Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
1998-10-20
2001-04-24
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S339000, C106S409000, C106S482000
Reexamination Certificate
active
06221326
ABSTRACT:
The subject-matter of the present invention is a method for preparing hollow particles comprising a dense silica shell by precipitation of active silica from an aqueous alkali metal silicate solution on a core composed of a material other than silica and removal of the said material without destruction of the silica shell. The said particles can be used as insulating materials, as hollow fillers for polymers, building materials, rubber, paper or paint, as absorbing agents, as dentifrice abrasives or additives, as supports for the absorption and/or the release of active material, the release of the said active material being achieved by destruction of the silica shell by mechanical rupture or by dissolution in basic medium of the silica shell or by diffusion, as sun protection agents or sun protection agent support, or for the formulation in solid form of liquid materials.
It is known to prepare heterogeneous particles composed of a dense silica shell deposited on a core composed of a charge other than silica by slow precipitation of active silica on the said core from an aqueous alkali metal silicate solution with adjustment of the pH using an acid (U.S. Pat. No. 2,885,366). According to this document, the precipitation operation must be carried out in a medium of low ionic strength with a rate of addition of silicate below a certain parameter S (expressed as weight of silica to be added per hour with respect to the weight of core to be coated) defined by the following equation
S=
(
A/
200)2
n
n being equal to (T-90)/10
A representing the specific surface, expressed in m
2
/g, of the support to be coated
and T the temperature in ° C., in order to avoid the formation of nuclei of dense particles of silica.
For this reason, the operation of precipitating the active silica is lengthy; thus, the deposition of the order of 20 parts by weight of silica on 100 parts by weight of calcium carbonate at a temperature of the order of 80 to 90° C. requires a precipitation reaction lasting between 3 and 5 hours.
When the core of the said silica particles thus obtained is composed of a compound which is sensitive to acids, hollow silica particles can then be obtained by removing the core by acid attack (U.S. Pat. No. 5,024,826).
The inventor has found a novel method which makes it possible to prepare hollow particles of dense silica by rapid precipitation of active silica on a core composed of a material other than silica, without the risk of formation of nuclei of silica particles, and then removing the said material without destruction of the dense active silica shell.
In a simplified way, “dense” is understood to mean a silica shell formed of a continuous layer composed of a silica lattice, in contrast to a layer composed of a porous assembly of individual silica particles.
The present invention thus consists of a method for preparing hollow particles comprising a dense silica shell by precipitation of active silica from an aqueous alkali metal M silicate solution, with an SiO
2
/Na
2
O ratio of at least 2, preferably of the order of 2.5 to 4, with adjustment of the pH using an acidifying agent, on a support made of a material other than silica, separation of the silica slurry formed and drying of the silica suspension recovered, with removal of the support, the said method being characterized in that
the operation of formation of silica slurry by precipitation is carried out according to the following stages:
a first stage consisting in employing an initial vessel heel with a pH of the order of 8 to 10, comprising
water
at least one organic or inorganic support other than silica, which support is insoluble in water under the pH and temperature conditions of the slurry formation operation but which is capable of being at least partially removable without dissolution or destruction of the silica shell during the subsequent removal operation,
an electrolyte salt from the group of alkali metals, the amount of electrolyte present being at least approximately 0.4 mol, preferably of the order of 0.4 to 1.5 mol, of alkali metal ion per liter of vessel heel,
optionally a buffer or basic agent, at a temperature of the order of 80 to 98° C.;
a second stage consisting in introducing, into the said vessel heel,
the alkali metal silicate in the form of an aqueous solution comprising at least approximately 100 grams of SiO
2
/liter, preferably of the order of 100 to 330 grams of SiO
2
/liter,
and the acidifying agent, under conditions such that the kinetics K of formation of active silica, expressed in grams of silica/hour/gram of support, corresponds to a value
K≧
3(
A/
200)2
n
, preferably
K≧
4(
A/
200)2
n
and very particularly
K≧
6(
A/
200)2
n
n being equal to (T-90)/10
A representing the specific surface, expressed in m
2
/g, of the support to be coated
and T the temperature in °C., the reaction mixture exhibiting a substantially constant pH of the order of 8 to 10 and being maintained at a temperature of the order of 80 to 98° C., until the desired amount of silica has been formed,
and in that particles obtained comprising an active silica shell and a core composed of the said support are subjected to an operation for the removal of the material constituting the support, without dissolution or destruction of the active silica shell.
The choice of the silicate and the acidifying agent for implementing the method of the invention is carried out in a way well known per se. The alkali metal silicate is advantageously a sodium or potassium silicate. Mention may very particularly be made of sodium silicate.
Use is generally made, as acidifying agent in the second stage of slurry formation, of an inorganic acid, such as sulphuric acid, nitric acid or hydrochloric acid, or an organic acid, such as acetic acid, formic acid or carbonic acid. It is preferably sulphuric acid. The latter can be employed in the dilute or concentrated form, preferably in the form of an aqueous solution exhibiting a concentration of the order of 60 to 400 g/l. Carbonic acid is preferably employed in the gaseous form.
Mention may be made, among materials which can be employed as support in implementing the method of the invention, of any inorganic or organic, solid or liquid compound which is inert with respect to active silica (hydroxylated silica), which is insoluble in water under the pH and temperature conditions of the slurry formation operation, which has any shape (spherical, acicular, and the like), and which is capable of being removed from the particles containing it by a treatment which depends on its chemical and physical nature.
The said support is preferably in solid form. “Compound which is inert with respect to silica” is understood to mean any compound which remains stable under the conditions for precipitation of silica. “Compound which is insoluble in water” is understood to mean any compound exhibiting a solubility in water of less than approximately 0.5% by weight at 25° C.
As regards the operation of removing the material constituting the support (core), this can be, according to the nature of the said material, a treatment by dissolution (using an acid or a non-polar solvent) for solids, followed by a separation operation; a heat treatment, whatever the material; or a simple separation operation, when it is a liquid. Whatever the nature of the material (solid, liquid, organic or inorganic), the latter can be removable by heat treatment at a temperature which is at least equal to that of vaporization or of decomposition of the said material, but which does not damage the silica shell. The solid materials which can be employed include those which can be removed by dissolution using an inorganic or organic acid at a pH of less than 8, preferably of the order of 2 to 7, or using a non-polar solvent, this dissolution operation being followed by separation, for example by centrifuging, filtration, distillation, evaporation, dialysis, osmosis, and the like. Mention may be made, among solid materials capable of being removed by dissolution using an acid, preferably in aqueous solution, of inorgan
Nguyen Ngoc-Yen
Rhodia Chimie
Seugnet Jean-Louis
LandOfFree
Method for preparing hollow silica particles does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for preparing hollow silica particles, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for preparing hollow silica particles will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2472682