5 Must-Have Features in a Aerogel Coating

As a leading silica aerogel manufacturer in China since , we get new inquiries about aerogel every day. People all around the world are curious about the amazing properties of aerogel. Therefore, we are happy to share the basic knowledge below to help you get the right insight into this wonderful material.

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Question 1: What is aerogel?

Answer: Aerogel is a general and collective term for a diverse class of nanoporous structured solid materials, including silica aerogel, which is currently the most widely applied. Most of the time, when we talk about ‘aerogel’, we mean Silica(SiO2) Aerogel.

The 3D network structure of aerogel is composed of nano-size particles, and filled with air or other gases. A typical type of aerogel contains 95-99% air in volume. They are the lowest-density solid materials that have been produced in the world.
Currently, there are more than 30 kinds of materials that can be processed into aerogel structures, but the most commonly seen is Silica(SiO2) Aerogel.

Check this video below to learn more about aerogel.

Question 2: Is aerogel a sticky or adhesive ‘Gel’ material?

Answer: No, aerogel is not sticky or adhesive at all. It is a dry and solid material called ‘solid smoke’ or ‘blue smoke’. 

So why do people call this dry and solid material ‘GEL’? Well, because after the supercritical drying process in aerogel production, the original ‘GEL’ becomes dry and solid, and the finished product is what we call AEROGEL.

Question 3: Is aerogel toxic or harmful to human health?

Answer: No, aerogel is non-toxic and has been proven to be safe for human health. Just it can cause dryness and short-term irritation to human body.

As the most commonly applied aerogel is Silica (SiO2) Aerogel, most of the time, when we talk about ‘Aerogel’, we normally mean Silica (SiO2) Aerogel. Silica aerogel is composed of air and silica(SiO2), no more others. It is 100% inorganic and non-toxic, and has been proven to be safe for human health.
The only thing you need to worry about when handling aerogel is the short-term irritation that aerogel powder(dust) may cause to human body.
Silica aerogel is hydrophobic and may cause dryness and short-term irritation irritation to the skin, eyes, and mucous membranes. Therefore, when handling aerogel, it is recommended to wear respiratory masks, gloves, long-sleeved clothes, safety glasses and even chemical goggles for greater protection. It is also recommended to rinse your body thoroughly after handling aerogel. Single-use overalls are recommended to minimize skin contact.

Click here to contact us and get the MSDS document of our silica aerogel material

Question 4: Why is aerogel so special that people say it is future material?

Answer: Because aerogel breaks 15 Guinness World Records for its magical properties, like the lowest thermal conductivity, the lowest density, etc., and people are still studying about different properties of different aerogels. Now more and more people come to realize the super insulation performance of aerogel and choose aerogel insulation as a better thermal insulation solution.

Click this blog here to learn more about the magical properties of aerogel

Question 5: What is aerogel material?

Answer: When we talk about ‘Aerogel Material’, normally we mean silica aerogel composite materials like flexible aerogel blankets, rigid aerogel panels, encapsulated aerogel thermal barriers, etc., as well as pure silica aerogel powder/particle.

Click here to learn more about our aerogel materials.

Question 6: Is aerogel insulation blanket pure aerogel or aerogel composite material?

Answer: Aerogel insulation blanket is a type of aerogel composite material.

When aerogel composited with fiberglass blanket or ceramic fiber blanket, the material shows good mechanical performance and extremely low thermal conductivity, which is better than pure aerogel alone or fiberglass/ceramic fiber blanket alone.

Click here to learn more about our aerogel insulation blanket.

Question 7: Why do people use aerogel composite material instead of 100% pure aerogel for high-efficiency thermal insulation?

Answer: Because 100% pure aerogel is too brittle to be used. It has to be composited with base materials of good mechanical performance such as fiberglass blanket, ceramic fiber blanket, etc.

Check the video below to have a general idea about our silica aerogel composite materials.

Question 8: Why is silica aerogel material called ‘the ultimate thermal insulation material in the world’?

Answer: Because silica aerogel features the lowest thermal conductivity. Silica aerogel contains 95% ~ 97% of air, which is trapped in the convoluted matrix of silica. So aerogel can effectively insulate heat against the Three Basic heat flow mechanisms: heat conduction, heat radiation, and heat convection.

Heat Insulation against Heat Conduction
The porous and convoluted nano-structure of aerogels creates extremely long heat transfer path, resulting in high thermal resistance in heat conduction.

Heat Insulation against Heat Radiation
The vast number of nano cells form gas-solid interfaces that prevent heat radiation by reflection, adsorption, transmission, and re-radiation.

Heat Insulation against Heat Convection
The pore size of aerogel is smaller than 70nm in diameter which restricts motion of gas molecules. Therefore, effectively preventing heat transfer through gas convection

Question 9: Is aerogel mass production tech already mature enough for civilian uses?

Answer: Absolutely, yes. Since decades ago, mass production technology of silica(SiO2) aerogel has been mature enough for civilian uses.

The most widely applied property is silica aerogel’s lowest thermal conductivity.
In , our factory Namate created a unique production technique and successfully realized cost-effective industrialization of high-quality silica aerogel and aerogel composites. This unique production technique solved the long-lasting severe dust-drop problem of silica aerogel composites and enhanced the thermal insulation performance to a new level.
Now silica aerogel materials have been widely applied in various civilian sectors like oil&gas, pulp&paper, power generation, building construction, e-vehicles, lightweight and high-performance thermal clothing and equipment, etc.

Check the video below to have a general idea about our factory.

Click here to learn more about our factory facilities.

Question 10: Why are aerogel materials so expensive? Are they cheaper than 10 years ago?

Answer: Yes, aerogel materials have become much cheaper than 10 years ago. They are expensive because of the high-cost and complicated production process.

For example, in China, aerogel price today is only half that 10 years ago. However, aerogel materials are still much more expensive than conventional thermal insulation materials like rockwool, fiberglass, PU foam, etc. The key reason lies in the high-cost and complicated production process of aerogel.
Generally, there are two steps to make high-quality aerogel:
Step 1. Sol-Gel, and Step 2. Supercritical Drying.
As for the first step, to get a perfect Gel from the Sol, high quality raw materials and highly sophisticated technical operators are essential while the both are quite costly.
As for the second step, the Supercritical Drying process requires high temperature and high pressure during the interaction. The initial investment of such specially designed reactors and the related equipment and devices is very costly. What’s more, to eliminate fluid from the Gel while causing no shrinkage, each small adjustment matters a lot, so it requires very strict and cautious operation. Currently, all around the world, only a few sophisticated and expert-level operators can successfully and consistently handle such delicate operations during daily aerogel mass production.

Check this video below to learn about how aerogel is made

Question 11: Currently what are the main applications of aerogel?

Answer: Thermal insulation and fire prevention in industrial pipelines and equipment, building construction, energy storage, e-vehicle battery thermal management, etc.

With so many magical properties, aerogel can be widely applied in various industries for the purposes of thermal insulation, fire protection, acoustic insulation, environment protection, biomedical engineering, etc. Scientists are still doing researches on aerogel’s different properties, and currently, aerogel materials are mainly and widely applied in the fields below:

With a working temperature range from -200℃to ℃, aerogel materials are widely applied in various heavy industries.

1. Industrial Insulation Applications:

  1.1 High-temperature Industrial Applications (≤650℃)

• Oil pipelines, and equipment in the petrochemical industry
• Steam turbines, boilers, long-distance thermal pipelines, and high and medium-pressure pipelines in the power generation industry
• Steam pipelines and smoke exhaust pipes in vessels and ships
• Body insulation for high-speed railways and subways
• Fire protection and thermal insulation of electric vehicles, etc.

  1.2 Cryogenic Industrial Applications 

• Cryogenic storage and transportation of LNG, liquid nitrogen, liquid oxygen, and others
• Cryogenic insulation in cold storage, chemical refrigeration, etc
• Thermal insulation in high-clean environments of the pharmacy industry, electronics industry, etc

  1.3 Super-high-temperature Industrial Applications(≥ ℃) 

• Industrial equipment like kilns
• Super-high-temperature industrial pipelines like smoke exhaust pipelines
• Fire protection and body thermal insulation for special military equipment

2. Building Construction Applications

• Fire protection and thermal insulation for walls, roofs, and windows
• Fire protection and thermal insulation for HVAC systems

3. Energy Storage

• Battery storage system fire protection and thermal insulation
• Limited-space thermal insulation

4. E-vehicle Battery Thermal Management Applications

• E-vehicle battery system fire protection and thermal insulation  
• Battery cell-to-cell thermal-runaway prevention

Click here to learn more about aerogel solutions in different application scenarios

Question 12: What kind of aerogel materials are widely applied in civilian scenarios?

Answer: Flexible aerogel blanket, rigid aerogel panel, and silica aerogel powder/particles.

12.1 Flexible Aerogel Blanket

• Flexible high-temperature aerogel insulation blanket(≤650℃) is the most commonly-seen aerogel insulation material. It is widely applied for the purposes of energy-saving, fire-prevention, and anti-scald in various industries like the oil&gas, chemical industry, power generation, building construction, etc.
• Cryogenic aerogel insulation blanket(-200℃~+400℃) is widely used for the purpose of energy-saving in cryogenic storage and transportation of LNG, liquid nitrogen, liquid oxygen, etc, and high-clean environments of the pharmacy industry, electronics industry, etc., especially in scenarios where there is only limited space for insulation material installation.

12.2 Rigid Aerogel Panel

• Rigid high-temperature aerogel insulation panel(≤650℃) is also commonly-seen in the thermal insulation of industrial equipment and building construction.
• Rigid super-high-temperature aerogel insulation panels(≤℃/℃) are mainly applied in the thermal insulation of super-high-temperature industrial equipment.

12.3 Silica Aerogel Powder/Particles

Goto Nano to know more.

Suggested reading:
What is the Advantage and Disadvantage of gear oil additives

• Silica aerogel powder/particles mainly used as the functional additive or raw material in thermal insulation coating, pre-mixed mortar, special plastic formula, etc.

Click here to learn more about our aerogel materials

Question 13: What are the advantages and disadvantages of aerogel materials?

Answer:

Advantages of aerogel materials:

• Excellent thermal insulation performance under high temperatures
  With nanoporous silica(SiO2) aerogel as the core material, its thermal conductivity is extremely low even under temperatures up to 650℃.
• Easy installation and extremely low thermal bridge
  Its thermal insulation layer thickness is only 1/5~1/3 of conventional thermal insulation materials. So it is easy to cut and install this insulation blanket and easily lapping over the edges can avoid thermal bridge to the utmost. 
• Hydrophobic 
  Hydrophobic rate≥98%. No water absorption and no moisture absorption. The insulation blanket can prevent pipelines and equipment from CUI and extend their service life.
• Flame retardant
  Flame retardant grade: A grade. It can effectively prevent pipelines and equipment from flame and fire.
• Durable and long service life
  As an aging-resistant inorganic material with a compact and stable material structure and high compression resilience and high hydrophobic rate, this insulation blanket service life can be the same as that of the pipelines and equipment.
• Non-toxic and environmentally friendly
  It does not contain harmful substances such as asbestos, benzene, and formaldehyde, and the whole production process and application process are non-toxic and environmentally friendly. It can be reused after removal so no worry about industrial waste disposal.

Disadvantages of aerogel materials:

• High price
  With the high-cost and complicated production process of aerogel materials, currently, aerogel material price is still much higher than conventional insulation materials like rookwool, fiberglass, PU foam, etc., despite the thinner aerogel insulation thickness in applications.
• Dust-drop problem
  Currently, dust-drop remains a problem in aerogel material applications. Especially in those clean-room requirement working scenarios. To get high thermal insulation performance and solve the dust-drop problem, people have to wrap the aerogel material thoroughly to avoid the influence of aerogel dust on the operators and working environment.

Question 14: What are the most important performances to care about when choosing aerogel materials?

Answer: Thermal conductivity, working temperature range, dust-drop rate, hydrophobicity, burning characteristics, density, flexibility/stiffness, etc.

Thermal conductivity

Low thermal conductivity is the most important reason why people choose areogel materials. Most silica aerogel material thermal conductivity is now around 0.020 at 25℃. While some high-quality aerogel material can even reach ≤0.015 at 25℃. When applied in high-temperature scenarios, aerogel thermal conductivities at different temperatures are considered as the most important.

Working temperature range

There are different aerogel materials for different working temperature ranges. The most popular one is for the working temperature range up to 650℃(Click here to check details about our ThermFelt HT650 series). There is also aerogel material specially designed for cryogenic insulation, which can withstand a temperature range -200℃ to +500℃(Click here to check details about our CryoFelt LD). Another special aerogel material is meant for super-high-temperature applications above ℃(Click here to check details about our ThermFelt SHT series)

Dust-drop Rate

Another important performance people care about is the dust-drop rate of aerogel material. Now all the silica aerogel composite materials are faced with the aerogel-dust-drop problem. Because after the super-critical drying process, there is inevitably some aerogel dust residue on the surface. Aerogel dust can cause dryness and irritation problems to human skin and make the working environment unclean. Fortunately, years ago, we successfully developed the low-dust-drop version aerogel insulation material, and after years of development, now the dust-drop rate has been proven to be the lowest among all the aerogel brands in the world. Click here to know more about our low-dust-drop aerogel materials

Hydrophobicity

Generally, for working temperatures below 200℃, normal aerogel materials(qualified aerogel materials, not defective or fake ones) can remain hydrophobic. When the temperature goes up to 300℃ or even above, only the ASTM standard aerogel materials can remain hydrophobic.

Burning characteristics

Qualified aerogel materials must be tested as non-flammable and non-smoke when burning or heating. Check our ASTM testing report

Density

Density is another specification to care about. Most aerogel material density is between 150 to 200kg/m3. If with the same base material, the lower density, the higher aerogel percentage, which means the better aerogel material.

Flexibility/stiffness

Flexibility/stiffness is another performance to care about. Application scenarios like small-size pipeline thermal insulation normally require ultra-flexible aerogel material for easy installation. Many high-temperature equipment insulation scenarios require higher stiffness for better mechanical strength.

Question 15: What’s the relation between insulation thickness, thermal conductivity, and thermal insulation effect of aerogel materials?

Answer:

Thermal conductivity is the property of the aerogel material type itself, and has nothing to do with insulation thickness;

With the same aerogel material type(the same thermal conductivity), the thicker insulation thickness, the better thermal insulation effect;

According to literature reports, aerogels can be used to develop diverse new high-performance products, such as jackets used to protect human body from extreme cold weather, constituents of space suits, specialized buildings, insulations for pipe and sound as well as heat-insulated blankets and aerogel-based textile composites, etc. Aerogel coatings on different substrates for different applications are quite common in recent years. The coating of aerogel on different substrates is conducted by a sol–gel process including techniques such as dip-coating, spin-coating, spray-coating, etc.

1. (1)

   Sol–gel coating process is a well-recognized method to prepare the gels and nanoparticles [59, 60]. When the sol is coated on a substrate, the dispersing medium constituting the sol is evaporated to form the gel layer. Many researchers have applied silica aerogel coatings onto the fibers and fabrics as functional agents through sol–gel method [26, 27]. In fabricating hydrophobic surface coatings with the sol–gel method, the surface roughness obtained can be easily regulated by varying the method conditions and the formula of the reactants. The sol–gel coating process to construct superhydrophobic surfaces has been studied widely in the last few decades. For example, a transparent and self-cleaning superhydrophobic coating can be fabricated using a very simple one-step sol–gel approach via the processing of long-chain fluoroalkylsilane [60]. This coating layer could exhibit a rough, wrinkled, hill-like surface morphology. Besides, the as-prepared coating could not only have super-hydrophobicity, but also show outstanding self-cleaning property. After the attack of a high-speed water jet, the superhydrophobic wetting state could be also well reserved.

2. (2)

   Dip-coating is the most commonly used and versatile approach to fabricate super-hydrophobic coatings on different textile substrates with a layer of hydrophobic micro-nano materials, i.e. titanium dioxide, silicon dioxide and zinc oxide [61]. It usually needs three key processing steps, including dipping in the coating slurry, drying and curing. Generally, the coating slurry is composed of: (1) nano/micro-scale dual-scale particles that can increase the roughness of the coating layer; (2) organic solvent that can wet the fabric and disperse the particles; (3) polymer that can particularly improve the bonding strength of the coating layer. Sometimes, the coating slurry may also contain a hydrophobic agent, such as fluorocarbon silane that can reduce the surface energy of the coating layer. Due to the toughness of the polymer binder and the layered roughness of the stable particle coating, some ultra-wet resistant fabrics made by the dip-coating process can perform outstanding mechanical durability. The schematic diagram illustrated in Fig. 4 shows the typical dip-coating process.

3. (3)

   Spray-coating is a simple, rapid and universal way to construct a rough multi-scale hierarchal structure, and this technique can be used in coating a layer of low surface energy polymer on any substrate [63, 64]. For example, the superhydrophobic or superoleophilic epoxy/attapulgite nanocomposite coatings on the stainless steel meshes could be constructed via a simple spray-coating process [65]. The coated mesh was also proved to maintain a highly superhydrophobic property after treating in a variety of harsh conditions, such as high temperature, mechanical scratch, corrosive substance, and humid atmospheres. A simple and typical spray-coating scheme is illustrated in Fig. 5

4. (4)

   Spin-coating is the simplest technique to construct coating layers on a substrate. The spin-coating can be used in the deposition of sol-gels on different substrates. Firstly, a gelatinous network on the substrate surface can be formed using this technique. Subsequently, a solid film can be obtained by removing the solvent from the gel. The typical spin-coating process on the substrate surface is illustrated in Fig. 6.

4.1 Aerogel coatings on textiles

Aerogels can be used for developing novel anti-wetting, self-cleaning and thermal insulating coatings on textiles. For instance, the silica biopolymer coatings can impart a high hydrophobicity degree to the fabric or impart flame retardancy to the fabric. In practice, most oxide-based aerogel coatings have poor durability due to their extreme brittleness, and adhesive or binder are often used to overcome these issues. However, the coating process with polymeric dispersions containing aerogel often results in the aerogel fragmentation and loss of insulation properties due to excessive pore wetting. As a result, the versatile functionality of oxide-based inorganic aerogel coatings on textiles is generally limited by the poor durability [68]. This has been the main issue and challenge in aerogel coating. Some reports show that constructing the hybrid organic–inorganic aerogel composite coatings is an effective way to enhance the coating durability [69, 70]. Thus, in recent years, several studies have been conducted on the coating of aerogels on fabrics to induce hydrophobicity through different coating techniques. For instance, a superhydrophobic cotton fabric with a water contact angle of 155.6 ± 0.9° for a 5 µL water droplet could be prepared via a dip-coating process using the PDMS/SiO2 aerogel composite solution [71]. Briefly, the authors prepared the PDMS/SiO2 aerogel dip-coating solution by ultrasonically dispersing SiO2 aerogel powder (4%), PDMS (4%) and curing agent (PDMS/curing agent weight ratio of 10:1) in isopropyl alcohol for 30 min with an ultrasonic frequency of 50 kHz and ultrasonic power of 130 W. The raw cotton fabrics were immersed in the as-prepared PDMS/SiO2 aerogel coating solution for 30 min and then compressing using an automatic padder with a nip pressure of 2 kg/cm2 and finally dried at 80 °C for 5 min and cured at 160 °C for 60 min in an oven. The excellent hydrophobicity of the coated fabric could be attributed to the combination of SiO2 aerogel particles with porous rough microstructure, high specific surface area and PDMS adhesive layer with low surface energy. As a confirmation, the authors made a comparison of the ability to induce hydrophobicity on the fabric surface with PDMS and SiO2 alone and in combination (as seen in Fig. 7). The cotton fabric coated by PDMS alone (as seen in Fig. 7c) showed a water contact angle of 133.4 ± 0.6°, indicating the difficulty in achieving superhydrophobicity only by reducing the surface tension without specific surface topography. Whereas, the cotton fabric sample coated by PDMS/SiO2 composite aerogel displayed excellent superhydrophobicity with a water contact angle of 155.6 ± 0.9° in (as seen in Fig. 7d). The effect of PDMS/SiO2 aerogel coating treatment on the mechanical strength properties of the cotton fabrics was negligible. This simple approach may pave the potential way for practical applications. In addition to the aerogel composites prepared by using the organic binder, some inorganic substrates, such as attapulgite, ceramic fibers, glass fiber, carbon et al., could also be used to reinforce the silica aerogel structure [72], and the resultant aerogel composites could be used in the fabrication of protective clothing, endowing it with high-temperature stability and excellent thermal insulation property [73, 74]. However, the use of inorganic aerogels to construct promising flexible coatings while reserving their natural characteristics still requires in-depth research.

Multiple special functionalities can be induced to textiles through sol–gel spin coating of aerogel nanoparticles [75]. In this regard, Shaban, et al. reported the sol–gel spin coating of cotton fibers with ZnO aerogel nanoparticles to induce both self-cleaning property and photocatalytic removal of Methyl Orange dye [75]. As the authors briefed, zinc acetate dihydrate and monoethanolamine (in 1:1 molar ratio) in 2-methoxyethanol were stirred at 60 °C for 2 h to prepare the ZnO nanoparticles sol. Then the as-prepared ZnO sol was aged for 24 h and spin-coated onto cotton fibers at  rpm for 60 s; finally dried at 50 °C for 30 min. The coating process was repeated for 10 times and finally annealed in a furnace at 150 °C in air for 2 h. As per the report, the ZnO-coated cotton showed self-cleaning property against methyl orange dye and decomposed the dye by 73% and 30.7% under the sunlight and lamp illumination, respectively.

Recent years, there are few studies involving the use of aerogels to coat wool-aramid blended fabrics to provide thermo-physiological comfort of protective clothing for firefighters [76]. The silica gel composite reinforced by aramid fibers with high thermal insulation property have been successfully fabricated. Its thermal conductivity was 22 mW/mK and its fiber content was 1.5–6.6% [77]. Furthermore, it has been reported that a doubling of the thermal insulation could be observed in silicon dioxide and fabric composite based on polyurethane [78]. The method of needle-free electro-spinning and electro-spraying of polymer mixture onto textiles has also been tried to overcome the difficulties associated with other coating methods on textile fabric substrate [79].

Farzaneh et al. reported a hydrophobic and thermal insulating polyester woven fabric using an electro-spraying coating of nanoporous silica powder [80]. As reported, a scoured polyester fabric could be coated using a mixture containing silica aerogel (3%, w/v) in RUCO-COAT FC (20%, w/v) via an electro-spraying device (as seen in Fig. 8). The coating mixture was prepared by adding silica aerogel powder and a small amount of emulsifier (for better dispersion) into the aqueous solution of the fluorocarbon material and stirring for 12 h using a magnetic stirrer. The parameters of electro-spraying process mainly include the voltage, flow rate, and distance from the needle tip to the collector, which could be set as 17 kV, 0.85 mL/h, and 5 cm, respectively. A needle with 0.6 mm inner diameter was used for the electro-spraying coating of the sample fabrics. Finally, the coated fabrics were dried at 100 °C for 5 min and then cured at 170 °C in a lab dryer.

In light of the reported results, it was observed that the reduction in heat transfer from 71.65% of the unsprayed fabric to 38.35% and 30.99% of the electro-sprayed fabric required 16 and 24 h, respectively. A similar change trend for air permeability was also observed. The samples with higher content of aerogel had lower air permeability. Moreover, the presence of aerogel particles could be in favor of improving the hydrophobicity of electro-sprayed samples. After being electro-sprayed with the aerogel/fluorocarbon mixture for 24 h, the samples exhibited the highest hydrophobicity with a contact angle of 152.2°. In addition, after the abrasion test, the weight loss of the sprayed sample was negligibly less than 5%, which resulted in low dust-removal performance of the aerogel particles from the fiber surface.

As previously reported, thermo-physiological comfort was related to the thermal properties of the fabric, moisture permeability, sweat absorption and drying capacity [81]. These characteristics are affected by many main parameters, such as molecular structure, fiber geometry, fabric structure, cross-section, pore-distribution, channels, surface tension, thickness, density, etc. [82, 83]. Similarly, the functionality and durability of coated fabric are also facilitated by the finishing treatments [84]. Many researchers have studied aerogel coatings on fabrics to inherit thermo-physiological comfort through different coating techniques including the famous sol–gel method [26, 27].

Jabbari, et al. reported the fabrication of a kind of polyester woven fabric coated by a novel lightweight and highly thermal insulative silica aerogel-doped poly(vinyl chloride) using blade-coating method [85]. As per the report, the coating solution was prepared by mixing different proportions (0, 2, 3 and 4 wt.%) of silica aerogel with PVC using a mechanical mixer having 3-blade propellers at different speeds. The as-prepared homogenous coating solution was coated on both sides of the polyester fabric with a laboratory-scale blade-coating method. Finally, both sides (first side and second side) were cured at 180 °C and 190 °C for 2.5 min and 1.5 min, respectively. A general illustration of the blade-coating technique is shown in Fig. 9. The report showed that thermal insulation performances were improved by 26% (from 205 to 152 mW/mK) compared with the unmodified coated fabric. The report investigated the dependency of the thermal insulation characteristics of the coating on the percentage of the aerogel content. For this purpose, four proportions of PVC/aerogel composite coatings such as 0, 2, 3 and 4 wt.% were prepared and coated on the fabric. The analyses proved that composite coating with 4 wt.% of PVA/aerogel showed the best heat-insulating property. According to the report, the dosage of 4 wt.% was the critical percentage. Besides, when the aerogel content exceeded 4 wt.%, the preparation of composites would be limited due to the high viscosity. The authors suggested the potential applications of the as-prepared coated fabric in developing textile bioreactors for the production of ethanol/biogas based on waste materials, tents, exterior wall coverings, temporary houses, tarpaulins and container linings, etc.

In high heat protection clothing, e.g., firefighter’s protective clothing (FFPC), aerogel coatings can both resist the incoming heat fluxes and block the dissipation of heat from the body. Therefore, the body temperature of the wearer will increase. Shaid, et al. [87] attempted to rectify the situation by introducing phase change materials (PCMs) into the aerogel coating. The work investigated the simultaneously coating of facial clothing linings using aerogel and PCM for firefighters' protective clothing. The ambient side of the thermal interlining was coated using silica aerogel particles. Meanwhile, the PCM/aerogel composite powder was coated on the side close to the skin. The new thermal insulation lining had excellent thermal insulation protection and comfort. It prolonged the time to reach the pain threshold and increased the time for pain prompts. When the composite powder was heated to a temperature three times higher than the melting temperature of pure PCM, no dripping or deformation was observed.

The thermal insulation characteristics of the aerogel coated fabrics depend on the aerogel microstructure and concentration on one hand and the fabric weave or knit structure on the other [88, 89]. Rosace, et al. investigated the impacts of weave structures and silica aerogel coatings obtained by the sol–gel method on the thermal insulation performances of woven cotton textile fabrics [90]. To this end, three kinds of important cotton fabric weaving structures were selected, including plain weave, satin weave and pile, which had different yarn counts, threads per centimeter and mass values per square meter. As it is observed from the thermal property versus density profile of the three weave structures in Fig. 10, the fabric weave and density strongly influenced the thermal properties as such: pique always showed the lowest value, while satin showed the highest value, and plain weave lied in between. Pique thermal resistance Ʀ values (as seen in Fig. 10b) decreased linearly with increasing the density, while the data for satin and plain weave fabrics actually remained the same and decreased by an order of magnitude. Therefore, the Pico cotton fabric had quite favorable thermal insulation performances, accounting for its higher thickness and weight, however, disadvantaged from its lower density, when compared to the plain and satin cotton fabrics. The thermal absorption Ь (as seen in Fig. 10c), on the other hand, increased from pique (warmer feeling) to plain weave up to satin (cooler feeling) as expected.

Imparting fire-retardant characteristics on fabrics is also a recent experience as many studies have been reported in recent years [91,92,93,94]. A good fire retardant (FR) fabric generally have the following features: (1) it will not ignite easily when exposed to flame or continue to burn after removing ignition source; (2) it will provide a barrier or an insulating layer on exposure to intense heat; (3) it will not melt or shrink when exposed to high heat flux and have good anti-static property.

Halogen, phosphorus and phosphorus-nitrogen compounds as traditional flame retardants used for almost all the textiles, and metallic salts applicable to protein fibers [95, 96] are limited by the toxicity and bioaccumulation and environmental persistence [97]. Hence, the development of environmentally benign fire retardants is an urgent need. In the textile industry, heat-stable condensates of tannic acid and terephthaloyl chloride were used to construct environmentally friendly flame-retardant coatings on nylon 66 fabrics, and the fabrics obtained exhibited rapid self-extinguishing properties [98]. Aerogel coatings for flame retardancy applications are alternative, nano-sized objects that can be easily synthesized via a bottom-up approach such as the sol–gel process [91]. The modification of eco-friendly fire retardants with aerogels enhances the flame resistance of the coatings. The preparation of aerogel modified fire retardant coating consisting of montmorillonite, epoxy resin, and tannic acid is a recent experience in this regard [99].

Aerogel coatings are also promising in the preparation of protective clothing. Bhuiyan, et al. developed a novel protective fabric accompanied with a favorable resistances to radiant heat and chemiosmosis of liquid [22]. The protective fabric can be tailored to achieve both barrier performance and thermal comfort by integrating the electrospun polyacrylonitrile (PAN)-silica aerogel nanofiber membrane with the needle punched viscose non-woven fabric. As demonstrated in Fig. 11, the PAN-Silica aerogel solution of different aerogel concentrations was electrospun coated on a viscose non-woven fabric. Next, another layer of non-woven material of similar thickness was used to cover the coated non-woven film. Under the temperature of 140 °C and the pressure of 6.0 kPa, the edge of the fabric was heat-set with an adhesive interlining for 3 min.

The incorporation (coating) of the extremely hydrophobic silica aerogel with exceptional porosity on to the naturally hydrophobic PAN non-woven fabric enhanced the overall hydrophobicity of the film to resist the penetration of chemicals with high surface tension (as seen in Fig. 12 b1, c1 and d1). When the liquid had a low surface tension, the chemicals were first absorbed on the surface of the fabric through the fibers, and then absorbed through the pores of the aerogel particles embedded in the film (as seen in Fig. 12 c2 and d2). Silica aerogels have pore sizes widely ranging from 5 to 100 nm. The interconnected network of open pores allows the chemicals to flow from one pore to another through limited restrictions, thereby being able to absorb liquids. Due to the adsorption of aerogel particles, the liquid chemicals are mainly dispersed on the surface of the film and cannot penetrate the nanofibers. More aerogel particles are adsorbed, the overall liquid adsorption capacity will be stronger, so more liquid will be absorbed, resulting in more chemicals being retained in the fabric. Finally, the protection against the penetration of liquid chemicals will be greatly improved.

The newly developed fabric also showed improved heat and moisture vapor transmission rates, and a higher evaporative cooling index indicated that the thermal comfort of the clothes was acceptable due to the reduced accumulation of sweat on the material. In addition, the proper air permeability and moisture management property suggested the diffusion of sweat vapor through the film, thereby affording good thermal comfort in the clothing-skin microenvironment.

In another study, Bhuiyan, et al. demonstrated PU-Silica aerogel coated cotton fabric having simultaneous thermal comfort and protective performance through a knife over roll coating method [100]. The protective properties, including the surface wettability and chemical resistance of the coated fabric, were evaluated based on the measured values of water contact angle, water repellency level and chemical resistance. Due to the increased hydrophobicity, PU-silica aerogel coated fabrics could exhibit higher water repellency rating. Similarly, after integrating porous aerogel particles, the chemical resistance was observed without a decrease in protective performance.

4.2 Aerogel coatings on leather

Until recently, little has been reported on the introduction of aerogels in the conventional leather finishing chemicals to impart special properties on the leather surface as has been tremendously reported on textile fabrics. Efforts to utilize the exceptional properties of aerogels by modifying acrylic resin in the preparation of exceptional leather finishing agents was made by Hu and associates [101, 102]. As the authors reported, the acrylic resin (AR)/nano-SiO2 leather finishing agent could be prepared by physically blending AR and nano-SiO2 sol. This nano-SiO2 sol was prepared via the TEOS sol–gel method catalyzed by ammonia or HCl [101]. Furthermore, the AR/nano-SiO2 leather finishing agent can also be prepared through the emulsion polymerization without adding emulsifier [102]. The water vapor permeability of leather treated with the AR/nano-SiO2 finishing agent was increased by 9.15% and the finish adhesion was increased by 10.35%. The modified AR finishing agents (nano-coatings) exhibit unusual properties [103, 104], which were related to the uniform dispersion of nano-SiO2 in the polymer, thus probably improving the resistances to abrasion, aging, climate, and the strength of the polymer. It has also been reported that the polyacrylate (PA)/nano-SiO2 composite could be synthesized via semicontinuous emulsion polymerization stabilized with polymerizable surfactant [105]. The application of this composite in leather finishing showed that, compared with conventional surfactants containing latex, the latex stabilized with polymerizable surfactants showed significantly higher water–vapor permeability (increased by 2.06%) and water absorption resistance (increased by 7.88%).

At present, the aerogel-modified coatings for leather are still in the early stages of exploration, and tremendous studies on advanced leather finishing agents to induce special properties have been widely carried out, for example, preparing novel sulfanilamide-conjugated polyurethane coatings with enzymatically-switchable antimicrobial capability for leather finishing [106], fabricating nano-scale core–shell type particle of caprolactam-butylacrylate co-modified casein (CA-CPL-BA) leather finishing agent with enhanced hydrophobicity as well as higher thermo-stability and biodegradability [107]. Moreover, photosensitive silicone-containing polyurethane acrylate leather finishing agent [108], alkali-soluble butyl acrylate/acrylic acid copolymer leather finishing agent with excellent air permeability, water-resistance and resistance to wet rub fastness [109] are being explored extensively. Hence, more studies on exploiting the special functionalities of aerogels coatings on leather substrates shall be expected in future, and there is also much work to be done in the future if the various functional features of aerogels are to be brought into play on leather coatings.

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4.3 Aerogel coatings on other substrates