1. Field of the Invention
The present invention relates to the technical field of reinforcing materials adapted to making up composite parts. More precisely, the invention relates to a novel intermediate material for producing composite parts by subsequent injection or infusion of thermosetting resin, in the form of a veiled tape with an improved resistance to delamination.
2. Description of Related Art
Composite parts or articles, i.e. comprising both one or more pieces of reinforcement or fibrous sheets and also a matrix made principally of the thermosetting (resin) type and that may include thermoplastics, may, for example, be produced by using a “direct” or “LCM” (liquid composite molding) method. A direct method is defined by the fact that one or more pieces of fibrous reinforcement are used in the “dry” state (i.e. without the final matrix), the resin or matrix being employed separately, for example by injection into the mold containing the fibrous reinforcement (“RTM” method, resin transfer molding), by infusion through the thickness of the fibrous reinforcement (the “LRI” or liquid resin infusion method, or the “RFI” or resin film infusion method) or by manual coating/impregnation, using a roller or brush, onto each of the individual layers of fibrous reinforcement, applied in succession to the form.
Other methods known as indirect methods use pre-impregnated materials that already comprise a sufficient quantity of resin to make up the desired composite part. Such materials are in particular those described in documents US 2005/048280, WO 92/20521 and EP 0 554 950.
For RTM, LRI, or RFI methods, in general a fibrous preform needs to be produced in the shape of the desired finished article, and then that preform is impregnated with a resin. The resin is injected or infused by pressure or temperature differentials, and then once all of the necessary quantity of resin is contained in the preform, the impregnated preform is heated to a higher temperature in order to carry out the cycle of polymerization/curing and thus cause it to harden.
The composite parts used in the automotive, aviation, or shipbuilding industries in particular are governed by very strict regulations, in particular in terms of mechanical properties. It is thus particularly important to have access to materials that are both extremely regular and also easy to handle and use.
In those sectors, a large number of preforms are produced that are based on reinforcing materials, formed from carbon fibers, in particular of the unidirectional type. In order to satisfy the high standards in terms of quality and productivity demanded in the aviation sector in particular, it is becoming ever more necessary to use automated methods.
The prior art proposes unidirectional sheets of reinforcing yarns in which the cohesiveness between the yarns is ensured by thermoplastic or glass/thermoplastic binding yarns, which may be woven or nonwoven, that extend transversely to the reinforcing yarns. Such sheets are, for example, supplied under the references PW-BUD by SIGMATEX UK Limited, Runcorn, Cheshire WA7 1TE, United Kingdom) or with other denominations (the TeXtreme® range from OXEON (Norrby Langata 4S, SE-50435, Boras, Sweden).
Other documents such as Japanese patent application JP 2009-235175, proposes depositing a thermosetting resin powder on a fabric type or unidirectional fibrous type support surface in order to provide a base material for producing a preform that has excellent deformability, form stability, good permeability to air, and satisfactory impregnability by resin. Japanese patent application JP 2009 235175 also proposes the same type of intermediate material adapted to the RTM method that can be readily molded and shaped to produce a preform and that can avoid the loss of properties suffered by an interply preform.
In order to provide yarns with better cohesiveness, the Applicant has proposed associating each face of a unidirectional reinforcing sheet with a nonwoven that could result in a more continuous bond compared with the spot bonds obtained with binding yarns or powder. Such materials are described in particular in patent application WO 2010/046609. Next, in patent application WO 2010/061114, the Applicant has described a method of producing unidirectional sheets with a given width that have high regularity, adapted to direct methods of producing composite parts from one or more yarns, while limiting material losses. Such a method can in particular be used to obtain controlled width veiled yarns that can then be used to manufacture preforms with a plurality of layers directly using an automated lay-up device. Another route consists in using them in order to make up woven or braided reinforcements that can then be stacked to produce composite preforms or parts using a direct method.
When using the veiled tapes previously proposed by the Applicant, it has been observed that during automated lay-up of a veiled tape, it is bonded to the preceding ply by a combination of a pressure and heating action followed by cooling, where cooling is possibly accomplished without adding a specific coolant, by using a “natural” route. The tape is thus bonded to the preceding ply via its lower face, and that mechanical bond, which is subjected to shear all the time the tape is being laid-up, is of an intensity proportional to the (lay-up tension)/(bonded length) ratio. The lay-up tension is generally assumed to be constant, and so the shear stress is higher during the first centimeters of lay-up and decreases as the length of the laid-up tape increases. The shear force is distributed over the whole thickness of the tape and if the lay-up tension is too high, delamination of the tape in its central zone has been observed in some cases by the Applicant, during the first centimeters of laying-up. In fact, the Applicant has observed that in such materials associating a tape of unidirectional fibers with each of its two faces carrying a thermoplastic veil, a preferential mechanical bond is established between the filaments located on the main faces of the tape and the veil, while the central zone of the tape, made up solely of filaments, corresponds to the zone with a lower shear strength.
That phenomenon may also be accentuated when a roller is used to deposit the tape. Under such circumstances, during the very first millimeters of bonding of the tape, the face in contact with the roller has a tendency to adhere to it, which further encourages delamination of the tape when its other face is then bonded to the preceding ply.
The Applicant has also observed the same phenomenon of shear of the yarn when laying up the tape along a trajectory that is curved in its plane, also known as “steering lay-up”. During this lay-up, the deposited tape undulates with an intensity that increases as the radius of lay-up decreases under the shear stress due to the surplus length of the filaments present on the internal radius of the tape compared with the filaments located on the outer edge of said tape.