HIGH PERFORMANCE THERMOPLASTIC COMPOSITE PROCESSING AND RECYCLING: FROM CRADLE TO CRADLE

Transcription

1 HIGH PERFORMANCE THERMOPLASTIC COMPOSITE PROCESSING AND RECYCLING: FROM CRADLE TO CRADLE MAXIME ROUX 1, NICOLAS EGUÉMANN, LIAN GIGER, CLEMENS DRANSFELD Fachhochschule Nordwestschweiz (FHNW), Institut für Kunststofftechnik (IKT), Klosterzelgstrasse 2, CH-5210 Windisch, SWITZERLAND SUMMARY Carbon-fiber reinforced polymer (CFRP) structures are widely implemented in the aerospace industry, where the need for efficient and lightweight structural components is paramount. However, the end-of-life represents a major challenge to the industry because European regulations require solutions to recycle of aircrafts components: CFRP are complicated and expensive to recycle. This research addresses new ways of recycling those materials. At the Institute of Polymer of Engineering (IKT) in Switzerland, a novel solution has been developed, allowing replacing metallic components with weight saving recyclable thermoplastic composites. The recycled materials were obtained through a technique called high voltage fragmentation. This method exhibits several benefits, especially for very high fiber volume content; common in aerospace. After recycling, the fragments were characterized and reused to produce recycled parts. Structural testing revealed a 20% decrease of the mechanical properties when compared to the first generation material. The combination of these manufacturing and recycling techniques closes the cradle to cradle loop for these thermoplastic composites. INTRODUCTION Aircraft manufacturers are facing many challenges. Not only the cost, the performance and the weight of airplanes are important parameters but also the environmental impact and end-of-life treatment of their structures. Aircraft manufacturers have reduced the weight and the fuel consumption of the last aircraft generation by using more and more composite materials. Carbon fibers reinforced polymers (CFRP) are used for replacing steel, titanium or aluminum components. Recycling in Aerospace Thermoset composites (TSC) are the most widely used in the aerospace for the production of large components. Their actual end-of-life solution is to landfill the 1 Corresponding author: maxime.roux@fhnw.ch

2 components or dispose the retired aircrafts in desert graveyards. One recycling solution is to remove the thermosetting matrix from the carbon fibers by thermal degradation (thermal pyrolysis) as low economy product (carbon powder). New methods are being developed in order to increase the quality of the recovered carbon fibers (critical water oxidation and micro-wave pyrolysis). Numerous reasons push to recycle CFRP wastes, namely environmental (European directives) and economic (disposal cost, price of raw materials). In contrast with TSC, carbon fiber reinforced thermoplastic polymers (TPC) have an industrial manufacturing and recyclability close to metal components. The low density, mechanical properties and the absence of corrosion enable the replacement of small metallic parts [1]. After use, the TPC can be chopped into small fragments, re-extruded, injected and processed into a new product. Thermoplastic polymers can be easily chopped using a mechanical shredder. The problem is when the thermoplastic is reinforced with high contents of carbon fibers such as airplanes components, the blades of the shredder are quickly worn and have to be frequently changed. No industrial solution has been found to grind down these aerospace composite parts. High Voltage Fragmentation High voltage fragmentation is traditionally used for the disintegration of rocks in the mining industry. In the 1960's, scientists in Russia and then in Germany developed this process to extract crystals and precious materials from rocks. A brittle material is placed between two electrodes in water; an electric current at high voltages ( kv) is applied over a short time (less than 5 μs). An electric arc is created and will hit and fragment the part ( Figure 1). The spark penetrating the material creates high pressure and high temperature (up to Pa and C) [2]. A pressure wave (shock wave) is created along the spark (plasma channel) that leads to the disintegration and the cracking of all the surrounding materials (the pressure is almost always greater than the tensile strength of the material). As soon as cracks are created, the sparks will travel through them and so on. Figure 1: Diagram of an electrodynamical fragmentation setup - Batch process MATERIALS AND METHODS Material The thermoplastic matrix used in this study is a polyether ether ketone (PEEK). The reinforcement is high modulus carbon fibers. The composite is supplied in unidirectional directional 55 vol% AS4 carbon fiber tapes pre-impregnated and chopped into chips (15 mm) by SUPREM AG, Switzerland (see Figure 3).

3 Composite Processing - Door Hinges A lightweight rotorcraft door hinge made of chips has been developed at the FHNW (Figure 2). The weight of the part is only 22 g, compared to 134 g of the initial door hinge made of steel. Eguémann et al [3] performed a complete study of the influence of the process parameters and the size and type of material used. The material is deposited in the mold cavity and pressed at high temperature (360 C) for several minutes. Recycled chips were processed using the same parameters as the chopped tapes and injection granules (24 vol% CF - Fiber length below 1 mm) [3]. Figure 2: Tooling for the processing of rotorcraft door hinges Recycling Process The equipment used for the recycling is a high voltage fragmentation lab unit manufactured by SELFRAG AG, Switzerland. This machine is a lab-scale operating in a closed vessel (continuous processes are available for minerals and larger amounts of material). The vessel contained from 3 to 4 liters of water. The procedure is to let the fragmentation run for 100 to 200 pulses, then to filter the water and start the cycle again. The filtration is performed in order to recover the fragments with the required size and to remove the carbon powder. The operation is run again until the part is fully fragmented. RESULTS AND DISCUSSION Recycling and Processing of the Recycled Chips The fragmentation of composites is similar to mineral products; the electrical sparks will travel through the material using the principal boundaries existing. In the case of the composite material presented here, there are two main interfaces, the interfaces between chips which is the weakest, and the interface between the polymer and the single fibers inside the chips. The main reason for shortening the cycle is to remove the carbon powder produced during the process which at a certain level will create a flashover in the water bypassing the part. When a fragment is Figure 3: High voltage fragmentation of a TPC; Right: chopped tapes, Left: fragmented chips detached from the part; the sparks can also hit and reduce their critical size beyond requirements. The goal of using short cycles is to simulate a continuous process.

4 Size Distribution of the Fragments The size of the fragments is controlled by filtering the water after every cycle of fragmentation. The size of the fragments varies as a function of the number of pulses at every cycle. The longer is the cycle, the smaller are the fragments obtained. Processing of the Door Hinge Following the same process parameters as the industrial tapes, the recycled door hinges are built using selected fragments between 0.5 mm and 4 mm. This selection decreases the amount of fine carbon powder and thick fragments which are more difficult to process and consequently will have a weak adhesion with surrounding fragments, especially at the thinnest position (ring, arm ) and will lead to Figure 4: Door hinges made with 20mm chopped tapes (left) and recycled chips (right) early breakage of the part in a brittle manner. The exterior aspect of the door hinge is similar to the one made of chopped tapes mainly due to the smallest fragments which create a smooth and homogeneous surface (Figure 4). Assessing the Quality of the Recycled Chips In the original chips supplied by the manufacturer, the fibers are well-embedded in the polymer (see Figure 5). On the recycled fragments, most of the fibers are not broken or damaged. After the fragmentation, a lot of polymer has been removed from surface of the fibers; some fibers are entirely free of polymer. The consequences of high pressures waves and high temperatures along the sparks are clearly visible; the matrix has been melted and mechanically torn up at different positions around the fibers (Figure 6). Inside the fragment, the fibers are still embedded in the polymer. 100x 100x Figure 5: SEM picture of the chopped tape surface (3kx) Figure 6: SEM picture of the recycled chips surface (3kx)

5 Assessing the Quality of the Recycled Door Hinge Mechanical tests were performed on the door hinges (Figure 7). The door hinge made of recycled chips exhibits a slow plastic deformation and better results than the one made of injection molding material granules. The reduction in strength of the recycled door hinge was only of 20 % compared to the one manufactured with chopped tapes. The fracture of the door hinges after mechanical testing was visualised using a scanning electron microscopy (SEM) in order to understand the mechanisms of rupture. In the case of a door-hinge made of chopped tapes, the fracture is mainly going around the tapes (Figure 8). At some positions, the fracture is also passing Figure 7: Graphic of the maximal load of door hinge made with granules [3], recycled chips and chopped tapes inside the chips especially along the fibers and at the extremities. The same behavior is observed with the door hinge made of recycled chips (Figure 9). In addition, two phases are detected, the first one represents the old chips which were fragmented and still have oriented and longer fibers. In this phase, fibers on the surface are either still fully covered or cleaned of polymer. The polymeric matrix is also very present between the fibers in the bundles (Figure 10). Some bundles exhibit a very small amount of matrix and a small adhesion of the fibers together. The second phase is made of randomly oriented and shorter fibers. A lot of polymer matrix phase is between the fibers (Figure 11). Lots of fibers were pulled out during mechanical testing. When having a closer look at those fibers, the surfaces are cleaned of residues and poorly coated in PEEK. Figure 8: SEM - Fracture of the original door hinge - 61x Figure 9: SEM - Fracture of the recycled door hinge - 75x

6 Figure 10: SEM - Recycled door hinge fracture : randomly oriented fibers in polymer - 500x Figure 11: SEM - Recycled door hinge fracture : Surface of a fragment with aligned fibers - 990x CONCLUSION AND OUTLOOK TPCs are good candidates for replacing complex metallic parts in aerospace applications, not only due to their low density, excellent mechanical properties and processability, but also regarding the new perspective of recycling that offers the electrodynamic fragmentation. The quality of those fragments has to be improved by adapting the process equipment and parameters for a given TPC. A continuous working machine dedicated to CFRP recycling is under-development in collaboration of the European research project CLEAN SKY JTI and SELFRAG AG. ACKNOWLEDGEMENTS The development of recycling activities has been supported by the European Union within the 7 th Framework Programme in the CLEAN SKY JTI Eco-design Project. Industrial support has been provided by SUPREM SA, Switzerland and SELFRAG AG, Switzerland. REFERENCES 1. P. Parleviet, C.P., C. Weimer Ecodesigned thermoplastic composite helicopter airframes structures, SETEC 12, LUCERNE 7th. INTERNATIONAL TECHNICAL CONFERENCE, 2012: p Bluhm, H., et al., Application of pulsed HV discharges to material fragmentation and recycling. Ieee Transactions on Dielectrics and Electrical Insulation, (5): p N. Eguémann, L.G., M. Roux, Prof. C.Dransfeld, Prof. Dr. F. and P.D.D.P. Thiébaud, Manufacturing and recycling of complex composite thermoplastic parts for aerospace application, SETEC 12, LUCERNE 7th. INTERNATIONAL TECHNICAL CONFERENCE, 2012: p. 34.