Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
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Mood: Chemistry: the subtle science.
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stirrer shaft seals
Biphasic (liquid-solid, liquid-liquid, etc) reactant mixes can be difficult to stir well. Without good stirring yields will suffer. To obtain
adequate mixing Org Syn, Vogel, etc, often call for the use of a mechanical overhead stirrer. A great stirrer for these situations uses a stepper
motor w/speed control. I have made one of these and show its construction and operation elsewhere on this forum.
When no contact with the atmosphere is required the procedure will often call for a mercury shaft seal. These don't seem to be readily available
anymore. Also I really don't want to use a mercury seal for safety, waste disposal, and cost reasons. So I've been looking for a good mechanical
seal. I feel that I have found it as shown in the pictures below.
The seal consists of a packing gland made of ptfe. A rubber O-ring provides the packing. I grease up the O-ring with Corning high vacuum grease and
place it over the stirrer shaft. The packing nut is then screwed down tight. This provides a very tight seal. A sitirrer capable of high torque is
required which is provided admirably by the stepper motor.
I really doubt if much air gets past this seal. To be doubly sure an argon or N2 purge of the reaction vessel could be provided.
The stirrers shown are ptfe 1/4" shaft folding prop types suitable for 19/22 or larger ports. The seal is for 19/22 tapers but 24/40 systems are also
available. The seals and stirrers are available on eBay.
If you have any questions, comments, or suggestions please post them.
 
The single most important condition for a successful synthesis is good mixing - Nicodem
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Dr.Bob
International Hazard
Posts: 2736
Registered: 26-1-2011
Location: USA - NC
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I have scads of stirring shafts, seals, paddles, and other related items available, if people are looking for similar things. The shafts are mostly
10 mm OD, but some smaller and some larger OD, and I have the seals in glass, teflon, and other variations in 14/20, 19/22, 24/40, 29/42, and several
larger sizes. The paddles are from tiny to large, most teflon, but some in glass, plus some larger glass ones with complex stirring paddles all in
one piece. There are a few photos posted on the thread, but I can get more for specific inquiries. Here is a photo of some of the shafts, bearings
and paddles I have, plenty more sizes as well.
 
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byko3y
National Hazard
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Registered: 16-3-2015
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I hate stepper motors, I think no modern device nowadays use it, except positioning system for optical drive (HD uses no stepping motors).
DC brushless motor is a best choice for high torque applications.
Another option for sealing is to use some another liquid instead of mercury in the trap. Pretty much any nonvolatile liquid will do the job, e.g.
silicon oil.
Of course it cannot withstand a pressure difference, but it might be an advantage as well by releaving some unneeded pressure difference. And because
there's no solid-solid friction in the seal, it can last virtually forever.
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Dr.Bob
International Hazard
Posts: 2736
Registered: 26-1-2011
Location: USA - NC
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Mood: No Mood
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Most glass stirrer shafts will hold a minor positive pressure (eg, N2, or Argon) if greased/oiled properly. The Teflon ones and o-ring ones can also
stand up to minor pressures. Making one work under vacuum is very tough, it can happen with minor vacuum, but hard under high vacuum, which is why
rotovap motors and seals cost so much, they have to be made to high and very consistent specs. The glass ones can last for a long time if handled
well.
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