Abstract:
The present invention provides a fireproof bulkhead, having a highly porous structure with an intumescent coating, the highly porous structure being formed as a reticulated foam or as a highly porous pimple or nub structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/842,688, filed Sep. 6, 2006, German Patent Application No. 10 2006 042 220.1 filed on Sep. 7, 2006 and German Patent Application No. 10 2006 055 377.2 filed on Nov. 23, 2006, the complete disclosures of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a highly porous structure with an intumescent coating, which is used as a fireproof bulkhead in aircraft construction, and to the production of such a fireproof bulkhead. 
     BACKGROUND OF THE INVENTION 
     The requirements for materials that are used in modern aircraft construction are continually increasing as a result of ever new regulations and changing requirements for the aircraft. For example, the requirements that an aircraft has to meet after a crash landing in which a fire breaks out, known as a post-crash fire, are becoming more stringent as a result of new regulations. The occupants must be protected from the flames attacking from the outside for a predetermined period of time that is sufficient to allow the passenger compartment to be evacuated. 
     The improved protection of the passenger compartment may be obtained by reinforcing the fuselage and also the secondary structure. However, this usually entails an increase in weight. However, a weight increase is undesired, in particular in the case of aircraft construction. Especially the hold in the lower region of an aircraft, produced from lightweight materials such as glass fibers and phenolic resins, offers only little protection to flames attacking from the outside. In order to improve the protection of the passenger compartment here, additional fireproof bulkheads can be introduced. These fireproof bulkheads are fitted between the lower region of the passenger compartment and the hold. 
     However, fireproof bulkheads that are to be fitted in this region have to meet special requirements. On the one hand, they must provide effective protection of the passenger compartment in the event of a fire. On the other hand, the free circulation of air around the hold must be ensured. This is necessary in particular when there is a sudden drop in pressure in the aircraft. If a high circulation of air cannot be ensured, there is the risk of the aircraft being torn apart if there is a sudden drop in pressure caused by damage to the outer skin of the aircraft, due to the great differences in pressure that cannot be equalized quickly enough. 
     The structures used until now as fireproof bulkheads in the aircraft substantially comprise mechanical flap structures, which close in the event of a fire, or deflector plates arranged in the form of a cascade. 
     Of these, flap structures have the disadvantage that, in the case of complicated geometries, they can only be realized with difficulty and, moreover, are accompanied by high weight and, on account of their mechanics, are susceptible to malfunctions or failures. 
     Deflector plates arranged in the form of a cascade have the disadvantage that they cannot completely separate the different regions. 
     A construction which normally ensures a high level of air permeability but at the same time can serve effectively as a fireproof bulkhead in the event of a fire, and along with that achieves a low weight, cannot be provided by either of the two types of fireproof bulkhead used so far in aircraft construction. 
     SUMMARY OF THE INVENTION 
     The present invention therefore has the object of providing a material which has a high level of air permeability but in the event of a fire can serve as a fireproof bulkhead and, moreover, has a low weight. 
     This object is achieved according to the invention by a fireproof bulkhead with features according to the present disclosure and appended claims. 
     A first aspect of the present invention concerns a fireproof bulkhead, comprising a highly porous structure with an intumescent coating, the highly porous structure being a reticulated foam or a highly porous pimple or nub structure. The highly porous structure must in this case perform various tasks. 
     Firstly, the highly porous structure must have very good permeability to gases, such as for example air. A high level of permeability to gases ensures that gases, such as for example air, can flow unhindered through the highly porous structure. The highly porous structure does not represent any obstacle to a flowing gas, in particular a rapidly flowing gas. This is important in particular in the event of a drop in pressure in the aircraft, in order to avoid bursting of the aircraft. Ideally, the highly porous structure does not offer any resistance, or any appreciable resistance, to an air stream. 
     Secondly, the highly porous structure must not have any appreciable weight. In particular in the case of aircraft construction, the greatest attention must be paid to weight saving. Structures which, while of a low weight, can at the same time still undertake a load-bearing function are used in aircraft construction. 
     A further, important feature that the highly porous structure must have is the bearing of an intumescent coating. It must be possible for the intumescent coating to be easily applied to the highly porous structure. In particular in the intumesced state, i.e. when the intumescent coating has turned into a thermal foam in the event of a fire, the highly porous structure must offer sufficient restraint and stability to the swollen, i.e. intumesced, coating in order that it can be kept in situ by the pressure of the flames. The flames must not detach the thermal foam from the structure by the pressure which they build up, and thereby separate the structure and the foam, since effective damping of the flames is then not possible. 
     A reticulated metallic foam is particularly advantageous here as the highly porous structure. A reticulated foam differs decisively in its properties from a normal foam. When a foam is reticulated, the cell membranes of the foam are removed virtually completely. This makes the cells as open as possible, which ensures extremely low flow resistance to gases, such as for example air. 
     For the reticulation, a conventional foam of polymer material is introduced into a steel chamber. After exchanging the air for a fuel gas mixture, the latter is ignited. When it is ignited, the cell membranes are torn apart by the heat and pressure wave produced and melt down to the cell walls. As a result, the cell walls are strengthened and the flow resistance to gases is reduced on account of the absence of the cell membranes. 
     On account of the reduced flow resistance to gases and the nevertheless stable structure, reticulated foams are particularly suitable for serving as a carrier for an intumescent coating and keeping the swollen thermal foam in situ in the event of a fire. A simple grid structure, such as for example a wire grid, cannot provide an adequate structure for a thermal foam to support it in the event of a fire and to ensure that the thermal foam is not blown away by the pressure of the fire occurring. 
     According to a further development, the reticulated foam is a reticulated metallic foam. The applicant is so far only familiar with such foams as heat exchangers, filters, catalyst surfaces, weight-saving structural elements or deformable energy absorbers. It is not, however, familiar so far with them being used as a carrier for an intumescent coating which is fixed in the swollen state by the reticulated metal structure. The coated, reticulated metallic structures advantageously have excellent permeability properties, in particular with respect to gases, such as for example air. 
     Serving for example as a starting point for metallic foams are reticulated polyurethane foams. These foams can be easily brought into the desired outer form. To transform the reticulated polyurethane foams into reticulated metallic foams, a special casting method is used. The reticulated polyurethane foam is filled with a ceramic material. After removing the organic component, a negative form remains, which is filled with liquid metal, such as for example aluminum, copper or zinc. After careful removal of the ceramic materials, an articulated metallic foam remains. A high-quality cell structure with 2-12 cells per centimeter is obtained. 
     Alternatively, an articulated foam of polymer material may be coated with a metal to produce a metallic structure. The coating may be performed for example by means of a chemical vapour deposition. It is optionally also possible, after an initial coating or other way of ionizing the surface, to use a galvanic method for depositing metals on the articulated structure of polymer material. A highly porous, reticulated metal foam is also obtained in this way. 
     According to a further development, the articulated metal foam may be coated with a ceramic. The ceramic coating provides the metal foam with further fire resistance. Moreover, the ceramic surface can be coated more easily with an intumescent coating. 
     Suitable metallic materials for a reticulated foam are aluminum, copper, zinc and titanium or their alloys. 
     Apart from reticulated foams, pimple or nub structures may also be used as carrier materials for an intumescent coating. The pimple or nub structures have the advantage here that they can be additionally used as structural elements that transfer relatively great forces. 
     To produce a pimple or nub structure, woven, knitted and/or laid fabrics of fibers are thermoformed over templates. Fibers which can easily be stretched can be used suitably. However, other fibers, such as for example glass fibers, in particular quartz glass fibers, or aramid fibers may also be used. To produce nubbed honeycombs, for example, first a knitted or laid fabric of partially stretched fibers, for example polyester, is thermoformed at an elevated temperature to form a nub structure. At the same time, the fibers are stretched. The thermoformed knitted or woven fabric may subsequently be impregnated with a reactive resin. After the thermoforming, it is already dimensionally stable sufficiently to retain its form during this working step. Alternatively, the knitted fabric may also be impregnated before the thermoforming, whereby a prepreg is obtained. The impregnated nubbed honeycombs may be provided with further outer layers and be cured together with them to form sandwich components. 
     By placing two layers of a pimple or nub structure one inside the other in an opposing fashion and adhesively bonding them, the specific properties of the structures can be further improved significantly. 
     Further working steps are necessary to produce such interlaced nub structures. Two pimple or nub structures are placed one inside the other, offset in relation to each other, the two layers being pressed one into the other with the aid of two tools with rubber studs, which engage in the nubs or pimples of the impregnated structure. The rubber studs become shorter and thicker during the pressing, so that they can reliably bond the nubs or pimples to one another even at the flanks. The tools are heated in order to cure the previously applied reactive resin and consequently adhesively bond the structures. 
     The bonding of two pimple or nub structures has the effect that the specific compressive stiffness is more than trebled and the specific compressive strength more than doubled in comparison with the single pimple or nub structure. 
     Such structures are therefore advantageous in particular whenever relatively great forces have to be transferred. When quartz glass fibers are used, for example, an increased fire resistance can be provided. Moreover, a double-layered pimple or nub structure can easily keep an intumesced coating in situ, in order in this way to form an effective fireproof bulkhead. The great spacing between the fibers also allows a high level of permeability with respect to gases, in particular air, to be ensured. 
     To provide a fireproof bulkhead, the highly porous structures described above must be coated with an intumescent coating. Used for this according to the invention are coatings which are suitable on account of their low viscosity for the impregnating of highly porous structures without impairing the advantageous properties of the highly porous structure, in particular the high level of permeability to gases. 
     In the event of a fire, it is advantageous if the intumesced coating, i.e. the thermal foam produced, fills the highly porous structure completely. In the event of a fire, the carrier material merely serves the purpose of fixing this thermal foam. The intumesced coating, i.e. the thermal foam, protects the structural elements and similarly the areas lying behind the fireproof bulkhead from the flames. 
     According to a further development, the intumescent coating is resistant to hydrolysis. In an aircraft, materials are subjected to great differences in temperature. At the same time, in particular, water deposits may occur on solid structures as a result of condensation of moisture in the air. It can be ensured by the use of hydrolysis-resistant, intumescent coatings that the intumescent coating is not damaged by condensed water vapour as a result of hydrolysis. 
     The intumescent coating develops its intumescent effect (only) at a temperature above 200° C., preferably (only) above 400° C. and most preferably (only) above 600° C. 
     A further aspect of the present invention concerns an aircraft or spacecraft with a fireproof bulkhead. 
     According to one embodiment, the fireproof bulkhead in the aircraft or spacecraft is formed as a fire barrier for a baggage compartment. 
     In particular, the fireproof bulkhead of the present invention may be fitted to the sides of a hold of an aircraft or spacecraft. 
     A further aspect of the present invention concerns a method for producing a fireproof bulkhead such as that described above, with the steps of providing a highly porous structure, the highly porous structure being a reticulated foam or a highly porous pimple or nub structure, and coating the highly porous structure with an intumescent material. 
     According to one embodiment, the intumescent material is applied by spraying or by dip coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of a single-layered pimple structure according to an exemplary embodiment of the present invention; 
         FIG. 2  shows a perspective view of a two-layered pimple structure according to an exemplary embodiment of the present invention; and 
         FIG. 3  shows a cross-sectional view through an aircraft fuselage with fireproof bulkheads according to an exemplary embodiment of the present invention. 
     
    
    
     In the figures, the same reference numerals designate components that are the same or functionally the same, unless otherwise indicated. 
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLES 
     Example 1 
     On a very open-cell, reticulated foam of aluminum, a low-viscosity, hydrolysis-resistant, intumescent lacquer is applied, for example by spraying. After drying of the intumescent coating, the foam can be used as a fireproof bulkhead. During the intumescence of the coating into a thermal foam, the intumescent coating is anchored in the foam microstructure and completely closes the spaces lying between the cell walls. 
     According to the above example, a fireproof bulkhead according to the invention can likewise be produced by the open-cell foam of aluminum being replaced by an open-cell foam of slip-treated aluminum with a ceramic coating. 
     Example 2 
     An open-cell knitted fabric of a partially stretched polyester is thermoformed at elevated temperature to form a nub structure. After impregnating with a reactive resin, this reactive resin is cured at elevated temperature.  FIG. 1  shows a nub structure  1  produced in such a way. Regularly arranged nubs are formed in the nub structure  1 . The nub structure  1  is coated with an intumescent coating for example by immersion in a low-viscosity, hydrolysis-resistant, intumescent coating solution. After drying of the coating, the coated nub structure can be used as a fireproof bulkhead. 
       FIG. 2  shows two interlaced nub structures  1 ,  1 ′ as they have been produced in accordance with the above example. After coating with a reactive resin, the nub structures  1  and  1 ′ are interlaced inversely with one another. The nubs of the nub structure  1 ′ thereby engage in the cavities that lie between the nubs of the nub structure  1 . In this arrangement, the nubs of the two structures touch one another. The nubs or pimples of the respective nub structure are pressed one into the other by rubber studs in such a way that the structures are bonded to one another at elevated temperatures as the reactive resin cures. After coating the interlaced nub structure with a hydrolysis-resistant, intumescent coating by tip coating and drying of the coating, the structure produced in this way can be used as a fireproof bulkhead. 
     In a further exemplary embodiment, interlaced pimple structures of quartz glass fibers are produced in accordance with the above Example 2 and coated with an intumescent coating. 
     The fireproof bulkheads of the present invention can be used in aircraft construction.  FIG. 3  shows by way of example a cross section through a fuselage  10  of an aircraft. Integrated in the fuselage  10  are a cabin  11  and a hold  12 . The hold  12  in this case lies underneath the cabin  11 . Neither the cabin  11  nor the hold  12  is in contact with the fuselage  10  and so they allow a circulation of air on an air circulation path, indicated by the arrows  14  in  FIG. 3 , around the cabin  11  and the hold  12 . In the event of a leak within the fuselage  10 , a rapid pressure equalization must be possible. The cabin  11  and the hold  12  together form a unit around which a rapid circulation of air is to be made possible in the event of a leak occurring in the fuselage  10 . 
     Furthermore, in the event of a fire occurring from the outside, the cabin  11  with the passengers in it must be protected from a fire. For this purpose, for example, the floor  13  of the hold is of a fire-retardant design. A possible source of fire  16  at the lower part of the fuselage is consequently hindered from advancing further in the direction of the cabin  11  by fire barrier devices in the floor  13  of the hold. 
     Since, however, areas to the side of the hold must be kept clear for possible air circulation  14 , spreading of flames can take place along these air circulation paths represented by the arrows  14 . Consequently, rapid advancement of the flames to the cabin  11  can no longer be prevented. According to the present exemplary embodiment, fireproof bulkheads  15  are fitted to the sides of the hold, in order to prevent advancement of the flames in the event of a fire. Since the fireproof bulkheads  15  also have a low resistance to air flowing through when there is no fire, the paths for the air circulation  14  are not affected, or only insignificantly, by the fireproof bulkheads  15 . In the event of a fire, however, spreading of the flames on the paths provided for the air circulation is prevented or delayed by the fireproof bulkheads  15 . 
     Although the present invention has been described in the present case on the basis of exemplary embodiments, it is not restricted to them but can be modified in various ways. 
     For example, it is conceivable to fit the fireproof bulkheads at a number of locations at which circulation of air is necessary when there is no fire, but effective protection from flames is to be provided in the event of a fire. However, it is also possible to fit the fireproof bulkheads at other locations in the aircraft or spacecraft.