Information regarding Change in permitivity of poleyster substrate in water?

While working on washable RFID Tag using UHF meander dipole antenna, I need to make it operable in water. How much permittivity of polyester will change when it is completely dipped in water. Substrate's permittivity increases as it absorbs water, I need to know the exact amount by which it will increase.
Thanks

With Best Regards :

Permittivity of water is a function of frequency. The real part is close to 80, the imaginary is widely varying with frequency.
Soaking a substrate in water is a bad idea, rather cover the substrate with a hygrophobic material, then run a test to see how the permittivity changed.
Water is also variable material, depends on whether it is distilled or a salty solution.
The "exact amount" of the effect can be found by experiments only.

If the RFID is only "washable", not to operate immersed, use a Teflon-like coating to remove water fast, then the effects of water can be neglected.