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Recycling of Epoxy Thermoset and Composites via Good Solvent Assisted and Small Molecules Participated Exchange Reactions Xiao Kuang, Yunying Zhou, Qian Shi, Tiejun Wang, and H. Jerry Qi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01538 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018
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ACS Sustainable Chemistry & Engineering
Recycling of Epoxy Thermoset and Composites via Good Solvent Assisted and Small Molecules Participated Exchange Reactions Xiao Kuanga,b, Yunying Zhoua,c, Qian Shia,d, Tiejun Wangd, H. Jerry Qia,b* a. The George W. Woodruff School of Mechanical Engineering b. Renewable Bioproduct Institute Georgia Institute of Technology, Atlanta, GA 30332, USA c. Department of Architectural Engineering North China Institute of Aerospace Engineering, Langfang 065000, China d. State Key Lab for Strength and Vibration of Mechanical Structures School of Aerospace Engineering, Xian Jiaotong University, Xian 710049, China
* Corresponding author. E-mail:
[email protected] (H.J.Q.).
KEYWORDS: Epoxy thermosets, carbon fiber reinforced composites, decomposition, recycling, transesterification 1
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ABSTRACT Thermosetting polymers and composites are a class of high-performance materials with significant industrial applications. However, recycling of thermosets and their filling matters are significantly challenging. Here, we propose a method to recycle epoxy thermosetting polymer and composites efficiently by a synergistic effect of a solvent mixture using a highly efficient organic catalyst at an ordinary pressure and mild temperatures. The anhydride-epoxy network depolymerization enabled by selective ester bond cleavage process is substantially enhanced by a good solvent assisted and alcohol participated transesterification reaction. The epoxy thermoset can be dissolved in 28 min with 50 % mass loss, and 70 min with 95 % mass loss at 170 oC under ambient pressure. We demonstrate that this method can be used to reclaim carbon fibers from industrial reinforced epoxy composite products and embedded metal parts from commercial electronic products with undiminished properties at a mild temperature (~170oC) under an ordinary pressure in a short time (1.5 hours). Moreover, the decomposed epoxy oligomer can be reused as a reactive ingredient for the preparation of new epoxy materials with high strength and modulus. This work provides a new insight into the thermosets dissolution and recycling.
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INTRODUCTION Since their first manufacturing and application over 100 years ago, thermosetting polymers enjoy tremendous industrial applications, such as high-performance composites and electronics packaging. In recent decades, carbon fiber reinforced polymer (CFRP) composites, with a combination of an excellent specific strength, size stability, and durability, have experienced explosive growth in a large array of applications ranging from aerospace, wind energy to ground transportation1-2. The wide industrial adoption of thermosetting polymers also produces a large amount of waste during manufacturing and end-of-service-life because these materials are known as nonrecyclable materials. With the prevailing awareness of environmental protection and energy-saving, the recycling and reuse of the high-value filling matters in thermosetting polymer composites, such as carbon fibers (CFs) and precious metals, become an intensively researched area in both academy and industry3-8. Currently, recovery of industrial thermosetting polymers and their filling matters still present significant challenges. On one hand, the highly cross-linked thermosetting polymer matrix possesses superior thermal and chemical stabilities, which usually require strong chemicals or high temperature for dissolution or decomposition. On the other hand, the embedded CFs or precious metals are prone to be harmed by high temperature and strong chemicals. Several recycling methods were investigated, including mechanical9-10, thermal (or pyrolysis)11-12, and chemical approaches13-20. The chemical recycling approach is among the most promising ones since the filling matters, such as continuous long CFs together with valuable building blocks of the epoxy resin can be recycled in the same process. However, conventional methods of chemically recycling high-performance epoxy thermosets may involve strong chemicals or high pressure and relatively high temperature (>200oC) for a long time21-22. These issues limited the large-scale applications of industrial thermosets and composites recycling.
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The recent development of thermosetting resins bearing dynamic exchange reactions (ERs), such as Diels-Alder reaction23-26, thiol-disulfide exchange reaction27-29, Schiff-base reversible reaction3 and transesterification reactions 30-31, or degradable bonds32-36 were shown to be very versatile for thermoset and composite recycling under relatively mild conditions. However, most of these resins need complex synthesis procedures or have compromised overall properties at high temperature; in addition and more importantly, the industrial adoption of these new resins would need significant efforts due to the requirements for certifications and thus their immediate impacts to recycling and environmental protection are limited. To-date, to our best knowledge, none of the current recycling approaches can enable reclamation of industrial epoxy composites and filling matters with undiminished properties at a relatively mild temperature (20 wt%) would make it difficult to obtain sufficient mixing, which led to the formation of defects in the materials and hence will harm to the overall properties. The recycled oligomer with multi-hydroxyl groups can be used as a reactive ingredient for other applications. For example, the oligomer as multifunctional alcohol can react with a diisocyanate to fabricate polyurethane51. The current dissolution method based on the selective cleavage of ester bond is applicable to anhydride cured epoxy and polyester resins based thermosets and composites. After directly separating the filling matters (CFs or metals), the depolymerized oligomer as an oil phase can be separated by precipitating the dissolution solution in water. Moreover, the organic solvents of NMP and alcohol in water phase can be potentially reclaimed by fractional distillation. However, the current organic catalyst cannot be reclaimed for reuse because the catalyst would be gradually oxidized under elevated temperatures for an extended time. Comparing with the solid form recycling, this solvent based chemical recycling is efficient to reclaim the filling matter, such as CF, with undiminished properties as well as the decomposed oligomers. Therefore, this approach is relatively environmental-friendly without involving highly evaporative, oxidative and corrosive chemicals. A glass container or steel reactor of large size can be used for larger scale of epoxy dissolution without any damages to the container. The main issue for the up-scaling application of this strategy is the higher cost of the organic catalyst, which is more expensive than the inorganic-based or salt catalyst. In the future, using a more efficient, stable and low-cost catalyst would help to enhance the dissolution rate, which can be also recycled for reuse.
CONCLUSION
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This work has tackled the challenges of depolymerizing a highly cross-linked industrial epoxy and composites under mild conditions, which involve mild temperature treatment (
