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Polymer-based (nano)composite foams containing conductive (nano)fillers limit electromagnetic interference (EMI) pollution, and have been shown to act as good shielding materials in electronic devices. However, due to their high (micro)structural complexity, there is still a great deal to learn about the shielding mechanisms in these materials; understanding this is necessary to study the relationship between the properties of the microstructure and the porous structure, especially their EMI shielding efficiency (EMI SE). Targeting and controlling the electrical conductivity through a controlled distribution of conductive nanofillers are two of the main objectives when combining foaming with the addition of nanofillers; to achieve this, both single or combined nanofillers (nanohybrids) are used (as there is a direct relationship between electrical conductivity and EMI SE), as are the main shielding mechanisms working on the foams (which are expected to be absorption-dominated). The present review considers the most significant developments over the last three years concerning polymer-based foams containing conductive nanofillers, especially carbon-based nanofillers, as well as other porous structures created using new technologies such as 3D printing for EMI shielding applications. It starts by detailing the microcellular foaming strategy, which develops polymer foams with enhanced EMI shielding, and it particularly focuses on technologies using supercritical CO2 (sCO2). It also notes the use of polymer foams as templates to prepare carbon foams with high EMI shielding performances for high temperature applications, as well as a recent strategy which combines different functional (nano)fillers to create nanohybrids. This review also explains the control and selective distribution of the nanofillers, which favor an effective conductive network formation, which thus promotes the enhancement of the EMI SE. The recent use of computational approaches to tailor the EMI shielding properties are given, as are new possibilities for creating components with varied porous structures using the abovementioned materials and 3D printing. Finally, future perspectives are discussed. [ABSTRACT FROM AUTHOR]

Polyurethane foam (PUF)'s porous structure, light weight, flexibility, and low‐cost properties make it useful in various cutting‐edge technologies. However, time‐consuming, costly, and complicated surface modification methods severely hinder its commercial applications. Herein, an ultrafast, simple, and cost‐effective surface modification method based on the evaporation of a low boiling point solvent to prepare a multifunctional graphene nanoplatelet (GNP)‐decorated PUF (GNP@PUF) is proposed. Due to the passive heat barrier of GNP sheets, the resulting sponge exhibits excellent flame retardancy by reducing the critical fire retardancy metrics, that is, peak heat release rate, total heat release, and total smoke release by 72%, 50%, and 81%, respectively. In addition, GNP@PUF can function as a piezoresistive sensor and electromagnetic interference (EMI)‐shielding material. As a piezoresistive sensor, it exhibits a wide‐compressive pressure (2.4–112 kPa)/strain (5–70%) range and ultra‐fast response/relaxation time (48/35 ms), wide‐stretching strain (5–100%) range, and it can detect minute human motions by being attached to different parts of the human body. Meanwhile, the composite foam displays good absorption‐dominant EMI shielding performance (≈38 dB), possibly due to conductive dissipation and multiple reflections/scattering of EM waves inside the 3D conductive graphene network. This study provides a simple coating technique for developing multifunctional lightweight foam materials. [ABSTRACT FROM AUTHOR]

Abstract Polyurethane foam (PUF)’s porous structure, light weight, flexibility, and low‐cost properties make it useful in various cutting‐edge technologies. However, time‐consuming, costly, and complicated surface modification methods severely hinder its commercial applications. Herein, an ultrafast, simple, and cost‐effective surface modification method based on the evaporation of a low boiling point solvent to prepare a multifunctional graphene nanoplatelet (GNP)‐decorated PUF (GNP@PUF) is proposed. Due to the passive heat barrier of GNP sheets, the resulting sponge exhibits excellent flame retardancy by reducing the critical fire retardancy metrics, that is, peak heat release rate, total heat release, and total smoke release by 72%, 50%, and 81%, respectively. In addition, GNP@PUF can function as a piezoresistive sensor and electromagnetic interference (EMI)‐shielding material. As a piezoresistive sensor, it exhibits a wide‐compressive pressure (2.4–112 kPa)/strain (5–70%) range and ultra‐fast response/relaxation time (48/35 ms), wide‐stretching strain (5–100%) range, and it can detect minute human motions by being attached to different parts of the human body. Meanwhile, the composite foam displays good absorption‐dominant EMI shielding performance (≈38 dB), possibly due to conductive dissipation and multiple reflections/scattering of EM waves inside the 3D conductive graphene network. This study provides a simple coating technique for developing multifunctional lightweight foam materials.

Polyurethane foams (PUFs) are a significant group of polymeric foam materials. Thanks to their outstanding mechanical, chemical, and physical properties, they are implemented successfully in a wide range of applications. Conventionally, PUFs are obtained in polyaddition reactions between polyols, diisoycyanate, and water to get a CO2 foaming agent. The toxicity of isocyanate has attracted considerable attention from both scientists and industry professionals to explore cleaner synthesis routes for polyurethanes excluding the use of isocyanate. The polyaddition of cyclic carbonates (CCs) and polyfunctional amines in the presence of an external blowing agent or by self-blowing appears to be the most promising route to substitute the conventional PUFs process and to produce isocyanate-free polyurethane foams (NIPUFs). Especially for polyhydroxyurethane foams (PHUFs), the use of a blowing agent is essential to regenerate the gas responsible for the creation of the cells that are the basis of the foam. In this review, we report on the use of different blowing agents, such as Poly(methylhydrogensiloxane) (PHMS) and liquid fluorohydrocarbons for the preparation of NIPUFs. Furthermore, the preparation of NIPUFs using the self-blowing technique to produce gas without external blowing agents is assessed. Finally, various biologically derived NIPUFs are presented, including self-blown NIPUFs and NIPUFs with an external blowing agent. [ABSTRACT FROM AUTHOR]

The evolution of technologies applied to surfboards has shown an accelerated growth in recent years. Not only has the design of the boards changed, but the materials used in the surfboards production have also evolved greatly. In this sense, in this work an investigative and comparative study was carried out between different types of materials used for the production of composites with sandwich structures used in surfboards. It was evaluated two thermoset polymeric matrices (unsaturated polyester and epoxy), 4 types of fibers in the form of blankets (glass, carbon, polyaramid and cotton) and 3 types of cores in the form of foams (polyurethane, expanded polystyrene and cork). The composite processing was carried out on a laboratory scale, but following the same procedures used in the surfboard production, that is, manual lamination by hand lay up. For the production of the sandwich structure, the composites (epoxy) were laminated on the foam surface, using two layers of glass fabrics and different cores. The main results indicate a better efficiency of the polyurethane core. In laminated composites, among the evaluated formulations, epoxy with carbon sample showed superior mechanical properties. [ABSTRACT FROM AUTHOR]

Cellular solid materials are progressively becoming more predominant in lightweight structural applications as more technologies realize these materials can be improved in terms of performance, quality control, repeatability and production costs, when allied with fast developing manufacturing technologies such as Additive Manufacturing. In parallel, the rapid advances in computational power and the use of new numerical methods, such as Meshless Methods, in addition to the Finite Element Method (FEM) are highly beneficial and allow for more accurate studies of a wide range of topologies associated with the architecture of cellular solid materials. Since these materials are commonly used as the cores of sandwich panels, in this work, two different topologies were designed — conventional honeycombs and re-entrant honeycombs — for 7 different values of relative density, and tested on the linear-elastic domain, in both in-plane directions, using the Natural Neighbor Radial Point Interpolation Method (NNRPIM), a newly developed meshless method, and the Finite Element Method (FEM) for comparison purposes. [ABSTRACT FROM AUTHOR]

The loss of energy especially in industrial and residential buildings is one of the main reasons of increased energy consumption. Improving the thermal insulation properties of materials is a fundamental method for reducing the energy losses. Polymeric foams are introduced as materials with excellent thermal insulation properties for this purpose. In the present study, a deep theoretical investigation is performed on the overall thermal conductivity of low-density polyethylene (LDPE) foams. The thermal conductivity by radiation is predicted using two different methods. The most appropriate model is selected in comparison with experimental results. The results show that the theoretical model has an appropriate agreement with the experimental results. The effects of foam characteristics including foam density, cell size, and cell wall thickness on the overall thermal conductivity are investigated. The results indicate that by decreasing the cell size and increasing the cell wall thickness, the overall thermal conductivity is decreased significantly. Also, there is an optimum foam density in order to achieve the smallest thermal conductivity. The lowest overall thermal conductivity achieved in the studied ranges is 30 mW/mK at foam density of 37.5 kg.m-3, cell size of 100 μm, and cell wall thickness of 6 μm.

Polymeric foams have good capacity of absorbing energy in compression but are brittle in tension. Linear elastic fracture mechanics is successfully applied to assess the integrity of polymeric foam‐based composite structures. The fracture toughness represents an important parameter. The different approaches to estimate the fracture toughness of polymeric foams are reviewed: analytical and numerical micromechanical models and experimental investigations. Focus is given on the parameters influencing the fracture toughness of polymeric foams like specimen type, solid material, density, loading speed, size effect and temperature. Data on mixed‐mode loading and dynamic fracture toughness are also presented. The last part of the paper presents some results to increase the fracture toughness by reinforcing of polymeric foams. [ABSTRACT FROM AUTHOR]

Cellular multistable architected material has been widely studied for its promising energy absorption applications for personnel protection, protective packaging, and crash mitigation. Despite the tailorable properties enabled by their geometric features, a major issue in the actual application is their lower strengths in the elastic behavior of the multistable architected material. Herein, a novel design strategy of strain rate-dependent architected cellular composite material (FACCM) is proposed, aiming to achieve enhanced energy-absorbing properties with the proposed composite system than the single material constituent. Guided by numerical simulations and experimental tests, the compressive behaviour of FACCM at multiple strain rates and the effect of filled foam on it with specific geometric parameters are characterized. The quasi-static performances of the FACCM specimens were evaluated both at the cell level and planar array level. Our results indicate that the strength, stiffness, and snap-through behaviour of multistable architected material can be significantly improved by the filled foam at elevated loading rates regardless of quasi-static loading or impact tests. The current study offers a new strategy for developing novel packing, shock absorption, and impact protection systems. [ABSTRACT FROM AUTHOR]

Polymeric foams are used for impact protection due to their ability to absorb large amounts of strain energy. In this work, the compressive response of an open cell polyurethane foam currently used as liner in the advanced combat helmet is examined across strain rates. A traditional load frame is used to investigate the quasi-static behavior, and two different modifications of a conventional Kolsky (split-Hopkinson) bar configuration are used to probe the dynamic response. A unique, independent method not relying on strain gage signals is presented that leverages high-speed full-field imaging to track the velocity on each side of the sample-bar interface and used to extract the dynamic stress-strain response; the results are compared against traditional strain gage measurements. X-ray tomography is used to examine the global morphological characteristics of the foam. The foam is found to be strongly rate dependent, where the characteristic properties vary logarithmically with strain rate. An analytical expression is presented to describe the rate dependency that collapses all stress-strain curves on a master curve. Full-field kinematic data from digital image correlation taken during loading is used to extract a nonlinear Poisson's ratio as a function of strain, which is found to be strain rate insensitive. A tangent Poisson function is used to explore the foam's auxetic behavior. These findings provide insight on physically-based constitutive modeling of foams, crucial to predictive brain injury simulations, as well as motivate the need to probe local heterogenous behavior across strain rates moving forward. [ABSTRACT FROM AUTHOR]

Featured Application: Mitigating vibrations of flat, large-scale structures, for example, car front wall or ceiling, as well as structural and/or cover parts for rail vehicles or noise barriers of different kinds. Furthermore adaptable compact dampers, for example, machine feet are also a promising application field for the proposed novel damping approach. Evanescent morphing in combination with an original concept of Compressible Constrained Layer Damping (CCLD) is a novel and promising approach for dynamic behaviour adaptation. The crucial component of the CCLD is a compressible intermediate layer with its thickness and material properties controlled by fluid actuation, enabling the adjustment of the damping and stiffness of the overall system. To estimate the potential of the CCLD, an analytical model was developed which describes the vibration behaviour of the overall structure, taking into account the compression-driven properties of the intermediate layer. The results confirm the principal correctness of the initial theoretical assumptions regarding the adaptive dynamic behaviour of structures with CCLD treatment. A significant vibration mitigation as well as a high adaptability of dynamic behaviour were observed, however, they show a complex dependence on the system configuration. Nevertheless, the developed analytical modelling approach can already be used for a preliminary system design. Besides the analysed polymer-based foams as the intermediate layer, nonwovens also exhibit compression-dependent shear properties and can therefore be used in CCLD. First preliminary investigations show that the damping performance is on average about ten times better than that of the polymeric foams. [ABSTRACT FROM AUTHOR]

Evanescent morphing in combination with an original concept of Compressible Constrained Layer Damping (CCLD) is a novel and promising approach for dynamic behaviour adaptation. The crucial component of the CCLD is a compressible intermediate layer with its thickness and material properties controlled by fluid actuation, enabling the adjustment of the damping and stiffness of the overall system. To estimate the potential of the CCLD, an analytical model was developed which describes the vibration behaviour of the overall structure, taking into account the compression-driven properties of the intermediate layer. The results confirm the principal correctness of the initial theoretical assumptions regarding the adaptive dynamic behaviour of structures with CCLD treatment. A significant vibration mitigation as well as a high adaptability of dynamic behaviour were observed, however, they show a complex dependence on the system configuration. Nevertheless, the developed analytical modelling approach can already be used for a preliminary system design. Besides the analysed polymer-based foams as the intermediate layer, nonwovens also exhibit compression-dependent shear properties and can therefore be used in CCLD. First preliminary investigations show that the damping performance is on average about ten times better than that of the polymeric foams.

An inverse radiation analysis is performed for identification of spectral radiative properties for a class of highly porous closed cell polymeric foams using coupled Monte Carlo-Particle Swarm Optimization method. An Open Message Passing Interface (MPI) based parallel Monte Carlo method is adopted to simulate thermal radiation within the foam. The Particle Swarm Optimization (PSO) with adaptive weighted delay velocity is employed to inversely identify the spectrally dependent transport extinction coefficient and scattering albedo of expanded polystyrene and polyethylene foams from measured spectral transmittance. The transmittance measurements were performed with a Fourier-transform infrared spectrometer across the IR wavelength range of 2.0–16.7 μm. The developed inverse model used transmittance measurements of two samples of the same foam having different thicknesses to successfully retrieve the unknown radiative parameters of the foams. Both of the foams demonstrated consistently high transport extinction coefficients at all wavelengths in a range of 6400–400 m−1. Spectral transport scattering albedos of both the foams were discovered to be in the range 0.70–1.0. The accuracy of the proposed inverse model is checked by numerically predicting the spectral transmittances for a third sample of different thickness using the retrieved radiative properties. A good agreement is observed between the experimental and the numerically predicted transmittance values for the third thickness. [ABSTRACT FROM AUTHOR]

Bike and ski helmets are mainly made up of two layers: the external shell and the foam liner. The foam liner, typically made of expanded polystyrene (EPS) or polypropylene (EPP), is asked to provide energy absorption in case of impacts. Standard helmet design requires the foam to maximize this energy absorption, thus achieving large deformations (up to 25% in compression) while maintaining the stress level below a threshold value. To optimize the helmet construction in terms of foam composition, structure and density, reliable numerical models are required, which in turn need to be fed with accurate experimental data. A characterisation of several foams was performed, including EPS and EPP having varying densities, under tensile and compressive stress states at varying strain rates. Typical mechanical parameters (elastic moduli and plateau stress in compression, Poisson's ratio) were compared with literature data and applicability of existing models to experimental results was discussed. A marked strain rate dependence – very important for impact applications – was accurately described using the Nagy phenomenological model. The foam microstructure was investigated using scanning electron microscopy (SEM) to assess structural changes before and after compression. The aforementioned mechanical features were then adopted in a rate-dependent constitutive law for crushable foams, to model the shock attenuation properties of helmets and validate the approach against available data. Finally, a microelectromechanical system based in-helmet wireless micro monitoring system was developed and inserted in a helmet prototype. The system is capable of acquiring impact load spectra, providing valuable information to investigate generic impacts with varying angles and energy. In particular, it can monitor the effect of repeated micro-impacts on the residual energy absorption characteristics of the foam. [ABSTRACT FROM AUTHOR]

Developments in design and technology in the engineering and medical fields necessitate the use of smart and high-performance materials to meet higher engineering specifications. The general requirements of such materials include a combination of high stiffness and strength with significant weight savings, resistance to corrosion, chemical resistance, low maintenance, and reduced costs. Over the last three decades, it has been demonstrated that auxetic materials offer a huge potential for the fields of engineering, natural sciences, and biomedical engineering, and for many other industries, including the aerospace and defense industries, through their unique deformation mechanism and measured enhancements in mechanical properties. To meet future engineering challenges, auxetic materials are increasingly being recognized as integral components of smart and advanced materials. Although materials with a negative Poisson's ratio have been known since the early 1900s, they did not capture researchers' attention until the late 1980s. Since 1991, these materials have been known as auxetic materials. Since then, their benefits and applications have been expanded to all major classes of materials such as metals, ceramics, polymers, and composites, and they are also now being used in engineering applications. The goal of this review was to present the development of auxetic polymers, which were first fabricated in the form of polyurethane foam approximately three decades ago and are now used in the fabrication of non-woven nano/micropolymeric structures. This review could provide useful information for the future development of auxetic polymers. [ABSTRACT FROM AUTHOR]

Portable, water filled road safety barriers are used to provide protection and reduce the potential hazard due to errant vehicles in areas where the road conditions change frequently (e.g. near road work sites). As part of an effort to reduce excessive working widths typical of these systems, a study was conducted to assess the effectiveness of introducing polymeric foam filled panels into the design. Surrogate impact tests of a design typical of such as barrier system were conducted utilising a pneumatically powered horizontal impact testing machine up to impact energies of 7.40 kJ. Results of these tests are utilised to examine the barrier behaviour, in addition to being used to validate a couple FE/SPH model of the barrier system. Once validated, the FE/SPH model it utilised as the basis for a parametric study into the efficacy and effects of the inclusion of polymeric foam filled panels on the performance of portable water filled road safety barriers. It was found that extruded polystyrene foam functioned well, with a greater thickness of the foam panel significantly reducing the impacting body velocity as the barrier began to translate. [ABSTRACT FROM AUTHOR]

Of interest, in this work, is the behavior of open-cell polymer foams under compression. In a recent work by Del Piero and Pampolini (Continuum Mech Thermodyn 24:181-199, ), a model coupling nonlinear elasticity and viscosity was introduced to describe the response of a polyurethane foam subjected to uniaxial cyclic compression. Inelastic effects were attributed to the viscous properties of the foam, while strain localization and hysteresis were attributed to the nonconvexity of the strain energy density. But the model did not reproduce the response curves after the first loading-unloading cycle. Here, the model is extended by taking into account the damage of the foam. A simple phenomenological one parameter damage law is used to describe the damage occurring during the first loading cycle. An accurate identification procedure for the material constants is also developed, and the interaction of viscosity and damage in both monotonic and cyclic deformation processes is discussed. In spite of the one dimensional formulation, the model permits a precise analysis of the main phenomena which determine the complex behavior of the foam. [ABSTRACT FROM AUTHOR]

Abstract: Heat insulating properties of polymeric foams at ambient temperature were investigated using the model of coupled heat transfer by conduction and radiation through computer-reconstructed domains. The proposed model considers absorbing and emitting gas and solid phases and partial photon reflection on phase interfaces. The P 1-approximation is used for the description of radiation. For optically thin polymeric walls, the interface conditions are adjusted to account for specular reflection and wave interference effects. The developed model was validated using experimental data and compared with other approaches, including state-of-the-art homogeneous phase approach (HPA). Contrary to HPA, our approach doesn't treat polymeric foams as homogeneous materials with effective properties and provides temperature and radiation gradient fields, which respect foam morphology. The model predicts that the radiative heat flux can account for more than one third of the total heat flux in high porosity expanded polystyrene (EPS) or polyethylene (PE) foams and that the equivalent conductivity of polymeric foam can be significantly reduced by the careful balancing of porosity, cell size, wall thickness and strut content. The presented model doesn't use any empirical correlations and can be used for the optimization of heat transfer in any type of polymeric foam. [Copyright &y& Elsevier]

Abstract: The thermo-mechanical and impact properties of materials used for hard-shell and soft-shell back protectors have been analysed in order to understand the mechanism of action of the foams used for protective equipment. Dynamical mechanical analysis has shown that materials used for soft-shell protectors present frequency-sensitive properties that permit to have a soft response when stressed at low speed and a hard response when subjected to a high-speed impact. Furthermore, by means of drop weight impact tests, the shock absorbing characteristics of the materials have been investigated at two temperatures pointing out the differences between soft and hard-shell protectors; in addition it has been demonstrated that the materials used for soft-shell protectors maintain their protective properties after multi-impacts on the same point. [Copyright &y& Elsevier]

Abstract: The choice of the appropriate material for the construction of ski-boot liners is of fundamental importance in order to achieve ergonomic comfort and high thermal insulation. In this work the effect on thermal comfort of different materials used for commercial ski-boot liners has been analysed. The thermal insulation and the moisture management of liners made of different materials have been tested both in a climatic chamber and in outdoor conditions using wireless sensors combined with infrared thermography. The results obtained show substantial differences in terms of thermal comfort between the liners in the same environmental conditions, showing that closed cell ethylene vinyl acetate foams provide the best thermal comfort. [Copyright &y& Elsevier]

Structural foams are a relatively new class of materials with peculiar characteristics that make them very attractive in some energy absorption applications. They are currently used for packaging to protect goods from damage during transportation in the case of accidental impacts. Structural foams, in fact, have sufficient mechanical strength even with reduced weight: the balance between the two antagonist requirements demonstrates that these materials are profitable. Structural foams are generally made of microcellular materials, obtained by polymers where voids at the microscopic level are created. Although the processing technologies and some of the material properties, including mechanical, are well known, very little is established for what concerns dynamic impact properties, for the design of energy absorbing components made of microcellular foams. The paper reports a number of experimental results, in different loading conditions and loading speed, which will be a basis for the structural modeling. [ABSTRACT FROM AUTHOR]

This paper shows in detail the modelling of anisotropic polymeric foam under compression and tension loadings, including discussions on isotropic material models and the entire procedure to calibrate the parameters involved. First, specimens of poly(vinyl chloride) (PVC) foam were investigated through experimental analyses in order to understand the mechanical behavior of this anisotropic material. Then, isotropic material models available in the commercial software Abaqus™ were investigated in order to verify their ability to model anisotropic foams and how the parameters involved can influence the results. Due to anisotropy, it is possible to obtain different values for the same parameter in the calibration process. The obtained set of parameters are used to calibrate the model according to the application of the structure. The models investigated showed minor and major limitations to simulate the mechanical behavior of anisotropic PVC foams under compression, tension and multi-axial loadings. Results show that the calibration process and the choice of the material model applied to the polymeric foam can provide good quantitative results and save project time. Results also indicate what kind and order of error one will get if certain choices are made throughout the modelling process. Finally, even though the developed calibration procedure is applied to specific PVC foam, it still outlines a very broad drill to analyze other anisotropic cellular materials. [ABSTRACT FROM AUTHOR]

An orthotropic polymeric foam with transverse isotropy (Divinycell H250) used in composite sandwich structures was characterized at various strain rates. Uniaxial experiments were conducted along principal material axes as well as along off-axis directions under tension, compression, and shear to determine engineering constants, such as Young's and shear moduli. Uniaxial strain experiments were conducted to determine mathematical stiffness constants, i. e., C. An optimum specimen aspect ratio for these tests was selected by means of finite element analysis. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson Pressure Bar system built for the purpose using polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals and results in lower noise to signal ratios and longer loading pulses. It was determined by analysis and verified experimentally that the loading pulses applied, propagated along the polycarbonate rods at nearly constant phase velocity with very low attenuation and dispersion. Material properties of the foam were obtained at three strain rates, quasi-static (10 s), intermediate (1 s), and high (10 s) strain rates. A simple model proposed for the Young's modulus of the foam was in very good agreement with the present and published experimental results. [ABSTRACT FROM AUTHOR]

Abstract: A method to model high strain rate compressive properties of PU foams was investigated. The nonlinear mechanical response of the foam was split into three independent contributions: foam morphology, time-dependent polymer response, gas entrapped in the cells. The foam morphology contribution was estimated by a simple uniaxial compression test at very low strain rate. The time (temperature)-dependent behavior of the polymeric foam was evaluated by stress relaxation tests. Due to the low degree of cell interconnections of the studied foam, at low strain rates the gas contribution was predicted by using a gas flow model derived from Darcy''s law. Starting from 2s−1 compressive strain rate, the foam exhibited a transition and the gas contribution was evaluated by a scaled closed cell model. The proposed method is able to predict the mechanical response of foams at high strain rates using data obtained by few low strain rates mechanical tests. [Copyright &y& Elsevier]

Abstract: Porous polymeric materials have a good impact strength, light weight and high porosity. Therefore, these foams are used in a wide range of applications, including insulation, separation processes and packaging. Since it is important to have an objective comparison of polymeric foams, this paper discusses a quantitative method using Voronoi diagrams, which has been developed to analyze and compare different morphologies of polymers foamed with supercritical carbon dioxide, based on the average cell area, cell area distribution and homogeneity. In contrast to SEM analysis only, different foam morphologies can be compared with this technique in an objective manner. The analysis is based on the centers of the cells and is therefore to a large extent independent of the sample preparation. Using the Voronoi-approach, it is possible to extrapolate the 2D view of the SEM picture to a 3D representation of the foam with the thickness of one cell layer. [Copyright &y& Elsevier]

In addition to lightweight and moldable characteristics, polymeric foams possess an excellent energy-absorbing capability that can be used for a wide range of commercial applications, especially for crash protection in automobiles. The purpose of the present study is to develop an experimental methodology to characterize the pressure dependent yield behavior of energy-absorbing polymeric foams. A device was designed to perform a compression test in a triaxial stress state. For the test material, polyurethane foams of two different densities were used. The displacement of the specimen, the load applied to the specimen, and hydrostatic pressure applied to the specimen were measured and controlled. Stress–strain curves and yield stresses for different hydrostatic pressures were obtained. The experiments conducted in this study revealed that the polyurethane foams exhibited significant increase in yield stress with applied hydrostatic pressure or mean normal stress. Based on this observation, a yield criterion which included the effect of the stress invariant was established for the foams. The experimental constants obtained which constituted the pressure-dependent yield criterion were verified. [ABSTRACT FROM AUTHOR]

Fabricating polymeric scaffolds using cost-effective manufacturing processes is still challenging. Gas foaming techniques using supercritical carbon dioxide (scCO2) have attracted attention for producing synthetic polymer matrices; however, the high-pressure requirements are often a technological barrier for its widespread use. Compressed 1,1,1,2-tetrafluoroethane, known as Freon R134a, offers advantages over CO2 in manufacturing processes in terms of lower pressure and temperature conditions and the use of low-cost equipment. Here, we report for the first time the use of Freon R134a for generating porous polymer matrices, specifically polylactide (PLA). PLA scaffolds processed with Freon R134a exhibited larger pore sizes, and total porosity, and appropriate mechanical properties compared with those achieved by scCO2 processing. PLGA scaffolds processed with Freon R134a were highly porous and showed a relatively fragile structure. Human mesenchymal stem cells (MSCs) attached to PLA scaffolds processed with Freon R134a, and their metabolic activity increased during culturing. In addition, MSCs displayed spread morphology on the PLA scaffolds processed with Freon R134a, with a well-organized actin cytoskeleton and a dense matrix of fibronectin fibrils. Functionalization of Freon R134a-processed PLA scaffolds with protein nanoparticles, used as bioactive factors, enhanced the scaffolds' cytocompatibility. These findings indicate that gas foaming using compressed Freon R134a could represent a cost-effective and environmentally friendly fabrication technology to produce polymeric scaffolds for tissue engineering approaches. [ABSTRACT FROM AUTHOR]

The elevated strain rate compressive response of closed-cell polyvinyl chloride (PVC) foam at various densities is investigated. Two loading directions, (i.e., parallel and perpendicular to foam rise direction) were considered to investigate structural anisotropy. The elevated strain rates tests (up to 200 s−1) were performed using a customized drop tower device. Engineering stress/strain behavior, energy dissipation, and maximum stress capacity were obtained for each density and compared against each other. Except for the lowest density of 45 kg/m3, strain rate effects were clearly observed through increased compressive strength and plateau stress when loading in the foam rise direction. The strain rate effect is more evident at higher densities. However, no significant strain rate effect was observed when loading perpendicular to the foam rise direction. Scanning electron microscopy (SEM) analysis showed that plastic hinges are the primary deformation mechanism for PVC foam cells. An analytical model has been calibrated using the experimental results and successfully predicted the mechanical response of the foam. Shape anisotropy has been measured employing the SEM images. The analytical approach was also able to predict the foam's anisotropic mechanical response. Image 1 • Intermediate strain rate (up to 200 s−1) compression tests on PVC foams. • Strain rate effects observed when loading in the foam rise direction. • No significant rate effects observed when loading perpendicular to rise direction. • SEM analysis illustrated plastic hinges are main deformation mechanism. • Anisotropic behavior accurately predicted using an analytical approach. [ABSTRACT FROM AUTHOR]

In this work, high strength, flexible and cycling-stable piezoresistive thermoplastic polyurethane (TPU)/multi-wall carbon nanotubes (MWCNTs) composite foams were fabricated by a solution-blending method and supercritical carbon dioxide (ScCO 2) foaming process. The microcellular structure (1–10 μm) of the TPU/MWCNTs composites enhanced the energy storage and reduced the conductive percolation threshold. The compressive strength and compressive modulus of the composites were 1.05 MPa and 3.79 MPa. Percolation threshold of the foamed composites decreased from 0.90 vol% to 0.31 vol%. The prepared TPUC5.0 foam (the content of MWCNTs is 5%) had high sensitivity, wide strain range, and stable resistance. The piezoresistive current was relatively stable, and showed high repetitive force-electric conversion behavior in 1000 cycles. Thus, polymeric foams are suitable for real-time monitoring of human foot movement using wearable strain sensors. [ABSTRACT FROM AUTHOR]

The material behavior and damage of polymeric foams (e.g., polyurethane foam and polystyrene foam) are extremely complex under compression, depending on strain rates and temperatures. In the present study, the elasto-viscoplastic-damage behavior of polymeric foams over a wide range of strain rates and temperature is evaluated and predicted using the modified Gurson-Tvergaard-Needleman-Lemaitre (GTNL) model and the modified Khan-Huang-Liang (KHL) model. A modified GTNL model is introduced to describe phenomena of decreasing void volume fraction and elastic modulus for polymeric foams under uniaxial compression. The modified KHL model is proposed to evaluate the nonlinear strain rate- and temperature-dependent material behavior of polymeric foams. The full derivation of the implicit integration algorithm of the proposed modified GTNL-KHL model is introduced, and a user-defined material (UMAT) subroutine for commercial finite element analysis code (i.e., ABAQUS) is established. Using the UMAT, the damage characteristics (e.g., void volume fraction and elastic modulus changes) and the nonlinear material behavior (e.g., linear elastic, softening/plateau and densification) of polymeric foams under uniaxial compression with various strain rates and temperatures are successfully simulated. • Unified elasto-viscoplastic-damage model for polymeric foams was developed. • Stress-strain curve, void volume fraction change and damage growth were evaluated. • Full derivation for implicit integration algorithm of present model was introduced. • ABAQUS user-defined subroutine (UMAT) for polymeric foams were developed. • Proposed method was validated by comparing a series of experimental results. [ABSTRACT FROM AUTHOR]