REFLECTOR: epoxy cure time and temperature
romott at roadrunner.com
Thu Jan 24 17:57:24 CST 2008
I built my strakes in the winter, (but not at 1 deg F!!!).
I used 130-140 degrees (almost too warm to touch) for my cures. I used mostly halogen work lights (two 500 watt lamps). It cures quicker that way, and typically I would let it cure over night or as much as 24 hours depending on how cold it was in the hangar. Epoxy is supposed to be self heating but most of our lay-ups are thin and there is not enough density of the epoxy to get this self heating. In our case, most of the lay-ups are thin and 70 degrees in the very minimum that will effect a cure - as your research found.
There has been a lot of discussion about post heating - Alan Shaw said NOT to do it with his wings. Some folks have wrapped the fuselage with black plastic and put it out in the sun to get a post cure. I wouldn't go over 150 degrees. Radio Shack and other electronic stores have temperature readouts with remote sensors. I bought a couple of these to monitor my cures.
----- Original Message -----
From: Grigore Rosu
To: Velocity Aircraft Owners and Builders list
Sent: Thursday, January 24, 2008 6:38 PM
Subject: REFLECTOR: epoxy cure time and temperature
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 menion 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?
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