Recycling BenzoxazineâEpoxy Composites via Catalytic Oxidation

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Recycling Benzoxazine-Epoxy Composites via Catalytic Oxidation Jonathan N. Lo, Steven Nutt, and Travis J. Williams ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01790 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Recycling Benzoxazine-Epoxy Composites via Catalytic Oxidation Jonathan N. Lo,1 Steven R. Nutt,*,1 and Travis J. Williams*,2 1

Mork Family Department of Chemical Engineering and Material Science, University of Southern California, 925 Bloom Walk, Los Angeles, 90089-1211, United States 2 Donald P. and Katherine B. Loker Hydrocarbon Institute and Department of Chemistry, University of Southern California, 837 Bloom Walk, Los Angeles, California, 90089-1661, United States. * [email protected], [email protected]

ABSTRACT: Carbon fiber reinforced polymers (CFRPs) are structural composites used in the aerospace and sporting goods industries. Their chief appeal lies in their high specific properties, which generally outperform metallic counterparts. Ther e is a contemporary need for viable methods for recycling CRFPs at the end of their lifecycles and for utilizing the considerable production waste (c.a. 30%) of CFRP part manufacturing. The cost associated with these waste streams is a principal economic driver inhi biting the penetration of CRFPs into larger-scale manufacturing, particularly in the automotive industry. Reported techniques for CRFP degradation involve pyrolysis or mechanical grinding of the CFRP, processes which are outlawed in some jurisdictions and can reduce the thermomechanical properties of the recycled products. In this study, we report a conceptually different approach to degrading a commercial blended benzoxazine/epoxy resin under mild, oxidative conditions. The thermosetting resin is polymerized, characterized, and then catalytically de-polymerized via hydride abstraction with a ruthenium catalyst. These results demonstrate a concept for sustainable recycling of CFRP composites. KEYWORDS: Benzoxazine, catalyst, recycling, composites INTRODUCTION CFRP composites are used increasingly in industry due to the characteristic high strength-to-weight ratio. 1 , 2 Plainweave CFRP composites can exhibit specific stiffness and strength > 300% greater than aluminum or steel. They have fatigue resistance, toughness, and thermal expansion properties typically superior to metallic alloys. 3 Due to these properties, composites are widely used in aerospace, recreational equipment, and wind energy industries, and are emerging in automotive manufacturing. Despite this widespread use, strategies for recycling of composites at the end of their life remain inefficient.4,5 This creates a serious sustainability problem that impedes wider-scale adoption and must be addressed. Current CRFP recycling strategies involve thermal or mechanical means to remove the polymer matrix and recover the fibers. 6,7 While these strategies work to some extent, they damage the fibers and can present pollution emission problems. A combination of high temperature and low temperature solvolytic approaches involving chemical dissolution of the matrix have been reported, but these techniques are also problematic. 8 , 9 , 10 , 11 Low-temperature methods involve concentrated corrosives such as nitric acid near or above boiling, which pose health and safety risks, particularly when performed at industrial scales. 12 , 13 , 14 Hydrogen peroxide is also effective with some CRFP matrices, 15, 16 but this introduces safety and sustainability issues; moreover, benzoxazines do not degrade under these conditions in our hands. Additionally, the resin systems

suitable for low-temperature recycling have low service temperature, exhibit high moisture uptake and high cure shrinkage, ultimately limiting their broader industrial or commercial applicability. Recent effort has produced resin systems that will depolymerize on demand, although these systems typically exhibit inferior thermomechanical properties.17 Additionally, a “degrade on demand” strategy does not address the mass of CRFPs already in use. Our groups have recently begun a project in which we use catalytic oxidation to depolymerize CRFP matrices in ways that retain the value of both the fibers and the matric monomers.18 We report here a new recycling strategy that is applicable to benzoxazines, a class of optimized CFRP composites that is presently in use. The method demonstrates two unexpected observations - that (1) it is possible to overcome the inertness of benzoxazine resins, which are among the most thermally stable and chemically resistant CRFP matrices known, and (2) it is possible for a homogeneous catalyst to effect bond cleavage within the CRFP superstructure without a pre-treatment or forcing condition. RESULTS AND DISCUSSION Chemical structures for the commercial benzoxazine resin system used in this study are shown Figure 1. This system is used for aerospace and automotive purposes. 19 , 20 , 21 The polymerization sequence (the resin curing process) proceeds via intermolecular Friedel-Craft alkylation by a thermally-generated iminium intermediate (figure 1). 21, 22 The role of the epoxy is to increase the crosslink density and

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the thermomechanical properties of the cured polymer, as benzoxazine/epoxy copolymers have greater Tg, strength, and ductility than the benzoxazine homopolymer. 23 In our tests, CRFP composites of this system exhibit excellent thermomechanical properties.

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The recovered fibers retain long-range order. Note that while the fibers have been freed from the polymer matrix and are now unsupported, they remain arranged as fabric. This indicates that although the fibers are released from their support, long-range order of the individual bundles, or fiber tows, is preserved. The organization of the recovered fibers will ultimately facilitate reuse, as it obviates any subsequent post-processing to reorganize and align them. 4

Figure 1. Chemical structures of the monomers and the crosslinked product.

The di-N-benzylaniline linkage of the polymerized matrix maps well on to N-phenyltetrahydroiosquiniline system popularized about 20 years ago as substrates for catalytic C—H oxidation via mild, oxidative iminium formation, thus making it an attractive target for cleavage under analogous conditions.24 To test this strategy, we fashioned a screen of oxidants and catalysts (Table 1) in which we measured the mass of polymer dissolved from solid (fiber-free) benzoxazine polymer grounds. Among oxidants, hydrogen peroxide was selected, because it is found in some known CFRP digestion schemes; oxone was selected as a peroxide source. Ceric ammonium nitrate (CAN)25,26 and periodate27 were chosen because they are known to be effective terminal oxidants for catalytic water splitting where they are used as surrogates 28 for electrolysis. Somewhat surprisingly, the benzoxazine matrix is stable to most of these conditions, which attests to the chemical inertness of benzoxazine composites, but CAN was able to effect dissolution of the polymer matrix in the presence of RuCl3. Table 1. Homogenization Conditions for Benzoxazine Resin Grounds Entry 1 2 3 4

Catalyst RuCl3 CuOAc CuI KRuO4

H2 O2 0% 0% 0% 0%

Oxone 0% 0% 0% 0%

CAN 98% ~5% ~5% ~5%

Figure 2. Composite panel pre- and post-depolymerization.

The fibers are not damaged by the catalytic process. Figure 3 shows images of recycled carbon fibers. After matrix depolymerization, these are washed by soxhlet extraction to remove any organic residue. Images of recovered fibers match virgin fibers from which the benzoxazine panels were manufactured (see supporting information). The long-range order of the recovered fibers is evident in Figure 3, left. The images in Figure 3 show that the recycling process causes no apparent damage to the recovered carbon fibers, despite the high oxidative potential of CAN (+1.6 V v. NHE) and the acidity of the solvent (pKa = 0). Preserving fibers undamaged will result in superior thermomechanical properties of recycled materials relative to those made of fibers recovered via pyrolysis or highpressure/temperature solvolysis: fibers treated with these processes are often damaged during recovery, causing surface pits that weaken the resulting composite products.8,29

NaIO4 0% 0% 0% 0%

Conditions: (80°C, 1.3 M [oxidant], 0.05 mol% catalyst in 50% AcOH/H2O, ambient atmosphere). Matrix dissolution is quantified by weighing neat resin blocks before and after treatment. See Supporting Information. Oxone = KOSO4H. CAN = (NH4)2Ce(NO3)6.

With effective conditions for homogenization of neat polymer grounds, our next step was to degrade resin cleanly away from carbon fiber in CFRP parts. This was demonstrated by treating a sample measuring ca. 30 × 15 mm excised from a larger composite part (Figure 2, left) with our conditions from table 1, entry 1. We found that degradation of solvent (acetic acid) by CAN limited the efficiency of oxidation; switching to triflouroacetic acid removed this limitation and enabled complete homogenization of the polymer matrix without damage to the reinforcing fibers (Figure 2, right).

Figure 3. Recycled carbon fibers at (left) 1,000x magnification (right) 10,000x magnification.

A most curious aspect of the overall oxidation is the mechanism by which the homogeneous reactants move through the heterogeneous composite material. Prior work in composites recycling has shown that high temperature and pressure are needed to soften the composite matrix and reduce the Tg. Our system is mechanistically different: the reactive materials move through the multi-layer composite panel one ply at a time. We view our reactive conditions as a homogeneous solution of acid (TFA), oxidant (CeIV), and catalyst (Ru), that degrades a solid, laminar material. Among these, the cerium is easily detected by energy-dispersive X-ray spectroscopy (EDS). Thus, we designed an experiment in which we degraded a panel to partial


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thickness, then cut a slice through it and examined the cross section by electron microscopy and EDS. Figure 4 (left) shows the cross section, with the partially degraded layer on the bottom of the image. The same figure (right) shows [Ce] by EDS recorded along a red trace in the image. One sees lowest [Ce] in the interior ply of the laminate and high [Ce] immediately at the boundary of the interior ply and the degrading ply, with the greatest [Ce] at the interface between the intact composite and the region where the matrix has been digested. At this interface, individual fibers have started pulling away from the bulk as the matrix is being digested. The cerium concentration decreases away from the interface where there is less matrix to digest. Figure 4 also shows the selectivity for benzoxazine/epoxy matrix degradation over fiber damage. This cross section of a partially digested composite panel shows, in the bottom half, individual carbon fibers that separated from the surrounding matrix as it depolymerized and dissolved. The top half of the image shows a partial ply of carbon fibers that remain bound in the matrix. Because the matrix is more reactive than the carbon fibers, it reacts preferentially. In contrast, recycling by pyrolysis attacks both the matrix and the fibers. 0

Distance (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 100 150 200 250 300 350 0

20

40

60

80

100

[Ce] (K-ratio)

Figure 4. Energy dispersive X-ray spectroscopy (EDS) on a partially digested fiber. Left: Micrograph. Each hatch is 50 μm. Right: corresponding [Ce] as a function of depth.

Figure 5 shows a proposed mechanism of polymer cleavage that is based on literature. 26–30 We posit that the depolymerization occurs via hydride abstraction from captodative methylene groups in the polymer’s di-Nbenzylaniline linkages, whereby the cerium oxidizes the complex to a ruthenium oxo species represented diagrammatically as a Ru=O structure. This species pulls a hydride from the polymer linkage to give a reduced metal species and an iminium cation. The ruthenium is reoxidized by cerium. Polymer cleavage occurs by hydrolysis of the intermediate iminium ion. By depriving the reaction of cerium, we show that turnover stops in its absence. By re-charging the reactor with additional CAN, we see that the reaction restarts. We therefore know that the ruthenium catalyst could be re-used if the oxidant were regenerated. Otherwise, it can be collected in aqueous solution at the end of the reaction by extracting away organics. If the mechanism proposed in Figure 5 is correct, then a product of the depolymerization reaction should be a bisphenol-F tri- or tetra-carboxylate, with each of the carboxylate groups deriving from a methylene group that previously served as a polymeric linkage; this is sketched as 3. The degradation should also release aniline (or nitrobenzene) from the cleavage of the linking aniline groups.

Figure 5. A potential mechanism for the depolymerization of a benzoxazine/epoxy resin via hydride abstraction and its predicted oxidative polymer degradation products.

We observe aniline, nitrobenzene, and tetracarboxylate 3 at early stages in our depolymerization process. We ran our degradation to the point of polymer homogenization, then stopped it and decanted off the resulting solution. Solvents were distilled form this solution for potential re-use, and aniline and nitrobenzene were found in this volatile fraction. The remaining non-volatile syrup was stirred in acidic methanol, and organics were extracted (see Supporting Information). 1H NMR data on the recovered organics are shown in Figure 6. We find 4, the tetramethyl ester of 3 and a collection of peaks appropriate for degradation products of epoxide 2, each marked according to their assignments. 3 is not stable to our degradation conditions and cannot be isolated when the solutions exposed to the conditions for a prolonged period. This is expected, due to the high oxidation potential of cerium(IV). Because conditions are known for decarboxylation of salicylates such as 3 and the conversion of the decarboxylated bis(phenol) back to 1, this discovery shows that our technology will be a viable approach to recovery of both the reinforcing fibers and the matrix itself.30,31

Figure 6. NMR data of the extract showing tetraester 4 and degradation products of epoxide 2.

CONCLUSIONS In sum, this work reports three key findings. Composite material recycling can be achieved under conditions of homogeneous redox catalysis. This result alone is significant, as it demonstrates (1) the intercalation of the homogeneous reaction milieu into the composite material in a layered fashion that results in complete homogenization of the polymer matrix in a short time, and (2) the conditions required of the catalysis are strikingly mild, compared to those of current techniques for recycling composites. Lastly, (3) despite the incorporation of a high-potential oxidant in this first proof-of concept



demonstration, the recovered carbon fibers emerge undamaged and ready for reuse. We further show that even the solvents and resin monomers can be recovered in principle, and ultimately these can be reused, which is a unique feature brought to the field by the use of catalysis under mild conditions.

AUTHOR INFORMATION

These results demonstrate that oxidative recycling of benzoxazine-based composite materials is potentially a viable concept, although the chemistry of benzoxazine/epoxy blends (such as the one studied here) is different from that of other composites, such as amine-linked epoxies, which require different catalytic conditions for efficient degradation in our hands. A key drawback of this demonstration technology is the large quantity of cerium required to drive this reaction to completion. We chose cerium as a demonstration reagent, because it can sometimes be a surrogate for electrolysis. Once made electrolytic, this technology defines the basis for a process potentially suitable for recycling industrial-scale quantities. In total, the results provide a proof of concept, demonstrating that the recycling of some epoxy formulations is possible under very mild conditions through a homogeneous catalysis approach.

Corresponding Authors

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/**. Experimental procedures, electron micrographs, and graphical spectral data (PDF)

(1) Campbell Jr, F. C. Manufacturing technology for aerospace structural materials; Elsevier, 2011. (2) Mallick, P. K. Fiber-Reinforced Composites: Materials, Manufacturing, and Design, Third Edition, 3rd ed.; Mechanical Engineering; CRC Press: Boca Raton, 2007. (3) Ashby, M.; Shercliff, H.; Cebon, D. Materials: Engineering, Science, Processing and Design, 2nd ed.; Elsevier: Oxford, 2010. (4) Pimenta, S.; Pinho, S. T. Recycling Carbon Fibre Reinforced Polymers for Structural Applications: Technology Review and Market Outlook. Waste Manag. 2011, 31, 378–392, DOI 10.1016/j.wasman.2010.09.019 (5) Asmatulu, E.; Twomey, J.; Overcash, M. Recycling of FiberReinforced Composites and Direct Structural Composite Recycling Concept. J. Compos. Mater. 2014, 48, 593–608, DOI 10.1177/0021998313476325. (6) Oliveux, G.; Dandy, L. O.; Leeke, G. A. Current Status of Recycling of Fibre Reinforced Polymers: Review of Technologies, Reuse and Resulting Properties. Prog. Mater. Sci. 2015, 72, 61–99, DOI 10.1016/j.pmatsci.2015.01.004. (7) Nahil, M. A.; Williams, P. T. Recycling of Carbon Fibre Reinforced Polymeric Waste for the Production of Activated Carbon Fibres. J. Anal. Appl. Pyrolysis 2011, 91, 67–75, DOI 10.1016/j.jaap.2011.01.005. (8) Jiang, G.; Pickering, S. J.; Lester, E. H.; Turner, T. A.; Wong, K. H.; Warrior, N. A. Characterisation of Carbon Fibres Recycled from Carbon Fibre/Epoxy Resin Composites Using Supercritical nPropanol. Compos. Sci. Technol. 2009, 69, 192–198, DOI 10.1016/j.compscitech.2008.10.007. (9) Morin, C.; Loppinet-Serani, A.; Cansell, F.; Aymonier, C. Near- and Supercritical Solvolysis of Carbon Fibre Reinforced Polymers (CFRPs) for Recycling Carbon Fibres as a Valuable Resource: State of the Art. J. Supercrit. Fluids 2012, 66, 232–240, DOI 10.1016/j.supflu.2012.02.001. (10) Prinçaud, M.; Aymonier, C.; Loppinet-Serani, A.; Perry, N.; Sonnemann, G. Environmental Feasibility of the Recycling of Carbon Fibers from CFRPs by Solvolysis Using Supercritical Water. ACS Sustain. Chem. Eng. 2014, 2, 1498–1502, DOI 10.1021/sc500174m. (11) Shibata, K. JEC Compos. Mag. 2011, 48 (66), 50–52. (12) Dang, W.; Kubouchi, M.; Yamamoto, S.; Sembokuya, H.; Tsuda, K. An Approach to Chemical Recycling of Epoxy Resin Cured with Amine

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Corresponding Authors *Steven R. Nutt, E-mail: [email protected] *Travis J. Williams, E-mail: [email protected]

Jonathan Lo 0000-0002-8369-8170 Steven R. Nutt 0000-0001-9877-1978 Travis J. Williams 0000-0001-6299-3747

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is sponsored by the M.C. Gill Composites Center at USC, the NSF (CHE-1566167), and the Hydrocarbon Research Foundation. We thank the NSF (DBI- 0821671, CHE0840366) and the NIH (S10 RR25432) for analytical instrumentation. We thank Matthew Thomas, for resin characterization experiments; Erynn Naccarelli, who assisted with the preparation and collection of data; and Mark Anders, who manufactured some of the composite resin panels.

REFERENCES

using Nitric Acid. Polymer 2002, 43, 2953–2958, DOI 10.1016/S00323861(02)00100-3. (13) Dang, W.; Kubouchi, M.; Sembokuya, H.; Tsuda, K. Chemical Recycling of Glass Fiber Reinforced Epoxy Resin Cured with Amine using Nitric Acid. Polymer 2005, 46, 1905–1912, DOI 10.1016/j.polymer.2004.12.035. (14) Pastine, S. Can Epoxy Composites be 100% Recyclable? Reinf. Plast. 2012, 56, 26–28. (15) Li, J.; Xu, P.-L.; Zhu, Y.-K.; Ding, J.-P.; Xuea, L.-X.; Wang, Y.-Z. A Promising Strategy for Chemical Recycling of Carbon Fiber/Thermoset Composites: Self-Accelerating Decomposition in a Mild Oxidative System. Green Chem. 2012, 14, 3260-3263, DOI 10.1039/C2GC36294E. (16) Liu, Y.; Liu, J.; Jiang, Z.; Tang, T. Chemical Recycling of Carbon Fibre Reinforced Epoxy Resin Composites in Subcritical Water: Synergistic Effect of Phenol and KOH on the Decomposition Efficiency. Polymer Degradation and Stability 2012, 97, 214-220, DOI 10.1016/j.polymdegradstab.2011.12.028. (17) La Rosa, A. D.; Banatao, D. R.; Pastine, S. J.; Latteri, A.; Cicala, G. Recycling Treatment of Carbon Fibre/Epoxy Composites: Materials Recovery and Characterization and Environmental Impacts through Life Cycle Assessment. Compos. Part B Eng. 2016, 104, 17–25, DOI 10.1016/j.compositesb.2016.08.015. (18) Navarro, C.; Kedzie, E. A.; Ma, Y.; Michael, K.; Nutt, S. R.; Williams, T. J. Catalytic, Oxidative Epoxy Depolymerization in Fiber-Reinforced Composites. Top. Catal. 2018, ASAP, DOI 10.1007/s1124-018-0917-2 (19) Dogan Demir, K.; Kiskan, B.; Yagci, Y. Thermally Curable Acetylene-Containing Main-Chain Benzoxazine Polymers via Sonogashira Coupling Reaction. Macromolecules 2011, 44, 1801– 1807, DOI 10.1021/ma1029746. (20) Kim, H.-D.; Ishida, H. Study on the Chemical Stability of Benzoxazine-Based Phenolic Resins in Carboxylic Acids. J. Appl. Polym. Sci. 2001, 79, 1207–1219, DOI 10.1002/10974628(20010214)79:73.0.CO;2-3. (21) Ghosh, N. N.; Kiskan, B.; Yagci, Y. Polybenzoxazines: New High Performance Thermosetting Resins: Synthesis and Properties. Prog. Polym. Sci. 2007, 32, 1344–1391, DOI 10.1016/j.progpolymsci.2007.07.002. (22) Ning, X.; Ishida, H. Phenolic Materials via Ring‐Opening Polymerization of Benzoxazines: Effect of Molecular Structure on


Page 5 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mechanical and Dynamic Mechanical Properties. J. Polym. Sci. Part B Polym. Phys. 1994, 32, 921–927, DOI 10.1002/polb.1994.090320515. (23) Ishida, H.; Allen, D. Mechanical Characterization of Copolymers Based on Benzoxazine and Epoxy. J. Polymer 1996, 37, 4487–4495, DOI 10.1016/0032-3861(96)00303-5. (24) Murahashi, S.-I.; Zhang, D. Ruthenium Catalyzed Biomimetic Oxidation in Organic Synthesis Inspired by Cytochrome P450. Chem. Soc. Rev. 2008, 37, 1490–1501, DOI 10.1039/B706709G. (25) Nair, V.; Deepthi, A. Cerium(IV) Ammonium Nitrate-A Versatile Single-Electron Oxidant. Chem. Rev. 2007, 107, 1862-1891, DOI 10.1021/cr068408n. (26) Regarding reactivity of periodate with late metals, see Grotjahn, D. B.; Brown, D. B.; Martin, J. K.; Marelius, D. C.; Abadjian, M. C.; Tran, H. N.; Kalyuzhny, G.; Vecchio, K. S.; Specht, Z. G.; Cortes-Llamas, S. A.; Miranda-Soto, V.; van Niekerk, C.; Moore, C. E.; Rheingold, A. L. Evolution of Iridium-Based Molecular Catalysts during Water Oxidation with Ceric Ammonium Nitrate. J. Am. Chem. Soc. 2011, 133, 19024-19027, DOI 10.1021/ja203095k. (27) Regarding reactivity of periodate with late metals, see Hintermair, U.; Sheehan, S. W.; Parent, A. R.; Ess, D. H.; Richens, D. T.;

Vaccaro, P. H.; Brudvig, G. W.; Crabtree, R. H. Precursor Transformation during Molecular Oxidation Catalysis with Organometallic Iridium Complexes. J. Am. Chem. Soc. 2013, 135, 10837–10851, DOI 10.1021/ja4048762. (28) DePasquale, J.; Nieto, I.; Reuther, L. E.; Herbst-Gervasoni, C. J.; Paul, J. J.; Mochalin, V.; Zeller, M.; Thomas, C. M.; Addison, A. W.; Papish, E. T. Iridium Dihydroxybipyridine Complexes Show That Ligand Deprotonation Dramatically Speeds Rates of Catalytic Water Oxidation. Inorg. Chem. 2013, 52, 9175–9183, DOI 10.1021/ic302448d. (29) Pickering, S. J. Recycling Technologies for Thermoset Composite Materials—Current Status. Compos. Part A Appl. Sci. Manuf. 2006, 37, 1206–1215, DOI 10.1016/j.compositesa.2005.05.030. (30) Kaeding, W. W. Oxidation of Aromatic Acids. IV. Decarboxylation of Salicylic Acids. J. Org. Chem. 1964, 29, 2556–2559. (31) Fu, Z.; Liu, H.; Cai, H.; Liu, X.; Ying, G.; Xu, K.; Chen, M. Synthesis, Thermal Polymerization, and Properties of Benzoxazine Resins Containing Fluorenyl Moiety. Polym. Eng. Sci. 2012, 52, 2473–2481.



For Table of Contents Use Only

Oxidation Catalysis

OPhN PhN

OH

CO2H O N Ph

OH CO2H

Ph NO2

HO2C

CO2H

Ph NH2

TOC: A catalytic method to depolymerize benzoxazine matrices away from carbon fiber reinforced polymer composites without damaging the fibers.


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Oxidation Catalysis

OPhN PhN

OH

CO2H O N Ph

OH CO2H

Ph NO2 Ph NH2


HO2C

CO2H



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Recycling BenzoxazineâEpoxy Composites via Catalytic Oxidation

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Recycling BenzoxazineâEpoxy Composites via Catalytic Oxidation