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According to Wikipedia, Tetrahydrofuran dissolves PLA. It also apparently dissolves PET.

THF is considered a relatively nontoxic solvent, with the median lethal dose (LD50) comparable to that for acetone.

Wikipedia further states that it's not particularly dangerous provided you keep it away from air circulation, so that it does not form peroxides.

One danger posed by THF follows from its tendency to form highly explosive peroxides on storage in air.

So well, this seems acceptable compared to other possible solvents which are all ridiculously dangerous. The question at hand is now of practical nature:

  • Is Tetrahudrofuran actually viable replacement for acetone, which only works with ABS?

I am asking because the fact that it does dissolve PLA does not really at all mean that it will work well. It could damage PLA structure, be more toxic than wikipedia says or not dry well.

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    $\begingroup$ Personally I'd either use epoxy (since there are plenty of microvoids in the PLA) or a "3D pen" to extrude a little fluid PLA and bond the parts, sort of like a hot-glue gun. $\endgroup$ – Carl Witthoft Feb 14 '17 at 19:00
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    $\begingroup$ I curious how well it works as well, but it was rather expensive at the couple places I was able to find it for sale at. The prices ranged from 70 to well over 100 USD for 100ml (the cheapest I was able to find any quantity), though the price didn't increase nearly as fast as the quantity went up. The price depended on how carefully it was processed to dry it and what preservative treatments it had. And last, just because the lethal dose is comparable to acetone, it doesn't mean its overall as benign as acetone. $\endgroup$ – T. M. Jan 3 '19 at 1:10
  • $\begingroup$ I don't think I'd call THF "not particularly dangerous". From the "storage" section of an MSDS: "Under normal storage conditions, peroxidizable compounds can form and accumulate peroxides which may explode when subjected to heat or shock. This material is most hazardous when peroxide levels are concentrated by distillation or evaporation. THF should never be distilled to dryness. Conditions to Avoid: Light, ignition sources, moisture, excess heat, evaporating to near dryness, confined spaces." $\endgroup$ – Mark Jan 3 '19 at 21:21
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According to Shuichi Sato, Daiki Gondo, Takayuki Wada, Shinji Kanehashi & Kazukiyo Nagai: Effects of various liquid organic solvents on solvent‐induced crystallization of amorphous poly(lactic acid) film in Journal of Applied Polymer Science, Volume 129 Issue 3 (2013), p1607-1617source, Tetrahydrofuran is classified as a solvent for PLA. The specific entry on page 1608:

Group  Solvent          Solvent type    dd   dp   dh   dt   Result
Ether  Tetrahydrofuran  Polar aprotic  16.8  5.7  8   19.4  soluble

The values dd dp dh and dt are explained on page 1607:

The effects of 60 liquid organic solvents on PLA are systematically investigated using the Hansen solubility parameter (HSP). The HSP is one of the digitizing methods for analyzing the interaction between polymer materials and organic solvents. In HSP analysis, all solvents have three parameters: energy from dispersion bonds between molecules (dd), dipolar intermolecular force between molecules (dp), and the hydrogen bonds between molecules (dh). All solvents were characterized by a point in a three-dimensional structure at which dd, dp, and dh are plotted on three mutually perpendicular axes. Generally, if the HSP values of the various organic solvents are near that of the given polymer, the solvent is considered compatible with the polymer material.

The factor dt is the total Hansen solubility parameter - the bigger this is, the better it is a solvent.

19.4 is a rather good solvent but extremely explosive: in air 20000 ppm (2%) are explosive and thus the allowable concentration in an area is 2000 ppmsource

Alternatives

A similar potent solvent would be Benzene (dt = 18.6) which more easily available and less explosive, but more deadly (10000-20000 ppm fumes) and has an allowable concentration of 500 ppmsource

Ethyl-acetate is also a solvent (dt = 18.2) and more available. It is explosive at an equal concentration as Tetrahydrofuransource, but it has just about half of its vapor pressure (73 mmHgsource vs. 132 mmHgsource), so can be stored more safely, and is less aggressive on the body. It is sometimes used to Smooth PLA via vapor chambersource, and only comes at a price tag of ca. 90 €/l for the pure stuff and also is used in some nail polish removers, put into a safe mix.

Acetone is classified as a better solvent (dt = 20.1), and it is known to act as a glue and to soften PLA with some exposure time, but from experience, it can't smooth it. It is available most easy (nail polish remover and in the home depot), and is the least deadly option.

Propylene-1,2-carbonate is classed as solvent and a far better at it with dt = 27.2. It has been used as an alternative to Ethyl-Acetatesource and its MSDS is rather gentlesource. It comes at a price tag of 130 €/l for the chemical-grade liquid.

Conclusion

Tetrahydrofuran is not a viable replacement due to its explosive properties. It is not an improvement above Benzene, which at least can be stored safely.

Using Acetone as a benchmark, Tetrahydrofuran should not smooth the surface in a vapor chamber, as it is a worse solvent than Acetone. It should also take longer to soften and dissolve objects than Acetone, but a heated bath or coating the surface with it could help to generate the needed exposure times.

However, its comparable ethyl-acetate has been claimed to be used as a cleaning, vapor smoothing, and brush on surface smoothing agent successfully and can be used better by helping the solubility via heating, which can be done much safer with ethyl-acetate than THF. In a proper chemical mix, its storage problem can be solved too.

A better alternative is propylene-1,2-carbonate, which is a better solvent, and much less dangerous.

tl;dr: No, Tetrahydrofuran is not able to dissolve PLA in a reasonable time1 without heat activation and it has worse characteristics than Acetone. It could arguably be used to weld parts, but Dichloromethane would be more effective.


1 - Sato, Gondo et al. in the aforementioned paper: Solubility tests were performed for 24 h at 35+-1 °C. which means we have a prolonged exposure of a thin film to a huge amount of solvent. how thin a film? Very thin: The PLA films were prepared by casting 2 wt % dichloromethane solution onto a flat-bottomed glass Petri dish in a glass bell-type vessel and by drying under atmospheric pressure at room temperature. Each solvent was allowed to evaporate for 48 h. The dried PLA films were then thermally treated under a vacuum for 48 h at 70 °C to eliminate the residual solvent and to obtain amorphous PLA films. Afterwards, the thermally treated PLA films were cooled at room temperature under atmospheric pressure.

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  • $\begingroup$ The first MSDS I looked at listed a toxicity-based maximum permissible exposure level of 200 ppm, less than 1% of the minimum explosive level. If you're using THF safely from a toxicity standpoint, it's not going to blow up. (Storage, on the other hand, is still a problem.) $\endgroup$ – Mark Jan 3 '19 at 21:17
  • $\begingroup$ @Mark Being more than the toxic dose doesn't help as much as you suggest. If you put some in a bottle, nobody cares if the air gap above the solvent contains a toxic level of THF vapour, because you don't stick your head in the bottle. But if the air gap contains an explosive level of THF vapour, you have a big problem. It's not just storage: most prints are porous and will absorb the vapour of whatever you use to finish them. What concentration of THF would end up inside the part? $\endgroup$ – Dan Hulme Jan 4 '19 at 9:57
  • $\begingroup$ Acetone does not dissolve PLA at all. The explosion risk is complicated issue. Generally the inside of the flask does not contain an explosive mix of gas because the oxygen molecules end up in the peroxide compound that naturally forms from THF. However, that compound is quite explosive on it's own. $\endgroup$ – Tomáš Zato - Reinstate Monica Jan 4 '19 at 11:20
  • $\begingroup$ @TomášZato I have read the paper. Acetone is able to dissolve the film they tested with within the setup, as did THF. from the paper: Solubility tests were performed for 24 h at 35+-1 °C., which means both are a solvent, but neither is a good one. You could use either of the mentioned solvents to weaken the surface layer and allow cross bonding - read the word reasonable time, which refers to the rather slow process. $\endgroup$ – Trish Jan 4 '19 at 12:24
  • $\begingroup$ @DanHulme, referring to the MSDS again, the air space at the top of the bottle won't have an explosive level either: the vapor pressure of THF is rather high, and a rough estimate is that it'll be about 15% THF. The upper explosive limit is 11.9%, so there's too much THF to explode. Similarly, that high vapor pressure means the finished part won't have enough trapped THF to be a problem: it will quickly escape into the larger atmosphere and be diluted below the lower explosive limit. THF is dangerous, but not for the reasons you're focusing on. $\endgroup$ – Mark Jan 4 '19 at 20:42
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From what I have read, THF is not super effective, it seems that Dichloromethane or DCM is actually a better solvent. Unfortunately I have no personal experience with either.

It is rather nasty though, as the LD50 is just 0.5 to 5 g/kg[1][2].

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  • $\begingroup$ I'm going to do more research, but so far I can tell for sure that THF disolves PLA much better than how acetone dissolves ABS. So it's super-effective compared to what I've ever seen. However, it's also kinda nasty. Just few minutes of worh with it resulted in nausea and headache. Precautions are advised! $\endgroup$ – Tomáš Zato - Reinstate Monica Feb 27 '17 at 21:51

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