Is heat shrink permeable? - Page 1
Forum section: electronics / general

author: jfiresto
author_rank: Frequent Contributor
timestamp: May 16, 2020, 08:11:37 am
content: I extended some uncoated, unshrunk, polyolefin heat shrink off the end of a small connector, and poured into it, some moisture-setting MS polymer adhesive, to pot the connections. It appears the adhesive is uniformly setting over the entire length, even deep within the tubing, far from the open end.Am I witnessing a fortuitous deficiency of plain old, (unshrunk) heat shrink – that it is permeable to water vapor?

author: fcb
author_rank: Super Contributor
timestamp: May 16, 2020, 02:13:07 pm
content: You need to bake out the heatshrink at 125C for a couple of hours.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2020, 01:12:18 pm
content: Just about every polymer is somewhat permeable to small molecules: gases, water vapor, etc. I don't know offhand how polyolefin (most common type of heatshrink) performs, in comparison with others. Polyolefins should generally be pretty good against water, I would think (they are nonpolar). I could see if it's still expanded (un-shrunk), it might have unusually high porosity -- heatshrink is prepared very simply by stretching it; it relaxes to normal size when heated.Compare to nylon for example, which is polar, and absorbs moisture well enough to run afoul of dimensional tolerances.Even PCB stock (heat-cure epoxy resins, among other types) can absorb enough moisture to have problems during soldering (mainly related to delamination as the steam expands between impermeable copper planes).Or Teflon, which tends to be porous, thread-seal tape especially, which a lot of people may not realize. It works because it's crushed by the threads, not because it has much sealing power by itself. Heat-sealed metallized pouches work the same way: the metallization or foil layer is thick enough to block diffusion through the sheet; the seal is plastic, but diffusion is lengthwise along the narrow sealing joint, so takes enough time to be a practical seal.If you need a true hermetic seal, there are some options: if you can arrange a thin and long seal (like the heat-sealed foil, the pipe thread, or a flanged joint), that helps greatly. An appropriate epoxy may work (something rated for hard vacuum?). Metal, ceramic or glass materials are the gold standard, and in modest thicknesses, can be nearly completely impermeable, even to hydrogen or helium when hot.If your goal is more just to exclude standing liquid water, you're probably fine. In that case, the potting doesn't need to be impermeable (and certainly, your material isn't, else how would it set in the first place!), just that it fills voids. Bubbles trapped against corrodable surfaces need to be avoided carefully (preferably with vacuum), as moisture can condense inside them. Water vapor isn't ionic or conductive, so isn't a bad thing in and of itself, for most electronic and environmental purposes.Also, if you have an enclosure that's supposed to be sealed, but could be permeable (indeed, over a long enough time scale, is guaranteed to be), you can gain a little time by putting some dry silica gel inside there, for example. Fight permeability with permeability, of a sort.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 28, 2020, 11:37:58 pm
content: A trick I heard from gadget mers.They sometimes use regular silicone chaulk, which is also moisture setting, and when they want to apply it in thick layers it usually cures very slowly.To remedy that they mix some moisted flour through the chaulk, which provides for the water to cure quickly throughout the product.But for heat shrink, you can buy versions with a layer of hot melt glue on the inside. Instant waterproofing with no messin' with liquid glues or other time consuming stuff.

author: jfiresto
author_rank: Frequent Contributor
timestamp: August 29, 2020, 03:35:10 pm
content: Quote from: Doctorandus_P on August 28, 2020, 11:37:58 pmOhhhh, I like that. The last time I potted with MS polymer I water-misted the non-stick surfaces of my mold so that the casting would cure inward from all sides rather than just the one, open side.