Formulating, assessing and using shear thickening fluids for cork helmet liners

Abstract

In this work, an assessment of different environmentally friendly formulations for shear thickening fluids (STFs) is performed. These formulations consist of concentrated dispersions of particles of precipitated calcium carbonate (PCC) in glycerin varying mass concentrations combined with four particle aspect ratios. A full rheological characterization is developed under simple shear (steady and oscillatory tests, i.e., SAOS and LAOS) and elongational flows. The study allows not only to select the most convenient particle aspect ratio and concentration of PCC, but also to observe a clear viscoelastic behaviour that shows unexpectedly “Beads On A String” (BOAS) structures under extensional flow, which is characteristic of polymer solutions. It is reported for the very first time that a continuous shear thickening fluid exhibits an increasing relaxation time with the extension rate within the thickening region. Based on the rheological information, the best formulation was used to develop reinforced cork composites that were implemented in cork helmet liners. The impact performance of the STF-cork helmet liners was better than commercial helmet liners without hard shell and for a reduction of 36% in thickness.