Plastic materials used in wlan antennas must be low lossed in nominal frequency. The frequency used in microwave ovens is in same band as 2,4 GHz wlan. So all microwave proof materials are suitable in use. It is simple to test it:
Put a piece of plastic material into the microwave oven. Also put a
cup of water in the other side of the oven to prevent oven overload. Let
the oven heat a couple of minutes so water in the cup becomes boil. If
the temperature of plastic material don't rises at all it is passed the
test and probably is suitable to wlan antennas.