Properties of laminated silica aerogel fibrous matting composites for footwear applications

Abstract

This paper describes a lamination method of commercially produced silica aerogel composite and investigates its suitability for thermal insulation within protective footwear applications for severe cold and extreme high-altitude environments. A silica aerogel composite was used with a thickness of 2.7 mm and mass per unit area of 500 gm^–2. Silica aerogel dust, which is generated during the crushing of brittle silica aerogel, was prevented from spreading into the environment by two-sided lamination of silica aerogel composite. A solid 5 µm thick membrane was used, reinforced with an abrasive resistant polyester knitted fabric. The thermal conductivity of laminated silica aerogel composite was comparable to that of the non-laminated one and amounted to 0.016 Wm^–1K^–1. Water vapor permeability of the laminated silica aerogel composite was 1.31 mgcm^–2h^–1. The silica aerogel composite was subjected to 30,000 cycles of flexing in order to study the impact of its irreversible crushed structure on the water vapor permeability and thermal resistances of laminated samples. It was observed that flexing did not damage the membrane of the laminated composite and had no statistically significant effect on its thermal resistance and water vapor permeability. During this study it was confirmed that the newly developed laminate has potential applications within protective footwear for extreme cold temperature environments.

Keywords

laminated silica aerogel composite personal protective equipment footwear thermal resistance thermal conductivity water vapor permeability

Protection of the human body within cold/hot environments requires a clothing ensemble and footwear that incorporate materials with high thermal insulation properties.¹ Effective personal protective equipment should also provide appropriate water vapor permeability (WVP),^2,3 low weight, flexibility, waterproofing, a good fit and other specific requirements related to personal protection at work,⁴ including fire⁵ and mechanical and chemical resistance.^6,7 Footwear and clothing used within cold environments, that is, under −5℃,⁸ are complemented using thermal insulation materials, which at present include a range of different traditional natural materials like wool and fur, and modern lightweight fleece materials like Thinsulate™ (3M), Primaloft® (PrimaLoft, Inc.), Heatseeker™ (The North Face), Opti-Warm™ (Merrell), etc., made from synthetic superfine fibers or microfibers. In addition, new functional materials like phase-change materials (PCMs) and fabrics with ceramic powder display a potential for use as good insulation materials.⁹ Most of the mentioned insulation materials are only effective at high thicknesses, which make clothing and shoes bulky and inflexible. There is still a lack of commercially available thin highly thermal insulating materials, especially for personal protective shoes. For those reasons silica aerogel composite, a recent development in thermal insulation materials with a potential for use within personal protective clothing and footwear within extremely cold environments, was studied in this research.

Silica aerogel was discovered by Samuel S Kistler in 1931.¹⁰ Today it is prepared using a sol-gel technique.¹¹⁻¹³ Pure silica aerogel has a nanoporous structure with approximately 80−99.8% of air nanopores, thus resulting in very low density (ca. 3 kgm^–3) and the lowest thermal conductivity (0.010 Wm^–1K^–1) of any known solid material.¹³ It is also distinguishable by its good WVP, hydrophobic and oleophilic properties, electrical non-conductivity and fire resistance.¹⁴ The disadvantages of pure silica aerogel limiting its usability are brittleness and dusting. Silica aerogel dust in itself does not pose any health risk.¹⁵ It absorbs the natural fat of the skin and induces an unpleasant feeling when in direct skin contact.¹³

Wider practical applicability can be gained by silica aerogel in the forms of flexible silica aerogel composites.¹⁶ Silica aerogel composites are textile-like blankets made from a silica aerogel matrix and a fibrous matting reinforcing material.

In 2010 Aspen Aerogels patented an advanced continuous industrial method of producing silica aerogel composites, whereby a solvent-filled gel precursor is embedded within a fibrous matting material and then converted into aerogel.¹⁷ In regard to fire protection applications, the fibrous matting materials are made from non-combustible carbon or glass fibers. As for other applications of silica aerogel composites, different synthetic fibers are used, such as polyester and polyethylene fibers.¹⁷

Silica aerogel composites with a thickness of 5 and 10 mm have already been used as an effective thermal insulation material throughout the building and construction industry.^18–20

In the area of personal protective equipment, NASA was the first to use a silica aerogel composite for the thermal insulations of space suits in 2006.²¹ During the study, a 4.2–7 mm thick silica aerogel composite was encapsulated within an aluminum foil and exposed to cyclic testing, which included a combination of bending, shearing and tensile deformations. They found out that after approximately 500 cycles, some aerogel dusting was observed between aerogel composites and the encapsulation layer. The best samples managed to retain about 80% of their insulation properties after 250,000 flexing cycles.

Kraner²² experimentally used silica aerogel composite with a thickness of 2–3 mm and thermal conductivity of 0.015 Wm^–1K^–1 within personal protective equipment for pilots. Preša et al.²³ laminated silica aerogel composite with a solid membrane during a continuous lamination process for restraining the spread of aerogel dust within the environment. They reported that bending silica aerogel when passing between cylinders caused greater silica aerogel matrix cracking and accumulation of silica aerogel dust inside the laminate. Prevolnik et al.²⁴ studied the properties of continuously laminated aerogel composites.

According to previous studies^21–24 dealing with applications of nanoporous silica aerogel composite within clothing and footwear, beside the problem of silica aerogel dust spreading into the environment, questions have arisen about the thermal insulation effectiveness of crushed silica aerogel composite.

This study suggests a new approach towards the lamination process of silica aerogel fibrous matting composite. Its suitability for personal protective footwear application in a severe cold environment has been investigated in comparison to contemporary thermal insulation materials used in this area. Since shoe soles undergo flexing, the influence of this on the material’s morphology and its thermal insulation properties and WVP is presented.

Experimental work

Materials

All materials used in the research are described in Table 1.

Table 1.

Materials

Abbreviation	Description
AC	Silica aerogel composite Pyrogel® 2250, produced by Aspen Aerogels, Inc., USA
LM	Laminated membrane Sympatex 2093-3T with a warp-knitted carrier fabric, produced by Sympatex GmbH, Germany
AC-LM^a	One-sided laminated silica aerogel composite with a LM
LAC	Five-layer laminate made of AC and two layers of LM
Thinsulate	Thinsulate™ material
eVent	eVent® material

Used only for measurements of water vapor permeability.

Silica aerogel composite Pyrogel® 2250, designated as AC in this paper, was selected as potential material for footwear and personal protective garments suitable for use within severe cold environments. Pyrogel® 2250 is made from a silica aerogel matrix embedded within a non-combustible reinforced fibrous matting material.²⁵ Its declared thickness of two millimeters provides, together with excellent thermal insulation properties, low dusting, high tensile strength and hydrophobic properties. It was primarily designed for applications, including transportation, power generation (thermal and fire protection), tube bundles and small diameter tubing.²⁵ AC was chosen for our research particularly because of its high thermal resistance at low thickness, which ensured an excellent insulating component for the newly developed insulation material.

When used within certain areas of footwear and garments, AC can be exposed to repetitive bending where a fragile silica aerogel matrix would crumble into dust. Thus, for this purpose we protected the AC with a membrane for preventing the spreading of silica aerogel dust into the environment. Micro-porous membranes were evaluated as less suitable than solid non-porous membranes because silica aerogel dust could smudge the membranes’ micro-pores and reduce their effective WVP. Among solid membranes Sympatex 2093-3T was chosen, which is designated as LM. This product is composed of a 5 µm thick solid membrane that is reinforced by a warp-knitted polyester fabric with a stitch density of 221 loops per square centimeter. On the other side of the membrane a thermoplastic polyamide adhesive is adhered. This membrane is characterized as being the thinnest among Sympatex membranes, which range up to 25 µm. It has a high abrasion resistance and good elastic characteristics. This membrane is chemically a polyether-ester block copolymer. The hydrophilic ether component absorbs moisture and expels water molecules throughout the evaporation process. The WVP effectiveness of the membrane depends on the temperature and moisture differences between the inner and outer sides of it. The Sympatex membrane is usually laminated to leather to provide waterproofing and WVP of leather footwear.²⁶

A laminated silica aerogel composite, designated as LAC (Figure 1), was prepared from AC and LM with a discontinuous lamination process in order to minimize crushing of silica aerogel and diminishing the formation of dust.

Figure 1.

The scheme of the laminated aerogel composite. 1 and 5: wrap-knitted fabric; 2 and 4: membrane; 3: aerogel composite.

A discontinuous process (Figure 2) was carried out on a heated plate-press machine Etipresor 590 (Ino d.o.o., Slovenia). The upperplate was heated to a temperature of 150℃ and pressed with a pressure of 500 kPa. The AC was firstly laminated with the LM on one side and then on the other side. Each lamination step lasted for 15 seconds.

Figure 2.

The scheme of the lamination process. 1: upper press-plate; 2: Sympatex 2093-3 T (LM); 3: aerogel composite (AC); 4: lower press plate; 5: second layer of LM; 6: one-side laminated AC.

Two commercially available materials, Thinsulate and eVent, used for thermal insulation in footwear were chosen for comparison with LAC. Thinsulate is a synthetic nonwoven fabric composed from polyester fibers of an average diameter of 15 µm. Its thermal insulation properties results from an air trapped between fine fibers.²⁷ eVent is a four-layered laminated material, composed from a polyamide warp-knitted fabric laminated with a micro-porous membrane eVent on one side, a polyurethane foam in the middle and a polyester woven fabric on the other side. eVent is used as a waterproof and thermal insulation layer in footwear. Its thermal insulation originates mainly from the polyurethane foam.²⁸

Methods

Cyclic flexing

The newly developed LAC and also the AC were exposed to cyclic flexing to simulate those mechanical stresses that occur during normal usages of footwear. Before flexing, the samples were placed into sealed plastic bags and then exposed to cyclic flexing on a Bennewart machine. Normally it is used for testing shoe sole materials in accordance with ISO 17707:2005 (E),²⁹ but during this research it was used to simulate mechanically caused structural changes of AC and LAC. Cyclic flexing was performed according to the standard, a sample of dimensions 10 cm × 15 cm was bent around a mandrel with a radius of 15 mm over an angle of 90° (Figure 3). During each cycle the flexing device moved from a neutral to a flexed position, thus bending the material without additional tensile deformation. According to the standard each sample was exposed to 30,000 cycles of flexing at a constant frequency of 150 cycles per minute.

Figure 3.

Scheme of a Bennewart flexing machine in the neutral position (a) and in the flexed position (b).

Mass per unit area

The mass per unit area of materials was measured in accordance with standard EN 12127:1999.³⁰ Ten measurements were made for each material.

Thickness

Measurements were conducted according to standard EN ISO 5084:1999,³¹ which defines the thickness of material as a perpendicular distance between two plates at a certain pressure. Thickness was measured under a pressure of material of 2.5 kPa. Ten measurements were made for each material.

Stereo-microscope

A Leica EZ4 D (Leica Mycrosystems, DE) was used for studying the surfaces of the materials.

Scanning electron microscope

The morphologies of the materials were studied from microphotographs obtained using a Joel JSM-6060-LV (JP) scanning electron microscope. The material was firstly attached to brass cylinders with diameters of 10 mm and then covered with a fine layer of colloidal gold.

Thermal conductivity and thermal resistance

The thermal insulation properties of silica aerogel and laminated silica aerogel were measured on a Lambdameter Stirolab LM 305 (Stirolab, SI). This single sample heat-flow meter apparatus with a symmetrical configuration (Figure 4), originally designed for testing the thermal performance of building materials and products. The apparatus was chosen because it enables measurements under non-convective measuring conditions that simulate conditions of heat transport regarding intermediate materials in footwear or garments. Those methods that measure thermal resistance on models, which include convective heat transfer, such as the skin model, are highly influenced by air velocity and air flows that result from natural convection. Conductivity was measured at high accuracy in accordance with the standard EN 12667:2002.³² Within the standard atmosphere a conditioned sample of dimensions 30 cm × 30 cm was inserted between a heated and a cooled unit of the heat-flow meter apparatus. The method is based on the monitoring of electromotive force output, mean temperature and the temperature gradient across the sample under stationary conditions. The average temperature gradient between the upper (20℃) and lower plate (0℃) was maintained at 20℃, and the mean temperature of the sample was around 10℃. Ten measurements were performed for each material.

Figure 4.

The scheme of a single sample heat-flow meter apparatus with symmetrical configuration. V: measuring sample; P″: heated unit; P′: cooled unit, M′, M″: heat-flow meters.

Average values of the observed steady-state data were used to make all computations, as given in Equations (1)–(4).

Thermal conductivity, λ, was calculated according to the following equation:

λ = \frac{j d}{T^{'} - T^{″}} (\frac{W}{mK})

(1)

where j is the heat flux density (Wm^–2), d is the average sample thickness (m), T′ is the average temperature (K) of the hotter side of the sample and T′′ is the average temperature (K) of the colder side of the sample.

Heat flux density, j, was calculated by Equation (2):

j = \frac{Φ}{A} (\frac{W}{m^{2}})

(2)

where Φ is the average power supplied to the metering section of the heating unit (W) and A is the tested sample area (m²).

Thermal resistance R, was calculated by Equation (3):

R = \frac{T^{'} - T^{″}}{j} (\frac{m^{2} K}{W})

(3)

where T′ is the average sample hot side temperature (℃), T′′ is the average sample cold side temperature (℃) and j is the heat flux density (Wm^–2).

The thermal resistance of LM is presented as part of serial-connected resistance elements in LAC. The thermal resistance of LM was calculated using Equation (4):

R_{LM} = \frac{R_{AC} - R_{LAC}}{2} (\frac{m^{2} K}{W})

(4)

where

R_{LM}

is the calculated thermal resistance of LM,

R_{AC}

is the average measured thermal resistance of AC and

R_{LAC}

is the average measured thermal resistance of LAC.

Water vapor permeability

WVP was determined according to EN 13515:2001,³³ which is used for footwear uppers and lining materials and is appropriate for thicker materials. A circular test sample was clamped across the open end of a test bottle containing a moisture-absorbing desiccant (dry silica gel). Air with a relative humidity of 50% and temperature of 20℃ was blown over the test sample at a set velocity. The air within the bottle was circulated by rotating the bottle, which then agitated the desiccant. The mass of water vapor transmitted through the sample was determined after a set period of time of 48 hours. The WVP of the material was calculated by Equation (5):

WVP = \frac{Δ m}{A Δ t} (\frac{mg}{{cm}^{2} h})

(5)

where Δm is the increase in the mass of the bottle (mg), Δt is the time of measuring (h) and A is the area of the open end of the bottle (cm²).

Statistical analysis

The Statgraphics Centurion version XVI computer program was used for statistical analysis. The F-test was used for verifying the effect of flexing on AC and LAC properties. The calculated P-values lower than 0.05 mean flexing had a statistically significant effect on the material’s property at the 95.0% confidence interval. P-values that are higher than 0.05 indicated no significant effect of flexing on the AC or LAC properties.

Results and discussion

Properties before cyclic flexing

The thickness of LAC is 9.33% higher than the thickness of AC. The mass per unit area of LAC is 23.8% higher than the mass per unit area of AC. The calculated sum of masses per unit area of the individual layers forming LAC was 660 gm^–2 and 1.2% higher than for the average measured mass per unit area for LAC (Table 2). In addition, the calculated sum of the thicknesses of the individual components of LAC amounted to 3.26 mm, being 8% higher than the average measured thickness of LAC (Table 2). This difference between measured and calculated thicknesses can be attributed to irreversible changes of the material during the lamination.

Table 2.

Thickness and mass per unit area

Sample	Thickness		Mass per unit area
Sample	Mean (mm)	CV (%)	Mean (gm^–2)	CV (%)
AC	2.72	2.4	496.5	6.9
LM	0.27	3.0	81.75	1.8
LAC	3.00	3.9	651.8	5.0
Thinsulate	2.10	3.8	215.4	7.7
eVent	2.85	1.6	319.3	1.0

Since the thickness of LAC is comparable to that of eVent, which is already used in footwear, we assume that thickness of LAC is also suitable to be used in footwear.

The mass per unit area of LAC is two times higher than that of eVent and three times higher than that of Thinsulate. In order to retain a suitable weight of the final product, lighter synthetic lining and upper material instead of leather should be chosen when designing footwear with LAC.

The measured average thermal conductivity of AC of 0.015 Wm^–1K^–1 (Table 3) at a mean temperature of 10℃ is comparable to the the manufacturer’s declared value, which is around 0.015 Wm^–1K¹.²⁵ The measured average thermal conductivity of LAC is 1.6% higher than that of AC. It can be concluded from these results that the lamination process did not cause any change of thermal conductivity of AC. The thermal resistance of LAC was higher than that of AC (Table 3) because of the higher thickness of LAC compared to AC.

Table 3.

Thermal conductivity and thermal resistances of the samples

Sample	Thermal conductivity, λ		Thermal resistance, $\bar{R}$
Sample	Mean (Wm^–1K^–1)	CV (%)	Mean (m²KW^–1)	CV (%)
AC	0.015	3.2	0.1705	1.5
LM	0.080^a	/	0.003^a	/
LAC	0.016	5.9	0.177	3.8
T	0.031	1.8	0.066	1.5
E	0.035	1.3	0.082	2.7

Calculated value.

The thermal conductivity of eVent and Thinsulate are more than 100% higher than that of LAC (Table 3), but the thermal resistance of eVent is 54% and of Thinsulate is 63% lower than for LAC. Thinsulate and eVent can reach the thermal resistance of LAC only at much higher thicknesses and consequently higher masses per unit area. This would lead to a higher bulkiness of end-products, which would restrict the freedom of movement and comfort much more than using LAC.

The WVP (Table 4) of AC amounted to 7.19 mg·cm^–2·h^–1. This value is much higher than the aniline leather, 4.1 mg·cm^–2·h^–1,³⁴ used for shoes. However, the WVP of Thinsulate is much higher than that of AC and aniline leather.

Table 4.

Water vapor permeability

Sample	Water vapor permeability, $\bar{WVP}$
Sample	Mean (mg·cm^–2·h^–1)	CV (%)
AC	7.19	4.2
LM	2.86	3.8
LM two layers	1.29	5.6
AC-LM with LM facing the outside of the pot	2.58	3.1
AC-LM with LM facing the inside of the pot	2.04	3.6
LAC	1.31	5.3
Thinsulate	17.1	4.3
eVent	16.35	3.5

The LM that provides dustproofing of the LAC has a WVP 2.9 mg·cm^–2·h^–1, which is typical for non-porous hydrophilic membranes.³⁵ The WVP of LAC depends on the permeability of the two layers of membrane and is 81.8% lower than the WVP of the non-laminated composite, AC. The WVP of LAC is comparable to the WVP of a 10 µm thick Sympatex membrane, which is also used for personal protective shoes.²⁶ Comparison of WVP of eVent and LAC (Table 4) also shows that the micro-porous membrane is more effective than the solid membrane. As was already mentioned, a micro-porous membrane was not chosen for lamination of AC because a significant lowering of WVP was expected as micro-pores would be filled with aerogel dust.

It is also evident from Table 4 that the WVP of the two layers of LM is 55% lower than the WVP of one layer of LM. Mukhopadhyay and Kumar Midha³⁵ indicated that increasing the thickness of a material leads to a decrease in WVP, which is similar to the situation when combining more layers of the same material. The effect on WVP was also assessed regarding combinations of AC and LM. The WVPs of the AC samples laminated with one layer of LM were assessed in two different ways: in the first test the laminated AC was placed on a test pot so that the LM was facing the outside, while in the second test the LM was facing the inside of the test pot. A 21% higher value of WVP was obtained when the tested AC laminated with LM was facing the outside than in the case of the AC laminated with LM facing the inside of the pot (Table 4). According to Mukhopadhyay and Kumar Midha,³⁵ the hydrophilic layer requires a reservoir of a certain level of moisture before effective moisture vapor transmission is initiated. It can be concluded that AC works as a reducer of water vapor concentration and therefore decreases water vapor pressure. The WVP of LAC was equal to the WVP of the two layers of LM. It can be concluded that the WVP of the multilayer material depends on the least permeable layer, the number of layers and also on their combination.

Morphological structure of materials before cyclic flexing and after it

The morphological structure of the AC material’s surface and cross-section before cyclic flexing and after it is presented in Figures 5 –8. The AC is composed of fibers embedded within a non-fibrous silica aerogel matrix. A lofty fibrous structure is used for preparing a matting made from many layers that works as a reinforcing material of the silica aerogel matrix.¹⁸ The longitudinal appearances of the reinforcing fibers show (Figure 5) a grooved surface morphology that is typical for acrylic fibers. Longitudinally crimped fibers with a circular cross-section have an average thickness of 13.5 µm and length of about 20 mm. On the basis of the longitudinal view and the fact that the fibers are non-combustible, we estimated that the used fibrous matting material in the AC was made from carbonized acrylic fibers.

Figure 5.

Scanning electron microscope appearance of longitudinal view of extracted fiber from the aerogel composite.

Figure 6.

Surface appearance of silica aerogel composite before flexing (a) and after it (b), made by a stereomicroscope.

Figure 7.

Scanning electron microscopic appearance of the silica aerogel composite surface before flexing ((a), (c)) and after it ((b), (d)) (× 150 above, × 350 below).

Figure 8.

Scanning electron microscopic appearance of the cross-section of the silica aerogel composite before flexing (a) and after it (b) (× 50).

The surface microscopic appearance of the non-flexed AC (Figure 6(a)) shows a homogeneous fibrous structure with no preferred fiber orientation. After flexing (Figure 6(b)) white areas could be seen on the surface of the AC, which indicates the presence of silica aerogel dust. The dust was incurred from the broken particles of aerogel migrated from the inner of the composite to its surface. After cyclic flexing only a small amount of the aerogel dust was noticed within a sealed transparent plastic bag into which silica aerogel composite was wrapped while being flexed. The quantity of the dust collected inside the bag was too small to be measured.

The scanning electron microscope images (Figure 7) reveal a more accurate insight into the solid silica aerogel matrix with minor cracks of the non-flexed material (Figures 7(a) and 7(c)) and with more and larger cracks after cyclic flexing (Figures 7(b) and 7(d)). Crushing a silica aerogel matrix produces particles of dimensions smaller than 50 µm. Silica aerogel dust usually contains particles larger than 100 nm.¹⁶

A cross-section of the AC (Figure 8(a)) reveals a non-homogenous layered structure (pointed with arrows) of silica aerogel composite that results from the preparation process of aerogel composite material.^18,36 After flexing (Figure 8(b)) the AC shows a uniform structure with no visible layers.

The cross-section’s microscopic appearance of the silica aerogel composite inside the LAC shows a similar morphological structure as that observed on the cross-sections of the AC (Figure 8). After flexing of the LAC no delamination between membrane and aerogel composite, no visible damage to the membrane and no dust accumulation outside of the LAC were observed. Crushing of silica aerogel at LAC flexing was confirmed by scanning electron microscopic images (Figure 9). No silica aerogel dust particles on the surface of the non-flexed LAC were observed (Figure 9(a)), but it had been observed in the case of the flexed LAC (Figure 9(b)). The aerogel particles did not penetrate through the membrane; they migrated from the edge of the LAC to its surface when preparing the samples for scanning electron microscopy because the silica aerogel composite inside the LAC was partially crushed.

Figure 9.

Scanning electron microscopic appearance of the surface of the laminated silica aerogel composite before flexing (a) and after it (b) (× 27).

Effect of flexing

For the purposes of the present discussion, the flexed AC samples are designated as AC - F and the flexed LAC as LAC - F.

No statistically significant difference was detected between the thicknesses (P-value 0.567) and masses per unit area (P-value 0.911) of flexed and non-flexed materials (Figure 10). This demonstrates that for the given flexing conditions the removed silica aerogel dust was minimal from the AC and LAC samples during testing.

Figure 10.

Effect of flexing (F) on thickness (a) and mass per unit area (b) of aerogel composite (AC) and laminated aerogel composite (LAC).

The influence of flexing on the thermal resistances of the AC and LAC were studied on the same set of samples before flexing and after it. The results are given in Figure 11(a). No statistically significant difference was observed between the means of the thermal resistances of the flexed and non-flexed samples. This demonstrates that at given flexing conditions insignificant quantities of silica aerogel were removed from the AC and LAC during testing with no effect on the samples’ thermal resistances.

Figure 11.

Effect of flexing on thermal resistance (a) and water vapor permeability (b) of silica aerogel composite (AC) and laminated silica aerogel composite (LAC).

The WVP of AC - F was 7% higher than for AC (Figure 11(b)). Flexing caused cracks in the silica aerogel matrix, which allowed better transmission of water vapor through the AC. However, no statistically significant difference in WVP was noted between the flexed and non-flexed samples of LAC. This proves that the chosen membrane did not suffer any damages during flexing with AC and would still provide waterproofing.

Conclusion

In conclusion, we claim that the discontinuous lamination process is a more appropriate way for preparing laminated aerogel composites for footwear than a continuous one. LAC was produced from the thinnest available silica aerogel composite Pyrogel® 2250 and thinnest Sympatex membrane. The both-sided lamination of the AC effectively prevents the spreading of aerogel dust into the environment. The thickness of this newly developed material amounted to 3 mm and its mass per unit area to 651.8 gm^–2. LAC excels by a low thermal conductivity of 0.016 Wm^–1K^–1 and a high thermal resistance of 0.177 m²KW^–1. The two layers of membrane have decreased the WVP of LAC in comparison to AC, but the WVP value of LAC is still comparable to other laminated materials with solid membranes that are used to provide waterproofing of footwear. The durability study revealed that 30,000 cycles of flexing had no significant effect on the thermal resistance and WVP of LAC. LAC surplus contemporary thermal insulation materials like laminated foams and Thinsulate™. Those materials would reach the thermal resistance of LAC only when using much thicker layers than used for LAC. Laminated aerogel composite provides waterproofed and breathable footwear with a competitive weight and a much higher thermal resistance at lower thickness of footwear than other advanced materials in this area. LAC could also be used in personal protective footwear for extremely cold environments. LAC, together with an upper material and lining built into protective footwear, could reach thermal resistances of 0.25 m²KW^–1 or higher. This equals an insulation of value of 1.6 clo. Such clo values can be used for protection in cold environments at temperatures around –25℃ during physical activity of 4 MET and up to a temperature of 5℃ during activity of 2.3 MET.³⁷

Footnotes

Acknowledgment

The measuring device was kindly loaned by Aerogel CARD.

Declaration of Conflicting Interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partly supported by European Union’s European Social Fund. It was implemented in the framework of the Operational Programme for Human Resources Development for the Period 2007–2013, Priority axis 1: Promoting entrepreneurship and adaptability, Main type of activity 1.1.: Experts and Researchers for competitive enterprises.

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