Thermoplastic foams have several advantages in comparison with unfoamed polymers such as lightweight, high strength to weight ratio, excellent insulation property, high thermal stability, high impact strength and toughness, as well as high fatigue life. These outstanding properties lead cellular plastics to various industrial applications in packaging, automotive parts, absorbents, and sporting equipment. Nowadays, polypropylene (PP), because of its outstanding characteristics such as low material cost, high service temperature, high melting point, high tensile modulus, low density, and excellent chemical resistance, is a major resin in the foaming industry. However, foaming of conventional PP is limited by its low melt strength leading to poor cell morphology, cell rupture/coalescence and limited density reduction. To improve PP melt strength, several strategies including particle addition as nucleating agent, introduction of long chain branching, blending with high melt strength polymers and crosslinking have been proposed. In this review, these issues are discussed and analyzed in terms of mechanical, thermal, and rheological characterizations.

Because of their ability to show ferroelectret behavior when exposed to an external electric field, cellular polymers have been recently considered for ferroelectret applications. These cellular polymer films can be produced by stretching or foaming, but depending on the application and conditions, different polymers, such as polypropylene (PP), poly(ethylene terephthalate), poly(ethylene naphthalate), poly(tetrafluoro ethylene), cross-linked PP, and some cyclo-olefines, have been considered. Nevertheless, cellular PP was the most investigated material because of its outstanding properties such as high piezoelectric d 33 coefficient, flexibility, good fatigue resistance, good charge trapping properties, and low cost. In this review, recent advances related to the materials used for ferroelectret applications and their processing are discussed. The effect of different parameters such as pressure, electrical breakdown strength of the gas phase, presence of fillers, and service temperature on the d 33 coefficient is presented and discussed. C

In this work, the effect of post-processing parameters (time, temperature, and pressure) on the morphology as well as mechanical and piezoelectric properties of foamed polypropylene (PP) films were studied. Two different post-processing methods, based on the saturation of a foamed film with supercritical nitrogen (N 2 ), were used to obtain an optimized eye-like cellular structure with a high cell aspect ratio (AR). The results showed that, when the PP-foamed films were exposed to a gradual temperature and pressure increase, an appropriate cellular structure with high AR value (about 6.6) was obtained. This structure led to a high quasistatic piezoelectric d 33 coefficient of 800 pC/N (45% higher than for untreated ones) indicating the importance of the post-processing treatment on the piezoelectric behavior of these films. On the other hand, when the treatment was performed in steps, cell morphology changed from an eye-like to a less elongated shape, resulting in lower d 33 values. The tensile characterization showed that higher cell aspect ratio led to lower Young's modulus, which is suitable to achieve higher piezoelectric properties. Finally, dynamic mechanical analysis (DMA) was used as a simple method to correlate mechanical and piezoelectric properties of cellular PP. This was done via the ratio of the storage moduli in the longitudinal and transverse directions, which is directly related to film anisotropy (AR value) and thus to the piezoelectric behavior.

A continuous method without any solvent is proposed to produce porous hollow fibers for membrane (HFM) applications. In this case, linear low-density polyethylene was combined with azodicarbonamide to produce samples via extrusion. In particular, the processing (chemical blowing agent content and temperature profile) and post-processing (stretching velocity) conditions were optimized to obtain a cellular structure having a high cell density and uniform cell size distribution. From the samples obtained, a complete set of characterization was performed (morphological, mechanical, physical, and gas transport). The results show that HFM having a higher cell density can increase gas permeability, especially for hydrogen. Overall, it is shown that low-cost polyolefins having a suitable cellular structure can be used for gas separation membranes.

Thin polypropylene (PP) foam films were produced by continuous extrusion using supercritical nitrogen (N2) and then charged via corona discharge. The samples were characterized by dynamic mechanical analysis as a simple method to predict the piezoelectric properties of the cellular PP obtained. The results were then related to morphological analysis based on scanning electron microscopy and mechanical properties in tension. The results showed that the presence of a nucleating agent (CaCO3) substantially improved the morphology (in terms of cell size and cell density) of the produced foam. Also, an optimization of the extrusion (screw design, temperature profile, blowing agent, and nucleating agent content) and post‐extrusion (calendering temperature and speed) conditions led to the development of a stretched eye‐like cellular structure with uniform cell size distribution. This morphology produced higher storage and loss moduli in the machine (longitudinal) direction than for the transverse direction, as well as higher piezoelectric properties. The morphological and mechanical results showed that higher cell aspect ratio led to lower Young's modulus, which is suitable to achieve higher piezoelectric properties. Finally, the best quasi‐static piezoelectric d33 coefficient was 550 pC/N for a cellular PP ferroelectret having a uniform eye‐like cellular structure using N2 as the ionizing gas inside the cells, while the highest value was only 250 pC/N when air was used. Hence, the value of d33 can be improved by more than 100% just by replacing air with N2 as the ionizing gas. Copyright © 2016 John Wiley & Sons, Ltd.

In this work, continuous extrusion-calendering was used to produce polypropylene (PP) foam films for piezoelectric applications. The setup is based on physical foaming using supercritical nitrogen (SC-N2) and calcium carbonate (CaCO3) as nucleating agent. In particular, the extrusion parameters (screw design, temperature profile, blowing agent and nucleating agent content) and post-extrusion conditions (calendaring temperature and speed) were optimized to achieve a specific stretched eye-like cellular structure with uniform cell size distribution. The morphology in both machine and transverse directions, as well as tensile properties were characterized. The results show that a cellular structure with a higher cell aspect ratio has a lower Young's modulus, which is appropriate for piezoelectric cellular films. Generally, the developed foam morphology presents high potential for the production of ferroelectret PP films used in different piezoelectric applications.

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The production of foamed hollow fiber membranes (HFMs) is presented based on polymer blends using various concentrations of linear low-density polyethylene (LLDPE) and low-density polyethylene (LPDE) combined with azodicarbonamide (chemical blowing agent) to prepare samples via twin-screw extrusion. In particular, the blowing agent concentration as well as the stretching speed were found to be the most important parameters to achieve a good cellular structure for membrane application. From the samples obtained, a complete set of morphological, thermal, and gas transport characterization was performed. The results show that LLDPE/LDPE blends compared to neat LLDPE lead to higher cell density at high stretching speed, which is appropriate for membranes having higher gas permeability and selectivity due to lower cell wall thickness. The results also show that the developed cellular structure has high potential for the continuous production of HFMs for different gas separation, especially for hydrogen recovery.