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As naturally derived material, cellulose aerogels have excellent thermal insulation properties due to their unique high porosity and three-dimensional mesoporous structure. However, its hydrophilic properties limit its application in the field of building insulation. Here, we propose a method to prepare high hydrophobicity by adopting the sol-gel method and chemical vapor reaction strategy using cellulose acetate type II as raw material and 2,4-toluene diisocyanate as the cross-linking agent. Thermal properties of cellulose acetate aerogels (CAAs) were measured, where pyridine was the catalyst, acetone was the solvent, and perfluorodecyltriethoxysilane (PFDS), hexamethyldisilazane (HMDS), and methyltriethoxysilane (MTES) were used as hydrophobic agents (by process hydrophobic test). Compared with MTES-modified cellulose acetate aerogels (M-CAAs) and HMDS (H-CAAs)-modified cellulose acetate aerogels, PFDS-modified (P-CAAs) cellulose acetate aerogels are the most hydrophobic. By implementing hydrophobic modification of PFDS both inside and outside the structure of cellulose acetate aerogels, the water contact angle can reach up to 136°, strongly demonstrating the potential of PFDS as a hydrophobic agent. The results show that the thermal conductivity and compressive strength of cellulose acetate aerogel with the best hydrophobic properties are 0.035 W m−1 K−1 at normal pressure and 0.39 MPa at 3% strain, respectively. This work shows that the highly hydrophobic cellulose acetate aerogel has potential as a waterproof material in the field of building thermal-insulation materials. [ABSTRACT FROM AUTHOR]

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, CO 2 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. [Display omitted] • Polybenzoxazine (PBz) aerogels present a wide-ranging potential for applications as emerging high-performance materials. • Elaborate summaries of preparation routes for PBz aerogels. • The microscopic morphology of PBz aerogels can be modulated through colloid chemistry principles. • Comprehensive introduction of composite and hybrid materials based on PBz aerogels. • Challenges and potential development directions faced by PBz aerogels. [ABSTRACT FROM AUTHOR]

High‐performance thermal protective composites with lightweight, micro‐ ablation and high‐efficient thermal insulation are urgently required for thermal protection systems in advanced hypersonic speed vehicles. However, the practical applications of thermal protective composites have long been hampered by the main issues such as low mass residual rate and poor long‐term antioxidation of the matrix in high‐temperature aerobic environments. Here, we report a novel silica‐polybenz oxazine (SiO2‐PBO) aerogels with interpenetrated networks, possessing the ability to antioxidation, thermal insulation, and flame‐retardant properties. The resulting SiO2‐PBO aerogels exhibit low density (0.25 g/cm3), low thermal conductivity (0.035 W/(m·K)), and superior peak heat release rate value (15.3 W/g). Moreover, the mass residual rate is up to 70.46 wt% in the N2 atmosphere and remains 57.83 wt% despite existing in the air atmosphere and experiencing the highest temperature of 800°C. Briefly, SiO2‐PBO aerogels as‐prepared could be a potential matrix for a new gene ration of high‐performance thermal protective composites in the future. [ABSTRACT FROM AUTHOR]

• A boric acid-regulated sol–gel transition strategy is used to achieve modulation of the morphology. • Ethanol is employed as a green solvent for the preparation of boron-doped polybenzoxazine (BPBz) aerogels. • The introduction of boric acid provides multifunctional properties to BPBz aerogels. • The BPBz aerogels exhibit intrinsic and alterable wettability that correlates with their morphologies. 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 kN·m·kg−1), low bulk density (0.203 g·cm−3), and low thermal conductivity (0.043 W·m−1·K−1), exhibiting excellent fire resistance when exposed to a 1200 °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. [ABSTRACT FROM AUTHOR]

Materials with high‐performance thermal insulation and excellent flame‐resistance are incredibly desirable for energy conservation and fire safety. In this study, a novel hybrid nanostructure network polybenzoxazine/silica (PBO/SiO2) aerogels were fabricated through facile in situ co‐polymerization sol‐gel methods with ambient pressure drying using benzoxazine (BO) monomers and SiO2 sol as reaction source, N, N‐dimethylformamide (DMF) as the solvent and hydrochloric acid (HCl) as the catalyst. The hybrid nanostructure network was retained by polymerization–induced nanoscale phase separation of PBO and SiO2. The resulting PBO/SiO2 aerogels exhibited finer microstructure, lower density (0.18 g/cm3), thermal conductivity (0.031 W/m·K), and better flame‐resistance in comparison with PBO aerogels. They demonstrated an excellent compressive strength of 0.81 to 1.12 MPa at 10% deformation. The remarkable improvement in thermal insulation and flame‐resistance of PBO/SiO2 aerogels could be attributed to the combined effects of finer microstructure and formation of SiO2 that was "in‐situ" interpenetrated and interacted with silanols (Si‐OH) in the PBO network during the combustion process. The successful synthesis of PBO/SiO2 aerogels highlights the possibility of fabricating a novel high‐performance thermal insulation and excellent flame‐resistance used for energy‐efficient buildings. [ABSTRACT FROM AUTHOR]

The lightweight ablation materials that have been successfully applied in thermal protection systems (TPS) are the focus of attention owing to the urgent demand for aerospace vehicles to have efficient thermal insulation materials with lightweight, long-term antioxidant, and micro-ablation. However, improving the antioxidant ablation properties in a long-term aerobic atmosphere is still a significant challenge. Herein, a novel strategy to fabricate needle quartz fiber (NQF) reinforced SiO 2 aerogel interpenetrating polybenzoxazine (PBO) aerogel thermal protection composites (NQF/SiO 2 –PBOs) with a binary network structure is proposed. The as-prepared NQF/SiO 2 –PBOs perfectly inherited their porous nanostructure and captivating properties, including lightweight (0.53 g/cm3), high mechanical strengths, and low thermal conductivity of 0.048 W/(m·K) at 25 °C and 0.079 W/(m·K) at 1100 °C. Moreover, the NQF/SiO 2 –PBOs exhibited outstanding high-temperature thermal insulation and long-time antioxidant ablation with low linear and mass ablation rates. The cold surface temperature peaked at approximately 307.2 °C within 1800 s when the hot surface temperature exceeded 1100 °C. The ablation/thermal insulation mechanism was also discussed through the analysis of the microstructure, chemical structure, and crystal structure. This research provides a meaningful reference for the development and exploitation of new advanced lightweight, long-term antioxidant, and micro-ablative thermal protective materials. [ABSTRACT FROM AUTHOR]

Polybenzoxazine (PBO) aerogels were prepared from 4,4′-diamino-diphenylmethane (MDA) benzoxazine monomer by HCl-catalyzed ring-opening polymerization at room temperature after CO2 supercritical drying. This study systematically analyzes the microstructures, thermal insulation properties, compressive properties, and thermal stabilities of the PBO aerogels. The aerogel structure was a cross-linked three-dimensional network with a pore size of 10–140 nm. The conductivity of the aerogels was 0.035–0.057 W/m K under ambient conditions, reducing to 0.009 W/m K at 3 Pa. The compressive strength reached 14.42 MPa under 10% strains, much greater than in SiO2 aerogels of similar density (0.40 g/cm3) reinforced by high-silica glass fiber, which is only 1.30 MPa. Hydrogen bonding interaction was found to be critical for constructing the three-dimensional network structure of the aerogels. This work provides a valuable reference for exploring new structures and expanding the thermal insulation applications of PBO aerogels. [ABSTRACT FROM AUTHOR]

[Display omitted] • The facile preparation of PBz aerogel with excellent thermal insulation performance is realized. • The char yields of PBz aerogel reach 55.00 % at 600 °C. • The obtained PBz aerogel demonstrates a low thermal conductivity of 0.0313 W m−1 K−1 at 25 °C. • The obtained PBz aerogel exhibits super-hydrophobicity and outstanding mechanical properties. Polybenzoxazine (PBz) aerogels with outstanding mechanical properties and thermal insulation have attracted significant attention because of their classification as the most advanced and efficient thermal insulation material. However, the complex and environmentally unfriendly preparation technology has consistently hindered the performance improvement and broad practical applications of PBz aerogels. Herein, we report a facile preparation method of PBz aerogels through an aqueous solution system and ambient pressure drying (APD), thereby overcoming the limitations of the conventional methods. The resulting PBz aerogel exhibits lightweight (0.194 g cm−3), efficient thermal insulation, low thermal conductivity (0.0313 W m−1 K−1), self-extinguishing, super-hydrophobicity (contact angle of 148°), and remarkable mechanical properties. This study proposes a cutting-edge approach to fabricate PBz aerogels, which further amplifies their potential application in the field of thermal insulation. [ABSTRACT FROM AUTHOR]

Polybenzoxazine (PBO) aerogels with low densities and low thermal conductivities are prepared from Bisphenol A (BPA) benzoxazine monomers by ring‐opened polymerisation using HCl as a catalyser at 10 °C. The obtained PBO aerogels have cross‐linked and 3D network structures with the densities ranging from 0.084 to 0.526 g cm−3. The thermal conductivities under different pressures (3–105 Pa, air) and different atmospheres (N2, Ar, and CO2, 105 Pa) are investigated. The thermal conductivities are in the range of 0.0335–0.0652 W m K−1 under ambient pressure and 0.0098–0.0571 W m K−1 at 3 Pa. The thermal transfer mechanism under different gas pressures is analyzed with increasing pressure. Under different atmospheres, the thermal conductivities decrease as the molecular weight of the gas increases. Compared with the traditional organic foam insulating materials of phenolic foam, polyurethane and polystyrene, which have similar apparent densities, PBO aerogels exhibit lower thermal conductivity of 0.0335 W m K−1 than that of traditional organic foam at room temperature. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Benzoxazine/melamine composite aerogel has a double network structure. • Aerogels was prepared through drying at normal temperature and pressure. • Aerogel owned the properties of flame retardancy, smoke suppression, resilience, heat insulation and hydrophobic properties. • The composite aerogel has good thermal insulation at 200 ℃. [ABSTRACT FROM AUTHOR]

Chitosan aerogels could be applied potentially in thermal insulation for energy-saving buildings, separation/adsorption, and catalysis. However, disadvantages of chitosan aerogels include their hydrophilicity and low insufficient mechanical strength. Here we propose a silica-phase hybriding route to create chitosan/silica hybrid aerogels with a synergistic capability for favourable hydrophobicity and superior mechanical strength, demonstrating an emergent finding (hydrophobicity optimised with the improved mechanical strength). The aerogels exhibit low drying shrinkage (as low as 13.41 %), lightweight (lowest to 0.149 g cm−1), high-efficient thermal insulation (thermal conductivity as low as to 0.024 W m−1 K−1 at room temperature and normal pressure) either under cryogenic (−196 °C) or high-temperature conditions, exceptional fire-retardancy (self-extinguishing in 1.8 s) and environmentally friendly characteristic (initial mineralisation after 10 d). High hydrophobic property (water contact angle up to 142°) of the aerogels were achieved depending upon 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane of vapor deposition, presenting a discovery concerning substantial improvement of mechanical properties (up to 0.188 MPa at 5 % strain, increased by 25 %). Furthermore, we demonstrate that a plausible mechanism for simultaneous hydrophobic and mechanical enhancement is depending upon the modulation of networking skeletons at the nanoscale. [Display omitted] [ABSTRACT FROM AUTHOR]

Polybenzoxazine (PBZ) aerogels are potential thermal insulation and ablative material for thermal protection system in aerospace. However, obtaining the excellent properties of thermal insulation and compression strength remain a formidable challenge at an extreme temperature. Here we proposed a new strategy by optimizing the nanoporous networking structure and pore size distribution to prepare PBZ aerogel (PAs-Fx), which could be self-extinguished in 0.24 s only once departing the outer flame, and the compressive stress of PAs–F10 reached up to 6.62 MPa at the strain of 10%. PAs–F10 presented great advantage in thermal insulation property (although the hot surface existed in the atmosphere at 302.4 °C, the temperature of cold surface remained at only 112.4 °C when heat flow was stable). Even burning the hot surface with 1200 °C outer flames for 30 min, the highest temperature of the cold surface only reached 254 °C at the heat balance state, moreover, the prominent shape-keeping property was confirmed after ultra-high temperature burning. We also found that PAs–F10 has superior compressive strengths which are 5.52 and 9.38 MPa at 10% strain in the conditions of ambient temperatures for 150 and −50 °C, respectively. PAs-Fx could also retain the excellent compression property after treating cryogenically in liquid nitrogen (−196 °C). The work would be a great promise in the application of ablative thermal insulation materials in a great measure in the future. [Display omitted] • The PAs-Fx aerogels behave abundant nanoporous networking structure, good thermal insulation properties, flame retardancy feature. • PAs–F10 behave excellent thermal insulation property in extreme high temperature environment (1200 °C). • This is the first report on the compression properties of PBZ aerogels in extreme environments (150 and −50 °C). • PAs-Fx aerogels could also retain the excellent compression property after treating cryogenically in liquid nitrogen (−196 °C). [ABSTRACT FROM AUTHOR]

[Display omitted] • Polybenzoxazine aerogels with excellent thermal insulation property were fabricated by Lewis acid catalysis. • The self-extinguishing performance of PBz aerogels could be obtained apparently. • The aerogels as-prepared possessed a three-dimensional networking structure in the range of 20–140 nm. Polybenzoxazine aerogel is a new polymer aerogel material developed in recent years, which the ring-opening polymerization of benzoxazine monomer under high temperature or HCl could achieve the formation of cross-linked three-dimensional network structure so that to further reduce the cost of preparation. However, HCl as a catalyst has the disadvantages of easy volatilization and corrosion. Here we introduced Lewis acid as the catalyst for fabricating nanoporous polybenzoxazine aerogels with low density and self-extinguishing properties depending upon the evolution law for the influence of the mixed ratio of Lewis acid and hydrochloric acid (volume ratio) on the corresponding aerogels. The results show that PBz aerogels prepared in the mixed acid environment (with n -hexane as the exchange solvent) have a good three-dimensional nanopore network structure, and the pore size distribution is in the range of 20–140 nm. Additionally, the obvious self-extinguishing performance (even exposing to a flame at 1200 °C), low density and excellent mechanical properties are reflected as well, potentially suggesting the broad application prospects in the field of aerospace. [ABSTRACT FROM AUTHOR]

Silica aerogels with different particle size were prepared by monodispersed silica sol for the first time. The properties of the silica aerogels under normal and high temperature were systematically studied. Among the four different particle size aerogels, the SA-2 sample consisted of 20.31 ± 1.21 nm particles with narrow size distribution had high temperature resistance and low thermal conductivity. Compared with the traditional acid-base two-step prepared silica aerogels, the monodisperse silica aerogels can significantly increase the temperature resistance while maintaining a low thermal conductivity (0.02723 W·m−1·K−1). The sturdy skeleton structure formed by the interconnection of large particles can effectively inhibit the viscous flow between aerogel particles and avoid pore collapse caused by skeleton failure which can maintain a stable structure until 900 °C and retain a relatively complete structure at 1000 °C with only about 35% volume shrinkage after 2 h heat treatment. At 1100 °C, the viscous flow between aerogel particles and pore collapse cannot be effectively suppressed, and the pure silica aerogels can only be used during a short period at 1100 °C. The results will be meaningful for the design of super thermal insulation materials with high temperature resistance and low thermal conductivity. Unlabelled Image • SiO 2 aerogels with high temperature resistance are prepared by controllable particle size monodispersed silica sol. • Thermal conductivity of SiO 2 aerogels is as low as 0.02723 W·m−1·K−1 despite forming large particle structure. • The aerogel large particle skeleton structure can inhibit the viscous flow between aerogel particles. • Pore collapse caused by skeleton failure can be obviously inhibited depending on formed sturdy skeletons. [ABSTRACT FROM AUTHOR]