April 03, 2020, 08:36:31 PM
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Topic: Behaviour of molten metal interacting with polylactic acid  (Read 529 times)

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Offline tunkor

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Behaviour of molten metal interacting with polylactic acid
« on: March 24, 2020, 06:08:25 AM »
Hello all!

I am kinda shooting in the dark here. I'm a mechanical engineering student with very little knowledge of chemistry. I'm trying to figure out a proces for casting a 3d printed model by directly pouring molten iron (1400C) on top of it.

The model is coated by a ceramic coating and put in a sand container. The molten metal 'interacts' with the PLA model by melting and evaporating(?) the PLA. The metal will take the shape of the 3D printed model inside the ceramic coating shell, leaving a casted product. I have done a few tests which came out decent, I am however eager to know about what exactly happens when these two materials (fluid metal, and PLA) interact with eachother.

What happens to the PLA, how fast does this happen, what kind of rest products are left behind, how much gas does the PLA produce? Information like this would be useful for me to know, to maybe screen for other printing materials other than PLA. I'm basically trying to analytically see what happens with the material.

For example, I want as little gas production from the material, because this gas needs to leave via the ceramic coating. So if I can screen a material beforehand this would be useful.

Would anyone know how I can figure this out, maybe some useful online tools/databases/literature?

I hope I am asking this in the right place!

Thanks in advance.

Kind regards.

Offline Borek

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #1 on: March 24, 2020, 06:01:27 PM »
By the look of it I would expect decomposition to a mixture of gases (CO, CO2, ethylene, water, some of them at these temperatures would immediately burn on contact with air), and perhaps some elemental carbon (which can easily dissolve in the iron). Assuming simple decomposition to CO2 and ethylene simple stoichiometry is all you need to calculate amounts.

The exact outcome is insanely difficult to predict, this is one of these cases where the experimental approach is the only viable way of getting into details.
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Offline Enthalpy

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #2 on: March 24, 2020, 06:47:26 PM »
This process uses a foam traditionally, polystyrene foam. It works because the same volume holds little foam mass or much iron mass. Polystyrene leaves uncontrolled dirty things (mostly carbon) whose small proportion in iron is tolerable.

I can't tell if the same is done with plain polymer like PLA nor if it can succeed.
  • The amount of PLA needs much heat to decompose, possibly more than hot iron brings.
  • The decomposition of PLA would leave a huge amount of carbon. Probably, iron can't fill the mold, but if iron achieves to decompose the PLA, too much carbon in iron will spoil its properties.
It that process already used? What lets you suppose that the dirty things will escape through the ceramic?

What the reaction is: mainly a pyrolysis, which is a pedant word for "makes a bunch of unknown dirty things". The big heat breaks all big molecules, the atoms rearrange in small molecules and often in graphite. From the C3H4O2 period, 2×CO may escape and possibly carry some hydrogen away. From the remaining C, some may leave as small hydrocarbons, but I suppose some remains as graphite.

By the way, pyrolysis products are always a bit toxic. Nothing tragic, but better do it outside, or in a well ventilated area, without making a chicha of it. If this shall become a regular activity, you need some protective measures.

Offline Borek

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #3 on: March 24, 2020, 08:17:12 PM »
It that process already used?

I have done a few tests which came out decent
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Offline tunkor

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #4 on: March 25, 2020, 06:18:45 AM »
This process uses a foam traditionally, polystyrene foam. It works because the same volume holds little foam mass or much iron mass. Polystyrene leaves uncontrolled dirty things (mostly carbon) whose small proportion in iron is tolerable.

I can't tell if the same is done with plain polymer like PLA nor if it can succeed.
  • The amount of PLA needs much heat to decompose, possibly more than hot iron brings.
  • The decomposition of PLA would leave a huge amount of carbon. Probably, iron can't fill the mold, but if iron achieves to decompose the PLA, too much carbon in iron will spoil its properties.
It that process already used? What lets you suppose that the dirty things will escape through the ceramic?

What the reaction is: mainly a pyrolysis, which is a pedant word for "makes a bunch of unknown dirty things". The big heat breaks all big molecules, the atoms rearrange in small molecules and often in graphite. From the C3H4O2 period, 2×CO may escape and possibly carry some hydrogen away. From the remaining C, some may leave as small hydrocarbons, but I suppose some remains as graphite.

By the way, pyrolysis products are always a bit toxic. Nothing tragic, but better do it outside, or in a well ventilated area, without making a chicha of it. If this shall become a regular activity, you need some protective measures.

The lost foam proces is what I'm 'copying' from. I am using a PLA which has a foaming additive to it, achieving very low weight printed parts. This was kinda the basis for starting this experiment, because the total density of the part can come pretty close to what a foam model would be. The difference being that the PLA model is mostly hollow, whereas the foam is homogeneous. This brings a difference in how the metal will fill the void (like the metal flowing too fast), but that's another story I am trying to experiment with.

How much energy does it take te decompose PLA compared to PS? Is this what activation energy means? How much carbon does PLA leave compared to PS?

I tried a few tests, but haven't checked the casted part for any properties (but this is in the works). The parts were pretty small and simple (a rounded off cube) but mostly casted fine, some parts did not fill up nicely. But I will be testing new PLA models soon which hopefully have even less mass which has to be burned.

By the way thank you so much for taking the time to reply, I really appreciate it!

Offline tunkor

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #5 on: March 25, 2020, 06:21:45 AM »
By the look of it I would expect decomposition to a mixture of gases (CO, CO2, ethylene, water, some of them at these temperatures would immediately burn on contact with air), and perhaps some elemental carbon (which can easily dissolve in the iron). Assuming simple decomposition to CO2 and ethylene simple stoichiometry is all you need to calculate amounts.

The exact outcome is insanely difficult to predict, this is one of these cases where the experimental approach is the only viable way of getting into details.

Thank you for your reply!

So if you simplify the reaction, the PLA would decompose into CO2 and ethylene?

Offline Borek

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #6 on: March 25, 2020, 09:04:14 AM »
So if you simplify the reaction, the PLA would decompose into CO2 and ethylene?

Yes, at first stage. Just remember - this is just an educated guess with all possible disclaimers, experiment can easily prove it wrong. Search for a thermogravimetric curve for PLA, it should give some insight into what is happening.

How much energy does it take te decompose PLA compared to PS?

You can try to use Hess law to estimate the numbers.

Quote
Is this what activation energy means?

No, activation energy is related to the reaction speed, you are looking for reaction enthalpy.

Quote
How much carbon does PLA leave compared to PS?

No way to tell - thermal decomposition (pyrolysis) is a process consisting of many quite random reactions, with lots of side products.

I tried a few tests, but haven't checked the casted part for any properties (but this is in the works). The parts were pretty small and simple (a rounded off cube) but mostly casted fine, some parts did not fill up nicely. But I will be testing new PLA models soon which hopefully have even less mass which has to be burned.

By the way thank you so much for taking the time to reply, I really appreciate it!
[/quote]
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Offline Enthalpy

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #7 on: March 25, 2020, 09:20:22 AM »
PLA foam is an excellent start. It could be better than polystyrene foam, because PLA contains more oxygen to form gaseous compound with carbon. I could only imagine POM-H (polyoxomethylene [-OCH2-]n) foam leaving less solid residues.

Within a ceramic mould deposited on the foam, I expect very little air mass is available, so the compounds present are together very reductive. I expect CO rather than CO2, which will pick some hydrogen to form things like H2CO. The rest of PLA [-C3H4O2-]n would be four H for one C if no hydrogen combined with CO.

If the foam has closed cells, the foaming gas contributes to the gross composition. With open cells, it will be air. The amount isn't huge, but it can increase radically the toxicity of the evolved gas. If polystyrene is foamed by a halogenated gas, this makes toxic fumes.

What do the up to four H for one C do? I won't risk a prediction. There will be some graphite and high-carbon compounds, but the amount doesn't result from a simple reaction equation. The rest can evolve as CH4, C2H4 and thousands more.

==========

Out of curiosity, I've let Propep compute the equilibrium products of hot polymers brought to 1400°C and 1atm. This is NOT what happens in your case, because the casting process is very far from an equilibrium. Also, iron will catch some oxygen. The evolved gases recombine as they cool down, for instance CH3, H, HCO disappear for sure. These are molar fractions. Propep hides the least abundant species. I've take polybutadiene for its small H/C ratio. Even smaller at polystyrene, but Propep failed.

----- POM

7.2750e-007  CH3                 
2.2516e-004  CH4                 
4.9990e-001  CO                   
5.9635e-005  CO2                 
1.4937e-005  C2H2,acetylene       
1.4589e-008  CH2CO,ketene         
4.6162e-007  C2H4                 
8.0184e-005  H                   
3.4620e-008  HCO                 
4.9952e-001  H2                   
1.0892e-007  HCHO,formaldehy     
1.9706e-004  H2O   

----- PLA

8.0500e-007  CH3                 
2.4911e-004  CH4                 
4.0002e-001  CO                   
3.4496e-005  CO2                 
2.2871e-005  C2H2,acetylene       
1.6148e-008  CH2CO,ketene         
7.0665e-007  C2H4                 
6.4143e-005  H                   
2.7699e-008  HCO                 
3.9954e-001  H2                   
8.7137e-008  HCHO,formaldehy     
1.1394e-004  H2O                 
Condensed species
1.9995e-001  C(gr)
                   

----- Polybutadiene

1.2192e-006  CH3                 
5.3350e-004  CH4                 
2.4496e-005  C2H2,acetylene       
1.5134e-006  C2H4                 
4.8583e-005  H                   
4.2792e-001  H2                   
Condensed species
5.7147e-001  C(gr)   
             

----- PS

Propep failed to find an equilibrium.

Offline tunkor

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #8 on: March 25, 2020, 12:10:30 PM »
So if you simplify the reaction, the PLA would decompose into CO2 and ethylene?

Yes, at first stage. Just remember - this is just an educated guess with all possible disclaimers, experiment can easily prove it wrong. Search for a thermogravimetric curve for PLA, it should give some insight into what is happening.

How much energy does it take te decompose PLA compared to PS?

You can try to use Hess law to estimate the numbers.

Quote
Is this what activation energy means?

No, activation energy is related to the reaction speed, you are looking for reaction enthalpy.

Quote
How much carbon does PLA leave compared to PS?

No way to tell - thermal decomposition (pyrolysis) is a process consisting of many quite random reactions, with lots of side products.


Thanks, can you elaborate on the law of Hess? The last chemistry I had was in high school 7 years ago, so I apologize for being so ignorant.

Offline Borek

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #9 on: March 25, 2020, 02:27:56 PM »
Thanks, can you elaborate on the law of Hess? The last chemistry I had was in high school 7 years ago, so I apologize for being so ignorant.

Any general chemistry 101 textbook should have an introduction, this is a relatively basic concept (which I am saying not to stress your lack of knowledge but to signal that you should be able to grasp the idea even without having a solid chemistry background :) ).

You may have problems finding enthalpy of formation for PLA, it can be estimated from bond energies though. Enthalpy of formation of a compound is equal to sum of bond energies, these can be found tabulated. Far from perfect, better than nothing.
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Offline Enthalpy

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #10 on: March 26, 2020, 07:12:03 AM »
I doubted that the heat brought by molten iron would suffice to destroy plain polymer. But with polymer foam, I could easily drop numerical checking.

Offline tunkor

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #11 on: March 26, 2020, 07:20:39 AM »
I doubted that the heat brought by molten iron would suffice to destroy plain polymer. But with polymer foam, I could easily drop numerical checking.

It's filament with a foaming agent, at a certain temperature the solid PLA filament will foam a little bit.

In that picture on the left is a single wall of normal PLA, and on the right is a single wall of foaming PLA. Volume can increase by 3x using this material according to the manufacturer, by increasing temperature and decreasing material flow lightweight parts are possible. So this would mean less mass to burn.

Offline Enthalpy

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #12 on: March 27, 2020, 12:23:10 PM »
So PLA is still rather dense. My estimation of the decomposition heat follows.

Liquid ethyl acetate has ΔHf=-479kJ/mol. One C-C bond more and H2 less add 44kJ/mol, so PLA has ΔHf=-435kJ/mol of -C3H4O2-.

If we were to believe Propep, it decomposes to 1*C + 2*CO + 2*H2 whose ΔHf=-221kJ/mol. The heat of the rest is negligible.

So the pyrolysis of 1 mol of -C3H4O2-, or 72g, absorbs 214kJ/mol. That's 3MJ/kg. If the foam is 440kg/m3 dense, it absorbs 1.3GJ/m3.

The heat capacity of molten Fe is around 45J/mol/K = 800J/kg/K = 5.6MJ/m3/K. Iron being supposedly less than 230K above its melting point, some iron will solidify.

The heat of fusion of Fe is roughly 13kJ/mol = 230kJ/kg = 1.6GJ/m3. This suggests that most iron solidifies by decomposing PLA. Bad.

A much lighter foam would be welcome.

Offline tunkor

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #13 on: March 27, 2020, 03:44:31 PM »
So PLA is still rather dense. My estimation of the decomposition heat follows.

Liquid ethyl acetate has ΔHf=-479kJ/mol. One C-C bond more and H2 less add 44kJ/mol, so PLA has ΔHf=-435kJ/mol of -C3H4O2-.

If we were to believe Propep, it decomposes to 1*C + 2*CO + 2*H2 whose ΔHf=-221kJ/mol. The heat of the rest is negligible.

So the pyrolysis of 1 mol of -C3H4O2-, or 72g, absorbs 214kJ/mol. That's 3MJ/kg. If the foam is 440kg/m3 dense, it absorbs 1.3GJ/m3.

The heat capacity of molten Fe is around 45J/mol/K = 800J/kg/K = 5.6MJ/m3/K. Iron being supposedly less than 230K above its melting point, some iron will solidify.

The heat of fusion of Fe is roughly 13kJ/mol = 230kJ/kg = 1.6GJ/m3. This suggests that most iron solidifies by decomposing PLA. Bad.

A much lighter foam would be welcome.

Guys thanks so much to both of you! This has given me a lot more insight already :)

Offline Enthalpy

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Re: Behaviour of molten metal interacting with polylactic acid
« Reply #14 on: April 01, 2020, 07:18:36 AM »
Please don't consider my estimate as a prediction! Propep computes equilibria and your process is very far from equilibrium. So if you observe a too fast solidification, you have a qualitative explanation, nothing more. Enough to seek a lighter foam. If less carbon in iron is desired, POM-H foam could be interesting.

And: double-checking my computation wouldn't hurt.

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