Degradation mechanism of FKM during thermo-oxidative aging from mechanical and network structure correlations

The thermal weak point of a peroxide-cured fluoroelastomer (FKM) in presence of triallylisocyanurate (TAIC) has been identified. To that purpose, the thermal stability was assessed by TGA measurements. In parallel, thermo-oxidative degradation was characterized after the aging of several samples at 250 °C for various durations. Specific attention has been directed to the structural evolution, evaluated by both FTIR and equilibrium swelling experiments. Experimental results highlight that TAIC, the system's crosslink node, is the thermal weak point of a peroxide-cured fluoroelastomer (FKM). Nonetheless, crosslinks are not entirely broken since the crosslinking density decreases with aging time before stabilizing for one week at 250 °C. This allows the material to retain some of its mechanical properties such as good elongation at break. For longest exposure times at 250 °C (beyond one week), chain scission mechanism becomes predominant leading to a loss of the mechanical properties. Regarding the degradation mechanism, TGA-FTIR results are rather in favor of a dehydrofluorination mechanism as hydrogen fluoride (HF) is detected upon the first times of thermo-oxidative aging. In a second step, chain scission starts to occur. Recombination reactions between double bonds provided by dehydrofluorination and macroradicals formed during chain scission could explain that FKM is still crosslinked after one week at 250 °C under air.