Separation and reclamation of automotive hybrid structures made of metal and fibre-reinforced plastic
Introduction
The lowering fuel consumption and the reduction of CO2 emissions is one of the central tasks of the future that can be met by lightweight automotive construction. In this context, multi-material or hybrid structures have been increasingly used in recent years to realize the required lightweight construction potential (Steigemann, 2012). These can be executed in many ways for example with a combination of metals and fiber composite plastics (FRP) such as carbon fiber reinforced plastics (CFRP) results in a hybrid structure (Jäschke and Dajek, 2004). Such combinations of materials can be used to realize automotive structures with very good mechanical properties at low weight and moderate costs (Lauter et al., 2011).
However, the firm bonding of foreign materials cannot be reconciled with the legally required recycling rates (Bundesrechtsverordnung, 2002). For the hybrid structures described, there are currently no industrially implemented concepts for separating these hybrid structures into individual materials.
Large-scale/high-volume recycling plants exist for the individual material components of hybrid materials, a breakdown of the recycling process is required. First, the hybrid structures have to be separated by a method, which was described in patent DE 10 2012 207 359 A1 (University Paderborn, 2013). Then the separated metal and FRP components can be brought to the specialized recycling processes. For both metallic materials and fibre-reinforced plastics established recycling methods already exist. Steel products can be completely recycled by returning them as steel scrap to the material cycle. There is no degradation so that the recycling process can be repeated endlessly (Stahlinstitut VDEh, 2013). Within the conventional recycling process, the carbon fibre gets destroyed even though carbon fibres are expensive and require an energy-intensive production (Niemeyer and Ziegmann, 2012). The only industrially implemented method for recovering the valuable carbon fibres from FRP is pyrolysis. In the process, organic material (the plastic matrix) is decomposed at high temperatures in a low-oxygen atmosphere. The mineral fibres (the carbon fibres) are preserved and can be reused (CFK CFK Valley, 2011, Meyer, 2011).
Pure fibres can be obtained from FRP components by a specialized recycling process. The carbon fibres embedded in plastic are released from the matrix in a pyrolysis process by a thermal treatment in the absence of oxygen (Pimenta and Pinho, 2011). The dissolution of the plastic produces gases that are used in a thermal post-combustion process. The recovered energy is thus returned to the pyrolysis process. After pyrolysis, the fibre surface is refined and the fibres are cut to length or ground for various applications (CFK CFK Valley, 2016, Meyer, 2011). The recovered fibres can be reprocessed in CFRP again.
The present research task introduces methods for separating and recycling automotive hybrid metal-CFRP structures. In the development process, the heat was specifically introduced into hybrid structures. The adhesion between metal and CFRP will be destroyed to such an extent that two pure individual materials remain.
For the evaluation and a possible industrial application of this novel process, further insights into the types of energy input into the structures, the process windows, and the damage mechanisms of the connection between single components of a hybrid structure are necessary. Detailed investigations were carried out at the University of Paderborn. This study focuses on developing suitable separation concepts for the recycling of hybrid structures.
For FRP, the component was select on the epoxy resin-based carbon fibre reinforced plastic. Thermoplastic FRP can be easily removed from the metallic component by remelting the plastic component. Of all the thermosetting plastics, the epoxy resins are preferably investigated/used for hybrid structures in the automotive industry. A possible effect based on the type of fibres on the separation of hybrid structures could not be mapped at the beginning of the study. Since the connection of the single components involves only plastic or an adhesive between them, it is probably not essential which type of fibre is embedded in the plastic. Especially with carbon fibres, because of their own high cost along with the high production cost, the aspect of recyclability is of most importance.
For the metallic component, soft deep-drawing steel was selected as a representative of the steel group of metals to investigate the heating methods. The main reason for this is the comparatively simple processing of this material concerning its cutability.
One of the main aspects of the present research task was to differentiate various thermal separation processes. For each thermal process, the determination of suitable process parameters and limitations in terms of an industrial application is analysed. Three alternative heating methods were selected in addition to the conventional furnace heating namely induction heating, resistance heating, and infrared heating.
Furnace heating is a conventional method for introducing thermal energy into structures for different material configurations. The technologies for the efficient operation of such systems are already noticed in the state of art and can be implemented with low investment. For these reasons, this heating method has been investigated in the current paper. One of the disadvantages of this method is the long heating time, due to the process of the necessary heat conduction in a gaseous medium.
Other methods for heating preferably metallic (iron-containing) components are induction and resistance heating. Among the main advantages (benefits, pros) of these methods is especially the low heating time. Within a few seconds, metallic elements can be heated to several hundred degrees. The application of these heating methods to complex structures turns out to be complicated or even unworkable. Although some limitations, these heating methods belong to already established methods in the automotive industry and are considered in this study.
The technology of infrared heating also usually offers the possibility of faster heating than in conventional furnace heating. Thus, with all the literature analysed gives the information about the possibility of partial or one-sided heating of the structures.
Section snippets
Preliminary investigation
First, a thermogravimetric analysis (TGA) of the carbon fibre reinforced plastic was carried out. From this, the temperature window for the separation of hybrid structures was roughly limited.
The test was performed in a synthetic air atmosphere at 85 ml/min gas flow rate and the heating interval ranged from 25 °C to 1000 °C. The TGA is using dynamic heating rates with a nominal heating rate of 5 °C/min. The sample weight was chosen at about 27 mg. The results of the TGA are shown in Fig. 1.
Fig.
Specimen and methods for testing heating methods
The following material combination was specified for the investigations: a 2 mm thick metal sheet (uncoated deep-drawn steel DD11) and a carbon fibre reinforced plastic based on the epoxy resin (SIGRAPREG® C U230-0/NF-E320/39% from SGL GmbH) with unidirectional fibre orientation. These material combinations are often used for structural components due to their excellent properties.
The CFRP was delivered as prepregs (pre-impregnated fibres). The density for the prepreg is 1.5 g/cm3 (manufacturer
Separation of hybrid structures
In the following tests for the separation of hybrid components, the samples were thermally treated without any external mechanical impact. For the investigation of the different heat methods, steel-CFRP-specimen were used without an external adhesive. The decomposition temperature of CFRP determined in the preliminary investigations is 300 °C. A temperature range below and above this temperature (200 °C – 400 °C) was therefore selected for the separation tests.
Conclusion
For several years, multi-material or hybrid structures are increasingly being used in the automotive industry to realize the required lightweight potentials. These materials often consist of a combination of metals and fibre-reinforced plastics (FRP) such as carbon fibre-reinforced plastics (CFRP).
However, structures, which are made of different materials, cannot be brought in line with the legally required recycling rates. Currently, there are no industrially implemented concepts for the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Special thanks to the Federal Ministry of Education and Research for the financial support under the funding initiative “KMU-innovativ” with a focus on “resource and energy efficiency” and all project partners for their excellent cooperation.
References (22)
- et al.
Thermal decomposition of fire retardant brominated epoxy resins
J. Anal. Appl. Pyrol.
(2002) - et al.
Evaluation of anisotropic thermal conductivity for unidirectional FRP in laser machining
Compos. A
(2001) - et al.
Recycling carbon fibre reinforced polymers for structural applications: Technology review and market outlook
Waste Manage.
(2011) - et al.
Recycling of carbon fibre-reinforced epoxy resin composites under various oxygen concentrations in nitrogen–oxygen atmosphere
J. Anal. Appl. Pyrol.
(2015) - Bundesrechtsverordnung, 2002. Verordnung über die Überlassung, Rücknahme und umweltverträgliche Entsorgung von...
- CFK Valley, 2011. Recycling für starke Fasern. CFK Valley Stade Recycling GmbH & Co. KG. Innovation Report 2/2011,...
- CFK Valley, 2016. CFK Recycling Center. CFK Valley Stade Recycling GmbH & Co. KG....
- et al.
Induktive Erwärmung – physikalische Grundlagen und technische Anwendungen
(1991) - Frauenhofer, M., 2010. Schnellhärtung struktureller Verbundklebungen mittels elektromagnetischer Wechselfelder....