Nonwoven

Abstract

The present invention relates to a nonwoven that includes superposed plies of fibers, such that a binder web is positioned between two plies and heat sealed to the at least two plies, and that it is not stitched. The heat sealing of the plies makes the nonwoven extremely solid and sets the orientation of the fibers, which can no longer move during the processing of the nonwoven. The fibers are, in fact, held by weld points that are non-continuous and are distributed over the entire surface, unlike stitching.

Description

This application claims priority to French patent Application No. 1560839 filed on Nov. 12, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a nonwoven composed of several superposed fiber plies.

The holding together of these various plies may pose a problem, especially if the nonwoven is used for producing pultruded profiles.

This problem is even more critical when the plies of fibers consist of fibers of different orientations, specifically if the pultrusion process is used for producing profiles with unidirectional fibers in the length direction of the profile, the orientation of the fibers is unlikely to be modified, but when the fibers have an orientation different from the direction of the pultrusion operation they have a tendency to move in an uncontrollable manner.

In particular, if it is desired to reinforce the profiles in the other directions, such as for example at 45° and/or at 90°, use is made of commercially available multiaxial reinforcements which are produced either by weaving, or by laying down of plies stitched together in order to form multiaxial nonwovens, in this case, the solidity (cohesion and maintaining of the angles during the processing) of the nonwoven depends on the stitching. These multiaxial reinforcements have the drawback of being fragile during their processing and particularly for the processing by pultrusion where it is necessary to pull on the multiaxial reinforcements and make them pass through a die. In particular, these multiaxial reinforcements become distorted and when the angles are initially 45° and 90°, these angles are no longer maintained after the processing. The mechanical characteristics of these profiles are then greatly reduced. Moreover, when there are no fibers in the length direction)(0°) in the nonwoven the tensile strength decreases greatly and is very low in the case where there are only fibers oriented at + and −45°. This may be particularly critical for certain applications such as aeronautics.

The yarns are bound at the stitch point but may move easily between the stitch points (spaced for example 5 to 10 mm apart), since there are few bonding points between the fibers. In addition, the stitches give rise to a deformation of the fiber ply to allow the needle to pass through, thus creating a hole which may be annoying on the one hand if a certain surface protection is desired for the possible electrical performance and/or the surface homogeneity, and on the other hand which may misalign the fibers leading to losses of the mechanical characteristics.

It is possible to join plies of transverse yarns by welding by means of a grid, these yarns being bound together by binding yarns to enable to be kept yarns parallel one another in the same plies, however the weld points will then be distributed in accordance with the grid, that is to say spaced apart according to a predetermined distance, thus the hold of the yarns will be insufficient.

In order to produce preformed parts, it is known to use a multiaxial fabric comprising reinforcing layers of unidirectional fibers with nonwoven intermediate layers comprising a spunbond or spunlace fabric or mesh of thermoplastic fibers positioned between the reinforcing layers. This process does not make it possible to guarantee the maintaining of the direction of the fibers during preforming or forming operations.

It is also possible to pre-impregnate the reinforcing fibers in order to give them an ability to better maintain their alignment than the same unimpregnated fibers leading to a significant increase in the production costs and the complexity of the processing (transport and storage condition, storage and operating temperature).

The fibers may also be covered with thermoplastic or thermosetting powder in order to bind the yarns by points. However, the use of powder makes it necessary to calibrate the amount of product to use and to eliminate the surplus.

SUMMARY

The invention proposes to produce nonwovens that eliminate these drawbacks, a process for producing this type of nonwoven and also the profile obtained with this nonwoven.

The nonwoven according to the invention consists of superposed plies of fibers, it is characterized in that a binder web is positioned between two plies and heat sealed, and that it is not stitched. The heat sealing of the plies makes the nonwoven extremely solid and sets the orientation of the fibers which can no longer move during the processing of the nonwoven. The fibers are, in fact, held by weld points that are non-continuous and are distributed over the entire surface, unlike stitching. The fact that the nonwoven is not stitched makes it possible to guarantee a constant orientation of the fibers with no discontinuity.

Advantageously, the nonwoven is a pultrusion nonwoven. This nonwoven is particularly suitable for pultrusion since it is not pierced by stitching and has a dense distribution of weld points of the fibers which are thus particularly well immobilized.

Advantageously, the plies consist of oriented fibers. The heat sealing makes it possible to have more bonding points between the fibers and therefore to guarantee a better hold of these fibers in particular when these fibers are oriented.

Advantageously, two superposed plies have different fiber orientations. The profiles have a better strength if nonwovens with fibers of opposite orientations are used.

Advantageously, the binder web is a thermoplastic. The advantage provided by these thermoplastic webs is that of improving the impact resistance of the composite obtained.

According to one particular arrangement, the nonwoven comprises a surface web. The heat sealing of a surface web (which may be a glass mat or a polymer web) makes it possible to provide functions of protection (against galvanic corrosion, against abrasion, etc.) during the production of profiles made of carbon. In general, for a better protection, or even a better insulation, the surface web should not be pierced and cannot therefore be stitched.

Advantageously, the binder web has a melting point above a pultrusion temperature. The binder web, which may be a thermoplastic, will be chosen to have a melting point above the temperature used during the pultrusion process so as to avoid a degradation of the nonwoven during the pultrusion operation.

Advantageously, the binder web has a basis weight less than or equal to the basis weight of the fibers of the plies. The nonwoven should remain permeable in order to enable the pultrusion. The basis weight of the binder webs or of the thermoplastics should be low enough to enable a good impregnation of the fibers but high enough to avoid any movement of the fibers after heat sealing. The basis weight of the binder web or of the thermoplastic depends on the basis weight of the fibers; it should be less than 10% of the basis weight of the fibers, preferably less than 5%. For example, a basis weight between 4 and 8 g/m² makes it possible to heat seal plies of 100 to 300 g/m² of carbon.

Advantageously, certain yarns are reinforcing fibers. These fibers may be made of carbon, of glass fibers, of aramid (Kevlar™), etc.

The invention also relates to a process for producing a nonwoven that consists in superposing plies of fibers and at least one binder web between two plies of fibers, then in heat sealing the assembly.

Advantageously, a surface web is positioned on the plies of fibers before the heat sealing. The surface web should be counted in the layers to be taken into account during the heat sealing operation. The reinforcing fiber layers are all heat sealed in a single go. However it is possible to repeat this operation several times in order to attain the desired stack of plies, for example a reinforcing web may thus be heat sealed to a pre-existing multiaxial reinforcement.

The invention also relates to a pultruded profile comprising a nonwoven with at least one of the preceding features. The pultruded profile uses a nonwoven obtained according to the process. The profile thus obtained is particularly suitable for aeronautical uses which are particularly demanding.

Other advantages may also become apparent to those skilled in the art on reading the examples below, illustrated by the appended figures, given by way of example:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a stitched nonwoven according to the prior art,

FIG. 2 is a schematic view of a nonwoven according to the invention,

FIG. 3 is a top view of a nonwoven according to the invention.

DETAILED DESCRIPTION

The nonwoven from the prior art illustrated in FIG. 1 here consists of four plies 1, 2, 3 and 4 of fibers 10, 20, 30 and 40 that are superposed and stitched together by stitching 5. The fibers of a same ply are parallel to one another. Here, the plies are oriented differently on each layer.

It will be considered that the vertical fibers in FIG. 1 are in the pull direction, that is to say that they make an angle of 0° . The fibers 20 of the ply 2 therefore make an angle of 90°. The fibers 20 of the second ply 2 are positioned on the fibers 10 of the first ply 1 oriented with an angle of 45°. The third ply 3 is positioned on the second ply 2 and its fibers 30 are positioned with an angle of −45° relative to the fibers 40. The fourth ply 4 has fibers 40 positioned at 0°, i.e. along the length of the nonwoven.

The stitching 5 creates bonding points between the plies in a linear and spaced out manner. The stitching 5′ is more sinuous. The fibers are bound at the stitch point but may move easily between the stitch points which are in general spaced 5 to 10 mm apart.

In this figure, the nonwoven is quadraxial (with angles of 45°, 90°, −45° and 0°) and its solidity depends on the stitching. But when there is no 0° in the nonwoven, the tensile strength decreases greatly and is very low in the case where there is only +and −45°.

In FIG. 2, there are three oriented plies 1, 2, 3: the ply 2 consists of fibers 20 having an angle of 0° , whereas the plies 1 and 3 consist of fibers 10 and 30 having angles of +or −45°. Each ply is separated from the next by a binder web 6.

For example, in order to fix a four-ply nonwoven a single binder web between two plies is sufficient, and in order to fix the other plies it is necessary either to insert binder webs or to add stitching as in the prior art.

The nonwoven illustrated in FIG. 3 consists of two plies 1 and 3 oriented at + or −45°, and an inserted binder web 6. As can be seen in the figure, this web 6 which will be heat sealed, covers a large portion of each ply 1 and 3, there will therefore be a large number of bonding points of the fibers of each ply rendering the various layers firmly attached.

The web 6 will preferably be very thin in order to make it possible to produce a multitude of micro-bondings preventing the fibers 10 and 30 from moving.

In this example, the nonwoven has only two layers (2 plies) but it is possible to add other layers by (optionally) inserting other binder webs 6.

The principle remains the same if the last layer is replaced by a surface web.

Claims

1. A nonwoven comprising at least two superposed plies of fibers; a binder web positioned between the at least two plies and being heat sealed, wherein the nonwoven is not stitched.

2. The nonwoven according to claim 1, wherein it is a pultrusion nonwoven.

3. The nonwoven according to claim 1, wherein the plies comprise oriented fibers.

4. The nonwoven according to claim 1, wherein two superposed plies have different fiber orientations.

5. The nonwoven according to claim 1, wherein the binder web is a thermoplastic.

6. The nonwoven according to claim 1, further comprising a surface web.

7. The nonwoven according to claim 2, wherein the binder web has a melting point above a pultrusion temperature.

8. The nonwoven according to claim 3, wherein the binder web has a basis weight less than or equal to the basis weight of the fibers of the plies.

9. The nonwoven according to claim 3, wherein certain fibers are reinforcing fibers.

10. The nonwoven according to claim 1, wherein the plies comprise carbon fibers.

11. A process for producing a nonwoven comprising superposing plies of fibers and at least one binder web between two plies of fibers to form an assembly; and, heating sealing the assembly.

12. The process according to claim 11, further comprising positioning a surface web on the plies of fibers before heat sealing.

13. A pultruded profile comprising a nonwoven comprising at least two superposed plies of fibers; a binder web positioned between the at least two plies and being heat sealed, wherein the nonwoven is not stitched.