Abstract:
A fire retardant, transparent panel and method of making the panel, that is especially well suited for use on mobile platforms, and particularly on aircrafts. The panel is a composite of a transparent matrix and a plurality of fire-retardant nanoparticles. The nanoparticles have a diameter less than the visible spectrum of light, and in one form between about 0.1 nm to about 400 nm in diameter. The nanoparticles may be arranged randomly or in predetermined patterns within the matrix during manufacturing of the panel. The panel is lightweight, transparent and yet highly fire retardant.

Description:
FIELD 
     The present invention relates to composite transparencies and more particularly to composite transparencies utilized to provide improved fire resistance for optically transparent thermoplastics that can be incorporated into a mobile platform. 
     BACKGROUND 
     Fire safety is important because of the destructive nature of an uncontrolled fire. Fire is a self-propagating cycle of reactions where matter changes form as indicated by the visible and tangible side effects of heat, light, and gas emission. Fire is caused by raising a fuel (such as plastic or wood, for example) to its combustion temperature. The heat decomposes the complex molecules of the fuel into smaller molecules that recombine with oxygen in the air to produce different fuels, generate more heat, and release various gases. The burning fuel releases free radicals that emit light or produce a flame. The fuel decomposition and radical emission contribute to the fire cycle until only the nearly pure carbonaceous base of the fuel, known as char, remains. The fire is accompanied by the release of volatile gas compounds, or smoke, from the fuel components that do not char. As long as there is a source of oxygen, free radicals, and fuel, the fire will continue and may become uncontrollable. 
     Technologies such as flame retardants and flame resistant materials, for example, have been developed to control fire by raising the combustion temperature, reducing the rate of burning, reducing flame propagation, and reducing smoke generation. Flame retardants include inorganic minerals, phosphorous compounds, halogenated organic compounds, and others for the inclusion in various fuels or substrates. The inorganic minerals reduce heat generated to below the combustion temperature of the fuel by endothermically decomposing and releasing water vapor upon decomposition. The phosphorous compounds stop fire by expediting charring of the outer layers of the fuel to insulate any remaining fuel from burning and to prevent the release of combustible gases. The halogenated organic compounds stop fire by eliminating the free radicals that contribute to the combustion process. 
     Choosing a flame retardant may be based on the desired mechanism of protection or may be based on non-fire related criteria as demonstrated in the transportation industry where the multiple safety variables are considered in order to best protect equipment and/or passengers. In mobile platforms such as aircrafts, trains, buses, or ships that are operator driven, it is necessary that visibility through transparencies of the mobile platform such as windows does not interfere with the visual range needed to safely maneuver the mobile platform. When selecting flame retardants for the transparency, particular care must be used because upon combustion of transparency polymers, the initial polymer material burns to undergo crosslinking, aromatization, fusion of aromatics, polymer chain scission, polymer chain stripping, and the like. These changes in polymer structure prolong the burning, provide innumerable fuel sources, increase flame propagation, and increase the generation of toxic gases. 
     Current methods of preventing fire and enhancing fire resistance of transparencies have included incorporating phosphorous and halogenated organic compounds into the transparency material, for example, by applying coatings of the materials onto the transparencies. Despite the benefits, the phosphorous and halogenated organic compounds produce corrosive smoke and/or various environmentally detrimental and toxic emissions. Furthermore, the coated transparencies have a different refraction index (RI) than the uncoated transparency material causing light bending and image distorting through the coated transparency or cause clouding and opacity of the transparent material. 
     Accordingly, there is still a need for a fire-resistant transparency that can provide fire resistance, reduce flame spreading, reduce the rate of heat release, facilitate extinction of the fire, minimize smoke evolution, and does not distort the refractive index of the transparency or impair the visibility through the transparency. The present invention is illustrated in connection with an aircraft window, however it is applicable to any transparency where fire resistance and undistorted visibility are of paramount importance. 
     SUMMARY 
     In various embodiments, the present invention provides a fire-retardant transparent panel comprising a transparent matrix and a plurality of fire-retardant nanoparticles integrated within the transparent matrix. The fire-retardant nanoparticles have a diameter less than the wavelength of visible light. The fire-retardant nanoparticles can be in particulate form or various types of fibers. In various embodiments, the fire-retardant nanoparticles are endothermic materials or dehydrators, or are independently selected from alumina trihydrate, magnesium hydroxide, calcium carbonate, ferrocene, and combinations thereof. 
     Methods for providing a fire-retardant transparent panel are also provided. The methods generally comprise providing a transparent matrix, providing a plurality of fire-retardant nanoparticles having a diameter less than the wavelength of visible light, and integrating the fire-retardant nanoparticles within the transparent matrix such that the panel is substantially insensitive to a difference between the refractive index of the transparent matrix and a refractive index of the fire-retardant nanoparticles. The arrangement of the fire-retardant nanoparticles in the transparent matrix can be selected to enhance the overall fire-retardant properties of the panel and/or to minimize impacts of the nanoparticles on visibility. 
     An aircraft having a fire-retardant window is also provided. The aircraft generally comprises a fuselage having an opening. A window is placed in the opening. The window comprises a transparent panel and a plurality of fire-retardant nanoparticles having a diameter less than the wavelength of visible light. 
     The features, functions, and advantages can be achieved independently in various embodiments of the present inventions or may be combined in yet other embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is an illustration of an exemplary mobile platform including a fire-retardant transparent panel according to various embodiments of the present invention; 
         FIG. 2  is a sectional view of the fire-resistant transparent panel shown in  FIG. 1 ; 
         FIG. 3A  is a schematic view of an injection mold used to construct the fire-retardant transparent panel, shown in  FIG. 1 , in accordance with various embodiments of the present invention; 
         FIG. 3B  is a schematic view of a fire-retardant nanoparticle pre-impregnated tape used to construct the fire-retardant transparent panel, shown in  FIG. 1 , in accordance with various embodiments of the present invention; and 
         FIG. 3C  is a sectional view of a fire-retardant transparent panel according to various embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to  FIG. 1 , a fire-retardant transparent panel  10  according to various embodiments of the present invention is depicted in operative association with a mobile platform  12 . In various embodiments, the mobile platform  12  is an aircraft. Although the mobile platform  12  is shown as an aircraft, the mobile platform  12  could also be represented in the form of other mobile platforms, including, but not limited to a ship, a train, a bus, or an automobile. Additionally, although various embodiments of the present invention will be described below as particularly applicable for use in association with mobile platforms, the invention should not be so limited in application. It is envisioned that the invention is equally applicable to aircrafts, trains, buses, ships, buildings, masks, respirators, goggles, or any application where a fire-retardant panel having high transparency and uninterrupted visibility is of paramount importance. 
     In the particular example provided, the fire-retardant transparent panel  10  is shown as a window in an opening in a fuselage  14  of the mobile platform  12 . It should be appreciated, however, that the fire-retardant transparent panel  10  can be used in any interior or exterior portion of the mobile platform  12 , and may include the cockpit window or a door window. Moreover, the fire-retardant transparent panel  10  can be used in any number of environments not strictly limited to conventional “windows”. For example, skylights, running light covers, satellite dome covers, view ports, and various other environments can employ the fire-retardant transparent panel  10  of the present invention. 
       FIG. 2  illustrates a portion of the fire-retardant transparent panel  10 . The fire-retardant transparent panel  10  generally includes a plurality of fire-retardant nanoparticles  16  integrated within a transparent matrix  18 . In a preferred form, the transparent matrix  18  is formed from polycarbonate, a transparent epoxy resin, or polyvinyl fluoride. Any suitable polymeric material may also be used for the transparent matrix  18  including, but not limited to thermoplastics. 
     The fire-retardant nanoparticles  16  are materials that reduce the spread of fire or prevent fire. Example fire-retardant materials include, but are not limited to, alumina trihydrate, magnesium hydroxide, calcium carbonate, ferrocene, and combinations thereof. The fire-retardant nanoparticles  16  may be any other suitable material. For example, endothermic materials or materials that absorb heat generated by the surroundings or dehydrators that remove sources of oxygen from a burning system may be employed. Suitable fire-retardant materials also include various transition metals. The fire-retardant materials may be selected based the desired level of heat control, the relative amount of the fire-retardant material for incorporation in the transparent matrix  18 , the end use of the fire-retardant transparent panel  10 , and any structural and physical limitations of the system into which the fire-retardant transparent panel  10  is incorporated. While not intending to be bound by a particular theory, it is believed that select fire-retardant nanoparticles  16  reduce fire by endothermic means such that when the fire-retardant material combusts, it absorbs heat from the surroundings, thereby reducing the temperature of the fire. After a succession of these endothermic reactions, the fire becomes extinguished as the combustion temperature of the fuel cannot be maintained by the burning system. 
     Although the fire-retardant nanoparticles  16  are depicted as individual fibers, the fire-retardant nanoparticles  16  can be of any suitable shape including particles, continuous fibers, chopped fibers, bundled fibers, intertwined fibers, and combinations thereof. In embodiments utilizing bundled fibers or weaved fibers, or fibers where there is overlap, it is understood that the total thickness of the overlapped regions is to not exceed the wavelength of light as to prevent any interference with visibility due to light scattering or refraction from the fibers. 
     The fire-retardant nanoparticles  16  can also be a mixture of fibers and particles. The integration of the fire-retardant nanoparticles  16  can be tailored to extinguish the fire in a distinct pattern or a time frame. For example, a dense region of fire-retardant particles can be placed on the outer surfaces of the transparent panel  10  while a single continuous fiber loops the core of the transparent panel. The dense region of particles could cause the surface of the fire-retardant transparent panel  10  to char to quickly block the remaining fuel source and stop the spread of fire. In other embodiments, dense regions could be used to isolate the fire. For example, strips of the fire-retardant transparent panel  10  could have a dense population of the fire-retardant nanoparticles  16  such that the fire is blockaded and cannot spread to other regions of the fire-retardant transparent panel  10 . 
     In one embodiment the fire-retardant nanoparticles are in fiber form and comprise alumina trihydrate. The alumina trihydrate is prepared using a solution synthesis or a precipitation method. An aluminum source, such as aluminum sulfate, is combined with a solution that provides the requisite oxidation of the aluminum sulfate and/or provides the hydrating component until the aluminum oxide trihydrate or alumina trihydrate begins to precipitate from the solution. The solids can be removed to stop precipitation. Increasing the length of time the precipitate remains in the solution leads to a greater diameter of the nanofiber or particulate or a longer nanofiber, depending on the agitation of the system and other reaction controls such as temperature. Additional synthesis techniques include laser induced dehydroxylation of Al(OH) 4   −  to Al(OH) 3 , hydrolysis of Al(CH 3 ) 3  in a turbulent subsonic gas jet reactor, sonochemical processing, microemulsion processing, high-energy ball milling, or any other techniques known in the art used to produce nanoparticulates. 
     The dependence of light scattering upon particles in a coating can be described by the following relationship:
 
I s α(Nd 6 /λ 4 )(Δn) 2 ,
 
     where N is the total number of nanoparticles, d is the diameter of the nanoparticles or nanoparticle aggregates, λ is the wavelength of light that is incident upon a coating with the nanoparticles, and Δn is the difference in the index of refraction of the nanoparticles and the transparent matrix. In a one embodiment the diameter “d” of the fire-retardant nanoparticles  16  is less than the wavelength of visible light, i.e., less than from about 400 nm to about 600 nm. Preferably, the fire-retardant nanoparticles  16  have a diameter d of from about 0.1 nm to about 400 nm, and more preferably from about 1 nm to about 5 nm. In embodiments where the fire-retardant nanoparticles  16  are in fiber form, the fibers can range from about 10 nm in length to about 500 nm in length. The small diameter insures that any light absorbed by the alumina trihydrate will be in the ultraviolet range and therefore invisible to the human eye. 
     Preferably the fire-retardant nanoparticles  16  are distributed within the transparent matrix  18  at from about 1% by volume to 60% by volume, and more preferably from about 20% by volume to about 40% by volume. 
     In accordance with a preferred implementation of the present invention, due to the diameter d being less than the wavelength of visible light, the fire-retardant nanoparticles  16  provide an uninterrupted view. The transmittance is on the order of at least 90% and preferably at least about 95%. Moreover, because the light absorption of the fire-retardant nanoparticles  16  is in the ultraviolet range and not visible to the human eye, it is possible to allow dissimilar RIs between the fire-retardant nanoparticles  16 , such as the white pigment alumina trihydrate, and the transparent matrix  18 , such as polycarbonate, without the fire-retardant transparent panel  10  becoming opaque. 
     In an exemplary implementation, the fire-retardant nanoparticles  16  are integrated with the transparent matrix  18  utilizing an injection molding process as illustrated in  FIG. 3A . A mold  20  generally includes mold halves  22  that combine to form a designated shape, such as, for example, a window shape. The epoxy resin  24  and fire-retardant nanoparticles  16  are then injected into the mold  20 . The fluid flow of the epoxy resin  24  can be varied to direct mixing and direction of the nanoparticles  16 . In an alternate preferred embodiment, not depicted, the inlets for the epoxy resin  24  and the fire-retardant nanoparticles  16  can be on the same side of the mold half  22 . A cycle of higher and slower flow rates of the epoxy resin  24  can also be implemented to achieve proper mixing of the epoxy resin  24  and the fire-retardant nanoparticles  16  or selectively deposit the fire-retardant nanoparticles  16  across the panel  10 . It is understood that the epoxy resin  24  and the fire-retardant nanoparticles  16  may be mixed prior to disposition of the mixture into the mold half  22  or after the components are placed in the mold half  22 . Once the epoxy resin  24  has set or cured, the fire-retardant transparent panel  10  may then be removed from the mold  20 . It is possible for the mold  20  to take on any shape desired, thereby allowing windows that have complex surfaces. 
     Turning to  FIG. 3B , in an alternative preferred embodiment, the fire-retardant nanoparticles  16  are used to form a reinforced pre-impregnated tape. For example, the fire-retardant nanoparticles  16  may be arranged in a resin that, after solidification, forms the transparent matrix  18  in the form of strips of pre-impregnated tape  26 . Successive layers of the pre-impregnated tape  26  may then be laminated to form the fire-retardant transparent panel  10 . Due to the random orientation of the fire-retardant nanoparticles  16  on the pre-impregnated tape  26  of various embodiments, the pre-impregnated tape  26  may not need to be aligned in any particular manner when laminated with layers of other pre-impregnated tape  26  to form the fire-retardant transparent panel  10 . 
     In addition to the random orientation of the injection molding, the integration of the fire-retardant nanoparticles  16  can also be in an ordered form such as lined patterns, such as strips or grids, regular shaped patterns, such as circles, rectangles, squares, or ellipses, irregular shaped patterns or free-form patterns, overlapping patterns, such as an interconnected series of circles, weaved patterns, such as cloth, and various combinations thereof. As depicted in  FIG. 3C , the fire-retardant nanoparticles  16  are arranged in a substantially linear pattern. The linear pattern may be achieved by forming a laminate of the transparent matrix  18  and the nanoparticles  16 . The pressure and directional force resultant from the laminate process can be manipulated to align the nanoparticles  16  in rows. In various embodiments, pultrusion techniques that generally include pulling a fiber reinforced resin through a shaping die, can be used to orient the fire-retardant nanoparticles  16  and provide the desired linear or regular shaped pattern. 
     Regardless of the particular technique employed, it is desirable that the implementation of the fire-retardant nanoparticles  16  be performed to prevent clumping or undesired aggregation of the fibers. In various embodiments, it may be desirable to integrate the fire-retardant nanoparticulates  16  in a manner to provide dense regions or a gradient of the fire-retardant nanoparticles  16 . 
     By employing fire-retardant nanoparticles  16  having a diameter less than the wavelength of light integrated within a transparent matrix  18 , the fire-retardant transparent panel  10  is substantially insensitive to RI ‘mismatch’, e.g. ‘mismatch’ caused by the integration of materials having different refractive indices. This surprising result is especially beneficial because some fire-retardant materials are actually pigments, such as the white coloring of alumina trihydrate. The fire-retardant nanoparticles  16  prevent light scattering and refraction that occurs from particles that can deflect light. That is, the fire-retardant transparent panel  10  will maintain a high level of transparency, e.g. 90% or greater, in the presence of the fire-retardant nanoparticles  16 . Moreover, the fire-retardant transparent panel  10  has an increased hardness which can be useful in the mobile platform  12 . 
     Preferably, the fire-retardant nanoparticles  16  are incorporated into the fire-retardant transparent panel in an amount sufficient to provide at least one of flame spread reduction, heat reduction, smoke reduction, rapid charring, and safety to the panel and its surroundings. The concentration of the fire-retardant nanoparticles incorporated can be altered based on the end use of the fire-retardant transparent panel, for example, maximizing the concentration of flame-retardant nanoparticles  16  to provide flame resistance to distinct regions of the fire-retardant transparent panel  10  where the flame may be able to propagate more as compared to other regions. 
     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations, which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.