Synthetic oil gels with organoclays in the formulation of magnetorheological fluids

Magnetorheological fluids (MRF) are smart composite materials that, under an external magnetic field, show a reversible solid-liquid transition in less than 10 ms. This study aimed to evaluate which organoclays would jellify a synthetic oil for the formulation of MRF. Three dispersant additives for carbonyl iron powder were evaluated. Fifteen different gelling additives from four clay families, bentonites, hectorites, montmorillonites, and mixed mineral thixotropes (MMT), were dispersed in oil only, keeping the same concentration, without iron particles. The gels were then tested through amplitude and frequency sweeps in oscillatory rheometry to evaluate their viscoelastic behavior. The thixotropy of the gels was measured through the “three-interval” test in a rheometer. After selecting the best gelling additive to prepare the MRF, three dispersing additives had their rheology evaluated to determine the best magnetorheological effect and redispersibility after 1 year of sample preparation. In the linear viscoelastic region, all MMT clays resulted in a weak viscoelastic gel (G′∼100 to 300 Pa and G″∼30 to 50 Pa). Some of the bentonite clays jellified, and others did not. The best organoclays were montmorillonites and hectorites, which formed consistent viscoelastic gels (G′∼1 to 5 kPa and G″∼70 to 250 Pa). The best organoclay presented a yield stress σ0 = (42 ± 3) Pa, a storage modulus G′ = (2690 ± 201) Pa, and a cohesive energy density (CED) = 98 mJ/m3, and it was selected to explore the rheology of MRF with three dispersant additives: octan-1-ol, octan-1-amine, and L-α-Phosphatidylcholine. All the MRFs were prepared using carbonyl iron powder HS (BASF SE) in oil gels and with the same organoclay. All three dispersant additives showed a thixotropic recovery above 100% in the three-interval test. Regarding the redispersibility after 1 year, the MRF formulations with octan-1-amine and lecithin were reproved, as they reached normal force peaks of 19 and 24 N, while the work was 28 and 415 mJ, respectively. The best MRF was formulated with octan-1-ol, and resulted in a normal force of 0.33 N and 3.4 mJ at 35 mm of vane penetration. Therefore, we conclude that the MRF with octan-1-ol and montmorillonite #6 showed a better balance between thixotropy, MR effect, and, above all, good redispersibility.