There are three main components to economically viable fiber recycling: low cost recovery of the fibers, aggregating the fibers in one or more formats that can be used in composites processing and designing composites processes that can utilize the recycled fibers in the manufacture of commercially valuable end products.
In all these stages, it is of key importance that the fibers incur minimal damage and attrition of mechanical properties. Various fiber recycling technologies have been developed and adopted by several companies over the last few years, including re-sizing and chopping scrapped virgin tow (Barnet, ELG); reclaiming fiber from uncured thermoset prepregs by pyrolysis (Carbon Conversions, ELG) or by solvolysis (Vartega, R&M International, Shocker Composites); and recovery of fibers from cured end-of-life composite parts by pyrolytic methods (Carbon Conversions, ELG, CHZ Technologies) or wet-chemistry approaches (Adherent Technologies).
Immediately after the recovery process, the fibers will usually be in the form of fluffy, entangled bundles, which cannot be directly fed to a composite manufacturing process. Typically, the fibers are chopped (to lengths ranging from 4 mm to 12 mm) and either processed into fiber webs, which can be infused or impregnated with a resin, or aggregated into pellet-like bundles in which the fibers are held together by a few weight percent of binder and/or sizing.
The “pellets” can then be fed by hopper to an injection molding machine or other thermoplastic compounding process. In practice, the formation of a pellet that may be fed consistently at commercial throughput rates can be challenging, if fiber damage and the addition of excessive sizing is to be avoided, but technical developments over the past two to three years have largely resolved this issue.
An important route to lowering the costs of composites – and increasing their competitiveness against metals – involves the utilization of recycled fibers in thermoplastic matrices. Whereas injection molding is a valuable option, limitations of part size and control of properties mean that there is a range of high-value applications for which this technology is not suited.
Fiber-reinforced additive manufacturing has great potential, but is currently a low throughput technology, not yet well-adapted to high-volume commercial manufacturing.
An attractive alternative is the incorporation of recycled fibers in thermoplastic sheets, which can then be laminated to different thicknesses and formed into final parts by short-cycle molding operations, similar to metal sheet stamping, widely used to produce automotive parts at high volumes.
However, methods which attempt to impregnate molten thermoplastic polymers in nonwoven fiber mats generally result in poor fiber wet out, which cannot be remedied in short cycle-time molding processes, and sheet casting processes using traditional extrusion methods lead to considerable attrition of fiber length (with fibers typically below 1mm) and/or non-uniform fiber dispersion.