Abstract:
An asphalt-based sheet roofing material includes capsules on its upper surface. When struck, as by hailstones, the capsules break to release a film forming fluid that spreads over the surface to heal the damage created by the hailstones.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a division of pending U.S. patent application Ser. No. 12/056,010, filed Mar. 26, 2008; which claims the priority of U.S. Provisional Application No. 60/910,051 filed Apr. 4, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to asphalt-based roofing materials such as shingles and processes for the manufacture of asphalt-based roofing materials. 
         [0004]    2. Brief Description of the Prior Art 
         [0005]    Asphalt-based roofing materials are ubiquitous in North America. Typically, these materials take the form of sheets or shingles which include a fibrous web embedded in a layer of a bituminous, asphalt-based composition to form a membrane which provides water resistance. Mineral-based granules are typically adhered to the exposed portions of the upper side of the sheet or shingle to provide a desirable aesthetic effect and to protect the underlying asphalt-based composition from harmful ultraviolet radiation. 
         [0006]    It is important to ensure that the roofing granules adhere to the asphalt substrate during the life of a shingle as it exposed to outdoor environments. However, adhesion of the roofing granules can be disrupted by impact forces, particularly those arising from hail storms. As a result, asphalt shingles can suffer from excessive granule loss due to the impact damage of hail or other projectiles. This not only can cause undesirable appearance of black spots on the roof, but also can result in premature failure in shingles by exposing the underlying asphalt to harmful UV radiation. 
         [0007]    Also, some shingles or roofing membranes may suffer from cracking or puncture damage during an extreme severe hail storm. This can result in leaking or further water damage to the roof assembly or interior of a house. Thus, it is advantageous to have asphalt shingles with increased resistance to damages due to impact events. 
         [0008]    Improved impact resistance is disclosed in U.S. Pat. No. 6,426,309 for asphalt-based roofing materials which include a protective coating adhered to the upper surface of the asphalt coating, a layer of granules adhered to the protective coating, and a web bonded to the lower region of the asphalt coating. The impact resistance of these roofing materials is improved, as the protective coating is disclosed to prevent excessive granule loss while the web is disclosed to act to stop crack propagation. 
         [0009]    A wind-resistant shingle and a method of making it is disclosed in U.S. Pat. No. 6,758,019 in which the rear surface of the shingle is provided with an attached reinforcement layer, which resists upwardly wind-applied bending torque when the shingle is installed on a roof, such that the failure of the shingle when it is bent beyond its elastic limit, is resisted until the shingle has absorbed a high percentage of applied torque. 
         [0010]    A method of repairing cracks in a paved asphaltic surface is disclosed by U.S. Pat. No. 7,059,800. A layer of liquefied asphalt is applied to a cracked, paved surface, and a reinforcement mat is then applied over the liquefied asphalt, which penetrates and soaks the reinforcement mat to form a water barrier. Finally, a layer of paving material is applied over the mat. Although it is known to apply a surface coating onto a roof after the roofing shingles have been installed to protect the shingles from granule loss and other damage, such surface coatings can be expensive and require additional labor to apply after the roofing shingles have been installed. 
         [0011]    Improved granule adhesion is disclosed in U.S. Pat. No. 7,125,601. An integrated granule product includes a film having a plurality of ceramic coated granules bonded to the film by a cured adhesive. The film can then be applied over a roofing substrate. For example, the integrated granule product can be applied onto an asphalt-based substrate to form a roofing shingle. 
         [0012]    There is a continuing need to extend the effective service life of roofs surfaced with asphalt-based roofing materials, and in particular to extend the life of roofs experiencing impact damage from hail and like circumstances. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention provides an improved, self-healing asphalt-based sheet roofing material, in the form of roofing shingles, roll roofing, and the like. The asphalt-based sheet roofing material of the present invention comprises a bituminous binder and having an upper surface, at least a portion of which is provided with a plurality of capsules. Preferably, at least a portion of the upper surface of the roofing material is also provided with roofing granules. Preferably, the portion of the upper surface that is provided with capsules is coextensive with the portion of the upper surface that is provided with roofing granules. Preferably, the capsules have an average size that is smaller than the average size of the roofing granules. Preferably, the capsules range in size from US mesh #18 to US mesh #50. The capsules have an outer wall and an inner cavity, with the outer wall being susceptible to being ruptured by mechanical impact. The inner cavity is provided with at least one functional material. In one embodiment of the present invention, the at least one functional material preferably comprises a liquid film-forming composition. In another embodiment of the present invention, the at least one functional material comprises at least one solvent component, the bituminous binder being soluble in the at least one solvent component of the functional material. Optionally, the functional material comprises at least one pigment. Optionally, the functional material comprises at least one biocide. Preferably, the functional material is selected from the group consisting of adhesives, adhesion promoters, coating compositions, such as asphalt-based coating compositions, asphalt-based emulsions, and plasticizers. Preferably, the material forming the outer wall of the capsules has a compressive strength greater than about 100 psi and less than about 30,000 psi. Preferably, the outer wall is formed from a material selected from the group consisting of glass, ceramic materials, and polymeric materials. In one embodiment of the present invention, it is preferred that the outer wall of the capsules comprises a capsule binder and at least one pigment. Preferably, the at least one pigment is a solar reflective pigment. 
         [0014]    The present invention also provides a process for preparing a bituminous asphalt-based sheet roofing material. The process comprises providing a base sheet comprising an upper layer of a bituminous composition. Preferably, the base sheet comprises a bituminous composition reinforced with fiber, preferably in the form of a web of glass fiber. Preferably, the base sheet is provided at a temperature above the softening point of the bituminous composition. The process further comprises depositing a plurality of roofing granules on at least a portion of the upper surface of the base sheet. The process further comprises depositing a plurality of capsules on at least a portion of the upper surface of the base sheet. In one embodiment of the process of the present invention, the process further comprises mixing roofing granules with capsules and depositing the mixture of the roofing granules and the capsules on at least a portion of the upper surface of the base sheet. In another embodiment of the process of the present invention, capsules are deposited before the roofing granules are deposited. In yet another embodiment of the process of the present invention, roofing granules are deposited before capsules are deposited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic sectional elevation view of an asphalt-based sheet roofing material according to a first embodiment of the present invention. 
           [0016]      FIG. 2  is a schematic sectional elevation view of an asphalt-based sheet roofing material according to a second embodiment of the present invention. 
           [0017]      FIG. 3  is a schematic sectional elevation view of an asphalt-based sheet roofing material according to a third embodiment of the present invention. 
           [0018]      FIG. 4  is a schematic sectional elevation view of an asphalt-based sheet roofing material according to a fourth embodiment of the present invention. 
           [0019]      FIG. 5  is a schematic, fragmentary section view of the asphalt-based sheet roofing material of  FIG. 1  shown after experiencing mechanical damage from contact with hail. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present invention provides a solution to the problem of impact-induced damage of asphalt-based, mineral surfaced shingles or roofing membranes. Such damage can be reduced or eliminated through the use of “self-healing” capsules that will rupture upon impact and dispense at least one functional material to help repair the damage to the shingles or roofing membranes. The “self-healing” capsules preferably have a shell or wall that has enough strength to endure the manufacturing operation of shingle making and normal foot traffic without rupture, and yet is weak enough to be easily broken upon impact of sizable hail stones. The shell can also be pigmented to contribute to or enhance the shingle color or surface solar reflectance. Preferably, the shell comprises a hard material such as glass, ceramics, or suitable polymers that are durable and inert toward the encapsulated multifunctional agents. The at least one functional material may comprise adhesives, adhesion promoters, coatings, asphalt-based coatings or emulsions, or plasticizers, or a mixture thereof, to prevent excessive granule loss or to mend cracks resulting from impact. The at least one functional material can further comprise pigments to enhance appearance or solar reflectance, or biocides to control undesirable fungi or algae growth. 
         [0021]    The “self-healing” capsules can be deposited onto asphalt coating surface prior to dropping of colored roofing granules, or premixed with roofing granules then followed by typical granule dropping and pressing operation. The self-healing capsules can also be deposited onto hot asphalt surface immediately after granule dropping to fill in the gaps between granules. Such manufacturing techniques will become more apparent to those who are skilled in the art. 
         [0022]    Referring now to the drawings, in which like reference numerals refer to like elements in each of the several views, there are shown schematically in  FIGS. 1 ,  2 ,  3  and  4  examples of asphalt-based sheet roofing material according to the present invention, while  FIG. 5  illustrates the “self-healing” of the roofing material after a damaging impact by hail. 
         [0023]      FIG. 1  is a schematic cross-sectional representation of a first embodiment of asphalt-based sheet roofing material  10  according to the present invention. The roofing material  10  includes an asphalt-based bituminous membrane  12  which is reinforced with an embedded web  14  of glass fibers. The roofing material  10  has a lower surface or back  16  which is covered with a surfacing material  18  and an upper surface  20 . Embedded in at least a portion of the upper surface  20  is a plurality of colored roofing granules  22 , each formed from a mineral core covered with a ceramic coating layer colored with a metal oxide colorant. Extending on and partially embedded in the upper surface in between the colored roofing granules  22  is a plurality of capsules  24  each having an outer wall and filled with a liquid film forming composition. 
         [0024]    In manufacturing the asphalt-based sheet roofing material  10  a continuous sheet of the glass fiber web  14  is fed through one or more tanks containing a molten asphalt-based bituminous composition at an elevated temperature to coat and impregnate the web  14  with the asphalt-based bituminous composition (not shown) in order to form the bituminous membrane  12 . While the bituminous composition is still warm and soft, the roofing granules  22  are dropped onto at least a portion of the upper surface of the bituminous membrane  12  and become partially embedded therein (not shown). Subsequently, while the bituminous membrane remains warm and soft, the capsules  24  are dropped onto at least a portion of the upper surface of the bituminous membrane, and the capsules adhere to and become partially embedded in the portion of the bituminous membrane that is not covered by the previously deposited roofing granules, as shown in  FIG. 1 . 
         [0025]      FIG. 2  is a schematic cross-sectional representation of a second embodiment of asphalt-based sheet roofing material  30  according to the present invention. As in the case of the above-described first embodiment, the roofing material  30  includes an asphalt-based bituminous membrane  32  which is reinforced with an embedded web  34  of glass fibers. The roofing material  30  has a lower surface or back  36  which is covered with a surfacing material  38  and an upper surface  40 . Embedded in at least a portion of the upper surface  40  is a plurality of colored roofing granules  42 , each formed from a mineral core covered with a ceramic coating layer colored with a metal oxide colorant. Extending on and partially embedded in the upper surface in between and beneath the colored roofing granules  42  is a plurality of capsules  44  each having an outer wall and filled with a liquid film forming composition. Thus, in this embodiment, a greater density of capsules  44  can be embedded in the bituminous membrane  32  than in the case of the first embodiment, because the capsules  44  lie both in between the roofing granule  42 , as in the case of the first embodiment, but also underneath the roofing granules  42 , unlike the case of the first embodiment. 
         [0026]    In manufacturing the asphalt-based sheet roofing material  30  of this second embodiment, a continuous sheet of the glass fiber web  34  is fed through one or more tanks containing a molten asphalt-based bituminous composition at an elevated temperature to coat and impregnate the web  34  with the asphalt-based bituminous composition (not shown) in order to form the bituminous membrane  32 , as in the case of the first embodiment. However, in the case of this second embodiment, while the bituminous composition is still warm and soft, the capsules  44  are dropped onto at least a portion of the upper surface of the bituminous membrane, and the capsules adhere to and become partially embedded in the bituminous membrane (not shown). Subsequently, while the bituminous membrane remains warm and soft, the roofing granules  42  are dropped onto at least a portion of the upper surface of the bituminous membrane  42  and become partially embedded therein, and in the process push down underlying capsule deeper into the membrane, to provide the asphalt-based sheet roofing material shown in  FIG. 2 . 
         [0027]      FIG. 3  is a schematic cross-sectional representation of a third embodiment of asphalt-based sheet roofing material  50  according to the present invention. As in the cases of the above-described first and second embodiments, the roofing material  50  includes an asphalt-based bituminous membrane  52  which is reinforced with an embedded web  54  of glass fibers. The roofing material  50  has a lower surface or back  56  which is covered with a surfacing material  58  and an upper surface  60 . Embedded in at least a portion of the upper surface  60  is a plurality of colored roofing granules  62 , each formed from a mineral core covered with a ceramic coating layer colored with a metal oxide colorant. Extending within and completely embedded in the bituminous membrane  52  beneath both the upper surface  60  and the colored roofing granules  62  is a plurality of capsules  64  each having an outer wall and filled with a liquid film forming composition. The capsules  62  lie generally in a plane parallel to the upper surface  60  within the bituminous membrane  52 . Thus, in this third embodiment, a greater density of capsules  64  can be embedded in the bituminous membrane  52  than in the case of the first embodiment, because the capsules  64  lie underneath the roofing granules  62 , unlike the case of the first embodiment. 
         [0028]    In manufacturing the asphalt-based sheet roofing material  50  of this third embodiment, a continuous sheet of the glass fiber web  54  is fed through one or more tanks containing a molten asphalt-based bituminous composition at an elevated temperature to coat and impregnate the web  54  with the asphalt-based bituminous composition (not shown) in order to form the bituminous membrane  52 , as in the case of the first and second embodiments. However, in the case of this third embodiment, while the bituminous composition is still warm and soft, the capsules  64  are dropped onto at least a portion of the upper surface of the bituminous membrane, and the capsules adhere to and become partially embedded in the bituminous membrane (not shown). Subsequently, another layer of bituminous coating composition is applied over the capsules, and then, while the bituminous membrane remains warm and soft, the roofing granules  62  are dropped onto at least a portion of the upper surface of the bituminous membrane  52  and become partially embedded therein, to provide the asphalt-based sheet roofing material shown in  FIG. 3 . The additional layer of bituminous composition overlying the capsules  64  protects the capsules  64  against mechanical damage through contact with roofing granules  62  when the roofing granules  62  are dropped. 
         [0029]      FIG. 4  is a schematic cross-sectional representation of a fourth embodiment of asphalt-based sheet roofing material  70  according to the present invention. As in the cases of the above-described first, second and third embodiments, the roofing material  70  includes an asphalt-based bituminous membrane  72  which is reinforced with an embedded web  74  of glass fibers, the web  74  dividing the membrane  72  into a layer  73  above the web  74  and a layer  75  below the web. The roofing material  70  has a lower surface or back  76  which is covered with a surfacing material  78  and an upper surface  80 . Embedded in at least a portion of the upper surface  80  is a plurality of colored roofing granules  82 , each formed from a mineral core covered with a ceramic coating layer colored with a metal oxide colorant. Extending within and completely embedded in the bituminous membrane  72  beneath both the upper surface  80  and the colored roofing granules  82  is a plurality of capsules  84  each having an outer wall and filled with a liquid film forming composition. The capsules  84  are randomly distributed in the layer of bituminous material  73  above the glass fiber web  74 . Thus, as in the case of the third embodiment, in this fourth embodiment, a greater density of capsules  84  can be embedded in the bituminous membrane  72  than in the case of the first embodiment, because the capsules  84  lie underneath the roofing granules  82 , unlike the case of the first embodiment. Furthermore, because the capsules  84  are not confined to lying in a plane as in the case of the third embodiment, a higher density of capsules  84  can be achieved than in the case of the third embodiment. 
         [0030]    In manufacturing the asphalt-based sheet roofing material  70  of this fourth embodiment, a continuous sheet of the glass fiber web  74  is fed through one or more tanks containing a molten asphalt-based bituminous composition at an elevated temperature to coat and impregnate the web  74  with the asphalt-based bituminous composition (not shown) in order to form the bituminous membrane  72 , as in the case of the first, second and third embodiments. However, in the case of this fourth embodiment, while the capsules  84  are mixed with molten bituminous material to form a suspension and the web  74  is passed through the suspension to coat the upper side of the web with the bituminous material containing the suspended capsules  84  and form the upper layer  73  of the bituminous membrane  72  (not shown). Subsequently, while the upper layer  73  remains warm and soft, the roofing granules  82  are dropped onto at least a portion of the upper surface of the bituminous membrane  72  and become partially embedded therein, to provide the asphalt-based sheet roofing material shown in  FIG. 4 . 
         [0031]      FIG. 5  is an enlarged, fragmentary schematic sectional view of the asphalt-based sheet roofing material  10  shown in  FIG. 1 , after the roofing material  10  has sustained mechanical damage in the form of a crack  90  in the upper surface  20  after being impacted by a large hailstone (not shown). The hailstone has ruptured several of the capsules  24 , which comprise, when intact, an outer wall or shell  25  filled a film-forming composition  27 , comprising an acrylic latex polymer emulsion in which is suspended a solar-reflective pigment such as titanium dioxide. The fluid film-forming composition  27  from the ruptured capsules  24  has flowed into the crack  90  to form a liquid layer covering the crack  90 , and the liquid layer has subsequently cured to provide a protective film  92  over the crack  90 , thus “healing” the crack  90 . The protective film  92  includes a solar-reflective pigment which helps protect the upper surface  20  from environmental degradation resulting from exposure to solar radiation, as does the colored coating  23  covering the exterior of the mineral cores  21  of the roofing granules  22 . 
         [0032]    The present invention provides asphalt-based sheet roofing materials provided with capsules including at least one functional material. Preferably, the at least one functional material is selected or formulated to provide good long-term stability, comparable to the anticipated service life of the roofing material in which the capsules are to be incorporated. In addition, it is preferred that the at least one functional material be selected or formulated to display good adhesion to the surface of bituminous membranes. In one aspect of the present invention, the at least one functional material comprises a liquid film-forming composition. Liquid film-forming compositions are well-known in the coatings art, and include solvent and aqueous coating compositions. Preferably, such film-forming compositions are formulated to have surface energy characteristics promoting their “wetting out” and spreading on the surface of bituminous membranes. Preferably, such film-forming compositions include at least one polymeric material, the at least one polymeric material preferably being selected from those polymeric materials that are resistant to photodegradation by exposure to solar radiation, such as acrylic polymeric materials. In one aspect, the at least one functional material includes at least one solvent component, such that the bituminous binder is at least partially soluble in the solvent component. In another aspect, the at least one functional material is selected from the group consisting of adhesives, adhesion promoters, coating compositions, asphalt-based emulsions and plasticizers. In this aspect, the coating composition is preferably an asphalt-based coating composition. Preferably, the at least one functional material includes at least one pigment. Preferably, the at least one pigment is a solar reflective pigment, such as titanium dioxide. Examples of solar reflective pigments that can be employed are disclosed, for example, in U.S. Pat. No. 7,241,500, incorporated herein by reference. In another aspect, the at least one functional material can include an inorganic or organic biocidal material, such as an algaecide, for example, cuprous oxide, zinc oxide, or a mixture thereof. 
         [0033]    The at least one functional material can be encapsulated to form the capsules using conventional techniques for forming capsules, including such techniques as interfacial polymerization, phase separation/coacervation, spray drying, spray coating, fluid bed coating, supercritical anti-solvent precipitation, and the like. Techniques for encapsulating liquids are disclosed, for example, in U.S. Pat. No. 6,703,127. The “self-healing” capsules can be prepared by various methods, for example, by the method disclosed in the U.S. Patent Application Publication No. 2003/0060569 A1. Other methods will be apparent to those who are skilled in the art. Preferably, the capsules are formed with sufficient mechanical strength so that the capsules will withstand the process of manufacturing the asphalt-based sheet roofing material, storage and transportation of the asphalt-based sheet roofing material to a jobsite, installation of the asphalt-based sheet roofing material on a roof, and the impact of foot traffic on the installed asphalt-based roofing material, but are sufficiently fragile so that the capsule wall will rupture on impact by a hailstone sufficient to damage the roofing membrane. Thus, the capsule wall preferable has a compressive strength of from about 100 psi to 30,000 psi. 
         [0034]    The preferred size of the capsules depends on whether they are to be adhered to the upper surface of the bituminous membrane or embedded under the surface of the bituminous membrane. When the capsules are to be adhered to the upper surface of the bituminous membrane, the capsules are preferably smaller in average size than the average size of the roofing granules, and more preferably sized to fit in between the roofing granules on the portion of the upper surface of the roofing membrane that would be otherwise exposed. In this case, the “self-healing” capsules preferably are smaller than typical roofing granules and larger than typical fillers. Thus, the capsules preferably range in size from about U.S. mesh #18 to U.S. mesh #50. 
         [0035]    The capsules can be prepared from a composition including one or more colorant materials or pigments, to provide capsules having walls that are colored or pigmented. The capsules can include, for example, one or more solar reflective pigments, such as titanium dioxide. Alternatively, the capsules can be coated with one or more coating compositions which may include pigments such as solar reflective pigments. 
         [0036]    When the capsules are distributed in a roofing material membrane, the capsules preferably have an average size that is less than the membrane thickness. Thus, when capsules are distributed in a roofing material membrane, the capsules preferably have an average size of from about 1 micrometer to 100 micrometers, and more preferably from about 2 micrometers to 50 micrometers. 
         [0037]    The asphalt-based sheet roofing materials of the present invention can be manufactured using conventional roofing production processes. Typically, bituminous roofing products are sheet goods that include a non-woven base or scrim formed of a fibrous material, such as a glass fiber scrim. Bituminous roofing products are typically manufactured in continuous processes in which a continuous substrate sheet of a fibrous material such as a continuous felt sheet or glass fiber mat is immersed in a bath of hot, fluid bituminous coating material so that the bituminous material saturates the substrate sheet and coats at least one side of the substrate. Thus, the substrate is coated with one or more layers of a bituminous material such as asphalt to provide water and weather resistance to the roofing product. The reverse side of the substrate sheet can be coated with an anti-stick material such as a suitable mineral powder or fine sand. The upper side of the roofing product is typically coated with mineral granules to provide durability, reflect heat and solar radiation, and to protect the bituminous binder from environmental degradation. The roofing granules are typically distributed over selected portions of the upper side of the substrate, and the bituminous material serves as an adhesive to bind the roofing granules to the sheet when the bituminous material has cooled. 
         [0038]    Roofing granules are generally used in asphalt-based roofing shingles or in roofing membranes to protect asphalt from harmful UV radiation and to add aesthetic values to a roof. Typically, roofing granules are produced by using inert mineral particles that are colored by pigments, clay, and alkali metal silicate binders in the processes as described by the U.S. Pat. Nos. 2,981,636, 4,378,408, 5,411,803, or 5,723,516. 
         [0039]    In the asphalt-based roofing products of the present invention, conventional roofing granules can be employed, or one or more types of specialized roofing granules, such as algae-resistant roofing granules, such as disclosed in U.S. Patent Publications 2004/0255548 A1, 2004/0258835 A1, 2007/0148340 A1, and 2007/0148342 A1, all incorporated herein by reference, or solar-heat resistant roofing granules, such as disclosed in U.S. Pat. No. 7,241,500 and U.S. Patent Application Publications 2005/0072114 A1 and 2008/0008832 A1, all incorporated herein by reference, can be mixed with conventional roofing granules, and the granule mixture can be embedded in the surface of such bituminous roofing products using conventional methods. 
         [0040]    Alternatively, one or more types of specialized roofing granules can be substituted for conventional roofing granules in the manufacture of bituminous roofing products to provide those roofing products with superior properties, such as resistance to biological discoloration and degradation, fire retardancy, or solar heat resistance. One or more classes of specialized roofing granules can be applied sequentially to the roofing product surface, optionally followed by application of conventional roofing granules. In one embodiment of the process of the present invention, a first class of specialized roofing granules is first applied to the surface of the roofing product, followed by application of a second class of specialized roofing granules, followed finally by application of conventional roofing granules. In another embodiment of the present invention, a mixture of two or more classes of specialized roofing granules is first applied to the surface of the roofing product, followed by application of conventional roofing granules. Given the order of application, any excess granules that are not successfully embedded in the surface of the roofing product are likely to be conventional granules. Thus, the order of application of these embodiments of the process of the present invention is likely to permit more precise loading of the roofing product surface with the classes of specialized roofing granules than otherwise. In yet another embodiment, one or more classes of specialized roofing granules are applied to the surface of the roofing product. 
         [0041]    The roofing product sheet can be cut into conventional shingle sizes and shapes (such as one foot by three feet rectangles), slots can be cut in the shingles to provide a plurality of “tabs” for ease of installation and aesthetic effects, additional bituminous adhesive can be applied in strategic locations and covered with release paper to provide for securing successive courses of shingles during roof installation, and the finished shingles can be packaged. More complex methods of shingle construction can also be employed, such as building up multiple layers of sheets in selected portions of the shingle to provide an enhanced visual appearance, or to simulate other types of roofing products. Release strips can also be strategically applied to the shingles so as to line up with sealing adhesive so that stacked shingles can be packaged without the need for separate release paper covers for the additional adhesive. 
         [0042]    The bituminous material used in manufacturing asphalt-based sheet roofing products according to the present invention is derived from a petroleum processing by-product such as pitch, “straight-run” bitumen, or “blown” bitumen. The bituminous material can be modified with extender materials such as oils, petroleum extracts, and/or petroleum residues. The bituminous material can include various modifying ingredients such as polymeric materials, such as SBS (styrene-butadiene-styrene) block copolymers, resins, oils, flame-retardant materials, oils, stabilizing materials, anti-static compounds, and the like. Preferably, the total amount by weight of such modifying ingredients is not more than about 15 percent of the total weight of the bituminous material. The bituminous material can also include amorphous polyolefins, up to about 25 percent by weight. Examples of suitable amorphous polyolefins include atactic polypropylene, ethylene-propylene rubber, etc. Preferably, the amorphous polyolefins employed have a softening point of from about 130 degrees C. to about 160 degrees C. The bituminous composition can also include a suitable filler, such as calcium carbonate, talc, carbon black, stone dust, or fly ash, preferably in an amount from about 10 percent to 70 percent by weight of the bituminous composite material. 
         [0043]    In asphalt shingles, the mass of roofing granules per unit of area generally lies between 0.5 and 2.5 kg/m 2 , preferably between 1 and 2 kg/m 2 . 
         [0044]    It will be apparent from the foregoing that various modifications may be made in the details of the sheet roofing materials and the processes of this invention, all within the spirit and scope of the invention as defined in the appended claims.