Uniform Cell Size Research Articles

Lightweight thermoplastic polyurethane/multi-wall carbon nanotube foams with a continuous gradient cell structure for electromagnetic interference shielding

With the rapid development of portable microelectronic devices, materials with tunable electromagnetic shielding (EMI) performance and comfort of use have become urgently necessary. A lightweight, conductive thermoplastic polyurethane/multi-walled carbon nanotube (TPU/MWCNT) composite foam with a continuous gradient cell structure has been developed to impede electromagnetic wave reflection induced by excessive impedance mismatch. The continuous gradient distribution of cell structure and conductivity provides the TPU/MWCNT composite foams with distinct shielding effects of electromagnetic waves in both front and back gradient directions. The maximum front absorption coefficient (A) reached 0.93, which is higher than any foam with a uniform cell size within the cell size range of the gradient composite foams. At the same time, the gradient composite foams possess the characteristics of lightweight, high elasticity, and extensibility. The weight was decreased by 67.27–85.45 %, the elastic recovery ratio of compression reached 95 %, and the toughness was also excellent. Numerical simulation of the gradient foam model was conducted using CST Microwave Studio to uncover the gradient asymptotic dissipation mechanisms. The lightweight and stretchable continuous gradient foams show great promises for the next-generation portable electronic devices that generate no radiation pollution.

Effects of carboxymethyl cellulose concentration on mechanical, viscoelastic properties, and thermal properties of starch/plant fiber foaming tableware materials with foam structure

In this study, a novel bio-based composite of starch/plant fiber foaming tableware material was prepared by the hot-pressing foaming method. The influence of the content of carboxymethyl cellulos (CMC) as a new type of film-forming agent filler on the properties of foam materials was systematically investigated. The results showed that the foam material with 10 wt% CMC had excellent cell morphology and uniform cell size (the average cell size reached the minimum value of 465.7 μm), relatively high porosity (57.01%), and excellent thermal insulation properties (thermal conductivity: 0.0541 W/m·K). The tensile strength of foam increased from 4.289 MPa for 0 wt% CMC to 5.714 MPa for 10 wt% CMC loading. Similarly, the flexural strength also increased from 11.609 MPa to 15.719 MPa for the foam material with 10 wt% CMC. The thermal stability and viscoelastic properties of the foam material reached the highest after adding 10 wt% CMC. In addition, the increase of CMC content is beneficial to improve the oil resistance and biodegradability of the material. The research shows that CMC is an effective filler to improve the properties of starch/plant fiber foaming tableware materials, which can greatly promote the development of bio-based polymer foam materials.

Optimization of natural rubber foams: Effect of foaming agent content and processing conditions on the cellular structure and mechanical properties

In the past decades, natural rubber (NR) foams became popular in the automotive, construction and aerospace industries because of their lightweight, flexibility and shock-absorbing properties. The selection of optimal formulation and processing parameters is critical to produce foam with specific properties depending on the application. In this study, the effect of foaming agent concentration, foaming temperature and time on the morphological and mechanical properties of NR foams was investigated. First, increasing the foaming agent content from 5 to 9 phr (parts per hundred rubber) increased the cell size (16%), while decreasing the compression modulus (28%). In the second part, increasing the foaming temperature (145 to 155°C) resulted in larger cell size (163%); while decreasing the cell density (28%), compression modulus (2%), and hardness (1%). In the third part, increasing the foaming time (25 to 45 min) led to smaller cell size (63%) combined with higher cell density (100%), compression modulus (16%), and hardness (3%). Based on all the results obtained, the best NR foam was obtained with 7 phr of foaming agent and produced at 150°C for 35 min leading to superior morphological and mechanical performance: the smallest cell size (25 µm) and the most uniform cell size distribution ( Đ = 1.03) generating the highest compression modulus (3.36 MPa). Finally, the experimental compression results were combined to build a nonlinear regression model to optimize the formulation and processing conditions leading to 6.5 phr of OBSH molded at 150°C for 36 min. The model showed good agreement with a validation test with less than 2% deviation observed for both compression modulus and strength.

Production, stability and properties of ultrafine MgAl2O4 (spinel) particles stabilized Mg–3Ca alloy foams

The present work reports the synthesis and mechanical behavior studies of Mg–3Ca alloy foams stabilized by ultrafine MgAl2O4 (spinel) particles. The MgAl2O4 particles was created in-situ in the Mg–3Ca alloy melt through the reaction of Mg, Al and O. Foaming was done by adding dolomite (CaMg(CO3)2) as blowing agent in the melt. The foaming behaviour was studied for different MgAl2O4 content in Mg–3Ca and holding times (10 and 15 min). The study reveals that the presence of MgAl2O4 significantly influences the foaming behavior of Mg–3Ca alloy resulting in equiaxed cell structure, uniform cell size distribution, higher expansion in comparison to the Mg–3Ca alloy foam which contains only MgO and CaO. An in-depth phase and microstructural analysis were performed to investigate the particles present in the gas-solid interface of the foam that contributes to foam stabilization. The quasi-static compression studies of foams exhibited better compressive strength (≈3–11 MPa) and energy absorption capacity (≈1.3–5.7 MJ/m3) in comparison to the Mg foams reported in the literature. The ductility of the Mg foams was also measured and compared with that of existing aluminium foams.

Structural and topological design of conformal bilayered scaffolds for bone tissue engineering

A fundamental outcome of bone tissue engineering is the regeneration of bone defects presenting large dimensions. A promising solution in biomedical practice is the implantation of biomimetic scaffolds, i.e. porous structures mimicking the natural shapes of healthy bone tissues, which are colonized by mesenchymal stem cells and that support the growth of the regenerating tissues, until the complete healing process is realized. This work presented a workflow for the geometrical modeling and the mechanical design of beam-based, bilayered, and conformal scaffolds, mimicking the human cortico-cancellous bone structure for filling a large dimension defect in the mandibular bone of an injured patient. An isotropic Voronoi topology built on a refined point set generated by a high-quality meshing algorithm was adopted for lattice generation, which led to an open-cell architecture characterized by full connectivity and uniform cell size. Such geometrical and structural features represent crucially important requirements for maximizing the osteointegration and the vascularization of the implanted scaffold. An irregular scaffold was modelled, including a cortical and a sponge layer. The beam radii of both layers were determined by matching the elastic properties of the corresponding bone tissues, thus minimizing the stress shielding effects. Interestingly, several scaffold properties deriving from the proposed procedure, such as the porosity and the pore size, were in good agreement with those reported in the literature.

The influence of dynamic crosslinked topology and bond exchange on thermal properties, viscoelasticity, and foaming behavior of multibranched TPEE vitrimer

Thermoplastic polyester ether elastomer (TPEE) vitrimers with different dynamic crosslinked networks were synthesized via zinc (II) acetylacetonate-catalyzed epoxy chemistry. The effects of bond exchange within networks, characterized by number of catalytic centers per crosslink (CC), and the dynamic crosslinked topology, characterized by number of entanglement and crosslinking points per crosslink (EC), on thermal, rheological, and foaming properties of TPEE vitrimer were compared. The results show that increasing EC is more favorable for lower crystallization temperature than improving bond exchange owing to the bad structural symmetry but almost constant crystallinity owing to the branches. TPEE vitrimer with a complex dynamic crosslinked topology exhibited slow relaxation with a stronger relaxing response and steeper strain hardening than that with more bond exchanges, indicating better viscoelasticity. Moreover, the good foaming behavior of TPEE vitrimers was ascribed to the dynamic crosslinked networks, which promoted heterogeneous nucleation to increase the cell density, and strain hardening good for cell growth as it suppressed cell coalescence and rupture. Furthermore, increasing CC facilitates bond exchange, promoting more uniform heterogeneous nucleation and cell size dispersity than those afforded by a complex dynamic crosslinked topology achieved by increasing EC. The improvement of foaming behavior for TPEE vitrimer is quite different from that of branched polymer by enhancing cell growth rather than cell density.

Extrusion foaming of multiphasic polyethylene/ethylene-vinyl acetate copolymer/carbon nanotube mixtures: Tailoring foam properties by selective localization of nanoparticles

Semi-conductive foams based on low-density polyethylene/ethylene-vinyl acetate copolymer (LDPE/EVA) blends in the presence of carbon nanotubes (CNTs) were prepared using a twin-screw extrusion process. The effects of CNTs content and localization state in the binary mixture on the physical and structural properties of LDPE/EVA/CNT foams were investigated. The results confirmed that the void fraction, cell density, bubble size and cell size distribution of foams are optimal against CNT loading. The lightest LDPE/EVA/CNT foam was obtained by the CNT localization in the LDPE matrix. This foam containing 2.5 phr of CNT had smaller cells and more uniform cell size comparing to the pure blend foam. The cell density of this foam was 1.598 × 106 cells/cm3, which is much larger than that for the blend foam, 8.64 × 105 cells/cm3. However, the CNT localization state in the dispersed EVA domains resulted in lower void fractions and cell densities comparing with the LDPE/EVA blend foam. The findings clarify the profound impact of the nanofiller localization state on the foam properties of the binary polymeric systems. Light semiconductive LDPE/EVA foams with small cells, uniform cell size and high cell densities were achieved by localizing and dispersing the CNT nanoparticles in the LDPE matrix phase.

Utilizing ANN for Predicting the Cauchy Stress and Lateral Stretch of Random Elastomeric Foams under Uniaxial Loading.

As a result of their cell structures, elastomeric foams exhibit high compressibility and are frequently used as buffer cushions in energy absorption. Foam pads between two surfaces typically withstand uniaxial loads. In this paper, we considered the effects of porosity and cell size on the mechanical behavior of random elastomeric foams, and proposed a constitutive model based on an artificial neural network (ANN). Uniform cell size distribution was used to represent monodisperse foam. The constitutive relationship between Cauchy stress and the four input variables of axial stretch λU, lateral stretch λL, porosity φ, and cell size θ was given by con-ANN. The mechanical responses of 500 different foam structures (20% < φ < 60%, 0.1 mm < θ < 0.5 mm) under compression and tension loads (0.4 < λU < 3) were simulated, and a dataset containing 100,000 samples was constructed. We also introduced a pre-ANN to predict lateral stretch to address the issue of missing lateral strain data in practical applications. By combining physical experience, we chose appropriate input forms and activation functions to improve ANN's extrapolation capability. The results showed that pre-ANN and con-ANN could provide reasonable predictions for λU outside the dataset. We can obtain accurate lateral stretch and axial stress predictions from two ANNs. The porosity affects the stress and λL, while the cell size only affects the stress during foam compression.

Anisotropic nucleation and compatibilization of SiO2@PS Janus particles on expandable polystyrene

Inorganic particles usually used as nucleation agent to promote the polymer foaming of supercritical carbon dioxide. However, they are apt to agglomerating in the matrix, which is not conducive to obtaining foamed materials with homogeneous cells. Herein, the organic-inorganic Janus particles possess anisotropic chemical compositions were introduced into expandable polymer, in which two sides can perform compatibilization and nucleation independently. The dispersion of silica@polystyrene (SiO2 @PS) Janus particles (JPs) in PS matrix and the resulting improvements in rheological property, foaming performance, compressive strength of PS was discussed systematically. JPs could be uniformly dispersed in PS matrix attributed to the PS end of JPs. While the SiO2 end of JPs could act as heterogeneous nucleation agent and promote cell formation. Compared with PS foams, PS/JPs foams had smaller and more uniform cell sizes, leading to better compressive properties, i.e. the compressive strength of PS/JPs-5% foams at 100 °C, 110 °C and 120 °C increased by 37.4%, 60.6% and 57.9%, respectively. This work demonstrates that JPs are a novel nucleation and compatibilizer for making polymer foams with optimal cell structure and compressive property.

Near-wall modeling of forests for atmosphere boundary layers using lattice Boltzmann method on GPU

In this paper, the simulation and modeling of the turbulent atmospheric boundary layers (ABLs) in the presence of forests are studied using a lattice Boltzmann method with large eddy simulation, which was implemented in the open-source program GASCANS with the use of Graphic Processing Units (GPU). A method of modeling forests in the form of body forces injected near the wall is revisited, while the effects of leaf area density (LAD) on the model accuracy is further addressed. Since a uniform cell size is applied throughout the computational domain, the wall-normal height of the near-wall cells is very large, theoretically requiring a wall function to model the boundary layer. However, the wall function is disregarded here when the forest is modeled. This approximation is validated based on the comparison with previous experimental and numerical data. It concludes that for the ABL conditions specified in this study as well as a large body of literature, the forest forces overwhelm the wall friction so that the modeling of the latter effect is trivial. Constant and varying LAD profiles across the forest zone are defined with the same total leaf area despite the varying one being studied previously. It is found that the two LAD profiles provide consistent predictions. The present forest modeling can therefore be simplified with the use of the constant LAD without degrading the model accuracy remarkably.

Role of hydrocolloids in improving the rheology, quality characteristics, and microstructure of gluten‐free proso millet bread

SummaryThe effect of xanthan gum (XG), guar gum (GG), and hydroxypropyl methylcellulose (HPMC) at different concentrations (0.5, 1.0, 1.5, 2.0 g/100 g) on the development of gluten‐free proso millet bread was studied. HPMC and GG addition showed a remarkable increase in the loss and storage modulus, indicating the viscoelastic behaviour of batter. With increasing frequency, loss tangent values decreased &lt;1, denoting gel‐like behaviour. Shear stress increased, and apparent viscosity decreased with shear rate denoting pseudoplastic non‐Newtonian fluid behaviour. HPMC and GG added batter showed improvement in stickiness, adhesion, cohesiveness, and vice versa observed in XG. HPMC2.0 and GG2.0 showed higher loaf and specific volume (450, 446.67 cm3 and 2.88, 2.85 cm3 g−1). It signifies increased gas holding capacity, which reduces the hardness of the crumb than control. HPMC breads showed higher number of cells, uniform cell size, thin cell wall, and lower porosity with the highest overall acceptability (7.33). Microstructure exhibited continuous network formation in HPMC2.0 crumb compared to web‐like structures in XG and GG breads. Immunological validation showed that the developed bread was gluten free. The results implied that the addition of 2 g/100 g HPMC improved the overall quality of proso millet bread.

Microcellular foaming and mechanical properties of iPP-iPPF using supercritical CO2

Isotactic polypropylene (iPP) is one of the most widely used commercial polymers in the industry. However, it is difficult to foam iPP due to its low melt strength. In this work, the foamability and mechanical properties of iPP were investigated by incorporating isotactic polypropylene fiber (iPPF), using a batch foaming method. Compared to the neat iPP foams, smaller cell size and more uniform cell size distribution were obtained for iPP–iPPF. When 30 % iPPF was added, the cell density of iPP-iPPF foam was 1.1 × 1010 cells/cm3 which was twice that of the neat iPP foam. It was attributed to the improved melt strength and heterogeneous nucleation of iPPF in the iPP matrix. In addition, the impact strength of foamed iPP-30iPPF was found to be increased by 136 % compared to its neat iPP counterpart. Furthermore, the tensile strength of the foamed samples also increased with the increasing iPPF content.

Effects of inorganic fillers on the performance of the water‐based intumescent fire‐retardant coating

SummaryThe purpose of this research was to evaluate the influences of filler type and its content on the performance of a water‐based intumescent fire‐retardant coating. Three fillers (vermiculite, celite, and aluminum hydroxide) were added to the intumescent paint formulation. The thermal and fire protective properties were studied with thermogravimetric analysis (TGA), torch test, electrical furnace, scanning electron microscopy (SEM), and Fourier transform infrared analysis (FTIR). The results showed that adding fillers into coatings up to 3% could improve the intumescent coating's behavior and increase its endurance against flames. Of the three fillers used, vermiculite showed a better performance in the torch test, attributed to its chemical and physical structure. Vermiculite has low thermal conductivity and is considered an appropriate filler for heat‐insulation. The final back‐plate temperatures in the torch test for the vermiculite‐containing samples were around 100°C–150°C lower than that of other samples. Moreover, vermiculite's addition improved the coating's expansion by 10% compared with the control sample's. The vermiculite sample's char layer morphology showed a uniform cell size distribution, indicating structural robustness. The coating samples successfully transformed polypropylene flammability from highly flammable to V0 level in the UL 94 vertical burning test standard. The results showed that vermiculite could improve intumescent paint's fire resistance and be used as an enhancer in intumescent coating formulations.

Green rigid polyurethane foam from hydroxyl liquid natural rubbers as macro-hydroxyl polyols

The novel renewable source precursors from hydroxyl liquid natural rubbers (HLNRs) with various secondary hydroxyl content of 22% (HLNR22), 35% (HLNR35), and 50% (HLNR50) (or naming macro-hydroxyl polyols) were used to prepare rigid polyurethane foam. The aim of this study was to investigate the effect of hydroxyl content of HLNR precursors and the ratio of HLNRs and commercial polyols on physico-mechanical properties of rigid polyurethane foams in comparison to foams made from commercial polyols. The increase in hydroxyl content of HLNRs resulted in the foams with larger cell size while the increase in the HLNR portion caused a small and more uniform cell size, which is related to their density and compressive strength. Thermal stability of polyurethane foams was analyzed by thermogravimetric analysis and the results have demonstrated that the use of HLNR polyols improved thermal stability of polyurethane foams in comparison to commercial foam.

Comparative study of Inplane gradient cellular pattern honeycombs with Uniform compliant honeycombs

Cellular structures have gained attention in recent years due to its wide range of applications across various fields. One of its categories are the “Honeycombs”, honeycomb structures that came into existence at the end of 1930’s. The shapes of these structures are taken from nature, that is, the shape of a beehive. Honeycomb structures have a huge potential in terms of mechanical properties, enhancing the performance of structures and many other traits which have gained the attention of researchers switching from the conventional materials. This paper compares the energy absorption and stability properties of gradient honeycomb structures (with varying cell size across the width) with conventional or compliant structures (uniform cell size throughout the structure). Analysis is done on structures with “In-plane orientation” by varying design parameters. Using numerical modelling and FEA analysis in ANSYS software, the results are validated.

Partially Implicit FDTD (PI-FDTD) Method for Lower Dispersion and Anisotropic Errors

A new partially implicit 3D FDTD (PI-FDTD) method is introduced. The method is not unconditionally stable, but its stability limit is 6 times higher than that of the classic Yee&#x2019;s FDTD method. The method has only one implicit update equation and uses four split time steps. A study has been carried out to compare the performances of the new method and the ADI-FDTD, LOD-FDTD, and HIE-FDTD methods. ADI-FDTD and LOD-FDTD methods are unconditionally stable and the HIE-FDTD method is conditionally stable with a stability limit of 3.46 times above the Yee&#x2019;s FDTD method. The dispersion and anisotropic errors of the PI-FDTD method are considerably lower than that of these methods and for small cell sizes, the new method is almost isotropic. For example at the stability limit of the PI-FDTD method with a uniform cell size of <inline-formula> <tex-math notation="LaTeX">$\lambda \mathord {\left /{ {{ {50}}} }\right. }$ </tex-math></inline-formula> the maximum dispersion errors and anisotropic errors are 0.15&#x0025; and 0.031&#x0025; for the new method and 1.66&#x0025; and 0.64&#x0025; for the ADI-FDTD and LOD-FDTD methods. At the stability limit of the HIE-FDTD method, the maximum dispersion errors and anisotropic errors of the PI-FDTD method are also less than HIE-FDTD, ADI-FDTD, and LOD-FDTD methods. The differences in errors are more pronounced for larger cell sizes. Although the new method uses a four time split step scheme and the ADI-FDTD and LOD-FDTD methods use a two time split step scheme, the computational time of the PI-FDTD method is less, as it uses an implicit scheme for only one of the field components when updating. The computational time of the PI-FDTD method is the same as that of the HIE-FDTD method. The study has revealed that the new method can be used over much wider frequency bandwidths.

Foaming of Polylactic Acid/Cellulose Nanocrystal Composites: Pickering Emulsion Templating for High-Homogeneity Filler Dispersions

Polylactic acid (PLA)/cellulose nanocrystal (CNC) composite foams have attracted much attention due to their biodegradability and potential to replace petroleum-based foams. However, the dispersion of (unmodified) CNCs in PLA remains challenging. Here, we used the Pickering emulsion templating method of unmodified CNCs to stabilize PLA microspheres and prepare a concentrated masterbatch. This masterbatch was used in conventional melt blending of pristine PLA to obtain well-dispersed CNCs in PLA with a CNC loading of up to 5 wt %. In comparison, PLA/CNC composites prepared by direct melt blending of CNCs in PLA showed visual agglomerates of CNCs. PLA/CNC composites were batch foamed with CO2 as a physical blowing agent. We demonstrate that the good CNC dispersion obtained by the Pickering emulsion route is favorable for preparing PLA foams with smaller and more uniform cell sizes and with a higher cell density. These results demonstrate that the Pickering emulsion approach to prepare CNC masterbatches for subsequent dispersion in thermoplastics by conventional processing seems promising for numerous industrial applications.

Microbubble technology for natural rubber latex foam production: The use of various gas-filled microbubbles

In recent years, microbubble technology has attracted great attention in many application fields including water treatment, food processing, oil recovery, surface cleaning, and therapeutic applications. In this paper, microbubbles (MBs) of air, nitrogen, and argon were applied to produce natural rubber latex foams (NRLFs). The bubbles were generated by flowing the gas through a porous diffuser and latex. The effect of gas source on cellular structure, density, elasticity, indentation hardness, and flammability of the bubbled foams was discussed. Argon MBs offered the latex foams with fine cell diameters and uniform cell size distribution resulting in enhanced elasticity and physical properties of the foams. Indentation hardness index and limiting oxygen index value depended significantly on the gas used. By using the microbubble technique, the future prospects in NRLF production can be expected due to its ability in controllable cellular structure.

Novel Pb alloys based composite foams containing hybrid pores produced by liquid metallurgy for lightweight batteries

High-quality novel Pb alloys based composite foams reinforced with fly ash cenospheres of different diameters in a variety of volume fractions were successfully synthesized by stir casting. Several crucial parameters such as wetting between the cenospheres and molten Pb alloy, volume fraction and size of the cenospheres, melt temperature, rate of cenosphere addition, stirring time and speed, foaming agent amount, foaming temperature and foaming time were optimized to produce high-quality composite foams. The microstructure of the composite foams revealed uniform cell distribution and size with mostly closed cell structure. The cells were measured to be of the size range of 290–430 μm. Microscopy and spectroscopy of the composite foams indicated that limited or no interfacial reaction products are formed between fly ash cenospheres and Pb alloys. The specific discharge electrical capacity of the composite foams was higher than those of the Pb alloys used in conventional Pb-acid batteries. Moreover, the weights of the composite foam electrodes were much lower than those of the conventional lead alloys, providing benefits in transportation applications. The cyclic voltammetric and galvanostatic polarization results indicated that the rates of oxidation of Pb into PbSO4 in the composite foam were much lower than those in Pb-grids and the oxidation of the Pb in the positive and negative grids occurred at much lower voltage than that in composite foams.

Optically transparent bio‐based polyamides with microcellular foaming properties derived from renewable difunctional aminoamides

AbstractA green approach for the synthesis of bio‐based polyamides (PAs) with a new macromolecular structure was developed. These bio‐based PAs were synthesized by solvent and catalyst‐free polycondensation combining fatty dimer acid Pripol™ 1009 with new difunctional aminoamides (DAAs). The later DAAs building blocks were synthesized by aminolysis of renewable dimethyl sebacate (DMS) and dimethyl adipate (DMA) with three different aliphatic diamines having diverse chain lengths, 2, 4, and 6 carbons. Bio‐based PAs showedMwup to 30,581 g/mol, initial thermal degradation over 380°C,Tmfrom 168.3–202.4°C,Tgfrom 28.3–37.5°C, strain from 36.4 ± 4.0–313.5 ± 2.0%, and tensile strength up to 27.4 ± 0.1 MPa. Interestingly, all the PAs films had excellent optical transparency with cut‐off wavelengths of 276.0–283.0 nm, transmittance from 82.7%–90.7% at 600 nm. As a proof‐of‐concept of their usage, one of the PA was shown to be form microcellular foam by the green scCO2method. The foams possessed uniform (cell size; 40 μm, foam density; 0.265 g/cm3, expansion rate; 7.2) and bimodal (cell size; 24/38 μm, foam density; 0.302 g/cm3, expansion rate; 6.5) cell textures depending on their foaming temperatures. This is the first report of bio‐based PA foams and transparent films manufacturing without toxic solvents.