ABSTRACT Polybenzoxazine (PBz) aerogels fabricated via a water–ethanol mixed green system are attractive for thermal insulation but are limited by inherent brittleness and the environmental burden of their conventional fabrication. To address this, we developed a high‐strength hybrid aerogel through a water‐ethanol mixed green strategy, employing melamine (Ma) as a chemical crosslinker. The reaction between Ma and the PBz precursor constructs a dense, chemically bonded network, which yields a remarkable compressive strength of 6.80 MPa at 10% strain—approximately 377% that of the PBz aerogel. This marked mechanical enhancement is achieved without compromising its overall performance, featuring a low density of 0.42 g cm −3 , a thermal conductivity of only 0.059 W m −1 K −1 , and improved flame resistance. This work successfully overcomes the traditional compromise between environmentally friendly processing and high performance, offering a feasible and scalable route to produce strong, lightweight, and fire‐resistant PBz‐based aerogels for advanced thermal insulation applications.

Polybenzoxazine (PBz) aerogels have garnered significant attention as advanced thermal insulation materials, primarily attributed to their superior thermal stability and lightweight nature. However, their mechanical properties need further improvement for high-load scenarios. Furthermore, the reliance on toxic solvents, cumbersome preparation processes, and high costs has significantly impeded the advancement of PBz aerogels. Herein, we present a novel methodology for fabricating polybenzoxazine-based (PBz/KH562) hybrid aerogels with enhanced mechanical properties. Notably, the integration of 3-(2,3-epoxypropoxy)propylmethyldimethoxysilane (KH562) through a sustainable process yields aerogels exhibiting a low density (0.39 g center dot cm-3), low thermal conductivity (0.0556 W center dot m-1 center dot K-1), and outstanding fire resistance. The optimized PBz/KH562 hybrid aerogels demonstrate a compressive strength of 8.45 MPa at 10 % strain and 5.11 MPa at 5 % strain, representing remarkable increases of 451.87 % and 477.57 % compared to those of pristine PBz aerogels, respectively. Remarkably, the compression modulus is up to 101.15 MPa. This research establishes an advanced synthesis method that incorporates a novel dual-crosslinked architecture, facilitating the creation of high-performance insulating materials.

Polybenzoxazine aerogels have emerged as promising candidates for applications in thermal insulation and adsorption owing to their porous structure, low density, and outstanding thermal stability. However, current preparation methods often rely on toxic or highly polar solvents, and the resulting products exhibit inadequate mechanical properties, hindering practical application. Herein, a novel and eco-friendly strategy of constructing high-strength composite aerogels with eco-friendly water-soluble benzoxazine monomers and phenolic fibers is presented. The resulting aerogels, exhibiting a nanoporous framework at a bulk density of 0.387 g cm(-3) and demonstrate outstanding mechanical performance. Specifically, the aerogels achieve a compressive strength of 6.11 MPa at 5 % strain, a compressive modulus of 126 MPa, and a flexural modulus of 136 MPa, all of which collectively confer excellent machinability. Meanwhile, with a low thermal conductivity of 0.0552 W m(-1) K-1 and a limiting oxygen index of 30.7 %, PF/PBz composite aerogels can maintain the overall structure under the butane torch (similar to 1200 degrees C) and quickly self-extinguish after the flame is removed, showing superior thermal insulation and flame retardancy. This research provides valuable scientific insights for the design of advanced composite aerogels that possess superior mechanical strength and thermal insulation with environmental sustainability.

Polybenzoxazine (PBz) aerogels, as a new class of high-performance polymer aerogels, have attracted extensive attention from the scientific and industrial communities. However, the use of toxic solvents in the process of monomer synthesis and ring-opening polymerization to form polybenzoxazine and relatively large thermal conductivity hinder their further development and practical application. Herein, a novel poly(resol-co-benzoxazine) (P-ReBz) aerogel was constructed by using a mixed solvent of water and ethanol as a green medium and a sodium base catalyst to catalyze the induction of 3-aminophenol and formaldehyde, which provided a feasible method to overcome the above situation. The prepared aerogels with alkaline aqueous solution as a catalyst have a density range of 0.106-0.225 gcm-3, low thermal conductivity of 0.0263 Wm-1K-1, and excellent flame retardancy (butane lance at 1200 degrees C three times, maintaining structural integrity with a self-extinguishing time of 1-3 s). The P-ReBz aerogels with lightweight, low thermal conductivity combined with environmentally friendly preparation strategies are expected to be used for thermal management and fire protection applications in energy-saving buildings, industry, and other thermal insulation fields.

The preparation of nanoporous polybenzoxazine aerogels with environmentally benign solvents has drawn increasing interest in recent years, but the strategies for enhancing their mechanical strength and the mechanisms by which microstructural evolution influences skeleton strength are not clear yet. Silicon functionalization, particularly using silane coupling agents, provides a practical route to tailor the microstructure and improve the mechanical properties of organic aerogels while retaining their thermal insulation. Drawing inspiration from this mechanism, we herein propose the rational design of a multifunctional boron-doped polybenzoxazine aerogel modified by methyltrimethoxysilane and 3-glycidoxypropyldimethoxymethylsilane (BPBz/MK hybrid aerogel) with a special nanostructure evolution and high mechanical strength through an eco-friendly solvothermal method based on ethanol-soluble benzoxazine (Bz) monomer. The BPBz/MK hybrid aerogel exhibits a remarkable integration of properties, including exceptional thermal insulation (0.0325 W m-1K-1), outstanding fire retardant properties, and intrinsic hydrophobicity with a water contact angle of 145 degrees. The mechanical strength of BPBz/MK hybrid aerogels was substantially improved, with modulus increasing from 0.11 to 4.22 MPa, an enhancement by a factor of approximately 38.36. Collectively, these characteristics position the material as an excellent thermal insulation candidate across a broad range of uses in demanding environments.

Polybenzoxazine-based aerogel composites have attracted much attention as intrinsic fire-retardant thermal insulation materials. However, their promotion in the thermal management field is currently constrained by the prevalent use of hazardous solvents and the requirement of high temperatures or high-active catalysts during preparation. Herein, a novel needle quartz fiber felt-enhanced polybenzoxazine aerogel composite (NQF/ WPBzAC) was prepared using deionized water as the solvent combined with ambient pressure drying, wherein a binary monomer gelation strategy was employed to induce the sol-gel transition occurred under mild and catalyst-free conditions. The resultant NQF/WPBzAC exhibited integrated characterizes of lightweight (0.324 g cm- 3), efficient thermal insulation (0.055 W m-1 K-1), good compressive and flexural resistance, and excellent fire retardance. Repeated water immersion tests and high-temperature exposure tests were carried out to assess the durability of NQF/WPBzAC in harsh environments. This research provides insights into the green and gentle preparation of polybenzoxazine-based aerogel composites applicable for fire resistance and thermal insulation.

High-temperature resistance polymer composites are crucial for application in aerospace field. Polybenzoxazine (PBz) aerogel is a novel insulation material with excellent mechanical properties, low bulk density, and low thermal conductivity, which has potential application in thermal protection systems. However, the improvement of high residual char rates for shape-keeping purposes at high-temperature is still a critical challenge. Herein, a novel kind of PBz aerogel composite reinforced with mullite-fiber fabric (PBZ-MF) is prepared by sol-immersion-gel strategy. The prepared composite integrates the nanoporous structure of aerogel with the fascinating properties of mullite-fiber fabric, resulting in relatively low thermal conductivity (0.0557-0.0647 W m-1 K-1 at 25 degrees C and normal pressure), lightweight (0.41-0.47 g cm-3), outstanding flame resistance (self-extinguishing time as short as 0.7 s), and high compressive modulus (20.611-32.656 MPa). Furthermore, PBz-MF demonstrates exceptional high-temperature resistance and shape-keeping properties even at 1100 degrees C. This work provides valuable insights into the development of lightweight insulation materials and expands the potential applications of aerogel composites in thermal protection systems within the aerospace field.

Polybenzoxazine (PBz) aerogels are promising candidates for high-performance thermal insulation due to their lightweight nature and excellent thermal stability. However, the preparation of non-green solvents and their non-hydrophobic nature result in poor thermal insulation properties during application, which seriously hinders the development of PBz aerogels. Herein, we report a green, one-pot strategy to fabricate a novel polybenzoxazine-based (BPBz/SiO2) hybrid aerogels with inherent hydrophobicity by incorporating boric acid and methyltrimethoxysilane. The prepared aerogels exhibit low density (0.144 g·cm−3), low thermal conductivity (0.0355 W·m−1·K−1), and excellent fire resistance. Remarkably, the BPBz/SiO2 hybrid aerogels exhibit outstanding intrinsic hydrophobicity (a water contact angle of 144°) and effectively prevent the adhesion of contaminants on the surface. This work is expected to play a significant role in advancing green synthesis technologies, innovating aerogel structural design, and developing high-performance thermal insulation materials.

Bio-based aerogel is a functionalized nanoporous material with environmentally friendly, high surface area, ultra-low density, high porosity, and low thermal conductivity, making it suitable for various applications such as energy-saving buildings, electronic information, separation, adsorption, catalysis, biomedicine, and others. However, the current bio-based chitosan aerogel still faces great challenges in reaching multifunctional improvement to address its intrinsic shortcomings. Herein, we propose a new approach depending upon supramolecular interactions for constructing chitosan/bacterial cellulose aerogels that simultaneously possess superior moisture resistance/fatigue, anti-thermal-shock, and flame retardancy. Specifically, the aerogels demonstrate remarkable characteristics, namely high strength (self-standing itself weight beyond 10,676 times), low thermal conductivity (lowest to 22 mW m-1 K-1 under normal pressure and room temperature), and excellent fatigue resistance (almost negligible permanent deformation at 1 % strain even undergoing compressive cycles up to 10,000 times). On the other hand, the aerogels display exceptional moisture resistance with superhydrophobicity (moisture absorption rate <0.88 % for 160 h at 70 °C and 85 % relative humidity), excellent thermal shock property (withstand cold-hot shock up to 200 cycles with rapid temperature changes between -30 °C and 60 °C), and remarkable fire retardancy (swiftly self-extinguishing in 0.6 s). Additionally, the compressive stress increases to 0.223 MPa at 3 % strain after hydrophobic treatment, representing a 27 % enhancement in mechanical robustness. Further, the mechanism responsible for microstructural evolution has been also established in different strain conditions. This work may provide rich possibilities for developing multifunctional bio-based aerogel for energy-saving buildings.

The rapid advancement of modern industries has imposed demands on thermal insulators that possess not only low thermal conductivities but also multifunctional properties. Polybenzoxazine (PBz) aerogels with several advantages have garnered considerable research interest in recent years. Nevertheless, PBz aerogels acquired from ambient pressure drying still confront the challenge of exhibiting relatively high densities and thermal conductivities. In this study, to enrich the category and propose practical ideas for overcoming the above plight, hexamethylene diisocyanate (HDI) was selected as an additive to prepare lightweight and efficient thermalinsulating PBz aerogels through chemical modification. Compared with the pristine specimens (0.268 g center dot cm- 3, 0.0387 W center dot m- 1 center dot K- 1), the HDI-modified PBz aerogels exhibited lower densities (0.237-0.256 g center dot cm-3) and thermal conductivities (0.0354-0.0370 W center dot m- 1 center dot K- 1). Moreover, the introduction of HDI has been demonstrated to not impact their superior intrinsic flame retardance, despite an observed reduction in the residual char yields in the HDI-modified groups. Given these excellent properties, PBz-HDI aerogels are expected to be promising candidates for thermal insulation and fire resistance applications.

Polybenzoxazine aerogels have gained widespread attention in recent years, but the prevalent use of hazardous solvents in the preparation process has severely impeded their further development and promotion for practical applications. Herein, using ethanol as a solvent, a strategy of boric acid-induced microstructure regulation is proposed to construct boron-doped polybenzoxazine (BPBz) aerogels, which provides a feasible approach to overcome the above situation. The introduction of boric acid enhances the backbone strength by forming intermolecular bridged structures between PBz chains and allows the use of ambient pressure drying. The resulting BPBz aerogel demonstrates high stiffness (specific modulus up to 255.97 kNmkg(-1)), low bulk density (0.203 gcm(-3)), and low thermal conductivity (0.043 Wm(-1)K-1), exhibiting excellent fire resistance when exposed to a 1200 degrees C flame (the self-extinguishing interval less than 1 s). More interestingly, the aerogel demonstrates intrinsic and tunable wettability that correlates with the morphology. Combining the environmentally friendly preparation strategy and these advantages, BPBz aerogels hold promise for thermal management and fire protection applications in energy-saving construction and other harsh conditions.

The host-guest composite that integrates a porous scaffold and organic phase change materials (PCMs) features high energy density and customizable function, promising for advanced thermal storage/utilization. However, highly flammable organic PCMs are prone to severe combustion in porous structures, making it challenging for traditional flame-retardant methods to balance fire safety and latent heat. Herein, a high-temperature-triggered crosslinking reaction between the host and guest is designed using a polybenzoxazine-based aerogel (PB-1) and benzoxazine-based PCMs (C-dad). At high temperatures, the ring-opening polymerization (ROP) of C-dad can be initiated by and reacted with the phenolic groups of PB-1 to form a polybenzoxazine copolymer monolith with an improved char yield and intrinsic low flammability and without using the typical flame-retardant components. This enables the obtained composite (PB-1/C-dad) to well balance latent heat (145.3 J g-1), char yield (a char residue of 13.1% at 600 degrees C), and flame retardancy (a peak heat release rate of 231 W g-1), outperforming the representative flame-retardant modified polymer/organic PCM complexes reported in the literature. This thermal-triggered mechanism allows PB-1/C-dad to be repeatedly and stably used within the working temperature and activates its flame retardancy when exposed to open flames. The proposed host-guest crosslinking strategy is believed to inspire the development of inherently nonflammable phase change composites for safer thermal management. A latent crosslinking reaction between the polymer aerogel and organic PCMs was designed, enabling the PCC to form a copolymer monolith with improved charring ability and intrinsic fire safety when overheated, without flame-retardant modification.

The unremitting pursuit of high-performance and multifunctional materials has consistently propelled modern industries forward, stimulating research and motivating progress in related fields. In such materials, polybenzoxazine (PBz) aerogel, which combines the virtues of PBz and aerogel, has attracted salient attention recently, emerging as a novel research focus in the realm of advanced materials. In this review, the preparation scheme, microscopic morphology, and fundamental characteristics of PBz aerogels are comprehensively summarized and discussed in anticipation of providing a clear understanding of the correlation between preparation process, structure, and properties. The effective strategies for enhancing the performance of PBz aerogels including composite fabrication and hybridization are highlighted. Moreover, the applications of PBz-based aerogels in various domains such as adsorption (including wastewater treatment, CO2 capture, and microwave adsorption), thermal insulation, energy storage as well as sensors are covered in detail. Furthermore, several obstacles and potential directions for subsequent research are delineated with a view to surmounting the prevailing constraints and achieving a realization of the shift from experimental exploration to practical applications.

Polybenzoxazine (PBz) aerogels have garnered considerable attention as an innovative and excellent thermal insulation material, celebrated for its lightweight, low thermal conductivity, and outstanding mechanical properties. However, the use of high boiling point toxic solvents in the preparation process requires a cumbersome solvent exchange procedure coupled with insufficient flame-retardant properties, which hinder the potential application of PBz aerogels. Herein, we employed tetraethyl orthosilicate (TEOS) into PBz using an easy, eco-friendly, and cost-less process to achieve hybrid structure polybenzoxazine/silica (PBz/SiO2) aerogels, by thermal catalysis (72 degrees C) ring-opening polymerization and polycondensation in water-ethanol solvent without any catalysts. The resulting PBz/SiO2 aerogels were characterized with low density (0.181 g/cm(3)), low thermal conductivity (0.0315 W/(mK)), excellent flame-retardancy (PHRR value of 32.3 W/g and THR value of 6.3 kJ/g), and superhydrophobicity (the water contact angle up to 155 degrees). Environment-friendly preparation strategy for the PBz/SiO2 aerogels with excellent comprehensive performance, poised to play a pivotal role in energy-saving buildings and fire-resistant applications.

Polybenzoxazine (PBz) aerogels are promising high-performance, halogen-free flame-retardant thermal insulation materials in aerospace applications. But their widespread use is hindered by high costs, significant drying shrinkage, and poor machinability. Herein, we successfully addressed these challenges by developing PBz aerogel composites using a cost-effective ambient pressure drying method that reduces energy consumption and shortening the preparation cycle. This approach expands the range of available monomers, reduces the inherent rigidity of the network structure, and enhances processability. The resulting PBz aerogels demonstrate low drying shrinkage (as low as 5.68 %), lightweight properties (lowest to 0.322 g cm(-3)), excellent fire-retardant (self-extinguishing in 1.8 s), and exceptional thermal insulation performance (as low as 0.0402 W m(-1) K-1 at room temperature and normal pressure). Further studies under various pressures show that at an atmospheric pressure of 10 Pa, the thermal conductivity at room temperature can reach as low as 0.027 W m(-1) K-1. Moreover, cryogenic treatment at -196 degrees C significantly enhances the compressive properties of PBz aerogels without inducing any noticeable shrinkage. Notably, PBz aerogels exhibit outstanding flame resistance, rated as nonflammable rating in vertical burning tests (UL-94, V-1 class), and showing a limiting oxygen index (LOI) as high as 33.7 %. Overall, these remarkable features underscore the exceptional potential of PBz aerogels as advanced thermal insulation materials in the aerospace industry.

Polybenzoxazine/Melamine (PBa/MS) double network aerogels were prepared using melamine foam (MS) as the backbone and bisphenol A-type benzoxazine (Ba) as the maxtrix. The effect of Ba concentration in the sol-gel on the performance of the double network aerogel was investigated. During the combustion, the PBa/MS composite aerogel exhibited self-extinguishing performance. Compared with PBa-18%, the contact angle of PBa-18%/MS composite aerogel was 18° higher, and the heat release peak (PHRR) and total smoke production (TSP) of the composite aerogel decreased by 7% and 39%, respectively. These values indicate that MS can significantly reduce the release of heat and smoke from the PBa aerogel, thus improving its flame retardancy. In addition, the composite aerogel formed a denser and more complete carbon layer during combustion, which blocks both heat and mass transfer. Thus, the resulting PBa/MS dual-network composite aerogels obtained in the current work are expected to meet the demands for flame-retardant, hydrophobic thermal insulation materials in many fields.

Polyimide aerogels (PIAs) are thermally insulating materials that possess various advantages, such as exceptionally high temperature resistance and low thermal conductivity, which make them great potential candidate materials to be applied in the area of thermal protection. However, under harsh mechanical stresses, the macro-and microscopic structural stability of PIAs may be compromised due to shrinkage/collapse, leading to performance degradation. Herein, we propose a homogeneous organic/ inorganic hybrid formation strategy for constructing nanoporous polyimide/ silica aerogels (PIAs-A) with exceptional mechanical strength and ultralow thermal conductivity. The results obtained indicate that PIAs-A have an optimal three-dimensional nanoporous network structure with a pore size distribution mainly within the range of 10-20 nm. Monitoring the temperature evolution of the material's cold surface showed that it remained at a low temperature of 37.5 degrees C even when the material was placed against a hot surface of 150 degrees C. The residual mechanical strengths of the aerogels remained at a superior level (with strength degradation being less than 9%) after exposure to ultralow and high-temperature atmospheres. Furthermore, compressive stress values at 3% strain exceeded previously reported values for PIAs by 625 and 733% at -50 and 100 degrees C, respectively. Meanwhile, our aerogels displayed flame resistance even when exposed to a heat source of approximately 1200 degrees C, possessed favorable hydrophobic properties, and maintained dimensional stability up to 504 degrees C. The robust mechanical and thermal insulation properties of PIAs-A make them a promising substitute material for thermal superinsulation in aircraft.

Nanoporous phenolic resins have been widely used in the thermal protection system for the aerospace field owing to their ability to withstand high temperatures during ablation. Polybenzoxazine among other phenolic materials is considered as a potential candidate for thermal protection in re-entry capsules. However, polybenzoxazine aerogels often exhibit relatively increased thermal conductivity and limited high-temperature resistance, resulting in an adverse effect. Herein, we report a strategy of introducing a nanoporous silica-phase structure into a network of polybenzoxazine aerogels with an outstanding thermally insulated property and high-temperature (1100 degrees C) resistance. As-prepared aerogels possess a residual mass rate of up to 61.14% at 800 degrees C in oxygen. Even at 800 degrees C for 30 min, they can maintain the original shape, reflecting their dimensional ability, and the compressive stress of the aerogels under 2% strain is maintained at 1.323 MPa. After hydrophobic treatment, the water contact angle increased from 0 degrees to 134 degrees, which significantly improved the loss of thermal insulation performance caused by the absorption of water in the air. The study provides insight into the design of thermal protection material matrices with high quality residuals and excellent thermal insulation performance for re-entry capsules in the aerospace field.