PROCESS FOR THE PREPARATION OF THERMOPLASTIC HONEYCOMB SANDWICH STRUCTURES

Abstract: Rapid production of honeycomb sandwich structures containing fiber-reinforced or non-reinforced thermoplastic honeycomb (1) adhered to skins or facing sheets (2) is accomplished by fusion bonding the skins or facing sheets (2) to the core (1) by means of thermoplastic film(s) (3) heated to the melt temperature (4) and applying sufficient pressure (5) fusion bond the skins or facing sheets to the core, followed, when necessary, by cooling the assembly (6) to prevent overheating of the core and/or skins or facing sheets.

Description: PROCESS FOR THE PREPARATION OF THERMOPLASTIC HONEYCOMB SANDWICH STRUCTURES Background of the Invention Field of the Invention The subject invention pertains to the preparation of honeycomb sandwich structures wherein facing sheets or skins are applied to a fiber-reinforced or non-reinforced thermoplastic honeycomb core in a rapid and cost-effective manner. Description of the Related Art Honeycomb panels are used in the transportation and aerospace industries where a stiff planar or modestly curvilinear structure which is also exceptionally light is needed. In the past, most honeycomb was prepared by impregnating fiber reinforcement with a thermosetting resin and laying up the honeycomb by hand. One cubic foot of such honeycomb often took greater than 24 hours to prepare and cure. Recently, thermoplastic honeycomb, both fiber-reinforced and non-reinforced have also been utilized, however heretofore, its production was also time and labor intensive. In copending application Serial No. 07/531,184, a continuous method for the production of thermoplastic honeycomb is disclosed which should enable increased utilization of these structural components. Following production of honeycomb by whatever method, the finished core is sawn or slit into planar segments of the required thickness and a heat-curable thermosetting film adhesive, for example a bismaleimide or epoxy resin is used to bond the skin, generally a fiber reinforced prepreg or laminate, to the core. The core assembly, with facing sheets and film adhesives, is heated while pressure is applied to the assembly. Following a curing time which is generally several hours, the honeycomb sandwich panel may be cut into appropriate size for its intended application. The processes of bonding the honeycomb to the facing sheets as practiced by prior art processes are time consuming and utilize relatively expensive film adhesives whose adhesion, viscosity, and cure cycles must often be tailored for each specific application. Objects of the Invention It has now been unexpectedly discovered that thermoplastic honeycomb core may be bonded to facing sheets by the use of a thermoplastic film which may be previously adhered to the facing sheets or is preferably supplied as a separate element. By this process, relatively inexpensive and readily available thermoplastic films may be used and cycle time for bonding of the face sheets reduced from several hours to only seconds. Brief Description of the Drawings Figure 1 - A view from the side of a honeycomb core being bonded to facing sheets having a thermoplastic bonding film attached thereto. Figure 2 - A view from the side of a honeycomb core being bonded to facing sheets utilizing a separate thermoplastic bonding film. Figure 3 - Side view of the process employing radiant heat bank and platens. Figure 4 - Top view of the process illustrated by Figure 3. Description of the Preferred Embodiments The thermoplastic honeycomb useful in the process of the subject invention may be virtually any type, including those prepared from resin systems containing a minor amount of thermosetting resins, i.e. not greater than 30 weight percent. Among the thermoplastics useful, and not by limitation, are the polyethylenes, polypropylenes, polyoxy ethylenes, polyarylene ethers, polyetherketones, polysulfones, polyethersulfones, polyamides, polyara ids, polybenzimidazoles, polyimides, thermoplastic polyurethanes, and the like. The minor portion of thermosetting resin may be, for example, bismaleimide resins, epoxy resins, cyanate resins, isocyanate resins, heat curable polyimides, and the like. The thermoplastic honeycomb may be reinforced with fibrous reinforcement. The reinforcement may be in the form of spun bonded fibers, random continuous or chopped fibers which may be optionally needled and/or sized to provide integrity, woven fabric, unidirectional tape, or combinations of these. Among the fibers useful are inorganic fibers such as glass, quartz, carbon (carbon/graphite) , boron nitride, silicon carbide, ceramics, and the like; or organic synthetic fibers such as those prepared from the thermoplastics mentioned earlier. The facing sheets may be of metal, fiber reinforced thermosetting resins or thermoplastics, or non-reinforced thermoplastics. The facing sheets and honeycomb core must be capable of adhering to the thermoplastic bonding film. Preferably, the thermoplastic utilized in the honeycomb, facing sheets, and bonding film are the same. The thermoplastic film must be compatible with the materials to be joined. Preferably, as previously stated, the thermoplastic film is the same thermoplastic as the honeycomb material. Compatibility may be assessed by determining the force necessary to remove the skin from the honeycomb. The thickness of the film may vary widely, but is preferably between about 0.05 mm and 1.0 mm, more ' preferably between 0.10 mm and 0.60 mm. The film may be first fusion bonded to the facing sheets or, if incompatible with the facing sheets, adhesively bonded thereto. It is further within the spirit of the invention to utilize instead of a separate film which is fusion or adhesively bonded to the skins, a skin which contains sufficient thermoplastic to be able to fuse to the honeycomb. Such skins may also be viewed as an integral film/skin unit. While the purpose of the subject invention is to avoid the use of thermosetting resins in their entirety when bonding skin to core, it is yet within the spirit of the invention to utilize thermoplastic bonding films containing limited amounts of thermosetting resins, i.e. less then about 10 percent by weight. Such films would be expected to behave like all-thermoplastic materials. In the process of the invention, the bonding film, whether or not previously attached to the facing sheets, or skins as they are often called, are heated to their fusion temperature by a heating means, for example by infrared radiation or hot gas jets; placed over the honeycomb; and pressed down thereby bonding the various components. The latent heat of fusion of the thermoplastic is sufficient to cause the thermoplastic in the honeycomb to melt and fuse to the bonding film and face sheets. Often, the latent heat of fusion is high enough that cooling of the assembly by a cool air blast or water or other liquid is required. Instead of the thermoplastic film being heated to its melt temperature, a film may be melt extruded at the point of bonding. In Figure 1, the honeycomb (1) is faced with skins (2) by means of a bonding thermoplastic film (3) which has been previously melt fused or adhesively bonded to the skins (2) . Infrared radiation from source (4) is used to heat the film (3) to its melt temperature. Bonding is effectuated by pressure rollers (5) and Optionally cooled by water jets (6) . Instead of the pressurizing rollers, the assembly may be pressurized utilizing a standard platen press. The surface of the platens should generally be cooled so as to remove heat thugs preventing overheating of the core and blooming of the skins. Figure 2 illustrates the preferred method of bonding skin to honeycomb. The numbering is as in Figure 1 except in this case, the film is a separate thermoplastic film not previously bonded to the skin. Figure 3 is a side view of one embodiment of the process. The honeycomb core (1) is guided by guide rails (2) into the area between pressure platens (9, 10) . The upper skin (3) and lower skin (4) are bonded to the honeycomb (1) by means of upper film layer (5) and lower film layer (6) respectively. The film layers are heated by infra-red heating banks (7, 8) and the platens closed and pressure maintained till cool. Cooling may be assisted by water spray or air impingement onto or into the platens (not shown) . Figure 4 is a view of the process of Figure 3 from the top. Honeycomb core (1) travels along its process guide rails (2) inbetween upper and lower platens (9, 10 (only top platen shown)). Upper and lower adhesive films (5, 6) are heated by upper and lower infra-red heating banks (7, 8 (only upper bank shown)) and shuttled along their guide rails (2) into the platen press area between the honeycomb (1) and upper and lower skins (3, 4) which are also shuttled along their guide rails into the platen area. The press is closed and pressure maintained until below the fusion temperature of the thermoplastic adhesive. The final faced honeycomb product exits at (11) and the process repeated. Example 1 A 19mm thick core measuring 30 cm by 30 cm and made of Tyvel^ spun bonded polyolefin was bonded to two 1.8 mm skins of glass fiber polypropylene impregnated mat supplied by Azdel, Inc. The bonding film was a 0.50 mm polypropylene film heated to its melting point by a ceramic infrared radiant heating element. Total time to assemble the honeycomb sandwich was 15 seconds of which 5 seconds was utilized to heat the polypropylene film, 5 seconds to pressure bond the assembly, pressure was applied by both platens and pressure rollers. Minimal pressure was required for bonding and pressure was maintained until cool and 5 seconds to cool by spraying the sandwich with a water spray. Example 2 The process of Example 1 was repeated but with a core made of glass fiber impregnated with polyethersulfone and facing sheets or skins of 0.38 mm polyethersulfone impregnated glass fiber. The bonding film utilized was a 0.13 mm film of polyetherimide thermoplastic (ULTEI ) . Total bonding time was 15 seconds as before. Example 3 Example 2 was repeated but heat was supplied exterior to the face sheets by a platen heated to 350C over a cool core. Immediately following bonding, the assembly was cooled by a water spray under pressure. Total time to produce the finished sandwich was approximately 30 seconds.

Claims: C L A I M S 1. A process for the preparation of honeycomb sandwich materials by adhering one or more skins to a thermoplastic honeycomb core without the use of thermosetting adhesives, comprising: (a) positioning a thermoplastic honeycomb adjacent a skin to be bonded thereto but spaced apart therefrom; (b) positioning a thermoplastic film between the honeycomb and the skin to be joined thereto; (c) heating said thermoplastic film to its melt temperature; (d) applying pressure in an amount sufficient to cause said honeycomb and said skin to be fusion bonded together. 2. The process of claim 1 wherein said thermoplastic film of step (b) is integral with the skin. 3. The process of claim 1 or 2, further comprising: (e) cooling the honeycomb/skin product thereby obtained by a stream of air or liquid. 4. The process of claim 1, 2 or 3, wherein the heating of the thermoplastic film (c) is accomplished by means of radiant heat.

Inventors: FELL BARRY MICHAEL, FELL, BARRY, MICHAEL
Applicants: BASF CORP, BASF CORPORATION
Application year: 1991
Main classification: B32B31
IPC codes: B29C35, B29C65, B32B3, B32B37