DE102011115966B4 - A method of making a CFRP molding material and method of making a particulate carbon fiber recyclate - Google Patents

Abstract

A method for producing a carbon fiber recyclate for CFRP components, comprising the steps of: comminuting a CFRP material which has carbon fiber bundles (CFC) in a matrix material into a recyclate starting material in which carbon fiber bundles surrounded by matrix material Sections (BA) are present, - subjecting the recyclate starting material to a decomposition treatment in the form of pyrolysis or carbonization of the matrix material, the matrix resin losing 15 to 40% by weight of its original weight and binding the carbon fibers of the bundle sections (BA) remain in that the pyrolyzed or carbonized matrix material continues to act as a binder between the fibers and the carbon fibers (1) are essentially exposed on a circumference of the bundle sections (BA), and thereby form a particulate carbon fiber recyclate (KFR)

Description

The invention relates to a process for producing a particulate carbon fiber recyclate and also to a process for producing a CFRP molding material therefrom. Furthermore, a CFK molding compound is disclosed.

The recycling of recyclable materials is becoming increasingly important in view of the scarcity of raw materials. For example, in many areas different materials are recycled in order to produce new objects. The recycling of fiber-containing materials, in particular fiber-reinforced plastics, is known.

So describes EP 0 407 925 B1 a recycled plastic composition of an unsaturated polyester resin and crushed plastic waste, which consists essentially of crosslinked polyester resin, reinforcing fibers and optionally fillers and contains more than 50 wt .-% fiber content with a length of more than 0.5 mm. The plastic waste is first coarsely crushed and then smashed so that individual glass fibers are herauspräpariert, the fibers are still largely covered with plastic. The prepared fibers are essentially in the form of elongated tufts or bundles of fibers, which are held together by the remaining plastic of the original component. Unsaturated polyester resins are added to the shredded plastic waste. In addition to the unsaturated polyester resin and the shredded plastic waste, inert plastic fillers and conventional synthetic resin additives can be added to the recycled plastic molding compound.

EP 0 566 830 B1 refers to a method and apparatus for making a thermosetting molding compound that includes a shredded and thus spreadable recycled fiberglass (FRP) recycled material. In this process, a resin paste is premixed with thickening agent and then continuously mixed with the spreadable GRP recyclate based on cross-linked fiber-reinforced thermosetting plastic. The recycled-resin mass is then applied in web form. The recycled material is shredded fiber-resin materials such as SMC mats (prepregs), premix molding compounds (BMC), injection molding compounds or continuously impregnated compounds, which have been processed into granules by defined comminution.

DE 10 2008 002 846 A1 is directed to a waste treatment process and an arrangement therefor, in particular to a recycling process for fiber-reinforced or fiber-containing semi-finished products or components. As a separation process for fiber separation, solvolysis, catalysis, hydrolysis and pyrolysis are mentioned. In the preferred pyrolysis, the matrix material is completely separated from the fibers and free fibers are obtained which, after separation, are wetted with a binder; to feed the recycled fibers to a new manufacturing process. A continuous oven process is also proposed to recycle fiber-reinforced or fiber-containing semi-finished products or components.

In the DE 43 25 775 A1 Waste pieces of CFRP or CFC composites shall be made using new C-fiber composites, with maximum reuse of the carbon content contained in the composites. It is intended to carbonize the starting materials and then, optionally after a comminution step, to mix with a cokeable matrix precursor, to form the mixture into bodies and to coke the shaped bodies. The DE 91 08 065 U1 discloses that can be prepared from regrind of glass fiber reinforced thermoplastic material and textile fibers and optionally up to 40 wt .-% of curable resins molding compounds that can be supplied to new, high-quality applications.

If fibers thus obtained are further processed using matrix material to form new semifinished products using mixing devices, the use of kneaders, for example, causes high shear stresses, which disadvantageously largely dissolve remaining fiber bundles and damage the fibers. D. H. especially severely shorten, whereby the mechanical performance of the material suffers. Due to the destruction of the fiber bundles and the fiber shortening only small volume contents of reinforcing fibers in the semifinished products thus produced can be achieved. In the further processing of the semifinished product to the finished component occurs by further shear forces, which arise for example in injection molding, a further damage to the reinforcing fibers.

To solve this problem, it has been proposed to embed the fiber bundles in a sufficiently solid matrix of a thermoplastic or a crosslinked thermosetting material and incorporated in the form of cut fiber bundles ("rods") in the semifinished product. As a result, damage to the fibers can be avoided and better mechanical properties of the finished component can be achieved.

The disadvantage of this procedure is that the adhesion of the matrix material to the impregnation of thermoplastic or crosslinked thermoset is too low and also only one relatively low surface area for attachment of matrix to the fiber is available.

It is an object of the present invention to provide a process for producing an improved carbon fiber recyclate which is a suitable carbon fiber recyclate as a starting material for processing into a semifinished product. Next, the task of producing a likewise suitable for semi-finished and continue to make the component CFRP molding compound.

• – Zerkleinern eines CFK-Materials, das Kohlenstofffaser-Bündel in einem Matrixmaterial aufweist, in ein Rezyklat-Ausgangsmaterial, in dem von Matrixmaterial umgebene Kohlenstofffaser-Bündel-Abschnitte vorliegen,
• – Unterziehen des Rezyklat-Ausgangsmaterials einer Zersetzungsbehandlung des Matrixmaterials bis zu einem Grad, bei dem die Kohlenstofffasern der Bündel-Abschnitte gebunden bleiben und die Kohlenstofffasern an einem Umfang der Bündel-Abschnitte im Wesentlichen frei liegen, und dabei Bilden des partikulären Kohlenstofffaser-Rezyklats.

These objects are achieved by the methods having the features of claims 1 and 6. Advantageous developments are set forth in the subclaims. In a first aspect, a process for producing a particulate carbon fiber recyclate according to the invention comprises the steps of:

• Crushing a CFRP material comprising carbon fiber bundles in a matrix material into a recycled raw material in which carbon fiber bundle sections surrounded by matrix material are present,
• Subjecting the recycled raw material to a decomposition treatment of the matrix material to a degree that the carbon fibers of the bundle portions remain bonded and the carbon fibers are substantially exposed at a periphery of the bundle portions, thereby forming the particulate carbon fiber recyclate.

The term "recycled raw material" used here means an educt of carbon fiber bundles comminuted in (longitudinal) sections, wherein quasi fiber sections as ordered fiber pieces form the bundles and are surrounded by matrix material and optionally by fillers and other additives. The recycled raw material is used to produce the intermediate "carbon fiber recyclate", which in turn is an educt for CFRP molding compounds and components thereof.

The decomposition treatment may be a thermal treatment in which the matrix material is at least partially converted to a char which holds the fibers of the bundle sections bound. The thermal treatment according to the present invention is therefore not pyrolysis in the classical sense, but a carbonization of the plastic matrix, which in addition to volatile hydrocarbons and other carbon-containing gases free carbon and the char produced. In doing so, the matrix material is decomposed by a bundle such that fibers exposed to the outside at the periphery of the bundle are present, while the "core" of the bundle of charred product remains bound. For the purposes of the present invention, "charring product" means that the matrix material is carbonized to such an extent that the properties of the matrix resin are only present so far that it continues to act as a binder between the fibers. The thermal treatment is preferably carried out in the absence of air or under non-oxidizing conditions. Preferably, the pyrolysis or carbonation is adjusted so that the matrix resin loses about 15 to 40% by weight of its original weight.

The particles thus obtained may be pellet-shaped. The marginal free fibers predestine this carbon fiber recyclate for use in CFRP molding compounds, since these fibers support the uptake of fresh matrix material during further processing. By means of the decomposition treatment according to the invention, 2 to 5 exposed fiber layers can advantageously be created with a suitable choice of the decomposition conditions.

In order to adjust the number of fiber layers, the conditions of temperature, pressure, and duration can be adjusted accordingly, for instance during the thermal decomposition.

Depending on the type of plastic matrix after the thermal treatment, or pyrolysis or carbonization, the charring product is microporous, which likewise promotes the penetration of fresh or "pure" matrix material into CFRP molding compound during further processing. This leads to an improved connection between matrix and fiber bundles.

As an alternative to the thermal decomposition treatment, this decomposition treatment may also be a wet-chemical or an etching treatment of the near-surface matrix components of the bundle portions. Again, the exposed fiber ply number can be adjusted by the duration of the treatment time or the choice of chemical.

It has proved to be advantageous if the carbon fiber bundles have a width of 1 mm to 4 mm, a thickness of 0.1 mm to 0.3 mm and a length of 5 mm to 30 mm. This defined ratio of width to thickness to length (which, among other things, results from the Tex number of the CF rovings originally used) serves to ensure uniform processing conditions and a consistent material quality.

• – Bereitstellen eines Kohlenstofffaser-Rezyklats, das durch das vorstehende Verfahren zur Herstellung eines partikulären Kohlenstofffaser-Rezyklats erhältlich ist,
• – Vermischen des Kohlenstofffaser-Rezyklats mit zumindest einem aushärtbaren Matrixmaterial, dabei Imprägnieren zumindest der freiliegenden Fasern am Umfang der Bündel-Abschnitte und Erhalten der CFK-Formmasse, wobei der Volumengehalt des Kohlenstofffaser-Rezyklats an der CFK-Formmasse bis zu 70% beträgt.

In another aspect, the present invention relates to a process for producing a CFRP molding material. It includes the steps:

• Providing a carbon fiber recyclate obtainable by the above process for producing a particulate carbon fiber recyclate,
• Mixing the carbon fiber recyclate with at least one curable matrix material, thereby impregnating at least the exposed fibers at the periphery of the bundle sections and obtaining the CFK molding compound, wherein the volume content of the carbon fiber recycled material in the CFRP molding compound is up to 70%.

With this method, it is possible to produce a CFRP molding compound from recycled materials, where otherwise expensive raw materials would have to be used. The penetration of the matrix resin into the exposed fibers at the periphery of the bundle sections and into the porous material holding the bundle sections together provides a very good bond between the virgin matrix material and the carbon fiber recyclate, and thus a high quality CFK molding compound with a high content of organized fibers.

In a development, the carbon fiber recyclate can be pre-impregnated with a pure resin, resulting in more stable bundles, so that lower shear loads occur during the kneading step.

The higher the volume content of the fiber bundles in the CFK molding compound, the higher the rigidity properties can be achieved later in the component produced therefrom. Therefore, the volume content of the carbon fiber recyclate is preferably up to 50%. Incidentally, the matrix resin may be a low-matrix matrix resin. Depending on the choice of low-matrix matrix resin, it may be possible to dispense with the addition of mineral fillers, which otherwise influence the shrinkage. Furthermore, a wetting agent and / or fillers may be present in the matrix resin. The wetting of the fiber bundles or the exposed individual fibers can be further improved by the use of a suitable wetting agent. In this respect, the relevant properties for the production of BMC materials of the matrix resin and the molding compound can be maintained.

Mixing by kneading can be carried out by means of a blade kneader or a co-rotating twin-screw extruder. Blade kneaders have a moderate shearing action on the CFRP molding compound, so that the contained reinforcing fibers are not excessively damaged. Even synchronous twin-screw extruders work with low shear.

A third aspect of the present invention relates to a CFRP molding composition comprising a carbon fiber recyclate obtainable by the method of the invention. It contains at least one curable matrix material which penetrates at least the exposed fibers at the periphery of the bundle sections. The volume content of the carbon fiber recyclate in the CFK molding compound is typically 40 to 60%, and preferably 60 to 70%.

CFRP molding compounds are used, for example, for the production of BMC materials; produced in accordance with the invention they are, and thus the components obtained, more efficient. Furthermore, the use of recycled material results in a significant cost reduction compared with the use of "new" fiber materials.

These and other advantages are set forth by the following description with reference to the accompanying figures. The reference to the figures in the description is to aid in the description and understanding of the subject matter. The figures are merely a schematic representation of an embodiment of the invention.

1 shows a process flow diagram.

2 shows a cross section through pellets of a carbon fiber recyclate.

The process flow diagram in 1 shows the essential steps in the production of a particulate carbon fiber recyclate: By crushing a CFRP material, which is incurred as waste, for example, as a blend in the production of CFRP components, and the carbon fiber bundle KFB in a matrix material, arises a recycled raw material in which carbon fiber bundle sections BA surrounded by matrix material are present. The bundle sections 1 that are retained in the product (see 2 ) should have a width of 1 mm to 4 mm, a thickness of 0.1 mm to 0.3 mm and a length of 5 mm to 30 mm.

Then, the recycled raw material is subjected to a decomposition treatment, whereby the matrix material decomposes to a degree where the carbon fibers of the bundle portions BA remain bound and the carbon fibers 1 be exposed on the outside of the circumference of the bundle sections BA. Thus, the carbon fiber recyclate KFR is obtained.

2 schematically shows microporous matrix material 2 which remains in the carbon fiber recyclate KFR after a thermal treatment, marginally exposed fibers 1 of the fiber bundle portion BA and the core forming fibers three , The fibers 1 . three in the sections BA are aligned substantially parallel to each other. Due to incomplete thermal decomposition, the char remains three as a binder between the carbon fibers 2 and causes their cohesion.

Optionally, the resulting carbon fiber recyclate may first be preimpregnated with a pure resin before it is impregnated in the next step of the process with a matrix resin which may additionally contain a wetting agent and optionally fillers.

By mixing the carbon fiber recyclate KFR with at least one curable matrix material, the CFRP raw material is obtained in a further process. Mixing may be done by kneading in a co-rotating twin screw extruder or a paddle kneader. Thus, the kneading process is carried out applying as little shear as possible. The CFRP molding compound is further processed to CFRP components using methods known to those skilled in the art.

If it is desired to achieve an even better connection between fiber bundles of the carbon fiber recyclate KFR and the freshly added matrix material, the carbon fiber recyclate KFR can be pre-impregnated with a pure resin, ie a filler and additive-free matrix resin. This matrix material may be the same as that which is mixed and kneaded with the KFR carbon fiber recyclate.

By using the carbon fiber recyclates according to the invention, both the technical properties of a carbon fiber-reinforced BMC material can be improved, as well as significantly reduced by the material costs for the reinforcing fibers.

In principle, other conventional reinforcing fibers such as glass fibers, ceramic fibers and metal fibers are recyclable according to the invention.

Claims (7)

A process for producing a carbon fiber recyclate for CFRP components, comprising the steps of: - crushing a CFRP material comprising carbon fiber bundles (KFB) in a matrix material into a recycled raw material in which carbon fiber bundles surrounded by matrix material Sections (BA) are present, subjecting the recycled material to decomposition treatment in the form of pyrolysis or carbonization of the matrix material, wherein the matrix resin loses 15% to 40% by weight of its original weight and the carbon fibers bind to the bundle sections (BA) remain by the pyrolyzed or carbonized matrix material continues to act as a binder between the fibers, and the carbon fibers ( 1 ) are substantially exposed at a periphery of the bundle sections (BA), - thereby forming a particulate carbon fiber recyclate (KFR) The method of claim 1, wherein the char product ( three ) is microporous and in particular the particulate carbon fiber recyclate (KFR) is pellet-shaped. Method according to at least one of claims 1 or 2, wherein the bundle sections ( 1 ) have a width of 1 mm to 4 mm, a thickness of 0.1 mm to 0.3 mm and a length of 5 mm to 30 mm. A process for producing a CFRP molding composition, comprising the steps of: - producing a carbon fiber recyclate (KFR) by the process according to at least one of claims 1 to 3 - mixing the carbon fiber recyclate (KFR) with at least one hardenable matrix material, thereby impregnating at least the exposed fibers ( 1 ) at the periphery of the bundle sections (BA), thereby obtaining the CFRP molding compound, wherein the volume content of the carbon fiber recyclate in the CFRP molding compound is up to 70%. The method of claim 4, wherein upon impregnation the matrix material comprises the microporous char ( three ) at least partially penetrates. Method according to claim 4 or 5, comprising the step: - Preimpregnating the carbon fiber recyclate (KFR) with a pure resin. Method according to at least one of claims 4 to 6, in which the mixing is carried out by kneading by means of a blade kneader or a synchronous twin-screw extruder.

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