Thermal degradation analysis of innovative PEKK-based carbon composites for high-temperature aeronautical components

Nowadays, composite materials find a large application in several engineering fields, spanning from automotive to aerospace sectors. In the latter, especially in aircraft civil transportation, severe fireproof requirements must be accomplished, taking into account that the second most frequent cause of fatal accidents involving airplanes, was the post-impact fire/smoke, as reported by the European Aviation Safety Agency (EASA) in 2014. In the light of this, experimental research is of crucial importance in the understanding thermal behavior of composites for aircraft components, when exposed to high-temperature and fire conditions. In this context, a thermal degradation study is carried out for two carbon-reinforced resins: the well known thermosetting phenolic and a thermoplastic polyether-ketone-ketone (PEKK), recently developed specifically for this kind of application. The aim is to evaluate the PEKK behavior and to understand the impact of composite nature in terms of structural strength under fire. To this end, thermogravimetric analyses were performed for three different non-isothermal heating programs, between 30 and 1000 °C. Under inert atmosphere one single global reaction is observed for carbon-PEKK between 500-700 °C, while two for carbon-phenolic, whose pyrolysis begins around 200 °C. This better PEKK strengthening is attributed to the ether and ketone bonds between the three aromatic groups of the monomer. As expected, under oxidative atmosphere, the kinetic process becomes more complex, making more difficult the detecting of single-step reactions, especially for carbon-phenolic. Nevertheless, the oxidative process of carbon-PEKK seems to be driven by three consecutive global reactions. The activation energy is estimated by means of both integral (Starink) and differential (Friedman) isoconversional methods, as a function of the extent of conversion, corresponding to the identified reaction intervals. For carbon-PEKK in inert conditions, with Starink a mean value of 207.71 ± 6.57 kJ/mol was estimated, while 213.88 ± 20.04 kJ/mol with Friedman. This expected slight difference depends on the nature of the considered mathematical approaches. The difficulty in activation energy estimation for polymeric materials prefers the use of at least two different methods, allowing for the identification of an activation energy range, for a resin of which no data are available in the literature. For the decomposition model evaluation, the so-called compensation effect method was implemented, as well as the single-step-based approach proposed by Friedman. The evaluation of a possible decomposition expression has been achieved only for carbon-PEKK under inert conditions, since the considered methods are valid and applicable only for well defined single-step reactions. In fact, the three reactions of the oxidative case cannot be considered as single-step processes. Moreover, the higher difference in the estimated activation energy between Starink and Friedman suggests to check the achieved results by implementing further isoconversional methods, to understand the most reliable for polymer-based carbon composites degradation analysis under oxidative atmosphere. However, the observed higher thermal performance of PEKK resin, attributed to its chemical structure, increases the interest toward its use as matrix for aerospace composite materials, that can be subjected to hazardous environments.