Reutilization and stabilization of wastes by the production of glass foams

Glass foams are known to represent highly valuable products for thermal and acoustic insulation, often produced by employing wastes. Although the usage of recycled glass is widely reported for developing the glass matrix, little research has been due to the usage of wastes for the foaming reaction. In this work the cellular structure is achieved after oxidation of SiC-based wastes coming from the polishing of glass articles. The foamed recycled soda-lime glass incorporated the residues from oxidation and provided a reasonably good chemical stability. The addition of MnO₂ to the starting mixtures of wastes led to a certain improvement of the oxidation of SiC, and a complex effect on the correlation between density and mechanical strength. For selected additions, a more homogeneous foaming was found to provide a stronger cellular structure.     

The use of egg shells to produce Cathode Ray Tube (CRT) glass foams

Cleaned Cathode Ray Tube (CRT) (panel and funnel) waste glasses produced from dismantling TV and PC colour kinescopes were used to prepare glass foams by a simple and economic processing route, consisting of a direct heating of glass powders at relatively low temperatures (600–800 °C). This study reports on the feasibility of producing glass foams using waste egg shells as an alternative calcium carbonate-based (95 wt%) foaming agent derived from food industry. The foaming process was found to depend on a combination of composition, processing temperature and mixture of raw materials (glass wastes). Hot stage microscopy (HSM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize foams and evaluate the foaming ability and the sintering process. The experimental compositions allowed producing well sintered glass foams with suitable properties for some functional applications with environmental benefits such as: (1) reduced energy consumption because of the low heat treatment temperatures used; and (2) materials produced exclusively from residues.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Foaming of flat glass cullet using Si3N4 and MnO2 powders

The introduction of a compound capable of releasing oxygen, such as MnO₂, greatly improves the foaming ability of Si₃N₄ used as foaming agent in soda-lime glass powder, leading to expansion at a relatively low temperature (800–850 °C) and short processing time (7–30 min). The effect is based on the supply of oxygen, in addition to that in the furnace atmosphere. At the highest level of porosity, however, the strength of foams is negatively affected by a coarse microstructure, determined by cell coalescence. The reduction of firing temperature or, above all, the reduction of the processing time, was found to limit the coalescence and significantly improve the strength of the foams.     

Fabrication of in-situ self-reinforced Si3N4 ceramic foams for high-temperature thermal insulation by protein foaming method

To meet demand for lightweight and high-strength ceramic foams, in-situ self-reinforced Si₃N₄ ceramic foams, with compressive strength of 13.2–45.9 MPa, were fabricated by protein foaming method combined with sintered reaction-bonded method. For comparison, ordinary protein foamed ceramics with irregular block microstructure were fabricated via reaction-bonded method, which had compressive strength of 3.6–20.5 MPa. Physical properties of these two types of samples were systematically compared. When open porosity was about 80%, both types of Si₃N₄ ceramic foams had excellent thermal insulation properties (<0.15 W m^?1 K^?1), while compressive strength of in-situ self-reinforced samples increased by more than 158% compared with ordinary samples. Under high-temperature oxidation conditions, microstructures of both types of samples were deformed with increase in oxidation temperature. Moreover, after oxidation temperature was increased to 1400 °C, oxidation weight gain decreased from 18.07% for ordinary samples to only 2.18% for self-reinforced samples. Thus, high-temperature oxidation resistance of Si₃N₄ ceramic foams was greatly improved.     

Challenge to fortyfold expansion of biodegradable polyester foams by using carbon dioxide as a blowing agent

This paper presents a new foaming technology using supercritical carbon dioxide as a blowing agent to obtain large volume expansions of biodegradable polyester foams of over fortyfold. The basic approach for the promotion of a large volume expansion ratio with carbon dioxide was to prevent cell coalescence by using a branched material, to dissolve carbon dioxide completely in the melt by promoting convective diffusion under a high processing pressure, to reduce the diffusivity of gas by lowering the melt temperature, and to optimize the processing conditions in the die to maximize volume expansion. The desirable composition of the materials includes dehydrated branched biodegradable polyester (polybutylene succinate), CO₂ (blowing agent), and tale (nucleating agent). A single‐screw extrusion system was used for foam processing. A large volume expansion ratio of up to forty‐fivefold was achieved from the biodegradable polyester foams. The morphologies and volume expansion ratios of biodegradable polyester foams at various processing temperatures and pressures were studied.     

CO oxidation over structured carriers: A comparison of ceramic foams, honeycombs and beads

This work aims an experimental comparison of different packings on the basis of their pressure drop, mass and heat transfer properties. Ceramic foams, beads and a honeycomb monolith were used as carriers in the oxidation of carbon monoxide. The carriers were coated with active Pt/SnO₂. The CO oxidation rate was measured in the regime of external diffusion control at superficial gas velocities between 1 and 10 m/s. The volumetric rate coefficients and the pressure drop of packings with similar geometric surface area decreased in the sequence particles > foams > honeycomb. The magnitude of the temperature gradient along the catalytic bed decreased as going from honeycomb over larger particles to foams and small particles. Foams were superior over particle beds from the viewpoint of combined high mass transfer and low-pressure drop. The main advantage of foams as compared to honeycomb resided in the radial mixing enabling a better heat transfer to the reactor walls.     

Control of micro‐ and nanocellular structures in CO2 foamed PES/PEN blends

High performance thermoplastic blends based on polyethersulfone (PES) and poly(ethylene 2,6 naphthalate) (PEN) were foamed with supercritical CO₂ to develop high‐performance foams with improved heat deflection temperature, extended processing range, and controlled cellular morphology. The cellular morphology resulted to be strongly influenced by the initial morphology of the blend. The presence of small amount of PES as dispersed phase in PEN based blends acted as blowing agent reservoir and allowed to extend the processing temperature range for obtaining low density foams. The presence of PEN droplets in PES based systems extended the foaming temperatures towards lower values and allowed the development of a bimodal micro/nanocellular morphology at a foaming temperature 60°C lower than the PES glass transition temperature. Furthermore, the presence of PEN droplets compensated for the reduced capability of the host matrix to generate a fine and diffuse porosity at low foaming temperatures. POLYM. ENG. SCI., 55:1281–1289, 2015. © 2015 Society of Plastics Engineers     

Cell morphology and mechanical properties of microcellular mucell® injection molded polyetherimide and polyetherimide/fillers composite foams

Microcellular polyetherimide (PEI) foams were prepared by microcellular injection molding using supercritical nitrogen (SC‐N₂) as foaming agent. The effects of four different processing parameters including shot size, injection speed, SC‐N₂ content, and mold temperature on cell morphology and material properties were studied. Meanwhile, multiwalled carbon nanotube (MWCNT), nano‐montmorillonoid (NMMT), and talcum powder (Talc) were introduced into PEI matrix as heterogeneous nucleation agents in order to further improve the cell morphology and mechanical properties of microcellular PEI foams. The results showed that the processing parameters had great influence on cell morphology. The lowest cell size can reach to 18.2 μm by optimizing the parameters of microcellular injection molding. Moreover, MWCNT can remarkably improve the cell morphology of microcellular PEI foams. It was worth mentioning that when the MWCNT content was 1 wt %, the microcellular PEI/MWCNT foams displayed optimum mechanical properties and the cell size decreased by 28.3% compared with microcellular PEI foams prepared by the same processing parameters. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4171–4181, 2013     

High-strength ceramic foams with ultralow dielectric constant and loss in terahertz frequency region

Sixth generation (6G) wireless networks operating in terahertz frequency region are significant solutions to the increasing number of telecommunication devices and the demand for faster data transmission. However, few dielectric materials exhibiting an ultralow relative dielectric constant (ε_r), loss tangent (tanδ), and high mechanical strength are found suited for 6G telecommunication. In this study, we developed lightweight ceramic foams by directly sintering glass hollow spheres. The effects of sintering temperatures on their mechanical performance, and terahertz optical and dielectric properties were systematically investigated. The final compositions of ceramic foams contain glass, cristobalite, quartz, and wollastonite. The crystalline content increases with increasing sintering temperature. The ceramic foams sintered at low temperatures of 700–850°C exhibit a high porosity ranging from 74.6% to 87.3%, ultralow ε_r ranging from 1.41 to 1.83, and tanδ of ∼0.011 at 0.5 THz, as well as high compressive strength ranging from 10.1 to 27.4 MPa, which could be promising materials for 6G telecommunication.     

The preparation and thermomechanical properties of high-temperature foams based on thermoplastic poly(phthalazinone ether ketone)

Poly(phthalazinone ether ketone) (PPEK) is an amorphous thermoplastic polymer with a high glass transition temperature (T_g) exceeding 250°C. We describe the preparation of foams from PPEK and characterize their properties. PPEK foams were prepared using dichloromethane as a foaming agent. The foaming agent was swollen into discs of the PPEK, which were then foamed by heating. Foams could be prepared at temperatures far below the T_g of the PPEK due to plasticization of the polymer by the foaming agent. Foams with densities ranging from 0.1 to 0.65 g/cm³ were prepared. Their thermal conductivity and modulus (measured approximately by indentation tests) were found to decrease with density, and the trends were similar to those expected from existing models. The foams could be annealed at 200°C without collapse suggesting that they may be useful in structural or insulation applications where stability at high temperature is essential.     

High‐temperature‐resistant polyimide/montmorillonite nanocomposite foams by solid blending

A series of high‐temperature‐resistant polyimide/montmorillonite (PI/MMT) nanocomposite foams were prepared by solid blending method. The dispersion of MMT and effects of MMT content on the properties of the PI/MMT nanocomposite foams were investigated. Results indicated that MMT could be exfoliated effectively and dispersed uniformly in the PI matrix by the solid blending method. The introduction of MMT could considerably increase the reduced compressive strength, thermal resistance, and decrease the dielectric constant of the PI/MMT nanocomposite foams. The reduced compressive strength of nanocomposite foams showed a maximum value at the MMT content of 5 wt%, which was 197% higher than that of pure PI foams. It was worth noting that a significant increase in glass‐transition temperature (T _g) could be achieved with the increase of MMT content, and the maximum T _g was as high as 436°C at the MMT content of 7 wt%. This study may provide a useful method to prepare PI/MMT nanocomposite foams with improved properties for targeted high‐temperature applications. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers     

Extrusion foaming of polystyrene/carbon particles using carbon dioxide and water as co-blowing agents

Carbon particles such as platelet-like graphite (GR), spherically shaped activated carbon (AC), and tubular carbon nanofiber (CNF) were used as additives in extruded polystyrene (PS) foams with carbon dioxide (CO₂) and water as co-blowing agents. It was found that GR is the best additive for improving the thermal insulation performance of CO₂ based foam samples because of GR’s good absorption and reflectivity of infrared (IR) radiation. However, when the GR concentration was higher than 0.5 wt.%, the extruded foams exhibited large bubbles in the center of the foam and the extrusion line became unstable. By adding water carried by AC as a co-blowing agent, it was able to decrease the temperature in the center of the extruded foam, which successfully eliminated the bubble problem and achieved stable foam extrusion with good control of the foam density and cell morphology. Moreover, water carried by AC could also improve the mechanical performance of extruded foams containing CNF or GR. Water was not found in the extruded foams and the presence of water during extrusion did not affect the molecular weight and glass transition temperature of PS. Our results showed that a combination of AC as a water carrier and GR as an absorber and reflector of IR radiation can produce CO₂ based PS foams with good thermal insulation and mechanical properties, particularly with the presence of a small amount of CNF nanoparticles.     

A simple method for preparation of polymer microcellular foams by in situ generation of supercritical carbon dioxide from dry ice

In this work, poly(methyl methacrylate) (PMMA) and PMMA/nanoclay nanocomposite microcellular foams were successfully prepared using a simple method based on in situ generation of supercritical carbon dioxide (CO₂) from dry ice. The method was compared with conventional methods exempted from high pressure pump and a separate CO₂ tank. Effect of various processing conditions such as saturation temperature and pressure and clay concentration on cellular morphology and hardness of the prepared microcellular foams was examined. State of the clay dispersion in the prepared PMMA/clay nanocomposites was characterized using X-ray diffraction and transmission electron microscopy techniques. Field emission scanning electron microscopy was used to study cellular morphology of the prepared foams. It was observed that elevation of saturation temperature from 85 to 105 °C at constant saturation pressure increased cell density and decreased average cell size of the prepared PMMA foams. Furthermore, an increase in saturation pressure from 120 to 180 bar resulted in a reduction in average cell diameter and an increase in cell density of the prepared PMMA foams. On the basis of the gathered results, optimum conditions for preparation of PMMA microcellular foams were determined and applied for preparation of PMMA/nanoclay microcellular foams. It was shown that incorporation of clay into the polymer matrix resulted in a finer and more uniform cellular morphology in the final microcellular foams. It was also observed that incorporation of nanoclay into the prepared foams, up to 3 wt%, led to a moderate increase in the foam hardness.     

Application of glass soot catalysts on metal supports to achieve low soot oxidation temperature

K–Ca–Si–O glass was applied to metal supports for use as a catalyst for diesel soot combustion. Glasses were processed from the melt and by a sol–gel route. Catalyst activity for the oxidation of diesel exhaust soot and flame soot from an oil lamp was compared by thermogravimetric analysis (TGA). The results show that a K-based catalytic glass coating on metal substrates can reduce the temperature where half of the engine soot is oxidized (T₅₀) to as low as 360 °C under loose contact conditions, and offers catalytic stability for long term combustion cycling. Scanning electron microscopy observations show that sol–gel glass processing is effective for coating complex wire mesh shapes without pore clogging.     

Preparation and Characterization of Sustainable Polyurethane Foams from Soybean Oils

Polyol derived from soybean oil was made from crude soybean oil by epoxidization and hydroxylation. Soy-based polyurethane (PU) foams were prepared by the in-situ reaction of methylene diphenyl diisocyanate (MDI) polyurea prepolymer and soy-based polyol. A free-rise method was developed to prepare the sustainable PU foams for use in automotive and bedding cushions. In this study, three petroleum-based PU foams were compared with two soy-based PU foams in terms of their foam characterizations and properties. Soy-based PU foams were made with soy-based polyols with different hydroxyl values. Soy-based PU foams had higher T _g (glass transition temperature) and worse cryogenic properties than petroleum-based PU foams. Bio-foams had lower thermal degradation temperatures in the urethane degradation due to natural molecular chains with lower thermal stability than petroleum skeletons. However, these foams had good thermal degradation at a high temperature stage because of MDI polyurea prepolymer, which had superior thermal stability than toluene diisocyanate adducts in petroleum-based PU foams. In addition, soy-based polyol, with high hydroxyl value, contributed PU foam with superior tensile and higher elongation, but lower compressive strength and modulus. Nonetheless, bio-foam made with high hydroxyl valued soy-based polyol had smaller and better distributed cell size than that using low hydroxyl soy-based polyol. Soy-based polyol with high hydroxyl value also contributed the bio-foam with thinner cell walls compared to that with low hydroxyl value, whereas, petroleum-based PU foams had no variations in cell thickness and cell distributions.     

Rigid interpenetrating polymer network foams prepared from a rosin-based polyurethane and an epoxy resin

A series of rigid interpenetrating polymer network (IPN) foams, based on a rosin-based polyurethane and an epoxy resin, were prepared by a simultaneous polymerization technique. The changes in the chemical structure, dynamic mechanical properties, and morphology of the rigid IPN foams were investigated by Fourier transform infrared (FTIR) spectroscopy, dynamic mechanical thermal analysis, and scanning electron microscopy. The FTIR analysis showed clearly that the cure rate of the rosin-based rigid polyurethane foam and the epoxy resin were different and, as a result, these two networks formed sequentially in the final rigid IPN foams. All of the rigid IPN foams exhibited a single, broad glass transition that shifted to lower temperature as the epoxy resin content increased. The experimental composition dependence of T_g's of the rigid IPN foams showed slight positive deviation from the Fox equation for homogeneous polymer systems. No phase separation was observed from the scanning electron microscopy investigation. It could be concluded that these two component networks were compatible in the final rigid IPN foams. This compatibility could be attributed to a graft structure in the polyurethane and the epoxy resin networks arising from the reaction of the hydroxyl groups of the epoxy resin with the isocyanate groups of MDI, and from the reaction of the hydroxyl groups of the polyols with the epoxide groups of the epoxy resin, as suggested by FTIR analysis. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 271–281, 1998     

Investigation on polyimide/silica hybrid foams and their erosion resistance to atomic oxygen

A series of polyimide/silica (PI/SiO₂) hybrid foams were prepared by the sol–gel process. Aminopropyltriethoxysilane was used as the coupling agent to enhance the compatibility between PI matrix and SiO₂. Fourier transform infrared spectroscopy and scanning electron microscopy were used to analyze the chemical structure and cellular structure of PI/SiO₂ hybrid foams. The results indicated that the three‐dimensional network of Si O Si was formed in the hybrid foams, and the hybrid foam presented the uniform cellular structure when the SiO₂ content was less than 6 wt%. The thermal stability, dynamic mechanical property, and dielectric property of PI/SiO₂ hybrid foams were investigated by dynamic mechanical analysis, thermogravimetric analysis, and vector network analyzer, respectively. The introduction of SiO₂ improved the thermal stability and increased the storage modulus and glass‐transition temperature. The hybrid foams showed higher dielectric constants compared with the neat PI foam. The erosion resistance to atomic oxygen (AO) of PI/SiO₂ hybrid foams was also evaluated in a ground‐based AO simulator. The surface morphology and chemical structure of PI/SiO₂ hybrid foams before and after AO exposure were investigated by scanning electron microscopy, atomic force microscopy, and X‐ray photoelectron spectroscopy. The results revealed that the inorganic SiO₂ protective layers were formed on the surface of PI/SiO₂ hybrid foams after AO exposure, which could effectively improve the AO erosion resistance of PI/SiO₂ hybrid foams. POLYM. COMPOS., 36:713–721, 2015. © 2014 Society of Plastics Engineers     

Synergy of supercritical CO2 and superheated H2O for enhanced processability of polyethersulfone towards open cell foams

Processing of a high glass transition (T_g) polymer such as polyethersulfone (PES; T_g = 225°C) poses a challenge as it requires high processing temperatures or sometimes toxic solvents. In this work, we report of a facile process using superheated water (shH₂O) and supercritical carbon dioxide (scCO₂) co‐media. PES solids were foamed in the scCO₂/shH₂O co‐media or scCO₂ alone in a batch process at different temperatures. The scCO₂/shH₂O produced a synergistic effect and achieved PES foams even at processing temperatures as low as 85°C below the nominal T_g; whereas, scCO₂ alone required higher processing temperatures. Moreover, the scCO₂/shH₂O co‐media produced highly porous PES foams that were at least 23% higher in porosity than what was obtained using scCO₂ alone. In addition, the scCO₂/shH₂O produced open cell foams at some processing conditions; whereas, scCO₂ produced closed cell morphologies. Since both CO₂ and H₂O are innocuous, this approach has potential for use in the preparation of ultrafiltration membranes, which currently require the use of toxic solvents for their fabrication by way of the phase inversion process. Moreover, the use of scCO₂/shH₂O is a cost‐effective approach for the processing of high T_g polymers at significantly lower temperatures. POLYM. ENG. SCI., 58:1108–1114, 2018. © 2017 Society of Plastics Engineers     

A study of cell nucleation in the extrusion of polypropylene foams

An investigation has been performed of the cell nucleation and initial growth behaviors in the foam processing of polypropylene (PP) in both the linear and branched forms. These materials were foamed in extrusion with the two blowing agents, CO₂ and isopentane. The cell density generally increased with an increased content of the blowing agent, for both CO₂ and isopentane. The effect of processing pressure on the cell density was distinct when CO₂ was used, whereas no pressure effect was observed in the foam processing with isopentane. The cell morphologies for the two PPs were found to be significantly different. A slightly lower nuclei density was observed in the branched PP foams than in the linear PP foams. However, the phenomenon of cell coalescence was observed much less in the branched PP foams. Most cells in the branched PP foams were closed, whereas in the linear PP foams they were connected to each other. The experimental results indicated that the branched structure played an important role in determining the cell morphologies through its effects on the melt strength and/or melt elasticity.