Glass foams produced by glass waste,sodium hydroxide,and borax with several pore structures using factorial designs

Glass foams have great potential for several technological applications, for example, filters and thermal or acoustic insulators. Sodium hydroxide is an efficient foaming agent to obtain glass foams with high level of porosity. However, the control of variables that influences on structure, type, and size of pores of glass foams is necessary. This study evaluates the influence of composition and process parameters on glass foams based on soda-lime glass waste, sodium hydroxide, and borax. Experiments were conducted using factorial designs. According to the experimental conditions, bulk density varied from 0.16 to 0.79 g cm⁻³ and maximum porosity of 92%. Amounts of NaOH and borax in addition to sintering temperature are the main variables of the foaming process. The role of NaOH content is to reduce the density and closed porosity of glass foams associated with an increase in their open porosity. The addition of borax with low NaOH amount promoted densification and pore closure in glass foams. NaOH and borax content allows controlling the type of predominant porosity on foams (open or closed porosity). Glass foams were resistant to sulfuric acid, hydrochloric acid, and nitric acid in diluted solutions. These results allow controlling the pore structure of glass foams for different applications.     

Mullite ceramic foams with tunable pores from dual-phase sol nanoparticle-stabilized foams

Particle-stabilized foams employing dual-phase sol of boehmite in combination with silica to prepare mullite ceramic foams has been proposed for the first time. The obtained mullite ceramic foams possess hierarchical pores, that is micropores derived from the air bubble templates and open windows formed by grain growth at thin area of pore wall according to the mullitization reaction. Furthermore, nanoparticles favor the improvement of specific surface area of ceramic foams, and wrinkles caused by drying shrinkage would retain when sintering at low temperature of 900℃-1100℃, leading to high specific surface area of 94.4-219.2 m²/g. The achieved mullite ceramic foams present relatively high compressive strength of 6.6?40.4 MPa at a high open porosity of 64.0 %–87.0 %, and their thermal conductivity could reach as low as 0.10 W/(m·K), which would make them promising lightweight materials applied in broad fields including thermal insulations, filters, bio-scaffolds, catalyst supports and the like.     

Ultrahigh-strength alumina ceramic foams via gelation of foamed boehmite sol

Ceramic foams with significant porosity and robust mechanical properties have received extensive attentions. However, it is still difficult to achieve excellent compressive strength at high porosity levels. In this work, a gelation of foamed boehmite sol method is proposed to settle this issue. The technological parameters during preparation process are systematically investigated. As-prepared alumina ceramic foams possess unprecedentedly high compressive strength of 34.1-89.1 MPa at high porosity levels of 66.0%-87.2%, which is attributed to the present of hierarchical pore structure, small grain size and pore size. This work demonstrates a facile and novel method for the fabrication of high-performance alumina ceramic foams toward practical applications.     

3D printing boehmite gel foams into lightweight porous ceramics with hierarchical pore structure

We herein report a novel hierarchically porous ceramic foams derived from boehmite gel foams, which possess both high porosity and superior strength. The gel foams show excellent printability due to its predominant stability, high yield stress and storage modulus, which endows such foam material ideal ink for 3D printing lightweight and complex-shape materials via direct ink writing approach. The 3D printed ceramic foams possess programmable architecture assembled by porous filaments, uniform macro-pores with tunable size in the range of 4∼70 μm, as well as nanoscale pores in cell wall, after sintering at relatively low temperature of 1200–1300 °C. In this way, ceramic foams with high strength were achieved, attributed to the tiny grains, large amount of grain boundaries, uniform pores and hierarchical pore structure. Notably, the foams sintered below 1200 °C have significant advantage on specific surface area, which could reach up to 300-400 m²/g.     

Highly porous barium strontium titanate (BST) ceramic foams with low dielectric constant from particle‐stabilized foams

A novel method for fabrication of highly porous barium strontium titanate (BST) ceramic foams based on particle‐stabilized foaming method was developed for the first time, in which propyl gallate (PG) was employed as BST particle modifier. The results showed that the stability of wet BST foams closely depends on the pH value and PG concentration, which could be explained by the adsorption behavior of PG on BST particle surface. BST ceramic foams with dense, uniform, and closed pore and defect‐free wall were obtained. The pore size and porosity can be well controlled by adjusting solid loading and sintering temperature. It was revealed that not only sintering temperature but also solid loading significantly influenced the growth of BST grain. The BST ceramic foams exhibited high porosity in the range of 81%‐95%, low dielectric constant in the range of 47‐150, and low dielectric loss below 0.0025. The BST ceramic foams with higher porosity presented a tendency of lower dielectric constant and the fitting results indicated that the natural logarithm of dielectric constant was linear correlated with porosity.     

Epoxy‐based gelcasting of machinable hydroxyapatite foams for medical applications

Machinable hydroxyapatite foams for subtractive manufacturing of customized bone scaffolds have been developed. The foams were prepared by direct foaming of water‐based hydroxyapatite suspension with dissolved epoxy resin. The foams were consolidated by gelation of the epoxy‐based suspension in air atmosphere. The effect of processing conditions on the foam structure was investigated. The foams had a cellular‐like structure with interconnected pores. The sintered foam with 78% open porosity, the most frequent pore size 430 μm, and the most frequent interconnecting pore window size 150 μm, has been chosen as the foam with the optimal structure from the viewpoint of an application in bone regeneration. The compressive strength of this sintered foam reached 3.3 MPa. The machinability of the optimal foam was investigated using computer numerical controlled (CNC) milling of a test pattern. The milling tests were carried out with the foam at different processing stages and after impregnation with paraffin wax. The best milling result was obtained for a dried foam impregnated with paraffin. The applicability of the whole processing chain was demonstrated and a customized scaffold was manufactured.     

Hierarchically porous ceria with tunable pore structure from particle-stabilized foams

We firstly fabricated CeO₂ ceramic foams with tunable structure by using particle-stabilized bubbles as template, and designed their interconnected porous structure and even hierarchically porous structure, which endows them the penetration ability for gases or liquids. Hollow spheres with single-layer shell were innovatively selected as the pore-former, which allows for the formation of open pores on the cell wall. Moreover, 3D printing CeO₂ particle-stabilized foams are realized with the aid of direct ink writing, which enables the production of CeO₂ ceramic foams with complex shape. Highly porous CeO₂ with relatively high compressive strength have been fabricated, the porosity of which varies from 81.0% to 92.0% while their excellent compressive strength ranges from 5.0 MPa to 20.0 MPa. Attributed to the hierarchical porous structure, uniform pore size distribution as well as densely assembled cell wall, 3D printing CeO₂ ceramic foams possess superior mechanical performance at high porosity level.     

In situ synthesis of three-dimensional nanofiber-knitted ceramic foams via reactive sintering silicon foams

Three-dimensional ceramic nanofiber-assembled materials with large specific surface area and excellent thermal insulation properties are attracting increasing interests for their unique structure and promising applications. In this paper, we propose a facile methodology to fabricate three-dimensional silicon nitride nanofiber-knitted ceramic foams via in situ reactive synthesis from silicon foams. Silicon particle-stabilized foams are fabricated for the first time using long-chain surfactant cetyltrimethyl ammonium bromide as a hydrophobic modifier. First, the fabrication and stability of silicon foams are investigated. Based on the stable silicon foams, silicon nitride-based nanofiber-knitted ceramic foams are synthesized via in situ reactive sintering in nitrogen atmosphere. The novel ceramic foam materials consist of three-dimensional nanofiber-assembled strut wall and nanofiber-spheres in the pores. The diameter of obtained silicon nitride nanofibers ranges from 15 to 100 nm. The unique nanofiber-knitted foams may have potential applications in specific fields, including catalysis, adsorption, separation, and thermal insulation.     

Highly porous and hierarchical MgAl2O4 ceramics with improved pore connectivity prepared from gelled particle-stabilized foams

Ultralight ceramics with striking mechanical properties and improved pore connectivity could have wide applications in areas ranging from catalyst support to hot gas filtration. However, creating such materials has proven to be a challenging target. This work demonstrated a novel methodology to prepare porous MgAl₂O₄ ceramics by calcining gelled MgO–Al₂O₃–SiO₂ particle-stabilized foams. The striking green strength of dried foams can be achieved as a consequence of MgO hydration and subsequent formation of gelled Mg(OH)₂ and MgO–SiO₂–H₂O skeleton. The decomposition of colloidal substance at elevated temperature resulted in the formation of small pores on the cell wall, thus forming the hierarchical porous architecture and improving the pore connectivity. The highly porous MgAl₂O₄ ceramics fired at 1600°C possessed the integrated properties of ultrahigh porosity (87.0%), improved pore connectivity and satisfactory compressive strength (7.93 MPa), showing great potential to be used in multiple industrial fields.     

Preparation and characterization of foam glass from waste container glasses and water glass for application in thermal insulations

Glass foams are modern developed building materials which are now favorably competing with conventional materials for applications in thermal insulation. In this study, glass foams are synthesized solely from waste container glasses of mixed colors using sodium silicate (water glass) as foaming agent. Several glass foams of 150 × 150 × 30 mm were prepared from waste glasses of 75 μm, 150 μm and 250 μm size with addition of 15 wt % sodium silicate respectively and pressed uniaxially under a pressure of 10 MPa. The prepared glass foams were then sintered at temperatures of 800 °C and 850 °C respectively. Tests such as bulk density, estimated porosity, flexural strength, compressive strength and microstructure evaluation were used to assess the performance of the developed glass foams. The results showed that with increasing temperature and grain sizes, the percent porosity of the developed foams increased while the bulk density decreased. The microstructure evaluation showed that the finer the grain sizes used, the more homogenized are the pores formed and the higher the temperature, the larger the pores but are mostly closed. Both compressive and flexural strength were found to decrease with grain sizes and higher temperatures. The thermal conductivities of all the developed foam glasses satisfy the standard requirement to be used as an insulating material as their thermal conductivities did not exceed 0.25 W/m.K.     

Hierarchically structured glass monoliths based on polyurethane foams as template

Hierarchically porous glass foams were prepared via a combination of a replication technique and a phase separation of sodium borosilicate glasses. Open-pore polyurethane foams were impregnated with a slurry containing sodium borosilicate glass powder, binders, solvents and stabilizers. The composite was calcinated and sintered whereupon the organic polymer was decomposed and the monolith was compacted. A phase separation was initiated by an additional thermal treatment. The sodium-rich borate phase in the phase separated glass was removed with hydrochloric acid. Finally, the secondary silica species within the pores generated by the acid treatment were removed with sodium hydroxide solution. The monoliths were characterized by electron microscopy, nitrogen sorption, μ-CT and mercury porosimetry. Pore diameters – obtained from the template structure – were achieved in a range of 0.4–1.0 mm. The following phase separation and the coupled acid-alkaline leaching lead to an additional pore system within the glass framework.     

A novel method of making PVF porous foam without using the pore forming agent

Different with the conventional method of manufacturing poly(vinyl formal) (PVF) porous foam by using the pore‐forming agents such as wheat or potato starches, a novel method without using the pore‐forming agent is introduced in this article. Through the help of images taken by a scanning electron microscope, the formation process of the present PVF foam will be discussed in terms of the spinodal decomposition (SD) phase separation principle. Additionally, the effect of poly(vinyl alcohol) concentration and reaction temperature on the pore structure of the PVF foam will be investigated. Moreover, the water adsorption capacities of the PVF foams obtained by the present method will be studied in details through the analyses of pore‐size distribution, mechanical modulus, and thermal property. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41270.     

Hierarchical porous ceramics with multiple open pores from boehmite gel emulsions

Ceramic foam materials with highly porous microstructure are playing vital role in increasing areas, especially for those with requirements for open channels and superior specific surface area. In this work, a simple and versatile approach to prepare ceramic foams with open pores has been proposed, that is gelation of boehmite nanoparticle-assembled emulsions. Notably, hierarchical porous microstructure with open channels and uniform pore structure has been built. High specific surface area up to389.4 m²/g is attainable, making it excellent adsorption material when combining the merit of hierarchical pore structure. Furthermore, lattice-shaped ceramics are prepared via direct ink writing gelled emulsion, displaying the potential of forming lightweight material with complex shape and designable macrostructure. The three-dimensional (3D) printed foams exhibit multiple open pores, which cover length scale from mm scale, to μm scale and nm scale, making them promising materials in several fields like adsorption and gas filtrations, etc.     

Reuse of mineral wool waste and recycled glass in ceramic foams

Novel ceramic foams have been prepared by high temperature sintering of waste mineral wool and waste glass using SiC as a foaming agent. The aim of the research was to understand the effects of composition and sintering conditions on the properties and microstructure and produce commercially exploitable ceramic foams. Optimum ceramic foams were formed from 40 wt% mineral wool waste and 2 wt% SiC, sintered at 1170 °C using a heating rate of 20 °C/min with a 20 min hold at peak temperature. The ceramic foams produced had a bulk density of 0.71 g/cm³ and a uniform pore size distribution. The research shows that ceramic foams can be formed from waste mineral wool and these can be used for thermal insulation with associated economic and environmental benefits.     

A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Open‐cell polypropylene/polyolefin elastomer blend foams fabricated for reusable oil‐sorption materials

The fabrication of high‐performance oil sorbents is of great significance for oil spill cleanup. The main objective of this study was to prepare open‐cell polypropylene/polyolefin elastomer (PP/POE) blend foams for fabrication of reusable sorbents for oil sorption. Open‐cell PP/POE blend foams were prepared via continuous‐extrusion foaming using supercritical carbon dioxide as the blowing agent. The interconnected open‐cell structure was characterized by scanning electron microscopy. The hydrophobicity and lipophilicity of PP/POE open‐cell foams were revealed by tests of contact‐angle measurement, water and cyclohexane sorption on the foam surface, CCl₄ and cyclohexane sorption in water, and oil/water separation. Further, the sorption tests indicated that PP/POE blend foams showed larger oil‐uptake capacities than pure PP foams. In addition, cyclic compression tests showed that PP/POE open‐cell foams had excellent ductility and significantly improved recoverability compared to pure PP foams. In cyclic sorption–desorption tests, the sorption kinetics was studied in terms of capacity and saturation time, showing that PP/POE foams kept larger sorption capacities for 10 cycles, with larger sorption rates and good reusability. Based on the high open‐cell content, the good hydrophobic and oleophilic properties, the high oil‐sorption capacity, the improved recoverability, the large sorption rate, and the good reusability in cyclic oil‐sorption performance, the PP/POE open‐cell foams have shown promise as potential oil sorbents in applications for oil spill cleanup. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43812.     

Micro-tomographic characterization of composite recycled glass-silicone foams for applications in civil engineering

Noninvasive X-ray micro-computed tomography was applied for a complete quantitative and qualitative analysis of the cellular structure of composite foams constituted by a silicone matrix and a glass production waste filler. Composite foams with different glass filler weight content in the range 0–80% were synthesized and characterized. The tomographic analysis was employed in order to assess the structural heterogeneities, void fraction values, and bubble size distribution for all composite foams. The 3D micro-CT images analysis, performed at different cross-sections, highlighted heterogeneous cell growth or more elongated cells in the case of low and high filler content foams, respectively. Batch with 60% of glass filler was identified as a composite foam with effective void fraction values and relatively uniform spheroidal cellular structure (average sphericity 0.723 and diameter 0.107 mm). © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48718.     

Novel Foams Based on Freeze‐Dried Renewable Vital Wheat Gluten

A new way of producing rigid or semi‐rigid foams from vital wheat gluten using a freeze‐drying process is reported. Water/gluten‐based mixtures were frozen and freeze‐dried. Different foam structures were obtained by varying the mixing process and wheat gluten concentration, or by adding glycerol or bacterial cellulose nanofibers. MIP revealed that the foams had mainly an open porosity peaking at 93%. The average pore diameter ranged between 20 and 73 µm; the sample with the highest wheat gluten concentration and no plasticizer had the smallest pores. Immersion tests with limonene revealed that the foams rapidly soaked up the liquid. An especially interesting feature of the low‐wheat‐concentration foams was the “in situ” created soft‐top‐rigid‐bottom foams.

     

Sustained release of dexamethasone from drug‐loading PLGA scaffolds with specific pore structure fabricated by supercritical CO2 foaming

Inducing differentiation of bone marrow stem cells to generate new bone tissue is highly desirable by controlling the release of some osteoinductive or osteoconductive factors from porous scaffolds. In this study, dexamethasone was selected as a representative of small molecule drugs and dexamethasone‐loading porous poly(lactide‐co‐glycolide) (PLGA) scaffolds were successfully fabricated by supercritical CO₂ foaming. Scanning electron microscopy images showed that scaffolds had rough and relatively interconnected pores facilitating cells adhesion and growth. Specially, dexamethasone which was incorporated into PLGA matrix in a molecularly dispersed state could serve as a nucleation agent to be helpful for the formation of interconnected pores. Dexamethasone‐loading porous PLGA scaffolds exhibited sustained release profile, and the delivery of dexamethasone from porous scaffolds could last for up to 2 months. The cumulative released amount of dexamethasone was relevant with drug loading capacity (1.66%–2.95%) and pore structure of scaffolds; while the release behavior was anomalous (non‐Fickian) transport by fitting with the simple exponential equation, which had a diffusional exponent n higher than 0.5. It is feasible to fabricate drug‐loading porous scaffolds by supercritical CO₂ foaming with specific pore structure and sustained release profile, which can be well applied in bone tissue engineering. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46207.     

Novel preparative method for porous hydrogels using overrun process

Porous poly(vinyl alcohol) (PVA) hydrogels were prepared using the overrun process which is usually used in manufacturing ice cream. The pores in the hydrogel formed exhibit dual‐pore structure due to the injection of air bubbles and ice recrystallization. Morphological investigation revealed that overrun‐processed hydrogels had closed pore structures and that their pore size and size distribution had been influenced by the impeller rate and concentration of polymer solution. The closed‐pore structure was reformed into interconnected open‐pore structure at lower concentrations of the solution. The freeze–thawing process, which takes place in PVA cross‐linking, has no effect on the bubble structure, but removes the small pores formed during ice recrystallization. Besides the swelling ratio of overrun‐processed PVA hydrogels is increased tenfold in comparison with non‐porous hydrogels. Overrun‐processed hydrogels showed more rapid swelling kinetics than freeze‐dried and film‐like hydrogels due to their larger surface area. In the future, the overrun process can be applied to prepare porous scaffolds containing proteins, such as growth factors and other cytokines, without denaturation, because it operates at a low temperature. Copyright © 2004 Society of Chemical Industry