Abstract:
There is disclosed a method of adhering a flexible solar panel to a PVC-based roofing membrane that interposes a certain plasticizer-containing PVC membrane between the solar panel and the roofing membrane.

Description:
BACKGROUND OF THE INVENTION 
     The use of polyvinyl chloride (PVC)-based membranes as a commercial roofing material has come into widespread application in the roofing industry inasmuch as the material is flexible, waterproof and may be rapidly applied over a large area as new roofing, re-roofing or over existing roofing by a wide variety of methods, including mechanical attachment or gluing, with adjacent panels being joined by seams formed by heat-welding or solvent-welding. Such PVC-based roofing membranes typically comprise a three-ply composite of a layer of a polyester, e.g., polyethylene terephthalate (PET) or fiberglass fabric reinforcement sandwiched between two PVC films, the overall composite typically being 30 to 90 mils thick. 
     In the late 1990&#39;s waterproof flexible photovoltaic solar panels were developed that could be secured to roofing materials, including the aforesaid composite reinforced PVC membranes, thereby permitting the passive generation of electrical energy from rooftops by exposure to the sun, in essence giving roofs a dual utility of conventional protection from the elements and the generation of power. 
     A fundamental problem with securing such flexible photovoltaic solar panels to PVC roofing membranes lies in the incompatibility between (1) the adhesive typically used to glue the solar panels to the roofing membranes and (2) the conventional plasticizers used in PVC roofing membranes. More specifically, the PVC in the PVC roofing membrane contains certain plasticizers to enhance the membrane&#39;s flexibility, and the most widely used adhesive is a butyl rubber-containing asphalt adhesive either applied to the back side of the solar panels or in the form of two-sided tape. Over a period of two to four years the conventional plasticizers in the PVC layers of the reinforced PVC roofing membrane migrate from the PVC into the butyl rubber-containing asphalt adhesive, which softens the adhesive bond and compromises the elasticity of the PVC layer of the roofing membrane, which in turn weakens and/or destroys the bond between the solar panel and the roofing membrane and seriously compromises the flexibility that allows for expansion and contraction of both the PVC roofing membrane and the flexible solar panel. Although other classes of adhesives have been investigated for securing such solar panels to such PVC roofing membranes, they have more drawbacks than such a butyl-based adhesive. For example, acrylics lack sufficient bond flexibility as soon as they have been applied and are costly, while ethylene vinyl acetate hot melts require special dedicated factory equipment and narrow temperature ranges, making them essentially useless on a job site. 
     Accordingly, there is a need in the art for a method of securing conventional flexible solar panels to PVC roofing membranes that provides a strong, elastic, long-lasting bond between the two. This need is met by the present invention, which is summarized and described in detail below. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method of installing a photovoltaic solar panel on a roof comprising the steps:
         (a) securing a PVC membrane to a roofing support;   (b) securing a plasticizer-containing PVC membrane to the PVC membrane of step (a); and   (c) securing a flexible photovoltaic solar panel membrane to the plasticizer-containing PVC membrane of step (b)
 
wherein the plasticizer-containing PVC membrane contains PVC-compatible, butyl-resistant plasticizers, and optional additives such as antioxidants, flame retardants, stabilizers, colorants and other conventional additives known in the art.
       

    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an exemplary roof assembly of the invention that includes a flexible photovoltaic solar panel. 
         FIG. 2  is a perspective view of the roof assembly of  FIG. 1  with elements of the assembly turned up on one corner. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein the same numerals refer to the same elements, there is shown in  FIGS. 1-2  a roof assembly  1  comprising a polymeric roofing membrane  10 , typically consisting of two PVC layers  12  with a polyester or fiberglass reinforcement layer  14  interposed between the two PVC layers. Roofing membrane  10  may be secured in conventional fashion to a roof deck of virtually any construction, including frames or flat or curved surfaces of any material, including preexisting roofing or roofing membranes. 
     A second element of the roofing assembly is a plasticizer-containing PVC membrane  16  containing PVC-compatible, butyl-resistant plasticizers and optional antioxidants, flame retardants, stabilizers, colorants and other conventional additives known in the art. Exemplary such additives are phenolic antioxidants, antimony oxide and calcium carbonate as flame retardants and minor amounts of cadmium, barium and/or zinc as stabilizers. By “PVC-compatible and butyl-resistant” plasticizers is generally meant plasticizers that do not degrade PVC and that resist or minimize migration from PVC into butyl-based adhesives. Exemplary classes of such plasticizers include polyester-based plasticizers, such as phthalate-based esters, nitrile butyl rubbers, ketone/ethylene ester terpolymers (commercially available form DuPont as ELVALOY® and known polymeric plasticizers. Such a membrane  16  is commercially available as Asphalt-Resistant Vinyl Copolymer/Fabric Laminate from Canadian General Tower, Ltd. of Cambridge, Ontatrio. Membrane  16  may be secured to roofing membrane  10  by conventional heat-welding, solvent-welding or gluing. Heat-welding may be conducted by a heated platen, by the use of a hot air gun or by a dielectric welder. Solvent-welding may be conducted by applying a solvent in which PVC is soluble to roofing membrane  10  or to plasticizer-containing PVC membrane  16  or to both, followed by joining the two membranes; preferred solvents are tetrahydrofuran (THF) and methyl ethyl ketone (MEK). Gluing is preferably conducted by applying an ethylene/propylene copolymer-based adhesive to both membrane  10  and membrane  16 , then joining the two; a preferred adhesive of this type is an epoxy resin-based contact adhesive such as IB Vertibond from IB Roof Systems of Eugene, Oreg.; Sarnacol 2170 from Sika Canada, Inc.; or Pliobond 1746 from Bio-Rad Laboratories, Inc. 
     The third and final element of the roofing assembly is a flexible photovoltaic solar panel  20 , typically comprising a flexible substrate  20   a  to which is adhered a solar module  20   b . Such panels are commercially available pre-assembled, typically with amorphous silicon photovoltaic cells encased within flexible, water-tight and transparent industrial fabrics and/or polymeric membranes and optionally including output cables and by-pass diodes. Suitable such solar panels  20  are the IB SolarWise 272 Watt and 544 Watt panels available from IB Roof Systems, Inc. of Eugene, Oreg. and the UNI-SOLAR® PVL 68 Watt and 544 Watt panels from United Solar Systems Corporation of Auburn Hills, Mich. Solar panel  20  is preferably glued to membrane  16  by a water-resistant contact adhesive such as a butyl rubber-containing asphalt adhesive or an ethylene/propylene copolymer adhesive containing butyl rubber, the latter being commercially available as a two-sided tape sold as SIKALASTOMER®-68 by Sika Corporation of Madison Heights, Mich. Butyl rubber-containing asphalt adhesives, commonly referred to as “rubberized asphalts,” typically comprise 40-60 wt % asphalt, 10-20 wt % of a rubber such as a styrene/butadiene block copolymer and up to 10 wt % of a plasticizer. Application of such an adhesive forms contact adhesive layer  18  between solar panel  20  and membrane  16 . In securing solar panel  20  to membrane  16 , it is preferred to leave suitable-sized longitudinal gaps, on the order of 2 cm wide, in the bond between solar panel  20  and membrane  16 , said gaps being in communication with the atmosphere, so as to permit the release of water vapor and gases and thereby help prevent delamination and maintain flexibility between panel  20  and membrane  16  upon expansion and contraction of the two. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.