Hmmm.... never having taken them in hand, I may be wrong then. If it were possible to measure their wieght vs. displacement you could probably get a better idea by determining their specific gravity.

I can't the property tables without checking for sure, but it seems to me that Bakelite has a SG of much more than 1.0 if so they would not float. Unfilled PE, PP have a SG of less than 1.0 and will float. Polycarbonate (Lexan) is greater than 1.0, but only slightly so...