Solar-reflective roofing granules, roofing products including them, and methods for making them
Provided are roofing granules, such as solar-reflective roofing granules, methods for making them and their use in roofing products. In one aspect, the disclosure provides a collection of solar-reflective roofing granules having a solar reflectivity of at least 70%, wherein substantially each roofing granule comprises an inner layer, an outer layer disposed on an outer surface of the inner layer and substantially surrounding the inner layer, wherein the inner layer and the outer layer are formed from substantially the same composition but have substantially different porosities, the outer layer having a porosity substantially lower than the porosity of the outer layer. Methods for making such roofing granules can include forming layers of substantially the same first and second compositions, wherein the forming of the layer of the second composition is performed under substantially different conditions from the forming of the layer of the first composition.
1. A collection of solar-reflective roofing granules having a solar reflectivity of at least 70%, wherein substantially each roofing granule comprises an inner layer, an outer layer disposed on an outer surface of the inner layer and substantially surrounding the inner layer, wherein the inner layer and the outer layer are formed from substantially the same composition but have substantially different porosities, the outer layer having a porosity substantially lower than the porosity of the inner layer, wherein the inner layer has a porosity of at least 20%.
2. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the composition of the outer layer is at least 98% common with the composition of the inner layer.
3. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the porosity of the inner layer is in the range of 20-50%.
4. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the porosity of the outer layer is no more than 25% of the porosity of the inner layer.
5. The collection of solar-reflective roofing granules according to claim 1 , wherein the inner layer is configured as a granule core, and wherein in substantially each roofing granule the inner layer has a thickness of at least 250 microns and no more than 5 mm.
6. The collection of solar-reflective roofing granules according to claim 1 , wherein the inner layer is configured as a coating on a base particle, and wherein in substantially each roofing granule the inner layer has a thickness of at least 20 microns and no more than 200 microns.
7. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the outer layer has a thickness of at least 5 microns and no more than 100 microns.
8. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the outer layer is a discrete layer from the inner layer, having a discrete boundary therebetween.
9. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the outer layer forms the outermost layer of the roofing granule.
10. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule a coating is formed on the outer layer.
11. The collection of solar-reflective roofing granules according to claim 1 , wherein the collection of solar-reflective roofing granules has a solar reflectivity of at least 75%.
12. The collection of solar-reflective roofing granules according to claim 1 , wherein the solar-reflective roofing granules of the collection are white in color, and the collection of solar-reflective roofing granules has a (a* 2 +b* 2 ) 1/2 less than 6.
13. The collection of solar-reflective roofing granules according to claim 1 , wherein the solar-reflective roofing granules of the collection substantially comprise a base particle having the inner layer disposed thereon, and the outer layer disposed on the outer surface of and surrounding the inner layer.
14. The collection of solar-reflective roofing granules according to claim 1 , wherein the solar-reflective roofing granules of the collection substantially comprise the inner layer as a granule core, and the outer layer disposed as a coating on the inner layer.
15. The collection of solar-reflective roofing granules according to claim 1 , wherein the composition of the inner layer comprises a binder and a solar reflective pigment, and wherein the composition of the outer layer comprises the binder and the solar reflective pigment.
16. The collection of solar-reflective roofing granules according to claim 15 , wherein in substantially each roofing granule the porosity of the outer later layer is less than 10%.
17. The collection of solar-reflective roofing granules according to claim 15 , wherein the binder is selected from the group consisting of metal silicates, aluminosilicates, clays, fluoropolymers, metal phosphates, silica coating binders, sol-gel coating binders, polysiloxanes, silicones, polyurethanes, and polyacrylates, and wherein the solar-reflective pigment is selected from the group consisting of titanium dioxide, calcium carbonate, zinc oxide, lithopone, zinc sulphide, white lead, glass microspheres, glass microbubbles, microvoid pigments, and synthetic polymeric opacifiers.
18. The collection of solar-reflective roofing granules according to claim 1 , wherein the composition of the inner layer comprises an aluminosilicate clay and the composition of the outer layer comprises the aluminosilicate clay.
19. The collection of solar-reflective roofing granules according to claim 1 , wherein each of the first layer and the second layer comprises sodium silicate and titanium dioxide.
20. The collection of solar-reflective roofing granules according to claim 1 , wherein in substantially each roofing granule the composition of the outer layer is at least 95% common with the composition of the inner layer.
21. A roofing product comprising
a substrate;
a bituminous material coated on the substrate, the bituminous material having a top surface; and
the collection of solar-reflective roofing granules according to claim 1 disposed on the top surface of the bituminous material, thereby substantially coating the bituminous material in a region thereof.
22. The roofing product according to claim 21 , having a solar reflectivity of no less than 60%.
23. A method for making a collection of solar-reflective roofing granules according to claim 1 , the method comprising, for substantially each roofing granule,
forming a layer of a first composition; and
forming a layer of a second composition disposed on an outer surface of and substantially surrounding the layer of the first composition,
wherein the second composition is substantially the same composition as the first composition, and
wherein the forming of the layer of the second composition is performed under substantially different conditions from the forming of the layer of the first composition.