SUSTAINABLE 3D PRINTING MATERIAL WITH MICRO-CARBON FIBER RECYCLED FROM INDUSTRIAL WASTE: A PRELIMINARY RESULTS OF EFFECT OF FIBER CONTENT ON MECHANICAL PERFORMANCE IN PA6,6 COMPOSITES

Preliminary characterization of the composite material manufactured by injection molding was carried out with a PA6,6 polymeric matrix and recycled carbon fibers (rCF) reinforcement, with the prospect of making a wire for 3D printing. Thus understanding if the addition of reinforcement can improve interlayer adhesion and obtaining a performing innovative composite through Additive Manufacturing. Standard PA6,6 specimens have been manufactured and characterized both neat and with reinforcement starting from BASF Ultramid 1000-11 NF2001 (dry) polymer through Thermo Scientific Process 11 single screw extruder, and Thermo Scientific Haake Minijet injection chamber Pro. The polymer has a density of 1.14 g/c3, a moisture content of 8.5% in equilibrium at Troom [1], a melting temperature of 257 °C. The recycled carbon microfibers are the result of a double recovery process, starting with pyrolysis from which long fibers are obtained, used to produce TNT fabrics and, subsequently, the waste obtained from this last process are shorter fibers that are ground in a ball mill. The powder obtained is the reinforcement used in our work which is extremely suitable for 3D applications. Therefore, rCF were previously characterized by SEM, EDX and density analysis with pycnometer. The reinforcement content of 5% and 10% within the polymer matrix has a significant effect on the mechanical properties of the material. The average values of Young's modulus and maximum force both in bending and traction were obtained. The neat specimens have a flexural stiffness and strength value equal to E=1,32 GPa, Fmax=61,21 MPa, tensile values equal to E=1,28 GPa, Fmax=56,43 MPa. An increase is visible from the results obtained by adding the microfibers, i.e., at bending E5%=1,98 GPa, Fmax5%=78 MPa, E10% =2,08 GPa, Fmax10%=75 MPa and traction equal to E5%=2,03 GPa, Fmax5%=58,7 MPa, E10%=2,16 GPa, Fmax10%=59,7 MPa. From scanning electron microscopy of reinforcement emerged fragments of carbon fibers with a very varied grain size with an average value of 31.26 μm, due to the recycling and crushing processes of the original material. Furthermore, the surfaces are sprinkled with adhered matrix residues. The focus of our work is on the recycling of end-of-life carbon fiber and reusing them in a sustainable 3D printing process. It was shown that the recovered microfibers can be effective in enhancing the structural stiffness of the material. The SEM images showed traces of polymer on the surface of the fibers, a phenomenon that can allows for favorable interaction between the molecules of PA6,6 and polymer. The density tests on composite specimens manufactured by injection molding have shown a considerable loss of reinforcement in the machinery, going from 5% by weight inserted into the extruder to 3% of the actual reinforcement. This is a problem that undoubtedly needs attention.