[Raveling]

Hi everyone,

I want to make a mixture of a density of 1.15 ans a melting point of about 240℃ I'm thinking about using baking soda and nacl.  Any insights and help would be appreciated.  Btw I hoping for a fusion phase change mixture.

Thanks,
               Tim

[Borek]

Finding an ionic salt that melts that low will be quite difficult. And the density of 1.15 (I am assuming g/mL) can be tricky, solids are rather move heavy, 1.15 g/mL sounds more like a water solution.

[Raveling]

Thank you for your input Borek,

My goal is to heat polish a low melting point plastic without deforming it.

I'm open to anything that will heat to around 260°C and is a liquid at that temperature.  That said I would MUCH prefer it wasn't oily or otherwise hard to clean off of plastic. 

I noticed that NaHCO3 has a surprisingly low melting point of 50°C, and ( compared to 1.15g/ml ) is high density.  Some quick searches suggested that a mixture may have an intermediate melting point, so I was thinking of using NaCl &/or NaHCO3 &/or hydrated or anhydrous C2H3NaO2  in the right mixture to fuse around my target temperature.  That said, I know the density is still way too high,  so I thought about a slurry with the salts and water.

I don't have enough chemistry to know whether it could produce a reaction, achieve my ideal density, work or not. 

Looking online, it seemed even supersaturated aqueous salt solutions would have a boiling point that's way too low.

The density thing isn't critical as long as the target material is supported in all directions at room temp, a powder is fine.  But I want it surrounded/ supported by liquid ( NaHCO3 ...? ) at 260°C.  The goal is to leave no marks on the target material, and I think being immersed in a hot liquid is an ideal method.

The ability to adjust the mixture density to use with other plastics or materials would be a bonus.

My goal is to find something to heat a material to around 260°C without leaving marks while still allowing in buoyancy/support in all directions but NOT having it float to the top and distort.

Ideally, it should start solid, heat to a liquid, but not boil, not become turbulent, fluid or flow so it neither distorts nor leave marks while allowing the target material to be neutrally buoyant or otherwise supported in all directions.

Any ideas, reaction notes, safety issues please let me know.

Thanks,
              Tim

[Borek]

I noticed that NaHCO3 has a surprisingly low melting point

That's not rue, it starts to decompose at the temperature, and it probably starts to dissolve in the water (being a decomposition product).

Some quick searches suggested that a mixture may have an intermediate melting point

It is quite tricky to find a correct mixture and a correct composition. As I said before - 250 °C is rather low for ionic salts.

so I was thinking of using NaCl &/or NaHCO3 &/or hydrated or anhydrous C2H3NaO2  in the right mixture to fuse around my target temperature.  That said, I know the density is still way too high,  so I thought about a slurry with the salts and water.

With water present you won't get to that high temperatures.

[Enthalpy]

I don't quite understand why the liquid should melt at +240°C. From the explanations, it seems that a liquid working at +240°C is fine, and that's much easier. The temperature should be made by a heater regulation instead. Or are there better reasons?

A mixture of solids has very few chances of melting at a fixed temperature (it's called a eutectic then). Almost always, you get one composition in the liquid and an other in the solid. The mixture of solids looks like one design constraint too much.

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If you accept a liquid, things get less difficult.

Vegetable oil is used up to that temperature for cooking. Choose one with a high smoke point. Lighter than 1150 kg/m³.

Silicone oils are stable at +240°C if protected from air or if you accept some oxidation. Many alkanes (and mineral oils) are stable. Both are lighter than 1150 kg/m³ but this can improve, say with a few oxygen atoms more in the molecule, or my mixing some talcum. By the way, is there already a powder in the liquid as a polish?

Some brake fluids (dot 5, dot 5.1) boil over +240°C
https://en.wikipedia.org/wiki/Brake_fluid
Some hydraulic fluids too resist to the temperature
https://en.wikipedia.org/wiki/Hydraulic_fluid
generally not meant to operate permanently at that temperature.

Tri- and tetraethylene glycol seem good liquids for +240°C, PEG-400 too
http://www.dow.com/ethyleneglycol/about/properties.htm
https://en.wikipedia.org/wiki/PEG_400
and they weigh 1125, 1124 and 1128 kg/m³ at room temperature. Longer molecules improve the flash point but get viscous; methyl ends reduce the viscosity.

All liquids will be much lighter at +240°C than at room temperature but the plastic part too. By the way, a molten solid would differ even more.

[clarkstill]

If you don't want it to be oily an ionic liquid might be the best bet:

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Aldrich/Brochure/al_chemfile_v5_n6.pdf

[Raveling]

Borek,

I'm very confused with your reply,  I said, or at least I meant to say, and am fairly certain I typed 240°C not "-240°C".

I suspected that water in any concentration would just boil off,  so do you have any solutions to the problem???  Any usable alternative that will allow me to do as I've described?

[Raveling]

  As for the melting point of Sodium Bicarbonate being untrue...please tell google,  I was assuming they had accurate information.

https://www.google.co.jp/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=melting+point+of+sodium+bicarbonate&*

That said, what IS the melting point of baking soda?

Thanks for your suggestion clarkstill, I'm hoping not have to special order anything and use commonly accessible materials or chemicals.  Being in Japan if there is any question of the intention or use of said chemical my lack of Japanese could make getting it a nightmare.

Antifreeze might be ideal.

Thank you for your ideas Enthalpy, what's preferable is that it be solid at room temperature and a liquid at 240°C exactly when it melts, doesn't matter.  It just needs to be melted and a liquid at 240°C while not reaching a turbulent state.  Being liquid at room temp. is acceptable,  being oily, hard to clean off of plastic, toxic or flamable at the temperature I want to use it isn't.

Let me clarify. 

It MUST:

1) be something that is liquid at or around 240°C
2) it's phase above or below 240°C temperature doesn't matter so long as it's a stable liquid that's not turbulent at the target temperature.

3) NOT be toxic flamable or otherwise dangerous, or it's not a practical option.

4) by virtue of being the same density and producing neutral bouancy support the plastic part

5) OR give 360° of support in all directions / axes while NOT leaving marks on the plastic by some other means

It doesn't matter:

1) whether it's a solid or liquid at room temperature.

2) at which temperature it fuses.

[Borek]

That said, what IS the melting point of baking soda?

Some substances don't a well defined melting point, as they decompose before melting. Yes, they may have a number reported in the "melting column", but what it really means is "this is the temperature we observed some change that looked a bit like melting". Sadly, such numbers are often reused without a second thought by people not understanding fine details.

[Raveling]

Fair enough,

Can it, or other substances be used in solo,  or as a mixture to achieve my goal?

If so which materials and mixtures meet my criteria?

[Borek]

I'm very confused with your reply,  I said, or at least I meant to say, and am fairly certain I typed 240°C not "-240°C".

Sorry for not seeing your post earlier, and sorry for a lousy punctuation, it wasn't a minus, it was a dash. Still, +240°C is way too low for ionic salts.

I suspected that water in any concentration would just boil off

Yes. And what is left will solidify. You can heat it further only to discover it decomposes again at about 850°C. This is more or less what will happen with many other salts. Hydrates may seem to be melting low, but once the water is removed you will be either left with a solid, or you will need to heat the sample up to several hundreds degs to keep it melted (at which stage some will decompose, leaving something that requires even higher temperature to become a liquid), neither suits your needs.

Can it, or other substances be used in solo,  or as a mixture to achieve my goal?

There is nothing prohibiting it in principle, but even if such mixtures exist they are extremely hard to find. I would test other routes.

[Raveling]

Thank you Borek,

That is disappointing, but a definitive answer I can act on. Worst case I cal live with burying the parts in a fine powder, even flour, cornstarch, talc, plaster, powdered clay or the like as it will leave only a fine powder on the outside of the parts while supporting it in 3 dimensions at the target temps.

Thanks for your input Borek,
                                           Tim

[Enthalpy]

"Heat-polish plastic" suggests 3D-printed plastic parts, which get small corrugations from the line-wise additive process, while injected plastic parts are as smooth as the mould.

Hobbyists polish printed parts with local heat and skilful hands, avoiding to soften the whole part. But sinking the part in hot liquid would be easier and more industrial, if indeed buoyancy provides support for the softened part. Very nice.

Though, this needs buoyancy to go on until the part has cooled down enough, which - if I grasp the goal properly - implies that the liquid can't solidify too early. It must stay liquid down to room temperature or at least down to moderate heat.

At least at Aldrich, I see prices like 500€/50g for ionic liquids. Even if the amounts and the supplier differ, I fear that would be expensive for such a use, where 50kg are little. Polyethylene glycol, preferably permethylated, would at least be cheap. It is flammable at heat, yes, similarly to frying oil. Not or little toxic, little polluting. Oily but washable in water. The long-term stability at +240°C isn't broadly documented.

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To save heat in a series production, the region of the liquid where the plastic part softens should better stay hot. Stratification would be a solution, with hot liquid on top, colder one at the bottom where the part is carefully introduced. A second column of liquid can communicate by their bottom with the stratified one to introduce the parts - or have some lock instead.

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I would be fantastic to sinter plastic parts after 3D-printing using a similar process. Presently, these parts stay more fragile than their injected equivalents as the successive lines of polymer don't aggregate perfectly, be it because the previous ones have cooled down or because of remaining voids.

Softening the polymer by heat and applying isostatic pressure should remedy both, while buoyancy from a liquid with similar density would avoid the most trivial deformations. This needs much pressure but probably less heat. For parts of moulded titanium it's done in silicone, hence far below the melting point, but at many 100 bar.

Raveling doesn't mention this and I didn't check the prior art, so I can't tell whether it's his intention, my idea, or common practice.

Depending on the chosen pressure, the liquid should better be stiff ("bulk modulus"), as for instance silicone oil increases its density by 15% at 3000 bar. Again polyethylene glycol is stiffer than most liquids, nearing solid polymers.

Air bubbles aren't desired when pressure is applied. Vacuum, possibly when the part is submerged, helps this.

For series production, the chamber seal rings are an interesting challenge. Standard parts exist up to 1500 bar at room temperature.

Marc Schaefer, aka Enthalpy

[Raveling]

Thank you for the detailed options list Enthalpy.

In simplest terms, I am just trying to melt the outside layer of 3d printed plastic parts.  Any process I use will have to be cost effective and safe,  this is home DIY not a research facility with fume hoods and lab equipment...

Pressurization, expensive chems... are out of the question,  but antifreeze,  provided it's flammability is comparable to cooking oil at the same temperature is probably acceptable.  That said would it deliver anything more advantageous than cooking oil?  If they're both oily and the same density,  even antifreeze may be pointless as I don't own a car,  but I do eat food which I sometimes cook with oil;-)

I've also considered using a 1-2 Watt laser since it directs the energy precisely,  but that still becomes just a higher tech version of polishing with a rotary tool or heat gun, so it's not ideal.  I have 60 plus pieces to polish and it's more cosmetic than functional,  so if it's too much work they can remain unpolished. 

[Enthalpy]

For 60 pieces I'd use the hair dryer. In a sunny place you could try a magnifier.

Polyethylene glycol (PEG) is denser than cooking oils (~920kg/m³) and easier to wash away, since it's soluble in water. It's not any banal antifreeze, which uses to be (ethylene) glycol.

Polyethylene glycol seems to be useable at +260°C, which is more or less the limit. PEG 1500 from Huntsmann has a flash point >260°C but has +53°C pour point (it's a solid at room temperature, in normal words). The thinner PEG 400 has 90cSt viscosity (a thick motor oil) at room temperature at a flash point >215°C (cooking oils ~315°C).
https://en.wikipedia.org/wiki/Polyethylene_glycol
https://en.wikipedia.org/wiki/PEG_400