Review of chemical recycling and reuse of carbon fiber reinforced epoxy resin composites
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摘要: 炭纤维增强环氧树脂复合材料以其优异的力学性能被广泛应用于交通运输、航空航天等领域。近年来,炭纤维增强环氧树脂复合材料的回收利用引起了全世界的关注。化学回收是一种有前景的方法,它可以选择性地破坏环氧树脂的特定化学键,实现环氧树脂的可控化学降解。复合材料中的环氧树脂被降解为单体或低聚物,高价值炭纤维可回收利用。首先,本文综述了炭纤维增强环氧树脂复合材料化学回收方法的研究进展,主要包括超临界和亚临界流体、氧化降解、醇解、电化学回收等。然后,简要介绍了可回收热固性树脂的合成及解聚机理,其应用有利于炭纤维增强环氧树脂复合材料中各组分的回收和再利用。最后,提出了化学回收炭纤维增强环氧树脂复合材料和制备高性能可回收环氧树脂材料可能的发展方向。Abstract: Carbon fiber-reinforced epoxy resin composites (CFRCs) have been used in the transportation and aerospace fields because of their excellent mechanical properties. The recycling of CFRCs has attracted attention worldwide in recent years. Chemical recycling is a promising method, which can selectively destroy specific resin bonds to achieve controllable degradation. Matrix epoxy resins are degraded into monomers or oligomers, and the high-value carbon fibers can be recycled. First, we summarize progress on chemical recovery methods, mainly super- and subcritical fluids, oxidation, alcoholysis and electrochemical recycling etc. Then, we briefly introduce the synthesis and depolymerization mechanism of recyclable thermosetting resins by the insertion of reversible chemical bonds into the resin to prepare recyclable resins, which is beneficial for the recycling and reuse of components in CFRCs. Finally, possible developments in the chemical recycling of CFRCs and the preparation of high-performance recyclable epoxy resins are proposed.
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Key words:
- Chemical recycling /
- Chemical degradation /
- Thermoset epoxy resin /
- Carbon fiber /
- Vitrimers
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Figure 1. (a) Schematic diagram of swelling and rapid degradation of thermosetting epoxy resin. (b) SEM images of swollen epoxy resin[59]. Reprinted with permission by American Chemical Society
Figure 3. Schematic diagram of electrochemical recovery of CFRCs[74]. Reprinted with permission by Elsevier
Figure 4. SEM images of (a) vCFs, (b) rCFs with a current of 4 mA, (c) rCFs with a current of 20 mA and (d) rCFs with a current of 25 mA[71]. Reprinted with permission by Elsevier
Figure 5. Photos of (a) CFRP waste and (b) rCFs. SEM images of (c) vCFs and (d) rCFs[79]. Reprinted with permission by American Chemical Society
Figure 6. Schematic diagram of preparation of oil absorbing material[81]. Reprinted with permission by Elsevier
Figure 7. Transesterification reaction in vitrimer[24]
Figure 8. The repair process of CFRCs with surface damage[82]. Reprinted with permission by WILEY-VCH
Figure 10. Synthetic route of polyimines[94]
Figure 11. Synthetic route and 3 repeating units of MB-PACM[95]
Figure 12. Synthetic route and 3 repeating units of PBE-DDM or VBE-DDM[96]
Figure 13. Hydrolysis of acetal bonds[95]
Figure 14. Synthetic route of HBA-CHDMVG or HOBA-CHDMVG[99]
Table 1. The main recycling methods of CFRCs
Methods Secondary classification Products Advantages Disadvantages Mechanical recycling - Powder, fibers Low processing cost, easy operation Low value-added products Thermal recycling Pyrolysis (anaerobic) Fibers, pyrolytic oil or tar, pyrolysis gas Easy operation, fibers can be recycled Toxic gases are produced,
e.g. NOx, SOx, COfluidized bed technique
(aerobic)Thermal energy, combustion gas Easy operation, heat can be recovered Toxic gases are produced,
e.g. NOx or SOxChemical recycling Supercritical liquids
(water, alcohols, etc.)Fibers, small molecule organic chemicals Fibers can be reclaimed The matrix is decomposed into
complex small moleculesStrong oxidants (nitric acid,
hydrogen peroxide, etc.)Fibers, small molecule organic chemicals Fibers can be reclaimed The matrix is oxidized into
complex small moleculesTable 2. Summary of degradation conditions of CFRCs with supercritical water
Catalyst Conditions Mechanical characteristics Ref. No catalyst 405 ± 2 °C, 28 ± 1 MPa, t = 10, 30, 60 and 120 min Tensile strength: 18%-36% reduction;
Modulus elasticity: 7.2%-20.2% reduction[19] 0.5 mol·L−1 KOH 523-673 K, 4.0-27.0 MPa, t = 1-30 min Tensile strength: 2%-10% reduction [26] 0.05 mol·L−1 KOH 395 °C, 27.0 MPa, t = 15, 30 and 60 min Flexural strength retained 80%-95% [27] Oxygen 440 ± 10 °C, 30 ± 1 MPa, t = 25-35 min Tensile strength : 3.13 GPa (rCFs),
3.11 GPa (vCFs)[28] No catalyst 350 °C, 30 min, water, CO2-expanded water
or water/acetone ratio of 20∶80 (v/v)- [29] No catalyst 320 °C, 18 ± 1 MPa, 2 h, water/acetone ratio of 20∶80 (v/v) - [30] No catalyst 300-380 °C, 16-30 MPa, t = 0-150 min,
water/acetone ratio of 80∶20 (v/v)Clean fibers can be recycled above
320 °C. Under extreme conditions,
fibers were severely damaged[20] 0.05 mol·L−1 ZnCl2, MgCl2 and
0.005 mol·L−1 AlCl3300 °C, 45 min, water/acetone ratio of 80∶20 (v/v) - [31] Table 3. Conditions of CFRCs degradation with subcritical/near-critical water
Catalyst Conditions Mechanical characteristics Ref. No catalyst 260 °C, 3.0 MPa, 75 min - [33] 1 mol·L−1 sulfuric acid 260 °C, 6.0 MPa, 90 min Tensile strength decreased by 1.8% [34] 0.4 mol·L−1, H2SO4 or
0.5-1.0 mol·L−1 KOH270 °C, 45 min - [35] KOH and phenol 315 °C, 9.0 MPa, 45 min Tensile strength: 2.67 GPa (rCFs), 2.62 GPa
(vCFs after size removal)[36] No catalyst a. 260-280 °C, t = 30-60 min b. 300-340 °C, t = 30-60 min - [32] No catalyst 260-300 °C, 6-30 MPa Tensile strength: 3.27 GPa (vCFs), 3.63 GPa (rCFs)
at 300 °C for 60 min[37] Table 4. A comparison of degradation conditions of CFRCs with super- and subcritical alcohols
Catalyst Solvent Conditions Mechanical characteristics Ref. No catalyst n-propanol 450 °C, 5 MPa Tensile strength: 3.90 GPa (rCFs),
4.09 GPa (vCFs);
Tensile modulus: 230 GPa (rCFs),
242 GPa (vCFs)[40] No catalyst n-propanol 310 °C, 5.2 MPa, 20 min Tensile strength: rCFs (4325 MPa (T600S), 5203 MPa (T700S), 4322 MPa (STS 5631)); vCFs (4338 MPa (T600S), 5220 MPa (T700S), 4415 MPa (STS 5631)) [42] No catalyst Methanol, ethanol, n-propanol, iso-propanol, n-butanol, and acetone 280-360 °C The tensile strength of rCFs by supercritical n-butanol and n-propanol can maintain 98% of that of the vCFs [25] No catalyst Methanol, ethanol, n-propanol, iso-propanol, n-butanol, and acetone 280-360 °C The tensile strength of rCFs by supercritical n-butanol and n-propanol can maintain 98% of that of the vCFs [43] No catalyst Methanol Batch reactor: 270 °C, 8 MPa, 90 min
Semi-flow reactor: 285 °C,
8 MPa, 80 minCompared with the tensile strength of the vCFs, the tensile strength of the rCFs in the batch reactor was reduced by 7%, and that in the semi-flow reactor was reduced by 9% [38] No catalyst Methanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol, acetone, and methyl ethyl ketone 320 °C, 1 MPa, 20 min The tensile strength of the rCFs did not decrease significantly [44] No catalyst Acetone 350 °C, 2-14 MPa - [21] 0.016-0.50 mol·L−1 KOH, NaOH or CsOH Methanol, ethanol, n-propanol, acetone 200-450 °C, 15.5 min The rCFs retained 85%-99% of the tensile strength of the vCFs [39] 0.2% KOH iso-propanol 300 °C, 5 MPa, 20 min - [45] 1% KOH 1-propanol 320 °C, 60 min - [41] 1% KOH 1-propanol 320 °C, 60 min Tensile strength: 3.47 GPa (rCFs) [46] 1% KOH 1-propanol 320 °C, 180 min The tensile strength of the rCFs retained 90% of that of the vCFs when the reaction time was less than 90 min, but the tensile strength decreased by 15% after 180 min [47] 0.02 mol·L−1 KOH n-butanol 330 °C, 30 min - [48] 0.36 mol·L−1 KOH Methanol 210 °C, 120 min - [49] No catalyst Ethylene glycol/water ratio of 5 400 °C Tensile strength: 3.4 GPa (rCFs),
3.5 GPa (vCFs)[50] Table 5. Oxidation recovery of CFRCs and its degradation products
Material Conditions Products Mechanical characteristics Ref. CF, bisphenol A epoxy resin, isophoronediamine curing agent (IPDA) 8 mol·L−1 nitric acid solution, material/nitric acid (6 g∶100 mL),
90 °C, 12 hMixtures consisting of mainly of 2,4-dinitrophenol and 2-nitro-4-carboxylphenol - [51] 52% CF, 8% GF, epoxy resin cured by amine 12 mol·L−1 nitric acid solution, material/nitric acid (100 g∶1.8 L), liquid flow of 1.0 cm/s, 90 °C, 6 h The main components were 2,4-dinitrophenol, O-nitrophenol and organic acids The tensile strength of the rCFs was reduced by 2.91% [55] Bisphenol F epoxy resin, amine curing agent (MDA) 4 mol·L−1 nitric acid solution, 80 °C, normal pressure, 150 h Monomers or dimers of the resin - [57] GF, Bisphenol F epoxy resin, DDM 4 mol·L−1 nitric acid solution, 80 °C, normal pressure, 450 h Similar to the monomers and dimers of resin - [58] Bisphenol F epoxy resin (EPICLON 830), aromatic and alicyclic amine curing agent (MXDA, BAC, MDA, IPDA, NBDA, MPDA) 4 mol·L−1 nitric acid solution,
80 °C, t = 30-168 hSmall molecules of low molecular weight - [56] Bisphenol F epoxy resin (EPICLON 830), alicyclic amine curing agent (BAC) 4 mol·L−1 nitric acid solution,
80 °C, 168 hMixtures of 2-amino-4-nitrophenol, 2,6-diamino-4-nitrophenol and picramic acid - [2] Prepreg 7.1 mol·L−1 nitric acid solution,
80 °C, 30 min- The tensile strength and tensile modulus of the rCFs were close to those of the vCFs [3] Bisphenol A epoxy resin (ER-51), DDM 4 mol·L−1 nitric acid solution,
60 °C, 3 hOligomers of high macromolecular weight - [59] CF, epoxy resin (JR-236 EP), acyclic amine (JH-239) Acetone/30% H2O2 (1∶2, v/v),
60 °C, 30 minBisphenol A and its derivatives The tensile strength of the rCFs retained above 90% of the tensile strength of the vCFs when the temperature was below 100 °C [52] CF, epoxy resin (JR-236 EP), acyclic amine (JH-239) DMF/H2O2 (1∶1, v/v),
90 °C, 30 min- The tensile strength of the rCFs remained more than 95% of the vCFs [60] DGEBA/amine 2 mol·L−1 nitric acid/9 mol·L−1 H2O2 (98/2, v/v), 65 °C, 9 h,
material/solvent (1∶60)Products contained molecules similar in structure to the DGEBA monomer as well as other substances of low molecular weight The tensile strength of the rCFs was slightly reduced [9] DGEBA/amine 14 mol·L−1 acetic acid/9 mol·L−1 H2O2 (95∶5, v/v), 65 °C, 5 h Mixtures of phenols and phenolic derivatives and substances containing C=O groups The rCFs is clean and long, and its tensile strength is comparable to that of the vCFs [54] CF, epoxy resin (RIM935 grade), aliphatic amine curing agent (RIM936 grade) 50% H2O2/TA (2, v/w), 120 °C,
1 min of microwave irradiation- The rCFs retained more than 92% of the tensile strength and 94% of the strain-to-failure retentions of the vCFs [53] Bi-functional epoxy resin (Araldite GY 6010), tri-functional epoxy (Araldite MY 0510), and tetra-functional epoxy (Araldite MY 721), curing agent (3,3’-diaminodiphenyl sulfone, 3,3’-DDS) a. K3PO4/benzyl alcohol system, 200 °C
b. Acetic acid/30% H2O2 (10/1, v/v), 110 °C, 1 h- The rCFs by acid digestion had no resin residue and no defects [10] Bisphenol A epoxy resin, 3,3’-DDS Acetic acid/30% H2O2 (6/1, v/v),
110 °C, 6 h- - [61] Bisphenol A epoxy resin, 3,3’-DDS Acetic acid/30% H2O2 (20/1, w/w),
110 °C, 4 hBisphenol A and tertiary amines - [62] CF, Bisphenol A epoxy resin, 3,3’-DDS 5% MnCl2 and AlCl3/acetic acid,
10 1010 kPa of O2, 180 °C, 43 hDDS monomer, bisphenol A The rCFs remain woven structure [1] Table 6. Alcoholysis of CFRCs and their degradation products
Material Conditions Products Mechanical characteristics Ref. Bisphenol A epoxy resin, methyl tetrahydrophthalic anhydride (MeTHPA) 0.2 mol·L−1 N-methyl-4-piperidinol/EG, 180 °C, 1 h, argon environment, without stirring An oligomer containing multifunctional hydroxyl groups - [63] CF, Epon resin 828, MeHHPA TBD/EG/NMP, 170 °C, 1.5 h, ordinary pressure An oligomer is 2,2-bis[4-(2,3-dihydroxypropoxy) phenyl] propane Tensile strength: 5.49 ± 0.74 GPa (rCFs), 4.81 ± 0.72 GPa (vCFs);
Young’s modulus: 278.6 ± 14.4 GPa (rCFs), 280.3 ± 13.5 (vCFs)[64] CF, bisphenol A epoxy resin, MeHHPA NaOH/benzyl alcohol or K3PO4/benzyl alcohol (1/10, w/w), 195 °C, 40 min Dicarboxylic acid salt and linear resin macromolecules The tensile strength of rCFs maintained over 90% of that of the vCFs [65] Bisphenol A epoxy resin (ER-51), MeTHPA NaOH/PEG200, 180 °C, atmospheric pressure, 50 min - - [66] CF, GF, bisphenol A epoxy resin (ER-51), MeTHPA NaOH/PEG200, 200 °C, atmospheric pressure, 4 h - The tensile strength of the rCFs was 94%-96% of the original fibers [67] CF, bisphenol F epoxy resin, curing agent (polyoxyalkyleneamine, 4,4’-methylenebis, and 2,2’-dimethyl-4,4’-methylenebis) KOH/PEG400, 160 °C, atmospheric pressure, 200 min A mixture of monomer and dimer of bisphenol F-epichlorohydrin epoxy resin Tensile strength: 3.89 GPa (rCFs),
4.07 GPa (vCFs);
Elastic modulus: 173.79 GPa (rCF), 179.27 GPa (vCF)[68] Tetraglycidyl 4,4’-diaminodiphenyl methane (TGDDM), 4,4’-DDS 20% ZnCl2/ethanol,
190 °C, 5 hAn oligomer containing amine and hydroxyl groups The surface of the rCFs was clean and smooth [69] Bisphenol A epoxy resin (ER-51),
MeTHPAK3PO4/ethanol, 120 °C, 3 h;
1 mol·L−1 hydrochloric acid
solution, 60 °C, 1 hProducts contained carboxylic groups - [70] Table 7. Specific conditions of electrochemical recovery of CFRCs
Year Conditions Mechanical characteristics Ref. 2015 Electrolyte concentration: 3%, 10% and 20%;
Applied current: 4, 10, 20 and 25 mA;
t = 21 daysTensile strength: 4382 MPa (vCFs), 3515 MPa (rCFs) at 3% NaCl solution and 25 mA applied current [71] 2016 Electrolyte concentration: 40 g/L NaOH solution or simulated pore water solution;
Applied current: 0, 0.5 and 4 mA;
t = 50 days- [72] 2019 Electrolyte concentration: 0.5%, 1.0%, 2.0% and 3.0%;
KOH catalyst: 0.5, 1.0 and 1.5 g/L;
Applied current: 20, 40, 62.5, 78.1, 104.2 and 156.3 mA;
Temperature: 40, 60 and 75 °C;
t = 36-72 hTensile strength: 4.641 GPa (vCFs), 4.083 GPa (rCFs) at 2% NaCl solution, 20 mA current, 1% KOH solution and 75 °C;
IFSS: 31.00 MPa (vCFs), 37.43 MPa (rCFs) at 2% NaCl solution, 20 mA current and 1% KOH solution[73] 2020 Electrolyte: NaCl, KCl, NaOH, KOH and Na2CO3;
Electrolyte concentration: 0.01-1.0 mol·L−1;
voltage: 2.5-15.0 V;
t = 0-20 hA few carbon fibers were fractured and had irregular fiber waviness [74] 2020 Electrolyte: pH 6.86 phosphoric acid aqueous solution;
Voltage: 15 V;
t = 0-180 minThere were small voids on the surface of the rCFs [75] Table 8. Recycling methods and degradation products of CFRCs
Material System Products Mechanical characteristics Ref. CF, bisphenol A epoxy resin, PA tetraline or 9,10-dihydroanthracene, 340 °C, 2 h The main products were bisphenol A, phenol, p-isopropylphenol, phthalic anhydride, benzoic acid, and benzene Tensile strength: 3950 MPa (rCFs) [76] Epoxy resin (DER 331), nadic methyl anhydride (NMA) 1% HPW aqueous solution, material/HPW aqueous solution (6 g/20 g), 190 °C, 5 h Oligomer contained muti-functional active groups - [77] Bisphenol A epoxy resin (ER-51), MeTHPA DETA, 130 °C, 50 min Oligomers contained a large of reactive groups (amide and hydroxyl groups) - [78] CF, bisphenol A epoxy resin, 4,4’-methylene dimethyl cyclohexylamine 60% ZnCl2 solution,
220 °C, 9 hProduct was similar to the DGEBA dimer The rCFs retained more than 90% of the tensile strength of vCFs [22] CF, bisphenol A epoxy resin, 4,4’-methylene dimethyl cyclohexylamine 15% AlCl3/CH3COOH,
180 °C, 6 hOligomer contained carbon skeleton structures of cured epoxy resin Tensile strength: 2871.96 MPa (rCFs),
2937.29 MPa (vCFs)
Tensile modulus: 168.46 GPa (rCFs),
171.77 GPa (vCFs)[79] -
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