Aramid fiber
Aramid fiber has high strength, high modulus, high temperature resistance, acid and alkali resistance, light weight and good insulation and aging resistance. Its strength is 5 to 6 times that of steel wire, its modulus is 2 to 3 times that of steel wire or glass fiber, its toughness is 2 times that of steel wire, and its weight is only about 1/5 of steel wire. It is often used as The preparation of reinforcements for high-performance composite materials occupies an important position in high-performance fibers.
Aramid fibers mainly include fully aromatic polyamide fibers and heterocyclic aromatic polyamide fibers, which can be divided into ortho-aramid, para-aramid (PPTA) and meta-aramid. There are three types of polyamide (PMTA), among which the products that have been industrialized mainly include meta-aramid and para-aramid. The main difference between these two types of aramid lies in the different connection positions of the amide bond and the C atom on the benzene ring.
Para-aramid, that is, polyparaphenylene terephthalamide (PPTA) fiber, In China, it is called aramid 1414, a linear polymer in which more than 85% of the amide bonds in the molecular chain are directly bonded to the aromatic ring. Para-aramid fibers mainly include Kevlar fiber from DuPont of the United States, Twaron and Technora fibers from Teijin Company of Japan, and Taparan fiber from Yantai Taihe New Materials. It is the world’s first fiber made from polymer liquid crystal spinning solution, ushering in a new era of high-performance synthetic fibers. Among the existing high-performance fibers, para-aramid is one of the fibers with the best comprehensive performance. Its most prominent features are high strength, high modulus and high temperature resistance. Taking Kevlar fiber as an example, its strength is 3 times that of steel wire and 4 times that of polyester industrial yarn. Its initial modulus is 4 to 10 times that of polyester industrial yarn and 10 times that of nylon. The mechanical properties of several para-aramid fibers are shown in Table 2.
Para-aramid has a high glass transition temperature (Tg) and thermal decomposition temperature (Table 3), and has good thermal stability. It can still maintain high strength under high temperature; the fiber has a high limiting oxygen index (LOI) and good flame retardancy. It was experimentally measured that the shrinkage rate of para-aramid fiber at 150℃ is 0, and it can still maintain 65% of its original strength at a high temperature of 260℃. When exposed to an environment of 300℃ for a short period of time, the mechanical properties of the fiber are almost unaffected. Embrittlement and degradation will not occur at low temperatures.
Although para-aramid has a variety of excellent properties, there are also some shortcomings. Due to the weak intermolecular force of para-aramid, the fiber surface lacks active groups, resulting in low compressive strength and compressive modulus, and low bonding strength at the fiber-resin interface. There are polar groups such as amide groups in the molecular structure of para-aramid fiber, and the fiber has poor hygroscopicity. After moisture absorption, the intrusion of water molecules destroys the hydrogen bonding between molecules, causing the fiber strength to decrease, and the bending and compression properties of the composite material to decrease. All decreased. In addition, para-aramid is not resistant to strong acids and alkalis, and has poor light resistance and UV resistance.
Meta-aramid, that is, polym-phenylene isophthalamide (PMTA) fiber, also known as aramid 1313 in my country, was first developed by Nomex by DuPont in the United States, and in 1967 Industrialized production was achieved in 2011, including Japan’s Teijin’s Conex, Yantai Taihe New Material’s Tametar, etc. Meta-aramid is the fastest growing seed crystal among organic high-temperature resistant fibers. The fiber molecules are composed of amide groups interconnected with meta-phenyl groups. The molecular chain is linearly zigzag. Due to the strong hydrogen bonding between molecules, the fiber It has excellent flame retardancy, thermal stability, radiation resistance and other properties.
Meta-aramid is a flame-retardant fiber that will not spontaneously ignite or melt in the air and will self-extinguish after leaving the flame. When the fiber encounters high temperatures, it will rapidly expand and carbonize, forming a heat insulation layer, which can block the transfer of heat and play an effective protective role. After continuous long-term use at high temperatures above 250°C, it still has high strength and dimensional stability. No obvious decomposition and carbonization will occur below 350°C. When the temperature exceeds 400°C, the fiber gradually becomes brittle and carbonized until it decomposes, but no melt droplets are produced. Meta-aramid has a very low dielectric constant and can maintain excellent electrical insulation under low temperature, high temperature, and high humidity conditions. It is recognized globally as the best insulation material. Meta-aramid fabric has excellent radiation resistance. Experiments show that it can still maintain 49% of its original strength after being continuously irradiated with 50kV X-rays for 250 hours. Meta-aramid has good stability in acids, alkalis, organic solvents, reducing agents and bleaching agents, but it cannot interact with strong acids and alkalis for a long time. Meta-aramid has poor stability to sunlight and is difficult to dye.
