COMPOSITE PARTS AND PROCESSES OF MANUFACTURE

Abstract

The disclosure relates to polymer-containing composite materials and to unusually low-pressure consolidation methods for forming composite parts from such materials. More specifically, for example, the disclosure relates to use of a certain “PEEK-PEDEK” copolymer in low-pressure consolidation methods, using as little as 1 bar pressure (or using atmospheric pressure acting on a consolidation that is held under vacuum) to provide composite parts that are substantially void-free.

Claims

1. A process for forming a composite part, the process comprising: selecting a composite material comprising a polymeric material having a repeat unit of formula (I):
—O-Ph-O-Ph-CO-Ph-  (I) and a repeat unit of formula (II):
—O-Ph-Ph-O-Ph-CO-Ph-  (II) wherein Ph represents a phenylene moiety and wherein the shear viscosity, SV, of the polymeric material, measured by a Standard method as defined in ISO11443:2014, is in the range from around 100 Pa.Math.s to around 200 Pa.Math.s, at 400° C., at a shear rate of 1000 s.sup.−1; and unidirectional fibres, wherein the unidirectional fibres are submerged in a dispersion of particles of the polymeric material to form a pre-impregnated tape; then arranging the pre-impregnated tape into a laminate comprising a plurality of layers of pre-impregnated tape to form a kitted stack; and heating the kitted stack at a consolidation temperature of at least 305° C. for at least 15 minutes at a consolidation pressure of less than 10 bar.

2. A process according to claim 1, further comprising heating the kitted stack at a consolidation temperature of at least 340° C.

3. A process according to claim 1, wherein the consolidation pressure is between atmospheric pressure and 8 bar.

4. A process according to claim 1, wherein the kitted stack is held under vacuum within a flexible membrane allowing atmospheric pressure to consolidate the stack.

5. A process according to claim 1, wherein the process further comprises arranging the pre-impregnated tape using hand lay-up.

6. A process according to claim 1, wherein the process further comprises arranging the pre-impregnated tape using automated fibre placement.

7. A process according to claim 6, wherein the process further comprises in-situ consolidation of the part.

8. A process according to claim 1, wherein the plurality of layers of pre-impregnated tape is arranged such that the direction of the pre-impregnated tape in a subsequent layer is positioned at 45 degrees to the direction of the pre-impregnated tape of a preceding layer.

9. A process according to claim 1, wherein in the arranging step, a subsequent layer of pre-impregnated tape is spot-welded to a preceding layer.

10. A process according to claim 1, wherein the heating step is carried out using a hot iron to tack the layers together.

11. A process according to claim 1, wherein the process further comprises arranging at least one functional layer within the kitted stack, wherein the at least one functional layer is selected from the following: an electrical isolating layer, an aesthetic layer such as a decorative layer, and a UV-protective layer.

12. A composite part, the composite part comprising a polymeric material having a repeat unit of formula (I):
—O-Ph-O-Ph-CO-Ph-  (I) and a repeat unit of formula (II):
—O-Ph-Ph-O-Ph-CO-Ph-  (II) wherein Ph represents a phenylene moiety and wherein the shear viscosity, SV, of the polymeric material, measured by a Standard method as defined in ISO11443:2014, is in the range from around 100 Pa.Math.s to around 200 Pa.Math.s, at 400° C., at a shear rate of 1000 s.sup.−1; and unidirectional fibres, wherein the composite part is formed using the process defined in claim 1.

13. (canceled)

14. A process according to claim 3, wherein the consolidation pressure is between atmospheric pressure and less than 5 bar.

15. A process according to claim 3, wherein the consolidation pressure is between atmospheric pressure and less than 3 bar.

16. A process according to claim 3, wherein the consolidation pressure is around 1 bar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0100] Embodiments of the invention are further described by reference to the accompanying figures, in which:

[0101] FIG. 1 shows a first run of Differential Scanning calorimetry (DSC) of the raw polymer;

[0102] FIG. 2 shows the resulting DSC cooling curve of the raw polymer;

[0103] FIG. 3 shows a second run of DSC of the raw polymer;

[0104] FIG. 4 shows a press cycle for tapes laid by automated fibre placement (AFP);

[0105] FIG. 5 shows a two-dimensional presentation of data from an AFP laid laminate consolidated in a press.

[0106] FIG. 6 shows a two-dimensional presentation of data from an oven consolidated laminate;

[0107] FIG. 7 shows a two-dimensional presentation of data from press consolidated laminate;

[0108] FIG. 8 (a to c) shows polished optical micro-sections of laminates consolidated out of autoclave where a) corresponds to a unidirectional laminate, b) corresponds to a ±45 laminate and c) corresponds to a quasi-isotropic laminate;

[0109] FIG. 9 shows an example DSC thermograph for oven processed laminate; and

[0110] FIG. 10 shows an example DSC thermograph for platen pressed laminate.

DETAILED DESCRIPTION

[0111] The polymer described herein is a copolymer based on polyketone chemistry which falls into the general class of polymers called polyaryletherketones (PAEKs). In the world of chemistry, this terminology covers a wide range of polymers comprising aromatic moieties connected by ketone and ether linkages at various ether to ketone ratios and with defined sequencing. The PAEK copolymer described herein is PEEK/PEDEK copolymer, having a melting temperature of T.sub.m 305° C., and an SV 130 Pa.Math.s (measured by a Standard method as defined in ISO11443:2014, at 400° C., at a shear rate of 1000 s.sup.−1). This particular PAEK copolymer, has a lower crystalline melting temperature than PEEK, or (polyetherketoneketone) PEKK, whilst substantially retaining mechanical, physical and chemical resistance properties typical of PEEK. The mechanical properties the PEEK:PEDEK polymer are shown in Table 1.

TABLE-US-00001 TABLE 1 Mechanical properties of PEEK:PEDEK copolymer. Property description Test Units Value Tensile strength Yield, 23° C. ISO 527 MPa 90 Tensile elongation Break, 23° C. ISO 527 % 15 Tensile modulus 23° C. ISO 527 GPa 3.5 Flexural strength 23° C. ISO 178 MPa 150 Flexural modulus 23° C. ISO 178 GPa 3.3 Izod impact strength Notched, 23° C. ISO 180/A kJm.sup.−2 5.0

[0112] Further details of the PEEK:PEDEK copolymer used herein are described in WO2014207458 A1 which is hereby incorporated by reference.

[0113] The copolymer is semi-crystalline (25-30% typically) at cooling rates consistent with oven or press consolidation (5-10° C./minute). The melting temperature is around 305° C. as measured using differential scanning calorimetry (DSC).

[0114] The Glass Transition Temperature (T.sub.g), the Cold Crystallisation Temperature (T.sub.n), the Melting Temperature (T.sub.m) and the recrystallization temperature for the polymer were determined using the four-step DSC method described hereinbefore on page 7.

[0115] DSC thermographs for amorphous pressed films made from raw polymer (no carbon fibres) are shown in FIGS. 1 to 3 after the first temperature cycle (a), the cooling phase (b) and the second cycle (c) respectively. These confirm key thermal transitions over the temperature range with a cold crystallisation peak at 189° C., crystalline melting point in this example of 303° C. and recrystallization peak at 271° C. The degree of crystallisation upon cooling reached 28% and the glass transition temperature on the second cycle was 148° C.

[0116] Composite prepreg tapes were prepared comprising unidirectional carbon fibres (Hexcel AS4 and AS7, supplied by Hexcel Composites) impregnated with PEEK:PEDEK copolymer. Tapes for AFP were prepared by slitting to 6.35 mm±0.125 mm. Tapes for hand layup were prepared at a width of 50 mm±0.125 mm. The details are for these tapes are shown in Table 2.

[0117] Composite prepreg tapes are formed by submerging the fibres in a dispersion of particles of polymeric materials as described in ‘Thermoplastic Aromatic Polymer Composites: A Study of the Structure, Processing and Properties of Carbon Fibre Reinforced Polyetheretherketone and Related Materials’ by Cogswell, 1992.

TABLE-US-00002 TABLE 2 Prepreg tapes for AFP and hand layup Fibre Area Weight Prepreg Process (FAW tape (layup and Prepreg Fibre g/m.sup.2) width (mm) consolidation) PEEK:PEDEK/CF AS4 134 6.35 ± 0.125 AFP/Press PEEK:PEDEK/CF AS7 192 50.0 ± 0.125 Hand layup/Press PEEK:PEDEK/CF AS7 192 50.0 ± 0.125 Hand layup/Oven

[0118] The composite laminates were formed using different processing techniques.

[0119] Automated Fibre Placement (AFP)

[0120] Tapes with a fibre area weight (FAW) of 134 gsm (Hexcel™ AS4 carbon fibre) nominally 6.35 mm wide (within the width tolerance) were laid by a Coriolis™ AFP machine (CORIOLIS COMPOSITES TECHNOLOGIES S.A.S, France) to create partially consolidated laminates with fibre orientations and laminate thicknesses suitable for mechanical testing. These were fully consolidated by pressing between steel caul plates and polyimide film treated with Frekote™ 55 mould release in a computer controlled Lauffer hydraulic press (Maschinenfabrik Lauffer GmbH & Co. KG, Germany) fitted with electrical platen heating and air/water cooling. The press was programmed with the requisite temperature and pressure sequence, as described below.

[0121] Hand Layup

[0122] Panels made from hand laid tapes were prepared as follows. Strips of prepreg tape (50 mm wide with AS7 Hexcel carbon fibres at a FAW of 192 g/m.sup.2) were cut and stacked by hand using a hot iron operating above T.sub.m, to tack weld and/or seam weld the tapes together. Welding was done at multiple points along and across the tape to form kitted ply lay-ups with ply stacking sequences and orientations that met the requirements for mechanical testing as detailed in Table 3.

[0123] In-Situ Layup

[0124] Panels made using in-situ layer were also prepared. In-situ layup comprises arranging the PEEK:PEDEK composite tape in a plurality of layers and heating the arranged composite tape immediately after laying to consolidate the composite tape to the preceding layer or surface. Typically, in-situ layup comprises the use of a laser to heat the composite tape. Other heat sources are available. The localised heating from the laser causes the polymeric material in the composite tape to melt and thereby consolidate with the previous layer.

TABLE-US-00003 TABLE 3 Layup sequence for hand-laid 50 mm wide prepreg tapes Number Property Lay-up of Plies In plane shear strength and modulus [+45/−45].sub.4S 16 Compression after impact (CAI) [+45/0/−45/90].sub.3S 24 Mode I (G1c) Fracture Toughness [0].sub.18 18 Plain tensile strength and modulus [+45/0/−45/90].sub.3S 24 Filled hole tensile strength (FHT) [+45/0/−45/90].sub.3S 24 Plain compression strength and modulus [+45/0/−45/90].sub.3S 24 Open Hole Compression (OHC) [+45/0/−45/90].sub.3S 24 Filled Hole Compression (FHC) [+45/0/−45/90].sub.3S 24 Bearing strength [+45/0/−45/90].sub.3S 24

[0125] These kitted stacks were then processed either by compression moulding in the Lauffer platen press, or by oven processing using only vacuum and temperature to achieve consolidation. In practice an autoclave was used for this purpose [Thermoplastic Composite Research Centre (TPRC) in Enchede, Holland] although without any applied pressure the autoclave was used only as an oven to heat the prepreg. This process is referred to here as Out-of-Autoclave (OoA) processing, in common with industry standard nomenclature. Kitted prepreg stacks were positioned on a steel plate and covered with a breather layer (glass cloth) separated with Frekote 55 treated polyimide film. The whole stack and associated layers of breather and film layers were sealed within a covering layer of polyimide film, which was bonded to the base plate using a high temperature resistant sealing adhesive around the perimeter.

[0126] Process Thermal Cycles

[0127] The process cycles used in the preparation of test laminates are shown in Tables 4, 5 and 6. Table 4 illustrates the press cycle used to consolidate the prepreg material laid by AFP which is also illustrated for clarity in FIG. 4.

TABLE-US-00004 TABLE 4 Press consolidation cycle for AFP laid panels Press Hydraulic Platen Heating Electrical Cooling Water/Air mix Heat up rate 7° C./minute Hold temperature 350° C. Hold time at temp. 15 minutes Hold pressure Two step: ramp 2 Bar then 6 Bar at max. temp. Cool down rate −4.5° C./minute

TABLE-US-00005 TABLE 5 Press consolidation cycle for hand laid panels Press Cycle for hand laid panels Press Hydraulic Platen Heating Electrical Cooling Water/Air mix Heat up rate 7° C./minute Hold temperature 360° C. Hold time at temp. 30 minutes Hold pressure 1 bar Cool down rate −5° C./minute

TABLE-US-00006 TABLE 6 Oven consolidation cycle (using autoclave without added pressure) Oven Cycle for hand laid panels Heating Electrical Cooling Air Heat up rate 7° C./minute Hold temperature 360° C. Hold time at temp. 30 minutes Hold pressure 1 bar Cool down rate −6° C./minute

[0128] Post Process Analyses

[0129] Physical and mechanical testing of all laminates was undertaken by GMA-WERKSTOFFPRÜFUNG GmbH (Stade, Germany). Laminates were ultrasonically C-scanned with an Omniscan MX unit using water coupling and then sectioned and imaged by optical microscopy to assess consolidation quality. Samples were analysed for fibre content, matrix content and porosity content by acid digestion. Density was measured and the thermal characteristics were recorded by DSC to ensure that the laminates met expectations.

[0130] Mechanical Testing—Hand Laid Laminates

[0131] Specimens were removed from both oven and press consolidated laminates using a diamond saw and in accordance with the relevant test methods. These were tested in triplicate under room temperature/dry and in some cases elevated temperature wet conditions, as summarised in Table 7.

TABLE-US-00007 TABLE 7 Mechanical test matrix Specimen Dry Wet Properties Geometry (R.T.) (70° C.) In plane shear strength and 230 mm × 25 mm 3 modulus Compression after impact (CAI)  150 mm × 100 mm 3 35J Mode I (G.sub.1c) Fracture 250 mm × 25 mm 3 Toughness Plain tensile strength and 340 mm × 32 mm 3 modulus Filled hole tensile strength 280 mm × 32 mm 3 3 (FHT) Plain compression strength 162 mm × 32 mm 3 3 and modulus Open Hole Compression (OHC) 162 mm × 32 mm 3 Filled Hole Compression (FHC) 162 mm × 32 mm 3 3 Bearing strength 150 mm × 45 mm 3

[0132] Results

[0133] Consolidation

[0134] AFP panel

[0135] An example C-scan of an AFP laid/press consolidated laminate is illustrated in FIG. 5. This shows that the laminate was fully consolidated without any areas of delamination and is typical of all laminates made by this process during this work.

[0136] Density measurements according to ASTM D 3171 method 1B-15 did not detect any porosity in the consolidated laminates.

[0137] FIGS. 6 and 7 illustrate a typical C-scan result for laminates made by oven consolidation (FIG. 6) and press consolidation (FIG. 7). PEEK:PEDEK composite tape processed well and was fully consolidated in each case with no delamination evident.

[0138] For the oven and press consolidated laminates made using hand layup, sections of laminate removed, polished and imaged by optical microscopy confirmed that full consolidation had occurred in all cases. Example micro-cuts for the oven processed laminates are shown in FIG. 8 a to c.

[0139] Differential Scanning Calorimetry (DSC)

[0140] Thermographic analysis of the consolidated laminates by DSC (Universal V4.5A TA Instruments) confirmed that in all cases (AFP laid/pressed and hand laid OoA and pressed laminates) the material was fully crystallised after processing. This was evidenced by the absence of a ‘cold’ crystallisation peak (at 189° C. in amorphous polymer) for any of the laminates produced. FIGS. 9 and 10 are example DSC thermographs for hand laid oven processed and pressed laminates respectively. By calculation, using a heat of crystallisation of 130 J/g, the area under the melt peak in each case gave crystallinity levels of between 25-28% for all laminates.

[0141] Mechanical Properties Test Results—Hand Laid Laminates Only

[0142] As this is a comparison between oven consolidated and press consolidated laminates only their relative performance is shown in Table 8. Here the press consolidated laminates represent the baseline level of performance with oven consolidated laminates being compared with the baseline values on a percentage basis.

TABLE-US-00008 TABLE 8 Comparative performance of oven-consolidated vs press-consolidated (baseline) laminates. Oven Consolidated Compared with Press Consolidated (% Retained Properties) Properties R.T. 70° C. Wet In plane shear strength Strength  95% — and modulus Modulus 104% — Compression after impact (CAI) 103% Mode I (G.sub.1c) Fracture Toughness  83% Plain tensile strength Strength 100% — and modulus Modulus 104% — Filled hole tensile strength (FHT) 103% — Plain compression strength Strength  98% 101% and modulus Modulus 105% 103% Open Hole Compression (OHC) 102% — Filled Hole Compression (FHC) 103%  93% Bearing strength 102% —

[0143] The C-scan image presented in FIG. 4 confirms that PEEK:PEDEK composite tapes can be laid by AFP and pressed to create fully consolidated laminates. Density measurements (ASTM D 3171 method 1B-15) confirm that laminates are fully dense, with no measurable void content.

[0144] Also this work has shown that PEEK:PEDEK composite tape can be fully consolidated by low pressure out of autoclave (OoA) processing, or with a low pressure platen press, using as little as 1 bar, as illustrated in sectional micrographs exemplified in FIGS. 6a to c, creating fully dense composites with minimal voids (below detection). Laminate compaction quality has been confirmed by C-scanning, as shown in FIGS. 6 and 7.

[0145] With its lower melting temperature compared with PEEK (305° C. vs 343° C.) PEEK:PEDEK unidirectional tape offers a wider processing window. DSC results (FIGS. 9 and 10) show that cooling at moderate rates, as with press and oven cycles (i.e. −5° C./minute) allows full crystallinity to be achieved at levels of about 28% which imparts maximum mechanical properties, heat and chemical resistance.

[0146] The mechanical properties of composite laminates made by either process are very comparable as shown in Table 8, illustrating that the material can be processed by either route. The low pressure processing ability of PEEK:PEDEK composite prepreg tape is a major advantage for processors and opens the gates to the production of high quality parts utilising out of autoclave processing.

[0147] Preformed material can be pressed by hot stamping processes to produce parts rapidly, meeting the demands for higher throughput rates required to meet aircraft build targets discussed in the introduction.

[0148] The conclusion from this work is that PEEK:PEDEK copolymer processes equally well under oven consolidation, press consolidation and in-situ consolidation, using hand layup or automated fibre placement to create highly consolidated laminates with substantially identical physical and mechanical properties, opening a broader range of manufacturing options for aerospace parts. The lower melting temperature of PEEK:PEDEK copolymer widens the processing window whilst still allowing fully crystalline morphology to develop through the cooling phase.

[0149] It has also been found that in-situ consolidation of PEEK:PEDEK composite tape provides additional benefits including parts having very low porosity levels while also exhibiting improved speed of manufacture due to increasing layup speeds. Further benefits include the reduction in the cost of manufacture of composite parts as the energy consumption of consolidation is reduced while still providing composite parts having low levels of porosity.

[0150] The present invention may also be applied to the use of comingled fibres formed from PEEK:PEDEK copolymer fibre and carbon fibre, that s comingled to form a PEEK:PEDEK and carbon fibre composite that is then processed according to the present invention.

[0151] Furthermore, additional benefits of the invention include a reduction in the energy required to make composite parts according to the process creating up to a 30% energy saving during oven consolidation over composite parts made using other PAEK tapes. A reduction in the processing temperature during tape placement was also achievable, and when using an auto fibre placement method, improvements in lay up speed may be realised, resulting in a higher throughput of composite parts. Irrespective of the method used to lay the tapes, a reduction in consolidation time was obtained compare with tapes made from other PAEKs.

[0152] Surprisingly, composite parts made according to the process exhibit a reduction in the porosity of the composite part indicating that the composite parts consolidate well without any voids or delamination. This surprising result was obtained despite the reduction in energy required by the process and despite an increase in speed of the process.