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Instead of cold, however, the actual
cross-linking reaction generates heat,
and lots of it. Temperatures close to
300° Fahrenheit (F) are common in a
small, 100-gram (g) volume. This is
called an exothermic reaction. A low
catalyst level generates less heat than
a high one, because the latter condition
speeds up the reaction and, in so
doing, gives off more heat. The hotter
the mix gets, the faster it cures; the
faster it cures, the hotter it gets. This
is why resin, if spread out in a thin
layer, will warm up only slightly,
whereas a larger mass—in a mixing
bucket, say, or in a thick lamination—
will get much hotter.
As the reactive sites on the resin
molecules become linked with the
monomers, the rate of cross-linking
tapers off, and the entire process
gradually calms down.
The ambient, or surrounding, temperature
also affects the reaction rate
to the extent that reactivity doubles
with every 10° Celsius (C) temperature
increase above 77°F. Thus, a catalyst
ratio that yields 30-minute gel
times at 77°F will give only 15-minute
gel times at 95°F.
These are rapid reaction rates, and
there are routine shop floor temperatures
above 100°F during much of the
year in a place like Florida, so how
can anyone possibly build glass boats
under those circumstances? They can,
and they do, and we''ll get to that in a
minute. First, though, let''s take up the
matter of a product data sheet (PDS).
Reading the Fine Print
Most builders tend to ignore the
PDS information for the catalysts and
resins they''re using. Too bad, because
these sheets are the best way to learn
the manufacturers'' recommendations
for handling their products, and they
are full of clues about potential problems.
The best stuff, and the scariest,
is in the fine print. Look for statements
such as: "Use of non-approved catalyst
will require requalification of resin
physical properties." In plain language,
that means if you change catalysts,
all bets are off about how this
particular resin will perform. Not all
catalysts give the same results, even if
their gel times are identical.
Figure 1—The presence of water in a
catalyst can have a dramatic effect on
resin viscosity—especially in gelcoats,
where the resulting porosity will quickly
lead to problems.
About the author:
In addition to being Professional
BoatBuilder''s technical
editor, Bruce Pfund acts
as a consultant to material
manufacturers and fabricators
on process controls, testing,
and vacuum bagging.
He has 25 years of experience
with marine composites
and boatbuilding.
Catalyst Selection
Certain types of resin—especially
vinylesters, or the isophthalic neopentyl
glycol (NPG) formulations
blended for gelcoats—are very sensitive
to catalyst specifications. Some
vinylesters will even foam violently
with incompatible catalysts. Similarly,
a porous gelcoat is a phenomenon often
traceable to a mismatched catalyst
containing water. The chart (Figure 1)
shows how little water it takes in a
catalyst (less than 1%) to double a
gelcoat''s viscosity.
There''s not much leeway here. Be
certain to use the recommended catalyst
with these sensitive resins. It''s also
good shop practice, in any case, to
abide by the manufacturer''s recommendations
with less expensive resins
as well.
Two key statements are often
tucked away somewhere in the text of
a PDS. One may read: "Recommended
working temperature range for this
resin is 65°F to 85°F," while the second
might say: "Do not use less than 1%
OCTOBER/NOVEMBER 7
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