REFLECTOR: epoxy cure time and temperature

Fri Jan 25 07:06:37 CST 2008

Grigore,
1F...your a brave man, Grigore.  As you heat your work area, just be cautious about using kerosene heaters.  Besides a potential for carbon monoxide in a tight space, the starting/stopping puffing may add to potential to lay out a light oily film from partially combusted fuel.  The observation is qualitative as I have no idea if there would be enough to affect the bonding between layers but its always worth considering.  Others may have a useful opinion on the matter instead of just a speculative concern.

Chuck Jensen 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On Behalf Of Grigore Rosu
Sent: Thursday, January 24, 2008 11:50 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: epoxy cure time and temperature

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

_______________________________________________
To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector

Visit the gallery!   www.tvbf.org/gallery
user:pw = tvbf:jamaicangoose
Check new archives: www.tvbf.org/pipermail
Check old archives: http://www.tvbf.org/archives/velocity/maillist.html

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.tvbf.org/mailman/private/reflector/attachments/20080125/620c1a31/attachment-0001.htm 