Solid-state microcellular polycarbonate foams. II. The effect of cell size on tensile properties

Polybutylene adipate terephthalate/polylactic acid interface enhanced compatibilization and its bead-foaming characteristics

Bead foaming technique is regarded as a highly promising method for preparing foams with complex geometries and high expansion ratios. The biodegradability of poly(butylene adipate-co-terephthalate) (PBAT) has garnered significant attention in the field of foam materials. However, due to inherent disadvantages such as low melt strength and low modulus, PBAT faces challenges during bead foaming. In this study, a small amount of polylactic acid (PLA) was incorporated into PBAT. Utilizing the differential melting points of PLA and PBAT, PLA served as physical cross-linking points. The epoxy-based chain extender ADR4370S was used as a chain extender and compatibilizer. By varying its content, the compatibility and foaming performance of the PBAT/PLA blend were regulated. Finally, the foaming process employed supercritical carbon dioxide (scCO2) impregnation followed by heating to address the hydrolysis issue of the PBAT/PLA blend during bead foaming. The results demonstrated that the introduction of ADR could initiate reactions between its epoxy groups and PBAT and PLA, resulting in grafting and chain extension. When the ADR content reached 0.6 wt%, the cell structure evolved from a bimodal to a uniform cell structure, with a minimum average cell size of 12.3 μm and a maximum foaming ratio of 10.3 times.

A Review on Mechanical Models for Cellular Media: Investigation on Material Characterization and Numerical Simulation

Cellular media materials are used for automobiles, aircrafts, energy-efficient buildings, transportation, and other fields due to their light weight, designability, and good impact resistance. To devise a buffer structure reasonably and avoid resource and economic loss, it is necessary to completely comprehend the constitutive relationship of the buffer structure. This paper introduces the progress on research of the mechanical properties characterization, constitutive equations, and numerical simulation of porous structures. Currently, various methods can be used to construct cellular media mechanical models including simplified phenomenological constitutive models, homogenization algorithm models, single cell models, and multi-cell models. This paper reviews current key mechanical models for cellular media, attempting to track their evolution from their inception to their latest development. These models are categorized in terms of their mechanical modeling methods. This paper focuses on the importance of constitutive relationships and microstructure models in studying mechanical properties and optimizing structural design. The key issues concerning this topic and future directions for research are also discussed.

Expanded Polycarbonate (EPC)-A New Generation of High-Temperature Engineering Bead Foams

Bead foams serve in a wide variety of applications, from insulation and packaging to midsoles in shoes. However, the currently used materials are limited to somewhat low temperature or exhibit significant changes in modulus in the temperature range of many applications due to their glass transition. By comparison, polycarbonate (PC) exhibits almost constant mechanics for temperatures up to 130 °C. Therefore, it appears as an advantageous base material for bead foams. The aim of the publication is to provide comprehensive data on the properties of expanded PC (EPC) in comparison to already commercially available expanded polypropylene, EPP, and expanded polyethylene-terephthalate, EPET. A special focus is set on the thermo-mechanical properties as these are the most lacking features in current materials. In this frame, dynamic mechanical analysis, and tensile, bending, compression and impact tests at room temperature (RT), 80 °C, and 110 °C are conducted for the three materials of the same density. Already at RT, EPC exhibits superior mechanics compared to its peers, which becomes more pronounced toward higher temperature. This comes from the low sensitivity of properties to temperature as EPC is used below its glass transition. In summary, EPC proves to be an outstanding foam material over a broad range of temperatures for structural applications.

Foaming of PLA Composites by Supercritical Fluid-Assisted Processes: A Review

Polylactic acid (PLA) is a well-known and commercially available biopolymer that can be produced from different sources. Its different characteristics generated a great deal of interest in various industrial fields. Besides, its use as a polymer matrix for foam production has increased in recent years. With the rise of technologies that seek to reduce the negative environmental impact of processes, chemical foaming agents are being substituted by physical agents, primarily supercritical fluids (SCFs). Currently, the mass production of low-density PLA foams with a uniform cell morphology using SCFs as blowing agents is a challenge. This is mainly due to the low melt strength of PLA and its slow crystallization kinetics. Among the different options to improve the PLA characteristics, compounding it with different types of fillers has great potential. This strategy does not only have foaming advantages, but can also improve the performances of the final composites, regardless of the implemented foaming process, i.e., batch, injection molding, and extrusion. In addition, the operating conditions and the characteristics of the fillers, such as their size, shape factor, and surface chemistry, play an important role in the final foam morphology. This article proposes a critical review on the different SCF-assisted processes and effects of operating conditions and fillers on foaming of PLA composites.

Cell structure and mechanical properties of microcellular PLA foams prepared via autoclave constrained foaming

Microcellular polylactic acid (PLA) foams with various cell size and cell morphologies were prepared using supercritical carbon dioxide (sc-CO2) solid-state foaming to investigate the relationship between the cell structure and mechanical properties. Constrained foaming was used and a wide range of cell structures with a constant porosity of ∼75% by tuning saturation pressure (8–24 MPa) was developed. Experiments varying the saturation pressure while holding other variables’ constant show that the mean cell size and the mean cell wall thickness decreased, while the cell density and the open porosity increased with increase of pressure. Tensile modulus of PLA foams decreased with increasing the saturation pressure, but the specific tensile modulus of PLA foams was still 15–80% higher than that of solid PLA. Tensile strength and elongation at break first increased with increasing saturation pressure up to 16 MPa and then decreased with further increasing saturation pressure (20 MPa and 24 MPa) at which opened-cell structure produced. Compressive modulus, compressive strength, and compressive yield stress also followed the same variation trend. The results indicated that not only cell size plays an important role in properties of PLA foams but also cell morphology can influence these properties significantly.

Correlation Between the Structure and Compressive Property of PMMA Microcellular Foams Fabricated by Supercritical CO2 Foaming Method

In this study, we fabricated poly (methyl methacrylate) (PMMA) microcellular foams featuring tunable cellular structures and porosity, through adjusting the supercritical CO₂ foaming conditions. Experimental testing and finite element model (FEM) simulations were conducted to systematically elucidate the influence of the foaming parameters and structure on compressive properties of the foam. The correlation between the cellular structure and mechanical properties was acquired by separating the effects of the cell size and foam porosity. It was found that cell size reduction contributes to improved mechanical properties, which can be attributed to the dispersion of stress and decreasing stress concentration.

Low Density Non-crosslinked Closed/Open Cell Polypropylene Foams with High Mechanical Properties

Low density polypropylene based foams with different cellular structures have been produced by the improved compression molding route using a high melt strength polypropylene as polymer matrix. In addition, different types of nanoparticles have been introduced in the formulation (multi-wall carbon nanotubes, organomodified nanoclays and natural nanoclays) to modify the structure and properties. The results have showed a clear correlation between the open cell content of the foams and the mechanical properties in compression. In the unfilled polypropylene high specific mechanical properties are only achievable with low values of open cell content. In comparison, for an equal value of the interconnectivity between cells, the samples containing nanoclays present much higher specific properties. This result is attributed to the reinforcement of these nanoparticles in the solid matrix, due to an improved exfoliation during the foaming process and the presence of a bimodal cellular structure. The produced foams have interesting properties with stiffness similar to those of commercial polymer foams used for the core of sandwich panels.

Mechanical and dielectric properties of microcellular polycarbonate foams with unimodal or bimodal cell-size distributions

This article reports on the compressive, dynamic mechanical and dielectric properties of microcellular polycarbonate foams with unimodal or bimodal cell-size distributions fabricated using the environment-friendly supercritical carbon dioxide. The effects of cell morphologies such as relative density, cell-size distribution and porosity on the compressive strength, Young’s modulus, storage modulus, loss modulus, dielectric constant and loss tangent of the microcellular polycarbonate foams are investigated quantitatively. Experimental values of the compressive strength and Young’s modulus fit very well with the theoretical values calculated from the Gibson–Ashby model at low relative densities. The higher relative density leads to higher storage modulus and loss modulus. The bimodal foams significantly improve the compressive and dynamic mechanical properties compared to the unimodal foams with the same relative density. The dielectric properties of microcellular foams depend only on the total porosity, but not on the cell-size distribution or microstructure of the foams. With increasing porosity, the dielectric constant of the microcellular foams gradually decreases, and agrees very well with the curve calculated from the Maxwell-Garnett-spheres model.