A detailed study of the production of polysaccharide aerogel (bio-aerogel) particles from lab to pilot scale is surveyed in this article. An introduction to various droplets techniques available in ...the market is given and compared with the lab scale production of droplets using pipettes and syringes. An overview of the mechanisms of gelation of polysaccharide solutions together with non-solvent induced phase separation option is then discussed in the view of making wet particles. The main steps of particle recovery and solvent exchange are briefly described in order to pass through the final drying process. Various drying processes are overviewed and the importance of supercritical drying is highlighted. In addition, we present the characterization techniques to analyse the morphology and properties of the aerogels. The case studies of bio-aerogel (agar, alginate, cellulose, chitin, κ-carrageenan, pectin and starch) particles are reviewed. Potential applications of polysaccharide aerogel particles are briefly given. Finally, the conclusions summarize the prospects of the potential scale-up methods for producing bio-aerogel particles.
Aerogels of polysaccharides with chelating functions can be useful as supports in many applications because of their hosting properties. This work demonstrates a facile method for the preparation of ...aerogels of chitosan derivative “ureido‐chitin”, containing ureido functional groups. The nanofibrillar networks of “ureido‐chitin” were produced by the nucleophilic addition of amine groups of chitosan with isocyanic acid prepared in situ. The presence of ureido functional groups was confirmed by FTIR, solid‐state CP‐MAS 13C and 15N NMR analyses. No crosslinking of molecular chains was observed. The maximum degree of ureido functional groups was estimated to be about 66 %. Characterization by powder XRD confirmed that the polymer chains self‐assembled, with the chitin polymers oriented in a highly ordered crystalline structure. The nanofibrillar networks showed no solubility under either aqueous acidic or alkaline conditions. Bio‐based hosting materials of this kind with two different hosting functional groups, ureido and amine, can potentially be utilized for a wide range of applications in aqueous medium, including filters, catalysis and biomedicine.
Adding function: This work demonstrates a facile method for the preparation of “ureido‐chitin” aerogels containing ureido (neutral) and amine (polarizable) moieties as chelating functional groups (see figure).
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•First systematic cellulose aerogel fiber extrusion experiments were carried out.•A twin screw extrusion facility was used for preparation of cellulose aerogel fibers.•The ...microstructure can be described as an open porous network of nano-fibrils.•The higher the extrusion temperature, the higher the specific surface area.•Systematic study on aerogel filaments was made for textile and non-textile applications.
The preparation, production and properties of light weight, porous cellulose aerogels in the form of thin extruded fibers is compared to monolithic pieces. The cellulose aerogels were synthesized from microcrystalline cellulose in a hydrated calciumthiocyanate salt melt, which upon cooling forms a gel at around 80°C. Twin screw extrusion experiments were performed systematically yielding thin and wet cellulose filaments. Washing and coagulation of the wet gels in ethanol was followed by supercritical drying with CO2 yielding cellulose aerogel filaments. These were characterized with regard to envelope density, nitrogen adsorption-desorption (BET) analysis, thermal conductivity measurements, tensile and compression tests and scanning electron microscopy (SEM). The microstructure can be described as an open porous network of nano-fibrils with pore sizes ranging from 10 to 100nm and fibril diameters of around 10 to 25nm. The densities of supercritically dried (SCD) cellulose aerogels were in the range of 0.009–0.137g/cm3 and the BET specific surface areas (SSABET) were between 120 and 230m2/g. The cellulose aerogel possessed thermal conductivities from 0.04 to 0.075W/m.K and compressive moduli up to 16.2MPa. The tensile strength of aerogel filaments increases with the increasing cellulose amount in the spin dope. Extruded cellulose aerogel filaments show a dependency of their specific surface area on the extrusion temperature: the higher the spinning temperature the higher the surface area.
The properties of silica aerogels, and the ability to prepare them by ambient pressure drying, hinge on their chemical surface modification. Until now, quantitative analysis of the surface chemistry ...has not been possible by state-of-the-art analytical methods. Here, we determine the surface chemistry of six archetypal, ambient pressure dried, hydrophobic silica aerogels and open-porous foams with quantitative 1H, 13C, and 29Si and 1H–29Si heteronuclear solid-state NMR spectroscopy. The quality of the external calibration, the validation by elemental analysis, and the consistency among the 1H, 13C, and 29Si MAS NMR data enable us to robustly quantify the surface chemistry of these classical silica materials. Four aerogels were derived from tetraethoxysilane (TEOS), polyethoxydisiloxane (PEDS), or waterglass precursors and hydrophobized with trimethylsilyl (TMS) groups through immersion in hexamethyldisilazane (HMDZ) in heptane, trimethylchlorosilane (TMCS) in heptane, or hexamethyldisiloxane (HMDSO) in ethanol. The resulting aerogels display remarkably similar chemistries despite their very different precursors (alkoxides, waterglass), hydrophobization agents (HMDZ, TMCS, HMDSO), and gelation and hydrophobization solvents (water, ethanol, heptane). The TMS content is nearly the same (22–27 wt %), the ethoxy content is low (<4 wt %), and the silica speciation displays a narrow range for all four aerogels. The 1H–29Si heteronuclear correlation spectra provide unambiguous evidence for the grafting of the TMS groups on the silica surfaces. Remarkably, the NMR data enable the prediction of the silica particle size to within 1 to 2 nm of that derived by TEM analysis. Two additional, inherently hydrophobic, ambient pressure dried, open-porous silica foams were synthesized from methyltrimethoxysilane (MTMS) and from a MTMS–dimethyldimethoxysilane (DMDMS) mixture. The reduction in the number of bridging oxygens per Si atom, from 3.76 to 3.90 for TEOS, PEDS, or waterglass based aerogels, to 2.86 for the MTMS based open-porous foam, and to 2.48 for the MTMS–DMDMS derived open-porous foam, results in a brittle-to-superflexible transition. Our results highlight the importance of the chemical treatment of the wet gels (hydrophobization and solvent exchange) on the final surface chemistry of silica aerogels.
To address the challenge of reconstructing or designing the three-dimensional microstructure of nanoporous materials, we develop a computational approach by combining the random closed packing of ...polydisperse spheres together with the Laguerre-Voronoi tessellation. Open-porous cellular network structures that adhere to the real pore-size distributions of the nanoporous materials are generated. As an example, κ-carrageenan aerogels are considered. The mechanical structure-property relationships are further explored by means of finite elements. Here we show that one can predict the macroscopic stress-strain curve of the bulk porous material if only the pore-size distributions, solid fractions, and Young's modulus of the pore-wall fibres are known a priori. The objective of such reconstruction and predictive modelling is to reverse engineer the parameters of their synthesis process for tailored applications. Structural and mechanical property predictions of the proposed modelling approach are shown to be in good agreement with the available experimental data. The presented approach is free of parameter-fitting and is capable of generating dispersed Voronoi structures.
The aim of this work is to develop a method to produce spherical biopolymer-based aerogel particles, which is capable for scale-up in the future. Therefore, the jet cutting method is suggested. ...Amidated pectin, sodium alginate, and chitosan are used as a precursor (a 1⁻3 wt. % solution) for particle production via jet cutting. Gelation is realized via two methods: the internal setting method (using calcium carbonate particles as cross-linkers and citric and acidic acid for pH adjustment) and the diffusion method (in calcium chloride solutions). Gel particles are subjected to solvent exchange to ethanol and consequent supercritical drying with CO₂. Spherical aerogel particles with narrow particle size distributions in the range of 400 to 1500 µm and a specific surface area of around 500 m²/g are produced. Overall, it can be concluded that the jet cutting method is suitable for aerogel particle production, although the shape of the particles is not perfectly spherical in all cases. However, parameter adjustment might lead to even better shaped particles in further work. Moreover, the biopolymer-based aerogel particles synthesized in this study are tested as humidity absorbers in drying units for home appliances, particularly for dishwashers. It has been shown that for several cycles of absorption and desorption of humidity, aerogel particles are stable with an absorption capacity of around 20 wt. %.
Within this study, monolithic three-dimensional silica aerogel (SA) composite parts with super insulating properties are presented. A generic part based on fiber-reinforced (FR) silica aerogel for ...thermal insulation of the exhaust tubing system—to keep the exhaust gases as hot as possible to improve the efficiency of the catalyst system—was produced via a sol-gel-based molding process in combination with a supercritical drying using scCO2. A thermal conductivity of 16 mW m−1 K−1 was measured via a heat flow meter technique. In this manuscript, we present a full cycle of the material compound design, starting with fundamental material evaluation including aerogel optimization, opacifier influence, and casting process. The obtained generic part in shape of a half-shell for pipe insulation is characterized under real conditions.
Organic aerogels in the form of powder, microgranules and microsized particles receive considerable attention due to their easy fabrication, low process time and costs compared to their monolithic ...form. Here, we developed resorcinol-formaldehyde (RF) aerogel microparticles by using an emulsion-gelation method. The main objective of this study is to investigate the influence of curing time, stirring rate, RF sol:oil ratio and initial pH of the sol in order to control the size and properties of the microparticles produced. The emulsion-gelation of RF sol prepared with sodium carbonate catalyst in an oil phase at 60 °C was explored. RF microparticles were washed with ethanol to remove the oil phase followed by supercritical and ambient pressure drying. The properties of the dried RF microparticles were analyzed using FT-IR, N2 adsorption isotherm, gas pycnometry, wide angle X-ray scattering and scanning electron microscope. RF microparticles with high surface area up to 543 m2/g and large pore volume of 1.75 cm3/g with particle sizes ranging from 50–425 µm were obtained.
The solvent exchange as one of the most important steps during the manufacturing process of organic aerogels was investigated. This step is crucial as a preparatory step for the supercritical drying, ...since the pore solvent must be soluble in supercritical carbon dioxide to enable solvent extraction. The development and subsequent optimization of a suitable system with a peristaltic pump for automatic solvent exchange proved to be a suitable approach. In addition, the influence of zeolites on the acceleration of the process was found to be beneficial. To investigate the process, the water content in acetone was determined at different times using Karl Fischer titration. The shrinkage, densities, as well as the surface areas of the aerogels were analyzed. Based on these, the influence of various process parameters on the final structure of the obtained aerogels was investigated and evaluated. Modeling on diffusion in porous materials completes this study.
In the past decade, biopolymer aerogels have gained significant research attention due to their typical properties, such as low density and thermal insulation, which are reinforced with excellent ...biocompatibility, biodegradability, and ease of functionalization. Mechanical properties of these aerogels play an important role in several applications and should be evaluated based on synthesis parameters. To this end, preparation and characterization of polysaccharide-based aerogels, such as pectin, cellulose and k-carrageenan, is first discussed. An interrelationship between their synthesis parameters and morphological entities is established. Such aerogels are usually characterized by a cellular morphology, and under compression undergo large deformations. Therefore, a nonlinear constitutive model is proposed based on large deflections in microcell walls of the aerogel network. Different sizes of the microcells within the network are identified via nitrogen desorption isotherms. Damage is initiated upon pore collapse, which is shown to result from the failure of the microcell wall fibrils. Finally, the model predictions are validated against experimental data of pectin, cellulose, and k-carrageenan aerogels. Given the micromechanical nature of the model, a clear correlation-qualitative and quantitative-between synthesis parameters and the model parameters is also substantiated. The proposed model is shown to be useful in tailoring the mechanical properties of biopolymer aerogels subject to changes in synthesis parameters.
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