REFLECTOR: epoxy cure time and temperature

gattenby at gattenby at
Fri Jan 25 10:40:06 CST 2008

To add to the collective urban legend...

Those quarts lamps put out a fair amt of UV light as well.
Most expoies don't do well with "extended" UV light.
I doubt that a few hours would make any diff,
but I did / do limit how much I use those quarts lamps.\
(but not as much as I limit how much work I do at single 
digit temps)

Noel Gattenby

On Thu, 24 Jan 2008 22:50:03 -0600
  "Grigore Rosu" <grigore.rosu at> wrote:
> 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> 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 
>> 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 
>> Alex
>>  ------------------------------
>> *From:* reflector-bounces at 
>>[mailto:reflector-bounces at] *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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