Aramid fiber is the abbreviation of aromatic polyamide fiber. There are two main types: one is polyparaphenylene terephthalamide (PPDA) fiber, such as Kevlar-49 from DuPont in the United States, TwaronHM from Enka from the Netherlands, China’s aramid 1414, etc.; another type is polyparabenzamide (PBA) fiber, such as Kevlar-29, aramid 14, etc. Kevlar-49 is an organic fiber successfully developed by DuPont in the late 1960s and commercialized in the 1970s. 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 its application range, the application field will continue to be promoted.
The mechanical properties of aramid fibers are different from other organic fibers, with high tensile strength and initial modulus, but low elongation. Among organic fibers, the mechanical properties of aramid fibers are excellent. The molecular chain of aramid is composed of benzene rings and amide groups arranged according to certain rules. The positions of the amide groups are all in the straight position of the benzene ring, so the polymer has good regularity, resulting in a high degree of crystallinity of the aramid fiber. This rigid aggregated molecular chain is highly oriented in the fiber axis. The hydrogen atoms on the molecular chain will combine with the carbonyl groups of the amide pairs on other molecular chains to form a hydrogen bond, which becomes a lateral connection between the 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. Furthermore, when Kevlar-49 and Aramid 1414 unidirectional composites were tested in tension, the obtained stress-strain curves before fracture were straight lines, but when tested in compression, they were elastic at low stress and elastic at high stress Plasticity, the unique compressive properties of Kevlar-49 and Aramid 1414 composites are very similar to the toughness of metals, and have certain application significance under specific conditions.
Aramid fibers and other organic fibers, like glass fibers, are easily woven into various 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. Short fiber reinforced thermoplastic composites, mainly due to the pull out of the short fibers from the matrix material. When the fiber content is relatively low, a ductile matrix can be made into a tough composite. When the fiber content increases, the toughness of the composite material increases. According to reports, when the matrix material contains 20% aramid fiber, the mechanical properties of the composite material can be significantly improved.
Aramid composites have poor compressive properties, about half that of glass fiber composites. Adding another fiber to make a hybrid composite can significantly improve its compressive properties. Since the thermal expansion coefficients of aramid fiber and carbon fiber are very close, these two fibers are especially suitable for mixed use in different proportions. However, the composite materials mixed with aramid and graphite can overcome the main disadvantages of the high price of graphite composite materials and the sudden fracture due to poor toughness. The mixed use of aramid and glass fiber can overcome the disadvantage of poor rigidity of glass fiber composites. When encountering special purposes, 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 fiber with carbon, boron and other high modulus fibers can obtain the compressive strength required in the application structure, and its unique performance is unmatched by other fiber reinforcement materials. For example, a hybrid material composed of 50% aramid fiber and 50% high-strength carbon fiber and epoxy resin has a flexural strength of over 620MPa. The impact strength of the hybrid composite material is about twice that of the high-strength carbon fiber used alone. If the high-modulus graphite fiber is used in combination, the impact strength will be greatly improved.
