Water-resistant and vapor-permeable textile laminates containing nanofiber membrane as footwear lining

Abstract

This paper aims to test the suitability of two nanofibrous membranes as a lining in outdoor shoes, especially leather ones. The motivation is that leather shoes offer some water resistance but are not fully waterproof. By inserting a membrane, higher hydrostatic resistance is achieved while maintaining the vapor permeability of the upper material - leather. Two nanofibrous membranes were developed, and three-layer and four-layer laminates were made from them. In addition to the membrane, the lining, and the top material, the four-layer laminate also contains a thermal insulation layer. These laminates were investigated concerning the following properties: hydrostatic resistance, water vapor permeability, thermal insulation, and air permeability, while the first two properties were chosen as crucial. For the study to be complete, the sample set was supplemented with commercially available membranes. It was found that laminates containing nanofiber membranes provide interesting properties: hydrostatic resistance higher than 10 m and water vapor permeability around 5 Pa.m².W⁻¹, which is promising, especially considering the vapor permeability of the shoe’s upper material – leather. From the selected laminate, a cut was designed, and an inner shoe was realized, which was subsequently incorporated into outdoor leather footwear.

Keywords

Nanofiber membrane footwear outdoor hydrostatic resistance water vapor permeability

Introduction

Footwear is crucial in meeting various human needs and can positively and negatively impact foot health. Orthopedic experts highlight that improper footwear can lead to irreversible foot deformities and diseases. Footwear should fulfill several essential functions: (a) protective - shielding the foot from mechanical injuries, environmental factors, chemicals, and harmful radiation; (b) hygiene and comfort - ensuring well-being by maintaining a healthy foot environment, providing ventilation, antimicrobial protection, biomechanical support, and proper anatomical design, (c) social and aesthetic: aligning with fashion trends, maintaining appearance, and being affordable.¹

In today’s world, with advancing technology and increased use of chemicals in manufacturing, the comfort and safety of footwear are paramount. Overall comfort is subjective and difficult to measure but is essential for all footwear types. It is closely linked to the proper fit, material properties, and the individual user’s foot characteristics.¹ Thermo-physiological comfort is closely related to the overall comfort of footwear, as it directly influences the wearer’s experience and satisfaction.

Thermo-physiological comfort refers to the ability of the footwear to maintain an optimal microclimate around the foot by regulating temperature and moisture levels, which means effectively managing heat and moisture produced by the feet during activity. When footwear provides good thermo-physiological comfort, it helps prevent the accumulation of heat and sweat, reducing the risk of discomfort, blisters, and foot odor. Proper ventilation, moisture absorption, and breathability contribute to this comfort aspect. Thermo-physiological comfort of footwear is significantly influenced by external conditions such as rain and very low air temperatures. In rainy conditions, footwear must effectively manage moisture from the outside and inside, which means being waterproof. In low air temperatures, footwear must retain heat to keep the feet warm while allowing moisture to escape. Adequate footwear generally ensures the feet remain dry, warm, and comfortable.

Each activity requires different footwear specifications. A study² assessed footwear preferences and microclimates of shoes made from leather versus mesh spacer fabric. Above mentioned study, which involved 80 respondents, revealed that most preferred sports shoes are made of mesh fabric, as leather sports shoes retained more heat and moisture. On the other hand, footwear with an upper made of leather is an excellent choice for outdoor use because it offers durability, satisfactory water resistance, comfort, breathability (water vapor permeability), acceptable heat insulation, and protection. As mentioned above, leather shoes offer some water resistance. Still, they are not fully waterproof and may absorb moisture in wet conditions like wet tall grass unless treated with waterproofing products. However, even with treatment, they may eventually allow water to seep during extended exposure to moist conditions.

Textile laminates with breathable membranes are used extensively in waterproof footwear to ensure permeability for water vapor from the inside and, simultaneously, waterproofness from the outside. A wide variety of semi-permeable polymer membranes allows the creation of multilayered textile laminates with distinctive properties.³

In the following text, the following two quantities will be used to characterize the water vapor permeability of the material: (a) water vapor permeability (VP) which refers to the ability of a material to allow water vapor to pass through it. A higher water vapor permeability means that the material allows more vapor to pass through; (b) Ret value in contrast, or resistance to evaporative heat transfer, is a measurement used to indicate water vapor resistance of a material. It quantifies how much a material resists water vapor moving through it, with lower Ret values representing higher breathability (and thus lower resistance to vapor transfer).

Waterproof breathable membranes (next to densely woven and coated waterproof breathable layers) can be classified into two large groups of hydrophilic non-porous and hydrophobic porous layers. These layers can be produced by various continuous and non-continuous processes such as coating, film stretching, casting, electrospinning, etc. In hydrophilic non-porous or dense membranes, water vapor transfer primarily occurs through the solution-diffusion mechanism, facilitated by hydrophilic segments or side groups (for example, Sympatex® membrane).^4,5 In contrast, gas and water vapor are mainly transmitted through the pores in porous hydrophobic membranes. However, the solution-diffusion mechanism still plays a role and cannot be entirely disregarded.⁶ Common materials used for these membranes include polytetrafluoroethylene (PTFE) and polyurethane (PU).⁷ PTFE, known commercially as Gore-Tex® or eVent®, is widely used in high-performance footwear due to its excellent breathability and waterproofing capabilities. OutDry® and Puratex® are patented waterproofing technologies made of PU.⁸ These membranes are frequently combined with a protective layer of polyamide or polyester fabrics, which may be woven, knitted, or non-woven using the lamination technique. Figure 1 shows the water vapor transport through a waterproof layer via the abovementioned mechanisms. Three types of flow throught pores based on the pore radius can be distinguished (valid for hydrophobic porous membranes) and solution-diffusion flow (valid for hydrophilic non-porous membranes) is visible from the Figure 1. The main parameters influencing water vapor permeability are the thickness of the layer (t), capillary pore radius (r), and vapor pressures on both sides of the membrane surface (ΔP).⁶

Figure 1.

Scheme of four mechanisms of water vapor transport via a waterproof layer depending on the pore size.⁶

Footwear heat loss and moisture retention using various waterproofing solutions was quantified in paper.⁹ It found that Gore-Tex with IQ Tex led to higher heat loss during walking without sweating compared to other membranes. Additionally, Gore-Tex with IQ Tex retained more moisture in the sock and insole than the OutDry membrane. The repeated process of water vapor absorption and release affects the hygienic properties of textile footwear lining laminates was examined in paper.³ The study found that the semi-permeable polymeric membrane Puratex reduces the rate of water vapor desorption and slows down the drying process of the footwear textile laminate. Thermal-insulation properties of laminates containing hydrophobic porous membranes were studied in.¹⁰ The authors summarize that the thermal insulation effect is mainly influenced by other textile layers that make up the laminate; the membrane has an entirely minor effect. The article¹¹ confirms the same phenomenon. Thermal properties of laminates intended for protective footwear for freezing environments were studied in paper.¹² These laminates contained knitted fabric, membrane (eVent®, Sympatex®), aerogel, or another thermal insulation layer (Thinsulate^TM). Laminated aerogel composite (sample marked LAC) provides waterproof and breathable footwear with a competitive weight and a much higher thermal resistance at a lower thickness of footwear than other advanced materials in this area, having thermal resistances of 0.25 m²·K·W⁻¹.

Electrospinning is a relatively new and promising technology for producing waterproof, breathable fabrics based on porous hydrophobic structures. Although the setup is simple, optimizing parameters like polymer type, solvent, concentration, voltage, spinneret-to-collector distance, temperature, humidity, and feed rate is crucial for achieving the desired fiber diameter, porosity, and pore size distribution. Recent advancements include needleless electrospinning setups.⁶

Electrospinning was used to explore the production of micro- and nanofibers and non-woven materials from polyether-urethane (PU) solutions in paper.¹³ It examines how adding hydrophilic polymers like polyvinyl alcohol (PVA) and polyacrylic acid (PAA) to PU solutions affects the structure and average diameter of the fibers. The study finds that incorporating these hydrophilic polymers improves the hygienic properties of the resulting non-woven materials, allowing the use of these membranes in bags for clothing and footwear. Following papers^14,15,16 introduce laminates containing nanofiber layers for filtration purposes mainly (face masks, window ventilation barriers, etc.). The effect of lamination temperature of laminate intended for clothing containing PAN nanofiber web was studied in.¹⁷ It was found that the increased lamination temperature has a statistically significant adverse effect on the air permeability of the laminate. Microporous hydrophobic fluoroplastic membranes were electrospun and described in paper¹⁸ using lab scale conditions. Nevertheless, the vapor permeability (VP = 1000 g/m²/24 h) of produced membranes exceeds Gore-Tex values and have exciting water resistance (WR = 16 m). Laminate containing PU nanofiber web was subjected to repeated washing to investigate the effect of maintenance on its mechanical properties. The authors concluded that the maintenance of laminate does not have a significant negative impact on mechanical properties, and this laminate has the potential to be used for outdoor wear.¹⁹ PU nanofiber layer was combined with one or two traditional textile fabrics to produce two-layered and three-layered laminate to check their suitability for sport and outdoor clothing.²⁰ The water vapor permeability was relatively high (Ret = 3 Pa·m²·W⁻¹), whereas waterproofness was excellent (WR = 24 m) for three-layered laminate. Another paper²¹ presents the transport properties of two and three-layered laminates, including the PU nanofiber layer, and studies the effect of other textile layers, whereas authors emphasize the possibility of designing the laminate to achieve a combination of high barrier and comfort performance. The most interesting thing seems to be that laminate marked C1 has WR = 7 m and VP = 5000 g/m²/24 h. The waterproof breathable performance of laminated fabric containing PU electrospun nanoweb and PTFE membrane was compared under different environments in the study.²² It can be concluded that the water vapor transmission rate of the nanoweb laminate was remarkably higher than that of the PTFE laminate, but the nanoweb laminate had lower water resistance than the PTFE laminate.

From the conducted literature research, it follows that the nanofibrous membrane could be an interesting competitor to traditional membranes (e.g., PTFE type). In contrast, its use in footwear has not yet been investigated.

This paper aims to test the use of a nanofibrous membrane as a lining for leather shoes for outdoor use. Traditional leather shoes of this type do not have sufficiently high waterproofing. In addition to surface treatments, the nanofiber membrane could provide permanent waterproofing (at least WR = 10 m) while maintaining the water vapor permeability of the upper material. Generally, for footwear leather, the Ret value typically falls within the range of 5 to 15 Pa·m²/W (equal to VP = 192 to 480 g/m²/24 h), depending on the specific type and treatment of the leather. This range indicates that footwear leather generally offers moderate to good breathability and is suitable for comfort in outdoor conditions.

Two laminates containing PU nanofiber membranes of different mass per unit area were fabricated. The properties of laminates made of these fabricated membranes (three- and four-layered) were then compared with those currently used in commercial shoes meant for outdoor activities. The laminates used for comparison were made of hydrophobic microporous membrane (PTFE) and hydrophilic membrane (Sympatex®). Based on heat and mass transfer (air permeability, water vapor permeability, hydrostatic resistance), which are essential for achieving the desired thermo-physiological comfort, a laminate with an optimal material composition and construction was chosen. An inner shoe was designed and made from optimized material (laminate), which can be inserted into the leather shoe as a functional lining.

Materials

Nanofiber layers - membranes

The nanofiber layers were made of polyurethane (PU). The production was performed in a laboratory using the Nanospider machine. This technology enables the mass production of individual continuous nanofibers (diameters from 100 to 500 nm) from various polymers. The method is based on the fact that Taylor cones can be created on a thin layer of polymer solution. This method does not use any nozzle or spinning capillary but uses a rotating cylinder as an electrode. This electrode is partially immersed in a polymer solution and rotates slowly. The opposite electrode, the collector, is placed above it, and a substrate moves across this collector electrode. The high voltage causes Taylor cones to appear on the surface of the rotating electrode, which in turn produces nanofibers. The nanofibers are directed toward the collector, forming a non-woven nanofiber mat on the substrate. More information about the roller electrospinning technique can be found for example in paper.²³

The PU polymer solution was added to a basin with a rotating roller. This solution was exposed to an electro-field voltage of U = 76.1 kV. A collector was placed above the basin at a distance of 175 mm. The speed at which the substrate was moving was set at v = 0.1 m/min. The relative humidity in the spinning chamber was regulated at 21%. The humidity sensor was not placed directly in the spinning chamber but in the tube, bringing the air to the chamber to ensure proper sealing. The turning of the roller (driven by a rotor) in the basin created a thin polymer solution layer on its surface from which, in turn, nanofibers were formed due to the high voltage and collected on the substrate (spun-bond non-woven fabric, GSM = 30 g/m²). The nanofiber layer was prepared at two different levels of areal density, namely 7 g/m² and 14 g/m². The basic properties of layers are shown in Table 1. Microscopic images of both nanofiber layers are shown in Figure 2.

Table 1.

Basic properties of nanofiber layers - mean values ( $\bar{x})$ and standard deviations (std) are shown.

	N7		N14
	$\bar{x}$	Std	$\bar{x}$	Std
GSM (g/m²)	7		14
Number of pores [1/cm²]	87 700 000	6 600 000	95 400 000	7 100 000
Surface porosity [-]	0.168	0.016	0.175	0.18
Equivalent pore diameter (nm)	0.453	0.179	0.415	0.150
Fiber diameter (nm)	215	0.014	213	0.016
Thickness (µm)	3	-	6	-

Figure 2.

Microscopic images of nanofiber layers using magnification 2000× (a) 7 g/m², (b) 14 g/m².

Traditional membranes

In this study, two representatives of commercially available membranes were chosen for comparison. The first one (M1) is a hydrophobic porous membrane made of PTFE, and the second one (M2) is a hydrophilic membrane made of polyester/polyethylene (70/30) mixture. The basic properties of these commercial membranes are shown in Table 2. Microscopic images of these membranes are shown in Figure 3.

Table 2.

Thickness and GSM of commercially available membranes.

	M1	M2
GSM (g/m²)	12	17
Thickness (µm)	13	20

Figure 3.

Microscopic images of commercially available membranes: (a) M1, (b) M2.

Production of laminates

In addition to using membranes, three types of fabrics were chosen to produce laminates: upper fabric, lining, and thermal insulation layer, see macroscopic images shown in Figure 4. Details of these fabrics are shown in Table 3.

Figure 4.

Macrocopic images of accompanying fabrics: (a) upper fabric, (b) thermal insulation fabric and (c) lining.

Table 3.

Details of accompanying fabrics (upper fabric, thermal insulation fabric, and lining).

	Upper fabric	Thermal insulation fabric	Lining
Type	Knitted	Non-woven	Knitted
Structure	Warp KF	Needle punched	Warp KF
Material composition	80% PET/+ 20% PA	100% PA6	100% PET
GSM (g/m²)	180	120	30
Thickness (mm)	0.59	1.46	0.40

Polyurethane adhesive points having an areal density of 7 g/m² were applied where desirable (upper fabric, membrane, thermal layer) to create a two-layered laminate. After 24 h, additional adhesive was transferred onto this two-layer laminate to create a three-layered laminate. The same procedure fabricated four-layered laminate (containing an additional thermal insulation layer). The lamination process parameters are listed in the Table 4. The scheme of the laminates is shown in Figure 5.

Table 4.

Setting of lamination process.

Type of the line	Lacome laminating line
Glazing roller temperature (°C)	115
Pressure between rollers (bar)	4
Speed (m/min)	2
Type of adhesive	PU based (Henkel) – Technomelt 9350

Figure 5.

Diagram of: (a) three-layered and (b) four-layered laminate, where: U – upper fabric, M – membrane, T – thermal insulation fabric, L – lining and half spheres represent lamination points.

The upper fabric, lining, and membrane (or additionally as well thermal insulation layer) were connected by lamination technology, and the following three-layer and four-layer laminates were fabricated (see Table 5 for labeling, structure, thickness, and areal density of laminates).

Table 5.

Details of the composition of all laminates.

Sample label	Upper fabric	Thermal insulation fabric	Lining	Membrane type	Thickness (mm)	GSM (g/m²)
L3N7	Y	N	Y	N7	1.47	236.0
L3N14	Y	N	Y	N14	1.47	238.3
L3M1	Y	N	Y	M1	1.51	235.3
L3M2	Y	N	Y	M2	1.53	247.0
L4N7	Y	Y	Y	N7	2.90	435.7
L4N14	Y	Y	Y	N14	2.90	450.7
L4M1	Y	Y	Y	M1	2.92	447.3
L4M2	Y	Y	Y	M2	3.10	448.3

Note: Y – the given fabric is present, N – the given fabric is not present.

Methods

As mentioned above, the main goal is to get a laminate that will have a higher waterproofness than the shoe’s upper material (leather, in this case) while maintaining water vapor permeability. That is why the resistance to water vapor transmission (Ret), which serves as an indicator of water vapor permeability (higher resistance to water vapor transmission causes lower water vapor permeability), and the water resistance (WR), an indicator of water column level, were measured. A material with high water vapor permeability (low Ret value) would have a Ret value below 6 Pa·m²/W. Moderate breathability is in the range of 6 to 13 Pa·m²/W. Poor water vapor permeability (high Ret value) is above 13 Pa·m²/W according to.²⁴ The minimum hydrostatic pressure at which fabric is deemed rainproof is 5000 mm water column, with waterproof materials able to withstand pressures of 10,000 to 15,000 mm. The hydrostatic pressure ranges from 15,000 to 30,000 mm water column for high-quality waterproof materials intended for harsh conditions. These fabrics remain fully waterproof even under extremely high pressure.²⁵ Moreover, the air permeability (AP) was measured. To increase the thermal insulation ability of the lining, an additional layer was added to the laminate to create a four-layered laminate, and resistance (TR) was also evaluated.

The measurements were conducted on the following equipment under the stated standards. Air permeability AP [l/m²/s] was measured by instrument Textest 3300 following ISO 9237,²⁶ test pressure: 100 Pa, number of repeated measurements per sample n = 5. Water vapor permeability Ret [Pa.m².W⁻¹] was established using instrument Permetest per ISO 11 092,²⁷ n = 5. Hydrostatic resistance, HR [cm] was evaluated by instrument HydroPro Hydrostatic Head Tester by ISO 20811.²⁸ The measurement was terminated as soon as the first 3 drops appeared on the fabric’s surface, n = 3. Thermal resistance TR [m²K/W] was measured with the help of instrument Alambeta,²⁹ n = 3. More details can be found in paper.³⁰ Mass per unit area GSM [g/m²] of all samples and thickness t [mm] of knitted and non-woven samples was evaluated per EN 12127³¹ and ASTM D1777-96³² (pressure: 1 kPa, measuring head area: 20 cm²), respectively, n = 5. Fiber diameter d [nm], porosity P [-], and thickness of the nanofiber layer t [µm] were evaluated based on SEM images and NIS Elements imagining software.³³ In the case of thickness and porosity evaluation, images of membrane sections were taken at 2000× magnification. For each membrane, the statistical quantities were determined from 100 measurements from one image.

Results and discussion

This paper aimed to test the use of a nanofibrous membrane as a lining for leather shoes for outdoor use because traditional leather shoes of this type do not have sufficiently high waterproofing. The nano fibre membrane laminate could provide permanent waterproofing (requirement: WR >10 m) while maintaining the water vapor permeability of the upper material. Moreover, laminates containing additional thermal insulation layer were fabricated. The results – mean values and standard deviations are shown in Table 6. The results were evaluated using QCExpert statistical software. Analysis of variance (ANOVA) was used with a significance level chosen at α = 0.05. Scheffé's method of pairwise comparison was used for a more detailed analysis.

Table 6.

Measured values of hydrostatic resistance, water vapor resistance, air permeability, and thermal resistance (mean values and standard deviations are shown).

Sample label	HR (m)		Ret (Pa.m².W⁻¹)		AP (l.m⁻².s⁻¹)		TR (m²·K.W⁻¹)
Sample label	$\bar{x}$	Std	$\bar{x}$	Std	$\bar{x}$	Std	$\bar{x}$	Std
L3N7	15.1	1.4	5.02	0.08	5.53	0.38	0.0289	0.00015
L3N14	15.2	0.6	5.14	0.11	4.12	0.11	0.0297	0.00058
L3M1	18.0	2.50	7.24	0.17	3.32	0.15	0.0267	0.00058
L3M2	16.1	6.7	12.54	0.26	0	0	0.031	0.001
L4N7	31.6	3.5	11.98	0.08	5.57	0.16	0.0907	0.00153
L4N14	32.0	1.2	12.08	0.08	3.99	0.08	0.091	0.001
L4M1	31.0	8.4	15.48	0.31	1.92	0.09	0.0907	0.00058
L4M2	30.2	3.7	21.82	0.47	0	0	0.0807	0.00153

Figure 6 shows the mean values and 95 % confidence intervals of means for all measured variables – hydrostatic resistance, water vapor permeability, air permeability, and thermal resistance. Using a grouped bar graph, a comparison of three-layer and four-layer laminates with the same composition was made (the only difference is the presence of a thermal insulating layer).

Figure 6.

Comparison of: (a) hydrostatic resistance, (b) water vapor permeability, (c) air permeability, and (d) thermal resistance for all types of laminates (shown are mean values and 95% confidence intervals).

As mentioned above, HR and Ret are the most crucial properties of the laminate. The HR of 3-layer laminates reaches at least 15 m of the water column, which is similar or even higher compared to results presented in the following papers.^18,21 The difference between the 3-layer laminates (the effect of the membrane type) is not statistically significant. Four-layer laminates reach a waterproof rating of about 30 m of water column. Also, in the case of 4-layer laminates, the effect of membrane type is negligible. From the comparison, adding a thermal insulation layer to the laminate significantly increases its waterproofness, caused by both the extra layer and the lamination points. The comparison is done, and ratios of measured mean values of properties between four- and three-layer laminates are shown in Table 7. A statistically insignificant effect of the area weight of the nanofibrous membrane on HR was found, which means using a membrane with a higher area weight does not achieve a statistically significant increase in waterproofness, see Table 8, where p-values for two-sample t-tests are shown. This test returns a test decision for the null hypothesis that the data come from independent random samples from normal distributions with equal means and equal but unknown variances. p-value of the test is a scalar value in the range <0,1>. p-value <0.05 indicates the rejection of the null hypothesis at the α = 0.05 significance level. The alternative hypothesis is that the data come from populations with unequal means.³⁴ It can be summarized that the condition HR >10 m was therefore fulfilled.

Table 7.

Ratios of measured mean values of properties between four- and three-layer laminates.

	HR (mm)	Ret (Pa.m².W⁻¹)	AP (l.m⁻².s⁻¹)	TR (m²·K/W)
L4N7 vs L3N7	2.09	2.39	1.01	3.14
L4N14 vs L3N14	2.11	2.35	0.97	3.06
L4M1 vs L3M1	1.72	2.14	0.58	3.40
L4M2 vs L3M2	1.88	1.74	N	2.60

Table 8.

p-values of t‐test (effect of nanomembrane GSM on three output variables).

(N)	HR (mm)	Ret [Pa.m².W⁻¹]	AP (l.m⁻².s⁻¹)
L3N7 vs L314	0.455	0.099	<0.001
L4N7 vs L414	0.169	0.095	<0.001

When researching water vapor permeability, it can be seen that laminates with nanofiber layers reach a Ret of around 5 Pa.m².W⁻¹, which is excellent, especially considering the vapor permeability of the upper material of the shoe - leather. It is interesting to note that laminates containing a nanofiber membrane for clothing purposes typically achieve a water vapor permeability of approximately 3 Pa.m².W^{−1 20}. Laminates containing traditional membranes (PTFE, Sympatex) show a statistically significantly higher Ret (7 and 12 Pa.m².W⁻¹), i.e., lower water vapor permeability. If a thermal insulation layer is added, the vapor permeability is reduced to approx. 50 % of the original value for all laminates; see Table 7 containing the ratios. Even in this case, the reduction in permeability for water vapor is caused by a thermally insulating layer of relatively high thickness, which prevents water vapor from passing through. The t test confirmed no effect of nanofiber membrane GSM on Ret; see Table 8, which shows the p-values of this test. It can be summarized that three-layer laminate containing nanofiber membrane fully meets the requirement to maintain the vapor permeability of the upper material - leather. In the case of the four-layer nanofiber membrane laminate, its vapor permeability (Ret = 12 Pa.m².W⁻¹) is also within the vapor permeability limits estimated for different types of shoe leather (5 < Ret < 15 Pa.m².W⁻¹).

Air permeability was evaluated rather informatively. The three-layer laminate containing the N7 nanofibrous membrane exhibits the highest breathability values (AP ∼ 6 L.m².s⁻¹). The laminate with the N14 membrane shows breathability 1.4 L.m².s⁻¹ lower than the laminate with the N7 membrane; therefore, a statistically significant effect of nanomembrane GSM on the air permeability of laminate was confirmed both using a visual comparison of the confidence intervals of the means and a t test, see Table 8. A three-layer laminate with a PTFE membrane achieves a permeability of only about 2 L.m².s⁻¹, and a three-layer laminate with a hydrophilic membrane has even zero air permeability, which agrees with previously published results.³⁵ A thermal insulation layer statistically significantly reduces air permeability only when using the M1 (PTFE) membrane.

The thermal comfort of footwear designed for extremely low-temperature conditions is ensured by the use of a thermal insulation layer and, thus, the use of four-layer laminates in this study. It can be observed that the TR of three-layer laminates is around 0.03 m²KW⁻¹, while the influence of the membrane type is extremely low. TR of four-layer laminates is around 0.09 m²KW⁻¹. The thermal insulation ability of four-layer laminate developed in this study is close to the thermal insulation ability of laminate made of silica aerogel and Sympatex membrane¹² (TR = 0.177 m²KW⁻¹, t = 3 mm, GSM = 652 g/m²) having slightly higher thickness and significantly higher GSM compared to L4N7 or L4N17 sample. The thermal resistance of the four-layer laminate was increased by approx. three times compared to the three-layer laminate, see Table 7.

Inner shoe design

Based on previous testing and data analysis, the laminate L3N7 and L4N7 containing nanofiber membrane with the lower GSM was chosen. Laminate L4N7 contains an additional thermal insulation layer compared to L3N7 laminate. When considering all thermo-physiological properties, the L3N7 laminate excels in its very high water vapor permeability and improved hydrostatic resistance compared to the upper material. Therefore, its use is prospective for summer - autumn shoes for wet environments - wet grass, etc. High water vapor permeability will ensure leg comfort, especially during higher physical activity and increased sweating. For this use and higher air temperatures, thermal insulation is not required. Laminate L4N7 could be used in boots designed for winter use, especially in wet or snowy conditions, as this laminate provides both high thermal insulation and extremely high water column (HR ∼ 30 m).

Based on heat and mass transfer (air permeability, water vapor permeability, hydrostatic resistance), which are essential for achieving the desired thermo-physiological comfort, a laminate with an optimal material composition was chosen. The main goal was to improve the thermo-physiological properties of outdoor shoes, especially for hiking, where additional thermal insulation is not required. An inner shoe was designed and made from optimized material (laminate L3N7), which can be inserted into the leather shoe as a functional lining, ensuring significantly increased hydrostatic resistance while maintaining vapor permeability of the upper material – leather. The photo of the inner shoe made of L3N7 laminate is shown in Figure 7. From the picture, you can see that the seams are taped not to reduce the product’s waterproofness. This inner shoe is then inserted into the leather shoe (the lower part of the inner shoe is in contact with the outsole and remaining part with the leather shoe) before the shoe is finally finished - stitching the shoe’s upper ankle part and the shoe’s tongue. The photos of the final product – leather boots, including functional lining-are shown in Figure 8. These boot prototypes were produced by a company called Prabos Plus a.s., Czech Republic specializing in boots for alpine hiking.

Figure 7.

Photo of the inner show made of L3N7 laminate.

Figure 8.

Photos of leather boots with functional lining made of L3N7 included: (a) view from the outside, (b) detail of sewing the functional laminate into the shoe.

Conclusion

The main aim of this paper was to test laminates with a nanofiber membrane used as a lining in shoe production for use in outdoor activities, mainly hiking and to evaluate its thermo-physiological properties. Hydrostatic resistance, water vapor permeability, air permeability, and thermal resistance were chosen as the most essential properties. Laminates containing nanofiber layers were compared with laminates containing microporous and hydrophilic membranes, which are commercially available. During the development of the laminates, two types of nanofibrous membranes were produced, differing in the areal weight – 7 g/m² and 14 g/m². Using nanofiber membranes and commercially available membranes, three-layer and four-layer laminates were prepared using the same accompanying fabrics. Four-layer laminate contains a thermal insulation layer (in addition to the lining, top material, and membrane). All laminates were evaluated based on the selected properties mentioned above.

The hydrostatic of 3-layer laminates containing nanofiber membranes reaches at least 15 m of water column, comparable to laminates containing commercially available membranes. Laminates with a nanofiber layer reach a resistance to water vapor of around 5 Pa.m².W⁻¹, which is less than satisfactory, especially considering the vapor permeability of the shoe’s upper material - leather. Laminates containing traditional membranes (PTFE, Sympatex) show a statistically significantly higher resistance to water vapor (7 and 12 Pa.m².W⁻¹), i.e., lower water vapor permeability.

It was found that adding a heat-insulating layer significantly increases the laminate’s heat-insulating ability and the laminate’s hydrostatic resistance. The addition of this layer has a negative effect on the vapor permeability of the laminate. Therefore, the thermal insulating laminate could be used to design winter boots.

The results confirmed that laminate containing nanofiber membrane can be used to produce shoes for outdoor activities. Its main advantage lies in its high vapor permeability compared to commercially available membranes. Future research directions of authors will focus on the stability, durability, and maintenance of prepared laminates containing nanofiber membranes with very positive results expected based on preliminary tests conducted so far.

It should be mentioned that research in this area is still current because the global market for waterproof breathable layers was valued at about $2.6 billion in 2023.³⁶ This market is growing steadily, driven by increased demand for advanced outdoor and sportswear fabrics and greater awareness of comfort and protection in clothing. It is expected to expand at a compound annual growth rate of around 6% in the coming years.

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: this work was supported by the Ministry of Education, Youth and Sports of the Czech Republic and the European Union – European Structural and Investment Funds in the framework of Operational Programme Research, Development and Education – project Hybrid Materials for Hierarchical Structures (HyHi, Reg. No. CZ.02.1.01/0.0/0.0/16_019/0000843).

ORCID iDs

Vladimír Bajzík

Veronika Tunáková

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