Canadian researchers employ recycled carbon-fiber mats to produce thermoset composites

  • 25-Oct-2013 01:07 EDT
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SEM micrographs of the recycled carbon fibers from a) prepregs, b) tooling, show that the fibers were generally clean and appeared relatively undamaged having no obvious cracks.

In the past decade, the use of carbon-fiber-reinforced plastics (CFRPs) has increased significantly in aerospace with the development of the Boeing 787 Dreamliner and the Airbus A380 programs, containing approximately 50 wt % and 20 wt % of CFRPs, respectively. This increase, however, has generated a significant amount of CFRP waste estimated at 3000 ton per year in North America and Europe. Currently, the vast majority of composite waste in North America is sent to landfills; therefore, aircraft manufacturers are under pressure to reduce waste through recycling or reuse, affecting both in-process waste and end-of-life disposal.

Recycling composite waste has been the subject of much investigation over the past 15 years. Yet, carbon-fiber-reinforced thermoset composites continue to be difficult to recycle, as they are a complex mixture of different materials such as thermosetting polymers, carbon fibers, and fillers; they may also contain foam cores, metal inserts, wire meshing, paints, and other contaminants. Furthermore, the cross-linked nature of thermosetting polymers prevents remolding.

Several recycling technologies have been proposed and developed for CFRPs. They can be classified in two broad categories: mechanical grinding and fiber reclamation. Currently, fiber reclamation has gained momentum because of its ability to recover high-quality carbon fibers by employing either a chemical or thermal process to break down thermosetting resin. Fiber reclamation is particularly suitable for CFRPs due to the high chemical and thermal stability of carbon fibers. The most common fiber-reclamation process is pyrolysis. It is defined as the thermal degradation (400-700°C) of organic material (i.e., polymeric matrix) in an inert environment (usually CO, CO2, or N2). Other fiber-reclamation technologies include catalytic conversion, fluidized bed reactor, and supercritical fluids.

In addition to the legislative “push” factor, another driving force for developing fiber-reclamation technologies is the savings in terms of energy for production. Manufacturing virgin carbon fibers requires a tremendous amount of energy estimated at 55 to 165 kW·h/kg. Recovering carbon fibers from CFRPs, on the other hand, requires only around 3 to 10 kW·h/kg. Therefore, economic viability of recycling CFRPs can be achieved if applications can be found for recycled carbon fibers.

To this end, researchers from the Universite Du Quebec at Montreal, Bell Helicopter Textron Canada, and the National Research Council Canada investigated the feasibility of using carded recycled carbon-fiber mats and epoxy resin to fabricate thermoset composite plates by infusion/compression molding.

Carbon fibers were recovered from a 100-kg sample of carbon-fiber-reinforced thermoset composite waste donated by Bell Helicopter Textron Canada. It contained carbon-fiber prepregs (cured and uncured) based on epoxy and bismaleimide resins, and tooling parts made from carbon-fiber-reinforced epoxy with trace amounts of silicone.

Pyrolysis was performed at Materials Innovation Technologies in Lake City, SC, using its commercial-scale batch pyrolysis oven. Before pyrolysis, the waste was sorted and cut into 2.5- x 2.5-cm fragments to limit the fiber length distribution, thereby controlling the overall uniformity of the recycled fibers. Pyrolysis was performed at low temperatures (<400°C) under a controlled atmosphere, and the residence time was adjusted as a function of the material to be pyrolyzed. Recycled carbon fibers were obtained from prepregs and tooling.

Toray T700SC (sized, tensile modulus: 230 GPa, tensile strength: 4900 MPa) virgin carbon fibers were chosen as a reference material, and cut into lengths of 2.5 cm. PRISM EP2400, produced by Cytec Engineered Materials, is an infusion-grade epoxy used to fabricate the carbon fibers/epoxy plates.

Scanning electron microscopy, density measurements, mono-filament tensile testing, and micro-droplet testing determined that the recycled carbon fibers retain most of their mechanical and surface adhesion properties.

Prepreg fibers exhibited surface striations designed to improve their adhesion with epoxy, and residual resin was often observed, probably due to a lack of optimization of the pyrolysis process (the process was optimized for epoxy composites, whereas the prepreg waste contained both epoxy and bismaleimide resins). The tooling fibers showed some loose debris and no surface striation. The average density of the prepreg fibers was determined by pycnometry to be 1.75 g/cm³, close to the value of the Toray fibers (1.80 g/cm³).

The recycled fibers from prepregs and tooling had a tensile modulus of 160 ± 20 GPa and 170 ± 10 GPa, and a maximum tensile strength of 3400 ± 400 MPa and 3800 ± 200 MPa, respectively. When compared to the virgin fibers, average differences of around -10% to -5% in tensile modulus and -22% to -13% in maximum tensile strength were observed. Tooling fibers offered a better mechanical performance than prepreg fibers, although prepregs contain high-performance aerospace grades of carbon fibers, while tooling composites generally contain industrial grades of carbon fibers. This result may be due to process variations that can occur during recycling. Therefore, further optimization of the pyrolysis conditions (temperature, residence time, and atmosphere), with respect to the type of composite waste being treated, should contribute to further improving the properties of the fibers.

The feasibility of fabricating recycled carbon-fiber-reinforced thermoset composites was also demonstrated by the researchers. A carding process produced homogeneous recycled fiber mats that were subsequently manufactured into composite plates by infusion/compression molding with epoxy. Fiber volume fractions up to 40% were obtained.

Tensile and flexural testing determined that plates containing recycled fibers exhibited better mechanical properties than pure epoxy, and similar properties to a plate containing virgin fibers, despite the presence of porosities. This demonstrates the potential of reintegrating recycled carbon fibers as reinforcement in new thermoset composites for a variety of applications.

This article is based on SAE International technical paper 2013-01-2208 written by Stefan Andjelic and Steen Brian Schougaard of Universite Du Quebec at Montreal, Judith Roberge of Bell Helicopter Textron Canada, and Nathalie Legros and Lolei Khoun of the National Research Council Canada.

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