Hyperbright white roofing granules with high solar reflectance

Abstract

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 L-value of at least 85.0, together with processes for making and using the same.

What is claimed is: 
     
         1 . 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, said bright white refractory roofing granule having a total solar reflectance of at least 0.80 and a Hunter Color L-value of at least 85.0. 
     
     
         2 . The granule of  claim 1  wherein said granules have a translucency of 5% or less, a Barrett Hardness value of at least 70, and a thermal emittance of at least 0.8. 
     
     
         3 . The granule of  claim 2 , wherein the granule has a porosity (mercury intrusion) of between 20-50%, a cristobalite content of less than 3%, a total iron content less than 1%, and no detectable asbestiform minerals. 
     
     
         4 . The granule of  claim 1  further comprising a post-treatment coating. 
     
     
         5 . The granule of  claim 4  wherein the post treatment coating is selected from: (i) a mixture comprising process oil and polysiloxane, (ii) aqueous polysiloxane, (iii) polymer emulsions, and (iv) mixtures thereof. 
     
     
         6 . The granule of  claim 1 , wherein when manufactured by a high-temperature sintering of a ceramic mixture comprising clay and one or more of the following components: (i) a white pigment such as titanium dioxide, magnesium oxide, barium sulfate, zinc oxide, or other materials that are color-stable at high temperatures; (ii) a filler or lightening agent such as finely ground silica, alumina, talc, or gypsum; (iii) a sintering aid which is a fusible binder selected from calcium hydroxide, sodium bicarbonate, sodium carbonate, sodium silicate, feldspar, nephaline syenite, and mixtures thereof; (iv.) a porosity enhancer selected from carbon black or other finely powdered carbonaceous combustible materials that will burn out to form light-scattering voids to enhance porosity and reflectance. 
     
     
         7 . The granule of any of the preceding claims when produced using a ceramic mixture comprising 50-85% white clay; 10-30% silica; and 0-25% sintering aids selected from calcium hydroxide, feldspar, nephaline syenite, and mixtures thereof. 
     
     
         8 . The granule of any of  claim 1  when coated with an aqueous emulsion post-treatment coating, wherein the coating comprises polysiloxane in an amount of 0.05-0.1% of oil by weight of the granules. 
     
     
         9 . A granule of  claim 1  having a composition of Al2O3 20-50%; SiO2 40-80%; Fe2O3 0-1%; and other components 0-10%. 
     
     
         10 . A granule of any of  claim 1  having an overall crystallinity of 30%-60%, e.g., comprising 25%-40% Mullite, 5%-15%, Quartz, 0%-7.0% Cristobalite. 
     
     
         11 . A process for making a bright white refractory roofing granule, comprising the step of firing a consolidated substantially homogenous mixture of a ceramic-forming clay, sintering material, and optionally including silica particles, such that said bright white refractory roofing granule has a total solar reflectance of at least 0.80 and a Hunter Color L-value of at least 85.0. 
     
     
         12 . The process of  claim 11 , comprising:
 i.) forming a mixture comprising clay, sintering material, and optionally one or more additional ingredients selected from silica particles, pigment, fillers, lightening agents, porosity enhancers, and mixtures thereof;   ii.) solidifying the mixture by means of compaction, extrusion, or pelletization;   iii.) optionally reducing the size of the solidified mixture by means of crushing and screening to form granule-sized aggregates   iv.) firing the mixture for a time and temperature sufficient to vitrify the sintering agent but not the clay, to obtain a conglomerate;   v.) breaking the conglomerate thus formed into granules, as necessary;   vi.) coating the granules with a post-treatment coating (e.g., an oil) to control dust and promote adhesion.   
     
     
         13 . The process of  claim 12  wherein the sintering material is selected from calcium hydroxide, sodium carbonate, sodium bicarbonate, sodium silicate, feldspar, nephaline syenite, and mixtures thereof. 
     
     
         14 . The process of  claim 12  wherein the firing temperature is between about 1000° C. to about 1400° C. 
     
     
         15 . The process of  claim 11  further comprising the step of forming granules, wherein the granules are formed by pelletizing or by compaction and size reduction prior to filing. 
     
     
         16 . The process of  claim 11  wherein the mixture of step (i) comprises white pigment selected from titanium dioxide, magnesium oxide, barium sulfate, zinc oxide, and mixtures thereof. 
     
     
         17 . The process of  claim 11  wherein the mixture of step (i) comprises a filler and lightening agent selected from finely ground silica, alumina, talc, gypsum, and mixtures thereof. 
     
     
         18 . The process of  claim 11  wherein the clay of step (i) is a white clay selected from kaolin, ball clay, montmorillonite, or combinations thereof. 
     
     
         19 . The process of  claim 11  wherein the components of step (i) are mixed with water to produce a homogeneous and uniformly dampened mass. 
     
     
         20 . The process of  claim 19  wherein the homogeneous and uniformly dampened mass is subsequently extruded through a die or a screen to produce fragments or strips of green refractory material. 
     
     
         21 . The process of  claim 20  wherein the green refractory material is predried at a time and temperature sufficient to reduce the internal moisture of the mixture to about 1% to about 5%. 
     
     
         22 . The process of  claim 11  wherein the conglomerate produced by step (ii) is crushed and screened to a grading suitable for use as a granular coating for a roofing membrane. 
     
     
         23 . The process of  claim 11  wherein the post-treatment coating of step (vi) is selected from: i) a mixture comprising process oil and polysiloxane ii.) aqueous polysiloxane, iii.) polymer emulsions, and iv.) mixtures thereof. 
     
     
         24 . A granule produced by the process of  claim 11 . 
     
     
         25 . A roofing material comprising (i) hyperbright white granules according to  claim 1 , attached to (ii) a base material comprising a nonwoven mat, coated and/or impregnated with asphalt. 
     
     
         26 . A method of reducing heat absorption of a roof, wherein said method comprises covering the roof with the roofing material of  claim 23 .