REFLECTOR: epoxy cure time and temperature
gattenby at tulsaconnect.com
gattenby at tulsaconnect.com
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 gmail.com> 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 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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