New stable, thermosensitive magnetorheological (MR) materials with a high MR response at the boundary between conventional MR fluids and elastomers
MR fluids are colloidal systems in which one phase has a magnetic response, in other words is magnetised in the presence of an external magnetic field. Generally speaking, MR fluids comprise a solid and magnetisable particulate phase dispersed in a non-magnetic carrier medium. When a magnetic field is applied to this dispersion, the magnetic microparticles become magnetised and aggregate in the direction of the magnetic field lines. The resulting particulate structures cause changes in the flow (rheological) properties such that, for example, the shear viscosity increases with the intensity of the applied field. If the field is sufficiently strong, the appearance of a threshold force on the time scale explored is even possible. MR elastomers are solid analogues of MR fluids. They tend to be manufactured by dispersing iron microparticles in a polymeric matrix, which is then cured in the presence of a magnetic field. Whereas MR fluids operate in the flow region, their elastomer counterparts do so in the pre-flow region.
This invention proposes the use of thermosensitive carrier media that allow the MR material to behave as a liquid, solid or viscoelastically depending on the temperature to which it is subjected. In other words, its mechanical behaviour can be controlled externally (by varying the temperature) and comprises a behaviour range bounded by the extremes liquid (MR fluid) and solid (MR elastomer). In a first example of the invention, triblock copolymer solutions which evolve towards a liquid-crystal crystalline phase upon heating are used, whereas in a second example concentrated microgel dispersions that achieve maximum packing when the temperature is reduced, thereby forming a repulsive colloidal gel, are used.
The new MR materials subject matter of this invention are directly applicable in the automotive and mechanical sectors, specifically in torque transmission devices, with particular emphasis on shock absorbers, brakes and clutches, allowing regulation of the hardness thereof. They may also be used as smart drug-release systems.
- The new materials exhibit viscoelastic behaviours intermediate between those of liquids (MR fluids) and solids (MR elastomers) due to the use of thermosensitive carrier media, specifically triblock copolymers and thermosensitive microgels.
- Sedimentation of the dispersed iron microparticles when the carrier medium operates in the solid regime is prevented.
- Very strong MR effects are achieved when the carrier medium operates in the liquid regime.
These thermosensitive MR materials exhibit a strong MR effect when the carrier medium operates in the liquid regime and the iron microparticles contained in them do not sediment due to gravity when the carrier medium operates in the solid regime. These new MR materials have a behaviour intermediate between MR fluids and MR elastomers. Moreover, they are cheap and easy to prepare as their composition is much simpler than that of a convention MR material, they are non-toxic and environmentally friendly.