This invention relates in general to structural panels and, more specifically, to a metal-surfaced polyimide foam structural panel in which the metal layer is diffused into interstices or discontinuities in the foam surface.
Metal surface layers have been applied to a very large variety of substrates. Typically, metals have been applied by electroless plating, electroforming, flame spraying, sputtering, plasma deposition and by simply bonding a metal film or foil to a substrate with an adhesive. While these techniques have generally produced acceptable results for many applications, when a composite panel produced by these methods is used in structural applications, problems of peeling of the metal layer from the substrate often occurs. Also, the metal is present only as a planar, parallel-surfaced, film and adds little to the structural strength of the panel. Further, these techniques have been less successful where the substrate is a foam material. Bonding to foam surfaces hav been found to be especially difficult, with low peel strength often resulting. Also, many of these application methods use temperatures or other conditions which tend to damage many insulating foam materials.
Attempts have been made to apply a metal surface layer to solid plastics or other substrates by chemical vapor deposition, such as is described by Popley in U.S. Pat. No. 3,519,473. Unfortunately, the coatings produced by these teachings do not have the desired adhesive strength since it is dependent upon simple adhesion. Ideally, where the composite structure is subject to stress causing catastrophic failure, the structure should fail by destruction of the substrate rather than failure along the metal-to-substrate bond line.
Complex techniques, such as the multi-layer, multi-metal, method described by Corwin in U.S. Pat. No. 3,537,881, have been used in an attempt to improve bonding. While some improvement results, peel strength is still not as great as is desirable. Attempts have been made to inter-lock sequential layers together, such as by using a layer of glass fibers extending both into a substrate and into a plastic surface coating, as detailed by El Bouhnini et al. in U.S. Pat. No. 4,242,406. This requires a complex and cumbersome mold and method in which the substrate and coating are produced at substantially the same time. This is not applicable to forming a metal coating or layer on a pre-existing substrate which may be highly contoured from previous shaping or re-shaping operations.
Thus, there is a continuing need for improved structural insulating panels having high insulating properties with a well-bonded metal surface layer and for methods of making such panels.