8.1)	97.1 (3.9)	.366 (.082)	5.8 (.1)^a	9.5 (0.1)^a	31.1 (2.3)^b
Significant difference (p value)	.417	.127	.161	.021	.216	.180	<.001	.997
No. 2-A	21.1 (9.1)	51.6 (5.0)	42.8 (2.6)	98.85 (12.5)	.199 (.014)	7.6 (.2)^b	19.8 (1.1)^c	25.0 (0.2)^a
No. 2-B	20.5 (2.1)	43.8 (6.0)	47.2 (5.1)	94.7 (5.5)	.253 (.002)	7.0 (.5)^b	15.2 (2.7)^b	25.7 (1.3)^a
Significant difference (p value)	.941	.228	.363	.725	.114	.241	.118	.497

Note. RHE = radiant heat exposure; HSC = hot surface contact.

Effect of Spring Position

Performance under RHE

Figure 4A presents the temperature distribution of fabric assemblies with No. 1 and No. 2 SMA springs at two positions under the RHE condition. All curves exhibited similar change trends (i.e., temperature rapidly increased during the exposure and gradually decreased after 77 s of exposure). The T ₇₇ of samples at Position B was 2–2.8 °C higher than that at Position A, and T ₇₇ with the No. 1 spring was 8.8–9.6 °C higher than with the No. 2 spring.

Figure 4.

Temperature of fabric assemblies with No. 1 and No. 2 at Positions A and B under thermal hazards: (A) radiant heat exposure condition; (B) hot surface contact condition.

The data for different dependent variables (ht12, ht24, ΔT _max, t _max, and ST) are displayed in Table 3 to examine the effect of spring position. For No. 1 and No. 2 springs, there was no significant difference in indices of ht12, ht24, ΔT _max, and ST between Positions A and B. But the t _max was obviously prolonged in No. 1-B compared with No. 1-A. The properties of samples incorporated with the No. 2 spring performed better in ht24 and ΔT _max than with the No. 1 spring.

Performance under HSC

As shown in Figure 4B, temperatures gradually increased for the first 3 s and then rose sharply from approximately 4 to 10 s; subsequently, the temperatures at Position A increased with a low rate, but the temperatures at Position B reached a plateau. The final temperature at Position A was lower than that at Position B for specimens with both No. 1 and No. 2 SMA springs. The temperature of fabric that incorporated the No. 2 spring was 4.7–5.0 °C higher than that of No. 1 spring.

According to Table 3, there was no significant difference in ht12 between Positions A and B. The ht24 of Position A was significantly different from that of Position B (p < .001) in No. 1 specimens, whereas there was no obvious difference in No. 2. For both types of SMA spring, there was no significant difference in ΔT _max between Positions A and B. Generally, the No. 2 SMA spring provided better performance than the No. 1 SMA spring, except for the ht12 in Position A.

Discussion

Effect of Moisture Content on TPP

RHE

During daily wear, the thermal liner will absorb different amounts of moisture when the wearer profusely sweats. We found that the temperature of samples in wet conditions exhibited obviously lower values than in the dry scenario under the RHE condition. One explanation for this might be that moisture increases the specific heat capacity (SHC) of fabric systems, which requires more internal energy to increase the equivalent temperature, and therefore, the heat transfer rate will decrease. Another possible explanation is that water evaporation in fabrics is accelerated by thermal hazards, and some energy will be removed by the phase change of a portion of water from liquid to gas. Therefore, the total heat transferred to the internal surface of thermal liners is reduced. Due to the influence of moisture evaporation, the temperature in the wet scenarios fluctuated greatly compared to the dry scenario.

For traditional three-layer protective clothing fabric, the moisture in the thermal liner greatly decreased T ₇₇ by approximately 50% compared with the dry scenario under the RHE condition. But the significant difference in ht12 and ht24 was only found between CON-0% and CON-100%. Within the first 40 s, heat transfer through the thermal liner decreased as the moisture content increased; however, the moisture less than 50% had an indistinct impact on thermal protection and did not significantly extend the ht12 and ht24. As the exposure time increased, the differences were gradually enlarged between wet and dry fabrics, detected by ΔT _max. This may be attributed to the fact that positive effects of SHC and water evaporation on thermal protection are not obvious at the beginning of exposure in scenarios of moisture content less than 50%. Subsequently, water evaporation increases as temperature rises, and more heat is transferred from fabric to environment, exhibiting significant discrepancies in ΔT _max between wet and dry samples. Interestingly, there was no significant difference in ΔT _max among wet samples in the CON group. This means that the impacts of SHC and water evaporation of internal fabric are similar among samples in three moisture contents under the RHE condition. These results were consistent with the findings of Zhang et al. (2018) and Barker et al. (2006) that 15%–20% moisture exhibited a negative impact on thermal protection, and in some extreme situations, such as 100% moisture, the performance was improved.

When the fabric system was embedded with an SMA spring, the ht12 and ht24 of No. 1-50% and No. 1-100% were significantly higher than that of No. 1-0% and No. 1-25%, and the differences in ΔT _max among SMA samples were also remarkable. It means that the influence of more than 50% moisture on the thermal protection of SMA samples is noticeable. That was different from the condition of CON. The air gap due to SMA deformation enhanced the positive effect of moisture on protective performance. According to relationship between moisture content and ΔT _max under the RHE condition, the moisture effectively decreased the fabric temperature; however, the performance of the sample in moisture content of 100% was not much superior to that of 50%. The reason may be that 100% moisture content obviously increases the thermal conductivity of the innermost fabric layer, offsetting the advantage of SHC in reducing heat transfer. Similarly, the presence of SMA enhanced the effect of moisture to reduce ΔT _max.

Comparing the SMA with the CON scenario, the temperatures of the SMA sample were always lower than that of CON in each moisture content under the RHE condition. In addition, ht12 and ht24 of wet SMA samples were significantly higher than that of the wet CON group. This was because the SMA springs formed a steady air gap between the moisture barrier and the thermal liner when the fabric assembly was exposed to thermal hazard. The heat conductivity of steady air is 0.027 W/m/°C, which approximates one sixth of the fibers (Ghazy, 2017). Therefore, the heat transfer of the SMA group is lower than that of the CON scenario. However, an interesting phenomenon was found: The difference of wet samples in ΔT _max was not significant between SMA and CON scenarios in three moisture contents. This indicates that moisture weakened the positive effect of the air gap on protective performance as exposure time increased. This phenomenon was in accordance with the study of Wang et al. (2012), though the radiation intensity was much higher than that in this study.

Although the increase of moisture has a positive effect on the TPP of a fabric system under RHE, the average ST of the wet fabric assemblies decreased as the moisture content increased. This is because the SHC of fabric will increase as moisture rises. The higher SHC causes lower heat dissipation in wet fabric systems. Meanwhile, it was detected that the ST of the CON group was higher than that of the SMA group. As the air gap within the fabric system is reduced, the energy storage in the air layer decreases and that in the thermal liner increases. However, the high energy storage in the internal fabric of the CON group has the potential to release more heat after exposure. That is the reason why the temperature of CON decreased dramatically after exposure.

HSC

The CON group under the HSC condition showed different results with RHE. The temperatures of CON-25% and CON-50% increased rapidly and gradually approached to CON-0%. The likely reason was that the environmental temperature was higher than that of the RHE condition, increasing the rate of water evaporation of fabric. As the remaining moisture amount decreased, the temperatures of CON-25% and CON-50% gradually approached that of CON-0%. Each wet sample of the CON group was significantly different in ht24 from the dry condition. For ΔT _max, CON-100% was distinctly superior to other CON samples. Further, the effect of moisture content above 25% on TPP was more significant, and the discrepancies among three moisture contents were more obvious than that of the RHE condition. The environmental temperature of HSC was much higher than that of the RHE condition. This caused the remarkable effect of water evaporation on thermal protection, especially when the moisture content was 100%.

The fabric assembly that incorporated springs under the HSC condition also showed some differences compared with that under the RHE condition. Under an HSC hazardous environment, the low moisture content approximated to 25% had a negative impact on TPP. But when moisture content increased up to 50% and 100%, the thermal protection of fabric assemblies improved substantially. Meanwhile, the ΔT _max between the CON and SMA group in four moisture contents was significantly different. This meant that the air gap generated by the spring effectively reduced the thermal conduction, providing superior protective performance. Compared with the RHE condition, the protective performance decreased in low moisture content but increased in high moisture content among the SMA group; the discrepancies of indices were more significant between the CON and SMA scenario in the HSC condition.

Effect of Spring Position

The heat source was placed beneath the specimen, and the external surface of the outer shell was positioned to face thermal exposure. In Position B, when the spring deformation occurs, the outer shell beneath the spring is more likely to be pressed, which causes the outer shell to be closer to heat source. Therefore, the temperatures of specimens in Position B under two hazardous conditions were higher than in Position A. And the TPP of No. 2 samples was superior to that of No. 1 because of higher air gap generated by the No. 2 SMA spring.

Under the RHE condition, the position had no significant impact on thermal protection according to indices of ht12, ht24, and ΔT _max. But after exposure, the temperature in Position B continued to rise more than 10 s than that in Position A when samples were incorporated with a No. 1 spring. The SMA spring produced an air layer and stored much heat during exposure that was discharged to the internal liner after exposure. The thermal liner was close to the air gap when SMA was in Position B, which caused a longer temperature rise after exposure. However, there was no distinct difference in t _max in No. 2 samples between the two positions, probably because the heat transfer during exposure and heat discharge after exposure were decreased by the larger air gap.

Under the HSC condition, position had a significant impact on ht24 of No. 1 samples, whereas it had no significant effect on ht12 and ΔT _max. Samples in Position A, compared with Position B, showed superior performance at the beginning but lost the advantage as exposure continued. That may be explained by the fact that the air gap in Position B was formed earlier due to the higher temperature, which enabled a balance of heat absorption and heat dissipation after 15 s. However, for samples with SMA in Position A, heat absorption was higher than heat dissipation during the exposure, and thus, temperature continually increased and gradually approached to that of Position B at the end of exposure. Compared with the RHE condition, the impact of position and spring size on TPP was more obvious in the HSC condition. The samples in Position A and incorporated with a No. 2 SMA spring showed better performance.

Conclusions

Moisture within the thermal liner had an influence on the TPP of both the traditional three-layer protective fabric and the SMA spring fabric assembly under two hazardous environments. In the radiant heat condition, moisture content of less than 50% in the traditional fabric system did not increase ht12 and ht24 but significantly reduced the temperature difference at the end of exposure. Conversely, moisture higher than 25% in the SMA fabric assembly remarkably extended ht24. In both the traditional and SMA fabric systems, 100% moisture content exhibited superior protective performance; however, there was no significant difference among wet samples. Due to the impact of moisture, the SMA group had no obvious advantage in reducing the heat transfer to skin during the exposure compared with the traditional fabric system under radiant heat condition. After exposure, the rate of heat dissipation decreased as moisture content increased, and the SMA group showed a lower rate than the traditional fabric system. In the HSC condition, the differences in TPP among four moisture contents were more noticeable than in the radiant heat condition. The low moisture content of approximately 25% within fabric had no obvious positive impact on TPP. The TPP of the spring fabric assembly was distinctly superior to the traditional fabric system. Under two hazardous conditions, the optimized position of the SMA spring was in the outside of the thermal liner. The samples in Position A and incorporated with the No. 2 SMA spring showed better performance. The result of this study will guide the engineering of temperature-responsive protective clothing with SMA.

Footnotes

Authors’ Note

Yehu Lu and Lijun Wang contributed equally to this work.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by a grant from the National Natural Science Foundation of China (51606131), the Technology Innovation Project of Key Industries in Suzhou (SYG201812), the Nantong Municipal Science and Technology Project (JC2018039), and the Open Project Program of Fujian Province University Engineering Research Center of Textile and Clothing, Minjiang University, China (MJFZ18104).

ORCID iDs

Yehu Lu

Wenfang Song

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