[Rhodes22-list] Re: CB trunk delam update

[Roger Pihlaja]

Lou,

Ok, what's going on as epoxy cures?  Epoxy comes from the manufacturer in
two containers, the resin and the hardener.  The resin consists of the
monomer or single chemical units that are going to be chemically reacted
together into a gigantic 3-D molecule.  When about a million monomer units
join together, the result is a solid polymer of cured epoxy.  This chemical
reaction is caused by the chemical in the other can, the hardener, which is
actually a catalyst.  As the term catalyst implies, the hardener accelerates
the rate of joining or chemical reaction between the monomer units and the
developing polymer.  However, the catalyst itself is not consumed or used up
in this process.  The most commonly used hardeners for epoxy are small
molecules called amines.  The funny thing about the epoxy system is that
amines & epoxy monomer are very soluble in each other.  This mutual
solubility makes it relatively easy to blend the resin and hardener together
to start the batch.  But, amines are not very soluble in the epoxy polymer.
We won't get into the reasons why, it's a physical chemistry thing; but,
trust me on this one.  So, as the epoxy monomers steadily turn into a giant
epoxy polymer, the molecules of amine hardener are repelled from the
interior of the developing solid.  Just like oil on water, the hardener
tends to accumulate at interfaces or surfaces as it becomes insoluble in the
cured epoxy.  However, when the epoxy has fully cured, most of the amine
molecules end up trapped somewhere inside the bulk solid & have to diffuse
thru a solid polymer in order to reach a surface.  Solid state diffusion is
a relatively slow process, requiring 6-12 hours at room temperature.
Macroscopically, we observe the accumulation of a waxy or oily layer on the
surface of the cured epoxy over the 1st day after initial setup.  This waxy
layer is commonly referred to as "amine blush".  This waxy layer will tend
to mechanically interfere with achieving a good bond with another layer of
epoxy on top of the old layer.  This is why we have to chemically wash away
the amine blush with a solvent like acetone before applying another layer of
epoxy.  If we keep adding new epoxy on top of the old layers before the
amines have had sufficient time to diffuse to the surface; then, we can
avoid this intermediate surface preparation solvent wash step.  There will
still be reactive epoxy monomer units on the old surface and the fresh epoxy
will actually bond at the molecular level as well as mechanically interlock
or adhere to the old layer.  By adding fresh epoxy to this green epoxy, we
achieve a much stronger bond between the two layers than if we allow the
amine blush to develop & then solvent wash it away.

One of the nice properties of  West System epoxy is the relatively large
time window available for adding the next layer of fresh epoxy over the old
epoxy without having to worry about solvent washing the old surface.  At
room temperature, the so called "pot life" of the reacting epoxy will be
about 10 minutes using the 205 Fast Hardener.  From that point out to about
6 hours, fresh epoxy can be directly added over the old epoxy.  If you can
press your fingerprint into the green epoxy & not peel off any of the
relatively soft polymer when you remove your finger; then, the epoxy is
sufficiently cured to add the next layer.

So, to answer your question regarding the cure time for the fiberglass tape
flange on the G-10 patch, the best thing would be to laminate the G-10 patch
into the centerboard wall while the epoxy on the fiberglass tape is still
green as I've described above.

Epoxy tie layers should NEVER be allowed to fully cure before adding the 2nd
layer.  Epoxy tie layers should always be used to bond with the next layer
of epoxy while still green for the reasons explained above.

There, that's probably more than you ever wanted to know about the physical
chemistry of epoxy.  Aren't you sorry you asked?

Roger Pihlaja
S/V Dynamic Equilibrium


----- Original Message -----
From: "Lou Rosenberg" <lsr3 at MAIL.nyu.edu>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Tuesday, December 14, 2004 10:16 AM
Subject: Re: [Rhodes22-list] Re: CB trunk delam update


>   Roger,
>   Although I just did cut out a 10x5swath of  cb wall, there was a
> hole from the previous weekend where I was trying  to sand down the
> bulging area and cracked through the laminate.
>   Which made me aware that the laminate is of varing thicknesses
> throughout the trunk.  Actually I mis spoke as it has only been a
> week since that piece cracked not two wks.
>
>
>
> >Lou,
> >
> >Right off the top of my head, the only other cutting tool that comes to
mind
> >would be a small end mill that would be chucked up into the Dremel
Multitool
> >using the flex drive.  These cutters resemble the small burrs the dentist
> >uses on your teeth when you have a cavity.
>
>
>
>     I know what you are referring to and will investigate.  The flex
> drive unit itself is almost the width of  the narrow aft section ,  I
> ve mounted the flex drive on a strong pc of wood but will have to go
> to a steel rod next so I can  manipulate the blade into the tight
> corners.
>
>
>
> >If you just cut the bad FRP laminate out this past weekend, how can the
damp
> >cement behind it have been exposed to the atmosphere & drying out since
> >Thanksgiving?
> >
> >I'm not sure you understood my suggestion re the fiberglass tape.  Let me
> >take another run at explaining it.  Imagine you are going to build a
> >"flange" on the centerboard well side of the G-10 patch.  Up on the
> >workbench, where everything is easy to get at & nice to work with, you
> >laminate half the width of some fiberglass tape all around the perimeter
of
> >the G-10 patch.  In effect, you will have built a flange of fiberglass
tape
> >that extends beyond the edge of the G-10 patch.  You let the epoxy cure
to
> >the green state so the fiberglass tape is good & stuck down & starting to
> >get fairly stiff.  With half the width of the fiberglass tape stuck down
to
> >the G-10 patch, the fiberglass tape should behave nicely when you
laminate
> >it into position in the centerboard well.
>
>
>   GOT IT NOW!! thanks for the more descriptive picture.
>
> >
> >After putting an epoxy tie layer on the cement, I would fill in the rest
of
> >the void space between the G-10 patch and the cement with thickened
epoxy.
> >Press the XS epoxy out around the sides of the G-10 patch as you push the
> >G-10 patch home.  The epoxy squeezing out around the sides of the G-10
patch
> >will provide the adhesive you will need for laminating the fiberglass
tape
> >down to the centerboard well.  As long as you can get in there with some
> >sort of small tool, like say a tongue depresser, to wet out the
fiberglass
> >tape as the epoxy squeezes out underneath it; then, this technique should
> >work slick as a whistle!
>
>
>
> NEW QUESTION!!*************
>
>   Roger,
>    DOES the tie layer of epoxy has to be cured before I proceed with
> the G10 patch?
>    That part of epoxy/glass work is a bit confusing to me..
>   I will most likely be working in 40 - 45F temps under my tarp here
> so I am guessing it may take a while for that tie layer to even begin
> to  kick.   And I assume I need to place plastic under the glass tape
> so it will not adhere to the worktable wood under it?
>   sorry to be stating the obvious but you can see we are writing a
> book here on  "FIBRE GLASS LAY UP FOR DUMMIES!"
>
>
>    I think I might just sand down the other narrow aft bulges with my
> small grinding tool if I cannot cut it out as that will just take
> care of the bevel at the same time.
>
> >
> >Your idea of a piece of plastic & a board to hold the G-10 patch in
> >alignment while the epoxy cures will probably work OK.
>
>   Thanks!
>
> >
> >I like the idea of the fiberglass tape around the edge of the trunk vs. a
> >simple fillet of thickened epoxy.  With the fiberglass tape, there will
be
> >no concerns over strength & it will be absolutely waterproof.
>
>
>    Yeah thats gonna be  an easier job than the trunk but getting the
> bad FRP our from around the edge is still a big job!  I might see the
> voids come into play there and will be pokin around to see how deep
> my driver goes before I hit the hull... yipes7^$#@!!
>
>   thanks again Roger!
>   Lou
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