Home Engineering Thermal Protective Clothing for Firefighters
Assessment of thermal resistance, evaporative resistance, and THL
Many researchers evaluated the thermal resistance, evaporative resistance, and THL of fabrics using the equipment/methods described in various standards (eg, ASTM F 1868, ISO 11092, ISO 5085-1, BS 4745:2005, CGSB 1977, ASTM D 1518) or by employing their own customized instruments/procedures. To further understand evaporative resistance, a group of researchers also evaluated the WVP or MVTR of fabrics using standardized (eg, ASTM E 96, JIS L 1099, CGSB 49, BS 7209, ASTM 2298) or nonstandardized equipments/methods [393-424]. These studies have identified that fabric features (eg, fiber types, weaves, design, weight, thickness, porosity) and ambient environmental variables (air, temperature, relative humidity) affect heat and/or moisture/water vapor transfer (convective/conductive/radiative/diffusive) through fabrics, which ultimately affect thermal resistance, evaporative resistance, and/or THL. However, it was very difficult to obtain a clear relationship between the individual fabric features and thermal resistance, evaporative resistance, and/or THL This is because most of these features are so profoundly interrelated it is impossible to separate them.
Black and Matthew  and Rees  studied the thermal resistance of fabrics. They examined the effect of environmental air relative humidity on the thermal resistance of fabrics, and found that thermal resistance is highly dependent upon relative humidity. They concluded that fabrics gain different levels of moisture under different levels of humidity, and this gained moisture affects thermal resistance. Black and Matthew  proved through experimentation that a marked reduction of thermal resistance takes place when the moisture content increases from 0% to 75% of the dry weight of fabrics. Furthermore, Farnworth  evaluated the thermal resistance of several commercial synthetic fabrics at a compression of 0.16 kPa under varied air temperatures. He found that the fiber used to manufacture the fabrics, along with the fabrics’ thickness and weight, has a significant effect on thermal resistance; a fabric which comprises a high percent fiber volume, a small fiber diameter, or a low thermal conductive fiber will possess high thermal resistance. Additionally, a fabric with high thickness and weight always traps air inside the fabric structure; as this air prevents heat transfer through fabrics, a fabric with high thickness and weight possesses higher thermal resistance than a fabric with low thickness and weight (when the same fiber is used to manufacture both the fabrics). For the same reason, heat transfer through a low- density fabric is reduced, and this type of fabric has a higher thermal resistance than high-density fabric. Hes, Araujo, and Djulay  analyzed the thermal resistance of multilayered woven fabric assemblies under steady-state and transient-state thermal conditions. In this study, two types of woven fabric assemblies were used: in the first, each layer of woven fabric was free from its subsequent layer(s), and in the second each layer of woven fabric was spot-bonded with its subsequent layer(s) by thermal fusion with polymer dots. They found that the free air layers (caused by raised surface fibers) between fabrics in the first type of assembly have a significant effect on thermal resistance. In fact, the free air layer-based fabric assembly exhibited up to 50% higher thermal resistance than the spot-bonded fabric assembly. For the spot-bonded fabric assembly, the thermal absorptivity increased up to 32% when compared to the free air layer-based fabric assembly. Similarly, Matusiak  investigated the thermal resistance of single and multilayered fabrics of woven and/or nonwoven structures. In this study, it was found that a highly porous nonwoven fabric possesses greater thermal resistance than a less porous and tightly woven fabric; this is because the tightly woven structure possesses less air than the nonwoven structure; as a consequence, the heat flow through the woven fabric is much higher than the nonwoven fabric. Additionally, the configuration of each layer in a multilayered fabric has an impact on the thermal resistance. A multilayered combined woven and nonwoven fabric assembly generally has higher thermal resistance than a multilayered woven fabric assembly, because the combined woven and nonwoven assembly possesses higher amounts of air than the woven fabric assembly alone. Moreover, Bhattacharjee and Kothari  modeled the thermal resistance of woven fabrics by considering conductive and radiative heat transfer through the fabrics in normal ambient environments. They considered that the basic weaves in the woven fabrics can be represented as a repeating unit consisting of stacked yarn, unsupported yarn between interlaced warp and weft, and air pores. In this study, it was found that the air pores affect both the conductive and radiative heat transfer through the fabrics, which, in turn, affect the thermal resistance of the fabric. Additionally, the stacked yarn and unsupported yarn mainly affect the conductive heat transfer through the fabrics or the thermal resistance of the fabrics . In another paper, Bhattacharjee and Kothari  modeled the thermal resistance of woven fabrics by considering natural and forced convective heat transfer through the fabrics. It was observed that the fabric thickness and porosity mainly control convective heat transfer through the fabrics or the thermal resistance of the fabrics. Here, a fabric with high thickness and porosity trapped a large amount of air inside its structure and enhanced its thermal resistance . Based on this result, it was found that the size of the air pores in a fabric is also important for its thermal resistance. If the size of the air pores is smaller, the fabric will not trap sufficient air in its structure, which will reduce thermal resistance. Additionally, Bhattacharjee and Kothari  concluded that the surface heat transfer coefficient of a fabric is not significant to thermal resistance, especially in the case of natural convection. Barker and Heniford  evaluated the thermal resistance of various inherently fire-resistant woven/nonwoven fabrics or multilayered fabric systems. In these studies, it has been found that fabric weight and thickness definitely affect thermal resistance; however, some other fabric features (eg, air permeability, porosity, surface area) are also equally important for thermal resistance [403,404]. Shekar et al.  found that environmental air velocity may cause the thermal resistance loss in a fabric system; however, the presence of an impervious outer layer in the fabric system helps to reduce the loss in thermal resistance under high wind velocity conditions. They also found that the thermal resistance of a fabric system is independent from the nature of the outer layer (pervious or impervious) under normal environmental conditions. Additionally, it was determined that nonwoven fabrics possess high thermal resistance due to their bulkiness, compression recovery, and porosity compared to woven fabrics [24,29]. However, wet nonwoven fabrics possess lower thermal resistance than dry nonwoven fabrics; this is because the thermal conductivity of the wet fabric is much higher than that of the dry fabric. Barker and Heniford  stated that the construction and thickness of fire-resistant nonwoven fabrics mainly affect thermal resistance. This study demonstrated that the effective layering of fiber web in nonwoven fabrics is a potential means to enhance thermal resistance because it contributes air layers and thickness without any increase in weight to the nonwoven fabrics.
Many fire-resistant fabrics are used in thermal/cold-weather clothing. One of the main requirements for thermal/cold-weather clothing is that it must possess a high thermal resistance in order to provide protection to wearers from thermal/cold exposures. However, it is necessary to remember that there must be a balance in thermal resistance to provide protection and comfort to wearers [28,406,379]. Recently, Matusiak and Sikorski  examined the impact of fabric structures (different types of weaves, linear densities of weft yarn, different weft densities) on thermal resistance. It has been found that the weave of woven fabrics significantly affects thermal resistance; plain weave fabrics were characterized by a lower thermal resistance than twill, rep, and hopsack weave fabrics, with the same linear and nominal densities of warp and weft yarn. It was also revealed that the linear density of weft yarn significantly affects thermal resistance, and the influence of the weave on the thermal resistance of woven fabric can be modified by the influence of the linear density of the weft yarn . Additionally, a strong and statistically significant correlation exists between the thickness/weight of fabrics and their thermal resistance; similarly, the correlation between the fabric cover factor and thermal resistance is weaker than the correlation between the fabric structural factor and thermal resistance. This is because the fabric structural factor is an integrated parameter (by considering the average percentages of weft or weft densities) of the fabric cover factor; eventually, it has more correlation with thermal resistance. Dias and Delkumburewatte  found that thermal resistance of a knitted fabric is inversely related to its thermal conductivity; hence, a thorough study on thermal conductivity may develop an understanding regarding thermal resistance. They established that thermal conductivity can be calculated by considering (1) thermal conductivity of the fibers used to manufacture the fabric, (2) the porosity of the fabric structure, and (3) water content in the fabric pores depending upon fabric hygroscopicity and environmental relative humidity. Dias and Delkumburewatte  concluded that the thermal conductivity of fabric increases due to three reasons: (1) the thermal conductivity of the fibers increases, (2) the fabric porosity decreases, and (3) the water content in the fabric pores is high. This is because (1) a highly thermal conductive fiber enhances the thermal conductivity of the solid yarn phase of a fabric; (2) a less porous fabric can trap less amounts of highly insulative air; and (3) water has a very high thermal conductivity. Bogaty, Hollies, and Harris  concluded that the thermal conductivity of a fabric is dependent on its bulk density. It is insensitive when the fibers/yarn are arranged parallel to the surface at higher bulk density, but becomes sensitive to this arrangement when fiber conductivity and bulk density are very high. In Ozcelic, Cay, and Kirtay’s  study of thermal resistance in structured knitted fabrics, interlock knitted fabrics produced with air-jet textured, false-twist textured, and nontextured filament yarns were compared. It was found that the thermal resistance of textured fabrics is higher than fabrics produced with nontextured filaments, due to increased interfiber pore dimensions and consequent thickness. Additionally, false-twist textured fabrics contain higher thermal resistance compared to air-jet textured fabrics. This is because the false-twist textured fabrics have more crimps and surface roughness, which ultimately enhances the thermal resistance by trapping more air on the boundary layer of the fabric.
Farnworth and Dolhan  described the evaporative resistance or water vapor transport behavior of cotton and polypropylene fabrics. In this study, it was established that there is no significant difference in the evaporative resistance of cotton and polypropylene fabrics under cold temperatures. It was also evident that water vapor transport through fabrics is mainly affected by their water absorptivity and wickability. A fabric with high moisture absorptivity and low wickability transports less water vapor, resulting in high evaporative resistance. Farnworth, Lotens, and Wittgen  analyzed the evaporative resistance of textiles under variable conditions of relative humidity. In most clothing applications, when the wearer is sweating, or when the ambient air temperature is low, or if it is raining on the garment, a high average relative humidity value is likely to be appropriate; whereas, if the wearer is only perspiring minimally in warm, dry conditions, a low average relative humidity is appropriate. Farnworth, Lotens, and Wittgen  found that the evaporative resistance of microporous polytetrafluoroethylene and polyurethane fabrics/films varied insignificantly with relative humidity; however, fabrics/films with hydrophilic coating showed a strong variation of evaporative resistances under different relative humidity conditions. In the hydrophilic case, evaporative resistance decreases substantially with increasing relative humidity.
Gibson  explored the evaporative resistance of various woven and nonwoven fabric materials. These materials included the permeable and impermeable types tested as single-layered, laminates, and composites. It has been found that the evaporative resistance of permeable materials is very low, while the evaporative resistance of impermeable materials is significantly higher. This is because impermeable materials do not allow the transfer of moisture vapor through their structure, whereas permeable materials allow moisture vapor-transfer through their structure at a high rate. Here, the evaporative resistances of permeable materials were evaluated in a variety of conditions: (1) under the varying directions and velocity of airflow over the materials, and (2) by providing an air gap between the material sample and the sweating skin simulant hot plate. It has been found that airflow conditions have a significant effect on evaporative resistance, and the open structure of the material becomes particularly important for evaporative resistance, especially when an air gap exists between the material sample and the sweating skin simulant hot plate. Gibson  concluded that the correlation of the open structure and evaporative resistance can be altered by varying the thickness of the materials at an air velocity of 1-2 m/s . McCullough  studied the evaporative resistance of fabrics used in various types of regular clothing. It was found that evaporative resistance is dependent upon the porosity and bulk density of fabrics. If any fabric is less porous or has a low bulk density, it may not allow for transfer of moisture vapor, which causes high evaporative resistance. However, evaporative resistance can be altered by using different types of fibers and finishing processes while manufacturing the fabrics. For example, the use of hydrophilic fibers and coatings in/on fabrics results in lower evaporative resistance than fabrics with hydrophobic fibers and coatings. Additionally, the ambient air velocity can also drastically lower evaporative resistance. Wang and Yasuda  investigated the evaporative resistance of layered fabrics. They concluded that the modification of fabric surfaces can change evaporative resistance, and the wicking ability of fabric turned out to be the dominant factor governing evaporative resistance. Generally, a fabric with high wicking ability has lower evaporative resistance. Additionally, Wang and Yasuda  found that the temperature of the air gap between two layers of fabrics increased when water vapor transport was present, and the temperature growth was almost proportional to the water absorption rate of the fabric; this temperature growth can change the evaporative resistance of fabric. Gretton et al.  studied the moisture vapor transmission through waterproof breathable fabrics under several temperature gradients along the fabric thickness. It was found that the presence of an accurate temperature gradient reduces the differences between the transmission rates of the hydrophilic fabrics and microporous waterproof breathable fabrics. Incorporation of a highly insulating fabric system with microporous breathable fabric may significantly lower the moisture transmission rates through microporous fabric. The fabric thickness of microporous waterproof breathable fabric also maintains a higher temperature gradient compared to hydrophilic fabric at a particular vapor pressure. This high-temperature gradient enhances the relative humidity gradient across the microporous fabrics so less condensation occurs.
Gibson  determined the WVP of different polymer membranes and mem- brane/textile laminates under different ambient air temperatures. It was found that the changes in water vapor flux through the membranes and laminates over the temperature range of 30-40°C were primarily due to the fundamental physical relationship between temperature and saturation vapor pressure of water; in this case, fabric structure does not play an important role. Here, the WVP was mainly influenced by the water concentration gradient along the thickness of the membrane and laminates, and the water vapor mainly transferred through the gas phases present in the membrane and laminate structures. Zhou, Wang, and Yuan  analyzed the evaporative resistance of conventional tightly woven, microporous film, and hydrophilic film-fabrics. It has been found that these fabrics generally possess a high evaporative resistance, and increase the temperature and water vapor pressure inside the fabric structure under certain environmental condition. The water vapor condensation is more prominent within a fabric with high evaporative resistance than a low evaporative resistance fabric at a particular relative humidity. Fukazawa et al.  studied the evaporative resistance of textiles under the combined influence of temperature and pressure simulating high altitude. In this study, it has been found that temperature and pressure have an impact on the moisture vapor transport through textiles, hence, the evaporative resistance of textiles; however, temperature has less effect on evaporative resistance than pressure. Fukazawa et al.  also observed that evaporative resistance decreases with increasing simulated altitude, due to an increase in the water vapor diffusion coefficient with increasing altitude. Additionally, the water vapor condensation in the fabrics tended to increase with increasing simulated altitude; as a consequence, the evaporative resistance appreciably decreases in the long run. Rossi and Gross  studied the water vapor resistance of multilayered fabric systems at several moderately cold temperatures. It was evident that the evaporative resistance increased in cold environments because the moisture partly condensed within individual layers of the fabric systems. Here, the evaporative resistance and moisture condensation rates strongly depended upon the ambient temperature and hydrophilicity of the outer layer of the system. The differences in effective evaporative resistance between the systems were small at an ambient climate of 20°C and 65% relative humidity, but the difference became larger with decreasing ambient temperature. The formation of moisture condensation was the smallest for the fabric systems with a hydrophilic membrane laminated on the inside of the systems, and the moisture condensation occurred more when the hydrophilic layers were placed underneath the outer layer of the systems.
Weder et al.  studied the evaporative resistance of various synthetic fiber-based fabrics under different relative humidities of ambient air. It has been found that the evaporative resistance of a fabric becomes lower in the presence of low relative humidity and vice-versa. This is because low air humidity causes a higher water vapor pressure gradient across fabric thickness; due to this gradient, the water vapor can transfer efficiently through the fabric. In this context, it was evident that the water vapor can be stored within the fabric structure in the presence of high relative humidity, which can affect the evaporative resistance of the fabric. Havenith, Hartog, and Martini  compared the evaporative resistance of the membrane and woven fabrics used in protective clothing. It has been found that a membrane fabric possesses higher evaporative resistance than a woven fabric; this is because the openness/porosity of the membrane fabric is much lower than the openness/porosity of the woven fabric. As a consequence, the protective clothing made by incorporating the membrane fabric systems can cause heat stress/strain to wearers. However, a membrane is always required in protective clothing to provide protection from various hot liquids, chemicals, etc. Thus, the membrane should be designed in such a way that it can be impermeable to liquids/chemicals, but breathable/permeable to water vapor to provide better comfort to wearers . In this context, Bartels and Umbach  quantified the MVTR through an ordinary membrane and a breathable membrane used in protective textiles at low ambient temperatures. It was found that moisture vapor transmission through the breathable membrane is usually higher than the ordinary membrane. There is also no relationship identified between the ambient air temperature and moisture vapor transport through the breathable membrane; it was observed that moisture vapor transmission through the breathable membrane remained the same in between the normal ambient temperature and —20°C temperature. Ding et al.  modeled the evaporative resistance of singlelayered fabrics used in thermal/cold-weather protective clothing. In this study, it was found that moisture diffuses in fabrics through the air spaces between fibers or yarn and it is affected by yarn and fabric structures as well as size and number of interstices (with warp and weft) developed in a certain area of the fabric; here, fabric count, yarn twist, and yarn linear density are the main features that affect the size and number of interstices. Ding et al.  also found a decreasing trend in evaporative resistance with increasing air velocity for all fabrics, with a relatively large decrease occurring in a range of 0-5m/s air velocity . It was observed that evaporative resistance decreases at a faster speed in the presence of turbulent air than laminar air, and evaporative resistance can be greatly increased by increasing fabric thickness at a particular air velocity. In this study, a small (4.54%) increase in evaporative resistance was observed when relative humidity varied from 0% to 100%; and, the smallest evaporative resistance was observed when the fabric porosity approached unity at a particular thickness . Additionally, it was found that the surface diffusivity of the fabric determines the rate of moisture transfer through the fabric; thus, the evaporative resistance of the fabric. Wang et al.  investigated the WVP of the multilayered thermal protective fabric systems (composed of shell fabrics, moisture barriers, thermal liners, and comfort liners) used in firefighters’ protective clothing. The experimental results demonstrated that thermal liner played a different role in the WVP of the multilayered fabric system; however, the shell fabrics, moisture barrier, and comfort lining showed no distinct dissimilarity. In this study, the WVP of the multilayered fabric system were correlated with the WVP of the systems’ individual layer (shell fabrics, moisture barrier, thermal liner, and comfort liner); and it was found that the WVP of the multilayered fabric system was highly correlated with the WVP of the moisture barrier, meaning that moisture barriers have the greatest effect on the WVP. Additionally, a combined interaction between the shell fabric and thermal liner also moderately affected the WVP. Prahsarn, Barker, and Gupta  evaluated the evaporative resistance and MVTR of synthetic fiber-based open knitted fabrics in the steady-state and transient conditions. They observed that evaporative resistance and MVTR through largely open knitted fabrics are predominantly controlled by the fiber, yarn, and fabric variables that determine the thickness and permeability of the fabrics. It was also found that evaporative resistance and MVTR can be controlled by a moisture vapor concentration gradient, coupled with a temperature gradient along the fabric thickness. It seems that fabric thickness governs the magnitude of the gradient, which is the main driving force for controlling evaporative resistance and MVTR. In the case of transient conditions, the openness of the fabric is most important, and in this condition the researchers concluded that a fabric with thin and open structure possesses high MVTR. In related research, Yoon and Buckley  showed the importance of knitted fabric constructional variables on evaporative resistance. They reported that evaporative resistance is dependent on fabric thickness, optical porosity, and water vapor diffusivity of the ambient air. Their findings indicate that steady-state moisture vapor transport through fabrics is controlled by a diffusion process that is strongly influenced by fabric structure, especially fabric thickness and openness.
Based on the preceding discussion, it is confirmed that thermal and evaporative resistance are affected by many direct or indirect parameters: namely, fabrics’ constructional (eg, fiber types, weaves, design) and physical (eg, weight, thickness, porosity) features, and/or ambient environmental variables (eg, air, temperature, relative humidity) [393-424]. These parameters can also be important for THL because it is a combined interpretation of thermal and evaporative resistance. Many researchers corroborated that heat loss through fabrics may occur through combined heat and moisture/ water vapor transfer by conduction, convection, radiation, evaporation, and/or diffusion [24,31,307,379,403,404,425]. Farnworth  studied heat loss by modeling the combined heat and water vapor transfer through multilayered fabrics. The heat transfer was administered by conduction and radiation; whereas, the water vapor transfer was delivered by diffusion. In this study, it was evident that hygroscopic and nonhygroscopic fabrics behave differently to transfer heat and water vapor. Thicker fabrics do not allow the transfer of heat and water vapor through their structure, which ultimately reduces heat loss. Additionally, hygroscopic fabrics absorb vapor and transfer it to the ambient environment; this phenomenon ultimately enhances heat loss through the fabrics. It was also found that the water-impermeable but vapor-permeable fabrics possess an excellent heat loss characteristic at high ambient temperature; however, this characteristic is not prominent at low ambient temperature. Farnworth  concluded that a layered fabric may not allow the transfer of heat and water vapor through its structure, which ultimately reduces heat loss. Ghali, Ghaddar, and Jones  studied heat loss by heat and moisture transfer through thin cotton fibrous media. They inferred that heat and moisture transfer mainly occurs through fabric by convection. Heat and moisture transfers are controlled by air pores present in any fabric. The air trapped in the pores may not allow the transfer of heat through the fabric and the heat loss is reduced; however, air passing through the pores may significantly enhance the moisture transfer through the fabric so that heat loss is increased. In this study, it was concluded that ambient air temperature and humidity mainly control heat and moisture transfer through clothing. Generally, high temperature and relative humidity may lower heat and moisture transfer through fabrics and reduce heat losses through fabrics. However, this heat and moisture transfer and/or heat loss through fabrics can be altered by changing moisture regain and absorptivity of the fabric. Cao et al.  investigated heat loss by studying the heat and moisture transfer through various synthetic woven and knit fabrics. It was evident that knitted fabrics possess higher wicking than woven fabrics; as a consequence, moisture transfer through knitted fabrics is significantly higher than in woven fabrics. However, knitted fabric possesses air loops within its structure, which ultimately resist the transfer of heat through the fabric and lower heat loss. Cao et al.  also stated that contaminating metal (if any) within a fabric structure may not significantly affect the heat and moisture transfer through the fabrics; as a result, heat loss will not be significantly affected. They suggested that the attachment of a liquid cooling device to the fabric may enhance its heat and moisture transport features, which can significantly enhance heat loss through fabrics. Weder et al.  studied wet heat loss through different underwear fabrics under different ambient air relative humidity. They confirmed that heat loss through fabrics is mainly dependent upon sweat-vapor generated by wearers. For low sweat rates (50-70g/h), the heat loss difference was insignificant under different relative humidities in the ambient air. With high-relative humidities and low-sweat rates, the sweat did not fully transfer through the fabrics and stored inside the fabrics thus inducing a higher heat loss by wet thermal conductivity. Wet thermal conductivity was also a dominant factor and caused higher amounts of heat loss through fabrics for high-sweat rates in high relative humidities. At low relative humidity, wet heat loss increased proportionally to the increase in sweat rate. However, wet heat loss increased with a much lower rate in dependency of the sweat rate when the relative humidity in the environment was raised.
Fanglong, Weiyuan, and Minzhi  analyzed heat loss through chemically modified fire-retardant and inherently fire-resistant fabrics used in firefighters’ clothing.
In this study, heat and moisture transfer through multilayered fabric systems (shell fabrics, moisture barrier, thermal liner) was observed. It was reported that the heat transfer through the shell fabric occurred at a much higher rate than for the thermal liner because the shell fabric did not trap as much air as the thermal liner did. It seems that heat loss can occur at greater rates through the shell fabric than thermal liner; hence, there is a need to design the thermal liner in such a way that it can balance the heat protection and metabolic heat loss. Additionally, it was observed that a fabric system with high thickness and weight causes a high evaporative resistance, which in turn lowers heat loss through the fabric system . In this context, Ding et al.  studied heat loss through single-layered thermal/cold-weather protective fabric by analyzing heat and moisture transfer through the fabrics. Results indicated that the heat and moisture transfer through fabric can be controlled, depending upon the thickness and porosity of the fabric. A highly thick fabric did not allow the transfer of heat and moisture through the fabrics, so that eventually, the heat loss is lower. Furthermore, a fabric with high porosity transfers the moisture (by diffusion) and heat (by radiation) through the fabrics, which may cause heat loss through fabrics . Recently, Tian et al.  analyzed the heat loss behavior of multilayered fabric systems used in thermal protective clothing. As usual, the multilayered fabric systems were composed of three different fabrics in this study; however, the composed fabrics were used in different layering sequences. Altogether, six different three-layered fabric systems were prepared using the different layer stacking sequences. In this study, it was observed that heat loss occurred through three-layered fabric systems mainly in transient condition, and the stacking sequence of the three-layered fabric systems played an important role in heat loss. It was found that the layer in contact with the heat source is the most important layer for the heat loss; here, the volumetric heat capacity of the layer contacting the heat source is the prime parameter for heat loss through three-layered fabric systems.
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