GB2114055A

GB2114055A - Manufacturing fibre-reinforced composites

Info

Publication number: GB2114055A
Application number: GB08302616A
Authority: GB
Inventors: Leslie Nathon Phillips; David Joseph Murphy
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1982-02-02
Filing date: 1983-01-31
Publication date: 1983-08-17
Also published as: GB2114055B; GB8302616D0

Abstract

Void-free fibre reinforced thermoplastics composites are produced by: a. impregnating at least one layer of reinforcing fibres throughout with a solution of thermoplastics material to form a layer which is not completely stiff; b. stacking alternately layers of an insoluble or a soluble thermoplastics material with layers of the impregnated reinforcing fibres; c. subjecting the stack to heat and pressure in a preheated mould or press for a time sufficient to cause the soluble and the insoluble thermoplastics material to soften and flow and to cause the thermoplastics materials to bond together to form a continuous thermoplastics matrix and to cause the at least one layer of reinforcing fibres to bond to the thermoplastics matrix material and to produce a stiff fibre reinforced composite; and d. allowing the mould or press to cool sufficiently to avoid distortion of the composite before releasing the pressure. Various fibres and thermoplastics both soluble and insoluble, are disclosed.

Description

SPECIFICATION Fibre reinforced composites The present invention relates to fibre reinforced composites and to methods of producing them.

The matrix material of a composite, in which reinforcing fibres are embedded, may be either a thermosetting plastics material or a thermoplastics material. In general thermoplastics soften and eventually melt under the influence of heat, resolidifying on cooling and are generally soluble in suitable solvents. The present invention is concerned with thermoplastics.

A fibre reinforced thermoplastics composite may be produced by the process described in UKP 1570000 whereby a stack consisting of alternate layers of thermoplastics materials and layers of reinforcing fibres which have been impregnated with thermoplastics material prior to stacking, is subjected to heat and pressure. However, a thermoplastics material which is insoluble presents a problem in that the reinforcing fibres cannot be impregnated with the thermoplastics material in question because a solvent solution of the insoluble thermoplastics material cannot be made.Merely stacking layers of unimpregnated reinforcing fibres with layers of the insoluble thermoplastics material and subsequently applying heat and pressure to such a stack results in a weak laminate in which there are many voids around the reinforcing fibres due to inadequate flow of the insoluble thermoplastics material into the interior of the tows of fibres that make up the layers of reinforcement. Although an increase in the duration of the application of heat and pressure to the stack alleviates the problem to some extent, such increases do not sufficiently improve the laminate so as substantially to eliminate voids; the interiors of the fibre tows remain essentially devoid of thermoplastics material. Consequently a composite produced in this manner has poor mechanical properties.

According to the present invention a method of producing a substantially stiff and substantially void-free fibre reinforced thermoplastics composite includes the steps of: a. impregnating at least one layer of reinforcing fibres throughout with a soluble thermoplastics material which is applied in a solvent solution, the solvent of the solvent solution being allowed to evaporate after impregnation, the level of impregnation being such that the reinforcing fibres are impregnated with insufficient thermoplastics material for a strong stiff composite to result from the impregnation, the product of the impregnation being substantially void free;; b. stacking alternately layers of an insoluble thermoplastics material in the form of sheets with at least one layer of reinforcing fibres impregnated by the impregnation step (a) the amount of the insoluble thermoplastics material included in the stack being such that the total amount of thermoplastics matrix material present in the stack is sufficient to enable a strong and substantially stiff fibre reinforced composite to be produced;; c. subjecting the stack to heat and pressure in a preheated mould or press for a time sufficient to cause the soluble and the insoluble thermoplastics materials to soften and flow and to cause the thermoplastics materials to bond together to form a continuous thermoplastics matrix and to cause the at least one layer of reinforcing fibres to bond to the thermoplastics matrix material and to cause a strong, substantially void free and substantially stiff fibre reinforced composite to be produced and thereafter d. and allowing the mould or press to cool sufficiently to avoid distortion of the composite produced before releasing the pressure.

The term 'impregnated' is to be understood as referring to the state of a layer of reinforcing fibres which has been treated with and retains, thermoplastics material throughout its thickness as distinct from merely on its surface.

It is preferable that the reinforcing fibres are impregnated with a weak solvent solution of the soluble thermoplastics material. Preferably the solvent solution has a concentration of between about 7.5% and about 20% by weight of the thermoplastics material.

A concentration of about 7.5% by weight is preferred. The level of impregnation of thermoplastics material in the reinforcing fibres is preferably between 12% and 40% by weight of thermoplastics material. Impregnation may be about 20% by weight but a 16% by weight impregnation is preferred.

Such levels of impregnation are insufficient for a strong, substantially stiff composite to result from the impregnation of the reinforcing fibres. Consequently the impregnated reinforcing fibres or pre-peg may be handled with ease and may be bent round complex shapes as required. The impregnated reinforcing fibres or pre-peg may be held as stock until such time that they are required, there being none of the problems normally associated with storage of thermo-setting plastics pre-pegs.

It is essential that sufficient time is allowed during the heat and pressure stage of the process for the thermoplastics materials to flow and bond together so that they form a continuous thermoplastics matrix, with the tows of the reinforcing fibres being thoroughly impregnated throughout with the thermoplastics materials, obviating a significant number of voids. For most thermoplastics at least 1 0 minutes is required for the application of heat and pressure. Preferably heat and pressure are applied for about 30 minutes, for thermoplastics of high softening point and high melt viscosity.

It will be realised that several layers of impregnated reinforcing fibres may be included in the stack. Any number of layers of the various materials may be included in the stack. It is preferred that the outer layers of the stack comprise unreinforced thermoplastics material to ensure that the outer layer or skin of the final composite comprises thermoplastic material. Additional layers of the soluble thermoplastics material, in sheet form, may be included in the stack.

In one embodiment of the present invention a composite is produced by applying heat and pressure to a stack comprising alternate layers of reinforcing fibres impregnated with a soluble thermoplastics material and layers of an insoluble thermoplastics material in sheet form.

In another embodiment of the present invention a composite is produced by applying heat and pressure to a stack comprising alternate layers of reinforcing fibres impregnated with a soluble thermoplastics material and layers of the soluble thermoplastics material in sheet form, the outer layers of the stack comprising layers of an insoluble thermoplastics material in sheet form.

The various layers of materials may be stacked prior to being placed in a preheated mould or press. Alternatively they may be stacked in the mould or press.

The reinforcing fibres may be any that are conventionally used in the production of fibrereinforced composites. For example they may be carbon fibres, aramid fibres, glass fibres, silica fibres or asbestos fibres and they may be woven. Carbon fibre cloth or tape or hybrid glass fibre/carbon fibre cloth or tape may be used for example. Preferably the reinforcing fibres are carbon fibres and preferably are in the form of a woven cloth. Glass fibres or carbon fibres or asbestos fibres can also be used in the form of a felt or a randomly oriented "chopped strand mat".

Thermoplastics materials suitable for use as the soluble thermoplastics material in the process of the present invention include polyethersulphones, polycarbonates, polysulphones and polyimides.

Thermoplastics material sutiable for use as the insoluble thermoplastics material in the process of the present invention include polyether-ether-ketone (PEEK) and polyphenylene sulphide (PPS).

A fibre-reinforced laminate produced by the process of the present invention may be in its final form. However it may be held as stock and moulded again to a desired shape for final use.

If the reinforcing fibres incorporated in the laminate include carbon fibres in a form capable of providing a continuous conducting path through the laminate, heating of the laminate so as to facilitate shaping or moulding of the laminate the final use may be achieved by passing a current through the laminate via the carbon fibres to heat and soften the laminate.

The process of the present invention can lead to the reduction or possibly substantial elimination of an unacceptably high proportion of voids in fibre reinforced composites of which the matrix includes insoluble thermoplastics materials.

It will be realised that composites produced by the process of the present invention may be tailored for particular purposes by judicial selection of materials and by selection of the appropriate number of layers of the various materials in the stack prior to the application of heat and pressure.

Furthermore the process of the present invention allows thermoplastics such as PEEK and PPS, which are insoluble, have high softening temperatures and high melt viscosities to be moulded into useful objects. Such materials as these are extremely difficult to mould by conventional plastics moulding techniques.

However such materials have good-chemical and erosion resistance as well as resistance to high temperatures and thus they are excellent for use in hostile environments. The process of the present invention allows the production of composites, which contain such materials, and which therefore may be used in such environments.

The present invention will now be described by way of example.

Example 1 A unidirectional Type III carbon fibre fabric, consisting of unidirectional warps of 6000 filament XAS (trade name) Courtauld carbon fibres held together by 44 tex glass fibre wefts was impregnated with a 73% by weight solution of polyethersulphone (grade 1 00P supplied by ICI) in equal proportions of dichloromethane and 1,1 2-trichloroethane. The impregnated fabric was dried in air at room temperature followed by about 2 hours at about 1200 C. The level of polyethersulphone impregnation was 16% by weight of polymer on the total weight of impregnated fabric.Nine impregnated layers of carbon fibre fabric, with the carbon fibres orientated in the same direction were then interleaved with 10 layers of polyether-ether-ketone (PEEK) film supplied by ICI (approximately 0.1 mm thick), with PEEK layers on the outside and placed in a mould which had been coated with Frekote 34 (trade name) release agent. The mould was placed in a press preheated to 3850C. Contact pressure was applied for about 5 minutes after which a pressure of 12 MPa was applied for about 30 minutes. Heating was then turned off and the press was allowed to cool under pressure before removing the composite which was about 2 mm in thickness. Two composite panels were made by the above process and were cut into specimens for testing. The composite panels had a fibre volume fraction (Vf) of 0.48. The flexural strength, flexural modulus and interlaminar shear strength of the composite specimens were measured. The results are given in Table 1. Each value is the mean of 5 determinations.

Example 2 A unidirectional Type Ill carbon fibre fabric consisting of unidirectional warps of 3000 filament XAS (trade name) Courtauld carbon fibres held together by 1 1 tex glass wefts was impregnated with a 739/0 by weight solution of polyethersulphone (grade 1 OOP supplied by ICI) in equal proportions of dichloromethane and 1,1 ,2-trichloroethane. The impregnated fabric was dried in air at room temperature followed by about 2 hours at about 12000. The level of polyethersulphone impregnation was 1 6% by weight of polymer on the total weight of impregnated fabric.Fourteen impregnated layers of carbon fibre fabric, with the carbon fibres orientated in the same direction were then interleaved with 15 layers of polyether-ether-ketone (PEEK) film (approximately 0.05 mm in thickness) supplied by ICI, with PEEK layers on the outside and placed in a mould which had been coated with Frekote 34 (trade name) release agent. The mould was placed in a press pre-heated to 3850 C. Contact pressure was applied for about 5 minutes after which a pressure of 12 MPa was applied for about 30 minutes.

Heating was then turned off and the press was allowed to cool under pressure before removing the composite which was about 2 mm in thickness. Two composite panels were made in this way and cut into specimens for testing. The composite panels had a fibre volume fraction (Vf) of 0.55. The flexural strength, flexural modulus and interlaminar shear strength of the composite specimens were measured. The results are given in Tables 1 and line 2 of Table 2. Each value is the mean of 5 determinations.

Example 3 Composite panels were produced in the same way as that described in Example 2 except that the grade of polyethersulphone was 600P supplied by ICI. The carbon fibre volume fraction (Vf) was 0.57 (line 3, Table 2). The experiment was repeated using desized carbon fibres. Desizing was effected by washing in acetone and dichloromethane. The grade of polyethersulphone was 1 00P supplied by ICI.

The carbon fibre volume fraction was 0.54. The test results in Table 2 (line 4) are mean values (of 5 determinations).

Example 4 Composite panels were produced in the same way as that described in Example 1 except that a five-shaft satin-weave fabric of 3000 filament carbon fibres was impregnated with polyethersulphone instead of the 6000 filament unidirectional fabric, and eight layers of fabric were interleaved with nine layers of PEEK. The fibre volume fraction was 0.55. The test results in Table 2 (line 1) are mean values (of 5 determinations).

Example 5 Composite panels were produced in the same way as that described in Example 1 except that the unidirectional carbon fibre fabric was not impregnated with polyethersulphone (PES) prior to interleaving the layers of carbon fibre fabric with PEEK film. The fibre volume fraction was 0.51.

Although the resultant composite panel appeared to be visually acceptable it had poor mechanical properties. The test results in Table 1 (line 1) are mean values (of 5 determinations).

Example 6 Sheets of unidirectional carbon fibre fabric (3000 filament tow, Courtaulds Grade XAS) were impregnated with polyethersulphone (ICI Victrex Grad 1 00P) solution to give a final weight pick-up of about 16% wt/wt.

Six layers of this impregnated cloth were stacked alternately with a double layer of 0.025 mm thick Ryton (trade name) polyphenylene sulphide film. Double thicknesses were used because the available film was too thin to allow single thicknesses to be used ie single thicknesses of film would not have provided sufficient material. The stack of materials was placed in a mould and the mould placed in a pre-heated platen press, thermostatically controlled at about 31500 and allowed to warm up for about 5 minutes. A pressure of about 10 MPa was then applied for a further 20 minutes.

The heat was then turned off and the mould and platens were cooled while still under pressure, by forced air cooling for several minutes until the mould temperature had fallen below about 10000.

The resultant laminate was of excellent appearance and had an estimated volume fraction of 63%. It was about 0.75 mm in thickness.

Mechanical properties of unidirectional carbon fibre laminates with PEEK/PES matrices Table 1

Interlaminar Flexural Flexural shear Flexural Flexural Fibre type strength at strength at strength at modulus at modulus at and room temp 2000C room temp room temp 2000C matrix MPa MPa MPa GPa GPa Example 1 6000 filament 1344 185 74 76 63 PEEK/PES Vf=0.48 CV=23.1% CV=7.7% CV=5.0% CV=6.7% CV=6.3% Example 2 3000 filament 1482 175 65 114 PEEK/PES Vf=0.55 CV=12.3% CV=24.0% CV=1 9.6% CV=9.6% ~ Example 5 6000 filament 573 167 37 64 53 PEEK only Vf=0.5l CV=12.9% CV=31.5% CV=9.2% CV=15.5% CV=17.5% Fibre volume fraction.

CV=coefficient of variation.

Table 2

Flexural strength (MPa) at given temperature fOCI Fabric type (all PEEK/PES Room matrices) temp 80 120 140 150 160 180 5-shaft satin weave 639 710 566 - - 415 297 3000 filament V,=0.55 (150 C) (170 C) Unidirectional 1482 1280 943 587 - 316 224 3000 filament Vf=0.55 CV=12.3% Unidirectional 1456 1524 806 612 - 385 228 3000 filament Vf=0.57 CV=1 29% (PES Grade 600P) Unidirectional 1716 - - 996 - 850 428 Carbon fibre desized by washing.

3000 filament Vf=0.54 CV=7.6% Fibre volume fraction.

CV=coefficient of variation.

Claims

1. A method of producing a substantially stiff and substantially void-free fibre reinforced thermoplastics composite including the steps of: a. impregnating at least one layer of reinforcing fibres throughout with a soluble thermoplastics material which is applied in a solvent solution, the solvent of the solvent solution being allowed to evaporate after impregnation, the level of impregnation being such that the reinforcing fibres are impregnated with insufficient thermoplastics material for a strong stiff composite to result from the impregnation, the product of the impregnation being substantially void free;; b. stacking alternately layers of an insoluble thermoplastics material in the form of sheets with at least one layer of reinforcing fibres impregnated by the impregnation step (a) the amount of the insoluble thermoplastics material included in the stack being such that the total amount of thermoplastics matrix material present in the stack is sufficient to enable a strong and substantially stiff fibre reinforced composite to be produced;; c. subjecting the stack to heat and pressure in a preheated mould or press for a time sufficient to cause the soluble and the insoluble thermoplastics materials to soften and flow and to cause the thermoplastics materials to bond together to form a continuous thermoplastics matrix and to cause the at least one layer of reinforcing fibres to bond to the thermoplastics matrix material and to cause a strong, substantially void-free and substantially stiff fibre reinforced composite to be produced and thereafter; d. allowing the mould or press to cool sufficiently to avoid distortion of the composite before releasing the pressure.

2. A method as claimed in claim 1 and wherein the solvent solution of the soluble thermoplastics material has a concentration of between about 7.5% and about 20% by weight of the soluble thermoplastics material.

3. A method as claimed in claim 1 or claim 2 and wherein the solvent solution has a concentration of about 7.5% by weight of the soluble thermoplastics material.

4. A method as claimed in any one preceding claim and wherein the level of impregnation of the soluble thermoplastics material in the reinforcing fibres is between about 12% and about 40% by weight of soluble thermoplastics material.

5. A method as claimed in claim 4 and wherein the level of impregnation is about 20% by weight of soluble thermoplastics material.

6. A method as claimed in claim 4 and wherein the level of impregnation is about 16% by weight of soluble thermoplastics material.

7. A method as claimed in any preceding claim and wherein the impregnated reinforcing fibres are held as stock until such time that they are required.

8. A method as claimed in claim 1 and wherein heat and pressure are applied to the stack of materials for a time in excess of ten minutes.

9. A method as claimed in claim 8 and wherein heat and pressure are applied for about 30 minutes.

10. A method as claimed in any one preceding claim and wherein the outer layers of the stack comprise unreinforced thermoplastics material.

11. A method as claimed in any preceding claim and wherein the stack comprises alternate layers of reinforcing fibres impregnated with a soluble thermoplastics material and layers of an insoluble thermoplastics in sheet form.

12. A method as'claimed in any one preceding claim and wherein the stack comprises alternate layers of reinforcing fibres impregnated with a soluble thermoplastics material and layers of the soluble thermoplastics material in sheet form, the outer layers of the stack comprising layers of an insoluble thermoplastics material in sheet form.

13. A method as claimed in any one preceding claim and wherein the reinforcing fibres are carbon fibres, aramid fibres, glass fibres, silica fibres or asbestos fibres.

14. A method as claimed in claim 13 and wherein the reinforcing fibres are carbon fibres or a hybrid of glass fibres and carbon fibres.

15. A method as claimed in claim 13 or 14 and wherein the reinforcement fibres are in the form of a woven cloth.

16. A method as claimed in any one preceding claim and wherein the soluble thermoplastics material is a polyethersulphone, a polycarbonate, a polysulphone or a poiyimide.

17. A method as claimed in any one preceding claim and wherein the insoluble thermoplastics material is polyether-ether-ketone or polyphenylene sulphide.

18. A method as claimed in any preceding claim, substantially as hereinbefore described and with reference to the examples.

19. A fibre reinforced thermoplastics composite made by the method claimed in any one preceding claim.