Polyurethane-silica hybrid foam by sol–gel approach: Chemical and functional properties

Enhancing Flame-Retardant Properties of Polyurethane Aerogels Doped with Silica-Based Particles

In this work, polyurethane (PUR) aerogels doped with different SiO2 particles, derived from a renewable source, were successfully synthesized, and the effects of SiO2 content on the properties of PUR aerogels were investigated. Specifically, three types of SiO2-based particles obtained from rice husk through different procedures were evaluated to enhance the thermal stability of the composites with special attention given to flame-retardant properties. With the optimal SiO2 particles, obtained through acid digestion, the influence of their content between 0.5 and 3 wt.% on the physicochemical characteristics of the synthesized aerogels was thoroughly examined. The results showed that increasing the doping agent content improved the lightness, thermal stability, and flame-retardant properties of the resulting PUR aerogels, with the best performance observed at a 2 wt.% doping level. The doped aerogel samples with non-modified SiO2 particles significantly enhanced the fire safety performance of the material, exhibiting up to an eightfold increase in flame retardancy. However, modification of the SiO2 particles with phytic acid did not slow down the combustion velocity when filling the aerogels. This research highlights the promising potential of doped PUR/SiO2 aerogels in advancing materials science and engineering applications for withstanding high temperatures and improving fire safety.

Balanced Thermal Insulation, Flame-Retardant and Mechanical Properties of PU Foam Constructed via Cost-Effective EG/APP/SA Ternary Synergistic Modification

To address the challenge of balancing the mechanical, thermal insulation, and flame-retardant properties of building insulation materials, this study presented a facile approach to modify the rigid polyurethane foam composites (RPUFs) via commercial expandable graphite (EG), ammonium polyphosphate (APP), and silica aerogel (SA). The resulting EG/APP/SA/RPUFs exhibited low thermal conductivity close to neat RPUF. However, the compressive strength of the 6EG/2APP/SA/RPUF increased by 49% along with achieving a V-0 flame retardant rating. The residual weight at 700 °C increased from 19.2 wt.% to 30.9 wt.%. Results from cone calorimetry test (CCT) revealed a 9.2% reduction in total heat release (THR) and a 17.5% decrease in total smoke production (TSP). The synergistic flame-retardant mechanism of APP/EG made significant contribution to the excellent flame retardant properties of EG/APP/SA/RPUFs. The addition of SA played a vital role in reducing thermal conductivity and enhancing mechanical performance, effectively compensating for the shortcomings of APP/EG. The cost-effective EG/APP/SA system demonstrates a positive ternary synergistic effect in achieving a balance in RPUFs properties. This study provides a novel strategy aimed at developing affordable building wall insulation material with enhanced safety features.

Polyols and Polyurethane Foams Obtained from Mixture of Metasilicic Acid and Cellulose

Hydroxyalkylation of the mixture of metasilicic acid and cellulose with glycidol and ethylene carbonate leads to a polyol suitable to obtain rigid polyurethane foams. The composition, structure, and physical properties of the polyol were studied in detail. The obtained foams have apparent density, water absorption, and polymerization shrinkage, as well as heat conduction coefficients similar to conventional, rigid polyurethane foams. The polyols and foams obtained from environmentally unobtrusive substrates are easily biodegradable. Additionally, the obtained foams have high thermal resistance and are self-extinguishing. Thermal exposure of the foams leads to an increase of the compressive strength of the material and further reduces their flammability, which renders them suitable for use as heat insulating materials.

Multifunctional SiC@SiO2 Nanofiber Aerogel with Ultrabroadband Electromagnetic Wave Absorption

Traditional ceramic materials are generally brittle and not flexible with high production costs, which seriously hinders their practical applications. Multifunctional nanofiber ceramic aerogels are highly desirable for applications in extreme environments, however, the integration of multiple functions in their preparation is extremely challenging. To tackle these challenges, we fabricated a multifunctional SiC@SiO₂ nanofiber aerogel (SiC@SiO₂ NFA) with a three-dimensional (3D) porous cross-linked structure through a simple chemical vapor deposition method and subsequent heat-treatment process. The as-prepared SiC@SiO₂ NFA exhibits an ultralow density (~ 11 mg cm^- 3), ultra-elastic, fatigue-resistant and refractory performance, high temperature thermal stability, thermal insulation properties, and significant strain-dependent piezoresistive sensing behavior. Furthermore, the SiC@SiO₂ NFA shows a superior electromagnetic wave absorption performance with a minimum refection loss (RL_min) value of - 50.36 dB and a maximum effective absorption bandwidth (EAB_max) of 8.6 GHz. The successful preparation of this multifunctional aerogel material provides a promising prospect for the design and fabrication of the cutting-edge ceramic materials.

Preparation and Properties of Polyurethane Composite Foams with Silica-Based Fillers

Polyurethane composite foams were prepared by adding three different types of silica materials as a filler to improve the mechanical and thermal insulation properties. The first type of filler consists of silica aerogels with high-volume pores, with the expectation of improving the thermal insulation of PU foams because silica aerogel itself has superior thermal insulation properties. Silica nanoparticle is used for the second type that has a size very similar to the pore size of silica aerogels for comparison. The last type to produce polyurethane composite foam uses a sol–gel reaction to produce polysiloxane that reacts with polyols during the urethane reaction and forming process. In particular, in the case of silica aerogels and nanoparticles, their surfaces are modified with APTES and then polymeric methylene diphenylene diisocyanate (PMDI) to increase the interaction between the polymer matrix and inorganic fillers. The polyurethane foam structure was successfully produced in all cases of composite foams. As expected, the mechanical properties and the thermal insulation effect were enhanced by the addition of silica fillers, but found to be closely related to the cell structure of polyurethane foams. The addition of small amounts of inorganic fillers improves the mechanical and thermal properties, but the higher the amount of filler, the worse they are due to the agglomeration of fillers on the cell walls. The dispersion of added inorganic fillers within the foam cells should be controlled effectively. Surface-modified silica fillers exhibit better enhancement of mechanical and thermal insulation properties.

Development and Characterization of Tailored Polyurethane Foams for Shock Absorption

In this paper, different types of polyurethane foams (PUR) having various chemical compositions have been produced with a specific density to monitor the microstructure as much as possible. The foam may have a preferential orientation in the cell structure. The cellular polyurethane tends to have stubborn, typical cellular systems with strong overlap reversibility. Free expansion under atmospheric pressure enables formulas to grow until they are refined. Moreover, the physicochemical characterization of the developed foams was carried out. They later are described by apparent density, Shore hardness, Raman spectroscopy analysis, X-ray diffraction analysis, FTIR, TGA, DSC, and compression tests. The detailed structural characterization was used by scanning electron microscope (SEM) and an optical microscope (MO) to visualize the alveolar polymer’s semi-opened cells, highlighting the opened-cell morphology and chemical irregularities. Polyurethane foams with different structural variables have a spectrum characterization that influences the phase separation and topography of polyurethane foam areas because their bonding capability with hydrogen depends on chain extender nature. These studies may aid in shock absorption production; a methodology of elaboration and characterization of filled polyurethane foams is proposed.

Anisotropic porous ceramic material with hierarchical architecture for thermal insulation

Porous ceramic materials are attractive candidates for thermal insulation. However, effective ways to develop porous ceramics with high mechanical and thermal insulation performances are still lacking. Herein, an anisotropic porous silica ceramic with hierarchical architecture, i.e. long-range aligned lamellar layers composed of hollow silica spheres, was fabricated applying a facile bidirectional freezing method. Due to such anisotropic structure, the as-prepared porous silica ceramic displays low thermal conductivity across the layers and high compressive strength along the layers. Additionally, the anisotropic porous silica ceramic is fire-resistant. As a proof of concept, a mini-house was roofed with the anisotropic porous silica ceramic, showing that the indoor temperature could be stabilized against environmental temperature change, making this porous ceramic a promising candidate for energy efficient buildings and other industrial applications. Our study highlights the possibility of combining intrinsically exclusive properties in engineering materials through constructing biomimetic porous structures.

Si3N4 Nanofibrous Aerogel with In Situ Growth of SiOx Coating and Nanowires for Oil/Water Separation and Thermal Insulation

Nanofibrous aerogels constructed by ceramic fiber components (CNFAs) feature lightweight, compressibility, and high-temperature resistance, which are superior to brittle ceramic aerogels assembled from nanoparticles. Up to now, in order to obtain CNFAs with stable framework and multifunctionality such as hydrophobicity and gas absorption, it is necessary to perform binding and surface modification processes, respectively. However, the microstructure as well as properties of CNFAs are deteriorated by the direct addition of binders and modifiers. To tackle these problems, we introduced a unique low-temperature (100 °C) chemical vapor deposition method (LTCVD) to achieve the cross-linking and hydrophobization of Si₃N₄ CNFA in only one step. More importantly, during the LTCVD process, SiO_x coatings and nanowire arrays were in situ formed via vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms on the surface and intersection of Si₃N₄ nanofibers, which cemented the aerogel framework, endowed it with hydrophobicity, and improved its oxidation resistance at high temperature. Compared to most of its counterparts, the Si₃N₄/SiO_x CNFA exhibited better mechanical properties, higher capability of oil/water separation (33-76 g·g^-1), lower thermal conductivity (0.0157 W/m·K^-1), and superior structural stability in a wide temperature range of -196-1200 °C. This work not only presents an excellent Si₃N₄/SiO_x CNFA for the first time but also provides fresh insights for the exquisite preparation strategy of CNFAs.

Advances in precursor system for silica-based aerogel production toward improved mechanical properties, customized morphology, and multifunctionality: A review

Conventional silica-based aerogels are among the most promising materials considering their special properties, such as extremely low thermal conductivity (~15 mW/mK) and low-density (∼0.003-0.5 g.cm^-3) as well as high surface area (500-1200 m². g^-1). However, they have relatively low mechanical properties and entail extensive and energy-consuming processing steps. Silica-based aerogels are mostly fragile and possess minimal mechanical properties as well as a long processing procedure which hinders their application range. The key point in improving the mechanical properties of such a material is to increase the connectivity in the aerogel backbone. Several methods of mechanical improvement of silica-based aerogels have been explored by researchers such as (i) use of flexible silica precursors in silica gel backbone, (ii) surface-crosslinking of silica particles with a polymer, (iii) prolonged aging step in different solutions, (iv) distribution of flexible nanofillers into the silica solution prior to gelation, and, most recently, (v) polymerizing the silica precursor prior to gelation. The polymerized silica precursor, as in the most recent approach, can be gelled either by binodal decomposition (nucleation and growth), resulting in a particulate structure, or by spinodal decomposition, resulting in a non-particulate structure. By optimizing the material composition and processing conditions of materials, the aerogel can be tailored with different functional capabilities. This review paper presents a literature survey of precursor modification toward increased connectivity in the backbone, and the synthesis of inorganic and hybrid systems containing siloxane in the backbone of the silica-based aerogels and its composite version with carbon nanofillers. This review also explains the novel properties and applications of these material systems in a wide area. The relationship among the materials-processing-structure-properties in these kinds of aerogels is the most important factor in the development of aerogel products with given morphologies (particulate, fiber-like, or non-particulate) and their resultant properties. This approach to advancing precursor systems leads to the next-generation, multifunctional silica-based aerogel materials.

Greener Nanocomposite Polyurethane Foam Based on Sustainable Polyol and Natural Fillers: Investigation of Chemico-Physical and Mechanical Properties

Nowadays, the chemical industry is looking for sustainable chemicals to synthesize nanocomposite bio-based polyurethane foams, PUs, with the aim to replace the conventional petrochemical precursors. Some possibilities to increase the environmental sustainability in the synthesis of nanocomposite PUs include the use of chemicals and additives derived from renewable sources (such as vegetable oils or biomass wastes), which comprise increasingly wider base raw materials. Generally, sustainable PUs exhibit chemico-physical, mechanical and functional properties, which are not comparable with those of PUs produced from petrochemical precursors. In order to enhance the performances, as well as the bio-based aspect, the addition in the polyurethane formulation of renewable or natural fillers can be considered. Among these, walnut shells and cellulose are very popular wood-based waste, and due to their chemical composition, carbohydrate, protein and/or fatty acid, can be used as reactive fillers in the synthesis of Pus. Diatomite, as a natural inorganic nanoporous filler, can also be evaluated to improve mechanical and thermal insulation properties of rigid PUs. In this respect, sustainable nanocomposite rigid PU foams are synthesized by using a cardanol-based Mannich polyol, MDI (Methylene diphenyl isocyanate) as an isocyanate source, catalysts and surfactant to regulate the polymerization and blowing reactions, H₂O as a sustainable blowing agent and a suitable amount (5 wt%) of ultramilled walnut shell, cellulose and diatomite as filler. The effect of these fillers on the chemico-physical, morphological, mechanical and functional performances on PU foams has been analyzed.

Nanofibrous Aerogel Bulk Assembled by Cross-Linked SiC/SiOx Core-Shell Nanofibers with Multifunctionality and Temperature-Invariant Hyperelasticity

Nanofibrous aerogels constructed solely by ceramic components with temperature-invariant hyperelasticity could have broad technological implications in extreme environments. However, creating such materials has proven to be extremely challenging. Despite the results from laboratory, those aerogels are, unfortunately, still plagued with issues that would retard their further application: inferior structural integrity, failure at large compressive deformation, high production cost, and inability to withstand rigorous working conditions. To tackle these challenges, we report a facile strategy combining the chemical vapor deposition process and layer-by-layer self-assembly to construct hyperelastic SiC nanofibrous aerogels with three-dimensional porous architecture and improved structural integrity. The resultant aerogels outperform their natural counterparts and most state-of-the-art ceramic nanofibrous aerogels in their capability to quickly recover from large compressive deformation (50% strain), function in a wide range of temperatures, from -196 °C to 1100 °C in air, maintain high particle matter removal efficiency of >99.96%, and rapidly absorb various organic solvents and oils with high capacity and robust recoverability. Nanofibrous aerogels constructed by such a versatile method could provide fresh insights into the exploration of multifunctional nanofibrous aerogels for a variety of applications in extreme environments.

The role of hollow silica nanospheres and rigid silica nanoparticles on acoustic wave absorption of flexible polyurethane foam nanocomposites

Pure polyurethane foam and nanocomposite foam are used to absorb sound. In this study, hollow silica nanospheres and rigid silica nanoparticles were added to the polyurethane matrix and their sound absorption properties were investigated by impedance tube and compared with pure polyurethane foam. Reinforcement phase influences on the morphology of the matrix were studied by scanning electron microscopy. Due to greater effects of the rigid silica nanoparticles on the morphology of the matrix, it was expected to increase the sound absorption coefficient of the rigid silica nanoparticles/polyurethane, more than hollow silica nanospheres/polyurethane, but the results show that the hollow silica nanospheres increased absorption coefficient of the composite more efficiently. The crust of hollow silica nanospheres increases the number of boundaries in a sound wave, and the air gap inside them cause the sound wave to damp. So the intrinsic property of the hollow silica nanospheres is more effective than the matrix morphology. Thus, by the same content of reinforcement in the matrix, hollow silica nanosphere/polyurethane sample with sound absorption coefficient of 0.87 for a thickness of 9 cm has the highest sound absorption coefficient compared to the rigid silica nanoparticles/polyurethane sample and pure polyurethane foam. In pure and nanocomposite samples, sound absorption coefficient increased by increasing the thickness of samples.

Fabrication of polysiloxane foam with a pendent phenyl group for improved thermal insulation capacity and thermal stability

This work reports on polysiloxane foam with a pendent phenyl group, which exhibits improved thermal insulation capacity and thermal stability.

Cold-Cured Epoxy-Based Organic⁻Inorganic Hybrid Resins Containing Deep Eutectic Solvents

The development of improved cold-cured resins, to be used as either adhesives or matrices for FRP (fiber reinforced polymer) composites employed in the construction industry, has become the focus of several academic and industrial research projects. It is expected that the use of nano-structured organic–inorganic hybrid materials could represent a realistic alternative to commercial epoxy-based resins due to their superior properties, especially in terms of higher durability against: moisture, temperatures, harsh environments, and fire. In this context, organic–inorganic epoxy hybrids were synthesized by a modified sol–gel method without the addition of water. The experimental formulations were prepared starting from a mixture of a silane-functionalized epoxy resin, alkoxysilane components and a deep eutectic solvent (DES) based on a blend of choline chloride and urea. The latter was added in two different loads in order to analyze in depth its effect as a promoter for an effective dispersion of silica nano-phases, formed through hydrolysis and condensation reactions, into the cross-linked epoxy network. The produced formulations were cold-cured for different time spans in the presence of two hardeners, both suitable for a curing process at ambient temperature. In this first part of a wider experimental program, several analyses were carried out on the liquid (rheological and calorimetric) and cold-cured (calorimetric, thermogravimetric, dynamic-mechanical, flexural mechanical, and morphological) systems to evaluate and quantify the improvement in properties brought about by the presence of two different phases (organic and inorganic) in the same epoxy-based hybrid system.

Hierarchical Porous Polyamide 6 by Solution Foaming: Synthesis, Characterization and Properties

Porous polym er materials have received great interest in both academic and industrial fields due to their wide range of applications. In this work, a porous polyamide 6 (PA6) material was prepared by a facile solution foaming strategy. In this approach, a sodium carbonate (SC) aqueous solution acted as the foaming agent that reacted with formic acid (FA), generating CO₂ and causing phase separation of polyamide (PA). The influence of the PA/FA solution concentration and Na₂CO₃ concentration on the microstructures and physical properties of prepared PA foams were investigated, respectively. PA foams showed a hierarchical porous structure along the foaming direction. The mean pore dimension ranged from hundreds of nanometers to several microns. Low amounts of sodium salt generated from a neutralization reaction played an important role of heterogeneous nucleation, which increased the crystalline degree of PA foams. The porous PA materials exhibited low thermal conductivity, high crystallinity and good mechanical properties. The novel strategy in this work could produce PA foams on a large scale for potential engineering applications.

Bio-based Large Tablet Controlled-Release Urea: Synthesis, Characterization, and Controlled-Released Mechanisms

To improve nitrogen (N) use efficiency and minimize environmental pollution caused by fertilizer overuse, novel bio-based large tablet controlled-release urea (LTCRU) was prepared using bio-based coating materials to coat large tablet urea (LTU) derived from urea prills (U). Nano fumed silica (NFS) was added to the bio-based coating materials to improve the slow-release properties. The surface area of the LTU and U was measured by three-dimensional scanning. In comparison to U, LTU had a smaller surface area/weight ratio, which can reduce the coating materials. Scanning electron microscopy analysis showed that the addition of NFS in bio-based coating materials reduced the porosity of the coating shells of LTCRUs and, thus, enhanced the N release longevity of the controlled-released fertilizer. Dependent upon the pores on the coating shells of LTCRU, two N release patterns were revealed. Because of the good release characteristics, the novel LTCRU shows great potential to support sustainable agricultural production.

In-Situ Incorporation of Alkyl-Grafted Silica into Waterborne Polyurethane with High Solid Content for Enhanced Physical Properties of Coatings

Waterborne polyurethane (WPU) with high solid content (45%) was obtained by utilizing dimethylol propionic acid (DMPA) and ethoxylated capped polymeric diol as complex hydrophilic groups. Alkyl-grafted silica was incorporated into polymer matrix through in situ polymerization to improve the performance of coatings casted from WPU dispersions. The addition of alkyl-grafted silica enlarged the particle size distribution whilst increased emulsion viscosity, which showed little influence on attainment of high solid content for WPU. The properties of obtained WPU/Silica coatings were investigated. Results showed that the functionalized surface of silica provides good compatibility with the WPU matrix, which promoted the homogeneous dispersion of silica particles. This facilitated the formation of nanosized silica papillae on coatings, contributing to surface roughness and hydrophobicity. Solvent resistance of WPU was enhanced with existence of alkyl-grafted silica particles. The WPU/Silica coatings also displayed improved thermal stability due to the thermal insulation ability and tortuous path effect of silica. Besides this, valid interactions between silica and WPU resulted in hybrid microphase of which the synergistic effect imparted superior mechanical properties at relatively low loadings of silica (2%). The facile technique presented here will provide an effective and promising method for preparing WPU hybrids with enhanced performance.

Ultralight and fire-resistant ceramic nanofibrous aerogels with temperature-invariant superelasticity

Ultralight aerogels that are both highly resilient and compressible have been fabricated from various materials including polymer, carbon, and metal. However, it has remained a great challenge to realize high elasticity in aerogels solely based on ceramic components. We report a scalable strategy to create superelastic lamellar-structured ceramic nanofibrous aerogels (CNFAs) by combining SiO2 nanofibers with aluminoborosilicate matrices. This approach causes the random-deposited SiO2 nanofibers to assemble into elastic ceramic aerogels with tunable densities and desired shapes on a large scale. The resulting CNFAs exhibit the integrated properties of flyweight densities of >0.15 mg cm-3, rapid recovery from 80% strain, zero Poisson's ratio, and temperature-invariant superelasticity to 1100°C. The integral ceramic nature also provided the CNFAs with robust fire resistance and thermal insulation performance. The successful synthesis of these fascinating materials may provide new insights into the development of ceramics in a lightweight, resilient, and structurally adaptive form.

Thermal insulation and stability of polysiloxane foams containing hydroxyl-terminated polydimethylsiloxanes

An effective method was described here to improve the thermal insulation and stability of polysiloxane foam (SIF) by controlling the chain length of hydroxyl-terminated polydimethylsiloxane (OH-PDMS). A series of SIFs were prepared through foaming and cross-linking processes with different cross-linking densities. The morphology of SIF was investigated by environmental scanning electron microscopy. The results demonstrated that increasing the chain length of OH-PDMS reduced the average cell size from 932 μm to 220 μm. Cell density ranged from 4.92 × 10⁶ cells per cm³ to 1.64 × 10⁸ cells per cm³. The thermal insulation capability was significantly enhanced, and the SIF derived from the long-chain OH-PDMSs yielded a minimum thermal conductivity of 0.077 W mK⁻¹. Cell size reduction and an increase in cell density were considered to be the main factors to reduce thermal conductivity. Thermal stability, which was also improved, mainly depended on the free motion rate of the polysiloxane chains and cross-linking density of the polysiloxane networks.

The thermal insulation and stability of polysiloxane foam was improved by an easy operating method.

Silica Foams for Fire Prevention and Firefighting

We report the new development of fire-extinguishing agents employing the latest technology of fighting and preventing fires. The in situ technology of fighting fires and explosions involves using large-scale ultrafast-gelated foams, which possess new properties and unique characteristics, in particular, exceptional thermal stability, mechanical durability, and full biocompatibility. We provide a detailed description of the physicochemical processes of silica foam formation at the molecular level and functional comparison with current fire-extinguishing and fire-fighting agents. The new method allows to produce controllable gelation silica hybrid foams in the range from 2 to 30 s up to 100 Pa·s viscosity. Chemical structure and hierarchical morphology obtained by scanning electron microscopy and transmission electron microscopy images develop thermal insulation capabilities of the foams, reaching a specific heat value of more than 2.5 kJ/(kg·°C). The produced foam consists of organized silica nanoparticles as determined by X-ray photoelectron spectroscopy and X-ray diffraction analysis with a narrow particle size distribution of ∼10-20 nm. As a result of fire-extinguishing tests, it is shown that the extinguishing efficiency exhibited by silica-based sol-gel foams is almost 50 times higher than that for ordinary water and 15 times better than that for state-of-the-art firefighting agent aqueous film forming foam. The biodegradation index determined by the time of the induction period was only 3 d, while even for conventional foaming agents this index is several times higher.

Preparation of a liquefied banana pseudo-stem based PVAc-nanosilica hybrid membrane and its modification by octadecyltrichlorosilane

A hydrophobic LBP–PVAc nanocomposite membrane with enhanced mechanical properties was prepared by adding silica sol and treating with OTS.