New Shear Thickening Fluid (STF) enables flexible, comfortable armor
By Mike Hanlon
August 12, 2006
August 13, 2006 Since warfare began, developing armor has been a balance between the need for protection and the need for comfort, flexibility and light weight. A new nanotechnology known as Shear Thickening Fluid (STF) created by scientists at ARL and UDTC looks set to provide the next generation of armor. STF has the ability to make ballistic fabrics highly resistant to penetration when impacted by a spike, knife or bullet without compromising their weight, comfort or flexibility. The potential applications of STF include a wide range of products such as body armor, vehicle armor, helmets, gloves and bomb blankets to protect soldiers and law enforcement officials plus myriad industrial safety applications all thr way through to protective clothing for motorcyclists. When the first products become available later this year, soldiers can expect to be much safer as the liquid body armor can be used in sleeves and pants, which are not usually protected by ballistic vests because they must stay flexible.
Under active development for the past five years, STFs are special materials with nano-particles that exhibit properties normally associated with both solids and liquids, but are rarely found in the same material. Sometimes referred to as "liquid armor," the material is actually a nanotechnology that exists in a flexible, fluid-like state under normal conditions but adopts seemingly rigid qualities and becomes less penetrable when impacted.
As a result, this special material can be applied to conventional ballistic fabrics or other materials used in armor applications, allowing them to remain flexible under normal wear, but simultaneously becoming resistant to penetration when impacted by a spike, knife or high velocity projectile or fragment. STF treated fabrics effectively spread the energy over a larger area.
Developed by the University of Delaware's Center for Composite Materials, in partnership with the Weapons and Materials Research Directorate of the U.S. Army Research Laboratory, STF has been exclusively licensed to Armor Holdings.
Extensive testing conducted by UDTC and ARL has demonstrated that when treated with STFs, a conventional ballistic fabric can resist penetration from an ice pick that would otherwise easily penetrate the fabric. However, further testing and applications in the field may be needed to understand fully the properties of STFs. In addition, fabrics treated with STF have been shown to reduce "back face deformation" (an indication of blunt trauma) from high energy ballistic impacts. Importantly, treating the fabric with this material has little or no effect on the look, feel, texture, weight or flexibility of the fabric.
A Shear Thickening Fluid (STF) is the key component of the liquid body armor. Hard particles (nano-particles of silica) are suspended in polyethylene glycol, an inert, non-toxic thickening agent used in a variety of common products, like some ice creams. To make liquid armor, STF is soaked into all layers of a Kevlar vest. The Kevlar fabric holds the STF in place, and also helps to stop the bullet. The saturated fabric can be soaked, draped, and sewn just like any other fabric. Under normal conditions the particles flow with the liquid, but when heavily strained, the particles become rigid. When the stress is relieved, it goes back to a liquid.
In a small glass vial, the light blue liquid is easily stirred with a small plastic stick – as long as the stick is moving in slow, easy motion. When sudden, rapid or forceful motion is applied, the liquid instantly hardens, preventing any movement.
“When the movement is slow, the glass particles can flow around each other,” explains Eric Wetzel, who heads the STF project team in the Weapons and Materials Research Directorate of the U.S. Army Research Laboratory. “But when the movement is fast, the particles bump into each other, preventing any flow of movement.”
When STF is applied to regular Kevlar, the fabric’s texture doesn’t change; it looks and feels the same as if it hadn’t been treated. Using a test swatch of four layers of untreated Kevlar – the normal thickness of body armor – Wetzel is able to stab an ice pick through the fabric. But when stabbing a treated section of fabric with all the force he can muster, the ice pick dents the fabric but can’t penetrate through.
Armor Holdings, which will be the sole commercial provider of this technology in applications related to body armor vests and extremity protection, helmets and gloves for protective use worldwide, anticipates fielding the first products later this year.
Armor Holdings has selected Barrday as a partner for development and production of STF-based ballistic fabrics. Barrday has strong complimentary experience in weaving fabrics from high strength fibers as well as applying films, resins, finishes and coatings for both soft and hard armor applications.
Background on Shear Thickening Fluid
Under the direction of Professor Norman Wagner, the University of Delaware, Center for Composite Materials began investigating shear thickening fluids in the mid 1990s. Beginning in 2000, UD CCM began working in partnership with the Army Research Lab's Materials Research, led by Dr. Eric Wetzel, to create a new armor material. The first promising ballistic results were achieved in 2002, presented publicly at the U.S. Army Science Conference in Orlando, FL, in 2002. The U.S. Army recognized the significance of this new technology by awarding the UD/ARL research team the Siple Award as the best paper at the 23rd Army Research Conference in December 2002. Work on the technology continued throughout 2003 and 2004, with important discoveries of the stab and puncture resistance of the fabric and further refinements in processing and fabrication. A U.S. patent application was filed in May of 2003.