Non-crimp fabrics

Abstract

A method of producing a non-crimp fabric (10) and a non-crimp fabric comprising a layup of fabric plies (12) stitched together by a thread (18), at least part of which is fusible, is provided. The method can include heating the stitched plies (12) to soften or melt the fusible thread (18). The use of fusible thread (18) can act as an in situ binder within the non-crimp fabric (10) and tension created by the unfused stitching may create channels for matrix resin infusion during manufacture.

Description

[0001] The invention relates to non-crimp fabrics and is particularly concerned with non-crimp fabrics with improved drape characteristics.

[0002] It is known to produce fabrics, by laying several plies of fibres into a stack of required thickness. The fabrics may be of a non-crimped or warp-knitted type. The fabrics maybe unidirectional (fibres in a single orientation) or more typically multi-axial. The term “multi-axial” means that alternate plies of fibres will be constructed in several different directions to produce a fabric with optimum strength and stiffness in required directions. Commonly used directions are 0°, +45°, −45° and 90°. Normally, and for the purposes of this invention, the 0° plies are in the warp direction and the 90° plies in the weft direction, but other layers may contain fibres at different angles, typically at 45° or −45°. The term “Non-crimp” refers to fabrics where one or multiple layers of fibres are laid upon each other and transformed into a fabric by stitching or application of a binder such that the fibres remain straight and without substantial crimp. As well as offering performance advantages relative to traditional woven materials, non-crimp fabrics (NCFs) may be cheaper to produce per unit mass and be faster to manufacture. Such composites have potential for use in the production of wing skins for example.

[0003] However, as the number of layers in a non-crimped fabric blanket increases, the ability to conform to changes in shape-of the component being made (drape) decreases due to the restrictions imposed by the stitching. Consequently, the drape characteristics of unidirectional and biaxial NCFs are generally comparable to that of woven fabrics whereas drape is increasingly restricted with triaxials, quadraxials, etc. hence restricting possible applications.

[0004] The present application is concerned with non-crimp fabrics having improved drape properties.

[0005] The prime role of the stitching is to hold the plies of the non-crimp fabric together during handling in a predominantly un-crimped manner. However, if the density of stitches is too high or the stitching is under too much tension or too thick then disruption and crimping of the fibres may occur.

[0006] It is also known to use a binder powder for pre-form manufacture using non-crimp fabrics. However, the use of such fine powders may be hazardous as the powder may inhaled or irritate the skin of an operative.

[0007] According to the invention there is provided a method of producing a non-crimp fabric which comprises providing a laminate of plies and stitching the plies together using a fusible thread.

[0008] In use, the primary function of the stitch is to hold the fibres together during the non-crimp fabric manufacture and to aid resin infusion. Whilst the stitch initially restricts drape, the eventual fusion of the stitch overcomes that restriction. In the present application, the term fusible means that the thread not only melts by itself when heated, but may also dissolve into a resin during processing at temperature. Therefore, by using a fusible thread, the stitched plies can be heated so as to soften or melt the fusible thread so as to improve the drape of the fabric.

[0009] Preferably, the plies are warp and weft dominated. Examples of warp and weft dominated plies are described in EP-A-0826438 to which the reader is directed for details. Fibres of each ply used in the composite may be constructed in several different directions. In the present application they are preferably 0°, +45°, −45° and 90°, but angles of ±20° to 90° can be used on the weft orientation.

[0010] The non-crimp fabric may comprise from 0.5% to 10% fusible thread by mass. More preferably, the non-crimp fabric comprises from 1% to 3% fusible thread by mass.

[0011] The fusible thread will preferably melt at a temperature of between 40° C. to 150° C., depending on application and processing route used. The fused thread may react with the curing resin or remain as a separate phase, preferably causing minimal decrease in mechanical properties and improving damage tolerance. Preferably, after curing, the fusible thread should have a glass transition temperature (Tg) of greater than 120° C. for aerospace applications. To balance the melting and Tg temperature, some reaction between the fused stitch and resin is desirable. A Tg greater than 120° C. would improve hot-wet performance, especially in compression. A Tg of less than 120° C. may help damage tolerance.

[0012] The fusible thread may be constructed from one or more different materials. Where a plurality of materials is used, the threads may fuse at different temperatures or contain both fusible and infusible materials. If one material is infusible, the fibres are preferably not continuous, thus allowing movement when the fusible material melts. Infusible thread materials at normal processing temperatures include: polyester, acrylamide polymers, e.g. Kevlar®, carbon, acrylonitrile polymers e.g. Panox®, or PBO. Fusible thread materials may include polyamides, lower molecular weight polymers of polyethersulphone e.g. Grillon K-110 from EMS Chemie AG or blends of epoxy resins.

[0013] The fusible thread may be constructed from two or more different materials, each having a different fusion temperature.

[0014] Preferably, the stitching runs substantially transversely through the plies and follows a predetermined pattern. The pattern may be tricot closed, open pillar stitch, closed pillar stitch, open tricot-pillar stitch, or closed tricot-pillar stitch or variants thereof.

[0015] The method may also include heating the plies as aforesaid to improve the drape of the fabric. In one embodiment, the plies are heated to a temperature not greater than the fusion temperature of the thread. However, in another embodiment, the plies are heated to a temperature at or above the fusion temperature of the fusible thread, so that the thread fuses and allows the fabric to move. Where this occurs without the presence of a matrix resin, cooling under pressure below the fusion temperature substantially maintains the shape formed creating a pre-form, due to the fusible stitch acting as an in-situ binder.

[0016] Tension in the un-fused stitching tends to create channels in the non-crimp fabric which are normally undesirable. However, it has been found that the method may facilitate easier matrix resin infusion into the fabric through the channels, and then heating the fabric to a temperature above the fusion temperature of the stitching whereby the stitching fuses and relieves the tension. In that way fibres in the non-crimp fabric, which have been crimped due to tension in the stitching, will move to minimise crimp, reduce resin rich areas and improve ultrasonic NDT inspection.

[0017] Where the method includes infusing the non-crimp fabric with a matrix resin, the fusible thread will preferably be chemically compatible with the matrix resin and be of a similar modulus.

[0018] The fusible thread may melt and remain as a separate phase from the matrix resin, dissolve in the matrix or precipitate out later as the matrix is cured or allowed to set.

[0019] The matrix may be a resin.

[0020] The fused stitching may also act as a toughening phase within resin infused non-crimp fabric. Such a secondary benefit would be normally at fusible stitch levels of 5-10% of fabric mass

[0021] A hairy yarn as disclosed in applicants co-pending UK Patent Application filed simultaneously herewith and entitled ‘Composites’ may be introduced into the plies so as to improve the shear properties between the plies and improve damage tolerance.

[0022] A non-crimp fabric in accordance with the invention will now be described by way of example and with reference to the accompanying drawings in which:

[0023] FIG. 1 is a schematic cutaway view of a non-crimp fabric being stitched with a fusible thread; and

[0024] FIG. 2 is a table showing different types of stitch patterns.

[0025] Referring to FIG. 1, a non-crimp fabric 10 is constructed from a laminate 12 of plies 14a-14e. The plies are multi-axial with fibres in respective plies 14a to 14e at angles of 0°, 90°, +45°, 90° and −45° as shown in FIG. 1.

[0026] A set of needles 16 is used to facilitate the stitching of the plies 12 together. The needles 16 move transversely up and down through the laminate 12 in a predetermined pattern whilst the laminate is moved in a direction shown by the arrow ‘A’. Examples of predetermined stitch patterns used can be seen in FIG. 2.

[0027] A fusible thread 18, for example made of Grillon K-110®, is threaded into the needles 16. The thread 18 is then stitched into the laminate 12 in the predetermined pattern.

[0028] The non-crimp fabric 10 can then be used to produce structural components. The fabric 10 is draped over a pre-form tool to be copied (not shown), with the fabric being cut to shape first. The fabric 10 may be heated to a temperature at or in excess of the fusion temperature of the thread so as to relieve some of the tension within the thread thereby improving the drape of the fabric.

[0029] In another embodiment of the invention, a resin (not shown) is infused in to the non-crimped fabric 10. During the infusion process the resin passes through channels (not shown) resulting from tension in the stitching. After infusion, the fabric 10 is heated to a temperature above the fusion temperature of the thread. This causes the stitching to fuse and thereby relieve the tension. As a result, fibres in the non-crimp fabric 10, which have been crimped due to tension in the stitching, can move to minimise crimp.

[0030] In still another embodiment of the invention a ‘hairy yarn’ (not shown) may be used. Strands of hairy yarn are substituted for fibres within the plies 14. The strands of hairy yarn in one ply interact with those in another thereby improving the shear strength properties of the non-crimp fabric 10 as described in the aforementioned UK Patent Application to which the reader is directed for full details.

Claims

1. A method of draping a non crimp fabric over a forming tool, the method including the steps of:

providing a non crimp fabric comprising a layup of plies stitched together using a thread at least part of which is fusible;

placing the non crimp fabric on the forming tool, and

heating the non crimp fabric sufficiently at least to soften the fusible thread as the non crimp fibric drapes over the tool, whereby to reduce the restriction to drape caused by the stitching.

2. A method according to claim 1 in which the plies are heated to a temperature not greater than the fusion temperature of the thread.

3. A method according to claim 1 in which the plies are heated to a temperature above the fusion temperature of the thread.

4. A method according to claim 3 in which pressure is applied to conform the plies to the shape of the tool and the plies are cooled, while under pressure, whereby to create a preform with the fused thread acting as an in situ binder within the non-crimp fabric.

5. A method according to any one of claims 1 to 4 including creating tension in the un-fused stitching to create resin transfer channels in the non-crimp fabric.

6. A method according to any preceding claim in which matrix resin is infused through the non-crimp fabric.

7. A method according to claims 5 or 6 including selecting a matrix resin which is chemically compatible with the fusible thread.

8. A method according to any one of claims 5 to 7 including selecting a matrix resin and a fusible thread with a similar modulus.

9. A method according to any one of claims 5 to 8 in which the fusible thread remains as a separate phase from the matrix resin, dissolves in the matrix resin or precipitates out as the matrix resin is cured or allowed to set.

10. A method according to any one of claims 5 to 9 in which the stitching is selected so that the fused stitching acts as a toughening phase within the resin infused non-crimp fabric.

11. A method according to any preceding claim in which the non crimp fabric provided comprises from 0.5% to 10% fusible thread by mass.

12. A method according to claim 11 in which the non-crimp fabric provided comprises from 1% to 3% fusible thread by mass.

13. A method according to any preceding claim including melting or dissolving the fusible thread at a temperature between 40° C. and 250° C.

14. A method according to any preceding claim including selecting a fusible thread with a glass transition temperature (Tg) of greater than 120° C. after curing.

15. A method according to any preceding claim in which the fusible thread is constructed from two or more parts comprising different materials, each having a different fusion temperature.

16. A method according to claim 15 in which at least one of the materials from which the thread is constructed is infusible.

17. A method according to claim 16 in which the said infusible part of the thread is non-continuous.

18. A method according to claim 16 or 17 in which the said infusible part of the thread is of polyester.

19. A method according to any preceding claim in which the stitching is run substantially transversely through the plies.

20. A method according to claim 19 in which the stitching is carried out in a predetermined pattern and is selected from the group: tricot closed, open pillar stitch, closed pillar stitch, open tricot-pillar stitch or closed tricot-pillar stitch.

21. A method according to any preceding claim including providing a said non crimp fabric in which the plies are multi-axial.

22. A method according to any preceding claim in which fibres of each ply are placed in directions of 0°, +45°, −45° and 90°.

23. A method according to any preceding claim in which fibres of each ply are placed in directions of ±20° to 90°.

24. A method according to any preceding claim in which a hairy yam is introduced into the plies.