It is a well known that nucleation by gases in the supercritical state lead to high-density nanocellular foams, but the aim of this article was to find a route to microcellular low-density closed-cell foam with expected low heat conductivity and high mechanical properties. Polystyrene was impregnated with carbon dioxide or propane at room temperature and 70 bar pressure. Pentane served as a reference substance. The samples were investigated by electron microscopy. The specimens comprising carbon dioxide and propane showed bubbles surrounded by a ring of lamellas. The samples were foamed in hot bathes of silicone, of saturated NaNO3 water solution, and in water vapor under stretching conditions at 100℃ or 110℃. The foaming was registered continuously on a balance by buoyancy experimentally and calculated in diffusion as well as viscosity-controlled foaming models. A microcellular foam with 35 kg/m3 density, 1.5 µm or 1500 nm cell diameter, 5.5 MPa tear strength, and 0.027 W/m.K heat conductivity was obtained, which looked like porcelain. The continuous manufacture of low-density carbon dioxide or propane foam by extrusion did not lead to microcellular foam, because the necessary high-pressure, low-temperature nucleation conditions could not be performed.

A series of linear controlled rheology polypropylenes were produced through reaction with dicumyl peroxide in the melt state, to investigate the effect of molecular weight and viscosity on foaming. Foaming experiments by compression molding, using a chemical blowing agent (azodicarbonamide), and by batch foaming using nitrogen-blowing agent, revealed that lower viscosity promoted higher expansion ratios and larger cells, due to reduced resistance to cell growth. Linear polypropylene was further modified using trimethylolpropane trimethacrylate and triallyl trimesate coagents. Coagent modification resulted in pronounced changes in the shear thinning and elasticity of the modified polypropylenes. However, evidence of long-chain branching was only present in polypropylene modified by triallyl trimesate. Foams based on coagent-modified polypropylenes had higher expansion ratios than their degraded counterparts. This was ascribed in part to the lower viscosities, and to a nucleating effect, arising from the presence of a finely dispersed phase of coagent-rich nanoparticles. Strain hardening in polypropylenes modified by triallyl trimesate further resulted in a finer cell structure, due to suppressed coalescence.

This study investigated the impact of foaming on the broadband dielectric properties of multi-walled carbon nanotube/polystyrene (MWCNT/PS) nanocomposites. Different carbon nanotube concentrations were prepared by blending of a 20 wt.% MWCNT/PS masterbatch and pure PS using a twin-screw extruder. A chemical blowing agent was used to foam the nanocomposites in a micro injection molding machine. Compression molding was applied to fabricate unfoamed nanocomposites for comparison purposes. Comparing the dielectric properties of unfoamed and foamed nanocomposites showed that foaming increased the percolation threshold, reduced DC and AC conductivities, widened the insulator–conductor transition window, and reduced the dissipation factor of the MWCNT/PS composites. These were attributed to deteriorated conductive network and inferior dispersion and distribution of MWCNTs coming from the presence of foam cells in the nanocomposites. The obtained results propose foaming as a promising technique to improve the dielectric properties of MWCNT/polymer composites.

The preparation of lightweight materials with electromagnetic interference-shielding effectiveness higher than 30 dB is critical for most industrial and consumer applications. Compounding polymer resin with conductive filler can generate excellent electromagnetic interference-shielding effectiveness but usually leads to a high-sample density, while the foaming of polymer composite suffers from the significant-reduced electromagnetic interference-shielding effectiveness. In this study, polyetherimide composite foams with loading of 10–80 phr (parts per hundred of resins) nickel particles were fabricated to meet the gap. The polyetherimide/nickel composite foams possessed uniform cell structure and low-sample density such as 0.86 g/cm3 at 70 phr nickel. The coupling effects of gravity settlement and cell-structure solidification led to the formation of gradient distribution of nickel particles across the foams. The formed novel structure facilitated the enhancement of multi-reflection and multi-scattering among nickel particles and cells. As a consequence, polyetherimide/nickel foam with 70 phr nickel (PEIN70) possessed a high-electromagnetic interference shielding effectiveness of 86.7–106.5 dB over a frequency range of 50–3000 MHz. When the sample density was considered, the specific electromagnetic interference shielding effectiveness of PEIN70 foam was as high as 121.3 dB/(g/cm3) at 1 GHz, which was higher than the reported electromagnetic interference-shielding materials. The excellent electromagnetic interference-shielding properties, lightweight, well-defined resin properties ensure polyetherimide/nickel composite foams useful in many advanced applications.

Poly(lactic acid)/polycarbonate blends were prepared by melt mixing. The transesterification reaction between poly(lactic acid) and polycarbonate was promoted by using tetrabutyl titanate as a catalyst. For poly(lactic acid)/polycarbonate (weight ratio of 75/25) blend with catalyst content up to 0.5 wt%, polycarbonate particles finely dispersed in the poly(lactic acid) matrix and the adhesion between the phases were improved, due to the enhanced compatibility by transesterification reaction. The blends were foamed by using batch-foaming process with CO2 as the blowing agent. Cell density of poly(lactic acid)/polycarbonate blend increased at low-catalyst content, while decreased at high-catalyst content, which was due to the changes of interfacial properties of blend phases through transesterification reaction and crystallinity of polycarbonate component. Cell types of poly(lactic acid)/polycarbonate blends were transferred from submicro-sized and even nanoscale cells to microscale cells by the deceased crystallinity of poly(lactic acid) component, which was caused by the increased degree of transesterification reaction and temperature. The declined viscosity of poly(lactic acid)/polycarbonate blend because of degradation during blend processing led to large cell size and low-cell density. However, the improvement of elasticity and viscosity of poly(lactic acid)/polycarbonate blend by transesterification reaction could decrease the cell size.

The microstructure of cellular materials, in particular microcellular polymers, is commonly characterized by the mean cell size and cell population density. Although the physical properties of foamed polymers are highly dependent on cell distribution, very insignificant work has been carried out to quantify the distribution of cells. This experimental study correlates the influential processing parameters of microcellular injection molding with the cell distribution, mean cell size and population density. For the first time, “Cell Distribution Index (CDI)”, a quantitative parameter for characterization of foams has been defined in this paper. Foamed samples of polystyrene were prepared via microcellular injection molding process. The design of experiment includes five influential processing parameters (factors) at five different levels. Molded samples were fractured cryogenically and micrographs were obtained using a table top scanning electron microscope. With the help of image processing package, mean cell size, cell population density, frequency and newly defined term CDI were calculated for each micrograph. Finally, the effect of processing parameters on cell distribution was analyzed by plotting the CDI against the processing parameters.

The expanded polypropylene beads with microcellular structure were prepared by an autoclave-based batch foaming process using non-supercritical CO2 as the foaming agent. Herein, smaller polypropylene micropellets (∼0.6 mm) were obtained from three kinds of polypropylene resin by underwater micro-pelletizer system. The results from differential scanning calorimetry indicated that the double melting peaks of beads moved to higher temperature with increasing saturation temperature and pressure. The kind of comonomer did not exert noticeable influence on the double melting behaviors. The X-ray diffraction of expanded polypropylene bead revealed that characteristic peaks of α-type crystal did not change compared with that of original polypropylene micropellets. The relationship between cell morphology and saturation temperature/pressure of expanded polypropylene beads are discussed preliminarily by scanning electron microscope.

To gain insight into the foamability of polystyrene-based polymer blends containing different types of polyethylene close to the experimental boundary conditions of temperature, batch pressure quench foaming experiments were conducted using CO2 as a physical foaming agent. The blends were prepared by melt-mixing in a twin screw extruder and then physically foamed by depressurizing CO2 from 10 MPa in an autoclave at various foaming temperatures in the range of 90–140℃. At low temperatures, where viscosity of polyethylene (PE) was lower than that of PS, microcellular foams with bubble sizes of approx. 100 µm in diameter were obtained, and the dispersed PE enhanced bubble nucleation. It was also found that the expandability of bubbles was correlated with complex viscosity PE. At the highest foaming temperature, where the viscosity of PE became lower than that of the matrix, the dispersed PE worked as a cell opening agent.

Stochastic cellular models of rigid foam based on Voronoi spatial partitioning are generated and investigated for potential use in numerical analysis using finite element methods. Such partitions are deterministic once a distribution of cell nuclei has been defined. A drawback is that the models tend to exhibit a significant share of short edges and small faces. Such small geometrical features are not likely to occur in real foams since they are unfavorable from a surface energy point of view and they also generate problems in numerical analysis due to associated meshing challenges. Through minimization of the surface area, using the computer software Surface Evolver, the Voronoi models are brought to better resemblance with ideal dry foam and the occurrence of small geometrical features is strongly reduced. It is generally seen that different seed point distribution algorithms result in different model topologies. The presented methodology is systematic, parameterized and the results are very promising. Good grounds are provided for modeling of real rigid foam materials, that do not necessarily fully resemble ideal dry foam.

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.