WO2016128505A1

WO2016128505A1 - Consolidation cycle

Info

Publication number: WO2016128505A1
Application number: PCT/EP2016/052893
Authority: WO
Inventors: Johannes Willem LUINGE; Sander Jozefus Maria HENDRIKSEN
Original assignee: Ten Cate Advanced Composites B.V.
Priority date: 2015-02-12
Filing date: 2016-02-11
Publication date: 2016-08-18
Also published as: NL2014282B1

Abstract

A method of manufacturing a fibre reinforced thermoplastic product, the method comprising: (a) heating a thermoplastic and fibre composition to a first temperature T₁under a first pressure p₁ for a first period of time t₁; (b) reducing the pressure applied from p₁ to a second pressure p₂ and heating the composition to a second temperature T2 under p₂ for a second period of time t₂; and (c) increasing the pressure from p₂ to a third pressure p₃ and applying p₃for a third period of time t₃; wherein: the temperatures T₁ and T₂ are equal to or greater than the melt temperature T_m of the thermoplastic; the temperature cools from T₂ to a temperature below the melt temperature T_m of the thermoplastic in t₃.

Description

Consolidation Cycle

Field of Invention

The present invention relates to a method of making a fibre reinforced thermoplastic product. Background

A fibre reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. Suitable fibres for use in a fibre reinforced plastic include carbon fibres, glass fibres, basalt fibres and aramid fibres (such as Kevlar® fibres), as well as natural fibres such as flax, hemp or jute. These fibres can be provided in many different forms, for example, as a uni-directional tape, a non- crimp fabric, a woven fabric or mat or as chopped strands. In order to form a fibre -reinforced product, the fibres must be mixed with an appropriate polymer matrix material. The polymer matrix can be both heat-curable (thermosetting) and heat-meltable (thermoplastic). The polymer matrix can also be a combination of both thermosetting and thermoplastic material. Typically, fibre reinforced plastic materials use a thermosetting (thermoset) plastic such as an epoxy resin, a phenol resin or a polyester resin as the polymer matrix. In order to form a product from a thermoset, the thermosetting plastic resin must be cured. Thermosetting plastics are typically cured by applying heat and pressure to the material when it is positioned within a mold. During the curing process, the polymer chains of thermosetting plastics cross-link together to form chemical bonds. This cross-linking process means that the thermoset cannot re-melt when heat is applied. Therefore, a thermosetting material cannot be melted and re-shaped after it has been cured.

Fibre reinforced plastic materials can also be formed from thermoplastics, for example, polyimides such as polyetherimide (PEI), poly amides, poly ether sulphones, polyaryl ether ketones (PAEK) such as polyether ether ketone (PEEK) and polyether ketone ketone (PEKK), polyurethane, polyethylene, polypropylene, polyphenylene sulphides, polysulphones such as polyphenylene sulphone (PPS), polyamide-imides, acrylonitrile butadiene styrene (ABS), styrene/maleic anhydride (SMA), polycarbonate, polyphenylene oxide blend (PPO), thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, as well as mixtures and copolymers of one or more of the above polymers. Thermoplastics also include liquid crystalline polymers.

Unlike thermosetting plastics, thermoplastics do not need to be cured. This means that thermoplastics can be continuously re-melted and re-shaped.

Thermoplastic polymers can be either semi-crystalline or amorphous. In the solid state, semi- crystalline thermoplastics have an ordered molecular structure with sharp melt points. They do not gradually soften with a temperature increase. Instead, semi-crystalline thermoplastics remain solid until a given quantity of heat is absorbed and then rapidly change into a liquid. Semi -crystalline thermoplastics include most of the polyamides, polyethylene, polypropylene, polyaryl ether ketones (PAEK) and the thermoplastic polyesters.

In contrast, amorphous thermoplastics have a random and disordered molecular structure in the solid state. This means that amorphous thermoplastics not have a sharp melt point. Instead amorphous materials soften gradually as the temperature rises once the required flow temperature (i.e. the temperature at which the material softens) has been reached. Amorphous thermoplastics include polystyrene, polycarbonate, acrylics, acrylonitrile-butadiene styrene and polyvinyl chloride.

Fibre -reinforced plastic products are formed by consolidating the fabric and resin into either single-layer products or a multi-layer laminates.

Before the product is consolidated, the fibres can be formed into a semipreg material. A semipreg material is usually produced by providing first a central reinforcement layer which is partially impregnated on one or both sides with resin material. For example, a semipreg may be formed from a layer of fabric with a film of the resin provided on one or both sides of this fabric.

Alternatively, the semipreg may be provided with a powder coating of the resin on one or both sides of the fabric.

Fibre -reinforced plastic products can be obtained from semipreg materials using a consolidation process. This consolidation process involves fully impregnating the reinforcing fibres with an appropriate polymer matrix material to create either a single -layer product or a multi-layer laminate. Alternatively, a multi-layer fibre -reinforced plastic material can be formed from layers in which the fibre has already been fully impregnated with the polymer matrix material (i.e. from prepreg materials). For example, these prepreg plies may be formed by mixing the fibres with the desired polymer material in molten state, as a solution or dispersion. In the latter cases, the solvent is then allowed to evaporate to form the prepreg.

It is an object of the present invention to provide an improved method (i.e. an improved consolidation process) for making a fibre reinforced thermoplastic material. Summary

In a first aspect of the present invention there is a method of manufacturing a fibre reinforced thermoplastic product. This method comprises:

(a) heating a thermoplastic and fibre composition to a first temperature T_x under a first pressure p_x for a first period of time t_x ;

(b) reducing the pressure applied from p_x to a second pressure p₂ and heating the composition to a second temperature T₂ under p₂ for a second period of time t₂; and (c) increasing the pressure from p₂ to a third pressure p₃ and applying p₃ for a third period of time t₃;

wherein:

the temperatures T_x and T₂ are equal to or greater than the melt temperature T_m of the thermoplastic; and

the temperature cools from T₂ to a temperature below the melt temperature T_m of the thermoplastic in t₃. The word "heating" is being used to describe the application of heat to the composition to bring or maintain the temperature above room temperature.

This method can be used to produce a fibre reinforced thermoplastic material with more homogenous fibre distribution, i.e. the method of the present invention can be used to form a product in which the fibres are more evenly distributed throughout the thermoplastic resin compared to conventional thermoplastic materials. It appears that the reduction of the pressure between steps (a) and (b) of the method, i.e. the reduction of the pressure applied from p_x to p₂, may allow the fibres to relax during the manufacturing process. When the fibres relax, they seem to then distribute themselves throughout the resin in a more homogenous manner. Furthermore, reducing the pressure from pi to p₂ additionally appears to allow the thermoplastic resin to "balloon" (i.e. to expand outwards). This also seems to promote a more homogenous fibre distribution throughout the resin. If the fibre reinforced thermoplastic product formed by the method of the present invention is a multi-layer laminate, the method promotes fibre re -distribution throughout each ply of the laminate. Furthermore, if there are predominately resin layers at the top and bottom of each ply, the method of the present invention enables the fibre content of these areas to be increased. In other words, the method of the present invention decreases the resin-rich areas which are weak spots in the case of mechanical loading.

A fibre reinforced thermoplastic material with a more homogenous fibre distribution will generally have a better mechanical performance than the same material with a less even distribution of fibres throughout the resin. If the fibres are more evenly spread through the material, any stress applied to the fibre reinforced plastic will also be more evenly spread throughout the material making the material less prone to a stress fracture. Consequently, a more homogenous fibre distribution results in a material with an increased mechanical performance.

A reduction in the pressure applied to the fibre reinforced thermoplastic composition between steps (a) and (b) of the method also appears to result in a resultant fibre reinforced thermoplastic with fewer resin voids (i.e. areas with no resin penetration). Preferably, the resultant composite has no resin free voids. These resin micro-voids are generally formed by entrapped air. It appears that when the pressure is reduced, filaments in the fibre bundles relax, allowing the entrapped to escape out of the fibre bundles.

A fibre bundle, also called a fibre tow, consists of multiple fibres or filaments. For example, in a woven fabric, the fabric will be formed by weaving the fibre bundles of multiple fibres/filaments into a specific construction such as a 5-harness satin weave style, a plain weave or a 2x2 twill. Other fibre reinforcement constructions, for example, uni-directional tapes and non-crimp fabrics are also formed from fibre bundles of multiple filaments. For carbon fibres, for example, fibre bundles typically have a tow size between 3000 (3k) and 50,000 (50k) filaments.

When a stress is applied to a fibre reinforced thermoplastic composite, this stress can be absorbed by fibres embedded in the resin. Therefore, if the fibres are located within voids instead of within the resin, this will prevent the fibres from absorbing the applied stress, reducing the strength of the composite. The voids will, therefore, cause stress concentrations. This means that the micro-voids can act as a crack initiator. Therefore, fibre reinforced thermoplastics with fewer resin-free voids will have increased mechanical performance compared to composites with voids. Furthermore, the relaxation step (i.e. the pressure relief step) appears to cause a redistribution of the filaments with the fibre bundles. This reduces the number of filament-filament contracts, reducing the stress concentration in the bundles (i.e. the stress is distributed more evenly throughout the bundle). This effect also increases the mechanical performance of the fibre- reinforced product.

In summary, the method of manufacturing a fibre reinforced thermoplastic product of the present invention produces a fibre reinforced thermoplastic product with improved mechanical properties as a result of various different effects. These are: (i) Reduction in the number of voids in the fibre reinforced thermoplastic product;

(ii) A more homogenous fibre distribution resulting in fewer resin-rich areas or resin pockets in the fibre reinforced thermoplastic product; and (iii) Fewer filament-filament contacts in the fibre bundles of the fibre reinforced thermoplastic product

Precisely which of these effects are observed seems to depend on the materials chosen to form the fibre reinforced thermoplastic product. In some embodiments, all effects may be observed.

However, in other embodiments, only one or two of these effects may be seen.

It is only required for one of these effects to take place for the fibre reinforced thermoplastic product to be formed with improved mechanical properties. For example, the mechanical properties of a prepreg material that already comprises fibre fully impregnated by resin can still be improved by consolidation with the method of the present invention due to a resultant more homogeneous fibre distribution throughout the material.

In the method of the present invention, preferably the first period of time t_x and/or the second period of time t₂ are at least one 1 minute. For example, the first period of time t_x and/or the second period of time t₂ may be between 1 and 60 minutes, preferably between 10 and 60 minutes, most preferably between 15 and 30 minutes. For example, t_x may be around 20 minutes and/or t₂ may be approximately 30 minutes.

Preferably, the third period of time t₃ is at least one minute. The third period of time is the time required for the temperature to cool from T₂ to a temperature below the melt temperature T_m. The exact time required will depend on the starting temperature and the rate of cooling of the thermoplastic and fibre composition. Furthermore, the rate of cooling will depend on whether an elevated temperature is applied during step (c) of the method and whether any cooling means are used to reduce the temperature of the composition. Typically, times of at least 30 minutes will be required to cool the composition. For example, the third period of time t₃ may be between 30 and 120 minutes, for example, around 100 minutes.

The method of the present invention requires the thermoplastic and fibre composition to be heated to at least the melt temperature of the thermoplastic. Preferably, the thermoplastic and fibre composition is heated to a temperature above the melt temperature. If the thermoplastic has a semi-crystalline structure, the material will have a defined melt point T_m. However, if the thermoplastic is instead amorphous, it will not have a defined melt point. Instead, amorphous thermoplastics gradually soften as the temperature rises. Therefore, for amorphous thermoplastics, the melt point T_m will actually be the softening/flow temperature (i.e. the temperature at which the thermoplastic begins to soften).

In step (c) of the method of the present invention, the thermoplastic and fibre composition is allowed to cool whilst a pressure (p₃) is still applied. The composition is allowed to cool to a temperature below the melt temperature T_m of the thermoplastic (i.e. the composition is allowed to cool to below the softening temperature for an amorphous thermoplastic) so that a solid product is formed. At the start of step (c), an elevated temperature such as temperature T₂ may be initially maintained for a period of time, for example, by applying a heat source. However, the composition will then be allowed to cool to a temperature below the melt temperature T_m. Preferably, the composition is allowed to cool to a temperature of around 70°C and the rate of cooling is approximately -3°C per minute. Thermoplastics products are usually cooled to temperatures of around 70°C as, at this temperature, the thermoplastic is cool enough to handle. The cooling rate may be between 1 and 10°C per minute, preferably between 1 and 5°C per minute. Preferably, the method of the present invention comprises a single step (a) and a single step (b) before step (c). However, in some embodiments of the invention, steps (a) and (b) are repeated one or more times before step (c) is carried out. This means that after the first relaxation step (b), the thermoplastic and fibre composition are once again heated to a first temperature Ti under a first pressure p_! for a first period of time ti. Then, in the next step, the pressure is reduced from p_! to a second pressure p₂ and the composition is heated to a second temperature T₂ under p₂ for a second period of time t₂. After this second relaxation step, the pressure can then be increased from p₂ to p₃ and the temperature is allowed to cool from T₂ to a temperature below the melt temperature T_m. In step (a) of the method of the present invention, the elevated temperature and pressure are applied to a thermoplastic and fibre composition. The wording "thermoplastic and fibre composition" describes any type of thermoplastic and fibre mixture which can be used to form a fibre-reinforced thermoplastic product. For example, the thermoplastic and fibre composition may be a woven fabric with a thermoplastic provided on at least one surface. The thermoplastic provided on a surface of the woven fabric may be in powder, pellet or sheet form, for example.

Alternatively, the fibre and thermoplastic composition may be a semipreg material (i.e. a material comprising a first a central fabric layer which is partially impregnated on one or both sides with resin), or a ply of prepreg material.

The present invention may be used to form consolidated products in the form of single-layer products or a multi-layer laminates. Preferably, the fibre reinforced product formed by the method of the present invention is a laminate of two or more layers. For example, the product may be a two ply, a four ply, an eight ply a 16 ply laminate or a 124 ply laminate. The thermoplastic and fibre composition preferably has a form that is able to form such a laminate. For example, the thermoplastic and fibre composition may be a laminate of two or more layers, and these layers may be formed from a semipreg or prepreg material.

Preferably, the pressures p_x and p₃ are between 400 kPa (4 bar) and 3000 kPa (30 bar), more preferably between 1000 kPa (10 bar) and 2000 kPa (20 bar). For example, p_x could be 2000 kPa and p₂ could be 1000 kPa. In some embodiments of the invention, the first pressure pi is greater than the third pressure p₃. Alternatively, the first and third pressures may be the same, or the third pressure p₃ may be greater than the first pressure p_x. Preferably, the second pressure p₂ is between 0 kPa and 400 kPa. More preferably, the second pressure p₂ is between 0 kPa and 150 kPa. The second pressure p₂ may be atmospheric pressure (i.e. 101.325 kPa) or the pressure may be removed entirely to give a pressure of 0 kPa.

The first, second and third pressures p_{1 ;} p₂ and p₃ may be provided by any suitable system. For example, the pressures may be provided by press molding. Examples of press molding include a plate press comprising two plates which sandwich the thermoplastic and fibre composition between them. In other embodiments, the pressures may be provided by a double belt press or by a continuous compression molding process (CCM). Alternatively, the pressure may be provided by an autoclave.

If the pressures are provided by a plate system, the decrease in pressure between p_x and p₂ may be provided by releasing the plates so that they no longer contact the fibre and thermoplastic composition. If such a system is used, the plates may be used to apply a p_x of 1000 kPa, then released so that the composition only experiences atmospheric pressure (p₂). The plate may then be brought back into contact with the composition so that they can apply a final pressure (p₃) of 1000 kPa. The plates can also be used to heat the fibre and thermoplastic composition.

Alternatively, the pressures may be provided by an autoclave. In an autoclave system the pressures may be 600 kPa for p_! and p₃, and 100 kPa for p₂. As well as providing the required pressure, the autoclave system would also heat the composition.

An autoclave set-up typically uses a vacuum bag. When a composition is positioned within this vacuum bag and no external pressure is applied, the pressure may be around -100 kPa (-1 bar). When pressure is applied, the pressure applied to the composition may be increased to 600 kPa. In step (b) of the method, the applied pressure may be reduced so that the composition is subjected to reduced pressure of around 100 kPa.

Preferably, the temperatures T_x and T₂ are between 100°C and 450°C, most preferably between 250°C and 450°C. For example, T_x and T₂ may be around 385°C. In some embodiments, T_x and T₂ may be the same (i.e. T_x = T₂). Preferably, the thermoplastic has a melt viscosity between 10² Pa.s and 10⁴ Pa.s. The viscosity of the thermoplastic may be measured using a rheometer. In embodiments of the invention the method may further comprise an additional step (a preconsolidation step) before step (a), wherein in the additional step the thermoplastic and fibre composition is heated to a temperature equal to or greater than the melt temperature T_m of the thermoplastic and a pressure lower than p_! is applied. The method of the present invention may be used to manufacture a fibre-reinforced thermoplastic material where the thermoplastic is a polyimides such as polyethylenimide (PEI), polyamides, polyether sulphones, polyaryl ether ketones (PAEK) such as polyether ether ketone (PEEK) and polyether ketone ketone (PEKK), polyurethane, polyethylene, polypropylene, polyphenylene sulphides, polysulphones such as polyphenylene sulphone (PPS), polyamide-imides, acrylonitrile butadiene styrene (ABS), styrene/maleic anhydride (SMA), polycarbonate, polyphenylene oxide blend (PPO), thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, as well as mixtures and copolymers of one or more of the above polymers.

In particular, the present invention may be used to manufacture a fibre-reinforced thermoplastic material where the thermoplastic is a polyaryl ether ketone (PAEK) such as polyether ether ketone (PEEK) or polyether ketone ketone (PEKK), polyphenylene sulphide (PPS), polyethylenimide (PEI), polyethylene, polypropylene, or a thermoplastic polyester. Preferably, the thermoplastic is a polyaryl ether ketone (PAEK) such as polyether ether ketone (PEEK) or polyether ketone ketone (PEKK).

The thermoplastic used in the method of the present invention preferably has a semi-crystalline structure.

The method of the present invention may be used to manufacture a fibre-reinforced thermoplastic material where the fibre of the fibre reinforced thermoplastic is selected from carbon, glass, basalt or aramid fibre, or natural fibres such as flax, hemp or jute. Preferably, the fibre is glass fibre or carbon fibre. Most preferably, the fibre is carbon fibre. For example, the fibre may be standard modulus carbon fibres, such as T300JB or AS4 or high modulus carbon fibres such as IM7 or T800. Preferably, the fibre is provided as a woven fabric, for example, in a 5-harness satin weave style. Alternatively, the fibre may be woven into a plain weave or a 2x2 twill, or formed into a unidirectional tape or a non-crimp fabric. According to a second aspect of the present invention there is a fibre reinforced thermoplastic product produced by the method of the first aspect of the present invention, wherein the fibre reinforced thermoplastic has a zero void content, and/or a substantially homogenous fibre distribution throughout the product, and/or a reduced number of filament-filament contacts in the fibre bundles of product compared to conventional products.

Brief description of the figures

The accompanying figures are used to illustrate a non-limiting exemplary embodiments of the present invention. The above and other advantages of the features and objects of the invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

Figure 1 is a graph illustrating the different stages of a standard method used when forming a fibre reinforced product;

Figure 2 is a graph illustrating the different stages of a method of the present invention used when forming a fibre reinforced product;

Figure 3a is a schematic drawing of a cross-section of the fibre reinforced product at the end of the preconsolidation step of the methods of Figures 1 and 2;

Figure 3b is a micrograph of a cross-section of the fibre reinforced product at the end of the preconsolidation step of the methods of Figures 1 and 2;

Figure 3c is another micrograph of a cross-section of the fibre reinforced product at the end of the preconsolidation step of the methods of Figures 1 and 2;

Figure 4a is a schematic drawing of a cross-section of the fibre reinforced product at the end of a consolidation step of the methods of Figures 1 and 2;

Figure 4b is a micrograph of a cross-section of the fibre reinforced product at the end of a consolidation step of the methods of Figures 1 and 2;

Figure 4c is another micrograph of a cross-section of the fibre reinforced product at the end of a consolidation step of the methods of Figures 1 and 2;

Figure 5a is a schematic drawing of a cross-section of the fibre reinforced product at the end of the second consolidation step of the method of Figure 2; Figure 5b is a micrograph of a cross-section of the fibre reinforced product at the end of the second consolidation step of the method of Figure 2;

Figure 5c is another micrograph of a cross-section of the fibre reinforced product at the end of the second consolidation step of the methods of Figure 2;

Description of embodiments

Figures 1 and 2 are graphs showing the various stages of a method used to produce a fibre reinforced polyether ether ketone (PEEK) material. The temperature and the pressure are given on the y-axis and the time is given on the x-axis.

The consolidation cycle methods illustrated by Figures 1 and 2 are for a fibre reinforced material has been formed from the thermoplastic polyether ether ketone (PEEK). In this fibre reinforced material, the PEEK could be reinforced by carbon fibres woven into a 5-harness satin weave style. In addition, the fibre reinforced material could be an 8 ply laminate with a lay-up of [(0,90)]4s, or a 6-ply laminate with a lay-up of [(0,90)] 3s, for example.

There are two types of plots on the graphs of Figures 1 and 2. The first type of plot 1 shows the temperature of the laminate PEEK material. The second type of plot 2 illustrates the pressure change.

The standard method of Figure 1 is known in the art. This method (consolidation process) has three distinct stages: a preconsolidation stage 11, a consolidation stage 12 and a cooling stage 13. In the preconsolidation stage 11 of Figure 1 , the system is heated to a temperature of 385°C and a pressure of 200 kPa (2 bar) is applied for 30 minutes. The system is heated to a temperature above the melt temperature T_m of the thermoplastic. PEEK has a melting temperature of around 343°C. After 30 minutes, the pressure is increased to 1000 kPa (10 bar) for the consolidation stage 12. In the consolidation stage 12, the temperature of 385°C is maintained and the pressure of 1000 kPa (10 bar) is applied for 20 minutes. After the consolidation stage 12 is complete, the system then enters the cooling stage 13. In the cooling stage 13, the pressure of 1000 kPa (10 bar) is maintained for around a further 120 minutes. However, the heat applied to the system is reduced allowing the fibre reinforced PEEK product to cool to 70°C, a temperature below the melt temperature of the PEEK. Figure 2 illustrates an embodiment of the method of forming a fibre-reinforced thermoplastic product of the present invention.

Unlike the standard method of Figure 1 , the method (consolidation cycle) of the embodiment of the present invention illustrated by Figure 2 comprises four stages: a preconsolidation stage 21, a first consolidation stage 22, a second consolidation stage 23 and a cooling stage 24.

In the preconsolidation stage 21 of Figure 2, the system is heated to a temperature of 385°C and a pressure of 200 kPa (2 bar) is applied for 30 minutes. As with Figure 1 , the system is heated to a temperature above the melt temperature T_m of the thermoplastic. After 30 minutes, the pressure is increased to 2000 kPa (20 bar) for the first consolidation stage 22.

In the first consolidation stage 22, the temperature of 385°C (T_x) is maintained and a pressure (pi) of 2000 kPa (20 bar) is applied for 20 minutes (tj). After the first consolidation stage 22, the system then enters a second consolidation stage 23.

In the second consolidation stage 23, the temperature of 385°C (T₂) is maintained. However, the pressure (p₂) is reduced to 150 kPa (1.5 bar). The temperature of 385°C (T₂) and the pressure (p₂) of 150 kPa are applied for 30 minutes (t₂). After this second consolidation stage 23 is complete, the system then enters the cooling stage 24.

In the cooling stage 24, the pressure (p₃) is reduced to 1000 kPa (10 bar). This pressure is then applied for around 120 minutes (t₃). In addition, the heat applied to the system is reduced allowing the fibre reinforced PEEK product to cool to 70°C, a temperature below the melt temperature T_m of the thermoplastic.

Figures 3a, 3b and 3c show cross sections of the fibre reinforced PEEK product at the end of the preconsolidation step 11, 21 of the consolidation cycles illustrated by Figures 1 and 2.

As can be most clearly seen from the schematic drawing of Figure 3a, after the preconsolidation step 11, 21, not all of the fibres 33 are surrounded by the resin 32. Instead, the resin 32 surrounds some of the outer fibres 33a but not the more central fibres 33b. This is because the resin 32 has not penetrated the centre of the product. There are, therefore, one or more large resin voids 34 (i.e. areas without any resin 32) which contain many of the central fibres 33b. This arrangement is also shown in the micrographs of Figures 3b and 3c.

Figures 4a is schematic drawing illustrating the fibre distribution of the fibre reinforced PEEK product at the end of either the single consolidation step 12 of the method of Figure 1 or the first consolidation step 22 of the method of Figure 2. In Figure 4a, the resin 32 has penetrated further into the reinforced fibre product. Both the outer fibres 33a and the inner fibres 33b are, therefore, at least partially surrounded by the resin 32. However, the product now contains a plurality of smaller resin voids 35 (i.e. areas without any resin 32). In the standard consolidation cycle of Figure 1, the final product has the structure of the product formed after the single consolidation cycle.

This configuration of Figure 4a is also shown in the micrographs of Figures 4b and 4c. In Figure 4c, the product formed had so micro-voids these voids acted as a crack initiator and a micro-crack 36 has been formed in the product.

Figure 5 a is a schematic drawing illustrating the fibre distribution of the fibre reinforced PEEK product at the end of the second consolidation step of the method of Figure 2. As is clearly shown in Figure 5a and the micrographs of Figures 5b and 5c, the resin 32 has now fully penetrated the product so that the fibres 33 are completely surrounded by the resin. There are no visible voids in the fibre reinforced PEEK.

Figure 5b shows a cross section through a single layer whereas Figure 5c shows a cross section through two layers 40, 41, where each layer comprises one ply of fabric with fibres in the 0° and 90° direction. In Figure 5c, the fibre orientation differs between adjacent layers.

Tests were performed on fibre reinforced PEEK products produced by the consolidation cycle methods of Figures 1 and 2. The percentage increase in the mechanical strength of the products formed by the consolidation process of the present invention is shown in Table 1 below. The consolidation cycles of Figures 1 and 2 were used to produce fibre reinforced PEEK products. In these products, the thermoplastic is reinforced by carbon fibres woven into a 5-harness satin weave style.

Table 1

The shear strength and the shear modulus of the fibre reinforced products were measured using the ASTM test D3518. This standard test method determines the in-plane shear response of polymer matrix composite materials. Using this test method, the maximum in-plane shear strength and shear stress for a 45° product was determined.

As can be seen from Table 1 , the shear strength of the product of the consolidation cycle of the present invention is 20% greater than the strength of the standard product. In addition, the shear modulus of the product of the consolidation cycle of the present invention is 7% higher than the shear modulus of the product of the standard cycle.

The compression strength and modulus were measured using the standard ASTM test D6641 (also known as EN2850). This test method determines the maximum compressive strength and modulus of polymer matrix composite materials. For tests at 90°, the compressive strength is 12% higher for products of the consolidation cycle of the present invention than products of the standard cycle.

The flex strength and modulus were measured using the standard test method EN2562A. This standard specifies the method for the determination of the maximum flexural strength and flexural modulus of fibre reinforced plastics. As can be seen from Table 1, the flexural strength is 21/22% larger for the fibre reinforced product of the consolidation cycle of the present invention. Overall, the use of the consolidation cycle of the present invention has greatly increased the strength of the fibre reinforced plastic composite. The modulus has also increased but to a smaller extent than the strength.

Claims

1. A method of manufacturing a fibre reinforced thermoplastic product, the method comprising:

(a) heating a thermoplastic and fibre composition to a first temperature Ί\ under a first pressure p_! for a first period of time ti ;

(b) reducing the pressure applied from p_! to a second pressure p₂ and heating the composition to a second temperature T₂ under p₂ for a second period of time t₂; and

(c) increasing the pressure from p₂ to a third pressure p₃ and applying p₃ for a third period of time t₃;

wherein:

the temperatures Ί\ and T₂ are equal to or greater than the melt temperature T_m of the thermoplastic; and

the temperature cools from T₂ to a temperature below the melt temperature T_m of the thermoplastic in t₃.

2. The method of Claim 1 , wherein the first period of time t_x and/or the second period of time t₂ are at least 1 minute.

3. The method of any preceding claim, wherein the first period of time t_x and/or the second period of time t₂ is between 1 and 60 minutes, preferably between 15 and 30 minutes.

4. The method of any preceding claim, wherein the third period of time t₃ is at least 1 minute.

5. The method of any preceding claim, wherein the pressures p_! and p₃ are between 500 kPa and 3000 kPa.

6. The method of Claim 5, wherein p_! and p₃ are between 1000 kPa and 2000 kPa.

7. The method of any preceding claim, wherein the second pressure p₂ is between 0 kPa and 400 kPa.

8. The method of Claim 7, wherein p₂ is between 0 kPa and 150 kPa.

9. The method of any preceding claim, wherein the temperatures T_x and T₂ are between 100°C and 450°C, preferably between 250°C and 450°C.

10. The method of any preceding claim, wherein the temperatures Ti and T₂ are the same.

11. The method of any preceding claim, wherein the thermoplastic and fibre composition is a laminate of two or more layers.

12. The method of Claim 11, wherein the layers are a semipreg or prepreg material.

13. The method of any of Claims 1 to 10 wherein the thermoplastic and fibre composition is a single ply of semipreg or prepreg material.

14. The method of any preceding claim, wherein the thermoplastic has a melt viscosity between 10² Pa.s and 10⁴ Pa.s.

15. The method of any preceding claim, wherein the method further comprises an additional step before step (a), wherein in the additional step the thermoplastic and fibre composition is heated to a temperature equal to or greater than the melt temperature T_m of the thermoplastic and a pressure lower than p_x is applied.

16. The method of any preceding claim, wherein the first pressure p_{1 ;} the second pressure p₂ and the third pressure p₃ are applied using a plate press, a double belt press, a continuous compression moulding machine or an autoclave.