I need to use a custom part inside high vacuum (cryo vacuum), for example as a shadow mask. If PLA/ABS are not volatile, I could just print the part I need.
The question is, are ABS or PLA too volatile to be used in cryo vacuum chambers?
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$\begingroup$ How is the 3D printed part going be to used? Do you have the ability to just test it? $\endgroup$ – GisMofx Dec 16 '15 at 0:11
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$\begingroup$ @GisMofx, I don't want to test it because if volatility is significant, the species would evaporate and deposit on the cyro surfaces... $\endgroup$ – Sparkler Dec 16 '15 at 0:16
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$\begingroup$ I was faced with a similar question myself recently. I found this answer on Kurt J Lesker's website: lesker.com/newweb/technical_info/questions/materials.cfm#7 $\endgroup$ – Spartacus Oct 26 '17 at 2:45
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It is unlikely polymers will be an issue, but the additives used may pose problems. Polymers should have negligible volatility due to their long chain nature. Polymer additives may still pose an issue. You may need to test any particular filament material to ensure it meets your needs.
Polymer Chain Mobility
Polymers do not behave like small molecules in terms of mobility. Whereas some small molecules are relatively free to leave the bulk at any temperature due to the nature of neighbor-neighbor bonding, the polymer molecules of interest have purely covalent bonding along their backbones. For a molecule to leave the bulk and enter a gas phase, not only does the molecule have to mechanically disentangle from its neighbors, the entire chain must simultaneously defeat secondary bonding along its entire length, which can be many thousands of atoms long. While it is possible for chains to disentangle, that only occurs above the glass transition temperature, or $T_g$, of the polymer in question. Above the $T_g$ is when reptation - relative, cooperative chain motion - becomes possible. Reptation time is inversely proportional to temperature, as noted in the Wikipedia article.
As a result, polymers should have negligible volatility, especially below their glass transition temperatures. The lowest $T_g$ is of the butadiene rubber in the ABS copolymer (acrylonitrile-butadiene-styrene), at about $-90\ \textrm{°C}$. The styrene has a $T_g$ of about $105\ \textrm{°C}$. Because ABS is a block or graft copolymer, meaning the different components reside on the same chain, the molecules are effectively "locked in" below the $T_g$ of the styrene component. PLA, or poly(lactic acid), typically has a $T_g$ of about $60\ \textrm{°C}$. If your cryo chamber is below these temperatures then the molecules have virtually no freedom to move relative to one another, let alone leave the bulk. Note that this is not a guarantee due to the stochastic nature of kinetics, and the dearth of studies on polymer volatility.
Additives
What you more likely need to be worried about are polymer modifiers:
- diluents: decreases melt viscosity, reduces $T_g$
- compatibilizers: allows better polymer blending
- trace solvents: remaining from production, or water from the atmosphere
all of which are small molecules. Some of these are almost certainly present in printed polymers due to the special requirements of printing. Diluents are likely in some filament-printer systems to achieve optimal high-resolution flow. Compatibilizers are unlikely in "pure" ABS and PLA, but are possible depending on the specifics of the filament material. Trace solvents are possible especially if the filament is of low quality. Additionally, some forms of PLA are mildly hydrophilic, and may absorb water. Drying printed PLA may be useful.
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$\begingroup$ It would likely help drive off any additives more rapidly, but without testing, how would you know when the sample is clean? $\endgroup$ – wwarriner Dec 16 '15 at 5:52
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$\begingroup$ if there's a typical baking time, waiting 10x the time could be safe, for example $\endgroup$ – Sparkler Dec 16 '15 at 12:39
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$\begingroup$ "You may need to test any particular filament material to ensure it meets your needs." -- what kind of test? $\endgroup$ – Sparkler Dec 16 '15 at 14:10
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$\begingroup$ There are several different approaches with varying levels of economy and suitability for your specific problem. The best way I can think of would be to mock up a cryo vacuum chamber like what you intend to use, run it at various temperatures well above the operating temperature (below $T_g$), each for the same amount of time, and see how much stuff comes out of the polymer at each temperature. Then extrapolate the results down to the operating temperature and see if that level of emission is acceptable. Or ask the filament supplier for support, if available. $\endgroup$ – wwarriner Dec 16 '15 at 17:31
