I have a polymer composite with Magnesium Hydroxide (MDH), CaCO3 and glass fibers. I would like to determine exact amount of glass fiber using conventional TGA. I sue N2 up to 650 C and then air to 850 C. Finding the amount of CaCO3 is not a problem which decompose at much higher temperature than polymer and MDH. The decomposition of MDH and polymer overlaps so finding exact beginning and end point for dehydration of MDH using DTA peak is not accurate. Also, perhaps MgO from MDH reacts with CO2 from CaCo3 and forms MgCO3. I tried different heating rates to see if improves accuracy but it does not. Would TGA with auto step work better?
On the thermal dehydration of Mg(OH)2: R.C. Turner, I. Hoffman, D. Chen, "Thermogravimetry of the dehydration of Mg(OH)2", Can. J. of Chem., 41(2) 1963, 243-251: http://www.nrcresearchpress.com/doi/abs/10.1139/v63-039#.VcAlljhRHZ4
The question is not the mechanism of decomposition of Mg(OH)2, but how to determine the amount of three different fillers using TGA. Decomposition of MDH and glass filled PP are too close in conventional TGA.
You might perform TGA run under oxidative atmosphere to decompose PP earlier than Mg(OH)2 and hopefully seperate thermograms of these two components. Another option could be the help of DSC. Make a standard DSC analysis and inversely estimate the PP amount by assuming that the degree of crystallinity for i-PP is generally about 30-35 wt%. Then you can determine the amounts of inorganics with the remaining part.
What heating ramp rate are you using? You may try very slow heating rate (2 C/min or slower) in addition to the running under oxidative conditions. Once your thermogram has been obtained, the derivative of the resulting curve can often capture minute transitions that may not be completely obvious from the %mass vs. T curve.
Running at 10 C/ minute. 2 C/ minute is not practical as there are several samples.
I tried to run in Air instead of N2 but peak resolution was not much different. Here is the problem. Glass fiber reinforces polymer and hence increases its decomposition onset temperature. There is an overlap between MDH and polymer decomposition. So even though DTG shows two separate events, it is difficult to pinpoint where to select the end of MDH decomposition.
See attached scan with % weight loss and DTG signals.
Running at 10 C/ minute. 2 C/ minute is not practical as there are several samples.
I tried to run in Air instead of N2 but peak resolution was not much different. Here is the problem. Glass fiber reinforces polymer and hence increases its decomposition onset temperature. There is an overlap between MDH and polymer decomposition. So even though DTG shows two separate events, it is difficult to pinpoint where to select the end of MDH decomposition.
See attached scan with % weight loss and DTG signals.
You can't really get away from extending the time spent per sample with this issue. You either have to slow your heating rate as was previously recommended or you place isothermals at 390-ish and 450-ish to allow the reactions to be complete for each step. That is assuming the reactions are not related in some way. To save some time you could narrow your temperature range so you only focus on the region of interest.
Good luck.
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