Thermal Degradation CaCO3

[Moazie]

I'm trying to determine the CaCO3 amount in a polyacrylate sealant by using Thermogravimetric Analysis (TGA). Before I started testing the polyacrylate sample, I first did an analysis of pure CaCO3. The result proved my thoughts: one single mass drop due to CaCO3 degradation where CO2 leaves and CaO remains.

 

But now, when I test the polyacrylate sample: the TGA-curve shows me 2 mass drops. I already figured that due to the inert atmosphere used for the TGA (N2 gas) and the polyacrylate is an aqueous system, I'm dealing with the following reaction:

 

CaCO3 + H2O -> Ca(OH)2 + CO2 : Because of the N2-atmosphere I think that the equilibrium is pushed to the right side.

 

 

When you look at the TGA-curve, the last 2 mass drops are caused by CaCO3/Ca(OH)2. I would think the first mass drop is caused by Ca(OH)2-degradation into CaO and H2O, where H2O instantly evaporates. Unfortunately the amount of water evaporating (about 15% of total mass) is too large , so there must be a mistake in my way of thinking.

 

For the second mass drop, I think it's caused by excess CaCO3-degradation to CaO and CO2.

 

Can somebody help me out? Many Thanks!

Edited August 12, 2016 by Moazie

[studiot]

You are testing a carbonate/polyacrylate mixture are you not

I don't know what the decomposition temperature of the polyacrylate is but for silicone engine sealant it is around 300^o C and you are testing much higher than this?

Edited August 12, 2016 by studiot

[Moazie]

Well, CaCO3 is used as filler for the polyacrylate sealant. So, my goal is to determine the filler amount in this polyacrylate sample. The polyacrylate decomposition temperature is around 400°C. In my opinion, every mass loss before 400°C is caused by plasticizer and water, the mass loss at 400°C is caused by polyacrylate and every mass loss above 400°C is caused by CaCO3/Ca(OH)2. I've used the TGA to analyze an acrylate/water (50/50) dispersion and a pure CaCO3-sample. Here you can see how polyacrylate decomposes, and how CaO is formed out of CaCO3. But clearly, when combined in the polyacrylate mixture, CaCO3 doesn't decompose in the same way it does in pure form.

[studiot]

I know next to nothing about calcium acrylate chemistry, but there is clearly significant knowledge about.

 

https://www.google.co.uk/search?q=calcium+acyrylate&rlz=1C1AVNG_enGB673GB673&oq=calcium+acyrylate&aqs=chrome..69i57.9069j0j8&sourceid=chrome&ie=UTF-8

 

I would hazard a guess that some intermediate reactions as with this TG analysis of calcium salicylate

 

Edited August 12, 2016 by studiot

[Moazie]

Okay, Thank you, I will look into this matter

[Sensei]

CaCO₃ reacts with acid f.e. acetic acid giving off CO₂ gaseous which could be gathered and its volume measured (wait for reaching room temperature).
If polyacrylate does not react with acetic acid or other weak acid but reacts with CaCO₃,
you could determine amount of CaCO₃ this way (if measuring its amount is your the main aim).

Another possibility is (at least temporarily) making Calcium Bicarbonate
Ca(HCO₃)₂ as intermediate byproduct.

Have it been in hermetic container? Had it access to air (so could absorb CO₂ from it)?
https://en.wikipedia.org/wiki/Calcium_bicarbonate
" The term does not refer to a known solid compound; it exists only in aqueous solution containing the calcium (Ca2+), bicarbonate (HCO3−), and carbonate (CO32−) ions, together with dissolved carbon dioxide (CO2). "

 

Calcium Bicarbonate after heating turns to Calcium Carbonate water and Carbon Dioxide

Ca(HCO₃)₂->CaCO₃+H₂O+CO₂

then CaCO₃ undergoes further decomposition.

CaCO₃->CaO+CO₂

(so double as much Carbon Dioxide produced than what you would expect from pure CaCO₃)

Edited August 12, 2016 by Sensei

[John Cuthber]

"Another possibility is (at least temporarily) making Calcium Bicarbonate

Ca(HCO₃)₂ as intermediate byproduct.

Have it been in hermetic container? Had it access to air (so could absorb CO₂ from it)?"

Will someone please tell the relevant authorities that the famed "white cliffs of Dover" and, by analogy, the Dolomites need to be hermetically sealed.

It's possible that you temporarily make something that's even less stable- but the odds are against it.

What you really need is a thermogravimetry scan of some of the same polyacrylamide but without the calcite.

Do you have a solvent that will dissolve the polymer, but not the filler?

There's a simple solution if all you have is calcite and polymer.

Weigh some of the mixture.

heat it in air to some high temperature, to burn off the polymer.

Weigh it again- what will be produced is CaO and that's what you will weigh at this point (technically, I'd add a drop of water then re-heat it to ensure destruction of things like calcium cyanamide)

The calcium oxide weighs 56/100 of the weight of the CaCO3 originally present.