This invention relates to barium sulfate having an average particle diameter d50 of from 0.1 to 10 μm and precipitated by mixing of barium- or of sulfate-containing solution, a process for its preparation and its use.
Such barium sulfate is used in particular in the production of photographic paper, paints and plastics as well as in medical diagnosis and in medical plastics. For use in the medical field, in addition to the high standards as regards purity, there are also legal requirements for pharmaceutical products which have to be complied with. Barium sulfate is a high-quality, pure white, inert, acid- and alkali-resistant filler which is completely weather-resistant.
Very finely-divided barium sulfate imparts colour stability and gloss to photographic paper. When used in paints, the very finely-divided barium sulfate produces resistance to flocculation in organic and inorganic pigments, so that segregation phenomena are largely prevented, depth of colour and purity of colour are increased and brightness and fogging are improved. Moreover, the mechanical properties of the plastics are improved as a result of the nucleating action of the barium sulfate powder.
In stretched films made from thermoplastics, in particular polymers containing incorporated very finely-divided barium sulfate, the latter acts as a white pigment, scattering light at the boundaries of the barium sulfate particles to the air, so that the films have an opaque colouring and a high gloss. A suitable thermoplastic is, in particular polyester, in which glycol suspension containing barium-sulfate has been incorporated, generally during the polycondensation. Here an adequate dispersion of the barium sulfate particles in the polyester is an essential precondition for opacity and high gloss in the stretched films. In the event of inadequate dispersion of the barium sulfate particles in the polymer, specks—these are agglomerates of barium sulfate particles—are formed after the stretching process and these can considerably impair the quality of the film surface.
DE-A 26 47 713 describes an opaque, simple thermoplastic film support made of a molecularly oriented film of a synthetic, linear polyester, in which 5 to 50 wt. %, based on the weight of the polyester, of barium sulfate particles having an average particle size of from 0.5 to 10 μm is dispersed, with the proviso that the actual particle size of 99.9% of the particles does not exceed 50 μm and that at least one light-sensitive image layer is disposed on at least one surface of the film support.
These film supports are highly impermeable to moisture and are therefore suitable for use as supports for photographic prints and copies for which it is possible to avoid a considerable part of the washing process which normally has to be carried out in the treatment of prints and copies on supports made of paper.
DE-C 44 31 735 discloses the coating of barium sulfate particles with phosphorus pentoxide, aluminium oxide and silicon dioxide in order to achieve an excellent adhesion between the barium sulfate particles serving as filler and the polymer, so that the light scattering does not occur.
DE-A 22 46 818 provides a continuous process for the preparation of crystalline barium sulfate powder having an average particle size d50>2 μm to 20 μm by precipitating barium sulfate from barium chloride solution using sulfuric acid, wherein the barium sulfate particles having the intended particle size are deposited in the outlet of the precipitation vessel and the smaller barium sulfate particles remain in the upper region of the precipitation vessel.
DE-A 36 35 050 provides a process for the continuous preparation of barium sulfate having a defined particle-size range, wherein barium sulfate is precipitated in a first step by the reaction of barium ions with sulfate ions. The suspension containing barium sulfate is withdrawn and transferred to a single-stage or multistage wet-grinding step, in which the grinding to the required particle-size range is carried out. The barium sulfate and mother liquor are subsequently separated.
The barium sulfate powder chemically precipitated by the known methods has an undesirable tendency to form agglomerates which prevent a homogeneous dispersion of the barium sulfate particles as white pigment and/or filler in the thermoplastics.
The object of the present invention is to provide barium sulfate precipitated by mixing of barium- and of sulfate-containing solution and, having an average particle diameter d50 of from 0.1 to 10 μm; this barium sulphate is to show no tendency to form agglomerates and consequently is to have excellent properties as regards dispersibility and freedom from specks and is to produce an optimal film surface.
This object is achieved by covering the barium sulfate with at least one organic additive, the organic additive being selected from one or more of the substances or groups of substances: carboxylic acids, soaps, metallic soaps, alcohols; polyhydric alcohols, such as pentaerythritol, neopentyl glycol and trimethylolpropane; polyalcohols, such as polypropylene glycol and polyethylene glycol; organic esters, such as neopentyl glycol dibenzoate; silanes, siloxanes, silicone oils, organic sulfones corresponding to the formula RSO2R, organic ketones (R—(C═O)—R), organic nitriles (RCN), organic sulfoxides (R2—SO2), organic amides (R—(C═O)—NR′R or R—(S═O)—ONR′R).
The organic additive content is given in claims 2 and 3.
The organic additives can be used in liquid or in solid form.
The barium sulfate particles can be covered with organic additive by a process described in claims 4 to 6. Further developments of this process are given in claims 7 to 9.
To achieve an optimal distribution of liquid organic additives on the surface of the barium sulfate particles, the solution containing organic additive is sprayed onto the barium sulfate particles with an even-speed mixer, or the solution is added to a suspension of BaSO4 and the mixture is homogenised by means of a dispersing apparatus.
When solid organic additives are used, first of all an aqueous or organic solution or suspension is prepared and is then sprayed onto the barium sulfate particles. In this way a very homogeneous distribution of the organic substances on the surfaces of the barium sulfate particles is achieved.
A particular advantage is that the barium sulfate particles aftertreated according to the invention render possible the preparation of masterbatches (pigment and filler concentrates) and compounds (mixtures) of thermoplastics in which the barium sulfate particles are so excellently dispersed that high-quality products can be made from them without special additional expense. The superior dispersibility in plastics of the barium sulfate particles aftertreated according to the invention is clearly shown by the fact that no specks arise in these plastics containing barium sulfate and the filter does not clog during a pressure filter test.
The barium sulfate particles according to the invention are suitable in particular for dispersion in plastics compositions, such as thermoplastics, thermosetting plastics and elastomers. The plastics thus treated are used in the production of fibres, coating materials, adhesives, films, film supports and the like.
The subject matter of the invention is explained in more detail by the following Examples.
First of all, a BaSO4 suspension was prepared. For this purpose, an aqueous solution of Na2SO4 and an aqueous solution of BaCl2 in a molar ratio of 1:0.98 were simultaneously introduced into a precipitation apparatus. The precipitation conditions were established as in the prior art, in such a way that the precipitated BaSO4 in the aqueous solution had an average particle size d50=0.52 μm. The precipitated BaSO4 was then filtered and washed several times with deionised water. The filter cake obtained was dispersed in water. The suspension of barium sulfate had a solids content of approximately 250 g/l and was further processed in the following manner.