Gradient foaming of polycarbonate/carbon nanotube based nanocomposites with supercritical carbon dioxide and their EMI shielding performances

The Study of Crystallization Behavior, Microcellular Structure and Thermal Properties of Glass-Fiber/Polycarbonate Composites

Polycarbonate (PC) foam is a versatile material with excellent properties, but its low thermal stability limits its application in high-temperature environments. The aim of this study was to improve the thermal stability of PC foam by adding glass fibers (GF) and to investigate the effect of GF on PC crystallization behavior and PC foam cell morphology. This study was motivated by the need to improve the performance of PC foams in various industries, such as construction, automotive, and medical. To achieve this goal, PC/GF composites were prepared by extrusion, and PC/GF composite foams were produced using a batch foaming process with supercritical carbon dioxide (SC-CO₂) as the blowing agent. The results showed that the addition of GF accelerated the SC-CO₂-induced crystallization stability of PC and significantly increased the cell density to 4.6 cells/cm³. In addition, the thermal stability of PC/GF foam was improved, with a significant increase in the residual carbon rate at 700 °C and a lower weight loss rate than PC matrix. Overall, this study highlights the potential of GF as a PC foam reinforcement and its effect on thermal and structural properties, providing guidance for industrial production and applications.

Absorption Dominated Directional Electromagnetic Interference Shielding through Asymmetry in a Multilayered Construct with an Exceptionally High Green Index

Fabricating green electromagnetic interference (EMI) shields is the need of the hour because strong secondary reflections in the vicinity of the shield adversely affect the environment and the reliability of the neighboring devices. To this end, the present work aims to maximize the absorption-based EMI shielding through a multilayered construct comprising a porous structure (pore size less than λ/5), a highly conducting entity, and a layer to match the impedance. The elements of this construct were positioned so that the incoming electromagnetic (EM) radiation interacts with the other layers of the construct before the conducting entity. This positioning of the layers in the construct offers a high green shielding index (g_s) and low reflection coefficient (R ∼ 0.1) with an exceptionally high percent absorption (up to 99%). Polyurethane (PU) foams were fabricated using the salt-leaching technique and strategically positioned with carbon nanotube (CNT) papers and polycarbonate (PC)-based films to obtain symmetric and asymmetric constructs. These structures were then employed to gain mechanistic insight into the directional dependency of shielding performance, g_s, and heat dissipation ability. Interestingly, maximum total shielding effectiveness (SE_T) of -52 dB (88% absorption @8.2 GHz) and specific shielding effectiveness/thickness (SSE_t) of -373 dB/cm²g were achieved for a symmetric construct whereas, for the asymmetric construct, the SE_T and SSE_t were -37 dB and -280 dB/cm²g, respectively, with an exceptionally high g_s of 8.6, the highest reported so far. The asymmetricity in the construct led to directional dependence of the absorption component (% SE_A, shielding effectiveness due to absorption) and heat dissipation, primarily governed by the electrical and thermal conductivity gradient, respectively. This study opens new avenues in this field and reports constructs with an exceptionally high green index.

Lightweight electromagnetic interference shielding poly(L-lactic acid)/poly(D-lactic acid)/carbon nanotubes composite foams prepared by supercritical CO2 foaming

Lightweight and biodegradable polymer composites with efficient electromagnetic interference (EMI) shielding performance are of great significance for controlling pollution caused by plastic waste and electromagnetic radiation. Herein, poly(lactic acid) (PLA)/carbon nanotubes (CNTs) composites were prepared through a melt blending method. By adding a small amount of poly(D-lactic acid) to poly(L-lactic acid) (PLLA), the EMI shielding performance of the composites was improved because an enhanced viscoelasticity and suitable crystallinity could help to construct fine CNT conductive networks. When the PDLA content was 2 wt%, the EMI shielding effectiveness (SE) of the PLLA-2PDLA-10CNTs reached 27.1 dB at 26.5 GHz. Based on these findings, a green supercritical CO₂ foaming method was employed to prepare lightweight PLLA/PDLA/CNTs composites. For the PLLA-2PDLA-10CNTs foams, when the expansion ratio was 1.24, the EMI SE was 20.1 dB at 26.5 GHz. In addition, the EMI shielding mechanism of the foams was dominated by absorption. This paper provides a facile way to prepare lightweight and environmentally friendly materials for EMI shielding applications.

Lightweight Epoxy/Cotton Fiber-Based Nanocomposites with Carbon and Fe3O4 for Electromagnetic Interference Shielding

Cotton fiber (CF)-based electroconductive papers were prepared by facile aqueous dispersion and drying processes combined with carbon nanotubes (CNTs) or graphene nanosheets (GNPs). To enhance the electromagnetic interference (EMI) shielding performance of the manufactured nanocomposites, the electroconductive papers were soaked with epoxy resin, which cooperated with the inner sprayed Fe₃O₄ nanoparticles. The EMI shielding effectiveness of Epoxy/CF-30-Fe₃O₄-30GNPs reached 33.1 dB, of which over 85.0% is attributed to absorption, which is mainly believed to be caused by the combination of GNPs and Fe₃O₄ nanoparticles due to their special structures and synergetic effects. Moreover, the infiltration of epoxy between the randomly distributed loose CFs and the multiple reflections inside the interconnected networks could also help to improve the EMI shielding performance of GNP-added samples. The prepared lightweight and stiff Epoxy/CF-30-Fe₃O₄-30GNP composites have promising applications in civil or military fields.

Effect of the Molecular Structure of TPU on the Cellular Structure of Nanocellular Polymers Based on PMMA/TPU Blends

In this work, the effects of thermoplastic polyurethane (TPU) chemistry and concentration on the cellular structure of nanocellular polymers based on poly(methyl-methacrylate) (PMMA) are presented. Three grades of TPU with different fractions of hard segments (HS) (60%, 70%, and 80%) have been synthesized by the prepolymer method. Nanocellular polymers based on PMMA have been produced by gas dissolution foaming using TPU as a nucleating agent in different contents (0.5 wt%, 2 wt%, and 5 wt%). TPU characterization shows that as the content of HS increases, the density, hardness, and molecular weight of the TPU are higher. PMMA/TPU cellular materials show a gradient cell size distribution from the edge of the sample towards the nanocellular core. In the core region, the addition of TPU has a strong nucleating effect in PMMA. Core structure depends on the HS content and the TPU content. As the HS or TPU content increases, the cell nucleation density increases, and the cell size is reduced. Then, the use of TPUs with different characteristics allows controlling the cellular structure. Nanocellular polymers have been obtained with a core relative density between 0.15 and 0.20 and cell sizes between 220 and 640 nm.

A composite with a gradient distribution of graphene and its anisotropic electromagnetic reflection

An electrochemical method was introduced to prepare graphene/polyurethane foams with gradient graphene distribution, and this composite shows obvious anisotropic reflection of electromagnetic waves.

An ultra-thin carbon-fabric/graphene/poly(vinylidene fluoride) film for enhanced electromagnetic interference shielding

Highly conductive carbon-based fibrous composites have become one of the most sought-after components in the field of electromagnetic interference (EMI) shielding due to their excellent comprehensive performance. In this work, a flexible nonwoven fabric consisting of carbon fibers (CFs) and polypropylene/polyethylene (PP/PE) core/sheath bicomponent fibers (ESFs), known as CEF-NF, is introduced into the graphene (GE)/poly(vinylidene fluoride) (PVDF) nanocomposite obtained by a solution casting method to fabricate a CEF-NF/GE/PVDF film. Disparate microstructures can be clearly observed in CEF-NF/GE/PVDF films with different graphene contents. Thanks to an internal porous network structure formed when the graphene content is high, this film exhibits better electrical conductivity. In the frequency range of 30-1500 MHz, this film can achieve a significantly high EMI shielding effectiveness (EMI-SE) value of about 48.5 dB at tiny thickness and density (1731.40 dB cm² g^-1), which are far better than many competitive materials. Moreover, this film exhibits adequate tensile strength and excellent flexibility, as the film's structural form can be retained even after multiple folding processes. In addition, by combining two-dimensional (2D) graphene and one-dimensional (1D) CF, the CEF-NF/GE/PVDF film achieves a remarkable in-plane thermal conductivity of 25.702 W m^-1 K^-1, making it an exceptional heat conductor. In summary, our results demonstrate that CEF-NF/GE/PVDF film is an excellent EMI shielding material that is light weight, highly flexible, and mechanically robust with outstanding thermal conductivity, which positions it superbly for applications in next-generation commercial portable electronics.

Retracted Article: The influence of gradient and porous configurations on the microwave absorbing performance of multilayered graphene/thermoplastic polyurethane composite foams

Single-layer graphene/TPU composite foams with different graphene content were prepared through a thermally induced phase separation (TIPS) process. Multilayer graphene/TPU composite foams were fabricated by bonding single-layer foams together. The arrangement of single-layer graphene/TPU composite foams in different orders could realize a gradient distribution of the graphene to endow the multilayer foams with good impedance matching characteristics. Facile regulation of the effective absorption bandwidth (EB) value and minimum reflection loss (RLmin) have been realized by adjusting the thickness and layer number or altering the combinatorial mode of single-layer foams with different graphene contents to endow these multilayered composite foams with optimal microwave-absorbing (MA) properties. In addition, the mechanism of microwave dissipation by gradient multilayers and porous structures has been elucidated. The EB values of the multilayer foams were all wider than those of their corresponding single-layer foams with the same graphene content and multilayer foams displayed much lower RLmin than single-layer foams. Among all the multilayer foams, 2L graphene/TPU composite foams with a thickness of 5 mm exhibit the widest EB value of 9.9 GHz and the lower RLmin (-36.7 dB) while 5L graphene/TPU composite foams with a thickness of only 2.5 mm show the lowest RLmin of -43.7 dB and wider EB values (5.3 GHz).

Processing-Mediated Different States of Dispersion of Multiwalled Carbon Nanotubes in PDMS Nanocomposites Influence EMI Shielding Performance

Advancement in wireless technology has increased the usage of wireless devices extensively in the past few years, which led to an increase in electromagnetic interference (EMI) in the environment. Extensive research on fabrication of EMI shielding materials has been done. However, the role of processing method of polymer composites in EMI shielding has been neglected. In this work, we investigate the role of two polymer processing methods, spin coating and compression molding, in EMI shielding application. Poly(dimethylsiloxane) (PDMS) nanocomposites with multiwalled carbon nanotube (MWCNT) were spin-coated onto glass slides and compression-molded to a similar thickness. The processing method that exhibited the best shielding was employed to fabricate multiple PDMS composites comprising different compositions of MWCNT and Fe₃O₄ and stacked to form a multilayered EMI shielding PDMS composite. Scanning electron micrographs revealed that MWCNT in spin-coated composites are significantly more agglomerated than in the compression-molded film. Direct current conductivity and curing temperature were higher in compression-molded films as the filler formed a well-percolated network and hindered cross-linking of polymer chains. EMI shielding results revealed that spin-coated films demonstrated greater shielding effectiveness than compression-molded composites in the Ku-band (12-18 GHz). Individual agglomerates of MWCNT in spin-coated film attenuated incoming electromagnetic radiation more effectively than well-dispersed MWCNT in compression-molded films. Therefore, PDMS composites of different compositions of MWCNT and Fe₃O₄ nanoparticles were prepared through spin coating and stacked with a gradient of filler concentration, which resulted in maximum shielding of -28 dB, i.e., shielding more than 99% of incoming EM radiation by a 0.9 mm film.

Electromagnetic Shielding Materials in GHz Range

The state-of-the art in the design and the manufacture methods of the different electromagnetic shielding materials has been reviewed. This topic has become a mainstream field of research because of the electromagnetic pollution generated by telecommunication technology development. The review is centred in absorbent materials and shows a general overview of how the absorption properties of such composites can be tailored through changes in geometry, composition, morphology, and the filler particles content. Although different types of materials are explained, the text is mainly focused on carbon materials such as graphene and carbon nanotubes. In this way, the importance of the dispersion of the conductive fillers in different polymer matrices is discussed. In addition, an extensive study on new complex architectures such as foam-based materials is presented. Finally, the combination of carbon fillers with other constituents such as metallic nanoparticles is mentioned. In all these studies, the efficiency of the composites as absorbent or reflective of electromagnetic radiation is discussed.

Efficient electromagnetic interference shielding of lightweight carbon nanotube/polyethylene compositesviacompression molding plus salt-leaching

Carbon nanotube/high density polyethylene (CNT/HDPE) foam composites with high electrical conductivity and electromagnetic interference (EMI) shielding performance were developed by means of compression molding plus salt-leaching. The uniform porous structure and interconnected CNT networks throughout the cell backbones endowed the as-prepared foam composites with a significantly lower electrical percolation threshold (0.22 vol%) than that of the solid composites (0.84 vol%). Owing to the multiple reflections and scattering between the cell–matrix interfaces, the foam composites presented a superior specific EMI shielding effectiveness (EMI SE) of 104.3 dB cm³ g⁻¹, 2.2 times higher than that of their solid counterpart. Besides this, the pore sizes of the CNT/HDPE foam composites could be easily tuned by controlling the particle size of the porogen. Also, the electrical conductivity and specific EMI SE increased with an increase in the cell diameter, which was attributed to the formation of a more perfect conductive network in the cell backbones. Our approach provides a novel idea for fabricating new lightweight EMI shielding materials, especially for aircraft and spacecraft applications.

Carbon nanotube/high density polyethylene foam composites with superior specific electromagnetic interference shielding effectiveness values of 104.3 dB cm³ g⁻¹ were developed via a compression molding plus salt-leaching method.

Grafting polycarbonate onto graphene nanosheets: synthesis and characterization of high performance polycarbonate–graphene nanocomposites for ESD/EMI applications

High performance polycarbonate–graphene nanocomposites were synthesized by grafting polycarbonate onto graphene nanosheets with outstanding mechanical and electrical properties.

Cell characteristics of epoxy resin foamed by step temperature-rising process using supercritical carbon dioxide as blowing agent

A temperature-rising batch foaming process with supercritical carbon dioxide (ScCO2) as blowing agent was used to prepare epoxy resin foams consisting of diglycidyl ether of bisphenol A and m-xylylenediamine. The dissolution and diffusion behaviors of CO2 in pre-cured epoxy resin were investigated, as well as the parameter effect of CO2 saturation step and foaming step on the cell characteristics. It was proved that closed-cells could be generated for CO2 unsaturated samples and the cell characteristics with the same dissolved CO2 concentration were similar. The merged and cracked bubble morphologies were usually obtained for CO2-saturated epoxy resin samples. With increasing CO2 concentration from 0.021 g CO2/g epoxy resin to 0.061 g CO2/g epoxy resin in the unsaturated samples, the cell size increased from 170.2 µm to 262.6 µm and the cell density decreased from 6.8 × 105/cm3 to 3.1 × 105/cm3. Bubble nucleation and growth occurred simultaneously with curing reaction in temperature-rising step. As the final foaming temperature increased from 60℃ to 120℃, the cell size of samples with dissolved CO2 concentration of 0.021 g CO2/g epoxy resin increased from 172.7 µm to 369.0 µm, while the cell density first increased from 6.8 to 7.3 and then decreased to 3.5. The cell size of samples with CO2 concentration of 0.031 g CO2/g epoxy resin increased from 145.3 µm to 180.5 µm with foaming time from 5 min to 20 min, but changed slightly when curing reaction almost finished and CO2 was depleted after 20 min.