I am trying to determine OIT of PE using DSC and TGA. Process is heat 5 mg, 1mm thick unstabilized HDPE on open Al Pan under Nitrogen (200 mL/miniute, 50 psi, introduced in DSC) to 190 C, maintain for 5 minutes and switch to Oxygen (50 mL/minute, 50 psi pressure). I am not seeing shift in base line or increase in exotherm . Iw as expecting OIT of 5-10 minutes for unstabilized HDPE but I do not see it even after 180 minutes. Something wrong with test. However, rotometer or both gases are stable indicating no gas leaks.
Is 50 psi too low a pressure? is 50 mL/minute not enough?
You performed an isothermal TGA run and followed the DTA signal. Although it is almost the same concept with an isothermal DSC run, I suggest you to make OIT analysis in a DSC due to the difference in signal sensitivity of devices. The ASTM probably recommends to perform isothermal OIT runs at a particular tempretature between 190-220 C (?). But, you might first make a non-isothermal run between a temp. range of 180-300 with a slow heating rate (3 or 5 C/min.) after switching to O2 in DSC to observe onset temperature of oxidation exotherm. Then you can set up your test conditions. You may also increase the O2 flow rate.
Good Luck.
I also feel what Ali Durmus says is quite valid in this case, normally OIT is done by DSC and pretty simple and dependable too.
I ran on both. TGA has two issues. ASTM method is at 200 C and 50 mL/minute rate with 14-15 Kpa pressure. TGA induces thermal degradation as well as oxidation . Upon oxidation, sample weight increased slightly. Sample temperature increased upon oxidation. I will use DSC which I was avoiding as use of pure oxygen had damaged my DSC once before.
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