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On 2/8/21 10:56 AM, Dave Cole wrote:

Hi,
Thanks for the clarification, do you think this is due to the PVC itself absorbing water, or just moisture in the air?

A little of both - moisture adsorbing into the surface will increase surface leakage for HV, but after wiping it down with alcohol, the leakage resistance was still higher, so I think there's some bulk absorption into the plastic.? Another indication is that blowing warm dry air on it takes a while to get the leakage back down.? This is in low current HV applications (Electrostatic machines like Van de Graaff generators or Kelvin water droppers), where 1 microamp is a lot of current, so driving leakages low is important.
I'll have to go look, but someone did a study measuring the Q of tesla coil secondaries on various form materials as the weather varied. Cardboard (sonotube) varies a lot, but I can't remember how PVC pipe worked.
My "take home" from this was "choose a different kind of plastic for those applications"
For the "plastic pipe in a microwave" I think it's more bulk absorption into the porosity - plastic pipe is not pure PVC, after all. It's PVC + fillers + dye, and with no attention to bulk dielectric properties.
In most RF applications (e.g. antennas) using PVC pipe is probably fine - there's a lot of other things that will dominate over any small dielectric losses, and whether the leakage is microamps or 10s of microamps is probably not worth worrying about.? I've not worried about it much. I don't think I'd use it as a substrate for a conformal patch antenna, though.? It's too easy and cheap to get something you know will work better.

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 2/8/21 10:34 AM, Jim Lux wrote:

The leakage resistance changes noticeably with humidity.