All PVC is not created equal. Put on your experimenter's hat and step into
the laboratory, er, kitchen. Put some of the PVC in question into the
microwave and fire it up. How long is up to you, after all it's your
experiment and your microwave. When the bell rings touch the PVC. Cool?
Slightly warm? Or too hot to touch? The hotter it is the more energy it
absorbed and the less suitable it is for RF. If you goofed and set it for
half an hour and walked away, you may find a microwave full of boilling goo.
In which case you get to buy a new microwave and take the old one apart for
more experiments.
73, Steve KW3A