Impact properties of shear thickening fluid impregnated foams

Concentrated colloidal suspensions of silica particles in polyethylene glycol exhibit a shear thickening behavior: above a critical shear rate in a confined environment, they show a steep increase of viscosity. This reversible transition from a low to a high viscosity state is associated with a large energy absorption that could be harnessed for impact protection. As these suspensions are liquid at rest, however, shear thickening fluids (STFs) are difficult to use in practical applications. Furthermore, their specific rheological properties exist within a narrow range of concentration, so they tend to disappear when the material is in contact with air and humidity. In this work, a soft foam scaffold was impregnated with STF to provide a three-dimensional shape to the assembly at rest, while a silicone was cast around it to serve as a physical barrier to the external environment. A method to quickly impregnate the foam was proposed. Impact tests were carried out on the STF/foam/silicone composite pads using a free fall impact tower. Compared to rubber or pure silicone, larger energy absorptions, up to 85%, were observed, which could be repeated for multiple impacts. The transmitted shock waves were also reduced, showing the potential of this system for impact protection of structures.

Shear Thickening Fluid Impregnated Foam Composites for Vibration Control and Impact Protection

Mathieu Soutrenon

Highly concentrated suspensions of silica particles present shear thickening properties, shifting from a low to a high viscosity state, instantly and reversibly at a critical shear rate. Their viscosity increases by up to 3 orders of magnitude, changing from a honey-like liquid to a brittle solid material. This sharp rise in viscosity is associated with a large absorption of energy that can be used in damping applications. These shear thickening fluids (STF) are among the few passive materials that can simultaneously bring both an increase in damping and in stiffness properties. In mechanical design, this leads potentially to a weight reduction of the overall system. This thesis work is aimed to develop and test a composite material solution for vibration control and impact protection, using STF as the damping element. A main challenge was to overcome the fact that STF are liquid at rest, and very sensitive to their environment. The proposed damping material was therefore constituted of foam impregnated with STF and encapsulated with silicone (STF/foam). A comparison of properties with other damping materials such as Smactane®was emphasized along the thesis. Storage and processing conditions of STF constituted of highly concentrated suspensions were shown to affect their rheological properties similarly to other sensitive parameters such as particle concentration. Rigorous processing methods were thus required to produce STF with defined and stable rheological properties. The use of sonication was preferred to disperse the aggregates of particles in concentrated colloidal suspensions due to the packing of the dry powder. Contact with air or humidity deteriorates the shear thickening properties of STF. Storage of STF in a freezer at −24 ◦C or under nitrogen prevents this. The use of a physical barrier to air and humidity such as silicone to protect STF is also shown to greatly reduce the ageing problem. The energy dissipated during a cyclic strain cycle was used as a measure of the damping property of the STF, and compared to that of other materials. It was measured with dynamic rheological measurements performed on suspensions with variations of particle concentration, size and test frequency. STF were shown to dissipate a large amount of energy during their transition, which increases with the increase of frequency and particle concentration and decrease with the increase of particle size. In specific cases of large strain amplitude, they potentially dissipate more energy than viscoelastic materials such as rubber. An open cell melamine resin foam was used as a lightweight scaffold to provide a three dimensional structure to the STF. The foam distributes the load and locally shears the STF. The shape of the foam is easily adjustable. To impregnate the foam with the STF, a system similar to vacuum infusion was used. The foam was placed under vacuum in a mold and the STF heated at 70 ◦C to reduce its viscosity and shear thickening properties was sucked in the foam with the depressurization. The STF/foam systems were then encapsulated in silicone to protect the STF/foam damping pads from contamination and avoid outgassing. During compression tests at low frequencies and large deformations, we demonstrated that STF could be activated in this type of structure, with mechanical properties comparable to those of a viscous honey-like fluid at rest and silicone when the STF is in the solid state. The same behavior was observed during impact solicitations. STF/foam presents a very low coefficient of restitution and absorbs the shock wave associated with impacts. Shock tests done separately confirm the shock absorption properties of STF/foam even if the STF is not activated. The combination of mechanical and damping properties of the foam/STF systems remain lower than for very high performance damping materials like Smactane®. The reasons are their low mechanical compression and shear properties at rest and the need to overcome a threshold strain or stress to activate the STF. However, the large dissimilarity between the states before and after the transition makes these materials interesting for impact or large strain amplitude applications such as sole for running shoes or several space structures.

EPFL2013

Structural damping using encapsulated Shear Thickening Fluids

Véronique Michaud, Mathieu Soutrenon

Smart structures with tunable damping and stiffness characteristics are of high interest to aerospace applications, but often require an external power source to be activated. This can be avoided by using highly concentrated silica suspensions, which exhibit a shear-thickening behavior, linked to a dramatic increase in viscous dissipation. These materials are however liquid at rest, and sensitive to humidity, so they are difficult to implement as such into structural applications. In the present work, highly concentrated solutions of monodisperse silica particles in PEG were selected for their strong thickening effect at rather low critical shear strain. Damping properties were characterized by measuring the energy dissipated per cycle at low frequency (

Spie-Int Soc Optical Engineering, Po Box 10, Bellingham, Wa 98227-0010 Usa2012

Highly branched polymer/montmorillonite clay nanocomposites

Marlene Rodlert

The aim of this thesis has been to assess the potential of branched polymers as novel exfoliants for layered silicates and their application in polyurethanes. Three different types of –OH functional branched polymers have been studied: dendrimers, hyperbranched polymers (HBPs) and star branched polymers (SBPs), with a range of structures and molecular weights. The dendrimers and their HBP analogues were synthesized in-house, while the other HBPs and SBPs were obtained from commercial suppliers. Various types of montmorillonite (MMT) layered aluminosilicate clays have been used as modifiers. The different branched polymers were either melt processed or solution processed with the MMT, using water and THF as dispersants. Whether intercalated or exfoliated nanocomposites were obtained depended on the choice of MMT, the presence of solvent during processing as well as the characteristics of the interface between the polymer and the MMT, with exfoliation being favored when the polymer-MMT interactions were strong. Very high degrees of exfoliation were obtained from dendrimers and HBPs processed in water at loadings as high as 20 wt% unmodified Na+MMT, with intercalation only becoming dominant at higher loadings. The MMT layer spacing in the intercalated HBP/Na+MMT nanocomposites depended directly on the pseudo-generation number, that is, on the size of the HBP, at intermediate loadings, consistent with adsorption of a monolayer of unperturbed HBP molecules at the layer surface in aqueous suspension. Dispersions of the HBPs with a range of organically modified MMTs in THF, on the other hand, showed mainly intercalation on drying, with layer spacings of the order of 3.8 nm at intermediate clay contents, which were apparently independent of the pseudo-generation. The difference between the final MMT layer spacing and its value prior to mixing was found to increase significantly with the polarity of the organic modifier. The contrasting behavior of Na+MMT and the organically modified MMTs was partly attributed to the tendency of Na+MMT to exfoliate in aqueous dispersion. The HBPs and dendrimers are then able to coat the individual MMT layers and stabilize the exfoliated structure during drying. The organically modified MMTs, on the other hand, did not exfoliate in THF dispersions and intercalation consequently dominated. Predominantly intercalated nanocomposites were also obtained with the SBPs, although increasing degrees of exfoliation were observed as the strength of the interactions between the SBP and the clay increased, as a result of systematic modification of the polarity of the SBP. The rheological properties of a range of exfoliated and unexfoliated highly branched polymer/MMT nanocomposites were investigated in small strain oscillating shear. The exfoliated nanocomposites showed a sharp increase in shear viscosity at very low MMT contents, accompanied by a transition from Newtonian behavior to solid-like behavior, characterized in terms of a limiting shear modulus at low frequencies and a limiting viscosity at high frequencies, both of which were strongly dependent on MMT content. Unexfoliated nanocomposites showed a more gradual increase in viscosity with MMT content, although they showed qualitatively similar behavior to the exfoliated nanocomposites at sufficiently high loadings. The rheological behavior was analyzed in terms of models for conventional filled polymers, leading to estimates of the effective particle aspect ratio that were approximately consistent with direct morphological observations by transmission electron microscopy.To investigate the influence of the MMT on solid state properties, the HBP/MMT and SBP/MMT nanocomposites were incorporated into model polyurethane formulations. In the case of the HBP/MMT, a reactive low molar mass polyol or an un-reactive solvent (THF) was used to reduce the viscosity sufficiently for room temperature processing. Exfoliated HBP/Na+MMT led to a 60% increase in the rubbery plateau modulus relative to that of the corresponding unfilled matrix at clay contents as low as 0.5 vol%, whereas more modest relative increases were observed with unexfoliated or partly exfoliated MMT. The same materials also showed a 2-fold increase in stress at break and elongation at break in the presence of 0.5 vol% exfoliated Na+MMT, although no improvement was observed for the corresponding intercalated polyurethane nanocomposites obtained in the absence of HBP. Much higher exfoliated clay contents led to processing problems owing to their high viscosities. Polyurethanes containing unexfoliated MMT, such as obtained by using SBPs rather than HBPs as the matrix, could be prepared up to relatively high MMT contents, but the improvements in mechanical properties were modest at any given MMT content. The semi-empirical Halpin-Tsai model and Mori-Tanaka's average stress theory, which are both based on a classical approach to particle reinforcement, accounted well for the evolution of the mechanical properties of the polyurethanes as the MMT content was varied and they led to estimates of the particle aspect ratio that correlated well with those inferred from the rheological models and direct observations. This in turn allowed correlations to be established between the degree of exfoliation, the shear viscosity of the nanocomposite precursors and the mechanical properties of the final polyurethanes.