Abstract
The degradation capability of the different supercritical fluids on carbon fiber-reinforced polymer (CFRP) was analyzed based on the impact of reaction temperature and time on degradation rate; the chain scission reaction of cross-linked network in CFRP occurred in supercritical fluid was investigated based on the analysis of liquid phase products; and the recycled carbon fiber under supercritical n-butanol and n-propanol were characterized. The results indicated that supercritical n-butanol had the excellent degradation capability on CFRP, followed by supercritical acetone. The degradation capability of supercritical ethanol and n-propanol on CFRP had little difference in temperature ranged from 280 °C to 340 °C, while supercritical n-propanol was superior to ethanol under the temperature ranged from 340 °C to 360 °C. The supercritical methanol and isopropanol were disadvantageous to CFRP degradation. The liquid phase products were main the benzene derivatives and phenol derivatives by the scission of C-C, C-O and -O- bond in linear chain segment, as well as that of C-N bond in cross-linked segment of epoxy resin cure system. In comparison with the original carbon fiber, the content of N, O and Si from the recycled carbon fiber surface decreased, while the content of C increased, and the tension strength can retain above 98 % of that of the original carbon fiber.
Introduction
Carbon fiber-reinforced polymer (CFRP) had a wide application prospect in many fields such as the aerospace, automobile and wind blade, etc., owing to their performance advantages such as the excellent corrosion resistance, thermal stability, high strength and shock resistance. The high price of the carbon fiber had limited the wide usage of CFRP. The leftover material and defective products generated in the production, damaged structures in the course use and the waste products exceed the period of use contained a great deal of high-performance carbon fibers, so recycle and reuse of the waste CFRP was considered to be a key for achieving the wide application of CFRP. CFRP cannot be re- melted and used for secondary molding, as well as its heating- resistant, chemical-resistant and resistant to biodegradation due to its three-dimensional cross-linked network, so the recycle and reuse of CFRP had become a difficult issue.
The current recycling methods of CFRP included the mechanical recycling, energy recycling, thermal recycling and chemical recycling [1]. In mechanical recycling method, the interfacial binding force between the carbon fiber and resin matrix was broken down by grinding CFRP under the action of mechanical force. The fibers can be stripped from the cross-linked resin, and the power rich in the resin and short fiber-shaped products can be obtained, while it is difficult to recycle the fibers with high performance. In energy recycling method, the generated heat quantity was converted into other usable energy during incinerating of CFRP, and the usable carbon fiber and other chemical materials cannot be obtained. The thermal recycling included the pyrolysis and fluidized bed method. In pyrolysis method, the resin matrix in CFRP can be decomposed to the gas products and low-molecular weight organics, and the mechanical performances of the recycled carbon fiber, which had the carbon deposition, was easy to be affected by pyrolysis parameters. Meanwhile, the resin materials cannot be completely removed, and the harmful gas was generated in pyrolysis. In fluidized bed method, the air can be regarded as the fluidized gas in reactor, and the short carbon fiber without carbon deposition can be obtained under reaction temperature ranged from 450 °C to 700 °C. The strength of the recycled carbon fiber had a loss of 25-50 % compared with that of the original carbon fiber, while it is difficult to recycle usable chemical materials from the resin in CFRP.
The chemical recycling as the current main recycling method for CFRP, the resin matrix can be converted to the small molecules by chemical reagents, thereby achieving the goal of dissociation of CFRP and recycling carbon fiber. The chemical recycling included the low temperature dissolution and supercritical fluid dissociation. In low temperature dissolution method, the nitric acid, alcohol, ammonia and glycol can be regarded as the reactants to degradation of CFRP by breaking down the chemical bond in resin matrix [2]. Lee et al. decomposed CFRP completely by hydrogen nitrate under temperature as 90 °C, time as 6 h, concentration as 12 M and flow as 1.0 cm/s [3]. Iwaya et al. decomposed CFRP by benzyl carbinol under temperature as 190-350 °C, time as 1-8 h and catalyst as potassium phosphate [4]. Tao et al. decomposed epoxy resin by NaOH or KOH under temperature as 200-350 °C, time as 5-120 min and catalyst as azotate [5]. Although the long carbon fiber with the excellent mechanical performance can be obtained, the used a larger number of the solvents for degradation of CFRP had a negative impact on environment. Supercritical fluid as a new reaction medium reduced the effects of the reaction solvents on the environment, and had the excellent degradation capability for the resin matrix in CFRP, and had the excellent mass transfer capability for once degradation products. Supercritical fluid was expected to achieve high efficiency and value recycling of CFRP.
Supercritical fluid can be obtained when temperature and pressure above the critical point of pure substance (T/Tc > 1 and P/Pc > 1). The density of supercritical fluid was close to that of liquid, while its viscosity and diffusion coefficient were close to that of gas. Supercritical fluid was much easy to diffuse towards porous material due to its surface tension was zero. The density, viscosity, diffusion coefficient, dielectric coefficient and solvability of supercritical fluid were very sensitive with the change of temperature and pressure near the critical point (1.00 < T/Tc < 1.10 and 1.00 < P/Pc < 1.10). The physical and chemical properties of supercritical fluid would be greatly changed by minute changes in pressure under 1.00 < T/Tc < 1.10. So Recycling waste CFRP by supercritical fluids with excellent dissolving capability and diffusion capability can obtain the high-performance carbon fiber. Recycling of CFRP by supercritical fluids was shown in Figure 1.
The cage effect was generated when the cage surrounding composites was formed due to the aggregation of fluid molecules, which formed the supercritical fluid stagnant layer.
University of Nottingham in collaboration with University of Valladolid, they had performed on research on recycling CFRP by sub-critical and supercritical water. The strength of the recycled carbon fiber can retain 98 % of that of original carbon fiber, and the yield of elimination of the resin can reach up to above 95 % [6]. Okajima et al. utilized sub- critical and supercritical water to degradation of CFRP, and the clean carbon fibers can be obtained under temperature as 380 °C and pressure as 25 MPa [7]. Liu et al. used supercritical water for degradation of CFRP, and the clean carbon fibers with a monofilament tension loss of 1.8 % were obtained under temperature as 290 °C, reaction time as 70 min and feed ration as 1:5 g/ml [8]. Pickering et al. used supercritical n-propanol for degradation of CFRP to obtain the carbon fiber, which had smooth surface, the lower residual resin content and the excellent mechanical performance [9]. Piñero-Hernanz et al. used sub-critical and supercritical alcohols for recycling CFRP. The strength of the recycled carbon fiber can retain 85-99 % of that of the original carbon fiber, while the degradation rate of epoxy resin in CFRP can reach up to 98 % [10]. Idzumi et al. used supercritical methanol for recycling CFRP. The thermosetting resin in CFRP can be completely decomposed to obtain the carbon fibers without thermal damage under reaction temperature as 270 C, reaction time as 90 min and pressure as 8 MPa [11].
Supercritical water and n-propanol were most used for degradation of CFRP to obtain high-performance carbon fibers, and the degradation parameters optimum and products characterization were discussed in above-mentioned literatures. However, it was still unknown to the selection method of supercritical fluid used for degradation of CFRP and the chain scission reaction of cross-linked network in CFRP occurred in supercritical fluid. This paper analyzed degradation capability of different supercritical fluids on CFRP based on the physical parameters of supercritical fluid, including Hildebrand parameter, dielectric coefficient and dipole moment, and proposed the principle for choosing supercritical fluids used for degradation of CFRP. Research results could solve the problem of how to choose supercritical fluids for the specific resin matrix. Meanwhile, the chain scission reaction of cross-linked network in CFRP occurred in supercritical fluid was investigated based on analysis of the components of the liquid phase products. Finally, degradation mechanism of CFRP in supercritical fluid was inferred, and the mechanical performance, surface morphology and surface chemistry of the recycled carbon fiber were analyzed.
Figure 1. Supercritical fluid used for recycling CFRP.
Experimental
Establishment of Recycling Test Plant
Recycling plant for CFRP was mainly composed of the pressurization unit and supercritical fluid degradation unit, as shown in Figure 2. Supercritical fluid can be obtained by heating reaction medium provided by pressurization unit in reactor. The solvent storage, mass flow meter, high-pressure plunger pump, gate valve and one-way valve were sequentially connected to form the pressurization unit by stainless steel tube. Supercritical fluid degradation unit was composed of reaction vessel, constant temperature heating device, monitoring device of temperature and pressure inside reaction vessel, safety valve and gate valve. The cooling coil was installed inside reaction vessel, which can bear the highest temperature as 500 °C and pressure as 40 MPa. The auxiliary pressure can be provided by CO2 and N2. The liquid catalyst such as H2O2 was provided to the reaction vessel by H2O2 pump, and solid catalyst including KOH or CsOH was usually dissolved in alcohol solvent to make alcohol-alkali solution (catalyst concentration: 6×10 -5×10 mol/ml). Alcohol-alkali solution could be pumped to reaction vessel by the pressurization unit.
Materials and Recycling Process
The uncured CFRP pre-preg material was supplied by Weihai Guangwei Composites Co., Ltd. The resin matrix was bisphenol-A epoxy resin (DGEBA/7901), hardened with dicyanodiamide as curing agent (DICY). The molecular structure of bisphenol-A epoxy resin, dicyanodiamide and its curing system were shown in Figure 3. The reinforcement was the carbon fiber (12K-A42). The content of epoxy resin in CFRP was determined to be 33.3 % (wt.%) according to GB/T3855-2005. CFRP pre-preg material was cured at 120 °C for three hours, then cured pre-preg material was carried on degradation test under supercritical alcohols and supercritical acetone respectively. The solid products after degradation of CFRP were collected from the reactor, and then were cleaned in 100 ml of equivalent acetone for three times. The cleaned solid products was dried until its mass became constant, followed by weighing the dried solid products, then degradation rate of epoxy in CFRP can be calculated according to equation (1).
Figure 2. Recycling test platform of CFRP; (a) schematic diagram and (b) recycling plant.
Figure 3. The molecular structure; (a) Bisphenol-A epoxy resin, (b)Dicyanodiamide (DICY), and (c) curing system.
Characterization
10 mg of CFRP sample was performed to thermogravimetric analysis (TGA) by STA449F3 Simultaneous Thermal Analyzer. The sample was heated at 10 °C/min in N2 atmosphere (100 cm /min) up to maximum temperature 900 °C for 5 min.
where L was the likelihood function, l was the testing length. n represented the number of the tested carbon fiber, m represented Weibull modulus, which reflected the uniformity and reliability of material. σi represented the tensile strength of the single carbon fiber (σi=4Fi/πd2), and Fi represented withstanding the maximum tension of the single carbon fiber, d was average diameter of the tested carbon fiber. σ0 was Weibull scale parameter, which represented the tensile strength of carbon fiber.
Setting ∂lnL /∂m = 0 and ∂lnL / ∂𝞼0 = 0, the following formula can be calculated:
The surface microstructure of the recycled carbon fiber was observed by Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). The surface chemistry of the recycled carbon fiber was examined by X-ray photoelectron spectroscopy (XPS).
The components of liquid-phase products from CFRP degradation can be determined by GC-MS (VARIAN CP- 3800), then chain scission reaction of cross-linked network in CFRP occurred in supercritical n-propanol was investigated. The chromatographic column was CP-5806 quartz capillary column (30 mm×0.25 mm×0.25 mm). The inlet temperature was was 280 °C; the column temperature was from 50 °C to 280 °C (10 °C/min); and the ion source temperature was 150 °C. The MS scan range was 35-650 amu. The electroimpact source was 70 eV.
The monofilament tensile strength of the recycled carbon fiber can be calculated by Weibull probability density function (2) according to the testing standard BS ISO 11566:1996.Setting ∂lnL /∂m = 0 and ∂lnL / ∂𝞼0 = 0, the following formula can be calculated:
Results and Discussion
CFRP Degradation in Different Supercritical Fluids
Initial mass of CFRP can be controlled in 1.992-2.0058 g in degradation test. When the content of solvent was 350 ml and reaction time was 60 min, the degradation rate difference of different supercritical fluids on the epoxy resin in CFRP was analyzed under different reaction temperatures, as shown in Figure 4(a). Meanwhile, when the reaction temperature was 320 °C and content of solvent was 350 ml, the degradation rate difference was analyzed under different reaction time, as shown in Figure 4(b).
Degradation rate difference of supercritical fluids on epoxy resin in CFRP can reflect degradation capability of supercritical fluids. CFRP can be surrounded by aggregation of supercritical fluid around CFRP was far greater than that of the bulk of fluid. Since the degradation capability of supercritical fluids was correlated positively with their dissolution capability, it can be analyzed based on the physical parameters of fluid, including Hildebrand parameter, dielectric coefficient and dipole moment.
Hildebrand parameter of different reaction mediums in supercritical state can be calculated according to the empirical formula (2) or (3) [12,13]. The n-butanol had a value of 23.4 MPa0.5 ; the n-propanol had a value of 24.6 MPa0.5 ; the isopropanol had a value of 23.7 MPa0.5 ; the methanol had a value of 29.6 MPa0.5 far from Hildebrand parameter of epoxy resin (11.5 MPa0.5 ); the ethanol had a value of 26.5 MP0.5 ;and the acetone had a value of 20.0 MPa0.5 much closed to Hildebrand parameter of epoxy resin. Dipole moment of acetone was 2.88 D, while other alcohols had the similar dipole moment as about 1.7 D, so acetone had the largest polarity relative to other alcohols. Supercritical acetone had the better dissolution capability on CFRP and once degradation products according to the similarity and intermiscibility principle of polymer. Hildebrand parameter of supercritical alcohols usually decreased with the increasing of the number of carbon atom in molecular structure. According to the similarity and intermiscibility principle of polymer, supercritical alcohols with the more carbon atom number had the better dissolution capability on CFRP and once degradation products, on account of the lower Hildebrand parameter was much closed to that of epoxy resin.
Figure 4. Degradation rate difference of different supercritical fluids on the epoxy resin in CFRP; (a) under different reaction temperatures and (b) under different reaction time.
There was the greater difference between dielectric coefficient of supercritical fluid and that of normal fluid. Dielectric coefficient of fluid often decreased with the increase of temperature. The dissolution capability of supercritical fluid on CFRP related to dielectric coefficient of fluid itself. Supercritical fluid with the larger dielectric coefficient was more disadvantageous to the proceeding of degradation of CFRP. Dielectric coefficient of supercritical alcohols decreased with the increase of the number of carbon atom in the molecular structure, so supercritical alcohol with the more carbon atoms number had the better dissolution capability for CFRP and once degradation products. Supercritical n-butanol with the smaller dielectric coefficient had the excellent degradation capability for CFRP relative to supercritical methanol, while Supercritical methanol with the larger dielectric coefficient was dis- advantageous to degradation of CFRP.
The pressure insider reactor caused by the content of alcohols solvent and temperature decreased with the increase of relative molecular weight of alcohols, as shown in Figure 5. From the impact of reaction temperature on degradation rate of the epoxy resin knowable (Figure 4(a)), the pressure condition demand for degradation CFRP lowered with the increase of relative molecular weight of alcohols. Though the n-butanol was in sub-critical state under reaction temperature as 280 °C or 300 °C and corresponding pressure as 3.2 MPa or 4.2 MPa, sub-critical n-butanol had still the higher degradation rate for CFRP relative to other fluid reaction medium. Therefore, supercritical n-butanol had the lower pressure condition demand for degradation of CFRP relative to other supercritical alcohols.
Figure 5. The pressure change with temperature in different fluids.
Table 1. Degradation products under different supercritical fluids (T=360 °C, V=350 ml and t=60 min)
Comprehensive consideration the effect of dissolution parameters including Hildebrand parameter, dielectric coefficient and dipole moment on dissolution capability of fluid itself, from degradation capability difference of supercritical fluids on CFRP knowable, supercritical n- butanol had excellent degradation capability for CFRP, followed by supercritical acetone. Degradation capability of supercritical ethanol on CFRP was similar to that of supercritical n-propanol under reaction temperature ranged from 280 °C to 340 °C, while degradation capability of supercritical n-propanol on CFRP was superior to that of supercritical ethanol under reaction temperature ranged from 340 °C to 360 °C. Supercritical methanol and isopropanol were disadvantageous to degradation of CFRP. The solid products from CFRP degradation under different supercritical fluids were as shown in Table 1, and a great quantity of the carbon fibers can be stripped from CFRP in supercritical n-butanol, n-propanol and acetone under reaction temperature as 360 °C, solvent content as 350 ml and reaction time as 60 min, then the epoxy resin in CFRP can be completely decomposed. The dissolution parameters of fluids and pressure condition demand for degradation of CFRP caused degradation capability difference. Therefore, the solvent with the small dielectric coefficient, the larger dipole moment and Hildebrand parameter much closed to that of epoxy resin in supercritical state was much easier to degrade CFRP.
Degradation Mechanism
DTG-TG curve as shown in Figure 6. Mass loss of CFRP was less than 2 % when temperature was below 300 °C. This stage mainly occurred to evaporation of internal water in sample, as well as overflow and diffusion of internal volatile matter in sample. The initial pyrolysis temperature of CFRP was Tf =310 °C, and there was the maximum weight loss rate at 375-450 °C.
Figure 6. Pyrolysis curve of CFRP.
The cured CFRP was degraded by supercritical n-propanol at t=360 min, T=300 °C and V=350 ml, then the liquid phase products was analyzed by GC-MS. The main name, molecular structure and relative peak area of the components of liquid phase products can be determined according to Saturn database, as shown in Table 2.
The components of supercritical n-propanol (blank sample) were analyzed by GC-MS under without adding CFRP to reactor. The name, molecular structure and relative peak area of components of supercritical n-propanol can be determined according to Saturn database, as shown in Table 3. Known from components of blank sample, there was the Guerbet reaction under supercritical n-propanol. The reaction process of Guerbet was shown in Figure 7.
Table 2. The component of liquid phase products
Table 3. Components of supercritical n-propanol (T=300 °C, V=350 ml and t=360 min)
The product 1' was generated by intramolecular dehydration of component e. The hydroxyl of component a' (isomer of component a) was oxidized to the ketone group under condition of high temperature and pressure, and aldehyde group was easy to generate reduction reaction comparing with ketone group, then the product 2' can be generated by intramolecular dehydration after the aldehyde group was reduced to the hydroxyl. The double bond in molecular structure of component b may generate addition reaction to form component d, and product 3' was isomer of component d.
During the n-propanol was oxidized to the propanal in Guerbet reaction, the content of generated H free radicals was much larger than the content of consumed H free radicals in Guerbet reaction. The excess H free radicals can combine with the free radicals formed by molecular chain scission of the epoxy resin cure system, which can generate stabilize the monomers. The degradation of epoxy resin cure system in CFRP under supercritical n-propanol can be inferred according to the components of liquid phase products, as shown in Figure 8.
Figure 7. Reaction process of Guerbet.
Figure 8. The degradation of DGEBA/DICY cure system under supercritical n-propanol.
The molecular chain segment of epoxy resin cure system was broken down to small molecules by the scission of C-N, C-O and C-C bond during CFRP degradation, and these small molecules can combine with H free radicals provided by Guerbet reaction to generate component a (product 6), b, c, d, e and f. The product 7 can be generated by rearrangement reaction of component b under high temperature and pressure. The component c was the alkane or alkene. The alkane can generate the alkene and H free radicals under high temperature cracking, and the alkene can be addition reaction with H free radicals to generate the alkane. So the two substances of component c can be converted each other under high temperature and pressure. The product 3 can be generated by displacement reaction between the alkane and propanol removed a H free radical, as well as addition reaction between the alkene and propanol removed a H free radical. The C-C bond of component d (product 9) was easy to break down to generate the product 5 and phenol. But hydroxyl free radical obtained from phenol under high temperature and pressure can react with H· to dehydration, the phenol can not be determined in the liquid phase products. The C=C and C≡N bond of component e was easy to be addition reaction to generate C-C and C-N bond, and the free radical formed by the scission of C-C and C-N bond can combine with each other to generate product 1. The n- propanol was oxidized to the propanal in Guerbet reaction. The active hydrogen atom in α-C of the propanal was easy to react with the component f to dehydration, then the formed component after dehydration occurred addition reaction to generate product 2.
Supercritical n-propanol acted as the solvent and reactant in CFRP degradation process. On the one hand, once degradation products can be transferred into the bulk of fluid from the reaction zone through mass transfer of supercritical n-propanol, which can reduce the possibility of secondary reaction for degradation products. On the other hand, supercritical n-propanol had the hydrogen donor ability owing to the Guerbet reaction. The degradation products were main the cumene, para-isopropylphenol, bisphenol A and 4-methoxystyrene, etc. when the reaction temperature was the lower than the initial pyrolysis temperature of CFRP, the solvolysis was predominated in CFRP degradation reaction. The liquid phase products were main the benzene derivatives and phenol derivatives by the scission of C-C, C- O and -O- bond in linear chain segment, as well as C-N bond in cross-linked segment of epoxy resin cure system.
Characterization of the Recycled Carbon Fiber
Supercritical n-butanol, acetone and n-propanol had the excellent degradation capability for CFRP. The liquid phase products contained a great quantity of alcohol components, which can be used repeatedly for degradation of CFRP under its supercritical state. The recycled carbon fiber under supercritical n-butanol and n-propanol was shown in Figure 9.
The number of tested carbon fiber was 20. The Weibull modulus and monofilament tensile strength of the recycled carbon fiber can be calculated according to d and Fi, as shown in Table 4. The tensile strength of recycled carbon fiber can retain 98 % of that of the original carbon fiber.
As shown in Figure 10, the surface of the recycled carbon fiber had a small amount of residual resin. The residual resin on the recycled carbon fiber under supercritical n-butanol with much higher molecular kinetic and mass transfer ability was less than that of the recycled carbon fiber under supercritical n-propanol. The stripping of epoxy resin on the surface of carbon fiber included three processes: (1) the fluid medium diffused toward the carbon fiber surface; (2) degradation reaction of epoxy resin in CFRP; (3) the degradation products diffused toward the fluid bulk. Therefore, the residual resin content can be reduced when strengthening the mass transfer of fluid by two ways: the higher density of reaction medium, and forced-convection flow. The carbon fiber with clean surface can be obtained in semi-continuous reactor due to the scouring action of supercritical fluid on the carbon fiber surface. Or, the residual resin on the recycled carbon fiber in batch reactor can be removed by further using the ultrasonic acetone.
Table 4. The calculation of Weibull parameters
Figure 9. The CFRP and recycled carbon fiber (T=360 oC, V=350 ml and t=60 min); (a) CFRP, (b) supercritical n-butanol, and (c)supercritical n-propanol.
Figure 10. The scanning electron microscopic image of the carbon fiber (T=360 C, V=350 ml and t=60 min); (a) the original carbon fiber, (b) supercritical n-butanol, and (c) supercritical n-propanol.
Figure 11. The microscopic image of the carbon fiber (T=360 °C, V=350 ml and t=60 min); (a) the original carbon fiber, (b) supercritical n- butanol and (c) supercritical n-propanol.
The surface microstructure of the recycled carbon fiber was as shown in Figure 11. The original carbon fiber surface had only a little quantity of the raised and shallower grooves, while the recycled carbon fiber under supercritical n-butanol and n-propanol had a little quantity residual resin and different extent of surface grooves. The generated surface grooves showed a certain physical etching action of supercritical fluid on the carbon fiber surface. The mechanical property aspect: the recycled carbon fiber had a certain strength loss owing to surface grooves. The reuse aspect: the wetting characteristics between the recycled carbon fiber and new resin can be improved.
The surface spectrum of the carbon fiber had the three main peaks, which represented C (284.08 eV), O (532.08 eV), N (401.08 eV) and Si (102.08 eV) respectively. The partial oxygen functional groups of carbon fiber surface was taken away duce to the physical etching action of supercritical fluid on carbon fiber, so the content of N, O and Si on the surface of the recycled carbon fiber decreased, while the content of C increased, as shown in Figure 12 and Table 5. Meanwhile, the increasing C content also showed that the residual resin on the recycled carbon fiber can be basically removed to clean. The content ratio of O and C reduction of carbon fiber surface was not obvious, so the chemical bonding effect between the recycled carbon fiber and new resin matrix can be basically guaranteed.
Figure 12. XPS images of the carbon fiber surface.
Table 5. The elementary composition of the recycled carbon fiber surface
Conclusion
- The solvent with the small dielectric coefficient, the larger dipole moment and Hildebrand parameter much closed to that of epoxy resin in supercritical state was much easier to degradation of CFRP. Supercritical n-butanol had the excellent degradation capability on the epoxy resin in CFRP, followed by supercritical acetone. The degradation capability of supercritical ethanol and n-propanol on CFRP had little difference under reaction temperature ranged from 280 °C to 340 °C, while supercritical n-propanol was superior to ethanol under the temperature ranged from 340 °C to 360 °C. The supercritical methanol and isopropanol were disadvantageous to CFRP degradation.
- The degradation products were analyzed by GC-MS. Supercritical n-propanol had the hydrogen donor ability owing to the Guerbet reaction, and acted as the solvent and reactant during degradation of CFRP. The stable monomers were generated when the excess H free radicals provided by Guerbet reaction combined with the free radicals from the molecular chain scission of the epoxy resin cure system. The liquid phase products were main the benzene derivatives and phenol derivatives by the scission of C-C, C-O and -O- bond in linear chain segment, as well as C-N bond in cross-linked segment of epoxy resin cure system.
- The recycled carbon fibers were characterized by the monofilament tension, SEM, AFM, XPS. The epoxy resin in CFRP can be rapidly removed to obtain the clean carbon fibers in supercritical n-butanol when reaction temperature was higher than the initial pyrolysis temperature of CFRP. Supercritical fluids had a certain physical etching for the recycled carbon fibers. In comparison with the original carbon fiber, the content of N, O and Si on the recycled carbon fiber surface decreased, while the content of C increased, and the tension strength of the recycled carbon fiber can retain above 98 % of that of the original carbon fiber.
1 School of Mechanical Engineering, Hefei University of Technology, PR China
2 Institute of Coal Chemistry, Chinses Academy of Science, PR China
2 Institute of Coal Chemistry, Chinses Academy of Science, PR China
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