WO2009150298A1

WO2009150298A1 - A method to produce engineered composite articles comprising epoxy and carbon nano tubes

Info

Publication number: WO2009150298A1
Application number: PCT/FI2009/050492
Authority: WO
Inventors: Joachim KARTHÄUSER
Original assignee: Eagle Tuulivoima Oy
Priority date: 2008-06-10
Filing date: 2009-06-10
Publication date: 2009-12-17
Also published as: EP2300219A1; FI20085570A0

Abstract

The invention relates to a method for producing composite articles containing epoxy and functionalized carbon nano tubes (CNT). The method comprises the following steps: a closable mould (10) is provided the mould having open and close positions, the inner parts of the open mould (10) is equipped with a film (3), which has compatibility with said epoxy and it is impermeable at least to the extent that resin is prevented from migrating through the film to the mould, the open mould (10) is further filled with glue containing epoxy and said functionalized carbon nano tubes, solid materials and elements enabling the mechanical connection to other machine elements, the mould (10) is closed and heated until the finished article is sufficiently stable.

Description

A METHOD TO PRODUCE ENGINEERED COMPOSITE ARTICLES COMPRISING EPOXY AND CARBON NANO TUBES

Background of the invention

Field of the invention

This invention relates to a method according to the preamble of claim 1. The technical field is characterized by the keywords: composites, thermosets, reinforcement, carbon fibers (CF) , carbon nano tubes (CNT), production of composites, windmill blades, propellers, sporting goods such as skis, automotive and industrial parts, production methods. The invention relates also to a windmill blade or similar composite article.

Description of the related art

Document US 3873654 presents a process of forming a rigid foam airfoil section. An oversized foam core is compressed between slit molds, which have partially cured epoxy-coated fibreglass cloth laid up in both halves of the molds. Thus, this assures a good bond between the cloth and foam core during the final curing process. Similar process has been presented in document US 4,639,284.

Epoxy mixtures with CNT are known, in particular under the trade name Hybtonite® by the company Amroy (FI) . Descriptions can be found e.g. in the prior art WO 2006/040398 Al. Hybtonite® is already used in the manufacture of large wind mill blades and other composite articles. The production process for wind mill blades typically involves one mould into which solids are placed such as long glass fibers. Thereafter, a liquid epoxy formulation is pumped into the mould, and typically vacuum has to be applied to ensure that the liquid fills all empty spaces prior to heating up the complete mould. EP 1859920 Al discloses a method to vent residual gas through at least one venting duct which may contain a semi-permeable membrane. The usually employed technique to produce long composite articles such as windmill blades is (vacuum-assisted) resin transfer moulding (RTM) . After curing at elevated temperature, the wind mill blade is painted or gel-coated in order to provide a surface finish and to protect the structure from weather influences and sunlight.

Epoxy or polyepoxide is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives. Epoxy is a copolymer; that is, it is formed from two different chemicals. These are referred to as the "resin" and the "hardener". The resin consists of monomers or short chain polymers with an epoxide group at either end. Most common epoxy resins are produced from a reaction between epichlorohydrin and bisphenol-A, though the latter may be replaced by similar chemicals. The hardener consists of polyamine monomers, for example Triethylenetetramine (TETA) . When these compounds are mixed together, the amine groups react with the epoxide groups to form a covalent bond. Each NH group can react with an epoxide group, so that the resulting polymer is heavily crosslinked, and is thus rigid and strong.

The process of polymerization is called "curing", and can be controlled through temperature and choice of resin and hardener compounds; the process can take minutes to hours. Some formulations benefit from heating during the cure period, whereas others simply require time, and ambient temperatures.

Problems with prior art

Disadvantages of the compositions and the manufacturing process for wind mill blades and similar composites, are: The costs for raw materials can be high. The costs for moulding are usually high because either pressure or vacuum equipment or both is needed in order first pump liquid raw material into the mould and to secondly ensure that no gas is trapped within the finished article. The surface quality and appearance of the finished article is often of low quality, and this requires after-work such as gel-coating or painting. In particular the stability and mechanical properties, or the weight/strength ratio is insufficient. In order to generate electricity from wind power especially at low wind speeds and with smaller installations such as cottage houses in the countryside, blades of extremely low weight such as weighing 2,5 kg or less for a blade length of 2 m are required. These are not available today.

Usually there is a need for mould release agents, which cause costs and environmental problem.

Finally, it is desirable that composite articles show a favorable life-cycle analysis (LCA) . After use, composite articles should contain high concentrations of materials, which either are biologically degradable, or combustible leaving a minimum amount of ash. From the foregoing it should be apparent that there is a need to provide methods to produce light weight windmill blades and composite articles at lower cost than possible today, combined with improvements in terms of LCA.

Summary of the invention The object of this invention is to achieve an improved method for producing engineered composite articles comprising epoxy resins and carbon nano tubes. This object is achieved with the features described in accompanying claim 1. The traditional need for evacuating the mould or the need for transferring liquid resin using pressure pumps is obviated.

The film is under any circumstances fully polymerized ("fully cured") . A good bonding is achieved because a) The film is highly compatible with the epoxy system, and/or b) Excess resin fills the film at least partly - whereas the resin, however, will not migrate through the whole film.

The good compability means that the epoxy resin wets the surface of the film. This will be true for thermoplastic elastomer, polyamide, polyacryle, polyvinylchloride (PVC) , polystyrene, and many other thermoplastics or their mixtures. In other words compatibility is defined here in terms of surface tension. On the contrary silicone and water, or polyester to take a plastic, are NOT compatible.

Certain thermoplastic may need a pre-treatment for reaching said compability. For example, PVC-layer isn't directly compatible with epoxy. PVC may be provide first a layer like paper, which is compatible both PVC and epoxy.

The surface quality is given by the surface quality of the film, and by the contact between film and the mould halves. It is not expected that a gel coat or any further finishing step apart from trimming edges (where excess resin has left the mould) is needed.

The need for mould release agents is also obviated, alternatively non-fluorinated release agents can be used, thereby reducing costs and environmental performance. For example, to allow the easy release of polyolefin films from a mould surface, paraffinic waxes are suitable. Liquid epoxy resin will not permeate through such films and will therefore not adhere later to the mould surface. A further benefit is that the films can be equipped directly with company logos or colour patterns. The film has also the function of enclosing the resin containing functionalized CNT. Without a film, there would be a risk of contamination of the mould with CNT, a rougher surface might result because resin/CNT would stick to the mould and create an irregular surface.

In this invention, windmill blades and similar composite articles are produced in the following manner: a mould is provided, equipped with a film which later shall at least partly form part of the windmill blade. The film is wetted with glue. The mould is further loaded with reinforcing structures such as glass fiber mats, light- weight core material and elements which allow the windmill blades to be connected to the rotor. The mould is closed whereby the core material may be oversized in comparison to the mould volume. The subsequent partial crushing of the core structure ensures that all remaining liquid epoxy fills all voids in the mould. After closure of the mould, the complete mould is heated until the material is cured. No mould evacuation is necessary. Also, the need for using pressure pumps to transfer liquid resin into the mould is obviated. Advantages

Advantages of the invention can be summarized as follows:

Improved mechanical properties: strength, elongation at break, elasticity, fatigue resistance, and the like are greatly improved when comparing articles of the same weight.

Low investment costs: the production process is cheaper because an investment in vacuum or pressure technology is not needed. The production process is not at all complex and does not require highly skilled workforce.

Life-cycle analysis: as the windmill blade contains less solid materials and more combustible materials, the product can be burned after use leaving less ash and solid products.

Brief description of Figures

Figure 1 presents open mould halves for receiving film and other required materials

Figure 2 presents mould halves in a closing stage Figure 3 presents another mould in the heating phase

Detailed description of preferred embodiments

The following description of a preferred embodiment shall only illustrate the principle of the invention and is not meant to be restrictive .

A mould (10) comprising two mould halves (1) is provided which are connected by an axis (2), Fig. 1. Film and solid components and resin are laid into one mould half whereupon the mould (10) is closed for curing. Film (3) may cover both mould halves (1) . The film covers at least 50%, but preferably at least 90% of the surface of the open mould (10) . Thermoplastic, like polyamide of film has preferably high melting point and pre-treated to reach said compability. If multi-layer film is used, the outer layer is impermeable, while others allow unpolymerized epoxy migrate through to the outer layer.

Referring now to Fig. 2 film (3) is used to fill at least part of the inner mould surface, ideally on both sides. Solid light-weight components (4) are used to fill the mould, as well as liquid resin (5) to fill voids and to provide binding between the solids and the film. After filling, in this case the right mould half, the mould is closed as indicated by arrow (6) . The film (3) is impermeable at least to the extent that unpolymerized epoxy is prevented from migrating through the film to the mould. The film has preferably with excellent compatibility with and adhesion to epoxy resins.

Preferably epoxy formulations containing functionalized Carbon Nano Tubes (CNT) are used in amounts of at least 1% of the final product weight, such as at least 2% or such as at least 5% or more. Aerogel, honeycomb structures, or other materials of a specific weight not exceeding 100 kg/m³ is used as light weight core material, preferably the specific weight not exceeding 60 kg/m³.

The closed mould is heated to afford polymerisation or curing of the resin system used, Fig. 3. To allow fastening of the article (e.g. a wind mill blade), connectors (6) have been inserted. They extend well into the wind mill blade as indicated by extensions (7) . The outer part (6) may be detachable from the inner part (7) e.g. by a screw connection.

These examples shall only illustrate the principle of the invention and are not meant to be restrictive. The important features are that film is forming an integral part of a rigid composite structure, and that a part of the film used in the composite is covering a part of the outer surface of the composite. The exceptional strength of film and its tolerance to defects such as mechanical damage helps to produce durable light-weight composites on the basis of sustainable raw materials.

A mould is provided, ideally in two halves made of metal such as aluminium. The mould is equipped with a film or a foil which later at least partly forms an integral part of the composite. The film material may be e.g. a thermoplastic polyurethane or polyamide and may be e.g. 0,75 mm in thickness (generally each layer 0,2 - 1,0 mm) .

In a variation, the film may consist of more than one layer whereby the top layer can be removed after the moulding process and serves otherwise as protective film. Glue containing epoxy and functionalized CNT, commercially available as Hybtonite®, is spread on the foil or the film. Thus, epoxy formulations contain preferably carbon nano tubes (CNT) with covalent bondings.

The film may be partly porous or microporous such that liquid glue impregnates the film, such that a better adhesion is achieved between the film and the glue.

Said porous or microporous film (3) which is used to cover the inner surfaces of the mould halves (1) has a thickness of at least 30 micrometer, but preferably at least 200 micrometer and most preferably 750 micrometer. It is made of thermoplastic urethane, thermoplastic elastomer or thermoplastic such as polyamide (particularly with melting point about 100 ⁰C) . When the film is provided as multi-layer film, one or more of such layers improving the heat transfer from the mould, or one or more of such layers serving as protective films being detachable after formation of the product, this operation enables the production of windmill blades with optical structures such as logos at excellent surface quality

A laminate such as consisting of glass or carbon fiber, in the form of woven or non-woven laminate or pre-impregnated (pre-cured) structures is placed in the mould. A light weight core is placed in the mould. Preferably Divinycell® HP 60 having a specific weight of 60 kg m³ is used. A bar which shall connect the core or the blade to the rotor of the wind power installation is placed in the mould in such a way that a good mechanical connection is achieved. For this purpose, the core may be cut out appropriately leaving space for the bar which in itself preferably is a cured glass fiber composite material. The said laminate structures or mats preferably cover the bar completely such that additional stability and fastening between the blade and the rotor is achieved. When the mould is fully loaded, the mould halves are closed.

In one embodiment, it is advantageous to build up the structure in one half of the mould and to cover the complete structure with the CAP film before the second (empty) half of the mould is placed onto the first half. Excess film may be cut off before or after heating. Preferably the core material is oversize compared with the mould volume such that closure of the mould leads to partial crushing of the core material. This is intentional as liquid material is pressed into remaining open spaces of the mould, and also a good cross- linking between the core and the thermosetting epoxy is achieved. Furthermore, any trapped air, or most of the trapped air, is pressed into the light weight core. The moulds may be held together by simple clamps.

The closed moulds are heated e.g. for 30 min at 75-100 ⁰C whereby the lower temperature often results in a better surface quality. After production and removal from the mould, the blades may provided with stickers such as advertising emblems. Providing gel-coat or paint is only optionally and usually avoided due to the perfect surface of the film.

The production process is highly economical and suitable for windmill blades of any length.

The connecting bar, i.e. the bar which connects the blade to the rotor of the wind power generator, may consist of pre-cured glassfiber and epoxy or similar hard material, and the connection to the rotor may be preferably by at least two metal rods having diameters of at least 10 mm for blades of 2 m length (and accordingly stronger for longer blades) . Preferably the solid bar is connected to the light weight core using glue such as containing epoxy and CNT.

To avoid a too high load onto the blade at high wind speeds, it is preferable that the blade can turn automatically away from the wind or expose only a minimum part of its surface to the wind. This can be achieved by fastening the rods into an arrangement which can rotate in the axis or parallel to the axis given by the long blade axis. Elastic components such as rubber parts or springs ensure that the blade is either moving away from the wind, or is returning to its default position of maximum wind power utilization.

Claims

1. A method to produce composite articles of an absolute length to maximum thickness ratio of at least 20:1 such as windmill blades, said articles containing polymerized epoxy and functionalized carbon nano tubes (CNT), said method comprising the following steps: a closable mould (10) is provided, the mould having open and close positions, the inner parts of the open mould (10) is equipped with a film (3) covering at least 50%, but preferably at least 90% of the surface of the open mould (10) , the open mould (10) is further filled with glue containing unpolymerized epoxy and said functionalized carbon nano tubes, solid materials such as glass or carbon fiber materials, laminates, light weight core material and elements enabling the mechanical connection to other machine elements such as a rotor, the mould (10) is closed and heated until epoxy has been polymerized and the finished article is sufficiently stable, such as at 75 - 100 ⁰C for about 30 minutes, - characterized in that the film (3) has compatibility with said epoxy and it is impermeable at least to the extent that unpolymerized epoxy is prevented from migrating through the film to the mould.

2. A method according to claim 1, wherein aerogel, honeycomb structures, or other materials of a specific weight not exceeding 100 kg/m³ is used as light weight core material, preferably the specific weight not exceeding 60 kg/m³.

3. A method according to claim 1 or claim 2, wherein the mould (10) is filled with an excess of at least 1% (volume/volume) of light weight core material is used, and where the closure of the mould leads to a partial crushing of said light weight core material, said crushing enabling partly a better bonding of the light weight core to the epoxy resin, and enabling partly the filling of all empty spaces by liquid resin.

4. A method according to any of the preceding claims, wherein said epoxy formulations contains carbon nano tubes (CNT) with covalent bondings .

5. A method according to any of the preceding claims, wherein the optionally porous or microporous film (3) which is used to cover the inner surfaces of the mould halves (1) is having a thickness of at least 30 micrometer, but preferably at least 200 micrometer and most preferably 750 micrometer, and is made of thermoplastic urethane, thermoplastic elastomer or thermoplastic such as polyamide.

6. A method according to any of the preceding claims, wherein the film is provided as multi-layer film, where the outer layer is impermeable, while others allow unpolymerized epoxy migrate through to the outer layer.

7. A method according to claim 6, wherein one or more of such layers improving the heat transfer from the mould.

8. A method according to claim 6, wherein one or more of such layers are serving as protective films being detachable after formation of the product, said mode of operation enabling the production of windmill blades with optical structures such as logos at excellent surface quality.

9. A method according to any of the preceding claims, wherein epoxy formulations containing functionalized Carbon Nano Tubes (CNT) are used in amounts of at least 1% of the final product weight, such as at least 2% or such as at least 5% or more.

10. A method according to any of the preceding claims, wherein a solid bar, e.g. consisting of a cured glass or carbon fiber structure containing one or more, preferably two metal rods, is used to connect the light weight core of the windmill blade and the rotor of the wind mill.

11. A method according to claim 10 wherein, said solid bar is connected to the light weight core using glue such as containing epoxy and functionalized CNT.

12. A method according to any of the preceding claims, wherein the solid bar is covered during the production process with laminates, woven or non-woven mats of glass or carbon fibers, or cured or non- cured reinforcing materials ensuring adequate and long-lasting connection of the bar to the blade.

13. A method according to any of the preceding claims, wherein the rods which connect the windmill blade and the rotor are fastened in the rotor such that the blade can rotate in the axis or parallel to the axis given by the long axis of the blade, such that the blade can turn away from the wind at high wind speeds, whereby the rotation of the blade is reversible and controlled e.g. by elastic components such as springs .

14. A method according to any of the preceding claims, wherein the mould (10) has two halves (1) , preferably made of metal such as aluminium.

15. A windmill blade or similar composite article such as propeller, aeronautic, automotive or industrial part or the like made according to any of the preceding claims.