EP0566931A1 - Magnetodilatant suspensions - Google Patents

Abstract

The invention relates to magnetodilatant suspensions, which are stable with regard to sedimentation, on the basis of magnetic particles having a particle size of less than 1 mu m, a dispersant substance and a solvent having a boiling point of at least 100 DEG C, which have a magnetorheological effect, measured in a magnetic field of 100 kA/m, of at least 500 Pa.

Description

The invention relates to sedimentation-stable, magnetodilatant suspensions and their use.

Magnetorheological fluids and suspensions, i.e. Liquids and suspensions, the viscosity of which can be adjusted by the action of an external magnetic field, are known (including V.I. Kordonsky et al., J. Magnetism and Magnetic Materials 85 (1990) 114-120). However, these systems have a pseudoplastic, i.e. shear thinning or Newtonian flow behavior. On the other hand, systems have been described that show the so-called dilated flow behavior. (E.J.W. Verwey et al., Rec. Trav. Chim. 57 (1939) 383 to 389). This refers to an increase in viscosity which occurs in certain shear rate ranges with increasing shear rate, as Verwey found in suspensions of oleic acid-coated carbonyl iron powder with an average diameter of 3 μm in inert organic liquids. However, the suspensions constructed in this way have the major disadvantage that they sediment after a short time and lead to a solid sediment. The parameters influencing the dilatant behavior were developed by H.A. Barnes in J. Rheol. 33 (2), 329-366 (1989). For example, the dependence of the critical shear rate, that is the shear rate above which the viscosity increases, is shown on the particle size or on the solvent viscosity.

A disadvantage of the known dilatant suspensions, however, is that they are either not stable to sedimentation or that their viscosity cannot be influenced within wide limits by applying an external magnetic field.

It was therefore an object to provide sedimentation-stable dilatant suspensions which do not have the disadvantages mentioned and in particular by one of parameters that are independent of the respective suspension can be influenced.

It has now been found that sedimentation-stable dispersions, consisting essentially of magnetic particles with a particle size of less than 1 μm of a dispersing substance and a solvent with a boiling point of at least 100 ° C., have a magnetorheological effect, measured in a magnetic field of 100 kA / m , show at least 500 Pa and show dilated flow behavior.

Suspensions in the sense of the invention are suspensions in which the quotient dlnη / dlnγ exceeds the value of 0.3 without the action of an external magnetic field in at least a partial range of the shear rate interval from 0.1 to 100,000 s⁻¹, with η the shear viscosity and γ mean the shear rate.

The suspensions according to the invention contain the magnetic particles with a volume fraction of at least 10, preferably at least 15% and up to 80% by volume, a range between 20 and 65% by volume being particularly advantageous. The volume of the magnetic particles used is the total volume, which means that the layer of dispersing substance must also be added to the respective particle. Suitable magnetic particles for the suspensions according to the invention with an average particle size of less than 1 μm, usually with a particle size of 10 to 100 nm, are preferably those with a high specific saturation magnetization. Preferred classes of substances are superparamagnetic iron oxides such as Fe₃O₄, γ-Fe₂O₃, Berthollide and in particular the cubic ferrites of the composition M _v Mn _w Zn _x Fe _y O _z described in US Pat. No. 4,810,401.

In a further embodiment of the invention, the magnetodilatant suspensions contain magnetic particles which, in the sense of a core-shell arrangement, consist only partially of the magnetic substance. For example, systems are considered in which only the shell consists of magnetic material that has been struck on an inorganic or organic core. Part of the Replace magnetic particles with an admixture of non-magnetic particles, taking into account the intended volume filling level. In this context, hard magnetic particles of this size can also be used. Particularly suitable non-magnetic admixture pigments are those that can even be processed into dilatant suspensions, such as the latex particles described in DE-A 30 25 562. While the particle size of the magnetic particles is always less than 1 μm, the particle size of the non-magnetic pigment admixtures should be 2 nm to 200 μm.

These magnetic particles or mixtures containing these particles are coated with polyelectrolytes in the magnetodilatant suspensions according to the invention. The polyelectrolytes not only effect a suitable steric stabilization but also an increase in the surface charge of the solid particles. What is important here is the pH, by which the rheological properties of the suspension are influenced. The charge carrier concentration in the polyelectrolytes and the volume of the adsorbate layer can be adjusted in a suitable manner by varying the pH. When using anionic polyelectrolytes, it has been found to be expedient for the pH to be greater than their acid constant (pKa), while for cationic polyelectrolytes the pH is preferably less than the pKa. Furthermore, the flow properties can be influenced favorably by adjusting the ionic strength. A large number of polyelectrolytes with a molecular weight between 500 and 250,000 are suitable. These polymers preferably have 5 to 1000 charges in the molecular structure. Polyelectrolytes from the group of polyacrylate acrylic acid / acrylamide copolymers, modified polyacrylates, phosphonomethylated polycarboxylates, polyvinylphosphonic acids, polyvinylphosphoric acids, polyamines, polyvinylamines, polysulfonic acids, polyphosphoric acids are particularly suitable. A pH range between 2 and 12 has proven to be particularly advantageous for the polyacrylates. In addition to the polyelectrolytes mentioned, other ligands which increase the surface charge can also be used.

The solvents contained in the magnetodilatant suspensions according to the invention can be polar or non-polar, water also being suitable. Particularly suitable solvents are ethylene glycol, triethylene glycol, tetraethylene glycol and mixtures thereof. The polarity can be varied either by choosing a different solvent or by mixing different solvents. The viscosity of the solvent (mixture) at 25 ° C. is expediently in the range from 1 to 10,000 mPas, preferably from 1 to 1000 mPas. If magnetodilatant suspensions are produced in water, a certain surface charge of the magnetizable particles must be set by choosing the appropriate pH in the range from 2 to 12, preferably from 5 to 11.

The magnetodilatant suspensions according to the invention remain flowable almost indefinitely because of the extremely low tendency to sedimentation. Their dilatant flow behavior and the possibility of reversibly influencing the viscosity by an external magnetic field are properties that make these suspensions useful for numerous applications. These magnetodilatant suspensions are particularly suitable for the controllable damping of vibrations and movements within shorter time intervals, e.g. for shock absorbers, engine vibration dampers, and also couplings. Magnetodilatant suspensions are also suitable for controlling forces and motion sequences in automotive auxiliary units such as air conditioning systems, alternators, power steering, motion or acceleration sensors, and small motor clutches. There are also possible uses for vibration damping in washing machines, centrifuges, sensitive electronic devices and for accelerating weighing processes.

The BET surface area measured in the following examples was measured in accordance with DIN 66 132 using a Ströhlein Areameter from Ströhlein, Düsseldorf, using the single point difference method according to Haul and Dümbgen. The magnetic properties were determined in a vibration magnetometer in an external magnetic field of 400 kA / m. The viscosity measurements and the determination of the magnetorheological effect were carried out with a Couette rheometer (CRM) with switchable magnetic field of 100 kA / m performed at room temperature.

Die scheinbare Wandschergeschwindigkeit γ (Wandgleiten vernachlässigt) ergibt sich zu

Durch die Geometrie der Polschuhe 3 und 3' bedingt, befinden sich nur Teile der Probe innerhalb der beiden Sektoren mit dem Winkel α (in rad) im Magnetfeld. Erhöht sich unter dem Einfluß eines senkrecht zur Scherebene stehenden Magnetfeldes die bei konstant gehaltener Schergeschwindigkeit gemessene Schubspannung vom feldfreien Wert τ auf den Wert ${\text{τ}}_{\text{M}}{\text{= τ + Δ τ}}_{\text{M}}$

, so führt dies bei der in Figur 1 gezeigten Geometrie zu einer Erhöhung des Drehmomentes vom Wert M auf M_M:

${\text{M}}_{\text{M}}{\text{/M = 1 + (α / π) (Δ τ}}_{\text{M}}\text{/ τ)}$

Der magnetorheologische Effekt folgt aus dem Drehmomentverhältnis zu

${\text{Δ τ}}_{\text{M}}{\text{/ τ = (M}}_{\text{M}}\text{/M - 1) / (α / π).}$

The Couette measuring arrangement with superimposed magnetic field used is shown in FIGS. 1a (side view) and 1b (top view). The test sample is located in the gap between a cylindrical, fixed iron stator 1 with the radius R 1 and a polyamide cup 2 rotating at the angular velocity Ω with the inner radius R 2. The torque M acting on the stator is measured. If H is the height of the stator, the wall shear stress τ on the stator follows in the field-free case

The apparent wall shear rate γ (neglected wall sliding) results in

Due to the geometry of the pole pieces 3 and 3 ', only parts of the sample are within the two sectors with the angle α (in rad) in the magnetic field. Under the influence of a magnetic field perpendicular to the shear plane, the shear stress measured at a constant shear rate increases from the field-free value τ to the value ${\text{τ}}_{\text{M}}{\text{= τ + Δ τ}}_{\text{M}}$

, this leads to an increase in the torque from the value M to M _{M in the} geometry shown in FIG. 1:

${\text{M}}_{\text{M}}{\text{/ M = 1 + (α / π) (Δ τ}}_{\text{M}}\text{/ τ)}$

The magnetorheological effect follows from the torque ratio

${\text{Δ τ}}_{\text{M}}{\text{/ τ = (M}}_{\text{M}}\text{/ M - 1) / (α / π).}$

example 1

.An aqueous suspension of soft magnetic MnZn ferrite pigments with a stoichiometry as described in Example 12 of US Pat. No. 4,810,401 was prepared. The content of the suspension was 25% by weight of Mn ₀ , ₃Zn ₀ , ₂Fe₂, ₅O₄, the BET surface area of the pigment was 92 m² / g, the specific magnetization was $\text{Mm / ρ = 78 nTm³ / g}$

.

51 g of a 45% aqueous solution of the sodium salt of polyacrylic acid with an average molecular weight of 4000 (BASF commercial product Sokalan CP10) and 115 g of triethylene glycol were added to 456 g of this suspension. A pH of 11.3 was measured. It was dispersed with an Ultra Turrax device for 1 hour. The water was drawn off on a rotary evaporator (water jet vacuum, 68 ° C.). 258 g of a flowable, non-sedimenting product were obtained, on which the dilatant and magnetorheological properties shown in FIG. 2 were found. Figure 2 shows the dependence of the viscosity η [Pas] of the suspension against the shear rate γ [s⁻¹] without a magnetic field (a) and in the magnetic field (b) and the dependence of the magnetic rheological effect MR [Pa] also against the shear rate.

Example 2

, und 17 g Wasser dazugegeben und mit 3 g 7,5 %iger NaOH Lösung ein pH von 7,6 in der Suspension eingestellt. Schließlich wurde am Rotationsverdampfer das Wasser abgezogen und nach erneutem Homogenisieren ein fließfähiges, nicht sedimentierenden Produkt erhalten, an dem die in Figur 3 dargestellten dilatanten und magnetorheologischen Eigenschaften gefunden wurden (Dimensions-Angaben gemäß Figur 2).30 g of freeze-dried latex particles with a particle diameter of 200 nm, which were produced according to DE-A 30 25 562, were stirred with 30 g of ethylene glycol. Then a suspension consisting of 6 g of soft magnetic MnZn ferrite of the formula Mn ₀ , ₃Zn ₀ , ₂Fe₂, ₅O₄, prepared as indicated in Example 1, characterized by BET = 92 m² / g and $\text{Mm / ρ = 78 nTm³ / g}$

, and 17 g of water were added and the pH was adjusted to 7.6 in the suspension with 3 g of 7.5% NaOH solution. Finally, the water was drawn off on a rotary evaporator and, after renewed homogenization, a flowable, non-sedimenting product was obtained, on which the dilatant and magnetorheological properties shown in FIG. 3 were found (dimensional information according to FIG. 2).

Example 3

60 g of freeze-dried latex particles used in Example 2 were mixed with 60 g of ethylene glycol, 12 g of soft magnetic ferrite according to Example 1 and 235 g of water on the Ultra-Turrax disperser for half an hour. A pH of 9 was set by adding 1.2 g of NaOH. The water was drawn off on a rotary evaporator and a flowable, non-sedimenting suspension was obtained, which was characterized as follows: minimum viscosity (at a shear rate of approx. 30 s⁻¹): 1.2 Pas, viscosity at 140 s⁻¹: 6 Pas, magnetorheological Effect (at 140 s⁻¹): 2000 Pa.

Claims (4)

Magnetodilatant suspensions, consisting essentially of magnetic particles with a particle size of less than 1 µm, a dispersing substance and a solvent with a boiling point of at least 100 ° C, which have a magnetorheological effect, measured in a magnetic field of 100 kA / m, of at least 500 Pa. Magnetodilatant suspension according to claim 1, characterized in that the magnetic particles are a cubic ferrite with a BET of 2 to 200 m² / g and a saturation magnetization of more than 10 nTm³ / g, the dispersing substance has at least one compound from the group of polyelectrolytes Charge number greater than 5 and a molecular weight between 500 and 250,000 and the solvent is an organic solvent with a viscosity in the range of 1 to 10,000 mPas (at 25 ° C) and the volume filling degree of the magnetic particles in the suspension 10 to 80 vol. -%. Magnetodilatant suspension according to claim 1, characterized in that some of the magnetic particles are replaced by latex particles with dilatant flow behavior with an average particle size of 2 nm to 200 µm. Use of the magnetodilatant suspensions according to one of Claims 1 to 3 for the controllable damping of vibrations and movement sequences.

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