The Composite and Nanocomposite Advanced Manufacturing Center (CNAM Center) at the South Dakota School of Mines and Technology (SD Mines) has developed a patent-pending extrusion-based process which incorporates discontinuous recycled fibers in continuous thermoplastic sheets (called DiFTS – Discontinuous Fiber Thermoplastic Sheets).
In the DiFTS process, the fibers are thoroughly encapsulated by the resin, are uniformly dispersed and retain fiber lengths in the range of a few millimeters, depending on feedstock length and the fiber-resin system. Moreover, the fibers in the sheets are preferentially oriented, conferring high anisotropic strength and stiffness properties to the composite.
Table 1 shows the measured 0o direction tensile and flexural properties (modulus and strength) of laminates produced from PA6 DiFTS incorporating 30 wt % carbon fibers from different sources: recycled standard modulus, virgin standard modulus and recycled intermediate modulus.
Given that the virgin fiber included a sizing for thermoplastic resins and the recycled fiber did not, the properties of the two laminates with standard modulus are quite close, whereas the properties of the laminates comprising recycled intermediate modulus fibers are significantly higher.
Using measured DiFTS mechanical properties, a truck-bed case study was undertaken using a laminate model with quasi-isotropic layup of the sheets. The results demonstrated that, in comparison with 16 Ga. high strength sheet steel, PA6 DiFTS with 30 wt % recycled carbon fiber gives weight savings of 56% and an increase in bending strength of 30% at an equivalent bending stiffness.
Another important feature of DiFTS technology is that the sheet format permits facile hybridization with continuous fiber sheets and tapes, and/or different fiber types, for tailoring cost/performance to meet the specifications of a given application. In addition, the laminated sheets can be combined with lightweight core materials.
The DiFTS process can be scaled-up to produce sheets several feet wide, and a laminator can be used to seamlessly join sheets (laterally) and to laminate any number of sheets (vertically), as demonstrated at the PolyOne Advanced Composites facility.
Successful industrial-scale molding trials of laminated DiFTS have been carried out by the CNAM Center and various partners using a range of processes, including thermoforming, vacuum thermoforming and compression molding. Cycle time is typically one or two minutes. Importantly, the sheets do not depend on the molding process to complete fiber wet-out.
To compete effectively with metals, where the vast majority of materials are recycled, it is crucial that the composites industry implements commercially attractive recycling strategies from reclaimed fiber to new composite end products.