Magpie
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luting compounds
Luting compounds are joint sealants normally used at high temperatures.
I have been attempting to find, or develop, a luting compound for use in sealing borosilicate glass to ceramic (homemade mullite).
My criteria are as follows:
1. Must retain gas tight integrity for 1 hour at 500C.
2. Must be compatible with borosilicate glass and mullite.
3. Must be removable following use without significantly damaging the glass and ceramic pieces.
I have found some commercial products that claim they are suitable as a metal joint sealant up to 510C. One, and likely the other also, are phenolic
based and therefore should be removable by burn out at a sufficiently high temperature. If they expand significantly during curing they would be
unacceptable. I have not tried them in my lab as I don't have any of the sealants.
I have evaluated several sodium silicate based formulations in my lab. The test method I use is to seal a glass microscope slide (yes, I know, it's
soda glass) to a 5cm x 5cm ceramic tile. This is then heated in my muffle furnace for 1 hour at 500C.
Results indicate that the pottery grade sodium silicate will meet the above criteria, even at a level as low as 5% of the formulation. There is only
minor corrosion of the glass and ceramic surfaces if a low level of the sodium silicate is used. Soaking in water eventually loosens the seal. The
best formulation so far contains 200 mesh SiO2 as fill.
If anyone has some suggestions for a luting compound meeting the above criteria I would very much appreciate hearing them.
[Edited on 9-12-2009 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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entropy51
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Magpie, I think you're on the right track with sodium silicate, but you might want to take a (half-humorous) look at some of the old chemical
technique books on www.archive.org One that I recall is by either Faraday or Davy. These old books often give recipes for luting compounds, which were a necessity
in the olden days, prior to the advent of ground glass joints. You may be amazed at some of the stuff they recommend, the duct tape of the early 19th
century.
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watson.fawkes
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FYI. Historically, wheat paste was used as a luting compound, but that was back when every metal vessel was
hand-hammered and tolerances were poor, and back when even an ordinary steam distillation rig needed luting.
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Magpie
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Another interesting sealant, formerly used by plumbers, is a mixture of litharge (PbO), glycerin, and water. One reference said it was good to 275C.
This would be OK for the lower temperatures, but I don't know how difficult it would be to break the seal. IIRC PbO melts at 888C but it might flux
the ceramic and melt it too.
I thought that RTV silicone rubber would make a good low temperature luting compound. My testing showed that it had integrity up to 300C. However,
above that temperature it decomposed into carbon and what looked like very fine SiO2, and for some reason, badly warped the glass slide.
The single most important condition for a successful synthesis is good mixing - Nicodem
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watson.fawkes
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Quote: Originally posted by Magpie  | | PbO melts at 888C but it might flux the ceramic and melt it too. [...] RTV silicone rubber [...] had integrity up to 300C. However, above that
temperature it decomposed into carbon and what looked like very fine SiO2, and for some reason, badly warped the glass slide. |
The first effect of PbO is to give you a layer of leaded glass on the surface of the glass vessel. The traditional flux for lead
glass is sodium and/or potassium oxides, but boric oxide would also do. Materially, it's going to lower the melting point and increase the COE
(coefficient of expansion, thermal). This has the effect of making a graded glass seal on the surface of the glass. This is likely not too much of a
problem, but may cause a greater likelihood of breakage. It also may causing spalling and cause a taper joint no longer to fit correctly without
luting. Therefore, I would recommend using a glass adapter piece for the transition. One side would be luted. The other would have a taper joint, or
perhaps better, one of the standard ball-and-cup joints, to better deal with overall thermal expansion on the hot side.
PbO in a reducing environment will happily reduce to elemental lead. Generally this results in crystal formation, which would interfere with the
plastic and glassine material properties of your luting. So there's some amount of process vs. sealing compatibility to worry about.
As for PbO fluxing the ceramic, that's only likely if the ceramic has significant alkali flux elements. One formulation of lead glass I looked up had
8% K2O and 4.6% Na2O as flux, so "significant" doesn't mean "large". Good quality mullite, which is just alumina and silica in composition, shouldn't
have this problem. For any homemade ceramic, though, I would definitely advise a test.
The warping on the glass slide is completely typical of incompatible crystal lattices and/or COE. This is a dominant issue in, for example, silicon
wafer processing, where if you get a pair of incompatible layers, you get exactly the kind of warping you observed. It was almost certainly
crystalline SiO2, as you saw, that was the origin of the problem. My guess is that if you looked hard enough, you'd fine a film of SiO2 on the surface
of the slide.
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bbartlog
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Would any of the carbonaceous luting compositions work for you? Graphite made into a paste with one of any number of binders? I'm not so sure that
such gunk would be at all easy to get off but I think it could handle the temperature at any rate (unlike some other things that have been mentioned).
See e.g. http://www.freepatentsonline.com/4192730.html.
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Magpie
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| Quote: Originally posted by bbartlog | | Would any of the carbonaceous luting compositions work for you? Graphite made into a paste with one of any number of binders? I'm not so sure that
such gunk would be at all easy to get off but I think it could handle the temperature at any rate (unlike some other things that have been mentioned).
See e.g. http://www.freepatentsonline.com/4192730.html. |
I tried that particular luting compound. See:
http://www.sciencemadness.org/talk/viewthread.php?tid=12958#...
The first time I tried it, where I'm sure it was kept below 300C, it seemed to work fine. On my second run, where I heated it to 500C to keep the Zn
in a vapor state, it failed. That's the trouble with carbonaceous binders at temperatures over 300C - they carbonize. Below 300C I believe some
carbonaceous binders will work fine. The one you cite has a big advantage in that it cleans up easily and causes no damage to the mating parts,
according to my test. RTV silicone rubber would also be a good candidate if it can be released and cleaned off after use. Same goes for a phenolic
resin. However, some tars are notoriously difficult to dislodge. Testing would be advised. Also, does it expand on curing. If so this would crack
the glass on a ground glass fitting, not something that would show up in my parallel plates testing method.
Actually the ceramic joint is relatively easy to seal if no joint heater is used, even if the retort is operated as high as 1300C. That's because
ceramic is such a good insulator that the joint likely never comes close to 300C. It's only for applications like making Zn where I need a luting
compound for use at 500C. For this I have to provide a separate heater to keep the ceramic retort joint at 500C.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sedit
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I don't think anyone has mentioned it but why not furnace cement? Its relatively cheep and should do the job nicely. We use it many time on boilers
and I don't see any reason it shouldn't work here. If the temperature gets great enough the Sodium Silicate in it melts and turns to Silica Dioxide.
Its sticky as hell to but I do not know if 500C is enough to "activate" it turning it into SiO2.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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Magpie
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I have used furnace cement to repair cracks in my muffle furnace. It's holding up well. I definitely think it's worth evaluating.
The single most important condition for a successful synthesis is good mixing - Nicodem
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12AX7
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A paste of sodium silicate and silica was mentioned: as far as I can tell, that's almost all that furnace cement is. The fillers are probably a
little more specific, but I know the binder is the same.
I've noticed it can take a long time to dry, and if heat is applied before it's dry, it will bubble up.
Tim
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Magpie
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| Quote: Originally posted by 12AX7 | | A paste of sodium silicate and silica was mentioned: as far as I can tell, that's almost all that furnace cement is. The fillers are probably a
little more specific, but I know the binder is the same. |
I think you've got it right. Here's the MSDS info on my Rutland's Chimney Sweep furnace cement:
Section II - Hazardous Ingredients/Identity Information
Identity CAS #
Sodium Hydroxide (Caustic Soda) 01310-73-2
Silicon Dioxide, Silica,Quartz,Sand 14808-60-7
Sodium Silicate 01344-09-8
If there's any more fillers they don't list them.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Texium
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