Kevlar. This word has been heard at least once by anyone who is more or less interested in science. A beautiful material used to build body armor, motorcycle suits, tennis rackets, kayaks and even aircraft carriers. What’s so special about it? Can things get even better in the future?
Kevlar is a polymer of the polyamide group that was invented in 1965 in DuPont labs by a team of researchers led by Stephanie Kwolek. It is a material strong enough to stop spheres or blades, often described as “five times stronger than steel”. Although not an ordinary fabric, it is difficult to imagine the modern world without Kevlar (trade name).
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What is Kevlar?
Kevlar is a super durable plastic. It is formed during the polymerization process (joining molecules with long chains) and has amazing properties – they are due partly to its internal structure and partly to the method of producing fibers that are closely intertwined.
Kevlar is not a material like cotton – not everyone can produce it with the right raw materials. It is a patented material manufactured exclusively by DuPont chemists. It comes in two variants: Kevlar 29 and Kevlar 49 (other variants are available by special request). Chemically, Kevlar is similar to another protective material known as Nomex. Kevlar and Nomex are examples of synthetic aromatic polyamides or aramids. But what does that actually mean?
Synthetic materials are materials that are created under laboratory conditions – they are not produced in nature (like cotton or wool). The term “aromatic” means that Kevlar molecules have a ring structure similar to that of benzene. The term “polyamide”, on the other hand, means that the ring-shaped particles stick together to form long chains – they are arranged parallel to the Kevlar fibers, somewhat like rebar. Kevlar is a polymer because it is made of many identical molecules (monomers). Like Nomex, Kevlar is a distant “relationship” to nylon, the first commercially available superpolyamide, also developed by DuPont chemists in the 1930s.
Kevlar was originally developed as a lightweight convertible steel rim for car wheels, but today it is used in very different circumstances.
Kevlar Strengths and Weaknesses
The properties of Kevlar are unique in many ways. It is a strong material, but relatively light. The tensile strength of both Kevlar 29 and Kevlar 49 is more than eight times that of steel wire. Unlike most plastics, Kevlar does not melt quickly – it can withstand high temperatures and only decomposes about 450aboutC. The material is flammable but combustion usually stops when the heat source is removed.
Kevlar is also impervious to extremely low temperatures. DuPont was not “brittle or chipped” at temperatures down to -196aboutC. However, it is susceptible to prolonged exposure to UV rays (eg sunlight), discoloration and some (but not complete) degradation of the fibers.
Kevlar is resistant to many chemicals, but even Kevlar will not survive prolonged exposure to strong acids or bases. Tests by DuPont engineers showed no change in Kevlar when exposed to hot water for more than 200 days.
However, it is not a perfect material. The highest is the very low compressive strength (despite high tensile strength). For this reason, Kevlar is not used in place of steel in architecture, such as building foundations, bridges, etc. – wherever compressive forces are common.
Applications of Kevlar
Kevlar can be used alone or as part of a composite material (one material combined with another) for added strength. Kevlar may be best known for its use in body armor and knife armor, but it has dozens of other uses as well. It is used to strengthen tires, brakes and bodies, boats and even aircraft carriers.
Kevlar is also used to increase the strength of fiber optic cables. This material was also used in the production of loudspeaker membranes, as well as the sails of racing yachts. It is also used in aerospace to reinforce composites with a polymer matrix.
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Xuetong Zhang’s team of the Chinese Academy of Sciences in Suzhou has developed a simple method for making Kevlar airgel fibers. This is a very important achievement, because aerogels are materials that are almost completely filled with air and exhibit poor mechanical strength. This can now be changed. Chinese researchers first made a suspension of Kevlar nanofibers by mixing a few grams of the material in dimethyl sulfoxide. They then formed gel fibers by extruding the slurry into a coagulation tub with a pump-driven syringe. Finally, the product was treated with alcohol and dried with a lyophilisate to remove the solvent. When frozen, the material expanded into a spongy matrix that transformed the Kevlar fibers into aerogels. Subsequently, airgel textiles were woven from it. Compared with various organic and inorganic aerogels, Kevlar airgel has similar surface area and same thermal conductivity, but greater mechanical strength. It also exhibits good thermal and fire resistance and can be painted and made hydrophobic.