MULTI-DIMENSIONAL TAILORED LAMINATE

Abstract: A fibrous core material (24) is disposed between a portion of two opposed metal skins (22) and bonded thereto to form a structural laminate (20) having comparable strength to steel sheets of greater weight and having a profile which varies compositionally in different regions of the laminate.

Description: MULTI-DIMENSIONAL TAILORED LAMINATE TECHNICAL FIELD The present invention relates to laminate structural panels and, more specifically, to light-weight laminates having desirable structural characteristics. BACKGROUND OF THE INVENTION Vehicle bodies commonly include metal panels which are selected for properties such as strength and stiffness, formability, corrosion resistance, weldability, impact resistance, and the like, according to the intended location of the panel on a vehicle. Where the application requires a mixture of properties, the selected metal panel may be a specialized alloy, have had custom heat treatments and may have to be joined, for example by welding or with fasteners to other metals in order to provide the desired properties in selected zones. More than any other material, steel is used extensively in motor vehicles to form panels and other structures. It will be appreciated by those skilled in the art that the required structural characteristics, such as stiffness, vary depending upon the specific application. When higher stiffness values are required for steel, the thickness is typically increased. Increasing steel thickness, however, produces a panel or part which is not only heavier, but is also more expensive. In United States Patent 5,985,457, STRUCTURAL PANEL WITH KRAFT PAPER CORE BETWEEN METAL SKINS, the entire disclosure of which is incorporated herein by reference, the inventor of the present invention discloses structural panels which are laminate structures having metals skins separated by and bonded to an intervening fibrous core. The laminates described therein have very high specific stiffness and are comparatively light weight. As described therein, however, the placement of the fibrous core is continuous with the metal skins. In some applications, the presence of the core may interfere with fasteners, welds or clearance openings and their methods of formation. It would be desirable to provide a light weight structural laminate which could be customized to accommodate specific fastening, welding and clearance opening operations. The present invention meets these and other desirable goals. SUMMARY OF THE INVENTION In one aspect, a structural laminate is provided having a profile which varies compositionally along the laminate body. The laminate has first and second skins of sheet metal. A fibrous paper core layer is provided between the sheet metal skins and is bonded to the skins. In one aspect, the fibrous paper core layer is impregnated with an adhesive resin which bonds the core layer directly to the skins. In another aspect layers of adhesive are placed between the core material and the skins to bond the core to the skins. Along the edge of the laminates there is a gap or space between the two metal skins; that is, the paper core does not extend completely to the edges of the steel skins in at least some spots. The core is non-uniform in cross- section, having a profile which is "tailored" for a desired application. For example, this non-uniform profile may allow fasteners or fixtures to pass through the laminate or facilitate the placement of clearance holes. The resulting tailored laminate structure is ext remely lightweight compared to a single steel sheet of comparable thickness and strength. In still another aspect of the invention, a method of forming a tailored structural laminate is provided. The method includes the steps of placing a fibrous core of paper material between two metal skins, the fibrous core covering less than the entire adjacent skin surfaces, i.e., along the edge of the laminates there is a gap or space between the two metal skins as described in the preceding paragraph. Pressure is applied to the tailored laminate to promote bonding between the core and the metal skins. In one aspect a number of laminates are prepared which are then stacked one upon another and are then pressed to simultaneously bond the individual layers. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-section of the laminate of the present invention. Figure 2 is a plan view of the laminate of Figure 1. Figure 3 is the laminate of the invention in another configuration. Figure 4 is a diagram depicting a die press forming the laminate of the invention. Figure 5 illustrates a door panel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Referring now to Figure 1 of the drawings, non-uniform or tailored laminate panel 20 is shown having metal skins 22 and intervening region 23 in which fibrous core 24 is seen. As best shown in Figure 2, fibrous core 24, in this embodiment, is recessed from each edge 25 of panel 20. Thus an unfilled gap or space 27 exists between portions of opposed metal skins 22. In this embodiment, the space is in the shape of a channel extending completely around the perimeter of panel 20. In plan view, the area oi space 27 may vary widely, but will typically be at least 1 square inch. It will be seen that the composition of panel 20 varies along axis 21 as indicated by arrows A and B. In other words, along the path of arrow A panel 20 is metal-space-metal; at arrow B panel 20 is metal-paper-metal. It is to be understood that the placement of gap 27 in panel 20 will vary depending on the intended application for the laminate panel or part; gap 27 may occur at one or more edges of panel 20, may be located solely in the interior regions of panel 20 (i.e., as a hole cut in the center of core 24 or may have an irregular shape to accommodate a specific application. Core 24 may be discontinous, for example forming separate strips or the like. One material for use in forming fibrous core 24 is fully described in the aforementioned United States Patent No. 5,985,457, the entire disclosure of which is incorporated herein by reference. The individual layers shown in Figure 1 will now be described in more detail. As stated, metal skins 22 generally will be flat having planar surfaces on each side. Metals which may be utilized to form skins 22 are preferably selected from the group consisting of steel, aluminum, copper alloys and various combinations thereof. Metals which provide adequate structural and (if required) corrosion resistance properties in the specific environment in which panel 20 is used, at the lowest cost, are most desirable. In one preferred construction, skins 22 are most preferably formed of zinc coated steel, with each layer 22 having a thickness of about .005 inch or greater, preferably from between about .005 inch and about .030 inch and more preferably from about .005 inch to about 0.012 inch in thickness. In one embodiment interveningfibrous layer 24 preferably has a thickness of from about .01 inch and greater and preferably from about .01 inch to about .05 inch. Thus, the overall thickness of panel 20 in one embodiment will typically be between about 0.020 inch and about 0.1 10 inch. A panel having the dimensions set forth above in the description of Figure 1 of the drawings and having the preferred layer thicknesses just described will typically have weight that is about 40 - 70% of the weight of a single sheet of steel of comparable dimensions and stiffness. As will be appreciated by those ski I led in the art, steel comes in a number of grades based on the amount of carbon and other elements which it contains. Broadly these grades can be described as low carbon steel, medium steel and high carbon steel. Preferred for use herein are low carbon steel and low carbon micro-alloyed high- strength steel (HSLA). The most preferred metal skins for use in the present invention are cold rolled steel, galvanized steel, tin coated steel, and stainless steel. It may be desirable to utilize single sided galvanized sheet with a galvanized surface comprising the outer surface of skins 22 with the inner surfaces of the skins being bare metal for adhesion purposes. In one embodiment differential zinc coating is preferred, i.e. a light zinc coating on the inside surface and a heavier zinc coating on the outer surface. In one embodiment, zinc coated steel is cold rolled with zinc on the surface to the final thickness. As stated, layer 24 is a fibrous material. As used herein, the term "fibrous," without limiting its scope in the context of the invention, is intended to mean a generally homogeneous collection of fibers, either natural or synthetic, which can be formed into a sheet product. The most preferred fibrous material for use herein as layer 24, a material which is believed to be unique among fibrous sheets, is paper. As will be appreciated by those skilled in the art, paper is essentially a matted or felted structure of fibrous material formed into a relatively thin sheet through the medium of a dilute suspension of pulp and water. It is composed essentially of cellulose fibers. Pulp for paper making can be prepared by grinding wood or other plant matter mechanically, by chemical processing (sulfite, kraft, or soda) and also by chemically treating cotton, linen and hemp rags, waste, straw, and the like. In the present invention, paper formed using the kraft process is most preferred. It will be appreciated by those skilled in the art that the kraft process (which may also be referred to as sulphate pulping or alkaline process) results in papers of high physical strength and bulk. One preferred paper is sold as saturating kraft paper, by Westvaco, of Charleston, S.C. Also, as will be appreciated by those skilled in the art, the average alignment of cellulose fibers in paper is controlled somewhat by the "machine direction" during production of the paper. It is believed that in the present invention the orientation of the paper in the laminates is a factor which may affect the stiffness and strength of the laminate. Most preferred are laminates where the machine direction of the kraft paper is a line parallel to a neutral axis of bending of the laminate. Another class of fibrous materials for use herein is plastic-fiber paper. ln one preferred embodiment of the invention, layer 24 is provided as a resin-impregnated fibrous material. Where layer 24 is kraft paper, the paper is saturated with a resin which is then dried. Most preferred for use herein is phenolic resin-impregnated kraft paper. Polyester resin impregnation may also be suitable in some applications. Methods of impregnating paper with resin will be well-known to those skilled in the art. In essence, the preferred resin-impregnated paper is formed by immersing a substrate paper web in liquid phenolic resin. Typically, layers of saturated impregnated paper are layered together to form a single layer of semi-cured impregnated paper. Although it may be suitable or desirable in specific applications to go beyond the ranges set forth hereinafter with respect to the resin content layer 24, in a preferred embodiment of the present invention resin constitutes from about 15% to about 45% by weight of resin-impregnated layer 24. In most instances thermosetting resins are preferred for use in impregnating paper layer 24, although in some applications thermoplastic resins may be acceptable. In the case of thermosetting resins, as stated the resin will generally be cured to B-stage prior to forming panel 20, but it may be possible to fully cure the impregnated paper prior to the laminate pressing operation (controlled heat and pressure) described below. In the case of a phenolic resin, the resin is cured to B-stage prior to lamination. It is then fully cured as skins 22 and impregnated paper core 24 are laminated together using the press. It may be suitable in some applications to include a number of standard additives in the resin such as curing agents, fillers and the like. With_or without resin impregnation of fibrous layer 24, it may be desirable or required to use a layer of adhesive to bond skins 22 to fibrous core 24. A number of adhesives may be suitable in specific applications, including epoxies, phenolics, isocyanates, polyurethanes, and hot-melts. A particularly preferred adhesive for this purpose is a nitrile phenolic sold as "Arofene 1 166" from Ashland Chemical. The adhesive may be applied directly to layer 24 or to metal skins 22 or both by any number of methods. It is preferred to pretreat the steel with a conversion coating such as a complex oxide or zinc phosphate to improve bond integrity and corrosion resistance. In another embodiment, as shown in Figure 3 edge strips 28 are provided which are of a different material than that of fibrous core 24. Various materials may be used for edge strips 28, such as a thermoplastic or an adhesive. Strips 28 may constitute a single strip aroundthe outer perimeter of panel 20. In another embodiment, fibrous layer 24 may have a plurality of holes (not shown) extending therethrough; these holes may provide adhesive "bridges" as more fully described in the aforereferenced U.S. Patent No. 5,985,457. Referring now to Figure 4 of the drawings, one method of assembling skins 22 and layer 24 is shown using press 30. Press 30 includes platens 32 which move towards one another in the customary manner using hydraulics or the like. Platens 32 are preferably heatable so both heat and pressure can be applied to the laminates to cure the resin and bond the adhesive as more fully described in copending United States Patent Application Serial No. 09/373,298 filed August 12, 1999, entitled, "IMPROVED STRUCTURAL PANEL AND METHOD OF MANUFACTURE", the entire disclosure of which is incorporated herein by reference. Several metal/fiber/metal laminates may be stacked and pressed as described in the immediately preceding U.S. Patent Application Serial No. 373,298. After the pressed stack has cooled, it is removed from the press, individual panels are then separated. In some applications, the laminate panels of the present invention may substitute for a metal section which is not a flat sheet. Therefore, as described in the immediately preceding U.S. Patent Application Serial No. 373,298, parts having non-planar geometries my benefit from the tailored laminate approach of the present invention. Such a multi-dimensional laminate 20 may find application for use as a door panel 34 shown in Figure 4. The body of the door panel 34 designated by numeral 36 may comprise a stiff, lightweight composite material comprising outer metal skins made of steel and an inner core made of paper of the kind described herein. The door panel 34 has forward and rear edge portions designated 38, 40 which comprise outer skins of steel and an inner core made of high strength steel suitable for welding. Adjacent to an opening 42 formed in the door panel 34 to create a window, there is an area designated by reference numeral 44 which will be deformed to create a recess associated with a door handle (not shown) and comprises outer skins of steel and an inner core made of thermoplastic material which may sustain strains of up to 30%. While particular embodiments of this invention are shown and described herein, it will be understood, of course, that the invention is not to be limited thereto since many modifications may be made, particularly by those skilled in this art, in light of this disclosure. !t is contemplated, therefore, by the appended claims, to covetiany such modifications as fall within the true spirit and scope of this invention.

Claims: CLAIMS I claim: 1 . A structural laminate comprising: first and second skins of sheet metal; a fibrous core disposed between said skins of sheet metal, said fibrous core being bonded to said skins of sheet metal; wherein said fibrous core is less than continuous with said skins such that a gap is formed between a portion of said metal skins. 2. The structural laminate recited in claim 1 , wherein said gap has an area in contact with said skins of at least 1 square inch. 3. The structural laminate recited in claim 1 , wherein each of said skins has a thickness of at least about 0.005 in. 4. The structural laminate recited in claim 1, wherein said gap forms a channel along the entire length of at least one edge of said laminate. 5. The structural laminate recited in claim 1 , wherein said gap forms a channel along the entire perimeter of said laminate. 6. The structural laminate recited in claim 1 , wherein said gap is filled with a material which is different than that forms said fibrous core. 7. The structural laminate recited in clajm 1 , wherein said sheet metal is selected from the group consisting of cold rolled steel, galvanized steel, tin-coated steel and stainless steel. 8. The structural laminate recited in claim 1 , wherein said fibrous core is adhesively bonded to said skins of sheet metal. 9. The structural laminate recited in claim 1 , wherein each of said skins has a thickness of from about 0.005 in. to about 0.030 in. 10. The structural laminate recited in claim 1 , wherein said fibrous core is impregnated with a resin. 1 1. The structural laminate recited in claim 1 , wherein said fibrous core has a thickness of at least about 0.01 in. 12. The structural laminate recited in claim 1 , wherein said fibrous core has a thickness of from about 0.01 in. and 0.05 in. 13. The structural laminate recited in claim 1 , wherein said laminate is a structural panel. 14. The structural laminate recited in claim 1 , further including layers of adhesive disposed between said fibrous core and each of said skins. 15. The structural laminate recited in claim 1 , wherein said fibrous core is paper. 16. The structural laminate recited in claim 1 , wherein said sheet metal skins are zinc coated steel which has been cold rolled with zinc on the surface. 17. The structural laminate recited in claim 1 , wherein said fibrous core is a plurality of webs of fiber adhesively bonded to each other. 18. The structural laminate recited in claim 1 , wherein said laminate is non-planar. 19. The structural laminate recited in claim 1 , wherein said metal skins are steel which has been pretreated with a conversion coating to promote bond integrity and corrosion resistance. 20. The structural laminate recited in claim 1 , wherein said metal skins are formed of low carbon micro-alloyed high-strength steel. 21. The structural laminate recited in claim 1 , wherein said material in said gap is a thermoplastic.

Inventors: CLIFFORD DAVID D ARCY, CLIFFORD, DAVID, D'ARCY
Applicants: DOFASCO INC, DOFASCO INC.
Application year: 2001
Main classification: B32B15
IPC codes: B32B3, B32B15, B60J5