A Study on Low Energy Recycling Technique of Carbon Fibers

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Applied Chemistry

A Study on Low Energy Recycling Technique of Carbon Fibers-reinforced Epoxy Matrix Composites Kwan-Woo Kim, Jin-Soo Jeong, Kay-Hyeok An, and Byung-Joo Kim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02554 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Industrial & Engineering Chemistry Research 4th International Conference of Chemical Engineering & Industrial Biotechnology, 1st -2nd Aug 2018, Kuala Lumpur, Malaysia

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A Study on Low Energy Recycling Technique of Carbon

Fibers-reinforced

Epoxy

Matrix

Composites Kwan-Woo Kim1,2, Jin-Soo Jeong1,2,3, Kay-Hyeok An3,*, Byung-Joo Kim1,* 1

Research Laboratory for Multifunctional Carbon Materials, Korea Institute of Carbon Convergence Technology, Jeonju 54853, Korea

2

Department of Organic Materials & Fiber Engineering, Chonbuk National University, Jeonju 54896, Korea.

3

Department of Carbon and Nano Materials Engineering, Jeonju University, Jeonju 55069, Korea *

Corresponding author: [email protected] (B.-J. KIM)

Abstract. In this work, an energy-efficient recycling technique was developed using various reactive gases and pyrolysis periods. An activation energy study of the polymer matrix decomposition was employed in order to obtain optimal pyrolysis conditions as a function of reactive gases. From the results, the total energy consumption for matrix pyrolysis at low temperature region (under 400oC) was smallest with the carbon dioxide atmosphere. To remove all carbonaceous residue, a two-step pyrolysis technique was used, consisting of a low temperature decomposition (carbon dioxide at 400oC) and high temperature decomposition (super-heated steam method at 700oC). Properly recycled carbon fibers were obtained after more than 40 min of super-heated stream treatment. The mechanical strength of the recycled carbon fibers was measured using a single fiber tensile test, and it was found that all recycled fibers had over 80% of the strength of virgin carbon fibers.

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KEYWORDS: Recycled carbon fiber; Activation energy for decomposition; Carbon dioxide; Super-heated steam; Pyrolysis.

１ INTRODUCTION The carbon fibers-reinforced plastics (CFRPs) have begun to be used in a wide variety of products due to their high corrosion resistance, mechanical properties and light weight compared to metal parts and are being used in a wide range of applications such as aerospace, military, ships, sports and automotive industries.1-4 In particular, the CFRP material market for automobiles is a high-growth group in the material component sector with a growth rate exceeding 10% every year, due to the needs of environmental-friendly industries.5 Currently, the main application fields of carbon fibers (CFs) are sports, leisure and aerospace industries, and its application market is gradually expanding to civil engineering and construction and energy industries. In addition, annual demand for CF is approaching 40,000 tons.6 CFRPs are also widely used for aircraft wings and fuselage, especially in Boeing's B787, which was launched in 2011. 35 tons of CFRPs is used per aircraft. In the automobile industry, at the end of 2013, BMW first applied CFRPs to a mass-produced electric vehicle, the 'i3'.7-9 In a number of market reports,6,8,10,11 CFRPs are estimated to be increasingly used in automobiles. It is clear that the exponential growth of CFRP wastes in the full-scale production of CFRP parts is inevitable.12,13 For CFRPs made with thermoplastic resin, it is possible to separate the CF and resin by simple heat treatment.14,15 However, since most CFRPs use thermosetting resins, it is difficult to extract CFs from the matrix using heat treatment.13,16 Therefore, methods such as marine dumping, landfill, and incineration are mainly used for disposal.17 Sending conventional CFRPs to landfill should be avoided for both environmental and economic aspects. Recently, in Europe, recycling of more than 85% of the CFRPs used in automobiles is obligatory


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policy.18 Therefore, it is necessary to develop an energy-efficient recycling technology in order to expand CFRP markets. Additionally, the price of CF yarn is about $30/kg, and the energy required for manufacturing is about 150~300 kWh/kg.19 The CF recycling process is a method of selectively removing the polymer matrix by heat-treating the CFRP wastes, and it is known that recycled CF production (recovery) is possible at a cost of less than $15/kg since the cost of the precursor is not required during the fiber manufacturing process.6 Commercialized recycling techniques can be divided into two methods: chemical decomposition of the polymer matrix in organic solvents under supercritical conditions, and pyrolysis of the polymer matrix at high temperatures.13,20 The chemical decomposition method is mainly a technology for recovering high grade CF, which is relatively expensive. It is a harmful process because it uses an organic solvent.21-25 On the other hand, the pyrolysis method is an economical recycling technique which selectively decomposes only the polymer matrix in an atmosphere of nitrogen and oxygen.26-29 However, the conventional pyrolysis method consumes a great deal of energy to decompose the polymer matrix, about 4~6 h of decomposition time and 800~1000 oC of reaction temperature.30 In our previous report, we introduced a technique that successfully recovered CF with 2 h of decomposition time and 600oC of reaction temperature using steam instead of the well-known nitrogen atmosphere. At that time, it was also confirmed that the mechanical strength of the CF was more than 80% of virgin CF.31 In this study, we conducted research to reduce the pyrolysis time, to increase energy efficiency compared to existing processes. The total pyrolysis process was divided into two stages. In the low temperature stage, the rapid formation of carbon char was induced by the pyrolysis of polymer resin in various gas atmospheres. In the high temperature stage, steam was used as a reactive atmosphere for the efficient decomposition of the carbon char. In



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addition, a pyrolysis activation energy analysis for the polymer matrix was used to derive the optimal pyrolysis conditions.

２ Experiment details 2.1

Materials and preparation of standard CFRP waste

In order to obtain standard CFRP waste, normal CFRPs were prepared using the well-known vacuum-bag molding technique. The carbon fibers (CFs) used were polyacrylonitrile (PAN)based unidirectional CFs (T700, Toray Co., Japan). The epoxy resin that was used as a matrix was diglycidyl ether of bisphenol-A (DGEBA, YD-128, Kukdo Chem., Korea). The epoxide equivalent weight was 185-190 g/eq, and the viscosity was 11500-13500 cps at 25°C. Diaminodiphenylmethane (DDM, Tokyo Chem., Japan) was selected as a hardener, and methylethylketone (MEK, Daejung Chem., Korea) was used to reduce the high viscosity of the DGEBA. Formulation of the mixed resin was 100:26.5:100 (epoxy:DDM:MEK). The CFRP samples were prepared via hot pressing at 175°C, and the CFRPs had a resin content of 36% volume. And then the cured CFRPs were cut into sample pieces of 100 mm in length × 20 mm in width × and 3 mm thickness. 2.2

Recycling conditions

An alumina tube chamber having a length of 1,000 mm and an inner diameter of 80 mm was mounted in a SiC electrical resistance furnace (15 kW). The CFRP samples were then placed in a removable porcelain crucible and positioned in the center of the tube. For the first step of pyrolysis, in order to rapidly form carbon char from the CFRPs, the samples were heated at 5 °C/min to 400°C while CO2 gas was inserted at 200 ml/min into the chamber. In the second step, H2O was fed at a rate of 1.2 ml/min while the temperature was raised (at 5 °C/min) to 700°C and held for 20 to 100 min. Recycled CF samples (R-CFs) were obtained after slowly cooling down to room temperature. Table 1 lists the sample names according to their


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recycling conditions. Table 1. Recycling Condition and Sample Naming Recycling conditions Sample names Temp.

R-CF-C4-S7-20 R-CF-C4-S7-40 R-CF-C4-S7-60 R-CF-C4-S7-80 R-CF-C4-S7-100

STEP I Holding Atmosphere

400°C

0

CO2

Temp.

700°C

STEP II Holding Atmosphere 20 min 40 min 60 min H2O 80 min 100 min

*C4 means that first thermal treatments were done at 400oC in CO2 atmosphere. *S7-20~100 mean that second thermal treatments were done at 700oC in steam atmosphere for 20~100 min.

2.3

Decomposition activation energy analysis

The thermal decomposition behavior of the CFRP samples was studied in various reaction atmospheres (N2, Air, CO2, and H2O) at different heating rates (5, 10, 15, and 20 °C/min) up to 1000 °C using a thermogravimetric analyzer (TGA, TGA-50, SHIMADZU, Japan). Also, TGA results were used to calculate the pyrolyzed kinetics of the epoxy resin. The samples were purged with reaction gas to maintain an inert environment at a flow rate of 50 ml/min. Decomposition activation energy (Ea), integral procedure decomposition temperature (IPDT), polymer decomposition temperature (PDT), and maximum pyrolysis temperature (Tmax) were also obtained from the TGA results. 2.4

Morphology analysis

A normal scanning electron microscope (SEM, AIS2000C, Seron Tech. Inc., Korea) was employed to explore the morphology of the virgin CFs and R-CFs. Samples were first placed on a sample holder prior to being coated with a thin layer of gold microparticles. The base pressure of the analyzer chamber was about 5 × 10-5 Pa and the acceleration voltage was 20 kV.



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2.5

Mechanical properties

Single-fiber tensile tests were performed according to ASTM D1577. The mounting specimen was carefully aligned with the loading axis of the tensile testing machine (FAVIGRAPH, Textechno, Germany). The gauge length of the fiber was 20 mm and the draw-off clamp speed was set at 1 mm/min. The filament wassuspended between the grips of the testing machine. The CF was loaded until failure, and the force-displacement curve was recorded. At least 60 fibers were tested for each sample.

３ Experiment details 3.1

Characteristics of the pyrolysis conditions

The pyrolysis behavior of the epoxy resin in each reaction gas was measured using a thermogravimetric analyzer (TGA). The results are shown in Figure 1 along with the pyrolysis behavior diagram. Figure 1 (a) is a schematic diagram of pyrolysis behavior that can occur during the recycling of carbon fibers-reinforced plastics (CFRPs) waste. As the heat treatment progresses, oxidation behavior occurs in 4 stages. These are: decomposition of epoxy resin in CFRPs and the conversion to carbon char (step 1), the oxidation reaction of carbon char (step 2), surface cleaning by further oxidation of carbon char (step 3), and etching of the carbon fiber (CF) itself by excessive oxidation (step 4). Figure 1 (b) and (c) show the TGA analysis results for epoxy resin and CF under the various gas atmospheres, respectively. In Figure 1 (b), it was observed that rapid thermal decomposition of the epoxy resin started at about 400°C in each reaction gas atmosphere. In the atmosphere of nitrogen, which is a typical inert gas, rapid pyrolysis was observed up to about 500°C, and pyrolysis was terminated at about 550°C. At this time, about 18% of carbon char was observed and the R-CF had such a large amount of carbon char, it was impossible to get individual single fibers.


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With air and water vapor, decomposition behaviors similar to those of nitrogen were observed. However, unlike nitrogen, char was completely removed between 500 and 1000°C. With air, gentle char removal was observed, whereas in the case of water vapor, char was completely removed at about 750°C. From this, it was confirmed that the most active elimination reaction occurs in the steam atmosphere in the case of carbon char. Meanwhile, the most abrupt epoxy decomposition behavior was observed for carbon dioxide between 400 and 500°C. However, carbon char removal was observed to be minimal after 500°C, which is considered to be the formation point of carbon char. In Figure 1 (c), the CFs show rapid oxidation behavior at 600°C in the oxygen and carbon dioxide atmospheres, whereas almost no oxidation behavior was observed in the nitrogen and water vapor. From the above results, it can be concluded that the use of carbon dioxide for the rapid thermal decomposition of epoxy resin may lead to a decrease in the total oxidation time compared to the oxidative steam or air atmospheres. On the other hand, since the carbon dioxide is not oxidized after the formation of carbon char, it is necessary to design a two-step recycling process using steam, which has the highest oxidizing power for carbon char.

Figure 1. A schematic of CFRP decomposition behavior with increasing reaction temperature (a), TGA results of epoxy matrix with various gas atmosphere (b), TGA results of carbon fibers with various gas atmosphere (c); Step I~IV mean CFRP deformation during recycling. From the TGA results, a thermal analysis including integral procedure decomposition


Industrial & Engineering Chemistry Research

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4th International Conference of Chemical Engineering & Industrial Biotechnology, 1st -2nd Aug 2018, Kuala Lumpur, Malaysia

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temperature (IPDT) was conducted for a detailed analysis of the thermal properties during the thermal decomposition of the epoxy resin. Doyle32 proposed a formulation for the IPDT which was shown to be correlated to the volatile parts of polymeric materials, and it has been used to estimate the inherent thermal stability of polymeric materials.33 The IPDT was calculated using the following equations: IPDT ∗ ∙ ∗ 1 ∗ 2

∗ 3 , where A* is the area ratio of the total experimental curve defined by the total TGA thermogram; Ti is the initial experimental temperature, and Tf is the final the initial experimental temperature. Figure 2 shows a representation of S1, S2 and S3 for calculating A* and K*.

Figure 2. A schematic representation of S1, S2, and S3 for A* and K*. Table 2 shows the thermal properties based mainly on the results of the epoxy decomposition behavior in air, carbon dioxide, and steam atmospheres, where the pyrolysis reaction was most effective. PDT is the polymer decomposition temperature and Tmax is the maximum pyrolysis temperature. Table 2. Thermal Properties of Epoxy Decomposition with Various Gas Atmospheres. TGA

PDT1 (°°C)

Tmax2 (°°C)

A* • K*


IPDT3 (°°C)

yield (%)

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Epoxy (Air)

404.78

412.90

0.51

523.26

2.20

Epoxy (CO2)

398.11

407.65

0.63

643.34

8.95

Epoxy (H2O)

407.57

412.24

0.42

437.40

0.49

1

PDT: Polymer decomposition temperature Tmax: Maximum pyrolysis temperature 3 IPDT: Integral procedure decomposition temperature 2

In Table 2, the IPDT of epoxy resin showed the lowest value of 437.40°C when steam was used as the atmospheric gas, and the value was the highest, at 643.34°C, when carbon dioxide was used. This means that the thermal stability index for steam is the lowest in the pyrolysis behavior of epoxy resin from room temperature to 1000°C, and that steam is the most suitable pyrolysis gas for epoxy resin. However, this is the result of analysis from room temperature to 1000°C. As mentioned in Figure 1, different results can be observed in the low temperature region (below 400°C). Pyrolysis activation energy was calculated to design optimal pyrolysis conditions for the epoxy resin. In order to calculate pyrolysis activation energies, TGA analysis was performed for oxygen, carbon dioxide, and steam atmospheres at different heating rates. The results are shown in Figure 3. The correlation between Tmax and temperature was also obtained for each atmospheric gas and is shown in Figure 4.

Figure 3. TGA analyses of epoxy resins as functions of atmosphere gases and different heating rates to obtain Tmax.


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4th International Conference of Chemical Engineering & Industrial Biotechnology, 1st -2nd Aug 2018, Kuala Lumpur, Malaysia

-3.6 Epoxy (Air) Epoxy (CO2) Epoxy (H2O)

-3.8

2

ln (φ/Tmax )

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

-4.0

-4.2

-4.4 1.6

2.0

2.4

2.8

3.2

1000/T (1/°C)

Figure 4. Plots of ln[ϕ/Tmax2 ] vs. 1/Tmax for epoxy resin during pyrolysis by reaction gas. Figure 4 shows the influence of the various heating rates on the maximum pyrolysis temperatures (Tmax) of the epoxy resin with different reaction gases. As observed in the collected trends, the pyrolysis temperatures increase with the growth of the heating rate. Several methods have been developed to study the pyrolysis of polymers (Ozawa34, Kissinger35, Horowitz-Metzger36 and Avrami37 methods). In calculating the decomposition activation energy of polymer resin using TGA, Kissinger's equations are the most widely used. Therefore, the kinetic parameters of pyrolysis were obtained using Kissinger's equation, and the relationship between the heating rates and Tmax temperatures are listed in Table 3. Kissinger's method is represented by Eq. (4).

$ %! 1 # ln # ∙ 4 %! $ !" !" where ϕ represents the heating rate; Tmax is the maximum pyrolysis temperature; Ea is the ln

activation energy; A is the rate constant, and R is the gas constant order. Therefore, if ln(ϕ/Tmax2) is plotted against the 1/Tmax inclination curves, the activation energy can be determined. Table 3. Activation Energies Obtained by the Kissinger’s Equation Condition

Kinetic factor (Kissinger’s method) 1/Tmax('10 ) 3

Epoxy (Air)

2

In[фTmax ]

Heating rate (°C/min) 5 10 15 20 2.83

2.75

2.67

2.64

4.10

3.82

3.67

3.68

Ea (kJ/mol)

18.38


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1/Tmax('10 ) 3

Epoxy (CO2)

2.77

2.55

2.49

2.43

In[фTmax ]

4.11

3.89

3.73

3.75

1/Tmax('10 )

2.07

1.85

1.85

1.84

4.37

4.16

3.99

3.99

2

3

Epoxy (H2O)

2

In[фTmax ]

9.60

12.08

From the results shown in Table 3, the lowest decomposition activation energy (9.60 kJ/mol) was observed when the epoxy resin was pyrolyzed under a carbon dioxide atmosphere. This means that the energy efficiency for the pyrolysis of epoxy resin is highest when using carbon dioxide. However, decomposition activation energy is the value obtained using the Tmax, which represents the energy required when the epoxy resin is converted to carbon char, and does not mean the total amount of energy for the removal of the entire carbon char. Therefore, it is necessary to consider the value by including the carbon char removal part in the total energy efficiency, as IPDT confirmed in Table 2. It is also clear that a multi-stage pyrolysis process consisting of carbon dioxide and steam is necessary in view of the total energy value. 3.2

Physical characteristics of the recycled carbon fibers

Based on pyrolysis characteristics and decomposition activation energy results, the CFRP recycling process step was designed in this study. In the first step, the carbon dioxide atmosphere was used to rapidly decompose the epoxy resin of the CFRPs, and in the next step, the carbon char was removed using a steam atmosphere. In this way, R-CFs were obtained. The surface morphology and mechanical strength of the R-CFs in the two-step recycling process are shown in Figure 5 and Figure 6, respectively.



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Figure 5. Scanning electron microscopy micrographs of the virgin and recycled carbon fibers with varying pyrolysis conditions; (a) virgin carbon fibers, (b) R-CF-C4-S7-20, (c) R-CF-C4S7-40, (d) R-CF-C4-S7-60, (e) R-CF-C4-S7-80, (f) R-CF-C4-S7-100. Figure 5 exhibits SEM images of virgin and R-CF as a function of recycling conditions. A large amount of carbon char was observed on the surface of sample (R-CF-C4-S7-20) which was subjected to a high-temperature heat treatment in a steam atmosphere for 20 minutes after the low-temperature heat treatment in carbon dioxide. However, as the heat treatment time in steam atmosphere increased, carbon char was hardly observed beyond 60 min (S7-60). On the other hand, it was confirmed that the diameter of R-CF decreased slightly as the heat treatment time in the steam atmosphere was increased, compared with that of virgin CF.


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8

250

Tensile strength (GPa)

200 6 150 4 100 2

0

50

Tensile modulus (GPa)

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

0 Virgin CF R-CF-C4-S7-40 R-CF-C4-S7-80 R-CF-C4-S7-60 R-CF-C4-S7-100 R-CF-C4-S7-20

Figure 6. Single fiber tensile strength and modulus of virgin and R-CF with various recycling conditions. Figure 6 exhibits the single fiber mechanical properties of virgin and recycled CFs. The average tensile properties of virgin CF after desizing were about 3.95 GPa, and tensile modulus was 211.1 GPa. As the recycling process time increased, the tensile strength of the CF decreased in the range of 10-25%, and the tensile modulus finally increased by 2.5%. It can be concluded that tensile properties respond sensitively to the recycling process time. The slight reduction in tensile strength after recycling can be explained by the formation of surface damage due to heat treatment, and the increase in brittleness by the oxidation of amorphous domains. However, in the case of tensile modulus, due to the same effect, the amorphous structure on the surface is decomposed first, and the crystal structure is statistically increased, resulting in enhancement of the tensile modulus of the R-CFs. It was confirmed that the R-CF-C4-S7-40 retained a tensile strength of about 90% as compared with virgin CF, which was the best result in this work.

４ Conclusion In conclusion, an optimization process was designed using various gases to obtain clean recycled carbon fibers (R-CFs) from carbon fibers-reinforced plastics (CFRPs). CFRP wastes were pyrolyzed using a two-step recycling process with different recycling times. Recycling



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behaviors were affected by all conditions, including pyrolysis temperature, time, and reaction gases. The SEM image shows a clean carbon fiber (CF) surface with a completely-removedepoxy-resin after a specific pyrolysis time. Based on the results, it was favorable to shorten the recycling process time by injecting carbon dioxide in the first step and then feeding steam in the second step. The maximum tensile strength and tensile modulus of the R-CFs were observed to be 90% and 102.5%, respectively, compared to virgin CFs. According to the results, the conditions in this study may be appropriate for the recycling of CFRPs, since they can shorten and optimize the recycling process time, resulting in the development of a new energy-saving process. AUTHOR INFORMATION Corresponding Authors Byung-Joo Kim E-mail: [email protected] Tel: +82-63-219-3710 Kay-Hyeok An E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgement This study was supported by the “Institute of Civil military Technology Cooperation (Project No.17-CM-MA-24)” and Industrial Core Technology Development Program (Project


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No.10083615) funded by the Ministry of Trade, Industry Energy (MOTIE), Republic of Korea.

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A Study on Low Energy Recycling Technique of Carbon Fibers

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