REFLECTOR: epoxy cure time and temperature

[Grigore Rosu]

I tried hard to find info on the internet about the epoxy that we use (EZ-84
+ DER-324) and I had a hard time finding what I needed, so I rely again on
the collective great experience of this list.  Sombeody please let me know
if I ask too many questions.

Last night we had 1F in central Illnois; schools were interrupted because of
the cold weather.  That was not a reason for me not to work on my airplane
in my garage till late last night and today.  I just finished installing the
pilot side lower strake.  Used a heat gun all the time to keep the resin
flow and all the heat-producing devices that I could find in the house to
keep the BID layups heated at 70F or higher.  However, at some moment during
the night, a 500W lamp that I used as a heater on the inner skin of the
strake stopped working and the temperature of some parts of the BID layup
got down to 30F or less over the night.  I heated it right away in the
morning and it appears to cure properly now.  Yet, since the strakes are so
important and since I do not understand very well the chemical details
underlying the epoxies, I am a bit concerned.  I guess my questions are:

1) Can our epoxy cure at various temperatures, say anywhere between 30F to
85F, without loosing its strength?  The specs I found all mention that the
total cure time is 3 days at 77F, or 8 hours for tack-free at 77F, but say
little or nothing about varying the temperature.  How about the following
scenario (which is probably my case): 5 hours at 85F or more, then 5 hours
at 30F, then again back to 85F or more for a few more hours.  Has the 5-hour
spike down to 30F damaged the quality of the layups?

2) How long is one expected to maintain the "cure temperature" of 75F to
85F before one lets the temperature drop to 30F or lower without worrying
about a loss of layup strength?  Is it sufficient to maintain it until it is
tack-free (8 hours or so), or one really needs to do it for the entire cure
time of 3 days?

3) The specs mention that post-cure for 2 hours at 150F increases the
strength of the layups, but the specs read is if there is an implicit
assumption that the post-cure should be done after the normal cure.  Can one
let it cure at 150F from the very begining for, say 4-5 hours, skipping the
three days at 77F required for normal curing?  Some parts of my last-night
layups were closer to the heater and got for sure more than 100F during the
night, maybe even closer to 150F; those parts were hard like a rock in the
mornining; are these properly cured in spite of the fact that they were held
at a higher temperature than in the specs?

I guess there should be some simple answer to all these, but I was just not
able to find it on the internet.  Browsing through the reflector archive, I
found some info on a so-called  "scratch test" to see whether the epoxy is
properly cured; can I assume that if my layups pass the scretch test they
are OK, regardless of how they were cured?

Grigore
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