Solar heat-reflective roofing granules, solar heat-reflective shingles and process for producing same

Abstract

Solar-reflective roofing granules having improved solar heat-resistance are formed by coating colored mineral particles with a coating composition including titanium dioxide nanoparticles.

We claim: 
     
         1 . Solar heat-reflective roofing granules comprising:
 (a) a base particle comprising an inert mineral;   (b) a first coating on the base particle, and   (c) a second coating on the first coating,   
       wherein the first coating comprises a first coating binder and particles of at least one solar reflecting pigment having an average reflectance of greater than about 60 percent in the wavelength range of from about 700 to 2500 nanometers, and the second coating comprises a second coating binder and at least one nano-pigment having an average particle size of less than about 200 nanometers and an average absorbency of less than about 20 percent in the wavelength range of from 700 to 2500 nanometers. 
     
     
         2 . Solar heat-reflective roofing granules according to  claim 1  wherein the solar reflecting pigment particles have an average solar reflectivity of at least 80 percent in the wavelength range from 700 to 2500 nanometers. 
     
     
         3 . Solar heat-reflective roofing granules according to  claim 1  wherein the solar reflecting pigment particles are selected from the group consisting of titanium dioxide, zinc dioxide, and zinc sulfide. 
     
     
         4 . Solar heat-reflective roofing granules according to  claim 1  wherein the second coating binder comprises a metal silicate binder having a refractive index of less than about 1.50, the silicate coating binder including at least one low atomic weight element, other than oxygen or hydrogen, having an average atomic weight less than the average atomic weight of silicon, the at least one low atomic weight element being present in sufficient amount in the coating binder to reduce the refractive index of the binder by at least about 0.003 units. 
     
     
         5 . Solar heat-reflective roofing granules according to  claim 1 , the second coating further comprising at least one supplementary pigment having a particle size of greater than about 200 nanometers an average absorbency of less than about 20 percent in the wavelength range of from 700 to 2500 nanometers. 
     
     
         6 . Solar heat-reflective roofing granules according to  claim 5  wherein the at least one supplementary pigment is selected from the group consisting of pearlescent pigments, light-interference platelet pigments, ultramarine blue, ultramarine purple, cobalt chromite blue, cobalt aluminum blue, chrome titanate, nickel titanate, cadmium sulfide yellow, cadmium sulfoselenide orange, phthalo blue, phthalo green, quinacridone red, diarylide yellow, and dioxazine purple. 
     
     
         7 . Solar heat-reflective roofing granules according to  claim 1  wherein the at least one nano-pigment has an average particle size of from about 20 to 150 nanometers. 
     
     
         8 . Solar heat-reflective roofing granules according to  claim 7  wherein the nano-pigment is selected from the group consisting of iron oxides, metal titanates, chromium oxides, zinc ferrites, mixed metal oxides, titanium dioxide, zinc oxides, copper oxides, vanadium oxide, magnesium oxide and the halogen adducts. 
     
     
         9 . Solar heat-reflective roofing granules according to  claim 1  wherein the nano-pigment is selected from the group of pigments that have strong near infrared absorbency in macro-pigment form. 
     
     
         10 . Solar heat-reflective roofing granules according to  claim 9  wherein the at least one nano-pigment is selected from the group consisting of carbon black, bone black, copper chromite black, iron oxide black, and KFe 2 (CN) 6 .H 2 O. 
     
     
         11 . Solar heat-reflective roofing granules comprising:
 (a) a solar-reflective inert base particle; and   (b) a coating over the base particle, the coating comprising a binder and at least one nano-pigment having an average particle size of less than about 200 nanometers and an average absorbency of less than about 20 percent in the wavelength range of from 700 to 2500 nanometers.   
     
     
         12 . Solar heat-reflective roofing granules according to  claim 11  wherein the solar-reflective inert base particles have a solar reflectivity of at least 60 percent. 
     
     
         13 . Solar heat-reflective roofing granules according to  claim 11  wherein the solar-reflective inert base particles are selected from the group consisting of slate, feldspathic rock, plagioclase rock, chert rock, aluminum oxide, mullite, ceramic grog, crushed porcelain, white-pigmented glass, copper, and zinc. 
     
     
         14 . Solar heat-reflective roofing granules according to  claim 11  wherein the second coating binder comprises a metal silicate binder having a refractive index of less than about 1.50, the silicate coating binder including at least one low atomic weight element, other than oxygen or hydrogen, having an average atomic weight less than the average atomic weight of silicon, the at least one low atomic weight element being present in sufficient amount in the coating binder to reduce the refractive index of the binder by at least about 0.003 units. 
     
     
         15 . Solar heat-reflective roofing granules according to  claim 11 , the coating further comprising at least one supplementary pigment having a particle size of greater than about 200 nanometers and an average absorbency of less than about 20 percent in the wavelength range of from 700 to 2500 nanometers. 
     
     
         16 . Solar heat-reflective roofing granules according to  claim 15  wherein the at least one supplementary pigment is selected from the group consisting of pearlescent pigments, light-interference platelet pigments, ultramarine blue, ultramarine purple, cobalt chromite blue, cobalt aluminum blue, chrome titanate, nickel titanate, cadmium sulfide yellow, cadmium sulfoselenide orange, phthalo blue, phthalo green, quinacridone red, diarylide yellow, and dioxazine purple. 
     
     
         17 . Solar heat-reflective roofing granules according to  claim 11  wherein the at least one nano-pigment has an average particle size of from about 20 to 150 nanometers. 
     
     
         18 . Solar heat-reflective roofing granules according to  claim 17  wherein the nano-pigment is selected from the group consisting of iron oxides, metal titanates, chromium oxides, zinc ferrites, mixed metal oxides, titanium dioxide, zinc oxides, copper oxides, vanadium oxide, magnesium oxide and the halogen adducts. 
     
     
         19 . Solar heat-reflective roofing granules according to  claim 11  wherein the nano-pigment is selected from the group of pigments that have strong near infrared absorbency in macro-pigment form. 
     
     
         20 . Solar heat-reflective roofing granules according to  claim 19  wherein the at least one nano-pigment is selected from the group consisting of carbon black, bone black, copper chromite black, iron oxide black, and KFe 2 (CN) 6 .H 2 O.