Investigation of the Fire Behaviour of PEEK-based Polymers and Compounds

Abstract

Polyetheretherketone (PEEK) is a polymer with outstanding performance, particularly concerning temperature resistance, chemical resistance and mechanical characteristics. Literature shows a gap in the knowledge with regards to PEEK flammability and decomposition products.
The aim of this thesis is to describe the decomposition and flammability behaviour of PEEK, in order to develop new fire safe PEEK-based materials. By relating the measured thermal decomposition behaviour and changes in physical properties of PEEK materials to their performance in standard flammability tests, the dependence of these tests has been investigated with regards to orientation, thickness, presence of fillers, moisture absorption and absorption of infrared radiation. This understanding can inform the development of modified PEEK materials with enhanced fire safety.
Various industry standard tests have been utilised to examine PEEK such as the Cone Calorimeter (ISO 5660), UL-94 (EN 60695-10-11), Limiting Oxygen Index (LOI), Thermogravimetric Analysis (TGA) in both air and inert atmospheres and the Small Flame Ignitability Test (ISO 11925). PEEK decomposition has been investigated using Thermogravimetric Analysis (TGA), Simultaneous Thermal Analysis coupled with Fourier Transform Infrared (STA-FTIR) and Pyrolysis Gas Chromatography coupled with Mass Spectrometry (pyGC/MS). Residue analysis has been carried out using Diamond Attenuated Total Reflectance coupled with Fourier Transform Infrared (dATR-FTIR), Solid State Nuclear Magnetic Resonance (MAS-NMR) and Scanning Electron Microscopy coupled with Electron Dispersive X-ray Analysis (SEM-EDX).
Thermal analysis shows a rapid mass loss around 580°C followed by a slower mass loss of the resultant char. The stages of decomposition have been investigated by using FTIR and NMR on the condensed phase residues. Samples subjected to various temperature regimes have been observed in the dATR-FTIR and MAS-NMR and the results shows differences in the structures of the residue during decomposition.
Generally, filled PEEK materials (with glass fibre, carbon fibre and talc) tend to have lower flammability in the Cone Calorimeter with a longer time to ignition and a lower peak heat release rate compared to the unfilled materials. The same is true in the LOI, where filled materials give a higher oxygen index, and in the UL-94 test where shorter burn times are recorded. PEEK shows inconsistent behaviour in some flammability tests, possibly due to the critical heat flux for ignition of PEEK being so close to the heat fluxes employed in many industry standard tests. The presence of moisture within the samples also reduces the time to ignition of PEEK in the Cone Calorimeter and increases the burning time in the UL-94, possibly due to the formation of a foamed sample close to the melting temperature of the polymer.