Method for producing a fiber composite component and use of the method

Abstract

Method for producing a fiber composite component (10) containing recycled carbon fibers (2), in particular a fiber composite component (10) for a motor vehicle, comprising the steps: - Providing flat semi-finished fiber products (3) containing recycled carbon fibers (2), wherein the proportion of recycled carbon fibers (2) is at least 50 vol.%, based on the total volume of the fibers used in the semi-finished fiber product (3), - Production of a standard fibre composite component with a defined number of fibre semi-finished products (3), defined fibre material, defined matrix material and defined layer arrangement, - Determining the flexural rigidity of the fiber composite component (10) according to the following formula (1): B bv = φ ( t ) φ lit = E lit , where B bv (t) the flexural rigidity of the fiber composite component (10) to be produced, E bv the elastic modulus of the fiber composite component (10), t sollis the layer thickness of the fiber composite component (10) and v is the Poisson's ratio, where E bv is determined using the following formula (2): E bv = φ ( t ) φ lit E lit , where E lit the elastic modulus of the fiber composite standard component, φ lit is the fiber volume content of the fiber composite standard component and φ(t) is the fiber volume content in the fiber composite component (10), - Arranging a number of flat fiber semi-finished products (3) which have the desired bending stiffness B bv (t) results in - Impregnating the flat fiber semi-finished products (3) with matrix material (9) and - Hardening of the matrix material (9).

Description

[0001] The present invention relates to a method for producing fiber composite components from recycled carbon fiber material and to a use of the method according to the invention.

[0002] Fiber composite components are known in various designs from the state of the art. In automotive engineering, particular emphasis is placed on lightweight fiber composite components. For example, DE 102005026889 A1 describes door panels for motor vehicles with a center module made of a glass fiber-polyurethane fiber composite material, which can be manufactured, for example, through an injection molding process. Especially today, in light of resource conservation, waste reduction, and cost savings, the demand for recycled or recyclable fiber composite components, while simultaneously focusing on weight-reducing materials, is becoming a focus of the manufacturing industry. These requirements cannot be met with existing fiber composite materials.DE 10 2013 216 647 A1, DE 10 2012 206 384 A1, DE 10 2011 119 856 A1, DE 10 2011 079 525 A1, DE 10 2010 042 349 A1 and WO 2014 / 037724 A1 describe general processes for producing fiber composite components using (recycled) fibers.

[0003] Based on this prior art, it is therefore the object of the present invention to provide a method for producing fiber composite components which conserves raw material resources, reduces disposal costs, contributes to waste prevention and, at the same time, is easy to use without high technical expenditure and thus enables cost-effective production of fiber composite components in a lightweight design.

[0004] The object is achieved according to the invention by a method in which flat semi-finished fiber products containing recycled carbon fibers are specifically converted. Flat semi-finished fiber products that are used here are, for example, fiber mats, fiber scrims, fiber fleeces, fiber fabrics and the like, and thus such structures that usually have a small expansion in one dimension, the thickness direction or the wall thickness or layer thickness. The flat semi-finished fiber products contain recycled carbon fibers, wherein a proportion of recycled carbon fibers is at least 50 vol.%, in particular 100 vol.%, based on the total volume of the fibers used in the semi-finished fiber product. Such semi-finished fiber products are unbindered, i.e. resin-free, and can be produced by any desired process, wherein an exemplary production process i) a provision of dry fiber waste, such as offcuts, e.g.Fabric waste, production residues, preform offcuts, and the like; ii) fiber preparation by exposing and separating the fibers, e.g., by punching or cutting the fiber waste and opening it, essentially without damaging the fibers; and iii) the production of a flat fiber semi-finished product. Further or additional process steps may be provided.

[0005] Typically, high mechanical demands are placed on a fiber composite component. Fiber composite components must exhibit a certain flexural rigidity to be used as a supporting or stiffening structure. To also consider the requirements for low dead weight, the invention determines the expected flexural rigidity of the fiber composite component to be manufactured in advance. This is then used to determine a minimum number of flat fiber semi-finished products to be used or, given the flexural rigidity, a minimum wall thickness of the fiber composite component. The flexural rigidity of the fiber composite component to be manufactured is determined according to the following formula (1). Bbv(t)=Ebv⋅tsoll312(1−υ2) where B bv (t) the flexural stiffness of the fiber composite component to be manufactured, E bv the elastic modulus of the fiber composite component, t sollis the layer thickness of the fiber composite component, and v is the Poisson's ratio. v, the Poisson's ratio, is a dimensionless material constant. It indicates the ratio of the relative transverse strain to the relative longitudinal strain: v=εyyεxx

[0006] That is, it indicates how the cross-section changes when the length changes. For fiber composite components, the general rule in the literature is v=0.35. This value can also be determined analogously using the rule of mixtures.

[0007] To determine the elastic modulus of the fiber composite component E bvTo be able to determine the elasticity, a fiber composite standard component is manufactured according to the invention with a defined number of fiber semi-finished products, defined fiber material (quantity and chemical nature of the fiber material), defined matrix material (quantity and chemical nature of the matrix material), and defined layer arrangement (layering of the fiber semi-finished products). The fiber composite standard component serves to determine the parameters for the fiber composite component to be manufactured and has the same physical and chemical nature (matrix material, fiber material, layer arrangement, etc.) as the fiber composite component to be manufactured. Thus, the elastic modulus E bv of the fiber composite component to be manufactured is determined using the following formula (2): Ebv=φ(t)φlitElit,

[0008] Here, E lit the elastic modulus of the fiber composite standard component, φ litis the fiber volume content of the fiber composite standard component and φ(t) is the fiber volume content in the fiber composite component. The elastic modulus E lit of the standard fibre composite component can be determined by non-destructive experimental tests or by mathematical calculation.

[0009] Subsequently, the flexural rigidity determined for the fiber composite component to be manufactured can be related to a desired flexural rigidity. For example, if an existing fiber composite component with a certain flexural rigidity is to be replaced by the fiber composite component manufactured according to the invention with a minimum 50 vol. % recycled carbon fiber content, the flexural rigidity B determined for the fiber composite component manufactured according to the invention bv(t) provides a good starting point for estimating the mechanical equivalence of fiber composite components. Only by correlating the flexural rigidity of the fiber composite component containing recycled carbon fibers with the desired overall flexural rigidity can a fiber composite component with outstanding mechanical and thus high-quality properties be produced, while maintaining minimal dead weight due to the low dead weight of carbon fibers compared to conventional glass fibers.

[0010] From the determined bending stiffness of the fiber composite component to be manufactured, taking into account the wall thickness t soll(this corresponds to the layer thickness of the fiber composite component) the number of semi-finished fiber products to be used for the fiber composite component can be determined. Alternatively, the flexural stiffness of the reference fiber composite component can also be used to determine a minimum wall thickness of the fiber composite component to be manufactured for a given number of flat fiber semi-finished products.

[0011] Once the number of flat fiber semi-finished products required to achieve the desired flexural rigidity has been determined, the flat fiber semi-finished products are arranged, for example, stacked, and impregnated with matrix material. After the matrix material has cured, a highly rigid, lightweight fiber composite component is obtained. Due to the use of recycled carbon fiber materials, this component conserves raw materials, reduces disposal costs, and contributes to waste prevention. The process can be implemented without significant technical effort and is therefore cost-effective. Due to the determination of the minimum required number of fiber semi-finished products to be used, it enables the production of fiber composite components with very thin walls without compromising stability. This makes the process according to the invention particularly suitable for producing a fiber composite component for a motor vehicle.

[0012] The subclaims contain advantageous developments and refinements of the invention.

[0013] Due to their good processability and cost-effective production, flat semi-finished fiber products in the form of nonwovens are particularly suitable. All semi-finished fiber products used, i.e., arranged in a nonwoven fabric, are particularly advantageous.

[0014] Furthermore, the nonwovens advantageously have an isotropic or anisotropic fiber orientation. An anisotropic fiber orientation can be achieved, for example, through a carding process, in which the prepared fibers are aligned or oriented after opening the fiber waste materials, for example, by combing. The use of anisotropically aligned fiber semi-finished products has proven particularly advantageous in the case of potentially anisotropic load effects.

[0015] A further reduction in weight and cost can be advantageously achieved by using recycled carbon fibers as the sole fiber material in the flat fiber semi-finished products.

[0016] The stability of the fiber composite component to be manufactured can be advantageously improved by incorporating at least one reinforcing element on and / or between the flat fiber semi-finished products to be arranged. So-called UD tapes, i.e., unidirectionally arranged fiber strips that can be easily arranged between or on the fiber semi-finished products, are particularly suitable for this purpose.

[0017] A particularly simple process for converting flat fiber semi-finished products with matrix material is made possible by the use of an SRIM process. Structural reaction injection molding (SRIM) is a process in which fiber semi-finished products are placed in an open mold and then sprayed with matrix material, preferably polyurethane, using a spray head. After the matrix material has been applied, the mold is closed, and the matrix material is cured. The application of temperature and / or pressure can be advantageous here. The SRIM process enables homogeneous matrix material application and thus contributes to improving the quality of the fiber composite component being manufactured.

[0018] The invention also provides a use of the method described above. This use involves the production of trim components for a motor vehicle, in particular for a door panel.

[0019] Further details, features, and advantages of the invention will become apparent from the following description and the figures. It shows: Fig. 1 a schematic representation of the individual process steps of an embodiment of the process according to the invention for producing a fiber composite component and Fig. 2 a graph in which for different wall thicknesses t soll of a fiber composite component the flexural stiffness B bv (t) is plotted.

[0020] The present invention will be explained in detail using an embodiment. In Fig. 1 only the aspects of the method according to the invention that are of interest here are shown, all other aspects are omitted for the sake of clarity.

[0021] Fig. 1 illustrates a method for producing a fiber composite component according to an advantageous embodiment of the invention.

[0022] Only the steps essential to the invention are shown. Additional process steps can complete the method.

[0023] First, fiber waste 1 is provided. The fiber waste 1 is unbindered, i.e., resin-free or matrix-free. In other words, it is dry fiber waste, such as offcuts from fabric production, production residues, or preform offcuts. The fiber waste 1 contains at least one fiber material that preferably consists exclusively of carbon fibers.

[0024] In a first step A, the fiber waste 1 is prepared, whereby fibers 2 are exposed from the fiber waste 1 by separating them. This is done, for example, by punching or cutting the fiber waste 1 and opening them, whereby the fibers 2 are essentially undamaged. Prepared fibers 2 are obtained. These have an isotropic alignment and can be anisotropically aligned through a subsequent carding process.

[0025] In step B, the fibers 2 are further processed into a flat semi-finished fiber product 3, which is preferably in the form of a nonwoven. The flat semi-finished fiber product 3 has a flat extension in the xz direction and a smaller extension in the y direction. The extension of the semi-finished fiber product 3 in the y direction corresponds to the layer thickness or wall thickness of the semi-finished fiber product 3 and is determined perpendicular to the xz plane.

[0026] The flat semi-finished fiber product 3 thus contains recycled carbon fibers, with a proportion of recycled carbon fibers being at least 50 vol.%, preferably 100 vol.%, based on the total volume of the fibers 2 used in the semi-finished fiber product 3.

[0027] The flexural rigidity of the fiber composite component to be manufactured is then determined mathematically according to the following formula (1): Bbv(t)=Ebv⋅tsoll312(1−υ2) where B bv (t) the flexural stiffness of the fiber composite component to be manufactured, E bv the elastic modulus of the fiber composite component to be manufactured, t soll is the layer thickness of the fiber composite component to be produced and v is the Poisson's ratio (here: 0.35).

[0028] From the flexural rigidity of the fiber composite component to be manufactured, if a desired flexural rigidity is specified, for example, the minimum number n bvon flat fibre semi-finished products 3 to be used, which produce at least the same good mechanical properties with a reduced dead weight of the fibre composite component.

[0029] This will be explained in detail using the following example: A reference door panel for a motor vehicle made of a glass fiber-polyurethane composite material serves as the example. It has a flexural stiffness of 19915.97 Nmm.

[0030] A nonwoven fabric containing a fiber material made entirely of recycled carbon fibers is used as the semi-finished fiber. Five layers of the nonwoven fabric are to be used in the fiber composite component, as an example.

[0031] First, the fiber volume content φ(t) of the fiber composite component is determined, which, as described above, is to be manufactured from five fiber mats and a defined amount of matrix material (in this case: polyurethane). The fiber volume content φ(t) is determined as follows: φ(t)=nbv⋅Ybvtsoll⋅ρcf

[0032] Here, n bv the number of fiber fleeces to be arranged (in this case: five), Y bv the basis weight of a nonwoven fabric (this can be determined experimentally), ρ cr the density of the fiber material, in this case the density of the recycled carbon fibers (this value can be taken from the literature (see Schürmann, H.: Constructing with fiber-plastic composites, Springer-Verlag, Berlin / Heidelberg / New York, 2007) and t soll the desired wall thickness of the fiber composite component.

[0033] The following fiber volume content φ(t) results: φ(t)=5⋅0.0096gcm20.26cm⋅1.7gcm3=0.11.

[0034] It can be seen that the fiber volume content φ(t) is a function of two parameters, the number of fiber webs n bv and the wall thickness t soll of the fiber composite component to be manufactured. This means that the required number of fiber fleeces n bv for a given bending stiffness of the fiber composite component to be manufactured, or the minimum wall thickness t soll of the fiber composite component depending on a defined number of fiber fleeces.

[0035] Next, the elastic modulus (E-modulus) of the fiber composite component to be manufactured E bv determined: Ebv=φ(t)φlitElit Ebv=0.110.39⋅41500Nmm2=11556Nmm2.

[0036] The fiber volume content determined above is taken into account here.

[0037] The values ​​E lit and φ litoriginate from experiments. For this purpose, a fiber composite standard component was manufactured with a defined number of fiber semi-finished products, a defined fiber material (quantity and chemical nature of the fiber material), a defined matrix material (quantity and chemical nature of the matrix material), and a defined layer arrangement (layering of the fiber semi-finished products). The chemical nature (fiber material, matrix material, layer arrangement) of the fiber composite standard component was identical to the chemical nature of the fiber composite component to be manufactured. The fiber composite standard component serves as a template for determining the Young's modulus E lit at a defined fiber volume content φ lit , which can be determined through tensile / bending tests. In the equation above, the Young's modulus is taken from the literature (see Schürmann, H.: Designing with Fiber-Reinforced Plastics, Springer-Verlag, Berlin / Heidelberg / New York, 2007).

[0038] Next, the mass of recycled carbon fibers mcf in the fiber composite component to be manufactured. This is required for the total mass. The mass of the recycled carbon fibers m cf in the fiber composite component is as follows: mcf=nbvYbvA, where A is the area of ​​the fiber composite component: mcf=5⋅0.0096gcm2⋅8000cm2=384g.

[0039] The mass fraction of matrix material ψ (in this case: polyurethane) is then determined. The mass fraction of matrix material can then be used to determine the mass of the matrix material in the fiber composite component: ψ=ρcf⋅φ(t)ρcf⋅φ+(1−φ(t)) ψ=1.7gcm3⋅0.111.7gcm3⋅0.11+1.1gcm3⋅(1−0.11)=0.16.

[0040] Here, φ is the previously determined fiber volume content φ(t), ρ cf the density of the fiber material, for example the density of the recycled carbon fibers in the fiber semi-finished product 3 and ρ puthe density of the matrix material (these values ​​are taken from the literature; see e.g. Schürmann, H.: Constructing with fiber-plastic composites, Springer-Verlag, Berlin / Heidelberg / New York, 2007).

[0041] With the mass fraction ψ, the mass of matrix material m pu in the fiber composite component: mpu=mcf=mcf(1−ψ)ψ mpu=384g⋅1−0.160.16=2039.6g.

[0042] The values ​​are derived from the above equations. Thus, the respective masses of matrix material and recycled carbon fiber are determined, and the total mass of the reference fiber composite component can be determined from the sum of the two individual masses: mpl=mcf+mpu, mpl=384g+2039.6g=2423.5g.

[0043] At the end, the flexural stiffness of the reference fiber composite component B bv (t) is determined. Bbv(t)=Ebv⋅tsoll312(1−υ2) Bbv(t)=11556Nmm2⋅2.63mm312⋅(1−0.352)=19288.31Nmm.

[0044] Included here are the E-module E bv , the target wall thickness t soll , as well as the Poisson's ratio v of the fiber composite component (here: 0.35).

[0045] The calculation is then carried out for different combinations of different wall thicknesses t soll and different numbers of fiber fleeces n bv (Varying the wall thicknesses from 1 mm to 3.5 mm in 0.2 mm increments, as well as 1-7 layers of fiber fleece, for example) is repeated. The resulting flexural stiffnesses are then compared with the specified flexural stiffness for the corresponding reference component made of glass fiber-polyurethane composite material. This allows the possible combinations to be determined.

[0046] Fig. 2 is a graph showing for different wall thicknesses t soll of a fiber composite component the flexural stiffness B bv(t). The graph shows three curves. Curve A is the curve of a reference fiber composite component made of glass fibers as the fiber material and polyurethane as the matrix material. Curve B is the curve of a fiber composite component made of a fiber nonwoven according to the invention as the fiber material and polyurethane as the matrix material. Curve C is the curve of a fiber composite component made of two fiber nonwovens according to the invention as the fiber material and polyurethane as the matrix material. The flexural rigidity B increases with the number of fiber nonwovens. bv (t) of the fiber composite component. Fig. 2 shows that, for a wall thickness of, for example, 3.4 mm, the flexural rigidity of a fiber composite component made of one layer of fiber fleece corresponds to the flexural rigidity of the fiber composite component made of glass fibers. It is clear that when using two fiber fleeces instead of just one, the wall thickness t sollto about 2.4 mm (intersection on curve C).

[0047] For example, if the desired bending stiffness is 30000 Nmm (see horizontal line in the graph in Fig. 2), this is achieved in a fiber composite component with two fiber fleeces according to the invention and polyurethane as the matrix material with a wall thickness of about 2.65 mm. When using only one fiber fleece (see dashed line in the graphic in Fig. 2) a wall thickness of approximately 3.8 mm would have to be provided.

[0048] Here, the wall thickness t soll Use the following formula to determine the number of fiber fleeces to be used to achieve the desired flexural rigidity: nbv=tsollφ(t)ρφYbv

[0049] The calculations are not dependent on the matrix material used. Any other matrix material can be used. Furthermore, it does not have to be nonwovens; other semi-finished fiber products can also be calculated using this method.

[0050] The correspondingly determined number of semi-finished fiber products 3 is, as in Fig. 1, arranged one above the other to form a stack 4 in step C.

[0051] In step D, the stack 4 is inserted into a mold. The mold has an upper mold half 5 and a lower mold half 6, with the two mold halves 5, 6 defining a gap 7 between them. A spray head 8 for applying a matrix material 9 (resin, polymer) is provided on the upper mold half 5. The semi-finished fiber products 3 are then impregnated with matrix material 9.

[0052] Step E illustrates the forming process of the fiber composite component. The mold has been closed, so that a certain pressure and possibly also an elevated temperature act on the stack 4 inside. This deforms the semi-finished fiber stack 4 and cures the matrix material 9.

[0053] In step F, the mold is opened. The resulting fiber composite component 10 can be demolded.

[0054] Advantageously, the produced fiber composite component 10 is a trim component for a motor vehicle, in particular a door panel. The fiber composite component 10 produced according to the method described above contains recycled carbon fibers and therefore has very good mechanical properties while being lightweight. The use of recycled carbon fibers conserves raw material resources, reduces disposal costs, and avoids waste production. The process is simple, requires minimal technical effort, and is therefore cost-effective.

[0055] The foregoing description of the present invention is for illustrative purposes only and not for the purpose of limiting the invention. Various changes and modifications are possible within the scope of the invention and its equivalents. List of reference symbols: 1 Fiber waste 2 fibers 3 semi-finished fiber products 4 stacks 5 upper tool half 6 lower tool half 7 gap 8 spray head 9 Matrix material 10 Fiber composite component

Claims

[1] Method for producing a fiber composite component (10) containing recycled carbon fibers (2), in particular a fiber composite component (10) for a motor vehicle, comprising the steps: - Providing flat semi-finished fiber products (3) containing recycled carbon fibers (2), wherein the proportion of recycled carbon fibers (2) is at least 50 vol.%, based on the total volume of the fibers used in the semi-finished fiber product (3), - Production of a standard fibre composite component with a defined number of fibre semi-finished products (3), defined fibre material, defined matrix material and defined layer arrangement, - Determining the flexural rigidity of the fiber composite component (10) according to the following formula (1): Bbv=φ(t)φlit=Elit, where B bv (t) the flexural rigidity of the fiber composite component (10) to be produced, E bv the elastic modulus of the fiber composite component (10), t sollis the layer thickness of the fiber composite component (10) and v is the Poisson's ratio, where E bv is determined using the following formula (2): Ebv=φ(t)φlitElit, where E lit the elastic modulus of the fiber composite standard component, φ lit is the fiber volume content of the fiber composite standard component and φ(t) is the fiber volume content in the fiber composite component (10), - Arranging a number of flat fiber semi-finished products (3) which have the desired bending stiffness B bv (t) results in - Impregnating the flat fiber semi-finished products (3) with matrix material (9) and - Hardening of the matrix material (9). [2] Method according to claim 1, characterized by that the flat semi-finished fibre products (3) are fibre fleeces. [3] Method according to claim 2, characterized by that the nonwoven fabrics have an isotropic or anisotropic fiber orientation. [4] Method according to one of the preceding claims, characterized bythat the flat fiber semi-finished products (3) contain recycled carbon fibers (2) as the only fiber material. [5] Method according to one of the preceding claims, characterized by introducing at least one reinforcing element onto and / or between the flat fiber semi-finished products (3) to be arranged. [6] Method according to one of the preceding claims, characterized by that the fiber composite component (10) is manufactured using an SRIM process. [7] Use of the method according to one of the preceding claims for producing trim components for a motor vehicle, in particular for a door trim.

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