On 18-May-17 10:47 AM, Jim Wilkins wrote:
"Steve W." wrote in message
news
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?


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.

--
Steve W.

I haven't had much luck sealing tube feedthrus with O rings unless
there was a threaded packing nut to compress them.




I did read that to seal some potted sonar assemblies a couple of tight
large section o-rings are placed on the cable inside the potted volume.

The o-ring material is selected to bond well to the potting material.


The theory is the o-ring tension provides a seal against the cable
sheath and the potting compound to o-ring bond seals the ingress path
around the outside.

It sounds viable but I'm a little skeptical about long term sealing.