Flammability of Polymer/Clay Aerogel Composites: An Overview

ABSTRACT

Polymer/clay aerogel composites fabricated using the freeze-drying method and water as solvent has drawn extensive attentions during the past decade. Such aerogels possess layered or network microstructures, low thermal conductivities, and good thermal stabilities; of special interest, they generally have very low flammability, which could be influenced by the composition and microstructure of the aerogel composites. The fire performance of the aerogels can be further improved with flame retardant modifications. Polymer/clay aerogel composites can also serve as effective flame retardant coatings. The mechanisms of the flame retardancy of polymer/clay aerogel composites are also discussed herein. The thorough survey of the current literatures offers useful information to realize potential of polymer/clay aerogels and help guidance to design novel high-performance polymer/clay aerogel composites.     

Light weight,mechanically strong and biocompatible α-chitin aerogels from different aqueous alkali hydroxide/urea solutions

Light weight and mechanically strong α-chitin aerogels were fabricated using the sol-gel/self-assembly method from α-chitin in different aqueous alkali hydroxide(KOH, Na OH and Li OH)/urea solutions. All of the α-chitin solutions exhibited temperature-induced rapid gelation behavior. 13 C nuclear magnetic resonance(NMR) spectra revealed that the aqueous alkali hydroxide/urea solutions are non-derivatizing solvents for α-chitin. Fourier transform infrared(FT-IR), X-ray diffraction(XRD) and cross-polarization magic angle spinning(CP/MAS) 13 C NMR confirmed that α-chitin has a stable aggregate structure after undergoing dissolution and regeneration. Subsequently, nanostructured α-chitin aerogels were fabricated by regeneration from the chitin solutions in ethanol and then freeze-drying from t-Bu OH. These α-chitin aerogels exhibited high porosity(87% to 94%), low density(0.09 to 0.19 g/cm~3), high specific surface area(419 to 535 m~2/g) and large pore volume(2.7 to 3.8 cm~3/g). Moreover, the α-chitin aerogels exhibited good mechanical properties under compression and tension models. In vitro studies showed that m BMSCs cultured on chitin hydrogels have good biocompatibility. These nanostructured α-chitin aerogels may be useful for various applications, such as catalyst supports, carbon aerogel precursors and biomedical materials.     

Silica–titania aerogel monoliths with large pore volume and surface area by ambient pressure drying

Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels show densities in the range 0.34–0.38 g/cm³. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones. The surface area for 5wt% titania aerogel has been found to be as high as 685 m²/g with a pore volume of 2.34 cm³/g and the 10wt% titania aerogel has a surface area of 620 m²/g with a pore volume of 2.36 cm³/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The surface modified aerogels possess high surface areas in the range of 540–640 m²/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity of the aerogel.     

Ultra porous nanocellulose aerogels as separation medium for mixtures of oil/water liquids

A novel type of sponge-like material for the separation of mixed oil and water liquids has been prepared by the vapour deposition of hydrophobic silanes on ultra-porous nanocellulose aerogels. To achieve this, a highly porous (>99%) nanocellulose aerogel with high structural flexibility and robustness is first formed by freeze-drying an aqueous dispersion of the nanocellulose. The density, pore size distribution and wetting properties of the aerogel can be tuned by selecting the concentration of the nanocellulose dispersion before freeze-drying. The hydrophobic light- weight aerogels are almost instantly filled with the oil phase when selectively absorbing oil from water, with a capacity to absorb up to 45 times their own weight in oil. The oil can also be drained from the aerogel and the aerogel can then be reused for a second absorption cycle.     

Thermally stable and flame-retardant poly(vinyl alcohol)/montmorillonite aerogel via a facile heat treatment

The PVA/MMT aerogels prepared by a simple freeze-drying and heat treatment process exhibited a significant improvement of thermal stability and flame retardant property.     

Porous structure of polybenzoxazine-based organic aerogel prepared by sol–gel process and their carbon aerogels

New organic aerogels were successfully prepared from a new class of phenolic resins called polybenzoxazines synthesized via conventional thermal curing reaction of a benzoxazine monomer using xylene as a solvent. Without the need for using supercritical conditions to remove the solvent during the process, the carbon aerogels were obtained with a much shortened time. From two different concentrations of benzoxazine solution, 20 and 40 wt%, the resulting polybenzoxazine aerogels, having densities of 260 and 590 kg/m³, respectively, were obtained after the curing process. The subsequent carbon aerogels were prepared by the carbonization of polybenzoxazine aerogels. The corresponding carbon aerogels exhibited a microporous structure with pore diameters less than 2 nm, the densities of 300 and 830 kg/m³, and surface area of 384 and 391 m²/g, respectively. The texture of the carbon aerogels was denser than that of their organic aerogel precursor, as evidenced by scanning electron microscopy. The transformation of the polybenzoxazine aerogel to the carbon aerogel was clearly observed using fourier transform infrared spectroscopy.     

Ambient pressure dried tetrapropoxysilane-based silica aerogels with high specific surface area

In the present paper, we report the synthesis of tetrapropoxysilane (TPOS)-based silica aerogels with high surface area and large pore volume. The silica aerogels were prepared by a two-step sol-gel process followed by surface modification via a simple ambient pressure drying approach. In order to minimize drying shrinkage and obtain hydrophobic aerogels, the surface of the alcogels was modified using trichloromethylsilane as a silylating agent. The effect of the sol-gel compositional parameters on the polymerization of aerogels prepared by TPOS, one of the precursors belonging to the Si(OR)₄ family, was reported for the first time. The oxalic acid and NH₄OH concentrations were adjusted to achieve good-quality aerogels with high surface area, low density, and high transparency. Controlling the hydrolysis and condensation reactions of the TPOS precursor turned out to be the most important factor to determine the pore characteristics of the aerogel. Highly transparent aerogels with high specific surface area (938 m²/g) and low density (0.047 g/cm³) could be obtained using an optimized TPOS/MeOH molar ratio with appropriate concentrations of oxalic acid and NH₄OH.     

Synthesis of freestanding silica and titania-silica aerogels with ordered and disordered mesopores

Freestanding blocks of silica and titania-silica aerogels were prepared by the sol-gel method. It is possible to prepare crack-free, titania-silica aerogels with high titanium content by a careful control of the synthesis conditions. Prehydrolysis, complexation and polymer addition were used to adjust the hydrolysis and condensation rates of the silicon and titanium alkoxide precursors. Photoactive anatase TiO₂ nanocrystals with a large surface area (i.e., up to 300m²g⁻¹) were crystallized from the gel network by the high-temperature ethanol supercritical drying, and the resulting aerogel blocks were gas permeable and display a transition-regime diffusion behavior. Pore and volume shrinkages were observed in samples prepared by ethanol supercritical drying when the titanium content was increased resulting in a lower flux. Adding Pluronic P123 creates ordered mesopore domains and produces large pore aerogels even at high titanium contents. The photocatalytic oxidation reaction of trichloroethylene was performed by flowing the reactant gas mixture through the UV-irradiated aerogel blocks with excellent results.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

Preparation and characterization of silica aerogels from diatomite via ambient pressure drying

The silica aerogels were successfully fabricated under ambient pressure from diatomite. The influence of different dilution ratios of diatomite filtrate on physical properties of aerogels were studied. The microstructure, surface functional groups, thermal stability, morphology and mechanical properties of silica aerogels based on diatomite were investigated by BET adsorption, FT-IR, DTA-TG, FESEM, TEM, and nanoindentation methods. The results indicate that the filtrate diluted with distilled water in a proportion of 1: 2 could give silica aerogels in the largest size with highest transparency. The obtained aerogels with density of 0.122–0.203 g/m³ and specific surface area of 655.5–790.7 m²/g are crack free amorphous solids and exhibited a sponge-like structure. Moreover, the peak pore size resided at 9 nm. The initial aerogels were hydrophobic, when being heat-treated around 400°C, the aerogels were transformed into hydrophilic ones. The obtained aerogel has good mechanical properties.     

Alumina aerogels prepared via rapid supercritical extraction

Alumina aerogels with surface areas from 460 to 840 m²/g and bulk densities from 0.025 to 0.079 g/cm³ were successfully fabricated using variations of an aluminum isopropoxide-based recipe developed by Armor and Carlson and the rapid supercritical extraction (RSCE) process developed at Union College. By utilizing the Union College RSCE method, it is possible to convert an alumina aerogel precursor mixture into aerogel monoliths in as little as 7.5 h. This process is safer than methanol extraction in an autoclave and faster and simpler than liquid CO₂ solvent exchange and extraction. By increasing the concentration of aqueous HNO₃ used in the precursor mixture, we were able to fabricate aerogels with significantly increased surface area, decreased bulk density, and altered microstructure. We attribute the observed variation in these aerogel properties at a given HNO₃ concentration to environmental factors such as humidity. The ability to more easily fabricate alumina aerogels with desirable properties will assist in making them a viable option for catalytic and other applications.     

Hydrophobic Silica Aerogels Strengthened with Nonwoven Fibers

Hydrophobic silica aerogels were prepared via a sol‐gel process by surface modification at ambient pressure. Nonwoven fibers were distributed inside the silica aerogels as a composite to act as a supporting skeleton which increased the mechanical property of the silica aerogels. The morphology and pore structure of the composites were characterized by scanning electron microscopy (SEM) and N₂ adsorption analyzer. The contact angle and the adsorption capacities of the composites were also determined. The results show that silica aerogels dispersed uniformly and maintained high porosity in the aerogel‐fiber composites. They have excellent hydrophobic properties and are excellent adsorptive materials.     

Soy protein–nanocellulose composite aerogels

Organic aerogels based on two important and widely abundant renewable resources, soy proteins (SP) and nanofibrillar cellulose (NFC) are developed from precursor aqueous dispersions and a facile method conducive of channel- and defect-free systems after cooling and freeze-drying cycles that yielded apparent densities on the order of 0.1 g/cm³. NFC loading drives the internal morphology of the composite aerogels to transition from network- to fibrillar-like, with high density of interconnected cells. Composite aerogels with SP loadings as high as ca. 70 % display a compression modulus of 4.4 MPa very close to that obtained from reference, pure NFC aerogels. Thus, the high compression modulus of the composite system is not compromised as long as a relatively low amount of reinforcing NFC is present. The composite materials gain moisture (up to 5 %) in equilibrium with 50 % RH air, independent of SP content. Furthermore, their physical integrity is unchanged upon immersion in polar and non-polar solvents. Fast liquid sorption rates are observed in the case of composite aerogels in contact with hexane. In contrast, water sorption is modulated by the chemical composition of the aerogel, with an important contribution from swelling. The potential functionalities of the newly developed SP–NFC composite green materials can benefit from the reduced material cost and the chemical features brought about the amino acids present in SPs.     

Shrinkage and pore structure in preparation of carbon aerogels

To aim at thermal insulator applications, the shrinkage and the pore structure of resorcinol–formaldehyde (RF) aerogels and carbon aerogels were investigated during the supercritical drying and the carbonization process. The water (W) molar ratio has small effects on the surface area or the particle size, but has significant effects on the density of the aerogel. Higher W/R ratio leads to lower density and larger pore size, and leads to less shrinkage during the carbonization process. The molar ratio of catalyst sodium carbonate (C) has significant effects on the shrinkage, pore size, and particle size of the aerogel. Lower R/C ratio leads to smaller particle size and smaller pore size, and thus induces more shrinkage both in the supercritical drying and in the carbonization, the obtained CA is much denser. The R/C ratio should be higher than 300 to prevent excessive shrinkage. In order to synthesize carbon aerogels combining with small shrinkage, low density (less than 0.1 g/cm³), and small pore size (less than 150 nm) for thermal insulators, the preferred W/R ratio is between 90 and 100, and the preferred R/C ratio is between 300 and 600.     

Characterization of divinyl benzene aerogels with density gradient using X-ray tomography technique

Production of density gradient aerogels with predetermined density steps or gradient is challenging, particularly when the experiments would demand a prescribed gradient in density along the axis of cylindrical shaped aerogel. In order to achieve this, it is vital to characterize and accurately measure the density gradient in order to “design” synthetic routes to achieve the desired density gradient that can be used in plasma physics experiments using high-energy lasers. X-ray tomography was used for the characterization of these aerogels and it is demonstrated that it is the most reliable and quick method for characterization of gradient density aerogels. Divinyl benzene aerogels samples were synthesized by Lewis acid catalysis and samples were supercritically dried, characterized and their parameters measured to realize that the necessary properties were achieved. The change in density from solid density to 100 mg/cm³ is registered and the accuracy is evaluated.     

化学改性氧化石墨烯交联的聚酰亚胺气凝胶

聚酰亚胺(PI)气凝胶是一类密度低、机械性能好、隔热性能优异的多孔材料, 通常使用昂贵的化学交联剂进行交联. 氧化石墨烯(GO)是近年来广受关注的用于聚合物增强的纳米功能填料. 以前报道的PI/GO 复合材料多是纤维或膜的形式. 为了获得PI/GO 复合气凝胶, 本文采用化学改性氧化石墨烯(m-GO)替代1,3,5-三(4-氨基苯氧基)苯(TAB)等常规的交联剂, 使之与4,4'-二氨基二苯基醚(ODA)和3,3',4,4'-联苯四羧酸二酐(BPDA)反应, 制得了m-GO交联的PI 气凝胶. GO的化学改性通过其与过量ODA在水热条件下反应实现. 通过扫描电子显微镜(SEM)研究了PI/m-GO气凝胶的微观结构. 分别通过氮气吸脱附测试、热重分析和热线法研究了m-GO对气凝胶的孔特性、热稳定性和热导率的影响. 测试结果表明, 所获得的PI/m-GO气凝胶保持了高的孔隙率、热稳定性和绝热性. 压缩测试结果显示, 与采用1.8% (质量分数, w)的TAB进行交联的PI 气凝胶相比,仅用0.6% (w)的m-GO交联所获得的气凝胶具有更高的比杨氏模量(杨氏模量/密度)、比屈服强度(屈服强度/密度)和更小的体积收缩率.     

Characterization of the microstructures of copper-based aerogels on the sol–gel process

In this thesis, we will elaborate on the sol–gel process during the preparation of monolithic copper-based aerogel. The microstructure of the copper-based aerogel appears to be various due to the different amounts of raw materials, such as polyacrylic acid, propene oxide, deionized water (H₂O) and copper(II) chloride (CuCl₂) in the sol–gel process. The proper molar ratios between these reactants play a crucial factor in mediating the morphology of the aerogel. The aerogels are characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy and Brunauer–Emmett–Teller methods. The combined results indicate that the copper-based aerogel shows a typical three-dimensional porous structure with a large surface areas about 568 m²/g, and the skeleton structure of the aerogel is composed of a large number of primary particles with the size about a few nanometers.     

环氧化物法制备锑掺杂氧化锡气凝胶

以无机金属盐为前驱体,采用环氧丙烷添加法结合CO2超临界流体干燥和热处理工艺,制备了不同锑掺杂浓度的二氧化锡(ATO)气凝胶.所得气凝胶为深蓝色块体,平均密度约为600 mg?cm-3,锑掺杂浓度在5%到20%(x)之间.电子显微镜图片显示ATO气凝胶的骨架由粒径约为数十纳米的颗粒堆积而成,而这些颗粒又由数纳米的初级球形颗粒构成.X射线衍射谱表明,样品的主要晶相为SnO2四方相金红石结构,锑的掺杂仅引起微小的晶格畸变.X射线光电子谱显示锡元素以+4价态存在,而锑则具有+3和+5的混合价态.四探针电阻率测试仪的结果表明,ATO气凝胶的电阻率在2.7-40Ω?cm之间变化,其中在锑掺杂浓度(x)为12%时具有最低电阻率.     

Physical Properties of Silica Aerogels Prepared with Polyethoxydisiloxanes

Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, E-40), followed by supercritical drying with ethanol. The morphology and microstructure of the silica aerogels were investigated by using specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The thermal conductivity was also measured as a function of air pressure. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is 20–80 nm. The specific surface area of the silica aerogel is about 470 m²/g and the thermal conductivity of the silica aerogel prepared with E-40 is 0.014 w m^–1 K^–1 at room temperature and 1 atm.     

Gels dried under supercritical and ambient conditions: a comparative study and their subsequent conversion to silica–carbon composite aerogels

In present work, we have prepared gels with various compositions of methyltrimethoxysilane—3-(2,3-epoxypropoxy) propyltrimethoxysilane (MTMS-GPTMS) using a two-step acid base sol–gel process. To make a comparative study between the two common drying routes, we prepared gels under supercritical and also under ambient conditions. The density of the supercritically dried hybrid aerogels lies between 0.18 and 0.31 gcm^?3, while the density of the ambient dried ones ranges between 0.35 and 0.42 gcm^?3. The surface area of MTMS-0.25 GPTMS aerogel dried under supercritical conditions, has been found to be 464 m² g^?1 with a pore volume and average pore diameter of 1.24 cm³ g^?1 and 11 nm respectively. The same composition dried under ambient conditions is found to have similar properties i.e. a BET surface area of 439 m² g^?1, pore volume of 1.22 cm³ g^?1 and average pore diameter of 11 nm. The aerogels were later pyrolyzed yielding silica/carbon composite aerogels. The pyrolized aerogels possessed a surface area as high as 207 m² g^?1 with a total pore volume of 0.98 cm³ g^?1. The pyrolysed aerogels were also calcined to yield carbon free materials.