WO2018055063A1

WO2018055063A1 - A method of forming a component

Info

Publication number: WO2018055063A1
Application number: PCT/EP2017/073960
Authority: WO
Inventors: Yuk Loon LEE
Original assignee: Suzlon Energy Limited
Priority date: 2016-09-21
Filing date: 2017-09-21
Publication date: 2018-03-29
Also published as: GB2554476A; GB201702541D0

Abstract

Method of forming a component comprising: (a) stacking a plurality of layers (2) of polymer onto one another; (b) heating the stack of layers to the glass transition-heat deflection temperature range of the polymer; (c) applying a force (6) to the stack of layers over a period of time while within or above the glass transition-heat deflection temperature range so as to deform the layer; and (d) bonding the plurality of layers within the deformed stack to one another to form the component.

Description

A METHOD OF FORMING A COMPONENT

The invention relates to a method of forming a component and particularly, but not exclusively, to a method of forming a component part of a blade of a wind turbine (also known as an aerodynamically-powered generator or, simply, an aerogenerator).

Renewable energy sources are becoming increasingly popular due to the environmental impact and cost of fossil fuels and nuclear power. In particular, wind energy production is growing rapidly and has the potential to easily satisfy worldwide energy demand. Of the many types and configurations of aerogenerators, most are typically horizontal axis wind turbines (HAWTs). Such turbines comprise a rotor located at the top of a tower. The rotor comprises a plurality of blades each having an aerofoil profile. The airflow over the blades cause the rotor to rotate, thereby generating electricity .

Larger blades produce more energy and are more efficient compared to shorter blades and so there has been a drive to use blades of ever increasing length. However, increasing the length of the blades also increases their weight. Consequently, blades are now being manufactured from composite materials, such as Polymer Matrix Composites (PMCs), which are lightweight, but exhibit high strength and stiffness.

For example, the blades may comprise a reinforcing spar formed from a stack of pultruded PMC planks which are bonded to one another and then shaped to comply with the external aerodynamic profiles. However, it has been found that it is difficult to achieve the required shape when working with such materials and so the resulting profile of the blade may not be fully optimised in order to maximise efficiency.

It is therefore desirable to provide a method of forming a component which provides an accurate and lasting shape combined with great strength and stiffness.

Accordingly, in accordance with an aspect of the invention there is provided a method of forming a component comprising: (a) stacking a plurality of layers of polymer onto one another; (b) heating the stack of layers to the glass transition-heat deflection temperature range (glass transition temperature range)_of the polymer; (c) applying a force to the stack of layers over a period of time while within or above the glass transition-heat deflection temperature range (glass transition temperature range)_ so as to deform the layer; and (c) bonding the plurality of layers within the deformed stack to one another to form the component.

Each layer of polymer may be a polymer matrix composite.

The polymer may be a thermoset polymer.

The polymer may be a high performance engineering polymer. The process works whether or not it contains an embedded second-phase material, such as flakes, fibres, particles, etc. to reinforce it for further strength and/or stiffness and/or thermal and/or viscoelastic (i.e. creep or relaxation) resistance; and crucially, regardless of whether the polymer is thermoplastic or thermoset.

Each layer may be a pultruded plank.

A veil sheet may be interleaved between each pair of adjacent layers. The veil sheet may be interleaved during the step of stacking the plurality of layers. The stack of layers may be forced against a mould. The force may be applied by a vacuum.

The force may be applied by a compressible solid, such as foam or sand.

The force may be applied by a pressurised incompressible liquid, such as oil or water.

In accordance with another aspect of the invention there is provided a component formed using the method described above.

The component may be a component of a wind turbine. For example, the component may be a leading edge panel, a trailing edge panel, a suction surface panel, a pressure surface panel, a spar cap, a shear web, a blade root fairing, or a blade tip moulding. For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure 1 is a flowchart of a method of forming a component according to an embodiment of the invention;

Figure 2 is a graph of temperature against thermal expansion a, specific heat capacity c and free volume Vfree;

Figure 3 is a schematic side view showing a stack of layers for forming a component in a first stage of the method of forming;

Figure 4 is a schematic side view showing the stack of layers in a second stage of the method of forming;

Figure 5 is a schematic side view showing the stack of layers after joining to form the component; Figure 6 is a schematic side view showing a component which is formed using a plurality of layers assembled with intermediate layers; and

Figure 7 is an enlarged view of a portion of the component showing the layer structure. Figure 1 shows a flowchart of a method of forming a component according to an embodiment of the invention. In step S2, a plurality of layers 2 of a polymer are stacked onto one another. For example, each layer 2 may be a planar pultruded plank of polymer matrix composite (PMC), particularly a thermoset polymer such as an epoxy matrix with turbostratic (high strength) or graphitic (high stiffness) carbon fibres. Each layer 2 is pre-cured, fully-hardened and fully chemically crosslinked. In step S4, the stack of layers 2 is heated to a temperature which lies within the glass transition (Tg) temperature range of the polymer. The layers 2 used to form the stack need not have identical compositions, but should at least have glass transition temperature ranges which overlap with one another such that all layers within the stack can be within their glass transition temperature ranges simultaneously. That is, to maximise the success of an all-in-one-stack thermoforming process, the Tg ranges of the involved materials must be reasonably matched.

As shown in Figure 2, the glass transition temperature range is a quantifiable range of temperatures in which the coefficient of thermal expansion a, specific heat capacity c and free volume Vfree change in a broadly smooth 'step' manner, as a characteristic identifiable physical, thermodynamic and chemical property of the substance. Essentially, the additional heat allows sufficient physical 'wriggle room' - a minute extra space at the size level of the entangled molecular chain - to permit the soft i.e. residual stress-free repositioning of the molecules. All polymers have a glass transition temperature range, clearly defining its observable change in thermal expansion, specific heat capacity, free volume and viscoelastic behaviours, which varies with the specific polymer and its processing before it is released and shipped to a materials user, such as a manufacturer of aerogenerators, aircraft, automotives, buses, trains or maritime transport systems.

As shown in Figure 2, a single glass transition temperature, Tg, may be derived for the material using a number of methods, such as differential scanning calorimetry (DSC - heat flow based), dynamic mechanical analysis (DMA - stress/strain response based), thermal mechanical analysis (TMA - coefficient of expansion based), or dynamic thermal mechanical analysis (DTMA - multiple indicator based), Such measurements are typically centred within the range with the onset (start) of the transition shown in Figure 2 occurring approximately 35°C below the defined Tg value and the end of the transition occurring approximately 35°C above the Tg value.

The polymer properties a, c, Vfree all change abruptly across a similar temperature range. Conventionally, this range and thus the Tg measurement is reliably determined using DSC techniques. For example, the specific branded epoxy material used has a Tg value measured using DSC of 107°C. As described below, the method is therefore performed at 80°C which lies within the glass transition temperature range (based on the 35°C figure given above). Advantageously, the temperature is selected to be at the lower end of the glass transition temperature range (i.e. below the observed Tg value) so as to avoid chemical degradation, minimise energy usage, and based on the other parameters used during the method (i.e. time/duration, force). The temperature must also exceed the heat deflection (also referred to as heat distortion) temperature (HDT) of the layers 2. The temperature is therefore in a combined glass transition-heat deflection temperature range. Tg may be considered to be a physical property of the material itself which operates at a micro/nano-scopic level in relation to the repositioning of the molecular chain. In contrast, the HDT is dependent on the geometry of the component and other factors which influence the distortion of the material under load and so may be considered to operate on a macroscopic, component level. The HDT is frequently located in the near vicinity to the Tg, particularly for uncomplicated polymer systems, such as those with no reinforcements or filler particles. However, for polymers which are provided with secondary or tertiary phases of other solid materials in order to modify the material's functional properties, the HDT may differ from the Tg. Many structural polymer materials are of the secondary, tertiary, etc. phase type so as to raise structural or processing (extrusion stage) functionality. For example, continuous long fibres of carbon, glass, para-aramid, ultra-high molecular weight polyolefins, or other very strong stiff filaments are used to reinforce pultruded planks and are commonly used in both the renewable energy and aerospace industries. The HDT is also affected by the size of the component. For example, longer beams will exhibit lower resistance to load and reduced HDT compared to shorter beams, including even at the laboratory test scale, which is a necessarily smaller but still significant size. This is in contrast to Tg which is largely independent of geometry, shape and size.

The HDT is defined as an experimentally measured temperature point (sometimes a range) which characterises the temperature point (or more strictly a range) across which the effective 'sagging strength' or 'pseudo-stiffness' of the material changes, achieving a pre-defined deflection or distortion under a known pre-defined load or stress level (which can vary depending on the required specification), as the temperature changes at a predefined rate.

While HDT may be considered to be more accurate, its measurement may prove more difficult. Therefore, if the HDT is not known or is impractical to measure, then the Tg range alone, which provides a good indicator of the required processing temperature, may be used as a starting point to begin the thermoforming process. This may allow the HDT range to be derived through experimentation.

As shown in Figure 3, a mould 4 may be used to shape the stack of layers 2 while it is heated to within the glass transition temperature range. At step S6 of the method, a force 6 is applied to the layer so as to deform the layer 2. Specifically, as shown in Figure 4, the stack of layer 2 is forced against the mould 4 so that it conforms to the curvature of the mould 4. The force 6 may be applied by a weight, which may form another half of the mould 4, or through suction applied via the mould 4 or through a vacuum bag arrangement which draws the layer 2 against the mould 4, or through positive pressure exerted by a pumped in gas (e.g. air) or liquid (like water or oil), a mechanical spring or compressible material like foam or sand pushing an exerting force on the polymer workpiece. Where a gas or a liquid is used to exert the shaping force, the process may be performed using a pre-heated fluid (gas or liquid) so as to impart additional heat to maintain a constant forming temperature.

The force 6 may be applied prior to the layers 2 reaching the glass transition temperature range or only once the layers 2 are at the correct temperature. The mould 4 itself may comprise heating elements which heat the layers 2 to the glass transition temperature range. The heating elements may be embedded below the surface of the mould 4. Alternatively, the mould 4 may be positioned within a heating vessel, such as a thermal oven or autoclave, or subjected to microwave energy indirect heating methods using radio frequency beams projected through the mould and absorbed in the stacked materials. An insulating blanket, which may be formed by a layer of glass wool, may be laid over the stack of layers 2 in order to retain and homogenise the applied heat.

The stack of layers 2 may be soaked for a period of time with the force 6 applied and the temperature maintained within the glass transition temperature range. For the epoxy thermoset matrix example described above, the stack of layers 2 may be heated to 80°C which lies within the material's glass transition temperature range shown in Figure 2. A soak time (i.e. the amount of time that the material requires once it reaches the desired temperature) of 4 hours may be used with an applied force of 1 atmosphere. The temperature and force conditions are suitable for harnessing the viscoelastic properties of the material to enable a 'hard' resetting of the solid polymer's shape, while maintaining the polymer material in the solid phase throughout the process. As a result, upon cooling down to ambient conditions the shape of the stack of layers 2 is retained and does not revert towards its original planar shape. The viscoelastic properties and accumulated tiny molecular movements (including minute entanglement displacements) of the polymer allow for permanent creep and subsequent relaxation to relieve the internal strains and stresses of the new profile. As a result, the process is a stress-relieving procedure with the layers 2 exhibiting no stresses or strains in the new profile. There is a variable range of temperatures, durations and forces specific to each thermoforming candidate material, whereby cooler temperatures will within reasonable limits still form the material at longer durations or higher forces. The size of the variable range depends on the specific material chemistry and shaping geometry.

This heating regimen generates the final effect in two broadly observable parts, separated by their chronological order. During the initial forming stage, polymer 'creep' is exploited to deform it relatively slowly to the required final shape. The optimal minimal processing energy consumption required will help dictate precisely how slowly or rapidly and this will depend on the specific polymer characteristics, the initial dimensions and the depth/severity of shaping required. For example, a thicker pultruded plank will necessarily require more energy and thus time to 'slip' its internal nano- and micro- scale molecular chain structure to achieve a given external radius of curvature or prescribed angle of bending. During the second of the observable chronological parts, the force, temperature and time are maintained - significantly, beyond the instant the final shape is achieved by creep in the initial first stage - to exploit polymer 'relaxation', whereby the internal stresses and residual strains within the material itself are permitted to dwell over a period of prescribed time to zero via the continued internal molecular readjustment and inter-atomic 'slipping' at the ends of the chains and in between them, driven by the same continued heat (temperature) and pressure (applied force).

As all polymers - whether thermoplastics or thermosets - have a glass transition temperature range and thus a free volume characteristic, the invention works for all plastics, including high-temperature high-modulus relatively expensive and exotic thermoplastics like PEEK (polyetehertherketone), PAI (polyamideimide), PPS (polyphenylsulphone), PS (polysulphone), PEI (polyetherimide), PUM/PUMA (polyurethanemethylacrylate), etc..

Thus, the invention allows for the radical external shaping of completely solid high performance engineering polymer-containing materials - even fully-crosslinked fully- cured thermosets - with no or negligible risk of 'springback' as observed in other methods as a result of residual internal stresses which cause the product to return to its undesirable initial shape at service temperatures after a period of time, and generates undesirable locked-in residual internal stresses leading to nucleation of flaws and the growth of cracks during operating service at cooler ambient temperatures.

The forming method allows, for the very first time in composites and polymer engineering, the elimination or substantial reduction of unintended built-in residual stresses and strains compared against other less optimal forming methods. Such stresses and strains may be generated by: thermal expansion/contraction due to temperature changes in the same phase of (solid) of material during processing; and moulding directly from curing a liquid polymer or resin to a stable solid due to shrinkage stresses from the liquid-solid phase change from the molten or unreacted fluid polymer constituents.

This inherent stress relief is permanent in its nature, and a useful by-product is reduced susceptibility to stress crazing in harsh chemical or irradiation environments (e.g. ultraviolet, sunlight, gases, ozone, ionising radiation, liquids, seawater, sewage, acids, solvents, etc.). The stress relief may also increase mechanical toughness and strength to shocks, impacts, dropping, etc. The process eliminates the issue of 'springback' in polymers or polymer matrix composite components of all kinds, whereby a conventionally stress-formed component returns towards its original shape, eg flat laminated pultrusions cold-bent then bonded to 'fix' their unstable forced-shape with infused resin. As a result, it is not necessary for manufacturers to engage in long and expensive experimental trials or computer simulations to determine the necessary over-shaping ('over-forming') required to produce a component of the correct shape and dimensions following this springback.

In step S8, the plurality of layers are bonded to one another using a suitable adhesive or resin to form a component 8 of desired thickness, as shown in Figure 5. For example, the layers may be joined using sheet film adhesive, or toughened (anti- fatigue) adhesive paste, or via a matrix resin infusion procedure. A matrix resin infusion procedure allows the layers 2 to be bonded to one another without first unstacking the layers 2. Figures 6 and 7 show an alternative arrangement where sheets 10 of a veil material are interleaved between adjacent pairs of layers 2 during step S2 prior to heating and deforming. Alternatively, the sheets 10 may be inserted between the layers 2 after they have been deformed and prior to bonding in step S8. The method described above ensures that each layer 2 has precisely the same shape. As a result, a consistent and almost negligible gap is generated between the layers 2. This enables the veil sheet 10 to be inserted between the layers 2. The veil sheets 10 acts as spacers between the layers 2. The veil sheet 10 is a thin, porous and flexible layer which may be formed from an engineering felt or random orientated fibre tissue, i.e. a non-woven textile, comprising short fine fibres that toughen, reinforce and homogenise the adhesive or infused matrix resin bondline, by virtue of fibre capillary surface-tension action. The veil sheets 10 therefore improve fracture toughness and fatigue strength. In particular, crack initiation at the tip of any impending flaw is delayed, due to the physical toughening mechanisms of the embedded fibres such as crack fibre bridging, fibre pull- out, etc. Once crack growth begins, the same toughening mechanisms delay the crack speed and increase the intrinsic crack opening load so that fast fracture is avoided. The veil sheets 10 also improve wetting of the adhesive or resin across the layers 2 and improve the uniformity of the bondlines. For example, for many epoxy structural adhesives, the bondline thickness is approximately 0.3mm. The veil sheets 10 have been shown to improve both the fatigue properties of the component 8 (repetitive cyclic loading), as well as the maximum static properties. They may therefore be used in applications where bondline inter-layer toughness, high static strength and/or high fatigue strength is required. The component 8 may be used within a blade of a wind turbine. In particular, the component 8 may be used as a structural reinforcement for the blade. In particular, the component 8 may form a structural spar cap which is located within the aerofoil cross- section and extends along part of or all of the blade span. The component may also form a structural shear web within the aerofoil cross-section which joins the upper and lower spar caps together, and extends along part of or all of the blade span. The component may also form a 'closing out' panel for the upper suction and/or lower pressure skins of the blade aerofoil cross-section, a leading edge panel for the forward skins of the blade aerofoil cross-section, or a trailing edge panel for the rear skins of the blade aerofoil cross-section. Although the specific example of an epoxy matrix with turbostratic carbon has been described above, it will be appreciated that the method may be used with any polymer matrix material, with any kind of continuous fibre, short fibre, particulate, powder, flake, or filler reinforcing phase. As examples, the polymer may be a thermosetting epoxy, polyester, vinyl ester, polyurethane, bismaleimide, or cyanate ester which is combined with any fibre, such as para-aramid (for high stiffness, toughness and strength, ultra low mass), graphitic carbon (for high stiffness), turbostratic carbon (for high strength), E glass (low cost), S, T or R glass (stronger and stiffer than E glass), quartz, or ultra high molecular weight polyolefin (low mass). The method may also be employed with unreinforced thermosetting polymers. In addition, the method may be used with high- performance thermoplastics, characterised by ultra-high melting points and rigid ring- /cyclic- shaped molecular groups, such as PEEK, PPS, PS, PEI, PAI, which have similar mechanical properties (particularly stiffness and strength) to thermosetting polymers and can be heated to the glass transition-heat deflection temperature range without the layers inadvertently adhering to one another, by virtue of the veil interleaves, functioning together as spacers and as infusion substrates for subsequent resin bonding consolidation of the planks.

Although the invention has been described with reference to the glass transition-heat distortion temperature range, it will be appreciated that the material may be heated beyond this temperature range, provided that it does not exceed the chemical degradation temperature. In particular, it has been found that a forming temperature of 10^°C above the glass transition temperature range is particularly effective and successfully preserved all measurable mechanical and chemical properties for unidirectional graphite-epoxy 2mm thick pultrusion planks. A temperature of 15^°C above the glass transition temperature range has also proved satisfactory for thicker material including 3mm and 5mm, as well as for a stack of multiple plies of 2mm planks.

Although the invention has been described with reference to blades of a wind turbine, such as a horizontal axis wind turbine, it will be appreciated that it may be used in other applications. In particular, the invention may be used to form components of vertical axis wind turbines and in any other application. Although it has been described that the invention takes a planar layer and forms it into a contoured layer, it will be appreciated that it may also be used to form contoured layers, either changing their contours or straightening to form a planar layer. The method may therefore utilise a pair of flat platens, rather than contoured moulds.

Claims

1. A method of forming a component comprising:

(a) stacking a plurality of fully-cured and hardened layers of polymer onto one another;

(b) heating the stack of layers to the glass transition-heat deflection temperature range of the polymer;

(c) applying a force to the stack of layers over a period of time while within or above the glass transition-heat deflection temperature range so as to deform the layer; and

(d) bonding the plurality of layers within the deformed stack to one another to form the component.

2. A method as claimed in claim 1 , wherein each layer of polymer is a polymer matrix composite.

3. A method as claimed in claim 1 or 2, wherein the polymer is a thermoset polymer.

4. A method as claimed in claim 1 or 2, wherein the polymer is a high-performance thermoplastic.

5. A method as claimed in any preceding claim, wherein each layer is a pultruded plank.

6. A method as claimed in any preceding claim, wherein a veil sheet is interleaved between each pair of adjacent layers.

7. A method as claimed in claim 6, wherein the veil sheet is interleaved during the step of stacking the plurality of layers.

8. A method as claimed in any preceding claim, wherein the stack of layers is forced against a mould.

9. A method as claimed in any preceding claim, wherein the force is applied by a vacuum.

10. A method as claimed in any preceding claim, wherein the force is applied by a compressible solid.

1 1. A method as claimed in any preceding claim, wherein the force is applied by a pressurised incompressible liquid.

12. A method substantially as described herein with reference to and as shown in the accompanying drawings.

13. A component formed using the method as claimed in any preceding claim.

14. A component as claimed in claim 13, wherein the component is a component of a wind turbine.

15. A component as claimed in claim 14, wherein the component is a leading edge panel, a trailing edge panel, a suction surface panel, a pressure surface panel, a spar cap, a shear web, a blade root fairing, or a blade tip moulding.