amaurer
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Adhesive that boils/decomposes under 250C
I'm looking for a substance, polymer, or whatever that I can use to hold objects together temporarily. The critical property is that it must be
"deactivated" and release the objects (either by vaporizing, or by otherwise changing, somehow) after exposure to elevated temperature... but no
hotter than ~250C.
A lot of hot-melt glues fit this specification, but they readhere once the temperature is lowered again... thats not what I want, I want the adhesive
to be "broken" after exposure to temp.
One thought I had was something that simply boils away, but I'm having trouble finding anything with a boiling point that low. Do you all know of
anything??
Another thought was maybe something that thermally decomposes...?
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amaurer
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Another thought I had was perhaps a substance/film which forms a stable hydrate which is robust and flexible, but which becomes powdery and brittle
when dehydrated?
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Swede
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Cyanoacrylate adhesives? I know they are often used in machine shops as temporary holding fixtures for machining operations, and released with heat.
How about an adhesive that can be simply dissolved with an aggressive solvent like acetone? Rubber cement, contact adhesive, that sort of thing.
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amaurer
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The problem is that the two parts need to be held very close together, like under 10-thousandths of an inch. Adhesives that melt don't flow out due
to capillary action. Solvents, likewise, don't penetrate well. Thats why I'm hoping for a gaseous option.
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bahamuth
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If you want real help you will need to elaborate..
Distances of "like under 10-thousandths of an inch", calculated into metric, which most of the world uses btw, is about 2.5 micrometers, which is damn
well close.
I figure most amateurs have problems getting something with a flatness and surface roughness to accommodate something like that if it at all have a
relatively large surface area.
Anyways, you could use a high viscosity oil to build a film that thick, if you only want the "spacing action" of the "glue"..
Any sufficiently advanced technology is indistinguishable from magic.
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Swede
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1/10,000th of an inch may as well be in contact. That is an insanely small value. There is no glue or adhesive on earth that will allow two parts to
be 0.000,1" apart. There must be an adhesive film, and that adhesive film is going to be at least 0.001" thick.
Loctite adhesives are the gold standard, IMO, and their sleeve adhesive, which can be thought of as an adhesive which holds a precision pin in a hole,
REQUIRES 0.002" clearance for a bond. Any less than that, and there's no adhesive. It gets squished out of the interface.
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Wizzard
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Why not attach these things externally?
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Rosco Bodine
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An electrostatic agent is something which could possibly do the job. Some of these were noted in the MMO anode preparation research as being
inorganic polymers which produce an adherent film a few nanometers thick, and repel from their unbonded surface or have bonding characteristics which
can be altered by heating. Perhaps some scheme involving such molecular coatings, maybe in combination with silicones or fluorocarbon resins would
give you a scheme for a mold release agent that would "switch" modes on heating.
Paraffinic waxes can also be used where the wax is applied to the heated mold and the wax film solidifies on cooling, the composite is cast and cured
at below the melting point of tha wax ......and then the joined parts or just the mold is heated to above the mp of the wax and the film liquifies and
the mold separation can be forced .....although some considerable force or impact may still be required for the parting of such a "laminate". What
are the substrate materials that are involved will have definite bearing on what proposed release coatings may work or not.
[Edited on 27-11-2012 by Rosco Bodine]
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amaurer
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I may have been a bit ambiguous; the distances are on the order of 10 one thousandths, not 1 ten thousandth... e.g. 0.010".
The adhesive action is secondary, I really just need a substance to fill the gap and then leave it. Ammonium carbonate seems like a
possibility...?
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DJF90
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10-thou is a reasonable but close tolerance. The instruction leaflet for this bottle of super-adhesive (it glued steel to PTFE when I needed to
repair part of my KNF... I shit you not! Everything else failed, even plain ol' cyanoacrylate or 2-part epoxy) claims a film thickness of 0.15mm is
achievable and desirable for the best mechanical properties. A quick google reveals this is just under 6 thou of an inch. A similar value ought to be
achievable for cyanoacrylate, so Swede's suggestion may hold merit. How easy it'll fall apart at temperature is a whole other matter. Once it cures, I
have a hard time believing upon heating it will melt and become sticky again. It probably burns up (HCN anyone?) and becomes all crusty.
Upon reading your last post, it seems as if something that melts low and sublimes/boils below 250 may be applicable. The melting action allows easy
application to your small gap. First thing that comes to mind (because I have worked with it before) is p-methoxyphenol. It melts around 60*C (I
forget the exact temp) and has a significant vapor pressure- it ought to sublime readily under vacuum and its boiling point is somewhere in the region
of 250*C also.
[Edited on 27-11-2012 by DJF90]
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amaurer
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I read some papers on thermal decomposition of cyanoacrylate - HCN is an issue. Still I tried it last night and it does fall apart after several
hours at 400F+, but its pretty slow and don't think the CA or its products evaporate; rather it just looses adhesion as it decomposes.
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