Mineral roofing granules and methods for making them
The present disclosure relates to roofing granules, such as solar-reflective roofing granules having one or more of low crystalline silica content, high stain resistance and algae resistance. The present disclosure provides a mineral roofing granule having at its mineral outer surface a first fired mixture comprising an aluminosilicate clay, the first fired material having no more than 2 wt % crystalline silica. The present disclosure also provides a mineral roofing granule having a mineral body and a mineral outer surface, the mineral roofing granule having at its mineral outer surface a first fired material, the first fired material being a first fired mixture comprising an aluminosilicate clay; one or more of a feldspar, a sodium silicate and a nepheline syenite; and, optionally, a zinc source.
1. A mineral roofing granule comprising a mineral body and a mineral outer surface, the mineral roofing granule having at its mineral outer surface a first fired material, the first fired material being a fired product of a first fireable mixture comprising:
kaolin clay present in the first fireable mixture in an amount in the range of 40-90 wt %;
a zinc source present in the first fireable mixture in an amount in the range of 10-30 wt %; and
one or more of
a feldspar present in the first fireable mixture in an amount in the range of 5-40 wt % and
a nepheline syenite present in the first fireable mixture in an amount in the range of 5-40 wt %.
2. The mineral roofing granule of claim 1 , wherein the mineral outer surface has a surface porosity of no more than about 5% as measured by mercury porosimetry.
3. The mineral roofing granule of claim 1 , wherein the first fireable mixture comprises the feldspar.
4. The mineral roofing granule of claim 3 , wherein the feldspar is present in the first fireable mixture in an amount in the range of 10-40 wt %.
5. The mineral roofing granule of claim 1 , wherein the first fireable mixture comprises the nepheline syenite, the nepheline syenite being present in an amount in the range of 10-40 wt %.
6. The mineral roofing granule of claim 1 , wherein at least 50% of the zinc present in the first fired material is present as a zinc oxide, a zinc aluminosilicate or a zinc silicate.
7. The mineral roofing granule of claim 1 , wherein the first fired material comprises ZnAl 2 O 4 , wherein no more than 40% of the zinc present in the first fired material is present as ZnAl 2 O 4 .
8. The mineral roofing granule of claim 1 , having a density in the range of about 2.25 g/mL to about 3.25 g/mL.
9. The mineral roofing granule of claim 1 , wherein the mineral body comprises a mineral shell disposed about a mineral core, the outer surface of the mineral shell being the mineral outer surface of the mineral roofing granule, the mineral shell being of the first fired material.
10. The mineral roofing granule of claim 1 , having a solar reflectance of at least 80%.
11. The mineral roofing granule of claim 1 , having a stain resistance drop of no more than 3 percentage points.
12. The mineral roofing granule of claim 1 , lacking a coating disposed on its mineral outer surface.
13. The mineral roofing granule of claim 1 , wherein the first fired mixture comprises ZnAl 2 O 4 and Zn 2 SiO 4 .
14. The mineral roofing granule of claim 1 , wherein the mineral body has a porosity greater than the mineral outer surface.
15. A roofing membrane or shingle including a collection of mineral roofing granules of claim 1 embedded in a bituminous surface of the roofing membrane or shingle.
16. A mineral roofing granule comprising a mineral body and a mineral outer surface, the mineral roofing granule having at its mineral outer surface a first fired material, the first fired material being a fired product of a first fireable mixture comprising:
kaolin clay present in the first fireable mixture in an amount in the range of 40-90 wt %;
a zinc source present in the first fireable mixture in an amount in the range of 10-30 wt %; and
a nepheline syenite present in the first fireable mixture in an amount in the range of 5-40 wt %,
wherein the first fireable mixture has less than 0.5 wt % feldspar.
17. The mineral roofing granule of claim 16 , wherein the first fireable mixture comprises the nepheline syenite present in an amount in the range of 10-40 wt %, and wherein the first fireable mixture has less than 0.5 wt % feldspar and less than 0.5 wt % sodium silicate.
18. A mineral roofing granule comprising a mineral body and a mineral outer surface, the mineral roofing granule having at its mineral outer surface a first fired material, the first fired material being a fired product of a first fireable mixture comprising:
kaolin clay present in the first fireable mixture in an amount in the range of 40-90 wt %;
a zinc source present in the first fireable mixture in an amount in the range of 10-30 wt %; and
a feldspar present in the first fireable mixture in an amount in the range of 5-40 wt %,
wherein the first fireable mixture has less than 0.5 wt % nepheline syenite.
19. A method for making a mineral roofing granule of claim 1 , comprising
providing a green granule having at its outer surface a first fireable mixture comprising an kaolin clay and one or more of a feldspar, a sodium silicate and a nepheline syenite, and
firing the green granule to provide the mineral roofing granule, the firing converting the first fireable mixture to the first fired material.