Solar reflective composite granules and method of making solar reflective composite granules
A method of making a plurality of composite granules can include: forming green body granules comprising an aluminosilicate; heating the green body granules to form sintered granules; cooling the sintered granules according to a cooling regime, wherein the cooling regime comprises a temperature hold between 700° C. and 900° C. for at least one hour. In a particular embodiment, the aluminosilicate for making the composite granules can have a particle size less than 150 μm. The composite granules are particularly suitable as roofing granules and can have a desired combination of high solar reflectance SR and low lightness L*, a low bulk density, good weather resistance and strength.
1. A plurality of composite granules, comprising:
an amount of aluminosilicate of at least 70 wt % based on the total weight of the composite granules;
an amount of aluminum trihydrate of at least 2 wt % based on the total weight of the composite granules;
a solar reflectance (SR) of at least 25%; and
a L* value is not greater than 50; and,
wherein the composite granules are free of a coating which increases solar reflectance.
2. The plurality of composite granules of claim 1 , wherein the composite granules are essentially free of titanium dioxide, zinc oxide, magnesium hydroxide.
3. The plurality of composite granules of claim 1 , wherein a bulk density of the composite granules is not greater than 1.2 g/cm 3 .
4. The plurality of composite granules of claim 1 , wherein a crush strength of the composite granules is at least 45 N.
5. The plurality of composite granules of claim 1 , wherein the composite granules further comprise titanium dioxide.
6. The plurality of composite granules of claim 1 , wherein the composite granules further comprise zinc oxide.
7. The plurality of composite granules of claim 1 , wherein the composite granules further comprise magnesium hydroxide.
8. A method of making a plurality of composite granules, comprising:
forming green body granules comprising an aluminosilicate, water, and aluminum trihydrate, wherein the aluminosilicate has an average particle size of at least 30 microns and not greater than 150 microns; and an amount of the aluminosilicate is at least 70 wt % based on the total weight of the green body granules; and
heating the green body granules to a maximum sintering temperature of at least 1000° C. and not greater than 1300° C.,
wherein the plurality of composite granules comprise an amount of aluminum trihydrate of at least 2 wt % based on the total weight of the composite granules; an amount of aluminosilicate of at least 70 wt % based on the total weight of the composite granules; a solar reflectance (SR) of at least 25%; and a L* value not greater than 50; and wherein the composite granules are free of a coating which increases solar reflectance.
9. The method of claim 8 , wherein the aluminosilicate is a natural rock including rhyolite, andesite, basalt, meta-basalt, or any combination thereof.
10. The method of claim 9 , wherein a maximum sintering temperature is not less than 50° C. below a color drop temperature T cd of the aluminosilicate.
11. The method of claim 10 , wherein a maximum sintering temperature is not less than 25° C. below a color drop temperature T cd of the aluminosilicate.
12. The method of claim 8 , wherein the green body granules are essentially free of titanium dioxide, zinc oxide, and magnesium hydroxide.
13. The method of claim 8 , wherein the aluminum trihydrate has an average particle size of not greater than 30 microns.
14. The method of claim 8 , wherein the green body granules further comprise titanium dioxide, or zinc oxide, or magnesium hydroxide, or any combination thereof.
15. The method of claim 8 , further comprising controlled cooling the green body granules at a speed not greater than 2° C./minute.
16. The method of claim 8 , further comprising controlled cooling the green body granules, wherein controlled cooling comprises conducting a temperature hold between 700° C. and 900° C. for at least one hour.