Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Mon, 28 Oct 2002 11:17:23 -0500
Message-ID: <redirect-1846674@logan.com>
From: Newlan2dl@aol.com
Subject: Re: [LML] Post curing wet layups
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Hello Dan

Post cure is used to increase the cross link density of a material.  In other 
words, molecules of resin and cross linker (this is also somewhat arbitrary 
since which is the resin and which is the cross linker can be argued, however 
the amine is typically called the cross linker).  Also note that the cross 
linker is often incorrectly called "catalyst".  There are catalysts for 
epoxies but this is not the same!

At any rate, these molecules of part "A" and "B: have to "find" each other in 
order to harden.  In a perfect stoichiometeric ratio, one molecule of resin 
is matched to one molecule of cross linker.  However not all molecules are 
equally reactive.  At room temperature, there is perhaps about 96-97% cross 
link desity, i.e. there are still about 3-4% of the molecules that have not 
found there counterparts.  These are just some less  reactive molecules that 
need a little heat to finally cross link.  Note, too that most epoxies shrink 
about 3% when they cure (vs. about 9% for polyester/isopthalic/vinylesters) 
based on volume.  The reason you will see more print through with a part that 
you finish with a nice gloss is the shrinkage from the final cross linking.  
This happens because at some point when the part gets hot enough to post 
cure, this is the last cure phase adds some more shrinkage to the part.  
Hence the print through.

Anyway, post curing will add some extra strength and isn't a bad thing to do 
in the least.  It is also a time/temperature thing.  You can get it near the 
HDT (heat distortion temperature) of the resin and cure it up pretty quickly 
or have it below there for a bit longer.  If I rememebr correctly, the Jeffco 
has an HDT of 190 degrees F (with the glass transition temperature about 
15-20 degrees above that).  So say you had an oven and took it to 170 degrees 
for 4 hours should do it.  Or 150 degrees for 6 hours.  It will be something 
like that.  You shouldn't go above the HDT although I have it on good 
authority that it doesn't really hurt the resin much as long as there is no 
stress on the part at the time.  However the HDT is exactly that.  It's the 
temperature at which the resin becomes a bit plastic and can move around 
under stress.  Here again, it's a bit arbitrary since actually, it's a curve 
so you can see the beginning of the softening a bit before the HDT (whereas 
the glass transition temperature is actually a definate fixed number).

Last thing is make sure the PARTS see the temperature required, not just the 
air in the oven!  A great example is some guys I know in Australia were 
building a 78' all carbon uni prepreg boat, (Steinlager IV designed and built 
for the around the world sailboat race).  They built an ENORMOUS oven for the 
job.  When they built the hull, they did a beautiful job!  Anyway after 
cooking for X number of hours they popped it off and started work on it.  So  
a few days into it, they noticed that when they ground the carbon to mount 
bulkheads and such, they didn't get dust, they had little balls of resin and 
carbon rolling up. The layers inside were uncured!  It turns out that while 
the oven temperatures were adequate, they hadn't placed thermocouples on the 
hull and against the plug so they didn't realise that the inside never got 
hot enough to cure.  

A hard lesson but that was one of the first prepreg built large boats ever 
and the learning curve was steep and painful!

Anyway, the best thing is to check with Jeffco.  They can give you the 
time/temperature curves.

Good luck!

Dan Newland

Super ES #61
 Alameda, California