On Tue, 9 Jul 2002, John Carmack wrote:
> ...Mark Henry (ex Beal propulsion engineer) had mentioned to us once
> that not all Teflon is completely peroxide compatible, and this seems to be
> an example of that...
Hmm, yes, definitely noteworthy...
Yes, there are different kinds of Teflon, partly because they're made by
different processes, partly because there is some custom tailoring of the
polymer for specific properties. The two flavors most likely to be seen
in ordinary contexts are Teflon PTFE and Teflon FEP.
Teflon PTFE is the original Teflon, as stubbornly chemically inert as any
solid material can be (unless you heat it too far). PTFE is Poly Tetra
Fluoro Ethylene -- you make CF2=CF2 first (easier said than done because
the fluorine really wants to add to that double bond to make CF3-CF3), and
then polymerize it.
Teflon FEP is easier to make and almost as good. And as we all know,
"almost" means "not". FEP is Fluorinated Ethylene Polymer, i.e. they take
polyethylene and try to replace all the hydrogens with fluorines. I'm not
sure whether the problem is that they don't entirely succeed, or that they
weaken the polymer in the process of succeeding, or both or neither. It's
pretty good but not the equal of PTFE.
My guess would be that the usual valve hardware is Teflon FEP, and what
you want for long-term peroxide compatibility is Teflon PTFE. (It's also
possible that the valve hardware is some slightly-modified variant, custom
cooked to reduce problems with cold flow. Teflon is notorious for trying
to ooze out when put under pressure.)
> Unfortunately, industrial suppliers don't make a distinction.
Lab-equipment suppliers do -- that's where I first ran into it -- but
whether you can find an industrial valve supplier who does (specialist
chemical-industry suppliers?) is another question.
Henry Spencer
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