Electromagnetic wave shielding coating is a functional coating that is mixed with conductive particles in chemical solvents and can be sprayed on non-metallic materials such as ABS and other engineering plastics, fiberglass, wood, cement walls, etc. to shield electromagnetic waves. It has the advantages of room temperature curing and strong adhesion. It is the simplest way to treat the non-metallic shells of mobile phones, monitors, printers and various instruments for electromagnetic shielding.
Introduction With the rapid development of information technology, computer networks, information processing equipment, electronic communication equipment and various electrical equipment have been widely used in various industries as carriers of information technology, especially the miniaturization and high integration of electronic components and the lightweight, high-speed and digitalization of electronic instruments; electromagnetic signals are susceptible to external electromagnetic interference and malfunction, which brings serious consequences. Therefore, various effective protective measures must be taken to ensure that they are not interfered with and paralyzed. From the perspective of electromagnetic signal leakage, whether it is military secrets or commercial secrets, the leakage of electromagnetic waves will cause great losses to related units. Therefore, corresponding shielding measures must be taken to prevent electromagnetic signal leakage and detection to prevent loss of confidentiality; from the perspective of preventing electromagnetic wave pollution, the protection of various electromagnetic pollution hazards has attracted great attention from environmental protection departments and relevant parties. Shielding electromagnetic pollution to limit it to a certain area has become one of the most active research topics in the field of environmental protection.
Electromagnetic shielding coatings are prepared from synthetic resins, conductive fillers, and solvents. They are applied to the surface of the substrate to form a cured film, thereby producing a conductive shielding effect. The coating methods mainly include spraying, brushing, dipping, and roller coating. The biggest advantage of conductive coatings as electromagnetic shielding materials is low cost, simple and practical, and wide application. The most commonly used is silver-based conductive coatings, which are also one of the earliest developed varieties1.
Development status Commonly used electromagnetic shielding coatings are mainly made by composite methods. They are composed of resins, diluents, additives, and conductive fillers. Resins have adhesive properties, and commonly used resins include epoxy resins, polyurethanes, phenolic resins, polyimides, and acrylic acid. When using, it can be selected according to its curing conditions, temperature resistance, wear resistance, hardness, deflection and other requirements. Various resins can also be mixed to obtain comprehensive performance. Diluents and additives are used to reduce the viscosity of the resin and infiltrate the filler. Commonly used ones include methyl cellosolve, pine oil, ethylene glycol butyl ether acetate, etc. Diluents are generally not solvent-based to avoid bubbles and reduce conductivity and adhesion. Additives are used to improve the performance of conductive adhesives, such as dispersants that can fully disperse conductive fillers, and reinforcing agents that can increase adhesion.
Existing electromagnetic shielding coatings are mainly conductive coatings. The conductive fillers added to conductive coatings are generally metal powders such as gold, silver, copper, nickel, and non-metallic powders such as carbon black and graphite. Gold powder has the highest conductivity and good chemical stability, but it is expensive, so its use is limited. Silver powder also has excellent conductivity and is cheaper than gold powder. Although it is easy to precipitate after mixing with glue and has "migration" phenomenon, it is still more commonly used. The performance of copper and nickel is similar to that of silver, and their prices are much lower than that of silver, but they are easily oxidized, their conductivity is unstable, and the durability of the adhesive is poor. Carbon black and graphite powder are used as conductive fillers. They have good dispersibility and low price, but their conductivity is poor. They are ideal materials for electromagnetic shielding2.
Development and application As early as the 1960s, the US military used silver-based conductive coatings as electromagnetic shielding materials. Silver-based coatings have stable performance and excellent shielding effects (up to 65dB or more), but due to their high cost, they are only suitable for certain specific occasions; nickel-based coatings are moderately priced, have good shielding effects, and have stronger antioxidant capacity than copper, so they have become the mainstream of electromagnetic shielding coatings in Europe and the United States. When the coating thickness is 50 to 70 μm, the volume resistivity is 10-3 Ω·cm, and the shielding effect can reach 30 to 60dB (500 to 1000Hz)
