CIDCrazyJay
Member
There seems to be a lot of concern over the risk of epoxy exotherm in thick layups.
I conducted a test to evaluate the validity of those warnings, to determine just how thick one could go before this actually became a concern.
The basis for this experiment, was to determine if an E-Glass Epoxy wing spar cap of 3x1 inches (76.2x25.4mm) would pose any real concern for a DIY homebuilder.
As you can see, there is almost zero reason to avoid these sorts of layups with purpose made epoxy designed for thicker laminates. (Aeropoxy PR2023/PH3660).
The maximum temperature recorded was around 102F (39C) several hours after commencing, but before completion. Layups conducted in a climate controlled room, which held between 77-80F (26C) degrees, which is slightly warmer than the 75F which is suggested. The layup was a short 1ft section, and around 25 layers were put down rapidly without excess squeegee time. Total thickness came out to around 0.625" or 16mm.
Intended to go thicker, but there was no increase in temp, and I ran out of 5 pairs of nitril gloves which kept tearing. I also had to keep mixing epoxy, and also cut more fiberglass, which required pulling the thread, and became more hassle than it was worth to learn almost nothing new. So I stopped.
Peak temperature arrived several hours into the project, as the thickness built and epoxy cured. Temp held steady thereafter and began to cool around the time of completion. Unfortunately, the written record and time-temp graph went missing, as this was conducted over a year ago.
The test was underwhelming, and completely discredits anyone suggesting that extremely thick laminates pose any concern to homebuilders. If you had a larger layup, the temp would actually be lower. Because of the time for the earlier layer to cure while working with a longer length. This was pit down so quickly, that the middle layers actually started squirming around, as the epoxy warmed up and thinned out after it had already been squeegeed out at a lower temp and viscosity.
Aeropoxy seems to work well enough, although slightly lower viscosity while wetting out and squeegeeing would be beneficial for this thick E-Glass. E.g. lower viscosity epoxy or use a heat source to gently warm it up, as demonstrated in the Rutan composite video.
The largest concern for builders is actually what specific peel-ply is used. And what prep techniques are used between spar caps and wing skins. Most peel-ply contains silicone release agents (such as this exact peel ply utilized here), which substantially hinder further secondary Epoxy bonding without concerted cleaning efforts that are proven to remove that specific contaminate from Composite laminates. There is only one or two specific peel plys that do not contain it. This seems to be a common error in composite construction, and is probably to reason for skin/spar cap delaminations in those famous canard tests on an otherwise flawless built component. This is my own hypothesis, and I've not heard or read about it anywhere else in Experimental Aviation writings, which I find rather alarming.
Although the finer details are not available, I thought some builders on here might be interested in the outcome.
I conducted a test to evaluate the validity of those warnings, to determine just how thick one could go before this actually became a concern.
The basis for this experiment, was to determine if an E-Glass Epoxy wing spar cap of 3x1 inches (76.2x25.4mm) would pose any real concern for a DIY homebuilder.
As you can see, there is almost zero reason to avoid these sorts of layups with purpose made epoxy designed for thicker laminates. (Aeropoxy PR2023/PH3660).
The maximum temperature recorded was around 102F (39C) several hours after commencing, but before completion. Layups conducted in a climate controlled room, which held between 77-80F (26C) degrees, which is slightly warmer than the 75F which is suggested. The layup was a short 1ft section, and around 25 layers were put down rapidly without excess squeegee time. Total thickness came out to around 0.625" or 16mm.
Intended to go thicker, but there was no increase in temp, and I ran out of 5 pairs of nitril gloves which kept tearing. I also had to keep mixing epoxy, and also cut more fiberglass, which required pulling the thread, and became more hassle than it was worth to learn almost nothing new. So I stopped.
Peak temperature arrived several hours into the project, as the thickness built and epoxy cured. Temp held steady thereafter and began to cool around the time of completion. Unfortunately, the written record and time-temp graph went missing, as this was conducted over a year ago.
The test was underwhelming, and completely discredits anyone suggesting that extremely thick laminates pose any concern to homebuilders. If you had a larger layup, the temp would actually be lower. Because of the time for the earlier layer to cure while working with a longer length. This was pit down so quickly, that the middle layers actually started squirming around, as the epoxy warmed up and thinned out after it had already been squeegeed out at a lower temp and viscosity.
Aeropoxy seems to work well enough, although slightly lower viscosity while wetting out and squeegeeing would be beneficial for this thick E-Glass. E.g. lower viscosity epoxy or use a heat source to gently warm it up, as demonstrated in the Rutan composite video.
The largest concern for builders is actually what specific peel-ply is used. And what prep techniques are used between spar caps and wing skins. Most peel-ply contains silicone release agents (such as this exact peel ply utilized here), which substantially hinder further secondary Epoxy bonding without concerted cleaning efforts that are proven to remove that specific contaminate from Composite laminates. There is only one or two specific peel plys that do not contain it. This seems to be a common error in composite construction, and is probably to reason for skin/spar cap delaminations in those famous canard tests on an otherwise flawless built component. This is my own hypothesis, and I've not heard or read about it anywhere else in Experimental Aviation writings, which I find rather alarming.
Although the finer details are not available, I thought some builders on here might be interested in the outcome.
