On Wednesday, May 17, 2017 at 5:41:13 AM UTC-4, Aussie wrote:
On 16-May-17 11:59 PM, wrote:
On Tuesday, May 16, 2017 at 3:46:47 AM UTC-4, Aussie wrote:
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?

Watch out for silicone or any other intermediary. Silicone is used as a release surface for epoxy.

Epoxy-to-PVC was always a "medium-performance" plastic bond (I first reported on adhesive assembly in 1979, and the situation remained the same for decades). In recent years, epoxy formulators have developed a lot of specialized products, including for specific plastics. PVC is one of them.

Here's a brief article that talks about one of the newer ones:

http://epoxyworks.com/index.php/glui...h-gflex-epoxy/



Thanks, that's an interesting article.

I thought silicone RTV rubbers were a different thing to the silicone
(oil?) used for mold release?

Same chemical family, but someone with chemistry knowledge would have to weigh 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 molten metals. (The latter are not RTV, but rather a hard, higher-temperature-cure type of silicone rubber.) Unless it's formulated specifically as an adhesive, it won't stick well to much of anything.

--
Ed Huntress