A METHOD OF MAKING A SHAPED ARTICLE FROM A SANDWICH CONSTRUCTION

Abstract: This invention relates to a method of making a shaped article from at least one integrated multi­ layered sheet having excellent mechanical properties and being light in weight, comprising the steps of - providing a thermoplastic synthetic material reinforced by a fiber mat on at least one side of a substantially flat sheet of thermoplastic synthetic foam, said fiber mat, thermoplastic synthetic material and sheet of synthetic foam being integrated under the influence of elevated temperature and increased pressure, - providing deformability by entirely or locally heating the resulting integrated fiber reinforced sheet, - giving the heated sheet the desired structure and shape, and - fixing the article by cooling. and to such a process starting from a sheet of a thermo­ plastic foam having attached thereto at least one fiber mat impregnated with a thermoplastic synthetic material, comprising the steps of - providing deformability by entirely or locally heating the resulting integrated fiber reinforced sheet, - giving the heated sheet the desired structure and shape, and fixing the article by cooling.

Description: This invention relates to a method of making a shaped article from one or more integrated multi­ layered sheets. Especially for use in transport means, such as aircraft, motor-cars and trains, a search has long been made for materials which both are light in weight and possess high stiffness, impact resistance and strength. It has already been proposed to make sandwich constructions consisting of a completely or partly hollow core having provided thereon a fibrous synthetic layer. Such sandwich constructions possess clear advantages with regard to mechanical properties and weight when compared with massive materials. A commercially sold product consists of, e.g., a honeycomb structure having provided on its surface a thin fibrous layer. Although the prior art sandwich constructions possess clear advantages from a viewpoint of mechanical properties, it has been found rather difficult in practice to shape these constructions. Especially in those situa­ tions in which there is much variation in shape or in which the shape is very difficult, the problem occurs that either very difficult and time-consuming operations are required or the desired form cannot be made. It is proposed in Canadian patent 1,184,106 to use a sandwich material consisting of a core of a foam material having provided on its two surfaces a layer consisting of a fibrous structure impregnated with a mixture of epoxide and hardener being in the B-stage. This material can be shaped in a mould or press and hardened to obtain a product having the desired form. With this type of materials the problem occurs that the deformation can take place only once and is not reversible. Consequently, it is not possible to accommodate a standard panel or another article afterwards to specific forms, requirements or situations. The object underlying the present invention is to provide a method of making a shaped article from one or more integrated multi-layered sheets, which method does not have the drawbacks as described above. This invention is therefore characterized by a method of making a shaped article from at least one integrated multi-layered sheet, which method comprises the steps of - providing a thermoplastic synthetic material reinforced by a fiber mat on at least one side of a substantially flat sheet of thermoplastic synthetic foam, said fiber mat, thermoplastic synthetic material and sheet of synthetic foam being integrated under the influence of elevated temperature and increased pressure, - providing deformability by entirely or locally heating the resulting integrated fiber reinforced sheet, - giving the heated sheet the desired structure and shape, and - fixing the article by cooling. Another embodiment of the invention comprises a method of making a shaped article from a sandwich construction consisting at least of an integrated multi-­ layered sheet formed by a sheet of a thermoplastic foam having attached thereon at least one fiber mat impregnated with a thermoplastic synthetic material, comprising the steps of - providing deformability by entirely or locally heating the resulting intergrated fiber reinforced sheet, - giving the heated sheet the desired structure and shape, and - fixing the article by cooling. Surprisingly, it has been found that the invention as defined by the method described above does not have the drawbacks of the prior art. A suitable selection of the materials leads to a product having excellent mechanical properties and being very light in weight. An important advantage of the product according to the inventin is that this product is chemically stable unlike the prior art materials on the basis of thermosetting synthetics. This means that the product is not susceptible to ageing so that the mechanical properties will not decline in the course of time. The sandwich construction obtained using the method according to the invention can be further shaped after manufacture by repeating the deforming operations according to the invention, e.g., for the purpose of accommodation to specific situations departing from the standard, but also for providing places of attachment without the occurrence of mechanical weakening. In the products obtained according to the invention an additional layer of fiber reinforced synthetic material can in fact be provided without connecting means on the surface which will form one integrated whole therewith so that an integrated local reinforcement is obtained, e.g., around a point of attachment. It is observed that the terms "structure and shape", as used herein, can relate not only to the physical change of the form of the sheet but also to, e.g., adaptations of the sheet in order to meet specific requirements, such as local reinforcement, which do not immediately give rise to a change of the form. According to a preferred embodiment of the method according to the invention the operations can be performed at a sheet consisting of a thermoplastic synthetic foam provided on one side with a layer of fiber reinforced thermoplastic synthetic material. In a simple mould this so-called half sandwich can then be shaped to the desired final form. Optionally by using varying accessories it is possible in the same mould to make a mirror-inverted half-laminate, and to bond the two separately formed sheets together via the uncoated surfaces of the thermoplastic synthetic foam. If desired, an optionally fibrous core of thermo­ plastic synthetic material, e.g., a carbon fiber-containing synthetic film, may be interposed between these two sheets. Bonding may also be effected by means of glue or adhesive tape, if so desired. This last adhesive step, too, may be performed in the same mould. It will be clear that such a method makes a great saving in the required equipment. Moreover, the time required for shaping is very short when compared with prior art methods. The method according to the present invention offers clear advantages over the use of thermosetting materials. In the first place, the product obtained according to the present invention has greater flexibility resulting in a lower risk of hair cracks. At high load the thermosetting material may come to show hair cracks, especially after ageing, resulting from the elongation in the reinforcing fibers, so that moisture and dirt can penetrate into the fibers or the core. Moreover, the quality of the material is properly reproducible owing to the fact that the semifinished product, the sandwich sheet, can be manufactured on an industrial scale so that also the final product is less dependent on the manual skill of the person manufacturing the product. This means that there is less spoilage. Besides, no expensive and complicated equipment is required for fixing the form. An important advantage of the present invention is that both the semifinished product and the final product are chemically stable, i.e. not or substantially not susceptible to ageing. In contrast thereto, the thermosetting materials have the drawback that a reaction continuously takes place resulting in that the product ages. This problem occurs not only in the final product but also in the semifinished product in which the epoxy/hard­ ener mixture is in the B-stage. The production and storage of this thermosetting semifinished product requires very close attendance, because after a rather short time already the hardening reaction has proceeded to the extent that the semifinished product is no longer useful. According to a preferred embodiment of the method of the invention it is very easy to make edge connections between two sheets. In fact, the sheets to be connected can be easily bonded together by applying heat. A suitable selection of temperature, pressure and shaping even results in a connection showing no appreciable thickening: when the sheets are superimposed in the longitudinal direction in partially overlapping condition and then pressed together by applying heat, there is obtained by melting the synthetic foam at the joint a connection having substantially the same thickness as the sheets themselves. The materials to be used for the present invention may be the well-known thermoplastic synthetic foams, in which respect imide foams, such as foam of polyether­ imide or polymethacrylimide,are preferred, especially from the viewpoint of stiffness, strength and temperature resistance. The thermoplastic synthetic material used for bonding the sheets together, and for the surface layers, is in general polyetherimide or polycarbonate. Subject to the desired temperature and chemical resistance, however, other synthetics may also be used. The fiber reinforcement optionally incorporated in the thermoplastic synthetic material may consist of glass fibers, carbon fibers, aramide fibers and the like. These fibers may be arbitrarily oriented, oriented in one direction or oriented in two or more directions. It is preferred that the fibers are oriented in two directions, using more in particular a fabric having longer fibers. These fibers preferably have a length corresponding to the length of the sheet because this will lead to the best mechanical properties. It is also possible to use composite rather short fibers having properties which, owing to the mode of processing, are comparable to "endless" long fibers. An important advantage of the shaped articles using the method according to the invention, especially when using fiber reinforcement in the form of fabrics, is that the sheets have excellent flame retardant proper­ ties. The method according to the invention can be carried out in a number of separate steps. The first step comprises providing a thermoplastic synthetic material reinforced by a fiber mat on the thermoplastic synthetic foam sheet. Preferably, a pre-impregnated fiber mat is started from, but it is also possible to start from a fiber mat and a synthetic material separated therefrom, the fiber mat being impregnated during manufacture. The sheet used is substantially flat, because it is most easy then to carry out the method. Bonding of the pre-impregnated fiber mat to the synthetic foam may be effected in various ways, but elevated temperature and increased pressure are always used. However, in order to obtain optimum bonding additional steps may be taken, depending on the types of thermoplastic synthetic material. In case of using, e.g., a methacrylimide synthetic foam in combination with a fiber mat impregnated with polyetherimide or polycarbonate, an additional non-fibrous film is preferably interposed between the foam and the fiber reinforced synthetic material; preferably in combination with rather shallow grooves in the synthetic foam. By applying heat and pressure there is obtained optimum bonding of the fibrous synthetic film to the foam. However, it is also possible to provide only shallow grooves on the foam or to use only an additional film. Also, one of these variants may be combined using a softener which, prior to bonding, is provided on one or both surfaces to be bonded together. Such a softener preferably consists of a solvent for the synthetics used, if required having dissolved therein an amount of the synthetic material used for the film. In this connection thoughts may be given to dichloromethane, etc. In case of using a synthetic foam core having the same chemical composition as the synthetic material of the fiber reinforced film, it is in general not necessary to promote bonding by taking special steps. It may be advantageous indeed to apply a slight amount of softener, but other steps seem superfluous in general. The temperatures used in the various operations are entirely dependent on the materials used. In this connection it is also important that both the foam and the fibers used substantially do not melt, unless this is exactly intended, e.g., for the foam: in general, while duly observing the above-mentioned limiting conditions a temperature ranging from 80 to 350°C will be maintained during operations requiring heat for providing deformabil­ ity by heating the thermoplastic synthetics used. Heating may be effected in a conventional manner, e.g., by means of radiation or hot air. In specific situations induction heating can be applied already. After shaping the article shaped is cooled to below the softening point of the lowest softening synthetic material. After optionally repeated heating, shaping and cooling, the article is generally cooled to room temperature, or the service temperature of the article. In the manner as described above there is obtained a semifinished product consisting of a synthetic foam sheet on one or both sides of which a fiber reinforced film is provided. This semifinished product can be directly shaped further in the manner according to the invention, but it is also possible to store this semifinished product and to shape it only in a much later stage. In this connection the invention also relates to the further processing of a semifinished product made in a previous stage in the manner described above. The second and following steps of the invention therefore always comprise the heating and shaping of said semifinished product in which it is not necessary then to obtain the final form in one shaping step. Espe­ cially in the manufacture of very complicated articles this means that the shaping operation can be divided into several separate steps, which has a favourable effect on the cost, while also the extent to which the final product satisfies the specifications can be greater. More in particular, it is also possible to correct small deviations later, if required. In this connection it is also important that larger series of identical products can be produced rather easily individual specimen of which are adapted to specific conditions later on the spot. In this connection thoughts may be given to, e.g., aircraft wall panels, of which one or two out of 100 panels must have a slightly different form or a number of additional recesses. These last different sheets can be easily shaped from the standard sheets. The invention will now be illustrated by means of a few examples but is not restricted thereto. Example 1 A sandwich constructions consisting of a 4.8 mm thick polymethacrylimide foam and two outer layers of aramide fabric impregnated with polyetherimide was made by providing surfaces of the foam with shallow grooves and applying thereto on both sides an aramide fabric impregnated with polyetherimide, by means of heat and pressure. The surface of the film were softened by means of methylene chloride having dissolved therein 5% by weight of polyetherimide. The thickness of the total construction was 5.0 mm, and the weight was 654 g/m². Of the tensile characteristics the tensile strength was 429 MPa, the E modulus was 19 GPa and the elongation at break 2.3%. The compression tests gave a compressive strength of 35.5 MPa and an E modulus of 5.5 GPa. The bending strength was 33 x 10⁴ N mm²/cm (ASTM C 393-62). The resulting laminate was heated by means of radiation to a temperature (measured at the surface of the fiber reinforced synthetic material) of 300°C, after which the sandwich construction was reduced to the desired form. After cooling to room temperature there was obtained an article having a fixed form and showing no ageing, with no substantial alteration of the mechanical properties. The resulting product could be made deformable by means of heating. Example 2 In the same manner as described in Example 1, a sandwich construction was made using a glass fiber web instead of an aramide fabric. The mechanical properties of an article formed from such a sandwich construction were as follows. weight: 754 g/m² E modulus top layer 23,000 N/mm² tensile strength 1,070 N/mm² bending strength 360,000 N/mm²/cm Example 3 Example 1 was repeated using a carbon fiber web instead of an aramide fabric. The results were as follows: weight 1082 g/m² E modulus top layer 74,000 N/mm² tensile strength 1000 N/mm² bending strength 870,000 N/mm²/cm

Claims: 1. A method of making a shaped article from at least one integrated multi-layered sheet, which method comprises the steps of - providing a thermoplastic synthetic material reinforced by a fiber mat on at least one side of a substantially flat sheet of thermoplastic synthetic foam, said fiber mat, thermoplastic synthetic material and sheet of synthetic foam being integrated under the influence of elevated temperature and increased pressure, - providing deformability by entirely or locally heating the resulting integrated fiber reinforced sheet, - giving the heated sheet the desired structure and shape, and - fixing the article by cooling. 2. A method according to claim 1, in which two separately formed sheet each provided on one side with a fiber mat are bonded together with the uncoated side. 3. A method according to claim 2, in which bonding is effected using an optionally fibrous core of thermo­ plastic synthetic material. 4. A method according to claims 1-3, characterized in that the steps of heating, shaping and/or cooling are repeated at least once. 5. A method of making a shaped article from a sandwich construction consisting at least of an integrated multi-layered sheet formed by a sheet of a thermoplastic foam having attached thereto at least one fiber mat impregnated with a thermoplastic synthetic material, comprising the steps of - providing deformability by entirely or locally heating the resulting integrated fiber reinforced sheet, - giving the heated sheet the desired structure and shape, and - fixing the article by cooling. 6. A method according to claim 5, in which after heating and shaping, two sheets each provided on one side with a fiber mat are bonded together with the uncoated sides. 7. A method according to claim 6, in which bonding is effected using an optionally fibrous core of thermo­ plastic synthetic material. 8. A method according to claims 5-7, characterized in that the steps of heating, shaping and/or cooling are repeated at least once.

Inventors: BRAMBACH, JOHAN ARIE
Applicants: SCHREINER LUCHTVAART GROEP B.V.
Application year: 1987
Main classification: B32B5
IPC codes: B32B5