Fabrication Of Foams Research Articles

Layered polymer composite foams for broadband ultra-low reflectance EMI shielding: a computationally guided fabrication approach.

The development of layered polymer composites and foams offers a promising solution for achieving effective electromagnetic interference (EMI) shielding while minimizing secondary electromagnetic pollution. However, the current fabrication process is largely based on trial and error, with limited focus on optimizing geometry and microstructure. This often results in suboptimal electromagnetic wave reflection and the use of unnecessarily thick samples. In this study, an input impedance model was employed to guide the fabrication of layered PVDF composite foams. This approach optimized the void fraction (VF) and the thickness of each layer to achieve broadband low reflection. Moreover, hybrid heterostructures of SiCnw@MXene were incorporated into the PVDF composite foams as an absorption layer, while the conductive PVDF/CNT composite foams served as a shielding layer. Directed by theoretical computations, we found that combining 2.2 mm of PVDF/SiCnw@MXene composite foam (50% VF) and 1.6 mm of PVDF/CNT composite yielded EMI shielding effectiveness of 45 dB, with an average reflectivity (R) of 0.03 and an effective absorption bandwidth of 5.54 GHz (for R < 0.1) over the Ku-band (12.4-18 GHz). Importantly, the corresponding peak R was only 0.000017. Our work showcases a theoretically guided approach for developing absorption-dominant EMI shielding materials with broadband ultra-low reflection, paving the way for cutting-edge applications.

용접⋅용단 시 가연물별 화재발생 메커니즘에 관한 연구

In recent years, many fire incidents caused by carelessness toward welding and fusing safety have occurred at industrial sites in Korea. This negligence has led to large-scale fires, resulting in devastating and irreversible national social losses. To address this issue, a safety awareness investigation was conducted by surveying welders and analyzing the major fire accidents caused by welding. In addition, a more accurate fire-generation mechanism for sandwich panels, urethane foam, and nonwoven fabrics was scientifically derived by conducting fire reproduction tests in which combustibles were surrounded by sparks during welding. Measures to prevent accidental fires were also devised.

Towards tunable polymer foam fabrication: A case study to advance green materials development in limited data scenarios

AbstractEarly phases of green material development can be accelerated by identifying driving factors that control material properties to understand potential tradeoffs. Full investigation of fabrication variables is often prohibitively expensive. We propose a pared‐down design of experiments (DOE) approach to identify driving variables in limited data scenarios using tunable polydimethylsiloxane (PDMS) foams made via sacrificial templating as an example system. This new approach systematically determines the dependencies of porosity, transparency, and fluid flow by varying the template particle size and packing while using a more sustainable solvent. Factor screening identified template particle size and packing density as the driving factors for foam performance by controlling pore size and interconnectivity. The framework developed provides a robust, foundational understanding of how to green and tune a novel material's properties using an efficient and effective exploration of the design space. Recommendations for applying this method to a broad suite of experiments are provided.

Facile fabrication of three-dimensional nanofibrous foams of cellulose@g-C3N4@Cu2O with superior visible-light photocatalytic performance

In this work, a unique three-dimensional nanofibrous foam of cellulose@g-C3N4@Cu2O was prepared via electrospinning followed by a foaming process. A cellulose solution in DMAc/LiCl containing g-C3N4 and CuSO4 was applied for electrospinning, while aqueous alkali was used as the coagulation bath. The solidification of electrospun cellulose/g-C3N4 nanofibers would be accompanied with in-situ formation of Cu(OH)2 nanoparticles. Interestingly, the hydrogen gas (H2) generated from NaBH4 could transform the two-dimensional membrane into a three-dimensional foam, leading to the increased specific surface area and porosity of the material. Meanwhile, the Cu(OH)2 nanoparticles attached on the electrospun nanofibers were reduced to Cu2O to form a p-n heterostructure between Cu2O and g-C3N4. The as-prepared cellulose@g-C3N4@Cu2O foam exhibited a high degradation efficiency (99.5 %) for the dye of Congo Red under visible light radiation. And ·O2− was discovered to be the dominant reactive species responsive for dye degradation. Moreover, the cellulose@g-C3N4@Cu2O could maintain its initial degradation efficiency even after seven cycles of reuse, suggesting the excellent stability and cycling performance.

Fabricating Industry-Compatible Olefin-Linked COF Resins for Oxoanion Pollutant Scavenging.

Large-scale and low-cost synthesis of covalent organic frameworks (COFs) to meet the demands of industrial application remains formidably challenge. Here we report using 2,4,6-collidine as monomer to produce a series of highly crystalline olefin-linked COFs by a melt polymerization method. This method enables the kilogram-scale fabrication of self-shaped monolithic robust foams. The afforded COFs possess extremely low cost (<50 USD/kg), superior to all the reported COFs. Furthermore, using one-pot or post-modification methods can conveniently transform neutral COFs to ionic COFs, which can be applied as highly efficient ion-exchange sorbents for scavenging oxoanion pollutants. Remarkably, the superior adsorption capacity of a model oxoanion (ReO4 - ) is the highest among crystalline porous materials reported so far. This work not only expands the scopes of olefin-linked COFs but also enlightens the route for the industrial production of crystalline ion exchange sorbents.

Ambilateral convergent directional freeze casting meta-structured foams with a negative Poisson’s ratio for high-performance piezoresistive sensors

Flexible piezoresistive sensors show broad application value in wearable electronic skin, health monitoring and motion detection. The practical range of conventional piezoresistive sensors, however, is often severely limited by their sensitivity, measuring span, mechanical properties, and lateral expansion caused by positive Poisson's ratio (PPR) effects. Herein we introduce a generally applicable methodology, ambilateral convergent directional freeze casting technique, for the fabrication of anisotropic porous foams with a negative Poisson's ratio (NPR). Different from traditional freeze casting technique, this new design methodology enables precise control of pore types and pore orientations, thereby endowing artificial materials with special mechanical properties. Under uniaxial compression, synthetic foams exhibit macroscopic lateral shrinkage, with a Poisson's ratio below – 4.2% at the strain of 25%. These conductive meta-structured foams are used to construct piezoresistive sensors. Compared with traditional structures, metastructures with NPR effects significantly increase the number of wall-to-wall contacts and the formation of electrical paths, thereby enhancing the sensitivity and sensing reliability of sensors. In addition, the NPR effect markedly enhances the mechanical elasticity and compression resistance of sensors, enabling it to have excellent signal stability even in the face of reiterative compressions. These meta-structured sensors also exhibited smart resistivity repair behavior in the face of overload damage. Integrated sensors can be used for monitoring of pressure distribution and athlete's gait. Lateral contractions caused by NPR effects can avoid the contact of adjacent sensors and narrow the gap between two sensors, thus improving sensing stability and pressure distribution resolution of distributed sensors.

Tuning the Mechanical Behavior of Density-Graded Elastomeric Foam Structures via Interlayer Properties.

The concept of density-graded foams has been proposed to simultaneously enhance strain energy dissipation and the load-bearing capacities at a reduced structural weight. From a practical perspective, the fabrication of density-graded foams is often achieved by stacking different foam densities. Under such conditions, the adhesive interlayer significantly affects the mechanical performance and failure modes of the structure. This work investigates the role of different adhesive layers on the mechanical and energy absorption behaviors of graded flexible foams with distinct density layers. Three adhesive candidates with different chemical, physical, and mechanical characteristics are used to assemble density-graded polyurea foam structures. The mechanical load-bearing and energy absorption performances of the structures are evaluated under quasi-static and dynamic loading conditions. Mechanical tests are accompanied by digital image correlation (DIC) analyses to study the local strain fields developed in the vicinity of the interface. Experimental measurements are also supplemented by model predictions that reveal the interplay between the mechanical properties of an adhesive interlayer and the macroscale mechanical performance of the graded foam structures. The results obtained herein demonstrate that the deformation patterns and macroscale properties of graded foam composites can be tuned by selecting different bonding agents. It is also shown that the proper selection of an adhesive can be a practical way to address the strength–energy dissipation dichotomy in graded structures.

Compression Properties and Fabrication of Closed-Cell Metal Matrix Syntactic Foams Al2O3hs/AZ91D.

Closed-cell metal syntactic foam is a new material consisting of hollow spheres embedded in metal matrix syntactic foams. These foams have good physical and mechanical properties and are increasingly used worldwide in industrial and high-tech fields. Magnesium matrix syntactic foams containing hollow Al2O3 spheres ((Al2O3hs)/AZ91D) were successfully fabricated by hot press sintering at different temperatures. The fabrication of Al2O3hs/AZ91D and the effect of sintering temperature on the microstructure and properties are reported in this paper. Additionally, sandwiched magnesium matrix syntactic foams were prepared by placing magnesium plates on both sides of the syntactic foam. Some Al2O3hs particles became filled with matrix particles during preparation. Thus, the actual density was greater than the theoretically calculated value and increases with increasing sintering temperature. Above 723 K, a brittle phase MgAl2O4 formed in Al2O3hs/AZ91D. The quasistatic and dynamic compressive strengths of Al2O3hs/AZ91D first increased and then decreased with increasing sintering temperature, and the maximums were 162 MPa and 167.87 MPa, respectively. Thus, this paper reports a new strategy for the controlled preparation of metal matrix syntactic foams with predetermined porosity. The results show that this strategy improved the performance of lightweight and high-strength syntactic foam materials and shows potential for further research.

Response Surface Analysis (RSA) optimization of temperature-resistant gel foam fabrication and performance evaluation

For applications in high temperature environments, a high temperature resistant gel foam formulation with a novel semi-interpenetrating network structure was created. It was specifically determined experimentally what affects component concentration and temperature had on gelation time, gelation strength, and foam qualities by the use of a single-factor screening research and response surface analysis (RSA). The findings revealed that the foam volume grew from 370 ml to 390 ml before and after aging at 100 °C, the precipitation half-life was increased from 25 min to 50.4 min. The foam stabilization period was doubled by the optimized temperature-resistant gel foam, which also greatly improved temperature resistance. The gel foam performed best when the coordination ratios of NaOH, AA, AMPS, MBA, APS, sodium streptavidin sulfonate (AOS), alkyl glycoside (APG0810), alkyl amide betaine (FM-20AB), and silicone resin polyether emulsion (MPS) were 1.4 %, 5 %, 2.4 %, 0.7 %, 0.1 %, 1.6 %, and 0.2 %, respectively. The SEM examination and thermal weight loss investigation showed that the enhanced gel foam generated an enhanced mechanism with greater temperature resistance and stability and considerable potential application areas.

Gelatinized starch-furanic hybrid as a biodegradable thermosetting resin for fabrication of foams for building materials

Bio-based biodegradable resin was prepared by condensation of gelatinized starch and furfuryl alcohol, in presence of glyoxal as crosslinker. The resin was blown with different foaming agents and/or flame retardants such as phosphoric or boric acids, to produce environmentally friendly foam structures, given the bio-based nature of the main components, which are both derived from corn. Scanning electron microscopy (SEM) imaging of the samples revealed closed cell structure with a smooth surface. The excellent pulverization ratio and appreciable compression strength compared with the phenol-formaldehyde (PF)-based foam, render them strong candidates for building materials. Different investigations proved characteristics such as low thermal conductivity, good stability against thermal degradation and high limiting oxygen index (LOI) values, support the liability of such structures for application as heat insulating and fire-resistive materials.

Restricted assembly of ultralow loading of graphene oxide for lightweight, mechanically flexible and flame retardant polydimethylsiloxane foam composites

Lightweight, mechanically flexible and flame retardant polydimethylsiloxane (PDMS) foam materials modified with low content of nano-fillers are highly needed for versatile applications. Instead of the direct mechanical mixing technique that induced the embedment of nanofillers inside the matrix, herein, a facile, rapid and green restricted assembly strategy was established to prepare PDMS foam composites coated with an extremely low content of restricted graphene oxide (GO) sheets. A GO/water mixture in silicone oil emulsion was prepared to produce the restricted assembly of GO layers precisely and solely on the foam surface with no GO sheets inside the skeleton. Based on such restricted assembly induced by the amphiphilic surfactant molecules, the presence of only ∼0.04 wt% GO greatly improved the flame resistance (e.g., 40.6% reduction in heat release rate and 27.1% of limit oxygen index) of PDMS foams without affecting the low density (∼0.23 g/cm3) and mechanical flexibility (unchanged stress at strain 70% after 100 cycles under 200 °C). Observations and analysis of the burnt zones demonstrated that the restricted ultralow content GO sheets formed a compact protective layer facilitated by the pyrolysis of the PDMS molecules, thus protecting the inside of the PDMS material from the heat and oxygen attacks. These results confirmed that the restricted self-assembly of 2D nano-fillers approach is effective for the fabrication of flame-retardant PDMS foams and retention of other important properties.

B4C‐Reinforced Al–Zn Foams Having Superior Energy Absorption Efficiency

The low density of the aluminium foam and high energy absorption make them favourable for automobile, aerospace, and defence industries. However, the melt route for foam fabrication has challenges in achieving homogeneous distribution of pores. Hence, we have adopted the space holder technique using the powder metallurgy methodology to overcome such a challenge. In the current study, we fabricated Al–Zn alloy foams reinforced with varying volume fractions of B4C particles using NaCl as a space holder implemented by hot pressure‐assisted sintering and dissolution. X‐ray computed tomography revealed a homogeneous distribution of pores. The quasistatic compression studies showed that the samples containing a higher volume fraction of pores exhibited higher energy absorption efficiency in the fabricated foam. The maximum energy absorption efficiency (η) achieved was ≈93% for the pristine Al alloy foam with ≈50% porosity, which is ≈11% higher than the η value of 9 vol% B4C samples with similar porosity. Additionally, B4C particles delay the sudden collapse of cell walls and stabilize the compression behaviour. Adding B4C improves the η and strength at higher relative density. This fabrication methodology would help us develop foams with a homogenous pore distribution and regular geometry, achieving highly desirable mechanical properties.

Biomass chemical upcycling of waste rPET for the fabrication of formamide-free TPEE microcellular foams via scCO2 foaming

Currently, the fabrication of microcellular structure in thermoplastic polymer foams is heeded people’s attention owing to their extensive application prospects. In this work, we combine recycled poly(ethylene terephthalate) (rPET) with renewable bio-based monomers (namely, MPO, SeA, and PTMG) for the synthesis of value-added, long-lasting thermoplastic poly(ether ester) elastomers (TPEEs) via a novel and flexible polycondensation strategy. Following this, a facile, eco-friendly, effective, and green scCO2-based foaming methodology was applied to fabricate a series of lightweight and robust microcellular TPEE foams. The addition of MPO and SeA not only improves melt viscoelasticity and scCO2 uptake but also decreases TPEEs melting temperatures and improves the foaming temperature windows. The uniform microcellular foam structure with the smallest cell size (26–34 µm) and the highest cell density (>1011 cm3/cells) was successfully prepared. The TPEE foams unveiled a high expansion rate (6.1–8.5) and strong dimensional stability, and the creep shape recovery percentage of the foams ranged from 94.2% to 98.6%, respectively. Thanks to scCO2 foaming, TPEE foams' has zero formamides, meet safety requirements, and are suitable for commercial applications. Furthermore, the fabricated TPEE foams show excellent recyclability. Therefore, it reveals a promising future for the production of eco-friendly polymer foams through a simple, cost-effective, and green microcellular foaming technology.

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Sol-gel processing of a covalent organic framework for the generation of hierarchically porous monolithic adsorbents

Microstructure development of PEBA and its impact on autoclave foaming behavior and inter-bead bonding of EPEBA beads

Expanded poly(ether-block-amide) (EPEBA) bead foams are a new class of advanced thermoplastic elastomer foams with low density and high resilience. Steam-chest molding realizes the fabrication of the molded EPEBA bead foams with complex geometry. However, the evolution of polymer microstructure in autoclave foaming process and the bonding mechanism behind the steam-chest molding of EPEBA beads are still unclear. In this work, the microstructure development of PEBA was characterized by DSC, SAXS and rheological methods. Then, EPEBA beads with an expansion ratio higher than 25-fold were prepared by autoclave foaming, and the molded EPEBA bead foams with a density lower than 0.15 g/cm3 and a resilience higher than 70% were produced by steam-chest molding. According to the change about thermal properties of EPEBA beads before and after steam-chest molding, a possible inter-bead bonding mechanism during EPEBA beads molding was proposed. The amorphous soft segments and melted low-ordered hard segments promoted the welding process. However, the well-ordered hard segments hindered the interdiffusion of polymer chains at the interface, which leaded to the reduced tensile strength and the narrowed molding temperature window of EPEBA bead foams.

CO2-assisted fabrication of PLA foams with exceptional compressive property and heat resistance via introducing well-dispersed stereocomplex crystallites

Poly (lactic acid) (PLA) foams have been considered as one of the most promising and effective substitutes for traditional petroleum-based foams owing to their excellent biodegradability and biocompatibility. In order to enhance the compressive properties and heat resistance of PLA foams, ethylene-glycidyl methacrylate copolymer (EGMA) as a carrier was first blended with poly (D-lactic acid) (PDLA) to fabricate PDLA/EGMA masterbatches. Subsequently, the resultant masterbatches were added into poly (L-lactic acid) (PLLA) to obtain well-dispersed stereocomplex crystallites (SC). Finally, the obtained PLLA/EGMA/PDLA specimens were foamed using a confined batch foaming method with CO2. The formation of SC was verified by X-ray diffraction and differential scanning calorimetry as well as their well dispersion was proven by scanning electron microscope and rheological measurement. Compared with pure PLLA, the melt viscoelasticity and crystallization rate of PLLA/EGMA/PDLA specimens were significantly improved by virtue of the formed well-dispersed SC. For PLLA/EGMA/PDLA foams, an ameliorated cell morphology was achieved. At a PDLA content of 6 wt%, the density-specific compressive strength and modulus of PLLA/EGMA/PDLA foams were 187 % and 360 % higher than those of pure PLLA foams, respectively. Additionally, the heat resistance of the PLLA/EGMA/PDLA foams was evidently enhanced, and their volume shrinkage was only around 1 % at 90 °C for 6 h. This study provides a prospect strategy for the preparation of lightweight, environmentally friendly, biodegradable PLA foams with satisfactory compressive performance and heat resistance.

Fabrication of rigid polyimide foams by adopting active crosslinking strategy

Rigid polyimide foams (PIFs) with excellent mechanical and thermal properties were fabricated by thermal foaming of active cross-linkable units terminated polyester ammonium salts (PEAS) precursor powders. The active cross-linking strategy was realized by breaking CC bonds in maleic anhydride under heating treatment. Results showed that the formation of cross-linked structure before complete imidization effectively resisted matrix shrinkage and endowed PIFs with outstanding mechanical properties. The room temperature compressive strength and modulus of PIFs with four repeating units reached as high as 2.34 and 56.1 MPa, respectively. Moreover, the PIFs exhibited excellent thermal properties with onset decomposition temperatures ranging from 490 to 550 °C and the glass transition temperatures exceeding 290 °C. The PIFs displayed better thermal insulation, and the values of thermal conductivity fell within a range of 0.056–0.090 W m−1 K−1, which demonstrates potential application as high temperature resistant structural materials in aerospace and energy sectors.

Robust polymer foams from 2-hydroxyethyl cellulose: Fabrication, stability, and chemical functionalization

We describe the fabrication of polymer foams from aqueous solutions of 2-hydroxyethyl cellulose (HEC), using freezing-induced phase-separation and solvent removal methods. Freezing aqueous solutions of HEC produced 3D polymer networks by a mechanism involving polymer exclusion from ice crystals and concentration at grain boundaries. This phase-separation is viewed as a kinetically controlled process, with properties of the resultant foams reliant on the processing temperatures and other conditions employed. Specifically, HEC foams were obtained by removal of the ice phase either by lyophilization or solvent exchange techniques. The morphology and pore characteristics of the resultant foams were visualized by scanning electron microscopy (SEM), which showed fibrillar and flake-like polymer material embedded in an open-cell porous structure. Foam density and stiffness increased as a function of the initial HEC concentration employed, while the mechanical integrity of the resultant foams was improved by crosslinking with poly(acrylic acid) (PAA) or maleic anhydride (MA). Such crosslinking successfully enabled liquid water sorption into the foams without loss of structural integrity, thus providing opportunities for further incorporation of chemical moieties. Dynamic vapor sorption (DVS) was employed to assess water uptake into the HEC foams, which was generally greater for foams produced by lyophilization, and in all cases exceeded that achieved by conventional, commercially available cellulose foams.

Reticulated porous lanthanum strontium manganite structures for solar thermochemical hydrogen production

Porous ceramic foams are utilized in various fields and applications due to their advantageous properties such as high porosity, high specific surface area, and controlled permeability. Non-stoichiometric oxide foams are of particular interest for the solar thermochemical production of hydrogen in directly irradiated solar receivers due to their enhanced heat transfer capabilities and structural integrity. We report the fabrication of lanthanum strontium manganite replica foams and their solar thermochemical water splitting performance. Highly porous (>80%) foams were produced with controlled structure and tested in a high temperature water splitting furnace to demonstrate stability and overall performance. The temperature profile of the burnout and sintering of the foam structure were altered based on simultaneous differential scanning calorimetry and thermal gravimetry analysis. Two-step sintering was used both to retain micron-sized pores (D50 = 0.4 μm) on the foam's struts and to limit grain growth to a grain size of ∼1 μm. The structural characteristics were confirmed by porosimetry and scanning electron microscopy. The foams were repeatedly cycled, producing approximately 200 μmol/g of H2 per cycle. Water splitting performance was tested for 50 cycles and approximately 35 h at 1400 °C. No statistical difference in the effective hydrogen production per cycle was observed, which is promising for long-term redox cycle stability.

Polyurethane Foams Coated with Phosphorus-Doped Mesoporous Carbon for Flame-Retardant Triboelectric Nanogenerators

A phosphorus-doped mesoporous carbon (PMC)-filled coating was deposited on polyurethane foam (PUF) via a layer-by-layer assembly method. First, PMC was prepared from bio-sourced saccharose and phytic acid using mesoporous silica KIT-6 as a hard template. The coated PUF was then prepared by alternate dipping into a chitosan (CH) solution and dipping into an alginate-stabilized PMC (AL-PMC) aqueous dispersion. A few layers (three bilayers) of the CH/AL-PMC coating allowed the PUF to self-extinguish when exposed to a butane flame (≈1400 °C). Additionally, this coating layer enabled the foam to pass the UL-94 rating. Cone calorimetry revealed that this coating reduced the peak heat release rate, rate of smoke release, total smoke release, peak CO2 production rate, and peak CO production rate by 56, 48, 29, 35, and 35%, respectively. Using this foam, a flame retardant foam-based triboelectric nanogenerator (FRF-TENG) was fabricated. The FRF-TENG exhibited excellent energy harvesting performance, offering 158 V and 2.26 μA cm–2 open-circuit voltage and short-circuit current density, respectively. Furthermore, the FRF-TENG can be attached on a chair and serve as a self-powered sensor for the detection of back movement and sit-stand motion. This study may provide a promising potential for the design and fabrication of multifunctional smart foams.