Aerogel was first reported in a paper submitted to Nature by Kistler () as a low-density, porous solid gel derived from gel. Aerogel can be synthesized from a variety of organic and inorganic substances, such as silicon oxide, titanium oxide, aluminum oxide, and carbon (Venkataraman et al., ). Silica aerogel is a typical type of aerogel. It has drawn a lot of interest both in science and technology due to its low bulk density (up to 95% of its volume is air), hydrophobicity, low thermal conductivity, high surface area, and optical transparency (Gurav et al., ). This low-density lightweight structure has very good thermal insulation properties (Tang et al., ). However, from the viewpoint of applications, aerogels have the drawbacks of absorbing moisture from the atmosphere, being fragile, being difficult to handle, and being impossible to use to insulate complex shaped bodies (Katti et al., ). To make them flexible for wider applications, aerogels are usually used after being composited with other materials. However, while this improves the overall performance, the original performance of the aerogel will inevitably be weakened due to it mixing with other materials.
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Aerogel blanket is a typical aerogel composite material. It is flexible, drapable, and is formed by depositing aerogel particles into fibrous substrates. Aerogel blanket has attracted attention in the clothing industry due to its unique properties, such as fire resistance, water resistance, heat resistance, and flexibility. Consequently, researchers have investigated the application of aerogel blanket in protective clothing used in thermal hazardous environments, such as high-heat, flame, electrical arc discharge, molten substance, and firefighting environments (Chakraborty et al., ). In addition, its application in winter clothing and footwear used in cold environments, such as alpine and polar regions, has been investigated. Du and Kim () conducted market research on the application of aerogels in daily clothing and found that the application of aerogel blanket in daily clothing is becoming active and specialized.
However, the easy detachment and dispersion of aerogel particles attached to non-woven materials into the air have limited the further application of aerogel blanket. To address this problem, aerogel blanket is encapsulated before use; however, this negatively affects the flexibility and breathability of the material.
To address these problems, researchers are devoting efforts to develop new composites of aerogel and polymer materials. For example, NASA’s Glenn Research Center developed polyimide aerogels, which exhibit approximately 500 times more compressive and tensile strength than silica aerogel and equivalent insulation ability. Polyimide aerogels are thin films with promising properties, such as high flexibility, light weight, and porosity (NASA’s Technology Transfer Program, ). However, there are currently no studies on the application of this material to clothing. Another study employed a method known as “aerogel-infused closed cell polyfoam” to develop a composite material of aerogel and polymer. The researchers applied for a patent for this material under the name of SOLARCORE, claiming that it exhibits superior insulation properties to existing insulation materials and addresses the dispersion problem of aerogel particles in air. Particularly, they emphasized that the material exhibits good flexibility and breathability and enables direct cutting and sewing (Shiwanov, ).
However, despite several attempts, the application of aerogel materials in the clothing industry is still limited. Du and Kim () conducted market research on aerogel clothing and found that that consumers’ opinions on aerogel clothing products varied. For example, most consumers stated that aerogel clothing exhibits good insulation properties, particularly during strong wind. However, consumers also stated that it was difficult to clearly differentiate the performance of aerogel clothing from previous insulation clothing. In addition, the lack of breathability and flexibility of aerogel clothing have affected consumers’ opinions of it.
From a human development perspective, with the increase in severe natural disasters caused by environmental destruction and the future exploration of severe cold area, the demand for clothing materials with strong protective properties for overcoming extreme environments will increase. Although several factors limit the effective application of aerogels to clothing, it is still highly anticipated due to aerogel’s excellent irreplaceable properties. Therefore, it is important to conduct extensive research on the application of aerogel in the clothing industry.
To explore the application of aerogel in the clothing industry, it is essential to clarify the necessary direction of further research based on the current research stage and difficulties. Therefore, in this study, a literature survey of the application of aerogel in clothing was conducted to understand the current research focus on aerogel clothing, details on the development of aerogel materials, and effects of their application in clothing. Based on the analyses of the problems of aerogel application in clothing, we suggest further research directions on the application of aerogel in clothing.
The research focus on aerogel clothing was examined using the keywords in the surveyed studies. For studies containing few or no keywords, nouns related to application purposes, aerogel materials, and measurement items were supplemented according to the titles and conclusions of the studies. A total of 105 keywords were collected from the 22 surveyed studies. To highlight the research focus of these studies, the same or similar keywords were combined, and the number of studies containing each keyword were indicated after the keywords. Subsequently, the collected keywords were divided into four categories: aerogel clothing types, aerogel material types, experimental measurement items, and other contents, as summarized in Table 1. The research focus of aerogel clothing was analyzed based on the four keywords categories, and they are discussed below.
A total of 22 keywords were related to aerogel clothing types, with each study corresponding to a keyword. These keywords were divided into two categories: aerogel clothing and aerogel footwear. The aerogel clothing category consisted mainly of firefighter clothing, space suits, and wetsuits. Eleven keywords relating to firefighter clothing were observed, which indicates that firefighter clothing is the major type of aerogel clothing investigated in studies on aerogel clothing at present.
Similarly, a total of 22 keywords were related to aerogel material types, with each study corresponding to a keyword. These keywords could be broadly classified into aerogel fiber composite, aerogel coating fabric, and aerogel foam composite. Aerogel fiber composite is commonly known as “aerogel blanket,” and among the 22 keywords, there were 16 keywords relating to aerogel blanket, which indicates that it is the main type of aerogel material used in the clothing industry. The development details of these materials will be described later.
There were 36 keywords corresponding to the experimental measurement items. They could be broadly classified into thermal performance evaluation, mechanical properties evaluation, and comfort performance evaluation. Thermal performance here refers to the thermal conductivity and thermal protection performance of the fabric. Thermal conductivity (often denoted by k, λ, or κ) refers to a material’s intrinsic ability to transfer or conduct heat, and the thermal protective performance (TPP) values are the product of the incident heat flux and the recorded tolerance time to second degree burn (cal/cm2) (Stoll & Chianta, ). Keywords relating to thermal performance evaluation appeared in all surveyed studies, which implies that thermal performance is the most important factor to consider when applying aerogel to clothing.
Mechanical properties, according to the theme definition of Nature magazine, are physical properties that a material exhibits upon the application of forces. Examples of mechanical properties are the modulus of elasticity, tensile strength, elongation, hardness, and fatigue limit (Nature, n.d.). Keywords related to the measurement of mechanical properties in this study accounted for 16.7% of all keywords and were mainly related to research of durability and compressive properties. Because clothing requires repeated use, when aerogel materials are used in clothing, the enhancement of the mechanical properties of materials will inevitably become an important and even decisive factor.
Comfort performance is another important attribute of clothing. The comfort performance of this study included thermal comfort performance, but was not limited to thermal comfort. Thermal comfort is defined as the condition of mind that expresses satisfaction with the thermal environment (Choi & Loftness, ), and comfort is the result of many physical factors as well as the mental, subjective psychological reaction (De Looze et al., ). The content related to comfort performance in this study included subjectively comfort, thermophysiological comfort, water vapor permeability, and weight reduction. It is obvious that research on this aspect is lacking from the number and composition of keywords.
In addition to the above content, there were 25 other keywords, which were divided into four parts: “synergistic materials,” “problems and solutions,” “experimental methods,” and “application areas.” Through these keywords, the general content of previous research could be quickly understood.
According to the analysis results in “Research focus on aerogel clothing,” aerogel materials used in the surveyed studies were classified into three types: aerogel fiber composite, aerogel foam composite, and aerogel coating fabric. The details of their development are summarized and discussed based on the manufacturing process of experimental samples described in the surveyed studies, and are shown in Table 2.
Early research on aerogel fiber composites mainly focused on optimizing material properties by controlling the type of fiber combined with aerogels and the ratio of the chemical reagents. For example, Tang et al. () utilized five sets of fiber-reinforced silica aerogel composite with different fiber reinforcements and fiber additives as a space suit insulation layer, and they evaluated the thermal performance and dust generation of the samples after repeated mechanical cycling. In addition, Paul and Diller () numerically analyzed the thermal conductivity performance of three insulating fiber materials using aerogel as the interstitial void media in terms of various deniers (sizes), interstitial void fractions, and orientations to the applied temperature gradient to evaluate their applicability as a new type of insulation for Mars suits.
Recent research has focused on the direct utilization of commercially available aerogel fiber composites or the combination of aerogel fiber composites with other materials to fabricate experimental samples that can replace original thermal insulation materials. For example, Zhang et al. () utilized the commercially available aerogel fiber composites and microencapsulated phase change materials (PCMs) as a thermal liner for firefighting suits, and they investigated the six-material composition schemes and their effects on the thermal protection performance of the firefighting suits. These commercially available aerogel fiber composites are mainly developed using Aspen Systems. Although all the evaluations of this application method affirm the thermal protection performance of aerogel and its synergistic materials, the detachment and dispersion of the aerogel particles attached to the fabric have restricted further application of this method. This is because the detachment of aerogel particles not only deteriorates the properties of the aerogel materials, but the inhalation of aerogel dust dispersed in air can also cause health problems.
To prevent the dispersion of aerogel particles into the air, aerogel fiber composites are usually encapsulated or laminated before use. For example, Nuckols et al. () used a prototype aerogel liner fabricated using Aspen Aerogel AR panels, and they encapsulated the liner with Pertex nylon Oxford covering to replace the commercial M400 Thinsulate coverall undergarment in cold-water diving garment ensembles. In addition, Zrim et al. () described a two-sided lamination method for commercially produced aerogel fiber composites for the prevention of the dispersion of aerogel powder into the air and investigated its suitability for thermal insulation in protective footwear applications for severe cold and extreme high-altitude environments (Fig. 2).
The encapsulation or lamination of aerogel fiber composites prevents the dispersion of aerogel particles into the air; however, this reduces the flexibility and breathability of the material and restricts the cutting and sewing of the material. To solve these problems, researchers developed aerogel foam composite, which effectively locks aerogel particles in and increases the material’s flexibility.
Aerogel foam composites are prepared by mixing aerogel fabric composites and foam or aerogel particles and foam. Bardy et al. (b) attempted to utilize a composite of aerogel fiber composite and foam in a wetsuit. To achieve this, the aerogel fiber composite was cut into circular discs and square shaped cut-outs, and these cut-outs were spaced apart in a set pattern, with the gaps between the cut-outs being filled with syntactic foam, as shown in Fig. 3. In addition, Yang et al. () attempted to use a composite of aerogel particles and foam in footwear applications. To achieve this, they created a nanocellular structure within a polyurethane (PU) foam by incorporating silica aerogel particles in the PU foam.
Aerogel coating fabric was developed by Shaid et al. (Australia). They attempted to enhance the performance of firefighting suits using aerogel coating or hybrid coating of aerogel and PCM. Shaid et al. () coated the 65/35 wool/Aramid blended fabric with different percentages (2%, 4%, and 8%) of NANOGEL (superhydrophobic aerogel nanoparticle from Cabot) using an acrylic binder. They analyzed the thermophysiological comfort of the coating fabrics by determining air permeability, moisture management properties, and heat transfer properties, and they obtained positive results, thus identifying the potential use of aerogel in firefighter clothing. However, aerogel in firefighter clothing not only resists incoming heat fluxes, but also blocks outbound body heat, which increases the body temperature of the wearer. Shaid et al. () tried to resolve this problem by using a hybrid coating of aerogel and PCM. They coated the ambient side of a thermal liner face cloth of traditional firefighting suits with silica aerogel particles and coated the skin side with PCM/aerogel composite powder. This successfully extended the time to reach pain threshold [human skin reaches around 44 °C (Stoll & Greene, )] and increased the pain alarm time [the time gap between starting to feel pain and receiving second-degree burns (Rossi & Bolli, )]. In addition, since most PCMs used in firefighter clothing are flammable, wearing protective clothing containing PCM presents a direct risk to the wearer. Shaid et al. (a) wanted to demonstrate that using aerogel along with PCM can improve the flame resistance of PCM-containing firefighter clothing. They added aerogel to the PCM-coated thermal liner, and then the coated thermal liner was used with an outer layer fabric and moisture barrier in a similar fashion as traditional firefighter clothing. They confirmed that aerogel coating on a PCM-coated thermal liner slowed down the spreading of flames in PCM-containing fabric. Of course, the coating increased the overall weight, but compared with PCM as the only coating material for firefighter clothing, the addition of aerogel reduced the weight.
To understand the effects of the application of aerogel on clothing performance, the methodologies for the evaluation of the performance of aerogel clothing in the surveyed studies were summarized, and the application effect of aerogel to clothing were qualitatively analyzed using the experimental results and research conclusions. Finally, the main problems of the application of aerogels in clothing were discussed. The methodologies of the surveyed studies on the evaluation of the performance of aerogel clothing were summarized into experimental samples, experimental methods, and measurement items and are presented in Table 3. Studies in the table are classified by application purpose of aerogel and sorted by year within the classification, but studies by the same researcher are arranged together.
The experimental samples in the surveyed studies could be broadly divided into clothing samples and fabric samples. Clothing samples are clothes fabricated by the application of aerogel, whereas the fabric samples are fabrics of clothes fabricated by the application of aerogel. The experimental samples used for aerogel footwear products were fabricated in the form of insoles, so all the experimental samples related to footwear were organized into the fabric samples category. As shown in Table 3, the experimental samples were mostly composed of aerogel fabric samples (86.4%), with aerogel clothing samples only occupying a small percentage (18.2%). How aerogel was used for fabric sample development has been described in “Development details of aerogel materials” section. The Arabic numbers in parentheses in the “Experimental samples” category in Table 3 indicate the types of aerogel materials developed. There were three cases of clothing samples that were made from aerogel fabrics in all surveyed studies, and they appeared in the studies of Nuckols et al. (, ), Bardy et al. (b), and Jin et al. ().
Both Nuckols et al. and Bardy et al. have developed wetsuits and used clothing samples to carry out human-wearing experiments. Nuckols et al. () fabricated a prototype aerogel garment consisting of Aspen Aerogel AR panels encapsulated with Pertex nylon Oxford covering, and they compared the thermal performance of this garment with the baseline Thinsulate worn under a commercial dry suit. The prototype aerogel garment was a one-piece coverall with two plies of AR and only a single ply of AR in the arms. Nuckols et al. () used the same prototype aerogel garments to compare their thermal performance with those of commercial garments through infrared photography. Bardy et al. (b) designed and fabricated a hybrid wetsuit in collaboration with a wetsuit manufacturer. Since the hybrid insulation (Fig. 3) lacks the flexibility and stretching ability, portions of the hybrid wetsuit were made from syntactic foam at strategic stretch joints and along the seams.
While Jin et al. () developed firefighter clothing and used a manikin to test the thermal protective performance of clothing samples by flame heat transmission test. They first used different concentrations of aerogel dispersed in acetone to treat the thermal barrier of the existing firefighting clothing, and they laminated it on both sides with 25-µm thick polytetrafluoroethylene (PTFE) membrane to prevent the dusting of the aerogel powder. Then, they made prototype firefighter clothing using aerogel-treated specimens as a thermal barrier.
The experimental methods used in the surveyed studies could be classified into three types: human-wearing, manikin-wearing, and machine-measuring experiments. Machine-measuring experiments (86.4%) were the main experimental method used in the surveyed studies and were used to measure the aerogel fabric samples. In contrast, human- and manikin-wearing experiments (18.2% and 9.1%, respectively) used in the measurement of aerogel clothing samples were insufficient. Physiological data obtained through human-wearing experiments, as well as the evaluation of protective clothing in hazardous environments through manikin-wearing experiments, are crucial to the study of clothing performance. However, effective integration of aerogel materials into clothing is still challenging. The small number of available clothing samples has restricted the further application of these experimental methods. The numbers in parentheses in the “Experimental methods” category in Table 3 indicate how many subjects participated in the human-wearing experiments. Although the small number of test subjects reduced the statistical power of the study, in the Bardy et al.’s (b) study, only one subject was tested. They explained that the lack of flexibility of the wetsuit limited its use by many people. Considering the uniformity of thermal responses of normal subjects and the high cost of the hybrid wetsuit construction, they only tested one subject and assumed that the data would apply to the population in general. In comparison, Lee et al. () tested the thermal insulation performance of aerogel insoles, which allowed for the participation of more subjects.
As shown in Table 3, the main measurement items evaluated in the surveyed studies included thermal performance, mechanical properties, and comfort performance. Thermal performance measurements (100%) were conducted in all the surveyed studies, and only few studies evaluated the mechanical properties (27.3%) and comfort performance (22.7%). The plus and minus symbols in parentheses in the “Measurement items” category indicate that the evaluation result of the item is positive or negative, respectively. In other words, based on the purpose and method of that research, they indicate whether the application effect of aerogel is good or insufficient.
The 22 studies surveyed in this study investigated the thermal performance of the experimental samples containing aerogel. Among them, 20 studies reported that the samples exhibited excellent thermal performance; however, two studies reported negative poor thermal performance.
As an example of the positive conclusions, Nuckols et al. () found that replacing the M400 Thinsulate liner in diving suits with a prototype aerogel garment not only increased the dive duration time in cold water by 38% but also enhanced the thermal protection of the garment for the fingers and toes. In addition, 11 out of the 22 surveyed studies investigated the application of aerogel to firefighter clothing. Their findings revealed that the thermal performance of aerogel materials is superior to that of conventional materials. For example, Kim et al. () used an aerogel to replace the thermal liner of traditional firefighter clothing and tested the thermal properties of fabrics containing aerogel materials through flame, radiant heat, and a mixture of flame and radiation heat, and found that the application of aerogels enhanced the thermal protective performance of firefighter clothing. In addition, Jin et al. () reported that the heat-insulating properties of aerogel-containing fabrics increased with an increase in the aerogel content, and the developed aerogel garment exhibited higher TPP than the existing firefighter clothing in the instrumented manikin test. Furthermore, Shaid et al. () reported that the simultaneous use of PCM and aerogel can significantly enhance the thermal protection of such hybrid fabrics in terms of pain threshold and pain alarm time. In addition, all four studies that applied aerogel to footwear products reported the positive influence of aerogel on the thermal performance of footwear. For example, Oh and Park () reported that aerogel fiber composite exhibited promising potential as an insole material for cold winter shoes requiring good thermal insulation protection. In addition, Yang et al. () concluded that the loading of the aerogels into foams effectively improved the thermal insulation.
In contrast, two studies including Bardy et al. (b) and Crowell et al. () reported negative effects of aerogel on thermal performance. Bardy et al. (a) prepared the aerogel-syntactic foam hybrid insulation (Fig. 3) for wetsuit, and measured the thermal resistance of the prepared material in a previous study. The results have showed that the aerogel-syntactic foam hybrid insulation has a thermal resistance significantly higher than those of both foam neoprene and underwater pipeline insulation at atmospheric and elevated hydrostatic pressures (1.2 MPa). However, when they used the developed aerogel-syntactic foam hybrid insulation to make a wetsuit, they reported that the aerogel-syntactic foam hybrid insulation wetsuit cannot provide more thermal protection than the foam neoprene wetsuit (Bardy et al., b). According to their analysis, due to the lack of flexibility and stretchability of the hybrid insulation, the strategic stretch joints and the seams of the wetsuit were could only be made from syntactic foam. In addition, the aerogel-syntactic foam hybrid insulation does not fit the body very well due to its lack of flexibility, so the water flow over the skin and the formation of thermal bridges in the insulation could have been contributing factors for the decrease in performance. Therefore, the reason for the negative results of the aerogel-syntactic foam hybrid insulation wetsuit can be attributed to the unsatisfactory method of integrating aerogel materials with clothing. They believed that an aerogel-syntactic foam hybrid insulation wetsuit can provide more thermal protection when the presence of its surface depressions can be eliminated and alternative methods for a tighter fit can be achieved.
As another negative conclusion on thermal performance, Crowell et al. () reported that stitching the aerogel non-woven composites was not a viable option for space suits due to the increase in thermal conductivity and the difficult manufacturing. They evaluated the thermal conductivity of stitched, windowed, and packeted aerogel non-woven composites through Hot Disk Testing and Forward-Looking Infrared Imaging methods. They found that the thermal conductivity of the stitched material was significantly increased at the stitches, while the aerogel non-woven composites that were windowed and packeted showed a reduction in thermal conductivity. However, according to their experimental results, the largest increase in thermal conductivity in all test cases was only 0. W/m·K, meaning that the thermal conductivity of the material increased from 0. to 0. W/m·K after stitching. In other words, while stitching an outer layer to the aerogel materials was unsuitable for space suits, this does not mean that the degree of thermal conductivity increase was unacceptable for general thermal protective clothing.
From the distribution of the mechanical properties’ measurement items in Table 3, the mechanical properties evaluation mainly focuses on space suits and footwear, with five cases of aerogel fabric composites and one case of aerogel foam composite. All six studies that examined the mechanical properties of aerogel clothing reported positive conclusions. For example, Tang et al. (, ) reported that some aerogel materials retained good insulation performance after approximately 250,000 mechanical flex cycles and had less than 1.3% relative weight loss. In addition, Yang et al. () evaluated silica aerogel-reinforced polyurethane foams for footwear applications and found that the incorporation of aerogels enhanced the compressive modulus, compressive stress, and deformation recovery of the foams, while preserving their excellent flexibility. Crowell et al. () evaluated the mechanical properties of the aerogel non-woven composite stitched with an outer layer through tension tests and revealed that stitching increased the strength of the aerogel non-woven composite, although it also increased the thermal conductivity. In fact, for insole applications, the mechanical properties of aerogel materials have already met the standards for commercial use. Shoe insoles made with aerogel fabric composite have been marketed and sold in the United States since (Tang et al., ).
Comfort of the human body is very complicated and is affected by many factors, including the thermal and mechanical properties of clothing. Comfort can be thermal, physical, sensational or thermophysiological, thermophysiological comfort refers to the heat and moisture transport properties of clothing and the way that the clothing maintains the body’s heat balance (Huang, ). In this study, the research results of comfort performance are summarized mainly based on the theme discussed by the authors. In 22 studies surveyed, five of them presented research conclusions on the comfort performance of aerogel clothing, including subjectively comfort, thermophysiological comfort, skin–clothing microclimate, and weight reduction, and these research conclusions were positive.
The research of Nuckols et al. () is the only study of the effect of aerogel application through the quantification of physical and psychological characteristics. The researchers asked the subjects to wear a prototype aerogel garment or commercial wetsuit for 6 hours of diving, and they evaluated the thermal benefit and wearing comfort of the prototype aerogel garment using the thermal state data and psychological state data of the body during the period. The authors suggested that the significant increase in the thermal benefit of aerogel clothing brings more comfort, which is mainly reflected in the increased temperature of the hands and feet. In addition, two subjects expressed in their subjective comfort evaluations that the prototype aerogel garment was less flexible than the commercial wetsuit.
Aerogel can improve the thermal protection, but it reduces the water vapor transmission of the thermal barrier, which causes the internal temperature of the clothing to rise. In firefighter clothing applications, the benefits of the extraordinary flame protection and heat insulation properties of aerogel can only be effectively utilized when sufficient release of body heat can be assured. Shaid et al. () investigated the thermophysiological comfort of the aerogel coating fabrics by analyzing the air permeability, moisture management properties, and heat transfer properties of the fabrics. They concluded that aerogel coating fabrics have the possibility to improve the thermophysiological comfort of firefighting clothing by selecting suitable coating thickener to reduce the hydrophobic characteristic of aerogel.
Shaid et al. () subsequently presented another approach to use PCM with aerogel to achieve added thermal protection without sacrificing comfort. They coated aerogel particle as a thermal protection aid on the ambient-side of fabrics and an aerogel/PCM composite powder on the skin side to absorb metabolic heat to enhance thermal comfort. Their results show that this approach allows the temperature of the skin–clothing microclimate to remain in the comfort zone for a longer duration. They emphasized that the PCM/aerogel composite powder was stable than pure PCM at elevated temperature, and no dripping or form deterioration was observed when the composite powder was heated over a temperature three times above the melting temperature of the pure PCM.
In addition, some studies mentioned that utilizing the lighter specific gravity of aerogel materials can reduce the weight of existing firefighter clothing to improve the comfort of the clothing (Kim et al., ; Shaid et al., a). In general, the current research on the comfort performance of aerogel clothing is very limited due to the infeasibility of human-wearing experiments of high-heat protective clothing in high-risk environments and the immaturity of the technical means of integrating aerogel into clothing.
Through the above sorting, it can be observed that the improvement of the thermal performance of clothing and fabrics by aerogel materials has been widely affirmed within the scope of the surveyed literature. At present, the main problem is with how to apply this kind of high-efficiency thermal insulation material to clothing in a harmless and sustainable manner without losing too much thermal performance. The main reason for this problem is the rigidity of monolithic silica aerogel and the dust pollution of aerogel particles. Monolithic silica aerogel cannot be bent and must be compounded with other materials to be flexible when used in clothing. The aerogel fabric composite used in most research tends to fracture, and aerogel particles loosen from the fiber reinforcement after repeated handling. Although the aerogel itself is harmless to the body, the aerogel particles that fall into the air and make contact with the skin or are inhaled by the human body produce a desiccant effect and cause irritation due to aerogel’s excellent hydrophobicity. Tang et al. () stated, “the answer to the contamination hazard generated by the decomposition of current ‘fiber-reinforced silica aerogel composite fabric’ matrix still remains elusive.” From the “Development details of aerogel materials” section in this study, researchers have tried various methods to apply aerogel materials in clothing. However, the current applications are mainly limited to aerogel fabrics, and there is still a lack of verification of the effect of aerogel fabrics applied to clothing. Kim et al. () found that both weight reduction and thermal protective performance were improved by the use of aerogels, while due to the fragile nature of aerogels, a method of fixing them at a constant thickness between layers of firefighters’ protective clothing should be considered in the future.
It’s true!
There are now many different types of aerogels that are flexible and high-strength, some of which are so mechanically robust they can actually be used for structural applications!
Although it’s true that a typical silica aerogel could hold up to times its weight in applied force, this only holds if the force is gently and uniformly applied. Also, keep in mind that aerogels are also very light, and times the weight of an aerogel still might not be very much. Additionally, most aerogels as-produced are extremely brittle and friable (that is, they tend to fragment and pulverize). As a result, structural applications of aerogels were for a long time totally impractical.
But never fear! There are several ways aerogels can be made strong and even flexible, enough that aerogels can now be used as structural elements.
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There are four general ways to enhance the mechanical properties of aerogels:
Particles of oxides, such as silica, are frequently mixed into plastics to make plastics with different properties. This process is called “doping”, in which the oxide particles are called a “filler”.
One day, Prof. Nicholas Leventis at the University of Missouri-Rolla (now the Missouri University of Science and Technology) wondered,
“If you can dope a polymer with a filler, can you dope a filler with a polymer?”
So thinking about cohesive forms of fillers used for doping polymers, he thought of silica aerogels, which are effectively macroscopic assemblies of silica nanoparticles.
Starting with a preformed wet silica gel of the type used for making silica aerogels, Leventis soaked the gel in solutions containing diisocyanates-crosslinking agents used to make polyurethane varnish-and then heated the gels to get the diisocyanates to bond. Upon supercritical drying, a silica aerogel with remarkably improved mechanical properties resulted-an aerogel that can actually bend not unlike stiff rubber! Try doing this with an ordinary silica aerogel and you’ll be left with lots of little broken pieces.
Diisocyanates are linear molecules with two ends that can react with hydroxyl groups to form carbamate bonds. The hydroxyl groups lining the skeleton of silica gels are perfect candidates for reacting with diisocyanates, and since diisocyanates have two reactive ends, they can bond to the aerogel skeleton twice. The result-each diisocyanate molecules acts like a nano-sized piece of Scotch® tape bonded to the surface of the aerogel skeleton, resulting in a conformal polymer skin that ties together the spherical silica nanoparticles that make up the aerogel. This conformal polymer skin makes the resulting aerogel much stronger than a typical silica aerogel and allows the structure to flex without breaking-sort of like the marshmallow coating on a Rice Krispies® treat!
These strong, polymer-crosslinked aerogels are called “x-aerogels”. Importantly, the word x-aerogel (say “ex-air-o-jell”) is not the same thing as the word xerogel (say “zee-ro-jell”), which is a gel that has collapsed into a densified material as a result of evaporative drying.
Pretty much any gel suitable for making aerogels can be modified to produce an x-aerogel. Here’s what has to happen:
Lots of polymers can be used to crosslink aerogels, including:
and lots more. To help these polymers attach to the gel framework, a compound called 3-aminopropyltriethoxysilane, or APTES for short, can be mixed in as the gel sets. This puts amine functional groups (-NH2) over the surface of the gel in addition to the hydroxyl groups that are normally there. These amine groups can be used to bond a wide variety of polymers to the gel framework. Dr. Mary Ann Meador at the NASA Glenn Research Center in Cleveland, Ohio has done extensive work refining this method.
Since the crosslinking takes place separately from the gel formation step, you can crosslink pretty much any gel to make an x-aerogel. X-silica (crosslinked silica aerogel), x-alumina (crosslinked alumina aerogel), and x-RF (crosslinked resorcinol-formaldehyde polymer aerogel) are just a few that have been prepared. In fact, x-aerogels of almost all of the colorful lanthanide oxides have been made as well.
So why isn’t every aerogel made today an x-aerogel? Well there are some trade-offs to consider.
Advantages:
Disadvantages:
Because of their impressive strength-to-weight ratios, decent insulating ability, and nanostructural properties, there are some interesting applications for x-aerogels:
X-aerogels are still new. Who knows what’s soon to come!
X-aerogel research is currently underway in the Leventis group at the Missouri University of Science and Technology in Rolla, MO and in the x-aerogel research group at NASA Glenn Research Center in Cleveland, OH led by Dr. Mary Ann Meador. Ongoing work to extend x-aerogel compositions to additional polymers, optimize processes for getting desired materials properties, and simplify and reduce cost of manufacture are currently underway.
In addition to strengthening aerogels by crosslinking wet gels prior to supercritical drying, it’s possible to reinforce aerogels after they’ve been supercritically dried as well. This can be done through deposition of a conformal polymer coating throughout the interior porosity of an aerogel by means of chemical vapor deposition (CVD) or atomic layer deposition (ALD).
Boday et al. at Los Alamos National Laboratory demonstrated this principle through ambient-temperature CVD of methyl cyanoacrylate (the main ingredient in superglue) onto silica aerogels and found that the resulting conformal coating increased the strength of the aerogels 30-fold with only a three-fold increase in bulk density!
Based on the data for comparable untreated silica aerogels, this observed increase in strength due to the conformal coating is approximately three times greater than would be achieved by simply preparing a comparably dense aerogel using a more concentrated solution in preparing the precursor gel. Although they are made through a different way, these materials are also considered x-aerogels.
Just like rebar reinforces concrete in bridges, it’s possible to reinforce aerogels with microfibers.
Aspen Systems, a contract research and development company, began experimenting with aerogels in the late ’s. Around , they were experimenting with casting silica gel onto fibrous batting (a porous, flexible fiber mat) and supercritically drying to produce reinforced aerogels. To their surprise, the mat was totally flexible and as co-inventor Dr. George Gould described it, “almost as if it were a totally different material”. The flexible aerogel blanket showed to be almost as insulating as the plain aerogel, except for unlike a typical silica aerogel, it could be rolled up and bent over and over.
In , Aspen Aerogels spun off from Aspen Systems and since then has developed a range of aerogel blankets designed for different applications. Technically their materials are aerogel composites because they combine fibrous battings of inorganic or organic fibers with aerogels and, for high-temperature applications, carbon black as well. Numerous patents from Aspen detail production of mechanically robust, flexible aerogel blankets of both inorganic and organic aerogels supported by meshes of polyimides (i.e., Nylon®), glass fibers, and many other materials. Aspen’s products can be found in subsea oil pipelines, refineries, winter apparel, and even shoe insoles.
The blankets are made first by mixing up a sol as you would a normal silica aerogel. The sol is poured onto a roll of fibrous batting and heated until the gel sets. The mat is rolled up and then placed in a tank under liquid while the gel continues to set and strengthen and is made hydrophobic through chemical reactions. The roll is then moved into a giant supercritical dryer and supercritically dried from CO2. Finally, the roll is heated to drive off excess solvent and can be shipped out.
To the touch, aerogel blankets feel kind of like a soft Brillo Pad® and are a little crunchy in bending. The blankets give off a little bit of powder when handled, but as Aspen reports, the blankets can be flexed over 250,000 and lose only 2% mass. Handling the blankets will make your fingers feel slippery, as the dust from the aerogel is hydrophobic (water-repelling) and will stick to your fingers.
Despite the seemingly obvious potential of aerogels for all sorts of applications, it actually took quite a while for Aspen to figure out how to make a business of aerogel blankets. After exploring a number of markets and developing technologies for a number of applications with government funding, the killer app appeared-sub-sea oil pipelines. Slaggy oil carried in sub-sea pipelines needs to be kept warm to keep flowing, otherwise the cold temperatures of the surrounding water will freeze up the slag. To keep the oil warm, a “pipe-in-pipe” configuration is used, that is, an inner pipe surrounded by insulation in a larger outer pipe. Because of the thickness of polyurethane insulation required to keep the oil sufficiently warm, the outer pipe had to be large. And because of the size of the outer pipe required, only three ships in the world were capable of laying that kind of pipe. Enter flexible aerogel blankets. Because the thermal conductivity of aerogel blanket is about twice as low as polyurethane per unit thickness, a much thinner aerogel blanket can do the job of much thicker polyurethane, meaning the diameter of the outer pipe can be that much smaller. Suddenly, over 250 ships around the world could lay that diameter pipe, saving the oil industries billions of dollars!
With that market in place, Aspen has started to address insulating needs in refineries and is working to replace mineral wool. Although today aerogel blankets are more expensive than other forms of insulation, they are not significantly more expensive and in the future with increased scale, aerogel will displace many types of insulation.
Aspen Aerogels aerogel blankets boast:
Different aerogel blanket formulations are designed for different applications. Cryogel® is designed for cryogenic applications, Spaceloft® is designed for clothing and apparel, and Pyrogel® is designed for high-temperature applications-six times better than mineral wool at temperatures of 350°C!
Aerogel blankets have many potential applications. One recent product is called Toasty Feet® and contains aerogel blanket in an insole for shoes to keep your feet warm in the winter and cool in the summer. Another product is aerogel blanket strips and aerogel wall wraps as home insulation that can help reduce heat loss through studs in the walls of a house.
For more information about fiber-reinforced silica aerogel blankets, see US patents G. Gould, et al., United States Patent Application , , and J. Ryu, United States Patent 6,068,882, .
Prof. A. Venkateswara Rao (say “ven-cat-a-swar-a”) at Shivaji University in Kolhapur, India invented a really interesting way to make flexible silica aerogels that also strongly repel water-by reducing the amount of bonding in the aerogel!
Normally a silica aerogel is made with tetrafunctional silicon compounds such as TMOS (tetramethoxysilane), that is, compounds that will result in a silicon atom with four oxygen bridges connecting it to four other silicon atoms, with each of those silicon atoms having four oxygen bridges connecting them to four other silicon atoms, and so on and so forth. This makes a rigid structure in which each silicon atom is highly mechanically constrained.
But if you use a trifunctional silicon compound such as MTMS (methyltrimethoxysilane) instead, you get a structure in which each silicon atom only has three oxygen bridges to other silicon atoms and on the fourth bond (because silicon tends to form four bonds to stuff), one terminating methyl group that doesn’t connect to anything else. This makes a structure that has reduced overall bonding, with longer internal struts that are less mechanically constrained, which makes the overall aerogel flexible!
The methyl groups attached to each silicon atom in a flexible aerogel made with MTMS are highly hydrophobic, that is, repel water, and so where normal silica aerogels would have sticky, water-attracting hydroxyl groups on their surface, MTMS-based silica aerogels are highly water-repellant.
Mixing water with powdered silica aerogel made from MTMS and pouring the mixture out will result in an aerogel-coated liquid “marble”. Kind of like mixing oil and vinegar, the aerogel powder (which repels water like oil) separates out from the water, but because the droplet is small and has surface tension, the aerogel moves to the surface of the droplet and creates a shell around the water.
Here’s the wacky part-an aerogel-coated water droplet is itself water-repellant because of its aerogel shell, meaning it will float on water! That’s right, a water droplet that floats on water!
Combining both MTMS and TMOS, you can make silica aerogels with varying degrees of flexibility, optical transparency, and hydrophobicity.
This technique is called “reduced bonding” since compared to the typical way to make silica aerogel, each silicon atom makes fewer network bonds.
One exciting application for reduced bonding aerogels is absorption of toxic chemicals. The same thing that makes MTMS-derived silica aerogels water-repellant makes them good at absorbing non-polar organic compounds. MTMS-derived silica aerogels can absorb 13 times their weight in gasoline and 20 times their weight in toxic benzene!
For more information about reduced bonding, see:
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