Coated magnetizable microparticles, reversible suspensions thereof, and processes relating thereto

Claim 1

Magnetically-responsive discrete microparticles suitable for use in preparing selectively reversible suspensions thereof; each such microparticle having a core, a coating thereabout, and a diameter no greater than about one micron; said core being formed of a single particle of magnetically-responsive material having a Curie temperature within the range of about 5 to 65° C., a magnetic moment at saturation of at least 2.0 μ B at 4.2° K, and a specific gravity above 3.0; said coating comprising a water-insoluble cross-linked polymeric material having reactive groups at the surface thereof.

Claim 11

A method for magnetically aggregating and thereafter suspending sub-micron-sized magnetizable particles, comprising the steps of forming each of said particles to have (a) a core of magnetically-responsive material with a Curie temperature within the range of about 5 to 65° C. and a saturation magnetic moment of at least 2.0 μ B at 4.2° K and (b) a cross-linked polymeric coating about said core having reactive sites on the surface thereof; mixing said particles at a temperature not substantially below said Curie temperature with a first liquid medium containing a component capable of reacting with said coating; then exposing said particles to a magnetic field at a temperature not substantially exceeding said Curie temperature to cause magnetic aggregation of said particles; then separating the particles so aggregated magnetically from said first liquid medium, introducing said magnetically-aggregated particles into a second fluid medium, and subjecting the same to a temperature in excess of said Curie temperature in the absence of a magnetic field and with agitation to resuspend said particles in said second medium.

Claim 12

A method for magnetically aggregating and thereafter suspending sub-micron-sized magnetizable particles, comprising the steps of forming each of said particles to have (a) a core of magnetically-responsive material with a Curie temperature within the range of about 5 to 65° C. and a saturation magnetic moment of at least 2.0 μ B at 4.2° K and (b) a cross-linked polymeric coating about said core having reactive sites on the surface thereof; mixing said particles at a temperature not substantially below said Curie temperature with a first liquid medium containing a component capable of reacting with said coating; then exposing said particles to a magnetic field at a temperature not substantially exceeding said Curie temperature to cause magnetic aggregation of said particles; then separating the particles so aggregated magnetically from said first liquid medium, introducing said magnetically-aggregated particles into a second fluid medium, and subjecting the same to a temperature in excess of said Curie temperature in the absence of a magnetic field and with agitation to resuspend said particles in said second medium.

Claim 16

A method of preparing a reversible suspension of coated magnetically-responsive microparticles, in which a magnetically-responsive core material is pulverized to produce core particles of a size substantially under 1.0 micron and said core particles are then each coated with a nonmagnetic polymeric material to form discrete coated magnetic microparticles having reactive groups at the surface thereof, wherein the improvement comprises forming said core particles of a magnetically-responsive material having a Curie temperature within the range of about 5 to 65° C., a magnetic moment at saturation of at least 2.0 μ B at 4.2° K, and a specific gravity above 3.0; and carrying out said pulverizing and coating steps in a liquid medium at temperatures above the Curie temperature of said material.

Claim 17

A method of preparing a reversible suspension of coated magnetically-responsive microparticles, in which a magnetically-responsive core material is pulverized to produce core particles of a size substantially under 1.0 micron and said core particles are then each coated with a nonmagnetic polymeric material to form discrete coated magnetic microparticles having reactive groups at the surface thereof, wherein the improvement comprises forming said core particles of a magnetically-responsive material having a Curie temperature within the range of about 5 to 65° C., a magnetic moment at saturation of at least 2.0 μ B at 4.2° K, and a specific gravity above 3.0; and carrying out said pulverizing and coating steps in a liquid medium at temperatures above the Curie temperature of said material.

Claim 21

The method of claims 14, 15, or 16 in which there is a further step of fractionating said coated particles immediately following said coating step, said further step of fractionating said coated particles being carried out above said Curie temperature.