Development of Flexible Extremities Protection utilizing Shear Thickening Fluid/Fabric Composites.

摘要

A systematic approach was undertaken to enhance spike resistance of flexible body armor. A traditional STF composite is consisted of fumed silica particles (about 7 nm in diameter but highly agglomerated due to van der Waals forces), PEG, and Kevlar. PEG is added to an aqueous suspension of silica and the solution is evaporated to remove water. This process of adding PEG to silica suspension and evaporation is continued several times until the concentration of silica to PEG comes around 55:45 by weight. At this ratio, the mixture is usually termed as Shear Thickening Fluid (STF) since it shows a sudden spike in viscosity at a certain shear rate when tested in a Rheometer. This sudden rise in viscosity is due to hydroclustering of silica particles within the carrier fluid, PEG. Such hydroclustering requires some movement and orientation of particles which is only possible when the carrier fluid is in liquid or in semi-liquid condition. Once STF is impregnated with Kevlar (with the help of ethanol, and subsequently dried) movement of silica particles are restricted further. Chances of hydroclustering and any alignment of particles to provide an instantaneous rise in viscosity and offer resistance to spike penetration are remote. It is therefore believed that under dry condition, resistance to spike penetration mostly comes from the energy required to (i) separate the filaments in fiber tows, (ii) deform the fiber, which is directly related to the elastic energy of the fiber, and (iii) plow through the particle- polymer-fiber layers which is related to frictional energy. We believe, a considerable amount of energy is absorbed by steps (i) and (iii). Energy absorption in these two steps can be increased if we can increase the bonding between the three components; particle, polymer, and fiber. We cannot do much with the fiber deformation and fracture (step-ii) until its elastic energy and cut properties are improved. We focused on increasing the chemical bonding.