Recent advancements in manufacturing technologies of microcellular polymers: a review

Forefront Research of Foaming Strategies on Biodegradable Polymers and Their Composites by Thermal or Melt-Based Processing Technologies: Advances and Perspectives

The last few decades have witnessed significant advances in the development of polymeric-based foam materials. These materials find several practical applications in our daily lives due to their characteristic properties such as low density, thermal insulation, and porosity, which are important in packaging, in building construction, and in biomedical applications, respectively. The first foams with practical applications used polymeric materials of petrochemical origin. However, due to growing environmental concerns, considerable efforts have been made to replace some of these materials with biodegradable polymers. Foam processing has evolved greatly in recent years due to improvements in existing techniques, such as the use of supercritical fluids in extrusion foaming and foam injection moulding, as well as the advent or adaptation of existing techniques to produce foams, as in the case of the combination between additive manufacturing and foam technology. The use of supercritical CO2 is especially advantageous in the production of porous structures for biomedical applications, as CO2 is chemically inert and non-toxic; in addition, it allows for an easy tailoring of the pore structure through processing conditions. Biodegradable polymeric materials, despite their enormous advantages over petroleum-based materials, present some difficulties regarding their potential use in foaming, such as poor melt strength, slow crystallization rate, poor processability, low service temperature, low toughness, and high brittleness, which limits their field of application. Several strategies were developed to improve the melt strength, including the change in monomer composition and the use of chemical modifiers and chain extenders to extend the chain length or create a branched molecular structure, to increase the molecular weight and the viscosity of the polymer. The use of additives or fillers is also commonly used, as fillers can improve crystallization kinetics by acting as crystal-nucleating agents. Alternatively, biodegradable polymers can be blended with other biodegradable polymers to combine certain properties and to counteract certain limitations. This work therefore aims to provide the latest advances regarding the foaming of biodegradable polymers. It covers the main foaming techniques and their advances and reviews the uses of biodegradable polymers in foaming, focusing on the chemical changes of polymers that improve their foaming ability. Finally, the challenges as well as the main opportunities presented reinforce the market potential of the biodegradable polymer foam materials.

In Situ Changes in Mechanical Properties Based on Gas Saturation Inside Pressure Vessels

In previous studies, difficulties were encountered in measuring changes within high-pressure vessels owing to limitations such as sensor connectors and sensor failures under high-pressure conditions. In addition, polymer-gas mixtures experience instantaneous gas desorption upon exiting high-pressure vessels owing to pressure differentials, leading to measurement errors. In this study, a device using magnetic sensors was developed to measure the real-time changes in gas-saturated polymers inside pressure vessels. Experiments on polymethyl methacrylate gas adsorption were conducted with parameters including pressure at 5 MPa and temperatures ranging from -20 to 40 °C for 60 and 180 min. It was observed that at -20 °C, the maximum magnetic field force density and deflection were 391.53 μT and 5.83 mm, respectively, whereas at 40 °C, deflection did not occur, with a value of 321.79 μT. Based on gas saturation experiments, a new model for deflection in high-pressure atmospheres is proposed. Additionally, an ANSYS analysis was conducted to predict the changes in Young's modulus based on gas saturation. In previous studies, mechanical properties were measured outside the pressure vessel, resulting in an error due to a pressure difference, while the proposed method is characterized by the ability to directly measure polymer behavior according to gas saturation in high-pressure vessels using a magnetic sensor in real time. Therefore, it is possible to predict polymer behavior, making it easy to control variables in high-pressure polymer processes.

Recent Trends in Polymeric Foams and Porous Structures for Electromagnetic Interference Shielding Applications

Polymer-based (nano)composite foams containing conductive (nano)fillers limit electromagnetic interference (EMI) pollution, and have been shown to act as good shielding materials in electronic devices. However, due to their high (micro)structural complexity, there is still a great deal to learn about the shielding mechanisms in these materials; understanding this is necessary to study the relationship between the properties of the microstructure and the porous structure, especially their EMI shielding efficiency (EMI SE). Targeting and controlling the electrical conductivity through a controlled distribution of conductive nanofillers are two of the main objectives when combining foaming with the addition of nanofillers; to achieve this, both single or combined nanofillers (nanohybrids) are used (as there is a direct relationship between electrical conductivity and EMI SE), as are the main shielding mechanisms working on the foams (which are expected to be absorption-dominated). The present review considers the most significant developments over the last three years concerning polymer-based foams containing conductive nanofillers, especially carbon-based nanofillers, as well as other porous structures created using new technologies such as 3D printing for EMI shielding applications. It starts by detailing the microcellular foaming strategy, which develops polymer foams with enhanced EMI shielding, and it particularly focuses on technologies using supercritical CO2 (sCO2). It also notes the use of polymer foams as templates to prepare carbon foams with high EMI shielding performances for high temperature applications, as well as a recent strategy which combines different functional (nano)fillers to create nanohybrids. This review also explains the control and selective distribution of the nanofillers, which favor an effective conductive network formation, which thus promotes the enhancement of the EMI SE. The recent use of computational approaches to tailor the EMI shielding properties are given, as are new possibilities for creating components with varied porous structures using the abovementioned materials and 3D printing. Finally, future perspectives are discussed.

In Situ Nanofibrillar Polypropylene-Based Composite Microcellular Foams with Enhanced Mechanical and Flame-Retardant Performances

With the increasing demand for plastic components, the development of lightweight, high strength and functionalized polypropylene (PP) from a cost-effective and environmentally friendly process is critical for resource conservation. In situ fibrillation (INF) and supercritical CO2 (scCO2) foaming technology were combined in this work to fabricate PP foams. Polyethylene terephthalate (PET) and poly(diaryloxyphosphazene)(PDPP) particles were applied to fabricate in situ fibrillated PP/PET/PDPP composite foams with enhanced mechanical properties and favorable flame-retardant performance. The existence of PET nanofibrils with a diameter of 270 nm were uniformly dispersed in PP matrix and served multiple roles by tuning melt viscoelasticity for improving microcellular foaming behavior, enhancing crystallization of PP matrix and contributing to improving the uniformity of PDPP’s dispersion in INF composite. Compared to pure PP foam, PP/PET(F)/PDPP foam exhibited refined cellular structures, thus the cell size of PP/PET(F)/PDPP foam was decreased from 69 to 23 μm, and the cell density increased from 5.4 × 106 to 1.8 × 108 cells/cm3. Furthermore, PP/PET(F)/PDPP foam showed remarkable mechanical properties, including a 975% increase in compressive stress, which was attributed to the physical entangled PET nanofibrils and refined cellular structure. Moreover, the presence of PET nanofibrils also improved the intrinsic flame-retardant nature of PDPP. The synergistical effect of the PET nanofibrillar network and low loading of PDPP additives inhibited the combustion process. These gathered advantages of PP/PET(F)/PDPP foam make it promising for lightweight, strong, and fire-retardant polymeric foams.

Solid-State Surface Patterning on Polymer Using the Microcellular Foaming Process

This study proposes a novel process that integrates the molding and patterning of solid-state polymers with the force generated from the volume expansion of the microcellular-foaming process (MCP) and the softening of solid-state polymers due to gas adsorption. The batch-foaming process, which is one of the MCPs, is a useful process that can cause thermal, acoustic, and electrical characteristic changes in polymer materials. However, its development is limited due to low productivity. A pattern was imprinted on the surface using a polymer gas mixture with a 3D-printed polymer mold. The process was controlled with changing weight gain by controlling saturation time. A scanning electron microscope (SEM) and confocal laser scanning microscopy were used to obtain the results. The maximum depth could be formed in the same manner as the mold geometry (sample depth: 208.7 μm; mold depth: 200 μm). Furthermore, the same pattern could be imprinted as a layer thickness of 3D printing (sample pattern gap and mold layer gap: 0.4 mm), and surface roughness was increased according to increase in the foaming ratio. This process can be used as a novel method to expand the limited applications of the batch-foaming process considering that MCPs can impart various high-value-added characteristics to polymers.

Polymeric Materials, Advances and Applications in Tissue Engineering: A Review

Tissue Engineering (TE) is an interdisciplinary field that encompasses materials science in combination with biological and engineering sciences. In recent years, an increase in the demand for therapeutic strategies for improving quality of life has necessitated innovative approaches to designing intelligent biomaterials aimed at the regeneration of tissues and organs. Polymeric porous scaffolds play a critical role in TE strategies for providing a favorable environment for tissue restoration and establishing the interaction of the biomaterial with cells and inducing substances. This article reviewed the various polymeric scaffold materials and their production techniques, as well as the basic elements and principles of TE. Several interesting strategies in eight main TE application areas of epithelial, bone, uterine, vascular, nerve, cartilaginous, cardiac, and urinary tissue were included with the aim of learning about current approaches in TE. Different polymer-based medical devices approved for use in clinical trials and a wide variety of polymeric biomaterials are currently available as commercial products. However, there still are obstacles that limit the clinical translation of TE implants for use wide in humans, and much research work is still needed in the field of regenerative medicine.

Lightweight and High Impact Toughness PP/PET/POE Composite Foams Fabricated by In Situ Nanofibrillation and Microcellular Injection Molding

Polypropylene (PP) has become the most promising and candidate material for fabricating lightweight products. Microcellular injection molding (MIM) is a cost-effective technology for manufacturing porous plastic products. However, it is still challenging to fabricate high-performance PP microcellular components. Herein, we reported an efficient strategy to produce lightweight and high impact toughness foamed PP/polyethylene terephthalate (PET)/polyolefin-based elastomer (POE) components by combining in situ fibrillation (INF) and MIM technologies. First, the INF composite was prepared by integrating twin-screw compounding with melt spinning. SEM analysis showed PET nanofibrils with a diameter of 258 nm were achieved and distributed uniformly in the PP due to the POE's inducing elaboration effect. Rheological and DSC analysis demonstrated PET nanofibrils pronouncedly improved PP's viscoelasticity and crystal nucleation rate, respectively. Compared with PP foam, INF composite foam showed more stretched cells in the skin layer and refined spherical cells in the core layer. Due to the synergistic toughening effect of PET nanofibrils and POE elastic particles, the impact strength of INF composite foams was 295.3% higher than that of PP foam and 191.2% higher than that of melt-blended PP/PET foam. The results gathered in this study reveal potential applications for PP based INF composite foams in the manufacturing of lightweight automotive products with enhanced impact properties.

Large-Scale Preparation of Uniform Millet Bread-like Durable Benzoxazine-Phthalonitrile Foam with Outstanding Mechanical and Thermal Properties

It is essentially important to develop durable polymer foams for services in high-temperature conditions. The current study reported the preparations and properties of a high-performance benzoxazine-phthalonitrile (BZPN) foam by utilizing azodicarbonamide and tween-80 as the blowing agent and stabilizer, respectively. Rheological and curing studies indicated that the appropriate foaming temperature for BZPN foam is below 180 °C, and its foaming viscosity window is below 20 Pa·s. Guided by these results, uniform millet bread-like BZPN foams with decimeter leveling size were successfully realized, suggesting the high prospect of large-scale production. The structural, mechanical, and thermal properties of BZPN foams were then investigated in detail. BZPN foam involves a hierarchical fracture mechanism during the compressive test, and it shows a high compression strength of over 6 MPa. During a burning test over 380 °C, no visible smoke, softening, or droplet phenomena appeared and the macroscopic structure of BZPN foam was well maintained. Mechanically robust, flame-retardant, and uniform large-size BZPN foam are promising light durable materials with high service temperatures, i.e., as filling materials even in a very narrow pipette.

Modeling and Experiment for the Diffusion Coefficient of Subcritical Carbon Dioxide in Poly(methyl methacrylate) to Predict Gas Sorption and Desorption

Several researchers have investigated the phenomenon of polymer–gas mixtures, and a few have proposed diffusion coefficient values instead of a diffusion coefficient model. There is a paucity of studies focused on the continuous change in the diffusion coefficient corresponding to the overall pressure and temperature range of the mixture. In this study, the gas sorption and desorption experiments of poly(methyl methacrylate) (PMMA) were conducted via solid-state batch foaming, and the weight change was measured using the gravimetric method with a magnetic balance. The control parameters were temperature, which ranged from 290 to 370 K, and pressure, which ranged from 2 to 5 MPa in the subcritical regime. Based on the experimental data, the diffusion coefficient of the PMMA was calculated using Fick’s law. After calculating the diffusion coefficient in the range of the experiment, the diffusion coefficient model was employed using the least-squares method. Subsequently, the model was validated by comparing the obtained results with those in the literature, and the overall trend was found to be consistent. Therefore, it was confirmed that there were slight differences between the diffusion coefficient obtained using only Fick’s equation and the value using by a different method.

Morphological evaluation of microcellular foamed composites developed through gas batch foaming integrating Fused Deposition Modeling (FDM) 3D printing technique

In this article, the development of microcellular structure foams has developed by integrating the two successful and existing technologies, namely CO2 gas batch foaming and Fused Deposition Modeling (FDM) 3D printing technique. It is a novel approach to manufacture complex design porous products for customized applications. The eventual cell morphologies of the extruded 3D printing filament depends on the process parameters pertaining to both microcellular foaming and 3D printing processes. Further, morphological study has been conducted to evaluate the cell morphologies of the 3D printing filament developed through customized FDM setup. During this process, the significance of various process parameters including saturation pressure, saturation time, desorption time, feed rate and extrusion temperature were thoroughly studied. To pursue this study base material used was acrylonitrile butadiene styrene (ABS). The 3D printed filaments consisted of cells with an average cell size in the range of 2.3–276 µm and the average cell density in the range of 4.7 × 104 to 4.3 × 109 cells/cm3. Finally, it has found that by controlling the process parameters different cell morphologies can be developed as per the end application.

Solid-state foaming of acrylonitrile butadiene styrene through microcellular 3D printing process

The lightweight products with superior specific strength are in great demand in numerous applications such as automotive, aerospace, biomedical, sports, etc. This work focussed on the manufacturing of lightweight products using the cellular three dimensional (3D) printing process. In this work, the continuous microcellular morphology has been developed in a single foamed filament using 3 D printing of carbon-di-oxide (CO2) saturated acrylonitrile butadiene styrene (ABS) filaments. The microcellular structures with average cell size in the range of 6–1040 µm were developed. The influence of printing parameters; nozzle temperature, feed rate, and flow rate on the foam characteristics and cell morphology at different levels were investigated. The different kinds of observed foamed extrudate irregularities were discussed.

Fabrication of Biomedical Scaffolds Using Biodegradable Polymers

Degradable polymers are used widely in tissue engineering and regenerative medicine. Maturing capabilities in additive manufacturing coupled with advances in orthogonal chemical functionalization methodologies have enabled a rapid evolution of defect-specific form factors and strategies for designing and creating bioactive scaffolds. However, these defect-specific scaffolds, especially when utilizing degradable polymers as the base material, present processing challenges that are distinct and unique from other classes of materials. The goal of this review is to provide a guide for the fabrication of biodegradable polymer-based scaffolds that includes the complete pathway starting from selecting materials, choosing the correct fabrication method, and considering the requirements for tissue specific applications of the scaffold.

Kinked Bisamides as Efficient Supramolecular Foam Cell Nucleating Agents for Low-Density Polystyrene Foams with Homogeneous Microcellular Morphology

Polystyrene foams have become more and more important owing to their lightweight potential and their insulation properties. Progress in this field is expected to be realized by foams featuring a microcellular morphology. However, large-scale processing of low-density foams with a closed-cell structure and volume expansion ratio of larger than 10, exhibiting a homogenous morphology with a mean cell size of approximately 10 µm, remains challenging. Here, we report on a series of 4,4′-diphenylmethane substituted bisamides, which we refer to as kinked bisamides, acting as efficient supramolecular foam cell nucleating agents for polystyrene. Self-assembly experiments from solution showed that these bisamides form supramolecular fibrillary or ribbon-like nanoobjects. These kinked bisamides can be dissolved at elevated temperatures in a large concentration range, forming dispersed nano-objects upon cooling. Batch foaming experiments using 1.0 wt.% of a selected kinked bisamide revealed that the mean cell size can be as low as 3.5 µm. To demonstrate the applicability of kinked bisamides in a high-throughput continuous foam process, we performed foam extrusion. Using 0.5 wt.% of a kinked bisamide yielded polymer foams with a foam density of 71 kg/m³ and a homogeneous microcellular morphology with cell sizes of ≈10 µm, which is two orders of magnitude lower compared to the neat polystyrene reference foam with a comparable foam density.

Recent Progress in Processing Functionally Graded Polymer Foams

Polymer foams are an important class of engineering material that are finding diverse applications, including as structural parts in automotive industry, insulation in construction, core materials for sandwich composites, and cushioning in mattresses. The vast majority of these manufactured foams are homogeneous with respect to porosity and structural properties. In contrast, while cellular materials are also ubiquitous in nature, nature mostly fabricates heterogeneous foams, e.g., cellulosic plant stems like bamboo, or a human femur bone. Foams with such engineered porosity distribution (graded density structure) have useful property gradients and are referred to as functionally graded foams. Functionally graded polymer foams are one of the key emerging innovations in polymer foam technology. They allow enhancement in properties such as energy absorption, more efficient use of material, and better design for specific applications, such as helmets and tissue restorative scaffolds. Here, following an overview of key processing parameters for polymer foams, we explore recent developments in processing functionally graded polymer foams and their emerging structures and properties. Processes can be as simple as utilizing different surface materials from which the foam forms, to as complex as using microfluidics. We also highlight principal challenges that need addressing in future research, the key one being development of viable generic processes that allow (complete) control and tailoring of porosity distribution on an application-by-application basis.