Foam Cell Size Research Articles

Effects of cell size and cell wall thickness variations on the stiffness of closed-cell foams

This paper concerns with the micromechanical modelling of closed-cell polymeric foams (M130) using Laguerre tessellation models incorporated with realistic foam cell size and cell wall thickness distributions. The cell size and cell wall thickness distributions of the foam were measured from microscope images. The Young’s modulus of cell wall material of the foam was characterised by nanoindentation tests. It is found that when the cell size and cell wall thickness are assumed to be uniform in the models, the Kelvin, Weaire–Phelan and Laguerre models overpredict the stiffness of the foam. However, the Young’s modulus and shear modulus predicted by the Laguerre models incorporating measured foam cell size and cell wall thickness distributions agree well with the experimental data. This emphasizes the fact that the integration of realistic cell wall and cell size variations is vital for foam modelling. Subsequently the effects of cell size and cell wall thickness variations on the stiffness of closed-cell foams were investigated using Laguerre models. It is found that the Young’s modulus and shear modulus decrease with increasing cell size and cell wall thickness variations. The degree of stiffness variation of closed-cell foams resulting from the cell size dispersion and cell wall thickness dispersion are comparable. There is little interaction between the cell size variation and cell wall thickness variation as far as their effects on foam moduli are concerned. Based on the simulation results, expressions incorporating cell size and cell wall thickness variations were formulated for predicting the stiffness of closed-cell foams. Lastly, a simple spring system model was proposed to explain the effects of cell size and cell wall thickness variations on the stiffness of cellular structures.

Deformation behavior of open-cell stainless steel foams

This study presents the deformation and cell collapse behavior of open-cell stainless steel foams. 316L stainless-steel open-cell foams with two porosities (30 and 45 pores per inch, ppi) were produced with the pressureless powder metallurgical method, and tested in quasi-static compression. As a result of the manufacturing technique, 316L stainless steel open-cell foams have a high amount of microporosity. The deformation behavior was investigated on a macroscopic scale by digital image correlation (DIC) evaluation of light micrographs and on the microscopic scale by in situ loading of cells in the scanning electron microscope.The deformation behavior of the metal foams was highly affected by microstructural features, such as closed pores and their distribution throughout the foam specimen. Moreover, the closed pores made a contribution to the plateau stress of the foams through cell face stretching. Strut buckling and bending are the dominant mechanisms in cell collapse. Although there are edge defects on the struts, the struts have an enormous plastic deformation capability. The cell size of the steel foams had no significant effect on the mechanical properties. Due to the inhomogeneities in the microstructure, the measured plateau stresses of the foams showed about 20% scatter at the same relative density.

Mechanical and Thermal Properties of Phenolic Foams Reinforced by Hollow Glass Beads

Hollow glass beads / phenolic foam composites were prepared by molding method. The influence of HGB on thermal performance and mechanical properties of phenolic foams were investigated using thermal conductivity measurement, thermogravimetric analysis (TGA) and compression tester. The results show that the addition of hollow glass beads lead to a significant improvement in the compressive property of phenolic foams, with the compressive strength reaching the maximum adding 10% HGB and HGB pretreated by silicane coupling agent further enhance the compressive property. FT-IR spectroscopy shows the reaction between alcohol-OH groups on the surface of HGB and methoxy (-OCH3) groups on silane coupling agent (KH560). The morphology indicates the average cell size decreases with HGB content increasing up to 10%, and again the cell size of foams reinforced by pretreated HGB are better. Addition of HGB improved the thermal stability property of phenolic foams, due to the porosity was mainly responsible for thermal conductivity property of phenolic foams, so HGB filled materials achieved higher thermal conductivity.

Microcellular foams made from gliadin.

We have generated closed-cell microcellular foams from gliadin, an abundantly available wheat storage protein. The extraction procedure of gliadin from wheat gluten, which involves only the natural solvents water and ethanol, respectively, is described with emphasis on the precipitation step of gliadin which results in a fine dispersion of mostly spherical, submicron gliadin particles composed of myriad of protein molecules. A dense packing of these particles was hydrated and subjected to an atmosphere of carbon dioxide or nitrogen in a high-pressure cell at 250 bar. Subsequent heating to temperatures close to but still below 100 °C followed by sudden expansion and simultaneous cooling resulted in closed-cell microcellular foam. The spherical gliadin templates along with the resulting foam have been analyzed by scanning electron microscope (SEM) pictures. The size distribution of the primary particles shows diameters peaked around 0.54 μm, and the final foam cell size peaks around 1.2 μm, at a porosity of about 80 %. These are the smallest foam cell sizes ever reported for gliadin. Interestingly, the cell walls of these microcellular foams are remarkably thin with thicknesses in the lower nanometer range, thus nourishing the hope to be able to reach gliadin nanofoam.

Absorption and foaming of plastics using carbon dioxide

In this study, carbon dioxide was used as a foaming agent for common plastics, such as acrylonitrile–butadiene–styrene (ABS) polymer, polystyrene (PS), polypropylene (PP), high-density polyethylene (HDPE), and high impact polystyrene (HIPS). Carbon dioxide was first absorbed by the sample plastics placed within a pressure vessel at various pressure levels and absorption time intervals. The Henry’s constant of the absorbed carbon dioxide in the plastics was determined. The diffusion coefficient of carbon dioxide in polymer was also identified by curve-fitting with the relationship between the absorbed amount and time. The results showed that ABS, PS, and HIPS absorbed more gas than did PP and HDPE, because PP and HDPE exhibit higher crystallinity. Generally, a polymer can take up saturation absorption of gas under higher pressure. After absorption, the foaming process occurred at various temperatures and time intervals. The cell structure, density, and size of the plastic foams were then investigated using scanning electron microscopy. A longer foaming period and higher temperature increase the size of the cell and decrease the cell density (the number of bubbles per unit volume). A dense skin layer without bubbles appeared directly adjacent to the surface of the foamed plastics. Its thickness decreased if the foaming process took place at higher temperatures.

Effect of different secondary quinoline insoluble content on the cellular structure of carbon foam derived from coal tar pitch

Carbon foam was produced using mesophase pitches obtained under different temperatures as precursors, via foaming and carbonization process. The physicochemical properties of mesophase pitch, as well as the microstructure and physical properties of carbon foam were investigated by optical microscope, infrared spectrometer, thermograviment analyzer (TGA), X-ray diffractometer (XRD), scanning electron microscope (SEM) and universal testing machine, respectively. The results show that the amount of secondary quinoline insoluble in mesophase pitches increase with heat-treatment temperature increase, meanwhile, the cell size of carbon foams increased firstly and then reduced. Moreover, the compressive strength of carbon foams also exhibited the same variation trend. The cellular structure of carbon foam can be severely affected by the secondary quinoline insoluble content of mesophase pitch; thus it is critical to tailor the secondary quinoline insoluble content of mesophase pitch for obtaining carbon foam with high performance.

Geopolymer foams by gelcasting

Highly porous geopolymers, with homogeneous microstructure, open cells and porosity up to 80vol%, were fabricated by gel-casting, a process commonly used to produce ceramic foams. Geopolymer foams were prepared by stirring an activated blend of metakaolin and fly ash with a mixture of potassium hydroxide and potassium silicate with Si/K=1.66. The cell size and size distribution of the geopolymer foams could be efficiently adjusted by the control of some parameters such as solid content, surfactant type and content and mixing speed. The influence of each parameter on the porosity and other characteristics of the geopolymer foams were investigated. The foams were evaluated only after heat treatment at 80°C, which was conducted in order to complete the geopolymerization reactions. The produced components could be heat treated up to 1200°C in air without melting, if desired.The characteristics (morphology, strength, chemical and thermal resistance) of the geopolymer foams suggest that they could be employed as low cost replacement for highly porous ceramics in applications such as catalysis supports, adsorption and separation, filtration of hot gases and refractory insulation of furnaces. In addition, these components could be considered sustainable, because they reach their final properties after processing at temperatures not exceeding 100°C and part of the raw materials employed are industrial waste.

Effect of Chain Extension on the Rheological Property and Thermal Behaviour of Poly(lactic acid) Foams

Epoxy-based chain extended high-melt-strength poly (lactic acid) (PLA) was prepared. Rheological properties and the thermal behaviours of the chain extended PLA were studied. The rheological properties indicated that the melt elasticity and melt strength of the chain extended PLA samples were gradually improved by increasing the content of chain extender (CE). Moreover, the thermal results showed that the crystallinity and the spheruilite morphology of PLA were greatly influenced due to the introduction of CE. The effect of CE contents on foam morphology was also studied. Adding CE resulted in higher expansion ratio and smaller cell size of PLA foams.

Acoustic Study Based on Sustainable Green Polymer Treated with H<sub>2</sub>O

Green polymer foam was prepared by the reaction of green monomer based on vegetable oil with commercial Polymethane Polyphenyl Isocyanate (Modified Polymeric-MDI) and distilled water (H2O). The morphological study of green polymer foam was examined by Scanning Electron Microscope (SEM) and acoustic property by means of H2O composition ratio equivalent to weight of polyol. It was found that the cell size of green polymer foam has significantly increment as well as H2O loading increased. Increasing of H2O more than 50% equivalent to weight of polyol shows nonuniform pore distribution, large average pore size and smallest number of pore. Furthermore, the cell size of neat green polymer foam gives 400μm up to 1833.3μm with high loading of H2O. In addition, the cell size of green polymer foam influences by the increasing amount of H2O loading and enhanced the sound absorption property at low frequency level.

Blocking lymphatic vessel function with soluble VEGFR3 in atherosclerotic mouse model alters lipoprotein metabolism and enhances atherogenesis

Purpose: Lymphatic vessels take part in lipid absorption, inflammatory reactions and draining of large blood vessels. Even though these processes are involved in arterial diseases, the role of lymphatic vessels in atherosclerosis has not been clarified yet. In this study, we evaluated the effect of insufficient lymphatic function on the lipoprotein metabolism and development of atherosclerosis by blocking Vascular Endothelial Growth Factor Receptor 3 (VEGFR3), the key mediator of lymphangiogenesis. Methods: Mice expressing soluble VEGFR3 (sVEGFR3) were cross-bred with atherosclerotic LDLR-/-/ApoB100/100 mice. Mice were fed with chow diet for 3-4 months, 7-8 months and 11-12 months followed by high-fat diet (42% of calories from fat and 0.15% from cholesterol) up to 12 weeks. Lipid levels and lipoprotein profiles were analyzed from blood plasma. Atherosclerotic lesion size was determined from en face aortas and cross-sections of aortic arches and lesion composition was evaluated with modified Movat's staining and immunohistochemical stainings for macrophages (mMQ), blood vessels (CD31) and lymphatic vessels (podoplanin). Results: Compared to LDLR-/-/ApoB100/100 controls, sVEGFR3 x LDLR-/-/ApoB100/100 mice had significantly increased plasma cholesterol levels with chow diet (8.2±0.4 mmol/l vs. 16.3±1.2 mmol/l, respectively) and up to 6 weeks with high-fat diet (36.9±3.0 mmol/l vs. 50.8±4.4 mmol/l, respectively). In addition, sVEGFR3 x LDLR-/-/ApoB100/100 mice had higher VLDL and LDL cholesterol and triglyceride levels than controls. Especially in young sVEGFR3 x LDLR-/-/ApoB100/100 mice, elevated cholesterol levels induced accelerated atherogenesis characterized by increased lesion size and rapid accumulation of foam cells and cholesterol crystals into the lesions. Even though the number of vasa vasorum in the atherosclerotic lesions was at similar level in the study and control groups, lymphatic vessels were almost absent in the lesions of sVEGFR3 x LDLR-/-/ApoB100/100 mice. Conclusions: Here we show that lipoprotein metabolism and atherogenesis are affected by insufficient lymphatic draining. These results suggest that lymphatic vessels have a previously undiscovered role in the lipid transport and highlight a potential target for the treatment of lipid related diseases, such as atherosclerosis.

Preparation and characterization of kaolin/starch foam

The objective of this work was to study the effect of the kaolin content on the properties of starch foams. The kaolin/starch foams were made with kaolin contents that ranged from 0 to 15m% by baking in a hot mold. The starch and kaolin/starch foams were stored at room temperature with a relative humidity (RH) of 55% for 7days prior to testing. An increase in the kaolin content increased the foam density. The izod impact strength increased up to 1151.37J/m2 at the highest kaolin content (15m%). The improvement was about five times the izod impact strength of pure starch foam. Moreover, the presence of any kaolin reduced the water absorption ability of the starch foam. Scanning electron microscopy revealed that kaolin increased the size of the starch foam cells and was itself well dispersed. Kaolin/starch foams showed a higher thermal stability than pure starch foam.

Preparation of polystyrene/graphene oxide composites and their supercritical carbon dioxide foaming

We investigated the nucleation effect of well exfoliated and dispersed graphene oxide(GO) sheets on polystyrene(PS) supercritical carbon dioxide foaming. To get PS/GO composites with well exfoliated GO sheets, a method based on latex concept was employed. The characterizations based on X-ray diffraction and transmission electron microscope demonstrated the full exfoliation and uniform dispersion of GO sheets in polymer matrix. Thermal stability and dynamic mechanical properties of the compound materials were investigated by a series of measurements such as differential scanning calorimetry analysis, dynamic mechanical analysis and thermogravimetry analysis, which suggested the higher thermal stability and glass transition temperature of the composites. The supercritical carbon dioxide foaming of pure PS and composites was carried out in a batch foaming system, the results showed that the well exfoliated GO sheets could be a high efficient nucleation agent. For the pure PS foam and PS/GO composite foams produced under the foaming condition of 120 °C and 13.8 MPa, the average foam cell size was reduced from 7.1 um to 4.3 um and the cell density was increased from 4.52 × 109 to 1.84 × 1010 cells/cm3 by around 5.0 wt.% GO sheets.

Morphology and physical properties of composite foams based on low‐density polyethylene and ground tire rubber

Low‐density polyethylene (LDPE)/(ground tire rubber) (GTR) composite foams with an average cell size smaller than 100 µm were produced by compression molding. The effects of the mechanochemical pretreatment of the LDPE/GTR blends and their GTR content on the cell morphology and physical properties were investigated. The results indicated that the pretreatment of LDPE/GTR blends by using a pan‐mill type of mechanochemical reactor made in our laboratory improved the dispersion state and thereby enhanced the foamability of the blends. The average cell size of the composite foams decreased from 64.0 to 42.3 µm with an increase in GTR content from 0 to 50 phr (parts by weight per hundred parts of resin). Moreover, the addition of 20–40 phr of GTR into the LDPE foams increased their rebound resistance and decreased their compression set. J. VINYL ADDIT. TECHNOL., 19:105–112, 2013. © 2013 Society of Plastics Engineers

Heat transfer in one-dimensional micro- and nano-cellular foams

We investigate heat transfer in one-dimensional multi-layer models of polymer foams and determine their equivalent thermal conductivities. The proposed model considers the participating gas and solid phases, i.e., combined heat transfer by conduction and radiation in both phases and partial photon reflection on phase interfaces. A correction for possible non-Fourier heat conduction in small gas cells, occurring in nanocellular foams, is implemented by the reduction of gas thermal conductivity based on the Knudsen number. The developed model was used for the optimization of foam structure with respect to its insulation properties. We found that although radiation can account for more than one-third of the total heat flux, it can be reduced in foams with cell-sizes below 100μm and be almost nullified by decreasing the foam cell size to sub-micron range. The presented model is a starting point for the development of an advanced model of heat transfer in spatially three-dimensional polymer foams.

Microstructure and Properties of Microcellular Poly (phenylene sulfide) Foams by Mucell Injection Molding

A series of microcellular poly (phenylene sulfide) (PPS) foams were prepared by Mucell injection molding. The cell structure, mechanical properties, crystallization behavior and dielectric property of microcellular PPS foams were systemically investigated. The results showed that the longer the length of flow passage of injection mold, the larger cell size of microcellular PPS foams. The injection parameter of shot size played an important role in relative density of microcellular PPS foams. When the relative density of microcellular PPS foam reached to 0.658, the tensile strength, flexural strength and impact strength of PPS foam materials achieved 10.82 MPa, 52.99 MPa and 0.305 J/cm2, respectively. Meanwhile, with the relative density decreasing, the dielectric constant of PPS foam materials reduced, while the volume resistivity of its uprated.

Optimization of Process Parameters for Generation of Nanocellular Polymer Foams

High melt strength polypropylene nanocomposites, PPNC/Cloisite 20A (clay) with exfoliated and intercalated morphologies were prepared and subsequently foamed in a batch setup under different foaming conditions. The foaming parameters were varied to relate the foam cell structure to these parameters and determine the efficiency of clay in producing fine cell foams. A Box Benkhen design approach was used initially to determine the effect of processing parameters on foam cell morphology and also to perform optimization studies. The optimization process helped in identifying the range of operating conditions needed to minimize foam cell sizes. Saturation pressure and temperature and foaming time and temperature are the four processing variables used in these studies. Nanocellular foam cells were effectively generated for the first time in Polypropylene nanocomposites.

Thermoplastic polyurethane/clay nanocomposite foam made by batch foaming

In this study, the foaming of nonwoven fabrics and bulk polymers was investigated. Four different polymers that are commonly used for textiles including polyethylene terephthalate, thermoplastic polyurethane, thermoplastic polyether ester elastomer and polypropylene were foamed by batch foaming. Among the polymers, thermoplastic polyurethane seems to be the most promising material since it possesses the highest cell density and smallest cell size. In addition, nanoclay was added to thermoplastic polyurethane to promote cell nucleation and to increase the cell density. The results showed that well dispersed nanoclay in thermoplastic polyurethane served as an excellent nucleation agent and the cell structure was improved. The cell size of thermoplastic polyurethane nanocomposite foam decreased to 1 µm while the cell density increased to 3 × 1011 cells/cm3.

High performance structured platelet milli-reactor filled with supported cobalt open cell SiC foam catalyst for the Fischer–Tropsch synthesis

The Fischer–Tropsch synthesis (FTS) was evaluated on a 30wt.% Co catalyst promoted with 0.1wt.% of Ru supported on a silicon carbide (SiC) foam in a platelet structured milli-reactor (PSR). The FTS was tested under isothermal conditions according to the Mear’s criterion. The tuneable morphology of the open cell foam (cell size, macroscopic porosity, strut diameter) allows an adjustment of both axial and radial flow patterns in the reactor and ensures local fluid recirculation, which are favorable to heat and mass transfer. The large pore size, i.e. meso- and macro-pores of the support allows a rapid escaping of the liquid products leading to a high FTS activity as well a good C5+ selectivity. The catalytic results obtained in the platelet milli-reactor were compared with those obtained in a ‘conventional’ packed bed tubular reactor (TR). The PSR shows a better FTS activity compared to that obtained in the TR and the FTS activity steadily increases with increasing the GHSV. Finally, a remarkably high C5+ selectivity (∼93%) and a negligible amount of methane formation (∼1%) were also obtained at a GHSV of 1900h−1 in the PSR.

Physical Properties of Tannin/Furanic Resin Foamed With Different Blowing Agents

Tannin/furanic rigid foam made from raw materials of natural origin could be used to replace polyurethane and phenolic foams in many future applications. In this study, diethyl ether, pentane, and a mixture of the two were used to prepare tannin/furanic foams having different microstructures. It was found that the bulk density of the foam could be significantly extended, up to 180 kg/m3. The cell size of the pentane foam was larger than that of diethyl ether foam with comparable density; however, both presented a sudden increase in cell size when the density decreased. The thermal conductivity and mechanical properties were determined for the two foam types at the same density but different cell size. The minimum thermal conductivity for diethyl ether and pentane foams were presented, along with a suggested method for lowering the thermal conductivity of such foam. Additionally, the results of mechanical tests indicate that cell size does not play a role in compressive strength.

TiO2/β-SiC foam-structured photoreactor for continuous wastewater treatment

This study of photocatalytic degradation of wastewater was carried out in alveolar cell β-SiC foam-structured photocatalytic reactors working in a recirculation mode. The immobilization of TiO2 on β-SiC foams was efficiently obtained through a sol-gel technique in acidic conditions. In order to optimize degradation yields obtained by the foam-structured prototype reactor for the photocatalytic water treatment, the operating conditions of the photoreactor have been investigated and the efficiency of the process was evaluated by measuring the photocatalytic degradation of Diuron (3-(3,4-dichlorophenyl)-1,1-dimethyl-urea)) under UV irradiation. Kinetic studies were carried out by investigating the influence of different parameters controlling the reaction (TiO2 loading and β-SiC foam cell size). The ageing of TiO2/β-SiC foam photocatalytic materials and the mineralization (TOC, Cl-, NO3- and NH4+) of Diuron were investigated.