Title: Enhancing spike resistance of flexible body armor using silane coupling agent and a fixative cross-linker

Abstract

The idea of flexible body armor is not new, and it has been around for the last few decades. What is, however, new is how it has evolved to its current state. Initial development included shear thickening fluid (STF) as a backbone polymer to impregnate ballistic fibers. STF was made from a mixture of silica particles and polyethylene glycol (PEG) at 55 to 45% weight ratio. At the core of the spike and ballistic resistance was the instantaneous rise in viscosity of STF at the event of an impact. Viscosity rise was due to hydroclustering of fumed silica particles embedded within the PEG. When the armor system was dry, hydroclustering was not possible in absence of any fluidity in the polymer. Still, coating of silica particles around the fiber offered reasonable amount of resistance to spike and ballistic penetration. In the next step, when silica nanoparticles were functionalized with a silane coupling agent (Gelest), siloxane (Si-O Si) bonds were formed and it improved bonding of particles to the fiber. Subsequently, PEG was replaced by silane as the base polymer to develop N-H (amide) receptors on the surface of silica particles. To promote further bonding, a fixative cross-linker, Glutaraldehyde (Gluta) was added in the particle-polymer mixture. Gluta created strong bridges between distant pair of amide groups present in silated silica particles and kevlar. The particle-polymer mix was then used to impregnate multiple layers of woven kevlar fabrics to construct the armor. After fabrication, armor composites were tested using a drop tower following the NIJ (National Institute of Justice) Standard 0115.0.NIJ tests were performed with spikes having a drop weight of 1790g and drop heights ranging from 0.05m to 1.0m. that produced impact energies from 1 to 18J. Impact energy is normalized with aerial density of the armor to account for any variation in layer thickness during impregnation. It is observed that spike resistance for no penetration (i.e., 0 witness layer) increased from 12 J-cm2/g for STF to 200 J-cm2/g for the new silane-silica-gluta system. The improvement is almost 16 times. Silica nanoparticles bonded strongly with kevlar in uniform dispersion and offered unprecedented resistance to penetration.