FEM simulation of polymeric foam with random pore structure: Uniaxial compression with loading rate effect

This study establishes FEM modeling for compressive deformation behavior of polymeric foams with different loading rates. The polymeric foam used in this study was made from polypropylene (the base matrix of the polymer) with porosity of about 95%. The pore size and shape were randomly distributed in the foam. The X-ray CT method was first conducted to observe the microstructure, the geometric feature of which was reproduced in the FEM model. Uniaxial compression tests with different loading speeds were carried out to investigate an effect of loading rate (strain rate) dependency on the deformation behavior. By using the X-ray CT method, in situ observation of microscopic deformation was carried out. Furthermore, FEM computations were carried out to simulate macroscopic and microscopic deformation behaviors. The random porous structure was modeled using Surface Evolver. The elastoplastic property with strain rate dependency was used in this model. The established FEM framework may be useful for a porous polymer with a random pore structure and for deformation modeling with strain rate effect.     

The role of microstructure on the mechanical properties of polyurethane foams containing thermoregulating microcapsules

Rigid polyurethane foams with up to 50 wt% of microcapsules from LDPE-EVA containing Rubitherm^®RT27 were synthesized. The influence of microcapsules on the foams density, microstructure and mechanical resistance was studied. Cell size and strut and wall thicknesses were analyzed by SEM. The relationships between densities and foam microstructures with their Young's moduli and collapse stress were found by the Gibson and Ashby formulations and the Kerner equation for mechanical properties of composites. It was found a cell structure change from polyhedral closed-cells to spherical or amorphous open-cells. A good agreement between the experimental and theoretical data was observed but requiring a cell form factor. Thus, Fitting parameters confirmed the high trend of these microcapsules to be incorporated into the foam cell walls and the form factors depicted the abrupt change of cell morphology. Thus, these equations are suitable for predicting the mechanical properties of foams containing fillers of low mechanical resistance.     

Material properties of the cell walls in nanofibrillar cellulose foams from finite element modelling of tomography scans

The mechanical properties of the nanofibrillar cellulose foam depend on the microstructure of the foam and on the constituent solid properties. The latter are hard to extract experimentally due to difficulties in performing the experiments on the micro-scale. The aim of this work is to provide methodology for doing it indirectly using extracted geometry of the microstructure. X-ray computed tomography scans are used to reconstruct the microstructure of a nanofibrillar cellulose foam sample. By varying the levels of thresholding, structure of differing porosities of the same foam structure are obtained and their macroscopic properties of the uni-axial compression are computed by finite element simulations. A power law relation, equivalent to classical foam scaling laws, are fit to the data obtained from simulation at different relative densities for the same structure. The relation thus obtained, is used to determine the cell wall material properties, viz. elastic modulus and yield strength, by extrapolating it to the experimental porosity and using the measured response at this porosity. The simulations also provide qualitative insights into the nature of irreversible deformations, not only corroborating the experimental results, but also providing possible explanation to the mechanisms responsible for crushable behaviour of the nanofibrillar cellulose foams in compression.     

X-ray computed tomography-based modeling of polymeric foams: The effect of finite element model size on the large strain response

A hybrid numerical–experimental approach is used to characterize the macroscopic mechanical behavior of polymeric foams. The method is based on microstructural characterization of foams with X-ray computed tomography (CT) and conversion of the data to finite element (FE) models. The 2D models are created from a 3D close-celled foam and subjected to compression loads. Since the large strain regime is explored, contact between elements is incorporated. It is shown that, for calculating the effective Young's modulus, a model consisting of at least 11²–12² cells in the model should be used, whereas for the large strain regime 12²–14² cells in the model are needed. Discretization had a significant influence on the results, where relatively coarse elements caused loss of connectivity in the cell walls and thickening of the cell walls. It is shown that at least three to four elements should be taken over the thickness of the cell walls for these structures. Finally, a good qualitative agreement is observed between the deformations found with the FE models and in situ compression experiments of an open-celled foam during X-ray CT. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1473–1482, 2010     

A multi point conductivity measurement system for characterisation of protein foams

Protein foams play an important role in both food and biotechnological processes. A sound understanding of foaming properties of proteins relevant to such processes is useful e.g. to allow adequate control of unwanted foams and appropriate choice of protein-physical system when foams of certain characteristics are required. In general, measurements of changes in foam volume (volumetric method) are used for foam characterisation. However, recently there has been increased interest in the use of measurement methods based on conductivity and capacitance. Simple relative techniques based on electrical conductivity measurements provide information on both foamability and foam stability. A multi point conductivity measurement system has been designed and used for characterisation of model protein foams (0.1 and 1.0 mg ml⁻¹ Bovine serum albumin, BSA). The solution of BSA was sparged with nitrogen or carbon dioxide gas at constant flow rate (90 cm³ min⁻¹) via a stainless steel sinter (0.5 or 2.0 μm in pore size). A comparison of foaming properties determined by volumetric and conductimetric techniques is provided. Both methods show that more stable foams are obtained for solutions at higher BSA concentrations. At all BSA concentrations, higher foamability and stability are achieved with a smaller sinter pore size. When nitrogen rather than carbon dioxide is used as a dispersed phase, higher foamability and foam stability are obtained. The conductivity measurements indicate that foamability is dependent on gas type, whereas, volumetric measurements do not show such differences.     

Experimental and numerical investigations of the anisotropic deformation behavior of low-density polymeric foams

Low-density porous materials and foams have been widely used for a variety of applications, such as light structural components, impact energy absorption, thermal insulation and sound absorption. The macroscopic deformation of such materials is strongly dependent on their inherent micro-cellular structure. This study investigated the compressive anisotropic deformation behavior of low-density polymeric foam by using X-ray computed tomography (CT) and the finite element method (FEM) in order to understand both the microscopic and macroscopic deformation behavior. The foams used in this study have a closed cell structure, with pores that are elliptical in shape. Three different types of expansion ratios were employed. The porosities of these materials were 93.5, 95, and 96%. From the observations using the X-ray CT method, the averaged pore heights were 1 mm and the aspect ratios were 2, 2.5, and 2.25, respectively. The foam demonstrated anisotropic deformation, dependent on the uni-axial compression direction. It was found that the deformation rigidity in the longitudinal direction was larger than that in the transverse direction. By using the X-ray CT method in situ, the microscopic deformation behavior when subjected to compressive loading was observed. Deformation and collapse of pores was observed for both directions during the loading. In conjunction with this, FEM computations were carried out to elucidate how such pore geometry undergoes elastoplastic deformation and leads to macroscopic deformation behavior. The FEM-created three-dimensional spatial structures were based on elongated rhombic dodecahedrons. It is revealed that the FEM computation shows relatively good agreement with the experimental results. Thus, our experimental and computational models may be useful for microstructural design using anisotropic cellular materials.     

Constitutive modeling for characterizing the compressive behavior of PMMA open‐cell foams

A constitutive model for evaluating the compressive behavior of Poly(methyl‐methacrylate) (PMMA) open‐cell foams is herein proposed. Specifically, the study investigates the viscoelastic and viscoplastic behaviors of the PMMA open‐cell foams. The constitutive equation is expressed in terms of the following polymer and foam properties: elastic modulus, relative density, as well as the relaxation and densification constants. PMMA open‐cell foams are manufactured using a gas foaming/particulate leaching method and uniaxial compression tests are performed. The mechanical properties and compressive stress‐strain responses obtained from the experiments are compared with those predicted by the proposed constitutive model. The results suggest that the constitutive model is an apt one for assessing and evaluating the compressive behaviors of PMMA open‐cell foams. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 436–443, 2007     

Low density polyethylene,expanded polystyrene and expanded polypropylene: Strain rate and size effects on mechanical properties

Polymeric foam materials may be used as energy absorbing materials for protection in impact scenarios, and design with these materials requires the mechanical properties of foams across a range of deformation rates, where high deformation rate testing often requires small samples for testing. Owing to their cellular macrostructure, and the large deformations that occur during loading of foams, the measured stress-strain response of a foam material may be influenced by the sample size. In this study, the mechanical properties of three closed-cell polymeric foams (Low Density Polyethylene, Expanded Polystyrene and Expanded Polypropylene) at two different densities were investigated over a range of deformation rates from 0.01 s⁻¹ to 100 s⁻¹. For each foam material, three different nominal sample sizes (10 mm, 17 mm and 35 mm) were tested. On average, the polymeric foam materials exhibited increasing stress with increasing deformation rate, for a given amount of strain.Density variation was identified at the sample level, with smaller samples often exhibiting lower density. Expanded Polystyrene demonstrated the highest variability in sample density and corresponding variability in mechanical response, qualitatively supported by observed variations in the macrostructure of the foam. Expanded Polypropylene exhibited variability in density with sample size, and observable variability in the material macrostructure; however, the dependence of the measured mechanical properties on sample size was modest. Low Density Polyethylene was found to have a relatively consistent cell size at the macrostructure level, and the material density did not vary significantly with sample size. In a similar manner, the dependence of measured mechanical properties on sample size was modest. The effect of sample size was identified to be material specific, and it is recommended that this be assessed using sample-specific density measurements and considering different sized samples when testing foam materials.     

Open-pore biodegradable foams prepared via gas foaming and microparticulate templating

Open-pore biodegradable foams with controlled porous architectures were prepared by combining gas foaming and microparticulate templating. Microparticulate composites of poly(epsilon-caprolactone) (PCL) and micrometric sodium chloride particles (NaCl), in concentrations ranging from 70/30 to 20/80 wt.-% of PCL/NaCl were melt-mixed and gas-foamed using carbon dioxide as physical blowing agent. The effects of microparticle concentration, foaming temperature, and pressure drop rate on foam microstructure were surveyed and related to the viscoelastic properties of the polymer/microparticle composite melt. Results showed that foams with open-pore networks can be obtained and that porosity, pore size, and interconnectivity may be finely modulated by optimizing the processing parameters. Furthermore, the ability to obtain a spatial gradient of porosity embossed within the three-dimensional polymer structure was exploited by using a heterogeneous microparticle filling. Results indicated that by foaming composites with microparticle concentration gradients, it was also possible to control the porosity and pore-size spatial distribution of the open-pore PCL foams.     

Ultra-lightweight cellulose foam material: preparation and properties

Ultra-lightweight cellulose foams were prepared by regeneration of sodium dodecyl sulfate (SDS)/cellulose/NaOH/urea blend solution via mechanical agitation and then freeze-drying. The morphology and properties of the blend solutions and foams were investigated via optical microscope, rheometer, BET and SEM. As a result, it was found that the inclusion complex structure between cellulose macromolecules and the solvent molecules was not destroyed. Moreover, the bubbles were about 20–50 μm in the solutions and larger (>100 μm) in the foams. Not only the micropores (bubbles) but also the nanopores could be observed in the wet and dried foams. The cellulose foams possessed ultra-low density of about 30 mg/cm³ and high specific surface area. The result of X-ray diffraction and Fourier transform infrared spectroscopy indicated that the cellulose foams were transited from cellulose I to cellulose II after dissolution and gelation. Bubbles inside the wet foams weakened the mechanical properties, but inversely increased the mechanical properties in the dried foams. Typical “J”-shaped curves were observed during the mechanical test, which revealed good compressive strength of dried foams. In this work, cellulose foams with ultra-lightweight and good mechanical properties were obtained, which exhibited great potentials for further development and comprehensive utilization of cellulose.     

Exploiting the Use of the Decarboxylative S-Alkylation Reaction to Produce Self-Blowing,Recyclable Polycarbonate Foams

Polymeric foams are widely used in many industrial applications due to their light weight and superior thermal, mechanical, and optical properties. Currently, increasing research efforts is being directed towards the development of greener foam formulations that circumvent the use of isocyanates/blowing agents that are commonly used in the production of foam materials. Here, a straightforward, one-pot method is presented to prepare self-blown polycarbonate (PC) foams by exploiting the (decarboxylative) S-alkylation reaction for in situ generation of the blowing agent (CO₂). The concomitant formation of a reactive alcohol intermediate promotes a cascade ring-opening polymerization of the cyclic carbonates to yield a cross-linked polymer network. It is shown that these hydroxyl-functionalized polycarbonate-based foams can be easily recycled into films through thermal compression molding. Furthermore, it is demonstrated that complete hydrolytic degradation of the foams is possible, thus offering the potential for zero-waste materials. This straightforward and versatile process broadens the scope of isocyanate-free, self-foaming materials, opening a new pathway for next-generation environmentally friendly foams.     

Tailoring the microstructure of particle-stabilized wet foams

Inorganic colloidal particles which are in situ hydrophobized upon adsorption of short-chain amphiphilic molecules can be used as foam stabilizers. In this study, we tailor the microstructure of particle-stabilized wet foams, namely, the foam air content, average bubble size, and bubble size distribution, by changing the composition of the initial colloidal suspension. Wet foams featuring average bubble sizes between 10 and 200 microm and air contents between 45% and 90% were obtained by adjusting the amphiphile and particle concentration, pH, ionic strength, and particle size in the initial suspension. The influence of these parameters on the bubble size was satisfactorily described in terms of a balance between the shear stress applied during mixing and the counteracting Laplace pressure of the air bubbles. This model, originally developed for oil droplets in emulsions, can therefore be used to deliberately tailor the microstructure of particle-stabilized wet foams.     

Urea hard segment morphology in flexible polyurethane foam

A series of flexible polyurethane slabstock foam samples were prepared with varying water content and studied using transmission electron microscopy (TEM), video-enhanced optical microscopy (VEM), and small-angle X-ray scattering (SAXS). A new TEM sample preparation technique was developed in which the foam is impregnated with water, frozen, and microtomed, and the polyether soft segment is selectively degraded in the electron beam. Structures of two size scales were detected. A texture with grains (“urea aggregates”) 50–200 nm in size was imaged using both VEM and low-magnification TEM for foams with formulations containing more than 2 pphp water. For the first time, images of urea hard segment microdomains in polyurethane foam (approximately 5 nm in size) were obtained using high-magnification TEM. A microdomain spacing of approximately 6–8 nm was estimated from the SAXS scattering profiles. Glycerol was added to one of the formulations in order to modify the urea microphase separation and to give insight into morphology development in molded polyurethane foam systems. No structure was observed in low-magnification TEM images of the glycerol-modified foam, although smaller structures (hard segments) were detected at high magnification and by SAXS. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 573–581, 1998     

New Generation of MOF-Monoliths Based on Metal Foams

Herein, it has been developed a method to prepare metallic foams starting from Zamak5 (ZnAlCu alloy) with different pore sizes. The Zamak5 metallic foam is designed to serve as a support and metallic precursor of ZIF-8. In this way, composite materials MOF-metal can be prepared, these composites have a large number of application in energy exchange processe such as: adsorption or chemical reactions. Additionally, this method of sythesizing MOFs is environmentally friendly thanks to absence of solvents. Hanerssing the low melting point of the linker, the linker is infiltrated into the foam where the foam and the linker react to form the ZIF-8. In this way we have managed to transform part of the foam into ZIF-8 crystals that remain adhered to the foam. The foams have been characterized and modeled studying the mechanical and electrical properties, finding that both can be predected by various models. Among these, Ashby and Mortensen models for mechanical properties and Ashby and Percolation model for electrical properties stand.     

Compressive response of composite ceramic particle-reinforced polyurethane foam

Quasi-static and dynamic compressive tests are undertaken on the polyurethane (PU) foam and fumed silica reinforced polyurethane (PU/SiO₂) foam experimentally. The ceramic microspheres with varying mass fractions are adopted to mix with the PU/SiO₂ foam to fabricate the composite particle-reinforced foams. The effects of strain rate and particle mass fraction are discussed to identify and quantify the compressive response, energy-absorbing characteristic, and the associated mechanisms of the composite foams. The results show the initial collapse strength and plateau stress of the foams are improved significantly by reinforcing with the ceramic microsphere within 60 wt% at quasi-static compression. The rate sensitivity is observed on all the foams, but in different patterns due to the influence of ceramic microsphere. The compressive response affected by ceramic microsphere can be attributed to the particle cluster effect and stress wave propagation. Together with the deformation, the compressive characteristic experiences non-monotonic change from the low to high strain rates. The specific energy absorption (SEA) of the foam with 41 wt% ceramic microsphere show the largest magnitude at quasi-static compression. With the increasing strain rate, the ceramic reinforced foam exhibits superior energy absorption efficiency at high strain rates to that of the pure foams.     

Multiscale description of polymeric foam behavior: A new approach based on discrete element modeling

The mechanical behavior of polymeric foams depends on several parameters, such as temperature, material density, and strain rate. The studied foams are multiscale materials; agglomerated beads (bead scale is millimetric) are composed of microscopic closed cells (a few tens of microns). The response of the material to dynamic loading consists of three regions: an elastic phase, a plastic phase, and densification. The first part of this work has been the identification of the behavior of these multiscale foams in terms of density and strain rate. Some results are presented in this paper. From these first dynamic results, the second step has been the observation and the analysis of the physical phenomena initiated during the yield plateau. Buckling of the bead and cell wall and strong damage localization were studied with several devices and techniques such as high-speed camera, SEM, and microtomography. The final objective is the development of a model adapted to the multiscale structure of the foam. The first step of this numerical approach consists in the modeling of the microstructure. Due to the microscopic discrete aspect of the foam, a Discrete Element Model has been developed to study the relationship between microscopic properties and the macroscopic behavior of foam. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 6, pp. 1037–1050. This article was submitted by the authors in English.     

Characterisation of microcellular foams produced from semi-crystalline PCL using supercritical carbon dioxide

Microcellular foams were produced from polycaprolactone (PCL) using a method based on the rapid release of supercritical CO₂. The average pore size in the structure was found to vary with the selected vent time and the final microstructure was a combination of open and closed cells. Thermal analysis showed that the degree of crystallinity of all foams was in the region of 70%. This was found to be consistent with the observation of lamellar textures on the inner surface of the pores. There was also a pronounced decrease in compressive modulus with increasing vent time and average pore size.     

Sound absorption properties of polyurethane foams derived from crude glycerol and liquefied coffee grounds polyol

The main objective of this study was to evaluate the sound absorption properties of rigid polyurethane foams (PUFs) produced from crude glycerol (CG) and/or liquefied coffee grounds derived polyol (POL). The lignin content of POL proved to have a major influence on the structure and mechanical properties of the foams. Indeed, the POL content increased the cell size of the foams and their stiffness, which subsequently influenced the sound absorption coefficients. The POL derived foam has slightly higher sound absorption coefficient values at lower frequencies, while the CG foam has higher sound absorption coefficient values at higher frequencies. In turn, the foam prepared using a 50/50 mixture of polyols presents slightly higher sound absorption coefficient values in the medium frequencies range due to a balance between the cell structure and the mechanical properties. The results obtained seem to suggest that the mechanisms involved in sound wave absorption depend on the formulation used to prepare the foams. Additionally higher POL contents improved the thermal stability of PUFs as well as their mechanical properties. From this work the suitability of CG and/or POL derived PUFs as sound absorbing materials has been proven.     

The effect of post-consumer PET particles on the performance of flexible polyurethane foams

In this work, the use of post-consumer PET (polyethylene terephthalate), PET_pc, as reinforcement filler in flexible polyurethane foams was studied, with the aim of finding alternatives for the recycling of polymer packaging. Density, number of cells per linear centimeter, tensile resistance, strain at break and tear resistance of standard foams were compared to those of foams with PET_pc in the formulation, using 1.5 parts per hundred of polyol of PET_pc (granulometric range 0–297 μm). The produced foams were sectioned into top, mid-top, mid-bottom and bottom layers. Tensile resistance, strain at break and tear resistance of the reinforced foam surpassed those of the standard foam for all layers. The number of cells was constant but density increased towards the base of the block. In addition, the filled foams yielded better wear, compression set and compression resistance than the standard foam, whereas no significant variation in morphology (cell shape) was found.     

Effect of Surface-modified Clay on the Thermal Stability and Insulation of Polyorganosiloxane Foam

Polyorganosiloxane foam(SIF) nanocomposites reinforced with vinyl-modified montmorillonite(Mt-V) and hydrox-yl-modified montmorillonite(Mt-OH) were prepared through cross-linking and foaming. The effects of modified Mt on the density, pore morphology, and thermal and compressive properties of the prepared polyorganosiloxane foams were investi-gated. The structure of the polyorganosiloxane foam was studied by solid-state nuclear magnetic resonance analysis. Clay dispersion in polyorganosiloxane nanocomposites and pore morphology were investigated by X-ray diffraction and scan-ning electron microscopy analyses. The thermal and mechanical properties of the prepared materials were also evaluated by differential scanning calorimeter, thermogravimetric analysis, thermal diffusivity and compressive strength. The results show that Mt-V exhibits improved cell structure, thermal insulation, and crush compressive than Mt-OH. The addition of modified Mt reduces the density, cell size, and thermal conductivity but increases the high-temperature resistance and com-pressive strength of the nanocomposite. The amount of the residues of SIF/Mt-OH nanocomposites increases by 9% com-pared with that of the pure SIF. Furthermore, SIF/Mt-V decreases the thermal conductivity to 0.014 W/mK and the cell size to 98 μm. Those properties give the material potential application value in the aerospace and construction industry.