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I'm trying to bond PMMA to Nylon-6 for a submerged application, and the only thing that I've found that will reliably bond these parts is a 2-part polyurethane. (No epoxies work, I think because of the dimensional change of nylon when it absorbs water.) The PU glue is water-resistant but still not recommended for submerged applications. If I put a thin film (<0.5 mm) of silicone sealant on top of the glue line, is that going to enhance the durability of the glue? Or will the water just migrate through the silicone?

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  • $\begingroup$ Answering questrions posed below: The silicone sealant is rated for submersion, but I know that it's still quite permeable to water, just not damaged by it. Yes, the nylon absorbing water is an issue, I was also aiming to slow water entry into the nylon around the glue line. I think adhesives are the only option. These are very small parts and mechanical fasteners are out of the question. It needs to last months, not decades. (It's for a saturated soil column experimental apparatus that will run for 6 months.) It's not load bearing, just needs to stay watertight, so maybe the glue will last.\ $\endgroup$
    – user278411
    Commented Mar 17, 2023 at 18:20
  • $\begingroup$ Mechanical fastener doesn't just mean nuts, bolts, and screws. It includes things like pins, press fits, dovetails, etc. And of course things like solvent welding, ultrasonic welding, friction welding, etc. $\endgroup$
    – DKNguyen
    Commented Mar 17, 2023 at 18:23
  • $\begingroup$ You may need to just test it. Hang a block of PMMA glued to the nylon with the silicon sealant at the top of a column of water with a weight hanging off of it for a couple of months or something. Though I don't know if your timeline lets you wait that long. I don't know if something like dye will permeate the silicone but if it does you could perhaps speed things up by gluing two pieces of acrylic and submerging that and examining if any dye has passed through. $\endgroup$
    – DKNguyen
    Commented Mar 17, 2023 at 18:27
  • $\begingroup$ Do you have enough time to run a test? Hang a test block at the top of a column of water and leave it there weighed on the bottom for a couple of months $\endgroup$
    – DKNguyen
    Commented Mar 17, 2023 at 20:24

2 Answers 2

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No. If silicone conformal coatings are anything to go by, they are not rated for submersion. One of the reasons you don't want air pockets on a silicone conformal coating is that water will collect there and then can't escape, even when the silicone conformal coating otherwise protects the printed circuit board from moisture.

Isn't the fact that nylon 6 is hydrophilic and submerged going to cause a problem anyways?

Are there no other methods of fastening you can use that do not involve adhesives?

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The basic question is: can you buy any kind of paint at all that will keep moisture from diffusing into the glue line? Now you can see that you need to know how long you want the glue joint to last under water. If the answer is ~years, then you'll need something like the gloss coat or clear coat that car manufacturers use as a topcoat over metallic paint finishes. This lasts ~years (but not ~decades, unfortunately).

Note also that you may get enough water resistance in the 2-part urethane by curing it with heat or ultraviolet light. Experiments will tell you the answer.

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