Very enlightening. Thank you.
Ben
-----Original Message-----
From: Davy, Gordon [mailto:[log in to unmask]]
Sent: Tuesday, November 27, 2007 12:17 PM
Subject: Re: [TN] Vacuum Baking of MSDs
I'm not subscribed to TechNet, so I didn't know about Ben Gumpert's question a week ago until Richard Stadem put it on Leadfree yesterday. I just scanned the thread, but vacuum baking is a topic of interest to me because I see so many engineers who talk as if they believe that vacuum will somehow help to "suck" water molecules out of polymers. It doesn't. Neither does a purge by a dry gas such as nitrogen. These notions may appeal to the intuition, but they don't hold up to analysis. Ben's question can be handled entirely by appeal to basic physics - no experiments needed.
Consider a water molecule that is dissolved in a polymer. It moves randomly and "knows" nothing about what is going on at the surface of the polymer. If there is a net motion of water molecules from the bulk to the surface, it is because there is a concentration gradient in that direction. The gradient can be thought of as a driving force for diffusion.
The drying process involves two steps: dissolved (bulk) water molecules diffusing to the surface, and surface water molecules desorbing from the surface. There are two ways to speed the diffusion process: get the molecules in the bulk moving faster by heating the polymer, and maximize the gradient by minimizing the concentration of surface water molecules.
The concentration of surface water molecules depends on temperature and the relative humidity of the gas near the surface. The relative humidity in any ordinary oven (i.e., even without vacuum) at 125°C is roughly one percent. (If you want to calculate it, divide the partial pressure of water in the room - i.e., room-temperature vapor pressure times the RH - by the vapor pressure of water at the oven temperature. Incidentally, contrary to some people's intuition there is nothing special that happens at 100°C, because there is no liquid water to boil. Clothes dryers work just fine at much lower temperatures.)
Hence pulling a vacuum or purging with a dry gas cannot further reduce the RH significantly. Vacuum may make things worse because of its interference with heat transfer; purging is just wasteful. So just use a conventional (forced-air convection) oven set at the highest temperature that the product can withstand.
The only situation for which vacuum baking may be useful is an enclosure with a small opening, where attaching a vacuum line to that opening will reduce the concentration of water molecules (i.e., the RH) in the internal volume. (Even for that situation, the vacuum helps the most where the temperature to which the enclosure can be heated is limited.) The progress of drying can be monitored with a thermocouple gauge in the vacuum line, since by the time the pressure has dropped low enough for the gauge to function, virtually all of the remaining molecules are water molecules.
Hope this helps.
Gordon Davy
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