Closed-cell Polymer Foams Research Articles

An investigation on the dynamic response of polymeric, metallic, and biomaterial foams

Mechanical characterization of foams at varying strain rates is indispensable for the selection of foam as core material for the proficient sandwich structure design at dynamic loading application. Both servo-hydraulically controlled Material Testing System (MTS) and Instron machines are generally considered for quasi-static testing at strain rates on the order of 10 −3 s −1 . Split Hopkinson pressure bar (SHPB) with steel bars is typically utilized for characterizing metallic foams at high strain rates, however modified SHPB with polycarbonate or soft martial bars are used for characterizing polymeric and biomaterial foams at high strain rates on the order of 10 3 s −1 for impedance match between the foam specimens and bars. This paper reviews the effect of strain rate of loading, density, environmental temperature, and microstructure on compressive strength and energy absorption capacity of various closed-cell polymeric, metallic, and biomaterial foams. Compressive strength and energy absorption capacity increase with the increase in both strain rate of loading and density of foams, but decrease with the increase in surrounding temperature. Foams of same density can have different strength and can absorb unequal amount of energy at the same strain rate of loading due to the variation of microstructure.

Application of polymeric foams for separation, storage and absorption of hydrogen

To build and operate a “hydrogen economy” in future, it is essential to develop safe and reliable technologies to handle (produce, recover, concentrate, store, deliver, etc.) hydrogen gas. In this work, application possibilities of polystyrene foam were studied and the experiments have proven that closed cell polymeric foams are able not only to separate and store hydrogen, but even to absorb it from gaseous mixtures.

Tomography and Simulation of Microstructure Evolution of a Closed-Cell Polymer Foam in Compression

Closed-cell foams in compression exhibit complex deformation characteristics that remain incompletely understood. In this paper the microstructural evolution of closed-cell polymethacrylimide foam was simulated in compression undergoing elastic, compaction, and densification stages. The three-dimensional microstructure of the foam is determined using Micro-Computed Tomography (μ-CT), and is converted to material points for simulations using the material point method (MPM). The properties of the cell-walls are determined from nanoindentation on the wall of the foam. MPM simulations captured the three stages of deformations in foam compression. Features of the microstructures from simulations are compared qualitatively with the in-situ observations of the foam under compression using μ-CT. The stress-strain curve simulated from MPM compares reasonably with the experimental results. Based on the results from μ-CT and MPM simulations, it was found that elastic buckling of cell-walls occurs even in the elastic regime of compression. Within the elastic region, less than 35% of the cell-wall material carries the majority of the compressive load. In the experiment, a shear band was observed as a result of collapse of cells in a weak zone. From this collapsed weak zone a compaction (collapse) wave was seen traveling which eventually lead to the collapse of the entire foam cell-structure. Overall, this methodology will allow prediction of material properties for microstructures driving the optimization of processing and performance in foam materials.

Characterisation of nonlinear material parameters of foams based on indentation tests

Closed cell polymeric foams, for example EVA foams, are widely used in engineering, sport and biomedical fields. The mechanical behaviours of foam is highly nonlinear, and the foams are often under complex loading conditions in service over different spectrums of strain. Foam testing and determination of the nonlinear foam constitutive parameters are important to predict their performance in service and to aid materials development and product design. Current testing approaches normally involve the use of conventional compression or shear tests, which require well defined sample shapes and sizes. In this work, the use of continuous indentation test and inverse FE modelling method to determine the nonlinear material parameters was systematically investigated. This potentially could provide a much more simple testing approach with the capacity for in situ situation where standard testing specimens are not attainable. The results were directly compared to conventional compression test and combined compression–shear tests of EVA foams. The accuracy and validity of each method were comparatively studied and discussed. Results showed that the indentation approach produced comparable results to the combined compression–shear tests, while uniaxial compression test could not provide accurate material data to simulate complex loading conditions.

Finite element micromechanics model of impact compression of closed-cell polymer foams

Finite element analysis, of regular Kelvin foam models with all the material in uniform-thickness faces, was used to predict the compressive impact response of low-density closed-cell polyethylene and polystyrene foams. Cell air compression was analysed, treating cells as surface-based fluid cavities. For a typical 1 mm cell size and 50 s −1 impact strain rate, the elastic buckling of cell faces, and pop-in shape inversion of some buckled square faces, caused a non-linear stress strain response before yield. Pairs of plastic hinges formed across hexagonal faces, then yield occurred when trios of faces concertinaed. The predicted compressive yield stresses were close to experimental data, for a range of foam densities. Air compression was the hardening mechanism for engineering strains <0.6, with face-to-face contact also contributing for strains >0.7. Predictions of lateral expansion and residual strains after impact were reasonable. There were no significant changes in the predicted behavior at a compressive strain rate of 500 s −1.

Degradation of Elastic Modulus of Progressively Crushable Foams in Uniaxial Compression

The degradation of apparent modulus in progressively crushable foams in uniaxial compression is studied in this article based on an analysis of the measurement of engineering strain. It shows that the deformation mechanism of the progressive collapse of the foam causes the non-uniform deformation in the foam during uniaxial compression, which leads to the decrease of apparent elastic modulus with the increase of strain in the plateau stress regime. Theoretical predictions are compared with experimental results for different closed-cell polymeric foams with satisfactory agreement. It is also shown that this method is not applicable to the in-plane compression of honeycombs and the numerical modeling based on macroscopic constitutive equation is incapable of predicting the degradation of apparent modulus in progressively crushable foams.

Fitting three-dimensional Laguerre tessellations to foam structures

Foam models, especially random tessellations, are powerful tools to study the relations between the geometric structure of foams and their physical properties. In this paper, we propose the use of random Laguerre tessellations, weighted versions of the well-known Voronoi tessellations, as models for the microstructure of foams. Based on geometric characteristics estimated from a tomographic image of a closed-cell polymer foam, we fit a Laguerre tessellation model to the material. It is shown that this model allows for a better fit of the geometric structure of the foam than some classical Voronoi tessellation models.

Tension, compression and shear fatigue of a closed cell polymer foam

A closed cell foam of polymetacrylimide (Rohacell) with three different densities is studied. The foam is tested quasistatically in tension, compression and shear. The tensile properties scale very well with the relative density of the foam, but the compression and shear properties do not scale the same way. It is believed to be due to cell edge and cell wall buckling being the dominated deformation mechanism in compression and shear for lower densities that does not occur for higher densities. Fatigue testing is then performed in tension, compression and shear. It is seen that for all load cases and densities, the fatigue life can be plotted using Basquin’s law. The results also show that the different failure mechanisms found in the static tests are the same in fatigue. This means that the fatigue life for different load types exhibit different failure mechanisms. This shows not only as a clear difference in the stress levels for fatigue failure, but also on the slope in the fatigue life relation.

Effect of sea environment on interfacial delamination behavior of polymeric sandwich structures

Sandwich structures are utilized in naval craft and thereby are exposed to sea water environment and temperature fluctuations over extended periods. The sandwich layup consists of a closed cell polymeric foam layer placed between thin carbon or glass fiber reinforced polymeric composite facings. Attention in this paper is focused on sea water effects on the interfacial mechanical response between foam and facing due to sustained sea water exposure using carefully controlled laboratory conditions. Pre-cracked sandwich composite samples are soaked in sea water for extended periods and interfacial fracture behavior compared against dry specimens. Results indicate that the delamination crack propagates close to the interface in the wet case, while it stays within the foam in the dry case. Significant reduction in fracture toughness due to sea water exposure is observed and needs to be considered in the design of ship structures. The effect of sea water on values of energy release rate are determined experimentally and predicted using the J-integral concept. A good agreement between data and predictions is achieved, indicating a reduction in fracture toughness by 30% due to sustained exposure to sea water.

Polymeric foams and sandwich composites: Material properties, environmental effects, and shear-lag modeling

Marine composite sandwich structural materials, comprising of low density PVC foam core and carbon fiber reinforced vinyl ester based resin composite facings, are studied for associated degradation in mechanical behavior caused by sea water. This paper presents experimental and analytical results concerning the properties and response of closed cell polymeric foams (PVC H100) and their sandwich composites. Data regarding the elastic properties of foam (shear and Young’s modulus) are collected by means of novel custom made devices and interpreted by means of displacement based analytical models. Emphasis is placed on environmental effects and a novel approach of using expansional strain analogy to study the effects of both sea water and temperature are proposed. It is shown that the custom made shear testing apparatus employed herein offers a significant improvement over the device recommended by ASTM. Using torsional testing, the value of shear modulus in the water-saturated outer shell of soaked foam samples are obtained and show a degradation of 72% with long-term exposure to sea water.

Closed-cell polymeric foam for hydrogen separation and storage

Closed-cell polystyrene foam was utilized as a membrane buffer for separation of (bio)hydrogen. In the foam the cell walls form a structured complex of membranes. The cells serve as pressure containers of separated gases. The foam membrane can buffer the difference between the feed injection rate and the rate of consumption of the product. It can be utilized when biohydrogen feeds a fuel cell and consumption of electricity is not synchronous with hydrogen production. The function of the foam was assessed using a cubic model and the theoretical data were checked experimentally by using gases present in the bioreactor (H 2, N 2 and CO 2). It turned out that there is a considerable difference (hundreds of minutes) in time-lags of various gases in the polymeric foam, thus the foam is able to separate hydrogen and also to store it effectively.

A model for fluid ingress in closed cell polymeric foams

Foam cored composite sandwich panels are used in the marine structures. These structures are exposed to sea water environment, which may degrade their structural properties and diminish their performance. This article concerns the ingress of sea water into polymeric foams, which was observed to result in substantial weight gain ratios and noticeable damage to cellular foam walls. The water ingress phenomenon modeled herein is based on the assumption that it is governed by the cell wall breakage, rather than caused by diffusion. The foam structure is represented by means of Voronoi cell geometries. Numerical simulations using the proposed model are presented.

A non-quadratic yield function for polymeric foams

The yield behavior of a closed cell polymeric foam is investigated under multiaxial loadings. A phenomenological yield function is developed to characterize the initial yield behavior of the closed cell polymeric foam under a full range of loading conditions. The principal stresses of a relative stress tensor and the second invariant of the deviatoric stress tensor are the main parameters in the yield function. The yield function is a linear combination of non-quadratic functions of the relative principal stresses and the second invariant of the deviatoric stress tensor. The convexity of the yield surface based on the non-quadratic yield function is proved. The non-quadratic yield function is shown to well characterize the yield behavior of a closed cell polymeric foam in [Deshpande, V.S., Fleck, N.A., 2001. Multi-axial yield behavior of polymer foams. Acta Materialia 49, 1859–1866] under a full range of loading conditions. Finally, a comparison of different phenomenological yield functions to characterize the yield behavior of the foam is presented.

Size effect and asymptotic matching analysis of fracture of closed-cell polymeric foam

The effect of structure size on the nominal strength of closed-cell PVC foam (Divinycell H100) is investigated experimentally, theoretically and numerically. Two types of size effect are considered––Type I, characterizing failure of structures with large cracks or notches, and Type II, characterizing failure at crack initiation. Geometrically similar single edge-notched prismatic specimens of cross section widths 6.35, 43.9 and 305 mm, are tested under tension. The results are shown to agree with Bažant’s law for type I energetic (deterministic) size effect derived by asymptotic matching of a solution by equivalent linear elastic fracture mechanics for large sizes and plastic crack solution for small sizes (in the derivation, the statically indeterminate size-dependent lateral shift of the axial load resultant due to rotational end restrain is taken into account). Fitting this law, previously verified for many quasibrittle materials, to the test results furnishes the values of the fracture energy of the foam as well as the characteristic size of the fracture process zone of foam. The size effect method of measuring the fracture characteristics of foam is further supported by analysis of recent notched beam tests of Zenkert and Bäcklund. Furthermore, it is shown that compressed V-notched specimens exhibit no size effect. Subsequently, the size effect of Type II is studied using previous test data of Fleck, Olurin and co-workers for dissimilar long holed panels having different width and different diameter-width ratios. An asymptotic matching formula for this type of size effect (similar to a previously derived formula for kink band failure of fiber composites) is set up and is shown capable of matching the test data well. But its verification as a predictive tool cannot yet be claimed because of inaccurate asymptotic properties of the available energy release function. Finally, the size effect of Type I is analyzed using the eigenvalue method for the cohesive crack model and the numerical results are shown to agree again with both Bažant’s size effect law and the test results.

Dielectric barrier microdischarges: Mechanism for the charging of cellular piezoelectric polymers

Charged closed-cell polymer foams have been found to be highly sensitive piezoelectric materials. Charging is shown to arise from dielectric barrier microdischarges within the voids of the cellular polymer. Above the threshold voltage for breakdown in the voids, the microdischarges are evidenced by light emission from the polymer, as well as by displacement-voltage hysteresis loops. Monitoring light emission during breakdown is shown to provide a quick check for the suitability of foams for piezoelectric applications. Additionally it allows for the visualization of micropores in foams in a nondestructive way. The piezoelectric response of the foam can be switched by applying dc-voltage pulses of alternating polarity above the breakdown threshold, thereby showing the feasibility of patterning the piezoelectric properties within the film plane. Although piezoelectric foams are nonferroelectric, the experiments prove similarities to ferroelectric materials with respect to hysteresis behavior, as well as a threshold (coercive) field for switching of the polarization and piezoelectricity.

Properties of Rigid Polyurethane Foams Related to Their Use for Corrosion Control Inside Confined Spaces

Abstract Recent studies show that rigid, closed cell polymer foam blocks seawater penetration and provides corrosion protection to the internal surfaces of metal cavities exposed to aqueous salt en...

Modeling of Diffusion in Closed Cell Polymeric Foams

Closed-cell foams made of polymers have the lowest thermal conductivity of any currently available insulation material other than vacuum insulation systems. The increase of foam conductivity with age occurs as air diffuses into the foam while the blowing agent diffuses out, thus modifying the cell gas composition. Also, the change in cell gas composition influences the dimensional stability of the foams. To predict the long term aging behavior and dimensional stability of these foams, the diffusion characteristics of the different components need to be known. Several models exist in the literature which describe diffusion in foams. The most popular of these models are reviewed, and effective diffusivities predicted from one model are compared with experimental data. An unsteady state model is then proposed and solved numerically using a finite difference scheme. The numerical solution algorithm is developed to efficiently solve the large number of coupled equations resulting from this model. The uptake curves predicted from both the unsteady-state model and a discrete model (Bart and Du Cauze De Nazelle, 1993) are compared with available experimental data for the polystyrene-nitrogen system. From the analysis of uptake curves generated for different numbers of cells, the effective diffusivity of the PS/N2 system is predicted. Also, the effect of initial cell gas composition and cell size on both the long term aging profile and dimensional stability of polyurethane foam is considered. The proposed model can easily be extended to include the influence of blowing agent concentration on diffusivity in the polymer phase and the isotherm describing the distribution of blowing agent between the gas and polymer phases.

Closed-Cell Polymer Foams for Corrosion Control in Confined Metal Spaces

Abstract The use of closed-cell plastic polymer foams to control corrosion in hollow, irregular cavities of preexisting metallic structures or assemblages was explored. The method involved blocking...

The high strain compression of closed-cell polymer foams

A compressed closed-cell polymer foam was modelled using a BBC lattice model of tetrakaidecahedral cells, loaded in the [001] direction. The contributions of cell face tensions and edge bending were analysed, assuming that the faces act as membranes, for a linearly-elastic, or a yielding material. The moduli and tensile yield stresses of highly oriented polymer films were measured to provide data for modelling, and the amount of polymer in the foam cell faces found to be high. Tensile face strains are predicted to reach 40% of the foam compressive strain. The predicted Youngs moduli are slightly low, because compressive face stresses are ignored, but Poissons ratio is correctly predicted. The compressive foam yield stress is predicted to depend on tensile yielding of the cell faces. Predicted values are close to experimental values for polyethylene foams, but half those of polystyrene extruded foams. The latter foam may collapse in compression when face yielding commences, rather than by the collapse mechanism of the model.

Thermoplastic Vulcanizates from Isotactic Polypropylene and Ethylene−Propylene−Diene Terpolymer in Supercritical Propane: Synthesis and Morphology

New thermoplastic vulcanizates (TPV's) have been synthesized from isotactic polypropylene and ethylene−propylene−diene terpolymer (EPDM) in a supercritical propane solution. The ternary solution of polypropylene and EPDM in supercritical propane exhibited less solubility than the corresponding binary solutions of polypropylene or EPDM separately in propane. Higher pressure in the supercritical polymer solution enhanced the mutual solubility of the two polymers. EPDM was cross-linked with tert-butyl peroxide while in supercritical solution. On lowering the pressure while remaining above the melting temperature of the polymers, the supercritical TPV gel phase separated in a manner consistent with spinodal decomposition. On isobaric cooling of the supercritical TPV gel, the polypropylene crystallized, freezing the morphology. The cross-linking of EPDM in the homogeneous supercritical propane solutions was found to be nearly complete. The final TPV's were phase separated, exhibiting two melting transitions. The morphology of the TPV's was a microporous, apparently closed-cell polymeric foam. Phase contrast optical microscopy showed micro-heterogeneous EPDM domains dispersed in the polypropylene matrix for the TPV's. The phase domain sizes were much smaller, by a factor of 5−10 or so, than those of the corresponding melt blends.