With the continuous improvement of the running speed of rail transit vehicles, not only the requirements for vehicle running stability and ride comfort are further improved, but also higher requirements are put forward for flame retardancy, environment protection, lightweight, vibration, and noise reduction. Therefore, more and more polymer materials are used in rail track systems. Some advanced polymer materials commonly used in rail transit are listed below.
Compared with general-purposed rubber, polyurethane has more excellent wear resistance and toughness, and is known as “wear-resistant rubber”. Due to the customizability of polyurethane molecules, by controlling the composition and structure of polyurethane, it can be developed in the direction of functionalization and high performance. At present, MDI polyurethane microporous elastic damping pads and polyurethane elastomer expansion joints have been successfully applied to China’s high-speed railways.
Polyurethane composite sleepers have the advantages of both wooden sleepers and concrete sleepers, and at the same time make up for the shortcoming mechanical properties, bending strength, and shear strength, but also can reduce vibration, reduce noise and improve driving comfort through elastic deformation, safety and protection of the track road, etc. At the same time, polyurethane sleepers also have insulation resistance and no need for insulation backing plates.
NDI-based polyurethane contains a naphthalene ring structure, and its molecular chain rigidity is stronger than that of a naphthalene ring. Macroscopically, it has higher compressibility, higher flexibility, better dynamic fatigue resistance, and the advantage of lighter weight.
Silicone foam material
Silicone refers to a compound containing Si-O bonds and at least one organic group directly connected to a silicon atom. Its structure contains both “organic group” and inorganic properties. Silicone materials can be used for a long time at -70—150℃ and have excellent electrical insulation properties, oxidation resistance stability, weather resistance, flame retardant, water repellency, corrosion resistance, non-toxic and tasteless, etc., especially silicone materials have characteristics of flame retardancy and non-toxicity make it widely used in rail transportation. At present, China’s technology is developing rapidly and gradually realizing localization.
Nylon has been widely used as the basic material of high-speed rail fasteners. In order to ensure the safe operation of high-speed railways in alpine regions, fasteners must meet higher requirements, especially low-temperature fatigue resistance. Therefore, higher requirements are put forward for nylon’s low-temperature toughness, processing fluidity, and wettability and coating of fillers such as glass fiber. Low temperature-resistant copolymerized nylon can combine high toughness and high fluidity, and has been applied in rail transit.
Aramid fiber is the abbreviation of aromatic polyamide fiber, which was invented by DuPont. It has the advantages of heat resistance, flame retardancy, lightweight, high strength, and good wear resistance. Meta-aramid products are widely used in high-end electrical insulation and lightweight honeycomb structures and have become important basic materials in the fields of electrical appliances and power transmission. High-quality aramid fiber is the key main insulation material in high-power motors, transformers, and electrical equipment in rail transit. It is used in areas such as wrapping insulation, slot insulation, and inter-turn insulation. Especially in the motors and transformers of high-speed EMU, the amount of insulation system is huge. The meta-aramid paper commercialized abroad mainly includes DuPont’s Nomex and Japan’s Teijin. The development of aramid fiber products such as laminated sheets, insulating paper, and aramid honeycomb paper have been applied in rail transit and other fields.
Composite materials mainly refer to fiber-reinforced composite materials and sandwich structure composite materials, which have the characteristics of lightweight, high strength, fatigue resistance, and corrosion resistance. At present, domestic rail transit composite materials are mainly used in non-load-bearing structures such as interior parts, among which the interior of the driver’s cabin, the driver’s console, head cover, seat surface of the passenger compartment, side wall panels, roof panels, door column covers and other structures are all used fiberglass composite. With the development of railways in the direction of high speed, safety, and lightweight, the role of polymer materials is becoming more and more important. The rail transit system will become an important field for the development of materials that have become the most promising application products.