Bonding epoxy to PVC for water proofing

I'm playing around potting some electronics for immersion in water up to
10 metres.
A cable with a polyethylene sheath and PVC insulated wires connected to
a circuit board are embedded in some rigid setting epoxy.
I know I'm not going to get the epoxy to adhere to the polyethylene.
What can I do to enhance the bond between the epoxy and the PVC wire
insulation to keep the water from wicking along the wire to the circuit
board?
I've read that brushing with PVC pipe glue primer (MEK) can help, as
well as flame treating.
Flame treating is impractical as the job is too small & tight to get
into the area where the PVC insulation is.
Any suggestions?
Is the MEK priming likely to help?
Perhaps painting on some sort of low viscosity RTV silicone that might
form an intermediary seal?
Reply to
Aussie
Loading thread data ...
I have used a two part plastic glue to bond some hard to bond plastics. I'm not sure if the glue is an epoxy. I do know that it is waterproof though. I bought the glue at the local hardware store and it is made by Henkel (I think). I'm pretty sure the brand was Loctite (and Loctite is now owned by Henkel). To repeat, I have used this glue on some pretty hard to bond plastics and it worked very well. In fact, it exceeded the performance guaranteed on the package. Eric
Reply to
etpm
Watch out for silicone or any other intermediary. Silicone is used as a rel ease surface for epoxy.
Epoxy-to-PVC was always a "medium-performance" plastic bond (I first report ed on adhesive assembly in 1979, and the situation remained the same for de cades). In recent years, epoxy formulators have developed a lot of speciali zed products, including for specific plastics. PVC is one of them.
Here's a brief article that talks about one of the newer ones:
formatting link
Reply to
edhuntress2
Home Depot, a monster hardware store, has a dip type of elastic encapsulent in a spray can. I bought a can and sprayed it on an empty pcb and it looked good and felt slightly flexible. The can is now patiently awaiting further testing.
Two part polyurethane might work also. Mix it and apply with a paint brush. No idea how either of these handles immersion in water but it would be nice to hear...
Hul
Aussie wrote:
Reply to
Hul Tytus
A good many years ago I was involved in something similar. Instead of deal ing with water it was dealing with high voltages and vacuum. The first try was epoxy potting. It cracked in temperature cycling.
So somewhat flexible polyurethane potting was tried. It did not adhere wel l enough.
The solution was a metal box with a gasketed lid. It was great for reworki ng as in did not have any conformal coating to remove. The connectors did have to be a special design. They were kind of like morse tapers. Necess ary because the voltage was about 2000 volts. Surprise , surprise the conn ector was designed on paper and worked without having to be revised.
Dan
Reply to
dcaster
Thanks, that's an interesting article.
I thought silicone RTV rubbers were a different thing to the silicone (oil?) used for mold release?
Reply to
Aussie
release surface for epoxy.
ported on adhesive assembly in 1979, and the situation remained the same fo r decades). In recent years, epoxy formulators have developed a lot of spec ialized products, including for specific plastics. PVC is one of them.
Same chemical family, but someone with chemistry knowledge would have to we igh in here.
Silicone rubbers, RTV and otherwise, are widely used for making molds, for plaster, for thermoset plastics like epoxy, and even for low-temperature mo lten metals. (The latter are not RTV, but rather a hard, higher-temperature -cure type of silicone rubber.) Unless it's formulated specifically as an a dhesive, it won't stick well to much of anything.
Reply to
edhuntress2
Perhaps you could research high-performance professional and low-cost educational solutions:
formatting link
formatting link

I got a good deal on a vacuum oven that leaked through the wiring feedthru. I replaced it with a solid plug and haven't found how to make one. Here's one possibility for solid wires or sealed tubes:
formatting link

Light bulbs have wire leads that seal vacuum-tight in glass.
formatting link

-jsw
Reply to
Jim Wilkins
You can get cable that is filled with a hydrophobic gel, so that even if the cable does have a leak somewhere along the length, water won't wick along it.
If your device is being potted, you could put a short section of single strand bare copper wire between the circuit board and each stranded wire. The epoxy should form a good seal to the bare copper. Liquid water in the stranded wire ought to stop at the solder joint between the stranded and non-stranded wire. Probably some water will diffuse through all of the plastics and resins eventually though.
You could try corona-treating the polyethylene and PVC which is much like flame treating but may produce less heat.
Reply to
Chris Jones
My understanding is that most epoxy does not adhere to copper. I think I read that somewhere. But you can find epoxy that will adhere sold for glueing copper plumbing together.
Dan
Reply to
dcaster
I was thinking you could coat the PVC with the two part plastic glue and then bond your epoxy to it. Eric
Reply to
etpm
I've not been able to find jelly cable or water block treated cable at a viable price. The inexpensive type is solid core conductors and I need multi-strand unfortunately.
I've left the insulation length long to provide a long sealing path, three is an opportunity to strip the copper back so I will definitely do that.
Corona treating - I'll check it out.
Thanks
Reply to
Aussie
I read that somewhere. But you can find epoxy that will adhere sold for g lueing copper plumbing together.
The difficulty with epoxy bonding to many metals is that it's actually bond ing to surface oxide. Depending on the metal and the thickness and nature o f the oxide, this results in weak joints in stainless steel, aluminum, copp er alloys, and, of course, anything containing lead. There are other proble m metals.
There are ways around it. One is to abrade the surface of the metal while i t's coated with the mixed epoxy. This is sometimes called "scratch-in" prep aration. I use it all the time, and it works great on stainless and aluminu m.
I don't know for sure about copper. I've been asked about it before but I n ever tried it or looked into it, but I suspect it will be the same as for o ther metals.
This abrasion method is not practical in most commercial applications: alum inum aircraft wing skins being a prime example. To work fast and to get mor e reliable results, industrial users have worked out various conversion coa tings and etches for specific metals that need to be bonded with epoxy and other adhesives. On aluminum, for example,they use phosphoric acid anodizin g, PAA, and I've heard there are newer, better conversion coatings for alum inum.
There is an entire literature on this, but when I have a question, I just c all 3M or Loctite, or whomever, and ask to speak to an engineer. Or I did. I was a trade magazine editor and I could use that leverage to get through. Now that I'm retired, they might just tell me to go piss up a rope. d8-)
Reply to
edhuntress2
Use a heated plate to melt a rolled lip on the end of the poly tubing. The lip will give the epoxy a mechanical bond. To ensure it doesn't leak you could put a common O ring that fits the tubing above the lip. The epoxy then encapsulates the ring and the lip.
Reply to
Steve W.

Site Timeline

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.