Aramid fiber is the abbreviation of aromatic polyamide fiber, there are two main categories: one is polyterephthalyl-p-phenylenediamine (PPDA) fiber, such as Kevlar-49 of DuPont of the United States, TwaronHM of Enka Company of the Netherlands, aramid 1414 of China, etc.; The other type is polyparabenamide (PBA) fibers, such as Kevlar-29, aramid 14, etc. Kevlar-49 is an organic fiber developed by DuPont in the late 60s of the 20th century and commercialized in the 70s of the 20th century. This is a new type of material with excellent properties such as high strength, high modulus, high temperature resistance and low density. Kevlar-49 fiber is mainly used in composite parts such as aviation, aerospace, shipbuilding, medical equipment and sporting goods. Due to its excellent performance and the particularity of the application range, the application field will continue to be promoted.
The mechanical properties of aramid fibers differ from other organic fibers in that their tensile strength and initial modulus are high, while their elongation is low. Aramid fiber has excellent mechanical properties in organic fibers. The molecular chain of aramid is composed of benzene rings and amide groups arranged in a certain law. The position of the amide group is in the straight position of the benzene ring, so this polymer has good regularity, resulting in a high degree of crystallinity of aramid fibers. This rigid agglomerated molecular chain is highly oriented in the fiber axial direction, and the hydrogen atoms on the molecular chain will combine with the carbonyl groups of the amide pair on the other molecular chain to form hydrogen bonds, becoming a transverse link between polymer molecules.
It can also be seen that Kevlar-49 and aramid 1414 composites have significant advantages over glass fiber reinforced composites in terms of density and strength. In addition, when Kevlar-49 and aramid 1414 unidirectional composites are tested under tension, the stress-strain curve obtained before fracture is a straight line, but in the compression test, they are elastic under low stress and plastic under high stress, Kevlar-49 and aramid 1414 composite materials This unique compression property is very similar to the toughness of metals and has certain application significance under specific conditions.
Aramid fibers and other organic fibers, like glass fibers, are easily woven into a variety of fabrics. The use of these fabrics brings great convenience to the composite molding process, and aramid staple fibers are mainly used to strengthen thermoplastic composites to improve the breaking strength of thermoplastic composites. Staple fiber reinforced thermoplastic composites, mainly due to the pulling of the staple fibers from the matrix material. When the fiber content is quite small, the ductile matrix can be made into a ductile composite. As the fiber content increases, the toughness of the composite increases. According to data reports, when the matrix material contains 20% aramid fiber, it can significantly improve the mechanical properties of the composite.
Aramid composites have poor compressive properties, about half that of glass fiber composites. If another fiber is added to make a hybrid composite, its compressive resistance can be significantly improved. Because the coefficient of thermal expansion of aramid fibers and carbon fibers is very close, these two fibers are particularly suitable for mixed use in different proportions. The composite materials mixed with aramid and graphite can overcome the main drawbacks of the high price of graphite composites and the sudden fracture due to poor toughness. Aramid is mixed with glass fiber, which can overcome the shortcomings of poor rigidity of glass fiber composites. When encountering special uses, there are many ways to mix and use composite materials, which can be reasonably matched according to the requirements of use.
In addition, the mixing of aramid and carbon, boron and other high-modulus fibers can obtain the compressive strength required for the application structure, and its unique properties are incomparable with other fiber reinforcement materials. For example, a hybrid material consisting of 50% aramid fiber and 50% high-strength carbon fiber with epoxy resin has a bending strength of more than 620MPa. The impact strength of the composite material after mixing is about twice that of high-strength carbon fiber alone, and if high-modulus graphite fiber is mixed, the impact strength will be greatly improved.