Process for improving adhesion of polysiloxane coatings to polymeric substrates via reduced alkali metal cation content

DETAILED DESCRIPTION In accordance with this invention, alkali metal cations are removed by ion exchange either (1) on the silica hydrosol before solution preparation, or (2) after the solution has been prepared and aged to promote condensation, or (3) after the hydrosol/silane mixture is hydrolyzed and diluted followed by aging to promote condensation.
Of these, alternative (2) is preferred.
Solutions made according to (1) do not demonstrate as high a level of abrasion resistance as solutions produced according to alternative (2).
Solutions prepared according to alternative (2) demonstrate better abrasion resistance if the solution is coated soon after ion exchange is effected.
The polysiloxane coating compositions treated in accordance with this invention may be prepared by the procedures described in the above-mentioned patents and European Patent Application, the disclosures of which are incorporated herein by reference.
Preferably, the coating solution comprises (a) from about 5 to 50 weight percent solids, the solids comprising from about 10 to 70 weight percent silica and from about 90 to 30 weight percent of a partially polymerized organic silanol of the general formula RSi(OH).
sub.
3, wherein R is selected from methyl and up to about 40 percent of a radical selected from the group consisting of vinyl, phenyl, 3-glycidoxypropyl, and 3-methacryloxypropyl, amino-functional silanes and (b) from about 95 to 50 weight percent solvent comprised of from about 10 to 90 weight percent water and from about 90 to 10 weight percent lower aliphatic alcohol.
The alkali metal cations are removed best by ion exchange techniques.
One method is to pass the hydrosol or finished polysiloxane solution through a cation exchange resin in the H.
sup.
+ form to replace the alkali metal ions with an ion which does not cause blistering or adhesion loss.
It has been found advantageous to deionize the hydrosol or coating solution using a cationic ion exchange resin in the H.
sup.
+ form and an anionic resin in the OH