Glass units
BACKGROUND OF THE INVENTION 
     It has become conventional to employ glass laminates instead of glass for such applications as window panes, windshields for automobiles and other motor vehicles and the like. Since glass breaks easily and shatters while breaking, it presents a substantial risk of serious injury. 
     Many types of laminated glass are known in the prior art. One type includes two or more sheets of glass bonded together by means of an elastomeric interlayer material. Polyvinyl butyral is commonly used as the interlayer because of its adhesion to glass when subjected to suitable termperatures and pressure, because it is transparent when bounded between two sheets of glass, and because of its resistance to elongation whenever an object impacts such a laminated glazing. Other interlayer materials employed include polyurethane compositions which are sometimes introduced as partial polymers and sometimes cast in place. 
     Another type of glass laminate is the combination of a polycarbonate and glass. The polycarbonates are especially advantageous because of their high rigidity, high impact strength and stability under load at temperatures from well below -100° C. to as high as 150° C. and above. Additionally, there is very little light absorption by the polycarbonate layer of a safety pane coupled with high ultraviolet light and infrared radiation absorptions. One difficulty in polycarbonate laminates arises from the fact that the glass and polycarbonate have different coefficients of thermal expansion and therefore expand at different rates. It is suggested in U.S. Pat. No. 3,821,071 to mount the polycarbonate and glass sheets slidably against one another in face-to-face relationship in a frame. While this arrangement overcomes some of the problems, it is not entirely satisfactory. 
     Accordingly, it is the object of this invention to provide a new glass unit which includes one or more polycarbonate layers and which avoids the problems inherent in the differential heat expansion rates of glass and the plastic material. This and other objects of the invention will become apparent to those skilled in the art from following detailed description in which the sole figure is a cross-section of a glass unit constructed in accordance with this invention. 
     SUMMARY OF THE INVENTION 
     This invention relates to a glass unit and more particularly to a safety glass unit in which a central polycarbonate sheet is surrounded by and spaced apart from glass sheets and one or more of the peripheral edges of the polycarbonate sheet lies within a channel which, in turn, provides room for lateral expansion of the polycarbonate sheet. 
    
    
     DESCRIPTION OF THE INVENTION 
     The sole FIGURE shows a section of a glass unit constructed in accordance with the present invention. The unit includes first and second layers of glass 1, 2 which are usually, but not necessarily, positioned so that the opposing faces of layers 1 and 2 are substantially parallel. While each of layers 1 and 2 could be a unitary glass body, it is preferred that each layer itself be a safety glass laminate. Thus, layer 1 is preferably constructed of a first and second glass layer 3,4 bonded together by an interlayer of polyvinyl butyral 5. Similarly, layer 2 is preferably composed of two glass panes 6, 7 interconnected and bonded together by an interlayer of polyvinyl butyral 8. Typically, glass panels 3,4,6, and 7 have a thickness of about 0.125 inch but can range up to 0.375 inch and interlayers 5 and 8 have a thickness of about 0.03 inch and can range up to 0.06 inch. 
     A polycarbonate sheet 9 is positioned between layers 1 and 2 such that each lateral surface of polycarbonate layer 9 is substantially parallel to the lateral face of the adjacent surface of glass layer 1 or 2. Any suitable polycarbonate material can be use in the present invention. For example, some of the suitable plastics are prepared by reacting di(monohydroxaryl) alkanes with carbonic acid derivatives such as diesters, phosgene or bischlorocarbonic acid esters of di(monohydroxyaryl) alkanes. Suitable polycarbonates and processes for their production are described in, inter alia U.S. Pat. Nos. 3,028,365; 2,999,846; 2,970,131; 2,991,271; 2,999,835; 3,014,891; 3,248,414, Chemistry and Physics of Polycarbonates by Schnell and Polycarbonates by Christopher and Fox. 
     Polycarbonate layer 9 is maintained apart from i.e., spaced apart from the adjacent glass layers 1 and 2 by means of any suitables spacers such as, for example, aluminum spacers 10. Spacers 10 preferably have a hollowed out interior which communicates with the area between polycarbonate layer 9 and adjacent glass layers 1 or 2 by means of a channel 11 so that the interior of spacer 10 can be packed with a suitable desiccant 12 such as silica gel. In the preferred embodiment of this invention, the thickness of polycarbonate layer 9 is approximately 0.25 inch and the distance between the lateral face of layer 9 and the adjacent face of glass layers 1 or 2 is about 0.1875 inch so that the entire unit has a thickness of about 1.185 inches. The distance between the lateral face of layer 9 can range up to 0.5 inch and the entire unit thickness can be up to 1.5 inches. 
     At least one and preferably all of the peripheral or marginal edges 13 of polycarbonate sheet 9 is provided with a substantially U-shaped capping channel 14. Capping channel 14 has a substantially flat base 15 and two integral lateral walls 16 each of which partially contact one face of polycarbonate sheet 9. Capping channel 14 thus straddles marginal edge 13 of polycarbonate sheet 9. Additionally, marginal edge 13 of polycarbonate sheet 9 is spaced apart form said flat base 15 of capping channel 14 by a given dimension to permit lateral expansion of the polycarbonate. In the preferred embodiment of this invention the given dimension is about 0.125 inch. Capping channel 14 can be constructed of any suitable material such as, but not limited to, stainless steel. 
     The spatial relationship of glass layers 1 and 2 polycarbonate layer 9 and spacers 10 is maintained substantially constant by use of a suitable framing means 17. Framing means 17 can be in the form of a metal channel of U-shaped cross-section or an adhesive tape or a rigid plastic member (e.g. polyvinyl chloride) or other suitable material. The framing means can also be of a thermal non-conducting material and construction such as described, for example, in U.S. Pat. No. 4,030,263. 
     The area between framing means 17 on the one hand and glass layers 1 and 2, capping channel 14, and spacers 10 is filled with an appropriate sealant 18. Polysulfide sealants have been found particularly appropriate. 
     Preferably a second sealant for instance made of butyl rubber is provided, as shown by reference numeral 19. The butyl rubber serves as a vapor and/or gas barrier. The polysulfide sealant 18 serves most advantageously as protection against weathering. 
     The glass units of the invention find particular application as installation units where security is of prime importance, as for example, in hospitals, for example, mental hospitals, prisons and correctional facilities, banks, jewelry stores, and the like. As can be appreciated the units can be modified to include alarm systems. The glass can be conventional clear or colored glass as well as photosensitive glass. 
     The units can be made in any size depending on the intended application and/or builder or architect&#39;s specification. 
     Various changes and modifications can be made in the glass unit of the present invention without departing from the spirit and scope thereof. The various embodiments described herein have been for the purpose of further illustrating the invention but were not intended to limit it.