Abstract:
A shingle formed from a base sheet, with a layer of polymer modified asphalt coating the top of the base sheet, and a layer of conventional oxidized asphalt, free from polymer additives, coating the bottom of the base sheet. Both layers preferably directly contact the strands of the base sheet for good adhesion. This improves the strength, flexibility and ultraviolet resistance of the shingle, while reducing the cost as compared with a shingle which utilized only polymer modified asphalt, and in addition it increases the stiffness of the shingle as compared with one using only polymer modified asphalt.

Description:
FIELD OF THE INVENTION 
     This invention relates to asphalt-based roofing shingles for use on the roofs of buildings. 
     BACKGROUND OF THE INVENTION 
     Asphalt-based roofing shingles are commonly made by taking a continuous base sheet which may be of organic felt, or fiberglass mat, covering both sides of the base sheet with a coating asphalt, and then embedding granules on the top side of the coated sheet. The granules protect the asphalt from breaking down through oxidation caused by ultraviolet rays. The finished sheet is cut into lanes and then into desired lengths for shingles. 
     Since conventional oxidized asphalt which is normally used for roofing shingles has limited strength and life, and can be brittle, it has become common to use a form of asphalt which is referred to as “modified asphalt” for roofing shingles. Modified asphalt has an elastomeric polymer blended with ordinary asphalt. The elastomeric polymer imparts elasticity and greater strength to the asphalt and also improves its resistance to ultraviolet rays. 
     However, a disadvantage of modified asphalt is that it is extremely expensive as compared with ordinary oxidized asphalt. Therefore, for economic reasons the use of modified asphalt in roofing shingles has been relatively limited. In addition, roofing shingles which use modified asphalt may often be too flexible and can therefore be difficult to apply to a roof. Further, because of their lack of stiffness they may tend to bend upwardly in a strong wind, increasing the likelihood (despite their greater strength) that they may eventually break. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides shingles which, although they incorporate some modified asphalt, also utilize some oxidized asphalt, and are therefore less costly than shingles which utilize only modified asphalt. In addition, the shingles of the invention tend to be stiffer than those which use solely modified asphalt. The stiffer shingles are easier to apply. 
     In one aspect, the invention provides a shingle comprising a base sheet having a plurality of fibers, said base sheet having a top and a bottom, an upper layer of asphalt over at least a portion of said top, a lower layer of asphalt under said bottom, at least one of said layers coating said base sheet, said upper layer comprising a blend of asphalt and elastomeric polymer, said lower layer comprising an oxidized asphalt substantially free from elastomeric polymer additives, said upper layer contacting said lower layer. 
     In another aspect, the invention provides a method of making a shingle comprising: selecting a base sheet comprising a plurality of fibers, said base sheet having a top and a bottom, applying a first layer of asphalt coating over said top of said base sheet and a second layer of asphalt coating under said bottom of said base sheet, at least one of said layers coating said base sheet, said first layer comprising a blend of asphalt and an elastomeric polymer, said second layer comprising oxidized asphalt substantially free from elastomeric polymer additives, said first and second layers contacting each other. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be better understood from the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic view of a production line or shingles according to the invention; 
         FIG. 2  is a diagrammatic cross-section, enlarged, of a shingle according to the invention before granules have been applied; 
         FIG. 3  is a view similar to that of  FIG. 2  but with the granules in place on the shingle; 
         FIG. 4  is a plan view of a completed shingle according to the invention; 
         FIG. 5  is a plan view of a multi-layer shingle according to the invention; 
         FIG. 6  is a diagrammatic cross-section of a modified shingle according to the invention, before the granules have been applied; and 
         FIG. 7  is a cross-sectional view similar to  FIG. 6  but showing a further modification. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is first made to  FIG. 1 , which shows a production line for shingles. The production line shown in  FIG. 1  is conventional and therefore is described only briefly. 
     As shown, the production line includes a roll  10  of fiberglass mat or organic felt (normally the roll  10  will be fiberglass mat). A sheet  12  is unrolled from the roll  10  and passed over a conventional coater  14  containing conventional oxidized asphalt  16 . The coater  14  coats the bottom of the sheet  12  with the oxidized asphalt  16 . 
     The sheet  12  then passes beneath a top coater  18 , which coats the top of the sheet  12  with modified asphalt  20 . The differences between the modified asphalt  20  and the conventional asphalt  16  will be described below. 
     The sheet  12 , now coated on both sides, next passes beneath a granule applicator indicated at  22 , where granules  24  are applied to the top surface of the sheet  12 . The sheet  12  then passes through rollers  30 ,  32  allowing the bottom surface of the sheet to be coated with a parting material such as talc, at station  34 . The sheet  12  then passes through rollers  35  which press the granules  24  in place on the sheet, after which the sheet passes through a cooling section  36  and a slitting and cutting section  38  where it is slit into lanes and cut into individual shingles  39 . The shingles  39  are stacked and packaged for delivery. 
     The conventional oxidized asphalt which is used can be any conventional coating asphalt used as a roofing asphalt. Usually, such asphalts have a penetration index from about 50 dmm to about 400 dmm, preferably from about 85 dmm to about 300 dmm, and more preferably from about 85 dmm to about 200 dmm. (As is known, “dmm” means penetration in decimillimeters, which is the amount by which a needle weighted by 100 grams will penetrate the surface of the asphalt in five seconds at 25° C., as defined by standard test ASTM D 5 .) 
     Typically, such conventional asphalts also have a softening point, prior to oxidation, from about 25° C. to about 100° C., preferably from about 25° C. to about 50° C., and more preferably from about 25° C. to about 40° C. The softening point is defined by standard test ASTM D 36 . 
     The oxidized asphalt is prepared by elevating the temperature of the asphalt and exposing it to oxygen, for example by bubbling air or oxygen through the asphalt. The oxidation increases the softening point of the asphalt while lowering its penetration. The softening point of the oxidized coating asphalt is typically from about 90° C. to 110° C. after it has been oxidized. 
     The oxidized asphalt used in shingle coating also usually has a quantity of filler material added thereto. The filler material may be any of those which are known in the art, such as finely crushed limestone, slag, traprock or a mixture thereof. The filler extends the asphalt by increasing its volume and thus reducing its cost, and also increases the fire and weathering resistance of the asphalt. 
     The modified asphalt  20  used on the top surface of the shingle is a blend comprising an elastomeric polymer and an asphalt of the kind described above (not oxidized). The elastomeric polymer may be any of those known in the art to impart elasticity to asphalt. Polymer materials which are suitable for such use include, but are not limited to, natural rubber, synthetic rubber such as butadiene-styrene copolymers including styrene-butadiene-styrene, thermoplastic rubber such as block copolymers of styrene/ethylene-butylene, or a thermoplastic polymer such as atactic polypropylene, or blends of these polymers. 
     A typical styrene-butadiene-styrene (“SBS”) polymer suitable for use in the present invention is that sold by Fina Oil and Chemical Co. of Dallas, Tex. A typical actactic polypropylene which is suitable for use is that sold by Rexene Corp. of Odessa, Tex. 
     The polymer content of the blend of asphalt and elastomeric polymer will typically be 10% to 12% by weight, based on the total weight of the blend, when SBS is used, and about 22% to 26% by weight, based on the total weight of the blend, when actactic polypropylene is used. However these proportions may vary, depending on the specific polymer or blend of polymers used, the type of asphalt used, and the proportions desired. 
     A quantity of filler material may be added to the blend of elastomeric polymer and asphalt. The filler material will typically be similar to the filler material used in the oxidized asphalt. Once again, the use of the filler increases the fire resistance of the blend and also extends the blend. 
       FIG. 2  shows the sheet  12  after the sheet has been coated on its bottom and top. The coating of oxidized asphalt is indicated at  40  and the coating of modified asphalt is indicated at  42 . The glass fiber strands of the glass fiber sheet are indicated at  44 , consisting of warp and weft strands. It is assumed in  FIG. 2  that the glass fiber sheet is woven, but it is more usually non-woven, in which case the sheet  12  prior to coating will consist of randomly oriented glass fibers instead of warp and weft strands. 
     In the embodiment shown, the bottom coating  40  has been applied first, so coating  40  is shown as extending adjacent to (but not coating) the tops of strands  44 . The top coating  42  (whether it is applied first or second) is typically thicker than the bottom coating and extends upwardly from the tops of the strands  44 . By coating the bottom first (and thus by having the bottom coating extend upwardly through most of the thickness of the sheet  12 ), the amount of conventional oxidized asphalt used is maximized, reducing the cost of a shingle and also making it stiffer. However, if preferred, the top coating can be applied first (by reversing coaters  14 ,  18 ), in which case the top coating will extend closer to the bottom of strands  44 . This will use more modified asphalt and less ordinary oxidized asphalt, increasing the cost of the shingle but making it stronger (which is desirable) and more flexible (which in some cases is desirable and in some cases may not be desirable). In both cases, each layer  40 ,  42  preferably contacts at least some of the strands  44  directly, and does not simply adhere to the other layer. This reduces any tendency for the two layers to delaminate, which might otherwise tend to occur if one coating simply adhered to the other coating. Of course the two layers  40 ,  42  also directly contact each other. 
       FIG. 3  shows the shingle of  FIG. 2  with the granules  24  embedded in the top layer  42 . It will be realized that various kinds and colors of granules can be used, to achieve any desired effect. 
     While two classes of polymer additives have been described, other kinds of polymer additives may be used, as desired. Examples of various kinds of polymer additives which may be used are disclosed in U.S. Pat. No. 5,347,785. 
     In addition, while the sheet  12  has been described as formed from woven glass fibers, other materials or mixtures of materials can be used in either woven or non-woven form, also as described in U.S. Pat. No. 5,347,785. 
     Reference is next made to  FIG. 4 , which shows a conventional shingle  50  in plan view. It will be seen that the shingle  50  includes a headlap portion  52 , which, when the shingle is installed on a roof, is normally covered by an adjacent shingle, and an exposed portion  54  which is exposed to the elements. To reduce costs, the upper surface of the headlap portion  52 , as well as the entire lower surface of the shingle  50 , can be coated of ordinary oxidized asphalt, while only the exposed upper surface of exposed portion  54  is coated with modified asphalt. 
     The invention is also applicable to multi-layer shingles, for example as shown in  FIG. 5  which illustrates a two-layer shingle  60  having a top sheet  62  and a bottom sheet  64 . As is well-known, each sheet  62 ,  64  itself consists of a fiber-based sheet (typically glass fibers) coated top and bottom with asphalt, and having decorative granules applied at least to the exposed portions. Such shingles which may of course have more than two sheets forming multiple layers, are often used for their decorative effect, or for a longer life, or both. In such shingles, where the bottom sheet  64  is almost entirely covered by the top sheet  62 , then both the upper and lower surfaces of the bottom sheet may be coated with ordinary oxidized asphalt. Only the portion of the top surface of the top sheet  62  which will be exposed to the elements needs to be coated with modified asphalt. Thus, the headlap portion  66  of the upper sheet  62 , and the entire upper surface of the lower sheet  64  (as well as the lower surfaces of both sheets) can be coated with ordinary oxidized asphalt. 
     Another embodiment of the invention is shown in  FIG. 6 . In the  FIG. 6  embodiment, the fiber base sheet, shown at  70 , is again coated with an upper layer  72  of modified asphalt. However, in this case the upper layer  72  extends through the fibers of the base sheet and to a position below the base sheet  70 . The bottom surface of the upper layer  72  is coated with ordinary oxidized asphalt  74 . This version provides for the use of more modified asphalt, for example where greater flexibility is needed. If desired, the situation can be reversed, as shown in  FIG. 7 , and the bottom layer  74  of oxidized asphalt can extend from below the base sheet to a position above the top of the base sheet  70 , with the upper surface of the oxidized asphalt (located above the base sheet) being coated with modified asphalt. In both cases, as before, granules (not shown) are applied to the upper surface of upper layer  72 . 
     While preferred embodiments of the invention have been described, it will be realized that various changes can be made within the scope of the invention.