I'm a chemistry student and I'm researching the kinetic degradation of mixed plastic waste. I'm having trouble calculating the activation energy by the coats & Redfern method.
The data that I have is HDPE plastic waste from TGA heating rate 5 °C/min, 10°C/min, 20°C/min and 30°C/min with 100 ml flow rate of nitrogen. I plot the graph of ln(-ln(1-alpha)/T^2) and 1/T using linear fitting in Origin software. Alpha is (mi - mt)/(mi - mf), with mi as the initial mass, mt is the mass at time t, and mf the final mass. The slope (activation energy) will be times - R (8.314 J⋅K−1⋅mol−1) from -Ea/R = slope.
From the literature that I have read, the Ea of HDPE is around 150 - 300 kJ/mol. However, from my calculation, I got the Ea of 700 - 800 kJ/mol. There might be a miscalculation from my method. I wanted to ask,
what is the correct alpha?
Does mt mean mass at any given time?
Is linear fitting in origin software already correct?
Can I know the detailed steps to calculate activation energy from the coats & Redfern method?
It'll be such a huge help, thank you.
Your alpha definition seems correct, maybe you need to check the temperatures of decomposition steps. In calculations, you have to use Temperature in Kelvin. Also, you may check the written forms of the formulas in your excel, one more time.
The correlation factor (Linearity) of the plot should be higher than 0.99
To have examples, you can read my studies:
DOI: 10.1007/s11164-015-2326-5
DOI: 10.1007/s11164-015-1952-2
DOI: 10.1134/S1087659620010162
Good luck!
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