>Date: Wed, 17 Nov 1999 16:57:15 -0500
I thought this might be worth repeating here. Gary is a chemist working for
Shell and a very experienced homebuilder. Much respected in US.
Graham
>[The Canard Aviators's Mailing list]
>
>
>I have been contemplating a reply on this subject for some time. But,
>lately I have been forcing myself to wait and see what precipitates
>before I butt in. You can never tell what great ideas can spring forth
>from a forum of this type.
>
>First I want to discuss - Shop Conditions, then Shop Heat
>
>Temperature - During the laminating procedure and for at least 24 hrs
>after completion, the temperature should be at least 65?F - preferably
>70? - 85?F. All of the curing agents available for constructing
>composite aircraft do not like to react at temperatures below 65?F.
>
>Below that temperature, the resin and curing agent reacts so slowly that
>it will not develop the same "level", "percentage" or "degree of cure"
>as it will at warmer temperatures. If it is too cool, it may not cure
>at all. One of the methods to describe or determine the "degree of
>cure" is to measure the cured resins Heat Distortion Temperature (HDT)
>or the Glass Transition Temperature (Tg).
>
>Ideally, a resin cured at lower temperatures will not develop the same
>Tg as the same resin cured at higher temperatures. HOWEVER, over the
>coarse of time it takes to complete a typical project, a resin that was
>initially cured at lower than optimum temperature conditions will
>eventually see warmer conditions. And as it does, it will slowly
>continue to cure to about the same "degree of cure" as a resin that was
>initially cured at the optimum temperature conditions. POST CURING at
>elevated temperatures will drive the "degree of cure" even higher and
>provide the best overall Tg / physical properties and fuel resistance.
>
>Humidity - The relative humidity (RH) is not as important as
>temperature, but should be below 70% if you want to minimize the "amine
>blush" some curing agents can exhibit. EZ-Poxy will not exhibit amine
>blush.
>
>Amine blush is caused by the curing agent reacting with CO2 in the air,
>forming a carbamate on the surface of the curing resin. After the resin
>is hardened, this "exudate" is characterized as a greasy or oily film
>you can feel on the surface. During and after cure this carbamate is
>hydrophilic and under high humidity conditions it will absorb moisture
>from the air making the resin surface turn whitish or develop "milky"
>swirls - this is the amine blush.
>
>Usually, the whitish appearance goes away on its own as the water
>evaporates, leaving the oily surface. Amine blush is not detrimental to
>the effect of the cured resin. But, it can hinder the development of
>good secondary bonds. Usually, wiping with a warm wet rag will remove
>the oily film. After which - sanding of the surface as you normally do
>will provide for good secondary bonds.
>
>SHOP HEAT - From the above, you can deduce that combustion type shop
>heaters that can produce high levels of both moisture and CO2
>(byproducts of combustion) will aggravate the amine blush scenario
>directly.
>
>Gary Hunter
>
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