How does acetone "rejuvenate" PEI?
The Prusa3d knowledge base mentions that acetone will "rejuvenate" PEI in addition to cleaning oil and grease from the surface.
About once a week, or when prints stop sticking, use ACETONE to clean the bed. It removes grease better than IPA or Windex. It also rejuvenates the print surface. However, if you use acetone every day, PEI will become brittle and start cracking.
From a chemical or physical perspective, how does acetone affect the PEI surface?
Looks like marketingspeak. A cleaned surface should behave like a new surface, assuming the PEI is homogeneous material.
Thanks! I can verify that it's customersupportspeak, as that's what they advised me when I contacted them. Interestingly enough, it worked very well and restored my print surface to like-new condition. I don't know if their PEI surface is homogenous, although they don't say anything to the contrary.
Rejuvenate is probably a bit of an exaggerated term. The number one adhesion suppressor is grease. The stuff that comes off of the fingers used to handle the sheet. Even if you are careful and only handle the sides, the grease will be carried to the center of the plate next time its cleaned with less aggressive solvents.
Isopropyl alchohol does break down grease but not to the extent that acetone does. Acetone also attacks plastic particles that accumulate on the print surface over time.
PEI is resistant to a wide array of chemicals including acetone but it can become brittle if exposed to it too much especially when hot so acetone is not recommended for daily cleaning.
 Ultem Product Broshure table 4-3
Excellent reference. Your last point is the one I'm trying to understand in more detail. What physical or chemical change takes place that makes the PEI brittle? This is more for intellectual knowledge than for practical considerations, of course.
@mark-harrison unfortunately GE does not specify what happens to polyethermide when it's attacked by ketones, they only list the failure mode after immersion which is rupture. My wild guess is that this is due to loss of molecular mass. PEI molecule does contain a methyl group (CH3) so acetone may be able to eventually break that away from the molecule.I'm not a chemist though and I am really grasping at straws here. This is probably a question for chemistry.stackexchange.com