Abstract:
The present invention relates to fire resistant sustainable sandwich panels comprising a thermoplastic foam core in between outer skins made of natural fibres set within a natural thermoset biopolymer. The sandwich panels are provided with a fire resistant protective coating on an outer surface. This surface may be the surface facing the cabin when installed in an aircraft interior. Such fire resistant sustainable panels provide the required flame and heat resistance, have a high strength-to-weight ratio, low maintenance costs and are generally easily installed. Furthermore, the fire resistant sustainable sandwich panels allow easy recycling and are cheaper than conventional sandwich panels.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of European Patent Application No. EP 14382004.1, filed on Jan. 8, 2014, the entire disclosure of which is expressly incorporated by reference herein. 
       FIELD 
       [0002]    The present invention relates to fire resistant sustainable aircraft interior panels comprising a sandwich panel structure. The fire resistant sustainable aircraft interior panels may be used in applications like floors, ceilings, sidewalls and stowage bins. 
       BACKGROUND 
       [0003]    Sandwich panels are used in many aircraft interior applications, such as floors, sidewalls, ceilings and stowage compartments. These types of sandwich panels may be used in similar applications in other types of transport vehicles. In addition to providing a finishing function, the sandwich panels need to have adequate weight and thickness and possess certain mechanical properties and have sufficient fire resistance. 
         [0004]    Of particular interest to the applicant is the use of fire resistant sustainable sandwich panels in aircraft interiors. Consequently, the following description focuses on the application of novel sustainable, or environmentally friendly, sandwich panels in aircraft interiors. It will be abundantly clear that the present invention may extend to fire resistant sustainable sandwich panels in general having the composition of the sustainable aircraft interior panels according to the claims. Such general fire resistant sustainable sandwich panels enjoy far greater applicability than just aircraft interiors and would not require any modification. 
         [0005]    Conventional aircraft interior panels are sandwich structures comprising a core sandwiched between outer skins. The materials used in these panels are chosen primarily for their fire resistant properties. For commercial airliners, there are strict regulations governing the fire resistant properties of the materials used in the cabin, along with limits as to the heat and smoke released during combustion of such materials. This has led to the widespread use of glass fibre-reinforced composites based on phenolic resins in conventional aircraft interior parts. In addition to their appropriate fire resistance, the panels based on these composite materials may be moulded into complex shapes, they have a high strength-to-weight ratio, have appropriate flexural strength and impact resistance, have low maintenance costs and are generally easily installed. 
         [0006]    In conventional panels, the outer skins comprise phenolic resins and glass fibre pre-pregs. Alternatively, skins may be made from a composite of glass fibre with epoxy or carbon fibre with epoxy. All these skin materials have known environmental limitations. Phenolic resins are regarded as highly noxious and can cause skin problems, such as dermatitis. Glass fibres cause irritation of the skin, eyes and upper respiratory system producing skin eruption similar in appearance to poison ivy, pneumoconiosis and silicosis. If ingested, glass fibres can also cause gastrointestinal conditions. 
         [0007]    The core of a conventional panel is usually formed from a Nomex® honeycomb that contains aramide fibres. These fibres are a heat-resistant synthetic fibre, but have a known disadvantage in that upon fracturing, they produce small fibrils that are harmful to the lungs and cause skin irritation. 
         [0008]    The use of such noxious skin and core materials presents difficulties during manufacturing, while heating the resins and where fibres may be exposed after curing, such that careful handling is required. Personal protective equipment is therefore required during manufacturing such panels. This does not apply once the part is made and installed on the aircraft. However, more significant issues arise at the end of the service life of the aircraft where it is scrapped and parts are disposed of. This is of course true for removal and disposal of interior panels at any stage of the aircraft&#39;s life, for example during a refit or conversion process. Moreover, the noxious nature of the materials makes the panels poor candidates for recycling and so often end up being sent for burial at landfill. They do not leach but still constitute harmful residues. This is contrary to the aerospace industries current drive for products that achieve a better environmental performance. 
         [0009]    The ideal situation would therefore be that in which the sandwich panels are more environmentally friendly while maintaining an excellent technical performance. For example, sandwich panels that are easier to recycle or to dispose would be extremely advantageous. An improvement on conventional sandwich panels has been described in EP-A-2,463,083. This document discloses the use of sustainable materials in sandwich panels, namely a sandwich panel comprising skins formed from natural fibres set within an inorganic thermoset resin or a thermoplastic resins and a core formed from fire resistant balsa wood, a fire resistant paper honeycomb or a fire resistant thermoplastic foam. The present invention provides an alternative form of sustainable sandwich panel. 
       SUMMARY 
       [0010]    Against this background and from a first aspect, the present invention resides in an aircraft interior panel comprising a core sandwiched between first and second skins. The first and second skins both comprise a composite comprising natural fibres set within a biopolymeric resin thereby forming a sustainable aircraft interior panel. The aircraft interior panel further comprises a coating on an outer surface of at least one of the first and second skins to increase the fire resistance of the panel. Thus, a fire resistant sustainable aircraft interior panel is obtained. 
         [0011]    The use of a natural biopolymeric resin provides significant “sustainable” benefits in terms of ease of recycling, and also offers other advantages such as reduced weight and lower cost as will be described in more detail below. In addition, biopolymeric resins allow the sandwich panels to be made in ways similar to how conventional sandwich panels are made, and using conventional tooling with only minimal changes in existing infrastructure. The fire resistant protective coating provides the required fire resistance to meet certification requirements for use in aircraft. Optionally, the fire protective coating is halogen free. 
         [0012]    Optionally, the biopolymeric resin comprises a natural thermoset polymer. The thermoset polymer may be derived from linseed oil, although may be derived from materials such as soya oil resin or bio-based epoxy resins. The biopolymeric resin may comprise a viscosity-fixing agent, for example an acrylic acid, a methacrylic acid, a styrene or a hydroxyethyl methacrylate monomer. The biopolymeric resin may comprise an initiator for promoting polymerisation, for example an organic peroxide like methyl ethyl ketone peroxide, benzoyl peroxide or butanone peroxide. These components may be mixed to form the resin. For example, the biopolymeric resin may comprise a mixture of 50% to 80% by weight linseed oil derived thermoset polymer, 10% to 30% hydroxyethyl methacrylate monomer and 1% to 10% initiator. 
         [0013]    Optionally, the fibres are natural fibres. For example, the fibres may be flax although other natural fibres like hemp, sisal and jute may be used. The fibres may be woven into a fabric. The present invention has been found to have utility over a broad range of fibre densities. 
         [0014]    The core may comprise a thermoplastic polymer foam, optionally a polyetherimide foam. The core may be a fire resistant thermoplastic foam. An advantage of using a foam core over a conventional honeycomb structure is enhanced soundproofing. When used in aircraft interiors, this can provide a quieter, more pleasant environment for passengers. 
         [0015]    For certain applications, the aircraft interior panel may comprise more than three layers, for example if thickness and weight are not prohibitive for the application. For example, in addition to the core, first skin and second skin, the aircraft interior panel may comprise further skins or further cores, or both further skins and cores, or other layers. Other layers may include conventional finishes for decorative purposes or fire retardant coatings. The core may be sandwiched between the first and second skins in all configurations, with first and second skins being arranged outermost in the aircraft interior panel, i.e., the first and second skins provide the outer surfaces of the aircraft interior panel. 
         [0016]    The present invention also extends to an aircraft including any of the fire resistant sustainable aircraft interior panels described above. Optionally, the fire resistant sustainable panel is fixed in the aircraft interior such that the fire resistant coating is provided on a surface exposed to a cabin of the aircraft interior. 
         [0017]    The present invention also extends to a method of manufacturing any of the aircraft interior panels described above, comprising curing a stack of the natural fibre fabrics, the resin and the core so as to form the aircraft interior panel. Then, the fire resistant protective coating is applied to the outer surface, on the first skin or second skin, depending on the application. For example, the method may comprise spraying the fire resistant coating onto the first and/or second skin. This may be done using an air gun. 
         [0018]    The method may comprise forming the biopolymeric resin by mixing a thermoset polymer, for example a natural thermoset polymer, a viscosity-fixing agent and an initiator, impregnating the fibres with the biopolymeric resin, laying up the fibres impregnated with the resin on both sides of the core to form the stack, and curing the stack in one step to form the aircraft interior panel. 
         [0019]    Optionally, the method comprises curing the stack using a vacuum bag or a hot mould press. 
     
    
     
       DRAWINGS 
         [0020]    In order that the present invention may be more readily understood, preferred embodiments will now be described, by way of example only, with reference to the following drawings in which: 
           [0021]      FIG. 1  is a perspective view of a fire resistant sustainable aircraft interior panel according to a first embodiment of the current invention; 
           [0022]      FIG. 2  is a perspective view of a fire resistant sustainable aircraft interior panel according to a second embodiment of the present invention; and 
           [0023]      FIG. 3  is a schematic representation of a method of assembling a fire resistant sustainable aircraft interior panel according to a first embodiment of the method of the present invention. 
       
    
    
     DESCRIPTION 
       [0024]      FIG. 1  shows a fire resistant sustainable aircraft interior panel  20  according to a first embodiment of the present invention. The fire resistant sustainable aircraft interior panel  20  comprises a core  22  sandwiched between an upper skin  24  and a lower skin  26 . A fire resistant protective coating  28  is shown above the upper skin  24 . The fire resistant protective coating  28  should preferably be arranged to be the surface exposed to the cabin when the panel is fitted in an aircraft. 
         [0025]    The core  22  is a fire resistant thermoplastic foam, for example a polyetherimide foam. Joined to the core  22  are the corresponding upper and lower outer skins  24 ,  26 . Each skin  24 ,  26  comprises a natural composite material made from natural fibres set within a biopolymer resin. In this exemplary embodiment, flax fibres are woven into a fabric. Other natural fibres like hemp, sisal and jute may be used. The fabric is impregnated with the biopolymer resin, laid up to either side of the core  22  and cured such that the impregnated fabrics form the skins  24 ,  26  that bond to the core  22  during the curing process. 
         [0026]    The present invention is not limited to fire resistant sustainable aircraft interior panel structures comprising only four layers. More than a single core layer may be included, and more than a single skin layer may be included to any one side of the core if the thickness and weight are not prohibitive for the application. 
         [0027]    An example of a further fire resistant sustainable aircraft interior panel  30  is shown in  FIG. 2 . The aircraft interior panel  30  comprises six layers that are stacked as follows, from top to bottom: a fire resistant protective coating  42 , an outer upper skin  34 , an inner upper skin  38 , a core  32 , an inner lower skin  40  and an outer lower skin  36 . The core  32  corresponds to the core  22  described in  FIG. 1 . The fire resistant protective layer  42  corresponds to the fire resistant protective layer  28  described in  FIG. 1 . Also, the skins  34 ,  36 ,  38 ,  40  correspond to the skins  24 ,  26  described in  FIG. 1 . Pairs of upper and lower skins  34 ,  38  and  36 ,  40  may be provided to increase strength if the thickness and weight are not prohibitive for the application. The skins may be laid up in an aligned manner, or with their plies rotated (e.g., the warp and weft of the outer upper skin  34  may have its warp and weft rotated through 90 degrees relative to those of the inner upper skin  38 ) for improved mechanical properties. The fire resistant sustainable aircraft interior panel of this second embodiment has a fire resistant protective coating  42  on top of the surface exposed to the cabin. 
         [0028]    Methods of manufacture of fire resistant sustainable aircraft interior panels according to the present invention will now be described. For the sake of simplicity, four-layer fire resistant sustainable aircraft interior panels will be described, although it will be readily appreciated that the method may be simply extended to fire resistant sustainable panels having more than four layers. 
         [0029]    A method of manufacture is shown in  FIG. 3 . At  100 , the materials that form the skins  24 ,  26  are formed and arranged. This step  100  comprises laying up natural fibre fabrics, as indicated at  102 . For example, one layer of flax fabric is laid up for each skin  24 ,  26 . 
         [0030]    At  104 , a biopolymer resin impregnates the natural fibre fabrics. The biopolymer resin may be prepared as follows: a mixture is formed of a natural thermoset polymer, a viscosity-fixing agent and an initiator. The natural thermoset polymer may be a linseed oil polymer such as Mecryl LT. Other suitable choices for the natural thermoset polymer include soya oil resin or bio-based epoxy resins. The natural thermoset resin may be mixed to a proportion of 50% to 80% by weight. The viscosity-fixing agent may be a (hydroxyethyl) methacrylate monomer, also known as HEMA. Other suitable choices include acrylic acid, methacrylic acid or styrene. The viscosity-fixing agent may be mixed to a proportion of 10% to 30% by weight. The initiator is a chemical additive that promotes the polymerisation reaction of the biopolymer. A suitable choice is Initiator BK. Other suitable choices include organic peroxides like methyl ethyl ketone peroxide, benzoyl peroxide or butanone peroxide. The initiator may be mixed to a proportion of 1% to 10% by weight. 
         [0031]    The impregnated fibre fabrics that will form the skins  24 ,  26  are laid up on both sides of the core  22 , as shown at step  106 . The resin acts as an adhesive to bond the impregnated fibre fabrics to core  22 . At  108 , this assembly is transferred to a vacuum bag or a hot press such that the complete sandwich panel  20  may be formed when heated at 140-150° C. for 15 minutes while applying pressure either with a vacuum bag or a hot press. Thus, a panel  20  may be formed every 15 minutes according to this one step forming process. It will be appreciated that the method of manufacture described above is similar to the conventional crush core process. Hence, advantageously, only minimal changes are needed to tooling and production methods to accommodate manufacture of these novel fire resistant sustainable sandwich panels. 
         [0032]    Subsequently, the upper skin  24  of the panel formed in  108  is provided with a halogen-free fire resistant protective coating  110 . An advantage of using a fire resistant coating is that it removes the need to impregnate the natural fibres with a flame retardant solution prior to impregnating them with the biopolymer resin. That is, the natural fibres do not need to be soaked in a flame retardant. The resulting panel  20  is found to be lighter yet still offer the same high level of fire resistance. 
         [0033]    The coating  28  is sprayed onto the upper skin  24  of the cured panel  20  using an air gun. The coating  28  is sprayed to an amount of 300 to 400 g/m 2 , and typically takes only one or two minutes. The coating  28  is then dried at room temperature for 24 hours. The dried thickness of the coating  28  is approximately 150 nm. 
         [0034]    Furthermore, other coatings may be applied to the protective coating  28 , for example decorative coatings to provide a desired colour, pattern or texture. 
         [0035]    It will be clear to the skilled person that variations may be made to the above embodiments without necessarily departing from the scope of the invention that is defined by the appended claims. 
         [0036]    For example, the methods described above with respect to four-layer fire resistant sustainable aircraft interior panels  20  may be readily adapted to more than four-layer fire resistant sustainable aircraft interior panels. For example, the number of skin layers laid up on the core may be increased from one each side if thickness and weight are not prohibitive for the application. More than a single core layer may also be included. 
         [0037]    Various fire resistant sustainable aircraft interior panels and various methods of manufacture have been described. It will be appreciated that the different methods may be applied to make any of the different fire resistant sustainable panels described. 
         [0038]    Further, the disclosure comprises embodiments according to the following clauses: 
         [0039]    Clause 1. An aircraft interior panel comprising a core sandwiched between first and second skins, wherein the first and second skins both comprise a composite comprising natural fibres set within a biopolymeric resin thereby forming a sustainable aircraft interior panel, and wherein the aircraft interior panel further comprises a coating on an outer surface of at least one of the first and second skins to increase the fire resistance of the panel thereby providing a fire resistant sustainable panel. 
         [0040]    Clause 2. The fire resistant sustainable aircraft interior panel of Clause 1, wherein the biopolymeric resin comprises a natural thermoset polymer, optionally a linseed oil derived thermoset polymer. 
         [0041]    Clause 3. The fire resistant sustainable aircraft interior panel of Clause 1, wherein the biopolymeric resin comprises a viscosity-fixing agent, optionally a hydroxyethyl methacrylate monomer. 
         [0042]    Clause 4. The fire resistant sustainable aircraft interior panel of Clause 1, wherein the biopolymeric resin comprises an initiator for promoting polymerisation. 
         [0043]    Clause 5. The fire resistant sustainable aircraft interior panel of a Clause 1, wherein the fibres are natural fibres, optionally flax. 
         [0044]    Clause 6. The fire resistant sustainable aircraft interior panel of Clause 1, wherein the core comprises a thermoplastic polymer foam, optionally a polyetherimide foam. 
         [0045]    Clause 7. An aircraft comprising one or more fire resistant sustainable aircraft interior panels of Clause 1. 
         [0046]    Clause 8. The aircraft of Clause 7, wherein the fire resistant sustainable panel is fixed in the aircraft interior such that the fire resistant coating is provided on a surface exposed to a cabin of the aircraft interior. 
         [0047]    Clause 9.A method of manufacturing the fire resistant sustainable aircraft interior panel of any of Clause 1, comprising curing a stack of the natural fibre fabrics, the resin and the core so as to form the aircraft interior panel, and applying the fire resistant protective coating to the outer surface of the at least one of the first and second skins. 
         [0048]    Clause 10. The method of Clause 9, comprising mixing a thermoset polymer, a viscosity-fixing agent and an initiator to form the biopolymeric resin, impregnating the fibres with the biopolymeric resin, laying up the fibres impregnated with the resin on both sides of the core to form the stack, and curing the stack in one step to form the aircraft interior panel. 
         [0049]    Clause 11. The method of Clause 10, wherein the fibres comprise a woven fabric. 
         [0050]    Clause 12. The method of any Clause 9, comprising curing by using a vacuum bag or a hot press. 
         [0051]    All mentioned documents are incorporated by reference as if herein written. When introducing elements of the present invention or exemplary aspects or embodiment(s) thereof, the articles “a,” “an,” “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Although this invention has been described with respect to specific embodiments, the details of these embodiments are not to be construed as limitations. Different aspects, embodiments and features are defined in detail herein. Each aspect, embodiment or feature so defined may be combined with any other aspect(s), embodiment(s) or feature(s) (preferred, advantageous or otherwise) unless clearly indicated to the contrary.