Thermal ageing of PTFE in the melted state: Influence of interdiffusion on the physicochemical structure

PTFE is one of the most used polymer for electrical insulation. For future aircraft, PTFE will be exposed to thermal constraints above its melting temperature. Its high melt viscosity allows PTFE to be operable but it will be subjected to thermal-oxidative ageing. PTFE presents two initial states, associated with its thermal history corresponding to the interdiffusion phenomenon. In the case of thermo-oxidative ageing in the melt, interdiffusion impairs the thermal stability by shifting the thermal degradation towards lower temperature. While interdiffusion reduces the thermal stability for long time ageing, strong physical interactions reduce the impact of degradation on the mechanical behaviour for short time ageing. Chemical ageing induced by degradation promotes recrystallisation of PTFE shorter chains; this crystalline phase modifies the β anelastic relaxation mode. The tan δ thermograms allows us to identify the β1 and β2 components of the anelastic effects of respectively the triclinic/hexagonal and hexagonal/pseudo hexagonal transitions observed by X-Rays Diffraction. Upon chemical ageing, the evolution of the β mode is mainly governed by the decrease in the β2 component corresponding to the pseudo-hexagonal phase. •Polytetrafluoroethylene films were aged under thermal stress in the melted state.•Thermal analysis revealed that polytetrafluoroethylene after interdiffusion is more affected by long time ageing.•Dynamic Mechanical Analysis shows that the crystalline phase is more affected by ageing than the amorphous phase.