PROCESS FOR PRODUCING BORIC ACID SUSPENSIONS

The present invention relates to a process for pro¬ ducing suspensions containing finely divided boric acid.

Boric acid and its salts are frequently used as bio- cidic components in wood preservatives, as for example, wood- preserving salt pastes, and also for the fire protection treatment of wood and particle boards, particularly of chip boards, as well as in the production of fire protecting agents.

However, the low solubility of the boric acid and of the borates in water causes difficulties. Since a uniform distribution is thus kept within narrow limits, the aqueous boric acid solution or borate solution must be replaced by another composition.

According to DE-PS 21 40 451 highly concentrated solutions of boric acid in water-soluble amines or polyamines are used. However, an amino component which has a detrimental effect in many compositions is thus additionally put into cor¬ responding mixtures. Furthermore, after the application these amines are washed off or out or they evaporate on moulding the chip board and thus result in a burden on the environment.

This also analogously applies to the solution of boric acid and ethylene glycol used according to EP- B 00 46 380.

According to DE-OS 24 21 289, DE-OS 25 30 295 and DE-OS 26 21 7 39 fire-retarding chip boards are produced by a process in which the wood chips serving for producing the boards are sprayed with the aqueous solution of a boron com¬ pound and an inorganic acid prior to bonding and the boric acid thus is produced directly on the chips in the finest dis¬ tribution. .

The process is expensive with respect to the appara¬ tus and its application is not simple since the acid must be - 1 1302(313 dosed and sprayed exactly and the process thus can easily result in imperfect products.

Therefore, the present invention produces boric acid in a manner as simple as possible and at a favourable cost in a form of application that is free from solvents, easy on the environment and can be handled reliably and, even at high dosages, assures a homogeneous distribution.

According to the present invention therefore there is provided a composition containing boric acid and its salts, in which the boric acid is present in a concentrated, finely divided suspension in an aqueous medium apart from small amounts of a borate.

The present invention also provides a process for producing a boric acid suspension in which boric acid, sus¬ pended in water, is mixed with an alkaline aqueous solution and fluidized for a short period.

It has now been found that a concentrated suspension of finely divided boric acid with small amounts of borate in wood-preserving salt pastes or in the production of particle boards assures a distribution of boric acid as homogeneous as possible as obtained by corresponding solutions of the boric acid in organic liquids. In addition, these suspensions are safe to handle and do not burden either the environment or the agents containing them with undeslred organic compounds.

Furthermore, it has also been found that the compo¬ sitions according to the present invention can be produced in a simple manner by mixing a suspension of boric acid in water with an alkaline solution followed by fluidizing for a short period. By controlling the reaction in a suitable manner and.

when required, by adding dispersing agents a suspension that is stable on storage is obtained.

This process seems to be costly considering the fact that boric acid suspensions can also be produced from ground boric acid. However, this method would be disadvantageous for the following reasons:- The higher the degree of fineness the more expensive the grinding process will be and the fineness obtained by means of the process according to the present invention can be attained only with very great expenditure for grinding. Finely ground powders are dusty. Therefore, spe¬ cial safety regulations must be maintained for processing them.

Furthermore, no uniform particle size is obtained upon grinding but always a mixture of crystal fragments of varying size. A suspension produced therefrom does not result in a homogeneous distribution of the solid as is the case in the suspension according to the present invention. The broken crystal fragments have a thickening effect so that the suspen¬ sion requires a larger amount of water.

For these reasons not only is the suspension pro¬ duced by means of the process according to the present Inven¬ tion simpler in its production process, but in its properties it also is superior to the suspension of ground crystal frag¬ ments .

Technical coarse-crystalline boric acid, which is suspended with water in the weight ratio of 1:1 to 5:1, is used. This suspension is mixed with an alkaline aqueous solu¬ tion and fluidized for a short period. The alkaline solution is added in an amount such that the molar ratio of boric acid to the alkaline agent lies in the range from 1:0.01 to 1:0.2.

Since boric acid, though in small amounts, dissolves in water or with the alkaline solutions it causes salt forma- tion, efforts will have to be made to keep the amount of water and thus also the alkalisolution as low as possible. How¬ ever, the suspension must still contain an amount of water such that the boric acid can be fluidized- This depends on the method selected and, for example, in the case of the agitating operation also on the type and power of the agitator. The alkaline agent added to the aqueous solution reacts with the boric acid while forming a relatively small amount of the corresponding borate. For the intended uses of the suspension the borate content thus obtained has no detrimental effect. On the contrary, when applying the suspension to fire protection purposes a content of, e.g. , ammonium borate even is quite desirable.

In the "reaction" of the boric acid in accordance with the present invention a person skilled in the art merely expects an acid-base reaction in the sense of the neutralization of the small amount of alkali by the stoichiometrically corresponding amount of boric acid.

However, surprisingly enough, apart from this neutralization which can easily be determined macroanalytically, substantially more reactions proceed since on completion of the reaction the entire boric acid is present in the fine- crystalline suspended form.

A dilute solution of an alkali or alkaline earth metal hydroxide or of a basic ammonium, alkali or alkaline earth metal salt or a hot solution of hexamethylenetetramine can be used as alkaline solution. These basic salts are, for example, carbonates, phosphates, pyrophosphates or polyphosphates. A concentrated ammonia solution is preferably used.

Small amounts of commercial dispersing aids (0.1 to 1.0% by weight, relative to the total mixture) can also be added to the suspension.

Immediately upon adding the alkali, the boric acid is fluidized for a short period so that all the crystals can interact with the alkaline solution. This fluidization can be carried out by means of various conventional methods, for - 4a - <i 13030X3 example, by shaking, by blowing a gas through, by pump circu¬ lation or preferably by stirring.

The time during which this treatment is carried out is between 15 and 60 minutes depending on the temperature of the mixture, which can be in the range of 0 to 80 C, the amount of the liquor used and on the crystal size of the boric acid used.

For boric acid having an average crystal size of 0.2 mm the treatment time, i.e., the time during which the boric acid must be fluidized, is 30 minutes at room temperature and at a molar ratio of boric acid to liquor of 1:0.05. There¬ after, a suspension having the finest boric acid particles and borate crystals, whose size is primarily in the order of below 1000 nm, has been formed by a recrystallization process whose mechanism is not yet fully understood.

The boric acid suspensions obtained are outstand¬ ingly suitable for use in the production of chip boards or moulded fibre boards which are flame-resistant or are pro¬ tected against lignicidal organisms when they are admixed with the chips, fibres or the glue during the production process of these boards. Together with other materials they are also suitable for the production of wood preservatives and fire- protecting agents in the form of pastes in cases where dust formation presents a problem.

THE EMBODIMENTS OF THE INVENTION TO WHICH AN EXCLUSIVE