METHOD FOR PRODUCING A WEB-LIKE FIBROUS STRUCTURE

Abstract: The invention relates to a method for producing a web-like fibrous structure by, in a first step, forming a first partial fibrous structure (21) and then, in a second method step, forming a second partial fibrous structure (22) while at the same time joining it to the first partial fibrous structure using joining fibres (5) when forming the first partial fibrous structure and when joining the two partial fibrous structures. The invention furthermore relates to a covering material made from fibre-reinforced plastic, in which the plastic is reinforced with a web-like fibrous structure according to the invention. The invention also relates to a panel structure in sandwich form, which comprises at least one core layer with layers of covering material which bear against the top side and underside and are joined to the core layer, in which the covering material is a covering material according to the invention.

Description: METHOD FOR PRODUCING A WEB-LIKE FIBROUS STRUCTURE The invention relates to a method for producing a web-like fibrous structure, by building this structure up from a plurality of mutually adjoining layers of fibres, each layer comprising a large number of fibres which are arranged parallel to one another and are at substantially equal distances from one another, and the directions of the fibres of the various layers forming predetermined angles with one another, so that the fibres intersect one another, while the various layers are joined together at the loca- tions of intersections of the fibres by means of joining fibres . A method of this nature is generally known and is used to produce fibrous structure materials which exhibit sufficient cohesion for it to be possible to handle and use them as reinforcement material for thermoplastic or thermosetting polymers. The cohesion of such materials is obtained by the fact that the connection between the various layers of the fibrous structure is obtained by joining together the fibres of the various layers at the location of the intersections of the fibres, by means of joining fibres. A material of this nature is used, for example, to produce, inter alia, covering material for aircraft, to which end the fibrous structure is impregnated with a thermoplastic or thermosetting resin, after which, as a result of solidification or curing, a strong material which is easy to handle and is eminently suitable for the above purpose is obtained. A drawback of a known fibrous structure of this nature when employed as reinforcement material for, for example, material for the internal lining of aircraft is that, in view of current requirements, the material does not yet impart sufficient strength to covering materials of this nature. Owing to the nature of the types of machine used, it is not easy to increase the number of layers, and moreover the results in tests have shown that this does not result in an improvement which outweighs the complication of producing materials with large numbers of layers, for example more than four layers . The object of the present invention is to provide a method of the type indicated which makes it possible, as desired, to produce a web-like fibrous structure which is composed of more layers than has hitherto been customary, i.e. of more than four layers, which method can be carried out on existing equipment and provides a material with considerably improved properties. To this end, according to the invention, the method of the type indicated is characterized in that, in a first step, a first partial fibrous structure is produced, with a structure as indicated above, using first joining fibres and, in a second step, in a similar way, a second partial fibrous structure is formed bearing against the first partial fibrous structure using second joining fibres which also join together the first partial fibrous structure and the second partial fibrous structure, and the first and second partial fibrous structures bear against one another in a bearing plane. Surprisingly, it has been found that it is possible, in a known way and using a known device, to produce a web-like fibrous structure in the manner indicated in the preamble and then, using such a first partial fibrous stru- cture which has been produced in the first instance, to add to this second partial fibrous structure which, in view of the nature of the method, is joined to the first partial fibrous structure by using second joining fibres at the intersections . The web-like fibrous structure obtained using the method according to the invention can, by means of this method, be produced in a greater thickness than has hitherto been customary, specifically by, in a second operation, using the same machine which has been used for a previously formed partial fibrous structure to form a second partial fibrous structure which, on the basis of the method, is joined to the first partial fibrous structure with the aid of second joining fibres. It has been found that the material, which is obtained without particular problems using conventional devices, provides excellent strength, imparting a quasi- isotropic characteristic to the material. This means that when it is incorporated in thermoplastic or thermosetting polymer, a fibre-reinforced plastic material of this nature exhibits substantially the same tensile strength in virtually all directions in the horizontal plane, while the strength in the vertical direction, perpendicular to the surface of a fibre-reinforced plastic material of this nature, is considerably increased by the enlarged number of fibrous layers, an effect which is contributed to considerably by the numerous joins by means of the joining fibres at the locations of the intersections of the fibres. Advantageously, the first partial fibrous structure and the second partial fibrous structure are positioned in such a manner with respect to one another that the intersections of the first partial fibrous structure and the second partial fibrous structure, in perpendicular projection onto the bearing plane between them, coincide. A highly attractive embodiment of the method accor- ding to the invention involves arranging the layers of the first partial fibrous structure and the second partial fibrous structure mirror-symmetrically with respect to the bearing plane. This is because it has been found that this mirror-symmetrical arrangement of the layers with respect to the common bearing plane between the first partial fibrous structure and the second partial fibrous structure results in a symmetrical formation which, when the complete web-like fibrous structure is used to reinforce plastics, leads to extremely good results. The increased thickness with fibres in many directions firstly creates a substantially isotropic characteristic in the reinforced plastic material in the plane of a reinforced plastic material of this nature which is made in panel form. The symmetry around the bearing plane, in addition to providing an extremely high strength in a direction perpendicular to the surface of a material of this nature, also results in a very good absorption of forces in the plane of a reinforced plastic material of this nature, in all directions. The presence of a large number of joining elements in the form of joining fibres at the location of the intersections of the fibres of the two partial fibrous struc- tures furthermore produces a high resistance to delamination, and in particular to delamination in the event of a considerable increase in temperature, such as in the event of fire. The FAA (Federal Aviation Administration) provides regulations for withstanding tests under kerosene fires. A flame at a temperature of ± 1000°C is formed; the barrier action of a material which is being tested must be at least 5 minutes . The material according to the invention which comprises two coupled-together partial fibrous structures withstands a test of this nature without problems. The considerable improvements which are achieved by means of the material obtained using the invention, when summarized, are as follows: 1) the material imparts a quasi-isotropic characteristic to a plastic in which it is incorporated for reinforcement purposes; 2) the material according to 1) has an increased resistance to point/impact loads; 3) the material according to 1) is not delaminated in the event of fire. Improvement 1 results from the bundles of fibres being arranged at a number of different angles. Improvements 2 and 3 result from the presence of the stitch joins which are formed with the aid of second joining fibres. This arrangement considerably strengthens the so-called Z direction (i.e. the direction perpendicular to the surface of the reinforced material) , resulting in a resistance to delamination . In a further advantageous embodiment of the method according to the invention, the second joining fibres join the first partial fibrous structure and the second partial fibrous structure at the locations of the intersections, which coincide when projected onto the bearing plane. Obviously, the joining fibres may also join together the various layers of fibres or partial fibrous structures at any other locations outside the intersections of the fibres, by means of stitching; preferably, however, the joins are made, by means of second joining fibres, at the locations of the intersections, which coincide when projected, as mentioned above. When a partial fibrous structure is being formed, some intersections may comprise more fibres than other intersections. Preferably, the second joining fibres join the first and second partial fibrous structures at least at the locations of the intersections, which coincide in projection, of the highest number of mutually intersecting fibres . This means that the intersections with the highest numbers of intersecting fibres of the first partial fibrous structure and the second partial fibrous structure coincide in projection, and that a join is made at these intersections, preferably with the aid of the second joining fibres . When using joining fibres, it is obvious that, as desired, it is possible to provide one ore more joining fibres per intersection, in order to join the fibres which intersect one another at the intersection together securely over the entire thickness of the assembled partial fibrous structures. In a highly attractive embodiment of the method according to the invention, the first partial fibrous structure and the second partial fibrous structure each comprise four layers, while the orientation of the fibres in the layers is respectively 0, 90, +(30-60) and -(30-60) degrees with respect to the longitudinal axis of the web of assembled partial fibrous structures; in particular 0, 90, +45 and -45 degrees. The method according to the invention described above relates to the formation of a web-like fibrous structure which is composed of a first partial fibrous structure which is produced in a first step and a second partial fibrous structure which is formed thereon, in a second step, and is joined to the first partial fibrous structure with the aid of second joining fibres. The method is not limited to two partial fibrous structures; more than two partial fibrous structures may also be joined together in the manner described. Furthermore, it will be clear that the second partial fibrous structure does not per se have to be formed on a first partial fibrous structure which has already been formed. It is also possible for the two partial fibrous structures to be formed separately and then placed together and joined by means of joining fibres with the aid of stitching or any other suitable joining method. The invention also relates to a covering material made from fibre-reinforced plastic in which, according to the invention, the plastic is reinforced with a web-like fibrous structure as obtained using the method according to the invention. As indicated above, the material with a web-like fibrous structure as obtained according to the invention is suitable for numerous applications in reinforced plastic materials, in which the plastic may be of thermoplastic or thermosetting type. Suitable thermoplastics which may be mentioned are polyvinyl chloride, polyethylene, polypropylene, and numer- ous others. Suitable thermosetting plastics which may be mentioned are phenolic resins, polyimide resins, epoxy resins, etc . The covering material made from fibre-reinforced plastic in which a web-like fibrous structure according to the invention is incorporated may, of course, be in numerous forms, such as tubes, corrugated sheet, panel, section, etc . In an advantageous embodiment, the covering material is a flat panel and, as such, can be used as a covering material in the aeronautical industry, such as for wall and floor lining. The invention also relates to a panel structure in sandwich form, at least comprising a core layer with a top side and an underside, and two layers of covering material which bear against the top side and the underside and are joined to the core layer, in which structure, highly advan- tageously, the covering material is a covering material according to the invention and as described above. A sandwich-like panel structure of this nature is known per se, for example from Canadian Patent 1,339,567 in the name of the applicant. By incorporating covering material according to the invention, which is composed of two joined-together partial fibrous structures which function as a reinforcement for thermoplastic or thermosetting polymer in a panel structure in sandwich form, on the top side and underside, it is possible to obtain an extremely strong, resistant panel structure which is suitable for many uses, such as in aircraft construction as a cabin lining, for floor applications, while obviously it is also true that the covering material according to the invention, as well as the panel structure according to the invention, can be used with equal success in vehicles, vessels, buildings, houses, portable offices, etc. The fibres which can be used for producing the partial fibrous structures are the customary fibres, such as glass fibres, aramid fibres, thermoplastic fibres, such as polyester, carbon fibres, ceramic fibres, etc. The stitching with the aid of the joining fibres may generally be carried out with all kinds of fibres, stitched glass and polyester fibres being preferred. By way of example, 220 detex glass thread can be used to obtain very good results . The number of joining points per unit surface area is obviously dependent on the fibre thickness employed and the distance between the fibres. Typical fibre grades for glass fibre or polyester may, for example, be 2000 denier, while the joining threads are 60 denier, i.e. considerably thinner. A typical join density is, for example, 81 joining points per square inch; a typical thickness of a web-like fibrous structure composed of two partial fibrous structures, in which each partial fibrous structure is composed of four layers of fibres, is between 0.6 and 1.2 mm; the thickness is typi- cally approximately 0.8 mm. The invention will now be described with reference to the drawing, in which: Figs. 1A and IB respectively show a diagrammatic plan view and a diagrammatic side view of a partial fibrous structure which is composed of four layers of fibres. Fig. 2 diagrammatically shows a fibrous structure composed of two partial fibrous structures; and Fig. 3 diagrammatically shows the arrangement of the layers of two joined-together partial fibrous struc- tures, in a preferred embodiment. The fibres are denoted by 1, 2, 3 and 4, fibres 4, for example, being parallel to the longitudinal direction of the web-like fibrous structure; the fibres 3 are perpendicular thereto, and the fibres 1 and 2 form angles of +45 and -45 degrees, respectively, with the longitudinal direction of the web-like fibrous structure. 5 Denotes joining threads which are processed into a stitched arrangement which, at the locations of the intersections with the highest density, surrounds the entire stack of fibres (cf. Fig. IB) . The fibres are not penetrated during stitching, so that they retain their original strength. Fig. 2 diagrammatically shows an assembly of a first partial fibrous structure 21 and a second partial fibrous structure 22, with a bearing plane 23. Both partial fibrous structures are composed of four layers of fibres, as indicated diagrammatically. To produce the web-like fibrous structure, firstly the partial fibrous structure 21 is produced in a machine which is known per se, after which the partial fibrous structure 21 formed in this way is again introduced into the machine, in order to form the partial fibrous structure 22 which is joined to the partial fibrous structure 21. The partial fibrous structures bear against one another in the bearing plane 23. Fig. 3 diagrammatically shows the arrangement of the various layers for a web-like fibrous structure according to the invention, in a preferred embodiment. The first partial fibrous structure comprises layers of fibres 31, 32, 33 and 34 which, in a manner known per se, are joined together by means of vertical joins between the layers. The resultant partial fibrous structure is again introduced into the machine, and then the second partial fibrous stru- cture is formed on the first partial fibrous structure, the layers 31', 32', 33' and 34' being arranged mirror-symmetrically with respect to the layers of the first partial fibrous structure, and the bearing plane 35 functioning as the mirror plane . The resultant web-like fibrous structure which is composed of two partial fibrous structures, when used for reinforcing a thermoplastic or thermosetting polymer, imparts a substantially isotropic strength characteristic to this polymer in the plane of the material formed in this way, and/or imparts a very high strength in a direction perpendicular to the surface of the plastic material reinforced in this way. Due to the presence of the large number of joins over the thickness of the web-like fibrous structure, the resistance to delamination in the event of fire is extremely good, and the material is also highly resistant to impact loads when used as a floor lining material in aircraft . With regard to the sandwich-like panel material according to the invention, it is also noted, with regard to the core which is to be used, that the latter may take numerous forms, such as a foamed structure, a honeycomb structure, aluminium, paper, etc. The layers of covering material which bear against the top side and underside may be joined to the core layer in various ways, for example by means of adhesive bonding; the attachment may, of course, also be mechanical, by means of rivets, screws, etc.

Claims: CLAIMS 1. Method for producing a web- like fibrous structure, by building this structure up from a plurality of mutually adjoining layers of fibres, each layer comprising a large number of fibres which are arranged parallel to one another and are at substantially equal distances from one another, and the directions of the fibres of the various layers forming predetermined angles with one another, so that the fibres intersect one another, while the various layers are joined together at the locations of intersections of the fibres by means of joining fibres, characterized in that, in a first step, a first partial fibrous structure (21) is produced, with a structure as indicated above, using first joining fibres and, in a second step, in a similar way, a second partial fibrous structure (22) is formed bearing against the first partial fibrous structure using second joining fibres which also join together the first partial fibrous structure and the second partial fibrous structure, and the first partial fibrous structure and second partial fibrous structure bear against one another in a bearing plane (23) . 2. Method according to claim 1, characterized in that the intersections of the first partial fibrous structure (21) and the second partial fibrous structure (22) coincide when projected perpendicularly onto the bearing plane (23) . 3. Method according to claim 2, characterized in that the layers of the first partial fibrous structure (31-34) and the second partial fibrous structure (31' -34') are arranged mirror-symmetrically with respect to the bearing plane (35) . 4. Method according to one or more of claims 2-3, characterized in that the second joining fibres join the first partial fibrous structure (31-34) and the second partial fibrous structure (31' -34') at the location of the intersections, which coincide when projected. 5. Method according to claim 4, characterized in that the second joining fibres join the first partial fibrous structure (31-34) and the second partial fibrous structure (31' -34') at least at the locations of the intersections, which coincide when projected, of the largest number of mutually intersecting fibres. 6. Method according to one or more of claims 1-5, characterized in that the first partial fibrous structure (31-34) and the second partial fibrous structure (31' -34') each comprise four layers, and the orientation of the fibres in the layers is respectively 0, 90, +(30-60) and - (30-60) degrees with respect to the longitudinal axis of the web. 7. Method according to claim 6, characterized in that the orientation of the fibres in the layers is 0, 90, +45 and -45 degrees with respect to the longitudinal axis of the web. 8. Covering material made from fibre-reinforced plastic, characterized in that the plastic is reinforced with a web-like fibrous structure as obtained with the aid of the method according to one of claims 1-7. 9. Covering material according to claim 8, characterized in that the covering material is a flat panel. 10. Panel structure in sandwich form, at least comprising a core layer with a top side and an underside, and two layers of covering material which bear against the top side and the underside and are joined to the core layer, characterized in that the covering material is a covering material according to claim 9.

Inventors: HILDERS HENDRICUS GERARDUS, HILDERS, HENDRICUS, GERARDUS
Applicants: ADPROTECH B V, HILDERS HENDRICUS GERARDUS, ADPROTECH B.V., HILDERS, HENDRICUS, GERARDUS
Application year: 1999
Main classification: B32B5
IPC codes: B32B5