Evaluation of the Moisture Transfer Property of Waterproof Breathable Fabric Under Low-Temperature Conditions Depending on the Pore Size and Distribution

Abstract

The moisture transfer properties of four types of waterproof breathable fabrics with different pore sizes and distributions under low temperatures were examined. The structure, thickness, and pore shape of the fabric by scanning electron microscopy and capillary flow porometer compared depending on the manufacturing method. The effects of these parameters on the wearing comfort as determined by water vapor transmission rate and analyzing the temperature/humidity changes in the microclimate using the human–clothing–environment simulator. The coating type membrane was the thickest, whereas the nano web specimen was the thinnest. The results showed that at subzero temperatures, there was little difference in the vapor pressure change of the microclimate depending on the pore size. In the case of the waterproof breathable fabric produced in various forms depending on the function, however, the evaluation should performed based on the actual clothing wearing conditions, rather than in the standard state, is needed.

Keywords

waterproof/breathable fabric water vapor transmission rate pore size and distribution microclimate low temperature

With the growing interest in health, various sports and leisure industries are expanding. Functional fiber materials and products that improve health, comfort, and hygiene are leading to an increased focus on the outdoor market (Cheng & Cheung, 1994).

Water vapor transmission rate (WVTR) is affected significantly by fiber type, fabric thickness, density, and porosity, according to Gibson (2000); the WVTR increases abruptly with increasing temperature. This is due to the difference in vapor pressure (VP) within the clothing depending on environmental conditions; when the outdoor temperature is high, extra sweat is not condensed and is instead diffused directly through the pores of the fabric in the gaseous state, whereas moisture is condensed on the fabric and diffused to the outside when the outdoor temperature is low (Kim, Park, Shin, & Oh, 1997). Gretton, Brook, Dyson, and Harlock (1998) measured WVTR using a heated and unheated dish and found that the WVTR based on the heated dish was higher. This suggests that WVTR varies according to temperature. On the other hand, when WVTR was examined under low-temperature conditions, it was reported that the diffusion resistance increased exponentially with decreasing temperature, reducing vapor diffusion, and making it difficult to discharge the vapor within the clothing (Bartels & Umbach, 2002).

Waterproof breathable fabric, one of the various functional fabrics that has been developed, eliminates discomfort by discharging sweat from the human body and blocking moisture from the outside. This property is accomplished by utilizing the different sizes present between gaseous vapor and liquid water. Among these, a film layer of microporous-type waterproof breathable fabric has micropores of 0.1–12 µm, which vapor, with a 0.003 µm diameter (Lomax, 2007), can pass through; however, raindrops, with a 100–3,000 µm diameter, cannot. The fabric is manufactured by laminating a microporous membrane using a polytetrafluoroethylene (ePTFE) membrane or by coating with resin (Lotens & Havenith, 1991). On the other hand, while hydrophilic nonporous fabric has no pores, the amorphous region of the hydrophilic polymers within the membrane acts as intermolecular pores. Therefore, vapor permeation (VPm) occurs through the absorption and gradual diffusion of vapor due to transient hydrogen bonding (Son, Kim, & Kwon, 2000). The fabric is manufactured by the direct coating of a hydrophilic nonporous membrane on the fabric or by laminating after manufacturing a film using release paper. Unlike porous fabric, it consists of a continuous nonporous solid coating layer. The hydrophilic nonporous membrane is an inexpensive raw material with a fast processing speed (due to the simplicity of the process) and an outstanding hydraulic pressure resistance with respect to water.

With the development of techniques and the increase in demand, waterproof breathable fabrics have been manufactured in various forms depending on their function. In developing a test method for evaluating the material properties of these fabrics, each standard has different measurement conditions and water vapor transmission mechanisms. Therefore, it is difficult to predict the wearing comfort (Hollies, Custer, Morin, & Howard, 1979). Currently, the American Society for Testing and Materials (ASTM E 96) specifies the temperature condition of 23°C. In contrast, the Korean Industrial Standards (KS K 0594) specify the temperature condition of 40°C, while the Japanese Industrial Standards (JIS L 1099) specify the temperature condition of 30°C. The representative evaluation methods for WVTR include the evaporation method, dynamic moisture permeation cell method, and moisture absorption method (Ding, 2008). Different temperature conditions are also specified by the evaluation methods. In addition, the actual wearing condition of waterproof breathable clothing is more diverse, and it generally worn in low-temperature conditions. In particular, the abovementioned test method cannot be used at subzero temperatures. When the porosity is low, vapor does not pass through the pores as it condenses or freezes before being discharged to the outside, which can deteriorate the WVTR and heat retention properties.

In addition, the pore shapes of waterproof breathable fabrics are irregular with various forms (Jena & Gupta, 2003). Molecular diffusion through the pores is a major movement path within the clothing that also affects its comfort (Gretton et al., 1998). The speed and amount of moisture transfer through the fabric are determined by the thickness and porosity of the fabric (Crow & Osczevski, 1998). In general, moisture transfer becomes easy when the pore size is large, the distribution is uniform, and there are many pores; this is an important element for determining the performance of waterproof breathable fabric (Frey, Richter, & Görlich, 2006). The thickness determines the moving distance of moisture on one side of the fabric and affects the VPm rate (Li, 2001). Accordingly, thin fabric generally shows high WVTR, as moisture is diffused quickly to the outside and the drying speed is fast. On the other hand, the total porosity of a fabric is the ratio of the air volume within the fabric to the total volume of the fabric, and the permeability of the fabric is not considered. Therefore, the pore size and distribution need to be measured to evaluate the permeability of various fabrics (Kim, 2014).

Sweating from the human body changes the VP and temperature of the microclimate within the clothing. This moisture discharged in the liquid and gaseous states depends on the amount and environmental conditions, and the human body can feel comfort when the moisture is transferred efficiently to the outside. Moisture (in a liquid state) is transferred by wetting and capillary action. Vapor (in a gaseous state) diffuses through the pores of fabric from high to low concentrations or transfers through the interior of the fiber (Kothari, 2000). Owing to the high humidity around the human body, hypothermia can occur when heat and moisture not discharged promptly. Accordingly, to minimize such risk, selection of the appropriate clothing is important, and the microclimate within the clothing under low-temperature conditions needs to be controlled more aggressively.

Therefore, the authors analyzed the microclimate within the clothing under similar conditions to actual wearing conditions and examined the effects of the porosity on the WVTR under low-temperature conditions. In particular, the purpose of the study was to predict the wearing performance by measuring the pore size and distribution depending on the manufacturing method, comparing the WVTR depending on the temperature, analyzing the correlation with the porosity characteristics, and examining the effects of the porosity on the microclimate within the clothing under low-temperature conditions.

Experimental Method

Materials

For the specimen, four waterproof breathable fabrics were selected: hydrophilic nonporous polyurethane (PU) laminating fabric (SP), microporous PU wet coating fabric (SW), ePTFE membrane laminating fabric (SF), and nano fiber web-layered fabric (SN). The test fabrics were supplied from Hyosung Co. Ltd., Korea. In terms of basic characteristics, the fabrics were thin, lightweight materials with a thickness of 0.17–0.25 mm and a weight of 96–175 g/m², as listed in Table 1. The ground fabric that combined with the membrane consisted of polyester and nylon. The thickness and the weight of the specimens were tested 5 times, and an average was taken.

Table 1.

Characteristics of the Test Specimens.

Specimen	Ground Fabric	Weight (g/m²)	Thickness (mm)	Yarn Count (5 cm × 5 cm)
Hydrophilic nonporous PU lamination (SP)	PET 100%	175 (.45)	0.21 (.01)	118 × 98 (2.39 × 1.52)
Microporous PU wet coating (SW)	Nylon 100%	98 (.26)	0.19 (.02)	144 × 98 (1.58 × 2.45)
ePTFE membrane lamination (SF)	Nylon 100%	99 (.12)	0.17 (.03)	170 × 118 (2.3 × 2.58)
Nano fiber web-layered fabric (SN)	Nylon 100%	96 (.13)	0.25 (.01)	178 × 118 (0.55 × 1.82)

Note. Standard deviation is given in parentheses. PU = polyurethane; ePTFE = polytetrafluoroethylene.

Scanning Electron Microscopy (SEM)

The surface characteristics of the fabric and the structure/thickness of the membrane and ground fabric were compared using a scanning electron microscope. The author coated samples with platinum for 150 s using the Cressington sputter coater 208HR. The surface was observed at 5,000× magnification and the cross-section at 50× magnification by field emission SEM (JEOL-6701F, JEOL Ltd., USA). The accurate thickness was measured by AxioVision SE64 software (Carl Zeiss, Germany), and the mean thickness of the membrane was calculated based on the minimum and maximum values.

Pore Size and Distribution

The liquid extrusion technique was applied to measure the pore size and distribution using a capillary flow porometer (1100-AEHXL, Porous Media Inc., Ithaca, NY). Among several pore types within the fabrics, such as through pore, blind pore, and closed pore, it is only possible to measure through pore sized 0.013–500 µm (Jena & Gupta, 2003). Galwick (Porous Media Inc., Ithaca, NY), with a surface tension of 15.4 dynes/cm, was used as the wetting agent, and the measurements of pore size were taken at a pressure range of 0–20 psi and a maximum flow limit of 200,000 cc/min. The analysis was based on three curve graphs: dry curve (data obtained from dry specimen), wet curve (wet specimen), and half curve (1/2 of the flow for the dry specimen; Tan & Obendorf, 2006). The smallest pore diameter, largest pore diameter, and pore distribution were measured automatically using Porous Media Inc. (Ithaca, NY) software based on data obtained from the dry and wet curves; the average pore size was calculated based on this (Frey et al., 2006).

WVTR

WVTR were measured using a Trul dish according to the KS K 0594:2008 (test methods for water vapor permeability of textile fabrics, Korean Standards Association) evaporation method at a rotation speed of 2 rpm; temperatures of 20°C, 15°C, 10°C, and 5°C ± 1°C; and a relative humidity of 50% ± 5%. In a synthetic conditioning room (Walk-in Environmental Test Chamber, ESPEC, US) with air circulation, the weight of the raw specimen and the weight of the specimen were measured after 6 hr. The WVTR was calculated based on the average of three measurements using the methods reported by Gibson (2000), which converted to g/m²/24 hr.

Microclimate Within the Clothing

To simulate the wearing conditions, the temperature and humidity in the microclimate were measured using a human–clothing–environment (HCE) simulator (Korea patent pending: 03-19136; Park & Kim, 2012). The simulator consisted of low- and high-temperature chamber environments. When a specimen is exposed to high and low temperatures and a specific amount of sweating is applied by setting the necessary conditions, it is possible to measure the skin temperature, the temperature/humidity changes of the microclimate within the specimen, and the electric energy. A sensor was installed between a heating plate and the waterproof breathable fabric. For the environmental conditions, the temperature was set to 20°C ± 0.5°C to −10°C at 5°C ± 0.5°C intervals, and the humidity was set to 50% ± 5%. For the experimental method, to evaluate the WVTR of the fabric under low-temperature conditions depending on the temperature in accordance with the protocol, a sensor was attached between the heating plate and specimen, and the humidity change measured every 10 s. After stabilization in the standard state for 10 min, the specimen was exposed to a low-temperature condition for 20 min with a sweat pulse of 140 µl/min. The temperature and humidity of the microclimate were measured for a total of 60 min, and the VP was calculated using the measured temperature and humidity.

Experimental Results and Discussion

Surface and Cross-Section Characteristics

The images of the surface (a–d) and cross sections (e–h) of the four types of waterproof breathable fabrics for SP, SW, SF, and SN are shown in Figure 1. In Figure 1(a), you can easily see the surface of the SP fabric, where there are no pores; the microporous fabrics types in (b)–(d) present different forms of pores. For the wet-coated SW (b), round and oval micropores were distributed irregularly with a size of 0.08–0.1 µm. For the SF (c), many expanded micropores were observed due to drawing; the drawing thickness of the ePTFE was 0.26–0.32 µm. For SN (d), the fiber thickness was 0.31–0.67 µm, and the distance between the fibers was 1.83–2.35 µm, which was not uniform. In addition, when the cross section of the fabric was filmed at a 50× magnification, the average thicknesses of the SP (e) and SW (f) fabrics were 57.97 and 46.365 µm, respectively. In the case of SF (g) and SN (h), the membrane formed a layered structure on the ground fabric. The membrane thicknesses of SF and SN were 33.89 and 19.44 µm, respectively. The three layers of SN (h), where another fabric was added on top of the membrane, can be observed. The sum of the thicknesses of the ground fabric and the membrane is in order of thickest to thinnest: SN, SP, SW, and SF; the membrane thickness, in order, is SP, SW, SF, and SN.

Figure 1.

Scanning electron microscopy images of the waterproof breathable fabric: surface section (a–d) and cross-sectional (e and f) images of hydrophilic nonporous polyurethane (PU) lamination (a and e), microporous PU wet coating (b and f), ePTFE membrane lamination (c and g), and nanofiber web-layered fabric (d and h). PTFE = polytetrafluoroethylene.

The Characteristics of Pore Size and Distribution

The pore size and distribution are proposed in Table 2 and Figure 2, respectively. Pore measurements were not possible for the SP. For the SW, the mean pore diameter was 0.094 µm. Within the range of 0.044–0.283 µm, the pore distribution was concentrated between 0.044 µm and 0.12 µm, and measurements above 0.12 µm accounted for less than 3%. Therefore, SW had the smallest pore size and most diverse distribution among the waterproof breathable fabrics used in this experiment. For the SF, the mean pore diameter was 0.226 µm within the range of 0.118–0.398 µm. The measurement of 0.23 µm accounted for 37.7%, and 0.25 µm for 13.8%. For the SN, the pores were distributed in the range, 0.305–1.824 µm. The mean pore diameter was 0.745 µm, and 0.7–0.8 µm of the pores accounted for approximately 85.2%. In other words, the pore size in order of largest to smallest is as follows: SN, SF, SW, and SP. The deviation of the smallest pore diameter and the largest pore diameter increased with increasing pore size. In the case of the actual dispersed distribution, however, the specimen with a small pore size showed a wide distribution.

Table 2.

Various Dimensional Parameters and Pore Size of SP, SW, SF, and SN.

Specimen	Pore Diameter (µm)
Specimen	Smallest	Mean	Largest
SP	—	—	—
SW	.04 (.05)	.09 (.02)	0.28 (.12)
SF	.12 (.07)	.23 (.07)	0.40 (.11)
SN	.31 (.12)	.75 (.17)	1.82 (.06)

Note. Standard deviation is given in parentheses.

Figure 2.

Pore size distribution for the microporous SW, SF, and SN specimen.

WVTR Depending on the Pore Size and Distribution of the Fabric and Membrane

The WVTR in the standard state based on pore size is shown in Figure 3. The WVTR increased with increasing pore size, and the SF and SN, where pores with a uniform distribution accounted for 85%, showed high WVTR (1,375 g/m²/24 hr and 1,437 g/m²/24 hr, respectively), while the SP was 707 g/m²/24 hr. This means that the porosity varied according to the manufacturing process, which affected the WVTR. In addition, for the SP, swelling occurred over time, which could be identified by the naked eye. The hydrophilic part of the membrane is believed to have expanded as the pores absorbed moisture. This is because the porosity, thickness, and fabric flexibility of hydrophilic fabrics changed when the fiber swelled with moisture absorption, which affects vapor diffusion through the pores by changing the geometric structure of the fabric (Benltoufa, Fayala, Cheikhrouhou, & Nasrallah, 2007). Accordingly, the moving path within the fabric is interrupted by moisture absorption and swelling as the vapor concentration difference between both sides of the specimen decreases with time, which could decrease the level of VPm.

Figure 3.

Water vapor transmission rate in accordance with the mean flow pore diameter.

WVTR Depending on the Thicknesses of the Fabric and Membrane

The thicknesses of the specimens used in this study were 0.17–0.21 mm, and there was no correlation between WVTR and fabric thickness, as summarized in Table 3. Unlike the SP, SW, and SF, consisting of the ground fabric and the membrane, the SN consisted of three layers with an added lining. Accordingly, when the ground fabric + membrane thickness excluding the lining was measured, the values were 0.21, 0.19, 0.17, and 0.16 for the SP, SW, SF, and SN, respectively. Therefore, the WVTR increased with decreasing thickness, where a negative correlation (r = −.9997**) was observed. In addition, when the thickness of only the membrane was measured, the SP, SW, SF, and SN had values of 57.965, 46.365, 33.89, and 19.44 µm, respectively. Therefore, the VPm was efficient when the membrane was thin. The membrane thickness and WVTR had a strong negative correlation (r = −.9989**).

Table 3.

Thickness and WVTR of the SP, SW, SF and SN Specimen.

Specimen	Thickness			WVTR (g/m²/24 hr)
Specimen	Fabric (mm)	Ground Fabric + Membrane (mm)	Membrane (µm)	WVTR (g/m²/24 hr)
SP	.21 (.01)	0.21 (.01)	57.97 (.08)	707 (0.31)
SW	.19 (.00)	0.19 (.00)	46.37 (.23)	1,144 (0.29)
SF	.17 (.03)	0.17 (.03)	33.89 (.07)	1,375 (1.71)
SN	.25 (.01)	0.16 (.01)	19.44 (.17)	1,437 (1.70)

Note. Standard deviation is given in parentheses. WVTR = water vapor transmission rate

WVTR Depending on the Temperature Change of the Fabric and Membrane

In the present study, to examine the effect of temperature on the moisture transfer of different waterproof breathable fabrics, the WVTR values of the fabrics were measured at different temperatures (20°C, 15°C, 10°C, and 5°C ± 2°C) at a relative humidity of 50% ± 5%. The results are presented in Figure 4. The WVTR decreased with decreasing temperature. When this was expressed as a percentage, it decreased by 11%, 15%, and 34% on average when the temperature decreased by 5°C under range of 20°C–5°C. The WVTR of all the specimens decreased with decreasing temperature. At a lower temperature, both the amount of reduced VPm and the reduced slope increased. The decrease in WVTR is due to the decrease in VP induced by the relative humidity, which has a proportional relationship with temperature. This is because the movement of gas molecules in the still air layer below the fabric slows as the temperature decreases. Moreover, the vapor diffusion through the fabric slows as the VP difference between both sides of the specimen decreases. In addition, the temperature change is believed to alter the relative humidity and moisture regain of the specimen, which affects the characteristics of the fabric.

Figure 4.

Water vapor transmission rate in accordance with the temperature change of the SP, SW, SF, and SN specimen.

Microclimate Temperature Change Under Low-Temperature Conditions

To examine the relationship between the temperature and moisture transfer of the waterproof breathable specimen, the temperature change in the microclimate within the clothing was measured using an HCE simulator at 20°C to −10°C, and the results for each specimen are presented in Figure 5. For the SP, the temperature of the microclimate was 32.7°C when the outdoor temperature was 20°C, which was maintained at 32.5°C after sweating, as shown in Figure 5(a). A similar temperature was maintained because VPm was efficient at an outdoor temperature of 20°C despite sweating. On the other hand, at outdoor temperatures of 10°C, 5°C, −5°C, and −10°C, the temperature of the microclimate decreased with sweating, and a constant temperature was maintained 10 min after sweating. At −10°C, the temperature decreased gradually after 10 min before the end of the experiment. In addition, the initial temperature decrease was larger at a lower outdoor temperature, which could be because the VPm speed decreased with decreasing temperature. On the other hand, the temperature did not decrease continuously and was maintained after the end of sweating. Therefore, it appears that VPm occurred even at subzero temperatures. On the other hand, the microporous specimens with different pore sizes and distributions—SW (b), SF (c), and SN (d)—presented patterns similar to that of SP depending on the outdoor temperature. Therefore, it is believed that although the temperature decreased during sweating, the temperature was maintained constantly without a further decrease over time due to efficient VPm.

Figure 5.

Microclimate temperature change in hydrophilic non-porous polyurethane (PU) lamination (a), microporous PU wet coating (b), ePTFE membrane lamination (c), and nanofiber web-layered fabric (d). PTFE = polytetrafluoroethylene.

Microclimate VP Change Under Low-Temperature Conditions

The VP changes of the microclimate for each specimen are shown in Figure 6(a)–(d). As the outdoor temperature decreased after stabilization of the specimen for 10 min, the VP decreased temporarily. On the other hand, the VP displayed a rising curve due to the effects of sweating and decreased gradually from 30 min when sweating had ended. In addition, the VP generally decreased with decreasing outdoor temperature. The vapor molecules in the microclimate could not discharge, as the temperature decreased abruptly from the standard state and was maintained at a low temperature. In addition, for the SN at 20°C, the VPm performance was outstanding, and thus, the VP maintained a constant without change. At room temperature, the VP of the microporous fabrics varied according to pore size and membrane thickness, although, at subzero temperature, the moisture transfer effect by pore size decreased significantly, regardless of the thickness of the fabric or membrane. The nonporous SP had a high VP, even at −10°C. Although the VPm was less than that of the other microporous fabrics, there was no difference depending on the pore size between each fabric. The VPm speed is believed to have decreased because the movement of the gas molecules within the clothing slowed as the outdoor temperature decreased. More outstanding VPm performance was observed when there were pores, but it was unaffected by the micropore size. At the same time, it does not seem to correlate with fabric or membrane thickness.

Figure 6.

Change in the microclimate vapor pressure under various temperatures for hydrophilic nonporous polyurethane (PU) lamination (a), microporous PU wet coating (b), ePTFE membrane lamination (c), and nanofiber web-layered fabric (d). PTFE = polytetrafluoroethylene.

Microclimate VP Change Within the Clothing Under the Various Temperatures

To examine the differences in the microclimate VP within the clothing depending on the porosity based on temperature, the relative humidity of the microclimate within the clothing was measured in the range of 20°C to −10°C. The measured relative humidity was converted to VP, as shown in Figure 7. The high VP shown in the graph could be due to the inefficient VPm during sweating. The microclimate VP within the clothing was low when the WVTR was outstanding, which is consistent with the results of other research (Stankovic & Bizjak, 2014). As can be seen in Figure 7(a), depending on the pore size, the VP was in the order of SP, SW, SF, and SN. Therefore, the VPm speed was slow when the pores were small, which is consistent with the result of the WVTR based on the WVT cup method, as can be seen in Figure 4. For SF, SW, and SP, the VP increased from the start of sweating, reached the peak value just before the end of sweating (30 min), decreased gradually, and almost recovered the initial VP at 60 min. The different rising degree and descending speed of the VP at the start of sweating could be due to the effects of pore size and thickness. On the other hand, for SN, with the largest pore size, the VP maintained at approximately 13 mbar without an increase despite the start of sweating. The humidity within the clothing was maintained at a constant, as VPm occurred immediately in the case of sweating because the pores were large and the membrane was thin.

Figure 7.

Microclimate vapor pressure change according to porosity in waterproof and breathable fabric under above zero temperatures at 20°C, 50% RH (a), 10, 50% RH (b), 5°C, 50% RH (c), and subzero temperatures at −5°C (d) and −10°C (e).

By varying the environmental conditions, the microclimate within the clothing was measured at a humidity of 50% and temperatures of 10°C and 5°C, as shown in Figures 7(b) and (c), respectively. Similar to the result in the 20°C, the VP was in the order of SP, SW, SF, and SN, indicating that the VP was high when the pores were small. At 20°C and 10°C, there was a distinct change, and the fabric at 10°C and 5°C showed similar trends. Moreover, the 5°C condition showed a smaller VP deviation between the specimens than the 10°C environment. The effect of the outdoor temperature was believed to be larger than those of the pore size and membrane, and the difference decreased gradually with increasing temperature. On the other hand, unlike the result at 20°C, the VP decreased for 8–10 min with the start of sweating and then increased. After reaching the maximum VP, it decreased. The VP within the human body fabric was believed to have decreased abruptly as the specimen was conditioned in the standard state for 10 min for stabilization, and the temperature of the specimen was decreased immediately to 10°C at the start of the experiment. This could be identified by the increase in the descending rate for the gradient of the graph at subzero temperatures. In other words, the change in VP at the subzero temperature was different from that at the above-zero temperature, as shown in Figure 7(d) and (e). The VP at −5°C was still proportional to the pore size in the order of SP, SW, SF, and SN. In other words, at −5°C, VPm was still efficient when the pore size was large, but SN and SF showed no difference. In addition, considering that the membrane thicknesses of SN and SF were 17.54–21.34 µm and 30.56–37.22 µm, respectively, it is believed that despite the difference in the thickness, the VP was solely affected by temperature regardless of the thickness and porosity. In contrast, SW and SP showed different VP, and outstanding WVTR was observed depending on the pore size. In addition, before and after the maximum VP, the VP decreased by as much as the increased amount and reached the initial VPmin status. Therefore, moisture transfer was efficient. When the microclimate within the clothing was measured at −10°C, the hydrophilic nonporous SP still showed the highest maximum VP, and the VP was high even after 60 min, showing the poorest VPm performance. Similar to the result at −5°C, SF and SN showed consistent trends. The degree of VPm was identical because the VP was identical, and the deviation from SW with the third largest pore size decreased. From the end of sweating to the completion of the experiment, they also showed an overlapping pattern, and the VP was identical. They were affected more by the temperature over time, and the VP decreased with temperature rather than by the pore size, distribution, and membrane thickness.

WVTR and VP of the Microclimate Depending on the Outdoor Temperature

The minimum VP (VPmin) and maximum VP (VPmax) used to examine the correlation between the VP and WVTR of the microclimate are compared in Figure 8. The high VPmin and VPmax of the microclimate indicate an inefficient VPm. At a specific temperature, the WVTR was low when the VPmin was high, and the gradient increased with decreasing outdoor temperature. In addition, the abrupt increase in gradient for the 20°C/15°C condition and the 10°C/5°C condition indicates that the WVTR decreased abruptly with decreasing temperature. At 20°C, 15°C, 10°C, and 5°C, the WVTR and VPmin had strong negative correlations. The WVTR was low when the VPmax was high, and the gradient increased with decreasing outdoor temperature. At 20°C, 15°C, 10°C, and 5°C, the WVTR and VPmax had strong negative correlations. In addition, the fabric with a high WVTR showed the lowest VPmax, indicating an inversely proportional relationship.

Figure 8.

Relationship between the WVTR with the minimum vapor pressure (a) and maximum vapor pressure (b). WVTR = water vapor transmission rate.

Conclusions

The authors examined the effects of porosity on the moisture transfer properties of waterproof breathable fabric under low-temperature conditions using the water vapor transmission cup method and the HCE simulator. The WVTR of the waterproof breathable fabric was measured at room temperature; the WVTR was then evaluated under sweating conditions at low temperatures to examine the effects on the microclimate within the clothing. The analysis was conducted based on the fabric thickness and pore size/distribution, and the following conclusions were obtained.

First, the membrane structure, pore shape, and thickness of the waterproof breathable fabric varied according to the manufacturing method. The membrane thickness of the SW was the thickest and that of the SN was thinnest. In addition, the microporous fabrics had different pore sizes, distributions, and shapes. The SW had the smallest pore size with a widely dispersed distribution, and the SN had the largest pore size with the porosity of 85%. Second, the WVTR depending on temperature was measured, and the results showed that the WVTR decreased when temperature was reduced. Therefore, the deviation between the specimens decreased. Under the same environmental conditions, the WVTR was excellent when the pores of the membrane were large and uniformly distributed. Third, porosity and environmental conditions affected the VP depending on the microclimate change within the clothing as well as the WVTR, which is the moisture transfer property of the waterproof breathable fabric. For the VP of the microclimate, which was proportional to the pore size in the standard state, the level of decrease in the VPmin decreased with decreasing outdoor temperatures, which reduced the VPm rate. At subzero temperatures, the deviation of the VPmax between the specimens decreased, and there was almost no difference in VP between the microporous fabrics. It was discovered that pore size, distribution, and environmental temperature affect the WVTR and microclimate within the clothing at the same time. The WVTR had a proportional relationship with porosity and temperature. On the other hand, the VP of the microclimate within the clothing was proportional to the porosity at above-zero temperatures when the temperature was identical, but there was no correlation at subzero temperatures. Therefore, in the case of the waterproof breathable fabrics for outdoor sportswear that are produced in various forms depending on function, a moisture transfer evaluation needs to be performed based on the actual clothing wearing season rather than the standard state.

Footnotes

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) received no financial support for the research, authorship, and/or publication of this article.

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