DRAIN CHANNEL FOR A COMPOSITE

BACKGROUND OF THE INVENTION

The invention relates to a new process for impregnation of fibrous structures by a resin for the production of composite materials. The invention involves a drain channel which is housed within the fibrous structure and which remains in the final composite.

SUMMARY OF THE INVENTION

All the processes for production of composite materials involve impregnation of fibers by a resin. Some of these processes, such as that using prepregs in sheet form (called "SMC" from the English Sheet Molding Compound), do not require time- consuming migration of the resin within the fibrous structure. Others, in contrast, require extensive migration of the resin, the difficulty of this migration then varying depending on the permeability and the size of the fibrous structure. Two of these processes can be mentioned in particular: the injection process (called "RTM" from the English "Resin

Transfer Molding") and the infusion process. These processes, in their principles, are well known to the expert in the field.

According to the RTM process, a fibrous structure is placed between a rigid mold and a rigid counter-mold (both generally made of metal or composite), and after possible application of a vacuum between the two parts of the mold, resin is injected so that it migrates and fills the whole space between the mold and counter-mold (space called "gap"). Under the effect of the vacuum, certain deformable molds and/or counter-molds (particularly those made of composite) can then crush the fibrous structure, which can considerably impede migration of the resin within said structure. According to the infusion process, the structure is placed between a rigid mold and a plastic film (or cover); the latter serving as a counter-mold compresses the fibrous structure under the effect of the vacuum before impregnation. In this case, the capability of the fibrous structure to resist crushing under the effect of the vacuum can be a huge influence on the flow speeds of the resin. In the case of large pieces, the operator places drains (or tubes) over the mat in order to aid in the migration of the resin. These tubes are removed at the same time as the cover after polymerization of the resin.

A new impregnation process has now been found which facilitates and accelerates impregnation of the fibrous structures by a resin. According to the invention, a channel is

placed within the fibrous structure, parallel to it, and drains the resin. The channel has a longitudinal axis. The channel is placed within the fibrous structure so that its longitudinal axis is essentially in the plane of the fibrous structure. There are fibers of the fibrous structure on either side of this longitudinal axis of the channel. The channel constitutes a preferred path for the resin, inside of which it rapidly travels, more rapidly than directly through the fibrous structure. The channel is provided with at least one opening (or hole) in its longitudinal wall (parallel to the longitudinal axis of the channel) enabling the resin to run at least partially transversely (with respect to the channel) after having traveled a certain distance longitudinally in the channel itself. Thus, the invention relates in the first place to a fibrous structure which has within it a channel provided with at least one hole in its longitudinal wall. The channel can, for example, be provided with a number of openings arranged regularly over its length. These openings can be actual holes produced by removal of material from a channel initially without holes. The hole can in particular be a cut. It has been found in effect that cutting of the channel, without removal of material, could also be suitable, taking into account the fact that the cut is often naturally slightly separated at least at certain places, or the fact that the pressure of the resin inside of the channel has been sufficient to separate the edges of said cut in order to allow the resin to run towards the exterior of the channel. In particular, excellent results have been observed with a spiral cut going around the channel with a regular pitch. In this case, a single cut is sufficient and extends the whole length of the channel, presenting a spiral around and the length of the axis of the channel. Generally, the channel is of a tubular type (annular cross section). Such channels with a spiral cut are marketed (for example, by the Compagnie Generale d'Electricite) for a completely different use from that provided for by the present invention, in that it pertains to gathering electrical cables. These channels on the market are generally made of polyolefm (such as polyethylene, polypropylene), vinyl-acetate copolymer, polyester (Nylon) or a fluorinated polymer such as PTFE (Teflon). The channel can also be produced out of the same material as that which is to serve as the matrix for the final composite. The channel can also consist of a band of material with a high modulus (for example, glass fiber) wound in the form of a spiral (giving it flexibility) and provided with a covering (which can be a lubricant or a binder) which is compatible with the matrix of the final composite. In this case, the channel can better contribute towards reinforcement of the composite.

The channel is chosen so that its flow section (inner cross section perpendicular to the axis of the channel through which the resin runs) is sufficiently large so that the resin runs more rapidly in the channel (in the longitudinal direction) than through the fibrous structure under the conditions of impregnation (that is, possibly compressed). The outer diameter of the channel can, for example, range from 2 mm to 95% of the thickness of the final composite. The wall of the channel is sufficiently thin so that the flow section is as large as possible and allows the resin to flow rapidly. The wall of the channel is sufficiently strong so that the channel resists crushing during its use. Thus, the inner diameter of the channel is generally greater than 1 mm. The channel is generally made out of a material of polymer type which is sufficiently flexible so that it can be wound, preferably with a winding diameter as small as 100 times the outer diameter of the channel, or even as small as 50 times the outer diameter of the channel.

Thus, the fibrous structure provided with the drain channel according to the invention keeps an excellent flexibility in all directions in spite of the presence of the channel which nevertheless resists crushing.

The channel can be arranged within the fibrous structure after formation of the fibrous structure itself or during its formation. In particular, if the fibrous structure is of the mat type with sufficient resistance to be manipulated, it is generally possible to slide the channel, by hand in particular, into the structure, parallel to its plane and over the desired length, after production of the mat. This mat can, for example, be made out of cut yarns or filament yarns bound by needle bonding (and without chemical binder) as described in WO2005/054559. This mat can also be made of non-woven material of polypropylene fibers, for example, crimped polypropylene fibers, in particular, as marketed under the brand Rovicore by Chomarat. The mat can also be bound by a chemical binder, seeing that this does not impede the insertion and the progression of the channel within the fibrous structure. Thus, the invention relates in particular to a process for manufacture of a fibrous structure which has at least one impregnation channel within it, entailing the production of a fibrous structure followed by insertion of the channel by pressure into said structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to produce the fibrous structure containing the channel, it is also possible quite simply to arrange the channel between two identical or different layers of fibrous structures. This operation can in particular be carried out directly in the mold in the following way: a first layer of fibrous structure is first of all placed in the mold; then, at least one channel length, or even two or three channel lengths (depending on the size of the piece) are arranged over the first layer of fibrous structure; then, the second layer of fibrous structure is arranged, the channels thus being sandwiched in between. The counter- mold is then put in place, and one can proceed with the injection after having created the vacuum, according to the traditional impregnation methods.

It is also possible to incorporate the channel in the fibrous structure at the time of production of said fibrous structure. For example, if the fibrous structure is a fabric, at least one yarn of this fabric (of warp or weft type) can be replaced by the channel. The fibrous structure according to the invention can contain cut yarns or filament yarns.

If the fibrous structure is a mat with cut yarns, it is possible to proceed as follows. It is known that these mats are generally produced by projection of cut yarns onto a belt traveling past, said projection taking place in a chamber for deposition of cut yarns. The cut yarns are blown into the chamber and fly randomly in this chamber and fall flat just as randomly as is their position and orientation in the plane of the mat on the traveling belt. A spool of channel can simply be unwound into the chamber through a hole made in the ceiling of this chamber. The channel then falls onto the traveling belt after a first thickness of cut yarns has covered said belt, a second thickness of cut yarns then falls on top of said channel. The invention also relates to a process for manufacturing of a fibrous structure with cut yarns which has at least one impregnation channel within it, said process entailing the deposition of the channel in a chamber for deposition of cut yarns. According to this process, cut yarns are deposited on either side of the channel in order to form a sheet, said sheet then being generally brought to a bonding unit. The sheet of cut yarns covering the traveling belt and containing the channel is removed from the deposition chamber and brought to the bonding unit, which applies to the sheet a bonding of chemical type (a third substance applied on the sheet in order to serve as binder generally after a thermal treatment) and/or a bonding of mechanical type

(by needle bonding or stitching). In particular, it has been observed that needle bonding or stitching is compatible with the presence of the channels, since the perforation of the channel by the needle bonding needles generates no problems. The invention also relates to the process according to which the bonding of stitching or needle bonding type produces holes in the channel. These holes promote impregnation of the fibrous structure by the resin.

The fibrous structure can also be of the type with filament yarns. In this case, filament yarns are projected onto a traveling belt by unwinding from spools owing to an arm performing a rocking movement across the traveling belt. For more specifics concerning this projection technique, one can refer in particular to WO2005054559. It is then sufficient to place a spool of channel among the spools of filament yarn so that the channel is deposited on the traveling belt after a first thickness of filament yarn has been deposited, a second thickness of filament yarn then being deposited on the channel. Generally, the channel is not given a transverse rocking movement because it is generally deposited a fixed distance from the edge of the sheet. The filament yarn can come from a spool or can be formed directly under dies.

The invention also relates to a process for manufacturing of a fibrous structure with filament yarns which has at least one impregnation channel within it, said process entailing the following successive unwinding operations on a planar surface: - at least one first filament yarn distributed across the planar surface, and then

- at least one impregnation channel, and then

- at least one other filament yarn distributed across the planar surface and covering at least partially said impregnation channel and said first filament yarn of which there is at least one. Depending on the width of the fibrous structure, one or more channels (parallel to one another or not) can be placed across a width of fibrous structure.

The fibrous structure provided with the channel can be integrated in a complex with a number of juxtaposed layers. In particular, the structure containing the channel, in the variant using filament yarns, can be the layer with randomly distributed filament yarns of the fibrous structure which is the object of WO03/060218. The fibrous structure containing the channel can be incorporated in a multilayer complex with the following structure: fibrous structure containing the channel + layer of cut yarns on a single side of said fibrous structure, or fibrous structure containing the channel + layer of cut yarns on

both sides of said fibrous structure (complex with 2 or 3 layers). Thus, it is possible to deposit on a traveling belt a first layer of fibers (for example: chopped fibers, for example, with a length between 12 and 100 mm) and then to deposit the fibrous structure containing the channel on this layer and then to proceed with needle bonding, and in that way bind the two layers together by needle bonding. It is also possible to add another layer (for example: cut yarns, for example, with a length between 12 and 100 mm) before the needle bonding. It is also possible to place the channel between two layers of fibrous structure, each being of the fabric or mat type. The needle bonding can be that ordinarily used by the expert in the field. However, if one wishes to reduce the formation of needle holes visible to the naked eye, a needle bonding can be carried out using needles moving at the same time as the structure to be bound, with essentially the same speed as the structure to be bound in a direction parallel to the direction of movement of the structure to be bound. Furthermore, the number of needle impacts can be reduced and can, for example, be at most 25 impacts per cm ² , and in particular at most 15 impacts per cm ² . Generally, the number of needle impacts is at least 1 impact per cm ² and preferably at least 2 impacts per cm ² .

The channel has sufficient flexibility to allow the fibrous structure provided with the channel to be wound over itself in the form of a roll as is ordinarily done in the case of mats. It is possible to proceed with the production of the composite materials in a manner known to the expert in the field, provided that the appropriate tool for cutting of the channel has been provided. In effect, for the production of the composite material, the fibrous structure is cut to the desired dimensions taking into account the foreseen size of the final piece, the structure being then placed in the impregnation mold for application of the RTM process or infusion process. Seeing that the channel is made out of a polymer, its cutting at the time of cutting of the fibrous structure poses no particular difficulty.

In a general manner, an end of the channel is placed in the vicinity of the point of injection so that it can fill with resin as quickly as possible and can quickly convey this resin farther into the fibrous structure.

According to the invention, the channel is incorporated in the final composite material. Since the channel has been deposited within a fibrous structure, there are fibers (or yarns) both above and below the channel and this channel is not visible in the final composite.

The final composite material includes a matrix and a fibrous structure according to the invention. The matrix is obtained by curing of a resin after impregnation of the fibrous structure by the resin. The impregnation resin for production of the composite material can be of thermoplastic type but is generally of the thermosetting type (polyester, vinyl ester or epoxy). Thus, the invention relates also to a process for the manufacturing of a composite material entailing the impregnation of a fibrous structure by a resin, the resin being conveyed at least partially through a channel placed within said structure, said channel being provided with at least one hole in its longitudinal wall. The impregnation process can be of the injection or infusion type. The fibrous structure can contain fibers or yarns (set of contiguous fibers) of any nature, and particularly polypropylene fibers or glass fibers or yarns.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 represents channel 1 of the flexible tubular type which can be incorporated in a fibrous structure. This channel is provided with openings 2 in its longitudinal wall 17. The resin runs preferentially inside of the channel in an axial (or longitudinal) direction AA' and also runs transversely through holes 2.

Figure 2 represents channel 1 of the flexible tubular type which can be incorporated in a fibrous structure. In this case the opening allowing the resin to escape transversely comes from continuous spiral cut 3 in longitudinal wall 17 which goes around according to a regular pitch the whole length of said channel. The cut describes a spiral around longitudinal axis AA' of the channel.

Figure 3 represents a device for production of a fibrous structure containing an impregnation channel. This device has chamber 4 in which cut yarns 5 are blown throughout the whole volume of the chamber in order to fall in the form of a sheet onto belt 6 traveling through the chamber. Spool 7 of channel 8 is unwound through the ceiling of the chamber so that the channel is deposited on traveling belt 6. Because the channel falls on the belt roughly in the middle of the chamber, the belt is covered with first layer 9 of cut yarns before the channel touches the belt, and second layer 10 of cut yarns then falls on the sheet of cut yarns so that the channel leaves the chamber surrounded by cut yarns, and it is in this sense that it is "within" the fibrous structure leaving the chamber. This structure can then be brought to a bonding device which is not represented in Figure 3.

Figure 4 very diagrammatically represents an installation for manufacturing of a mat with filament yarns in which an impregnation channel in the sense of the invention is inserted. First, filament yarn 12 is projected onto traveling belt 11 owing to oscillating arm 13 which performs a rocking movement 14 as indicated by arrow 14 so as to distribute the filament yarn across traveling belt 11. Channel 15 is then unwound from a spool in the middle of the width of the traveling belt, in the plane of the final mat and in the longitudinal direction. This channel is deposited in the middle of the traveling belt, and no rocking movement modifies its position on the belt. A second projection of filament yarn 16 onto the traveling belt is then carried out, exactly like filament yarn 12. This second filament yarn covers the first filament yarn and the impregnation channel. Figure 4 represents a manufacturing principle without unnecessary details. In practice, there can be a number of spools of filament yarn placed before the channel and a number of spools of filament yarn placed after the channel and a number of spools of tubes placed at different points over the width of the traveling belt. Figure 5 represents the production of a composite in the course of impregnation.

The technique used is infusion. A needle bonded mat of filament yarns (glass yarns) is seen from the top through the counter-mold which is a transparent plastic film. The progression of the resin can be seen through the transparent film. Within the mat, in the middle of its thickness, three impregnation channels 21 of the type of Figure 2 have been slid. These channels are parallel to one another and parallel to the longitudinal edges of the mat. They are invisible because they are covered by yarns, and their location is indicated by dotted lines. The resin is injected through transverse edge 22. The hatch marked zone corresponds to the part of the mat that has already been impregnated. It is seen that the front of feed control 23 of the resin has peaks corresponding to the locations of the channels. This shows that the progression of the resin is greatly facilitated by the presence of the channels.