A PROCESS FOR PRODUCING A COMPOSITE ARTICLE

Abstract

Process for producing a composite material comprising depositing a prepreg containing a radiation initiated curing agent onto a mould using automated apparatus and applying heat and second source radiation to at least partially cure the prepreg at least simultaneously with the deposition of the prepreg. Also epoxy resin formulations of a mixture of a liquid epoxy resin and a solid or semi-solid epoxy resin containing a photoinitiator are used as the matrix in prepregs which are cured or partially cured by radiation to avoid the need for thermal cure in an oven. The formulation is particularly useful in the production of wind turbine blades especially in an automated process. Additionally an automated tape laying apparatus comprising compaction device, a heat source and a second radiation source.

Claims

1. A process for producing a composite article comprising the steps of: a) depositing a prepreg containing a radiation initiated curing agent onto a mould; and b) applying heat and second source radiation to at least partially cure the prepreg at least simultaneously with deposition of the prepreg.

2. A process according to claim 1 wherein said radiation initiated curing agent comprises at least two initiators configured to be initiated at different frequencies of radiation.

3. A process according to claim 1 wherein said second source radiation is ultra violet radiation.

4. A process according to claim 1 wherein heat is applied using infra-red lamps or xenon bulbs.

5. A process according to claim 1 wherein pressure is applied to the prepreg during deposition.

6. A process according to claim 1 wherein the composite article is partially cured and then demoulded.

7. A process according claim 6 comprising the additional step of completing cure of the demoulded composite article in an oven.

8. A process according to claim 1 for the production of wind turbine blades.

9. A prepreg capable of at least partial cure without the use of an oven, comprising a resin and a fibrous material impregnated with the resin, the resin comprising a mixture of a liquid epoxy resin, a solid epoxy resin and at least one radical photoinitiator and a radical (photo) cure accelerator.

10. A prepreg according to claim 9 wherein the resin further comprises an encapsulated amine curative.

11. A prepreg according to claim 9 in which the fibrous material comprises carbon fibre, glass fibre or aramid fibre.

12. (canceled)

13. A prepreg according to claim 9 in which the liquid epoxy resin has an epoxy equivalent weight in the range of from 50 to 500.

14. (canceled)

15. (canceled)

16. A prepreg according to claim 9 in which the photoinitiator is selected from the group consisting of alkyl sulphonium salts, alkyl iodonium salts, sulphonium salts comprising fluorophosphates and fluoroanitmonate.

17. A prepreg according to claim 9 in which the photoinitiator is present in an amount from 0.25 to 10 wt % based on the weight of the resin.

18. A prepreg according to claim 9 in which the resin contains a thermally activatable curing agent such as a polycyandiamide, a urea based curing agent or an imidazole.

19. (canceled)

20. An automated tape laying apparatus for automated deposition and simultaneous partial cure of prepreg, the apparatus comprising a compaction device, a heat source, and a second radiation source.

21. An automated tape laying apparatus according to claim 20 wherein the second radiation source is an ultra violet source.

22. An automated tape laying apparatus according to claim 21 wherein the ultra violet source comprises an array of sources configured to provide ultraviolet radiation at two or more target frequencies.

23. An automated tape laying apparatus according to claim 20 wherein the compaction device comprises a heated shoe.

24. (canceled)

25. (canceled)

26. (canceled)

Description

[0056] The process of the invention is highly tailorable, with many possible parameters that can be modified giving enhanced automated control over properties, product shape, and fibre orientation. Parameters including shoe temperature, shoe pressure, compaction roller temperature, distance of UV lamp from the prepreg, speed of robot movement, as well as a number of additional passes of UV light allowing for final and full cure can be varied according to the nature of the prepreg and the nature of the finished article to be produced.

[0057] If the article is to have a final thermal cure, the epoxy resin composition may also comprise one or conventional non-radiation activated curing agents. These can be selected from aliphatic or aromatic amines or their respective adducts, amidoamines, polyamides, cycloaliphatic amines, anhydrides, polycarboxylic polyesters, isocyanates, phenol-based resins (e.g., phenol or cresol novolak resins, copolymers such as those of phenol terpene, polyvinyl phenol, or bisphenol-A formaldehyde copolymers, bishydroxyphenyl alkanes or the like), dihydrazides, sulfonamides, sulfones such as diamino diphenyl sulfone, anhydrides, mercaptans, imidazoles, ureas, tertiary amines, BF3 complexes or mixtures thereof. Particular preferred curing agents include modified and unmodified polyamines or polyamides such as triethylenetetramine, diethylenetriamine tetraethylenepentamine, cyanoguanidine, dicyandiamides and the like. Particularly preferred curing agents are those that are encapsulated so as to prevent them from poisoning the cationic cure from the radiation initiated curing agent, one such example of a particularly suited encapsulated curing agent is TEP (1,1,2,2-Tetrakis(p-hydroxyphenyl)ethane).

[0058] It is preferred to use from 0.5 to 10 wt % based on the weight of the epoxy resin of a curing agent, more preferably 1 to 8 wt %, more preferably 2 to 8 wt %, more preferably 0.5 to 5 wt %, more preferably 0.5 to 4 wt % inclusive, or most preferably 1.3 to 4 wt % inclusive.

[0059] When used the urea curing agent may comprise a bis urea curing agent, such as 2,4 toluene bis dimethyl urea or 2,6 toluene bis dimethyl urea and/or combinations of the aforesaid curing agents. Urea based curing agents may also be referred to as “urones”.

[0060] Preferred urea based materials are the range of materials available under the commercial name DYHARD® the trademark of Alzchem, urea derivatives, which include bis ureas such as UR500 and UR505.

[0061] When used the thermally activated curing agent should preferably have an onset temperature in the range of from 115 to 125° C., and/or a peak temperature in the range of from 140 to 150° C., and an enthalpy in the range of from 80 to 120 J/g (Tonset, Tpeak and. Onset temperature is defined as the temperature at which curing of the resin occurs during the differential scanning calorimetry (DSC) scan, whilst peak temperature is the peak temperature during curing of the resin during a (DSC) scan. Typically these are measured by DSC in accordance with ISO 11357, over temperatures of from −40 to 270° C. at 10° C./min).

[0062] In an embodiment of the present invention, the heat source and second radiation source may be provided by the same apparatus. In alternative embodiments both the heat and the second source of radiation may be provided as continuous or pulsed radiation.

[0063] The heat or radiation source according to embodiments of the present invention can employ a pulsed electromagnetic radiation source (or simply a ‘pulsed radiation source’). As will be described, some embodiments of the present invention employ a Xenon flashlamp of generally known kind, which can emit a relatively broadband radiation spectrum including one or more of IR, visible light and ultra-violet (UV) radiation components. Unless otherwise indicated, the terms ‘flash’ and ‘pulse’ will be used interchangeably herein at least in respect of flashlamp embodiments. In general terms, however, any other suitable pulsed or non-pulsed radiation source may be employed according to alternative embodiments of the invention. For example, according to some embodiments, a pulsed laser source may be employed.

[0064] As used herein, a flashlamp is a type of electric arc lamp designed to provide short pulses (or flashes) of high energy, incoherent radiation with a relatively wide spectral content. Flashlamps have been used in photographic applications, as well as in a number of scientific, industrial and medical applications. The use of a pulsed radiation system, rather than a continuous heating system, opens up a number of new options for controlling heating temperature, as will be described herein. For the heating of contact surfaces herein, the process may be optimised by adjusting one or more of a number of system parameters, including but not limited to: the number of pulses, pulse width (or flash duration), pulse intensity and pulse frequency. As will be described, shaped or 3D reflectors can also be employed to focus and control the direction of emitted radiation. Appropriate 3D reflectors may comprise flat, singly curved or doubly curved surfaces.

[0065] Xenon flashlamps are particular suited for use as a heat source with the present invention, they are capable of heating contact surfaces, for example of composite material samples, very quickly, consistently and controllably, typically exceeding the performance of other heat sources, such as known IR heat sources. Moreover, after a pulse, gasses cool relatively quickly—that is, they retain less residual heat than filament-based heaters (after ‘switch-off)—which means flashlamps afford far greater control over heating and cooling speed during operation, compared with filament-based heaters, and may obviate entirely supplemental heating and cooling sub-systems that are taught in the prior art. This greater heating and cooling control capability also supports increased manufacturing speeds, for example, whereby relative speeds between a heater and a contact surface being heated can be increased.

[0066] A sequence of pulses (flashes) in quick succession can be employed to raise the surface temperature of a layer of prepreg (or any other contact surface, such as a tool) in an extremely controlled manner. The temperature can be controlled, for example, according to the number of pulses and the time between pulses, which, in the illustrative example shown, is one pulse approximately every five seconds. Higher and lower pulse frequencies can of course be employed depending on the heating profile required. Once the surface has reached a target temperature, the time between pulses can be increased to maintain the desired temperature. Of course, other pulse parameters, such as pulse intensity, may be modified instead of or in addition to pulse frequency in order to control and maintain target temperatures.

[0067] Multiple pulses can be used to achieve and then maintain a target temperature. The combination of fast heating (during the pulses) and relatively slow cooling (between the pulses) provides a novel method of temperature control during the manufacture of composite articles. For example, according to embodiments of the present invention, as the surface temperature varies between the higher peaks and the lower cooling areas, the time delay between heating the surfaces and bringing the surfaces together may be varied to target the optimal temperature for the process. Consequently, advantage can be taken of the surface temperature peaks, without having to heat the bulk of a material to that high temperature.

[0068] In further alternative embodiments of the invention, plural flashlamps (or other radiation sources) may be mounted and arranged to heat substantially simultaneously both unlaid and previously laid layers of prepreg. Of course, one or more flashlamps (or other radiation sources) may instead or in addition be mounted and arranged to heat any other element or surface of the system, as the need dictates.

[0069] The radiation source output can be controlled according to a required head speed—that is, the speed the head moves across the tool or previously laid tows—to reach and maintain a target temperature and extent of cure. In particular, as head speed is increased the output of radiation is increased as well (or vice versa). The degree of heating and cure may in addition, or alternatively, be controlled by varying at least one of the distance of the source(s) from the contact surface and the angle of the incident radiation in relation to the prepreg's surface. In addition (or alternatively) a radiation filter may be placed between the source and contact surface. Such a filter may be formed as part of the source itself or as an intermediate structure between the source and the contact surface being heated.

[0070] Preferred UV sources are UV LEDS providing radiation of wavelength between 340 and 430 nm. Exemplary UV sources include Phoseon® Fireline LED UV lamps and Heraeus Noblelight Fusion UV F300s. Preferred wavelengths may be selected from 365 nm and 395 nm.

[0071] The structural fibres employed in the prepregs may be in the form of random, knitted, non-woven, multi-axial fibres or any other suitable pattern. For structural applications, it is generally preferred that the fibres be unidirectional in orientation. When unidirectional fibre layers are used, the orientation of the fibre can vary throughout the prepreg stack. However, this is only one of many possible orientations for stacks of unidirectional fibre layers. For example, unidirectional fibres in neighbouring layers may be arranged orthogonal to each other in a so-called 0/90 arrangement, which signifies the angles between neighbouring fibre layers. Other arrangements, such as 0/+45/−45/90 are of course possible, among many other arrangements.

[0072] The structural fibres may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous fibres. The structural fibres may be made from a wide variety of materials, such as carbon, graphite, glass, metalized polymers, aramid and mixtures thereof. Glass and carbon fibres are preferred, carbon fibre being preferred for wind turbine shells of length above 40 metres such as from 50 to 60 metres. The structural fibres, may be individual tows made up of a multiplicity of individual fibres and they may be woven or non-woven fabrics. The fibres may be unidirectional, bidirectional or multidirectional according to the properties required in the final laminate. Typically the fibres will have a circular or almost circular cross-section with a diameter in the range of from 3 to 20 μm, preferably from 5 to 12 μm. Different fibres may be used in different prepregs used to produce a cured laminate.

[0073] The structural fibres of the prepregs will be substantially impregnated with the epoxy resin and prepregs with a resin content of from 20 to 45 wt %, preferably 28 to 40 wt %, and more preferably from 30 to 38 wt % based on the total prepreg weight.

[0074] Upon curing, the stack becomes a composite laminate, suitable for use in a structural application, such as for example an automotive, marine vehicle or an aerospace structure or a wind turbine structure such as a shell for a blade or a spar. Such composite laminates can comprise structural fibres at a level of from 80% to 15% by volume, preferably from 58% to 65% by volume.

[0075] The invention has applicability in the production of a wide variety of materials. One particular use is in the production of wind turbine blades. Typical wind turbine blades comprise two long shells which come together to form the outer surface of the blade and a supporting spar within the blade and which extends at least partially along the length of the blade. The length and shape of the shells vary but the trend is to use longer blades (requiring longer shells) which in turn can require thicker shells and a special sequence of materials within the stack to be cured. This imposes special requirements on the materials from which they are prepared. Carbon fibre based prepregs are preferred for blades of length 30 metres or more, particularly those of length 40 metres or more such as 45 to 65 metres whilst the dry fibre is preferably a glass fibre. The length and shape of the shells may also lead to the use of different prepregs/dry fibre materials within the stack from which the shells are produced and may also lead to the use of different prepregs/dry fibre combinations along the length of the shell.

[0076] The invention is illustrated by way of example only and with reference to the following Example.

[0077] The following formulation was prepared.

TABLE-US-00001 Semi-solid bisphenol A epoxy resin LY1589 92.0% Liquid epoxy resin of EEW 175-205 DER 736 1.5% Mixed amyl sulphonic salt Photoinitiator 5.5% CPI 6976 Catalyst UV 9390 1.0% total 100.0% by weight of formulation

[0078] The formulation was mixed in a 10 L Winkworth mixer, and then filmed into two 65 gsm films, which were in turn impregnated into Ahlstrom R344 glass fibre with a nominal fibre aerial weight of 300 gsm. The prepregs were cut to size and then cured.

[0079] The cure process used a Fanuc Mi16B/20 robot, with a robotic head comprising a chute into which the pre-cut prepreg was fed. This chute directs the prepreg under a compaction roller which causes the prepreg to adhere to the substrate and then pulled the prepreg past a UV array by moving the robot head. The UV array provided UV radiation at 395 nm at an intensity of 15 W/cm.sup.2 measured using a Dymax LED/UV radiometer. The UV source allowed partial curing of the prepreg at a curing rate of approximately 15 mm/s which equates to 2.3 kg/h.

[0080] Panels were manufactured by laying down multiple piles in any direction.

[0081] The panels were also subjected to microscopic analysis for void content. Void content of laminates of the invention was less than <1% which is comparable with current hand lay-up processes.

[0082] The unidirectional panels of 10 ply thickness were subjected to ILSS (interlaminar shear strength) testing (in accordance with ASTM EN2563) providing values of >30 MPa, using a Fanuc Mi16B/20 with a shoe temperature of 200° C., shoe pressure of 5 Bar, 100% UV intensity (395 nm), with the robot set to move at a rate of 15 mm/s.

[0083] Unidirectional panels 10 plies thick have been produced and subjected to inter-laminar shear strength testing providing values of >30 MPa, using a shoe temperature of 200° C., shoe consolidation pressure of 5 Bar, employing 100% UV intensity (395 nm) for cure, with the robot head set to move at a rate of 15 mm/s. A triaxial ‘like’ panel was also tested for ILSS, returning an average value of 39.1 MPa, this panel was also tested for flexural strength and modulus returning average values of 660 MPa and 28 GPa respectively. These results indicate that the properties of the experimental photocured materials are close to the strength of commercially available systems produced by thermal cure in ovens.

EXAMPLE 6

[0084] In this example the components are as follows.

LY1589 bisphenol A epoxy resin (Huntsman)
TASHFP triaryl sulphonium hexafluorophosphate (50% by weight in solution in propylene carbonate)
TASHFA triaryl sulphonium hexfluoroantimonate (50% by weight in solution in propylene carbonate)

[0085] A formulation is prepared using the following formulations:

TABLE-US-00002 TABLE 1 6a 6b 6c 6d 6e 6f 6g 6h 6i Component (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) LY1589 97 98 99 96 94 95 99.5 99 99.5 TASHFP 3 2 — — — — 1 .5 TASHFA — —  1  4  6  5 .5

[0086] These formulations were analysed using differential scanning calorimetry (DSC, using a PerkinElmer DSC6000) and dielectric analysis (DEA, using a Nietzsche DEA 288 Epsilon) to measure the heat of reaction and the rate of cure as well as the level or extent of cure using the method described in the paper Heat of reaction, degree of cure, and viscosity of Hercules 3501-6 resin, Lee W I et al., J Composite Materials, Vol. 16, p 150, November 1982. In addition, the glass transition temperature (T.sub.g) in accordance with ASTM E1356 and the enthalpy and transition temperatures were determined in accordance with ASTM3418 and ASTM E2038 and E2039.

[0087] The results are presented in below Table 2.

TABLE-US-00003 TABLE 2 Total Enthalpy Time to Peak Time to Maximum enthalpy 80% cure 80% cure enthalpy Peak cure rate Formulation (J/g) (J/g) (mins) (W/g) (mins) (W/g/min) 6a 196 157 .5 19 .1 842 6b 209 1403 6c 206 .4 818 6d 185 .6 292 6e 187 .9 290 6f 192 .7 6g 186 .6 629 6h 212 .6 970 6i 199 .9 726

[0088] The formulation of Example 6c provides an advantageous time to cure to 80% of 0.4 mins whilst the maximum rate of cure is of a desired level to allow thermal management of the exotherm heat release in lay-ups containing multiple prepreg layers.

[0089] The formulation of Example 6c was used to impregnate a unidirectional glass fiber reinforcement material of 2400 tex fiber tows and of 600 g/m.sup.2 weight. Lay-ups were prepared from 4 layers of this prepreg material.

[0090] The lay-ups were subjected to the following cure schedule. Each formulation was heated to 50° C. and a light source producing light at a wavelength of 365 nm with an output of 8 W/cm.sup.2. The material was exposed to the light source at varying speeds as follows: 5 mm/s, 12.5 mm/s, 25 mm/s, 37.5 mm/s and 50 mm/s. We found that the lay-ups can be fully cured for processing speeds up to 25 mm/s in a single pass.