Rheological Behavior Of Highly Concentrated Silica Suspensions And Its Impregnated Kevlar Fabrics Stab Resistance

With the development of society,human beings' own values and substance life level,the individual self-protection awareness is brought in constant renewal.For the increase of emergent incidents and great quantities of public and safe problem,there are vast market potential and important theoretical values to develop new protective materials.The key of personal armor protective material design is to look for the balance between specific strength and mobility.How to ensure dresser's easiness and provide furthermore defence is the problem and emphasis in the domain.Shear thickening is the behavior of non-Newtonian fluid,and most shear thickening fluids(STFs)are created using a concentrated colloidal suspensions composed of nonaggregating solid particles suspended in fluids,which is marked by a rapid,sometimes discontinuous increase in viscosity with small increase in shear rate.Because of its unique energy-absorbing ability,shear thickening fluids have recently gained a lot of attention in its application research.Recent studies have been reported that it can be applied in defensive and protective equipment.In this dissertation,a self-made drop-hammer tester was developed,and the rheological behavior of prepared concentrated suspensions,the stab resistance of Kevlar composites impregnated with STF and the dynamic performance of STF were studied.The response process of transient force received and the penetration depth of STF-Kevlar composites with SiO2 particles prepared by Stober method or chemical precipitation method were studied in this paper,and the energy-absorbing mechanism during penetration was analyzed in detail.The results show that,the stab resistance of impregnated Kevlar cloth increased obviously to about 400N and the impact response time decreased to 3.5 millisecond while the penetration depth had little reduction.The composite impregnated with middle concentration dispersion prepared with 120nm SiO2 nanoparticles showed superior stab resistance,which increased to 2.7 times of pure Kevlar cloth,and its penetration depth reduces by ten percent;The composite impregnated with stober silica particle dispersion shows lower stab resistance,which is raised to 1.6?1.8 times of pure Kevlar cloth,and its penetration depth reduces by fifteen percent.The difference could be attributed to the different dynamic response of two kinds of fluid.Monodisperse concentrated suspensions of different concentrationon were prepared with Stober method and centrifugation/solvent exchange procedure.The rheology behavior of different concentration were studied.The results show that when the mass percent of prepared suspension is lower than 50%,suspension shows weak shear thickening behavior,which viscosity increases and critical shear rate reduces with increasing concentration;when it is about 55%,suspension shows shear thickening at first and then transfer to shear thining;when it is bigger than 65%,shear thinning comes first and then appears obviously shear thickening.The FTIR spectra for concentrated dispersions with precipitation silica particles shows that the increasing of SiO2 mass percent led to the hydroxyl peak shift to a lower wavenumber.This result represents the interaction between PEG and silica nanoparticles predominantly occurs in the PEG/silica dispersions,resulted in formation of increasing amount of hydrogen bonds.XPS,FTIR and rheological measurements show that silica particles with different ratio hydroxyl groups could be prepared,and the rheological behaviors of suspensions prepared with different silica particles vary remarkably as increasing ratio of hydroxyl group under the same mass fraction.Monodipersed SiO2 particles were successfully modified with silicane coupling agent KH550,and TG analysis shows about 4.47%weight KH550 was grafted on surface of SiO2 particles.FTIR shows that chemical grafting reaction occurred in SiO2/PEG suspension.Both suspension systems before and after modification have obviously reversible shear thickening;but the suspension system after modification has stronger shear thickening effect,which could be better explained by the introduction of polymer chain and the clustering theory.The dynamic behaviors of STF were studied with SHPB and low impact velocity testing devices.Under high strain rate,the stress of STF correlates with the magnitude and change of strain rate and the response time is tens of microseconds.The STF internal stress increases with increasing strain rate and dramatically drops down when strain rate becomes plateau or begans to decrease.Under low strain rate,STF behaves with different viscoelastic effect and completely reponses with about 0.82ms.The Kevlar composite impregnated with suspension prepared with non-modified SiO2 particles has better stab resistance performace because of stronger adsorption for Kevlar cloth.In terms of force-displace plot,STF behaves more like a coating,which provides higher interyarn friction,the tensile modulus and break strength of fibre to absorb more energy.The stab resistance and penetration depth of 4 layer impregnated Kevlar cloths correspond to or gain the advantage over that of 6 layer pure Kevlar cloths,which suggests that the impregnating technique could take lower cost of personal protection.