What are the different categories of enameled wire?

　Enamelled wire is a key raw material for products such as motors, electrical appliances, and household appliances. Particularly in recent years, with the sustained rapid growth of the electric power industry and the swift development of household appliances, the application of enamelled wire has expanded into broader fields, thereby placing increasingly higher demands on this product. As a result, it has become inevitable to adjust the product structure of enamelled wire. Consequently, the associated raw materials (copper and enamel), enamelling processes, process equipment, and testing methods all urgently require further research and development. There are quite a few categories involved; details are provided below:

　　1) Acetal enameled wire

　　Acetal enameled wire is one of the earliest varieties developed in the world. As early as 1930, it was introduced to the market by Germany and the United States respectively, and the Soviet Union also saw rapid development in this field. Two types were produced there: polyvinyl alcohol acetal and polyvinyl alcohol acetaldehyde. In China, research into this type of wire was successfully completed in the 1960s. Although this enameled wire has a relatively low temperature rating (105°C and 120°C), it is widely used in oil-immersed transformers due to its excellent resistance to high-temperature hydrolysis. This characteristic has been officially recognized worldwide. Currently, China still produces small quantities of this wire, particularly acetal enameled flat wire, which is used to manufacture transposition conductors for large-scale transformers.

　　2) Polyester enameled wire

　　In the mid-1950s, West Germany was the first to successfully develop polyester enamels for magnet wire based on dimethyl terephthalate. Thanks to their excellent heat resistance and mechanical strength, wide processing flexibility, and low cost, these enamels became the dominant product in the magnet wire market starting from the 1950s. However, due to their poor resistance to thermal shock and susceptibility to hydrolysis under high-temperature and high-humidity conditions, by the late 1970s, polyester enamels used as single-layer coatings had already been discontinued in production in West Germany and the United States. In contrast, they continued to be widely produced and used in Japan, China, and Southeast Asia. According to statistics from 1986, polyester enamels accounted for 96.4% of China's total magnet wire enamel production. After ten years of effort, the variety of magnet wire enamels has expanded, yet the gap remains substantial compared to that of developed countries [1].

　　Considerable efforts have also been made in domestic polyester modification. Both THEIC modification and imide modification have been studied and developed. However, due to the relatively slow pace of adjustments in the structure of enameled wire, the production volumes of these two types of enamel coatings remain modest at present. To date, the issue of voltage drop in modified polyester enameled wires still requires widespread attention.

　　3) Polyurethane enameled wire

　　Polyurethane enameled wire enamel was developed and introduced by Bayer of Germany in 1937. Due to its direct solderability, high-frequency resistance, and colorability, it is widely used in fields such as electronics and electrical appliances. Currently, overseas research is focusing on enhancing the heat resistance rating of polyurethane enameled wire without compromising its direct solderability. In Europe, the United States, and Japan, polyurethane enameled wires of Class F and Class H have been successively developed. Thanks to the rapid development of color televisions, a long-length polyurethane enameled wire with no salt-water pinholes, developed by Japan for use in color TV flyback transformers (FBTs), has attracted worldwide attention and remains a key product from Japan [1].

　　The development of polyurethane enameled wire in China has been slow. Although some manufacturers produce conventional polyurethane enamel, due to issues such as poor processability and subpar surface quality, most of the enamel still relies on imports. Class F polyurethane enamel has also been developed domestically, but production capacity has yet to be established. A high-quality, large-length polyurethane enamel free of pinholes has now been successfully developed and is being marketed, primarily for use in manufacturing FBT coils for black-and-white televisions.

　　4) Polyester-imide enameled wire

　　Thanks to the improved heat resistance achieved by modifying polyester with imide, the global demand for polyester-imide enameled wire has surged since the 1970s. In Europe and the U.S., this type of enameled wire has already replaced single-coat polyester enameled wire. Currently, the most representative brands worldwide are Germany’s TerebeFH series and the U.S.’s Isomid series. Meanwhile, straight-solderable polyester-imide enameled wires have been developed one after another and are now widely used as windings in small motors, simplifying the soldering process and reducing motor manufacturing costs. In Japan, some manufacturers also use straight-solderable polyester-imide enamel as a primer for self-adhesive enameled wire used in color television deflection coils, further streamlining the manufacturing process. Domestically, manufacturing technologies for polyester-imide enamel were initially introduced from Germany and Italy, and some Chinese companies have since successfully developed their own versions. However, due to factors such as the instability of raw materials, China still largely relies on imports for polyester-imide enamel used as a primer for refrigerant-resistant composite enameled wires. Only a small quantity of single-coat polyester-imide enameled wire made with domestically produced enamel is currently in use; yet voltage instability remains a major concern for manufacturers. The straight-solderable polyester-imide enamel has already been successfully developed by the Cable Research Institute.

　　5) Polyimide enameled wire

　　Polyimide is currently the highest-class heat-resistant varnish used in organic enameled wires, with a long-term operating temperature exceeding 220°C. This varnish was successfully developed in the United States in 1958. Polyimide enameled wires exhibit high thermal resistance as well as excellent resistance to solvents and refrigerants. However, due to its high cost, poor storage stability, and toxicity, its widespread use has been limited. At present, these enameled wires are primarily used in specialized applications, such as wiring for coal mine motors and instruments used in aerospace equipment.

　　6) Polyamide-imide varnish

　　Polyamide-imide enamel boasts high thermal resistance, excellent mechanical properties, outstanding resistance to refrigerants, and superior chemical resistance, earning it the well-deserved reputation of “enamel for magnet wire.” Currently, this enamel is primarily utilized for its exceptional performance characteristics. It is widely employed as the topcoat in composite-coated magnet wires, enhancing their thermal resistance while reducing costs. At present, domestically produced magnet wires coated with this enamel are mainly used for applications requiring resistance to refrigerants. Although domestic production of this enamel remains limited, it is primarily imported from the United States, Italy, and Germany.

　　7) Composite-coated enameled wire

　　To enhance temperature resistance and develop special-purpose enameled wires, composite insulation layers are generally employed. Compared with single-layer enameled wires, composite-coated enameled wires exhibit the following advantages: (1) They can meet specific application requirements—for example, self-adhesive enameled wires used for complex, frameless shaping, as well as enameled wires resistant to refrigerants for refrigerator and air-conditioner compressors—can be achieved through a composite coating structure; (2) By combining various insulation layers, their respective strengths are leveraged while compensating for weaknesses, thereby improving and enhancing performance to meet application demands—for instance, polyester/nylon composite-coated enameled wires offer superior thermal shock resistance and winding performance, making them suitable for hot-dip impregnation processes and ideal for motor windings subjected to transient overheating caused by overload; (3) They can reduce the cost of certain enameled wires—for example, polyester-imide or polyamide-imide composite-coated enameled wires can replace single-layer polyamide-imide enameled wires, resulting in significant cost savings.