Khai Trung Le looks at failures of various ballistic body armour, resulting in Kevlar maintaining its position as the king of personal protection.
In June 2003, Californian police officer Tony Zeppetella was shot 13 times and killed during a traffic stop – a death some claim was preventable but for the failure of the Zylon ballistic body armour he was wearing. This event sparked a series of longstanding disputes surrounding the Zylon armour, developed by Second Chance Body Armor, USA, as a number of police departments across California, Massachusetts, Georgia, Illinois and Connecticut also began legal proceedings against Second Chance, citing further instances of failure-related injuries.
The company declared bankruptcy in 2004, before being acquired by Armor Holdings, USA, in 2005. But the reasons behind the failure of Zylon, created with polybenzoxazole (PBO) fibre – considered one of the world’s strongest synthetic fibres – have been difficult to ascertain until recently, following work by the National Institute of Standards and Technology (NIST), USA.
Solutions in the subatomic
The question of whether PBO failed is undeniable. Christopher Soles, Group Leader of the Functional Polymers Group at NIST and co-author of the paper, Nanostructural evidence of mechanical aging and performance loss in ballistic fibers, published in the Journal of Polymer Science, told Materials World, ‘PBO wouldn’t withstand flexing very well. It would have really good properties out of the box, much higher than Kevlar, but after a few rounds of cycling it would quickly start to lose these properties.’
Gale Holmes, Materials Research Engineer at the NIST Material Science and Engineering Division, added, ‘With repeated folding around a 6.35cm stainless steel rod around 5,000 times, we began to see surface changes in the PBO that were not seen in Kevlar. In the field, the PBO fibres started degrading within six months. We actually performed one severe, acute bend on the PBO and saw a 10% loss in mechanical properties.’
But Soles noted that, looking for chemical signs of degradation, there were no visible changes in the fibres. ‘It wasn’t clear what was breaking down, what the damage was or how it was occurring,’ he added. Nor did the NIST team have an analytical technique to characterise the structural or chemical differences that would account for performance loss. However, the use of positron annihilation spectroscopy helped provide a molecular-level view of the structure of PBO. Positrons are inserted into the fibres, whereby they collide and dislodge electrons to form neutral positronium. Soles explained, ‘Because the particle is neutral, it wants to minimise its interaction with the material, so it searches low-density regions to localise, such as the chains in polymers. You have covalently bonded linear chains and these are pretty stiff chains in ballistic fibre materials, and the positronium will seek out the regions of frustrated packing between the chains.’
The team was surprised to see how sensitive the positrons were to voids – molecular packaging inefficiencies as small as five nanometres – created between the chains from bending. ‘That’s what we found after bending, damage being accumulated at these length scales,’ said Soles.
Holmes said that this result ‘turned out to be a really surprising piece of research. But it’s a nice way of showing the relationship between molecular structure and durability.’ Previously, there was no secure means of determining why some materials broke during folding tests and others did not. Soles added, ‘This is the first materials characterisation tool that gives insight into why some materials can be folded and still maintain their strength.’
Kevlar is king
Since the proliferation of firearms, mankind has experimented with materials as diverse as silk and ballistic nylon in the hopes of withstanding the impact forces produced by these weapons. But the DuPont ballistic fibre, developed by Stephanie Kwolek, known as Kevlar has remained the standard for personal body armour for more than three decades.
Ballistic body armour needs three properties, according to Soles. ‘You want a very high tensile strength fibre, to stop the bullet. You want a stiff, high modulus fibre to help restrain the bullet as it’s flying into the system. And, finally, you want a high strain-to-failure, because that’s how much energy a system can absorb before rupture. Brittle materials, despite being high strength, fail.’
Much of the suitability of Kevlar comes from strong hydrogen bonding between well-packed chains with an absence of voids, which support mechanical transfer of loads. While PBO has a higher initial tensile strength than Kevlar, it quickly degrades and the void issue eliminates its suitability as a ballistic body armour material. According to Holmes, ‘There is no fundamental solution to the problem because it’s inherent in the structure. There is very low hydrogen bonding and interaction between chains.’
Regardless of Second Chance’s high-profile failure, the ballistic body armour market continues to escalate, and according to Grand View Research, USA, is expected to reach US$5.6bln by 2024 as companies work on advancing the bulletproof vest. These range from the US Army Research Laboratory’s work on the hyperplastic behaviour of polyurethane urea elastomers to Rice University, USA, exploring the use of graphene.
Chris Hare, Research and Technology Manager at Coventive Composites (for more on his talk at Advanced Engineering 2017 click here), told Materials World, ‘Design in ballistic body armour hasn’t developed in decades. It’s still essentially a plate in a vest. Perhaps it’s because the industry is still pretty fragmented, so there is not a cohesive path towards solutions.’
The company is a partner in LightArmour, a dual-use technology exploitation project, exploring armour based on self-reinforced polymers (SRP), a thermoplastic composite material in which both the reinforcing fibre and polymer matrix belong to the same polymer family. This helps create lightweight impact-resistant materials that are affordable and quickly processed through compression moulding, and Coventive Composites aim to pair SRPs with complementary materials including ceramics and ultra-high molecular weight polyethylene.
The affordability and speed of work with SRPs means Coventive Composites hope to bring military-grade technology to emergency service first responders. ‘In this period of perceived heightened danger, first responders often face the same kinds of threats [as soldiers]. But, because they’re not military-trained, ballistic body armour needs to be more comfortable and to not restrict things like driving or even sitting down.’
Much of this approach is fed into the team’s drive to look beyond material choice, with heightened focus on design. Hare said, ‘Looking at design means going back to fundamentals. Comfort, weight, maneuverability and flexibility, among others, are things we’re thinking about.’
But in the meantime, if you’re expecting to face down firearms in the near future, Kevlar remains king.
To read Nanostructural evidence of mechanical aging and performance loss in ballistic fibers, visit bit.ly/2hxxdF