WO1999047620A1

WO1999047620A1 - Prepreg for joining honeycomb structures

Info

Publication number: WO1999047620A1
Application number: PCT/GB1999/000704
Authority: WO
Inventors: Roger Mark Sloman
Original assignee: Advanced Composites Group Ltd.
Priority date: 1998-03-18
Filing date: 1999-03-09
Publication date: 1999-09-23
Also published as: AU2737699A

Abstract

An adhesive film includes two fully formulated uncured resins, one having different properties, for example, viscosity, from the other. The film may be used to form a prepreg by laying it on a sheet of fibrous reinforcing material so that one of the resins penetrates into the reinforcing material.

Description

1

PREPREG FOR JOIN ING HONEYCOMB SRUCTURES

The present invention concerns improvements in or relating to adhesive film.

According to the present invention there is provided an adhesive film having at least two components each including fully formulated uncured resin, the resin of one component having different properties from the other.

Preferably the viscosity of the resin of one component is different from that of the other component.

Preferably the film is applied to a carrier sheet.

Preferably the carrier sheet is a release paper, for example a silicone coated release paper.

Preferably a removable separator sheet may cover the side of the film opposite to the carrier sheet.

Preferably a substrate is incorporated in the film. The substrate may be a fine woven or non-woven scrim cloth, or a fine felt (tissue) or a felt, or a knitted fibrous material.

Preferably the first component is applied as a layer on the release paper and the second component is applied as a layer on the first component.

Preferably, the viscosity of the second component is less than the viscosity of the first component.

Preferably each component exhibits most or all of the desirable features of existing adhesives film and at least one of the components is initially curable at the initial cure temperature of a skin formed from a prepreg attachable to a lightweight core by the adhesive film.

Preferably the resins of the adhesive film are further curable to a desired or maximum cure state without undesired distortion, failure of the skin to core bond or other form of damage.

Preferably one component of the adhesive film has an existing adhesive film composition and is coated with a layer of resin-based adhesive which is capable of partial curing at a temperature in the range 20°- 60° C to a sufficient degree of cure which enables the moulded component to be removed from the mould without distortion or damage for further curing at an elevated temperature.

Alternatively the two components of the final adhesive film are applied separately.

Alternatively the two components of adhesive film could be laid down as interlocking patterns. The patterns may be stripes or discreet areas enclosed within a network.

Further alternatively, where the two different resin components of the adhesive film are immiscible they are mixed together by emulsifying techniques and applied in a single coating operation to provide a film with one adhesive suspended in the other.

Alternatively, one resin takes the form of encapsulated components mixed in the other. Preferably, in this example the higher temperature curing resin adhesive is encapsulated.

According to another aspect of the present invention there is provided a prepreg comprising a sheet of fibrous reinforcing material and an adhesive film of the type disclosed in the second to sixth paragraphs of this specification laid thereon. Preferably the second low viscosity resin layer is laid on the outer face of the reinforcing material.

The viscosity of the second layer is sufficiently low that resin of the layer penetrates into the reinforcing material by capillary action or relatively light pressure from rollers leaving the layer of higher viscosity on or adjacent to one surface of the reinforcing layer.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying diagrams in which:-

Fig. 1 shows a cross-section through part of a moulded component, and

Fig. 2 shows a cross section through a sheet of a fibrous reinforcing material with an adhesive film thereon.

The advantages of fibre reinforced resin matrix composites are well known to those skilled in the art of producing lightweight components and structures.

Whenever high quality lightweight, stiff and strong components are required it has become standard practice to specify fibre reinforced preimpregnated materials combined with low density core materials such as aluminium or NOMEX (DuPonts Registered Trade Mark) honeycomb core. The reinforcing fibre used is often selected from a group which includes carbon, aramid, glass, boron, amongst others.

Prepreg is an intermediate product consisting of an array of reinforcing fibres in a specified form, for example, a woven cloth, which is impregnated with a resinous matrix to form a flexible, tacky sheet material with a controlled ratio of resin and reinforcement. The process for converting the flexible tacky intermediate prepreg material to a rigid structural part involves laying sheets of prepreg onto the core , normally in a mould which is then sealed within a tough membrane. The membrane is evacuated and the application of heat, with the pressure differential resulting from evacuation is then used to cause the resin to flow and the individual layers to coalesce and consolidate prior to resin gellation and the formation of the cured composite component.

When a low density core material is used, an adhesive film is often used between the skin (comprising the lamination of sheets of prepreg) and the core to increase the strength of the bond between the two elements.

The resistance to peeling of the skin from the core, or peel strength, is an important parameter, and can depend on a number of factors, one of which is the performance of the adhesive film.

The resin in the adhesive film is preferably relatively tough compared to the resin matrix in the skin material, and it must also flow in a controlled fashion in order to form fillets on the edges of the honeycomb core. It must also form a strong adhesive bond to both the core and the skin.

The uncured adhesive film should also be flexible and sufficiently tacky to stick to the other components in the component during the process of laying up the materials into the mould, prior to carrying out the moulding cure process. It must also have sufficient "grab" to hold the sandwich panel components in place firmly enough to allow accurate positioning and retention in position of different elements of the structure during the lay-up process, and the application of the membrane.

Presently attempts to achieve these desirable characteristics in one adhesive composition involve the selection of complex formulations often including many different ingredients in order to obtain all the properties required.

The initial curing temperature of most existing film adhesives is in the range 120°C to 180°C, although other temperatures, both higher and lower have been used with certain adhesives. Therefore there is normally an increase in temperature above ambient of 100°C to 160°C which causes a large reduction in viscosity of the resin in the adhesive film, thereby allowing it to flow and form fillets on the edges of the honeycomb cells and thus produce a strong skin-to- core bond.

U.K. Patent No. 2108038 taught that there were considerable previously unrecognised advantages to be gained by using prepreg formulations which could be initially cured between 20°C and 80°C, the initial cure to be followed by a second cure (referred to as a postcure) carried out at a higher temperature than the initial cure, to develop the desired maximum level of heat resistance, strength, or stability in the moulded article. A principal advantage of the LTM (Registered Trade Mark, Advanced Composite Components Ltd) low temperature moulding technique is that it allows the use of a mould that is only capable of resisting relatively low elevated temperatures in the 20°C - 80°C range without distortion or failure. LTM provides a method of achieving considerable cost reduction, energy saving, and improvements in accuracy, which are all highly desirable benefits compared with using conventional 120°C - 180°C curing prepregs.

Another PCT Patent Application No. PCT/GB98/00603 of the present applicants teaches a method of achieving low void content, high quality, (and potentially toughened) laminates using LTM prepregs and process technology, which enables the production of aerospace quality structures using the vacuum bag oven curing process only, thus avoiding the high costs and the size limitations inherent in current industry-standard autoclave processes. This new process is applicable to all types of composites structures of any size but at present there is no suitable film adhesive available which can be used in conjunction with the process when a honeycomb core is used to manufacture sandwich LTM prepreg structures.

As illustrated in Fig. 1 of the drawings a component comprises a low density honeycomb core 10 formed from aluminium, NOMEX or any other 6

appropriate lightweight core material. The core 10 is covered by a plurality of sheets 12 (only some of which are shown) of a prepreg, that is a reinforcing fibre sheet comprising glass, carbon, aramid, boron or any other suitable fibres which are normally woven into a cloth like material but which can take other convenient forms into which has previously been impregnated a resinous matrix to form a flexible sheet with a controlled ratio of resin and fibrous reinforcement. The component is formed in a mould (not shown) in which, typically, a first layer of prepreg sheets 16 is laid on the mould surface to follow the contours of the mould and thereafter the suitably shaped honeycomb core 10 is placed on the uncured first layer to be subsequently covered by a second layer 14 of sheets of prepreg 12. The mould, with the assembled prepreg layers and core thereon is then covered by a tough membrane which is sealed at its edges and thereafter evacuated so that on the application of heat the resin gellates to form a homogenous mass and thereafter cures to a desired degree which may be less than a full cure.

To assist in the adhesion of the prepreg layers 14, 16 to the honeycomb core 10, an adhesive film layer 18 is interposed at the interface between the core and prepreg layers during the layup of the different materials.

In one embodiment the resin of the prepreg is a low temperature moulding resin which partly cures at a temperature in the range 20°C to 60°C and on partial cure is sufficiently rigid to allow the component to be removed from the mould such that it can be subsequently post-cured to the desired cured state while unsupported by the mould.

According to one example of the present invention the adhesive film 18 is an adhesive film of an existing composition coated with a second layer of an LTM (Registered Trade Mark of Advanced Composite Components Ltd.) resin- based adhesive which is capable of partial curing at a temperature in the range 20°C to 60°C to a sufficient degree of cure which enables the moulded component to be removed from the mould without distortion for further curing at an elevated temperature while unsupported by the mould. The formulation 7

of the existing adhesive film could be modified such that it cures initially at temperatures between 20°C and 120°C.

In a further example the adhesive film comprises two separate components, that is the existing or modified existing adhesive film described in the preceding paragraph and a separate LTM adhesive film component applied during a separate step to the core/prepreg laminate during the construction of the component.

Alternatively the two different adhesive films could be brought together and combined into a single hybrid film which may then be used independently as a normal adhesive film or combined with a fibrous reinforcing material to produce a prepreg, or be used in a resin film infusion moulding process.

In another example two different types of adhesive film could be laid down as interlocking patterns in any form suitable for production and use, for example as alternating strips or as discrete areas enclosed within a network.

In another example employing two different immiscible resin formulations for the adhesive film emulsifying techniques may be employed to mix the formulation which would then be coated in a single coating operation to produce a film in which one adhesive is suspended in the other. In this example, by varying the ratio of the types of adhesives, a range of properties for the film would be obtained.

In a still further example at least one of the adhesive resins may be encapsulated and the encapsulated resin mixed in the other. In this example it is preferably that the higher temperature curing, tougher adhesive resin is encapsulated and mixed in the lower temperature curing adhesive resin in due proportions. As in the example described in the preceding paragraph the resin/encapsulated resin mix is then coated as a single film.

It will be realised that whatever method of combining two different 8

adhesives at the region of bond of the prepreg laminate to the core, the advantageous principles of using the lower temperature curing and the higher temperature postcuring stages remain the same in all cases, whilst not precluding the option of continuing the initial cure into the postcure phase either with or without any break in the cure cycle and possibly leaving the initially cured component on the mould during the postcure if the mould can withstand the postcure temperature and pressure cycle.

When the low temperature curing prepreg structure is initially cured the low temperature curing resin adhesive will be at least partially cured and provide sufficient adhesive bond strength to enable the sandwich structure to be removed from the mould and then postcured, essentially free standing without a mould, without damage. During the postcure stage the second tougher higher temperature cure adhesive resin cures and provides the full potential structural strength of the component. The lower temperature curing adhesive resin may become fully cured during the postcure process. The adhesive film may include additional adhesive resin types combined with the two types described in the preceding examples according to any of the techniques described above. The initial curing of the component may be a two or three stage cure/postcure process or a process involving more than three curing stages, if required by the specific requirements of the application and materials selected.

In the embodiment shown in Fig. 2 adhesive film is used in the manufacture of a prepreg comprising a sheet of fibrous reinforcing material 20 which may comprise a woven, felted or any other known reinforcing sheet structure.

There are situations where it is desirable to incorporate two differing resin compositions into the prepreg and one method of achieving this is the use of alternating prepreg layers of different compositions as disclosed, for example, in our copending PCT application GB 98/00603. In the present embodiment, both resin compositions are incorporated into a single sheet of reinforcing material.

By way of example only, and especially applicable to thick, bulky or multi-layered reinforcing materials, this may be achieved by applying an adhesive film to a sheet of "dry" reinforcing material. The film, as illustrated in Fig. 2 comprises a carrier sheet of a silicone coated release paper 22 which is coated with a first layer 24 of highly viscous resin, that is resin which will not flow easily under normal ambient temperature conditions. To the surface of this first resin layer 24 remote from the release paper 22 there is applied a second layer 26 of resin of considerably lower viscosity than the first layer, the viscosity being such that in normal conditions the resin of the second layer is fluid.

The film is applied to one or both surfaces of the reinforcing sheet 20 and as a result of the low viscosity of the layer 26 adjacent to the surface of the reinforcing sheet 20, capillary action and/or light roller action causes the resin of the first layer to permeate into and through the sheet. The more viscous second layer, then lying on and adjacent to the surface of the sheet, provides a good level of tack for adherence to a mould surface. Subsequent similar reinforcing resin sheets laid thereon will have their tacky viscous resin side laid down on to the drier top side of the previous layer of cloth. For normal thickness reinforcement material the less viscous resin can be the correct viscosity for normal prepreg tack and drape properties to be obtained and may be forced into the reinforcement by the use of higher roller pressures and temperatures.

The film described with reference to Fig. 2 is often supplied in roll form and if the layer remote from the carrier sheet is very tacky it may adhere to the free surface of the carrier sheet. This can be prevented by applying a removeable sheet, for example a polythene sheet, to the outer surface.

In a modification the resin layers may be applied to a fine woven or non woven scrim cloth, or a fine felt (tissue) or a felt, or knitted fibrous material 10

which remains incorporated in the film as part of it when it is applied to the reinforcing material.

Claims

11CLAIMS

1. An adhesive film characterised in that it comprises at least two components each including fully formulated uncured resin, the resin of one component having different properties from the other.

2. An adhesive film as claimed in claim 1, characterised in that the viscosity of the resin of one component is different from that of the other component.

3. An adhesive film as claimed in claim 1, or claim 2, characterised in that the film is applied to a carrier sheet (22).

4. An adhesive film as claimed in claim 3, characterised in that the carrier sheet is a release paper (22), for example a silicone coated release paper.

5. An adhesive film as claimed in claim 3 or claim 4, characterised in that a removable separator sheet covers the side of the film opposite to the carrier sheet (22).

6. An adhesive film as claimed in any preceding claim, characterised in that a substrate is incorporated in the film.

7. An adhesive film as claimed in claim 6, characterised in that the substrate is a fine woven or non-woven scrim cloth, or a tissue or a felt, or a knitted fibrous material.

8. An adhesive film as claimed in any of claims 4 to 8, characterised in that the first component is applied as a layer on the release paper and the second component is applied as a layer on the first component.

9. An adhesive film as claimed in any preceding claim, characterised in that the viscosity of the second component is less than the viscosity of the first 12

component.

10. An adhesive film as claimed in any preceding claim, characterised in that each component exhibits most or all of the desirable features of existing adhesives film and at least one of the components is initially curable at the initial cure temperature of a skin formed from a prepreg attachable to a lightweight core by the adhesive film.

11. An adhesive film as claimed in claim 10, characterised in that the resins of the adhesive film are further curable to a desired or maximum cure state without undesired distortion, failure of the skin to core bond or other form of damage.

12. An adhesive film as claimed in any of the preceding claims, characterised in that one component of the adhesive film has an existing adhesive film composition and is coated with a layer of resin-based adhesive which is capable of partial curing at a temperature in the range 20┬░ - 60┬░ C to a sufficient degree of cure which enables the moulded component to be removed from the mould without distortion or damage for further curing at an elevated temperature.

13. An adhesive film as claimed in any of claims 1 to 11, characterised in that the two components of the final adhesive film are applied separately.

14. An adhesive film as claimed in any of claims 1 to 11, characterised in that the two components of adhesive film are laid down as interlocking patterns.

15. An adhesive film as claimed in claim 14, characterised in that the patterns are stripes or discreet areas enclosed within a network.

16. An adhesive film as claimed in any of claims 1 to 11, characterised in that where the two different resin components of the adhesive film are not miscible, they are mixed together by emulsifying techniques and applied in a 13

single coating operation to provide a film with one adhesive suspended in the other.

17. An adhesive film as claimed in any of claims 1 to 11, characterised in that one resin takes the form of encapsulated components mixed in the other.

18. An adhesive film as claimed in claim 17, characterised in that the encapsulated resin adhesive has the higher curing temperature.

19. A prepreg characterised in that it comprises a sheet of fibrous reinforcing material and an adhesive film as claimed in any of claims 2 to 9.

20. A prepreg as claimed in claim 19, characterised in that the second low viscosity resin layer is laid on the outer face of the reinforcing material.

21. A prepreg as claimed in claim 19 or claim 20, characterised in that the viscosity of the second layer is sufficiently low that resin of the layer penetrates into the reinforcing material by capillary action or relatively light pressure from rollers leaving the layer of higher viscosity on or adjacent to one surface of the reinforcing layer.