Hyperbright white roofing granules with high solar reflectance
The invention provides a bright white refractory roofing granule, comprising a ceramic material formed from a substantially homogenous mixture of a ceramic-forming clay, sintering material, and optionally comprising silica particles, and other potential additives, said bright white refractory roofing granule having a total solar reflectance of at least 0.80 and a Hunter Color Lvalue of at least 85.0, together with processes for making and using the same.
1. An article comprising:
clay;
crystalline silica particles,
wherein the crystalline silica particles are present in an amount of 10 wt % to 40 wt % by weight of the article; and
a sintering aid selected from calcium hydroxide, sodium bicarbonate, sodium carbonate, sodium silicate, feldspar, nepheline syenite, or any combination thereof;
wherein the article comprises a plurality of light-scattering voids;
wherein the article has a porosity of 30% to 50% when measured by mercury intrusion; and
wherein the article is a white refractory roofing granule having a total solar reflectance of at least 0.8; and
wherein the article, when tested without a colored coating, having:
a Hunter Color L-value of at least 85,
a Hunter Color a-value of less than 0.4, and
a Hunter Color b-value of less than 11.
2. The article of claim 1 , wherein the article has a total solar reflectance of from 0.8 to 0.845.
3. The article of claim 1 , wherein the article has a Hunter Color L-value of 85 to 89.7.
4. The article of claim 1 , wherein the article has a translucency of 5% or less, a Barrett Hardness value of at least 70, and a thermal emittance of at least 0.8.
5. The article of claim 1 , wherein the article has a cristobalite content of less than 3%, a total iron content of less than 1%, and no detectable asbestiform minerals.
6. The article of claim 1 , further comprising a coating selected from: a mixture comprising process oil and polysiloxane, aqueous polysiloxane, a polymer emulsion, or any combination thereof.
7. The article of claim 1 , further comprising a composition comprising aluminum (III) oxide (Al 2 O 3 ) in an amount of 20-50 wt % based on a total weight of the article.
8. The article of claim 1 , wherein the article having an overall crystallinity of 30%-60%.
9. A process comprising:
firing a mixture comprising a ceramic-forming clay, crystalline silica particles, and a sintering material, so as to form a white refractory roofing granule,
wherein the white refractor roofing granule comprises a plurality of light-scattering voids;
wherein the white refractory roofing granule has a porosity of 30% to 50% when measured by mercury intrusion;
wherein the white refractory roofing granule has a total solar reflectance of at least 0.8;
wherein the white refractory roofing granule, when tested without a colored coating, having:
a Hunter Color L-value of at least 85,
a Hunter Color a-value of less than 0.4, and
a Hunter Color b-value of less than 11;
wherein the crystalline silica particles are present in the white refractory roofing granule in an amount of 10 wt % to 40 wt % by weight of the white refractory roofing granule;
wherein the sintering material comprises at least one of calcium hydroxide, sodium bicarbonate, sodium carbonate, sodium silicate, feldspar, nepheline syenite, or any combination thereof.
10. The process of claim 9 , further comprising:
forming the mixture, wherein the mixture comprises clay, the sintering material, and the crystalline silica particles;
solidifying the mixture by compaction, extrusion, palletization, or any combination thereof;
firing the mixture for a time and temperature sufficient to vitrify the sintering material but not the clay, to obtain a conglomerate;
breaking the conglomerate into granules; and
coating the granules with a post-treatment coating.
11. The process of claim 10 , wherein the sintering material is selected from calcium hydroxide, sodium carbonate, sodium bicarbonate, sodium silicate, feldspar, nepheline syenite, or any combination thereof; and wherein the firing temperature is from 1000° C. to about 1400° C.
12. The process of claim 10 , further comprising forming granules, wherein the granules are formed by pelletizing or by compaction and size reduction prior to firing.
13. The process of claim 10 , wherein:
the mixture of the forming step further comprises white pigment selected from titanium dioxide, magnesium oxide, barium sulfate, zinc oxide, or any combination thereof;
the mixture of the forming step further comprises a filler selected from alumina, talc, gypsum, or any combination thereof; and
the clay of the forming step is a white clay selected from kaolin, ball clay, montmorillonite, or any combination thereof.
14. The process of claim 10 , wherein:
the mixture of the forming step is mixed with water to produce a homogeneous and uniformly dampened mass; and
the homogeneous and uniformly dampened mass is subsequently extruded through a die or a screen to produce fragments or strips of green refractory material.
15. The process of claim 10 , wherein the post-treatment coating is selected from: a mixture comprising process oil and polysiloxane aqueous polysiloxane, a polymer emulsion, or any combination thereof.
16. A roofing material comprising an article according to claim 1 , and a base material comprising a nonwoven mat, wherein the nonwoven mat is coated with asphalt, impregnated with asphalt, or any combination thereof.
17. A method of reducing heat absorption of a roof, wherein the method comprises covering the roof with the roofing material of claim 16 .
18. The article of claim 1 , further comprising a white pigment selected from: titanium dioxide, magnesium oxide, barium sulfate, zinc oxide, or any combination thereof.
19. The article of claim 1 , wherein the crystalline silica particles are quartz particles.
20. The article of claim 1 , wherein
the Hunter Color L-value is 85 to 100,
the Hunter Color a-value is −1.5 to 0.3, and
the Hunter Color b-value is −5 to 10.