Influence of TPU/EVA Phase Morphology Evolution on Supercritical Carbon Dioxide Extrusion Foaming

Ethylene-vinyl acetate copolymer (EVA) was added at different contents to the thermoplastic polyurethane (TPU) matrix to form a non-compatible blending system, and foaming materials with high pore density were prepared using the supercritical carbon dioxide extrusion method. The influence of the phase morphology and crystal morphology of the TPU/EVA blend on its foaming behavior was studied. The results show that EVA changed the phase morphology and crystal morphology of the blends, leading to the improved melt viscosity and crystallinity of the blend system. At the same time, interfacial nucleation increases the density of cells and decreases the cell thickness and size, which is beneficial for improving the foaming properties of the blends. For the EVA content of 10% (mass fraction), the cell size is small (105.29 μm) and the cell density is the highest (3.74 × 10⁶ cells/cm³). Based on the TPU/EVA phase morphology and crystal morphology, it is found that the sea-island structure of the blend has better foaming properties than the bicontinuous structure.

A Sensitive and Flexible Capacitive Pressure Sensor Based on a Porous Hollow Hemisphere Dielectric Layer

Capacitive pressure sensors based on porous structures have been widely researched and applied to a variety of practical applications. To date, it remains a big challenge to develop a capacitive pressure sensor with a high sensitivity and good linearity over a wide pressure range. In this paper, a sensitive, flexible, porous capacitive pressure sensor was designed and manufactured by means of the "salt template method" and man-made grooves. To this aim, the size of the salt particles used for forming pores/air voids, time taken for thorough dissolution of salt particles, and the depth of the man-made groove by a pin were taken into consideration to achieve a better effect. With pores and the groove, the sensor is more liable be compressed, which will result in a dramatic decrease in distance between the two electrodes and a conspicuous increase of the effective dielectric constant. The optimize-designed sensor represents a sensitivity 6-8 times more than the sensor without the groove in the pressure range of 0-10 kPa, not to mention the sensor without pores or the groove, and it can keep good linearity within the measurement range (0-50 kPa). Besides, the sensor shows a low detection limit of 3.5 Pa and a fast response speed (≈50 ms), which makes it possible to detect a tiny applied pressure immediately. The fabricated sensor can be applied to wearable devices to monitor finger and wrist bending, and it can be used in the object identification of mechanical claws and object cutting of mechanical arms, and so on.

Overview and comparison of modelling methods for foams

Cellular materials, especially foams, are widely used in several applications because of their special mechanical, electrical and thermal properties. Their properties are determined by three factors: bulk material properties, cell topology and shape as well as relative density. The bulk material properties include the mechanical, thermal and electrical properties of the matrix. The cell topology determines if the foam exhibits stretch or bending dominated behaviour. The relative density corresponds to the foaming degree. It is defined by the cell edge length and cell wall thickness. Especially for the linear elastic properties there are many different modelling approaches. In general, these methods can be divided into two groups namely direct modelling, e.g. analytical and finite element models and constitutive modelling, e.g. models which are generated through homogenization methods. This paper presents an overview of the different modelling methods for foams. Furthermore, sensitivity studies are presented which enable the comparison of the models with regard to the estimation of the elastic properties, show the limits of those models and enable the investigation of the influence of the above mentioned factors on the elastic properties. Selected models are validated with experimental data of a low density foam regarding the Young’s modulus.

Poly(ether imide)/Epoxy Foam Composites with a Microcellular Structure and Ultralow Density: Bead Foam Fabrication, Compression Molding, Mechanical Properties, Thermal Stability, and Flame-Retardant Properties

It is challenging to prepare ultralow-density microcellular foams based on high-performance polymers due to their low gas solubility and rigid polymer matrix. In this study, by applying microcellular foaming technology using CO₂/acetone as the blowing agent, ultralow-density poly(ether imide) (PEI) bead foams with an expansion ratio of 30-56 times and cell density of 10⁷-10⁹ cells/cm³ were fabricated, resulting from the enhanced plasticization effect of the mixed fluid. The slow diffusivity of acetone at room temperature ensured the saturated PEI beads to foam after desorption for more than 6 days, which potentially reduces the transportation cost of PEI bead foams significantly. A novel compression molding process was developed to prepare the molded PEI bead foams (MPEIs) using epoxy as a coating agent. The good infiltration character of epoxy on bead foams endowed the MPEIs with excellent mechanical properties, together with an ultralow density of 80-200 kg/m³, long-term dimensional stability at 160 °C, and excellent flame-retardant properties of V0 rating. These features made the MPEIs very promising for many advanced applications.