Investigation of thermal and thermo-oxidative degradation of poly(ether ether ketone) by gas chromatography

Albert S. SABAEV, Azamat A. ZANSITOV, Zhanna I. KURDANOVA, Leana Kh. KUCENOVA, Svetlana Yu. KASIROVA(- . . , 360004, )

Poly(ether ether ketone) (PEEK) is a semicrystalline aromatic polymer with excellent thermal and mechanical properties including good chemical resistance, low flammability, excellent wear, and fatigue resistance [1,2].

The thermal characteristics of PEEK and the products of its thermal degradation have been studied previously via gas chromatography-mass spectrometry [3-5]. Among the gaseous pyrolysis products, CO and CO2were detected and their total yields were determined. Various fragments were then identified using a mass spectrometer, which included benzene, phenol, hydroquinone, and larger fragments.

The purpose of this investigation was to study the kinetics of the evolution of the main gaseous products of the thermal and thermo-oxidative degradation of PEEK.

1 Experimental

PEEK was synthesized by a nucleophilic substitution reaction of 4,4′-difluorobenzophenone with hydroquinone at a molar ratio of 1.01∶1 in the presence of anhydrous potassium carbonate (K2CO3) in diphenyl sulfone (Fig. 1). The reaction was carried out under a stream of nitrogen with continuous stirring and a stepwise temperature increase from 30 ℃ to 320 ℃. After 5 h at 320 ℃, the reaction mixture was discharged, cooled and the resulting solid was milled. The diphenyl sulfone solvent and inorganic salts were removed by washing successively with acetone (twice), water (three times), and isopropyl alcohol (twice). The resulting solid polymer was dried at 150 ℃ under vacuum.

Fig. 1 Scheme for the synthesis of PEEK

Thermogravimetric investigations were carried out on a PerkinElmer TGA 4000 instrument in air and nitrogen atmospheres. The temperature increase rate was 5 ℃/min over the range of 30-750 ℃.

Analysis of the main gaseous products of pyrolysis of PEEK was carried out using a Tsvet-800 gas chromatograph with a thermal conductivity detector, according to a procedure described previously [6].

2 Results and discussion

Thermogravimetric analysis showed that in an inert atmosphere (Fig. 2, in nitrogen), 2% and 5% of the mass loss occurred at temperatures of 550 and 560 ℃, respectively. Decomposition of the polymer occurred in one stage with the formation of a coke residue accounting for approximately 50% of the original polymer weight.

Fig. 2 Thermogravimetric decomposition curves of PEEK

In air (Fig. 2), 2% and 5% of the mass loss occurred at 530 and 550 ℃, respectively. In this case, the mass loss curve exhibits two clearly expressed stages. The first stage corresponds to the reactions involved in the rupture of the main polymer chain, the speed of which indicates a radical-chain mechanism of decomposition. In the second stage, the rate of mass loss clearly decreased, indicating a transition from the radical chain mechanism to simple combustion reactions (wherein the polymer combusted completely). To further investigate the nature of the processes occurring during the pyrolysis of PEEK, trends in the formation of the main gaseous products were studied. The investigations were carried out using gas chromatograph with a thermal conductivity detector at temperatures of 400-500 ℃ under isothermal conditions with a pyrolysis time of 100 min. Each investigation used a fresh sample of the polymer (30 mg).

Results indicated that gaseous degradation products were not formed in significant amounts at temperatures up to 425 ℃, except for trace amounts of H2and CO which appeared after pyrolysis for 100 min at 400 ℃.

Fig. 3 displays the curves of hydrogen evolution at temperatures above 400 ℃.

Fig. 3 Kinetic curves of hydrogen evolution during thermal degradation of PEEK

Hydrogen evolution at 425 and 450 ℃ was insignificant, and was associated with the processes of structuring (cross-linking). At 500 ℃, hydrogen evolution increased approximately five times, implying the destruction of not only the main polymer chain but also the benzene ring. Based on the structure of PEEK, the main mechanism of decomposition was assumed to primarily involve exposed ketone groups with the liberation of CO as shown in Fig. 4.

Fig. 4 Scheme of the decomposition of the ketone moieties

However, approximately equivalent amounts of CO2were produced, as seen in Fig. 5. The formation of CO2indicates that, at temperatures above 475 ℃, decomposition of the ether bond releases oxygen, which then oxidizes CO to CO2.

Fig. 5 Kinetic curves for CO and CO2 evolution during thermal degradation of PEEK

A further increase in pyrolysis temperature is accompanied by the formation of CH4(beginning at 525 ℃, as seen in Fig. 6).

Appreciable quantities of CH4were observed at temperatures of 525 ℃. Under such high temperatures, destruction of benzene rings occurred and was accompanied by a coking process. Thus, the thermal destruction of PEEK begins at lower temperatures with destruction of the ketone and ether bonds and proceeds to processes of coke formation at higher temperatures.

Further, the formation of water during thermal decomposition was investigated and is shown in Fig. 7.

Fig. 6 Kinetic curves of methane formation during thermal degradation of PEEK

Fig. 7 Kinetic curves for H2O evolution during thermal degradation of PEEK

Water formation began at a temperature of 425 ℃ and after a pyrolysis time of 20 min. For temperatures up to 475 ℃, the amount of H2O produced increased over time. However, a further increase in temperature (to 500 ℃) led to a decrease in water formation. We associated this behavior with the thermal hydrolysis of PEEK. In general, to clarify the mechanism of the effect of water on the thermal characteristics of a given polymer, it is necessary to investigate the formation of decomposition products with forced dosing of H2O into the reaction mixture at lower temperatures (375-425 ℃). The results obtained in this way helped us to separate thermohydrolysis from thermal degradation, which further allowed us to evaluate both the quality of cleaning and the quality of polymer drying. The method of investigation of the thermal and thermo-oxidative degradation of polymers published previously [6] allowed us to study the kinetics of oxygen absorption with a simultaneous analysis of the gaseous products of thermo-oxidative degradation. As seen in Fig. 8, the nature of oxygen uptake at temperatures up to 400 ℃ did not change and the curves exhibited S-shapes.

Fig. 8 Kinetic curves of oxygen absorption

This behavior was attributed to the ability of PEEK to cross-link at these temperatures, which hinders the oxidation process. At a higher temperature (400 ℃), thermal degradation significantly prevailed over the crosslinking processes, which inevitably affected the rate of oxygen uptake.

In the thermo-oxidative degradation of polymers, gaseous products are released, which mainly comprise carbon oxide and carbon dioxide. Other products released in the thermo-oxidative degradation of PEEK included insignificant amounts of hydrogen (as displayed in Fig. 9) and trace amounts of methane.

Preliminary experiments suggested that significant absorption of oxygen occurred at temperatures as low as 300 ℃, although according to the thermogravimetric analysis, PEEK began to lose weight in air at a temperature of 500 ℃ (Fig. 2). The kinetic curves of oxygen absorption as functions of time and temperature are displayed in Fig. 8.

The amounts of hydrogen formed at temperatures of 325-375 ℃ approximately corresponded to the amounts formed during thermal degradation. However, at temperatures of 400-425 ℃, hydrogen was released at 3 times during thermo-oxidative degradation (Fig. 9) more than during thermal pyrolysis (Fig. 3). An additional source of hydrogen production under these conditions was thought to be the oxidation of aromatic atoms, which is accompanied by the detachment of a hydrogen atom as shown in Fig. 10.

Fig. 9 Kinetic curves of hydrogen evolution during the thermo-oxidative degradation of PEEK

Fig. 10 Scheme of the oxidation of the aromatic ring

This process proceeds with the formation of hydroperoxide radicals, the isomerization and decomposition of which is accompanied by the release of various other oxidation products.

Fig. 11 shows the kinetic curves of CO and CO2evolution during thermo-oxidative degradation.

Fig. 11 Kinetic curves of evolution for CO and CO2 during thermo-oxidative degradation

As seen in the graphs, at all temperatures studied for thermo-oxidation, CO2was formed in higher amounts than CO; the yield of the latter became significant starting at 375 ℃. Based on the structure of the polymer and the sample weight of the test samples, it was calculated that the maximum total yield of CO and CO2could not exceed 2.4 mL, which would signify the complete destruction of ketone groups. Consequently, at the investigated temperatures, it was found that there was no oxidation of the carbon atoms of the benzene rings to CO or CO2.

3 Conclusions

In this paper the thermal and thermo-oxidative degradation mechanisms of PEEK were investigated. It was concluded that during thermal degradation, the decomposition of the polymer starts with the rupture of ketone and ether bonds and proceeds to destruction of the benzene ring at higher temperatures, which is accompanied by the formation of H2O and CH4. Above 500 ℃, the polymer degradation further involved thermohydrolysis. The thermo-oxidation of PEEK, which was accompanied mainly by the formation of CO2and H2, was noticeable beginning at 325 ℃. The total yield of the latter indicated oxidation of fragments of the benzene ring.

Acknowledgment

This work was carried out within the framework of the agreement No. 14.577.21.0240 with the Ministry of Education and Science of the Russian Federation of September 26, 2017 (project identifier RFMEFI57717X0240).

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