On Fri, 07 Nov 2014 08:48:37 +0700, John B. Slocomb
wrote:

On Thu, 06 Nov 2014 10:01:09 -0500, Ed Huntress
wrote:

On Thu, 06 Nov 2014 08:41:26 -0600, Richard
wrote:

On 11/6/2014 7:28 AM, Ed Huntress wrote:
John, this has been bugging the heck out of me -- I really hate it
when I don't have a precise answer, so I did some checking to see that
the story is on polyester gelcooat resin.

Quality commercial gelcoats are made from an entirely different resin
than laminating resin. Gelcoats are based on ISO/NPG
(Isophthalic/Neopentyl Glychol) resins, which are much stronger,
clearer, and more scratch-resistant than ordinary laminating resin,
which is orthophthalic resin. In the course of checking this out I
read that high-quality boatbuilders are switching to ISO/NPG resin for
laminating, too, but that wasn't true when I was involved with it, so
I don't know anything about that.

You no doubt know that common plastic laminating resins, particularly
epoxy, have no natural thixotropic properties. Spray or brush them on
a vertical surface, and they drool. Commerical polyester gelcoat
resins are loaded with thixotropic agents so you can lay on a fairly
thick, even coat that doesn't drool. If you cut all the way through
the gelcoat on a cheap boat, you may find that the gelcoat is very
uneven in thickness. Sometimes you can see this as ripples on the
outside surface, because shrinkage upon curing makes the
thick-and-thin stand out. But most builders conquered that problem
decades ago. In the US, they use commercial gelcoat resin from the
start.

FWIW, even ISO/NPG resin, compared to most other resin systems, is
weak and brittle. Without the glass fiber in it for reinforcement, a
good gelcoat resin is mechanically much weaker than the laminate
beneath it. But it does the job acceptably well. And, as you pointed
out, it's cheap. g


Excellent research, Ed.

After all this comes the actual mix.

The resin to catalyst ratio is a few drops of MEKP
(methylethelkeytoneperoxide) per pound of resin.
So how big is a drop?
Rule of thumb? Roughly 50 CCs per gallon?
So you can see that a little bit goes a LONG way.

What might be the results from variations in the mix?

Or how well mixed the mix is in the first place?

At what temperature is this all done?

Humidity?

Alignment of the stars and planets?

Lots of possible very small variations that have rather profound
effects in the end product.

It's certainly a lot more complex than we've discussed so far. At
Ranger, laminating resin was mixed, IIRC, 5 gallons at a time. There
was a schedule for desired gel times, and a time/temperature chart
that told you how much MEK would be required to meet those specs. They
used a little graduated cup to measure out the MEK and it was mixed
with a mechanical mixer.

As I mentioned, I started there as a furniture bonder. We mixed much
smaller quantities. We had a rule of thumb, but it was up to us to
make small adjustments. My partner, who spoke no English, liked to get
it over with fast. Some of his mixes actually bubbled and turned
brown. He was overjoyed. The people who bought our boats may not have
been. g

In terms of end properties, polyester resins actually are quite
flexible. You can get away with a pretty wide range of MEK
percentages. The biggest danger, when you're working with large
quantities and going slowly, is to use too little. The resin may never
harden if you do that.

If there's money involved and if that happens, sometimes they'll try
to rescue the job by brushing on some kind of accelerator. Maybe it's
MEK, but I don't think so. Sometimes it works, but it's really
problematic.

Way back when I was in the Air Force the sheetmetal guys used what was
likely polyester to make certain repairs on the B-52's (and probably
other airplanes but I was in a B-52 wing :-) and in those days it was
a three part mix, resin, hardener and an accelerator that they
referred to as "cobalt". As I remember it the resin/hardener ratio was
a constant and the cobalt was used for a longer or shorter hardening
time.

This wasn't my area of expertise but I've seen the Tin-benders mix
stuff that set the paper mixing cup on fire.

Rather impressive I thought :-)

Yeah, a hot mix can be pretty exciting. Someone who's used to mixing a
half-pint or less at a time, who mixes a gollon or two at the same
ratio they used for quick cures in small batches, is in for a real
surprise. g

You can't make hot mixes of polyesterin big quantities without risking
a fire and a complete loss of the batch. And a really hot mix can
start a fire even in much smaller quantities. If you have a paper cup
full and it catches on fire, that's 'way too hot to produce good
mechanical properties in the cured resin.

About the cobalt: Yes, and that's already pre-mixed in resins made for
laminating boats. I've seen contradictory words used in the
terminology, so I don't know which is really the catalyst, and which
is the accelerator. But the cobalt compound percentage is fixed in
ordinary resins. Military applications probably are much different.

I'm surprised they used polyester, though. Its adhesive properties are
pretty lousy, and a very small fraction of the adhesion of epoxy. And
if it's military, it sure isn't because they're trying to save money.
It may be the speed of cure that was required. Epoxies that cure at
room temperature, in the time range of typical polyesters, are not
strong epoxy resins, even if they set to an initial gel just as
quickly.

--
Ed Huntress