1.  Field of the Invention
The present application relates to roofing granules and roofing products including roofing granules, such as roofing shingles, and to processes for making such roofing granules. 
2.  Brief Description of the Prior Art
Asphalt shingles are conventionally used in the United States and Canada as roofing and siding materials.  Mineral surfaced asphalt shingles, such as those described in ASTM D225 or D3462, are generally used in steep-sloped roofs to provide water-shedding function while adding an aesthetically pleasing appearance to the roofs.  The asphalt shingles are generally constructed from asphalt-saturated roofing felts and surfaced with pigmented color granules, such as those described in U.S. Pat.  No.  4,717,614.  Roofing granules are typically distributed over the upper or outer face of such shingles.  The roofing granules, in general, are formed from crushed and screened mineral materials, and serve to provide the shingle with durability.  They protect the asphalt from the effects of the solar radiation, in particular from the degradative effects of ultraviolet rays, and of the environment, including wind, precipitation, pollution, and the like, and contribute to better reflection of incident radiation.  The granules, moreover, are typically colored, naturally or artificially by way of the application of pigments, to meet the aesthetic requirements of the user.  Roofing granules usually are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. 
The mineral particles customarily used for making roofing granules, such as talc, slag, limestone, granite, syenite, diabase, greystone, slate, trap rock, basalt, greenstone, andesite, porphyry, rhyolite, and greystone, generally have low solar heat reflectance, that is, low reflectance of near infrared radiation.  Further, the pigments employed for coloring roofing granules have usually been selected to provide shingles having an attractive appearance, with little thought to the thermal stresses encountered on shingled roofs.  As a result, the colored roofing granules themselves usually have low solar heat reflectance. 
Other mineral particles, such as calcite, feldspar, quartz, white rock, plagioclase, or zeolite, may have high solar heat reflectance; however, they are less opaque to UV radiation and hence are not suitable for roofing granules.  Other types of highly reflective synthetic particles, such as aluminum oxide, recycled ceramic particle, ceramic grog, or porous silica, are also less opaque to UV radiation and will not be suitable for roofing granules for asphalt-based roofing membranes. 
The binder for the coating applied to color roofing granules can be a soluble alkali metal silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkali metal silicate, resulting in an insoluble colored coating on the mineral particles.  For example, U.S. Pat.  No.  1,898,345 to Deming discloses coating a granular material with a coating composition including a sodium silicate, a coloring pigment, and a colloidal clay, heating below the fusing temperature of sodium silicate, and subsequently treating with a solution, such as a solution of calcium or magnesium chloride, or aluminum sulphate, that will react with the sodium silicate to form an insoluble compound.  Similarly, U.S. Pat.  No.  2,378,927 to Jewett discloses a coating composition for roofing granules consisting of sodium silicate, and clay or another aluminum-bearing compound such as sodium aluminate or cryolite or other insoluble fluorides such as sodium silicofluoride, and a color pigment.  The coating is then heat cured at a temperature above the dehydration temperature of the coating materials but below the fusion temperature at which the combination of materials fuses, thus producing a non-porous, insoluble weather-resistant cement.  Roofing granules are customarily produced using inert mineral particles with metal-silicate binders and clays as a latent heat reactant at an elevated temperature, for example, such as those described in U.S. Pat.  No.  2,981,636.  The granules are employed to provide a protective layer on asphaltic roofing materials such as shingles, and to add aesthetic values to a roof. 
Depending on location and climate, shingled roofs can experience very challenging environmental conditions, which tend to reduce the effective service life of such roofs.  One significant environmental stress is the elevated temperature experienced by roofing shingles under sunny, summer conditions, especially roofing shingles coated with dark-colored roofing granules.  Although such roofs can be coated with solar reflective paint or coating material, such as a composition containing a significant amount of titanium dioxide pigment, in order to reduce such thermal stresses, this utilitarian approach will often prove to be aesthetically undesirable, especially for residential roofs. 
Asphalt shingles coated with conventional roofing granules are known to have low solar heat reflectance, and hence will absorb solar heat especially through the near infrared range (700 nm-2500 nm) of the solar spectrum.  This phenomenon is increased as the granules covering the surface become dark in color.  For example, while white-colored asphalt shingles can have solar reflectance in the range of 25-35%, dark-colored asphalt shingles can only have solar reflectance of 5-15%.  Furthermore, except in the white or very light colors, there is typically only a very small amount of pigment in the conventional granule's color coating that reflects solar radiation well.  As a result, it is common to measure temperatures as high as 77° C. on the surface of black roofing shingles on a sunny day with 21° C. ambient temperature.  Absorption of solar heat may result in elevated temperatures at the shingle's surroundings, which can contribute to the so-called heat-island effects and increase the cooling load to its surroundings.  It is, therefore, advantageous to have roofing shingles that have high solar reflectivity in order to reduce the solar heat absorption.  The surface reflectivity of an asphalt shingle largely depends on the solar reflectance of the granules that are used to cover the bitumen. 
In recent years, the state of California has implemented a building code requiring low-sloped roofs to have roof coverings with solar reflectance greater than seventy percent.  To achieve such high levels of solar reflectance, it is necessary to coat the roof with a reflective coating over granulated roofing products, since the granules with current coloring technology are not capable of achieving such high levels of solar reflectance.  However, polymeric coatings applied have only a limited amount of service life and will require re-coat after several years of service.  Also, the cost of adding such a coating on roof coverings can be prohibitive. 
In order to reduce the solar heat absorption, one may use light colored roofing granules which are inherently more reflective towards the solar radiation.  White pigment-containing latex coatings have been proposed and evaluated by various manufacturers.  However, consumers and homeowners often prefer darker or earth tone colors for their roofs.  In recent years, there have been commercially available roofing granules that feature a reflective base coat (i.e., a white coat) and a partially coated top color coat allowing the reflective base coat to be partially revealed to increase solar reflectance.  Unfortunately, the colors of these granules have a “washed-out” appearance due to the partially revealed white base coat. 
Other manufacturers have also proposed the use of exterior-grade coatings that were colored by infrared-reflective pigments for deep-tone colors and sprayed onto the roof in the field.  U.S. Patent Application Publication No.  2003/0068469 A1 discloses an asphalt-based roofing material comprising a mat saturated with an asphalt coating, and a top coating having a top surface layer that has a solar reflectance of at least seventy percent.  U.S. Patent Application Publication No.  2003/0152747 A1 discloses the use of granules with solar reflectance greater than 55% and hardness greater than 4 on the Moh's scale to enhance the solar reflectivity of asphalt-based roofing products.  However, there is no control of color blends and the novel granules are typically available only in white or buff colors.  U.S. Pat.  No.  7,455,899 discloses a non-white construction surface comprising a first reflective coating and a second reflective coating with total direct solar reflectance of at least twenty percent. 
Also, there have been attempts to use special near-infrared-reflective pigments in earth-tone colors to color roofing granules for increased solar reflectance.  However, the addition of kaolin clays, which are used to make the metal-silicate binder durable through heat curing, inevitably reduces the color strength or the color intensity of the pigment. 
Colored roofing granules can also be prepared using a metal silicate binder without adding clay and curing the binder at temperatures greater than the glass sintering temperature, or through a “pickling” process by applying acid.  However, these alternatives require either very high temperatures, or the use of corrosive chemicals, and in many cases could result in loss of color due to pigment degradation by the acid. 
In the alternative, a non-silicate binder, such as a synthetic polymeric binder, can be used to coat the inert mineral materials in order to produce roofing granules with dark colors and high solar reflectance.  However, the long-term durability and cost for polymeric coatings are not as advantageous as the silicate binders. 
Another approach is provided by solar control films that contain either thin layers of metal/metal oxides or dielectric layers through vacuum deposition, and which have been commercially available for use in architectural glasses. 
There is a continuing need for roofing materials, and especially asphalt shingles, that have improved resistance to thermal stresses while providing an attractive appearance, and providing good resistance to the degradative effects of ultraviolet radiation.