Mechanical properties of CRETE,a polyurethane foam

The room-temperature mechanical properties of a closed-cell, polyurethane encapsulant foam were measured as a function of foam density. Over the range of densities examined, the modulus could be described by a power-law relationship with respect to density. This power-law relationship was the same for both tension and compression testing. The basis for this power-law relationship is explained in terms of the elastic compliance of the cellular structure of the foam using a simple geometric model put forth by Gibson and Ashby. The elastic collapse stress, a property relevant to compression testing, also is found to exhibit a power-law relationship with respect to density. The density dependence of this property is also found in the work of Gibson and Ashby and is explained in terms of the Euler buckling of the struts that comprise the cellular structure. Energy absorption during deformation is also reported for both tension and compression testing. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1045–1055, 1998     

Characterization of epoxy foams

Epoxy foams were prepared from the commercial system (LY 5054 epoxy resin, HY 5054 amine as curing agent, and DY 5054 siloxane as foaming agent) supplied by Ciba‐Geigy. From the differential scanning calorimeter results the optimal epoxy–amine ratio was determined. A maximum T_g∞ value of 85°C was found for an epoxy–amine ratio of 100:35 by weight. In this system, the siloxane reacts with the amine releasing hydrogen that acts as the real foaming agent. The density decreased from 490 to 215 kg/m³ as the epoxy:blowing agent ratio increased from 100:1 to 100:3 by weight of the reactive mixture. Under the synthesizing conditions, the glass transition temperature (T_g∞) of the foam did not vary significantly as the blowing agent increased. The modulus and compressive strength of the foam exhibited a power‐law dependence with respect to density of the form: E∝(ρ)ⁿ, where n=1.8. Scanning electron microscopy analysis verified that the foam have a closed cell structure. The relation between composition, final morphology, density, and properties of the foam was analyzed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2992–2996, 2003     

Thermosetting foam with a high bio‐based content from acrylated epoxidized soybean oil and carbon dioxide

A resilient, thermosetting foam system with a bio‐based content of 96 wt % (resulting in 81% of C₁₄) was successfully developed. We implemented a pressurized carbon dioxide foaming process that produces polymeric foams from acrylated epoxidized soybean oil (AESO). A study of the cell dynamics of uncured CO₂/ AESO foams proved useful to optimize cure conditions. During collapse, the foam's bulk density increased linearly with time, and the cell size and cell density exhibited power‐law degradation rates. Also, low temperature foaming and cure (i.e. high viscosity) are desirable to minimize foam cell degradation. The AESO was cured with a free‐radical initiator (tert‐butyl peroxy‐2‐ethyl hexanoate, T_i ～ 60°C). Cobalt naphtenate was used as an accelerator to promote quick foam cure at lower temperature (40–50°C). The foam's density was controlled by the carbon dioxide pressure inside the reactor and by the vacuum applied during cure. The viscosity increased linearly during polymerization. The viscosity was proportional to the extent of reaction before gelation, and the cured foam's structure showed a dependence on the time of vacuum application. The average cell size increased and the cell density decreased with foam expansion at a low extent of cure; however, the foam expansion became limited and unhomogeneous with advanced reaction. When vacuum was applied at an intermediate viscosity, samples with densities ～ 0.25 g/cm³ were obtained with small (<1 mm) homogeneous cells. The mechanical properties were promising, with a compressive strength of ～ 1 MPa and a compressive modulus of ～ 20 MPa. The new foams are biocompatible. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Using organic montormollontrite to reinforce the structure and flexural property of denture base nanocomposites

The denture bases nanocomposites were prepared by adding organic montormollontrite(OMMT) into poly(methyl methacrylate) matrix. The resulting composites were characterized by XRD, TEM, and ISTRON 3365 flexural strength testing systems. One‐way analysis of variance and Tukey's test were used to compare the difference in flexural strength and flexural modulus. The results showed the formation of exfoliated‐intercalated and intercalated denture base nanocmposite when the content of OMMT was 3 wt % and 5 wt % in the powder of denture base resin, respectively. The maximum ultimate flexural strength was observed in denture base nanocomposite containing 3 wt % of OMMT and flexural strength decreased with further increases of OMMT content. The flexural modulus of denture base nanocomposite increased continually with OMMT content increasing from 1 wt % to 10 wt %. Significant difference in both flexural strength and flexural modulus was observed between the nanocomposites with 3 wt % OMMT and controls (n = 5, P < 0.05). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Cell morphology,bubbles migration,and flexural properties of non‐uniform epoxy foams using chemical foaming agent

Epoxy foams with different densities and microstructures were prepared by changing the process parameters including the foaming temperature, chemical foaming agent (CFA) content and precuring extent. The microstructure of foams reveals a smaller cell size, higher cell density, and more homogeneous distribution of cells at higher precuring extent. However, the cell size and distribution are not affected by the foaming temperature and CFA content without precuring process. In addition, the bubbles migration, which resulted in non‐uniform cell density distribution, was promoted by increasing the foaming temperature and depressed by increasing the CFA content and precuring extent. The flexural properties of the non‐uniform epoxy foams were also studied. Results showed that the flexural modulus was related to the cell morphology, while the flexural strength was affected by both the cell morphology and the position of the specimens during test. It was also found that the relative flexural modulus and strength exhibited a power‐law dependence with respect to the relative density. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41175.     

Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

Optimization of the structure of integral skin foams for maximal flexural properties

The effective flexural properties of integral skin foams (ISF), are modeled using Euler-Bernouli beam theory along with a power law empirical equation relating the properties of a homogeneous foam to its density. The optimal density profile that maximizes the effective flexural modulus of an ISF beam of fixed overall density, and with the density constrained to lie in a given range, is continuous when the power law exponent (n) is less than 1. For n > 1, the optimal density profile is discontinuous with a low density core and a high density skin. The effective flexural modulus of such sandwich beams is maximized for a fixed density ratio (ratio of the core density to the skin density) and fixed overall density. The maximal flexural modulus is found to increase monotonically with decreasing density ratios and increasing values of n. The flexural strength of the sandwich beam is also maximized considering failure to occur by tensile fracture or buckling of the skin. In this case an optimal skin thickness and an optimal density ratio are obtained for a fixed overall density. The results are useful for the design and evaluation of flat ISF panels.     

Structures and properties of closed‐cell polyimide rigid foams

Closed‐cell polyimide rigid foams with different Calculated Molecular Weight (Calc'd M_n) and foam density (ρ) have been prepared by thermal foaming of nadimide‐endcapped imideoligomers (NAIO) powder. The NAIO powder was obtained by thermally treating of a PMR poly(amide ester) solution derived from the reaction of diethyl ester of 2,3,3′,4′‐biphenyltetracarboxylic dianhydride (α‐BPDE) and p‐phenylenediamine (p‐PDA) using monoethyl ester of cis‐5‐norbornene‐ endo‐2,3‐dicarboxylic acid (NE) as reactive endcapping agent in ethyl alcohol. Effect of Calc'd M_n and foam density (ρ) on mechanical and thermal properties of the polyimide rigid foams have been systematically investigated. It was found that the thermal foaming properties of NAIO powders were affected by the Calc'd M_n. The appreciate Calc'd M_n could yield polyimide foams with both high closed‐cell content (C_c) (>80%) and outstanding mechanical properties. Moreover, the thermal properties were reduced by increasing of Calc'd M_n and the mechanical properties improved gradually by increasing foam densities. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3282–3291, 2013     

Uniaxial Strength Behavior of Brittle Cellular Materials

Two types of brittle reticulated materials were evaluated under uniaxial tensile and compressive loading and analyzed in terms of the Gibson and Ashby model for brittle open-cell solids. The samples consisted of an open-cell alumina–mullite material which was tested as a function of density at a constant cell size and a reticulated vitreous carbon tested at one density and two cell sizes. The samples were mounted such that only the loading direction was varied in the tests. A combination of video photography and acoustic emission was critical to interpreting the results. The model assumes that identical deformation modes, bending failure of the struts, are responsible for failure of the bulk foam in tension and compression. The results of this work indicate a significant difference between the density dependence in tension and compression. Tensile failure in both materials appeared to be characterized by the catastrophic propagation of a single crack. Compressive failure was significantly different between the alumina and glassy carbon foams. The alumina foam failed by a damage accumulation process, whereas the carbon foam failed by the catastrophic collapse of a band of cells perpendicular to the loading direction.     

Behaviour of open-cell cordierite foams under compression

Cordierite foams were produced using the polymer foam replication method. The effects of both the structure of the polymeric foam template and the slurry's solid loads on the compressive strength and the sintered structure of the ceramic foam were evaluated in order to achieve the optimal manufacturing conditions. The compressive strengths of cordierite foams were measured at room temperature. Polyurethane (PU) foams were used as templates. Aqueous ceramic suspensions were prepared with solids weight fraction ranging from 50 to 65%. The effects of both the PU density and the solids volume fraction on porosity and strength of the developed cordierite foams were evaluated. The cordierite foams produced are of semiclosed-cell type. Some impregnation difficulties were experienced with increasing of the PU density. The compressive strength of the cordierite foams increased (from 0.1 to 2 MPa) with increasing solids volume fraction. These data are in agreement with the predictions of the model developed by Gibson and Ashby. However, the exponent of the model was half of the measured one (≈3) over the range of relative densities investigated (80–90%). Such discrepancy might be related to several factors such as the morphological differences in the structural unit of the developed foams with respect to a cubic open-cell foam or to the mixture of both open and closed cells or to the presence of non-periodic cells. In addition, it was found that the compressive strengths depended on the cell size for foams of similar relative densities and generally decreased with increasing of the cell size, which deviates from the theoretical predictions. When the starting polymeric substrate contained a higher fraction of closed cell windows, however, the ceramic material present on the cellular structure was not only distributed on the struts but also filled the cell walls. This contributed to an increase of the relative density of the cordierite foams and consequently to higher compressive strengths.     

Filamentous green algae additions to isocyanate based foams

Filamentous green algae, Cladophora glomerata, obtained from rivers and streams in California was used as a filler in isocyanate‐based foams. Up to 40 wt % dried and chopped algal fibers were added to the isocyanate–polyol mix. The algal fibers and foam samples were examined by scanning electron microscopy. The effective hydroxyl number of the algae and the tensile and compressive properties of the composite foam were measured. The results indicate that foams with a uniform cell size were obtained for algal fiber lengths between 500 and 600 μm. Peak mechanical properties were obtained for algae concentrations between 5 and 10 wt %. The elastic modulus of the foam follows a power law relationship with density, with an exponent of 1.6. The overall hydroxyl number for the algae is around 300. The hydroxyl groups in the algae can serve as efficient sites for reactions with isocyanate and hence algae fibers can act as reinforcing fillers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2469–2477, 2004     

Development and characterization of green composites from bio‐based polyethylene and peanut shell

In the present work, different compatibilizers, namely polyethylene‐graft‐maleic anhydride (PE‐g‐MA), polypropylene‐graft‐maleic anhydride (PP‐g‐MA), and polystyrene‐block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene‐graft‐maleic anhydride (SEBS‐g‐MA) were used on green composites derived from biobased polyethylene and peanut shell (PNS) flour to improve particle–polymer interaction. Composites of high‐density polyethylene/peanut shell powder (HDPE/PNS) with 10 wt % PNS flour were compatibilized with 3 wt % of the abovementioned compatibilizers. As per the results, PP‐g‐MA copolymer lead to best optimized properties as evidenced by mechanical characterization. In addition, best particle–matrix interface interactions with PP‐g‐MA were observed by scanning electron microscopy (SEM). Subsequently HDPE/PNS composites with varying PNS flour content in the 5–30 wt % range with PP‐g‐MA compatibilizer were obtained by melt extrusion and compounding followed by injection molding and were characterized by mechanical, thermal, and morphological techniques. The results showed that PNS powder, leads to an increase in mechanical resistant properties (mainly, flexural modulus, and strength) while a decrease in mechanical ductile properties, that is, elongation at break and impact absorbed energy is observed with increasing PNS flour content. Furthermore, PNS flour provides an increase in thermal stability due to the natural antioxidant properties of PNS. In particular, composites containing 30 wt % PNS powder present a flexural strength 24% and a flexural modulus 72% higher than the unfilled polyethylene and the thermo‐oxidative onset degradation temperature is increased from 232 °C up to 254 °C thus indicating a marked thermal stabilization effect. Resultant composites can show a great deal of potential as base materials for wood plastic composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43940.     

Synergistic Effects of Carbon Fillers of Phenolic Resin Based Composite Bipolar Plates on the Performance of PEM Fuel Cell

The most essential and costly component of polymer electrolyte membrane fuel cells is the bipolar plate. The production of suitable composite bipolar plates for polymer electrolyte membrane fuel cell with good mechanical properties and high electrical conductivity is scientifically and technically very challenging. This paper reports the development of composite bipolar plates using exfoliated graphite, carbon black, and graphite powder in resole‐typed phenol formaldehyde. The exfoliated graphite with maximum exfoliated volume of 570 ± 10 mL g⁻¹ used in this study was prepared by microwave irradiation of chemically intercalated natural flake graphite in a few minutes. The composite plates were prepared by varying exfoliated graphite content from 10 to 35 wt.% in phenolic resin along with fixed weight percentage of carbon black (5 wt.%) and graphite powder (3 wt.%) by compression molding. The composite plates with filler weight percentage of 35/5/3/exfoliated graphite/carbon black/graphite powder offer in‐plane and trough‐plane electrical conductivities of 374.42 and 97.32 S cm⁻¹, bulk density 1.58 g cm⁻³, compressive strength 70.43 MPa, flexural strength 61.82 MPa, storage modulus 10.25 GPa, microhardness 73.23 HV and water absorption 0.22%. Further, I–V characteristics notify that exfoliated graphite/carbon black/graphite powder/resin composite bipolar plates in unit fuel cell shows better cell performance compared exfoliated graphite/resin composite bipolar plates. The composite plates own desired mechanical properties with low bulk density, high electrical conductivity, and good thermal stability as per the U.S. department of energy targets at low filler concentration and can be used as bipolar plates for proton exchange membrane fuel cells.     

Pulverized polyurethane foam particles reinforced rigid polyurethane foam and phenolic foam

Polyurethane consumption has been increasing in recent years, raising concerns about how to deal with the polymer waste. Post‐consumer rigid polyurethane foams or polyurethane foam scraps (PPU) ground into particles were utilized to strengthen mechanical properties of rigid polyurethane foam (PUF) and phenolic foam (PF). Viscosity of prepolymer with PUF was measured and PPU was well dispersed in prepolymer, as observed by optical microscope. Microstructures and morphologies of the reinforced foam were examined with scanning electron microscope (SEM) while cell diameter and density were measured by Scion Image software. Universal testing machine was employed to optimize compressive properties at various weight ratios of PPU. Both PUF and PF with 5 wt % PPU, respectively, exhibited considerable improvement in mechanical properties especially compressive property. The compressive modulus of PUF with 5 wt % PPU was 12.07 MPa, almost 20% higher than pure PUF while compressive strength of PF with 5 wt % PPU reached 0.48 MPa. The thermal stability of the reinforced foam was tested by thermal gravity analysis (TGA) and the result shows no obvious impact with PPU. The decomposition temperatures of PUF with PPU and PF with PPU were 280°C, because PPU has relatively weak thermal stability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39734.     

Anisotropy Effects in the Shape-Memory Performance of Polymer Foams

Isotropic and anisotropic shape-memory polymer foams are prepared by supercritical carbon dioxide foaming from a multiblock copolymer (PDLCL) consisting of poly(ω-pentadecalactone) and poly(ε-caprolactone) segments. Analysis by micro-computed tomography reveals for the anisotropic PDLCL foam cells a high shape anisotropy ratio of R = 1.72 ± 0.62 with a corresponding Young's compression moduli ratio between longitudinal and transversal direction of 4.3. The experimental compression data in the linear elastic range can be well described by the anisotropic open foam model of Gibson and Ashby. A micro-morphological analysis for single pores using scanning electron microscopy images permits the correlation between the macroscopic stress-compression behavior and microscale structural changes.     

Air‐hole properties of calcite‐filled polypropylene copolymer films

This work was designed to study the effects of inorganic calcite powder on structurally different copolymer [poly(propylene‐co‐ethylene)] and terpolymer [poly (propylene‐co‐ethylene‐co‐1‐butene)] matrices and the possibility of making a suitable porous composite film. The yield stress of the composites did not improve, but the modulus increased gradually with the filler loading. The theoretical and experimental modulus and yield stress of the composites provided evidence of filler and polymer adhesion behavior. The impact strength showed little enhancement up to a 20 wt % loading for the poly(propylene‐co‐ethylene‐co‐1‐butene) system. The number‐average, weight‐average, and z‐average air‐hole diameters were compared with respect to the draw ratio as well as the calcite loading. The morphology of a micromechanically deformed composite, studied with an image analyzer, revealed that the aspect ratio and area of the air holes increased linearly as a function of the draw ratio, but the change in the aspect ratio upon filler loading was not remarkable. A suitable loading of a filler up to 30 wt % was good for controlling the porosity in the composite films. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Processing and properties of an ethylene–vinyl acetate blend foam incorporating ethylene–vinyl acetate and polyurethane waste foams

Waste polyurethane foam (w‐PU) and waste ethylene–vinyl acetate foam (w‐EVA) were used as fillers for the production of an ethylene–vinyl acetate (EVA) blend foam. Two different foaming techniques (single‐stage and heat–chill processes) were used for this purpose. The waste foam concentration was varied up to 30 wt % of the original EVA. The physical, mechanical, and morphological properties of the filled foam were studied. The single‐stage process produced blend foams with a lower density and a greater cell size than the foams obtained by the heat–chill process. The density and compression strength of the blend foam increased as the percentage of w‐PU foam increased. However, for the w‐EVA/EVA blend foams, the addition of w‐EVA foam did not significantly affect the density or compression strength compared to the original EVA foams. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44708.     

Additive manufacturing,quasi-static and dynamic compressive behaviours of ceramic lattice structures

Ceramic lattice structures (CLSs) are used for construction in common and extreme environments because of the extraordinary properties of ceramics. In this study, we designed and additively manufactured CLSs with distinct structural parameters to explore their quasi-static and dynamic compressive behaviours in detail. It was demonstrated that both the relative density (?ρ) and inclination angle (ω) had a significant impact on the quasi-static and dynamic mechanical properties of the CLSs. Furthermore, the mathematical relationships between the quasi-static compressive properties, including quasi-static compressive strength (QS), quasi-static Young’s modulus (QY), and quasi-static energy absorption (QE), versus ?ρ and ω obeyed the Gibson–Ashby and Deshpande and Fleck models, respectively. It was revealed by experiment and simulation that as the stiffness increased, the quasi-static failure mode of the CLSs changed from a parallel-vertical-inclined mixed mode to a parallel-vertical mode. In addition, the relationship between the dynamic mechanical properties of the CLSs versus ?ρ and ω also followed the Gibson–Ashby and Deshpande and Fleck models. The exceptional dynamic increase factor indicated that CLSs are highly suitable for extreme environments. These findings will aid in the research and development of customised additively manufactured CLSs.     

Impact of doping on the mechanical properties of acicular mullite

Acicular mullite (ACM) is a highly porous ceramic with a needlelike microstructure. Next-generation ACM-based diesel particulate filters will require porosities >60%, making optimizing ACM's mechanical properties a key area of interest. A prior study determined that, for the range of microstructures evaluated, the elastic modulus, strength, and fracture toughness were largely functions of total porosity and not needle or pore size, consistent with the Gibson–Ashby foam model. Therefore, alternate strengthening and toughening methods were sought. Doping the ACM precursor with either MgO or Nd₂O₃ produced ACM microstructures that appeared similar but had differing bulk mechanical properties. The mechanical properties of the mullite needles, the intergranular glassy phase, and the mullite–glass interface of the ACMs were investigated, but no major differences were found. Using X-ray computed tomography, a 3D imaging technique, it was found that MgO-doping of the ACM created a less uniform, and thus weaker, microstructure than Nd₂O₃-doping.     

Enhanced foamability of isotactic polypropylene composites by polypropylene‐graft‐carbon nanotube

In this article, utilizing a nucleophilic substitution reaction between epoxy group in polypropylene‐graft‐glycidyl methacrylate (PP‐g‐GMA) and carboxyl groups in oxidized carbon nanotubes (O‐CNTs), PP‐g‐CNT was fabricated for reinforcing the interfacial adhesion between CNTs and polypropylene (PP) matrix, favoring the enhancement of melt strength and elastic modulus, i.e., enhancing the foaming ability of PP composites. Cellular structure and thermo‐mechanical properties of PP foams were characterized by scanning electron microscopy and dynamic mechanical analysis, respectively. The average cell diameter of PP foams decreased from 289.2 (PP‐g‐GMA) to 96.7 μm (PP‐g‐CNT foams with 2.0 wt % O‐CNT) and the distribution of cell size also became more uniform. The storage modulus of PP‐g‐CNT foams increased by nearly 62.5% at ?40°C, compared with that of PP‐g‐GMA foams. This work also provided a new procedure for improving the foam ability and thermo‐mechanical property of PP composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 961‐968, 2013