I carried out multiple TGA tests on a carbon-epoxy composite. It was performed in nitrogen, in the temp range 20-800C at heating rates of 5, 10 and 20C/min. For 5C/min, above 600C, a slow, continuous mass loss is observed, whereas at 10 and 20C/min, the curve remains flat after 600C. What might be the explanation for that? Can this be a second step of decomposition? The graph is attached
I think you need to look into the state of the decomposed polymer at high temperatures. In your case, maybe it reached the range of glass transition and decomposition temperatures.
Dear Ewa Mikinka,
The thermal decomposition of Epoxy matrices is described with the following equation which β denotes the heating rate (𝑑𝑇⁄𝑑𝑡, ˚C/min).
𝐷 = ∫ (𝐴/ 𝛽 exp (− 𝐸𝑎 /𝑅 𝑇 ) (1 − 𝐷)^𝑛)𝑑𝑇
So, the heating rate affects the decomposition rate. For more details, I have attached one article. I hope it would be useful.
I expect there is the second mass-loss process after 600 degC. This process shifts to higher temperatures with increasing heating rate. The expected second mass-loss process is also evident in your TG-DTG curves even at heating rates of 10 and 20 degC/min, which is observed at higher than 700 decC. Please try to extend the maximum temperature for these measurements.
I found in your TG-DTG curves that the mass-loss ratio for the first mass-loss process changes depending on the measurements. This does not systematically change with heating rate. Thus, I expect, if the initial sample mass was correctly measured, your composite sample is not homogeneous enough to trace the thermal degradation behavior, especially for discussing its kinetics using TG-DTG curves at different heating rates. Please check your sample when you study the thermal degradation kinetics.
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