Application of Coating Mixture Based on Silica Aerogel to Improve Thermal Protective Performance of Fabrics

The sol-gel processing method is very popular and used to synthesize materials, especially metal oxides. It consists of the system transition from a liquid sol to a solid phase in the form of a gel. The sol-gel process includes the following stages: solution formation, gelling, aging, drying, and thickening [1,2,3]. The most important advantages of the sol-gel process are its simplicity and the fact that the process is an effective method for the production of high-quality materials such as aerogels. Aerogel is a gel made up of a microporous solid, in which the dispersed phase is gas. Among different kinds of aerogels [3, 4], silica aerogels SiO₂ are popular, because they exhibit unique properties. Silica is a natural environmentally-friendly ingredient. The aerogels are characterized by small particle size, from 2 nm to 4,000 mm (depending on a form, powder, or granules) [1, 3] and pore diameter, ~0.02 mm [1, 5], and contain 80–99.8% of air [6]. The materials have a very low density, 120–150 kg/m³ [7,8,9]. Specific surface area is in the range of 500–1,000 m²/g [1, 6, 10]. Silica aerogels have a melting point of ~1,200°C [11]. They have low thermal conductivity, 0.004–0.018 W/(m∙K) [7,8,9, 12]; a low refractive index, 1.0–1.1; a low dielectric constant, ~1.1 for a density of 0.1 g/cm³ [5, 11]; and also low speed of sound, 100 m/s for a density of 0.07 g/cm³ [11].

Generally, silica aerogels are fragile and brittle because of the interparticle connections and make them inapt for load-bearing applications [11]. Aerogels cannot be easily applied due to their weak strength resulting from their high porosity and cracking along the drying process. A quite effective solution is combining aerogels with textile structures [8].

Due to the wide potential of aerogel-based materials, its applications can be found in all industrial sectors. Aerogels can be used in construction as thermal and acoustic materials [12,13,14]. Properties of aerogels have made them a new material for aerospace applications, sensors and coatings, energy generation and storage, and biomedical devices and implants [11, 13, 15, 16]. They were also used in the textile sector [7, 9, 16,17,18,19]. This material, created as a result of the use of aerogel granules between glass fiber and Kevlar fabrics, enables protection against laser radiation [7]. Aerogel blankets were produced as a composite consisting of silica aerogel and fibrous reinforcement [16]. The applied reinforcement made it possible to change the fragile aerogel into a durable and flexible material. The blanket manufactured by Aspen Aerogels Inc. was produced by depositing a thin layer of aerogel on the fabric surface.

The most commonly used solution, especially in the protective clothing protecting against extremely low temperatures, is that of nonwoven composites consisting of a silica aerogel matrix and a reinforcing material, which is a nonwoven fabric [20]. The manufactured products are sometimes laminated to eliminate undesirable aerogel dust particles. Scientists have shown that by incorporating silica-based aerogels into nonwovens, the thermal properties of firefighters’ protective clothing can be significantly improved. Another example of the use of aerogel in textiles is insoles for shoes, which were used by climbers [1]. Low thermal conductivity makes silica aerogels desirable for insulating applications such as cover layers for the protective clothing used in a hot work environment. As research was shown [6] aerogels can be used successfully in the construction of garments protecting against thermal factors to improve their protective properties by increasing thermal insulation. The silica aerogel was used in textile packaging intended for implementation in protective clothing [21]. Three different packaging designs were made in the form of removable inserts filled with aerogel and assessed for thermal resistance. The increase of resistance to radiant heat and convection was observed. Silica aerogel was also used for the modification of the basalt fabric surface [22]. The aerogel layer improved resistance to contact heat and thermal radiation.

Working in a hot environment is often associated with heat loads resulting from both high temperatures and increased physical activity of the employee. At workplaces, the employee is exposed to hot factors in the form of a flame, thermal radiation, or contact heat. There is a hot microclimate, which, in combination with the protective clothing and physical effort used, results in an unfavorable hydrothermal condition under the clothing, which causes a feeling of discomfort. Reduction of discomfort can be achieved through appropriately selected undergarments and underwear, taking into account materials supporting the collection of heat, water vapor, and liquid. Scientists are constantly striving to produce innovative textile materials intended for personal protective equipment, which will show good protective and mechanical properties, and are also characterized by good comfort of use [8, 10, 23]. Therefore, the aerogel layer is a promising solution for thermal insulation and protection [9, 10, 18].

The main aim of the study is to improve the thermal protective performance of fabrics using a coating mixture based on the silica aerogel. Flame-resistant fabrics characterizing comparable thermal properties were chosen. However, they don’t meet the requirements described in the standards [24,25,26]. To improve thermal parameters (thermal conductivity, resistance to contact, and radiant heat) the coating mixture based on aerogel was applied on the sample surface.

Two twill-weave fabrics used in the production of protective clothing intended for use in a hot work environment were selected for the tests. The Nomex^® fabric by DuPont (Wilmington, USA) and the cotton fabric with a flame-retardant finish by XinXiang YuLong Textile (Xinxiang, China) are flame-retardant and arc-proof materials. The raw material composition and selected parameters are presented in Table 1.

Three samples of each fabric with dimensions of 24 cm × 10 cm were prepared and acclimated in the temperature of 20°C ± 2°C and the relative humidity of 65% ± 5% [27].

A coating mixture was prepared, which consisted of aerogel, adhesive mean, and flame retardant. The base of the mixture was the Enova^® Aerogel by Cabot Co. USA. The selected silica aerogel is described in detail in Table 2. The aerogel enables the production of coatings with a smooth and uniform finish, high abrasion resistance, low thermal conductivity, and safe contact with the skin.

The Bonatex^® KCB adhesive mean was used to improve the adhesion of the coating mixture to the substrate. It is a dispersion adhesive mean based on water dispersions of special synthetic resins and polymers. The adhesive mean does not contain any organic solvents. It is designed for bonding textiles, natural and synthetic leather, some plastics, as well as cork, cardboard, and similar materials. It contains a mixture of 5-chloro-2-methyl-4-isothiazol-3-one and 2-methyl-4-isothiazolin-3-one and may produce an allergic reaction.

The Burnblock^® fire-retardant preparation was used to improve the thermal properties of the coating mixture. It is a harmless, non-toxic, ecological, and colorless product. It protects material capable of absorbing various types of liquid, such as fabrics, carpets, carpets, paper, and raw wood. The impregnated material becomes stiffer. Burnblock^® firestop fluid has a limited ability to be absorbed by artificial materials, made of 100% PP, PE, PVC. It protects fabrics for five years if the fabric is not subjected to any wear processes (abrasion, chemical, or mechanical processes).

The coating mixture was prepared in two variants, which differed from each other in the content of individual ingredients. It was assumed that the percentage of aerogel in the mixture is 45 wt% and 60 wt% for variants A and B, respectively, and the particle density equals 120 kg/m³. The remaining part is adhesive mean and flame-retardant preparation in a 75:25 ratio composition of adhesive mean/flame-retardant preparation. The variants of the coating mixture are presented in Table 3.

Three ingredients of coating mixture —aerogel, adhesive mean, and flame-retardant—were prepared in the amount specified in Table 3 for chosen variant. Next, the ingredients were mixed in the vessel until a homogeneous mixture was obtained. The coating mixture of 90 ml was divided into three equal parts. The mixture was applied to one side of the surface of each of the three fabric samples using a woody spatula. The designations of all obtained samples variants are summarized in Table 4.

The drying process was carried out at a temperature of 40°C and for 20 minutes as recommended [28].

The image of the Nomex^® fabric and cotton fabric with a flame-retardant finish before and after the application of the coating mixture was taken with the laser confocal microscope Olympus LEXT OLS5100 (Tokyo, Japan) at the total visual magnification of 100 x and shown in Figure 1.

Figure 1

Thermal parameters obtained for woven fabrics before applying the coating mixture are presented in Table 5. Next to the mean values of quantities is their standard deviation, given in parentheses.

As can be seen from Table 5, the parameters characterizing the thermal properties of the fabrics before the mixture is applied to their surface are comparable.

The relative heat transfer index RHTI₂₄ obtained for fabrics before the coating mixture above 12 s is applied indicates that the fabrics meet the first efficiency level of protection. Due to the low value of the threshold time t_t, fabrics can’t be used as materials protected against contact heat.

Samples on the surfaces of which the mixture in variant A or B was applied were tested. Results of thermal conductivity obtained for samples are presented in Figure 2. The coefficient of variation did not exceed 2.7%.

Figure 2

A decrease in thermal conductivity was observed (Figure 2). The parameter is directly related to the structure of textile material.

The thicker the material, the greater its ability to protect against heat loss. An additional layer, in this case, is the coating mixture applied to the sample surface. Values of thermal conductivity of modified samples are comparable due to the application of the same amount of coating mixture to the sample surface. Similar conclusions were reached by other researchers [19], claiming that the thickness of aerogel is important. They found a better ability to retain heat during crossflow for the nonwoven fabric with higher thickness and aerogel content.

The results of measurements of contact heat resistance for all samples for contact temperature 250°C are shown in Figure 3. The coefficient of variation did not exceed 3.1%.

Figure 3

In the case of resistance to contact heat, all samples modified with coating mixture obtained the first efficiency level of protection (Figure 3). It is the minimum performance requirement for clothing protected against contact heat. The best result reached the modified cotton sample (CB) using the coating mixture in variant B containing 60 wt% of silica aerogel. The mean threshold time was equal to 6.99 s for the sample.

We noticed that application of the coating mixture in variant B caused a greater increase in the threshold time value compared with the coating mixture in variant A assuming the same substrate. Therefore, the level of improvement depends on the textile substrate. The results show that cotton fabric (C) with a flame-retardant finish is a better substrate from the point of view of resistance to contact heat than Nomex^® fabric (N). The cotton fabric with a flame-retardant finish has greater thickness and surface mass than the Nomex^® fabric. Moreover, due to material composition, the cotton fabric is a hydrophilic material, which may cause this phenomenon. The faster absorption of the coating mixture during application to the surface was observed just for cotton samples. On both the cotton and Nomex^® fabrics surfaces, the aerogel layer was found (Figure 1b).

Results of measurements of the radiant heat resistance for all samples are shown in Figure 4. The coefficient of variation did not exceed 2.1%.

Figure 4

The resistance to thermal radiation was improved for all of the samples. Both unmodified and modified fabrics showed the first efficiency level of protection. The highest index RHTI₂₄ was obtained for modified cotton fabric (CA) and equal to 19.93 s (with a coefficient of variation of 0.5%). The result obtained for modified Nomex^® fabric is also promising (RHTI₂₄ = 18.13 s with a coefficient of variation of 1.0%). The samples were modified with the coating mixture in variant A containing 45 wt% of silica aerogel.

The profile and areal methods to compare unmodified woven fabrics surface and fabrics modified with coating mixture were applied in compliance with ISO 4287 [31] and ISO 25178-2 [32]. The laser confocal microscope Olympus LEXT OLS5100 (Tokyo, Japan) at the total visual magnification of 100 x was used for this purpose. The analysis was subjected to the unmodified Nomex^® and cotton fabrics, and the woven fabrics modified with coating mixture in one variant, A, due to the same total amount of the mixture used in variants A and B. In the case of modified fabrics, both sides of the sample were analyzed.

The sampling length l_r and the characteristic wavelength of the profile filter λ_c were equal to 1,000 μm and assumed in the surface profile method. The definition area Ω ≅ 22 mm², in the form of the square, was assumed in the areal surface method. Moreover, high-pass filtering of the surface was applied in the method, wherein the value of the L-filter was equal to 1,000 μm. Results of the confocal microscopy performed are presented in Table 6. Next to the determined R-parameters, the coefficient of variation is given. The measurements were repeated in three places for each sample. In the case of the areal surface method, S-parameters were determined for one definition area indicated on the sample surface.

The values of the R- and S-parameters obtained for N and C samples confirm that the cotton fabric is thicker than Nomex^®. Moreover, if we compare the variation coefficients of R_a, we can conclude that the cotton fabric has a rougher surface.

Woven fabrics modified with a coating mixture (the modified side of the coated sample) are characterized by higher values of R- and S-parameters. The differences in the roughness of the samples NA-ms and CA-ms increased relative to the original samples, comparing parameters R_a and S_a. The cotton fabric CA-ms remained thicker than the Nomex^®. It can be assumed that the coating mixture was not completely absorbed by the cotton fabric substrate. Better adhesion of the coating mixture to the cotton fabric substrate compared to Nomex^® fabric was observed. It also results from the raw material composition of the used textile substrates.

The unmodified sides of the coated samples, NA-ums and CA-ums, are characterized by comparable values of R- and S-parameters. The surfaces of fabrics became smoother compared with the original samples due to the substrate on which the samples were placed after the modification process. This indicates that the mixture soaked through the cotton fabric smoothing its unmodified side. The value of the parameter S_z obtained for sample CA-ums is less than the value obtained for the original, unmodified cotton fabric with the flame-retardant finish (C).

Aerogel is quickly becoming one of the most attractive materials of the future. Traditional clothing protecting against the high temperature and heat is usually made of thick, multilayer material systems, the effectiveness of which increases with increasing thickness, while materials with aerogels can provide adequate protection against heat and flame without an excessive increase in their thickness and hence the mass. The coating mixture based on silica aerogel causes a decrease in thermal conductivity.

In the case of resistance to contact heat for the contact temperature of 250°C, all fabrics modified with coating mixture in variants A and B showed the first efficiency level of protection.

The minimum performance requirement for clothing protected against contact heat was fulfilled.

The resistance to thermal radiation was improved for all of the modified samples and the first efficiency level of protection against radiant heat was obtained.

The best sample from the point of view of protection against contact and radiant heat was CA. The following parameters were obtained: t_t = 5.87 s and RHTI₂₄ = 19.93 s.

The cotton sample with the flame-retardant finish was modified using the coating mixture in variant A containing 45 wt% of silica aerogel. The amount of aerogel turned out to be sufficient to achieve the first efficiency level of protection against contact and radiant heat by the sample.

The differences in the roughness of the modified side of the samples decreased relative to the original samples; better adhesion of the coating mixture to the cotton fabric (C) compared with Nomex^® fabric (N) was observed.

Protective clothing is subjected to maintenance. Therefore, before the clothing reaches the end user, testing of the resistance to radiant heat and contact heat after maintenance cycles is necessary.