This review summarizes the electromagnetic shielding mechanism, the classification of functional rubber materials, and their research progress. It focuses on discussing the performance optimization and application of metal fillers, carbon-based fillers, and intrinsic conductive polymers in electromagnetic interference (EMI) shielding rubbers. With the advancement of aviation technology, the increase in high-power electronic equipment such as radar and communication systems on aircraft has raised the requirements for electromagnetic compatibility and shielding technology. To reduce electromagnetic wave and radiation interference, and to improve the reliability and operational accuracy of electronic devices, high-performance electromagnetic shielding materials, including advanced Conductive Rubber Formulation, are often used in areas like equipment bays, skins, and access panels. Electromagnetic shielding rubber, a key EMI Shielding Elastomer, is one such material. This type of rubber offers flexibility, processability, environmental stability, a diversity of material choices, stable electromagnetic shielding performance, excellent mechanical properties, and water vapor sealing capability, making it a current research hotspot. By compounding conductive fillers (such as Conductive Carbon Nanotubes, graphene, metal powders, conductive polymers, etc.) with a rubber matrix, often via a specialized Conductive Filler Masterbatch, functional rubber materials with excellent electromagnetic shielding performance can be prepared. In recent years, significant progress has been made in the preparation, performance optimization, and application of EMI shielding functional rubber materials, offering effective Solutions for electromagnetic interference in electronics.
1 Electromagnetic Shielding Mechanism
Electromagnetic interference (EMI) shielding refers to the phenomenon where specifically designed materials reflect or absorb electromagnetic waves. This design allows the material to act as a shield, preventing electromagnetic radiation from penetrating electronic equipment, which is the core principle behind creating ESD Safe Rubber components. Furthermore, this shielding phenomenon can also be understood as using a shielding layer made of conductive or magnetic materials to block electromagnetic radiation
The interaction between shielding materials and electromagnetic interference waves is illustrated in Figure 1. According to Schelkunoff’s theory, there are three primary shielding mechanisms for materials against electromagnetic waves: (1) Reflection loss (SE_R) occurring when waves contact the material surface; (2) Absorption loss (SE_A) as waves penetrate the material interior; (3) Loss due to multiple reflections (SE_M) of waves inside the material. The sum of these three losses is called the Shielding Effectiveness (SE), as shown in formula (1). Therefore, the Shielding Effectiveness (SE) value is used to evaluate a material’s electromagnetic shielding capability, measured in decibels (dB). When the SE value of an EMI shielding material is 20 dB and 30 dB, it means it can block 99% and 99.9% of electromagnetic waves, respectively. Moreover, a higher SE value indicates a stronger electromagnetic shielding capability
SE_R + SE_A + SE_M = SE (1)
Figure 1 Interaction between electromagnetic interference and shielding materials
The shielding capability of EMI functional materials primarily depends on the material’s dielectric constant, magnetic permeability, and thickness. Therefore, when designing and preparing EMI functional materials, it is necessary to endow them with high electrical conductivity, high magnetic permeability, and appropriate thickness. Generally, high electrical conductivity and magnetic permeability positively affect the material’s SE_A but have different impacts on SE_R; high conductivity increases SE_R, while high magnetic permeability decreases SE_R.
2 Electromagnetic Shielding Functional Rubber Materials
EMI shielding functional rubbers mainly include silicone rubber (SR), nitrile rubber (NBR), natural rubber (NR), and ethylene propylene diene monomer rubber (EPDM). They can be classified based on conductive filler type into metal fillers, carbon-based fillers, and intrinsic conductive polymers.
2.1 Metal Fillers
Metal fillers hold significant research and practical value in EMI shielding rubber materials. Metals possess excellent electrical conductivity and electromagnetic wave reflection characteristics. Adding them as a Conductive filler for rubber compound can significantly enhance the composite’s SE. Commonly used metal fillers include silver, copper, nickel, aluminum, and their alloys, which can build conductive networks and effectively attenuate electromagnetic wave propagation. Silver fillers have high conductivity and outstanding shielding performance at both low and high frequencies; nickel fillers are magnetic and exhibit good absorption loss capability in the mid-to-low frequency range.
Studies on the effect of identical particle sizes of silver and nickel powder addition levels on EMI rubber composites show that, at the same addition level, silver powder’s conductivity is much higher than nickel powder’s; furthermore, the shielding effect is optimal when the metal filler addition is sufficient to form a conductive network within the rubber matrix. However, excessive metal filler addition leads to a rapid decline in mechanical properties. Additionally, the shape of metal fillers (e.g., particles, fibers, or flakes) and the filling amount significantly impact shielding performance, crucial for Improving rubber shielding effectiveness. Flake fillers, due to their large specific surface area and easier formation of conductive pathways, often result in higher shielding effectiveness. Guan et al. prepared a High performance EMI shielding rubber material composite coating using flake silver-coated copper powder as filler. When the coating thickness was 300 μm, the resistivity reached 0.03 Ω·cm, and the shielding effectiveness reached 70.15–77.46 dB in the 0.3–1000 MHz frequency band. However, the high density and easy oxidizability of metal fillers can affect the material’s processing performance and durability. Therefore, researchers are exploring methods like surface modification, optimizing filler dispersion, and compounding with other functional fillers (e.g., carbon materials) to balance electromagnetic shielding performance with mechanical properties and stability, sometimes seeking Custom electromagnetic shielding elastomer solutions for specific applications.
2.2 Carbon-Based Fillers
Carbon-based fillers added to EMI shielding rubbers mainly include conductive carbon black, Multi walled Carbon Nanotubes, carbon fibers, and graphene. These fillers offer good conductivity, large specific surface area, low density, and excellent chemical stability. Their multi-level molecular structure, particle size, and micro-morphology contribute to both reflection and absorption of electromagnetic waves.
2.2.1 Conductive Carbon Black
Research on the influence of conductive carbon black addition amount and material thickness on the electromagnetic shielding performance of silicone rubber shows that when the mass fraction of conductive carbon black is 5% and the material thickness is 1.9–2.7 mm, the SE value is 10 dB. When the mass fraction of conductive carbon black is 15% and the thickness is 2.8 mm, at the same frequency, the SE value can increase to 40 dB. Studies on the influence of conductive carbon black EC600JD’s particle size, structure, and addition amount on the conductivity and EMI shielding performance of EPDM indicate that carbon black with smaller particle size and higher structure more easily forms a conductive network. As the carbon black dosage increases, the material’s EMI shielding performance improves. When the carbon black dosage is 20 phr, the shielding effectiveness can reach 26.8 dB.
2.2.2 Nanotubos de carbono
Carbon Nanotubes (CNTs), with their unique one-dimensional nanostructure, exhibit broad application prospects in EMI shielding functional rubbers due to their excellent electrical, mechanical, and thermal properties. SaneZen Group, through its subsidiary Powerflex New Material Co., Ltd., specializes in the production of high-performance Multi walled Carbon Nanotubes under the GreenThinking® brand, such as CNT44G These products are engineered to offer superior dispersion and integration into various rubber matrices, including Fluorocarbon Rubber (FKM), Ethylene Propylene Diene Monomer (EPDM), and Silicone Rubber, providing a comprehensive Conductive Rubber Formulation solution.
CNT44G, a vertically aligned and easy-dispersion CNT material, is specifically designed to enhance rubber performance. It not only possesses extremely high electrical conductivity, enabling the effective construction of conductive networks, but also offers significant mechanical reinforcement and thermal conductivity. For instance, in an FKM system, adding just 2 phr of CNT44G can bring the material to an antistatic level (surface resistivity 10^6–10^7 Ω), while increasing the loading to 5 phr reduces the surface resistivity to <10^2 Ω and the volume resistivity drops as low as 5 Ω, demonstrating excellent EMI shielding capability. Similarly outstanding performance is observed in EPDM; after adding 5 phr of CNT44G, the volume resistivity is merely 3 Ω and the surface resistivity falls below 10^3 Ω, meeting the requirements for most EMI shielding and ESD Safe Rubber applications.Beyond exceptional conductive and shielding properties, CNT44G significantly enhances the mechanical properties of rubbers. Test data from the company shows that adding CNT44G to FKM and EPDM noticeably improves the composites’ tensile strength, modulus, and tear strength. For example, in EPDM, the tear strength can be increased by over 60% with 5 phr loading. It also leads to higher strength retention after aging and a significant reduction in compression set, indicating superior sealing performance for Low compression set conductive rubber seal. Furthermore, the incorporation of CNT44G improves the wear resistance and thermal conductivity of the rubber, helping to extend product life.
In tire applications, SaneZen Group’s High-dispersion multi-wall CNT products like CNT44G demonstrate significant advantages. As shown in the company’s test data, incorporating CNT44G into tire tread compounds can effectively as masilla de caucho replace conductive carbon black, improving abrasion resistance and tear strength while maintaining or even enhancing other key properties. This makes CNT44G an ideal Conductive Materials for Tires, contributing to the development of High performance, next-gen tires with improved safety and durability.
In summary, SaneZen Group’s Carbon Nanotubes, such as CNT44G, serve as multifunctional nanofillers. They not only impart excellent electromagnetic shielding and antistatic properties to rubbers but also simultaneously enhance their mechanical strength, heat resistance, and durability. These products are particularly suitable for high-end applications such as aerospace, Conductive Tires, electronic seals, and antistatic conveyor belts, representing an important Cost effective electromagnetic shielding material and performance-enhancing solution for high-performance rubber materials.
2.2.3 Carbon Fiber
Carbon fibers were added to silicone rubber, and the effect of different carbon fiber contents on the conductivity and SE of silicone rubber composites was studied. The results indicated that within the frequency range of 2.6–3.95 GHz, the carbon fiber-filled silicone rubber composites exhibited threshold conductivity and SE values. When the carbon fiber content reached 50 phr, the SE value of the composite nearly peaked at 63 dB. Further increasing the carbon fiber content resulted in almost no change in the SE value.
2.2.4 Graphene
Studies on the electromagnetic interference (EMI) shielding performance of composites based on polydimethylsiloxane (PDMS) incorporating carbon black (CB) and high-specific-surface-area reduced graphene oxide (rGO) showed that the hybrid materials significantly enhanced the composite’s conductivity and dielectric loss, thereby improving shielding efficiency. A three-dimensional layered structure formed inside the composite, causing multiple reflections of microwaves at the layer interfaces. The composite exhibited excellent absorption-dominated shielding performance, with an SE reaching 28 dB. Liao et al. selectively localized graphene in the surface layer of silicone rubber, successfully preparing a novel flexible sandwich-structured silicone rubber/graphene composite. This material exhibited unique frequency-selective EMI Shielding Effectiveness (SE) and insulating characteristics in the shielding direction. When the graphene mass fraction was 3.00%, the composite’s average and maximum SE values were 30.42 dB and 34.72 dB, respectively, representing increases of 59.60% and 72.39% compared to the homogeneous structure composite. Hao et al. prepared a high-loading graphene-coated ferroferric oxide/silicone rubber composite (Fe₃O₄@RGO). The results showed that the composite’s electrical conductivity increased with increasing filler content. When the filler mass fraction reached 25%, the composite exhibited good curing rate and maximum torque value, although further increases in filler content led to more internal defects. Simultaneously, the filler orientation degree significantly improved with increasing magnetic field strength. The composite prepared under a 180 mT magnetic field strength showed the optimal filler orientation structure, effectively constructing a conductive network. This optimized structure resulted in a 400% increase in the composite’s SE compared to pure silicone rubber, and about a 40% increase compared to the composite without magnetic field orientation treatment.
2.3 Intrinsic Conductive Polymers
Intrinsic Conductive Polymers (ICPs) are a class of polymer materials with unique conductive properties. Unlike traditional conductive materials (e.g., metals), ICPs achieve charge transport under an applied electric field through the conjugated structures in their molecular chains – π-electron systems with alternating single and double bonds – allowing electron delocalization along the chain, without the need for external conductive fillers. Their conductivity can be further enhanced by chemical or electrochemical doping (e.g., oxidation or reduction). Commonly used ICPs include polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and polyacetylene (PA), among which PANI and PPy are frequently used for electromagnetic shielding. Research on one-dimensional nanostructured (nanotubes, nanoribbons, and nanofibers) polypyrrole materials found that, with very low conductive filler content (5% mass fraction) in silicone rubber, 2 mm thick polypyrrole nanotube and nanoribbon silicone rubber composites could shield nearly 80% of incident radiation in the C-band. The resulting lightweight, flexible polypyrrole composites show good application prospects for EMI shielding in sensitive biological and electronic systems. Another study added polyaniline to silicone rubber and tested changes in the composite’s resistivity and EMI shielding capability by varying the amount. The results showed that when the polyaniline content was 100 phr, the material’s resistivity dropped by 12 orders of magnitude, and the SE reached up to 19.3 dB in the low-frequency range of 3–1500 MHz. Currently, there is a flexible electromagnetic wave absorption shielding composite material based on NR/NBR blending rubber, using hydrothermally synthesized modified polyaniline nanofibers (PANI) and strontium ferrite (SrFe₁₂O₁₉) as functional fillers, achieving a maximum SE of 36 dB.
3 ConclusionEMI shielding functional rubber materials can effectively suppress electromagnetic wave propagation and achieve efficient EMI shielding by adding conductive or magnetic fillers. Metal fillers (e.g., silver, copper, nickel) significantly enhance the composite’s shielding effectiveness due to their high conductivity and electromagnetic wave reflection characteristics, but their high density and susceptibility to oxidation limit their application. Carbon-based fillers (e.g., conductive carbon black, carbon nanotubes, graphene) offer low density, high conductivity, and good chemical stability, enabling reflection and absorption of electromagnetic waves through their multi-level structures and micro-morphologies. The development of new Conductive Rubber Formulation using Conductive Carbon Nanotubes from leading Carbon Nanotube Manufacturers China y Carbon Nanotube Suppliers China is pivotal for progress in this field. Intrinsic conductive polymers (e.g., polyaniline, polypyrrole) achieve efficient charge transport through conjugated structures in their molecular chains, providing excellent EMI shielding performance without relying on external conductive fillers. Future research should focus on filler surface modification, composite interface optimization, and multifunctional integrated design to further improve electromagnetic shielding performance and application scope. These materials hold broad application prospects in fields such as electronic equipment, aerospace, and medical devices, providing comprehensive Solutions for electromagnetic interference in electronics.
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