![]() The initial work on polysiloxane chemistry dates back almost 60 years and it continues today, so "getting the formulation right" is hardly a meaningful expression. ![]() The chemistry of a particular polymer determines it characteristics, from hard and brittle varnishes, to soft and flexible rubbers. To begin this discussion, however, it should be stated that silicone polymer is a term much like "plastic" - it covers a rather wide range of materials and properties. The corrosion is entirely dependent on the particular type of silicone sealant used, and most common ones generally available to the public still release acetic acid when curing. Ornitz of Eastman Chemical Company Research (1997): This is a more thorough contribution by Dr. Though your nose will give you plenty of warning that something isnt right. Mind the fumes from all of them, but if you have a large surface area covered, Acetic acid fumes can be pretty nasty. The most desirable for electronics is Alkoxy cure, which releases methanol. This can corrode copper on contact, but isnt as bad to use near electronics, just not directly on copper. Oxime cure which releases a ketone on curing. The other two kinds are neutral cure, but still slightly different. Even sealing an enclosure with it, the trapped fumes inside can corrode the items. Acetoxy cure, which is the cheapest, bonds well, but releases acetic acid on curing (the acid ingredient in vinegar) And corrodes copper and lots of other metal. Your normal every day RTV silicone is 'Condensation cured' where atmospheric moisture is what makes it cure. This is a related and relevant answer, by "ConKbot" (2014): But for metal conduit (galvanized steel) and in junction boxes with exposed metal these sealants must be avoided, or -in the case of boxes- left to cure 48hrs with the box lid open. ![]() This likely poses no problem in short open ended stubs of PVC conduit, commonly used to penetrate a house wall from an outside box. The fumes released during the curing process may corrode the conduit or the wiring. Most silicone type sealers (this in contrast to acrylic or putty sealers) are corrosive curing unless noted otherwise on the label. Those with a curing agent or accelerator will indicate so on the label, e.g. So capping a conduit should be done with at most 1in deep plug, allowing cure depths from both sides, the open end and the tube side. Most sealers cure only to a limited depth, perhaps 0.5in. When selecting a silicone sealer, note that Wire that is THHN should not be in a wet location, and therefore the conduit is best kept open at the interior end. The conduit does not have to breathe or be open if the wiring is rated for exterior/wet, e.g. You can seal the inside of the conduit with silicone caulking or duct seal putty (as you suggest). You can seal the outside of conduit with closed cell expanding foam (as you suggest) or silicone caulking. Is this reasonable and Code-compliant? (Anecdotes suggest that nobody worries about sealing conduit interiors these days) I would use duct seal to seal the outside of the conduit where it goes through holes in the wall. ![]() My current plan is thus to apply some of that spray foam inside each liquidtite neck wherever it enters an electrode cap enclosure or exterior junction box or the first enclosure after going through the exterior wall. I have also read that spray cans of low-expanding closed cell urethane foam make good vapor blocks and are removable for future service. I have duct seal "putty" but have read that over time it does not make a good water vapor block especially with wires running through it. What should I use to seal the inside of the conduit, and where? ![]() I've got 1/2" liquidtite flexible PVC conduit (1) going down to protective "cap" shields on top of buried grounding electrodes, (2) running along exterior of garage wall between a few exterior surface-mounted junction boxes and (3) coming through the exterior garage wall into the unfinished garage interior. I've recently become aware that Code requires not just the outside of conduit penetrations of exterior walls to be sealed, but also the interior. ![]()
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