Foam Cell Walls Research Articles

Boosting Impedance Matching by Depositing Gradiently Conductive Atomic Layers on Porous Polyimide for Lightweight, Flexible, Broadband, and Strong Microwave Absorption.

Gradient structures are effective for microwave absorbing but suffer from inadequate lightweight and poor flexibility, making them fall behind the comprehensive requirements of electromagnetic protection. Herein, we propose a hierarchical gradient structure by integration with porous and sandwich structures. Specifically, polyimide (PI) foams are used as a robust and flexible skeleton, in which the foam cell walls are sandwiched by Ti3C2Tx, ZnO, and ZrO2 atomic layers in sequence. Owing to the decreasing conductivity of Ti3C2Tx, ZnO, and ZrO2, they form gradient impedance matching layers on both sides of the PI foam cell walls, significantly enhancing the absorbing intensity for microwaves. In addition, the porous and sandwich structures can synergistically facilitate multiple reflections, increasing the number of interactions between microwave and foam cell walls. Therefore, the resulting lightweight ZrO2@ZnO@Ti3C2Tx@PI (ZrZnTP) composite foams reach a minimum reflection loss of -68.4 dB with an effective absorbing bandwidth covering the whole X band (8.2-12.4 GHz). The ZrZnTP also exhibits outstanding flexibility even at an extremely low temperature of -196 °C (i.e., liquid nitrogen). This work offers a general approach to realizing hierarchically integrated structures of gradient, porousness, and sandwich structures for lightweight, flexible, broadband, and strong microwave absorbing materials.

In-Plane mechanical and failure responses of honeycombs with syntactic foam cell walls

This work investigates the load-bearing and energy absorption capacities of hexagonal honeycombs with syntactic cell walls and spatial gradation of cell densities. Structures are processed and property-tuned by varying the volume fraction of hollow microspheres (microballoons), cell wall thickness, and spatial gradation of cell densities. The mechanical behavior of the structures is characterized by subjecting them to in-plane compression. Full-field deformation and failure responses of these structures at meso- and macro-scales are characterized by digital image correlation (DIC) and postmortem fracture analyses, respectively. Mesoscale analyses reveal heterogeneous strain accumulation at the cell hinges, which leads to a fracture in the vicinity of the hinge. Failure is characterized by a brittle mode in all samples. It is shown that the energy absorption capacity of the structures can be improved with the spatially-controlled incorporation of microballoons into the cell struts, at the penalty of reduced overall strength. In addition, cell-density gradation offers a notable improvement to the energy absorption performance over uniform density structures and a mechanism to lower structural density while achieving high mechanical performance.

Preparation of a Ceramifiable Phenolic Foam and Its Ceramization Behavior.

Ceramifiable phenolic foam (GC-PF) with a low ceramization temperature has been prepared by incorporation of low melting point glass frits (LMG) containing B2O3 and Na2O as main components into a phenolic resin matrix. Fourier transform infrared spectrometry, X-ray diffractometry, and scanning electron microscopy were used for assessment of the structure, phase composition, and morphology of GC-PF before and after combustion analysis, respectively. A glassy ceramic protective layer is formed when GC-PF is exposed to flame or a high temperature environment. The presence of LMG not only reduces the level of defects in the phenolic foam cell wall (gas escape pore), but also promotes the generation of a glassy ceramic protective layer that could inhibit heat feedback from the combustion zone and reduce the rate of formation of volatile fuel fragments. Thermogravimetric analysis and differential scanning calorimetry were used to establish that GC-PF exhibits excellent thermal stability. Limiting oxygen index (LOI) determination suggests that GC-PF displays good flame retardancy. The LOI of GC-PF was as high as 45.6%, and the char residue at 900 °C was six times greater than that for ordinary phenolic foam (O-PF). The area of the raw material matrix of GC-PF after combustion for 60 s was about 1.7 times larger than that for O-PF. A possible mode of formation of glassy ceramics has been proposed.

Fluorescence assisted visualization and destruction of particles embedded thin cell walls in polymeric foams via supercritical foaming

Supercritical CO2 foaming is widely used to prepare microcellular foams and particles are frequently added to regulate cellular structure. Nevertheless, microcellular foams present a complicated structure (three-dimension cells and two-dimension cell walls embedded with particles), which is a huge challenge to enlighten scCO2 foaming behavior and corresponding properties. Therefore, a strategy was proposed to take fluorescence’s advantage of real-time and field visualization in scCO2 foaming: (1) cell wall rupture process was studied via in-situ visualization; (2) dual fluorescence design was used to construct particle dispersive state in a cell; (3) polystyrene particles were chemical fluorescence modified to resist strong extraction of scCO2 fluid. This work reveals that effect of particles on bubble nucleation is two folds: interface promoted bubble nucleation, but elastic strain energy inhibited bubble nucleation. By in-situ visualization, irregular particle dominates in initiating cracks in cell walls due to the much larger stress concentration compared with spherical particles.

3D structured graphene fluoride-based epoxy composites with high thermal conductivity and electrical insulation

Graphene-based polymer composites possess high thermal conductivity, unfortunately, the high electrical conductivity of these composites limits their application in electronic devices. To address this issue, we report on the thermally conducting yet electrically insulating graphene fluoride (GF) based epoxy composites. GF was coated on cell walls of polyurethane foam (PUF) by a cyclic assembly deposition process, and the PUF@GF foam was subsequently compressed and infiltrated with epoxy resin. The resultant PUF@GF/epoxy composite exhibited a thermal conductivity of 9.68 W·m−1·K−1 at 8.04 vol% of GF loading which was 51-folds higher than that of neat epoxy. The significant enhancement in thermal conductivity is attributed to the effective formation of 3D thermoconductive pathways of GF in epoxy matrix. Additionally, the PUF@GF/epoxy composites possess an electrical insulating property (10−9 S·m−1). This approach exhibits a cost-effective method to fabricate the thermoconductive composites with electrical insulation that could be applied for thermal management in electronic applications.

Effect of operating temperature conditions in 21-year-old insulated pipe for a district heating network

Since the insulated pipe in the district heating network should have a service life of more than 30 years based on EN253, the long-term thermal performance of PU foam is substantially important to guarantee sufficient shear strength. To examine the effect of operating temperature conditions on the degradation of PU foam in 21-year-old insulated pipe, two types of aged pipes were prepared from public office and general residence, since a large fluctuation in temperature profile with low average temperature is noted at the public office, while a small variation in temperature profile and high average temperature is noted at the general residence. The rate of thickness reduction in the cell wall of the aged PU foam is relatively faster at the public office than at the general residence since it underwent large fluctuation in operating temperature despite being exposed to low average temperature. In terms of the shear strength of the aged pipe, it exhibits relatively low shear strength at the public office compared to general residence, which reveals that the temperature fluctuation in the operating condition for the DH network tends to influence the degradation rate of the insulated pipe, substantially, compared to average exposed temperature.

Development of Coating Technology for Antibacterial Ag Zeolite Powder on Open Cell Type Ni Foam Cell Walls

Development of Coating Technology for Antibacterial Ag Zeolite Powder on Open Cell Type Ni Foam Cell Walls

Percolation of Primary Crystals in Cell Walls of Aluminum Alloy Foam via Semi-Solid Route

Herein, a uniform aluminum alloy foam was fabricated by the addition of TiH2 as a blowing agent to Al-6.4 mass % Si in the semi-solid state and subsequent solidification. This was aimed at propounding the stabilization mechanism of the proposed foaming process. The microscopic images, which were the cross section on the center of the foam etched with Weck’s reagent, showed the primary crystals in the semi-solid state and solidifying segregation surrounding the crystals. Thus, it became evident that the area ratio of primary crystals in the semi-solid state approximately equals to the set solid fraction. According to the percolation theory for the cell wall model, the drainage in the cell walls with primary crystals above the percolation threshold was found to be inhibited. By considering that each cell wall is a flow path of the foam, the percentage of the cell walls with inhibited drainage to all the other cell walls was observed to exceed the percolation threshold of the lattice model (0.33) as per the percolation theory. Therefore, it can be concluded that the primary crystals inhibit drainage in some cell walls, ensuring that the stability of the foam is maintained.

Exploitation influence on compressible polyurethane flexographic sleeve properties

Abstract Flexographic sleeves are made using various materials, including a sub-group with extremely durable polyurethane foam shock-absorbing layer. During exploitation, the sleeves are exposed to cyclic dynamic loading, and the flexographic printing process is highly sensitive to the changes in pressure. Deformation of printing elements occurs due to the almost two times higher residual strain of the exploited sleeves. Changes in the residual strain induce occurrence of the hysteresis losses, which lead to heat generation. The forces impacting the material are not strong enough to induce permanent deformation in the microstructure. Therefore, the leading cause of the change lies in the molecular structure of the parent polymer of the polyurethane foam cell walls, whose resilience is declining. The thickness of the exploited sleeves tends to be around 8 % lesser. In addition to high-frequency cyclic loads during printing, the adhesive layer of self-adhesive sleeves undergoes reduction in the share of acrylates, phthalates and rosin, thereby reducing the adhesive strength and the force needed to initiate the de-adhesion by half. The knowledge of mechanisms of change in certain characteristics of the sleeves enables predicting their service life and increasing the stability of the printing process through possible corrections of other process parameters.

Effect of carbon nanotube doping on the energy dissipation and rate dependent deformation behavior of polyurethane foams

An experimental investigation is performed to characterize the effect of carbon nanotubes on the average mechanical properties of polyurethane foams. Polyurethane foams are doped with as-grown and oxidized carbon nanotubes at varying carbon nanotube concentrations. It is observed that the inclusion of carbon nanotubes up to a threshold concentration decreases the density of freely expanding polyurethane foams. Uniaxial and cyclic compression testing of foam samples is carried out to study their energy dissipation and rate dependent deformation behavior. While energy dissipation is observed to be higher in neat polyurethane foams, carbon nanotube reinforced foams show better recovery when compressed beyond elastic limit due to their stiffer foam cell walls. It is shown that incorporation of oxidized carbon nanotubes should be preferred over as grown carbon nanotubes to improve flexural, thermal and acoustic response of polyurethane foams. Scanning electron microscopy analysis of compressed samples reveals that cell shearing; cell bending and fracture at nodes are the predominant mode of deformation in all foam samples.

Microstructure Aspects of 7075 Al-SiO2 Composite Foams Produced by Direct Melt Foaming Method

Direct melt foaming method was used to fabricate 7075 Al-SiO2 composite foams from recycled beverage aluminum cans and SiO2 waste particles. The microstructures of the produced composite foams revealed typical characteristic features of the conventional foams, where large dark areas (pores), curved cell walls, cell edges, nodes on the cell edges, cell walls and plateau regions were observed. The microstructures of the composite foams showed fine and irregular shape of SiO2 particles, which were uniformly distributed in the cell wall and plateau region of composite foams. The grain size of α-Al was reduced by the addition of SiO2, indicating that the refining effect of the SiO2 is due to heterogeneous nucleation of the α-Al grain by SiO2 particles or restriction of the α-Al grain growth by the SiO2 particles, or both. SEM and EDX indicated that the SiO2 particles reacted with the molten 7075 Al during melting and foaming process, forming MgAl2O4, MgO, Al2O3 and Mg2Si.

Inverse identification of cell-wall material properties of closed-cell aluminum foams based upon Vickers nano-indentation tests

This study aims to develop an inverse approach to identifying the cell-wall material properties of closed-cell aluminum (Al) foams, which is based upon indentation tests, numerical simulation and optimization technique. Vickers nano-indentation tests are conducted on the cell-wall of foams, and the residual surface deformation is measured by a high definition confocal microscopy. To enhance identification efficiency, a combined 2D with 3D micromechanics modeling strategy is proposed here, in which the load-depth curve and residual imprint (pile-up) of the indentation tests are considered. In this study, the cell-wall material properties such as the yield strength and strain-hardening exponent are identified using this proposed inverse approach, while the Young's modulus is obtained directly for the unloading part of the indentation load-depth curves. The identification results indicate that the proposed inverse procedure can provide a well-posed solution to the cell-wall material properties by introducing the pile-up information. Microscopic computed tomography (μCT) technique is also utilized to reconstruct accurate 3D image-based representative volume element (RVE) model of foam structure, through which the numerical simulation is conducted for uniaxial compression. The identified cell-wall material parameters are validated by comparing the numerical simulation with the experimental data in terms of the deformation/failure modes and quantitative results. It is noted that the cell-wall of aluminum foams that are commonly made of melt foaming processes exhibits a quite different stress-strain relation when compared with the raw material of cell-wall. This study is expected to provide an effective approach for identifying the material properties that may be not easily determined through conventional testing methods.

Thermal Conductivity of Low-Density Polyethylene Foams Part II: Deep Investigation using Response Surface Methodology

Here, we conducted a deep study on the thermal insulation performance of polymeric foams using response surface methodology (RSM). Cell size, foam density, and cell wall thickness were considered as variable parameters. Analysis of variance (ANOVA) tool was utilized to recognize the effective parameters on the different mechanisms of heat transfer. Regression models were presented to forecast the different mechanisms of heat transfer and their validities were checked using ANOVA tool as well as compared to the thermal conductivity results. Surface plots were used to study the interaction effect of significant parameters. The optimization procedure was performed using RSM. Foam density and cell wall thickness are effective parameters on the solid thermal conductivity whereas cell size and foam density were significant parameters on the thermal radiation. By decreasing foam density, gaseous thermal conductivity and thermal radiation were increased and solid thermal conductivity was reduced. The regression model predicted the overall thermal conductivity with an average error smaller than 3%. The results illuminated that the overall thermal conductivity in the optimum conditions was as small as 29 mW/mK.

The effect of pore numbers in the cell walls of soybean oil polyurethane foam on sound absorption performance

Since environmental protection is of increasing importance, the sound absorption of polyurethane foams in which the normal petroleum content is partly replaced by soybean oil has been studied through simulations and experiments. In this study, the theoretical sound absorption coefficients of the foam with different microstructures were calculated by using Johnson-Champoux-Allard (JCA) method. The parameters required in the JCA method were calculated based on the specific finite element analysis of periodic unit cells (PUCs) under ideal conditions. The PUCs were modeled as tetrakaidecahedrons with different numbers of pores in the wall cells. Both the experimental results and simulation results show that the sound absorption performance of PU foam is influenced by pore numbers, and the more pores the better sound absorption performance. Simulations show that the number of pores influences the JCA parameters including air-flow resistivity, tortuosity and characteristic lengths.

Preparation and properties of negative ion/polyurethane flexible foam composites

In this study, negative ionpowder was modified with a silane coupling agent and then added to the polyurethane flexible foam to prepare NI/PU flexible foam composites by the one-step foaming method. The effects of the amount of negative ion powder on the mechanical properties, thermal properties and release of negative ions were investigated using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and negative ion detectors. The SEM results showed that modified negative ion powder could be more uniformly distributed around the cell walls of the polyurethane flexible foam. The thermal stability, tensile strength and resilience of the NI/PU flexible foam composite were improved with the increase of the amount of modified negative ion powder. Increasing the amount of modified negative ion powder could also result in an increase in the release of negative ions, and it reached 5500/cm3 or higher at a negative ion content of 3%.

Nanostructural approach to the thickening behavior and oxidation of calcium-stabilized aluminum foams

The influences of calcium content on thickening stage, dynamic oxidation and subsequent formation of cell walls in closed-cell aluminum foams were investigated by both precise thermodynamic and experimental studies. A novel technique was also used for the sample preparation, and the compendious characterization by transmission electron microscopy. Based on nanostructural investigations, the results indicated that in low calcium amounts (0.5 wt.%), the formation of complex oxide nanoparticles plays a key role in cell wall formation and bubble stabilization, when ignoring the effect of foaming agents. The higher calcium contents (1.5 wt.%) result in different thickening mechanism in which, relatively thick and brittle complex oxide layers are formed. Breaking of these oxide layers due to mechanical stirring causes the entrapment of several fragments in the interface of bubbles and consequently lead to cell wall stabilization. Due discussions were carried out about the effects of different thickening mechanism on the mechanical behavior of cell walls.

Electroless Ag deposition on the cell walls of carbon foam by a displacement method

To obtain efficient metalization of the surface and cell walls of carbon foam, an electroless silver displacement deposition method was investigated, in which a copper layer was first electroless-deposited and then displaced by silver. The purity of the silver coating, and the morphology, microstructure, mechanical strength and electrical conductivity of the resulting carbon foams were characterized by SEM, EDS, XRD, mechanical and conductivity tests. Results indicate that a uniform and continuous silver film is formed on the cell walls of the carbon foam with a better adhesion to the carbon compared to that obtained using the traditional one-step silver plating method. The coating is crystalline silver without copper or silver oxide. The conductivity and mechanical strength of these carbon foams increase from 700 to 2055 S/cm and 0.54 to 1.05 MPa, respectively when the silver content is increased from 0 to15.5 wt%.

High temperature deformation behavior of closed cell ZnAl27 hybrid foam made through stir casting technique

Closed cell ZnAl27 hybrid foam was synthesized through stir casting technique using CaH2 (0.5 wt%) as foaming agent and cenosphere particles were used as thickening agent and to create microporosity into the final structure. The relative density of the synthesized hybrid foam was found in the range of 0.20–0.30. Microstructural characterization was carried out to find distribution of cenosphere particles into the cell wall of hybrid foam. Compression tests were carried out to find out the effect of temperature, relative density and strain rate on the deformation behavior of the foam. It was found that each foamed sample displays three regions i.e., linear elastic, plateau and densification respectively. On the basis of stress-strain curves obtained at varying relative density, strain rate and temperature the values of plateau stress, energy absorption capacity and densification strain were calculated. It was found that according to the increase in relative density and strain rate the plateau stress and energy absorption capacity also increased. The effect of temperature on plateau stress and energy absorption capacity for lower relative density was more as compared to the higher relative density. The densification strain was almost invariant (0.02–0.03) on varying strain rate for the same relative density.

Failure strength predictions for closed-cell polyvinyl chloride foams

This paper describes an effort to model mechanical strength of closed-cell polyvinyl chloride foams under static loading. The study presented here is a continuation of an earlier study to model elastic stiffness of closed-cell polyvinyl chloride foams as effective transversely isotropic materials. An engineering approach is used in the study and governing equations are developed for predicting the strength of polyvinyl chloride foams. To account for foam microstructure and cell-shape anisotropy on foam strength, a unit cell representation of the polyvinyl chloride foam microstructure is used to derive equations to assess tensile and shear strengths of polyvinyl chloride foams. The differential stretching of polyvinyl chloride foam cell walls (in the rise direction and in the in-plane directions) on the strength of the foam-matrix polymer is also taken into account in modeling the mechanical strength of polyvinyl chloride closed-cell foams. The behavior of closed-cell polyvinyl chloride foams under compression is different from that under tension. In the paper, the equations for predicting compressive strength of closed-cell polyvinyl chloride foams are based on an approximate theory developed in an earlier study of compressive strength of unidirectional composites. The validity of the foam strength predictive equations, derived in the paper, is first demonstrated through comparison of the predictions with the results on Divinycell H (DIAB) foams obtained from a systematic in-house test program. A comparison is also carried out between the strength predictions and the test results published by two polyvinyl chloride foam manufacturers for different density polyvinyl chloride foams. Good agreements are found for all the different density foams studied.

Nanoplatelet reinforcement of cavity cell walls in polymer foams using carbon dioxide supercritical fluid

ABSTRACTReinforcing the cavity cell walls of polymer foams using nanoparticles can offer a new era for the property‐structure‐processing field in the development of functionalized ultra‐light components and devices manufactured from foam. When the nanoparticles are exfoliated in polymers, the viscosity substantially increases and thus mixing or foaming usually becomes almost impossible. We use CO2 supercritical fluid (CO2 SCF) for the mixing and foaming of poly(ethylene‐vinyl acetate) copolymer (EVA) with montmorillonite (MMT) nanoplatelets. The in situ evaporation of CO2 induces robust cavity cells of the EVA/MMT nanocomposite foam in a stable form of spherical shapes, which are seldom achieved by other methods. As the bubble grows and becomes stabilized in CO2 SCF, the exfoliated MMT nanoparticles are aligned at the cell walls by the Gibbs adsorption principle to minimize the surface energy at the gas–liquid interface and increase the rupture strength of the cavity walls. It is demonstrated that the developed methodology can be successfully used for foaming EVA containing high vinyl acetate (VA) content (>40%). Since EVA is too soft to construct cell walls of foam using conventional methods, the applicability of the developed methodology is extensively broadened for superior adhesion and compatibility with other materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46615.