Process for preparing finely divided hydrophobic oxide particles

#This invention relates to a process for preparing hydrophobic finely divided particles of oxides of metals and oxides of silicon by chemically bonding hydrocarbon radicals to the surface of the oxide particles. More particularly, the oxides are treated with hydrolysable metal or metalloid compounds containing OR groups.

#It is known in the art that particles can be rendered hydrophobic by treatment with organo -silanes to form metals or metalloids on the surface of the particles. Such products are particularly useful when the hydrocarbon radicals (preferably methyl) are chemically bonded to the surface of finely distributed oxides, which renders the oxides, depending upon the quantity of hydrocarbon radicals present, either partially or completely hydrophobic.

#It is characteristic of this type of treated particle that the transition from entirely hydrophilic to entirely hydrophobic behavior first occurs with a certain "degree of hydrophobing", more or less immediately without any distinct intermediate phase. Thus for example, products of this type with only 0.3% C or less possess hydrophilic characteristics and behave like the untreated materials. On the other hand, products with at least about 0.5 - 0.7% C are practically entirely hydrophobic depending on the specific surface characteristics of the starting oxides. It would be desirable to be able to prepare products having a degree of hydrophobing lying between these extremes, which behave like mixtures of hydrophobic and hydrophilic oxides. However, experience has shown that it is not possible to obtain products which are hydrophobic and cannot be wetted with water, but which nevertheless retain certain hydrophilic characteristics. For certain purposes, for example, reinforcing agents for silicon rubber and other elastomers, it is desirable to have fillers which are hydrophobic and distinctly organophilic, but which also have an affinity for protons. The previously mentioned hydrophobic products do not possess this latter characteristic since the alkyl or aryl radicals, bonded to silicon atoms, possess no affinity for protons. Also, small residues of, for example, silanol groups, on the oxide particles (such as occur in a truly hydrophobic product) are screened by large numbers of alkyl groups in such a way that the particle does not exhibit proton affinity. This latter effect has actually become a quality characteristic of such products.

#The process of this invention will be more clearly understood by reference to the following examples:

#a. 500 g of a pyrogenic silicic acid (surface area 120 m.sup.2 /g BET) are placed in a vessel after activation, and put under a vacuum. 50 g (of 10% by weight) tetra-n-butoxy silane are dripped in at room temperature within a few minutes, while it is briskly agitated. After the vacuum is neutralized with N.sub.2, a dry NH.sub.3 stream of gas is introduced into the product for 2 - 3 seconds. A sample taken 30 minutes after conducting ammonia through it remains completely unwettable even after intensive shaking with water; the aqueous phase is completely clear.

#b. The experiment described in Example 1(a) is carried out in the same manner, but without the treatment with NH.sub.3.