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FIELD OF THE INVENTION 
     The present invention relates to a shutter for closing over a window, door or other opening of a building, for protecting the window, door or opening during extreme temperature conditions, as can occur during fires, in particular bushfires. The invention has been developed particularly in relation to the protection of windows and it will therefore be convenient to describe the invention in that context. However, it will be appreciated that the invention has wider application to doors or other openings, such as chimneys, flues or air vents. 
     BACKGROUND OF THE INVENTION 
     Windows, doors or other openings in buildings form parts of the building structure which can fail during extreme temperature conditions and which thereafter allow entry into the building of flames and embers, and of oxygen which fuel the flames and embers. Once a fire is established within a building, it is difficult to save the building from complete destruction. Accordingly, it is recognised that protection of windows, doors and other openings in a building is important in order to protect buildings against destruction by fire. 
     Windows can be protected by shutters which typically are positioned to form a cover over the external side of the window. Shutters exist already to close over a window and certain shutters have been developed for protection of windows in bushfire conditions. However, shutters of which the applicant is aware typically are aluminium roller shutters and a disadvantage with these shutters is that the melting temperature of aluminium can be lower than the temperature to which the shutter is exposed during a bushfire, so that the aluminium shutter could melt in such extreme temperature conditions. For that reason, recent amendments in Australia to building standards require shutters used for protection in bushfire conditions to continue to operate in a protective manner in temperatures exceeding the melting point of aluminium, i.e. in temperatures beyond 700° C. 
     Some existing shutters have been constructed in steel, which has a higher melting temperature than aluminium and so does not suffer the same drawbacks as aluminium. However, these shutters do not prevent transmission of radiant heat from the external or fire side of the shutter to the internal or non-fireside, and because of that radiant heat transmission, it is often the case that the window frame or the glass of the window fails even though the shutter is in a position covering the window. These forms of shutters also have sealing issues and therefore can leave gaps between the shutter and the surrounds of the window and this allows ingress of embers and oxygen. 
     There are also flame and smoke control ‘curtain’ type products, typically used in indoor environments to prevent the spread of fire from one area of a building to another. These products however have limited benefit when applied externally over windows or doors, as they can be deflected or shifted by wind, or if hit by flying embers and other debris for example, causing the glass of the window to break or allowing ember and heat access to the frame of the window or door. Such curtain type products are also not primarily designed for deflecting the heat, so that they can allow the cavity between the curtain and the window or door to get excessively hot and thus cause the window glass or window or door frame to fail. 
     Some curtain fabrics exist that do have fire retardant or heat reflective properties, but these fabrics are not necessarily capable of long term external use. In addition, they can also present difficulties for mounting, so that prohibitively expensive and difficult mounting arrangements are required. 
     Accordingly, applicant is not aware of a shutter which operates successfully under extreme temperature conditions as can occur during some extreme bushfire events. The applicant has therefore developed a new and unique shutter which aims to overcome or at least alleviate some of the disadvantages with shutters of the prior art. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a shutter comprising: 
     an outer sheet of generally square or rectangular shape, 
     a non-combustible insulating panel of generally the same shape as the outer sheet, and 
     a frame, 
     the panel being positioned between the outer sheet and the frame so that the outer sheet overlies a first broad face of the panel and the frame is attached adjacent to a second broad face of the panel which is opposite the first face, 
     the outer sheet having a melting point of greater than or equal to about 840° C., the panel being operable to retard heat transmission from the first face thereof adjacent to the outer sheet to the second face opposite the first face, so that when the first face is exposed to a temperature of 730° C. for a period of 15 minutes, the temperature at the second face does not exceed 250° C., 
     each of the outer sheet and the panel being secured to the frame and the frame being substantially resistant to distortion up to a temperature of about 250° C. 
     A shutter of the above kind advantageously can protect a window, door or other opening from both direct flame and from radiant heat, thereby increasing the likelihood of the window, door or other opening surviving extreme temperature conditions. Moreover, the shutter can limit the progression of heat through the window, door or other opening, so that occupants of a building which is subject to an extreme temperature condition, for example a bushfire, can be subject to reduced temperature within the building than would otherwise be the case if the shutter was not fitted to the window, door or other opening. Still further, a shutter according to the invention can be made to have a aesthetically pleasing appearance despite its required construction, which is important given that the shutter is an external fitting which is on view at all times. 
     The outer sheet of a shutter according to the invention can be of any suitable material, although a metal outer sheet is considered at this stage to be most appropriate, in particular a steel sheet. Testing to date has employed successfully a 0.5 mm “Colourbond” steel sheet. 
     Other materials suitable for adoption for the outer sheet could be employed subject to satisfying the requirement of providing a resistance to melting up to 840° C. Such materials could include metals or fabrics having suitable fire resistance. In the testing to date, a 0.5 mm “Colourbond” steel sheet has provided a non-combustible layer which has resisted melting at temperatures of up to 840° C. Advantageously, such a steel sheet has also provided a suitable barrier against penetration of flame and oxygen to the internal side of the shutter. In addition, that material also is cost effective compared to other materials that could be employed. 
     The non-combustible insulating panel can also be manufactured from any suitable material, but in testing to date, a suitable panel has been found to comprise a 13 mm thick plasterboard which is supplied by Lafarge Plasterboard Ltd under the product name “Firestop”. However, it is envisaged that various other materials could satisfy the requirements of the insulating panel of the invention, for example fibrous materials or foam materials, and it is expected that panel thicknesses of between 10 to 16 mm could be employed. Panels of greater or lesser thickness could be employed, but greater thickness panels could increase the bulk of the shutter beyond acceptable levels, while panels of reduced thickness could require more expensive materials that increase the cost of the shutter prohibitively. 
     The outer sheet overlies the insulating panel and each of the outer sheet and the insulating panel are attached to or supported by the frame. In some forms of the invention, the outer sheet and the insulating panel can be fixed together and in one arrangement, an adhesive is employed for that purpose. In some forms of the invention, the adhesive can be selected to fail at a certain upper temperature, with the outer sheet then being supported by the frame when adhesive failure takes place. The benefit of selecting an adhesive which will fail at a particular temperature is to allow expansion of the outer sheet during an extreme temperature event. By this mechanism, the adhesive fails which then allows the outer sheet to expand under the extreme temperature, but the outer sheet is maintained in position, albeit less precisely, by the frame. Thus, while allowance is made for some shifting or movement of the outer sheet, that movement is not sufficient to expose the insulating panel to direct flame, and the outer sheet thus continues to perform the function of providing a barrier against flame and oxygen penetration through the shutter. Accordingly, while the aesthetic appearance of the shutter might deteriorate upon failure of the adhesive, the structural integrity of the shutter remains intact and the shutter continues to form an effective barrier and temperature retarder, protecting the window, door or opening over which the shutter has been placed. 
     Many suitable adhesives are likely to be available which meet the requirements for fixing the outer sheet and panel in the shutter and for failing at a selected temperature if required. In testing conducted to date, a construction adhesive, Selleys Silicone 401 industrial engineering adhesive sealer, has been successfully employed, having a 205° C. failure temperature. 
     Screws can be employed for various fastening requirements. For example, screws can be employed for fastening the outer sheet and the panel to the frame, whereby the screws extend through the outer sheet and the panel and into engagement with the frame. However, it is preferred to minimise the number of screws used because during an extreme temperature event such as a bushfire, heat can be conducted through a screw which projects from the external side of the shutter through to the internal side of the shutter. This conduction can raise the temperature to which the window is exposed and thus excessive conduction can detract from the performance of the shutter and potentially lead to window failure. Additionally, where the screws are fixed to the frame, conduction through the screws can result in heating of the frame and excessive heating can distort the frame and again, detract from the performance of the shutter. Accordingly, by minimising the number of screws which are employed, heat transmission of this kind is minimised and the likelihood of window failure or of frame distortion occurring is likewise minimised. 
     For further fixing of the outer sheet and the panel, the frame can include or define a lip or flange, or a channel, within which edge regions of the outer sheet can be captured or located. In this arrangement, edge regions of the outer sheet can be adhesively fixed to the lip, flange or channel, or fixed by suitable fasteners, such as rivets, or they can simply be positioned within the lip, flange or channel. The panel can also be adhesively fixed to the frame, or it can be fixed to the frame by suitable fasteners, or both. The panel can also be positioned within the lip, flange or channel in the same manner as the outer sheet. The lip, flange or channel can extend completely or partially about the periphery of the outer sheet and the panel. 
     The frame can be of any suitable shape, construction and material. Testing to date has been conducted with a steel frame, partly of square hollow section (SHS), with dimensions 20×20×2.5 mm. However, it is clearly possible that alternative sections could be used, such as rectangular hollow section (RHS), or right-angle section. 
     The frame can have a generally rectangular or square configuration and be located about the periphery or edge regions of the insulating panel, on the opposite side to the outer sheet. However, the frame could be positioned inboard of the edges, or it could extend diagonally across the second face of the panel from each upper corner of the panel to an opposite lower corner. Other frame configurations are possible. 
     The frame can thus consist of a portion that is positioned adjacent to the second face of the panel and a lip, flange or channel portion that extends about the edges of the panel and the outer sheet to capture or confine the edges. 
     As indicated above, the frame is required to be substantially resistant to distortion up to a temperature of about 250° C., which is the maximum temperature expected at the second face of the panel if the extreme temperature conditions do not exceed 730° C. for a period of 15 minutes and the maximum temperature does not exceed 840° C. Thus, upon distortion of the outer sheet under extreme temperature conditions, the frame is not caused to distort other than slight or minor distortion. The selection of steel for the frame is considered appropriate for the temperature limit discussed above, while steel also advantageously is capable of gentle distribution of heat throughout the frame structure as the temperature on the internal side of the shutter increases, rather than abrupt distribution or uneven distribution. By this gentle overall increase of the frame temperature, distortion of the frame is minimised. 
     A seal can be disposed between the side edge regions of the shutter and facing surfaces of the surrounds or frame of the window, door or opening within which the shutter is mounted. The seal can be provided to minimise air exchange from the external side of the shutter to the internal side, and to prevent passage of embers and gases from the external side. 
     An effective form of seal is an intumescent seal, which increases in volume as the ambient temperature increases. Accordingly, during a fire event, the seal will expand and more firmly engage between the shutter and the frame of the window, door or opening, forming a barrier against air, embers or gases. The advantage of an intumescent seal is that the seal has minimum volume at ambient temperature so that it can be arranged not to interfere with the operation of the shutter in normal temperature conditions. However, the seal expands and forms an interference fit with facing surfaces when the temperature rises to extreme levels. In testing which has been conducted to date, a seal under the name Lorient HP1602AS has been successfully employed. 
     A seal can also be employed between adjacent shutter leaves and between adjacent sections of a shutter. In fact, a seal can be employed at all joins and openings within the shutter and between the shutter and the surrounds or body within which the shutter is mounted. 
     A shutter according to the invention can provide an effective barrier against ingress of heat and embers or direct flame to a window, door or other opening to protect the window, door or other opening from failure and thus to protect the building in which the window, door or other opening is installed. A shutter according to the invention can also reduce the temperature increase within the building during an external extreme temperature event, by limiting the transfer of heat from outside the building to inside through the window, door or other opening. Thus, any occupants of the building are likely to be exposed to reduced temperature and are more likely to survive the extreme temperature event. It is to be noted that in bushfires, the fire tends to move through an area relatively quickly and so the period in which building and the building occupants must survive is often a period of minutes rather than hours, but the intensity of the fire is often extremely high for that short period. In testing of a shutter according to the invention undertaken to date, the shutter has survived under simulated extreme bushfire conditions for a typical period under which a building would be subject to the bushfire. 
     For a better understanding of the invention and to show how it may be performed, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 to 3  illustrate three different prior art shutter arrangements. 
         FIG. 4  is a horizontal section of a shutter according to one embodiment of the invention. 
         FIG. 5  is a vertical section of the shutter of  FIG. 4 . 
         FIGS. 6 to 9  illustrate variations of portions of the shutter illustrated in  FIGS. 4 and 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1   a  and  1   b  illustrate a 4 panel bi folding “casement” shutter  10  in a respective fully open position ( FIG. 1   a ) and a fully closed position ( FIG. 1   b ).  FIG. 1   a  illustrates a pair of bi-fold shutters sections  11  and  12 , each formed by a pair of shutter leaves  13  and  14  which are fitted to cover an opening represented by broken line  15 . The opening can be closed by a window or door (not illustrated). 
     The shutter leaves  13  and  14  are of equal dimension and each of the shutter leaves  13  is connected to the associated shutter leaf  14  by hinges  16 . Likewise, each of the shutter sections  11  and  12  is connected by hinges to the frame or surrounds of the window by hinged connection of the leaves  13  with the frame or surrounds. 
     In  FIG. 1   a , the shutter leaves  13  and  14  are folded open completely, so that shutter leaf  14  overlies shutter leaf  13 , and each shutter section  11  and  12  is fully hinged so that the opening  15  is fully exposed. 
     Suitable latching arrangements can be employed to retain the shutter sections  11  and  12  in the fully open position of  FIG. 1   a , while the same latching arrangements or different latching arrangements can be employed to retain the shutter sections  11  and  12  closed in the  FIG. 1   b  illustration. 
     The casement shutter  10  is a form of shutter which exists already and which is easily moved between open and closed positions. Such shutters are therefore popular as covers for windows. However, the casement shutter  10  has not heretofore been used as a fire barrier. 
       FIGS. 2   a  and  2   b  illustrate a double panel casement shutter  17 , while  FIGS. 3   a  and  3   b  illustrate a single panel casement shutter  18 , each in closed and open conditions respectively. In each case, the shutter leaves  19  are hinged to the window frame for movement between open and closed positions. 
     Applicant has developed a shutter arrangement which can form a barrier across a window, door or other opening or the like to protect the window, door or opening against exposure to extreme high temperatures, such as those experienced during an intense bushfire. As explained earlier, openings such as windows and doors are prone to fail during an extreme temperature conditions and allow ingress of flame and embers, and oxygen. Accordingly, protecting windows and doors against failure is important in protecting a building against destruction by fire. 
     A shutter according to the invention can be formed as a casement shutter of the styles depicted in  FIGS. 1 to 3 . Alternatively, a shutter according to the invention can be similar to that depicted in  FIGS. 1   a  and  1   b , but with a tri-fold arrangement, or greater. Moreover, while the leaves of the shutters illustrated in  FIGS. 1 to 3  are hinged along a vertical line, the leaves could be hinged along a horizontal line so that the leaves fold vertically. 
       FIGS. 4 and 5  illustrate cross-sectional views of a shutter according to the invention through horizontal and vertical sections respectively. Referring first to the horizontal cross-section of  FIG. 4 , this depicts a shutter  20  which is fixed over or in front of a window assembly  21 . The window assembly consists of a double glazed window pane  22  which is mounted within side styles  23 . No further discussion will be made in relation to the window assembly  21  given that the window assembly  21  is not important in relation to describing the invention, although it will be appreciated that the shutter  20  of the invention is provided for the purpose of protecting the pane  22  against failure, and for resisting ingress of flame and embers to the window assembly  21 . 
     The shutter  20  includes a pair of shutter sections  25  and  26  each of which could be formed in a single or bi-fold manner, as illustrated in  FIGS. 1 and 2 . The shutter sections  25  and  26  thus include separate shutter leaves  27  and  28 . The shutter leaves  27  and  28  would be connected by one or more hinges (not shown) to further shutter leaves if the shutter sections  25  and  26  were bi-fold sections. 
     The shutter sections  25  and  26  are connected to opposite vertical frame assemblies  35  and  36 . Each of the frame assemblies includes an angle section  37  which is fixed to the window surround  38  in any suitable manner. The frame assemblies  35  and  36  include hinges (not shown) to which the shutter sections  25  and  26  are connected. The frame assemblies  35  and  36  include a metal frame  42  which cooperates with the angle section  37 . The frame assemblies  35  and  36  can include an infill  41  within the metal frame  42  to support a screw  43  which extends through the frame  42  and the infill  41  and into the angle section  37  to secure the frame  42  to the angle section  37 . The infill can be of any suitable material. An alternative arrangement employs a metal box section, ie 30×30×2.5 mm SHS, to replace the frame  42  and the infill  41 . 
     The shutter leaves  27  and  28  each comprise an outer metal sheet  50  and a non-combustible insulating panel  51 . The outer sheet  50  is disposed on the fire-side or external side of the shutter  20 , and it can be seen from both  FIGS. 4 and 5 , that the outer sheet  50  provides complete coverage for the facing surface of the panel  51 . 
     On the opposite or internal side of the panel  51 , a frame  52  is located and this comprises a square frame formed of 20×20×2.5 mm SHS section. The frame  52  is formed as a rectangle, about the periphery of the panel  51 . 
     A rear metal panel  53  extends across the internal side of the shutter  20  and is formed of 0.5 mm steel sheet. The metal panel  53  is attached to the rear side of the frame  52 . 
     The frame  52  includes a flange or channel  54  which defines a front lip  55 , a rear lip  56  and a base  57 . The flange or channel  54  accepts the periphery of the outer sheet  50 , the insulating panel  51 , and the rear panel  53 . The flange or channel  54  extends fully about the periphery of the respective outer sheet  50 , the insulating panel  51  and the rear panel  53 . 
     A seal  58  is disposed between the flange or channel  54  and the metal frame  42  of the frame assemblies  35  and  36  of  FIG. 4  and the further frame assemblies  59  and  60  of  FIG. 5 . The frame assemblies  35  and  36  extend along the side edges of the shutter sections  25  and  26 , while the frame assemblies  59  and  60  extend across the top and bottom edges of the shutter sections  25  and  26 . The frame assemblies  59  and  60  are formed in the same manner as the frame assemblies  35  and  36  and therefore the same reference numerals are employed for the same parts. 
     The seals  58  are intumescent seals as described earlier. A further intumescent seal  61  is positioned between the angle section  37  and the frame  42 . The seals  58  are prepared seals whereas the seals  61  are a liquid sealant which is applied as one of the last installation steps during installation of shutters according to the invention. 
     The shutter  20  is easily fitted to the reveal of an existing window, door or other opening.  FIG. 5  illustrates a screw  62  which extends through the window surround  38  and it is the case that this form of fixing can be employed about the complete periphery of the shutter  20 . The method of assembly, is that the angle sections  37  are first secured to the window surround  38 , where after the remaining shutter components are fixed to the angle section  37  via the screw  43 . Once that fixing has taken place, the intumescent sealant  61  can be applied to finalise the installation process. The use of the sealant  61  provides some flexibility with tolerances in fitting the shutter  22  a window, as the gap into which the sealant  61  is applied might vary between different windows. 
     Once installed, it will be appreciated that with the various seals  58  and  61 , that the shutter  20  in a closed condition forms a complete barrier against ingress of embers and direct flames to the window assembly  21 . Referring to  FIG. 4 , it can be seen that the seals  58  close all of the gaps in the shutter structure, including between shutter sections  25  and  26 . While not illustrated in  FIG. 4 , similar seals  58  can be employed between respective shutter leaves in a bi-fold shutter arrangement. 
     Moreover, the resistance to conduction of heat from an external side of the shutter to an internal side, protects the window assembly  21  from the extreme heat on the external side of the shutter  20  during an extreme temperature event, such as a bushfire. 
     To maintain the shutter  20  in a closed condition, suitable latches can be employed and in testing conducted to date, zinc plated steel padbolts have been employed. However, it is clear that various other latching arrangements could be employed, but what is required is that the padbolt, if applied to the external side of the shutter  20 , be able to survive temperatures of the kind that the outer sheet  50  is required to survive and for the same timeframes. 
     Several variations of the shutter  20  illustrated in  FIGS. 4 and 5  have been devised at this stage and include variations illustrated in  FIGS. 6 to 9 . Referring to  FIG. 6 , this variation involves the extension of the rear panel  53  of  FIGS. 4 and 5  about the side edges of the frame  52 , the insulating panel  51  and the outer sheet  50 . Thus, instead of the arrangement of the shutter  20 , in which a separate channel  54  is provided, in the  FIG. 6  arrangement, the rear panel  65  extends to a side portion  66  and to a front lip portion  67 . The side and front lip portions  66  and  67  are formed integrally with the rear panel  65 . 
     In  FIG. 7 , a variation is provided in relation to the frame  42  and the infill  41  of the shutter  20 . Instead of the frame  42  and the infill  41 , a SHS  70  is provided through which the screw  43  extends. It is expected that this variation will be employed in practice, although testing to date has not been conducted in relation to this variation and therefore it remains an option only. 
     The variation illustrated in  FIG. 8  is similar to the variation of  FIG. 7 , except that a screw  71  extends through the angle section  37  and into only one portion of the SHS  70 . 
     The variation of  FIG. 9  shows the SHS  70  being fixed directly to the wall face  72  which surrounds a window by a screw  73 . 
     The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the present disclosure. 
     Throughout the description of this specification the word “comprise” and variations of that word, such as “comprises” and “comprising”, are not intended to exclude other additives or components or integers.

Summary:
A shutter  20  including an outer sheet  50  and a non-combustible insulating panel  51 . The panel  51  is positioned between the outer sheet  50  and a frame  52 . The outer sheet  50  overlies a broad face of the panel  51  and the frame  52  is attached adjacent to a second broad face of the panel  51  opposite the first broad face. The outer sheet  50  has a melting point of greater than or equal to about 840° C. The panel  51  is operable to retard heat transmission from the first face to the second face so that when the first face is exposed to a temperature of 730° C. for a period of  15  minutes, the temperature of the second face does not exceed 250° C. Each of the outer sheet  50  and the panel  51  are secured to the frame  52  and the frame is substantially resistant to distortion of up to a temperature of about 250° C.