scCO2 Foaming Research Articles

Preparation and Antibacterial Properties of Poly (l-Lactic Acid)-Oriented Microporous Materials

In this manuscript, an efficient self-reinforcing technology—solid hot drawing (SHD) technology—was combined with green processing supercritical carbon dioxide (SC-CO2) foaming technology to promote poly (l-lactic acid) (PLLA) to form an oriented micropore structure. In addition, Polydimethylsiloxane (PDMS), with a high affinity of CO2 and biological safety, was introduced to enhance the nucleation effect in SC-CO2 foaming and co-regulate the uniformity of oriented micropores’ structure. The results showed that orientation induced PLLA crystallization, so the tensile strength was improved; the maximum tensile strength of the oriented micropores’ PLLA reached 151.2 MPa. Furthermore, the micropores mainly improved the toughness; the maximum elongation at break reached 148.3%. It is worth mentioning that PDMS can form an antibacterial film on the surface of the material, so that the material has a continuous antibacterial effect.

Multistage microcellular waterborne polyurethane composite with optionally low-reflection behavior for ultra-efficient electromagnetic interference shielding

Electromagnetic interference (EMI) shielding materials with superior shielding efficiency and low-reflection properties hold promising potential for utilization across electronic components, precision instruments, and fifth-generation communication equipment. In this study, multistage microcellular waterborne polyurethane (WPU) composites were constructed via gradient induction, layer-by-layer casting, and supercritical carbon dioxide foaming. The gradient-structured WPU/iron-cobalt loaded reduced graphene oxide (FeCo@rGO) foam serves as an impedance-matched absorption layer, while the highly conductive WPU/silver loaded glass microspheres (Ag@GM) layer is employed as a reflection layer. Thanks to the incorporation of an asymmetric structure, as well as the introduction of gradient and porous configurations, the composite foam demonstrates excellent conductivity, outstanding EMI SE (74.9 dB), and minimal reflection characteristics (35.28 %) in 8.2–12.4 GHz, implying that more than 99.99999 % of electromagnetic (EM) waves were blocked and only 35.28 % were reflected to the external environment. Interestingly, the reflectivity of the composite foam is reduced to 0.41 % at 10.88 GHz due to the resonance for incident and reflected EM waves. Beyond that, the composite foam is characterized by low density (0.47 g/cm3) and great stability of EMI shielding properties. This work offers a viable approach for crafting lightweight, highly shielding, and minimally reflective EMI shielding composites.

Enhanced thermal conductivity in thermoplastic polyether ester elastomer vitrimer composite foams via cell growth-induced orientation and lap of hybrid fillers

Lightweight thermal conductive polymer foams own a great potential application in some weight sensitive fields such as aerospace and encapsulation. Herein, the enhancement of thermal conductivity of thermoplastic polyether ester elastomer (TPEE) vitrimer composites foams caused by cell growth inducing fillers orientation and lap was investigated with hybrid fillers of fixed 15 vol% h-BN and various SiC nanowires. First, the hybrid fillers orientation in TPEE vitrimer composites (TVC) characterized by SEM and XRD patterns showed that the hybrid fillers were induced to be through-plane orientation and no interfacial defects were observed owing to the increasing contact area among fillers caused by large specific area and length of SiC nanowires. Based on that, cells were generated into TVC through scCO2 foaming. The effect of through-plane orientation and lap of hybrid fillers caused by cell growth of different cell structures on the thermal conductivity of TVC foams was investigated using extended Hashin-Shtrikman model and Maxwell-Eucken I model. The results showed that the thermal conductivity of TVC foams could greatly be enhanced to be higher than that of TVC by cell growth of continuous cell structures as it contributed to the through-plane orientation and lap of hybrid fillers to increase the contact and lap ratio among fillers. Thereby, the thermal conductive networks favorable to heat transfer were constructed in TVC foams to enhance the thermal conductivity as well as merit of lightweight.

Constructing structure-gradient silicone rubber/CNTs foam with desirable resilience and strength via green supercritical CO2 foaming of non-equilibrium gas concentration profiles

In comparison to uniform foams, structure-gradient foams can offer practical solutions that optimize their resilience and strength. However, controlling the gradient structure is a challenge, especially in silicone rubber (PMVS) based foams. Herein, a facile and eco-friendly one-step supercritical CO2 (scCO2) foaming technique is presented for fabricating structure-gradient PMVS/CNTs composite foams utilizing a non-equilibrium gas concentration profile. Compared with conventional scCO2 foaming, this method features a more energy-efficient process (lower saturation temperature and time) and results in lighter foam products (density as low as 0.11 g/cm3). Moreover, the formation mechanism of gradient cellular structures in PMVS/CNTs foams and the means of adjusting the cell gradient degree are further discussed. Finally, cyclic compression results demonstrate that PMVS/CNTs foams can be optimized to achieve desirable mechanical strength and resilience by adjusting cell gradient degree.

Swelling of rubbers of different chemical natures in supercritical carbon dioxide

Objectives. To investigate the swelling of the main types of rubbers used in the rubber industry in carbon dioxide in a supercritical state (SC-CO2), in order to assess the possibility of obtaining elastomeric materials with porous structures using fluid technology, based on them.Methods. The process of swelling of rubbers in SC-CO2 and subsequent foaming was carried out according to a specially developed technique using the original installation. This is a high-pressure apparatus with transparent windows, allowing for the use of an optical technique to directly measure the geometric dimensions of samples during swelling and foaming using a digital video camera. The study of the porous structure of foamed rubbers was carried out using scanning electron microscopy.Results. The study established experimental curves of the swelling kinetics in SC-CO2 of isoprene, butadiene, styrene butadiene, ethylene propylene, chloroprene, ethylene acrylate, siloxane, and organofluorine rubbers. The influence of temperature and pressure on the rate and equilibrium degree of swelling was studied. The diffusion coefficients of SC-CO2 in rubbers of various chemical natures were also determined.Conclusions. It was shown that the equilibrium swelling degree of rubbers in SC-CO2 depends on the chemical nature of rubbers. It does not correlate with the value of their solubility parameters, changes directly proportional to the diffusion coefficient and increases with increasing temperature and pressure. It was found that irrespective of the degree of swelling in SC-CO2, all the rubbers studied are intensively foamed at a sharp pressure drop. The size of the pores formed is tens of microns: significantly smaller than the size of pores formed when chemical pore formers are used.

One-pot preparing EVA elastomer auxetic foam through stress relaxation of molecular chains by heat annealing

It is a challenge to prepare closed-cell auxetic foam by a simple method without destroying the properties of the foam. In this paper, auxetic foams were prepared directly from ethylene-vinyl acetate copolymer (EVA) resin in one step by annealing immediately after sc-CO2 foaming. The effects of annealing time and foaming temperature on the cell structure and Poisson's ratio were investigated using scanning electron microscopy (SEM) and Poisson's ratio testing. The reasons for the formation of EVA foams with negative Poisson's ratio were analyzed by dynamic mechanical thermal analysis (DMA) and differential scanning calorimetry (DSC). It was found that annealing treatment maintained the high movement ability of molecular chains and provided sufficient time for the molecular chains to relax, so that the stress generated by the foaming process can relax quickly and reach a new stable state, thus weakening the elastic shrinking of EVA foam. Under the action of the pressure difference between the inside and outside of the cell formed by the different diffusion rates of air and CO2, the cell concaves inward to produce a reentrant structure, and then the foam shape is fixed with the continuous improvement of crystallization. The obtained EVA auxetic foam has excellent resilience. Finally, it is proved that the method has good universality for the preparation of auxetic foams with most elastomers, so this study has important implications for the preparation of closed-cell elastomer polymer auxetic foams.

Layered Structural PBAT Composite Foams for Efficient Electromagnetic Interference Shielding

HighlightsA layered segregated shielding network was organized in porous PBAT/Fe3O4@MWCNTs/Ag composite by scCO2 foaming and scraping techniques.The composite foam achieved an electromagnetic interference (EMI) shielding effectiveness (SE) of up to 68.0 dB and a reflectivity of as low as 23% due to the “absorption-reflection-re-absorption” shielding mechanism.The solid and foamed PBAT/Fe3O4@MWCNTs/Ag composites displayed superior retention (> 92%) of EMI SE even after peeling experiment of 500 times under 100 g weight pressure.

Super thermal-insulation PS/PMMA/CNTs composite foams with shape recovery property formed by the synergy of ultrasound and H2O in scCO2 foaming

For polymer foams, a high expansion rate is often required to obtain a low thermal conductivity. However, polymer foams with high expansion ratio faces a difficulty of shape recovery. In this work, polystyrene (PS)/polymethyl methacrylate (PMMA)/carbon nanotubes (CNTs) composite foams were prepared by the synergy of ultrasound and H2O in supercritical carbon dioxide (scCO2) foaming. The introduction of ultrasound and H2O is beneficial to improving expansion ratio. CNTs can enhance the melt strength of PS/PMMA, which also absorb part of the thermal radiation. Therefore, the PS/PMMA/CNTs foams with 0.5 wt% CNTs exhibits the high expansion ratio (80.7 times), low thermal conductivity (25.98 mW/mK) and 100% compression recovery in water at 85 °C after one compression. In addition, with the increasing of filler contents, the reusability of the foam is further improved. After ten compressions, the PS/PMMA/CNTs foams with 2 wt% CNTs still achieve the 99% recovery. This work provides a new way to prepare polymer foams with super-insulation and reusability.

High-pressure drop rates in solid-state batch one-step scCO2 foaming of acrylic polymers: A way to stabilize the structure of micro-nano foams

One-step solid-state batch scCO2 foaming is used with the target of achieving acrylic polymer micro-nano foams. Foaming is triggered by an average pressure drop (APDR), covering two decades, from 0.3 to 30 MPa.s−1. This study principally addresses the combined beneficial effects of block copolymer addition (BCP, here denoted as MAM) and high APDR. Numerous subtle kinetic parameters actually interplay and compete in the production of the final foams. In particular, the material effective temperature, the effective glass transition temperature of the plasticized system and the instantaneous PDR are physical quantities each having their own kinetics during foaming. The resulting foam morphologies are quantified by SEM microscopy and image analysis. A high APDR and the presence of BCP are shown to play a key role in the final structure of the foams. Over the scrutinized range of saturation temperature (40 °C to 60 °C i.e. rather ‘low’ temperatures in the CO2 supercritical state), the APDR is the main factor for significantly reducing cell size and increasing nuclei density in foams from neat PMMA. In the block copolymer approach, increasing the APDR is of secondary importance as the targeted reduction of the porosity dimensions and augmentation of nuclei density are mostly the consequence of MAM presence. In this latter case, increasing the APDR still promotes the ‘efficiency’ of the BCP nucleants. A real efficient nucleation activity of MAM additive is observed at a very high APDR (30 MPa.s−1), leading to monomodal homogeneous distribution of tiny pores in nearly nanosized foams. At lower APDR, an interesting reproducible double porosity (foams containing intra-wall and inter-wall pores) is detected in PMMA/MAM systems. In such double porosity foams, benefits from the Knudsen effect achieved within well expanded local domains (showing micron-sized pores) may remain meaningful thanks to a locally poorly expanded nanoporous thick solid skeleton encapsulating these local domains. Thereby, the radiative thermal conduction can be minimized and does not override the conductive component at the sample scale. This work provides further insight on acrylic polymer BCP foams influenced by different kinetics.

Unexpected super anti-compressive styrene-acrylonitrile copolymer/polyurea nanocomposite foam with excellent solvent resistance, re-processability and shape memory performance

High-performance, recycling and environmental protection are the goal of the development of polymer materials in the future. The introduction of crosslinking and composite are two main ways to improve the comprehensive properties of materials, however, which leads to the non-recyclability of materials or brings difficulties to the recovery process. Here, we report a polymer nanocomposite foam with superior compression properties to most existing commercial foams, just originating from common commercial styrene-acrylonitrile copolymer (SAN) and polyurea (PUA), and develop a green and energy-efficient method to fabricate high-performance rigid foam by reactive plasticizing and reaction induced phase-separation combining with supercritical CO2 (sc-CO2) foaming. Reaction induced phase-separation results in the in-situ formation of nanoscale structure in the cell walls of the resultant foams, in which the crosslinked PUA is the dispersed phase with the size of 10–40 nm in the matrix of SAN. Optimizing crosslinking network structure of PUA dramatically improve the comprehensive performance of SAN/PUA nanocomposite foams. The resultant SAN/PUA nanocomposite foam shows some performance characteristic of thermosetting polymers, such as excellent heat resistance and solvent resistance than the SAN matrix, meanwhile the performance characteristic of thermoplastic polymers, such as re-processability. More interestingly, the SAN/PUA nanocomposite foam presents excellent shape memory performance. With the above exceptional performances, this polymer nanocomposite foam will be a perfect substitute for the existing commercial rigid foams.

Skinless Polyphenylene Sulfide Foam with Enhanced Thermal Insulation Properties Fabricated by Constructing Aligned Gas Barrier Layers for Surface-Constrained sc-CO2 Foaming.

A solid skin layer inevitably forms on the foam surface for supercritical carbon dioxide (sc-CO2) foaming technology, leading to deterioration of some inherent properties of polymeric foams. In this work, skinless polyphenylene sulfide (PPS) foam was fabricated with a surface-constrained sc-CO2 foaming method by innovatively constructing aligned epoxy resin/ferromagnetic graphene oxide composites (EP/GO@Fe3O4) as a CO2 barrier layer under a magnetic field. Introduction of GO@Fe3O4 and its ordered alignment led to an obvious decrease in the CO2 permeability coefficient of the barrier layer, a significant increase of the CO2 concentration in the PPS matrix, and a decrease of desorption diffusivity in the depressurization stage, suggesting that the composite layers effectively inhibited the escape of CO2 dissolved in the matrix. Meanwhile, the strong interfacial interaction between the composite layer and the PPS matrix remarkably enhanced the heterogeneous nucleation of cells at the interface, resulting in elimination of the solid skin layer and formation of an obvious cellular structure on the foam surface. Moreover, by the alignment of GO@Fe3O4 in EP, the CO2 permeability coefficient of the barrier layer became much lower, and the cell density on the foam surface further increased with decreasing cell size, which was even higher than that of the cross section of foam, attributed to stronger heterogeneous nucleation at the interface than the homogeneous nucleation in the core region of the sample. As a result, the thermal conductivity of the skinless PPS foam reached as low as 0.0365 W/m·k, decreasing by 49.5% compared with that of regular PPS foam, showing a remarkable improvement in the thermal insulation properties of PPS foam. This work provided a novel and effective method for fabricating skinless PPS foam with enhanced thermal insulation properties.

Developing sustainable and recyclable high-efficiency electromagnetic interference shielding nanocomposite foams from the upcycling of recycled poly(ethylene terephthalate)

The demand for high-performance electromagnetic interference (EMI) shielding composite foams with satisfactory environmental friendliness and high sustainability has increased considerably. However, considerable obstacles are frequently encountered in the practical development of incorporating upcycling of plastic wastes and environmentally friendly foaming technologies into the manufacture of such EMI shielding systems. In this research, we developed a lightweight sustainable nanocomposite foam system with outstanding EMI shielding performance from the integration of chemical upcycling of recycled poly(ethylene terephthalate) (PET) and supercritical carbon dioxide (sc-CO2) microcellular foaming. First, a green thermoplastic polyamide elastomer (TPAE) system was synthesized using a renewable bis(6-aminohexyl)terephthalamide (BAHT) monomer derived through the aminolysis of recycled PET, followed by adding single-walled carbon nanotubes (SWCNTs) into the TPAE system for the manufacture of TPAE/SWCNT nanocomposites. The π–π stacking of BAHT efficiently enhanced the melt strength of the material system, which was conducive for microcellular foaming process. After sc-CO2 foaming, a series of highly expanded nanocomposite foams with uniform microporous structure that could enable multiple reflections of electromagnetic waves were fabricated. Upon adding only 2 wt% SWCNTs, the foam system achieved a favorable conductivity of 0.15 S/cm and an extremely high specific shielding effectiveness (SSE) of 213.26 (dB cm3/g). Furthermore, the green nanocomposite foam possessed excellent durability. After being bent or twisted 1000 times, the foam maintained EMI shielding effectiveness of > 90% of the original value. Moreover, the foam could be easily recycled, reprocessed, and refoamed. For developing sustainable EMI shielding products, we believe that this study provides a promising direction with benefits of alleviating waste accumulation and reducing the use of virgin plastics in manufacture of foams, showing potential for advanced sustainable electronic applications.

Progress in the Foaming of Polymer-based Electromagnetic Interference Shielding Composites by Supercritical CO2.

As a critical action plan formulated for peaking carbon dioxide emissions, polymeric electromagnetic interference (EMI) shielding materials based on CO2 foaming technology have recently been attracting widespread attention in both research and industry, attributable to their efficient use of CO2 , high specific strength, corrosion resistance and low-cost characteristics. In the past decade, the emergence of novel design concepts and preparation techniques for CO2 foaming technology has led to the development of new high-performance EMI shielding materials in this field. This review summarizes the research progress made to date on the fabrication of EMI shielding composite foams by supercritical carbon dioxide (scCO2 ) foaming. We also explore the structure-activity relationships between the component/distribution and EMI shielding properties. Additionally, the application prospects and development challenges of new EMI shielding composite foams are described.

Interfacial stabilizing and reinforcing effects of silsesquioxane-based hybrid Janus molecules in extrusion supercritical CO2 foaming of polypropylene

High-melt-strength (HMS) polypropylene (PP) has enabled easy supercritical-CO2 (scCO2) foaming of PP. However, producing microcellular PP foams via simple scCO2 extrusion foaming still remains challenging, whereas macrocellular PP foams with high expansion ratios (ER) generally exhibit poor mechanical strengths. In this study, to simultaneously enhance both ER and compressive strengths of macrocellular PP foams, a type of low-cost Janus molecules, heptaisobutyl open-cage silsesquioxane (HOS), is incorporated into HMS-PP. Since HOS has PP-philic groups and a partially condensed silsesquioxane cage that is relatively CO2-philic, it could diffuse like a surfactant to the PP-CO2 interface during scCO2 extrusion foaming to stabilise the cells in their growth stage and reinforce the cell surface upon solidification. The surfactant effect is validated via X-ray Photoelectron Spectroscopy. Moreover, with 1 wt% HOS, the PP/HOS foam could offer 38 % ER improvement, 23 % density reduction, and 20 % compressive strength increase over the neat PP foam. HOS could also slightly improve the extensional viscosity and melt strength of PP at ≤1 wt% loadings and reduce the shear viscosity of PP by acting as a lubricant. This approach provides a simple and economically viable materials design guideline for alleviating ER-compression strength trade-offs for macrocellular polymer foams.

Thermal insulative 2D-oriented bimodal micro-foams and high strength photonic thermoplastic elastomers by assembling olefin multiblock copolymers

Hierarchical 2D-oriented micro-foams and photonic elastomers are produced by assembling the mesophase separated and crystallized superstructures of olefin multiblock copolymers (OBCs). Highly 2D-oriented monomodal and bimodal micro-foams are prepared via compression molding of as-received nascent OBC pellets, irradiation crosslinking, and a subsequent moderate-to-high-pressure supercritical CO2 foaming. Randomly short range ordered bimodal micro-foams are generated from the sheared counterparts. These micro-foams show excellent lightweight properties with foam density of 0.061–0.188 g cm−3 and outstanding thermal conductivity of 20–36 mW m−1 K−1 by laser flash analysis. Moreover, the order-disorder and order-order transitions of nascent and sheared bulk OBCs are characterized by a sequential annealing and subsequent crystallization procedure. The self-seeded nascent and sheared OBCs show hierarchical mesophase separation with domain spacings of 80–258 nm, measured by ultra-small X-ray scattering. The selective visible-light transmitted, transparent, and high-strength thermoplastic elastomers are then made via solvent-free compression molding of these OBC pellets. Meanwhile, low-temperature Sc-CO2 foaming shows that 2D lamellar crystals and 2D, 3D-growing spherulitic crystals with sharp interfaces are individually organized within 2D-oriented lamellar domains of photonic and opaque sheets, which account for their high tensile strength and low strain hardening properties, respectively. The 2D-oriented bimodal micro-foams and assembled photonic elastomers possess potential applications in lightweight sound and thermal insulation, auto-parts, shape memory light actuators, and soft sensors.

Microwave-assisted foaming and sintering high-strength and antistatic polypropylene bead foams by constructing conductive 3D skeleton structure

Polypropylene (PP) bead foam parts, one of the most widely used automotive interiors, are traditionally welded by high-pressure steam, while their large inter-bead gaps result in the poor mechanical properties. Herein, syndiotactic polypropylene (sPP) powders, with low melting point, coated with carbon nanotubes (CNTs) were adopted as the microwave absorbing medium to weld the surface of expanded isotactic polypropylene (EiPP) beads, which were initially prepared by supercritical carbon dioxide (scCO2) foaming. Then, the modified EiPP (EiPP coated with CNTs@sPP powders, denoted as mEiPP) composite foams were successfully fabricated by microwave-assisted foaming and sintering. The obtained foam part consisted of porous iPP bead structure (body) and interconnected CNTs@sPP pathway (3D skeleton), which showed superior mechanical properties with tensile strength of 4.07 MPa and compressive strength of 4.64 MPa (10 % strain). Due to the conductive 3D skeleton, the requirements of antistatic PP bead foams could be sufficed with the lower CNTs loading.

Biomass chemical upcycling of waste rPET for the fabrication of formamide-free TPEE microcellular foams via scCO2 foaming

Currently, the fabrication of microcellular structure in thermoplastic polymer foams is heeded people’s attention owing to their extensive application prospects. In this work, we combine recycled poly(ethylene terephthalate) (rPET) with renewable bio-based monomers (namely, MPO, SeA, and PTMG) for the synthesis of value-added, long-lasting thermoplastic poly(ether ester) elastomers (TPEEs) via a novel and flexible polycondensation strategy. Following this, a facile, eco-friendly, effective, and green scCO2-based foaming methodology was applied to fabricate a series of lightweight and robust microcellular TPEE foams. The addition of MPO and SeA not only improves melt viscoelasticity and scCO2 uptake but also decreases TPEEs melting temperatures and improves the foaming temperature windows. The uniform microcellular foam structure with the smallest cell size (26–34 µm) and the highest cell density (>1011 cm3/cells) was successfully prepared. The TPEE foams unveiled a high expansion rate (6.1–8.5) and strong dimensional stability, and the creep shape recovery percentage of the foams ranged from 94.2% to 98.6%, respectively. Thanks to scCO2 foaming, TPEE foams' has zero formamides, meet safety requirements, and are suitable for commercial applications. Furthermore, the fabricated TPEE foams show excellent recyclability. Therefore, it reveals a promising future for the production of eco-friendly polymer foams through a simple, cost-effective, and green microcellular foaming technology.

Biomass upcycling of waste rPET to higher-value new-easy-recyclable microcellular thermoplastic (co)polyamide foams and hot-melt adhesives

Poly(ethylene terephthalate) (PET) is one of the extremely used fossil-based products in the chemical industry, accounting for about 13% of the world's production. Today, most PET waste accumulates in landscapes or the environment, causing severe pollution. From the perspective of sustainability, scientists have tried to recycle waste PET through chemical and mechanical methods. Mechanical recycling offers some limitations because of the properties of the final product decrease from the second cycle. Rather, chemical recycling has attracted much attention because it can selectively convert plastic waste into high-value-added products including fuel, monomer, and refinery feedstock. Therefore, in this study, the chemical depolymerization of PET was investigated experimentally. After successfully deconstructing waste PET into polymerizable N1, N4-bis(6-aminohexyl)terephthalamide (BAHT) monomer, the obtained BAHT was combined with a bioderived dimer fatty acid (DA; renewable content 100%) building block for the synthesis of bio-based thermoplastic (co)polyamides (TPCPAs) via solvent-free melt polycondensation. The obtained TPCPAs underwent a scCO2 foaming process to give the higher value-added microcellular foams. Interestingly, scCO2 foaming has zero formamides, which meets the safety requirements and is suited to commercial uses. Moreover, TPCPA foams showed an excellent shape recovery property and recyclability due to the entangled DA structure. Adhesive tests also show that TPCPAs exhibit a higher Tpeel strength (185.2 N/cm–420.5 N/cm) than commercial PA glues. This strategy to recycle PET into value-added biomass TPCPAs has a promising application prospect.

Fabrication of lightweight flexible thermoplastic polyurethane/multiwalled carbon nanotubes composite foams for adjustable frequency-selective electromagnetic interference shielding by supercritical carbon dioxide

In this study, the thermoplastic polyurethane (TPU)/multiwalled carbon nanotubes (MWCNTs) lightweight flexible foam with sandwich structure was achieved by supercritical carbon dioxide (scCO2) foaming. This work studied the relationship between foaming behavior and electromagnetic interference (EMI) shielding performance. The results showed that the formation of cell structure could not only achieve the purpose of lightweight, but also control the position of electromagnetic interference shielding efficiency (EMI SE) peak by adjusting foaming temperatures. The EMI SE peak could shift to high frequencies as the foaming temperature increases, and the adjustable frequency ranged from 9.76 GHz to 14.72 GHz. The maximum EMI SE of TPU/MWCNTs composite foams increased to 44.86 dB under 2.5 wt% filler content with a density of 0.61 g/cm3. This work provided a feasible idea for the preparation of lightweight and adjustable frequency-selective EMI shielding composite materials by supercritical fluid foaming.

High‐strength and antistatic PET/CNTs bead foams prepared by scCO2 foaming and microwave sintering

AbstractThis paper reported the high strength and antistatic polyethylene terephthalate (PET) bead foam with a typical segregated structure prepared by supercritical carbon dioxide (scCO2) foaming and microwave selective sintering. Firstly, carbon nanotubes (CNTs) were coated on the surface of PET pellets, and then the composite foam with segregated structure was prepared via scCO2 foaming in a confined mold. The interfaces among PET foamed beads were further sintered by utilizing the selective heating behaviors of microwave, which greatly enhanced the interfacial strength and thus the overall mechanical strength of bead foam. The final composite foam showed an electrical conductivity of 0.004 S/m with an extremely low CNTs loading of 0.138 vol%, and its mechanical strength reached as high as 10.44 MPa, far superior to the composite without microwave treatment. Thus, the problem in the welding of PET bead foams and antistatic properties could be solved. The effect of the foaming conditions on the structure and properties of PET foam were systematically investigated, providing a novel method for the preparation of high strength and antistatic engineering plastics composite foam.