Foam Research Articles

Synergistic enhancement of poly(lactide) melt strength via copolymerization and multi-arm branching strategy

The lower melt strength of poly(lactide) (PLA) limits its broader applications. Here, a strategy combining copolymerization with multi-arm branching was propose to enhance the melt strength of PLA. Initially, stereoisomeric cyclic ester monomers (CEM) synthesized via zeolite catalysis were copolymerized into PLA chains. Subsequently, rheological testing revealed that the zero-shear viscosity (η0) of linear PLA increased by 467 % with only 1 mol% of CEM units. Our study further systematically explored the relationship between the side group structure and chirality of the comonomers and the rheological properties of the copolymers. CEMs with long-chain branched structures and opposite chirality had the best enhancement effect. In order to further enhance the melt strength, we successfully achieved alterations in polymer topology by employing trimethylolpropane as an initiator, corresponding three-arm copolymers achieve up to a 67-fold increase in η0 (1.0 kPa∙s to 68.1 kPa∙s). Tensile tests indicated that the mechanical properties of the copolymers were comparable to those of PLA, with a tensile strength of approximately 65 MPa. Additionally, due to the high melt strength, we successfully produced closed-cell PLA-based foam materials with uniform pore sizes. In summary, this study furnishes a feasible method for designing polymer materials possessing the desired melt strength.

Active vibration control of metal foam reinforced agglomerated graphene nanoplatelets nanocomposite truncated conical shells with piezoelectric layers

This study examines active vibration control (AVC) in a truncated conical shell composed of metal foam reinforced with agglomerated graphene nanoplatelets and integrated with piezoelectric layers functioning as sensors and actuators. Both complete and partial agglomeration states are considered based on the Eshelby–Mori–Tanaka approach, and the mechanical properties of the porous core are characterized using a closed-cell metal foam model. The governing equations of motion are derived using Hamilton’s principle, based on the two-dimensional axisymmetric elasticity theory, and solved through the finite element method coupled with the Newmark method. Vibration mitigation is achieved using a fractional-order fuzzy proportional–integral–derivative (PID) controller. A comprehensive parametric study is conducted, examining the effects of geometric dimensions, various boundary conditions, reinforcement parameters (including weight fraction, distribution patterns, and agglomeration parameters), and porosity parameters (including pore size, and porosity pattern) on the vibration properties of the shell. Numerical simulations are performed to assess the effectiveness of the proposed AVC system in reducing structural vibrations compared to a constant velocity feedback control approach. The velocity feedback controller reduced central deflection from 12.65 to 5.089 μ m , while a fractional-order fuzzy PID controller further improved it to 2.348 μ m , representing a 53.86% enhancement over the velocity feedback controller.

Comparison of sound insulation performance between closed-cell foam and automotive carpet

In developing sound insulation materials for automobiles, multiple sound insulation and sound absorption materials are laminated to achieve high performance. Generally, steel sheets, needle felts, and carpets are used to create sound insulation properties. In a luxury car, multiple layers with sound absorption and sound insulation properties are placed on the carpet to add higher sound insulation properties, but this increases the mass of the carpet. To solve this problem, we are investigating the the potential to realize both lightweight and high sound insulation properties using closed-cell form. In this study, we experimentally evaluated the sound transmission loss of closed-cell foam by comparing it with those of carpets with the same configuration used in standard and luxury cars.Furthermore, a numerical study is conducted. As a result, the closed-cell foam obtained higher sound transmission loss than the automobile specification carpet in a wide frequency range from 400 Hz to 5000 Hz. On the other hand, the sound transmission loss decreased by the same degree for both the automotive carpets and the closed-cell foam at 250 Hz.

Observation of micropolar modes in the transmission of acoustic waves at oblique incidence by polystyrene plates.

This study investigates micropolar plates through the analysis of transmitted acoustic waves at various incident angles. Using a combined theoretical and experimental approach, we explore the non-classical elasto-dynamic behavior of these materials. Theoretical transmission coefficients are tailored to highlight generalized Lamb modes and a new type of vibration mode called micropolar modes, considering the constant thickness of the plate and the effects of material micropolarity on linear elasticity. Overlaying diverse transmission coefficients for different angles provides insight into micropolar behavior, aiding in the understanding of mode evolution in the frequency domain and the estimation of the critical angle. We employ two types of low-density closed-cell foams made from polystyrene (expanded in one case, extruded in the other), covering a frequency range from 4 to 60 kHz using a tweeter loudspeaker and a microphone integrated with a National Instruments acquisition system. Identification of the first vibration mode of the plates is facilitated by the frequency range rich in low frequencies. Additionally, all responses exhibit supersonic velocities with substantial attenuations. The originality of this paper is twofold: it provides theoretical predictions of micropolar modes, along with their experimental observations above a critical angle of incidence. Consequently, this research enriches our understanding of micropolar materials and their potential contribution to effective noise reduction strategies in acoustic engineering.

Potential of Soft-Shelled Rugby Headgear to Lower Regional Brain Strain Metrics During Standard Drop Tests

BackgroundThe growing concern for player safety in rugby has led to an increased focus on head impacts. Previous laboratory studies have shown that rugby headgear significantly reduces peak linear and rotational accelerations compared to no headgear. However, these metrics may have limited relevance in assessing the effectiveness of headgear in preventing strain-based brain injuries like concussions. This study used an instantaneous deep-learning brain injury model to quantify regional brain strain mitigation of rugby headgear during drop tests. Tests were conducted on flat and angled impact surfaces across different heights, using a Hybrid III headform and neck.ResultsHeadgear presence generally reduced the peak rotational velocities, with some headgear outperforming others. However, the effect on peak regional brain strains was less consistent. Of the 5 headgear tested, only the newer models that use open cell foams at densities above 45 kg/m3 consistently reduced the peak strain in the cerebrum, corpus callosum, and brainstem. The 3 conventional headgear that use closed cell foams at or below 45 kg/m3 showed no consistent reduction in the peak strain in the cerebrum, corpus callosum, and brainstem.ConclusionsThe presence of rugby headgear may be able to reduce the severity of head impact exposure during rugby. However, to understand how these findings relate to brain strain mitigation in the field, further investigation into the relationship between the impact conditions in this study and those encountered during actual gameplay is necessary.

Direct Writing of graphene/graphitic foam through picosecond pulsed laser-induced transformation of soluble polyimide suspension

We report the direct writing of graphene/graphitic foam with high electrical conductivity using laser-induced-transformation of polyimide (PI) resin films on glass surfaces. Raman spectroscopy of the treated surfaces indicated that average laser power irradiation between 900 and 1500 kW/cm2 transformed the PI film into a few layered graphene-dominated film, and the increase in irradiation power above 1500 kW/cm2 led to the formation of graphitic (multilayered graphene) material. The electrical conductivity of the transformed film was between 5800±750 S m-1 (lower power irradiation) and 1250±300 S m-1 (higher laser power irradiation). SEM imaging showed that the transformed material has a closed cell foam morphology enclosed between the smooth top and bottom layers. The results indicate that heat treatment of the polyimide suspension films, and subsequent ultra-short, pulsed laser irradiation resulted in a closed-cell graphene/graphitic foam with high electrical conductivity. The pore aspect ratio, density, and film conductivity are used to estimate the conductivity of the solid phases in the laser-treated films at different powers. Laser-induced transformation of the PI suspension into graphene/graphitic foam is conducive to additive manufacturing and may enable the direct printing of graphitic foam-based three-dimensional components.

Preparation, characterisation, physical, and compression testing of sucrose based closed-cell aluminium foam

ABSTRACT Aluminium foam can be used as a porous and lightweight material having comparatively high impact energy absorption. To reduce the death rate caused by collisions of vehicles, there is a need of analysing the crash-worthiness properties of materials. In this present work, the Aluminium foam was prepared using the powder metallurgy method for the crash-worthiness analysis. The Al6061 powder as a matrix material having particle sizes ranging from 5 to 30 µm and Sucrose particles as a space-holding material were mixed. The compaction process was done up to a pressure of 450 MPa to make the green compact and was further subjected to sintering up to a temperature of 500 °C for 1 hour resulting into the Aluminium foam. The compressive strength significantly decreased by 26.56%, 51.39%, 72.49%, and 64.34% approximately while volume fractions of sucrose particles varied as 20%, 30%, 40%, and 50%. On the other hand, the measured maximum porosity was found as 51.8% approximately in the Aluminium foam corresponding to the addition of 50% sucrose particles by volume. Out of all the examined samples, the foam developed by containing 20% (vol.) sucrose has exhibited the maximum strain energy absorption, specific energy absorption (SEA), and crushing force efficiency (CFE).

Elastomer-based soft syntactic foam with broadly tunable mechanical properties and shapability

Fabrication of lightweight composites using thermally responsive expandable microspheres (EM) embedded in an elastomeric matrix to form syntactic foams offers significant opportunities to create functional and structural composites for diverse applications. The morphology of the EM-elastomer composite on the micro- and macro-scales is significantly influenced by the fabrication sequence and parameters (composition, expansion time/temperature, etc.). Herein, we report the morphological evolution of an EM-elastomer composite through the fabrication processes and elucidate the interaction of relevant fabrication parameters. Unlike the majority of published reports on hard EM composites, the in-situ expansion of the closed-cell foam structure within a soft matrix leads to larger void sizes, and the concomitant expansive force of the EM impacts the polymer chain mobility within the soft composite. At the same time, higher EM loading improves the mechanical properties; the evolved microstructure leads to a lightweight but stiff structure. For the first time the report details the thermal expansion of EM under applied pretension, leading to irreversible prestrain-dependent morphological changes toward anisotropic mechanical properties. Additionally, the composites are shaped under constrained expansion. The study provides a comprehensive guide and expands the pathways for the practical application of EM-embedded soft syntactic foams in aerospace, electronics, wearables, robotics, etc.

Pressure-Relieving Effect of Different Insole Top Covers in People with Diabetes at High Risk of Foot Ulceration.

Pressure-relieving footwear helps prevent foot ulcers in people with diabetes. The footwear design contributes to this effect and includes the insole top cover. We aimed to assess the offloading effect of materials commonly used as insole top cover. We measured 20 participants with diabetes and peripheral neuropathy for in-shoe peak pressures while walking in their prescribed footwear with the insole covered with eight different materials, tested in randomized order. Top covers were a 3 mm or 6 mm thick open or closed-cell foam or a 6 mm thick combination of open- and closed-cell foams. We re-assessed pressures after one month of using the top cover. Peak pressures were assessed per anatomical foot region and a region of interest (i.e., previous ulceration or high barefoot pressure). Walking comfort was assessed using a 10-point Likert scale. Mean peak pressure at the region of interest varied between 167 (SD:56) and 186 (SD:65) kPa across top covers (p < 0.001) and was significantly higher for the 3 mm thick PPT than for four of the seven 6 mm thick top covers. Across 6 mm thick top covers, only two showed a significant peak pressure difference between them. Over time, peak pressures changed non-significantly from -2.7 to +47.8 kPa across top cover conditions. Comfort ratings were 8.0 to 8.4 across top covers (p = 0.863). The 6 mm thick foams provided more pressure relief than the 3 mm thick foam during walking in high-risk people with diabetes. Between the 6 mm thick foams and over time, only small differences exist. The choice of which 6 mm thick insole top cover to use may be determined more by availability, durability, ease of use, costs, or hygienic properties than by superiority in pressure-relief capacity.

Cryogenic Insulation-Towards Environmentally Friendly Polyurethane Foams.

Cryogenics is the science and technology of very low temperatures, typically below 120 K. The most common applications are liquified natural gas carriers, ground-based tanks, and propellant tanks for space launchers. A crucial aspect of cryogenic technology is effective insulation to minimise boil-off from storage tanks and prevent frost build-up. Rigid closed-cell foams are prominent in various applications, including cryogenic insulation, due to their balance between thermal and mechanical properties. Polyurethane (PU) foam is widely used for internal insulation in cryogenic tanks, providing durability under thermal shocks and operational loads. External insulation, used in liquified natural gas carriers and ground-based tanks, generally demands less compressive strength and can utilise lower-density foams. The evolution of cryogenic insulation materials has seen the incorporation of environmentally friendly blowing agents and bio-based polyols to enhance sustainability. Fourth-generation physical blowing agents, such as HFO-1233zd(E) and HFO-1336mzz(Z), offer low global warming potential and improved thermal conductivity. Additionally, bio-based polyols from renewable resources like different natural oils and recycled polyethylene terephthalate (PET) are being integrated into rigid PU foams, showing promising properties for cryogenic applications. Research continues to optimise these materials for better mechanical performance and environmental impact.

Optimized PI-PDF active structural acoustic control of smart FG GPL-reinforced closed-cell metallic foam sandwich plate

This study investigates Active Structural Acoustic Control (ASAC) applied to sandwich plates featuring Functionally Grade (FG) porous Graphene-Platelet-Reinforced Piezoelectric (GPLRP) materials. These plates incorporate a core layer with internal pores and GPLs dispersed in a metal matrix, along with piezoelectric sensors and actuators. Using a spatial state-space formulation based on linear 3D piezoelasticity theory, a semi-analytical solution is derived for the vibroacoustic response of these sandwich plates. The mechanical properties of the porous core are modeled using a closed-cell metal foam. The effects of various parameters, including porosity distributions, porosity coefficient, weight fractions of nanofiller, and geometric parameters on the radiation efficiency and radiated sound power have been investigated. Then, the validation of the proposed model is examined by comparing the natural frequencies calculated in the present study to those from the literature. This comparison allows us to assess the accuracy and reliability of our model against established findings. Radiated sound power reduction from mechanically excited structures is achieved by employing a dual-stage Proportional Integral–Proportional Derivative with Filter (PI-PDF) controller, optimized using Grey Wolf Optimization (GWO). Finally, several numerical simulations are conducted to validate the effectiveness and accuracy of the proposed active control strategy.

Friction Investigation of Closed-Cell Aluminium Foam during Radial-Constrained Test.

The energy-absorbing capacity and friction phenomena of different closed-cell aluminium foam-filled Al tube types are investigated through experimental compression tests. Concerning the kind of investigation, free, radial-constrained and friction tests occurred. The radial-constrained compression test results confirm that the process requires significantly more compression energy than without the constrain. Pushing away different pre-compressed foams inside the aluminium tube, the static and kinematic frictional resistances can be determined and the energy required to move them can be calculated. Knowing the value of the energy required for the frictional resistance, we can obtain how much of the energy surplus in radially inhibited compression is caused by the friction phenomena. The main goal present study is to reveal the magnitude of friction between the foam and the wall of the tube during the radially constrained test. The investigation used 0.4 and 0.7 g/cm3 density closed-cell aluminium foam whilst a compressive test was applied where the force-displacement data were recorded to calculate the absorbed energy due to friction. Considering the results of the test, it can be stated that 18% of the invested energy was used to overcome friction in the case of lighter foam and almost 23% with 0.7 g/cm3 foam during the radial-constrained test.

Effect of the silanization process on the fire resistance and thermal properties of closed-cell foams with sunflower husk ash

The topics covered in this paper are relevant to the further development of polyurethane materials and propose a new method for producing sustainable and innovative foams. In this study, the effect of filler silanization on the flammability and thermal stability of rigid polyurethane foam (RPUF) was investigated. The RPUF composites study was preceded by an analysis of sunflower husk ash (SHA). Thermal decomposition behaviour and particle size distribution were carried out for the SHA samples. More, the elemental composition and structure of the fillers were investigated. The use of silanization of the filler results in homogenization of its structure. Sunflower husk ash has higher characteristic melting points than most biomass ashes and good thermal stability. The foam flammability results were preceded by an analysis of density and cell morphology. The use of silanized filler in RPUFs results in a homogenized structure and a 14 % reduction in apparent density. A wide range of tests were carried out as part of the flammability tests, which included: limiting oxygen index (LOI), gross calorific value, cone calorimeter, and optical density test. Additionally, thermal analysis methods were used to investigate the thermal stability of the foams. Flammability tests carried out confirm the positive effect of filler salinization on flammability. Heat release during combustion is reduced by 10 % and the release of toxic gases is also reduced. On the other hand, the thermal stability results are similar for foam with unmodified filler and those with silanized ash. Based on the comprehensive flammability and smoke results, a trend towards improved flame-retardant properties of RPUFs was observed.

Microstructural characterization of bimodal composite metal foams under compression with machine learning

Cellular materials are gaining popularity in today’s major sectors. The aim is to develop high-performance materials to meet customer and application demands. This study revolves around the beginning of the failure; computed tomography and statistical image analysis assisted with machine learning were employed to quantitatively characterize the occurring processes within the structure at the beginning of the well-known plateau effect. Simple techniques (e.g., easily obtainable shape parameters, forest decision, 2D image slices) were employed to sort the structural elements into seven classes based on their visual appearances. The presented method can provide additional information about the fracture mode of the different foams, possibly applying/extending them for other types of closed-cell (composite or not) foams.

Preparation and energy absorption of flexible polyurethane foam with hollow glass microsphere

This paper presents the preparation of a partially open-cell and partially closed-cell flexible polyurethane foam material (flex-PUF), which exhibits improved cushioning performance compared to conventional protective materials. Hollow Glass Microspheres (HGM) were used as a filling material to enhance the compressibility of the material. In order to investigate the effects of HGM on the multi-impact protection and vibration damping performance of flex-PUF with different thicknesses, flex-PUF samples filled with varying volume fractions of HGM were subjected to multi-impact testing and dynamic viscoelasticity experiments. The destructive mechanism of HGM under impact was observed using scanning electron microscopy (SEM). The experimental results revealed that, under the same impact energy conditions, filling flex-PUF with HGM reduced the maximum impact displacement while enhancing energy absorption, although at the expense of cushioning performance. As the number of impact increases, the stiffness of flex-PUF decreased. In the vibration experiments, as the frequency increased, the proportion of flex-PUF’s viscous damping energy dissipation decreases.

Influence of the cell size and wall thickness on the compressive behaviour of fused filament fabricated PLA gyroid structures

The development of Additive Manufacturing (AM) has greatly facilitated the fabrication of cellular and lattice materials. Gyroid-based lattice structures, known for their distinctive properties such as interconnected porosity, high surface-to-volume ratio, and remarkable structural stiffness combined with specific energy absorption, have been extensively explored. Many studies examining the impact of design parameters on the mechanical properties of Gyroid lattice materials have utilized metal AM techniques. This research aims to evaluate the influence of two design parameters on the compressive properties of Gyroid structures obtained by fused filament fabrication (FFF). A full factorial analysis was employed to assess the effects of cell size and wall thickness on the compressive properties of polylactic acid (PLA) Gyroid lattices. Cell sizes were varied between 4 mm, 5 mm, and 10 mm, while wall thickness ranged from 0.4 mm, 0.6 mm, to 0.8 mm. After 3D printing, the print quality was assessed, samples were weighted and then subjected to compression testing. During compression, the lattices with 10 mm cells exhibited successive layer collapse, whereas the lattice with 4 mm and 5 mm cells displayed plastic deformation, marked by a plateau in the stress-strain curve. These behaviours were mostly independent of wall thickness, except for the 5 mm cell lattice with 0.4 mm wall thickness. The elastic modulus, yield stress and absorbed energy per volume aligned with the apparent density of the lattices, ranging between 1% and 12% of the bulk 3D printed material for both the stiffness and yield stress, and between 1% and 22% for the energy absorbed. Analysis of the fitted means indicated that doubling the cell size had a more significant impact on the measured properties than doubling the wall thickness, while doubling both the cell size and wall thickness exerted a more pronounced influence on the yield stress and strain. Notably, under the conditions of this study, the 3D printed PLA Gyroids behaved similarly to closed cell foams, despite their interconnected channels. Their compressive mechanical properties comparable to those of rigid polyurethane foams with closed cells.

Bayesian calibration of constitutive models for polymeric foams

Porous materials are traditionally used for their sound absorption and insulation properties. Over the past decade, more attention has been given to their elastic and damping properties. There is a particular interest in the automotive industry to replace heavy layers (consisting of constrained viscoelastic rubber layers) with felts or foams evidencing high damping capabilities. Hence, characterizing efficiently the viscoelastic properties of porous materials is crucial for quality control and further improvements in product development and fabrication. In this sense, the main contribution of this work is to propose a stochastic strategy to determine the constitutive equations describing the viscoelastic behaviour of foams. Vibration tests are carried out on a two-layer panel containing a free-layer of a foam. A finite element model is developed considering only the viscoelasticity of the porous skeleton, neglecting the influence of the fluid phase. A fractional derivative model is calibrated and validated for two types o foam based on a Bayesian framework. The models indicate that the reconstitute porous rubber presents absolute values of the complex modulus greater than the ones provided by the closed-cell polyurethane foam and its energy dissipation is greater the one provided by the closed-cell polyurethane one.

Curved Polymeric Sandwich Composites Subjected to Air Shock: An Experimental Investigation

BackgroundThe vulnerability of polymeric composite sandwich structures in marine applications to air explosions highlights a significant gap in our understanding of the dynamic behavior of the curved sandwich structures, which is essential for design improvements.ObjectiveThis study aims to explore the dynamic response and failure mechanisms of curved sandwich composite panels subjected to air-blast loading, providing insights into their structural integrity under such conditions.MethodsExperiments were performed using laboratory-simulated air shocks generated by a shock tube, employing high-speed photography and digital image correlation to measure deflections on the back surface of the panels. The panels, made with PVC closed-cell foam cores of two densities (H45 and H130), were tested across three curved geometries (radii of 112 mm, 305 mm, and infinity) under various boundary conditions.ResultsFindings indicate an increase in deformation with a decreased radius of curvature under simple support conditions, a trend that reverses under arrested displacement conditions. Moreover, a reduced radius significantly enhances panel strength and resistance to interfacial damage, with the primary failure mode transitioning from core shear cracking to interfacial debonding as core density increases.ConclusionsThe study reveals that the radius of curvature, boundary conditions, and core density significantly affect curved sandwich panels’ dynamic response and performance. Panels with smaller radii and higher core densities exhibit increased strength, though boundary conditions introduce variable effects on deformation behavior.

Energy quantification framework for underwater explosive loading into PVC foam cladded composite plates

This paper presents a novel approach for analyzing the effects of near-field underwater blast loading on composite marine structures. The operational requirements of these structures often expose them to blast or shock loading, which can lead to significant damage. The study focuses on the propagation of spherical blast waves and the subsequent secondary bubble collapse pulse that affects the structure under near-field underwater blast loading events. An analytical framework is developed to calculate the blast energies and structural energies during this complex loading event. The study further investigates the effect of applying a sacrificial cladding made of low-density closed-cell foam to the loading face of a composite panel. Experiments were conducted in a specialized facility to characterize the explosive-driven blast-loading and the subsequent interaction between the shock pressure front and the structure. Dynamic pressure measurements and high-speed imaging were utilized to capture the behavior of the composite panel under these extreme conditions. 3D digital image correlation was employed to analyze strain and deformation of the composite panel, while high-speed side view cameras captured the fluid-structure interaction during the blast and bubble collapse loading event. The results strongly indicate that the collapse of the explosion bubble is the dominant failure source in near-field underwater blast loadings. The analytical quantification of energy further corroborates the significant damaging effects of bubble collapse due to the buildup of after-flow energies during the bubble collapse duration. Furthermore, the inclusion of low-impedance elastic-plastic PVC foam cladding is shown to significantly mitigate the effect of blast loading on the composite structure.

Innovative Cellular Insulation Barrier on the Basis of Voronoi Tessellation-Influence of Internal Structure Optimization on Thermal Performance.

The optimization of structure and thermal properties in 3D-printed insulation materials remains an underexplored area in the literature. This study aims to address this gap by investigating the impact of 3D printing on the thermal properties of manufactured cellular composites. The materials studied were closed-cell foams with a complex cell structure based on the Voronoi cell model, manufactured using incremental technology (3D printing). The influence of the cellular structure of the composite, the type of material used, and the number of layers in the composite structure on its thermal properties, i.e., thermal conductivity coefficient, thermal resistance, and coefficient of heat transfer, was analyzed. Samples of different types of thermosetting resins, characterized by different values of emissivity coefficient, were analyzed. It was shown that both the type of material, the number of layers of the composite, and the number of pores in its structure significantly affect its thermal insulating properties. Thermal conductivity and permeability depended on the number of layers and decreased up to 30% as the number of layers increased from one to four, while thermal resistance increased to 35%. The results indicate that material structure is key in regulating thermal conduction. Controlling the number of cells in a given volume of composite (and thus the size of the air cells) and the number of layers in the composite can be an effective tool in designing materials with high insulation performance. Among the prototype composites produced, the best thermal performance was that of the metalized four-layer cellular composites (λ = 0.035 ± 0.002 W/m·K, Rc = 1.15 ± 0.02 K·m2/W, U = 0.76 ± 0.01 W/m2·K).