I just got this message from Frank Grunthaner who seems to know. I thought
the group would like to know that I goofed by identifying my shims
incorrectly. Oops, sorry for the confusion, y'all.

William,

Polyethylene is just fine. The flow properties of polytetrafluoroethylene
(and other members of the Teflon family) are what I was referring to. This
should point out the importance of correctly identifying materials!

For example the difference between lipids, proteins, and carbohydrates on
one side and silicones on the other!

Frank Grunthaner