REPAIR STRIP AND PROCESS FOR WIND TURBINE LEADING EDGE PROTECTION TAPE

[0001] The present disclosure relates to a method for repairing the surface of rotor blades, in particular of wind turbines, or aircraft wings. In particular, the present disclosure relates to a method for repairing rotor blades comprising a protection layer, in particular on their leading edges. In addition, the present disclosure further relates to a rotor blade or aircraft wing comprising a protection layer obtained by the method as described herein.

[0002] It has long been known that objects made of polymeric or composite materials travelling at high speeds may suffer from erosion due to the contact with dust, sand or even rain. In fact, this is well-observed with regard to the leading edges of rotor blades of helicopters, wings of aircrafts, and, since their introduction, rotor blades of wind turbines. Erosion, in particular rain erosion, can not only be detrimental to the structural integrity or optical appearance of the objects concerned, but also have negative impact on the aerodynamic profile of rotor blades and wings. While it is obvious that this means an undesired deterioration of aerodynamic efficiency of wings and helicopter rotor blades, it must be stressed that this is also of great concern when running a wind turbine. This circumstance is even aggravated due to the ever-increasing size of wind turbines and the corresponding rotor blades, since an increase in size has the consequence of an increase in speed at which a particular point on the leading edge of the rotor blade travels. In particular, at the leading edges closer the tip of the blade speeds are achieved at which rain erosion represents a problem.

[0003] Thus, counter measures were developed, inter alia, the application of protection layers. The protection layers may for example be protection tapes that get applied over the leading edges of rotor blades of wind turbines and helicopters. These protection tapes are well-known in the art. Often, polyurethane tapes are used, which are also in most cases equipped with a pressure-sensitive adhesive (PSA) layer for affixing the tape to the rotor blade. However, these tapes are also prone to erosions such as rain erosion and therefore need to be replaced from time to time. Clearly, when undergoing repair, the operation of helicopters and particularly wind turbines must be suspended, which directly means a loss of output of electric energy and a corresponding economic loss. Thus, repair times should be as short as possible, giving rise to a demand for cost- and time-efficient repair measures and equipment which is easy to apply. Moreover, erosion or damage may not only appear on the complete leading edge of rotor blades or wings, but may also be present at small or limited localizations on the edge. Hence, it may be desirable not to remove all of the protection tape, but only repair the damaged or eroded parts. Finally, it may also be desirable that the repaired area exhibits the same erosion protection as the original protection layer. Also, repair of small areas may be necessary because of transportation damages.

[0004] Other known protection layers are protection coatings that get applied over the leading edges of rotor blades or aircraft wings. These protection coatings are designed to help protect a leading edge of rotor blades or aircraft wings from damage caused by for example sand and rain erosion and minor impact. The coating may for example be a two component polyurethane coating that provides excellent erosion protection in a single layer. These coatings can be easily applied via brush or casting. These coatings may—depending on the application—provide certain advantages in the application process, the aerodynamics and the handling compared to protection tapes.

[0005] In summary, without wanting to deny the advantages and progresses in the art in this regard, there still exists a need for new methods for repairing erosion protection layers, in particular on leading edges of rotors of wind turbines and helicopters as well as aircraft wings.

[0006] In one aspect of the present disclosure, there is provided a method for repairing the surface of rotor blades or aircraft wings, comprising

• • [0007] (a) a composite substrate; and
  • [0008] (b) a protection layer having at least one area of at least partial damage,
  • [0009] the method comprising
    • [0010] (i) optionally, removing at least part of the damaged part of the protection tape;
    • [0011] (ii) optionally, reconstructing the surface of damaged composite substrate;
    • [0012] (iii) optionally, applying primer solution to the surface of the composite substrate;
    • [0013] (iv) optionally, applying a polymer solution, preferably in at least one organic solvent;
    • [0014] (v) applying a repair patch of a tape onto at least a part of the composite surface;
    • [0015] (vi) applying heat so that the polymer tape melts and covers the damaged area, thereby forming a protection layer or applying a repair solution, preferably in at least on organic solvent, so that the repair patch at least partially resolves and covers the damaged area.

[0016] In another aspect of the present disclosure, there is provided a rotor blade or aircraft wing comprising a protection layer, obtained by the method as described herein.

[0017] Before any embodiments of this disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. As used herein, the term “a”, “an”, and “the” are used interchangeably and mean one or more; and “and/or” is used to indicate one or both stated cases may occur, for example A and/or B includes, (A and B) and (A or B). Also herein, recitation of ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 10 includes 1.4, 1.9, 2.33, 5.75, 9.98, etc.). Also herein, recitation of “at least one” includes all numbers of one and greater (e.g., at least 2, at least 4, at least 6, at least 8, at least 10, at least 25, at least 50, at least 100, etc.). Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Contrary to the use of “consisting”, which is meant to be limiting, the use of “including,” “containing”, “comprising,” or “having” and variations thereof is meant to be not limiting and to encompass the items listed thereafter as well as additional items.

[0018] Amounts of ingredients of a composition may be indicated by % by weight (or “% wt”. or “wt.-%”) unless specified otherwise. The amounts of all ingredients give 100% wt unless specified otherwise. If the amounts of ingredients are identified by % mole the amount of all ingredients gives 100% mole unless specified otherwise.

[0019] Unless explicitly stated otherwise, all embodiments of the present disclosure can be combined freely.

[0020] The first aspect of the present disclosure is a method for repairing the surface of rotor blades or aircraft wings, comprising

• • [0021] (a) a composite substrate; and
  • [0022] (b) a (leading edge) protection layer having at least one area of at least partial damage,
  • [0023] the method comprising:
    • [0024] (i) optionally, removing at least part of the damaged part of the protection tape;
    • [0025] (ii) optionally, reconstructing the surface of damaged composite substrate;
    • [0026] (iii) optionally, applying primer solution to the surface of the composite substrate;
    • [0027] (iv) optionally, applying a polymer solution, preferably in at least one organic solvent;
    • [0028] (v) applying a repair patch of a tape onto at least a part of the composite surface;
    • [0029] (vi) applying heat so that the polymer tape melts and covers the damaged area, thereby forming a protection layer or applying a repair solution, preferably in at least one organic solvent, so that the repair patch at least partially resolves and covers the damaged area.

[0030] Accordingly, the method gives rise to a combination of effects desirable for the repair and maintenance of rotor blades or aircraft wings, such as rotor blades of wind turbines. In particular, the method as described herein enables the skilled person to quickly repair localized parts of damaged protective layers on substrates. In addition, the repaired part of the protective layer may exhibit the same erosion protective properties as the original undamaged part. Thus, the presently described method is suited for efficiently repairing damaged parts of erosion protection layers of aircraft wings or rotors of helicopters and wind turbines, respectively.

[0031] Generally, the present method relates to repairing the surface of aircraft wings and rotor blades of helicopters and wind turbines, respectively. Preferably, the method according to the present disclosure is applied for repairing the surface of rotor blades of wind turbines, in particular the leading edges of wind turbines.

[0032] The rotor blades and the aircraft wing as used herein generally comprise a composite substrate, one or several coatings, and a protection layer disposed thereon. In most cases, the protection layer will be located on the leading edge of these substrates, since erosion and rain erosion takes part mostly on these areas. The composite substrates itself consist of composite materials commonly known and used in the art for rotor blades of helicopters and wind turbines as well as aircraft wings. For example, a composite material may be obtained by curing an epoxy or phenoxy resin enforced by fibers or a woven or non-woven fabric, or may contain polyurethane enforced with non-woven or woven fabrics. Fibers and fabrics obtained therefrom may be selected from polymeric fibers, glass fibers, metal fibers, and carbon fibers, dependent on the selected property and price of the desired application. In general, on top of the composite substrate, at least one coating is applied. These coatings may be referred to as top coat or hard coat and may comprise at least one filler material to obtain an even surface and at least one top coat obtained from polyurethanes (often applied as two-component system) and epoxy coating compositions known in the art for these purposes.

[0033] The protection layer disposed on the composite substrate may be obtained from an erosion protection tape known in the art. Such a tape may comprise a polymeric layer and a pressure sensitive adhesive (PSA) layer used for affixing the tape onto the substrate. The polymeric layer of the tape forming the protection layer may comprise, for example, polyurethane or a thermoplastic polyurethane (TPU). Erosion protection tapes are, for example, commercially available from 3M Germany GmbH under the trade designation Wind Protection Tape W8607 or W8750. The protection layer disposed on the composite substrate may also be obtained from an erosion protection coating known in the art. Such coatings may comprise a polyurethane coating, e.g. a two component polyurethane coating that provides excellent erosion protection in a single layer. The coating may be designed for application in OEM facilities and can be easily applied via brush or casting. Erosion protection coatings are, for example, commercially available from 3M Germany GmbH under the trade designation 3M™ Wind Blade Protection Coating W4600 or W4601.

[0034] The protection layer of the substrate may exhibit at least one area of at least partial damage. Damage is either or both physical and chemical damage, however, in the present context physical damage is predominant. Physical damage is in most cases erosion caused by rain, dust, sand, or other particles. In any event, the nature and appearance of such damage is well-known to the skilled person. Moreover, also the coatings and substrate beneath the protection layer may be affected by physical damage such as rain erosion and the like. Damage comprises all kind of damages, in particular, small, localized damage such as cracks or punctual damage, as well as damage in larger areas of several square centimetres. Also, damage comprises damage only to a certain thickness of the protective layer or to the complete thickness of the protective layer and the surface under the protective layer, i.e. the hard coat of the composite substrate itself.

[0035] In the first step of the method according to the present disclosure, at least part of the damaged part of the protection layer, i.e. protection tape affixed to the substrate surface or protection coating affixed to the substrate surface, may optionally be removed. Preferably, the complete damaged part of the protection layer is removed. Also, damaged top coatings, fillers or other underlying layers may need to be removed. It is also preferred that more of the damaged area of the protective layer is removed. That is, some of the non-damaged protection layer adjacent to the damages area may be removed. As appreciated by the skilled person, areas are cut with a knife or other cutlery and then the damaged parts are removed in stripes or patches. Finally, removal of protection layers or part or protection layers, or parts of or complete protection tapes is well-known in the art. Depending on the seize of the damaged area it is also possible to not remove any parts of the protection layer.

[0036] In the above-mentioned case that the substrate, i.e. the surface of the composite substrate, is also affected by damage, it is preferred that this surface is reconstructed. This may be achieved by means commonly known in the art, e.g. by the application of structural repair patches and fillers. In addition, a top coating could then be applied as known in the art. Similarly, if only the top coating under said protection layer is damaged, it is understood that this top coating could then be coated onto the substrate. Reconstructing the damaged part of the surface of the composite substrate may further comprise grinding and/or polishing the surface of the part.

[0037] It may further be preferable to apply an adhesion promoter (which may also be referred to as primer) onto the composite substrate surface and/or the reconstructed composite surface or any other coating or layer which represents the top layer of the construction obtained in the previous optional repair step. This may have the effect of an improved adhesion of the repair patch of the polymer tape to the composite substrate, which may also give rise to an improved erosion resistance of the repaired area covered by the repair patch. In general, the adhesion promoter or primer is coated to a thickness dependent on the constitution of the underlying substrate surface. That is, a more porous surface requires a thicker coating of primer or adhesion promoter. For example, the adhesion promoter may be coated to a thickness in the range of from 10 to 50 μm. Primer solutions and methods for their applications are well-known in the art. For example, polyamide solutions in isopropanol are available e.g., under the trade designation W9910 primer from 3M Germany GmbH. It is preferred that the solvent of the adhesion promoter solution is evaporated before the next step, i.e. the substrate coated with said adhesion promoter solution is dried. This may be achieved by either allowing the coating to dry under ambient conditions or by heating it, e.g. by using a heat gun or the like.

[0038] Instead or in addition to a primer it is possible to apply a solution of at least one polymer in at least one organic solvent onto the surface of the composite substrate. In this regard, it is understood that the above described area of the composite surface is meant of which the protective layer was removed in a previous step. Applying the polymer solution may have the effect that the adhesion of a repair patch to the composite substrate surface is improved, which may further enhance the effects described herein such as improved erosion protection of the repaired protection layer of the composite substrate. In this regard, it is preferred that the at least one organic solvent is selected from tetrahydrofurane, diethylether, dichloromethane, trichloromethane, ethyl acetate, dimethylformamide, ethanol, cyclohexane, butanol, pentanol, hexanol, diethylene glycol, diethylene glycol dimethyl ether, methyl tert-butylether, methylene chloride, pentane, hexane, petroleum ether, xylene, and mixtures thereof. Of these solvents, tetrahydrofurane, isopropanol and ethanol, dichloromethane and mixtures thereof are particularly preferred, with particular preference of tetrahydrofurane. Alternatively, aqueous dispersions of e.g. thermoplastic polyurethanes may be used. With regard to the at least one polymer in the polymer solution, it is preferred that it is the same polymer as in the polymer in the repair patch, i.e., it is preferred that the at least one polymer in the polymer solution is at least one thermoplastic elastomer, preferably the same as in the repair patch. This will have the effect of an improved bonding of the repair patch to the composite surface, with may further add to an improved erosion resistance. In general, the polymer solution is coated to a thickness dependent on the constitution of the underlying substrate surface. That is, a more porous surface requires a thicker coating of polymer solution. For example, the polymer of the polymer solution may be coated to a thickness in the range of from 10 to 50 μm. Again, it is preferred that the solvent of said polymer solution is evaporated before the next step. This may also be achieved by either allowing the substrate coated with the polymer solution to dry under ambient conditions or by applying heat such as by using a heat gun as known in the art.

[0039] Next, a repair patch of a polymer tape is applied onto at least part of the composite substrate surface. The repair patch of the polymer tape may comprise the same polymer as the protection layer. Such an embodiment may provide the advantage that the adhesion of the repair patch of a polymer tape to the protection layer is enhanced because they are based on the same polymer chemistry. Through the welding process the polymer of the patch and the protection layer get the possibility to melt into each other. It may also comprise at least one polyurethane polymer. It may further comprise at least one thermoplastic elastomer.

[0040] The use of at least one thermoplastic elastomer has the advantage that an effective erosion protection layer may be obtained. Furthermore, since the repair patch is molten or dissolved onto the substrate and the protection layer and forms an intimate adhesion with the substrate surface, there is no need for a PSA layer or using other adhesives for affixing the repair patch of a polymer tape to said surface. The repair patch of a polymer tape may have any given sizes and shapes which may be obtained by cutting or otherwise shaping a polymer tape. Generally, the shape and size is determined by the shape and size of the damaged area of the protection layer of the composite substrate. In most cases, the repair patch of a polymer tape will have the form of a strip. In this regard, the repair patch is applied that it may also cover the edges of the protection layer surrounding the damaged area. It is also preferred to squeeze out air that may be entrapped under the repair patch, preferably by means of a roller, preferably a silicone or rubber roller. Air or air bubbles entrapped under the repair patch may be detrimental to the adhesion of the repair patch to the substrate and/or to the erosion resistance of the finished protection layer. In general, it is preferred that the repair patch does not comprise an adhesive layer such as a PSA layer for affixing to the composite substrate surface. However, in the case that large areas of the protection layer of the composite substrate are affected by damage, it may also be preferred to use a large repair patch having a PSA layer, and cut it to a size and shape smaller than the affected damaged area. This patch may be then affixed to the damaged and preferably pre-treated composite substrate surface area such that a gap between the affixed repair patch having a PSA and the surrounding protection layer is formed. In this case, it is preferred that this gap is covered by a repair patch without having a PSA as described herein, preferably that also covers the edges of both repair patch having a PSA and surrounding edges, i.e. at least part of the adjacent protection layer.

[0041] If the repair patch of a polymer tape is dissolved a repair solution, preferably in at least one solvent, may be used. The repair solution may be applied onto the repair patch. The repair solution may be a solution of at least one polymer in at least one organic solvent. Applying the polymer solution onto the repair patch may have the effect that the adhesion of a repair patch to the composite substrate surface is improved, which may further enhance the effects described herein such as improved erosion protection of the repaired protection layer of the composite substrate. It may also have the effect, that the repair patch gets at least partially dissolved thereby adapting its shape to the at least one area of at least partial damage. It is preferred that the at least one organic solvent of the repair solution is selected from tetrahydrofurane, diethylether, dichloromethane, trichloromethane, ethyl acetate, dimethylformamide, ethanol, cyclohexane, butanol, pentanol, hexanol, diethylene glycol, diethylene glycol dimethyl ether, methyl tert-butylether, methylene chloride, pentane, hexane, petroleum ether, xylene, and mixtures thereof. Of these solvents, tetrahydrofurane, isopropanol and ethanol, dichloromethane and mixtures thereof are particularly preferred, with particular preference for tetrahydrofurane. Alternatively, aqueous dispersions of e.g. thermoplastic polyurethanes may be used. With regard to the at least one polymer in the polymer solution, it is preferred that it is the same polymer as in the polymer in the repair patch, i.e., it is preferred that the at least one polymer in the polymer solution is at least one thermoplastic elastomer, preferably the same as in the repair patch. This will have the effect of an improved bonding of the repair patch to the composite surface, with may further add to an improved erosion resistance. In general, the polymer solution is coated to a thickness dependent on the constitution of the underlying substrate surface and repair patch constitution. That is, a more porous surface requires a thicker coating of polymer solution. For example, the repair solution may be coated to a thickness in the range of from 10 to 50 μm.

[0042] According to one embodiment the at least one thermoplastic elastomer in the repair patch is selected from thermoplastic polyurethane, styrenic block copolymers, thermoplastic olefins, elastomeric alloys, thermoplastic copolyesters, thermoplastic polyamides and combinations thereof, and preferably is thermoplastic polyurethane. With regard to the thermoplastic elastomer, any thermoplastic elastomer may be used. The term “thermoplastic elastomers” as used herein has the meaning common in the art, i.e., compounds behaving at room temperature similar to classic elastomers, but are moldable into shape under addition of heat, i.e. which exhibit in addition to elastomeric properties thermoplastic behaviour. Of further particular importance to the present disclosure is the elastomeric behaviour under ambient conditions, i.e. the ability to stretch or deform under physical influence and return to its original or near original shape. This has the benefit of creating a longer life and better physical range than other materials, which means that these materials may act as protection layer for the composite substrate against physical influence such as raindrops, sand or dust particles impinging the substrate surface at high speeds. In summary, the repair patch comprising thermoplastic elastomers may have the advantages of being affixed under the addition of heat, thereby saving additional adhesive, and offer erosion protection for the composite substrate.

[0043] Preferably, the at least one thermoplastic elastomer in the repair patch is selected from thermoplastic polyurethane, styrenic block copolymers, thermoplastic olefins, elastomeric alloys, thermoplastic polyesters, thermoplastic polyamides and combinations thereof. Exemplary, commercially available thermoplastic elastomers are these under the trade designations Thermolast, Santoprene (block copolymers), Thermolast A, Forprene, Termoton-V (elastomeric alloys), Sofprene (SBS), Laprene (SBS), Thermolast K (SEBS) (styrenic block copolymers), For-Tec E (thermoplastic olefins), Desmopan, Elastollan, Avalon, Irogran (thermoplastic polyurethane). Since thermoplastic polyurethanes exhibit properties such as elasticity, transparency, and a certain resistance to oil, grease and abrasion, they are preferred as thermoplastic elastomers in the present disclosure.

[0044] The repair patch of the polymer tape may also comprise at least one polyurethane polymer, e.g. a two component polyurethane. A repair patch comprising at least one polyurethane polymer provides the advantage of providing provides excellent erosion protection in a single layer as well as easy application.

[0045] The repair patch may be polyurethane-based as mentioned above. They may provide the same polyurethane chemistry as the coating (protection layer). The following description refers to a possible chemistry of the repair patch and the coating. They are prepared from a precursor composition. The precursor composition is a reactive composition and typically contains an isocyanate-reactive component and an isocyanate-functional component. These components react with each other (cure) to form the coating composition (the cured composition). The precursor composition typically is a two-component (2K) composition. This means the compositions comprising the reactive components (the composition comprising the isocyanate-reactive component on the one hand and the composition comprising the isocyanate-functional component on the other hand) are kept separated from each other and are only combined to form a precursor composition. The compositions provided herein thus comprise or are the reaction product of the reaction of the isocyanate-reactive component with the isocyanate-functional component. Suitable isocyanate-reactive components and suitable isocyanate-functional components will be described in greater detail below.

[0046] To increase the effectiveness as an anti-erosion coating, the polyurethane-based coatings or repair patches preferably have a combination of mechanical properties; in particular, if they are meant to be effective at a low thickness, for example, having a thickness of from about 150 up to about 700 μm. Low thickness may be desired for economical reasons and also aerodynamic reasons. The inventors realized that good rain erosion properties may be achieved by polyurethane-based coatings when the coating is highly elastic, for example having an elongation at break of at least about 400%. Preferably, the coatings or repair patches have an elongation at break of at least about 500% and typically of at least about 600%. Without wishing to be bound by theory it is believed that high elasticity of the protective material dampens the impact of rain droplets hitting the blade.

[0047] Next to the high elasticity the coating or repair patch favorably has a substantial non-elastic component and a sufficient tensile strength to counter-balance the highly elastic behavior. It is believed that otherwise the protection gained by the increased elasticity may be lost again by a too elastic material having insufficient resilience. In this case particles or rain drops may make an impact on the surface to be protected if the material is only elastic and does not offer sufficient resilience. The coatings favorably have a tensile strength of at least about 20 MPa, for example from about 31 MPa to about 65 MPa.

[0048] The coatings or repair patches desirably have a considerable non-elastic component. This means they do not retain their original strength after having been stretched (for example to 300% of their original length). The non-elastic component may be determined by the permanent set E. Suitable coatings have a permanent set of E from about 15% to about 60%, preferably from about 24% to about 45%. This means after the elongation to 300% followed by complete relaxation, the material does not retain its original length but has a length that is from about 15% to about 60%, preferably from about 24 to about 45% greater than its original length. Such permanent set E is similar to that observed in effective commercial erosion protection tapes (e.g. protection tape W8067 from 3M Company).

[0049] The above described mechanical properties of the coatings or repair patches can be achieved by selecting the reactive components of the coating precursor compositions and adjusting their relative amounts.

[0050] The following components of the coatings or repair patches and its precursor composition are provided herein as guidance to prepare precursor compositions that will cure to coating compositions having the desired mechanical properties described above. However, it may be possible to use other combinations to provide coatings with the same properties.

[0051] The isocyanate-reactive component typically contains a combination of several isocyanate-reactive materials. As understood by one of ordinary skill in the art, an isocyanate-reactive material includes at least one active hydrogen. Those of ordinary skill in the polyurethane chemistry art will understand that a wide variety of materials are suitable for this component. For example, amines, thiols, and polyols are isocyanate-reactive materials. However, it is preferred that the isocyanate-reactive material be a hydroxyl-functional material. Polyols are the preferred hydroxyl-functional material used in the present disclosure. Polyols provide urethane linkages when reacted with an isocyanate-functional component, such as a polyisocyanate.

[0052] Suitable isocyanate-reactive materials to prepare the coatings according to the present disclosure comprise a combination of a short chain hydroxyl-functional compound, typically an □-□ hydroxyl compound (i.e. a compound comprising two terminal hydroxyl groups) and a high molecular weight hydroxyl functional compound, typically a compound comprising two terminal □-□□ hydroxyl groups and further comprising one or more oxyalkylene or polyoxyalkylene units.

[0053] Preferably, the short chain hydroxyl-functional materials are compounds having two terminal (α-ω) hydroxyl groups. Typically, the material has a molecular weight of less than 250 g/mole, preferably less than about 220 g/mole. Such material includes dihydroxyl-compounds having a carbon chain of from 2 to 12 carbon atoms, preferably from 3 to 10 and more preferably from 4 to 8 carbon atoms. In some embodiments, the carbon chain may be interrupted by one or more single oxygen atoms, while in other embodiments the carbon chain may not be interrupted. The latter embodiment is preferred. Preferably the short chain hydroxyl-functional material may be linear, cyclic or branched, although linear materials are preferred. The hydroxyl functional material includes compounds may be selected from alkane diols, alkane ether diols, alkane polyether diols and alkane ester diols containing from about 2 to 12, preferably 3 to 12 carbon atoms. Such compounds are preferably α-ω diols. Preferably the diols are linear α-ω diols, i.e. diols where the hydroxyl functions are at the terminal positions of the molecule. Typical examples of short chain α-ω diols include but are not limited to 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentane-diol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol and combinations thereof.

[0054] The isocyanate-reactive component may typically contain from about 20 to 40 ppw (parts per weight) of the short chain hydroxyl-functional materials (based on 100 parts of the high molecular weight hydroxyl-functional component).

[0055] The high molecular weight hydroxyl-functional material has a molecular weight of from at least 250 g/mole. Typically, the high molecular weight hydroxyl-functional material has a molecular weight of from about 250 to about 10,000 g/mole, preferably from at least 250 g/mole up to about 2,500 g/mole. Preferably, the high molecular weight materials are polyols, more preferably diols, most preferably α-ω diols and more particularly α-ω diols comprising one or more units selected from oxyalkylenes or polyoxyalkylenes. The high molecular weight hydroxyl-functional materials are preferably aliphatic and may be branched, cyclic or linear. Examples of high molecular weight hydroxyl-functional material include but are not limited to alkylene oxide diols (also referred to as oxyalkylene diols or ether diols) like diols containing an alkylene oxide unit selected from ethylene oxide, propylene oxide and butyleneoxide to name just a few. Other examples include polyether diols (also referred to as polyoxyalkylene diols) e.g. diols containing one or more polyoxyalkylene units, including but not limited to propylene oxide units, polyethylene oxide units, polyoxytetramethylenes and combinations thereof.

[0056] The high molecular weight hydroxyl-functional material may be a blend of several compounds, in which case the molecular weight ranges may be average molecular weight ranges, typically weight averaged molecular weight ranges.

[0057] The polyurethane may comprise a reaction product of a reactive composition comprising an isocyanate-reactive component and an isocyanate-functional component and wherein the isocyanate-reactive component comprises

• • [0058] a first component i) being a short chain hydroxyl-functional compound having two terminal (α-ω) hydroxyl groups having a molecular weight of less than 250 g/mole and having at least 2 carbon atoms, and
  • [0059] a second component ii) comprising a high molecular weight hydroxyl-functional compound having two terminal (α-ω) hydroxyl groups and a molecular weight of at least 250 g/mol and comprising one or more units selected from oxyalkylene units and polyoxyalkylene units,
    and wherein the isocyanate-functional component is an isocyanate prepolymer of the general formula NCO—Z—NCO, wherein Z is a linking group comprising at least two urethane (—NH—CO—O—) units and additionally one or more units selected from alkylenes, oxyalkylenes, polyoxyalkylenes, alkylene esters, oxyalkylene esters, polyoxyalkylene esters and combinations thereof.

[0060] According to another embodiment, the protection layer of the rotor blade or aircraft wing may comprise at least one thermoplastic elastomer. The protection layer of the rotor blade or aircraft wing may also comprise any other known and suitable material, such as for example a polyurethane as already described above.

[0061] According to another embodiment, the polymer in the polymer solution may be the same polymer as in the repair patch of the polymer tape. It is also possible that the polymer in the repair solution is the same polymer as in the repair patch of the polymer tape.

[0062] With regard to good adhesion and resistance to erosion, it is preferred that the polymer in the polymer solution, the repair solution, the protection layer and the polymer in the repair patch is the same. It is also preferred that the polymer in the polymer solution, the repair solution, the protection layer and the repair patch is a thermoplastic elastomer, with the same disclosure and preference as discussed with regard to the thermoplastic elastomer in the repair patch applying. It may also be preferred that the polymer in the polymer solution, the repair solution, the protection layer and the repair patch is a two component polyurethane.

[0063] If the protection layer is a coating, such as for example a 3M™ Wind Blade Protection Coating W4600 or W4601 or any other suitable coating, the method according to the invention discloses the repair of such a damaged coating with repair patches or stripes. The composition of the repair patch or strip of polymer tape may be identical or similar to the protection coating. The repair patches or stripes can either be provided as ready to use patch or stripe or can be made (with individual thickness and shape) by the user prior to application. The repair patches will be placed on the damaged coating and heat may be applied so that the repair patch melts and covers the damaged area. The new method may enable (in case of moderate damage) a durable repair without following a complex procedure of removal of the damaged coating and subsequent reapplication. The repair patch may also be applied with the help of a repair solution, preferably in at least one organic solvent, so that the repair patch of the polymer tape at least partially dissolves and covers the damaged area. The repair patches of polymer tape may for example be made by the user by applying small quantities of the protection coating onto a low surface energy substrate such as polypropylene and coated with a quadrupole film applicator (applicator film). The recommended coating quantity may for example be 1 to 10 ml and the recommended coating thickness is 50 to 300 μm followed by a curing step according the technical instructions as given in the technical data sheet of the coating.

[0064] Next, heat is applied to the repair patch so that the polymer tape melts and covers the damaged area, thereby forming a protection layer. Applying heat is preferably carried out by the addition of hot air. This may be achieved, dependent on the size of the substrate, in a hot air oven or by means of a heat gun. In this regard, it is preferred that the air temperature used in this step has temperature in the range of from 120 to 500° C., preferably in the range of from 200 to 450° C., more preferably from 250 to 350° C. Melting and covering the damaged area may be made sure by visual inspection of the area. However, in particular, when using thermoplastic polyurethane in either or all of protection layer, polymer solution and repair patch, it is preferred to apply heat until the surface temperature of the repair patch reaches 140 to 200° C. This temperature may be measured by means of a contactless thermometer, e. g. an IR thermometer used for measuring temperatures of surfaces from a distance.

[0065] After the repair patch has molten and the protection layer has been established, it may be allowed to cool down. Alternatively, in an additional step a roller, preferably a silicone or rubber roller, is rolled over the molten repair patch so as to flatten the edges and fuse the patch to the composite substrate and/or to the surrounding part of the protection layer. It is also preferred that rolling the roller over the repair patch is done parallel to applying heat to the repair patch. This may be advantageous in that a closer connection between the molten repair patch and the composite substrate and/or the surrounding part of the protection layer may be established. In particular, this may be particularly important when larger parts of repair patches are being used in the method prescribed herein.

[0066] As an alternative to using heat to fuse the edges of the repair patch with the protection layer a repair solvent can be used. The repair solvent may be applied onto the repair patch of a polymer tape so that the repair patch at least partially dissolves to cover the damaged area of the substrate, thereby forming a protection layer. The repair solution may be applied into the gap between two edges of the protection layer or the gap between the repair patch and a protection layer and the surrounding area for example by means of a small paint brush. The solution will slightly swell the underlying polymer tape and provide a good bond. It will also provide good adhesion to the surface of the substrate. Melting and covering the damaged area may be made sure by visual inspection of the area.

[0067] After the repair patch has been dissolved and the protection layer has been established, the solvent needs to be evaporated. Depending on the temperature it might take some hours until the repaired areas will achieve and regain its final mechanical properties. Alternatively, in an additional step a roller, preferably a silicone or rubber roller, is rolled over the molten repair patch so as to flatten the edges and fuse the patch to the composite substrate and/or to the surrounding part of the protection layer. It is also preferred that rolling the roller over the repair patch is done parallel to applying heat to the repair patch. This may be advantageous in that a closer connection between the molten repair patch and the composite substrate and/or the surrounding part of the protection layer may be established. In particular, this may be particularly important when larger parts of repair patches are being used in the method prescribed herein.

[0068] According to one embodiment of the invention, the repair patch covers the edges of the protection layer surrounding the damaged area. Covering the edges of the protection layer surrounding the damaged area according to the claimed invention may be interpreted as such that the repair patch overlaps the protection layer at least partially in the area of the edges of the protection layer surrounding the damaged area. Overlapping of protection layer and repair patch is a convenient and fast method when replacing sections of a protection layer. In order to make sure that the overlapping areas are sufficiently bonded to the protection layer an additional welding process may be applied. This may especially be necessary, when the protection layer is a protection tape including an adhesive layer and when the repair patch is a tape as well including an adhesive layer. The welding process may either be a heat welding process or a solvent welding process. The heat welding process may for example be conducted with the help of a heat gun and a silicon rubber roller. The solvent welding process may for example be conducted by applying a solvent and a silicon rubber roller. The solvent may be selected such that it is able to solve the polymer of the repair patch and/or the protection layer.

[0069] Since the method as described herein is able to provide repaired protective coatings exhibiting desirable erosion protection properties, the present disclosure further provides rotor blades of helicopters or wind turbines as well as aircraft wings obtained by the present method.

[0070] In particular, since the present method is excellently suited for repairing rain erosion protection layers of rotor blades of wind turbines, a rotor blade of a wind turbine comprising a protection layer obtained by the method described herein is preferred. Preferably, the protection layer is a rain erosion protection layer.

[0071] The present disclosure is further described without however wanting to limit the disclosure thereto. The following examples are provided to illustrate certain embodiments but are not meant to be limited in any way. Prior to that some test methods used to characterize materials and their properties will be described.

[0072] RT: room temperature

[0073] h: hour(s)

[0074] min: minute(s)

[0075] s: seconds

[0076] Ex.: Example

[0077] TPU : thermoplastic polyurethane

[0078] 3M W8750 tape: thermoplastic polyurethane tape having acrylic PSA layer

[0079] 3M W8750 tape without acrylic PSA layer

[0080] 3M W9910 primer

[0081] 3M W4600 coating

[0082] 10 wt.% solution of Krystalgran (PE102-201) in tetrahydrofurane (THF)

[0083] Isopropyl Alcohol

[0084] Application Solution (mixture of 75 wt.-% demineralized water and 25 wt.-% Isopropyl Alcohol)

[0085] Rubber Squeegee (e. g. 3M WETORDRY Rubber Squeegee Part. No. 05517)

[0086] Grit 320 sand paper

[0087] Heat Gun (Hot Air Blower) with adjustable temperature setting

[0088] Small Paint Brush

[0089] Box Sealing Tape (e. g. 3M 371)

[0090] Rubber Roller (e. g. 3M Safety-Walk Rubber Hand Roller 903)

[0091] The test blade profiles surfaces having a length of 225 mm simulating the leading edges of wind turbines in the rain erosion text as described herein were ground with grit 320 abrasive paper, cleaned with isopropyl alcohol and allowed to dry under ambient conditions. Two sheets of wind protection tape for each profile were prepared. The size of each sheet was selected in such a way that there was a 5-10 mm excess of the sheet on the long as well on both of the short sides of the profile. Also, there was a gap of 8-12 mm between the two pieces at the front edge of the profile.

[0092] For example 1, both sheets were made of 3M Wind Protection Tape W8750. The liner was removed from the first tape sheet and the PSA side of the tape as well as the profile surface were sprayed with the application solution. Next, the tape sheet was affixed with its PSA side onto one side of the profile. The tape was positioned in such a way that there was a 5-10 mm excess of the sheet on the long as well on both of the short sides of the profile and a distance of approx. 5 mm to the very outer leading edge of the profile. The outer surface of the tape sheet was then sprayed with application solution. A rubber squeegee was used to carefully squeeze out the application solution without moving or stretching the tape sheet. Accordingly, all air bubbles and water pockets which were visible under the tape sheet were removed. These steps were repeated with the second tape sheet on the other side of the profile. After that, the profile was allowed to dry for at least 1 hour under ambient conditions. Next, excess tape material was sliced off from the long and short edges of the profile. A Heat Gun was used to further carefully dry the gap between the two tape sheets on the profile. After that, W9910 adhesion promoter was applied onto the surface of the profile in the gap between the two tape sheets by means of a small paint brush. Again, a Heat Gun was used to evaporate the adhesion promoter solvent. Prepare an appropriate piece of the repair tape to cover the gap on top of the profile, align it on top of the gap (overlapping the previously applied tape sheets) and fix it on one side by means of a piece of box sealing tape. A Heat Gun with a temperature setting of 270° C. was used to gently heat-up the repair tape starting from the side which was fixed with the box sealing tape. At the same time, the rubber roller was used in order to push the repair tape towards the surface. In order to compensate for thermal expansion, the roller was moved towards the non-fixed end of the tape. Melting of the tape was observed in that the repair tape softened, became glossy and attached to the underlying tape and gap surface. Additional heat and pressure was applied in order to finalize the bonding process and flatten the edges of the repair tape. The test profiles were allowed to cool down under ambient conditions. The box sealing tape and any other excess material was removed. Finally, the profiles were stored for 1 week under ambient conditions to obtain final performance.

[0093] For example 2, both sheets were made of 3M Wind Protection Tape W8750. The liner was removed from the first tape sheet and the PSA side of the tape as well as the profile surface were sprayed with the application solution. Next, the tape sheet was affixed with its PSA side onto one side of the profile. The tape was positioned in such a way that there was a 5-10 mm excess of the sheet on the long as well on both of the short sides of the profile and a distance of approx. 5 mm to the very outer leading edge of the profile. The outer surface of the tape sheet was then sprayed with application solution. A rubber squeegee was used to carefully squeeze out the application solution without moving or stretching the tape sheet. Accordingly, all air bubbles and water pockets which were visible under the tape sheet were removed. These steps were repeated with the second tape sheet on the other side of the profile. After that, the profile was allowed to dry for at least 1 hour under ambient conditions. Next, excess tape material was sliced off from the long and short edges of the profile. A Heat Gun was used to further carefully dry the gap between the two tape sheets on the profile. After that, W9910 adhesion promoter was applied onto the surface of the profile in the gap between the two tape sheets by means of a small paint brush. Again, a Heat Gun was used to evaporate the adhesion promoter solvent. In the next step, the solution of Krystalgran PE102-200 in THF was applied into the gap and the surrounding areas by means of a small paint brush. The solution slightly swelled the underlying tape and provided a good bond. Prepare an appropriate piece of the repair tape to cover the gap on top of the profile, align it on top of the gap (overlapping the previously applied tape sheets) and fix it on one side by means of a piece of box sealing tape. This needs to be done before the Krystalgran solution evaporates. At the same time, the silicone rubber roller was used in order to push the repair tape towards the surface and to remove all air bubbles and flatten the edges of the repair tape. The repair strip as well as the underlying tape contained a relatively large amount of solvent which needed to be evaporated. That took several hours. As soon as the final mechanical properties of the protection layer is achieved the box sealing tape and any other excess material was removed. Finally, the profiles were stored for 1 week under ambient conditions to obtain final performance.

[0094] For example 3, the protection layer consisted of a W4600 coating. A repair patch was prepared that consisted of the same polymer as the coating. In order to prepare a repair patch the W4600 coating was applied onto a low surface energy substrate, such as polypropylene, and coated with a quadrupole film applicator (applicator film). The coating quantity was between 1 to 10 ml and had a thickness of 50 to 300 μm followed. A curing step according the instructions as given in the technical data sheet of the coating was followed.

[0095] After the repair patch was cured it was applied with the help of a repair solution, preferably in at least one organic solvent, so that the repair patch of the polymer tape at least partially dissolves and covers the damaged area.

[0096] As example 4, the overlap welding process is described for a W8750 tape. The example has been prepared by simple lamination of W8750 tape with a 10 mm overlap on a polypropylene plate which allows easy removal or the laminate. The overlap was heated by a hot air gun and simultaneously rolled over using a silicon rubber roller. The temperature setting of the heat gun was 300° C. and the process was visually controlled. During the welding process, the surface turns smooth and glossy. This allows to control the process and indicates, when the material starts to melt. The process can be stopped when the tape edges show a smooth appearance and no sharp edges are visible anymore. After preparation, the example was conditioned for 48 hours at 50° C.

[0097] In contrary to a simple overlap the resulting construction provides the following advantages: the repair provides a shape that is perfectly adapted to the underlying surface without any entrapped air. The polymer layer of the repair patch covers the adhesive layer. The edges of the repair patch are smoothened or rounded. There is no visible breakthrough or mixing of the adhesive with the polymer layer.

[0098] The anti-erosion properties were measured with the rain erosion test method according to ASTM G73-10.

[0099] The test profiles obtained as described above were mounted on the blades of a rotor, which was rotated to provide a speed ranging from test velocity of 160 m/s at the tip of the blades to a test velocity of 143 m/s in the center and 126 m/s at root.

[0100] Rainfall was simulated by spraying water (23° C.), having a droplet size of about 2 mm, with a velocity of 30 mm/hour onto the rotating blades inside the rig. The test was stopped every 30 minutes after which the coated surfaces were visually inspected. The test was run during 18 hours.

[0101] The examples were visually inspected and any damage or erosion to the tape surfaces were determined. For all examples, no visible damage or breakthrough of the repair tapes could be observed. Thus, it can be concluded that the method as described herein is excellently suited for repairing erosion tape surfaces of rotor blades such as rotor blades of wind turbines.