Mechanism Identification and Kinetics Analysis of Thermal Degradation For

polymers

Article Mechanism Identiﬁcation and Kinetics Analysis of Thermal Degradation for Carbon Fiber/Epoxy Resin

Han Li, Nasidan Wang, Xuefei Han, Haoran Yuan and Jiang Xie *

College of Airworthiness, Civil Aviation University of China, Tianjin 300300, China; [email protected] (H.L.); [email protected] (N.W.); [email protected] (X.H.); [email protected] (H.Y.) * Correspondence: [email protected] or [email protected]

Abstract: For carbon fiber epoxy resin used in aerostructure, thermal degradation mechanism and kinetics play an important role in the evaluation of thermal response and combustion characteristics. However, the thermal decomposition process and mechanism are difficult to unify strictly due to the complexity of the components from different suppliers. In the present study, a product of carbon fiber epoxy resin made by AVIC (Aviation Industry Corporation of China) composite corporation is examined to identify its thermal degradation mechanism and pyrolysis products by measurements, including simultaneous thermal analysis, Fourier transform infrared spectroscopy and mass spectrometry, establish the kinetic model by Kissinger/Friedman/Ozawa/Coats-Redfern methods. The results show thermal degradation occurs in three steps under the inert atmosphere, but in four steps under air atmosphere, respectively. The first two steps in both environments are almost the same, including drying, carbon dioxide escape and decomposition of the epoxy resin. In the third step of inert atmosphere, phenol is formed, methane decreases, carbon monoxide basically disappears

and carbon dioxide production increases. However, in air, thermal oxidation of the carbonaceous residues and intermolecular carbonization are observed. Furthermore, thermal degradation reaction

Citation: Li, H.; Wang, N.; Han, X.; mechanism submits to the F4 model. These results provide fundamental and comprehensive support Yuan, H.; Xie, J. Mechanism for the application of carbon fiber epoxy resin in aircraft industry. Identification and Kinetics Analysis of Thermal Degradation for Carbon Keywords: thermal degradation; kinetics; gas analysis; carbon fiber/epoxy resin Fiber/Epoxy Resin. Polymers 2021, 13, 569. https://doi.org/10.3390/ polym13040569 1. Introduction Academic Editor: Bon-Cheol Ku Attributed to high resistance to corrosion, high specific strength and stiffness of FRP (Fiber Reinforced Polymer) material, its utilization in infrastructures and manufacturing is Received: 10 January 2021 more and more popular. For example, concrete slabs with GFRP(Glass Fiber Reinforced Accepted: 9 February 2021 Published: 14 February 2021 Polymer) bars and hallow composite reinforcing system [1] and a novel prefabricated FRP composite jacket with an easy-fit and self-locking mechanical joining system [2] have been

Publisher’s Note: MDPI stays neutral developed. In addition, carbon fiber epoxy resin has been widely used in the design and with regard to jurisdictional claims in manufacture of aircraft load-bearing and functional structures. However, thermal behavior published maps and institutional affil- of FRP is completely different from that of aluminum, because it can soften, decompose iations. and burn to release fumes, especially in fire, leading to threaten the safety of occupants. As a result, both the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) require us to consider fire protection, flame retardancy, combustion product toxicity, and other thermal issues of composite aircraft structures in airworthiness certification [3]. For this reason, the generic material combustion computational model, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. ThermaKin [4–7] and a toxicity assessment model [8–10] have been developed by FAA. In This article is an open access article addition, Tranchard [11] developed a material thermal response model based on SEMCEF distributed under the terms and for fire environments and validated it with the T700/M21 composite material used in conditions of the Creative Commons Airbus 350. Li [12,13] also developed a thermal response model for composites in the fire Attribution (CC BY) license (https:// environment based on ABAQUS. creativecommons.org/licenses/by/ Thermal response and combustion characteristics of carbon fiber epoxy resin in the 4.0/). fire environment mainly depends on the thermal decomposition of resin matrix. For this

Polymers 2021, 13, 569. https://doi.org/10.3390/polym13040569 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 569 2 of 16

reason, Quintiere [14] has conducted reaction kinetic parameter tests, thermophysical property tests and flame retardant performance tests on fiber epoxy resins conforming to the Boeing Material Specification. Tranchard [15,16] also carried out similar tests on T700/M21 to obtain the reaction kinetic parameters of the material and to establish the reaction kinetic model, and to explore decomposition mechanism and the combustion products. Reaction kinetic parameters, including activation energy, reaction order and the pre-exponential factor, are typically obtained by the thermal analysis. The combination of thermal analysis and infrared or mass spectrometry analysis can more accurately analyze the reaction process and mechanism. Considerable progress has been made in modeling of kinetic [17–23], including multi- or single-step kinetic models that can be utilized in thermal–mechanical model [24], lightning strikes [25], fire behavior [11], curing process design, fiber recycling [26,27], etc. Moreover, the complexity and even uncertainty of components of composites lead to a large amount of kinetics and mechanism research. Ahamad and Alshehri [28] studied the thermal degradation behavior of UF/epoxy resin blends (UFE) using a thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectra and mass spectrometry (MS) to identify decomposition products such as CO2, CO, H2O, HCN, HNCO and NH3, and established a reaction kinetic model using the Coats–Redfern method to clarify the main decomposition mechanisms. Li and Wu [29] studied the pyrolysis performance and flame-retardant mechanism of flame retardant EP with respect to the synthetic amino resin intumescent flame retardant (AIFR), showing that AIFR advances the pyrolysis of EP, reduces the activation energy of pyrolysis, increases the residual carbon and decreases the intensity of the absorption peak in the infrared spectrum. Liu et al. [30] investigated the curing reaction of bisphenol A formaldehyde phenolic epoxy resin (bisANER) with diaminodiphenyl ether (DDE) and the thermal degradation properties of its cured products, and calculated the activation energy of bis-ANER/DDE non-isothermal curing reaction by the Kissinger method and Ozawa–Flynn–Wall method, also determined the thermal degradation mechanism under different atmospheres. Huang et al. [31] used thermogravimetric mass spectrometry to investigate the pyrolysis reaction behavior of the amino-phenolic resin. The kinetic parameters of the pyrolysis reaction of amino-phenolic resin were obtained by the kinetic analysis of the experimental data using the Coats–Redfern integra- tion method, and the decomposition mechanism and pyrolysis products were clarified. Thomas et al. [32] used the standard Flynn–Wall–Ozawa (FWO) method to deduce the Arrhenius parameters from thermograms of biosourced substrates, hence, the energy of activation (Ea) was obtained. Balart et al. [33] studied systematic kinetics of the thermal degradation of recycled ABS in inert atmosphere with using Flynn–Wall–Ozawa (FWO), the Kissinger–Akahira–Sunose (KAS) and the Starink methods, and suggested the degradation process of ABS takes place in a single-step process. Franco-Urquiza et al. [34] performed the thermogravimetric analysis at 5, 10 and 20 ◦C/min allowed determining the degradation kinetic parameters based on the Friedman and Kissinger models for neat polyester and nanocomposites, which showed that heating rates promoted an increase in the temperature degradation. Krawiec et al. [35] used a testing methodology to determine the toxicometric indicators on belts made of classically used fabric–rubber composite material, and evalu- ated the degradation kinetics of the polymeric belts by (TGA). Zhang et al. [36] investigated the effect of basalt fiber content in HDPE matrix composites on the thermal decomposition process using dynamic thermogravimetric analysis and utilized improved Coats–Redfern (C-R), Flynn–Wall–Ozawa (F-W-O), Friedman and Kissinger methods to ascertain the specific apparent activation energy (Ea) of each component and composite material. Therefore, for whether a new material or material modification, it is necessary to carry out thermal analysis, identify the thermal behavior or mechanism of degradation and establish the kinetic model. For carbon fiber epoxy resin used in aerostructure, it usually consists of a fiber, epoxy monomer, curing agent, catalyst and flame retardant, with complex composition and content. Therefore, the thermal decomposition process and mechanism of products from different suppliers are difficult to unify strictly. In this paper, Polymers 2021, 13, 569 3 of 16

and establish the kinetic model. For carbon fiber epoxy resin used in aerostructure, it usually consists of a fiber, epoxy monomer, curing agent, catalyst and flame retardant, with complex composition and content. Therefore, the thermal decomposition process and mechanism of products from different suppliers are difficult to unify strictly. In this paper, for a new product of carbon fiber reinforced epoxy resin composite, BA3202, developed by AVIC (Aviation Industry Corporation of China) composite corporation for aerostructures, simultaneous thermal analysis (STA), FTIR spectra and mass spectrometry (MS) were conducted to obtain the reaction kinetic parameters, and a degradation kinetic model was established based on the Kissinger/Friedman/Ozawa/Coats–Redfern methods. More- over, the thermal decomposition products of the material were identified to clarify the reaction mechanism. Compared to the published study in literature, a more comprehensive and thorough insight into thermal behavior of CFRP used in aerostructures is presented, and detailed data can support thermal response modeling, combustion modeling and toxicity assessment of material in fire.

2. Materials and Methods Polymers 2021, 13, 569 3 of 16 Samples of carbon fiber reinforced epoxy resin matrix composite material used for aerostructures were provided by AVIC composite corporation (Beijing, China), in which matrix is mainly DGEBA. The preparation process was prepreg cutting-lay-up-autoclave molding–machiningfor a new product of. The carbon size fiber of sample reinforced was epoxy 4 mm resin × 4 composite,mm × 1.2 mm BA3202, with developed the lamina by thicknessAVIC (Aviation of prepreg Industry of 0.15 Corporation mm, shown of as China) Figure composite 1. Stacking corporation sequence for is aerostructures,[0/45/90/135]s withsimultaneous the fiber volume thermal of analysis54% and (STA),the density FTIR of spectra 1.55 g/cm and3. mass spectrometry (MS) were conductedThermal to analysis obtain theexperiments reaction kinetic were perfor parameters,med using and athe degradation simultaneous kinetic thermal model ana- was lyzerestablished (NETZSCH based STA on the449 Kissinger/Friedman/Ozawa/Coats–Redfern F3, NETZSCH, Selb, Germany), FTIR spectrometer methods. (BRUKER Moreover, VERTEXthe thermal 80, BRUKER decomposition,Billerica, products MA, USA) of the and material mass spectrometry were identified (NETZSCH to clarify theQMS403C, reaction NETZSCH,mechanism. Selb, Compared Germany) to to the investigate published the study mass in changes literature, (TG, a moreThermogravimetric), comprehensive de- and rivativethorough of insightmass changes into thermal (DTG, behavior Derivative of CFRP Thermogravimetric) used in aerostructures and gas is presented, composition and (FTIR/MS),detailed data referring can support to the test thermal standards response [37–40]. modeling, The sample combustion used in the modeling simultaneous and toxicity ther- malassessment analyzer ofwas material heated infrom fire. room temperature to 1100 °C at the heating rates of 5, 10, 20 and 40 °C/min, under the atmosphere of air and inert gas, respectively, with the purge flow 2. Materials and Methods rate set to 20 mL/min. The primary aim of the TG analyses was to know the thermal degra- dationSamples characteristics of carbon of materials, fiber reinforced and to obta epoxyin adequate resin matrix data composite to perform material kinetic analysis used for inaerostructures order to obtain were the Arrhenius provided byparameters. AVIC composite IR spectra corporation were recorded (Beijing, in the China), spectral in range which ofmatrix 4500–650 is mainly cm−1 with DGEBA. a resolution The preparation of 4 cm−1process and an wasaverage prepreg number cutting-lay-up-autoclave of scans of 16. The × × intensitiesmolding–machining. of 17 ions (m/ Thez = 14, size 16, of 17, sample 18, 26, was28, 30, 4mm 42, 44, 446, mm 56, 60,1.2 64 mmand with94) were the laminadeter- thickness of prepreg of 0.15 mm, shown as Figure1. Stacking sequence is [0/45/90/135] mined according to the previous work [16,41]. s with the fiber volume of 54% and the density of 1.55 g/cm3.

FigureFigure 1. 1. SpecimenSpecimen in in the the cruc crucibleible and and placement placement in in the the equipment. equipment.

Thermal analysis experiments were performed using the simultaneous thermal analyzer (NETZSCH STA 449 F3, NETZSCH, Selb, Germany), FTIR spectrometer (BRUKER VERTEX 80, BRUKER, Billerica, MA, USA) and mass spectrometry (NETZSCH QMS403C, NETZSCH, Selb, Germany) to investigate the mass changes (TG, Thermogravimetric), derivative of mass changes (DTG, Derivative Thermogravimetric) and gas composition (FTIR/MS), referring to the test standards [37–40]. The sample used in the simultaneous thermal analyzer was heated from room temperature to 1100 ◦C at the heating rates of 5, 10, 20 and 40 ◦C/min, under the atmosphere of air and inert gas, respectively, with the purge ﬂow rate set to 20 mL/min. The primary aim of the TG analyses was to know the thermal degradation characteristics of materials, and to obtain adequate data to perform kinetic analysis in order to obtain the Arrhenius parameters. IR spectra were recorded in the spectral range of 4500–650 cm−1 with a resolution of 4 cm−1 and an average number of scans of 16. The intensities of 17 ions (m/z = 14, 16, 17, 18, 26, 28, 30, 42, 44, 46, 56, 60, 64 and 94) were determined according to the previous work [16,41]. Polymers 2021, 13, 569 4 of 16

3. Results and Discussion 3.1. Thermal Degradation Figure2 shows the TG and DTG curves of carbon fiber reinforced epoxy resin matrix composite heated from room temperature to 1100 ◦C at 20 ◦C/min under air and inert atmosphere, respectively. Under inert atmosphere, the TG curve shows three mass loss stages, corresponding to three DTG curve peaks. The first stage of weight loss ranged from room temperature to 286 ◦C with the mass loss of 0.69%, and the maximum weight loss rate was 0.17%/min with the peak temperature of 244 ◦C. At this stage, physical escape of low- molecular-weight gases occurs, with the drying effect caused by the temperature rise from Polymers 2021, 13, 569 room temperature, where water was evaporated rapidly, justified by mass spectrometry in 5 of 16 Section 3.3.2. The same drying process is also observed in thermos-gravimetric analysis of epoxy resins by Ahamad [28] and Shao [42].

0.045 4 TG-N DTG-N 100 2 2 TG-Air DTG-Air 0.036 2 80

0.027 0 60

248.5℃ 0.018 -2 Weight (%) 40 Gram–Schmidt Deriv.Weight (%/min)

20 684℃ 745.5℃ 0.009 -4

401.7℃ 0 0.000 -6 0 200 400 600 800 1000 Temperature (℃) FigureFigure 2. 2.TG TG and and DTG DTG curves curves for the for carbon/epoxy the carbon/epoxy composite comp materialosite carried material out under carried inert andout under inert and ◦ airair atmosphere atmosphere at 20 at C/min.20 °C/min. The second stage ranged from 286 to 515 ◦C with the mass loss of 23.29%, where the maximum3.2. Kinetic weight Model loss rate was 4.99%/min at the peak temperature of 410 ◦C. This range is close toFigure the pyrolysis 3 shows interval the of TG epoxy and resin, DTG where curves the ether at different bond is broken heating to generate rates under BPA inert and air (Bisphenol-A),atmosphere. andTwo the inflection phenol comes points from appeared the further decompositionin the TG curves of BPA for and/or different the heating rates breakage of the C-C bond connecting the benzene ring after the breakage of unilateral ether bondunder on inert the epoxy atmosphere, resin [43], which which is an corresponded obvious degradation to the behavior two peaks of the resinof the matrix. DTG curve. As the Thetemperature weight loss rise of 4.54% rate inincreases, the third stagethe weight occurred lo inss a curve wide temperature shifts to the range higher from temperature re- 515gion to 1100[46].◦ However,C, which was in relatively air atmosphere, gentle, and the the maximumeffect of the loss temperature rate was 0.64%/min rise rate on weight- ◦ correspondingloss behavior to appears the peak temperaturemore complex of 537.4 dueC. to According the oxidative to the decomposition literature [44], the of carbon fiber. pyrolysis temperature of BPA epoxy resin is higher than 660 ◦C, so this stage was considered asThe further lower pyrolysis the temperature of the polymer rise [28 ].rate is, the longer the time is to reach the same temperature,Under and theso the air atmosphere, more sufficient the specimen the reaction was almost is. As completely a result, degraded the two withlower the heating rates (5 massand loss10 °C/min) of 90.64% make reached the at temperature 1100 ◦C. The existence at which of four the peaksweight in theloss DTG rate curve reached indi- the peak value catedlower, that so the that weight two loss more process peaks was could divided be into seen four below stages, 1000 and the °C. peak However, temperature under the heating was shown in Figure1. The characteristics of the ﬁrst weight loss stage in air atmosphere rates of 20 and 40 °C/min, the degradation was not completed within 1100 °C, and no was almost consistent with that in inert gas, which was a typical drying stage. The peak temperaturecorresponding in the peaks second were stage observed. was only slightly lower, but the pyrolysis interval was also similar to that under the inert atmosphere, indicating that the degradation of the resin matrix also occurred at this stage. Therefore, it showed that the reaction atmosphere has less inﬂuence on the degradation reaction of the resin matrix. However, the percentage of

weight loss and weight loss rate in the inert atmosphere were greater. The temperature range of the third stage was from 497 to 710 ◦C with the mass loss of 100 100 18.38%, the maximum weight loss rate of 2.53%/min and the peak temperature of 684 ◦ 5C. ℃/min 5 ℃/min 10 ℃/min 95 10 ℃/min 20 ℃/min 80 20 ℃/min 40 ℃/min 90 40 ℃/min 60 85

TG (%) TG (%) 80 40

75 20

70 0 0 250 500 750 1000 1250 0 200 400 600 800 1000 Temperature (℃) Temperature (℃)

(a) (b)

Polymers 2021, 13, 569 5 of 16

0.045 4 TG-N DTG-N 100 2 2 TG-Air DTG-Air 0.036 2 80

0.027 0 60

248.5℃ 0.018 -2 Weight (%) 40

Polymers 2021, 13, 569 Gram–Schmidt 5 of 16 Deriv.Weight (%/min)

20 684℃ 745.5℃ 0.009 -4

401.7℃ 0 0.000 -6 Thermal0 oxidation 200 of intermediate 400 600 transient 800 1000 carbonaceous residues [16], which is a process of intermolecular cross-linking,Temperature (℃ carbonization,) and further removal of small molecules, occurred at this stage [29]. The weight loss in the fourth stage ranged from 710 to 1100 ◦C Figure 2. TG and DTG curves for the carbon/epoxy composite material carried out under inert and with the mass loss of 52.65%; the maximum weight loss rate was 2.55%/min, and the air atmosphere at 20 °C/min. peak temperature was 745.5 ◦C. This was due to the reaction of the carbon fiber reinforced material3.2. Kinetic with Model oxygen and the nearly complete oxidative decomposition of the carbon fiber caused by prolonged heating, which was supported by literature [45]. Figure 3 shows the TG and DTG curves at different heating rates under inert and air 3.2.atmosphere. Kinetic Model Two inflection points appeared in the TG curves for different heating rates underFigure inert3 shows atmosphere, the TG which and DTG corresponded curves at to different the two heating peaks of rates the underDTG curve. inert andAs the airtemperature atmosphere. rise Two rate inflection increases, points the weight appeared loss curve in the shifts TG curves to the forhigher different temperature heating re- ratesgion under [46]. inertHowever, atmosphere, in air atmosphere, which corresponded the effect toof the the two temperature peaks of the rise DTG rate curve.on weight- As theloss temperature behavior appears rise rate more increases, complex the weightdue to the loss oxidative curve shifts decomposition to the higher of temperature carbon fiber. regionThe lower [46]. However,the temperature in air atmosphere, rise rate is, thethe effectlonger of the the time temperature is to reach rise the rate same on weight-tempera- lossture, behavior and so appearsthe more more sufficient complex the duereaction to the is. oxidativeAs a result, decomposition the two lower of heating carbon rates fiber. (5 Theand lower 10 °C/min) the temperature make the risetemperature rate is, the at longerwhich thethe timeweight is to loss reach rate the reached same temperature,the peak value andlower, so the so morethat two sufficient more peaks the reaction could be is. s Aseen a below result, 1000 the two °C. lowerHowever, heating under rates the (5 heating and 10rates◦C/min) of 20 make and 40 the °C/min, temperature the degradation at which the was weight not completed loss rate reached within the1100 peak °C, valueand no lower,corresponding so that two peaks more were peaks observed. could be seen below 1000 ◦C. However, under the heating rates of 20 and 40 ◦C/min, the degradation was not completed within 1100 ◦C, and no corresponding peaks were observed.

100 100 5 ℃/min 5 ℃/min 10 ℃/min 95 10 ℃/min 20 ℃/min 80 20 ℃/min 40 ℃/min 90 40 ℃/min 60 85

TG (%) TG (%) 80 40

75 20

Polymers 202170, 13, 569 6 of 16

0 0 250 500 750 1000 1250 0 200 400 600 800 1000 Temperature (℃) Temperature (℃)

(a) (b)

0 0

-2 5 ℃/min -2 -4 10 ℃/min 20 ℃/min 40 ℃/min -4

-6 5 ℃/min 10 ℃/min

DTG (%/min) DTG (%/min) 20 ℃/min -8 -6 40 ℃/min

-10 -8

-12 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature (℃) Temperature (℃)

FigureFigure 3. 3.( a(a)) TG TG curves curves and and (c )(c DTG) DTG curves curves under under inert inert atmosphere; atmosphere; (b) TG(b) curvesTG curves and (andd) DTG (d) DTG curves curves under under air atmosphere air atmos- forphere T300/Epoxy for T300/Epoxy composites composites at 5, 10, at 20 5, and10, 20 40 and◦C/min. 40 °C/min.

The different methods for solving the kinetic constants of reactions are based on the same basic theoretical formula derived from the Arrhenius formula, as shown in the following equation: dα −E = A ()α Afexp (1) dt RT , α ()()−− where = M iieMMM is the conversion degree, M i , M , M e are the initial mass, the mass of the specimen at the current moment, and the final mass at the end of the reaction, respectively. f ()α is a function of the kinetic mechanism, which is a function of the conversion degree, depending on the specific degradation mechanism. R is the ideal gas constant, with a value of 8.314 J/(mol K). In this paper, the kinetic parameters of the reaction were solved by the Kissinger [47] differential method, the Friedman [20] differential method and the Ozawa [22] integral method, respectively. These three methods did not require the degradation reaction mechanism of the material to obtain the activation energy. As to calculate the reaction order, it f ()αα=− (1 )n can be assumed that the mechanism function can be expressed as follows: . The Kissinger method [47] assumes the maximum reaction rate at the peak temperature. The Friedman method [20] assumes that the reaction kinetic constant A, EA and n are independent of the heating rate. While the Ozawa method [22] assumes that the conversion degree at the peak temperature at different heating rates is a constant (iso-conversion method). Additionally, the Coats–Redfern method was used to determine the reaction mechanism function. The data obtained under inert atmosphere are shown in the following Table 1.

Table 1. Kinetic parameters by Kissinger, Friedman and Ozawa methods.

α −1 Method EA /(kJ/mol) A /min 264.671 3.088 × 1024 Kissinger 216.508 3.14 × 1013 0.2 211.957 0.3 215.000 Friedman 1.343 × 1014 0.4 217.153 0.5 222.009

Polymers 2021, 13, 569 6 of 16

The different methods for solving the kinetic constants of reactions are based on the same basic theoretical formula derived from the Arrhenius formula, as shown in the following equation: dα −E = A exp A f (α), (1) dt RT

where α = (Mi − M)/(Mi − Me) is the conversion degree, Mi, M, Me are the initial mass, the mass of the specimen at the current moment, and the ﬁnal mass at the end of the reaction, respectively. f (α) is a function of the kinetic mechanism, which is a function of the conversion degree, depending on the speciﬁc degradation mechanism. R is the ideal gas constant, with a value of 8.314 J/(mol K). In this paper, the kinetic parameters of the reaction were solved by the Kissinger [47] differential method, the Friedman [20] differential method and the Ozawa [22] integral method, respectively. These three methods did not require the degradation reaction mechanism of the material to obtain the activation energy. As to calculate the reaction order, it can be assumed that the mechanism function can be expressed as follows: f (α) = (1 − α)n. The Kissinger method [47] assumes the maximum reaction rate at the peak temperature. The Friedman method [20] assumes that the reaction kinetic constant A, EA and n are independent of the heating rate. While the Ozawa method [22] assumes that the conversion degree at the peak temperature at different heating rates is a constant (iso-conversion method). Additionally, the Coats–Redfern method was used to determine the reaction mechanism function. The data obtained under inert atmosphere are shown in the following Table1.

Table 1. Kinetic parameters by Kissinger, Friedman and Ozawa methods.

-1 Method α EA/(kJ/mol) A/min 264.671 3.088 × 1024 Kissinger 216.508 3.14 × 1013 0.2 211.957 0.3 215.000 0.4 217.153 Friedman × 14 0.5 222.009 1.343 10 0.6 221.851 0.7 285.503 0.2 181.670 1.775 × 1014 0.3 194.279 9.12 × 1014 0.4 198.611 5.1 × 1014 Ozawa 0.5 202.817 2.89 × 1014 0.6 208.223 1.56 × 1014 0.7 225.263 7.54 × 1013

3.2.1. Kissinger Method Differentiation on both sides of (1) yields the following equation:

E β −E A = ( − )n−1 A 2 An 1 αm exp , (2) RTm RTm

where Tm is the peak temperature from the DTG curve, αm, is the peak conversion corre- dT sponding to Tm, which is obtained from the TG curve, and β = dt is the constant heating n−1 rate during the experiment. Kissinger considers n(1 − αm) is independent of β and its value is approximately equal to 1. The activation energy can be obtained from the above equation by logarithm and derivation: d ln β/T2 E m = − A , (3) d(1/Tm) R Polymers 2021, 13, 569 7 of 16

0.6 221.851 0.7 285.503 0.2 181.670 1.775 × 1014 0.3 194.279 9.12 × 1014 0.4 198.611 5.1 × 1014 Ozawa 0.5 202.817 2.89 × 1014 0.6 208.223 1.56 × 1014 0.7 225.263 7.54 × 1013

3.2.1. Kissinger Method Differentiation on both sides of (1) yields the following equation:

EEβ − − AA=−α n 1 An(1m ) exp  RT2 RT (2) mm, α where Tm is the peak temperature from the DTG curve, m , is the peak conversion cor- T β = dT responding to m , which is obtained from the TG curve, and dt is the constant heat- −α n−1 β ing rate during the experiment. Kissinger considers n(1m ) is independent of and its value is approximately equal to 1. The activation energy can be obtained from the above equation by logarithm and derivation: Polymers 2021, 13, 569 ()β 2 7 of 16 dT()ln / m E =− A (3) dT()1/ R m ,

The thermalthermal weightweight lossloss processprocess ofof carboncarbon ﬁberfiber reinforcedreinforced epoxyepoxy resinresin compositescomposites under inertinert atmosphere hashas twotwo mainmain stages.stages. AsAs shownshown inin Figure4 4,, the the intercept intercept and and slope slope data areare obtained.obtained.

-8.5

-9.0 Ar-step1 -9.5 Ar-step2 Air-step1 Air-step2

) -10.0 Linear fit of Ar-step 1 2 m

/T Linear fit of Ar-step 2

β Linear fit of Air-step 1

ln( -10.5 Linear fit of Air-step 2

-11.0

-11.5

1.5 1.6 1.7 1.8 1.9 2.0 3 1/T ×10 m β 2 2 Figure 4.4. Plots of lnln()β/T / mTm − −1/ 1/TmTmby by Kissinger Kissinger method method under under inert inert and and air air atmosphere. atmosphere.

The relationship between the pre-exponential factor A, and the slope and intercept of The relationship between the pre-exponential factor A, and the slope and intercept of the curve is as follows: the curve is as follows: ln(A/(−k)) = a, (4) ln()Ak / ()−= a When calculating the reaction order n, consider, the mechanism function to be ex-(4) pressed as f (α) = (1 − α)n and n is calculated by the following equation: When calculating the reaction order n, consider the mechanism function to be ex- αα=− n pressed as f () (1 )and n is calculatedn−1 by the following2RTm equation: − n(1 − αm) = 1 + (n − 1) , (5) EA The results are shown in the Table2 below:

Table 2. Kinetic parameters by Kissinger method.

Reactions Step 1 Step 2 Atmosphere Ar Air Ar Air Activation E /(kJ·mol−1) 264.671 301.884 216.508 313.307 energy A Pre-exponential A/min−1 3.088 × 1024 1.96 × 1028 3.14 × 1013 1.75 × 1021 factor Order n 0.94 0.93 0.96 0.945

3.2.2. Friedman Method Taking the logarithm of (1) gives the following equation

dα E ln β = ln(A) + n ln(1 − α) − A , (6) dT RT

dα dα where ln β dT = ln dt . dα The relationship between ln dt and 1/T at different conversion degrees was calculated as shown in Figure5. Polymers 2021, 13, 569 8 of 16

n−1 2RT −−nn()111α =+− ()m m E (5) A , The results are shown in the Table 2 below:

Table 2. Kinetic parameters by Kissinger method.

Reactions Step 1 Step 2 Atmosphere Ar Air Ar Air

Activation energy EA/(kJ·mol−1） 264.671 301.884 216.508 313.307 Pre-exponential factor A/min−1 3.088 × 1024 1.96 × 1028 3.14 × 1013 1.75 × 1021 Order n 0.94 0.93 0.96 0.945

3.2.2. Friedman Method Taking the logarithm of (1) gives the following equation dα E βα=+−−A ln ln()An ln ( 1 )  (6) dT RT , ddαα lnβ = ln where dT dt . dα The relationship between ln  and 1/T at different conversion degrees was cal- dt Polymers 2021, 13, 569 8 of 16 culated as shown in Figure 5.

-1.5 α=0.2 -2.0 α=0.3 α=0.4 α=0. -2.5 5 α=0.6 α=0.7 -3.0 /dt)

α -3.5 ln(d -4.0

-4.5

-5.0 1.35 1.40 1.45 1.50 1.55 1.60 1/T∗103 (k-1) Figure 5. Plots of ln dα -1/T by the Friedman method under inert atmosphere. dα dt Figure 5. Plots of ln -1/T by the Friedman method under inert atmosphere. dt Figure6 shows a line chart of the activation energy with conversion degree, which illustrates that the activation energy is not a constant, but changes as the thermal decompo- Figure 6 shows a line chart of the activation energy with conversion degree, which sition reaction proceeds and the conversion degree increases, showing an overall upward illustrates that the activation energy is not a constant, but changes as the thermal decom- Polymers 2021, 13, 569 trend. Therefore, the thermal decomposition reaction of carbon ﬁber reinforced epoxy9 of 16 position reaction proceeds and the conversion degree increases, showing an overall up- composite under inert gas is a multi-step. Furthermore, the predominant factor was found ward trend. Therefore, the thermal decomposition reaction of carbon fiber reinforced to be 1.34287 × 1014. epoxy composite under inert gas is a multi-step. Furthermore, the predominant factor was found to be 1.34287 × 1014. 290 E 280 A

270

260

250

240

230

220 Activation energy/(kJ/mol) 210

0.2 0.3 0.4 0.5 0.6 0.7 Degree of conversion Figure 6. ActivationActivation energy energy as as a afunction function of of fraction fractionalal mass mass loss loss by bythe the Friedman Friedman method method under under inertinert atmosphere. atmosphere.

3.2.3. Ozawa Ozawa Method Method E The activation energy EA waswas calculated calculated by by the the following following formula: formula: RR dd(log()logββ) E E==−− , (7) A A ( ) (7) 0.45670.4567ddT1/ (1/T ) , The relationship relationship between between log log β βandand 1/ 1/T wasT was calculated calculated for fordifferent different conversion conversion de- greesdegrees as asshown shown in inthe the following following Figure Figure 7: 7:

1.75 α=0.2 α=0.3 1.50 α=0.4 α=0.5 α=0.6 1.25 α=0.7 ) β

Log( 1.00

0.75

0.50 1.35 1.40 1.45 1.50 1.55 1.60 1/T∗103 (k-1)

β Figure 7. Plots of log -1/T by the Ozawa method under inert atmosphere.

The pre-exponential factor was calculated by the following formula [48,49]: logA =+ logβ logEERTRT + 0.434 / −− log 2log AA , (8) At the same conversion degree, yielding the values of the corresponding pre-exponential factor for different heating rates, as shown in Figure 8. And the mean values were finally obtained. The activation energies obtained by Ozawa’s method were similar to those obtained by Friedman’s method, which verified the accuracy of the results.

Polymers 2021, 13, 569 9 of 16

290 E 280 A

270

260

250

240

230

220 Activation energy/(kJ/mol) 210

0.2 0.3 0.4 0.5 0.6 0.7 Degree of conversion Figure 6. Activation energy as a function of fractional mass loss by the Friedman method under inert atmosphere.

3.2.3. Ozawa Method

The activation energy EA was calculated by the following formula: d ()log β =− R EA (7) 0.4567dT (1/ ) , The relationship between log β and 1/T was calculated for different conversion de- Polymers 2021, 13, 569 grees as shown in the following Figure 7: 9 of 16

1.75 α=0.2 α=0.3 1.50 α=0.4 α=0.5 α=0.6 1.25 α=0.7 ) β

Log( 1.00

0.75

0.50 1.35 1.40 1.45 1.50 1.55 1.60 1/T∗103 (k-1)

Figure 7. Plots ofβ log β-1/T by the Ozawa method under inert atmosphere. Figure 7. Plots of log -1/T by the Ozawa method under inert atmosphere. The pre-exponential factor was calculated by the following formula [48,49]: The pre-exponential factor was calculated by the following formula [48,49]: log A = log β + log E + 0.434E /RT − log R − 2 log T, (8) logA =+ logβ logEERTRT +A 0.434 /A −− log 2log AA , (8) At the same conversion degree, yielding the values of the corresponding pre-exponential Polymers 2021, 13, 569 At the same conversion degree, yielding the values of the corresponding pre-expo-10 of 16 factor for different heating rates, as shown in Figure8. And the mean values were ﬁnally nentialobtained. factor for The different activation heating energies rates, obtainedas shown byin Figure Ozawa’s 8. And method the mean were similarvalues were to those finallyobtained obtained. by Friedman’sThe activation method, energies which obtained veriﬁed by the Ozawa’s accuracy method of the results. were similar to those obtained by Friedman’s method, which verified the accuracy of the results.

230 1x1015

E 9x1014 220 A A 8x1014

14 210 7x10 (kJ/mol) A 6x1014

200 14 5x10

14 190 4x10 3x1014

180 Pre-exponentialA factor 2x1014

E energy Activation 1x1014 170 0.2 0.3 0.4 0.5 0.6 0.7 Degree of conversion α

FigureFigure 8. 8. ActivationActivation energy energy and pre-exponential factor as functionfunction ofof fractionalfractional massmass lossloss byby Ozawa Ozawamethod method under inertunder atmosphere. inert atmosphere. 3.2.4. Coats–Redfern Method 3.2.4. Coats–Redfern Method Coats and Redfern assumed the function of the kinetic mechanism is f (α) = (1 − α)n, f ()αα=− (1 )n and derivedCoats and the Redfern following assumed expression: the function of the kinetic mechanism is , and derived the following expression: α AR 2RT EA ln α = ln 1− − E , (9) ln( 2 ) =−− lnAR 1 2 RT A T T 2 βEAβ EERTEA RT (9) AA, where α in the logarithmic term of the left equation is the result of integrating 1/ f (α) to α α 1/f ()α whereand then ignoring in the logarithmic higher-order term terms of underthe left the equation condition is ofthe low result conversion of integrating rate after poly- α tonomial and expansion. then ignoring In order higher-order to study the terms kinetic un modelder the of thermalcondition decomposition, of low conversion assuming rate afterthat thepolynomial mechanism expansion. function In is order unknown, to study the the alternative kinetic model term to ofα thermalin the logarithmic decomposition, term α assumingis the integral that formthe mechanismF(α) of 1/ ffunction(α), and is considering unknown, thatthe alternative2RT approaches term zero,to the in formulathe log- EA F ()α 1/f ()α 2RT arithmicis as follows: term is the integral form of , and considering that EA ap- F(α) AR E proaches zero, the formula is as follows:= − A ln 2 ln , (10) T βEA RT F ()α =−AR EA Different mechanism modelsln were2 established ln β by this method, as shown in Table(10)3. T ERTA  ,

Different mechanism models were established by this method, as shown in Table 3.

Table 3. Thirteen kinds of commonly used pyrolysis kinetics models.

Mechanism F ()α Reaction Mechanism −−()α F1 ln 1 One-dimensional random nuclear reaction 1 F2 1−α Two-dimensional random nucleation reaction 1 F3 ()1−α 2 Three-dimensional random nuclear reaction

1n− ()111−−α ()n − th Fn  n order −−()α 1/2 A2 ln 1 Avrami random nucleation n = 2 −−()α 1/3 A3 ln 1 Avrami random nucleation n = 3 −−()α 1/4 A4 ln 1 Avrami random nucleation n = 4 1/2 R2 11−−()α Phase boundary reaction, cylindrical symmetry 1/3 R3 11−−()α Phase boundary reaction, spherical symmetry 2 D1 α One-dimensional diffusion ()()−−+ααα D2 1ln1 Two-dimensional diffusion, cylindrical symmetry, (Valensi equation)

Polymers 2021, 13, 569 10 of 16

Table 3. Thirteen kinds of commonly used pyrolysis kinetics models.

Mechanism F(α) Reaction Mechanism

F1 − ln(1 − α) One-dimensional random nuclear reaction 1 F2 1−α Two-dimensional random nucleation reaction 1 F3 2 Three-dimensional random nuclear reaction (1−α) h 1−n i Fn (1 − α) − 1 /(n − 1) nth order

1/2 A2 [− ln(1 − α)] Avrami random nucleation n = 2 1/3 A3 [− ln(1 − α)] Avrami random nucleation n = 3 1/4 A4 [− ln(1 − α)] Avrami random nucleation n = 4 1/2 R2 1 − (1 − α) Phase boundary reaction, cylindrical symmetry 1/3 R3 1 − (1 − α) Phase boundary reaction, spherical symmetry 2 D1 α One-dimensional diffusion Two-dimensional diffusion, cylindrical D (1 − α) ln(1 − α) + α 2 symmetry, (Valensi equation)

h i2 Three-dimensional diffusion, spherical D 1/3 3 1 − (1 − α) symmetry Jander equation Three-dimensional diffusion, spherical D (1 − 2α/3) − (1 − α)2/3 4 symmetry, Ginstling–Brounshtein equation

F(α) − The ln T2 1/T curves for the conversion rates from 0.2 to 0.7 at different heating rates were made to obtain the activation energy values. Pearson’s correlation coefﬁcients were calculated to measure the correlation between the variables, as shown in Table4. Comparing the activation energy values in Table4 with those calculated by the three kinetic methods in Table1. The average E A of F4 is the closest to the previous calculation, which was 211.691 kJ/mol. It is concluded that the pyrolysis reaction of carbon ﬁber reinforced epoxy resin composite in the inert atmosphere is the closest to that predicted 4 by the F4 model. The expression of the mechanism function is: (1 − α) . After averaging the activation energies and pre-exponential factors of the four heating rates, the reaction mechanism model was obtained as follows dα −211691 = 8.26 × 1011 exp (1 − α)4, (11) dt RT

Table 4. Correlation coefficients corresponding to various mechanism function for the plot ln(F/T2)-1/T at different heating rates.

Activation Energy β = 5 ◦C/min β = 10 ◦C/min β = 20 ◦C/min β = 40 ◦C/min R (kJ/mol)

F1 85.9009 90.2059 93.7321 97.1478 −0.98982 F2 46.0787 48.1592 50.0159 52.5525 −0.98913 F3 103.327 107.678 111.615 116.918 −0.99107 F4 199.137 207.792 215.374 224.461 −0.99922 A2 37.3654 39.4233 41.0745 42.6674 −0.98643 A3 21.1869 22.4958 23.52189 24.5073 −0.98121 A4 13.0976 14.0320 14.7456 15.4273 −0.97261 R2 72.4191 76.1849 79.2192 81.9901 −0.98259 R3 76.7291 80.6684 83.8604 86.8359 −0.98531 D1 132.311 139.064 144.484 149.150 −0.97677 D2 147.134 154.499 160.467 165.816 −0.98217 D3 164.628 172.696 179.304 185.485 −0.98728 D4 152.932 160.530 166.711 172.335 −0.98407 Polymers 2021, 13, 569 11 of 16

3.3. Gas Phase Analysis 3.3.1. FTIR Analysis The Gram–Schmidt spectrum under inert atmosphere in Figure2 represented the FTIR absorption intensity of the escaping gas during pyrolysis of the material, showing that there were only two peaks at 425 ◦C and 557 ◦C, respectively, which were delayed compared to the corresponding DTG curve. This was due to the time delay between gas detection and gas generation in the FTIR spectrometer. The larger peak at 425 ◦C indicated that the amount of gas escaping at this stage was higher, and had a higher infrared extinction coefﬁcient. Figure9 shows the infrared spectral data of the gas at the characteristic temperatures determined by the DTG curve. At the characteristic temperature of 244 ◦C, which was −1 the ﬁrst stage of mass loss, the products were carbon dioxide (CO2: 2300–2400 cm ) Polymers 2021, 13, 569 and water (H O: 3500–4000 cm−1). The same phenomenon is observed by Ahamad12 of [28 16] 2 in the study of urea-formaldehyde/epoxy resin blends. Thus, this stage was not only a drying stage, but also a physical escape of small molecule gases. Obvious degradation 410behavior °C. Since of thethe resinstretching matrix vibration occurred of in saturated the second hydrocarbon stage, corresponding C-H is usually to theclose infrared to the spectral data at 410 ◦C. Since the stretching vibration of saturated hydrocarbon C-H is frequency absorption of 3000 cm−1, it is speculated that there was methane gas (CH4) ac- −1 cordingusually to close the tothree the peaks frequency in the absorption range of 2900–3170 of 3000 cm cm−1,: it3045 is speculated cm−1, 3016 thatcm−1 thereand 2970 was −1 −1 cmmethane−1. In addition gas (CH to4) water according and tocarbon the three dioxide, peaks carbon in the monoxide range of 2900–3170 (CO: 2000–2200 cm : 3045 cm−1 cm with , 3016 cm−1 and 2970 cm−1. In addition to water and carbon dioxide, carbon monoxide double peaks at 2048 cm−1 and 2070 cm−1) and phenol (main peaks of C6H5OH: 3650, 1608, (CO: 2000–2200 cm−1 with double peaks at 2048 cm−1 and 2070 cm−1) and phenol (main 1510, 1260 and 1176 cm−1) were also observed. The above products were considered to be peaks of C H OH: 3650, 1608, 1510, 1260 and 1176 cm−1) were also observed. The above the chain scission6 5 and depolymerization reactions of BPA epoxy resins [31], where CO products were considered to be the chain scission and depolymerization reactions of BPA and CO2 are mainly derived from the ether group (R-O-R’), methyl group (-CH3), meth- epoxy resins [31], where CO and CO2 are mainly derived from the ether group (R-O-R’), ylene group (-CH2) and other carbon-containing functional groups. At 537 °C, carbon methyl group (-CH ), methylene group (-CH ) and other carbon-containing functional monoxide almost disappeared3 and turned into2 carbon dioxide, while the production of groups. At 537 ◦C, carbon monoxide almost disappeared and turned into carbon dioxide, the other gas products decreased compared with that at 410 °C. while the production of the other gas products decreased compared with that at 410 ◦C.

C H OH 244 ℃ 6 5 410 ℃ 537 ℃

CH4 C6H5OH CO H2O Absorbance

H2O

CO2

4500 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) FigureFigure 9. 9. SpectraSpectra collected collected at at 244, 410 and 537 ◦°CC of the degradation productsproducts ofof compositescomposites released re- leasedduring during TGA (20 TGA◦C/min–inert (20 °C/min–inert atmosphere). atmosphere).

3.3.2.3.3.2. MS MS Analysis Analysis TheThe following following gas gas molecules molecules were were detected detected by by mass mass spectrometry spectrometry at ata heating a heating rate rate of ◦ 20of °C/min 20 C/min under under inert inert atmosphere: atmosphere: CH2 CH (m2/z( m= /14),z = CH 14),4 CH(m/4z (=m 16),/z = H 16),2O ( Hm2/O(z = m18),/z =C2 18),H2 (Cm2/Hz =2 (26),m/ zCO= 26), (m/ COz = 28), (m/ zC=2H 28),4 (m C/z2 H= 428),(m /Cz3H=6 28), (m/ Cz 3=H 42),6 (m CO/z =2 ( 42),m/z CO= 44),2 ( mC/3Hz =8 ( 44),m/z C= 344)H8 and(m/ zphenol= 44) andC6H phenol5OH (m C/z6 =H 94).5OH (m/z = 94). AccordingAccording to the order ofof magnitudemagnitude of of ion ion current current intensity, intensity, the the classiﬁcation classification analysis anal- ysiswas was carried carried out out from from high high to low.to low. As As shown shown in in Figure Figure 10 10,, thethe leftleft ordinateordinate shows the the massmass spectra spectra of of gas gas products products with with a a mass-to-charge mass-to-charge ratio ratio of of 28, 28, which which was was supposed supposed to to be be COCO and/or and/or C C2H2H4. 4Additionally,. Additionally, there there were were three three peaks peaks in in the the curve, curve, which which respectively respectively occurred at 125 °C, 437 °C and 920 °C. The peak with the largest ion current intensity appeared between 300 and 500 °C, corresponding to the main decomposition peak in the DTG analysis, which was presumed to be the precipitation of CO gas, also observed by Tranchard [16] and Ahamad [28] and related to the certain amount of oxygen-containing groups in the polymer. The mass spectrum of phenol with the mass-to-charge ratio of 94 was shown on the right ordinate. Its precipitation range was 300–750 °C, which covered the second and third mass loss stages, also corresponded to the peaks of infrared spectrum at 410 °C and 537 °C.

Polymers 2021, 13, 569 13 of 16

4.20 3.0

4.18 CO/C H 2 4 2.5 4.16

Polymers 2021, 13, 569 4.14 12 of 16 2.0 A) A) 4.12 -8 -12 4.10 1.5

occurred4.08 at 125 ◦C, 437 ◦C and 920 ◦C. The peak with the largest ion current intensity 1.0 4.06 ◦ Intensity (10

appeared between 300 and 500 C, corresponding to theIntensity (10 main decomposition peak in the 4.04 C H OH DTG analysis, which was presumed to be the6 5 precipitation0.5 of CO gas, also observed by Tranchard4.02 [16] and Ahamad [28] and related to the certain amount of oxygen-containing groups4.00 in the polymer. The mass spectrum of phenol0.0 with the mass-to-charge ratio of 0 125 250 375 500 625 750 875 1000 1125 94 was shown on the right ordinate. Its precipitation range was 300–750 ◦C, which covered Polymers 2021, 13, 569 Temperature (℃) 13 of 16 the second and third mass loss stages, also corresponded to the peaks of infrared spectrum atFigure 410 ◦10.C andIntensity 537 ◦curvesC. of m/z = 28 and 94 degradation products of composite released during TGA (20 °C/min-inert gas). 4.20 3.0 For the detection result of water molecules in Figure 11, a trough appeared before 4.18 CO/C H 2 4 2.5 250 °C,4.16 which corresponded to the first peak in the DTG curve, verifying that this was the

drying4.14 stage. In addition, due to the pyrolysis reaction of the resin, the peak of water mol- 2.0 A) eculesA) 4.12 appeared at 406 °C, which was ahead of the precipitation of CO, indicating that the -8 -12 decomposition4.10 of the epoxy resin would preferentially1.5 produce water molecules, but

mainly4.08 precipitated CO. At this stage, the water produced by the thermal decomposition 1.0 of epoxy4.06 resin mainly come from the breaking of the bonds of -OH functional groups. Intensity (10 For the gas mass-to-charge ratio of 44, CO2, N2O Intensity (10 and C3H8 were considered. How- 4.04 C H OH ever, the peak of the curve occurred at 410 °C,6 5ahead0.5 of 435 °C, which was the precipitation 4.02 temperature of the CO peak. Considering the sequence of reactions, CO mainly comes 4.00 0.0 from the0 reforming 125 250 of 375 the 500 C-O 625 group 750 in 875 the 1000bisphenol 1125 A epoxy resin, and then combined with oxygen ions to formTemp COerature2. Therefore, (℃) it was speculated that the m/z = 44 mass spec- FigureFiguretrum 10.curve 10. IntensityIntensity was mainly curves curves ofrepresented of mm/z/ =z 28=28 and and by 94 94the degradation degradation C3H8 gas products molecule. products of ofcompositeSince composite the releasedreaction released during was during car- TGATGAried (20 (20out °C/min-inert◦ underC/min-inert an inert gas). gas). atmosphere, oxygen ions were supplied only by the material itself, which was also verified in the order of magnitude. It was speculated that the relatively smallForFor increasement the the detection detection of resultthe result amplitude of of water water of molecules molecules this curve in inafterwards Figure Figure 11,11 might ,a a trough trough correspond appeared appeared to beforethe before for- ◦ 250250mation °C,C, which of which CO corresponded2.corresponded to tothe the first ﬁrst peak peak in the in the DTG DTG curve, curve, verifying verifying thatthat this thiswas wasthe dryingthe dryingThe stage. peak stage. In temperature addition, In addition, due of to dueC 2theH2 to gaspyrolysis the molecules pyrolysis reaction was reaction ofclose the of toresin, thethe resin,temperaturethe peak the of peak water of ofthe watermol- third eculesmoleculespeak inappeared the appeared DTG at curve,406 at °C, 406 which which◦C, which was was supposed ahead was ahead of theto of be precipitation the the precipitation product of of CO, the of CO,indicating second indicating step that of thatthe de- decompositionthecomposition. decomposition It of can the of be theepoxy verified epoxy resin that resin would the would de prefcomposition preferentiallyerentially ofproduce producecarbon waterfiber water reinforced molecules, molecules, epoxy but but mainlymainlyresin matrix precipitated precipitated composite CO. CO. Atunder At this this inertstage, stage, atmosphere the the water water produced producedexhibits atby by least the the thermal thermaltwo steps decomposition decomposition (a major step ofofand epoxy epoxy a secondary resin resin mainly mainly step). come come from from the the breaki breakingng of of the the bonds bonds of of -OH -OH functional functional groups. groups. For the gas mass-to-charge ratio of 44, CO2, N2O and C3H8 were considered. How- ever, the peak of the curve occurred at 410 °C, ahead of 435 °C, which was the precipitation 1.2x10-10 temperature of the CO peak. Considering the sequence of reactions, CO mainly comes m/z=18 CH4 from the reforming-10 of the C-O group in the bisphenol A epoxy resin, and then combined 1.0x10 C2H2 with oxygen ions to form CO2. Therefore, it was speculated that the m/z = 44 mass spec- CO2/N2O/C3H8 -11 trum curve8.0x10 was mainly represented by the C3H8 gas molecule. Since the reaction was car- H2O ried out under an inert atmosphere, oxygen ions were supplied only by the material itself, -11 which6.0x10 was also verified in the order of magnitude. It was speculated that the relatively m/z=26 small increasement of the amplitude of this curve afterwards might correspond to the for- Intensity (A) 4.0x10-11 mation of CO2. m/z=17 The2.0x10 peak-11 temperaturem/z=44 of C2H2 gas molecules was close to the temperature of the third peak in the DTG curve, which was supposed to be the product of the second step of de- m/z=16 composition. It can be verified that the decomposition of carbon fiber reinforced epoxy 0 250 500 750 1000 resin matrix composite under inert atmosphere exhibits at least two steps (a major step Temperature (℃) and a secondary step). FigureFigure 11.11. IntensityIntensity curvescurves ofofm m//z = 16, 18, 26 andand 4444 degradationdegradationproducts products of of the the composite composite released re- duringleased during TGA (20 TGA◦C/min-inert (20 °C/min-inert atmosphere). atmosphere). 1.2x10-10 m/z=18 CH For the gas mass-to-charge ratio of 44, CO4 ,N O and C H were considered. However, -10 C H2 2 3 8 1.0x10 ◦ 2 2 ◦ the peak of the curve occurred at 410 C, ahead of 435 C, which was the precipitation CO2/N2O/C3H8 temperature-11 of the CO peak. Considering the sequence of reactions, CO mainly comes from 8.0x10 H O the reforming of the C-O group in the bisphenol2 A epoxy resin, and then combined with oxygen6.0x10 ions-11 to form CO2. Therefore, it was speculated that the m/z = 44 mass spectrum m/z=26

Intensity (A) 4.0x10-11 m/z=17 2.0x10-11 m/z=44 m/z=16 0 250 500 750 1000 Temperature (℃) Figure 11. Intensity curves of m/z = 16, 18, 26 and 44 degradation products of the composite released during TGA (20 °C/min-inert atmosphere).

Polymers 2021, 13, 569 13 of 16

curve was mainly represented by the C3H8 gas molecule. Since the reaction was carried out under an inert atmosphere, oxygen ions were supplied only by the material itself, which was also verified in the order of magnitude. It was speculated that the relatively small increasement of the amplitude of this curve afterwards might correspond to the formation of CO2. The peak temperature of C2H2 gas molecules was close to the temperature of the third peak in the DTG curve, which was supposed to be the product of the second step of Polymers 2021, 13, 569 14 of 16 decomposition. It can be verified that the decomposition of carbon fiber reinforced epoxy resin matrix composite under inert atmosphere exhibits at least two steps (a major step and a secondary step). 3.3.3. Effect of the Heating Rate on Gas Phase Products 3.3.3.The Effect increase of the of Heating the heating Rate rate on Gas leads Phase to the Products phenomenon of thermal hysteresis, which is specificallyThe increase manifested of the heating as the ratepeak leads temperatures to the phenomenon of CO, H2O, of thermalCO2 and hysteresis, CH4 ion flow which all moveis specifically toward the manifested high temperature as the peak region temperatures with the increase of CO, H of2O, the CO heating2 and rate. CH4 Theion flowpro- ductionall move of toward H2O, CO the2 highand CH temperature4 all increase region with with the theincrease increase of heating of the heatingrate as shown rate. The in Figureproduction 12, which of H2 indicatesO, CO2 and that CH faster4 all temperature increase with rise the promotes increase the of heating reaction. rate The as produc- shown tionin Figure of CO 12 had, which a relatively indicates obvious that faster weak temperature value at the rise low promotes heating rate, the reaction.but had little The changeproduction at the of heating CO had rates a relatively of 10 °C, obvious 20 °C weakand 40 value °C/min. at the low heating rate, but had little change at the heating rates of 10 ◦C, 20 ◦C and 40 ◦C/min.

4.4 22

(454.7, 4.35024) 20 4.2 (410.36479, 4.26732) 18 5 ℃/min (434.63196, 4.15923) 16 10 ℃/min 4.0 20 ℃/min 14 A) A) 5 ℃/min 40 ℃/min -8 -11 10 ℃/min 12 3.8 20 ℃/min 40 ℃/min 10 3.6 8 Intensity (10 (377.2, 3.47334) Intensity (10 6 3.4 4

2 3.2 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Temperature (℃) Temperature (℃) (a) (b)

5 30 5 ℃/min 10 ℃/min 4 25 20 ℃/min 5 ℃/min 40 ℃/min 10 ℃/min 20 ℃/min A) 20 A) 3 -12 -11 40 ℃/min

15 2

10 Intensity (10 Intensity (10

1 5

0 0 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Temperature (℃) Temperature (℃) (c) (d)

FigureFigure 12.12. ((aa)) CO,CO, ((bb)H) H22O, (c) CO2 andand ( d)) CH CH44 releaserelease against against temperature temperature with with th thee heating heating rate rate during during pyrolysis.

4. Conclusions Thermal degradationdegradation ofof aa newnew product product of of carbon carbon ﬁber fiber reinforced reinforced epoxy epoxy resin resin compos- com- posite,ite, namely namely BA3202, BA3202, occurs occurs in three in three steps step unders under inert atmosphere,inert atmosphere, but in but four in steps four under steps under air atmosphere, respectively. The first two steps were almost the same. The first step is the physical changes, mainly water drying and carbon dioxide escape. The second step is the decomposition of the epoxy resin matrix, and the products are mainly methane, water, carbon monoxide, carbon dioxide and phenol. The third step in the inert atmosphere is the further pyrolysis of the polymer, in which phenol is formed, methane decreases, carbon monoxide basically disappears and carbon dioxide production increases. However, in air, the thermal oxidation of the carbonaceous residues and the intermolecular carbonization are observed. Furthermore, the last step in air is the oxidative decomposition of the carbon fibers. In addition, it is concluded that faster temperature rise can promote the reaction by the experimental comparison in different heating rates. These

Polymers 2021, 13, 569 14 of 16

air atmosphere, respectively. The first two steps were almost the same. The first step is the physical changes, mainly water drying and carbon dioxide escape. The second step is the decomposition of the epoxy resin matrix, and the products are mainly methane, water, carbon monoxide, carbon dioxide and phenol. The third step in the inert atmosphere is the further pyrolysis of the polymer, in which phenol is formed, methane decreases, carbon monoxide basically disappears and carbon dioxide production increases. However, in air, the thermal oxidation of the carbonaceous residues and the intermolecular carbonization are observed. Furthermore, the last step in air is the oxidative decomposition of the carbon fibers. In addition, it is concluded that faster temperature rise can promote the reaction by the experimental comparison in different heating rates. These results provide a comprehensive and thorough understanding into the thermal behavior of CFRP in the thermal environment or fire, and can be used to support material selection and airworthiness certification of aviation structure. Moreover, the activation energy value obtained by mechanism F4 is closed to that of the Kissinger/Friedman/Ozawa’s method. Therefore, it can be inferred that the thermal degradation reaction mechanism of CFRP for this study obeys the F4 model, which indicates the order of reaction is 4. As a result, the kinetic equation is obtained to support the application of carbon fiber reinforced epoxy matrix composite, such as modeling and assessment of thermal protection, fire resistance and lightning strikes.

Author Contributions: Methodology, H.L.; formal analysis, N.W.; investigation, X.H.; resources, H.L.; data curation, X.H.; writing—original draft preparation, H.Y.; writing—review and editing, H.L.; supervision, J.X. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Al-Rubaye, M.; Manalo, A.; Alajarmeh, O.; Ferdous, W.; Lokuge, W.; Benmokrane, B.; Edoo, A. Flexural behaviour of concrete slabs reinforced with GFRP bars and hollow composite reinforcing systems. Compos. Struct. 2020, 236, 111836. [CrossRef] 2. Mohammed, A.A.; Manalo, A.C.; Ferdous, W.; Zhuge, Y.; Vijay, P.V.; Pettigrew, J. Experimental and numerical evaluations on the behaviour of structures repaired using prefabricated FRP composites jacket. Eng. Struct. 2020, 210, 110358. [CrossRef] 3. Composite Aircraft Structure; FAA AC 20-107B; Federal Aviation Administration: Washington, DC, USA, 2010. 4. Stoliarov, S.I.; Crowley, S.; Walters, R.N.; Lyon, R.E. Prediction of the burning rates of charring polymers. Combust. Flame 2010, 157, 2024–2034. [CrossRef] 5. Linteris, G.T.; Lyon, R.E.; Stoliarov, S.I. Prediction of the gasification rate of thermoplastic polymers in fire-like environments. Fire Saf. J. 2013, 60, 14–24. [CrossRef] 6. Stoliarov, S.I.; Leventon, I.T.; Lyon, R.E. Two-dimensional model of burning for pyrolyzable solids. Fire Mater. 2014, 38, 391–408. [CrossRef] 7. Mckinnon, M.B.; Ding, Y.; Stoliarov, S.I.; Crowley, S.; Lyon, R.E. Pyrolysis model for a carbon fiber/epoxy structural aerospace composite. J. Fire Sci. 2016, 35, 36–61. [CrossRef] 8. Mouritz, A.P. Review of Smoke Toxicity of Fiber-Polymer Composites Used in Aircraft. J. Aircr. 2009, 46, 737–745. [CrossRef] 9. Speitel, L.C. Toxicity Assessment of Combustion Gases and Development of a Survival Model; DOT/FAA/AR-95/5; Federal Aviation Administration: Washington, DC, USA, 1995. 10. Speitel, L.C. Fractional effective dose model for post-crash aircraft survivability. Toxicology 1996, 115, 167–177. [CrossRef] 11. Tranchard, P.; Samyn, F.; Duquesne, S.; Estèbe, B.; Bourbigot, S. Modelling Behaviour of a Carbon Epoxy Composite Exposed to Fire: Part II—Comparison with Experimental Results. Materials 2017, 10, 470. [CrossRef] 12. Li, H.; Wang, N.; Han, X.; Fan, B.; Feng, Z.; Guo, S. Simulation of thermal behavior of glass fiber/phenolic composites exposed to heat flux on one side. Materials 2020, 13, 421. [CrossRef][PubMed] 13. Li, H.; Fan, B.; Wang, N.; Han, X.; Feng, Z.; Guo, S. Thermal response study of carbon epoxy laminates exposed to fire. Polym. Compos. 2020, 41, 4757–4770. [CrossRef] Polymers 2021, 13, 569 15 of 16

14. Quintiere, J.G.; Walters, R.N.; Crowley, S. Flammability Properties of Aircraft Carbon-Fiber Structural Composite; DOT/FAA/AR-07/57; Federal Aviation Administration: Washington, DC, USA, 2007. 15. Tranchard, P.; Samyn, F.; Duquesne, S.; Estèbe, B.; Bourbigot, S. Modelling Behaviour of a Carbon Epoxy Composite Exposed to Fire: Part I—Characterisation of Thermophysical Properties. Materials 2017, 10, 494. [CrossRef][PubMed] 16. Tranchard, P.; Duquesne, S.; Samyn, F.; Estèbe, B.; Bourbigot, S. Kinetic analysis of the thermal decomposition of a carbon fibre-reinforced epoxy resin laminate. J. Anal. Appl. Pyrolysis 2017, 126, 14–21. [CrossRef] 17. Brown, M.E.; Maciejewski, M.; Vyazovkin, S.; Nomen, R.; Sempere, J.; Burnham, A.; Opfermann, J.; Strey, R.; Anderson, H.L.; Kemmler, A.; et al. Computational aspects of kinetic analysis: Part a: The ictac kinetics project-data, methods and results. Thermochim. Acta 2000, 355, 125–143. [CrossRef] 18. Coats, A.W.; Redfern, J.P. Kinetic parameters from thermogravimetric data. Nature 1964, 201, 68–69. [CrossRef] 19. Simon, P. The isoconversional method for determination of energy of activation at constant heating rates. J. Anal 1983, 27, 95–102. 20. Friedman, H.L. Kinetics of thermal degradation of char-forming plastics from thermogravimetry: Application to a phenolic plastic. J. Polym. Sci. Part C Polym. Symp. 1964, 6, 183–195. [CrossRef] 21. Opfermann, J.; Kaisersberger, E. An advantageous variant of the ozawa-flynn-wall analysis. Thermochim. Acta 1992, 203, 167–175. [CrossRef] 22. Ozawa, T. A new method of analyzing thermogravimetric data. Bull. Chem. Soc. Jpn. 1965, 38, 1881–1886. [CrossRef] 23. Šesták, J.; Berggren, G. Study of the kinetics of the mechanism of solid-state reactions at increasing temperatures. Thermochim. Acta 1971, 3, 1–12. [CrossRef] 24. Burns, L.A.; Feih, S.; Mouritz, P.A. Fire-under-load Testing of Carbon Epoxy Composites. In Proceedings of the 47th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Orlando, FL, USA, 5–8 January 2009. 25. Chippendale, R.D. Modelling of the Thermal Chemical Damage Caused to Carbon Fibre Composites. Ph.D. Thesis, University of Southampton, Southampton, UK, 2013. 26. Deng, J.; Xu, L.; Liu, J.; Peng, J.; Han, Z.; Shen, Z.; Guo, S. Efficient method of recycling carbon fiber from the waste of carbon fiber reinforced polymer composites. Polym. Degrad. Stab. 2020, 182, 109419. [CrossRef] 27. Wang, B.; Wang, X.; Xu, N.; Shen, Y.; Lu, F.; Liu, Y.; Huang, Y.; Hu, Z. Recycling of carbon fibers from unsaturated polyester composites via a hydrolysis-oxidation synergistic catalytic strategy. Compos. Sci. Technol. 2021, 203, 108589. [CrossRef] 28. Ahamad, T.; Alshehri, S.M. Thermal degradation and evolved gas analysis: A polymeric blend of urea formaldehyde (UF) and epoxy (DGEBA) resin. Arab. J. Chem. 2013, 7, 1140–1147. [CrossRef] 29. Li, G.; Wu, F. Flame-Retardant Mechanism of Epoxy Resin Treated with Amino Resinous Intumescent Flame Retardant. Plast. Sci. Technol. 2010, 15–18. 30. Liu, Y.; Du, Z.; Zhang, C.; Li, H. Curing Reaction and Thermal Degradation of Bisphenol A Type Novolac Epoxy Resin with 4, 40-Diaminodiphenyl Ether. Chin. J. Appl. Chem. 2007, 3, 256–260. 31. Huang, N.; Liu, L.; Wang, X. Pyrolysis and Kinetics of Phenolic Resin by TG-MS Analysis. Aerosp. Mater. Technol. 2012, 2, 109–112. 32. Thomas, A.; Moinuddin, K.; Tretsiakova-Mcnally, S.; Joseph, P. A Kinetic Analysis of the Thermal Degradation Behaviours of Some Bio-Based Substrates. Polymers 2020, 12, 1830. [CrossRef] 33. Balart, R.; Garcia-Sanoguera, D.; Quiles-Carrillo, L.; Montanes, N.; Torres-Giner, S. Kinetic Analysis of the Thermal Degradation of Recycled Acrylonitrile-Butadiene-Styrene by non-Isothermal Thermogravimetry. Polymers 2019, 11, 281. [CrossRef] 34. Franco-Urquiza, E.A.; May-Crespo, J.F.; Escalante Velázquez, C.A.; Mora, R.P.; González García, P. Thermal Degradation Kinetics of ZnO/polyester Nanocomposites. Polymers 2020, 12, 1753. [CrossRef][PubMed] 35. Krawiec, P.; Wargula, L.; Małoziec, D.; Kaczmarzyk, P.; Dziechciarz, A.; Czarnecka-Komorowska, D. The Toxicological Testing and Thermal Decomposition of Drive and Transport Belts Made of Thermoplastic Multilayer Polymer Materials. Polymers 2020, 12, 2232. [CrossRef][PubMed] 36. Zhang, X.; Huang, R. Thermal Decomposition Kinetics of Basalt Fiber-Reinforced Wood Polymer Composites. Polymers 2020, 12, 2283. [CrossRef] 37. GB/T 19267.12-2008. Physical and Chemical Examination of Trace Evidence in Forensic Sciences—Part 12: Thermoanalysis; China National Standardization Administration: Beijing, China, 14 August 2008. 38. ASTM E2550-17. Standard Test Method for Thermal Stability by Thermogravimetry; ASTM International: West Conshohocken, PA, USA, 2017. 39. GB/T 19267.1-2008. Physical and Chemical Examination of Trace Evidence in Forensic Sciences—Part 1: Infrared Absorption Spectrometry; China National Standardization Administration: Beijing, China, 14 August 2008. 40. GB/T 19267.7-2008. Physical and Chemical Examination of Trace Evidence in Forensic Sciences—Part 7: Gas Chromatography/Mass Spectrometry; China National Standardization Administration: Beijing, China, 14 August 2008. 41. Marker, T.R.; Speitel, L.C. Development of a Laboratory-Scale Test for Evaluating the Decomposition Products Generated Inside an Intact Fuselage during a Simulated Postcrash Fuel Fire; DOT/FAA/AR-TN07/15; Federal Aviation Administration: Washington, DC, USA, 2008. 42. Shao, J.; Yan, R.; Chen, H.; Wang, B.; Lee, D.; Liang, D. Pyrolysis Characteristics and Kinetics of Sewage Sludge by Thermo- gravimetry Fourier Transform Infrared Analysis. Energy Fuels 2008, 2, 38–45. [CrossRef] 43. Ding, H.; Zhang, J.; Cai, P. Thermal Degradation of DGEBA/EDA Epoxy Resin. Polym. Mater. Sci. Eng. 2011, 27, 83–85. Polymers 2021, 13, 569 16 of 16

44. Yang, X.; Chen, R. Study on curing kinetics of bisphenol a epoxy resin by pyrolysis gas chromatography (I)—Determination of epoxy value. China Adhes. 1987, 3, 24–25. 45. Feih, S.; Mouritz, A.P. Tensile properties of carbon fibres and carbon fibre–polymer composites in fire. Compos. Part A 2012, 43, 765–772. [CrossRef] 46. Xu, Y.; Yang, Y.; Zhang, Y.; Wang, Z. Pyrolysis characteristics of unidirectional carbon fiber/epoxy prepreg. Acta Mater. Compos. Sin. 2018, 35, 2442–2448. 47. Kissinger, H.E. Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957, 29, 1702–1706. [CrossRef] 48. Bai, Y. Material and Structural Performance of Fiber-Reinforced Polymer Composites at Elevated and High Temperatures. Ph.D. Thesis, School of Architecture, Civil and Environmental Engineering, Lausanne, Switzerland, 2009. 49. Lee, J.Y.; Shim, M.J.; Kim, S.W. Thermal decomposition kinetics of an epoxy resin with rubber-modified curing agent. J. Appl. Polym. Sci. 2001, 81, 479–485. [CrossRef]