Carbon fiber reinforced plastics (CFRP) are composite materials that consist of carbon fibers embedded in a polymer matrix, a combination that yields materials with properties exceeding the individual properties of each component. CFRP have several advantages over metals: they offer superior strength to weight ratios and superior resistance to corrosion and chemical attack. These advantages, along with continuing improvement in manufacturing processes, have resulted in rapid growth in the number of CFRP products and applications especially in the aerospace/aviation, wind energy, automotive, and sporting goods industries. Due to theses well-documented benefits and advancements in manufacturing capabilities, CFRP will continue to replace traditional materials of construction throughout several industries. However, some of the same properties that make CFRP outstanding materials also pose a major problem once these materials reach the end of service life. They become difficult to recycle. With composite consumption in North America growing by almost 5 times the rate of the US GDP in 2012, this lack of recyclability is a growing concern. As consumption increases, more waste will inevitably be generated. Current composite recycling technologies include mechanical recycling, thermal processing, and chemical processing. The major challenge of CFRP recycling is the ability to recover materials of high-value and preserve their properties. To this end, the most suitable technology is chemical processing, where the polymer matrix can be broken down and removed from the fiber, with limited damage to the fibers. This can be achieved using high concentration acids, but such a process is undesirable due to the toxicity of such materials. A viable alternative to acid is water in the sub-critical and supercritical region. Under these conditions, the behavior of this abundant and most environmentally friendly solvent resembles that of an organic compound, facilitating the breakdown of the polymer matrix. To date, very few studies have been reported in this area and the studies thus far have only focused on small scale feasibility and have only shown the recovery of random fibers. The goal of this research is to advance the knowledge in the field of sub-critical and supercritical fluid recycling by providing fundamental information that will be necessary to move this process forward to an industrial scale. This dissertation work consists of several phases of studies. In the first phase of this research, the feasibility of recycling woven CFRP was established on a scale approximately 30 times larger than previously reported. The industrial relevance was also conveyed, as the process was shown to remove up 99% of a highly cross-linked resin from an aerospace grade composite system with 100% retention of the single filament tensile strength and modulus whilst also retaining the highly valuable woven fiber structure. The second phase of research demonstrated the power of this technology to recycle multi-layer composites and provide the ability to reuse the highly valuable materials. Up to 99% resin elimination was achieved for a woven 12-layer aerospace grade composite. The recycled woven fabric layers, with excellent retention of the fiber architecture, were directly reused to fabricate reclaimed fiber composites (RFC). Manufacturing issues associated with the use of the recycled fiber were investigated. Several fabrication technologies were used to fabricate the composite, and the composites show moderate short beam shear strength and may be suitable for certain industrial applications. Moreover, fresh composites were also recycled, recovered, and reused to investigate the retention of flexural properties of the fibers after recycling. Up to 95% of the flexural strength and 98% of the flexural modulus was retained in the reclaimed fiber composites. The recycled resin residual can be incorporated into fresh resin and cured, demonstrating a near complete recycling loop. After showing the feasibility and power of this technology, the third phase of the study was focused on the fundamentals on the degradation of highly cross-linked polymer network by sub- and near-critical water. A methodology framework was established to study the apparent kinetics of the degradation of epoxy in sub-critical water. The reaction rate was modeled by a phenomenological rate model of nth order, and the rate constant was modeled by taking into account of the contributions of important physical parameters, e.g., pressure, temperature and dielectric constants. The applicability of the established model to describe the degradation kinetics was confirmed by the validation runs. This model is a suitable starting point to gain the knowledge required for eventual industrial process design. The final phase of this research consisted of a preliminary foray into investigating the economic feasibility of this technology. A process model was designed around a reactor which was sized according to considerations of industrial relevancy. The simulation of the process was done using Aspen Plus, powerful and comprehensive process simulation software. Economic analysis of this pseudo-realistic process suggested that such technology was economically viable and competitive comparing to other recycling technologies. In summary, this dissertation work represents the first comprehensive investigation on recycling aerospace-grade, multilayer woven fabric composites using supercritical and sub-critical water. The fundamental knowledge gained and process technology developed during this research is anticipated to play an important role in advancing this recycling technology toward potential adoption and implementation by the recycling and composite industry. / A Dissertation submitted to the Department of Industrial Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester, 2013. / June 28, 2013. / Carbon, Composites, Fiber, Recycling, Supercritical, Woven / Includes bibliographical references. / Changchun Zeng, Professor Directing Dissertation; Chuck Zhang, Professor Co-Directing Dissertation; Ravindran Chella, University Representative; Richard Liang, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_183778 |
| Contributors | Knight, Chase C. (authoraut), Zeng, Changchun (professor directing dissertation), Zhang, Chuck (professor co-directing dissertation), Chella, Ravindran (university representative), Liang, Richard (committee member), Department of Industrial and Manufacturing Engineering (degree granting department), Florida State University (degree granting institution) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource, computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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