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 method of making a plurality of composite granules, comprising:
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,
wherein a difference in solar reflectance ΔSR of composite granules subjected to the temperature hold (SR TH ) in comparison to composite granules not subjected the temperature hold (SR F ) is at least 3%, with ΔSR=SR TH −SR F .
2. The method of claim 1 , wherein the temperature hold is at least two hours.
3. The method of claim 1 , wherein the solar reflectance of the composite granules is at least 25%.
4. The method of claim 1 , wherein the L* value of the composite granules is not greater than 50.
5. The method of claim 1 , wherein forming of the green body granules includes adding of a solvent and a binder.
6. The method of claim 5 , wherein the binder is modified starch and the solvent is water.
7. The method of claim 1 , wherein forming of the green body granules includes adding of ATH in an amount of at least 2 wt % based on the total dry weight of the green body granules.
8. The method of claim 7 , wherein the ATH has a median particles size not greater than 30 μm.
9. The method of claim 1 , further including sizing the green body granules to a size of at least 0.3 mm and not greater than 3.0 mm.
10. The method of claim 1 , wherein heating is conducted up to a maximum sintering temperature of at least 1000° C.
11. The method of claim 1 , wherein heating is conducted up to a maximum sintering temperature of not greater than 1300° C.
12. The method of claim 1 , wherein the aluminosilicate is a natural rock including rhyolite, andesite, basalt, meta-basalt, or any combination thereof.
13. The method of claim 1 , wherein an amount of the aluminosilicate in the green body granules is at least 50 wt % based on the total weight of the green body granules.
14. The method of claim 1 , wherein the aluminosilicate of the green body granules has an average particle size not greater than 150 μm.
15. The method of claim 1 , wherein a bulk density of the composite granules is not greater than 1.0 g/cm 3 .
16. The method of claim 1 , wherein the composite granules have an average crush strength of at least 45 N.
17. The method of claim 1 , wherein the green body granules are essentially free of TiO 2 , ZnO 2 , and Mg(OH) 2 .
18. A plurality of composite granules, comprising:
a bulk density not greater then 1.0 g/cm 3 ;
an amount of aluminosilicate of at least 50 wt % based on a 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 of at least 25%;
an L* value of not greater than 50; and
an average crush strength of at least 45 N.
19. The plurality of composite granules of claim 18 , wherein the composite granules are free of a coating.