Improving the properties of Kevlar reinforced composites

Could extra-long carbon nanotubes give us stronger Kevlar bullet-proof vests?

By National Photo Company [Public domain], via Wikimedia Commons

Discovered by DuPont™ chemist Stephanie Kwolek in 1964, Kevlar® is one of the world’s most famous fibres. Found in everything from parachutes and skis, to tyres and bullet-proof vests, it combines low weight with high-strength, toughness and thermal stability. However, when used in high-performance structural composites, it can suffer from failure, due to poor adhesion between the fibres and the surrounding matrix. Modifying the surface of the fibre to improve adhesion usually requires multiple processing steps, but researchers from India’s CSIR-National Physical Laboratory claim that they’ve developed stronger Kevlar composites, simply by adding extra-long carbon nanotubes.

Published in Carbon [DOI: 10.1016/j.carbon.2018.05.017], their study explores the use of multi-wall carbon nanotubes (MWCNTs) as a secondary reinforcement in Kevlar-epoxy composite materials. The team measured properties such as the maximum tensile strength and Young’s modulus of a series of material tapes – pristine Kevlar, Kevlar-epoxy composite, and five Kevlar-MWCNT-epoxy composites, which each contained a different weight-percentage (wt %) of nanotubes. These nanotubes were synthesised in-house, with an average length of 450 µm.
In tensile tests, they found that the Kevlar fabric gradually failed through fibre slippage in the direction of the applied load. In contrast, when used in a composite, with or without nanotubes, Kevlar’s failure was found to focus on a single point. The authors argue that the addition of a resin restricts the relative motion of the fibres, causing a catastrophic point-failure that depends on the mechanical properties of the epoxy resin, rather than the fibre.

The addition of nanotubes to the composite was shown to improve the mechanical properties in all cases, but the best-performing composite contained 0.3 wt % MWCNTs – its maximum tensile strength was ~81% higher than the Kevlar-epoxy composite. To investigate the impact of the nanotube length on the composite’s properties, they made identical tape samples with commercial MWCNTs that were 300 times shorter than those produced at CSIR-NPL. In terms of both Young’s modulus and storage modulus, the long-length nanotube composites outperformed those made with shorter MWCNTs.

The authors also measured the mechanical properties of laminar composites that were produced by stacking the tapes (Kevlar-epoxy and Kevlar-0.3%MWCNT-epoxy), and compressing them under vacuum, at high-pressures and temperatures. The load carrying capacity of the nanotube composite was found to be significantly higher than that of the Kevlar-epoxy, and flexural modulus, Young’s modulus and storage modulus were improved by ~33%, ~50%, and ~233%, respectively.
The authors conclude that “long length MWCNTs helped in load sharing”, by improving the matrix stiffness through a “bridging effect”.


This news story originally appeared on the Materials Today website: https://www.materialstoday.com/carbon/news/extra-strong-kevlar-composites/

Research paper ($): Sushant Sharma, Abhishek K. Pathak, Vidya Nand Singh, Satish Teotia, S.R. Dhakate, B.P. Singh. “Excellent mechanical properties of long length multiwalled carbon nanotube bridged Kevlar fabric” Carbon 137 (2018) 104-117. DOI: 10.1016/j.carbon.2018.05.017