Effects of graphene nanoplatelets on the morphology of polycarbonate–graphene composite foams prepared by supercritical carbon dioxide two-step foaming

Foaming of PCL-Based Composites Using scCO2—Biocompatibility and Evaluation for Biomedical Applications

The process of foaming poly(caprolactone)-based composite materials using supercritical carbon dioxide was analyzed, especially in terms of the biocompatibility of the resultant materials. The influence of foaming process conditions and composite material properties on the functional properties of polymer solid foams, intended for artificial scaffolds for bone cell culture, was investigated. The relationship between wettability (contact angle) and water absorption rate as a result of the application of variable conditions for the production of porous structures was presented. For the evaluation of potential cytotoxicity, the MTT and PrestoBlue tests were carried out, and animal cells (mouse fibroblasts) were cultured on the materials for nine days. There was no toxic effect of composite materials made of poly(caprolactone) containing porogen particles: hydroxyapatite, crystalline nanocellulose, and graphene oxide on cells. The desired effect of the porogens used in the foaming process on the affinity of cells to the resultant material was demonstrated. The tested materials have been shown to be biocompatible and suitable for applications in biomedical engineering.

Rational assembly of GO-based heterocyclic sulfur- and nitrogen-containing aerogels and their adsorption properties toward rare earth elementals

An aromatic heterocyclic compound, 2-aminobenzothiazole (ABT), was used to decorate graphene oxide (GO) by a facile hydrothermal self-assembly procedure. The developed three-dimensional (3D) GO-ABT composite aerogels could be utilized as high-powered and sustainable adsorbents for the enrichment and recovery of low concentration rare earth elements (REEs) from aqueous solutions. The composition and microstructure of GO-ABT composites were explored various characterization methods. The enrichment properties of GO-ABT composites for REEs were investigated in detail, revealing the existence of S-, N- and -NH₂ in ABT, as well as the carboxyl and hydroxyl groups of GO which might act as the major REE binding sites. The adsorption of GO-ABT composites for low concentration REEs could reach equilibrium in 30 min. Our investigations confirmed that the optimal pH value of GO-ABT composites for REEs was pH 4.0-5.0. For the adsorbent regeneration study, 50.0 mg of GO-ABT_{15:1/120 °C/6 h} composite was used toward 20.0 mL of Er³⁺ solutions. After ten regeneration cycles, the adsorption rates of GO-ABT composites for Er³⁺ remained around 100%, and the desorption rates maintained over 90%. The long-term storage of the adsorbent did not affect its adsorption ability, while desorption rates increased, indicating it possessed relatively higher stability.

Application of Fluids in Supercritical Conditions in the Polymer Industry

This article reviews the use of fluids under supercritical conditions in processes related to the modern and innovative polymer industry. The most important processes using supercritical fluids are: extraction, particle formation, micronization, encapsulation, impregnation, polymerization and foaming. This review article briefly describes and characterizes the individual processes, with a focus on extraction, micronization, particle formation and encapsulation. The methods mentioned focus on modifications in the scope of conducting processes in a more ecological manner and showing higher quality efficiency. Nowadays, due to the growing trend of ecological solutions in the chemical industry, we see more and more advanced technological solutions. Less toxic fluids under supercritical conditions can be used as an ecological alternative to organic solvents widely used in the polymer industry. The use of supercritical conditions to conduct these processes creates new opportunities for obtaining materials and products with specialized applications, in particular in the medical, pharmacological, cosmetic and food industries, based on substances of natural sources. The considerations contained in this article are intended to increase the awareness of the need to change the existing techniques. In particular, the importance of using supercritical fluids in more industrial methods and for the development of already known processes, as well as creating new solutions with their use, should be emphasized.

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.

Comparison of Experimental and Modeled EMI Shielding Properties of Periodic Porous xGNP/PLA Composites

Microwave absorbing materials, particularly ones that can achieve high electromagnetic interference (EMI) absorption while minimizing weight and thickness are in high demand for many applications. Herein we present an approach that relies on the introduction of periodically placed air-filled pores into polymer composites in order to reduce material requirements and maximize microwave absorption. In this study, graphene nano platelet (xGNP)/poly-lactic acid (PLA) composites with different aspect ratio fillers were characterized and their complex electromagnetic properties were extracted. Using these materials, we fabricated non-perfect electrical conductor (PEC) backed, porous composites and explored the effect of filler aspect ratio and pore geometry on EMI shielding properties. Furthermore, we developed and experimentally verified a computational model that allows for rigorous, high-throughput optimization of absorbers with periodic porous geometries. Finally, we extend the modeling approach to explore the effect of pore addition on PEC-backed composites. Our composite structures demonstrated decreased fractions of reflected power and increased fractions of absorbed power over the majority of the X Band due to the addition of periodically arranged cylindrical pores. Furthermore, we showed that for xGNP/PLA composite material, reflection loss can be increased by as much as 13 dB through the addition of spherical pores. The ability to adjust shielding properties through the fabrication of polymer composites with periodically arranged pores opens new strategies for the modeling and development of new microwave absorption materials.

Compressive Behavior of Aluminum Microfibers Reinforced Semi-Rigid Polyurethane Foams

Unreinforced and reinforced semi-rigid polyurethane (PU) foams were prepared and their compressive behavior was investigated. Aluminum microfibers (AMs) were added to the formulations to investigate their effect on mechanical properties and crush performances of closed-cell semi-rigid PU foams. Physical and mechanical properties of foams, including foam density, quasi-elastic gradient, compressive strength, densification strain, and energy absorption capability, were determined. The quasi-static compression tests were carried out at room temperature on cubic samples with a loading speed of 10 mm/min. Experimental results showed that the elastic properties and compressive strengths of reinforced semi-rigid PU foams were increased by addition of AMs into the foams. This increase in properties (61.81%-compressive strength and 71.29%-energy absorption) was obtained by adding up to 1.5% (of the foam liquid mass) aluminum microfibers. Above this upper limit of 1.5% AMs (e.g., 2% AMs), the compressive behavior changes and the energy absorption increases only by 12.68%; while the strength properties decreases by about 14.58% compared to unreinforced semi-rigid PU foam. The energy absorption performances of AMs reinforced semi-rigid PU foams were also found to be dependent on the percentage of microfiber in the same manner as the elastic and strength properties.

Preparation of microcellular poly(lactic acid) composites foams with improved flame retardancy

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.