[tunkor]

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.

Currently I am trying to apply the decomposition of PLA to simple models EPS decomposing. Then I hope to verify these predictions in real life scenarios.

What I need for these models is the energy required for decomposing PLA. From what I gathered in an earlier reply you calculated the enthalpy of formation (an indicator for the energy required for decomposition?) by looking at the enthalpy of formation of ethyl acetate and compare the energy bonds with PLA.

I don't understand how you calculated this exaclty however, if I compared PLA with ethyl acetate I see that PLA has a C = C bond, one C less and 2 H2 less. Could you clarify this for me? It would help me a lot!

And the other thing I have to find out is the amount of gas generated, to see what the pressure will be in the gap between the PLA and the metal. But I am still looking for info about that.

[Enthalpy]

I compare the heat of formation of the "reactants" (PLA) and the "products" (CO, H₂, C) to obtain the heat of reaction (here a pyrolysis or decomposition). It's a difference, not forgetting numbers of moles. Sign conventions exist, the useful part is that the present decomposition absorbs heat.

You're absolutely right that ethyl acetate has one C too much. I botched that. "Double check" was more than rhetoric here: it's the base of science.

Methyl acetate would be the right starting material

to compare with the PLA period, gratefully pinched from Wiki and appended here. Methyl acetate resembles the PLA period: same ester group, as many carbons and bonds, just two C-H becoming a C-C and an H-H, so few changes of small energy suffice.

From tables, the heat of formation of methyl acetate is ΔH=-446kJ/mol. For the liquid, not the solid, slight inaccuracy. With ethyl acetate, my error was 33kJ/mol, over the previous 214kJ/mol heat of decomposition.

The formal transformation to PLA period is the same as from two octanes becoming hexadecane and hydrogen. 2* (-250kJ/mol) becomes -456kJ/mol and 0kJ/mol, this absorbs 44kJ/mol. We could skimp on primary and secondary carbons, but I won't since this makes 5kJ/mol difference. Here we get a heat of formation ΔH=-402kJ/mol for the PLA period.

Again from tables, CO has ΔH=-110.5kJ/mol while H₂ and solid C have zero by convention. So the decomposition to 1*C + 2*CO + 2*H₂ absorbs 402-2*110.5=181kJ/mol, not 214.

However, this is at 298K, but here the reaction products exit at about 1500°C - I shouldn't have botched that one. Misusing at heat the capacity at room temperature (OK for H₂ and CO, not for CO₂ or H₂O)
https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities
8,5 + 2×29,1 + 2×28,8 = 124J/mol/K for 1*C + 2*CO + 2*H₂
heating from 0 to 1500°C absorbs further 186kJ/mol.

So from one PLA period at room temperature to 1*C + 2*CO + 2*H₂ at 1500°C, it takes about 367kJ/mol of C₃H₄O₂ weighing 72g, or 5MJ/kg.

==========

The amount of gas is easy. 2*CO + 2*H₂ at 1500°C, you can apply the ideal gas law.

The pressure is about impossible to evaluate. Your mould needs vents or it will explode, and the foam must be weak enough to open passages. The pressure results from very dynamic processes, not from an equilibrium.

[tunkor]

I compare the heat of formation of the "reactants" (PLA) and the "products" (CO, H₂, C) to obtain the heat of reaction (here a pyrolysis or decomposition). It's a difference, not forgetting numbers of moles. Sign conventions exist, the useful part is that the present decomposition absorbs heat.

You're absolutely right that ethyl acetate has one C too much. I botched that. "Double check" was more than rhetoric here: it's the base of science.

Methyl acetate would be the right starting material

to compare with the PLA period, gratefully pinched from Wiki and appended here. Methyl acetate resembles the PLA period: same ester group, as many carbons and bonds, just two C-H becoming a C-C and an H-H, so few changes of small energy suffice.

From tables, the heat of formation of methyl acetate is ΔH=-446kJ/mol. For the liquid, not the solid, slight inaccuracy. With ethyl acetate, my error was 33kJ/mol, over the previous 214kJ/mol heat of decomposition.

The formal transformation to PLA period is the same as from two octanes becoming hexadecane and hydrogen. 2* (-250kJ/mol) becomes -456kJ/mol and 0kJ/mol, this absorbs 44kJ/mol. We could skimp on primary and secondary carbons, but I won't since this makes 5kJ/mol difference. Here we get a heat of formation ΔH=-402kJ/mol for the PLA period.

Again from tables, CO has ΔH=-110.5kJ/mol while H₂ and solid C have zero by convention. So the decomposition to 1*C + 2*CO + 2*H₂ absorbs 402-2*110.5=181kJ/mol, not 214.

However, this is at 298K, but here the reaction products exit at about 1500°C - I shouldn't have botched that one. Misusing at heat the capacity at room temperature (OK for H₂ and CO, not for CO₂ or H₂O)
https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities
8,5 + 2×29,1 + 2×28,8 = 124J/mol/K for 1*C + 2*CO + 2*H₂
heating from 0 to 1500°C absorbs further 186kJ/mol.

So from one PLA period at room temperature to 1*C + 2*CO + 2*H₂ at 1500°C, it takes about 367kJ/mol of C₃H₄O₂ weighing 72g, or 5MJ/kg.

==========

The amount of gas is easy. 2*CO + 2*H₂ at 1500°C, you can apply the ideal gas law.

The pressure is about impossible to evaluate. Your mould needs vents or it will explode, and the foam must be weak enough to open passages. The pressure results from very dynamic processes, not from an equilibrium.

Thank you so much! I really appreciate you taking the time to help me with this.

I have one another question, I've been looking at articles about the decomposition of PLA. I see a lot about different types of hydrocarbons being emissions of PLA decomposition, but these are from TGA done at 400C for example. You mention the products are CO, H2 and C. Is this what the products of decomposition are after reaching 1500C? Any partical reason the products decompose to those ones?

Thanks again for the tremendous *delete me*

[Enthalpy]

The decomposition products depends fundamentally on the temperature, and also on the pressure. At not too high temperature, like 400°C, they depend also on the heat history, because the detailed processes still matter: heat doesn't suffice to break and rearrange any compound, instead, the reactions with the least hurdles proceed more easily.

1500°C or 1600°C suffice to break most bonds, especially C-C and C-H. No organic material operates at this temperature, only ceramics and metals. This lets the atoms rearrange in species (not even molecules) according to the thermodynamic equilibrium. Propep, a combustion estimation software for rocket engines, computes exactly that: it neglects how the atoms were combined prior to heat, and seeks a mathematical equilibrium among the hundreds of species known from tables and capable to form from the available atoms.

If PLA were brought uniformly to 1500°C, I'm confident the decomposition products would match Propep's estimation. The difference is that your process is very dynamic: some PLA not in direct contact with iron decomposes well before reaching the final temperature, and the decomposition products escape by deforming the surrounding material. These products have a different composition.

[billnotgatez]

I have been following this thread and wonder if you considered using the standard lost wax procedure of heating the investment first before pouring in the final metal. We do it at 1000 F for overnight. The pour hole and sprues allow the wax to flow out and/or burn out (investments are holes down). We vacuum out the pour hole and sprues to remove any residual burn products and pour while the investment is somewhere below 500 F. Due to the lock down I am unable to reach out to my fellow casters to get their thoughts (which I would have added here).

[tunkor]

The decomposition products depends fundamentally on the temperature, and also on the pressure. At not too high temperature, like 400°C, they depend also on the heat history, because the detailed processes still matter: heat doesn't suffice to break and rearrange any compound, instead, the reactions with the least hurdles proceed more easily.

1500°C or 1600°C suffice to break most bonds, especially C-C and C-H. No organic material operates at this temperature, only ceramics and metals. This lets the atoms rearrange in species (not even molecules) according to the thermodynamic equilibrium. Propep, a combustion estimation software for rocket engines, computes exactly that: it neglects how the atoms were combined prior to heat, and seeks a mathematical equilibrium among the hundreds of species known from tables and capable to form from the available atoms.

If PLA were brought uniformly to 1500°C, I'm confident the decomposition products would match Propep's estimation. The difference is that your process is very dynamic: some PLA not in direct contact with iron decomposes well before reaching the final temperature, and the decomposition products escape by deforming the surrounding material. These products have a different composition.

That makes sense, man there's a lot to this! How can I compute the reaction products myself in Propep? I see there's no option for PLA in the list.

[tunkor]

I have been following this thread and wonder if you considered using the standard lost wax procedure of heating the investment first before pouring in the final metal. We do it at 1000 F for overnight. The pour hole and sprues allow the wax to flow out and/or burn out (investments are holes down). We vacuum out the pour hole and sprues to remove any residual burn products and pour while the investment is somewhere below 500 F. Due to the lock down I am unable to reach out to my fellow casters to get their thoughts (which I would have added here).

Good question, investment casting with melting out the investment has some downsides from what I understand. With the investment still being inside the coating it will add to the total strength. So you can use a thinner coating, which also means a shorter lead time before you can start casting. I think lost foam casting also allows for larger casting products due to this reason.

[Enthalpy]

Propep and variants aren't exactly easy to use. Kudos if you managed in within reasonable time. I use CPropepShell which offers a graphical user interface for Windows 95. I believe the website for the programme is closed, but I can mail it, it's like 500kB. More recent programmes are easier to use for rockets but can't be misused so nicely.

To compute an equilibrium, I check or fill in CPropepShell
Fixed pressure and temperature
Fixed chamber pressure
Fixed chamber temperature
and I give twice PLA as an ingredient, 500g+500g, because CPropepShell doesn't understand monopropellants.

I added PLA with a text editor anywhere in the propellants list propellant.dat as:
    8043 Polylactic acid (zero Hf!)      3C   4H   2O   0    0    0      0 .0000]
The density .0000 in Lb/in³ (avoirdupois) is unnecessary for most purposes, and I kept 0 cal/g heat of formation in but-last column because it serves to compute the chamber temperature, but here we impose it.
The number of spaces matters in the line format and this forum may suppress some. Check that the new line aligns with the others. The line number 8043 plays no role known to me. C H O can be other elements and in a different sequence, apparently the usual chemical symbols.

POM exists already in the list as Delrin.

----------

Removing the foam before casting is a different process. Both serve. Removal would be difficult for some shapes and it takes more steps. Only some alloys like cast iron accept the carbon pollution. POM could be an answer to that, if a foam light enough is or can be made.