What's going on with ABS is mixed with acetone (chemically)?

I've commonly heard of acetone being called a "solvent" for ABS, but what does it actually do to the ABS on the polymer chain level? I assume it's not breaking down the polymer chains becuase ABS disolved in acetone seems to be recoverable once the acetone evaporates.

On a similar note, does THF break down the polymer chains of PLA?

I'm interested in this because I want to find a scientific explanation to why vaporsmoothing methods with ABS and acetone work out so much better than the same methods used with PLA and THF. So far my personal expirmentation (inspired by this article: [hackaday.com] ) has just made it clear that if you want to chemically smooth your prints, don't use PLA.

Thanks!

As you said, It is a solvent.

Think of the polymer chains like fresh cooked spaghetti, they curl and wind and hook to each other physically, not chemically. (Actually some of them really stick to each other in an organized way, which is called semi-crystalline zones, you can observe that with spaghetti too, but its not too important here) If you add lots of water (solvent) and stir a little,the space between the spaghetti gets bigger and bigger depending on how much water you add.
The spaghetti don't break down it's just that the space between them is filled with solvent and thus gets bigger.
At some point they don't adhere anymore to each other and you get first a sticky mass, then a liquid, depending on the solvent ammount.

Analogy: Ok, sugar is totally crystaline, but if you add sugar to water, the same thing happens, when you evaporate the water again, the remain is sugar.
No breakdown occuring. Almost the same happens with polymers, it's just that the molecules are MUCH bigger.

I haven't used ethers like THF with PLA, but I did try Haloalkanes like Dichloromethane with PLA which also dissolved the PLA very good.
I could imagine that smoothing of the surface could also be achieved by this stuff.
Itś a bit unhealthy to work with DCM, but it smells fantastic :-)

Edited 1 time(s). Last edit at 06/27/2014 05:18AM by Yellobello.