Post-functionalized roofing granules and process for preparing same

1. A process for preparing functionalized roofing granules, the process comprising:

(a) providing base roofing granules having a first coating layer disposed on an inert mineral core, the first coating layer including at least one metal oxide colorant disposed in a binder based on an alkali metal silicate; then

(b) coating the base roofing granules with a second coating composition to form a second coating layer over the first coating layer, the second coating layer being a sodium ion barrier formed from silica glass or crystalline silicaceous material;

(c) curing the second coating layer on the base roofing granules to form intermediate granules;

(d) coating the intermediate granules with a transparent or translucent outer coating composition, the outer coating composition comprising an outer coating binder and at least one functional material, the outer coating binder being one or more of colloidal silica and organic polymer, the outer coating not including a binder based on an alkali metal silicate, an alkaline earth metal silicate, or a metal phosphate, the at least one functional material being photocatalytic anatase titanium dioxide having an average particle size less than 0.2 microns; and

(e) curing the outer coating composition to form a transparent or translucent outer coating layer on the base roofing granules.

2. A process according to claim 1 wherein the second coating layer is formed from silica glass.

3. A process according to claim 1 wherein the outer coating composition comprises a colloidal silica binder.

4. A process according to claim 3 , wherein the colloidal silica is prepared by treating a tetralkoxysilane, an organotrialkoxysilane and/or a siloxane oligomer with acid.

5. A process according to claim 4 , wherein the colloidal silica is prepared by treating one or more of tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetra-n-propoxysilane, tetra-n-butoxysilane, tetrakis(2-methoxyethoxy)silane, methyltriethoxysilane (MTEOS), methyltrimethoxysilane, methyl tri-n-propoxysilane, phenyl triethoxysilane, vinyl triethoxysilane, hexamethoxydisiloxane, and octamethoxytrisiloxane.

6. A process according to claim 1 , wherein the outer coating binder includes colloidal silica and wherein the outer coating does not include an organic polymer.

7. A process according to claim 1 , wherein the outer coating binder includes at least one organic polymer.

8. A process according to claim 7 , wherein the at least one organic polymer is selected from the group consisting of polyurethane polymers, acrylic polymers, polyurea polymers, silicone polymers, siliconized polymers, and sulfo-urethane silanol-based polymers.

9. A process according to claim 7 , wherein the outer coating binder is an acrylic polymer.

10. A process according to claim 1 , wherein the outer coating layer is mesoporous.

11. A process according to claim 1 , wherein the outer coating binder includes colloidal silica and wherein the outer coating does not include an organic polymer, and wherein the outer coating is dried at a temperature between about 20 and 80 degrees C.

12. The process according to claim 1 , wherein the first coating layer has a color, and wherein an apparent color of the first coating layer in the functionalized roofing granules is not significantly altered by the outer coating.