Method for preparing particles comprising a core and a silica sh

Details Method for preparing particles comprising a core and a silica sh Method for preparing particles comprising a core and a silica sh

1998-10-20

2000-10-17

Page, Thurman K.

Drug, bio-affecting and body treating compositions

Preparations characterized by special physical form

Particulate form

424403, 424489, 42840224, A01N 2534, A61K 914, A61K 916

Patent

active

061327735

DESCRIPTION:

BRIEF SUMMARY
This application is an application under 35 U.S.C. Section 371 of International Application Number PCT/FR97/00720, filed on Apr. 22, 1997.
The subject-matter of the present invention is a method for preparing particles comprising a dense silica shell and a core composed of a support made of a material other than silica by precipitation on the said core of active silica from an aqueous alkali metal silicate solution. Another subject-matter of the invention, as novel industrial products, is composite particles composed of a shell formed of dense silica and of a core made of a non-polymer organic material possessing biological activity. The particles obtained according to the method of the invention can be used as fillers for rubber, polymers, concretes or papers or as vehicles for solid active materials or active principles, in particular possessing biological activity, constituting the core or included in the core, with fast or slow release of the active material or of the active principle by destruction of the shell by mechanical or chemical action or by diffusion.
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 can be carried out in a medium of low ionic strength with a rate of addition of silicate lower than 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 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.degree. C. requires a precipitation reaction lasting between 3 and 5 hours.
The Inventor has found a novel method which makes it possible rapidly to precipitate dense active silica on a core composed of a charge other than silica, without the risk of formation of nuclei of silica particles.
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 particles comprising a dense silica shell by precipitation of active silica from an aqueous alkali metal M silicate solution, with an SiO.sub.2 /Na.sub.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, the said method being characterized in that the operation of formation of silica slurry by precipitation is carried out according to the following stages: the order of 8 to 10 comprising water, at least one organic or inorganic support other than silica which is insoluble in water under the pH and temperature conditions of the slurry formation 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 litre of vessel heel, and optionally a buffer or basic agent, at a temperature of the order of 80 to 98.degree. C.; alkali metal silicate in the form of an aqueous solution containing at least approximately 100 grams of SiO.sub.2 /litre, preferably of the order of 100 to 330 grams of SiO.sub.2 /litre, 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 8 to 10 and being maintained at a temperature of the order of 80 to 98.degree. C., until the desired amount of active silica has been formed.
The choice of the silicate and of the acidifying agent for carrying out the meth

REFERENCES:
patent: 2885366 (1959-05-01), Iler
patent: 5628932 (1997-05-01), Linton

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