Compositions – Magnetic – Iron-oxygen compound containing
Reexamination Certificate
1998-10-09
2001-02-06
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Iron-oxygen compound containing
C252S062540, C252S062520, C106S460000, C106S456000, C427S600000, C427S601000, C427S220000, C428S405000
Reexamination Certificate
active
06183658
ABSTRACT:
The present invention relates to a process for producing agglomerate-free nanosize iron-containing oxide particles, i.e. iron-containing oxide particles having a particle size (a mean particle diameter) of the primary particles of not more than 100 nm, preferably not more than 40 nm and in particular not more than 30 nm, which have a very hydrolysis resistant coating based on silane (core-shell structure).
Particularly for the production of superparamagnetic iron oxide particles, nanosize particles are usually produced via a precipitation process and attempts are subsequently made, by means of surface modifiers, to prevent a growing together of the primary particles (superparamagnetism requires a particle size of at most about 30 nm) or to break up agglomerates which have already formed and to prevent the primary particles thus formed from reagglomerating. As can be seen from the literature, surface modification alone, e.g. using functional silanes, achieves only a moderately deagglomerated product. This makes probable a strong bond between nanosize primary particles which is not broken up by the surface modifier. The additional input of energy by means of ultrasound, as already described in the literature, also does not change this in any fundamental way.
However, the use as surface modifier of functional silanes, i.e. silanes which have not only hydrolysable or condensable groups (e.g. alkoxy groups) via which both bonding of the silane to the surface of the metal oxide particles and condensation of silane molecules already bound to the surface with one another to form Si—O—Si bonds can take place but also contain functional groups bound to nonhydrolysable hydrocarbon groups which functional groups make it possible, after the metal oxide particles have been coated, for a variety of species (e.g. via molecules) to be bound to the particles is highly desirable for the reasons just mentioned provided that a way can be found of breaking up agglomerates into the primary particles of which they are composed during surface modification (coating) and providing these primary particles with a stable coating which does not become detached again from the surface of the particles even under unfavourable conditions, thus reliably preventing agglomeration of the primary particles.
Monomeric or oligomeric silanes bound to the surface of metal oxide particles are, however, quite generally very hydrolysis-sensitive. This problem is increased when, in particular, the functional silane used as surface modifier has functional groups which can catalyse hydrolytic cleavage of the (in any case relatively weak) metal—O—Si bond, as occurs, for example, in the case of aminosilanes, so that the silane species already present on the primary particles are detached again from the particle surface under very mild conditions (by hydrolysis) and thus leave an (at least partly) uncoated primary particle which can form an agglomerate with other particles of this type and generally will also do so since nanosize particles quite generally have a strong tendency to agglomerate formation if they are not prevented from doing so by appropriate measures.
In this context, it may be pointed out that agglomerates present a problem for many applications even if their dimensions are within the desired or still acceptable (nanosize range) because of their irregular shape (in contrast thereto, primary particles are essentially spherical) and their susceptibility to mechanical stress (disintegration into smaller agglomerates and/or primary particles) This applies even to (stably) coated agglomerates, since should they disintegrate they leave smaller agglomerates and/or primary particles which are not coated or only partly coated and can thus result in the formation of new (possibly even larger) agglomerates.
It is thus an object of the present invention to provide a process for producing (essentially) agglomerate-free nanosize iron oxide particles (in particular superparamagnetic oxide particles) which not only makes it possible to break up existing primary particle agglomerates efficiently but also leads to primary particles which have a hydrolysis-resistant coating based on functional silanes (in particular based on aminosilanes) so as to reliably prevent them from (re)agglomerating.
According to the invention, it has surprisingly been found that the above object can be achieved by a process for producing an (essentially) agglomerate-free suspension of stably coated nanosize iron-containing oxide particles, which comprises the following steps in the order indicated:
(1) preparation of an aqueous suspension of nanosize iron-containing oxide particles which are partly or completely present in the form of agglomerates;
(
2
) addition (i) of a trialkoxysilane which has a hydrocarbon group which is directly bound to Si and to which at least one amino, carboxyl, epoxy, mercapto, cyano, hydroxy and/or (meth)acrylic group is bound, and (ii) of a water-miscible polar organic solvent whose boiling point is at least 10° C. above that of water;
(3) treatment of the resulting suspension with ultrasound until at least 70% of the particles present have a size within the range from 20% below to 20% above the mean particle diameter;
(4) removal of the water by distillation under the action of ultrasound; and
(5) removal of the agglomerates which have not been broken up.
The above step (5) is preferably followed by, as step (6), removal of salts from the (essentially nonaqueous) suspension which has been freed of agglomerates.
In particular, it has been found according to the invention that the polycondensation of the silane species used on the particle surface which is necessary for the stable coating of the (primary) particles can be achieved efficiently under the above conditions.
The iron-containing oxide particles used according to the invention are usually ones which may, if desired, contain not only iron ions but also other (preferably divalent) metal ions, preferably of metals selected from the group Zn, Cu, Co, Ni and/or Mn, where the content of these metals preferably does not exceed 70 metal atom % and in particular 35 metal atom %. Metals other than those mentioned may also be present in the iron-containing oxide particles, e.g. alkaline earth metals such as Ca and Mg. However, the iron-containing oxide particles are preferably pure iron oxide particles and in particular ones containing both Fe(III) and Fe(II), with the Fe(II)/Fe(III) ratio preferably being from 1/1 to 1/3. Superparamagnetic iron oxide particles are particularly preferred.
The above steps of the process of the invention are described in more detail below.
Step (1)
In step (1), an aqueous suspension of nanosize iron-containing oxide particles which are partly or completely present in the form of agglomerates is prepared. This suspension normally has a solids content of from 1 to 30% by weight, preferably from 2 to 20% by weight and particularly preferably from 3 to 10% by weight and its pH is generally from slightly acid to neutral, e.g. pH 4.5 to 7.
The origin of the agglomerated nanosize iron-containing oxide particles is of no importance, but they will usually originate from a precipitation process as is described below by means of a preferred embodiment. However, it is also possible to use, for example, iron-containing oxide particles prepared in a micro emulsion as starting materials for the process of the invention.
Finally, it may be remarked that the aqueous suspension can of course also contain (water-soluble) species which originate from the preceding steps of the iron-containing oxide preparation as long as these do not adversely affect the process of the invention, i.e. in particular do not prevent or hinder the bonding of the silane to the particle surface and the condensation between silane molecules. Such species are, for example, ions derived from inorganic and organic acids and bases, oxidizing agents, etc.
Step (2)
In step (2), a particular trialkoxysilane and also a particular water-miscible polar organic solvent are added to the above aqueous
Lesniak Christoph
Nass Rudiger
Schiestel Thomas
Schmidt Helmut
Heller Ehrman White & McAuliffe LLP
Institut für Neue Materialien gem. GmbH
Koslow C. Melissa
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