A Case Study of Polyether Ether Ketone (I): Investigating the Thermal and Fire Behavior of a High-Performance Material

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Article A Case Study of Polyether Ether Ketone (I): Investigating the Thermal and Fire Behavior of a High-Performance Material

Aditya Ramgobin , Gaëlle Fontaine and Serge Bourbigot * CNRS, INRAE, Centrale Lille, UMR 8207—UMET—Unité Matériaux et Transformations, Univ. Lille, F-59000 Lille, France; [email protected] (A.R.); [email protected] (G.F.) * Correspondence: [email protected]

Received: 14 June 2020; Accepted: 2 August 2020; Published: 10 August 2020

Abstract: The thermal and fire behaviors of a high-performance polymeric material—polyether ether ketone (PEEK) was investigated. The TG plots of PEEK under different oxygen concentrations revealed that the initial step of thermal decomposition does not greatly depend on the oxygen level. However, oxygen concentration plays a major role in the subsequent decomposition steps. In order to understand the thermal decomposition mechanism of PEEK several methods were employed, i.e., pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS), thermogravimetric analysis (TGA) coupled with a Fourier-transform infrared spectrometer (FTIR). It was observed that the initial decomposition step of the material may lead to the release of noncombustible gases and the formation of a highly crosslinked graphite-like carbonaceous structure. Moreover, during the mass loss cone calorimetry test, PEEK has shown excellent charring and fire resistance when it is subjected to an incident heat flux of 50 kW/m2. Based on the fire behavior and the identification of pyrolysis gases evolved during the decomposition of PEEK, the enhanced fire resistance of PEEK was assigned to the dilution of the flammable decomposition gases as well as the formation of a protective graphite-like charred structure during its decomposition. Moreover, at 60 kW/m2, ignition occurred more quickly. This is because a higher rate of release of decomposition products is achieved at such a heat flux, causing a higher concentration of combustibles, thus an earlier ignition. However, the peak of heat release rate of the material did not exceed 125 kW/m2.

Keywords: high-performance polymer; polyether ether ketone (PEEK); reaction to ﬁre; thermal stability; decomposition mechanism

1. Introduction For more than half a century, plastics have been increasingly used as alternatives to traditional materials such as wood or metals in various applications. They versatility and ease with which they can be processed make them valuable assets for their use in everyday life. Despite possessing outstanding properties and numerous applications, most of the polymeric materials (plastics) have one undeniable downside: they are highly flammable. In the event of a fire, most plastics, in their virgin form, degrade and release fuel and toxic gases. This can cause severe material damage as well as human casualties and deaths. In Europe, 4000 fire-related deaths are reported every year. Ninety percent of fires in the European Union happen in buildings and because of the use of an increasing amount of flammable materials. Recent statistics have shown that it takes only three minutes for a fire to involve an entire room [1]. Therefore, most plastics need to be flame retarded in order to be used in most environments. Advances in polymeric materials have allowed the development of high-performance plastics that resist high temperatures and release only little fuel and are therefore virtually non-flammable.

Polymers 2020, 12, 1789; doi:10.3390/polym12081789 www.mdpi.com/journal/polymers Polymers 2020, 12, 1789 2 of 22

Some of these materials are used in industries whereby extreme conditions are met. Such industries include the aeronautics industry, microelectronics or fireproof overalls. Examples of such materials are polyimide (Kapton®), polyether ether ketone (PEEK) or aromatic polyamides such as Kevlar®. These materials all exhibit extreme fire resistance and thermal stability [2]. As a result, high performance polymeric organic materials are being increasingly used in a wide array of applications varying from the automobile industry, the construction industry as well as aerospace. These applications require that the materials in question display a high resistance to extreme conditions such as high temperature or fire. However, in a fire scenario with extreme temperatures, such materials can still decompose, releasing combustibles that can fuel a fire. This decomposition may be due to an incident heat flux or the combination of heat and oxygen. Indeed, there are reports suggesting that the presence of oxygen can play a role in speeding up the decomposition of a material, increasing the rate of released compounds available to eventually feed the fire. This role has long been assumed to be catalytic [3] whereby the degradation temperature of a material is lowered and the rate of degradation is increased. However, other reports have shown that oxygen may, on the contrary, delay the intermediate stage of the thermal decomposition [4,5]. Furthermore, in a fire scenario, the availability of oxygen to the burning materials may vary greatly. In a well-ventilated system, the oxygen concentration may be considered constant, however, in the closed room on fire, the level of oxygen decreases drastically. While the presence of oxygen shows the tendency of decreasing the decomposition temperature of most polymeric materials, some seem to be unaffected to it. Indeed, some high performance materials such as polyether ether ketone (PEEK) exhibit a thermal decomposition onset at the same temperature be it in air or under inert atmosphere [2]. This makes PEEK a highly interesting material in the sense that the catalytic effect of oxygen is not noticeable when observing solely the decomposition temperatures. It is therefore of interest to attempt at understanding the underlying properties that bring about such a resistance to the oxidation under high temperatures to PEEK. Therefore, in this study, the influence of oxygen on the thermal decomposition and fire behavior of a high-performance polymeric material (polyether ether ketone, PEEK) was investigated. One of the most widely method adopted to investigate the thermal stability of the materials was thermogravimetric analysis (TGA) in different atmospheres (atmospheres containing different oxygen levels). This provided insight to the effect that the nature of the surrounding atmosphere has on the modes of decomposition of the materials. Moreover, attempts to elucidate the thermal decomposition pathway of the polymer when it is subjected to an elevated temperature were made. To achieve this, the gases evolved during the thermal decomposition of PEEK were identified via two different methods: pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) and TGA coupled with a Fourier-transform infrared spectrometer (TGA–FTIR). However, before going into the elaboration of a thermal decomposition pathway, the fire behavior of this material was also studied. Finally, attempts at elucidating the thermal decomposition mechanism were made. This furthered the comprehension of the fire behavior and thermal stability of this material.

2. Materials and Methods

2.1. Materials Polyether ether ketone (PEEK) was kindly supplied by Quadrant Engineering Polymers and Plastics (Dagneux, France).

2.2. Thermogravimetric Analysis Thermogravimetric analyses (TGA) were conducted on a Netzsch Libra instrument. Samples of 9–10 mg were placed in open alumina pans and heated under diﬀerent percentages of oxygen (0%, 2%, 4%, 8%, 12% and 20%) at a heating rate of 10 ◦C/min up to 900 ◦C. Samples were ground in liquid nitrogen and placed into open alumina pans. Polymers 2020, 12, 1789 3 of 22

TGA–FTIR measurements were carried out to study the evolved pyrolysis gases. This was conducted using a TA instruments (TGA Discovery) coupled with a Nicolet iS-10 infrared spectrometer. Samples (~10 mg) were heated in a 250 µL alumina crucible from 50 ◦C to 800 ◦C with a heating rate of 20 ◦C/min under nitrogen atmosphere. A balance purge ﬂow of 15 mL/min and a sample purge ﬂow of 50 mL/min was maintained. A transfer line with an inner diameter of 1 mm was used to connect TGA to the infrared cell. The temperature of the transfer line and the gas cell was kept at 225 ◦C. Prior to 1 this, samples were kept for 2 h under nitrogen stream. IR spectra were collected in 400–4000-cm− 1 spectral range (resolution: 4, laser frequency: 15,798.7 cm− ).

2.3. Simultaneous Thermal Analysis (STA) Simultaneous thermal analyses were performed on a Netzsch Jupiter instrument. A blank was performed with an empty crucible. The temperature program was as follows: cool down to 30 C; − ◦ ramp-up of temperature to 900 ◦C at a heating rate of 10 ◦C/min. Heat ﬂow curves and TGA curves were obtained simultaneously.

2.4. Py–GC–MS Samples (approximately 200 µg) were analyzed by pyrolysis GC–MS (Shimadzu, GC–MS-QP2010 SE). GC preparation was carried out with a fused silica capillary column (SLB 5 ms) of 30-m length and 0.25-mm thickness. Analyses were carried out both in direct pyrolysis mode and desorption method. The temperature selection for desorption is based on the TGA program used for the PEEK samples, i.e., with a heating rate of 20 ◦C/min. The initial column temperature was held at 35 ◦C for a period that corresponds to desorption time followed by a temperature ramp at 10 ◦C/min to a final temperature of 300 ◦C and isotherm for 5 min. For direct pyrolysis, the furnace is set for the degradation temperature (700 ◦C which corresponds to completion of second major degradation step) and sample is pyrolyzed for 0.5 min. Column oven temperature is programmed in the following way. The initial column temperature was held at 35 ◦C for 1 min. followed by a temperature ramp at 1 10 ◦C min− to a final temperature of 300 ◦C and isotherm for 20 min. Helium was used as a carrier gas at pressure of 94.1 kPa with a split ratio of 100. The transfer line was maintained at 280 ◦C. The MS was operated under electron ionization (EI) mode. An online computer using GC–MS real time analysis and PY-2020i software-controlled GC–MS system. The eluted components were identified by library search and only significant peaks observed in the total ion chromatograms were studied and compared to a mass spectral database (GC–MS postrun analysis and NIST). This allowed for the separation of the decomposition products that are evolved when PEEK is heated at 20 K/min. The decomposition gases analyzed using Py–GC–MS were coherent with those identified with TGA–FTIR. However, this did not provide a real time analysis of the evolved gases. Indeed, using this technique, all the evolved gases during thermal decomposition are kept at 35 ◦C the beginning of the chromatography column. It is only after the heating process that the separation of the gases is initiated. The advantage of this method is that it provides the possibility of identifying individual mass spectra per decomposition product. In order to have a real time analysis of the evolved gases during thermal decomposition, EGA–MS was performed. The sample was left for 1 min @ 50 ◦C, then heated up at a heating rate of 20 ◦C/min up to a temperature of 750 ◦C. The GC column was kept at 300 ◦C for 42 min. This allowed the real time analysis of the evolved gases during thermal decomposition as they travel straight to the MS analyzer as soon as they are evolved. This allowed for the plotting of the intensity profiles of specific m/z values, which correspond to decomposition products that were identified by TGA–FTIR and Py–GC–MS. It also provides insight regarding different steps of the thermal decomposition of PEEK under pyrolytic conditions. Polymers 2020, 12, 1789 4 of 22

2.5. Fire Testing

PolymersIn 2020 order, 12 to, x FOR evaluate PEER REVIEW the burning behavior of the flame retarded materials, the mass loss4 cone of 22 calorimeter (MLC) was used. The MLCMLC isis a a bench-scale bench-scale reaction reaction to fireto fire test test which which provides provides a combustion a combustion scenario scenario that is typicalthat is oftypical developing of developing or developed or developed fires. The fires. measurements The measurements were carried were out carried on a fire out testing on a technologyfire testing (FTT)technology mass loss(FTT) calorimeter mass loss devicecalorimeter (ISO 13927device [6 ],(ISO ASTM 13927 E906 [6], [7 ]).ASTM The E906 schematic [7]). representationThe schematic ofrepresentation the device is of shown the device in Figure is shown1. The in procedureFigure 1. The involved procedure exposing involved plates exposing (100 plates100 (1003.7mm × 1003) × × in× 3.7 horizontal mm3) in horizontal orientation orientation with heating with elements. heating elem Samplesents. wereSamples wrapped were wrapped in aluminum in aluminum foil leaving foil theleaving upper the surface upper exposedsurface exposed to the heater to the (external heater (external heat flux heat of 35 flux kW /ofm 235corresponding kW/m2 corresponding to a mild to fire a scenariomild fire or scenario 50 kW/ mor2 50and kW/m² 60 kW and/m2 60corresponding kW/m² corresponding to a well-developed to a well-developed fire scenario) fire and scenario) placed onand a ceramicplaced on backing a ceramic board backing at 35 mmboard from at 35 the mm cone from base. the cone base.

Figure 1. Schematic representation of the mass loss cone calorimeter.

The MLCMLC measuredmeasured thethe temperature temperature of of the the evolved evolved gases gases using using a thermopilea thermopile located located at theat the top top of theof the chimney. chimney. The The calibration calibration of the of heat the releaseheat re ratelease (HRR) rate was(HRR) performed was performed with methane. with methane. A methane A flowmethane of 0 toflow 6.7 of mL 0 /tomin 6.7 wasmL/min burnt was above burnt the above sample the holder sample to holder obtain to a calibrationobtain a calibration curve of thecurve heat of releasethe heat as release a function as a function of methane of methane flow [8]. flow [8]. The values measured measured by by MLC MLC were were the the heat heat release release rate rate (HRR), (HRR), the the peak peak of heat of heat release release rate rate(pH (pHRR), RR),the total the total heat heat release release (THR), (THR), the the time time to toigni ignitiontion (TTI) (TTI) and and the the mass mass loss loss of of the the sample sample during combustioncombustion (ML).(ML). All All measurements measurements were were performed performed at least at least thrice. thrice. The presentedThe presented curves curves are the are worst the caseworst of case repeatable of repeatable results. results. The acceptable The acceptable error of error measurement of measurement was estimated was estimated at 10% for at all10% values. for all values.To identify the gases released during the burning of the polymeric materials tested, the MLC was coupledTo identify with the a Fourier-transform gases released during infrared the usingburning a heated of the transferpolymeric line materials (to avoid tested, condensation the MLC of combustionwas coupled products). with a Fourier-transform infrared using a heated transfer line (to avoid condensation of combustionThe gas-picking products). pistol and transfer line were provided by M&C Tech Group. FTIR, AntarisTM IndustrialThe gas-picking Gas System, pistol was provided and transfer by Thermo line were Fisher. provided The transfer by M&C line Tech between Group. MLC FTIR, and Antaris FTIR wasTM 2Industrial m long and Gas was System, heated was up toprovided 180 ◦C. Beforeby Thermo analyzing Fisher. the The gases transfer by FTIR, line soot between particles MLC were and filtered FTIR bywas two 2 m filters long (2and and was 0.1 heatedµm) made up to of 180 glass °C. fibersBefore and analyzing ceramic, the respectively. gases by FTIR, The soot FTIR particles gas cell were was setfiltered to 185 by◦ Ctwo and filters 652 (2 Torr. and The 0.1 opticalμm) made pathway of glass was fibers 2 m and long ceramic, and the respectively. chamber of theThe spectrometerFTIR gas cell waswas filledset to with 185 dry °C air.and FTIR 652 spectraTorr. The obtained optical using pathway MLC–FTIR was were2 m treatedlong and using the OMNIC chamber software of the 1 (laserspectrometer frequency: was 15,798.0 filled with cm− dry, resolution: air. FTIR 0.500).spectra To obtained quantify using gases, MLC–FTIR spectra need were to be treated recorded using at OMNIC software (laser frequency: 15,798.0 cm−1, resolution: 0.500). To quantify gases, spectra need to be recorded at different concentrations for targeted gases and a quantification method needs to be created using TQ Analyst. In creating the method, representative regions in the FTIR spectra of the selected gas needed to be chosen and interactions with other gases needed to be taken into account.

Polymers 2020, 12, 1789 5 of 22 different concentrations for targeted gases and a quantification method needs to be created using TQ Analyst. In creating the method, representative regions in the FTIR spectra of the selected gas needed to be chosen and interactions with other gases needed to be taken into account. Using MLC–FTIR, the following gases could be quantified: water, carbon monoxide, carbon dioxide, nitrogen dioxide and hydrogen cyanide. Quantification was reproducible within 10%.

3. Results and Discussion

3.1. Thermal Stability TGA is an efficient method to study the thermal decomposition temperatures of a material using milligram scale samples. By applying a thermal stress on a material in a controlled atmosphere, the decomposition temperature of the sample, for the thermal stress applied, can be deduced by analyzing the temperature at which the onset of mass loss is observed. Under pyrolytic conditions, temperature is the only factor that affects the decomposition of the material. As mentioned previously, in a fire scenario, the bulk of a burning material undergoes pyrolysis in absence of oxygen. However, while many reports suggest that in a fire scenario, a burning polymeric material is only subjected to pyrolytic thermal stress, the presence of even a small amount of oxygen may influence its decomposition behavior [9] and, as such, on its burning behavior. Therefore, the impact of low oxygen concentrations (0%–12% oxygen) on the decomposition PEEK was studied. Furthermore, even though the oxygen concentration is often considered as limited in the fire scenario in a closed room, this is not the case in a well ventilated one, where oxygen can be brought by convection. Additionally, when a material is far from a flame, it can still be subjected to an incident heat flux. In this case, the material is subjected to thermo-oxidative degradation. It is therefore equally important to evaluate the thermal stability of the material in air. Therefore, the thermal stability of PEEK was studied under different atmospheres to intricately understand its thermal decomposition behavior under different types of fire scenarios. The TG plots and their DTG (derivative of the thermogravimetric curve) of PEEK when heated at 10 ◦C/min under different atmospheres are shown in Figure2. The data are summarized on Table1. Under nitrogen, the temperature at the onset of PEEK thermal decomposition (T95%) was evaluated at 574 ◦C and the maximum mass loss rate (11.0 wt%/min) occurs at 592 ◦C. A nonzero DTG value is also observed at temperatures ranging from 615 ◦C to 800 ◦C corresponding to a mass loss ranging from around 60 wt% at 615 ◦C to approximately 50 wt% at 800 ◦C. At higher temperatures, there is little mass loss recorded. Indeed, the final residual mass of PEEK at 900 ◦C under nitrogen is 48%. This suggests that the product formed during the pyrolysis of PEEK is quite stable under pyrolytic conditions and at this temperature. This also suggests that the pyrolysis of PEEK is at least a two-step process, one occurring at the onset of the decomposition and another that overlaps with the first degradation process. Furthermore, it was reported that the PEEK is a highly charring polymer. This suggests that during the first step of the decomposition, the reaction occurring leads to a thermodynamically stable char structure [10,11]. Following this char-formation step, a second decomposition step is observed. This step corresponds to the slow degradation of the char at high temperatures. It explains the low, albeit nonzero, values on the DTG curve above 650 ◦C. With increasing oxygen concentrations, the TG plots show that there is little change in terms of the temperature at the onset of the decomposition. This is evidenced by the temperature at which the sample records 5% mass loss, which ranges from 565 ◦C (at 4% O2) to 575 ◦C (at 8% O2). This suggests that the inertness or oxidative nature of the atmosphere in which PEEK is heated has little effect on the temperature at which it starts to decompose. This is even more deducible by looking at the temperature at which the peak mass loss rate is recorded (589–593 ◦C). It is therefore likely that this first step of thermal decomposition corresponds to the formation of a charred structure Polymers 2020, 12, x FOR PEER REVIEW 5 of 22

Using MLC–FTIR, the following gases could be quantified: water, carbon monoxide, carbon dioxide, nitrogen dioxide and hydrogen cyanide. Quantification was reproducible within 10%.

3. Results and Discussion

3.1. Thermal Stability TGA is an efficient method to study the thermal decomposition temperatures of a material using milligram scale samples. By applying a thermal stress on a material in a controlled atmosphere, the decomposition temperature of the sample, for the thermal stress applied, can be deduced by analyzing the temperature at which the onset of mass loss is observed. Under pyrolytic conditions, temperature is the only factor that affects the decomposition of the material. As mentioned previously, in a fire scenario, the bulk of a burning material undergoes pyrolysis in absence of oxygen. However, while many reports suggest that in a fire scenario, a burning polymeric material is only subjected to pyrolytic thermal stress, the presence of even a small amount of oxygen may influence its decomposition behavior [9] and, as such, on its burning behavior. Therefore, the impact of low oxygen concentrations (0%–12% oxygen) on the decomposition PEEK was studied. Furthermore, even though the oxygen concentration is often considered as limited in the fire scenario in a closed room, this is not the case in a well ventilated one, where oxygen can be brought by convection. Additionally, when a material is far from a flame, it can still be subjected to an incident heat flux. In this case, the material is subjected to thermo-oxidative degradation. It is therefore equally important to evaluate the thermal stability of the material in air. Therefore, the thermal stability of PEEK was studied under different atmospheres to intricately understand its thermal decomposition behavior under different types of fire scenarios. The TG plots and their DTG (derivative of the thermogravimetric curve) of PEEK when heated at 10 °C/min under different atmospheres are shown in Figure 2. The data are summarized on Table 1. Polymers 2020, 12, 1789 6 of 22

Figure 2. TGA and DTG plots of polyether ether ketone (PEEK) when heated at 10 ◦C/min under pyrolytic conditions (black, square), 2% oxygen (red, circle), 4% oxygen (blue, upwards triangle), 8% oxygen (magenta, downwards triangle), 12% oxygen (green, diamond) and in synthetic air (brown, left-pointing triangle).

Table 1. Summary of the data from the TGA of PEEK under diﬀerent oxygen concentrations.

O2%T95% (◦C) T@ MLRMAX (◦C) MLRMAX (%/min) Residue @ 900 ◦C (wt%) 0% 574 592 11.0 48 2% 573 593 10.0 22 4% 565 589 11.4 0 8% 575 589 9.2 0 12% 572 590 10.5 0 Air 568 592 11.0 0

Furthermore, the TG plots also show that the oxygen concentration has a significant effect on the second part of the decomposition occurring at temperatures above 650 ◦C. Indeed, after the first decomposition step, whereby the MLRMAX is recorded, there is a second slower decomposition step, which can also be seen on the DTG curves. These DTG curves evidence that for the second stage of the decomposition, a higher mass loss rate is associated with a higher oxygen concentration. From this, it can be deduced that that this decomposition stage corresponds to the oxidation of the charred structure formed in the first step. This would explain the oxygen concentration dependence of the mass loss rate where oxygen is a limiting factor in the thermo-oxidative decomposition of the char formed in the first step. Moreover, it is interesting to note that at 900 ◦C, only the TG plots of PEEK under nitrogen, and 2% oxygen show some residual mass. This means that the charred structure formed during the initial thermal decomposition stage is completely oxidized at 900 ◦C even at oxygen concentrations as low as 4% while in inert atmospheres, the residual structure corresponds to almost half of the initial mass of the material. Polymers 2020, 12, 1789 7 of 22

Polymers 2020, 12, x FOR PEER REVIEW 7 of 22 This means that in certain fire scenarios, especially those where the material is subjected to pyrolytic stress, flaming PEEK can form a char which would act as a thermal and physical barrier. This char can limit the access of heat to the bulk of the material as well as prevent further This char can limit the access of heat to the bulk of the material as well as prevent further decomposition decomposition products from leaving the bulk to reach the flame to feed the fire. products from leaving the bulk to reach the flame to feed the fire. While the key temperatures relating to the thermal stability and decomposition behavior of While the key temperatures relating to the thermal stability and decomposition behavior of PEEK PEEK under inert atmosphere were extracted using TGA, enthalpic data concerning the thermal under inert atmosphere were extracted using TGA, enthalpic data concerning the thermal behavior of behavior of PEEK under nitrogen were evaluated using STA (Figure 3). PEEK under nitrogen were evaluated using STA (Figure3).

Figure 3. Thermogravimetric (TG) and differential scanning calorimetry DSC plot for PEEK with a heating rate of 10 C/min under nitrogen atmosphere. Figure 3. Thermogravimetric◦ (TG) and differential scanning calorimetry DSC plot for PEEK with a heating rate of 10 °C/min under nitrogen atmosphere. From the DSC curve (Figure3), a small exothermic peak can be observed at around 170 ◦C. This corresponds to the glass transition temperature of the material. At around 343 C, a larger, From the DSC curve (Figure 3), a small exothermic peak can be observed at around 170◦ °C. This endothermic, peak is observed, corresponding to the melting temperature of the polymer. Two corresponds to the glass transition temperature of the material. At around 343 °C, a larger, exothermic peaks are observed at temperatures corresponding to the decomposition of the material endothermic, peak is observed, corresponding to the melting temperature of the polymer. Two (580 C). This confirms that the decomposition of PEEK is at least a two-step process when it is heated exothermic◦ peaks are observed at temperatures corresponding to the decomposition of the material at 10 C/min under nitrogen. (580 °C).◦ This confirms that the decomposition of PEEK is at least a two-step process when it is heated The enthalpy of decomposition measured was 417 J/g. Moreover, the exothermic nature of at 10 °C/min under nitrogen. − these peaks is unusual in the sense that common polymers show an endothermic peak during their The enthalpy of decomposition measured was −417 J/g. Moreover, the exothermic nature of these decomposition. This can be explained by the fact that PEEK is a highly charring polymer [10]. peaks is unusual in the sense that common polymers show an endothermic peak during their This means that during its thermal decomposition, it forms a highly stable carbonaceous structure, decomposition. This can be explained by the fact that PEEK is a highly charring polymer[10]. This implying that the heat released during bond formation in the charred structure is higher than that means that during its thermal decomposition, it forms a highly stable carbonaceous structure, released during the bond breaking process during the decomposition. implying that the heat released during bond formation in the charred structure is higher than that This exothermic nature of the thermal decomposition of PEEK means that it releases heat when it released during the bond breaking process during the decomposition. degrades. While this property is not highly enviable in a fire scenario, it also suggests that the char This exothermic nature of the thermal decomposition of PEEK means that it releases heat when structure that is formed during its initial decomposition is highly stable and can possibly act as a fire it degrades. While this property is not highly enviable in a fire scenario, it also suggests that the char barrier, protecting the underlying material while the exposed surface decomposes. structure that is formed during its initial decomposition is highly stable and can possibly act as a fire Studying the thermal stability and behavior of PEEK under different oxygen concentrations has barrier, protecting the underlying material while the exposed surface decomposes. provided insight on the dependence of the oxygen concentration on the different steps of the thermal Studying the thermal stability and behavior of PEEK under different oxygen concentrations has decomposition of PEEK. Indeed, whether a thermal stress is applied in the absence of oxygen or provided insight on the dependence of the oxygen concentration on the different steps of the thermal under air, the temperature at which the first step of decomposition occurs is only slightly affected. decomposition of PEEK. Indeed, whether a thermal stress is applied in the absence of oxygen or This implies that the reigning reaction that occurs as the polymer first starts to degrade and decompose under air, the temperature at which the first step of decomposition occurs is only slightly affected. thermally is the action of heat only. It is therefore of interest to study the nature of the decomposition This implies that the reigning reaction that occurs as the polymer first starts to degrade and gases that are evolved during the thermal decomposition of this material. decompose thermally is the action of heat only. It is therefore of interest to study the nature of the decomposition gases that are evolved during the thermal decomposition of this material.

3.2. Identification of Decomposition Products of PEEK

Polymers 2020, 12, x FOR PEER REVIEW 8 of 22

3.2.1. Pyrolysis Products of PEEK To identify the products released during its pyrolysis, PEEK was subjected to TGA coupled with FTIR. TGA–FTIR of PEEK under nitrogen yielded many different pyrolysis products. The main decomposition products identified correspond to carbon dioxide and carbon monoxide. In order to qualitatively compare the gases evolved, intensity profiles corresponding to characteristic peaks of some degradation products (CO2 at 2356 cm−1, CO at 2184 cm−1 and CH4 at 3018 cm−1) were plotted (Figure 4). The peak intensity of CO2 remains stable until around 30 min (approximately 530 °C). At this temperature, there is a negligible mass loss recorded on the TGA plot. After this, the peak intensity of CO2 increases gradually until it reaches a maximum at around 35 min (600 °C). It then decreases to a local minimum at 40 min (737 °C) before increasing again. These maxima/minima suggest that there may be more than two steps involved in the thermal decomposition mechanism of PEEK. In the case of CO, the peak intensity profile shares a similar shape to that of CO2 at the beginning. Polymers 2020, 12, 1789 8 of 22 However, as opposed to the CO2 intensity plot, the CO plot shows a second maximum peak at 42 min. This suggests that there may be different reactions producing CO at different temperatures. 3.2.From Identification the intensity of Decomposition profile of methane, Products ofit can PEEK be seen that methane is released at a slightly higher temperature than CO and CO2. Indeed, the intensity of the methane peak starts to increase at 32 min (5703.2.1. °C) Pyrolysis and remains Products relatively of PEEK constant until around 42 min. This means methane is released shortly after theTo main identify pyrolysis the products step (first released decomposition) during its of pyrolysis, PEEK. This PEEK can wasbe explained subjected by to the TGA secondary coupled pyrolysiswith FTIR. of TGA–FTIR volatile aromatics of PEEK under and nitrogenthe decomposition yielded many of dithefferent charred pyrolysis structure products. during The mainthe graphitizationdecomposition process products (see identified mechanism correspond described, to Scheme carbon dioxide1, page and17) [12]. carbon Moreover, monoxide. the Inpyrolysis order to ofqualitatively a charred structure compare also the leads gases to evolved, the formation intensity of profiles hydrogen corresponding gas, which can, to characteristic in turn reactpeaks with a of carbonized structure to form methane [13].1 Furthermore,1 secondary pyrolysis1 of volatile some degradation products (CO2 at 2356 cm− , CO at 2184 cm− and CH4 at 3018 cm− ) were plotted decomposition(Figure4). products (such as phenol) may also lead to methane formation [14–16].

Figure 4. TGA curve of PEEK under nitrogen at a heating rate of 20 ◦C/min (top) and intensity proﬁles 1 ) 1 Figureof characteristic 4. TGA curve peaks of PEEK of CO under (2184 nitrogen cm− ), CO at a2 heating(2356 cm rate1 ofand 20 CH°C/min4 (3018 (top cm) and− ) intensity obtained profiles through − ofFTIR characteristic (bottom). peaks of CO (2184 cm−1), CO2 (2356 cm−1) and CH4 (3018 cm−1) obtained through FTIR (bottom). The peak intensity of CO2 remains stable until around 30 min (approximately 530 ◦C). At this temperature,In addition there to the is a previously negligible mass mentioned loss recorded decomposition on the TGA gases, plot. other After evolved this, the gases peak intensitywere also of identified.CO2 increases However, gradually due until to overlapping it reaches a maximumof some peaks at around in the 35 FTIR min (600spectra,◦C). Itcharacteristic then decreases peak to a profileslocal minimum of the identified at 40 min gases (737 were◦C) beforenot singled increasing out. By again. comparing These maximawith reference/minima FTIR suggest spectra, that other there may be more than two steps involved in the thermal decomposition mechanism of PEEK.

In the case of CO, the peak intensity profile shares a similar shape to that of CO2 at the beginning. However, as opposed to the CO2 intensity plot, the CO plot shows a second maximum peak at 42 min. This suggests that there may be different reactions producing CO at different temperatures. From the intensity profile of methane, it can be seen that methane is released at a slightly higher temperature than CO and CO2. Indeed, the intensity of the methane peak starts to increase at 32 min (570 ◦C) and remains relatively constant until around 42 min. This means methane is released shortly after the main pyrolysis step (first decomposition) of PEEK. This can be explained by the secondary pyrolysis of volatile aromatics and the decomposition of the charred structure during the graphitization process (see mechanism described, Scheme1, page 17) [ 12]. Moreover, the pyrolysis of a charred structure also leads to the formation of hydrogen gas, which can, in turn react with a carbonized structure to form methane [13]. Furthermore, secondary pyrolysis of volatile decomposition products (such as phenol) may also lead to methane formation [14–16]. Polymers 2020, 12, 1789 9 of 22

In addition to the previously mentioned decomposition gases, other evolved gases were also

Polymersidentified. 2020, 12, However, x FOR PEER dueREVIEW to overlapping of some peaks in the FTIR spectra, characteristic9 of peak 22 Polymers 20202020,, 1212,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 2222 Polymersprofiles 2020 of,, 12 the,, xx FORFOR identified PEERPEER REVIEWREVIEW gases were not singled out. By comparing with reference FTIR spectra,9 of 22 Polymers 2020, 12, x FOR PEER REVIEW 9 of 22 Polymers 2020, 12, x FOR PEER REVIEW 9 of 22 decompositionother decomposition products products were identified were identified and are andreported are reported in Table in 2. TableOf the2. decomposition Of the decomposition gases, decomposition products were identified and are reported in Table 2. Of the decomposition gases, decompositionaromaticgases, aromatic ethers, products phenols, ethers, phenols,were and identifiedfurans and furanswere and werethe are main reported the main groups groupsin Tableof products of 2. products Of the identified, decomposition identified, suggesting suggesting gases, a a aromaticdecomposition ethers, products phenols, were and identifiedfurans were and the are main reported groups in Tableof products 2. Of the identified, decomposition suggesting gases, a aromaticdecompositiondecomposition ethers, mechanism mechanismphenols, and whereby whereby furans the thewere polymer polymer the main chains chains groups are are cut cut of randomly. randomly.products identified, suggesting a decompositionaromatic ethers, mechanism phenols, andwhereby furans the were polymer the mainchains groups are cut of randomly. products identified, suggesting a decompositionTheThe main main decompositionmechanism decomposition whereby products products the polymeridentified identified chains at at720 720 are °C◦ cut Cwere were randomly. methane, methane, phenol, phenol, carbon carbon dioxide dioxide decompositionThe main decompositionmechanism whereby products the identifiedpolymer chains at 720 are °C cutwere randomly. methane, phenol, carbon dioxide andand carbonThe carbon main monoxide. monoxide.decomposition The The products products products identified identified identified were at were 720previously °C previously were reported methane, reported in phenol,a study in a study carbonconcerning concerning dioxide the and carbonThe main monoxide. decomposition The products products identified identified were at previouslypreviously720 °C were reportedreported methane, inin phenol,aa studystudy carbonconcerningconcerning dioxide thethe andmechanismthe carbon mechanism monoxide.of the ofthermal the The thermal decompositionproducts decomposition identified of PEEK were of PEEK[12]. previouslypreviously However, [12]. However, reportedreported the release in thein aa release studystudy of methane concerningconcerning of methane was notthethe was mechanismand carbon monoxide.of the thermal The decompositionproducts identified of PEEK were [12].[12]. previously However,However, reported thethe releaserelease in a study ofof methanemethane concerning waswas notnotthe mechanismreported.not reported. of the thermal decomposition of PEEK [12]. However, the release of methane was not reported.mechanismreported. of the thermal decomposition of PEEK [12]. However, the release of methane was not reported. reported. TableTable 2. 2.DecompositionDecomposition gases gases identified identified with with TGA–FTIR TGA–FTIR at attwo two decomposition decomposition steps steps of ofPEEK PEEK (600 (600 °C◦ C Table 2. Decomposition gases identified with TGA–FTIR at two decomposition steps of PEEK (600 °C Tableand 2. 720 DecompositionC), under nitrogen gases identified and a heating with rate TGA–FTIR of 20 K/ min.at two decomposition steps of PEEK (600 °C Tableand 720 2. °C),Decomposition◦ under nitrogen gases and identified a heating with rate TGA–FTIR of 20 K/min. at two decomposition steps of PEEK (600 °C andTable 720 2. °C),Decomposition under nitrogen gases and identified a heating with rate TGA–FTIR of 20 K/min. at two decomposition steps of PEEK (600 °C and 720 °C), under nitrogen and a heating rate of 20 K/min. Temperatureand 720 °C), under nitrogen Product and a heating rate of Structure 20 K/min. Wavenumbers (cm−11) Temperatureand 720 °C), under nitrogenProduct and a heating rate of Structure 20 K/min. Wavenumbers (cm−−11) Temperature Product Structure Wavenumbers (cm−1) Temperature Product Structure Wavenumbers (cm−−11) Temperature CarbonCarbonProduct monoxidemonoxide Structure Wavenumbers2184,2184, 2114 2114 (cm −1) Temperature CarbonProduct monoxide Structure Wavenumbers2184, 2114 (cm −1) CarbonCarbon monoxide dioxide 2184,671, 2114 2356 CarbonCarbon monoxide dioxide 2184,671, 2356 2114 CarbonCarbon monoxide dioxide 2184,671, 2356 2114 Carbon dioxide 671, 2356 Carbon dioxide 3652, 3056,671, 1605, 2356 1497, 1336, Carbon dioxide 3652, 3056,671, 1605, 2356 1497, 1336, Phenol 3652,3652, 3056, 3056, 1605, 1605, 1497, 1497, 1336, 1336, 1263, 1182 Phenol 3652, 3056,1263, 1605, 1182 1497, 1336, 600 ◦C Phenol 3652, 3056,1263, 1605, 1182 1497, 1336, Phenol 1263, 1182 1263, 1182 Benzoquinone 1678, 1303, 1069, 1055, 881 BenzoquinoneBenzoquinone 1678,1678, 1303, 1303, 1069, 1069, 1055, 1055, 881 881 Benzoquinone 1678, 1303, 1069, 1055, 881 Benzoquinone 3073,1678, 1684, 1303, 1603, 1069, 1449, 1055, 1313, 881 3073, 1684, 1603, 1449, 1313, Benzophenone 3073,3073, 1684, 1684, 1603, 1603, 1449, 1449, 1313, 1313, 1273, 937, BenzophenoneBenzophenone 3073,1273, 1684, 937, 1603, 700, 1449, 640 1313, Benzophenone 3073,1273, 1684, 937, 1603,700, 700, 640 1449, 640 1313, 600 °C Benzophenone 1273, 937, 700, 640 600 °C 3042,1273, 1585, 937, 1490, 700, 1237, 640 1164, 600 °C Diphenylether 3042,1273, 1585, 937, 1490, 700, 1237, 640 1164, 600 °C Diphenylether 1074,3042,3042, 1585,1025,1585, 1490, 9*8,1490, 799, 1237, 1237, 750, 1164, 1164, 693 1074, 600 °C DiphenyletherDiphenylether 1074,3042, 1025,1585, 9*8,1490, 799, 1237, 750, 1164, 693 Diphenylether 1074,3042, 1025, 1025,1585, 9*8, 9*8,1490, 799, 799, 1237, 750, 750, 6931164, 693 Diphenylether 3078,1074, 1593, 1025, 1488, 9*8, 1222,799, 750, 880, 693 753, 1,4-diphenoxybenzene 3078,1074, 1593, 1025, 1488, 9*8, 1222,799, 750, 880, 693 753, 1,4-diphenoxybenzene 3078, 1593, 1488,690 1222, 880, 753, 1,4-diphenoxybenzene1,4-diphenoxybenzene 3078,3078, 1593, 1593, 1488,1488,690 1222, 1222, 880, 880, 753, 753, 690 1,4-diphenoxybenzene 690 3076, 1451, 1195,690 1110, 840, 750, Dibenzofuran 3076, 1451, 1195, 1110, 840, 750, Dibenzofuran 3076, 1451, 1195,722 1110, 840, 750, Dibenzofuran 3076, 1451, 1195,722 1110, 840, 750, DibenzofuranDibenzofuran 3076,3076, 1451, 1451, 1195,1195,722 1110, 1110, 840, 840, 750, 750, 722 Dibenzofuran 722 2- 3656, 1480, 1445,722 1189, 1163, 807, 2- 3656, 1480, 1445, 1189, 1163, 807, hydroxydibenzofuran2- 3656, 1480, 1445,745 1189, 1163, 807, hydroxydibenzofuran2- 3656, 1480, 1445,745 1189, 1163, 807, 2-hydroxydibenzofuranhydroxydibenzofuran 3656, 1480, 1445,745 1189, 1163, 807, 745 hydroxydibenzofuranCarbon monoxide 2184,745 2114 Carbon monoxide 2184, 2114 CarbonCarbon monoxide dioxide 2184,2356, 2114 671 CarbonCarbon monoxide dioxide 2184,2356, 2114671 CarbonCarbonCarbon monoxidemonoxide dioxide 2184,2356,2184, 2114671 2114 CarbonMethane dioxide CH4 3018,2356, 1305671 720 °C CarbonMethane dioxide CH44 3018,2356, 1305 671 720 °C CarbonCarbonMethane dioxidedioxide CH4 3018,2356,2356, 1305 671 671 720720 °CC Methane CH44 3018, 1305 720 °C◦ Methane CH4 3652, 3056,3018, 1605, 1305 1497, 1336, 720 °C MethanePhenol CH44 3652, 3056,3018,3018, 1605, 1305 1305 1497, 1336, Phenol 3652, 3056, 1605, 1497, 1336, Phenol 3652, 3056,1263, 1605, 1182 1497, 1336, Phenol 3652, 3056,1263, 1605, 1182 1497, 1336, Phenol 1263, 1182 Phenol 3652, 3056, 1605,1263, 1497, 1182 1336, 1263, 1182 1263, 1182 In order to have better accuracy in the identification of evolved gases during the pyrolysis of InIn orderorder toto havehave betterbetter accuracy in the identification of evolved gases during the pyrolysis of PEEK,In pyrolysis–gasorder to have chromatography–massbetter accuracy in the identificationspectrometry of(Py–GC–MS) evolved gases was during used. theBy subjectingpyrolysis of a PEEK,In pyrolysis–gasorder to have chromatography–massbetter accuracy in the identificationspectrometry of(Py–GC–MS) evolved gases was during used. theBy subjectingpyrolysis ofa PEEK,PEEK sampleInpyrolysis–gas order to to the have same chromatography–mass better heating accuracy rate as in for the th spectrometrye identification TGA–FTIR measurements,(Py–GC–MS) of evolved gases was the used. duringevolved By the gasessubjecting pyrolysis during a of PEEKPEEK, sample pyrolysis–gas to the same chromatography–mass heating rate as for th spectrometrye TGA–FTIR measurements,(Py–GC–MS) was the used.evolved By gasessubjecting during a PEEKitsPEEK, thermal sample pyrolysis–gas decomposition to the same chromatography–mass heating are collected rate as and for thseparatede spectrometryTGA–FTIR via a measurements, gas (Py–GC–MS) chromatography the was evolved used. column, By gases subjectingwhich during are a itsPEEKits thermalthermal sample decompositiondecomposition to the same heating areare collectedcollected rate as andand for separatedthseparatede TGA–FTIR viavia aa measurements, gasgas chromatographychromatography the evolved column,column, gases whichwhich during areare itsthenitsPEEK thermalthermal analyzed sample decompositiondecomposition using to the a mass same areare spectrometer. heating collectedcollected rate andand as However, for separatedseparated the TGA–FTIR due viavia to aathe gasgas measurements, nature chromatographychromatography of the instrument, theevolved column,column, the gases whichwhich pyrolysis during areare thenitsthen thermal analyzedanalyzed decomposition usingusing aa massmass are spectrometer.spectrometer. collected and However,However, separated due via to athe gas nature chromatography of the instrument, column, the which pyrolysis are thenprogramthenits thermalanalyzedanalyzed can decompositiononly usingusing be aa used massmass in spectrometer.spectrometer. arehelium, collected therefore, However,However, and separated this dueanalysis to via the a“only” gasnature chromatography provides of the instrument, information column, the regarding pyrolysis which are programthen analyzed can only using be a used mass in spectrometer. helium, therefore, However, this dueanalysis to the “only” nature provides of the instrument, information the regarding pyrolysis programthethen pyrolysis analyzed can onlyproducts using be used aof mass the in thermalhelium, spectrometer. therefore,decomposition However, this analysis of due the to material. the“only” nature provides of the instrument,information the regarding pyrolysis theprogramthe pyrolysispyrolysis can productsonlyproducts be used ofof thethe in thermalthermalhelium, decompositiondecompositiontherefore, this analysis ofof thethe material.material. “only” provides information regarding thetheprogram pyrolysispyrolysisFigure can5 productsproductsshows only bethe ofof used intensity thethe in thermalthermal helium, profiles decompositiondecomposition therefore, of carbon this dioxideofof analysis thethe material.material. and “only” phenol, provides measured information using EGA–MS regarding the pyrolysisFigure 5 productsshows the of intensity the thermal profiles decomposition of carbon dioxideof the material. and phenol, measured using EGA–MS method.theFigure pyrolysis Phenol 5 shows products (Figure the intensity of5, theviolet) thermal profileshas the decomposition ofhighest carbon intensity, dioxide of the material.andfollowed phenol, by measuredcarbon dioxide using (FigureEGA–MS 5, method.Figure Phenol 5 shows (Figure the intensity5, violet) profileshas the ofhighest carbon intensity, dioxide followedand phenol, by measuredcarbon dioxide using (FigureEGA–MS 5, method.gold). ThisFigure Phenol suggests5 shows (Figure that the 5,there intensity violet) is a hassignificant profiles the highest of amount carbon intensity, dioxideof decarboxylation followed and phenol, by reactionscarbon measured dioxide occurring using (Figure EGA–MS during 5, gold).method. This Phenol suggests (Figure that there5, violet) is aa significantsignificanthas the highest amountamount intensity, ofof decarboxylationdecarboxylation followed by reactions reactionscarbon dioxide occurringoccurring (Figure duringduring 5, gold).method. This Phenol suggests (Figure that there5, violet) is a significanthas the highest amount intensity, of decarboxylation followed by reactions carbon dioxide occurring (Figure during 5, thegold).method. thermal This suggestsPhenol decomposition (Figure that there5 ,of violet) PEEK.is a significant has Moreover, the highest amount it has intensity, of been decarboxylation reported followed that by reactions the carbon release occurring dioxide of CO (Figure2 duringis also5 , thegold). thermal This suggests decomposition that there of PEEK.is a significant Moreover, amount it has of been decarboxylation reported that reactions the release occurring of CO2 duringis also the thermal decomposition of PEEK. Moreover, it has been reported that the release of CO2 is also associatedthegold). thermal This with decomposition suggests the pyrolytic that there of decompositionPEEK. is a significant Moreover, of amount carbonylsit has been of decarboxylation[17]. reported The CO that2 profile the reactions release also occurring evidencesof CO2 is during alsothat associatedthe thermal with decomposition the pyrolytic of decomposition PEEK. Moreover, of carbonylsit has been [17]. reported The CO that2 profile the release also evidences of CO2 is thatalso associated with the pyrolytic decomposition of carbonyls [17]. The CO2 profile also evidences that thereassociated are two with major the stepspyrolytic occurring decomposition when PEEK of carbonylsis decomposing: [17]. The two CO peaks2 profile are observed, also evidences one at that620 thereassociated are two with major the stepspyrolytic occurring decomposition when PEEK of iscarbonyls decomposing: [17]. The two CO peaks2 profile are observed, also evidences one at that620 there°Cthere and areare the twotwo other majormajor at around stepssteps occurringoccurring 680 °C. whenwhen PEEKPEEK isis decomposing:decomposing: twotwo peakspeaks areare observed,observed, oneone atat 620620 °Cthere°C andand are thethe two otherother major atat aroundaround steps occurring 680680 °C.°C. when PEEK is decomposing: two peaks are observed, one at 620 °C and the other at around 680 °C. °C and the other at around 680 °C.

Polymers 2020, 12, 1789 10 of 22

the thermal decomposition of PEEK. Moreover, it has been reported that the release of CO2 is also associated with the pyrolytic decomposition of carbonyls [17]. The CO2 proﬁle also evidences that there are two major steps occurring when PEEK is decomposing: two peaks are observed, one at 620 ◦C Polymers 2020, 12, x FOR PEER REVIEW 10 of 22 and the other at around 680 ◦C.

Polymers 2020, 12, x FOR PEER REVIEW 10 of 22

Figure 5. Intensity profiles of carbon dioxide (gold, m/z = 44) and phenol (violet, m/z = 94) using EGA– FigureFigure 5. 5. IntensityIntensity profiles profiles of ofcarbon carbon dioxide dioxide (gold, (gold, m/z m= 44)/z = and44) phenol and phenol (violet, (violet, m/z = 94)m/z using= 94) EGA– using MS at a heating rate of 20 °C/min under helium. The dashed lines indicate the moment where the MSEGA–MS atcorresponding a heating at a heating rate product of rate 20 is of°C/min detected. 20 ◦C/ minunder The under filled helium. lines helium. correspondThe The dashed dashed to thelines linestemperature indicate indicate the(or the time)moment moment whereby where where the the correspondingcorrespondingthe maximum product product intensity is is ofdetected. detected. a product TheThe is analyzed. filled filled lines lines correspond correspond to to the the temperature temperature (or (or time) time) whereby whereby thethe maximum maximum intensity intensity of of a a product product is is analyzed. analyzed. EGA–MS of PEEK at 20 °C/min shows that phenol has the intensity profile with the highest EGA–MSintensityEGA–MS (>150,000, of of PEEK PEEK Figure at at 20 20 5) °C/minthan◦C/min the showsother shows decomposition that that phen phenolol products has has the the (<15,000,intensity intensity Figure profile profile 6). with with the the highest highest intensityintensity (>150,000, (>150,000, Figure Figure 5)5) than than the the other other decomposition products (<15,000, ( <15,000, Figure Figure 6).6).

FigureFigure 6. 6.Intensity Intensity proﬁles profiles ofof biphenylbiphenyl (blue), (blue), 2-ph 2-phenoxydibenzo[b,d]furanenoxydibenzo[b,d]furan (olive), (olive), benzoquinone benzoquinone (black), 4-diphenoxybenzene (pale blue), 4-hydroxybenzophenone (purple), hydroquinone (red), diphenylether (pale brown), dibenzo[b,d]furan-2-ol (green), 4-phenoxyphenol (Prussian blue) and

Figure 6. Intensity profiles of biphenyl (blue), 2-phenoxydibenzo[b,d]furan (olive), benzoquinone (black), 4-diphenoxybenzene (pale blue), 4-hydroxybenzophenone (purple), hydroquinone (red), diphenylether (pale brown), dibenzo[b,d]furan-2-ol (green), 4-phenoxyphenol (Prussian blue) and

Polymers 2020, 12, 1789 11 of 22

Polymers 2020, 12, x FOR PEER REVIEW 11 of 22 (black), 4-diphenoxybenzene (pale blue), 4-hydroxybenzophenone (purple), hydroquinone (red), dibenzofuran (magenta), analyzed using EGA–MS at a heating rate of 20 °C/min. Dashed lines diphenylether (pale brown), dibenzo[b,d]furan-2-ol (green), 4-phenoxyphenol (Prussian blue) and indicate the moment where the corresponding product is detected. The filled lines correspond to the dibenzofuran (magenta), analyzed using EGA–MS at a heating rate of 20 ◦C/min. Dashed lines indicate temperature (or time) whereby the maximum intensity of a product is analyzed. the moment where the corresponding product is detected. The filled lines correspond to the temperature (or time) whereby the maximum intensity of a product is analyzed. From the intensity profiles on Figure 5 and Figure 6, it can be observed that the onset of dibenzofuranFrom the release intensity occurs profiles at the on lowest Figures temperature5 and6, it can (575 be °C). observed CO2 is thatthe next the onset gas to of be dibenzofuran observed at aroundrelease 600 occurs °C. atMost the lowestof the other temperature decomposition (575 ◦C). ga COses2 is start the nextto be gas released to be observed within the at aroundtemperature 600 ◦C. rangeMost 620–650 of the other °C. The decomposition maximum intensities gases start observed to be released are within within the the range temperature 640–670 °C. range 620–650 ◦C. The maximum intensities observed are within the range 640–670 ◦C. 3.2.2. Thermal Decomposition Products of PEEK under Low Oxygen Concentration 3.2.2. Thermal Decomposition Products of PEEK under Low Oxygen Concentration In order to better understand the effect of oxygen on the thermal decomposition products releasedIn orderby PEEK, to better TGA–FTIR understand was performed the effect of with oxygen low on (<2%) the thermal oxygen decomposition concentration. productsFigure 7 releasedshows theby intensity PEEK, TGA–FTIR profiles of wasthe different performed products with low that (< were2%) oxygenidentified concentration. during the TGA–FTIR Figure7 shows analysis the withintensity 2% oxygen. profiles of the different products that were identified during the TGA–FTIR analysis with 2% oxygen.

Figure 7. TGA plot (top) of PEEK at 2% O2 at heating rate 20 ◦C/min and peak intensity profiles Figure 7. TGA plot (top) of PEEK at 2% O2 at heating rate 20 °C/min and peak intensity profiles (bottom) corresponding to gases evolved during the TGA measurement. (bottom) corresponding to gases evolved during the TGA measurement. It can be observed on the peak intensity profiles that the highest intensity corresponds to that It can be observed on the peak intensity profiles that the highest intensity corresponds to that of of CO2 peak. However, at the beginning of the decomposition (33 min), phenol, carbon monoxide, COmethane2 peak. andHowever, 4-phenoxyphenol at the beginning are also of identified. the decomposition Similar to (33 the thermalmin), phen decompositionol, carbon monoxide, of PEEK in methanenitrogen, and the 4-phenoxyphenol release of methane are occursalso identified. at a slightly Similar higher to the temperature thermal decomposition than that of the of PEEK release in of nitrogen,phenol andthe carbonrelease monoxide.of methane occurs at a slightly higher temperature than that of the release of phenolFrom and carbon the TGA, monoxide. it can be deduced that the first step of the decomposition is not greatly influenced by theFrom presence the TGA, of it oxygen. can be deduced However, that the the second first st step,ep of which the decomposition corresponds is to not the greatly degradation influenced of the bychar the formed,presence undergoes of oxygen. thermal However, oxidation the second when step, the which atmosphere corresponds contains to 2%the oxygen.degradation This of is eventhe char formed, undergoes thermal oxidation when the atmosphere contains 2% oxygen. This is even clearer when comparing the CO2 intensities. clearerTGA–FTIR when comparing was also the performed CO2 intensities. in a synthetic air atmosphere so as to get further information concerningTGA–FTIR its firstwas stepalso ofperformed degradation in a (Figure synthetic8) air atmosphere so as to get further information concerning its first step of degradation (Figure 8)

PolymersPolymers 20202020, 12, 12, x, 1789FOR PEER REVIEW 1212 of of 22 22

Figure 8. (top) TGA plot of PEEK in synthetic air at a heating rate of 20 ◦C/min and (bottom) peak Figureintensity 8. (top profiles) TGA corresponding plot of PEEK to in gases synthetic evolved air at during a heating the TGA rate measurement.of 20 °C/min and (bottom) peak intensity profiles corresponding to gases evolved during the TGA measurement. Only a few decomposition products were identified, as most pyrolysis products are probably thermo-oxidizedOnly a few decomposition before reaching products the spectrometer. were identified, However, as most the pyrolysis presence products of methane are and probably phenol thermo-oxidized before reaching the spectrometer. However, the presence of methane and phenol were still recorded... This means that at low temperatures (600 ◦C), despite the abundance of oxygen, werethese still decomposition recorded... This gases means are notthat oxidized. at low temperatures This is coherent (600 °C), with despite our previous the abundance hypothesis: of oxygen, the first thesedecomposition decomposition step hasgases little are or not no oxidized. dependence This on is thecoherent oxidative with or our inert previous nature ofhypothesis: the atmosphere the first it is decompositionin and at the heating step has rate little that or was no dependence used. on the oxidative or inert nature of the atmosphere it is in andFrom at the a fire heating behavior rate understanding, that was used. the decomposition products identified are potential flammable volatilesFrom that a fire can contributebehavior understa to feedingnding, a flame the during decomposition a fire scenario. products However, identified since a largeare proportionpotential flammableof the decomposition volatiles that gases can contribute correspond to tofeeding carbon a flame dioxide during (and a possibly fire scenario. carbon However, monoxide), since it a is largepossible proportion that dilution of the of decomposition flammables may gases occur corr in theespond gas phase to carbon when dioxide the material (and is possibly subjected carbon to heat. monoxide),Indeed, this it is was possible evidenced that dilution by comparing of flammables the heat may of combustion occur in the of gas PEEK phase to when that of the the material gas phase is subjecteddecomposition to heat. products Indeed, usingthis was a combustion evidenced by flow comparing calorimeter. theThe heat total of combustion heat of combustion of PEEK oftoPEEK, that ofmeasured the gas phase using decomposition an oxygen bomb products calorimeter using [18 a] wascombustion found to flow be around calorimeter. 31 kJ/g, The whereas total heat the heatof combustionof combustion of PEEK, of its decompositionmeasured using products, an oxygen measured bomb calorimeter using a microscale [18] was combustionfound to be around calorimeter 31 kJ/g,summed whereas up tothe around heat of 12 combustion kJ/g. This of means its deco thatmposition during a products, fire scenario, measured more than using 50% a microscale of the heat combustionreleased during calorimeter the burning summed of PEEK up to comes around from 12 kJ/g. the thermal This means oxidation that during of the chara fire structure scenario, and more not thanfrom 50% the of decomposition the heat released gases during themselves. the burning of PEEK comes from the thermal oxidation of the char structureThese results and not show from that the the decomposition thermal decomposition gases themselves. of PEEK leads to a plethora of decomposition products,These results whatever show the that atmosphere the thermal in decomposition which it is heated of PEEK in. However, leads to a itplethora can be of hypothesized decomposition that products,in an oxidative whatever environment, the atmosphere the gasesin which released it is heated during in. the However, initial decomposition it can be hypothesized step are, that for in the anmost oxidative part, thermo-oxidizedenvironment, the togases form released mostly during carbon the dioxide. initial Thisdecomposition means whatever step are, the for atmosphere, the most part,the thermo-oxidized first reactions occurring to form mostly would carbon correspond dioxid toe. theThis formation means whatever of a char the andatmosphere, the release the of first the reactionsdecomposition occurring gases. would However, correspond the fireto the behavior formatio ofn theof a materialchar and willthe release depend of on the the decomposition concentration gases.of these However, decomposition the fire gases behavior as a material of the willmateri onlyal ignitewill depend if there ison enough the concentration combustibles of as wellthese as decompositionoxygen availability. gases as Moreover, a material the will diff onlyerent ignite stages if ofthere a fire is scenarioenough combustibles correspond to as di wellfferent as oxygen kinds of availability.thermal stress: Moreover, before ignitionthe different (thermo-oxidation), stages of a fire transientscenario stagecorrespond (low oxygen to different concentration kinds of orthermal oxygen stress:deprived), before during ignition flaming (thermo-oxidation), combustion (oxygen transient deprived) stage and (low after oxygen flameout concentration (thermo-oxidation). or oxygen deprived), during flaming combustion (oxygen deprived) and after flameout (thermo-oxidation).

PolymersPolymers2020 2020, 12,, 1789 12, x FOR PEER REVIEW 13 of13 22 of 22

3.3. Fire Behavior of PEEK 3.3. Fire Behavior of PEEK In an attempt at linking the decomposition behavior of PEEK in the previous section to its fire In an attempt at linking the decomposition behavior of PEEK in the previous section to its fire behavior, the latter has been studied using the mass loss cone calorimetry (MLC). behavior, the latter has been studied using the mass loss cone calorimetry (MLC). Three different incident heat fluxes were used: 35 kW/m², 50 kW/m² and 60 kW/m² (Figure 9) on Three different incident heat fluxes were used: 35 kW/m2, 50 kW/m2 and 60 kW/m2 (Figure9) on PEEK plaques to study its fire behavior. PEEK plaques to study its fire behavior.

140

120 PEEK - 60 kW/m² PEEK - 50 kW/m² 100 PEEK - 35 kW/m²

Heat Release Rate (kW/m²) 20

0 0 500 1000 1500 2000 Time (s)

Figure 9. HRR profile of PEEK samples subjected to an incident heat flux of 50 kW/m2 using the MLC. SampleFigure dimensions: 9. HRR profile (100 of 100PEEK3.7) samples mm3 .subjected Distance to from an incident the heating heat cone: flux of 35 50 mm. kW/m² using the MLC. Sample dimensions: (100× × 100× × 3.7) mm3. Distance from the heating cone: 35 mm. When subjected to an incident heat flux of 35 kW/m2, PEEK does not ignite, even after 1000 s. Visual observationsWhen subjected made to includedan incident the heat formation flux of of 35 small kW/m², bubbles PEEK on does the surfacenot ignite, of theeven material after 1000 as s. wellVisual as slight observations melting. made included the formation of small bubbles on the surface of the material as wellWhen as slight subjected melting. to an incident heat flux of 50 kW/m2, the HRR of the material remains zero until it ignitesWhen (843 s).subjected After ignition, to an incident there is heat a sudden flux of increase 50 kW/m², in HRR, the HRR which of the peaks material at 125 remains kW/m2 (990zero s).until it Itignites is worth (843 noting s). After that, ignition, right before there ignition, is a sudden the materialincrease hadin HRR, swollen which by morepeaks than at 125 1000% kW/m² (visual (990 s). observations,It is worth Figure noting 10). that, This right means before that ignition, the gases the released material during had swollen the thermal by more decomposition than 1000% (visual of PEEKobservations, at this incident Figure heat 10). flux This do notmeans readily that ignite, the gases however, released upon during their release, the thermal they pull decomposition up the char of formed.PEEK Thisat this protects incident the heat underlying flux do polymericnot readily material ignite, however, by preventing upon thetheir incident release, heat they flux pull from up the reachingPolymerschar 2020 formed. it., 12, x FOR This PEER protects REVIEW the underlying polymeric material by preventing the incident 14heat of 22 flux from reaching it. The swelling phenomenon can be observed on Figure 10, which corresponds to pictures taken at different times during the test. Indeed, well before ignition occurs, at 307 s, significant swelling is already observed. As the swelling increases, the material gets increasingly close to the heating resistance. This means that the actual incident heat flux reaching the material is higher than the one measured at the initial sample position. Moreover, it should be noted that the igniter (red circle, Figure 10) does not contribute to the ignition. In fact, the region below the igniter is less swollen, possibly because of the lower heat flux incident on that part of the material due to a shielding effect brought about by the presence of the igniter. Once the polyme r ignited, the exposed surface smoldered (Figure 10, middle) before properly flaming up (Figure 10, right). It is interesting to note Figure 10. Pictures ofof PEEKPEEK takentaken atat (left(left)) 307 307 s, s, ( middle(middle)) 852 852 s s and and (right (right) 889) 889 s whens when it isit subjectedis subjected to that the ignition occurs on the inside2 of the material. anto an incident incident heat heat flux flux of 50of 50 kW kW/m²/m using using the the MLC. MLC.

The swelling phenomenon can be observed on Figure 10, which corresponds to pictures taken In order to dig deeper in the reactions that may be occurring during the thermal decomposition at diﬀerent times during the test. Indeed, well before ignition occurs, at 307 s, signiﬁcant swelling of PEEK under the incident heat flux, an FTIR was connected to the exhaust to analyze the gases evolved during the whole test. This method allowed for the quantitative analysis of water, carbon dioxide and carbon monoxide evolved during the test. Figure 11 shows the HRR curve as well as the profiles of the aforementioned gases with respect to time during the MLC test.

Figure 11. (red) HRR curve and intensity profiles of (black) CO, (blue) carbon dioxide (CO2] vol% =

[CO2] ppm/1000) and (magenta) water when tested under the MLC at 50 kW/m².

From the intensity profile of water, it can be observed that there is no water released before ignition. During the burning process, the intensity curve of water has a similar shape as the HRR and CO2 curve, as it can be expected for a combustion scenario. Furthermore, the intensity profile of CO and CO2 reveal that before ignition, there is a non- negligible amount of these gases that is produced. This means that the swelling that is observed during the charring phenomenon is partly led by the release of CO and CO2, among other decomposition gases. At 60 kW/m², PEEK exhibits a quite different behavior than at 50 kW/m². The HRR curve concerning the burning behavior of PEEK can be split into two parts, the first one corresponds to the

Polymers 2020, 12, x FOR PEER REVIEW 14 of 22

Polymers 2020, 12, 1789 14 of 22 is already observed. As the swelling increases, the material gets increasingly close to the heating resistance. This means that the actual incident heat flux reaching the material is higher than the one measured at the initial sample position. Moreover, it should be noted that the igniter (red circle, Figure 10) does not contribute to the ignition. In fact, the region below the igniter is less swollen, possibly because of the lower heat flux incident on that part of the material due to a shielding effect brought about by the presence of the igniter. Once the polymer ignited, the exposed surface smoldered Figure 10. Pictures of PEEK taken at (left) 307 s, (middle) 852 s and (right) 889 s when it is subjected (Figureto 10an, incident middle) heat before flux of properly 50 kW/m² flaming using the up MLC. (Figure 10, right). It is interesting to note that the ignition occurs on the inside of the material. InIn order order to to dig dig deeper deeper in in the the reactions reactions that that may ma bey be occurring occurring during during the the thermal thermal decomposition decomposition of PEEKof PEEK under under the incidentthe incident heat flux,heat anflux, FTIR an wasFTIR connected was connected to the exhaust to the exhaust to analyze to theanalyze gases the evolved gases duringevolved the during whole the test. whole This method test. This allowed method for allowed the quantitative for the quantitative analysis of water, analysis carbon of water, dioxide carbon and carbondioxide monoxide and carbon evolved monoxide during evolved the test. during Figure the 11 teshowsst. Figure the HRR 11 shows curve the as wellHRR as curve the profiles as well of as the the aforementionedprofiles of the aforementioned gases with respect gases to timewith duringrespectthe to time MLC during test. the MLC test.

FigureFigure 11.11. (red(red) HRR) HRR curve curve and intensity and intensity profiles proﬁles of (black of) CO, (black (blue) CO,) carbon (blue dioxide) carbon (CO dioxide2] vol% = 2 (CO[CO2]2] vol% ppm/1000)= [CO 2and] ppm (magenta/1000) and) water (magenta when) tested water under when testedthe MLC under at 50 the kW/m². MLC at 50 kW/m .

FromFrom the the intensity intensity profile profile of of water, water, it it can can be be observed observed that that there there is is no no water water released released before before ignition.ignition. During During the the burning burning process, process, the the intensity intensity curve curve of of water water has has a a similar similar shape shape as as the the HRR HRR and and COCO2 2curve, curve, as as it it can can be be expected expected for for a a combustion combustion scenario. scenario. Furthermore,Furthermore, thethe intensity profile profile of of CO CO and and CO CO2 reveal2 reveal that that before before ignition, ignition, there there is a isnon- a non-negligiblenegligible amount amount of these of these gases gases that is that produced. is produced. This means This that means the thatswelling the swellingthat is observed that is observedduring the during charring the charringphenomenon phenomenon is partly is led partly by ledthe byrelease the release of CO of and CO CO and2, COamong2, among other otherdecomposition decomposition gases. gases. 2 2 AtAt 6060 kW kW/m²,/m , PEEKPEEK exhibitsexhibits a a quite quite di differentfferent behavior behavior than than at at 50 50 kW kW/m²./m . TheThe HRRHRR curvecurve concerningconcerning the the burning burning behavior behavior of of PEEK PEEK can can be be split split into into two two parts, parts, the the first first one one corresponds corresponds to to the the rapid increase of HRR from its ignition to the first peak of HRR (Figure9). The second part, whereby the heat release rate starts to increase again and plateaus before slowly decreasing to zero.

Polymers 2020, 12, x FOR PEER REVIEW 15 of 22 Polymers 2020, 12, x FOR PEER REVIEW 15 of 22 Polymers 2020, 12, 1789 15 of 22 rapidrapid increaseincrease ofof HRRHRR fromfrom itsits ignitionignition toto thethe firstfirst peakpeak ofof HRRHRR (Figure(Figure 9).9). TheThe secondsecond part,part, wherebywhereby thethe heatheat releaserelease raterate startsstarts toto increaseincrease againagain andand plateausplateaus beforebefore slowlyslowly decreasingdecreasing toto zero.zero. SimilarSimilar toto thethe test test carried carried out out at 50at kW50 /kW/m²,m2, HRR H ofRR PEEK of PEEK when when it is subjected it is subjected to 60 kW to/m 602 remainskW/m² remainsremainsclose to zerocloseclose until toto zerozero ignition untiluntil ignitionignition at 113 s. atat After 113113 s. ignition,s. AfterAfter ignition,ignition, the heat thethe release heatheat ratereleaserelease increases raterate increasesincreases rapidly rapidlyrapidly until around untiluntil around105 kW /105m2 kW/m²before decreasing before decreasing again. This again. decrease This decrease in the HRR in the may HRR be may explained be explained by the limitedby the limited supply supplysupplyof fuel byofof fuelfuel the polymer,byby thethe polymer,polymer, due to the duedue char toto thethe structure charchar stst thatructureructure is formed thatthat isis duringformedformed this duringduring first thisthis phase firstfirst of phasephase burning ofof burningwhich may which have may temporarily have temporarily protected the protected inner layer th ofe inner the material. layer of However, the material. this protective However, layer this is protectivequickly smoldered layer is quickly into ashes, smoldered and the into fire ashes, on the and polymer the fire rises on again. the polymer This is rises visible again. on the This HRR is visible curve onwhereby the HRR the curve HRR startswhereby to increase the HRR again starts at to 309 increa s. Thesese againagain peak HRRatat 309309 reached s.s. TheThe peakpeak after HRRHRR this, isreachedreached 107 kW afterafter/m2 . this,this,It remains isis 107107 relatively kW/m².kW/m². ItIt constant remainsremains around relativelyrelatively this valueconstantconstant until around 480 s, wherethis value it starts until to gradually480 s, where decrease it starts again to graduallyuntil the material decrease flames again out.until the material flames out. FromFrom the the pictures pictures of of PEEK PEEK taken taken just just before before ig ignition,nition, some some swelling swelling ca cann be be observed observed (Figure (Figure 12,12, left).left). FumesFumes Fumes areare are escapingescaping escaping thethe the bulkbulk bulk materialmaterial material onon on thethe the left-handleft-hand left-hand sideside side ofof of thethe the materialmaterial material justjust just beforebefore before ignition.ignition. ignition. These fumes are ignited, leading to the subsequent burning of the material. This This ignition causes the whole polymer to start burning, causing a a rapid increase in the HRR, which peaks at 105 kWkW/m²/m2 (Figure(Figure 12,1212,, middle). middle). However,However, However, atat at thth thisisis stage,stage, stage, asas as PEEK PEEK burns,burns, signisigni significantficantficant swellingswelling isis still still observed (Figure(Figure 12,1212,, right). right). This This swellingswelling correspondscorresponds toto to sisi significantgnificant char char formation formation reactions reactions and and is coupled with a slight decrease in the heat release rate.rate.

Figure 12. Photographs of PEEK when subjected to 60 kW/m² under the MLC at different times. (left) FigureFigure 12. 12. PhotographsPhotographs of of PEEK PEEK when when subjected subjected to to 60 60 kW/m² kW/m2 underunder the the MLC MLC at at different diﬀerent times. times. ( (leftleft)) 79 s; (middle) 150 s; (right) 252 s. 7979 s; s; ( (middle)) 150 150 s; s; ( (rightright)) 252 252 s. s.

2 TheThe second part ofof thethe burningburning processprocess of of PEEK PEEK under under 60 60 kW kW/m²/m is is also also interesting. interesting. Indeed, Indeed, after after a aslight slight decrease decrease in in HRR HRR at at 150 150 s, s, the the HRR HRR starts starts toto riserise again.again. A relatively small flame flame remains remains present present 2 forfor aroundaround 3 3 min min and and subsides. subsides. DuringDuring this this time, time, the the HRR HRR stagnates stagnates at at around around 107107 kWkW/m².kW/m²./m . AfterAfter thethe attenuationattenuation of of the the flame, flame, the the heat heat release release rate rate does not instantaneously reach reach a a lower value but exhibitsexhibits a a gradual gradual decrease. decrease. During During this this time, time, the the ma materialterialterial isis inin aa a smolderingsmoldering smoldering state,state, state, wherebywhereby whereby itit slowlyslowly degrades,degrades, corresponding toto thethe incandescenceincandescence observedobserved atat ttt > >> 400400 s. s. ItIt meansmeans means thatthat thethe materialmaterial material isis beingbeing thermo-oxidizedthermo-oxidized duedue due toto to thethe highhigh incidentincident heatheat flux flux.flux.. TheThe The degradationdegradation degradation effecteffect effect isis clearlyclearly visiblevisible onon thethe materialmaterial (Figure 13,13, right) as there is hard hardlylyly anyany ofof itit leftleft atat thethe endend ofof thethethe test.test.test.

Figure 13. Photographs of PEEK under 60 kW/m² kW/m2 atat ( (leftleft)) 382 382 s, s, ( (middle)) 756 756 s s and ( right) at the end Figure 13. Photographs of PEEK under 60 kW/m² at (left) 382 s, (middle) 756 s and (right) at the end of the test. of the test. From the investigation of its ﬁre behavior, PEEK has proved to exhibit excellent resistance to ﬁre. This is in accordance with the high thermal stability that was observed in the previous section. Indeed,

Polymers 2020, 12, 1789 16 of 22 the first step that was observed when PEEK was subjected to an incident heat flux corresponded with the formation of a char. However, the swelling behavior was not predictable from the TGA analyses. There seems to a correlation between the thermal decomposition products and the charring behavior of the polymeric material that enhances the fire behavior of PEEK, especially at 50 kW/m2. The next section attempts to elucidate this by determining the thermal decomposition pathway of the material when it is subjected to a thermal stress.

3.4. Thermal Decomposition Reactions of PEEK under Pyrolytic Conditions From the STA of PEEK, a highly exothermic decomposition peak was observed ( 417 J/g). This was − attributed to the fact that a large energetic contribution goes to the formation of a stable char. In order to better understand this char formation, the decomposition mechanism of PEEK is investigated hereafter. From the TGA analyses, it was deduced that the thermal decomposition process was at least a two-step one. This was confirmed by the TGA–FTIR and pyrolysis GC–MS whereby different sets of decomposition products were detected at different temperatures. Moreover, the thermal decomposition products evolved during the MLC test have also provided insight as to the different stages of thermal decomposition occurring in PEEK when it is subjected to an incident heat flux. From the MLC–FTIR and TGA–FTIR results, it was observed that the first degradation products correspond to the formation of carbon monoxide and carbon dioxide. The formation of carbon monoxide is related to the graphitization mechanism (Scheme1). This corroborates with the MLC test as both swelling and charring were observed during the test (Figure 10, page 14). Previous reports have suggested that the charring process occurs at temperatures 2 above 750 ◦C. However, in the case of the MLC test at 50 kW/m , charring was observed while the temperature at the back of the sample was at around 500 ◦C. This means that the temperature on the surface is higher than 500 ◦C. Indeed, at this temperature, random scission of the polymer chain may release aromatic radicals. These radicals, when reacting with adjacent benzene rings on a benzophenone moiety leads to the formation of a fluorenone-like structure. Succeeding this reaction, the release of CO leads an aromatic diradical, two (or more) of which can react together to form a crosslinked aromatic structure. Scheme1 is a summary of this charring process [12]. Another possible pathway explaining the formation of carbon monoxide is illustrated in the first step of the decomposition mechanism in Scheme2. Homolytic scission of the carbonyl between two aromatic phenyls leads to the formation of carbon monoxide and two aromatic radicals. These aromatic radicals may combine to contribute to the carbonization process described in Scheme1. Moreover, from the EGA–MS measurements, it was observed that dibenzofuran was one of the first components to be detected. This suggests that it is released relatively early during its degradation. However, a direct mechanism for its formation seems improbable. Indeed, these two molecules involve a furan ring, which is unlikely to form at high temperatures because decomposition products such as phenol or phenoxyphenol are much more likely to occur [12]. One suggested pathway which could explain the formation of dibenzofuran and dibenzofuranol involves a two-step process. One, which occurs at low temperature, leading to the formation of the furan moiety on the polymer backbone, and another whereby random scission of the backbone leads to the release of dibenzofuran and dibenzofuranol (Scheme2). From the relative intensity profiles on the EGA–MS of PEEK, it can be deduced that the major product of such a decomposition is the dibenzofuran rather than dibenzofuranol. Indeed, dibenzofuranol is detected at a higher temperature with a lower relative intensity than dibenzofuran (Figure6). As for the formation of phenoxydibenzofuran, a similar mechanism is probable, whereby the carbonyl–phenyl bond is cleaved rather than an ether–phenyl one (Scheme2). Polymers 2020, 12, 1789 17 of 22 Polymers 2020, 12, x FOR PEER REVIEW 17 of 22

SchemeScheme 1. Proposed 1. Proposed mechanism mechanism for for the the graphitization graphitization processprocess of of PEEK PEEK at highat high temperature temperature [12]. [12].

Another possible pathway explaining the formation of carbon monoxide is illustrated in the first step of the decomposition mechanism in Scheme 2. Homolytic scission of the carbonyl between two aromatic phenyls leads to the formation of carbon monoxide and two aromatic radicals. These aromatic radicals may combine to contribute to the carbonization process described in Scheme 1. Moreover, from the EGA–MS measurements, it was observed that dibenzofuran was one of the first components to be detected. This suggests that it is released relatively early during its degradation. However, a direct mechanism for its formation seems improbable. Indeed, these two molecules involve a furan ring, which is unlikely to form at high temperatures because decomposition products such as phenol or phenoxyphenol are much more likely to occur [12]. One suggested pathway which could explain the formation of dibenzofuran and dibenzofuranol involves a two-step process. One, which occurs at low temperature, leading to the formation of the furan moiety on the polymer backbone, and another whereby random scission of the backbone leads to the release of dibenzofuran and dibenzofuranol (Scheme 2). From the relative intensity profiles on the EGA–MS of PEEK, it can be deduced that the major product of such a decomposition is the dibenzofuran rather than dibenzofuranol. Indeed, dibenzofuranol is detected at a higher temperature with a lower relative intensity than dibenzofuran (Figure 6). As for the formation of phenoxydibenzofuran, a similar mechanism is probable, whereby the carbonyl–phenyl bond is cleaved rather than an ether–phenyl one (Scheme 2).

Polymers 2020, 12, 1789 18 of 22 Polymers 2020, 12, x FOR PEER REVIEW 18 of 22

SchemeScheme 2.2. DecompositionDecomposition mechanism mechanism explaining explaining the theformat formationion of carbon of carbon monoxide monoxide as well as as well furan as furanderivatives. derivatives.

Moreover,Moreover, another another bond bond that that is susceptibleis susceptible to be to cleaved be cleaved at high at temperatureshigh temperatures is the ether–aromaticis the ether– one.aromatic Indeed, one. it wouldIndeed, lead it would to the lead formation to the offormation a phenoxy of radicala phenoxy that isradical stabilized that byis stabilized an aromatic by ring.an Therefore,aromatic ring. following Therefore, the release following of CO, the it isrelease likely thatof CO, the it cleavage is likely of that the ether–aromaticthe cleavage of linkage the ether– leads toaromatic the formation linkage of leads phenol to the (Scheme formation3). of phenol (Scheme 3).

Polymers 2020, 12, 1789 19 of 22 Polymers 2020, 12, x FOR PEER REVIEW 19 of 22

Scheme 3. Decomposition mechanism explaining the formation of aromatic decomposition gases and benzoquinoneScheme 3. Decomposition (adapted from mechanism [12]). explaining the formation of aromatic decomposition gases and benzoquinone (adapted from [12]).

Polymers 2020, 12, 1789 20 of 22 Polymers 2020, 12, x FOR PEER REVIEW 20 of 22

However, when the cleavage occurs over two adjacent ether–phenyl bonds, it leads to the formation of a benzoquinone (diphenoxy radical me mesomeresomere shown in Scheme3 3).). ThisThis diradicaldiradical cancan abstract two hydrogens to form a hydroquinone. During the pyrolysis GC–MS, both hydroquinone and benzoquinone benzoquinone were were observed, observed, suggesting suggesting that thatboth bothreactions reactions are occurring are occurring during the during thermal the decomposition.thermal decomposition. To explainexplain the the formation formation of carbonof carbon dioxide, dioxide, possible possible rearrangement rearrangement reactions reactions need to be need considered. to be considered.Indeed, during Indeed, thermal during decomposition, thermal decomposition, CO2 may be generatedCO2 may frombe generated the formation from ofthe carboxylic formation acid of carboxylicderivatives acid such derivatives as aromatic such ester as as aromatic described ester in Schemeas described4. This in canScheme be initiated 4. This bycan the be cleavageinitiated ofby thea bond cleavage between of a anbond aromatic between ring anand aromatic a carbonyl ring and function a carbonyl and an function ether bond, and an which ethersubsequently bond, which subsequentlyrearrange to form rearrange an ester. to form The latter an ester. can thenThe latte undergor can athen decarboxylation, undergo a decarboxylation, to release carbon to dioxiderelease carbonand two dioxide parts of and polymer two parts chain of radicals.polymer chain radicals.

Scheme 4. PossiblePossible r reactioneaction pathway involving the formation of carbon dioxide.

Furthermore, at around 620 °C,◦C, phenol and diph diphenyletherenylether were detected both by TGA–FTIR and by Py–GC–MS. Py–GC–MS. The The same same two two bonds bonds as as for for the the expl explanationanation for for the the formation formation of 4-phenoxyphenol of 4-phenoxyphenol are involved.are involved. However, However, the placement the placement of the of scission the scission and andthe subsequent the subsequent reactions reactions occurring occurring are not are notthe same.the same. Moreover, a cleavage of two adjacent carbonyl–phenyl bonds would lead to the formation of 1,4-diphenoxybenzene diradical.diradical. It can abstract a hydrogen on its either side leading to the formation of 1,4-diphenoxybenzene [[12].12]. Despite the extensive graphitization assumed during the thermal decomposition of of PEEK, hydroxybenzophenone was was also also detected detected during during EGA– EGA–MS.MS. Its Itsformation formation can can be explained be explained by the by fact the thatfact thatat high at hightemperature, temperature, two twoether–phenyl ether–phenyl bond bondss are cleaved are cleaved in such in such a way a waythat thatthey they form form the hydroxybenzophenonethe hydroxybenzophenone diradical. diradical. By abstracting By abstracting two two hydrogens, hydrogens, volatile volatile hydroxybenzophenone hydroxybenzophenone is formed.is formed.

4. Conclusions The thermal stability investigation on PEEK under under different diﬀerent oxygen concentrations have brought to light that the onset of the thermal decompositiondecomposition of PEEK is mostly independent of the oxidative or inert nature of the atmosphere.atmosphere. This suggests that initial step of the decomposition may involve the same reactions. However, when looking at the decomposition products at the beginning of the decomposition under low oxygen concentrations, a higher relative amount of carbon dioxide is

Polymers 2020, 12, 1789 21 of 22 the same reactions. However, when looking at the decomposition products at the beginning of the decomposition under low oxygen concentrations, a higher relative amount of carbon dioxide is observed than inert atmosphere, meaning that thermo-oxidation of the decomposition products occurs in the presence of oxygen. This step probably corresponds to the formation of a charred structure. However, when the char formed is in the presence of oxygen at high temperature, it starts to degrade thermally, showing a greater effect on the second step of the thermal decomposition. Understanding the thermal decomposition of PEEK was essential in explaining its enhanced fire behavior. First, the initial stage of its thermal decomposition leads to the formation of a highly stable charred structure. This structure has the ability of protecting the bulk of the polymer from incident heat flux. Adding to this, the combustible decomposition products released by PEEK is probably diluted by the high amount of carbon dioxide and carbon monoxide released during the early decomposition stages. Furthermore, the swelling behavior observed during the mass loss cone calorimetry test also plays a role in retarding the ignition of PEEK by, once again, protecting the underlying undecomposed polymer, leading to a slow release of combustibles. This charring of PEEK was possible due to the presence of a high amount of aromatic structures in the backbone of the polymer. Further evidence from the mechanism of char formation shows that having adjacent phenoxy radicals (stabilized because of the oxygen connected to the aromatic ring) is a valuable property to look for if char formation is to be promoted. However, this phenomenon is not observed to the same extent when the incident heat flux is increased. Indeed, under a higher heat flux (60 kW/m2), the release of combustibles occurs at a higher rate (higher temperature). There is less time for the polymer to start melting, form a char and for the released gases to rise and drive with them the surface of the polymer. This also means that the decomposition occurs faster and that the concentration of decomposition products is higher. Py–GC–MS and EGA–MS have allowed us to identify the decomposition products that are released at high temperatures, some of which are highly inflammable. It is probable that the concentration of these products becomes high enough to cause ignition. As ignition occurs, the decomposition mechanism of the material continues at a higher rate as there is little protection of the bulk material by the charred surface. Despite catching fire when subjected to high incident heat flux, the heat release rate of PEEK remains relatively low (<125 kW/m2 both with an incident heat flux of 50 and 60 kW/m2) which is an impressively low value for a wholly organic polymeric material.

Author Contributions: Conceptualization S.B.; Formal analysis A.R.; Investigation A.R., G.F., S.B.; Methodology A.R. and G.F.; Project administration S.B.; Supervision S.B.; Writing—original draft A.R.; Writing—review & editing G.F. and S.B. All authors have read and agreed to the published version of the manuscript. Funding: This work has received funding from the European Research Council’s Advanced Grant (Proposal N◦670747) for FireBar Concept project. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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