Research progress on preparation, modification, and application of phenolic aerogel

Chengfan Yuan; Degang Wang; Yijun Zhang; Kai Li; Jie Ding

doi:10.1515/ntrev-2023-0109

Open Access Published by De Gruyter August 23, 2023

Research progress on preparation, modification, and application of phenolic aerogel

Chengfan Yuan , Degang Wang , Yijun Zhang , Kai Li and Jie Ding

From the journal Nanotechnology Reviews

https://doi.org/10.1515/ntrev-2023-0109

Abstract

Phenolic aerogel is a type of nanoporous resin-based material with low density, high specific surface area, excellent thermal insulation performance, and a high carbon residue rate which has been widely used in the field of thermal protection. However, the development of phenolic aerogel is limited by the complex and expensive preparation technique, inadequate oxidation/ablation resistance, and excessive brittleness. As a result, academic research is constantly concentrated on low-cost preparation methods and efficient modification of phenolic aerogel. This review gives a description of the preparation technology, modification techniques, and application fields of phenolic aerogel and summarizes the limitations. Now, phenolic aerogel is not applied widely due to its complex and expensive preparation technique. Moreover, phenolic aerogel shows greater brittleness and insufficient mechanical property. The prospective future developments of the materials were prospected, and the phenolic aerogel with environmental protection, efficient thermal protection, and low cost will be the mainstream research direction.

Keywords: phenolic aerogel; nanoporous; preparation; modification; application

1 Introduction

As a novel type of nanoporous functional material with a three-dimensional network structure, aerogel offers excellent physical properties [1]. For instance, aerogel is known to be the solid material with the lowest density and thermal conductivity in the world currently. Its density and thermal conductivity can be as low as 0.003 g/cm³ and 0.001 W/m K, respectively. Moreover, aerogel offers high specific surface area (400–1,500 m²/g) and exceptionally high porosity (80–99.8%) [2–4]. In recent years, aerogel materials have received much attention and are regarded as an excellent functional material with low density. Actually, aerogel has been widely used in efficient thermal insulation material, catalyst support, electrochemical device, biomass materials, and other applications due to its exceptional properties [5–11].

Among the aerogel materials, phenolic aerogels are frequently applied in the thermal insulation of hypersonic vehicles on account of unique advantages such as high carbon residue rate, high-temperature resistance, and good thermal insulation performance [12–15]. Phenolic aerogel was prepared by Pekala from Lawrence Livermore National Laboratory for the first time [16], which had a nanoporous structure with low density (0.03–0.079 cm³/g). However, there are some limitations to the method. For instance, phenolic aerogels are synthesized from small-molecule phenols and aldehydes, which prolong the preparation period beyond 7 days. Moreover, significant capillary pressure will be generated in the wet gel during the drying process, causing the aerogel to collapse and wrinkle or shrink. Therefore, the wet gel must undergo expensive supercritical drying. Due to the high cost and difficult preparation process, the large-scale industrial manufacturing of phenolic aerogels has been severely hampered. Besides, the compressive strength of phenolic aerogel is usually less than 3.5 MPa [17,18], and the carbon residue rate at 800°C in air is below 1% [19,20]. On account of the poor mechanical properties and oxidation/ablation resistance, it is hard for phenolic aerogel to adapt to the complex and challenging working environment.

As shown in Figure 1, in recent years, researchers have succeeded in regulating and controlling the properties of phenolic aerogels by changing the raw materials, catalyst, and reaction parameters. In addition, numerous drying procedures have been created to achieve simple and low-cost preparation while preserving as much of the network structure as possible [21–23]. To improve the oxidation resistance of phenolic aerogel, many modification approaches have been proposed. For example, phenolic aerogels modified by silicon are reported to have better oxidation/ablation resistance [24]. In recent years, fiber reinforced phenolic aerogel has attracted lots of attention. The introduction of fiber can not only improve the mechanical properties of phenolic aerogel but also improve its oxidation/ablation resistance of phenolic aerogel in a way. Among numerous phenolic aerogels reinforced by fiber, phenolic impregnated carbon ablator (PICA) stands out and has been widely used in the thermal protection of spacecraft [25]. This work summarizes the preparation process, modification, and application, as well as existing issues and future developments, of phenolic aerogel.

Figure 1

Methods for regulating properties of phenolic aerogel [16–20].

2 Preparation of phenolic aerogel

2.1 Sol–gel reaction

Sol–gel reaction process is crucial for the establishment of three-dimensional network in aerogel [26,27]. There are currently two major reaction systems for preparing phenolic aerogel through the sol–gel technique, respectively, using small-molecule monomers and phenolic resin as precursors. As shown in Figure 2, when small molecules are selected as raw material to prepare phenolic aerogel, they are transformed into polymer gel with a three-dimensional network structure via three stages of sol–gel reaction. First, materials undergo addition and polycondensation reaction to form oligomer under the effect of catalysts [29]. Then, the oligomer with multiple functional groups crosslinks together to generate tiny particles while monomers continue to polymerize. Finally, these particles further grow, aggregate, crosslink, and merge to form a stable sol–gel system [28,30].

Figure 2

Reaction mechanism of preparation with small-molecule monomers as precursor [28].

In addition, to reduce production costs, commercial phenolic resin is chosen to prepare phenolic aerogels. As shown in Figure 3, phenolic resin oligomers were formed and then aggregated into sol particles under the action of crosslinking agent. At last, the sol particles piled up on each other to form a phenolic wet gel [17]. During the sol–gel process, reaction parameters will affect the particles and pore structure of phenolic aerogels, causing changes in macroscopic characteristics. Consequently, many researchers are trying to establish the relationship between microstructure and properties of phenolic aerogels, as well as to explore the regulating mechanism of pore structure [21,26,31].

Figure 3

Reaction mechanism of preparation with phenolic resin as precursor [17].

2.1.1 Raw materials

Traditional phenolic aerogels were made from resorcinol and formaldehyde [16]. Subsequently, scientists are continually attempting to use phenols (phenol [32], phloroglucinol [33], hydroquinone [34], cresol mixture [35], etc.) and aldehydes (formaldehyde [36], furfural [37], acetaldehyde [38], etc.) as raw materials to prepare phenolic aerogels. Since expensive resorcinol in the traditional system prevents the commercialization of phenolic aerogel, Zhu et al. [39] used low-cost mixed cresol instead of resorcinol and successfully prepared phenolic aerogel blocks with a 5% shrinkage rate, greatly reducing the production cost of phenolic aerogel. To expedite the sol–gel reaction, Barral [40] substituted phloroglucinol which has a higher activity for resorcinol, and prepared a low-density phenolic aerogel (only 0.013 g/cm³). The method shortened the preparation time from 7 to 4 days. Research shows that phloroglucinol can shorten gelation time by 10–20 times compared with resorcinol [41]. Wu et al. [26] used resorcinol and furfural as precursors and found that alcohols could be used to replace deionized water as the solvent in the reaction system, leading to a substantial boost in gelling rate. On this basis, Zheng et al. [37] used orcinol (5-methylresorcinol) instead of resorcinol and discovered that phenolic aerogel prepared by the orcinol-furfural system had a richer micropore structure and greater specific surface area than that prepared by the resorcinol-furfural system (549 vs 440).

Besides, the molar ratio of phenols and aldehydes plays a crucial role in the sol–gel process. When the phenol level is excessive, phenol and aldehyde can form linear novellas under the action of an acid catalyst. This kind of resin needs to be crosslinked to a network structure by a curing agent to achieve curing. When the aldehyde level is excessive, under the action of a base catalyst, phenol and aldehyde can form resoles with a crosslinked structure that can be cured under heating conditions [30]. As shown in Figure 4, in the resorcinol-formaldehyde reaction system, formaldehyde is added to benzene by an acid or base catalyst to form methylene, which acts as a “bridging” agent for subsequent polycondensation reaction [31]. Therefore, when the molar ratio of resorcinol and formaldehyde (R/F) is low, the molecular weight of the phenolic resin generated is low, resulting in a decrease in nanoporous structure. When R/F is high, there is not enough formaldehyde in the system to function as a “bridging” effect, thus the connection of gel structure is unsteady, causing the collapse of three-dimensional network [33,34]. To obtain phenolic aerogels with a complete and stable nanoporous structure, the ratio of phenols and aldehydes must be controlled within a reasonable range [35].

Figure 4

“Bridging” mechanism of formaldehyde in sol–gel reaction [31].

In addition to the process mentioned above, phenolic aerogels can be directly prepared through the crosslinking, aggregation, and accumulation of phenolic resin. Shi et al. [17] obtained phenolic aerogel with an average particle size of up to 1 μm using commercial phenolic resin as raw material and ethanol as an organic solvent catalyzed by hexamethylenetetramine (HMTA). Jia et al. [42,43] believed that the good compatibility of phenolic resin with ethanol leads to the formation of large-size particles that are tightly connected. Though selecting phenolic resin as raw material can reduce reaction time and costs, it results in higher shrinkage of phenolic aerogel (10–20% linear shrinkage) [17,18]. To address the problem, Zhang et al. [44] added furfural to the phenolic reaction system and gained phenolic-furfural aerogel catalyzed by hydrochloric acid. Furfural was discovered to not only effectively lower the shrinkage rate of phenolic-furfural aerogel but also to improve the pore volume of macropores and mesopores.

2.1.2 Catalysts

Catalysts play a significant part in gelling rate and microstructure of phenolic aerogel. Currently, acid and base catalysts are the most commonly utilized. Despite slight variations in catalytic mechanisms, the primary reaction stages are addition and condensation [45]. Common base catalysts cover Na₂CO₃ [11], NaOH [46], NaHCO₃ [47], Ca(OH)₂ [47], and HMTA [43], among which Na₂CO₃ is the most extensively used. The advantages embody that phenolic aerogels catalyzed by base catalysts possess a compact stack of uniform gel particles. However, it spends a long time to react in base catalyzed systems (hours to days) [30]. Common acid catalysts include HNO₃ [48], HCl [40], and HClO₄ [49]. In comparison to base catalysts, acid catalysts can accelerate the reaction rate (gelation time is merely several minutes), and the gelation time is inversely proportional to the catalyst concentration. Nevertheless, the gel particles catalyzed by base are not nearly as homogeneous as that catalyzed by acid [50,51]. To contrast the influence of acid and base catalysts on the microstructure of phenolic aerogel, Barral [40], respectively, used HCl and Ca(OH)₂ to catalyze the phloroglucinol-formaldehyde precursor system. By characterizing and analyzing the microstructure, it was revealed that the pore size distribution is relatively large, and the gel structure shows a huge colloidal network in the case of acid catalyst, while the aerogel catalyzed by base shows a dense network structure similar to fiber. In response to the phenomenon, Barral analyzed the mechanisms of acid and base catalytic reactions. As shown in Figure 5, under base catalysis, the electrophilic addition between phenols and aldehydes has a high reactivity, whereas subsequent polycondensation between phenolic substitutes has a low reactivity. The gel structure is rough and irregular like fiber due to the fast electrophilic addition process. Under acid catalysis, the electrophilic addition has a low reactivity, while the polycondensation has a high reactivity. Slow addition rate makes the reaction more sufficient resulting in a complete and exquisite gel network, while quick polycondensation makes the particle stack uneven, resulting in a broad particle size distribution of aerogels. On this premise, Barral proposed an acid-base catalyzed system, in which base catalyst was employed to branch the phenols and then acid catalyst was added to dramatically accelerate the polycondensation reaction. Since the rate of polymer particle formation was much faster than the rate of crosslinking, the method played a role in expanding the pore of phenolic aerogels [40].

Figure 5

Mechanism of phenolic resin formation catalyzed by base/acid [40].

2.1.3 pH of solution

In addition to catalysts, the pH of solution has a significant impact on the sol–gel process. In a base catalyzed system, high pH (higher than 7.5) will block the condensation reaction, preventing the formation of a crosslinked network structure. Low pH (lower than 5.5) will result in an excessively long gelation time [52]. In an acid catalyzed system, excessively low pH (lower than 0.85) will lead to instantaneous gelling and precipitation [45]. In conclusion, it is crucial to control the pH of reaction system within a reasonable range. Generally, pH is controlled by adjusting the molar ratio of reactant and catalyst, which regulates the microstructure of phenolic aerogels. As shown in Figure 6, Yang et al. [22] explored the effect of catalyst content (the ratio of resorcinol to sodium carbonate, referred to as R/C) on the pore structure of aerogels over a wide range (300–2,000). It was observed that when the catalyst level is high, the particles of phenolic aerogel pile up rather densely so as to form inconspicuous small nanopores. When the catalyst level is low, the stacking of particles is loose, leading to the formation of a mass of macropores and mesopores. As shown in Figure 7, Yang et al. analyzed that catalysts could provide active nucleation sites for the reaction. When the catalyst concentration is low, there are fewer active nucleation sites, which means that the system needs more time to accomplish the sol–gel process. Within a sufficient reaction period, more opportunities are given for phenolic particles to aggregate into gel clusters with large sizes and vast space. Conversely, high catalyst concentration endowed the system with enough nucleation sites, causing the phenolic particles to aggregate into gel clusters with small size and space [22].

Figure 6

SEM images of carbon aerogels with different concentrations: (a) R/C = 300, (b) R/C = 500, (c) R/C = 1000, (d) R/C = 1500, and (e) R/C = 2000 [22].

Figure 7

The effect of catalysts amount on the particle size and pore diameter of phenolic aerogels.

As a common curing agent for thermoplastic phenolic resin, the content of HMTA also has a crucial influence on the pore structure of phenolic aerogels. Distinct from using small-molecule monomers as precursors, Shi et al. [17] observed that the pore size of phenolic aerogel is small (160–260 nm) and of uniform distribution when HMTA content is low. As the content increases, the pore size and distribution come to become larger, while the connection between phenolic particles gets looser and less compact. The reason why HMTA produces different catalytic laws than the catalysts mentioned above is that HMTA is both a catalyst and a crosslinking agent. Under high HMTA concentration, molecular chains are quickly crosslinked into polymer networks and there is no time for chain segments to adjust and move, resulting in large sol particles. On the contrary, the crosslinking rate is slow under low concentration. So, it is easy for chain segments to adjust and rearrange to generate smaller sol particles.

2.1.4 Selection of reaction solvent

Deionized water is normally used as a reaction solvent for small-molecule phenols and aldehydes in the traditional preparation process. Compared with deionized water, alcohol can react with formaldehyde to generate hemiformal, which slows down the rate of polymerization [53]. Moreover, choosing alcohols as solvent increases the density and shrinkage rate of phenolic aerogel [54]. However, many materials used to prepare phenolic aerogels cannot dissolve well in deionized water. Furthermore, as shown in Table 1, deionized water has superior surface tension (about 72.7 mN/m) than alcohol solvents (generally 20–50 mN/m), which will raise greater capillary pressure during the drying process and then damage the gel skeleton. Therefore, both supercritical drying and ambient pressure drying undergo laborious solvent replacement for wet gels prepared with deionized water. To save reaction time and cost, many researchers use organic solvents with low surface tension to simplify the drying process directly [18,43]. Currently, the sol–gel reaction can be carried out using methanol [53], ethanol [55], ethylene glycol [56], n-propanol [44], n-butanol [28], isopropanol [57], and others as solvent. Xia et al. [54] used methanol, ethanol, and isopropyl alcohol as solvents to prepare phenolic aerogel, respectively, and discovered that three distinct solvents had diverse influences on the phenolic particles and pore structures. As shown in Figure 8, due to its small molecular size, the aerogel prepared using methanol as solvent had a dense structure and its particles were shaped in clumps. Oppositely, the aerogels prepared using ethanol and isopropyl alcohol as solvents were composed of loose and small cluster particles which led to larger pore diameters. In comparison to isopropyl alcohol and methanol, the particles of phenolic aerogels prepared by ethanol solvent were more regular and uniform and possessed a developed network structure, which may be related to the moderate molecular size and low surface tension [54].

Table 1

Surface tension of deionized water and common alcohol solvents

Solvent	Deionized water	Ethanol	Ethylene glycol	n-Propanol	Methanol	Hexane
Surface tension (mN/m)	72.7	22.27	48.4	23.78	20.14	23.8

Figure 8

SEM images of phenolic aerogels prepared with different solvents: (a) methanol, (b) ethanol, and (c) isopropyl alcohol [54].

2.1.5 Reaction time and temperature

The traditional preparation technique normally takes 7 days to prepare phenolic aerogels [16], and the long production period is not conducive to the development of industrialization. Research shows that gelation time can be greatly reduced with the increase in the sol–gel temperature [22]. In addition, the wet gel usually requires aging at the end of sol–gel reaction, which can enrich the microstructure of nanopores and enhance the strength of gel skeleton. The nanoporous structure grows more complete as the wet gel ages. Coincidentally, the aging process is tremendously affected by temperature and time [58]. Therefore, it is essential to control the temperature and time of reaction system within a reasonable range. By contrasting different temperature experimental groups, Wiener et al. [59] discovered that the synthesis time of phenolic aerogels could be shortened from 3 to 1 day when the temperature was controlled at 90°C. Yang et al. [22] further explored the influence of different temperatures on the microstructure of phenolic aerogels and found that the sizes of particles and pores decreased significantly when the temperature climbed from 30 to 45°C. Moreover, increasing temperature can significantly improve the uniformity of particles and pores. However, with the temperature increasing from 45 to 90°C sequentially, the effects of temperature on particles and pores became too tiny to observe. Yang et al. analyzed that catalysts could not gain enough energy to activate the polymerization reaction and then the nucleation sites were less, leading to the formation of large pores and particles. Conversely, with the increase in temperature, there was more energy for the system to overcome the reaction barrier. So, reaction system would generate more nucleation sites, and the particles of phenolic aerogels tended to be small and homogeneous. Restricted by the catalyst concentration, the number of nucleation sites could not expand unceasingly with the further increase in temperature. That is why the sizes of particles and pores change a little between 45 and 90°C [22].

Although increasing the temperature can improve the gel rate, it also results in smaller pores of phenolic aerogels, which is not conducive to the subsequent drying process of wet gel. Whereupon, Hong et al. [60–62] proposed a technique to prepare phenolic aerogels by controlling temperature gradient (treatment at 90, 120, and 180°C for 1, 3, and 3 h) using phenolic resin as raw material and ethylene glycol as a solvent and pore-forming agent. Afterward, ethanol with lower surface tension was utilized to replace the ethylene glycol solvent. The technique realized the preparation of phenolic aerogels with large pores (the average pore diameter is 100 nm) by ambient pressure drying. At the initial stage of the reaction, the temperature is kept at 90°C so that the aerogels could form a three-dimensional skeleton. When the skeleton is built basically, the temperature rises to 120°C to accelerate the aging process and reinforce the gel skeleton. Finally, the temperature increases to 180°C to expedite the phase separation of ethylene glycol and gel that facilitated the subsequent drying process, while the gel could be further cured and solidified. Phenolic aerogels prepared in this method can form a solid three-dimensional network structure rich with macropores.

2.2 Drying of wet gels

The purpose of drying aerogel is to remove the residual solvent in the gel while preserving its original three-dimensional network structure to the greatest extent. On account of the specific nanoporous structure, if the aerogel is directly heated to vaporize the solvent, the original structure will be destroyed seriously due to capillary pressure, surface tension, and osmotic pressure, causing significant collapse, wrinkle shrinkage, and cracking [63,64]. Kistler [65] from Stanford University developed organic aerogels by the sol–gel technique and found that the gels would be broken or powdered by direct drying. There are always two mainstream solutions to this problem. The first is to strengthen the skeleton of the aerogel so that it can overcome the impacts of the surface tension without ruining the nanoporous structure. Another solution is to explore a new special drying technique that can effectively reduce or even eliminate the effects of surface tension on the aerogel skeleton [64,66]. Researchers have carried out a lot of studies on the drying of aerogel in recent years. Currently, there are three primary methods used widely: supercritical drying, freeze-drying, and ambient pressure drying [16,18,67].

2.2.1 Supercritical drying

Supercritical drying is the most effective method to dry aerogels. By regulating the pressure and temperature inside the reaction vessel, the transition from liquid phase to supercritical fluid phase of the solvent can be achieved when reaching its critical point. At this time, there is nearly no interface between gas and liquid, thus no capillary pressure is generated. The skeleton of aerogel remains complete without collapse or shrinkage [68,69]. At present, deionized water is used as a preparation medium for many phenolic aerogels. As shown in Table 2, the critical temperature and pressure of deionized water are respectively 374.1°C and 21.8 MPa. High temperature and pressure environments not only have higher requirements for equipment but also have potential safety hazard for experimenters. So it is necessary to replace the deionized water with organic solvents with low surface energy such as acetone and ethanol, which can reduce temperature and pressure to reasonable range (approximately 250°C and 4–7 MPa) [71,72]. Szczurek et al. [72] investigated the effects of ethanol and acetone as drying mediums on phenolic aerogels and found that the phenolic aerogel obtained through supercritical acetone drying had a lower shrinkage rate and density than that obtained through supercritical ethanol drying. In addition, the effect of acetone as a drying medium was comparable to that of CO₂. Nevertheless, solution replacement is generally a laborious procedure, so many researchers use ethanol directly as the solvent in the sol–gel reaction, simplifying the preparation process and lowering the cost [21,43,73].

Table 2

Critical temperature and pressure of common drying media [70]

Drying medium	Deionized water	CO₂	Ethanol	Acetone
Critical temperature (°C)	374.1	31.3	243	235
Critical pressure (MPa)	21.8	7.36	6.36	4.7

In general, supercritical drying is a particularly effective method to dry phenolic aerogel since it can preserve the nanoporous structure to the greatest extent possible. However, there are many obvious problems to be addressed in supercritical drying. For example, the experimental environment of supercritical drying method is usually high temperature and pressure, which brings considerable safety risks to operators. Moreover, the purchase and maintenance costs of the equipment are extremely steep, and the solution replacement process takes a long time (8–48 h), which hinders currently the industrial mass production of phenolic aerogel produced by supercritical drying [68,74].

2.2.2 Freeze drying

As an effective method to prepare nanoporous aerogels, freeze drying has developed rapidly in recent years [75,76]. In contrast to supercritical drying, freeze drying transforms the liquid–gas interface into the solid–gas interface at a low temperature and pressure, reducing the capillary pressure by eliminating the discrepancy between gas and liquid phase. Then, the wet gel is dried with the sublimation of solvent at an appropriate temperature and vacuum degree and the microstructure of gel can be integrally preserved [67,77]. Since the volume change of tert-butanol is much smaller than that of water during the freezing process, Tamon et al. [78] used tert-butanol to replace the water in the phenolic wet gel and ultimately gained phenolic aerogel with enriched mesopores and complete network structure. Simultaneously, Pons et al. [79] soaked the wet gel in tert-butanol for 209 days and the volume of the final consumption of tertiary butanol was approximately 5,000 times the gel volume. Even after sufficient replacement, the volume of wet gel still changed by 2% when frozen, leading to gel splitting and skeleton destruction easily at low temperatures. As shown in Figure 9, Job et al. [80] found that when the solvent level in the gel block was high, ice crystals grew and could not be controlled during the freezing process. The disoriented growth resulted in uneven distribution of internal stress in the gel, which cause the break of the gel after drying finally. Therefore, freeze-drying is mostly employed to dry powdered aerogel. In general, freeze-drying consumes less energy than supercritical drying, but it still has some problems such as extensive preparation periods, harsh conditions, and complex equipment operations. Thus, there is still a distance from large-scale industrial production [75].

Figure 9

Ice growth mechanism during freezing. (a) wet gel; (b) ice crystals appearance; (c) ice crystal growth and texture reorganization; and (d) dried polymer after ice removal by sublimation [80].

2.2.3 Ambient pressure drying

Ambient pressure drying is to dry wet gel slowly at atmospheric pressure and low temperature, which is considered as a simple and low-cost method. However, directly drying aerogel will result in the collapse of the network structure. Wu et al. [26] believed that three aspects can be considered to realize ambient pressure drying: enhancing the strength of the gel network, improving the uniformity of pores in gel while increasing the size of pores and particles appropriately, and reducing the surface tension of the gel. Nonetheless, ambient pressure drying inevitably leads to the loss of some properties of phenolic aerogel while pursuing low cost, short preparation cycles, and operational convenience. Therefore, ambient pressure drying is a compromise method between supercritical drying and direct drying.

At present, many researchers are seeking methods to achieve ambient pressure drying without dramatically sacrificing the properties of the aerogel [18,62]. Fischer et al. [81] and Petričević et al. [82] both regulated the concentration of the catalyst and found that the low concentration of catalyst was conducive to the formation of larger pores, which was beneficial to reduce the surface tension to achieve drying at ambient pressure. Yang et al. [22] found that appropriate gel particle attachment and complete skeleton could be acquired by adjusting the concentration of the catalyst and temperature within a specified range. Because the particles and pores were larger and uniformly distributed, the aerogel skeleton was strengthened while the surface tension was reduced. As a consequence, the phenolic aerogel was successfully dried at ambient pressure. Hong et al. [60–62] achieved ambient drying of phenolic aerogel by designing a temperature gradient to gain large pores and a solid three-dimensional skeleton to overcome the effect of surface tension. Zhang et al. [18] proposed a preparation method for phenolic aerogel under ambient pressure that carried out simultaneous drying during the curing process of phenolic gel, greatly shortening the preparation cycle and lowering the reaction risk without significantly affecting the pore structure of the aerogel.

As shown in Table 3, phenolic aerogel prepared by supercritical drying shows higher specific surface area (400–900 m²/g) than that by freeze drying (350–550 m²/g) and ambient pressure drying (less than 150 m²/g). High specific surface area means supercritical drying can retain the original structure full of macropores and mesopores to the greatest extent. Compared with ambient pressure drying, freeze-drying can also retain more of the original pore structure. Despite the operating convenience of ambient pressure drying, the specific surface area of phenolic aerogel prepared is much smaller which indicates that most of the nanopore structures have been destroyed.

Table 3

Comparison of phenolic aerogels prepared by different drying methods

Ref.	Raw materials	Catalyst	Solvent	Drying	Density (g/cm³)	S _BET (m²/g)
[40]	Phloroglucinol-formaldehyde	Ca(OH)₂ + HCl	Deionized water	SD	0.013	—
[83]	Resorcinol-formaldehyde	Na₂CO₃	Deionized water	SD	0.036–0.207	389–905
[21]	Resorcinol-furfural	NaOH	Ethanol	SD	0.26–0.52	589
[78]	Resorcinol-formaldehyde	Na₂CO₃	Deionized water	FD	0.19–0.31	349–513
[84]	Resorcinol-formaldehyde	Na₂CO₃	Deionized water	FD	—	468–542
[26]	Resorcinol-furfural	HMTA	Ethanol	APD	0.22–0.42	25–135
[22]	Resorcinol-formaldehyde	Na₂CO₃	Deionized water	APD	0.31–0.92	—
[17]	Phenolic resin	HMTA	Ethanol	APD	0.20–0.50	0.53–44.39
[18]	Phenolic resin	HMTA	Ethanol	APD	0.25–0.33	54.42–88.95

Note: SD is supercritical drying; FD is freeze drying; APD is ambient pressure drying.

2.3 Summary and discussion

In summary, as shown in Figure 10, the preparation process of phenolic aerogel can be divided into two main steps: sol–gel reaction and drying of wet gel. Now, the mechanism and technology of the sol–gel reaction are mature, and it is regarded as an excellent approach for preparing phenolic aerogel. In recent years, influence factors of the reaction such as catalysts, solvents, temperatures, and reaction time on the micro and macro features of phenolic aerogel have been thoroughly researched, and some conclusions and regularities have been drawn. However, there are many issues to be investigated in response to the need for environmental protection and energy conservation. The raw material of phenolic aerogel is polluting and expensive, so it is required to develop new low-cost and eco-friendly raw materials. More crucially, a low-cost and efficient drying method has not yet been developed, which is not conducive to the industrial production of phenolic aerogel.

Figure 10

Preparation process of phenolic aerogel.

3 Modification of phenolic aerogels

With the development of phenolic aerogels, researchers have noticed that many defects exist in phenolic aerogel in recent years, which makes it hard to apply to various complex working circumstances directly. For instance, the mechanical properties of pure phenolic aerogels are poor (the compressive strength is generally less than 3.5 MPa) due to the loose connection of resin molecules, and the materials have high brittleness [18]. Moreover, phenolic aerogels are inadequate in ablation/oxidation resistance and thermal stability. In nitrogen, the thermal weight loss rate reaches 10% at 300°C and the carbon residue rate is merely 50% at 800°C [17]. Because of the loose and porous structure, oxygen can easily infiltrate the carbon layer formed during the hyperthermal ablation process, oxidizing the materials to failure. In actual working conditions, routine phenolic aerogel has some problems such as brittleness, poor mechanical properties, and insufficient ablation/oxidation resistance. Therefore, it is particularly important to modify and reinforce the phenolic aerogels to adapt to the harsh practical environment.

3.1 Modification by surfactants

According to research, the size distribution of gel particles prepared in the conventional reaction system of raw materials, solvent, and catalysts is generally large, and the stacking of particles is irregular, which implies the network structure is unstable and easy to collapse during the drying process [85]. Coincidentally, the surfactant is a good solution to the problem. In the sol–gel reaction, the surfactant is primarily served as a “template” for raw materials. Typically, the surfactant is transformed into spherical or rod-shaped micelles, and then the polymer particles grow along the micelles, which achieves regulating accurately the particle sizes of phenolic aerogels [86]. Additionally, surfactants can decrease the free energy of the reaction system, as well as adsorb and cover aerogel particles to inhibit nuclear development, promoting the uniformity and regularity of gel particles [87].

Common surfactants can be divided into three categories: nonionic, cationic, and anionic surfactants. Now, cationic surfactants represented by cetyltrimethylammonium bromide (CTAB) are frequently used [88]. Lee and Oh [89] studied the regulating mechanism of CTAB, and found that CTAB can generate spherical micelle templates in solution and then phenolic resin forms gel clusters around the templates. In the end, the gel clusters are crosslinked together to form a three-dimensional network. After the sol–gel reaction, CTAB is still adsorbed in the gel pore structure due to its good thermal stability combined with the electrostatic interaction between ionic surfactant and polymer, which effectively reduces the surface tension between solvent and gel. On this basis, Wu et al. [90] investigated the effects of CTAB concentration on aerogel further. It was discovered that CTAB could reduce the surface tension of the reaction system and the sol particle size gradually decreased as CTAB concentration increased within limits. However, excessive concentration of surfactant will lead to phase separation in the polymerization process, affecting the properties of phenolic aerogel greatly. Wu et al. observed that the concentration of CTAB decreased continuously as the reaction progressed, and subsequently the surface tension of the system immediately increased when the concentration dropped to a particular level. Concerning this phenomenon, Zhao and Wang [91] believed that the surfactant would be associated with a colloidal polymer from a single ion or molecule when its concentration exceeded the critical micelle concentration. Whereafter, the surface tension does not decrease with the increase in concentration and remains a constant value. Contrary to cationic surfactants, the particle size of phenolic aerogels increases as anionic surfactant concentration increases. However, ionic surfactants are often difficult to remove due to their good thermal stability. Besides, Wu et al. [90] found that ionic surfactants would consume the active site of the reaction to some extent, leading to a decrease in the curing degree of phenolic aerogels. To deal with these problems, Hasegawa et al. [92] took F127 (PEO-PPO-PEO) as the templating agent which enabled raw materials to self-assemble in the sol–gel reaction, and gained aerogels with large pores and network structure. Since PEO and PPO are both soluble in water, they can be removed easily by washing and heating. Subsequently, Thepphankulngarm et al. [93] studied the effect of P123 and CTAB on the microstructure of polybenzoxazine (PBO) porous carbon, and found that the pore size of materials prepared by P123 was larger than that of CTAB. As shown in Figure 11, the cationic CTAB formed spherical micelles by ion-dipole interactions and then benzoxazine was assembled inside the spherical micelles to polymerize and grow into spherical clusters (50 nm). Compared with CTAB, non-ionic P123 formed spherical micelles by dipole–dipole interactions, resulting in larger micelle sizes (80 nm). Moreover, nanoporous carbon using P123 showed higher specific surface areas than that using CTAB (703 vs 633).

Figure 11

Schematic diagram showing the formation of PBZ-derived carbon modified with (a) CTAB and (b) P123 [93].

3.2 Modification of phenolic resin matrix

Modification of phenolic resin matrix refers to improving its properties by introducing functional elements, groups, or molecular chains into phenolic resin, so as to improve the properties of phenolic aerogel. Among multitudinous modified phenolic resins, boron phenolic resin stands out and is often utilized as a matrix material to resist ablation in the aerospace field [94,95]. Wang et al. [96] modified phenolic resin with B₂O₃ and prepared aerogel-like phenolic foam composite with a porous structure. The compressive strength of boron-modified phenolic foam composite was increased by 5.18% compared with that of pure phenolic foam. The carbon residue rate was 66.37% at 800℃ in nitrogen, which was 16.05% higher than that of pure phenolic foam. Research has found that these might be related to the B–O bond in boron phenolic resin. The B–O bond has a high bond energy and enables phenolic resin to form a three-phase crosslinked structure during the curing process, which can improve the mechanical properties and ablation performance of phenolic aerogels. Furthermore, boron in the phenolic resin can form borate at high temperatures, and the dense borate covers the surface of phenolic aerogels to slow down the entry of heat flow, thus greatly improving the ablation resistance of phenolic aerogels [97]. He et al. [98] found that boron modified phenolic aerogel had good flexibility that it could basically revert to the original shape within 20% of deformation. Even if a large irreversible deformation occurred, its rebound rate could reach around 80%, which alleviated great brittleness and greatly improved machinability of phenolic aerogel.

In addition to modification by boron, silicon-modified phenolic aerogels with good thermal stability, ablation resistance, and flexibility have received a lot of attention [99]. Zhao et al. [100] used RF solution and polysiloxane prepolymer synthesized by polycondensation of methyltriethoxysilane, dimethyldiethoxylsilane, and phenyltriethoxysilane to prepare RF–SP composite aerogel monolith. At 1,000℃ in argon, the carbon residue rate of the composite aerogels can reach 66.7%, which increases by 15% approximately in comparison to that of the pure phenolic aerogels. To improve the oxidation resistance of phenolic aerogels at high temperatures, Xiao et al. [19] prepared SiO₂–PBO composite aerogel by modifying PBO with SiO₂ aerogel. At 800℃ in air, the carbon residue rate of SiO₂–PBO composite aerogel increased from 0.49 to 57.83%. Xiao found that the residual carbon is relatively loose and there are many holes in it, which expand the contact area of the aerogel with oxygen and heat flow. That is why the carbon residue rate of PBO aerogel is merely 0.49%. By contrast, SiO₂ nanoparticles of SiO₂–PBO composite aerogel would fill in the residual carbon layer and form a denser protective layer that can efficiently inhibit the diffusion of heat flow and oxygen into the interior of aerogel, improving the oxidation resistance greatly [19]. Shi et al. [20] prepared PF/SiO₂ hybrid aerogels using tetraethyl orthosilicate and phenolic resin through the sol–gel reaction. It was observed that SiO₂ uniformly filled in the large pore of phenolic aerogel skeleton and formed a hybrid gel network with an interpenetrating structure, enhancing the strength of gel aerogel skeleton and the flexibility due to the lubrication effect of SiO₂. In addition, many elements such as barium, magnesium, aluminum, and tungsten can also be used to modify phenolic resin, and they can improve the ablation and heat resistance more or less [93].

3.3 Modification by reinforcement

In the 1990s, NASA-Ames Research Center developed a low-density ablative material with nanoporous structure–PICA employing phenolic resin as matrix and carbon fiber as reinforcement [25]. In recent years, with the development of fiber-reinforced phenolic aerogels represented by PICA, PICA materials have been applied to the heatshield of re-entry capsules of spacecraft such as “Stardust” and “Crew Dragon” [101,102]. Jia et al. [43] prepared PICA composite material by impregnating carbon fiber mat with phenolic resin. The PICA retained the low density (0.27–0.47 g/cm³) and good heat insulation (0.056–0.062 W/m K) of pure phenolic aerogels while considerably improving the mechanical properties (flexural strength could reach 16.5 MPa) because of the carbon fiber mat. Zhang et al. [103] studied the mechanism of fiber-reinforced phenolic aerogels. As shown in Figure 12, phenolic particles fill in the fiber network equably and form a good interface with fiber. Thus, the force is transferred to the fiber through the interface when the composite material is loaded, achieving the reinforcement of phenolic aerogel. However, materials reinforced with carbon fiber mat may be delaminated to failure due to the poor interlayer interaction. Whereupon, Zhu et al. [4] substituted pierced carbon fiber mat for normal carbon fiber mat to prepare PICA and discovered that its flexural strength can reach 35.9 MPa which is twice as high as that prepared by carbon fiber mat. Considering the poor oxidation resistance of carbon fiber, Zhang et al. [104] used silane coupling agent and Si–B–C preceramic solution to treat successively the surface of 3D carbon fiber preform. Then, the carbon fiber preform was impregnated with phenolic resin to prepare an anti-oxidation 3D fiber preform-reinforced phenolic aerogel. Zhang et al. [104] found that the ceramic precursor could undergo the ceramization reaction and generate dense SiC, SiO₂, and C (graphite) composite ceramics on the surface of the material under high-temperature environments, considerably improving the ablation and anti-oxidation ability. Moreover, this treatment method significantly improved the mechanical properties of phenolic aerogel. In comparison to the pure phenolic aerogel, the flexural, tensile, and compressive strengths of the material increased from 12, 10, and 2 to 57, 46, and 18 MPa.

Figure 12

Sketch of fiber reinforced phenolic aerogel composite [103].

Besides, there are numerous reports on quartz fiber, glass fiber, and high silica fiber reinforced phenolic aerogels. Zhang et al. [103] used glass fiber and quartz fiber to reinforce phenolic aerogels, respectively, and obtained low-density composites with low thermal conductivities of 0.045 and 0.046 W/m K. To exert the effect of fibers adequately, Wang et al. [105] selected quartz fiber, glass fiber, and high silica fiber to design functionally gradient fiber reinforcements and prepared multi-functional phenolic aerogels. At high temperatures, the quartz fiber reinforced layer melts, oxidizes, and forms the carbon layer, resisting ablation and heat flow. The high silica fiber reinforced layer absorbs heat to form the pyrolysis layer, which insulates the material from heat. The glass fiber reinforced layer has no ablative phenomenon and acts as heat insulation to protect the internal structure. Research shows that the thermal insulation effect of the multi-functional aerogel is superior to that of carbon fiber composite at 1,600°C.

In addition to common fiber reinforcement, Wu et al. [106] innovatively used melamine foam (MF) to reinforce phenolic aerogel. It was surprisingly discovered that the composite gel with macroporous porosity (0.18–0.42 μm) could be dried at ambient pressure without any volume shrinkage, which is conducive to the development of ambient pressure drying. Moreover, the introduction of MF could not only improve the mechanical properties but also endowed the aerogel with excellent flexibility (high compressibility under the strain of 90%).

3.4 Modification by fillers

Filler particles are introduced to phenolic aerogels to improve rigidity, thermal stability, and dimensional stability [107]. After long-term research, the Key Laboratory of Advanced Materials for Special Functional Materials developed the carbon-based ceramifiable theory. The theory acknowledges that the polymer matrix composite modified by ceramic fillers will undergo the transformation of ceramicization at high temperatures, inhibiting oxygen diffusion and improving the ablation resistance of the material [108]. Ding et al. [109] modified the carbon–phenolic composite with ZrSi₂. Experiments suggested that the residual rate of the material was 77.07% at 1,200°C, and the linear ablation rate was only 0.017 mm/s under oxyacetylene ablation above 2,000°C. Ding et al. [109] believed that ZrSi₂ reacted with oxyacetylene molecules at high temperatures to form the SiO₂–ZrO₂ layer and molten SiO₂ cover, which could prevent the entry of oxygen and heat flow, improving the ablation and oxidation resistance effectively. Similarly, Wang et al. [110] used SiC to modify carbon–phenolic composite, and found that when the mass content of SiC was less than 5% of resin content the ablation resistance was improved because SiC would be oxidized to SiO₂ and covered the materials to protect the inner structure. However, the ablation resistance would decrease when the content exceeded 5%. Wang et al. [110] analyzed the oxidation process change from passive to active oxidation and the product changes from SiO₂ to SiO, accelerating the ablation rate. Natali et al. [111] used nanoclays to modify the PICA and found that nanoclays could improve thermostability and rigidity. Good rigidity allowed the materials to better accommodate the thermal expansion and reduce the effect of thermal stress.

3.5 Summary and discussion

As shown in Figure 13, numerous modification methods are proposed to improve overall properties of phenolic aerogel. Flexible groups and macromolecule chains are frequently used to improve brittleness. To overcome the problem of uneven particles, surfactants are often utilized in the sol–gel reaction. Poor mechanical properties have been effectively conquered by reinforcing it with fiber. Moreover, to improve the ablative and oxidation resistance, inorganic elements or fillers are often introduced to the phenolic aerogel. The approaches described above may have a synergistic effect on other characteristics, but the effect is usually minor. Now, many modification strategies have yielded positive outcomes, but the mechanism remains unknown. In addition, most approaches are still in the theoretical and experimental stages, and the costs are high, making them unsuitable for practical application.

Figure 13

Modification methods to the defects of phenolic aerogel [106,112–114].

4 Application of phenolic aerogels

As shown in Figure 14, phenolic aerogel material has developed rapidly in recent years. Due to its low density, low thermal conductivity, porosity, and other characteristics, phenolic aerogel is widely employed in thermal protection systems, catalyst carriers, precursor material of carbon aerogel, pollutant adsorption, and other applications. The following are the primary applications for phenolic aerogels.

Figure 14

Applications of phenolic aerogels.

4.1 Precursor materials for carbon aerogel

Phenolic aerogel is normally selected as an organic precursor to prepare carbon aerogel due to its high carbon residue rate and the compact and stable carbon layer formed by the pyrolysis of phenolic aerogel [57]. After carbonization, glass-like carbon aerogel is formed. Retaining the three-dimensional network and nanoporous structure of organic aerogel, carbon particles endow the material with excellent electrical, thermal, and mechanical properties [90]. Mayer et al. [115] and Pekala et al. [116] discovered that carbon aerogels can be utilized as electrode materials for supercapacitors. Large specific surface area (100–700 m²/g) of carbon aerogels offers electrode material a huge specific capacitance of 10⁵ F/kg and a high energy density of 25 kW/kg. Meanwhile, the electrode material has a good durability that it still performs well after being recharged 4,000 times. Aside from the electrical field, high stability and porosity of carbon aerogel make it a superior candidate for catalyst and carrier materials. Du et al. [117] prepared Pt catalysts supported on carbon aerogel and Vulcan XC-72, respectively, and found that the Pt catalyst supported on carbon aerogel exhibited higher electrical conductivity and catalytic activity compared to that supported in Vulcan XC-72. In addition, carbon aerogel has a good performance in the fields of adsorption materials, hydrogen storage materials, and biological materials [118].

4.2 Thermal protection materials of aerospace

With excellent thermal insulation performance, ablation resistance, and low cost, phenolic resin is preferably applied to the thermal protection system. Meanwhile, phenolic aerogel realizes lightweight based on maintaining these properties, which gains lots of attention of many researchers [119,120]. Actually, phenolic aerogels represented by PICA do have an outstanding performance in the aerospace field. For example, PICA was applied to the heatshield of the re-entry capsule of “Stardust” and successfully withstood a high-speed aerothermal environment of 12 MW/m² [101], and then was subsequently used as the heatshield for the Mars Science Laboratory (Figure 15a). The density of the PICA heatshield of MSL is reported to be 0.224–0.321 g/cm³ and it can endure an instantaneous high temperature of 3,200°C [121]. Then, NASA-Ames Research Center and Space Exploration Technologies Corp. collaborated to further optimize the PICA and developed PICA-X. Compared with PICA, PICA-X has superior ablation/oxidation resistance and thermal insulation performance, while the preparation costs are lowered. Now, PICA-X has been applied to the “Crew Dragon” (Figure 15b) and returned successfully 18 times since the first flight in 2012 [102].

Figure 15

Tiled PICA heatshield [102]. (a) PICA heatshield of MSL and (b) PICA heatshield of Dragon capsule.

4.3 Sewage treatment, cryogenic target materials, and other fields

The material has been employed in numerous fields since the development of phenolic aerogel. Environmental pollution and sewage discharge have long been difficult issues. Current study is focusing on how to deal with contaminants without causing significant environmental harm. Due to its high specific surface area and porosity, phenolic aerogel has been regarded an appropriate material to overcome this problem [122]. Li et al. [123] developed a phenolic resin-based nanofiber aerogel material and discovered it had a good SO₂ capturing property (SO₂ adsorption capacity is 9.16 mmol/g at a short time of 3.1 min). Moreover, the aerogel displayed a very high SO₂/N₂ selectivity (7,271 at 298.2 K), which is conducive to treat polluting gases in air. Phenolic aerogel has also been used in adsorption studies of CO₂, hydrogen, deuterium, organic pollutants, and metal ions [122,124–127]. In addition, phenolic aerogels are mainly composed of low-z elements such as carbon, hydrogen, and oxygen. The density of phenolic aerogel is low and the distribution of micropores is relatively uniform. So, it can be used for cryogenic targets to adsorb D-T fuel. Besides, it can serve as the filling material of multi-layer target to interact with laser plasma [125]. In medicine, compatible with biosome well, phenolic aerogels can be used to fabricate artificial biotissues, artificial organs, and drug carriers [128].

5 Conclusion

Not only does phenolic aerogel have the high temperature and ablative resistance of phenolic resin, but also has the nanoporous structure of aerogels, thus it is a promising functional material. Although phenolic aerogel has made great progress in recent years, there are still some problems in the preparation process of phenolic aerogel such as high cost and long cycle. Moreover, high brittleness, poor oxidation resistance, and poor mechanical properties when used in actual working conditions are still the pain points of phenolic aerogel. The above problems also lead to the fact that phenolic aerogels are mainly used in aerospace thermal protection and as precursors of carbon aerogel. Large-scale industrial production is still a long way off. The following suggestions are put forward in this article:

The synthetic raw materials of phenolic aerogel are harmful to the environment, so it is significant to find a large number of cheap biomass materials with good environmental benefits.
Aerospace thermal protection is an important application field of phenolic aerogels. To improve its performance in extreme service environments, it is required to design the phenolic aerogel with multi-component and multi-scale. First, research may be concentrated on gradient design and hybrid weaving of phenolic aerogel to improve overall functionality. Second, by introducing new reinforcement components, the phenolic aerogel will develop the structure of organic and inorganic bicomponent network, ameliorating the brittleness and improving the oxidation resistance further.
Although phenolic aerogel is commonly used as an ablative insulation material in the aerospace industry, it is rarely reported in the civil industry. As a result, it will be a significant tendency to develop new low-cost, short cycle, and rapid preparation processes, hoping for phenolic aerogels to be used in our life.

Funding information: The authors state no funding involved.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2023-05-16

Revised: 2023-07-13

Accepted: 2023-07-29

Published Online: 2023-08-23

This work is licensed under the Creative Commons Attribution 4.0 International License.

Research progress on preparation, modification, and application of phenolic aerogel

Abstract

1 Introduction

2 Preparation of phenolic aerogel

2.1 Sol–gel reaction

2.1.1 Raw materials

2.1.2 Catalysts

2.1.3 pH of solution

2.1.4 Selection of reaction solvent

2.1.5 Reaction time and temperature

2.2 Drying of wet gels

2.2.1 Supercritical drying

2.2.2 Freeze drying

2.2.3 Ambient pressure drying

2.3 Summary and discussion

3 Modification of phenolic aerogels

3.1 Modification by surfactants

3.2 Modification of phenolic resin matrix

3.3 Modification by reinforcement

3.4 Modification by fillers

3.5 Summary and discussion

4 Application of phenolic aerogels

4.1 Precursor materials for carbon aerogel

4.2 Thermal protection materials of aerospace

4.3 Sewage treatment, cryogenic target materials, and other fields

5 Conclusion

References

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