The present invention relates to superparamagnetic liquids having desirably low viscosity and low corrosivity.
Superparamagnetic liquids, sometimes referred to as "ferrofluids" or magnetic colloids, are colloidal dispersions or suspensions of sub domain sized magnetic particles in a carrier liquid. The magnetic particles are maintained in stable colloidal suspension by one or more dispersing agents.
Superparamagnetic liquids can be positioned and held in space, without a container, by a magnetic field. This unique property has led to their use as liquid seals having very low drag torque and which do not generate particles during dynamic operation as conventional lip seals may do. Liquid seals using superparamagnetic liquid have found wide use as exclusion seals for computer disc drives and as pressure seals in devices with a multiplicity of liquid seals, or stages. Superparamagnetic liquids are also used as heat transfer fluids between voice coils and magnets of loudspeakers. Certain superparamagnetic liquids and their compositions are described in U.S. Pat. Nos. 3,700,595, 3,764,540, 3,843,540, 3,917,538, 4,208,294, 4,285,801, 4,315,827, 4,333,988 and 4,701,276.
The dispersant is a critical component in magnetic fluids which remain stable suspensions in the presence of a magnetic field yet which have desirable viscosity characteristics. Fatty acids, such as oleic acid, have been used as dispersing agents to stabilize magnetic particle suspensions in some low molecular weight non-polar hydrocarbon liquids such as kerosene. Use of fatty acids, however, has not proven satisfactory for dispersing magnetic particles in polar organic carrier liquids or hydrocarbon oils which are high molecular weight non-polar carrier liquids.
Viscosity is an important characteristic of superparamagnetic liquids. In many dynamic applications such as in exclusion seals, the viscosity of the superparamagnetic liquid corresponds to the friction of the seal. The higher the viscosity, the greater the energy loss, the higher the temperature of the superparamagnetic liquid in the dynamic mode. Moreover, the higher the temperature of the superparamagnetic liquid the higher the evaporation rate of the carrier liquid and the shorter will be the operating life of the device. U.S. Pat. No. 4,430,239 describes superparamagnetic liquids with low viscosity, high solids content and good magnetization which use acid phosphoric acid esters as dispersants for the magnetite particles. According to U.S. Pat. No. 4,430,239, the use of strong phosphoric acid-type surfactants as dispersing agents, particularly use of an excess of the usual or normal dispersing amount needed to disperse the magnetic particles, materially reduces the viscosity of the "ferrofluid". The excess amount of acid phosphoric acid ester used in U.S. Pat. No. 4,430,239 is about 10 percent by weight more than the usual or normal dispersing amount of the dispersing agent and, more preferably, 30-60% by weight more than the usual or normal dispersing amount.
Acid phosphoric acid ester dispersing agents described in U.S. Pat. No. 4,430,239, however, tend to lower the viscosity of the "ferrofluid", in part, by dissolving the smaller magnetite particles in the "ferrofluid". This is shown by a shift of the particle size distribution from log-normal distribution toward a Gaussian distribution when acid phosphoric acid ester dispersants are used. The corrosive character of acid phosphoric acid ester dispersing agents is apparently responsible for dissolving small magnetic particles. An excess of strong acid-type dispersant also tends to dissolve and corrode metallic components of systems with which these "ferrofluids" are used. In addition, it is known that acid phosphoric acid esters of aliphatic alcohols undergo thermal decomposition at temperatures above about 100.degree. C. and form acid phosphoric acid as one of the decomposition products. The thermal decomposition of an phosphoric acid ester is illustrated by the following equation: ##STR1## Phosphoric acid, of course, is a stronger acid than the acid phosphoric acid ester and it also tends to corrode metallic components of systems in which the "ferrofluids" are used and to dissolve some of the finely divided magnetite in suspension thereby lowering the saturation magnetization value of the "ferrofluid". The magnetization value of the superparamagnetic liquid, of course, is a measure of the quantity of magnetic particles in the superparamagnetic liquid stabilized by the dispersant. Therefore, although use of acid phosphoric acid ester dispersants provides "ferrofluids" with desirably low viscosity, the corrosive character of the dispersant itself and the byproduct of its thermal decomposition, creates drawbacks to the use of "ferrofluids" using acid phosphate acid ester dispersants. In accordance with the present invention, stable, superparamagnetic liquids with desirably low viscosity are provided using dispersants for the magnetic particles which are substantially less acidic and less corrosive than those used in the superparamagnetic liquids described in U.S. Pat. No. 4,430,239.
A further problem with magnetic fluids using acid phosphoric acid esters of long chain alcohols is the oxidative degradation of the dispersant when the magnetic fluids are heated in air. Oxidative degradation of the dispersant, in addition to its thermal decomposition, results in gellation of the magnetic colloid more rapidly than would occur in the absence of oxidative degradation. Practice of the present invention can provide magnetic colloids having diminished oxidative degradation relative to magnetic colloids using acid phosphoric acid esters of long chain alcohols as the dispersant.