REFLECTOR: epoxy cure time and temperature

Grigore Rosu grigore.rosu at gmail.com
Thu Jan 24 22:50:03 CST 2008


Ron and Alex,

Many thanks for your answers, they are very useful.  I am happy to know that
nothing is wrong with my layups so far; they just need more heat on them and
they will be fine.

I will probably keep some more heat on the strake till tomorow, just to make
sure that it does not change its position in case I need to remove the wing
for some reason before the summer comes (I actually hope that I will need to
remove the wing sooner than that, to put on the other wing to install the
other spar).

Grigore


On Jan 24, 2008 8:58 PM, Alex Balic <velocity_pilot at verizon.net> wrote:

>  Hi Grigore,
>
> all epoxies will slow down in cure with depressed temperature- the
> aerospace industry uses "prepreg" material, which is pre- saturated cloth
> that has to be kept refrigerated to prevent the epoxy from curing- it can be
> kept this way for months and still used- after molding, it is heated in an
> autoclave oven to effect cure of the resins- our epoxy dues much of the
> same- if you mix it, then wet the glass, then refrigerate it- you are in
> effect creating prepreg material- it will begin to cure when the temperature
> goes back in range- we found no loss of strength in carbon prepeg material
> due to reduced cure temp, only extended cure time- although material that
> had become "stale" in the refrigerator, did not bond as well to the other
> layers in the layups.  I  can't remember exact times- but on my Velocity-
> the work that I did during the cold weather would take days to get hard-
> probably didn't cure fully until they got some good heat on them weeks
> later- the cross polymerization rate drops as less un-cured material is
> available to react with the catalizers, kind of like concrete, it can still
> cure for some time after it looks done- for example, you can re- heat some
> of the parts with a heat gun a week or so after cure (normal temps) and you
> can still get some give out of the parts, but after a few months, the
> material will just start to de-compose with the heat application.
> Basically, the lay-up will cure when it is warm enough to cure, and sleep
> when it is too cold to cure- when it warms back up- it will start to cure
> again-  probably good to limit the heat and cold cycles as it might tend to
> form microscopic stress layers in the resin matrix, but I would not think it
> would significantly affect the strength since the aircraft is pretty well
> over designed. I have no data on the temperature cure schedules for the
> Velocity epoxies, the factory might have access to this information.
>
>
>
> Alex
>
>
>  ------------------------------
>
> *From:* reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] *On
> Behalf Of *Grigore Rosu
> *Sent:* Thursday, January 24, 2008 5:39 PM
> *To:* Velocity Aircraft Owners and Builders list
> *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 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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