The invention relates to a method for drying a core material used in a polyurethane composite material.

Polyurethane composite materials are widely used in various fields, such as: pultrusion, window frames, household appliances and wind turbine blades. In recent years, the superiority of polyurethane composites in the preparation of turbine blades has received increasing attention. Wind energy is considered to be one of the cleanest and most environmentally friendly energy sources currently available. Therefore, wind turbines have been continuously demanded by the market. Compared with the turbine blades made of traditional epoxy resins, the turbine blades made of polyurethane composite materials have the advantages of lower cost and better mechanical properties. However, polyurethane is sensitive to water, and the core material used to produce turbine blades, such as balsa wood, PVC foam, etc., usually contain a certain amount of moisture, and therefore need to be dried prior to infusing the polyurethane liquid raw materials.

US 6264877 B l discloses a method of manufacturing a composite part, in particular a wind turbine blade of great length, which is made of a polymer-containing fabric or fiber, such as glass fiber or polypropylene fiber. A fabric is placed in two molds having a part shape, and a hollow envelope-shaped body is placed in the middle of the fabric within one of the mold sections. The molds are closed, and heated to 20°C under high pressure to melt the polypropylene of the fabric, so that the glass fiber is embedded. Then, it is cooled and demolded.

CN 102076485 B discloses a rotor blade and a method for manufacturing a rotor blade for a wind turbine as well as its manufacturing mold. The rotor blade extends longitudinally from the blade root area for connection to the rotor hub of the wind turbine up to the blade tip in a ready-to-use state and is divided into at least two sections for its manufacture, wherein there is at least one division between the blade root area and the blade tip in the direction approximately transverse to its longitudinal extension. The purpose of this invention is to simplify the manufacture of a rotor blade, in particular its mass production, and to shorten the production period, but still to supply a rotor blade like a conventional monolithic rotor blade.

At present, it is the common practice in the industry to heat the mold to dry the core material during the manufacture of turbine blades or their components. However, this method is inefficient, and it can only heat the bottom of the core material, and thereby it is difficult to completely dry its middle and upper parts. Hence, an effective drying method is still urgently needed by the industry.

According to the first aspect of the present invention, there is provided a method for drying a core material used for a polyurethane composite material, which comprises the following steps:

placing a core material on a support device, covering the core material with at least one film, sealing the periphery of the film and the support device with at least one channel reserved; heating the core material and evacuating the sealed space enclosed by the film and the device via the channel, until the core material is dry.

The support device may be a device for heating the core material, such as a mold that can be heated; it may also be a non-heating device, such as a platform or a mold for resting the core material. The device is preferably a mold, for example: a mold for a turbine blade and/or its components, a mold for an aircraft and/or its components, a mold for a hull and/or its components, a mold for a vehicle body and/or its components, and the like.

Preferably, at least one flow media is placed between the film and the core material prior to sealing the periphery of the film and the device. The flow media refers to a material having a porous structure, which may be a wattled, woven, knitted, extruded or crocheted material, foam, or a material having a mesh or mesh structure itself. Specifically, it includes, but is not limited to, woven flow mesh, pressed flow mesh, continuous fiber mats; and mixed flow mesh such as those made by mixing two or more fiber fabrics of woven flow mesh, pressed flow mesh and continuous felts and cut felts. The person skilled in the art is aware of that the material used as a flow media may include, but is not limited to, polystyrene (PS), polyurethane (PUR), polyphenylene oxide (PPO), polypropylene, ABS, and glass fiber fabrics and the like. The surface density of the material having a porous structure is preferably from 100 g/m ² to 500 g/m ² . The flow media is mainly used to assist in evacuating during the drying process and guiding the flow during the introduction of a polyurethane liquid material.

The core material is preferably heated by means of one, two or more heating elements selected of the group containing electric blanket heating, electric film heating, microwave heating, infrared heating, and hot air heating. The core material which is placed on the support device is covered preferably with at least two films, said at least two films are sealed at the periphery and provided with at least one inlet channel used to introduce hot air flow to heat the core material and one outlet channel used to expel the air flow from the space sealed by the two films. More preferably, the support device is also heated or the device further includes a heating element.

Prior to placing the core material on the device, a fiber reinforced material is preferably laid on the device and then the core material is laid on the fiber reinforced material.

More preferably, prior to placing the core material on the device, at least one fiber reinforced material is laid on the device, and then at least another fiber reinforced material is laid on the core material after placing the core material and before placing the film.

The fiber reinforced material is preferably selected from the group consisting of layers of randomly oriented glass fibers, glass fiber fabrics and glass fiber webs, cut or ground glass fibers or mineral fibers, and fiber mats, fiber non-wovens and fiber knitted fabrics based on polymer fiber, mineral fiber, carbon fiber, glass fiber or aramid fiber, and mixtures thereof.

The core material is preferably selected from the group consisting of balsa wood, PVC foam, SAN foam, polyurethane foam, PS foam, PMI foam, and PET foam.

Preferably, a peel ply is disposed between the core material or the fiber reinforced material and the flow media. The peel ply is used to assist in demolding and surface treatment of polyurethane resin. If there is no fiber reinforced material, it is placed between the core material and the flow media; if there is a fiber reinforced material, it is usually placed between the fiber reinforcing material and the flow media.

The device is preferably heated to 40-80°C for 1 -6 hours.

The surface temperature of the core material during heating preferably reaches 30-75°C, more preferably 35-70°C, particularly preferably 35-65°C. The thickness of the film is preferably 40-100 pm.

It has been surprisingly found, by means of experiments, that the method of the invention effectively dries the core material in a relatively short period of time, thereby effectively improving the production efficiency and quality of the polyurethane composite material.

According to a second aspect of the present invention, there is provided a method for preparing a polyurethane composite material containing a core material, comprising the following steps:

drying the core material by using any one of the aforementioned drying methods;

cooling the core material to room temperature, placing it in a mold and carrying out vacuum infusion of polyurethane, and curing the polyurethane resin to obtain the polyurethane composite material.

In the method for preparing a polyurethane composite material according to this specification, , the core material is selected from the group consisting of balsa wood, PVC foam, SAN foam, polyurethane foam, PS foam, PMI foam, and PET foam, with balsa wood being particularly preferred.

According to another aspect of the present invention, there is provided the use of a polyurethane composite material in a turbine blade.

The present invention will be described below by way of examples with reference to the accompanying drawings, in which:

Figure 1 : 1 indicates a mold; 2 indicates a sealing tape; 3 indicates a polyurethane infusion runner; 4 indicates the first film; 5 indicates the second film; 6 indicates hot air; 7 indicates a peel ply and a flow media; 8 indicates a core material; 9 indicates a fiber reinforced material; and 10 indicates a vacuum runner.

Figure 2: 1 shows a photograph of the appearance of a polyurethane composite material obtained by drying a core material according to the method of the present invention; 2 shows a photograph of the appearance of a polyurethane composite material obtained by drying a core material according to the method commonly used in the art.

Various aspects of the present invention will now be described in detail.

According to the first aspect of the present invention, there is provided a method for drying a core material used for a polyurethane composite material, which comprises the following steps:

placing a core material on a support device, covering the core material with at least one film, sealing the periphery of the film and the device with at least one channel reserved; heating the core material and evacuating the sealed space enclosed by the film and the device via the channel, until the core material is dry.

The support device may be a device for heating the core material, or a non-heating device, such as a platform or a mold for resting the core material. The device is preferably a mold. The mold includes, but not limited to, a mold for a turbine blade and/or its components, a mold for an aircraft and/or its components, a mold for a hull and/or its components, a mold for a vehicle body and/or its components, and the like. In an embodiment of the invention, the device is a mold that can be used to produce a turbine blade and/or its components by means of a polyurethane vacuum infusion method.

In an embodiment of the invention, a flow media is placed between the film and the core material prior to sealing the periphery of the film and the device.

In an embodiment of the invention, the core material is heated by means of one, two or more heating element devices selected from electric blanket heating, electric film heating, microwave heating, infrared heating, and hot air heating. Electric blanket or electric film heating means that the heating is carried out by providing a current to an electric blanket or an electric film under the mold or over the film. Other conventional heating methods in the art can also be used in the present invention.

According to a specific preferred embodiment of the invention, the core material which is placed on the support device is covered with at least two films, said at least two films are sealed at the periphery and provided with at least one inlet channel used to introduce hot air flow to heat the core material and one outlet channel used to expel the air flow from the space sealed by the two films. More preferably, the support device is heated simultaneously.

In an embodiment of the invention, prior to placing the core material on the device, a fiber reinforced material is laid on the device and then the core material is laid on the fiber reinforced material.

In a preferred embodiment of the present invention, prior to placing the core material on the support device, at least one fiber reinforced material is laid on the device, and at least another fiber reinforced material is laid on the core material after placing the core material and before placing the film.

Optionally, the fiber reinforced material is selected from the group consisting of layers of randomly oriented glass fibers, glass fiber fabrics and glass fiber webs, cut or ground glass fibers or mineral fibers, and fiber mats, fiber non-wovens and fiber knitted fabrics based on polymer fiber, mineral fiber, carbon fiber, glass fiber or aramid fiber, and mixtures thereof.

According to the method of the present invention, the core material is preferably selected from the group consisting of balsa wood, PVC foam, SAN foam, polyurethane foam, PS foam, PMI foam, and PET foam.

Optionally, a peel ply is disposed between the core material or the fiber reinforced material and the flow media. In an embodiment of the invention, the peel ply is disposed between the fiber reinforced material and the flow media.

According to the method of the present invention, the device is preferably heated to 40-80°C for 1-6 hours.

Preferably, the surface temperature of the core material during heating reaches 30-75°C, more preferably 35-70°C, particularly preferably 35-65°C.

The thickness of the film is preferably 40-100 pm. The experimental results show that the drying method of the present invention can increase the surface temperature of a core material and remove the moisture of a core material more quickly than the drying method of the prior art within an identical time period. In this way, polyurethane composite materials with better quality can be produced more efficiently.

According to the second aspect of the present invention, there is provided a method for preparing a polyurethane composite material containing a core material, comprising the following steps:

drying the core material by using any one of the aforementioned drying methods;

introducing a polyurethane liquid material into a mold and curing it to obtain the polyurethane composite material.

In the method for preparing a polyurethane composite material containing a core material according to the present invention, the polyurethane liquid material is introduced preferably by means of vacuum infusion.

According to the third aspect of the present invention, there is provided the use of the inventive polyurethane composite material in a turbine blade. The polyurethane composite material is produced by the aforementioned method for preparing a polyurethane composite material containing a core material.

Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by the person skilled in the art. In case that the definitions of terms herein are inconsistent with those commonly understood by the person skilled in the art, the definitions set forth herein shall prevail.

Unless otherwise indicated, all numbers for indicating the amounts of components, reaction conditions and the like used herein are understood to be modified by the term "about".

As used herein, the term "and/or" refers to one or all of the elements mentioned.

The terms "include" and "comprise" as used herein encompasses both the case that only the mentioned elements are present and the case that there are also other elements not mentioned in addition to the recited elements.

All percentages herein are by weight unless otherwise indicated.

The examples which follow provide additional illustration of the present invention without restricting the subject matter.

Examples

Raw materials:

Balsa wood (density: 160 g/m ² ; thickness: 1 inch): purchased from sino-compsite Company; Film: purchased from Leadgo-Tech Co., Ltd., thickness: 50 pm;

Sealing tape (article number: WD209): purchased from Shanghai Kangda New Materials Co., Ltd.;

Glass fabric (biaxial): purchased from Chongqing International Composite Materials Co., Ltd.;

Peel ply (gram weight: 95 g/m ² ): purchased from Leadgo-Tech Co., Ltd.;

Insulation blanket (specification: a width of 1 m, a length of 2 m, a thickness of 30 mm): purchased from a sponge factory;

Flow media: purchased from related market;

Polyurethane liquid raw material (article numbers: Baydur 78BD085 and Desmodur 44CP20): purchased from Covestro Polymers (China) Co., Ltd..

Test method:

Temperature measurement: using an infrared temperature gun to monitor the surface temperature.

Example 1 : A glass fabric was laid on a mold, and then balsa wood, another glass fabric, a peel ply, a flow media, and a film was sequentially placed thereon. The periphery of the film and the device were sealed and the film was tightened using a vacuum pump. Then, the second film was laid thereon and fixed by using sealing tape, with an inlet channel and an outlet channel reserved. The mold was heated while hot air was filled in between the first film and the second film, and the upper surface of the first film was provided with a temperature close to the mold temperature (the mold temperature was set to 50°C). After 2 hours, the heating and the hot air blowing were stopped, and the surface temperature of the balsa wood was measured as 40°C with a temperature gun. After cooling to room temperature, a polyurethane liquid raw material was infused and cured to obtain a polyurethane composite material (the surface condition is shown in FIG. 2-1). Comparative Example 1 :

A glass fabric was laid on a mold, and then balsa wood, another glass fabric, a peel ply, and a flow media were sequentially placed thereon, and they are finally covered with an insulation blanket. The mold was heated (the mold temperature was set to 50°C). After 2 hours, the heating was stopped, and the surface temperature of the balsa wood was measured as 33 °C with a temperature gun. After cooling to room temperature, a polyurethane liquid raw material was infused and cured to obtain a polyurethane composite material (the surface condition is shown in FIG. 2-2).

When comparing the polyurethane composite materials prepared from balsa wood treated with said two drying methods, it can be seen that the balsa wood treated with the drying method of the present invention (Example 1 , Fig. 2- 1) has only a slight and partial bulging of the polyurethane resin after polymerization, while the entire surface area of the polyurethane resin produced in Comparative Example 1 was completely bulging throughout the balsa wood (Fig. 2-2). Obviously, in case of carrying out the pre-dehumidifying drying for 2 hours under the same mold temperature, the drying method of the present invention can increase the surface temperature of the balsa wood more effectively than that in the prior art, so that the balsa wood is better heated and the moisture is more effectively removed.

Although the present invention has been described in detail in term of the purpose of the present invention, it is to be understood that this detailed description is merely illustrative. Besides the contents defined by the claims, various changes can be made by those skilled in the art without departing from the spirit and scope of the present invention.