Last edited 03 Jul 2020

Kevlar

[edit] Introduction

Kevlar is a strong, heat-resistant nylon-like polymer invented in 1963 by Stephanie Kwolek, a US-based scientist, as part of her work for chemical giant DuPont. Kwolek had been researching strong, synthetic textile fibres since 1950, particularly their manufacture from long chains of molecules for use in light yet strong tyres.

Kevlar (poly-para-phenylene terephthalamide) is a low weight, high strength material that is stable at high temperature and whose fibres are claimed to be five times stronger than steel per unit weight (in tension). It is used for heat- or flame-resistant fabrics, radial and bicycle tyres, bulletproof vests, walking boots and fibre-reinforced composite materials for aircraft panels, boat hulls and golf-club shafts. It has also been used as a substrate for membranes used in fabric structures.

Like many innovations, Kevlar was discovered almost by accident. Kwolek was looking to create a better way of joining monomers (simple molecules) to polymers (longer chains) through low-temperature fusion. Instead of the clear solution she thought she would achieve, the result was an opaque liquid of low viscosity. When subsequently spun into fibres (ropes or fabric sheets) the liquid created a new type of synthetic material which, unlike nylon, did not break easily. In the patent application, Kwolek described it as a ‘highly orientable, crystallisable, filament-forming polyamide’.

Typically, Kevlar spun-fibres have a tensile strength of around 3,620mPA due to the many inter-chain bonds. Its relatively rigid molecules form mostly planar sheet-like structures which keep their strength and resilience down to around -196°C. At higher temperatures, the strength is slightly reduced.

In 1971, Kevlar was marketed commercially and first used as a steel substitute in racing tyres. More recent uses have included armoured walls for panic rooms, marine current turbines and wind turbines, smartphones, expansion joints and hoses.

In building construction, Kevlar can be formed into a sheet material for roofing, cladding and reinforcing. It was used unsuccessfully on the roof of the Olympic Stadium in Montreal which only lasted 10 years before it had to be renewed. It can be used to protect bank counters and reinforce seismic shear walls. However, it can be problematic to install and when used structurally, the structural calculations can be difficult. It is also very expensive.

Three types of Kevlar and their uses:

Kevlar – for car- and bicycle-tyre reinforcement to help resist punctures.
Kevlar 29 – armour for lightweight military vehicles and protection of personnel inside them.
Kevlar 49 – used in aerospace and marine applications. In boat hulls, it can withstand tensile, torque and twisting forces and is stronger than fibreglass.