Bulk Modulus (E*) is also a good approximation, but requires you to know the Loss Modulus (E") in addition to the Storage Modulus (E').
Since E* = E' + iE",
E* can be calculated by:
E* = SQRT( (E')^2 + (E")^2 )
Silicones typically have very low surface energy. The presence of silicone may change the wettability of the valve by other materials such as non-silicone paints. Another characteristic of silicone is that they out-gas quite a bit; i.e., they contain a lot of volatile smaller molecular weight...