Method for improving adhesion of silicone resin coating composition

DETAILED DESCRIPTION OF THE INVENTION The coating compositions used in this invention are prepared by hydrolyzing an alkyltrialkoxysilane or aryltrialkoxysilane of the formula RSi(OR).
sub.
3, wherein R is alkyl of from 1 to 3 carbons or aryl, such as phenyl, in an aqueous dispersion of colloidal silica, and then admixing an aggressive solvent compound with the resultant reaction product.
Preferably they are aged at least about 5 days at a temperature of at least about 18.
degree.
C.
, before the admixing step.
In the practice of the present invention, suitable aqueous colloidal silica dispersions generally have a particle size of from 5 to about 150 millimicrons in diameter.
These silica dispersions are well known in the art and commercially available ones include, for example, those sold under the trademarks of Ludox (duPont) and Nalcoag (NALCO Chemical Co.
).
Such colloidal silicas are available as both acidic and basic hydrosols.
For the purposes of this invention, wherein the pH of the coating compositions is on the basic side, basic colloidal silica sols are preferred.
However, acidic colloidal silicas, wherein the pH is adjusted to a basic level, are also contemplated.
In addition, it has been found that colloidal silicas having a low alkali content (e.
g.
, Na.
sub.
2 O) yield a more stable coating composition.
Thus, colloidal silica having an alkali content of less than 0.
35% (calculated as Na.
sub.
2 O) are preferred.
Moreover, colloidal silicas having average particle size of from 10 to 30 millimicrons are also preferred.
A particularly preferred one for the purposes herein is known as Ludox LS, sold by duPont Company.
In accordance with this invention, the aqueous colloidal silica dispersion is added to a solution of a small amount of alkyltriacetoxysilane in alkyltrialkoxysilane or aryltrialkoxysilane.
The temperature of the reaction mixture is maintained at about 20.
degree.
C.
to about 40.
degree.
C.
and preferably below 25.
degree.
C.
It has been found that in about six to eight hours sufficient trialkoxysilane has reacted to reduce the initial two-phase liquid mixture to one liquid phase in which the now treated silica (i