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Research on nanocellular foams is motivated in part by the promise of physical properties, in particular mechanical properties, that can go beyond the classical mechanical framework. However, due to the difficulty in obtaining foams of a given density but different cell sizes, determining the effect of cell size on the mechanical properties of polymer foams remains a challenge. To overcome this difficulty, studies on the mechanical behaviour of mesocellular, microcellular and nanocellular polymer foams have been compiled in this review article. After describing the different cellular structures between meso-, micro- and nanocellular foams, the mechanical properties are examined as a function of relative density and cell size. It is shown that for small strains and at low strain rates, nanocellular foams exhibit mechanical behaviour predicted by the Gibson and Ashby model. Relative density remains the first important factor to be taken into account when studying the Young's modulus and buckling stress of nanocellular foams. The focus then shifts to fracture properties, as microcellular foams have already been shown to be far superior to more conventional foams. As studies are still scarce and different methodologies have been used, no general conclusions can be drawn. However, the fracture and impact properties could be greatly improved by this change in scale. The local confinement of molecular chains in polymeric nanocellular foams or the relaxation of the triaxial stress state in front of the crack tip could explain these observations. [ABSTRACT FROM AUTHOR]

The cross-section of an injection-molded plate of foamed long glass fiber reinforced polypropylene was analyzed using scanning electron microscopy. The distribution of the glass fiber orientations and the microcellular structure in the thickness direction were also studied. A multilayer representative volume element was constructed based on the fiber orientation tensors and the cell distribution. Nested and two-step homogenization methods based on the Mori–Tanaka and Voigt models were used to homogenize each layer of the representative volume element. Finally, classic laminate theory was used to obtain the effective elastic modulus of the material. The computed elastic moduli of the single-layer and multilayer representative volume element models with different loading directions predicted by the homogenization and finite element methods were compared with the experimental results. We found that the constructed multilayer representative volume element model can predict the elastic moduli of the foamed glass fiber reinforced polypropylene effectively and that the predicted results were accurate and stable. [ABSTRACT FROM AUTHOR]

The microcellular foaming of chain-extended polyethylene terephthalate (CPET) by crystallization induction method was reported in this article. The crystallization behaviors of various polyethylene terephthalate (PET) samples which were affected by the combined effect of pyromellitic dianhydride, Surlyn, and CO2 were investigated. After Surlyn was added to CPET, the crystal nucleation of various CPET samples was improved, and numerous but small spherulites were generated. Two kinds of CPET samples with the content of 0 phr and 1 phr Surlyn were foamed at various temperature by batch foaming method. Changing the saturation temperature could adjust the appearance of high-temperature melting crystals which would affect the final cellular structures in various CPET foams. With the decrease of saturation temperature, the cell size decreased while cell density increased. At the saturation temperature of 265°C and 250°C, the cell density of CPET foam with Surlyn was one magnitude larger than CPET foam without Surlyn. At the saturation temperature of 247°C, the microcellular PET foams with the cell density of 109 cells cm−3 and the cell size less than 10 µm had been developed successfully. [ABSTRACT FROM AUTHOR]

In this work, the foaming process was employed to achieve lightweight thermoplastic polyurethane materials, and then the hysteresis and residual strain of corresponding materials in the tensile process were quantitatively calculated. In order to study the deformed mechanism, the influences of cell type and skin-core structure on the tensile elasticity of thermoplastic polyurethane foam were investigated. The open-cell thermoplastic polyurethane foam exhibited much lower hysteresis and residual strain compared to thermoplastic polyurethane film without cell structure, which demonstrated that the open-cell structure benefited to the tensile elasticity. In the case of closed-cell thermoplastic polyurethane foam, it had lower hysteresis and residual strain than thermoplastic polyurethane film; however, higher value than the thermoplastic polyurethane film can be observed beyond 100% strain, resulting from the stress concentration in the skin-core structure. Consequently, the hysteresis phenomenon can be improved by adjusting the ratio of skin-core structure. Moreover, the influence of density on the elasticity of the open-cell thermoplastic polyurethane foam was also discussed in this study. [ABSTRACT FROM AUTHOR]

A mold foaming process for preparing thermoplastic polyurethane foam sheets was proposed and realized in pilot scale hot-press machine based on the mechanism of solid-state polymer foaming. Thermoplastic polyurethane microcellular sheets with volume expansion ratios of 2.2–7.0 were prepared by adjusting foaming temperature of 95–120°C at the fixed CO2 pressure of 12 MPa with natural post-curing. The good foam dimensional stability and the uniform cell morphology of these thermoplastic polyurethane foams suggested that the mold foaming of thermoplastic polyurethane sheet was feasible. Regardless of volume expansion ratio, a linear shrinkage of 17–19% occurred to all thermoplastic polyurethane foams after post-curing. The flexible properties of these post-cured thermoplastic polyurethane foams, such as hardness, rebound resilience, compression set, and dynamic shock absorption, were investigated. The test results indicated that these thermoplastic polyurethane foams were excellent flexible materials. [ABSTRACT FROM AUTHOR]

A mold foaming process for preparing thermoplastic polyurethane foam sheets was proposed and realized in pilot scale hot-press machine based on the mechanism of solid-state polymer foaming. Thermoplastic polyurethane microcellular sheets with volume expansion ratios of 2.2–7.0 were prepared by adjusting foaming temperature of 95–120°C at the fixed CO2 pressure of 12 MPa with natural post-curing. The good foam dimensional stability and the uniform cell morphology of these thermoplastic polyurethane foams suggested that the mold foaming of thermoplastic polyurethane sheet was feasible. Regardless of volume expansion ratio, a linear shrinkage of 17–19% occurred to all thermoplastic polyurethane foams after post-curing. The flexible properties of these post-cured thermoplastic polyurethane foams, such as hardness, rebound resilience, compression set, and dynamic shock absorption, were investigated. The test results indicated that these thermoplastic polyurethane foams were excellent flexible materials. [ABSTRACT FROM AUTHOR]

Mixed matrix membranes made from silica nanoparticles and microcellular polymers were prepared from Matrimid® 5218 combined with tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane via the sol–gel method. The nanoparticles were prepared in situ during membrane casting yielding a homogeneous distribution inside a foamed polyimide structure. Mixed matrix membranes with SiO2 contents up to 16% wt. were treated at 60℃, 100℃, 150℃, and 200℃. Thermal gravimetric analysis and Fourier transform infrared spectroscopy analyses were performed providing information on chemical composition and thermal stability, while the porous structure (average cell diameter and cell density) was studied by scanning electron micrograph. Also, dynamic mechanical analysis was used to determine the glass transition temperature (Tg) and elastic modulus. Finally, the gas transport properties were studied in terms of treatment temperature, feed pressure, SiO2 loading, and testing temperature. CO2 permeability was found to increase by a factor of 3–4 at 3% SiO2 content using tetraethoxysilane in Matrimid, while ideal selectivity for CO2/CH4 separation was constant. Finally, the plasticization effect was practically eliminated by the introduction of SiO2 nanoparticles. [ABSTRACT FROM AUTHOR]

Microcellular polystyrene foam sheets prepared by extrusion foaming with CO2 as the blowing agent were found to exhibit a significant anisotropy in mechanical behavior which could not be explained solely by the shape anisotropy of the cells and/or the preferential orientation of polystyrene molecules in the machine direction. Surprisingly, samples allowed to fully relax were still found to maintain part of their mechanical anisotropy, which was attributed to the weakness of the cell walls perpendicular to machine direction. [ABSTRACT FROM AUTHOR]

Thermoplastic polyurethane possesses many special characteristics. Its flexibility, rigidity, and elasticity can be adjusted by controlling the ratio of soft segments to hard segments. Due to its versatile physical properties, thermoplastic polyurethane is commonly used in transportation, construction, and biomaterials. However, methods for thermoplastic polyurethane foam production using CO2 are still under investigation. We have previously prepared nanoporous thermoplastic polyurethane foam using commercially available thermoplastic polyurethane; however, in this study, thermoplastic polyurethane was synthesized using 4,40-methylenebis(phenyl isocyanate), poly(propylene glycol) and 1,4-butanediol, without solvents, using a pre-polymer method. The properties of the synthesized thermoplastic polyurethane were characterized by Fourier transform infrared spectroscopy, thermal analysis, and their mechanical properties were measured. The synthesized thermoplastic polyurethane was foamed by batch foaming using supercritical CO2 as the blowing agent. The effect of saturation temperature and saturation time on the cell morphology of the thermoplastic polyurethane foam was examined. [ABSTRACT FROM AUTHOR]

In this study, flame-retardant poly(lactic acid) foams with satisfactory cell structures were prepared by microcellular foaming technology using phosphorus-containing flame retardant and graphene as the charring agent. The introduction of 5–30 wt% flame retardant increased the limited oxygen index value of poly(lactic acid) from 19.0 to 26.5–37.8% and simultaneously increased the foam expansion of poly(lactic acid) foams from 4.4 to 5.8–17.5. In addition, all the prepared poly(lactic acid)/flame-retardant composites passed the UL-94 V-0 rating. The addition of 0.5 wt% graphene increased the limited oxygen index value of poly(lactic acid)/flame-retardant composite with flame-retardant content of 15 wt% from 27.9 to 29.2%, and more graphene additions improved the antidripping behavior of poly(lactic acid) composites. The possible mechanisms of the effects of the resultant cellular structure on the flame-retardant properties of poly(lactic acid) composites were also discussed. [ABSTRACT FROM AUTHOR]

A systematized calculation of foaming parameters in producing microcellular polymethyl methacrylate (PMMA) with supercritical carbon dioxide (SCCO2) was proposed. The calculated optimal results were saturation temperature = 313 K, pressure = 25 MPa and time = 15 h, which were achieved by the Chow model, classical nucleation theory and Fick diffusion law, respectively. Additionally, the Center-Composite Design (CCD) statistical approach in Response Surface Method (RSM) was employed to evaluate the validity of the calculated parameters. The error between the optimal parameters predicted by CCD statistical approach and the calculated ones was acceptable, demonstrating the accuracy of the calculated parameters. [ABSTRACT FROM AUTHOR]

A systematized calculation of foaming parameters in producing microcellular polymethyl methacrylate (PMMA) with supercritical carbon dioxide (SCCO2) was proposed. The calculated optimal results were saturation temperature = 313 K, pressure = 25 MPa and time = 15 h, which were achieved by the Chow model, classical nucleation theory and Fick diffusion law, respectively. Additionally, the Center-Composite Design (CCD) statistical approach in Response Surface Method (RSM) was employed to evaluate the validity of the calculated parameters. The error between the optimal parameters predicted by CCD statistical approach and the calculated ones was acceptable, demonstrating the accuracy of the calculated parameters. [ABSTRACT FROM AUTHOR]

A regression model constructed by response surface methodology was employed to optimize the relationships between the cell density of microcellular polymethyl methacrylate foam and three independent variables: foaming pressure, temperature, and time. A Box–Behnken Design statistical approach was employed to fit the available response data to a second-order polynomial response surface model. The analysis of variance of the model indicated that the interactions between the foaming pressure and temperature, and that between the foaming temperature and the saturation time, both positively affect the cell density. Experimental verification of the predicted optimum conditions of foaming pressure = 21 MPa, foaming temperature = 313 K, and saturation time = 6.9 h gave an actual maximum cell density of 20.86 × 109 cells/cm3, which is close to the data predicted by the regression model. [ABSTRACT FROM AUTHOR]

A regression model constructed by response surface methodology was employed to optimize the relationships between the cell density of microcellular polymethyl methacrylate foam and three independent variables: foaming pressure, temperature, and time. A Box–Behnken Design statistical approach was employed to fit the available response data to a second-order polynomial response surface model. The analysis of variance of the model indicated that the interactions between the foaming pressure and temperature, and that between the foaming temperature and the saturation time, both positively affect the cell density. Experimental verification of the predicted optimum conditions of foaming pressure = 21 MPa, foaming temperature = 313 K, and saturation time = 6.9 h gave an actual maximum cell density of 20.86 × 109 cells/cm3, which is close to the data predicted by the regression model. [ABSTRACT FROM AUTHOR]

In this study, microcellular poly(lactic acid) foams with various crystallinities, cell morphologies, and densities were prepared using CO2 as the physical blowing agent. The evolution of crystallinity developments of four types of poly(lactic acid) samples during the saturation, foaming, and annealing processes was investigated. Crystallization of about 20% was reached in poly(lactic acid) samples after CO2 saturation, a high crystallinity of about 38.2% could be achieved for the foamed poly(lactic acid) that has the highest crystallization ability. Poly(lactic acid) samples had low elongation at break of 3.6–15.1%. After foaming, however, poly(lactic acid) foam presented a significant increase in the elongation at break up to 15.1 times compared with that of the unfoamed counterpart. On the other hand, microcellular foaming endowed poly(lactic acid) foams with a maximum increase in specific tensile strength of 53.1%. The influences of crystallinity, foam density, and cell morphology on the tensile properties of poly(lactic acid) foams were investigated. [ABSTRACT FROM AUTHOR]

Polycarbonate has the reputation of having a tough breaking behavior, but it is widely unknown that this applies only to special conditions. The impact strength of polycarbonate depends on the temperature, thickness (with a tough brittle transition as thickness increases), contribution of notch tip radius, impact speed, physical blowing agent, molecular weight of the polymer, and processing parameters. Research results indicate that microcellular foams produced by injection molding with physical blowing agent (MuCell™ Technology by Trexel) show a significantly higher notched impact strength than compact polycarbonate if the compact material is brittle under the same testing parameters. However, if the compact polycarbonate breaks toughly, the notched impact strength of the foamed material is always lower. Therefore, it is highly important to pay attention to the testing parameters and conditions when comparing the toughness of the foam with that of the compact material. The toughness of microcellular foams has similar properties like PC/ABS and PC/PP blend systems, which provides the possibility to combine the higher impact strength with the advantages of microcellular foaming such as weight reduction, lower shrinkage, shorter cycle times, lower clamp forces, and reduced melt viscosity. In order to use technologies and conditions, which are applied in the polymer industry as well, all materials were produced by an injection molding process. Special processing technologies such as gas counter pressure and precision mold opening were used in order to reach microcellular foam structures with cell diameters around 10 µm. These technologies yield exactly adjustable foam morphologies. Special morphologies are required to improve the notched impact strength of the foamed material. Two different equivalent models were extracted from the analyses, which indicate a significantly higher notched impact strength than the compact material under the same testing conditions. The knowledge of the ideal foam morphologies enables the industry to produce foamed materials with improved mechanical properties. [ABSTRACT FROM AUTHOR]

This article presents an extrusion-based, open-cell foaming process using thermoplastic polymers such as polystyrene (PSI and polycarbonate (PC) with supercritical COP. Our previous studies have indicated that a cell opening can be promoted by inducing: (i) a nonhomogeneous melt structure by cross-linking, polymer blending, or filler compounding, (ii) cell-wall thinning by a high volume expansion ratio while maintaining soft cell walls, (iii) cell-wall thinning by a high cell-population density, and (iv) plasticization of the soft region of the cell walls with a secondary blowing agent. Until now, the foam extrudate temperature across the cross-section was maintained uniformly for the simplicity of the experiments. In this study, the significant temperature difference between the core and surface of the foam extrudate was induced by surface cooling method. This method increased the chance of cell opening by: (i) increasing the core temperature of the foam extrudate and thereby softening the cell walls, and (ii) decreasing the foam surface temperature to prevent gas loss and thereby increasing the internal gas pressure within the cells. The effects of CO2 content, surface quenching, die geometry, and temperature on foam morphologies were investigated. Low-density, microcellular, open-cell foams were successfully produced. The large intercellular pores were observed from micrographs for both PS and PC foams at optimum processing conditions. [ABSTRACT FROM AUTHOR]

This article demonstrates the effect of exfoliated nano-clay on the microcellular foam processing of amorphous and crystalline nylon. Amorphous and crystalline nylon 6 nanocomposites are prepared using a twin-screw extruder. The exfoliated nanocomposite structures are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nylon and its nanocomposites are foamed in extrusion using supercritical CO2. Subsequently, the cell morphologies of nylon and its nanocomposite foams are investigated. It appeared that the nano-clay not only enhanced cell nucleation, but also suppressed cell deterioration during the microcellular foaming of nylon. [ABSTRACT FROM AUTHOR]

Focuses on the introduction of a novel family of processable rigid-rod polymers with outstanding mechanical properties in the market in January 2004. Implications for the field of high-performance, rigid structural foam; Effects of die temperature on the cell size, cell density, cell morphology and volume expansion ratios; Blowing agent concentrations and their effects on foam creation.

In this work, we have shown for the first time that assemblies of up to 64 alternating foam and film layers can be successfully made via the microlayer coextrusion technology. We have investigated the effect of some important processing variables on the foam-film structure, and have demonstrated how they can be optimized. With the optimum concentration of the right type of chemical blowing agent, the cell size can be significantly reduced for the polypropylene foam-film systems by increasing the number of layers. The foam layer density does not change significantly with increasing number of layers. Measurements of the cell density and cell volume indicate that enhanced nucleation and/or reduced coalescence is responsible for the reduction in cell size with an increasing number of layers. Constrained cell growth comes into play when a single cell/layer morphology is observed. Different types of polymers can be used as the film layer to alter the flexibility of the foam/film composite. The compressive behavior of the hard/hard polypropylene foam/polypropylene film system is similar to that of cork in terms of the compressive modulus, collapse stress, and densification strain. The tensile and compressive moduli increase with the increasing number of layers and this could be explained on the basis of an increase in the fraction of material in the cell walls, using the conventional Gibson-Ashby model. The soft/soft polyethylene foam/ethylene—styrene copolymer film system exhibits enhanced toughness in tension. The effect of the foam/film composition on the tensile and compressive moduli can be explained using the composite series and parallel models. [ABSTRACT FROM AUTHOR]