A Nanoparticle Film to Stop a Speeding Bullet

Bullet-proofing materials such as glass, body armour, and military hardware is about to enter a new phase thanks to developments in nanotechnology.

The breakthrough was made by researchers from the National Institute of Standards and Technology (NIST) and Columbia Engineering who discovered a way to toughen traditional raw materials with the application of silica nanoparticles covered in polymer strands.

As nanomaterials have an incredibly small size they have an exceptionally large surface area in relation to their weight. This makes them a highly efficient industrial ingredient, requiring an extremely low input quantity to improve a material’s properties, such as impact resistance.

While the ability of nanomaterials to increase tensile, compressive, shear, torisonal, and yield strength has long been known, the latest research has shown the capacity to dissipate energy away from the impact site – a much sought-after property.

Current technology for ballistic armour involves combining polymers with other materials which can prove problematic.

“Mixing together plastics with some solid particles is like trying to mix oil and water. They want to separate,” explains Prof. Sanat Kumar, the study’s co-author. “The realization we’ve made in my group is: One way to fix that is to chemically tether the plastics to the particles. It’s like they hate each other but they can’t get away.”

Tying polymers to other materials at the nanoscale is a completely innovative approach to more traditional body armour design.

“The polymers that constitute most of the high-impact plastics today consist of linear chains of repeating synthetic molecules that either physically intertwine or form chemical bonds with each other, forming a highly entangled network,” explains the online journal Science Daily. “The same principle applies to most polymer composites, which are often strengthened or toughened by having some nonpolymer material mixed in.”

However, these new protective films provide a far better barrier against a bullet or other high-speed projectile. A technology which could be applied to body armour or even space craft and satellites which are under constant bombardment from high-speed debris.

They are made of tiny glass spheres, each measuring less than 1/10,000th the width of a human red blood cell, which are connected with polymer chains called polymethacrylate (PMA). When combined, the silica balls and polymer chains form a barrier of polymer-grafted nanoparticles (PGNs).

The researchers found that if the polymer chains were too short, they were limited in movement by neighbouring chains and so did not interact with each other. If they were too long, then they would twist together in a tangled mass.

However, at a sweet spot in length the researchers could create chains with a “intermediate molecular mass” where the chains would connect to form a network yet would still leave enough space for gases to pass through (the research team’s first goal). But the real surprise came when they began to examine what effect PGNs had on toughness.

“More tangles translate to greater toughness, up to the point where the material is completely tied up,” the report states. Further noting that, “… the films could defy traditional polymer behavior. The toughest samples had chains far shorter than the length for full entanglement, meaning that tangles were not the only factor driving toughness.”

Most significantly, testing proved that PGN composite films were generally tougher than films composed only of PMA materials. With those made from chains of ‘intermediate molecular mass’ length yielding the toughest film. Better still, was that as the polymer chains were neither inactive nor completely entwined, they had ‘wriggle room’ to dissipate energy from any impact.

“Because this material doesn't follow traditional concepts of toughening that you see in classical polymers,” says researcher Edwin Chain – the study’s co-author, “it opens up new ways to design materials for impact mitigation.”

The full study can be found at the scientific journal Soft Matter.

While still at an early stage of development, the ability of nanomaterials to provide added toughness to regular materials is already well known. By expanding on this knowledge, this research is creating better ways to protect. Offering traditional construction materials such as glass, plastic, metals, and even Kevlar with an added nanotechnology film which could provide battlefield protection to soldiers or improved impact defence against all kinds of high-speed projectiles.

Photo credit: 4924546 from pixabay, cottonbro on pexels, Andre Moura, & SpaceX on unsplash