Abstract:
Composite sheet material comprises a core ( 12 ) formed from cellular material having a respective skin of continuous sheet material on each side thereof. Both the core and the skins are formed of thermoplastic materials and the skins are attached to the core by a thermoplastic adhesive having a fusion temperature less than that of both the core and the skins.

Description:
FIELD 
     This invention relates to composite sheet material of the type comprising a core formed from cellular material having a respective skin of continuous sheet material on each side thereof. 
     The term “cellular material” is used herein to mean a solid material containing air-filled or gas filled voids, for example, honeycomb or foam material. 
     RELATED ART 
     In a known composite material of this type, the honeycomb material comprises paper honeycomb impregnated with a phenolic lacquer resin. Each skin comprises a glass fibre mat also impregnated with phenolic resin. The skins are secured to the core using an epoxy based film adhesive. When the resin is cured, a piece of such composite material comprises a lightweight rigid panel that is particularly suitable for use in aircraft cabin interiors. 
     However, this material suffers from the disadvantage that, once the resin has cured, the resulting panel is permanently rigid. If it is desired to form radiuses or rebates in the material, it is necessary to make cuts in the skin on the inside of such a radius in order to allow the material to be bent. After the required radius has been formed, it is necessary for the cuts in the skin to be “glassed over” with phenolic resin. 
     Another disadvantage is that the skins tend to delaminate in use, particularly when adhesive tape is used to secure carpet to such material. When the tape is pulled off, the surface of the skin is pulled away, exposing some of the glass fibres. Subsequent application of adhesive tape to the damaged area results in such tape adhering directly to the exposed glass fibres. On the next occasion time when the carpet is removed, such exposed glass fibres tend to pull away with the tape, thus causing further delamination. 
     SUMMARY OF THE INVENTION 
     It is one object of the invention to provide a composite material that is not subject to these disadvantages. 
     According to one aspect of the invention, in a composite material of the type described above, both the core and the skins are formed of thermoplastic materials and the skins are attached to the core by a thermoplastic adhesive having a fusion temperature less than that of both the core and the skins. 
     The effect of this is that if the material is heated to above the fusion temperature of the adhesive and then bent to form a radius therein, the skins move relative to the core rather than the skin on the outside of the radius being stretched and the skin on the inside of the radius being compressed. When the material has cooled, the two skins form continuous smooth surfaces round the radius and no subsequent glassing is necessary. 
     According to another aspect of the invention, in a composite material of the type described above, both the core and the skins are formed of thermoplastic materials and the material is compressed to a thickness less than the sum of the thicknesses of the core and the skins while at a temperature higher than the softening temperature of the core. 
     This compression causes the edges of the cells of the core to distort so as to increase their contact area with the skins and thus increase the strength of the bond therebetween. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an edge view of a panel formed from composite material in accordance with a first embodiment of the invention; 
         FIG. 2  is a plan view of the panel shown in  FIG. 1  with part of the upper skin removed; 
         FIG. 3  is a scrap view on an enlarged scale of part of the panel shown inn  FIG. 2 ; 
         FIG. 4  is a side view of a press for forming a panel of composite material in accordance with a second embodiment of the invention, the press being in its open position; 
         FIG. 5  is a side view of the press shown in  FIG. 4 , in its closed position; and 
         FIG. 6  is a scrap view, similar to  FIG. 3 , of a panel formed on the press shown in  FIGS. 4 and 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1 to 3  show a panel  10  formed from a honeycomb core  12  having the ends of the honeycomb cells covered by respective skins  14  and  16  which are secured to the core  12  by respective layers  18  and  20  of thermoplastic adhesive. 
     The core  12  is formed from extruded tubes, of circular cross-section. The material of the core  12  is polyetherimid, which may, for example comprise Trauma-Lite Honeycombs Type PEI 3.5-70 supplied by Trauma-Lite Limited, PO Box 456, Manchester M62 LL, United Kingdom. In this material, the diameter of the tubes is 3.5 mm and the material density is 70 kilos per cubic meter. This material can be thermoformed at temperatures up to 170° C. 
     The skins  14  and  16  may comprise Cetex GI0303 reinforced thermoplastic laminate, supplied by Ten Cate Advanced Composites, Campbellweg 30, 7443 PV Nijverdal, Netherlands. This material is in accordance with Boeing Aircraft Corporation&#39;s Material Specification BMS8-353 and can be thermoformed at temperatures in the range 150° C. to 300° C. 
     The thermoplastic adhesive may be a polyester based thermoplastic web such as Sharnet Web Sh4275 supplied by Bostic Findley Limited, Alderscote Road, Leicester LE4 68W, England. This has a fusion temperature of 130° C. 
     Consequently, the preferred temperature for bending and shaping composite material made from these materials is 150° C. 
     When it is desired to bend the material, an entire panel is heated (for example in an oven) heated to a temperature between the fusion temperature of the adhesive and the fusion temperatures of the core and the skins. The panel is then bent to form a radius therein. The skins move relative to the core so that the skin on the outside of the radius no longer reaches the edge of the core while the skin on the inside of the radius projects beyond the edge of the core. When the material has cooled, the two skins form continuous smooth surfaces round the radius and no subsequent glassing is necessary. Finally, the edges of the panel are cut away to the required peripheral shape, thereby removing surplus skin and those parts of the core that are no longer covered by skin on both sides. 
     If a component formed from a composite material in accordance with the invention is damaged, the damaged material may be cut away and replaced by fresh material which is bonded into place with the use of a hot iron. This type of repair can be performed within an aircraft cabin, avoiding the need to remove the relevant equipment from the aircraft. 
       FIG. 4  shows a press having a lower platen  20  carrying spacers  22  and  24 , and an upper platen  26  carrying spacers  28  and  30 . The lower spacers  22  and  24  are aligned with respective upper spacer  28  and  30 . 
     In order to form a panel of composite material, a skin  32  of reinforced thermoplastic laminate, such as Cetex GI0303, is placed on the lower platen  20  and a layer of thermoplastic adhesive such as Sharnet Web Sh4275 (not shown) is place on top of the skin  32 . Next a core  34  formed of polyetherimid circular cross-section tubes, for example Trauma-Lite Honeycombs Type PEI 3.5-70 is placed on the layer of adhesive. A second layer of adhesive (not shown) is placed on top of the core  34  and finally a second skin  36  of reinforced thermoplastic laminate is placed on the second layer of adhesive. The overall thickness of the core and the two skins is greater than the combined height of each pair of spacers  22 ,  28  and  24 ,  30 . 
     The platens  20  and  26  are then heated to a temperature above the fusion temperature of the adhesive layers (150° C. for the materials described above) and the press is closed to the position shown in  FIG. 5 . The force exerted by the platens  20  and  26  causes each end of the tubes forming the core  34  to be distorted so as to bridge the interstices between adjacent tubes and increase the surface area available to make contact with the adjacent skin, as shown in  FIG. 6 . This produces a stronger bond between the core and its skins than is obtained with a panel of the type shown in  FIGS. 1 to 3 .