Playing with fire helps the NRC enhance the resistance of bullet-proof fabric to flames—and leads to a featherweight nanotube-based solution for lightening heavy protective equipment.
Whether defending international borders or fighting wars abroad, defence personnel often find themselves in the line of fire. Their protective garments packed with armour and gear could weigh more than 40 kg—a heavy load for one person to carry in situations that require a swift response.
Materials used in bullet-resistant vests, military helmets—even armour panels on helicopters and ground vehicles—are good at stopping projectiles, but they must also be lightweight and resistant to fire hazards. The lightest armour used today is based on ultra-high-molecular-weight polyethylene (UHMWPE) fabric, marketed under brand names such as Dyneema® and Spectra®. While it is as effective as highly popular fabrics such as Kevlar® and almost 50% lighter, it is much more heat-sensitive and highly flammable.
In 2016, 2 major Canadian manufacturers of personal protective equipment (PPE) designed for first responders and the Canadian Armed Forces reached out to the National Research Council of Canada (NRC). They were searching for innovative solutions to help them meet safety and flammability requirements. "Because polyethylene is so flammable, you must apply a fire-resistant coating to prevent it from burning rapidly when exposed to flames," says Michael Jakubinek, Research Officer, Nanocomposites at the NRC's Security and Disruptive Technologies Research Centre. "The coating adds not only an extra step in the manufacturing process, but also weight to the armour."
He explains that the challenge was to devise a surface treatment that enabled the UHMWPE-based material to pass industry flammability requirements without adding substantial weight. After evaluating 3 different approaches, an expert team from 3 NRC research centres—Security and Disruptive Technologies, Aerospace, and Automotive and Surface Transportation—settled on a lightweight NRC-developed fabric made from carbon nanotubes (CNTs). These fabrics are also compatible with typical manufacturing processes used for armour laminates, unlike coatings that are applied separately as add-ons.
NRC nanomaterial packs power against fire
According to Vladimir Pankov, Senior Research Officer, Coatings and Metallic Products Group at the Aerospace Research Centre, the CNT-based fabrics studied met essential flame-resistance requirements against an established standard. "The client's goal was to have us improve fire resistance without adding more than 50 mg of weight per square centimetre," he says. "Our featherweight nanotube-based solution exceeded that tenfold, coming in at just 5 mg."
Damien Maillard, Research Officer, Advanced Polymer Composites at the Automotive and Surface Transportation Research Centre adds, "We applied the most difficult tests possible to the samples, using equipment at the NRC lab in Boucherville, Quebec, that is unique in Canada." One of these was to gauge the material's reaction to fire by applying flame to the sample for 30 seconds and removing it for 10 seconds, 4 consecutive times. "This was huge, because during previous tests the fabric was exposed to flame for only 5 seconds." The next research phases could include conducting fire tests on larger sheets of fabric, or equipment and clothing.
In the NRC lab in Boucherville, Quebec, flammability tests were performed to assess the use of novel flame-resistant materials applied to modern, high-performance armour materials. In the time-lapse sequence on the bottom right, when a flame comes close to the panel without a carbon nanotube-based protective layer, the sample catches fire. It clearly does not meet flammability requirements. The images on the left differ only in one key factor: the application of a protective layer to the surface. Using the same time-lapse sequence, the panel with the carbon nanotube-based fabric applied to the surface of the armour resists catching fire (top right).
The CNT-based fabric, which contains a higher proportion of CNTs than typical nanocomposites, was first produced at the NRC in 2014 during a small-scale research study investigating the interaction between polyurethane polymers and CNTs. "Once we had fabricated the material and seen how easily we could tailor properties to make it stronger, more conductive or stretchable, we realized that it could be used in countless applications," says Yadienka Martinez-Rubi, Research Officer, Nanocomposites, at the Security and Disruptive Technologies Research Centre. "Just by tuning a few parameters, it can be applied to anything from protective clothing and automotive textiles to aircraft de-icing and composite structures in space."
Due to their tunable physical, chemical and biological properties, as well as better performance over their bulk counterparts, nanomaterials are becoming increasingly important in all areas of life. Ongoing research and development into nanotube fabric technology will lead to novel applications in a broad range of engineering and industrial fields—not just for armour and fire protection, but also electromagnetic shielding and heating. The nanotube fabric approach can also be applied with other nanomaterials, including the NRC's world-leading boron nitride nanotube technology.
The Security and Disruptive Technologies Research Centre's Jakubinek and Martinez-Rubi are now engaging with industry partners through an Innovative Solutions Canada challenge to address manufacturing and scale-up. The NRC welcomes industry partners to help commercialize and apply this promising technology—a one-of-a-kind opportunity for Canadian manufacturers to gain competitive firepower.