HK1216091B - Pyrolysis system and method of recovering carbon fibres from carbon-fibre-containing plastics - Google Patents

Description

Pyrolysis system and method for recovering carbon fibers from carbon fiber-containing plastics

Technical Field

The present invention relates to the field of recovering (recycling) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre reinforced plastics (CFP), preferably from carbon fibre-containing or carbon fibre reinforced composites.

In particular, the invention relates to a pyrolysis device for recovering (reusing) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibre-containing and/or carbon fibre-reinforced composites (composite materials), and to the use thereof for recovering (reusing) carbon fibres from carbon fibre-containing plastics.

Furthermore, the present invention relates to a method for recovering (reusing) carbon fibers from carbon fiber-containing plastics, in particular from carbon fiber reinforced plastics (CFP), preferably from carbon fiber-containing or carbon fiber reinforced composites (composite materials), as well as the recovered carbon fibers obtained by this method and their use.

The invention also relates to a plastic, building material or cement-containing system comprising or prepared using the recycled carbon fibres obtained by the process of the invention.

Finally, the present invention relates to shaped bodies (e.g. parts), moulds and sheet-like materials (such as nonwovens), in particular in the form of composites or compounds, comprising the recycled carbon fibers obtained by the process of the present invention or which are prepared using the recycled carbon fibers obtained by the process of the present invention.

Background

In general, carbon fiber reinforced plastics (also referred to as CFPs) may be referred to as fiber plastic composites, wherein a plurality of carbon fibers are preferably embedded in a plurality of layers as a reinforcing material in a matrix, such as plastic. As the polymer matrix, thermosetting materials such as epoxy resin, acrylate and polyurethane, and thermoplastic materials such as acrylonitrile-butadiene-styrene (ABS), Polyamide (PA), polylactic acid (PLA), polymethyl methacrylate (PMMA), Polycarbonate (PC), polyethylene terephthalate (PET), Polyethylene (PE), polypropylene (PP), Polystyrene (PS), polyether ether ketone (PEEK) and polyvinyl chloride (PVC) can be used together. However, it is also possible to embed carbon fibers in a matrix consisting of ceramic (also called ceramic fiber composite) in order to obtain a very thermally stable component, such as a brake disc.

Carbon fiber reinforced plastics (CFP) have high strength and rigidity and low weight, and are preferably used in fields requiring high weight ratio of strength and rigidity. For example, CFPs are used in the aerospace industry, wind power industry, automotive manufacturing, or sports equipment (e.g., bicycle frames, speed skates, tennis rackets, sports arrows, and fishing rods). In construction and construction, carbon fiber reinforced plastic (CFP) may be bonded in sheets to the surfaces of the components to reinforce the structure.

In the case of other fiber matrix composites, the strength and stiffness in the fiber direction of a material or component produced from carbon fiber reinforced plastic (CFP) is typically significantly higher than the strength and stiffness perpendicular to the fiber direction. Thus, for example, the strength perpendicular to the carbon fibers may be lower than the strength of the matrix material used. To ensure uniform strength and stiffness in all directions in space of a material or assembly consisting of CFPs, a single fiber layer is arranged in each direction. For example, in the case of high performance structural components, the fiber direction can be determined by computer calculations, such as classical ply theory, to achieve a specified strength and stiffness.

The primary carbon fibers (also called fibrils) used in CFP production are produced industrially mainly from carbon-containing raw materials, in particular Polyacrylonitrile (PAN), by a stabilization reaction in air, followed by pyrolysis in an inert atmosphere and subsequent graphitization. The stiffness and strength of the raw carbon fibers can be controlled in a targeted manner during the production process by means of the temperature of the pre-stretching and also of the carbonization and graphitization, so that various fiber types are commercially available. HT fibers (high tensile fibers) and IM fibers (medium modulus fibers) are mainly used as the primary carbon fibers due to their inexpensive manufacture. In order to improve the adhesion of the raw carbon fiber after graphitization, the oxidation of the surface of the raw carbon fiber may be performed by electrochemical treatment. Typically, the virgin carbon fibers are then provided with a sizing, such as an epoxy, and collected together to form a roving. These rovings are wound onto a conventional textile spindle in a final step.

Carbon fiber reinforced plastic (CFP) can be manufactured using various methods depending on the length of the original carbon fibers used. CFP parts with long virgin carbon fibers can generally be manufactured by a resin injection molding process, also known as Resin Transfer Molding (RTM). In the first step of the resin injection molding process, a preform is made, which is composed of one or more layers of woven virgin carbon fibers to ensure constant strength and stiffness in all directions in space. In a second step, these preforms are mixed in a closed casting mold with a liquefied matrix consisting of plastic and optionally a hardener. After the matrix is cured and excess edge material is removed, a corresponding CFP assembly is obtained.

The manufacture of carbon fiber reinforced plastics (CFPs) with short raw carbon fibers, in particular chopped raw carbon fibers, is usually carried out by injection molding. For this purpose, chopped virgin carbon fibers are mixed in batches with a liquefied matrix composed of one or more plastics, extruded and then processed by injection molding to produce a CFP assembly.

However, the use of carbon fiber reinforced plastic (CFP) results in a considerably higher cost of the final product than the use of similar components composed of light metals, such as aluminum, magnesium and titanium. In particular, this involves complex and expensive manufacturing of raw carbon fibers using carbon-containing raw materials, in particular Polyacrylonitrile (PAN). Furthermore, the worldwide consumption of virgin carbon fibers for manufacturing CFP components has increased significantly, and due to the global high demand for virgin carbon fibers, it is expected that the cost of using carbon fiber reinforced plastics has not significantly decreased.

Despite the high demand for virgin carbon fibers, large quantities of virgin carbon fibers (known as prepreg or preimpregnated fibers) that have not been treated with plastic that has cured or that has passed the storage date are disposed of as CFP-containing waste.

Furthermore, large amounts of CFP-containing waste plastic to be processed are obtained in the manufacture of aircraft parts and parts for wind turbines and also due to the processing of moulding dies, production waste, prototypes, incorrect batches and "scrapped" components.

However, the disposal of CFP-containing waste plastics in landfills is uneconomical because of the presence of valuable carbon fibers therein. Furthermore, due to its chemical inertness, it is generally expected that the CFP-containing waste plastics will remain undisturbed for a long period of time and will not degrade in landfills. Furthermore, unlimited disposal of CFP-containing waste plastics is not easy to do, even prohibited, because of legal requirements in many european countries.

There is therefore a great need for an inexpensive and efficient pyrolysis apparatus and method for recovering or reusing carbon fibers from CFP-containing waste, particularly in view of the global demand for carbon fibers for manufacturing CFP components.

In the prior art, carbon fibers are recovered or reused from CFP-containing materials (CFP materials) by pyrolysis. For the purposes of the present invention, pyrolysis is, in particular, the thermal decomposition of organic compounds, in which large organic molecules are split into smaller organic molecules by high temperatures, for example in the range from 300 to 1000 ℃. In general, no oxygen is introduced during pyrolysis. Therefore, it has hitherto been necessary to use a sealed and complicated pyrolysis apparatus and a complicated process to ensure an inert atmosphere or use a reduced pressure during the removal of the polymer matrix. However, the pyrolysis process is sometimes also operated in an oxygen-containing atmosphere, especially under controlled conditions.

Such a pyrolysis device is described in document EP0636428a 1. There, a protective gas furnace is used for the pyrolysis, in which the material containing CFP is pyrolyzed under a protective gas atmosphere. However, pyrolysis is carried out for a long period of time, with the result that the recovery is uneconomical and unsuitable for industrial scale. Furthermore, in order to obtain carbon-containing shaped bodies, a complicated aftertreatment of the regenerated material with a further pyrolysis step is provided.

Furthermore, in known pyrolysis devices using belt furnaces and processes performed therein for recovering carbon fibers from CFP material, the CFP material cannot be recycled. Thus, the CFP material is not mixed and accordingly all areas of the CFP material present on the conveyor belt are not heated uniformly. The result is considerable pyrolysis and resin residue on the surface of the recycled carbon fibers, which can have a detrimental effect on incorporation into the polymer matrix.

Such pyrolysis plants are described, for example, in document DE102008002846B4 and in equivalent family EP2282879a 1. Pyrolysis of polymer matrices is carried out in a pyrolysis unit comprising a belt furnace, the CFP-containing waste material first having to be pre-sorted and subsequently comminuted to small sizes. Furthermore, it is necessary to perform post-treatment of the recycled carbon fibers to avoid entanglement of the recycled carbon fibers.

In addition, document WO2010/075952a1 describes a pyrolysis apparatus comprising a treatment chamber in the form of a belt furnace or a rotary tube furnace. The process chamber has a hot air inlet and a heating source in the form of a resistive heating element and a source of microwave radiation, which is why a complex device is eventually required for the recovery.

Finally, document EP2152487B1 describes a pyrolysis apparatus with a belt furnace, the oxygen proportion of which is controlled in a targeted manner by means of a control device, so that the polymer matrix is substantially pyrolyzed without being burnt or vaporized.

In addition, the pyrolysis apparatus and process described above not only result in a large amount of pyrolysis residue on the surface of the recycled carbon fibers, but also result in high cost of recycling (reusing) the carbon fibers from the CFP-containing waste material because of their complexity. Furthermore, because of non-optimal mixing, the surface of the CFP-containing waste material is not uniformly treated in the pyrolysis apparatus and process described above. The recovered carbon fiber also frequently shows quality fluctuation.

Furthermore, CFP-containing waste materials have been pretreated in a complex manner, in particular by mechanical and/or chemical methods, before being recycled (re-used).

For this reason, the use of recycled carbon fibres in CFP assemblies has hitherto been possible only to a limited extent, because of the above-mentioned disadvantages.

Methods for recovering carbon fibers from CFP-containing waste on a laboratory scale are also known in the art. However, these processes tend to be complex and are not suitable for industrial scale carbon fiber recovery.

Disclosure of Invention

It is therefore an object of the present invention to provide a pyrolysis device and a corresponding method for recovering (reusing) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibres and/or carbon fibre-reinforced composites (composites), which avoid or at least reduce to a large extent the above-mentioned disadvantages associated with the prior art.

In particular, it is an object of the present invention to provide a pyrolysis device and a corresponding method for recovering (re-using) carbon fibres from carbon fibre-containing plastics, which pyrolysis device or method ensures a uniform heating of the CFP-containing material during decomposition of the polymer matrix. Furthermore, the pyrolysis apparatus or process will allow for inexpensive recovery (reuse) of carbon fibers that can be carried out on an industrial scale. Furthermore, the use of the pyrolysis device or process results in a recycled carbon fiber which is at least substantially free of pyrolysis or carbonization residues and which exhibits good bondability to plastics.

The applicant has now surprisingly found that the above-mentioned objects can be achieved in an efficient manner when a pyrolysis apparatus, in particular having an indirectly heated rotary tube furnace with an outlet opening, is used for recovering (re-using) carbon fibres from a carbon fibre-containing plastic. Using an indirectly heated rotary tube furnace with an outlet opening, a precisely defined atmosphere can be obtained in the rotary tube furnace, since the pyrolysis gas formed can be removed in a controlled manner through the outlet opening and, in particular, the oxygen content in the indirectly heated rotary tube furnace can be controlled by this removal. Furthermore, a homogeneous heating and thus a homogeneous pyrolysis of the polymer matrix is ensured because of the homogeneous mixing of the CFP material of the rotary tube furnace. Due to the homogeneous pyrolysis of the polymer matrix, the recycled carbon fibers, which have been at least substantially completely freed from the matrix material and are substantially free of pyrolysis residues (hereinafter referred to as pyrolytic carbon residues) on the surface of the recycled carbon fibers, are obtained with a better bondability to plastics than the virgin carbon fibers and the conventional recycled carbon fibers.

In order to solve the above-mentioned problems, the present invention therefore proposes a pyrolysis device for recovering (reusing) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibres and/or carbon fibre-reinforced composites (composite material), as claimed in claim 1. Further, advantageous properties of the pyrolysis apparatus of the invention are the subject matter of the dependent claims directed to the pyrolysis apparatus.

The invention also provides the use of a pyrolysis device according to the invention as claimed in claim 10 for recovering (reusing) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibres and/or carbon fibre-reinforced composites (composite material).

The invention also provides a process as claimed in claim 11 for recovering (reusing) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibres and/or carbon fibre-reinforced composites (composite materials). Further, advantageous properties of the method according to the invention are the subject matter of the dependent method claims.

Furthermore, the present invention provides recycled carbon fibres obtained by the process of the invention as claimed in claims 13 and 14. Furthermore, advantageous properties of the recycled carbon fibers according to the invention are the subject matter of the relevant dependent claims directed to the recycled carbon fibers.

The invention furthermore provides the use according to the invention of recycled carbon fibres, as described and defined in the respective use claims.

The present invention also provides a plastic, building material or cement-containing system as claimed in claim 17, which comprises or is produced using the recycled carbon fibres obtained by the process of the present invention.

Finally, the present invention provides shaped bodies (e.g. modules), moulds and sheet-like materials (e.g. nonwovens), in particular in the form of composites or compounds, as claimed in claim 18, which comprise or are prepared using the recycled carbon fibers obtained by the process of the present invention.

It is to be understood that specific configurations and embodiments described in the context of only one aspect of the invention are equally applicable to other aspects of the invention, as such are not explicitly indicated.

Further, those skilled in the art may deviate from the numbers, values, or ranges described below for particular applications or individual cases without departing from the scope of the present invention.

Thus according to the inventionFirst of allIn one aspect, the invention provides a method for removing carbon from a substrateA pyrolysis device for recovering (recycling) carbon fibres from a fibrous plastic, in particular from a carbon fibre reinforced plastic (CFP or CFP material), preferably from a carbon fibre-containing and/or carbon fibre reinforced composite (composite material),

having an elongated pyrolysis furnace for continuous pyrolysis of CFP material, which is continuously operated during operation,

having an input station for introducing the CFP material to be treated into the pyrolysis furnace at one end thereof,

having an output station for discharging the recovered carbon fiber material from the pyrolysis furnace at the other end of the pyrolysis furnace,

having a gas duct arrangement for the pyrolysis gas generated in the pyrolysis furnace, and

having a control device, in particular for regulating at least individual components of the gas in the pyrolysis furnace, in particular for regulating the oxygen proportion (oxygen content) in the gas in the pyrolysis furnace,

wherein the pyrolysis furnace is an indirectly heated rotary tube furnace having at least the following components:

an elongated rotating tube forming a receiving space for the CFP material to be treated and connected to the input and output stations, the rotating tube being provided with an outlet opening in its cylindrical wall for discharging pyrolysis gases formed during pyrolysis over at least a part of its length, and

a housing which is insulated from the outside and at least partially encloses the rotating tube, has an opening for the inlet station and optionally also for the outlet station, and has an exhaust line, in particular for pyrolysis gases.

For the purposes of the present invention, a rotary tube furnace is, in particular, a furnace for continuous processes to which heat is supplied in the process engineering. In contrast to a directly heated rotary tube furnace, in which the introduction of heat takes place within the furnace, in the case of an indirectly heated rotary tube furnace heat is transferred from the outside of the reaction space via the rotary tube wall. In general, an indirect heating rotary tube furnace has: an input station, often in the form of a feed screw or a feed belt, a rotary tube forming the core of the rotary tube furnace, an output station at the end of the rotary tube furnace for discharging the treated material, bearings for the rotary tube and a drive or drives for the rotary tube distributed over the length of said rotary tube, and finally a housing insulated from the outside and enclosing the rotary tube, wherein the amount of heat required inside the rotary tube is introduced or generated in any way.

The indirectly heated rotary tube furnace used according to the invention has at least the following components:

an elongated rotary tube forming a receiving space for the CFP material to be treated and connected to the input and output stations, the rotary tube being provided with an outlet opening in its cylindrical wall for discharging pyrolysis gases formed during pyrolysis over at least a part of its length, and a housing insulated from the outside and surrounding the rotary tube and having openings for the input and output stations and, in particular, an exhaust line for the pyrolysis gases.

According to the invention, the rotary tube of a rotary tube furnace used as a pyrolysis furnace is configured in this case in a particular manner: it has outlet openings (i.e. a plurality or a large number of outlet openings, for example in particular at least 5 outlet openings, preferably at least 10 outlet openings) on its cylindrical wall over at least a part of its length for discharging pyrolysis gases formed during the pyrolysis process. Pyrolysis furnaces in the form of rotary tube furnaces are therefore particularly suitable for the specificity of pyrolysis. The pyrolysis gases will be able to escape from the rotating pipe very quickly where they are formed, so as to hinder the pyrolysis process as little as possible.

In conventional belt furnaces of the prior art, the CFP material in the pyrolysis furnace is open towards the top and the formed pyrolysis gases may escape upwards at any point, but the circulation and mixing of the CFP material to be treated may be insufficient. In contrast, when using conventional rotating tubes in the prior art rotary tube furnace, the pyrolysis gases formed cannot escape upwards, so that they will surround the CFP material and hinder further pyrolysis. Although the conventional rotary tube furnaces of the prior art may have some effect with respect to the mixing and circulation of CFP material, this may be offset by a considerable disadvantage related to the lack of ability to discharge pyrolysis gases formed during pyrolysis. Only the use according to the invention or the provision of an outlet opening in the cylindrical wall over at least a part of its length, i.e. where at least a particularly large amount of pyrolysis gas is formed, according to the invention, leads to a suitable solution for a pyrolysis apparatus for the recovery (reuse) of carbon fibre-containing plastics.

With the inventive pyrolysis apparatus comprising an indirectly heated rotary tube furnace with outlet openings, it is possible firstly to allow homogeneous mixing, circulation and heating of the CFP material in the pyrolysis furnace and secondly at the same time to efficiently discharge the pyrolysis gas through the outlet openings in the wall of the rotary tube. This results in optimal pyrolysis conditions, resulting in selective removal of only the polymer matrix without damaging the carbon fibers to be recovered anywhere in the rotary tube furnace, so that after complete removal of the polymer matrix, at least substantially no pyrolysis residue remains on the surface of the recovered carbon fibers.

Furthermore, the outlet opening in the indirectly heated rotary tube furnace of the pyrolysis apparatus of the present invention allows to control the atmosphere, in particular the oxygen content, in the rotary tube furnace. In combination with the indirect heating of the rotary tube furnace, by means of which temperature fluctuations can be avoided, the polymer matrix can be removed selectively and at least substantially completely without damaging the recovered carbon fibers, in particular excessively damaging and/or excessively oxidizing, since this leads to a significant reduction in the mechanical properties of the recovered carbon fibers.

The homogeneous heating and mixing of the CFP-containing material in combination with the targeted control of the oxygen content and/or temperature is made possible by using the pyrolysis device according to the invention with an indirectly heated rotary tube furnace with an outlet opening, which results in stable high-quality recycled carbon fibers which are at first at least substantially free of pyrolysis or carbonization residues on their surface. Secondly, the above combination results in only partial oxidation of the surface of the recycled carbon fiber, i.e., on a rough surface, particularly a hydrophilic surface, which shows more adhesive affinity, and in an increase of oxygen-containing groups, such as hydroxyl groups, aldehyde groups, carboxyl groups, etc., on the surface of the resulting recycled carbon fiber. The local oxidation of the surface of the recycled carbon fibers, which results from the combination of the pyrolysis device according to the invention and the process conditions according to the invention, leads to an increased wettability due to the rougher and functionalized, in particular more hydrophilic, surface of the recycled carbon fibers and thus to an improved bondability to plastics compared to the virgin carbon fibers or conventional recycled carbon fibers.

Thus, due to the combination of the use of a pyrolysis apparatus according to the invention with an indirectly heated rotary tube furnace with outlet openings on the wall of the rotary tube and a targeted control of the oxygen content and/or temperature, the recycled carbon fibers are at least substantially free of pyrolysis residues, but the mechanical and electrical properties of the recycled carbon fibers are not or not significantly impaired by the recycling. Furthermore, the recycled carbon fiber has improved wettability due to partial oxidation of the surface of the recycled carbon fiber, thereby having improved bondability with plastics.

Drawings

Further advantages, features, aspects and characteristics of the present invention can be derived from the following description of preferred illustrative embodiments shown in the drawings. The features mentioned above and/or those disclosed in the claims and/or in the following description of the figures may also be combined with each other, if desired, even if not explicitly described in detail.

In the drawings:

FIG. 1 schematically shows a longitudinal section of the rotating tube, without cut, according to a preferred embodiment of the pyrolysis apparatus according to the present invention;

fig. 2 shows a part of the pyrolysis apparatus of fig. 1 in the region of its right-hand end, i.e. in the region of the input station, in an enlarged illustration;

FIG. 3 shows schematically a representation of a cross section through the pyrolysis apparatus of FIG. 1 in a starting region of a pyrolysis furnace;

FIG. 3A schematically illustrates a diagram of a mixing and conveying element in accordance with a preferred embodiment of the present invention;

FIG. 3B schematically illustrates an enlarged cross-sectional view of the mixing and conveying element shown in FIG. 3A;

FIG. 4 schematically illustrates a locking arrangement for an input station in accordance with a preferred embodiment of the pyrolysis apparatus of the present invention;

FIG. 5 schematically shows a flow diagram of the method according to the invention for recovering (reusing) carbon fibers from fiber-containing plastics, according to a preferred embodiment of the method according to the invention;

FIG. 6A schematically illustrates a representation of a prior art virgin carbon fiber having a smooth surface;

FIG. 6B schematically shows a representation of recycled carbon fibers of the prior art, obtained by a non-inventive method, with pyrolysed or carbonized residues and grooves; and

figure 6C schematically shows a representation of recycled carbon fibres, obtained by the method of the invention, with grooves.

The pyrolysis apparatus shown in fig. 1 to 3 is used in the described illustrative embodiment for recovering carbon fibers from carbon fiber reinforced plastic (CFP). For a general background on this recycling technique, reference may be made to the sources mentioned at the outset in the prior art.

Detailed Description

Fig. 1 to 3 show a pyrolysis device according to the invention for recovering (reusing) carbon fibers from carbon fiber-containing plastics, in particular from carbon fiber-reinforced plastics (CFP or CFP material), preferably carbon fiber-containing and/or carbon fiber-reinforced composites (composite materials), with an elongated pyrolysis furnace 1 for the continuous pyrolysis of CFP material 2, which runs continuously during operation, with an input station 3 for introducing CFP material 2 to be treated into the pyrolysis furnace 1 at one end of the pyrolysis furnace, with an output station 6 for discharging recovered carbon fiber material 7 from the pyrolysis furnace 1 at the other end of the pyrolysis furnace, with gas duct means 8 for pyrolysis gas 9 generated in the pyrolysis furnace 1, which pyrolysis gas 9 can then be conveyed into an exhaust gas treatment device, in particular after combustion, and with control means 10, in particular for regulating at least the individual components of the gas in the pyrolysis furnace 1, in particular for regulating the oxygen proportion (oxygen content) in the gas in the pyrolysis furnace 1. The pyrolysis furnace 1 is an indirectly heated rotary tube furnace having at least the following components: an elongated rotating tube 11 forming a receiving space for the CFP material 2 to be treated and connected to the input station 3 and the output station 6, which rotating tube 11 is provided with an outlet opening 12 in its cylindrical wall for discharging pyrolysis gases 9 formed during pyrolysis over at least a part of its length, and a housing 13 insulated from the outside and at least partly enclosing the rotating tube 11 and having an opening for the input station 3 and optionally also for the output station 6 and having an exhaust line 14, in particular for the pyrolysis gases 9.

The pyrolysis apparatus shown schematically in longitudinal section in fig. 1 has as its core an elongated, continuously operating pyrolysis furnace 1 for the continuous pyrolysis of CFP material 2. On the right-hand side of fig. 1, an input station 3 for introducing CFP material 2 to be processed can be seen. In fig. 1, the CFP material 2 is poured onto a continuous conveyor belt 5 through a chute 4, and then the CFP material 2 is transported into the pyrolysis furnace 1 at one end thereof.

On the left hand side of fig. 1, there is an output station 6 for discharging recycled carbon fibre material or re-used carbon fibres 7 from the pyrolysis furnace 1. There, the output station 6 is depicted as a simple movable collecting bucket. The pyrolysis device has an air duct arrangement 8 for the pyrolysis gases 9 generated in the pyrolysis furnace 1. In fig. 1, the flowing pyrolysis gas 9 is indicated by curved arrows.

Finally, the pyrolysis device has a control device 10 for regulating the atmosphere in the pyrolysis furnace 1, in particular for regulating the individual components of the gas, preferably for regulating the oxygen proportion or the oxygen content, in the pyrolysis furnace 1.

For the purposes of the present invention, it is preferred that the oxygen proportion of the gas in the pyrolysis furnace 1 is set to a sub-stoichiometric value relative to the amount required for complete decomposition of the polymer matrix of the CFP material during pyrolysis thereof. Furthermore, it is preferred to set the oxygen content to a superstoichiometric value relative to the amount required for the decomposition of the polymer matrix in a further region of the pyrolysis furnace 1 or after the occurrence of the thermal decomposition of the polymer matrix, in order to at least substantially completely remove any pyrolysis residue on the surface of the recycled carbon fibers or to at least partially oxidize the surface.

For an understanding of the teachings of the present invention, fig. 1, 2 and 3 should now be viewed together as they allow the various aspects of the teachings of the present invention to be readily seen.

According to the invention, the pyrolysis furnace 1 is an indirectly heated rotary tube furnace. Such an indirectly heated rotary tube furnace has first of all an elongated rotary tube 11 forming a receiving space for the CFP material 2 to be treated. This connects the input station 3 and the output station 6. The rotary tube is mounted as is usual in rotary tube furnaces (see, for example, DE102004036109a1) so as to be rotatable, for example by means of rollers or by means of suitable ball bearings. Furthermore, there is a rotation drive for rotating the tube 11, which is not shown in fig. 1. It can be seen that the input station 3 with its conveyor belt 5 at the right-hand end projects a little into the rotary pipe 11, and that the collecting container at the left-hand end output station 6 is directly below the outlet of the rotary pipe 11.

It is essential for the purpose of the invention that the rotating tube 11 is provided with an outlet opening 12 in its cylindrical wall for discharging pyrolysis gases 9 formed during pyrolysis over at least a part of its length. These outlet openings 12 can be seen in different places in fig. 1. In fig. 3, the outlet openings 12 in the rotary tube 11 are indicated by the flow arrows shown there for the pyrolysis gas 9.

It is important that the pyrolysis gas 9 can be continuously discharged from the rotating pipe 11 close to where it appears. In this way, the oxygen content in the rotary tube 11 can be controlled in a targeted manner, as explained at the outset.

Fig. 2 shows an enlarged view of the right-hand end of the pyrolysis furnace 1 closest to the input station 3. In addition to what is shown in fig. 1, roller bearings 11' of the rotary tube 11 can be seen at the bottom of fig. 2. These roller bearings 11' support the rotating tube in such a way that a temperature-dependent expansion in the longitudinal direction can be achieved. In each case, between these roller bearings 11', there is a gear wheel 11 "with which a suitable rotary drive is engaged.

Another important component of the pyrolysis apparatus according to the invention is a housing 13, which is insulated from the outside and surrounds the rotary pipe 11 and has an opening for the input station 3 in the pyrolysis furnace 1. In fig. 1, the opening for the input station 3, which is highly insulated from the rotary tube 11, can be seen on the right. The insulation takes into account the fact that this is the "hot" end of the rotating tube 11. On the left side of fig. 1, the opposite output station 6 is outside the housing 13. This feature will be discussed in detail below.

Finally, in particular, an exhaust line 14 in which the pyrolysis gases 9 are discharged is also present in the housing 13.

According to a further feature, the preferred embodiment of the pyrolysis apparatus according to the invention shown in fig. 1 is characterized in that the rotating duct 11 has a first heating section 15 extending from the input station 3 and a second adjacent cooling section 16 leading to the output station 6. In fig. 1, the heating portion 15 of the rotary pipe 11 can be seen on the right side, where the housing 13 is also provided. This surrounds the rotary pipe 11 only in the heating section 15. Insulation from the outside is only required here, since only here a high heat input is required. A discharge line 14 for pyrolysis gases 9 is also located there. Somewhat to the left of the illustrated middle of the pyrolysis apparatus in fig. 1, openings for the entry of the rotary tubes 11 into the cooling section 16 can be seen. The opening in the housing 13 is therefore not at the output station 6, but inside the pyrolysis furnace 1 between the heating section 15 and the cooling section 16.

Fig. 2 shows the right-hand end of the heating portion 15 of the pyrolysis furnace 1.

The depicted preferred embodiment clearly shows that in the shown embodiment the rotating pipe 11 is cooled by water or can be cooled by water in the cooling section 16; the water is spread from above through the nozzles 17 and collected in a collecting tray 18 below the cooling section 16 of the rotating pipe 11. This enables the recycled carbon fibres 7 to be discharged at a relatively low temperature at the output station 6.

It is preferable for the rotary pipe 11 to have no outlet openings in the cooling section 16 in any case.

With the arrangement of the rotary pipe 11 in the heating section 15 of the pyrolysis furnace 1, the outlet openings 12 are substantially evenly distributed over the circumference of the rotary pipe 11. Only a few outlet openings 12 distributed over the circumference are shown in fig. 1 and 2. However, the entire circumference of the pair of rotary tubes 11 should be considered as illustrative only. The distribution over the entire circumference is useful because the rotary pipe 11 is continuously rotated if there is no other reason.

As can be seen in fig. 1 and 2, in the illustrated embodiment the size of the outlet opening 12 is not the same throughout. In the rightmost first part, the outlet opening 12 is still relatively small. There, the CFP material 2 has not yet been heated very strongly, so that only small amounts of pyrolysis gases 9 are evolved. A large amount of pyrolysis gas 9 is formed only in the next two sections of the heating section 15, so that the diameter of the outlet opening 12 in the rotary pipe 11 is also significantly larger there. In the next section, the outlet openings are again reduced in density and size, since the pyrolysis is already largely completed there.

According to the invention, the size of the outlet opening 12 or the different sizes of the outlet opening 12 is matched to the size of the CFP material 2 and also the carbon fibres present therein, or the size of the outlet opening 12 or the different sizes of the outlet opening 12 may be matched to the size of the CFP material 2 and the carbon fibres present therein and/or may be adjusted so that the CFP material 2 or the recycled carbon fibres do not fall out of the rotary pipe 11 into the housing 13. This should not be so to any significant extent, since otherwise the recovery aspect would be offset.

In a variation of the present teaching, not shown, the rotary tube 11 can be matched to different CFP materials 2 by means of an adjustable outlet opening 12 in the rotary tube 11.

Fig. 1 and 2 show a further feature of the construction of the pyrolysis apparatus according to the invention in this illustrative embodiment. It can be seen that the discharge line 14 for pyrolysis gases is located within a heating section 15 in the housing 13, again at the end closest to the input station 3. With this first third arrangement in the housing 13 closest to the input station 3, in particular the hot pyrolysis gases 9 formed are further back into the housing, counter-flow to the flow direction of the CFP material 2 in the direction of the input station 3 and additionally contribute to heating the CFP material 2. Due to this "backflow", a more uniform heat distribution is thus achieved through the input of the pyrolysis furnace 1.

It has been pointed out above that the rotary tube 11 in the pyrolysis furnace 1 of the pyrolysis apparatus of the present invention should also be configured in a particular manner with respect to heating. According to a preferred teaching of the present invention, a plurality of sections 19 with different gas temperatures are provided in the housing 13 along the length of the rotary pipe 11, or a plurality of sections 19 with different adjustable gas temperatures are provided (i.e. the gas temperatures in the individual sections 19 can be adjusted differently or differently from each other), and the outlet openings 12 in the rotary pipe 11 are provided at least in the section with the highest gas temperature.

Some things about the different formation of pyrolysis gas 9 in the different sections 19 of the heated section 15 of the rotating tube 11 have been explained above. In a further embodiment of the inventive teaching explained at the end, it is advantageous for the pyrolysis furnace 1 to have a plurality of sections 19, in particular at least one heating zone 19.1, a first pyrolysis zone 19.2, a second pyrolysis zone 19.3 and a cooling zone 19.4.

Furthermore, in a preferred embodiment of the invention, the gas composition within the pyrolysis furnace 1 is adjusted differently in the sections 19 of the rotary tube 11 (i.e. the gas composition is different in the pyrolysis furnace 1, or may be adjusted differently in the sections 19 of the rotary tube 11), in particular a low proportion of oxygen (oxygen content) in the first pyrolysis zone 19.2 and a higher proportion of oxygen (oxygen content) in the second pyrolysis zone 19.3 compared to the first pyrolysis zone 19.2.

Furthermore, it may be preferred according to the invention that the gas composition and/or the temperature within the pyrolysis furnace 1 can be adjusted differently or can be adjusted differently in the sections 19 of the rotary tube 11, preferably with a defined proportion of oxygen (oxygen content) G (B1) and/or a defined temperature T (B1) in the first pyrolysis zone 19.2 and with a defined proportion of oxygen (oxygen content) G (B2) and/or a defined temperature T (B2) in the second pyrolysis zone 19.3. In particular, the oxygen content G (B2) in the second pyrolysis zone B2 is increased compared to the oxygen content G (B1) in the first pyrolysis zone B1 and/or the temperature T (B2) in the second pyrolysis zone B2 is increased compared to the temperature T (B1) in the first pyrolysis zone B1.

In particular, the temperatures indicated, in particular the temperatures indicated T (B1) and T (B2) and the specific temperature values or temperature value ranges given herein below relate to the temperatures to be reached in the object to be treated or to be recovered.

As fig. 3 shows a cross section in the heating section 15 of the pyrolysis furnace 1 close to the input station 3, an air inlet 20 with a control valve 21 on the right side is used for introducing air and thus oxygen into the first section 19.1 of the heating section 15.

It is only indicated in fig. 1 and 2 that in the depicted preferred embodiment, the rotary pipe 11 is inclined downwards from the input station 3 to the output station 6. The rotation of the inclined and rotating tube 11 about its longitudinal axis produces an axial transport movement of the CFP material 2 in the rotating tube 11. In the depicted preferred embodiment, the rotating pipe 11 is additionally provided with mixing elements 22 in its interior, i.e. with deflection plates which can be easily seen in fig. 3.

The depiction in fig. 3A and 3B shows features that are preferably used in the case of a rotating tube 11 used according to the invention. The combination of firstly the mixing element 22 and secondly the transport element 26 is particularly preferred, the transport element 26 preferably being in the form of a feed screw, in particular a wound, helical or screw-like feed screw, particularly preferably an archimedes screw. According to the invention, as depicted in the enlarged view of said transport element 26 in fig. 3B, the mixing element 22 is preferably arranged between the individual windings of the feed screw, in particular of the transport element 26 in the form of an archimedean screw. The CFP material 2 is thus forcibly conveyed in the longitudinal direction of the rotary pipe 11.

Fig. 4 shows another feature not shown in the other figures. In a variant of the invention, the input station 3 can be configured not as disclosed with a conveyor belt 5 as shown in fig. 1, but as an input lock. The input lock 3 according to fig. 4 operates like a kind of gas chamber barrier, by which it is ensured that the inevitable introduction of oxygen and the introduction of CFP material 2 into the first section 19.1 of the heating portion 15 of the furnace 1 can be controlled in terms of quantity. The lock still allows for the continual spurious introduction of CFP material 2 into individual "sections". However, this is still a great advantage for accurate control by the control device 10.

In an embodiment of the invention, which is not shown in the drawings, the pyrolysis device of the invention can also have a comminution device for comminuting the CFP material 2 to be treated, preferably arranged upstream of the input station 3. In other words, the pyrolysis device according to the invention in this embodiment has a comminution device for comminuting the CFP material 2 to be processed, which is arranged upstream or in front of the input station 3 in the processing direction (i.e. the processing or running direction). This ensures that the CFP material 2 to be treated and subsequently introduced into the input station has an optimal size or dimension for pyrolysis. By way of non-limiting example, suitable comminution apparatus are in the form of shredders, choppers, grinders, tearing and/or cutting equipment.

In a further embodiment of the invention, which is also not shown in the drawings, the pyrolysis device may additionally have a post-treatment device, preferably arranged downstream of the output station 6, for post-treating, in particular sorting and/or comminuting, the recycled carbon fibers obtained from the CFP material 2. In other words, in this embodiment, the pyrolysis apparatus of the invention has a post-treatment device for post-treating, in particular sorting and/or comminuting, the recycled carbon fibres obtained from the CFP material 2, which is arranged after or downstream of the output station 6 in the treatment direction (i.e. treatment or operating direction). This ensures that the recycled carbon fibres obtained from the CFP material 2 are optimally post-treated for subsequent processing.

In this respect, fig. 1 to 3 also show that the heating of the pyrolysis furnace 1 is effected by at least one external gas burner 23 via a heating gas line 24 in the housing 13. In fig. 3, the connection for the external burner 23 can be seen at the lower left, and the arrows indicate that the heating gas flows into the heating gas line 24 inside the housing 13, with the rotary pipe 11 rotating. These gases then leave the housing 13 again via the discharge line 14 together with the pyrolysis gases. As can be seen in fig. 1, this introduction is via a heated gas line 24 which is arranged transversely to the longitudinal extension of the rotary tube 11 in the bottom of the housing 13.

In fig. 1, also the connection 25 for the afterburner normally connected thereto can be seen at the upper right of the discharge line 14 there. This greatly increases the temperature of the gases in the discharge line 14, so that post-combustion of the pyrolysis gases 9 takes place there.

In fig. 3, the pre-combustion at the inlet side leading from the gas burner 23 can also be seen. Here, the post-combustion starts directly in the discharge line 14 at the outlet of the receiving space of the rotary tube 11 in the housing 13.

Typical temperatures in the rotating tube 11 are in the temperature range from 200 ℃ to 750 ℃ and typical temperatures downstream of the tip of the connection 25 are from 1000 to 1200 ℃. In this respect, reference is also made to the full explanations in the prior art discussed at the outset.

According to the inventionSecond oneIn a further aspect, the invention provides a pyrolysis unit according to the invention for the pyrolysis of carbon fibres from carbon fibre-containing plastics,in particular from carbon fibre-reinforced plastics (CFP or CFP materials), preferably carbon fibres from carbon fibres and/or carbon fibre-reinforced composites (composites).

For more detailed information on this aspect of the invention, reference may be made to the above description of the pyrolysis apparatus of the invention, which is similarly applicable to this aspect of the invention.

According to the inventionThird stepIn an aspect, the invention additionally provides a process for recovering (recycling) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibres and/or carbon fibre-reinforced composites (composite materials),

wherein the object of a carbon fiber-containing plastic based on carbon fibers contained in a polymer matrix is subjected to a multistage pyrolysis in the presence of oxygen, the polymer of the polymer matrix being decomposed during the pyrolysis to give carbon fibers,

and the pyrolysis is carried out in a pyrolysis apparatus according to the invention.

Fig. 5 shows a specific embodiment of the process according to the invention which can be carried out in the pyrolysis apparatus according to fig. 1. This embodiment relates to a method for recovering (reusing) carbon fibres from carbon fibre-containing plastics, in particular from carbon fibre-reinforced plastics (CFP or CFP material), preferably from carbon fibres and/or carbon fibre-reinforced composites (composite material), wherein a multi-stage pyrolysis is carried out in the presence of oxygen on the basis of an object of the carbon fibre-containing plastics comprising carbon fibres in a polymer matrix, the polymer of the polymer matrix being decomposed in the pyrolysis process to obtain carbon fibres, wherein the pyrolysis is carried out in a pyrolysis device as claimed in any of the preceding claims with a pyrolysis device P, in particular a pyrolysis furnace 1, and the pyrolysis device P comprises at least the following process zones, in particular the following sections 19 of a rotary tube 11, in the order specified below, and the object is passed through the following process zones in this order:

(A) a heating zone A (corresponding to reference numeral 19.1 in FIG. 1), in which the objects to be treated and/or recovered are heated to a defined temperature T (A),

(B1) a subsequent first pyrolysis zone B1 (corresponding to reference numeral 19.2 in FIG. 1), in which pyrolysis of the polymer matrix of the object to be treated takes place and/or takes place at a defined temperature T (B1) and a defined oxygen content G (B1),

(B2) a subsequent second pyrolysis zone B2 (corresponding to reference numeral 19.3 in FIG. 1), in which the final pyrolysis of the polymer matrix of the object to be treated still present after the pyrolysis zone B1 (corresponding to reference numeral 19.2 in FIG. 1) is carried out at a defined temperature T (B2) and a defined oxygen content G (B2) to at least substantially complete removal,

(C) a subsequent cooling zone C (corresponding to reference numeral 19.4 in FIG. 1) for cooling the recovered carbon fibers RF (corresponding to reference numeral 7 in FIG. 1) obtained from said second pyrolysis zone B2 (corresponding to reference numeral 19.3 in FIG. 1),

wherein the oxygen content G (B2) of the second pyrolysis zone B2 (corresponding to reference numeral 19.3 in fig. 1) is increased compared to the oxygen content G (B1) of the first pyrolysis zone B1 (corresponding to reference numeral 19.2 in fig. 1), and/or the temperature T (B2) of the second pyrolysis zone B2 (corresponding to reference numeral 19.3 in fig. 1) is increased compared to the temperature T (B1) of the first pyrolysis zone B1 (corresponding to reference numeral 19.2 in fig. 1).

The temperatures indicated, in particular the temperatures indicated T (B1) and T (B2) and the specific temperature values or temperature value ranges indicated herein below relate in particular to the temperatures to be reached in the object to be treated or to be recovered.

Fig. 5 shows a schematic flow diagram of the method according to the invention for recovering (reusing) carbon fibers from carbon fiber-containing plastics, which is carried out in a pyrolysis device according to fig. 1, according to a preferred embodiment of the invention. The polymer matrix of the carbon fiber-containing plastic CFP to be treated is first heated to a defined temperature in a heating zone a (corresponding to reference numeral 19.1 in fig. 1) of the pyrolysis device P and is subjected in a subsequent step to selective pyrolysis in a first pyrolysis zone B1 (corresponding to reference numeral 19.2 in fig. 1) at a defined temperature and a defined oxygen content. In a second pyrolysis zone B2 (corresponding to reference numeral 19.3 in fig. 1) following the first pyrolysis zone B1, the remaining polymer matrix of the carbon fiber-containing plastic CFP is subjected to a final pyrolysis at a defined temperature and a defined oxygen content until at least substantially complete removal of the polymer matrix and also complete removal of the pyrolysis residues from the surface of the recycled carbon fibers. Subsequently, cooling is carried out in a cooling zone C (corresponding to reference numeral 19.4 in fig. 1) so that the recycled carbon fibers RF (corresponding to reference numeral 7 in fig. 1) obtained in this way are finally produced.

Fig. 6A schematically shows a prior art original carbon fiber 1 having a smooth surface structure.

Fig. 6B schematically shows a prior art recycled carbon fiber 1' obtained by a non-inventive method. The surface of the recycled carbon fibers 1' has grooves 2, which are formed by oxidation of the surface of the carbon fibers during the recycling process. In addition, a large amount of pyrolysis or carbonization residue 3 exists on the surface of the conventionally recovered carbon fiber 1'. The bondability of the conventional recycled carbon fibers 1' obtained by the non-inventive method to plastics is not as good or significantly better than that of the original carbon fibers 1 due to the large amount of pyrolysis or carbonization residues 3.

Fig. 6C schematically shows a recycled carbon fibre 1 "obtained by the method of the invention. The surface of the recycled carbon fibres has grooves 2' as a result of oxidation of the carbon fibres during the recycling process. However, there is no pyrolysis or carbonization residue on the surface of the recycled carbon fiber 1 ″ according to the present invention due to the process conditions according to the present invention. Thus, the recycled carbon fiber 1 ″ of the present invention has significantly improved bondability to plastics compared to the virgin carbon fiber 1 and the recycled carbon fiber 1' not according to the present invention.

The schematic representations in fig. 6A, 6B and 6C correspond to microscopic analyses performed by the applicant on the respective products.

In particular, it is a feature of the present invention that the recycled carbon fibers are surprisingly at least substantially free of pyrolysis residues, and furthermore have a better wettability than virgin carbon fibers and conventional recycled carbon fibers, in particular due to local oxidation of the surface of the carbon fibers, which are obtained as a result of the combination of the pyrolysis apparatus according to the invention comprising an indirectly heated rotary tube furnace with an outlet opening and the process conditions according to the invention, in particular the increased oxygen content and/or temperature in the second pyrolysis zone.

The combination of the pyrolysis apparatus according to the invention and the process according to the invention thus allows the selective removal of the polymer matrix without destroying the recovered carbon fibers and, consequently, without a significant deterioration of the mechanical properties, in particular of the tensile strength and of the modulus of elasticity, and of the electrical properties, so that the material properties of the recovered carbon fibers at least substantially correspond to those of the original carbon fibers.

Furthermore, the combination of the pyrolysis apparatus according to the invention and the method according to the invention results in a slight oxidation of the surface of the recovered carbon fibers, i.e. the surface is rougher than in the case of the original carbon fibers or conventional recovered carbon fibers, and the number of oxygen-containing functional groups, such as phenolic, carboxyl, carbonyl, aldehyde, keto, hydroxyl and/or oxo groups, on the surface of the carbon fibers is significantly greater.

The roughness and greater hydrophilicity of the recycled carbon fibers resulting from the oxidized surface results in improved wettability resulting in better bondability of the recycled carbon fibers to plastics, building materials, or cement-containing systems.

In a preferred embodiment of the invention, the oxygen content G (B2) of the second pyrolysis zone B2 is increased by at least 3%, in particular by at least 5%, preferably by at least 7.5%, particularly preferably by at least 10% by volume in comparison with the oxygen content G (B1) of the first pyrolysis zone B1. In order to avoid oxidation of the carbon fibers in the first pyrolysis zone B1, using a lesser amount of oxygen than in the second pyrolysis zone B2, removal of any pyrolysis residue on the surface of the recycled carbon fibers occurs in the second pyrolysis zone B2. So that decomposition of the polymer matrix takes place at least substantially in the first pyrolysis section B1 with a lower oxygen content than in the second pyrolysis zone B2 where the pyrolysis residue is removed. The presence of only a small amount of oxygen in the first pyrolysis zone B1 is achieved firstly by the input station 3, in particular in the form of an input lock, and secondly in particular by the substantially steam-saturated atmosphere in the first pyrolysis zone B1, which consists in particular of gaseous decomposition products produced in the pyrolysis of the polymer matrix and has only a small proportion of oxygen.

In this context, in particular, the oxygen content G (B2) of the second pyrolysis zone B2 is increased by at least 3% to 25%, in particular by 5% to 20%, preferably by 7.5% to 17.5%, particularly preferably by 10% to 15%, by volume in comparison with the oxygen content G (B1) of the first pyrolysis zone B1. As described above, the oxygen content of the second pyrolysis zone B2 is set higher than the oxygen content of the first pyrolysis zone B1 to at least substantially completely remove any pyrolysis residue on the surface of the recycled carbon fibers.

According to the invention, particularly good results are achieved when the value of the oxygen content G (B1) in the first pyrolysis zone B1 is set to 0.1% to 12%, in particular 0.5% to 10%, preferably 0.75% to 6%, particularly preferably 1% to 4% by volume, and when the value of the oxygen content G (B2) in the second pyrolysis zone B2 is set to 2% to 30%, in particular 3% to 20%, preferably 5% to 17%, particularly preferably 6% to 14% by volume, with the proviso, in particular, that the oxygen content G (B2) of the second pyrolysis zone B2 is increased by at least 3%, in particular 5%, preferably 7.5%, particularly preferably 10%, by volume compared to the oxygen content G (B1) of the first pyrolysis zone B1, and/or the oxygen content G (B2) of the second pyrolysis zone B2 is increased by at least 25% by volume compared to the oxygen content G (B1) of the first pyrolysis zone B1), in particular from 5% to 20%, preferably from 7.5% to 17.5%, particularly preferably from 10% to 15%.

In particular, for the purposes of the present invention, the oxygen content in the first and second pyrolysis zones B1 and B2 can be controlled and/or adjusted in such a way that the oxygen content G (B1) in the first pyrolysis zone B1 is set to a sub-stoichiometric value with respect to the polymer matrix to be decomposed and the oxygen content G (B2) in the second pyrolysis zone B2 is set to a super-stoichiometric value with respect to the carbon matrix to be decomposed. It is therefore preferred according to the invention that the decomposition of the polymer matrix is carried out in the presence of a relatively small amount of oxygen in the first pyrolysis zone B1 and that the removal of any pyrolysis residue is carried out in the presence of a large amount of oxygen in the second pyrolysis zone B2. In particular, the oxygen content in the first pyrolysis zone B1 is set so that it is present in an amount less than the amount of oxygen required for the combustion of the gaseous decomposition products of the polymer matrix; a small amount of oxygen in the first pyrolysis zone B1 is necessary to avoid oxidation of the carbon fibers, thereby partially or completely destroying the carbon fibers, which would result in significant deterioration of mechanical and electrical properties. However, the higher oxygen content in the atmosphere relative to the polymer matrix to be decomposed is necessary for complete combustion of any pyrolysis residue on the surface of the recycled carbon fibers in the second pyrolysis zone B2.

It is preferred according to the invention that the oxygen content, in particular the oxygen content G in the first pyrolysis zone B1 (B1) and the oxygen content G in the second pyrolysis zone B2 (B2), is controlled and/or regulated during the pyrolysis, preferably throughout the process, preferably by removing decomposition products, in particular gaseous decomposition products, which result from the decomposition of the polymer matrix, and/or by introducing oxygen, preferably in the form of air. According to the invention, the oxygen content in the first pyrolysis zone B1 and the second pyrolysis zone B2 is controlled by withdrawing gaseous decomposition products of the polymer matrix through the outlet opening of an indirectly heated rotary tube furnace in such a way that the oxygen content in the first pyrolysis zone B1 is sufficiently high to enable decomposition and partial combustion of the polymer matrix, but on the other hand is sufficiently low to limit damage to the oxidation of the carbon fibers to a minimum level or to avoid it altogether. Further, according to the present invention, the oxygen content in the second pyrolysis zone B2 is set so as to ensure combustion of any pyrolysis residue on the surface of the recovered carbon fibers, and partial oxidation of the surface of the recovered carbon fibers occurs without damaging the recovered carbon fibers. According to the invention, the oxygen content in the first pyrolysis zone B1 and the second pyrolysis zone B2 is set by taking off gaseous decomposition products of the polymer matrix via the outlet opening of the indirectly heated rotary tube furnace. The removal of the combustion gases leads to a suction effect, as a result of which air flows through the preferably open pyrolysis device into the corresponding region of the indirectly heated rotary tube furnace. Thus, from a process point of view, the control and/or regulation of the oxygen content in the individual pyrolysis zones is easy to carry out and does not require the use of expensive gases, such as oxygen.

In this connection, the oxygen content in the pyrolysis process, preferably in the entire process, in particular the oxygen content G (B1) in the first pyrolysis zone B1 and the oxygen content G (B2) in the second pyrolysis zone B2, can be measured by means of an oxygen measuring device, in particular an oxygen-sensitive sensor and/or a pressure sensor. In this case, in particular, the oxygen content can be controlled and/or regulated by the oxygen measuring device by removing decomposition products resulting from the decomposition of the polymer matrix, preferably by means of a gas-conducting tube device, and/or by introducing oxygen. For the purposes of the present invention, the oxygen content in the first pyrolysis zone B1 and the second pyrolysis zone B2 can be measured by sensors and/or pressure sensors, such as pitot tubes. In the context of the present invention, the determined oxygen content is used to control the airway device in accordance with the determined oxygen content and the desired value of the oxygen content in the respective pyrolysis zone. The oxygen content can thus be increased by taking off the gaseous combustion gases in the first and second pyrolysis zones through the outlet opening of the indirectly heated rotary tube furnace, since the removal of the gaseous decomposition products leads to a suction effect and to a flow of ambient air into the respective pyrolysis zone in the open pyrolysis apparatus used according to the invention. Increasing the oxygen content in the air of each pyrolysis zone promotes combustion of the polymer matrix and of any pyrolysis residue on the surface of the recycled carbon fibers. However, an increase in oxidation of the surface of the recovered carbon fiber also occurs.

In a preferred embodiment of the invention, the oxygen content in the pyrolysis process, preferably in the entire process, in particular the oxygen content G (B1) in the first pyrolysis zone B1 and the oxygen content G (B2) in the second pyrolysis zone B2, is controlled and/or regulated in such a way that only the polymer matrix is pyrolyzed at least substantially selectively in the first pyrolysis zone B1, the polymer matrix and pyrolysis (coke) residues still remaining only after the first pyrolysis zone B1 are removed at least substantially selectively, and the surface of the recovered carbon fibers is at least partially oxidized in the second pyrolysis zone B2 in such a way. The control of the oxygen content in the first and second pyrolysis zones B1 and B2 causes the polymer matrix to be selectively pyrolyzed, i.e., selectively thermally decomposed in the presence of a certain oxygen content, but without excessive oxidation on the surface of the recovered carbon fibers. The reduced amount of oxygen in the first pyrolysis zone B1 does not affect the thermal decomposition of the polymer matrix, but a combustion with reduced gaseous decomposition products of the polymer matrix and only a low oxidation of the carbon fibers takes place. In the second pyrolysis zone B2, the oxygen content was set in such a manner that the remaining polymer matrix and pyrolysis residue formed on the surface of the recovered carbon fibers in the first pyrolysis zone B1 were removed. Due to the higher amount of oxygen in the second pyrolysis zone B2, at least partial oxidation of the surface of the recycled carbon fibers also takes place there, resulting in improved wettability due to the more hydrophilic and rougher surface at the same time.

As mentioned above, according to the invention, in particular, the temperature T (B2) in the second pyrolysis zone B2 is increased compared to the temperature T (B1) in the first pyrolysis zone B1.

According to the invention, in this case the temperature T (B2) in the second pyrolysis zone B2 can be increased by at least 25 ℃, in particular by at least 50 ℃, preferably by at least 75 ℃, particularly preferably by at least 100 ℃, even more preferably by at least 125 ℃, very particularly preferably by at least 150 ℃ compared to the temperature T (B1) in the first pyrolysis zone B1. To ensure substantially complete removal of pyrolytic residue on the surface of the recycled carbon fibers in second pyrolysis zone B2, a higher temperature T (B2) may be selected in second pyrolysis zone B2 than in first pyrolysis zone B1, since at elevated temperatures, and in particular in combination with the higher oxygen content, at least substantially complete removal of any pyrolytic (coke) residue on the surface of the recycled carbon fibers may be ensured. However, said temperature should not exceed a certain maximum value, otherwise the recycled carbon fibers are excessively oxidized, so that at least partial destruction of the recycled carbon fibers which occurs can significantly reduce the mechanical stability of the fibers.

In particular, the temperature T (B2) in the second pyrolysis zone B2 can be increased by 25 ℃ to 300 ℃, in particular by 50 ℃ to 250 ℃, preferably by 75 ℃ to 200 ℃, particularly preferably by 100 ℃ to 175 ℃, compared to the temperature T (B1) in the first pyrolysis zone B1.

According to the invention, it is preferred that the temperature T (B1) in the first pyrolysis zone B1 is set in the range from 375 ℃ to 475 ℃, in particular 390 ℃ to 465 ℃, preferably 415 ℃ to 455 ℃, particularly preferably 430 ℃ to 445 ℃, and the temperature T (B2) in the second pyrolysis zone B2 is set in the range from 450 ℃ to 750 ℃, particularly 480 ℃ to 690 ℃, preferably 510 ℃ to 675 ℃, particularly preferably 515 ℃ to 650 ℃, with the proviso, however, that the temperature T (B2) in the second pyrolysis zone B2 is increased by at least 25 ℃, particularly by at least 50 ℃, preferably by at least 75 ℃, particularly preferably by at least 100 ℃, even more preferably by at least 125 ℃, very particularly preferably by at least 150 ℃ compared to the temperature T (B1) in the first pyrolysis zone B1, and/or the temperature T (B2) in the second pyrolysis zone B2 is increased by at least 25 ℃ to 300 ℃ compared to the temperature T (B1) in the first pyrolysis zone B1, in particular from 50 ℃ to 250 ℃, preferably from 75 ℃ to 200 ℃ and particularly preferably from 100 ℃ to 175 ℃. The first pyrolysis zone B1 is for selectively decomposing the polymer matrix and the gaseous decomposition products produced are at least partially combusted by oxygen present in the atmosphere of pyrolysis zone B1; due to the small amount of oxygen in the atmosphere of the first pyrolysis zone B1, slow decomposition of the polymer matrix occurs, so that pyrolysis residues may be formed on the surface of the carbon fiber. These pyrolysis residues have to be removed in the second pyrolysis zone B2 at a higher temperature than in the first pyrolysis zone B1, as described above, by the defined temperature and the defined oxygen content setting avoiding excessive oxidation of the recycled carbon fibers.

In this respect, the temperature in the pyrolysis process, preferably in the entire process, in particular the temperature T (B1) in the first pyrolysis zone B1 and the temperature T (B2) in the second pyrolysis zone B2, can preferably be controlled and/or regulated by means of temperature measuring devices, in particular by means of temperature-sensitive sensors. It is desirable to control the temperature in the first and second pyrolysis zones B1 and B2 by means of thermal sensors to ensure selective pyrolysis of the polymer matrix in the first pyrolysis zone B1 and also to ensure complete removal of any pyrolysis residue on the surface of the recycled carbon fibers in the second pyrolysis zone B2. In this case, a defined control of the temperatures T (B1) and T (B2) in the first and second pyrolysis zones B1 and B2 should be ensured to avoid excessive oxidation and/or decomposition of the carbon fibers, which would result in a significant reduction in the mechanical stability of the recovered carbon fibers.

According to the invention, it is preferred that the temperature in the pyrolysis process, preferably in the entire process, in particular the temperature T (B1) in the first pyrolysis zone B1 and the temperature T (B2) in the second pyrolysis zone B2, is controlled and/or regulated in such a way that only the polymer matrix is pyrolyzed at least substantially selectively in the first pyrolysis zone B1 and in the second pyrolysis zone B2 only the remaining polymer matrix and pyrolysis (coke) residues are removed at least substantially selectively after the first pyrolysis zone B1, in such a way that the surface of the recovered carbon fibers is at least partially oxidized.

In a preferred embodiment according to the invention, the oxygen content G (B2) in the second pyrolysis zone B2 is increased compared to the oxygen content G (B1) in the first pyrolysis zone B1,and isThe temperature T (B2) in the second pyrolysis zone B2 is increased compared to the temperature T (B1) in the first pyrolysis zone B1. In this connection, it is preferred according to the invention that the oxygen content G (B1) in the first pyrolysis zone B1 is set to a value in the range from 0.1% to 12%, in particular from 0.5% to 10%, preferably from 0.75% to 6%, particularly preferably from 1% to 4% by volume, and the oxygen content G (B2) in the second pyrolysis zone B2 is set to a value in the range from 2% to 30%, in particular from 3% to 20%, preferably from 5% to 17%, particularly preferably from 6% to 14% by volume, with the proviso that the oxygen content G (B2) in the second pyrolysis zone B2 is increased by at least 3%, in particular at least 5%, preferably at least 7.5%, particularly preferably at least 10%, by volume compared to the oxygen content G (B1) in the first pyrolysis zone B1 and/or that the oxygen content G (B2) in the second pyrolysis zone B2 is increased by at least 3% to 25% by volume compared to the oxygen content G (B2) in the first pyrolysis zone B1%, in particular from 5% to 20%, preferably from 7.5% to 17.5%, particularly preferably from 10% to 15%. In this respect, it is furthermore preferred according to the invention that in the first pyrolysis zone BThe temperature T (B1) in 1 is set to 375 ℃ to 475 ℃, in particular 390 ℃ to 465 ℃, preferably 415 ℃ to 455 ℃, particularly preferably 430 ℃ to 445 ℃, and the temperature T (B2) in the second pyrolysis zone B2 is set to 450 ℃ to 750 ℃, in particular 480 ℃ to 690 ℃, preferably 510 ℃ to 675 ℃, particularly preferably 515 ℃ to 650 ℃, with the proviso that the temperature T (B2) in the second pyrolysis zone B2 is increased by at least 25 ℃, in particular by at least 50 ℃, preferably by at least 75 ℃, particularly preferably by at least 100 ℃, even more preferably by at least 125 ℃, very particularly preferably by at least 150 ℃ compared to the temperature T (B1) in the first pyrolysis zone B1, and/or the temperature T (B2) in the second pyrolysis zone B2 is increased by 25 ℃ to 300 ℃, in particular by 50 ℃ to 250 ℃, preferably by 75 ℃ to 200 ℃ compared to the temperature T (B1) in the first pyrolysis zone B1, particularly preferably from 100 ℃ to 175 ℃.

In particular, for the purposes of the present invention, it is possible for the oxygen content G (B2) in the second pyrolysis zone B2 to be increased in comparison with the oxygen content G (B1) in the first pyrolysis zone B1,and isThe temperature T (B2) in the second pyrolysis zone B2 is increased compared to the temperature T (B1) in the first pyrolysis zone B1. In this case, it is also possible to set the oxygen content G (B1) in the first pyrolysis zone B1 to 0.75% to 6% by volume and to set the oxygen content G (B2) in the second pyrolysis zone B2 to 6% to 14% by volume, with the proviso that the oxygen content G (B2) in the second pyrolysis zone B2 is increased by 3% to 13% by volume compared to the oxygen content G (B1) in the first pyrolysis zone B1. In this case, it is preferred according to the invention that the temperature T (B1) in the first pyrolysis zone B1 is set to 375 ℃ to 475 ℃ and the temperature T (B2) in the second pyrolysis zone B2 is set to 450 ℃ to 750 ℃, with the proviso that the temperature T (B2) in the second pyrolysis zone B2 is increased by 50 ℃ to 250 ℃ compared to the temperature T (B1) in the first pyrolysis zone B1.

As indicated above, the combination of the pyrolysis apparatus according to the invention and the process conditions according to the invention, in particular the controlled increase of the oxygen content G (B2) and/or the temperature T (B2) in the second pyrolysis zone B2 of the indirectly heated rotary tube furnace with outlet openings, results in the recovered carbon fibers being at least substantially free of any pyrolysis residues and having a more hydrophilic and rougher surface than the original carbon fibers or the conventional recovered carbon fibers due to the partial oxidation of the surface. This results in the recycled carbon fibers resulting from the process conditions according to the present invention having better wettability and bondability than the virgin carbon fibers or the conventional recycled carbon fibers. At the same time, however, according to the invention, the surface oxidation is controlled in a targeted manner, so that the mechanical properties, in particular the mechanical stability, preferably the hardness and the tensile strength, of the recycled carbon fibers remain substantially unchanged compared to those of the original carbon fibers.

In general, the residence time of the objects to be recovered can vary within wide limits:

in particular, the residence time VD (B1) of the objects to be recovered in the first pyrolysis zone B1 is from 0.1 to 60 minutes, in particular from 0.5 to 30 minutes, preferably from 0.75 to 15 minutes, particularly preferably from 1 to 10 minutes, very particularly preferably from 1 to 8 minutes. In particular, the residence time in the first pyrolysis zone B1 is sufficient to ensure at least substantially complete removal of the polymer matrix; however, the residence time in the first pyrolysis zone B1 should not exceed a specific time in order to avoid oxidation of the carbon fibers and excessively long treatment times and thus uneconomical process durations. For example, the residence time VD (B1) may be set by the transport speed of the objects to be recovered in the first pyrolysis zone B1 and/or by the spatial length or extension of the first pyrolysis zone B1.

Furthermore, in particular, according to the invention, the residence time VD (B2) of the objects to be recovered in the second pyrolysis zone B2 is 0.01 to 30 minutes, in particular 0.1 to 12 minutes, preferably 0.5 to 7.5 minutes, particularly preferably 1 to 6 minutes, very particularly preferably 2 to 5 minutes. Advantageously according to the invention, the residence time in the second pyrolysis zone B2 is controlled in such a way as firstly to avoid excessive oxidation of the surface of the recycled carbon fibres at high temperatures and secondly to at least substantially remove any pyrolysis residues. In particular, since the residence time depends on the temperature and/or oxygen content, increased residence times should be used at low temperatures and/or low oxygen contents, while higher temperatures and/or higher oxygen contents result in less residence time. In addition, in particular, the residence time in the respective pyrolysis zones B1 and B2 also depends on the size or dimensions and also the composition of the objects to be recovered. In particular, the bulky objects or resin-impregnated objects may cause an increase in the residence time in said first and/or second pyrolysis zones B1 and B2, to ensure firstly a complete removal of the polymer matrix and secondly a complete removal of any pyrolysis residues on the surface of the recycled carbon fibers. For example, the residence time VD (B2) may be set by the transport speed of the objects to be recovered in the second pyrolysis zone B2 and/or by the spatial length or extension of the second pyrolysis zone B2.

In this case, according to the invention, the ratio Q of the residence time VD (B1) of the objects to be recovered in the first pyrolysis zone B1 to the residence time VD (B2) of the objects to be recovered in the second pyrolysis zone B2 is at least 1.05, in particular at least 1.1, preferably at least 1.2, more preferably at least 1.3, particularly preferably at least 1.5, and/or not more than 4, in particular not more than 3.5, preferably not more than 3, more preferably not more than 2.75, particularly preferably not more than 2.5. In particular, a greater residence time in first pyrolysis zone B1 than in second pyrolysis zone B2 can be achieved by first pyrolysis zone B1 having a greater spatial length or extension than in second pyrolysis zone B2, and/or by the transport velocity of the objects to be recovered in first pyrolysis zone B1 being greater than the transport velocity in second pyrolysis zone B2.

According to the present invention, the ratio Q of the stay time VD (B1) of the objects to be recovered in the first pyrolysis zone B1 to the stay time VD (B2) of the objects to be recovered in the second pyrolysis zone B2 may preferably satisfy the following inequality:

1.05. ltoreq. Q.ltoreq.4, in particular 1.1. ltoreq. Q.ltoreq.3.5, preferably 1.2. ltoreq. Q.ltoreq.3, more preferably 1.3. ltoreq. Q.ltoreq.2.75, particularly preferably 1.5. ltoreq. Q.ltoreq.2.5

Furthermore, it is advantageous according to the invention if the residence time of the object to be recovered in the heating zone a is from 0.05 to 20 minutes, in particular from 0.1 to 15 minutes, preferably from 0.5 to 10 minutes, particularly preferably from 1 to 5 minutes, very particularly preferably from 1.5 to 4 minutes. In this case, it is advantageous according to the invention if the temperature in the heating zone A is in the range from 50 ℃ to 350 ℃, in particular from 100 ℃ to 325 ℃, preferably from 150 ℃ to 300 ℃, particularly preferably from 205 ℃ to 295 ℃. The use of the heating zone a is advantageous in that the material to be recovered is preheated to a specific temperature, and thus a uniform temperature of the object to be recovered is very quickly achieved in the first pyrolysis zone B1. This ensures a uniform removal of the polymer matrix with a relatively short residence time and thus also a constant quality of the recovered carbon fibers, since a non-uniform removal of the polymer matrix due to large temperature gradients is avoided. Furthermore, the process time of the process of the invention can be significantly reduced by using the heating zone a, since a relatively short residence time in the first pyrolysis zone B1 can be made possible by heating the objects to be recovered to a specific temperature.

Furthermore, according to the invention, the residence time of the recycled carbon fibers in the cooling zone C may be from 0.1 to 30 minutes, in particular from 0.5 to 25 minutes, preferably from 1 to 20 minutes, particularly preferably from 5 to 18 minutes, very particularly preferably from 7.5 to 15 minutes. In this case, in particular, the temperature C of the cooling zone may be from 10 ℃ to 350 ℃, in particular from 20 ℃ to 250 ℃, preferably from 25 ℃ to 200 ℃ and particularly preferably from 30 ℃ to 150 ℃. The cooling zone C serves to pre-cool the recycled carbon fibers so that they are subsequently rapidly cooled in the cooling section 16, thereby ensuring rapid further processing, in particular comminution, packaging and/or storage. For example, the cooling zone C may be cooled by air blown into the cooling zone C.

With regard to further details of the process according to the invention, the process according to the invention can in principle be carried out continuously or batchwise, preferably continuously. The continuous implementation of the process of the invention allows for an energy-saving and thus economical process, since it is more economical to maintain a continuous temperature. Furthermore, thermal fluctuations in the pyrolysis zones B1 and B2, which may have an adverse effect on the quality of the recovered carbon fibers and also on the service life of the pyrolysis apparatus P, in particular of the pyrolysis furnace 1 with the indirectly heated rotary tube furnace 11, can be avoided by the continuous process. In addition, the continuous process allows direct processing of the objects to be recycled without complex storage of the materials to be recycled.

Furthermore, for the purposes of the present invention, the pyrolysis apparatus P, which is arranged in particular between the first pyrolysis zone B1 and the second pyrolysis zone B2, can have at least one further pyrolysis zone, in particular at least two further pyrolysis zones, preferably at least three further pyrolysis zones. In this case, the pyrolysis apparatus P can also have, according to the invention, 1 to 10 further pyrolysis zones, in particular 2 to 8 further pyrolysis zones, preferably 3 to 7 further pyrolysis zones, preferably 4 to 6 further pyrolysis zones, which are arranged in particular between the first pyrolysis zone B1 and the second pyrolysis zone B2.

In the context of the present invention, it is preferred that one or more zones of the pyrolysis unit P, preferably all zones of the pyrolysis unit, are not physically separated and/or pass over each other, or that one or more zones of the pyrolysis unit P, in particular the first pyrolysis zone B1 and the second pyrolysis zone B2, are physically separated, in particular by one or more locks.

Furthermore, for the purposes of the present invention, the objects to be treated and/or recovered can be pretreated, in particular comminuted, upstream of the heating zone a. The objects to be recovered are advantageously comminuted upstream of the heating zone a in order to match the maximum size of the objects to be recovered to the size of the opening of the input station 3 of the pyrolysis device. In particular, the size of the objects to be recovered in use therefore depends on the dimensions of the input station 3 of the pyrolysis device. However, the objects to be recovered can also be comminuted to a size smaller than that required for the input station 3 of the pyrolysis apparatus used.

In this case, the recycled fibres obtained from the process and/or after the cooling section following the cooling zone C can also be subjected to a post-treatment, in particular a crushing, preferably by cutting, chopping, grinding and/or chopping, and/or in particular a contact with at least one treating agent, preferably selected from sizing, dispersing, antifoaming and binding agents and mixtures or combinations thereof. The post-treatment of the recycled carbon fibers, in particular the comminution, can be carried out in a cutting device which is generally used for this purpose and is known per se to the person skilled in the art, the comminution being able in principle to be carried out by wet or dry processes. The multiple pulverization, particularly the multiple chopping, sets the fiber length of the recovered carbon fiber to a desired fiber length. In this case, the previously chopped carbon fibers can also be used to produce ground recycled carbon fibers; milled carbon fibers can be obtained by grinding chopped carbon fibers, in particular using a grinder, such as a hammer mill, impact plate mill, screen basket mill, or the like. In addition, the recycled carbon fibers, and particularly the surface of the recycled carbon fibers, may be treated with a treatment agent to match the properties of the recycled carbon fibers to the properties of the matrix, thereby improving its bondability to plastics, building materials, or cement-containing systems.

For further details regarding this aspect of the invention, reference may be made to the above description regarding other aspects of the invention, which are similarly applicable to this aspect of the invention.

According to the inventionFourth step ofIn aspects, the present invention still further provides recycled carbon fibers obtained by the method of the present invention.

The combination of the pyrolysis apparatus according to the present invention and the process conditions according to the present invention is directly reflected in the resulting recovered carbon fiber. Since the pyrolysis furnace used is in the form of an indirectly heated rotary tube furnace with an outlet opening and the process conditions according to the invention, in particular the partial oxidation of the surface of the recovered carbon fibers resulting from the pyrolysis, the recovered carbon fibers resulting from the combination of the pyrolysis apparatus according to the invention and the process conditions according to the invention are substantially free of pyrolysis residues and also have rougher surfaces, in particular grooves, trenches, furrows, recesses etc. Furthermore, because of the partial oxidation, the surface of the recovered carbon fiber of the present invention is more hydrophilic than the surface of the original carbon fiber or the conventional recovered carbon fiber. The rougher, more hydrophilic surface of the recycled carbon fibers of the present invention unexpectedly results in improved wettability compared to virgin or conventional recycled carbon fibers, thereby also improving bondability to plastics.

In a preferred embodiment of the invention, the recycled carbon fibers have a wettability with respect to water, as determined by the Wilhelmy method and by single fiber measurement at (23 ± 0.5) ° c, as a tension metric measuring the contact angle, which is not more than 75 °, in particular not more than 73 °, preferably not more than 70 °, particularly preferably not more than 68 °, even more preferably not more than 65 °, very particularly preferably not more than 60 °.

In this case, in particular, the recycled carbon fibers can have a wettability with respect to water, as determined by the Wilhelmy method and by single fiber measurement at (23 ± 0.5) ° c as a tensiometric measurement of tension, of a contact angle of 30 ° to 75 °, in particular 35 ° to 73 °, preferably 38 ° to 70 °, particularly preferably 40 ° to 68 °, even more preferably 45 ° to 65 °, very particularly preferably 50 ° to 60 °.

The wettability of the recovered carbon fiber was determined by a tensiometer using the Wilhelmy method as a single fiber measurement at (23 ± 0.5) ° c with respect to water. With regard to The Wilhelmy method, reference is made in particular to Abe k, ondishi s, akijama h, Takiguchi h, Tamada k, journal of The Surface Science Society of Japan,2000,21, pages 643 to 650, and Baskom w.d., The sitting Behavior of Fibers, in: schrader m, Loeb g model Applications to wetability, both thermal and Applications; plenum Press, New York 1992, pages 359 to 373. In addition, reference may also be made to the following working examples of the present invention, which serve to describe in detail the contact angle measurement by the Wilhelmy method.

For the purposes of the present invention, it is preferred that the recovered carbon fibers have a proportion of pyrolysis residues (charring residues) of less than 5% by weight, in particular less than 4% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, even more preferably less than 1% by weight, particularly preferably less than 0.9% by weight, most preferably less than 0.5% by weight, based on the recovered carbon fibers.

In particular, according to the invention, the recovered carbon fibers have a proportion of pyrolysis residue (charring residue) of less than 0.001 to 5 wt%, in particular 0.01 to 4 wt%, preferably 0.05 to 3 wt%, more preferably 0.1 to 0.95 wt%, as determined by weight analysis, in particular, by thermogravimetric analysis, based on the recovered carbon fibers. On the other hand, a high proportion of pyrolysis residues on the surface of the recycled carbon fibers will firstly lead to a loss of bondability to plastics, building materials or cement-containing systems and secondly to a significant deterioration of the electrical properties of the recycled carbon fibers. For this reason, according to the present invention, the recycled carbon fiber has a very small proportion of pyrolysis residue to obtain excellent bondability and excellent electrical properties. The pyrolysis residue can be determined gravimetrically, in particular by thermogravimetric analysis (TGA), with TGA being preferred. For a detailed description of the gravimetric and thermogravimetric analysis (TGA), reference is made to the working examples according to the present invention.

Furthermore, for the purposes of the present invention, the recycled carbon fibers may have oxygen-containing functional groups, in particular polar and/or hydrophilic groups, in particular selected from phenol, carboxyl, carbonyl, aldehyde, keto, hydroxyl and/or oxo groups, on their surface, in particular as determined by chemical analysis Electron Spectroscopy (ESCA), preferably by X-ray photoelectron spectroscopy (XPS). The oxygen-containing functional groups on the surface of the recycled carbon fibers resulting from the combination of the pyrolysis apparatus according to the present invention and the process conditions according to the present invention result in a more hydrophilic surface of the recycled carbon fibers, which therefore has better wettability compared to the more hydrophobic virgin carbon fibers or conventional recycled carbon fibers. In particular, the oxygen-containing functional groups on the surface of the recovered carbon fiber can be determined by chemical analytical Electron Spectroscopy (ESCA), preferably by X-ray photoelectron spectroscopy (XPS). For further information on the way in which ESCA and XPS measurements are carried out, reference may be made to Levsen K, Physikalische method der Chemie: ESCA, Chemie incoser Zeit, 10.1976, pages 48 to 53, as well as working examples according to the invention.

Further, the recycled carbon fiber may have grooves, dents, grooves, scratches, craters, etc. on the surface thereof. The combination of the pyrolysis apparatus according to the invention and the process conditions according to the invention results in recycled carbon fibres which, contrary to the original carbon fibres, do not have a smooth surface. In particular, this is due to the process conditions according to the invention, in particular the surface oxidation of the recycled carbon fibers during the removal of any pyrolysis residues. In addition to the presence of hydrophilic groups, the rougher surface of the recycled carbon fibers is one reason for the good wettability of the recycled carbon fibers obtained by the combination of the pyrolysis apparatus according to the present invention and the process conditions according to the present invention.

In general, the fiber length of the recycled carbon fibers in an uncrushed state can vary over a wide range. In particular, the recycled carbon fibers in the uncrushed state have a fiber length of 0.01 to 5 meters, in particular 0.05 to 3 meters, preferably 0.1 to 2 meters, more preferably 0.2 to 1 meter. In particular, the size of the recycled carbon fibres obtained by the method of the invention depends on the size of the input station 3 of the pyrolysis device for recycling and on any shredding step performed on the objects to be recycled before recycling. The measurement of the fiber length and the fiber diameter can be generally carried out by a method known to those skilled in the art. In particular, the fiber length and the fiber diameter are usually determined by measurement methods based on light scattering, in particular X-ray diffraction and/or laser light scattering, but also by optical microscopy, electron microscopy, etc. Furthermore, the determination of the fiber length and the fiber diameter in the millimeter range can also be carried out by sieve analysis in accordance with DIN 66165. In particular, the above-mentioned dimensions relate to an at least substantially fibrous basic structure. Further, the information of the sizing given below may be referred to.

Furthermore, the recycled carbon fibres may have a tensile strength of 1000 to 6000MPa, in particular 1500 to 5000MPa, preferably 2000 to 4000MPa, more preferably 2500 to 3500 MPa. In particular, the tensile strength can be determined according to EN ISO 527-1.

Furthermore, for the purposes of the present invention, the recycled carbon fibers may have an elastic modulus of 20 to 1000GPa, in particular 50 to 800GPa, preferably 75 to 600GPa, more preferably 100 to 400GPa, even more preferably 150 to 300 GPa. In particular, the modulus of elasticity can be determined in accordance with DIN EN 61.

Furthermore, the recovered carbon fiber may have an average fiber diameter of 0.1 to 100. mu.m, particularly 1 to 50 μm, preferably 2 to 25 μm, more preferably 3 to 15 μm, particularly preferably 4 to 10 μm. For example, the determination of the average fiber diameter can be performed by optical microscope and/or electron microscope based determination as described above.

In a preferred embodiment according to the invention, the recycled carbon fibres may contain on their surface at least one treatment agent, in particular selected from (i) thermosetting polymers, in particular epoxy resins; (ii) thermoplastic polymers, particularly polyolefin resins; (iii) dispersants, especially aliphatic amine ethanolates and/or dialkylene glycols; (iv) defoamers, in particular polydialkylsiloxanes; and also mixtures and combinations thereof. In order to bond the recycled carbon fibers obtained from the combination of the pyrolysis apparatus according to the invention and the process conditions according to the invention, the recycled carbon fibers may be modified, in particular on their surface, by at least one treatment agent in order to improve the surface properties and thus the bondability of the recycled carbon fibers to plastics, building materials and cement-containing systems, or to match the surface properties of the recycled carbon fibers according to the invention to the respective matrix. The adaptation of the surface properties to the respective matrix results in a homogeneous bonding of the recycled carbon fibers and thus in an effective improvement or reinforcement of the respective matrix.

Furthermore, for the purposes of the present invention, the recycled carbon fibers may be present in comminuted form, in particular in chopped and/or double chopped and/or ground form. As mentioned above, the chopping of the recycled carbon fibres can be carried out in a cutting device which is generally used for this purpose and which is known per se to the person skilled in the art, said chopping being able in principle to be carried out by wet or dry methods. The fiber length of the recovered carbon fiber may be appropriately set by multi-stage pulverization or multi-stage chopping. Further, the milled recycled carbon fibers may be obtained from previously chopped recycled carbon fibers, for example, using a mill, such as a hammer mill, impact plate mill, or basket mill, among others.

In this case, the comminuted recycled carbon fibers may also have an average fiber length of from 0.01 to 200mm, in particular from 0.1 to 150mm, preferably from 0.2 to 100mm, more preferably from 0.5 to 90mm, particularly preferably from 1 to 80mm, very particularly preferably from 2 to 70 mm. In particular, the determination of the fiber length can be carried out by the above-indicated measuring method. In addition, the above fiber length relates to the individually pulverized recovered carbon fiber, which has a larger fiber length than the multi-stage pulverized recovered carbon fiber. However, this will be apparent to those skilled in the art.

Furthermore, the comminuted recycled carbon fibres may have an average fibre length of from 0.1 to 70mm, in particular from 0.5 to 60mm, preferably from 1 to 50mm, more preferably from 2 to 40mm, particularly preferably from 3 to 30mm, very particularly preferably from 5 to 20 mm. The average fiber length of the recycled carbon fiber may be determined as described above. In this case, the above-mentioned fiber length relates to a double-pulverized recovered carbon fiber having a shorter fiber length than that of a single-pulverized recovered carbon fiber.

Furthermore, the comminuted recycled carbon fibers may have an average fiber length of from 0.1 to 1000 μm, in particular from 1 to 900 μm, preferably from 5 to 700 μm, more preferably from 10 to 500 μm, particularly preferably from 25 to 400 μm, very particularly preferably from 50 to 350 μm, even more preferably from 75 to 250 μm. The above-mentioned measurement of the average fiber length of the recovered carbon fiber can be performed by the measurement method as described above. In this case, the above average fiber length of the recycled carbon fibers relates to the ground recycled carbon fibers.

Further, as for the pulverized recycled carbon fiber, the pulverized recycled carbon fiber may have 200 to 5000kg/m³In particular from 300 to 4500kg/m³Preferably 500 to 4000kg/m³More preferably 700 to 3500kg/m³Particularly preferably 1000 to 3000kg/m³Very particularly preferably from 1200 to 2500kg/m³And even more preferably from 1500 to 2200kg/m³. In particular, the fiber density of the recycled carbon fibers can be determined in accordance with DIN 29971.

For further information regarding this aspect of the invention, reference may be made to the description above regarding other aspects of the invention, which are similarly applicable to this aspect of the invention.

Further, according to the inventionFifth aspect of the inventionIn an aspect, the invention also provides the use of the recycled carbon fibers according to the invention as additives, in particular as additives for plastics, building materials or cement-containing systems, or for producing or combining, in particular mixing, carbon fiber-containing plastics, or for producing carbon fiber-containing shaped bodies (e.g. components), moulds and sheet-like materials (e.g. nonwovens).

For the purposes of the present invention, thermoplastic polymers, mixtures of thermoplastic polymers and thermosetting polymers can preferably be used. In particular, the plastic (polymer) may be selected from polycarbonate resins, polyamide resins, saturated polyester resins, polyurethane resins, polyacetal resins, polysulfone resins, polyethersulfone resins (PES), polyphenylene sulfide resins (PPS), polystyrene resins (PS), polyolefin resins, polyvinyl chloride resins, polyether ether ketone resins (PEEK), polyetherimide resins (PEI), polyarylene oxide resins, polyamideimide resins, polyacrylate resins, polyimide resins, and mixtures and combinations thereof.

In this case, in particular, the recycled carbon fibres of the invention can be provided for mixing, in particular for incorporation into plastics. In particular, the incorporation of the recycled carbon fibres of the present invention leads to an upgrading of the plastic and/or an improvement of the mechanical properties in particular.

For further information regarding this aspect of the invention, reference may be made to the description above regarding other aspects of the invention, which are similarly applicable to this aspect of the invention.

In addition, according to the inventionSixth aspect of the inventionIn an aspect, the present invention also provides a plastic, building material or cement-containing system comprising recycled carbon fibres according to the present invention, which has been described in detail above, or which has been produced using recycled carbon fibres obtained by the process of the present invention, which has been described in detail above. For further information regarding this aspect of the invention, reference may be made to the description above regarding other aspects of the invention, which are similarly applicable to this aspect of the invention.

Finally, according to the inventionSeventh aspect of the inventionIn aspects, the invention further provides shaped bodies (e.g. modules), moulds and sheet-like materials (e.g. nonwovens), in particular in the form of composites or composites comprising recycled carbon fibers according to the invention, as has been described in detail above, or have been produced using recycled carbon fibers obtained by the process of the invention, as has been described in detail above. For further information regarding this aspect of the invention, reference may be made to the description above regarding other aspects of the invention, which are similarly applicable to this aspect of the invention.

As mentioned above, the present invention has many advantages and features, some of which are described below, but this is not meant to be limiting:

the use of the pyrolysis apparatus of the present invention comprising an indirectly heated rotary tube furnace with an outlet opening allows for uniform mixing, circulation and heating of the CFP material in the pyrolysis furnace while discharging pyrolysis gases. This results in optimal pyrolysis conditions to selectively remove the polymer matrix at any desired location in the rotary tube furnace such that substantially no pyrolysis (coke) residue remains on the surface of the recycled carbon fibers after complete removal of the polymer matrix.

Furthermore, by indirectly heating the outlet opening of the rotary tube furnace of the pyrolysis apparatus of the present invention, the atmosphere, in particular the oxygen content, in the rotary tube furnace can be controlled. Combined with the indirect heating of the rotary tube furnace, which makes it possible to avoid temperature fluctuations, it is possible to remove the polymer matrix selectively and substantially completely without excessive breakage of the recycled carbon fibers, in particular excessive strong oxidation, since this would lead to a significant reduction in the mechanical and electrical properties of the recycled carbon fibers.

By using the pyrolysis apparatus of the present invention and the process conditions according to the present invention, a targeted control of the uniform heating and mixing and oxygen content and/or temperature of the CFP material can be achieved, which in combination can result in recycled carbon fibers that are substantially free of pyrolysis or carbonization residues at least on their surface. In addition, the above combination leads to partial oxidation of the surface of the recovered carbon fiber, i.e., to a rough surface, particularly a hydrophilic surface, exhibiting stronger binding affinity, and to an increase in oxygen-containing groups, such as hydroxyl groups, aldehyde groups, carboxyl groups, and the like, on the surface of the resulting recovered carbon fiber. Due to the roughened and functionalized, in particular more hydrophilic, surface of the recycled carbon fibers, the partial oxidation of the surface of the recycled carbon fibers resulting from the combination of the pyrolysis device according to the invention and the process conditions according to the invention leads to an increased wettability and thus also to an improved bondability to plastics compared to the virgin carbon fibers or conventional recycled carbon fibers.

Furthermore, the combination of the pyrolysis apparatus of the present invention and the process conditions according to the present invention, particularly the control of the temperature and oxygen content throughout the pyrolysis process, avoids excessive oxidation of the recycled carbon fibers, thereby enabling the recycled carbon fibers of the present invention to have mechanical properties not inferior to those of the original carbon fibers.

Furthermore, the thermal energy required for carrying out the process of the invention is significantly reduced due to the strongly exothermic reaction of oxygen with the gaseous decomposition products of the polymer matrix, so that the process of the invention is very economical. Furthermore, because of the strongly exothermic reaction, the corresponding temperatures necessary for the pyrolysis can also be reached very quickly, so that the method according to the invention results in a short residence time of the object to be recovered. Thus, large amounts of carbon fiber-containing plastics can be recovered in a short time by the inventive pyrolysis apparatus in combination with the inventive process.

Before carrying out the process of the invention in the pyrolysis apparatus of the invention, it is not necessary to carry out any mechanical and/or chemical pretreatment of the carbon fiber-containing plastic to be recycled, so as to obtain recycled carbon fibers having long fiber lengths, which, for example, after addition of a resin, can produce prepregs. However, the recycled carbon fibers having a long fiber length may also be pulverized to a prescribed fiber length such as that used in blending.

Furthermore, it is possible to recover laminated strips of carbon fibre-containing plastics and other reinforcing materials, such as glass fibres, without the need for complex separation in the process of the invention, since the process of the invention results in the recovery of individual strip-like layers of carbon fibres from which other reinforcing materials can be easily removed.

Furthermore, the process of the invention as carried out in the pyrolysis apparatus of the invention also allows continuous operation and execution on an industrial scale.

Further embodiments, modifications and variations of the present invention may be readily recognized and effected by those skilled in the art upon reading the foregoing description without departing from the scope of the present invention.

The invention is illustrated by means of the following working examples, which are not intended to limit the invention.

Working examples are as follows:

particular advantages of the invention are described below, for embodiments for recovering (re-using) carbon fibers from carbon fiber-containing materials.

A)Method for recycling carbon fiber-containing plastics

For example, carbon fiber reinforced plastic waste (CFP waste) obtained in aircraft structures (e.g. aircraft wings) or from wind power turbines (e.g. wind blades) is used as carbon fiber containing plastic. If the pieces of CFP waste have a larger size than the opening of the input station of the pyrolysis device, the comminution of the CFP waste is carried out by cutting means known to the person skilled in the art before recycling.

The recovery of carbon fibers from the above carbon fiber-containing waste material in the form of a prepreg was carried out in a pyrolysis apparatus shown in table 1.

Table 1: pyrolysis device for recycling

According to the invention

The recovery of the carbon-fibre-containing plastics was carried out in the abovementioned pyrolysis units, the process conditions described in table 2 being set in each case in the respective pyrolysis unit.

Table 2: process conditions

According to the invention

The surface structure on the surface of the recovered carbon fiber obtained by recovery in various pyrolysis apparatuses, the proportion of pyrolysis residue, and the oxygen-containing group were determined by the measurement methods described below.

B)Measurement method and results

a) Optical microscopy of recycled carbon fibers

The nature of the surface and the presence of pyrolysis residues were examined by Scanning Electron Microscopy (SEM). Suitable scanning electron microscopes are well known to those skilled in the art. In this case, for example, a scanning electron microscope of JEOL 6400F or Hitachi S-3200 type can be used. The resolution of the scanning electron microscope data is determined by the resolution of the scanning electron microscope. The results of the various pyrolysis units obtained by scanning electron microscopy are listed in table 3 below.

Table 3: results of scanning electron microscopy

According to the invention

It has surprisingly been found that recycled carbon fibres without significant damage to the fibre surface can only be obtained using the pyrolysis apparatus of the invention with an indirectly heated rotary tube furnace with outlet openings in combination with the process conditions according to the invention, but which have a rough surface in the form of grooves and depressions as a result of oxidation during the recycling process. Furthermore, when the pyrolysis apparatus of the present invention is used in combination with the process conditions according to the present invention, the recycled carbon fibers are free of pyrolysis residues (see sample nos. 1 and 2), so that neither bondability nor mechanical properties, in particular hardness and elongation, nor electrical properties are significantly affected by recycling (see sample No. 2). In particular, when the process conditions according to the present invention (see sample No. 2) were used, the recovered carbon fiber had almost the same hardness and elongation, which was only about 5% to 10% lower than the original carbon fiber. The use of a pyrolysis apparatus comprising an indirectly heated rotary tube furnace without an outlet opening resulted in recovered carbon fibers having a large amount of pyrolysis residue (see sample No. 3); in particular, this is due to the low oxygen content, since the pyrolysis gases cannot escape from the rotating tube during the pyrolysis process because of the absence of an outlet opening. The use of a pyrolysis apparatus comprising an indirectly heated rotary tube furnace without an outlet opening but with two treatment zones also results in recovered carbon fibres with a large amount of pyrolysis residue (see sample No. 4). The large amount of pyrolysis gas directly present on the recycled carbon fibers prevents a high oxygen content on the surface of the recycled carbon fibers, but is necessary for removing the pyrolysis residue and partial oxidation of the surface of the recycled carbon fibers. Furthermore, the combustion of the pyrolysis gases takes place in the presence of relatively large amounts of oxygen, which, due to the strongly exothermic reaction, leads to local temperature increases and thus to damage to the recycled carbon fibres. The use of a pyrolysis unit with a belt furnace results in a large amount of pyrolysis residue of the recovered carbon fiber because of the too low temperature and the mixing that exists during pyrolysis (see sample No. 5).

b) Contact angle measurement by the Wilhelmy method

Contact angle measurements on recycled carbon fibers obtained using various pyrolysis devices are determined by tensiometers, e.g. from hamburger, germanyK100SF tensiometer from GmbH. The contact angle measurement was performed as a single fiber measurement with respect to water.

For this purpose, each recovered carbon fiber is first pulverized to a length of 0.8 to 1 cm. For example, the comminution of the individual recycled carbon fibers, which can in principle be carried out by wet or dry methods, can be effected by chopping in a cutting device known to the person skilled in the art and generally used for this purpose.

The comminuted recycled carbon fibres are fixed by means of a sample holder to the force sensor (weighing system) of the tensiometer, the test liquid, in the present case water, is introduced into a measuring vessel (glass, diameter 70mm, volume about 70ml) and positioned in the temperature control unit of the tensiometer under the force sensor.

The wet length of each individual fiber relative to n-heptane was first determined. The parameters of the n-heptane test liquid and water required for the measurements are shown in the table below.

Table 4: parameters of the test liquid

Test liquid	σ[mN/m]
			N-heptane	20.4	0.684
Water (W)	72.80	0.998

The contact angle and wetting length measurements were made at a temperature of (23 ℃. + -. 0.5). degree.C.and the dynamic contact angle was determined as the angle of approach. A double assay was performed for each recovered carbon fiber.

The detection speed was 6mm/min, the measurement speed was 1mm/min, the sensitivity was 0.0004mg, and the immersion depth (position) of the fiber was 5 mm.

Contact angle is a change in force of recycled carbon fiber in contact with water registered at the force sensor and is a function of location, surface tension of the water and by software (e.g. from hamburger, germany)Labdesk software from GmbH) the previously determined wetted length of the recycled carbon fiber, which in the present case is implemented as an on-line contact angle.

The wet length and contact angle of the recovered carbon fibers are shown in table 5 below.

Table 5: wet length and contact angle of recycled carbon fibers

Serial number	Length of wetting [ mm]	Contact angle [ ° [ ]]
			1*	0.025±0.000	69.26±0.22
2*	0.025±0.001	66.35±0.11
			3	0.023±0.001	83.45±0.04
5	0.025±0.001	76.16±0.82

According to the invention

The contact angle of sample No. 4 could not be determined because of partial destruction of the recycled carbon fibers. Furthermore, it was surprisingly found that when the pyrolysis apparatus of the present invention was used in combination with the recovery according to the process conditions of the present invention, oxidation of the surface occurred, resulting in hydrophilic surfaces (samples No. 1 and No. 2). This is clear, the contact angle is smaller relative to sample No. 3, which does not have a hydrophilic surface due to the lower oxygen content during pyrolysis. Sample No. 5 also had a hydrophilic surface, but because a belt furnace as described above was used, a large amount of pyrolysis residue was found on the surface of the recovered carbon fiber by a scanning electron microscope.

In general, it has surprisingly been found that surface oxidation of the recycled carbon fibers occurs only when the pyrolysis apparatus of the present invention is combined with the process conditions according to the present invention, without significantly damaging these, so that the mechanical properties of the recycled carbon fibers of the present invention are retained. A more hydrophilic surface will result in better bondability to plastics, building materials or cement-containing systems than a virgin carbon fiber having a contact angle above 75 c (i.e., more difficult to wet).

C) Gravimetric and thermogravimetric analysis (TGA)

Gravimetric determination of the pyrolysis residue can be carried out by suspending a precisely determined amount of the respective recovered carbon fibers in a solvent such as dichloromethane, subsequently treating the suspension in an ultrasonic bath, filtering the suspension through a coarse screen which only obstructs the carbon fibers, and reweighing the dried recovered carbon fibers. The proportion of pyrolysis residue is given by the weight difference before and after the treatment of the recovered carbon fiber with a solvent such as dichloromethane.

However, in the present case, the proportion of pyrolysis residue is determined by thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) can be performed using measurement devices known to those skilled in the art. In the present case, the recycled carbon fibres obtained by using various pyrolysis devices are first of all finely comminuted, which comminution can be carried out using comminution methods known to the person skilled in the art, in particular using cutting devices or grinding devices, such as hammer mills, impact plate mills, screen basket mills. After pulverizing each sample, 1mg of the pulverized recovered carbon fiber obtained using various pyrolysis apparatuses was transferred to a measuring apparatus and subjected to thermogravimetric analysis using the following parameters: air flow rate of 20cm³(ii) s, temperature heating rate of 10 ℃/min and recording speed of 1/s. The residue vaporized at about 550 ℃ is a pyrolysis residue, which was determined by weighing the sample before and after thermogravimetric analysis. For example, the determination may be made by a microbalance.

In the case of samples No. 1 and No. 2, no weight loss was found, so the combination of the pyrolysis apparatus of the present invention and the process conditions according to the present invention did not result in a large amount of pyrolysis residue. In particular, the recycled carbon fibers produced using the pyrolysis apparatus of the present invention and the process conditions according to the present invention have less than 0.1 wt% of pyrolysis residue (i.e., below the detection limit). Samples No. 3 to 5, which are not according to the invention, each have a large amount of pyrolysis residues, in each case more than 5 wt.%, which is why they have a poor bondability to plastics. In addition, the high proportion of pyrolysis residue in samples nos. 3 to 5 adversely affects the electrical properties of the recovered carbon fibers.

d) X-ray photoelectron spectroscopy (XPS)

Finally, the kind and amount of oxygen-containing groups on the surface of the recovered carbon fiber prepared by using various pyrolysis apparatuses were determined by X-ray photoelectron spectroscopy (XPS).

Each recovered carbon fiber was applied in a bundle to a gasket, which was made of stainless steel, had a diameter of 1cm, and was provided with a double-sided adhesive tape. The ends of the bundled recycled carbon fibers are then secured to the mat by using additional adhesive tape. All 6 samples were placed on the pad at a distance of about 5mm from each other.

X-ray photoelectron spectroscopy may be performed using a measuring instrument suitable for the purpose, such as Kratos AXISULTRA, a monochromatic Al-K.alpha.X-ray electron source (1486.6eV) using an emission current of 15mA and an anode potential of 10 kV. The recorded spectra are in the range 0 to 1100eV, the transmission energy is 80eV and the step size is set to 0.5 eV. All spectra were recorded using a reflection angle of 90 °. In each case 3 positions in the middle of the sample were measured, the surface area of the sample in each case being 300. mu. m.times.700. mu.m.

The composition of the surface is calculated by software such as CasaXPS.

The applicant has surprisingly determined that the carbon fibers of samples No. 1 and No. 2 recovered according to the invention have a higher concentration of ketone groups and carboxylate groups on the surface than samples No. 3 and No. 4 not according to the invention. Sample No. 5 also had a higher proportion of ketone groups and carboxylate groups than the recovered carbon fibers of samples No. 3 and 4 not according to the present invention, but it was found that there were a large amount of pyrolysis residues on the surface of the recovered carbon fibers of sample No. 5 due to the combination of the pyrolysis apparatus used and the process conditions used, and these pyrolysis residues made the bonding with plastics more difficult and had an adverse effect on the electrical properties of the recovered carbon fibers.

The increased proportion of oxygen-containing groups, particularly ketone groups and carboxylate groups, results in the surface of the recycled carbon fibers of the present invention being more hydrophilic. This more hydrophilic surface results in better bondability because of better wettability of the surface.

List of reference numerals:

1 pyrolysis furnace P pyrolysis device

2 CFP material A heating zone

3 input station B1 first pyrolysis zone

4 chute B2 second pyrolysis zone

5 conveyor belt C cooling zone

6 output station CFP carbon fiber-containing plastic

7 carbon fiber material

8 air duct device RF recycled carbon fiber

9 pyrolysis of gas 1 virgin carbon fibers (not according to the invention)

10 control unit

11 rotating tube

12 outlet opening 1' carbon fibre recovery (not according to the invention)

13 casing

14 discharge line

15 heating section 1' recovery of carbon fibers (according to the invention)

16 cooling part

17 nozzle

18 collecting tray 2 recovering grooves (not according to the invention) in carbon fibres 1

19 part (C)

19.1 heating zone A

19.2 first pyrolysis zone B1

19.3 second pyrolysis zone B22 'recovery of grooves on carbon fiber 1' (according to the invention)

19.4 Cooling zone C

20 air inlet

21 control valve

22 mixing element 3 pyrolysis or carbonization residue

23 gas burner

24 heating gas pipeline

25 connection

26 transfer element

Claims (10)

1. A pyrolysis apparatus for recovering carbon fibers from a carbon fiber-containing plastic, wherein the pyrolysis apparatus comprises:

an elongated pyrolysis furnace for continuous pyrolysis of CFP material, which is continuously operated during operation,

an input station for introducing CFP material to be processed into the pyrolysis furnace at one end of the pyrolysis furnace,

an output station for discharging the recovered carbon fiber material from the pyrolysis furnace at the other end of the pyrolysis furnace,

a gas duct device for pyrolysis gas generated in the pyrolysis furnace, an

Control means for regulating at least individual components of the gas in the pyrolysis furnace,

wherein the pyrolysis furnace is an indirectly heated rotary tube furnace having at least the following components:

an elongated rotating tube forming a receiving space for the CFP material to be treated and connected to the input and output stations, the rotating tube being provided with an outlet opening in its cylindrical wall for discharging pyrolysis gases formed during pyrolysis over at least a part of its length, and

a housing which is insulated from the outside and at least partially encloses the rotating tube, has an opening for the input station and optionally also for the output station, and has an exhaust line for pyrolysis gas, wherein a plurality of sections with different or separately adjustable gas temperatures are arranged in the housing along the length of the rotating tube;

wherein the outlet opening in the rotating tube is arranged at least in the section with the highest gas temperature;

wherein the pyrolysis furnace has different sections, namely at least one heating zone, a first pyrolysis zone, a second pyrolysis zone and a cooling zone; the gas composition and temperature inside the pyrolysis furnace in different sections of the rotating tube can be adjusted separately, i.e. with a defined oxygen proportion and a defined temperature in the first pyrolysis zone and a defined oxygen proportion and a defined temperature in the second pyrolysis zone; and is

Wherein the pyrolysis device additionally has a comminution device for comminuting the CFP material to be treated, which comminution device is arranged before or upstream of the input station.

2. The pyrolysis apparatus of claim 1, wherein the rotary pipe is disposed to be inclined downward from the input station to the output station, and the rotary pipe is provided with a mixing element inside thereof.

3. The pyrolysis apparatus of claim 1 or 2, wherein the pyrolysis apparatus additionally has a post-treatment device disposed after the output station for post-treating recycled carbon fibers obtained from the CFP material.

4. A method for recovering carbon fibers from carbon fiber-containing plastics, carbon fiber-reinforced plastics, carbon fiber-containing composites, and carbon fiber-reinforced composites, wherein the method comprises utilizing the pyrolysis apparatus of claim 1.

5. A method for recovering carbon fibres from a carbon fibre-containing plastic, wherein the method comprises the steps of:

based on the multistage pyrolysis of objects of carbon fiber-containing plastics comprising carbon fibers in a polymer matrix in the presence of oxygen, the polymer of the polymer matrix is decomposed during the pyrolysis process to give carbon fibers,

the pyrolysis is carried out in a pyrolysis apparatus having a pyrolysis furnace as recited in claim 1, wherein the pyrolysis furnace comprises at least the following treatment zones in the following specified order, and the object passes through the following treatment zones in this order:

(A) a heating zone A, in which the objects to be treated and/or recovered are heated to a defined temperature,

(B1) a subsequent first pyrolysis zone B1, in which pyrolysis of the polymer matrix of the object to be treated is carried out at a defined temperature T (B1) and a defined oxygen content G (B1),

(B2) a subsequent second pyrolysis zone B2, in which the final pyrolysis of the polymer matrix of the object to be treated still present after the pyrolysis zone B1 is carried out at a defined temperature T (B2) and a defined oxygen content G (B2) to at least substantially complete removal,

(C) a subsequent cooling zone C for cooling the recovered carbon fibers obtained from the second pyrolysis zone B2,

wherein the oxygen content G (B2) of the second pyrolysis zone B2 is increased by 3 to 25% by volume compared to the oxygen content G (B1) of the first pyrolysis zone B1, and

the temperature T (B2) of the second pyrolysis zone B2 is increased by 25-300 ℃ compared to the temperature T (B1) of the first pyrolysis zone B1.

6. Carbon fibers recovered from carbon fiber-containing plastics by pyrolysis,

wherein the recycled carbon fibers are obtainable by the process of claim 5, and

wherein the recycled carbon fibers have:

wettability with respect to water, contact angle measured as a degree of tension determined by the Wilhelmy method and by single fiber measurement at (23 + -0.5) ° C, which does not exceed 75 °,

a proportion of pyrolysis residue of less than 0.5 wt%, said proportion being based on the recovered carbon fibres and determined gravimetrically,

the oxygen-containing functional groups on the surface, i.e. polar and/or hydrophilic groups, are selected from the group consisting of phenol, carboxyl, carbonyl, aldehyde, keto, hydroxyl and oxo groups, as determined by chemical analysis of Electron Spectroscopy (ESCA), and

with grooves, trenches, recesses, trenches, scratches or craters on its surface.

7. The recycled carbon fiber as claimed in claim 6,

wherein the recycled carbon fiber has wettability with respect to water, a contact angle measured as a degree of tension determined by a Wilhelmy method and by single fiber measurement at (23 + -0.5) ° C, which is not more than 73 °, and

the recovered carbon fiber has a pyrolysis residue in a proportion of 0.001 to 0.5 wt% based on the recovered carbon fiber and determined by gravimetric analysis.

8. The recycled carbon fiber as claimed in claim 6,

the recovered carbon fibers have a tensile strength of 1000 to 6000MPa,

the recycled carbon fiber may have an elastic modulus of 20 to 1000GPa,

the recovered carbon fibers have an average fiber diameter of 0.1 to 100 μm.

9. A material selected from a plastic, a building material, or a cement-containing system comprising the recycled carbon fiber of claim 6.

10. A shaped body, a mold, or a sheet-like material in the form of a composite material comprising the recycled carbon fiber as claimed in claim 6.