Can carbon nanotubes give us stronger Kevlar bulletproof vests?
Kevlar®, discovered by DuPont ™ chemist Stephanie Kwolek in 1964, is one of the most famous fibers in the world. From parachutes and snowboards to tires and bulletproof vests, it has low weight, high strength, toughness and thermal stability. However, when it is used in high-performance structural composites, it may suffer damage due to poor adhesion between the fibers and the surrounding matrix. Changing the fiber surface to improve adhesion usually requires multiple processing steps, but researchers from India's CSIR-National Physics Laboratory claim that they have developed stronger Kevlar composites by adding ultra-long carbon nanotubes .
Their research, published in the journal Carbon, explores the secondary strengthening of multi-walled carbon nanotubes (MWCNTs) as Kevlar epoxy composites. The team measured properties such as maximum tensile strength and Young's modulus for a range of material strips, including original Kevlar, Kevlar epoxy composites, and five Kevlar-MWCNT-epoxy composites, each with different weight percentage (Wt%) nanotubes. These nanotubes are synthesized inside and have an average length of 450 μm.
In the tensile test, they found that the fiber slip of the Kevlar fabric in the direction of the applied load gradually failed. In contrast, when using or not using nanotubes in composites, Kevlar's failure focused on one point. The authors believe that the addition of resin will limit the relative movement of the fibers, leading to catastrophic point failure, which depends on the mechanical properties of the epoxy resin, not the mechanical properties of the fiber.
Adding nanotubes to the composite can improve its mechanical properties in all cases, but the best performing composite contains 0.3 wt% MWCNTs, and its maximum tensile strength is about 81% higher than that of Kevlar-epoxy composites. To study the effect of nanotube length on composite properties, they made the same tape samples as commercial MWCNTs, which were 300 times shorter than samples produced by CSIR-NPL. In terms of Young's modulus and storage modulus, the performance of long nanotube composites is better than that of nanotube composites made with shorter MWCNTs.
The authors also measured the mechanical properties of layered composites, which were formed by stacking tapes (Kevlar-epoxy and Kevlar-0.3% MWCNT-epoxy) and compressing them under vacuum, high pressure and high temperature. The results show that the bearing capacity of nanotube composites is significantly higher than that of Kevlar epoxy resin, and the flexural modulus, Young's modulus, and storage modulus are increased by ~ 33%, ~ 50%, and ~ 233%, respectively.
The authors conclude that "long MWCNTs contribute to load distribution" and that matrix stiffness can be improved through the "bridge effect".