Evaluation of the thermal oxidative stability of polyimides via TGA techniques

Abstract

Thermogravimetric analysis (TGA) has been used for many years to evaluate polymer thermal stability. The objective of this study is to determine if weight-loss curves from TGA and isothermal TGA (IGA) can be used to determine degradation activation energies and thus rank the thermal stability (TS) and thermooxidative stability (TOS) for selected polyimides. Two high-temperature stable addition-cured polyimides and two aromatic condensation polyimides, all four containing fluorinated connecting linkages in the dianhydride monomers, were compared. Three TGA kinetic methods (Coats/Redfern, Ingraham/Marier, Horowitz/Metzger) were used to determine the activation energy for decomposition in air. The results were then used to rank polyimide stability compared to more traditional rankings based on long-term isothermal air aging weight-loss (IWL) studies and thermal decomposition temperatures (T d ) from TGA data. Use of TGA coupled to a Fourier transform infrared (TGA-FTIR) spectrophotometer allowed for the simultaneous identification and relative quantification of evolved decomposition products (CO 2 , CO, ArNCO, and CHF 3 ) of the four polyimides degraded in air or nitrogen. Isothermal TGA-FTIR (IGA-FTIR) was also done in air to determine the relative rate of product evolution at a constant temperature. Activation energies using TGA and IGA data were determined and then compared with IWL values for the degradation of the polyimide to examine for correlations of real-life thermal oxidative aging to accelerated aging techniques. The Coats/Redfern method and T d were found to best reproduce stability rankings of those from long-term, high-temperature IWL studies. Together, they may provide a time-saving technique to evaluate polyimide thermal oxidative stability.

Carbon molecular sieve structure development and membrane performance relationships

“…1), consistent with other studies(34)(35)(36)(37)(38). On the other hand, CO and CO 2 evolution occurs even at relatively low temperatures to remove most, but not all, of the originally present oxygen atoms.…”

Thermal degradation of high‐temperature fluorinated polyimide and its carbon fiber composite

“…Furthermore, the resin has lost 5% of its weight at 529°C in air, whereas this same level of degradation does not occur until 538°C in nitrogen. The lower degradation temperature observed with an air atmosphere indicates a modest level of oxidation 19. This fluorinated polyimide can withstand up to 10 min at 510°C, just below its T d , with only a 4.5% weight loss in air.…”

“…Carbon fiber acts as a protective barrier for the resin in the composite to prevent exposure to destabilizing elements. This protection allows the resin to withstand higher temperatures while also increasing the strength, thermal oxidative stability, and higher strength to weight ratio than their metal counterparts 19. The barrier from the carbon fiber also delays the decomposition of the polyimide resin within the composite.…”

“…Previous studies have focused on how similar PMR (polymerization of monomer reactants) type polyimides degrade at high temperatures in an attempt to predict the long‐term, high‐temperature performance 8, 19. There are two different pathways that degradation occurs for these polyimides.…”

Novel acetylene‐terminated polyimide oligomers with excellent processability and high toughness of films

“…The temperatures of 5% weight loss in N 2 atmosphere were all above 468°C, the char yields reported at 800°C in N 2 atmosphere were all above 54%. Surprisingly, the T d and T d10 value of all the cured films in N 2 were much lower than in air which may caused by the unreacted ethynyl moieties were oxidized and made crosslinks . Therefore, the cured films exhibited better thermal stability, especially thermal‐oxidative stability.…”